2010

U.S. Department of the Interior

U.S. Geological Survey

Geologic Map of Lassen Volcanic National Park and 

Vicinity, California

By Michael A. Clynne and L.J. Patrick Muffler

Pamphlet to accompany

Scientific Investigations Map 2899

Lassen Peak and the Devastated Area

Aerial view of Lassen Peak and the proximal Devastated Area looking south. Area with sparse trees marks the paths of the avalanche and 

debris-flow deposits of May 19–20, 1915 (unit sw9) and the pyroclastic-flow and fluid debris-flow deposits of May 22, 1915 (unit pw2) 

(Clynne and others, 1999; Christiansen and others, 2002). Small dark crags just to right of the summit are remnants of the May 19–20, 

1915, lava flow (unit d9). The composite dacite dome of Lassen Peak (unit dl, 27±1 ka) dominates the upper part of the view. Lithic 

pyroclastic-flow deposit (unit pfl) from partial collapse of the dome of Lassen Peak is exposed in the canyon of the headwaters of Lost 

Creek in center of view. Ridges flanking central area are glacial moraines (unit Qta) thinly covered by deposits of the 1915 eruption of 

Lassen Peak (Christiansen and others, 2002). Small permanent snowfield is seen on the left lower slope of Lassen Peak. Area east of 

the snowfield is the rhyodacite lava flow of Kings Creek (unit rk, 35±1 ka, part of the Eagle Peak sequence). Dacite domes of Bumpass 

Mountain (unit db, 232±8 ka), Crescent Crater (unit dc, 236±1 ka), hill 8283 (unit d82, 261±5 ka), and Loomis Peak (unit rlm, ~300 ka) 

are part of the Bumpass sequence. Photograph by Michael A. Clynne. 
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CONTENTS

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

Regional Volcanism.......................................................................................................................................1

Calc-Alkaline Volcanoes......................................................................................................................1

Low-Potassium Olivine Tholeiite (LKOT)...........................................................................................2

Volcanic Centers............................................................................................................................................2

Tectonic Setting .............................................................................................................................................3

Eruptive History of the Lassen Area...........................................................................................................4

Regional Volcanic Rocks .....................................................................................................................4

Pliocene-Early Pleistocene Volcanic Centers .................................................................................4

Dittmar Volcanic Center..............................................................................................................5

Maidu Volcanic Center ...............................................................................................................5

Lassen Volcanic Center.......................................................................................................................6

Rockland Caldera Complex ........................................................................................................6

Brokeoff Volcano .........................................................................................................................7

Lassen Domefield.........................................................................................................................7

Holocene Activity................................................................................................................8

Chaos Crags................................................................................................................8

Cinder Cone ................................................................................................................8

1914-1917 Eruption of Lassen Peak.........................................................................9

Caribou Volcanic Field .......................................................................................................................10

Red Cinder Chain .......................................................................................................................10

Poison Lake Chain .....................................................................................................................10

Caribou Chain .............................................................................................................................10

Other Chains and Sequences in the Caribou Volcanic Field..............................................10

Glacial Geology ............................................................................................................................................11

Methods.........................................................................................................................................................11

Unit Names...........................................................................................................................................11

Mineral Names....................................................................................................................................12

Reacted Phenocrysts and Mafic Inclusions ..................................................................................12

Unit Ages..............................................................................................................................................12

Unit Labels............................................................................................................................................12

Acknowledgments.......................................................................................................................................13

Description of Map Units ...........................................................................................................................14

Glacial, sedimentary, and hydrothermal deposits.........................................................................14

Nonvolcanic sedimentary deposits ........................................................................................14

Alluvium and colluvium....................................................................................................14

Mass-wasting deposits ...................................................................................................14

Hydrothermal deposits.....................................................................................................16

Glacial deposits.................................................................................................................16

Till .......................................................................................................................16

Outwash ....................................................................................................................17

Sedimentary deposits ......................................................................................................18

Volcanic Centers of southernmost Cascades................................................................................18

Lassen Volcanic Center............................................................................................................18

Lassen domefield..............................................................................................................18

Twin Lakes sequence, younger.............................................................................18

Twin Lakes sequence, older...................................................................................24
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Eagle Peak sequence..............................................................................................25

Bumpass sequence.................................................................................................30

Brokeoff Volcano ..............................................................................................................34

Diller sequence ........................................................................................................34

Mill Canyon sequence ............................................................................................36

Rockland caldera complex..............................................................................................38

Maidu Volcanic Center .............................................................................................................40

Stage 3................................................................................................................................41

Stage 2................................................................................................................................42

Stage 1................................................................................................................................43

Dittmar Volcanic Center............................................................................................................44

Stage 3................................................................................................................................44

Stage 2................................................................................................................................45

Stage 1................................................................................................................................46

Latour Volcanic Center .............................................................................................................47

Tuscan Formation.......................................................................................................................47

Regional volcanic rocks.....................................................................................................................48

North and west of Lassen Volcanic Center...........................................................................48

Late Pleistocene volcanic rocks ....................................................................................48

Tumble Buttes chain................................................................................................49

Sugarloaf chain........................................................................................................52

Middle Pleistocene and older volcanic rocks .............................................................53

South and east of Lassen Volcanic Center............................................................................61

Regional volcanic rocks of the Caribou area........................................................................65

Caribou Volcanic Field .....................................................................................................66

Tuya chains ...............................................................................................................66

Red Cinder chain......................................................................................................66

Bidwell Spring chain ...............................................................................................69

Poison Lake chain....................................................................................................71

Cone Lake chain.......................................................................................................77

Caribou chain ...........................................................................................................78

Triangle Lake sequence..................................................................................78

Echo Lake sequence .......................................................................................79

Caribou sequence............................................................................................80

Sunrise Peak sequence..........................................................................................86

Island Lake sequence .............................................................................................87

Beauty Lake sequence ...........................................................................................87

Older volcanic rocks ........................................................................................................88

References cited..........................................................................................................................................93

Tables

1. Correlation of glacial terminology, age assignments, and depiction of glacial deposits in 

Lassen Volcanic National Park and vicinity, California...................................................96

2. K-Ar and 40Ar/39Ar Ages obtained for the geologic map of Lassen Volcanic National Park 

and vicinity, California ..........................................................................................................97

3. Alphabetical list of map unit names, labels, and ages ..........................................................101

Figures

1. Map of northeastern California showing location of geologic map and Lassen Volcanic 

National Park (sheet 3)

2. Map showing areas of published geologic mapping (sheet 3)

3. Shaded-relief map of the Lassen Volcanic National park and vicinity, California (sheet 3)
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4. Generalized geologic maps showing the location, extent, and ages of large volcanic 

centers, regional volcanic rocks, and stratigraphy of the Lassen Volcanic Center 

(sheet 3)

5. Map showing regional tectonic setting of area from Sacramento Valley to CaliforniaNevada border (sheet 3)

6. Generalized geologic map showing the location and extent of volcanic sequences and 

chains in the Caribou Volcanic Field (sheet 3)
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1

Introduction

The geologic map of Lassen Volcanic National Park 

(LVNP) and vicinity encompasses 1,905 km2 at the south end 

of the Cascade Range in Shasta, Lassen, Tehama, and Plumas 

Counties, northeastern California (fig. 1, sheet 3). The park 

includes 430 km2 of scenic volcanic features, glacially sculpted 

terrain, and the most spectacular array of thermal features in the 

Cascade Range. Interest in preserving the scenic wonders of the 

Lassen area as a national park arose in the early 1900s to protect 

it from commercial development and led to the establishment in 

1907 of two small national monuments centered on Lassen Peak 

and Cinder Cone. The eruptions of Lassen Peak in 1914–15 

were the first in the Cascade Range since widespread settling of 

the West in the late 1800s. Through the printed media, the eruptions aroused considerable public interest and inspired renewed 

efforts, which had languished since 1907, to establish a national 

park. In 1916, Lassen Volcanic National Park was established 

by combining the areas of the previously established national 

monuments and adjacent lands. 

The southernmost Cascade Range is bounded on the west 

by the Sacramento Valley and the Klamath Mountains, on the 

south by the Sierra Nevada, and on the east by the Basin and 

Range geologic provinces. Most of the map area is underlain 

by middle to late Pleistocene volcanic rocks; Holocene, early 

Pleistocene, and late Pliocene volcanic rocks (<3.5 m.y.) are less 

common. Paleozoic and Mesozoic rocks are inferred to underlie 

the volcanic deposits (Jachens and Saltus, 1983), but the nearest 

exposures of pre-Tertiary rocks are 15 km to the south, 9 km to 

the southwest, and 12 km to the west. Diller (1895) recognized 

the young volcanic geology and produced the first geologic map 

of the Lassen area. The map (sheet 1) builds on and extends 

geologic mapping by Williams (1932), Macdonald (1963, 1964, 

1965), and Wilson (1961). The Lassen Peak area mapped by 

Christiansen and others (2002) and published in greater detail 

(1:24,000) was modified for inclusion here. Figure 2 (sheet 3) 

shows the mapping credit for previous work; figure 3 (sheet 3) 

shows locations discussed throughout the text. 

A CD-ROM entitled Database for the Geologic Map of 

Lassen Volcanic National Park and Vicinity, California accompanies the printed map (Muffler and others, 2010). The CD-ROM 

contains ESRI compatible geographic information system data 

files used to create the 1:50,000-scale geologic map, both geologic and topographic data and their associated metadata files, 

and printable versions of the geologic map and pamphlet as PDF 

formatted files. The 1:50,000-scale geologic map was compiled 

from 1:24,000-scale geologic maps of individual quadrangles 

that are also included in the CD-ROM. It also contains ancillary 

data that support the map including locations of rock samples 

selected for chemical analysis (Clynne and others, 2008) and 

radiometric dating, photographs of geologic features, and links 

to related data or web sites. Data contained in the CD-ROM are 

also available at http://pubs.usgs.gov/sim/2899/.

The southernmost Cascade Range consists of a regional 

platform of basalt and basaltic andesite, with subordinate 

andesite and sparse dacite. Nested within these regional rocks 

are “volcanic centers”, defined as large, long-lived, composite, 

calc-alkaline edifices erupting the full range of compositions 

from basalt to rhyolite, but dominated by andesite and dacite. 

Volcanic centers are produced by the focusing of basaltic flux 

from the mantle and resultant enhanced interaction of mafic 

magma with the crust. Collectively, volcanic centers mark the 

axis of the southernmost Cascade Range. The map area includes 

the entire Lassen Volcanic Center, parts of three older volcanic centers (Maidu, Dittmar, and Latour), and the products of 

regional volcanism (fig. 4, sheet 3). Terminology used for subdivision of the Lassen Volcanic Center has been modified from 

Clynne (1984, 1990).

Regional Volcanism

Regional volcanism built a broad platform that covers the 

southernmost Cascade Range (fig. 4A, B). Two distinct parental 

magmas and their derivatives contribute to the suite of regional 

lavas in the Lassen area: calc-alkaline basalt and tholeiitic basalt 

(Clynne, 1993; Borg and others, 2002). Calc-alkaline compositions are related to Cascade Arc magmatism and dominate the 

regional suite in both volume and abundance. Small calc-alkaline volcanoes are cinder cones with fields of lava flows, and 

large calc-alkaline volcanoes are steep-sided cones or shields 

with gentle slopes. Low-potassium olivine tholeiitic basalt 

(LKOT; also called high-alumina olivine tholeiitic or HAOT) 

is a volumetrically minor but widespread component of the 

regional volcanism and is related to the Basin and Range geologic province. LKOT erupts from fissures between the shield 

volcanoes and the fluid lavas flood intervening valleys. Edifices 

containing both the calc-alkaline and LKOT suites are not 

found. On this map, LKOT is distinguished from calc-alkaline 

lavas by color and by including tholeiitic in the unit names. The 

overall result of regional volcanism is a broad platform of overlapping mafic to intermediate volcanoes. A seismic-refraction 

experiment reported by Berge and Stauber (1987) indicates that 

the young volcanic platform is about 4 km thick in the active 

southernmost Cascade Arc. 

Calc-alkaline Volcanoes

Calc-alkaline regional volcanoes have a wide range of 

sizes and forms. They comprise a continuum of sizes from 

cinder cones with lava flows through steep-sided lava cones to 

broad shield volcanoes. Typically, lava flows from regional volcanoes have blocky vesicular lava surfaces, although aa surfaces 

are also observed. Interiors have thick, slabby to thin, platy 

joint patterns; aphanitic textures are typical, and holocrystalline 

textures are generally confined to thick or ponded flows.

The smallest and most abundant volcanoes are cinder 

cones. Cinder cones are composed of bedded cinders or scoria 

and bombs piled up around a vent. Typically they have an associated lava flow that erupts from the base of the cone. Cinder 

cones have a single period of activity (monogenetic) and erupt 

small-volume aa to blocky lava flows. These small calc-alkaline 

volcanoes are usually compositionally and mineralogically 

homogeneous and tend to occur in groups of similar age and 
2

lithology. We use the term "sequence" for groups of volcanoes 

related by lithology and age and the term "chain" for sequences 

displaying a conspicuous linear geometry.

Lava cones and shield volcanoes are bigger than cinder 

cones and are more or less circular in plan view. Lava cones and 

shield volcanoes are constructed by eruptions from a central vent 

or flank vents high on the edifice to build a steep-sided lava cone 

or flatter shield volcano. They are usually capped by a cinder 

cone but generally lack widely distributed fragmental deposits. 

Typically they are active for a short period of decades to a few 

millennia and build a variety of edifice sizes of as much as a 

few cubic kilometers in volume. West Prospect Peak and the 

complex of Red Lake Mountain and Red Mountain are examples 

of lava cones in the map area. Sugarloaf Peak, mostly outside the 

map area, is another very symmetric lava cone. Prominent shield 

volcanoes in the map area are Prospect Peak, Sifford Mountain, 

Mount Harkness, Badger Mountain, and Table Mountain. 

Rocks from the longer-lived, larger regional volcanoes 

(shields or lava cones) are composed of a single composition 

or display minor ranges throughout the lifetime of the edifice. 

When heterogeneous, they typically evolve with time to higher 

silica and crystal contents with accompanying changes in 

phenocryst assemblage. These ranges are discussed in the unit 

descriptions, but the larger regional volcanoes are usually considered single entities and are generally mapped as single units. 

The calc-alkaline regional suite is dominated by a continuum of olivine basalt, olivine-augite basaltic andesite, and 

two-pyroxene andesite; rocks more silicic than andesite are 

rare. Phenocryst assemblages in calc-alkaline regional lavas are 

relatively simple. Primitive basalts contain olivine, commonly 

without a second phase. Evolved basalts add augite and plagioclase to the fractionating assemblage, and most basaltic andesites contain a combination of olivine, augite, and plagioclase 

phenocrysts. Complex zoning patterns, multiple populations of 

phenocrysts, and resorption or reaction features in phenocrysts 

are generally absent in mafic calc-alkaline rocks, but they are 

present in some andesites. Hypersthene replaces olivine in the 

fractionating assemblage in the range 56–58 percent SiO2

, and 

most andesites contain two pyroxenes. Glomeroporphyritic 

clots of crystals, generally of the same phases as present in the 

rock, are common in evolved rocks of the calc-alkaline suite. 

Individual calc-alkaline volcanoes erupt from a single or small 

number of compositionally similar magma batches with relatively little fractionation or evolution. 

The calc-alkaline lava suite displays significant geochemical diversity but has geochemical characteristics typical of arc 

volcanic rocks erupted through continental crust, for example, 

the Japan or Kamchatka Arcs (Gill, 1981; Thorpe, 1982). 

Geochemical diversity is greatest in the most primitive compositions, and fractionation reduces the range of geochemical 

diversity in more evolved rocks through interaction with the 

lower crust and mixing within the suite. Evolution of mafic 

magmas is dominated by crystal fractionation, but addition of a 

small-degree partial melt of mafic lower crust is also an important process (Clynne, 1993). Assimilation of upper crust plays 

a role in determining the geochemical composition of some 

andesitic regional volcanoes. 

Low-Potassium Olivine Tholeiite (LKOT)

Erupted volumes of LKOT are relatively large; volumes 

of as much as a few cubic kilometers are typical. LKOT flows 

erupt from fissures that build spatter ramparts and rarely 

construct cinder cones. These vents are inconspicuous in the 

geologic record because they are composed of glassy scoria that 

is easily weathered and eroded, and the low-relief vent profiles 

are often buried by younger volcanism. Vents for LKOT flows 

older than a few hundred thousand years are difficult to identify. 

LKOT lava flows form widespread sheets that have a characteristic surface morphology. The upper and lower surfaces of 

flow units contain abundant spherical vesicles, and flow tops 

are often ropy (pahoehoe) and have a characteristic joint pattern 

of crudely hexagonal blocks. Flow-unit interiors are generally 

holocrystalline and diktytaxitic and are massive with a widely 

spaced blocky joint pattern. The upper surfaces have little relief 

except for common to abundant tumuli. These characteristics 

are rapidly obscured by weathering and soil-forming processes 

so that within about 400 to 500 ka, LKOT flows in the Lassen 

area become flat surfaces with thick soil.

LKOT displays remarkable chemical and petrographic 

homogeneity. Most units contain 48 to 50 percent SiO2

 and are 

olivine basalts. LKOT lava typically contains sparse olivine 

phenocrysts accompanied by plagioclase in slightly more 

evolved examples. Augite phenocrysts and multiple phenocryst 

populations are rare, as are phenocrysts with strong zoning or 

resorption textures. Glomeroporphyritic clots of crystals like 

those found in calc-alkaline rocks are uncommon and generally confined to near-vent parts of lava flows. Borg and others 

(2002) summarized the lithologic, petrographic, and distinctive 

geochemical characteristics of LKOT. 

Volcanic Centers

Intercalated within the regional volcanic rocks are a few 

voluminous (as much as a few hundred cubic kilometers) longlived (0.5–1 m.y. or longer) foci of volcanism called volcanic 

centers. Volcanic centers erupt calc-alkaline compositions. The 

fundamental differences between regional calc-alkaline volcanoes and volcanic centers are the latter’s longevity, volume, 

and range of compositions in a single edifice or geographic 

locale. Five volcanic centers younger than about 3.5 Ma are 

recognized along the Cascade axis in the Lassen area (fig. 

4A). Each volcanic center experienced a unique evolution but 

generally consists of an initial phase of silicic volcanism followed by construction of a large composite cone dominated by 

andesite, which is flanked by a variety of younger, more silicic 

rocks. Late in the evolution of each volcanic center, an acidic 

hydrothermal system driven by heat from cooling subvolcanic 

silicic magma bodies caused alteration of the permeable rocks 

of the composite cone. Subsequent glacial and fluvial erosion of the altered interiors of the composite cones resulted in 

bowl-shaped depressions surrounded by more-resistant rims of 

thick, unaltered, flank lava flows. The Yana, Maidu, Dittmar, 

and Latour Volcanic Centers have reached this stage, and their 
3

hydrothermal systems are extinct. The Lassen Volcanic Center 

hosts continuing silicic volcanism and an active hydrothermal 

system (Muffler and others, 1982b; Muffler and others, 1983; 

Ingebritsen and Sorey, 1985; Clynne and others, 2003; Janik 

and McLaren, 2010). 

Volcanic centers contain a diverse assemblage of rock 

types from basalt or basaltic andesite to rhyolite but are dominated by intermediate calc-alkaline compositions (andesite–

dacite). Rocks in the volcanic centers are petrographically 

much more complex than rocks from regional volcanoes, and 

geochemical and petrographic evidence for magma mixing 

is abundant (Bullen and Clynne, 1990). Multiple phenocryst 

populations, reverse and strong normal zoning, compositional 

spikes in phenocryst compositions, and resorbed and disequilibrium phenocryst assemblages are typical of volcaniccenter rocks. Phenocrysts commonly are larger and more 

abundant than in regional lavas of equivalent silica content. 

Glomeroporphyritic clots of crystals, which we interpret to be 

derived from bodies of cumulate material fractionated from 

parental and evolved mafic magmas, are abundant in volcaniccenter andesites and attest to efficient recycling of crystals in 

the magmatic system. 

Volcanic centers result from a local increase in the amount 

of basaltic magma intruded into the crust from the mantle 

relative to the background calc-alkaline regional volcanism 

(Guffanti and others, 1996). Increased basaltic intrusion heats 

the crust and promotes interaction between mafic magma and 

the crust. A complex vertically zoned magmatic system with 

localized bodies of magma develops in the crust beneath the 

volcanic center. Magma mixing between andesitic magma 

batches is common, and eventually the additional heat causes 

crustal melting beneath volcanic centers leading to eruption of 

silicic lavas (Borg and Clynne, 1998) and interaction between 

mafic and silicic magma bodies. Geochemical diversity of rocks 

decreases with time in volcanic centers, attesting to the increasing importance of homogenization in the magmatic systems 

during their evolution (Clynne, 1990; Bullen and Clynne, 1990; 

Feeley and others, 2008). 

Volcanic centers are the surface expression of a magmatic 

cycle driven by a local increase in regional magmatic flux that 

waxes as the crust is heated and wanes as the easily fusible 

component of the lower crust is depleted. Early in the evolution 

of volcanic centers, magmatic fractionation is dominated by 

crystal fractionation of basalt with a relatively small contribution 

from the crust. As the evolution of volcanic centers proceeds, 

the crustal component becomes increasingly important until, in 

the later stages, it dominates the magmatic system. The origin 

of increased magmatic flux is unclear; it may be caused by 

structures that focus magma at the base of the crust or within the 

lower crust (Blakely and others, 1997). Alternatively, it could be 

related to inhomogeneities of magma production in the mantle 

wedge or even to heterogeneities at the slab-wedge interface. 

Volcanic centers do not significantly overlap in space, possibly 

as a result of the depletion of the easily fusible component in 

the lower crust that is required for production of large volumes 

of silicic magma. Given the uncertainties in dating the early 

volcanic centers, we believe that volcanism in volcanic centers 

has been nearly continuous somewhere in the Lassen area over 

the past 3.5 m.y. Typically, a single volcanic center is active and 

becomes extinct before or as the next center begins. However, 

during the late Pliocene to early Pleistocene, the life spans of the 

Maidu and Dittmar Volcanic Centers coincided (fig. 4A). 

Tectonic Setting

Figure 5 (sheet 3) illustrates the tectonic setting of the 

Lassen segment (Guffanti and Weaver, 1988) of the Cascade 

Volcanic Arc from the Sacramento Valley on the west to the 

California-Nevada border on the east. Volcanism in the area 

is a result of oblique subduction of the Juan de Fuca oceanic 

plate beneath the North American continental plate offshore of 

northern California. About 12 Ma, the axis of the Lassen segment was near the California-Nevada border. Since then, the 

arc axis has migrated westward to its present position (Guffanti 

and others, 1990). Simultaneously, the width of the arc narrowed, probably due to slab steepening. The southern terminus 

of active volcanism retreated roughly parallel to the northward 

extension of the San Andreas Fault system and migration of the 

Mendocino Triple Junction off the northern California coast. 

Also at 12 Ma, the south limit of Cascade volcanism was in the 

Sierran block approximately at the latitude of Lake Tahoe, 180 

km southeast of the Lassen area. At 3 Ma, the south limit of 

active volcanism was in the area of the Yana Volcanic Center, 

30 km south of Lassen Peak near Lake Almanor (fig. 5). At 

present, the south limit of active volcanism approximately 

corresponds to the south boundary of LVNP, and the youngest 

dated regional volcano in the area is Sifford Mountain at about 

170 ka. These relations suggest a long-term migration rate for 

the southern terminus of Cascade Arc volcanism of 15 to 20 km 

per m.y. (1.5 to 2 cm per yr) and a rate for at least the last 3 m.y. 

of about 10 km per m.y. (1 cm per yr). 

Two adjacent tectonic regimes affect the Lassen area. The 

extensional Basin and Range Province is expanding westward 

into the Cascade Arc (Guffanti and others, 1990), and the active 

Hat Creek and Lake Almanor Grabens are the westernmost 

major Basin and Range structures in the area (fig. 5). This 

impingement is manifested in numerous normal faults, which 

commonly provide pathways for magma to reach the surface. 

Some magmas traverse the crust rapidly, and relatively primitive mafic lavas are widespread, albeit not abundant (Borg and 

others 1997). Many regional volcanoes are aligned along fault 

traces and partially to completely bury them. The Lassen area 

also sits astride the Walker Lane, a broad linear zone of distributed strike-slip faulting, parallel to and inboard of the San 

Andreas Fault System, that accommodates about 20 percent of 

the relative motion between the North American and Pacific 

Plates (Blakely and others, 1997; Faulds and Henry, 2008). 

At the latitude of the Lassen area, the Walker Lane regime 

generally manifests itself as a small strike-slip component on 

the normal faults. On the map, faults are shown only where 

offset of volcanic stratigraphy can be documented in the field. 

Although the Hat Creek Graben-Lake Almanor Graben structure crosses the east half of LVNP, few faults are mapped there. 
4

Expression of the grabens is obscured by a combination of 

young volcanism and glaciation, and additional faults are likely 

present in the subsurface. 

Eruptive History of the Lassen Area

Regional Volcanic Rocks

Over the last 3 Ma, regional calc-alkaline volcanism in the 

Lassen segment (fig. 4A, B) of the Cascade Arc (Guffanti and 

Weaver, 1988) constructed a broad platform of volcanic rocks 

in a band straddling the arc axis. A variety of mostly older lavas 

erupted from shield volcanoes, lava cones, and cinder cones 

are exposed in the mostly unglaciated terrain surrounding the 

Lassen Volcanic Center. North of the Lassen Volcanic Center, 

these older rocks commonly are broken into linear blocks 

bounded by faults. The volume of regional volcanic rocks 

diminishes with distance in both directions from the arc axis, so 

that there are lesser volumes of fore-arc and back-arc volcanism. On a short time scale, regional volcanism is concentrated 

in local areas, for example, in the last 75,000 years regional 

volcanism produced the Tumble Buttes and Sugarloaf chains 

(fig. 4B). The Red Lake Mountain and Red Cinder areas were 

active in the interval from 75,000 to 100,000 years ago and 

the Poison Lake area from 100,000 to 110,000 years ago. Prior 

to that time regional volcanism was concentrated in different 

areas. Regional volcanism does not occur within the boundaries of active volcanic centers. There, rising regional magmas 

are intercepted and processed and contribute to the magmatic 

system of the volcanic center. Return of regional volcanism 

within the confines of a volcanic center indicates solidification 

of that part of its magmatic system. 

Within the regional lavas of the map area, young alignments of monogenetic cones and flows that share spatial, 

chemical, and lithologic characteristics are noteworthy. These 

are illustrated by the Tumble Buttes and Sugarloaf chains (discussed in the following paragraph) and several alignments in the 

Caribou Volcanic Field (discussed below). The Tumble Buttes 

chain consists of 13 units of monogenetic flows and cones 

(additional vents and flows occur north of the map area) correlated on the basis of similar spatial, temporal and compositional 

characteristics. Although not offset by faults, the Tumble Buttes 

chain forms a linear array of vents 15 km long and parallel to 

the pattern of regional faulting, and their location is clearly 

controlled by faults in the subsurface. Tumble Buttes lavas 

are olivine and pyroxene basaltic andesite and andesite. The 

Tumble Buttes chain is mostly about 10–15 to ~50 ka in age and 

is probably the youngest zone of concentrated mafic volcanism 

in the Lassen area.

The Sugarloaf chain consists of four units of monogenetic 

cones and flows in the area south of Sugarloaf Peak, the lava 

cone of Sugarloaf Peak, and a few additional units north of 

the map area. The units are correlated on the basis of similar 

spatial, temporal, and compositional characteristics. Lavas of 

the Sugarloaf chain are sparsely porphyritic olivine and pyroxene basaltic andesite and andesite. Although not offset by faults, 

this 12 km linear array of vents coincides with and obscures the 

western boundary fault of the Hat Creek Graben. The Sugarloaf 

chain is mostly about 45 to ~75 ka in age.

LKOT is related to Basin and Range tectonics (Clynne, 

1993), and it erupted across the Lassen region in a wave progressing from east to west that approximately coincides with 

progressive impingement of Basin and Range extension on 

the Cascade Arc (Guffanti and others, 1990). Although they 

produce relatively large volumes of lava, LKOT eruptions are 

relatively infrequent; only 13 are found in the map area, and all 

are less than about 1 m.y. Older tholeiites are common in the 

areas east and north of the map (Grose and others, 1991, 1992; 

Macdonald 1963, 1964, 1965; Macdonald and Lydon, 1972), 

and, because they are readily buried by younger volcanism, 

some are likely present in the subsurface.

Pliocene–Early Pleistocene Volcanic Centers

The oldest volcanic rocks in the vicinity of the present-day 

axis of the Cascade Range in the Lassen area were mapped as 

the Tuscan Formation by Anderson (1933) and Lydon (1968). 

The Tuscan Formation consists of a broad volcanic and volcanoclastic wedge of fragmental material, primarily volcanic 

conglomerate, volcanic sandstone, siltstone, and minor pumiceous tuff, emplaced on the west slope of the southernmost 

Cascade Range. The Tuscan Formation deposits were emplaced 

by a number of mechanisms, but lahars and debris flows dominate. Pyroclastic flows and interbedded lava flows are sparse. 

A few major ash-flow sheets and tuffs are also present (Helley 

and others, 1981). Adjacent to the Cascade Range, the Tuscan 

Formation is ~500 m thick (Lydon, 1968) where it interfingers 

with proximal deposits of the Latour and Yana Volcanic Centers. 

It thins dramatically toward the east margin of Sacramento 

Valley where it is intercalated with fluvial sediments (Harwood 

and others, 1981). Much of the Tuscan Formation was 

reworked by fluvial processes, especially in distal areas near 

the Sacramento Valley. In the Lassen area, these deposits were 

mostly derived from the Latour and Yana Volcanic Centers. 

The Tuscan Formation ranges in age from ~3,500 ka (Helley 

and others, 1981) to perhaps as young as 2,500 ka. Similar, but 

older, deposits related to the Miocene to Pliocene Cascade Arc 

are found south of the Yana Volcanic Center and are called the 

Mehrten Formation (Curtis, 1954).

Rocks of the Latour Volcanic Center (>3 Ma) are present 

along the northwest boundary of the map area. Rocks of the 

Yana Volcanic Center (~3.4–2.4 Ma) dominate the area southwest of Lake Almanor but do not crop out within the boundaries of the map area. Little is known about the evolution of the 

Yana Volcanic Center and even less is known about the Latour 

Volcanic Center. 

The next important volcanic events in the Lassen area 

were growth of the Dittmar (2.4–1.3 Ma) and Maidu (2.4–1.2 

Ma) Volcanic Centers (fig. 4A). Rocks of the Dittmar Volcanic 

Center are abundant in the southeastern part of LVNP and the 

adjacent area. The Maidu Volcanic Center is better preserved 

and its evolution is known in more detail, but it is mostly 

located beyond the southwest boundary of the map. 
5

Dittmar Volcanic Center

The Dittmar Volcanic Center lies in the area of the intersection between the Hat Creek Graben and Lake Almanor 

Graben (figs. 3, 4A), and the volcanic center has been chopped 

into a number of fault blocks. Many more faults probably exist 

than have been shown on the map. The detailed stratigraphy 

of the volcanic center cannot generally be traced across these 

faults or correlated between blocks. Consequently, the details of 

its evolution are poorly known. The largest preserved remnants 

of the Dittmar Volcanic Center are Mount Hoffman, Saddle 

Mountain, Pilot Mountain, Kelly Mountain, and Wild Cattle 

Mountain (fig. 3). The Dittmar Volcanic Center hosted a hydrothermal system in its waning stages of activity; thus many of the 

early Dittmar rocks are hydrothermally altered. Glaciation of 

weak, hydrothermally altered rocks has deeply eroded the central area, and rocks are generally poorly exposed. Radial dips 

of lava flows and fragmental deposits indicate that the Dittmar 

Volcanic Center was centered in the area southwest of Saddle 

Mountain. Deposits of the Dittmar Volcanic Center are grouped 

into three stratigraphic stages: Stage 1, early growth of the composite cone; Stage 2, later growth of the composite cone; and 

Stage 3, late silicic volcanism. 

Stage 1 of the Dittmar Volcanic Center consists of generally thin lava flows and interbedded fragmental deposits ranging from flow breccias, lithic fall deposits, and block and ash 

deposits to lahars. These lavas and volcanic deposits erupted 

from a central vent whose position was in the vicinity of 

Warner Valley between Kelly and Saddle Mountains but which 

has been completely removed by erosion. Best exposures of 

relatively fresh Stage 1 rocks are found on the north flank of 

Kelly Mountain, on the cliffs below Juniper Lake, on Mount 

Hoffman, and on Wild Cattle Mountain. Strongly altered lavas 

are well exposed in the valleys of Kings and Hot Springs 

Creeks on both sides of Flatiron Ridge. Olivine-bearing basaltic andesite and hypersthene-augite andesites dominate Stage 

1, and pyroxene±hornblende dacite to rhyolite are subordinate. 

Lavas are generally porphyritic, typically containing 15–30 

percent phenocrysts, although some sparsely porphyritic and 

a few nearly aphyric lavas are present. The duration and age 

of Stage 1 is poorly constrained; three K-Ar ages from near 

the top and bottom of the unaltered parts of the stratigraphic 

section are 1,650±35 and 1,785±35 ka, and 2,315±29 ka, 

respectively.

A debris avalanche derived from Stage 1 of the Dittmar 

Volcanic Center is exposed in roadcuts along Calif. Hwy 36 

near Inskip Hill (Muffler and others, 1989). Rocks in the debris 

avalanche are correlated to Stage 1 lithologies present on the 

north flank of Kelly Mountain above Warner Valley and are 

nearly 50 km from their source area. The age of this debris avalanche is poorly known. It postdates deposition of the Tuscan 

Formation in the Inskip Hill area and is probably younger than 

about 2,500 ka and probably older than the intervening Maidu 

Volcanic Center and, thus, younger than ~2,200 ka. 

Ingebritsen and Rojstaczer (1983) suggested from satellite 

imagery that several old calderas might be present in the area 

around Sifford Mountain. One of these is just south of LVNP in 

the area of Wild Cattle Mountain and Feather River Meadows. 

The area was partially filled by younger rocks from the Lassen 

Volcanic Center but contains rocks correlated with Stage 1 of 

the Dittmar volcanic center. A geothermal exploration well 

in Feather River Meadows penetrated several hundred feet of 

ash-flow tuff that is similar to tuffs exposed on Kelly Mountain. 

It is likely that the thick tuff ponded in a small caldera, and it 

is possible that the eruption was related to the debris avalanche 

exposed at Inskip Hill. 

Stage 2 of the Dittmar Volcanic Center consists of generally thick lava flows with sparse interbedded fragmental 

deposits. Good exposures of relatively fresh Stage 2 rocks are 

found on the south flank of Kelly Mountain and the upper parts 

of Saddle and Pilot Mountains. Stage 2 lavas erupted from 

flank vents whose locations are not preserved. Hyperstheneaugite andesites dominate Stage 2; olivine-bearing andesite and 

pyroxene±hornblende dacite are subordinate. Lavas are porphyritic, typically containing 20–30 percent phenocrysts, and some 

contain small mafic inclusions. The duration and age of Stage 2 

is poorly constrained; a single K-Ar age is 1,398±42 ka.

Stage 3 of the Dittmar Volcanic Center consists of a few 

lava flows and domes of hornblende-biotite rhyodacite and 

rhyolite that overlie Stage 2 lavas. No vents for these units are 

known, and the best exposures are in the Grassy Swale area and 

at the summit of Pilot Mountain. Additional Stage 3 lava flows 

and related deposits probably were buried beneath younger 

rocks of the Central Plateau of LVNP. The duration and age of 

Stage 3 is poorly constrained; a rhyolite lava flow exposed in 

Summit Creek is 1,273±7 ka. 

Maidu Volcanic Center

The Maidu Volcanic Center is located south of LVNP, and 

a small part of it lies within the southeast corner of the map area 

(fig. 4A). The largest preserved remnants of the Maidu Volcanic 

Center in the map area are Hampton Butte, Blue Ridge, and 

North Stover Mountain (fig. 3). Within the map area, Maidu 

Volcanic Center unconformably overlies the Tuscan Formation 

and, outside the map area, it unconformably overlies rocks of 

the Yana Volcanic Center. Its base is exposed in the canyons of 

Battle, Mill, and Deer Creeks. The core of the Maidu Volcanic 

Center was hydrothermally altered and deeply eroded and is 

well exposed in the cliffs surrounding Battle Creek Meadows. 

The summit and vent areas have been completely obscured 

by a combination of hydrothermal alteration and glacial and 

fluvial erosion. The preserved parts of the Maidu Volcanic 

Center dip radially away from Battle Creek Meadows, which is 

therefore presumed to have been the center of activity. Wilson 

(1961) attributed the depression of Battle Creek Meadows to 

explosive eruption of the Rockland tephra and formation of 

a small caldera at this site. The Rockland tephra, however, is 

much younger than the Maidu Volcanic Center, and no other 

widespread deposit of the appropriate age is known that could 

have come from the area (Clynne, 1984). It is more likely that 

the Battle Creek Meadows depression has an erosional origin. 

Although deeply eroded, the Maidu Volcanic Center is, in general, structurally intact. The total volume erupted at the Maidu 
6

Volcanic Center was in excess of 200 km3 and it was active 

from 2.4 to ~1.2 Ma.

Despite its age, hydrothermal alteration, and deep erosion, 

the stratigraphy of the Maidu Volcanic Center is fairly well 

understood. Deposits of the Maidu Volcanic Center are grouped 

into three stratigraphic stages: Stage 1, early growth of the composite cone; Stage 2, later growth of the composite cone; and 

Stage 3, late silicic volcanism. Stage 1 is mostly composed of 

thin lava flows and fragmental deposits erupted from a central 

vent during the period from ~2.4 to ~2.15 Ma.. Lithologies and 

compositions are diverse; olivine-augite, olivine-augite-hypersthene, augite-hypersthene, and hypersthene-augite assemblages 

occur. Hypersthene-augite andesites dominate, and olivinebearing basaltic andesite and pyroxene±hornblende dacite are 

subordinate. Stage 1 contains considerable intercalated pyroclastic material. Fragmental deposits are not mapped separately; 

they range from fallout to block-and-ash flow deposits, and 

lahar deposits are common. The permeable fragmental deposits 

and brecciated flow carapaces are particularly susceptible to 

hydrothermal alteration. Hydrothermal alteration was especially 

intense in the area between Christie Hill, Little Round Valley, 

and Mineral Summit. 

Stage 2 of the Maidu Volcanic Center is dominated by a 

small number of thick porphyritic augite-hypersthene dacite 

lava flows erupted from flank vents. These lava flows generally 

lack interbedded fragmental deposits. Stage 2 was active from 

~2.15 to ~2.05 Ma, and possibly later. Stage 3 consists of five 

large, very thick felsic lava flows erupted between ~1.3 and 

~1.2 Ma. One is composed of porphyritic rhyodacite, and the 

other four are sparsely porphyric rhyolite and are some of the 

largest silicic lava flows in the Cascade Range; the largest, the 

Blue Ridge rhyolite flow, has a volume of ~15 km3

.

Lassen Volcanic Center

Recently acquired geochronology (Lanphere and others, 

1999) and reinterpretation of rock units surrounding the Brokeoff 

Volcano provoked reorganization of the three-stage stratigraphic 

description of the Lassen Volcanic Center presented in previous 

publications (Clynne, 1984, 1990; Muffler and others, 1989). 

Herein, the major stratigraphic divisions of the Lassen Volcanic 

Center are designated the Rockland caldera complex, Brokeoff 

Volcano, and Lassen domefield (fig. 4C). These eruptive stages 

represent a continuum of lithologically correlated volcanism 

distinctly different from one another but linked by a common 

magmatic system. The Rockland caldera complex consists of a 

group of dacite to rhyolite domes, flows, and pyroclastic deposits 

and the Rockland tephra. Brokeoff Volcano was previously 

called Stages 1 and 2 of the Lassen Volcanic Center. The Lassen 

domefield consists of the dacite domefield and surrounding 

hybrid andesite lava flows on the Central Plateau area of LVNP 

and surrounding areas that we previously called Stage 3 of the 

Lassen Volcanic Center. Clynne (1990) and Bullen and Clynne 

(1990) discussed the geochemistry and magmatic evolution of 

the Lassen Volcanic Center using the old stage terminology. The 

total erupted volume of the Lassen Volcanic Center was estimated by Sherrod and Smith (1990) to be about 200 km3

. 

Rockland Caldera Complex

The Rockland caldera complex consists of the Rockland 

tephra and a group of geographically, stratigraphically, and lithologically correlative silicic volcanic units ranging in age from 

~825 to 609 ka. Dacite to rhyolite domes and flows exposed 

peripheral to Brokeoff Volcano (fig. 4C) erupted during this 

approximately 215,000-year-long period of silicic volcanism, 

which culminated in eruption of the Rockland tephra at 609±7 

ka (Lanphere and others, 1999, 2004). Total erupted volume of 

the Rockland caldera complex is at least 60–70 km3.

The Rockland tephra was first recognized by Wilson 

(1961) as the “Rockland rhyolite pumice tuff breccia”. The 

name derives from the Rockland School, which was then 

located 0.5 mile northwest of Camp Forward along the western margin of the map area. A coeval distal ash-fall deposit 

is widely distributed in northern and central California and 

northeast to Idaho and in deep-sea cores taken off the northern 

California coast (Sarna-Wojcicki and others, 1985). Meyer and 

others (1991) defined the term “Rockland tephra” to refer to 

both distal and proximal exposures, and this usage was continued in recent papers specifying the age of the unit (Lanphere 

and others, 1999, 2004). We accept the term as defined by 

Meyer and others (1991). 

Wilson (1961), Helley and others (1981), and Meyer and 

others (1991) attributed the Rockland deposits to the Maidu 

Volcanic Center; indeed, Helley and others (1981) called the 

unit the “ash of Mount Maidu”. Clynne (1984) recognized that 

the Rockland deposits were younger than the Maidu Volcanic 

Center and, based on a fission-track age of about 400 ka (Meyer 

and others, 1980, 1991), attributed the Rockland tephra to the 

Lassen Volcanic Center. This age was younger than Brokeoff 

Volcano, and thus the Rockland tephra was thought possibly 

to be the initial eruption of the Lassen domefield stage of the 

Lassen Volcanic Center. Subsequent precise dating determined 

the Rockland tephra to be 609±7 ka (Lanphere and others, 1999; 

2004), slightly older than the Brokeoff Volcano. The calculated volume of the Rockland tephra is ~50 km3

 (dense rock 

equivalent; Sarna-Wojcicki and others, 1985), which is similar 

to that of the Mazama ash from Crater Lake (Bacon, 1983). An 

eruption of this magnitude must have formed a collapse caldera. Projection of outcrop pattern of proximal deposits points 

toward Brokeoff Volcano, and it seems likely that eruption of 

the Rockland tephra produced a small caldera that has subsequently been filled by Brokeoff Volcano and younger products 

of Lassen Volcanic Center. 

Clynne (1984) also recognized a number of silicic rock 

units older than Brokeoff Volcano and the Rockland tephra, and 

he assigned them to the Maidu and Dittmar Volcanic Centers. 

Subsequent radiometric dating and improved understanding 

of the evolution of the Maidu and Dittmar Volcanic Centers 

showed these silicic rock units to be inconsistent with those 

centers. Instead, these silicic rock units form a ring around 

the inferred source of the Rockland tephra and thus are now 

grouped with the Rockland tephra as the Rockland caldera complex. Lithic fragments from several of these silicic rock units 

are present in the Rockland tephra. These relations support our 
7

hypothesis that the Rockland tephra was erupted from a caldera 

located at the present site of Brokeoff Volcano. The nature and 

sources of rocks occupying the area of the Rockland caldera 

complex prior to its formation are incompletely known, but the 

presence of lithic clasts of unit acs and possibly unit rd3 in 

the Rockland tephra suggests that a small andesitic composite 

volcano and a silicic domefield possibly related to the Dittmar 

Volcanic Center occupied the area before emplacement of the 

Rockland caldera complex and eruption of the Rockland tephra. 

Brokeoff Volcano

Brokeoff Volcano is the second of the major stratigraphic 

divisions of the Lassen Volcanic Center. Brokeoff Volcano, 

also designated “Mount Tehama” by the National Park Service, 

consists of a large, ~80-km3, composite volcano. Almost 

immediately after eruption of the Rockland tephra, the caldera 

began to fill as renewed activity formed Brokeoff Volcano. The 

stratigraphy of Brokeoff Volcano is described as two sequences 

of deposits: the Mill Canyon sequence and the Diller sequence.

The Mill Canyon sequence consists of dozens of smallvolume basaltic andesite to dacite lava flows and interbedded 

fragmental deposits erupted from a central vent between about 

590 and 470 ka. The sequence is dominated by packages of thin 

andesite lava flows and breccias of a single lithology. Adjacent 

flow packages are different lithologies, and a wide variety of 

porphyritic olivine and pyroxene basaltic andesites and andesites to pyroxene-hornblende dacites are present. Although 

mapped here as a single unit, the andesites of Mill Canyon 

were shown in more detail by Clynne (1984). Vent or near-vent 

facies of Brokeoff Volcano andesites are found along the ridge 

between Mount Diller and Brokeoff Mountain, on Diamond 

Peak, and on the ridge south of Bumpass Hell and demonstrate 

that the position of the central vent varied as the volcano grew. 

The Mill Canyon sequence is capped by thick dacite lava flows 

erupted at 470 ka and mapped as the dacite of Twin Meadows. 

The dacite of Twin Meadows is the only rock unit of significant 

lateral extent in the Brokeoff Volcano and provides a convenient 

stratigraphic boundary between the Mill Canyon sequence and 

the overlying Diller sequence. 

The Diller sequence consists of thick, large-volume, 

lithologically similar lava flows erupted between 470 and 385 

ka. These flows erupted from flank vents and generally lack 

interbedded pyroclastic deposits. The most extensive exposures 

of Diller sequence lava flows are on the south and west flanks 

of Brokeoff Mountain, on the ridge to the west of Mount Diller, 

and in the drainage of Rice Creek. Six large augite-hypersthene 

andesite to silicic andesite lava flows are mapped, and remnants 

of additional lava flows are combined as an additional unit. No 

volcanism is recognized in the Lassen Volcanic Center between 

about 385 and 300 ka. After extinction of the Brokeoff Volcano, 

the character and locus of volcanism in the Lassen Volcanic 

Center changed dramatically. 

Brokeoff Volcano rocks, especially the Mill Canyon 

sequence, have undergone variable degrees of hydrothermal 

alteration. Rocks exposed near the eroded core of Brokeoff 

Volcano are strongly altered, and the degree of alteration 

generally decreases away from the core. Permeable fragmental 

deposits and flow breccias tend to be more altered than dense 

lava flow cores. The most extensive sections of fresh Mill 

Canyon sequence andesites are found in the cliffs of Bailey 

Creek (between Brokeoff Mountain and Mount Diller) and in 

Mill Creek. Williams (1932) proposed that the eroded core of 

Brokeoff Volcano was a collapse caldera. However, no largevolume pyroclastic deposit of the appropriate age has been 

found in the Lassen area and no significant stratigraphic offsets 

between lavas in the core and flanks of Brokeoff Volcano have 

been found. The eviscerated core of Brokeoff Volcano is the 

product of enhanced glacial and fluvial erosion of weak, hydrothermally altered rock, and much of the core area now consists 

of material emplaced by landslides. The largest remnants of the 

Brokeoff Volcano (Mount Diller and Brokeoff Mountain) were 

protected from erosion by thick, unaltered, lava flows of the 

Diller sequence on their outer flanks. 

Lassen Domefield

The silicic rocks in the northwest corner of LVNP and 

hybrid andesite lavas on the Central Plateau (fig. 3) comprise 

the Lassen domefield, the final stage of the evolution of the 

Lassen Volcanic Center. The silicic rocks of the Lassen domefield erupted along the northern flank of Brokeoff Volcano and 

are divided on the basis of age into the Bumpass (~300–190 

ka) and Eagle Peak (~70–0 ka) sequences. The hybrid andesite 

units erupted in two groups called the older (~310–~240 ka) and 

younger (~90–0 ka) Twin Lakes sequence and are contemporaneous with the Bumpass and Eagle Peak sequences, respectively. No volcanism is known in the Lassen Volcanic Center 

during the period 190–90 ka. The erupted volumes of the silicic 

rocks of the Lassen domefield total ~40–50 km3 and the hybrid 

andesites ~10 km3. 

The Bumpass sequence is a group of 15 dacite to rhyodacite lava domes, flows, and a lithic pyroclastic flow deposit 

that comprise the older part of the dacite domefield. These 

were emplaced in the southern part of the Lassen domefield 

on the north flank of Brokeoff Volcano in a broad curvilinear 

array between about 300 and 190 ka. Repeated glaciation has 

removed glassy dome carapaces and most of the fragmental 

deposits associated with domes and flows. Bumpass sequence 

lavas are mostly porphyritic pyroxene and hornblende dacites; 

quartz and biotite are generally sparse. A few are porphyritic 

biotite-hornblende rhyodacite. Mafic inclusions are sparse to 

abundant. The Bumpass sequence rock units are characterized by weakly disequilibrium phenocryst assemblages that 

originated by mixing of a small proportion of regional mafic 

magma with rhyodacite of the Lassen domefield magmatic 

system. 

The Eagle Peak sequence consists of a group of seven 

dacite and rhyodacite lava domes and flows and their associated pyroclastic deposits erupted in the northern part of Lassen 

domefield between 66 and 1.1 ka. Vents are concentrated in a 

small area in the central part of the Lassen domefield. These 

include the most prominent young volcanic features in LVNP: 

Lassen Peak (27 ka) and Chaos Crags (1.1 ka) and the products 
8

of five older eruptions. Most units of the Eagle Peak sequence 

are glaciated, but the degree of glacial erosion is much less 

than on domes of the Bumpass sequence, and significant portions of glassy carapaces, primary hot talus, and pyroclastic 

deposits are preserved. Lavas of the Eagle Peak sequence are 

porphyritic hornblende and biotite dacite and rhyodacite; some 

also contain quartz. The rocks are characterized by weakly 

disequilibrium phenocryst assemblages and mafic inclusions 

that originate by mixing of a small proportion of regional 

mafic magma with rhyodacite of the active Lassen domefield 

magmatic system. 

The Twin Lakes sequence consists of two time-stratigraphic 

groups of mostly andesite and basaltic andesite lava flows and 

agglutinate cones erupted around the periphery of the Lassen 

domefield but primarily on the Central Plateau of LVNP. An older 

group of lava flows is correlative with the Bumpass sequence, 

and a younger group of lava flows is correlative with the Eagle 

Peak sequence. The two youngest eruptions in LVNP, the basaltic 

andesites of Cinder Cone and deposits of the 1914–17 eruption of 

Lassen Peak, are included in the younger Twin Lakes sequence. 

Rocks of the Twin Lakes sequence have strongly disequilibrium 

phenocryst assemblages that are characterized by coexistence 

of magnesian olivine and quartz. They originate by mixing of 

subequal proportions of regional mafic magma and rhyodacite of 

the Lassen domefield magmatic system. 

The Bumpass, Eagle Peak, and Twin Lakes sequences 

belong to a suite of lavas formed by a continuum of magmamixing processes that cause many rock units to be variable in 

appearance and composition. Thus, chemically different lavas 

may have a similar macroscopic appearance, or the same lava 

may range in appearance. Lassen dacites evolve by introduction of regional mafic magma into a silicic reservoir containing felsic melt and phenocrysts. Mixing of hot mafic magma 

and its phenocrysts with cool host dacite and its phenocrysts, 

partial resorption of the host dacite phenocrysts, quenching of 

the resultant mixed magma to form undercooled inclusions, 

and circulation and disaggregation of inclusions back into the 

host dacite create complex disequilibrium phenocryst assemblages (Clynne, 1999). Phenocryst abundance, character, and 

even assemblage can vary within individual dacite domes, and 

intradome variability can be as large as interdome variability. 

The two fundamental factors controlling the character of the 

erupted magma are the relative proportions of mafic and silicic 

components in the mixture and the length of time between the 

mixing event(s) and eruption. When the relative proportions of 

mafic and silicic components approach unity, inclusions are no 

longer formed, and the magmas mix homogeneously to form 

hybrid magmas. Hybrids are typically andesitic and are usually 

characterized by strongly resorbed phenocrysts and the coexistence of magnesian olivine and quartz as phenocrysts. 

Holocene Activity

The Lassen Volcanic Center is still active, and three 

eruptions of Holocene age occurred at Chaos Crags, Cinder 

Cone, and Lassen Peak. No other eruptions documented to be 

Holocene have occurred in the area covered by the map. 

Chaos Crags

The Chaos Crags erupted northwest of Lassen Peak about 

1,103±13 yr B.P. (Clynne and others, 2002) and is the youngest unit of the Eagle Peak sequence. The eruptive sequence 

consists of a group of six rhyodacite lava domes and associated 

pyroclastic deposits that define a typical silicic eruption in the 

Lassen Volcanic Center. The depiction of map units follows that 

of Christiansen and others (2002). Initial pyroclastic deposits 

are mapped as a single unit, and subsequent lava domes and collapse deposits are mapped separately. 

The Chaos Crags eruption began with a vent-opening 

phase followed by explosive eruption and emplacement of the 

lower and middle pyroclastic flow that cooled as a single unit. 

Fallout of pumice around the vent built a small tuff cone at the 

north end of the Chaos Crags. Extrusion of a small lava dome 

A and cooling of magma in the conduit then plugged the vent. 

Reopening of the conduit and explosive eruption of rhyodacite as the upper pyroclastic flow followed a short hiatus in the 

eruptive sequence and was much larger than the eruption that 

produced the lower and middle pyroclastic flows. This eruption 

partially destroyed dome A. Subsequent to eruption of the upper 

pyroclastic flow, domes B through F were emplaced. Although 

the relative ages of these domes are clear, the time interval over 

which they erupted is not. However, similar paleomagnetic 

direction for all units indicate that the entire eruption probably took less than a few decades (D.E. Champion, written 

commun., 2003). Domes D and E had small hot collapse events 

that emplaced short run-out lithic pyroclastic avalanches. Talus 

emplaced hot from the lava domes as they grow is also mapped. 

At a later time, dome C partially collapsed as a series of three, 

cold, rockfall avalanches that emplaced the Chaos Jumbles at 

278±28 yr BP. 

Chaos Crags lavas are porphyritic hornblende-biotite 

rhyodacite that contain a complex disequilibrium phenocryst 

assemblage characteristic of Lassen dacites (Clynne, 1999) 

and variable amounts of mafic inclusions. Early erupted lava 

contrasts with later erupted lava. Mafic inclusions are sparse to 

rare in the early rhyodacites (generally less than 1 percent to a 

few percent), whereas they form up to nearly 20 percent of late 

rhyodacites. The proportion of reacted phenocrysts and groundmass microphenocrysts disaggregated from inclusions mirrors 

the proportion of inclusions in the rocks. Bulk rock compositions 

(macroscopically inclusion-free) also reflect the proportion of 

disaggregated inclusions; early rhyodacites are a homogeneous 

group at about 69–70 percent SiO2, whereas late rhyodacites are 

heterogeneous and contain 67–68.5 percent SiO2. 

Cinder Cone

Cinder Cone is the youngest mafic volcano in the map 

area and consists of five lava flows, two cinder cones, and a 

widespread mafic ash fall deposit; it is the second youngest 

eruption in the Twin Lakes sequence. The calibrated radiocarbon age of Cinder Cone is 264±28 yr B.P., and stratigraphic and 

paleomagnetic data indicate that the lava and tephra erupted 

over a short interval about 1650 C.E. (Clynne and others, 2000; 
9

Clynne and others, 2002). Tree-ring dendrochronology indicates 

that Cinder Cone erupted in 1666 C.E. (Sheppard and others, 

2009). Subdivision of Cinder Cone deposits is based on eruptive sequence and composition; two compositional groups are 

present. An early cone, now a remnant, produced the Old Bench 

flow (olivine basaltic andesite) and the two Painted Dunes 

flows (olivine basaltic andesite and olivine andesite, respectively). Painted Dunes flows contain blocks of welded cinders 

and agglutinate from several to tens of meters across that 

were rafted from the remnant cinder cone, which was almost 

completely destroyed by their eruption. Painted Dunes flows 

blocked drainage from Central Plateau of LVNP and created 

Snag Lake. Renewed eruptions built Cinder Cone and emplaced 

the two Fantastic Lava Beds flows (augite-olivine basaltic 

andesite). A widespread ash deposit as much as 2.5 m thick is 

composed of ash of both the Painted Dunes and Fantastic Lava 

Beds compositions and has no discernable time break between 

them. Thickness of the ash is shown on the map as isopachs in 

centimeters. The isopachs were adapted from those presented by 

Heiken (1978) by combining the isopachs of his three units and 

showing them as total thickness. The map does not show a 0-cm 

isopach because ash extends locally for at least several kilometers beyond the 0-cm isopach as shown by Heiken (1978). 

Old Bench and Painted Dunes 1 flows have a thick cover of 

colorfully oxidized ash, and Painted Dunes 2 flow has a thin 

cover of ash with Fantastic Lava Beds composition. Oxidation 

of ash demonstrates that Old Bench and Painted Dunes lava 

flows were still hot when the ash that formed Cinder Cone 

erupted. Little ash was erupted during or after Fantastic Lava 

Beds flows were emplaced, and their surfaces are nearly ash 

free. Painted Dunes and Fantastic Lava Beds flows have rough, 

block-covered surfaces with considerable relief and many flow 

levees. Lava flows completely lack soil and are nearly devoid of 

vegetation; only a few mature but stunted trees grow on Painted 

Dunes ash deposits. Deposits of diatomite around the base of 

the Fantastic Lava Beds flows indicate that Butte Lake was 

considerably larger before the eruption of Cinder Cone and was 

partially filled in by the lava flows. 

1914–1917 Eruption of Lassen Peak

The 1914–17 eruption of Lassen Peak is the most recent 

eruption in the Twin Lakes sequence and is a complex eruptive sequence consisting of a dacite dome and lava flow; dacite 

pyroclastic flow and fall deposit; and phreatic, avalanche, 

debris-flow, and flood deposits. Magmatic activity was confined to the period from about May 14 to 22, 1915, and affected 

primarily the northeast flank and slope of Lassen Peak (now 

called the Devastated Area) and the valleys of Lost and Hat 

Creeks as far as about 50 km downstream. Division of deposits of the 1914–1917 eruption is based on stratigraphy and 

mode of emplacement and generally follows the depiction of 

Christiansen and others (2002). Rock types range from olivine 

andesite to several varieties of hornblende-biotite dacite with 

abundant mafic inclusions and complex mineralogy. The presence of spectacularly banded pumice blocks erupted on May 

22 serves to distinguish May 22 from May 14–19 deposits. See 

Clynne (1999) for more detailed description and interpretation of these rocks and Clynne and others (1999) for additional 

description of the 1915 eruption.

The eruptive sequence began on May 30, 1914, with a 

phreatic explosion at the summit of Lassen Peak. By mid-May 

of 1915, more than 180 steam explosions had blasted out a 300-

m-wide crater at the summit. Deposits of this precursory activity are thin and were mostly confined to the immediate vicinity 

of Lassen Peak or were buried by subsequent deposits and are 

not shown on the map. In the week before May 19, 1915, a 

small dacite lava dome (d4) filled the crater. On the late evening of May 19, 1915, the growing dacite dome was disrupted 

by a large phreatic explosion. Hot blocks of lava were thrown 

onto the snow-covered upper flanks and summit of Lassen Peak 

(p9), which initiated an avalanche of snow and rock that swept 

down the steep northeast face of Lassen Peak, over the low 

ridge northeast of Emigrant Pass, and into Hat Creek (sw9). 

The avalanche was quickly followed by a debris flow formed 

from melting of snow by hot lava blocks and underlying loose 

rock debris (f9). The debris flow followed the same path as 

the avalanche until it encountered the low ridge northeast of 

Emigrant Pass, where it was deflected west into Lost Creek. The 

debris flow continued down Lost Creek for another 7 km before 

coming to rest in the large flat area 3 km west of Twin Bridges. 

During the early morning hours of May 20, 1915, the debrisflow and avalanche deposits released large volumes of water 

that caused a flood along Hat Creek north of Old Station (f9). 

Also in the late evening and early morning of May 19–20, 1915, 

dacite lava, which was more fluid than that erupted during the 

previous week, welled up into and filled the newly excavated 

crater. It spilled over two low areas on the rim and emplaced 

two short lava flows (d9) on the steep west and northeast flanks 

of Lassen Peak.

The eruptive sequence culminated two days later at 

approximately 4:00 pm on May 22, 1915. A vertically directed 

column of pumice and gas blasted through the May 19–20 lava 

flow, created a new crater, and rose to 30,000 ft. Partial collapse of the column initiated a pyroclastic flow on the northeast 

slope of Lassen Peak, which rapidly incorporated and melted 

snow in its path. By the time it reached the lower Devastated 

Area, the pyroclastic flow transformed into a fluid debris flow 

(pw2) that traveled down Lost Creek to beyond Twin Bridges, 

where it released water and caused a second flood along Hat 

Creek north of Old Station. Continued fallout from the eruption cloud emplaced a pumice deposit on the upper slopes of 

Lassen Peak (p2). Pumice that fell on remaining snow-covered 

areas high on Lassen Peak generated six viscous debris flows 

that were emplaced on the west, north, and east flanks of Lassen 

Peak (w2). The May 19 lava flow on the northeast flank of 

Lassen Peak was removed in this eruption and incorporated into 

May 22 deposits, and the still-hot lava at the summit partially 

slumped back into the May 22 crater. For several years after this 

eruption, spring snowmelt percolating into Lassen Peak encountered hot rock and triggered steam explosions. Particularly vigorous phreatic explosions in May 1917 blasted out the western 

of the two craters at the summit of Lassen Peak and emplaced 

phreatic deposits (p17).
10

Caribou Volcanic Field

The Caribou Volcanic Field is an area of intense, young 

regional volcanism located 20–30 km east of the Lassen 

Volcanic Center (figs. 4A, 6; sheet 3). The field is composed of 

monogenetic cinder cones, flows, and small- to medium-sized 

lava cones, generally of mafic composition, and is distinguished from older regional mafic lavas around its periphery 

by substantial constructional morphology and the absence 

of significant structural disruption. The Caribou Volcanic 

Field (~425–0 ka) is contemporaneous with the later part of 

the Lassen Volcanic Center and is the result of a higher flux 

of magma from the mantle compared to the Lassen segment 

of the arc as a whole, but not as high as that at the Lassen 

Volcanic Center (Guffanti and others, 1996). The Caribou 

Volcanic Field may represent an incipient volcanic center 

that is not quite vigorous enough to initiate crustal melting 

and silicic volcanism. No single edifice has a long history of 

activity; Caribou volcanoes, however, tended to erupt in arrays 

that were probably active for periods as long as 100–200 ka. 

Emplacement of arrays is generally structurally controlled; 

that is, linear arrays of vents bury active or inactive faults that 

provided pathways exploited by ascending magma and acted 

to focus volcanism at the surface.

The volcanoes of the Caribou Volcanic Field were mapped 

in three larger and six smaller arrays that share similar spatial, temporal, and compositional characteristics. The major 

arrays are the Red Cinder chain, the Poison Lake chain, and 

the Caribou chain. The Caribou chain is further subdivided into 

the Triangle Lake sequence, the Echo Lake sequence, and the 

Caribou sequence. The six smaller arrays are the Tuya chains, 

the Bidwell Spring chain, the Cone Lake chain, the Sunrise 

Peak sequence, the Island Lake sequence, and the Beauty Lake 

sequence.

Red Cinder Chain

The Red Cinder chain includes 18 closely spaced vents 

whose eruptive products overlapped to build the substantial 

edifice of Red Cinder along the boundary between LVNP and 

the Caribou Wilderness Area. The edifice is nearly circular, but 

vents show a northwest–southeast alignment parallel to regional 

structure. Many vents are preserved as cinder cones. Several of 

the older units are cut by small faults, but in general the edifice 

is intact. The Red Cinder chain is dominated by porphyritic 

olivine basalt and olivine-augite basaltic andesite; one unit is 

pyroxene andesite. Only the andesite of Red Cinder (69±20 ka) 

is dated, but the Red Cinder chain is probably all less than ~100 

ka; the youngest units are ~25 ka. Volcanoes of the Red Cinder 

chain are not as heavily glaciated as the nearby Caribou chain; 

many lava flows partially retain their original surfaces. 

Poison Lake Chain

A broad zone of 39 young basalt and basaltic-andesite 

cinder cones and lava flows forms the Poison Lake chain. The 

entire chain lies outside the area of glaciation, and cinder cones 

and flow surfaces are well preserved. The cones and flows in 

the Poison Lake chain are combined into nine groups based 

on shared lithologic, chemical, and spatial characteristics. The 

vents are arranged in two parallel arrays only a kilometer apart, 

but no temporal or compositional differences can be discerned 

that are specific to one or the other of the lines of volcanoes. 

The chain coincides with and buries faults with small offsets 

that can be seen at the ends of the zone (south end off the map 

area). These faults have been used repeatedly as conduits for 

magma to erupt at the surface. Rocks that form these volcanoes 

are dominated by sparsely porphyritic to porphyritic olivine 

basalts that include some of the most primitive lavas in the 

Lassen area (Borg and others, 1997; Clynne and Borg, 1997). 

Eight of the nine groups have 40Ar/39Ar dates between ~100 and 

~110 ka; the ninth is undated.

Caribou Chain

The Caribou chain is the dominant constructional feature 

in Caribou Volcanic Field. The chain extends 14 km from Star 

Butte in the south to Triangle Lake in the north. Lavas of the 

Caribou chain are dominated by porphyritic olivine basaltic 

andesite and augite–olivine basaltic andesite with little lithologic and compositional diversity. 

The central part of the Caribou chain is designated the 

Caribou sequence and includes the large edifices of North, 

middle, and South Caribou, the platform of slightly older rocks 

upon which the edifices rest, and younger lava flows northwest 

of Silver Lake. The Echo Lake and Triangle Lake sequences, 

respectively south and north of the Caribou sequence, are 

similar but younger rocks. The Caribou sequence was active 

from ~425 ka to ~325 ka; the Echo Lake and Triangle Lake 

sequences extend the eruptive period of the entire Caribou 

chain to ~275 ka. The three Caribou edifices were high points 

throughout the late Pleistocene glacial episodes; the upper parts 

of the edifices are deeply eroded, exposing several intrusive 

masses. Vents, when found, are poorly preserved agglutinate 

cones, and the surfaces of lava flows are stripped of their block 

carapaces. 

Other Chains and Sequences in the Caribou Volcanic Field

The smaller groups are the Tuya chains, the Bidwell Spring 

chain, the Cone Lake chain, the Sunrise Peak sequence, the 

Island Lake sequence, and the Beauty Lake sequence. Each has 

been correlated on the basis of shared similar spatial, temporal, 

and compositional characteristics. 

Two arrays of unusual olivine basaltic-andesite volcanoes called tuyas exposed in the Caribou Wilderness Area are 

mapped as the Tuya chains. Tuyas have a distinctive shape that 

is steep-sided and flat-topped like a silicic lava dome and are 

interpreted to have erupted beneath glacial ice. Rock exposed on 

the sides of the tuyas has a distinctive pattern of closely spaced 

joints so that the rock breaks into angular roughly cubic blocks 

with slightly curved to almost conchoidal planar surfaces. Both 

chains erupted along distinct linear arrays, probably controlled 
11

by faults in the subsurface. The older chain probably erupted 

during a glacial advance at either 120–130 ka or about 60 ka. 

The younger chain probably erupted during the latest major glacial episode at 18–35 ka and is among the youngest volcanoes 

in the Caribou Volcanic Field. 

A line of small monogenetic mafic volcanoes that extends 

from Black Butte for about 5 km north is designated the Bidwell 

Spring chain. They are mostly outside the area of glaciation, but 

some have been weakly glaciated or partially covered by till. 

One radiometric date and the age of overlying glacial deposits 

indicate the vents in this chain to be between about 20 and 70 

ka. 

The Cone Lake chain comprises a group of basalt and 

basaltic andesite lava and cinder cones near Cone Lake. 

Although generally similar to flows of the Poison Lake chain, 

they are older (~200 to ~250 ka) and partially buried by units 

of Poison Lake chain and Bidwell Spring chain. The Cone Lake 

chain is mostly beyond the limit of glaciation, and the cinder 

cones are fairly well preserved. 

The Sunrise Peak sequence comprises a group of basalt 

and basaltic andesite lava flows and vents near Butte Lake. 

Although generally similar to the Red Cinder chain, they are 

older (~300–400 ka), more heavily glaciated, and cut by a 

prominent, old, till-covered fault that is part of the discontinuous fault zone between Lake Almanor Graben and Hat Creek 

Graben. 

The Island Lake sequence comprises two spatially and 

temporally related basalt lava flows near Bonte Peak. The Island 

Lake sequence is ~350 ka, heavily glaciated, and cut by faults 

along the discontinuous fault zone between the Lake Almanor 

Graben and the Hat Creek Graben. 

The Beauty Lake sequence includes glaciated lava flows 

lacking preserved vents south of Red Cinder. They are lithologically similar to mafic rocks in Caribou chain, but spatially 

offset from them, and approximately coeval with oldest rocks in 

Caribou chain (~425 ka).

Glacial Geology

Five episodes of Pleistocene glaciation are recognized 

in the Lassen area (Crandell, 1972; Kane, 1982; Colman and 

Pierce, 1992; Turrin and others, 1998). In most parts of the 

map, however, glacial deposits are divided simply into younger 

and older tills. In general, till, younger glaciations (Qty) has 

well-preserved moraines, whereas till, older glaciations (Qto)

has only moderately to poorly preserved moraines. The boundary between the two tills corresponds to the boundary between 

middle Wisconsin and early Wisconsin glacial stages as defined 

by Colman and Pierce (1992) or approximately to the Tioga 

and Tahoe glacial advances as these terms are used by Kane 

(1982). Our depictions of this boundary are based on our field 

observations, and our interpretations do not always agree with 

those of Crandell (1972) and Kane (1982). For example, in the 

Bailey Creek drainage northwest of Brokeoff Mountain, our 

contact lies between that shown by Crandell (1972) and that 

shown by Kane (1982). A comparison of different workers’ 

depictions of the Lassen area glacial geology is presented in 

table 1.

In the Lost, Hat, and Manzanita Creek drainages, 

Christiansen and others (2002) recognized and mapped six ages 

of glacial deposits, which were adapted for this map without 

modification. Tills designated late till of Anklin Meadows 

(Qtal), till of Anklin Meadows (Qta), and till of Raker Peak 

(Qtr) by Christiansen and others (2002) correspond to late Tioga, 

middle Tioga, and early Tioga tills of Kane (1982), respectively. 

These tills are equivalent to Qty in the rest of the map area. 

Post-maximum till of Raker Peak consisting of Lassen Peak 

avalanche debris (Qtrl) (Christiansen and others, 2002) is present 

only in the Lost and Hat Creek drainages. Till designated till of 

Badger Mountain (Qtb) in the Hat and Manzanita Creek drainages by Christiansen and others (2002) corresponds to the Tahoe 

advance of Kane (1982) and is included in Qto in all other drainages on this map. In the Lost, Hat, and Manzanita Creek drainages, the subdivided glacial units are intercalated with volcanic 

rocks of the Eagle Peak sequence. Use of the detailed glacial 

unit terminology was partially extended to the Bailey and Digger 

Creek drainages. In all areas other than the Hat, Manzanita, Lost, 

Bailey, and Digger Creek drainages, the contacts between Qty 

and Qto should be considered preliminary and approximate. 

Extension of the detailed glacial stratigraphy from the Lost, Hat, 

and Manzanita Creek drainages to the rest of the map area represents the most important remaining geologic mapping problem 

in the Lassen area. 

A similar scheme was adopted in mapping outwash gravels. 

Some till and outwash gravel mapped as Qto and outwash gravel 

(Qoo and Qou), particularly in the distal part of the Bailey 

Creek drainage in the northwestern part of the map and along the 

south boundary of the map area, is probably Illinoian in age. 

Methods

Unit Names

We base most volcanic rock unit names on SiO2 content 

of the dominant lithology. Rock units that were not chemically 

analyzed are named based on their phenocryst assemblage, the 

morphology of the rock unit, and the geologist’s experience 

with similar rock units. Because the composition of many rock 

units varies, we use the dominant composition to name the 

rock. However, the chemical composition of volcanic rocks is a 

continuum from basalt to rhyolite. When a lithology straddles or 

is close to a boundary, a future analysis may indicate a slightly 

different composition; thus, the name of some units is subject 

to revision. Major-element chemical analyses for samples from 

most rock units were published in Clynne and others (2008). 

Location of samples for chemical analysis and radiometric 

dating are included in the digital database on the accompanying CD-ROM (Muffler and others, 2010). Chemical analyses 

of Lassen rocks also were published by Clynne (1984), Bullen 

and Clynne (1990), Bacon and others (1997), Borg and others 

(1997), Borg and Clynne (1998), Clynne (1999), and Tepley and 

others (1999).
12

Mineral Names

Common mineral names, such as hornblende, hypersthene, 

and augite, are used for mafic minerals in the map descriptions even though they may not be compositionally precise. 

All magnesian orthopyroxenes are called hypersthene, and all 

magnesian clinopyroxenes are called augite. Hornblende is 

used for all calcic amphiboles, and biotite for all Fe–Mg micas. 

We use abbreviations for common minerals: pl, plagioclase; ol, 

olivine; aug, augite; hyp, hypersthene; pyr, pyroxene; hbl, hornblende; bt, biotite; and qtz, quartz. Mineral rock-name modifiers are used conventionally, in order of increasing abundance. 

Phenocryst abundances are estimated: the term "sparse" means 

less than about 1 percent and "trace" means much less than 

1 percent. When the rock type is variable, the most common 

lithology is estimated or ranges are given. Oxide minerals, 

primarily titanomagnetite, are ubiquitous but are not included in 

the names or lithologic descriptions.

Reacted Phenocrysts and Mafic Inclusions

The unit descriptions commonly do not take into account 

subtle variations in the macroscopic appearance of these rocks. 

For example, most Lassen dacites and many andesites have two 

populations of phenocrysts, one that is normal and a second 

that is reacted, but it may be difficult to distinguish reacted 

phenocrysts in hand specimen. Reacted crystals were subjected 

to significant thermal or chemical disequilibrium, usually as a 

result of mixing magmas. The terms resorbed, fritted, or sieved 

are used commonly in the literature to describe the resulting 

texture and are generally synonymous with our use of the term 

“reacted.”

Most Lassen dacites, and also some andesites and rhyolites, contain inclusions of more mafic chilled magma, ranging from a few millimeters to about 1.5 m in size. These are 

usually subrounded with sharp margins that are often crenulate 

against the host. In some dacites, many of the inclusions are 

broken and have angular margins. Inclusions have a finegrained, highly microvesicular texture in which plagioclase and 

hornblende or pyroxene form a network of acicular crystals 

with variable proportions of interstitial glass. Many rocks also 

contain partly recrystallized inclusions with a range of coarsergrained, weakly microvesicular to dense textures. Inclusions 

generally are formed from hybridized magma and commonly 

contain weakly to strongly reacted felsic phenocrysts derived 

from the host dacite. They may also contain mafic phenocrysts 

retained from their parental mafic magma. In addition to 

mafic inclusion, the terms "enclave," "quenched inclusion," 

"undercooled inclusion," and "andesitic inclusion" are used in 

the literature to describe this type of inclusion (Bacon, 1986). 

See Clynne (1999) for additional explanation and discussion of mafic inclusions and reacted phenocrysts and their 

significance. The abundance, distribution, and character of 

mafic inclusions, although commonly variable in individual 

dacites, can be used to distinguish dacite units. When useful, 

information on the mafic-inclusion population of rock units is 

provided in the unit descriptions. The abundance of inclusions 

is described semi-quantitatively: the term "abundant" means 

more than about 5 percent inclusions, "common" means from 1 

percent to about 5 percent, and "sparse" means less than about 

1 percent. 

Unit Ages

Ages of rock units reported in the Description of Map 

Units and Correlation of Map Units were acquired through a 

variety of radiometric dating techniques including radiocarbon, potassium-argon (K-Ar), and argon-argon (40Ar/39Ar). 

Age boundaries are based on Gradstein and others (2005) and 

Walker and others (2009). Paleomagnetic polarity boundaries 

are based on Berggren and others (1995). Ages for a few units 

are constrained by paleomagnetic directional data. References 

are given for published radiometric and paleomagnetic data, but 

most ages are unpublished. They are compiled in table 2 and 

details will be published elsewhere. Uncertainties given for all 

ages are 1 sigma. A few ages are weighted averages of several 

determinations. Ages of undated rock units are estimated using 

a variety of information, including stratigraphic position relative 

to dated units, preservation of volcanic morphology, degree of 

glacial erosion and relation to glacial deposits, degree of faulting, extent of soil development, and magnetic polarity; these 

ages are subject to revision. 

Unit Labels

We construct unit labels in the following order: age, rock 

type, unit name. An initial capital letter T is used for the known 

or inferred age of Tertiary map units and the capital letter Q for 

Quaternary surficial and glacial units (except hydrothermally 

altered rocks and travertine deposits). For Quaternary volcanic 

units, the initial Q is omitted.

Volcanic units:

b, basalt (<53% SiO2)

m, basaltic andesite (53–57% SiO2)

a, andesite (57–63% SiO2)

d, dacite (63–68% SiO2)

r, rhyodacite (68–72% SiO2) and rhyolite (>72% SiO2)

p, pyroclastic flow and fall deposits

Glacial and sedimentary units:

c, colluvium, including talus

d, diatomite

f, fluvial deposits, including all alluvium

h, hydrothermally altered rocks and travertine deposits

o, glacial outwash deposits

s, landslide and avalanche deposits

t, till

w, mudflow and other debris-flow deposits

Second (third for Tertiary units) and subsequent letters 

specify the individual rock units on the map. Rock units placed 

in groups and given similar names followed by a number, 

for example basaltic andesites of Silver Lake, units 1–11, are 

assigned numbers in known or inferred stratigraphic order, from 
13

oldest (unit 1) to youngest (unit 11). For example, unit bb10

represents Quaternary basalts and basaltic andesites of Bogard 

Buttes, unit 10.

The Correlation of Map Units is organized into five major 

groups as follows: glacial and sedimentary deposits, rocks of 

volcanic centers including the Tuscan Formation, and three 

groups of regional volcanic rocks. Rocks of each volcanic 

center and the Caribou Volcanic Field are enclosed in colored 

boxes to indicate all units belonging in those groups. Rock units 

related to regional volcanism are shown according to location; 

Regional volcanic rocks north and west of Lassen Volcanic 

Center: Regional volcanic rocks south and east of Lassen 

Volcanic Center, and Regional volcanic rocks of the Caribou 

area. Table 3 gives the page number of each unit’s description in 

the Description of Map Units and assistance in finding individual units on the Correlation of Map Units. Table 3 also gives 

the 1:24,000 quadrangle(s) within which each map unit occurs 

together with radiometric dates.

Acknowledgments

Our work builds on that of previous geologists (fig. 2) 

and would have been considerably more difficult without the 

context they provided. The mapping and publications of Howel 

Williams were, by far, the most insightful in this regard. His 

Geology of the Lassen Volcanic National Park (Williams, 1932) 

stands as a seminal work in interpretation of the geologic history of the Lassen area and the Cascade Range. 

A project of the scope and size of this geologic map cannot 

possibly be completed without the assistance and consultation 

of colleagues. We particularly acknowledge the significant collaborations of Robert L. Christiansen and Marianne Guffanti in 

successful completion of this geologic map. Clynne and Muffler 

were ably assisted over the course of geologic mapping by a 

number of field assistants: Lars Borg, Miranda Fram, Marianne 

Guffanti, Gordon Keating, Jacob Madden, Charles Melancon, 

and Deborah Trimble. 

Geochronologic data are essential to the satisfactory completion of any geologic map in volcanic terrane. The volcanic 

stratigraphy presented herein reflects the major contributions of 

G. Brent Dalrymple, Marvin Lanphere, Brent Turrin, Deborah 

Trimble, and Andrew Calvert, each of whom contributed dates 

on a variety of materials. Paleomagnetic data and correlations 

contributed by Duane Champion played a significant role in 

refining volcanic stratigraphy in some areas. 

Tracey Felger, David Ramsey, Joel Robinson, Dillon 

Dutton, Ellen Lougee, and Peggy Bruggman provided assistance with GIS compilation, training, and consultation. Wes 

Hildreth, Russell Evarts, James G. Smith, and Julie DonnellyNolan provided useful reviews or comments. Jan Zigler edited 

the map and text and significantly improved the presentation. 

Kathy Nimz skillfully assembled cartographic files.

This project would not have been possible without the 

access and permission to collect samples in Lassen Volcanic 

National Park that were provided by helpful park management 

staff. In particular, we recognize Al Dennison, Scott Isaacson, 

Elizabeth Knight, Russ Lesko, Marilyn Parris, Ellis Richard, 

Richard Vance, and Steve Zachary.

Finally, we would like to acknowledge Penelope Bowen, 

who began this project in 1975 under the direction of Muffler 

and introduced Clynne to geologic mapping at Lassen.
14

DESCRIPTION OF MAP UNITS

[Note that not all map units are labeled on the map. To identify units that are not labeled, because they 

are too small for color identification and (or) labeling will obscure data, see database (accompanying 

CD-ROM). See table 3 for an alphabetical listing of map unit labels, as well as unit name, page number of 

unit description in pamphlet, radiometric age, location by heading in Correlation of Map Units, and quadrangle location in database. See Methodology section for detailed information on methods used to describe 

and name map units. Unit labels are abbreviations for age (all units without a capital letter are Quaternary), 

rock type, and unit name: unit bb10 represents Quaternary basalts and basaltic andesites of Bogard Buttes, 

unit 10. Rock type abbreviations (volcanic units): b, basalt (<53% SiO2); m, basaltic andesite (53–57% 

SiO2); a, andesite (57–63% SiO2); d, dacite (63–68% SiO2); r, rhyodacite (68–72% SiO2) and rhyolite 

(>72% SiO2); p, pyroclastic flow and fall deposits. Rock type abbreviations (glacial and sedimentary 

units): c, colluvium, including talus; d, diatomite; f, fluvial deposits, including all alluvium; h; hydrothermally altered rocks and travertine deposits; o, glacial outwash deposits; s, landslide and avalanche deposits; 

t, till; w, mudflow and other debris-flow deposits. Common mineral abbreviations: pl, plagioclase; ol, 

olivine; aug, augite; hyp, hypersthene; pyr, pyroxene; hbl, hornblende; bt, biotite; and qtz, quartz. Abbreviations in brackets at end of unit description represent 1:24,000-scale quadrangle(s) where unit is mapped 

(see fig. 3)]

GLACIAL, SEDIMENTARY, AND HYDROTHERMAL DEPOSITS

NONVOLCANIC SEDIMENTARY DEPOSITS

Alluvium and Colluvium

Qf Alluvium (Holocene and Pleistocene)—Moderately well sorted, unconsolidated, lenticularbedded sand and gravel in modern stream channels. Includes stream terraces and reworked 

glacial outwash in some areas. Dominantly Holocene, but includes some older Pleistocene 

alluvium. Includes extensive marsh deposits at northeast end of Lake Almanor. [BB, CH, 

CM, GP, LP, MH, MN, MZ, OS, PL, PP, RC, RP, ST, SW, TH, VI, WP]

Qc Colluvium and talus (Holocene and Pleistocene)—Nonsorted to laterally sorted, unconsolidated, nonbedded, coarse to fine rubble. Consists mainly of talus at the base of cliffs or steep 

slopes of bedrock, but locally occurs as slopewash or thin, local debris flows. In northern 

Caribou Volcanic Field, includes flat, generally internally drained areas of windblown ash 

related to eruption of nearby cinder cones. Older colluvium generally has well-developed 

soil. Mapped only where it obscures underlying bedrock completely or is different lithology 

than underlying bedrock. Does not include debris emplaced hot from growing dacite domes. 

Age ranges throughout Holocene and, in areas outside the limits of glaciation, into Pleistocene. [BB, CH, LP, LY, MH, MN, MZ, OS, PL, PP, RC, RP, ST, SW, WP]

Qcl Talus on Lassen Peak (late Pleistocene)—Generally nonsorted, unconsolidated, nonbedded, 

coarse to fine rubble. Covers much of surface of the Lassen Peak dacite dome (dl). Some 

of talus was deposited as hot blocks during emplacement of dome and is recognized by 

presence of prismatically jointed blocks of dacite as large as 5 m across. Much talus was 

deposited later, during glaciation, weathering, and rockfalls from cliffs and other outcrops on 

Lassen Peak during late Pleistocene. [LP, MZ, RP]

Mass-Wasting Deposits

Qsh Landslide deposits in hydrothermally altered core of Brokeoff Volcano 

(Holocene)—Nonsorted, unconsolidated, nonbedded, fine to coarse rubble in small slumps, 

debris flows, and landslides on slopes underlain by unconsolidated or hydrothermally altered 

rocks. Mapped only where unit obscures bedrock or deposit is thick. Much of hydrothermally altered core of Brokeoff Volcano is unstable and subject to small landslides and debris 

flows, dozens of which occurred in East and West Sulphur Creek. Some slides subject to 

reactivation during spring snowmelt. Several major landslides of postglacial age originated 

from high, interior parts of eroded Brokeoff Volcano; largest landslide (not mapped separately) broke away from scarp above Forest Lake and flowed nearly 7 km down Mill Creek. 

It contains bedrock blocks as much as 100 m long and has radiocarbon date of 3,310±55 yr 

B.P. (Clynne and others, 2002). Another large landslide originated from northwest side of 
15

Pilot Pinnacle and flowed north into Blue Lake Canyon. Major rockfall of Holocene age 

originated from south face of Pilot Pinnacle. [LP, RP]

Qsj Avalanche deposits of Chaos Jumbles (Holocene)—Nonsorted, unconsolidated, nonbedded 

deposit of coarse to fine rubble with angular blocks as much as 5 m across. Consists almost 

entirely of pink, oxidized rhyodacite of Chaos Crags, dome C (rcc). Trains of blocks of 

weakly altered andesite of Brokeoff Volcano from septum between dome C and dome B 

(rcb) can be found in Chaos Jumbles. Deposit has lobate margins (with run-up on adjacent 

topographic highs, especially Table Mountain), transverse and longitudinal debris ridges 

on surface, and scarp-like margins generally 1–5 m high (see Eppler and others, 1987, for 

more details). Formed by catastrophic collapse of dome C as three rockfall avalanches were 

emplaced in quick succession (Crandell and others, 1974). Soils and forest are much better 

developed near margins of deposit relative to interior. Radiocarbon age of 278±28 yr B.P. 

was obtained from trees drowned by Manzanita Lake, which formed when first avalanche 

dammed Manzanita Creek (Clynne and others, 2002). Tree-ring correlation age for emplacement of Chaos Jumbles is 1644±14 C.E. [MZ]

Qwh Debris-flow deposits from the northeast side of Lassen Peak (Holocene)—Poorly sorted, 

generally unconsolidated, massively bedded rubble ranging from silt to large subrounded 

blocks. Consists entirely of dacite of Lassen Peak (dl). Commonly pink in color, but ranges 

from pink to gray, reflecting lithology of source dacite on different parts of Lassen Peak 

dome. Thickness, <1–6 m. Formed as mudflows from steep, high, northeast slopes of Lassen 

Peak and deposited in drainage of upper Lost Creek. Soil underlying deposit has radiocarbon 

age of 8,130±100 yr B.P. and soil developed on surface of deposit has radiocarbon age of 

7,550±50 yr B.P. (Turrin and others, 1998; Clynne and others, 2002). Thus, the age of the 

deposit is ~8,000 yr B.P. [LP, MZ, RP, WP]

Qsl Avalanche debris from Lassen Peak spread across glacial ice (late Pleistocene)—Nonsorted, 

unconsolidated, nonbedded, angular rubble with blocks commonly as large as 1–2 m across, 

locally (on the east side of Raker Peak) to 6 m; coarse grit matrix. Consists entirely of material derived from Lassen Peak (dl). Occurs high on east side of Raker Peak, north of Raker 

Peak on west side of Hat Creek drainage to as low as ~5,400 ft elev, and on east side of Hat 

Creek drainage at foot of Badger Mountain. Interpreted as debris spread across surface of 

glacier in valley of Hat Creek after partial glacial retreat from terminal position of Raker 

Peak glaciation (Qtr). [WP]

Qs82 Avalanche deposit from dacite of hill 8283 (middle Pleistocene)—Nonsorted, unconsolidated, 

nonbedded, angular rubble with blocks commonly as much as 1–2 m across in coarse grit 

matrix. In proximal areas, consists almost entirely of dacite of hill 8283 (d82). Occurs in 

valley of Manzanita Creek from west base of Lassen Peak to as low as ~4,000 ft elev. Best 

exposed near intersection of Calif. Hwys. 44 and 89, just west of park entrance at Manzanita 

Lake. West of this exposure, deposit is largely covered by pumiceous pyroclastic-flow 

deposits (pe) from Eagle Peak, and only large avalanche blocks protrude to surface. In 

area of McCumber Reservoir, deposit is mostly covered by outwash gravel and by tholeiitic basalt of Eagle Canyon (bec). In distal areas, deposit contains abundant fragments of 

andesite of Manzanita Creek (amz), which were probably picked up as avalanche crossed 

amz. Qs82 contains some prismatically jointed blocks, but it is unclear whether or not unit 

was emplaced hot by partial collapse of the d82 dacite dome. Age of deposit is uncertain, 

although Qs82 must be equivalent to or younger than d82 (261±5 ka) and older than bec

(199±22 ka). [MZ, VI]

Qwb Debris-flow deposit from Brokeoff Volcano (middle Pleistocene)—Sheetlike fragmental 

deposit, primarily in drainages of Rock and Digger Creeks, found only beyond limit of glaciation. Outcrops are sparse; good exposures occur only in road and stream cuts. Thickness 

is variable; thickest section measured is 8 m in Digger Creek. Deposit is matrix supported 

and light orange in color, weathering to dark orange. Matrix is composed of comminuted 

rock fragments, silt, and mud and is compacted but unconsolidated. Unweathered exposures 

of matrix are scarce. Debris flow contains as much as 20% angular to rounded clasts from a 

few millimeters to several meters in size. Clasts have dark-orange-brown weathering rinds 

as thick as 3 mm and are predominantly porphyritic aug-hyp andesite (andesite of Digger 

Creek, adc). Fragments of adc from all parts of lava flow (for example, vesicular, dense, 

platy, glassy, massive) can be found together. Matrix weathers away to leave a lag of clasts 
16

on ground surface. Distribution of debris flow and prominent lithologies indicate an origin 

from slopes of Brokeoff Volcano. Deposit surmounts topography in Blue Ridge area and 

may have been emplaced by a debris avalanche. Age of debris flow is unclear but may be 

related to eruption of adc, probably 350–450 ka. [GP]

Hydrothermal Deposits

h Hydrothermally altered rocks in active thermal areas (Holocene)—Rocks exposed in areas 

of active hydrothermal alteration and so intensively altered that their original lithology and 

stratigraphy are indeterminable. Typically, light-gray or orange-brown, altered andesite and 

dacite, without preserved mafic phenocrysts. Occasionally, areas of kaolinite and silica with 

scattered blocks of intensely altered rock. Mapped only in areas of intense active hydrothermal alteration at Bumpass Hell, Devils Kitchen, and Sulphur Works. [LP, RP]

ht Travertine (Holocene)—Deposits of calcium carbonate deposited on ground surface by flowing 

hot water. Around active hot-spring vents, forms mounds 1–4 m diam; commonly streaked 

with brightly colored green and orange algae. Deposits around modern but inactive vents 

are white, sometimes forming multiple terraces each 0.4 m high. Older travertine deposits 

are brown and commonly disaggregated by surface processes into 0.1 m angular blocks. 

In addition to two areas of dominantly old travertine mapped in upper Little Hot Springs 

Valley, springs currently depositing travertine occur on east bank of West Sulphur Creek 

at 6,850 ft elev, 550 ft at 176° azimuth from the roadcut at Sulphur Works, and on the east 

side of Little Hot Springs Valley at 7,360 ft elev, 4,600 ft at 79° azimuth from the summit 

of Diamond Peak. A travertine deposit too small to show at map scale occurs on the south 

side of Hot Springs Creek about 800 ft along trail from Warner Valley picnic area. Travertine 

deposits are all younger than till, younger glaciations (Qty). [LP]

Glacial Deposits

Two groups of glacial deposits are mapped: generalized deposits of glacial advances that apply to most 

of the map area and deposits intercalated with volcanic rocks in Hat, Manzanita, and Bailey Creeks that are 

subdivided and mapped separately.

Till

Qth Till or protalus-rampart debris (Holocene)—Poorly sorted, unconsolidated, nonbedded 

diamicton consisting of silt to boulders of dacite of Lassen Peak (dl). Equivalent to unit of 

Christiansen and others (2002). Forms small moraines or ramparts at ~7,800–9,000 ft elev 

near south and east base of Lassen Peak. Similar deposits on the north-facing upper slopes of 

Reading Peak and around several tuyas (basaltic andesite of Turnaround Lake, mtt) are not 

mapped. [RP]

Qty Till, younger glaciations (late Pleistocene)—Diamicton with boulders commonly as large as 

2 m across, locally to 4 m, and occasionally larger, consisting of a range of volcanic lithologies. Occurs as sheetlike deposits and large moraines as low as ~5,900–6,200 ft elev 

in Mineral area, 5,400 ft in valley of Warner Creek, 5,600 ft in area around Feather River 

Meadows and in valleys of Butte Creek and Bailey Creek (Chester quadrangle), 5,200 ft in 

Bunchgrass Valley, and 6,000–6,500 ft around Caribou Wilderness. Generally mapped only 

in areas where it obscures bedrock lithology or forms moraines. Typified by moraine complexes with well-preserved morphology and weakly oxidized soil zone ~50 cm thick. Subdivided as late till of Anklin Meadows (Qtal), till of Anklin Meadows (Qta), and till of Raker 

Peak (Qtr) in the Lost Creek, Hat Creek, and Manzanita Creek drainages (Christiansen and 

others, 2002) and in Bailey Creek (Viola quadrangle) and Digger Creek drainages. Equivalent in part to deposits of the early, middle, and late Tioga-age glaciations in Lassen area as 

defined by Kane (1982) and generally correlative with deposits of Tioga glaciations (late 

Wisconsin) in Sierra Nevada. [BB, CH, CM, LP, MH, MZ, PP, RC, RP, ST, TH, VI, WP]

Qtal Late till of Anklin Meadows (Holocene to late Pleistocene)—Diamicton with boulders of 

locally derived dacite as much as 4 m across. Occurs as small moraines at ~7,600–9,000 

ft elev on east and south sides of Lassen Peak. Also found buried beneath pumiceous 

pyroclastic-flow and fall deposits (rhyodacite of Chaos Crags, pc) in Crescent Crater 

and at ~7,600 ft elev in small cirque east of Chaos Crags. Equivalent to deposits of late 

Tioga age in Lassen area as defined by Kane (1982) and Crandell (1972). Unit overlain by 
17

debris-flow deposits from the northeast side of Lassen Peak (Qwh, ~8,000 yr B.P.). [LP, 

MZ, RP, WP]

Qta Till of Anklin Meadows (late Pleistocene)—Heterolithologic diamicton with boulders commonly as much as 2 m across, locally to 4 m, and occasionally larger. Occurs as sheetlike 

deposits and large moraines as low as 5,500–5,600 ft elev in Lost Creek, 6,100–6,200 ft in 

Hat Creek, 6,100 ft in Manzanita Creek, 5,600 ft in Bailey Creek (Viola quadrangle), and 

5,600–5,700 ft in Digger Creek. Generally mapped only in areas where unit obscures the 

bedrock lithology or forms moraines. Typified by moraine complexes with well-preserved 

morphology and a weakly oxidized soil zone ~50 cm thick. In Hat, Lost, and Manzanita 

Creek drainages, Qta is dominated by material derived from Lassen Peak and is distinguished from till of Raker Peak (Qtr) by presence of dacite of Lassen Peak (dl; Christiansen 

and others, 2002). Equivalent to deposits of middle Tioga glaciation in the Lassen area as 

defined by Kane (1982). [GP, LP, MZ, RP, WP]

Qtrl Post-maximum till of Raker Peak consisting of Lassen Peak avalanche debris (late 

Pleistocene)—Diamicton, with boulders as large as 1.5 m across, consisting almost entirely 

of dacite derived from avalanche debris from Lassen Peak spread across glacial ice (Qsl). 

Forms a small moraine at northeast base of Raker Peak. [WP]

Qtr Till of Raker Peak (late Pleistocene)—Heterolithologic diamicton containing boulders commonly as much as 2 m across, locally to 4 m, and occasionally larger. Occurs as sheetlike 

deposits and large moraines as low as ~5,800 ft elev in Manzanita Creek valley and 5,500 ft 

in Hat Creek valley. Typified by moraine complexes with well-preserved morphology and 

weakly oxidized soil zone ~50 cm thick. Generally mapped only where it obscures bedrock 

lithology or forms moraines. In Manzanita Creek drainage, prominent boulder lithologies 

include dacite of hill 8283 (d82) and rhyodacite of section 27 (r27). In Hat Creek drainage, Qtr contains abundant andesite of Hat Mountain (ah), dacite of Reading Peak (dr), and 

rhyodacite of Dersch Meadows (rd). Distinguished from till of Anklin Meadows (Qta) by 

lack of clasts derived from Lassen Peak (dl; Christiansen and others, 2002). Equivalent to 

deposits of early Tioga glaciation in Lassen area (Kane, 1982). [GP, MZ, WP] 

Qto Till, older glaciations (late Pleistocene)—Heterolithologic diamicton with boulders commonly 

as large as 2 m across, locally to 4 m, and occasionally larger. Occurs as sheetlike deposits 

and subdued moraines preserved beyond limits of till, younger glaciations (Qty) in the areas 

around Mineral and North Fork Feather River and valleys of Digger, Bailey (Grays Peak 

quadrangle), and Warner Creeks. Qto is extensive in area south and east of Caribou Wilderness, 5,500–6,500 ft elev. Younger part equivalent in age to till of Badger Mountain (Qtb) as 

mapped in Hat, Lost, and Manzanita Creek drainages (Christiansen and others, 2002). Generally mapped only in areas where it obscures bedrock lithologies or forms moraines. Qto is 

typified by moderately to poorly preserved and eroded moraines and strongly oxidized soil 

zone >1 m thick. Equivalent to Tahoe and pre-Tahoe glaciations in Lassen area (defined by 

Kane, 1982) and generally correlative with Tahoe and older glaciations (early Wisconsin and 

older) in Sierra Nevada. [BB, CH, CM, GP, LP, LY, MH, MN, MZ, PP, RC, RP, ST, VI]

Qtb Till of Badger Mountain (late Pleistocene)—Heterolithologic diamicton with boulders commonly as large as 2 m across, locally to 4 m, and occasionally larger. Occurs as sheetlike 

till deposits and subdued moraines preserved beyond the limits of younger tills (till of 

Anklin Meadows, Qta, and till of Raker Peak, Qtr) in the Hat, Lost, and Manzanita Creek 

drainages. Generally mapped only in areas where it obscures bedrock lithology. Occurs at 

5,500–6,500 ft elev. Typically has oxidized soil zone >1 m thick. Prominent boulder lithologies include andesite of Raker Peak (arp) and andesites of Badger Mountain (abm); less 

prominent but relatively common lithologies include tholeiitic basalt of Nobles Trail (bn) 

and various older dacites, including dacite of Reading Peak (dr). Equivalent to deposits of 

Tahoe glaciation in Lassen area (defined by Kane, 1982) and ~60 ka in age. [MZ, WP]

Outwash

Qoy Outwash gravel, younger glaciations (late Pleistocene)—Moderately sorted, unconsolidated 

gravel and sand, commonly containing boulders as much as 1–2 m across consisting of 

same lithologies as the correlative till, younger glaciations (Qty). Occurs as partial valley 

fills, thick sheets or alluvial fans in drainages of Butte Creek, Warner Creek, Willow Creek, 

Box Canyon, Bunchgrass Valley, Feather River Meadows, and North Fork Feather River. 
18

Equivalent to outwash gravel of Anklin Meadows (Qoa) and outwash gravel of Raker Peak 

(Qor) in Hat Creek and to part of outwash gravel, undivided (Qou) in Manzanita Creek and 

Battle Creek Meadows. Probably includes older outwash in some areas. [BB, CH, CM, LP, 

MH, MN, MZ, OS, PP, RC, RP, ST, TH, VI, WP]

Qou Outwash gravel, undivided (late Pleistocene)—Moderately sorted, unconsolidated gravel and 

sand, commonly containing boulders as much as 1–2 m across consisting of same lithologies as the correlative till, younger glaciations (Qty) and till, older glaciations (Qto). Occurs 

as partial valley fills and thick sheets in valleys of South Fork Battle Creek and Manzanita 

Creek where outwash gravels are not divided. Age is equivalent to Qty and Qto. [GP, LY, 

MN, PP, SW, VI]

Qoa Outwash gravel of Anklin Meadows (late Pleistocene)—Moderately sorted, unconsolidated 

gravel and sand, commonly containing boulders as much as 1–2 m across consisting of the 

same lithologies as the correlative till of Anklin Meadows (Qta). Occurs as partial valley 

fill, thick sheets, or alluvial fans in Hat Creek drainage. Distinguished from outwash gravel 

of Raker Peak (Qor) by presence of clasts derived from Lassen Peak (dl). Collectively, Qor

and Qoa are equivalent to outwash gravel, younger glaciations (Qoy) and form part of outwash gravel, undivided (Qou) in other drainages. [MZ, OS, WP]

Qor Outwash gravel of Raker Peak (late Pleistocene)—Moderately sorted, unconsolidated gravel 

and sand, commonly containing boulders as much as 1–2 m across consisting of the same 

lithologies as the correlative till of Raker Peak (Qtr). Occurs as partial valley fill, thick sheets, 

or alluvial fans in Hat Creek drainage. An extensive fan of Qor in Emigrant Ford area may, 

in part, be related to floods generated by the emplacement of Lassen Peak at 27 ka. In area 

between Twin Bridges and Big Spring, Qor is covered but not completely obscured by thin 

pyroclastic-flow and associated fluid debris-flow deposits, May 22 (pw2) from the 1915 

eruption of Lassen Peak (Christiansen and others, 2002). Qor is distinguished from outwash 

gravel of Anklin Meadows (Qoa) by the absence of clasts derived from Lassen Peak (dl). 

Collectively Qor and Qoa are equivalent to outwash gravel younger glaciations (Qoy) and 

comprise part of outwash gravel, undivided (Qou) in other drainages. [MZ, OS, WP]

Qoo Outwash gravel, older glaciations (late Pleistocene)—Moderately sorted, unconsolidated 

gravel and sand, commonly containing boulders as large as 1–2 m across consisting of the 

same lithologies as till, older glaciations (Qto). Occurs in valley of Gurnsey Creek and as 

partial basin fill south of Wilson Lake. Age is equivalent to the younger part of Qto. [CM, 

MN]

Sedimentary Deposits

Qd Diatomite (Holocene)—Deposits of diatomite occur peripheral to Cinder Cone lava flows for 

about 2 km west from present shores of Butte Lake, as kipukas in Fantastic Lava Beds flows 

from Cinder Cone, and on east shore of Butte Lake. Diatomite occurs primarily as disturbed 

sediment bulldozed by lava flows. Deposited in ancient Butte Lake; pattern of the deposits indicates that a much larger lake existed prior to eruption of Cinder Cone (Clynne and 

others, 2000). Many additional deposits of diatomite present in Butte Lake area are too small 

to show at map scale. [PP]

VOLCANIC CENTERS OF SOUTHERNMOST CASCADES

LASSEN VOLCANIC CENTER

Lassen domefield

Twin Lakes sequence, younger

Deposits of 1914–1917 eruption of Lassen Peak (Holocene)—Complex eruptive sequence 

consisting of a dacite dome and lava flow, dacite pyroclastic flow and fall deposit, and 

phreatic, avalanche, debris-flow, and flood deposits. Magmatic activity was confined to the 

period from about May 14 to 22, 1915, and affected primarily the northeast flank and slope 

of Lassen Peak (now called the Devastated Area) and the valleys of Lost and Hat Creeks as 

far as about 50 km downstream. Division of deposits of the 1914–1917 eruption is based on 

stratigraphy and mode of emplacement and generally follows the depiction of Christiansen 
19

and others (2002). Rock types range from ol andesite to several varieties of hbl-bt dacite 

with abundant mafic inclusions and complex mineralogy. The presence of spectacularly 

banded pumice erupted on May 22 serves to distinguish May 22 from May 19 deposits. See 

Clynne (1999) for more detailed description and interpretation of these rocks. [LP, MZ, OS, 

RP, WP]

1917 C.E.

p17 Phreatic deposit, May–June—Generally poorly sorted, partly indurated, thin to thick beds 

of lithic ash, lapilli, and blocks deposited in 1917. Matrix is generally very fine, indurated, 

and pale-yellowish-brown ash. Consists mainly of material derived from dacite dome of Lassen Peak (dl); thins abruptly from several meters on crater rim (north edge of Lassen Peak 

summit) to a few centimeters at mapped edge of deposit. Ejected in steam-blast eruptions 

from crater in the northwestern part of the Lassen Peak summit, mainly during May and 

June of 1917. [LP]

1915 C.E.

w2 Viscous debris-flow deposits, May 22—Largely nonsorted, unconsolidated, nonbedded 

debris-flow deposits with lapilli and blocks to >1 m and rarely to 2 m in a sandy to silty matrix. Dominated by banded and light-colored pumice of the May 22, 1915, eruption but also 

contains fragments of d9, d4, and dacite of Lassen Peak (dl). Generally lobate with scarplike margins as high as 1–2 m; thickness ranges from a few decimeters to ~3 m. Formed by 

debris flows from steep upper slopes of Lassen Peak after major pumice eruption of May 22, 

1915. The two largest debris flows are found in lower Devastated Area and upper Manzanita 

Creek. [LP, MZ, RP, WP]

p2 Pumice-fall deposit, May 22—Generally well sorted, unconsolidated, thick-to-thin beds 

consisting of blocks, lapilli, and ash of dacitic pumice; blocks prismatically fractured and 

commonly disintegrated in place. Erupted from Lassen Peak on afternoon of May 22, 1915. 

Bedding generally indistinct. Pumice blocks weakly banded, typically with yellowish-brown 

and light-gray to nearly white dacitic layers; lapilli and coarse ash commonly yellowish 

brown with faint color banding. Mapped only on north and east sides of Lassen Peak, where 

thickness generally exceeds ~2 m. Not mapped where preserved only in small patches or 

where thickness is only a few decimeters or less. Narrow lobe of scattered pumice lapilli can 

be traced from LassenPeak directly over Hat Mountain to vicinity of Cinder Cone. [LP, RP, 

WP]

pw2 Pyroclastic-flow and associated fluid debris-flow deposits, May 22—Consists of pyroclastic-flow and associated debris-flow deposits erupted on the afternoon of May 22, 1915, 

that grade into each other in middle Devastated Area (mapped separately in Christiansen 

and others, 2002). Pyroclastic-flow deposit is largely nonsorted, unconsolidated, nonbedded 

material with lapilli and blocks of d9, d4, and dacite of Lassen Peak (dl) as large as 3 m 

in a sandy to silty matrix. Blocks of banded pumice with dark-gray andesitic (59.7–61.0% 

SiO2) and light-gray to nearly white dacitic (64.2–66.8% SiO2) layers are conspicuous and 

diagnostic. Rhyodacite pumice from Chaos Crags eruption (pc) is minor compared to its 

abundance in sw9. Deposit contains abundant fragments of wood, some charred, including 

numerous decayed logs lying in positions pointing downslope away from summit of Lassen 

Peak. Ranges in thickness from feather edge to at least 3 m. Occurs in upper to middle Devastated Area on northeast side of Lassen Peak.

The fluid debris-flow deposits derived from the pyroclastic flow are largely nonsorted, 

unconsolidated, nonbedded deposits containing lapilli and blocks of d9, d4, and dl as large 

as 3 m in a sandy to silty matrix. Banded pumice blocks are conspicuous and diagnostic. 

Individual, apparently rootless, small flows emerge from pyroclastic-flow deposit and merge 

downslope into generally sheetlike deposit as thick as 2–3 m, with margins tapering to imperceptibility. Exposed in the middle to lower Devastated Area on northeast side of Lassen 

Peak and in drainages of Lost Creek and Hat Creek to near Old Station. [LP, MZ, OS, RP, 

WP]

d9 Dacite flow, May 19–20—Porphyritic bt-hbl dacite (64–65% SiO2) erupted from vent at 

summit of Lassen Peak that was reopened by phreatic explosion on evening of May 19, 

1915. Lava filled crater and flowed through notch on west side of summit and ~300 m down 

west flank. Another similar flow on northeast flank was removed during pyroclastic eruption of May 22. Lava flow was still fluid enough to partially slump back into crater created 
20

by pyroclastic eruption two days later on May 22. Lava flow has uneroded, spiny aa surface 

with 5–10 m of relief. Phenocrysts: 20% pl, from 3–5 mm, but composite crystals to 12 mm 

are conspicuous; 3% hbl, 1–5 mm, rarely to 12 mm; 3% bt, 1–3 mm; 2% qtz, 1–3 mm, rarely 

to 6 mm; trace ol, 0.75 mm. Contains abundant microphenocrysts of pl, aug, and hyp to 0.5 

mm. About 40% of pl, bt, hbl, and qtz phenocrysts are reacted. Groundmass is dark gray to 

black and typically glassy. Mafic inclusions from a few cm to ~50 cm form ~5% of rock. 

They contain reacted host-rock phenocrysts (large pyr-rimmed qtz particularly conspicuous) and ol to 0.75 mm. Most inclusions have fine-grained microvesicular texture of pyr and 

pl microphenocrysts with interstitial glass, are subangular, and lack crenulate margins. See 

Clynne (1999) for detailed description and interpretation of 1915 rocks. [LP]

f9 Flood deposits, May 19–20—Sorted to nonsorted, unconsolidated, generally nonbedded 

sand to boulders and other debris in Hat Creek between the bedrock gorge just north of 

Lassen Volcanic National Park boundary and Twin Bridges that were deposited by dewatering of the May 19–20, 1915, debris flows (sw9). Scoured by numerous braided stream 

channels, most abandoned. Amount of sediment decreases progressively downstream, but 

affected area remains complexly scoured and marked by chaotic jams of boulders, logs, and 

other debris. [WP]

sw9 Avalanche and debris-flow deposits, May 19–20—Composite unit mapped as two separate units by Christiansen and others (2002). Avalanche deposit is composed of nonsorted, 

unconsolidated, nonbedded debris from d4, dacite of Lassen Peak (dl), Chaos Crags pumice 

(pc), and much wood debris. Generally lies on soil developed in underlying deposits, including pumice from Chaos Crags (pc). Deposited from a mixed snow-and-debris avalanche 

originating in cirque at top of east face of Lassen Peak late in the evening of May 19 and 

(or) the early morning of May 20. Deposits occur in Devastated Area on northeast side of 

Lassen Peak and across low divide northeast of Emigrant Pass in Hat Creek drainage.

Debris-flow deposits from Lassen Peak are composed of nonsorted, unconsolidated, 

generally nonbedded sand to boulders as much as 3 m diam. They consist almost entirely 

of fragments of d4, dl, pc, and logs derived from underlying avalanche deposit of May 

19–20 or adjacent areas of forest. Deposits occur in Devastated Area on northeast side of 

Lassen Peak and down Lost Creek drainage to a few kilometers east of Twin Bridges. Deposits commonly are rooted within avalanche deposit of May 19–20. Emplaced during late 

evening of May 19 and (or) early morning of May 20 immediately following the avalanche.

Near margins, numerous standing trees predating deposits are scarred from collisions with 

boulders and logs carried in debris flows. Piles of boulders and logs behind large trees or 

boulders are common along margins of deposit. [WP]

p9 Pyroclastic deposit, May 19—Nonsorted, unconsolidated, nonbedded deposit of blocks to 

fine lithic ash, consisting entirely of dense clasts of d4 and dacite of Lassen Peak (dl), emplaced late in evening of May 19, 1915. Ranges from layer ~4 m thick to a field of discontinuous blocks. Ejected by phreatic explosion that opened summit crater through the dacite 

dome of May 14–19 and preserved only in Lassen Peak summit area. [LP]

d4 Dacite dome, May 14–19—Porphyritic bt-hbl dacite (64–65% SiO2), which forms remnants 

of a small dacite lava dome at summit of Lassen Peak, emplaced between about May 14 and 

evening of May 19, 1915. Dome filled crater excavated by phreatic explosions that began 

on May 30, 1914, and was partially disrupted by single large phreatic explosion on evening 

of May 19, 1915. Phenocrysts: 20% pl, from 3–5 mm, but composite crystals to 12 mm are 

conspicuous; 3% hbl, 1–5 mm, rarely to 12 mm; 3% bt, 1–3 mm; 2% qtz, 1–3 mm, rarely 

to 6 mm; trace ol, 0.75 mm. Contains abundant microphenocrysts of pl, aug, and hyp to 0.5 

mm. Pl, bt, hbl, and qtz phenocrysts are commonly reacted and more abundant than in d9. 

Groundmass is dark gray to black, typically glassy and microvesicular. Mafic inclusions 

from a few mm to ~50 cm form ~5% of rock and contain reacted host-rock phenocrysts 

(large pyr-rimmed qtz is particularly conspicuous) and ol to 0.75 mm. Most inclusions have 

fine-grained microvesicular texture of pyr and pl microphenocrysts and interstitial glass, are 

subangular, and lack crenulate margins. Millimeter- to centimeter-sized inclusions are more 

abundant in lava dome than lava flow (d9). [LP]

Basaltic andesites of Cinder Cone (Holocene)—Complex eruptive sequence consisting of two 

cinder cones (Cinder Cone and remnant cone at same location) and five block-lava flows 

south of Prospect Peak and west of Butte Lake on Central Plateau. Subdivision is based on 
21

eruptive sequence and composition; two compositional groups are present. Old Bench flow 

(mo) and Painted Dunes flows (mp1, mp2) erupted from remnant cone and comprise an 

early group of ol basaltic andesite to andesite deposits having the Painted Dunes composition. Painted Dunes flows (especially mp2) contain blocks of welded cinders and agglutinate 

from several to tens of meters across that were rafted from remnant cinder cone, which was 

almost completely destroyed by eruption of the flows. Painted Dunes flows blocked drainage into Butte Lake from south and created Snag Lake. Fantastic Lava Beds flows (mf1, 

mf2) erupted from Cinder Cone and form a late group of ol basaltic andesites having the 

Fantastic Lava Beds composition. Painted Dunes composition has higher MgO and lower 

K2O and TiO2 at similar SiO2 content than Fantastic Lava Beds composition. A widespread 

ash deposit as much as 2.5 m thick is composed of ash of both compositions, Fantastic Lava 

Beds overlying Painted Dunes with no discernable time break between; thickness is shown 

on the map as isopachs in centimeters (adapted from Heiken, 1978). Ash thinner than 5 cm 

extends locally for at least several kilometers beyond the 5 cm isopach. Old Bench flow and 

Painted Dunes flow 1 have thick cover of colorfully oxidized ash, and Painted Dunes flow 

2 has thin cover of ash; both ashes have Fantastic Lava Beds composition. Oxidation of ash 

demonstrates that Old Bench and Painted Dunes lava flows were still hot when the ash that 

formed Cinder Cone erupted. Little ash was erupted during or after Fantastic Lava Beds 

flows were emplaced, and their surfaces are nearly ash free. Painted Dunes and Fantastic 

Lava Beds flows have rough, block-covered surfaces with considerable relief and many 

flow levees. Lava flows completely lack soil and are nearly devoid of vegetation; only a few 

mature but stunted trees grow on Painted Dunes ash deposits.

Painted Dunes composition is sparsely porphyritic ol basaltic andesite and andesite, and 

Fantastic Lava Beds composition is sparsely porphyritic aug-ol basaltic andesite. Fantastic 

Lava Beds rocks also contain small pl phenocrysts, but the two types are difficult to discriminate in the field. Groundmasses are dark gray to black and strongly microvesicular and are 

typically glassy, but aphanitic lava is also common. Inclusions of clear and milky qtz, generally a few mm to 1 cm in size, are common to abundant in both Painted Dunes and Fantastic 

Lava Beds compositions; rare qtz inclusions to 10 cm have rhombohedral cleavage. Two additional types of inclusions are rare; one type consists of a variety of partially melted mafic 

to felsic granitic rocks, and the second type consists of mafic inclusions of basaltic andesite 

(53.3–53.4% SiO2

). Stratigraphic, paleomagnetic, and radiocarbon data indicate that the lava 

and tephra erupted over a short interval about 1650 C.E. (Clynne and others, 2000; Clynne 

and others, 2002). Tree-ring dendrochronology indicates that Cinder Cone erupted in 1666 

C.E. (Sheppard and others, 2009). Cinder Cone is the youngest mafic volcano in the map 

area. [PP]

Fantastic Lava Beds

mf2 Flow 2—Thin to thick, block-lava flow of sparsely porphyritic aug-ol basaltic andesite 

(55.1–56.2% SiO2) erupted from south base of Cinder Cone. Lava flowed in a channel 

across mf1 and entered Butte Lake. Phenocrysts: 5% ol, 0.25–1 mm, few larger; 2–3% pl, 

0.5 mm, few larger, mostly reacted; sparse aug, 0.25–0.5 mm. Groundmass is dark gray to 

black, glassy, and microvesicular. [PP]

mf1 Flow 1—Block-lava flow as thick as 20 m of sparsely porphyritic aug-ol basaltic andesite 

(56.4–57.3% SiO2) erupted from south base of Cinder Cone. Lava flowed south over Painted 

Dunes flows toward Snag Lake then veered north to Butte Lake and crops out in the area between Cinder Cone and Butte Lake. Small outcrop on east shore of Butte Lake demonstrates 

that mf1 lava underlies much of lake basin. Phenocrysts: 5% ol, 0.25–1 mm, few larger; 

2–3% pl, 0.5 mm, few larger, mostly reacted; sparse aug, 0.25–0.5 mm. Groundmass is dark 

gray to black, glassy, and microvesicular. Xenocrysts of feldspar to 5 mm disaggregated 

from granitic inclusions are sparse. Cinder Cone is constructed of scoria bombs, lapilli, and 

ash of Fantastic Lava Beds composition. Cone has a double rim, but there is no discernable 

difference in lithology or composition between inner and outer rims. Also includes a ring 

of dense agglutinate bombs as large as 3 m across at base of cone. Light-colored vesicular 

inclusions of partially melted felsic granitic rocks are rare but conspicuous in scoria. [PP]

Painted Dunes

mp2 Flow 2—Thick (to 30–40 m) block-lava flow of sparsely porphyritic ol andesite 

(57.1–59.7% SiO2) erupted from remnant cone at base of Cinder Cone. Lava flowed south 
22

from base of remnant cone over Painted Dunes flow 1 toward Snag Lake then veered north 

toward Butte Lake and is mapped primarily south of base of Cinder Cone to shore of Snag 

Lake. Ash-covered kipukas in area of Fantastic Lava Beds flows near Butte Lake indicate 

that Painted Dunes flow 2 is more extensive than present outcrop area and was partially 

buried by Fantastic Lava Beds flows. Ash from Cinder Cone is poorly preserved on surface 

of this flow, because it has sifted between jumbled blocks on surface. Phenocrysts: 5% ol, 

0.25–1 mm, few larger. Groundmass is medium to dark gray, aphanitic, and dense to black, 

glassy, and microvesicular. Commonly contains sparse feldspar xenocrysts. [PP]

mp1 Flow 1—Thick (to 30–40 m) block-lava flow of sparsely porphyritic ol basaltic andesite 

(54.1–56.1% SiO2) erupted from remnant cone at south base of Cinder Cone. Lava flowed 

south toward Snag Lake then veered north toward Butte Lake and is mapped primarily 

around south base of Cinder Cone. Characterized by a cover of ash as thick as several meters 

called the Painted Dunes. Ash-covered kipukas in area south of continuous outcrop indicate 

that flow is more extensive and was partially buried by the younger flows (mp2, mf1, mf2). 

Phenocrysts: 5% ol, 0.25–1 mm, few larger. Groundmass is medium to dark gray, aphanitic 

or dense, black, glassy, and microvesicular. Commonly contains sparse feldspar xenocrysts. 

One large and one small remnant of a cinder cone constructed of scoria bombs, lapilli, and 

ash are found under south flank of Cinder Cone. Most of cone was destroyed by eruption of 

the Painted Dunes lava flows, and much of remainder was buried by construction of Cinder 

Cone. Cone remnants are recognized by their irregular shape and Painted Dunes composition. [PP]

mo Old Bench flow—Relatively thin, as thick as 10 m, lava flow of sparsely porphyritic ol 

basaltic andesite (53.5–54.1% SiO2) erupted from remnant cone for the Painted Dunes 

flows. Found at the east base of Cinder Cone and is almost completely covered with a 

several-meter-thick deposit of weakly oxidized ash from later eruptions of Cinder Cone; 

only a few pinnacles of lava extend through the ash. Flow was more extensive than mapped 

and is mostly buried by Painted Dunes and Fantastic Lava Beds flows. Phenocrysts: 5% 

ol, 0.25–0.5 mm, few larger. Groundmass is black and glassy with abundant flattened to 

stretched vesicles. Qtz grains are less abundant and smaller than in Painted Dunes and 

Fantastic Lava Beds flows. [PP]

a74 Andesite of hill 7416 (late Pleistocene)—Thick (to 50 m) block-lava flow of porphyritic ol-aug 

andesite (57.5% SiO2) erupted from a vent marked by a scoria cone on the upper east flank 

of West Prospect Peak. Lava flowed northward down crease between Prospect Peak and 

West Prospect Peak and partially buried trace of Hat Creek Fault on north flank of Prospect 

Peak. Flow has not been glaciated, is unforested, and is completely lacking in soil; its 

original block surface is well preserved. Contains a disequilibrium phenocryst assemblage 

consisting of strongly reacted sodic pl, qtz, hbl, and bt (?) and unreacted ol, aug, and calcic 

pl. Phenocrysts: 12% pl, 0.5–6 mm, mostly 1–3 mm; 1–2% qtz, 1–3 mm; 1% aug, 0.5–2 

mm, aggregates of small crystals; 1% ol, to 2 mm, mostly 0.5–1 mm; trace hbl to 1 cm. 

Fragments of andesitic inclusions 5 mm to a few centimeters in diameter are abundant; 

larger inclusions to 10 cm are sparse. Groundmass is dark gray to black, glassy, and dense to 

vesicular. Unit not dated, but its age is well constrained by stratigraphy. Flow is not offset by 

Hat Creek Fault, which offsets 15–18 ka glacial deposits farther north (Muffler and others, 

1994). Unit erupted into a glacial cirque 18–25 ka but is older than Chaos Crags pumice 

(1.1 ka). It may be Holocene, but estimated to be about 12–15 ka. [OS, PP, SW]

ah Andesite of Hat Mountain (late Pleistocene)—Porphyritic aug andesite (58.0–61.6% SiO2) 

forming generally ~100-m-thick lava flows. Vent is marked by breached cone of agglutinated cinder and spatter. Entire edifice has been glaciated; original block surface of lava 

flows was removed by erosion. Lava is generally massively jointed and typically weathers 

into rounded boulders 0.5 to several meters in diameter. Flow contains a disequilibrium 

phenocryst assemblage consisting of reacted group of felsic phenocrysts (pl, qtz, hbl, and 

bt?) and group of mafic phenocrysts (aug, calcic pl). These groups of phenocrysts are present in variable proportions. Typical flow contains strongly reacted felsic phenocrysts: 4–8% 

pl, 1–2 mm; sparse–1% aug to 1 mm; sparse–1% qtz, 1–2 mm rimmed by tiny aug crystals 

and often containing black to brownish specks of glass and bt; sparse–1%, 1–4 mm pseudomorphs of hbl (and bt?). Less-common type of flow found primarily around Twin Lakes is 

more porphyritic (~15% total phenocrysts) and contains same phenocryst assemblage but 
23

with many felsic phenocrysts less strongly reacted. Medium- to dark-gray groundmass is 

aphanitic (less porphyritic samples) to glassy (more porphyritic samples), and both types are 

microvesicular. Mafic inclusions are generally small, less than ~10 cm and sparse, although 

more common locally. They contain same phenocryst assemblage found in host rock, but 

pl, hbl, and qtz phenocrysts are sparse and strongly reacted or rimmed, whereas small aug 

phenocrysts are abundant. Chemical composition is variable; typical samples have 58–59% 

SiO2, and more porphyritic samples have as much as ~62% SiO2. Flow overlies rhyolites 

of Dittmar Volcanic Center, Stage 3 (rd3), basalt of Badger Flat (bbf), and rhyodacite of 

Dersch Meadows (rd). Attempts to date this unit failed, and its precise age is unknown. 

Unit overlies and therefore must be younger than basaltic andesite of Fairfield Peak (mfp, 

82±14 ka) and andesite of Crater Butte (ac, 93±13 ka). Clasts of ah are not present in till 

of Badger Mountain (Qtb) suggesting that ah is younger than ~60 ka. Clasts of ah in till of 

Anklin Meadows (Qta) and till of Raker Peak (Qtr) demonstrate that it is older than ~27 ka 

and suggest that it is older than ~35 ka. The flow is estimated to be ~40–50 ka. [MH, PP, RP, 

WP]

ae Andesite of Eagle Peak (late Pleistocene)—Macroscopically heterogeneous unit formed by 

mixing of rhyodacite of Eagle Peak (re) and aug-hyp andesite magmas. Color-banded and 

compositionally banded to blotchy, microvesicular to pumiceous rock of variable lithology 

(61–68% SiO2) is poorly exposed in small area of mostly oxidized and altered rock between 

Ski Heil Peak and Eagle Peak. The most mafic lithology is porphyritic andesite, containing 

phenocrysts of pl, hbl, bt, and qtz that are similar in character to those in re, although hbl 

and bt tend to be slightly oxidized. Also contains a few percent of 0.5–1 mm hyp and aug 

(hyp>aug) and abundant 0.5-mm calcic pl phenocrysts from andesitic magma. Total phenocryst content is ~10%. Also contains glomeroporphyritic clots of aug+hyp+pl. Groundmass is dark brownish gray, glassy, and microvesicular. Least mafic lithology resembles 

re but contains sparse phenocrysts from andesite contaminant and has brownish, glassy 

groundmass. Majority of the unit is intermediate between the two lithologies. Precise age is 

unknown, but it is probably contemporaneous with the end of the eruption of re at 66±4 ka.

[LP]

mfp Basaltic andesite of Fairfield Peak (late Pleistocene)—Short stubby lava flow of sparsely 

porphyritic, ol basaltic andesite (54.4% SiO2) erupted from vent marked by agglutinate cone 

of Fairfield Peak. Original block surface of flow has been removed by glaciation. Outliers 

along base of Prospect Peak demonstrate that till, younger glaciations (Qty) and ash and 

lava from Cinder Cone bury part of mfp. Cone is composed of scoria, cinders, and blocks of 

vesicular lava armored by agglutinated surface. Phenocryst assemblage is in strong disequilibrium; pl is reacted and has thick overgrowths of more calcic pl, and qtz is rimmed by aug. 

Phenocrysts: 3% ol, 0.5–2 mm, mostly 0.5 mm; 2% pl, 0.5–5 mm, mostly 1–3 mm; sparse 

qtz, 1–3 mm. Trace pseudomorphs of hbl to 1 cm but generally only a few millimeters. 

Groundmass is usually light to medium gray and aphanitic but is often oxidized. Dominant 

joint pattern is thick and slabby. Overlies andesite and basaltic andesite of Cluster Lakes 

(acl) and andesite of Crater Butte (ac), and is overlain by andesite of Hat Mountain (ah). 

K-Ar age is 82±14 ka. [PP]

ac Andesite of Crater Butte (late Pleistocene)—Lava flow as much as 150 m thick of porphyritic 

aug-ol andesite (60.4–60.5% SiO2) between Hat Mountain and Fairfield Peak. Erupted from 

vent marked by agglutinate cone; lobes of lava flowed northeast to Butte Lake and south to 

Horseshoe Lake. Original block surface of flow has been removed by glaciation, exposing 

the massively jointed interior. Phenocryst assemblage is in weak to strong disequilibrium; 

pl is reacted, hbl and bt are pseudomorphed by pl, pyr, and oxide, and qtz is rimmed by aug. 

Abundance of phenocrysts is variable: a lighter-colored, relatively phenocryst-rich phase has 

weakly reacted felsic phenocrysts, and a darker-colored, relatively phenocryst-poor phase 

has more strongly reacted felsic phenocrysts. Phenocrysts: 5–10% pl, 0.5–5 mm, mostly 1–3 

mm; 1–2% ol, 0.5–2 mm; sparse–2% qtz, 1–2 mm; 1–2% aug, 0.5–3 mm; sparse pseudomorphs of hbl and bt to 1 cm, but generally a few millimeters. Groundmass is light to dark 

gray and aphanitic. Small andesitic inclusions to 10 cm are uncommon, but sporadically 

present, especially in lighter-colored phase. The darker-colored phase is often somewhat oxidized, massively jointed, and breaks into large blocks; lighter-colored phase is fresher, often 

platy jointed, and breaks into blocks with rough, uneven surfaces. Variety of lithologically 
24

distinct, fresh and oxidized, vesicular and dense blocks of lava and scoria form the agglutinate cone. Both phases of the lava flow, as well as composite blocks, are present. Lower part 

of north rim of cone is composed of a variant of the lighter phase that is porphyritic hbl-bt 

dacite (65% SiO2); it is glassy and contains 20% phenocrysts, andesitic inclusions to 30 cm, 

and abundant small fragments of andesitic inclusions. Cone is capped by sparsely porphyritic, more mafic lava (56.7% SiO2

) with aphanitic groundmass and small aug, reacted ol, hbl, 

and pl, and pyr-rimmed qtz phenocrysts. Composition is similar to the andesitic inclusions 

and probably represents hybridized inclusion-forming magma. Overlies rhyolites of Dittmar 

Volcanic Center, Stage 3 (rd3), andesite of section 22 (a22), and basaltic andesites and basalt of Snag Lake (msn), and underlies basaltic andesite of Fairfield Peak (mfp). K-Ar age is 

93±13 ka. [MH, PP]

Twin Lakes sequence, older

acl Andesite and basaltic andesite of Cluster Lakes (middle Pleistocene)—Two lava flows of 

porphyritic ol andesite and ol-aug andesite (56.6–57.9% SiO2) erupted from vent marked by 

agglutinate cone. Original block surfaces have been removed by glaciation. Two mapped 

lava flows, one near vent and another in Box Canyon, are slightly different. Phenocryst assemblage is in strong disequilibrium; pl is resorbed and thickly rimmed, and qtz is rimmed 

by aug. Phenocrysts (flow near vent, 56.6% SiO2): 8% pl, 0.5–6 mm, mostly 1–3 mm; 4–5% 

ol, 0.5–2 mm, mostly 0.5–1 mm; 1% qtz, 1–3 mm. Phenocrysts (flow in Box Canyon, 57.9% 

SiO2): 6% pl, 0.5–5 mm, mostly 0.5–2 mm; 1% ol, 0.5–2 mm; 1–2% aug, 0.5 mm; sparse 

qtz, 1–2 mm. Pseudomorphs of hbl to 1 cm, but generally a few millimeters, are trace component of both flows. Groundmass of both flows is light gray and aphanitic; dominant joint 

pattern is slabby. Overlies basalt of Badger Flat (bbf) and andesites of West Prospect Peak 

(awp) and underlies basaltic andesite of Fairfield Peak (mfp). Age unknown, but underlies 

and must be older than andesite and basaltic andesite of Prospect Peak (ap, 247±56 ka) and 

is estimated to be 250–300 ka. [PP, WP]

arp Andesite of Raker Peak (middle Pleistocene)—Thick hyp-ol-aug andesite (57.4–58.7% SiO2) 

lava flows forming a lava cone with agglutinated scoria cone at vent. Nearly entire unit, 

except northern distal portions of lava flows and summit area, has been glaciated. Unit has 

a disequilibrium phenocryst assemblage and is lithologically heterogeneous. Most common rock type consists of thick lava flows (50–100 m) of porphyritic hyp-ol-aug andesite. 

Phenocrysts: 10–15% pl, 1–6 mm, usually reacted; 4–6% aug, 0.5–3 mm; 2–4% ol, 1–5 mm; 

1–2% hyp, 0.5–1 mm; 1% hbl 0.5–3 mm; sparse qtz to 2 mm generally with pyr rim. Aug 

and hyp also occur with pl in 5-mm and smaller glomeroporphyritic clots. Hbl phenocrysts 

are replaced by dark-colored aggregates of pl, pyr, and Fe-Ti oxide; pl is strongly reacted. 

Groundmass is light gray and microvesicular when aphanitic and dark gray to black when 

glassy. Typical exposures of flow surfaces are rounded boulders or angular joint-bounded 

blocks, whereas flow interiors are dense with massive to platy jointing. A few earlier flows 

at base of volcano are much less porphyritic than stratigraphically higher flows and contain 

same phenocryst assemblage as later flows but only 2–3% total phenocrysts. Groundmass 

is medium gray, dense, and aphanitic. Mafic inclusions are absent in both flow types, but 

xenoliths of volcanic and metamorphic crustal rocks are found in several places. Unit is 

much younger than underlying rhyolite of Raker Peak (rr). A sample of earlier flows yielded 

a K-Ar age of 270±18 ka. [MZ, WP]

av Andesite of Viola (middle Pleistocene)—Thick block-lava flow of sparsely porphyritic aug-ol 

andesite (57.3–59.0% SiO2). Vent is marked by eroded agglutinate cone at hill 6924 ~1.5 km 

southeast of Deep Hole. Lava flowed northwest ~7 km from vent. Original, poorly preserved 

flow surface is marked by subrounded vesicular boulders. Flow cores, where exposed, 

are slabby to platy jointed. Good exposures are present along Calif. Hwy 44 in Manzanita 

Creek. Lava flow has been glaciated in vent area and along its south margin. It has a variable 

composition and distinctly disequilibrium phenocryst assemblage consisting of ol, aug, hbl, 

pl, and qtz. Phenocrysts: 4–8% pl, generally 2–5 mm, many strongly reacted; trace–2% aug, 

0.25–1 mm; 1% hbl (+bt?), 3–5 mm, completely pseudomorphed by fine-grained aggregate 

of pl, pyr, and Fe-Ti oxide; trace ol, 0.1–0.35 mm, usually corroded and partially converted 

to iddingsite; trace qtz to a few millimeters usually has pyr rims. Proximal portions of flow 

contain higher abundances of phenocrysts, whereas distal portions have lower abundances. 
25

Groundmass is light to medium gray and aphanitic. Flow is covered with thick orange soil. 

Overlies andesites of Diller sequence, undivided (ad), andesite of Digger Creek (adc), 

andesite of Manzanita Creek (amz), and rhyodacite of Manzanita Chute (rmz), and underlies basaltic andesite and andesite of Red Lake Mountain (mrd) and lithic tephra from Deep 

Hole (pdh). K-Ar age is 313±8 ka. [LP, MZ, VI]

Eagle Peak sequence

Rhyodacite of Chaos Crags (Holocene)—Chaos Crags is composed of porphyritic hbl-bt dacite 

and rhyodacite (67–70% SiO2) erupted as two tuff cones, three pumiceous pyroclastic-flow 

deposits, a fall deposit, six lava domes, and two lithic pyroclastic-flow deposits. Lava domes 

(rca, rcb, rcc, rcd, rce, rcf) and lithic pyroclastic-flow deposits (pcd, pce) are mapped 

individually, whereas the other pyroclastic deposits are mapped as a single unit (pc). Talus 

emplaced hot from growing domes is also mapped (cc). The eruption initiating the sequence was explosive and emplaced a thin fall deposit (mostly buried by later deposits), 

two pyroclastic-flow deposits, and a 40-m-high tuff cone at the south end of Chaos Crags. 

Effusive eruption of lava filled the tuff cone and built a small lava dome A (rca). Renewed 

explosive activity blasted away most of dome A, emplaced a third pyroclastic flow deposit, 

and built a second tuff cone 20 m high adjacent to the first cone. Pumice from the eruptive 

cloud formed a widespread fall deposit. Effusive activity constructed five additional domes 

(rcb, rcc, rcd, rce, rcf), emplaced sequentially B through F, two of which had hot collapses 

that resulted in emplacement of lithic pyroclastic-flow deposits (pcd, pce). 

Rhyodacite of Chaos Crags has a slightly variable lithology. Phenocrysts: 25% pl, 2–5 

mm (composite crystals as much as 1 cm); 4% bt, 1–3 mm (rarely larger); 4% hbl, 2–5 mm, 

occasionally as much as 1.5 cm; 2% qtz, 2–5 mm. Qtz is conspicuous and more abundant 

than in all Lassen Volcanic Center units except dacite of Lassen Peak (dl). Rhyodacite ranges from white or light gray and glassy to light to medium gray and devitrified. Abundance 

of reacted phenocrysts, millimeter- to centimeter-sized fragments of mafic inclusions, and 

larger mafic inclusions increases systematically from the early pumiceous pyroclastic-flow 

and fall deposits through domes A–F. Pyr-rimmed qtz phenocrysts are particularly conspicuous in younger domes. Ol and calcic pl xenocrysts from disaggregation of quenched 

inclusions are sparse. Mafic inclusions are texturally variable; both porphyritic and sparsely 

phyric varieties are present. Sparsely phyric inclusions generally contain 1–2 mm ol and calcic pl phenocrysts from mafic parent. Inclusions with hbl+pl groundmasses are most common, but inclusions with aug+hyp+hbl+pl groundmasses are also present. Mafic inclusions 

sparse in the lower and middle pyroclastic flows of unit pc; slightly more abundant in the 

upper pyroclastic flow and domes A (rca) and B (rcb); abundant in domes C (rcc), D (rcd), 

E (rce), and F (rcf), forming as much as 20% of the rock. Inclusions generally small, rarely 

>10–20 cm in the pyroclastic flows (pc) and domes A and B; often as large as 20–50 cm and 

occasionally as much as 1 m in domes C, D, E, and F. Pumice in pyroclastic flows of unit pc

and domes A and B contains 69.5 to 70% SiO2

. Domes C through F are more variable and 

slightly less felsic (67–69% SiO2). Domes A and B have weakly pumiceous outer carapaces; 

domes C, D, E, and F are composed of denser rock. Chaos Crags domes have the same 

paleomagnetic direction as pyroclastic deposits (pc, pcd, pce) and, thus, are essentially the 

same age, 1,103±13 yr B.P. (Clynne and others, 2002). [MZ, OS, VI, WP]

cc Talus, emplaced hot from domes B–F—Laterally sorted deposits of nonbedded blocks from 

Chaos Crags domes B–F, (rcb, rcc, rcd, rce, and rcf). Largest blocks, ~4 m across (rarely as 

large as 10 m), are prismatically jointed and commonly disintegrated in place after deposition. Formed by hot rockfalls from domes during emplacement. Locally includes some talus 

formed by normal mass-wasting processes. [MZ]

rcf Dome F—Lava dome of porphyritic hbl-bt rhyodacite (67–68% SiO2). Dome F erupted into 

the scar created by partial collapse of dome E (rce) and covered its vent. [MZ]

pce Lithic pyroclastic-flow deposit from partial collapse of dome E—Nonsorted, unconsolidated, 

nonbedded deposit of fine rubble to blocks commonly as large as ~2 m across, a few as large 

as 10 m. Deposit on east side of Chaos Crags consists entirely of fragments from dome E 

(rce). Deposit was emplaced hot; many large blocks are prismatically jointed or internally 

fractured. Formed by partial collapse of rce before emplacement of dome F (rcf) in resulting 

avalanche scar. Radiocarbon ages are 230±40 and 730±40 yr B.P. (Clynne and others, 2002), 
26

but paleomagnetic direction is similar to other Chaos Crags deposits, and pce is probably the 

same age (see discussion of pumiceous pyroclastic-flow and fall deposits, unit pc). [MZ, WP]

rce Dome E—Lava dome of porphyritic hbl-bt rhyodacite (67–68% SiO2). Dome E erupted from 

a vent that it now covers. Partial collapse of dome E produced a hot lithic pyroclastic flow 

(pce). [MZ]

pcd Lithic pyroclastic-flow deposit from partial collapse of dome D—Nonsorted, unconsolidated, nonbedded deposit of fine rubble to blocks as large as about 2 m, a few to 10 m. Deposit 

on west side of Chaos Crags consists of material derived from dome B (rcb) and dome D 

(rcd). Deposit was emplaced hot; many large blocks are prismatically jointed. Formed by 

partial collapse of dome D (rcd). Resultant avalanche swept across surface and incorporated 

pumiceous blocks from underlying dome B (rcb). Blocks derived from rcb were either still 

hot from emplacement of the dome or were reheated to above the Curie Point in the hot 

deposit. Radiocarbon date of 430±30 yr B.P. was obtained from deposit (Clynne and others, 

2002), but paleomagnetic direction is similar to other Chaos Crags deposits, and pcd is 

probably same age (see unit pc). [MZ]

rcd Dome D—Lava dome of porphyritic hbl-bt rhyodacite (69% SiO2). Dome D of the Chaos 

Crags erupted from a vent now covered by the dome and has the highest elevation of the six 

lava domes. Partial collapse of dome D produced a hot lithic pyroclastic flow (pcd). [MZ]

rcc Dome C—Lava dome of porphyritic hbl-bt dacite (68% SiO2). Dome C of the Chaos Crags 

erupted from a vent now covered by the dome. A septum of Brokeoff Volcano andesites 

(andesites of Diller sequence, undivided, ad) several tens of meters wide is present along 

southwest boundary with dome B lava; Brokeoff Volcano rocks at summit of dome C indicate dome grew as a solid plug. Small area of partially welded hot talus just east of saddle 

between domes D and E is inferred to be related to dome C (Christiansen and others, 2002). 

Nearly vertical, northwest-facing cliff of dome C breakaway scar of Chaos Jumbles avalanche deposits (Qsj) exposes strongly oxidized pink dacite of the dome interior. [MZ]

rcb Dome B—Lava dome of porphyritic hbl-bt rhyodacite (69.5–70% SiO2). Dome B erupted from 

and partially filled and buried the tuff cone of upper pyroclastic flow of unit pc. This relatively flat-topped dome flowed sluggishly north. In contrast to other domes, lava of dome B 

often shows crude or vague flow layering and, locally, thin sheets of smeared mafic inclusions. [MZ]

pc Pumiceous pyroclastic-flow and fall deposits—Deposits of three pumiceous pyroclastic 

flows, a coignimbrite fall deposit, and two tuff cones are combined on the map. Fall deposits 

at the surface are generally well-sorted unconsolidated beds of nearly white rhyodacite pumice ranging in size from blocks as large as ~1 m across near the source vent to lapilli and ash 

farther away. The deposit mantles the ground surface and is locally reworked as colluvium. 

Thickness ranges from <1 m distally, through several meters near Chaos Crags, to as much 

as 35 m in the vent cones at south margin of Chaos Crags dome cluster. Mostly postdates 

emplacement of dome A (rca) but predates dome B (rcb). Fall deposits underlie and are 

intercalated with nonsorted, unconsolidated, pumiceous pyroclastic flows. Three distinct 

pyroclastic-flow units are recognized: lower and middle flow units are similar and have finecelled pumice blocks as much as ~30 cm across in a pumiceous ash matrix, each generally 

gray to pale yellow in color but with a pink top ~1–2 m thick. A pink, laminated, fine-ash 

cloud deposit ~5–20 cm thick locally overlies the middle flow unit. Lower and middle 

pyroclastic flows cooled as a single unit and predate rca; charcoal samples yielded weighted 

average radiocarbon age (n=7) of 1,118±17 yr B.P. (Clynne and others, 2002). Upper 

pyroclastic-flow unit is characterized by coarse-celled pumice in breadcrust blocks as large 

as 2 m across (rarely to 4 m); glassy ash matrix is white to gray. This flow unit postdates 

emplacement of rca but predates rcb; incorporated charcoal samples yielded a weighted 

average (n=6) radiocarbon age of 1,083±19 yr B.P. (Clynne and others, 2002). Eruption of 

upper pyroclastic flow partially destroyed dome A, and blocks of that dome are common in 

proximal exposures of the pyroclastic flow. Two radiocarbon ages discussed above are not 

statistically distinguishable; although there may be a short hiatus between eruption of dome 

A and upper pyroclastic flow, a weighted average of all ages, 1,103±13 yr B.P., is used for 

the Chaos Crags eruption. 

Unit pc occurs widely in area around Chaos Crags. Exposures of lower and middle pyroclastic flows are confined to vicinity of Lost and Manzanita Creeks within a few kilometers 
27

of Chaos Crags. Upper pyroclastic flow traveled down Lost Creek nearly to Twin Bridges, 

a distance of ~20 km. In Manzanita Creek, upper pyroclastic flow transformed into a lahar 

by incorporation of water and flowed nearly to McCumber Reservoir; the transition from 

pyroclastic flow to lahar is gradual, and a boundary was not mapped, but occurs in area of 

Deer Flat. Scattered pumice lapilli, mostly coignimbrite debris associated with eruption 

column that produced upper pyroclastic flow, can be found scattered over most of Lassen 

Volcanic National Park and east to Bogard Buttes area, but unit pc is mapped only where it 

masks underlying units. Lithology and composition of pc is identical to rca and rcb. [MZ, 

OS, VI, WP]

rca Dome A—Lava dome of porphyritic hbl-bt rhyodacite (69.5–70% SiO2). Dome A erupted into 

and partially filled tuff cone from which the lower and middle pyroclastic flows (pumiceous 

pyroclastic-flow and fall deposits, unit pc) erupted. Dome A was mostly destroyed by the 

eruption of upper pyroclastic flow (pc), and only a remnant of the original dome is preserved. Fragments of dome A are abundant in the upper pyroclastic flow within ~1–2 km of 

vent. [MZ]

Dacite of Lassen Peak (late Pleistocene)—Dome and pyroclastic flow of porphyritic hbl-bt 

dacite and rhyodacite (63.5–70.2% SiO2). Phenocrysts: 20% pl, 1–5 mm with some composite crystals to 1 cm; 4% qtz (more abundant than in other dacites in map area), typically 1–2 

mm, rarely to 5 mm; 3% bt, 1–2 mm; 2% hbl, 1–5 mm, rarely to 15 mm; sparse aug, typically 0.25–1 mm, composite crystals to 3 mm; trace ol, generally 1 mm. Ol is more prominent in the most silicic rock from the northeast ridge and is often obscure or absent elsewhere. Combination of abundant qtz and aug phenocrysts is diagnostic of dacite of Lassen 

Peak. Partially reacted phenocrysts of pl, hbl, bt, and qtz are common. Microphenocrysts 

of pl, aug, hyp, and hbl (disaggregated from inclusions) and tiny microlites are abundant in 

light- to medium-gray, glassy to aphanitic groundmass. Fragments of mafic inclusions from 

a few millimeters to a few centimeters are abundant. Mafic inclusions are particularly large 

and abundant, as large as 1.5 m; inclusions in 20–50 cm range are more abundant than in 

any other dacite in the map area except dacite of Mount Helen (dh). Mafic inclusions have 

a wide range of textures from fine grained and porphyritic to coarse grained and aphyric; 

multiple-generation inclusions are sparse. Porphyritic mafic inclusions contain resorbed 

phenocrysts from host dacite; some also contain aug and ol phenocrysts to a few millimeters. 

Fine-grained inclusions typically have hbl-pl groundmass but may have pyr-hbl-pl groundmass. Most aphyric inclusions contain remnants of aug and ol phenocrysts in hbl-pl groundmass. 40Ar/39Ar age of pfl is 28.3±2.7 ka; paleomagnetic correlation suggests 27±1 ka (Turrin 

and others, 1998). [LP, MZ, RP, WP]

pfl Lithic pyroclastic-flow deposit from partial collapse of dome—Poorly sorted, unconsolidated, nonbedded volcanic ash to blocks as large as 3 m across, consisting of dense, porphyritic, 

gray, hbl-bt dacite (68.0% SiO2) lithologically identical to the dacite dome of Lassen Peak 

(dl); blocks commonly prismatically jointed. Matrix-supported, block-rich deposit with matrix of comminuted dacite. Forms sheet at northeast base of Lassen Peak, incised by upper 

Lost Creek. Also exposed in the north-facing toe of Survivors Hill and present in subsurface 

at Emigrant Pass (north of the Devastated Area parking lot). Thickness ranges from feather 

edge to as much as 50 m. Formed by hot, dome-collapse, pyroclastic flows during emplacement of Lassen Peak dacite dome. [WP]

dl Dome—Large (2 km3) volcanic dome of porphyritic hbl-bt dacite that covers its vent. Where 

exposed, the dome interior is platy to massively jointed. Single magnetic direction (Turrin 

and others, 1998) and lack of internal contacts suggest a single emplacement unit. Outer 

part of dome is commonly oxidized to grayish-pink. Parts of dome are covered by breccia 

carapace; few areas of primary hot talus are preserved, recognized by presence of prismatically jointed blocks of dacite as much as 5 m across. Large spines are prominent on 

south flank of dome, and a prominent cirque formed on northeast side during glaciation of 

Lassen Peak (Christiansen and others, 2002). Dome composition ranges considerably from 

~70% SiO2

 in northeast-projecting ridge to ~63.5% SiO2 in spine low on northwest side 

above upper Manzanita Creek, but bulk of dome is 66–68% SiO2. Despite chemical heterogeneity, petrography of dome is nearly uniform. Chemical range is attributed to varying 

amount of disaggregated andesitic inclusions not readily apparent in hand specimen. [LP, 

MZ, RP]
28

Rhyodacite of Kings Creek (late Pleistocene)—A sequence of at least four pumiceous pyroclastic flows, tuff cone, and thick lava flows of porphyritic hbl-bt rhyodacite (69.1–70.3% 

SiO2) erupted from vent now partially buried by Lassen Peak. Phenocrysts: 15% pl, 1–5 

mm (largest are composite grains); 2% bt, 1–2 mm; 1% hbl, 1–15 mm; sparse qtz, 1–2 mm; 

sparse hyp, 0.5 mm. The sparse 1–1.5 cm hbl phenocrysts are distinctive of this rock unit. 

Resorbed equivalents of dacitic phenocrysts, broken fragments of inclusions, and crystals 

derived from disaggregation of mafic inclusions are sparse. Mafic inclusions are rare and 

typically smaller than ~10 cm, although a few are as large as 20 cm. Inclusions have coarsegrained hbl-pl groundmass and contain reacted phenocrysts derived from host magma and 

small ol phenocrysts derived from mafic parent. Age of pk is 35±1 ka (paleomagnetically 

calibrated radiocarbon age, Clynne and others, 2002; Turrin and others, 1998) and coeval 

with the lava flows (rk). [LP, RP, WP]

rk Flows—Thick lava flows of porphyritic hbl-bt rhyodacite (69.1–70.3% SiO2). Small, poorly 

preserved tuff cone (pk) at east end of Lassen Peak parking lot marks vent. Lava flowed 

eastward toward Reading Peak, where a ridge divided it into two lobes. One lobe flowed 

southeast down Kings Creek drainage for 2 km to Kings Creek Meadows, and the second 

lobe flowed northeast down headwaters of Hat Creek for ~3 km. Lobe in Kings Creek is 

approximately 75 m thick with steep lateral margins; lobe in Hat Creek is somewhat thinner. 

Although glaciated, flow morphology is well preserved. Basal and lateral margins are glassy, 

typically perlitic, and locally brecciated; top of flow is variably pumiceous. Flow banding, 

spherulitic zones, and lithophysal zones are common. Devitrified interior of flow is rarely 

exposed. Lava flow overlies the pyroclastic flow deposits (pk) and is in turn overlain by 

dacite of Lassen Peak (dl). [LP, RP]

pk Pumiceous pyroclastic-flow deposits—Poorly sorted, unconsolidated volcanic ash, with 

pumice blocks to ~1 m across. Pumice is light gray to white, glassy, coarsely pumiceous, and 

weathers to pale yellow. Pumice consists of porphyritic bt-hbl rhyodacite (69.1-70.6% SiO2) 

lithologically identical to rk. Tuff cone just east of Lassen Peak parking lot marks the vent. 

Pumiceous pyroclastic flows inundated the Kings Creek and West Fork Hat Creek valleys. 

Flows immediately preceded the emplacement of lava flows (rk). Four separate pyroclastic 

flows or flow units emplaced in rapid succession are exposed in banks of Hat Creek just 

southeast of Emigrant Pass; total thickness ~10 m. Similar deposits are exposed in West Fork 

Hat Creek and Kings Creek Meadows and may have been emplaced by separate flows. Original extent of pk was markedly reduced by glacial erosion. Age of pk is 35±1 ka. [LP, RP, WP]

Rhyodacite of Sunflower Flat (late Pleistocene)—Pumiceous pyroclastic-flow deposits 

and domes of porphyritic bt-hbl rhyodacite (68.3–70.0% SiO2) north of Chaos Crags. 

Phenocrysts: 12% pl, 1–5 mm, a few to 8–10 cm are composite crystals; 2% hbl, 1–12 mm 

(large phenocrysts sparse but conspicuous); sparse bt, 1–2 mm; sparse qtz, 1–2 mm. Microphenocrysts of hyp and aug (hyp>aug) are abundant but generally <0.5 mm, although a few 

as large as ~1 mm are present. Reacted pl phenocrysts and pl phenocrysts with vermicular 

glass inclusions are abundant and distinctive. Groundmass is dark gray to black when dense 

and glassy to light gray or pink when microvesicular or partially devitrified. Broken fragments of mafic inclusions from a few millimeters to a few centimeters in size are common, 

but larger inclusions (10 cm max) are rare. Age is 41±1 ka (calibrated radiocarbon age from 

psf, Turrin and others, 1998). [MZ, WP]

rsf Domes—Overlapping complex of eight domes of porphyritic bt-hbl rhyodacite (68.3–70.0% 

SiO2) that erupted in a linear trend north of Chaos Crags. Domes overlie the associated 

pyroclastic-flow deposits mapped as psf. On north flank, overlain by lateral moraines of till 

of Anklin Meadows (Qta) but not glaciated; domes are heavily forested and retain taluscovered, primary morphology. Interiors of domes are not exposed. [MZ, WP]

psf Pumiceous pyroclastic-flow deposits—Poorly to moderately sorted, unconsolidated, volcanic 

ash with pumice blocks as large as ~50 cm. Occurs as pyroclastic flows from Sunflower Flat 

to Lost Creek and as proximal fall deposits (not mapped separately) forming a pumice crater 

southeast of Sunflower Flat. Pumice is porphyritic, bt-hbl rhyodacite (69.0–69.3% SiO2) 

lithologically identical to rsf. Pumice blocks with coarse cell structure; lithic blocks of dense 

dome or conduit rock sparse. Fresh pumice is white, and matrix weathers to pale yellow; 

blocks often oxidized to pink and yellow. Calibrated radiocarbon age is 41±1 ka (Turrin and 

others, 1998). [MZ]
29

rkr Rhyodacite of Krummholz (late Pleistocene)—Thick flow of porphyritic bt-hbl rhyodacite 

(68.3–69.3% SiO2); small dome marks the vent. Dome and flow are glaciated and mostly obscured by thick blanket of pumice from Chaos Crags eruption (pc). Outer parts of flow are 

characteristically dense and glassy, although devitrified rock is present where flow surface 

was removed by glaciation. Dome rock is massively jointed and weathers into large rectangular blocks. Lava flow locally displays vague flow banding and abundant thin platy jointing. Phenocrysts: 10% pl, mostly 2–3 mm (composite crystals as large as 1 cm common); 

1% hbl, generally 1–2 mm (sparse hbl to ~1 cm); sparse bt, 1–2 mm; sparse qtz, to 2 mm; 

sparse hyp, to 0.5 mm. Reacted pl phenocrysts and pl phenocrysts with vermicular glass 

inclusions are abundant. Groundmass is light to dark gray and glassy, although perlitic and 

spherulitic varieties occur. Distal and deeply eroded parts of flow have an aphanitic groundmass. Mafic inclusions are sparse and small, generally <10 cm in size. Inclusions contain 

reacted felsic phenocrysts derived from host, sparse small ol phenocrysts, and sparse aug 

phenocrysts. Groundmasses of mafic inclusions contain acicular microphenocrysts of pyr, 

hbl, and pl, with glass. 40Ar/39Ar age of 43±2 ka is consistent with its unusual transitional 

remanent magnetic direction (Laschamp Geomagnetic Excursion, D.E. Champion, written 

commun., 1998). [MZ, WP]

r27 Rhyodacite of section 27 (late Pleistocene)—Thick lava flows of porphyritic bt-hbl rhyodacite (69.7–69.8% SiO2

) on the north flank of Lassen Peak. Vent was probably in area now 

occupied by Lassen Peak. Flows are slightly to moderately glaciated, and original flow 

morphology and pumiceous carapace are preserved locally. Flows are spherulitic, flow 

banded, and brecciated at the base. Several flow lobes fill canyons on tributaries of upper 

Manzanita Creek south of hill 8283. Phenocrysts: 12% pl, 1–3 mm (composite crystals to 6 

mm common); 3% hbl, 1–3 mm (rarely larger); 1% hyp, to 1 mm; sparse bt, generally 1–2 

mm; trace qtz, 0.5–2 mm. Aug xenocrysts are rare. Rock is dark gray to black with perlitic 

groundmass where dense and white to colorless where vesicular or pumiceous. Broken fragments of mafic inclusions a few millimeters to ~1 cm in size are common. Mafic inclusions 

are generally small and sparse, ranging to 10–12 cm in size, typically containing aug and 

calcic pl phenocrysts in addition to phenocrysts derived from host rhyodacite. Age is not 

closely constrained. Unit overlies but is much younger than dacite of Crescent Crater (dc, 

236±1 ka) and dacite of hill 8283 (d82, 261±5 ka); r27 is overlain by and slightly older than 

rhyodacite of Krummholz (rkr, 43±2 ka). Age estimated to be ~50-60 ka. [MZ, WP]

Rhyodacite of Eagle Peak (late Pleistocene)—Pumiceous pyroclastic-flow deposits and 

thick lava flow and summit dome of porphyritic hbl-bt rhyodacite (71.0–71.6% SiO2). 

Phenocrysts: 12% pl, 1–3 mm (few to 6 mm); 1–2% bt, 0.5–2 mm; 1–2% hbl, generally 

1–4 mm (rarely to 8 mm); 1% qtz, to 4 mm. Reacted pl, hbl, and bt phenocrysts are sparse. 

Groundmass is light gray to white and typically glassy or perlitic with sparse to abundant 

spherulites. Mafic inclusions are abundant and have a wide variety of textures, although 

majority are fine to coarse grained and nearly aphyric. Most are 10–15 cm in size, but range 

to ~50 cm. Finer-grained inclusions often contain small 0.5–1 mm phenocrysts of calcic 

pl and phenocrysts or remnants of resorbed phenocrysts of ol and aug, as well as reacted 

host rhyodacite phenocrysts. The inclusions generally have hbl+pl groundmasses, although 

inclusions with aug+hyp+hbl+pl groundmasses are also present. The unit overlies but is 

much younger than dacite of Ski Heil Peak (ds, 244±10 ka) and dacite of Vulcans Castle 

(dv). 40Ar/39Ar age of 66±4 ka indicates that re is probably the oldest unit in Eagle Peak 

sequence. [GP, LP, MZ]

re Dome and flow—Thick lava flow with summit dome of porphyritic hbl-bt rhyodacite (71.0–

71.6% SiO2) that overlies its vent. Lava flow exposed at Crescent Cliff is ~100 m thick with 

steep lateral margins. Although glaciated, flow morphology is well preserved. Basal and 

lateral margins are glassy and locally brecciated; top of the flow is weakly pumiceous. Flow 

banding, spherulitic zones, and lithophysal zones are common. The devitrified interior of 

dome and flow not exposed. 40Ar/39Ar age is 66±4 ka. [LP]

pe Pumiceous pyroclastic-flow deposits—White, poorly sorted, matrix-supported, unconsolidated ash, lapilli, and pumice blocks. Erupted from a vent now covered by dome and lava 

flow (re) and preserved only beyond limits of late Pleistocene glaciation. Most extensive 

area of outcrop is west of Manzanita Lake, where deposit occurs as a single ~2-m-thick bed. 

A several-meter-thick, similar deposit, including dense fragments of re, occurs on summit 
30

of Ski Heil Peak. Several small areas of pumiceous pyroclastic-flow deposit correlated with 

pe are found in Digger Creek drainage in the area east of Camp Forward. Pumice blocks as 

large as several meters in size are sparse but widespread. Contains loose mafic inclusions 

as lithic fragments. Pumice is porphyritic bt-hbl rhyodacite (70.9% SiO2) similar to re. The 

pyroclastic flows immediately preceded eruption of and are same age as the associated dome 

and flow (re). [GP, LP, MZ]

Bumpass sequence

rd Rhyodacite of Dersch Meadows (middle Pleistocene)—Thick lava flow of porphyritic bthyp-hbl rhyodacite (69.4% SiO2

). Vent location is unknown but probably east of Paradise 

Meadows; lava flowed north to Dersch Meadows. Five discrete outcrop areas are separated 

by extensive till cover. Phenocrysts: 15% pl, 1–3 mm, few to 7 mm; 2% hbl, 1–5 mm, rarely 

to 8 mm; 1% hyp, 0.25–1 mm; sparse bt, to 1 mm; trace qtz, 1 mm. Reacted phenocrysts and 

fragments of mafic inclusions are uncommon. Mafic inclusions are sparse, rarely exceed 5 

cm, and contain sparse, bright-green aug phenocrysts. Pumiceous upper surface of flow has 

been removed by glacial erosion, and massively jointed core of flow is exposed. Interior 

of lava flow is light gray, locally pink when oxidized, strongly flow banded, and usually 

devitrified. Base of flow is well exposed just east of Hat Lake and is dark gray to black, 

dense and glassy, often perlitic and spherulitic, and brecciated. Overlies andesites of Diller 

sequence, undivided (ad) in Hat Creek. 40Ar/39Ar age of 193±11 ka indicates that rd was the 

last unit of Bumpass sequence to be emplaced. [RP, WP]

dr Dacite of Reading Peak (middle Pleistocene)—Complex of porphyritic bt-hbl dacite (64.9–

68.9% SiO2) lava domes that have been extensively glaciated; massive-jointed, devitrified, 

and locally oxidized interiors are exposed. Includes protalus ramparts and small moraines 

on north-facing upper slopes of domes. Includes small area of dacite on east flank of 

Lassen Peak that is of generally similar lithology. Several dacite domes are present in the 

complex, but contacts between them could not be located. When emplaced, dome complex 

rivaled Lassen Peak in volume. Lithology and composition of dacite is slightly variable. 

Phenocrysts: 10–12% pl, 1–5 mm, few composite crystals to 1 cm; 2% hbl, mostly 1–3 mm, 

few to 5 mm; sparse–1% bt, to 1 mm; rare–sparse qtz, to 2 mm; rare aug, to 1 mm in small 

clusters. Hyp and aug microphenocrysts are common but rarely exceed 0.5 mm in size and 

are sometimes found as small glomeroporphyritic clots. Most pl phenocrysts are weakly to 

strongly reacted, and almost all hbl and bt has been pseudomorphed by aggregates of Fe-Ti 

oxide, pl, and pyr. Fragments of mafic inclusions are abundant. Groundmass is light to 

medium gray, dense and aphanitic. Mafic inclusions are abundant and display a wide variety 

of textures from porphyritic to coarse-grained aphyric; size and character vary. Most inclusions are <20 cm, but inclusions to 50–60 cm are common. Sparse phenocrysts of calcic pl, 

aug, and ol, as well as reacted host dacite phenocrysts, are present in some inclusions. Mafic 

inclusions have hbl-pyr-pl groundmasses. A sample from the National Park Service quarry at 

east end of the easternmost dome yielded a K-Ar age of 212±5 ka. [RP]

db Dacite of Bumpass Mountain (middle Pleistocene)—Dome and lava flow of porphyritic 

hbl-aug dacite (64.1–64.4% SiO2). A thick lava flow (>100 m) emerged from beneath the 

dome at Bumpass Mountain and flowed southeast for several kilometers, covering ~12 km2. 

Smaller flow extended northwest from Bumpass Mountain. Extensively glaciated, original 

glassy carapace of dome was completely stripped, and only massively jointed, devitrified, 

and locally oxidized interior of dome is exposed. Phenocrysts: 5% pl generally 1–5 mm, 

composite crystals to 8 mm; 1% aug, 0.5–5 mm, often in small glomeroporphyritic clots; 1% 

hbl, 1–3 mm, few to 5 mm; sparse bt, generally 1–2 mm; sparse hyp, 0.5–1 mm; rare qtz, 

generally 1–2 mm. Hbl, bt, and some pl phenocrysts are strongly resorbed and often difficult 

to see. Some qtz phenocrysts have rims of fine-grained aug. Groundmass is light to medium 

gray and aphanitic. Fragments of broken inclusions, 1–2 cm across, are common, but 

larger mafic inclusions are notably sparse and generally <20 cm. Inclusions have a pyr+pl 

groundmass and contain resorbed host-lava phenocrysts and sparse 0.25–1 mm ol and aug 

phenocrysts. Joint pattern is thickly platy to massive; columnar jointing is locally present in 

dome. Thick zones of thinly platy-jointed rock are exposed in glaciated margins of lava flow. 

Unit is younger than all Brokeoff Volcano rocks, dacite of Ski Heil Peak (ds), and dacite of 

Mount Helen (dh) and older than dacite of Reading Peak (dr). K-Ar age is 232±8 ka. 
31

Remnants of a fragmental deposit related to dacite of Bumpass Mountain are preserved in 

area south and west of Bumpass Mountain. Deposit is nonsorted, nonbedded, and composed 

of subrounded blocks of dense dacite of Bumpass Mountain in moderately consolidated 

matrix of comminuted dacite and ash. Matrix supported; blocks generally 10–50 cm in size, 

but some larger. Deposit is several tens of meters thick on south side of Bumpass Hell and 

thins rapidly away from Bumpass Mountain. Original extent of breccia was greatly reduced 

by glacial erosion. Deposit probably represents phreatic explosive disruption of small growing dome by magmatic gas or heated groundwater. The Bumpass Hell thermal area occupies 

the area of the Bumpass Mountain vent. Lithology of dacite blocks is identical to db, and 

eruption of the explosion breccia probably occurred early in the emplacement of db. In area 

of most intense hydrothermal activity at Bumpass Hell thermal area, explosion breccia is 

almost completely altered. [LP, RP]

dc Dacite of Crescent Crater (middle Pleistocene)—Lithologically and compositionally zoned 

unit composed of porphyritic hyp-hbl dacite dome (65.6% SiO2) and thick porphyritic 

bt-hyp-hbl rhyodacite lava flow (68.9% SiO2). Dome covers the vent. Depression at the 

summit is a cirque that exposes devitrified interior of dome. Lava flow at north base of dome 

probably predates it. Layer of ol-aug basaltic andesite scoria and ash that overlies dome and 

underlies pumiceous pyroclastic-flow and fall deposits (pc) from Chaos Crags is exposed at 

summit (Christiansen and others, 2002) but is too small to be depicted at the map scale. Origin of scoria and ash deposit is unknown. Phenocrysts: 15% pl, 2 mm to ~1 cm (largest are 

composite crystals); 3% hbl, 1–5 mm; 1% hyp, 0.5–1 mm; sparse bt, 1–2 mm; trace qtz, 1–2 

mm. Hbl and bt are fresh in lava flow but strongly to completely oxidized in dome. Reacted 

pl phenocrysts are more abundant in dome than in lava flow; dome contains abundant aug in 

addition to hyp microphenocrysts. Broken fragments of ol- and aug-bearing mafic inclusions 

are abundant, especially in dome. Groundmass of lava flow is dark gray to black, glassy, and 

typically spherulitic or perlitic. Where fresh, groundmass of dome is medium gray and aphanitic, but in most places it is oxidized to purplish pink. Lava flow locally displays flow banding; both flow and dome exhibit massive to blocky jointing. Abundant mafic inclusions as 

large as 50 cm are porphyritic and contain an assemblage of 1–2 mm ol, aug, hyp, and calcic 

pl, as well as reacted host-lava phenocrysts. Inclusions have hbl+pl or pyr+pl groundmasses. 

Another type of porphyritic inclusion contains 50% millimeter-sized phenocrysts of pl, hyp, 

aug, and hbl in a glassy groundmass. Overlain by dacite of Lassen Peak (dl), rhyodacite of 

section 27 (r27), and rhyodacite of Krummholz (rkr). 40Ar/39Ar age is 236±1 ka. [MZ, WP]

ds Dacite of Ski Heil Peak (middle Pleistocene)—Dome of porphyritic aug-hbl dacite (64.2–66.5% 

SiO2) that covers its vent. Extensively glaciated; only massively jointed, devitrified, and 

commonly oxidized interior of dome is exposed. Summit area of dome is covered by pumiceous pyroclastic-flow deposits (pe) from Eagle Peak. Phenocrysts: 10% pl, generally 1–5 

mm, composite crystals to 1 cm; 1% hbl, 1–3 mm, few to 5 mm; 1% aug, 0.5 to 2 mm, often 

in small glomeroporphyritic clots; 1% bt, generally 1–2 mm; sparse qtz, generally 1–2 mm. 

Hbl, bt, and some pl phenocrysts are strongly reacted. Groundmass is light gray to pinkish, 

dense, and aphanitic. Fragments of 1–2 cm mafic inclusions are common; larger mafic inclusions to 20 cm are sparse. West and south margins of dome are less oxidized and more mafic 

(64.2% SiO2) than interior (66.5% SiO2) and contain conspicuous aug phenocrysts to 5 mm. 

Unit is younger than dacite of Mount Helen (dh), dacite of Lassen Peak parking lot (dpl), 

and dacite of Vulcans Castle (dv) and older than dacite of Bumpass Mountain (db). K-Ar 

age is 244±10 ka. [LP]

dh Dacite of Mount Helen (middle Pleistocene)—Large dacite dome of porphyritic bt-hbl dacite 

(65.5% SiO2) forming Mount Helen. Vent was beneath Mount Helen on steep flank of 

Brokeoff Volcano, and some lava flowed eastward towards Kings Creek Meadows to 

produce short, thick lava flow. Extensively glaciated, original glassy carapace was almost 

completely stripped; massive-jointed, devitrified, and locally oxidized interior is exposed. 

Small area of original glassy carapace is locally preserved beneath rhyodacite of Kings 

Creek (rk) just below summit of Calif. Hwy 89 (near Lassen Peak parking lot). This remnant 

has a different composition (69.3% SiO2) and lithology and grades into dominant lithology, 

but has not been mapped separately. Small area of porphyritic bt-hbl dacite exposed in roadcut on Calif. Hwy 89 under rk is mapped as dh; although lithologically similar, this dacite 

lacks inclusions and may not be related. Phenocrysts (dome interior): 10% pl, generally 1–5 
32

mm, composite crystals to 8 mm; 1–2% hbl, 1–3 mm, few to 5 mm; 1% bt, generally 1–2 

mm; sparse qtz, generally 0.5–2 mm; sparse hyp, 0.5–1 mm. Rock also contains 1 mm aug 

(often in small glomeroporphyritic clots) and 0.5 mm ol crystals derived from disaggregation of mafic inclusions. Hbl, bt, and some pl phenocrysts are strongly reacted, and some qtz 

phenocrysts have rims of aug. Groundmass is light to medium gray, dense, and aphanitic. 

Joint pattern is massive to thick tabular and columnar. Characteristic are abundant sparsely 

porphyritic mafic inclusions (59% SiO2) that locally form as much as 20% of rock. Inclusions are generally to ~20 cm, locally larger, and contain reacted host-lava phenocrysts and 

sparse 0.25–1 mm ol and aug phenocrysts in pyr+pl groundmass. Broken fragments of inclusions, 1–2 cm in size, are also abundant. Phenocrysts (dome margin): 15% pl, generally 1–5 

mm, composite crystals to 1 cm; 3–4% hbl, 1–3 mm, few to 5 mm; 1–2% qtz, 0.5–2 mm; 

1% bt, generally 0.5–1 mm; sparse hyp, 0.5–1 mm. Hbl and bt phenocrysts are generally 

fresh, a few pl phenocrysts are reacted, and qtz phenocrysts lack aug rims. Groundmass is 

medium gray, dense, and glassy and perlitic. Inclusions in dome margin are similar to those 

in dome interior. Fragments of inclusions, 1–2 cm in size, are also abundant. Joint pattern is 

massive. Unit overlies dacite of Lassen Peak parking lot (dpl) and dacite under Mount Helen 

(duh) and underlies dacite of Bumpass Mountain (db). K-Ar age of 249±12 ka is consistent 

with stratigraphy. [LP, RP]

duh Dacite under Mount Helen (middle Pleistocene)—Sparsely porphyritic aug-hbl dacite (64.2% 

SiO2) forming small remnant of dome on north flank of Mount Helen. Dacite of Mount 

Helen (dh) mostly buried duh, and only a small area of extensively glaciated, massively 

jointed, and devitrified interior of dome is exposed. Phenocrysts: 5% pl, generally 0.5–5 

mm, a few larger grains are composite crystals; 1–2% hbl, 1–3 mm, reacted; sparse bt, 

generally 1 mm; sparse qtz, 1 mm. Small ol crystals and clots of ol and pl are conspicuous 

accessories. Groundmass is light to medium gray, aphanitic, and dense. Fragments of inclusions, 1–2 cm, and larger mafic inclusions to ~20 cm are common. Precise age is unknown, 

but unit is older than dh (249±12 ka), probably similar in age to nearby Bumpass sequence 

domes, and is estimated to be 250–260 ka. [RP]

dsc Dacite of Summit Creek (middle Pleistocene)—Thick flow and associated fragmental deposits of porphyritic hyp-hbl dacite (65.0% SiO2

) exposed in Kings Creek drainage. Source 

is unknown but is probably located near Reading Peak, buried beneath glacial deposits 

or younger dacites. Flow was extensively glaciated; its original form is poorly preserved. 

Massively jointed, devitrified, and somewhat oxidized interior of thick flow is exposed. In 

Summit Creek, fragmental deposits of similar lithology that underlie lava flow are poorly 

sorted, vaguely reversely graded, weakly to strongly consolidated, locally vaguely bedded 

deposits of subrounded to angular, dense blocks in a sand- to pebble-sized matrix of comminuted dacite. These deposits are monolithologic and clast supported, with blocks as large 

as 2 m. Largest blocks are slightly pumiceous, some blocks are pink, and some show vague 

prismatic jointing. The fragmental deposit is probably a lithic (dome-collapse) pyroclastic 

flow or hot talus from a nearby dome. Phenocrysts: 8–10% pl, generally 0.5–6 mm, larger 

crystals are composite; 2–4% hbl, 0.5–4 mm, mostly 1–3 mm; sparse–1% hyp, 0.5–2 mm, 

often in small glomeroporphyritic clots. Some pl crystals are reacted in both lava flow and 

fragmental deposits, but hbl is reacted only in lava flow. Lava has a light gray aphanitic 

groundmass; fragmental deposits are light to dark gray and glassy. Small, generally aphyric 

mafic inclusions and fragments of inclusions are abundant, and larger inclusions to 20–30 

cm are sparse. Unit unconformably overlies hydrothermally altered rocks related to the 

Dittmar Volcanic Center (Tad) and dacite of Bench Lake (dbl, 679±14 ka). Precise age 

unclear; correlated with Bumpass sequence, but could belong to Rockland caldera complex. 

Estimated to be ~250–260 ka. [RP]

dpl Dacite of Lassen Peak parking lot (middle Pleistocene)—Moderately porphyritic hbl-aug 

dacite lava dome (63.7% SiO2). Extensively glaciated, massive-jointed, devitrified, and 

typically oxidized interior is exposed. Flow banding is common, especially in exposures at 

Lassen Peak parking lot. Phenocrysts: 5% pl, 0.5–5 mm, few to 8 mm are generally composite crystals; 1% aug, to 3 mm, often in small glomeroporphyritic clots; 1% hbl, 1–2 mm, 

few to 5 mm; sparse bt, generally 1 mm; trace qtz, 1 mm. Most pl, hbl, and bt show reaction 

textures. Groundmass is medium gray, dense, and aphanitic and typically has splotches of 

vapor-phase devitrification. Broken fragments of mafic inclusions are abundant and range 
33

from a few millimeters to a few centimeters; most are smeared and merge with groundmass 

through disaggregation. Whole mafic inclusions to ~10 cm are sparse. Age is only partially 

constrained; older than dacite of Ski Heil Peak (ds, 244±10 ka), probably older than dacite 

of Mount Helen (dh, 249±12 ka), and estimated to be 250–260 ka. [LP]

dv Dacite of Vulcans Castle (middle Pleistocene)—Dome of porphyritic bt-hbl dacite (65.6-66.5% 

SiO2). Dome was highly sculpted by glacial erosion and mass wasting; massively jointed, 

devitrified, locally strongly flow banded, and oxidized interior is exposed. Brecciated and 

glassy margins are preserved along north flank, and a small area of partially welded domecollapse breccia is northwest of Crescent Cliff. Phenocrysts: 10% pl, 2–6 mm, largest are 

composite; 1% hbl, 1–3 mm; 1% bt, 1–2 mm; 1% qtz, 1–2 mm; sparse aug, 0.5–1 mm, as 

phenocrysts and small glomeroporphyritic clots of crystals; sparse hyp, 0.5 mm, as microphenocrysts. Groundmass is light to medium gray, often oxidized to pink, and is aphanitic 

and dense. Mafic inclusions with a variety of textures are abundant and generally <10 cm. 

Age is poorly constrained, but dv is clearly younger than Brokeoff Volcano (extinct at ~390 

ka). Unit is older than dacite of Ski Heil Peak (ds, 244±10 ka) and probably younger than 

rhyodacites of Loomis Peak (rlm) and Manzanita Chute (rmz, 297±1 ka) and is estimated to 

be 250–260 ka, but could be slightly older. [LP]

d82 Dacite of hill 8283 (middle Pleistocene)—Dome and thick lava flow of porphyritic pyr-hbl 

dacite (65.7–66.4% SiO2). Hill 8283 marks vent for thick lava flow 3.5 km long that flowed 

to north toward Lost Creek. Flow is heavily glaciated; carapace is entirely stripped, and 

massively jointed interior is exposed. Dome is also heavily glaciated, but remnants of its 

pumiceous carapace are preserved. Weak banding shown by aligned phenocrysts is common. Relative proportion of phenocrysts is somewhat variable, especially in lava dome. 

Phenocrysts: 12% pl, 1–6 mm, few composite crystals with dark cores to 1 cm; 1% hbl, 1–5 

mm; sparse bt, generally ~2 mm; sparse–1% qtz, generally ~2 mm. Hyp and aug microphenocrysts generally <0.5 mm are abundant. Both reacted and unreacted felsic phenocrysts are 

present. Aug and ol xenocrysts are common, especially on west flank of dome. Fragments 

of broken mafic inclusions 5 mm–2 cm across are abundant and often contain pyr microphenocrysts and glomeroporphyritic clots of pyr microphenocrysts. Groundmass is light to 

medium gray and aphanitic with local patchy microvesicular areas. Mafic inclusions from 10 

to ~50 cm are abundant. They have variable textures from fine to coarse grained and contain 

sparse to common, reacted host phenocrysts; sparsely porphyritic inclusions are most common. Hyp and aug microphenocrysts generally <1 mm are abundant. Both pyr-pl and hbl-pl 

groundmasses are present. K-Ar age is 261±5 ka. [MZ, WP]

dmz Dacite of upper Manzanita Creek (middle Pleistocene)—Dome of porphyritic pyr-bt-hbl 

dacite (66.0–67.0% SiO2). Dome is heavily glaciated, and the massively jointed and devitrified interior is exposed. Glacial and fluvial erosion along Manzanita Creek cut the single 

dome into several remnants. Phenocrysts: 12% pl, 2–5 mm, few composite crystals with 

dark cores to 1 cm; 2% hbl, 1–5 mm; 1% bt, to 2 mm; rare qtz, to 1 mm. Hyp and aug microphenocrysts (hyp>aug) are common but rarely exceed 0.5 mm and are sometimes found 

as small glomeroporphyritic clots. Groundmass is light to medium gray and aphanitic, with 

splotchy areas of devitrification. Fragments of mafic inclusions a few millimeters to a few 

centimeters in size are abundant. Larger mafic inclusions are abundant and to ~20 cm in 

diam. Contains coarse-grained inclusions that lack host-lava phenocrysts and fine-grained 

inclusions that contain resorbed host-lava phenocrysts. All inclusions contain sparse 0.5–1 

mm phenocrysts of aug and ol and have hbl-pl groundmasses. Age is unknown, but dmz underlies dacite of hill 8283 (d82, 261±5 ka) and is one of oldest domes of Bumpass sequence. 

Estimated to be ~265–280 ka. [MZ]

rmz Rhyodacite of Manzanita Chute (middle Pleistocene)—Thick lava flow and dome of moderately porphyritic bt-hbl rhyodacite (69.3% SiO2

). Erupted from vent beneath summit dome 

and flowed downhill toward northwest ~3 km. Although it is older than several glacial 

advances (Christiansen and others, 2002), flow is unglaciated and has well-preserved flow 

morphology; upper surface has well-defined crescentic flow ridges and a pumiceous carapace, whereas basal part is dense and perlitic. Phenocrysts: 10% pl, generally 1–3 mm, few 

to 5 mm; 2% hbl, generally 1–3 mm, few to 5 mm; 1% bt, 0.5–1 mm. Reacted phenocrysts 

are absent, and fragments of mafic inclusions are sparse. Groundmass ranges from white, 

pumiceous, perlitic glass with spherulites to dark-gray to black, dense perlitic glass. Mafic 
34

inclusions with variety of textures are generally sparse but locally common; majority are 

10–30 cm, aphyric, and fine to coarse grained with hbl-pl groundmasses. Porphyritic mafic 

inclusions are smaller, contain sparse 1-mm aug and ol phenocrysts and reacted host rhyodacite phenocrysts, and have fine-grained hbl+pl groundmasses. Unit overlies andesite of 

Manzanita Creek (amz), rhyodacite of Loomis Peak (rlm), andesite of Viola (av, 313±8 ka), 

and basalt of Onion Springs (bo) and underlies lithic tephra from Deep Hole (pdh). 40Ar/39Ar age is 297±1 ka. [LP, MZ]

rmc Rhyodacite of Mount Conard (middle Pleistocene)—Thick lava flows of porphyritic hyphbl rhyodacite (69.1-71.0% SiO2

) on north, west, and southeast flanks of Mount Conard, 

including remnant of lava flow at Terrace Lake. No vent has been located for these lava 

flows, but they appear to have flowed southward for an unknown distance from a vent that 

was high on east flank of Brokeoff Volcano. The 100-m-thick flow that forms a prominent 

cliff on the west face of Mount Conard is not part of Brokeoff Volcano but filled a canyon 

cut in its upper flank. Phenocrysts: 10–12% pl, 0.5–5 mm, generally 1–3 mm; 2–3% hbl, to 

5 mm, generally 1–3 mm; sparse–2% hyp, 0.5–2 mm; sparse–1% bt, 0.5–1 mm. Fragments 

of mafic inclusions and mafic inclusions to ~10 cm with a variety of textures are common. 

Groundmass is generally glassy and light colored, although some parts of thick flows are 

aphanitic. Flow banding is typically well developed, and spherulitic zones are common; flow 

margins are pumiceous or perlitic. Flows on Mount Conard overlie andesite of Rice Creek 

(ar) and are probably younger than all of Brokeoff Volcano. Flow remnant at Terrace Lake 

is overlain by dacite of Reading Peak (dr) and is probably older than all Bumpass sequence 

units except, perhaps, rhyodacite of Manzanita Chute (rmz) and rhyodacite of Loomis Peak 

(rlm). 40Ar/39Ar age of 298±9 ka for thick flow on west flank of Mount Conard indicates lava 

flows are associated with early part of the Bumpass sequence. [LP, RP]

rlm Rhyodacite of Loomis Peak (middle Pleistocene)—Thick lava flow of porphyritic bt-hyp-hbl 

rhyodacite (68.6% SiO2). Erupted from vent approximately located at Loomis Peak and 

flowed northwest. Upper surface has poorly preserved pumiceous carapace, whereas basal 

part is dense and perlitic. Extensive glacial erosion of flow margins exposed massively 

jointed, flow-banded, and devitrified interior where spherulitic, lithophysal, and oxidized 

zones are common. Phenocrysts: 12% pl, generally 1–3 mm, few to 5 mm, composite 

crystals to 8 mm; 2% hbl, 1–3 mm, few to 5 mm; 1% hyp, to 0.75 mm; trace bt, 0.5–1 mm. 

Fragments of mafic inclusions, reacted phenocrysts, and small crystals disaggregated from 

inclusions are abundant. Groundmass is variable: white to pinkish, dense to pumiceous, 

and devitrified to dark-gray to black, dense perlitic glass with spherulites and lithophysae. 

Mafic inclusions are abundant. Textures range from fine grained porphyritic, with resorbed 

host-rhyodacite phenocrysts, to coarse grained aphyric. The majority are 10–20 cm, but 

inclusions up to 50 cm occur. Coarse-grained aphyric inclusions are typically larger and 

have hbl+pl groundmasses, whereas porphyritic inclusions are smaller, contain sparse 1-mm 

ol, aug, calcic pl, and reacted host-rhyodacite phenocrysts, and have fine-grained pyr+pl or 

pyr+hbl+pl groundmasses. Mafic inclusions enclosing older generations of inclusions are 

relatively common. Age is poorly constrained. Unit conformably overlies basalt of Onion 

Springs (bo) and andesite of Mount Diller (amd) and, therefore, can be no older than ~390 

ka. It is overlain by rhyodacite of Manzanita Chute (rmz) and, therefore, must be older than 

297 ka. Age is estimated to be ~300 ka; it is one of oldest units of Lassen domefield stage of 

Lassen Volcanic Center. [LP, MZ]

Brokeoff Volcano

Diller sequence

amd Andesite of Mount Diller (middle Pleistocene)—Thin (10–30 m) lava flows of porphyritic 

aug-hyp andesite (61.2–63.0% SiO2) erupted from vent, which is not preserved, high on 

north flank of Brokeoff Volcano. Andesite of Mount Diller is the youngest lava of the Diller 

sequence and forms the uppermost slopes of Brokeoff Volcano on its west, north, and northeast flanks. Thickness is 300–400 m on ridge trending northwest from Mount Diller. Flows 

often have thick basal breccias and tops composed of subrounded vesicular blocks. Flow 

interiors usually have blocky joint patterns. Includes lava flows at the summit of Brokeoff 

Mountain with similar lithology but slightly more mafic composition (60.2% SiO2). 
35

Phenocrysts: 20-30% pl, 0.5–4 mm; 5% hyp, 0.5–2 mm; 3% aug, 0.5–2 mm. Xenocrystic ol 

is usually present in trace amounts. Glomeroporphyritic clots of pl+aug+hyp±ol generally 5 

mm–1 cm are abundant and characteristic of amd. Mafic inclusions containing pl and pyr 

phenocrysts are small and rare. Groundmass is typically dark gray to black and hyalopilitic 

but ranges to light gray and aphanitic. Flows bordering core of Brokeoff Volcano are hydrothermally altered to varying degrees. Overlies andesites of Mill Canyon (amc) and dacite 

of Twin Meadows (dt) on Brokeoff Mountain, andesite of Digger Creek (adc) in Blue Lake 

Canyon area, and andesite of Rice Creek (ar) in Cold Boiling Lake area. Flow near middle 

of amd section on Mount Diller yielded K-Ar age of 387±10 ka. This age is consistent with 

known stratigraphy of Lassen Volcanic Center but is ~50 ka younger than any other dated 

Brokeoff Volcano unit and may not be accurate. Overlain by rhyodacite of Loomis Peak (rlm) 

and younger Bumpass sequence dacites along north flank of Brokeoff Volcano. [LP, RP]

ad Andesites of Diller sequence, undivided (middle Pleistocene)—Small outcrop areas of porphyritic aug-hyp andesite lava flows (60–63% SiO2

) on north and west sides of Lassen domefield are correlated with Brokeoff Volcano on the basis of similar lithology and age. Younger 

glacial and volcanic deposits obscure sources and extents of these lava flows. Includes two 

small outcrops of a moderately welded aug-hyp andesite ash-flow tuff in South Fork Bailey 

Creek (unit Qapf of Clynne, 1984). Andesites are generally similar, typically medium to 

dark gray aphanitic rocks containing 10–30% phenocrysts of pl, hyp, and aug (generally 

hyp>aug); pl dominates assemblage. Phenocrysts typically 1–3 mm, locally as large as 5 

mm; ol sparsely present as partly resorbed crystals. Glomeroporphyritic clots of pl+pyr common in some flows. Unit is overlain by andesite of Viola (av, 313±8) and younger rocks of 

the Lassen domefield. Age estimated to be 400–470 ka. [GP, MZ, VI, WP]

adc Andesite of Digger Creek (middle Pleistocene)—Thick (30–60 m) lava flows of porphyritic 

aug-hyp andesite (61.3–61.9% SiO2) erupted from vent that is not preserved but was higher 

on west flank of Brokeoff Volcano than hill 8198. Unit is part of Diller sequence and forms 

the upper slopes of Brokeoff Volcano on its west flank. Lava flowed west and was diverted 

around dome of Red Rock Mountain. Lava flows underlie andesite of Mount Diller (amd) 

in Blue Lake Canyon area. Typical outcrops are glaciated cliffs exposing thinly platy jointed 

flow interiors. Flow tops are usually composed of oxidized and vesicular rounded blocks. 

Phenocrysts: 20–30% pl, mostly 1–3 mm but to 5 mm; 5–6% hyp, 0.5–2 mm; 3–4% aug, 

0.5–2 mm. Xenocrystic ol is locally present in trace amounts. Hbl is also present in trace 

amounts and is usually found as oxidized relict grains but occasionally as euhedral, Fe-Ti 

oxide-rimmed crystals. Glomeroporphyritic clots of pl+aug+hyp±ol generally 5 mm–1 cm 

are abundant and characteristic. Mafic inclusions containing pl and pyr phenocrysts are 

small and very sparse. Xenoliths of metamorphic and metavolcanic rocks occur in area of 

hill 8198. Groundmass is typically light gray and aphanitic with pl, pyr, and Fe-Ti oxide microlites but black and glassy at flow margins. Samples from flow interiors are aphanitic and 

usually show splotchy devitrification of glass to cryptofelsite. Overlies basaltic andesite of 

Hootman Ranch (mhr, 562±12 ka), dacite of Red Rock Mountain (drr, 672±20 ka), andesite 

of Cabin Spring (acs, 690±34 ka), andesites of Mill Canyon (amc, 591–472 ka), and dacite 

of Twin Meadows (dt, 470±10 ka) and other older rocks. It is overlain by andesite of Viola 

(av, 313±8 ka) and rhyodacite of Manzanita Chute (rmz, 297±1 ka). Sample from hill 8198 

yielded K-Ar age of 451±10 ka. [GP, LP, MZ, VI]

amz Andesite of Manzanita Creek (middle Pleistocene)—Thick block-lava flow of porphyritic 

aug-hyp andesite (62.6% SiO2) correlated on basis of lithology and age with Diller sequence 

and erupted from vent marked by small cinder cone south of Manzanita Lake. Eroded cinder 

cone is built of andesite scoria and has a thick soil cover. Lava flowed from cone to west for 

8 km. Lava flow is unglaciated and retains considerable original flow morphology. Upper 

surface is characterized by 0.5–1 m rounded vesicular boulders. Distal part of flow is locally flow banded, partially oxidized, and devitrified. Phenocrysts: 20% pl, 1–3 mm; 6% 

hyp, 0.5–2 mm; 3% aug, 0.5–2 mm. Sparse hbl phenocrysts, generally <1–2 mm are locally 

present. Glomeroporphyritic clots of pl+aug+hyp from 5 mm to 1 cm are abundant. Mafic 

inclusions are sparse and small, generally <5 cm. Groundmass, when fresh, is medium to 

dark gray and aphanitic; it generally weathers to light gray. Age poorly constrained by stratigraphy. Overlies basaltic andesite of Hootman Ranch (mhr, 562±12 ka; Lanphere and others, 2004) and is overlain by andesite of Viola (av, 313±8 ka) and rhyodacite of Manzanita 
36

Chute (rmz, 297±1 ka). Unit is probably older than andesite of Mount Diller (amd, 387±10 

ka) and about the same age to somewhat younger than andesite of Digger Creek (adc, 

451±10 ka); estimated to be ~425–450 ka. [MZ, VI]

ag Andesite of Glassburner Meadows (middle Pleistocene)—Thick (30–60 m) lava flows of 

porphyritic aug-hyp andesite (61.4–61.7% SiO2) erupted from unpreserved vent on south 

flank of Brokeoff Volcano. Part of Diller sequence and forms uppermost slopes of Brokeoff 

Volcano on part of its south flank. Lava flowed south over area covered by dacite of Twin 

Meadows (dt) and andesite of Bluff Falls quarry (abf). Upon reaching lower slopes, flow 

ponded against dacite of Rocky Peak (drp). Lava flow was glaciated, and typical outcrops 

are small cliffs exposing flow interiors that are thinly platy jointed. When exposed, flow tops 

are typically composed of oxidized, vesicular, rounded blocks. Phenocrysts: 25% pl, mostly 

1–3 mm, but to 5 mm; 4–5% hyp, mostly 0.5–1, but to 2 mm; 3–4% aug, mostly 0.5–1, 

but to 2 mm. Xenocrystic ol is locally present in trace amounts. Hbl also present in trace 

amounts and found as oxidized relict grains. Glomeroporphyritic clots of pl+aug+hyp±ol 

generally 5 mm–1 cm are abundant. Mafic inclusions containing pl and pyr phenocrysts are 

present but small and very sparse. Groundmass is typically light gray and aphanitic with pl, 

pyr, and Fe-Ti oxide microlites. Samples from flow interiors are aphanitic, dense, and usually show devitrification of glass to cryptofelsite. Unit not dated, but is slightly younger than 

andesite of Bluff Falls quarry (abf, 467±10 ka) and probably similar in age to andesite of 

Digger Creek (adc, 451±10 ka). Overlain by basaltic andesite of Huckleberry Lake (mhl), a 

post-Brokeoff Volcano lava flow that is ~300 ka in age. [LP]

abf Andesite of Bluff Falls quarry (middle Pleistocene)—Thick (30–60 m) lava flows of porphyritic aug-hyp andesite (59.0–59.1% SiO2

) erupted from unpreserved vent on south flank of 

Brokeoff Volcano. Lava flowed south over lava flows of Mill Canyon sequence. Typical 

outcrops are glacial cliffs exposing flow interiors with thin platy jointing. Large exposure 

at Bluff Falls quarry exhibits the jointing patterns of thick silicic andesite flows particularly 

well (Muffler and others, 1989). These thick flows typically have brecciated and oxidized 

zone at base, overlain progressively by zones of flow-banded and massively jointed rock, 

thinly platy jointed rock, and columnar-jointed rock. Where exposed, flow tops are typically composed of oxidized and vesicular, rounded blocks. Phenocrysts: 20% pl, mostly 

1–3 mm, but to 5 mm; 4–5% hyp, mostly 0.5–1 mm, but to 2 mm; 3–4% aug, mostly 0.5–1 

mm, but to 2 mm. Xenocrystic ol is usually present in trace amounts. Hbl is also present in 

trace amounts and is typically found as oxidized relict grains. Glomeroporphyritic clots of 

pl+aug+hyp±ol generally 5 mm–1 cm are abundant. Mafic inclusions containing pl and pyr 

phenocrysts are small and uncommon. Groundmass is typically light gray and aphanitic with 

pl, pyr, and Fe-Ti oxide microlites. K-Ar age is 467±10 ka. Overlain by basaltic andesite of 

Huckleberry Lake (mhl), a post-Brokeoff Volcano lava flow that is ~300 ka in age. [LP]

ar Andesite of Rice Creek (middle Pleistocene)—Thick (30–60 m) lava flows of porphyritic aughyp andesite (62.4–63.0% SiO2

) erupted from unpreserved vent on east flank of Brokeoff 

Volcano. Unit is oldest of Diller sequence lavas. On east flank of Brokeoff Volcano, ar lava 

flowed east and then south and covered 50 km2 of Mill Canyon sequence lavas and older 

rocks. Outlier of ar is found beneath dacite of Bumpass Mountain (db) at Kings Creek 

Falls. Typical outcrops are glacial cliffs exposing flow interiors that are thinly platy jointed. 

When exposed, flow tops are usually composed of oxidized, vesicular, rounded blocks. 

Phenocrysts: 20% pl, 0.5–2 mm; 4-5% hyp, mostly 0.5–1 mm, but up to 2 mm; 3–4% 

aug, mostly 0.5–1 mm, but up to 2 mm. Xenocrystic ol is locally present in trace amounts. 

Glomeroporphyritic clots of pl+aug+hyp±ol generally 5 mm–1 cm across are abundant. 

Mafic inclusions containing pl and pyr phenocrysts are small and very sparse. Groundmass 

is typically light gray, but often pinkish gray. Flow interiors are aphanitic and usually show 

devitrification of glass to cryptofelsite. Few flow remnants in Twin Meadows and Spencer 

Meadows areas are less porphyritic and have lower SiO2 (59–60%); they are similar to andesite of Bluff Falls quarry (abf), but are not mapped separately. Two different samples from 

north of Feather River Meadows yielded K-Ar ages of 485±12 and 477±14 ka. [CM, LP, RP]

Mill Canyon sequence

dt Dacite of Twin Meadows (middle Pleistocene)—Thick lava flows of porphyritic pyr-hbl dacite 

(66.1–68.0% SiO2) that cap Mill Canyon sequence and form the most important widespread 
37

stratigraphic marker of Brokeoff Volcano. Vent area(s) not preserved. Flows are generally 

100 m, but to 200 m, thick and massively to thickly platy jointed. Basal vitrophyres are 

locally exposed. Flows are found high on southwest flank of Brokeoff Mountain and form 

most of upper part of mountain. Another thick flow is found in Blue Lake Canyon (North 

Fork Bailey Creek). Flows in area below Mount Diller and Pilot Pinnacle are strongly 

altered, as are flows near Cold Boiling Lake. Unit is one of few rocks of Brokeoff Volcano 

that contains hbl as an essential phenocryst. Phenocrysts (Brokeoff Mountain flows): 8–10% 

pl, 1–3 mm, but mostly 1–2 mm; 2% hbl, 0.5–1.5 mm; 1% pyr, 0.5–1 mm (hyp>aug). Where 

fresh, groundmass is light to dark gray and aphanitic, but typical dacite of Twin Meadows 

is oxidized. Glomeroporphyritic clots of pl, hyp, and aug as much as 5 mm are common. 

Inclusions to 20 cm are abundant in flows exposed on Brokeoff Mountain and sparse in the 

flows at Blue Lake Canyon and near Cold Boiling Lake. Flows at Blue Lake Canyon and 

near Cold Boiling Lake are generally less porphyritic and contain less hbl than Brokeoff 

Mountain flows. Unit exposed in north face of Brokeoff Mountain has K-Ar age of 470±10 

ka. [LP, RP]

amc Andesites of Mill Canyon (middle Pleistocene)—Variety of generally thin ol basaltic andesite 

to pyr andesite lava flows and sparse dacite lava flows (55–65% SiO2) forming older part 

of Brokeoff Volcano. These flows and associated fragmental deposits erupted from central 

vents that cannot be located precisely. Overwhelming majority of lavas contain 57–61% 

SiO2, and the average of 40 Mill Canyon sequence andesites contains 58.5% SiO2. Base 

of Brokeoff Volcano section is exposed in Mill and Blue Lake Canyons, where flows tend 

to be more mafic (basaltic andesite to andesite) and are often ol-bearing. Higher in section, hyp+aug andesite lava flows are dominant, and flows containing ol are less common. 

Pyr±hbl dacites are sparse. Mill Canyon sequence andesites are generally porphyritic, typically containing 15–30% phenocrysts, although some sparsely porphyritic and a few nearly 

aphyric lavas are present. Phenocryst size is variable, but tends to be 0.5–2 mm. Phenocrysts 

typically are less abundant and smaller than in Diller sequence lavas. Mafic phenocryst 

assemblages are variable; ol-aug, ol-aug-hyp, hyp-aug, and aug-hyp assemblages occur. 

Pl is ubiquitous and typically dominates phenocryst assemblage, but pl-poor lavas also are 

present. Glomeroporphyritic clots of pl and mafic minerals are common. Many pyr andesite 

lavas contain ol xenocrysts, and a few contain hbl. In lower part of Mill Canyon sequence, it 

is common for adjacent flows to have different phenocryst assemblages and (or) abundances. 

Stratigraphically higher in volcano, andesites typically occur as packages of several similar 

flows, but adjacent packages are different. Groundmasses are generally medium to dark gray 

and aphanitic to glassy; aphanitic textures are most common. 

Includes a thick (to 150 m) lava flow of porphyritic pyr-hbl dacite (63.4% SiO2) at base 

of Brokeoff Volcano exposed in cliff of Mill Canyon overlying dacite of Morgan Mountain 

(dmm). This flow (dacite of Growler Spring of Clynne, 1984) and another at the base of 

Brokeoff Volcano in South Fork Bailey Creek are correlated with amc but could be related 

to Rockland caldera complex. Phenocrysts (Mill Canyon lava flow): 20% pl, mostly 0.5–2 

mm, some to 5 mm; 2% aug, 0.5 mm, few to 2 mm; 2% hyp, 0.5 mm, few to 2 mm; 1% oxidized hbl, 0.5–1 mm; sparse ol, 0.5–2 mm; rare qtz rimmed with aug. Groundmass is light 

gray, aphanitic, and dense to sparsely microvesicular. Small inclusions and glomeroporphyritic clots of pl+aug+hyp are common. 

In contrast to Diller sequence, Mill Canyon sequence lavas contain considerable pyroclastic material intercalated with the lavas. Fragmental deposits, not mapped separately, 

include fallout and lithic pyroclastic flow deposits; lahar deposits are common, and pumiceous pyroclastic-flow deposits are sparse in Mount Conard area. Thick accumulations of 

fragmental deposits are notable in East Fork Sulphur Creek area and area between Brokeoff 

Mountain and Mount Diller and probably mark proximity to vent areas. Core of Brokeoff 

Volcano is variably hydrothermally altered, and fresh rock is rare. Permeable fragmental 

deposits and brecciated flow carapaces are particularly susceptible to hydrothermal alteration. In areas of intense active hydrothermal alteration, rocks are so extensively altered that 

their original lithology is difficult to determine; typically, these are light-gray or orangebrown, altered andesite with no preserved mafic phenocrysts, but locally there are areas 

of kaolinite and silica with scattered blocks of intensely altered rock. The most extensive 

areas of unaltered to relatively unaltered Mill Canyon sequence rocks are exposed in walls 
38

of Mill Canyon, on lower slopes of Mount Conard, and in canyons of North and South Fork 

Bailey Creek. Mill Canyon sequence is capped by dacite of Twin Meadows (dt, 470±10 ka). 

Six samples of amc yielded K-Ar ages of 591±13, 555±18, 534±27, 506±14, 473±10, and 

472±9 ka and date the entire age span of the Mill Canyon sequence. [LP, RP,WP]

Rockland caldera complex

pr Rockland tephra (middle Pleistocene)—Consists of poorly exposed deposits of hyp-hbl rhyolite 

pumice (71.0–74.5% SiO2) emplaced as fallout facies and ash-flow facies in the area of Onion 

Creek, Canyon Creek, and Digger Creek near Camp Forward. Additional deposits of the ashflow facies are exposed in the extreme southwest corner of map area in roadcuts along Calif. 

Hwy 36 and outside the map area along the east bank of Guernsey Creek west of Deer Creek 

Meadows. Deposits are characterized by intense dissection and white to light-orange soil. 

Much better exposures occur west of the map area in the adjacent Manton quadrangle.

The ash-flow facies is nonsorted, nonwelded, and friable, although vapor-phase crystallization locally has weakly indurated the deposit. White to tan ash forms bulk of ash flows. 

Pumice lapilli and bombs to 20 cm form 10–20% of deposit. Lithic fragments are abundant, 

especially near base. Deposit weathers quickly, and fresh outcrops are limited to roadcuts 

and quarries. Thickness is poorly known; no complete section is exposed. Helley and others 

(1981) suggest thickness of at least 60 m near Manton. Incomplete exposures in Canyon 

Creek and Onion Creek areas are 30–80 m thick.

Phenocrysts: 8% pl, 1–3 mm; 1% hbl, 1–2 mm; sparse hyp, 0.1 mm; sparse qtz, 1–2 mm. 

Pumice has characteristic silky texture and very elongate vesicles. Small mafic inclusions 

are locally common; larger ones to 20 cm are sparse. Many types of fresh to weathered or 

hydrothermally altered, mafic to silicic volcanic rocks occur as lithic fragments in ash-flow 

deposits, but most of their sources have not been identified; however, a distinctive porphyritic, qtz-bearing, flow-banded, and devitrified rhyolite (possibly the type of rhyolites 

of Dittmar Volcanic Center, Stage 3 from Summit Creek, rd3) is ubiquitous and identifies 

Rockland tephra deposits. Clasts of the distinctive andesite of Cabin Spring (acs) are also 

occasionally found. Most abundant lithic types are porphyritic aug-hyp andesites similar to 

those found on large composite volcanoes in the region. 

The fallout facies of the Rockland tephra is composed of beds of ash and pumice lapilli 

that are from a few millimeters to a few centimeters in size; lithic fragments are sparse. 

Exposures of fallout facies are limited to isolated roadcuts and quarries west of the map area 

around Manton. Thickness is at least 2 m in exposures along Forwards Mill Road and 5 m 

or more in the quarries around Manton where the deposit has been thickened by reworking 

(Helley and others, 1981). 40Ar/39Ar age is 609±7 ka (Lanphere and others, 1999; 2004). 

[GP, LY, VI]

rr Rhyolite of Raker Peak (middle Pleistocene)—Steep-sided lava dome of moderately porphyritic hbl-bt rhyolite (73.5% SiO2

) erupted from an underlying vent. Glacial erosion along south 

margin of dome exposed prominent columnar joints, and dome has been stripped of pumiceous and blocky carapace. Phenocrysts: 10% pl, 1–3 mm, many resorbed; 2% bt, to 1 mm; 

1% hbl, to 1 mm; sparse qtz, to 0.25 mm. Groundmass is white to light gray and glassy but 

often spherulitic. Coarse, even-grained mafic inclusions (57.5% SiO2) are common, generally 10–30 cm. Small fragments of mafic inclusions a few millimeters in size are abundant. 

Inclusions have a pyr+hbl+pl mineral assemblage and sparse pl phenocrysts. In some places 

inclusions are smeared in thin, even layers a few millimeters thick giving appearance of flow 

banding in rhyolite. These layers commonly provide planar discontinuities upon which rock 

separates into large flat blocks. Overlain by much younger lava flows of andesite of Raker 

Peak (arp, 270±18 ka). K-Ar age is 588±69 ka, but it is probably older than the Rockland 

tephra. It is chemically very similar to Rockland tephra (pr) and may represent a pre-caldera 

leak of the Rockland magma chamber. [WP]

drr Dacite of Red Rock Mountain (middle Pleistocene)—Lava dome of porphyritic hyp-hbl dacite 

(63.7% SiO2) on west flank of Brokeoff Mountain. Tentatively correlated with Rockland 

caldera complex, although it may be unrelated. Red Rock Mountain was glaciated, and 

pumiceous and glassy carapace of the dome was completely removed; thinly platy jointed 

core of dome is exposed. Phenocrysts: 8% pl, 0.5–4 mm; 2% hbl, 1–2 mm; sparse qtz with 

aug rims, 1–2 mm; sparse hyp, 0.5 mm. Fresh rock is light gray, but it is typically oxidized 
39

to shades of red. Mafic inclusions to a few centimeters across are common. Overlies andesite 

of Cabin Spring (acs); predates Brokeoff Volcano and formed barrier to advancing Brokeoff 

andesites. K-Ar age is 672±20 ka. [GP, LP]

dbl Dacite of Bench Lake (middle Pleistocene)—Several small domes of porphyritic hyp-hbl dacite 

(65.7% SiO2) related to Rockland caldera complex are poorly exposed under glacial deposits 

and younger rocks in Kings Creek. Original pumiceous carapaces or hot talus covers were 

completely removed by erosion, exposing their crystalline, thick platy- to massive-jointed 

interiors. Lithology of domes is slightly variable; the following description is of the largest dome (hill 800 meters southeast of Kings Creek Falls). Phenocrysts: 8–10% pl, generally 0.5–6 mm, mostly 1–3 mm, larger crystals generally composite; 2–3% hbl, mostly 1–3 

mm, few to 6 mm; sparse hyp, 0.5 mm; traces of 1–3 mm bt and qtz. Some pl phenocrysts 

are strongly reacted. Groundmass is light to medium gray and glassy to aphanitic. Large 

quenched andesitic inclusions are sparse, but broken fragments of andesitic inclusions are 

common and generally <1–2 cm. Unit underlies rocks of Brokeoff Volcano (andesite of Rice 

Creek, ar, 477±14 and 485±12 ka) and overlies hydrothermally altered rocks of Dittmar 

Volcanic Center. K-Ar age is 679±14 ka. [RP]

dbc Dacite of Bailey Creek (middle Pleistocene)—Small body of porphyritic bt-hbl dacite (64.2% 

SiO2) with domical outcrop and massive jointing poorly exposed in the canyon of South 

Fork Bailey Creek. Dacite was tentatively correlated with the Maidu Volcanic Center by 

Clynne (1984), but it is now recognized as part of the Rockland caldera complex. Phenocrysts: 10% pl, to 5 mm; 2% oxidized hbl, 0.5–4 mm; 1% oxidized bt, 0.5–2 mm; 1% qtz, 

0.5–1 mm; sparse hyp, 0.5–1 mm; trace corroded ol, 1 mm; trace aug, 0.5 mm. Where fresh, 

groundmass is light gray and aphanitic, but typically it is oxidized pinkish. Age unknown, 

but dbc is overlain by andesites of Mill Canyon (amc) and, thus, is older than Brokeoff 

Volcano. Estimated to be ~700–825 ka. [LP]

drp Dacite of Rocky Peak (early Pleistocene)—Thick lava flow of porphyritic pyr-hbl dacite 

(66.6% SiO2) with a dome at Rocky Peak marking the vent. Extensive glaciation removed 

most of original carapace of glassy and vesicular rock. Dome has massive jointing and 

lacks flow structure. Upper part of lava flow is glassy and massively jointed and has vague 

flow structure marked by aligned phenocrysts and often zones of microvesicles, brecciation, oxidation, and devitrification. Interior of lava flow is more crystalline and has thin 

platy jointing. Phenocrysts: 15–20% pl, 1–4 mm; 1–3% oxidized hbl, 1–4 mm; 1–2% 

hyp, 0.5 mm; trace–3% aug, 0.5–1 mm. Lava flow is hyp-hbl dacite, whereas the dome is 

primarily hbl-aug dacite; transition between lithologies is gradational. Mafic inclusions 

to ~20 cm are abundant. Groundmass is light to medium gray and glassy to aphanitic. 

Unit overlies andesite of Martin Creek (amr) and basaltic andesite of Grays Peak (mgp, 

1,080±26 ka) and is overlain by andesites of Mill Canyon (amc, <591±13 ka), andesite of 

Digger Creek (adc, 451±10 ka), and andesite of Glassburner Meadows (ag). K-Ar age is 

803±27 ka. [GP, LP]

dfr Dacite of Flatiron Ridge (early Pleistocene)—Thick lava flow (>100 m) of porphyritic hbl-pyr 

dacite (65.9% SiO2) erupted from vent now buried by younger domes of Lassen Volcanic 

Center. Lava flowed southeast and filled a paleovalley of Warner Creek. Subsequently, 

Kings Creek and Hot Springs Creek were re-excavated by glaciers along north and south 

margins of flow to create Flatiron Ridge, and original carapace of flow was completely 

stripped. Phenocrysts: 8–10% pl, generally 1–3 mm, few to 6 mm; 1–2% hbl, 1–3 mm, 

few to 5 mm; sparse hyp, 0.5–1 mm; rare aug, 0.5–1 mm. Hbl and some pl phenocrysts are 

strongly reacted. Groundmass is medium to dark gray, dense, and aphanitic. Abundant 1–2 

cm broken fragments of andesitic inclusions with pyr+pl groundmass are characteristic; 

larger inclusions are rare. Thick zones of thinly platy jointed rock are exposed in glaciated 

margins of lava flow, and top is covered with till. In Kings Creek, unconformably overlies 

much older hydrothermally altered rocks of Dittmar Volcanic Center (Tad) and dacite of 

Panther Creek (dp, 827±18 ka). K-Ar age is 812±6 ka. [MH, RP]

dp Dacite of Panther Creek (early Pleistocene)—Small group of porphyritic hyp-hbl dacite domes 

(65.3% SiO2) exposed around and south of Devils Kitchen. Domes were eroded so that 

original pumiceous carapaces or hot talus covers were completely removed and crystalline thick platy to massive jointed interiors exposed. At Devils Kitchen, rocks are strongly 

altered. Largest of domes erupted from vent centered under hill 7139. Dome(s) and sparse 
40

dome-collapse or other breccia deposits exposed in canyon wall north of Devils Kitchen 

are similar. Phenocrysts: 12–15% pl, 0.5–6 mm, mostly 1–3 mm, larger crystals generally 

composite; 2–4% hbl, mostly 1–3 mm, few to 1 cm; sparse hyp, 0.5 mm; rare 1 mm crystals 

of ol and aug. Hbl and some pl phenocrysts are strongly reacted. Groundmass is light to 

medium gray and aphanitic. Porphyritic mafic inclusions are abundant but generally <15–25 

cm, and 1–2 cm fragments of mafic inclusions are abundant. Unit underlies rocks of Brokeoff Volcano and overlies hydrothermally altered rocks of Dittmar Volcanic Center. K-Ar age 

of sample from hill 7139 is 827±18 ka. [RP]

dpg Dacite of Plantation Gulch (early Pleistocene)—Dome of porphyritic bt-hbl dacite (66.9% 

SiO2) that forms twin-peaked ridge east of Plantation Gulch. Dome was probably emplaced 

as a solid mass that uplifted and distorted lava flows of the underlying andesite of Martin 

Creek (amr). Phenocrysts: 10–15% pl, 1–5 mm, mostly about 2 mm; 0–5% hbl, 1–6 mm; 

1–3% qtz, 1–3 mm; 1% oxidized bt, 1–2 mm; 1% hyp, 0.5 mm. Inclusions to 20–30 cm 

and fragments of disaggregated inclusions containing aug are abundant. Groundmass is 

light gray and aphanitic but often oxidized, and rock is massively jointed. Located between 

Brokeoff Volcano and Maidu Volcanic Center and related to earliest part of Rockland caldera 

complex. Age is poorly known and estimated to be about same age to slightly older than 

dacite of Rocky Peak (drp, 803±27 ka). [LP, MN]

dch Dacite of Christie Hill (early Pleistocene)—Porphyritic hyp-hbl dacite (67.3% SiO2) lava 

dome at Christie Hill and thick flow extending south. Phenocrysts: 20% pl, 1–3 mm; 4% 

hbl, 0.5–3 mm; 1% hyp, 0.5–1 mm; 1–2 % qtz, 1–5 mm. When fresh, rock is medium gray 

with devitrified aphanitic groundmass, but typically it is oxidized from pink to deep red. 

Inclusions to 20–30 cm with hbl-pl groundmass and aug and ol phenocrysts are abundant, 

and fragments of inclusions and 1 mm aug and ol disaggregated from them are common. 

Qtz more abundant, but inclusions, fragments of inclusions, and aug and ol much less 

abundant in lava flow than in dome. Dome is dense and massively jointed; flow is microvesicular and commonly flow banded. Unit overlies much older altered rocks of Maidu 

Volcanic Center, but is itself unaltered. Located between Brokeoff Volcano and Maidu 

Volcanic Center and related to earliest part of Rockland caldera complex. Age is poorly 

known and estimated to be about same age to slightly older than dacite of Rocky Peak 

(drp, 803±27 ka). [LP, MN]

dmm Dacite of Morgan Mountain (early Pleistocene)—Large, heterogeneous lava dome of porphyritic pyr-hbl dacite (66.6% SiO2

) at Morgan Mountain. Glaciated steep-sided dome has no 

original surface morphology preserved, is massively jointed, and lacks internal flow structure. Much of it is covered with talus and colluvium and, on lower slopes, till. Phenocrysts: 

15–20% pl, 0.5–2 mm, few to 5 mm; 0–3% oxidized hbl, 1–4 mm; 1–2% aug, 0.5–1 mm, 

few larger; sparse–1% hyp, 0.5–1 mm; 1–3% qtz, mostly 1 mm, few larger. Inclusions to 

~20 cm, both pyr-pl and hbl-pl groundmasses are abundant. Fresh rock is medium gray to 

pink with aphanitic, dense to sparsely microvesicular groundmass, but generally weathered. 

Located between Brokeoff Volcano and Maidu Volcanic Center and related to earliest part 

of Rockland caldera complex. Age is poorly known and estimated to be about same age to 

slightly older than dacite of Rocky Peak (drp, 803±27 ka). [LP, MN]

dcy Dacite of Canyon Creek (early Pleistocene)—Porphyritic pyr-hbl dacite (68.0% SiO2) buried by early Brokeoff Volcano lavas and exposed by erosion in Canyon Creek. Dacite has 

domical outcrop and massive to thick platy jointing and is probably a lava dome. Lacks 

an obvious vent. Phenocrysts: 10–12% pl, 0.5–4 mm, mostly 1–2 mm, many reacted and 

with gray cores; 1–2% hbl, 1–3 mm, mostly pseudomorphed by pyr and Fe-Ti oxide; 1–2% 

hyp, 0.25–0.5 mm. Groundmass is dark gray, aphanitic, and dense and contains spherulites. 

Located between Brokeoff Volcano and Maidu Volcanic Center and related to earliest part 

of Rockland caldera complex. Age is poorly known and estimated to be about same age to 

slightly older than dacite of Rocky Peak (drp, 803±27 ka). [LP, RP]

MAIDU VOLCANIC CENTER

Rocks of Maidu Volcanic Center rest unconformably on Tuscan Formation (Tt) and are abundant in 

southern and southwestern parts of map; most of the center lies south and west of map area. Maidu Volcanic 

Center is moderately well preserved, and its evolution is known in general. However, the interior of Maidu 

Volcanic Center is hydrothermally altered and deeply eroded; consequently, details of its evolution are poorly 
41

known. Three stages of Maidu Volcanic Center rocks correspond to early and late stages of growth of an 

andesitic stratovolcano (Stages 1 and 2) and subsequent silicic volcanism (Stage 3) peripheral to the andesitic 

stratocone. Several Stage 3 lava flows (rb, rmp, rlp, rsm) are among the largest rhyolite lava flows in the 

Cascade Range; their combined volume is ~37 km3. Maidu Volcanic Center was active 2.4–1.2 m.y.

Stage 3

rb Rhyolite of Blue Ridge (early Pleistocene)—Thick (to 150 m) flow of sparsely porphyritic 

hyp-hbl rhyolite lava (75.4-75.5% SiO2) that forms extensive plateau covering ~90 km2 on 

northwest flank of Maidu Volcanic Center. Lava flowed west from vent in vicinity of Rocky 

Peak or Grays Peak, but precise location of vent is unknown. Phenocrysts: 5% pl, to 4 mm, 

mostly 1–2 mm; 1% hbl, to 2 mm, mostly 0.5–1 mm; 1% hyp, 0.5–1 mm. Mafic inclusions 

absent. Where glassy, rb is light gray to black, flow banded, and perlitic, but more typical 

rock is devitrified, white to tan and deeply weathered, often strongly flow banded, spherulitic, and lithophysal. Flow morphology of surface and pumiceous carapace were completely 

obliterated by erosion. Flow is covered by thick, very light colored soil. Volume of lava 

flow is ~15 km3. K-Ar ages of 1,150±70 ka (whole rock) and 1,240±110 ka (pl) reported by 

Gilbert (1969) are consistent with known stratigraphic relations. [GP, LY]

rmp Rhyolite of Mill Creek Plateau (early Pleistocene)—Thick (to 150 m) lava flow of sparsely 

porphyritic hbl-bt rhyolite (75.1-75.9% SiO2). Erupted from buried vent near west margin 

of flow. Flowed southeast to form extensive plateau covering ~55 km2 on east flank of 

Maidu Volcanic Center; mostly south of map area. Surface flow morphology and pumiceous 

carapace completely removed by erosion. Flow is covered by a thick, very light colored 

soil. Includes two small remnants of hbl-bt rhyolite lava flow exposed just south of Morgan 

Mountain; no vent is apparent. Also includes small body of strongly flow banded and deeply 

weathered hbl rhyolite lava exposed 0.4 km northwest of Christie Hill, which is almost completely buried beneath younger rocks and till. Phenocrysts (Mill Creek Plateau): 5% pl, to 5 

mm, mostly 1–2 mm; 1% bt, 0.5–1 mm; sparse hbl, 1–2 mm, a few larger. Xenocrysts of 1–2 

mm hyp disaggregated from crystal clots of hyp+pl are present: mafic inclusions are absent. 

Locally, rmp is light gray to black, flow-banded, and perlitic, but typically is devitrified, 

white to tan, deeply weathered, generally strongly flow-banded, spherulitic, and lithophysal. 

Rare exposures of base of flow reveal light gray vitrophyre. Phenocrysts (flow remnants at 

Morgan Mountain): 6-8% pl, 0.5–2 mm, few larger; 1–2% bt, 0.5–1 mm; sparse hbl, 1–2 

mm, few to 5 mm; sparse qtz, 0.5–1 mm. Groundmass is white to light gray, microvesicular, and glassy to devitrified. Phenocrysts (flow remnant northwest of Christie Hill): 6% pl, 

0.5–2 mm; 4% hbl, 0.5–1 mm. Groundmass is flow banded, white to dark gray, and aphanitic. Volume of lava flow is ~7 km3

. Unit overlies andesites of Maidu Volcanic Center, Stage 

1, undivided (Tam), andesites of Wild Catle Mountain (Taw), and rhyodacite of Gurnsey 

Creek (rg); overlain by tholeiitic basalts of Mill Creek Plateau (bmc). Similar in age to 

rhyolite of Lost Creek Plateau (rlp, 1,305±10 ka) and rhyolite of Blue Ridge (rb, ~1,200 ka). 

[CM, LP, MN] 

rlp Rhyolite of Lost Creek Plateau (early Pleistocene)—Thick (to 150 m) lava flow of sparsely 

porphyritic bt-hbl rhyolite (74.3% SiO2). Erupted from vent near Wilson Lake and flowed 

east. Forms extensive plateau covering ~30 km2 on east flank of Maidu Volcanic Center; 

mostly south of map area. Surface flow morphology and pumiceous carapace completely 

removed by erosion. Flow was weakly glaciated, primarily on north flank and has sparse till 

cover. Flow is covered by a thick, very light colored soil. Phenocrysts: 10% pl, to 5 mm, few 

larger grains generally composite crystals; 2–3% hbl, to 4 mm; sparse bt, 0.5 mm. Mafic inclusions are absent. When glassy, lava flow is light gray to black, flow banded, and perlitic, 

but typically it is devitrified, white to tan, deeply weathered, generally strongly flow banded, 

spherulitic, and lithophysal. Volume of lava flow is ~5 km3. Unit overlies andesites of Wild 

Cattle Mountain (Taw) and rhyodacite of Gurnsey Creek (rg); overlain by tholeiitic basalt of 

Ice Cave Mountain (bic). 40Ar/39Ar age is 1,305±10 ka. [CM]

rsm Rhyolite of North Stover Mountain (early Pleistocene)—Thick (to 150 m) lava flow of 

sparsely porphyritic bt-hbl rhyolite (74.1% SiO2). Erupted from vent near Ice Cave 

Mountain and flowed southeast. Forms extensive plateau covering ~50 km2 on east flank of 

Maidu Volcanic Center; partly south of map area. Surface flow morphology and pumiceous 

carapace completely removed by erosion. Flow was glaciated, primarily on north side; 
42

covered by a thick, very light colored soil. Cut by major fault associated with Lake Almanor 

Graben. Phenocrysts: 5% pl, to 5 mm, mostly 1-3 mm; 1% hbl, 1-4 mm; trace bt, 0.5-1 mm. 

Xenocrysts of aug and hyp, 1-2 mm, are disaggregated from crystal clots of pyr+pl. Mafic inclusions are absent. When glassy, lava flow is light gray to black, flow banded, and 

perlitic, but typically it is devitrified, white to tan, deeply weathered, generally strongly 

flow banded, spherulitic, and lithophysal. Rare exposures of flow margins reveal light-gray 

vitrophyre. Volume of lava flow is ~8.5 km3. Unit overlies andesites of Dittmar Volcanic 

Center, Stage 2, undivided (ad2) and andesites of Wild Cattle Mountain (Taw); overlain by 

tholeiitic basalt of Warner Valley (bwv), basalts and basaltic andesites of Sifford Mountain 

(bsm), tholeiitic basalts of Buzzard Springs (bbz), and tholeiitic basalt of Ice Cave Mountain (bic). Similar in age to rhyolite of Lost Creek Plateau (rlp, 1,305±10 ka) and rhyolite of 

Blue Ridge (rb, ~1,200 ka). [CM, ST]

rg Rhyodacite of Gurnsey Creek (early Pleistocene)—Small exposure of thick lava flow of 

coarsely porphyritic aug-hyp rhyodacite at southeast end of Wild Cattle Mountain; bulk 

of flow is south of map area across Gurnsey Creek. Erupted from vent buried by rhyolite 

of Mill Creek Plateau (rmp) and flowed east from east flank of Maidu Volcanic Center. 

Includes small exposure of similar rock in Nanny Creek that probably represents a different 

flow. Phenocrysts: 15% pl, to 3 mm, mostly 1–2 mm; 2–3% hyp, 0.5–2 mm; sparse–1% aug, 

0.5–2 mm, mostly 0.5–1 mm; rare hbl, 0.5 mm. Glomeroporphyritic clots of pl+aug+hyp 

5 mm to 5 cm are common; small mafic inclusions are present. Groundmass is nearly white 

to light gray, aphanitic, and dense. Splotchy areas of spherulites and flow banding are common; jointing is massive. Base of flow dark gray, perlitic, and brecciated. Covered by thick 

tan to light-orange soil. Volume of lava flow is ~2 km3. Unit overlies andesites of Wild 

Cattle Mountain (Taw) and is overlain by rhyolite of Lost Creek Plateau (rlp). Age is similar 

to slightly older than rlp (1,305±10 ka). [CM, LP]

Stage 2

Tah Andesites of Hampton Butte (late Pliocene)—Two thick lava flows of coarsely porphyritic 

hyp-aug-ol andesite (58.2% SiO2) form summit of Hampton Butte and cap Stage 2 section 

of Maidu Volcanic Center in map area. Flowed toward northwest from vent on flank of 

Maidu Volcanic Center that is not preserved. Deeply weathered flow breccia separates two 

flows. Flow tops display massive, rounded blocks that contain abundant rounded to flattened elongate vesicles. Flow cores display thick slabby jointing. Unit has well-developed, 

deep-reddish soil; thin cover of glacial till is common, especially on north slope of Hampton 

Butte. Relative proportions of mafic phenocrysts are variable in both flows, so descriptions 

are generalized. Phenocrysts (lower flow): 20% pl, to 3 mm, mostly 1–2 mm; 1–3% ol, 

0.5–2 mm, few larger are composite; 4–5% hyp, 0.5–2 mm; 2–3% aug, 0.5–2 mm. Phenocrysts (upper flow): 20% pl, to 3 mm, mostly 1–2 mm; 5–7% ol, 0.5–2 mm, few larger 

are composite; 2–3% aug, 0.5–2 mm; sparse hyp, 0.5–2 mm. Pl phenocrysts with dark-gray 

reacted cores are present in both flows but are more abundant in upper flow. Glomeroporphyritic clots of pl+ol+aug+hyp to 1 cm are common in both flows. Ol-bearing mafic inclusions to ~5 cm are sparse but widely distributed in upper flow. Groundmasses are medium to 

dark gray and glassy to aphanitic. Splotchy areas of vapor-phase crystallization are common, 

especially in upper part of flows. Unit overlies andesites of Maidu Volcanic Center, Stage 1, 

undivided (Tam, ~2,150–2,375 ka), andesite of South Fork Battle Creek (Tab, 2,166±67 ka) 

and dacites of Maidu Volcanic Center, Stage 2, undivided (Tdm, ~2,100–2,150 ka). Unit 

underlies basaltic andesite of South Fork Battle Creek (mbc, 1,738±46 ka), andesites of 

Cowslip Campground (acc, 1,320±40 ka), and basaltic andesite of Grays Peak (mgp, 

1,080±26 ka). Estimated to be ~2,050–2,100 ka. [GP, LY]

Tdm Dacites of Maidu Volcanic Center, Stage 2, undivided (late Pliocene)—Thick (30–100+ m) 

porphyritic aug-hyp dacite (63.3–65.8% SiO2) lava flows erupted from vents on upper 

flanks of the composite volcano that are not preserved. In the map area, flows occur on the 

north and east flanks of Maidu Volcanic Center. Typical outcrops are glacial cliffs exposing thinly platy jointed flow interiors. Flow tops are composed of oxidized and vesicular, 

rounded blocks. Phenocrysts: 20–30% pl, mostly 1–3 mm, but to 5 mm; 3–6% hyp, mostly 

0.5–1 mm, but to 2 mm; 2–4% aug, mostly 0.5–1 mm, but to 2 mm. Xenocrystic ol is often 

present in trace amounts, and a few flows contain sparse hbl. Glomeroporphyritic clots of 
43

pl+aug+hyp±ol, generally 5 mm to 1 cm, are abundant. Small mafic inclusions containing pl+pyr phenocrysts are rare. Groundmass typically light gray and aphanitic and shows 

devitrification of glass to cryptofelsite. Unit overlies Tuscan Formation (Tt), basaltic 

andesite of Lassen Lodge (Tml), andesites of Maidu Volcanic Center, Stage 1, undivided 

(Tam, ~2,150–2,375 ka), and andesite of South Fork Battle Creek (Tab, 2,166±67 ka). 

Unit is overlain by andesites of Hampton Butte (Tah), basaltic andesite of South Fork 

Battle Creek (mbc, 1,738±46 ka), basalt of Panther Creek (bpa, 1,105±49 ka), rhyolite of 

Blue Ridge (rb, ~1,200 ka), basaltic andesite of Grays Peak (mgp, 1,080±26 ka), dacite of 

Morgan Mountain (dmm), and basaltic andesite of Huckleberry Lake (mhl). K-Ar age of 

sample from near Hampton Butte is 2,120±46 ka. K-Ar ages from samples collected outside 

map area are 2,105±25, 2,140±42, and 2,259±46 ka. Best estimate for the age of Tdm is 

~2,100–2,150 ka. [GP, LY, MN]

Stage 1

Tam Andesites of Maidu Volcanic Center, Stage 1, undivided (late Pliocene)—Variety of basaltic 

andesite, andesite, and dacite lava flows forming the older part of Maidu Volcanic Center. 

Flows and associated fragmental deposits erupted from central vents that cannot be precisely 

located. Analyzed samples range in composition from 55–65% SiO2; most are andesite. In 

the map area, flows are exposed in the cliffs surrounding Battle Creek Meadows, in the canyon of South Fork Battle Creek. Outside the map area, base of unit is exposed in Mill Creek 

and Deer Creek canyons. Stage 1 andesites are generally porphyritic, typically containing 

15–30% phenocrysts, although some sparsely porphyritic and few nearly aphyric lavas are 

present. Phenocryst size is variable but tends to be 0.5–2 mm. Phenocrysts typically less 

abundant and smaller than in dacites of Maidu Volcanic Center, Stage 2, undivided (Tdm). 

Phenocryst assemblages are variable; ol-aug, ol-aug-hyp, aug-hyp, and hyp-aug assemblages 

occur. Pl is nearly ubiquitous and typically dominates phenocryst assemblage. Glomeroporphyritic clots of pl and mafic minerals are common. Many pyr-andesite lavas contain ol 

xenocrysts, and hbl or pseudomorphs of hbl are occasionally present. Groundmasses are 

generally medium to dark gray and aphanitic to glassy; aphanitic textures are most common. In contrast to later Maidu Volcanic Center deposits (Tdm), unit contains considerable 

intercalated pyroclastic material. Fragmental deposits are not mapped separately; they range 

from fallout to block-and-ash flow deposits, and lahar deposits are common. Permeable fragmental deposits and brecciated flow carapaces are particularly susceptible to hydrothermal 

alteration, which was especially intense in area around Christie Hill, Little Round Valley, 

and Mineral Summit. Unit unconformably overlies Tuscan Formation (Tt). Unit is overlain 

by andesite of South Fork Battle Creek (Tab, 2,166±67 ka), dacites of Maidu Volcanic 

Center, Stage 2, undivided (Tdm, ~2,100–2,150 ka), andesites of Hampton Butte (Tah), 

basaltic andesites of South Fork Battle Creek (mbc, 1,738±46 ka), andesites of Wild Cattle 

Mountain (Taw), andesites of Cowslip Campground (acc, 1,320±40 ka), andesite of Martin 

Creek (amr), basalt of Panther Creek (bpa, 1,105±49 ka), dacite of Christie Hill (dch), 

Rockland tephra (pr, 609±7 ka), basaltic andesite of Huckleberry Lake (mhl), and basalts of 

Cold Creek Butte (bcc). K-Ar ages of two samples from incipiently hydrothermally altered 

rock near the top of unit on mountain just north of Mineral are 2,045±42 and 1,905±42 ka, 

which are probably slightly too young. Four additional K-Ar ages from south of map area 

are 2,375±49, 2,325±49, 2,250±49, and 2,152±47 ka. Best estimate for the age of Tam is 

~2,150–2,375 ka. [LP, LY, MN]

Tab Andesite of South Fork Battle Creek (late Pliocene)—Thick complex of finely porphyritic 

hyp-aug andesite (62.6% SiO2) lava flows from younger part of Maidu Volcanic Center 

Stage 1. Lava flow section ~300 m thick where exposed in cliffs above South Fork Battle 

Creek. Lava probably erupted from a flank vent, flowed generally northwest, and filled a 

canyon cut into the composite volcano. Flow contacts and interflow breccias are scarce. 

Flow interiors display thick slabby to thin platy jointing. Phenocrysts: 30% pl, to 1 mm; 

7% aug, 0.5–2 mm; 3% hyp, 0.5–1 mm; sparse ol, 1–2 mm. Aphanitic groundmass often 

has bluish cast but otherwise is very light to medium gray. Unit is youngest Stage 1 lava 

on northwest flank of the composite volcano. Unit overlies andesites of Maidu Volcanic 

Center, Stage 1, undivided (Tam, ~2,150–2,375 ka) and is overlain by dacites of Maidu 

Volcanic Center, Stage 2, undivided (Tdm, ~2,100-2,150 ka), andesites of Hampton Butte 
44

(Tah), basalt of Panther Creek (bpa, 1,105±49 ka), basaltic andesite of Grays Peak (mgp, 

1,080±26), and rhyolite of Blue Ridge (rb, ~1,200 ka). K-Ar age is 2,166±67 ka. [GP, LY]

Tml Basaltic andesite of Lassen Lodge (late Pliocene)—Porphyritic aug-ol basaltic andesite lava 

flows probably erupted from Maidu Volcanic Center are located ~5–6 km west of town 

of Mineral. Near Lassen Lodge, Tml comprises the lowermost lava flows of the volcano 

in this area. Similar flows outside map area near Judd Creek (Finley Butte and Lyonsville 

quadrangles) may be related. Phenocrysts: 10–15% pl, 0.5–3 mm, mostly 0.5–1 mm; 3–5% 

ol, 0.5–2 mm, few larger; 4% aug, 0.5–2 mm, few larger. Groundmass is dark gray to black, 

aphanitic to glassy, and microvesicular. Some flows have more abundant and larger phenocrysts: 15–20% pl, 0.5–5 mm, mostly 0.5–1 mm; 8% aug, 0.5–5 mm, mostly 0.5–1 mm, 

few larger; 4% ol, 0.5–3 mm, few larger. These contain abundant glomeroporphyritic clots 

of ol+pl+aug to 1 cm and probably owe their more abundant crystals to disaggregation of 

clots. Groundmass is medium gray, aphanitic, and microvesicular. Aug phenocrysts in both 

lithologies are bright green, indicating a chromian diopside composition. Age is very poorly 

constrained. Unit overlies Tuscan Formation (Tt), is at the base of Maidu Volcanic Center, 

and could be equivalent to the oldest part of andesites of Maidu Volcanic Center, Stage 1, 

undivided (Tam, ~2,150–2,375 ka); estimated to be ~2,350–2,375 ka. [LY]

DITTMAR VOLCANIC CENTER

Rocks of Dittmar Volcanic Center are abundant in southeastern Lassen Volcanic National Park and adjacent Lassen National Forest. Dittmar Volcanic Center sits astride the area connecting the Lake Almanor Graben 

and Hat Creek Fault and has been structurally dismembered by faulting. Interior of Dittmar Volcanic Center is 

hydrothermally altered and deeply eroded; consequently, details of its evolution are poorly known. Rocks of 

Dittmar Volcanic Center are divided into three stages corresponding to early and late stages of the growth of 

an andesitic stratovolcano (Stages 1 and 2) and subsequent silicic volcanism (Stage 3) peripheral to andesitic 

stratocone. Dittmar Volcanic Center was active from at least 2.4 to 1.3 m.y. and probably somewhat earlier. 

Stage 3

rd3 Rhyolites of Dittmar Volcanic Center, Stage 3 (early Pleistocene)—Isolated erosional remnants or buried lava domes and flows of porphyritic bt-, hbl-, and hyp-bearing rhyolite and 

rhyodacite (70.9–75.0% SiO2) on north flank of Dittmar Volcanic Center. Includes bodies 

on Pilot Mountain, in Summit Creek and Grassy Swale, and just east of Hidden Lake. Rocks 

related to this unit must have been more abundant prior to glaciation, and similar rocks 

undoubtedly lie buried beneath younger rocks of Lassen Volcanic Center on Central Plateau 

of LVNP and in Caribou Volcanic Field. 

Small area of porphyritic, qtz-bearing, hbl rhyolite (75.0% SiO2) is exposed in wall of 

tributary to Summit Creek ~1.6 km north of its juncture with Kings Creek. This rhyolite is 

strongly flow banded and is probably a thick lava flow; location of vent is unknown. Extent 

of lava flow is also unknown, but it likely covers large area in subsurface. Phenocrysts: 

8% pl, 1–3 mm; 2–3% qtz, 1–2 mm; 1–2% hbl, 1–3 mm; sparse bt, 1 mm. Groundmass is 

aphanitic and medium gray, sometimes pinkish in color, and spherulitic. Small, fine-grained, 

aphyric inclusions and fragments of inclusions are common. Flow is mostly buried beneath 

glacial deposits and probably by andesite of Hat Mountain (ah) lava flows. Lithology is 

similar to qtz-bearing rhyolite ubiquitously found as lithic fragments in Rockland tephra 

(pr). 

Small body of porphyritic bt-hbl rhyodacite (70.9% SiO2) caps Pilot Mountain. This 

rhyodacite has massive to thick platy jointing and is a thick lava flow or lava dome; location 

of vent is unknown but, perhaps, is buried beneath flow. Phenocrysts: 12% pl, 0.5–3 mm, 

mostly 0.5–1 mm; 2–3% hbl, 0.5–2 mm; sparse bt, 0.5–1 mm. Rock has a light-gray, almost 

white, glassy groundmass. Contains abundant inclusions of basaltic andesite composition 

(54% SiO2), 5–30 cm in long dimension, although most are 5–15 cm. Some inclusions have 

distinctive texture of as much as 30% clots of pl+ol+aug in a fine-grained matrix; others lack 

clusters but contain isolated crystals of same mineral phases. Fragments of inclusions are 

common in rhyolite, and 0.5–2 mm crystals of pl, ol, and aug were derived from disaggregation of mafic inclusions.

Body of porphyritic bt-hbl rhyodacite (71.6% SiO2) is exposed on ridge just north of 

Grassy Swale. This rhyodacite is generally flow banded and has perlitic glassy base with 
45

massive to thick platy jointing and appears to be a thick lava flow dipping north; location 

of vent is unknown, and flow is mostly buried beneath andesite of Hat Mountain (ah) lava 

flows. Phenocrysts: 10–12% pl, 1–3 mm; 2–3% hbl, 1–3 mm; 1% bt, 1 mm; sparse qtz, 

1–2 mm. Groundmass is glassy and very light gray to white, except where perlitic or spherulitic. Small, fine-grained, aphyric inclusions and fragments of inclusions are common.

Small body of porphyritic bt-hyp-hbl rhyolite or rhyodacite is exposed on ridge just east 

of Hidden Lake. This body has massive to thick platy jointing and is probably a thick lava 

flow; location of vent is unknown but perhaps is buried beneath flow. Some areas are flow 

banded, oxidized, and devitrified. Phenocrysts: 10% pl, 1–5 mm, mostly 1–2 mm; 3% hbl, 

1–5 mm, mostly 2–4 mm; 2% hyp, 1–2 mm; sparse bt, 0.5–2 mm. Small sparsely porphyritic 

inclusions and fragments of inclusions are common. Groundmass is glassy and very light 

gray to white. 

K-Ar age of rhyolite lava flow exposed in Summit Creek is 1,273±7 ka. Ages of other 

rhyolites in this unit are unknown, but they overlie andesites of Dittmar Volcanic Center, 

Stage 2, undivided (ad2) conformably and are probably 1,250–1,400 ka. [MH, RP]

Stage 2

ad2 Andesites of Dittmar Volcanic Center, Stage 2, undivided (early Pleistocene)—Thick 

(30–60 m) lava flows of porphyritic aug-hyp andesite (58–63% SiO2) and sparse dacite 

(63–64% SiO2) that covered the flanks of the volcano. Flows exposed in south face of upper part of Saddle Mountain are generally thinner (10–20 m). Erupted from vents probably 

located on upper slopes or at the summit of volcano and not preserved. These flows dip 

radially away from core area of Dittmar Volcanic Center, which was centered just southwest 

of Saddle Mountain. Intercalated pyroclastic material generally is lacking. Largest exposures 

are in fault blocks of Kelly Mountain and the ridge southwest of Willow Creek, and on north 

slopes of Saddle Mountain. Typical outcrops are glacial cliffs exposing flow interiors that are 

thinly platy jointed and locally columnar jointed. Flow tops are composed of oxidized and 

vesicular, rounded blocks. Phenocrysts: 20% pl, 0.5–2 mm; 6–8% hyp, mostly 0.5–1, but to 2 

mm; 2–4% aug, mostly 0.5–1, but to 2 mm. Xenocrystic ol locally present in trace amounts. 

Glomeroporphyritic clots of pl+aug+hyp±ol generally 5 mm–1 cm are abundant and diagnostic. Mafic inclusions containing pl+pyr phenocrysts are present in some flows but are generally small and sparse. Groundmass is typically light gray, but often pinkish. Samples from 

flow interiors are aphanitic and usually show devitrification of glass to cryptofelsite. Three 

related units, andesite of Benner Creek (ab), andesite of Doe Mountain (adm), and andesite 

of Domingo Spring (ads) are correlated with ad2. K-Ar age of one sample is 1,398±42 ka. 

[MH, PP, RP, ST]

adm Andesite of Doe Mountain (early Pleistocene)—Small dome-shaped mass of porphyritic 

aug-hyp andesite correlated with andesites of Dittmar Volcanic Center, Stage 2, undivided 

(ad2). Probably erupted from a vent now buried by dome. Glaciation and erosion modified 

original morphology of dome. Phenocrysts: 15% pl, 1–5 mm; 4% hyp, mostly 0.5–1, but to 

2 mm; 2% aug, mostly 0.5–1, but to 2 mm. Glomeroporphyritic clots of pl+aug+hyp, generally ~5 mm, are abundant. Mafic inclusions containing pl+pyr phenocrysts are present but 

generally small and sparse. Groundmass is light gray, glassy to aphanitic, and dense. Unit 

overlies andesites of Maidu Volcanic Center, Stage 1, undivided (Tam), andesites of Wild 

Cattle Mountain (Taw), and dacites of Maidu Volcanic Center, Stage 2, undivided (Tdm).

Although correlated with Dittmar Volcanic Center, that relation is not clear, and adm may 

be associated with Maidu Volcanic Center. Age is similar to ad2 (1,398±42 ka) and probably 

1,400–1,600 ka. [CM, MN]

ads Andesite of Domingo Spring (early Pleistocene)—Small dome-shaped mass of porphyritic 

aug-hyp andesite (61.9% SiO2) probably correlates with andesites of Dittmar Volcanic 

Center, Stage 2, undivided (ad2). Probably erupted from a vent now buried by dome. Faulting and glaciation modified original morphology of dome. Phenocrysts: 20% pl, 0.5–2 mm; 

6% hyp, mostly 0.5–1, but to 2 mm; 4% aug, mostly 0.5–1, but to 2 mm. Glomeroporphyritic clots of pl+aug+hyp, generally 5 mm–1 cm are abundant. Mafic inclusions containing 

pl+pyr phenocrysts are present but generally small (to 10 cm) and sparse. Groundmass is 

light gray, glassy to aphanitic, and sparsely microvesicular. Age is similar to ad2 (1,398±42 

ka) and probably 1,400–1,600 ka. [MH, ST]
46

ab Andesite of Benner Creek (early Pleistocene)—Thick (30–60 m) block lava flows of porphyritic hyp-aug andesite exposed in north end of graben occupied by Lake Almanor. Exposures are unglaciated and have conspicuous rounded vesicular boulders forming carapace 

of flow. Base of flow is black, glassy, dense vitrophyre. Small scattered exposures beneath 

till north of mapped exposures indicate additional flow is buried beneath till. Phenocrysts: 

10–15% pl, 0.5–4 mm, mostly 1–2 mm; 3% aug, 0.5–1 mm, few to 2 mm; 1% hyp, 0.5–1 

mm. Groundmass is medium to dark gray and glassy to aphanitic. Glomeroporphyritic clots 

of pl+aug+hyp to 5 mm are common; small andesitic inclusions are sparse. Correlated with 

andesites of Dittmar Volcanic Center, Stage 2, undivided (ad2, 1,398±42 ka) and is probably 

similar age (1,400–1,600 ka). [CH, ST]

Stage 1

Tad Andesites of Dittmar Volcanic Center, Stage 1, undivided (late Pliocene to early 

Pleistocene)—Early part of Dittmar composite cone is composed of lithologically diverse, 

generally thin, basaltic andesite to andesite lava flows and fragmental deposits and sparse 

rhyodacite pyroclastic deposits. Flows range in composition from 55.7 to 68% SiO2, but the 

majority contain 55.7–58.4 SiO2. Rhyodacite ash flow (68.4% SiO2) is exposed on north 

flank of Kelly Mountain. Base of Dittmar Volcanic Center section may be exposed in Hot 

Springs Creek and at base of Saddle Mountain, where andesite flows overlie hydrothermally altered, thick dacite and rhyodacite lava flows and domes(?) that may be older than 

Dittmar Volcanic Center. Relations among these rocks are obscure, and they have not been 

mapped separately. Andesite flows are generally porphyritic, typically containing 10–20% 

phenocrysts, although some sparsely porphyritic and a few nearly aphyric lavas are present. Phenocryst size is variable but tends to be 0.5–2 mm; ol-aug, ol-aug-hyp, aug-hyp, 

and hyp-aug assemblages are present. Pl is nearly ubiquitous and typically dominates the 

phenocryst assemblage, but pl-poor lavas also are present. Glomeroporphyritic clots of pl 

and mafic minerals are common. Many pyr andesite lavas contain ol xenocrysts; hbl is rare. 

Rhyodacite ash flow is sparsely porphyritic, contains aug, hyp, and pl phenocrysts, and is 

variably welded. In lower part of section, it is common for adjacent flows to have different 

phenocryst assemblages and (or) abundances. Groundmasses are generally medium to dark 

gray and aphanitic to glassy. Higher in section, andesites typically occur as packages of 

similar flows, but adjacent packages have different lithologies. Hydrothermal alteration and 

poor exposure limit recognition of fragmental deposits (which are not mapped separately). 

These range from fallout to lithic pyroclastic flow deposits; lahar deposits are common, and 

pumiceous pyroclastic flows are sparsely present in Kelly Mountain area. K-Ar ages of three 

samples are 1,650±35 ka, 1,785±35 ka and 2,315±29 and indicate a long eruptive history for 

early phase of Dittmar Volcanic Center. [MH, PP, RP, ST]

Taw Andesites of Wild Cattle Mountain (late Pliocene)—Variety of thin, porphyritic, hyp-aug-ol, 

hyp-ol-aug, and hyp-aug andesite (59.6% SiO2) lava flows correlated with Dittmar Volcanic 

Center. Wild Cattle Mountain comprises a series of south-dipping andesite lava flows; vent 

locations are unknown but presumably were to the northeast and are not preserved. Flow 

contacts and interflow breccias are rarely exposed. Includes bedded pyroclastic rocks near 

base of section on Wild Cattle Mountain. Abundance and size of phenocrysts is variable; 

composition more variable than single analysis indicates. Includes coarsely porphyritic, 

propylitically altered aug andesite lava flows north of Feather River Meadows. Phenocrysts 

(Wild Cattle Mountain): 6–12% pl, 0.5–5 mm, mostly 0.5–2 mm; 2–4% aug, 0.5–2 mm, 

mostly 0.5–1 mm; 1–3 % ol, 0.5–2 mm, mostly 0.5–1 mm; 1–3 % hyp, 0.5–2 mm, mostly 

0.5–1 mm. A few flows lack larger pl phenocrysts; some lack ol. Glomeroporphyritic 

clots of pl+aug+hyp±ol are common to abundant. In some flows, many phenocrysts were 

derived from disaggregated glomeroporphyritic clots. Small quenched andesitic inclusions are sparse in some flows. Groundmass is light to medium gray, aphanitic, and dense. 

Cores of lava flows generally have thin platy jointing, but some are massive. Phenocrysts 

(Feather River Meadows): 8–12% pl, 0.5–6 mm, mostly 0.5–3 mm; 2–3% aug, 0.5–2 mm, 

few to 5 mm. Glomeroporphyritic clots of pl+aug to 5 mm are common. Rock is weakly 

altered; groundmass and some phenocrysts are partially replaced by chlorite, calcite, 

and qtz. Groundmass is greenish dark gray, aphanitic and dense. Unit overlies andesites 

of Maidu Volcanic Center, Stage 1, undivided (Tam). Unit is overlain by rhyodacite of 
47

Gurnsey Creek (rg), rhyolite of Lost Creek Plateau (rlp, 1,305±10 ka), rhyolite of North 

Stover Mountain (rsm), andesite of Rice Creek (ar, 477±14 and 485±12 ka), and tholeiitic 

basalt of Ice Cave Mountain (bic). The lithology and composition are similar to lava flows 

exposed in cliffs of Saddle Mountain mapped as andesites of Dittmar Volcanic Center, 

Stage 1, undivided (Tad). Unit is interpreted to correlate with upper part of Tad and to be 

southwest flank of Dittmar Volcanic Center. Age is poorly constrained but is likely to be 

~1,600–2,000 ka. [CM, LP, MN, RP]

LATOUR VOLCANIC CENTER

Small part of Latour Volcanic Center (mostly outside map area north and west of North Battle Creek 

Reservoir) is located in north part of Viola quadrangle. Latour Volcanic Center appears to consist of an 

andesitic volcano at Latour Butte and flanking silicic rocks similar to other volcanic centers in Lassen area.

Tlv Rocks of Latour Volcanic Center (late Pliocene)—Dominated by porphyritic hbl-bt rhyolite, bthbl dacite, and hyp-aug andesite. Constructional morphology is poorly preserved, and vents 

have not been located. Obscured by thick soils. In area west of North Battle Creek Reservoir, 

most abundant rock is porphyritic hbl-bt rhyolite (74.9% SiO2); most of Latour Volcanic 

Center in map area consists of this lithology. Rhyolite is a thick flow that erupted from vicinity of Buck Butte (Viola quadrangle) and flowed south into map area. Phenocrysts: 12% pl, 

0.5–3 mm, mostly 1 mm; 6% qtz, 0.5–2 mm; 1% bt, 0.5–2 mm, mostly 0.5 mm; sparse hbl, 

1 mm. Small, quenched magmatic inclusions containing 1 mm aug phenocrysts are common. 

Groundmass is light gray, glassy, and dense when fresh, but typically light to medium gray, 

devitrified, and locally spherulitic. Often flow banded, massively jointed, and usually deeply 

weathered. 

Underlying rhyolite north of North Battle Creek Reservoir is poorly exposed porphyritic 

bt-hbl dacite. Phenocrysts: 15% pl, 0.5–4 mm, larger crystals are composite; 2–3% hbl, 

1–3 mm; 1% qtz, 0.5–1 mm; sparse bt, 1 mm; sparse hyp, 0.5–1 mm. Most hbl and bt are 

reacted and pseudomorphed by pyr, oxides, and pl; qtz crystals are rounded. Small quenched 

magmatic inclusions are common. Groundmass is aphanitic and microvesicular. When fresh, 

rock is light gray, but often it is oxidized to a pinkish-gray cast. 

Most common andesitic lithology is porphyritic hyp-aug andesite that probably underlies 

other Latour Volcanic Center rocks in map area. Phenocrysts: 15–25% pl, 0.5–4 mm, many 

with gray cores; 3–5% hyp, 0.5–1 mm; 2-4% aug, 0.5–2 mm. Glomeroporphyritic clots of 

pl+hyp+aug are common. Groundmass is dark gray to black, glassy to aphanitic, and dense. 

K-Ar age of andesite lava flow is 3,140±67 ka and indicates that Latour Volcanic Center is 

much older than the overlying lavas mapped as older regional andesites and basaltic andesites, undivided (am). [VI]

TUSCAN FORMATION

Tt Tuscan Formation (late Pliocene)—Within the map area, the Tuscan Formation is exposed only 

in extreme southwest corner, where the deposits were derived from Yana Volcanic Center 

(fig. 4A, south of map area). Deposits are predominantly debris-flow breccias composed of 

angular to subrounded fragments of volcanic rock, as much as 3 m in diam but mostly from 

a few to 20 cm, in a matrix of gray-tan volcanic sand and silt. Individual debris flows 0.5 to 

10 m thick deposited as sheets over gently sloping terrain or as lenses in drainages. Debrisflow deposits are poorly sorted and often reversely graded, and upper and lower contacts 

are usually sharp. Thin weathered zones occur between some debris-flow deposits. Tuscan 

debris-flow deposits are strongly to weakly consolidated. Most debris flows were generated as direct result of eruptive processes, and many were probably emplaced hot or warm. 

Consolidated debris-flow deposits are resistant to erosion and form benches on steep canyon 

walls. A variety of clasts are present, but porphyritic ol and ol-aug basaltic andesites and pyr 

andesites are most common. In and just south and west of map area, unit consists of only 

uppermost part of Tuscan Formation and is overlain by rocks of Maidu Volcanic Center. 

Equivalent rocks in Yana Volcanic Center range in age from ~3,300 to ~2,700 ka; overlying 

Maidu Volcanic Center rocks are as old as ~2,400 ka. Tuscan Formation in map area was 

probably deposited close to 2,700 ka. [LY]
48

REGIONAL VOLCANIC ROCKS

NORTH AND WEST OF LASSEN VOLCANIC CENTER

Includes rocks related to regional volcanism (calc-alkaline and tholeiitic) erupted from monogenetic 

volcanoes and small- to moderate-sized lava cones and shield volcanoes of all ages in axis of Cascade Arc in 

area north and west of Lassen Volcanic Center. Area approximately bounded on east by trace of Hat Creek 

Fault-Lake Almanor Graben system. Major features include Tumble Buttes and Sugarloaf chains of monogenetic cones and Prospect Peak, West Prospect Peak, Badger Mountain, Table Mountain, and Red Lake 

Mountain shield volcanoes.

Late Pleistocene volcanic rocks

Late Pleistocene volcanic rocks (11.7–125 ka) in the areas north and west of Lassen Volcanic Center. 

Includes the Tumble Buttes and Sugarloaf chains of small, monogenetic cinder cones and lava flows, a few 

larger lava cones and shield volcanoes like Red Mountain and Red Lake Mountain, and the voluminous tholeiitic Hat Creek Basalt lava flow. These young volcanoes are well-preserved constructional features that are 

not disrupted by faults and are unglaciated to weakly glaciated.

bhc Hat Creek Basalt (late Pleistocene)—Sparsely porphyritic, tholeiitic, ol basalt (48.1–49.0% 

SiO2) lava-flow complex. This tube-fed pahoehoe lava field consists of many flow units 

erupted from a 3 km-long fissure capped by spatter cones, just south of Old Station. Lava 

flowed north into and filled Hat Creek Graben for 20 km to Cassel area (north of map area; 

fig. 1). Phenocrysts: trace–2% ol, 0.5–2 mm, few larger; 0–5% pl, 0.5–2 mm, few larger. 

Bulk of lava is sparsely porphyritic; phenocrysts are more abundant in earliest-erupted lava 

found near vent fissure, and this lava locally contains glomeroporphyritic clots of ol+pl to 

5 mm. Groundmass is light to medium gray and holocrystalline and is typically a coarsely 

diktytaxitic intergrowth of pl, ol, pyr, and Fe-Ti oxide microphenocrysts. Lava typically has 

hexagonal block-joint pattern at top and massive jointing in interior of flow units. Flow has 

abundant round vesicles at upper and lower surfaces. Surface of flow is rough and sparsely 

vegetated, and tumuli and pahoehoe surfaces are common. Cut by most recent episode of 

movement on Hat Creek Fault, but scarps as high as 30 m high do not expose the base of the 

flow (Muffler and others, 1994). Unsubstantiated information from local water-well drillers 

suggests that flow may be as thick as 50 m in center of Hat Creek Graben; its volume is substantial, perhaps as much as 5 km3

. Outwash gravel of middle Wisconsin age (outwash gravel 

of Anklin Meadows, Qoa) overlies lava flow (Muffler and others, 1994). Outwash gravels 

related to older glacial advances are not present, and flow is constrained to between 15 and 

~40 ka. North of map area, flow overlies andesite of Sugarloaf Peak (ass, 46±7 ka) and lavas 

from Cinder Butte (north of map area) that were dated as 38±7 ka (Turrin and others, 2007). 

40Ar/39Ar age of 24±6 ka (Turrin and others, 2007) is consistent with the stratigraphy. [OS]

Tholeiitic basalts of Big Lake (late Pleistocene)—Two units comprise a zoned porphyritic ol 

basalt lava-flow and cinder-cone complex (48.6–50.6% SiO2), partly north of map area. 

Unit 1 (bk1) erupted from a vent in the vicinity of hill 6051. Subsequently, scoria of unit 

2 (bk2) built cinder cones and buried bk1 vent; followed by emplacement of several lava 

flows. Lava flows fill a small graben and dammed drainage to form Big Lake (just north of 

map area; see fig. 3). One flow crossed basin now filled by North Battle Creek Reservoir 

and flowed 2 km down North Fork Battle Creek. Lava flows cross several regional faults 

but are not offset by them. Cinder cones have a soil several meters thick. Both units overlie 

older regional andesites and basaltic andesites, undivided (am), but age is otherwise poorly 

constrained. Age is estimated to be 50–75 ka. [MZ, TH, VI]

bk2 Unit 2—Moderately thick lava flows (10–30 m) and cinder cones of porphyritic ol basalt 

(48.6% SiO2). Phenocrysts: 5% ol, 0.5–1 mm, few larger; 5% pl, 0.5–2 mm, mostly ~1 mm. 

Most phenocrysts are part of glomeroporphyritic clots of 0.5–1 mm ol+pl, which are abundant and as large as 1 cm. Groundmass is dark gray, aphanitic, and extremely microvesicular; jointing is blocky to massive. [MZ, TH, VI]

bk1 Unit 1—Moderately thick flow (10–20 m) and cinder cone of sparsely porphyritic ol basalt 

(50.6% SiO2). Phenocrysts: 5% ol, 0.5–1 mm, few larger; 2% pl, 0.5–1 mm. Glomeroporphyritic clots of 0.5–1 mm ol+pl are present and as large as 5 mm, but are much less abundant than 

in bk2. Groundmass is dark gray, aphanitic, and microvesicular; jointing is blocky. [MZ, VI]
49

meh Basaltic andesite of Eskimo Hill (late Pleistocene)—Porphyritic, ol-aug basaltic andesite 

(54.5% SiO2) lava flow and cinder cone satellitic to Red Lake Mountain. Lava flowed from 

cinder cone south for 1 km. Aa flow surface is unglaciated and sparsely vegetated. Phenocrysts: 25% pl, generally 0.5–1 mm, a few to 1.5 mm; 8% bright-green diopsidic aug, 

generally 1 mm, a few to 2 mm; 5% ol, mostly 1 mm, a few to 2 mm. Small clots of ol+aug 

are common. Groundmass is medium gray, glassy to aphanitic, and microvesicular. Unit 

is presumed to be coeval with lithologically identical early stage of basaltic andesite and

andesite of Red Lake Mountain (mrd). It is slightly older than pumiceous pyroclastic-flow 

deposits (pe, 66±4 ka) from Eagle Peak and is estimated to be ~75 ka. [MZ]

mrd Basaltic andesite and andesite of Red Lake Mountain (late Pleistocene)—Thin aa to blocklava flows and large scoria cone of porphyritic ol-aug basaltic andesite (54.0–54.4% SiO2

). 

Relatively small volume of late-stage hyp-ol-aug andesite (58.8% SiO2) forms bocca on 

northwest side of cone (not mapped separately). Lava erupted from cone and flowed nearly 

10 km west, buried faults in older terrane, and filled topography. Flow surfaces are rough 

and unglaciated with weakly developed soil and sparse vegetation. Phenocrysts (early stage): 

25% pl, 0.5–1 mm, few to 1.5 mm; 8% bright green diopsidic aug, 1 mm, few to 2 mm; 5% 

ol, 1 mm, few to 2 mm. Phenocrysts (late stage): 20% pl, 0.5–1 mm, few to 1.5 mm; 8% 

bright green diopsidic aug, 1 mm, few to 2 mm; 2% ol, 1 mm, few to 2 mm; 1% hyp, < 0.5 

mm. Small clots of ol+aug are common. Groundmass is medium gray, glassy to aphanitic, 

and microvesicular. Overlies older regional andesites and basaltic andesites, undivided (am), 

andesite of Table Mountain (at), andesite of Manzanita Creek (amz), tholeiitic basalt of 

Eagle Canyon (bec, 199±22 ka), avalanche deposit from dacite of hill 8283 (Qs82), basalt 

of Cherry Thicket (bct), andesite of Viola (av, 313±8 ka), and basaltic andesite of Red 

Mountain (mrm) and is overlain by slightly younger pumiceous pyroclastic-flow deposits 

(pe, 66±4 ka) from Eagle Peak; age estimated to be ~75 ka. [MZ, VI]

mrm Basaltic andesite of Red Mountain (late Pleistocene)—Thick aa to block-lava flows and scoria 

cone of porphyritic ol-aug basaltic andesite (54.5% SiO2). Lava flows have rough, unglaciated flow surfaces that have considerably better developed soil and more forest than overlying basaltic andesite and andesite of Red Lake Mountain (mrd). Lava erupted from cone 

of Red Mountain and flowed for as much as 6 km east. Lava buried faults in older regional 

andesites and basaltic andesites, undivided (am) and filled topography. Phenocrysts: 7% 

bright green diopsidic aug, 1 mm, few to 2 mm; 3% ol, 1 mm, few to 2 mm. Small clots of 

ol+aug are common; qtz xenocrysts are sparse. Pl microphenocrysts, generally to 0.5 mm, 

but with a few to 1 mm, compose ~30% of rock. Groundmass is medium gray, aphanitic, and 

microvesicular. Unit overlies andesite of Grayback Ridge (agr), andesite of Table Mountain 

(at), andesite of Ashpan Butte (aa), and andesite of Raker Peak (arp, 270±18 ka). Unit is 

overlain by basaltic andesite and andesite of Red Lake Mountain (mrd) and pumiceous pyroclastic-flow deposits (pe, 66±4 ka) from Eagle Peak. Its relation with andesite of Tumble 

Buttes (att) is unclear, but it is probably older. Age is estimated to be ~75–100 ka. [MZ] 

Tumble Buttes chain

The Tumble Buttes chain comprises a linear zone of cinder cones and lava flows 12 km long trending 

N. 15° W., approximately coincident with the Cascade Arc axis a few kilometers north of the Lassen Volcanic Center. The linear array of vents is parallel to nearby faults and probably reflects an underlying fault that 

served as a conduit for magma. Ten mapped units of basaltic andesite and andesite occur on the map, and 

three additional units are present north of the map area. The Tumble Buttes chain lies mostly outside the area 

of the youngest glaciation, and cinder cones and flow surfaces are well preserved. The similar lithology and 

composition of Tumble Buttes chain units suggest that the chain represents a relatively short lived episode 

of volcanism. Based on a 40Ar/39Ar age for the andesite of Bear Wallow Butte (abt, 35.1±3.1 ka), 9 of the 13 

units are estimated to be 35–50 ka in age. A few units are interpreted to be slightly older, 50–75 ka, and andesite of Mud Lake (mtm) may be as old as about 100 ka. The youngest unit, andesite of Devils Rock Garden 

(atd), is one of the youngest regional lava flows in the Lassen area and is probably 10–15 ka. The interpreted 

eruptive order of the Tumble Buttes chain units is based on known stratigraphic relations. However, some 

units do not have definitive stratigraphic relations, and the eruptive order may be subject to revision.

atd Andesite of Devils Rock Garden (late Pleistocene)—Thick (to 60 m), sparsely porphyritic olaug andesite (58.2% SiO2

) block-lava flow. Flow has conspicuous irregular flow levees and 
50

rugged surface with significant relief. Vent marked by small parasitic cinder cone on south 

flank of southern of two cones called Tumble Buttes, just north of map boundary. Phenocrysts: 3% pl, 0.5–1 mm; 1% aug, 0.5–1 mm; sparse ol, 0.5–1 mm. Glomeroporphyritic 

clots of all three phenocryst phases, dominated by pl, are sparse and as large as a few millimeters across. Groundmass is medium gray, glassy to aphanitic, and sparsely microvesicular. 

Rock has flow layering defined by thin oxidized zones of vapor-phase crystallization. Age is 

unknown, but it is one of youngest flows in Lassen area as indicated by lack of soil and forest cover; Macdonald (1963) suggested that it might be Holocene. Overlies all adjacent lava 

flows including andesite of Logan Mountain (alg) and basaltic andesites of Magee Volcano 

(mm), and other flows of Tumble Buttes chain: basaltic andesite of hill 6770 (mt67), basaltic andesite of Tumble Buttes (mtu), basaltic andesite of Mud Lake (mtm), basaltic andesite 

of section 5 (mt5), and basaltic andesite of hill 6138 (mt61). Relation to glacial deposits is 

unclear, but unit probably is younger than till, younger glaciations (Qty, ~17–35 ka) and outwash gravel, younger glaciations (Qoy, ~15–18 ka in this area). Estimated to be 10–15 ka.

[TH]

atb Andesite of Bear Wallow Butte (late Pleistocene)—Thick block-lava flow with as much as 

40-m-high flow fronts of porphyritic aug-ol andesite (57.0% SiO2). Unit is southernmost of 

youthful Tumble Buttes chain of volcanoes. Scoria cone marks vent, and lava flowed mostly 

to south and east for as much as 3 km. Lava flow is unglaciated and, although it supports a 

few mature trees near margins, is generally devoid of soil or vegetation. Phenocrysts: 5% 

pl, generally 0.5 mm but occasionally to 1.5 mm; 3% ol, 1–2 mm; 2% aug, 0.25–0.5 mm. 

Glomeroporphyritic clots of pl+ol+aug, 1–10 mm, are abundant. Groundmass is microvesicular, dark gray, and glassy to aphanitic. Overlies all adjacent lava flows including older regional andesites and basaltic andesites (am), tholeiitic basalt of Twin Bridges (btb), andesite 

of Ashpan Butte (aa), and basaltic andesite of Red Mountain (mrm). Overlies other Tumble 

Buttes chain flows including basaltic andesite of Mud Lake (mtm), basaltic andesite of section 5 (mt5), basaltic andesite of hill 6138 (mt61), basaltic andesite of Bear Wallow Butte 

(mtb), basaltic andesite of hill 5410 (mt54), and andesite of Tumble Buttes (att). 40Ar/39Ar 

age is 35.1±3.1 ka. [MZ, OS, TH, WP]

mt67 Basaltic andesite of hill 6770 (late Pleistocene)—Thick (20–40 m) aa to block lava flow of 

porphyritic ol-aug basaltic andesite (56.3% SiO2). Vent marked by the northern of two cinder 

cones labeled Tumble Buttes (hill 6770; off map area to north). Lava from cone flowed 4 km 

southwest. Has well-defined marginal flow levees and a rugged surface with significant relief. 

Phenocrysts: 7% pl, 0.5–1 mm, few to 2 mm; 1–2% ol, 0.5–1 mm; 1–2% aug, 0.5–1 mm. 

Glomeroporphyritic clots of all three phenocryst phases, but dominated by pl, are sparsely 

present and as large as 5 mm. Groundmass is medium gray, glassy to aphanitic, and microvesicular. Irregular vesicles are abundant. Age is not well constrained, but it underlies andesite 

of Devils Rock Garden (atd) and overlies basaltic andesite of Tumble Buttes (mtu). Lack of 

well-developed soil and forest cover indicates relative youth; estimated to be 35–50 ka. [TH]

mtu Basaltic andesite of Tumble Buttes (late Pleistocene)—Thin aa to block-lava flow (5–15 m) 

of porphyritic ol basaltic andesite (55.7–56.3% SiO2). Vent was in area of northern of two 

cinder cones labeled Tumble Buttes (hill with elevation 6770, north of map area) and was 

probably buried by basaltic andesite of hill 6770 (mt67). Lava flowed for 2 km from vent 

and has a rugged, sparsely forested surface. Phenocrysts: 5% ol, 0.5–3 mm; 3% pl, 0.5–2 

mm; sparse aug, 0.5–1 mm. Many phenocrysts occur as small glomeroporphyritic clots of 

ol+pl. Groundmass is dark gray to black, glassy to aphanitic, and sparsely microvesicular. 

Age is unknown, but lack of well-developed soil or forest cover indicates relative youth. It 

is clearly older than andesite of Devils Rock Garden (atd) and basaltic andesite of hill 6770 

(mt67) and is probably slightly older than andesite of Bear Wallow Butte (atb). Estimated 

age is 35–50 ka. [TH]

mt54 Basaltic andesite of hill 5410 (late Pleistocene)—Thin (5–10 m) aa to block-lava flow of porphyritic ol-aug basaltic andesite. Vent marked by cinder cone just east of Bear Wallow Butte. 

Second cone lies ~300 m to east. Lava flowed northeast for ~1 km and partially buried older 

faulted terrane of older regional andesites and basaltic andesites (am). Part of flow probably 

buried by andesite of Bear Wallow Butte (atb). Phenocrysts: 10% pl, 0.5–1 mm, few to 2 

mm; 6% aug, 0.5–2 mm; 2% ol, 0.5–1 mm, few to 2 mm. Glomeroporphyritic clots of all 

three phenocryst phases, dominated by pl, are common and as large as 5 mm. Groundmass 
51

is medium gray, glassy to aphanitic, and microvesicular. Vesicles with irregular shapes are 

abundant. Overlies older regional andesites and basaltic andesites (am), basaltic andesite of 

section 5 (mt5), and andesite of Tumble Buttes (att). Age is poorly constrained, but unit is 

overlain by atb (35.1±3.1 ka). Lack of well-developed soil and forest cover indicates relative youth; estimated to be 35–50 ka. [TH]

mtb Basaltic andesite of Bear Wallow Butte (late Pleistocene)—Thin (5–10 m), aa to block-lava 

flow of sparsely porphyritic aug-ol basaltic andesite (56.5% SiO2). Vent marked by cinder 

cone just north of Bear Wallow Butte. Lava flowed from vent to both east and west for ~3 

km. Andesite of Bear Wallow Butte (atb) partly buries the flow. Phenocrysts: 2% ol, 0.5–2 

mm; 2% pl, 0.5–2 mm; sparse aug, as large as ~0.5 mm. Glomeroporphyritic clots of all 

three phenocryst phases, dominated by pl, are common and as large as 5 mm. Groundmass is 

dark gray, glassy to aphanitic, and microvesicular. Vesicles with irregular shapes and smooth 

walls are abundant. Overlies basaltic andesite of Mud Lake (mtm), basaltic andesite of 

section 5 (mt5), and basaltic andesite of hill 6138 (mt61). Age is not well constrained, but 

unit is overlain by atb. Nevertheless, lack of well-developed soil and forest cover indicates 

relative youth; estimated age is 35–50 ka. [TH]

mt61 Basaltic andesite of hill 6138 (late Pleistocene)—Thin (5–15 m) aa to block lava flow of porphyritic ol-aug basaltic andesite (55.2–55.6% SiO2

). Vent marked by cinder cone just north 

of Bear Wallow Butte. Lava flowed for ~2 km southeast and was subsequently partially 

buried by andesite of Devils Rock Garden (atd). Phenocrysts: 14% pl, 0.5–2 mm; 4% aug, 

0.5–2 mm; 2% ol, 0.5–1 mm, few to 2 mm. Glomeroporphyritic clots of all three phenocryst 

phases, dominated by pl, are common and as large as 5 mm across. Groundmass is medium 

gray, glassy to aphanitic, and microvesicular. Irregular vesicles are abundant. Lava flow buries faulted older regional andesites and basaltic andesites (am) and is unglaciated. Overlies 

basaltic andesite of Mud Lake (mtm) and basaltic andesite of section 5 (mt5) and is overlain 

by basaltic andesite of Bear Wallow Butte (mtb), andesite of Bear Wallow Butte (atb), and 

andesite of Devils Rock Garden (atd). Lack of well-developed soil and forest cover indicates relative youth, and unit is estimated to be ~50 ka. [TH]

mt5 Basaltic andesite of section 5 (late Pleistocene)—Thick (20–30 m) aa to block lava flow of 

porphyritic aug-ol basaltic andesite (55.9% SiO2). Flow has well-defined marginal flow levees and rugged oxidized and clinkery surface with significant relief. Vent marked by cinder 

cone kipuka surrounded by andesite of Devils Rock Garden (atd). Lava from vent emerges 

from beneath atd and can be traced for 2 km southeast. Phenocrysts: 12% pl, mostly 0.5–2 

mm, few to 4 mm; 6% ol, 0.5–2 mm; 2–3% aug, 0.5–2 mm. Majority of phenocrysts occur 

in glomeroporphyritic clots of all three phenocryst phases, dominated by pl, and are as large 

as 8–10 mm. Groundmass is dark gray, aphanitic, and microvesicular. Unit overlies andesite 

of Logan Mountain (alg) and is overlain by andesite of Devils Rock Garden (atd), basaltic 

andesite of Bear Wallow Butte (mtb), and basaltic andesite of hill 6138 (mt61). Lack of 

well-developed soil and forest cover indicates relative youth; estimated to be ~50 ka. [TH]

att Andesite of Tumble Buttes (late Pleistocene)—Thin (5–10 m), porphyritic ol-aug andesite 

(57.7% SiO2) aa to block-lava flow erupted from vent subsequently buried by andesite of 

Bear Wallow Butte (atb). Lava flowed southeast for ~4 km. Large irregular vesicles that 

commonly contain opaline silica are characteristic. Flow is unglaciated and has a poorly 

developed soil and forest cover. Phenocrysts: 2–3% ol, 1–2 mm; 2–3% pl, mostly 0.5–1 mm, 

with a few to 1.5 mm; trace aug to ~0.5 mm. Glomeroporphyritic clots of all three phenocryst phases, dominated by pl, are abundant and as large as 5 mm across. Groundmass is 

black and glassy to aphanitic. Overlies older regional andesites and basaltic andesites (am) 

and basalt of Twin Bridges (btb) and is overlain by andesite of Bear Wallow Butte (atb) 

and basaltic andesite of hill 5410 (mt54). Probably younger than basaltic andesite of Red 

Mountain (mrm). Age is poorly constrained, but lack of well-developed soil and forest cover 

indicates relative youth. Unit is probably not substantially older than atb (35.1±3.1 ka); 

estimated to be 50–75 ka. [MZ, OS, TH, WP]

mtm Basaltic andesite of Mud Lake (late Pleistocene)—Thin (5–10 m) aa to block-lava flow of 

porphyritic, aug-ol basaltic andesite. Vent unlocated, probably buried beneath andesite of 

Devils Rock Garden (atd). Phenocrysts: 2% pl, 0.5–1 mm; 1% ol, 0.5–1 mm; sparse aug, 

0.5–1 mm. Glomeroporphyritic clots of all three phenocryst phases, dominated by pl, are 

sparse and as large as 5 mm across. Groundmass is medium gray, glassy to aphanitic, and 
52

microvesicular. Vesicles with irregular shapes are abundant. Unit overlain by all adjacent 

rock units: andesite of Devils Rock Garden (atd), basaltic andesite of hill 6138 (mt61), 

basaltic andesite of Bear Wallow Butte (mtb), and andesite of Bear Wallow Butte (atb). Unit 

is unglaciated. Age is poorly constrained, but better-developed soil indicates one of oldest 

lavas in Tumble Buttes chain. Estimated to be 75–100 ka. [TH]

Sugarloaf chain

The Sugarloaf chain comprises a linear zone of cinder cones and lava flows 12 km long trending N. 15° 

W. approximately coincident with the west boundary of the Hat Creek Graben. The linear array of vents is 

parallel to nearby faults and probably reflects an underlying fault that served as a conduit for magma. Five 

mapped units of basaltic andesite and andesite occur on the map, and three additional units are present north 

of the map area. The Sugarloaf chain lies mostly outside the area of the youngest glaciation, and cinder cones 

and flow surfaces are well preserved; some smaller cones are partly buried by the basalt of Hat Creek. The 

Sugarloaf chain is a young volcanic feature, and the similar lithology and composition of the eight units suggest that the chain represents a relatively short-lived episode of volcanism. 40Ar/39Ar ages (Turrin and others, 

2007) are available for andesite of Sugarloaf (ass, 46±7 ka), basaltic andesite of Little Potato Butte (msl, 

67±4 ka), and andesite of Potato Butte (asp, 77±11 ka). The remaining units are estimated to be slightly 

older, perhaps as old as 100 ka. The interpreted eruptive order of units of the Sugarloaf chain is based on the 

40Ar/39Ar ages and known stratigraphic relations. However, some units do not have definitive stratigraphic 

relations, and the eruptive order may be subject to revision.

ass Andesite of Sugarloaf Peak (late Pleistocene)—Thick block-lava flows of sparsely porphyritic 

aug-hyp andesite (59.3–60.6% SiO2) erupted from steep-sided lava cone of Sugarloaf Peak 

near Old Station. Flow fronts are steep and as high as 100 m. Scoria cone at the summit 

marks vent just north of map area (see fig. 3). Phenocryst content is variable from ~2–10%. 

Phenocrysts: 1–5% pl, mostly 0.5 mm, few larger; 1–3% hyp, mostly 0.5 mm, few larger; 

sparse–2% aug, 0.5 mm. Most phenocrysts form glomeroporphyritic clots of pyr+pl as large 

as 1 cm. Groundmass is black and glassy to dark gray and aphanitic. Vesicular flow tops 

have flattened and elongated or irregular-shaped vesicles. Flow interiors are dense with 

blocky jointing. Flows have little or no soil and support only sparse forest. Flow buries 

faults along the west side of the Hat Creek Graben, but is unfaulted. 40Ar/39Ar age is 46±7 ka 

(Turrin and others, 2007). [OS]

msl Basaltic andesite of Little Potato Butte (late Pleistocene)—Thick (25 m), short, block-lava 

flow of porphyritic ol-aug andesite (54.8% SiO2) near Old Station; two cinder cones mark 

vent. Scoria in both cones is mostly oxidized; one cone is partly buried by lava flow. Lava 

flowed to north for ~1 km. Unit is partly buried by vents and lava of Hat Creek Basalt (bhc). 

Erupted along west side of Hat Creek Graben, but flow is unfaulted. Phenocrysts: 5% aug, 

0.5–1.5 mm, mostly <1 mm; 5% pl, 0.5–1.5 mm, mostly <1 mm; 1% ol, 0.5–1.5 mm, mostly 

<1 mm. Most phenocrysts occur in 3–8 mm glomeroporphyritic clots of ol+aug+pl. Groundmass is light to medium gray and aphanitic and contains abundant flat amoeboid vesicles. 

Unit overlies basaltic andesite of hill 4709 (ms47), is overlain by andesite of Potato Butte 

(asp), and has a thin soil. 40Ar/39Ar age is 67±4 ka (Turrin and others, 2007). [OS]

asp Andesite of Potato Butte (late Pleistocene)—Thick (to 60 m) block-lava flows of porphyritic 

ol-aug andesite (57.9–58.4% SiO2) with two cinder cones that mark vent. Two flows are distinguished by phenocryst content and morphology. Older flow, exposed along north boundary of outcrop area, has less relief and fewer phenocrysts but composition similar to later 

flow. Younger flow forms most of outcrop area. Few small outcrops around east and south 

periphery of younger flow may be similar-appearing older lavas. Erupted along west side of 

Hat Creek Graben, but flow is unfaulted. Phenocrysts (older flow): sparse–2% ol, 0.5–2 mm; 

mostly <1 mm; sparse–2% aug, 0.5–1 mm; sparse–2% pl 0.5–1, mm. Phenocrysts (younger 

flow): 3–5% pl, 0.5–2 mm; 2–3% ol, 0.5–2 mm; 2–3% aug, 0.5–1 mm. Most phenocrysts 

present as abundant small glomeroporphyritic clots of ol+aug+pl, 2–5 mm in size. Groundmass is dark gray to black and glassy with abundant, flat, irregular vesicles. Block-lava 

flows have little or no soil and support only sparse forest. Unit overlain by Hat Creek Basalt 

(bhc, 24±6 ka). 40Ar/39Ar age is 77±11 ka (Turrin and others, 2007). [OS]

ms47 Basaltic andesite of hill 4709 (late Pleistocene)—Thin (10 m), short, block-lava flow of porphyritic aug-ol basaltic andesite (56.0% SiO2

) near Old Station. Cinder cone, consisting mostly 
53

of unoxidized scoria and dense blocks and bombs, marks vent. A deep orange soil is ~1 m 

thick on cone; block lava flows covered with a thin soil. Much of the lava flow is buried by 

flows of Hat Creek Basalt (bhc) and basaltic andesite of Little Potato Butte (msl). Erupted 

along west side of Hat Creek Graben, but flow is unfaulted. Phenocrysts: 3% ol, 0.5–2 mm; 

mostly <1 mm; 1% aug, 0.5–1 mm; sparse pl, 0.5–1 mm. Majority of phenocrysts occur in 

small (to 5 mm) glomeroporphyritic clots of ol+aug+pl. Groundmass is dark gray to black 

and glassy to aphanitic. Unit underlies andesite of Potato Butte (asp) but has not been 

dated; it is probably only slightly older than asp (77±11 ka), (msl, 67±4 ka), and andesite of 

Sugarloaf Peak (ass, 46±7 ka). [OS]

aso Andesites of Old Station (late Pleistocene)—Small group of poorly exposed, sparsely porphyritic ol-aug andesite block lava flows and scoria cones near Old Station that are mostly 

buried by Hat Creek Basalt (bhc). Description is for the largest flow in unit, but others are 

similar. Vent is small cinder cone cut by Calif. Hwy 44. Erupted along west side of Hat 

Creek Graben, but lava flows are unfaulted and have little or no soil cover. Phenocrysts: 

2–3% pl, 0.5–1 mm, few larger; 1–2% aug, 0.5–1 mm; sparse ol, 0.5–1 mm. Glomeroporphyritic clots of ol+aug+pl to 5 mm in size are sparse. Flow has a black glassy to dark gray 

aphanitic, dense to microvesicular groundmass. Age is probably similar to or slightly older 

than basaltic andesite of Little Potato Butte (msl, 67±4 ka), andesite of Potato Butte (asp, 

77±11 ka), and andesite of Sugarloaf Peak (ass, 46±7 ka) and is estimated to be 75–100 ka. 

[OS]o

Middle Pleistocene and older volcanic rocks

Middle Pleistocene and older volcanic rocks (125–1,800 ka) in the areas north and west of the 

Lassen Volcanic Center. Major middle Pleistocene features include Prospect Peak, West Prospect Peak, Badger Mountain, and Table Mountain shield volcanoes. Includes monogenetic cinder cones and lava flows and 

several tholeiitic basalt lava flows. These are mostly glaciated and are more likely to be faulted with increasing age. Early Pleistocene rocks are mostly preserved in fault blocks, including the Hat Creek Fault.

mbg Basaltic andesite of Little Bunchgrass Meadow (middle Pleistocene)—Thick lava flow and 

eroded cinder cone of porphyritic ol-aug basaltic andesite (54.2% SiO2). Lava flowed north 

from cone and filled a deep canyon cut by glacial erosion through andesite and basaltic 

andesite of Cluster Lakes (acl) and basalt of Badger Flat (bbf) and into andesites of Badger 

Mountain, (abm) and andesites of West Prospect Peak (awp). Original surface of proximal part of lava flow was removed by glacial erosion; dense core is exposed. Distal part 

of flow retains oxidized, clinkery upper surface; basal breccia is occasionally exposed. 

Phenocrysts: 10% aug, 0.5–6 mm, larger are composite; 5% ol, 0.5–2 mm, mostly 1 mm; 

1% pl, 0.5–3 mm, mostly 1 mm. Aug is distinctive bright green, and xenocrysts of qtz to 

3–4 mm are ubiquitous accessory. Groundmass is light to dark gray and aphanitic. Jointing ranges from thin platy to massive. Small cliff of similar but older, sparsely porphyritic, 

aug-ol basaltic andesite (55.9% SiO2) is exposed on south side of cinder cone by glacial 

erosion but is not mapped separately from cinder cone. Phenocrysts: 5% ol, 0.5–2 mm; 2% 

aug, 0.5–1 mm. Rock contains abundant pl microphenocrysts (few as large as 1 mm) and 

has medium gray, aphanitic groundmass. Unit is younger than bbf and acl. 40Ar/39Ar age is 

143±6 ka. [WP]

mbx Basaltic andesite of Box Canyon (middle Pleistocene)—Small cinder cone and short, thick 

lava flow of porphyritic ol-aug basaltic andesite (53.7% SiO2) on lower northwest flank of 

West Prospect Peak. Cinder cone of red to brown oxidized cinders and bombs. Lava flow 

erupted from west base of cone and flowed 1 km down flank of West Prospect Peak into Box 

Canyon. Lava filled an old glacial canyon, which was recut during late Wisconsin glaciation. 

Phenocrysts: 15% aug, 0.5–2 mm, composite crystals to 4 mm; 10% ol, 0.5–2 mm. Groundmass is light gray, aphanitic, and dense. Small qtz xenocrysts with aug rims are sparse. Age 

is poorly constrained but is probably slightly older than basaltic andesite of Little Bunchgrass Meadow (mbg) and is estimated to be ~150 ka. [WP]

b44 Tholeiitic basalt of Calif. Hwy 44 (middle Pleistocene)—Thin lava flow of sparsely porphyritic, tholeiitic, ol basalt (49.6% SiO2

) erupted from fissure north of Prospect Peak. This extensive lava flow was fed by lava tubes and flowed north out of map area along base of Butte 

Creek Rim for >15 km. It also flowed to west and barely spilled over Hat Creek Rim. Well-
54

preserved spatter rampart of welded cinders and agglutinate and sparse scoria was built over 

vent at hill 5885. Surface of flow is relatively flat and has low relief, although punctuated 

by widely spaced tumuli. Phenocrysts: 3–5% ol, 0.5–1 mm; trace pl, 0.5–1 mm. Few small 

glomeroporphyritic clots of ol+pl are locally present. Flow lobe to northwest is slightly more 

porphyritic than main part of lava flow and may be oldest part of flow. Groundmass is light 

to medium gray and is typically holocrystalline and diktytaxitic. Upper surface of flow is 

strongly vesicular and commonly ropy; top layer of flow breaks along joints into equant hexagonal blocks. Thick orange soil covers flow over much of its extent. Unit overlies andesites, basaltic andesites, and basalts of Hat Creek Rim (ahc), basaltic andesite of Signal Butte 

(msg), and andesite and basaltic andesite of Prospect Peak (ap, 247±56 ka). Unit is overlain 

by much younger andesite of hill 7416 (a74). 40Ar/39Ar age of 190±18 ka is consistent with 

its excursional magnetic direction (D.E. Champion, written commun., 2006). [OS, PP, SW]

b18 Tholeiitic basalt of hill 1879 (middle Pleistocene)—Short, thin pahoehoe lava flow and small 

cinder cone of sparsely porphyritic, tholeiitic ol basalt (49.2% SiO2) east of Butte Creek. 

Lava flowed north for ~2 km from cinder cone located ~5 km north of Twin Buttes. Surface 

of flow is relatively flat and has low relief. Short flow is unusually small for tholeiitic basalt 

in Lassen area. Phenocrysts: 3–5% ol, 0.5–1 mm; trace pl, 0.5–1 mm. Few small glomeroporphyritic clots of ol+pl locally present. Groundmass light to medium gray, typically 

holocrystalline, and weakly to strongly diktytaxitic. Upper surface of flow is vesicular and 

often ropy, and top layer of flow breaks along joints into equant hexagonal blocks. Thick 

orange soil covers flow over much of its extent. Age is poorly constrained. Unit overlies basalts and basaltic andesites of Swains Hole (bsh), basalt of section 30 (bl30), and basalt of 

hill 2109 (bl21). Probably older than Poison Lake chain, but probably not older than basalt 

of hill 2232 (bl22, 220±14 ka); chemically similar to tholeiitic basalt of Calif. Hwy 44 (b44, 

190±18 ka) and may be similar in age. [BB, PL, PP, SW]

bec Tholeiitic basalt of Eagle Canyon (middle Pleistocene)—Thin, widespread lava flows of 

sparsely porphyritic ol basalt (48.6% SiO2). Like other tube-fed pahoehoe lava flows, unit 

has tumuli on its upper surface and is composed of a number of flow units. Location of vent 

is unclear, but cinder and agglutinate exposed in roadcuts along U.S. Forest Service road 

17, slightly south of Calif. Hwy 44, and at east limit of outcrop of bec, suggest that vent 

was in that area. Lava flowed down Manzanita Creek drainage into North Fork Battle Creek 

and subsequently over and along base of Battle Creek Fault scarp for a total of ~40 km. 

MacDonald (1963) and MacDonald and Lydon (1972) called this and several other, much 

older, tholeiitic basalt flows “Shingletown Basalt”. Helley and others (1981) separated out 

the basalt of Eagle Canyon but did not recognize its entire distribution or source. Phenocrysts: 1–5% ol, 1–3 mm; 0–5% pl, 1–2 mm. Phenocryst abundance is higher in proximal 

localities. Locally contains distinctive clusters of 2–5 mm pl crystals to 2 cm diam that 

radiate from small ol+pl glomeroporphyritic clots. Groundmass is medium gray, holocrystalline, and diktytaxitic. Flows exhibit hexagonal-block joint pattern at top of flows and blocky 

jointing in interiors. Upper and lower portions of individual flow units contain abundant 

round vesicles. Unit overlies older regional andesites and basaltic andesites (am), Rockland 

tephra (pr), andesite of Manzanita Creek (amz), andesite of Viola (av), and probably rhyodacite of Manzanita Chute (rmc). Unit is overlain by avalanche deposit from dacite of hill 

8283 (Qs82), pumiceous pyroclastic-flow deposits (pe) from Eagle Peak, and pumiceous 

pyroclastic-flow and fall deposits (pc) from Chaos Crags. 40Ar/39Ar age is 199±22 ka. [GP, 

MZ, VI]

mm Basaltic andesites of Magee Volcano (middle Pleistocene)—Thin lava flows of porphyritic 

aug-ol basaltic andesite (56.3% SiO2) exposed on south flank of Magee Volcano. Most of 

lower south slopes of andesitic composite Magee Volcano (Borg, 1989), located north of 

map area, are obscured by till. However, two rock types are present beyond limits of glaciation and in stream valleys, roadcuts, and small fault scarps. Vents for these lava flows are unknown, but they must have been upslope on Magee Volcano, and probably were removed by 

glacial erosion. Phenocrysts (outcrop area adjacent to andesite of Devils Rock Garden, atd): 

25% pl, 0.25–1 mm, mostly <0.5 mm; 3% ol, 0.5–1 mm; 1% aug, 0.5–1 mm. Few strongly 

resorbed pl (3–5 mm) and aug megacrysts (to 5 mm) are present. Groundmass is light to 

medium gray, aphanitic, and microvesicular. Phenocrysts (remaining outcrop areas including 

analyzed sample): 4% ol, 0.5–1 mm, few larger; 4% pl, 0.5–1 mm, mostly 0.5 mm; 1% aug, 
55

0.5–1 mm, few larger. Most crystals are present as small clots of ol+pl+aug. Groundmass is 

dark gray to black, glassy to aphanitic, and microvesicular. Single low-precision K-Ar date 

of 210±120 ka is from the youngest part of Magee Volcano (L.E. Borg, written commun., 

1989); flows in map area are probably slightly older. [TH]

bn Tholeiitic basalt of Nobles Trail (middle Pleistocene)—Thin, aphyric basaltic (48.0% SiO2) 

lava flow in Hat Creek northeast of Raker Peak. Location of vent is unknown, but it is probably buried beneath younger lava flows of andesite of Hat Mountain (ah) on Central Plateau 

of LVNP. Unit consists of multiple flow units of tube-fed pahoehoe with hexagonal-block 

joint pattern at their tops and massively jointed interiors. Lava flow is weakly glaciated but 

well preserved, with abundant spherical vesicles at upper and lower surfaces. Phenocrysts: 

trace ol, to about 1 mm. Groundmass is medium gray, coarsely diktytaxitic, and consists of 

pl, ol, aug, and Fe-Ti oxide microlites. Age is poorly constrained, but unit overlies andesites 

of Badger Mountain (abm) and andesite of Raker Peak (arp, 270±18 ka) and underlies till 

of Raker Peak (Qtr). May be similar in age to tholeiitic basalt of Eagle Canyon (bec) and is 

estimated to be ~200–250 ka. [WP]

ap Andesite and basaltic andesite of Prospect Peak (middle Pleistocene)—Block-lava flows of 

porphyritic ol basaltic andesite, aug-ol andesite, and aug-hyp andesite (53–58% SiO2) forming Prospect Peak shield volcano. Shield is capped by cinder cone with several flank vents 

near summit. Summit crater contains pumice from 1.1 ka Chaos Crags eruptions. Lower 

flanks on south and west sides of volcano are buried by till and moraines. Three types of 

lava flows erupted to build Prospect Peak. Late-stage hyp-aug andesite lava flows that erupted from summit crater cover some flanks of volcano. Porphyritic aug-ol andesite lavas form 

bulk of edifice and area beyond north flank of volcano, north of Hat Creek Fault. Exposures 

of early-stage sparsely porphyritic ol basaltic-andesite lava are confined to extreme northwest outcrop area. Phenocrysts (late aug-hyp andesite): 25% pl, 0.5–2 mm, mostly 1 mm; 

4% hyp, 0.5–2 mm, mostly <1 mm; 2% aug, to 2 mm, mostly <1 mm; trace ol, <1 mm. 

Phenocrysts (aug-ol andesite): 35% pl, 0.5–2 mm, mostly 1 mm; 4% aug, 0.5–2 mm, mostly 

<1 mm; 3% ol, 0.5–2 mm, mostly <1 mm. Phenocrysts (early ol basaltic andesite): 3% ol, 

0.5–2 mm, mostly <1 mm; sparse pl, 0.5–2 mm. Groundmass of all flows is dark gray to 

black and glassy to aphanitic. Surfaces of lava flows consist of rounded vesicular blocks. 

Jointing is massive, forming rounded blocks, but interior of flows is rarely exposed. Hat 

Creek Fault slightly offsets lavas on north flank of Prospect Peak. Unit overlies andesites of 

Badger Mountain (abm), andesites of West Prospect Peak (awp), and andesite and basaltic 

andesite of Cluster Lakes (acl); overlain by tholeiitic basalt of Calif. Hwy 44 (b44, 190±18 

ka). K-Ar age of sample of late hyp-aug andesite is 247±56 ka; volcano was probably active 

for, at most, a few thousand years. [PP, SW, WP]

pdh Lithic tephra from Deep Hole (middle Pleistocene)—Nonsorted, nonbedded, unconsolidated, 

and ungraded deposit of lithic debris from andesite of Viola (av) and sparse fragments of 

andesite of Digger Creek (adc). Forms raised rim on the west, south, and east sides of Deep 

Hole explosion crater, located ~3 km west of Manzanita Lake. Fragments are angular to subrounded and from all parts of lava flows. Deposit is dominated by 0.5- to 10-cm-sized fragments, but blocks to 2 m are found. Matrix is comminuted ash-sized andesite. Deposit has 

thick orange soil; exposures of unweathered deposit were not found. Deposit was formed by 

one or more phreatic explosions, probably when an unknown magma body approached the 

surface and encountered groundwater. Explosion(s) was directed north. Walls of Deep Hole 

expose av and adc, and floor is covered with thick deposit of sand derived from rhyodacite 

of Manzanita Chute (rmc) lava flow. Deposit also found as thin sheet overlying av north of 

Deep Hole, but mapped only where it has geomorphic expression. Area surrounding Deep 

Hole has been extremely disturbed by bulldozing manzanita thickets into windrows; no 

undisturbed surface was found. Roadcut on U.S. Forest Service road 17 just south of Calif. 

Hwy 44 exposes 80–100 cm of weakly sorted, nonbedded, ungraded, and unconsolidated 

deposit. Clasts are mostly 1–2 cm; maximum clast size is 7 cm. Clasts are mostly dense, 

angular fragments of av; clasts of adc are rare. Matrix is comminuted ash-sized andesite, 

and deposit is deeply weathered. Age is unknown, but it is younger than av (313±8 ka) and 

rmc (297±1 ka) and much older than pe (66±4 ka); it underlies unmapped dacite pyroclastic flow exposed in road 17 roadcut, which is probably related to Bumpass sequence. Unit

may be related to eruption of tholeiitic basalt of Eagle Canyon (bec, 199±22 ka), the basalt 
56

lava thought to have vented near toe of rmc lava flow, although relation to bec is unclear; 

estimated to be ~200 ka. [MZ]

bct Basalt of Cherry Thicket (middle Pleistocene)—Poorly exposed lava flow of porphyritic ol 

basalt (51.2% SiO2) west of Red Lake Mountain. Vent has not been located and is probably 

buried by overlying basaltic andesite and andesite of Red Lake Mountain (mrd). Phenocrysts: 3% ol, 0.5–3 mm, mostly 0.5–1 mm; 1% pl, 0.5–2 mm, mostly 0.5–1 mm. Small 

glomeroporphyritic clots of pl+ol to 3 mm are common. Groundmass is light to medium 

gray, aphanitic, and microvesicular. Large round to ovoid vesicles are abundant in blocks at 

surface of flow; interior jointing is blocky. Age is poorly constrained as younger than andesite of Manzanita Creek (amz, 400–450 ka) and older than mrd (75–100 ka) and is estimated 

to be ~300 ka. [VI] 

bo Basalt of Onion Springs (middle Pleistocene)—Obscure, porphyritic aug-ol basalt (51.7% 

SiO2) lava flow and eroded cinder cone at toe of lava flow of rhyodacite of Loomis Peak 

(rlm). Small volume of lava erupted from cone in saddle between Loomis Peak and rhyodacite of Manzanita Chute (rmc) dome and puddled in saddle. Phenocrysts: 6% ol, 0.5–2 

mm; 4% pl, 0.5–5 mm, mostly 0.5–1 mm; 2% aug, 0.5–1 mm. Some pl phenocrysts have 

dark-gray cores; small glomeroporphyritic clots of pl+ol to 3 mm are abundant. Groundmass 

is medium gray, aphanitic and microvesicular. Large, round to ovoid vesicles are abundant 

in blocks at surface of flow; interior jointing is massive. Unit is younger than andesite of 

Mount Diller (amd, 387±10), but older than rlm and rmc (297±1 ka) and is estimated to be 

~300–325 ka. [LP, MZ]

awp Andesites of West Prospect Peak (middle Pleistocene)—Lava flows of porphyritic aug-hyp 

andesite and aug-ol andesite (59.3–61.2% SiO2) forming steep-sided lava cone. Small cirque 

occupies northwest flank just below the summit; summit area is eroded cinder cone. Most 

of volcano has thick mantle of colluvium and soil. Two rock types are present: most of outer 

surface of cone is composed of aug-hyp lava flows; porphyritic aug-ol andesite (59.3% 

SiO2) is exposed along base and part of north flank of lava cone and probably represents 

early phase of volcano’s evolution. Phenocrysts: (porphyritic aug-hyp andesite): 3–4% hyp, 

0.5–1 mm; 1–2% aug, 0.5–1 mm; 1–2% pl, 0.5–1 mm. Phenocrysts (porphyritic aug-ol 

andesite): 5–7% pl, 0.5–1 mm, mostly 0.5 mm; 2–3% ol, 0.5–1 mm; sparse aug, 0.5–1 

mm; trace hyp, 0.5 mm. Small glomeroporphyritic clots, 1–5 mm across, of aug+hyp+pl are 

abundant in aug-hyp andesite; similar but smaller and sparser clots are present in aug-ol andesite. Groundmasses are dark gray to black, glassy to aphanitic, and dense to vesicular. Flow 

surfaces are composed of massively jointed, rounded blocks. Vague flow banding is common, 

and smeared-out fragments of welded, oxidized lava are locally present. Age of West Prospect 

Peak is poorly constrained. It is older than andesite and basaltic andesite of Prospect Peak 

(ap, 247±56 ka) but unfaulted and is estimated to be 300–400 ka. [OS, PP, SW, WP]

bbf Basalt of Badger Flat (middle Pleistocene)—Lava flow and cinder cone of porphyritic ol 

basalt (51.3–52.2% SiO2), near Badger Flat. Flow and cone were glaciated, and much till 

was deposited on flow. Lava flowed down Box Canyon to Potato Butte area and is ~10 m 

thick. Phenocrysts: 10–12% ol, 0.5–2 mm, mostly 1 mm; 6–8% pl, 0.5–2 mm, mostly 1 

mm. Xenoliths of qtz to 2 cm are sparse. Groundmass is variable; in proximal areas, glacial 

erosion exposed core of flow, and lava is light gray, finely holocrystalline, and microvesicular to weakly diktytaxitic (although not tholeiitic). At distal end of flow, original flow 

surface is preserved, although weathered, and groundmass is dark gray and glassy. Flow top 

is vesicular, and jointing is thick slabby to massive in flow interior. Age poorly constrained. 

Unit overlies andesites of Badger Mountain (abm, 708±21 ka) and is overlain by andesites 

of West Prospect Peak (awp), andesite and basaltic andesite of Cluster Lakes (acl), basaltic andesite of Little Bunchgrass Meadow (mbg), and andesite of Hat Mountain (ah). It is 

slightly older but close in age to awp (300–400 ka). [OS, WP]

mhr Basaltic andesite of Hootman Ranch (middle Pleistocene)—Thick lava flow of porphyritic 

ol-aug basaltic andesite (55.5–56.2% SiO2). Location of vent is unknown but is probably 

in area between Red Rock Mountain and Red Lake Mountain. Macdonald (1963), Helley 

and others (1981), and Blake and others (1999) attributed this unit to Brokeoff Volcano, but 

based on the lithology and composition of the lava, this correlation is unlikely. Proximal part 

of flow is buried by glacial deposits, lavas of Lassen Volcanic Center, and regional lavas. 

Lava flowed westward and was focused into two narrow streams near Canyon Creek and 
57

Rock Creek. Rock Creek lava stream flowed along base of Battle Creek Fault scarp almost 

to Sacramento River (west of map area) for a total distance of ~28 km (Blake and others, 

1999). Unit is slightly offset by displacement on Battle Creek Fault scarp near Volta powerhouse (west of map area in Manton quadrangle; see fig. 3). Phenocrysts: 30% pl, 0.5–6 mm, 

mostly 1–2 mm; 7% aug, to 2 mm, distinctive bright-green color; 4% ol, 1–2 mm; 3% hyp, 

0.5–1 mm. Glomeroporphyritic clots, dominated by ol, but containing pyr and pl, to several centimeters but generally ~5 mm, are abundant and characteristic. Groundmass is dark 

gray and glassy to aphanitic. Flow tops contain abundant, large, spherical to ovoid vesicles. 

Joint pattern is massive; weathering forms rounded boulders. Thick, dark-orange to red soil 

covers most of flow. Unit overlies older regional andesites and basaltic andesites, undivided 

(am), basaltic andesites of Camp Forward (mcf), and andesite of Onion Creek (ao) and 

is overlain by andesite of Digger Creek (adc), andesite of Viola (av), and basalt of Eagle 

Canyon (bec). Unit directly overlies Rockland tephra (pr, 609±7 ka), an important regional 

stratigraphic marker. 40Ar/39Ar age is 562±12 ka (Lanphere and others, 2004), and is consistent with an excursional magnetic direction that was correlated with Big Lost paleomagnetic 

event at ~575 ka (Champion and others, 1996; Lanphere and others, 1999). [GP, VI]

aw Andesite of Wilcox Peak (middle Pleistocene)—Complex of thin, porphyritic, aug-hyp andesite 

(58.2% SiO2) lava flows from lava cone of Wilcox Peak (north of map area, fig. 3); only the 

southernmost part of unit appears on map. Phenocrysts: 20–25% pl, 0.5–1 mm, few larger; 

5% hyp, 0.5–2 mm, a few larger; sparse–1% aug, 0.5–1 mm. Glomeroporphyritic clots of 

pyr+pl are common and to 5 mm in size. Relative abundance of hyp and aug and general 

size of phenocrysts are variable among flows. Groundmass is medium gray, dense, glassy 

to aphanitic. Blocks on top of lava flows are vesicular with flattened and stretched vesicles. 

Rock displays thin-platy to thick-slabby jointing. Unit overlies older regional andesites and 

basaltic andesites, undivided (am), but age is otherwise poorly constrained. Wilcox Peak 

lava cone is less severely faulted than underlying am and has well-developed soil and mature forest. Has normal magnetic polarity and is estimated to be 500–700 ka. [OS]

alg Andesite of Logan Mountain (middle Pleistocene)—Complex of thin, porphyritic aug-hyp andesite (57.6% SiO2

) lava flows from lava cone of Logan Mountain (vent is north of map area, 

fig. 3). Phenocrysts: 25% pl, 0.5–2 mm, few larger; 3–4% hyp, 0.5–1 mm, few larger; 1–2% 

aug, 0.5–1 mm, few larger. Glomeroporphyritic clots of pyr+pl are generally present and to 

5 mm. Relative abundance of hyp and aug and general size of the phenocrysts is variable. 

Groundmass is medium gray, glassy to aphanitic and dense, and rock displays thick-slabby 

to blocky jointing. Flow tops are vesicular with flattened and stretched vesicles. Unit overlies 

older regional andesites and basaltic andesites, undivided (am), but age is otherwise poorly 

constrained. Unit is less severely faulted than the underlying am and has well-developed soil 

and mature forest. It has normal polarity, and its age is estimated to be 500–700 ka. [TH]

agr Andesite of Grayback Ridge (middle Pleistocene)—Thin lava flow of porphyritic aug-ol andesite. Partially eroded cinder cone at hill 6122 on Grayback Ridge marks vent. Lava flowed 

south, where it was confined by and partially filled fault-bounded valley between Grayback 

Ridge and adjacent unnamed ridge to southwest. Lava flow has a scoria-covered surface, 

but flow core is dense. Phenocrysts: 4% ol, 0.5–2 mm, mostly 0.5 mm; 4% pl, 0.5–2 mm, 

mostly 0.5 mm; sparse aug, 0.5–1 mm. Glomeroporphyritic clots of ol+pl or pyr+pl to 5 mm 

in size are common. Groundmass is medium gray, aphanitic, and sparsely microvesicular. 

Age is poorly constrained but is considerably younger than underlying older regional andesites and basaltic andesites, undivided (am). Lava flow buries old faults that cut am terrane 

into blocks but is not faulted itself. Vent is poorly preserved, and soil development is much 

less extensive than on am. Unit overlain by basaltic andesite of Red Mountain (mrm) and 

tholeiitic basalts of Big Lake, units 1 and 2 (bk1, bk2). Unit has normal magnetic polarity, 

and age is estimated to be 600–700 ka. [MZ]

acs Andesite of Cabin Spring (middle Pleistocene)—Lava flow of porphyritic hyp-aug andesite (61.6% SiO2

) that crops out west of Red Rock Mountain. This lava flowed westward 

from source subsequently obscured by dacite of Red Rock Mountain (dr) and (or) lavas of 

Brokeoff Volcano. Eastern part of lava flow was glaciated and stripped of its block carapace; 

western part of lava flow is beyond limit of glaciation, and thick soil obscures its surface. 

Phenocrysts: 5% pl, 0.5–1 mm; 5% aug, 1–2 mm; 5% hyp, 1–2 mm. Many aug phenocrysts 

are strongly zoned with light-green cores and dark-green rims and give rock a distinctive 
58

appearance. Much of pyr occurs as monomineralic or 2-pyr glomeroporphyritic clots to 5 

mm. Groundmass is medium gray, aphanitic, and microvesicular, with irregular bands and 

zones of devitrification. Joint pattern is thick-slabby to blocky. Inclusions of milky-white 

vein qtz are common. Unit is overlain by dacite of Red Rock Mountain (drr, 672±20 ka) and 

andesite of Digger Creek (adc, 451±10 ka). K-Ar age is 690±34 ka. [GP]

mr Basaltic andesite of Rock Spring (middle Pleistocene)—Small outcrop of vesicular flow front 

of porphyritic ol-aug basaltic andesite (55.8% SiO2) exposed at Rock Spring, ~5.5 km northeast of Camp Forward. It is almost completely buried by andesite of Digger Creek (adc) and 

debris-flow deposit from Brokeoff Volcano (Qwb). Little is known about source or affinity of this lava flow, but its lithology and composition suggest that it is a regional volcanic 

unit and not related to Lassen Volcanic Center. Phenocrysts: 2% aug, 1–4 mm; 2% ol, 1–2 

mm; 6% pl, 1 mm. Glomeroporphyritic clots of aug+pl and ol+pl are common. Groundmass 

is light to medium gray and aphanitic. Age is very poorly constrained. It is older than late 

Brokeoff Volcano lavas (andesite of Digger Creek, adc) and probably older than andesite of 

Cabin Spring (acs, 690±34 ka). It may be slightly younger than or similar in age to andesite 

of Onion Creek (ao) and is estimated to be ~700 ka. [GP]

ao Andesite of Onion Creek (middle Pleistocene)—Porphyritic ol-aug andesite (58.3% SiO2) lava 

flow. Emerges from beneath andesite of Digger Creek (adc) and forms a low cliff along 

south bank of Onion Creek. Flow is unglaciated and little eroded; its source is unknown. 

Lava flowed west following pre-existing drainage, burying a terrane of older regional andesites and basaltic andesites, undivided (am) and basaltic andesites of Camp Forward (mcf). 

Blocky vesicular flow top grades downward into a massively jointed flow core. Phenocrysts: 

25% pl, 0.5–2 mm; 4% aug, to 2 mm; 1% ol, to 1 mm; trace hyp, to 0.5 mm. Groundmass is 

dark gray and glassy. Flow is not dated, but is overlain by Rockland tephra (pr, 609±7 ka), 

basaltic andesite of Hootman Ranch (mhr, 562±12 ka), andesite of Digger Creek (adc, 

451±10 ka), and debris-flow deposit from Brokeoff Volcano (Qwb). It has normal magnetic 

polarity and is probably slightly older than andesite of Cabin Spring (acs, 690±34 ka). 

Estimated to be 700–780 ka. Clynne (1984) inferred ao to be part of Mill Canyon sequence 

of Brokeoff Volcano, but that interpretation now appears to be incorrect. [GP]

at Andesite of Table Mountain (middle Pleistocene)—Porphyritic aug-hyp andesite (61.4% SiO2) 

lava flows forming small shield volcano. Vent area is poorly preserved; edifice was not 

glaciated. Several faults oriented north-northwest slightly offset the edifice. Flow tops are 

conspicuously rubbly, whereas flow interiors are dense; typical exposures consist of rounded 

boulders. Phenocrysts: 20% pl, to 0.75 mm; 5% hyp, to 0.75 mm, few to 1.5 mm; 3% aug, 

to 0.75 mm, few to 1.5 mm. Glomeroporphyritic clots and crystal clusters consisting of all 

three phenocryst phases but dominated by pl are abundant and generally 2–5 mm in size. 

Groundmass is dark gray to black, glassy, and vesicular. Unit is older than all adjacent units. 

It has not been dated, but has normal polarity and is probably similar in age to andesites of 

Badger Mountain (abm, 708±21 ka), ~700 ka. [MZ]

abm Andesites of Badger Mountain (middle Pleistocene)—Moderately porphyritic to very porphyritic aug-hyp andesite and ol-hyp-aug andesite lava flows forming a small shield volcano. 

Flows are generally thin (to 10 m); interflow breccias are sparse. Shield is glaciated on 

northwest margin and cut by normal faults; vent area is poorly preserved. Two types of 

lava flows corresponding to upper (59.7–62.7% SiO2) and lower (56.9–57.3 SiO2) stratigraphic positions in Badger Mountain can be distinguished but are not mapped separately. 

Phenocrysts (upper): 10–20% pl, 0.5–1 mm, few larger to 2 mm; 3% hyp, 0.5–1 mm, few 2 

mm; 2% aug, 0.5–1 mm, few 2 mm. Phenocrysts (lower): 25–30% pl, 0.5–1 mm; 3–4% aug, 

0.5–1 mm; sparse–2% hyp, 0.5–1 mm; sparse–2% ol, 0.5–1 mm. Some lower flows contain 

more ol than hyp. Glomeroporphyritic clots of pl+aug+hyp±ol are common to abundant in 

both upper and lower flows, generally 2–5 mm. Groundmasses are dark gray to black and 

glassy to aphanitic. The two types of lavas correspond to two chemical groups: ol-hyp-aug 

andesites in lower section contain 56.9–57.3 SiO2

; upper aug-hyp andesites in remainder of 

volcano contain 59.7–62.7% SiO2. Unit overlies older regional andesites and basaltic andesites, undivided (am) and tholeiitic basalt of Twin Bridges (btb); overlain by adjacent units 

basalt of Badger Flat (bbf), andesite and basaltic andesite of Cluster Lakes (acl), andesite 

and basaltic andesite of Prospect Peak (ap), tholeiitic basalt of Nobles Trail (bn), andesite of 

Potato Butte (asp), and avalanche debris from Lassen Peak spread across glacial ice (Qsl). 
59

K-Ar age of 708±21 ka for upper flow is consistent with its normal magnetic polarity; entire 

volcano is probably about that age. [OS, WP]

btb Tholeiitic basalt of Twin Bridges (middle Pleistocene)—Nearly aphyric tholeiitic basalt 

(49.0% SiO2) flow forming flat surface north of Badger Mountain. Multiple thin flow units 

are typically from a few tens of centimeters to a few meters thick; flow tops are vesicular. 

Maximum exposed thickness is ~30 m in fault scarp near Big Spring, but base of unit is not 

exposed. Original flow surfaces are poorly preserved, and location of vent is unknown. One 

large and several smaller faults oriented north-northwest break flows into north-northwesttrending linear blocks. Phenocrysts: typically, sparse–1% ol, 0.5–1.5 mm, sometimes in 

small glomeroporphyritic clots to 3 mm. Groundmass is light gray, holocrystalline, and 

diktytaxitic. Unit overlies older andesites and basaltic andesites, undivided (am). Unit has 

not been dated, but is overlain by andesites of Badger Mountain (abm, 708±21 ka), basaltic 

andesite of Red Mountain (mrm), andesite of Tumble Buttes (att), and andesite of Potato 

Butte (asp). It has normal magnetic polarity, and is probably 730–780 ka. [OS, WP]

aa Andesite of Ashpan Butte (early Pleistocene)—Sparsely porphyritic aug-hyp andesite (60.8% 

SiO2) lava flow and lava cone at Ashpan Butte. Eroded scoria cone at summit marks vent. 

Phenocrysts: 1–4% hyp, mostly 0.5 mm, few larger; sparse aug, 0.5–1 mm; 1–4% pl, mostly 

0.5 mm, few larger. Phenocryst content is variable from 3–8%; most phenocrysts are in 

glomeroporphyritic clots of pyr+pl to 5 mm across. Flow has black glassy to medium-gray, 

dense, aphanitic groundmass and thin platy to thick slabby jointing. Groundmass is commonly mottled by thin, oxidized zones of vapor-phase crystallization. Vesicular flow tops 

have flat stretched vesicles and zones of roundish vesicles. Age is poorly constrained. Unit 

overlies older regional andesites and basaltic andesites, undivided (am) and is surrounded 

by much younger basaltic andesite of Red Mountain (mrm) and andesite of Bear Wallow 

Butte (atb). Lava cone was significantly modified by erosion; scoria cone is poorly preserved. Both have thick, well-developed soil. Although Ashpan Butte lava cone buried a 

major fault, it appears to be unfaulted. It has reversed polarity, and its age is estimated to be 

800–1,000 ka. [MZ]

msg Basaltic andesite of Signal Butte (early Pleistocene)—Thin lava flow of sparsely porphyritic ol 

basaltic andesite with large cinder cone marking vent. Cinder cone contains both fresh and 

oxidized cinders and has orange soil to 1 m thick. Lava from cone flowed as much as 3 km 

north and west toward, but not as far as, Hat Creek Fault scarp. Lava flow is cut by small 

faults oriented parallel to Hat Creek Fault. Flow has thick well-developed soil that obscures 

surface morphology. Phenocrysts: 5–8% pl, 0.5–3 mm, most weakly to strongly reacted; 

sparse ol, 0.5–1 mm. Contains sparse small andesitic inclusions, small glomeroporphyritic 

clots dominated by pl, reacted hbl, and qtz xenocrysts with rims of aug. Groundmass is medium gray, aphanitic to holocrystalline, and dense to microvesicular; jointing is massive. Age 

is very poorly constrained. Unit overlain by tholeiitic basalt of Calif. Hwy 44 (b44, 190±18 

ka), which is much younger. Unit has reversed magmatic polarity. It overlies andesites, basaltic andesites, and basalts of Hat Creek Rim (ahc, 924±24 ka) and is probably about the same 

age as initiation of movement on Hat Creek Fault (estimated to be ~900–1,000 ka). [OS, SW]

ahc Andesites, basaltic andesites, and basalts of Hat Creek Rim (early Pleistocene)—Variety of 

thin ol and pyr basalt, basaltic andesite, and andesite (mostly 60–63% SiO2) lava flows and 

vent material exposed in Hat Creek Fault scarp. Lavas exposed in scarp face have small 

lateral extent and interfinger and are probably broadly equivalent to rocks mapped as older 

regional andesites and basaltic andesites, undivided (am) and younger rocks on west side of 

Hat Creek and in Grayback Ridge area. Similar andesite lava flows in Butte Lake area are 

mapped as andesite of Butte Lake (abl) and andesite of section 22 (a22). Variety of lavas 

with phenocryst assemblages ranging from ol to 2 pyr-pl are present; most common are 

sparsely phyric to porphyritic ol andesite (5% ol, 0.5–1 mm), sparsely phyric to porphyritic 

pyr andesite (5% aug), and porphyritic aug-hyp andesite (3–4% hyp, 0.5–1 mm; 1–2% aug, 

0.5–1 mm; 5–15% pl, 0.5–2 mm, few glomeroporphyritic clots). Groundmasses are typically 

dark gray and aphanitic. K-Ar ages of 924±24 ka on basalt lava flow from top of Hat Creek 

Fault and 1,645±35 ka from flow within the section but north of map area, suggest a considerable age range for the unit. Estimated to be ~900 to >1,645 ka. [OS, PP, SW]

abl Andesite of Butte Lake (early Pleistocene)—Porphyritic aug-hyp andesite (60.5% SiO2) lava 

flow(s), exposed in upthrown fault scarp east of Butte Lake and mostly buried by till. Lava 
60

flows were heavily glaciated; surface features are not preserved. Vent location is unknown. 

Phenocrysts: 15% pl, 0.5 mm; 4% hyp, 0.5–1 mm; 1% aug, 0.5–2 mm. Groundmass is dark 

gray to black, aphanitic to glassy, and dense. Some flows have sparse glomeroporphyritic 

clots of pyr+pl to a few millimeters. Thin platy to massively jointed flow interiors are exposed. Similar lithologically and in stratigraphic position to andesites, basaltic andesites, and 

basalts of Hat Creek Rim (ahc), suggesting that it may be related to lavas exposed in Hat 

Creek Rim north of Prospect Peak. Nearby units basalt of Bathtub Lake (bsb) and basalts 

of Sunrise Peak, units 1 and 2 (bss1, bss2) that are interpreted to be much younger, drape 

over old fault scarp exposing abl. Age is estimated to be similar to youngest part of the ahc

section, ~900–1,000 ka. [PP]

a22 Andesite of section 22 (early Pleistocene)—Nearly aphyric ol andesite lava (59.9% SiO2) 

flows exposed in downthrown fault block just east of flows from Cinder Cone; mostly 

buried by till. Lava flows were heavily glaciated; surface features are not preserved. Thin, 

platy to massively jointed flow interiors are exposed. Outcrops are probably small fault 

blocks. Location of vent area is unknown. Phenocrysts: sparse ol, 0.5–1 mm; sparse pl, 

0.5–1 mm. Groundmass is dark gray to black and aphanitic to glassy, locally with irregular 

flow banding or banding defined by vapor-phase crystallization. Inclusions of partially 

melted granitic rocks and milky qtz xenocrysts from few millimeters to 1 centimeter are 

common. Similar lithologically and in stratigraphic position to andesites, basaltic andesites, 

and basalts of Hat Creek Rim (ahc); may be related to lavas exposed in Hat Creek Rim 

north of Prospect Peak. Age is estimated to be similar to youngest part of ahc, ~900–1,000 

ka. [PP]

dhm Dacite of Huckleberry Mountain (early Pleistocene)—Complex of thick, porphyritic, 

aug-hbl-hyp dacite (63.4% SiO2) lava flows erupted from vent near Huckleberry Mountain, 

(northwest of map area in Jacks Backbone quad, fig. 3). Flow complex was glaciated, and 

flow-margin features removed; vent is not preserved. Lithology is slightly variable; some 

flows may be andesites. Phenocrysts: 30–45% pl, 0.5–1 mm, few larger; 3–5% hyp, 0.5–1 

mm; sparse–1% hbl, 2 mm–1 cm. Sparse aug, generally 0.5–1 mm, is present in some flows. 

Amount of hbl is variable, and it is usually strongly reacted and replaced by an aggregate of 

Fe-Ti oxide, pyr, and pl. Mafic inclusions, generally <1 cm, present in some flows. Groundmass is light gray to almost white, glassy to aphanitic, and sparsely microvesicular. Rock 

has thick-slabby to blocky jointing. Age is unknown. Unit overlies rocks of Latour Volcanic 

Center (Tlv) and older regional andesites and basaltic andesites, undivided (am) suggesting 

it is younger than ~1,400 ka; age is otherwise poorly constrained. Exposure is poor, and colluvium and thick soil suggest that unit is relatively old. Unit has reversed magnetic polarity 

and is estimated to be 800–1,200 ka. [TH, VI]

am Older regional andesites and basaltic andesites, undivided (early Pleistocene)—Variety of 

older volcanic rocks exposed in swath across northwest edge of map area. Includes wide 

variety of lithologies and compositions from ol basaltic andesite to pyr andesite (55.2–58.3% 

SiO2) related to regional volcanism. Intercalated pyroclastic material and vent facies are locally present. East of Hat Creek, lavas mapped as andesites, basaltic andesites, and basalts of 

Hat Creek Rim (ahc) have similar lithology and stratigraphic position.

In Viola, Manzanita Lake, Thousand Lakes Valley, and Old Station quadrangles, composed of rocks that form platform of normally faulted and eroded lava flows. Largest 

exposure is between Grayback Ridge and North Battle Creek Reservoir at northwest corner 

of map. Lavas were erupted from numerous small centers; only a few vents are shown. 

Faulting and erosion disrupted these volcanoes and obscured their original constructional 

morphology. Range of their ages and relation to other units are poorly known; single K-Ar 

date of 1,400±37 ka was obtained from lava flow near top of pile southwest of North Battle 

Creek Reservoir. Some rocks may be late Pliocene in age. Wide variety of sparsely phyric 

to porphyritic ol basaltic andesite, ol-pyr basaltic andesite, and 2-pyr andesite lava flows 

are present. Three analyzed samples contained 55.2%, 56.7%, and 58.3% SiO2

, but range of 

compositions present is certainly broader. Groundmasses are medium to dark gray, aphanitic 

to glassy, and dense. Most outcrops are deeply weathered. In Little Logan Butte and Hat 

Creek Hill area, sparsely porphyritic pyr andesite and ol basaltic andesite are most common 

rock types. Groundmasses are generally dark gray to black, aphanitic, and microvesicular. 

Thick, well-developed, orange to red soils are common.
61

In Grays Peak quadrangle, patches of pyr andesite exposed beneath Rockland tephra (pr) 

in Canyon Creek area and basaltic andesites of Camp Forward (mcf) in Digger Creek represent eroded terrane covered by younger units. Flows that appear to come from north may be 

related to similar lavas in Viola, Manton, and Shingletown quadrangles. Their source, age, 

and relation to other units is poorly known. Most common lithology is porphyritic hyp-aug 

andesite (58.3% SiO2) containing 20–40% phenocrysts, dominantly pl to 5 mm, and occasionally sparse ol. Other lithologies similar to those from Grayback Ridge and North Battle 

Creek Reservoir areas are also present. [GP, MZ, OS, TH, VI]

mcf Basaltic andesites of Camp Forward (early Pleistocene)—Thin flows of porphyritic, aug-ol 

and ol-aug basaltic andesite (54.3% SiO2) that form deeply eroded remnants of small shield 

volcano at west boundary of map area. Lava flows 5–10 m thick dip gently away from vent 

(near hill 4110) that is not preserved. Rocks are deeply weathered; thick cover of bright-red 

soil covers most of flows. Two rock types are present. Lower flows are aug-ol basaltic andesite (54.3% SiO2

). Phenocrysts: 10% ol, 1 mm; sparse pl, 1 mm; sparse aug, 1 mm. Upper 

flows are ol-aug basaltic andesite; they have not been analyzed but may be less mafic than 

lower flows. Phenocrysts: 8–10% aug, 1–2 mm; 5% ol, 1 mm; 2% pl, 1 mm. Glomeroporphyritic clots of aug+ol and pl are abundant in upper flows; xenoliths of milky white vein 

qtz are common in all flows. Flow lineation and vapor-phase crystallization are common in 

light- to medium-gray groundmass. Age is poorly known. Unit overlies rocks assigned to 

older regional andesites and basaltic andesites, undivided (am, 1,400±37 ka), but the age 

of am in this area is probably older than date. Unit is overlain by rhyolite of Blue Ridge 

(rb, ~1,200 ka) but was already eroded before partial burial by rb lava. Unit has reversed 

magnetic polarity and is estimated to be 1,400–1,600 ka. [GP]

SOUTH AND EAST OF LASSEN VOLCANIC CENTER

Includes rocks related to regional volcanism and erupted from monogenetic volcanoes and small- to 

moderate-sized lava cones and shield volcanoes of all ages in axis of Cascade Arc south and east of Lassen 

Volcanic Center. Area approximately bounded on east by trace of Hat Creek Fault-Lake Almanor Graben 

system. Prominent features include Sifford Mountain and Mount Harkness shield volcanoes. 

bbz Tholeiitic basalts of Buzzard Springs (late Pleistocene)—Thin, sparsely porphyritic tholeiitic 

ol and aug-ol basalt (48.7–49.6% SiO2) lava flows erupted from line of at least six unnamed 

cinder cones south-southwest of Sifford Mountain. Related flow, tholeiitic basalt of Ice 

Cave Mountain (bic), is the southernmost and largest flow in this group, slightly older, and 

is mapped separately. Some lava mapped as bbz may belong to underlying, lithologically 

similar bic. Alignment of vents for bbz probably reflects a north-northwest-trending fault 

in subsurface. These flows are among youngest regional lavas in map area, are unfaulted, 

have little or no soil, and support only sparse forest. Ponding of these flows in North 

Fork Feather River channel partially blocked river and created Feather River Meadows. 

Phenocryst content 1–10%. Near-vent samples tend to be more porphyritic than those away 

from vents. Phenocrysts: 0–6% pl, mostly 0.5–1 mm, few larger; sparse–3% ol, mostly 

0.5–1 mm, few larger; rare aug, 0.5 mm. Many phenocrysts are in glomeroporphyritic 

ol+pl clots as large as 5 mm in size. Flows have aa surfaces, rarely pahoehoe, and mediumgray, aphanitic to holocrystalline and microvesicular to weakly diktytaxitic groundmasses. 

Vesicular flow tops have rounded to irregular-shaped vesicles. Flows overlie basaltic 

andesite of Small Butte (msm) and basalts and basaltic andesites of Sifford Mountain 

(bsm); glacial drift partially buries north end of cinder cone chain. K-Ar age of 65±45 ka is 

consistent with stratigraphy and probably more meaningful than implied by the analytical 

uncertainty. [CM, RP, ST]

bic Tholeiitic basalt of Ice Cave Mountain (late Pleistocene)—Lava flow of nearly aphyric to 

sparsely porphyritic tholeiitic basalt (48.6–48.8% SiO2) erupted from vent at summit of Ice 

Cave Mountain and flowed south out of map area. Vent is largest and southernmost of chain 

of at least six vents of similar age; the other vents and flows are mapped as tholeiitic basalts 

of Buzzard Spring (bbz). Lava on Ice Cave Mountain is mostly remobilized spatter and 

contains sparse small lava tubes. Large volume of pahoehoe basalt lava from this vent entered Lost Creek drainage and flowed south and west to Deer Creek Meadows and south and 

east to Soldier Meadows (both south of map area; see fig. 3). Small stream of lava flowed 
62

north and is buried by bbz. Phenocrysts: rare–2% ol, 0.5–2 mm; rare–6% pl, 0.5–2 mm. 

Phenocryst content variable; lava near vent is most porphyritic and contains sparse to common glomeroporphyritic clots of ol+pl to ~1 cm. Lava has light- to medium-gray aphanitic 

to holocrystalline, typically diktytaxitic groundmass; flow tops contain abundant round 

vesicles; jointing is blocky. Unit lies on much older rocks of Maidu and Dittmar (and south 

of the map area, Yana) Volcanic Centers, occupies present-day drainages, and is minimally 

incised. Although it is faulted in Deer Creek Meadows, flow lacks significant soil cover 

and has well-preserved flow features. Unit overlies till, older glaciations (Qto) and outwash 

gravel, older glaciations (Qoo). Age is equivalent to bbz (65±45 ka). [CM]

bcc Basalts of Cold Creek Butte (late Pleistocene)—Thick (30–60 m) block lava flows of early 

aug-ol basalt (52.4% SiO2) and late ol basalt (50.9% SiO2) erupted from cinder cone of 

Cold Creek Butte at west end of Battle Creek Meadows. Lava flowed down South Fork 

Battle Creek for 8 km and ponded in canyon. Backflow of early lava from vent flowed 3 

km toward Mineral, blocked drainage of South Fork Battle Creek, and created Battle Creek 

Meadows. Outer part of cinder cone is composed of scoria and bombs of late ol basalt lithology. Abundance of phenocrysts is variable in both lithologies. Phenocrysts (early aug-ol 

basalt): 8% ol, 0.5–2 mm, mostly 1 mm; 3–8% pl, 0.5–2 mm, mostly 0.5–1 mm; sparse–2% 

aug, 0.5–1 mm. Glomeroporphyritic clots of ol+aug+pl are abundant and as large as ~1 

cm in early flows but absent from late flows. Early flows have medium-gray, aphanitic 

to holocrystalline groundmass and tend to have vesicular flow tops. Phenocrysts (late ol 

basalt): 5–8% ol, 0.5–2 mm, mostly 0.5 mm. Late flows have light- to dark-gray, aphanitic 

to glassy groundmasses. Age is poorly constrained. One of youngest lava flows south of 

LVNP, it is exposed in South Fork Battle Creek canyon but was completely incised by 

creek along most of its length. Creation of Battle Creek Meadows by trapping of sediment 

indicates that bbc erupted before latest glacial advances. It is estimated to be 75–100 ka. 

[LY, MN]

bmc Tholeiitic basalts of Mill Creek Plateau (middle to late Pleistocene)—Lava flows of porphyritic tholeiitic basalt (48.5% SiO2

) erupted through rhyolite of Mill Creek Plateau (rmp). 

Spatter ramparts are preserved at four vents aligned north-south. Three southern vents and 

two southern flows are south of map area. Small volume of lava from two southern vents 

produced localized flows, whereas larger volume of lava from two northern vents produced 

more extensive flows. Lava from northernmost vent entered Mill Creek drainage and forms 

a discontinuous bench as much as 20 m thick for 10 km downstream, including bench on 

which the community of Mill Creek is located. Lava from second northernmost vent entered 

Guernsey Creek drainage and flowed 8 km downstream; most of flow is buried beneath 

alluvium and colluvium. Phenocrysts: 2–4% ol, 0.5–2 mm; 2–6% pl, 0.5–2 mm, mostly 

0.5–1 mm. Phenocryst content is variable; lava from northernmost vent is especially porphyritic. Glomeroporphyritic clots of ol+pl are sparse to abundant and as large as ~1 cm. Lava 

has light- to medium-gray, holocrystalline, often diktytaxitic groundmass; flow tops contain 

abundant round vesicles; jointing is massive to blocky. Age is poorly constrained. Unit lies 

on much older rocks of Maidu Volcanic Center and occupies present drainages. Significant 

post-emplacement erosion in Mill Creek indicates that bmc is pre-glacial. Estimated age is 

100–140 ka. [CM, MN]

bsm Basalts and basaltic andesites of Sifford Mountain (middle Pleistocene)—Variety of lithologically distinct, porphyritic, aug-ol basalt and basaltic andesite (51.5–57.4% SiO2

) block-lava 

flows that form the small shield volcano of Sifford Mountain. Eroded scoria cone at summit 

marks vent. Lava from Sifford Mountain flowed at least 10 km south and east and partially 

blocked North Fork Feather River. More Sifford lava may be present under till east of 

Domingo Spring. Contacts between flows are not mapped. Includes three common and two 

less-common lava types. 

(1) Porphyritic ol-hyp-aug basaltic andesite (55.8–57.4% SiO2). Phenocryst size and 

abundance is variable, especially pl and pyr. Phenocrysts: 5–15% pl, mostly 0.5 mm, few 

larger to 2 mm; 1–4% aug, 0.5 mm; 1–2% hyp, 0.5–1 mm; 1–2% ol, 0.5–4 mm, few larger. 

Rock is dark gray to black with a glassy to aphanitic groundmass and massive jointing. 

This lava covers much of south flank of Sifford Mountain and probably constitutes youngest flows on volcano. (2) Porphyritic aug-ol basalt (51.5% SiO2

). Phenocrysts: 10% pl, 

0.5–2 mm, mostly 0.5 mm; 7% ol, 0.5–2 mm; 3% aug, 0.5–2 mm. Glomeroporphyritic 
63

clots as large as ~5 mm composed of all three phenocryst phases are sparse. Groundmass is 

medium gray and holocrystalline; the joint pattern is massive. This lava is exposed in North 

Fork Feather River valley and Domingo Spring area and is probably intermediate in age. 

One of these flows is at least 20 m thick where it is cut by young fault near Domingo Spring. 

Higher on north flank of Sifford Mountain, these flows are buried by younger lava flows. 

(3) Porphyritic ol basalt. Phenocrysts: 25% pl, mostly 0.5 mm, few larger; 3–4% ol, 0.5–4 

mm, few larger; sparse aug, 0.5 mm. Medium- to dark-gray, aphanitic to holocrystalline 

groundmass; massive jointing. This lava forms most of west flank of volcano and is probably intermediate in age. (4) Porphyritic ol-aug basalt (53.0% SiO2

). Phenocrysts: 20% pl, 

mostly 0.5 mm, few larger to 1 mm; 8% aug, 1–5 mm, most as glomerocrysts to 8 mm; 2% 

ol, 0.5–1 mm. The lava also contains common glomeroporphyritic clots as large as 5 mm 

composed of all three phenocryst minerals. Groundmass is medium gray and holocrystalline 

and displays thick slabby to massive jointing. This basalt is exposed only in fault scarp along 

Hot Springs Creek and is probably oldest of exposed Sifford lavas. (5) Porphyritic ol basalt. 

Phenocrysts: 10% pl, 1–6 mm, mostly 2–3 mm; 8% ol, 0.5–4 mm, few larger are composite 

crystals. Groundmass is medium to dark gray and aphanitic; rock contains sparse glomeroporphyritic clots of pl+ol. This lava is found only in small area northeast of Domingo 

Spring. It is poorly exposed; its age relative to other Sifford lavas is unclear. 

Sifford Mountain shield is weakly glaciated and little eroded. Except for porphyritic 

ol-aug basalt, only flows forming surface of volcano are exposed; however, vesicular and 

rubbly upper surfaces of lava flows are mostly stripped. Several young faults with little displacement cut edifice. Faults near Boiling Springs Lake and along Hot Springs Creek offset 

late Wisconsin glacial deposits (not visible at map scale). 

Sifford Mountain is youngest regional shield volcano south of Lassen Volcanic Center 

and marks southern limit of young regional volcanism in Cascade Arc. 40Ar/39Ar ages are 

172±23 ka for aug-ol basalt lava flow exposed near Domingo Spring and 167±4 ka for porphyritic ol-aug basalt exposed in fault scarp along Hot Springs Creek. Entire shield volcano 

was probably emplaced within span of few thousand years. [CM, MH, RP, ST]

msm Basaltic andesite of Small Butte (middle Pleistocene)—Thick flow (>60 m) of porphyritic aug-ol 

basaltic andesite (55.6% SiO2). Small plug of partially oxidized lava and remnants of agglutinated cinder cone mark glaciated vent. Lava surrounds eroded vent for ~1 km, but more is 

probably buried beneath tholeiitic basalts of Buzzard Springs (bbz). Flow surface is heavily 

glaciated; thickly jointed core of flow is exposed. Phenocrysts: 10% pl, 0.5–2 mm, few larger; 

3% ol, 0.5–1 mm, few larger; sparse aug, 0.5–1mm. Some phenocrysts are small glomeroporphyritic clots, 2–4 mm. Groundmass is medium to dark gray, aphanitic, and microvesicular. 

Age is poorly constrained. It overlies andesite of Rice Creek (ar, 477±14 and 485±12 ka) and 

is overlain by bbz (65±45 ka). It is probably same age or somewhat older than basalts and 

basaltic andesites of Sifford Mountain (bsm, 172±23 and 167±4 ka). [CM, RP]

amh Andesite and basalt of Mount Harkness (middle Pleistocene)—Thin lava flows of porphyritic 

ol basalt (51.8% SiO2) and aug-ol andesite (57.9% SiO2) erupted from Mount Harkness 

shield volcano. Scoria cone at summit marks youngest vent. Mount Harkness volcano consists of older ol basalt lava flows and younger aug-ol andesite lava flows. The aug-ol andesite lava flows represent some of youngest regional volcanism in southern LVNP. These lavas 

cascaded over glacial cliffs of Warner Valley and partly filled valley bottom. Upper slopes 

of volcano are mantled by fields of boulders transported short distances from their original 

flow-top locations by periglacial action. Blocky-jointed lava on northeast margin of mountain indicates confinement of flowing lava by ice. A small cirque on the north side of summit 

exposes vent complex. Ol basalt lava flows are best exposed on glacially eroded north flank 

of volcano above Juniper Lake. Basaltic and andesitic parts of volcano are probably, but 

not necessarily, closely related in time. Phenocrysts (ol basalt): 5% ol, 0.5-3 mm, mostly 

1 mm; sparse aug, 0.5 mm. Flows have black to medium- to dark-gray, aphanitic, microvesicular groundmasses and blocky jointing. Phenocrysts (aug-ol andesite): 5% ol, mostly 0.5 

mm, few to 1 mm; 3% aug, 0.5–1 mm; 3% pl, mostly 0.5 mm, few larger to 1 mm; sparse 

hyp, 0.5–1 mm. Glomeroporphyritic clots to 5 mm of pl+ol+aug are common. Flows have 

block-lava surfaces and black to dark-gray, aphanitic, groundmasses and thick platy jointing. 

Vesicular flow tops have rounded to irregular-shaped vesicles. K-Ar age of 188±32 ka for 

younger ol-aug andesite is consistent with stratigraphy. Older ol basalt flows overlie rocks of 
64

Dittmar Volcanic Center and probably tholeiitic basalt of Warner Valley (bwv, 610±22 ka) 

and may be somewhat older than aug-ol andesite. [MH]

mhl Basaltic andesite of Huckleberry Lake (middle Pleistocene)—Thin lava flows of aug-ol 

basaltic andesite 53.1–54.0% SiO2) erupted from vent high on south flank of Brokeoff 

Volcano. This regional mafic lava is unrelated to Lassen Volcanic Center magmatic system, 

and its presence on flank of Brokeoff Volcano marks the end of a viable Brokeoff Volcano 

magmatic system. Flows covered at least 18 km2 of south flank of Brokeoff Volcano and 

flowed down stream courses into Battle Creek Meadows nearly to Mineral. Individual flow 

units near the vent are separated by layers of scoria. Vent is marked by small plug intruded 

into a cinder cone, most of which was removed by glacial erosion. Dikes intruded and 

partially melted underlying dacite of Twin Meadows (dt); contaminated basaltic andesite 

(not visible at map scale) is exposed in cliffs south of Forest Lake. Lava flows are heavily 

glaciated; typically, only flow interiors crop out. Lithology is variable. Phenocrysts: 5–7% 

ol, 0.5–2 mm, mostly 1 mm; 2–5% pl, 0.5–2 mm, mostly 0.5–1 mm; sparse aug, 0.5–1 mm. 

These early lava flows contain only ol, pl, and sparse aug phenocrysts. Most lava, however, 

has less ol but contains abundant xenocrysts of pl (to 5 mm), aug, and hyp, as well as glomeroporphyritic clots of same minerals, all derived from underlying Brokeoff Volcano dacite 

and andesite lava flows. Aug-ol basaltic andesite has light-gray holocrystalline groundmass, 

whereas contaminated basaltic andesite has medium- to dark-gray, aphanitic groundmass. 

Rarely preserved flow surfaces are blocky with stretched vesicles; flow interior is typically 

thick platy to massively jointed. Unit overlies andesite of Glassburner Meadows (ag) and 

andesite of Bluff Falls quarry (abf, 467±10 ka), but is significantly younger and estimated to 

be ~300 ka. [LP, MN]

bwv Tholeiitic basalt of Warner Valley (middle Pleistocene)—Extensive sheet of sparsely porphyritic, tholeiitic, ol basalt (48–49% SiO2

). Erupted from vent or vents in vicinity of and possibly under Mount Harkness. One stream of lava flowed southward through Lake Almanor 

Graben and into North Fork Feather River canyon (south of map area and fig. 3; see fig. 1) 

for at least 60 km. Second stream of lava flowed down Butt Creek (also south of fig. 3) into 

North Fork Feather River canyon. In Warner Valley, flows are offset at least 100 m by faults 

extending northwest from Lake Almanor Graben. West of Lake Almanor, cut by many faults 

with small offsets. Lava flows are tube-fed. In proximal area at cliffs of Warner Valley, at 

least 11 flow units are present. Unit is glaciated and has extensive till cover east of Warner 

Valley. Phenocrysts: rare to 2% ol, generally 0.5–2 mm, sparse ol to 5 mm in few flow units. 

Lava has light- to medium-gray, aphanitic to holocrystalline, often diktytaxitic, groundmass. 

Flow tops contain abundant round vesicles; jointing is massive to blocky. Vesicle sheets 

and cylinders are common where thick flow units are exposed. Groundmass of thick flow 

units often has greenish splotchy appearance caused by growth of large poikilitic aug. Flow 

interiors have thick blocky to massive joint pattern. Thin flow units and flow-unit tops 

often have crudely hexagonal, blocky joint pattern. Beyond limits of till cover, bwv has a 

well-developed orange to reddish soil; flow-top features are rarely observed. Unit overlies 

much older rocks of Dittmar and Maidu Volcanic Centers including andesites of Dittmar 

Volcanic Center, Stage 2, undivided (ad2), andesite of Benner Creek (ab), and rhyolite of 

North Stover Mountain (rsm), and it is overlain by Rockland tephra (pr, 609±7 ka) and the 

much younger andesite and basalt of Mount Harkness (amh, 188±32 ka). 40Ar/39Ar age is 

610±22 ka. [CH, MH, ST]

mgp Basaltic andesite of Grays Peak (early Pleistocene)—Sparsely porphyritic, ol basaltic andesite 

(56.1% SiO2) lava flows near Hampton Butte with a poorly preserved cinder cone at vent. 

Lava flowed down Panther Creek at least 6 km. Upper surface has well-developed orange 

soil cover; flow morphology is poorly preserved. Phenocrysts: 1–2% ol, to 1 mm. When 

fresh, groundmass is light to medium gray and aphanitic to holocrystalline, but typical rock 

is slightly vesicular and weakly oxidized to pinkish-gray. Unit overlies andesites of Cowslip 

Campground (acc, 1,320±40 ka) and rhyolite of Blue Ridge (rb, ~1,200 ka) and is overlain 

by dacite of Rocky Peak (drp) and debris-flow deposit from Brokeoff Volcano (Qwb). K-Ar 

age is 1,080±26 ka. [GP, LY]

bpa Basalt of Panther Creek (early Pleistocene)—Porphyritic aug-ol basalt (52.7% SiO2) found 

on ridge above intersection of Panther Creek and South Fork Battle Creek is an erosional 

remnant of canyon-filling lava flow that is much younger than underlying Maidu Volcanic 
65

Center rocks. Location of vent is unknown; relation to other regional mafic lavas is unknown. Phenocrysts: 10–12% ol, 0.5–1 mm, few larger; 2–3% pl, 0.5–2 mm; 1–2% brightgreen chromian aug, 0.5–3 mm. Groundmass is medium gray, aphanitic, and microvesicular. 

K-Ar age is 1,105±49 ka. [LY]

amr Andesite of Martin Creek (early Pleistocene)—Thick lava flows of porphyritic hyp-aug 

andesite (61% SiO2) and eroded volcanic neck that forms hill 6869. Lava flows dip gently south, indicating that they flowed toward Maidu Volcanic Center or were deformed by 

emplacement of younger dome. Phenocrysts: 25% pl, 0.25–1 mm; 6% aug, 0.5–3 mm; 4% 

hyp, 0.5–2 mm. Much of pyr occurs as small glomeroporphyritic clots to 3–4 mm. Milky qtz 

xenocrysts surrounded by pyr reaction rims are common; pseudomorphs of hbl are present 

but rare. Groundmass is light to medium gray. Vague flow lineation is sometimes apparent. 

Flow tops are moderately vesicular, flow cores are characterized by closely spaced platy 

joints, flow margins have massive jointing, and base is brecciated and oxidized. Unit is 

undated but overlies andesites of Maidu Volcanic Center, Stage 1, undivided (Tam) and underlies dacite of Rocky Peak (drp, 803±27 ka). Unit is reversely magnetized, and it appears 

to have been uplifted by intrusion of dacite of Plantation Gulch (dmg). Lithology is similar 

to nearby andesites of Cowslip Campground (acc, 1,320±40 ka), and amr may be of similar 

age. Estimated age 1,100–1,300 ka. [LP, MN]

acc Andesites of Cowslip Campground (early Pleistocene)—Thick lava flows of porphyritic 

hyp-aug andesite (58.1% SiO2). Lavas form crest of ridge culminating at hill 6964 east of 

Hampton Butte and are exposed as dip slopes northwest to Hazen Flat, indicating they came 

from vent located to south, probably on southwestern flank of Maidu Volcanic Center, that is 

not preserved. Lack of interflow breccias, massive jointing, and coarsely crystalline groundmass support interpretation that unnamed hill between Dry Lake and Cowslip Campground 

is thickly ponded flow. Contains two types of porphyritic hyp-aug andesite: one with small 

and another with large pl phenocrysts. Phenocrysts (large pl): 15% pl, 0.5 to 4 mm: 8–10% 

aug, 0.5–5 mm; 3–5% hyp, 0.5–3 mm; sparse ol, 1–4 mm. Phenocrysts (small pl): 30% pl, to 

1 mm; 8–10% aug, 0.5–5 mm; 3-5% hyp, 0.5–3 mm; sparse ol, 1–4 mm. Much of pyr occurs 

in glomeroporphyritic clots of aug and hyp to 1 cm. Ol is altered to iddingsite and is rimmed 

with pyr crystals. Large aug phenocrysts are color-zoned from light-green cores to dark-green 

rims. Groundmass is light to medium gray. Inclusions of milky vein qtz, 1–3 cm, are common 

in some flows, as well as altered fragments of volcanic and sedimentary rocks. Originally 

named andesite of Turner Mountain by Wilson (1961) and Clynne (1984) based on its superficial resemblance to lavas near summit of Turner Mountain (south of map area). Rocks in the 

Turner Mountain area are part of the Maidu Volcanic Center, but acc overlies and is much 

younger than the Maidu Volcanic Center. K-Ar age is 1,320±40 ka. [GP, LP, LY, MN]

mbc Basaltic andesites of South Fork Battle Creek (early Pleistocene)—Three erosional remnants 

of sparsely porphyritic ol basaltic andesite lava flows in South Fork Battle Creek. No vent 

areas are preserved; relation to other regional mafic lavas is unknown. Three outcrop areas 

are just west of Hampton Butte (55.5% SiO2), just north of Hampton Butte, and 5 km south 

of Hampton Butte on south side of South Fork Battle Creek. There is some lithologic variability among the three outcrop areas; they may not be same eruptive unit. Each of these lava 

flows overlies rocks of Maidu Volcanic Center. Phenocrysts: 4–6% ol, 0.5–1 mm, few larger; 

4–6% pl, 0.5–1 mm. Few larger pl and pyr crystals occur singly or in clots and are probably xenocrysts derived from underlying, coarsely porphyritic andesites of Maidu Volcanic 

Center. Groundmass is light gray, aphanitic, and microvesicular. K-Ar age is 1,738±46 ka for 

a sample from area just west of Hampton Butte. [LY]

REGIONAL VOLCANIC ROCKS OF THE CARIBOU AREA

Includes rocks of monogenetic volcanoes and small- to moderate-sized lava cones and shield volcanoes 

of all ages in Caribou area. Area approximately bounded on west by trace of Hat Creek Fault-Lake Almanor 

Graben system. Regional volcanic rocks of Caribou area are divided into two major groups based on age 

and character of volcanism: Caribou Volcanic Field, and older volcanic rocks in Caribou area. Volcanism in 

Caribou area was particularly intense during last 450,000 years and produced Caribou Volcanic Field, which 

dominates easternmost Lassen Volcanic National Park and adjacent areas to east and northeast. Before 450 

ka, volcanism in Caribou area was typical of dispersed regional volcanism; rocks of this period are generally 

buried by rocks of Caribou Volcanic Field and obscured by glacial deposits.
66

Caribou Volcanic Field

Tuya chains

mtt Basaltic andesite of Turnaround Lake (late Pleistocene)—Porphyritic ol basaltic andesite (55.6–56.3% SiO2

) lava flows erupted subglacially (tuyas) and aligned north-south, 

presumably reflecting subsurface fault control of vent locations. Includes tuyas at 

Turnaround Lake (55.6% SiO2) and hills 7576 (56.3% SiO2) and 7353 southwest of South 

Divide Lake. Forms steep-sided, oval to roughly circular, relatively flat-topped piles of lava 

as much as ~1 km diam and 100–200 m high. Sides of tuyas have characteristic curved and 

polygonal joint patterns that indicate chilling against ice. Phenocrysts: 10–20% pl, 0.5–2 

mm, mostly 0.25–1 mm; 2% ol, 0.25–1 mm, mostly 0.5 mm. Few small glomeroporphyritic clots of ol+pl are present. Groundmass is black, glassy, and coarsely microvesicular. 

Unit overlies basalts of Triangle Lake, unit 1 (brt1) and basalt of hill 8030 (br80). Unit 

erupted during last major glacial episode, probably 17–35 ka, and is one of youngest units in 

Caribou Volcanic Field. [BB, RC]

mte Basaltic andesites of Evelyn Lake (late Pleistocene)—Porphyritic, aug-ol and ol-aug basaltic 

andesite (53.1–55.1% SiO2) lava flows erupted subglacially (tuyas) in southern Caribou 

Volcanic Field. Includes hills 7275 (53.1% SiO2) and 7299 west of Evelyn Lake and hill 

7436 (55.1% SiO2) slightly farther west. Forms steep-sided, oval to elongate, relatively flattopped piles of lava 0.5–1 km long and as high as 100 m. Sides of tuyas have characteristic 

curved and polygonal joint patterns that indicate chilling against ice. Hill 7436 contains an 

intrusive plug and subaerial flows near summit. Phenocrysts (hills 7275 and 7299): 5% pl, 

0.5–2 mm, mostly 0.5–1 mm; 2% aug, 0.5–1 mm, few composite grains to 2 mm; 1% ol, 

0.5–1 mm. Groundmass is dark gray, glassy, and dense to microvesicular. Phenocrysts (hill 

7436): 5–10% pl, 0.25–1 mm, mostly <0.5 mm; 2% ol, 0.5–2 mm, mostly <1 mm; sparse 

aug, 0.5–1 mm. Groundmass of lava is dark gray, glassy, and coarsely microvesicular. 

Groundmass of plug is medium gray, aphanitic, and dense. Unit overlies basaltic andesite 

of Black Cinder Rock (mb, 667±24 ka) and basalt of Posey Lake (bbp). Evelen Lake tuyas 

are more eroded and older than Turnaround Lake (mtt) tuyas and probably erupted during 

early Wisconsin or Illinoian glaciation. Therefore, mte is estimated to be either ~60 ka or 

120–130 ka. [RC]

Red Cinder chain

The Red Cinder chain comprises a zone of vents l1 km long and 2.5 km wide trending N. 10° W. across 

the west-central part of the Caribou Volcanic Field, congruent with two young faults that cut several of the 

older units in the chain. The Red Cinder chain is dominated volumetrically by the complex edifice centered 

on Red Cinder (at 2,553 m the highest elevation in the eastern part of map area). Units of the Red Cinder 

chain are not as heavily glaciated as other nearby parts of the Caribou Volcanic Field, and many lava flows 

retain their original surfaces. 

mrc Basaltic andesite of Red Cinder Cone (late Pleistocene)—Thin block-lava flows of porphyritic 

ol basaltic andesite (53.1% SiO2) erupted from northerly of pair of vents collectively called 

Red Cinder Cone, 1 km northwest of Red Cinder. Lava flowed ~1 km northwest. Flows were 

only slightly eroded by glaciers. Phenocrysts: 3% ol, 0.5–1 mm; 3% pl, 0.5–2 mm. Small 

glomeroporphyritic clots of ol+pl to 5 mm are present but not abundant; qtz xenocrysts are 

sparse. Groundmass is medium gray, aphanitic, and microvesicular to dense. Jointing is thick 

slabby to massive; flow tops are vesicular. Unit mrc overlies basalt of Red Cinder Cone 

(brc), basaltic andesite of Red Cinder (mrr), andesite of Red Cinder (arr, 69±20 ka), and 

andesites of Dittmar Volcanic Center, Stage 1, undivided (Tad). Age is not well constrained;

estimated to be 20–25 ka. [MH, PP]

brc Basalt of Red Cinder Cone (late Pleistocene)—Lava flow of porphyritic aug-ol basalt (52.1% 

SiO2) erupted from southerly of pair of vents collectively called Red Cinder Cone. Flow 

is short and thick and may have been confined by glacial ice. Lava flow was only slightly 

eroded by glaciers. Most of cinder cone built over vent was destroyed by eruption of lava 

flow and by glaciation. Phenocrysts: 8% ol, mostly 0.5–2 mm, few larger; 6% aug, 0.5–3 

mm; 4% pl, 0.5–2 mm. Small glomeroporphyritic clots of ol+aug+pl to 5 mm are abundant. 

Groundmass is black when glassy and medium to dark gray and aphanitic in dense flow 
67

cores. Flow surfaces are vesicular and oxidized. Unit brc is overlain by basaltic andesite of 

Red Cinder Cone (mrc) and overlies basaltic andesite of Red Cinder (mrr), andesite of Red 

Cinder (arr, 69±20 ka), and andesites of Dittmar Volcanic Center, Stage 1, undivided (Tad). 

Age is not well constrained; estimated to be 25–40 ka. [MH, PP]

mrr Basaltic andesite of Red Cinder (late Pleistocene)—Thin block-lava flows and large cinder 

cone of porphyritic ol basaltic andesite (53.3 % SiO2) erupted from Red Cinder. Lava flowed 

3 km west-northwest and 3.5 km southwest from vent. Lava flow was glaciated, but surface 

morphology is mostly preserved. Flow tops are very vesicular to scoriaceous and often oxidized. Phenocrysts: 10% pl, 0.5–2 mm, mostly <1 mm; 5% ol, 0.5–1 mm. Glomeroporphyritic clots of pl+ol, 2–5 mm in size, are present but not abundant. Groundmass is typically 

light to medium gray, aphanitic, and sparsely microvesicular, but black where glassy; commonly flow banded. Unit mrr is overlain by basaltic andesite of Red Cinder Cone (mrc) and 

basalt of Red Cinder Cone (brc) and overlies basalt of hill 8030 (br80), basalt of Cameron 

Meadow (brm), andesite of Red Cinder (arr, 69±20 ka), basaltic andesites of Long Lake, 

unit 3 (mrg3), andesites of Dittmar Volcanic Center, Stage 2, undivided (ad2), and andesites of Dittmar Volcanic Center, Stage 1, undivided (Tad). Unit mrr is older than last glacial 

maximum; estimated to be 25–40 ka. [BB, MH, PP, RC]

br80 Basalt of hill 8030 (late Pleistocene)—Thin lava flows of porphyritic aug-ol basalt (52.1–52.3 % 

SiO2) erupted from vent marked by cinder cone 1 km southeast of Red Cinder at hill 8030. 

Lava flowed 4 km west and 3 km southeast. Flows are offset by small N. 10° W. faults along 

discontinuous fault zone between Lake Almanor Graben and Hat Creek Graben. Glaciated 

but not deeply eroded, although little original surface morphology is preserved. Phenocrysts: 

10% pl, 0.5–2 mm; 5% ol, 0.5–1 mm; sparse aug, 0.5–1 mm. Glomeroporphyritic clots of 

pl+ol, 2–5 mm in size, are abundant. Groundmass is dark gray, aphanitic, and microvesicular. Unit br80 overlies basalt of Cameron Meadow (brm), andesite of Red Cinder (arr, 

69±20 ka), basaltic andesites of Long Lake, units 1, 2, and 3 (mrg1, mrg2, mrg3), and 

andesites of Dittmar Volcanic Center, Stage 2, undivided (ad2). Unit br80 is overlain by 

basaltic andesite of Turnaround Lake (mtt) and basaltic andesite of Red Cinder (mrr). Unit 

br80 is older than last glacial maximum; estimated to be 25–40 ka. [MH, RC]

brm Basalt of Cameron Meadow (late Pleistocene)—Thin lava flows of porphyritic ol basalt erupted from vent to east now buried by younger lavas. Lava fills valley floor between Cameron 

Meadow and younger lavas from Red Cinder. Glaciated; crops out as small rounded hills 

of dense, thickly jointed, flow cores. Phenocrysts: 8–10% ol, mostly 0.5–1 but up to 2 mm; 

sparse pl, 1 mm; sparse aug, 1–2 mm. Groundmass is medium to dark gray and aphanitic 

with pink vapor-phase crystallization along flow planes. Unit brm overlies the much older 

basaltic andesite of Jakey Lake (mj) and andesites of Dittmar Volcanic Center, Stage 2, undivided (ad2) and is overlain by basalt of hill 8030 (br80) and basaltic andesite of Red Cinder 

(mrr). Unit brm is older than last glacial maximum; estimated to be 25–40 ka. [MH]

bra Basalt of Ash Butte (late Pleistocene)—Short, thick lava flow of porphyritic aug-ol basalt 

(52.0% SiO2) erupted from cinder cone of Ash Butte at north-northwest end of Red Cinder 

chain. Cone and lava flow were glaciated but generally are free of thick till deposits. Rubbly 

surface of flow and shape of cone are well preserved, although cone is sufficiently eroded 

to expose agglutinated core in some places. Cinders are generally oxidized; part of cone 

was rafted away by lava flow. Phenocrysts: 6% pl, 0.25–1.5 mm, mostly <1 mm; 4% ol, 

0.5–1 mm, mostly 0.5 mm. Glomeroporphyritic clots of ol+pl to a few millimeters and small 

xenocrysts of qtz are sometimes present. Groundmass is dark gray, aphanitic, and dense to 

microvesicular. Unit bra overlies andesite of Red Cinder (arr, 69±20 ka), basalt east of Ash 

Butte (bre), and basalt of Widow Lake (brw). Unit bra is older than last glacial episode and 

estimated to be 40–70 ka. [PP]

br22 Basalt of hill 2283 (late Pleistocene)—Relatively thin lava flow (to 20 m) of sparsely porphyritic ol basalt (52.5% SiO2

) erupted from well-preserved cinder cone at hill 2283. Cone is 

composed of agglutinated cinders that are mostly oxidized. Cone and lava flow glaciated, 

extensively buried by till, and poorly exposed, but cone shape is well preserved. Lava from 

cone flowed ~1 km west. Phenocrysts: 3% ol, 0.5–1 mm, mostly 0.5 mm. Groundmass is 

light to medium gray and aphanitic. Rafted, agglutinated cinders cover parts of flow; in 

other places, massively jointed flow interior is exposed. Unit br22 overlies basalt of section 

25 (br25), andesite of Red Cinder (arr, 69±20 ka), and basalt of Widow Lake (brw). Age is 
68

poorly constrained but is clearly older than last glacial episode; estimated to be 40–70 ka. 

[BB, PP]

br25 Basalt of section 25 (late Pleistocene)—Thin lava flow of porphyritic ol basalt (52.9% SiO2) 

erupted from cinder cone that forms hill 7711 at north-northwest end of Red Cinder chain. 

Lava flowed north for 3 km. Cone and lava flow were only weakly glaciated; brecciated 

surface of flow and conical shape of cone are fairly well preserved. Phenocrysts: 4% ol, 

0.5–1 mm, mostly 0.5 mm; 6% pl, 0.25–1.5 mm, mostly <1 mm. Groundmass is light to 

medium gray, holocrystalline, and microvesicular. Glomeroporphyritic clots of ol+pl to a 

few millimeters and small xenocrysts of qtz are sometimes present. Unit br25 is overlain 

by basalt of hill 2283 (br22) and overlies andesite of Red Cinder (arr, 69±20 ka) and basalt 

of Widow Lake (brw). Age is not well constrained, but is older than last glacial episode; 

estimated to be 40–70 ka. [PP]

arr Andesite of Red Cinder (late Pleistocene)—Thin, block-lava flows of porphyritic aug-hyp 

andesite (59.6–60.3% SiO2) that comprise the largest-volume unit of the Red Cinder chain. 

Erupted from vent now buried by younger lavas of basaltic andesite of Red Cinder (mrr). 

Lava flows were glaciated but are not significantly eroded. Phenocrysts: 10% pl, 0.5–1 mm, 

few to 2 mm; 3% hyp, 0.5–1 2 mm; 2% aug, mostly 0.5–1, few to 2 mm. Glomeroporphyritic clots of pl+hyp+aug, 2–5 mm in size, are abundant. Groundmass is dark-gray, dense, 

and aphanitic. Flattened vesicles are common in upper parts of flows. Unit arr is overlain 

by basaltic andesite of Red Cinder Cone (mrc), basalt of Red Cinder Cone (brc), basaltic 

andesite of Red Cinder (mrr), basalt of hill 8030 (br80), basalt of Ash Butte (bra), basalt of 

hill 2283 (br22), and basalt of section 25 (br25). Unit arr overlies basalt east of Ash Butte 

(bre), basalts of Triangle Lake, units 1 and 2 (brt1, brt2), basaltic andesites of Silver Lake, 

unit 4 (mc4), and andesites of Dittmar Volcanic Center, Stage 1, undivided (Tad). Oldest 

exposed unit of the complex edifice centered at Red Cinder; K-Ar age is 69±20 ka. [BB, 

MH, PP, RC]

bre Basalt east of Ash Butte (late Pleistocene)—Short, thick (20–30 m) lava flow of sparsely 

porphyritic ol basalt erupted from cinder cone 500 m east of Ash Butte. Cone and lava flow 

were glaciated and are partially covered by thick deposits of till. Cone is sufficiently eroded 

to expose its agglutinated core; cinders are generally oxidized. Phenocrysts: 3% pl, 0.25–1.0 

mm, many with overgrowth rims; 1% ol, 0.5–1 mm. Glomeroporphyritic clots of ol+pl to 4 

cm, but mostly 5 mm or less, are common, and small qtz xenocrysts are sometimes present. 

Groundmass is medium gray, aphanitic to holocrystalline, and microvesicular. Flow top is 

vesicular; large stretched vesicles occur throughout unit. Unit bre is overlain by basalt of 

section 25 (br25), basalt of Ash Butte (bra), and andesite of Red Cinder (arr, 69±20 ka). 

Age is poorly constrained; estimated to be 70–100 ka. [PP]

brw Basalt of Widow Lake (late Pleistocene)—Relatively thin lava flows (to 10 m) of porphyritic 

ol basalt (52.1% SiO2) at north-northwest end of Red Cinder chain. Erupted from two cinder 

cones (7,232 ft and 2,213 m elev) 0.5 km northeast of Widow Lake. The cones are aligned 

west-northwest and composed of agglutinated, mostly oxidized cinders. Both cones and lava 

flows were glaciated, are extensively buried by till, and are very poorly exposed. Third cone, 

~0.75 km southwest of Widow Lake, is completely buried by till, but outcrop 300 m farther 

west suggests that this cone is also brw. Phenocrysts: 5% ol, 0.5–2 mm; rare pl, 0.25–1.0 

mm. Groundmass is medium gray and aphanitic but commonly oxidized. Unit brw is overlain by basalt of Ash Butte (bra), basalt of hill 2232 (br22), and basalt of section 25 (br25). 

Age is poorly constrained; estimated to be ~100 ka. [BB, PP]

Basaltic andesites of Long Lake (late Pleistocene)—Thick lava flows of porphyritic ol basaltic 

andesite and ol-aug basaltic andesite (53.2-56.3% SiO2) on south flank of Red Cinder edifice. Age is poorly constrained, but flows are among oldest in Red Cinder chain; estimated 

to be ~100 ka. [MH, RC]

mrg3 Unit 3—Thick lava flow of porphyritic aug-ol basaltic andesite (53.2% SiO2) erupted from 

poorly preserved cinder cone that forms hill 7602, 5 km west-northwest of Posey Lake. 

Lava flowed 2.5 km west and is offset by two small faults along discontinuous fault zone 

between Lake Almanor Graben and Hat Creek Graben. Flow was heavily glaciated and is 

partly buried by till; surface features are not preserved. Phenocrysts: 8% pl, 0.5–1 mm, few 

larger; 6% ol, 0.5–1 mm, few larger; 3% aug, 0.5 mm. Groundmass is light to medium gray, 

aphanitic to holocrystalline, and dense. Unit mrg3 overlies unit mrg1, unit mrg2, basaltic 
69

andesite of Black Cinder Rock (mb, 667±24 ka), and andesite of Dittmar Volcanic Center, 

Stage 2, undivided (ad2) and is overlain by basaltic andesite of Red Cinder (mrr) and basalt 

of hill 8030 (br80). [MH, RC]

mrg2 Unit 2—Thick lava flow of porphyritic aug-ol basaltic andesite (55.5–56.3% SiO2) erupted 

from poorly preserved vent at hill 7603 north-northwest of Posey Lake. Lava flowed several 

kilometers west and is offset by one of two small faults along discontinuous fault zone between Lake Almanor Graben and Hat Creek Graben. Another outcrop is 1.5 km farther west; 

intervening area is buried by unit mrg3. Flow was heavily glaciated and is partly buried by 

till; surface features are not preserved. Lithology is somewhat variable. Phenocrysts: 8–12% 

pl, 0.5–3 mm; 4–8% ol, 0.5–3 mm; sparse to 2% aug, 0.5–2 mm. Many crystals occur in 

glomeroporphyritic clots of ol+pl±aug generally to 2–5 mm in size. Groundmass is light to 

medium gray, aphanitic to holocrystalline, and dense to microvesicular. Unit mrg2 overlies 

unit mrg1 and is overlain by unit mrg3 and basalt of hill 8030 (br80). [MH, RC]

mrg1 Unit 1—Thick lava flows of porphyritic ol basaltic andesite (53.6% SiO2) in area l–2 km 

north-northwest of Posey Lake. Vent not preserved; flows probably erupted from vent now 

buried by mrg2 vent or by younger lavas of Red Cinder chain. Lava flowed south from vent 

area ~3 km. Flows were heavily glaciated and are partly buried by till; surface features are 

not preserved. Lithology is somewhat variable. Phenocrysts: 4–8% ol, 0.5–3 mm, few larger; 

1–3% pl, 0.5–1 mm. Small glomeroporphyritic clots of ol+pl are sparse in some flows and 

common in others. Groundmass is medium gray, usually aphanitic and dense, but occasionally holocrystalline and microvesicular. Lava is massive to thick platy-jointed. Unit mrg1 is 

overlain by unit mrg2, unit mrg3, and basalt of hill 8030 (br80). [RC]

mrb Basaltic andesite of Caribou Wilderness (late Pleistocene)—Porphyritic, aug-ol basaltic 

andesite lava flows in six small outcrop areas southwest of Triangle Lake. Vent location uncertain, but probably located to southwest and buried by younger lavas. Flows were mostly 

stripped by glaciers; only a few remnants of flow cores are preserved. Somewhat variable 

in lithology. Phenocrysts: 4–6% ol, 0.5–2 mm; 5–8% pl, 0.5–1 mm; 4–6% aug, 0.5–1 mm. 

Small glomeroporphyritic clots of ol+pl+aug are common. Groundmass is medium to dark 

gray, holocrystalline, and dense; usually has abundant vapor-phase crystallization splotches 

and, generally, some purplish oxidation. Unit mrb overlies basalts of Triangle Lake, Units 

1 and 2 (brt1, brt2) but has not been dated; estimated to be ~100 ka, but could be older. 

[BB]

Basalts of Triangle Lake (late Pleistocene)—Lava flows of sparsely porphyritic and porphyritic 

ol basalt (52.4% SiO2) located immediately southwest of Triangle Lake. Vents not preserved, 

but probably erupted from vents located to southwest and buried by younger lava flows. 

Flows were heavily glaciated and are poorly exposed, typically only as flow cores. Basalts 

of Triangle Lake are not dated; estimated to be ~100 ka, but could be older. [BB]

brt2 Unit 2—Lava flows of porphyritic ol basalt displaying a limited range of lithologic variability. Phenocrysts: 5% ol, 0.5–2 mm; sparse to 2 % pl, 0.5–1 mm; rare aug, 0.5–1 mm. Few 

samples contain glomeroporphyritic clots of ol+pl+aug. Most pl and aug may be from disaggregated glomeroporphyritic clots. Groundmass is light to medium gray and usually holocrystalline, but locally aphanitic and dense to microvesicular. Unit brt2 overlies unit brt1

and is overlain by basaltic andesite of Caribou Wilderness (mrb) and andesite of Red Cinder 

(arr, 69±20 ka). [BB]

brt1 Unit 1—Lava flows of sparsely porphyritic ol basalt (52.4% SiO2) displaying a range of lithologic variability. Phenocrysts: 5–15% ol, 0.5–4 mm, mostly <2 mm (most flows have ~5% 

ol, 0.5–2 mm). Groundmass is light to medium gray and usually holocrystalline, but locally 

is aphanitic and dense to microvesicular. Unit brt1 is overlain by unit brt2, basaltic andesite 

of Caribou Wilderness (mrb), andesite of Red Cinder (arr, 69±20 ka), and basaltic andesite 

of Turnaround Lake (mtt). [BB]

Bidwell Spring chain

The Bidwell Spring chain comprises a zone of vents 8.5 km long and ~2 km wide trending 

N. 25° W. across the north-northwest part of the Caribou Volcanic Field, ~2 km northeast of Butte Lake. 

Units of the Bidwell Spring Chain erupted at the northern margin of young glaciation; only parts of the flows 

are overlain by till and outwash deposits, and the cinder cones are only slightly eroded. Age of units in the 

Bidwell Spring chain ranges from ~25 ka to ~70 ka. 
70

adb Andesite of Bidwell Spring (late Pleistocene)—Thin, block-lava flow of nearly aphyric andesite 

(59.0% SiO2) with small cinder cone forming its vent. Flow is displaced slightly to the 

southwest from the main N. 25° W. orientation of the Bidwell Spring chain. Proximal portion of lava flow is overlain by till, younger glaciations (Qty), whereas distal portion of flow 

is beyond extent of young glaciation, and a rough scoriaceous block-surface is preserved. 

Small cinder cone is surrounded by thin, marginal deposits of unit Qty, but only weakly 

eroded. Phenocrysts: trace ol, 0.5 mm. Contains a few large xenocrysts of pl. Groundmass 

is black and glassy; lava contains abundant irregularly shaped stretched vesicles. Age is 

constrained by overlying glacial deposits which indicate that it is older than ~25 ka, and it is 

probably younger than basalt of Twin Buttes (bdt, 46.3±3.4 ka). [PP, SW]

mdp Basaltic andesite of Pole Spring Road (late Pleistocene)—Short, thick, aa to block-lava flow 

of sparsely porphyritic ol basaltic andesite (53.7% SiO2) in northwest part of Bidwell Spring 

chain; vents are marked by two small cinder cones aligned north-northeast. Lava flowed 2 km 

northwest. Southwest edge of lava flow is overlain by till, younger glaciations (Qty), whereas 

distal portion and east side of lava flow are beyond glacial limit, and a rough scoriaceous 

surface is preserved. Phenocrysts: 1–2% ol, 0.25–1 mm; 1–2% pl, 0.25–1mm. Groundmass 

is dark gray to black, glassy to aphanitic, and coarsely microvesicular. Sparse xenocrysts of 

pl and qtz to 5 mm are present. Lava blocks on upper surface have stretched to irregularly 

shaped vesicles; flow interior is microvesicular. Lava flow has poorly developed soil. Unit 

overlies much older basalts and basaltic andesites of Swains Hole (bsh) and basalt of hill 

2109 (bl21). Age is well constrained; weakly affected by 18–25 ka glacial advance, but overlies basalt of Twin Buttes (bdt, 46.3±3.4 ka). Estimated age is between ~25 and 45 ka. [PP]

md36 Basaltic andesite of section 36 (late Pleistocene)—Short, thick, block-lava flow of porphyritic, ol basaltic andesite (53.6% SiO2

) in northwest part of Bidwell Spring chain; vent area 

marked by two cinder cones aligned north-south. Scoria and bombs in cones are oxidized; 

agglutinate rafted from cones covers flow in some places. Lava flowed ~2 km northwest of 

cones. Phenocrysts: 12% pl, 0.5–2 mm; 2% ol, 0.5–2 mm, mostly 1 mm. Glomeroporphyritic clots of pl+ol are present. Groundmass is medium gray, aphanitic, and microvesicular. 

Blocks at flow surface contain abundant stretched vesicles. Unit md36 is younger than 

basalt of Twin Buttes (bdt, 46.3±3.4 ka) but older than basaltic andesite of Pole Spring Road 

(mdp); estimated age is 25–40 ka. [BB, PP]

bdt Basalt of Twin Buttes (late Pleistocene)—Block lava flows and two large well-preserved northnorthwest-aligned cinder cones of sparsely porphyritic ol basalt (52.8% SiO2

) in central 

part of Bidwell Spring chain. Lava from cinder cones flowed in two separate streams as far 

as 7 km north. Lava flows cover nearly 25 km2 and have as much as 10 m of relief at their 

margins. Blocky flow tops are vesicular; interiors are dense. Cinders and ash from the cones 

of Twin Buttes blanket area east of cones. Flow has orange soil in some areas, but outcrop 

is still abundant. Phenocrysts: 3–4% ol, 0.5–1 mm; sparse pl, 0.5 mm. Small xenoliths 

(1–2 cm) of coarsely crystalline qtz are occasionally found. Groundmass is black to medium 

gray, glassy to aphanitic, and dense to microvesicular. Unit bdt overlies basaltic andesites 

of Black Butte, unit 1 (mdb1) and is probably older than all other units of Bidwell Spring 

chain except basaltic andesites of Black Butte. 40Ar/39Ar age is 46.3±3.4 ka, which is slightly 

older than last major glacial advance (18–25 ka). [BB, PL, PP]

Basaltic andesites of Black Butte (late Pleistocene)—Thick, block-lava flow and associated 

complex of cinder cones at southeast end of Bidwell Spring chain. Flows and cinder cones 

are all basaltic andesite. Age is probably 50–70 ka. [BB, PP]

mdb2 Unit 2—Prominent, well-preserved cinder cone that forms Black Butte (elev, 2437 m). Cinders 

are basaltic andesite identical in composition to unit mdb1. [BB]

mdb1 Unit 1—Thick, block-lava flow of sparsely porphyritic ol basaltic andesite (55.8–56.1% SiO2). 

Lava flowed as far as 3 km from vent, and additional lava is probably buried beneath basalt 

of Twin Buttes (bdt). Associated cinder cone 0.5 km west of Black Butte was partially 

destroyed when lava flows were extruded from its base; some areas of lava flow are covered 

by cinders and agglutinate rafted from the cone. Surface of lava flow has unusual topography characterized by fissures a few meters across and a few meters deep, oriented at various 

angles. These features were probably caused by a shallow underlying intrusion, perhaps 

related to aborted rise of magma that produced the later cone (unit mdb2). Phenocrysts: 

2% ol, 0.5–1 mm. Groundmass is dark gray, aphanitic, and dense to microvesicular, with va-
71

por-phase crystallization along planar fractures. Lava flow is partially buried by till, younger 

glaciations (Qty), but majority of lava flow is beyond the limits of glaciation. Unit mdb1

is overlain by the basalt of Twin Buttes (bdt, 46.3±3.4 ka); fresh flow surfaces suggest that 

unit mdb1 is only slightly older than unit bdt. [BB, PP]

Poison Lake chain

The Poison Lake chain comprises a zone of cinder cones and lava flows 18 km long and 2 km wide 

trending N. 40° W. across the northeast part of the Caribou Volcanic Field. Vents in the chain are concentrated 

in two rows that probably reflect underlying faults that served as conduits for magma. Thirty-nine mapped 

units of basalt and basaltic andesite are divided into nine groups based on shared lithologic, chemical, and 

spatial characteristics. The Poison Lake chain lies outside the area of glaciation, and cinder cones and flow 

surfaces are well preserved. Ten 40Ar/39Ar ages and one K-Ar age range from 110.8 ka to 97.6 ka, indicating 

that the Poison Lake chain units erupted during a short-lived episode of volcanism. The eruptive order of the 

Poison Lake chain units is based on known stratigraphic relations and the 40Ar/39Ar ages. However, some of 

the groups do not have definitive stratigraphic relations, and the uncertainties of many of the 40Ar/39Ar ages 

overlap. Consequently, the eruptive order may be subject to revision.

Basalts of Cone Lake Road (late Pleistocene)—Group of two ol basalt (49.0–49.5% SiO2) lava 

flows and cinder cones 3.5 km southwest of Bogard Buttes. Interpreted to be the youngest 

group in the Poison Lake chain based on 40Ar/39Ar age of 97.6±9.8 ka for unit bc2. [BB]

bc2 Unit 2—Thin, short, stubby block lava flow of porphyritic ol basalt (49.3–49.5% SiO2); vents 

marked by row of small cinder cones aligned north-south. Lava flowed ~2 km north and 1 km 

south. Phenocrysts: 10% ol, 0.5–3 mm, mostly 1 mm; trace pl, 1 mm. Groundmass is light to 

medium gray, aphanitic, and dense to microvesicular. Blocks on upper surface of flow contain 

abundant, large, rounded to irregular-shaped vesicles. Unit bc2 overlies basalts of Stephens 

Campground, units 2 and 3 (bs2, bs3) and unit bc1. 40Ar/39Ar age is 97.6±9.8 ka. [BB]

bc1 Unit 1—Thick, short, lobate lava flow of porphyritic ol basalt (49.0% SiO2); large, well-preserved cinder cone marks vent. Cinders and ash in cone are oxidized; aa surface of lava flow 

is covered in some places with thick layer of rafted cinders and agglutinate. Phenocrysts: 

10–12% ol, 0.5–5 mm, mostly 1–2 mm; sparse–2% pl, 0.5–3 mm, mostly ~1 mm. Small 

glomeroporphyritic clots of ol or pl and ol+pl are present. Groundmass is medium gray, 

aphanitic to holocrystalline, and extremely microvesicular. Unit overlies basalts of Stephens 

Campground, units 2 and 3 (bs2, bs3) and basalt of hill 2088 (bl20); overlain by unit bc2. 

[BB]

Basalts of old railroad grade (late Pleistocene)—Group of six cinder cones and three lava 

flows of porphyritic ol basalt (50.0–51.5% SiO2) in northwestern Poison Lake chain. Basalts 

of old railroad grade overlie basalts of Poison Butte, but their relation to other units is unclear. 40Ar/39Ar age of unit bg4 is 102.1±11.4 ka. [BB, PL, SW]

bg4 Unit 4—Cinder cone (hill 1791) and lava flow of sparsely porphyritic aug-ol basalt (51.2–

51.5% SiO2). Lava flowed as far as 1 km northwest and south. Top of flow is composed of 

vesicular aa and is covered by deep orange soil. Interior of flow is dense and flow banded. 

Phenocrysts: 5% ol, 0.5–3 mm, mostly <1 mm; 2% pl, 0.5–1 mm, mostly 0.5 mm; trace aug, 

0.5 mm. Groundmass is medium gray, aphanitic, and microvesicular. Unit bg4 overlies basalts and basaltic andesites of Swains Hole (bsh) and basalts of Poison Butte, unit 3 (bp3). 40Ar/39Ar age is 102.1±11.4 ka. [PL]

bg3 Unit 3—Cinder cone (hill 1784) and thick lava flow of porphyritic ol basalt (50.0% SiO2) located 2–3 km west of Poison Lake. Lava flowed northwest from cone ~3 km, mostly beyond 

north boundary of map area. Phenocrysts: 15% ol, 0.5–6 mm, mostly <3 mm; 5% pl, 0.5–3 

mm, mostly <1 mm. Groundmass is dark gray, aphanitic, and microvesicular. Unit bg3 overlies basalts and basaltic andesites of Swains Hole (bsh) and basalts of Poison Butte, units 1 

and 3 (bp1, bp3). [PL, SW]

bg2 Unit 2—Large, well-preserved cinder cone (hill 1816) and short stubby aa lava flow of porphyritic ol basalt (50.2% SiO2

). Flow has thick ash cover from eruption of nearby cones. 

Phenocrysts: 10% ol, 0.5–3 mm, mostly 1 mm; sparse pl, 0.5–1 mm. Small glomeroporphyritic clots of ol+pl are present. Groundmass is medium-gray, aphanitic, and dense. Unit bg2

overlies basalts and basaltic andesites of Swains Hole (bsh) and basaltic andesite of hill 

1868 (ml18). [PL]
72

bg1 Unit 1—Three small cinder cones of porphyritic ol basalt (52.9% SiO2) aligned N. 40° W. and 

located 3.2–3.7 km south of Poison Lake; no associated lava flows. Phenocrysts: 4–5% ol, 

0.5–2 mm, mostly 0.5–1 mm; 1–2% pl, 0.5–2 mm, mostly 0.5–1 mm. Small glomeroporphyritic clots of ol+pl are common. Where fresh, groundmass of dense blocks is dark gray, 

aphanitic, and microvesicular, but most blocks and scoria are oxidized to shades of red. Unit 

bg1 overlies basaltic andesite of hill 1868 (ml18). [BB, PL]

Basalts of Poison Butte (late Pleistocene)—Group of cinder cones and lava flows of sparsely 

porphyritic ol basalt (51.4–52.8% SiO2). Most units also contain sparse pl and (or) aug. 

Basalts of Poison Butte are older than basalts of old railroad grade and younger than basalts 

of Pittville Road. They are approximately the same age as basalts and basaltic andesites of 

Bogard Buttes, but there are no contact relations between the two groups. 40Ar/39Ar age of 

bp6 is 105.0±6.0 ka. [PL]

bp6 Unit 6—Short, thick, block lava flow of sparsely porphyritic aug-ol basalt (52.8% SiO2). Lava 

flowed ~1.5 km north from well-preserved but partially buried cinder cone between hill 

1925 and hill 1919. Phenocrysts: 2% ol, 0.5–2 mm, mostly 0.5–1 mm; 1% pl, 0.5–1 mm; 

sparse aug, 0.5–1 mm. Groundmass is medium gray, aphanitic, and microvesicular. Unit bp6

overlies basalts of Pittville Road, units 1 and 2 (bt1, bt2), basalts of Robbers Spring, unit 2 

(br2), unit bp4, and unit bp5. 40Ar/39Ar age is 105.0±6.0 ka. [PL]

bp5 Unit 5—Well-preserved cinder cone (hill 1925) of sparsely porphyritic aug-ol basalt (52.3–

52.4% SiO2). Cone is younger than most surrounding cinder cones and lava flows; it probably does not have associated lava flow. Includes remnant of older, largely buried cinder 

cone on its northeast flank. Both cones have similar composition, but slightly different 

petrography. Phenocrysts (main cone): 2–3% ol, 0.5–2 mm, mostly 0.5–1 mm; sparse aug, 

0.5–1 mm; rare pl, 0.5 mm. Groundmass is medium to dark gray, glassy to aphanitic, and 

microvesicular. Phenocrysts (older cone): 2–3% ol, 0.5–2 mm, mostly 0.5–1 mm; trace aug, 

0.25–0.5 mm. Groundmass is medium gray, glassy to aphanitic, and microvesicular. Unit 

bp5 overlies basalts of Pittville Road, unit 2 (bt2) and basaltic andesite of hill 1868 (ml18); 

overlain by unit bp6. [PL]

bp4 Unit 4—Short and thin lava flow of sparsely porphyritic ol basalt (52.6% SiO2) erupted from 

large, well-preserved cinder cone (hill 1942, ~1.5 km south of Poison Lake). Cone has multiple craters and is composed of oxidized scoria. Flow has oxidized, scoriaceous, and rubbly 

top. Phenocrysts: 3% ol, 0.5–1 mm; sparse aug, 0.5 mm. Groundmass is dark gray, aphanitic, 

and microvesicular. Unit bp4 overlies basaltic andesite of hill 1868 (ml18); overlain by unit 

bp6. [PL]

bp3 Unit 3—Lava flow of sparsely porphyritic ol basalt (52.8% SiO2); large cinder cone of Poison 

Butte marks vent. Most cinders in cone are oxidized to bright red. Lava erupted from cone 

flowed ~2 km, mostly west. Lava flow is cut by N. 40° W. fault with 10 m offset. Top 

of lava flow is composed of rubbly and vesicular aa and is covered by deep orange soil. 

Phenocrysts: 3% ol, 0.5–2 mm, mostly <1 mm. Fresh groundmass is medium gray, glassy 

to aphanitic, and microvesicular. Unit bp3 overlies basalts and basaltic andesites of Swains 

Hole (bsh), basaltic andesite of hill 1868 (ml18), and unit bp1; overlain by basalts of old 

railroad grade, units 3 and 4 (bg3, bg4). [PL]

bp2 Unit 2—Short, thick (15 m) lava flow of sparsely porphyritic ol basalt (51.4% SiO2) just 

north-northwest of Poison Lake. Lava flow erupted from same vent as bp1 but apparently 

with little or no additional pyroclastic activity, because no cinders of bp2 lithology were 

found on cone. Lava flowed only ~1 km from vent; most of unit is north of map area. Flow 

has rubbly and vesicular aa top and is often strongly oxidized. In some places, it is covered 

by cinders rafted from bp1 cone and by deep reddish soil. Phenocrysts: 2% ol, 0.5–2 mm; 

sparse pl, 0.5 mm. Groundmass is light to medium gray, glassy to aphanitic, and microvesicular. Unit contains fewer, but larger, ol phenocrysts than bp1. Unit bp2 overlies unit 

bp1. [PL]

bp1 Unit 1—Thick (15 m) lava flow and cinder cone of sparsely porphyritic ol basalt (52.2–52.3% 

SiO2). Vent is at large, well-preserved cinder cone (hill 1882) northwest of Poison Lake, just 

north of map area (see fig 3). Lava flowed north and northeast as far as 3 km; most of unit 

is north of map area. Lava bench on southwest side of cone is covered by 2-m-thick blanket 

of cinders; most cinders on bench and in cone are oxidized. Flow is covered by deep reddish 

soil. Phenocrysts: 4% ol, 0.5–2 mm, mostly <1 mm; sparse pl, 0.5 mm. Groundmass is light 
73

to medium gray, glassy to aphanitic, and microvesicular. Unit bp1 is overlain by basalts of 

old railroad grade, unit 3 (bg3), unit bp2, and unit bp3. [PL]

Basalts and basaltic andesites of Bogard Buttes (late Pleistocene)—Group of 12 thick (10–15 

m), short aa and block lava flows and cinder cones of sparsely porphyritic ol basalt and 

basaltic andesite (52.1–53.2% SiO2), comprising the largest group of flows and cinder cones 

in the Poison Lake chain. One cinder cone (just northwest of hill 2092, fig. 3) and associated 

flow (bb7) are east of map area in Pine Creek Valley quadrangle. Most cinders in cones are 

oxidized, but sporadic unoxidized blocks are found locally. Rough upper flow surfaces are 

well preserved but in some places covered by poorly developed orange soil and sparse forest. Some flows have distinct lateral flow levees. Flows fall into two lithologic groups: bb1–

bb7 are slightly more mafic than bb8–bb12, tend to be less porphyritic, and have ol>pl. In 

contrast, bb8–bb12 have ol<pl and are usually slightly more porphyritic. Phenocrysts (both 

groups): sparse–4% pl, 0.5–2 mm; 1–2% ol, 0.5–2 mm. Small glomeroporphyritic clots of 

ol+pl and rarely aug are sparse in most flows. Groundmasses are medium to dark gray, aphanitic, and generally microvesicular. 40Ar/39Ar ages of units bb12 and bb6 are 107.5±3.8 and 

100.1±8.5 ka, respectively; all twelve units are approximately the same age. [BB, PL] 

bb12 Unit 12—Large, composite cinder cone and lava flow of sparsely porphyritic ol basaltic andesite (53.1–53.2% SiO2

) located on upper west flank of Bogard Buttes. Early cone is primarily 

oxidized scoria, but also contains a few dense bombs. Small, well-preserved late cone was 

built on top of early cone; thick, short aa lava flow erupted through it and flowed down steep 

flank of Bogard Buttes ~1 km and constructed prominent flow levees. Phenocrysts: 1–2% 

pl, 0.5–2 mm, mostly 0.5–1 mm; sparse ol, 0.5–2 mm, mostly 0.5–1 mm. Small glomeroporphyritic clots of ol+pl are sparse. Unit overlies dacite of Bogard Buttes (Tdb), unit bb4, and 

unit bb8. 40Ar/39Ar age is 107.5±3.8 ka. [BB]

bb11 Unit 11—Thick, aa lava flow of sparsely porphyritic ol basaltic andesite (53.1% SiO2) that 

erupted from well-preserved cinder cone (hill 2023, 3.2 km south of Bogard Buttes). Lava 

flowed east ~2 km out of map area. Phenocrysts: 2% pl, 0.5–2 mm, mostly 0.5–1 mm; 

sparse ol, 0.5–2 mm, mostly 0.5–1 mm. Small glomeroporphyritic clots of pl+ol are sparse. 

Groundmass is medium to dark gray, aphanitic, and microvesicular. Unit bb11 overlies units 

bb1, bb2, bb6, and bb9. [BB]

bb10 Unit 10—Thick, short aa lava flow of sparsely porphyritic ol basalt (53.0% SiO2) erupted from 

cinder cone 2.5 km south-southwest of Bogard Buttes, just south of hill 2106, and flowed 

west and south ~1 km. Phenocrysts: 1–2% pl, 0.5–2 mm, mostly 0.5–1 mm; sparse ol, 0.5–2 

mm, mostly 0.5–1 mm. Small glomeroporphyritic clots of ol+pl are sparse. Groundmass is medium to dark gray, aphanitic, and dense to microvesicular. Unit bb10 overlies basalt of section 

11 (b11), basalts of Stephens Campground, unit 2 (bs2), and units bb3, bb8, and bb9. [BB]

bb9 Unit 9—Thick, short aa lava flow of sparsely porphyritic ol basalt (52.8% SiO2) that erupted 

from small, well-preserved cinder cone 3.2 km south of Bogard Buttes, 0.6 km west of hill 

2023. Lava flowed west ~0.5 km, rafting part of cinder cone. Phenocrysts: 1–2% pl, 0.5–2 

mm, mostly 0.5–1 mm; sparse ol, 0.5–2 mm, mostly 0.5–1 mm. Small glomeroporphyritic 

clots of ol+pl are sparse. Groundmass is medium to dark gray, aphanitic, and dense to microvesicular. Unit bb9 overlies units bb1, bb2, and bb3 and is overlain by units bb10 and 

bb11. [BB]

bb8 Unit 8—Thick, short block lava flow of sparsely porphyritic ol basalt (52.8% SiO2) erupted 

from small well-preserved cinder cone (hill 2106) on south flank of Bogard Buttes. Lava 

flowed 1 km west. Phenocrysts: 1–2% pl, 0.5–1 mm; sparse ol, 0.5–1 mm. Small glomeroporphyritic clots of pl are sparse. Groundmass is medium gray, aphanitic, and microvesicular. Unit bb8 overlies dacite of Bogard Buttes (Tdb), basalt of hill 2088 (bl20), and unit 

bb3; overlain by units bb10 and bb12. [BB]

bb6 Unit 6—Thick, short aa lava flow of sparsely porphyritic ol basalt (52.8% SiO2) erupted from 

small, well-preserved cinder cone (hill 2008) 4 km south of Bogard Buttes. Lava flowed east 

~2 km out of map area. Phenocrysts: 1% ol, 0.5–2 mm, mostly 0.5–1 mm; sparse pl, 0.5–1 

mm. Groundmass is medium to dark gray, aphanitic, and dense to microvesicular. Unit bb6

overlies basaltic andesites of Susan River (Tmsr), basalt of section 11 (b11), and unit bb2; 

overlain by unit bb11. 40Ar/39Ar age is 100.1±8.5 ka. [BB]

bb5 Unit 5—Thick, aa lava flow of porphyritic ol basalt (52.4% SiO2) erupted from vent marked 

by cinder cone at hill 2096 on northwest flank of Bogard Buttes. Lava flowed north 4 km. 
74

Phenocrysts: 4% pl, 0.5–1 mm; 2% ol, 0.5–2 mm, mostly 0.5–1 mm. Unit is more porphyritic than other bb units, but most crystals occur in small glomeroporphyritic clots of pl+ol. 

Groundmass is medium to dark gray, aphanitic, and microvesicular. Unit bb5 overlies dacite 

of Bogard Buttes (Tdb), basalts of Robbers Spring, unit 2 (br2), basalts of Stephens Campground, unit 5 (bs5), and unit bb4. [BB, PL]

bb4 Unit 4—Thick, short aa lava flow of sparsely porphyritic ol basalt (52.5% SiO2) that erupted 

from small, well-preserved cinder cone on west flank of Bogard Buttes and flowed west 

~1 km. Phenocrysts: 1–2% ol, 0.5–2 mm, mostly 0.5–1 mm; sparse pl, 0.5–1 mm. Small 

glomeroporphyritic clots of ol+pl and aug+pl are very sparse. Groundmass is medium gray, 

aphanitic, and dense to microvesicular. Unit bb4 overlies dacite of Bogard Buttes (Tdb) and 

basalts of Stephens Campground, units 4 and 5 (bs4, bs5); overlain by units bb5 and bb12. 

[BB]

bb3 Unit 3—Thick, short block lava flow of sparsely porphyritic ol basalt (52.3% SiO2) erupted 

from small well-preserved cinder cone just east of hill 2106 on south flank of Bogard Buttes 

and flowed east ~2 km just out of map area. Phenocrysts: 1–2% ol, 0.5–2 mm, mostly 0.5–1 

mm; sparse pl, 0.5–1 mm. Small glomeroporphyritic clots of aug are very sparse. Groundmass is medium to dark gray, aphanitic, and dense to microvesicular. Unit bb3 overlies 

dacite of Bogard Buttes (Tdb) and unit bb1; overlain by units bb8, bb9, and bb10. [BB]

bb2 Unit 2—Thick, short aa lava flow of sparsely porphyritic ol basalt (52.1% SiO2) located 3.5 

km south of Bogard Buttes. No obvious vent, although the flow may have erupted at site of 

bb9 cone. Phenocrysts: 1–2% ol, 0.5–2 mm, mostly 0.5–1 mm; sparse pl, 0.5–1 mm. Small 

glomeroporphyritic clots of ol+pl and rarely aug are sparse. Groundmass is medium gray, 

aphanitic, and microvesicular. Unit bb2 is overlain by units bb6, bb9, and bb11. [BB]

bb1 Unit 1—Short, thick aa lava flow of sparsely porphyritic ol basalt (52.1% SiO2) located 2.9 km 

south-southwest of Bogard Buttes. No obvious vent, although the flow may have erupted 

at site of bb10 cone south of Bogard Buttes. Lava flowed east ~1.5 km out of map area. 

Phenocrysts: 1–2% ol, 0.5–2 mm, few larger. Also contains few small glomeroporphyritic 

clots of aug and is only Bogard Buttes unit that lacks pl. Groundmass is dark gray, aphanitic, 

and microvesicular. Unit bb1 is correlated with other Bogard Buttes units based on its spatial relationship, similar lithology and composition, and similar apparent age; however, it is 

slightly different in lithology and composition and could be older or unrelated to them. Unit 

bb1 is overlain by units bb3, bb9, and bb11. [BB]

Basalts of Pine Creek (late Pleistocene)—Group of three cinder cones and lava flows of 

sparsely porphyritic aug-ol basalt (52.2–52.5% SiO2) located 4.5 to 6 km south of Bogard 

Buttes. Soil and forest are weakly developed on the lava flows. Basalt of Pine Creek units 

overlie much older rocks and are not in contact with any other Poison Lake chain units. They 

are placed stratigraphically below basalts and basaltic andesites of Bogard Buttes primarily 

based on their lithologic and compositional similarity and 40Ar/39Ar age of 106.2±3.6 ka for 

unit bpc1. [BB]

bpc3 Unit 3—Two short, thick (10–20 m), aa to block lava flows of sparsely porphyritic aug-ol 

basalt (52.2% SiO2) that erupted from small cinder cone (hill 2023, just north of Pine Creek) 

and flowed only a few hundred meters from vent. Phenocrysts: 1–2% ol, 0.5–1 mm, few 

larger; trace aug, 0.5–1 mm; trace pl, 0.5–1 mm. Lava has vesicular flow top and contains 

abundant microphenocrysts (to 0.25 mm) of pl and mafic minerals. Groundmass dark gray, 

aphanitic, and microvesicular. Unit bpc3 overlies basalt of section 11 (b11) and no other 

rock units. Relation to units bpc1 and bpc2 is unclear. [BB]

bpc2 Unit 2—Short, thick (10–20 m) aa to block lava flow of sparsely porphyritic aug-ol basalt 

(52.5% SiO2) that erupted from small cinder cone (hill 1986) and flowed 1 km northeast out 

of map area. Phenocrysts: 1–2% ol, 0.5–1 mm, few larger; trace aug, 0.5–1 mm; trace pl, 

0.5–1 mm. Lava has vesicular flow top and contains abundant microphenocrysts (to 0.25 

mm) of pl and mafic minerals. Groundmass dark gray, aphanitic, and microvesicular. Unit 

bpc2 overlies basalt of section 11 (b11) and unit bpc1. [BB]

bpc1 Unit 1—Short, thick (10–20 m) aa to block lava flow of sparsely porphyritic aug-ol basalt 

(52.4% SiO2) that erupted from small cinder cone (hill 2060) and flowed 1 km north and 

west. Phenocrysts: 1–2% ol, 0.5–1 mm, few larger; trace aug, 0.5–1 mm; trace pl, 0.5–1 mm. 

Lava has vesicular flow top and contains abundant microphenocrysts (to 0.25 mm) of pl and 

mafic minerals. Groundmass dark gray, aphanitic, and microvesicular. Unit bpc1 overlies 
75

basaltic andesites of Susan River (Tmsr) and is overlain by unit bpc2. 40Ar/39Ar age is 

106.2±3.6 ka. [BB]

Basalts of Pittville Road (late Pleistocene)—Group of cinder cones and lava flows of sparsely 

porphyritic aug-ol basalt (50.3–50.6% SiO2). Basalts of Pittville Road are older than basalts 

of old railroad grade, unit 4 (bg4, 102.1±11.4 ka) and younger than basalts of Stephens 

Campground, units 4 and 5 (bs4, 110.8±3.8 ka and bs5, 108.6±2.1 ka); 40Ar/39Ar age of unit 

bt4 is 101.0±1.9 ka and K-Ar age of unit bt1 is 108±14 ka. [BB, PL]

bt4 Unit 4—Thick, short stubby lava flow and complex of large, well-preserved cinder cones (hill 

1974 and hill just to south) of aug-ol basalt (50.4% SiO2), located 5 km south-southeast of 

Poison Lake. Flow moved ~1 km north and may be partly buried beneath younger units. 

Phenocrysts: 2% ol, 0.5–2 mm; 1–2% aug, 0.5–2 mm. Contains sparse, small glomeroporphyritic clots of aug. Groundmass is medium gray, aphanitic to holocrystalline, and dense 

to microvesicular. Unit bt4 overlies basalts and basaltic andesites of Swains Hole (bsh), 

basalts of Robbers Spring, units 1 and 2 (br1, br2), basalts of Stephens Campground, units 2 

and 4 (bs2, bs4), and units bt2 and bt3. 40Ar/39Ar age is 101.0±1.9 ka. [BB, PL]

bt3 Unit 3—Short, thick, block-lava flow and large cinder cone of sparsely porphyritic aug-ol 

basalt (50.3% SiO2). Lava flowed north ~1.5 km from vent at hill 2001, 3.3 km northwest of 

Bogard Buttes. Phenocrysts: 2% ol, 0.5–1 mm; 1% aug, 0.5–1 mm; rare pl, 0.5 mm. Most of 

phenocrysts are in small glomeroporphyritic clots of ol or aug. Groundmass is medium gray, 

aphanitic, and microvesicular. Unit bt3 overlies basalts of Stephens Campground, unit 5 

(bs5), basalts of Robbers Spring, unit 1 (br1), and unit bt2; overlain by unit bt4. [BB]

bt2 Unit 2—Short, 10-m-thick, block lava flow and small but well-preserved cinder cone of 

sparsely porphyritic ol basalt (50.4–50.6% SiO2), 3.5 km south-southeast of Poison Lake just 

southwest of hill 1919. Lava flowed west and east in two, 1-km-long, broad lobes. Western 

lobe has steep front, and top of flow is composed of microvesicular blocks. East lobe has 

more subdued flow front and is separated from cinder cone by basalts of Pittville Road, unit 

4 (bt4). Phenocrysts: 1–2% ol, 0.5–2 mm; sparse aug, 0.5–2 mm, mostly 0.5 mm. Contains 

sparse, small glomeroporphyritic clots of aug. Groundmass is dark gray, aphanitic to holocrystalline, and microvesicular. Unit bt2 overlies basalts and basaltic andesites of Swains 

Hole (bsh), basaltic andesite of hill 1868 (ml18), and basalts of Robbers Spring, unit 2 

(br2); overlain by basalts of Poison Butte, units 5 and 6 (bp5, bp6) and unit bt4. [BB, PL]

bt1 Unit 1—Lava flow of sparsely porphyritic aug-ol basalt (50.3% SiO2) located 1.5 km southeast of Poison Lake, mostly buried by younger deposits. Vent probably lies buried beneath 

younger basalts of Pittville Road (bt) or basalts of Poison Buttes (bp). Phenocrysts: 2% ol, 

0.5–2 mm; rare aug, 0.5–1 mm. Groundmass is medium gray, aphanitic, and microvesicular. 

Unit bt1 is overlain by basalts of Poison Butte, unit 6 (bp6) and overlies basalts of Robbers 

Spring, unit 2 (br2). K-Ar age is 108±14 ka. [PL]

m20 Basaltic andesite west of hill 2078 (late Pleistocene)—Cinder cone of sparsely porphyritic ol 

basaltic andesite (53.5% SiO2) located just west of hill 2978, 3.5 km west of Bogard Buttes. 

Correlated with the Poison Lake chain on the basis of geography and stratigraphy, but 

distinguished from the other units on the basis of petrographic and chemical characteristics. 

No associated lava flow, although one may be buried by basalts of Stephens Campground 

lava flows. Scoria in cone is mostly oxidized. Phenocrysts: 1–2% ol, 0.5–2 mm; rare aug, 

1–2 mm; sparse pl, 0.5 mm. Groundmass of unoxidized bombs is dark gray, glassy, and 

microvesicular to dense. Unit m20 overlies basalts of Stephens Campground, units 4 and 5 

(bs4, bs5), but stratigraphic relation to other Poison Lake units is uncertain. Age is poorly 

constrained, but probably similar to other units of Poison Lake chain. [BB]

Basalts of Stephens Campground (late Pleistocene)—Group of cinder cones and lava flows 

of porphyritic ol basalt (50.4–51.5% SiO2). Compared to other units of Poison Lake chain, 

this group is characterized by abundant pl (except bs1) and relatively thin, large-volume, 

far-traveled lava flows. Some flows (bs3, bs4, bs5) also contain aug phenocrysts. Basalts 

of Stephens Campground are older than basalts of Pittville Road and younger than basalts of 

Robbers Spring. 40Ar/39Ar ages of unit bs5 and unit bs4 are 108.6±2.1 ka and 110.8±3.8 ka, 

respectively. [BB, PL]

bs5 Unit 5—Thick (15–20 m) lava flow and large, well-preserved cinder cone (hill 2078) of 

porphyritic aug-ol basalt (51.4% SiO2), located 3 km west of Bogard Buttes. Lava flowed 

~3 km north. Phenocrysts: 8% ol, 0.5–3 mm; 4% pl, 0.5–1 mm; 1% aug, 0.5–1 mm. Glom-
76

eroporphyritic clots of ol+pl+aug to 5 mm are common. Groundmass is medium gray, aphanitic, and microvesicular. Top of flow is vesicular aa and is covered by poorly developed 

orange soil. Unit bs5 overlies dacite of Bogard Buttes (Tdb), basalts of Robbers Spring, unit 

2 (br2), and unit bs4; overlain by basalts of Pittville Road, units 2 and 3 (bt2, bt3), basaltic 

andesite west of hill 2078 (m20), and basalts and basaltic andesites of Bogard Buttes, units 

4 and 5 (bb4, bb5). 40Ar/39Ar age is 108.6±2.1 ka. [BB, PL]

bs4 Unit 4—Short lava flow of porphyritic aug-ol basalt (50.6% SiO2) with small, well-preserved 

cinder cone marking vent on west flank of Bogard Buttes. Lava flowed west and north ~3 

km and is separated from its cone by younger lava flows of basalts and basaltic andesites of 

Bogard Buttes, unit 4 (bb4). Phenocrysts: 8% pl, 0.5–2 mm, mostly 0.5–1 mm; 5% ol, 0.5–2 

mm; sparse aug, 0.5–1 mm. Small glomeroporphyritic clots of pl+ol+aug to 5 mm are common. Groundmass is medium gray, aphanitic to holocrystalline, and microvesicular. Large, 

round to ovoid vesicles are abundant in blocks at flow surface; flow has massively jointed 

interior. Unit bs4 overlies dacite of Bogard Buttes (Tdb), basalt of section 2 (bl2), basalts 

of Robbers Spring, unit 1 (br1), and unit bs2; overlain by basaltic andesite west of hill 2078 

(m20), bb4, and unit bs5. 40Ar/39Ar age is 110.8±3.8 ka. [BB]

bs3 Unit 3—Thin (5 m) block lava flow of porphyritic aug-ol basalt (51.4–51.5% SiO2). Small 

cinder cone at hill 2146 marks vent location, 3.7 km southwest of Bogard Buttes. Lava 

flowed northwest and south as far as 3 km from vent area; small lobe flowed few hundred 

meters east. Phenocrysts: 6–10% pl, 0.5–2 mm, mostly 0.5 mm; 4–5% ol, 0.5–2 mm, mostly 

0.5–1 mm; 1–2% aug, 0.5–1 mm. Glomeroporphyritic clots of ol+pl to 5 mm are common. 

Groundmass is medium gray, aphanitic to holocrystalline, and microvesicular. Blocks on 

flow surface are very vesicular; joint pattern is massive and flow has only thin soil cover. 

Unit overlies basalt of section 11 (b11), basalt of hill 2232 (bl22), and units bs1 and bs2; 

overlain by basalts of Cone Lake Road, units 1 and 2 (bc1, bc2). [BB]

bs2 Unit 2—Thin, block lava flow of porphyritic ol basalt (50.5–50.8% SiO2). Small cinder cone 

adjacent to bs3 cone, 3.7 km southwest of Bogard Buttes, marks vent location. Lava flowed 

west and north from vent area as far as 6 km and east for a few hundred meters. Phenocrysts: 

5–7% ol, 0.5–5 mm; mostly 0.5–1 mm; 2–3% pl, 0.5–2 mm, mostly 0.5–1 mm. Small glomeroporphyritic clots of ol+pl are present. Groundmass is medium to dark gray, aphanitic to 

holocrystalline, and microvesicular. Large, round to ovoid vesicles are abundant in blocks at 

surface of flow; joint pattern is massive. Unit bs2 overlies basalt of section 11 (b11), basalts 

and basaltic andesites of Swains Hole (bsh), basalt of hill 2232 (bl22), basalt of section 2 

(bl2), and basalts of Robbers Spring, unit 1 (br1); overlain by basalts of Pittville Road, unit 

4 (bt4), basalts of Cone Lake Road, units 1 and 2 (bc1, bc2), basalt of Twin Buttes (bdt), 

basalts and basaltic andesites of Bogard Buttes, unit 10 (bb10), unit bs3, and unit bs4. 

[BB]

bs1 Unit 1—Thin (5 m) block lava flow of porphyritic ol basalt (50.4% SiO2) located 6 km southwest of Bogard Buttes. Vent was probably to north near vents for units bs2, bs3, bc1, and 

bc2 and is now buried by younger lavas of Poison Lake chain. Lava flowed at least 3 km 

from inferred vent area. Phenocrysts: 5% ol, 0.5–3 mm; trace pl with dark-gray cores. Small 

glomeroporphyritic clots of ol are also present. Groundmass is medium gray, aphanitic, and 

microvesicular. Lava at flow surface contains abundant large, rounded to flattened vesicles; 

lava breaks along joints into thick tabular to squarish slabs. Flow has thin soil. Unit bs1

overlies basalt of section 11 (b11); overlain by unit bs3. [BB]

Basalts of Robbers Spring (middle Pleistocene)—Group of two cinder cones and lava flows of 

sparsely porphyritic to porphyritic ol basalt (51.1–51.4% SiO2). Oldest group of vents and 

flows in Poison Lake chain; overlain by basalts of Poison Butte, basalts of Pittville Road, 

and basalts of Stephens Campground. 40Ar/39Ar age of unit br2 is 101.5±8.7 ka. [BB, PL]

br2 Unit 2—An extensive, 10-m-thick, aa and block-lava flow and large cinder cone (hill 1919) 

of porphyritic ol basalt (51.1–51.4% SiO2), located 3.5 km south-southeast of Poison Lake. 

Lava flowed northeast and east 3 km and emplaced a broad lava flow. Top of flow composed 

of microvesicular blocks. Phenocrysts: 6% ol, 0.5–2 mm, mostly 0.5–1 mm. Groundmass is 

dark gray, aphanitic, and microvesicular. Unit br2 overlies tholeiitic basalt of Grays Valley 

(bgv) and is overlain by basalts of Stephens Campground, unit 5 (bs5), basalts of Pittville 

Road, units 1, 2, and 4 (bt1, bt2, bt4), basalts of Poison Butte, unit 6 (bp6), and basalts and 

basaltic andesites of Bogard Buttes, unit 5 (bb5).40Ar/39Ar age is 101.5±8.7 ka. [BB, PL]
77

br1 Unit 1—Thin lava flow and well-preserved cinder cone (hill 1921) of porphyritic ol basalt 

(51.3% SiO2), located 3.8 km west-northwest of Bogard Buttes. Lava has aa surface and 

flowed northwest for at least 1 km, but it is largely buried by younger lava flows of Poison Lake 

chain. Phenocrysts: 10% ol, 0.5–3 mm, largest generally composite grains; sparse pl, 0.5 mm. 

Few small glomeroporphyritic clots of ol present. Groundmass medium to dark gray, aphanitic, and strongly microvesicular. Unit br1 is overlain by basalts of Stephens Campground, 

units 2 and 4 (bs2, bs4) and by basalts of Pittville Road, units 3 and 4 (bt3, bt4). [BB]

Cone Lake chain

Cone Lake chain comprises a group of spatially and temporally related basalt and basaltic andesite lava and cinder cones in the north part of the Caribou Volcanic Field 4–5 km west of Bogard Buttes. 

Units assigned to the Cone Lake chain form two parallel arrays approximately coincident with the Poison 

Lake chain. Four of the eight units constitute a zone of vents 8 km long and ~1.5 km wide trending N. 30° 

W. from Cone Lake. The other four units form a parallel 8-km-long array displaced 3.5 km to the east. 

Although generally similar in character to the Poison Lake chain, the Cone Lake chain is older (~200–250 

ka) and is partially buried by units of the Poison Lake and the Bidwell Spring chains. Units of the Cone 

Lake chain are mostly beyond the north limit of glaciation; cones are fairly well preserved.

bl35 Basalt of section 35 (middle Pleistocene)—Short block-lava flow and large, poorly preserved 

cinder cone of sparsely porphyritic ol basalt (52.5% SiO2) located 2.9 km west-southwest of 

Bogard Buttes, just southeast of hill 2091. Phenocrysts: 2–3% ol, 0.5–1 mm. Small glomeroporphyritic clots of ol are common. Groundmass is medium gray, aphanitic to holocrystalline, and microvesicular and has vague flow lineation and interstitial vapor-phase silica. 

Unit bl35 overlies unit bl2. 40Ar/39Ar age is 202.0±2.2 ka. [BB]

bl2 Basalt of section 2 (middle Pleistocene)—Short block-lava flow and large, well-preserved 

cinder cone (hill 2091) of sparsely porphyritic ol basalt (51.5–52.5% SiO2) located 3.2 

km west-southwest of Bogard Buttes. Lavas in Poison Lake chain may bury much of lava 

flow. Phenocrysts: 2–3% ol, 0.5–1 mm. Small glomeroporphyritic clots of ol are common. 

Groundmass is medium gray, aphanitic to holocrystalline, and microvesicular and has a 

vague flow lineation and interstitial vapor-phase silica. Unit bl2 is overlain by basalts of 

Stephens Campground, units 2 and 4 (bs2, bs4) and by basalt of section 35 (bl35). [BB]

bl20 Basalt of hill 2088 (middle Pleistocene)—Cinder cone (hill 2088) of porphyritic ol basalt 

(52.2% SiO2) located 3.2 km southwest of Bogard Buttes. Large, well-preserved cone is 

composed of oxidized cinders and ash and has thick orange soil cover. It has no apparent lava flow, but one could be buried by younger flows and cones of Poison Lake chain. 

Phenocrysts: 4% ol, 0.5–3 mm, mostly 1 mm. Sparse, fresh bombs in cinder cone have black 

to dark-gray, glassy to aphanitic groundmass and are dense to microvesicular. Unit bl20 is 

overlain by basalts of Cone Lake Road, unit 1 (bc1) and basalts and basaltic andesites of 

Bogard Buttes, unit 8 (bb8). [BB]

bl30 Basalt of section 30 (middle Pleistocene)—Short, thin lava flow of porphyritic ol basalt at 

northwest end of Cone Lake chain, with small, well-preserved cinder cone just east of hill 

1879. Scoria in cinder cone is mostly oxidized. Lava flowed ~1 km north. Phenocrysts: 

3–4% ol, 0.5–2 mm, few larger; sparse pl, 0.25–0.5 mm. Small glomeroporphyritic clots of 

ol+pl to 5 mm are common. Groundmass is medium to dark gray, glassy to aphanitic, and 

microvesicular. Unit overlies basalts and basaltic andesites of Swains Hole (bsh) and basalt 

of hill 2109 (bl21); overlain by tholeiitic basalt of hill 1879 (b18) and basalt of Twin Buttes 

(bdt). Unit bl30 is not dated, nor is its relation to any dated unit known. It is, however, 

somewhat older than the Poison Lake chain and probably older than ~200 ka. It is probably 

younger than unit bl21, which is estimated to be 225–250 ka, and is probably younger than 

basalt of hill 2232 (bl22, 220±14 ka). Estimated to be 200–220 ka. [BB]

bl22 Basalt of hill 2232 (middle Pleistocene)—Small shield volcano in southeastern part of Cone 

Lake chain, composed of lava flows of porphyritic ol basalt (51.2% SiO2), with eroded cinder 

cone marking vent. Lava flowed in all directions from vent to produce small shield at least 4 

km diam. Younger flows and glacial deposits buried much of this flow. Phenocrysts: 10% pl, 

0.5–2 mm, mostly 0.5–1 mm; 4% ol, 0.5–1 mm. Glomeroporphyritic clots of ol+pl to 1 cm 

are common. Groundmass is medium gray, holocrystalline, and microvesicular. Locally contains xenoliths of milky vein qtz to 2–3 cm. Flows are massively jointed. Unit bl22 overlies 
78

basaltic andesites of Cone Lake (mlc); overlain by basalts of Stephens Campground, units 2 

and 3 (bs2, bs3) and basalt of Twin Buttes (bdt). K-Ar age is 220±14 ka. [BB]

bl21 Basalt of hill 2109 (middle Pleistocene)—Relatively thin, short, aa lava flow of porphyritic ol 

basalt (50.8% SiO2) with large cinder cone marking vent. Located in northwest part of Cone 

Lake chain. Small lava tubes are present; flow top is vesicular. Lava flowed northwest from 

vent ~3 km. Younger units have buried much of this flow. Cinder cone has at least two craters, neither of which is well preserved. Phenocrysts: 3% ol, 0.5–2 mm, mostly 0.5–1 mm; 

sparse pl, 0.5–1 mm, mostly in clots. Groundmass is medium gray, aphanitic, and microvesicular. Small glomeroporphyritic clots of ol+pl are sparse. Unit bl21 overlies basalts and 

basaltic andesites of Swains Hole (bsh); overlain by basalt of section 30 (bl30), tholeiitic 

basalt of hill 1879 (b18), basaltic andesite of Pole Spring Road (mdp), and basalt of Twin 

Buttes (bdt, 46.3±3.4 ka). Unit not dated and is much older than unit bdt. It is somewhat 

older than Poison Lake chain and appears to be slightly older than nearby basalt of hill 2232

(bl22, 220±14 ka). Estimated to be about 225–250 ka. [BB, PP]

mlc Basaltic andesites of Cone Lake (middle Pleistocene)—Two large and two small cinder cones 

of similar age located in southeast part of Cone Lake chain. Lava flows from these cones 

may exist buried beneath till, younger glaciations (Qty), basalt of Twin Buttes (bdt), or 

basalt of hill 2232 (bl22). Cones are moderately well preserved but have well-developed, 

thick, orange soils. Phenocrysts (big cone just east of Cone Lake): 2–3% ol, 0.25–1 mm, 

mostly <0.5 mm; sparse pl, 0.5 mm. Phenocrysts (big cone just northwest of Cone Lake): 

15% pl, 0.25–1 mm, mostly 0.5 mm; 2–3% ol, 0.25–2 mm, mostly 1.0 mm; trace aug, 0.5 

mm. Phenocrysts (small cone 1 km north of Cone Lake): 2–3% ol, 0.5–1 mm, mostly ~1 

mm; sparse pl, 0.5 mm. Phenocrysts (small cone north of Twin Buttes): 2% ol, 0.5–2 mm; 

sparse–2% pl, 0.5 mm. All have dark gray, glassy, microvesicular groundmasses. Unit mlc

is overlain by basalt of hill 2232 (bl32) and basalt of Twin Buttes (bdt, 46.3±3.4 ka). Age is 

poorly constrained; considerably older than unit bdt and may be slightly older than unit bl22

(220±14 ka). Estimated to be about 225–250 ka. [BB]

ml18 Basaltic andesite of hill 1868 (middle Pleistocene)—Lava flow and cinder cone of porphyritic 

aug-ol basaltic andesite (54.1–54.4% SiO2), located 2.5 km south of Poison Lake. Lava flow 

has very vesicular and scoriaceous top and is covered by deep soil. Edifice cut by small 

faults and mostly buried by younger lava flows of Poison Lake chain. Phenocrysts: 15% 

pl, 0.5–3 mm, mostly 0.5–1 mm; 8% ol, 0.5–2 mm, mostly 0.5–1 mm; aug 2%, 0.5–2 mm, 

mostly 0.5–1 mm. Most crystals present as glomeroporphyritic clots from few millimeters 

to 1 cm. Groundmass is medium to dark gray, aphanitic to holocrystalline, and dense to 

microvesicular. Flow interior is dense to microvesicular and massively jointed. Unit ml18 

overlies basalts and basaltic andesites of Swains Hole (bsh); overlain by basalts of old 

railroad grade, units 1 and 2 (bg1 and bg2), basalts of Pittville Road, unit 2 (bt2), basalts 

of Poison Butte, units 3, 4, and 5 (bp3, bp4, bp5), and basalt of Twin Buttes (bdt, 46.3±3.4 

ka). Age is poorly constrained; considerably older than bdt and considerably younger than 

bsh. It is older than Poison Lake chain and probably older than basalt of hill 2232 (bl22, 

220±14 ka). Estimated to be about 225–250 ka. [BB, PL]

Caribou chain

The Caribou chain comprises a zone of vents l4 km long and 1.5 km wide trending N. 25° W. across 

the east part of the Caribou Volcanic Field, extending from Star Butte in the southeast to Triangle Lake in 

the northwest. Rocks of the Caribou chain are primarily eroded lava flows of ol basaltic-andesite and aug–ol 

basaltic andesite with little lithologic or chemical diversity. Small gabbroic inclusions are distinctive. The 

Caribou chain has been highly glaciated. Cinder cones are generally absent, although some vents are marked 

by resistant agglutinate cones or plugs. The Caribou chain was active about 425–275 ka. 

The Caribou chain is divided into three sequences: (1) The Triangle Lake sequence to the north, (2) the 

Echo Lake sequence to the south, and (3) the Caribou sequence. The Caribou sequence comprises the highlands of the Caribou Wilderness and is dominated by the large North, middle, and South Caribou lava cones 

but includes many smaller volcanoes and lava flows. 

Triangle Lake sequence

A few basaltic andesite lava flows and cinder cones near Triangle Lake at the northern end of the 

Caribou chain are younger than the Caribou sequence. These lavas are heavily glaciated and obscured by a 
79

cover of drift. Their relation to other units is poorly known, but they are included in the Caribou chain on the 

basis of age and lithology. 

mtr Basaltic andesite of Triangle Lake (middle Pleistocene)—Short, thick lava flow of sparsely 

porphyritic aug-ol basaltic andesite (55.4% SiO2). Poorly preserved agglutinate cone at small 

hill east of Triangle Lake marks location of vent. Flows are heavily glaciated; typically, 

only flow cores crop out. Block carapaces and vesicular material generally are not found. 

Phenocrysts: 2% ol, 0.5–2 mm, mostly 0.5 mm; 1% pl, 0.5–2 mm, mostly 0.5 mm; trace 

aug, 0.5–1 mm. Variety of small inclusions and xenocrysts are common, including gabbroic 

clots of ol+aug+pl to 1 cm, qtz xenocrysts, and partially melted crustal inclusions. Many 

phenocrysts are disaggregated from gabbroic clots. Groundmass is medium to dark gray, 

aphanitic, and dense; jointing is massive. Unit mtr is completely surrounded by till, younger 

glaciations (Qty). K-Ar age is 277±22 ka. Unit mtr is thus one of youngest units in the 

Caribou chain. [BB]

mt20 Basaltic andesites of section 20 (middle Pleistocene)—Thick, short, porphyritic ol basaltic 

andesite lava flows in northwest part of Caribou chain. Poorly preserved vents are located at 

hill 2221 just southeast of Triangle Lake and at unnamed hill just northeast of Triangle Lake. 

Lava flows are heavily glaciated and mostly buried by till. Block carapaces or vesicular 

material generally are not found. Two flows have slightly different lithologies. Phenocrysts 

(hill 2221): 3–5% pl, 0.5–2 mm, few larger crystals usually composite; 3–4% ol, 0.5–3 

mm. Glomeroporphyritic clots of ol+pl to 1 cm are present. Groundmass is light gray, 

holocrystalline, and dense to weakly microvesicular. Phenocrysts (unnamed hill): 5–7% pl, 

0.5–2 mm, few larger crystals often composite; 3–4% ol, 0.5–1 mm, few larger; 1–2% aug, 

0.5–1 mm. Small glomeroporphyritic clots of ol+pl+aug are common; xenocrysts of qtz and 

hbl are rare. Most of pl and aug may have been derived from disaggregated glomeroporphyritic clots. Groundmass is medium to dark gray, holocrystalline, microvesicular, and often 

weakly oxidized. Unit mt20 overlies basalt of Caribou Wilderness (bcw) but is otherwise 

surrounded by till, younger glaciations (Qty). Unit mt20 has not been dated, but it is probably one of the younger units of the Caribou chain; it is estimated to be ~300 ka. [BB]

Echo Lake sequence

The Echo Lake sequence consists of a small group of lithologically similar basalt and basaltic andesite 

lava flows, domes, and cinder cones at the southeast end of the Caribou chain. The units are lithologically 

similar to rocks in the Caribou sequence but are slightly younger (~300 ka). Thick cover of till obscures most 

stratigraphic relations among units of Echo Lake sequence. 

Basalts of Bond Valley (middle Pleistocene)—Three poorly preserved cinder cones and lava 

flows of porphyritic ol basalt just northeast and southeast of Star Butte. Age is approximately equivalent to basaltic andesites of Echo Lake, unit 1 (mee1, 302±36 ka). The stratigraphic relationship among the three lava flows can be seen in the adjoining Swain Mountain 

quadrangle (off map to east). [RC]

beb3 Unit 3—Thick lava flow of porphyritic ol basalt (52.9 % SiO2). Cinder cone marking vent 

is very poorly preserved and mostly buried by till, older glaciations (Qto). Flow is heavily glaciated and mostly buried by till; flow top is scoriaceous. Lava flowed south ~0.3 km 

from vent along faulted margin of Swain Mountain. Phenocrysts: 10% pl, 0.5–2 mm, mostly 

<1 mm, many reacted; 2% ol, 0.5–1 mm. Small glomeroporphyritic clots of ol+pl are common. Groundmass is medium gray, aphanitic, and microvesicular. Unit beb3 overlies unit 

beb2. [RC]

beb2 Unit 2—Poorly preserved cinder cone partly buried by till marks vent for thick lava flow 

of porphyritic ol basalt. Lava flow is buried by till, older glaciations (Qto) and beb3 in 

the map area. Lava from this cone flowed south along faulted margin of Swain Mountain 

volcano for at least 6 km and crops out in adjacent Swain Mountain quadrangle. Lava flow 

may be slightly offset by faults along east margin of Swain Mountain. Phenocrysts: 4–6% 

ol, 0.5–2 mm, mostly <1 mm; 4–6% pl, 0.5–2 mm, mostly <1 mm. Glomeroporphyritic clots 

of pl+ol are abundant and as large as 1 cm; most phenocrysts occur in clots or were disaggregated from clots. Groundmass is light gray, holocrystalline, and coarsely microvesicular. 

Flow top is scoriaceous. Unit beb2 overlies unit beb1 and is overlain by unit beb3. [RC]
80

beb1 Unit 1—Poorly preserved cinder cone partly buried by till marks vent for thick lava flow of 

porphyritic ol basalt. Most cinders in cone are oxidized. Lava flow is buried by till, older 

glaciations (Qto) and by unit beb3 and crops out only east of map area in Swain Mountain quadrangle. Lava flowed south from this cone along west margin of Swain Mountain 

volcano for 6 km, where it is faulted. Phenocrysts: 8–10% ol, 0.5–4 mm; sparse pl, 1–2 mm; 

rare aug, 1–2 mm. Small glomeroporphyritic clots of pl+aug are rarely present. Groundmass 

is dark gray, aphanitic, and dense. Unit beb1 overlain by unit beb2. [RC]

Basaltic andesites of Echo Lake (middle Pleistocene)—Complex of lava domes, flows, and 

cinder cone of sparsely porphyritic ol basaltic andesite (53.9–55.6% SiO2) near Echo Lake. 

K-Ar age of unit mee1 is 302±36 ka; unit mee2 is approximately the same age. [RC]

mee2 Unit 2—Thick lava flow (30 m) of sparsely porphyritic ol basaltic andesite (55.2–55.6% SiO2). 

Cone of mostly oxidized cinders marks vent. Lava from cone flowed south in two thick 

lobes, west lobe ~3 km, and east lobe ~1 km. Lava flow outcrops along the contact between 

till, older glaciations (Qto) and till, younger glaciations (Qty) and is overlain by both. West 

lobe is weakly modified by glaciers but extensively buried by till; typically only steep flow 

margins crop out. East lobe is less extensively buried by till. Flow has a rough oxidized surface and is partially covered by agglutinate, probably rafted from cinder cone. Phenocrysts: 

3–4% pl, 0.5–1 mm, mostly 0.5 mm; 1–2% ol, 0.5–1 mm, mostly 0.5 mm; rare aug, 0.5 mm. 

Small qtz xenocrysts are sparse; some pl phenocrysts are strongly reacted. Groundmass is 

dark gray, glassy to aphanitic, and dense to microvesicular. Unit mee2 overlies older basalts 

and basaltic andesites south of Caribou Volcanic Field (boc) and unit mee1. [RC]

mee1 Unit 1—Lava domes and short, thick, lava flows of porphyritic, ol basaltic andesite (53.9% 

SiO2). Rocks are heavily glaciated; typically only dome and flow cores crop out. Five domes 

mark vents for lava flows; another vent is mostly buried by till. Some compositional variation is present. Phenocrysts: 10% pl, 0.5–2 mm, mostly 1 mm; 3–4% ol, 0.5–1 mm; sparse 

aug, 0.5–1 mm. Groundmass is medium gray, aphanitic, and dense to microvesicular. Unit 

mee1 is overlain by unit mee2. K-Ar age is 302±36 ka. [RC]

mes Basaltic andesite of Star Butte (middle Pleistocene)—Thick lava flow and cinder cone of 

sparsely porphyritic ol basaltic andesite near Echo Lake. Location of vent is marked by 

large cinder cone of Star Butte. Lava flowed ~1 km west, was heavily glaciated, and is 

almost completely buried by till. Phenocrysts: 4% ol, 0.25–1 mm; 1% pl, 0.25–1 mm. Contains few small glomeroporphyritic clots of ol+pl. Groundmass is medium gray, aphanitic 

to holocrystalline, and dense. Flow cores display thick slabby jointing. Unit mes overlies 

older basalts and basaltic andesites south of Caribou Volcanic Field (boc); overlain by 

basalts of Bond Valley, unit 2 (beb2), and by till, older glaciations (Qto). Approximately 

the same age as basalts of Bond Valley (beb) and basaltic andesites of Echo Lake (mee; 

302±36 ka). [RC]

Caribou sequence

The Caribou sequence consists of the volcanoes in the central part of the Caribou chain including the 

large edifices of North, middle, and South Caribou. 

Vents and lava flows northwest of Silver Lake and south of South Caribou form a platform upon which 

the three large Caribou edifices were built. Glaciers flowing from the Caribou edifices covered this platform 

during several episodes. Glacial erosion, lithologic homogeneity, textural variability, and abundant disaggregated inclusions combine to make these flows difficult to discriminate in the field. Although the stratigraphy 

is generally correct, there may be mistakes in mapping detail. In addition, some flows of relatively small 

volume or extent have been combined with larger flows.

mce Basaltic andesite of Eleanor Lake (middle Pleistocene)—Thick lava flow of porphyritic augol basaltic andesite (54.3% SiO2

) just east of Turnaround Lake. Lava erupted from poorly 

preserved vent at hill 2234 and flowed north and east ~1.5 km. Lava flow heavily glaciated; typically only flow cores crop out. Till and poorly developed soil cover much of flow. 

Phenocrysts: 20% pl, 0.5–2 mm, mostly 0.5–1 mm, many resorbed; 4% ol, 0.5–2 mm; sparse 

aug, 0.5–1 mm. Small glomeroporphyritic clots of ol+pl are abundant. Contains a variety 

of small (to 4–5 cm) coarse-grained granitic- to cumulate-textured inclusions, fragments of 

inclusions, and individual crystals derived from inclusions, which are more abundant than in 

similar units (basaltic andesite of Jewel Lake, mcj, and basaltic andesite of Rim Lake, mcr). 
81

Two most common types are fresh, but recrystallized, 2-pyr gabbro and partially melted gabbro containing pseudomorphs of hbl. Small xenoliths of metamorphic rock and qtz are also 

present. Black pseudomorphs of hbl are sparse. Groundmass is light to medium gray, generally holocrystalline and dense. Upper part of flow is microvesicular, oxidized, and massively 

jointed; interior of flow has thick tabular to platy jointing. Relation to similar units basaltic 

andesite of Jewel Lake (mcj) and basaltic andesite of Rim Lake (mcr) is unknown. Unit 

mce overlies basalt of Caribou Peak (bcp), basalt of Caribou Wilderness (bcw), basalt of 

Cowboy Lake (bcb), and basaltic andesites of Silver Lake, unit 8 (mc8, 400±14 ka). Age 

uncertain but probably younger than basaltic andesite of middle Caribou (mcm, 362±33 ka); 

estimated to be ~325–350 ka. [BB]

mcj Basaltic andesite of Jewel Lake (middle Pleistocene)—Thick lava flow of porphyritic aug-ol 

basaltic andesite (54.6–54.7% SiO2) 2.5 km west of Silver Lake. Lava erupted from poorly 

preserved vent at hill 2213 and flowed east ~2 km. Lava flow heavily glaciated; typically 

only flow cores crop out. Drift and poorly developed soil cover much of flow. Phenocrysts: 

8–10% pl, 0.5–2 mm, mostly 1 mm, few larger and many reacted; 2–3% ol, 0.5–1 mm; 

sparse–1% aug, 0.5–1 mm. Contains variety of small coarse-grained, granitic- to cumulatetextured inclusions, fragments of inclusions, and individual crystals derived from inclusions. 

These are generally partially melted or recrystallized, gabbroic in composition, and composed 

of pl+ol+pyr. Groundmass is light to medium gray, generally holocrystalline and dense. Upper part of flow is microvesicular, oxidized, and massively jointed; interior has thick tabular 

to platy jointing. Relation to similar units basaltic andesite of Rim Lake (mcr) and basaltic 

andesite of Eleanor Lake (mce) is unknown. Unit mcj overlies basaltic andesites of Silver 

Lake, units 6, 8, and 9 (mc6, mc8, mc9). Age uncertain but probably younger than basaltic 

andesite of middle Caribou (mcm, 362±33 ka); estimated to be ~325–350 ka. [BB, RC]

mcr Basaltic andesite of Rim Lake (middle Pleistocene)—Thick lava flow of porphyritic aug-ol basaltic andesite (55.7% SiO2

) 2 km west of Silver Lake. Lava erupted from vent at unnamed 

hill in western part of outcrop area and flowed east ~1.5 km. Lava flow heavily glaciated; 

typically only flow cores crop out. Till and poorly developed soil cover much of flow. Phenocrysts: 10% pl, 0.5–3 mm, mostly 1 mm, few larger composite grains; 2% aug, 0.5–1 mm; 

1% ol, 0.5–1 mm, few larger composite grains. Small glomeroporphyritic clots of aug+pl are 

present. Contains abundant, small, coarse-grained inclusions, fragments of inclusions, and 

individual crystals derived from inclusions. These are partially melted and microvesicular, 

gabbroic in composition, and composed of pl and pseudomorphs of hbl. Black pseudomorphs of resorbed hbl from a few millimeters to nearly 1 cm, derived from disaggregated 

gabbroic inclusions, are abundant and conspicuous. Groundmass is light to medium gray, 

aphanitic, and dense. Upper part of flow is microvesicular, oxidized, and massively jointed; 

interior of flow has thick tabular to platy jointing. Relation to similar units basaltic andesite 

of Jewel Lake (mcj) and basaltic andesite of Eleanor Lake (mce) is unknown. Unit mcr

overlies basaltic andesites of North Caribou, units 3 and 6 (mcn3, mcn6) and basaltic 

andesites of Silver Lake, units 6, 8, and 10 (mc6, mc8, mc10). Age uncertain but probably younger than basaltic andesite of middle Caribou (mcm, 362±33 ka); estimated to be 

~325–350 ka. [RC]

Basaltic andesites of North Caribou (middle Pleistocene)—Thick lava flows of porphyritic 

aug-ol basaltic andesite (54.4–56.9% SiO2

). Flows erupted from North Caribou and several 

associated vents and built large lava cone of North Caribou. Lava flows were heavily glaciated; typically only flow cores crop out. Till and poorly developed soil cover much of these 

flows. Flows contain a variety of small (to 4–5 cm) coarse-grained, granitic to cumulatetextured inclusions, fragments of inclusions, and individual crystals derived from inclusions. 

Also contain pseudomorphs of hbl, hbl pseudomorph-bearing inclusions, and qtz. Units 

overlie a number of basaltic andesites of Silver Lake units including mc8 (400±14 ka) and 

are probably slightly younger than basaltic andesite of middle Caribou (mcm, 362±33 ka). 

Estimated to be ~350 ka. [RC]

mcn8 Unit 8—Plug of porphyritic aug-ol basaltic andesite (55.0% SiO2) intruded into vent agglutinate and lava flows of mcn3 at older and northern of two vents at North Caribou. Phenocrysts: 20% pl, 0.5–2 mm, mostly 1 mm; 3–4% ol, 0.5–2 mm; sparse aug, 0.5–1 mm. 

Unoxidized ol displays unusual dark-green color. Contains variety of coarse-grained 

(to 10–12 cm), granitic to cumulate-textured inclusions, fragments of inclusions, and in-
82

dividual crystals derived from inclusions. Light-colored granitic inclusions lacking ol and 

pyr and containing qtz and potassium feldspar are also found. Pseudomorphs of hbl and hbl 

pseudomorph-bearing inclusions are also present, but not common. Groundmass light to 

medium gray, aphanitic at margins, and holocrystalline and dense in interior of plug. Interior 

of plug is massively jointed; margins have vertically oriented, widely spaced joints. Younger 

than units mcn1, mcn2, and mcn3; relation to other basaltic andesites of North Caribou is 

unknown. [RC]

mcn7 Unit 7—Small dome-like mass of aug-ol basaltic andesite lava 1.8 km north of North Caribou. Erupted through basaltic andesite of Silver Lake, unit 10 (mc10) from vent at small 

unnamed hill. Entire outcrop is oxidized and reddened. Phenocrysts: 10% pl, 0.5–3 mm, 

mostly 0.5–1 mm; 2–3% ol, 0.5–2 mm, mostly <1 mm; 1% aug, 0.5–1 mm. Small glomeroporphyritic clots are common. Groundmass is weakly oxidized, aphanitic, and microvesicular. [RC]

mcn6 Unit 6—Small dome-like mass of porphyritic aug-ol basaltic andesite at small hill 1 km 

north of North Caribou. Hill is an eroded and partially oxidized agglutinate cone intruded 

by a small plug of similar rock (not mapped separately). Phenocrysts: 15% pl, 0.5–2 mm, 

mostly 0.5–1 mm, few larger and reacted; 4% ol, 0.5–2 mm, mostly <1 mm; sparse aug, 

0.5 mm. Glomeroporphyritic clots of ol+pl up to 5 mm are present. Small pseudomorphs 

of hbl are sparse, and qtz xenocrysts are rare. Groundmass is light to medium gray, aphanitic to holocrystalline, and microvesicular to dense. Jointing is typically massive to thick 

platy. Unit mcn6 overlies unit mcn3 and is overlain by basaltic andesite of Rim Lake 

(mcr). [RC]

mcn5 Unit 5—Thick lava flows of porphyritic aug-ol basaltic andesite (55.0% SiO2) erupted at 

younger and southern of two vents at North Caribou; lava flowed east ~2 km. Forms summit 

of cone of North Caribou. Phenocrysts: 20% pl, 0.5–5 mm, mostly 1–3 mm; 2–4% ol, 0.5–2 

mm. Inclusions much less common than in basaltic andesite of middle Caribou (mcm) and 

in most other basaltic andesites of North Caribou. Hbl pseudomorphs and hbl pseudomorphbearing inclusions are absent. Groundmass is light to medium gray, aphanitic to generally 

holocrystalline, and typically dense. Flow interior is thickly platy jointed. Unit mcn5 overlies basaltic andesites of Silver Lake, unit 3 (mc3) and units mcn4 and mcn1. [RC]

mcn4 Unit 4—Thick lava flows of porphyritic aug-ol basaltic andesite (56.9% SiO2) erupted at 

younger and southern of two vents at North Caribou; lava flowed east ~3 km. Phenocrysts: 

5–8% pl, 0.5–2 mm, mostly 1 mm; 1% ol, 0.5–2 mm; sparse aug, 0.5–1 mm. Pseudomorphs 

of hbl to 1 cm and small hbl pseudomorph-bearing inclusions are particularly abundant 

(1–2%) and characteristic. Groundmass is light to medium gray, aphanitic to holocrystalline, 

and usually dense. Flow interior is thickly platy jointed; the flow top is massively jointed 

and often oxidized. Unit mcn4 overlies basaltic andesites of Silver Lake, units 1, 2, 3, and 

8 (mc1, mc2, mc3, mc8), basaltic andesite of middle Caribou (mcm), and units mcn1, 

mcn2, and mcn3. [RC]

mcn3 Unit 3—Thick lava flows of porphyritic aug-ol basaltic andesite erupted at older and northern of two vents at North Caribou; lava flowed both east and west. Lava flows were deeply 

eroded, exposing plug (mcn8) intruded into vent agglutinate. Phenocrysts: 20% pl, 0.5–2 

mm, mostly 1 mm; 3–4% ol, 0.5–2 mm; sparse aug, 0.5–1 mm. Contains light-colored granitic inclusions lacking ol and pyr and containing qtz and potassium feldspar. Groundmass is 

light to medium gray, aphanitic to holocrystalline, and dense. Flow interior has thick slabby 

jointing. Unit mcn3 overlies basaltic andesites of Silver Lake, unit 8 (mc8) and unit mcn4; 

overlain by units mcn6 and mcn8 and by basaltic andesite of Rim Lake (mcr). [RC]

mcn2 Unit 2—Thick lava flows of porphyritic aug-ol basaltic andesite (54.4% SiO2) probably erupted at older and northern of the two vents at North Caribou; lava flowed east. Phenocrysts: 

12% pl, 0.5–2 mm, mostly 1 mm; 2–3% ol, 0.5–2 mm; sparse aug, 0.5–1 mm. Glomeroporphyritic clots of ol+pl are present. Groundmass is light to medium gray, aphanitic to holocrystalline, and dense. Interior of flow has thick slabby jointing; top of flow is massively 

jointed and microvesicular. Unit mcn2 overlies unit mcn1; overlain by unit mcn4. [RC]

mcn1 Unit 1—Thick lava flows of porphyritic aug-ol basaltic andesite (54.5% SiO2) probably 

erupted at older and northern of two vents at North Caribou; lava flowed east. Phenocrysts: 

10% pl, 0.5–2 mm, mostly 1 mm; 2–3% ol, 0.5–2 mm; sparse aug, 0.5–1 mm. Groundmass 

is light to medium gray, aphanitic to holocrystalline, and dense. Interior of flow has thick 
83

slabby jointing; top of flow is massively jointed and microvesicular. Unit mcn1 overlies 

basaltic andesites of Silver Lake, units 3 and 8 (mc3 and mc8); overlain by units mcn2, 

mcn4, and mcn5. [RC]

bcs Basalt of South Caribou (middle Pleistocene)—Lava flows and plug of ol basalt (50.0–50.2% 

SiO2) located in southeastern Caribou Volcanic Field. Lava flows are heavily glaciated; typically only flow cores crop out. Erupted from vent at summit of South Caribou and covers most of 

west flank of South Caribou; vent is partially preserved as agglutinated cinders in summit area 

and is intruded by small plug. Phenocrysts: 3–5% ol, 0.5–2 mm, mostly <1 mm. Groundmass 

of lava flows is light to medium gray, holocrystalline, and microvesicular. Plug has a dark gray 

to black, aphanitic, and dense quenched margin and light gray, coarsely holocrystalline, and 

dense interior. Jointing in flows is variable from blocky to thick platy; plug is blocky to columnar jointed. Both plug and lava flow contain sparse, small qtz xenoliths. Unit bcs overlies 

basaltic andesite of middle Caribou (mcm, 362±33 ka) and basaltic andesite of Hay Meadow 

(mch). Estimated to be ~350 ka, but could be as young as ~300 ka. [RC]

mcm Basaltic andesite of middle Caribou (middle Pleistocene)—Thick lava flows of porphyritic 

aug-ol basaltic andesite (54.0% SiO2). Flows are heavily glaciated, and typically only flow 

cores crop out; till and poorly developed soil cover much of these flows. Lavas vented at 

both hill 7643 (informally designated middle Caribou; 0.5 km south of North Caribou) and 

at South Caribou (vent now covered by basalt of South Caribou, bcs). Minimal lithologic 

and compositional variation in lavas. Outlier, 1 km east of main outcrop area, has a similar 

but slightly different lithology and indicates that additional flow is buried by till. Phenocrysts: 12–15% pl, 0.5–2 mm, mostly 1 mm; 2–4% ol, 0.5–2 mm; sparse aug, 0.5–1 mm. 

Contains a variety of small (to a few centimeters) coarse-grained, granitic- to cumulatetextured inclusions, fragments of inclusions, and individual crystals derived from inclusions. 

These gabbroic inclusions are pl-dominated, typically ol and (or) aug bearing, and partially 

melted. Light-colored granitic inclusions lacking ol and pyr are also found. Inclusions are 

common in flows from middle Caribou vent and more abundant than in flows from South 

Caribou vent. Resorbed pseudomorphs of hbl to 5 mm are locally common; qtz xenocrysts 

are found. Groundmass is light to medium gray, aphanitic to typically holocrystalline, usually dense, but locally microvesicular. Flow interiors are thickly platy jointed; flow tops are 

massively jointed and oxidized. Unit mcm overlies basalt of Hidden Lakes (bbh), basaltic 

andesites of Silver Lake, unit 1 (mc1), basalt of Indian Meadow (bci), basaltic andesite 

of hill 7114 (mc71), and basaltic andesite of Hay Meadow (mch); overlain by basalt of 

South Caribou (bcs) and basaltic andesite of North Caribou, unit 4 (mcn4). K-Ar age is 

362±33 ka. [RC]

mch Basaltic andesite of Hay Meadow (middle Pleistocene)—Thick lava flow of ol basaltic andesite. Lava erupted from vent marked by poorly preserved cinder cone (hill 7143) at south end 

of Caribou sequence and flowed south over 3 km to near Heckle Ranch. Cone is composed 

of oxidized agglutinated cinders and has dike-like spine of oxidized and sheared basaltic andesite oriented north-northwest extending length of hill. Flow is heavily glaciated; typically 

only flow core crops out; till cover is abundant. Some compositional and lithologic variation 

is present. Phenocrysts: 5–8% ol, 0.5–2 mm, mostly 1 mm; sparse–1% pl, 0.5–1 mm, mostly 

0.5–1 mm. Groundmass is medium gray, aphanitic, and microvesicular; few glomeroporphyritic clots of ol+pl are present. Unit mch is overlain by basalt of South Caribou (bcs) 

and basaltic andesite of middle Caribou (mcm, 362±33 ka), and overlies basalt of Indian 

Meadow (bci). It is probably younger than the basaltic andesites of Silver Lake, units 1–11 

(mc1–mc11; ~400 ka). It is estimated to be 375–400 ka. [RC]

mc71 Basaltic andesite of hill 7114 (middle Pleistocene)—Thick lava flow of sparsely porphyritic ol 

basaltic andesite (55.5% SiO2) 1.5 km west-southwest of South Caribou. Although somewhat off the axis of the Caribou chain, mc71 correlates with it based on apparent age and 

lithology. Unit probably erupted from deeply eroded vent located at hill 7114 and flowed 

southwest only ~1 km. Flows are heavily glaciated; typically only flow cores crop out. 

Phenocrysts: 1% ol, 0.5–1 mm. Groundmass is dark gray, aphanitic, and dense with abundant microphenocrysts of pl. Unit mc71 is overlain by basaltic andesite of middle Caribou 

(mcm, 362±33 ka) and probably overlies the basaltic andesites of Silver Lake, units 1–11 

(mc1–mc11, ~400 ka). Like the basaltic andesite of Hay Meadow (mch), it is estimated to 

be 375–400 ka. [RC]
84

bci Basalt of Indian Meadow (middle Pleistocene)—Thick lava flow of aug-ol basalt (52.8% SiO2) 

in southwestern Caribou Volcanic Field. Vent location unknown but probably buried by basaltic andesite of middle Caribou (mcm). Flow is heavily glaciated; typically only flow core 

crops out. Phenocrysts: 8–10% ol, 0.5–3 mm, mostly 1–2 mm; 5–6% aug, 0.5–3 mm, mostly 

1 mm; 3–4% pl, 0.5–2 mm, mostly 0.5–1 mm, few larger are composite. Most crystals are 

in glomeroporphyritic clots of ol+pl+aug to 1 cm. Groundmass is medium gray, aphanitic, 

and dense. Unit bci is overlain by basaltic andesite of Hay Meadow (mch) and unit mcm

(362±33 ka) and is probably younger than the basaltic andesites of Silver Lake, units 1–11 

(mc1–mc11, ~400 ka). It estimated to be 375–400 ka. [RC]

Basaltic andesites of Silver Lake (middle Pleistocene)—Thick lava flows of porphyritic ol basaltic andesite and aug-ol basaltic andesite (53.4–54.8% SiO2

) in southern Caribou Volcanic 

Field. Flows are heavily glaciated; typically only flow cores crop out. Vent locations for 

most flows are unknown. Forms a platform upon which basaltic andesite of middle Caribou 

(mcm), basaltic andesites of North Caribou (mcn1–8), basaltic andesite of Eleanor Lake 

(mce), basaltic andesite of Jewel Lake (mcj), and basaltic andesite of Rim Lake (mcr) were 

emplaced. Lava flows grouped in some units are slightly variable and did not necessarily erupt from common vent. Flows overlie basalt of Cowboy Lake (bcb) and are younger 

than older basalts and basaltic andesites south of Caribou Volcanic Field (boc, 650±20 ka); 

overlain by unit mcm (362±33 ka). K-Ar age of mc8, 400±14 ka, approximates age of other 

units in basaltic andesites of Silver Lake. [BB, RC]

mc11 Unit 11—Thick lava flow of sparsely porphyritic ol basaltic andesite (53.8% SiO2) exposed 

near South Divide Lake. Erupted from poorly preserved vent at unnamed hill 0.5 km south 

of North Divide Lake and flowed south ~2 km. Phenocrysts: 2–3% ol, 0.5–3 mm, larger 

grains composite; sparse–1% pl, 0.5–1 mm, few larger, many reacted. Groundmass is medium gray, holocrystalline, and dense. Overlies units mc10 and mc8. [RC]

mc10 Unit 10—Thick lava flows of porphyritic ol basaltic andesite (53.5% SiO2) exposed southeast 

of North Divide Lake and probably erupted from vent at small hill 1 km southeast of North 

Divide Lake. Outlier forms another small hill just south of North Divide Lake. Lithology is 

somewhat variable. Phenocrysts: 5–10% ol, 0.5–4 mm, mostly 1 mm; sparse–1% pl, 0.5–2 

mm, generally 0.5–1 mm. Some flows, especially those in outlier, contain small, coarsegrained, granitic-textured inclusions, fragments of inclusions, and individual crystals 

derived from inclusions. These are gabbroic in composition and are composed of pl+pyr. 

Inclusions of qtz are also present. Groundmasses are light to dark gray, aphanitic to holocrystalline, and dense. Unit mc10 overlies units mc7 and mc8; overlain by unit mc11, 

basaltic andesite of North Caribou, unit 7 (mcn7), and basaltic andesite of Rim Lake 

(mcr). [RC]

mc9 Unit 9—Thick lava flows of ol basaltic andesite near Emerald Lake. Vent locations are uncertain, but one may be at small hill southwest of Emerald Lake. Phenocrysts: 10% ol, 0.5–1 

mm, few to 2 mm; sparse–1% pl, 0.5–1 mm. Groundmass is dark gray to black, aphanitic 

to holocrystalline, and dense. Unit mc9 overlies units mc8, mc6, and mc5; overlain by 

basaltic andesite of Jewel Lake (mcj). [BB, RC]

mc8 Unit 8—Thick lava flows of porphyritic aug-ol basaltic andesite (53.5–54.8% SiO2) occurring as continuous outcrop over substantial area. Unit probably erupted from several vents, 

but none were located with certainty. One may be beneath the vent for mcj (hill 2213) and 

another in the vicinity of mcr and mcn6. Lava flows have similar compositions and lithologies, although some variation is present. Phenocrysts: 10–15% pl, 0.5–2 mm, few larger 

are xenocrysts; 3–5% ol, 0.5–2 mm, sparse larger crystals are composite; sparse–1% aug, 

0.5 mm. Groundmasses are light to medium gray, aphanitic to holocrystalline, and typically 

dense. Contains small glomeroporphyritic clots, gabbroic xenoliths, and crystals (including 

pseudomorphs of hbl) disaggregated from them. Unit mc8 overlies units mc3, mc6, and 

mc7; overlain by units mc9, mc10, and mc11 and by basaltic andesite of Eleanor Lake 

(mce), basaltic andesite of Jewel Lake (mcj), and basaltic andesite of Rim Lake (mcr). 

K-Ar age is 400±14 ka. [BB, RC]

mc7 Unit 7—Thick lava flows of sparsely porphyritic ol basaltic andesite underlying mc8 and 

mc10 near North Divide Lake. Phenocrysts: 2–3% ol, 0.5–3 mm, larger crystals are composite; sparse–1% pl, 0.5–1 mm, few larger with gray cores. Groundmass is medium to dark 

gray, aphanitic, and dense. Unit mc7 is overlain by units mc8 and mc10. [RC]
85

mc6 Unit 6—Lava flows of porphyritic ol basaltic andesite occurring as small outcrop underlying mc8 and mc9 southeast of Gem Lake and east of Emerald Lake. Phenocrysts: 3–4% 

ol, 0.5–3 mm, mostly 0.5–1 mm; 1% pl, 0.5–1 mm. Few small glomeroporphyritic clots of 

ol+pl are present. Groundmass is medium gray, aphanitic, and dense. Unit mc6 overlies unit 

mc5; overlain by units mc8, mc9, and basaltic andesite of Rim Lake (mcr). [RC]

mc5 Unit 5—Lava flow of porphyritic ol basaltic andesite occurring as small outcrop 0.2 km southeast of Gem Lake. Phenocrysts: 3–4% ol, 0.5–3 mm, mostly 0.5–1 mm; 1% pl, 0.5–1 mm. 

Few small glomeroporphyritic clots of ol+pl are present. Groundmass is dark gray, aphanitic, and dense. Unit mc5 is overlain by units mc6 and mc9. [RC]

mc4 Unit 4—Lava flow of sparsely porphyritic ol basaltic andesite erupted from poorly preserved 

agglutinate cone at hill 2294, ~1.5 km southwest of Turnaround Lake. Lava flowed northeast ~2.5 km to beyond Turnaround Lake. Cone is mostly buried by andesite of Red Cinder 

(arr); small plug is exposed at its summit. Rocks are heterogeneous, because of a large, and 

variable component of disaggregated xenolithic material. Phenocrysts: 1–2% ol, 0.5–2 mm, 

mostly 0.5 mm; 1–3% pl, 0.5–2 mm, mostly 0.5 mm; trace–sparse aug, 0.5–1 mm. Variety of 

small inclusions and xenocrysts are common, including gabbroic clots to 1 cm of ol+aug+pl, 

qtz and hbl xenocrysts, small andesitic inclusions, and partially melted crustal inclusions. 

Many phenocryst-sized crystals are disaggregated from gabbroic clots or other inclusions; 

many pl are partially reacted. Groundmass is medium to dark gray, aphanitic and dense; 

jointing is massive. Unit mc4 is overlain by unit arr. [BB]

mc3 Unit 3—Thick lava flows of porphyritic ol basaltic andesite occurring as small outcrop 1 km 

southwest of Silver Lake. Phenocrysts: 5% pl, 0.5–1 mm, few with gray cores; 2–3% ol, 

0.5–2 mm, mostly 1 mm. Groundmass is light gray, aphanitic to holocrystalline, and dense. 

Unit mc3 is overlain by unit mc8 and by basaltic andesites of North Caribou, units 1, 4, and 

5 (mcn1, mcn4, mcn5). [RC]

mc2 Unit 2—Lava flows of sparsely porphyritic ol basaltic andesite occurring as bench 1–1.5 km 

south of Silver Lake. Phenocrysts: 1% ol, 0.5–2 mm, mostly 0.5 mm; 1% pl, 0.5–2 mm, 

mostly 0.5 mm. Groundmass is light gray, aphanitic, and dense. Unit mc2 overlies unit mc1

and is overlain by basaltic andesites of North Caribou, unit 4 (mcn4). [RC]

mc1 Unit 1—Lava flows of porphyritic ol basaltic andesite (53.4% SiO2) occurring as benches and 

isolated outcrops 1.6 km south of Silver Lake. All flows have ol>pl, although some variation is present. Phenocrysts: 5–8% ol, 0.5–4 mm, mostly 1–2 mm; sparse–3% pl, 0.5–2 mm, 

mostly 0.5–1 mm, some with gray cores. Glomeroporphyritic clots of ol+pl and occasionally aug from a few millimeters to a few centimeters are present. Groundmasses are light to 

medium gray, aphanitic to holocrystalline, and microvesicular to dense. Jointing is typically 

massive to thick platy. Unit mc1 is overlain by unit mc2, basaltic andesite of North Caribou, 

unit 4 (mcn4), and basaltic andesite of middle Caribou (mcm). [RC]

bcw Basalt of Caribou Wilderness (middle Pleistocene)—Small group of thick, porphyritic, ol 

basalt (49.7–50.4% SiO2) lava flows in northern Caribou chain, ~1.5 km east of Triangle 

Lake. Flows are heavily glaciated; typically only flow cores crop out. Poorly preserved vents 

are located at the till-covered hill 2213 (east of Triangle Lake) and a small unnamed hill 

to northeast. Small outcrop areas l km northeast of Triangle Lake may have erupted from 

another vent that is not preserved. Phenocrysts: 5–8% ol, 0.5–4 mm, mostly 0.5–2 mm; rock 

from hill 2213 has fewer ol phenocrysts. Groundmass is light to medium gray, aphanitic to 

holocrystalline, and dense to microvesicular. Unit bcw overlies basalt of Caribou Peak (bcp) 

and is overlain by basaltic andesite of Eleanor Lake (mce) and basaltic andesites of section 

20 (mt20). Age is probably older than unit mce and is estimated to be 375–400 ka. [BB]

bcp Basalt of Caribou Peak (middle Pleistocene)—Thick lava flows of sparsely porphyritic ol 

basalt in northern part of Caribou chain. Flows are heavily glaciated; typically only flow 

cores crop out. Poorly preserved vent of weakly agglutinated cinders is exposed at Caribou Peak, and other vents are probably buried by basaltic andesite of Eleanor Lake (mce). 

Phenocrysts: 4–6% ol, 0.5–2 mm, a few larger crystals are composite; sparse pl, 0.5 mm. 

Groundmass is medium to dark gray, aphanitic to finely holocrystalline, and dense to weakly 

microvesicular with blocky to thick platy jointing. Unit bcp overlies basalt of Cowboy Lake 

(bcb) and is overlain by basalt of Caribou Wilderness (bcw) and by unit mce. Age is similar 

to possibly somewhat older than basaltic andesites of Silver Lake, unit 8 (mc8, 400±14 ka) 

and is estimated to be 400–425 ka. [BB]
86

bcb Basalt of Cowboy Lake (middle Pleistocene)—Poorly exposed thick lava flows of porphyritic 

ol basalt (52.2% SiO2) between Silver Lake and Caribou Peak in central Caribou Volcanic 

Field. Lava flows are heavily glaciated; typically only flow cores are exposed, and they are 

mostly covered by extensive sheet of till. Flow morphology is better preserved than older 

basalts and basaltic andesites south of Caribou Volcanic Field (boc, 650±20 ka). Vents not 

located and probably buried beneath till or basaltic andesites of Silver Lake, units 1–11 

(mc1–11). Consists of unknown number of flows with similar lithology and composition to other lavas of Caribou Volcanic Field. Phenocrysts: 3–8% ol, 0.5–1 mm, mostly 0.5 

mm; sparse–5% pl, 0.5–2 mm, mostly 0.5–1 mm. Ol is generally more abundant than pl, 

but is of subequal abundance in some flows. Few flows contain sparse, larger ol and (or) pl 

phenocrysts to 3 mm; some contain trace of aug. Most flows have small glomeroporphyritic clots of ol+pl, some to a few centimeters across. Groundmasses are light to medium 

gray and generally aphanitic, but occasionally holocrystalline and weakly microvesicular to 

dense. Small xenoliths of qtz, commonly with thin aug rims, are widely distributed. Unit is 

overlain by basaltic andesites of Silver Lake, unit 8 (mc8, 400±14 ka), and is estimated to 

be 400–425 ka. [BB, RC]

Sunrise Peak sequence

The Sunrise Peak sequence comprises a group of spatially and temporally related basalt and basaltic 

andesite lava flows and vents east and north of Butte Lake in the northwest Caribou Volcanic Field. Although 

generally similar to the Red Cinder chain, the Sunrise Peak sequence is older (~300–400 ka), more heavily 

glaciated, and cut by a prominent, old, till-covered fault that is part of the discontinuous fault zone between 

Lake Almanor Graben and Hat Creek Graben. 

bsl Basalt of Lost Spring (middle Pleistocene)—Porphyritic aug-ol basalt (52.7–53.0% SiO2) lava 

flow and cinder cone. Exposed near Duck Lake, north of Butte Lake. Vent is 1 km southeast 

of Duck Lake. Lava from vent flowed at least 1.5 km northwest. Lava was heavily glaciated, surface features are not preserved, and it is mostly buried by till, younger glaciations 

(Qty). Cinder cone, located on edge of glaciated area, is partially eroded and buried by 

Qty. Phenocrysts: 8% pl, 0.5–2 mm, few larger; 5% ol, 0.5–2 mm, mostly 1 mm; 2% aug, 

0.5–2 mm, mostly 1 mm. Most crystals occur in glomeroporphyritic clots of ol+pyr+pl to 1 

cm. Groundmass is light gray, holocrystalline, and dense. Massively jointed flow interiors 

are exposed. Unit bsl probably overlies basalt of Duck Lake (bsd) and is overlain by the 

much younger basalt of Twin Buttes (bdt, 46.3±3.4 ka). 40Ar/39Ar age is 302±7 ka. [PP]

bsd Basalt of Duck Lake (middle Pleistocene)—Sparsely porphyritic ol basalt lava flow exposed 

around Duck Lake, in northern Caribou Volcanic Field. Lava flow was heavily glaciated and 

is mostly buried by till; surface features are not preserved. Vent location is unknown. Phenocrysts: 5% ol, 0.5–2 mm. Groundmass is light gray, aphanitic, and dense. Thick, slabbyjointed flow interiors are exposed. Locally displays flow banding defined by vapor-phase 

crystallization. Unit bsd is probably overlain by basalt of Lost Spring (bsl), but is otherwise 

completely surrounded by till, younger glaciations (Qty). Estimated to be ~300 ka. [PP]

bs72 Basalt of hill 7243 (middle Pleistocene)—Cinder cone of porphyritic ol basalt in northwestern 

Caribou Volcanic Field. Cone is almost completely buried by till; associated lava flow may 

exist beneath till. Exposed part of cone is composed of agglutinated, strongly oxidized scoria 

and bombs. Phenocrysts: 7–8% ol, 0.5–2 mm; sparse aug, 0.5–2 mm. When fresh, groundmass is dark gray, glassy to aphanitic, and microvesicular. Age is very poorly constrained, but 

it is probably slightly younger than basalts of Sunrise Peak, unit 1 (bss1, 393±4 ka). [PP]

Basalts of Sunrise Peak (middle Pleistocene)—Porphyritic aug-ol basalt lava flows (51.4–

52.3% SiO2) and agglutinated scoria cones, exposed in the prominent fault scarp east of 

Butte Lake. Vent located at Sunrise Peak. Eruptions built two overlapping scoria cones 

with bss1 cone buried by bss2 cone. The bss2 cone was eroded by glaciers and is 

mostly covered by till, younger glaciations (Qty). Lithologically similar, but distinct, lava 

flows were erupted from the scoria cones. Lava flows were heavily glaciated, and surface 

features are not preserved. 40Ar/39Ar age of unit bss1 is 393±4 ka; unit bss2 is slightly 

younger. [PP]

bss2 Unit 2—Porphyritic aug-ol basalt (52.3% SiO2) lava flows and agglutinated scoria cone. Thin 

platy to massively jointed flow interiors are exposed. Phenocrysts: 6% ol, 0.5–2 mm, mostly 
87

1–2 mm; 2% aug, 0.5–1 mm; 2% pl, 0.5–1 mm. Groundmass is dark gray to black, aphanitic 

to glassy, and dense to microvesicular. Some flows have sparse glomeroporphyritic clots of 

ol+aug+pl to few millimeters. Xenocrysts of qtz are present. Unit bss2 overlies unit bss1, 

but is otherwise completely surrounded by till, younger glaciations (Qty). Age is slightly 

younger than bss1 (393±4 ka) [PP]

bss1 Unit 1—Porphyritic aug-ol basalt (51.4% SiO2) lava flows and agglutinated scoria cone. 

Phenocrysts: 3% ol, 0.5–2 mm, mostly 0.5–1 mm; 2% aug, 0.5–1 mm; 5% pl, 0.5–2 mm, few 

larger. Groundmass is dark gray to black, aphanitic to glassy, and dense to microvesicular. 

Glomeroporphyritic clots of ol+aug+pl to few millimeters are common. Sparse xenocrysts of 

qtz are present. Unit bss1 overlies basalt of Bathtub Lake (bsb); overlain by bss2, but is otherwise completely surrounded by till, younger glaciations (Qty). 40Ar/39Ar age is 393±4 ka. [PP]

bsb Basalt of Bathtub Lake (middle Pleistocene)—Sparsely porphyritic ol basalt lava flows in 

Butte Creek, north of Butte Lake. Lithologies of several flows are somewhat variable. Flows 

were heavily glaciated; outcrops are discontinuous; and surface features are not preserved, 

being mostly buried by till or by ash from Cinder Cone eruption. Only massively jointed 

flow interiors are exposed. Vents are unknown and probably were removed by glaciation. 

Cut by small faults extending southeast from Hat Creek Fault. Phenocrysts: sparse to 5% 

ol, 0.5–2 mm, larger in some flows; 0–2% pl, 0.5–1 mm. Groundmasses are light to dark 

gray and holocrystalline to aphanitic. Unit bsb is overlain by basalt of Sunrise Peak, unit 1 

(bss1, 393±4 ka), but is otherwise completely surrounded by till, younger glaciations (Qty). 

Age is probably ~400 ka. [PP]

Island Lake sequence

Island Lake sequence comprises two spatially and temporally related basalt lava flows near Bonte Peak, 

in the southwest part of the Caribou Volcanic Field. Generally similar to other rocks in Caribou Volcanic 

Field, the Island Lake sequence is ~350 ka, heavily glaciated, and cut by one of the small N. 10° W. faults 

along the discontinuous fault zone between Lake Almanor Graben and Hat Creek Graben. K-Ar age of unit 

bie is 331±45 ka; unit bii is probably slightly younger. 

bii Basalt of Island Lake (middle Pleistocene)—Thick lava flow of porphyritic aug-ol basalt 

(52.7% SiO2) erupted from eroded vent at hill 7470, 2.2 km north-northeast of Bonte Peak 

and flowed southwest for ~2 km. Lava flow was heavily glaciated and is partially buried 

by till. Surface features are not preserved, and only flow cores crop out. Phenocrysts: 15% 

pl, 0.5–2 mm, mostly 1 mm; 10% ol, 0.5–2 mm, mostly 1 mm; 2% aug, 0.5–2 mm, mostly 

1 mm. Glomeroporphyritic clots of pl+ol+aug to 8 mm are abundant. Groundmass is light to 

medium gray, aphanitic to glassy, and dense. Thick platy to massively jointed flow interiors 

are exposed. Unit bii overlies older rock units: andesites of Dittmar Volcanic Center, Stage 

2, undivided (ad2), basaltic andesites and basalt of Snag Lake (msn), basalt of Bonte Peak 

(bnt), and basaltic andesite of Black Cinder Rock (mb, 667±24 ka). Age is poorly constrained but probably slightly younger than basalt of East Lake (bie, 331±45 ka). Estimated 

to be 300–350 ka. [MH]

bie Basalt of East Lake (middle Pleistocene)—Lava flows of porphyritic ol basalt (52.9% SiO2) 

erupted from poorly preserved vent at hill 7321, 1 km northeast of Bonte Peak. Lava flowed 

4.5 km south. Heavily glaciated. Typically only flow cores crop out; till obscures much 

of southern part of unit. On east boundary, faulted against much older basaltic andesite of 

Black Cinder Rock (mb). Phenocrysts: 5% pl, 0.5–2 mm, mostly 0.5–1 mm; 3% ol, 0.5–1 

mm, few larger. Glomeroporphyritic clots of pl+ol are abundant. Groundmass is light gray, 

holocrystalline, and microvesicular; lava near south limit of outcrop area is coarsely holocrystalline. Unit bie overlies basalt of Bonte Peak (bnt) and basaltic andesite of Black 

Cinder Rock (mb, 667±24 ka), and is probably slightly older than basalt of Island Lake (bii). 

K-Ar age is 331±45 ka. [MH, RC]

Beauty Lake sequence

The Beauty Lake sequence consists of highly glaciated lava flows immediately northwest of Hay 

Meadow in the southern part of the Caribou Volcanic Field. The flows are lithologically similar to mafic 

rocks in the Caribou chain, but are spatially offset from them. The Beauty Lake sequence is approximately 

coeval with the oldest rocks of the Caribou chain, ~425–450 ka. 
88

bbb Basalt of Beauty Lake (middle Pleistocene)—Thick lava flows of porphyritic aug-ol basalt 

(52.6% SiO2). Flows are heavily glaciated; typically only flow cores crop out. No vent is 

preserved. Phenocrysts: 9% ol, 0.5–2 mm, few larger; 5% aug, 0.5–2 mm, few larger; 1% 

pl, 0.5–1 mm. Small glomeroporphyritic clots of ol+aug+pl are sparse. Groundmass is light 

gray, holocrystalline, and microvesicular. Jointing is blocky to thick platy. Unit bbb overlies 

basalt of Hidden Lakes (bbh) and basalt of Posey Lake (bbp) and is youngest unit of Beauty 

Lake sequence. Absolute age is poorly constrained. It is probably older than basaltic andesites of Silver Lake, unit 8 (mc8, 400±14 ka) and is estimated to be ~425–450 ka. [RC]

bbp Basalt of Posey Lake (middle Pleistocene)—Thick lava flows of porphyritic ol basalt (51.7% 

SiO2). Flows are heavily glaciated; typically only flow cores are exposed. Location of vent 

is unknown but was probably north of Posey Lake. Phenocrysts: 6% ol, 0.5–2 mm, mostly 

<1 mm; 2% pl, 0.5–1 mm, few larger. Small glomeroporphyritic clots of ol+pl are sparsely 

present. Groundmass is light to medium gray, holocrystalline, and dense. Jointing is blocky 

to thick platy. Unit bbp is overlain by basalt of Beauty Lake (bbb) and overlies basalt of 

Hidden Lakes (bbh). Absolute age is poorly constrained. It is probably older than basaltic 

andesites of Silver Lake, unit 8 (mc8, 400±14 ka) and is estimated to be ~425–450 ka. [RC]

bbh Basalt of Hidden Lakes (middle Pleistocene)—Thick lava flows of ol basalt (51.5–51.9% 

SiO2) between Posey Lake and Hidden Lakes and south to Hay Meadow. Probably erupted 

from vent near Posey Lake that is not preserved and flowed south. Flows are heavily glaciated; typically only flow cores are exposed. Flows have similar lithologies, although some 

variation is present. Phenocrysts: 8–10% ol, 0.5–2 mm, mostly <1 mm, few larger to 4 mm. 

Groundmass is medium gray, holocrystalline, and dense. Unit bbh is overlain by basalt of 

Beauty Lake (bbb), basalt of Posey Lake (bbp), and basaltic andesite of middle Caribou 

(mcm, 362±33 ka). Absolute age is poorly constrained, but is probably older than basaltic 

andesites of Silver Lake, unit 8 (mc8, 400±14 ka) and is estimated to be 425–450 ka. [RC]

Older volcanic rocks

Older volcanic rocks (mostly >650 ka) surround the Caribou Volcanic Field on north, east, and south 

and form a base on which the volcanic field is built. Immediately south of the Caribou Volcanic Field, the 

rocks are heavily glaciated, mostly buried by till, and exposed only as obscure bits of lava flows. They are 

disrupted by faults extending northwest from the Lake Almanor Graben. A better-exposed group of old volcanoes occurs at higher elevations at Black Cinder Rock and Bonte Peak. 

Much older rocks are exposed around the Caribou Volcanic Field beyond the limits of glaciation. To 

the south, these include several extensive sheets of tholeiitic basalt, a thick stack of 2.2–2.7 Ma calc-alkaline 

regional lavas exposed in Mud Creek Rim, and the west edge of the Swain Mountain shield volcano (fig. 3). 

In the north, these older rocks include the tholeiitic basalt sheet filling Grays Valley and the shield volcanoes 

of Bogard Buttes, Cal Mountain, and Cone Mountain (fig. 3).

bgv Tholeiitic basalt of Grays Valley (middle Pleistocene)—Thin, extensive sheet of nearly aphyric, tholeiitic, ol basalt (48.4–48.6% SiO2

) erupted from spatter rampart just west of Bogard 

Buttes Ranger Station (east of map area in Pine Creek Valley quadrangle). Multiple flow 

units of tube-fed pahoehoe flowed north-northwest and filled valley between Bogard Buttes, 

Cal Mountain, Cone Mountain, and Crater Lake Mountain (fig. 3). Phenocrysts: sparse pl, 

1–2 mm; sparse ol, 0.5–2 mm. Few small glomeroporphyritic clots of ol+pl are present. 

Groundmass is light gray and coarsely diktytaxitic. Upper flow surface contains abundant 

spherical vesicles and has hexagonal-block, joint pattern with massively jointed flow interior. Age poorly constrained, but normal magnetic polarity indicates that unit bgv is <780 

ka. Well-developed soil and obscure flow-surface features suggest that unit bgv is older than 

~500 ka; estimated to be ~600 ka. [BB, PL]

bsh Basalts and basaltic andesites of Swains Hole (middle Pleistocene)—Group of lithologically 

similar, sparsely porphyritic ol basalt and basaltic andesite (52.1–53.3% SiO2) lava flows 

poorly exposed 4.5 km southwest of Poison Lake. Buried to south by younger rock units. 

Much of unit is covered by a thick blanket of windblown dust and ash from eruptions in 

Poison Lake chain. Vents were located to south and east and are now buried by younger 

lavas from Caribou Volcanic Field. North of map area, these same rocks are well exposed in 

Butte Creek Rim (fig. 3). Lithologically variable. Phenocrysts: 2–8% ol, 0.5–1 mm; 1–3% 

pl, 0.5–1 mm in most flows. One flow 1 km southwest of Poison Lake, has ~15% 0.5–1 mm 
89

phenocrysts of ol and pl with pl>>ol. Groundmasses are generally medium to dark gray, 

glassy to aphanitic; most are microvesicular. Many flows have vesicular tops. Age poorly 

constrained; clearly much older than Poison Lake chain, tholeiitic basalt of Calif. Hwy 44 

(b44, 190±18 ka), and andesite and basaltic andesite of Prospect Peak (ap, 247±56 ka) and 

therefore much older than ~300 ka. Broadly equivalent to older basalts and basaltic andesites 

south of Caribou Volcanic Field (boc, 650±20 ka). Unit bsh has normal polarity and thus is 

probably younger than 780 ka. Estimated to be ~600–725 ka. [BB, PL, PP, SW]

boc Older basalts and basaltic andesites south of Caribou Volcanic Field (middle Pleistocene)—

Sparsely porphyritic to porphyritic ol basalt and basaltic andesite (52.3–52.7% SiO2) lava 

flows scattered through southern Caribou Volcanic Field. Exposed in 60 small outcrops, 

mostly in Red Cinder quadrangle. These older flows are heavily glaciated, and constructional morphology is subdued or absent; typically, only small areas of flow core crop out 

beneath thick till cover. Vents are generally not located, and affinities of these flows to other 

units cannot be determined. Largest outcrop is lava flow of sparsely porphyritic ol basalt in 

extreme southeast part of map that filled canyon cut in basalt of Rock Creek (brk). Proximal 

part of this lava flow is buried by till, older glaciations (Qto); distal part has thick soil cover. 

Location of vent is unknown, but it was probably somewhere to northwest of flow near 

Bailey Creek. Four analyses are all basaltic, but some rocks probably are basaltic andesites. In general, these older basalts and basaltic andesites have compositions and lithologies 

similar to younger lavas in Caribou Volcanic Field. Typically, they contain sparse to ~5% 

small phenocrysts of ol, often accompanied by small pl crystals and more rarely by aug.

Some are more porphyritic and contain subequal amounts of larger ol and pl phenocrysts and 

glomeroporphyritic clots of ol and pl. Groundmasses are light to dark gray, aphanitic to holocrystalline, and microvesicular to dense. Lava flows are older than Caribou Volcanic Field, 

for example basaltic andesites of Silver Lake, unit 8 (mc8, 400±14 ka). Unit boc overlies 

tholeiitic basalt of Rock Creek (brk, 989±49 ka). A number of measurements suggest all 

lavas have normal polarity. K-Ar age of larger outcrop area ~3 km south of Heckle Ranch 

is 650±20 ka. Age of unit boc is estimated to be ~600 to ~725 ka, broadly stratigraphically 

equivalent to basaltic andesite of Black Cinder Rock (mb), basaltic andesites and basalt of 

Snag Lake (msn), basalt of Bonte Peak (bnt), basalts and basaltic andesites of Swains Hole 

(bsh), and a few other units. [BB, CH, MH, RC]

bhr Basaltic plug near Heckle Ranch (middle Pleistocene)—Sparsely porphyritic ol basalt (52.3% 

SiO2) plug in southern part of Caribou Volcanic Field. Plug intruded cinder cone of older 

basalts and basaltic andesites south of Caribou Volcanic Field (boc), which was subsequently heavily glaciated and mostly eroded away. Part of unit consists of dikes intruded into 

welded, agglutinated, and oxidized cinders. Adjacent lava flow of unit boc may be related 

to plug and (or) cinder cone. Phenocrysts: 5% ol, 0.5–1 mm, mostly 0.5 mm. Groundmass 

is medium gray, holocrystalline with light-green patches of poikilitic groundmass pyr, and 

sparsely microvesicular. Rock is massively jointed. Age poorly constrained but similar to 

older basalts and basaltic andesites south of Caribou Volcanic Field (boc, 650±20 ka). [RC]

mj Basaltic andesite of Jakey Lake (middle Pleistocene)—Thick lava flow of porphyritic ol basaltic andesite erupted from vent at hill 7741, 2.6 km northeast of Juniper Lake in southern 

Caribou Volcanic Field. Lava flowed ~1 km north from poorly preserved vent. Lava was 

heavily glaciated; surface features are not preserved; flow is partially buried by till. Phenocrysts: 8–10% ol, 0.5–3 mm, mostly 1 mm. Groundmass is light to medium gray, aphanitic to holocrystalline, and dense. Jointing is massive to thick platy. Age is poorly constrained. 

Unit mj overlies much older rocks of Dittmar Volcanic Center but is probably younger than 

basaltic andesites and basalt of Snag Lake (msn) and similar in age to basalt of Bonte Peak 

(bnt) and basaltic andesite of Black Cinder Rock (mb, 667±24 ka). Estimated to be ~650 ka. 

[MH]

bcg Basalt of Butte Creek Campground (middle Pleistocene)—Cinder cone of ol basalt in the Butte 

Creek drainage in the northern part of the Caribou Volcanic Field. A related lava flow may 

be buried beneath till or younger volcanic units. Phenocrysts: 10% ol, 0.5–2 mm, mostly 0.5 

mm; sparse pl, 0.5–2 mm, mostly 0.5 mm. The groundmass is dark-gray to black and aphanitic. Age poorly constrained; much older than the overlying andesite and basaltic andesite 

of Prospect Peak (ap, 247 ± 56 ka). Probably equivalent to the younger part of basalts and 

basaltic andesites of Swains Hole (bsh), and estimated to be about 600–650 ka. [PP]
90

mb Basaltic andesite of Black Cinder Rock (middle Pleistocene)—Lava flows of sparsely porphyritic ol basaltic andesite (54.2–55.0% SiO2

), south of Caribou Volcanic Field, that erupted 

from vent marked by scoria cone at Black Cinder Rock and at least one other vent. Lava 

flows are heavily glaciated; typically only flow cores crop out. Edifice of Black Cinder Rock 

was cut by faults of the discontinuous fault zone between Lake Almanor Graben and Hat 

Creek Graben. Lithology of lava flows is slightly variable. Phenocrysts: 1–5% ol, 0.5–1 mm, 

mostly <0.5 mm, some flows with a few to 2 mm; rare–sparse pl, 0.5–1 mm, mostly <0.5 

mm. Few small glomeroporphyritic clots of ol or ol+pl are present in some flows. Few 

larger, ragged crystals of pl are locally found and are probably xenocrysts. Groundmass is 

light to dark gray, aphanitic to holocrystalline, and typically dense. Unit mb overlies parts of 

older basalts and basaltic andesites south of Caribou Volcanic Field (boc, 650±20 ka), but is 

generally equivalent in age to that unit. K-Ar age is 667±24 ka. [MH, RC]

msn Basaltic andesites and basalt of Snag Lake (middle Pleistocene)—Ol basaltic andesite and 

ol basalt in five isolated areas near Juniper and Snag Lakes, southwest of Caribou Volcanic 

Field. Rocks are probably not directly related but are combined because of similar lithology 

and age. Lava flows are heavily glaciated; typically only flow cores crop out; vent locations 

are unknown. Outcrops have deep soil and thick cover of till. Most of the five outcrop areas 

consist of sparsely porphyritic ol basaltic andesite. Phenocrysts: 3–5% ol, 0.5–2 mm, few 

larger; sparse to 5% pl, many with dark cores. Many ol phenocrysts are weakly oxidized. 

Groundmasses are dark to medium gray, aphanitic, and dense to weakly microvesicular. 

Area 1 km northeast of Glen Lake contains ~10% ol and glomeroporphyritic clots of ol+pl. 

Southern part of area southwest of Snag Lake contains 5% pl and only 1–2% ol. Area 1 km 

east of Mount Hoffman contains fragmental material and may be a deeply eroded vent. Age 

of unit msn is very poorly constrained, but older than Caribou Volcanic Field. Unit msn

directly and unconformably overlies much older rocks of Dittmar Volcanic Center and is 

overlain by younger lava flows of Caribou Volcanic Field. Age is probably similar to older 

part of older basalts and basaltic andesites south of Caribou Volcanic Field and is estimated 

to be ~675–725 ka. [MH, PP]

bnt Basalt of Bonte Peak (middle Pleistocene)—Lava flows of sparsely porphyritic ol basalt (51.6–

53.1% SiO2) south of Caribou Volcanic Field. Vent was probably near summit of Bonte 

Peak. Lava flows are heavily glaciated; typically only flow cores crop out. Flanks of Bonte 

Peak were oversteepened by glacial erosion; much of lava away from summit is buried by 

till. Faulting along the discontinuous fault zone between Lake Almanor Graben and Hat 

Creek Graben may cut lavas, but relations are obscured by glacial erosion and till deposits. 

Phenocrysts: 2-4% ol, 0.5–1 mm, mostly 0.5 mm; sparse pl, 0.5–1 mm, mostly 0.5 mm. Few 

larger, ragged crystals of pl found in some flows are probably xenocrysts. Groundmass is 

variable from medium to dark gray and aphanitic to light gray and holocrystalline. Aphanitic 

lava is often oxidized to purplish or pinkish shades and holocrystalline lava to light brown 

or tan. Unit bnt is overlain by basaltic andesite of East Lake (bie, 331±45 ma) and probably by basaltic andesite of Black Cinder Rock (mb, 667±24 ka). Unit bnt overlies parts of 

older basalts and basaltic andesites south of Caribou Volcanic Field (boc, 650±20 ka), but is 

generally equivalent in age to it, and is estimated to be 675–700 ka. [MH, RC]

bcr Basalt of Butte Creek Ranch (early Pleistocene)—Porphyritic ol basalt lava and cinder cone 

in Butte Creek valley, 6.5 km southwest of Poison Lake. Cinder cone is somewhat eroded, 

leaving remnant of small crater-filling lava flow exposed at summit. Additional lava may 

be buried by younger lava flows of basalts and basaltic andesites of Swains Hole (bsh) and 

glacial outwash. Cinder cone is composed of oxidized scoria and blocks and bombs of ol basalt; covered by thick orange soil. Phenocrysts: 20% ol, 0.5–2 mm, few larger are composite 

grains; 5% pl, 0.5–1 mm. Small glomeroporphyritic clots of ol+pl are common. Groundmass 

is medium gray, aphanitic and microvesicular. Most of lava at summit is weakly oxidized. 

Age is very poorly constrained, but unit bcr has reversed magnetic polarity and must be 

older than 780 ka; estimated to be 800–850 ka. [SW]

a16 Andesite of section 16 (early Pleistocene)—Thin lava flow of porphyritic aug-hyp andesite 2.4 

km east of Black Butte. Heavily glaciated and poorly exposed; only a small area of flow 

core crops out. Vent location is unknown. Phenocrysts: 15% pl, 0.25–1 mm; 5% hyp, 0.25–1 

mm; 2% aug, 0.5–1 mm. Glomeroporphyritic clots of pl+pyr are abundant. Groundmass is 

dark gray and glassy to aphanitic. Unit a16 is older than all nearby units and is interpreted 
91

to be broadly equivalent to rocks exposed in upper part of Hat Creek Fault scarp. Unit not 

dated but is probably similar in age to andesite of Butte Lake (abt) and andesite of section 

22 (a22); estimated to be ~900–1,000 ka. [BB]

brk Tholeiitic basalt of Rock Creek (early Pleistocene)—Extensive sheet of sparsely porphyritic, 

tholeiitic ol basalt (48.9% SiO2) erupted from vent now buried beneath glacial till or younger 

rocks, probably near center of Red Cinder quadrangle. Number of small outcrops are southwest of Hay Meadow; larger continuous outcrops are beyond limit of till in southeast corner 

of map area. Lava flow extends southward into Lake Almanor Graben and down North Fork 

Feather River canyon, a distance of ~40 km. Lava sheet is at least 30 m thick near Lake Almanor. Offset at least 100 m east of Manzanita Mountain by faults extending northwest from 

Lake Almanor Graben; offset ~60 m by Clear Creek Fault in area south of Swain Mountain 

(out of map area; Westwood West quadrangle). Phenocrysts: sparse–2% ol, 0.5–2 mm. Lava 

has light to medium gray, aphanitic to holocrystalline, often coarsely diktytaxitic groundmass; flow tops contain abundant round vesicles; jointing is massive to blocky, often crudely 

hexagonal near upper surface of individual flow units. Lava flow has a well-developed 

orange to reddish soil; flow-top features are rarely observed. Unit brk can be discriminated 

from nearly identical but younger tholeiitic basalts by its reverse magnetic polarity. Paleomagnetic directional data (D.E. Champion, written commun., 1996) indicate that unit brk

was emplaced as single eruptive unit. 40Ar/39Ar age is 989±49 ka. [CH, RC]

msw Basaltic andesites of Swain Mountain (early Pleistocene)—Shield volcano composed of 

porphyritic ol basaltic andesite (53.2–56% SiO2) lava flows. Only westernmost part is 

in map area; most is in adjacent Swain Mountain quadrangle. Generalized description 

applies to entire Swain Mountain shield volcano. Volcano is ~8 km long and 6 km wide, 

and overall is well preserved. West flank of volcano (in map area) was oversteepened by 

faulting and glacial erosion; part of edifice is buried by till. Cirque is present on northeast 

side of summit; north flank is weakly glaciated. Scattered outcrops of agglutinated scoria 

in summit area are remnants of poorly preserved summit cone. Much of flanks of volcano, 

especially north flank, is covered by till and (or) thick soil. Volcano composed of two 

types of ol basaltic andesite, with and without pl phenocrysts, respectively. Phenocrysts 

(ol basaltic andesite, 53.2% SiO2, northeast base of volcano): 5–8% ol, 0.5–3 mm, mostly 

0.5–1 mm; rare–sparse pl, 0.5–1 mm. Groundmass is light to medium gray, aphanitic to 

holocrystalline, and dense to microvesicular. Cores of thick flow units, where ponded at 

north base of volcano, are very light gray and coarsely holocrystalline. Phenocrysts (ol-pl 

basaltic andesite, 56.3% SiO2, near summit): 2–4% ol, 0.5–3 mm, mostly 0.5–1 mm; 2–4% 

pl, 0.5–2 mm. Small glomeroporphyritic clots of ol+pl are common. Jointing is slabby in 

flow interiors. A number of much younger units are adjacent to the west flank of Swain 

Mountain, but till obscures contacts. Basaltic andesite flow from summit has K-Ar age of 

1,570±35 ka. [RC]

b11 Basalt of section 11 (early Pleistocene)—Lava flows of porphyritic ol basalt (52.2% SiO2) 5 km 

south of Bogard Buttes, with eroded lava cone marking vent. Lava flowed ~3 km southeast 

and is partly buried by younger units. Outcrop is sparse; flow is deeply weathered and covered by thick gravelly soil containing subrounded boulders of weathered lava. Phenocrysts: 

10% pl, 0.5–2 mm, mostly 0.5–1 mm; 5% ol, 0.5–2 mm, usually oxidized. Glomeroporphyritic clots of ol+pl to 1 cm are common. When fresh, groundmass is light gray, holocrystalline, and microvesicular. Rock is generally weathered, is brownish gray to pinkish gray, and 

has blocky jointing. Unit b11 is overlain by number of units from the much younger Poison 

Buttes chain. K-Ar age is 1,710±42 ka. [BB]

Tbm Basalts and basaltic andesites of Mud Creek Rim (late Pliocene)—Thick (>400 m) section of 

regional mafic lava flows and sparse interbedded pyroclastic material. Section is exposed 

primarily in fault scarps that form east boundary of north part of Lake Almanor Graben. 

Although variety of rocks is present, most abundant are ol basalts and basaltic andesites 

(51.6–57.2% SiO2). These lavas erupted from unknown number of small overlapping shield 

volcanoes north of Lake Almanor. In general, these lavas are sparsely to moderately porphyritic ol basalt to basaltic andesite with 1–10% ol, 0.5–2 mm; some flows contain aug 

and (or) pl and may lack ol. Groundmasses are generally light to medium gray and aphanitic. 40Ar/39Ar age of 2,785±16 ka and three K-Ar ages of 2,455±49 ka, 2,307±32 ka, and 

2,200±49 ka indicate significant range in age. [CH, RC]
92

Tdb Dacite of Bogard Buttes (late Pliocene)—Shield volcano composed of porphyritic aug-hyp 

dacite (63.4–63.5% SiO2) lava flows. Bogard Buttes volcano is ~5 km diam; its general 

form is preserved, although several small normal faults cut edifice and form small graben. 

Vent area(s) are not preserved. Rock is devitrified and (or) altered to light brown to tan. 

Phenocrysts: 15% pl, 0.5–2 mm, mostly 0.5–1 mm; 4% hyp, 0.5–2 mm, mostly 0.5–1 mm; 

4% aug, 0.5–2 mm, mostly 0.5–1 mm. Small glomeroporphyritic clots of pl+aug+hyp are 

common. Fresh groundmass is medium to dark gray, aphanitic, and dense and is locally 

flow banded. Jointing in flow interiors is slabby to platy. Unit Tdb is overlain on west flank 

by much younger vents and flows of Poison Lake chain. K-Ar ages of 2,350±70 ka and 

2,225±49 ka were obtained, but volcano was probably constructed over a shorter time interval than suggested by these ages. [BB]

Tac Andesite of Cal Mountain (late Pliocene)—Shield volcano composed of lava flows of sparsely 

porphyritic ol andesite (58.2–59.1% SiO2). This volcano is ~4 km diam and is well preserved; only south half lies within map area. Two samples, both from south flank of volcano, 

exhibit subtle variation in phenocryst content. Phenocrysts: 2% ol, 0.5–3 mm, mostly 0.5–1 

mm, sparse–trace pl, 0.5 mm. Groundmass is medium greenish gray, aphanitic, and dense. 

Splotchy vapor-phase crystallization parallel to flow banding is common in some flow interiors. Jointing is slabby in flow interiors. K-Ar age is 2,345±49 ka. [PL]

Tmc Basaltic andesite of Cone Mountain (late Pliocene)—Shield volcano composed of lava flows 

of sparsely porphyritic ol basaltic andesite (56.0–56.1% SiO2), north of Grays Valley at 

northeast corner of map area. Only extreme south flank of volcano is in map area. Volcano is 

~6 km diam and was somewhat dissected by erosion. Lava flows exhibit subtle variation in 

phenocryst content. Phenocrysts (south flank): 2% ol, 0.5–2 mm, mostly 0.5–1 mm. Groundmass is medium gray, contains abundant microlithic feldspar, and is microvesicular. K-Ar 

age is 2,575±57 ka. [PL]

Tmsr Basaltic andesites of Susan River (late Pliocene)—Sparsely to moderately porphyritic ol 

basaltic andesite (54.7% SiO2) lava flows forming extensive block-faulted terrane northeast 

of Silver Lake, mostly east of map area in Pine Creek Valley and Swain Mountain quadrangles. Constructional morphology is poorly preserved; individual lava flows and cones are 

not distinguished. Several poorly preserved vents are exposed east of map area. Rocks are 

generally deeply weathered and covered by extensive blanket of till and thick soil; outcrop 

is very sparse. Phenocrysts (ol basaltic andesite from map area): 10–12% pl, 0.5 mm; 3–4% 

ol, 0.5–1 mm. Groundmass is dark gray, aphanitic, and weakly microvesicular to dense. Age 

is very poorly constrained. Unit Tmsr is probably older than basaltic andesites of Swain 

Mountain (msw, 1,570±35 ka), basalt of section 11 (b11, 1,710±42 ka), and dacite of Bogard Buttes (Tdb, 2,225±49 ka and 2,350±70 ka). Flows in map area have normal magnetic 

polarity and probably fall into Gauss Chron of magnetic-polarity time scale and thus are 

probably older than 2,580 ka. [BB]
93

References cited

Anderson, C.A., 1933, The Tuscan formation of northern 

California, with a discussion concerning the origin of 

volcanic breccias: Berkeley, University of California, 

Department of Geological Sciences Bulletin, v. 23, no. 7, 

p. 215–276. 

Bacon, C.R., 1983, Eruptive history of Mount Mazama and 

Crater Lake caldera, Cascade Range, U.S.A.: Journal of 

Volcanology and Geothermal Research, v. 18, p. 57–115.

Bacon, C.R., 1986, Magmatic inclusions in intermediate and 

silicic volcanic rocks: Journal of Geophysical Research, v. 

91, p. 6091–6112.

Bacon, C.R., Bruggman, P.E., Christiansen, R.L., and 3 others, 

1997, Primitive magmas at five Cascade volcanic fields—

Melts from hot, heterogeneous sub-arc mantle: Canadian 

Mineralogist, v. 35, p. 397–423.

Berge, P.A., and Stauber, D.A., 1987, Seismic refraction study 

of upper crustal structure in the Lassen Peak area, northern 

California: Journal of Geophysical Research, v. 92, p. 

10,571–10,579.

Berggren, W.A., Hilgen, F.J., Langereis, C.G., and 5 others, 

1995, Late Neogene chronology—New perspectives 

in high-resolution stratigraphy: Geological Society of 

America Bulletin v. 107, p. 1272–1287.

Blake, M.C., Harwood, D.S., Helley, E.J., and 3 others, 

1999, Geologic map of the Red Bluff 30' x 60' 

quadrangle, California: U.S. Geological Survey Geologic 

Investigations Series I-2542.

Blakely, R.J., Christiansen, R.L., Guffanti, Marianne, and 

5 others, 1997, Gravity anomalies, Quaternary vents, 

and Quaternary faults in the southern Cascade Range, 

Oregon and California—Implications for arc and backarc 

evolution: Journal of Geophysical Research, v. 102, p. 

22,513–22,527.

Borg, L.E., 1989, Petrogenesis of Magee Composite Volcano, 

northern California: Austin, University of Texas, M.A. 

thesis, 128 p.

Borg, L.E., Blichert-Toft, Janne, and Clynne, M.A., 2002, 

Ancient and modern subduction zone contributions to 

the mantle sources of lavas from the Lassen Region of 

California inferred from Lu–Hf isotopic systematics: 

Journal of Petrology, v. 43, p. 705–723.

Borg, L.E., and Clynne, M.A., 1998, The petrogenesis of felsic 

calc-alkaline magmas from the southernmost Cascades, 

California—Origin by partial melting of basaltic lower 

crust: Journal of Petrology, v. 39, p. 1197–1222.

Borg, L.E., Clynne, M.A., and Bullen, T.D., 1997, The 

variable role of slab-derived fluids in the generation 

of a suite of primitive calc-alkaline lavas from the 

southernmost Cascade Range: Canadian Mineralogist, v. 

35, p. 425–452.

Bullen, T.D., and Clynne, M.A., 1990, Trace element and 

isotopic constraints on magmatic evolution at Lassen 

Volcanic Center, California: Journal of Geophysical 

Research, v. 95, p. 19,671–19,691.

Champion, D.E., Lanphere, M.A., and Anderson, S.R., 1996, 

Further verification and 40Ar/39Ar dating of the Big Lost 

Reversed Polarity Subchron from drill core subsurface 

samples of the Idaho National Engineering Laboratory, 

Idaho [abs.]: EOS Transactions of the American 

Geophysical Union, v. 77, p. F165.

Christiansen, R.L., Clynne, M.A, and Muffler, L.J.P., 2002, 

Geologic map of the Lassen Peak, Chaos Crags, and 

Upper Hat Creek area, California: U.S. Geological Survey 

Geologic Investigations Series I-2723, scale 1:24,000, 17 p..

Clynne, M.A., 1984, Stratigraphy and major-element 

geochemistry of the Lassen Volcanic Center, California: 

U.S. Geological Survey Open-File Report 84-224, 168 p.

Clynne, M.A., 1990, Stratigraphic, lithologic and major element 

geochemical constraints on magmatic evolution at Lassen 

Volcanic Center, California: Journal of Geophysical 

Research, v. 95, p. 19,651–19,669.

Clynne, M.A., 1993, Geologic Studies of the Lassen Volcanic 

Center, Cascade Range California: Santa Cruz, University 

of California, Ph.D. dissertation, 404 p.

Clynne, M.A., 1999, Complex magma mixing origin for rocks 

erupted in 1915, Lassen Peak, California: Journal of 

Petrology, v. 40, p. 105–132. 

Clynne, M.A., and Borg, L.E., 1997, Olivine and chromian 

spinel in primitive calc-alkaline and tholeiitic lavas from 

the southernmost Cascade Range—A reflection of relative 

fertility of the source: Canadian Mineralogist, v. 35, p. 

453–472.

Clynne, M.A., Champion, D.E., Trimble, D.A., Hendley, J.W., 

II, and Stauffer, P.H., 2000, How Old is “Cinder Cone”?—

Solving a Mystery in Lassen Volcanic National Park, 

California: U.S. Geological Survey Fact Sheet 023-00, 4 p.

Clynne, M.A., Christiansen, R.L., Felger, T.J., Stauffer, P.H., 

and Hendley, J.W., II, 1999, The 1915 Eruption of Lassen 

Peak: U.S. Geological Survey Fact Sheet 173-98, 2 p.

Clynne, M.A., Christiansen, R.L., Trimble, D.A., and 

McGeehin, J.P., 2002, Radiocarbon dates from volcanic 

deposits of the Chaos Crags and Cinder Cone eruptive 

sequences and other deposits, Lassen Volcanic National 

Park and vicinity, California: U.S. Geological Survey 

Open-File Report 02-290, 21 p.

Clynne, M. A., Janik, C.J., and Muffler, L.J.P., 2003, Hot water 

in Lassen Volcanic National Park—Fumaroles, steaming 

ground and boiling mudpots: U.S. Geological Survey Fact 

Sheet, 101-02, 4 p. 

Clynne, M.A., Muffler, L.J.P., Siems, D.F., Taggart, J.E. Jr., and 

Bruggman, Peggy, 2008, Major and EDXRF trace element 

chemical analyses of volcanic rocks from Lassen Volcanic 

National Park and vicinity: U.S. Geological Survey OpenFile Report 2008-1091, 11 p. and .xls table [http://pubs.

usgs.gov/of/2008/1091/].

Colman, S.M., and Pierce, K.L., 1992, Varied records of early 

Wisconsin alpine glaciation in the western United States 

derived from weathering-rind thicknesses, in Clark, P.U., 

and Lea, P.D., eds., The Last Interglacial-Glacial Transition 

in North America: Geological Society of America Special 

Paper 270, p. 269–278.
94

Crandell, D.R., 1972, Glaciation near Lassen Peak, northern 

California: U.S. Geological Survey Professional Paper 

800-C, p. C-179 to C188.

Crandell, D.R., Mullineaux, D.R., Sigafoos, R.S., and Rubin, 

Meyer, 1974, Chaos Crags eruptions and rockfallavalanches, Lassen Volcanic National Park, California: 

U.S. Geological Survey Journal of Research, v. 2, p. 

49–59.

Curtis, G. H., 1954, Mode of origin of pyroclastic debris in 

the Mehrten Formation of the Sierra Nevada: Berkeley, 

University of California, Department of Geological 

Sciences Bulletin, v. 29, no. 9, p. 453–502. 

Diller, J.S., 1895, Description of the Lassen Peak sheet, 

California: U.S. Geological Survey Geologic Atlas, Folio 

15, 14 p.

Eppler, D.B., Fink, Jonathan, and Fletcher, Raymond, 1987, 

Rheologic properties and kinematics of emplacement of 

the Chaos Jumbles rockfall avalanche, Lassen Volcanic 

National Park, California: Journal of Geophysical 

Research, v. 92, p. 3623–3633.

Faulds, J.E., and Henry, C.D., 2008, Tectonic influences on 

the spatial and temporal evolution of Walker Lane—An 

incipient transform fault along the evolving Pacific-North 

American plate boundary, in Spencer, J.E., and Titley, S.R., 

eds., Circum-Pacific tectonics, geologic evolution, and ore 

depostis: Tucson, Arizona Geological Society Digest 22, p. 

437–470.

Feeley, T.C., Clynne, M.A., Winer, G.S., and Grice, W.A., 

2008, Oxygen isotope geochemistry of the Lassen 

Volcanic Center, California: Resolving crustal and mantle 

contributions to continental arc magmatism: Journal of 

Petrology, v. 49, p. 971–997.

Gilbert, N.J., 1969, Chronology of post-Tuscan volcanism 

in the Manton area, California: Berkeley, University of 

California, M.A. thesis, 79 p.

Gill, J.B., 1981, Orogenic andesites and plate tectonics: New 

York, Springer-Verlag, 390 p.

Gradstein, F.M., Ogg, J.G., Smith, A.G., eds., 2005, A geologic 

time scale 2004: Cambridge, U.K., Cambridge University 

Press, 589 p. 

Grose, T.L.T., Saucedo, G.J., and Wagner, D.L., 1991, 

Geologic map of Susanville quadrangle, Lassen and 

Plumas Counties, California: California Department of 

Conservation, Division of Mines and Geology, Open-File 

Report 91-1.

Grose, T.L.T., Saucedo, G.J., and Wagner, D.L., 1992, Geologic 

map of Eagle Lake quadrangle, Lassen County, California: 

California Department of Conservation, Division of Mines 

and Geology, Open-File Report 92-14.

Guffanti, Marianne, Clynne, M.A., and Muffler, L.J.P., 1996, 

Thermal and mass implications of magmatic evolution 

in the Lassen volcanic region, California, and minimum 

constraints on basalt influx to the lower crust: Journal of 

Geophysical Research, v. 101, p. 3003–3013.

Guffanti, Marianne, Clynne, M.A., Smith, J.G., Muffler, 

L.J.P., and Bullen, T.D., 1990, Late Cenozoic volcanism, 

subduction and extension in the Lassen region of 

California, Southern Cascade Range: Journal of 

Geophysical Research, v. 95, p. 19,453–19,464.

Guffanti, Marianne, and Weaver, C.S., 1988, Distribution 

of late Cenozoic volcanic vents in the Cascade Range, 

volcanic arc segmentation and regional tectonic 

considerations: Journal of Geophysical Research, v. 93, p. 

6513–6529.

Harwood, D.S., Helley, E.J., and Doukas, M.P., 1981, Geologic 

Map of the Chico Monocline and northeastern part of the 

Sacramento Valley, California: U.S. Geological Survey 

Map I-1238.

Heiken, Grant, 1978, Characteristics of tephra from Cinder 

Cone, Lassen Volcanic National Park, California: Bulletin 

of Volcanology, v. 42, p. 119–131.

Helley, E.J., Harwood, D.S., Barker, J.A., and Griffin, E.A., 

1981, Geologic Map of the Battle Creek Fault Zone 

and adjacent parts of the Northern Sacramento Valley, 

California: U.S. Geological Survey Miscellaneous 

Investigations Map MF-1298.

Ingebritsen, S.E., and Rojstaczer, S., 1983, The Sifford Peak 

rings—a late Pliocene nested caldera complex [abs.]: Eos, 

Transactions of the American Geophysical Union, v. 64, p. 

879.

Ingebritsen, S.E., and Sorey, M.L., 1985, A quantitative 

analysis of the Lassen hydrothermal system, north central 

California: Water Resources Research v. 21, p. 853–868.

Jachens, R.C., and Saltus, R.W., 1983, Precision gravity 

network for monitoring the Lassen geothermal system, 

northern California: U.S. Geological Survey Open-File 

Report 83-193, 16 p.

Janik, C.J., and McLaren, M.K., 2010, Seismicity and 

fluid geochemistry at Lassen Volcanic National Park, 

California—Evidence for two circulation cells in the 

hydrothermal system: Journal of Volcanology and 

Geothermal Research, v. 189, p. 257–277.

Kane, Phillip, 1982, Pleistocene glaciation, Lassen Volcanic 

National Park: California Geology, v. 35, p. 95–105.

Lanphere, M.A, Champion, D.E., Clynne, M.A., and Muffler, 

L.J.P., 1999, Revised age of the Rockland tephra, northern 

California—Implications for climate and stratigraphic 

reconstructions in the western United States: Geology, v. 

27, p. 135–138.

Lanphere, M.A, Champion, D.E., Clynne, M.A., and 3 others, 

2004, Age of the Rockland tephra, western U.S.A.: 

Quaternary Research, v. 62, p. 94–104.

Lerch, F.G., 1987, Geology of the Red Cinder 7.5-minute 

quadrangle, Lassen and Plumas Counties, California: 

Golden, Colorado School of Mines, M.S. thesis, 274 p.

Lydon, P.A., 1968, Geology and lahars of the Tuscan Formation, 

northern California: Geological Society of America 

Memoir 116, p. 441–475.

Macdonald, G.A., 1963, Geologic map of the Manzanita Lake 

quadrangle, California: U.S. Geological Survey Geologic 

Quadrangle Map GQ-248, scale 1:62,500.

Macdonald, G.A., 1964, Geologic map of the Prospect Peak 

quadrangle, California: U.S. Geological Survey Geologic 

Quadrangle Map GQ-345, scale 1:62,500.
95

Macdonald, G.A., 1965, Geologic map of the Harvey Mountain 

quadrangle, California: U.S. Geological Survey Geologic 

Quadrangle Map GQ-443, scale 1:62,500.

Macdonald, G.A., and Lydon, P.A., 1972, Geologic map of 

the Whitmore quadrangle, California: U.S. Geological 

Survey Geologic Quadrangle Map GQ-993, scale 

1:62,500.

Meyer, C.E., Sarna-Wojcicki, A.M., Hillhouse, J.W., and 

3 others, 1991, Fission-track age (400,000 yr) of the 

Rockland tephra, based on inclusion of zircon grains 

lacking fossil fission tracks: Quaternary Research, v. 35, p. 

367–382. 

Meyer, C.E., Woodward, M.J., Sarna-Wojcicki, A.M., and 

Naeser, C.W., 1980, Zircon fission-track age of 0.45 

million years on ash in the type section of the Merced 

Formation, west-central California: U.S. Geological 

Survey Open-File Report 83-193, 16 p.

Muffler, L.J.P., Bacon, C.R., Christiansen, R.L., and 5 others, 

1989, South Cascades arc volcanism, California and 

southern Oregon, in Chapin, C.E., and Zidek, Jiri, eds., 

Field excursions to volcanic terranes in the western United 

States, Volume II, Cascades and Intermontane West: New 

Mexico Bureau of Mines and Mineral Resources Memoir 

47, p. 183–225.

Muffler, L.J.P., Clynne, M.A., and Champion, D.E., 1994, Late 

Quaternary normal faulting of Hat Creek Basalt, northern 

California: Geological Society of America Bulletin, v. 106, 

p. 195–200.

Muffler, L.J.P., Clynne, M.A., and Cook, A.L., 1982a, Mineral 

and geothermal resource potential of the Wild Cattle 

Mountain and Heart Lake Roadless Areas, Plumas, Shasta, 

and Tehama Counties, California: U.S. Geological Survey 

Open-File Report 82-846, 23 p.

Muffler, L.J.P., Jordan, Raymond, and Cook, A.L., 1983, 

Thermal features and topography of Bumpass Hell and 

Devils Kitchen, Lassen Volcanic National Park, California: 

U.S. Geological Survey Miscellaneous Field Studies Map 

1484, scale 1:2000.

Muffler, L.J.P., Nehring, N.L., Truesdell, A.H., Janik, C.J., 

Clynne, M.A., and Thompson, J.M., 1982b, The Lassen 

Geothermal System: Proceedings of Pacific Geothermal 

Conference, 1982, Part 2, p. 349–356.

Muffler, L.J.P., Robinson, J.E., Felger, T.J., Dutton, D.R., and 

Clynne, M.A., 2010, Database for the Geologic Map of 

Lassen Volcanic National Park and vicinity, California: 

U.S. Geological Survey Scientific Investigations Map 

I-2899, scale 1:50,000, [CD-ROM].

Sarna-Wojcicki, A.M., Meyer, C.E., Bowman, H.R., and 4 

others, 1985, Correlation of the Rockland Ash Bed, 

a 400,000-year-old stratigraphic marker in northern 

California and western Nevada, and implications for 

middle Pleistocene paleogeography of central California: 

Quaternary Geology, v. 23, p. 236–257.

Sheppard, P.R., Ort, M.H., Anderson, K.C., Clynne, M.A., 

and May, E.M., 2009, Multiple dendrochronological 

responses to the eruption of Cinder Cone, Lassen 

Volcanic National Park, California: Dendrochronologia, 

v. 27, p. 213–221.

Sherrod, D.R., and Smith, J.G., 1990, Quaternary extrusion 

rates of the Cascade Range, northwestern United States 

and southern British Columbia: Journal of Geophysical 

Research, v. 95, p. 19,465–19,474.

Tepley, F.J., III, Davidson, J.P., and Clynne, M.A., 1999, 

Magmatic interactions as recorded in plagioclase 

phenocrysts of Chaos Crags, Lassen Volcanic Center, 

California: Journal of Petrology, v. 40, p. 787–806. 

Thorpe, R.S., ed., 1982, Andesites—Orogenic andesites and 

related rocks: New York, John Wiley & Sons, 724 p.

Till, Alison, McHugh, E.L., Ritchey, J.L., Rumsey, C.M., and 

Cather, E.E., 1983, Mineral resource potential of the 

Caribou Wilderness Areas, Lassen and Plumas Counties, 

California: U.S. Geological Survey Open-File Report 

83-481, 13 p.

Turrin, B.D., Christiansen, R.L., Clynne, M.A., and 4 others, 

1998, Age of Lassen Peak, California, and implications 

for the ages of late Pleistocene glaciations in the southern 

Cascade Range: Geological Society of America Bulletin, v. 

110, p. 931–945.

Turrin, B.D., Muffler, L.J.P., Clynne, M.A., and Champion, 

D.E., 2007, Robust 24±6 ka 40Ar/39Ar age of a lowpotassium tholeiitic basalt in the Lassen region of NE 

California: Quaternary Research, v. 68, p. 96–110. 

Walker, Mike, Johnsen, S., Rasmussen, S.O., Popp, T., and 15 

others, 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, p. 3–17.

Williams, Howel, 1932, Geology of Lassen Volcanic National 

Park, California: Berkeley, University of California 

Department of Geological Sciences Bulletin, v. 21, p. 

195–385.

Wilson, T.A., 1961, The geology near Mineral, California: 

Berkeley, University of California, M.S. thesis, 92 p.
96

Table 1. Correlation of glacial terminology, age assignments, and depiction of glacial deposits in Lassen Volcanic National Park 

and vicinity, California.

[Map unit labels in parentheses; - - -, not indicated. Ages of glacial deposits for the Christiansen and others (2002) units and 

this map follow those of Turrin and others (1998): Tioga is Late to Middle Wisconsin in age and equivalent to Marine Oxygen 

Isotope Stage 2; Tahoe is Early Wisconsin in age and equivalent to Marine Oxygen Isotope Stage 4; and pre-Tahoe is Late 

Illinoian and older and equivalent to Marine Oxygen Isotope Stage 6 and older stages]

Crandell, 1972 Kane, 1982 Turrin and 

others, 1998

Christiansen and 

others, 2002, and 

detailed areas of 

this map

Additional 

generalized units 

on this map

Estimated age

- - - - - - Till of Anklin 

Meadows A4 - - - - - - 200–300 yrs

Moraines of late 

Tioga age Late Tioga till

Till of Anklin 

Meadows A3

Till or protalusrampart debris (Qth)

Till, younger 

glaciations (Qty)

8–12 ka

Late till of Anklin 

Meadows (Qtal)

Till of Anklin 

Meadows A2 Till of Anklin 

Meadows (Qta) 17–25 ka

Tioga Drift Middle Tioga till Till of Anklin 

Meadows A1

Tahoe Drift

Early Tioga till “Early Tioga” till

Post-maximum 

till of Raker Peak 

consisting of Lassen 

Peak avalanche 

debris (Qtrl)

~27 ka

Till of Raker Peak 

(Qtr) 27–35 ka

Tahoe till

Till of Badger 

Mountain B2 Till of Badger 

Mountain (Qtb) Till, older glaciations 

(Qto)

60–70 ka

Till of Badger 

Mountain B1

pre-Tahoe drift pre-Tahoe till - - - - - - >130 ka
97

Table 2. K-Ar and 40Ar/39Ar Ages obtained for the geologic map of Lassen Volcanic National Park and vicinity, California

[Unit labels are listed in the same order as shown in table 3 and Description of Map Units. See accompanying CD-ROM or files online (httpsample locations, given here as latitude and longitude in decimal degrees. A few samples were collected outside of the map area and are not taken from published sources and discussed in the Description of Map Units are not repeated here. Abbreviations: gl, glass; gm, groundmasUSGS analysts: ATC, A.T. Calvert; BDT, B.D. Turrin; GBD, G.B. Dalrymple; MAL, M.A. Lanphere. LEB, (L.E. Borg, written commun., 1

for William D. Page of Pacific Gas and Electric Company at Berkeley Geochronology Center, Berkeley, Calif., under the supervision of Paucommun., 1995)]

Unit label Map unit Sample No. Latitude Longitude Material dated Method 

Lassen Volcanic Center

mfp Basaltic andesite of Fairfield Peak LC86-928 40.5230° -121.3618° wr K-Ar 

ac Andesite of Crater Butte LC86-922 40.4915° -121.3527° wr K-Ar 

arp Andesite of Raker Peak LC85-748 40.5268° -121.4528° wr K-Ar 

av Andesite of Viola LC80-391 40.4976° -121.6086° wr K-Ar 

rkr Rhyodacite of Krummholz LC81-733 40.5036° -121.5003° wr 40Ar/39Ar 

re Rhyodacite of Eagle Peak, Dome and flow LC92-1752 40.4758° -121.5147° gl 40Ar/39Ar rd Rhyodacite of Dersch Meadows LC86-997 40.5099° -121.4619° gl 40Ar/39Ar dr Dacite of Reading Peak LC81-683 40.4685° -121.4415° wr K-Ar 

db Dacite of Bumpass Mountain B-331 40.4605° -121.4600° wr K-Ar 

dc Dacite of Crescent Crater LC87-1230 40.5073° -121.4900° gm 40Ar/39Ar 

ds Dacite of Ski Heil Peak LC83-355 40.4695° -121.5164° wr K-Ar 

dh Dacite of Mount Helen B-160 40.4707° -121.5048° wr K-Ar 

d82 Dacite of hill 8283 LC89-1514 40.5072° -121.5130° wr K-Ar 

rmz Rhyodacite of Manzanita Chute LC92-1754 40.5325° -121.5935° gl 40Ar/39Ar 

rmc Rhyodacite of Mount Conard LM80-899 40.4307° -121.5094° pl 40Ar/39Ar amd Andesite of Mount Diller LC80-449 40.4677° -121.5553° wr K-Ar 

adc Andesite of Digger Creek LC80-527 40.4497° -121.5797° wr K-Ar 

abf Andesite of Bluff Falls quarry 80G101 40.4144° -121.5319° wr K-Ar 

ar Andesite of Rice Creek B-316 40.4336° -121.4497° wr K-Ar 

80G112 40.3955° -121.4721° wr K-Ar 

dt Dacite of Twin Meadows LC82-865 40.4465° -121.5552° wr K-Ar 

amc Andesites of Mill Canyon B-338 40.4399° -121.5122° wr K-Ar 

LC84-605 40.4357° -121.5068° wr K-Ar 

LM80-884 40.4123° -121.5117° wr K-Ar 80G107 40.4749° -121.5624° wr K-Ar 

LM80-854 40.4223° -121.4971° wr K-Ar 

B-51 40.4657° -121.5738° wr K-Ar 

rr Rhyolite of Raker Peak LC82-210 40.5250° -121.4762° wr K-Ar 

drr Dacite of Red Rock Mountain 80G105 40.4704° -121.6181° wr K-Ar 

dbl Dacite of Bench Lake LC86-879 40.4597° -121.4258° wr K-Ar 

drp Dacite of Rocky Peak 80G102 40.4117° -121.5967° wr K-Ar 
98

Table 2. K-Ar and 40Ar/39Ar Ages—continued

Unit label Map unit Sample No. Latitude Longitude Material dated Method 

dfr Dacite of Flatiron Ridge LC81-629 40.4364° -121.3586° gm K-Ar 

dp Dacite of Panther Creek LC88-1427 40.4337° -121.4273° wr K-Ar 

Maidu Volcanic Center

rlp Rhyolite of Lost Creek Plateau LC93-1863 40.2798° -121.3993° wr 40Ar/39Ar 

Tdm Dacites of Maidu Volcanic Center, Stage 2, undivided LM93-3087 40.1962° -121.6313° wr K-Ar 

LC91-1611 40.3603° -121.6578° wr K-Ar 

LC88-1393 40.2542° -121.5447° wr K-Ar 

LC88-1372 40.2687° -121.6018° wr K-Ar 

Tam Andesites of Maidu Volcanic Center, Stage 1, 

undivided

LC80-504 40.3600° -121.6247° wr K-Ar 

LC79-54 40.3753° -121.5935° wr K-Ar 

LM93-2959 40.2422° -121.4737° wr K-Ar 

LC93-1814 40.2637° -121.4498° wr K-Ar 

LC88-1391 40.2733° -121.5395° wr K-Ar 

LC88-1382 40.2893° -121.5658° wr K-Ar 

Tab Andesite of South Fork Battle Creek LC91-1578 40.3562° -121.6573° wr K-Ar 

Dittmar Volcanic Center

rd3 Rhyolites of Dittmar Volcanic Center, Stage 3 LC86-865 40.4790° -121.4145° wr K-Ar 

ad2 Andesites of Dittmar Volcanic Center, Stage 2, 

undivided

LC81-576 40.3839° -121.3547° wr K-Ar 

Tad Andesites of Dittmar Volcanic Center, Stage 1, 

undivided

LC86-886 40.4777° -121.3057° wr K-Ar 

LC88-1413 40.4947° -121.2900° wr K-Ar 

LC93-1866 40.4273° -121.3322° wr K-Ar 

Latour Volcanic Center

Tlv Rocks of Latour Volcanic Center LC92-1696 40.6284° -121.6825° wr K-Ar 

Regional volcanic rocks

atb Andesite of Bear Wallow Butte LC83-263 40.6157° -121.5043° wr 40Ar/39Ar 

mbg Basaltic andesite of Little Bunchgrass Meadow LC83-276 40.5745° -121.4023° gm 40Ar/39Ar b44 Tholeiitic basalt of Calif. Hwy 44 LC84-569 40.6338° -121.3343° pl 40Ar/39Ar bec Tholeiitic basalt of Eagle Canyon LC84-563 40.5388° -121.7082° wr 40Ar/39Ar mm Basaltic andesites of Magee Volcano LB88-32 40.6895° -121.6182° wr K-Ar 

ap Andesite and basaltic andesite of Prospect Peak LM87-1443 40.5740° -121.3414° wr K-Ar 

acs Andesite of Cabin Spring 80G104 40.4614° -121.6303° wr K-Ar 

abm Andesites of Badger Mountain LC83-380 40.5755° -121.4013° wr K-Ar 

ahc Andesites, basaltic andesites, and basalts of Hat Creek 

Rim

LM87-1381 40.8295° -121.4557° wr K-Ar 

LM87-1378 40.7485° -121.4137° wr K-Ar 
99

Table 2. K-Ar and 40Ar/39Ar Ages—continued

Unit label Map unit Sample No. Latitude Longitude Material dated Method 

am Older regional andesites and basaltic andesites, 

undivided

LC92-1717 40.5723° -121.6842° wr K-Ar 

bbz Tholeiitic basalts of Buzzard Springs B-306 40.4025° -121.4301° wr K-Ar 

bsm Basalts and basaltic andesites of Sifford Mountain LC81-720 40.4357° -121.3743° wr 40Ar/39Ar 

LC81-649 40.3575° -121.3569° wr 40Ar/39Ar 

amh Andesite and basalt of Mount Harkness LC82-928 40.4099° -121.3071° wr K-Ar 

bwv Tholeiitic basalt of Warner Valley LC93-1970 40.2845° -121.2652° wr 40Ar/39Ar 

mgp Basaltic andesite of Grays Peak LM90-2422A 40.3790° -121.6592° wr K-Ar 

bpa Basalt of Panther Creek LM92-2438 40.3598° -121.7118° wr K-Ar 

acc Andesites of Cowslip Campground 80G108 40.3844° -121.6228° wr K-Ar 

mbc Basaltic andesites of South Fork Battle Creek LC91-1613 40.3613° -121.6508° wr K-Ar 

Caribou Volcanic Field

arr Andesite of Red Cinder LM87-1487 40.5200° -121.2693° wr K-Ar 

bdt Basalt of Twin Buttes LC86-831 40.6267° -121.2278° wr 40Ar/39Ar 

bc2 Basalts of Cone Lake Road, unit 2 LM03-4010 40.5573° -121.1708° gm 40Ar/39Ar 

bg4 Basalts of old railroad grade, unit 4 LM91-2116 40.6410° -121.2268° gm 40Ar/39Ar bp6 Basalts of Poison Butte, unit 6 LM03-4004 40.6370° -121.1740° gm 40Ar/39Ar bb12 Basalts and basaltic andesites of Bogard Buttes, unit 12 LC08-2433 40.5832° -121.1600° gm 40Ar/39Ar bb6 Basalts and basaltic andesites of Bogard Buttes, unit 6 LM90-1938 40.5565° -121.1260° gm 40Ar/39Ar bpc1 Basalts of Pine Creek, unit 1 LM07-4339 40.5350° -121.1313° gm 40Ar/39Ar bt4 Basalts of Pittville Road, unit 4 LM90-2070 40.6255° -121.1748° gm 40Ar/39Ar bt1 Basalts of Pittville Road, unit 1 LM90-2078 40.6490° -121.1798° gm K-Ar 

bs5 Basalts of Stephens Campground, unit 5 LM90-2027 40.6205° -121.1475° gm 40Ar/39Ar 

bs4 Basalts of Stephens Campground, unit 4 LM90-1983 40.5937° -121.1742° gm 40Ar/39Ar br2 Basalts of Robbers Spring, unit 2 LM90-2076 40.6374° -121.1711° gm 40Ar/39Ar bl35 Basalt of section 35 LM90-1981 40.5837° -121.1737° gm 40Ar/39Ar bl22 Basalt of hill 2232 LC85-655 40.5802° -121.2168° wr K-Ar 

mtr Basaltic andesite of Triangle Lake LM87-1351 40.5278° -121.2140° wr K-Ar 

mee1 Basaltic andesites of Echo Lake, unit 1 LM91-2233 40.4460° -121.1562° wr K-Ar 

mcm Basaltic andesite of middle Caribou LC88-1327 40.4653° -121.1770° wr K-Ar 

mc8 Basaltic andesites of Silver Lake, unit 8 LC82-898 40.4903° -121.1622° wr K-Ar 

bsl Basalt of Lost Spring LM87-1467 40.5766° -121.2828° gm 40Ar/39Ar 

bss1 Basalts of Sunrise Peak, unit 1 LM87-1548 40.5590° -121.2787° gm 40Ar/39Ar bie Basalt of East Lake LM87-1777 40.4417° -121.2454° wr K-Ar 

Older volcanic rocks

boc Older basalts and basaltic andesites south of Caribou 

Volcanic Field

LM91-2392 40.3970° -121.1735° wr K-Ar 
100

Table 2. K-Ar and 40Ar/39Ar Ages—continued

Unit label Map unit Sample No. Latitude Longitude Material dated Method 

mb Basaltic andesites of Black Cinder Rock LG92-50 40.3970° -121.1735 wr K-Ar 

brk Theoleiitic basalt of Rock Creek LC39-1980 40.4368° -121.2199 wr 40Ar/39Ar 

msw Basaltic andesites of Swain Mountain LM92-2746 40.4242° -121.1013° wr K-Ar 

b11 Basalt of section 11 LM90-1942 40.5478° -121.1325° wr K-Ar 

Tbm Basalts and basaltic andesites of Mud Creek Rim LM91-2379 40.3828° -121.2178° wr K-Ar 

LC82-879 40.3543° -121.2031° wr K-Ar 

LM91-2367 40.3348° -121.1770° wr K-Ar 

LM93-3150 40.2888° -121.1565° wr 40Ar/39Ar 

Tdb Dacite of Bogard Buttes LM90-1956 40.5777° -121.1333° wr K-Ar 

LM90-2018 40.5973° -121.1370° wr K-Ar 

Tac Andesite of Cal Mountain LM92-2802 40.6525° -121.1753° wr K-Ar 

Tmc Basaltic andesite of Cone Mountain LB90-85 40.6987° -121.1137° wr K-Ar 
101

Table 3. Alphabetical list of map unit names, labels, and ages.

[Correlation of Map Units (CMU) group abbreviations—CVF, Caribou Volcanic Field, may include additional abbreviations: D, Bidwell Spsequence; C, Caribou chain; CL, Cone Lake chain; I, Island Lake chain; P, Poison Lake chain; R, Red Cinder chain; S, Sunrise Peak sequen

Volcanic Center. LVC, Lassen Volcanic Center, may include additional abbreviations: B, Bumpass sequence; BV, Brokeoff Volcano; E, Eag

caldera complex; TO, Twin Lakes sequence; older, TY, Twin Lakes sequence, younger. LtVC, Latour Volcanic Center. MVC, Maidu Volcanvolcanic rocks north and west of Lassen Volcanic Center, may include additional abbreviations: T, Tumble Buttes chain, S, Sugarloaf chain. rocks in Caribou area. RSE, regional volcanic rocks south and east of Lassen Volcanic Center. Other units: G, glacial, sedimentary, and hydFormation. Ages from Table 2 or the Description of Map Units. See figure 3 (sheet 3) for quadrangle locations and abbreviations]

Unit label Unit name Text page CMU group* Age of dated units a16 Andesite of section 16 90 ROC a22 Andesite of section 22 60 RNW a74 Andesite of hill 7416 22 LVC-TY aa Andesite of Ashpan Butte 59 RNW ab Andesite of Benner Creek 46 DVC 

abf Andesite of Bluff Falls quarry 36 LVC-BV 467±10 ka 

abl Andesite of Butte Lake 59 RNW abm Andesites of Badger Mountain 58 RNW 708±21 ka ac Andesite of Crater Butte 23 LVC-TY 93±13 ka 

acc Andesites of Cowslip Campground 65 RSE 1,320±40 ka 

acl Andesite and basaltic andesite of Cluster Lakes 24 LVC-TO 

acs Andesite of Cabin Spring 57 RNW 690±34 ka ad Andesites of Diller sequence, undivided 35 LVC-BV 

ad2 Andesites of Dittmar Volcanic Center, Stage 2, undivided 45 DVC 1,398±42 ka 

adb Andesite of Bidwell Spring 70 CVF-D 

adc Andesite of Digger Creek 35 LVC-BV 451±10 ka 

adm Andesite of Doe Mountain 45 DVC 

ads Andesite of Domingo Spring 45 DVC 

ae Andesite of Eagle Peak 23 LVC-TY ag Andesite of Glassburner Meadows 36 LVC-BV 

agr Andesite of Grayback Ridge 57 RNW ah Andesite of Hat Mountain 22 LVC-TY ahc Andesites, basaltic andesites, and basalts of Hat Creek Rim 59 RNW 924±24 ka 

1,645±35 ka

alg Andesite of Logan Mountain 57 RNW 

am Older regional andesites and basaltic andesites, undivided 60 RNW 1,400±37 ka amc Andesites of Mill Canyon 37 LVC-BV 472±9 ka 

473±10 ka506±14 ka534±27 ka555±18 ka591±13 ka
102

Table 3. Alphabetical list of map unit names, labels, and ages—continued

Unit label Unit name Text page CMU group* Age of dated units 

amd Andesite of Mount Diller 34 LVC-BV 387±10 ka 

amh Andesite and basalt of Mount Harkness 63 RSE 188±32 ka 

amr Andesite of Martin Creek 65 RSE 

amz Andesite of Manzanita Creek 35 LVC-BV 

ao Andesite of Onion Creek 58 RNW ap Andesite and basaltic andesite of Prospect Peak 55 RNW 247±56 ka ar Andesite of Rice Creek 36 LVC-BV 477±14 ka 

485±12 ka

arp Andesite of Raker Peak 24 LVC-TO 270±18 ka 

arr Andesite of Red Cinder 68 CVF-R 69±20 ka 

aso Andesites of Old Station 53 RNW-S asp Andesite of Potato Butte 52 RNW-S 77±11 ka 

ass Andesite of Sugarloaf Peak 52 RNW-S 46±7 ka at Andesite of Table Mountain 58 RNW atb Andesite of Bear Wallow Butte 50 RNW-T 35.1±3.1 ka 

atd Andesite of Devils Rock Garden 49 RNW-T att Andesite of Tumble Buttes 51 RNW-T av Andesite of Viola 24 LVC-TO 313±8 ka 

aw Andesite of Wilcox Peak 57 RNW awp Andesites of West Prospect Peak 56 RNW b11 Basalt of section 11 91 ROC 1,710±42 ka 

b18 Tholeiitic basalt of hill 1879 54 RNW b44 Tholeiitic basalt of Calif. Hwy 44 53 RNW 190±18 ka 

bb1 Basalts and basaltic andesites of Bogard Buttes, unit 1 74 CVF-P bb2 Basalts and basaltic andesites of Bogard Buttes, unit 2 74 CVF-P bb3 Basalts and basaltic andesites of Bogard Buttes, unit 3 74 CVF-P bb4 Basalts and basaltic andesites of Bogard Buttes, unit 4 74 CVF-P bb5 Basalts and basaltic andesites of Bogard Buttes, unit 5 73 CVF-P bb6 Basalts and basaltic andesites of Bogard Buttes, unit 6 73 CVF-P 100.1±8.5 ka 

bb8 Basalts and basaltic andesites of Bogard Buttes, unit 8 73 CVF-P bb9 Basalts and basaltic andesites of Bogard Buttes, unit 9 73 CVF-P bb10 Basalts and basaltic andesites of Bogard Buttes, unit 10 73 CVF-P bb11 Basalts and basaltic andesites of Bogard Buttes, unit 11 73 CVF-P bb12 Basalts and basaltic andesites of Bogard Buttes, unit 12 73 CVF-P 107.5±3.8 ka 

bbb Basalt of Beauty Lake 88 CVF-B 

bbf Basalt of Badger Flat 56 RNW bbh Basalt of Hidden Lakes 88 CVF-B 

bbp Basalt of Posey Lake 88 CVF-B 

bbz Tholeiitic basalts of Buzzard Springs 61 RSE 65±45 ka 
103

Table 3. Alphabetical list of map unit names, labels, and ages—continued

Unit label Unit name Text page CMU group* Age of dated units 

bc1 Basalts of Cone Lake Road, unit 1 71 CVF-P bc2 Basalts of Cone Lake Road, unit 2 71 CVF-P 97.6±9.8 ka 

bcb Basalt of Cowboy Lake 86 CVF-C 

bcc Basalts of Cold Creek Butte 62 RSE 

bcg Basalt of Butte Creek Campground 89 ROC 

bci Basalt of Indian Meadow 84 CVF-C 

bcp Basalt of Caribou Peak 85 CVF-C 

bcr Basalt of Butte Creek Ranch 90 ROC bcs Basalt of South Caribou 83 CVF-C 

bct Basalt of Cherry Thicket 56 RNW bcw Basalt of Caribou Wilderness 85 CVF-C 

bdt Basalt of Twin Buttes 70 CVF-D 46±3 ka 

beb1 Basalts of Bond Valley, unit 1 80 CVF-C 

beb2 Basalts of Bond Valley, unit 2 79 CVF-C 

beb3 Basalts of Bond Valley, unit 3 79 CVF-C 

bec Tholeiitic basalt of Eagle Canyon 54 RNW 199±22 ka bg1 Basalts of old railroad grade, unit 1 72 CVF-P bg2 Basalts of old railroad grade, unit 2 71 CVF-P bg3 Basalts of old railroad grade, unit 3 71 CVF-P bg4 Basalts of old railroad grade, unit 4 71 CVF-P 102.1±11.4 ka bgv Tholeiitic basalt of Grays Valley 88 ROC bhc Hat Creek Basalt (tholeiitic) 48 RNW 24±6 ka bhr Basaltic plug near Heckle Ranch 89 ROC bic Tholeiitic basalt of Ice Cave Mountain 61 RSE 

bie Basalt of East Lake 87 CVF-I 331±45 ka 

bii Basalt of Island Lake 87 CVF-I bk1 Tholeiitic basalts of Big Lake, unit 1 48 RNW bk2 Tholeiitic basalts of Big Lake, unit 2 48 RNW bl2 Basalt of section 2 77 CVF-CL bl20 Basalt of hill 2088 77 CVF-CL bl21 Basalt of hill 2109 78 CVF-CL bl22 Basalt of hill 2232 77 CVF-CL 220±14 ka 

bl30 Basalt of section 30 77 CVF-CL bl35 Basalt of section 35 77 CVF-CL 202.0±2.2 ka 

bmc Tholeiitic basalts of Mill Creek Plateau 62 RSE bn Tholeiitic basalt of Nobles Trail 55 RNW bnt Basalt of Bonte Peak 90 ROC bo Basalt of Onion Springs 56 RNW boc Older basalts and basaltic andesites south of Caribou Volcanic Field 89 ROC 650±20 ka 
104

Table 3. Alphabetical list of map unit names, labels, and ages—continued

Unit label Unit name Text page CMU group* Age of dated units 

bp1 Basalts of Poison Butte, unit 1 72 CVF-P bp2 Basalts of Poison Butte, unit 2 72 CVF-P bp3 Basalts of Poison Butte, unit 3 72 CVF-P bp4 Basalts of Poison Butte, unit 4 72 CVF-P bp5 Basalts of Poison Butte, unit 5 72 CVF-P bp6 Basalts of Poison Butte, unit 6 72 CVF-P 105.0±6.0 ka 

bpa Basalt of Panther Creek 64 RSE 1,105±49 ka 

bpc1 Basalts of Pine Creek, unit 1 74 CVF-P 106.2±3.6 ka 

bpc2 Basalts of Pine Creek, unit 2 74 CVF-P bpc3 Basalts of Pine Creek, unit 3 74 CVF-P br1 Basalts of Robbers Spring, unit 1 77 CVF-P br2 Basalts of Robbers Spring, unit 2 76 CVF-P 101.5±8.7 ka 

br22 Basalt of hill 2283 67 CVF-R 

br25 Basalt of section 25 68 CVF-R 

br80 Basalt of hill 8030 67 CVF-R 

bra Basalt of Ash Butte 67 CVF-R 

brc Basalt of Red Cinder Cone 66 CVF-R 

bre Basalt east of Ash Butte 68 CVF-R 

brk Tholeiitic basalt of Rock Creek 91 ROC 989±49 ka 

brm Basalt of Cameron Meadow 67 CVF-R 

brt1 Basalts of Triangle Lake, unit 1 69 CVF-R 

brt2 Basalts of Triangle Lake, unit 2 69 CVF-R 

brw Basalt of Widow Lake 68 CVF-R 

bs1 Basalts of Stephens Campground, unit 1 76 CVF-P 

bs2 Basalts of Stephens Campground, unit 2 76 CVF-P 

bs3 Basalts of Stephens Campground, unit 3 76 CVF-P bs4 Basalts of Stephens Campground, unit 4 76 CVF-P 110.8±3.8 ka bs5 Basalts of Stephens Campground, unit 5 75 CVF-P 108.6±2.1 ka 

bs72 Basalt of hill 7243 86 CVF-S bsb Basalt of Bathtub Lake 87 CVF-S bsd Basalt of Duck Lake 86 CVF-S bsh Basalts and basaltic andesites of Swains Hole 88 ROC bsl Basalt of Lost Spring 86 CVF-S 302±7 ka bsm Basalts and basaltic andesites of Sifford Mountain 62 RSE 167±4 ka 

172±23 ka

bss1 Basalts of Sunrise Peak, unit 1 87 CVF-S 393±4 ka bss2 Basalts of Sunrise Peak, unit 2 86 CVF-S bt1 Basalts of Pittville Road, unit 1 75 CVF-P 108±14 ka 

bt2 Basalts of Pittville Road, unit 2 75 CVF-P 
105

Table 3. Alphabetical list of map unit names, labels, and ages—continued

Unit label Unit name Text page CMU group* Age of dated units 

bt3 Basalts of Pittville Road, unit 3 75 CVF-P bt4 Basalts of Pittville Road, unit 4 75 CVF-P 101.0±1.9 ka 

btb Tholeiitic basalt of Twin Bridges 59 RNW bwv Tholeiitic basalt of Warner Valley 64 RSE 610±22 ka 

cc Rhyodacite of Chaos Crags, Talus, emplaced hot from Domes B–F 25 LVC-E d4 Deposits of 1914–1917 eruption of Lassen Peak, Dacite dome, May 14–19 20 LVC-TY 1915 C.E. d82 Dacite of hill 8283 33 LVC-B 261±5 ka 

d9 Deposits of 1914–1917 eruption of Lassen Peak, Dacite flow, May 19–20 19 LVC-TY 1915 C.E. 

db Dacite of Bumpass Mountain 30 LVC-B 232±8 ka 

dbc Dacite of Bailey Creek 39 LVC-R 

dbl Dacite of Bench Lake 39 LVC-R 679±14 ka 

dc Dacite of Crescent Crater 31 LVC-B 236±1 ka dch Dacite of Christie Hill 40 LVC-R 

dcy Dacite of Canyon Creek 40 LVC-R dfr Dacite of Flatiron Ridge 39 LVC-R 812±6 ka 

dh Dacite of Mount Helen 31 LVC-B 249±12 ka dhm Dacite of Huckleberry Mountain 60 RNW dl Dacite of Lassen Peak, Dome 27 LVC-E 

dmm Dacite of Morgan Mountain 40 LVC-R dmz Dacite of upper Manzanita Creek 33 LVC-B dp Dacite of Panther Creek 39 LVC-R 827±18 ka 

dpg Dacite of Plantation Gulch 40 LVC-R 

dpl Dacite of Lassen Peak parking lot 32 LVC-B 

dr Dacite of Reading Peak 30 LVC-B 212±5 ka drp Dacite of Rocky Peak 39 LVC-R 803±27 ka drr Dacite of Red Rock Mountain 38 LVC-R 672±20 ka ds Dacite of Ski Heil Peak 31 LVC-B 244±10 ka dsc Dacite of Summit Creek 32 LVC-B dt Dacite of Twin Meadows 36 LVC-BV 470±10 ka duh Dacite under Mount Helen 32 LVC-B dv Dacite of Vulcans Castle 33 LVC-B f9 Deposits of 1914–1917 eruption of Lassen Peak, Flood deposits, May 19–20 20 LVC-TY 1915 C.E. h Hydrothermally altered rocks in active thermal areas 16 G ht Travertine 16 G m20 Basaltic andesite west of hill 2078 75 CVF-P mb Basaltic andesite of Black Cinder Rock 90 ROC 667±24 ka mbc Basaltic andesites of South Fork Battle Creek 65 RSE 1,738±46 ka mbg Basaltic andesite of Little Bunchgrass Meadow 53 RNW 143±6 ka 
106

Table 3. Alphabetical list of map unit names, labels, and ages—continued

Unit label Unit name Text page CMU group* Age of dated units 

mbx Basaltic andesite of Box Canyon 53 RNW mc1 Basaltic andesites of Silver Lake, unit 1 85 CVF-C 

mc2 Basaltic andesites of Silver Lake, unit 2 85 CVF-C 

mc3 Basaltic andesites of Silver Lake, unit 3 85 CVF-C 

mc4 Basaltic andesites of Silver Lake, unit 4 85 CVF-C 

mc5 Basaltic andesites of Silver Lake, unit 5 85 CVF-C 

mc6 Basaltic andesites of Silver Lake, unit 6 85 CVF-C 

mc7 Basaltic andesites of Silver Lake, unit 7 84 CVF-C 

mc8 Basaltic andesites of Silver Lake, unit 8 84 CVF-C 400±14 ka 

mc9 Basaltic andesites of Silver Lake, unit 9 84 CVF-C 

mc10 Basaltic andesites of Silver Lake, unit 10 84 CVF-C 

mc11 Basaltic andesites of Silver Lake, unit 11 84 CVF-C 

mc71 Basaltic andesite of hill 7114 83 CVF-C 

mce Basaltic andesite of Eleanor Lake 80 CVF-C 

mcf Basaltic andesites of Camp Forward 61 RNW mch Basaltic andesite of Hay Meadow 83 CVF-C 

mcj Basaltic andesite of Jewel Lake 81 CVF-C 

mcm Basaltic andesite of middle Caribou 83 CVF-C 362±33 ka 

mcn1 Basaltic andesites of North Caribou, unit 1 82 CVF-C 

mcn2 Basaltic andesites of North Caribou, unit 2 82 CVF-C 

mcn3 Basaltic andesites of North Caribou, unit 3 82 CVF-C 

mcn4 Basaltic andesites of North Caribou, unit 4 82 CVF-C 

mcn5 Basaltic andesites of North Caribou, unit 5 82 CVF-C 

mcn6 Basaltic andesites of North Caribou, unit 6 82 CVF-C 

mcn7 Basaltic andesites of North Caribou, unit 7 82 CVF-C 

mcn8 Basaltic andesites of North Caribou, unit 8 81 CVF-C 

mcr Basaltic andesite of Rim Lake 81 CVF-C 

md36 Basaltic andesite of section 36 70 CVF-D 

mdb1 Basaltic andesites of Black Butte, unit 1 70 CVF-D 

mdb2 Basaltic andesites of Black Butte, unit 2 70 CVF-D 

mdp Basaltic andesite of Pole Spring Road 70 CVF-D 

mee1 Basaltic andesites of Echo Lake, unit 1 80 CVF-C 302±36 ka 

mee2 Basaltic andesites of Echo Lake, unit 2 80 CVF-C 

meh Basaltic andesite of Eskimo Hill 49 RNW mes Basaltic andesite of Star Butte 80 CVF-C 

mf1 Basaltic andesites of Cinder Cone, Fantastic Lava Beds, Flow 1 21 LVC-TY 1666 C.E. 

mf2 Basaltic andesites of Cinder Cone, Fantastic Lava Beds, Flow 2 21 LVC-TY 1666 C.E. mfp Basaltic andesite of Fairfield Peak 23 LVC-TY 82±14 ka 

mgp Basaltic andesite of Grays Peak 64 RSE 1,080±26 ka 
107

Table 3. Alphabetical list of map unit names, labels, and ages—continued

Unit label Unit name Text page CMU group* Age of dated units 

mhl Basaltic andesite of Huckleberry Lake 64 RSE 

mhr Basaltic andesite of Hootman Ranch 56 RNW 562±12 ka mj Basaltic andesite of Jakey Lake 89 ROC 

ml18 Basaltic andesite of hill 1868 78 CVF-CL mlc Basaltic andesites of Cone Lake 78 CVF-CL mm Basaltic andesites of Magee Volcano 54 RNW 210±120 ka mo Basaltic andesites of Cinder Cone, Old Bench flow 22 LVC-TY 1666 C.E. mp1 Basaltic andesites of Cinder Cone, Painted Dunes, Flow 1 22 LVC-TY 1666 C.E. mp2 Basaltic andesites of Cinder Cone, Painted Dunes, Flow 2 21 LVC-TY 1666 C.E. mr Basaltic andesite of Rock Spring 58 RNW mrb Basaltic andesite of Caribou Wilderness 69 CVF-R 

mrc Basaltic andesite of Red Cinder Cone 66 CVF-R 

mrd Basaltic andesite and andesite of Red Lake Mountain 49 RNW mrg1 Basaltic andesites of Long Lake, unit 1 69 CVF-R 

mrg2 Basaltic andesites of Long Lake, unit 2 69 CVF-R 

mrg3 Basaltic andesites of Long Lake, unit 3 68 CVF-R 

mrm Basaltic andesite of Red Mountain 49 RNW mrr Basaltic andesite of Red Cinder 67 CVF-R 

ms47 Basaltic andesite of hill 4709 52 RNW-S 

msg Basaltic andesite of Signal Butte 59 RNW msl Basaltic andesite of Little Potato Butte 52 RNW-S 67±4 ka msm Basaltic andesite of Small Butte 63 RSE 

msn Basaltic andesites and basalt of Snag Lake 90 ROC msw Basaltic andesites of Swain Mountain 91 ROC 1,570±35 ka 

mt20 Basaltic andesites of section 20 79 CVF-C mt5 Basaltic andesite of section 5 51 RNW-T mt54 Basaltic andesite of hill 5410 50 RNW-T mt61 Basaltic andesite of hill 6138 51 RNW-T mt67 Basaltic andesite of hill 6770 50 RNW-T mtb Basaltic andesite of Bear Wallow Butte 51 RNW-T mte Basaltic andesites of Evelyn Lake 66 CVF-T mtm Basaltic andesite of Mud Lake 51 RNW-T mtr Basaltic andesite of Triangle Lake 79 CVF-C 277±22 ka 

mtt Basaltic andesite of Turnaround Lake 66 CVF-T mtu Basaltic andesite of Tumble Buttes 50 RNW-T p17 Deposits of 1914–1917 eruption of Lassen Peak, Phreatic deposit, May–June 19 LVC-TY 1917 C.E. p2 Deposits of 1914–1917 eruption of Lassen Peak, Pumice-fall deposit, May 22 19 LVC-TY 1915 C.E. p9 Deposits of 1914–1917 eruption of Lassen Peak, Pyroclastic deposit, May 19 20 LVC-TY 1915 C.E. pc Rhyodacite of Chaos Crags, Pumiceous pyroclastic-flow and fall deposits 26 LVC-E 1,103±13 yr B.P 
108

Table 3. Alphabetical list of map unit names, labels, and ages—continued

Unit label Unit name Text page CMU group* Age of dated units 

pcd Rhyodacite of Chaos Crags, Lithic pyroclastic-flow deposit from partial collapse of 

dome D

26 LVC-E 

pce Rhyodacite of Chaos Crags, Lithic pyroclastic-flow deposit from partial collapse of 

dome E

25 LVC-E 

pdh Lithic tephra from Deep Hole 55 RNW pe Rhyodacite of Eagle Peak, Pumiceous pyroclastic-flow deposits 29 LVC-E pfl Dacite of Lassen Peak, Lithic pyroclastic-flow deposit from partial collapse of dome 27 LVC-E 27±1 ka 

pk Rhyodacite of Kings Creek, Pumiceous pyroclastic-flow deposits 28 LVC-E 35±1 ka pr Rockland tephra 38 LVC-R 609±7 ka psf Rhyodacite of Sunflower Flat, Pumiceous pyroclastic-flow deposits 28 LVC-E 41±1 ka pw2 Deposits of 1914–1917 eruption of Lassen Peak, Pyroclastic-flow and associated fluid 

debris-flow deposits, May 22

19 LVC-TY 1915 C.E. 

Qc Colluvium and talus 14 G Qcl Talus on Lassen Peak 14 G Qd Diatomite 18 G Qf Alluvium 14 G Qoa Outwash gravel of Anklin Meadows 18 G Qoo Outwash gravel, older glaciations 18 G Qor Outwash gravel of Raker Peak 18 G Qou Outwash gravel, undivided 18 G Qoy Outwash gravel, younger glaciations 17 G Qs82 Avalanche deposit from dacite of hill 8283 15 G Qsh Landslide deposits in hydrothermally altered core of Brokeoff Volcano 14 G 3,310±55 yr B.P. 

Qsj Avalanche deposits of Chaos Jumbles 15 G 278±28 yr B.P. 

Qsl Avalanche debris from Lassen Peak spread across glacial ice 15 G 

Qta Till of Anklin Meadows 17 G Qtal Late till of Anklin Meadows 16 G Qtb Till of Badger Mountain 17 G Qth Till or protalus-rampart debris 16 G Qto Till, older glaciations 17 G Qtr Till of Raker Peak 17 G Qtrl Post-maximum till of Raker Peak consisting of Lassen Peak avalanche debris 17 G 
109

Table 3. Alphabetical list of map unit names, labels, and ages—continued

Unit label Unit name Text page CMU group* Age of dated units 

Qty Till, younger glaciations 16 G Qwb Debris-flow deposit from Brokeoff Volcano 15 G Qwh Debris-flow deposits from the northeast side of Lassen Peak 15 G ~8,000 yrs B.P. 

r27 Rhyodacite of section 27 29 LVC-E 

rb Rhyolite of Blue Ridge 41 MVC 1,150±70 ka 

1,240±110 ka

rca Rhyodacite of Chaos Crags, Dome A 27 LVC-E 1,103±13 yr B.P. 

rcb Rhyodacite of Chaos Crags, Dome B 26 LVC-E 

rcc Rhyodacite of Chaos Crags, Dome C 26 LVC-E rcd Rhyodacite of Chaos Crags, Dome D 26 LVC-E rce Rhyodacite of Chaos Crags, Dome E 26 LVC-E rcf Rhyodacite of Chaos Crags, Dome F 25 LVC-E rd Rhyodacite of Dersch Meadows 30 LVC-B 193±11 ka 

rd3 Rhyolites of Dittmar Volcanic Center, Stage 3 44 DVC 1,273±7 ka 

re Rhyodacite of Eagle Peak, Dome and flow 29 LVC-E 66±4 ka 

rg Rhyodacite of Gurnsey Creek 42 MVC 

rk Rhyodacite of Kings Creek, Flows 28 LVC-E rkr Rhyodacite of Krummholz 29 LVC-E 43±2 ka 

rlm Rhyodacite of Loomis Peak 34 LVC-B 

rlp Rhyolite of Lost Creek Plateau 41 MVC 1,305±10 ka 

rmc Rhyodacite of Mount Conard 34 LVC-B 298±9 ka rmp Rhyolite of Mill Creek Plateau 41 MVC 

rmz Rhyodacite of Manzanita Chute 33 LVC-B 297±1 ka 

rr Rhyolite of Raker Peak 38 LVC-R 588±69 ka 

rsf Rhyodacite of Sunflower Flat, Domes 28 LVC-E 

rsm Rhyolite of North Stover Mountain 41 MVC 

sw9 Deposits of 1914–1917 eruption of Lassen Peak, Avalanche and debris-flow deposits, 

May 19–20

20 LVC-TY 1915 C.E. 

Tab Andesite of South Fork Battle Creek 43 MVC 2,166±67 ka 

Tac Andesite of Cal Mountain 92 ROC 2,345±49 ka 

Tad Andesites of Dittmar Volcanic Center, Stage 1, undivided 46 DVC 1,650±35 ka 

1,785±35 ka2,315±29 ka

Tah Andesites of Hampton Butte 42 MVC 
110

Table 3. Alphabetical list of map unit names, labels, and ages—continued

Unit label Unit name Text page CMU group* Age of dated units 

Tam Andesites of Maidu Volcanic Center, Stage 1, undivided 43 MVC 1,905±42 ka 

2,045±42 ka2,152±47 ka2,250±49 ka2,325±49 ka2,375±49 ka

Taw Andesites of Wild Cattle Mountain 46 DVC 

Tbm Basalts and basaltic andesites of Mud Creek Rim 91 ROC 2,200±49 ka 

2,307±32 ka2,455±49 ka2,785±16 ka

Tdb Dacite of Bogard Buttes 92 ROC 2,225±49 ka 

2,350±70 ka

Tdm Dacites of Maidu Volcanic Center, Stage 2, undivided 42 MVC 2,105±25 ka 

2,120±46 ka2,140±42 ka2,259±46 ka

Tlv Rocks of Latour Volcanic Center 47 LtVC 3,140±67 ka 

Tmc Basaltic andesite of Cone Mountain 92 ROC 2,575±57 ka 

Tml Basaltic andesite of Lassen Lodge 44 MVC 

Tmsr Basaltic andesites of Susan River 92 ROC Tt Tuscan Formation 47 TF 

w2 Deposits of 1914–1917 eruption of Lassen Peak, viscous debris-flow deposits, May 22 19 LVC-TY 1915 C.E.