U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1812

Eruptive History of Mammoth Mountain and 

its Mafic Periphery, California
Cover. Photograph of east side of dacitic Mammoth Mountain, showing steep former glacial 

headwall, several discrete lava domes, and numerous cleared ski runs. Complex edifice consists of 

24 separate extrusions, all erupted between 100 ka and 50 ka. Total relief in image ~970 m. Lincoln 

Peak is large dome at right. Part of residential district of Town of Mammoth Lakes at right center. See 

figure 1 for more details. 
Eruptive History of Mammoth Mountain and 

its Mafic Periphery, California

By Wes Hildreth and Judy Fierstein

Professional Paper 1812

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod/.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Hildreth, Wes, and Fierstein, Judy, 2016, Eruptive history of Mammoth Mountain and its mafic periphery, California: 

U.S. Geological Survey Professional Paper 1812, 128 p., 2 plates, scale 1:24,000,

http://dx.doi.org/10.3133/pp1812

ISSN 1044-9612 (print)

ISSN 2330-7102 (online)

ISBN 978-1-4113-4065-7
iii

Acknowledgments

Garniss Curtis introduced Wes Hildreth to Long Valley in 1960, Ian Carmichael motivated his 

work on the caldera-forming eruption in the 1970s, Gail Mahood and Colin Wilson kept him 

busy there for decades, and Dave Hill drew him into Mammoth Mountain investigations in 

2003. Our work built upon that of our late friend and colleague, Roy Bailey, whose 1:62,500-

scale geologic map provided a guide to our investigations. Hill and Margaret Mangan have ably 

overseen USGS research and monitoring at Long Valley, today as part of the California Volcano 

Observatory (http://volcanoes.usgs.gov/observatories/calvo/). Duane Champion led our paleomagnetic investigations, and Andy Calvert provided the new 40Ar/39Ar age determinations. We 

are grateful to Deanna Dulen and Wymond Eckhardt for engaging us in the geology, ecology, and 

history of Devils Postpile National Monument, which they have overseen with dedication and 

good judgment; to Clifford Mann for a wealth of information on the history and logistics of the 

Mammoth Mountain Ski Area; to Gene Suemnicht and Mike Sorey for advice on well logs and 

the geothermal system; to Brent Turrin and Dean Miller for contributions in the field and the lab; 

to John Gottwald and Scott Lee for access to the Arcularius and Alpers Ranch properties; and to 

Sally and Bob Drake of Old Mammoth for hospitality, companionship, and imagination and vigor 

on the mountainside. The manuscript was constructively reviewed by Charlie Bacon, Margaret 

Mangan, and Dave Hill.
iv

Contents

Acknowledgments........................................................................................................................................iii

Introduction.....................................................................................................................................................1

Recent Unrest.................................................................................................................................................1

Physiography and Access............................................................................................................................2

Settlement and Development ......................................................................................................................3

Previous Geological Work............................................................................................................................5

Methods...........................................................................................................................................................7

Geologic Setting.............................................................................................................................................7

Basement Rocks ...................................................................................................................................7

Neogene Volcanic Rocks ....................................................................................................................9

Long Valley Rhyolites .........................................................................................................................11

Precaldera Mafic and Dacitic Magmatism ...........................................................................11

Glass Mountain Rhyolites.........................................................................................................14

Climactic Eruption and Caldera Formation............................................................................14

Postcaldera Eruptive History of the Long Valley System....................................................15

Early Postcaldera Rhyolite ..............................................................................................15

Postcaldera Resurgent Uplift..........................................................................................16

Rhyodacite Dome 7403.....................................................................................................16

North-Central Rhyolite Chain..........................................................................................17

Southeastern Rhyolite Cluster........................................................................................17

West Moat Rhyolites ........................................................................................................17

Mammoth Mountain and Contemporaneous Peripheral Volcanism...................................................18

Mafic Periphery...................................................................................................................................18

230–150 ka...................................................................................................................................20

150–100 ka...................................................................................................................................20

100–50 ka.....................................................................................................................................20

50–0 ka.........................................................................................................................................23

Trachydacitic Focus at Mammoth Mountain .................................................................................24

Petrography ................................................................................................................................24

Eruptive Sequence ....................................................................................................................25

Vent Locations............................................................................................................................25

Postglacial Phreatic Activity....................................................................................................26

Diffuse CO2 Emissions ...............................................................................................................27

Postglacial Diamicts..................................................................................................................28

Volcanic Evidence for Glacial History ......................................................................................................29

Inyo Chain......................................................................................................................................................30

Faults ...........................................................................................................................................................31

Major Faults.........................................................................................................................................32

Composition of Eruptive Products.............................................................................................................33

Mammoth Mountain...........................................................................................................................35

Mafic Periphery...................................................................................................................................36

Neogene Periphery.............................................................................................................................37

Discussion.....................................................................................................................................................46
v

Mammoth System Versus Long Valley System..............................................................................47

Rhyolitic Versus Trachydacitic Culminations.................................................................................47

Mammoth System Magma Reservoir..............................................................................................49

Mammoth Mountain When Active...................................................................................................49

Volcano Hazards ..........................................................................................................................................50

Introduction to Description of Map Units ................................................................................................51

Description of Map Units............................................................................................................................54

Surficial Deposits................................................................................................................................54

Glacial Deposits ..................................................................................................................................61

Pre-Cenozoic Basement Rocks........................................................................................................66

Tertiary Volcanic Rocks .....................................................................................................................68

Quaternary Volcanic Rocks...............................................................................................................76

References Cited........................................................................................................................................114

Figures

1. East side of Mammoth Mountain, showing steep former glacial headwall and 

numerous cleared ski runs......................................................................................................2

2. Regional location map and volcanic setting of Mammoth Mountain and Long Valley 

Caldera .......................................................................................................................................3

3. Shaded relief map centered on Long Valley Caldera, showing Mammoth Mountain 

at caldera’s southwest margin ...............................................................................................5

4. Map of principal drainages, highways, and geographic names in and near 

geologic map area....................................................................................................................6

5. Simplified map of basement structure beneath late Pleistocene Mammoth 

Mountain....................................................................................................................................8

6. Well logs for selected drill holes within map area................................................................13

7. North side of Mammoth Mountain, viewed from caldera rim 5.5 km north-northwest 

of summit..................................................................................................................................18

8. Map showing distribution of volcanic vents exposed on and near Mammoth 

Mountain at southwest margin of Long Valley Caldera ...................................................19

9. Map of three major mafic lava-flow units in western half of Long Valley Caldera .........21

10. Photo of peripheral vent cluster 5–9 km south of Mammoth Mountain............................22

11. Devils Postpile, designated a National Monument in 1911, lies 4.5 km west of 

summit of Mammoth Mountain ............................................................................................23

12. The Chasm, glacially eroded notch below summit headwall of Mammoth Mountain, 

viewed toward southeast......................................................................................................26

13. Measured section of surficial deposits beneath Chair 11, ~300 m west-southwest 

of Main Lodge of Mammoth Mountain Ski Area ...............................................................27

Plates

[In pocket and available online at http://www.dx.doi.org/10.3133/pp1812]

1. Geologic Map

2. Correlation of Map Units and List of Map Units
vi

14. Schematic cross-section (cartoon) across summit of Mammoth Mountain, 

simplifying surface units and hypothesizing a permeable layer of buried rubble that 

drapes the granitic caldera wall against which the edifice was constructed.............28

15. Total alkalies (Na2O + K2O) versus SiO2 contents in weight percent for (A) Mammoth 

Mountain dome complex and (B) peripheral units of wider Mammoth magmatic 

system.......................................................................................................................................34

16. Plot of composition versus age for all dated units of the Mammoth system ...................35

17. Compositional plots for Mammoth Mountain.........................................................................39

18. Compositional plots for peripheral units of Mammoth magmatic system that crop 

out along south moat of Long Valley Caldera and in Lakes Basin..................................41

19. Compositional plots for peripheral units of Mammoth magmatic system that crop 

out along west moat of Long Valley Caldera......................................................................43

20. Compositional plots for eight peripheral units of Mammoth magmatic system that 

crop out along Middle Fork of San Joaquin River drainage system, along with 

data for 17 Tertiary units nearby ..........................................................................................45

21. Bare moraines of Laurel Creek and forested moraines of Sherwin Creek.......................65

22. Southeast side of Mammoth Mountain above glaciated trough floored by 

Twin Lakes ...............................................................................................................................85

23. Town of Mammoth Lakes and four adjacent late Pleistocene (~110–87 ka) lava 

domes: Units d81 and d61 consist of Mammoth Mountain trachydacite, whereas 

units rdc and rmk are Long Valley high-silica rhyolite .....................................................90

24. Trachydacite of Rainbow Falls, ~1 km downstream from Devils Postpile, showing abrupt change from columnar base (which rests on granite) to platy interior, 

which is characteristic of most outcrops...........................................................................91

25. Stratigraphic logs of pumice-fall deposit of unit rfp at Highway 203 roadcut near 

foot of Mammoth Mountain and at quarry near Cashbaugh Ranch, 17 km east .......107

26. Glass Creek flow (unit ric) draping caldera wall 3 km west of Highway 395 

at Crestview...........................................................................................................................109

Tables

1. Unit names and petrography ..................................................................................................123

2. 40Ar/39Ar ages for Mammoth Mountain and its mafic periphery .......................................126

3. Lengths of lava-flow aprons in Mammoth Lakes region....................................................128

Appendix

1. Chemical data for Mammoth Mountain and its mafic periphery, California. (Available 

online at http://dx.doi.org/10.3133/pp1812.)
Introduction

This report and accompanying geologic map portray 

the eruptive history of Mammoth Mountain and a surrounding array of contemporaneous volcanic units that erupted in 

its near periphery. The moderately alkaline Mammoth eruptive suite, basaltic to rhyodacitic, represents a discrete new 

magmatic system, less than 250,000 years old, that followed 

decline of the subalkaline rhyolitic system active beneath 

adjacent Long Valley Caldera since 2.2 Ma (Hildreth, 2004). 

The scattered vent array of the Mammoth system, 10 by 20 km 

wide, is unrelated to the rangefront fault zone, and its broad 

nonlinear footprint ignores both Long Valley Caldera and the 

younger Mono-Inyo rangefront vent alignment.

The Mammoth Lakes area of Mono County, owing to its 

spectacular alpine landscape, has become one of California’s 

busiest recreational playgrounds and a regional center of 

real estate development. The name applies to the town of 

Mammoth Lakes as well as to the cluster of lakes in a large 

cirque southwest of town that is now locally called the 

Lakes Basin. The town has spread around the eastern base 

of Mammoth Mountain, a late Pleistocene pile of silicic lava 

domes, and has locally expanded onto lower slopes of the 

mountain itself (fig. 1). Looming nearly 1,000 m above the 

downtown area, much of the 5-km-wide volcanic edifice has 

been laced with chair lifts, gondolas, ski runs, and bike paths 

by the Mammoth Mountain Ski Area, a corporate entity under 

permit from Inyo National Forest. In addition to skiing, longestablished, snowboarding and summertime mountain biking 

have recently become major activities. Tourism to Mammoth 

Lakes is estimated to be 1,300,000 visitors per winter and 

1,500,000 per summer. Some of America’s top long-distance 

runners also live and train in Mammoth Lakes, attracted by its 

elevation and its variety of challenging trails.

At the western base of Mammoth Mountain, along the 

canyon of the Middle Fork San Joaquin River, lies the Devils 

Postpile National Monument, a National Park Service enclave 

surrounded by extensive wilderness areas administered by 

the U.S. Forest Service. As many as 2,000 visitors per day 

enter the monument during the summer season. The area 

also contains several of the busiest trailheads in the Sierra 

Nevada, providing wilderness access for hikers, pack animals, 

mountaineers, and fishermen.

Many geographic names that appear in this report are 

informal despite having been in local use for decades. Most 

appear on maps distributed by the Town of Mammoth Lakes 

or the Mammoth Mountain Ski Area and can be found here on 

map figures 2–5, on several photo figures, and on the geologic 

map.

Recent Unrest

Geophysical unrest beneath Mammoth Mountain and in 

adjacent parts of the Sierra Nevada and Long Valley Caldera 

has generated concern among residents, stakeholders, and 

geoscientists since at least 1980, when four magnitude 6 

earthquakes shook the area. Extensive monitoring for three 

decades has documented numerous earthquake swarms, 

ground uplift and deformation, changes in hydrothermal 

systems, and emission of magmatic CO2 at several sites that 

semiencircle Mammoth Mountain (Evans and others, 2002; 

Farrar and others, 1995; Foulger and others, 2003, 2004; 

Gerlach and others, 2001; Hill, 1996, 2006; Hill and others, 

1990, 2003; Hill and Prejean, 2005; Julian and others, 1998; 

Langbein, 2003; Pitt and Hill, 1994; Pitt and others, 2002; 

Prejean and others, 2002, 2003; Rogie and others, 2001; 

Sorey and others, 1993, 1998; Tizzani and others, 2009). 

Such investigations led the U.S. Geological Survey, in cooperation with Federal, State, County, and local authorities, to 

prepare a response plan for any future episodes of unrest, 

calibrated for progressively more intense premonitory activity (Hill and others, 2002). 

Unrest specific to Mammoth Mountain, 1979–2003, 

was reviewed thoroughly by Hill and Prejean (2005), who 

included interpretation of the midcrustal mafic magma system 

inferred to drive deep long-period (LP) seismicity, CO2 exsolution, and fluid ascent into the brittle upper 4–6 km where 

swarms of brittle-failure earthquakes have been recurrent for 

more than three decades. The varied and persistent unrest also 

prompted scrutiny of volcanological data already in hand and 

critical review of our understanding of the region’s volcanic 

history (Hildreth, 2004). That review led us into intensified 

geological and laboratory investigations, the results of which 

are reported here.

Eruptive History of Mammoth Mountain and its 

Mafic Periphery, California

By Wes Hildreth and Judy Fierstein
2 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Physiography and Access 

Mammoth Mountain straddles the principal drainage 

divide (figs. 2–4) that separates the San Joaquin River system, 

which drains to the Pacific Ocean, from tributaries of the 

Owens River system, which is landlocked. The volcano grew 

atop one of the lowest saddles along the crest of the central 

Sierra Nevada, and the 2,800-m passes adjacent to the 3,369-m 

volcanic edifice remain among the lowest in the range. Here, 

the divide also marks the structural transition between the 

Basin and Range extensional region and the tectonically rigid 

Sierra Nevada microplate. Where adjoined by lavas of Mammoth Mountain, the divide is also the southwest topographic 

rim of Long Valley Caldera (although the ring-fault structural 

margin of the caldera lies buried ~5 km northeast). 

Major regional Highway 395, which extends from 

southern California to Reno (Nevada) and beyond, transects the eastern part of the map area. About 6 km east of 

the town of Mammoth Lakes, Highway 203 branches from 

395 and extends into town and an additional 9 km through 

the Mammoth Mountain Ski Area to Minaret Summit on the 

divide. From there, a narrow paved mountain road (open only 

in summer) descends into the San Joaquin River canyon for 

~11 km, reaching Devils Postpile National Monument and 

Reds Meadow Pack Station at road’s end. In town, Lake Mary 

Road and Old Mammoth Road branch from Highway 203 

and extend separately ~7 km southwest into the Lakes Basin, 

where they rejoin. Starting from the Lakes Basin, San Joaquin 

canyon, and Mammoth Mountain, numerous trails lead off into 

the John Muir and Ansel Adams Wilderness areas of the High 

Sierra. The Sierra has been called a “gentle wilderness,” and 

for experienced hikers this is generally true during the summer 

months, although black bears, cougars, precipitous terrain, and 

stream crossings do introduce risks.

Annual precipitation in downtown Mammoth Lakes 

(2,380 m elevation) averages ~59 cm (23 inches), and 

Bottomless

Pit

Dragons Back

flows

South Summit

Dome

Canyon Express

flow

Summit

Dome

Solitude

Dome

Facelift

Planèze Lincoln Peak

Dome

dbp

ddl

ddu

dsd

dsu

dfl rss dlp

rce

Figure 1. East side of Mammoth Mountain, showing steep former glacial headwall and numerous cleared ski runs. Several extrusive 

units are identified and are fully described in text. Dragons Back flows (ddl, ddu) and Solitude Dome (rss) are youngest eruptive units on 

edifice (~58–50 ka). 40Ar/39Ar ages include 64±7 ka for Lincoln Peak dome (dlp), 61.4±2 ka for Summit Dome (dsu), and 87±6 ka for South 

Summit Dome (dsd), one of oldest and largest units on edifice. Subhorizontal roadcut across center of image is Lake Mary Road, which 

links upper residential neighborhood of Mammoth Lakes (at right) to Lakes Basin (out of view to left). Housing development at right 

center is built on forested coulee of rhyodacitic unit rce (~80 ka). Total relief in image ~970 m. View is westward from moraine crest near 

Sherwin Creek.
Settlement and Development 3

average annual snowfall is ~533 cm (210 inches). Mammoth 

Mountain Ski Area (based 7 km farther west, at 2,700 m) 

has average annual snowfall of 884 cm (348 inches) and, in 

2005–06, it had a record snowfall of 1,468 cm (578 inches). 

Precipitation is greatest from November through April but 

scanty from June through September. East of the Sierra 

Nevada, precipitation falls off steeply to 19 cm (7.5 inches) 

at Benton and to only 13.5 cm (5.3 inches) at the Bishop airport. Location of Mammoth Mountain at a low point on the 

rangecrest and at the head of the major southwest-trending 

San Joaquin canyon system accounts for its high snowfall 

and unusually long skiing season, which commonly lasts 

from October through June. In downtown Mammoth Lakes, 

average daily maximum and minimum temperatures are 

78 oF (26 oC) and 46 oF (8 oC) in July and 39 oF (4 oC) and 

16 oF (–9 oC) in February.

Seven modest streams drain much of the map area east 

of the main divide. Largest is Mammoth Creek, which drains 

the Lakes Basin and the east flank of Mammoth Mountain. 

Sherwin, Laurel, and Convict Creeks drain northward from 

Sierran cirques and down the south wall of Long Valley 

Caldera into Long Valley. Deadman and Glass Creeks drain 

the northwest wall, flow generally eastward to their confluence near Crestview and join the Owens River near Big 

Springs. Dry Creek drains the north slope of Mammoth 

Mountain, flows (seasonally) northeastward for 18 km to join 

the Owens River northeast of Lookout Mountain. Although 

ephemeral today, Dry Creek cut gorges 40–50 m deep 

through the lavas of several units during the Pleistocene. 

West of the main divide, the southwest flank of Mammoth 

Mountain is drained steeply by Reds and Boundary Creeks, 

and the Pumice Butte area is drained more circuitously by 

Crater Creek; all three join the Middle Fork San Joaquin 

River. King Creek and Minaret Creek are western tributaries 

of the Middle Fork that cut through volcanic units at the west 

edge of the map area.

Total relief in the map area is ~1,500 m. The floor of Long 

Valley at the northeastern and southeastern margins of the map 

is lower in elevation than 2,150 m, and late Pleistocene lavas 

are preserved along the floor of the Middle Fork San Joaquin 

canyon to as low as 2,000 m. Bloody Mountain at the head 

of Laurel Creek stands 3,823 m high; the divide at the head 

of Sherwin Creek reaches 3,680 m; Mammoth Crest locally 

exceeds 3,500 m; Mammoth Mountain is 3,369 m; and San 

Joaquin Mountain on the principal divide reaches 3,536 m. 

Vegetation zones, accordingly, range from barren alpine 

through pine-fir forests to semiarid sagebrush steppe.

Settlement and Development

There are many cities on volcanoes (Auckland, 

Kagoshima, Napoli, Hilo), but few have expanded so fast 

and recently as Mammoth Lakes. Thousands of prospectors, 

miners, and camp followers first swarmed the Lakes Basin 

area in 1878–79 when gold was reported in the Mesozoic 

metavolcanic rocks of Red Mountain, stained reddish brown 

by oxidation of disseminated pyrite. The boom camps were 

largely abandoned after 1880, although small-scale operations 

persisted intermittently through the 1950s, as summarized 

by Rinehart and Ross (1964). Total production of Au and Ag 

from the several mines in the district probably did not exceed 

$1 million, but the ballyhoo to attract investors gave rise to 

the name “Mammoth Mine,” which spread with time to the 

Mammoth Rock, Lakes, Creek, and Mountain.

Homesteading began on Mammoth Meadow (also called 

Arcularius Meadow) by 1897, and the first hotel to accommodate tourists was built by 1905. Building a good road up 

Sherwin Grade in 1916 shortened to 2.5 hours the drive from 

Bishop to Mammoth Lakes, opening the way for construction 

of lodges and expansion of fishing, hiking, and pack-train 

recreation throughout the 1920s and 1930s. Inyo National 

Forest was established in 1907 and Devils Postpile National 

Monument in 1911. Mammoth Lakes opened its first post 

0 25 50 75 100 KILOMETERS

Bridgeport

Yosemite

Valley

Bishop

Big Pine

Fresno Independence

Tioga

Pass

Fish Lake Valley

White Mountains

AURORA

V.F.

ADOBE

HILLS

V.F.

LVC

KINGS

RIVER

V.F.

SALINE

RANGE

V.F.

BIG PINE

V.F.

SAN

JOAQUIN

V.F.

CHIATOVICH

CREEK

V.F. 

Mono

 Craters

Mammoth

Mountain

Glass

Mountain

Kennedy

Table

Sierra Nevada

drainage divide

NEVADA

CALIFORNIA

120°

38°

37°

119° 118°

Millerton

Lake

Mono

Lake

Owens River

Merced River

San Joaquin River

Kings

River

NV

CA

Study Area

Figure 2. Regional location map and volcanic setting of Mammoth 

Mountain and Long Valley Caldera (LVC). Fields in pink delimit 

areas having vents for Quaternary volcanic rocks. Shaded fields 

enclose vents for predominantly Pliocene volcanic fields (v.f.). 

Aurora field also contains a few Pleistocene vents (Lange and 

Carmichael, 1996), and San Joaquin field includes a few Miocene 

vents (Moore and Dodge, 1980). Late Miocene Kennedy Table and 

related intracanyon andesite lava-flow remnants are correlated 

with unit Tasj ~80 km upstream near Mammoth Mountain. Other 

Miocene and older volcanic rocks not shown.
395

395

203

120

158

119°15' 119°0' 118°45'

37°50' 37°40'

119°15'

37°50'

37°40'

119°' 118°45'

ML

Bloody Canyon

GrantLake

June

Lake

GemLake

ThousandIsland Lake

Glass Creek

Dry

Creek

Hot Creek

Convict Creek

Sherwin Cree

L

k

akes Basin

Fish

Creek

San Joaquin River Middle Fork

North Fork

Rush Creek

Lake

Crowley

O

wens Rive

McGee Ck

Dexter Canyon

Taylor Canyon

Mono Lake

GilbertPeak

CDHS

Benton

Crossing

Dome7403 

LookoutMountain

BaldMountain

DeerMountain

DeadmanCreek flow

Crestview

Deadman

Pass

ConePeak

CarsonPeak

San Joaquin

Mountain 

S O U T H M O A T

R e s u r g e n t D o m e

W E S T

M O A T

L o n g

V a l l e y

G l a

s

s

M o

u

n t a i n

B l a c k M t n

Adobe Valle

Granite

Mountain

CowtrackMountain

Pumice

Valley Mono

Craterschain

Wilson Butte

Obsidian flow

June Mountain

White Wing Mtn

Mammoth

Mountain

DevilsPostpile

Minaret

Summit

Iron Mtn

T h e M i n a r e t s

TomsPlace

BloodyMountain

 Mt Morrison

Laurel

Mountain

PumiceButte

LionPoint

McGee

Mountain

Mt Ritter

AgnewPass

KoipPeak

Mt Dana

Mt Gibbs

Mammoth Crest

rhc

bar

mjl

Tmcl

Tmel

Tmwc

120

TIOGA PASS RD

rwm

HSF

HCF

0
Previous Geological Work 5

office in 1923, but the town was not incorporated until 1984. 

In 1920–21, the Forest Service made the trails to Lakes Basin 

suitable for automobile traffic. In the 1930s and 1940s, the 

Forest Service supported growth of recreational skiing, leading to establishment of numerous rope tows along the Eastern 

Sierra, including some set up by the locally iconic entrepreneur Dave McCoy as early as 1946 along Minaret Road at the 

toe of Mammoth Mountain. 

In 1953, McCoy was awarded a 25-year permit to 

develop what became the Mammoth Mountain Ski Area 

(MMSA), building a warming hut at the site of the present 

Main Lodge and three rope tows in series to the top of North 

Knob. The first chair lift, Chair 1, was erected in 1955, and 

others were added or improved during succeeding decades, 

proceeding to Chair 27 in 1994. The gondola to the summit 

took three summers to construct, opening in 1967. By the 

1980s, MMSA had 40 snow “cats” to groom ski runs and 250 

ski instructors. In 2005, when McCoy (by then 90 years old) 

sold it to Starwood Capital Group, MMSA had 400 ski instructors and 3,000 permanent and seasonal employees. 

Growth of MMSA engendered a sustained boom in real 

estate development. The year-round population of Mammoth 

Lakes had been 26 in 1946 but grew to 7,100 by 2000 and 

8,234 in 2010. Tourism to Mammoth Lakes has expanded to 

reach an annual total of 2.8 million. There are about 9,000 

housing units in Mammoth Lakes and 4,800 rental units. 

Residential neighborhoods have crept as high as 2,630 m 

on the east flank of the volcano, and numerous cabins are 

at 2,600–2,800 m in the Lakes Basin at its southeastern toe. 

There are lodges at elevations of 2,450, 2,530, and 2,720 m 

around the base of the mountain as well as restaurants at 

McCoy Station (2,930 m) and on the summit (3,360 m). From 

the summit of Mammoth Mountain, it is 2.4 km to Mammoth 

Mountain Inn, 2.6 km to Tamarack Lodge, 3.1 km to Canyon 

Lodge, and 6 km to the downtown corner of Main Street and 

Old Mammoth Road. Nearly all of these facilities could be at 

risk were volcanic activity to resume on or near the edifice.

Previous Geological Work

Comprehensive geologic mapping of the current study 

area was first completed in the 1950s at a scale of 1:62,500 by 

Rinehart and Ross (1964) and Huber and Rinehart (1965a). At 

the same scale, Bailey (1989) adapted their regional mapping 

while elaborating in more detail the volcanic and structural 

history of Long Valley Caldera and its surroundings. A useful 

map of volcanic and surficial deposits in the Mammoth Lakes 

area is part of a fine thesis by Lipshie (1974). An excellent 

map of the volcanic rocks in and near Devils Postpile National 

Monument was prepared on behalf of the National Park 

Service as an undergraduate mapping project at Fresno State 

University (Clow and Collum, 1983).

Topical and preliminary studies in the area have been 

many and varied. Mayo (1930, 1934a) made a reconnaissance geologic map of a large region that included the 

Mammoth Lakes and Long Valley areas, and he coined the 

term “Mammoth embayment” for the rangefront reentrant 

now known to have been magnified by subsidence of Long 

Valley Caldera. Mayo (1934b) also drew attention to wavecut terraces and Pleistocene lake deposits produced by 

intracaldera Long Valley Lake. Mayo and others (1936) first 

described the alignment of Holocene vents (later called the 

Inyo Chain) and noted its proximity but ambiguous relationship to Sierran rangefront faults, an issue revisited by 

Bursik and others (2003). Erwin (1934) mapped the upper 

San Joaquin Basin and the Ritter Range, focusing on basement rocks but also contributing early observations on the 

volcanic rocks of Devils Postpile, Red Cones, and Pumice 

Butte. Benioff and Gutenberg (1939) investigated the young 

fissure called the “Earthquake Fault” near Mammoth Lakes. 

Chelikowski (1940), in the course of studying the early postcaldera rhyolites (our unit rer), also noted the sites of several 

mafic vents and the extent of Pleistocene intracaldera lake 

deposits; Chelikowski may have been first to propose explicitly that the “Mammoth embayment” is the collapsed caldera 

source of the Bishop Tuff, although Mayo (1935, p. 674–5; 

1937, p. 184) and Gilbert (1938, p. 1860) had already flirted 

with the idea. While concentrating on older volcanics to the 

north and east, Gilbert (1941) also made reconnaissance 

observations on the late Pleistocene volcanic rocks of the 

Mammoth Lakes area.

Matthes (1960) reconnoitered the geomorphology and 

glacial geology of the upper San Joaquin Basin. Kesseli 

(1941a, b) and Curry (1968, 1971) did the same for the 

Mammoth Lakes side of the main divide, both of them recognizing that lava flows near Mammoth Creek separated tills of 

Figure 3. (facing page) Shaded relief map centered on Long 

Valley Caldera, showing Mammoth Mountain at caldera’s 

southwest margin. Rugged terrain to west and south is glacially 

sculpted Sierra Nevada. Green line indicates Sierran drainage 

divide between Pacific slope and internally drained Great Basin. 

Selected peaks along main divide are Bloody Mountain (3,826 m), 

Mammoth Mountain (3,369 m), San Joaquin Mountain (3,536 m), 

Mount Gibbs (3,893 m), and Mount Dana (3,981 m). Other notable 

Sierran peaks are Mount Morrison (3,942 m), Iron Mountain 

(3,398 m), Mount Ritter (4,008 m), Carson Peak (3,325 m), and 

Koip Peak (3,951 m). Volcanic highs on caldera rim include Bald 

Mountain (2,775 m), Glass Mountain (3,390 m), and Cone Peak 

(3,094 m). High point on caldera’s resurgent uplift is Gilbert Peak 

(2,619 m). Surface level of Mono Lake fluctuates around 1,945 m, 

and that of Lake Crowley on the caldera floor around 2,062 m. 

Downtown Mammoth Lakes (ML) has an elevation of ~2,400 m. 

Four red stars indicate vents for important eruptive units outside 

the map area but described in the text. Additional abbreviations: 

CDHS, site of Casa Diablo Hot Springs, now replaced by a 

geothermal plant; HCF, Hilton Creek Fault; HSF, Hartley Springs 

Fault. Three nonglaciated Pleistocene lava flows prominent on 

image are basalt of Arcularius Ranch (bar), rhyolitic Hot Creek 

flow (rhc), and rhyolitic West Moat Coulee (rwm).
Sherwin Creek

Laurel Creek

Hot Creek

Little Hot 

Creek

GilbertPeak

CaRa

Dry Creek dome

MammothKnollsdome

Dome

2861

LookoutMountain

MammothMountain

MammothRock 

Crestview

CrystalCrag

White Wing

 Mountain

BannerPeak

MountRitter

MountMorrison

San JoaquinMountain 

DeadmanPass

37°45'

119°

AgnewPass

Minaret

Summit

town ofMammoth Lakes

Old Mammoth

Mammoth

Pass

Devils

Postpile Reds Meadow

Deer

Mountain

RedCones

Pumice

Butte

LionPoint

IronMountain

LaurelMountain

McGeeMountain

BloodyMountain

Inyocraters

geothermalplant 

fishhatchery

Chance

Ranch 

AIRPORT

WEST

MOAT

NORTH MOAT

SOUTH MOAT

The Minarets

Mammoth Crest

Hilt

0 1 2 3 4 5 KILOME

Hot

Creek

flow

Hartley Springs

Fault

LVEW

Glass Creek

EastPortal

Big

Springs

Clark

Lakes

Shadow

Creek

Middle Fork San Joaquin River

King

Creek

San Joaquin River

Lakes Basin

Crater Creek

WhHo

ConvictLake

DuckLake

RESURGENT

UPLIFT

LONG VALLE

SCENIC LOOP RD

Creek

Fish

Creek

Deer Creek

Deadman

Creek

Mammoth Creek

HWY 203

Dry Creek

Dry Creek foot of calder

foot of caldera wall

Regional Drainage Divide Regional Drainage Divide Minaret

Figure 4. Principal drainages, highways, and geographic names in and near geologic map area. Regional drainage divide, locally higher than separates streams flowing to Pacific Ocean via San Joaquin River from those flowing to Owens River system, which is ultimately landlocked wRange Province. Dashed line approximates topographic floor of Long Valley Caldera, equivalent to base of present-day topographic wall excephas been buried by Mammoth Mountain and several moraines. Structural wall (ring-fault zone) on which caldera subsidence took place is deeinboard, roughly along an arcuate trend linking vents indicated for Hot Creek flow, Dry Creek and Mammoth Knolls domes, and Lookout Mount2004). Selected vents are shown by stars. North-trending alignment of six vents north of Deer Mountain is Holocene Inyo Chain. The term "moaannular lowland that encircles the caldera's 10-km-wide resurgent uplift (outlined in purple), the center of which is marked by site of 3-km-deeLVEW (Long Valley Exploratory Well; McConnell and others, 1995). Geothermal plant is at site of former Casa Diablo Hot Springs.
Geologic Setting 7

different ages. Putnam (1949) mapped the June Lake district, 

just north of our map area, describing the north end of the 

Inyo Chain and relationships between mafic lavas northeast of 

June Lake and the glacial deposits that sandwich them. Huber 

(1981) summarized evidence for the timing and magnitude 

of uplift of the central Sierra Nevada, for incision of the San 

Joaquin River canyon, and for Pliocene closure of the local 

segment of the Sierran drainage divide.

Publications dealing with aspects of volcanic rocks investigated in the current report also include: Huber and Rinehart 

(1965b, 1967); Rinehart and Huber (1965), Bailey and others 

(1976), Chaudet (1986), Mankinen and others (1986), Vogel 

and others (1994), and Mahood and others (2010). Studies 

of such rocks that were principally geochemical-petrological 

include: Cousens (1996), Reid and others (1997), Heumann 

and Davies (1997), Heumann and others (2002), and Bailey 

(2004). Studies of the volcanology and petrology of the late 

Holocene Inyo Chain have been numerous: Eichelberger and 

others (1985, 1988), Gibson and Naney (1992), Mastin and 

Pollard (1988), Mastin (1991), Miller (1985), Nawotniak 

and Bursik (2010), Sampson and Cameron (1987), Sampson 

(1987), Varga and others (1990), Vogel and others (1989), and 

Wood (1977).

Investigations of the Long Valley hydrothermal system 

(Lachenbruch and others, 1976a; Sorey and others, 1978, 

1991, 1993, 1998; Sorey, 1985; Shevenell and others, 1987; 

Suemnicht and Varga, 1988; Farrar and others, 2003; Suemnicht and others, 2006) have made clear that the principal heat 

source lies not deep beneath the widespread surface manifestations in the south-central and southeastern parts of Long 

Valley Caldera but, instead, near the caldera’s western margin, 

within the area of younger volcanic rocks addressed here.

Methods

Sampling began in 1998 and 2003 in support of efforts at 

Stanford University to expand the Ar-geochronological database for Pleistocene volcanic rocks in the Mammoth Mountain 

area, culminating in the report by Mahood and others (2010). 

For an interdisciplinary workshop on volcanic processes in 

and near Long Valley Caldera, held at Mammoth Mountain in 

October 2003, Hildreth was asked to prepare a comprehensive 

volcanological overview that was presented orally and later 

published (Hildreth, 2004). The review led to recognition that 

Mammoth Mountain and its mafic periphery represent a new 

magmatic system, compositionally, temporally, and spatially 

separate from contiguous Long Valley Caldera. Field reconnaissance soon made clear that numerous eruptive units, both 

mafic and silicic, had been lumped in the course of previous 

mapping, despite being distinguishable in age, composition, 

and petrography. Recognition that a better understanding of 

the eruptive history would be readily achievable by means of 

field scrutiny and high-precision Ar geochronology nucleated 

the detailed remapping presented here at a scale of 1:24,000. 

From 2004 through 2012, 294 days of fieldwork were devoted 

to the project, generally in two-week intervals scattered each 

year between May and October.

Because most of the area is either glaciated and till-strewn 

or mantled with 1350 C.E. pumice-fall deposits, aerial photographs were of only minor utility, and our detailed mapping 

was done directly on USGS 1:24,000 topographic quadrangle 

maps (Bloody Mountain, Convict Lake, Crestview, Crystal 

Crag, Dexter Canyon, June Lake, Mammoth Mountain, Old 

Mammoth, and Whitmore Hot Springs). A hand-held GPS 

device was commonly useful in the woods. Much of the work 

was conducted as day hikes, but multinight backpack trips were 

made in the San Joaquin canyon and the Pumice Butte area. 

Most of the Quaternary map units were cored and their paleomagnetic directions measured in the USGS lab at Menlo Park, 

California, by D.E. Champion. The data were used to help correlate or distinguish scattered exposures. Mammoth Mountain 

Ski Area permitted vehicular access and paleomagnetic drilling 

of volcanic outcrops.

One or more thin sections of each volcanic map unit were 

examined microscopically. The observations are summarized 

in the unit descriptions and tabulated in table 1. All eruptive 

units are listed in table 1, along with the 3-letter unit labels by 

which each is identified on the geologic map and cited in the 

text. Most samples were analyzed chemically, and the data are 

listed in appendix 1. 

New radioisotopic ages were determined in the U.S. 

Geological Survey geochronology laboratory in Menlo 

Park, California, supervised by A.T. Calvert. Most samples 

in this study were dated using 40Ar/39Ar incremental heating techniques following methods described in Calvert and 

Lanphere (2006), and Muffler and others (2011), and the 

results are listed in table 2. Because most samples for which 

40Ar/39Ar ages reported by Mahood and others (2010) were 

collected with Hildreth’s participation, sites and stratigraphic 

relations cited here for those units are equally well known 

to us. Mahood and others (2010) showed that Mammoth 

Mountain biotite contains excess Ar and yields ages much 

older than those of coexisting feldspar, thus demonstrating 

that published middle Pleistocene K-Ar biotite ages (Bailey 

and others, 1976; Mankinen and others, 1986) for Mammoth 

Mountain are untenable.

Geologic Setting

Basement Rocks

Mammoth Mountain formed at the convergence of four 

significant structural boundaries (fig. 5). The volcanic edifice 

grew (1) within a scalloped reentrant of the topographic wall 

of 767-ka Long Valley Caldera (Bailey, 1989), and (2) it overlies the intrusive contact of a Late Cretaceous granite pluton 

against steeply foliated Mesozoic metavolcanic rocks (Huber 

and Rinehart, 1965a). The edifice also buries (3) a regional 
8 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

contact (Morgan and Rankin, 1972; Brook and others, 1974) 

between the metavolcanic belt and Paleozoic metasedimentary 

rocks of the Mount Morrison pendant (Rinehart and Ross, 

1964; Greene and Stevens, 2002). Finally, Mammoth Mountain overlies (4) the Rosy Finch shear zone segment of the 

Sierra Crest Shear Zone System (Tikoff and Teyssier, 1992; 

Greene and Schweickert, 1995; Tikoff and Greene, 1997), a 

north-northwest striking dextral transpressional structure of 

Late Cretaceous age that foliates both metavolcanic and granitic basement rocks nearly vertically. There may additionally 

be a right-stepping offset in the shear zone beneath the edifice 

(fig. 5), an inherited structure that might also help focus 

crustal transport of magmatic fluids.

Pre-Quaternary rocks beneath and near Mammoth 

Mountain can be summarized fourfold: Paleozoic metasedimentary rocks, Mesozoic metavolcanic rocks, Late Cretaceous 

granitoid plutons, and late Neogene volcanic rocks.

The Paleozoic metasedimentary rocks extend along the 

south rim of the caldera as what is called the Morrison block 

or Mount Morrison pendant, which was mapped in detail by 

Rinehart and Ross (1964) and Greene and Stevens (2002). 

The steeply dipping strata consist of Cambrian through 

Permian marine formations that extend as a northweststriking series of pendants and septa for ~115 km from 

Bishop Creek to Mono Lake. The sequence underlies the 

eastern part of Mammoth Mountain and western half of Long 

70

85

75 70 45

65

70

80

80 70

70

75

75

60

50

73

73

75

75

119°5’ 119°

37°40’

37°35’

0 1 2 3 4 5 KILOMETERS

Devils

Postpile

Summit MAMMOTH

MOUNTAIN

LP

ML

Bloody

Mtn 

LONG VALLEY CALDERA

RFSZ

L-C FAULT

Base of caldera wall

till-covered

-J

-D

M-P

M-P

Krv

Kmo

-D

Tv+ gr

ss-sh-ch-chert marble

sh+ch

sh

sh

Pb

Mb

Mb

Pld

Pml

*mb

mb

mb HL

RL

StL

SL

LG

brc

bmc

mcl

d81

bar d61 bmn

mss

Base of caldera wall

MMSA

75

-J

GLSZ

gondola

Fumarole

Tree-kill area

Vertical bedding or foliation

Strike and dip of bedding

Cretaceous shear zones

EXPLANATION

Figure 5. Simplified map of basement structure beneath late Pleistocene Mammoth Mountain. Two Late Cretaceous granitoid plutons 

(Kmo, Krv) and near-vertical belts of Paleozoic metasedimentary and Mesozoic metavolcanic strata strike northwest beneath the 

volcanic edifice. Gem Lake shear zone (GLSZ) and Rosy Finch shear zone (RFSZ), ductile belts of Late Cretaceous dextral transpression 

(Greene and Schweikert, 1995; Tikoff and Greene, 1997), also pass beneath Mammoth Mountain. The Laurel-Convict Fault (L-C) is a 

Permian-Triassic high-angle fault zone that juxtaposes sequences of lower and upper Paleozoic marine strata (Greene and others, 

1997). Mammoth Mountain edifice was built against topographic wall of 767-ka Long Valley Caldera, which may influence distribution of 

CO2 gas to tree-kill areas (10 gray patches) around western half of edifice (see fig. 14). Several vents peripheral to edifice are indicated 

by stars and identified by unit label (table 1). Abbreviations: HL, Horseshoe Lake; LG, Lake George; LP, Lincoln Peak; ML, downtown 

Mammoth Lakes; MMSA, Main Lodge of Mammoth Mountain Ski Area; RL, Reds Lake; SL, Shadow Lake; StL, Starkweather Lake. Late 

Paleozoic lithologies indicated: ch, calc-hornfels; sh, siliceous hornfels; ss, sandstone. Basement structure and simplified stratigraphy 

generalized from Greene and Stevens (2002), Huber and Rinehart (1965a), and Rinehart and Ross (1964).
Geologic Setting 9

Valley Caldera, as indicated by metasedimentary remnants 

on the north and west rims of the caldera (Bailey, 1989) as 

well as the major pendant that forms much of the south rim, 

from Mammoth Rock to the Hilton Creek Fault. The belt of 

near-vertical Paleozoic strata is ~10 km wide, represents a 

stratigraphic thickness of ~7 km, and was severely folded 

and faulted during the Permian and Mesozoic (Greene and 

Stevens, 2002). The rocks include siliceous, calcsilicate, and 

pelitic hornfels, argillite and slate, fine-to-coarse sandstone 

and quartzite, black chert, and limestone/marble, all metamorphosed in the hornblende-hornfels facies at ~200 MPa 

during pluton emplacement. The fine-grained clastic sediments are interpreted to have been deposited in deep water 

on the continental slope or rise, the chert hemipelagically, the 

sands on submarine fans, and the limestone on a carbonate 

platform (Stevens and Greene, 1999).

The Mesozoic metavolcanic rocks that strike beneath 

Mammoth Mountain form a steeply southwest-dipping 

sequence dominated by silicic pyroclastic rocks of Triassic and Jurassic age. South of Mammoth Mountain, they 

form a septum 2.5–4 km wide that extends 12 km southeast, 

separating the two large Cretaceous plutons described below 

(Rinehart and Ross, 1964). North of Mammoth Mountain, 

they form a 5-km-wide belt that extends more than 40 km 

northwest, separating the Paleozoic metasedimentary section 

on the east from the Cretaceous metavolcanic complex of the 

Ritter Range on the west (Huber and Rinehart, 1965a). Both 

belts are complexly stratified homoclines that young southwestward, having foliation and bedding that typically dip 

60o–80o SW. Lithologically, massive to stratified intermediateto-rhyolitic tuffs predominate, including a few quartz-bearing 

ignimbrites—some of them hundreds of meters thick. Stratified tuffs rich in crystals and lithic clasts include fallout, 

pyroclastic-flow, and reworked deposits. Also present are 

aphanitic and porphyritic lava flows, shallow intrusives, and 

subordinate interbeds and lenses of shale, sandstone, siltstone, 

calcsilicate hornfels, marble, and tuffaceous sedimentary 

rocks (Rinehart and Ross, 1964). Major ignimbrite sheets 

and plane-parallel bedding of intercalated sandy sediments 

and tuffs suggests subaerial and shallow subaqueous depositional environments of low relief. The entire sequence is 

in the hornblende-hornfels facies, locally schistose, and was 

deformed and metamorphosed before and during emplacement of the Late Cretaceous batholith. 

The east wall of upper Mammoth Creek from Arrowhead 

Lake to Lake Mary is a high ridge called Red Mountain (also 

known locally as Gold Mountain) that consists of hornfelsed 

Mesozoic metavolcanic rocks that are stained reddish 

brown by oxidation of disseminated pyrite. Associated 

weak mineralization, said to be principally vein-hosted 

gold and silver, has attracted intermittent prospecting 

and minor production (Rinehart and Ross, 1964). The 

northwest-trending upland surface of Red Mountain, 4 km 

long, 0.3–1 km wide, and 3,000–3,400 m in elevation, is an 

unglaciated remnant of a preglacial Sierran erosion surface 

of only modest relief. Called the “Solitude surface” by Curry 

(1971), Pleistocene erosion has isolated it 300 m above Lake 

Mary, 600 m above downtown Mammoth Lakes, and nearly 

1,000 m above the caldera floor. In addition to remnants of 

Pliocene basaltic units Tbmm and Tbrm that occupy shallow 

paleochannels atop the surface, there are pebbles, cobbles, 

and boulders of aphyric felsite and obsidian, hornblendebiotite dacite, and phenocryst-poor olivine-plagioclase basalt 

scattered across the surface, probably reflecting precaldera 

stream transport from a volcanic field that collapsed into 

Long Valley Caldera.

The Mono Creek Granite (Bateman, 1992a, b) is one 

of the great Late Cretaceous plutons of the Sierra Nevada, a 

50-km-long, northwest-elongate, medium-to-coarse-grained 

biotite granite, rich in alkali-feldspar megacrysts. Near 

Mammoth Mountain, the granite forms Mammoth Crest, 

Crystal Crag, the north wall of Fish Creek canyon below 

Pumice Butte, and both walls of the Middle Fork San Joaquin 

canyon, terminating against metavolcanic rocks 6 km north 

of Devils Postpile. The western half of Mammoth Mountain 

banks against and conceals a northeast-sloping granite wall 

that formed a steep scalloped alcove in the caldera margin. 

The scallop had probably been a middle Pleistocene cirque 

during Marine Isotope Stage 6 (MIS 6), before being filled 

by the trachydacite edifice in the late Pleistocene. Mammoth 

Mountain does not drape the caldera rim; it grew within the 

scallop and its lava flows bank against the granite wall. The 

granite crops out as far east as Lake George and Horseshoe 

Lake in Lakes Basin. West of Mammoth Mountain, most of 

the Tertiary and Quaternary volcanic units along the Middle 

Fork rest on Mono Creek Granite.

Two discrete plutons correlated lithologically (by Rinehart 

and Ross, 1964) with the Round Valley Peak Granodiorite 

(Bateman, 1965; 1992a, b) crop out on the caldera wall at 

the south edge of the map area. The western one, 3–5 km 

wide, encloses the canyon of Sherwin Creek, intrudes along 

the contact between the metavolcanic and metasedimentary 

sequences, and extends 15 km southeastward as far as Lee 

Lake. The smaller eastern pluton is roughly equant in plan 

view, only 3 km across, and intrudes the Paleozoic metasedimentary rocks on the north slope of Laurel Mountain. 

Both plutons consist of Late Cretaceous medium-grained, 

hornblende-biotite granodiorite.

Neogene Volcanic Rocks

Remnants of numerous Pliocene mafic and trachydacitic 

volcanic rocks (and a few of late Miocene age) survive within 

15 km north, west, and south of Mammoth Mountain (Huber 

and Rinehart, 1965a; Bailey, 1989, 2004). Northwest of Mammoth Mountain, 14 Pliocene vents were indicated by Bailey 

(1989) along the 12-km-long reach of the Sierran drainage 

divide between Minaret Summit and Gem Lake that includes 

Deadman Pass and San Joaquin Mountain (figs. 3, 4).

Across the northwest to north-central rim of Long Valley 

Caldera, Bailey mapped an additional 19 Pliocene mafic and 
10 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

intermediate precaldera vents. For these and other precaldera 

lavas east of the caldera, Bailey (2004, his fig. 3) compiled 

numerous ages that range from 4.5 to 2.3 Ma, predominantly 

3.7 to 2.7 Ma. Some of these have been reinvestigated by 

us, but many lie outside our map area, and all are far older 

than the largely late Pleistocene vent array around Mammoth 

Mountain. Judging from surviving circumcaldera outcrops, 

however, Pliocene volcanic rocks were probably distributed 

densely across what are now the western and northern parts 

of the caldera, prior to glaciation and caldera collapse. Such 

rocks are as thick as 466 m beneath the Long Valley Caldera 

floor just southeast of Inyo Craters, where they were penetrated by a geothermal well (fig. 6) at depths of 1,168 to 

1,634 m (Suemnicht, 1987), sandwiched between metasedimentary basement rocks and intracaldera Bishop Tuff. Sparse 

fragments of concealed intracaldera Pliocene basalt have also 

been brought up in the ejecta ring of unit rwm (~150 ka) adjacent to Mammoth Knolls. 

South and west of Mammoth Mountain, we have investigated the Neogene mafic and intermediate lavas in more 

detail (and we have dated some), because they are spatially 

associated with the Quaternary lavas and have not been clearly 

distinguished from them by previous investigators. 

Altogether, we have described 32 Neogene volcanic units 

around the periphery of the map area. They are summarized 

here in the following seven geographic groupings: 

1. Lakes Basin. Three units are olivine-rich remnants of 

mafic lava flows (units Tbmm, Tbrm, and Tmmc) on 

the cirque rim of Lakes Basin just south of Mammoth 

Mountain; one of them gave a K-Ar age of 3.15±0.1 Ma. 

Source vents are unidentified, and only small fractions of 

the flows are preserved.

2. Pumice Butte area. Four Neogene units (Tasj, Tacc, 

Tmcm, Tmsd) rest on Mesozoic granite in the Pumice 

Butte area, 8–10 km south of Mammoth Mountain. 

Unit Tasj is a thick stack of phenocryst-rich olivineclinopyroxene-plagioclase trachyandesite (9.16±0.02 

Ma), partly intrusive, that rims the canyon of the 

Middle Fork San Joaquin River and is preserved as 

intracanyon remnants as far as 80 km downstream, near 

Millerton Lake Reservoir, almost to the Great Valley. 

The second unit, Tacc, is a pair of phenocryst-poor, 

glacially scoured, trachyandesite domes (4.3±0.2 Ma) 

in the headwaters of Crater Creek. Third is unit Tmcm, 

a small remnant of olivine-rich basaltic trachyandesite 

(3.36±0.02 Ma) near Upper Crater Meadow. Last of the 

four, unit Tmsd, is a substantial remnant of an undated 

olivine-rich mafic shield on the divide between Deer 

and Fish Creeks, 4–5 km east of Pumice Butte.

3. High Sierra. High on the west wall of the Middle Fork 

are vents for four olivine-bearing mafic units, so glacially ravaged that all are inferred to be pre-Quaternary. 

Unit Tbld, resting on granite, consists of a pair of 

basaltic lava-flow remnants (3.35 Ma) and an agglutinated vent complex at Knoll 2836, 2.5 km west of Devils 

Postpile and ~7 km west of the summit of Mammoth 

Mountain. The other three are glaciated plugs of basaltic trachyandesite, each of which intrudes metavolcanic 

basement. Although all three lie just west of the map 

area (fig. 3), they were investigated, analyzed (appendix 

1), and described in order to eliminate them as possible 

vents for mafic lavas along the Middle Fork. Unit Tmwc

is a small plug (UTM 123/712) that forms Crag 10640 

(3,243 m), 700 m west of Castle Lake and 9.5 km northwest of the summit of Mammoth Mountain. Unit Tmcl

is a plug (UTM 099/731) that lies 300 m northwest of 

Cabin Lake on the south wall of upper Shadow Creek. 

Unit Tmel is a third plug (UTM 095/774), cropping out 

above the east shore of Emerald Lake (~1 km east of 

Thousand Island Lake) in the headwaters of the Middle 

Fork. None of these correlate with any of the intracanyon 

lava remnants preserved downstream within the map 

area. Our reconnaissance likewise excludes correlations 

with any of numerous mafic lava flows that vented and 

crop out near Agnew Pass (Huber and Rinehart, 1965a; 

Bailey, 1989) at the head of the Middle Fork.

4. Middle Fork. On and near the floor of the Middle Fork 

canyon southwest of Mammoth Mountain, glaciated 

remnants of mafic lava flows, rich in clinopyroxene and 

olivine (with or without plagioclase) and locally columnar, are preserved on both sides of the river. On the west 

wall, unit Tbtb is a thick stack of several conformable 

basaltic lava flows (The Buttresses) dated at 3.75±0.01 

Ma and having reversed paleomagnetic polarity. On 

the east wall, 2 km downstream, unit Tmcw is a pair 

of petrographically similar but slightly more evolved 

basaltic trachyandesite lava flows, undated but yielding a reversed paleomagnetic direction similar to that 

of unit Tbtb. Five kilometers still farther downstream 

on the lower southeast wall of the Middle Fork above 

its convergence with Fish Creek, a 300-m stack of ~20 

undated phenocryst-rich olivine-clinopyroxene basaltic 

lava flows (unit Tbpl) crops out extensively, just east of 

Pond Lily Lake. All three units rest directly on granite. 

Despite wide search, source vents have been located for 

none of them.

5. West Wall. The caldera’s west rim is capped by a thick 

stack of Pliocene mafic lava flows (unit Tbdp), which 

(along with overlying trachydacitic lavas and tuffs) 

extend ~11 km northwest along the San Joaquin Ridge 

as far as Agnew Pass and Gem Lake. The complex was 

mapped and described by Chaudet (1986), Bailey and 

others (1990), and Bailey (2004), but only its southern 

half is covered by the present map area. About 1 km 

south of Deadman Pass, the stack is locally as thick as 

450 m and apparently fills a paleochannel (cut in basement rocks) that crossed the modern Sierran drainage 
Geologic Setting 11

divide from northeast to southwest. The basal flow 

in the paleochannel yielded an 40Ar/39Ar age of 3.71 

Ma, and a flow capping the 450-m stack gave 3.28 Ma 

(table 2). It was proposed by Huber (1981) that this 

paleochannel linked the Neogene San Joaquin River 

to former tributaries that drained a catchment area 

within and east of the site of present-day Mono Basin. 

Extensional opening of Mono Basin and Adobe Valley 

and rangefront faulting younger than the Pliocene mafic 

lava flows created the Quaternary divide and beheaded 

the river system.

Overlying the mafic stack (unit Tbdp) along the crest 

of the San Joaquin Ridge is a set of hornblende-biotite 

trachydacite lava domes that includes San Joaquin 

Mountain and Two Teats (both just northwest of the 

map area; Bailey, 1989) as well as Dome 3321 (our 

unit Tdsd), which gave a K-Ar age of 2.8 Ma. Eruption of these domes was accompanied by emplacement 

of comagmatic pyroclastic-density-current deposits 

(units Tdpd, Tdsj) that are preserved extensively along 

the ridgecrest (Bailey and others, 1990). East of the 

ridgecrest, a chain of four more hornblende-biotite 

trachydacite domes (units Tdud, Td65, Tdww) extends 

4 km northeast toward White Wing Mountain, where 

the distal dome yielded a K-Ar age of 2.6 Ma. Although 

bulk compositions range from 64 to 71 percent SiO2, all 

7 domes and their associated pyroclastic deposits are 

lithologically similar in being crystal-rich and plagioclase-dominated. An additional Neogene eruptive unit 

on our map is a large scoria cone of clinopyroxene-olivine basaltic trachyandesite (unit Tmww) that lies high 

on the south slope of White Wing Mountain, southeast 

of the 2.6-Ma dome (unit Tdww). Just north of the map 

area, a few more Pliocene mafic and dacitic units crop 

out at June Mountain (Bailey, 1989). 

6. North Wall. Many Neogene volcanic units also crop out 

along and near the north wall of Long Valley Caldera 

(Bailey, 1989). We have mapped and described nine of 

them because of the need to distinguish them clearly 

from contiguous Quaternary volcanic units. Just west of 

the Obsidian Flow (unit ric), 2 km north of the caldera 

wall, glacially ravaged unit Tmwo includes a vent complex and lava-flow remnants of phenocryst-rich basaltic 

trachyandesite, which, though remarkably fresh, yielded 

a 40Ar/39Ar age of 3.15±0.03 Ma. A derivative lava flow 

that crops out along the canyon walls of Glass Creek 

1.7–3.0 km east of the vent is merely the shallow surface 

manifestation of a concealed stack of mafic lavas 300 m 

thick that rests on granitic basement and is recognized 

only in drill core (Eichelberger and others, 1985). 

Cone 8478, just north of Crestview, is a 300-m-high 

Pliocene edifice (unit Tb84; 3.44 Ma) around which 

varied mafic and andesitic lava flows (units Taec, 

Taww, and Tmcv) extend ~4 km southwest as well 

as south into the later site of Long Valley Caldera, 

whence they supplied abundant lithic fragments during ring-fracture eruption of the Bishop Tuff (Hildreth 

and Mahood, 1986). Just east of mafic Cone 8478, two 

more Pliocene domes of crystal-rich hornblende-biotite 

trachydacite (units Td83 [3.42 Ma] and Td78) crop out 

on the caldera wall; they are chemically similar (65–68 

percent SiO2) to parts of the large Bald Mountain dome 

complex just east of the map area. All these units are 

either lapped by the Bishop Tuff (unit rbt; 767±2 ka) or 

demonstrably older than it.

At the foot of the caldera’s north wall, between Big 

Springs and Alpers Canyon, densely welded crystalrich rhyolitic ignimbrite (unit Trac; 11.7 Ma) overlies 

severely weathered phenocryst-poor basalt (unit Tbbs), 

which is recognized only locally. The ignimbrite is a remnant of an extensive unit inferred to have erupted from a 

Miocene source far northeast of the map area.

7. South Rim. High on the south rim of Long Valley Caldera, a tiny precaldera remnant of a phenocryst-poor 

orthopyroxene-plagioclase-hornblende trachydacite 

lava flow, unit Tdlm (3.6 Ma), has been erosionally 

isolated at the top of the caldera wall. Its source vent is 

unknown but probably foundered with the caldera. Its 

similarity to some of the hornblende-bearing trachydacite domes on the north and west rims of the caldera 

supports the likelihood that a formerly continuous field 

of Pliocene mafic and dacitic lavas spanned what later 

became the western half of the caldera. The inference 

is strengthened by the presence of scattered blocks of 

hornblende-biotite dacite, aphyric rhyolite, and various 

mafic lavas strewn across the unglaciated low-relief 

Pliocene erosion surface cut on Mesozoic basement 

(units }mv and Krv) high above the southwest rim 

of the caldera. The source of such blocks can only 

have been a precaldera highland above the site of the 

present-day caldera.

The distribution of pre-Quaternary volcanic units and 

many more mapped by Bailey (1989) east of our map 

area shows that an extensive volcanic field (fig. 2) blanketed the later sites of Long Valley Caldera and Mammoth Mountain during the Pliocene. Starting in the early 

Quaternary, however, and for the next 2 million years 

(2.2 to 0.2 Ma), only rhyolite is known to have erupted in 

the Long Valley region.

Long Valley Rhyolites

Precaldera Mafic and Dacitic Magmatism

The Pliocene-to-Quaternary volcanic field that includes 

Long Valley and Mammoth Mountain (fig. 2) is part of an 

active regional transtensional zone at the Sierra Nevada–

Basin and Range transition. Its eruptive history overlaps in 

time with smaller mafic-to-silicic volcanic fields along this 
~bar ~ddc 

rwm aic mcl al rer rer rbt

Tv

|ms

rerrer

rer

|ms

bmn

mcv

s

mor

~bsr?

aic

al

rer

mcl

rerrer

rer

rer

rer

mdn, xr lava 

al

?

rer

} gr

rer

till (gst?)

xp mcl

xr bar ́

xp mnd

ol-pl lava (bed?)

till (gst?)

pe, Inyo pumice

xr lava

hb-pl lava

TD 92 m

SE 2500 m

CH-9

SE ~2525 m

Inyo-4

WEST MOAT 

SE 2450 m

IDFU, 44-16 

SE ~2400 m

OXY, OIP-1

SE 2357 m

PLV-2

SE 2593.5 m

PLV-1

SE 2291 m

DC-1

TD 810 m

TD 1799 m

TD 703 m

TD 635 m

TD 714 m

TD 236.5 m

050100150200

phreatic ejecta

Many mafic Pleistocene lava flows

Inyo ejecta

xm lava

mcl, vxp lavas

xp lava xp lavaxp lava(53.9 SiO2)

xp lava (52.6 SiO2)

xm lava (50.8 SiO2)

alluvial sand + gravel

Missing (no data)

sand + gravel

plutonic basement

mafic and intermediate lavas (466 m)

calc silicate  hornfels (>165 m)

gravelrhyolite xp lavas and tuffs (537 m)Bishop Tuff (253 m)

~5 flows

red soil

sandy

sediment

9-m scoria

3 xp flows3 mafic flows

22-m scoria

7 mafic flows7-m scoria

7-m breccia

49-m breccia~8 xp mafic flows

sand + gravelrhyolite vxp tuff

rhyolite vxp tuff

brecciated Quartzite

rhyolite vxp lavarhyolite vxp lava

rhyolite vxp lava + tuff

5-m sediment

319 m

131 ka

378 m

241 m

170 m

256 m

329 m

469 m

687 m

576 m

574 m

1375 

390 

18 m

350 m

512 m

664 m

915 m

1168 m1634 m

seven maficlava flows scoria scoria xp lava (53.9 SiO2) gravel, clay,

 pumice-bearing sand

pumice-rich tuff 

mafic tuff + breccia

mafic tuff + breccia

1

2

3

4

5

6

7

hb-bi xr tuffhb-bi xr tuff

vxp rhyolite tuff

hb-bi

 xr lava

tufftufftuff

lava

vxp rhyolite tuff

basal vitrophyre

sand+tuff

bedded vxp rhyolite tuff

vxp rhyolite lava flow

vxp rhyolite

massive vxp tuffvaguely layered vxp tuff

lava

avalanc breccia

meters

186 ka

230 ka

233 ka

A
321 m

260 m 269 m

88 m

47 m

82 m

107 m 

134 m

433 m

160 m

205 m

420 m 427 m

386 m

478 m

545 m

651 m

TD 663 m

TD 468 m

TD 491 m

TD 715 m

TD 488 m

TD 610 

TD 216 m

TD 98 m

amp amp

rer

rer

rer

rer

rwm

rer

rbt

rbt

rbt

mcl mcl

bfh

mkv?

rer

rer

rce gcd

gtl gtl

amp

aml

bsc

mcl

asr

gcd gcd

gcd gcd

gcd

gtl gtm

aml

rer

tuff

rer

rer

rer

rer

rer

~10 xp lava flows, each 1–7 m thick

~9 mafic lava

flows (b) 

~9 mafic lava

flows (b) 

1 bi-hb lava flow

scoria fall

till

till + gravel

till till

till

till till

sandsand

rhyolitic ash

till

xmm

xpb

hb-bi rhyolite pdc

till

till ?

till

mafic lava flows

scoriascoria

scoria

scoriascoria

scoria

SE 2750 m

HSL-1

SE 2430 m

18 M

SE 2464 m

OH-1

SE 2450 m

MLGRAP-1

SE 2395 m

MLGRAP-2

SE 2395 m

30 M

SE 2348 m

57-25

SE 2323 

BC 12-

SE 2374 m

SHADY REST RDO-8

fluvial

sediment

gravel

gravel + ss

pebbly ss, ashy silt, laminated clay

rhyolite-rich tuff and ss

b

lake

sediment

b

b

vxp rhyolite lava flow

pumice-bearing tuff

tuff

tuff

tuff

tuff

tuff

lava

hb-bi lava xp lava

 flow

~5 mafic xp lavas

(52% SiO2)

xp, rhyolite lava

(51% SiO2)

lava(50.5% SiO2)

(53% SiO2)

(51.5% SiO2)

colluvium

xp rhyolite  lava

basalt

xp rhyolite tuff + lava

vxp rhyolite pdc

may include ash-rich  lake silts 

rer lava TD 195 m

xp

gd

+

ms

xp

mafic

lavas

xp

mafic

lavas

xp

mafic

lavas

050100150200

METERS

SOUTHWEST MOAT

b

dacite

mafic lava  flows

B

Figure 6. Well logs for selected drill holes within map area. All drill sites are located on figure 8. Unit labels as in tables 1, 2, and in Description of Map Uidentified by Hildreth by microscopic examination and chemical analysis of cuttings or core. SE, surface elevation; TD, total depth of well. A, Wells in weuplift. Sources of well-log data: Inyo-4, Eichelberger and others, 1988; IDFU 44–16, Suemnicht, 1987; OXY OIP-1, geothermal corehole, 1984, logged and s1984, unpublished; PLV-1 and PLV-2, Benoit, 1984; DC-1, Sorey and others, 1978, logged and sampled by R.A. Bailey, 1973; CH-9, Sorey and others, 1978; M1988; LVEW, McConnell and others, 1995. For Inyo-4, 40Ar/39Ar ages (ka) for core samples are labeled in red and given in table 2. B, Wells in or near southwell-log data: HSL-1, cuttings taken by C. Farrar, logged by R.A. Bailey, 1996, unpublished; 18 M, cuttings logged for Mammoth Community Water DistricOH-1, cored for town of Mammoth Lakes, logged by E.V. Ciancanelli, 1991, unpublished; MLGRAP-1 and -2, Town of Mammoth Lakes geothermal corehopartially logged and sampled by R.A. Bailey, 1988; Shady Rest RDO-8, Wollenberg and others, 1987, partial log by R.A. Bailey, 1986; 30 M, water well for Mcollected by A.K. Ardell and T.D. McCarthy (MCWD), 2005, unpublished; 57–25, Ormat geothermal well, 2005, logged by R. Dunlap, J. Pollard, and K. Clark2007; BC 12–31, Mammoth Pacific geothermal corehole, 2002, Suemnicht and others, 2006, Fowler and others, 2011; 66–31, Mammoth Pacific geothermaand others, 2006, logged by R.A. Bailey, 1992, unpublished. Abbreviations: bi, biotite; hb, hornblende; ms, metasedimentary rocks; gd, granodiorite; ss, saand intermediate volcanic rocks; xr, crystal-rich; xm, crystal content intermediate; xp, crystal-poor; vxp, very crystal-poor (≤1 percent but not aphyric).
14 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

transition zone, as at Big Pine (Bateman, 1965) and Coso 

(Duffield and Bacon, 1981), respectively 50 and 110 km 

south-southeast of Long Valley. Around 4 Ma, decompressing 

upper mantle began leaking modest batches of mafic magma 

to the surface across a broad belt (Moore and Dodge, 1980) 

that extended from the later site of Long Valley for 40 km 

southwest into the Sierra Nevada and 30 km northeast into 

the Adobe Hills (fig. 2). Only close to Long Valley, which 

coincides with a major left-stepping offset in the Sierran 

rangefront, were the precaldera mafic products accompanied 

by eruptions of dacite (3.6–2.6 Ma). Precaldera magmatism 

close to Long Valley was reviewed by Bailey (2004), and 

all precaldera eruptive units included in our map area were 

summarized in the section above. Bailey noted that the mafic 

and dacitic magmas ceased erupting by ~2.5 Ma, somewhat 

before the onset of rhyolitic eruptions at 2.2 Ma. For ~2 

million years thereafter, an increasingly molten deep-crustal 

environment evidently favored entrapment of mantle-derived 

basaltic magma, which in turn amplified crustal melting, 

initiating a prolonged interval of rhyolitic magmatism (which 

included caldera collapse at 767 ka). Not until after 250 ka 

did mafic eruptions resume and, even then, their vents have 

been limited to an area west of the structural caldera (Hildreth, 2004), a new magmatic focus herein called the Mammoth system.

Glass Mountain Rhyolites

Glass Mountain (fig. 3) is a sprawling precaldera 

complex (2.2–0.79 Ma) of more than 60 overlapping eruptive units, exclusively high-silica rhyolite (76.6–77.7 percent 

SiO2), at the northeast periphery of Long Valley Caldera, 

where the rhyolites are exposed to a thickness greater than 

1,050 m on the caldera wall (Metz and Mahood, 1985, 1991; 

Metz and Bailey, 1993). At least 50 km3 of rhyolitic material 

is preserved as lava flows, domes, proximal tuff, and fans of 

mixed pyroclastic and reworked debris. Based on the pattern of intersection of caldera faults with the Glass Mountain 

edifice and the abundance of Glass Mountain lithic fragments 

in the Bishop Tuff (Hildreth and Mahood, 1986), we estimate 

that roughly 10–20 km3 more is downfaulted beneath the 

caldera floor. As many as 17 plinian and subplinian events 

distributed Glass Mountain fallout from the Pacific Ocean to 

Utah (Izett and others, 1988), though the total volume is hard 

to estimate from the scattered remnants. The aggregate thickness of fall units is several meters in pumice quarries 15–20 

km east of Glass Mountain (Sarna-Wojcicki and others, 2005), 

which, along with preservation of ash layers as far as 470 km 

south and 550 km east, suggests a dispersed volume of tephra 

in the range 50–100 km3 (equivalent to 25–50 km3 of hydrous 

high-silica-rhyolite magma). Total magma volume released by 

the Glass Mountain center may thus have been 100±20 km3, 

all of it high-silica rhyolite. A few individual fall units may 

have exceeded 10 km3, whereas only a few of the lavas are as 

voluminous as 1 km3. 

Compositional data for Glass Mountain lavas fall into 

two groups: (1) an older sequence (2.2–1.3 Ma) of at least 

24 eruptive units, all high-silica rhyolite but chemically 

varied, tapped sporadically at different stages of evolution, 

including some units considerably more evolved than the 

Bishop Tuff in trace-element contents; and (2) a younger 

sequence (1.2–0.79 Ma) of at least 35 eruptive units, all of 

them geochemically similar to the more evolved end of the 

compositionally zoned Bishop Tuff array, and presumably 

tapped from a common, by-then-integrated, expanding 

magma chamber (Metz and Mahood, 1991; Metz and Bailey, 

1993). All but one of the Glass Mountain rhyolites are 

crystal-poor; a dozen have 6–8 percent phenocrysts, but the 

great majority have 0–5 percent. Phenocryst abundances, 

species, and compositions (Metz, 1987) resemble those of 

the evolved, first-erupted part of the zoned Bishop Tuff, most 

units having quartz, sanidine, plagioclase, biotite, allanite, 

zircon, apatite, and Fe-Ti oxides. Glass Mountain lies just 

northeast of the present map area, but it represented a major 

component of the Long Valley rhyolitic system.

Climactic Eruption and Caldera Formation

The caldera-forming eruption of the Bishop Tuff at 

767 ka began as a plinian outburst along or near the Hilton 

Creek Fault in the south-central part of what soon became 

the caldera (Hildreth and Mahood, 1986). The roof of the 

growing chamber, then about 5 km deep (Wallace and others, 

1999), ultimately failed catastrophically, releasing more than 

600 km3 of gas-rich rhyolitic magma, compositionally and 

thermally zoned (Hildreth, 1979; Hildreth and Wilson, 2007), 

in a virtually continuous eruption about 6 days long (Wilson 

and Hildreth, 1997), thereby permitting 2–3 km subsidence 

of the roof, creating the caldera. About half the Bishop Tuff 

volume was emplaced radially as a set of sectorially distributed ignimbrite outflow sheets (unit rbt) along with concurrent 

plinian and coignimbrite fallout. The other half ponded inside 

the subsiding caldera, where welded intracaldera ignimbrite 

as thick as 1,500 m was subsequently buried by 500–800 m of 

postcaldera rhyolite tuffs, lavas, and sedimentary fill (Bailey, 

1989). Pumice clasts in the Bishop Tuff range from ~1 to 25 

weight percent phenocrysts and define compositional continua 

from 78 to 73 percent SiO2 and 2 to 600 ppm Ba (Hildreth, 

1979). The main suite of white pumice is accompanied by a 

sparse population of crystal-poor gray-to-black pumice that 

extends the ranges to 65 percent SiO2 and to 1,350 ppm Ba 

(Hildreth and Wilson, 2007). 

The 17 by 32 km depression called Long Valley Caldera 

owes its dimensions and the physiography of its walls to 

large-scale syneruptive slumping and to subsequent secular 

erosion. As inferred from gravity, drill holes, and vent distribution, the ring-fault zone—outlining the area of steep 

structural collapse of the cauldron (roof) block into the magma 

reservoir—encloses a subsided oval 12 by 22 km across. This 

encompasses an area of ~220 km2 or roughly 55 percent of the 
Geologic Setting 15

400-km2 floor of the topographic-hydrographic basin conventionally portrayed as Long Valley Caldera (fig. 3 of Hildreth, 

2004). The magma chamber had to be somewhat wider than 

the roof plate that sank into it. Nonetheless, clarity in definition of the structural caldera can help avoid misleading 

conceptualizations. For example, the Inyo rhyolites (unit ric) 

are commonly said to have invaded the caldera in 1350 C.E., 

and Mammoth Mountain is often said to straddle the caldera 

rim. In reality, both are extracaldera volcanoes, well outside 

the ring-fault zone. Despite proximity, Mammoth Mountain is 

compositionally and spatially independent of the Long Valley 

magma reservoir.

Seismic refraction profiles (Hill, 1976; Hill and others, 1985) and gravity models (Kane and others, 1976; Carle, 

1988) indicate that the caldera fill thickens substantially 

toward the north and east, as confirmed in drill holes that 

show the intracaldera Bishop Tuff thickening from 0.9–1.2 km 

centrally to ~1.5 km in the eastern third of the caldera (Bailey, 

1989). What fractions of the deepening may reflect precaldera 

topography, differential magma withdrawal, or tilting of the 

cauldron block during collapse remain uncertain. Where the 

caldera-forming eruption began, the Hilton Creek Fault system 

may have had several hundred meters of northeast-facing precaldera relief, and, where the caldera center is now, a left-stepping north-sloping ramp may have separated the en-echelon 

Hilton Creek and Hartley Springs Faults (figs. 3, 4). Greater 

subsidence in the north and east is also consistent with stratigraphic and petrological evidence that the final eruptive packages of the Bishop Tuff, which preferentially flowed toward 

those sectors, were withdrawn from deeper, hotter levels of the 

magma reservoir (Hildreth, 1979; Hildreth and Mahood, 1986; 

Wilson and Hildreth, 1997; Wallace and others, 1999; Hildreth 

and Wilson, 2007).

Postcaldera Eruptive History of the Long Valley 

System

Compositions of postcollapse eruptive units (760 to 

100 ka) that vented inside or near the caldera’s ring-fault zone 

are consistent with derivation from a reorganized, convectively mixed, and thermally restructured Long Valley magma 

reservoir. Compositions of silicic units on and near Mammoth 

Mountain, outside the ring-fault zone, are clearly distinct. 

Postcaldera units of Long Valley compositional affinity 

(Bailey, 1989; Hildreth, 2004) are (1) rhyodacite Dome 7403 

(east of map area), (2) the voluminous crystal-poor early postcaldera rhyolite (unit rer), and (3) three sets of what Bailey 

and others (1976) called "moat rhyolites"—the north-central 

rhyolite chain (unit rnc), the southeastern rhyolite cluster (unit 

rhc and associated lavas shown by Bailey, 1989, just outside 

our map area), and several rhyolites in the west moat (units 

rdc, rdm, rmk, and rwm). A caldera moat is a physiographic 

term for the annular trough in resurgent calderas (Smith and 

Bailey, 1968) that separates the intracaldera structural uplift 

from the caldera wall (see figure 3). Typically the site of 

postcaldera sedimentation and ring-fracture eruptions, a moat 

conceals and is broader than the structural zone of caldera ring 

faults.

Early Postcaldera Rhyolite 

What Bailey and others (1976) termed the “Early rhyolite” (our unit rer) consists of ~100 km3

 of fairly uniform, 

phenocryst-poor rhyolite (74–75 pecent SiO2; ~5.2 percent 

K2O) that erupted during the 100,000-year interval following 

caldera collapse. This enormous eruptive volume is comparable to that of precaldera Glass Mountain and an order of 

magnitude greater than the total of all subsequent Long Valley 

rhyolites erupted in the last 600,000 years. Released in many 

eruptions from several vents (Bailey, 1989), the Early rhyolite 

includes as many as 12 exposed lava flows and domes, a few 

more intersected by drilling, and a predominance of rhyolite 

tuffs—mostly nonwelded pyroclastic-flow deposits (subunits 

rtav and rtcd)—that make up as much as three-quarters of the 

assemblage. Eight lava flows (but no tuffs) were K-Ar dated 

(Mankinen and others, 1986), yielding ages from 751±16 ka to 

652±14 ka. The Early rhyolite extends far beyond its outcrop 

area, as documented in numerous wells (fig. 6; Suemnicht 

and Varga, 1988; Bailey, 1989; Sorey and others, 1991). At 

least 622 m thick near its center of outcrop, the Early rhyolite 

assemblage is still thicker than 350 m where deeply buried in 

the southeast moat and 230–537 m thick in wells in the west 

moat. 

Because no correlative layers of distal ash have been 

identified outside Long Valley, it seems likely that individual 

eruptions of Early rhyolite tephra, though numerous, were 

subplinian and moderate in volume. This might be interpreted 

to mean that the residual rhyolite magma had been relatively 

depleted in volatiles during the caldera-forming eruption. 

On the other hand, the observations that three-quarters of the 

assemblage is pyroclastic and that much of it is nearly aphyric 

but biotite-bearing suggest that the magma was not waterpoor. Perhaps the abundance of medium-scale Early rhyolite 

eruptions reflected relative ease of magma escape through the 

downfaulted and broken roof plate, thereby preventing by frequent eruptive release (and perhaps also by passive degassing) 

any postcaldera recurrence of severe gas overpressure.

Compositions of early postcaldera rhyolite lavas are 

similar in most respects to the last-erupted part of the zoned 

Bishop Tuff, except that Zr (170–200 ppm) and Ba (950–1,230 

ppm) significantly extend the range of Bishop zoning (fig. 4 of 

Hildreth, 2004). Phenocryst contents of Early rhyolite are low, 

only 0–2 percent, compared to 15–25 percent in the directly 

preceding, last-erupted part of the Bishop Tuff. The dominant 

minerals in the Bishop Tuff, sanidine and quartz, are absent 

in Early rhyolite, and the sparse crystals present appear to be 

new, euhedral to subhedral, and unresorbed. These include 

plagioclase, orthopyroxene, Fe-Ti oxides, and sparse biotite, 

as well as traces of apatite, zircon, and pyrrhotite (Bailey, 

1978, 1989). Processes that may have produced the contrast in 

crystal content between late Bishop Tuff and (compositionally 
16 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

similar) postcollapse Early rhyolite were discussed by Hildreth 

(2004). The greatly elevated Ba contents of Early rhyolites 

relative to the Bishop Tuff may be attributable, at least in part, 

to resorption of accumulative sanidine.

Postcaldera Resurgent Uplift

Intracaldera resurgence was defined by Smith and Bailey 

(1968) as structural uplift of a caldera floor by renewed 

buoyancy of the viscous magma remaining in the postcollapse 

reservoir (or replenishment thereof), but the term has sometimes been inappropriately applied to postcaldera eruptive 

activity that may or may not accompany such uplift. A resurgent dome is a structural and physiographic feature that need 

not be accompanied by extrusion of lava domes. Bailey and 

others (1976) argued that structural uplift at Long Valley was 

important during the 100,000-year interval of Early rhyolite 

eruptions and may have been largely over by ~600 ka. Some 

Early rhyolite vents may lie along or close to faults associated 

with the uplift, but the relative timing of individual eruptive 

units and offsets along long-active faults is seldom clear. The 

roughly circular area of uplift is ~10 km across (figs. 3, 4) and 

dips radially outward at 10–25o (as inferred by Bailey, 1989). 

Lookout Mountain (subunit rlm; 692±14 ka), a cratered shield 

of Early rhyolite in the northwest moat, is outside the uplift, 

as are thick sections of Early rhyolite concealed beneath other 

sectors of the moat. 

The high point of the uplift is Gilbert Peak (2,626 m 

elevation) and, if Early rhyolite thickness there is similar to 

that (622 m) in the Long Valley Exploratory Well (LVEW; 

fig. 6) ~2 km south, then the top of the subjacent Bishop Tuff 

would be at ~2,000 m, probably its maximum intracaldera 

elevation. This is 261 m higher than the top of the Bishop Tuff 

in the LVEW (a site downfaulted within the graben system 

atop the uplift), 469 m higher than in well 44-16 in the west 

moat, and 575 m higher than in well 66-29 in the southeast 

moat (Suemnicht and Varga, 1988; Bailey, 1989; McConnell 

and others, 1995). It seems likely that the (initially fluidized) 

primary surface of the Bishop Tuff that ponded inside the 

caldera was virtually horizontal at the close of its eruption. 

Therefore, even though part of the excess elevation of the 

resurgent structure owes to the constructional pile of proximal Early rhyolite, and part to differential compaction of the 

Bishop Tuff (which is much thicker in the low eastern third of 

the caldera; Hill, 1976; Bailey, 1989), doming of the top surface of the Bishop Tuff clearly demonstrates central uplift of 

at least 400 m. The net effect on surface elevation of transfer 

of ~100 km3 of Early rhyolite magma from chamber to surface 

is uncertain; unknowns include magma recharge volume and 

strength of the roof plate as well as relative aspect ratios of the 

magma domain withdrawn and the extrusive rhyolite pile and 

its moat-filling apron.

Much of the uplift appears to have taken place during 

Early rhyolite time, but what fraction has continued episodically is unquantified. As shown on our map, mafic lava flows 

as young as 180–125 ka are displaced by faults of the graben 

system that crosses the resurgent uplift. If most of the uplift 

had indeed been roughly contemporaneous with eruption of 

Early rhyolite, as proposed by Bailey (1989), then uplift of 

~400 m in ~100,000 years yields an average rate an order of 

magnitude smaller than that of the ~80 cm of renewed uplift 

that took place between 1980 and 2000 (Savage and Clark, 

1982; Langbein, 2003). 

Drill core from the LVEW (virtually central to the 

uplift) revealed within the 1.2-km-thick Bishop Tuff some 

ten phenocryst-poor intrusions, apparently sill-like and not 

present in wells drilled peripheral to the uplift (McConnell 

and others, 1995). Compositionally, the sills are Ba-rich 

rhyolite much like the Early rhyolite and, having a cumulative thickness of ~330 m, they could account for most of the 

resurgent uplift. For a 10-km-wide domical uplift of 400 m, 

the apparent volume of inflation is about 15.7 km3 or merely 

~16 percent of the volume of Early rhyolite erupted. The 

conventional model that a resurgent residual magma chamber buoyantly upwarps the cauldron block by reinflation or 

upward stoping is, therefore, for Long Valley, no more compelling than a model of central uplift by injection of shallow 

sills or laccoliths into the thick intracaldera fill (McConnell 

and others, 1995).

The fault system on the resurgent uplift is dominated 

by north-northwest trends essentially parallel to those of 

rangefront faults north and south of the caldera, but also 

parallel to the strike of steeply dipping structures and bedding in the metamorphic basement rocks (Rinehart and Ross, 

1964). Radial and dome-concentric faults are absent. Thus, 

the structure of the uplift may have been influenced more 

by regional precaldera structures and susceptibility of the 

shallow, subhorizontally layered Bishop Tuff to sill injection 

rather than by chamber-wide buoyancy. Location of the uplift 

nonetheless surely reflects the main locus of the reorganized postcaldera magma reservoir, which so voluminously 

supplied the Early rhyolite erupted. For the past 500,000 

years, however, despite paths provided by the complex fault 

system, there have been no more eruptions centered on the 

resurgent structure.

Rhyodacite Dome 7403 

In the northeast corner of the caldera floor (fig. 3), a 

small rhyodacite lava dome stands alone at the foot of the 

Glass Mountain scarp (Hildreth, 2004). Only 110 m high and 

~0.01 km3 in volume, this glassy, coarsely blocky to polygonally jointed dome, subcircular in plan, is not a downfaulted 

precaldera mass but an unequivocally postcaldera eruptive 

unit. It extruded in three pulses, the summit dome rising ~40 m 

higher than southwest and northwest lobes. Bailey (1989) suggested that its glassy exterior reflected eruption into a Pleistocene intracaldera lake, but we observe only angular indigenous 

scree, no rounded or exotic pebbles, and no wave-cut features 

on its widely micropumiceous surface. The dome is compositionally homogeneous (67.9 percent SiO2

 with ~1,500 ppm 

Ba and ~800 ppm Sr) but unique in being the only nonrhyolite 
Geologic Setting 17

postcaldera eruptive unit from the Long Valley reservoir. 

Unlike the trachydacites of similar SiO2 content at Mammoth 

Mountain, the dome is subalkaline (Hildreth, 2004). Containing abundant euhedral plagioclase (<1 mm) and hornblende 

needles as long as 1 mm, it contrasts with Glass Mountain, 

Bishop Tuff, and early postcaldera rhyolites in having hornblende (instead of biotite or pyroxene) as the principal mafic 

silicate phase. The lava is compositionally somewhat like the 

rare dacite pumice ejected toward the end of the Bishop Tuff 

eruption (Hildreth and Wilson, 2007), which was probably 

withdrawn from a level beneath the thick capping rhyolite in 

the magma reservoir. 

An attempt was made to determine its age by 40Ar/39Ar 

dating of clean euhedral plagioclase. Although sparse granitederived xenocrysts were avoided, excess Ar was indicated by 

an erratic incremental-fusion spectrum. An attempt to date 

its tiny acicular hornblende euhedra also failed. Although a 

few accordant steps in the middle of the Ar-release spectrum 

are consistent with a middle Pleistocene age, the absence of 

lakeshore features suggests extrusion after shrinkage of the 

intracaldera lake, probably after MIS 6. 

North-Central Rhyolite Chain

The earliest of three clusters of postresurgence rhyolites 

that Bailey (1989) termed moat rhyolite is a northwest-trending chain (unit rnc) that crosses the northeast sector of the 

Early rhyolite and is, therefore, not really in the caldera moat 

at all, nor aligned along the ring-fault zone. In contrast to the 

voluminous Early rhyolite, all components of unit rnc are phenocryst-rich and of small eruptive volume, totalling ~1 km3

. 

About 100 oC lower in Fe-Ti-oxide temperature than the 

nearly aphyric Early rhyolite (Bailey, 1978), the north-central 

rhyolites are rich in plagioclase, sanidine, quartz, hornblende, 

and biotite. SiO2 contents (73.9–75.3 percent) are similar to 

Early rhyolite, but K2O (4.7 percent) and Ba (680–715 ppm) 

contents are significantly lower, probably as a result of sanidine fractionation. Mafic enclaves (53.3–54.9 percent SiO2

) 

occur in lava flows and domes along the north-central chain, 

but none have been found in any of the younger (southeast and 

west moat) rhyolites. Four discrete components of unit rnc

yielded sanidine K-Ar ages of 527±12, 523±11, 505±15, and 

481±10 ka (Mankinen and others, 1986). Simon and others 

(2014) recently published sanidine ages of ~570 ka for one of 

the domes and a derivative lava flow, apparently lengthening 

the time interval spanned by eruptions of unit rnc.

Southeastern Rhyolite Cluster

After an apparent hiatus of ~120,000 years, another set 

of rhyolite lavas erupted over an interval as long as ~35,000 

years, from a cluster of six vents in the caldera’s low southeast moat (Bailey, 1989). Two of these vents arguably extend 

the trend of the north-central chain just discussed, and two 

lie along the ring-fault zone, the others inboard of it. The 

extensive (12 km2) Hot Creek flow (unit rhc) is the only one 

within our map area; the other five lie 0.5–5 km east of the 

Hot Creek flow (Bailey, 1989), where their limited outcrops 

are partly buried by caldera-lake sedimentary deposits. Four 

of the six lavas are crystal-poor (1–2 percent feldspars plus 

trace biotite, quartz, pyroxene, and Fe-Ti oxides), whereas 

two are phenocryst-rich hornblende-biotite rhyolites like 

the north-central chain. All six have 75–77 percent SiO2 and 

500–700 ppm Ba where fresh, although parts of several of 

them were altered by interaction with saline-alkaline lake 

water and sediment. 

Altogether, the lavas exposed add up to only ~1.5 km3, 

the Hot Creek flow being most of it. Five have been dated 

(Mankinen and others, 1986; Heumann, 1999; Simon and others. 2014). Sanidine yields ages of 362±8 ka and 333±10 ka 

for the two crystal-rich units. For crystal-poor units, sanidine 

gave ages of 329±23 ka and 329±3 ka, and for the Hot Creek 

flow, obsidian gave 288±31 ka and sanidine gave 333±2 ka. 

Whatever process promoted reversion to crystal-poor rhyolite 

at about 330 ka, it was unique in the post-Early rhyolite evolution of the Long Valley magma reservoir, because all other 

Long Valley rhyolites (527 to 100 ka) are rich in phenocrysts. 

Thermal rejuvenation by basalt injection is an unlikely explanation for these low-temperature rhyolites, because the crystalpoor units are marked by small euhedral phenocrysts and lack 

xenocrysts or partly resorbed relicts of an earlier generation. A 

more likely process, high-silica melt extraction from crystalrich felsic mush, was discussed by Hildreth (2004).

West Moat Rhyolites

After another hiatus of about 180,000 years, a third 

cluster of rhyolites erupted, this time west of the resurgent 

uplift. It consists of three modest lava domes (units rdm, rdc, 

and rmk) and the extensive West Moat Coulee (unit rwm; 8.5 

km2; as thick as 574 m; Benoit, 1984). The coulee represents 

~2 km3 of rhyolite lava but the domes add up to only ~1 km3

more. Three of the rhyolite vents are along the ring-fault 

zone, whereas that of Deer Mountain apparently lies ~2.5 km 

outboard of it. All four have been dated (Mankinen and others, 1986; Heumann, 1999; Mahood and others, 2010). Oldest 

is the West Moat Coulee, for which four ages range from 163 

to 147 ka, while the Dry Creek dome, the bilobate Mammoth 

Knolls dome, and the Deer Mountain dome yield overlapping 

ages in the range 115–97 ka. All are phenocryst-rich (15–30 

percent) low-temperature rhyolites containing plagioclase, 

sanidine, quartz, biotite, hornblende, Fe-Ti oxides, and trace 

amounts of zircon, apatite, allanite (± pyroxenes and pyrrhotite), but chemically they are of two kinds. Deer Mountain 

and the coulee have high Ba (700–860 ppm) and only 72–73 

percent SiO2, whereas the Dry Creek and Mammoth Knolls 

domes are more evolved, with 76–77 percent SiO2 and lower 

Ba (110–200 ppm). Like all Long Valley rhyolites, all four are 

subalkaline, in contrast to the Mammoth Mountain suite. That 

Deer Mountain and the West Moat Coulee are chemically 

less evolved than the voluminous Early rhyolite (unit rer; 
18 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

750–650 ka) suggests mafic input to the Long Valley magma 

reservoir during the long intervening interval.

Mammoth Mountain and Contemporaneous Peripheral Volcanism

Mafic Periphery

The central complex at Mammoth Mountain consists of 

25 distinguishable silicic units that erupted between 100 ka and 

50 ka, an interval ~50,000 years long. The silicic edifice (fig. 

7) accumulated near the center of a distributed field (fig. 8) of 

small monogenetic mafic and intermediate vents, the products 

of which we have divided into 38 separate map units (plus a 

few subunits). This peripheral monogenetic volcanic field, 

which consists predominantly of mildly alkalic basalt and its 

differentiates, produced a total eruptive volume in the range 

7–12 km3. Its activity began around 230 ka and has continued 

sporadically into the Holocene. Growth of the trachydacitic 

Mammoth Mountain edifice (~4±1 km3) thus began near the 

middle of the basalt-driven volcanic field, in time and in space. 

Vents for the volcanic units peripheral to Mammoth Mountain 

scatter within a well-defined field only 10 by 20 km across, 

evidently reflecting a new domain of mantle-derived melt 

ascent, compactly circumscribed and unrelated to the rangefront fault system. Before initiation of the Mammoth system, 

there had been no nonrhyolitic eruptions nearby since the Pliocene. Contemporaneous with the well-circumscribed Mammoth 

volcanic field, there has been no mafic or intermediate volcanism any closer than June Lake (20 km north of Mammoth 

Mountain) and Big Pine (85 km southeast). The Mammoth 

volcanic field is thus a new magmatic system that started up 

after 250 ka. It contrasts with the adjacent Long Valley system 

in that it lacks high-silica rhyolite and is consistently more 

alkaline (fig. 4 of Hildreth, 2004).

Vents for the 38 peripheral units are distributed within 

4.6 km west, 6.8 km northeast, 8.5 km north, and 9 km south 

Skyline Dome

Scottys Dome

Summit Dome

Hemlock

Ridge

granite

McCoy

Station

Main

Lodge

dlp dgr dms

rss ddu

dsd

dsu

dsu

dfl

dnk rmf

rce

bar

Scoria Cone

Tbdp

dsk

dwr

dwr

Figure 7. North side of Mammoth Mountain, viewed from caldera rim 5.5 km north-northwest of summit. Northwest-trending dome 

alignment supports crest, and several more silicic domes and flows are identified by unit label on northeast slope. All units visible 

erupted in interval 90–55 ka. In left foreground, red scoria cone, source of extensive lava flows of unit bar, erupted ~90 ka. Cliffs in 

near foreground are stack of Pliocene basalts of precaldera unit Tbdp. Summit of Mammoth Mountain is 650 m higher than MMSA 

Main Lodge, which can be seen at foot of edifice, just beyond scoria cone, where ski runs meet forest. Scottys Dome marks vent for 

extensive set of lava flows. Skyline Dome is widely acid-altered, as is notch to left of Summit Dome (see fig. 12).
Mammoth Mountain and Contemporaneous Peripheral Volcanism 19

119°05 ́ 119°

37°40 ́

37°35 ́

San Joaquin

River

0 1 2 3 4 5 KILOMETERS

CONTOUR INTERVAL 400 FEET

Middle Fork

Inyo chain

Mammoth Mountain trachydacites 

and alkalic rhyodacites

Basalts and trachyandesites 

Long Valley rhyolites (160–100 ka)

Drill sites mentioned in text

Crater

HL

LM

RM

DP

RF

MP

CC

MR

Casa Diablo

geothermal

plant

DC

GC

IC

ML

Mammoth

Mountain

Mammoth Crest

OD

Crestview

SR 57-25

18M 30M

OH-1 66-31

HSL-1

BC12-31

MLGRAP-1

MLGRAP-2

M-1

PLV-1

LVEW

PLV-2

DC-1

OIP-1

Inyo-4 44-16

CH-9

topographic margin

ring-fault zone

MS

MMSA

Dome

2861

Dome

2781 

The

Bluff

bar

bmn+

H

wy

395

EXPLANATION

8800

8800

10000

10000

10400

9600

9600

9200

9200

mss

bhl

bed

mcv

mnd

rnd

rcw

rcd

dnw

mdm

rdm

bcf

rdc

rwm

rmk

mkv

msd

mcl

bmc

a62

brc

apb

amp

Figure 8. Distribution of volcanic vents exposed on and near Mammoth Mountain at southwest margin of Long Valley Caldera. Mammoth 

Mountain edifice is colored green, and its exposed vents are indicated by black stars. Peripheral mafic and intermediate vents are 

represented by orange stars. Four subalkaline Long Valley rhyolites contemporaneous with the alkaline Mammoth system are indicated 

by red dots. Red triangles represent Holocene rhyolitic Inyo chain; those indicated by bold letters erupted in 1350 C.E. All other vents 

are identified by 3-letter unit labels (see tables 1 and 2). Logs for drill holes located on map given in figure 6. Abbreviations: CC, Crystal 

Crag; DC, Deadman Creek flow; DP, Devils Postpile; GC, Glass Creek flow; HL, Horseshoe Lake; IC, Inyo (phreatic) Craters; LM, Lookout 

Mountain; ML, downtown Mammoth Lakes; MP, Mammoth Pass; MR, Mammoth Rock; MS, Minaret Summit; OD, Obsidian Dome; RF, 

Rainbow Falls; RM, Reds Meadow.
20 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

of the summit of Mammoth Mountain (fig. 8). Some exposed 

vents are as close as 4.6 km southwest, 4.4 km north, 3 

km northeast, and 3 km southeast of the summit, a few of 

them right at the toe of the edifice. Vents for several other 

mafic and intermediate units are buried by the edifice itself, 

by adjacent surficial deposits, or by the cluster of 150- to 

87-ka rhyolite and trachydacite domes at the northeast toe 

of the edifice. A few additional vents indicated on the map 

of Bailey (1989) and on figure 4 of Hildreth (2004) have 

proven, on renewed investigation, to be either of Neogene 

age, rootless secondary vents on late Pleistocene lava fields, 

or imaginary. 

230–150 ka

The 38 peripheral units include 13 basalts, 16 mafic 

andesites, 6 andesites, and 3 dacites. Nine of them erupted 

before ~150 ka. The oldest exposed unit dated is unit mcl, an 

apron of lava flows that issued at the Canyon Lodge scoria 

cone and spread as far as 9 km northeast and 8 km east (fig. 9). 

Extensively concealed by younger lavas, its distal exposures 

have yielded ages as old as ~190 ka, but the most precise are 

175–180 ka. Beneath unit mcl, a window of undated lava-flow 

unit asr is exposed near Shady Rest Campground, and several 

wells in the west moat and in Mammoth Lakes township have 

penetrated units mcl, asr, and a few underlying mafic lava 

flows that are uncorrelated and nowhere exposed (fig. 6). In 

the Inyo-4 core hole near Inyo Craters (Eichelberger and others, 1988), a set of flows compositionally identical to unit mcl

(upper Group II of Vogel and others, 1994) yields a 40Ar/39Ar 

age of 186±2 ka. This is underlain by a 170-m-thick stack of 

~18 mafic lava flows (Groups II to IV of Vogel), nowhere 

exposed, for which three 40Ar/39Ar ages (table 2) are as old as 

233±4 ka. These unexposed mafic lavas below unit mcl are 

the oldest known products of the Mammoth volcanic system. 

Their vents are unknown but certainly buried in the west moat 

or beneath Mammoth Mountain.

On the south slopes of Knolls Vista (hill 2517), adjacent 

to downtown Mammoth Lakes, units asr and mcl are successively overlain by one lava flow of unit bsm (165±2 ka) 

and a set of fountain-fed flows of unit mkv (153±1 ka) from 

its nearby vent complex, against which lavas of unit msc

(154±2 ka) apparently banked. Vents for units bsm and msc

were buried by effusion of the rhyolitic West Moat Coulee 

(unit rwm; ~150 ka). Eastward along the south moat, exposures of lava-flow unit mmc (160±2 ka) are compositionally 

indistinguishable from those of vent complex mkv. Still farther east, near Chance Ranch, the stratigraphically lowest lava 

exposed in the south moat sequence (unit bsc) gave an age 

of 172±2 ka; it overlies units mcl and asr in water wells near 

town (fig. 6). In the west moat ~3 km north of the toe of Mammoth Mountain, a glacially scoured spatter cone (phenocrystpoor unit bcf) erupted at 164±2 ka.

The earliest of several mafic monogenetic eruptions south 

of Mammoth Mountain also took place during this opening 

interval. About 7 km south of the edifice, basaltic lava flows 

of unit bdc (155±2 ka), rich in olivine and plagioclase, form a 

platform that extends south from beneath andesitic Cone 2962 

(unit a62; 118±2 ka).

150–100 ka

The next 50,000-yr-long interval, from ~150 ka to ~100 

ka, was marked by eruption of Long Valley’s west moat rhyolites (units rwm, rdm, rdc, and rmk), the first phenocryst-rich 

Mammoth Mountain trachydacite (unit d81; 99±7 ka), and 9 

more peripheral mafic and andesitic units. Among these were 

a pair of 200-m-high andesite scoria cones, units apb (called 

Pumice Butte; 142±5 ka) and a62 (Cone 2962; 118±10 ka) 

that erupted 7–9 km south of Mammoth Mountain (fig. 10). A 

mafic scoria cone (unit mss; 121±2 ka) erupted through the 

bed of the Middle Fork San Joaquin River, which then incised 

its interior, exposing two dikes on the riverbank opposite Soda 

Springs Campground. 

In the south moat, phenocryst-rich lava-flow unit mlc

(130±1 ka) is exposed only at its rugged distal terminus, 

there sandwiched by basaltic lavas of units bsc and bfh; its 

source vent is buried by younger units and unlocated—but 

must lie many kilometers to the west. Also in the south moat, 

the flats around the geothermal plant and highway cloverleaf 

are floored by phenocryst-rich basaltic lavas of unit bcd

(125±2 ka), which appears to have erupted at spatter-cone 

unit bed (121±3 ka) in the west moat, near the toe of Mammoth Mountain, prior to extrusion of three lava domes (units 

d81, rmk, and d61) that now separate the cone from exposures of its apron lavas.

Also in the west moat, unit aic (131±1 ka) is a stack of 

trachyandesitic lavas that crops out near and on the walls of 

South Inyo Crater, but its source vent is buried by younger 

deposits and remains unlocated. An eroded mafic scoria cone 

(unit mdm) nearby is undated but apparently older than the 

adjacent Deer Mountain rhyolite dome (unit rdm; 101±8 ka). 

A fourth west moat unit that erupted during this time interval 

is a glacially scoured coulee of weakly porphyritic pyroxeneplagioclase trachydacite (unit ddc; 103±9 ka) that appears to 

have issued from a vent subsequently buried by the Mammoth 

Mountain edifice.

100–50 ka

The next 50,000-yr-long interval included incremental 

construction of the Mammoth Mountain edifice (~87 ka to 

~50 ka) and eruption of 14 peripheral units—5 basalts, 6 

mafic andesites, 2 andesites, and 1 dacite. Two of the oldest 

in this interval are extensive lava flows on opposite sides of 

Mammoth Mountain—unit bsr (99±1 ka) in the south moat 

and unit drf (98±1 ka) along the Middle Fork San Joaquin 

River; both may have erupted from vents now covered by 

Mammoth Mountain. Both were subsequently overrun by 
aprons of phenocryst-poor lava flows of unit amp (97±1 ka), 

which erupted on the drainage divide at the later site of Mammoth Mountain and poured both eastward along Mammoth 

Creek and westward into the Middle Fork. Undated unit aml

is a glaciated remnant of the phenocryst-poor orthopyroxenehornblende silicic andesite at McLeod Lake, just south of the 

Mammoth Mountain edifice; its vent is not exposed, but its 

identification directly beneath unit amp in wells drilled near 

Horseshoe Lake and downtown Mammoth Lakes suggests 

that it is not much older than 97 ka (fig. 6). The colonnade 

called Devils Postpile (fig. 11) is part of an intracanyon mafic 

lava flow (unit mdp) along the floor of the Middle Fork, 

where it overlies units mss, drf, and amp and yielded an age 

of 82±1 ka.

A basaltic scoria cone (unit bmc) atop the Sierran drainage divide at Mammoth Crest, above Lakes Basin and only 

4 km south of the summit of Mammoth Mountain, yielded an 

age of 92.7±2.4 ka. It was the source of an apron of lava flows 

(unit bfh) rich in large plagioclase phenocrysts that extended 

along Mammoth Creek as far as 18 km (fig. 9) but is exposed 

for only its distal 6 km (though also identified in intermediate water wells). Although the apron was glacially eroded 

and covered by till and younger lavas for the first 12 km 

away from the cone, the distal flow near the Hot Creek fish 

Mammoth Mountain and Contemporaneous Peripheral Volcanism 21

X

Inyo 4

OIP-1

119° 118°50’

37°45’

37°40’

0 1 2 3 4 5 KILOMETERS

Early rhyolite

750-650 ka

(resurgently uplifted)

North Dome

525 ka

Lookout

Mountain

690 ka

EP

~90 ka

92 ka

175 ka

Hot Creek flow

300 ka

Mammoth

Mountain

90-55 ka

IC

MS

DP

GP

FH

MMSA

West

Moat

Coulee

150 ka

HWY 395

HWY 395

Pliocene

tributary

Owens River

Base of caldera wall

Base of caldera wall

Base of caldera wall

Lakes Basin

ML2 66-31

ML1

30M

Middle 

Fork

bar

mcl

mcl

mcl

bar

bfh

bfh

Figure 9. Map of three major mafic lava-flow units in western half of Long Valley Caldera. Because each is extensively covered by 

younger volcanic and glacial deposits, this diagram is provided to illustrate their original continuity. Scoria cone at source of each is 

shown by red star: unit mcl (~175 ka) in blue, bfh (~92 ka) in gray, and bar (~90 ka) in green. Colors are strong where unit is exposed but 

pale where unit is covered or eroded away. Small two-colored rectangle north of mcl scoria cone is north-dipping slab of mafic lavas 

(units mcl and bar) uplifted on roof of trachydacite Dome 2861 (~87 ka). Six wells are shown in which one or more of the three mafic 

units has been identified in drill cores (see fig. 6). Long Valley rhyolites outlined in orange predate the three mafic units, except that 

unit mcl is older than West Moat Coulee (~150 ka). Trachydacitic Mammoth Mountain is largely younger than the three, but its growth 

began quasi-contemporaneously with two of them. On west rim of caldera, Pliocene tributary of Middle Fork San Joaquin River (incised 

through pre-Cenozoic rocks) was filled and permanently blocked by 3.7-to-3.2-Ma stack of basaltic lava flows. Abbreviations: DP, 

Devils Postpile; EP, East Portal; FH, Hot Creek Fish Hatchery; GP, Casa Diablo Geothermal Plant; IC, Inyo Craters; MMSA, Main Lodge of 

Mammoth Mountain Ski Area; MS, Minaret Summit, road crossing from Mammoth Lakes township to Middle Fork.
22 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

hatchery is chemically, petrographically, and paleomagnetically identical to the cone, and it yielded an indistinguishable 

age—92.4±2.4 ka.

Another basaltic scoria cone is well preserved on the 

caldera wall 2 km northeast of Minaret Summit and 4 km 

north of Mammoth Mountain (fig. 7). It was the source of 

two distinctive aprons of lava flows in the west moat, unit 

bmn (87±7 ka) and the overlying phenocryst-richer unit 

bar, which has given several ages, the most precise of which 

is 88±5 ka. A slab of unit bar was uplifted on the roof of 

trachydacite Dome 2861 (unit d61; 87±2 ka). In addition to 

spreading across much of the southwest moat, flows of unit 

bar extended 13 km northeast to bank against the north wall 

of the caldera, thence 10 km eastward to a terminus along the 

Owens River (fig. 9).

A multilobate flow field of mafic lavas extends 10 km 

northeast from the west moat near Dome 2861 to the north 

wall of the caldera near Big Springs. The southern half is 

exposed as a series of windows through late Pleistocene till 

of unit gdc and, beyond the glacial limit, the northern half 

branches into three lobes, the central one thickening and widening into a steep-sided coulee. We tried to separate the lobate 

field into two map units (mnd and mor)—variants that range 

in phenocryst content from aphyric to 4–5 percent plagioclase—but we found gradational areas and numerous rootless 

vents and fissures that complicate the outflow sequence. The 

mapped variants turned out to have overlapping ranges of 

composition (51.2–54.2 percent SiO2) and indistinguishable 

paleomagnetic directions. The coulee, unit mor, gave an age 

of 66±2 ka. Map unit mns is a poorly exposed patch of scoria 

lapilli and bombs as big as 50 cm that is largely covered by 

glacial and colluvial deposits in the saddle between trachydacitic Domes 2861 and 2781; similar to units mor and mnd

compositionally and mineralogically, the scoria may mark a 

concealed vent for those lava flows.

Unit msj is a small window of moderately porphyritic 

(clinopyroxene-plagioclase-olivine), subalkaline basaltic 

andesite lava that crops out directly beneath unit mnd at the 

Fish Creek

a62

apb

Tacc Tacc

Tasj

brc

brc

Middle Fork

Figure 10. Peripheral vent cluster 5–9 km south of Mammoth Mountain. Pair of small red scoria cones (unit brc) erupted in early 

Holocene. Pair of large forested scoria cones (units a62 and apb) erupted in interval 110–150 ka. Pair of off-white lava domes (unit 

Tacc) erupted in Pliocene. Low-relief forested foreground is Mammoth Pass, which is floored by lava-flow apron of unit amp (97 ka). 

Confluence of glacial canyons of Fish Creek and Middle Fork San Joaquin River is hidden behind Pumice Butte (apb). Canyon walls

and steep nose at left center (northwest end of Mammoth Crest) are Mesozoic granite. View is southward from summit of Mammoth 

Mountain. 
north end of the complex graben followed by Highway 395. 

Its source vent is unknown, and it crops out nowhere else. Nor 

has a source vent has been recognized for a lithic-rich mafic 

scoria-fall deposit (unit mic), several meters thick and exposed 

only on the walls of the Inyo Craters, where it rests directly 

atop unrelated unit aic (131±1 ka). Scoriae are mostly lapilli 

(but as big as 20 cm), contain 54.3–55.8 percent SiO2, and 

are overlain by Mono Craters ashfall, then by magmatic and 

phreatic ejecta of the 1350 C.E. Inyo episode. Unit mic may 

be a phreatomagmatic or purely phreatic deposit; its vent was 

probably nearby but not identical to Inyo Craters.

50–0 ka

Subsequent to growth of the Mammoth Mountain edifice, 

there have been four mafic eruptions in its periphery as well as 

extrusion of a chain of five trachydacite lavas in the caldera’s 

northwest moat. One is a glaciated sheet of phenocryst-rich 

olivine-clinopyroxene-plagioclase basaltic lava (unit bhl; 

31±1 ka) that banked against the southeast toe of Mammoth 

Mountain near Horseshoe Lake. Remnants floor part of Lakes 

Basin, and abundant erratics of unit bhl occur in all of the 

numerous nested moraines along Mammoth Creek (unit glb), 

demonstrating that the till was deposited during MIS 2.

A large mafic scoria cone just west of Crater Flat and 6 

km north of the summit of Mammoth Mountain produced a 

lava-flow apron (unt mcv; 31±2 ka, 34±1 ka) that extends 9 

km northeast and banks against the caldera wall at Crestview. 

All but the distal 3 km of the apron lavas are concealed by 

younger lavas and surficial deposits.

A northwest-trending chain of three glacially ravaged 

remnants of comagmatic mafic scoria, agglutinate, and 

fountain-fed lava (unit mdn) is the product of aligned vents, 

Mammoth Mountain and Contemporaneous Peripheral Volcanism 23

Figure 11. Devils Postpile, designated a National Monument in 1911, lies 4.5 km west of summit of Mammoth Mountain. Colonnade of 

phenocryst-rich basaltic trachyandesite, in this photo as high as 20 m, consists of prismatic columns 50–100 cm thick. Mapped as unit 

mdp, single intracanyon lava flow is altogether as thick as 110 m and may have taken 20–25 years for fractures to advance inward while 

it cooled and solidified. Eruption age 82±1 ka.
24 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

probably dike-fed, on the caldera wall, 2 km northeast of 

Deadman Pass and 8 km north of the summit of Mammoth 

Mountain. Banked against the toe of Lookout Mountain 9 km 

northeast, a surficial deposit (2 km beyond the glacial limit) 

consists entirely of angular clasts of unit mdn. One of many 

meter-sized blocks in the deposit gave an age of 17±1 ka, and 

a dense vent-proximal lava gave an age of 16±2 ka.

Red Cones is a pair of 120-m-high basaltic scoria cones, 

3–4 km southwest of Mammoth Mountain (fig. 10). They 

jointly fed a 1.2-km2 lava-flow field southwest of the cones 

(unit brc). The most recent eruption in the Mammoth Mountain periphery, this episode released ~0.025 km3

 of magma, 

equally divided between lavas and fragmental deposits. Small 

bits of charcoal in sediment beneath the ashfall gave a radiocarbon age of about 8.5 ka (Browne and others, 2010).

A southeast-trending nonglaciated chain of five comagmatic trachydacite domes and coulees (unit dnw) crosses the 

northwest moat, 11–12 km north of Mammoth Mountain. All 

five are glassy and rich in phenocrysts of plagioclase, sanidine, hornblende, and biotite, but they are variably contaminated with mafic enclaves, microdioritic blebs, and disequilibrium crystals, thus ranging in bulk composition from 67.3 to 

60.4 percent SiO2. Although the five vents are roughly aligned, 

their 40Ar/39Ar ages range from 42±1 ka to 27±1 ka. The two 

southeastern coulees, despite being the most contaminated, 

gave younger ages than the three northwestern units.

Trachydacitic Focus at Mammoth Mountain

Mammoth Mountain is a pile of overlapping silicic 

domes and coulees (figs. 1, 7) that extruded from a vent cluster only ~2.5 km across and spread laterally into a 5-km-wide 

volcanic edifice that has ~850 m of relief. The edifice exposed 

consists of 22 compositionally distinguishable extrusive units, 

among which only five (units dfl, dnh, dsd, dwr, and rce) represent more than a single flow. In addition, two comagmatic 

extrusions (units d81 and d61) form contiguous off-edifice 

domes, centered respectively 3.5 and 4.5 km northeast of the 

summit of Mammoth Mountain. The 24 extrusive units range 

in SiO2 content from 63 to 71 percent, and all are mildly 

alkaline, widely glassy, and generally flow foliated. Nineteen 

are phenocryst-rich trachydacites, and five that have 70–71 

percent SiO2 and fewer phenocrysts (units rce, rmf, rrc, rsq, 

and rss) are designated alkalic rhyodacites. A thick, complex 

pumice-fall deposit (unit rfp) preserved at several sites scattered northeast and east of the glaciated edifice is compositionally similar to edifice unit rce.

Every bit of Mammoth Mountain was ice-covered and 

thus either surficially scoured or deeply excavated during late 

Pleistocene glaciation. As a result, most of the silicic lava 

flows and domes that make up the edifice were substantially 

reduced in size. Confirming correlations among lava remnants 

has thus required accumulation of chemical, petrographic, and 

paleomagnetic data. Except for unit rfp, any pyroclastic flow 

and fall deposits that accompanied extrusion of the 24 silicic 

lava units exposed have been eroded away or concealed by 

younger units.

Petrography

All 25 units (table 1) contain Fe-Ti oxides, biotite, 

sanidine, and plagioclase; the dominant phenocryst phase in 

every sample is plagioclase. Although it is more abundant than 

sanidine in all Mammoth Mountain products, unit descriptions 

give only the percent of total feldspar in each unit, owing to 

intrasample inhomogeneities in plagioclase/sanidine proportions, multigrain feldspar intergrowths, and wide ranges in 

sizes and disequilibrium morphologies of coexisting feldspar 

populations. Hornblende is the dominant mafic phenocryst 

in units dbp, ddl, ddu, and dom (all east of the summit) 

and in off-edifice unit d61. On the other hand, hornblende is 

sparse in units dgr, dnh, dsk, dsu, dwr, and virtually absent 

in all the rhyodacites (units rce, rfp, rmf, rrc, rsq,and rss). 

In the remaining trachydacite units (dfl, dlp, dml, dms, dnk,

drc, dsd, dtl, and d81), biotite and hornblende are similar in 

abundance and either one may exceed the other in particular 

samples. Although subordinate to other mafic phases, pyroxene microphenocrysts (and sparse prisms 0.5–2 mm long) 

are common (0.5–2 percent) in units dfl, dgr, dlp, dml, dms, 

dnh, drc, dsd, dsu, dtl, d61, and d81. Pyroxenes appear to 

be absent in units dbp, dnk, dom, rce, and rfp, and they have 

been observed in merely trace amounts in the remaining eight 

silicic units at Mammoth Mountain. Most pyroxenes occur in 

clusters or in clots with other phases, suggesting entrainment 

from crystallizing rinds at the margins of conduits or magma 

pods. Orthopyroxene is more abundant than clinopyroxene, 

but both are present in some units. Many of the clinopyoxene 

crystals may have been released from andesitic enclaves that 

disaggregated in host dacites. Sparse enclaves (1–5 cm) were 

seen at some outcrops, and fine-grained mafic blebs, typically 2–5 mm across, were observed in thin sections of 13 

of the 24 units; they are especially common in units dml and 

dtl. Groundmasses of all 100-odd Mammoth Mountain silicic 

samples sectioned are at least partly glassy and charged with 

microlites. 

Some petrographic data based on thin-section examination are given in unit descriptions and summarized in table 1. 

In addition, the following generalizations can be noted. 

• Mammoth Mountain trachydacites carry 15–35 percent 

crystals and the alkalic rhyodacites only 5–15 percent. 

Size ranges of the several mineral species are given in 

each unit description. 

• Crystal clots (mostly 1–3 mm), containing interstitial 

glass and any combination of the phases present, occur 

in all samples. 

• More than one population of plagioclase is typically present in every sample. Crystals with sieve 
textures, internal resorption surfaces, and more sodic 

(or sometimes potassic) overgrowths are common in 

all trachydacites and present even in most samples of 

alkalic rhyodacite. 

• Sanidine is more often inclusion-free and euhedral, 

especially in the most evolved units. Microprobe 

reconnaissance on about 10 samples from Mammoth 

Mountain by Ring (2000) showed all or most of the 

sanidine to be sodic (along with lesser anorthoclase), 

contrasting with the potassic sanidine present in the 

subalkaline rhyolites of the Long Valley suite. 

• Ring (2000) also identified abrupt increases in Ba 

content of feldspars just outboard of resorption 

surfaces, possibly reflecting episodes of mixing 

with Ba-enriched hotter magma that had induced the 

resorption.

Eruptive Sequence

The 25 silicic units were erupted in several episodes 

between ~100 ka and ~50 ka, an interval ~50,000 years long 

(1) First was Dome 2781 (unit d81) at the northeast toe of the 

subsequent Mammoth Mountain edifice. There is no evidence 

of further activity after its extrusion at 99±7 ka until (2) an 

interval around 85±5 ka when off-edifice Dome 2861 (unit 

d61) and the oldest units exposed on the edifice erupted. The 

latter group includes the earliest alkalic rhyodacites (units rrc,

rce, and rfp) and the large South Summit Dome (unit dsd; 

~68 percent SiO2), which may be the most voluminous eruptive unit on Mammoth Mountain. Initiation of edifice growth 

closely followed (or may have overlapped in time with) the 

interval of elevated peripheral activity that produced units 

amp, bar, bfh, bmc, bmn, bsr, ddc, and drf, all of which 

erupted close to or beneath the subsequent site of the Mammoth Mountain edifice.

(3) There followed extrusion of relatively low-silica 

(63–64 percent SiO2), pyroxene-bearing trachydacite (unit dtl

[76±1 ka]), now exposed at the east base of the edifice along 

Mammoth Creek. 

(4) An episode of higher-silica trachydacite extrusions 

(66–70 percent SiO2) ensued, producing the western flowdome complex (unit dwr, 73±2 ka), the easternmost coulee at 

The Bluff (unit dom, 73±1 ka), and the Skyline Dome on the 

crest of the mountain (unit dsk, 71±1.5 ka).

(5) Another silicic trachydacite episode (66–70 percent 

SiO2) subsequently produced two coulees on the north flank 

(units dml [67±1 ka] and dms [68±1 ka]). The age of the small 

dome, unit dgr (68.8 percent SiO2), is poorly constrained but 

likely to have erupted during this episode or the next. Poorly 

exposed unit dnh (65±1 ka) crops out low on the southeast 

slope of the edifice, where it is overlain by the undated Bottomless Pit flow (unit dbp; 67 percent SiO2

), which forms much of 

the 150-m-high scarp above Twin Lakes.

(6) After a respite of a few thousand years, a short interval of renewed activity yielded a variety of silicic lavas, all of 

which erupted near or north of the summit. Low-silica (63–65 

percent SiO2) pyroxene-bearing flows of units drc and dfl

(61±3 ka); pyroxene-free North Knob dome (unit dnk; 60±1 

ka; 66.5 percent SiO2); Lincoln Peak dome (unit dlp, 64±7 

ka); and two alkalic rhyodacite lava flows (units rmf [61±1 

ka] and rsq [63.7±4 ka]). These rhyodacites erupted from 

separate vents, but unit rsq issued atop the same site where 

rhyodacite unit rce had erupted ~15,000 years earlier and 

where unit rss subsequently built Solitude Dome (~50 ka). 

All four rhyodacites are readily distinguishable compositionally and are thus inferred to represent four separately evolved 

magma batches.

(7) After a brief interval, the Summit Dome (unit dsu, 

68–70 percent SiO2) extruded on the crest, where it overlies units dsd, dsk, drc, and dfl (61±3 ka). It rests directly 

atop unit dfl (fig. 12), and its radioisotopic age (61±2 ka) is 

indistinguishable.

(8) The pair of coulees capping the Dragons Back arete 

(units ddl [58±2 ka] and overlying ddu), both thick trachydacites (66–67 percent SiO2

), are among the youngest lavas 

erupted at Mammoth Mountain. Both flows continue eastward 

across the narrow glacial outlet of Lakes Basin, beyond which 

they were ice-sculpted to form the smooth till-mantled hill 

called Panorama Dome.

(9) Youngest of all is apparently Solitude Dome (rhyodacite unit rss), which grew atop the vent for older units rce

(80±1 ka) and rsq (63.7±4 ka); it yields a sanidine age of 50±3 

ka. The entire multiepisode growth of Mammoth Mountain thus lasted about 50,000 years, and it has been another 

~50,000 years since the final eruption of the edifice.

Vent Locations

Vents for many of the 25 eruptive units distinguished at 

Mammoth Mountain are obscure or have been deeply eroded 

glacially. Dome 2781 (unit d81), Dome 2861 (unit d61), Skyline Dome (unit dsk), North Knob (unit dnk), Solitude Dome 

(unit rss), Gold Rush Express knob (unit dgr), and Lincoln 

Peak (unit dlp) are lava domes that emerged from subjacent 

conduits, and Scottys Dome likewise covers the vent for the 

White Bark Ridge complex (unit dwr). In common with Skyline 

Dome, the crestal regions of South Summit Dome (unit dsd) 

and Summit Dome (unit dsu) are extensively acid altered, 

indicating fluid flux from below, which is also consistent with 

subjacent conduits. The apron of rhyodacite lava flows (unit 

rce) that covers much of the northeast slope of the mountain 

apparently issued from a vent that was later covered by Solitude 

Dome (unit rss), which had also been the site from which the 

lava-flow unit rsq issued ~15,000 years after unit rce.

The glacially excavated trough (The Chasm) that separates the summit ridge of Mammoth Mountain from both Solitude Dome and the north-dipping planèze named "Face Lift" 

by the ski area (fig. 12) is apparently where several of the 

Mammoth Mountain and Contemporaneous Peripheral Volcanism 25
26 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

remaining lavas originated. The upper end of the planèze and 

the narrow north-trending rib (of unit dsd) that closes the east 

end of the trough are both severely acid altered, as are parts 

of the colluvial fill that floors the trough. The steep northeast 

face of the summit ridge was the principal glacial headwall, 

away from which ice flowed east, north, and northwest during the late Pleistocene (MIS 2). Glacial action excavated a 

trough ~200 m deep and more than 1 km long, removing as 

much as 20 million cubic meters of lava, some large fraction 

of which had been altered fumarolically. From the distribution 

of orphan lava flows that extend north and southeast from the 

trough, it is inferred that the excavated region was the central 

hub, a multivent complex for units dbp, ddl, ddu, dml, dms,

dfl, dnh, drc, and rmf. Adjacent to the inferred vent complex 

is the Summit Dome (unit dsu), lobes of which rest directly 

atop unit dfl on both walls of The Chasm (fig. 12 and geologic map).

Trachydacite units dtl and dom are exposed only 

distally, extending eastward from beneath the Dragons Back 

coulees. Less certainly linked to the excavated near-summit 

vent complex, they are nonetheless likely to have originated 

there, because both are younger than the massive South 

Summit Dome, which precludes outflow along alternatively 

plausible azimuths. Unit rrc is one of the oldest on the edifice, and, because it is exposed only locally beneath the toe 

of South Summit Dome, its vent is thoroughly concealed and 

thus unlocated.

Postglacial Phreatic Activity

Four small postglacial pits interpreted as phreatic craters 

were mapped by Huber and Rinehart (1965a, 1967) at elevations near 2,800 m just north of Lincoln Peak. Simplified to 

rmf

dfl

dfl

dsu

Summit Dome

Face Lift Planèze 

Figure 12. The Chasm, glacially eroded notch below summit headwall of Mammoth Mountain, viewed toward southeast. At upper right, 

thick flow of unit dfl spills northward from beneath Summit Dome (unit dsu), is erosionally cut out along glacial trough, and resumes 

on Face Lift planèze at left, where unit dfl is overlain by a flow of unit rmf. Original continuity of dfl flow, dipping 20o NNE., precludes 

invoking a fault. Summit crest at right is as much as 190 m above floor of notch. Aerial góndola spans notch and ends at visitor center, 

hidden behind cliff of false summit.
three pits, they were reproduced on the map of Bailey (1989), 

but they were never described. Although the sites have been 

partly bulldozed for ski runs, our repeated scrutiny of undisturbed areas around and between them has turned up no evidence for craters, ejected diamict, juvenile clasts, or thermal 

or fluid alteration. Undisturbed forested ground within and 

adjacent to the sites formerly depicted as craters are covered 

by combinations of thin colluvium, patchy ground moraine, 

bedrock and regolith of unit rce, and the 1350 C.E. Inyo 

pumice-fall deposit (with or without a thin aeolian sand layer 

beneath it). We conclude that explosion pits never existed 

there. Because the Mammoth Mountain Ski Area (MMSA) 

widely and repeatedly excavates, modifies, and replaces 

numerous shallow water-holding pits for erosion control and 

production of artificial snow, it seems likely that abandoned 

pits were interpreted as phreatic craters.

A contiguous pair of authentic phreatic craters beneath 

Chair 11 was mapped by Huber and Rinehart (1965a) and 

Bailey (1989), ~300 m southwest of the Main Lodge (at UTM 

200/685). Although partly bulldozed for ski runs and construction of a reservoir, their phreatic (nonjuvenile) apron deposits 

are locally intact, 1–10 m thick, fines-rich, and thermally 

discolored (see unit pe). The phreatic diamict directly underlies 1350 C.E. Inyo pumice-fall deposits and directly overlies 

a thin organic layer containing charcoal dated at 850±60 cal 

yr B.P. (fig. 13). A stump buried in the phreatic deposit gave 

radiocarbon ages in the range 659–737 cal yr B.P. (Sorey and 

others, 1998).

Present-day fumarolic discharge on and near Mammoth 

Mountain is weak, and sites are few but widely scattered. 

Diffuse emission of water vapor carrying CO2 and sulfur 

gases is detectable (1) along the upper gorge of Dry Creek 

(UTM 210/670), (2) along a small fault at the south toe of the 

edifice (UTM 211/652), and (3) more vigorously from cracks 

in bedrock of unit rmf at elevation 3,030 m on the Face Lift 

planèze (UTM 209/673) ~800 m north of the summit (Sorey 

and others, 1998). Several fumaroles along the summit ridge 

of Mammoth Mountain were said to have been active in the 

1950s (Huber and Rinehart, 1967), and patches of early snowmelt along that ridge (over acid-altered areas of units dsd, 

dsu, dsk, dwr) are today observed annually and attributed 

by MMSA personnel to warm ground. Off the edifice, ~1 km 

east of Twin Lakes, the persistent odor of H2S is detectable 

(without visible emission) along a zone of altered ground that 

extends 250 to 500 m northwest of Mammoth Rock, coinciding with a Mesozoic fault. 

Diffuse CO2 Emissions

About ten sites (fig. 5) around the base of the Mammoth 

Mountain edifice have, since 1990, developed cold soil-gas 

CO2 concentrations great enough to kill trees (Farrar and 

others, 1995; Sorey and others, 1998, 2000; Cook and others, 

2001; Evans and others, 2002). None of these have produced 

craters, visible orifices, or sulfur-bearing discharge, but some 

of the tree-kill areas do overlap the older Chair 11 phreatic 

craters and the pre-existing fumarolic area at the south toe of 

the edifice. Elevated CO2 was also detected above tree line in 

MMSA vaults on the summit ridge and along the trough below 

its northeast face. Emissions are diffuse but substantial (Sorey 

and others, 1998; Lewicki and others, 2012). 

Mammoth Mountain and Contemporaneous Peripheral Volcanism 27

THICKNESS

(CM)

5–30

20–50

50

95–105

2– 3

2– 4

45

1–2

DESCRIPTION

Vegetation

Soil and reworked fall material

Massive rhyolitic pumice fall (MP~4 cm); 

1350 C.E.; Glass Creek vent

Stratified rhyolitic fall (MP~3 cm); ash and 

lapilli layers (0.5–10 cm) alternating; 

1350 C.E.; Deadman Creek vent

Clayey diamict; all clasts Mammoth Mtn 

lavas; (1– 35 cm), subangular to 

subrounded; muddy matrix has swelling 

clays; vuggy porosity around clasts; 

thermal-oxidized discoloration 

lenticular. Phreatic crater deposit

Black charcoal-rich layer 

(930±20 14C yr B.P.)

White fine ash; from Mono Craters?

Massive tan sand (Md=0.2 mm; 11% silt; 

4% granules); charcoal-bearing lens 

(1,970±20 14C yr B.P.)

Charcoal at contact nearby 

(UTM3192/41684); (4,130±25 14C yr B.P.)

Coarse diamict; till with incipient soil at 

top

Measured Section beneath Chair 11 (UTM 32007/416871)

Figure 13. Measured section of surficial deposits beneath Chair 

11, ~300 m west-southwest of Main Lodge of Mammoth Mountain 

Ski Area. Late Holocene phreatic crater deposit, here ~1 m thick 

and as thick as 10 m nearby, is overlain, with slight unconformity, 

by two southward-dispersed rhyolitic fall deposits of multipulse 

Inyo eruption of 1350 C.E. (see units ric and p, table 1). Three 

uncalibrated radiocarbon ages are indicated. When calibrated by 

program of Stuiver and Reimer (1993; update of 2014), the 2s ages 

become 850±60, 1,910±40, and 4,690±130 yr B.P. Abbreviations: MP, 

maximum pumice size—average diameter of five largest clasts 

found in layer; Md, median diameter of sand by sieve analysis.
28 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

The tree-kill areas form a half-circle that extends from 

Horseshoe Lake on the southeast around the south and west 

margins of the edifice to the vicinity of the MMSA Main 

Lodge at the northwest toe. No tree-kill areas are found around 

the eastern half of the edifice. It seems likely that the principal gas flux ascends centrally beneath the edifice, as manifest 

in the high-elevation fumaroles and the near-summit diffuse 

discharge. Some fraction of the ascending gas is evidently 

diverted to the south and west periphery, probably along a 

highly permeable layer of rubble (till and talus) that separates 

the edifice lavas from the northeast-sloping scallop of the 

Long Valley Caldera wall, against which the edifice was built 

(fig. 14). The partly concealed wall here is largely granitic 

(unit Kmo) but includes Mesozoic metavolcanic components 

as well. Such a semiannular buried slope of permeable scree 

would explain why the tree-kill areas are distributed only on 

sides of the edifice that bank against basement rocks. The 

northeast-facing caldera wall had been glaciated repeatedly 

between its creation at 767 ka and burial by Mammoth Mountain lavas starting ~85 ka.

As recounted in the reports cited above, tree-killing 

emissions began soon after an 11-month interval of persistent earthquake swarms (attributed to dike intrusion; Hill and 

Prejean, 2005) beneath Mammoth Mountain in 1989. Evans 

and others (2002) identified a substantial magmatic component in the dissolved inorganic carbon of virtually every spring 

and groundwater source, mostly cold and dilute, in the vicinity 

of the mountain. They infer that CO2 now discharging in the 

tree-kill areas is gas that exceeds the dissolving capacity of the 

groundwater, and they hypothesize that the excess CO2 began 

to be released in 1989 owing to disruption of a low-permeability seal atop an intracrustal reservoir of secularly stored 

basalt-derived gas.

Postglacial Diamicts

In addition to the late Holocene ejecta sheet surrounding 

the twin phreatic craters near Chair 11, various thin colluvial 

diamicts have been exposed in numerous MMSA water pits, 

typically 2–4 m deep and 10–30 m wide, that are widely 

distributed across the northern base of Mammoth Mountain 

between Reds Lake and Chair 4 (UTM 228/683). Most such 

exposures are tan or orange-brown, 0.5–3 m thick, vaguely 

stratified in layers 5–50 cm thick defined by slight grainsize variations, and might at first glance be taken for thin 

nonwelded pyroclastic-flow deposits. They overlie Mammoth Mountain lavas or discontinuous sheets of Pleistocene 

ground moraine, and they are generally overlain by 1350 

C.E. Inyo pumice-fall deposits, either directly or with an 

intervening sheet of aeolian sand. Virtually all clasts are 

Mammoth Mountain dacite and rhyodacite, glassy or partly 

so, typically dense and nonvesicular, subrounded to subangular, and mostly 0.5–4 cm across but rarely and locally 10 

cm or larger. Neither prismatically jointed clasts nor thermal/

oxidation effects of clasts on the sandy-silty matrix were 

observed. 

In some pits, the diamicts also contain granules and lapilli 

as big as 4 cm of biotite-feldspar pumice. Pumice clasts analyzed are strongly hydrated (LOI >10 weight percent), deficient in silica and alkalies, and enriched in Al2

O3 (> 23 weight 

percent), suggesting long residence in wet colluvium. The 

pumices were probably reworked from unit rfp and soaked in 

groundwater for as long as 80,000 years. In 2004, two former 

pits (near UTM 211/688 and 226/684) on flats at the north 

toe of Mammoth Mountain exposed ashy diamicts, 0.5–1.5 m 

thick, rich in such hydrated/altered pumice lapilli; these may 

? ?

?

?

? ?

3300

3000

2700

2400

METERS

3300

3000

2700

2400

METERS

SOUTHWEST NORTHEAST NORTH

Mammoth

Mountain

summit

Lincoln Peak

Canyon

Lodge

gas

talus, till,

av

mafic lavas talus plus till

soil CO2

BEND INSECTION

Dome

2861

87 ka 

Dome

2781

100 ka 

Solitude

rhyodacite

dome

80–50 ka 

Dacite pile

(n~15)

85–58 ka

175 ka

(mcl)

Kmo 95 ka (bar)

msd

mcl

scoria cone

175 ka

NOT TO SCALE 0 1 2 KILOMETERS

Figure 14. Schematic cross-section (cartoon) across summit of Mammoth Mountain, simplifying surface units and hypothesizing a 

permeable layer of buried rubble that drapes the granitic caldera wall against which the edifice was constructed. Some fraction of 

the CO2 ascending from a deep basalt-fed reservoir toward near-summit fumaroles is diverted laterally toward the half-ring of tree-kill 

areas, which are limited to the side of the edifice in contact with basement rocks (fig. 5). Section bends northward at Canyon Lodge 

scoria cone to transect the two off-edifice domes of Mammoth Mountain lithology. No vertical exaggeration.
Volcanic Evidence for Glacial History 29

have been remnants of nonwelded pyroclastic-flow deposits 

related to units rce or rfp (~80 ka) or colluvially remobilized 

equivalents.

The diamicts as a whole we interpret as thin postglacial colluvium that, by creep, slumping, or sheetwash, has 

filled minor depressions on low-relief surfaces. As well 

as near the break-in-slope at the toe of the edifice, similar 

diamict is exposed in water pits on a 2,930-m plateau, ~300 

m southeast of McCoy Station, where sparse clasts of dense, 

variably rounded, dacite and rhyodacite are as big as 9 cm 

but mostly 0.5–4 cm across. Here, as at the lower sites, an 

orange-brown layer of massive or lenticular crystal-vitric 

sand, 30–110 cm thick, lies between the diamict and the 

capping 1350 C.E. Inyo pumice falls. From site to site, its 

median grainsize (Md) ranges from 0.3 to 0.7 mm—fine to 

medium sand. It contains granules and sparse pebbles as big 

as 4 cm of dacite, granite, and biotite-feldspar pumice. We 

interpret the sand to have resulted from aeolian reworking of 

postglacial diamicts. 

Volcanic Evidence for Glacial History

Glacial ice accumulated upon and blanketed Mammoth 

Mountain following its construction in the late Pleistocene. 

Although the volcanic edifice existed only for the last major 

glaciation (during Marine Isotope Stage [MIS] 2), glaciers had 

earlier advanced at least twice (and probably several times) 

into lowland parts of the map area from cirques on the Sierran 

rangecrest. Although the classic sequence of four glaciations 

defined for the eastern Sierra Nevada by Blackwelder (1931) 

has been scrutinized and provisionally amended many times, 

it remains remarkably robust in the Mammoth Lakes area. The 

McGee Till, which lies high on the south rim of the caldera 

just outside the map area, rests on basalt that gave a K-Ar age 

of 2.6±0.1 Ma, implying glaciation early in the Pleistocene 

(Putnam, 1962; Dalrymple, 1963). The Sherwin Till (probably MIS 22), which directly underlies the 767-ka Bishop 

Tuff, crops out in the northwest wall of the caldera (unit gst), 

more extensively just southeast of the caldera (Sharp, 1968), 

and may be buried by younger moraines at Convict Creek 

(Sharp, 1969). It was probably also deposited in the Mammoth 

embayment, where it was later engulfed by caldera collapse 

and buried during postcaldera volcanism. The Tahoe Till, 

inferred by Blackwelder to be of early Wisconsin age, has 

recently been shown to be complex (Phillips and others, 1996, 

2009; Gillespie and Zehfuss, 2004) but largely to represent 

MIS 6. The only correlative glacial deposit in the map area is 

the Casa Diablo Till (unit gcd; Curry, 1968, 1971), which is 

here bracketed by dated lava flows to the interval 160–125 ka 

(MIS 6). The Tioga Till, youngest of Blackwelder’s sequence, 

dominates glacial deposits in the Mammoth Lakes area, 

principally as a piedmont lobe of ~40 nested morainal ridges. 

Because they all include clasts of basaltic unit bhl (31±1 ka), 

the entire Tioga assemblage had to be deposited during MIS 2, 

irrespective of how many ice-front fluctuations and stillstands 

are represented. In the eastern Sierra, north and south of the 

Mammoth embayment, Phillips and others (1996, 2009) have 

dated by surface exposure methods moraines representing four 

Tioga advances between 28 ka and 14.5 ka, terminating in 

rapid retreat at 15–14.5 ka.

The Pliocene “Deadman Pass till” of Curry (1971) was 

shown to be a nonglacial diamict by Bailey and others (1990). 

Deposits of any glacial advances between MIS 22 and MIS 6 

were obliterated here by the later advances that deposited the 

Casa Diablo and Tioga Tills. No deposits assignable to MIS 

4, to a minor Recess Peak readvance (14.2–13.1 ka; Clark and 

Gillespie, 1997), to a Younger Dryas readvance (12.9–11.5 ka; 

Gillespie and Clark, 2011), or to Little Ice Age (~1350–1850 

C.E.) cirque glaciers have been identified in the map area. 

Rock glaciers (see unit rg) here are thought to be of Holocene 

age, although parts of the heavily debris-laden moraines of 

Sherwin Creek (see unit gud) may have moved like rock glaciers in the late Pleistocene.

Erratic boulders of Sierran origin have been found 

overlying unglaciated Early rhyolite (unit rer) at many sites 

along the southwest side of the caldera’s resurgent uplift. On 

ridge 2362, ~3 km east of downtown Mammoth Lakes (UTM 

299/681), boulders of porphyritic granite (unit Kmo), granodiorite (unit Krv), leucogranite, and metavolcanics (unit }mv) 

are scattered in abundance at elevations as high as 2,345 m. 

As the site is slightly above but adjacent to the highest distal 

deposits of Casa Diablo Till (unit gcd), the boulders may have 

tumbled from the glacier front at its maximal advance. Accompanying boulders of phenocryst-poor mafic lavas (almost 

certainly younger than 200 ka) support emplacement of the 

erratics during MIS 6.

Erratics found at several other sites are well outboard of 

exposed moraines, however, and erratics of Quaternary lavas 

are absent. The highest site is on the east side of Peak 2506 

(BM 8224 ft on older maps), ~3 km northeast of downtown 

Mammoth Lakes (UTM 293/696); boulders of the same four 

Mesozoic rock types just mentioned form an isolated cluster 

~20 m across at an elevation of 2,495 m. If these were transported by an iceberg, as seems likely, it was prior to resurgent 

uplift of the Early rhyolite lava upon which they rest, because 

the closest lakeshore gravels are 270 m lower, at an elevation 

of 2,225 m near the geothermal plant, 3 km southeast. 

Abundant erratics are also scattered along the ridge 

east of Sawmill Cutoff (road), 1.5–2 km northwest of Peak 

2506, at elevations of 2,430–2,465 m, as far north as UTM 

283/713. Boulders of granite, leucogranite, and metavolcanics 

are common, but the granodiorite (from unit Krv) is absent 

at these northerly sites. Another area of scattered erratics is 

within a 700-m-long strip at 2,300–2,365 m elevation along 

the ridge ~1 km east-northeast of the geothermal plant (UTM 

318–323/681–688), where clusters of boulders of leucogranite, granodiorite, and metavolcanics are common. They are 

accompanied there by blocks of welded Bishop Tuff, many 

20–60 cm across, which were certainly not transported by ice. 

As the Bishop Tuff is not preserved around the Mammoth 
30 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

embayment, and intracaldera Bishop Tuff is buried by >400 

m of caldera fill (fig. 6), the welded blocks were probably 

excavated from below during explosive phases of Early 

rhyolite (unit rer) activity (760–650 ka). Explosive ejection 

of the granitoid and metavolcanic boulders from buried till 

is less likely than a glacial origin, owing to the great size of 

many (1–2.5 m), to their small patchy occurrences, and to 

concentration of sites on the southwest side of the resurgent 

uplift, which faces Sierran sources of the Pleistocene glaciers. 

Our preferred interpretation is that the basement boulders were 

transported by icebergs from calving glaciers across an arm of 

Long Valley Lake and beached against the Early rhyolite prior 

to full structural uplift of the resurgent dome.

Inyo Chain

The 10-km-long Holocene Inyo chain (Bailey, 1989; 

Hildreth, 2004) consists of seven rhyolitic lava flows and 

domes, several phreatic craters, and a composite apron of 

pyroclastic fall and flow deposits (Miller, 1985; Sampson and 

Cameron, 1987; Nawotniak and Bursik, 2010). The oldest 

unit is North Deadman dome (unit rnd; ~0.04 km3), undated 

but probably middle Holocene, followed by Wilson Butte 

(~0.05 km3), which erupted just north of the map area about 

1.3 ka. Both domes are aphyric glassy rhyolite. Wilson Butte 

(76.6 percent SiO2) is similar compositionally and petrographically to the Mono Craters domes (Kelleher and Cameron, 1990) 

and should be called a Mono dome were it not for its position 

on the north-south trend of the Inyo chain rather than the arcuate 

trend of the Mono chain (fig. 2 of Hildreth, 2004). North Deadman dome (74.7 percent SiO2

) is compositionally intermediate 

between Wilson Butte and the crystal-poor lower-silica rhyolite 

(Sampson and Cameron, 1987) that dominated the youngest 

Inyo eruptive episode in 1350 C.E. Two phenocryst-poor minidomes (each <0.001 km3

) just north and south of the large Glass 

Creek flow erupted after Wilson Butte but prior to the 1350 C.E. 

Inyo eruption, the most evolved products of which they compositionally resemble (Sampson and Cameron, 1987). These 

are units rcd (73.2 percent SiO2) and rcw (72.2 percent SiO2), 

which both carry 1–2 percent feldspar phenocrysts and trace 

amounts of biotite, pyroxene, hornblende, and Fe-Ti oxides.

Injection of the Inyo dike (Eichelberger and others, 1985) 

in late summer of 1350 C.E. (Millar and others, 2006) led 

to sequential eruption of the Deadman Creek, Obsidian, and 

Glass Creek flows (all unit ric), each preceded by substantial 

pyroclastic outbursts (Miller, 1985), the last of which was 

followed by phreatic eruptions (unit pe) at nearby Inyo Craters 

(Mastin, 1991). Total magma volume erupted during the multipulse episode was estimated by Miller (1985) to be 0.4 km3

 as 

lava, 0.17 km3 as fallout, and >0.05 km3 as pyroclastic density 

currents. 

Compositionally, the mid-14th century Inyo eruption 

was unusually complex (Sampson and Cameron, 1987; Vogel 

and others, 1989). In addition to the crystal-poor (2–3 percent 

crystals; “finely porphyritic”) zoned rhyolite (70–74 percent 

SiO2) that dominated the eruptive products, a very crystalrich rhyodacite (71.3±1 percent SiO2

; 25–40 percent crystals; 

“coarsely porphyritic”) piled up over the vents of the Deadman 

Creek and Glass Creek flows late in their extrusive episodes 

and mingled (to a limited extent) locally with the coerupted 

crystal-poor magma. Evidently having been stored separately, 

the crystal-rich magma was ~100 oC cooler and chemically 

unrelated to the crystal-poor one, which has much higher K, 

Rb, Zr, Y, and REE (and lower Ti, Mg, Ca, and Sr) at equivalent SiO2

 contents. In addition to the silicic magmas, andesitic 

enclaves (~60 percent SiO2) are present in the crystal-rich 

central parts of both flows (Varga and others, 1990). 

The main crystal-poor magma was itself zoned (70–74 

percent SiO2), yielding smoothly linear compositional arrays 

(for example, 1.3–2.6 FeO and 265–1420 ppm Ba), which are 

continuous but show an apparent tendency toward volumetric 

bimodalism (Sampson and Cameron, 1987; Vogel and others, 

1989). To explain the arrays, Sampson and Cameron (1987) 

suggested back-mixing between two slightly zoned silicic 

magmas that had earlier fractionated at higher pressure, while 

Vogel and others (1989) called for mixing between dacitic 

and rhyolitic end-members. Bailey and others (1976) had 

suggested that the crystal-poor phase might be Mono Craters 

magma (which may thus be partly right) and that the crystalrich phase is Long Valley magma (which appears to be wholly 

right). The compositional and petrographic similarity of the 

crystal-rich Inyo phase and the nearby Long Valley rhyolite of 

Deer Mountain (unit rdm) was pointed out by Sampson and 

Cameron (1987). Moreover, Reid and others (1997) identified, 

in both the 1350 C.E. crystal-rich Inyo phase and 100-ka Deer 

Mountain lava, zircon populations with crystallization ages 

that cluster around 230 ka. Residual or thermally rejuvenated 

Long Valley magmatic mush is apparently implicated. 

During the 1350 C.E. sequence, four closely spaced 

subplinian pumice eruptions issued from the three vents 

now covered by the lava flows (Miller, 1985), mantling the 

western part of the map area with substantial pumice-fall and 

derivative reworked deposits (unit p). Isopachs were drawn 

by Miller (1985), and grain size and componentry were given 

by Nawotniak and Bursik (2010). Along Glass Creek gorge 

between the Obsidian and Glass Creek flows, the cumulative 

sheet of coarse ejecta is >50 m thick, and it may also be as 

thick adjacent to the Deadman Creek flow where it is little 

incised. Many rhyolitic ejecta of varied textures are larger 

than 1 m proximally, and abundant rounded boulders of granite and basalt entrained from glacial deposits (buried during 

the eruption) are as big as 1–2.5 m. Poorly sorted tackwelded agglutinate is several meters thick in proximal fall 

deposits along Glass Creek. The fall deposits thin to 10–15 

m by 1–2 km away from the vents, and the combined thickness of the pair of fall units that dispersed southward (fig. 

13) is still ~1 m at Mammoth Mountain and Devils Postpile, 

7–10 km south of the Deadman Creek flow. No paleosols or 

erosion surfaces separate the four fall units, and no fallout 

mantles the lava flows.
Faults 31

Faults

Several small faults have displaced young volcanic rocks 

within the map area. All appear to be normal faults with scarps 

generally 1–10 m high, and most are north-striking, consistent 

with ongoing Basin-and-Range extension and with continuation of the Hartley Springs rangefront fault, buried beneath the 

west moat. Exposure is poor owing to the mantle of pumiceous 

fallout from the Inyo eruptions of 1350 C.E., but it is clear that 

most of the faults displace late Pleistocene glacial deposits and 

numerous lava flows dated between 180 and 60 ka.

The Upper Dry Creek Fault, located ~1 km west of 

Dome 2861, extends for ~2 km, striking north in its southern 

third before bending toward N. 10o E. Displacement, which is 

normal and east-side-down, ranges from 5–10 m in unit ddc

(103±9 ka) to 1–6 m where its northern splays cut units bmn

(87±7 ka) and bar (88±5 ka). In several places along its strike, 

there are open clefts 1–5 m wide.

The Inyo Craters Fault System is a family of subparallel 

extensional faults and fissures, encompassing the Inyo Craters 

alignment. The principal fault, the westernmost, produced 

an east-side-down scarp, 5–14 m high where it cuts unit aic

(131±1 ka) and as high as 19 m where mantled by the sheet 

of Inyo phreatic ejecta (unit pe) ~100 m northwest of South 

Inyo Crater. This fault strikes north-south for ~1 km southward from the west rim of that crater, then bends to a strike 

of S. 20o W. for another 1.5 km; along the latter reach it drops 

unit bar (88±5 ka) 3–10 m down to the east. Just north of 

South Inyo Crater, it bends northwestward and splays into 

a set of minor faults and fissures mapped at larger scale by 

Mastin (1991). Another north-striking fault drops the southeast 

rim of South Inyo Crater 13 m down to the east, and within 

150 m east of this fault a parallel set of lesser faults outlines 

a pair of shallow graben. Additional extensional faults extend 

northward across the summit of Deer Mountain where another 

phreatic crater erupted during the Inyo episode of 1350 C.E. 

Some small fraction of the east-west extension here was probably promoted by southward propagation of the rhyolitic dike 

that drove the magmatic and phreatic eruptions of that year 

(Mastin and Pollard, 1988).

The Dry Creek moraine faults cut the blanket of late 

Pleistocene till (unit gdc), 2–5 km north of Dome 2861. The 

principal fault extends ~4 km northward from the Dry Creek 

Dome, striking N. 5o–15o W., displacing the till (MIS 2) as 

well as windows of units mnd and mor (66±2 ka) along a 

west-facing scarp typically 5–6 m high. At its south end, the 

fault cuts the Dry Creek Dome (unit rdc, 112±6 ka), atop 

which a graben-like trough may reflect vertical displacement 

as great as 10 m. About 1.5 km east of this fault, a set of three 

small north-striking normal faults likewise displaced the till 

and underlying unit mor (66±2 ka). Two of them produced 

west-facing scarps only a few meters high, whereas the easternmost of the three drops unit mor ~10 m, east-side-down. 

Although the set can be traced for ~1 km, and two of them 

cross Dry Creek, none propagated farther southward into the 

West Moat Coulee (unit rwm, ~150 ka).

The so-called Earthquake Fault is an open fissure that 

crosses Highway 203 just west of downtown Mammoth Lakes. 

Cutting dacite lava of unit d81 (99±7 ka), the fissure has long 

been highlighted as a tourist attraction and provided with its 

own carpark. A southern segment strikes N. 15o E. for ~900 

m, then bends into a northern segment that strikes 5o W. for 

600 m before ending in a set of three small splays just south 

of the base of Dome 2861. Its south end descends the flow 

front of unit d81 and disappears into landscaped rubble (till of 

unit gmm and scoria of unit mcl) at the site of Canyon Lodge. 

Width and depth vary along strike, but major parts of the 

open fissure are as deep as 15 m and as wide as 2 m. Vertical 

displacement is small or absent, and in any case hard to measure precisely. Benioff and Gutenberg (1939) estimated that 

relative elevation of the east rim could locally be ~0.5 m. On 

the south rim of Dome 2861, to the north of the fault termination, about ten vertical joints are eroded into gullies, but there 

is no ground breakage atop the dome, and we see no sign of a 

fracture continuous with the Earthquake Fault. Unlike Bailey (1989), we agree with Rinehart and Ross (1964) that the 

Earthquake Fault terminates just south of the dome.

The inferred Sawmill Cutoff Fault may extend 2 km 

northward from Shady Rest campground, striking N. 10o E. 

We think a poorly exposed fault may be required here to 

account for the eastern escarpments of units mkv (153±1 

ka) and msc (155±28 ka), each more than 100 m high. The 

steep eastern scarps of these mafic phenocryst-poor lavas 

are unglaciated, were never subjected to stream erosion, 

and were unconstrained to the east when erupted. Apparent 

displacement may exceed 50 m, east-side-down, but probably 

diminishes toward the north end where part of unit msc banks 

against early postcaldera rhyolite (unit rer).

The South Mammoth Mountain Fault strikes northwest 

across the lower south slope of the edifice, displacing colluvium vertically by as much as 3 m, mountain-side-down. 

It can be mapped clearly for 1.7 km, and it might project 

an additional 1 km southeast to Horseshoe Lake (Hill and 

Prejean, 2005), but no surface displacement is discerned 

across the pumice-mantled slope of unit amp. The fault strikes 

principally across dacite lava of unit dsd (87±6 ka), although 

only thick colluvium that conceals the lava along the fault is 

demonstrably displaced. The southeastern end of the fault (as 

here mapped) is marked by weak fumarolic alteration and a 

small tree-kill area that reflects diffuse CO2 discharge. The 

larger area of CO2-induced tree kill at Horseshoe Lake (Farrar 

and others, 1995) is consistent with the possibility that the 

fault (or equivalent fracture) continues that far. The unusual 

strike and sense of displacement may reflect loading by the 

Mammoth Mountain edifice where it buries the caldera’s topographic wall, which is, here, probably a steeply sloping contact 

between granitic bedrock and a pre-Mammoth Mountain apron 

of surficial debris (fig. 13).

The McLeod Lake Fault is similarly modest, striking north-south past the east side of that lake for ~1.5 km. 

Apparent vertical displacement of the granite basement, 

west-side-down, is only a few meters near and north of the 
32 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

lake but may be ~10 m some 500 m south of the lake. Units 

aml and amp (97±1 ka) appear to be offset by little more than 

a meter. South of McLeod Lake, from Mammoth Crest to Fish 

Creek, we have been unable to verify the existence of several 

other north-striking faults portrayed by Bailey (1989) as cutting the granite (unit Kmo).

The Chalets Fault, ~250 m west of Mammoth Mountain Inn, strikes north or slightly east of north and vertically 

displaces pumice and till 4–9 m, east-side-down. It can be 

followed for ~700 m, but we were unable to verify its continuation south of Highway 203 as portrayed by Bailey (1989). 

The Mammoth Rock Fault, a near-vertical fault, separates 

Paleozoic metasedimentary strata at Mammoth Rock from 

adjacent Mesozoic metavolcanic rocks. According to Rinehart 

and Ross (1964), the stratigraphic separation and thus the 

inferred displacement is 1.4 to 1.8 km, southwest side down. 

Although they inferred (and we concur) that all or most slip 

was pre-Cenozoic, the northwest end of the fault zone today 

controls weak fumarolic (H2S-bearing) discharge and attendant 

alteration (Bailey, 1989).

The Chasm, a northwest-trending trough ~1.5 km long 

(fig. 12) that separates the steep northeast face of Mammoth 

Mountain’s trachydacite summit ridge from the Face Lift 

planèze and Solitude Dome, was previously mapped as a fault 

(Bailey, 1989), inappropriately in our analysis. The elongate 

cleft was glacially excavated, facilitated by altered rocks 

at dome margins and by cornice-fed excess snow from the 

ridgecrest. The rhyodacites (units rmf, rce, rsq, and rss) are 

not present on the summit side of The Chasm, and the relative 

elevations of the four trachydacite units (dsd, ddu, dfl, and 

dsu) that do cross The Chasm (and are 20 m to 110 m lower 

on the northeast side) are consistent with simple downslope 

flow of the lavas, without fault displacement. Ice flowed away 

broadly to the north and northeast but preferentially to the 

northwest and east along the trough excavated between lava 

domes. With its floor today as high as 3,170 m, The Chasm 

was the main glacier headwall on Mammoth Mountain during 

the late Pleistocene, and it still retains snow slopes later each 

summer than anywhere else on the mountain.

Major Faults

The Long Valley Caldera ring-fault zone lies well inboard 

of the caldera’s topographic walls (Hildreth, 2004; Bailey, 

1989, cross-sections; Carle, 1988; Hill and others, 1985). 

Although it is everywhere deeply buried by postcaldera volcanic and sedimentary fill, the zone probably influenced eruption 

locations for Lookout Mountain (subunit rlm, ~690 ka), the 

Hot Creek flow (unit rhc, ~330 ka), and west-moat rhyolitic 

units rwm, rdc, and rmk (~150-100 ka). The zone is likely 

to be a family of nested faults and sheared breccia. Intense 

seismicity since 1980 along the caldera’s south moat has been 

resolved into a 1-km-wide multifault shear zone outboard and 

several steeply inward-dipping fault planes just inboard of 

it (Prejean and others, 2002). Most seismicity is at depths of 

3–9 km, and much of it is dextral, apparently reflecting reactivation of the ring-fault system by regional tectonic stresses 

(Riley and others, 2012) rather than by either magma intrusion 

or renewed normal-fault subsidence.

The ring-fault zone may not be as smoothly elliptical as 

commonly portrayed, particularly beneath the west moat, where 

subsidence may have utilized precaldera faults related to the 

left-stepping rangefront system. Offsets along that inherited en 

echelon fault system may have conveyed a jigsaw pattern to the 

western structural margin, which could, in turn, have influenced (1) the outboard dike that fed 100-ka rhyolite to the Deer 

Mountain vent; (2) the path of crystal-rich Long Valley subalkaline rhyolite that reached the northwest moat where it mixed 

with the 42–27-ka hybrid dacites (unit dnw) and the 1350 C.E. 

Inyo rhyolite (unit ric); and (3) ascent of deep thermal water that 

thence flows eastward in shallow aquifers to the active geothermal areas in south-central parts of the caldera.

Rangefront faults north and south of the caldera were not 

remapped as part of this investigation, but the Hartley Springs 

and Hilton Creek Faults (figs. 3, 4) are certainly important 

in having influenced intracaldera structural development in 

postcaldera time. Each is an important member of a family of 

rangefront faults, respectively north and south of the caldera, 

that represent Quaternary encroachment of Basin-and-Range 

extension (and locally transtension) into the Sierra Nevada 

microplate. 

The Hartley Springs Fault Zone is a multistrand array 

~10 km long that includes a steep east-facing scarp where 

it disappears into the northwest corner of the caldera. The 

family of anastomosing strands is ~2 km wide, trends 

~N. 15–20° W., and includes a few large antithetic (west-sidedown) faults that block out prominent granitic horsts (Bailey, 

1989; Bursik and Sieh, 1989). Total relief across the fault 

zone is ~450 m, down to the east. The Silver Lake Fault Zone, 

~6 km farther west, brings total basement relief across the 

rangefront fault system to ~750 m. The easternmost strand, 

which bounds a steep granitic scarp (unit Kjl) near Hartley 

Springs, locally drops the Bishop Tuff by 100–120 m, yielding an average vertical slip rate of ~0.15 m/kyr since 767 ka. 

Bursik and Sieh (1989) estimated ~30 m combined vertical offset for two strands (their F1

 and F2) that cut a Tahoe 

moraine just northeast of June Lake; if the moraine represents MIS 6, the slip rate would be 0.23 m/kyr since 130 ka, 

or if MIS 4 (less likely), then ~0.4 m/kyr since ~70 ka. The 

Reversed Peak strand, 2 km northwest of June Lake, could 

also be considered part of the family; for that fault, Bursik 

and Sieh (1989) estimated 21 m vertical offset of a Tahoe 

moraine, which would give a slip rate of 0.16 m/kyr since 

130 ka. Aggregate down-to-east displacement across the zone 

since the middle Pleistocene thus appears to be little more 

than 0.5 m/kyr (0.5 mm/yr). Bursik and others (2003) suggested that slip on the Hartley Springs Fault and southward 

propagation of the Inyo Dike (Eichelberger and others, 1984, 

1985) may have exerted mutual influence in 1350 C.E. 

The Hartley Springs Fault has no equivalent on the south 

wall of the caldera. Its buried extension beneath the west 
Composition of Eruptive Products 33

moat may have influenced the several small young faults 

described there, but such structures terminate at the foot of the 

caldera wall buried by Mammoth Mountain. Faults portrayed 

by Bailey (1989) southward across granitic basement from 

Mammoth Crest to Fish Creek cannot be verified and, in our 

judgment, do not exist. The precaldera rangefront fault system 

had stepped left within the area of the subsequent caldera and 

did not continue south through the later site of Mammoth 

Mountain. 

The Hilton Creek Fault extends S. 20° E. for ~18 km 

south of the caldera margin and is marked by a great east-facing bedrock scarp roughly 1,000 m high. Its displacement of 

Tioga (MIS 2) moraines by 15–26 m at McGee Creek (Berry, 

1997) yields a vertical slip rate of at least 1 mm/yr for postglacial time. Along its northerly 6-km-long reach from McGee 

Creek to Convict Creek, its postglacial offset decreases, and 

its trace degrades into small scarplets of varied orientation 

in surficial deposits (Rinehart and Ross, 1964; Clark and 

Gillespie, 1981). Although minor ground cracking of colluvium by M6 earthquakes in May 1980 was mapped along its 

projection, both south and north of Convict Creek (Taylor and 

Bryant, 1980; Clark and others, 1982), we are unable to verify 

measurable fault displacements on various notional lineaments 

crossing the 330-ka Hot Creek Flow that were interpreted by 

Bailey (1989) as intracaldera splays of the Hilton Creek Fault. 

Both the Hartley Springs and Hilton Creek Faults certainly 

extended into basement rocks now buried beneath the caldera 

fill, but there is little evidence that their intracaldera segments, 

cut off by the caldera ring fault, have produced postcaldera 

surface displacements. Just outside the caldera, however, postcaldera slip on the Hilton Creek fault remained vigorous, helping to form the lowland basin now occupied by Lake Crowley 

and to downwarping the highstand strandline of Pleistocene 

Long Valley Lake south-southwestward toward the toe of the 

fault scarp. 

On the north wall of the caldera, if the several lesser 

(non-range-bounding) faults mapped by Bailey (1989) in the 

Bald Mountain-McLaughlin Creek area were precaldera splays 

or extensions of the Hilton Creek Fault, then displacement 

had diminished greatly northward across the terrain that later 

became the caldera. Among the faults north of Bald Mountain, 

the largest scarp (up to 60 m high) is west-facing, and those 

(like the Hilton Creek Fault) that face east form scarps only 

10–25 m high. Already in precaldera time, therefore, the two 

great rangefront en echelon faults died out or attenuated over 

the later site of the caldera, and between their dying ends was 

a 15-km left stepover, which, in postcaldera time, became the 

focus for eruption of the intracaldera Early rhyolite and for 

resurgent uplift. As Bailey (1989) reasoned, the stepover was 

probably the site of a northwest-sloping topographic ramp or 

warp in precaldera time.

Faults on the resurgent uplift. Both the Hilton Creek and 

Hartley Springs Faults strike N. 20o W. where they approach 

the caldera, and strikes of faults that cut the resurgent uplift 

range between N. and N. 35o W., averaging about N. 20o W. 

The main extensional belt defined by these faults crosses the 

western half of the resurgent uplift and is neither medial nor 

apical; it is ~9 km long, strikes N. 20o W., and is centered 

within the left stepover that separates the dead ends of the 

great rangefront faults. 

The eastern half of the resurgent uplift is far less faulted, 

but it has a few north-striking faults of modest displacement. 

Three such faults, all west-facing, are mapped near Little 

Hot Creek and Little Antelope Valley, where they influenced 

distribution of widespread hydrothermal alteration during the 

middle Pleistocene (Cleveland, 1962; Bailey, 1989; Sorey and 

others, 1991).

The resurgent uplift has none of the radial faults expected 

for a structural dome. The main belt of normal faults striking 

N. 20o W. across the uplift likely reflects structural inheritance 

controlled by the precaldera stepover rather than faulting in 

response to doming. Across the principal graben (Smokey 

Bear Flat) within the belt, displacement of 700-ka rhyolites 

yields an average vertical slip rate of ~0.25 mm/yr, and graben 

subsidence between outcrops of rhyolite unit rwm yields 

essentially the same rate for the last 150,000 years.

Where Dry Creek crosses Highway 395, the graben 

displaces mafic lavas of unit mcl (~175 ka) as well as rhyolite 

lava of unit rwm (~150 ka). Near the Casa Diablo geothermal 

plant, faults of the graben system cut mafic lavas of units mcl

and unit bcd (125±2 ka). The intracaldera extensional faulting 

thus persisted here for at least half a million years after the 

final eruptions of Early rhyolite (unit rer, ~750–650 ka), which 

constitute nearly all exposures on the resurgent uplift.

Composition of Eruptive Products

Major- and trace-element data for all eruptive units 

mapped are given in appendix 1 and plotted in figures 15–20. 

Chemical data for several postcaldera Long Valley rhyolites 

were also published by Heumann and Davies (1997), Heumann (1999), and Heumann and others (2002), along with Nd, 

Sr, Pb and U-Th isotope data for many of them. For numerous mafic lavas, as mapped by Bailey (1989), chemical data 

and Nd, Sr, and Pb isotope data were published by Cousens 

(1996). 

A conventional alkali-silica diagram (fig. 15) illustrates 

the contrast between the alkaline Mammoth Mountain rocks 

(100–50 ka) and the postcaldera subalkaline rhyolites (750–

100 ka) that erupted from the residual Long Valley system a 

few kilometers to the east. Relative to postcaldera Long Valley 

rhyolites, eruptive products of Mammoth Mountain are less 

silicic but more alkalic; this generalization likewise applies 

to all precaldera and caldera-forming Long Valley rhyolites 

(fig. 4 of Hildreth, 2004). Figure 15B shows that, except for 

a few basalts, nearly all of the many peripheral eruptive units 

(230–8 ka) of the Mammoth system are likewise alkaline by 

the criteria of LeBas and others (1986). The most strikingly 

subalkaline exception is the Holocene basalt of Red Cones, 

youngest eruptive unit in the whole Mammoth system.
34 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Peripheral Units

alkaline

subalkaline

EXPLANATION

a62

aic

aml

amp

apb

asr

bar

bar'

bcd

bcf

bed

bfh

bhl

bmc

bmn

bmn'

brc

bsc

bsm

bsr

ddc

dnw

drf

mcl

mcv

mdm

mdn

mdp

mic

mkv

mlc

mmc

mnd

mor

msc

msd

msj

mss

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

48 50 52 54 56 58 60 62 64 66 68

SiO2, in weight percent

Total alkalies, in weight percent

Mammoth Mountain alkaline suite

Long Valley subalkaline rhyolites

8.0

8.5

9.0

9.5

10.0

10.5

62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

Si02, in weight percent

Total alkalies, in weight percent

dbp

ddl

ddu

dfl

dgr

dlp

dml

dms

dnh

dnk

dom

drc

dsd

dsk

dsu

dtl

dwr

d61

d81

rce

rfp

rmf

rrc

rsq

rss

rdc

rer

rdm

rhc

rmk

rwm

A EXPLANATION

B

Figure 15. Total alkalies (Na2O + K2O) versus SiO2 contents in weight percent for (A) Mammoth Mountain dome complex and (B) 

peripheral units of wider Mammoth magmatic system. Unit labels as in table 1, appendix, and Description of Map Units. Also plotted in 

panel A are data for six units of subalkaline Long Valley system, four of which erupted 150–100 ka, contemporaneous with Mammoth 

system. Alkaline/subalkaline boundary in panel B follows LeBas and others (1986). Dashed enclosure in panel B is field of Mammoth 

Mountain trachydacites (63–68 percent SiO2) from panel A.
Composition of Eruptive Products 35

Figure 16 provides a compositional overview of the 

eruptive sequence documented, focal and peripheral, showing 

apparently unsystematic variation in SiO2 content with eruption age since ~230 ka. It can be noted, however, that the five 

silicic andesite units (a62, aic, aml, amp, apb), all phenocryst-poor and widely scattered, erupted during the interval 

140–90 ka, which overlaps both the final episode of Long Valley rhyolitic extrusions and initiation of Mammoth Mountain. 

The bulk of the silicic edifice was constructed in about 20 

eruptive pulses between 90 ka and 60 ka. In contrast, eruption 

of mafic magmas (48–55 percent SiO2) was fairly continuous 

over the interval 230–8 ka, with the possible exception of an 

eruptive lull at 60–30 ka, which coincided with termination 

of the silicic eruptive activity that had built the Mammoth 

Mountain edifice.

Figures 17–20 illustrate, for a large number of samples 

from all units of Mammoth Mountain and its periphery, the 

extent of compositional variation within each eruptive unit 

and the chemical grounds for correlating mutually isolated 

exposures of many such units. Figure 20 further compares 

compositions of the Mammoth system (largely late Pleistocene) with those of the many Neogene lavas preserved nearby. 

Mammoth Mountain

The 25 eruptive units distinguished for Mammoth Mountain range continuously from 63 to 71 percent SiO2 and from 

3.6 to 5.1 percent K2O (fig. 17A). Contents of MgO (0.2–1.7 

percent), FeO* (1.4–4.7 percent), TiO2 (0.30–1.04), and CaO 

(0.6–3.7 percent) all decrease fairly linearly with SiO2 (figs. 

17B, C). Broader, more scattered arrays are exhibited by 

Na2O (4.5–5.7 percent), owing in part to hydration of generally glass-rich samples, and by Al2

O3 (15–17 percent) and 

Ba (1,250–1,900 ppm), P2O5 (0.03–0.40 percent), and Zr 

(270–480 ppm), presumably owing, respectively, to heterogeneous distribution of feldspars, apatite, and zircon. Among the 

phenocryst-poorer rhyodacites alone, Ba ranges from 1,250 to 

76

72

68

64

60

56

52

48

20 40 60 80 100 120 140 160 180 200 220 240

Eruptive age, ka

SiO2, in weight percent

461

918 824

1046

Inyo Chain West Moat rhyolites

West Moat core

Mammoth Mountain

?

rnd

rcd

rcw

ric (1350 CE)

rss rsq rmf rce rrc

rfp

rdm

dms

dsu

dlp dwr

dml

dnw-I

dnw-II

ddl

dfl

dsk

dnk

dom d61

dsd ddc

drf

dtl

ddu

rmk

rdc

d81

rwm

a62 aic apb

asr

mdn mcv

brc

bhl

mor

mnd

msd

bmn

bar

bsr

bfh

bmc

mss mlc

bed

bcd bdc

mkv mmc

msc

bcf

bsc

bsm

mcl

mdp

amp

aml

?

Figure 16. Plot of composition versus age for all dated units of the Mammoth system. Also plotted are Holocene Inyo chain and four 

subalkaline Long Valley rhyolites contemporaneous with alkaline Mammoth system. SiO2 contents in weight percent. Unit labels as in 

text and table 1. Intraunit ranges in SiO2 from appendix; error bars on ages express analytical precision from table 2. Five undated units 

in table 1 are not plotted but are probably all late Pleistocene: units dbp and dgr are constrained between 80 ka and 60 ka; unit mdm is 

older than 101 ka; mic is younger than 131 ka; and msj is older than 66 ka. Oldest four ages are core samples of lava flows from Inyo-4 

drill hole (Eichelberger and others, 1985), identified by core depth, in feet; nowhere exposed at surface, samples represent Groups II, III, 

and IV of Vogel and others (1994).
36 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

1,620 ppm and Zr from 270 to 480 ppm (fig. 17D), evidently 

reflecting wide differences in degrees of sanidine and zircon 

retention in the crystal mush from which successive rhyodacitic melt batches were extracted. A Sr-Rb plot (fig. 17E) 

shows a moderately coherent trend dominated by plagioclase 

fractionation until reaching the rhyodacite range where sanidine retention (or removal) suppressed Rb enrichment in the 

most evolved melts. No systematic compositional trends with 

time are recognized among eruptive products of Mammoth 

Mountain, as more and less evolved units were emplaced both 

early and late during construction of the edifice (figs. 16, 17). 

The longest arrays (1.5–2.9 percent SiO2), although narrow, are shown by eruptive units dfl, dml, dms, and dwr. In 

contrast, units dlp, dsd, dtl, rce, rmf, rsq, and rss all exhibit 

tight arrays that extend through less than one percent SiO2. 

Samples of units dbp, dom, ddl, and ddu overlap linearly 

within the limited SiO2 range 65.7–67.1 percent, consistent 

with their stratigraphic stacking on the southeast side of the 

edifice. The steep Dome 2861 (unit d61) stands out among 

trachydacites in its enrichment in Ba and Zr (fig. 17D) 

and in its high Na content, which is reflected in its relative 

isolation on figure 15.The alkalic rhyodacites, poorer in 

phenocrysts than the trachydacites, tend to be compositionally bimodal (fig. 17); units rce, rfp, and rrc are richer in K 

and Nb than units rss and rsq but poorer in Ti, Fe, Mg, Ca, 

Sr, and P. Among the rhyodacites, unit rmf, slightly richer in 

phenocrysts and erupted at a different vent, is compositionally intermediate in some respects but unique in its low Fe 

and high Rb and Ba contents. Considering the Mammoth 

Mountain suite as a whole, there is remarkable compositional 

variety among the 25 silicic units that built a compact edifice 

over a lifetime of ~50,000 years, a variety (fig. 17) that 

presumably reflects both persistent mafic recharge and effective crystal-melt fractionation of its crystal-rich, multiphase 

phenocryst assemblage.

Mafic Periphery

Of the 38 late Pleistocene eruptive units peripheral to 

Mammoth Mountain, most are alkaline (fig. 15) by the IUGS 

criteria of LeBas and others (1986); six are transitional (units

bcd–bed, bsm, mdn, mlc, and msj), and four are subalkaline olivine-rich basalts (units bfh–bmc, bhl, and Holocene 

brc). The six andesitic and three dacitic peripheral units are 

all unequivocally alkaline. The least silicic units (48.4–50.4 

percent SiO2) are bsm, bfh–bmc, bhl, and brc, the latter pair 

also being among the youngest basaltic units in the map area. 

The most magnesian units are bhl (9.9–10.5 percent MgO) and 

brc (8.1–8.3 percent MgO), and units bcd–bed, bfh–bmc, 

and bsm are next with 6–7 percent MgO. For clarity, plots of 

data for the many peripheral units are separated into three sets 

of figures, for units principally in the South Moat, West Moat, 

and San Joaquin drainage (figs. 18–20, respectively).

Tight compositional arrays are exhibited by units asr, 

bfh–bmc, bhl, brc, bsc, bsm, mcv, mdm, mdn, mdp, 

mkv–mmc, mlc, msc, and mss, each ranging less than one 

percent in SiO2 content (figs. 18–20). The widest arrays (each 

ranging 2–3 percent in SiO2) are shown by units aic, amp, 

mcl, and mor, all of which are phenocryst-poor, and by the 

contaminated hybrid dacite chain of unit dnw (60.4–67.3 

percent SiO2). 

Many noteworthy points are illustrated in figures 18–20. 

All of the following concern the distribution, sequence, compositions, affinities, and distinctions among the 38 peripheral 

units of the Quaternary Mammoth system.

• Lava-flow aprons bcd, bfh, and mmc correlate well 

compositionally with their respective vent cones, units 

bed, bmc, and mkv, from which they are surficially 

separated by younger units. 

• Spatially associated units mnd and mor and scoria from 

their inferred (but poorly exposed) vent complex, unit 

msd, together yield an overlapping continuous array. 

• Units mss and mdp, directly superimposed on the 

canyon floor of the Middle Fork, yield tightly adjacent (but nonoverlapping) compositional groupings, 

centered respectively at 53 and 54 percent SiO2 (fig. 

20). Despite a measured age difference of ~40,000 

years, the apparent chemical affinity of the units suggests location of the unknown vent for younger unit 

mdp somewhere not far from that of unit mss, which 

is exposed near Soda Springs Campground. Distinguishably different paleomagnetic directions (Hildreth 

and others, 2014) and Sr, Nd, and Pb isotope ratios 

(Cousens, 1996) confirm independence of the two 

units.

• Subunit bar' (51.1–52.4 percent SiO2), which crops 

out in two areas within the primary apron of extensive 

phenocryst-rich unit bar (50.6–51.2 percent SiO2), is 

a low-Mg, high-Al variant that shares similar concentrations of Ti, Fe, Ca, and Na and has only slightly 

lower K and P than unit bar. Although petrographically similar to most exposures of unit bar, the subunit 

carries slightly more plagioclase and only half as much 

olivine. It is inferred to be a late flow within the poorly 

exposed medial apron of bar flows. 

• TiO2 concentration generally exceeds 1.2 percent in all 

the mafic units (though not in the dacites), but it peaks 

at >2 percent in intermediate units mic and mkv–mmc, 

in the intermediate suite mor–mnd–msd, and in basaltic unit bsr, all of which are phenocryst-poor. 

• Phenocryst-rich unit bhl is far more enriched in CaO 

(10.1–10.7 percent) than any other unit mapped here 

(fig. 18B). Carrying 5–7 percent clinopyroxene, 1–2 

percent olivine, and 10–15 percent plagioclase, the 

unit is also the richest in MgO (9.9–10.5 percent) and 
Composition of Eruptive Products 37

Sr, unusually rich in P, and relatively poor in Al. For 

its low SiO2 content, the unit has moderate K2O but 

unusually low Na2O, accounting for its contrasting 

positions on figures 15B and 18A. 

• Notably enriched in Al2O3 are units mlc (20.1–20.9 

percent), bar' (19.1–19.9 percent), and mdn (18.6–

19.5 percent). Each carries 15–20 percent plagioclase 

phenocrysts, somewhat more than other crystal-rich 

units like bcd, bfh, and bar but not greatly so. The 

latter three and most other mafic units here have peak 

Al2O3 values in the range 17.0–18.5 percent, while 

the andesites and dacites generally have still lower 

peak values. Noteworthy for its unusually low Al2O3

content is unit bhl (15.0–15.7 percent), probably 

owing to its abundance of clinopyroxene and olivine 

phenocrysts.

• Among the mafic units (49–55 percent SiO2), a wide 

range in alkalinity, especially in K2O, tends toward 

bimodality (figs. 15B, 18A, 19A). Scrutiny of the units 

composing the contrasting arrays apparent in those 

diagrams reveals no simple correlation with age or 

geographic distribution.

• Units ddc and drf, at 66–67 percent SiO2 among the 

most evolved units peripheral to Mammoth Mountain 

(figs. 15B, 18, 19), are both crystal-poor trachydacites. 

Although their outcrops are now separated by >6 km 

on opposite sides of the Sierran divide, both appear to 

have erupted from vents later buried by the Mammoth 

Mountain edifice, and both yield radioisotopic ages of 

~100 ka.

• Although most of the peripheral units have yielded 

compositional ranges of ≤1 percent SiO2, several have 

ranges of ~2 percent (units a62, aic, bar, bmn, and 

mcl), and a few have ranges larger still (units mor

3.1 percent and amp 4.4 percent). Several trachydacites of the Mammoth Mountain edifice yielded comparably wide compositional ranges. The hybrid dacite 

chain of the northwest moat (unit dnw), with a range of 

7 percent SiO2, is the special case of a contamination 

series (as discussed in its unit description). Such ranges 

illustrate the importance of analyzing several samples 

from most units. Single analyses could misrepresent a 

unit, would miss evidence for fractionation or mixing trends within units, and might tempt unwarranted 

modelling of affinities between unrelated units.

• Sr, Nd, and Pb isotope ratios have been measured by 

others for 28 samples of the monogenetic peripheral 

units of the Mammoth system (Cousens, 1996; Bailey, 

2004), representing eruptive units we can now identify 

as a62, aic, amp, bar, bcd, bfh, bhl, brc, bsr, drf, 

mcl, mcv, mdp, mor, and mss.

Most samples cluster narrowly: 87Sr/86Sr 

0.70613–0.70638; 143Nd/144Nd 0.51243–0.51259; and 206Pb/204Pb 19.16–19.27. Two 87Sr/86Sr values determined for Mammoth Mountain dacites (units d81 and 

rce) plot within the tight mafic cluster, as does a value 

for unit drf. Units bfh (0.70591), mss (0.70596), and 

bhl (0.7067) fall outside the 87Sr/86Sr cluster (although 

not outside for Nd or Pb). The only conspicuously 

anomalous outlier is the Holocene basalt of Red Cones 

(0.70516–0.70531 and 0.51277), unit brc, which is one 

of the few subalkaline peripheral units as well as one 

of the most primitive and the youngest. 

Although basement xenocrysts plagued early 

efforts to determine K-Ar ages of several of these 

units (Curry, 1971; Bailey and others, 1976; Mahood 

and others, 2010), Sr-Nd-Pb isotope data for nearby 

basement rocks show that upper-crustal contributions 

to mafic and dacitic products of the Mammoth system 

are small. For example, a metavolcanic schist of unit 

}mv near Agnew Meadows gave values of 0.72396 

for 87Sr/86Sr and 18.94 for 206Pb/204Pb. Metavolcanic 

rocks from the Ritter Range gave 87Sr/86Sr values that 

range from 0.7053 to 0.7264, most of them >0.709 

(Kistler and Swanson, 1981). Metasedimentary rocks 

just south of Long Valley Caldera gave 87Sr/86Sr values 

that range from 0.7090 to 0.7250 (Goff and others, 

1991). Granitoid plutons beneath the volcanic field 

have given 87Sr/86Sr values ranging from 0.70666 to 

0.70987, and other nearby Sierran plutons range from 

~0.707 to ~0.715 (Cousens, 1996; Bailey, 2004). The 

narrow isotopic ranges for products of the Mammoth 

system are incompatible with significant contributions 

from such basement rocks.

• Sampled only in the Inyo-4 corehole near Inyo Craters 

is a 170-m-thick stack of ~18 unexposed mafic lava 

flows that underlies ~150 m of flows correlated with 

units aic and mcl. All flows were analyzed chemically 

by Vogel and others (1994), and Sr, Nd, and Pb isotope 

ratios were determined for many by Cousens (1996). 

The unexposed units (~190–230 ka; figs. 6, 16) were 

grouped chemically by Vogel as follows: Lower Group 

II is basaltic trachyandesite (54.6–55.6 percent SiO2); 

Group III is basaltic trachyandesite (52.3–53.2 percent 

SiO2); and Group IV is transitional basalt (50.1–50.9 

percent SiO2). Vogel’s Group I is our unit aic, and his 

upper Group II is our unit mcl. Because his Group V 

is not in place (Vogel and others, 1994, p. 19,832) and 

thus lacks stratigraphic context, we ignored it.

Neogene Periphery

We do not attempt in this report to discuss in detail the 

many Neogene basaltic, andesitic, and dacitic units preserved 

around the periphery of the map area, on the rim of Long 
38 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

MgO, in weight percent

FeO*, in weight percent

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10

CaO, in weight percent

TiO2, in weight percent

Mammoth Mountain

3.5

4.0

4.5

5.0

5.5

62 63 64 65 66 67 68 69 70 71 72

K O, in weight percent

2

SiO2, in weight percent

EXPLANATION

dbp

ddl

ddu

dfl

dgr

dlp

dml

dms

dnh

dnk

dom

drc

dsd

dsk

dsu

dtl

dwr

d61

d81

rce

rfp

rmf

rrc

rsq

rss

A

B

C
Composition of Eruptive Products 39

0

100

200

300

400

500

600

70 80 90 100 110 120 130

Rb, in parts per million

Sr, in parts per million

200

250

300

350

400

450

500

1,200 1,300 1,400 1,500 1,600 1,700 1,800 1,900

Zr, in parts per million

Ba, in parts per million

Mammoth Mountain

EXPLANATION

dbp

ddl

ddu

dfl

dgr

dlp

dml

dms

dnh

dnk

dom

drc

dsd

dsk

dsu

dtl

dwr

d61

d81

rce

rfp

rmf

rrc

rsq

rss

D

E

Figure 17. Compositional plots for Mammoth Mountain. A, K2O versus SiO2; B, CaO versus TiO2; C, MgO 

versus FeO* (total iron calculated as FeO); D, Zr versus Ba; E, Sr versus Rb. Data for all samples in 

appendix.
40 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

1

2

3

4

5

6

7

8

9

10

11

5 6 7 8 9 10 11

FeO*, in weight percent

MgO, in weight percent 

C

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

15 16 17 18 19 20 21

Al2O3, in weight percent

CaO, in weight percent

B

A

South Moat

0.5

1.0

1.5

2.0

2.5

3.0

3.5

48 50 52 54 56 58 60 62 64

K O, in weight percent

2

SiO2, in weight percent

aml

amp

asr

bcd

bed

bfh

bhl

bmc

bsc

bsr

mcl

mkv

mlc

mmc

msc

EXPLANATION
Composition of Eruptive Products 41

600

800

1000

1200

1400

1600

1800

2000

0 10 20 30 40 50 60 70 80

Rb, in parts per million

Sr, in parts per million

E

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

TiO2, in weight percent

P O2 5, in weight percent

D

South Moat

aml

amp

asr

bcd

bed

bfh

bhl

bmc

bsc

bsr

mcl

mkv

mlc

mmc

msc

EXPLANATION

Figure 18. Compositional plots for peripheral units of Mammoth magmatic system that crop out along 

south moat of Long Valley Caldera and in Lakes Basin. A, K2O versus SiO2; B, CaO versus Al2O3; C, MgO 

versus FeO*; D, P2O5 versus TiO2; E, Sr versus Rb. Data in appendix.
42 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

West Moat

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

48 50 52 54 56 58 60 62 64 66 68

SiO2 in weight percent

K O, in weight percent

2

aic

bar

bar'

bcf

bmn

bmn'

bsm

ddc

dnw

mcl

mcv

mdm

mdn

mic

mnd

mor

msd

msj

EXPLANATION

2

3

4

5

6

7

8

9

10

15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5

Al2O3, in weight percent

CaO, in weight percent

0

1

2

3

4

5

6

7

8

3 4 5 6 7 8 9 10 11

FeO*, in weight percent

MgO, in weight percent

A

B

C
Composition of Eruptive Products 43

0.15

0.25

0.35

0.45

0.55

0.65

0.75

0.85

0.95

1.05

0.7 0.9 1.1 1.1 1.5 1.7 1.9 2.1 2.3 2.5

TiO2, in weight percent

P O2 5, in weight percent

250

450

650

850

1,050

1,250

1,450

0 20 40 60 80 100 120

Rb, in parts per million

Sr, in parts per million

West Moat

aic

bar

bar'

bcf

bmn

bmn'

bsm

ddc

dnw

mcl

mcv

mdm

mdn

mic

mnd

mor

msd

msj

EXPLANATION

D

E

Figure 19. Compositional plots for peripheral units of Mammoth magmatic system that crop out along 

west moat of Long Valley Caldera. A, K2O versus SiO2; B, CaO versus Al2O3; C, MgO versus FeO*; D, 

P2O5 versus TiO2; E, Sr versus Rb. Data in appendix.
44 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

0

1

2

3

4

5

6

46 48 50 52 54 56 58 60 62 64 66 68

SiO2, in weight percent

K O, in weight percent

2

a62

amp

apb

bdc

brc

drf

mdp

mss

Tacc

Tasj

Tbdp

Tbld

Tbmm

Tbpl

Tbrm

Tbtb

Tdsd

Tdsj

Tmcl

Tmcm

Tmcr

Tmcw

Tmel

Tmsd

Tmwc

Tbtb

brc

bdc mss

mdp

Tasj

a62

apb

amp

Tacc

drf

Tdsj Tdsd

San Joaquin

EXPLANATION

1

2

3

4

5

6

7

8

9

10

11

12

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

TiO2, in weight percent

CaO, in weight percent

Tv

Qv

bdc

brc

0

1

2

3

4

5

6

7

8

9

10

11

12

2 3 4 5 6 7 8 9 10 11 12

FeO*, in weight percent

MgO, in weight percent

Tv

Qv

brc (8 ka)

Tasj (9.1 Ma)

A

B

C
Composition of Eruptive Products 45

0

50

100

150

200

250

300

350

400

0 500 1,000 1,500 2000 2,500 3,000 3,500 4,000

Ba, in parts per million

Zr, in parts per million

Tv

Qv

Qv

Tv

0

500

1,000

1,500

2,000

2,500

0 20 40 60 80 100 120 140

Rb, in parts per million

Sr, in parts per million

Tv

Qv

Tdsj Tdsd

Tbmm

D

E

San Joaquin

a62

amp

apb

bdc

brc

drf

mdp

mss

Tacc

Tasj

Tbdp

Tbld

Tbmm

Tbpl

Tbrm

Tbtb

Tdsd

Tdsj

Tmcl

Tmcm

Tmcr

Tmcw

Tmel

Tmsd

Tmwc

EXPLANATION

Figure 20. Compositional plots for eight peripheral units of Mammoth magmatic system that crop out along 

Middle Fork of San Joaquin River drainage system, along with data for 17 Tertiary units nearby. A, K2O versus 

SiO2; B, CaO versus TiO2; C, MgO versus FeO; D, Sr versus Rb; E, Zr versus Ba. Data for all samples given 

in appendix. Dividing lines in each panel separate Tertiary (Tv) from Quaternary (Qv) volcanic units; a few 

exceptions are labelled. Dashed lines in panel C indicate compositional arrays of units Tacc and Tbtb, as 

discussed in text. Of four analyses for unit Tasj, two are samples from the map area and two are from near 

Millerton Lake (fig. 2), 80 km downstream.
46 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Valley Caldera and within the Middle Fork San Joaquin 

drainage system. Their eruptive volume was at least five 

times greater than that of the entire late Pleistocene Mammoth system, and most of them erupted during the Pliocene. 

However, most of such pre-Quaternary eruptive products have 

been eroded away. No mafic or intermediate unit is known to 

have erupted between the end of the Pliocene and ~230 ka. We 

anticipate that ongoing work will comprehensively compare 

the many precaldera eruptive units north and east of the caldera with the westerly units described here (see Description of 

Map Units). Because Pliocene units Tacc, Tbtb, and Tmcw

were previously mapped as Quaternary (Bailey, 1989), however, it seems necessary to comment here on their ages and 

compositions. We also summarize results of our search within 

the Middle Fork drainage basin for the unknown source vents 

of units Tbtb, Tmcw, and mdp.

The pair of glaciated domes combined as unit Tacc

(fig. 10) on the granitic rim high above Crater Creek is compositionally bimodal (fig. 20). Although both are phenocrystpoor and similarly contaminated with granite-derived xenocrysts, the southwest dome has 58.3–58.7 percent SiO2

 and 

the northeast dome 61.3–62.6 percent. They differ likewise in 

Fe, Mg, and Ca contents, but what confirms their affinity is 

their similar and unusually low concentrations of TiO2 (~0.6 

percent) and P2O5 (~0.3 percent). The southwest dome yielded 

a 40Ar/39Ar age of 4.29±0.20 Ma.

The basalt of The Buttresses (unit Tbtb) is a severely 

glaciated stack of many lava flows preserved as five discrete 

remnants that span an elevation range of 470 m on the west 

wall of the Middle Fork canyon. As the largest remnant still 

extends down to river level, they appear to represent a conformable set of flows that once filled the canyon to a depth 

as great as 500 m. A columnar flow resting on granite gave 

a 40Ar/39Ar age of 3,754±7 ka, consistent with its reversed 

paleomagnetic polarity. Although all the flows examined are 

rich in olivine and clinopyroxene, the stack is compositionally bimodal, and the two compositions are interlayered in 

all sectors. Five flows sampled contain 46.5–47.5 percent 

SiO2 and lack plagioclase phenocrysts; five other flows have 

49.8–50.6 percent SiO2 and carry a few percent plagioclase. 

Remarkably, Mg and Al increase systematically with SiO2

while Fe, Ti, and P decrease steeply and Ca modestly within 

the 10-sample array (fig. 20). Preservation of the base of 

the Tbtb stack on granitic basement at river level indicates 

that the floor of this south-flowing reach of the Middle Fork 

canyon had already attained its present depth in the Pliocene. Pleistocene glaciation widened but did not deepen the 

canyon here.

Across the river and 2 km farther downstream along the 

east wall, a remnant pair of columnar lava flows (unit Tmcw) 

resembles some flows of unit Tbtb in carrying subordinate 

plagioclase and abundant olivine and clinopyroxene. The unit 

yields a reversed paleomagnetic direction similar to that of 

Tbtb. Nonetheless, the flows are more silicic (52.3–52.6 percent SiO2

) and far less magnesian than any flows of unit Tbtb

(fig. 20). They probably represent a Pliocene eruptive episode 

independent of that of unit Tbtb, but vents have been located for 

neither unit.

In trying (unsuccessfully) to locate vents for units Tbtb

and Tmcw (and for late Pleistocene unit mdp), we sampled 

nine more mafic remnants along the Middle Fork canyon 

(units Tasj, Tbdp, Tbld, Tbpl, Tmcl, Tmcm, Tmel, Tmsd, 

and Tmwc), for six of which we recognize vents. As none of 

the nine units provided satisfactory chemical or petrographic 

matches, the search was extended farther up-canyon, to Agnew 

Pass and Clark Lakes north of the map area, equally unsuccessfully. A rewarding result of the search for source vents, 

nonetheless, was finding that unit Tasj, which underlies unit 

Tacc on the Crater Creek rim, is compositionally and petrographically identical (fig. 20) to the intracanyon andesite 

of Kennedy Table (fig. 2), 80 km downstream along the San 

Joaquin River canyon (Huber, 1981). Distal and proximal 

outcrops both yield 40Ar/39Ar ages ~9.2 Ma.

Three more Pliocene mafic-lava remnants, units Tbmm, 

Tbrm, and Tmcr, also plotted in figure 20, rest on Mesozoic 

basement along the rim of Lakes Basin. They neither correlate 

with each other nor with Pliocene units recognized elsewhere, 

and source vents have not been located for any of them. Unit 

Tbmm is the type locality of the Mammoth Reverse Subchron 

(Doell and others, 1966).

The Tertiary samples plotted in figure 20 include three 

plugs high on the west wall of the Middle Fork drainage. That 

these units, Tmcl, Tmel, and Tmwc (all west of the map area; 

see fig. 3), had been reduced to nothing but massive vent-filling lavas draws attention to the severity of Pleistocene glacial 

erosion on the east flank of the Ritter Range (figs. 3, 4). Such 

upland erosion highlights even more strongly the contrasting 

evidence from unit Tbtb that the floor of this granitic segment 

of the Middle Fork had been cut to its present depth already in 

the Pliocene.

Of the eight late Pleistocene peripheral units along the 

Middle Fork (fig. 20), all are alkaline except the Holocene 

basalt of Red Cones (unit brc). The Neogene units plotted in 

figure 20 are similar to the Pleistocene alkaline units in total 

alkali contents, but almost all are more potassic (fig. 20A) 

as well as richer in Ca, Mg, Sr, and Ba (figs. 20B–E). At 5.2 

percent K2O, olivine-rich Pliocene unit Tmcm is shoshonitic (fig. 20A); and in common with unit Tmcm, unit Tbpl is 

unusually rich in Sr (fig. 20D).

Discussion

Were it not for its contiguity with Long Valley Caldera, 

the Mammoth volcanic field might long ago have been recognized as an independent magmatic system. Like the larger 

Lassen Volcanic Center in the Cascade arc (Clynne and 

Muffler, 2010), it consists of a central cluster of phenocrystrich dacite lava domes surrounded by a contemporaneous 

array of mafic volcanoes in an extensional tectonic setting. 

The Long Valley system had erupted ~850 km3 of rhyolite 
Discussion 47

magma, all of it subalkaline and most of it phenocryst-poor. 

In contrast, eruptive products of the Mammoth system are 

two orders of magnitude less voluminous and almost entirely 

alkaline. All of its silicic products are phenocryst-rich and 

none are rhyolitic. 

Mammoth System Versus Long Valley System

The Long Valley vent array was 30 km across, and for 

nearly 2 million years (2.2–0.3 Ma) it erupted nothing but rhyolite, centrally or peripherally. Its large silicic magma reservoir evidently prevented eruption of the mafic magma inferred 

to have sustained it thermally; mafic enclaves have been found 

in only three of ~100 rhyolitic eruptive units. In contrast, the 

vent array of the Mammoth system is predominantly mafic 

and 10 by 20 km wide; its central trachydacite edifice is only 

5 km across and carries abundant relatively mafic enclaves 

and xenocrysts. The footprint of the Mammoth vent array is 

compactly circumscribed, extending west from the caldera’s 

ring-fault zone to the San Joaquin River and north-south from 

Deadman Creek to Pumice Butte. Beyond the limits of the 

well-defined Mammoth system, the nearest Quaternary mafic 

vents are in Mono Basin (at June Lake and Black Point) and 

85 km southeast near Big Pine. 

Inception of Mammoth mafic volcanism at 233±4 ka 

culminated the 2-million-year-long west-southwestward 

migration of the mantle-derived focus of deep crustal intrusion 

that had successively energized the rhyolitic systems manifest 

as Glass Mountain, the Bishop Tuff, and the early postcaldera 

rhyolite. Growth of the Mammoth system sustained that trend, 

yielding a secular drift of the melting anomaly toward 240o at 

a long-term average rate of ~15 m/kyr from Glass Mountain to 

Mammoth Mountain.

The Mammoth Mountain edifice and almost all of the 38 

monogenetic peripheral units vented outside (west or southwest of) the caldera’s ring-fault zone. However, proximity 

to the ring-fault zone of a few of the mafic vents (units bed, 

mdm, mkv; possibly also bsm and msc) inspires the attractive hypothesis that their intrusive counterparts (165–120 ka) 

reenergized part of the crystallizing Long Valley reservoir 

to yield eruption of the four subalkaline rhyolite lavas in the 

west moat at 150–100 ka (units rwm, rdc, rmk, and rdm). 

Moreover, glass and mineral Rb-Sr isochrons for these four 

rhyolites suggest a feldspar fractionation episode at 257±39 ka 

(Heumann and others, 2002); a 230Th-238U mineral isochron 

gives an age of 236±1 ka for rhyolitic unit rdm (Heumann and 

others, 2002), similar to the weighted mean (230Th/238U) model 

age of the main zircon population in that unit (Reid and others, 

1997). These mineral-crystallization ages are 100–150 kyr 

older than eruption ages of the four west moat rhyolites, but 

they are essentially synchronous with initiation of the adjacent Mammoth system at 233±4 ka. Outside of the west moat, 

there have been no Long Valley eruptions since ~330 ka, and 

those between ~570 ka and 330 ka had been rhyolites of small 

volume.

A few of the Mammoth peripheral monogenetic vents 

are within the west moat, their locations thus conceivably 

influenced by proximity to a buried segment of the rangefront (basement-hosted) Hartley Springs Fault Zone that 

foundered with the 767-ka caldera. Most of the 38 peripheral 

units, however, vented in the Sierra Nevada, or high on the 

caldera wall, or beneath the Mammoth Mountain edifice. 

The footprint they define is broadly scattered, ovoid in plan, 

nonlinear, and (in contrast to the Holocene Inyo alignment) 

clearly not controlled by the rangefront fault zone. In any 

case, the rangefront fault zone terminated beneath what later 

became the caldera’s west moat, never having extended as 

far south as the site of Mammoth Mountain and, instead, 

stepped left ~15 km across the later site of the caldera, 

resuming as the range-bounding Hilton Creek Fault south of 

the caldera margin (fig. 3).

In contrast to the wide mafic array, the Mammoth Mountain trachydacite edifice is focussed over a structural singularity (fig. 5). It banks against Long Valley caldera’s steep 

topographic wall; it overlies the near-vertical regional contact 

that separates Paleozoic metasedimentary from Mezozoic 

metavolcanic sections; it overlies the margins of one or more 

Cretaceous granitoid plutons that intrude the pre-Cenozoic 

sections; and it overlies a segment of the near-vertical Sierra 

Crest Shear Zone System (fig. 5). Reflecting the field of 

mantle-derived magma ascent, the peripheral array of monogenetic vents is much broader than this structural nexus, but 

the intracrustal site of the Mammoth Mountain silicic magma 

body that developed at its core was probably localized by this 

convergence of older structures.

Rhyolitic Versus Trachydacitic Culminations

Why does the edifice that extruded piecemeal from a 

silicic magma body at the center of the Mammoth monogenetic volcanic field consist predominantly of crystal-rich 

trachydacite? Why does it lack the rhyolite characteristic of 

Long Valley, Mono Craters, Coso, and many more Neogene 

and Quaternary volcanic fields in the extensional Basin 

and Range Province? In common with many such fields, 

the Mammoth volcanic field tends toward compositional 

bimodality (fig. 16), but the modes here are 48–55 percent 

and 64–70 percent SiO2, not basalt and rhyolite. The few 

silicic andesites (units a62, aic, aml, amp, apb) that occupy 

the gap between modes (fig. 16) are all phenocryst-poor, 

thus presumed to have been melt batches that separated 

from crystal-richer basaltic representatives of the mafic 

mode. Of the 25 silicic units exposed on the silicic edifice, 

19 are crystal-rich trachydacites (64–69 percent SiO2) and 

six are alkalic rhyodacites (70–71 percent SiO2) that carry 

only half as many phenocrysts. Six times between ~85 and 

50 ka, melt-enriched fractions separated from trachydacitic 

mush as small rhyodacite batches that were buoyant and 

voluminous enough to erupt. Most of the trachydacite extrusions carry relatively mafic blebs and enclaves and scattered 
48 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

clinopyroxene crystals likely to represent persistent mafic 

recharge of a recurrently stirred crystal-rich reservoir. As 

crystal-depleted extracts from the top of the reservoir, the 

rhyodacites lack evidence of such mafic cargo.

Products of the Pliocene volcanic field (~4.0–2.6 Ma) 

that preceded the Long Valley rhyolitic episode are well 

exposed around the caldera rim (Bailey, 1989, 2004). The data 

define an alkalic compositional array that is fairly continuous from 49 to 69 percent SiO2

. Like the late Quaternary 

Mammoth suite, however, rhyolites are absent, and most of 

the silicic units are crystal-rich hornblende-biotite-feldspar 

trachydacites. The extended Long Valley rhyolitic interval 

(2.2–0.3 Ma), which was accompanied by no mafic eruptions, 

was thus bracketed by the Pliocene and Mammoth eruptive 

intervals, when alkalic basalts and their intermediate alkalic 

derivatives were accompanied principally by phenocryst-rich 

trachydacites but no rhyolites.

What accounts for the contrast between the exclusively 

rhyolitic episode and the basalt-to-dacite episodes that temporally bracket it? It cannot be presumed that primitive magma 

input from the mantle remained mildly alkalic basalt throughout the entire 4-million-year-long eruptive history. It seems 

likely that the contrasting patterns of intracrustal magma evolution reflect far more intensive mantle melting during the rhyolite interval. This would have entailed higher-melt-fraction 

subalkaline basalts, elevated rates of basaltic intrusion into the 

crust, and concomitantly greater degrees of deep-crustal partial 

melting, feeding back as an expanding barrier of mushy ductile crust that prevented further ascent of the basalts and thus 

intensified intracrustal melting. No mafic batches were erupted 

during the long rhyolitic interval, but mafic enclaves found in 

a few postcaldera rhyolitic lava flows are indeed subalkaline 

basalt in unit rer (~680 ka) and transitional basaltic andesite in 

unit rnc (~570 ka). 

Simon and others (2007) inferred from U-Pb zircon ages 

and Pb-isotope ratios of phenocrysts and host glasses that, 

during the Glass Mountain through Bishop Tuff rhyolitic 

interval (2.2–0.76 Ma), the Long Valley system had undergone 

a broadly increasing rate of rhyolite production, decreasing 

magma residence times, declining crustal contributions, and an 

increasing proportion of mantle input. Such trends are unlikely 

to have reversed during the postcaldera eruption of ~100 km3

of phenocryst-poor Early rhyolite (~750–650 ka), but the 

rhyolite eruption rate dropped drastically thereafter.

By contrast, during the low-flux dacite-culminating 

intervals, lower-melt-fraction alkalic basalts might ascend 

from the mantle in smaller batches, lodge at deeper crustal 

depths, induce less crustal melting, and produce derivative 

products dominated by fractionation and remelting of mafic 

intrusions. Smaller batches crystallize faster, and reduced 

contributions from dehydration melting of crustal wall rocks 

would limit the water contents of fractionating magmatic 

hybrids, favoring crystal-rich viscous intermediates rather 

than accumulation of water-enriched, low-temperature, lowcrystallinity rhyolite. During the Mammoth episode and the 

much longer Pliocene episode of basalt-to-dacite eruptions, 

the volumetric eruption rate was an order of magnitude 

lower than during the extended Long Valley rhyolite interval 

(2,200–650 ka). Assuming similar rate ratios for fluxes of 

mantle-derived basalt, deep-crustal reservoirs should have 

remained relatively modest during the low-flux episodes, 

sporadically releasing small batches of dacite melt that could 

ascend to upper-crustal subvolcanic chambers where they 

could partially degas and crystallize extensively (Annen and 

others, 2006). Dacite and silicic andesite batches contemporaneous but peripheral to the Mammoth Mountain chamber 

evidently ascended to the surface with little interruption, 

erupting as crystal-poor units drf, ddc, aml, and amp. During the long high-flux interval between, deep-crustal melting 

was areally and volumetrically far more extensive, enabling 

the melting region to expand into the middle crust, inducing dehydration melting of hydrous protoliths, and releasing large batches of water-rich intermediate melt to shallow 

chambers where they could fractionate thick roof-zone layers 

of low-temperature crystal-poor rhyolite melt (Hildreth, 

2004).

In summary, the 4-million-year-long magmatic history 

here is envisaged to include a 2-million-year-long episode 

of vigorous mantle upwelling and subalkaline basaltic 

magma production sandwiched by low-flux episodes of 

lower-melt-fraction alkalic basalt production. Derivatives of 

the main phase culminate in voluminous crystal-poor subalkaline rhyolite, whereas those of the less vigorous bracketing episodes culminate in crystal-rich trachydacite. The 

succession is not unlike that of oceanic hotspot volcanoes 

that move progressively across fixed melting columns, thus 

producing early and late alkalic episodes that sandwich far 

more voluminous tholeiitic shield stages. The GPS velocity 

of the Long Valley region (in a fixed North American reference frame) is 8–12 mm/yr northwestward (Oldow, 2003), in 

contrast to the southwesterly drift of the rhyolite focus and 

thus in complete conflict with the model of plate migration 

over a fixed melting column. Here, it is the mantle melting 

anomaly itself that waxed and then waned. 

During the Pliocene, the basaltic flux was widely distributed but moderate, culminating in a field of trachydacite 

domes scattered from Crater Creek to San Joaquin Mountain, 

Bald Mountain, and Laurel Mountain, and probably spanning 

the later site of Long Valley Caldera. The mantle-derived 

flux intensified and focussed more locally by 2.2 Ma, first 

beneath Glass Mountain, then migrated west-southwest at 

~15 mm/yr beneath the sites of Long Valley and the Early 

rhyolite, and finally diminished after ~650 ka. How regional 

redistribution of intraplate extension may have led to a refocusing of intense mantle upwelling ~20 km north, beneath 

subalkaline rhyolitic Mono Craters around 60 ka, is a speculation we need not address here. Adjacent to the southwest 

margin of the Early rhyolite, however, and on the trend of 

the secular migration of the magmatic focus, a relocated 

but diminished and more distributed flux of mantle magma 

ascent initiated the alkaline Mammoth basalt-to-dacite system by 230 ka. 
Discussion 49

Mammoth System Magma Reservoir

Although the vent field for Mammoth Mountain and 

its 38 peripheral units covers nearly 90 km2, the total eruptive volume is small. The poorly constrained total eruptive 

volume for the peripheral array is between 7 and 12 km3, the 

most voluminous units being mcl, amp, bar, bfh, and mormnd. The scattered peripheral vent array can be compared 

to the present-day midcrustal distribution of scattered long 

period (LP) seismicity, which extends ~10 km east-west from 

beneath Mammoth Mountain to the Middle Fork at depths 

of 10–18 km (Pitt and others, 2002; Hill and Prejean, 2005). 

The numerous LP earthquakes are likely to represent injection 

of basaltic dikes and ascent of CO2-rich fluids derived from 

them. Beneath the LP array, at lower-crustal depths of 19–31 

km, a few ascending swarms of brittle-failure earthquakes 

(2006–2009) were likewise interpreted (Shelly and Hill, 2011) 

as slip induced in otherwise ductile crust by pressurized fluids. 

Only a few dozen times in the 230,000-year-long Mammoth 

episode did magmatic dikes attain the surface and feed 

monogenetic eruptions, but ascent of magma-derived CO2-rich 

hydrous fluids along fractures may be much more common. 

An 11-month-long earthquake swarm beneath Mammoth 

Mountain in 1989 (Hill and others, 1990) produced a vertical planar distribution of hypocenters at 6–9 km depth that 

was interpreted as fracture-bound fluid injection, potentially 

related to deeper dike intrusion beneath the brittle crust (Hill 

and Prejean, 2005). The several tree-kill sites peripheral to 

the edifice (figs. 5, 14), caused by elevated CO2 emission, was 

first observed in 1990.

Mammoth Mountain itself, despite uncertainties about 

glacial losses and depth of its concealed base, erupted no 

more than 4±1 km3 of silicic lavas and pyroclastics (including the two off-edifice domes d61 and d81). The consistent 

similarity of eruptive products at Mammoth Mountain—their 

high crystal content, multiphase mineralogy, and continuous 

range of bulk composition—throughout a 50,000-year active 

lifetime suggests a stably located, relatively simple magma 

reservoir. Vents on the edifice are confined within a footprint 

only ~2 km2 in area, and most vents fall along a northwesterly 

trend (fig. 8) that crudely approximates that of the nearvertical bedding and foliation of the basement (fig. 5). The 

narrowly elongate vent corridor on the edifice may reflect 

northwest elongation of the silicic magma reservoir, as influenced by the basement structure, which might likewise favor a 

vertically prolate chamber and disfavor sill emplacement. The 

widespread occurrence in the trachydacites of well-distributed 

relatively mafic cargo—enclaves, blebs, streaks, plagioclaseclinopyroxene clots—attests to persistent mafic recharge 

and convective stirring within a compact unitary chamber. 

Nonetheless, over the course of a multiepisode 50,000-year 

eruptive history, it should be expected that small intrusive 

masses, rinds, and dikes would crystallize, probably coexisting 

transiently with active pods of crystal mush that wax and wane 

in crystallinity, sporadically erupt, and less often fractionate 

melt-enriched rhyodacitic lenses. 

The 87Sr/86Sr values for Mammoth Mountain trachydacites 

(units d81 and rce), along with that for peripheral crystal-poor 

trachydacite unit drf, are virtually the same (0.7063±0.0001) 

as for most units of the contemporaneous mafic periphery, 

suggesting comagmatic descent (whether by direct fractionation or by also entailing partial remelting of forerunning 

crustal mafic intrusions). Sr-Nd-Pb isotope data for nearby 

basement rocks confirm that upper-crustal contributions 

to mafic and dacitic products of the Mammoth system are 

small. For example, a metavolcanic schist of unit }mv just 

northwest of Mammoth Mountain gave 0.72396 for 87Sr/86Sr. 

Granitoid plutons beneath the Mammoth volcanic field have 

given 87Sr/86Sr values ranging from 0.70666 to 0.70987, and 

other nearby plutons in the eastern Sierra range from ~0.707 to 

~0.715 (Cousens, 1996; Bailey, 2004).

Although the silicic magma reservoir beneath Mammoth 

Mountain may well have crystallized by ~50 ka, abundant 

geophysical evidence shows that mantle-derived mafic magma 

still intrudes the crust beneath the Mammoth volcanic field 

today, as it has since ~230 ka. The eruption ages summarized 

in figure 16 show that time intervals between eruptions here 

have rarely been as long as 10,000 years. The most recent 

eruption was ~8,000 years ago.

Mammoth Mountain When Active

The silicic edifice itself was intermittently active for 

several tens of thousands of years but has been quiescent since 

~50 ka. Late Pleistocene glaciers mantled the entire edifice, 

quarried substantial fractions of its many domes and coulees, and removed almost all of its pyroclastic apron. Despite 

the glacial erosion, however, the 5-km-wide footprint of the 

lava-dome complex has been reduced only marginally, and its 

~1 km of relief is unlikely to have been reduced by more than 

100 m. What can be inferred about the volcano when it was 

active? 

Historically active dome complexes comparable 

to Mammoth Mountain include Soufrière Hills, Unzen, 

Santiaguito, Augustine, and Mount St. Helens (Druitt and 

Kokelaar, 2002; Hoshizumi and others, 1999; Nakada and 

others, 1999; Harris and others, 2003; Waitt and Begét, 2009; 

Hopson, 2008; Clynne and others, 2008). Although each differs from Mammoth Mountain in particular characteristics, 

all were constructed by repeated extrusion of phenocryst-rich 

domes and derivative thick lobes and tongues of viscous lava. 

Bulk compositions of the crystal-rich domes and coulees range 

from silicic andesite to rhyodacite (58–71 percent SiO2), overlapping those of Mammoth Mountain (63–71 percent SiO2

). 

All are amphibole-bearing, and all produced aprons of pyroclastic-flow deposits, principally from block-and-ash flows 

generated by gravitational collapse of oversteepened parts of 

growing lava domes and coulees. There can be little doubt 

that such aprons widely surrounded Mammoth Mountain, 

too, and that the fragmental deposits were glacially stripped 

during the late Pleistocene. Pumice-rich pyroclastic flows and 
50 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

falls generated by column collapse (though common at Mount 

St. Helens) are generally subordinate at these volcanoes and 

likewise at Mammoth Mountain, where only one such eruptive 

unit (rfp) has been preserved.

As at Mammoth Mountain, most of the extrusions that 

built each edifice issued from either a central vent or a fairly 

compact vent complex. Patterns include tight clusters of interfering domes at summit vents (Augustine, Soufrière Hills), 

vent alignments 1–3 km long (Mammoth, Santiaguito), nearsummit dome clusters (Fugendake cone at Unzen), and scatterings of flank domes 1–3 km outboard of a summit cluster 

(Augustine, St. Helens, Mammoth). Like the pair of off-edifice 

domes at Mammoth Mountain, two large middle-Holocene 

domes grew 4 km east of the compositionally similar and 

contemporaneous Fugendake edifice at Unzen.

At various stages of development, most of the comparative volcanoes named above were marked by craters or sectorcollapse amphitheaters, which were commonly refilled by 

younger domes, but no evidence for such features survives at 

Mammoth Mountain. Neither are the tops of conduits or dikes 

exposed beneath the lavas of Mammoth Mountain, but studies 

at comparative volcanoes suggest that conduits for comparable 

viscous domes were 30–70 m in diameter, flaring to 100–250 m 

near the surface. Pulses of dome extrusion at the historically 

observed comparators were commonly punctuated by vulcanian 

explosions of short duration that produced ash plumes that rose 

several kilometers and modest density currents that extended a 

few kilometers radially, but no pyroclastic deposits from such 

events have survived glacial scour at Mammoth Mountain. 

Millennium-long sequences of silicic dome extrusions are 

sometimes interrupted by central eruption of relatively mafic 

(or andesitic) magma (Unzen, St. Helens), but the last such 

penetration at the site of the silicic Mammoth edifice was by 

the voluminous unit amp at ~97 ka. Nonetheless, ubiquity 

of mafic enclaves in the silicic lavas of Mammoth Mountain 

and of most of its comparators provides evidence of frequent 

recharge and convective redistribution within crystallizing/

fractionating/mixing upper-crustal magma chambers.

Extrusive episodes at these volcanoes have been shown to 

represent contrasting timescales. At one extreme, ~20 episodes 

of dome growth took place in six years in the crater of Mount 

St. Helens (1980–1986); following the first three, small domes 

were destroyed explosively. Similarly, at Soufrière Hills pulses 

of dome growth and partial collapse (1995–2012) have been 

numerous. At Unzen, in 1991–1995, a complex dacite dome 

produced 13 flow lobes in four years, each lobe advancing 

1–10 m/day for several months. A typical lobe there is 350 m 

long, 250 m wide, and 50–100 m thick, smaller than most 

(but not all) at Mammoth Mountain. As each lobe cooled and 

stalled at Unzen, a new piggyback lobe sometimes formed 

over the vent. If successive flow units at Mammoth Mountain 

had extruded so quickly, they could be expected to have identical paleomagnetic directions (contrary to our measurements).

At Santiaguito, eight extrusive episodes since 1922 have 

produced lava lobes and flows 0.5–7 km long, and at Augustine, seven extrusive events have taken place since 1812. On 

such decadal timescales, paleomagnetic directions of successive lavas might be distinguishable or even describe a coherent secular path among dome fragments that survive, but in 

blocky silicic lavas the measurements might seldom be precise 

enough to confirm such patterns.

The eruptive history of Mount St. Helens since ~28 ka 

(Mullineaux, 1996; Clynne and others, 2008) may offer the 

closest analogue for the episodic extrusive record at Mammoth 

Mountain. Several eruptive stages, each a few thousand years 

long, were separated by dormant intervals that also lasted for 

thousands of years. During the past 4,000 years, the detailed 

stratigraphic record at Mount St. Helens indicates numerous 

eruptive events with a millennial to centennial (and sometimes 

decadal) frequency. Extrusion of dacite domes at Mount St. 

Helens was more commonly accompanied by pumice-rich 

eruption columns than at Mammoth Mountain, but the unsystematic timescales and the millennial episodes of dormancy 

appear to have been similar at both volcanoes.

Volcano Hazards

Since 1980, seismicity, deformation, CO2 discharge, 

and changes in local hydrothermal systems have stimulated 

scientific and societal concern in the Mammoth Lakes area, 

prompting intensive and continuous monitoring by scientists 

of the Volcano Hazards Program of the U.S. Geological Survey (USGS). The unrest and monitoring have been interpreted 

and summarized by Hill and others (2002), Hill and Prejean 

(2005), and Hill (2006). Several Fact Sheets were prepared to 

interpret the ongoing processes for the general public (Sorey 

and others, 2000; Hill and others, 1997, 2014; Farrar and others, 2007). The USGS California Volcano Observatory Web 

site provides background, maps, images, monitoring data, and 

frequent updates for Long Valley and Mammoth Mountain 

(http://volcanoes.usgs.gov/observatories/calvo/).

The Mammoth Mountain trachydacitic system erupted 

many times during a 50,000-year-long interval (100 ka to 

50 ka) but not at all during the last 50,000 years, suggesting 

that it is moribund. The dozens of basaltic and intermediate vent sites (~230 ka to ~8 ka) peripheral to Mammoth 

Mountain, however, outline a 10-by-20-km footprint for the 

mantle-derived magmatic flux that was intrinsic to generating 

the Mammoth Mountain silicic anomaly at its center. If the 

1989 seismic swarm directly beneath the Mammoth Mountain 

edifice represented mafic dike emplacement at 7–9 km depth 

(Hill and others, 1990), and if the ensuing thousands of brittlefailure earthquakes that extended from 6 km to as shallow as 

1 km below the summit represented propagation of a pressure front of magmatic fluid through a pre-existing fracture 

network (Prejean and others, 2003; Hill and Prejean, 2005), 

then this supports our inference that the trachydacitic magma 

reservoir had indeed solidified during the past 50,000 years. 

Moreover, it is implicit that the source of the ongoing CO2

discharge is mafic magma, not silicic.
Introduction to Description of Map Units 51

Midcrustal seismicity and CO2 discharge under and 

around the Mammoth Mountain edifice persist in 2014, 

providing convincing evidence for mafic magma storage deep 

beneath the footprint today. In addition to abundant shallow 

brittle-failure seismicity (Prejean and others, 2003) under and 

within the edifice, numerous long-period (LP) earthquakes 

take place at depths of 10–18 km beneath the mafic vent array, 

from Mammoth Mountain to Red Cones and Devils Postpile 

(Pitt and Hill, 1994; Hill, 1996; Pitt and others, 2002; Foulger 

and others, 2003; Hill and Prejean, 2005). The area underlain 

by the LP array is marked by an extraordinary conductive 

heat-flow anomaly (3.75 heat-flow units in a 250-m hole 

drilled in granitic unit Kmo near Devils Postpile; Lachenbruch 

and others, 1976b), and it coincides with an extracaldera 

salient in the gravity low (Carle, 1988) that extends from 

Mammoth Mountain to Pumice Butte and Devils Postpile, an 

area uniformly underlain by the same Cretaceous granite.

All the evidence thus suggests that the next eruption 

on or near Mammoth Mountain should be of mafic magma, 

which has here typically produced scoria cones and derivative lava flows. Examples close to the edifice include the 

Red Cones (unit brc) eruption of ~8 ka and several more vent 

cones—near Deadman Pass (unit mdn, ~17 ka), Crater Flat 

(mcv, ~33 ka), Minaret Summit (bar, ~90 ka), and Mammoth 

Crest (bmc, ~92 ka). The vent cones for units mcl (~175 ka) 

and mkv (~153 ka) are directly adjacent to densely developed parts of Mammoth Lakes township, as is the Mammoth 

Knolls rhyolitic edifice (rmk, 100 ka). A new mafic eruption 

in the area from Red Cones to Pumice Butte is unlikely to do 

more than ignite a forest fire and rain fine ash on downwind 

settlements, but a recurrence in the Lakes Basin or around the 

eastern half of Mammoth Mountain could cause severe damage to property and the local economy. An eruption in the bed 

of the Middle Fork (like unit mss, ~121 ka) or an intracanyon 

lava flow (like units drf, amp, and mdp; 100–80 ka) would 

disrupt and temporarily impound the river, probably resulting 

in flooding, both locally and downstream.

Table 3 summarizes the measured lengths of some lava 

flows in the Mammoth Lakes region. Several mafic lavas 

flowed through what are now developed areas, and a few 

extended as far as 10 km or more. The aprons of mafic flows 

have typically been only ~1 km wide, but units amp, bar, bfh, 

bsr, and mor broadened to ~2 km locally. The rhyolitic and 

dacitic lava flows are generally shorter, thicker, and slower 

moving, but their broader hazard results from associated pyroclastic currents, not from the lava flows themselves.

The Mono Craters domes and their southern extension, 

the Inyo chain (figs. 2–4), represent a more clear and present 

threat of eruption, as reviewed by Hildreth (2004). Magmatic components of the late Holocene hybrid Inyo chain 

have compositional affinities with both the Mono domes 

and 100-ka rhyolite domes in the west moat of Long Valley 

Caldera, but they are unrelated (compositionally, spatially, 

or structurally) to Mammoth Mountain, which shut down 

~50,000 years before the first Inyo eruption. The greater Mammoth vent system has a broad footprint that widely straddles 

the Sierran rangecrest, and the Inyo chain is a narrow linear 

vent array along the rangefront fault system. It can be inferred 

that the most likely eruptive recurrence of the Mono-Inyo 

system would be yet another rhyolite extrusion (with preceding pyroclastic outbursts) along the same alignment. Because 

a dozen or more such rhyolite eruptions have taken place 

there in the last 3,000 years, another in our time should not be 

unanticipated.

Kaye and others (2008) reported that a January 2006 

face-to-face survey of employers and employees in the business and tourism sectors at Mammoth Lakes demonstrated 

only a low to moderate level of volcanic-hazards awareness, 

despite the long-term engagement of USGS personnel with 

the community and local authorities. Kaye and others (2009) 

went on to compare impacts of a dacite extrusion at Mammoth Mountain and a rhyolite extrusion at the south end of 

the Inyo chain, laying emphasis on effects of pyroclastic 

density currents associated with dome collapse. Impacts 

modelled by Kaye and others (2009) are, of course, far greater 

on and around heavily developed Mammoth Mountain than 

for an Inyo eruption. Deposits of such currents emplaced in 

1350 C.E. extend several kilometers from the Inyo domes, 

and heavy pumice falls were regionwide (Miller, 1985). For 

Mammoth Mountain, most or all of its 24 identified extrusions 

were certainly accompanied by dispersal of pyroclastic ejecta, 

but nearly all evidence for this has been stripped by glacial 

erosion. 

In the very unlikely event of a revival of dacitic magmatism at Mammoth Mountain, extrusion of another lava 

dome would probably generate pyroclastic density currents 

that could overrun parts of the residential, downtown, and 

ski areas. Timely evacuation would be wise and, no doubt, 

controversial. Recurrence of an eruption plume like that which 

deposited pumice-fall unit rfp (~80 ka) might again mantle the 

township with a few meters of pumiceous fallout. 

The most certain things that can be said, realistically, 

about volcanic hazards here are (1) that a future dome extrusion on Mammoth Mountain is exceedingly unlikely; (2) that 

a basaltic scoria and lava-flow eruption is of greater likelihood 

but threatens rather limited downwind and downslope swaths 

with slow-paced devastation; and (3) that a Mono-Inyo rhyolite eruption is virtually inevitable on a millennial timescale 

and would impact areas with important highways but few 

habitations.

Introduction to Description of Map 

Units

The trachydacite complex of Mammoth Mountain and 

the array of contemporaneous mafic volcanoes in its periphery form a discrete magmatic system that is thermally and 

compositionally independent of the adjacent subalkaline Long 

Valley system (Hildreth, 2004). The Mammoth system first 
52 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

erupted about 230 ka, last erupted about 8 ka, and remains 

restless and potentially active. Magmas of the Mammoth 

system extruded through Mesozoic plutonic rocks of the Sierra 

Nevada composite batholith as well as through extensive 

remnants of prebatholithic metasedimentary wall rocks and 

synbatholithic metavolcanic rocks (Bailey, 1989, 2004). All 

of the many mafic and silicic vents of the Mammoth system 

are west or southwest of the concealed structural boundary of 

Long Valley Caldera; none lie inboard of the caldera’s buried 

ring-fault zone, and only one Mammoth-related vent appears 

to lie within that zone. Mammoth Mountain has sometimes 

been called part of the Mono-Inyo volcanic chain, but we 

regard that assignment inappropriate and misleading. Although 

the rhyolitic Inyo alignment (which is directed by the rangefront fault system) trends toward Mammoth Mountain, the 

basalt-to-dacite Mammoth vent array is scattered, nonlinear, 

far broader, and much older. The 10 by 20 km footprint of the 

Mammoth vent array essentially ignores existence of Long 

Valley Caldera and the Mono-Inyo alignment. Moreover, the 

Mammoth Mountain dacite dome complex ended its period 

of eruptive activity (100–50 ka) well before the principal 

Holocene interval of Mono-Inyo rhyolitic volcanism.

Here we describe 63 map units that compose the Mammoth basalt-to-rhyodacite system, along with numerous 

other rock units that crop out within the map area—including 8 rhyolitic units of the Long Valley system, 4 units of the 

Holocene Inyo Chain, and 32 Tertiary volcanic units. Of the 

63 Mammoth units, 17 are phenocryst-rich trachydacite lavas 

of the main edifice, two are similar trachydacite lava domes 

(units d61 and d81) that erupted at vents just north of the 

edifice, and 6 are edifice-based alkalic rhyodacites that carry 

fewer phenocrysts. Units of the Mammoth system peripheral 

to the edifice include 13 basalts, 16 mafic andesites, 6 silicic 

andesites, and 3 distinctly different dacites. All units are listed 

in table 1.

Usually we prefer to keep volcanic-rock nomenclature 

simple by basing it entirely on SiO2 contents: basalt has 47–52 

percent SiO2, basaltic andesite 52–57 percent, andesite 57–63 

percent, dacite 63–68 percent, rhyodacite 68–72 percent, and 

rhyolite more than 72 percent SiO2. Because the Mammoth 

suite straddles the alkaline/subalkaline boundary of LeBas and 

others (1986), however, we largely adopt their nomenclature 

in the unit descriptions below. Of the basaltic units, some 

are subalkaline basalts and others are trachybasalts; all more 

silicic units of the Mammoth suite are alkaline—basaltic trachyandesites, trachyandesites, and trachydacites. Because the 

six most evolved map units erupted at the Mammoth Mountain edifice have fewer phenocrysts and higher silica contents 

(70–71 percent) than the volumetricaly dominant trachydacites 

(63–69 percent), we distinguish them as alkalic rhyodacites. 

Rhyolites of the Long Valley system are subalkaline, and products of the Inyo chain are complex hybrids.

In the unit descriptions, we abbreviate names of the common volcanic phenocrysts clinopyroxene and orthopyroxene 

as cpx and opx, respectively. Phenocrysts are defined here as 

0.5 mm and larger; microphenocrysts (mph) as 0.1 to 0.4 mm; 

and microlites smaller still. The modifier phenocryst-rich

means that a rock has ≥12 percent crystals larger than 0.4 mm; 

phenocryst-poor signifies ≤5 percent; aphyric means what it 

says—none. Rocks containing 6 to 12 percent phenocrysts we 

typically characterize as moderately porphyritic or of moderate phenocryst content. Because crystal sizes range widely for 

each species within most thin sections (and more so within 

map units), we give estimates of combined abundance of 

phenocrysts and microphenocrysts for each species in each 

unit, followed parenthetically by the size range measured in 

thin section for each—from microphenocrysts (>0.1 mm) 

to the largest crystal observed. [For example, 7–10 percent 

plagioclase (mph to 4 mm); 2–3 percent olivine (mph to 

1.5 mm); and sparse Fe-Ti-oxide mph]. Antecrysts are cognate 

crystals out of equilibrium with their eruptive igneous host 

but inferred to have been inherited from an earlier stage of the 

same (thermally or compositionally) evolving magma system. 

Xenocrysts are crystals foreign to the system.

Most of the volcanic map units described here are newly 

defined, although equivalents of several were defined by Bailey 

(1989). Each is an eruptive unit derived from a single vent or 

dike-fed alignment of vents. Some are simple flow units, but 

many consist of scoria cones and one or several lava flows 

that have chemical and mineralogical coherence. Most units 

of the Mammoth Mountain edifice are lava domes, some of 

which also issued thick coulees 1–2 km long. Each map unit 

was delineated by field mapping on foot and its integrity confirmed, challenged, or revised by chemical, paleomagnetic, and 

microscopic work in the laboratory. Definition of a few units 

required iterative acquisition of field and lab data over a period 

of years, providing a firm basis for subdividing, lumping, or 

correlating somewhat heterogeneous packages of lavas. Most 

units have narrow compositional ranges, but some show zoning 

or heterogeneity spanning ranges of a few percent SiO2.

SiO2 contents reported in the description of each volcanic 

unit are based on major-element analyses (appendix) normalized on an anhydrous basis, determined by x-ray fluorescence 

methods in the U.S. Geological Survey laboratory at Lakewood, Colorado, supervised by J.E. Taggart, or in the Washington State University GeoAnalytical Laboratory, supervised 

by J.A. Wolff and R.M. Conrey. FeO* is total iron calculated 

as FeO.

Geographic names cited in the text are located on the 

geologic map, on map figures 2–5, and on some photo-figures. 

Many place names are informal but have long been used 

locally by the Town of Mammoth Lakes and the Mammoth 

Mountain Ski Area. Measurements are metric, as are elevations (except along north fringe of our map where contours 

are available only in feet). Grid references to site locations 

mentioned in the text are given to 100 m using the Universal 

Transverse Mercator (UTM) grid, zone 11 (North American 

datum of 1927), which is shown on U.S. Geological Survey 

topographic maps of the area. The first three digits are easting, 

and the second three are northing. For example, the summit 

of Mammoth Mountain is approximated as 208/665, 20.8 km 

east, 66.5 km north. Occasionally, a fourth digit is added for 
Introduction to Description of Map Units 53

precision to 10 m. The U.S. Geological Survey 7.5-minute 

quadrangles (1:24,000) on which the map is based are Bloody 

Mtn., Convict Lake, Crestview, Crystal Crag, Dexter Canyon, 

June Lake, Mammoth Mtn., Old Mammoth, and Whitmore 

Hot Sprs, all in California. 

The basaltic trachyandesite of June Lake (unit mjl), 

which is described here and depicted on the map of Bailey 

(1989 ), lies just north of our map area near June Lake (fig. 

3). Three more units of basaltic trachyandesite (Tmcl, Tmel, 

Tmwc), deemed significant to volcanic investigations in the 

San Joaquin River drainage system, are plugs that lie just west 

of our map (fig. 3).

For three mafic eruptive complexes, pyroclastic vent cones 

and lava-flow aprons are physically separated by erosion or 

younger deposits. Vent-cone units bed, bmc, and mkv are correlated with lava-flow units bcd, bfh, and mmc, respectively. 

Although several lines of evidence support the correlations, 

we retain independent designations to avoid the appearance of 

dismissing uncertainties explained in the unit descriptions.

Ages are estimated for all units, on the basis of mutual 

stratigraphic positions, numerous new 40Ar/39Ar age determinations, and relationship to glacial erosion and deposits. 

In unit descriptions, ages not credited to other sources were 

determined in this study (table 2). The radioisotopic age 

determinations were made in the U.S. Geological Survey geochronology laboratory at Menlo Park, California, supervised 

by A.T. Calvert, following methods described in Calvert and 

Lanphere (2006) and Muffler and others (2011).

Apart from the pre-Cenozoic basement rocks (not fully 

remapped by us), most units mapped are Pliocene or Quaternary. The Pliocene-Pleistocene boundary has recently been 

redefined at 2.588 Ma (Gibbard and others, 2009). Boundaries 

between early, middle, and late Pleistocene are widely agreed 

to be ~780 ka and ~126 ka (Gradstein and others, 2004). 

The formally defined boundary between the Pleistocene and 

Holocene is not very useful here, even as modified to 11.7 ka 

(Walker and others, 2009), because deglaciation took place 

several thousand years earlier. 
54 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

Surficial deposits are subordinate for the purposes of this volcanological map. They are mapped principally where such deposits form discrete landforms or are extensive enough to prevent inference of 

underlying rocks. Talus and colluvium are commonly omitted where subjacent volcanic units are identifiable or inferable with confidence. Surficial deposits are depicted extensively on the geologic map of 

Bailey (1989), and some are discussed in detail by Kesseli (1941a, b), Curry (1968, 1971) and Lipshie 

(1974).

pe Phreatic ejecta (late Holocene)—Poorly sorted fines-rich debris ejected in steam explosions 

from Inyo Craters in 1350 C.E. and, at uncertain times, from crater clusters west of 

Inyo Craters (Mastin, 1991) and from small craters beneath Chair 11 at northwest toe 

of Mammoth Mountain. Ejecta blankets extend 50–300 m radially from lesser craters 

but as far as 1 km northeastward from three principal Inyo Craters, which erupted in 

continuous sequence from north to south—Summit Crater (on Deer Mountain), North 

Inyo Crater, then South Inyo Crater (Mastin, 1991). Deposits are typically a few meters 

thick proximally but as thick as 13 m on east rim of South Inyo Crater and >20 m on 

walls of North Inyo Crater. Around Inyo Craters, ballistic ejecta are as big as 70 cm and 

up to 1 m around Summit Crater. Most block-sized fragments are crystal-poor mafic 

lavas (largely from unit aic) around South and North Inyo Craters. On floor of North Inyo 

Crater, sparse crystal-rich basaltic boulders are as big as 50 cm and granitoids are as big 

as 2.5 m, both presumed to have been entrained from unexposed underlying till. Around 

Summit Crater, white biotite rhyolite (from unit rdm) provided both ejected blocks and 

much of the cream to buff deposit matrix. Smaller and less abundant fragments from 

Inyo Craters include Early rhyolite tuff and lava (from unit rer) as well as granitoids 

and metavolcanics (both probably from subsurface gravels or till); Mastin (1991) 

tabulated lithologic proportions of finer ejecta. Ejecta around Summit Crater (3–10 m 

thick) likewise include granitoids and mafic lavas (crystal-poor and crystal-rich) but no 

fragments of oxidized crystal-poor mafic agglutinate from nearby unit mdm. Where 

exposed on crater walls, deposits are crudely stratified in meter-scale plane-parallel beds 

that show no evidence of erosion between successive eruptive pulses. Phreatic deposits 

around the three principal craters rest directly upon the pumice-fall deposits of the 

magmatic Inyo eruptions; because alignment of the phreatic craters extends that of the 

magmatic vents (unit ric), all are inferred to have resulted from southward propagation of 

a rhyolite dike (Miller, 1985; Eichelberger and others, 1984, 1985, 1988). 

The West Crater and Middle Crater clusters of Mastin (1991) lie 0.5–1 km west of 

the three principal craters and, although undated, likewise erupted soon after the 1350 

C.E. Inyo pumice falls, upon which their phreatic ejecta blankets rest. Ejected blocks 

include granitoids, metavolcanics, and Pliocene mafic lavas (all probably from till) as 

well as crystal-poor and crystal-rich Quaternary mafic lavas. Predominance of crystalpoor mafic lavas in the ejecta blanket, many meters thick, around the Middle Crater 

cluster suggests presence of buried lava flows like those exposed in South Inyo Crater 

(unit aic). 

Phreatic ejecta beneath Chair 11, ~200–400 m southwest of Main Lodge in Mammoth Mountain Ski Area, issued from a contiguous pair of small craters (Bailey, 1989) 

now bulldozed for ski runs and a reservoir. Where undisturbed, deposit is >10 m thick, 

thinning to ~1 m within 100 m laterally. It directly underlies 1350 C.E. Inyo pumice falls 

(fig. 13) and rests on a layer of white crystal-poor rhyolitic ash (2–4 cm thick), which 

is thought to have erupted at Mono Craters. The poorly sorted, unconsolidated phreatic 

deposit consists of medium-gray mud that encloses subangular-to-subrounded matrixsupported clasts—exclusively of Mammoth Mountain lavas. Where deposit is ~1 m 

thick, clasts are 1–35 cm across; where deposit is proximally thickest, clasts are as big as 

1 m and many are in mutual contact. Internally, deposit has modest porosity (especially 

around big clasts) that facilitated interstitial steam flux, which produced irregular lenses 

and patches of red, orange, and yellow alteration of matrix mud. Swelling clays impart 
puffy texture to pale gray dried external surfaces. Matrix and clasts are thought to represent underlying till and colluvium remobilized by phreatic blasts. 

Additional phreatic craters were depicted 500–700 m north of Lincoln Peak on 

northeast slope of Mammoth Mountain (three craters by Bailey [1989], four by Huber 

and Rinehart [1965a]), but our repeated scrutiny has identified neither explosive fragmental deposits nor thermal or fluid alteration effects. The sites have been groomed for 

several ski runs, but undisturbed areas between such runs consist only of rhyolitic unit 

rce, derivative regolith and colluvium, and limited patches of till

p Pumice deposits (late Holocene)—Pumice falls, local proximal pyroclastic-flow deposits, and derivative reworked deposits, all resulting from Inyo Chain eruptions of unit 

ric. Shown only where thick enough (≥2 m) to obscure underlying rocks sufficiently to 

prevent inference of their extent. Deposited by series of subplinian eruptions in 1350 C.E. 

(Millar and others, 2006). Within western half of map area, pumice mantle was originally 

ubiquitous, combined product of two successive outbursts from Deadman Creek vent 

and a third from Glass Creek vent (Miller, 1985; Nawotniak and Bursik, 2010; see unit 

ric). Deposits are unconsolidated and consist of pale-gray to cream, biotite-plagioclase 

rhyolite pumice lapilli, along with crystal-vitric ash and minor accidental lithic fragments. 

Combined primary thickness as great as 10 m near vents, 8 m on White Wing Mountain, 

and ~1 m near Mammoth Mountain and Devils Postpile. Widely sloughed from slopes 

and redeposited in areas of low local relief

rg Rock glacier deposits (Holocene)—Lobate masses of coarse angular debris that originated 

as talus, became impregnated with interstitial ice, and flowed glacier-like downslope 

(Wahrhaftig and Cox, 1959). Of many such masses nearby in the Sierra Nevada (Kesseli, 

1941a; Rinehart and Ross, 1964; Millar and Westfall, 2008), typically on north-facing 

scarps that shed copious blocky rockfall debris, two are mapped here, east of Mammoth 

Rock, where each extends ~1.5 km north from headwalls of Mesozoic granodiorite (unit 

Krv). Both consist mostly of angular to subangular granodiorite blocks—commonly 

up to 1 m and as big as 5 m. Eastern deposit is forested and apparently inactive (rg'); it 

overlaps Tioga-age moraines, which (unlike these rock glaciers) contain abundant blocks 

of metavolcanic rocks in addition to granodiorite. Western deposit is more barren but 

divisible into two parts: (1) a lower subdued apron that supports scattered trees, has lost its 

flow morphology by collapse into chaotic jumbles that overlap Tioga moraines, and has 

probably been long inactive (rg'); and (2) an active upper lobe (rg) that has barren rugged surface, arcuate transverse flow ridges and furrows, and steep flow front 60 m high 

that probably dates from the Little Ice Age (~1350–1850 C.E.). Surface of active segment 

slopes 20o and has 32o front; inactive surfaces of both rock glaciers slope ~10o and have 

subdued convex fronts. Unusually low elevations (2,500–2,700 m) make it unlikely that 

either originated as (or evolved from) a debris-mantled ice-cored glacier (see Clark and 

others, 1994). Debris mantle insulates interstitial ice, permitting its perennial survival and 

flowage at elevations well below regional equilibrium-line altitude (ELA) for true glaciers. 

Till (unit glb) subjacent to talus from degrading fronts of both rock-glacier deposits can be 

distinguished by presence of blocks of unit bhl (31±1 ka) derived from Lakes Basin

sd Spring deposits (Holocene and late Pleistocene)—Postglacial aprons of travertine, tufa, 

and siliceous sinter, principally at southwest end of Mammoth Meadow below Mammoth 

Rock and The Bluff. Low-relief, sparsely vegetated prairie capped by surficial spring 

deposits crops out over 250 by 300 m area that slopes ~3o eastward toward marshy 

flatland and golf course. Springs are apparently inactive recently. Surface is largely 

calcareous, along with patchy exposures of sinter best revealed in gullies 1–2 m deep. 

Both lithologies are porous, mostly soft and crumbly deposits rich in fossil plant fragments; sinter is typically white and vaguely stratified on gully walls; travertine generally 

pale gray or off-white; both erode to flakes and slabs. Deposits postdate and overlap till 

along prairie margins; they are overlain by thin alluvium encroaching from mouth of 

steep ravine to west, and they interfinger distally with subhorizontal silty pond and marsh 

deposits. An organics-rich dark-gray silt layer, 3.5 cm thick, exposed along shallow 

arroyo cut in deposit of spring carbonate and white silt (at UTM 032505/416520), yields 

an uncalibrated radiocarbon age of 1,640±20 14C yr B.P.

Description of Map Units—Surficial Deposits 55
56 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Isolated travertine deposits a few meters thick, ~250 m southwest of prairie just 

described, drape bedrock and colluvial slopes below Mammoth Rock. Hidden Lake, 

adjacent to spring-deposit prairie, is not a maar or volcanic crater but simply a shallow 

pond impounded by a minor berm of meadow mud and turf bound together by willows; 

no ejecta surround it.

Sinter deposits also crop out along fault zone north of Hot Creek, across from fish 

hatchery, where they overlie units ss and oal as well as rhyolite-rich colluvium. Deposits 

are irregular and porous or laminated and dense. About 2.5 km northwest of there, in 

lower Fumarole Valley (UTM 341/690), an eroded sinter deposit rich in silicified marsh 

vegetation crops out as slabby ledge ~5 m high. Crude layers are 5–40 cm thick; some 

layers preserve fibrous vegetation in upright growth positions. At northwest end of ledge, 

intercalated layers of rhyolite-rich sandstone suggest proximity of paleoshoreline of 

Pleistocene Long Valley Lake (at elevation 2,170 m)

al Alluvium, undivided (Holocene and late Pleistocene)—Unconsolidated, water-transported 

mud, sand, gravel, and bouldery debris deposited in or adjacent to active, ephemeral, or 

recently abandoned streams; includes local marsh and pond deposits 

fan Alluvial fan deposits (Holocene and late Pleistocene)—Sheets of boulder-bearing gravelly 

detritus deposited intermittently below mouths of steep gulches that drain southern wall 

of Long Valley Caldera east of Laurel Creek. Merges uphill into talus cones and distally 

into lower-gradient alluvium of various ages (mostly oal). Deposition of fans has continued since caldera formation at 767 ka and remains potentially active at times of torrential 

runoff or major rockfall, but exposed surfaces are dominated by late Pleistocene detritus. 

East of map area, numerous similar fans along east and northeast margins of Long Valley 

postdate, overrun, and widely obscure highstand strandlines of Long Valley Lake that 

were cut in later part of middle Pleistocene

oal Older alluvium (late and middle Pleistocene)—Stream deposits of various ages now 

undergoing erosion and dissection; largely unconsolidated. Includes glacial outwash, 

terrace gravels, and low-relief distal reaches of some alluvial-fan deposits. In northeast 

and southeast areas of map, unit is subdivided (where exposure permits) into successively 

younger (inset) stages—oal1, 2, 3. Their absolute ages are poorly constrained, but all are 

inferred to be middle or late Pleistocene, MIS 6 or younger. Alluviated lowlands are 

commonly veneered by aeolian material rich in sand and pumice, still subject to recurrent 

reworking, that widely conceals subjacent alluvium.

Postcaldera sources of unit oal within map area include the following: (1) north 

moat, collecting contributions from Clark and Alpers Canyons, Glass Creek, and northwest moat via Deadman Creek; (2) Dry Creek, which flowed vigorously during Pleistocene, draining Mammoth Mountain and west moat; (3) “Powerline Canyon” (UTM 

370/760), draining northeast sector of resurgent uplift; (4) Little Hot Creek, draining east 

sector of resurgent uplift; (5) Mammoth Creek/Hot Creek, which drains glaciated terrain 

east and southeast of Mammoth Mountain; (6) Sherwin and Laurel Creeks, which drain 

caldera’s southern wall and south moat; (7) Convict Creek and Tobacco Flat, which drain 

glacial deposits at Sierran front, feeding alluvial apron between Hot Creek flow and Lake 

Crowley. Contrasting alluvial clast assemblages reflect different basement rocks, volcanic 

units, and glacial deposits exposed in each sector.

Alluvium of all ages (units al and oal) transported by Little Hot Creek and “Powerline Canyon” is almost exclusively phenocryst-poor rhyolite because only Early rhyolite 

(unit rer) crops out in their drainage systems.

Wide swath of older alluvium extending from Laurel Creek to Convict Creek is 

late Pleistocene glacial outwash, supplemented by slopewash near rangefront. Exposures 

are as thick as 10 m in quarries north and west of Mammoth airport, where fluvial beds 

are 0.1–1 m thick and include subordinate pebble-rich matrix-supported debris-flow 

deposits and lenses of silt and sand. Rounded pebbles, cobbles, and sparse boulders are 

predominantly granitic and metasedimentary but include sparse dacites from Mammoth 

Mountain, rare clasts of (west moat) biotite rhyolite, and sparse basalts, among which are 

clasts from units bfh (92±2 ka) and bcd (125±2 ka). These gravels overlie units bfh, bsr,

and mlc (130±1 ka). Lack of Early rhyolite (unit rer) clasts indicates stream transport 
was from west, principally from Laurel and Sherwin Creeks, with little contribution from 

north. Farther east, oal from Laurel Creek merges with Tioga-age outwash from Convict 

Creek, which consists of metasedimentary, granitic, and carbonate clasts and lacks clasts 

of Quaternary volcanic rocks.

East of Convict Creek, 1-km-wide, gently sloping gravel bench (designated oal1; 

2,140–2,180 m elevation) consists of glacial outwash (>40 m thick) associated with high 

lateral moraine of Convict Creek (see unit gud) assigned to Tahoe glaciation by Sharp 

(1969) and now inferred to have been deposited during MIS 6. Pebble-cobble stream 

gravel in abandoned quarry (UTM 400/648), 1.3 km south of Whitmore Hot Springs, 

consists predominantly of metasedimentary clasts accompanied by sparse granitoids and 

lacks clasts of Quaternary volcanic rocks. Adjacent fan draining Tobacco Flat, designated 

oal3, though morphologically younger, has same clast asemblage and shares same morainal source but is probably of Tioga age (MIS 2).

Swath of older alluvium (oal1) that extends 8 km east from Arcularius Ranch along 

north moat of caldera is thicker than 12 m in quarry at UTM 362/785. Includes abundant 

pebbles and cobbles of metasedimentary, granitic, and metavolcanic rocks, phenocrystpoor mafic lavas, and Early rhyolite (unit rer; ~700 ka), plus common clasts of unit rnc

(~500 ka), along with sparser clasts of Pliocene dacites, and Pliocene olivine basalts. 

Lack of dacite clasts from Mammoth Mountain or clasts of nearby unit bar (~90 ka) 

suggests that this extensive outwash sheet dates from MIS 6. Most alluvium along north 

moat underlies lava flows of unit bar (~90 ka). Inset swaths designated oal2 carry same 

clast assemblage but may date from MIS 2. Gravels and pebbly sands in quarry (UTM 

403/717), 400 m south of Cashbaugh Ranch, are as thick as 8 m. Pebbles, cobbles, and 

rare boulders enclosed in sand-silt matrix were deposited in thin to thick beds, some 

clast-supported but mostly matrix-supported. Predominant clasts are hornblende-biotite 

(west moat) rhyolite, Sierran metasediments, and phenocryst-poor basalt, along with 

sparser olivine-plagioclase basalt, Early rhyolite, granites, metavolcanics, and sandstone. 

Few clasts are larger than 5 cm, except scoriaceous basalts, which range up to 12 cm. 

Site is directly east of resurgent uplift and close to Mammoth Creek/Hot Creek, the latter 

being the principal contributor of its gravels. On quarry floor, gravels rest on tan sand and 

white stratified lake silt as well as primary pumice-fall deposit of unit rfp (70.4% SiO2; 

80±8 ka), here as thick as 91 cm. Gravel sheet carrying same clast suite extends 200 m 

north and 1 km south of Cashbaugh Ranch buildings, capping bluffs of sandstone and 

siltstone.

Coarse oal gravels at Dry Creek Sand Flat (UTM 303/762) are exposed to a depth 

of ~7 m where incised by an intermittent Holocene alluvial wash. Most clasts are Early 

rhyolite (unit rer), locally derived and subangular to subrounded. Well-rounded pebbles 

and cobbles are predominantly crystal-poor mafic lavas and scoria but include crystal-rich 

mafic lavas, Mammoth Mountain trachydacite, and rare granodiorite. Chemical analyses 

of mafic clasts confirm contributions from units mcl (175±3 ka), mnd (66±2 ka), and mdn

(16–17 ka). Derivation of clasts from west moat along course of intermittent Dry Creek, 

which seldom flows vigorously today, suggests transport during Last Glacial Maximum 

(MIS 2). Presence of rhyolite and basalt boulders as large as 1–3 m suggests intermittent flood flow down late Pleistocene Dry Creek, which incised narrow 50-m-deep gorge 

through dense rhyolite of subunit rlm just downstream from Dry Creek Sand Flat 

pal Precaldera alluvium (early Pleistocene)—Fluvial gravels that crop out just north of Long 

Valley Caldera in a belt that extends ~3 km from west of lower Alpers Canyon to upper 

Clark Canyon. Largely unconsolidated but locally cemented, the clast-supported gravels are seriate from pebbles to boulders and have a gritty matrix of coarse sand; some 

exposures are crudely stratified and gently dipping. Most clasts are rounded or subrounded, many are 20–50 cm across, and some are 1–2 m. Predominant clast types are 

varied metasedimentary rocks and various mafic lavas, while granitoids, quartzite, and 

hornblende-biotite dacite are also common; subordinate are clasts of phenocryst-poor 

felsite and welded tuff of unit Trac. Clasts of nearby Bishop Tuff (unit rbt, 767±2 ka) 

are absent. Gravels here represent oldest stream deposits exposed in map area, directly 

underlying Bishop Tuff and resting principally on Pliocene basalts and locally on units 

Description of Map Units—Surficial Deposits 57
58 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Jbs and Trac. Clasts are of northerly derivation and not outwash from Sierran sources. 

A local scattering of 1–2 m boulders (UTM 299/802), interpreted by Bailey (1989) as 

Sherwin Till, is more likely a lag of deflated stream gravel, as it lies 5 km east of rangefront till deposits (unit gst), and its clast assemblage is far more varied than the till near 

Crestview. Erosion of the pal gravel sheet, widely 50–70 m thick, facilitated headward 

retreat of overlying cliffs of Bishop Tuff along Alpers and Clark Canyons, producing 

deep embayments lined by scarps of welded tuff. Gravels probably represent main 

course of precaldera Owens River, which extended southeast across later site of caldera 

to Owens Gorge, where river had cut deep canyon in granodioritic basement rocks long 

before emplacement of Bishop Tuff

av Rockfall avalanche deposits (Holocene and late Pleistocene)—Sheets or tongues of monolithologic rubble fallen from steep scarps. Principal example here is apron of debris that 

wraps around east base of Dome 2861 (unit d61) and extends into 1-km-long tongues at 

northeastern and southeastern toes of dome. Unstratified monolithologic deposits consist 

of pale gray dacite blocks, some tinted pinkish and mostly angular, typically 10–50 cm 

across but rarely as big as 2 m, in a gritty pale gray matrix, which is nonindurated 

and seriate from gravel to fine sand. Blocks weather tan, and most are dense or finely 

vesicular, and glassy, partly glassy, or devitrified. Where southeast tongue overlaps till 

of unit gmm, it lacks the black and brick-red vitrophyre blocks (from Mammoth Mountain sources) predominant in the till. Northeast apron banks against Dry Creek Dome 

(unit rdc), exhibits subdued hummocks with 10–20 m relief, and appears to be modified 

and truncated by lateral moraine of late Pleistocene Dry Creek glacier. Roadcut in northeast apron is exceptionally matrix-rich and conceivably exposes a pyroclastic facies of 

unit d61, but no prismatic blocks or thermal effects of blocks on matrix were observed. 

Deposits were probably emplaced piecemeal from steep-sided dome over extended time 

interval.

Lesser rockfall deposits mapped on southwest wall of caldera near Mammoth Rock 

consist of angular blocks of granitoids and metavolcanics. Such deposits grade into 

piecemeal talus slopes, which we largely ignore on this map. A small rockfall avalanche 

from steep flow front of one of the unit dnw coulees emplaced a narrow blocky deposit 

that armors right-bank terrace of Deadman Creek for 200 m eastward (UTM 237/7775); 

many blocks exceed 1 m, but there is no equivalent deposit on left-bank terrace. Another 

avalanche mass, ~100 m wide, fell from east scarp of unit bmc and lodged just southwest 

of Crystal Lake

s Surficial deposits, undivided (Holocene and late Pleistocene)—Generally combinations 

of glacial, fluvial, pond, aeolian, and reworked fallout deposits or locally mixed accumulations of till, talus, protalus, or other colluvium. Widely omitted; mapped principally 

where such deposits form discrete landforms or are extensive enough to prevent inference 

of underlying rocks. Bulldozed ski runs on Mammoth Mountain combine debris that had 

naturally been colluvium, till (unit gmm), aeolian sand, and Inyo pumice falls (unit p).

Numerous pits, typically 2–4 m deep and 10–30 m across, excavated by Mammoth 

Mountain Ski Area to control storm and snowmelt runoff, currently entrap slurries that 

consist principally of sand and silt flushed from loose surficial debris and carry sparse 

dispersed clasts of Inyo pumice or Mammoth Mountain lavas and pumice. In low-relief 

areas around toe of Mammoth Mountain and on benches and swales higher on the edifice, 

such excavations commonly expose older slurry deposits beneath layers of 1350 C.E. 

Inyo fallout. Such diamicts could be misinterpreted as small pyroclastic-flow deposits. 

Sparse clasts in such diamicts are principally subangular to subrounded dense fragments 

(mostly 1–10 cm across) of texturally varied lavas, along with subordinate pumice, all 

exclusively trachydacites and alkalic rhyodacites of Mammoth Mountain. In most pits, 

pumiceous and hydrothermally altered clasts are sparse or absent, and sand-silt matrix is 

fresh and nonindurated. 

In a few pits beneath 1350 C.E. fallout, there is also a massive sandy layer (30–112 cm 

thick), probably aeolian, that carries sparse pumice granules and sparser lapilli (as big as 

4 cm) of Mammoth Mountain biotite-bearing rhyodacite pumice (like those of unit rfp). 

Such tan to orange-brown layers of massive sand overlie coarser diamicts thought to 
be ground moraine. Sieve analyses at four sites yield median diameters (Md) that range 

0.2–0.4 mm (medium sand) with 4–11 wt% silt and only 4–10 wt% coarser than 2 mm. A 

charcoal lens within a 45-cm-thick sand layer excavated at UTM 200/687 (near Chair 11) 

gave an age of 1,970±20 14C yr B.P. Charcoal layer that separates 112-cm-thick layer of 

massive sand from an underlying paleosol developed on diamict (probably till) at UTM 

192/683 (along Reds Lake Road) gave 4,130±25 14C yr B.P. (fig. 13).

Along resurgent graben system that cuts Early rhyolite (unit rer), several shallow 

depressions crossed by Highway 395 contain multicomponent surficial deposits (unit s) 

eroded into small ridges with 10–20 m of local relief. Exposure is poor, owing to aeolian 

veneer, Inyo pumice falls, and near-absence of Holocene streamflow. At north end where 

graben is crossed by course of ephemeral Dry Creek, such ridges reveal small exposures 

of Pleistocene gravel (UTM 275/749 and 282/745) that includes cobbles of crystal-poor 

mafic lavas, hornblende-biotite rhyolite, and trachydacite of Mammoth Mountain, indicating late Pleistocene transport from west moat. Near junction of Sawmill Cutoff with 

Highway 395 (UTM 280/740), another 15-m-high ridge has scattered clasts of nearby 

units mcl and rer as well as sparser clasts of metasedimentary and granitic rocks that 

probably originated at west wall of caldera. As indicated by their clast contents and volcanic units they bank against, these depression-filling deposits are younger than 66±2 ka 

and their relief owes to late Pleistocene streamflow. A few kilometers farther south, 

Smokey Bear Flat (UTM 290/730) is a large flat-floored depression surrounded by Early 

rhyolite; lacking relief and streamflow, its depth of fill is unknown and its floor is covered by aeolian deposits, Inyo pumice, and rhyolitic pea gravel. At south end of graben, 

another depression (UTM 303/683; west of geothermal plant) includes a 10-m-high eastwest ridge of surficial fill incised by Highway 395; in addition to pebbles of local units 

rer and bcd (125±2 ka), fill has sparse clasts of granitic unit Kmo and common clasts of 

hornblende-biotite rhyolite (100–150 ka). 

eb Explosion breccia deposit of Bailey (late Pleistocene)—Unconsolidated heterogeneous assemblage of erratic blocks atop low-relief crest of granitic ridge 3044 (Hemlock Ridge; 

fig. 7), which buttresses west flank of Mammoth Mountain. Along most of 600-m-long 

crest of ridge, rocks exposed through Inyo pumice cover are exclusively loose clasts of 

porphyritic granite (unit Kmo), rounded to subangular, pebbles seriate to 2-m boulders, 

plucked and redistributed from local bedrock by ice. Limited to a 50-m-long strip (~20 m 

wide) on southwest slope of saddle along crest, however, abundant granite boulders are 

joined by blocks of crystal-poor mafic lavas, biotite rhyodacite, and various felsite and 

vitrophyre clasts containing hornblende, biotite, and feldspars—each clast type as big 

as 30–75 cm. Seven chemically analyzed clasts are identified as Mammoth Mountain 

rhyodacites and trachydacites and andesite of Mammoth Pass (unit amp). Interpreted tentatively as accidental ballistic ejecta from a (phreatic?) outburst on Mammoth Mountain; 

unit was probably more widely distributed but survived glacial scour only in saddle. 

South of our map area, Bailey (1989) also assigned to this unit comparable lithic assemblages sparsely scattered on upland surfaces; those atop the Pliocene erosion surfaces 

of Mammoth Crest and Red Mountain, however, are not ejecta from Mammoth Mountain 

but are, instead, clasts of rhyolite, dacite, and basalt derived from a precaldera volcanic 

highland on the later site of Long Valley Caldera.

lmd Diamict of lower Mammoth Creek (late Pleistocene)—Unconsolidated, unstratified, poorly 

sorted deposit ~10 m thick that makes up hill 2200+, just south of Mammoth Creek, 

~1 km east-southeast of Casa Diablo geothermal plant. Hill is 400 m long, 200 m wide, 

and poorly exposed; though possibly matrix-dominant, many large blocks protrude 

from sideslopes in clusters. Hilltop is veneered with fine sandy-pebbly aeolian sediment, through which very few blocks are exposed. Counts of clasts larger than ~10 cm 

(n=300) yield 43% phenocryst-poor mafic lavas, 14% phenocryst-rich mafic lavas, 

28% hornblende-biotite-feldspar rhyolite, 2% phenocryst-poor Early rhyolite (unit rer), 

4% dacites of Mammoth Mountain, 8% Mesozoic metavolcanics, 1% granitoids, and 

<1% schistose metasedimentary rocks. Clasts 10–50 cm across are abundant and those 

50–200 cm across are not uncommon; many are rounded to subrounded, but numerous 

blocks of mafic lava (whether massive or scoriaceous, crystal-rich or crystal-poor) are 

Description of Map Units—Surficial Deposits 59
60 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

subangular to angular. Blocks of mafic lava in the deposit are correlated chemically with 

units mkv, mmc, bcd, bsr, and possibly asr—all within the local drainage area and none 

farther than 6 km from the deposit. Occurrence of clasts of youngest local units (bsr, 

bcd; both ≤125 ka) shows deposit is not MIS 6 Casa Diablo Till (unit gcd), nor derived 

therefrom. Deposit is neither alluvium nor till, though subordinate rounded cobbles 

may have been entrained from such deposits. Probably some kind of mass-flow deposit, 

but it entrained little or no material from nearby moraines, which are rich in granitoids, 

metavolcanics, and Mammoth Mountain trachydacites. Overlies unit bfh (92±2 ka)

ss Sandstone of Long Valley Lake (middle Pleistocene)—Littoral apron of well-indurated 

sandstone, grit, and pebble conglomerate that sloped gently eastward and northeastward 

into former Long Valley Lake (Mayo, 1934b; Rinehart and Ross, 1964; Bailey, 1989). 

Unit crops out over ~18 km2 north and west of Hot Creek and also covers ~9 km2 east of 

the Hot Creek flow (unit rhc) beyond our map area. Strata range from laminae to beds as 

thick as 30 cm; most are planar or broadly low-angle lenticular but locally cross-bedded 

or deltaic. They are silica-cemented, seldom carbonate-cemented. Sorting is moderate 

to good, but average grain size of alternating layers ranges widely from medium sand 

to pebble gravel. Subordinate layers of fine sand and silt are more common distally 

and conglomeratic layers abundant proximally. Within map area, sediment was derived 

predominantly from phenocryst-poor Early rhyolite (unit rer); accordingly, pebbles and 

sand grains are glassy, devitrified, or pumiceous rhyolite plus minor derivative feldspar 

and biotite (and locally quartz, presumably from Sierran sources). Many rhyolitic clasts 

are angular or subangular, reflecting nearby provenance. East of Hot Creek flow, Sierran 

sources provide a greater contribution of sand and gravel of granitic and metasedimentary 

origin. Exposed thickness of unit is as great as 25 m, but drill holes north and east of Hot 

Creek flow penetrated >300 m of lake sediments, a significant but unquantified fraction 

of which is sandstone and rhyolite-pebble conglomerate (Smith and Rex, 1977; Sorey 

and others, 1978; Lewis, 1975; Fournier, 1989). Overlies and interfingers distally with 

ash-rich, fine-grained, diatom-and-ostracod-bearing lacustrine silt and siltstone. Interfingers proximally with pumiceous lava-flow breccias of unit rer and in distal drillholes 

with pyroclastic facies of Early rhyolite (subunit rtav) beneath caldera’s southeast moat. 

Deposition presumed to have begun in early postcaldera time (~760–650 ka) and persisted until well after 330-ka emplacement of Hot Creek flow, which both overlies and is 

overlain by sandstone strata. Sandstone surrounds (and locally coats) ledge of unit mcl

(179.5±2.4 ka) near Casa Diablo geothermal plant. At slightly lower elevation ~1.2 km 

southeast of that ledge, no sandstone or other lake sediment overlies extensive lava flow 

of unit bsc (172±2 ka), suggesting withdrawal of lake in that time interval. Unit is widely 

overlain and concealed by aprons of late Pleistocene alluvium (unit oal). Sandstone is 

overlain by subaerial pumice fall of unit rfp (80±8 ka) near Cashbaugh Ranch

st Siltstone of Long Valley Lake (middle Pleistocene)—Fine-grained lacustrine basin facies 

of Pleistocene Long Valley Lake (Mayo, 1934b; Rinehart and Ross, 1964; Bailey, 

1989). White to pale gray silt, clay, and fine sand is horizontally layered, laminated to 

thin bedded, and weakly indurated. Puffy surface crusts decimeters thick, promoted by 

swelling clays, typically conceal well-bedded deposits. Unit is exposed discontinuously 

over ~60 km2 area, largely in Long Valley lowland east of map area but also in dissected 

terrain between Hot Creek and Little Hot Creek. Exposures are generally 10–20 m thick, 

their upper surfaces armored by sandstone or veneers of pebbly alluvium. Drill holes 

north and east of Hot Creek flow penetrated >300 m of lake sediments, a significant but 

unquantified fraction of which is siltstone (Smith and Rex, 1977; Sorey and others, 1978; 

Lewis, 1975; Fournier, 1989). Large proportion of the fine sediment is derived from 

rhyolitic ash of unit rer. Unit contains widely dispersed diatoms and ostracods, as well 

as common tan layers 1–3 cm thick that consist almost entirely of ostracod shells and 

thin white laminae of nearly pure diatomite. Intercalated tephra layers 1–30 mm thick are 

also widely exposed. Unit is overlain by littoral sandstone/conglomerate facies of Long 

Valley Lake (unit ss) and is interbedded with it at a few exposures (more extensively so 

in subsurface); also overlain by units oal and al. Hot Creek and Owens River have cut 

broad shallow floodplains on the siltstone unit, which there widely floors marshes, wet 
meadows, and alkali flats. Where preserved on hillocks in Long Valley, the primary top 

surface of siltstone is generally near 2,100 m elevation; in reentrant between Hot Creek 

and Little Hot Creek, however, sandstone/siltstone contact climbs to 2,150 m. Lake 

Crowley is not a remnant of the Pleistocene lake but a reservoir created by completion of 

Long Valley Dam in 1941

GLACIAL DEPOSITS

Late Pleistocene glacial deposits are here divided into four lithologic assemblages 

that reflect provenance and a fifth undifferentiated catch-all that includes moraines 

peripheral to the study area, poorly exposed deposits, and patchy remnants. All or nearly 

all are thought to be products of the last Pleistocene glacial episode (Marine Isotope 

Stage 2; MIS 2), conventionally called the Tioga glaciation in eastern Sierra Nevada 

(Blackwelder, 1931). A few modest distal moraines near Laurel and Sherwin Creeks 

were assigned older ages by Bailey (1989), but it seems prudent to leave them undifferentiated until surface exposure dating can confirm pre-Tioga ages. The 40-odd nested 

moraines north and south of Mammoth Creek, including the most distal, contain basaltic 

blocks of unit bhl (31±1 ka), so they are clearly not of MIS 4 age or older. No evidence is 

recognized here for MIS 4 or “early Wisconsin” glacial deposits (historically assigned to 

“Tahoe glaciation”). Elsewhere in the eastern Sierra Nevada, many deposits long called 

Tahoe Till (Blackwelder, 1931) have been shown by surface exposure dating (Phillips and 

others, 1996, 2009; Rood and others, 2011) to be pre-Wisconsin, deposited at 165–130 ka 

(during MIS 6). Useful discussions and illustrations of glacial deposits in and near the 

map area were provided by Kesseli (1941b) and Rinehart and Ross (1964).

Tioga advances ranged between 28 and 15 ka (Phillips and others, 1996, 2009); 

retreat from Last Glacial Maximum positions began by 19 ka (Rood and others, 2011); 

and glaciers had retreated to the Sierran crest by 15–14.5 ka (Clark and Gillespie, 1997). 

Recess Peak moraines (bracketed between 14.2 and 13.1 ka; Clark and Gillespie, 1997; 

Gillespie and Zehfuss, 2004), which occur in Sierran cirques higher than 3,300 m, are not 

present within the map area, as only a few summits reach such an elevation. Nor are there 

locally any Little Ice Age cirque moraines (roughly 1350–1850 C.E.), although some 

are present at higher elevations elsewhere in the Sierra. Nor has evidence been found in 

the Sierra for glacial deposits of Younger Dryas age (12.9–11.5 ka; Gillespie and Clark, 

2011) nor of early Holocene age. 

Degraded glacial deposits of MIS 6, locally called Casa Diablo Till (Curry, 1968, 

1971), are preserved on both sides of Mammoth Creek, 1–2 km outboard of the most 

distal Tioga-age moraines. Sherwin Till, older than the 767-ka Bishop Tuff and probably 

deposited during MIS 22, crops out northwest and southeast of Long Valley Caldera.

gcc Till of Cold Water Canyon (late Pleistocene)—Glacial deposit dominated by clasts of 

metavolcanic rocks (unit }mv) transported northwestward down Cold Water Canyon to 

Lake Mary and Panorama Dome. During major ice advances, such material merged with 

and medially mixed with granite-dominant debris (units glb and gcd) quarried by ice 

from Mammoth Crest. Clast counts at Panorama Dome (n=450) give 80–90% metavolcanics, 10–20% unit bhl (31±1 ka), and <3% small granitoids; around Lake Mary, metavolcanic clasts exceed 95%, and in Cold Water Canyon, they approach 100%. By contrast, 

adjacent till of unit glb near Twin Lakes outlet is granitoid-dominant and has common 

clasts of Mammoth Mountain dacites as well as subordinate clasts of units }mv and 

bhl. Deposit contains many 2–3 m boulders and represents final glacial advance along 

Cold Water and upper Mammoth Creeks; ice followed a deep narrow canyon from cirque 

headwalls at elevations as high as 3,350 m (11,000 ft). Overlies units bhl, ddl, ddu, dom, 

and dtl; younger than all contiguous units except al and sd. Banks against, overlies, and 

locally grades into unit glb, deposited contemporaneously during MIS 2

gdc Till of Dry Creek (late Pleistocene)—Lobe of glacial till as wide as 3 km that extends along 

both sides of Dry Creek from base of Mammoth Mountain to sharp terminus 8 km northeast. Marked by few prominent moraines, hummocky sheet appears to reflect ablation 

of dead ice of a debris-laden piedmont lobe. West margin obscured by Inyo pumice-fall 

Description of Map Units—Glacial Deposits 61
62 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

deposit (unit p) related to unit ric (1350 C.E.). Clasts in till near Dry Creek are predominantly basaltic but include abundant trachydacite from Mammoth Mountain (100–50 ka). 

Clast count (n=374) in Scenic Loop roadcut (UTM 259/743) gave 51% crystal-poor mafic 

lavas, 20% Mammoth Mountain lavas, ~8% each of units ddc, bar, and bmn, and 4% 

hornblende-biotite rhyolite. Along fault scarp 1.5 km farther west (UTM 245/738), clast 

count (n=175) gave 53% crystal-poor mafic lavas, 12% Mammoth Mountain lavas, 18% 

ddc, 13% bar, 4% bmn, and no rhyolite. Few stones are larger than 1 m. Lack of basement 

clasts shows that ice came from Mammoth Mountain, not from caldera’s southwest wall. 

Deposit overlies units d61 (87±2 ka) and mor (66±2 ka); till is younger than all contiguous 

units except Inyo pumice fall and phreatic ejecta of 1350 C.E. Deposited during MIS 2

glb Till derived from Lakes Basin (late Pleistocene)—Set of 40-odd nested morainal ridges 

and associated ground moraine that extends 6 km eastward from outlet of Mammoth 

Lakes Basin to area of Sherwin Creek campground, where its most distal end moraine 

is breached by Mammoth Creek. Till was deposited during MIS 2 by a broad piedmont glacier that terminated ~1.5 km short of the Casa Diablo moraines of MIS 6 (unit 

gcd). Some morainal ridges are as high as 40–50 m, and numerous drill holes adjacent 

to such ridges penetrated 10–90 m of till (fig. 6). Moraine surfaces are boulder-rich, 

sharp-crested, and little weathered. Boulders are dominantly granitoids but include 

metavolcanics, mafic lavas, Mammoth Mountain dacites, and rare metasedimentary 

rocks. Source of ice-transported debris was principally Lakes Basin with lesser but 

important contributions from south slope of Mammoth Mountain and granodiorite and 

metavolcanic debris from rangefront scarp south and east of Mammoth Rock. North of 

Mammoth Creek, stones in till are 80–90% porphyritic granite (unit Kmo) as big as 5 m, 

~5% crystal-rich basalt of unit bhl (31±1 ka) as big as 3 m, and 3–5% metavolcanics, 

also as big as 3 m; sparser constituents are stones from big-feldspar basaltic unit bmc, 

crystal-poor scoria and lava clasts of units mcl and amp, fine-grained granitoids, and rare 

hornblende-biotite rhyolites. Northernmost moraines are poor in metavolcanics and carry 

only sparse Mammoth Mountain dacites, but both of these clast groups are present in 

inner moraines close to Mammoth Creek. South of Mammoth Creek, granodiorite stones 

(unit Krv) derived from south wall commonly outnumber porphyritic granites (unit Kmo) 

derived from Lakes Basin; in southernmost moraines granodiorite typically dominates, 

but metavolcanics are also abundant and are locally dominant; sparser constituents are 

boulders of units bhl, amp, and various Mammoth Mountain dacites. Few clasts of unit 

bmc are present south of Mammoth Creek. Innermost moraines south of Mammoth 

Creek are poor in Mammoth Mountain dacite but relatively enriched in porphyritic granite; during recession, shrinking axial ice from Lakes Basin flowed principally through 

a Twin Lakes outlet trough already deeply incised through several Mammoth Mountain 

dacite units. Whether entire nested moraine set represents Last Glacial Maximum and its 

recessional stillstands awaits testing by surface-exposure dating; presence of clasts of unit 

bhl (31±1 ka) in all moraines on both sides of Mammoth Creek as far east as confluence 

of Sherwin Creek nonetheless proves that all were deposited during MIS 2, not in early 

or middle Wisconsin time. Deposits overlie units amp (97±1 ka), bhl (31±1 ka), ddl

(58±2 ka), dtl (76±1 ka), asr, bsr, and mcl

gmm Till derived from Mammoth Mountain (late Pleistocene)—Two tongues of glacial till that 

extend eastward from sources exclusively on Mammoth Mountain. One extends 3 km 

from northeast base of Mammoth Mountain into downtown Mammoth Lakes; second 

extends 3 km along valley between Dragons Back and Lincoln Peak as far as Juniper 

Ridge. Linking the tongues, till west of Camp High Sierra grades up steep east-facing 

scarp of Timber Ridge into colluvium rich in rhyodacite of unit rce. Sandy-gravelly 

fines-poor matrix dominates and encloses boulders and smaller stones that are nearly 

all (>99%) Mammoth Mountain dacites and rhyodacites (100–50 ka), accompanied by 

sparse mafic lavas and rare metavolcanic and granitoid clasts. Stones range from subrounded to subangular, and many are 10–50 cm across but rarely 1–2 m. In contrast to 

granitoid boulders, ice transport of Mammoth Mountain dacites tended to re-fracture 

rather than round them. Major outcrop area in residential neighborhood that extends from 

Canyon Lodge to downtown is an irregular terrain of hills and short ridges with 15–30 m 
local relief; abundance of small moraines and weak development of lateral moraines may 

reflect short runout of glaciers and overloading of ice by debris from numerous brecciated silicic lava domes. A few elongate moraines, all east-trending, include Juniper Ridge, 

ridges just east of The Village, a ridge just north of Forest Trail, and another along valley 

1 km south of Lincoln Peak (Gold Hill ski run); the last is actually a medial moraine 

(without laterals) squeezed between convergent ice sheets that draped slopes of Dragons 

Back and Lincoln Peak. Although they were deposited in large part contemporaneously, 

distal till of unit gmm overlies till of unit glb along an abrupt contact, from which we 

infer that ice advancing down Mammoth Mountain outlasted retreat of the Lakes Basin 

glacier from distal lowlands. Their contrasting compositions are readily compared on 

adjacent moraines of Juniper Ridge (gmm) and Lone Tree Hill (glb). Northern tongue 

was previously interpreted as deposit of a debris-avalanche from Lincoln Peak (Bailey, 

1989), but it lacks hummocks, megablocks, and composite blocks; its varied Mammoth 

Mountain clasts (64.0–70.5% SiO2; n=12), moreover, are not derived principally from 

Lincoln Peak, which in any case lacks evidence for an amphitheater or other avalanche 

scar. Volume of 3-km2 northern tongue of unit gmm (~0.1 km3) alone exceeds that of 

Lincoln Peak (unit dlp). Very sparse boulders of mafic lava in unit gmm are correlated 

chemically with units bhl (31±1 ka), aml, amp, and mcl. Deposits overlie units d81, dtl, 

rce, rss, glb, scoria cone mcl, and bank against units mkv, bsm, mcl, and rmk. Overlain 

by rockfall avalanche deposits from Dome 2861 (unit av) and by Inyo pumice-fall deposits (unit p; 1350 C.E.). Deposited during MIS 2

gud Undivided glacial deposits (late and middle Pleistocene)—Moraines at Sherwin, Laurel, 

and Convict Creeks, moraines buried by Holocene pumice-fall deposits in caldera’s west 

moat, and discontinuous till remnants near Middle Fork San Joaquin River canyon, are 

largely or entirely of Tioga age—broadly of last major glacial episode during MIS 2. 

Several distal lobes of Sherwin, Laurel, and Convict Creek moraine complexes may be 

pre-Tioga (fig. 21), as suggested by Rinehart and Ross (1964), Sharp (1969), and Bailey 

(1989), but this is unproven. Scattered patches of till along ice-scoured Middle Fork 

canyon and ground moraine west of Mammoth Pass consist largely of locally derived 

material. Speculation by Fullerton (1986) about middle Pleistocene glaciation near Reds 

Meadow was based on a K-Ar age far too old for intracanyon lava of Devils Postpile 

(unit mdp), now dated at 82±1 ka. Main occurrences of unit gud are the following.

Till of west moat. Glacial deposits in caldera’s west moat, poorly exposed beneath 

late Holocene pumice-fall deposits (unit p), form several buried moraines between Crater 

Flat and Deadman Creek. Scattered erratic blocks and sparse gully exposures through 

pumice cover indicate that clasts from Mammoth Mountain are uncommon, granitoid 

clasts are abundant, and clasts of metamorphic rocks, Pliocene lavas, and units ddc, bar, 

and bmn are all ubiquitous but subordinate. Dacite erratics from Mammoth Mountain 

on divide northeast of Minaret Summit (UTM 188/695) provide evidence that some late 

Pleistocene ice flowed north-northwestward from edifice along ridgecrest, as well as 

northward toward Dry Creek. 

Till of Convict Creek. Main components are (1) north-trending lobate complex, 

2 km wide and 3 km long, constructed of many nested loop and lateral moraines, and 

(2) older northeast-trending double-crested lateral moraine, 3 km long and >300 m high; 

both were characterized in detail by Sharp (1969) and not restudied by us. Sharp (1969) 

reviewed several previous interpretations of moraine-emplacement sequence and concluded that entire lobate complex (plus set of seven lateral moraines just east of Convict 

Lake and north-trending lateral moraine along east side of Convict Creek) are all of Tioga 

age (MIS 2). Sharp (1969) showed the enormous northeast-trending lateral moraine to be 

older and, by regional comparison, inferred it to be of Tahoe (early Wisconsin) age, but 

he suggested that it might conceal still older buried till. Surface exposure dating (Phillips 

and others, 2009) has shown many eastern Sierra Nevada moraines long called Tahoe to 

have been deposited 165–130 ka (MIS 6). Stones are predominantly metasedimentary and 

granodioritic, along with subordinate metavolcanics.

Till of Laurel Creek. Moraine complex, ~3 km long and as wide as 1.5 km, that 

extends north from canyon of Laurel Creek to caldera floor near Sherwin Creek Road; 

Description of Map Units—Glacial Deposits 63
64 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

characterized by Rinehart and Ross (1964) and not restudied in detail by us. Treeless 

Laurel Creek moraines contain subordinate granodioritic debris but consist predominantly 

of metasedimentary material, in contrast to adjacent Sherwin Creek moraines, which are 

densely forested (see fig. 36 of Rinehart and Ross, 1964) and consist almost exclusively 

of granodiorite till, suggesting lithologic or edaphic control of vegetation. A sharp-crested 

lateral-terminal moraine loop that closes at ~2,560 m elevation is generally thought to 

be of Tioga age (MIS 2). Older lateral moraines that extend ~1 km farther north and 

300 m lower (fig. 21) are morphologically more subdued though lithologically similar; 

their termini at 2,260–2,300 m are similar in elevation to Casa Diablo Till (unit gcd) of 

Mammoth Creek, only 2–4 km north, suggesting that they too could have been deposited 

during MIS 6 (or alternatively during early MIS 2). Prominent alluvial fans issuing from 

these distal moraines may in part be comparably old.

Till of Sherwin Creek. Massive apron, 3 km wide, of unusually coarse till that 

emerges from two rangefront canyons and extends ~3 km farther northward on both sides 

of Sherwin Creek to caldera floor near Sherwin Creek Campground. At least 200 m thick, 

complex deposit includes a few modest longitudinal ridges distally but lacks high lateral 

moraines typical of most canyon-mouth glacial deposits in eastern Sierra Nevada. Till 

consists almost entirely of granodiorite (from unit Krv) derived from steep canyon walls; 

abundant boulders armoring deposit are commonly as big as 2–5 m. Jointed bedrock of 

narrow cirque-canyons so overloaded glaciers with coarse debris that glacier rheology 

(and morphology of resulting deposit) approached those typical of rock glaciers, as noted 

by Kesseli (1941b) and Rinehart and Ross (1964). At northeast extremity of deposit, two 

morainal ridges, each ~40 m high, diverge ~700 m beyond main lobe; although emplaced 

before massive main lobe, neither appears definitively older on basis of boulder weathering or soil development. Nonetheless, they could be either early MIS 2 lateral moraines 

that predated overloading of ice that led to construction of the massive rock-glacier-like 

lobe atop them or, alternatively, they could represent MIS 4 or 6. Eastern distal ridge of 

Sherwin Creek moraine complex (UTM 301/650) is relatively unforested (unlike other 

Sherwin Creek glacial deposits); although armored by granodiorite boulders like rest of 

complex, its matrix contains abundant pebbles and cobbles of metasedimentary rocks, 

which apparently inhibited forest development as on adjacent Laurel Creek moraines 

(see fig. 36 of Rinehart and Ross, 1964). Like distal Laurel Creek moraines, it probably 

predates main advance of LGM (fig. 21). Excepting this anomalous northeasternmost 

moraine, distal till of Sherwin Creek laps against units bsr (99±1 ka) and amp (97±1 ka), 

showing that voluminous Sherwin moraines were deposited in late Pleistocene (not during MIS 6)

gcd Casa Diablo Till (latest middle Pleistocene)—Subdued glacial deposits preserved in two 

swaths—a northerly one along and north of Sawmill Road and a southeasterly one 

that extends across Highway 203 and Mammoth Creek. Sawmill Road swath extends 

3 km eastward from near Shady Rest Park, locally includes a low moraine crest, and is 

exposed to thicknesses as great as 20 m. Boulder counts (n=328) yield 80% granitoids, 

9% metavolcanics, and 11% mafic lavas; abundant granitoids are 0.5–2.5 m in diameter; metavolcanics are typically 10–40 cm but as big as 1 m; mafic lavas are typically 

10–80 cm, massive or vesicular, and of low to moderate phenocryst content. Southeast 

swath, which extends 1.2 km southeastward, is lithologically different, and its thickness 

exceeds 30 m at Mammoth Creek. Boulder counts (n=836) on a prominent knoll 120 m 

north of Highway 203 (UTM 300/675) gave 27% granitoids, 56% metavolcanics, 16.5% 

mafic lavas, and two clasts of hornblende-biotite-feldspar rhyolite. A second site (n=183) 

on a ridgetop 200 m south of Highway 203 (UTM 301/6715) gave only 5% granitoids, 

10% mafic lavas, and 85% metavolcanics. At both southeasterly sites, average clast size 

is somewhat smaller than in Sawmill swath: granitoids are typically 10–50 cm and rarely 

up to 2 m; metavolcanics typically 5–50 cm but up to 1 m; and mafic lavas 5–50 cm. 

Clasts of Mammoth Mountain dacites (100–50 ka) are absent at all sites. The two swaths 

were probably distal lateral moraines of a single 10-km-long valley glacier that descended from accumulation areas in and near present-day Mammoth Lakes Basin; there is no 

need to appeal to Ritter Range sources. Right-lateral moraine was supplied predominantly 
by metavolcanic terrain south and east of Lake Mary; left-lateral moraine mainly by 

granitic headwall sources between Lake George and Minaret Summit (now partly covered 

by Mammoth Mountain). Sawmill swath overlies units rer, asr, mcl, mkv, and mmc

(160±2 ka); overlain by units bcd (125±2 ka) and bsr (99±1 ka). Southeast swath overlies unit mmc and is overlain by units bcd and bsr. Weathering of granitoid boulders is 

more advanced than on nearby late Pleistocene moraines. Boulders of mafic lava in Casa 

Diablo Till are correlated chemically with units bsc (172±2 ka), mkv (153±1 ka), mmc

(160±2 ka), msc (154±2 ka), mcl (175±3 ka), and aic (156±15 ka). Unit bsc today crops 

out only distally relative to gcd glacial deposits. Rare rhyolite clasts in till are probably 

from unit rwm (~150 ka). Casa Diablo glacial advance took place during MIS 6. Former 

speculations on possibly older ages for this till were based on K-Ar ages (far too old) for 

xenocryst-bearing mafic lavas that sandwich Casa Diablo Till and for Mammoth Mountain biotite that carries excess Ar

Description of Map Units—Glacial Deposits 65

Krv

Krv

y

o y

o

mv

mv

bsr

Figure 21. Bare moraines of Laurel Creek and forested moraines of Sherwin Creek. View southwest from Sherwin Creek Road toward 

south wall of Long Valley Caldera. Green swath at left is Laurel Creek. Rangefront scarp, here as high as 1,200 m, is largely granodiorite 

(unit Krv), though reddish rocks along right skyline are pre-batholith Mesozoic metavolcanic rocks along Solitude surface. Predominant 

material in Sherwin moraines is granodiorite but in Laurel moraines is Paleozoic metasedimentary rocks of Morrison pendant; both high 

moraines (y, younger) are thought to have achieved present morphology in Tioga glaciation (Marine Isotope Stage [MIS] 2). Lower pair 

of bare distal moraines in center middle ground (o, older), however, may have formed in MIS 6; they appear to be paired lateral moraines 

deposited by a glacier with a trajectory intermediate between those of their higher successors. Clasts in low moraine at left are mostly 

metasedimentary; those in low moraine at right are mixed, uniquely reflecting combined granodiorite and metasedimentary provenance. 

Basaltic flow front, ~8 m high, on sagebrush flat at right is unit bsr (99±1 ka; 103.5±1.4 ka).
66 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

gst Sherwin Till (late early Pleistocene)—Boulder-rich glacial diamict that crops out near 

Crestview and (southeast of map area) at Rock Creek, Sherwin Hill, and Owens Gorge 

(Blackwelder, 1931; Putnam, 1960; Sharp, 1968; Bailey, 1989). Not present at Sherwin 

Creek (see unit gud, above). Moraine ridges are not preserved, but till is as thick as 

200 m near Rock Creek and 60 m northwest of Crestview. Till is poorly sorted and unstratified, containing sandy-silty matrix and stones that are everywhere dominantly Sierran granitoids. Metasedimentary clasts are sparse, and, north of caldera, clasts of Pliocene 

mafic lavas are common. In till northwest of Crestview, many granitic clasts are 0.5–3 m 

across, mafic lava clasts are generally 5–40 cm, and sparse cobbles of hornblende-biotiteplagioclase dacite probably came from June Mountain. Many granitic stones are grussy, 

far more deeply weathered than in other tills in map area. Till sheet extends farther out 

from Sierran rangefront than that of any younger glaciation. Remnant on west rim of 

Alpers Canyon, shown as till by Bailey (1989), is probably lag of coarse stream gravel 

(unit pal). Unit overlies Mesozoic granitoids and Tertiary mafic lavas. Overlain directly 

by Bishop Tuff (767±2 ka), both north and southeast of caldera. Age 50–70 ky older 

than Bishop Tuff was inferred from weathering of till beneath ignimbrite (Gillespie and 

Zehfuss, 2004), suggesting correlation of Sherwin glacial advance with MIS 22

PRE-CENOZOIC BASEMENT ROCKS

[Listed in order of increasing age]

Kmo Mono Creek Granite (Late Cretaceous)—One of several extensive plutons of megacrystic granite emplaced during terminal stages of 140-million-year-long accumulation of 

Sierra Nevada composite batholith (Bateman, 1992a, b). Elongate pluton extends >50 km 

northwest from Pine Creek to Minaret Summit and, within map area, forms Mammoth 

Crest and underlies parts of Mammoth Mountain and valley of Middle Fork San Joaquin 

River. Light-colored granite carries several percent alkali-feldspar megacrysts (1–3 cm) 

in medium-grained (1–5 mm) groundmass. Ranges of mineral contents: 20–35% total 

alkali feldspar; 20–40% quartz; 30–50% plagioclase; ~5% biotite; and trace amphibole 

(Rinehart and Ross, 1964; Huber and Rinehart, 1965a; Bateman, 1992a); mafic enclaves 

uncommon. Foliation is generally inconspicuous except along Rosy Finch Shear Zone 

(Tikoff and Saint Blanquat, 1997), a steeply foliated belt that encompasses both pluton 

margin and its metavolcanic wall rocks; zone strikes beneath Mammoth Mountain and is 

well exposed south of Lake George. Pluton intrudes unit }mv and, south of map area, 

units |ms and Krv

Krv Round Valley Peak Granodiorite (Late Cretaceous)—Three discrete plutons, lithologically similar, were assigned to this unit (Rinehart and Ross, 1964; Bateman, 1992a, b). 

Two are in map area, whereas third and largest intrudes Mount Morrison Pendant (|ms) 

along Rock Creek farther southeast (Bateman, 1965; 1992b). Within map area, western 

pluton extends 15 km southeast from Mammoth Rock and forms a northwest-elongate 

mass 3–5 km wide that separates wall-rock units |ms and }mv. Eastern pluton, only 

3 km wide and equant in plan view, intrudes |ms sequence on Long Valley Caldera rim 

north of Laurel Mountain. Homogeneous and equigranular, rocks have average grain size 

of 1–2 mm, and euhedral hornblende and biotite are evenly distributed. Mineral contents: 

20–35% quartz; 15– 25% alkali feldspar; 45–60% plagioclase; 5–15% biotite; and 1–5% 

hornblende (Rinehart and Ross, 1964; Bateman, 1992a). Mafic enclaves are not abundant 

but are far more common than in nearby granitic unit Kmo. 

Subunit Krv' represents dikes of leucogranite (70% SiO2) inferred to have issued 

from the western Krv pluton. One cuts |ms strata at ~2,575 m elevation just north of 

Mammoth Rock, and another cuts unit }mv at 3,100 m on the crest of Red Mountain. 

Each is 20–25 m wide and crops out along strike for ~250 m. Both are coarse grained and 

carry only sparse mafic crystals

Kjl Granite of June Lake (Late Cretaceous)—Elongate granitic pluton that forms White Wing 

Mountain at northwest rim of Long Valley Caldera and extends at least 13 km northwest through June Lake to Grant Lake. Medium-grained hornblende-biotite granite 

is not strongly porphyritic but contains scattered alkali-feldspar megacrysts—seldom 
bigger than 1 cm. Ranges of mineral contents (Kistler, 1966): 11–25% quartz; 22–42% 

alkali feldspar; 27–55% plagioclase; 5–12% biotite; and 1–6% hornblende. Formerly 

correlated with Late Triassic megacrystic pluton of Wheeler Crest by Huber and 

Rinehart (1965a) and Kistler (1966), but reassigned to Cretaceous by Bateman (1992a). 

Two isolated exposures that intrude Paleozoic metasedimentary rocks along caldera 

wall east and northeast of Deadman Pass are apparently correlative; quite felsic and 

medium-to-fine-grained, they have ~74% SiO2, carry only ~10% mafic minerals, and 

contain aplitic masses and dikes and locally thick quartz veins. Third and southernmost 

plutonic outcrop along west wall of caldera, ~2.5 km southeast of Deadman Pass, is 

different (see unit }gd)

Jbs Granite of Big Springs (Jurassic?)—One or more plutons that crop out along north margin 

of Long Valley Caldera, east and west of Big Springs and north and east of Crestview. 

Exposures locally extend 2–3 km north of caldera margin. Rocks are medium-grained 

and range from biotite leucogranite to hornblende-biotite granodiorite. Correlated tenuously with faraway Leucogranite of Casa Diablo Mountain (Rinehart and Ross, 1957) by 

Bateman (1992a), who cited a U-Pb age of 161 Ma for that pluton; unit was described 

as “quartz monzonite of Big Springs” by Rinehart and Ross (1964), who had mapped 

both plutons. Exposures north and east of Crestview are more mafic than those near Big 

Springs, containing 25–30% mafic and accessory minerals. Intrudes unit |ms

}gd Granodiorite northeast of Minaret Summit (Mesozoic)—Cliffy exposure of mafic granodiorite, ~1 km long, on west wall of Long Valley Caldera 2 km northeast of Minaret 

Summit and just northwest of unit bar scoria cone. Medium-grained equigranular rock 

carries 25–30% mafic and accessory minerals, including anhedral biotite and hornblendebiotite aggregates. Intrudes unit |ms, includes abundant wall-rock fragments, and injects 

aplite dikes (1–2 m thick) into metasedimentary host rocks. Pluton is cut, in turn, by two 

porphyritic granitic dikes (74% SiO2), each 5–10 m thick, which trend east-northeast 

and carry quartz and feldspar phenocrysts (1–10 mm) in dark-gray, finely crystalline 

matrix laced locally by epidote veinlets. A similar porphyritic granitoid dike, 2 km northnorthwest of Deadman Pass (UTM 165/754), is 5–15 m thick, strikes ~25° NW, and 

intrudes vertically foliated unit |ms for ~150 m; arbitrarily assigned to unit }gd, it may 

or may not be correlative. Unit was correlated improbably by Huber and Rinehart (1965a) 

and Bailey (1989) with Late Triassic Granite of Lee Vining Canyon (Kistler, 1966; 

Bateman, 1992a), which has only 2–7% mafic minerals. Main southern outcrop overlain 

by Pliocene basaltic unit Tbdp. Age unknown but certainly Mesozoic

}mv Metavolcanic rocks of Ritter Range pendant (Mesozoic)—Heterogeneous assemblage of 

subaerial (and sparser subaqueous) arc-volcanic rocks of Triassic through Cretaceous age, 

moderately metamorphosed during intrusion by plutons of Sierra Nevada batholith in 

Cretaceous. Thicker than 10 km, assemblage was deposited as felsic ignimbrites, varied pyroclastic and volcaniclastic deposits, andesite-dacite lava flows, and subordinate 

shallow-water sedimentary intercalations (mostly tuffaceous), as described by Rinehart 

and Ross (1964) and Huber and Rinehart (1965a); see summary by Bateman (1992a, 

p. 10–13). Deformed during Cretaceous plutonism, sequence now forms steeply westdipping homoclinal section that extends west to Ritter Range and strikes northwest from 

south of Mammoth Rock through Minaret Summit, west of caldera rim. Within map area, 

intruded by major Late Cretaceous plutons Kmo and Krv

|ms Metasedimentary rocks of Mount Morrison pendant (Paleozoic)—Deep-water continental-slope and submarine-fan sedimentary deposits of Cambrian through Permian 

age, moderately metamorphosed during intrusion by plutons of Sierra Nevada batholith in Mesozoic. Assemblage thicker than 7 km consists of calc-silicate and siliceous 

hornfels, argillite, chert, limestone and marble, siltstone, sandstone, and quartzite, as 

mapped and described by Rinehart and Ross (1964), Stevens and Greene (1999), and 

Greene and Stevens (2002). Sequence was coeval with shallow-water continental-shelf 

and platform deposits now exposed in White-Inyo Mountains to southeast. Assemblage 

makes up much of south wall of Long Valley caldera; components of it also crop out 

on west and north walls (Bailey, 1989) and have been penetrated in drill holes deep 

beneath caldera floor

Description of Map Units—Pre-Cenozoic Basement Rocks 67
68 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

TERTIARY VOLCANIC ROCKS

[Listed alphabetically by unit label]

A few Miocene and numerous Pliocene volcanic units crop out beneath or around the periphery of the 

Mammoth Mountain volcanic field, principally around the rim of Long Valley Caldera and on valley 

walls of the Middle Fork San Joaquin River.

Tacc Trachyandesite of Crater Creek (Pliocene)—Pair of glacially eroded knobs of phenocrystpoor andesite (58.3–62.6% SiO2

) on east rim of Crater Creek canyon, 2 km north of 

Pumice Butte (fig. 10). Centered ~1 km apart, both knobs are elongate northeastsouthwest. Larger northeast knob is 600 m long with 90 m relief, whereas southwest 

knob is 300 m long with 50 m relief. Both consist of pale-to-medium-gray massive 

lava, locally mottled, that weathers tan or pale pink and disintegrates into chaotic talus 

of blocks, plates, and slabs that reflect crude flow foliation. Rocks are nearly aphyric 

but contain hornblende needles (0.2–2 mm long), some with swallowtail morphology; 

trace amounts of olivine (0.5–1 mm), opx (0.5–1 mm), and cpx (2–4 mm long, some 

in clusters) are present. Both knobs also contain common xenocrysts of 1-mm quartz, 

singly or in clusters, with reaction rims; less common are composite xenocrysts of 

quartz and cloudy feldspar; free feldspar crystals (1–4 mm) are rare and probably also 

xenocrystic. Lack of exposure in saddle between knobs leaves unresolved whether 

they are separate domes or eroded parts of a once-continuous coulee. Two analyses of northeast knob yield 61.3–62.6% SiO2

 whereas two of southwest knob yield 

58.3–58.7% SiO2, suggesting separate domes; the four analyses, however, define linear 

compositional arrays for most elements. Steep north face of southwest knob exhibits 

near-vertical jointing, but lack of oxidation, ejecta, or breccia provides no suggestion of 

vent features. Unit rests on Mesozoic granite and lavas of unit Tasj (9.164±0.017 Ma). 40Ar/39Ar age: 4.29±0.20 Ma (G.A. Mahood, unpublished data, 2010)

Taec Trachyandesite east of Crestview (Pliocene)—One or more phenocryst-poor lava flows 

(57.1% SiO2) exposed for ~600 m along both walls of distal reach of southeast-draining 

gulch at southwest toe of Dome 8325 (unit Td83), ~1 km east-northeast of Crestview. 

Crops out as massive, block-jointed ledges typically 3 m high, with locally exposed relief 

as great as ~8 m. Phenocrysts: None. Sparse olivine microphenocrysts (0.1–0.4 mm), 

many iddingsitized; rare feldspar xenocrysts broken, sieved, and resorbed. Banks against 

unit Td83 (3.42±0.08 Ma). Overlain by flows of units Tb84 and Tmcv. Source vent buried; may be an early product of Cone 8478. Undated

Tasj Trachyandesite of San Joaquin River (Miocene)—Glaciated phenocryst-rich lava flows 

(58.0–58.1% SiO2) and subjacent intrusive mass that extend for ~1.1 km along east wall 

and rim of Middle Fork canyon, north and west of Pumice Butte and ~7 km southwest 

of Mammoth Mountain. On plateau near rim, as high as 2,820 m, massive lavas are 

ice-scoured into smooth swells, nubbins, and ledges, their surfaces typically polygonally 

jointed, locally columnar, and widely glassy or partly so. Cliffy outcrop is as thick as 

250 m at its north end where it fills a reentrant in the granitic canyon wall, there displaying glassy chunky structure with local tiers of short irregular columns; lower half is probably intrusive. Phenocrysts: 15–20% plagioclase (0.5–2.5 mm); ~1% cpx (0.5–2 mm); 

<1% olivine (0.2–1mm); and scattered Fe-Ti oxide mph. Unit intrudes and rests on 

Mesozoic granite (unit Kmo). Overlain by units Tacc (4.29±0.20 Ma) and a62. 40Ar/39Ar 

age: 9,164±17 ka. A trachyandesite lava flow as thick as 75–85 m at Kennedy Table near 

Millerton Lake Reservoir (Huber, 1981; Bateman and Busacca, 1982) on San Joaquin 

River (fig. 2), 66–85 km downstream and 2,400 m lower in elevation, is mineralogically 

and chemically similar and yields a 40Ar/39Ar age of 9,192±15 ka

Taww Trachyandesite north of White Wing Camp (Pliocene)—Phenocryst-poor lava flows 

(59.0–59.7% SiO2) that form southeast-facing caldera-wall scarp east of Glass Creek 

flow and ~700 m north of White Wing Camp. Unit crops out in a few ledges and buttresses for ~1 km along 80-m-high scarp. Ledges suggest presence of several flows, 

but mantle of pumiceous scree from 1350 C.E. Inyo ejecta obscures most of unit. 

Unit may be thicker than 80 m, but upland slope above scarp rim is covered by Inyo 
ejecta. Outcrops are flow-foliated, nonvesicular, slabby or irregularly block-jointed, 

and pale gray, weathering orange-brown or gray-brown. Phenocrysts: 1–3% olivine 

(0.5–2.5 mm); sparser cpx (0.5–1 mm); and 1–2% microphenocrystic plagioclase laths, 

rarely longer than 0.5 mm. Overlies unit Tmwo (3,153±26 ka) near Glass Creek. Overlain on caldera rim by unit Tmcv. K-Ar age: 2,950±90 ka for outcrop at UTM 241/790 

(Mankinen and others, 1986) 

Tbbs Basalt east of Big Springs (Miocene)—Phenocryst-poor mafic lava flows (49.8–51.9% 

SiO2) preserved as 500-m-wide remnant south of uppermost Owens River, midway 

between Big Springs and Arcularius Ranch. Deeply weathered rock disintegrates into 

dark gray corestones with crumbly brown rinds. Unit has ~40 m relief on south wall 

of river gorge, but base is not exposed there. Poorly exposed remnant of vent cone, 

1.2 km north-northwest of Big Springs, includes oxidized scoria bombs as big as 30 cm. 

Contains only sparse olivine mph, contrasting with phenocryst-rich Pliocene mafic 

lava flows to east and west. Banks against granite of unit Jbs; overlain by units Trac

(~11.7 Ma) and rer. Undated

Tbdp Trachybasalt of Deadman Pass (Pliocene)—Stack of numerous mafic lava flows on San 

Joaquin Ridge between Minaret Summit and Deadman Pass (figs. 3, 4, 7). Samples 

analyzed range in SiO2 from 50.4 to 53.2%. Flows are subset of sequence that continues 

along ridge beyond map area, altogether for 12 km northwest as far as Clark Lakes and 

Agnew Pass (Huber and Rinehart, 1965a; Chaudet, 1986; Bailey, 1989, 2004). Within 

map area, at least 30 flows range in thickness from ~3 m to >70 m, supporting benches 

and cliffs widely frost-riven into slabby ledges. Cliffs, 200 m high, ~1 km east of Deadman Pass, expose stack of ~20 thin rubbly flows, capped by a few flows 15–40 m thick. 

Entire sequence is as thick as 300 m on east-facing caldera wall and as thick as 450 m 

where it filled a paleotributary of Middle Fork on southwest-facing side of San Joaquin 

Ridge, ~1 km south of Deadman Pass (fig. 9). Dark gray in fresh interiors, most exposures are weathered and resinous; whether massive or vesicular, slabs and hackly blocks 

typically carry orange-brown weathering rinds 1–2 cm thick. Unit includes degraded 

scoria cone ~250 m wide, on west rim of main ridge (UTM 182/715); cone consists of 

brick-red lapilli and sparse bombs in palagonitized coarse ash matrix, arranged in decimeter-to-meter-scale fall layers that dip 15o–20oW or NW; east half of cone is covered by 

surficial debris. Flows contain 5–15% olivine (0.5–2 mm, commonly altered); some 

(generally thicker) flows also have 1–5% cpx (1–5 mm) and abundant Fe–Ti oxide mph; 

but none carry plagioclase phenocrysts. Zeolites and veinlets of silica and carbonate are 

common. Overlies pre-Cenozoic basement rocks. Overlain by Pliocene trachydacite pyroclastic deposits of units Tdpd and Tdsj (Bailey, 1989, 2004; Bailey and others, 1990). 

K-Ar age: 3.15±0.1 Ma (Dalrymple, 1964a; recalculated by 40K decay constants of Steiger and Jäger, 1977) for flow at east rim of stack. 40Ar/39Ar ages: 3,700±12 ka for lowest 

flow in channel-filling stack (UTM 170/719) just east of Agnew Meadows; 3,277±7 ka 

for top flow of stack, on caldera rim (UTM 191/720) ~2 km east and 400 m higher

Tbld Trachybasalt south of Lost Dog Lake (Pliocene)—Two glaciated remnants of olivine-rich 

basalt (51.9–52.1% SiO2) 0.6–1.3 km south of Lost Dog Lake and ~7 km west of Mammoth Mountain summit. North remnant is east-trending lava ridge, 100 m wide, 350 m 

long, and as thick as 20 m at its eastern scarp. South remnant is 200 x 400 m across and 

likewise thickest at its eastern scarp, which exposes 10 m of massive lava atop 10 m of 

agglutinated red scoria fall and a lower 10 m slope concealed by its own blocky talus. 

Vent complex, centered on south remnant, consists predominantly of agglutinated red 

scoria lapilli but includes scoria bombs as big as 30 cm and angular lava fragments up to 

40 cm. Lava flows are dense, massive, and dark gray, weathering orange-brown or pale 

gray-brown; rock splits on irregular fractures into angular blocks on scarps and on sets of 

parallel hairline fractures into plates and flakes on ice-scoured ridgetop surfaces. Weathering rinds are commonly 1–1.5 cm thick. Phenocrysts: 5–7% olivine (0.1–1.2 mm); no 

plagioclase or pyroxene phenocrysts; Fe-Ti oxides limited to finely crystalline groundmass. Also contains sparse quartz xenocrysts as big as 5 mm and scattered anhedral feldspar xenocrysts (1–4 mm). Compositionally unrelated to nearby basalt of The Buttresses 

(unit btb). Rests on Mesozoic granite. 40Ar/39Ar age: 3,353±31 ka

Description of Map Units—Tertiary Volcanic Rocks 69
70 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Tbmm Trachybasalt of Mammoth Mine (Pliocene)—Olivine-rich basaltic lava flow (51.8% SiO2; 

9.1% MgO) preserved as 300-m-wide remnant, high on northwest nose of Red Mountain, 

~1 km northeast of Lake Mary and 400 m southwest of Mammoth Rock. Single flow, 

inset along gently inclined paleoswale cut in Mesozoic metavolcanic rocks, wedges out 

southeastward but is as thick as 80 m at its northwest limit on sidewall cliff of glacially 

excavated outlet of Lakes Basin. A gulch cut into basalt surface by a successor stream 

trends northwestward, apparently parallel to the paleochannel filled by the lava flow. Cliffforming lava breaks into meter-scale massive blocks and slabs that form long steep talus. 

Source vent unknown. Unit is type locality of Mammoth Reversed-Polarity Subchron of 

the Gauss Normal-Polarity Chron (Doell and others, 1966). Phenocrysts: 7–10% olivine 

(mph to 2.5 mm); plagioclase, pyroxene, and oxides limited to groundmass, which is 

holocrystalline. K-Ar age: 3.15±0.1 Ma (Dalrymple, 1964a; Doell and others, 1966; recalculated by 40K decay constants of Steiger and Jäger, 1977). Four small basalt remnants, 

1–2 km southeast along southwest rim of Red Mountain, first correlated by Rinehart 

and Ross (1964) with unit Tbmm, are compositionally different and are here assigned 

to unit Tbrm

Tbpl Trachybasalt east of Pond Lily Lake (Pliocene)—Glaciated stack of phenocryst-rich 

lava flows (48.8–51.7% SiO2) that extends 2 km along east wall of Middle Fork San 

Joaquin River canyon as far as nose above its convergence with Fish Creek, ~10 km 

south-southwest of Mammoth Mountain. Above exposed base at rim of 2,100-m shelf 

traversed by Pacific Crest Trail, ledgy stack is as thick as 300 m. About ten ledges crop 

out in particular sections, and total number of flows is ~20. Massive zones are 5–15 m 

thick, slabby or hackly, locally vesicular, and commonly polygonally jointed to columnar; one flow at north limit of outcrop is ~30 m thick. Flows are dark gray, weathering pale gray to tan and typically having 1-cm-thick weathering rinds. Within stack 

of flows, two unstratified intervals of coarsely blocky, oxidized autobreccia are each 

~20 m thick; most blocks are nonvesicular, suggesting fragmentation during downslope 

flow rather than proximity to vent. Vent location unknown, but attitudes of flows and 

breccia sheets and interflow unconformities indicate flow directions toward west and 

southwest, not southward along Middle Fork canyon. Phenocrysts: all flows examined 

have 5–10% olivine (0.3–1.5 mm); most contain 1–5% cpx (0.1–2 mm, commonly in 

clusters); plagioclase is limited to groundmass and rare xenocrysts (as big as 3 mm); 

rimmed quartz xenocrysts (also up to 3 mm) are not uncommon. Samples analyzed are 

unusually rich in Sr (fig. 20D). Unit rests directly on Mesozoic granite (unit Kmo). 

Overlain by unit rbt (767±1 ka). Undated

Tbrm Basalt of Red Mountain (Pliocene)—Glaciated remnants of single flow of olivine-rich 

basalt (49.9–50.3% SiO2) aligned along southwest rim of Pliocene erosion surface just 

east of Lake Mary. Four remnants, each 15–25 m thick, extend linearly but discontinuously for ~1 km, banked against Red Mountain sidewall of Lakes Basin, along and just 

below scarp rim, apparently channeled by what was then a shallow paleovalley. Base of 

remnants is ~100 m higher than that of compositionally different Pliocene basaltic unit 

Tbmm, which lies ~1 km northwest. Compared to unit Tbmm, unit is richer in Fe, Ca, 

and Sr and poorer in SiO2, K2O, P, Ba, Rb, and Zr. Remnants form dark gray ledges, 

slabby or block-jointed, and mostly massive but locally retaining upper vesicular facies. 

Phenocrysts: 8–10% olivine (mph to 3 mm); common Fe-Ti-oxide mph. Unit rests exclusively on Mesozoic metavolcanic basement. Undated

Tbtb Basalt of The Buttresses (Pliocene)—Stack of severely glaciated phenocryst-rich lava 

flows (46.5–50.6% SiO2) that form stairstep benches just west of Middle Fork San 

Joaquin River, southwest of Devils Postpile. Main remnant is ~1 km wide and extends 

2 km north-south; consists of several flows, individually 15–40 m thick, altogether at 

least 100 m thick, but draping granitic basement to produce local relief of ~170 m. A 

300-m-long columnar outlier lies south of King Creek at southwest surviving limit of 

unit. Three more outlying remnants, ~1 km west of main one, extend to elevation of 2,750 

m, ~470 m higher than base of unit along Middle Fork. Several ledge-forming flows have 

tiers of bold vertical columns, typically 30–80 cm wide and locally as tall as 10 m; elsewhere flows are block-jointed, or intricately hackly jointed with spacings of 10–30 cm, or 
locally exhibit crude variously oriented columns. Except for rubbly breccia zones locally 

exposed, flows are massive, nonvesicular, and (though phenocryst-rich) have aphanitic 

crystalline groundmass. Rocks are dark gray, weathering gray-brown; weathered surfaces 

are speckled by crystals (1–3 mm) of olivine and cpx (or clusters thereof) that stand out 

in relief. Along 300-m-long reach of Middle Fork that begins 400 m south of Devils 

Postpile, unit Tbtb drapes steep west-bank granite wall all the way to riverside, indicating that river has here simply re-incised its pre-basalt channel and cut no deeper during 

entire Pleistocene. Above river and elsewhere, zones of interflow autobreccia as thick 

as 8 m are conspicuous; angular dense clasts and subrounded vesicular clasts coexist 

within clast-supported pale-gray matrix of unstratified coarse ash; yellow-orange alteration is superficial, confined to fracture films and patchy domains in ash-rich breccia, not 

penetrative into lava blocks. There is no explosively ejected facies nor evidence for a vent 

here or elsewhere; vent is inferred to have been far upstream, north or northwest of map 

area. Phenocrysts: some flows contain 2–3% plagioclase (0.5–1.5 mm, rarely to 3 mm); 

7–10% olivine (1–3 mm); and 10–12% cpx (1–2.5 mm), commonly in clusters. Other 

flows contain no plagioclase phenocrysts but have 5–10% each of olivine and cpx (both 

as big as 3 mm), including abundant cpx clusters, in olivine-rich groundmass. Contains 

sparse quartz xenocrysts as big as 6 mm. Sites in north-central and King Creek sectors 

of unit yield same (reversed) paleomagnetic direction (Hildreth and others, 2014). Rests 

exclusively on Mesozoic granite. 40Ar/39Ar age: 3,754±7 ka

Tb84 Trachybasalt and basaltic trachyandesite of Cone 8478 (Pliocene)—Stack of varied mafic 

lava flows (49.0–55.5% SiO2) above northwest wall of Long Valley caldera, north and 

east of Crestview. Vent complex at Cone 8478 is centered 1.5 km north of Crestview. 

Total exposed relief and apparent thickness ~300 m. Several flows, 5–20 m thick and 

irregularly block-jointed, are exposed discontinuously as ledges and buttresses through 

thick mantle of Inyo pumice fall. Interflow breccias zones, 1–5 m thick, are exposed only 

in roadcuts. Most exposures are massive, only locally vesicular, and pale to medium gray, 

weathering gray-brown or orange-brown, commonly with rinds 1–2 cm thick. Summit 

of Cone 8478 exposes coarse scoria and agglutinate, black to brick red, interlayered with 

dense sheets of fountain-fed lava. Phenocrysts: 5–7% olivine (0.3–3 mm); 1–4% cpx 

(0.5–2.5 mm); sparse Fe-Ti oxide mph. Different flows range widely in content (1–7%) 

of plagioclase (0.5–2.5 mm), some of it phenocrystic, much of it antecrystic, broken, or 

resorbed. Some flows also contain xenocrysts of quartz and feldspar and small cognate 

microdiorite inclusions; disequilibrium cargo may account for wide chemical range of 

unit. Overlies granitic unit Jbs and phenocryst-poor lava flows of unit Taec, which in 

turn banks against dacite Dome 8325 (unit Td83; 3.42±0.1 Ma). Overlain by units gst, 

rbt (767±2 ka), Tmcv, and coarse ejecta of 1350 C.E. Inyo eruptions. 40Ar/39Ar age: 

3,436±44 ka

Tdlm Trachydacite of Laurel Mountain (Pliocene)—Phenocryst-poor lava-flow remnant (63.0% 

SiO2) at 2,900 m on high rim of Long Valley Caldera, 2 km west of Convict Lake and 

2 km northeast of summit of Laurel Mountain. Frost-wedged, massive to flow-foliated 

lava is largely devitrified and splits slabby. Remnant is only ~120 m wide and 50 m 

thick. Phenocrysts: <1% plagioclase (0.3–1 mm), rare opx (0.2–1.5 mm), and ~5% 

opacitized hornblende (0.2–2 mm long). Crystalline groundmass contains abundant 

plagioclase-lath mph and sparse Fe-Ti oxide mph. Overlies several meters of alluvium 

rich in metasedimentary and granitoid cobbles that rests in turn on Devonian quartz sandstone (Greene and Stevens, 2002). No evidence for vent recognized. Lava was emplaced 

on a precaldera surface of only moderate relief. Its isolated presence on south rim suggests that a precaldera dacite dome field could have extended across present site of Long 

Valley Caldera, continuous with numerous Pliocene dacites on north and northwest rims. 

40Ar/39Ar age: 3,600±43 ka

Tdpd Trachydacite pyroclastic-density-current deposit of Deadman Pass (Pliocene)—Stratified 

assemblage of phenocryst-rich pumiceous pyroclastic-flow and surge deposits rich in 

accidental lithic fragments; best exposed along northeast face of San Joaquin Ridge 

between Dome 3321 and Deadman Pass. Equivalent in part to “lower pyroclastic member” of Bailey and others (1990). Interpreted here as vent-opening explosive phase of 

Description of Map Units—Tertiary Volcanic Rocks 71
72 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

eruption that ended with extrusion of Dome 3321 (unit Tdsd), just east of which pyroclastic assemblage exposes its maximum thickness of ~150 m and is eroded into spires 

and fluted cliffs. Thins to ~30 m at Deadman Pass, 2 km southeast. Dominant lithology 

of thick cliff sections is white nonwelded ignimbrite, indurated by weak silicification and 

crudely stratified by varied abundances of lithics. Massive lithic-poor intervals are as 

thick as 10 m, separated by numerous lithic-richer layers 0.5–3 m thick and an interval of 

thin-bedded fall and surge deposits, together ~7 m thick (see fig. 5 of Bailey and others, 

1990). Most layers are ash-dominant, carrying abundant pumice (1–5 cm) and highly 

varied contents (5–30%) of lithics (<1 to 30 cm). In some layers, most lithics are comagmatic trachydacite lava; others carry abundant clasts of metasedimentary, basaltic, and 

granitoid rocks derived from basement through which tuff vented explosively. Despite 

dozens of varied layers and high-energy emplacement, no significant unconformities are 

exposed. Uppermost 20–30 m of proximal section (near base of Dome 3321) consists of 

ash-rich coarse breccia in layers as thick as 5 m, dominated by angular basement clasts 

commonly 20–40 cm across and rarely >1 m. Unit is observed to rest only on metasedimentary rocks (unit |ms), but plutonic, hypabyssal, and basaltic rocks are inferred to be 

concealed nearby, possibly beneath dome 3321. Not directly dated but probably same age 

as inferred vent-filling lava dome, 2.8-Ma unit Tdsd.

Diamicton at Deadman Pass (Bailey and others, 1990) is deflated colluvial lag 

residual from this pyroclastic unit; much of ash and pumice has been stripped from top 

few meters, concentrating coarse lithics in wind-scoured ridgecrest saddle. Clast suite is 

same as within intact pyroclastic assemblage north of pass: Felsic and mafic granitoids > 

metapelite, hornfels, and quartzite > porphyritic Mesozoic dike rocks; only ~5% each of 

clasts are Pliocene basalt (as big as 70 cm) and (juvenile) Pliocene trachydacite (as big 

as 25 cm). No metavolcanic clasts from Ritter Range or upper Middle Fork are present. 

Most blocks larger than 1 m are granitoids or quartzite. Most clasts are angular or subangular except some subrounded grussy granitoids

Tdsd Trachydacite of Dome 3321 (Pliocene)—Phenocryst-rich lava dome (67.3% SiO2) on crest 

of San Joaquin Ridge 2 km northwest of Deadman Pass. Equivalent to “South Dome” of 

Bailey and others (1990), 500-by-700-m dome is partly covered by dacitic pyroclastic 

deposits but has ~120 m relief on its steeply exposed east face. Exposures are coarsely 

block-jointed, finely crystalline or vitrophyric, medium to dark gray, weathering brown 

with oxidized dark-brown rinds on joint faces; lower parts of east face are strongly 

flow foliated. Phenocrysts: 25–30% plagioclase (mph to 2.5 mm), 5–7% biotite (mph to 

2 mm), ~1% hornblende (0.5–2 mm), and traces of opx and Fe-Ti oxide mph. Dome lava 

overlies trachydacitic pyroclastic complex (unit Tdpd), which is probably its own ventopening eruptive phase that, in turn, rests on unit |ms. Dome is partly surrounded and 

overlapped by trachydacitic block-and-ash-flow deposits (unit Tdsj), which were in part 

derived from Two Teats dome (1 km northwest) but in part produced during fitful extrusion of Dome 3321 itself. K-Ar age: ~2.8±0.1 Ma (Curry, 1966)

Tdsj Trachydacite block-and-ash-flow deposits of San Joaquin Ridge (Pliocene)—

Phenocryst-rich block-and-ash-flow deposits (66.8–67.0% SiO2) that cap San Joaquin 

Ridge (Sierran drainage divide) as 700-m-wide ridgecrest swath extending 3 km southeast from Dome 3321 (unit Tdsd). As thick here as 120 m, unit is equivalent to “upper 

pyroclastic member” of Bailey and others (1990). Clasts are almost exclusively juvenile 

dacite shed during extrusive dome growth, in contrast to underlying “lower pyroclastic 

member” of those authors (our unit Tdpd), which is rich in basement clasts excavated 

during explosive reaming of vent later filled by dome lava. Clasts are angular to subrounded, mostly glassy, and dense or weakly vesicular; many are prismatically jointed. 

Clast sizes are seriate from ashy matrix to meter-scale blocks, a few as large as 5 m. 

On wind-scoured ridgecrest, deposit is widely deflated, concentrating coarse blocks on 

surface, which, with thick colluvium, obscures any internal stratification in the deposit. 

Bailey and others (1990) reported similar deposits surrounding lithologically similar 

trachydacite domes (San Joaquin Mountain and Two Teats) farther northwest, beyond our 

map area; what fractions of such deposits had issued from each of those and from Dome 

3321 has not been determined. Phenocrysts: ~25% plagioclase (mph to 4 mm), 5–7% 
biotite (mph to 1.5 mm), ~1% hornblende (0.5–1 mm), and traces of opx and Fe-Ti oxide 

mph. Overlies units Tbdp (~3.28 Ma), Tdpd, and |ms. Not directly dated but probably 

equivalent in age to inferred source lava dome, 2.8-Ma unit Tdsd

Tdud Trachydacite complex of upper Deadman Creek (Pliocene)—Phenocryst-rich pyroclastic 

vent complex with two lava domes centered on knoll 3025 between headwaters forks of 

Deadman Creek. Tuff (stippled) crops out over rugged area of 0.3 km2 with >300 m of 

relief. Domes, which erupted through and overlap tuff complex, have maximum dimensions of 350 m and 600 m and relief of 150 m and 300 m, respectively. Lavas are glassy 

or finely crystalline, massive to finely vesicular, flow foliated, block jointed, and generally pale to medium gray, weathering tan to orange-brown. Tuff is massive to chaotic, 

locally crudely stratified, is nonwelded but widely indurated by silicification, and carries 

abundant dacitic and basement lithics and abundant punky dacitic pumice; generally white, 

it weathers tan or yellow, hackly, pitted, or cavernous, and erodes into spires and hoodoos. 

Tuff lithology is unlike that of pyroclastic units Tdpd or Tdsj ~1 km south. Tuff is cut by 

dikes and irregular intrusive masses of crystal-rich hornblende-biotite-plagioclase trachydacite (67.4–70.7% SiO2

), which are presumed comagmatic. Both domes have 20-25% 

plagioclase (1–4 mm), 5–7% biotite (mph to 1.5 mm), trace hornblende (mph to 12 mm), 

and trace Fe-Ti oxide mph. Larger dome (68.5% SiO2) contains common ovoid inclusions 

(5–20 mm) of similar micropumiceous feldspar-rich dacite in which mafic phenocrysts are 

smaller and generally oxidized. Smaller southwest dome has 67.9% SiO2. Domes and tuff 

all erupted through and rest upon (or steeply against) unit |ms. Undated but presumed 

Pliocene in common with several other trachydacite domes nearby

Tdww Trachydacite of White Wing Mountain (Pliocene)—Phenocryst-rich lava dome (66.8% 

SiO2) and associated fragmental deposits that cap western quarter of White Wing Mountain, which consists predominantly of Mesozoic granite and forms steep northwestern 

segment of Long Valley Caldera wall. Glaciated dome is 1 by 1.5 km in area and has 

100–200 m of relief, much of it blanketed by 1350 C.E. Inyo pumice-fall deposits. 

Summit of White Wing Mountain (peak 3051), which stands 50 m higher than any 

exposure of dacite lava, consists almost entirely of fragments of same dacite, angular to 

subrounded, many as big as 1–2 m, that form a deflated lag deposit that armors summit 

and its west ridge. Rare blocks of granite suggest deposit could have been glacially transported from adjacent dome. However, remnant deposits of similar angular dacite blocks 

down at 2,660–2,775 m elevation on south ridge of White Wing Mountain lack basement clasts and include fragments of hydrothermally altered dacite, suggesting emplacement as explosively driven fragmental flows. Blocks and dome lava are glassy or finely 

crystalline, dense to finely vesicular, and flow foliated. Phenocrysts: 25–30% plagioclase 

(1–6 mm); 6–8% biotite (mph to 1.5 mm); ~1% hornblende (0.3–1 mm); traces of quartz 

(0.5–1.5 mm) and Fe-Ti oxide mph. Overlies granitic unit Kjl. K-Ar age: 2.6±0.4 Ma 

(Jenny Metz, analyst; reported by Chaudet, 1986)

Td65 Trachydacite of Dome 2965 (Pliocene)—Phenocryst-rich lava dome (64.4% SiO2) just 

southwest of White Wing Mountain on divide between headwaters of Glass and Deadman 

Creeks. Subcircular in plan view, dome is ~1.3 km in diameter and has as much as 350 m 

of relief. Outcrops are limited, as lower slopes of dome are brush-covered, but crest and 

south ridge are largely bare, covered by coarsely blocky to platy scree. Although dome 

was locally glaciated and is lapped on west by till deposits, material exposed consists 

mostly of glassy to finely crystalline, pale-gray blocks and slabs that weather tan to 

orange-brown. Phenocrysts: ~30% plagioclase (1–5 mm); 6–8% biotite (mph to 1.5 mm); 

1–2% hornblende (0.5–3 mm); traces of opx (mph to 1 mm) and Fe-Ti oxide mph. Base 

not exposed, but unit is inferred to intrude and overlie granitic and metasedimentary 

basement rocks. Undated but may be similar in age to lithologically similar lava dome, 

adjacent unit Tdww

Td78 Trachydacite of Dome 7835 (Pliocene)—Small dome of phenocryst-rich dacite (65.2% SiO2) 

500 m east of lithologically similar large Dome 8325 (unit Td83) on northwest wall of 

Long Valley Caldera. Poorly exposed, ~1 km northwest of Big Springs, domelet is 200 

by 300 m in plan view and has maximum exposed relief of ~35 m. Unit is nonvesicular, pale gray, weathers tan and punky, and disintegrates into irregular angular blocks. 

Description of Map Units—Tertiary Volcanic Rocks 73
74 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Phenocrysts: 5–7% biotite (0.2–2 mm); 3–4% amphibole prisms (1–4 mm long); feldspar 

present only as mph; tiny Fe–Ti oxides only in groundmass. Overlies units |ms and 

mafic granodiorite assigned to unit Jbs. Overlain by unit rbt (767 ka). Undated

Td83 Trachydacite of Dome 8325 (Pliocene)—Phenocryst-rich dacite dome (66.0–66.5% SiO2) on 

northwest wall of Long Valley Caldera ~1 km northeast of Crestview. Dome is ~1 km in 

diameter, has 280 m of exposed relief, and is heavily mantled by colluvium and Inyo fallout except near windscoured summit. Massive lava is strongly flow foliated, splits slabby, 

and is pervasively pink. Phenocrysts: 5–8% biotite (mph to 2.5 mm); 3–5% hornblende 

(mph to 1.5 mm, rims commonly opacitized); trace cpx (mph to 1.2 mm); ~1% plagioclase (0.2–1.5 mm, rims commonly resorbed). Partly glassy groundmass rich in mph and 

microlites. Overlies Mesozoic granodiorite. Banked against it are units Taec, Tb84, rbt

(767±1 ka), and mor (66±2 ka). K-Ar age: 3.42±0.08 Ma (Metz, 1987)

Tmcl Basaltic trachyandesite west of Cabin Lake (Pliocene?)—Olivine-rich mafic plug (52.9% 

SiO2) that intrudes Mesozoic metavolcanic basement at elevation 2,920 m on south wall 

of Shadow Creek (UTM 099/731), 300 m west of Cabin Lake, 700 m east of Ediza Lake, 

and 3.7 km east of summit of Mount Ritter (west of map area; see fig. 3). Ovoid in plan, 

plug is 25 x 35 m across and has no relief above gently sloping surface of glacially scoured 

host rock. Rock is nonvesicular and dark gray, weathering tan to orange-brown. Generally displays irregular joint blocks 10–50 cm across, but steep joints and flow lamination 

locally parallel contact. No ejecta or agglutinate preserved. Phenocrysts: 8–10% olivine 

(0.5–1.5 mm). No xenoliths seen. No outflow equivalents known to be preserved. Undated

Tmcm Basaltic trachyandesite south of Upper Crater Meadow (Pliocene)—Remnants of single 

glaciated olivine-rich lava flow (53.2% SiO2; 7.5% MgO; 5.25% K2O) on both rims of a 

gorge tributary to Crater Creek (UTM 190/608), ~600 m south of Upper Crater Meadow. 

Rim ledges are 1–4 m thick and overlain by thin till deposits and Inyo pumice. Phenocrysts: ~7% olivine (0.5–2 mm), many with tiny spinel inclusions; also contains sparse 

quartz xenocrysts with reaction rims. Holocrystalline groundmass, rich in flow-aligned 

plagioclase-lath mph, also carries abundant olivine mph and rare Fe-Ti oxides (both 

~0.1 mm). Rests on and armors 20-m-thick remnant of massive white grus exposed on 

gorge walls where grus grades down into fresh Mesozoic granite. Unit is unusually rich in 

Sr (2,050 ppm) and Ba (3,600 ppm) (fig. 20). 40Ar/39Ar age: 3,363±15 ka

Tmcr Basaltic trachyandesite of Mammoth Crest (Pliocene)—Olivine-rich mafic lava flow 

remnant (53.3% SiO2, 7.5% MgO) preserved as hill 3228 atop Mammoth Crest, ~1 km 

southwest of Crystal Crag. Remnant of single flow, resting on Cretaceous granite, is 250 

by 400 m in plan view and only ~25 m thick. Ice-scoured, finely crystalline, massive 

lava is frost-riven into whaleback pile of slabs. Phenocrysts: ~7% olivine (mph to 2 mm); 

common Fe-Ti oxide mph. Source vent unknown. Lower MgO content than olivine-rich 

3.15-Ma basalt (unit Tbmm) at Mammoth Mine, base of which lies 300 m lower in elevation and 3.5 km northeast across Lakes Basin, but the two are lithologically and petrographically similar. Undated

Tmcv Basaltic trachyandesite of Crestview (Pliocene)—Olivine-rich lava flows notably enriched 

in K and P (53.1–54.9% SiO2; ~4.5% K2O; ~0.85% P2O5) that crop out discontinuously 

on scarps east, north, and west of confluence of Glass and Deadman Creeks. South-facing 

scarp 600 m southeast of Crestview is stack of flows ~40 m thick that rests on Mesozoic 

granodiorite. On steep nose 500 m northwest of Crestview, unit is ~60 m thick. Along 

rim of southeast-facing scarp 1–2 km southwest of Crestview, unit may be single flow; its 

exposure there is at least 10–30 m thick, but its upland slope continuing northwest above 

rim is wholly concealed by 1350 C.E. Inyo ejecta. Exposures are block-jointed and massive to sparsely vesicular. Phenocrysts: 7–10% olivine (mostly 0.2–3 mm; locally hoppershaped crystals as big as 4–7 mm); sparse cpx (0.3–1 mm); sparse plagioclase-lath mph; 

and cpx-plagioclase microdioritic inclusions. Also carries quartz and feldspar xenocrysts. 

Overlies Mesozoic granodiorite and units Taww (2,950±90 ka), Tmwo (3,153±26 ka), 

Taec, and Tb84. Overlain by Sherwin Till (unit gst). Relative elevations of three main 

outcrop strips suggest a westerly source vent, now concealed. Undated

Tmcw Basaltic trachyandesite west of Crater Creek (Pliocene)—Glacially eroded remnant of 

two moderately porphyritic lava flows (52.3–52.6% SiO2) that form 200-by-300-m 
steep-sided bench banked against granitic canyon sidewall, 2 km west of Red Cones and 

1.5 km south of Rainbow Falls. Remnant consists of two similar lava flows, lower one 

15–20 m thick and upper one 8–10 m thick. Basal colonnade of lower flow has slender 

vertical columns 2–12 m high and 25–70 cm thick, resting directly on granite. Upper 

flow has irregular vertical joints, and both have hackly entablatures and locally inclined 

or subhorizontal columns. Both have gray-brown or orange-brown weathering rinds, 

extensive patches of colorful lichens, sparse vesicles filled with secondary minerals, and 

sparse veinlets of silica. Phenocrysts: 1–3% plagioclase (0.5–1.5 mm), generally corroded 

or tinted pale soapy green; 5–8% apple-green olivine and black cpx (both 0.5–3 mm), 

commonly clustered and many with alteration rims. In contact only with Mesozoic 

granite. Petrographically similar to basalt of The Buttresses (unit Tbtb), 2 km upstream, 

which, despite its more mafic composition, shares with unit Tmcw a similar (reversed) 

paleomagnetic direction (Hildreth and others, 2014). Undated but presumed to be similar 

in age to lava flows of unit Tbtb

Tmel Basaltic trachyandesite of Emerald Lake (Pliocene?)—Moderately porphyritic mafic plug 

(52.3% SiO2) that intrudes Mesozoic metavolcanic basement at east shore of Emerald 

Lake (UTM 095/774), near John Muir Trail ~1 km east of Thousand Island Lake in 

headwaters of Middle Fork (west of map area; see fig. 3). Glacially scoured remnant is 30 

by 70 m across and has only 15 m of relief. Rock is nonvesicular and dark gray, weathering pale gray to brown; generally hackly or chunk-jointed, it locally displays inclined 

columns. West end of plug has 3-m-thick, flow-foliated rind in which vertical laminae 

parallel conduit margin against wall rock. No ejecta or agglutinate preserved. Phenocrysts: 5–8% olivine (1–3 mm) and 1–2% cpx (1 mm). No outflow equivalents known to 

be preserved. Undated

Tmsd Basaltic trachyandesite south of Deer Creek (Pliocene?)—Moderately porphyritic lava 

flows (53.2% SiO2) that cap high divide between upper Deer Creek and Fish Creek 

canyon. Stack of flows is ~150 m thick where exposed on cirque headwall at head of 

Deer Creek. Glaciated unit extends 2 km west from vent remnant at hill 3206, which lies 

5 km east of Pumice Butte and 9 km south of Mammoth Mountain. Upland surface of 

unit is blocky colluvial rubble with a few ledgy exposures. Phenocrysts: 5–7% olivine 

(0.2–3 mm); trace cpx but no feldspar. Overlies Mesozoic granite (unit Kmo). Undated

Tmwc Basaltic trachyandesite west of Castle Lake (Pliocene?)—Mafic plug and adhering remnants of lava and agglutinated scoria (52.9% SiO2

) that form Crag 10640 (3,243 m) on 

crest of southeast arm of Volcanic Ridge, 600 m west of Castle Lake and 9.5 km northwest of Mammoth Mountain summit (UTM 1236/7121). Unit is west of map area; see 

fig. 3. Glaciated remnant forms column atop pedestal, together ~25 m high, ~15 m wide 

at top, and ~25 m wide at base, which is surrounded by apron of mafic scree. Blocks 

of massive lava and scoria bombs are both as big as 50 cm. Fragments of metavolcanic 

basement rocks are common in both agglutinate and homogenized lava. Phenocrysts: 

~7% olivine (0.2–1.2 mm), many with tiny spinel inclusions. No plagioclase or cpx phenocrysts. Rare tiny oxides in finely crystalline groundmass. Rests on deformed Mesozoic 

metavolcanic rocks (Huber and Rinehart, 1965a). No remnants of outflow lavas known to 

be preserved. Undated

Tmwo Basaltic trachyandesite west and east of Obsidian flow (Pliocene)—Phenocryst-rich lava 

flows and agglutinate (53.1-54.5% SiO2) that crop out as glaciated near-vent remnants 

just west of Obsidian flow (unit ric) and along walls of Glass Creek from beneath Obsidian flow for ~1.3 km eastward. Three proximal remnants form north-south alignment only 

200 m long, each only 30–70 m in maximum dimension. Northern remnant is largest, a 

15-m-high steep pile of brick-red agglutinated spatter and scoria with ejecta as large as 

30 cm; this is invaded by thin dikes and on its north and west walls by irregular masses of 

dark-gray block-jointed lava. Middle remnant is 6-m-high ledge of coarsely block-jointed 

massive lava ~30 m long. Southern remnant is another ledge of similar lava ~70 m long, 

somewhat more vesicular and strewn with glacial erratics and Inyo ejecta. Lavas are 

locally streaky but generally dense and massive or weakly flow-foliated, their surfaces 

weathering dark brown to orange brown. Outcrops on walls of Glass Creek are irregularly block-jointed ledges as thick as 10 m, but exposure is limited by Inyo ejecta and 

Description of Map Units—Tertiary Volcanic Rocks 75
76 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

derivative scree; ledges suggest possibility of more than one flow. Drill holes beneath and 

adjacent to Obsidian flow (Eichelberger and others, 1985) penetrated 250–350 m of unexposed mafic lavas sandwiched between till and granitic basement; unit Tmwo represents 

at least upper part of mafic section. Phenocrysts: 3–7% olivine (0.2–1 mm), iddingsite 

rims common; 1–4% plagioclase (0.3–1 mm long, mostly laths); 1–3% cpx (0.5–1.5 mm), 

many in clusters; also carries sparse quartz xenocrysts. Finely crystalline groundmass is 

rich in tiny plagioclase laths and clusters of pyroxene mph. Distal exposure on south wall 

of Glass Creek is least evolved chemically and poorest in phenocrysts, with plagioclase 

limited to microphenocrysts. Base not exposed, but inferred to rest on Mesozoic granite 

(exposed nearby). Overlain by units Taww (2,950±90 ka), Tmcv, and gst, which is in 

turn overlain by unit rbt. Fresh condition of lava led Putnam (1949) to infer Pleistocene 

age, but unit was called Pliocene by Bailey (1989). 40Ar/39Ar age: 3,153±26 ka 

Tmww Basaltic trachyandesite of White Wing Mountain (Pliocene?)—Moderately porphyritic 

scoria cone, agglutinate, and subordinate lava (52.3–52.4% SiO2) preserved as two 

remnants on southwest ridge of White Wing Mountain. Major remnant is 300 by 500 m 

across, extending downslope just south of summit from elevation 2,990 m to 2,830 m. 

Massive lava exposed at base is pale gray, weathers orange-brown, splits slabby, and 

contains vesicle sheets. Principal outcrops are cliffs, 30–50 m high, of crudely stratified 

ejecta, tack-welded to moderately agglutinated. Ejecta are dominantly lapilli but seriate 

from coarse ash to 30 cm, enclosing scattered scoria bombs as big as 50–200 cm. Most 

clasts are vesicular scoria, but many are dense juvenile fragments. Cliffy pyroclastic 

section is black below but grades upward to brick-red, then orange-brown with yellow 

palagonitic matrix locally. Major ridge-capping remnant erodes craggy; huge agglutinate 

blocks were ice-transported into a hollow 250 m east. Smaller remnant, down ridge at 

elevation 2,700 m, is only 100 m across and consists of ledges of scoria lapilli and tackwelded agglutinate. Phenocrysts: 5–8% olivine (mostly mph but a few as big as 1.5 mm), 

and rare cpx (<1 mm). Overlies granitic unit Kjl. Undated

Trac Rhyolite tuff of Alpers Canyon (Miocene)—Phenocryst-rich rhyolitic ignimbrite exposed 

on north and south walls of uppermost Owens River at its confluence with Alpers Canyon. Outcrops are densely welded, crystal-rich, poor in fiamme, and contain only sparse 

lithic fragments (1–5 cm). Lithics are mostly mafic and rhyolitic lavas, crystal-poor to 

crystal-rich, and rare hornblende dacite. Tuff is pale gray (almost white) but weathers tan 

to orange-brown or dark gray-brown; irregularly block-jointed, many outcrops are crumbly or cavernously weathered, and some erode into knobs and spires. Unit is 60–90 m 

thick on canyon walls, but base is not exposed where thickest. Rare outcrop of silicified 

nonwelded tuff contains abundant fragments (2–10 mm) of crystal-poor pumice. Source 

unknown, but Bailey (1989) reported additional remnants 9–20 km to northeast. Once a 

regional outflow sheet, surviving remnants are small and scattered. Phenocrysts: 15–20% 

each of sanidine and quartz (many broken); 1–2% each of biotite and plagioclase; sparse 

Fe-Ti oxide mph; quartz and feldspars mostly 0.5–3 mm, biotite 0.1–1 mm; oxides 

≤0.4 mm. Easily distinguished from Bishop Tuff (unit rbt), which contains abundant 

pumice lapilli and fiamme. Overlies Mesozoic granodiorite and phenocryst-poor Miocene 

basalt (unit Tbbs). Overlain north of Owens River canyon by coarse gravel (unit pal), 

which lacks clasts of nearby Bishop Tuff (767±2 ka). Lithic-free whole-rock composition 

71.5% SiO2. K-Ar age: 11.7±0.1 Ma (Huber, 1981; Bailey, 1989)

QUATERNARY VOLCANIC ROCKS

[Listed alphabetically by 3-letter unit label]

a62 Trachyandesite of Cone 2962 (late Pleistocene)—Scoria cone and adjacent lava-flow apron 

(57.1–59.2% SiO2) on divide between Deer and Crater Creeks, 7 km south of Mammoth 

Mountain. Cone is 160 m high, ~1 km wide, elongate northeast, and is centered 2 km 

northeast of Pumice Butte (fig. 10). From its southwest toe emerges a 1-km-wide apron 

of blocky lava flows that extends 3 km southwest to rim of San Joaquin River canyon. 

Vent-proximal half of apron is plateau where lavas were lightly ice-scoured and striated 

but remain block-jointed and vesicular. Southwest-sloping distal half of apron, however, 
was ice-free and remains ruggedly blocky, glassy, and scoriaceous (although weathered and frost-damaged). Longitudinal troughs between flow ridges and knolls provide 

10–20 m local relief, and several stairstep flow fronts are 10–15 m high. At San Joaquin 

River canyon rim, apron drapes granitic wall, exposing at least ten flows in a steeply 

dipping stack as thick as 200 m. Scoria cone is poorly exposed except on its nonglaciated summit flat, around margins of which arcuate ridges of coarse red scoria (loose or 

tack-welded) appear to be rim remnants of a filled crater; scoria bombs there are as big as 

70 cm. True summit is a 10-m-high pile, ~25 m in diameter, of frost-shattered lava blocks, 

massive to vesicular, that was a knob-like extrusion; its southeast margin adjoins a stack of 

red scoria bombs, some >1 m in diameter. Lower slopes of cone locally expose glaciated 

ledges of massive platy lava. Phenocrysts: bombs and lapilli on cone and lava flows alike 

are crystal-poor, containing ≤1% plagioclase (0.5–1.5 mm), still sparser olivine mph, and 

trace cpx (mph to 1 mm). Scoria cone overlies Mesozoic granite and basaltic lava flows of 

unit bdc (155±2 ka); lava-flow apron overies units Tasj and rbt and banks around Pumice 

Butte (unit apb; 142±5 ka). 40Ar/39Ar age: 118±10 ka (Mahood and others, 2010) 

aic Trachyandesite of Inyo Craters (late middle Pleistocene)—Stack of at least five phenocryst-poor lava flows (57.5–59.4% SiO2

) exposed on walls of South Inyo Crater (at south 

toe of Deer Mountain) and along south-striking fault scarp for ~1 km south from crater. 

Flows are thin and rubbly, but their (modestly vesicular) more massive interior zones 

support ledges 3–6 m thick. All have sparse plagioclase and still sparser olivine and cpx 

(both ≤1 mm), and some carry rare feldspar and quartz xenocrysts as large as 10 mm. 

Relations on crater walls suggest flow direction was toward northeast. Base not exposed, 

but a set of at least five (compositionally correlative) flows, each 5–13 m thick, was 

cored by Inyo-4 slant drill hole at slant depths of ~21–62 m (fig. 6), where they overlie a 

contrasting set of basaltic trachyandesite lavas (see unit mcl) and cinders (Eichelberger 

and others, 1988; Vogel and others, 1994). In drill hole DC-1 (Sorey and others, 1978), 

located at UTM 259/781 (fig. 8), ~6 km northeast of South Inyo Crater, a 15-m lava flow 

of unit aic was penetrated at depth of ~90–105 m (fig. 6), beneath flows of units mcv, 

mor, and another flow compositionally indistinguishable from unit bsr (99±1 ka). Flows 

compositionally correlative with unit aic have not been identified elsewhere.

Flows exposed on crater wall are directly covered by 8–12 m of red mafic pyroclastic strata (unit mic), suggesting presence of an unexposed vent nearby; these in turn 

are overlain by ~1 m of Inyo pumice fall (1350 C.E.), atop which lies as much as 13 m 

of phreatic ejecta (unit pe) that extends to crater rim and was explosively emplaced at 

end of 1350 C.E. eruptive sequence (Mastin, 1991). Three flows drilled at top, middle, 

and bottom of stack exposed on crater wall gave nearly identical paleomagnetic directions (Mankinen and others, 1986). Absence of rhyolitic ejecta or float from nearby 

Deer Mountain intercalated between lava flows (or within or beneath subjacent pile of 

mafic lavas cored by Inyo-4 strongly suggests that unit aic is older than Deer Mountain 

(unit rdm; 101±8 ka). Vogel and others (1994) gave 40Ar/39Ar ages of 151±17 ka and 

161±14 ka for two flows in the uppermost set of lava flows in Inyo-4 core, flows chemically similar to those exposed on crater walls, for which we obtained a 40Ar/39Ar age of 

131±1 ka

aml Trachyandesite of McLeod Lake (late Pleistocene)—Phenocryst-poor lava flow (61.5–

62.3% SiO2) exposed as narrow sliver along north-striking fault 40 m east of McLeod 

Lake (also variously rendered McCloud or McClowd, but named for U.S. Forest Service 

Ranger Malcolm McLeod), ~1 km south of southern toe of Mammoth Mountain. Crops 

out as 1-m-high ledge of black massive aphanitic lava, locally weakly vesicular and 

partly glassy; block-jointed or locally slabby; weathers tan and carries pink films along 

joint planes. Unit banked against Mesozoic granite (unit Kmo), and both are severely 

glacially eroded; more remnants of lava may be buried by glacial deposits or beneath 

lake. Phenocrysts: 1–2% plagioclase (0.5–2 mm); 1–2% amphibole (0.1–1.3 mm long; 

rare opx microphenocrysts (in clots with oxides and plagioclase); and sparse Fe-Ti-oxide 

microphenocrysts (0.1–0.4 mm). At bottom of 98-m-deep drill hole (HSL-1; fig. 6) near 

Horseshoe Lake carpark, scoria lapilli (oxidized, not rounded) of a 10-m-thick stratified 

fall deposit (rich also in granitoid and metavolcanic lithic lapilli) are compositionally 

Description of Map Units—Quaternary Volcanic Rocks 77
78 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

similar to lava of unit aml; scoria fall directly underlies a 15-m-thick trachydacite lava 

flow similar to unit dsd, which is in turn overlain by 73-m-thick stack of 7–10 phenocryst-poor vesicular lava flows of unit amp (97±1 ka). It can be inferred that unit aml

predates all or much of Mammoth Mountain and that its vent lay within a few kilometers 

of Horseshoe Lake. In water well 30M (fig. 6), 2 km southeast of downtown Mammoth 

Lakes, a 15-m-thick lava flow of unit aml was penetrated at a depth of 45–60 m, where 

overlain by a thin flow of unit amp and ~35 m of late Pleistocene till (unit glb, which 

extends to surface). Lava of unit aml thus flowed as far east as Sherwin Creek, but none 

is exposed farther east nor recognized in other south moat drill holes. Undated

amp Trachyandesite of Mammoth Pass (late Pleistocene)—Aprons of phenocryst-poor lava 

flows (57.2–61.6% SiO2) that erupted on high divide north of Mammoth Pass at vent 

later covered by construction of Mammoth Mountain. Flows emerge from beneath south 

toe of Mammoth Mountain edifice and turn both east and west, extending 9 km east to 

terminus near Sherwin Creek Campground and at least 7 km southwest where remnants 

survive on both sides of lower Crater Creek. Entire western apron is glacially scoured 

from Mammoth Pass down to Sotcher Lake, Reds Meadow, and Boundary and Crater 

Creeks. Lavas of eastern tongue have been more severely eroded and widely concealed 

by late Pleistocene till, although windows are exposed along Mammoth Creek, and 

a 1-km-wide blocky terminal lobe just beyond distal moraines of unit glb is virtually 

uneroded. Lavas form glaciated ribs, nubbins, and ledges, block-jointed or slabby, rarely 

platy, and typically massive and aphanitic with pale-gray mottling; locally oxidized, 

especially in healed flow-breccia zones. Vesicular zones are widely stripped. Thickest 

exposures are typically massive with vertical joints, irregularly prismatic. On west slope, 

belt between 2,750 and 2,850 m elevation was glacially eroded into west-trending noses, 

each sculpted from a single lava flow, each having 10–25 m local relief, and scoured free 

of till and all but sparse erratics. Thickest exposures are 30 m near upper Twin Lakes, 

100 m near Horseshoe Lake, and >140 m along Boundary Creek where at least 5 cliffy 

flows are exposed, each 10–30 m thick. Drill hole HSL-1 (fig. 6), near Horseshoe Lake, 

penetrated 7–10 flows in a total thickness of 73 m of unit amp, which rests at depth on an 

unexposed trachydacite lava flow of typical Mammoth Mountain lithology. On northfacing scarp of Hill 2938 (high point of unit), a glacially excavated trough at south toe 

of Mammoth Mountain exposes a pair of 8-m-thick platy lava flows of unit amp; minor 

scoriaceous lava-flow rubble is present but no ejected scoria exposed. Vent is inferred to 

have been farther north, subsequently covered by Mammoth Mountain trachydacite pile. 

Vents shown by Bailey (1989) at hill 2938 and Sherwin Creek Campground do not exist. 

Phenocrysts: typically ~1% plagioclase (0.5–1.5, rarely as big as 3 mm), uncommonly in 

clots; traces of olivine and cpx (both ≤1 mm); sparse Fe-Ti-oxide mph (0.1–0.3 mm); rare 

plagioclase antecrysts (3–5 mm). Among many flows, plagioclase content ranges from 

almost none to as much as 3%. Groundmass is grainier, slightly coarser than in subjacent 

unit drf, with which unit can easily be confused, especially where unit is locally platy and 

mottled. Olivine crystals, where oxidized light to dark brown, can resemble opx prisms of 

unit drf, but olivine is generally equant, not elongate. 

Unit overlies Mesozoic granite, Bishop Tuff (unit rbt; 767±2 ka), unit drf (98±1 ka) 

near western limit, and unit bsr (99±1 ka) at eastern limit. Overlies unit aml in drilholes 

at Horseshoe Lake and southeast of downtown Mammoth Lakes (fig. 6). Overlain by 

unit mdp (82±1 ka) near Sotcher Lake, by units dbp, dnh, and bhl near Twin Lakes, 

and widely overlain by late Pleistocene till (unit glb). Flows of unit amp emerge from 

beneath base of South Summit Dome (unit dsd; 87±6 ka) at drainage divide, but they 

overlie an unexposed trachydacite lava flow in drill hole HSL-1 (fig. 6). Southwest distal 

remnants overlie unit drf and fill paleochannels cut in granite. Steep slope extending 

2 km north of Boundary Creek at 2,500–2,600 m elevation reflects draping and concealment of previously glaciated granitic canyon sidewall by flows of unit amp. Low-relief 

belt along western slope at 2,700–2,750 m is poorly exposed, as is southwest corner of 

apron where lavas bank against Bishop Tuff; both areas are mantled by Inyo pumice and 

ground moraine. Sites sampled near Reds Meadow and in Valentine Reserve Ecological Study Area on Mammoth Creek yield similar paleomagnetic directions (Mankinen 
and others, 1986; Hildreth and others, 2014). 40Ar/39Ar ages: 97±1 ka (table 2); 81±1 ka 

(Mahood and others, 2010); K-Ar age: 86±10 ka (Mankinen and others, 1986)

apb Trachyandesite of Pumice Butte (middle Pleistocene)—Scoria cone and moderately porphyritic lava flows (58.4–59.0% SiO2

) atop plateau between canyons of Fish Creek and 

San Joaquin River, 6 km east of their confluence and 8 km south-southwest of Mammoth Mountain (fig. 10). Pumice Butte scoria cone, ~750 m wide and 150 m high, stood 

above Pleistocene ice and is only moderately eroded, its slopes strewn with red and black 

sloughing scoria. Lava flows form a stubby lobe at north toe of cone and a narrow tongue 

that extends southwestward from midway up west slope. Lavas are glacially scoured, 

coarsely blocky, partly glassy, and massive or vesicular. Summit preserves a low-relief 

dish-like crater, largely filled; below its northwest and east rims are steep faces, 20–30 m 

high, of coarse red scoria—some loose but mostly tack-welded. Scoria bombs are as big 

as 1 m. Phenocrysts: 2–3% plagioclase (0.5–3.5 mm; mostly ≤1 mm); ~1% olivine (mostly mph but rarely as big as 1.5 mm); ~1% cpx (mph, some clustered with plagioclase and 

(or) oxides); trace Fe-Ti-oxide mph. In thin section, glassy groundmass is choked with 

microlites and plagioclase-lath mph. Overlies Mesozoic granite (unit Kmo) and Bishop 

Tuff (unit rbt; 767±2 ka). Overlain by lava flows of unit a62 (118±10 ka) from nearby 

scoria cone 2962. 40Ar/39Ar age: 142±5 ka (Mahood and others, 2010)

asr Trachyandesite of Shady Rest Campground (middle Pleistocene)—Phenocryst-poor lava 

flow (56.4–56.6% SiO2) exposed poorly for ~400 m along southeast toe of Knolls Vista 

(hill 2517), from elevation ~2,375 m at road near campground entrance to as high as 

~2,410 m. Subrounded to angular joint blocks are finely vesicular to massive and weakly 

flow-foliated. Phenocrysts: Almost aphyric; <1% plagioclase (0.5–1.3 mm), rare cpx 

(0.1–0.8 mm), anhedral plagioclase antecrysts (~4 mm), and tiny rare Fe-Ti oxides. Partly 

glassy groundmass rich in plagioclase laths. Base not exposed. Overlain by units mcl

(175±3 ka), bsr (99±1 ka), and till of units gcd and glb. Slope is lightly strewn with unit 

gcd erratics that include abundant granitoids but no dacites from Mammoth Mountain. 

In water well 30M (fig. 6), 3 km south of Knolls Vista, a 17-m-thick flow of unit asr was 

penetrated at 143–160 m depth, directly beneath flows of unit mcl but atop uncorrelated 

(and nowhere exposed) mafic lavas (51.5–53.2% SiO2) at 160–195 m depth. Undated

bar Trachybasalt of Arcularius Ranch (late Pleistocene)—Scoria cone (fig. 7) and extensive 

phenocryst-rich lava flows (50.6–52.4% SiO2) that erupted on southwest wall of Long 

Valley Caldera, 2 km northeast of Minaret Summit and 4 km north of summit of Mammoth Mountain. Lava flows covered much of caldera’s southwest moat, next poured 

13 km northeastward between Deer Mountain and West Moat Coulee (unit rwm), 

wrapped west side of Lookout Mountain, then turned eastward at north wall of caldera 

near Big Springs, and extended additional 10 km along north moat to terminus along 

Owens River (fig. 9). Scoria cone, 160 m high and 700 m wide, consists of lapilli and 

bombs as big as 70 cm, mostly oxidized, generally loose but locally agglutinated. Stratified cinders at least 12 m thick exposed along gulch just south of cone are palagonitized 

and capped by a unit bar lava-flow remnant ~10 m thick; bombs in fall deposit include 

common representatives of phenocryst-poorer unit bmn as well as predominant phenocryst-rich bombs of unit bar. Cone banks high against wall of precaldera basement and, 

though degraded, was not devastated by late Pleistocene glaciation. Proximal lava-flow 

apron and windows through till south of Crater Flat are glacially scoured, but exposures 

west, north, and east of Lookout Mountain lie outside the glacial limit and thus remain 

ruggedly scoriaceous, even craggy. Wherever exposed, individual flows are 3–10 m thick, 

both proximally and along north moat, where two (virtually identical) flows extend together almost to ultimate terminus. Block joints are common, and crudely prismatic joints 

locally mark vesicular surfaces. Phenocrysts: 15–25% plagioclase (1–7 mm, rarely to 10 

mm); 2–5% olivine (0.5–2 mm, rarely to 3 mm), commonly in clusters; cpx (0.5–2.5 mm) 

is sparse proximally and rare distally. Scoria bombs on cone contain scattered silicic 

xenoliths (1–5 cm), some partially melted, probably of metavolcanic basement. Overlies units bcf, bmn (87±7 ka), and ddc (103±9 ka). Overlain by units mcv, mnd, mor

(66±2 ka), and gdc. Locally uplifted on roof of large dacite dome, unit d61 (87±2 ka). 

Subunit bar' includes this uplifted fragment as well as a medial exposure west of Lookout 

Description of Map Units—Quaternary Volcanic Rocks 79
80 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Mountain; compositionally, lavas at the two sites are identical (n=3), but they differ 

slightly from rest of unit bar (n=17) in being somewhat richer in Al and slightly poorer 

in Mg, K, and P. Subunit bar' is inferred to be a single flow, slightly poorer in olivine 

(1–2%) and a bit richer in plagioclase (20–25%), much of which is more sieve textured 

than in associated flows of bar. Six sites drilled, proximally, medially, and distally, all 

yield similar paleomagnetic directions for bar and bar' (Mankinen and others, 1986; 

Hildreth and others, 2014). K-Ar ages: 108±12 ka (distal at Big Springs) and 95±13 ka 

(proximal) (Mankinen and others, 1986). 40Ar/39Ar ages: 88±5 ka (quarry north of MMSA 

Main Lodge) and 94±9 ka (toe of scoria cone) (Mahood and others, 2010)

bcd Basalt of Casa Diablo Hot Springs (late Pleistocene)—Apron of phenocryst-rich lava flows 

(51.1–52.1% SiO2) that emerges from beneath younger unit bsr ~1 km east of Shady 

Rest Park and extends 3.5 km southeastward to its terminus just south of Mammoth 

Creek and Highway 395. Forms a 1-km-wide expanse that spans cloverleaf junction of 

Highways 203 and 395 and adjoins geothermal plant at former site of Casa Diablo Hot 

Springs. Near plant, lava apron is cut and moderately displaced by faults of the resurgent graben system. Source vent is probably spatter cone 2580+ (unit bed; 121±3 ka) 

just west of Dome 2861 (unit d61; 87±2 ka); cone agglutinate of unit bed is similar to 

unit bcd petrographically and in major- and trace-element composition, but paleomagnetic directions differ. Apron outcrops are gently rolling scoriaceous surfaces or (where 

stream-incised) block-jointed ledges, commonly crudely columnar; nearly all exposures 

are vesicular except at lowest levels exposed along arroyos. No exposure is thicker than 

15 m. Phenocrysts: 7–12% plagioclase (0.5–7 mm); 5–8% olivine (0.5–2.5 mm); 1–2% 

cpx (1–2.5 mm); Fe-Ti oxides limited to crystalline groundmass. Overlies units mmc and 

gcd. Overlain by units bfh (92±2 ka) and bsr (99±1 ka). Crops out only beyond limit of 

late Pleistocene till. 40Ar/39Ar age: 125±2 ka 

bcf Trachybasalt south of Crater Flat (middle Pleistocene)—Spatter cone 2545, consisting of 

phenocryst-poor agglutinate (51.7–51.8% SiO2), 500 m south of Crater Flat and 5 km 

north of summit of Mammoth Mountain. Low cone is glacially scoured, 500 m wide, 

and only 40 m high. Mantled by late Holocene Inyo pumice-fall deposit, cone is poorly 

exposed, cropping out only as brick-red ledges of dense agglutinate on south slope, a 

few reaches along shallow draw at southeast toe of cone, and one massive lava ledge 

just southeast of that draw. Phenocrysts: ~1% plagioclase (0.5–4 mm); traces of olivine 

and cpx mph (both 0.1–0.2 mm); Fe-Ti oxides limited to finely crystalline groundmass; 

also contains sparse quartz and feldspar xenocrysts with reaction rims. Base not exposed. 

Overlain by units bmn (87±7 ka) and bar (88±5 ka). 40Ar/39Ar age: 164±2 ka

bdc Trachybasalt west of Deer Creek (middle Pleistocene)—Moderately porphyritic lava flows 

(51.6% SiO2) that crop out as a pair of 10-m-high benches that extend for ~1 km along 

southeast toe of Cone 2962, just west of Deer Creek. Fresh angular joint blocks are massive, nonvesicular, and moderately ice-scoured. Contains 3–5% olivine (0.5–1 mm) and 

8–10% plagioclase (mostly 0.5–1 mm but as big as 4 mm). Not related to Pliocene mafic 

center 3206, 2–4 km east (Bailey, 1989), which consists of weathered lava flows of cpxolivine basaltic trachyandesite (unit Tmsd) that lacks plagioclase phenocrysts. Veneer of 

till obscures contact with closely underlying granite of unit Kmo. Unit is directly overlain 

by scoria Cone 2962 (unit a62; 118±10 ka). 40Ar/39Ar age: 155±2 ka

bed Basalt east of Dry Creek (late Pleistocene)—Poorly exposed, glacially eroded, phenocrystrich spatter cone (52.0–52.1% SiO2

) between Dry Creek and Dome 2861. Cone 2580+ is 

~500 m wide, ~50 m high, and moderately degraded, with a subdued crater open to north. 

Ejecta are almost entirely hidden by late Holocene Inyo pumice-fall deposit, and cone is 

also strewn with glacial erratics of unit ddc and trachydacites of Mammoth Mountain. 

Lone significant outcrop is on east rim of summit where ledges of oxidized agglutinate 

are 1–2 m thick and enclose thin dark-gray massive lenses. Most is tack-welded with 

oxidized lapilli and bombs still discernible, but this facies grades abruptly into massive 

domains, somewhat homogenized though still streaky. No lava flows associated with 

cone are exposed proximally, but flows of unit bcd (125±2 ka), ~8 km east-southeast, 

are chemically and petrographically identical; although paleomagnetic directions differ, 

this may reflect mobility of cone agglutinate. Phenocrysts: 7–10% plagioclase (1–5 mm); 
5% olivine (1–2 mm); 1–2% cpx (1–3 mm); Fe-Ti oxides limited to crystalline groundmass. Base not exposed; unit is in exposed contact with no other except till. 40Ar/39Ar age: 

121±3 ka (Mahood and others, 2010)

bfh Basalt of Fish Hatchery (late Pleistocene)—Apron of phenocryst-rich lava flows (49.4–

49.7% SiO2) that emerges from beneath younger unit bsr ~1.5 km south of Casa Diablo 

geothermal plant and extends 5.5 km east along south side of Mammoth and Hot Creeks 

to its terminus against rhyolitic Hot Creek flow (unit rhc). Flow surfaces are rough and 

scoriaceous, marked by polygonally jointed or block-jointed swells, tumuli, and crags. 

All exposures lie beyond limits of late Pleistocene till and Casa Diablo Till. Parts of 

southwest sector are covered by marsh deposits and postglacial alluvium of Laurel Creek. 

Near Fish Hatchery, lava flow is both underlain and overlain by Pleistocene alluvium 

(unit oal). Source vent is Mammoth Crest scoria cone (unit bmc), which is compositionally, petrographically, and paleomagnetically identical. Phenocrysts: 5–10% plagioclase 

(1–15 mm); 1–3% olivine (0.5–2 mm); and sparse to common cpx (1–8 mm); unusually large feldspar crystals are diagnostic of unit. Overlies units bcd (125±2 ka), mlc

(130±1 ka) and bsc (172±2 ka), and banks against rhyolitic Hot Creek flow (333±2 ka). 

Overlain by unit bsr (99±1 ka) and diamict unit lmd. In MLGRAP-2 drill core (fig. 6; 

downtown Mammoth Lakes), 40 m of unit bfh directly underlies late Pleistocene till (unit 

glb) and rests on 17 m of alluvial and glacial deposits (probably unit gcd), which in turn 

overlies unit mcl (175±3 ka). 40Ar/39Ar age: 92.4±2.4 ka (cf. 92.7±2.4 ka for source scoria 

cone, unit bmc)

bhl Basalt of Horseshoe Lake (late Pleistocene)—Phenocryst-rich lava flows (48.9–49.3% 

SiO2) that floor 1-km-wide shelf between Lake Mary and Horseshoe Lake in Mammoth 

Lakes Basin. Thickest exposure of severely eroded unit is ~60 m on glacially excavated 

cliffs overlooking Twin Lakes. Ice-scoured ledges and swells are block-jointed to slabby, 

locally flow-foliated, and typically massive but slightly to moderately vesicular in 

places; some lava outcrops adjacent to lake are heterogeneously streaky, as if fountainfed. Glacially ravaged source vent was at site of Horseshoe Lake and is partly preserved 

on its east peninsula where a remnant of block-jointed lava caps and thus preserved a 

proximal scoria-fall deposit. Primary basaltic ejecta are mostly 5–25 cm, but a few are 

as big as 50–80 cm, and some have oxidized rinds. A smaller exposure on northeast 

rim of lake (UTM 032189/416440) likewise consists of scoria fall armored by a lava 

remnant. Elsewhere, southeast and east shores of lake are strewn with scoria and angular 

vesicular blocks of bhl lava, all ice mobilized and mixed with granitic erratics (many as 

big as 1–3 m) and sparser erratics of unit amp. Stratified surficial deposit that dips into 

lake at its southwest corner is largely a scree cone of scoria lapilli that washed down the 

cirque wall from unit bmc scoria cone atop Mammoth Crest. Phenocrysts in unit bhl: 

10–15% plagioclase (0.5–5 mm); 5–7% cpx (1–3.5 mm); and 1–2% olivine (1–2.5 mm). 

Lava flows are notably rich in Mg and Ca but relatively poor in Al. Overlies units dbp

and amp (97±1 ka). Overlain only by late Pleistocene till. Clasts of unit bhl are present 

in all late Pleistocene moraines of unit glb on both sides of Mammoth Creek. 40Ar/39Ar 

age: 31±1 ka 

bmc Basalt of Mammoth Crest (late Pleistocene)—Eroded scoria cone (49.5–51.1% SiO2) 

preserved as 1-km-long remnant, 500 m wide, atop north end of Mammoth Crest, an 

arête on main regional drainage divide, 1 km west of Crystal Crag. Upper half of cone 

remnant consists of stratified scoria, mostly brick-red but locally black, that dips radially 

and includes a few tack-welded layers; lapilli dominate but bombs as big as 1.5 m are 

common along crest. Gutted crater is enlarged and modified by erosion. Lower half of 

120-m-thick unit consists of stratified agglutinate and fountain-fed lavas in layers 1–5 m 

thick that drape granite rim of arête; centrally these thicken downslope eastward to fill 

a paleoswale, where they are now eroded into a 40–50 m-high ledgy cliff. Massive cliff 

section is hackly or block-jointed but grades up into stratified agglutinate at its 3,100-m 

rim. Directly downslope from toe of cliff, at ~3,000 m, a bench-forming outlier of partly 

glassy massive lava is 70 m long, 25 m wide, and >8 m thick on its steep downslope 

face, which lies only ~55 m above Crystal Lake; outlier is in place, as shown by its intact 

jointing, consistently north-trending glacial striae, and paleomagnetic direction identical 

Description of Map Units—Quaternary Volcanic Rocks 81
82 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

to cliff above. A second outlier, 150 m southwest of Crystal Lake, is a chaotic slide mass 

(unit av on map). Lava evidently flowed down floor of Lakes Basin, from which it was 

subsequently stripped by glacial erosion, and extended 18 km to terminus near Hot Creek 

Fish Hatchery, where unit bfh is chemically, petrographically, paleomagnetically, and 

geochronologically identical. It is inferred that Lakes Basin contained little or no ice at 

time of eruption. Nonetheless, it seems required that stripping of unit bmc/bfh from most 

of Lakes Basin took place before emplacement of unit bhl (31±1 ka), which survives 

on the basin floor. This inference suggests that bmc/bfh lavas were removed during the 

interval 90–30 ka, presumably during MIS 4, by cirque glaciation within Lakes Basin. 

Scoriae reworked from the cone postglacially form a 5-m-thick stratified deposit of wellrounded lapilli (1–10 cm) at southwest shore of Horseshoe Lake. Lowest 1.5 m of this 

reworked section dips 25o NE into lake, and its clasts are less rounded than those higher 

in deposit, where wave reworking continues during seasonal highstands. Section represents a scree fan of sloughed cinders, in which many lapilli layers are separated by thin 

partings of buff ash.

Phenocrysts: Unit ranges from moderately to highly porphyritic; 8–15% plagioclase 

(1–22 mm, mostly 3–10 mm); 1–2% olivine (0.5–3 mm, plus sparse crystals as long as 

8 mm); and sparse cpx (1 mm). Also contains olivine clusters of as many as 20 grains, 

rare olivine-plagioclase intergrowths, sparse xenocrysts of feldspar and quartz, and 

scattered granitoid xenoliths (1–15 cm), both rounded and angular. Scoria cone rests on 

Mesozoic granite and contacts no other volcanic unit. 40Ar/39Ar age: 105±7 ka (Mahood 

and others, 2010); 92.7±2.4 ka (cf. 92.4±2.4 ka for distal lava flow of unit bfh)

bmn Trachybasalt northeast of Minaret Summit (late Pleistocene)—Apron of modestly porphyritic lava flows (51.3–53.1% SiO2

) that extends for 2.8 km northeast from basin east 

of Minaret Summit; exposed discontinuously (through thick mantle of late Holocene Inyo 

pumice). Source vent is scoria cone 2 km northeast of Minaret Summit, same cone that 

later produced lavas of phenocryst-rich unit bar. Proximal scoria-fall deposit at south 

base of cone, 12 m thick where dissected by stream, includes ballistic bombs, 10–70 cm 

across and dense to scoriaceous, that are predominantly of bar type but commonly also 

of bmn petrography and composition. Lava apron, ~1 km wide, is everywhere glacially 

scoured and crops out as jointed ledges and platy floors of shallow draws; only distally 

is it eroded into cliffs and noses with 15–20 m relief. Southwesternmost proximal lava 

exposure (UTM 1993/6998) is compositionally most evolved part of unit (52.4–53.1% 

SiO2, designated subunit bmn') and carries about half as many phenocrysts as the rest. 

Phenocrysts in most of unit: 3–5% plagioclase (1–3 mm, rarely 4–8 mm); 0.5–3% olivine 

(0.5–2 mm), and rare Fe-Ti-oxide mph (0.1–0.2 mm); cpx is absent or rare, found only in 

clots with plagioclase. Overlies unit ddc (103±9 ka). Overlain by unit bar (88±5 ka), late 

Pleistocene till, and 1350 C.E. Inyo pumice-fall deposit. Proximal, medial, and distal drill 

sites yield similar paleomagnetic directions (Mankinen and others, 1986; Hildreth and 

others, 2014). 40Ar/39Ar age: 87±7 ka (Mahood and others, 2010)

brc Basalt of Red Cones (Holocene)—Pair of scoria cones, agglutinate, and associated apron 

of subalkaline basaltic lava flows (49.9–51.2% SiO2; 7.9–8.3% MgO; 0.6–0.8% K2O), 

centered 3–4 km southwest of southern toe of Mammoth Mountain (fig. 10). Each cone 

is ~120 m high, 500 m wide, and consists of loose brick-red scoria and lesser spatter; 

lapilli dominate and bombs as big as 50 cm are uncommon. Scoria fall extends northeastward, thinning to a few cm within 2 km. Cone summits are 700 m apart, defining a 

north-northeasterly trend that strikes toward summit of Mammoth Mountain. Each has a 

small breached crater; that of northern cone opens southwest and that of southern cone 

northwest. Both breaches funnel into fans of agglutinate and thin fountain-fed lava flows, 

and these in turn merge into a 1.2-km2 lava-flow field that steeply drapes the canyon wall 

for ~1 km west, thence extends 1.5 km southward to an uneroded terminus. Surface of 

lava apron is rubbly and scoriaceous, marked by ridges, steps, knobs, and swales that 

impart 3–8 m of local relief. Apron lavas are numerous and thin, 1–2 m thick proximally 

and 3–6 m distally; massive interior zones, which are typically <1 m thick, are only rarely 

exposed. Magma volume erupted is ~0.01 km3, approximately equally divided between 

lavas and fragmental deposits. Unit has lowest alkali content in entire volcanic field. 
Phenocrysts: 5–12% plagioclase (0.5–3 mm, rarely to 6 mm) seriate to groundmass laths; 

5–8% olivine (0.5–2 mm, rarely 3 mm); cpx phenocrysts (0.5–1 mm) rare but cpx mph 

common. Also present are spinel inclusions in olivine, free titanomagnetite microphenocrysts, and sparse corroded plagioclase antecrysts. Xenocrysts and lithic ejecta derived 

from granitoids are common. Overlies Mesozoic granite (unit Kmo). Age: ~8 ka, from 14C data for small charcoal fragments in sediments a few centimeters beneath ashfall at 

site 4.5 km northeast of Red Cones (Deming, 2002; Browne and others, 2010)

bsc Trachybasalt of Sherwin Creek Road (middle Pleistocene)—Low-relief sheet of phenocryst-poor lava (50.6–51.1% SiO2

) that extends 2 km eastward in two tongues near 

junction of Sherwin Creek Road with Highway 395, south of Mammoth Creek. Rolling 

surface is marked by low block-jointed swells and is highly vesicular. Unit is only 

patchily exposed, nowhere stream-incised, and widely mantled by sagebrush, aeolian 

sand, and marsh deposits. Phenocrysts: <1% plagioclase (0.5–1 mm, rarely up to 3 mm), 

many resorbed and corroded; rare olivine (mostly 0.1–0.3 mm; rarely 0.5–1 mm). Also 

contains sparse quartz and feldspar xenocrysts. Base not exposed. Overlain by units mlc

(130±1 ka) and bfh (92±2 ka) as well as by alluvial, aeolian, and marsh deposits. Source 

vent unknown, buried by younger units far to west. Despite distal elevation as low as 

2,170 m, lava flow was not inundated by Long Valley Lake. In water well 30M (fig. 6), 

2 km southeast of downtown Mammoth Lakes, a 15-m-thick lava flow of unit bsc was 

penetrated at 92–107 m depth, where it directly overlies unit mcl (175±3 ka) and underlies ~30 m of glacial and alluvial deposits (inferred to be unit gcd). Clasts of unit in 

exposures of Casa Diablo Till (unit gcd) demonstrate an age greater than that of MIS 6. 40Ar/39Ar age: 172±2 ka

bsm Basalt of Sawmill Cutoff (middle Pleistocene)—Poorly exposed phenocryst-poor subalkaline lava flows (48.4–48.9% SiO2

, 10.3% FeO*, 6.4% MgO, 1.2% K2O) that make up 

1-km-wide steep ridge that extends 1.3 km eastward from beneath overlying margin of 

West Moat Coulee (unit rwm), 3–4 km north of Mammoth Lakes business district. Also 

includes an exposure on south slope of Knolls Vista and two isolated low-relief exposures 

north and south of Dry Creek near southwest foot of Lookout Mountain, all correlated on 

basis of petrography and distinctive chemical composition. Exposures are block-jointed, 

massive, only locally vesicular or vuggy, and have fine-grained groundmass. Unit is 

presumed to include several flows, at least on ridge where >90 m thick, but mantle of 

Inyo pumice and colluvium obscures almost all outcrops. Eastern nose of ridge abuts 

Early rhyolite (unit rer), but steepness of its north and south slopes is enigmatic. Glacial 

confinement of its low-viscosity lava flows during MIS 6 is conceivable, but till deposits 

are absent and exposure is too poor to provide evidence.

Phenocrysts: sparse plagioclase (1–2 mm, plus rare shattered megacrysts as big as 

10 mm); sparser olivine (0.5–1 mm, rarely 3 mm, some in clots with plagioclase); rare 

cpx (1–3 mm); and rare plagioclase antecrysts, rounded and corroded. Also contains 

sparse quartz and feldspar xenocrysts. Unit banks against unit rer (~700 ka here) and 

overlies unit mcl (175±3 ka). Overlain by units rwm (~150 ka) and mkv (153±1 ka). 40Ar/39Ar age: 165±2 ka

bsr Trachybasalt of Shady Rest (late Pleistocene)—Apron of phenocryst-poor lava flows 

(50.6–51.7% SiO2) that emerges from beneath late Pleistocene till at Shady Rest Campground and extends 5.5 km southeast to its terminus along and north of Laurel Creek 

(fig. 21). Apron has rolling surface marked by swells, ridges, and scattered tumuli; as 

thick as 8–12 m along its eastern flow front and medially where incised by Mammoth 

Creek. Exposures are generally vesicular and block-jointed except in a few places that 

are deeply incised where massive and slabby. Phenocrysts: 1–3% plagioclase (0.5–2 mm, 

rarely 3–4 mm); <1% olivine (0.5–1.5 mm; mostly mph); and traces of cpx (<1 mm) and 

Fe-Ti oxides (0.1–0.5 mm). North-central flow lobe that terminates just south of Sawmill 

Road is chemically identical but carries slightly more phenocrysts—3–5% plagioclase 

(1–4 mm), ~1% olivine (1 mm), and scattered cpx (1–2 mm). Small granitic xenoliths and 

derivative xenocrysts are sparse but widespread. Rare gabbroic intergrowths of olivine, 

cpx, and plagioclase as big as 10 mm are also present. Unit contains more and larger 

plagioclase and olivine than comparably phenocryst-poor unit amp near their mutual 

Description of Map Units—Quaternary Volcanic Rocks 83
84 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

contact east of Sherwin Creek Campground. Flows are notably rich in Fe and Ti. Overlies 

units asr, mmc, bcd, bfh, and Casa Diablo Till (gcd). Overlain by unit amp (97±1 ka) 

and by late Pleistocene till (unit glb). Source vent is buried somewhere to west, beneath 

either Mammoth Mountain or glacial deposits. In drill hole DC-1 (fig. 6; Sorey and others, 1978), located at UTM 259/781 (fig. 8), ~12 km north of Shady Rest Campground, 

a 20-m-thick lava flow compositionally and petrographically identical to unit bsr occurs 

beneath a flow of unit mor (66±2 ka) and atop a flow of unit aic (131±1 ka); if indeed 

coerupted with exposed unit bsr, this flow took a separate path, down west moat from a 

presumed vent near present site of Mammoth Mountain. Three sites sampled—proximal, 

medial, and distal—yield virtually identical paleomagnetic directions (Mankinen and others, 1986; Hildreth and others, 2014). 40Ar/39Ar ages: 99±1 ka (Mahood and others, 2010); 

103.5±1.4 ka (table 2). K-Ar age: 64±14 ka (Mankinen and others, 1986). Overlies unit 

bfh (~bmc), which provided separate ages of 92.4±2.4 ka and 92.7±2.4 ka. Age conflict 

between units bfh and bsr is unresolved

dbp Trachydacite of Bottomless Pit (late Pleistocene)—Phenocryst-rich coulee (66.8–67.1% 

SiO2) as thick as 160 m that forms east-facing bluff that rises above middle of the three 

Twin Lakes at southeast toe of Mammoth Mountain (fig. 22). Name derives from erosional tunnel, a rocky sloping passageway that descends through flow from high rim to talus 

at foot of bluff. Tunnel is flanked by several open chutes nearby, all eroded into bluff rim 

parallel to near-vertical northwest-striking flow foliation and jointing. Elsewhere, unit is 

block-jointed and glacially scoured into rounded knobs and buttresses. Flow is pervasively oxidized, atypically strongly so for Mammoth Mountain; crops out pale pinkish brown 

to reddish brown or locally tan with streaks and layers of black vitrophyre. Phenocrysts: 

10–15% feldspar (0.5–6.5 mm); 3–4% hornblende (1–2 mm, rarely as long as 10 mm); 

1–3% biotite (mph to 2 mm); common Fe-Ti-oxide mph, mostly in clots with feldspar. 

Overlies units dnh (64.5±0.8 ka), dsd (87±6 ka) and amp (97±1 ka). Overlain by units

ddl (58±2 ka), ddu, and bhl (31±1 ka). Undated

ddc Trachydacite of Dry Creek (late Pleistocene)—Moderately porphyritic lava flow 

(66.9–67.4% SiO2) that crops out in a north-northwest-trending belt 2.5 km long in upper Dry Creek basin, west of Dome 2861. As thick as 40–50 m, flow was everywhere 

glacially eroded into ridges, knobs, bluffs, and ledges; its width of exposure is only 

~1 km because it was overrun by younger lavas. Exposures range from glassy to finely 

devitrified with pale-gray mottling, and from massive and block-jointed to (more widely) 

flow-foliated and thinly platy; platy jointing is commonly ramped or even near-vertical. 

Phenocrysts: 3–5% plagioclase (≤1 mm, rarely to 2 mm), a minority coarsely sieved 

internally; ≤1% opx (0.5–1 mm); and abundant Fe-Ti oxide mph (0.1–0.3 mm). Not 

coarsely porphyritic like hornblende-biotite trachydacites of units d61, d81, and Mammoth Mountain. Source vent buried by younger units to south or southwest, probably 

beneath Mammoth Mountain. Base not exposed, but may bank around base of unit d81

(99±7 ka). Overlain by units bar (88±5 ka) and bmn (87±7 ka). K-Ar age: 103±9 ka 

(Mankinen and others, 1986)

ddl Trachydacite of lower Dragons Back (late Pleistocene)—Phenocryst-rich coulee (66.0–

66.8% SiO2) as thick as 180 m that caps distal 600 m of Dragons Back, southeast ridge 

of Mammoth Mountain (fig. 22). Flow forms steep east-facing bluff above Twin Lakes as 

well as north- and south-facing scarps of ridge itself. Narrow ridgetop plateau is capped 

by blocky to craggy unglaciated vitrophyre that survived as cleaver above adjacent late 

Pleistocene glaciers. Eroded scarps are block-jointed ledges and buttresses of dense 

vitrophyre with locally pronounced flow foliation; blocks typically have thin brown 

scoriaceous rinds while freshly broken surfaces are sheer, black, and shiny. Where ddl

flow overlaps oxidized unit dbp, it locally exposes a 10-m-thick massive red-brown 

lower zone that grades down into oxidized basal breccia, both of which pinch out toward 

eastern cliff. At north foot of steep Dragons Back ridge, chemically identical lava forms 

a glaciated shelf 30 m wide and 80 m long (UTM 225/659), which is block jointed on 

top but platy to slabby on its 15-m-high north-facing scarp, with oxidized flow breccia at base of flow. Glacial excavation of Twin Lakes Basin separated Dragons Back 

segment from its eastward extension, which survives as till-mantled Panorama Dome 
(2,723 m on top); this segment crops out only in roadcuts on Lake Mary Road (2,640 

to 2,660 m), where exposures are block jointed to slabby, moderately flow-foliated 

vitrophyre and felsite, widely iron-stained along partings. Phenocrysts: 10–15% feldspar 

(0.5–10 mm), 3–5% hornblende (1–5 mm); 1–2% biotite (mph to 1.5 mm); trace pyroxene mph; common Fe-Ti oxide mph; also contains oxide-amphibole-feldspar clots and 

cpx-oxide-feldspar clots. Overlies units dbp, dom (73±1 ka), and dtl (76±1 ka). Overlain 

by unit ddu. Paleomagnetic direction differs from those of unit dbp below and unit ddl

above.40Ar/39Ar age: 58±2 ka (Mahood and others, 2010)

ddu Trachydacite of upper Dragons Back (late Pleistocene)— Phenocryst-rich coulee 

(66.0–66.9% SiO2) as thick as 160 m that caps uppermost 1-km-long segment of cleaver 

called Dragons Back, southeast of Mammoth Mountain summit; flow extends along 

ridgecrest from ~3,270 m down to eroded flow front at ~2,960 m. Much of crest is pale 

gray or iron-stained, crumbly vitrophyre or partly devitrified lava, but blocky remnants 

of massive dark gray vitrophyre, some of it vesicular, survive locally; flow foliation is 

locally conspicuous and many blocks retain thin brown scoriaceous crusts. Ice-scoured 

buttresses on north slope of flow are slabby to platy near base of unit. An erosional outlier 

<100 m wide that forms nubbin 3120+ (UTM 210/669), ~700 m north of upper end of 

Dragons Back and ~400 m northeast of Mammoth Mountain summit, is a precise match 

compositionally. Distal remnants also crop out through till on northeast side of Panorama 

Dome and in vicinity of 2,610-m switchback on Old Mammoth Road (UTM 240/652). 

Phenocrysts: 12–15% feldspar (0.5–7 mm); ~5% hornblende (1–5 mm, rarely 10 mm 

long); 1–2% biotite (mph to 1.5 mm); sparse Fe-Ti oxides; also contains common clots of 

Description of Map Units—Quaternary Volcanic Rocks 85

dsd

ddu

dsd

rss

dlp

ddl

dbp

South Summit Dome

Lincoln Peak

Figure 22. Southeast side of Mammoth Mountain above glaciated trough floored by Twin Lakes. Pervasively oxidized Bottomless Pit 

flow (unit dbp) is overrun by lower Dragons Back flow (unit ddl), which is, here, as thick as 180 m. Scree-mantled South Summit Dome 

(unit dsd), one of largest units on edifice, is widely altered hydrothermally.
86 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

all combinations of these phases. Overlies units dsd (87±6 ka), dbp, and ddl (58±2 ka). 

Chemically almost identical to subjacent flow unit ddl but consistently slightly richer in 

Al (n=17 analyses; ddl and ddu together). Undated

dfl Trachydacite of Face Lift (late Pleistocene)—Phenocryst-rich lava flows (63.0–65.9% 

SiO2) that extend from north face of Mammoth Mountain summit for ~1.5 km northward 

beneath unit rmf on Face Lift planèze (fig. 12) and bank against unit dms of McCoy 

Station plateau. Base not exposed but has as much as 80 m of relief along east margin, 

50 m along west margin, and ~60 m at foot of summit scarp along The Chasm. Generally 

hackly or block jointed; flow foliation locally pronounced. Everywhere glacially eroded, 

but exposures are mostly glassy; oxidation and iron-staining along joint planes only modest and local. Phenocrysts: 15–20% feldspar (0.5–5 mm); 5–7% biotite plus amphibole in 

varied but subequal amounts (each as big as 2.5 mm); 1–2% pyroxene (0.3–2 mm); common Fe-Ti oxide mph. Also carries mafic blebs, oxide-pyroxene clots, feldspar clusters, 

and sparse angular xenoliths of pale-gray or tan, moderately porphyritic felsite (1–4 mm). 

Apparently extruded beneath summit or where The Chasm was eroded (fig. 12). Several 

dfl flows underlie unit rmf (61±1.5 ka) along both flanks of Face Lift planèze. Proximally, unit underlies summit-capping unit dsu (61.4±2 ka); distally, it banks against both 

sides of unit dms (68±1 ka) as well as apparently against unit dnk (60.4±1.2 ka). Several 

adjacent units here yield eruptive ages that are similar, but unit dfl is distinctive among 

them in being chemically less evolved and richer in pyroxene. Unit is similar in composition and paleomagnetic direction to unit drc, which crops out to south, on opposite side 

of Mammoth Mountain. 40Ar/39Ar age: 61±3 ka 

dgr Trachydacite of Gold Rush Express (late Pleistocene)—Ice-scoured remnant of phenocrystrich lava dome (68.7–68.8% SiO2

) that forms north-trending ridge 350 m long and only 

150 m wide, just west of Lincoln Peak. Flat summit of ridge is current terminus of Gold 

Rush Express (Chair 10); lava nose that descends to north has ~100 m relief. Ridgecrest 

consists of pale-to-dark gray vitrophyre, block-jointed and flow-foliated; nose below to 

north is partly devitrified, widely oxidized, and strongly flow-laminated. Phenocrysts: 

15–20% feldspar (0.5–4 mm, many atypically small for Mammoth Mountain); 5–6% 

biotite (mph to 2 mm); sparse hornblende (≤1 mm); ≤1% pyroxene (0.2–0.7 mm); common Fe-Ti oxide mph. Although slightly more evolved, this unit is chemically similar to 

Lincoln Peak (unit dlp); however, it contains far less hornblende and has a paleomagnetic 

direction distinct from that of adjacent Lincoln Peak. Contact relations with units dlp

(64±7 ka) and rce (80±1 ka) are obscured by colluvium, but dgr is probably younger than 

rce and older than dlp. Undated.

Preserved atop dgr dome (UTM 217/670) is a 50-m-long remnant of poorly 

sorted, unstratified diamict 6–7 m thick. Enclosed in sandy matrix are angular clasts of 

phenocryst-rich dacite (typical of Mammoth Mountain) seriate in size from granules to 

blocks as big as 50–80 cm. Absence of thermal effects and prismatic jointing, along with 

contrasting textures among blocks (fresh vs altered; vesicular vs dense; glassy vs devitrified), suggests deposit is glacial or colluvial, not pyroclastic. One large block analyzed is 

petrographically and chemically indistinguishable from unit dsd (87±6 ka), which crops 

out ~1 km uphill

dlp Trachydacite of Lincoln Peak (late Pleistocene)—Phenocryst-rich Dome 3079 (67.7–68.5% 

SiO2) centered 1.5 km northeast of Mammoth Mountain summit (fig. 1). In plan view, 

glaciated lava dome is 500 by 700 m, elongate to east-northeast. Steep north face has 

160 m of relief and east face as much as 220 m; south and west slopes are gentler. Many 

exposures are black to medium-gray vitrophyre that splits blocky to slabby. Flow foliation conspicuous and nearly ubiquitous; common for pale gray or reddish brown layers 

to interleave with dark glassier ones. Eastern cliffs dominated by buttresses of shiny 

black vitrophyre; northern cliff includes such vitrophyre but also has medium-gray partly 

devitrified domains that weather dull gray and, locally, flow-laminated domains that split 

into thin plates with tan and pink films. Phenocrysts: 15–20% feldspar (0.5–8 mm, few 

>5 mm); 2–4% biotite (mph to 1.5 mm); 1–3% hornblende (mph to 2.5 mm); trace cpx 

(0.4–0.8 mm); common Fe-Ti oxide mph. Flow-foliated rocks of summit and nearby 

southeast spur (UTM 222/672) are physically disturbed, as inferred from anomalous 
paleomagnetic vectors; after partial cooling, upper crags were probably heaved and 

rotated by renewed inflation of dome. Overlies units rce (80±1 ka) and probably banks 

against unit dgr. Unit rsq (63.7±4 ka) banks against south toe of Lincoln Peak dome. 40Ar/39Ar age: 64±7 ka

dml Trachydacite of Main Lodge Coulee (late Pleistocene)—Phenocryst-rich coulee 

(65.6–68.6% SiO2) that emerges from beneath unit dms at 2,900 m elevation and extends 

1 km northward to an eroded flow front at 2,710 m, just east of MMSA Main Lodge. 

Outcrops of glaciated flow are patchy because of heavy mantle of till and colluvium, 

but relief of at least 50 m is exposed along east margin. Exposures are weakly to moderately flow-foliated vitrophyre, medium gray to black with subordinate layers vesicular, 

sheared, or weathered reddish brown; splits slabby but more commonly coarsely blocky; 

weathered outcrops crumbly. Where bulldozed for ski runs, pale gray or pinkish gray 

devitrified interior facies is locally exposed. Phenocrysts: 12–20% feldspar (mph to 

5 mm); 2–4% biotite (mph to 2.5 mm); 1–4% hornblende (mph to 2 mm; ~1% pyroxene 

(0.5–2.5 mm); common Fe-Ti oxide mph. Also contains rare 1–3 mm inclusions of intergrown feldspar and black pyroxene. Base not exposed. Overlain by unit dms (68±1 ka). 40Ar/39Ar age: 67±1 ka (Mahood and others, 2010)

dms Trachydacite of McCoy Station (late Pleistocene)—Phenocryst-rich coulee (68.5–69.6% 

SiO2) that emerges from beneath sloping apron of unit rmf and extends ~1 km northward, 

forming plateau site of McCoy Station (fig. 7) and distal lobe farther downslope atop unit 

dml. Although thinned substantially by glacial erosion, flow is still as thick as 60 m on 

northeast-facing scarp of plateau. Farther north, unit pinches out as a few deeply scoured 

remnants exposed patchily through till, colluvium, and Inyo pumice covering surface 

of underlying dml coulee. Moderately to strongly flow-foliated vitrophyre; dense black 

glassy layers alternate with pale gray partly devitrified layers and lenses, some lithophysal or spherulitic; glassy parts often disintegrate crumbly where weathered. Phenocrysts: 

10–12% feldspar (0.5–5 mm); 2–5% biotite (mph to 3 mm); ~1% opx (mph to 2 mm); 

sparse hornblende (0.3–1 mm; rarely as long as 3 mm); common Fe-Ti oxide mph; also 

carries sparse mafic blebs and common pyroxene-oxide-feldspar clots. Composition 

significantly more silicic than adjacent units dfl and dml. Overlies unit dml (67±1 ka); 

overlain by units dfl (61±3 ka ) and rmf (61±1.5 ka). 40Ar/39Ar age: 68±1 ka (Mahood and 

others, 2010)

dnh Trachydacite north of Horseshoe Lake (late Pleistocene)—Two phenocryst-rich lava flows 

(67.9–68.4% SiO2) in poorly exposed window between units amp and dbp, ~700 m north 

of Horseshoe Lake. Widely covered by colluvium, unit crops out as minor southeasttrending rib ~500 m long and ~150 m wide, expressed mainly as two 10-m-high glaciated 

ledges (at elevations 2,760 and 2,860 m) that probably represent two southeast-sloping 

lava flows. Both ledges are block-jointed vitrophyre, black to pale gray or locally oxidized. Phenocrysts: 12–15% feldspar (0.5–5 mm); 2–3% biotite (mph to 3 mm); 0.5–2% 

hornblende (mph to 1.2 mm); ~1% opx (mph to 2.5 mm); common Fe-Ti oxide mph. Also 

carries sparse mafic blebs. Paleomagnetic directions of both flows are apparently excursional, contrasting with those for units dsd, dbp, and others nearby (Hildreth and others, 

2014). Lower ledge contains abundant gray fine-grained enclaves (56.7% SiO2), 1–20 cm 

across and variously rounded, angular, or crenulate; they carry fewer crystals than host 

lava but their crystal cargo consists of same species plus common cpx. Base of unit not 

exposed but inferred to bank against unit amp (97±1 ka). Overlain by unit dbp. 40Ar/39Ar 

age: 64.5±0.8 ka

dnk Trachydacite of North Knob (late Pleistocene)—Phenocryst-rich lava dome (66.4–66.5% 

SiO2) centered 450 m west of McCoy Station at midslope on north flank of Mammoth 

Mountain (fig. 7). Terminus of Broadway Express (Chair 1) is on summit of dome 

(3,016 m). Dome exposure is altogether 700 m in diameter and includes 400-m-long 

ice-scoured rib separated by scree chute just west of main knob; relief is ~200 m on steep 

north slope of dome. Outcrops are mostly medium-gray vitrophyre, massive or flowfoliated, and widely oxidized reddish brown (especially upper parts); extensive domains 

are partly devitrified (owing to glacial removal of original glassy carapace), and southeast 

slope is hydrothermally altered. Phenocrysts: 12–15% feldspar (0.5–8 mm); 3–5% biotite 

Description of Map Units—Quaternary Volcanic Rocks 87
88 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

(mph to 2 mm, rarely to 5 mm); 2–3% hornblende (mph to 2 mm); common Fe-Ti oxide 

mph. Also carries plagioclase clusters and biotite-oxide-feldspar clots. Almost surrounded 

by surficial deposits but probably older than unit dfl along its east margin. Northerly 

paleomagnetic declination and moderate (40°) inclination differ from steep northwesterly 

directions of flow sequence that built adjacent Face Lift planèze (units dml, dms, dfl, 

rmf, and dsu). 40Ar/39Ar age: 60.4±1.2 ka

dnw Trachydacites of northwest moat (late Pleistocene)—Five discrete nonglaciated extrusions of related but variably contaminated magma (60.4–67.3% SiO2

) in northwest moat 

of Long Valley Caldera, 9 km north of Mammoth Mountain; their vents are aligned 

roughly northwest-southeast along a 2.5-km-long trend. From northwest to southeast, 

the five are generally more voluminous and more contaminated. All are glassy but 

choked with microlites and carry 15–20% crystals larger than 0.5 mm—phenocrysts, 

antecrysts, and xenocrysts. All have abundant biotite and sparser amphibole (both mph to 

1.5 mm), both euhedral but ranging from fresh to opacitized. All contain rounded quartz 

(1–3 mm), many of them embayed. Intergrown clots (1–4 mm) of cpx, Fe-Ti oxides, and 

plagioclase (all generally mph) are common in all five. Free cpx (and rare opx) euhedra 

(mph to 1 mm) are probably disaggregated from such clots. Feldspars (1–25 mm) dominate all assemblages, include both sanidine and plagioclase, and are mostly sieved and 

partly resorbed; some have clear overgrown rims, and some were in advanced stages of 

dissolution when eruptively quenched. Subordinate plagioclase populations (0.5–3 mm) 

are euhedral. Sparse mafic blebs and enclaves are crystal-poor but olivine-bearing, and 

some enclose sieved feldspars from host dacite. From north to south, the five subunits 

mapped are:

1. Small coulee (65.7% SiO2), 20–30 m thick, that crops out 500 m long and 400 

m wide below snout of 1350 C.E. Glass Creek flow (unit ric), which conceals its vent at 

caldera wall. Coulee is glassy and rugged, its knolls and ridges consisting of joint blocks 

with rough scoriaceous surfaces; rich in hornblende, biotite, and plagioclase, but few 

phenocrysts >5 mm. Contains mafic blebs as big as 1 mm; larger enclaves were sought 

but none found. 

2. Small steep-sided dome (66.0% SiO2), nearly circular in plan, intrudes east 

margin of subunit 1 near base of caldera wall. Dome is 250 m in diameter, 50 m high, 

largely glassy, coarsely blocky, and little eroded. Rocks are mostly massive, with scattered irrregular vugs and a few internal shear planes of oxidized breccia; some blocks 

have scoriaceous surfaces. Contains abundant feldspar (both plagioclase and sanidine; 

mostly 1–3 mm, fewer 6–10 mm); Carries sparse cpx+olivine+plagioclase-bearing mafic 

enclaves (1–10 cm), but fewer than in subunits to southeast. 

3. Adjacent to subunits 1 and 2, knob 2430 caps a coulee that spread radially from 

a vent marked by a steep dome 30 m high. Knobby lobate coulee is 750 by 1000 m wide 

and has relief of 20–80 m, greatest at its eastward-descending lobe. This subunit, too, is 

glassy and block jointed but, in addition, is strongly flow foliated, its foliation ramping 

radially upward toward coulee margins. Crystal cargo is like that of subunit 2, including 

abundant biotite, but its mafic enclaves are more abundant, mostly 1–5 cm across, and 

irregularly distributed; a sampled enclave has 56.4% SiO2. Vent dome and adjacent southeast surface of coulee differ from rest of subunit in containing abundant and conspicuous 

large feldspar crystals (10–25 mm). At 67.3% SiO2, vent dome is most silicic part of unit 

dnw sampled; subjacent coulee has 65.2–65.7% SiO2 (n=5). 

4. Fourth in line is a bilobate lava flow (62.5–64.3% SiO2—strongly hybridized) 

that spread radially from its apparent vent at knob 2400+. Sections of its chilled carapace were dragged and spread by its then still-mobile interior to produce a labyrinth of 

rifted troughs on its surface; such fissures are typically 10–15 m wide, as deep as 10 

m, and extend for several hundred meters. Flow surface is glassy and coarsely blocky, 

and northeast flow front is 25–30 m high. Surfaces of blocks are commonly scoriaceous 

and oxidized, though smooth on walls of fissures. Contains abundant plagioclase and 

sanidine crystals (mostly 5–15 mm but overall 1–25 mm), many equant but some elongate or broken; also a varied cargo of biotite, hornblende, opx, cpx, rounded or wormy 

quartz, cpx-olivine-oxide-plagioclase clots, and rare olivine xenocrysts. Also contains 
abundant mafic enclaves (1–10 cm), typically fine-grained, phenocryst-poor, olivinebearing, and oxidized. 

5. Most voluminous, most contaminated (60.4–61.7% SiO2), and southeasternmost 

of the five subunits is a major coulee, 1–1.5 km wide, that flowed 2 km northeastward 

from its vent. Its surface is a rough craggy plateau that descends gently northeastward to 

a steep lobate flow front as high as 20 m, which overlies lava field of unit mcv (~33 ka). 

Rugged block-jointed ridges and swells on its surface have 3–8 m of local relief. Like 

subunit 4, flow surface has rifted troughs, fewer and smaller than on its neighbor, variously oriented, 100–200 m long, their floors 5–12 m wide and their walls 3–7 m high. 

Nearly all outcrops are black crystal-rich vitrophyre carrying scattered irregular vesicles; 

block surfaces are commonly scoriaceous and oxidized brown. Contains same crystal 

cargo as accompanying subunits, including abundant plagioclase and sanidine as big 

as 25 mm; coexisting quartz and magnesian olivine; opacitized biotite and hornblende; 

numerous rounded, embayed, or strongly sieved crystals; microdioritic inclusions; and 

abundant mafic enclaves (1–10 cm). Xenoliths of granitic and metamorphic rocks are also 

present (Rinehart and Ross, 1964; Bailey, 2004). Vent for coulee, near its southwest limit, 

is marked by tack-welded to dense agglutinate and lineated rolls of extrusive lava. 

Unit dnw overlies units bar, mnd, and mcv; probably also rests on Mesozoic 

granite and Pliocene mafic lavas, both of which crop out within a few hundred meters 

of north margins of dnw. Overlain by Inyo pumice-fall deposit and rhyolitic Glass 

Creek flow (both 1350 C.E.). 40Ar/39Ar ages (sanidine from each subunit): (1) 40±1 ka; 

(2) 42±1 ka; (3) 40±1 ka; (4) 30±1 ka; (5) 27±1 ka (1–2 from table 2; 3–5 from 

Mahood and others, 2010) 

dom Trachydacite of Old Mammoth (late Pleistocene)—Most distal and topographically lowest 

of four dacite coulees east of Twin Lakes that step down eastward toward Old Mammoth 

district. Phenocryst-rich flow (65.7–66.0% SiO2) emerges from under till and unit ddu

near 2,570 m elevation along Old Mammoth Road grade and extends ~1 km eastward as 

flat-topped till-strewn bench to its eroded terminus at steep east-facing bluff above Mammoth Meadow (fig. 23). Glacially scoured flow is at least 90 m thick as exposed on north, 

east, and south scarps, locally called The Bluff. Rock is predominantly flow-foliated 

vitrophyre, patchily oxidized, with some layers and domains partly devitrified (especially 

on deeply scoured rim of The Bluff). Phenocrysts: 10–12% feldspar (0.5–10 mm); 3–5% 

hornblende (mph to 2 mm); 1–2% biotite (mph to 2 mm); common Fe-Ti oxide mph. 

Base not exposed, but south margin banks against pre-Cenozoic rocks. Overlain by units 

ddu and ddl (58±2 ka). 40Ar/39Ar age: 73±1 ka

drc Trachydacite of upper Reds Creek (late Pleistocene)—Phenocryst-rich lava flow (63.8–

65.7% SiO2) on upper southwest slope of Mammoth Mountain; exposed as 250-m-wide 

swath ~600 m long that descends steeply from beneath summit dome (unit dsu) at ~3,260 

m to an eroded flow front at 3,040 m on west wall of Reds Creek. Rock is flow-foliated 

and block-jointed vitrophyre, black to medium gray; only locally vesicular or oxidized. 

Uppermost outcrops are ribs and ledges widely concealed by scree; distal outcrops are 

steep rugged cliffs that weather dark brown. Phenocrysts: 15% feldspar (0.5–7 mm); 

3–5% hornblende (mph to 2 mm, rarely to 3.5 mm); ~1% biotite (mph to 1.5 mm); 

~1% pyroxene (mph to 2.5 mm); common Fe-Ti oxide mph. Also carries mafic blebs, 

oxide+cpx±plagioclase clots, and brick-red dacitic autoliths. Base not well exposed 

but fresh drc overlies adjacent unit dsd (87±6 ka), which is acid-altered at and near 

contact. Also overlies units dwr (73±2 ka) and dsk (71±1.5 ka), but is overlain by unit 

dsu (61.4±2 ka). Unit is similar in composition and paleomagnetic direction to unit dfl

(61±3 ka), which crops out to north, on opposite flank of Mammoth Mountain. Undated

drf Trachydacite of Rainbow Falls (late Pleistocene)—Phenocryst-poor coulee (65.7–67.1% 

SiO2) that extends from near Reds Meadow for 6 km southward along or near course of 

Middle Fork San Joaquin River. Severely glaciated flow is 30–40 m thick in north and as 

thick as 80 m in south; its surviving width is ~600 m but was originally more expansive 

and may have extended much farther downstream. Unit fills paleochannels cut in Mesozoic granite, and it supports two prominent waterfalls along Middle Fork (the eponymous 

fall being 30 m high). Striking meander of Middle Fork, 250 m in amplitude and ~800 m 

Description of Map Units—Quaternary Volcanic Rocks 89
90 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

upstream from Rainbow Falls, produced channel entrenched in dacite and abandonment of shallower channel along west edge of unit that had temporarily accommodated 

overflow when intracanyon dacite lava flow displaced river. Where exposed atop granite, 

basal zone includes vertical prismatic joints that locally form slender polygonal columns 

3–8 m high, overlain by abrupt transition to platy interior of flow (fig. 24). Surface is 

rolling terrain of ice-scoured hillocks, ridges, ribs, and ledges. Some outcrops are massive, block-jointed, hackly, chunky, or slabby, but most are strongly flow-foliated and 

thinly platy. A few glassy domains survive, notably basal vitrophyre zones locally, but 

most exposures are devitrified, aphanitic, pale gray, and commonly mottled. Phenocrysts: 

1–3% plagioclase (mostly 0.5–1 mm; some slender laths as long as 3 mm); rare biotite 

(mph to 1 mm); sparse opx (0.3–1 mm; either equant or prismatic); trace Fe-Ti oxides 

(0.1–0.3 mm); with or without sparse hornblende needles (mph to 2 mm long, some with 

rounded resorbed ends), unevenly distributed. Also contains pale gray, tan-weathering 

sugary enclaves (1–30 mm), some vesicular and either angular or rounded; in thin 

section, enclaves are meshwork of slender plagioclase and hornblende prisms, studded with tiny equant pyroxenes and oxides. Unit overlies Mesozoic granite (unit Kmo) 

d81 d61 rdc rmk

rce



marble

rg

sd

dom

Figure 23. Town of Mammoth Lakes and four adjacent late Pleistocene (~110–87 ka) lava domes: Units d81 and d61 consist of 

Mammoth Mountain trachydacite, whereas units rdc and rmk are Long Valley high-silica rhyolite. Background scarp is northwest wall 

of Long Valley Caldera, in the west moat of which the four domes erupted. In left foreground, Mammoth Rock is white spire of Paleozoic 

marble. Near center is 90-m-high scarp (called The Bluff), which is easternmost coulee of Mammoth Mountain trachydacite (73-ka unit 

dom). Avalanche scar in lower foreground; toe of rock glacier (unit rg) at lower right. Barren part of Mammoth Meadow in center-right 

foreground is covered in part by postglacial travertine and sinter (unit sd). View northward from Solitude surface on Red Mountain.
and banks against unit Tbtb (3,754±7 ka). Overlain by units mdp (82±1 ka) and amp

(97±1 ka). Contact of massive unit amp atop platy unit drf is well exposed on south wall 

of gorge just below Rainbow Falls. 40Ar/39Ar ages: 98±1 ka (Mahood and others, 2010); 

98.8±0.6 ka (table 2)

dsd Trachydacite of South Summit Dome (late Pleistocene)—Phenocryst-rich lava dome 

(67.3–68.4% SiO2) that forms 500-m-wide plateau just south of summit dome (unit dsu) 

and most of south slope of Mammoth Mountain (figs. 1, 22)—from 3,320 m down to 

2,900 m. One of most extensive on edifice, unit also crops out as altered arete 3180+ 

(500 m east of summit) and in windows through till as low as 2,860 m in valley north of 

Dragons Back. Long south slope is mantled by thick colluvium, and steep northeast face 

is widely draped by till. Upper slopes, northeast arete 3180+, and Reds Creek area are extensively acid altered (cream-white to yellow) and locally also oxidized brick red. Alteration facilitated glacial excavation of northwest-trending trough (The Chasm, which is not 

a fault zone) along base of steep northeast face of Mammoth Mountain. Deep erosion has 

exposed varied lithologies, ranging from devitrified through pale-gray or black vitrophyre 

and from massive to weakly or strongly flow foliated. Best outcrops are block-jointed 

ledges. Phenocrysts: 10–15% feldspar (0.5–7 mm); 3–5% biotite (mph to 2.5 mm); 

2–5% hornblende (mph to needles as long as 10 mm); sparse pyroxene (mph to 2.5 mm); 

Description of Map Units—Quaternary Volcanic Rocks 91

Figure 24. Trachydacite of Rainbow Falls, ~1 km downstream from Devils Postpile, showing abrupt change from columnar base (which 

rests on granite) to platy interior, which is characteristic of most outcrops. Glacially scoured intracanyon flow, here only ~12 m thick, 

is elsewhere as thick as 80 m and is preserved for ~6 km along Middle Fork San Joaquin canyon. Phenocryst-poor lava flow erupted at 

98±1 ka.
92 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

common Fe-Ti oxide mph; also has mafic blebs, pyroxene-oxide-plagioclase clots, and 

clots as big as 20 mm of oxides, amphibole, biotite, and feldspars in varied proportions. 

Overlies units rrc (83±1 ka) and amp (97±1 ka). Overlain by units dbp, ddu, drc, and 

dsu (61.4±2 ka). 40Ar/39Ar age (at south toe of unit): 87±6 ka.

In drill hole HSL-1 near Horseshoe Lake (fig. 6), a trachydacite flow ~15 m thick 

(nowhere exposed) is similar to unit dsd chemically and petrographically, but it underlies 

stack of thin flows of unit amp, which predates dsd. It may be the oldest trachydacite 

known on Mammoth Mountain

dsk Trachydacite of Skyline Dome (late Pleistocene)—Phenocryst-rich lava dome (67.2–69.8% 

SiO2), ~700 m wide, on crest of Mammoth Mountain (fig. 7) just southeast of Scottys 

Dome (unit dwr). On precipitous north rim is conspicuous terminal of Chair 23; northface has more than 200 m exposure, its toe concealed by talus. Unit is block-jointed or 

slabby, and flow foliation is subdued (relative to lavas of adjacent domes). Upper part of 

unit is widely acid-altered, its surfaces cream-white, yellow, or rusty reddish brown, but 

interiors of some thick slabs are pale to medium gray and fairly fresh. Lower part of north 

face is fresher, eroding into coarsely block-jointed ledges. Erosional outlier, similar in 

composition and paleomagnetic declination, forms ice-scoured rib at 3,100 m elevation 

(UTM 204/672), 600 m north of summit and 270 m lower; northwest-trending rib, ~80 m 

wide and 250 m long with <20 m local relief, consists of flow-foliated black and reddishbrown vitrophyre with a slabby devitrified domain exposed at its lower end. Phenocrysts: 

12–15% feldspar (0.5–5 mm); 3–5% biotite (mph to 1.5 mm); sparse Fe-Ti oxide mph; 

trace pyroxene (0.3–0.8 mm); also contains biotite-oxide-feldspar clots and rare clusters 

of pyroxene. Overlies unit dwr (73±2 ka). Overlain by units drc and dsu (61.4±2 ka). 40Ar/39Ar age: 71±1.5 ka

dsu Trachydacite of Mammoth Mountain summit (late Pleistocene)—Phenocryst-rich 

summit dome (68.0–69.6% SiO2) of Mammoth Mountain (elevation 3,369 m; figs. 1, 7). 

Ice-scoured dome-flow complex extends ~700 m along crest and has ~150 m of relief. A 

large but uncertain fraction has been glacially stripped. Summit is site of café, interpretive center, and terminus of góndola that ascends from north toe of edifice. Most exposures are ledges and cliffs of flow-foliated vitrophyre, black to pale gray, widely exhibiting reddish brown films on joint surfaces. Cliffs also expose shear zones of flow breccia 

as thick as 10 m. Much of upper surface and south slope are acid altered, whereas freshest 

rocks are cliffs at southeast end of unit and a strongly flow-foliated lava flow along its 

northwest rim. An outlying remnant, only ~300 m long and 50 m wide, banks against 

unit rmf along upper east side of glaciated Face Lift planèze, where it is youngest unit 

preserved. Phenocrysts: 10–15% feldspar (0.5–7 mm); 3–5% biotite (mph to 2.5 mm); 

1–2% hornblende (mph to 2.5 mm); sparse pyroxene (0.3–1 mm); sparse Fe-Ti oxide 

mph; also carries mafic blebs, biotite-oxide-feldspar clots, hornblende-feldspar clots, 

and angular biotite-felsite xenoliths (1–15 cm). Intrudes and overlies unit dsd (87±6 ka. 

Overlies units drc, dfl (61±3 ka) and rmf (61±1.5 ka); apparently banks against unit dsk

(71±1.5 ka). 40Ar/39Ar age: 61.4±2 ka

dtl Trachydacite of Twin Lakes outlet (late Pleistocene)—Phenocryst-rich lava flow of 

low-silica trachydacite (63.0–64.0% SiO2) that extends ~1 km eastward from Twin 

Lakes outlet bridge. Crops out as a 50-m-high ledge above Lake Mary Road just north 

of outlet and extends downstream along both banks of Mammoth Creek, supporting a 

scenic waterfall at 2,550 m. Elsewhere extensively covered by glacial deposits, unit is 

also exposed in 2,530-m roadcut on Old Mammoth Road (240/6575) and as three small 

windows at 2,760–2,810 m on floor of till-mantled valley north of Dragons Back. Everywhere glaciated, outcrops are partly glassy or widely devitrified, block-jointed or slabby, 

patchily oxidized, variously massive, vuggy, or weakly vesicular, with flow foliation 

typically poorly developed. Phenocrysts: 15–25% feldspar (0.5–7 mm); 3–5% hornblende 

(0.5–7 mm); 3–5% biotite (mph to 1.5 mm); 1–2% pyroxene (mph to 2 mm); common 

Fe-Ti-oxide mph. Also contains mafic blebs, fine-grained cpx-oxide-plagioclase clots, 

intergrowths of plagioclase and hornblende (1–2 cm), and finely porous, fine-grained 

inclusions (1–25 mm, rounded to angular) that carry mph of plagioclase, hornblende, 

biotite, and pyroxene—all typically ≤0.5 mm. Base not exposed but probably overlies 
unit amp (97±1 ka). Inferred to be overlain by nearby units dbp and ddl (58±2 ka), but 

all contacts are covered by surficial deposits. 40Ar/39Ar age: 76±1 ka

dwr Trachydacite of White Bark Ridge (late Pleistocene)—Phenocryst-rich lava dome-flow 

complex (67.9–69.5% SiO2) that makes up entire west flank of Mammoth Mountain 

(fig. 7). Outcrop belt is ~1 km wide and extends ~2.5 km north-south, including White 

Bark Ridge (UTM 192/677), Scottys Dome (UTM 197/671), and southwest flank west of 

Reds Creek. Exposed up to 3,175 m atop steep-walled dome, down to 2,850 m on White 

Bark Ridge, and as low as 2,780 m at southwest extremity. From 2,800 m to 2,960 m, 

pumice-mantled southwest lobe consists of four ledges of block-jointed vitrophyre, which 

appear to be discrete (but compositionally similar) lava flows. Most outcrops are black to 

medium-gray vitrophyre, strongly to weakly flow-foliated, block-jointed or slabby, and 

only locally either vesicular or partly devitrified. Craggy rim of dome retains reddishbrown scoriaceous crusts and may have stood above ice; unit is everywhere else glacially 

eroded. Although unit consists of several physiographic components, no simple subdivision of sprawling complex can be made on basis of broadly similar compositional data; 

for example, TiO2 (0.50–0.58%) and K2O (4.25–4.46%) exhibit no systematic spatial 

pattern (n=15). Nonetheless, two extrusive episodes are indicated by paleomagnetic data: 

northern coulee and western flow lobe share modest inclination and northeasterly declination, whereas younger components (Scottys Dome and topmost coulee to southwest) 

share steep inclination and northerly declination (Hildreth and others, 2014).

Phenocrysts: 10–15% feldspar (0.5–6 mm); 3–5% biotite (mph to 2.3 mm); <1% 

hornblende (mph to 1.5 mm); sparse pyroxene (0.2–1 mm); sparse Fe-Ti oxide mph; 

also carries varied clots of these phases in all combinations. Overlies units rrc, amp

(97±1 ka), and dsd (87±6 ka), and banks against Mesozoic granite (unit Kmo) at topographic wall of Long Valley Caldera, thereby enclosing 1-km-long hollow that now 

contains Reds Lake. Overlain by units drc and dsk (71±1.5 ka). 40Ar/39Ar ages: 79±12 ka 

for western flow lobe; 73±2 ka for top of Scottys Dome

d61 Trachydacite of Dome 2861 (late Pleistocene)—Large dome of phenocryst-rich lava 

(66.1–67.6% SiO2) of Mammoth Mountain type centered 2 km northeast of toe of 

Mammoth Mountain edifice and 3 km northwest of Mammoth Lakes business district 

(fig. 23). Steep- sided dome is 1.5–2 km wide and >300 m high. Most exposures are 

flow-foliated and variously block-jointed, hackly, or slabby. Rolling unglaciated upper surface of dome remains glassy and micropumiceous, but steep faces have at least 

been frost riven, as much of their glassy carapace has been removed. Ten steep (55o) 

chutes are cut into upper 120 m of south face; because there is little catchment area 

above, their excavation along parallel vertical joints may have been accomplished by 

late Pleistocene frost action beneath a thin mantle of ice or nevé. Dome was called 

“Mammoth Shoulder dome” by Suemnicht and Varga (1988). Inappropriately called 

“Earthquake Dome” (for example, by Bailey, 1989) because that name associates dome 

with “Earthquake Fault” (a tourist attraction and open fissure), which cuts adjacent 

Dome 2781 (see unit d81) but not this one. Low-relief aprons adjacent to southeast 

and northeast toes of dome consist of monolithologic fragmental deposits derived from 

it, including angular to subrounded clasts (1–50 cm) enclosed in a dominant matrix of 

pale-gray fine-to-coarse ash; blocks are pale purplish-gray, tan-weathering, glassy, and 

finely to non- vesicular. Deposits are unstratified, tan-weathering, and at least 2–7 m 

thick. Hummocky surfaces suggest deposition as rockfall avalanches (unit av) from 

steep faces of dome, but a few increments may have originated as pyroclastic flows. 

Phenocrysts: 10–15% feldspar (0.5– 4 mm); 1–2% biotite (mostly mph but as big as 

1.5 mm); 2–3% hornblende (mph to 3.5 mm); trace pyroxene (0.2–0.7 mm); common 

Fe-Ti-oxide mph. Roof of piston-like dome lifted a 500-m-long slab containing mafic 

lavas of units mcl (175±3 ka) and bar (88±5 ka). Dome appears to overlap adjacent 

dome (unit d81; 99±7 ka). 40Ar/39Ar age: 87±2 ka (Mahood and others, 2010)

d81 Trachydacite of Dome 2781 (late Pleistocene)—The “real” Earthquake Dome, a sprawling phenocryst-rich flow-dome complex (68.6–69.1% SiO2

) that lies between adjacent 

younger edifices of Dome 2861 (unit d61) and Mammoth Mountain. Gently sloping 

edifice is 2 by 2.5 km wide, has total relief of 180 m to north and 300 m to south, and 

Description of Map Units—Quaternary Volcanic Rocks 93
94 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

is centered 3.5 km northeast of Mammoth Mountain summit (fig. 23). Poorly exposed 

slopes are deeply mantled by colluvium and by 1350 C.E. Inyo pumice-fall deposit. 

Good exposures are present at summit, along west and southeast margins, and along open 

fissure nicknamed “Earthquake Fault” (Benioff and Gutenberg, 1939). Exposures are 

generally crumbly, thinly flow-foliated vitrophyre, black or locally oxidized, but include 

pale-gray, brown-weathering pumiceous carapace preserved near summit. Surface around 

summit consists of knolls of block-jointed and flow-foliated vitrophyre, commonly frostriven but not glaciated. Moderately sloping morphology of edifice contrasts with cliffy 

form of adjacent Dome 2861, which is of similar (Mammoth Mountain type) lithology 

and slightly less silicic. Phenocrysts: 10–12% feldspar (0.5–4 mm); 2–3% biotite (mostly 

mph but as big as 1.5 mm); ≤1% hornblende (mph to 2 mm); sparse pyroxene (mph to 

1 mm); common Fe-Ti-oxide mph. Overlies unit mcl scoria cone (175±3 ka). Overlain 

by units msd and d61 (87±2 ka). Margin of edifice is lapped or abutted by units ddc

and rce, and by late Pleistocene till on its northwest (unit gdc) and southeast (unit gmm) 

sides. 40Ar/39Ar age: 99±7 ka. Oldest unit exposed having Mammoth Mountain lithology

mcl Basaltic trachyandesite of Canyon Lodge (middle Pleistocene)—Phenocryst-poor scoria 

cone and lava flows (53.8–55.9% SiO2; n=22) that crop out at several isolated locations 

(fig. 9), correlated on basis of composition, petrography, stratigraphy, and paleomagnetism. Scoria cone, ~600 m wide, was bulldozed for chairlifts in front of Canyon Lodge; 

it consists predominantly of brick-red scoria lapilli in layers 10–100 cm thick, ranging 

from loose to tack-welded to moderately agglutinated. Sparse bombs are as big as 1.5 m, 

a few of which have dense unoxidized interiors. Most extensive lava outcrops are several 

block-jointed knolls and ledges, east and west of Highway 395 near its crossing of Dry 

Creek, 7–9 km from vent, where Inyo pumice-fall deposit obscures continuity among 

numerous local exposures. From Canyon Lodge cone, lavas flowed northeast along west 

moat prior to emplacement of rhyolite lavas there (units rwm, rdc, rmk). An intricately 

shattered slab of unit mcl lava, as thick as 25 m and 250 by 450 m in plan view, was uplifted on roof of Dome 2861 (unit d61) during its plug-like extrusion (86±2 ka) and now 

dips 25o–30o NNE. Lavas from the cone also flowed at least 8.5 km eastward along south 

moat, where unit is identified at depths of 110–190 m (beneath till in and near the town 

of Mammoth Lakes) in at least three water wells (fig. 6), as a ledge near southern base 

of Knolls Vista (UTM 265/6875), and as a faulted remnant that forms 10-m-high bench 

(UTM 319/678) ~300 m southeast of Casa Diablo geothermal plant. Near the geothermal 

plant, outcrop of unit mcl is surrounded and lapped by rhyolite-derived sandstone and 

pebble conglomerate (unit ss) of Pleistocene Long Valley Lake.

Lava exposures are generally massive or locally vesicular, block-jointed or hackly, 

rarely slabby, and commonly erode as ledges 3–7 m high. Scoria cone and Knolls Vista 

exposure were glaciated, and exposures beyond glacial limits are fairly degraded; many 

joint blocks have been rounded and stripped of primary scoriaceous carapace. Phenocrysts: Plagioclase (0.5–2 mm) content is 0.5–2% at most exposures and up to 5% in a 

few intercalated flows, which additionally carry sparse plagioclase crystals as long as 

4 mm (many larger ones sieved or rounded) and feldspar xenocrysts as large as 10 mm 

with reaction rims. Olivine is sparse and tiny (0.1–0.7 mm), seldom in clusters. Cpx is 

typically absent but present rarely as microphenocrysts. Granitic xenoliths occur sparsely. 

Within compositional range of unit mcl (7.5-6.8% FeO*; 1.8-2.2% K2O), phenocrystpoorer flows (or domains within them) tend to be slightly more evolved chemically. 

Overlain by units bsm (165±2 ka), rwm (~150 ka), mkv (153±1 ka), mor, mnd, and bar

(in slab uplifted atop Dome 2861). 

In south moat water wells (fig. 6), unit is also overlain by units amp, bcd, bsc, 

and gcd. Along south moat, unit is inferred older than units mmc and bfh. Drill hole 

SF66–31 on Sawmill Road [UTM 302/6765], ~1 km west of geothermal plant, is collared in unit bcd, which caps a 67-m-thick section of mafic lava flows; lowest flow is 

chemically and petrographically identified as unit mcl, which rests on ~9 m of crystalrich ash that overlies Early rhyolite tuff (unit rer) hundreds of meters thick. In water 

well 30M, 2 km southeast of downtown Mammoth Lakes, a 27-m-thick section of 

two or three mcl lava flows penetrated at 110–137 m depth directly underlies unit bsc
(172±2 ka) and overlies unit asr. In Inyo-4 slant drill hole beneath South Inyo Crater 

(Eichelberger and others, 1988), unit mcl closely underlies unit aic (131±1 ka) at slant 

depths of ~70–245 m; several flows comprising upper ~75 m of Group II of Vogel and 

others (1994) are compositionally indistinguishable from unit mcl, and several more 

comprising the lower ~55 m of Group II are similar but not identical. In drill hole OIP-1 

(UTM 237/749), ~1 km northeast of Deer Mountain, phenocryst-poor lava flows correlated chemically with unit mcl were penetrated at depths between 90 and 190 m, where 

they underlie one flow of unit mdn and another of unit bmn. At exposures southwest 

of Lookout Mountain, hilly local relief of 20–40 m, multiledge knolls, steep bluffs, and 

rhyolite remnant (unit rwm) atop 40-m-high knoll of unit mcl indicate that unit mcl is 

significantly eroded. Relief probably owes to combination of displacements on resurgent 

graben fault system and Pleistocene incision by Dry Creek drainage system, which issued 

from glacial limit only 2 km to west. 40Ar/39Ar ages: 175±3 ka (UTM 278/742); 172±2 ka 

(UTM 282/755); 179.5±2.4 ka (UTM 319/678). Inyo-4 core sample at slant depth of 

461.5 ft (140.6 m), upper Group II, gave 186±2 ka. One 40Ar/39Ar age of 190±9 ka (UTM 

281/757) was determined by Mahood and others (2010) 

mcv Basaltic trachyandesite south of Crestview (late Pleistocene)—Scoria cone west of Crater 

Flat and lava flow field (53.3–54.2% SiO2) that extends northward across Deadman 

Creek to terminus near Crestview highway maintenance station, 9 km from vent. Cone is 

130 m high, 1 km wide, and heavily covered by Inyo pumice-fall deposit. Its unusually 

deep and steep-walled crater is breached and open to north. Outcrops on cone are limited 

to east and south rims, consisting of brick-red ejecta, both loose scoria and tack-welded 

stratified agglutinate. Coarse scoriae armoring crater rim are abundantly 30–50 cm and 

as big as 1 m. Lithic clasts among rim ejecta include a 5-m block of unit bar; Mesozoic 

metavolcanics; various mafic and felsic lavas (many probably from subjacent till); and 

abundant fragments (1–8 cm) of uncorrelated black, glassy, moderately porphyritic, 

hornblende-biotite-plagioclase rhyolite (72.1% SiO2) that is somewhat more evolved than 

rhyolite of nearby Deer Mountain (unit rdm). One mcv lava flow crops out poorly along 

draw at south base of cone, but lava apron north of cone is wholly concealed by Inyo 

pumice. Lavas flowed northeast around Deer Mountain, beneath later sites of rhyolitic 

Deadman Creek flow (unit ric) and coulee of unit dnw, emerging from beneath latter 

~7 km from vent cone. North of flow front of dnw coulee, unit mcv forms a low-relief 

sagebrush-covered lava flow field that extends 2–2.5 km northward to end against north 

wall of Long Valley Caldera. Its rough, little-eroded surface is marked by low swells 

and hillocks, a few crags, and scattered small tumuli. Lavas are generally block-jointed, 

vesicular, commonly scoriaceous and oxidized reddish brown, and seldom expose more 

massive internal zones. Phenocrysts: 1–2% plagioclase (0.5–2 mm); traces of olivine and 

cpx (each mostly mph but as big as 1 mm); much of cpx occurs in clots with plagioclase. 

Overlies units bar and mor. Overlain by unit dnw (27±1 ka), by Inyo pumice deposits 

(primary and reworked), and by alluvium of Deadman Creek. Drill hole DC-1 (fig. 6; 

Sorey and others, 1978), located at UTM 259/781, ~800 m northwest of Crestview 

roadside rest, penetrated to base of unit mcv at ~25 m depth, then successively downward 

through a 30-m-thick flow of unit mor (66±2 ka), a 20-m flow compositionally identical 

to unit bsr (99±1 ka), a 15-m flow of unit aic, and a 10-m uncorrelated phenocryst-poor 

flow, before passing into ~75 m of gravels and ashy sediments, and finally, at a depth of 

191 m, into tuff and lava of Early rhyolite (unit rer; ~700 ka). 40Ar/39Ar ages of unit mcv: 

31±2 ka (Mahood and others, 2010); 33.8±1 ka (table 2)

mdm Basaltic trachyandesite of Deer Mountain (late or middle Pleistocene)—Poorly exposed 

scoria cone (55.7–56.0% SiO2) partly overlapped by northeast base of rhyolitic Deer 

Mountain dome in west moat of Long Valley Caldera. Exposures limited to loose scoria 

and tack-welded agglutinate, all brick-red, that form a ledge, 1–3 m high, around east 

to southeast rim of cone and to a 15-m-high stack of agglutinate ledges lower on east 

slope. Subdued crater, open to north, is filled and obscured by 1350 C.E. Inyo pumicefall deposits (see units ric and p) and ensuing lithic ejecta (unit pe) from phreatic eruptions of nearby Inyo Craters. Abundant lithics here include granitoids, rhyolite of Deer 

Mountain, and varied mafic and silicic lavas (many from underlying gravels and till), but 

Description of Map Units—Quaternary Volcanic Rocks 95
96 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

they were ejected in 1350 C.E., not with mdm scoria. These late Holocene fall deposits 

blanket >90% of the mafic cone and obscure its contacts with contiguous rhyolite of Deer 

Mountain (unit rdm; 101±8 ka), which is probably younger. Cone is at least ~90 m high 

and ~400 m wide. Scoriae are oxidized, predominantly lapilli but as big as 30 cm, and 

almost aphyric, containing rare plagioclase (0.5–2 mm) and still rarer oxidized mafic 

mph. Crude layers are 20–100 cm thick; within coarser layers, dominant lapilli (1–6 cm) 

are accompanied by subordinate 10–15 cm scoriae and by rare spindle bombs. Compositions of scoria are unlike those of units mic and aic (131±1 ka) exposed in nearby South 

Inyo Crater; their relative ages are not established, but all three units appear to be older 

than unit rdm. Undated

mdn Basaltic trachyandesite northeast of Deadman Pass (late Pleistocene)—Glacially eroded 

remnants of phenocryst-rich scoria and agglutinate (52.4–53.7% SiO2) surrounding each 

of three vents, aligned northwest-southeast partway up west wall of Long Valley Caldera, 

south of upper Deadman Creek and 1.5–2 km northeast of Deadman Pass. Northwest 

vent is shallow crater, 100 m across, surrounded by 300-m-wide ejecta ring of loose 

brick-red scoria. Ejecta are mostly lapilli but include finely to coarsely vesicular bombs, 

abundantly 20–50 cm, sparsely 1–2 m, and as big as 3.5 m; many large bombs have dense 

interiors. Two ledges of tack-welded to dense agglutinate crop out on eastern outer slope, 

15–25 m below rim. Circular crater, heavily mantled by Inyo pumice-fall deposits, has 

13 m relief on its southeast inner wall and at least twice that much on its north wall.

Middle exposure is a northeast-facing agglutinate cliff, 8–15 m high and ~200 m 

long. Lowest 1 m of exposure is brick-red agglutinate, dense but streaky, that grades up 

into ~10 m of massive gray fountain-fed lava, almost homogenized but locally displaying 

subtle flow foliation and carrying cream-white metavolcanic xenoliths (1–7 cm) that tend 

to be elongate and oriented along flow-foliation planes. Lava sheet and its foliation dip 

~20o NE. Top 3 m of cliff is brick-red massive agglutinate with faint streaky or blobby 

structure patchily discernible. Sparse scoria bombs as big as 50 cm near cliff rim suggest 

that vent lies nearby, probably concealed by adjacent moraine that extends from Deadman 

Pass cirque on southwest and, in turn, by Inyo pumice-fall deposit. 

Southeast exposure includes a 600-m-long ridge of lightly welded brick-red agglutinate and an adjacent steep-walled gorge, both walls of which expose denser agglutinate 

50–80 m thick. Ridgecrest becomes narrow arête to northeast and consists mostly of tackwelded scoria lapilli sprinkled with sparse scoria blocks (20–70 cm) and spindle bombs 

up to 1 m. On gorge walls, deposit is dense red-brown agglutinate with a few thin lava 

ledges intercalated, some of which are block-jointed and dark gray. Wall sections grade 

down into dense fountain-fed lava near gorge floor. Narrow gorge through vent complex 

was probably cut glacially and later deepened by glacial outlet stream; its walls remain 

glacially striated, but stream now draining it is feeble and intermittent. 

Products of all three vents are chemically and petrographically alike and presumed 

to have been erupted contemporaneously from a common dike; vents align along local 

rangefront scarp . A lone distal exposure crops out 2.8 km northeast of those vents as a 

water-smoothed lava ledge ~30 m long at 2,445 m elevation on floor of Deadman Creek 

(UTM 2115/7618); outflow lavas are elsewhere wholly concealed by surficial deposits. 

Products are not related to Cone 2652 (large scoria cone of phenocryst-poor unit mcv), 

toward which the mdn vent alignment roughly strikes. 

Phenocrysts: 10–15% plagioclase (1–3 mm, rarely 4–5 mm); 1–3% olivine 

(0.5–1 mm); 1–2% cpx (0.5–2 mm); and common 5–10 mm intergrowths of 10–15 

grains each of cpx and plagioclase (both 0.5–2 mm). Northwestern vent ejected metavolcanic xenoliths (1–4 cm, rarely to 8 cm), some partially melted; middle vent ejected 

partially melted xenoliths of both granitoids and silicic metavolcanics. Base not exposed 

but presumed to overlie Mesozoic granitic rocks and still older metamorphic rocks, both 

exposed nearby but covered locally by till, colluvium, and Inyo pumice-fall deposit. All 

three proximal exposures were overtopped and eroded by glacial ice and strewn with 

basement erratics. Scoria lapilli (1–6 cm), chemically and petrographically indistinguishable from unit mdn, are scattered atop Dome 2861, 6 km southeast of mdn vents. 40Ar/39Ar age: 16±2 ka. 
The undated basaltic trachyandesite of June Lake (unit mjl), which lies on the rangefront fault system 10 km north of unit mdn, is similar to it in composition, mineralogy, 

and unusually steep paleomagnetic direction (Hildreth and others, 2014). As both are 

overlain by MIS 2 glacial deposits, they may have erupted contemporaneously. 

Blocky rubble, designated subunit mdn', banked against southwest base of Lookout 

Mountain (at UTM 03277/41769), consists exclusively of coarsely vesicular blocks of 

unit mdn, indistinguishable from near-vent outcrops chemically, petrographically, and 

in age (40Ar/39Ar age: 16.9±1.3 ka). Angular clasts (10–300 cm) protrude from thick 

mantle of Inyo pumice-fall deposit over an area 65 m along slope (north-south) and 35 m 

upslope (east-west), forming a flat-topped bench ~10 m high. Of 140 clasts counted, all 

are of unit mdn lithology. In drill hole OIP-1 (fig. 6), ~1 km northeast of Deer Mountain, 

a 10-m-thick lava flow chemically and petrographically similar to unit mdn is topmost 

of seven mafic lava flows penetrated and is covered there by ~18 m of Inyo ejecta (unit 

pe) and glacial deposits. Material was apparently transported from caldera-wall vents, 

first as periglacial or englacial lava flows, subsequently as fragmental flows. Straight-line 

path north of Deer Mountain would be 7–8 km from vents to moraine limit and additional 

2 km to rubble deposit banked against Lookout Mountain 

mdp Basaltic trachyandesite of Devils Postpile (late Pleistocene)—Glacially eroded sheet of 

phenocryst-rich lava (53.9–54.4% SiO2) preserved discontinuously along floor of Middle 

Fork San Joaquin River canyon in two major remnants and several small ones, distributed for 7 km downstream from near (upper) Soda Springs Campground. Vent location 

unknown, but unit is inferred to have erupted near or upstream from that campground, 

where scoria cone and dikes of subjacent unit mss are exposed. No dikes or near-vent 

features of unit mdp are recognized. Largest remnant measures 2 km north-south, ~1 km 

wide, and includes near its south end the spectacular columnar jointing that constitutes 

the Devils Postpile, which was designated a National Monument in 1911. At base of 

columnar cliff is a remarkable postglacial talus of fallen columns (fig. 11). Far more than 

half of original unit has been lost to erosion, as shown by 3.5-km-long gap between cliffy 

outcrops near Reds Meadow and small remnants southeast of Rainbow Falls. As with unit 

Tbtb, just downstream, river re-incised through intracanyon lava only to granitic floor of 

pre-lava channel, and no deeper. Ice-polished polygonal mosaic on bench at top of iconic 

columnar cliff is 50 m higher than basal colonnade at 2,295-m river level. Just east of 

that bench, however, glacially scoured top of flow extends up to elevation 2,405 m, 60 m 

higher than top of main colonnade and 110 m above flow base at riverside. At three additional locations, one 300 m north of Sotcher Lake and two on west side of river (opposite 

Pumice Flat and Devils Postpile Campgrounds), scoured top of flow is also at 2,400– 

2,410 m elevation. Vesicular lava and scoriaceous joint blocks on these glaciated surfaces 

suggest proximity to primary flow top. By coincidence, ~110-m original flow thickness is 

same as that of 1959 Kilauea Iki lava lake (Hawaii), which took 20–25 years to solidify 

fully and produced audible cracking as polygonal contraction joints advanced incrementally into thickening crust (Peck and Minakami, 1968; Ryan and Sammis, 1978).

Lava is widely striated and plucked, eroded into knolls, ridges, and sidewall 

benches; surviving exposures are nearly all massive and have only sparse scattered 

vesicles. Slender columns of Devils Postpile colonnade are 15–20 m high, polygonal, 

typically 0.5–1 m thick, and variously vertical, curved, inclined, or subhorizontal (Huber 

and Rinehart, 1965b); elsewhere in unit, sets of stouter or less regular columns are 

widespread, but hackly and block jointing is common, too. Partly glassy chunk jointing 

is present at riverside, and locally fanned or chevron sets of columns reflect localized 

control of cooling geometry by major fractures. Evidence for ice-contact emplacement 

has not been observed and would not be expected here at time of eruption. Some column 

faces exhibit wavy undulations and, although widely obscured by weathering and glacial 

scour, bands (“chisel marks”) are present locally, representing incremental advance and 

stoppage as contraction fractures penetrated stepwise into solidifying interior (DeGraff 

and Aydin, 1987). 

Phenocrysts: 7–10% plagioclase (0.5–4 mm); 2–3% olivine (0.5–1.5 mm), commonly clustered; and ~1% cpx (1–3 mm), both free and intergrown with plagioclase. Overlies 

Description of Map Units—Quaternary Volcanic Rocks 97
98 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

units amp, mss, and drf. Widely rests on and wedges out laterally against Mesozoic 

granite on both walls of canyon. At its northwesternmost outcrop, unit mdp lava rests on 

~9 m of fluvial sand and gravel. 40Ar/39Ar ages: 82±1 ka (near Minaret Falls Campground; 

Mahood and others, 2010); 82±2 ka (distal downstream outlier at UTM 03121/41161; 

table 2). K-Ar age of 940±160 ka reported by Dalrymple (1964b) suggests either an abundance of excess Ar in the plagioclase fused or an error in decimal-place transcription

mic Mafic pyroclastic deposit of Inyo Craters (Holocene or late Pleistocene)—Vaguely stratified ash-rich fragmental deposit exposed on walls of South Inyo Crater (Rinehart and 

Huber, 1965). Those authors reported it on walls of North Inyo Crater as well, but, if 

present, it was scree-covered during our investigations. Weakly to moderately indurated 

deposit is slope-forming and widely scree-mantled, may be as thick as 8–12 m, and is best 

observed on east wall of South Inyo Crater. Gritty reddish-brown matrix is volumetrically 

dominant and appears to be seriate from sand grade through granules to lapilli. Among 

lapilli, scoriae are outnumbered by lithic fragments, which are mostly 1–8 cm but rarely as 

big as 25 cm. Commonest lithics are (in order of declining abundance) crystal-poor mafic 

lavas, rounded granitoids, crystal-rich mafic lavas, and porphyritic trachydacite; among the 

trachydacites, one clast analyzed chemically is identical to unit dwr on Mammoth Mountain, and was presumably entrained from till or alluvium. Scoriae—not necessarily juvenile—are black or brick red, mostly 3–6 cm across but as large as 20 cm, and crystal-poor, 

containing sparse olivine and plagioclase phenocrysts. Three scoriae analyzed have 54.3, 

54.5, and 55.8% SiO2; they are far less evolved and notably richer in FeO and TiO2 than 

underlying lava flows of unit aic and are compositionally unlike any other unit exposed 

in map area. Unit may be a phreatic or phreatomagmatic deposit that entrained clasts 

from unexposed scoria layers as well as from till; nine layers of cinders were identified 

between mafic lava flows in top 306 m of nearby Inyo-4 core (Eichelberger and others, 

1988). Deposit rests directly upon unit aic (131±1 ka). According to Mastin (1991), top 

2 m of pyroclastic section is weathered and capped by a thin organic mat, which is in turn 

overlain by 1 cm of Mono Craters “tephra 2” of Wood (1977), then by >1 m of 1350 C.E. 

pumice-fall deposits (unit p) associated with Inyo Chain eruptions (unit ric), and finally by 

thick phreatic deposit ejected from Inyo Craters (unit pe). Undated

mjl Basaltic trachyandesite of June Lake (late Pleistocene)—Scoria cone and associated moderately porphyritic lava flows (53.5–54.1% SiO2

) just west of Mono Craters. Although 

cone is ~5 km north of our map area, it is described here because of its proximity, 

compositional similarity, and contemporaneity with mafic eruptive units of the Mammoth magmatic system. Cone is 700 m wide, 75 m high, and is centered 1 km northeast 

of June Lake (fig. 3). The cone has loose scoria on its slopes and scoria blocks as big as 

1.2 m on top, but most exposures are brick-red agglutinate, largely tack welded, and some 

are dense enough to maintain glacial striae. Entire cone is till covered, best exposure 

being 30-m-high wall of stratified agglutinate inside breached and eroded crater, which is 

open to north. Lava-flow field forked into two lobes, one terminating 3 km north of vent, 

another flowing 4 km northeast, where ponding produced a rootless secondary vent that 

built a 30-m-high craggy pile of coarse red agglutinate, beyond which lava flowed an additional 7 km northward around east side of Aeolian Buttes to a broad flow-front terminus 

at southeast margin of Pumice Valley. Lava flows rafted away parts of cone, distributing 

them atop both lobes as far as 3 km from vent as piles of red scoria and agglutinate that 

simulate rootless vents and might readily be confused as such. Lavas are massive and 

weakly vesicular internally but scoriaceous and blocky on unglaciated surfaces. Distal 

apron has scattered tumuli, some of which burst to produce rootless spatter mounds as 

high as 5 m; distal flow front is 2.5 km wide, commonly 5–15 m high, and block-jointed 

to rubbly. Cone and proximal 2 km of lava-flow field are overlain by Tioga Till (MIS 2); 

most of main tongue and distal 600 m of north lobe are unglaciated. Phenocrysts: 5–10% 

plagioclase (0.5–4 mm); 1–2% olivine (0.5–1 mm); trace cpx (0.5-1 mm). Plagioclase 

content may increase distally, although cone and lava lobes show little range in chemical composition. Lava flows locally contain abundant granitic xenoliths and derivative 

xenocrysts. Lavas bank against 767-ka Bishop Tuff and overlie pre-Tioga glacial deposits variously named or subdivided by others (Putnam, 1949; Bailey, 1989, 2004; Bursik 
and Gillespie, 1993). Overlain by Tioga Till, by rhyolitic South Coulee of Mono Craters 

(~2 ka), and by pumice deposits from Holocene vents at Mono Craters. Several 40Ar/39Ar 

age experiments have failed to yield interpretable plateaus or isochrons; data suggest a 

very late Pleistocene age, conceivably intra-Tioga.

The 17-ka basaltic trachyandesite northeast of Deadman Pass (unit mdn), which 

lies 10 km south of the June Lake cone, is similar to it in composition, mineralogy, and 

unusually steep paleomagnetic direction (Hildreth and others, 2014). As both units are 

overlain by MIS 2 glacial deposits, they may have erupted contemporaneously

mkv Basaltic trachyandesite of Knolls Vista (middle Pleistocene)—Phenocryst-poor agglutinate 

and fountain-fed lava flows (54.5–54.9% SiO2) that make up 1.3-km-long northeasttrending ridge, ~100 m high, that lies just east of Mammoth Knolls and just north of 

Mammoth Lakes business district. Cone 2517 at southwest end of ridge best preserves 

evidence of spatter-vent complex. Ledges poorly exposed on colluvium-mantled slopes 

suggest stacks of several lava flows, each 3–10 m thick. Sparse scoriaceous ejecta loose 

on slopes are finely to coarsely vesicular, mostly 20–40 cm but as large as 1 m. Summit 

of ridge is eroded into small nubbins and ledges of dense lava typically 2–3 m thick that 

are locally flow-foliated and widely exhibit streaky texture inherited from spatter-fed 

origin. Some ridgetop lavas have scoriaceous surfaces; others grade into brick-red agglutinate, which ranges from tack-welded to moderately welded lenticulite. Ridge-forming products are compositionally identical to lavas of unit mmc, 4 km southeast. Six 

poorly exposed block-jointed and hackly ledges on steep south slope of edifice are all 

phenocryst poor, but only the upper three belong to unit mkv; lower three are chemically 

identified as units bsm (165±2 ka), mcl (175±3 ka), and asr. Phenocrysts: <1% plagioclase (0.5–1.5 mm); <1% olivine mph (0.1–0.3 mm); rare cpx (0.5 mm); and sparse 

Fe-Ti oxide mph (0.1–0.4 mm). Foot of edifice is lapped by both Casa Diablo Till (unit 

gcd) and late Pleistocene till (units gmm and glb) and by lavas of unit bsr (99±1 ka). 

North margin of unit is apparently overlapped by unit msc (154±2 ka). Sites drilled atop 

cone yield excursional paleomagnetic direction (Mankinen and others, 1986; Hildreth 

and others, 2014). 40Ar/39Ar ages: 148±14 ka (Mahood and others, 2010); 153±1 ka 

(B. Turrin, unpublished data)

mlc Basaltic trachyandesite of lower Laurel Creek (middle Pleistocene)—Phenocryst-rich 

lava flow (51.8–52.9% SiO2; 20.1–20.9% Al2O3) exposed only south of Mammoth Creek 

at rugged terminal flow front that runs ~1 km south-southwest from junction of Sherwin 

Creek Road and Highway 395. Exposure is only 500 m long (along eastward direction of 

flow). Outcrops include ridges, hillocks, and tumuli, heaved and tilted surfaces of which 

are fissured, pervasively vesicular, and marked by crude polygonal joints, many of which 

extend into columns 1–2 m long. Crops out only beyond ice limit of Tioga and Casa Diablo 

glaciations. Source vent unknown, buried by younger units to west. Phenocrysts: 10–15% 

plagioclase (mostly 0.5–1.5 mm; few as big as 3.5 mm); 2–3% olivine (0.5–2 mm); sparse 

cpx (1 mm). Lacks large plagioclase of adjacent unit bfh; contains fewer olivine and cpx 

than nearby unit bcd but more abundant and smaller plagioclase. Bulk lava flow is unusually rich in Al. Overlies unit bsc (172±2 ka). Overlain by unit bfh (92±2 ka). Also overlain 

by alluvium and marshy sump of (artificially diverted) Laurel Creek and by sandy aeolian 

deposits that are widely as thick as 1 m. 40Ar/39Ar age: 130±1 ka

mmc Basaltic trachyandesite of Mammoth Creek (middle Pleistocene)—Phenocryst-poor lava 

flow (54.4–54.8% SiO2) exposed as 1-km-long window through younger units along floor 

and walls of shallow gulch tributary to Mammoth Creek and as 30-m-high hill between 

Highway 203 and Mammoth Creek. Only additional exposure is a small window through 

Casa Diablo Till 500 m farther north, just east of Sawmill Road. Crops out mostly as 

block-jointed ledges, massive below and vesicular to scoriaceous on or near flow surface. 

Phenocrysts: <1% plagioclase (0.5–2 mm); rare olivine (0.5 mm); trace cpx mph; and 

rare Fe-Ti-oxide mph (0.2–0.5 mm). Base not exposed. Overlain by units bcd, bsr, and 

Casa Diablo Till (gcd). 40Ar/39Ar age: 162±2 ka (Mahood and others, 2010); K-Ar age: 

129±26 ka (Mankinen and others, 1986). Although paleomagnetic directions differ and 

overlap of radiometric ages is equivocal, this flow is chemically and petrographically 

identical to agglutinated products of unit mkv vent complex, 4 km northwest

Description of Map Units—Quaternary Volcanic Rocks 99
100 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

mnd Basaltic trachyandesite northwest of Dry Creek (late Pleistocene)—Phenocryst-poor lava 

flows and agglutinate (53.3–54.0% SiO2) exposed in six different places along 6-km-long 

belt northwest of Dry Creek; correlated chemically, petrographically, and stratigraphically. Largest patch is 1.3-km-wide unglaciated lobe that extends 1 km beyond limit of 

late Pleistocene till (unit gdc) to terminus 4 km northeast of Deer Mountain; margins 

and flow front are little modified and have 10–20 m relief. Surface is marked by several tumuli and rootless vents that fed ragged piles and ridges (3–5 m high) of oxidized 

tack-welded agglutinate, rich in scoria bombs. Second large exposure is 1-km-wide 

unglaciated steep-sided lobate bench, just north of Dry Creek and west of Highway 395, 

that rises steeply 40 m higher than both; probably fault-bounded as well as eroded by 

Pleistocene Dry Creek. In both unglaciated areas, most surfaces are rubbly and scoriaceous, whereas sparse outcrops of flow interiors are block-jointed and massive to 

weakly vesicular. Several lesser exposures to southwest are glaciated windows through 

blanket of till. Narrow sliver of oxidized tack-welded agglutinate at north end of westfacing fault scarp, 2 km northeast of Deer Mountain, is probably part of a rootless vent. 

Three southernmost outcrops are a cluster of discrete ice-scoured swells that rise above 

till blanket within 1 km west of rhyolitic Dry Creek Dome and only 100–400 m west 

of Dry Creek; each is ~100 m wide, has only a few meters of ledgy relief, and consists 

of streaky to flow-foliated, aphanitic lava. Sites drilled at several of the outcrops yield 

similar paleomagnetic directions (Hildreth and others, 2014). Phenocrysts: <1% plagioclase (0.5–1 mm); trace olivine mph (rarely as big as 0.5–1 mm). Also contains rare 

but widespread xenocrysts or antecrysts of rounded or corroded plagioclase (1–6 mm). 

Overlies units mcl (~175 ka) and bar (~95 ka). Overlain by units mor (66±2 ka), dnw, 

and gdc. Geothermal corehole PLV-2 (Benoit, 1984), collared in northernmost area of 

the unit, penetrated 169 m of mafic lavas (perhaps as many as 8 flows), which overlie at 

least 467 m of rhyolitic tuffs and lavas that extend to bottom of hole (fig. 6). Many distal 

samples of extensive flow field of unit mor are chemically and petrographically indistinguishable from those of unit mnd, whereas steep-sided, well-defined, proximal-to-medial 

part of mor, which directly overlies mnd, carries significantly more phenocrysts and 

has 1–2 percent less SiO2 than unit mnd. Medial and distal sites drilled in unit mor and 

sites in unit mnd (both east and west of mor) all yield similar paleomagnetic directions 

(Hildreth and others, 2014). It is thus inferred that unit mnd represents early phase of 

more extensive (and chemically more varied) subsequent effusion of unit mor (66±2 ka), 

probably from same source vent (southwest of Dome 2861; see unit msd) and of virtually 

same age. Not independently dated

mor Basaltic trachyandesite of Owens River Road (late Pleistocene)—Phenocryst-poor lava 

flows (51.2–54.2% SiO2) along and north of Dry Creek that condense west of Lookout 

Mountain into a single steep-sided coulee that is exposed continuously beyond till limit 

(unit gdc) near Dry Creek for 5.5 km to flow terminus at north wall of Long Valley 

Caldera along Deadman Creek. Coulee is 1 km wide in south, broadening distally to 

2 km, and has steep lobate margins, commonly as high as 30 m. Full thickness exceeds 

50 m at north toe of Lookout Mountain and along flow front at Deadman Creek, west 

of Big Springs. Unglaciated surface is rolling, hilly, and rubbly, marked by scoriaceous 

knobs, tumuli, and several rootless vents for craggy piles and fissure-fed ridges of red 

agglutinate and scoria. For 6 km southwest of glacial limit, unit is covered by till unit 

gdc and exposed only along and adjacent to a 5-m- high west-facing fault scarp north of 

rhyolitic Dry Creek Dome, where concealed unit is probably at least 1 km wide. Source 

vent appears to be poorly exposed scoria cone in saddle between Domes 2861 and 2781 

(see unit msd). Phenocrysts: Unit ranges from almost aphyric through as much as 4–5% 

plagioclase, most commonly 0.5–2%. Plagioclase phenocrysts are mostly 0.5–3 mm 

but rarely 4–5 mm, though scattered antecrysts or xenocrysts as big as 10 mm are also 

present. Sparse olivine occurs mostly as microphenocrysts, rarely as big as 1 mm. Cpx is 

generally absent but rare microphenocrysts occur in trace amounts. Sparse Fe-Ti oxides 

are typically 0.1–0.3 mm, rarely 0.5 mm. Granitic xenoliths and derivative xenocrysts 

are uncommon but widespread. Glaciated or stream-worn exposures are massive or 

weakly flow-foliated and mostly block-jointed; little eroded surfaces are blocky or 
rubbly, vesicular to scoriaceous, and block-jointed or hackly. Overlies units mnd, mcl, 

and bar (88±5 ka) and banks against Early rhyolite of Lookout Mountain (subunit rlm; 

692±14 ka). Units mnd and mor overlap chemically and have indistinguishable paleomagnetic directions (see unit mnd). Unit mcv (31±2 ka) banks against northwest margin 

of unit mor. 40Ar/39Ar age: 66±2 ka (Mahood and others, 2010)

msc Basaltic trachyandesite of Sawmill Cutoff (middle Pleistocene)—Poorly exposed bench 

of phenocryst-poor lava flows (52.3–52.5% SiO2) that extends 1.5 km north from 

Knolls Vista edifice (unit mkv) between Sawmill Cutoff Road and West Moat Coulee 

(unit rwm). Bench is 600 m wide and 100–140 m high; neither of two hillocks along its 

crest provides evidence of a vent. Exposure is inadequate to determine how many flows 

comprise section, which emerges eastward from beneath steep margin of overlying rhyolite coulee and is abruptly truncated by an inferred fault at a scarp with 50–100 m relief. 

Scattered outcrops are block-jointed ledges and knobs of vesicular or locally massive 

lava. Phenocrysts: 1–3% plagioclase (0.5–2 mm), both equant and laths (the latter seriate 

to microphenocrysts and groundmass); 1–2% olivine (0.3–1 mm); cpx microphenocrysts 

rare. More and larger plagioclase and olivine than in adjacent (chemically more evolved) 

unit mkv, which is abutted and apparently draped by msc lavas. Banks against postcaldera Early rhyolite (unit rer; 694±17 ka; Mankinen and others, 1986) as well as against 

unit mkv. Overlain by unit rwm (~150 ka). Clasts of unit msc are present in Casa Diablo 

Till (unit gcd) of MIS 6. 40Ar/39Ar age: 154±2 ka. K-Ar ages: 155±28 ka and 154±72 ka 

at two separate sites. Drilled at same two sites, unit has anomalously steep northerly 

paleomagnetic direction (Mankinen and others, 1986). 

Scattered on surface of unit msc are sparse clasts of aphyric, hydrothermally 

altered Early rhyolite (unit rer; 1–25 cm), of hornblende-biotite-plagioclase vitrophyre 

(unit rmk; 4–13 cm), and of similarly crystal-rich rhyolitic pumice (unit rwm, as big as 

12 cm). All were probably expelled during explosive phases of rhyolitic units rwm and 

rmk, both of which vented ~1.5 km southwest

msd Basaltic trachyandesite scoria southeast of Dry Creek (late Pleistocene)—Phenocrystpoor scoria (51.8–53.0% SiO2

) exposed poorly in 100-m-wide window at northwest end 

of saddle between Domes 2861 and 2781. Largely obscured by colluvium from adjacent 

domes; no stratified cinders or lava flows are exposed; instead, site is a scattering of 

scoria blocks and bombs as big as 50 cm, finely to coarsely vesicular, mostly dark gray 

but some oxidized brick- red. Phenocrysts: <1% plagioclase (0.5–5 mm; many probably 

xenocrysts) and trace of olivine (<1 mm). Base not exposed. Overlain by glacial erratics 

and by colluvial debris from both domes (units d61 and d81), which appear to be older. 

Chemically and petrographically similar to presumptively comagmatic units mnd and 

mor (66±2 ka), for which unit is inferred to mark source vent. Undated

msj Basaltic andesite of Scenic Loop Road junction (late or middle Pleistocene)—Poorly 

exposed, moderately porphyritic lava flow (52.8% SiO2) that forms a pumice-mantled 

shelf (UTM 2785/754) adjacent to west lane of Highway 395, ~300 m north-northwest 

of its junction with Scenic Loop Road. Shelf, with only ~15 m relief, protrudes ~200 m 

eastward from beneath high bluff of unit mnd. A few weathered blocks of lava are 

exposed through pumice cover. Phenocrysts: ~3% olivine (0.5–1 mm, some with reaction 

rims); sparse plagioclase (1–4 mm); rare cpx mph. Compositionally and petrographically 

unique in map area, lava is transitional between subalkaline and alkaline fields; contains 

7% MgO, only 5.2% alkalies, and low TiO2 (1.29%). Overlain by unit mnd. Base not 

exposed. Undated

mss Basaltic trachyandesite of upper Soda Springs (late Pleistocene)—Phenocryst-poor 

lava flows (52.7–53.2% SiO2) that erupted at severely eroded scoria cone exposed 

on west bank of Middle Fork San Joaquin River, opposite upper Soda Springs Campground. Riverbank exposure of stratified scoria is 8–25 m high, black to yellow-orange, 

thin-to-thick bedded, and includes three layers of dense agglutinate (each 2–3 m thick); 

scoria are predominantly lapilli but include blocks as big as 35 cm. Because at least half 

of cone has been eroded away by river, all layering dips 15–20o W; base of scoria is not 

exposed. Two vertical dikes, respectively <1 m and ~5 m thick, strike east-northeast 

and cut scoria section on riverbank; at rim, thicker southern dike grades into dense 

Description of Map Units—Quaternary Volcanic Rocks 101
102 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

agglutinate, which in turn grades into a thin lava flow. From cone, eroded remnants of 

lavas extend 300 m upstream and ~1 km downstream, preserved as a narrow ice-scoured 

strip (100–250 m wide) between Pacific Crest Trail and river. Lava flow surface is rolling 

glaciated plateau marked by knobs and ledges cut on a single downstream flow. Unit 

is as thick as 40 m near vent but thins (largely owing to erosion) to <10 m both upstream and downstream. Lavas and dikes are massive, hackly, chunky, or block-jointed. 

Phenocrysts: 1–3% plagioclase (1–3 mm) and <1% olivine (0.5–1 mm, rarely 2 mm). 

Some exposures ~400 m downstream from dikes carry as much as 5% plagioclase but 

are chemically indistinguishable from rest of unit. Cpx is absent (or extremely rare) 

in contrast to overlying unit mdp. Overlies Mesozoic granite (unit Kmo). Overlain 

by phenocryst-richer unit mdp (82±1 ka), which has certain compositional affinities 

(fig. 20) with lower-SiO2 unit mss. Nonetheless, in addition to differences in age, chemistry, mineral content, and paleomagnetic direction, units mdp and mss differ in Pb, Nd, 

and Sr isotope ratios (Cousens, 1996); for example, 87Sr/86Sr = 0.70613 and 0.70596, 

respectively. 40Ar/39Ar age: 121±2 ka

rbt Bishop Tuff (middle Pleistocene)—Rhyolitic ignimbrite, voluminous compositionally zoned 

pyroclastic unit (71–78% SiO2), eruption of which permitted collapse of Long Valley 

caldera (Gilbert, 1938; Hildreth, 1979; Bailey, 1989; Wilson and Hildreth, 1997; Hildreth 

and Wilson, 2007). Variably welded outflow sheets crop out radially around caldera; 

concurrently erupted fall deposit is preserved patchily from Pacific Ocean to Nebraska. 

Intracaldera thickness beneath postcaldera volcanics and sediments exceeds 1 km, as 

recorded in many drill holes (McConnell and others, 1995). Total eruptive volume of 

equivalent magma estimated to have been 600–650 km3 (Hildreth, 2004). Within map 

area, main outcrops are (1) nonwelded to densely welded outflow sheet along north wall 

of caldera; and (2) glaciated remnants along walls of Middle Fork San Joaquin River 

canyon—including nonwelded to sintered exposures east of Sotcher Lake, sintered to 

densely welded ledges southwest of Pumice Butte, and densely welded (vitric and devitrified) zones near Reds Meadow, Boundary Creek, Crater Creek, King Creek, and Fish 

Creek. Remnants are generally steep, and some welded zones are columnar jointed. More 

than 95 percent of the ignimbrite originally deposited along San Joaquin canyon has 

been removed by glacial and fluvial erosion. Thicknesses preserved generally increase 

downcanyon from ~160 m near Reds Meadow to ~195 m near King Creek and 230 m 

southwest of Pumice Butte. Unit is also exposed along north edge of map area where 

many outcrops range from sintered to densely welded; nonwelded base is exposed in 

Highway 395 roadcut at UTM 242/813. Accidental clasts of welded Bishop Tuff as big 

as 50 cm are locally abundant in intracaldera pyroclastic deposits (subunits rtav, rtcd) of 

early postcaldera rhyolite (unit rer), which had excavated them explosively from the caldera fill hundreds of meters deeper. Phenocrysts: total content zoned from ~1% to ~25%, 

including sanidine, quartz, plagioclase, biotite, allanite, Fe-Ti oxides, zircon, and (in late 

subunits) antecrysts of cpx and opx (Hildreth, 1979; Hildreth and Wilson, 2007). Rests 

on Mesozoic granitoids, Paleozoic metasedimentary rocks, glacial till (unit gst), alluvium 

(unit pal), and various Tertiary mafic and intermediate lavas. Overlain in map area by 

units a62, amp, apb, drf, and gud (and by many more outside map area and beneath 

caldera floor). 40Ar/39Ar age: 767±2 ka (Rivera and others, 2011)

rcd Rhyolite of Cratered Dome (Holocene)—Disrupted rhyolite minidome (73.2% SiO2) on 

north bank of Glass Creek, 150 m north of Glass Creek flow (unit ric); extruded through 

Mesozoic granitoid at foot of Sierran escarpment on trace of Hartley Springs Fault. Dome 

is <150 m in diameter, and its craggy rim stands only 10–15 m above its external base. 

Most of dome center, however, is now a closed funnel-shaped crater 25–30 m deep and 

~100 m wide at its rim. An annular terrace 5–10 m wide rings much of inner pit, ~10 m 

below its rim and ~18 m above its floor. Coarsely block-jointed flow-foliated rhyolite 

forms ragged narrow rim and steep inner walls of crater; foliation was mapped by Mayo 

and others (1936). Rock is glassy, micropumiceous, pale gray, and weathers tan to pale 

orange-brown; its groundmass is rich in microlites. Phenocrysts: 1–2% feldspar (mph to 

1.5 mm), some in clusters; trace amounts of biotite, opx, cpx, and Fe-Ti oxides (all mph). 

Unit is chemically similar to evolved end of zoned array of phenocryst-poor member of 
unit ric erupted in 1350 C.E. (Sampson and Cameron, 1987). Cratered Dome is older, 

however, as its floor and parts of its rim are covered by 1350 C.E. pumice-fall deposit. 

Apparent absence of tephra from nearby South Mono and Wilson Butte eruptions, each 

~1.3 ka (Bursik and Sieh, 1989), suggests that cratering (if not necessarily eruption) 

of Cratered Dome postdates those events. Coarse ejecta around crater, dominated by 

a sheet that extends ~150 m south and southeast of rim, include granitoids, Mesozoic 

metavolcanic rocks, and mafic lavas as well as juvenile rhyolite. Some blocks are as big 

as 50 cm; abundance of rounded ones suggests (at least partial) derivation from underlying till. Rhyolite debris appears inadequate to account for crater volume, suggesting that 

magma withdrawal could have contributed to crater formation. Undated but apparently 

bracketed between 700 C.E. and 1350 C.E.

rce Alkalic rhyodacite of Canyon Express (late Pleistocene)—Moderately porphyritic lava-flow 

apron (70.3–70.8% SiO2) that emerges from beneath unit rss west of Lincoln Peak and 

spreads northward for 2 km to foot of Dome 2781(unit d81) and eastward for 2.5 km to 

east-facing bluffs (Timber Ridge) southeast of Canyon Lodge. Upper terminus of Canyon 

Express (Chair 16) is built on this rhyodacite at 2,910 m elevation near northwest base 

of Lincoln Peak. Exposures include black to gray flow-foliated vitrophyre and extensive 

tracts of cream-white massive felsite; locally oxidized pink or weathered brown, especially along vuggy foliation planes. Some outcrops form bold ledges and nubbins; others 

are crumbly vitrophyre surfaces bulldozed smooth for ski runs. Unit everywhere glacially 

eroded, strewn with till and colluvium, and widely concealed by such deposits distally. 

Phenocrysts: 4–7% feldspar (0.5–3 mm); 1–2% biotite (mph to 2 mm); trace Fe-Ti-oxide 

mph. Also carries rare mafic blebs, feldspar clusters, and biotite-oxide-feldspar clots. Vent 

inferred to underlie unit rss at Solitude Dome. Consistently distinguished from nearby 

units rmf and rss by lower Fe and Sr contents and higher Nb, Zr, K, and Na. Overlies or 

banks against unit d81 (99±7 ka) and Canyon Lodge scoria cone (unit mcl; 175±3 ka). 

Overlain by units rsq (63.7±4 ka), rss (50±3 ka), dfl (61±3 ka), and dlp (64±7 ka). 40Ar/39Ar ages: 81±1 ka (Mahood and others, 2010); 80±1 ka (table 2)

rcw Rhyolite of caldera wall (Holocene)—Sparsely porphyritic rhyolite minidome (72.2% SiO2) 

on northwest wall of Long Valley Caldera, <100 m west of southwestern snout of Glass 

Creek flow (unit ric). Tiny forested protrusion, only ~50 m wide and 20 m high, perched 

partway up wall at 2,440–2,460 m elevation, erupted through scree-mantled Mesozoic 

granite (unit Kjl) just west of trace of Hartley Springs Fault. Rock is pale gray, glassy, 

micropumiceous, and carries abundant microlites. Phenocrysts: 1–2% feldspar (mph to 

1 mm), some in clusters; <1% biotite (mph to 1 mm); trace hornblende (mph to 1.5 mm 

long); trace cpx (mph to 0.7 mm long); trace tiny Fe-Ti-oxide mph; also contains rare 

mafic blebs as big as 5 mm that carry pyroxene and oxide mph. Unit is chemically similar 

to evolved end of zoned array of phenocryst-poor member of unit ric that erupted in 1350 

C.E. (Sampson and Cameron, 1987). Age unknown, but mantle of Inyo tephra and absence 

of South Mono and Wilson Butte tephras appear to bracket its extrusion (like that of unit 

rcd) to between 700 and 1350 C.E. (Sampson, 1987; Sampson and Cameron, 1987)

rdc Rhyolite of Dry Creek Dome (late Pleistocene)—Phenocryst-rich steep-sided rhyolite lava 

dome (76.0–76.4% SiO2) between West Moat Coulee (unit rwm) and Dry Creek, centered 

6.5 km northeast of Mammoth Mountain summit (fig. 23). Dome is circular in plan, 1.2 

km in diameter, and has as much as 240 m relief—greatest on its northwest side. Summit 

plateau, ~700 m wide, is marked by small rifts and knobs that provide 10–40 m of local 

relief. Unglaciated surface consists of cream-to-white, tan-weathering, micropumiceous 

carapace, locally block jointed. Phenocrysts: 10–15% feldspar (0.5–1.5 mm; plagioclase 

and sanidine); ~3% quartz (0.5–1.5 mm), many embayed; 2–3% biotite (0.1–1.5 mm); 

sparse hornblende (≤0.5 mm); trace amounts of Fe-Ti oxides, zircon, apatite, and allanite. 

Intrudes and overlies unit rwm. Late Pleistocene till (unit gdc) and avalanche deposits 

(unit av) from Dome 2861 (unit d61) bank against west toe of dome. K-Ar age: 110±11 

ka (Mankinen and others, 1986). 40Ar/39Ar age: 112±6 ka (Heumann and others, 2002)

rdm Rhyolite of Deer Mountain (late Pleistocene)—Phenocryst-rich lava dome (71.9–72.1% 

SiO2) centered 7.5 km north of Mammoth Mountain summit and 2.5 km inside northwest topographic wall of Long Valley Caldera. Steep-sided dome is nearly circular in 

Description of Map Units—Quaternary Volcanic Rocks 103
104 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

plan, 900 m in diameter, and ~180 m high. Exposures are largely of glassy, white, finely 

pumiceous carapace, but outcrops are limited to areas near summit and upper west slope 

of dome, owing to extensive cover by late Holocene phreatic ejecta from Inyo Craters; 

denser vitrophyre layers and lenses are present locally. North and South Inyo Craters 

lie at south toe of Deer Mountain, and two additional phreatic craters (roughly contemporaneous with them) were excavated through its summit and its lower north flank 

(Mastin, 1991). Total phenocrysts 25–35%. Contains 10–15% feldspar (both plagioclase 

and sanidine); 10–12% quartz (rounded and embayed, commonly broken, 0.2–4 mm); 

~3% biotite (mph to 1.5 mm); ~1% hornblende (mph to 1.5 mm); trace tiny Fe-Ti-oxide 

mph. Also contains trace amounts of cpx, opx, zircon, allanite, apatite, and pyrrhotite 

(Mayo and others, 1936; Reid and others, 1997). Appears to overlap and partly cover 

adjacent, severely eroded, scoria cone of unit mdm. 40Ar/39Ar age: 101±8 ka (Heumann 

and others, 2002). K-Ar age: 115±3 ka (Mankinen and others, 1986)

rer Early rhyolite of Bailey (middle Pleistocene)—Aphyric to sparsely porphyritic lava flows, 

domes, and predominant pyroclastic deposits (74–75% SiO2) that represent earliest 

episode of postcaldera volcanism in Long Valley Caldera. Erupted from several separate 

vents and exposed principally on resurgent uplift and Lookout Mountain, as depicted by 

Bailey (1989). 

Vents mostly obscured by lavas that retain little primary morphology; several are 

indicated on map sheet by stars, and others are probably downfaulted beneath graben 

system and Fumarole Valley.

Drilling shows assemblage to be as thick as 622 m near center of its outcrop area; 

and even where deeply buried by younger units, wells prove it as thick as 537 m beneath 

west moat and >350 m beneath southeast moat. Volume roughly 100 km3 (McConnell and 

others, 1995; Hildreth, 2004). Phenocrysts: 0–2%, including combinations (in various 

eruptive units) of plagioclase (0.5–1.5 mm), opx (0.2–0.7 mm), rare biotite (<1 mm), 

and traces of apatite, zircon, titanomagnetite, ilmenite, and pyrrhotite (Bailey, 1978). 

Relatively mafic enclaves and blebs (0.5–6 cm; 51–59% SiO2) are very rare but found in 

four different lava flows of unit rer. Eight K-Ar ages, all for glass from obsidian flows, 

range from 751±16 ka to 652±14 ka (Mankinen and others, 1986). McConnell and others 

(1995) gave a 40Ar/39Ar plateau age of 675±9 ka for an obsidian, corroborating the K-Ar 

age of 680±29 ka for same lava flow. Areas of acid alteration and strong silicification are 

patterned on maps of Cleveland (1962) and Bailey (1989) and not repeated here

rlm Lookout Mountain subunit is a unique element of Early rhyolite, forming a discrete rhyolitic shield 3–4 km in diameter with as much as 340 m of relief that separates caldera’s 

west moat from its north moat and is not deformed by resurgent uplift, lying just outboard 

of the resurgent structure. Extraordinarily for a rhyolitic volcano, edifice has shallow 

summit crater ~1 km wide. Outcrops along crater rim are predominantly obsidian that 

form a series of knolls and ridges separated by five shallow saddles and notches. Devitrified exposures are few, and pumiceous carapace facies is preserved on rim only locally, 

just east of northeast rim notch. Sparse felsite (exposed principally on east rim) is flowfoliated, pale gray or tan, and mostly dense, although locally vuggy or brecciated. Outer 

slopes of edifice are largely mantled by thick colluvium, but sparse exposures are mostly 

obsidian. Pumiceous carapace crops out locally on north slope and north lobe; pyroclastic 

deposits are rare on Lookout Mountain, which is armored by lava flows in all sectors. 

Southeast lava-flow lobe that extends 2 km beyond Dry Creek gorge is compositionally 

identical to edifice. All rhyolite facies on Lookout Mountain are aphyric; rare plagioclase 

microphenocrysts (<0.5 mm) are present locally, as are 0.5-mm aggregates of 0.1-mm 

crystals of orthopyroxene and plagioclase; mafic enclaves were sought but none found. 

Low-relief crater floor is wholly covered by colluvium, Inyo pumice, and forest litter. 

No explosion breccia is exposed that might account for crater. Edifice was not glaciated, 

crater floor is not gullied, and narrow rim notches are only weakly incised, rendering 

unlikely an erosional origin for crater. Creation of a summit depression by subsurface 

magma withdrawal or by construction of a peripheral ring of several discrete extrusions 

would be mechanisms extraordinary for a rhyolite volcano. A speculative possiblity is 

that effusion of the southeast lava lobe from a flank vent permitted summit subsidence. 
K-Ar ages: 692±14 ka; 677±14 ka (Mankinen and others, 1986). 40Ar/39Ar age: 678±9 ka 

(Simon and others, 2014)

rtav Tuff of Little Antelope Valley subunit is a major component of Early rhyolite (partly 

equivalent to unit Qet of Bailey, 1989). Consists predominantly of white nonwelded 

high-silica-rhyolite ignimbrite, massive to vaguely stratified, but locally includes intervals a few meters thick in which thin pumice-fall and hybrid-fall layers (Wilson and 

Hildreth, 1998) are intercalated. Although widely obscured by colluvium, unit extends 

9 km from Dry Creek Sand Flat nearly to Hot Creek, covering ~20 km2 of map area. 

From map pattern and drillhole data, tuff is inferred to be present over a far greater subsurface area (in all directions), where buried by associated rhyolite lava flows (unit rer), 

by various mafic lava flows, and by lacustrine siltstones that were in large part reworked 

from it. Exposed tuff is widely 50–300 m thick, although few individual outcrops 

exceed 40 m. In several drill holes, tuff is ~150–560 m thick, but proportions of primary 

ignimbrite and reworked rhyolitic siltstone are seldom clear from core logs. Outcrops are 

white to cream, weakly to moderately indurated but locally case-hardened or pervasively 

silicified, and uncommonly medium-gray or green where altered. Exposures are nonwelded, but partial welding is reported in some cores (McConnell and others, 1995). Tuff 

is poorly sorted, ash-dominant, and rich in small pumice fragments— mostly 0.2–2 cm 

and rarely 3–10 cm. Where tuff is not massive, vague gradational layers, 3–150 cm thick, 

are defined by alternating pumice-poor and pumice-bearing intervals. Pumice lapilli and 

granules are concentrated into lenses or thin layers only locally. Locally exposed within 

otherwise massive tuff sections, however, are packages 1–2 m thick of thin (2–20 cm) 

planar beds rich in pumice granules separated by 1–2 cm laminae of medium ash. All 

pumice is aphyric or nearly so, containing only trace amounts of opx, biotite, and plagioclase. Lithic fragments are typically sparse to rare, and nearly all are cognate aphyric rhyolite lava, mostly glassy, and smaller than 2 cm; exceptions include rare (vent-proximal?) 

outcrops of massive ignimbrite that carry distributed rhyolitic lithics as large as 8 cm and 

of stratified ignimbrite containing clast-rich layers 1–2 m thick that are rich in 5–15 cm 

fragments of rhyolitic lava. Clasts of densely welded phenocryst-rich Bishop Tuff, 

5–50 cm and subrounded to subangular, are sparsely distributed but locally abundant in 

nonwelded massive ignimbrite, evidently explosively excavated from intracaldera welded 

tuff several hundred meters below modern surface. Base of unit is nowhere exposed but is 

shown by drilling to rest on intracaldera Bishop Tuff in several wells. Tuff is overlain by 

rhyolite lava flows of units rer and rlm and by rhyolitic sandstone of Long Valley Lake 

(unit ss). Along eastern limits of its outcrop, near Clay Pit and Little Hot Creek, tuff is 

overlapped by and may grade into sedimentary pumice breccia and ash-grade lacustrine 

siltstone, both of exclusively rhyolitic derivation. Not directly dated, but bracketed by 

Bishop Tuff (767±2 ka) and an Early rhyolite obsidian flow (751±16 ka)

r68 Pumice Cone 2368 subunit is a vent knoll of nearly aphyric Early rhyolite (UTM 340/740) 

~2 km north of Little Antelope Valley. Exposed cone is 800 m long, 450 m wide, elongate 

northeast, and has ~100 m maximum relief at its southwest end. Pumice clasts (74.7% 

SiO2) are closely packed, mostly subangular, and seriate from granules to predominant 

lapilli, with subordinate blocks. Pumice is white, weathering tan, and frothy to finely 

vesicular with both equant and fibrous vesicles. Exposures lack interstitial ash and layering. Pumiceous carapace of flow-foliated lava is locally exposed on southwest rim and 

in excavation near summit. Most clasts are 1–5 cm (rarely 10 cm) along northeast ridge, 

but commonly 10–35 cm on pair of low knolls along crestal plateau; there, rare shattered pumice blocks were originally as large as 1 m. By contrast, few pumice clasts in 

surrounding unit rtav, which underlies the cone, are larger than 2 cm. Total phenocrysts 

<1%; plagioclase (0.2–0.7 mm), opx (0.3–0.6 mm long), biotite (0.2–0.5 mm), and trace 

Fe-Ti oxide mph (~0.1 mm). Undated but presumed to be in age range of Early rhyolite 

(unit rer)

rtcd Tuff east of Casa Diablo subunit is discrete exposure of Early rhyolite ignimbrite, 700 m 

by 2,000 m across, between Fumarole Valley and Casa Diablo geothermal plant. Total 

exposed relief ~150 m. Dominantly massive white ash, unit lacks obvious layering and 

carries sparse aphyric pumice clasts, few of which exceed 2 cm. Abundant lithic content 

Description of Map Units—Quaternary Volcanic Rocks 105
106 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

distinguishes unit from unit rtav, which is generally lithic-poor. Contains subangular to 

subrounded clasts (10–100 cm) of welded Bishop Tuff (unit rbt); abundant cognate clasts 

(1– 30 cm) of flow-foliated Early rhyolite lavas—felsitic, perlitic, and obsidian; and 

sparse blocks of Mesozoic metavolcanic rock (as big as 40 cm) and of grussy granodiorite (as big as 65 cm). All accidental lithics appear to be ejecta, not glacial erratics, 

although some may have come from Sherwin Till beneath intracaldera Bishop Tuff. Unit 

is thought to represent explosive opening phase of Early rhyolite eruption that produced 

obsidian flow that overlies its north margin. Tuff is also overlain by another Early rhyolite lava flow to its south. Tuff not directly dated, but obsidian flow interpreted as coeruptive gave K-Ar age of 688±14 ka (Mankinen and others, 1986)

rfp Alkalic rhyodacite pumice fall of Highway 203 (late Pleistocene)—Lapilli-fall deposits of 

well-inflated pumice (69.5–70.9% SiO2) exposed in roadcuts north and east of Mammoth 

Mountain. Sparse, isolated exposures dominated by white pumice lapilli are 3–6 m thick. 

Excavation of complete section at location A (see below) provided evidence for several 

eruptive pulses and one substantial time break (fig. 25). (1) lapilli fall, 63 cm thick, complexly bedded; capped by a pyroclastic density-current deposit 20 cm thick. (2) Lapilli 

fall, 77 cm thick, with grain-size fluctuations indicating six eruptive pulses; capped by a 

14-cm-thick paleosol that contains colluvial blocks of lava-flow unit d81. (3) Lapilli fall, 

>4 m thick, divisible into 35-cm basal fall, 12-cm-thick dark-colored fall layer with ~30% 

lithic fragments, and >367-cm pumice fall that is crudely inversely graded, vaguely stratified, and far coarser than any layers below. Base of section rests on ~0.5 m of crudely 

stratified colluvium, which, in turn, overlies pumiceous lava carapace of unit d81 dome. 

Most pumice clasts are 1–5 cm across, although small fraction in top unit are 10–29 cm; 

most are pale gray or white to buff, oxidized pale pink only in cores of large clasts. Lithic 

fragments, which make up 2–3% of total deposit, are predominantly phenocryst-rich 

trachydacite and common but subordinate alkalic rhyodacite of Mammoth Mountain; 

clast counts yield 94% Mammoth Mountain lavas, 4% mafic lavas, and 2% metavolcanics (n=321). Most lithics are <1 to 8 cm across, rarely 9–16 cm. Phenocrysts: 4–6% 

plagioclase and sanidine (0.5–3 mm); 1–2% biotite mph; sparse tiny Fe-Ti oxide mph. 

Outcrops are too small to show on geologic map. Principal locations are: (A) roadcut on eastward salient of Highway 203 at elevation 2,545 m (UTM 2405/69135; best 

complete section; fig. 25); (B) three roadcuts along north side of Highway 203 at elevations 2,560 m (UTM 2393/6892), 2,665 m (UTM 223/686), and 2,683 m (UTM 220/687); 

(C) roadcut on north side of Forest Trail (residential street) at elevation 2,480 m, ~350 m 

west of Highway 203 (UTM 248/688); (D) disturbed (possibly transported) pumice 

lapilli strewn around abandoned former maintenance yard (UTM 295/675) at 2,295 m 

along north side of Highway 203, ~1 km east of its junction with Meridian Boulevard; 

and (E) primary pumice fall (fig. 25), weakly inversely graded, as thick as 91 cm, on 

floor of old gravel quarry (UTM 403/717) ~300 m south of Cashbaugh Ranch buildings; 

here, 19 km east of source vent, deposit underlies ~8 m of unit oal, and its white pumice 

weathers buff-yellow and ranges in grain size from 0.2 to 4 cm. (Another pumice-fall 

deposit atop roadcut on northbound side of Highway 395 [UTM 2727/7668; assigned to 

this unit by Bailey, 1989; his unit rmp] is 1350 C.E. Inyo fallout). Vent is certainly on 

Mammoth Mountain edifice, probably beneath Solitude Dome, along with that for unit 

rce, lavas of which are chemically and mineralogically similar to unit rfp pumice lapilli. 40Ar/39Ar ages: 80±8 ka (table 2); 88±6 ka, 91±2 ka (Mahood and others, 2010)

rhc Rhyolite of Hot Creek flow (middle Pleistocene)—Phenocryst-poor coulee (76.0–76.2% 

SiO2), as wide as 2.5 km and 5 km long, in south moat of Long Valley Caldera. Equivalent to part of unit Qmrh of Bailey (1989). Erupted at subaerial vent west of Whitmore 

Hot Springs, coulee flowed northward where much of it invaded Pleistocene Long Valley 

Lake (Mayo, 1934b; Bailey and others, 1976), producing widespread flow breccia and 

perlite. Steep-sided coulee has 100 m of relief proximally, and its upper surface is hilly 

and craggy, but distal exposures have low relief and are partly lapped by lake sediments (unit ss). Lithologically, rhyolite is flow-foliated and variously includes black 

vitrophyre, gray to pink felsite, coarsely pumiceous carapace, pale gray perlite, and flow 

breccia. Strongly oxidized and hydrothermally altered exposures are common along Hot 
Description of Map Units—Quaternary Volcanic Rocks 107

Hwy 203 roadcut

(UTM 3241/41691)

Gravel pit south of 

Cashbaugh Ranch

(UTM 34038/417186)

Pumice lapilli fall;

coarsest at top 

(MP~4 cm): lithic-rich 

at base

Yellow-brown sand

Top eroded

>367

Thickness

(cm)

>91 cm

35

14

25

38

20

15

9

5

5.5

42

48

lithic-rich

pink

grey

vf pink ash 

coating lapilli

Top eroded

Coarse pumice lapilli fall with 

very lithic-rich base: 

MP~25 cm

3-part lithic-rich pumice lapilli 

fall: top has MP~3 cm

Paleosol: with colluvial clasts

Lithic-rich pumice lapilli fall: 

reversely graded, MP~5 cm 

3 lithic-rich pumice fall 

intervals: nongraded with fine 

sand tops; lowest interval MP 

~5 cm

Sand+granule base, fine sand 

top

2 pumice lapilli falls with fine 

ash tops: lower lithic-poor, 

MP~ 1 cm; upper lithic-rich, 

MP~2 cm

Pyroclastic flows?

Fine sand layers

2 pulses lithic-rich pumice 

lapilli fall: lower MP~ 2–4 cm, 

sandy matrix in top 2 cm; 

upper MP ~0.5 cm

Lithic-rich pumice lapilli fall: 

layering from fluctuations in 

grain size (MP~ 2 cm); sandy 

matrix in top 2 cm

Colluvium, crudely stratified

Pumiceous carapace; 

unit d81

?

Figure 25. Stratigraphic logs of pumicefall deposit of unit rfp at Highway 203 

roadcut near foot of Mammoth Mountain 

and at quarry near Cashbaugh Ranch, 

17 km east. Principal fall unit thins 

eastward from >4 m to ~1 m. Large 

pumice clasts from Cashbaugh site and 

from 15-cm, 25-cm, 35-cm, and thick 

topmost fall layers at Highway 203 site 

are all compositionally similar: 70.2–70.7% 

SiO2, 0.20–0.28% MgO, 99–114 ppm Rb, 

and 372–430 ppm Zr. Lithic fragments 

throughout fall deposits are nearly all 

Mammoth Mountain lavas. MP, maximum 

pumice size—average diameter of five 

largest clasts found in layer.
108 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Creek gorge, which incises coulee distally and remains present-day site of boiling pools, 

geysers, and fumaroles (Farrar and others, 2007). Phenocrysts: ~1% total, of which about 

half is plagioclase (0.5–1.5 mm), along with trace amounts of sanidine, biotite, and cpx, 

as well as microphenocrysts of Fe-Ti oxides, zircon, and apatite. Base not exposed, but 

drill hole CH-10B penetrated lake sediments beneath north-distal part of lava flow at a 

depth of 60 m. Overlain by rhyolitic sandstone and pebble conglomerate of Pleistocene 

Long Valley Lake (unit ss). Basalt of Fish Hatchery (unit bfh; 92±2 ka) and late Pleistocene alluvium (unit oal) bank against unit rhc coulee. K-Ar age (for glass): 288±31 ka 

(Mankinen and others, 1986). U-Pb age for zircon ~300 ka (J.A. Vazquez, unpublished 

data). 40Ar/39Ar age for sanidine: 333±2 ka (Simon and others, 2014).

 Subunit rhc' represents two small knolls of brecciated high-silica rhyolite that crop 

out near the Hot Creek coulee and are compositionally identical to it. (1) Knob 2122 

(UTM 394/714), on north side of Antelope Springs Road southwest of Cashbaugh Ranch, 

is 4-m-high mound, 25-m-wide, of unstratified all-rhyolite breccia (76.2% SiO2) ~1.3 km 

north of apparent terminus of main coulee. Breccia may have avalanched across shallow 

lake or mudflat from advancing flow front. Alternatively, because outcrop is surrounded 

by lake silt, it could be an exposure of the coulee itself if its distal 1.3 km reach is now 

sediment-covered. (2) A small knoll (UTM 401/6615) next to carpark at Whitmore Hot 

Springs, 100 m east of margin of Hot Creek coulee, is similar rhyolite breccia (76.1% 

SiO2) mixed with lake sediment. Exposure is ~20 m wide and has only 2–3 m of relief. 

Because it is unlikely that such tiny masses are independent extrusions, we attribute them 

to the Hot Creek eruptive episode

ric Rhyolite and rhyodacite of Inyo chain (1350 C.E.)—Compositionally complex lava flows 

and pumiceous ejecta (70–74% SiO2) that erupted from a dike-fed, 5-km-long, northsouth chain of three separate vents (Miller, 1985) in late summer of 1350 C.E. (Millar 

and others, 2006). (1) Southern vent released ~0.05 km3 of magma as pyroclastic flows 

(>10 m thick proximally), which extend as far as 6 km from vent, as well as a similar 

volume as pumiceous fall deposits distributed in two narrow sequential lobes that extend 

>20 km northeast and >25 km south-southwest. These were followed by extrusion of 

~0.13 km3 of lava as Deadman Creek flow, pancake-shaped, weakly lobate flow on 

south bank of Deadman Creek, ~1 km inside northwest topographic wall of Long Valley 

Caldera. Barren, rugged, sprawling flow is nearly circular in plan, 1.3 km in diameter, 

and has flow fronts as high as 40 m and total relief of 170 m. Pyroclastic-flow deposits 

near Deadman Creek flow have poorly sorted, tan matrix of fine-to-coarse ash that carries 

scattered clasts, mostly lapilli but commonly 10–20 cm across, of wide textural variety, 

ranging from pumiceous to finely microvesicular to dense black vitrophyre. 

 (2) Northern vent was second to erupt, releasing ~0.02 km3 of magma as a narrow lobe of pumiceous fallout that extends >25 km northeast, followed by thin proximal 

pyroclastic flows. These were followed by extrusion of ~0.17 km3 of lava as Obsidian 

flow, just north of Glass Creek at foot of Sierran rangefront fault scarp. Rugged barren 

lava flow is 1.5 by 1.8 km across, and its steep margins are 30–50 m high, thickening to 

more than 100 m centrally. Because emplaced on a slope, its total exposed relief reaches 

~190 m. Its conduit lava remains as wide as 33 m where intersected by drilling at a depth 

of ~500 m (Eichelberger and others, 1984, 1985). 

 (3) Last of three magmatic vents to open was high on topographic rim of Long 

Valley Caldera, ~1.5 km south of vent for Obsidian flow and 3.5 km north of that for 

Deadman Creek flow. It released ~0.1 km3 of pumiceous fallout, as thick as 10 m proximally, that extends more than 190 km southward, by far most widespread and voluminous explosive product of 1350 C.E. sequence (“Tephra 1” of Wood, 1977; Miller, 1985). 

Along crest of White Wing Mountain (fig. 26), where fallout is 8 m thick, 2–4 km southwest of Glass Creek vent, tree-ring study of numerous trees killed by the eruption yielded 

an eruption age of 1350 C.E. (Millar and others, 2006). This outburst was followed by 

remobilized flows of pumiceous proximal ejecta (Miller, 1985), which are poorly exposed 

for several kilometers eastward along Glass Creek, and finally by extrusion of ~0.1 km3

of lava as rugged, blocky Glass Creek flow (fig. 26). From its vent on caldera rim, this 

700-to-900-m-wide flow spread ~500 m northward (to a steep northerly flow-front as 
high as 75 m) as well as ~1 km southward, descending 220 m in elevation to foot of 

caldera wall. 

 None of the four pumice-fall deposits (Nawotniak and Bursik, 2010) overlie any 

of the three still-barren lavas, although they blanket Devils Postpile region, Mammoth 

Mountain, west moat of caldera, and plateaus north of caldera, widely obscuring contacts among older units. Although one vent lies within topographic basin of Long Valley 

Caldera, all three vents are well outside structural ring-fault margin of caldera (Hildreth, 

2004). Although sometimes referred to as “Inyo Domes” or popularly called “Inyo 

Craters” (presumably in deference to phreatic craters that formed soon after magmatic 

sequence; see unit pe), all three 1350 C.E. effusive units are pancake-style flow-foliated 

lava flows with extensive low-angle surfaces of shattered craggy glass. Phenocrysts: 

products of crystal-poor magma (2–3% phenocrysts) dominate fall deposits and Obsidian 

flow. Crystal-rich lava (25–40% phenocrysts) makes up 30–40% of Glass Creek flow and 

more than 50% of Deadman Creek flow, its proportion thus increasing southward along 

chain. Central areas of latter two flows consist of relatively homogeneous, white, moderately vesicular crystal-rich phase (71.5±0.5% SiO2

), which grades radially into peripheral zones where it is either interlayered with crystal-poor lava or broken up into lenses, 

blobs, and streaks that were disaggregating into it when frozen (Sampson, 1987). Crystalpoor phase is texturally more heterogeneous, ranging from coarsely to finely vesicular 

to dense obsidian or microcrystalline felsite; moreover, it is chemically zoned (70–74% 

SiO2) as described by Sampson and Cameron (1987) and Vogel and others (1989) and 

Description of Map Units—Quaternary Volcanic Rocks 109

White Wing Mountain

Glass Creek flow

Figure 26. Glass Creek flow (unit ric) draping caldera wall 3 km west of Highway 395 at Crestview. Rugged rhyolitic coulee covers trace 

of Hartley Springs Fault, which separates Pliocene intermediate lavas that form caldera wall at right from Meozoic granite that forms 

wall at left. On skyline is granitic White Wing Mountain, thickly mantled with pumiceous fallout, much of it ejected from the nearby vent 

that subsequently extruded the Glass Creek flow. Tree-ring study of logs and stumps killed by the eruption atop White Wing Mountain 

pinned the event to late summer 1350 C.E. (Millar and others, 2006). View westward from surface of unit mcv (33 ka). Forested ridge at 

lower left is unit dnw.
110 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

reviewed by Hildreth (2004). Crystal-rich lava contains 15–20% plagioclase, 10–15% 

sanidine, 2% biotite, ~2% hornblende, ~1% quartz, ~1% titanomagnetite, and trace 

amounts of ilmenite, cpx, opx, apatite, zircon, and allanite. Feldspars are as big as 10 mm 

and biotite and hornblende are commonly >3 mm. Crystal-poor lava contains most of 

these phases but lacks quartz and allanite, and higher-silica samples along its zoned array 

also lack hornblende; few of its mafic phenocrysts are larger than 0.5 mm, and most of 

its feldspars are smaller than 2 mm (though rarely as big as 5 mm). Sparse fine-grained 

enclaves, dense to vesicular, <1 to 10 cm across, are distributed within crystal-rich 

rhyolite in high central parts of Deadman Creek and Glass Creek flows, where lavas can 

be inferred to represent latest material extruded from directly subjacent vents (Varga 

and others, 1990). Bulk enclave compositions are intermediate (59.5–60.7% SiO2), but 

their cargo of abundant disequilibrated phenocrysts from host rhyolite indicates that they 

are magmatic mixtures of mafic and silicic magma. Mafic contribution may represent 

seldom-seen source of long-lived thermal flux beneath west moat of Long Valley Caldera

rmf Alkalic rhyodacite of Mammoth Mountain fumarole (late Pleistocene)—Moderately 

porphyritic lava flow (69.9–70.7% SiO2) that forms 800-m-long dipslope that is referred 

to as Face Lift planèze (figs. 7, 12). It descends northward from 3,208-m knob to eroded 

flow front facing McCoy Station reservoir at 2,940 m. Erosionally isolated knob, 400 m 

north of Mammoth Mountain summit, is upper terminus of Face Lift Express (Chair 3). 

Dipslope forms glacially eroded planèze 200–350 m wide. Midway up dipslope surface, 

wispy fumarole issues from joints in lava (and from small artificial pipes) at ~3,030 

elevation. Below fumarole, strip of modest acid alteration extends ~200 m downslope, 

largely confined to surficial debris along a gulch and to films on fractures in the rhyodacitic lava. Flow is mostly dark- or medium-gray vitrophyre, strongly flow foliated, with 

subordinate layers devitrified, spherulitic, or locally lithophysal; layers are commonly 

2–10 cm thick, locally folded convolutely, and ramped into steep inward dips around 

flow margins. South-facing 25-m-high scarp of knob 3208 is acid-altered, which facilitated glacial excavation of 150-m-wide notch (The Chasm; fig. 12) that separates knob 

from north scarp of summit dome (units dfl and dsu). Phenocrysts: 10–12% feldspar 

(0.5–3 mm, rarely 5 mm); ~2% biotite (mph to 2 mm); ≤1% opx (0.3–1.2 mm); sparse 

Fe-Ti-oxide mph. Also carries mafic blebs and sparse clots of all these minerals in varied 

proportions. Overlies units dms (68±1 ka) and dfl (61±3 ka). Overlain by remnant outlier 

of unit dsu (61.4±2 ka). 40Ar/39Ar age: 61±1.5 ka

rmk Rhyolite of Mammoth Knolls (late Pleistocene)—Bilobate rhyolite dome (76.4–76.8% 

SiO2) centered 5.5 km east-northeast of Mammoth Mountain summit. Steep-sided 

unglaciated dome is 1 by 1.5 km wide and towers 200 m above north edge of Mammoth 

Lakes business district (fig. 23). Knobs on its rolling upper surface provide 10–20 m local 

relief. Southeast half has been described as a discrete contiguous dome but, being slightly 

lower, it may instead be a bulging protrusion rather than having its own independent 

vent. Exposures are glassy, finely vesicular to micropumiceous, and white to pale gray, 

weathering tan. At northeast side of dome, pumiceous ejecta form a 50-m-high half-ring, 

500 by 800 m across and ~300 m wide at its rim, which was largely constructed during 

eruption of unit rwm ~50 kyr earlier. Pumice lapilli (and blocks as big as 25 cm) of unit 

rmk are scattered on surface of older ejecta ring; they also form a linear rib ~3 m high, 

adjacent to toe of rmk dome, along crater floor inside rwm ring. Phenocrysts: ~15% 

feldspars (0.2–1.5 mm; more sanidine than plagioclase); 2–3% quartz (0.2–1.5 mm), 

many embayed; 1–2% biotite (mostly <0.5 mm, rarely up to 1.5 mm); sparse hornblende 

(<0.5 mm); and trace amounts of Fe-Ti oxides, zircon, apatite, and allanite. Intrudes and 

marginally overlies unit rwm. Late Pleistocene till of unit gmm banks against south base 

of dome. K-Ar ages: 97±6 ka (northwest lobe) and 106±2 ka (southeast lobe) (Mankinen 

and others, 1986). 40Ar/39Ar ages: 109±3 ka and 113±1 ka (northwest lobe) and 111±5 ka 

(southeast lobe) (Heumann and others, 2002; Mahood and others, 2010)

rnc Rhyolite of north-central chain (middle Pleistocene)—Northwest-trending alignment of 

phenocryst-rich rhyolitic domes and one lava flow (73.9–75.3% SiO2) that crosses north 

flank of resurgent uplift of Long Valley Caldera. Equivalent to unit Qmr1 of Bailey 

(1989). Called “moat rhyolite” by Bailey and others (1976) but erupted through Early 
rhyolite (unit rer) on central uplift, not in caldera moat. Like other Long Valley rhyolites, 

unit is subalkaline, contrasting with mildly alkaline domes of Mammoth Mountain. Four 

extrusive units are defined. (1) Minidome, exposed 2 km north of Little Antelope Valley; 

crops out only as a 5-m-high subdued ridge within an area 30 by 60 m (UTM 355/7435). 

Consists of glassy rounded blocks (75.3% SiO2), micropumiceous and pale gray, weathering tan. Near-primary surface has been exhumed from rhyolite-dominated ashy colluvium 

that surrounds it. (2) Dome 2395 (UTM 344/755) is 200 m in diameter and has 50 m 

of relief exposed on its south side. Blocky lava (75.2% SiO2) is medium-to-pale gray, 

weathers tan to orange-brown, and (in contrast to subjacent unit rer) displays little flow 

foliation. Brecciated marginal facies is exposed along steep southwest side. Contains 

phenocryst-poor mafic enclaves (2–15 cm; 54.9% SiO2) on its southeast nose. (3) Dome 

2415 (UTM 340/764) is 700 m in diameter, nearly circular in plan, and has ~145 m of 

relief exposed on its northwest slope. Blocky lava (74.6% SiO2) is glassy, massive or 

finely pumiceous, and pale gray, weathering tan to pink. (4) Coulee (73.9–74.3% SiO2), 

covering ~3 km2, extends 2 km west from Dome 2415 (its inferred vent site), then 4 km 

northeastward, descending north slope of caldera’s resurgent uplift into its north moat. 

Maximum relief exposed is ~60 m along gorge just west of dome; far western scarp of 

coulee, cut by canyon of Dry Creek, has 40 m of relief. Northeast tongue in moat generally has <10 m of exposed relief because lapped by alluvium that further covers most of 

its distal 1.3-km reach. Outlying knob (UTM 355/787) in north moat, 70 m diameter and 

8 m high, is distal extremity of coulee; massive to finely vesicular, glassy outlier is locally 

flow foliated or brecciated and carries sparse crystal-poor mafic enclaves (5–25 cm; 

54.7% SiO2). Parts of low-relief northeast tongue are strongly hydrated and disintegrate 

into glassy kernels, suggesting interaction with shallow caldera lake. Lower 10 m of coulee, well exposed along gorge of Dry Creek, is coarse lava-flow breccia (not agglutinated 

spatter fall as interpreted by Bailey, 1989). Milled during flow, rhyolite blocks are angular 

to subrounded, commonly 10–50 cm across, and as big as 1 m. Blocks are separated by 

crush breccia of smaller angular clasts, which are generally weakly to moderately adherent but locally strongly healed to ghost-breccia texture. Mafic enclaves (53.3% SiO2

) are 

common within rhyolite blocks, in interblock breccia, and widely elsewhere in nonbrecciated parts of coulee; enclaves are angular to rounded, 1–20 cm across, and contain sparse 

plagioclase and olivine crystals. 

Phenocrysts: each rhyolite subunit contains 10–15% feldspar (0.5–5 mm; both 

sanidine and plagioclase), variously euhedral, sieved, or composite; 1–3% hornblende 

(0.5–3 mm long); 1–2% biotite (0.2–1.5 mm); sparse rounded quartz as big as 1.5 mm; 

and rare tiny Fe-Ti oxide mph. Groundmass glassy, dense to finely vesicular; commonly having wavy, collapsed pumiceous texture. All subunits erupted through resurgently deformed unit rer or its pyroclastic subunit rtav. K-Ar age of sanidine from each 

subunit (Mankinen and others, 1986): (1) 505±15 ka; (2) 527±12 ka; (3) 523±11 ka; 

(4) 481±10 ka. 40Ar/39Ar ages of 570±8 ka were reported by Simon and others (2014) for 

sanidine from both (3) and (4).

A quartz-rich biotite-granite porphyry of bulk composition (73.9% SiO2) similar to that of unit rnc was penetrated beneath intracaldera Bishop Tuff in well CP-1 

(UTM 360/728) at depths from 1,420 m to 1,847 m (Suemnicht and Varga, 1988), 

~1.7 km south of minidome (1). At least 427 m thick (to bottom of hole), porphyry 

contains euhedral quartz (0.5–1.5 mm), shaggy chloritized biotite (0.5–5 mm long), clayaltered feldspar (mph to 2.5 mm), and sparse tiny Fe-Ti oxide mph. Lacking hornblende, 

it might be magmatically related to extrusive unit rer rather than to unit rnc, but it fails to 

provide a convincing chemical match for either 

rnd Rhyolite of North Deadman Creek Dome (Holocene)—Compact middle Holocene lava 

dome (74.7% SiO2) at foot of northwest topographic wall of Long Valley Caldera, on 

north bank of Deadman Creek, just north of barren and rugged Deadman Creek flow 

(southernmost of 1350 C.E. chain of Inyo lavas; unit ric). Forested dome is steep-sided, 

craggy, nearly circular in plan, 600 m in diameter, and has 130 m relief. Pale gray aphyric 

lava is glassy, block-jointed, flow-foliated, and micropumiceous to massive; sheds coarse 

talus of angular blocks. Volume of virtually uneroded dome is ~0.03 km3, and coerupted 

Description of Map Units—Quaternary Volcanic Rocks 111
112 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

ejecta (if any) are not exposed. Phenocrysts: none; but glassy groundmass is rich in 

feldspar and biotite microlites (<0.1 mm) with sparse feldspar-lath mph (0.1–0.3 mm). 

Apparently erupted through Mesozoic granite (unit Kjl), exposed in adjacent caldera wall. 

Overlain by Inyo pumice-fall deposits of 1350 C.E. Undated, but estimated to be roughly 

6,000 years old by Miller (1985) 

rrc Alkalic rhyodacite of Reds Creek (late Pleistocene)—Moderately porphyritic lava flow 

(70.9–71.2% SiO2) that crops out only in 200-by-300-m area along gorge of Reds Creek 

at southwest toe of Mammoth Mountain. Most of exposure is highly flow-foliated, 

even laminated, cream-white felsite that breaks slabby to platy and forms 20-m-high 

cliffs on both walls of creek; rusty films are widespread on foliation planes and cross 

joints, but only upstream-most end of unit is hydrothermally altered like overlying 

trachydacite (unit dsd) along Reds Creek. Lowermost parts of exposures on both banks 

are black vitrophyre, 3–8 m thick, hydrated and crumbly, but generally massive with 

sparse devitrified layers. Phenocrysts: 4–5% feldspar (0.5–2 mm); 1–2% biotite (mph to 

1 mm); trace pyroxene mph; sparse Fe-Ti- oxide mph; also contains sparse clusters of 

feldspar and biotite-feldspar clots. Base not exposed, but probably overlies nearby unit 

amp (97±1 ka), contact with which is obscured by scree rich in 1350 C.E. Inyo pumice. 

Overlain by units dsd (87±6 ka) and dwr (73±2 ka). Unit is richest in K2O and SiO2 and 

poorest in MgO and FeO of all units of Mammoth system. 40Ar/39Ar age: 83±1 ka

rsq Alkalic rhyodacite of Quicksilver ski run (late Pleistocene)—Moderately porphyritic lava 

flow (70.0–70.3% SiO2) that issues from beneath Solitude Dome (unit rss) and extends 

1.5 km eastward (south of Lincoln Peak) to flow front at ~2,750 m. Crops out patchily 

through colluvium and bulldozed ski-slope rubble as flow-foliated block-jointed vitrophyre, black to pale gray, but irregularly streaked tan or pink and locally lithophysal. 

Flow was glacially scoured and plucked distally into a few block-jointed ledges 5–20 m 

high. Phenocrysts: ~10% feldspar (mph to 3.5 mm); ~2% biotite (0.5–1.5 mm); sparse 

pyroxenes (0.2–0.7 mm); sparse Fe-Ti oxide mph; and rare hornblende mph. Flow is 

distinguished from Solitude Dome (unit rss) by paleomagnetic directions; flow also has 

consistently greater Ba and Zr abundances than unit rss and lower Ti, Mg, Rb, and Sr. 

Flow overlies unit rce (80±1 ka); also apparently overlies toe of adjacent Lincoln Peak 

dome (unit dlp; 64±7 ka), but thick colluvial wedge obscures contact. Flow is overlain by 

dome unit rss (50±3 ka). 40Ar/39Ar age of lava flow: 63.7±4 ka

rss Alkalic rhyodacite of Solitude ski run (late Pleistocene)—Moderately porphyritic Solitude 

Dome, knob 3160+, between Lincoln Peak and Mammoth Mountain summit. Dome 

(70.0– 70.6% SiO2) lies 500 m northeast of summit (figs. 1, 7), just across ice-excavated 

trough (The Chasm) that separates it from summit’s northeast face (which consists of 

units dfl and dsu). Flow foliation dips radially on slopes of dome and defines a broad 

open fold on dome’s south face; top central part of dome is more disrupted and may 

include a final breccia-enveloped protrusion, ~20 m wide and ~80 m long. Exposures are 

black to pale-gray vitrophyre, only locally oxidized, strongly flow-foliated but also commonly block-jointed; some domains are cream-white and microvesicular; spherulitic, 

lithophysal, and tan-to-pink flow layers are subordinate. Phenocrysts: 7–12% feldspar 

(0.5-5 mm); ~2% biotite (mph to 1.5 mm); ≤1% opx (0.5–1.5 mm, rarely as long as 

3 mm); sparse Fe-Ti-oxide mph. Also contains sparse feldspar clusters and biotiteoxide-pyroxene-feldspar clots. Overlies units rce (81±1 ka) and dsd (87±6 ka). Dome 

is distinguished from rhyodacite units rce and rmf by its higher Sr, Ca, Mg, Fe, Ti, and P 

abundances and lower K, Na, Nb, and Zr. Dome is distinguished from lava-flow unit rsq

by greater Mg, Ti, Rb, and Sr abundances, lower Ba and Zr, and by different paleomagnetic directions. Despite chemical and magnetic differences, alkalic rhyodacite units rce

(80±1 ka), rsq (63.7±4 ka), and rss evidently issued sequentially at a common vent site. 40Ar/39Ar age: 50±3 ka

rwm Rhyolite of West Moat Coulee (middle Pleistocene)—Major rhyolite lava flow (72.1–73.4% 

SiO2) that forms knobby plateau extending 5 km north-northeast from Mammoth Knolls 

to Dry Creek. As wide as 2.5 km proximally and 1.5 km distally, flow has steep margins, 

generally 50–80 m high. Knobs and ridges on its rolling unglaciated surface have 20 

m or more of local relief, much of it primary, although its roughness has been subdued 
by thick mantle of late Holocene Inyo pumice-fall deposit. Most outcrops are pale gray 

to white micropumiceous carapace that weathers tan; massive interior felsite is rarely 

exposed. In drillhole PLV-1 (fig. 6; Benoit, 1984), which was collared on mid-flow 

surface (550 m southeast of toe of Dry Creek Dome), flow is 574 m thick (more than 

5 times its visible thickness), suggesting either that it filled a deep paleovalley (unlike 

any drainage nearby) or, more likely, that drillhole extended down into intrusive domain 

that fed surface coulee. Terminal area of flow was cut by resurgent graben faults and 

eroded by Pleistocene Dry Creek. Outlying remnant of rhyolite unit (~50 m wide) caps 

mafic lava knoll 2351 (UTM 283/7483), ~750 m northeast of flow’s eroded terminus and 

40 m higher than alluviated graben floor that separates them. At surface, vent is marked 

by a 50-m-high half ring of pumiceous ejecta, 500 by 800 m across and ~300 m wide 

at its rim. Its juvenile ejecta are dominantly ash and lapilli but include finely vesicular 

blocks up to 30 cm across; accompanying blocks of pumiceous carapace (pale gray but 

commonly pink internally) are as big as 70 cm. Cognate lithic fragments are angular to 

subrounded, as big as 55 cm, and include dense gray vitrophyre and flow-foliated felsite. 

Sparser lithic fragments of mafic lavas, crystal-poor to crystal-rich and as big as 30 cm, 

range from subrounded to angular or slabby; some correlate chemically with units mkv, 

msc, and mcl. Absence among lithic ejecta of well-rounded cobbles or of granitoid and 

metavolcanic clasts suggests that mafic fragments are not derived from till or alluvium 

but from underlying lava flows. Pumice lapilli (and blocks as big as 25 cm) of unit rmk

are scattered across ejecta-ring surface; they also form a linear rib ~3 m high, adjacent to 

toe of rmk dome, along crater floor inside rwm ring. 

 Phenocrysts: 15% feldspar (plagioclase and sanidine; 0.3–3 mm); ~7% quartz 

(0.3–2.5 mm); 2–3% biotite (0.1–1 mm); ~1% hornblende (mostly 0.3–0.6 mm, rarely up 

to 2 mm long); sparse Fe-Ti oxides (0.1–0.2 mm); and rare pyroxene, zircon, apatite, and 

allanite mph. Conspicuously contains more hornblende than adjacent rhyolite units rdc

and rmk. Overlies units msc (154±2 ka), mcl (~175 ka), and bsm (165±2 ka). Intruded 

by and overlain by units rmk and rdc. Late Pleistocene till (unit gdc) banks against 

northwest margin of coulee. 40Ar/39Ar ages (all for sanidine): 163±2 ka (proximal) and 

161±2 ka (distal) (Mahood and others, 2010); 147±4 ka (proximal) and 151±4 ka (distal 

outlier) (Heumann and others, 2002). Trough (now traversed by Highway 395) that separates snout of coulee from compositionally identical outlier is part of resurgent graben 

system, which thus exhibits fault displacement younger than 150,000 years

Description of Map Units—Quaternary Volcanic Rocks 113
114 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

References Cited

Annen, C., Blundy, J.D., and Sparks, R.S.J., 2006, The genesis 

of intermediate and silicic magmas in deep crustal hot 

zones: Journal of Petrology, v. 47, p. 505–539.

Bailey, R.A., 1978, Volcanism, structure, and petrology of 

Long Valley caldera, California: Baltimore, Md., Johns 

Hopkins University, Ph.D. dissertation, 112 p.

Bailey, R.A., 1989, Geologic map of Long Valley caldera, 

Mono-Inyo Craters volcanic chain, and vicinity, eastern 

California: U.S. Geological Survey Map I-1933, pamphlet 

11 p., 2 sheets, scale 1:62,500. 

Bailey, R.A., 2004, Eruptive history and chemical evolution 

of the precaldera and postcaldera basalt-dacite sequences, 

Long Valley, California; Implications for magma sources, 

current seismic unrest, and future volcanism: U.S. Geological Survey Professional Paper 1692, 75 p.

Bailey, R.A., Dalrymple, G.B., and Lanphere, M.A., 1976, 

Volcanism, structure, and geochronology of Long Valley 

caldera, Mono County, California: Journal of Geophysical 

Research, v. 81, no. 5, p. 725–744.

Bailey, R.A., Huber, N.K., and Curry, R.R., 1990, The 

diamicton at Deadman Pass, central Sierra Nevada, California; A residual lag and colluvial deposit, not a 3 Ma 

glacial till: Geological Society of America Bulletin, v. 102, 

p. 1165–1173.

Bateman, P.C., 1965, Geology and tungsten mineralization 

of the Bishop District, Califonia: U.S. Geological Survey 

Professional Paper 470, 208 p.

Bateman, P.C., 1992a, Plutonism in the central part of the 

Sierra Nevada batholith, California: U.S. Geological Survey 

Professional Paper 1483, 186 p.

Bateman, P.C., 1992b, Pre-Tertiary bedrock geologic map of 

the Mariposa 1o by 2o quadrangle, Sierra Nevada, California: U.S. Geological Survey Map I-1960, scale 1:250,000.

Bateman, P.C., and Busacca, A.J., 1982, Geologic map of the 

Millerton Lake quadrangle, west-central Sierra Nevada, 

California: U.S. Geological Survey Map GQ-1548, scale 

1:62,500.

Benioff, H., and Gutenberg, B., 1939, The Mammoth “Earthquake Fault” and related features in Mono County, California: Seismological Society of America Bulletin, v. 29, no. 2, 

p. 333–340.

Benoit, W.R., 1984, Initial results from drillholes PLV-1 and 

PLV-2 in the western moat of the Long Valley caldera: Geothermal Resources Council Transactions, v. 8, p. 397–402.

Berry, M.E., 1997, Geomorphic analysis of late Quaternary 

faulting on Hilton Creek, Round Valley and Coyote Warp 

faults, east-central Sierra Nevada, California, USA: Geomorphology, v. 20, p. 177–195.

Blackwelder, E., 1931, Pleistocene glaciation in the Sierra 

Nevada and Basin Ranges: Geological Society of America 

Bulletin, v. 42, p. 865–922.

Brook, C.A., Nokleberg, W.J., and Kistler, R.W., 1974, 

Nature of the angular unconformity between the Paleozoic 

metasedimentary rocks and the Mesozoic metavolcanic 

rocks in the eastern Sierra Nevada, California: Geological 

Society of America Bulletin, v. 95, p. 571–576.

Browne, B., Bursik, M., Deming, J., Louros, M., Martos, A., 

and Stine, S., 2010, Eruption chronology and petrologic 

reconstruction of the ca. 8500 yr B.P. eruption of Red 

Cones, southern Inyo chain, California: Geological Society 

of America Bulletin, v. 122, p. 1401–1422.

Bursik, M.I., and Gillespie, A.R., 1993, Late Pleistocene 

glaciation of Mono Basin, California: Quaternary Research, 

v. 39, p. 24–35.

Bursik, M., Renshaw, C., McCalpin, J., and Berry, M., 2003, A 

volcanotectonic cascade; Activation of range front faulting 

and eruptions by dike intrusion, Mono Basin-Long Valley 

caldera, California: Journal of Geophysical Research, v. 108, 

no. B8, 2393, 14 p.

Bursik, M., and Sieh, K., 1989, Range front faulting and 

volcanism in the Mono Basin, eastern California: Journal of 

Geophysical Research, v. 94, no. B11, p. 15,587–15,609.

Calvert, A.T., and Lanphere, M., 2006, Argon geochronology 

of Kilauea’s early submarine history: Journal of Volcanology and Geothermal Research, v. 151, p. 1–18.

Carle, S.F., 1988, Three-dimensional gravity modeling of the 

geologic structure of Long Valley caldera: Journal of Geophysical Research, v. 93, no. B11, p. 13,237–13,250.

Chaudet, R.E., 1986, The petrology and geochemistry of 

precaldera magmas, Long Valley caldera, eastern California: 

Blacksburg, Va., Virginia Polytechnic Institute and State 

University, M.S. thesis, 152 p.

Chelikowski, J.R., 1940, Tectonics of the rhyolite in the 

Mammoth embayment, California: Journal of Geology, v. 48, 

p. 421–435.

Clark, D.H., Clark, M.M., and Gillespie, A.R., 1994, Debriscovered glaciers in the Sierra Nevada, California, and their 

implications for snowline reconstructions: Quaternary 

Research, v. 41, p. 139–153.
References Cited 115

Clark, D.H., and Gillespie, A.R., 1997, Timing and significance of late-glacial and Holocene cirque glaciation in 

the Sierra Nevada, California: Quaternary International, 

v. 38–39, p. 21–38.

Clark, M.M., and Gillespie, A.R., 1981, Record of late Quaternary faulting along the Hilton Creek fault in the Sierra 

Nevada, California: Earthquake Notes, v. 52, no. 1, p. 46.

Clark, M.M., Yount, J.C., Vaughan, P.R., and Zepeda, R.L., 

1982, Map showing surface ruptures associated with the 

Mammoth Lakes, California, earthquakes of 1980: U.S. 

Geological Survey Map MF-1396, scale 1:24,000.

Cleveland, G.B., 1962, Geology of the Little Antelope Valley 

clay deposits, Mono County, California: California Division 

of Mines and Geology Special Report 72, 28 p.

Clow, D.W., and Collum, K.R., 1983, Geology of the volcanic 

rocks at Devils Postpile, California. [Unpublished report 

of a joint undergraduate mapping project for Fresno State 

University, 4 p., instigated by Wymond Eckhardt, then 

superintendent at Devils Postpile National Monument, the 

report remains in Monument files and its geologic map was 

published in Huber and Eckhardt (1985).]

Clynne, M.A., Calvert, A.T., Wolfe, E.W., Evarts, R.C., Fleck, 

R.J., and Lanphere, M.A., 2008, The Pleistocene eruptive 

history of Mount St. Helens, Washington, from 300,000 

to 12,800 years before present, chap. 28 of Sherrod, D.R., 

Scott, W.E., and Stauffer, P.H., eds., A volcano rekindled; 

the renewed eruption of Mount St. Helens, 2004–2006: U.S. 

Geological Survey Professional Paper 1750, p. 593–627.

Clynne, M.A., and Muffler, L.J.P., 2010, Geologic map of 

Lassen Volcanic National Park and vicinity, California: 

U.S. Geological Survey Scientific Investigations Map 2899, 

110 p., 3 sheets, scale 1:50,000. 

Cook, A.C., Hainsworth, L.J., Sorey, M.L., Evans, W.C., and 

Southon, J.R., 2001, Radiocarbon studies of plant leaves 

and tree rings from Mammoth Mountain, CA; a long-term 

record of magmatic CO2 release: Chemical Geology, v. 177, 

p. 117–131.

Cousens, B.L., 1996, Magmatic evolution of Quaternary mafic 

magmas at Long Valley caldera and the Devils Postpile, 

California; Effects of crustal contamination on lithospheric 

mantle-derived magmas: Journal of Geophysical Research, 

v. 101, no. B12, p. 27,673–27,689.

Curry, R.R., 1966, Glaciation about 3,000,000 years ago in the 

Sierra Nevada: Science, v. 154, p. 770–771.

Curry, R.R., 1968, Quaternary climatic and glacial history 

of the Sierra Nevada, California: Berkeley, University of 

California, Ph.D. dissertation, 219 p.

 Curry, R.R., 1971, Glacial and Pleistocene history of the 

Mammoth Lakes Sierra—a geologic guidebook: Missoula, 

University of Montana, Department of Geology, Geological 

Series Publication no. 11, 49 p.

Dalrymple, G.B., 1963, Potassium-argon dates of some 

Cenozoic volcanic rocks of the Sierra Nevada, California: 

Geological Society of America Bulletin, v. 74, p. 379–390.

Dalrymple, G.B., 1964a, Cenozoic chronology of the Sierra 

Nevada, California: University of California Publications in 

Geological Sciences, v. 47, 41 p.

Dalrymple, G.B., 1964b, Potassium-argon dates of three 

Pleistocene interglacial basalt flows from the Sierra Nevada, 

California: Geological Society of America Bulletin, v. 75, p. 

753–758.

DeGraff, J.M., and Aydin, A., 1987, Surface morphology 

of columnar joints and its significance to mechanics and 

direction of joint growth: Geological Society of America 

Bulletin, v. 99, p. 605–617.

Deming, J.H., 2002, Stratigraphic analysis of Paoha Island 

and the Red Cones, eastern California; integration with a 

volcanic database: Buffalo, State University of New York at 

Buffalo, M.S. thesis, 92 p.

Doell, R.R., Dalrymple, G.B., and Cox, A., 1966, Geomagnetic polarity epochs; Sierra Nevada data, 3: Journal of 

Geophysical Research, v. 71, no. 2, p. 531–541.

Druitt, T.H., and Kokelaar, B.P., eds., 2002, The eruption of 

Soufrière Hills Volcano, Montserrat, from 1995 to 1999: 

London, The Geological Society, Memoir no. 21, 645 p.

Duffield, W.A., and Bacon, C.R., 1981, Geologic map of 

the Coso volcanic field and adjacent areas, Inyo County, 

California: U.S. Geological Survey Map I-1200, scale 

1:50,000.

Eichelberger, J.C., Lysne, P.C., and Younker, L.W., 1984, 

Research drilling at Inyo domes, Long Valley, California: 

Eos, v. 65, p. 721–725.

Eichelberger, J.C., Lysne, P.C., Miller, C.D., and Younker, 

L.W., 1985, Research drilling at Inyo domes, California; 

1984 results: Eos, v. 66, no. 17, p. 186–187.

Eichelberger, J.C., Vogel, T.A., Younker, L.W., Miller, C.D., 

Heiken, G.H., and Wohletz, K.H., 1988, Structure and stratigraphy beneath a young phreatic vent; South Inyo Crater, 

Long Valley Caldera, California: Journal of Geophysical 

Research, v. 93, no. B11, p. 13,208–13,220.

Erwin, H.D., 1934, Geology and mineral resources of northeastern Madera County, California: California Journal of 

Mines and Geology, v. 30, p. 7–78.
116 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Evans, W.C., Sorey, M.L., Cook, A.C., Kennedy, B.M., Shuster, D.L., Colvard, E.M., White, L.D., and Huebner, M.A., 

2002, Tracing and quantifying magmatic carbon discharge 

in cold groundwaters; Lessons learned from Mammoth 

Mountain, USA: Journal of Volcanology and Geothermal 

Research, v. 114, p. 291–312.

Farrar, C.D., Sorey, M.L., Evans, W.C., Howle, J.F., Kerr, 

B.D., Kennedy, B.M., King, C.-Y., and Southon, J.R., 1995, 

Forest-killing diffuse CO2 emission at Mammoth Mountain 

as a sign of magmatic unrest: Nature, v. 376, p. 675–678.

Farrar, C.D., Sorey, M.L., Roeloffs, E., Galloway, D.L., 

Howle, J.F., and Jacobson, R., 2003, Inferences on the 

hydrothermal system beneath the resurgent dome in Long 

Valley caldera, east-central California, USA, from recent 

pumping tests and geochemical sampling: Journal of Volcanology and Geothermal Research, v. 127, p. 305–328.

Farrar, C.D., Evans, W.C., Venezky, D.Y., Hurwitz, S., and 

Oliver, L.K., 2007, Boiling water at Hot Creek—The 

dangerous and dynamic thermal springs in California’s 

Long Valley caldera: U.S. Geological Survey Fact Sheet 

2007–3045, 4 p.

Foulger, G.R., Julian, B.R., Hill, D.P., Pitt, A.M., Malin, P.E., 

and Shalev, E., 2004, Non-double-couple microearthquakes 

at Long Valley caldera, California, provide evidence for 

hydraulic fracturing: Journal of Volcanology and Geothermal Research, v. 132, p. 45–71.

Foulger, G.R., Julian, B.R., Pitt, A.M., and Hill, D.P., 2003, 

Three-dimensional crustal structure of Long Valley caldera, 

California, and evidence for the migration of CO2 under 

Mammoth Mountain: Journal of Geophysical Research, 

v. 108, no. B3, 2147, p. 1-16.

Fournier, R.B., 1989, Lithology, mineralogy, and paleontology 

of Quaternary lake deposits in Long Valley caldera, California: U.S. Geological Survey Open-File Report 89–413, 95 p.

Fowler, A., Wildgoose, M., Zierenberg, R., Schiffman, P., and 

Suemnicht, G., 2011, Hydrothermal alteration and geochemistry in Core Hole BC 12–31; Implications for segregation 

of transient flow regimes in the Long Valley geothermal 

system: Geothermal Resources Council Transactions, v. 35, 

p. 785–789.

Fullerton, D.S., 1986, Chronology and correlation of glacial 

deposits in the Sierra Nevada, California, p. 161-169 in

Sibrava, V., Bowen, D.Q., and Richmond, G.M., eds., Quaternary glaciations in the Northern Hemisphere: Quaternary 

Science Reviews, v. 5, p. 161–169.

Gerlach, T.M., Doukas, M.P., McGee, K.A., and Kessler, R., 

2001, Soil efflux and total emission rates of magmatic CO2

at the Horseshoe Lake tree kill, Mammoth Mountain, California: Chemical Geology, v. 177, p. 101, p. 116.

Gibbard, P.L., Head, M.J., Walker, M.J.C., and the Subcommission on Quaternary Stratigraphy, 2009, Formal ratification of the Quaternary System/Period and the Pleistocene 

Series/Epoch with a base at 2.58 Ma: Journal of Quaternary 

Science, v. 25, no. 2, p. 96–102. 

Gibson, R.G., and Naney, M.T., 1992, Textural development 

of mixed, finely porphyritic silicic volcanic rocks, Inyo 

domes, eastern California: Journal of Geophysical Research, 

v. 97, p. 4541–4559. 

Gilbert, C.M., 1938, Welded tuff in eastern California: Geological Society of America Bulletin, v. 49, p. 1829–1862.

Gilbert, C.M., 1941, Late Tertiary geology southeast of Mono 

Lake, California: Geological Society of America Bulletin, 

v. 52, p. 781–816.

Gillespie, A.R., and Clark, D.H., 2011, Glaciations of the 

Sierra Nevada, California, USA: Developments in Quaternary Science, v. 15, p. 447–462.

Gillespie, A.R., and Zehfuss, P.H., 2004, Glaciations of the 

Sierra Nevada, California, USA, in Ehlers, J., and Gibbard, 

P.L., eds., Quaternary glaciations—Extent and chronology; 

Part II; North America: Amsterdam, Elsevier, Developments 

in Quaternary Science, v. 2b, p. 51–62.

Goff, F., Wollenberg, H.A., Brookins, D.C., and Kistler, R.W., 

1991, A Sr-isotopic comparison between thermal waters, 

rocks, and hydrothermal calcites, Long Valley caldera, California: Journal of Volcanology and Geothermal Research, 

v. 48, p. 265–281.

Gradstein, F.M., Ogg, J.G., Smith, A.G., Bleeker, W., and 

Lourens, L.J., 2004, A new geologic time scale, with 

special reference to Precambrian and Neogene: Episodes, 

v. 27, p. 83–100.

Greene, D.C., and Schweickert, R.A., 1995, The Gem Lake 

shear zone; Cretaceous dextral transpression in the Northern 

Ritter Range pendant, eastern Sierra Nevada, California: 

Tectonics, v. 14, p. 945–961.

Greene, D.C., and Stevens, C.H., 2002, Geologic map of 

Paleozoic rocks in the Mount Morrison pendant, eastern 

Sierra Nevada, California: California Division of Mines and 

Geology Map Sheet 53; scale 1:24,000.

Greene, D.C., Stevens, C.H., and Wise, J.M., 1997, The 

Laurel-Convict fault, eastern Sierra Nevada, California; A 

Permo-Triassic left-lateral fault, not a Cretaceous intrabatholithic break: Geological Society of America Bulletin, 

v. 109, no. 4, p. 483–488.

Harris, A.J.L., Rose, W.I., and Flynn, L.P., 2003, Temporal 

trends in lava dome extrusion at Santiaguito 1922–2000: 

Bulletin of Volcanology, v. 65, p. 77–89.
References Cited 117

Heumann, A., 1999, Timescales of processes within silicic 

magma chambers: Amsterdam, Vrije Universiteit, Ph.D. 

dissertation, 200 p.

Heumann, A., and Davies, G.R., 1997, Isotopic and chemical 

evolution of the post-caldera rhyolitic system at Long Valley, California: Journal of Petrology, v. 38, p. 1661–1678.

Heumann, A., Davies, G.R., and Elliott, T., 2002, Crystallization history of rhyolites at Long Valley, California, 

inferred from combined U-series and Rb-Sr isotope systematics: Geochimica et Cosmochimica Acta, v. 66, no. 10, 

p. 1821–1837.

Hildreth, W., 1979, The Bishop Tuff; Evidence for the origin 

of compositional zonation in silicic magma chambers: Geological Society of America Special Paper 180, p. 43–75.

Hildreth, W., 2004, Volcanological perspectives on Long 

Valley, Mammoth Mountain, and Mono Craters; several 

contiguous but discrete systems: Journal of Volcanology and 

Geothermal Research, v. 136, p. 169–198.

Hildreth, W., and Mahood, G.A., 1986, Ring-fracture eruption 

of the Bishop Tuff: Geological Society of America Bulletin, 

v. 97, p. 396–403.

Hildreth, W., and Wilson, C.J.N., 2007, Compositional zoning 

of the Bishop Tuff: Journal of Petrology, v. 48, p. 951–999.

Hildreth, W., Fierstein, J., Champion, D., and Calvert, A., 

2014, Mammoth Mountain and its mafic periphery—A late 

Quaternary volcanic field in eastern California: Geosphere, 

v. 10, no. 6., p. 1315–1365.

Hill, D.P., 1976, Structure of Long Valley caldera, California, 

from a seismic refraction experiment: Journal of Geophysical Research, v. 81, p. 745–753.

Hill, D.P., 1996, Earthquakes and carbon dioxide beneath 

Mammoth Mountain, California: Seismological Research 

Letters, v. 67, p. 815.

Hill, D.P., 2006, Unrest in Long Valley caldera, California, 

1978-2004, in Troise. C., De Natale, G., and Kilburn, 

C.R.J., eds., Mechanisms of activity and unrest at large 

calderas: Geological Society of London, Special Publication 

269, p. 1–24.

Hill, D.P., Bailey, R.A., Miller, C.D., Hendley, J.W., II, and 

Stauffer, P.H., 1997, Future eruptions in California’s Long 

Valley area—What’s likely?: U.S. Geological Survey Fact 

Sheet 073–97, 2 p.

Hill, D.P., Bailey, R.A., Hendley, J.W., II, Stauffer, P.H., and 

Marcaida, M., 2014, California’s restless giant—the Long 

Valley Caldera: U.S. Geological Survey Fact Sheet 2014 

3056, 2 p.

Hill, D.P., Dzurisin, D., Ellsworth, W.L., Endo, E.T., Galloway, D.L., Gerlach, T.M., Johnston, M.J.S., Langbein, J., 

McGee, K.A., Miller, C.D., Oppenheimer, D., and Sorey, 

M.L., 2002: Response plan for volcano hazards in the Long 

Valley caldera and Mono Craters region, California: U.S. 

Geological Survey Bulletin 2185, 57 p.

Hill, D.P., Ellsworth, W.L., Johnston, M.J.S., Langbein, J.O., 

Oppenheimer, D.H., Pitt, A.M., Reasenberg, P.A., Sorey, 

M.L., and McNutt, S.R., 1990, The 1989 earthquake 

swarm beneath Mammoth Mountain, California; an initial 

look at the 4 May through 30 September activity: Bulletin of the Seismological Society of America, v. 80, no. 2, 

p. 325–339.

Hill, D.P., Kissling, E., Luetgert, J.H., and Kradolfer, U., 

1985, Constraints on the upper crustal structure of the Long 

Valley–Mono Craters volcanic complex, eastern California, 

from seismic refraction measurements: Journal of Geophysical Research, v. 90, p. 11,135–11,150.

Hill, D.P., Langbein, J.O., Prejean, S., 2003, Relations 

between seismicity and deformation during unrest in Long 

Valley Caldera, California, from 1995 through 1999: Journal of Volcanology and Geothermal Research, v. 127, p. 

175–193.

Hill, D.P., and Prejean, S., 2005, Magmatic unrest beneath 

Mammoth Mountain, California: Journal of Volcanology 

and Geothermal Research, v. 146, p. 257–283.

Hopson, C.A. 2008, Geologic map of Mount St. Helens, 

Washington, prior to the 1980 eruption: U.S. Geological 

Survey Open-File Report 02–468; 2 sheets, scale 1:31,250.

Hoshizumi, H., Uto, K., and Watanabe, K., 1999, Geology and 

eruptive history of Unzen volcano, Shimabara Peninsula, 

Kyushu, SW Japan: Journal of Volcanology and Geothermal 

Research, v. 89, p. 81–94.

Huber, N.K., 1981, Amount and timing of late Cenozoic uplift 

and tilt of the central Sierra Nevada; evidence from the 

upper San Joaquin River basin: U.S. Geological Survey 

Professional Paper 1197, 28 p.

Huber, N.K., and Eckhardt, W.W., 1985, Devils Postpile story: 

Three Rivers, California, Sequoia Natural History Association, 30 p.

Huber, N.K., and Rinehart, C.D., 1965a, Geologic map of the 

Devils Postpile quadrangle, Sierra Nevada, California: U.S. 

Geological Survey Map GQ-437, scale 1:62,500.

Huber, N.K., and Rinehart, C.D., 1965b, The Devils Postpile National Monument: California Division of Mines 

and Geology, Mineral Information Service, v. 18, no. 6, 

p. 109–118.
118 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Huber, N.K., and Rinehart, C.D., 1967, Cenozoic volcanic 

rocks of the Devils Postpile quadrangle, eastern Sierra 

Nevada, California: U.S. Geological Survey Professional 

Paper 554-D, 21 p., 1 plate.

Izett, G.A., Obradovich, J.D., and Mehnert, H.H., 1988, 

The Bishop ash bed (middle Pleistocene) and some older 

(Pliocene and Pleistocene) chemically and mineralogically 

similar ash beds in California, Nevada, and Utah: U.S. Geological Survey Bulletin 1675, 37 p.

Julian, B.R., Pitt, A.M., and Foulger, G.R., 1998, Seismic 

image of a CO2 reservoir beneath a seismically active volcano: Geophysical Journal International, v. 133, p. F7–F10.

Kane, M.F., Mabey, D.R., and Brace, R., 1976, A gravity and 

magnetic investigation of the Long Valley caldera, Mono 

County, California: Journal of Geophysical Research, v. 81, 

p. 754–762.

Kaye, G., Finnis, K.K., Johnston, D.M., and Paton, D., 2008, 

Volcanic hazard awareness in the tourism sector in Mammoth Lakes, California: Wellington, New Zealand, Institute 

of Geological and Nuclear Sciences, GNS Science Report 

2008/35, 13 p.

Kaye, G., Cole, J., King, A., and Johnston, D., 2009, Comparison of risk from pyroclastic density current hazards to 

critical infrastructure in Mammoth Lakes, California, USA, 

from a new Inyo Craters rhyolite dike eruption versus a 

dacite dome eruption on Mammoth Mountain: Natural Hazards (Springer), v. 49, p. 541–563.

Kelleher, P.C., and Cameron, K.L., 1990, The geochemistry 

of the Mono Craters–Mono Lake Islands volcanic complex, 

eastern California: Journal of Geophysical Research, v. 95, 

no. B11, p. 17643–17659.

Kesseli, J.E., 1941a, Ice streams in the Sierra Nevada, California: The Geographical Review, v. 31, no. 2, p. 203–227.

Kesseli, J.E., 1941b, Studies in the Pleistocene glaciation of 

the Sierra Nevada, California: University of California Publications in Geography, v. 6, no. 8, p. 315–362.

Kistler, R.W., 1966, Geologic map of the Mono Craters 

quadrangle, Mono and Tuolumne Counties, California: U.S. 

Geological Survey Map GQ-462, scale 1:62,500.

Kistler, R.W., and Swanson, S.E., 1981, Petrology and geochronology of metamorphosed volcanic rocks and a middle 

Cretaceous volcanic neck in the east-central Sierra Nevada, 

California: Journal of Geophysical Research, v. 86, p. 

10,489–10,501.

Lachenbruch, A.H., Sorey, M.L., Lewis, R.E., and Sass, J.H., 

1976a, The near-surface hydrothermal regime of Long Valley caldera: Journal of Geophysical Research, v. 81, no. 5, 

p. 763–768.

Lachenbruch, A.H., Sass, J.H., Munroe, R.J., and Moses, T.H., 

Jr., 1976b, Geothermal setting and simple heat conduction 

models for the Long Valley caldera: Journal of Geophysical 

Research, v. 81, no. 5, p. 769–784.

Langbein, J.O., 2003, Deformation of the Long Valley caldera, 

California; inferences from measurements from 1988 to 

2001: Journal of Volcanology and Geothermal Research, 

v. 127, p. 247–267.

Lange, R.A., and Carmichael, I.S.E., 1996, The Aurora volcanic field, California-Nevada: Oxygen fugacity constraints 

on the development of andesitic magma: Contributions to 

Mineralogy and Petrology, v. 125, p. 167–185.

LeBas, M.J., Lemaitre, R.W., Streckeisen, A., and Zanettin, B., 

1986, A chemical classification of volcanic rocks based on 

the total alkali–silica diagram: Journal of Petrology, v. 27, 

p. 745–750.

Lewicki, J.L., Hilley, G.E., Dobeck, L., and Marino, B.D.V., 

2012, Eddy covariance imaging of diffuse volcanic CO2

emissions at Mammoth Mountain, CA, USA: Bulletin of 

Volcanology, v. 74, p. 135–141.

Lewis, R.E., 1975, Data from a 1,000-foot (305-metre) core 

hole in the Long Valley caldera, Mono County, California: 

U.S. Geological Survey Open-File Report 75–683, 16 p.

Lipshie, S.R., 1974, Surficial and engineering geology of the 

Mammoth Creek area, Mono County, California: University 

of California at Los Angeles, M.S. thesis, 163 p.

Mahood, G.A., Ring, J.H., Manganelli, S., and McWilliams, 

M.O., 2010, New 40Ar/39Ar ages reveal contemporaneous 

mafic and silicic eruptions during the past 160,000 years 

at Mammoth Mountain and Long Valley caldera, California: Geological Society of America Bulletin, v. 122, 

p. 396–407.

Mankinen, E.A., Grommé, C.S., Dalrymple, G.B., Lanphere, 

M.A., and Bailey, R.A., 1986, Paleomagnetism and K-Ar 

ages of volcanic rocks from Long Valley, California: Journal 

of Geophysical Research, v. 91, no. B1, p. 633–652. 

Mastin, L.G., 1991, The roles of magma and groundwater 

in the phreatic eruptions at Inyo Craters, Long Valley 

Caldera, California: Bulletin of Volcanology, v. 53, no. 8, 

p. 579–596.

Mastin, L.G., and Pollard, D.D., 1988, Surface deformation 

and shallow dike intrusion processes at Inyo Craters, Long 

Valley, California: Journal of Geophysical Research, v. 93, 

no. B11, p. 13,221–13,235.

Matthes, F.E., 1960, Reconnaissance of the geomorphology and glacial geology of the San Joaquin basin, Sierra 

Nevada, California: U.S. Geological Survey Professional 

Paper 329, 62 p.
References Cited 119

Mayo, E.B., 1930, Preliminary report on the geology of southwestern Mono County, California: Mining in California, 

Report 26 of the State Mineralogist, p. 475–482.

Mayo, E.B., 1934a, Geology and mineral deposits of Laurel 

and Convict Basins, southwestern Mono County, California: 

California Journal of Mines and Geology, v. 30, p. 79–87.

Mayo, E.B., 1934b, The Pleistocene Long Valley Lake in eastern California: Science, v. 80, no. 2065, p. 95–96.

Mayo, E.B., 1935, Some intrusions and their wall rocks in the 

Sierra Nevada: Journal of Geology, v. 43, no. 7, p. 673–689.

Mayo, E.B., 1937, Sierra Nevada pluton and crustal movement: Journal of Geology, v. 45, no. 2, p. 169–192.

Mayo, E.B., Conant, L.C., and Chelikowsky, J.R., 1936, 

Southern extension of the Mono Craters, California: American Journal of Science, v. 32, p. 81–97.

McConnell, V.S., Shearer, C.K., Eichelberger, J.C., Keskinen, 

M.J., Layer, P.W., and Papike, J.J., 1995, Rhyolite intrusions 

in the intracaldera Bishop Tuff, Long Valley caldera, California: Journal of Volcanology and Geothermal Research, 

v. 67, p. 41–60. 

Metz, J., 1987, Physical and chemical evolution of Glass 

Mountain; Precaldera high-silica rhyolites from the Long 

Valley magma system: Stanford University, Ph.D. dissertation, 161 p.

Metz, J.M., and Bailey, R.A., 1993, Geologic map of Glass 

Mountain, Mono County, California: U.S. Geological Survey Map I-1995, scale 1:24,000.

Metz, J.M., and Mahood, G.A, 1985, Precursors to the Bishop 

Tuff eruption; Glass Mountain, Long Valley, California: 

Journal of Geophysical Research, v. 90, no. B13, p. 11,121–

11,126.

Metz, J.M., and Mahood, G.A, 1991, Development of the 

Long Valley, California, magma chamber recorded in precaldera rhyolite lavas of Glass Mountain: Contributions to 

Mineralogy and Petrology, v. 106, p. 379–397.

Millar, C.I., King, J.C., Westfall, R.D., Alden, H.A., and 

Delany, D.L., 2006, Late Holocene forest dynamics, volcanism, and climate change at White Wing Mountain and San 

Joaquin Ridge, Mono County, Sierra Nevada, CA, USA: 

Quaternary Research, v. 66, p. 273–287.

Millar, C.I., and Westfall, R.D., 2008, Rock glaciers and 

related periglacial landforms in the Sierra Nevada, CA, 

USA; inventory, distribution and climatic relationships: 

Quaternary International, v. 188, p. 90–104.

Miller, C.D., 1985, Holocene eruptions at the Inyo volcanic 

chain, California—Implications for possible eruptions in the 

Long Valley caldera: Geology, v. 13, p. 14–17.

Moore, J.G., and Dodge, F.C.W., 1980, Late Cenozoic volcanic rocks of the southern Sierra Nevada, California; I., 

Geology and petrology: Geological Society of America Bulletin, v. 91, no. 9, p. 515–518 and p. 1995–2038.

Morgan, B.A., and Rankin, D.W., 1972, Major structural break 

between Paleozoic and Mesozoic rocks in the eastern Sierra 

Nevada, California: Geological Society of America Bulletin, 

v. 83, p. 3,739–3,744.

Muffler, L.J.P., Clynne, M.A., Calvert, A.T., and Champion, 

D.E., 2011, Diverse, discrete, mantle-derived batches of 

basalt erupted along a short normal fault zone; the Poison 

Lake chain, southernmost Cascades: Geological Society of 

America Bulletin, v. 123, p. 2177–2200.

Mullineaux, D.R., 1996, Pre-1980 tephra-fall deposits erupted 

from Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1563, 99 p.

Nakada, S., Shimizu, H., and Ohta, K., 1999, Overview of the 

1990–1995 eruption at Unzen Volcano: Journal of Volcanology and Geothermal Research, v. 89, p. 1–22.

Nawotniak, S.E.K., and Bursik, M., 2010, Subplinian fall 

deposits of Inyo Craters, California: Journal of Volcanology 

and Geothermal Research, v. 198, p. 433–446.

Oldow, J.S., 2003, Active transtensional boundary zone 

between the western Great Basin and Sierra Nevada block, 

western U.S. Cordillera: Geology, v. 31, p. 1033–1036.

Peck, D.L., and Minakami, T., 1968, The formation of 

columnar joints in the upper part of Kilauean lava lakes, 

Hawaii: Geological Society of America Bulletin, v. 79, p. 

1151–1166.

Phillips, F.M., Zreda, M.G., Benson, L.V., Plummer, M.A., 

Elmore, D., and Sharma, P., 1996, Chronology for fluctuations in late Pleistocene Sierra Nevada glaciers and lakes: 

Science, v. 274, p. 749–751.

Phillips, F.M., Zreda, M., Plummer, M.A., Elmore, D., and 

Clark, D.H., 2009, Glacial geology and chronology of 

Bishop Creek and vicinity, eastern Sierra Nevada, California: Geological Society of America Bulletin, v. 121, p. 

1013–1033.

Pitt, A.M., and Hill, D.P., 1994, Long-period earthquakes in 

the Long Valley caldera region, eastern California: Geophysical Research Letters, v. 21, no. 16, p. 1679–1682.

Pitt, A.M., Hill, D.P., Walter, S.W., and Johnson, M.J.S., 2002, 

Midcrustal, long-period earthquakes beneath northern California volcanic areas: Seismological Research Letters, v. 73, 

p. 144–152.
120 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Prejean, S., Ellsworth, W.L., Zoback, M., and Waldhouser, 

F., 2002, Fault structure and kinematics of the Long Valley 

caldera region, CA, revealed by high-accuracy earthquake 

hypocenters and focal mechanism stress inversions: Journal 

of Geophysical Research, v. 107, no. B12, p. 1–19.

Prejean, S., Stork, A., Ellsworth, W., Hill, D., and Julian, B., 

2003, High precision earthquake locations reveal seismogenic structure beneath Mammoth Mountain, California: 

Geophysical Research Letters, v. 30, no. 24, 2247, p. 1–4.

Putnam, W.C., 1949, Quaternary geology of the June Lake 

district, California: Geological Society of America Bulletin, 

v. 60, p. 1281–1302.

Putnam, W.C., 1960, Origin of Rock Creek and Owens River 

gorges, Mono County, California: University of California 

Publications in Geological Sciences, v. 34, p. 221–280.

Putnam, W.C., 1962, Late Cenozoic geology of McGee 

Mountain, Mono County, California: University of California Publications in Geological Sciences, v. 40, no. 3, 

p. 181–207.

Reid, M.R., Coath, C.D., Harrison, T.M., and McKeegan, 

K.D., 1997, Prolonged residence times for the youngest 

rhyolites associated with Long Valley caldera; 230Th-238U ion 

microprobe dating of young zircons: Earth and Planetary 

Sciences Letters, v. 150, p. 27–39.

Riley, P., Tikoff, B., and Hildreth, W., 2012, Transtensional 

deformation and structural control of contiguous but independent magmatic systems; Mono-Inyo Craters, Mammoth 

Mountain, and Long Valley caldera, California: Geosphere, 

v. 8, no. 4, p. 740–751.

Rinehart, C.D., and Huber, N.K., 1965, The Inyo Crater 

Lakes—A blast in the past: California Division of Mines 

and Geology, Mineral Information Service, v. 18, no. 9, 

p. 169–172.

Rinehart, C.D., and Ross, D.C., 1957, Geology of the Casa 

Diablo Mountain quadrangle, California: U.S. Geological 

Survey Map GQ-99, scale 1:62,500.

Rinehart, C.D., and Ross, D.C., 1964, Geology and mineral 

deposits of the Mount Morrison Quadrangle, Sierra Nevada, 

California: U.S. Geological Survey Professional Paper 385, 

106 p.

Ring, J.H., 2000, Young volcanism in western Long Valley 

caldera: Stanford University, M.S. thesis, 128 p.

Rivera, T.A., Storey, M., Zeeden, C., Hilgen, F.J., and Kuiper, 

K., 2011, A refined astronomically calibrated 40Ar/39Ar age 

for Fish Canyon sanidine: Earth and Planetary Science Letters, v. 311, p. 420–426.

Rogie, J.D., Kerrick, D.M., Sorey, M.L., Chiodini, G., and 

Galloway, D.L., 2001, Dynamics of carbon dioxide emission at Mammoth Mountain, California: Earth and Planetary 

Science Letters, v. 188, p. 535–541.

Rood, D.H., Burbank, D.W., and Finkel, R.C., 2011, Chronology of glaciations in the Sierra Nevada, California, from 

10Be surface exposure dating: Quaternary Science Reviews, 

v. 30, p. 646–661.

Ryan, M.P., and Sammis, C.G., 1978, Cyclic fracture mechanisms in cooling basalt: Geological Society of America 

Bulletin, v. 89, p. 1295–1308.

Sampson, D.E., 1987, Textural heterogeneities and vent area 

structures in the 600-year-old lavas of the Inyo volcanic 

chain, eastern California, in Fink, J.H., ed., The emplacement of silicic domes and lava flows: Geological Society of 

America Special Paper 212, p. 89–101.

Sampson, D.E., and Cameron, K.L., 1987, The geochemistry 

of the Inyo volcanic chain; Multiple magma systems in the 

Long Valley region, eastern California: Journal of Geophysical Research, v. 92, no. B10, p. 10,403–10,421.

Sarna-Wojcicki, A.M., Reheis, M.C., Pringle, M.S., Fleck, 

R.J., Burbank, D., Meyer, C.E., Slate, J.L., Wan, E., 

Budahn, J.R., Troxel, B., and Walker, J.P., 2005, Tephra 

layers of Blind Spring Valley and related upper Pliocene 

and Pleistocene tephra layers, California, Nevada, and Utah; 

Isotopic ages, correlation, and magnetostratigraphy: U.S.; 

Geological Survey Professional Paper 1701, 63 p.

Savage, J.C., and Clark, M.M., 1982, Magmatic resurgence in 

Long Valley caldera, California; Possible cause of the 1980 

Mammoth Lakes earthquakes: Science, v. 217, no. 4559, p. 

531–533.

Sharp, R.P., 1968, Sherwin Till–Bishop Tuff geological relationships, Sierra Nevada, California: Geological Society of 

America Bulletin, v. 79, p. 351–364.

Sharp, R.P., 1969, Semiquantitative differentiation of glacial 

moraines near Convict Lake, Sierra Nevada, California: 

Journal of Geology, v. 77, p. 68–91.

Shelly, D.R., and Hill, D.P., 2011, Migrating swarms of brittlefailure earthquakes in the lower crust beneath Mammoth 

Mountain, California: Geophysical Research Letters, v. 38, 

L20307, doi:10.1029/2011GL049336.

Shevenell, L., Goff, F., Grigsby, C.O., Janik, C.J., Trujillo, 

P.E., Jr., and Counce, D., 1987, Chemical and isotopic 

characteristics of thermal fluids in the Long Valley caldera 

lateral flow system, California: Geothermal Resources 

Council Transactions, v. 11, p. 195–201.
References Cited 121

Simon, J.I., Reid, M.R., and Young, E.D., 2007, Lead isotopes 

by LA-MC-ICPMS; Tracking the emergence of mantle signatures in an evolving silicic magma system: Geochimica et 

Cosmochimica Acta, v. 71, no. 8, p. 2014–2035.

Simon, J.I., Weis, D., DePaolo, D.J., Renne, P.R., Mundil, 

R., and Schmitt, A.K., 2014, Assimilation of preexisting 

Pleistocene intrusions at Long Valley by periodic magma 

recharge accelerates rhyolite generation: rethinking the 

remelting model: Contributions to Mineralogy and Petrology, v. 167:955, 35 p.

Smith, R.L., and Bailey, R.A., 1968, Resurgent cauldrons: 

Geological Society of America Memoir 116, p. 613–662.

Smith, J.L., and Rex, R.W., 1977, Drilling results from the 

eastern Long Valley caldera: American Nuclear Society Meeting on Energy and Mineral Recovery Research, 

Golden, Colorado, April 12–14, 1977, p. 529–540.

Sorey, M.L., 1985, Evolution and present state of the hydrothermal system in Long Valley caldera: Journal of Geophysical Research, v. 90, no. B13, p. 11,219–11,228.

Sorey, M.L., Evans, W.C., Kennedy, B.M., Farrar, C.D., Hainsworth, L.J., and Hausback, B., 1998, Carbon dioxide and 

helium emissions from a reservoir of magmatic gas beneath 

Mammoth Mountain, California: Journal of Geophysical 

Research, v. 103, no. B7, p. 15,303–15,323.

Sorey, M.L., Farrar, C.D., Gerlach, T.M., McGee, K.A., Evans, 

W.C., Colvard, E.M., Hill, D.P., Bailey, R.A., Rogie, J.D., 

Hendley, J.W., II, and Stauffer, P.H., 2000, Invisible CO2

gas killing trees at Mammoth Mountain, California: U.S. 

Geological Survey Fact Sheet 172–96, 2 p. 

Sorey, M.L., Kennedy, B.M., Evans, W.C., Farrar, C.D., and 

Suemnicht, G.A., 1993, Helium isotope and gas discharge 

variations associated with crustal unrest in Long Valley 

caldera, California, 1989–1992: Journal of Geophysical 

Research, v. 98, no. B9, p. 15,871–15,889.

Sorey, M.L., Lewis, R.E., and Olmsted, F.H., 1978, The 

hydrothermal system of Long Valley caldera, California: 

U.S. Geological Survey Professional Paper 1044-A, 60 p., 

1 plate.

Sorey, M.L., Suemnicht, G.A., Sturchio, N.C., and Nordquist, 

G.A., 1991, New evidence on the hydrothermal system in 

Long Valley caldera, California, from wells, fluid sampling, 

electrical geophysics, and age determinations of hot-spring 

deposits: Journal of Volcanology and Geothermal Research, 

v. 48, p. 229–263.

Steiger, R.H., and Jäger, E., 1977, Subcommission on Geochronology—Convention on the use of decay constants in 

geo- and cosmochronology: Earth and Planetary Science 

Letters, v. 36, p. 359–362.

Stevens, C.H., and Greene, D.C., 1999, Stratigraphy, depositional history, and tectonic evolution of Paleozoic continental-margin rocks in roof pendants of the eastern Sierra 

Nevada, California: Geological Society of America Bulletin, 

v. 111, p. 919–933.

Stuiver, M., and Reimer, P.J., 1993, Extended 14C data base 

and revised CALIB radiocarbon calibration program: 

Radiocarbon, v. 35, p. 215–230 (version 5.0).

Suemnicht, G.A., 1987, Results of deep drilling in the western 

moat of Long Valley, California: Eos, v. 68, no. 40, p. 785, 798.

Suemnicht, G.A., Sorey, M.L., Moore, J.N., and Sullivan, R., 

2006, The shallow hydrothermal system of Long Valley 

caldera, California: Geothermal Resources Council Transactions, v. 30, p. 465–469.

Suemnicht, G.A., and Sorey, M.L., 2007, Long Valley caldera 

geothermal and magmatic systems: Geothermal Resources 

Council, Field Trip #1; September 29–30, 2007 for the 

Annual Meeting, Reno, Nevada; 21 p.

Suemnicht, G.A., and Varga, R.J., 1988, Basement structure 

and implications for hydrothermal circulation patterns in the 

western moat of Long Valley caldera, California: Journal of 

Geophysical Research, v. 93, no. B11, p. 13,191–13,207.

Taylor, G.C., and Bryant, W.A., 1980, Surface rupture associated with the Mammoth Lakes earthquakes of 25 and 27 

May, 1980, in Sherburne, R.W., ed., Mammoth Lakes, 

California earthqukes of May 1980: California Division of 

Mines and Geology Special Report 150, p. 49–67.

Tikoff, B., and Greene, D., 1997, Stretching lineations in 

transpressional shear zones; An example from the Sierra 

Nevada Batholith, California: Journal of Structural Geology, 

v. 19, p. 29–39.

Tikoff, B., and Saint Blanquat, M., 1997, Transpressional 

shearing and strike-slip partitioning in the Late Cretaceous 

Sierra Nevada magmatic arc, California: Tectonics, v. 16, 

p. 442–459.

Tikoff, B., and Teyssier, C., 1992, Crustal-scale, en echelon 

“P-shear” tensional bridges: A possible solution to the 

batholithic room problem: Geology, v. 20, p. 927–930.

Tizzani, P., Battaglia, M., Zeni, G., Atzori, S., Berardino, P., 

and Lanari, R., 2009, Uplift and magma intrusion at Long 

Valley caldera from InSAR and gravity measurements: 

Geology, v. 37, p. 63–66.

Varga, R.J., Bailey, R.A., and Suemnicht, G.A., 1990, Evidence for 600-year-old basalt and magma mixing at Inyo 

Craters volcanic chain, Long Valley caldera, California: 

Journal of Geophysical Research, v. 95, no. B13, 

p. 21,441–21,450.
122 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Vogel, T.A., Eichelberger, J.C., Younker, L.W., Schuraytz, 

B.C., Horkowitz, J.P., Stockman, H.W., and Westrich, H.R., 

1989, Petrology and emplacement dynamics of intrusive 

and extrusive rhyolites of Obsidian Dome, Inyo Craters 

Volcanic Chain, eastern California: Journal of Geophysical 

Research, v. 94, p. 17,937–17,956.

Vogel, T.A., Woodburne, T.B., Eichelberger, J.C., and Layer, 

P.W., 1994, Chemical evolution and periodic eruption of 

mafic lava flows in the west moat of Long Valley Caldera, 

California: Journal of Geophysical Research, v. 99, no. B10, 

p. 19,829–19,842.

Wahrhaftig, C., and Cox, A., 1959, Rock glaciers in the Alaska 

Range: Geological Society of America Bulletin, v. 70, no. 4, 

p. 383–436.

Waitt, R.B., and Begét, J.E., 2009, Volcanic processes and 

geology of Augustine Volcano, Alaska: U.S. Geological 

Survey Professional Paper 1762, 78 p., 2 plates.

Walker, M., Johnsen, S., Rasmussen, S.O., Popp, T., Steffensen, J.-P., Gibbard, P., Hoek, W., Andrews, J., Björck, S., 

Cwynar, L.C., Hughen, K., Kershaw, P., Kromer, B., Litt, T., 

Lowe, D.J., Nakagawa, T., Newnham, R., and Schwander, 

J., 2009, Formal definition and dating of the GSSP (Global 

Stratotype Section and Point) for the base of the Holocene 

using the Greenland NGRIP ice core and selected auxiliary 

records: Journal of Quaternary Science, v. 24, no. 1, p. 

3–17.

Wallace, P.J., Anderson, A.T., Jr., and Davis, A.M., 1999, 

Gradients in H2O, CO2, and exsolved gas in a large-volume 

silicic magma system; Interpreting the record preserved in 

melt inclusions from the Bishop Tuff: Journal of Geophysical Research, v. 104, no. B9, p. 20,097–20,122.

Wilson, C.J.N., and Hildreth, W., 1997, The Bishop Tuff; New 

insights from eruptive stratigraphy: Journal of Geology, v. 

105, p. 407–439.

Wilson, C.J.N., and Hildreth, W., 1998, Hybrid fall deposits in 

the Bishop Tuff, California: A novel pyroclastic depositional 

mechanism: Geology, v. 26, no. 1, p. 7–10.

Wood, S.H., 1977, Distribution, correlation, and radiocarbon 

dating of late Holocene tephra, Mono and Inyo Craters, 

eastern California: Geological Society of America Bulletin, 

v. 88, p. 89–95.
Map unit Unit name Percent SiO2 Percent phenocrysts

Mammoth Mountain edifice (25 units)

dbp Trachydacite of Bottomless Pit 67 15–20 pl-san-hb-bi-ox

ddl Trachydacite of lower Dragons Back 66–67 15–22 pl-san-hb-bi-px-ox

ddu Trachydacite of upper Dragons Back 66–67 17–22 pl-san-hb-bi-px-ox

dfl Trachydacite of Face Lift 63–65 15–25 pl-san-bi-px-hb-ox

dgr Trachydacite of Gold Rush Express 68.5–69 20–25 pl-san-bi-hb-px-ox

dlp Trachydacite of Lincoln Peak 67.5–69 17–25 pl-san-bi-hb-px-ox

dml Trachydacite of Main Lodge coulee 65.5–68.5 17–27 pl-san-bi-hb-px-ox

dms Trachydacite of McCoy Station 68.5–69.5 15–20 pl-san-bi-px-hb-ox

dnh Trachydacite north of Horseshoe Lake 68–68.5 17–20 pl-san-bi-hb-px-ox

dnk Trachydacite of North Knob 66.5 18–23 pl-san-bi-hb-ox

dom Trachydacite of Old Mammoth 65.5–66 15–20 pl-san-hb-bi-ox

drc Trachydacite of upper Reds Creek 63.5–65.5 15–22 pl-san-hb-bi-px-ox

dsd Trachydacite of South Summit Dome 67–68.5 17–22 pl-san-bi-hb-px-ox

dsk Trachydacite of Skyline Dome 67–70 15–20 pl-san-bi-px-ox

dsu Trachydacite of Mammoth Mtn summit 68–69.5 15–20 pl-san-bi-hb-px-ox

dtl Trachydacite of Twin Lakes outlet 63–64 25–35 pl-san-hb-bi-px-ox

dwr Trachydacite of White Bark Ridge 68–69.5 13–17 pl-san-bi-hb-px-ox

d61 Trachydacite of Dome 2861 66–67.5 15–20 pl-san-hb-bi-px-ox

d81 Trachydacite of Dome 2781 68.5–69 13–17 pl-san-bi-hb-px-ox

rce Alkalic rhyodacite of Canyon Express 70–71 5–9 pl-san-bi-ox

rfp Alkalic rhyodacite pumice fall of Hwy 203 70–71 5–8 pl-san-bi-ox

rmf Alkalic rhyodacite of Mammoth Mtn fumarole 70–71 12–15 pl-san-bi-px-ox

rrc Alkalic rhyodacite of Reds Creek 71 5–7 pl-san-px-ox

rsq Alkalic rhyodacite of Quicksilver Ski Run 70–71 10–15 pl-san-bi-px-ox

rss Alkalic rhyodacite of Solitude Ski Run 70–71 10–15 pl-san-bi-px-ox

South moat peripheral eruptive units (15 units) 

aml Trachyandesite of McLeod Lake 61.5–62.5 3–5 pl-hb-opx-ox

amp Trachyandesite of Mammoth Pass 57–61.5 1–2 pl-ol-cpx-ox

asr Trachyandesite of Shady Rest Campground 56.5 <1 pl-cpx-ox

bcd Basalt of Casa Diablo Hot Springs 51–52 15–20 pl-ol-cpx±ox

bed Basalt east of Dry Creek (source of bcd) 52 13–18 pl-ol-cpx±ox

bfh Basalt of Fish Hatchery 49–50 10–15 pl-ol-cpx

bhl Basalt of Horseshoe Lake 49 18–24 pl-cpx-ol

bmc Basalt of Mammoth Crest (source of bfh) 49.5–51 10–15 pl-ol-cpx

bsc Trachybasalt of Sherwin Creek Road 50.5–51 <1 pl-ol-ox

bsr Trachybasalt of Shady Rest 50.5–51.5 2–4 pl-ol-cpx±ox

mcl Basaltic trachyandesite of Canyon Lodge 54–56 1–5 pl-ol-cpx±ox

mkv Basaltic trachyandesite of Knolls Vista 54.5–55 1 pl-ol-cpx-ox

mlc Basaltic trachyandesite of lower Laurel Creek 52–53 15–20 pl-ol-cpx-ox

mmc Basaltic trachyandesite of Mammoth Creek 54.5–55 1 pl-ol-cpx-ox

msc Basaltic trachyandesite of Sawmill Cutoff 52–52.5 2–5 pl-ol-cpx-ox

West moat peripheral eruptive units (16 units)

aic Trachyandesite of Inyo Craters 57.5–59.5 1–2 pl-ol-cpx-ox

bar Trachybasalt of Arcularius Ranch 50.5–52.5 20–30 pl-ol-cpx±ox

bcf Trachybasalt south of Crater Flat 51.8 1 pl-ol-cpx

Table 1. Unit names and petrography.

[Mammoth magmatic system includes the main silicic edifice and contemporaneous basaltic, andesitic, and dacitic units that erupted peripherally. Long Valley 

rhyolites and nearby Tertiary eruptive units are listed separately. SiO2 contents are from appendix. Volume percent phenocrysts were estimated visually in 

thin section. Mineral species, in declining order of abundance, are abbreviated: aln, allanite; bi, biotite; cpx, clinopyroxene; hb, hornblende; ol, olivine; opx, 

orthopyroxene; ox, Fe-Ti oxides; pl, plagioclase; px, pyroxenes (undivided); qz, quartz; san, sanidine. Other abbreviations: pdc, pyroclastic density current; xen, 

xenocryst. Three-letter labels appear on maps and diagrams; details of all are given in Description of Map Units. Units amp, bed, and mcl appear in more than 

one geographic set below]

Tables 123
124 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Map unit Unit name Percent SiO2 Percent phenocrysts

West moat peripheral eruptive units (16 units)—Contined

bed Basalt east of Dry Creek (source of bcd) 52 13–18 pl-ol-cpx±ox

bmn Trachybasalt northeast of Minaret Summit 51–53 3–8 pl-ol-cpx-ox

bsm Basalt of Sawmill Cutoff 48.5–49 1–3 pl-ol-cpx

ddc Trachydacite of Dry Creek 67–67.5 3–5 pl-opx-ox

dnw Trachydacites of Northwest Moat 60–67 15–25 pl-san-bi-hb-px-ox

mcv Basaltic trachyandesite south of Crestview 53–54 1–2 pl-ol-cpx-ox

mdm Basaltic trachyandesite of Deer Mountain 55.5–56 <1 pl-cpx-ol

mdn Basaltic trachyandesite NE of Deadman Pass 52.5–54 10–15 pl-ol-cpx

mic Mafic pyroclastic deposit of Inyo Craters 54–56 1–3 pl-ol

mnd Basaltic trachyandesite NW of Dry Creek 53–54 <1 pl-ol-ox

mor Basaltic trachyandesite of Owens River Road 51–54 1–5 pl-ol-cpx-ox

msd Basaltic trachyandesite scoria SE of Dry Creek 52–53 <1 pl-ol

msj Basaltic andesite of Scenic Loop Road junction 52.8 3–4 ol-pl-cpx

San Joaquin peripheral eruptive units (8 units) 

a62 Trachyandesite of Cone 2962 57–59 1 pl-ol-cpx-ox

amp Trachyandesite of Mammoth Pass 57–61 1–2 pl-ol-cpx-ox

apb Trachyandesite of Pumice Butte 58–59 3–5 pl-ol-cpx-ox

bdc Trachybasalt west of Deer Creek 51.5 10–15 pl-ol

brc Basalt of Red Cones 50–51 12–18 pl-ol-cpx-ox

drf Trachydacite of Rainbow Falls 65.5–67 1–3 pl-bi-opx-hb-ox

mdp Basaltic trachyandesite of Devils Postpile 54–54.5 9–13 pl-ol-cpx

mss Basaltic trachyandesite of upper Soda Springs 52.5–53.5 2–5 pl-ol-ox

Long Valley magmatic system (8 units in map area) 

rbt Bishop Tuff 71–78 5–25 san-qz-pl-bi-ox-px-aln

rdc Rhyolite of Dry Creek Dome 76–76.5 15–20 san-pl-qz-bi-hb-ox-aln

rdm Rhyolite of Deer Mountain 72 20–30 qz-pl-san-bi-hb-ox-px-aln

rer Early Rhyolite of Bailey 74.5–75.5 0–2 pl-opx-bi

rhc Rhyolite of Hot Creek flow 76 1 pl-san-bi-px-ox

rmk Rhyolite of Mammoth Knolls 76.5–77 20 san-qz-pl-bi-hb-ox-aln

rnc Rhyolite of north-central chain 74–75.5 15–20 pl-san-hb-bi-qz-ox

rwm Rhyolite of West Moat Coulee 72–73.5 17–25 pl-qz-san-bi-hb-ox-px-aln

Tertiary eruptive units (32 in map area)

Tacc Trachyandesite of Crater Creek 58–63 1 hb-cpx-(qz xens)

Taec Trachyandesite east of Crestview 57 <1 ol

Tasj Trachyandesite of San Joaquin River 58–58.3 15–20 pl-cpx-ol-ox

Taww Trachyandesite north of White Wing Camp 59–60 4–7 ol-pl-cpx

Tbbs Basalt east of Big Springs 50–51 <1 ol

Tbdp Trachybasalt of Deadman Pass 50–53 7–20 ol-cpx-ox

Tbld Basalt south of Lost Dog Lake 52 5–7 ol-ox

Tbmm Trachybasalt of Mammoth Mine 52 7–10 ol-ox 

Tbpl Trachybasalt east of Pond Lily Lake 49–52 7–12 ol-cpx

Tbrm Basalt of Red Mountain 50 5–8 ol-ox

Tbtb Basalt of The Buttresses 46.5–50.5 15–25 cpx-ol±pl

Tb84 Trachybasalt of Cone 8478 49–54 8–15 ol-cpx-pl-ox

Tdlm Trachydacite of Laurel Mountain 63 5 hb-pl-opx-ox

Table 1. Unit names and petrography.—Continued

[Mammoth magmatic system includes the main silicic edifice and contemporaneous basaltic, andesitic, and dacitic units that erupted peripherally. Long Valley 

rhyolites and nearby Tertiary eruptive units are listed separately. SiO2 contents are from appendix. Volume percent phenocrysts were estimated visually in 

thin section. Mineral species, in declining order of abundance, are abbreviated: aln, allanite; bi, biotite; cpx, clinopyroxene; hb, hornblende; ol, olivine; opx, 

orthopyroxene; ox, Fe-Ti oxides; pl, plagioclase; px, pyroxenes (undivided); qz, quartz; san, sanidine. Other abbreviations: pdc, pyroclastic density current; xen, 

xenocryst. Three-letter labels appear on maps and diagrams; details of all are given in Description of Map Units. Units amp, bed, and mcl appear in more than 

one geographic set below]
Map unit Unit name Percent SiO2 Percent phenocrysts

Tdpd Trachydacite pdc of Deadman Pass ~70 20–25 pl-bi-hb-opx-ox

Tdsd Trachydacite of Dome 3321 (South Dome) ~70 30–35 pl-bi-hb-opx-ox

Tdsj Trachydacite block-and-ash-flow deposit of San 

Joaquin Ridge

~70 30–35 pl-bi-hb-opx-ox

Tdud Trachydacite complex of upper Deadman Creek 68–71 25–35 pl-bi-hb-ox 

Tdww Trachydacite of White Wing Mountain 67 20–25 pl-bi-hb-ox

Td65 Trachydacite of Dome 2965 64.5 20–25 pl-bi-hb-opx-ox

Td78 Trachydacite of Dome 7835 65 10 bi-hb-pl-ox

Td83 Trachydacite of Dome 8325 66–66.5 10–13 bi-hb-pl-cpx-ox

Tmcl Basaltic trachyandesite west of Cabin Lake 53 7–10 ol

Tmcm Basaltic trachyandesite south of Crater Mdw 53 5–7 ol

Tmcr Basaltic trachyandesite of Mammoth Crest 53 7 ol-ox

Tmcv Basaltic trachyandesite of Crestview 53–55 7–10 ol-cpx-pl

Tmcw Basaltic trachyandesite west of Crater Creek 52–53 10 ol-cpx-pl

Tmel Basaltic trachyandesite of Emerald Lake 52 7–10 ol-cpx

Tmsd Basaltic trachyandesite south of Deer Creek 53 5–7 ol-cpx

Tmwc Basaltic trachyandesite west of Castle Lake 53 7 ol 

Tmw Basaltic trachyandesite near Obsidian Flow 53–54.5 10–15 ol-pl-cpx

Tmww Basaltic trachyandesite of White Wing Mtn 52 5–8 ol-cpx

Trac Rhyolite tuff of Alpers Canyon 72+ 30–45 san-qz-pl-bi-ox

Table 1. Unit names and petrography.—Continued

[Mammoth magmatic system includes the main silicic edifice and contemporaneous basaltic, andesitic, and dacitic units that erupted peripherally. Long Valley 

rhyolites and nearby Tertiary eruptive units are listed separately. SiO2 contents are from appendix. Volume percent phenocrysts were estimated visually in 

thin section. Mineral species, in declining order of abundance, are abbreviated: aln, allanite; bi, biotite; cpx, clinopyroxene; hb, hornblende; ol, olivine; opx, 

orthopyroxene; ox, Fe-Ti oxides; pl, plagioclase; px, pyroxenes (undivided); qz, quartz; san, sanidine. Other abbreviations: pdc, pyroclastic density current; xen, 

xenocryst. Three-letter labels appear on maps and diagrams; details of all are given in Description of Map Units. Units amp, bed, and mcl appear in more than 

one geographic set below]

Tables 125
126 Eruptive History of Mammoth Mountain and its Mafic Periphery, California Table 2. 40Ar/39Ar ages for Mammoth Mountain and its mafic periphery. [Samples irradiated at USGS TRIGA reactor using 9.6345 Ma Bodie Hills sanidine as a neutron flux monitor. MSWD, mean square weighted deviation. 40Ar/36Ar indicates initial ratio. Samples were ground- mass concentrates except those marked as plag (plagioclase) or fsp (sodic sanidine ± minor plagioclase). Preferred ages in bold font. All determinations in USGS lab at Menlo Park, supervised by A.T. Calvert. For details of methods, see Calvert and Lanphere (2006)] Sample Map unit 40Ar/39Ar weighted mean plateau age 40Ar/39Ar isotope correlation (isochron) age 40Ar/39Ar total gas age (ka) Recoil age (ka) Age (ka)

%39Ar [steps, °C] MSWD Age (ka)

%39Ar [steps, °C] MSWD 40Ar/36Ari

M-123 mdn 15.5 ± 1.8 62.8 [800–950] 0.55 19.8 ± 11.6 62.8 [800–950] 0.98 293.4 ± 12.5 4.2 ± 1.6M-124 mcl 177.2 ± 2.0 64.5 [700–950] 1.08 182.1 ± 14.8 64.5 [700–950] 1.31 294.2 ± 8.6 171.6 ± 2.2M-145 plag Tasj 9164 ± 17 41 [950–1150] 1.89 9188 ± 79 100 [650–1450] 35.7 279.2 ± 27.2 9102 ± 9 M-168 mdp 82.1 ± 1.6 79 [700–1050] 0.61 86.3 ± 4.1 79 [700–1050] 1.35 293.6 ± 3.1 79.9± 1.7M-173 bsr 103.5 ± 1.4 88 [550–925] 0.9 100.7 ± 6.5 88 [550–925] 1.02 297.6 ± 11.4 100.2 ± 1.5M-181 bfh 92.4 ± 2.5 100.0 [550–1175] 1.23 98.4 ± 3.1 100.0 [550–1175] 0.72 293.1 ± 1.9 87.5 ± 3.1M-182 bsc 171.8 ± 1.8 75.1 [650–900] 0.74 159.3 ± 7.1 75.1 [650–900] 0.17 303.5 ± 10.1 168.9 ± 1.7M-185 Tbtb 3754.4 ± 6.9 46.4 [625–795] 0.96 3754.3 ± 8.0 46.4 [625–795] 1.26 295.3 ± 15.6 3728.4 ± 6.7M-219 mcl 174.6 ± 3.2 73.0 [550–800] 0.61 166.2 ± 5.1 73.0 [550–800] 0.56 297.0 ± 1.7 161.2 ± 3.4M-225 fsp dnw 41.7± 0.8 85 [1125–1450] 1.16 43.8± 2.0 85 {1125–1450] 1.03 284.2± 21.3 38.3± 1.1M-240 Tdlm 3558.1 ± 5.8 52 [825–1025] 4.07 3574.5 ± 7.2 52 [825–1025] 1.96 295.9 ± 1.9 3605.6 ± 2.7 3600.2 ± 42.5M-243 bsm 164.9 ± 2.1 100 [550–1100] 0.67 169.2 ± 5.8 100 [550–1100] 0.79 294.0 ± 4.2 164.2 ± 2.0M-267 mcv 33.8 ± 1.0 72 [800–1150] 0.25 35.4 ± 2.9 72 [800–1150] 0.21 294.5 ± 3.7 28.2 ± 1.1M-282 bcf 163.7 ± 1.7 88 [550–900] 0.2 162.3 ± 3.7 88 [550–900] 0.21 295.9 ± 1.9 158.8 ± 2.1M-313 mlc 129.6 ± 1.2 92.6 [600–975] 0.95 131.9 ± 2.2 92.6 [600–975] 0.88 293.1 ± 4.1 127.1 ± 1.3M-315 mcl 179.5 ± 2.4 66 [750–1150] 0.67 183.0 ± 5.0 66 [750–1150] 0.68 294.7 ± 2.3 199.2 ± 2.4M-316 bcd 125.3 ± 1.9 82 [550–850] 0.67 125.9 ± 7.0 82 [550–850] 0.8 295.3 ± 4.8 115.3 ± 2.2M-319 dom 72.8 ± 0.6 88.5 [600–850] 0.82 70.5 ± 2.1 88.5 [600–850] 0.87 299.9 ± 9.3 70.6 ± 0.6M-322 fsp d81 98.9± 7.5 79 [800–1200] 0.47 72.8± 25.1 79 [800–1200] 0.26 307.7± 26.6 176.0± 7.3M-324 msc 154.2 ± 1.5 87 [600–1000] 0.96 153.2 ± 4.9 87 [600–1000] 1.08 295.9 ± 3.9 151.5 ± 2.0M-329 fsp rfp 80.1± 8.3 80 [900–1225] 0.36 65.3± 31.5 80 [900–1225] 0.34 326.6± 164.8 101.1±14.4M-335 mdn 16.9 ± 1.3 100 [550–1100] 0.65 16.4 ± 2.1 100 [550–1100] 0.72 295.7 ± 1.3 17.4 ± 1.5M-344 fsp dwr 73.2 ± 2.0 87.5 [700–1200] 0.64 75.1 ± 2.7 87.5 [700–1200] 0.5 293.1 ± 11.4 87.7 ± 2.4M-347 fsp dwr 78.6 ± 11.8 52.7 [700–1150] 0.34 79.0 ± 13.9 52.7 [700–1150] 0.4 295.3 ± 7.6 89.3 ± 10.3M-355 dnk 60.4 ± 1.2 88 [550–800] 0.26 62.5 ± 2.6 88 [550–800] 0.13 292.5 ± 7.7 57.3 ± 1.7M-359 fsp rss 50.3 ± 3.1 64.7 [600–1175] 0.25 50.4 ± 4.7 64.7 [600–1175] 0.3 295.4 ± 6.4 99.3 ±2.7M-365 mss 121.4 ± 2.1 96.9 [550–975] 1.49 120.2 ± 4.8 96.9 [550–975] 1.99 296.0 ± 4.5 124.8 ± 1.9M-374 fsp rrc 83.3 ± 1.1 67.1 [600–1000] 0.17 84.8 ± 2.2 67.1 [600–1000] 0.005 292.5 ± 8.7 111.1 ± 1.1M-375 fsp dsd 86.7 ± 6.1 79 [600–1100] 1.14 100.1 ± 10.1 79 [600–1100] 0.75 288.1 ± 10.9 158.9 ± 5.9M-392 fsp dfl 61.0 ± 2.7 78 [600–1175] 0.38 65.2 ± 9.0 78 [600–1175] 0.39 292.7 ± 12.6 73.5 ± 2.5M-395 fsp rce 79.9 ± 0.7 69.3 [800–1050] 0.22 80.3 ± 2.4 69.3 [800–1050] 0.29 294.4 ± 14.0 79.3 ± 0.9M-400 fsp rsq 63.7 ± 3.9 99 [700–1475] 0.44 61.0 ± 8.9 99 [700–1475] 0.47 297.0 ± 11.7 65.8 ± 4.4M-410 bhl 31.1 ± 1.0 100 [550–1150] 0.91 30.5 ± 2.1 100 [550–1150] 1.02 295.9 ± 3.8 31.2 ± 1.1M-411 fsp dnh 64.5± 0.8 73 [650–1250] 1.14 62.5± 1.2 73 [650–1250] 0.78 301.1± 6.0 68.7± 0.7M-416 fsp dnw 39.6 ± 0.5 63.5 [1125–1400] 2.95 40.5 ± 1.9 63.5 [1125–1400] 3.91 282.7 ± 52.8 37.3 ± 0.3
Table 2. 40Ar/39Ar ages for Mammoth Mountain and its mafic periphery.—Continued

[Samples irradiated at USGS TRIGA reactor using 9.6345 Ma Bodie Hills sanidine as a neutron flux monitor. MSWD, mean square weighted deviation. 40Ar/36Ar indicates initial ratio. Samples were groundmass concentrates except those marked as plag (plagioclase) or fsp (sodic sanidine ± minor plagioclase). Preferred ages in bold font. All determinations in USGS lab at Menlo Park, supervised by A.T. Calvert. For details of methods, see Calvert and Lanphere (2006)]

Sample Map unit 40Ar/39Ar weighted mean plateau age 40Ar/39Ar isotope correlation (isochron) age 40Ar/39Ar total 

gas age (ka)

Recoil

age (ka) Age (ka) %39Ar [steps, °C] MSWD Age (ka) %39Ar [steps, °C] MSWD 40Ar/36Ari

M-416 fsp dnw 34.0 ± 13.6 100 [11 crystals] 0.58 48.3 ± 11.7 100 [11 of 11] 4.4 164.6 ± 133.6 42.2 ± 14.4M-434 fsp dlp 63.7 ± 6.8 75 [850–1225] 1.62 49.4 ± 21.9 75 [850–1225] 1.8 300.1 ± 16.0 84.3 ± 5.4M-447 Tmwo 3125 ± 3 37 [725–820] 0.037 3123 ± 9 37 [725–820] 0.02 297.0 ± 13.2 3140 ± 2 3153 ± 26M-454 fsp rmf 61.4 ± 1.5 64.5 [700–1125] 1.93 62.9 ± 2.9 64.5 [700–1125] 2.18 290.2 ± 16.6 71.8 ± 0.9M-463 dtl 75.9 ± 0.6 97.0 [550–1000] 0.79 75.1 ± 1.1 97.0 [550–1000] 0.86 298.1 ± 7.3 77.0 ± 0.7M-483 aic 130.8 ± 1.4 52.1 [750–950] 0.34 133.7 ± 3.9 52.1 [750–950] 0.38 293.3 ± 6.6 132.6 ± 1.0 133.9 ± 1.9M-487 bdc 155.3 ± 2.0 95 [600–1150] 1.57 158.3 ± 4.7 95 [600–1150] 1.64 294.1 ± 4.2 156.8 ± 1.9M-494 Tmcm 3377.1 ± 8.8 60 [670–950] 12.28 3362.6 ± 14.7 60 [670–950] 8.55 342.7 ± 103.3 3311.9 ± 2.4M-523 fsp dsk 71.2 ± 1.5 94.5 [700–1225] 0.2 69.8 ± 5.0 94.5 [700–1225] 0.16 298.8 ± 22.9 79.6 ± 1.9M-550 bmc 92.7 ± 2.4 96.2 [550–1000] 0.82 92.2 ± 4.5 96.2 [550–1000] 1.04 295.7 ± 3.6 90.6 ± 2.5M-613 drf 98.8 ± 0.6 100 [550–1150] 1.17 97.6 ± 1.3 100 [550–1150] 1.17 297.1 ± 4.0 99.0 ± 0.6M-650 Tasj 9191.5 ± 14.5 47 [900–1075] 0.57 9216.4 ± 31.2 47 [900–1075] 0.13 264.6 ± 76.1 9217.2 ± 14.0M-815 Tbdp 3711.6 ± 5.5 37 [750–850] 0.25 3708.2 ± 7.9 37 [750–850] 0.14 298.0 ± 9.8 3699.0 ± 4.5 3700.2 ± 11.6M-816 Tbdp 3277.3 ± 6.7 88 [700–1025] 1.8 3271.9 ± 7.0 88 [700–1025] 1.68 301.6 ± 12.4 3281.2 ± 5.1M-869 dsu 61.4 ± 2.0 98 [600–1400] 0.91 60.8 ± 2.7 98 [600–1400] 1.04 295.9 ± 5.6 63.1 ± 2.5M-899 amp 97.3 ± 1.2 100 [550–1150] 1.18 90.1 ± 3.2 100 [550–1150] 0.67 301.5 ± 5.9 98.5 ± 1.2Inyo-4-461 mcl 

(upperGroup II)

185.9± 1.7 97 [600–1100] 0.77 185.6 ± 3.8 97 [600–1100] 0.85 295.6 ± 5.3 184.7 ± 2.0

Inyo-4-824 Group III 229.9± 6.5 94 [650–1200] 3.87 217.8 ± 5.2 94 [650–1200] 1.52 298.5 ± 1.9 251.4 ± 5.6Inyo-4-918 Group III 222.5± 5.5 56 [700–925] 0.35 211.8 ± 13.2 56 [700–925] 0.2 297.5 ± 5.2 220.6 ± 4.4Inyo-4-1046 Group IV 232.6± 4.2 96 [650–1300] 0.98 224.0 ± 6.9 96 [650–1300] 0.82 296.8 ± 1.9 240.2 ± 5.3

Tables 127
128 Eruptive History of Mammoth Mountain and its Mafic Periphery, California

Map unit Length in km 

Peripheral units

amp 9.2 (E)

>7.3 (SW)

a62 3.5

bar 22.5

bcd 10.5

bfh 18.1

brc 2

bsc >8

bsm >4.3

drf >6

mcl 9.5 (NE)

8.8 (E)

mcv 9.8

mdp >7.2

mkv-mmc >4.5

mor 11.8

Mammoth Mtn

dml 2.2

dom 4 (?)

dwr 1.5

rce 2.6

Long Valley Rhyolites

rlm 3.8 (ESE flow, Lookout Mtn)

rhc 4.6

rnc 3.8

rwm 5.3

Neogene units

Tasj 80

Tbtb >15 (?)

Table 3. Lengths of lava-flow aprons in Mammoth Lakes region.
Produced in the Menlo Park Publishing Service Center, California

Edited by Claire M. Landowski

Cartography and layout by Kathryn Nimz

Manuscript approved for publication January 27, 2015
Hildreth and Fierstein—Eruptive History of Mammoth Mountain and its Mafic Periphery, California—Professional Paper 1812

ISSN 1044-9612 (print)

ISSN 2330-7102 (online)

http://dx.doi.org/10.3133/pp1812