Geologic Map of Mount Mazama and Crater Lake Caldera, Oregon

By Charles R. Bacon

Pamphlet to accompany

Scientific Investigations Map 2832

View from the south-southwest rim of Crater Lake caldera showing the caldera wall from Hillman Peak on the west to Cleetwood 

Cove on the north. Crater Lake fills half of the 8- by 10-km-diameter caldera formed during the climactic eruption of Mount 

Mazama volcano approximately 7,700 years ago. Volcanic rocks exposed in the caldera walls and on the flanks record over 

400,000 years of eruptive history. The exposed cinder cone and andesite lava flows on Wizard Island represent only 2 percent 

of the total volume of postcaldera volcanic rock that is largely covered by Crater Lake. Beyond Wizard Island, the great cliff 

of Llao Rock, rhyodacite lava emplaced 100–200 years before the caldera-forming eruption, dominates the northwest caldera 

wall where andesite lava flows at the lakeshore are approximately 150,000 years old.

2008

U.S. Department of the Interior

U.S. Geological Survey
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i

CONTENTS

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

 Physiography and access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Geologic setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Eruptive history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

 Regional volcanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

 Pre-Mazama silicic rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

 Mount Mazama . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

 Preclimactic rhyodacites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

 The climactic eruption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

 Postcaldera volcanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Submerged caldera walls and floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Glaciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Geothermal phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

 Volcanic hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

 Earthquake hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Description of map units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

 Sedimentary deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

 Volcanic rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

 Regional volcanism, northwest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

 Regional volcanism, southwest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

 Mount Mazama . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

 Regional volcanism, east . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Figures

1. Shaded-relief map of map area showing Mount Mazama and Crater Lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Shaded-relief map of Crater Lake region showing Quaternary faults and maximum extent of pyroclastic-flow deposits

 of the ~7,700 yr B.P. climactic eruption of Mount Mazama. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
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INTRODUCTION

Crater Lake partly fills one of the most spectacular calderas 

of the world, an 8-by-10-km basin more than 1 km deep formed 

by collapse of the volcano known as Mount Mazama (fig. 1) 

during a rapid series of explosive eruptions about 7,700 years ago. 

Having a maximum depth of 594 m, Crater Lake is the deepest 

lake in the United States. Crater Lake National Park, dedicated in 

1902, encompasses 645 km2 of pristine forested and alpine terrain, 

including the lake itself, virtually all of Mount Mazama, and most 

of the area of the geologic map. The geology of the area was first 

described in detail by Diller and Patton (1902) and later by Williams 

(1942), whose vivid account led to international recognition of 

Crater Lake as the classic collapse caldera. Because of excellent 

preservation and access, Mount Mazama, Crater Lake caldera, and 

the deposits formed by the climactic eruption constitute a natural laboratory for study of volcanic and magmatic processes. For 

example, the climactic ejecta are renowned among volcanologists 

as evidence for systematic compositional zonation within a subterranean magma chamber. Mount Mazama’s climactic eruption also 

is important as the source of the widespread Mazama ash, a useful 

Holocene stratigraphic marker throughout the Pacific Northwest, 

adjacent Canada, and offshore. A detailed bathymetric survey of 

the floor of Crater Lake in 2000 (Bacon and others, 2002) provides 

a unique record of postcaldera eruptions, the interplay between 

volcanism and filling of the lake, and sediment transport within 

this closed basin. Knowledge of the geology and eruptive history 

of the Mount Mazama edifice, greatly enhanced by the caldera 

wall exposures, gives exceptional insight into how large volcanoes 

of magmatic arcs grow and evolve. Lastly, the many smaller volcanoes of the High Cascades beyond the limits of Mount Mazama 

are a source of information on the flux of mantle-derived magma 

through the region. General principles of magmatic and eruptive 

processes revealed by the present study have been incorporated 

not only in scientific investigations elsewhere, but in the practical 

evaluation of hazards (Bacon and others, 1997b) and geothermal 

resources (Bacon and Nathenson, 1996) in the Crater Lake region. 

In addition to papers in scientific journals, field trip guides, and 

the hazard and geothermal reports, the major product of this longterm study of Mount Mazama is the geologic map. The map is 

unusual because it portrays bedrock (outcrop), surficial, and lake 

floor geology. Caldera wall geology is depicted in detail on the 

accompanying geologic panoramas.

PHYSIOGRAPHY AND ACCESS

Crater Lake is located on the crest of the Cascade Range of 

southern Oregon about 90 km north of the city of Klamath Falls 

and about 100 km northeast of Medford. Crater Lake National 

Park is accessible on the east from U.S. Route 97 via Oregon 

Highways 62 and 138, on the southwest via Highway 62 from the 

Rogue River valley and Medford, and on the northwest from the 

Umpqua River drainage and Roseburg via Highway 138. Prior to 

its climactic eruption and collapse of Crater Lake caldera, Mount 

Mazama had a summit elevation of about 3,700 m. The present 

high point is Mount Scott at 2,277 m, 3 km east of the caldera 

rim. The caldera rim ranges in elevation from approximately 2,040 

m to 2,484 m, with maximum relief of about 600 m above the 

surface of Crater Lake, virtually equal to maximum water depth. 

Vertical cliffs, steep bedrock slopes, and talus piles define the caldera walls. In contrast, surviving flanks of Mount Mazama consist 

of lava that slopes gently away from the caldera rim, incised by 

deep glacial valleys partially filled with pyroclastic-flow deposits of the climactic eruption. Only postcaldera volcanic features 

and the Llao Rock and Cleetwood lava flows are too young to 

have been modified by glaciation. All but the steepest slopes are 

discontinuously mantled with sandy to pumiceous deposits of the 

climactic eruption. At higher elevations the sandy soils commonly 

form sparsely vegetated meadows, but elsewhere these deposits, 

bedrock outcrops, and areas of glacial till support conifer forests 

that are particularly dense at lower elevations in the west.

Much of the map area is within Crater Lake National Park, is 

classified as wilderness, and is administered by the National Park 

Service. Within the park, vehicles are allowed only on the Rim 

Drive, north and south entrance roads, Pinnacles road, Grayback 

Ridge road, and Oregon Highway 62. Several trails, mostly 

unpaved former roads, traverse parts of the backcountry within the 

park. Adjacent lands are within Rogue River and Winema National 

Forests where a network of unpaved logging roads provides access 

virtually to the park boundary. Park facilities are located on the 

south caldera rim at Rim Village, at the head of Munson Valley, 

and near Annie Spring. Nearby communities outside of the map 

area are resorts at Union Creek on the southwest and Diamond 

Lake on the north and the village of Fort Klamath on the south.

Sites of particular geologic interest are located and described 

in field guides (Bacon, 1987; Muffler and others, 1989). Significant 

features also are noted in the following text and in the Description 

of Map Units.

METHODS

The primary goal of mapping the geology of Mount Mazama 

and Crater Lake caldera has been to document the spatial distribution, volume, and composition of eruptive products through time 

in order to serve as a framework for investigations of volcanic and 

magmatic processes and to evaluate natural hazards and geothermal resource potential. Additional goals have been to enhance the 

experiences of visitors to Crater Lake National Park and to provide 

outcrop-scale information on earth materials to land managers 

and researchers in fields beyond volcanology. Geologic mapping 

took place in the summers of 1979–88, beginning with the caldera 

walls and supplemented by brief field seasons in subsequent years. 

The boundaries of the rectangular map do not coincide with park 

boundaries but were defined by the extent of Mount Mazama lavas 

and pre-Mazama rhyodacites, including as much terrain as could 

be depicted at a scale of 1:24,000 on a single sheet. In the same 
2

Figure 1. Shaded-relief map of map area showing Mount Mazama and Crater Lake. Extent of lavas of Mount Mazama indicated 

defined, by dotted white line where concealed. Topography from U.S. Geological Survey 10-m DEM; bathymetry from Gardner and 
3

Figure 2. Shaded-relief map of Crater Lake region showing Quaternary faults (heavy black lines) and maximum extent of pyroclastic-flow 

deposits (heavy white lines) of the ~7,700 yr B.P. climactic eruption of Mount Mazama (modified after Bacon and others, 1997).
4

project, several volcanoes within Crater Lake National Park and 

north of the map boundary were sampled, and the extent (fig. 2) 

and character of pyroclastic-flow deposits of the climactic eruption 

were determined. 

Geologic units were distinguished in the field by hand-lens 

examination of lava samples and by the physical characteristics 

and composition of fragmental deposits. Assignments to map 

units were further refined using chemical and petrographic data. 

Compositions of lava flows and clasts in fragmental deposits were 

determined by chemical analysis in U.S. Geological Survey laboratories. SiO2

 contents given in unit descriptions are from X-ray 

fluorescence analyses recalculated to sum to 100 percent volatilefree and rounded to the nearest 0.5 wt percent. Rock names are 

based on silica content: basalt, ≤52 wt percent; basaltic andesite, 

52–57; andesite, 57–63; dacite, 63–68; and rhyodacite, 68–72. 

Phenocryst contents in unit descriptions are visual estimates from 

low-magnification microscopic views of thin sections. K-Ar and 

40Ar/39Ar dates of volcanic rocks from the laboratory of M.A. 

Lanphere (Bacon and Lanphere, 2006) are critical to establishing 

a stratigraphic framework and quantitative eruptive history. Also 

of great utility have been determinations of paleomagnetic pole 

positions for eruptive units by D.E. Champion. Published 14C dates 

of organic materials constrain the ages of some late Pleistocene 

and Holocene pyroclastic deposits. The Holocene-Pleistocene 

boundary is 10 ka; late Pleistocene-middle Pleistocene boundary, 

128±1 ka (Stirling and others, 1998); and middle Pleistocene-early 

Pleistocene boundary, 775±10 ka (Bassinot and others, 1994).

Geologic mapping of the flanks of Mount Mazama and 

the surrounding area was recorded in the field on aerial photographs. Unit contacts and generalized caldera wall geology were 

transferred to a stable topographic base using a Kern PG-2 stereoplotter, scanned, and edited with ArcInfo GIS software using 

the ALACARTE interface (Fitzgibbon and Wentworth, 1991; 

Wentworth and Fitzgibbon, 1991). The caldera floor was mapped 

in ALACARTE/ArcInfo on a base created from the digital elevation model (DEM) obtained in the 2000 multibeam echo-sounding 

bathymetric survey (Gardner and others, 2001; Bacon and others, 

2002). The combined data (accompanying CD-ROM) form the 

map on sheet 1 and a Bedrock Map on sheet 3; a List of Map 

Units and a Correlation of Map Units are on sheet 2. The geology of the caldera walls was mapped on photographs taken from 

Crater Lake, Wizard Island, and helicopter. This information was 

transferred to a series of panoramic views (sheet 4) made from 

photomosaics and combined with topographic contours determined from a DEM.

Map units represent products of an episode of eruptions from 

a source vent or, uncommonly, groups of coeval vents (for example, units bx, dc). Such episodes may have lasted as little as a 

few months or as much as a few tens of thousands of years in the 

case of some of the more extensive, older units. Map units typically are restricted in composition but some are more complex and 

have a range of compositions (such as andesite of Applegate Peak, 

aa). The principal features of a volcanic map unit are stratigraphic 

position and compositional kinship; phenocryst mode, size distribution, and texture must be consistent with the other criteria, as 

well as the mineralogy and composition of microxenoliths (typically bits of holocrystalline gabbro or diorite), aggregates (crystal mush with intergranular melt), or enclaves (undercooled blobs 

of magma) when they are present. Many phenocrysts in lavas 

are derived from microxenoliths or aggregates, and some microphenocrysts probably originated in enclaves (Nakada and others, 

1994). Some older units are less restrictively defined than younger 

ones because of alteration of rocks low on the south caldera walls 

and (or) widely separated outcrops (such as dacite of Chaski Bay, 

db). Individual lava flows within map units were mapped where 

possible; flow bases are shown on the caldera wall panoramas and 

flow margins or, locally, bases in steep-walled canyons are indicated on the map.

GEOLOGIC SETTING

Mount Mazama is one of the major volcanoes of the Cascades 

Arc, a chain of about 16 prominent volcanic cones and hundreds 

of smaller shields and cinder cones extending from northern 

California to southern British Columbia. The Cascades Arc is a 

manifestation of the relatively slow, northeastward subduction of 

the young, hot Juan de Fuca and Gorda plates beneath the North 

American plate. The volcanic arc is broad from northern California 

to southern Washington, where plate convergence is oblique to the 

continental margin, and is narrow in northern Washington and 

southern British Columbia, where convergence is normal. 

Mount Mazama is in the broad part of the arc where there 

are many smaller, or “regional,” volcanoes that are the surface 

expression of melts born in the mantle and escaping to the surface, 

after undergoing varying amounts of differentiation and assimilation, because of a mildly extensional tectonic environment at 

the west edge of the Basin and Range province. These regional 

volcanoes erupted primitive magmas ranging from high-alumina 

olivine tholeiite (HAOT; for example, basalt of Castle Point, bc) 

to magnesian basaltic andesite (such as basaltic andesite of Red 

Cone, br) that reflect increasing amounts of subduction-related 

fluids or melts added to a depleted mantle source (Bacon, 1990; 

Bacon and others, 1994, 1997a). The basaltic andesites and more 

differentiated lavas of Mount Mazama and Crater Lake caldera 

contain geochemical signatures of most of the range of primitive 

magmas from the regional volcanoes. Mazama erupted differentiated magmas because the site has long been a magmatic focus 

and, thus, relatively warm because of repeated intrusions in the 

crust. Extensive pre-Mazama dacite and rhyodacite lavas underlie 

the southern and eastern parts of the edifice (Nakada and others, 

1994), the youngest of these being compositionally similar to 

rhyodacite of the Holocene climactic eruption. Mount Mazama is 

(somewhat arbitrarily) defined as the andesite-dacite edifice built 

upon these >400 ka silicic lavas.

The underpinnings of Mount Mazama are known from exposures in deep canyons on its south flank, samples of the submerged 

caldera walls, and drillcore from two geothermal exploration wells 
5

(Bacon and Nathenson, 1996). Basaltic andesite and andesitic lava 

flows and shallow intrusions that represent pre-Mazama regional 

volcanism occur west of long 122°06’ (approximately at the center 

of the caldera). Near the bottom of the lake off Eagle Point, similar 

rocks are hydrothermally altered to greenschist-facies assemblages 

(Bacon and others, 2002); but in the 867-m-deep MZII-1 exploration 

well about 12 km south of Crater Lake (east of Annie Creek, south 

of the map boundary), they are comparatively fresh. Pliocene(?) 

and (or) early Pleistocene basaltic andesite lava flows form a dissected tableland west of Crater Lake National Park. Beneath preMazama rhyodacite in the MZI-11A exploration well about 7 km 

southeast of the caldera, there is nearly 1 km of andesitic fragmental rocks and basaltic andesitic lava flows underlain by another 0.4 

km of greenish-gray intermediate to silicic tuffs with apparent dips 

of up to 36° (Bacon and Nathenson, 1996). The tuffs may correlate 

with Tertiary rocks of the Western Cascades exposed west of the 

Rogue River. The uppermost crust under Mount Mazama, then, 

consists of basaltic andesite and local andesite, probably underlain 

by silicic tuffs, all variably altered and metamorphosed by heat 

and fluid circulation related to intrusion of younger magmas of the 

Mazama focus. Among these intrusions at a depth of about 5 km 

is a late Pleistocene granodiorite porphyry sill or pluton at least 

as extensive as the subsided floor of Crater Lake caldera (Bacon 

and others, 2000; Bacon and Lowenstern, 2005). The nature of 

the deeper crust is uncertain. Isotopic data for Mazama area lavas 

are consistent with pre-Cenozoic Klamath-type basement rocks 

but also could be reconciled with younger mafic rocks intruded in 

response to extension (Bacon and others, 1994). Leaver and others 

(1984) reported a crustal thickness of 44 km about 30 km west of 

Crater Lake. Crustal thickness directly beneath Mazama, however, 

is not well known.

Active, generally north-south normal faults traverse the Crater 

Lake region (Bacon and others, 1997b, 1999; fig. 2), which is in 

the part of the Cascades characterized by oblique plate convergence and at the seismically active western margin of the Basin 

and Range province. The most prominent fault within the map area 

is the Annie Spring fault (fig. 1), which is part of the west Klamath 

Lake fault zone that defines the western boundary of the Klamath 

graben south of Mount Mazama. This particular fault zone continues northward as the Red Cone Spring fault and extends towards 

Diamond Lake and Mount Bailey north of the map area. Related, 

parallel faults west of Union Peak and north of Sphagnum Bog are 

probably connected by faults concealed beneath late Pleistocene 

lavas and climactic ignimbrite west of Mazama. Less prominent 

north-south- or northeast-southwest-trending faults are present 

southeast of Mount Mazama. All mapped tectonic faults, except 

those of the east Klamath Lake fault zone, have substantial downto-the-east normal displacement and apparently little strike-slip 

motion. Vent alignments, particularly of coeval vents, also tend 

to trend north-south, reflecting the regional maximum horizontal 

compressive stress (east-west-opening fractures) away from the 

immediate vicinity of the Mazama focus. Other faults and fractures, mapped on and near the south caldera wall between Castle 

Crest and Sun Notch, are boundaries of blocks that have moved 

down and into the caldera but have not failed completely to form 

debris avalanches (Bacon and others, 2002).

ERUPTIVE HISTORY

The geologic map and panoramas depict stratigraphic 

relations and constrain volumes of map units fundamental to 

establishing the eruptive history of Mount Mazama. Including 

chemical analyses and geochronologic data yields quantitative 

time-volume-composition information (Bacon and Lanphere, 

2006). The map delineates 111 eruptive units, excluding climactic ejecta; several of the units contain multiple facies. Five of the 

eruptive units are basalt, 32 are basaltic andesite, 46 are andesite, 19 are dacite, and 9 are rhyodacite. Many of these units are 

products of single, short-lived eruptions. Others, such as the dacite 

of Pumice Castle (dc), are coherent packages of lava flows, commonly associated with comagmatic pyroclastic deposits (dcp) 

that were erupted from one or more related vents over a period 

as brief as a few months(?) to as long as thousands of years (for 

example, dacite of Mount Scott, ds). Some units have constituent 

lavas that are outside the compositional range implied by the unit 

name, which reflects the dominant composition (such as andesite 

of Applegate Peak, aa).

More than half of the map units, including multiple samples 

from many, have been dated in the laboratory of M.A. Lanphere 

by K-Ar and (or) 40Ar/39Ar methods (Bacon and Lanphere, 2006). 

Ages of a few of the late Pleistocene and several Holocene units, 

including the climactic ejecta, are constrained by radiocarbon 

dating of associated carbonaceous material. A few correlations of 

undated units with dated units were made by paleomagnetic pole 

determinations (D.E. Champion, unpub. data, 2000). The geologic 

record is most complete for the recent past because older features 

are buried by younger lavas and pyroclastic deposits. Confidence 

in time-volume-composition information, thus, is greatest for the 

late Pleistocene and Holocene. Dated samples suggest regional 

volcanism has been active throughout at least the last 700,000 yr, 

but continuity of activity prior to approximately 200 ka is uncertain. Several regional volcanoes, some of them large shields, were 

active during the interval from about 200 to 100 ka. There was 

comparatively little regional volcanism between about 100 and 40 

ka, whereas during the subsequent growth of the climactic magma 

chamber, rather voluminous primitive basalt and magnesian basaltic andesite were erupted from several vents west of Mazama. 

Meanwhile, from about 420 ka to 40 ka, Mount Mazama itself 

produced andesite and dacite with few discernible gaps in activity 

but with notable pulses of high productivity (as at Mount Adams, 

Hildreth and Fierstein, 1995; Hildreth and Lanphere, 1994). The 

only rhyodacitic magmas were vented before construction of the 

Mazama edifice or after approximately 30 ka. Following leaks of 

various preclimactic rhyodacites, the climactic eruption vented 

much of the differentiated fraction of the shallow magma chamber, resetting the system at its focus approximately 7,700 calendar 

yr B.P. Postcaldera andesites were erupted within a few hundred 
6

years of caldera formation, followed by the sole postcaldera rhyodacite dome approximately 4,800 calendar yr B.P.

REGIONAL VOLCANISM

The Quaternary High Cascades at the latitude of Crater Lake 

are characterized by dominantly basaltic andesitic lava flows from 

isolated cinder cones and larger shield volcanoes (Bacon, 1990; 

Bacon and others, 1994; Bruggman and others, 1989). Although 

present, andesites and true basalts are less voluminous. These 

manifestations of regional volcanism apparently underlie the entire 

map area and continue both north and south, as well as interfinger 

with some distal Mazama lavas and overlie others. Regional volcanic units are divided into northwest, southwest, and east (of the 

longitude of Munson Valley) groups in the Description of Map 

Units. In the map area, exposures of ≥1 Ma rocks (for example, 

basalt north of Crater Creek, bck) are limited except in the northwest corner where a dissected upland, downdropped on the east 

along north-south normal faults, slopes gently toward the Rogue 

River west of the map area. A large, deeply eroded basaltic andesitic center (basaltic andesite of Union Creek, buc) thought to be ≥1 

Ma in age includes Rocktop Butte in the southwest corner of the 

map (buci). Successively younger, large shield volcanoes that are 

decreasingly eroded are Union Peak (basaltic andesite of Union 

Peak, bu), Timber Crater (immediately north of map area; fig. 2), 

and Scoria Cone (basaltic andesite of Scoria Cone, bsc; fig. 2). 

The largest shield volcano, Union Peak, has a 6- by 8-km base. 

Several regional volcanoes show effects of eruption during 

glacial times. These range in scale from local chilling against ice 

to form columnar joints (andesite of Arant Point, at) through chilling and fragmentation in intraglacial lakes producing hyaloclastite 

breccias (basaltic andesite of Whitehorse Creek, pyroclastic, bwcp; 

andesite south of Bear Bluff, tuff breccia, abt) to the gross morphology of volcanic constructs bounded by a regional ice cap. Several tuyas, or table mountains, are recognized (basaltic andesite of 

Whitehorse Bluff, bx; andesite of Arant Point, at; andesite south of 

Bear Bluff, ab). Glacial erosion modified nearly all regional lavas 

and their source vents, commonly exposing the intrusive cores of 

the vents. Exceptions are Pleistocene cinder cones east of Mount 

Mazama and the Holocene vents near Castle Point (bc, bcp).

Many vents that produced regional lavas form north-south 

alignments of similar age and composition. These vents are the surface expression of dikes that propagated parallel to the maximum 

horizontal compressive stress. North-south-elongated intrusions 

are exposed at Rocktop Butte (buci), south of Highway 62 east of 

Whitehorse Creek (bwci), and at Arant Point (ati). Prominent vent 

alignments are delineated by Red Cone and a fissure system about 

1.5 km to its north (units br, brw), several vents in a 6-km-long 

array west of Pumice Flat (bx), and cinder cones and fissure vents 

of the Scoria Cone group (bsc; Scoria Cone itself is immediately 

south of the map area). 

Over 40 source vents for regional lavas were identified within 

the map area. Notable among these are Crater Peak, Red Cone, 

Williams Crater, and Castle Point. Lava flows (andesite of Crater 

Peak, acr) from Crater Peak, an andesitic cinder cone south of 

Mount Mazama, cascaded into the canyons of Annie and Sun 

Creeks about 100 ka. Crater Peak lavas resemble some andesites 

of Mount Mazama, and this cone may be more properly termed 

parasitic than regional. Red Cone (br, brp) is noted for its primitive magnesian basaltic andesitic composition that is the closest representative of mantle-derived magma, highly enriched in 

incompatible trace elements associated with subduction-related 

magmatism (Bacon and others, 1994, 1997a). Williams Crater 

(basaltic andesite of Williams Crater, pyroclastic, bwp), named 

for Howel Williams, produced basaltic andesite contaminated 

with gabbro (bw) and mingled hybrids (mingled lava of Williams 

Crater, mw) of the basaltic andesite and genetically unrelated 

dacite (Bacon, 1990). An apparently coeval vent (basaltic andesite 

northwest of Williams Crater, pyroclastic, bwnp) erupted similar, 

but not identical, basaltic andesite (bwn) 2.6 km north of Williams 

Crater. The youngest regional volcano consists of three early 

post-glacial vents astride Castle Point and associated HAOT lava 

flows (bc). Some of these lavas are among the most primitive tholeiites in the Crater Lake region (Bacon and others, 1994). 

PRE-MAZAMA SILICIC ROCKS

Steep-sided domes and thick lava flows of rhyodacite and 

dacite underlie the edifice of Mount Mazama to its south and east. 

Nakada and others (1994) referred to the youngest three units 

of these silicic rocks as pre-Mazama rhyodacites (rhyodacite of 

Scott Creek, rsc; rhyodacite south of Crater Peak, rcs; rhyodacite of Pothole Butte, rpb). The pre-Mazama rhyodacites erupted 

between 500 and 400 ka, probably in a few short-lived episodes. 

Pre-Mazama rhyodacite lava flows and domes are exposed beyond 

the limits of andesitic and dacitic lavas of Mount Mazama from 

the East Fork of Annie Creek, counterclockwise to the vicinity of 

Bear Butte about 3.5 km northeast of the caldera rim. A rhyodacite 

sample recovered by manned submersible off Palisade Point may 

be from a pre-Mazama flow. Pre-Mazama rhyodacite lava flows 

are well exposed in the walls of the glacial valleys of Sun and 

Annie Creeks. Elsewhere glaciation has removed nearly all of the 

original pumiceous carapace from pre-Mazama rhyodacite lava 

flows. Domes north of Scott Creek, in the lee of Mount Scott, are 

the best preserved. Although commonly mantled by a thick blanket of Holocene pumice-fall deposits (climactic, Plinian and other 

Holocene pumice falls, cp) and (or) ignimbrite veneer (climactic, 

fine-grained lithic- and crystal-rich ignimbrite, cu), virtually every 

hill of pre-Mazama rhyodacite is exposed on the caldera-facing 

part of its summit, because pyroclastic flows of the climactic eruption removed the pumice-fall deposits and left insufficient veneer 

or lithic breccia to completely obscure the bedrock. 

Three map units of pre-Mazama rhyodacite (rsc, rcs, rpb) are 

defined on the basis of chemical composition and phenocryst content (Nakada and others, 1994; Bruggman and others, 1993). These 

units are about 460–410 ka in age, although sparse wholerock 
7

K-Ar dates are inconsistent with apparent stratigraphic relations. 

Paleomagnetic pole determinations by D.E. Champion (Nakada 

and others, 1994, fig. 5) at widely separated localities support the 

distinction between units rpb and rsc and suggest that each unit 

erupted over a short interval of time. The pre-Mazama rhyodacite 

units comprise as many as 40 lava flows and domes amounting to 

at least 20 km3. Vents for many of the flows are marked by domes, 

but some flows lack known vents because of partial burial by 

younger lavas. Related pyroclastic deposits have been identified 

in three settings: (1) the only known exposure of a fall deposit is 

a 1+ m thickness beneath the rpb basal vitrophyre where it lies on 

unit dw (dacite west of The Pinnacles) south of Maklaks Pass; (2) 

the MZI-11A geothermal exploration well about 4 km southeast 

of Mount Scott penetrated a total of 64 m of units cu and cp and 

the andesite of Scott Creek (asc) before passing through 123 m of 

rhyodacitic lava and 17 m of rhyodacitic(?) pyroclastics underlain 

by 256 m of hornblende dacite lava flows and breccias (Bacon and 

Nathenson, 1996) and a variety of volcanic rocks to a depth of 

1,423 m; and (3) tephra layers present above the Rockland ash bed 

in sediment cores, obtained from Buck Lake and Wocus Marsh 

about 75 km south of Crater Lake and from Mohawk Lake in the 

northern Sierra Nevada, are compositionally similar to glass from 

units rsc and rcs (A.M. Sarna-Wojcicki, written commun., 2002). 

At present, there is little evidence for voluminous pre-Mazama 

rhyodacitic pyroclastic deposits or a related buried caldera. 

Domes and lava flows of four other pre-Mazama silicic units 

occur southeast of Mount Mazama. The rhyodacite west of Cavern 

Creek (rcc) is older (724±5 ka) than the three units Nakada and 

others (1994) grouped as pre-Mazama rhyodacite (rsc, rcs, rpb), 

is chemically more evolved, and has a lower phenocryst content. 

Altered rhyodacite near Anderson Bluffs is now believed to belong 

to unit rcc. Rich in andesitic enclaves, the dacite west of The 

Pinnacles (dw) crops out on either side of Grayback Ridge, is dated 

at 612±8 ka, and is overlain by unit rpb. Although similar in field 

appearance to pre-Mazama rhyodacites, the dacite of Sand Creek 

(dsc) is chemically distinct and much older (1,058±16 ka). The 

dacite of Dry Butte (dd) has conspicuous hornblende phenocrysts. 

At 1,275±14 ka, unit dd is the oldest silicic unit in the map area.

MOUNT MAZAMA

Mount Mazama formerly rose to an elevation of 3,700 m as 

one of the major volcanoes of the Cascades before its collapse 

formed Crater Lake caldera (Williams, 1942). The remnants of 

Mount Mazama include the peaks outlining the caldera and the terrain sloping outward from them. Composed almost entirely of lava 

flows, the bulk of Mazama is andesite and low-silica dacite, while 

dacite and basaltic andesite are subordinate. The erupted magmas 

had a range of parents characterized by varying isotopic compositions and concentrations of elements incompatible in mantle 

sources (Bacon and others, 1994). Like other large, long-lived volcanoes, Mazama is made up of a complex of overlapping shields 

and stratovolcanoes, each of which probably was active for a comparatively brief interval. The character of the foundered peak can be 

inferred from exposures in the caldera walls and on the preserved 

flanks, although some cryptic units may not have left a tangible 

record. Lavas and volcanic deposits of the contiguous edifice that 

are younger than the pre-Mazama rhyodacites (rsc, rcs, rpb) are 

assigned to Mount Mazama, although older andesite and dacite are 

known from limited manned submersible traverses of the drowned 

caldera walls and from the exploration well MZI-11A east of the 

park. Clearly, a volcanic center has been producing intermediate to 

silicic magmas in this location for a long time. From the accessible 

part of Mazama, ages and stratigraphic relations indicate that the 

focus of activity migrated from east to west. By projecting flank 

slopes upward and from the directions of glacier motion recorded 

by scratches on outcrops, the summit of Mount Mazama in the 

latest Pleistocene and early Holocene is shown to have been above 

the south-central part of Crater Lake (Atwood, 1935). The caldera 

is thus eccentric to the Mazama edifice. Lavas assigned to Mount 

Mazama are exposed in a crudely elliptical pattern with the major 

axis oriented east-west. Most flows end within about 5 km of the 

caldera rim but a few can be traced as far as 11 km. 

The superb exposures in the caldera walls and cliffs of glacial 

valleys provide exceptional opportunities to interpret the origins of 

textures and structures that are useful tools for understanding eruptive processes and for mapping flank exposures. The multitude of 

cross-sectional and longitudinal views of lava flows and pyroclastic deposits reveals that most of Mount Mazama is composed 

of lavas that were fed from low fountains. Individual flow units 

can be traced from proximal agglutinated bomb deposits through 

streaky lava to distal homogeneous, commonly flow banded lava 

(for example, aa in Sun Notch and valley walls and ds on Mount 

Scott). Other examples are thin sheets of mafic andesite that may 

have great lateral extent (andesite of Cloudcap Bay, ac; andesite of 

Llao Bay, lower unit, all). All lavas of Mount Mazama initially had 

rubbly tops that may amount to as much as half of the total thickness of an eruptive unit. In the south caldera walls where hydrothermal alteration is most intense, these rubbly tops commonly 

appear yellowish from a distance, because their high primary permeability allowed relatively intense alteration to clays and minor 

pyrite, oxidation of the latter coloring the flow-top breccia. Bases 

of lava flows may be strikingly columnar jointed and vitric in their 

lower parts, especially where lava flowed over damp soil or till (for 

example, dacite of Sentinel Rock, dr, above Danger Bay). A typical lava flow has a zone of subhorizontal platy joints above basal 

columns or breccia (for example, andesite of the boat landing, abl, 

on the trail above the boat landing). Above the plates may be a 

relatively massive core surmounted by more platy joints and, at the 

top, blocky-jointed lava grading up into flow-top breccia or rubble. 

Platy joints may wrap around the interior of a flow onion-like, so 

that the plates are steep at the margins. Poles to plates tend to 

point toward the interior or source of a flow, a characteristic useful 

in mapping isolated exposures on the forested slopes of Mount 

Mazama. Lava of units erupted when ice was present displays icecontact and water-chilling features, such as closely spaced vitric 

columns that are 10–20 cm in cross section (polygonal joints; 
8

Lescinsky and Fink, 2000), often in unusually great thicknesses, 

and that may grade up into vitric breccia (for example, unit aa on 

Dutton and Grayback Ridges and at Wineglass; andesite of Llao 

Bay, upper unit, alu, near Pumice Point). Unlike many large composite volcanoes, Mount Mazama did not have an abundance of 

pyroclastic flowage deposits or an apron of lahars about its base 

prior to the climactic eruption. The near absence of volcanic fragmental deposits probably is a result of late Pleistocene flushing of 

drainages on the south and west slopes and burial elsewhere by 

ignimbrite of the climactic eruption. Extensive pyroclastic-flow 

or volcanic avalanche deposits, such as those formed by collapse 

of lava domes or steep-fronted flows, are represented in only two 

eruptive episodes (dr at Cloudcap Bay; dacite of Munson Valley, 

dv and dvb). The only mappable pumiceous pyroclastic deposits 

that are part of Mount Mazama itself are about 70–50 ka dacite 

(pyroclastic facies of the dacite of Pumice Castle, dcp; dacite 

below Llao Rock, dlp; dacite of The Watchman, dwp). Many dikes 

are visible in the caldera walls. Several can be traced upward into 

lava flows as indicated on the panoramas (sheet 4) or are correlative 

with lava on the flanks of Mount Mazama. The dikes commonly 

have glassy selvages with horizontal columnar joints. Irregular 

intrusions are present below Hillman and Applegate Peaks and in 

Phantom Cone (named by Williams, 1942). 

The eruptive history of Mount Mazama, prior to the climactic eruption, spans 400,000 yr. The following summarizes major 

units. A more detailed account appears in Bacon and Lanphere 

(2006). The oldest lavas assigned to Mount Mazama are the 

approximately 420–400 ka andesite of phantom cone (apn) and 

dacite of Mount Scott (ds). The andesite of phantom cone accumulated near its source vent in the wall southeast of Phantom Ship 

and is capped by a distinctive quartz-bearing lava flow (dacite of 

phantom cone, dpn) that also is found in the west wall of upper 

Kerr Valley. Mount Scott is a great pile of low-silica dacitic agglutinate locally rich in andesitic enclaves; lava from Mount Scott 

and satellite vents overlies pre-Mazama rhyodacite. Older intermediate-composition rocks from the Mazama focus may be present below lake level or in the subsurface. The dacite of Chaski Bay 

(db; approximately 380–350 ka), typically containing andesitic 

enclaves, forms the lower part of the south caldera wall and crops 

out at the edges of Kerr and Munson Valleys. Extensive, comparatively thin flows of the andesite of Kerr Notch (ak; approximately 

340–310 ka) also floor Sun Notch and are found from Grotto 

Cove to below Applegate Peak as far as 5 km from the caldera rim 

in Kerr Valley and Sun Creek valley (a single thick flow in Sun 

Creek valley south of the Grayback Ridge road). Fed by a dike, the 

dacite of Sentinel Rock (dr, about 300–340 ka) is exposed widely 

in the caldera wall, fills a glacial valley cut in unit ak south of 

Sentinel Rock, and lies on glaciated unit ak near Skell Head. The 

andesite below Rim Village (arv) and andesite west of Fumarole 

Bay (af), both recognized only in the southwest caldera wall, had 

sources northwest of Garfield Peak about 300 and 275 ka, respectively. South of the present caldera rim, a thick lava flow or set of 

domes known as the dacite of Munson Ridge (dm) apparently was 

emplaced between glaciers occupying the drainages of presentday Dutton and Munson Creeks about 275 ka. The relatively mafic 

andesite of Cloudcap Bay (ac), approximately coeval with unit 

dm, forms many thin but extensive agglutinate sheets near its 

source vent above its namesake where a vertical feeder dike (ac) is 

visible in the caldera wall. Unit ac transforms northward to fewer, 

thicker lava flows in exposures above Grotto Cove. 

Like other large Cascade volcanoes (such as Mount Adams, 

Hildreth and Lanphere, 1994), Mount Mazama had episodes of 

rapid cone building. The most obvious of these produced the 

andesite of Applegate Peak (aa), a compositionally diverse pile 

of fountain-fed lava flows erupted from the summit vent between 

about 270 and 210 ka (mainly about 250–230 ka). Lava of unit 

aa forms roughly the upper half of the caldera wall between Rim 

Village and Kerr Notch, is present in the wall as far as Wineglass 

where there also is an apparent unit aa dike, and can be traced 5 

km south and 7 km east and west of the caldera rim. A dike below 

Sun Notch is aa and a larger intrusion below Applegate Peak is 

aai. Ice was present during much or all of this episode, shown by 

common ice-contact columnar (polygonal) joints and breccias in 

unit aa, such as at Wineglass and high on Dutton Ridge. A distinctive but kindred dacite flow, erupted from a satellite vent and 

mapped separately as the dacite south of Garfield Peak (dg), is 

present 5 km from its source where it is displaced by the Annie 

Spring fault near Arant Point. Emplacement of unit aa flows 

southwest of the summit ended by about 225 ka when the hornblende-phyric andesite of Garfield Peak (ag) covered part of the 

south flank of Mazama.

A different type of eruption took place about 215 ka, near the 

close of Applegate and Garfield Peak activity. A flank vent 2 km 

west of Rim Village produced uniquely Ti-rich dacitic lava that 

flowed due west, forming the present divide between Bybee Creek 

and Castle Creek drainages, for at least 10 km before it descended 

into both canyons where it is covered by ignimbrite. This dacite 

north of Castle Creek (dcn) originated in the Mazama magmatic 

system, because dikes of identical composition (dcn) in the caldera wall northwest of Rim Village trend directly towards the vent. 

The magma apparently broke through to the surface when lateral 

feeder dikes intersected the Annie Spring fault, which now displaces vent agglutinate (dcnp).

Following about 40,000 yr for which there is no record of 

Mazama activity, andesitic magma issued from a flank vent south 

of Garfield Peak (andesite east of Munson Valley, amv) and, independently, different andesitic magma began to construct a large 

shield where the present Llao Rock is located. This shield volcano 

grew in two episodes (andesite of Llao Bay, all and alu) spanning 

the interval of about 170 to 120 ka; an unconformity separates the 

two packages of similar, fountain-fed sheets of relatively mafic 

andesite. At Pumice Point, the unconformity evidently represents 

an ice advance at the time of marine oxygen isotope stage (MIS) 

5d because thick unit alu lava, dated at 117±3 ka by 40Ar/39Ar, 

encountered ice and rests on a glaciated unit all flow dated by 

K-Ar at 122±20 ka. In contrast to the unit all shield, at about 160 

ka andesite of Roundtop (ar) formed a >130-m-thick lava flow at 

least 2.5 km long resting on about 115 m of till and glaciofluvial(?) 
9

sediment on the northeast flank of Mount Mazama. Although icecontact features have not been recognized in its few exposures, 

unit ar probably is an example of an ice-bounded lava flow as 

described from Mount Rainier by Lescinsky and Sisson (1998). 

Several andesite and dacite units, emplaced mainly as thick 

lava flows in the approximate interval 130–110 ka (dacite of Steel 

Bay, dsb; dacite of Palisade Point, dpt; andesite of the boat landing, abl; dacite east of Palisade Point, dpe; andesite west of Pumice 

Point, apw), are present in the north wall of the caldera but are not 

exposed on the low north flank of Mount Mazama. Connecting 

dikes show that at least some of these units (dsb, apw) erupted 

from flank vents. In the northwest wall, andesite of Merriam Point 

(am) forms a mound of thick flows at least 2.5 km wide and 250 m 

thick above its feeder neck. At the same time, an extensive pluton 

or sill of granodiorite porphyry, represented by clasts in deposits of 

the climactic eruption, solidified at a depth of about 5 km (Bacon 

and others, 2000; Bacon and Lowenstern, 2005). No evidence of 

eruptive activity is known from the ensuing approximately 100 

to 75 ka interval, which was followed by a pulse of vigorous and 

diverse volcanism. 

Beginning about 75 ka from a source west of the center of 

the present caldera, andesite of the west wall (aww) spread as 

relatively thin lava flows forming a pile about 300 m thick below 

Hillman Peak and extending from Steel Bay to below Discovery 

Point and at least 7 km west of the caldera rim in Bybee Creek. 

Probably in this period, olivine-phyric basaltic andesite of Steel 

Bay erupted from a vent near Steel Bay and deposited palagonitic 

tuff breccia (bsp) and thin lava flows (bs), now exposed in the caldera wall. By about 70 ka, Mazama volcanism had changed character dramatically, producing voluminous dacite of Pumice Castle 

as pyroclastic deposits (dcp) and lava flows (dc). Dikes (dc) and 

welded pumice-fall deposits (dcp) north of Pumice Castle mark 

a major vent area. Extensive lava flows (dc) issued from vents 

east of the caldera rim and flowed to either side of Mount Scott to 

form uppermost Anderson Bluffs and the extensive coulees east 

of Scott Bluffs. Similar dacite is present as lava at Steel Bay and 

above Merriam Point and in a probable dike sampled by manned 

submersible below Llao Rock. Immediately above unit dc lava 

at Steel Bay is a pumice-fall deposit (dlp) that is locally welded, 

indicating a nearby vent. The evidence for several vents for these 

dacitic units distributed over perhaps 8 km across the north side 

of Mount Mazama suggests the presence of a large, contiguous 

dacitic magma body. Andesite of Grotto Cove (agc) issued from 

a flank vent northeast of Mazama’s summit, forming two thick 

flows that traveled as far as 3 km northeast of the caldera rim. 

Joint patterns suggest that these flows banked against ice near the 

present caldera rim and, like unit ar, probably were ice-bounded 

lava flows. A dacitic pumice-fall deposit (dlp) is intercalated with 

andesite flows included in unit agc that overlie nonwelded pyroclastic-flow deposits of unit dcp near Cleetwood Cove. Between 

the dcp and dlp dacitic eruptions, hornblende-phyric basaltic 

andesite of Hillman Peak (bh) vented from a source exposed in the 

wall (bhi, bhp) below its namesake, notably west of the summit 

of Mount Mazama. Lava flows of unit bh spread west for at least 

5 km. These are partially buried by andesite of Hillman Peak (ah), 

which originated from a vent somewhat east of the unit bh vent. 

The 50±3 ka Watchman dacite flow (Williams, 1942; dacite of The 

Watchman, units dwv, dwf) is a prominent feature of the west rim 

of Crater Lake caldera. Its feeder dike (dwf) is obvious on the caldera wall. Pyroclastic-flow deposits (dwp) of an early phase of 

the eruption are present in the caldera wall to the south and in 

rare patches on the Castle-Bybee Creek divide about 4 km west 

of Rim Village. Signaling a return to andesite effusion, thick flows 

of andesite south of The Watchman lava (atw; 55±3 ka) overlie 

unit dwp at the caldera rim and also are present north of Devils 

Backbone, suggesting a source between Hillman Peak and the 

Mazama summit. A far more voluminous outpouring of andesite of 

Devils Backbone (ad) next took place from a vent or vents fed by 

the Devils Backbone dike system (ad), creating a flow field on the 

northwest flank of Mazama extending 11 km west and terminating 

in the drainage of Copeland Creek at an elevation of about 1,430 

m. Andesites of Lightning Spring (als), Pumice Point (apu), and 

Steel Bay (asb) are the youngest andesitic lavas known on Mount 

Mazama. Andesite of Lightning Spring (als) is widely exposed to 

about 5 km west of the caldera rim, and the other andesites crop 

out mainly in the northwest caldera wall. They probably originated 

at a vent or vents near or west of the summit of Mount Mazama 

about 50–40 ka. 

The youngest units assigned to the Mount Mazama edifice, 

both approximately 35 ka, are the dacite of Munson Valley (dv, 

dvb) and the dacitic component of the mingled lava of Williams 

Crater (mw). Units dv and dvb are avalanche deposits fed by 

a collapsing lava dome(s) or steep flow front(s) high on Mount 

Mazama. The source must have been immediately southwest of 

the summit, because these units are found only on the southwest 

flank and immediately south of Devils Backbone, which suggests avalanches flowed to either side of any remaining secondary western summit related to slightly earlier andesitic volcanism 

near Hillman Peak. Williams Crater is a basaltic andesitic cinder 

cone and lava flow complex just west of the caldera rim on the 

shoulder of Hillman Peak. Mingled dacite and hybrid-andesite 

lava also vented there and as a tiny dome adjacent to Rim Drive. 

Tephra from Williams Crater directly overlies unit dvb north of 

Devils Backbone, and compositional affinities of the dacites and 

their enclaves indicate that unit mw is only slightly younger than 

dvb (Bacon, 1990). Following eruption of these dacites, there is 

no record preserved of near-summit activity at Mount Mazama. 

Subsequent volcanism has a different compositional and eruptive 

character.

PRECLIMACTIC RHYODACITES

The caldera-forming eruption of Mount Mazama was preceded by emplacement of several preclimactic rhyodacitic lava 

domes and flows, at least three of which overlie related pyroclastic deposits (Bacon, 1983; Bacon and Druitt, 1988). The preclimactic rhyodacites are divided into four map units (re, rbb, rs, 
10

rh). The oldest, evolved Pleistocene rhyodacite (re, rec, rev) 

includes the Grouse Hill and Redcloud flows (Williams, 1942) 

and a small dome above Steel Bay (Bacon and Lowenstern, 2005). 

These hornblende rhyodacites were emplaced north and northeast 

of the summit of Mount Mazama during the last glaciation about 

30–27 ka. A related pumice-fall deposit (rep) is locally preserved 

under the Redcloud flow and has been correlated with the Trego 

Hot Springs Bed of Davis (1978) by Rieck and others (1992). 

Emplaced during the glacial maximum, a small dome informally 

named rhyodacite of Bear Bluff (rbb; 24±3 ka 40Ar/39Ar isochron 

age) was extruded where a dike, apparently from the Mazama climactic magma chamber, evidently intersected the Annie Spring 

fault about 8 km south-southwest of Garfield Peak. Coeval andesite 

south of Bear Bluff (ab) is compositionally similar to enclaves in 

unit re flows, suggesting that it, too, was derived from the Mazama 

system. The unit rbb composition can be explained by admixture 

of a small amount of ab magma to re rhyodacitic magma. Lightly 

glaciated domes about 4–7 km northeast of the caldera, mapped 

as rhyodacite of Sharp Peak (rs; 18±4 ka 40Ar/39Ar isochron age), 

were fed by a dike system emanating from the climactic chamber 

(Bacon and Druitt, 1988). 

The rhyodacitic Llao Rock and Cleetwood flows (Williams, 

1942) and associated dikes and pumice-fall deposits are Holocene 

preclimactic rhyodacite (rh, rhc, rhv, rhp). The Llao Rock products 

are older and more differentiated, having preceded the climactic 

eruption by about 200–100 yr. Llao Rock pumice-fall deposits are 

widespread southeast of Mazama and correlate with the Tsoyowata 

Bed of Davis (1978) in Nevada and eastern California (Rieck and 

others, 1992). A pumice-fall deposit from the Cleetwood vent 

underlies the Cleetwood flow. Identical in composition to climactic rhyodacite, the Cleetwood flow was still hot when the caldera 

collapsed and remobilized lava oozed down the caldera wall and 

also flowed northeast for at least an additional kilometer beyond 

its original terminus. Field relations between Cleetwood lava and 

various deposits of the climactic eruption demonstrate the precursory nature of the Cleetwood eruption and the timing of caldera 

collapse (Bacon, 1983). Detailed information about the Llao Rock 

and Cleetwood pumice-fall deposits near Crater Lake can be found 

in the thesis by Young (1990).

The preclimactic rhyodacites are interpreted as magma that 

leaked from the top of the climactic chamber as it grew. Enclaves 

in these lavas contain the chemical fingerprints of different varieties of new andesitic magma injected into the differentiating 

chamber (Bacon and Druitt, 1988; Bacon and others, 1994). These 

andesitic replenishments were parental to the rhyodacites and to 

complementary phenocryst-rich andesitic scoria and gabbroic 

crystal mush ejected late in the climactic eruption.

THE CLIMACTIC ERUPTION

The climactic eruption of Mount Mazama devastated the terrain for tens of kilometers from the volcano, sent pyroclastic flows 

over the slopes of Mazama and into every drainage to travel as much 

as 70 km from their source (fig. 2), and produced ash fall throughout 

much of the Pacific Northwest. At least 90 percent of the approximately 50 km3

 of magma erupted was uniform rhyodacitic pumice 

(70.5% SiO2). The remainder was crystal-rich andesitic scoria and 

mafic crystal mush (61–47% SiO2). The eruption took place in two 

phases (Bacon, 1983): (1) a single-vent phase in which a towering 

Plinian column rising from a vent northeast of the summit released 

pumice and ash at high altitude to be carried by winds, resulting in 

the widespread fall deposit (Young, 1990; included in unit cp, climactic Plinian and other Holocene pumice-fall deposits); vent widening and increasing eruption rate eventually caused the column 

to collapse to lower height, producing the pyroclastic flows that 

deposited the Wineglass Welded Tuff of Williams, 1942 (unit cw) 

in valleys on the north and east flanks of Mazama (Kamata and 

others, 1993) and (2) a ring-vent phase, which began at the onset 

of caldera collapse and produced energetic pyroclastic flows, fed 

by columns rising from a number of vents that circumscribed the 

foundering cauldron block and descending radially about Mount 

Mazama resulting in a compositionally-zoned deposit up to about 

100 m thick (units cf, cb, and cu, ring-vent-phase ignimbrite, lithic 

breccia, and fine-grained lithic- and crystal-rich ignimbrite, respectively; Druitt and Bacon, 1986; Suzuki-Kamata and others, 1993). 

The five units of the climactic eruption are mapped where they are 

exposed or can be inferred to be just below the ground surface. The 

petrology and geochemistry of the climactic ejecta and preclimactic 

rhyodacites are described in Bacon and Druitt (1988), Bacon and 

others (1992), Druitt and Bacon (1988, 1989), and Bruggman and 

others (1987). Throughout these deposits, but especially common 

in unit cb and at the top of cf, are variably fused granodiorite and 

related accidental lithic blocks (Bacon and others, 1989, 1994; 

Bacon, 1992) that were derived from an approximately 110 ka 

intrusion (Bacon and others, 2000; Bacon and Lowenstern, 2005) 

forming the walls of the climactic magma chamber at about 5 km 

depth (Bacon and others, 1992). 

Radiocarbon ages of charcoal associated with deposits of the 

climactic eruption (Bacon, 1983) have a weighted mean value of 

6,845±50 14C yr B.P., or a calendar age of approximately 7,700 

calendar yr B.P. (Stuiver and others, 1998). More recent work 

by Hallet and others (1997) suggests a somewhat younger age of 

6,730±40 14C yr B.P., or 7,470–7,620 calendar yr B.P. The climactic eruption probably lasted at most a few days. Although there 

is no written record of its constituent events, this eruption nevertheless has been of fundamental importance to volcanologists in 

understanding large explosive eruptions, compositional zonation 

in magma chambers, and collapse calderas (for example, Williams, 

1941, 1942). The Crater Lake region is a magnificent laboratory for 

the study of these natural phenomena because of its ease of access, 

completeness of geologic record, and excellent preservation of the 

deposits of the climactic eruption. The zoned ignimbrite in the valleys south and west of the caldera is world famous in volcanology. 

Exposures along and near Rim Drive at Cleetwood Cove may be 

unique in their documentation of the timing of caldera collapse 

during the various phases of a major explosive eruption. Finally, 

the fact that the rhyodacitic Cleetwood lava flow was still hot at 
11

the onset of the climactic eruption alerted volcanologists to the 

possibility of a catastrophic event following close on the heels of a 

smaller eruption from the same magma source.

POSTCALDERA VOLCANISM

Volcanic activity in the Crater Lake region since the climactic 

eruption of Mount Mazama, “postcaldera volcanism,” has been 

confined within the caldera. Most of the volcanic products are 

hidden from view beneath Crater Lake—Wizard Island amounts 

to but 2 percent of the total volume of about 4 km3 of postcaldera 

andesite. Knowledge of the postcaldera eruptive history was gained 

through acoustic mapping of the lake floor (Gardner and others, 

2001; Bacon and others, 2002) supplemented by dredged samples 

and by observation and sampling with a manned submersible 

(Nelson and others, 1994). A major conclusion is that all postcaldera andesitic volcanism took place within a few hundred years of 

caldera collapse, while Crater Lake was filling to nearly its present 

level. Products of four andesitic vents are recognized: andesite of 

the east basin (ae), andesite of the central platform (apc, apcb), 

andesite of Merriam Cone (amc, amcb), and andesite of Wizard 

Island (aw, awb, awp). Parts of the last three either vented under 

water or flowed from then-subaerial vents into the rising lake, 

leaving a record in foundered shorelines of lava deltas (Bacon and 

others, 2002; Nathenson and others, 2007). No tephras from these 

eruptions have yet been recognized on Mount Mazama. The final 

known postcaldera volcanism resulted in a rhyodacitic ash bed, 

recovered in a core taken in the lake floor, and extrusion of a subaqueous dome (r, rb) on the northeast flank of Wizard Island about 

4,800 calendar yr B.P. (Nelson and others, 1994). 

SUBMERGED CALDERA 

WALLS AND FLOOR

The multibeam echo-sounding survey of Crater Lake conducted in 2000 (Gardner and others, 2001) revealed the locations 

of bedrock outcrops of the submerged caldera walls, the surface 

morphology of debris slopes, and the boundaries of flat-floored 

basins containing ponded sediment (Bacon and others, 2002; 

Nathenson and others, 2007). Outcrops are characterized by steep 

cliffs and inclined benches that probably represent either individual lava flows or packages of flows, similar to those exposed 

on caldera walls. The outcrop areas are labeled with specific map 

units where they appear to be continuous with exposed units and 

(or) where samples collected by manned submersible (four traverses) unambiguously identify map units exposed above lake 

level. Other bedrock exposures are shown as undivided submerged caldera wall outcrops (su). Four dikes are identified 

(apn, dr, alu, dc). 

Crater Lake caldera has a scalloped outline that is typical of 

collapse calderas. The embayments forming the scalloped pattern 

are scars formed by landsliding of oversteepened walls, mainly 

during caldera collapse. Although the slide masses are largely 

buried in thick caldera fill hidden beneath the lake floor, the most 

recent ones appear on the lake floor as hummocky ground characterized by coherent blocks up to several hundred meters on a side 

(landslide deposits, ls). The south wall of the caldera above Chaski 

Bay is broken by caldera-parallel normal faults into several large 

blocks that did not slide completely to the caldera floor. Much of 

the submerged caldera wall is composed of talus and unconsolidated debris (talus, t), sloping at least 13°, that commonly is contiguous with subaerial talus (also mapped as unit t). These debris 

aprons give way at depth to sediment gravity-flow deposits of the 

three deep basins (modern sediment of the lake floor, sl). Sediment 

ponds on lava flows and landslides also are mapped as unit sl. 

GLACIATION

Glaciated outcrops on the slopes of Mount Mazama, U-shaped 

notches in the caldera rim at the heads of deep glaciated canyons, 

and lateral moraines in the lower reaches of the larger valleys 

have been noted since the classic account of J.S. Diller (Diller and 

Patton, 1902). Atwood (1935) additionally recognized evidence 

of multiple glacial advances in buried surfaces within the caldera 

walls. Williams (1942) promoted Mazama’s history of fire and 

ice. In the years since these pioneering studies, we have developed a refined glacial history of Mount Mazama using precise 

radiometric dating methods and detailed paleoclimatic records. In 

addition, volcanologists have gained a greater understanding of 

pyroclastic-flow deposits and how to distinguish them from nonvolcanic debris, such as glacial till. The deep canyons of Sand, 

Sun, and Annie Creeks were excavated by repeated ice advances, 

mainly over lavas no younger than about 210 ka; each advance 

moved debris left by earlier glaciers into Klamath Marsh and 

Basin (fig. 2) or into major rivers that have transported it out of 

the area. The north and west caldera rims are not deeply notched 

because the comparatively young lava flows present on those 

flanks have been eroded only by late Pleistocene glaciers. Virtually every hill outside the caldera bears some effect of glacial erosion, except Holocene units and a few Pleistocene lava domes and 

cinder cones at low elevations near the east boundary of the map. 

A modest glacial cirque is present on the northwest side of Mount 

Scott, and all flanks of that edifice have been sculpted by ice. The 

Union Peak volcano was entirely glaciated so that its intrusive 

core stands now as a pyramidal horn at the head of a cirque with 

a deep glacial valley descending northwestward into the valley 

of Castle Creek. Many pre-Mazama rhyodacite lava flows east of 

Mount Mazama appear to have been glaciated, perhaps not during 

the most recent ice advance(s), because most exposures are composed of thoroughly devitrified felsite that commonly forms cliffs 

at the margins of valleys such as the one south of Pothole Butte. 

Glacial till is preserved locally on the slopes of Mount 

Mazama and is widespread at lower elevations and in the western 

part of the map. Because of poor exposure, no attempt has been 
12

made to separate tills of different ages. Well-developed lateral 

and a few recessional moraines are present in some drainages (for 

example, recessional, 3 km west of Discovery Point), and their 

crests are indicated on the map where possible. Glacial till and 

associated fluvial(?) sediments are present in the caldera walls and 

are shown on the panoramas where they are not covered by talus. 

An especially thick accumulation below Roundtop and Wineglass 

appears to be a relatively permeable horizon in the caldera wall 

that may regulate the level of Crater Lake (Bacon and Lanphere, 

1990; Bacon and others, 2002). All of these glacial deposits are 

collectively mapped as unit g, which also includes thick colluvium on forested slopes in the southwestern part of the map. 

At many places in the caldera walls, glaciated lava surfaces 

have been buried by subsequent lava flows where K-Ar or 40Ar/39Ar 

dates bracket the time of ice presence (Bacon, 1983; Bacon and 

Lanphere, 2006). Similarly, ice contact features (Lescinsky and 

Fink, 2000) such as thick glassy margins with 10–30-cm-spaced 

columnar (polygonal) joints are common in some units (for example, aa). The table mountain or tuya morphology and structure 

(Mathews, 1947) of some monogenetic volcanoes near Mount 

Mazama also indicate the presence of ice of a specific minimum 

thickness at the time of eruption (units at, ab). Although it is feasible that alpine glaciers were present, at least at high elevations, 

on Mount Mazama at virtually any time between about 400 ka and 

caldera collapse, it seems likely that dated times of ice presence 

correlate with well-established glacial intervals tied to paleoclimatic chronologies such as marine oxygen isotope stages (MIS; 

Bowen and others, 1986; Martinson and others, 1987). Examples 

of such correlations include (1) unit dr on glaciated ak at Sentinel 

Rock and Grotto Cove, MIS 10; (2) tuya of unit at at Arant Point, 

MIS 8; (3) ice-contact features on unit dm on Munson Ridge, 

MIS 8; (4) widespread ice-contact features in unit aa, MIS 8; (5) 

emplacement of ag and dcn lava flows on glaciated unit aa lava, 

MIS 7; (6) unit ar ice-bounded lava flow, MIS 6; (7) ice-contact 

features in unit alu at Pumice Point and lying on glaciated unit 

all, MIS 5d; (8) ice-contact features in unit agc at caldera wall, 

MIS 4; (9) glaciated surface on unit aww overlain by dwp, MIS 4 

in the caldera wall between The Watchman and Discovery Point; 

(10) ice-contact features in unit re on Redcloud and Grouse Hill 

flows of Williams (1942), MIS 2 or 3; and (11) tuya of unit ab and 

coeval, dated rbb, MIS 2. Note that ice-contact structures have 

not been found in units that erupted during MIS 1, 3, 5a, 5c, 5e, 

7, 9, or 11, which presumably would have been ice free, except 

perhaps at the highest elevations. The correlations with MIS constrain the number of times ice advanced down the major canyons 

of the south flank of Mazama since eruption of unit aa, the last 

major unit to be emplaced on that side of the volcano, to as many 

as six (MIS stages 8, 6, 5d, 5b, 4, and 2). 

Reconstructions of Mount Mazama show a 3,700-melevation volcano (Diller and Patton, 1902; Atwood, 1935; 

Williams, 1942). There is not a compelling reason to revise this 

estimate. The three paintings by Paul Rockwood under direction of Howel Williams (for example, Briggs, 1962) depict (1) 

Pleistocene Mazama sheathed in ice, (2) the volcano at the onset 

of its climactic eruption, and (3) the freshly collapsed caldera in a 

pumice-mantled, devastated landscape. The middle painting in the 

sequence portrays Mazama with ice-clad upper slopes and glaciers 

descending to about 1,900 m in the southern valleys, while the last 

painting retains beheaded glaciers in Munson Valley and Sun and 

Kerr Notches. The Rockwood paintings are excellent representations of a modern view of the late history of Mount Mazama with 

the principal exceptions of the climactic eruption initiating at the 

summit, the extent of Holocene ice, and the presence of remnant 

glaciers following caldera collapse. These features evidently were 

included because Williams interpreted deposits now mapped as 

lithic breccia (unit cb) as probable glacial till. Improved knowledge of pyroclastic deposits negates the till hypothesis. Moreover, 

paleoclimatic reconstructions indicate that at the time of the climactic eruption, a relatively warm and dry period, any ice would 

have been restricted to the highest part of Mount Mazama, and its 

south flank, especially, would have been ice free at elevations of 

the caldera rim. 

GEOTHERMAL PHENOMENA

Hydrothermal systems within volcanoes of magmatic arcs 

are commonly driven by heat from magma conduits and shallow 

intrusions. Widespread alteration of rocks older than about 120 

ka exposed in the caldera walls attests to this type of circulation 

of heated water within ancestral Mount Mazama. Postcaldera 

thermal features probably are related to residual heat from the 

climactic magma chamber. The chemistry of water in Crater Lake 

requires input of thermal fluid through the caldera floor (Bacon 

and Nathenson, 1996). Areas of high convective heat flow caused 

by circulation of such fluid were delineated by heat flow measurements by D.L. Williams (Williams and Von Herzen, 1983). 

Exploration of these areas with a manned submersible documented numerous bacterial mats north of Eagle Point that have 

elevated internal temperatures as well as small pools of warm, 

solute-laden water below Cleetwood Cove (Dymond and Collier, 

1989; Bacon and others, 2002). Many independent lines of evidence indicate that thermal fluid enters Crater Lake through its 

floor, subtly increasing the temperature at the sediment-water 

interface and causing the lake to convectively mix on a time scale 

of about 3 years (Williams and Von Herzen, 1983; Wheat and 

others, 1998). One submersible traverse encountered fossil subaqueous thermal-spring deposits in the form of silica spires as 

high as 10 m at about 550 m depth off Skell Head (Bacon and 

others, 2002), which indicate that higher temperature fluid apparently vented through the lake floor in the past.

Two geothermal exploration wells were drilled by California 

Energy Company, MZI-11A (1,423 m deep) just east of the park 

boundary in the Scott Creek drainage and MZII-1 (867 m deep) 

south of the boundary and east of Annie Creek (fig. 1; Bacon and 

Nathenson, 1996). Drill core samples of regional mafic lavas 

from the southern well are relatively fresh and the maximum temperature measured was 40°C, whereas hydrothermal alteration is 
13

ubiquitous in core samples from the eastern well, which had a 

maximum temperature of 130°C and intersected more compositionally diverse rocks. Bacon and Nathenson (1996) suggested 

that the geothermal system responsible for the 130°C temperature from MZI-11A is small and may be powered by residual heat 

from intrusions related to the dacite of Pumice Castle (dc).

There are many springs in the map area. None of these discharge thermal waters, nor is there any evidence that water from 

Crater Lake escapes through these springs. Groundwater flow in 

Mount Mazama and in the surrounding volcanic terrain is controlled by gently inclined to horizontal, relatively permeable 

zones (perched aquifers) such as rubbly tops of lava flows or fragmental deposits of various kinds. Cold springs commonly occur 

where such permeability contrasts intersect the ground surface. 

Deep circulation of ground water is facilitated by the dominantly 

north-south normal faults that may act as conduits or may retard 

transverse flow because relatively impermeable gouge is present. 

Nathenson (1990) identified several springs in the Wood River 

Valley, 20–30 km south of Crater Lake along the east Klamath 

Lake fault zone, that have high flows and temperatures as high 

as about 10°C. These springs produce mixed waters with an 

unknown hot end-member composition. Their large convective 

heat discharge suggests a thermal source beneath Mount Mazama, 

presumably related to the climactic magma chamber (Bacon and 

Nathenson, 1996).

HAZARDS

The climactic eruption and collapse of Crater Lake caldera 

so dramatically changed the character of Mount Mazama volcano 

that many potential types of future eruptions have no precedent 

there. Because a lake is now present within the most likely site of 

future volcanic activity, many of the hazards at Crater Lake are 

different from those at other Cascade volcanoes. Also significant 

are many faults near Crater Lake that have been active in the recent 

past. These faults, and historic seismicity, indicate that damaging 

earthquakes can occur in the Crater Lake area in the future.

VOLCANIC HAZARDS

All volcanic eruptions in the Crater Lake area since the climactic eruption and formation of the caldera have taken place 

within the caldera itself. The most recent known eruption was 

about 4,800 years ago. Future eruptions may occur within the 

lake where interaction of magma and water may produce explosions that could eject ballistic blocks and ash outside of the caldera (Bacon and others, 1997b). This could result in downwind 

tephra fall or generation of pyroclastic surges. Surges have the 

potential to devastate not only the area within the caldera but also 

the valleys and upper slopes of Mount Mazama. Eruptions from 

vents in shallow water may be highly explosive, while those in 

the deep lake would be expected to be much less violent. An eruption from a vent in the caldera wall itself also might be explosive because of the abundant groundwater within the mountain. 

Waves several meters high on Crater Lake could be associated 

with explosive eruptions within the caldera. Because postcaldera 

volcanoes are concentrated in the west half of the caldera, it is 

considered the most likely site of future activity. The 30-year 

probability of renewed volcanic activity within or very near to 

the caldera is greater than 1 chance in 330, or 3x10-3 (Bacon and 

others, 1997b). 

Most large Cascade stratovolcanoes (for example, Mount 

Rainier) have produced lahars in the past and are likely to continue to do so. Mount Mazama differs from them, because no iceclad summit or fragile mountaintop remains as a source of water 

and debris at high elevation. However, should an eruption occur 

within Crater Lake near the shoreline with sufficient violence to 

eject lake water from the caldera, abundant loose debris left by 

the climactic eruption on the upper slopes of Mount Mazama and 

in the valleys might be mobilized to form lahars. Alternatively, 

an eruption outside of the caldera that resulted in rapid melting 

of a thick snowpack similarly might produce lahars. Such lahars 

would be localized in low-lying areas and would tend to be confined to narrow canyons.

Hazards from monogenetic volcanoes near Crater Lake 

include slow-moving lava flows and viscous domes and associated tephra falls, surges, and pyroclastic flows. If an eruption generates surges or pyroclastic flows, which might be expected for an 

eruption in a low-lying (wet) location, the area affected by them 

likely would be only a few square kilometers. Tephra falls may be 

significant near the vent and for a few kilometers downwind. Lava 

flows will advance slowly enough that they will pose a threat only 

to property and structures. Estimates of 30-year probabilities of 

an eruption occurring in a particular area are higher for the region 

west of the main axis of the Cascades and lower to the east. The 

30-year probability of eruption of a new volcanic vent near Crater 

Lake is 3x10-3 to 3x10-4 (Bacon and others, 1997b).

Three volcano-related events of high consequence are considered to have low probability. (1) A large pyroclastic eruption, 

such as the one during which the caldera formed or the (smaller) 

1991 eruption of Mount Pinatubo, Philippines, is not considered 

likely for many thousands of years because the magma reservoir 

that fed the climactic eruption of Mount Mazama has not had 

sufficient time to regenerate a large volume of gas-rich silicic 

magma. (2) Sudden gas release from Crater Lake, such as the 

lethal release of cold CO2 from Lake Nyos, Cameroon, in 1986, 

would seem to be a possibility. However, natural mixing of deep 

water with near-surface water in Crater Lake prevents volcanic 

CO2 that escapes from the lake floor from building up. As long 

as the natural mixing process continues, sudden gas release is not 

considered to be a significant hazard at Crater Lake. (3) Catastrophic draining of Crater Lake is an extremely unlikely event 

but one that would have disastrous consequences for downstream 

lowlands in the affected tributary drainages. No known mechanism, short of another large-volume eruption, could either eject 

most of the water in the lake or cause the caldera wall to fail.
14

EARTHQUAKE HAZARDS

The West Klamath Lake fault zone (WKLFZ), composed of 

several individual faults having lengths as much as 15 km and an 

aggregate length of 50 to 70 km, has been mapped through Crater 

Lake National Park west of the caldera (Bacon and others, 1997b). 

One of its constituent faults, the Annie Spring fault, passes less 

than 2 km west of Rim Village. All of the faults in the WKLFZ 

trend approximately north-south and have mainly dip-slip displacement that drops down the east side relative to the west side. 

By determining the ages of lava flows that have been offset by 

the faults, Bacon and others (1999) found the long-term rate of 

vertical displacement to be about 0.3 mm/year. The lengths of the 

faults and the measured displacements suggest that the WKLFZ is 

capable of tectonic earthquakes as large as magnitude (M) 7.25. 

The recurrence interval of large earthquakes in this fault zone 

is unknown but probably is between 3,000 and 10,000 years. 

Although few earthquakes have been recorded in the Crater Lake 

area, the known events are consistent with the WKLFZ being 

active. Moreover, the September 1993 Klamath Falls earthquakes 

(the two largest events were M ≈ 6.0) occurred farther south along 

the same general zone. Many other potentially active faults are 

present east of the Cascades, notably along the east side of Klamath 

valley (East Klamath Lake fault zone). Local volcanic earthquakes 

would produce ground motion at Crater Lake but the likely maximum magnitude of such events is about M = 5, which is significant 

but far smaller than expected for tectonic earthquakes. An additional source of seismic activity is the Cascadia subduction zone. 

Although distant, the potential for this zone to generate M = 8–9 

earthquakes means that as much as several minutes of continued 

shaking could occur at Crater Lake.

Earthquake hazards in the greater Crater Lake area are similar 

to those in other earthquake-prone areas, namely damage to structures, utilities, communication lines, and transportation systems. 

Rockfalls and landslides are significant hazards below steep canyon 

or caldera walls. Should a large mass of rock fall or slide rapidly 

from the caldera wall into Crater Lake, one or more large waves 

could be generated. Waves could be many meters high and travel 

across the lake in as little as two minutes, such as from Chaski Bay 

to the boat landing at Cleetwood Cove. Volcanic, local tectonic, or 

distant Cascadia subduction zone earthquakes all could produce 

shaking adequate to trigger sliding of the fractured and poorly consolidated rock of the caldera walls and talus slopes. Earthquake 

shaking alone, without rapid entry of slide material into Crater 

Lake, would not be expected to cause dangerous waves.

ACKNOWLEDGMENTS

A great many individuals have contributed to the body of 

knowledge that has made this geologic map possible. I thank Marvin 

Lanphere and those who have worked in his laboratory for K-Ar 

and 40Ar/39Ar age determinations. Duane Champion conducted the 

paleomagnetic studies to test correlations of several geologic units. 

Hundreds of samples critical to establishing map units were analyzed by X-ray fluorescence by U.S. Geological Survey chemists, 

among them Joseph Taggart, David Siems, and Peggy Bruggman. 

Assistants in geologic mapping of the caldera walls and of Mount 

Mazama include William Bartling (1979, 1980), Steven Novak 

(1981), Marcus Moench (1982), Carl Michelsen (1983), Marc 

Hirschmann (1984, 1985), Anne Gartner (1986, 1987), Maribeth 

Price (1986, 1987), Simon Young (1987, 1988), Maura Hanning 

(1988), Shir Filler (1989), Steve McKnight (1991, 1992), Patricia 

Weston (1993), Jacob Madden (1994), and Tracey Felger (1995). 

The topographic base for the caldera wall panoramas was made by 

Brian Bennett. The digital database containing the geologic map 

and caldera wall panoramas was constructed by Dillon Dutton and 

David Ramsey. The caldera floor geology was mapped on the base 

made by Joel Robinson and David Ramsey from multibeam echosounding data provided by James Gardner and Peter Dartnell. 

The geologic map of the lake floor would not have been possible 

without samples and observations collected by Hans Nelson, Mark 

Buktenica (Crater Lake National Park), Robert Collier (Oregon 

State University), and Jack Dymond (Oregon State University). 

Al Waibel (Columbia Geosciences) provided access to drill core 

from two exploration wells made available by Joseph La Fleur 

(California Energy Company). Superintendents of Crater Lake 

National Park permitted work on the caldera walls and collection 

of rock specimens. Park staff members, notably Mark Forbes, 

Mark Buktenica, and Scott Selleck, were instrumental in facilitating field work in the park and, especially, on the caldera walls. 

Although much field work was conducted from a mobile base, 

participants benefited in the later years from the hospitality of Tai 

Guimond and Adrian Aaby of Wilson’s Cottages, Fort Klamath. 

Important contributions to the geologic map were drawn from 

topical research by collaborators Timothy Druitt, Setsuya Nakada, 

and Simon Young. Thorough reviews of the map and text by Julie 

Donnelly-Nolan and Peter Lipman resulted in many improvements 

in presentation. Howel Williams’ classic account of the geology of 

the park provided a basis on which to build, and his endorsement at 

an early stage of this work is gratefully acknowledged. This study 

began on a suggestion from Wes Hildreth that I study the precaldera 

silicic domes of Mount Mazama. It has been accomplished thanks 

to the patience and support of Cynthia Dusel-Bacon.

DESCRIPTION OF MAP UNITS

[Labels ending in the letter “p” indicate pyroclastic units, 

ending in the letter “i” indicate intrusive units. Mineral abbreviations: pl, plagioclase; ol, olivine; aug, augite; opx, orthopyroxene; 

hbl, hornblende; ap, apatite. Fe-Ti oxides are titanomagnetite and, 

in many silicic rocks, ferrian ilmenite. Total phenocryst contents 

(estimated volume percent) typically dominated by plagioclase; 

phases listed in order of decreasing volume percent. Microxenoliths are rock fragments; aggregates are bits of crystal mush with 

intergranular melt; and enclaves are undercooled blobs of magma 

(Nakada and others, 1994). SiO2

 rounded to nearest 0.5 weight 
15

percent (see discussion). Rock names are based on silica content: 

basalt, ≤52 weight percent; basaltic andesite, 52–57; andesite, 

57–63; dacite, 63–68; and rhyodacite, 68–72. HAOT refers to 

high-alumina olivine tholeiite of Hart and others (1984). Marine 

oxygen isotope stages (MIS) after Bowen and others (1986) and 

Martinson and others (1987)]

SEDIMENTARY DEPOSITS

al Alluvium (Holocene)—Unconsolidated water-transported mud, sand, gravel, and coarser debris deposited in or adjacent to present-day streams. Typically 

contains a large fraction of material reworked from 

deposits of climactic eruption

sl Sediment gravity-flow deposits (Holocene)—Clastic 

sediment in the three major basins and in depressions 

on and between lava flows and landslide deposits on 

the floor of Crater Lake (Nelson and others, 1988; 

Bacon and others, 2002). Maximum thicknesses 

75 m in the east basin and <50 m in the southwest 

and northwest basins (Nelson and others, 1986). 

Uppermost layers consist of mud and fine-grained 

sand thought to have been deposited by sheet-flow 

turbidity currents

t Talus (Holocene and Pleistocene)—Unconsolidated talus 

and thick colluvium (especially in southwest corner 

of map). Beneath lake surface, includes post-7,700 

yr B.P. talus, scree, sand, and probable debris-flow 

and minor landslide material generally sloping at 

least 13°

ls Landslide deposits (Holocene)—Landslide and debrisavalanche deposits, mainly beneath the surface of 

Crater Lake (Bacon and others, 2002). Composed 

of unconsolidated, poorly sorted, typically heterolithologic debris derived from the caldera walls and 

transported into the lake by mass wasting. Debrisavalanche deposits have hummocky surfaces and 

contain lithic blocks as large as many tens of meters 

in size in a matrix that may contain a range of particle 

sizes from clay to boulders. Largest debris-avalanche 

deposit, below Chaski Bay, has a volume of ≥0.2 

km3

 and traveled 2–3 km from its source

g Glacial deposits, undivided (Pleistocene)—Till and 

minor associated outwash forming a discontinuous mantle on the slopes of Mount Mazama, lateral 

moraines, extensive deposits west of long ~122°13’

W., and layers exposed locally in the caldera walls. 

Till is characterized by a heterolithologic assemblage of dense, abraded or rounded volcanic clasts 

and presence of ultrafine material in unsorted matrix. 

No attempt was made to divide unit into deposits of 

different ages. Till, overlain by Holocene pumicefall deposits west of Cleetwood Cove, near Palisade 

Point, and at Skell Head, as well as most till on the 

slopes of Mount Mazama and till that forms distinct 

lateral moraines, dates from the latest Pleistocene glaciation about 22–16 ka (Rosenbaum and Reynolds, 

2004). Older glacial deposits are present low in the 

caldera walls between Pumice Point and Skell Head 

where their ages are bracketed by K-Ar dates on lava 

flows (see panoramas on sheet 4 and discussion)

s Sedimentary deposits, undivided (late and middle 

Pleistocene)—Deposits of clastic sediment exposed 

locally in the caldera walls. Typically inaccessible 

and poorly exposed, this unit probably consists of 

laharic or local fluvial sediment of various ages

VOLCANIC ROCKS

Regional volcanism, northwest

bwn

bwnp

Basaltic andesite northwest of Williams Crater (late 

Pleistocene)—Medium-dark-gray porphyritic basaltic andesite (52.5% SiO2

) lava flow (bwn) and small 

cinder cone remnant (bwnp) 2.6 km north-northwest of 

Williams Crater, west-northwest of the caldera. Vent 

coincides with probable trace of Red Cone Spring 

fault. Lava flow is locally exposed for ~2.5 km to 

west where it terminates near Pacific Crest National 

Scenic Trail. Rock contains abundant, conspicuous 

gabbroic aggregates (≤1 cm) of coarse pl + ol + 

aug. Compositionally and texturally similar but not 

identical to lava of unit bw (Bacon, 1990). Phenocrysts (~30%, including aggregates but excluding 

microphenocrysts): pl (≤4 mm, finely sieved), ol (≤3 

mm), and aug (≤3 mm) in an intersertal groundmass. 

Overlies units ah and ad. Undated. Paleomagnetic 

direction similar to that of bw and degree of preservation suggest age of about 35 ka

bw

bwp

Basaltic andesite of Williams Crater (late Pleistocene)—

Medium-gray porphyritic basaltic andesite (51.5% 

SiO2) lava flow (bw) and grayish-red cinders (bwp) 

of Williams Crater complex (see also unit mw) west 

of the caldera. Formerly known as Forgotten Crater 

(Williams, 1942), the cinder cone has been named 

in honor of volcanologist Howel Williams. The 

Williams Crater complex includes basaltic andesite 

contaminated with gabbro (resulting in bulk SiO2

content of a basalt), dacite, and hybrid andesite 

(Bacon, 1990). The basaltic andesite comprises the 

cinder cone, a lava flow locally exposed from 1 to 4 

km west of the west base of the cone and possibly 

vented from a fissure, and enclaves within the dacite 

and hybrid andesite. Ubiquitous within the basaltic 

andesite are gabbroic microxenoliths or aggregates 

(~2–10 mm) of various combinations of ol, pl, and 

aug. Phenocrysts (~30%, excluding seriate pl micro-
16

phenocrysts), many of which are derived from the 

grabbroic fragments: ol (typically ≤3 mm, rarely to 

6 mm), pl (typically ≤2.5 mm, commonly to 4 mm; 

larger crystals finely sieved), and aug (≤4 mm) in 

an intersertal to intergranular groundmass. Overlies 

units bh, ah, and dvb; overlain by mw. Undated. 

Smooth contact of bwp tephra on dvb at caldera 

wall south of Devils Backbone suggests similar age, 

or about 35 ka

brw Basaltic andesite northwest of Red Cone (late 

Pleistocene)—Medium-gray porphyritic basaltic 

andesite (53.5% SiO2) lava flows erupted from vent 

1.7 km north of Red Cone, northwest of the caldera. 

Similar to lavas of unit br but contains larger, sieved 

pl phenocrysts. Phenocrysts (15%; excluding abundant pl microphenocrysts): ol (≤4 mm), aug (≤3 

mm), and pl (≤3 mm; sieved) in an intergranular 

groundmass. Microxenoliths (≤5 mm) of aug ± ol 

± pl abundant. Overlies unit br. Compositional and 

paleomagnetic data indicate brw lava represents 

part of the same ~35 ka eruptive episode as does 

unit br

br

brp

Basaltic andesite of Red Cone (late Pleistocene)—

Light- to medium-gray porphyritic basaltic andesite 

(52.5–54% SiO2) lava flows (br), bombs, and cinders (brp) erupted from vent marked by Red Cone 

and from fissure vent system 1.5 km north of Red 

Cone, northwest of the caldera. Lava flowed to westnorthwest around source of unit arw to within ~1 

km of Oasis Butte. Unit is offset down to the east 

by Red Cone Springs fault. Composition is magnesian basaltic andesite, chemically primitive yet rich 

in incompatible trace elements. Olivine-phyric lavas 

of br represent the arc end member among primitive mafic lavas in the Crater Lake region (Bacon 

and others, 1997a). Recognized in the field by 

coarse, blocky olivine phenocrysts. Phenocrysts (5–

>20%; excluding abundant pl microphenocrysts): ol 

(≤5 mm), aug (≤4 mm), and pl (≤2 mm) in an intergranular groundmass. Phenocryst assemblages vary 

from ol only to ol + aug to ol + aug + pl, phenocryst 

content increasing in that order. Microxenoliths 

(≤5 mm) of aug ± ol ± pl abundant in samples with 

those phases as phenocrysts. Overlies units bo, boe, 

and arw; overlain by closely-related brw. K-Ar age: 

36±12 ka; 40Ar/39Ar plateau age: 35±4 ka. Samples 

with abundant aug apparently contain inherited 40Ar 

and do not yield meaningful K-Ar ages

boe Basalt east of Oasis Butte (late Pleistocene)—Mediumgray porphyritic basalt lava at north-northwest 

boundary of map; part of extensive field of similar 

basalt (HAOT; 48.5%, rarely to 50% SiO2) flows 

from vent marked by low cinder mound 1.5 km 

east-southeast of Bald Crater (north of mapped 

area). Phenocrysts (~5%): ol (≤2 mm, rarely 3 mm) 

and pl (≤2 mm) in an intergranular groundmass; ol 

phenocrysts more abundant than pl. Microxenoliths 

(≤5 mm) of ol + pl common. Undated but K-Ar and 

paleomagnetic data suggest similar in age to overlying unit br (40Ar/39Ar plateau age 35±4 ka)

atc Andesite of Timber Crater (late or middle Pleistocene)—Finely porphyritic seriate-textured mediumto dark-gray andesite (58–59% SiO2

) lava flows and 

summit cinder cone of Timber Crater shield volcano 

at north limit of map. Unit shown only on Bedrock 

Map (sheet 3) because it is covered by deposits of 

climactic eruption within map area; description 

applies to lavas exposed north of map area. Phenocrysts (~3–30%): ol (≤1 mm, rarely to 3 mm; 

commonly resorbed), pl (≤1 mm; commonly seriate; may show strong preferred orientation), minor 

aug (≤0.7 mm), with or without opx (≤0.7 mm) in a 

pilotaxitic to trachytic groundmass. Phenocryst-poor 

samples have ol and pl; phenocryst-rich samples are 

dominated by pl and contain aug and more abundant 

opx, while ol is strongly resorbed. Aggregates (≤3 

mm) of ol ± pl ± aug sparse. Capping cinder cone 

and shield-forming lava flows have been lightly glaciated. In size and compositional uniformity, Timber 

Crater volcano resembles the more heavily glaciated, somewhat older Union Peak volcano. 40Ar/39Ar 

isochron age: 137± 10 ka

bo

boi

bop

Basaltic andesite of Oasis Butte (middle Pleistocene)—Medium-gray porphyritic basaltic andesite 

(53–53.5% SiO2) plug (boi) at Oasis Butte and lava 

(bo) and cinders (bop) vented from Oasis Butte and 

dissected cinder cone 1 km to south near northwest 

corner of map. Lava outcrops traced as far southwest as 4,800 ft elev on a tributary of Crater Creek. 

Patches of olivine basaltic andesite mapped as bo

east-northeast of Oasis Butte may have had different 

source. Distinguished by relatively coarse and abundant ol and common microxenoliths of pl + aug. 

Phenocrysts (~5%): pl (≤2 mm, finely sieved; rarely 

to 4 mm), ol (≤2 mm, commonly to 3 or 4 mm; may 

be skeletal), and aug (≤3 mm) with or without opx 

(<1 mm) or Fe-Ti oxide (≤0.2 mm) in an intergranular or pilotaxitic groundmass. Microxenoliths (≤3 

mm, rarely to 1 cm) of pl + aug ± ol or opx relatively 

abundant. Microxenoliths commonly have diabasic 

texture or form stellate clusters dominated by pl. 

Overlies units bck, bow, and ao; overlain by asl, 

arw, and br. K-Ar age: 201±13 ka

bn Basaltic andesite north of Red Cone (middle? Pleistocene)—Medium-gray coarsely porphyritic basaltic 

andesite (52.5% SiO2

) lava flows erupted from fissure system now marked by low north-south ridge, 

north-central map boundary. May be related to flows 
17

from Desert Cone (north of map area; K-Ar age 

213±26 ka). Phenocrysts (~15%): ol (≤3 mm), pl 

(≤3 mm; cores finely sieved), and aug (≤3 mm) in an 

intergranular groundmass. Common microxenoliths 

(≤6 mm) of ol + pl ± aug may be source of many 

phenocrysts. Undated

ao Andesite southwest of Oasis Butte (middle Pleistocene)—Porphyritic medium-gray andesite lava 

exposed in block between parallel north-southtrending normal faults ~1–2 km southwest of Oasis 

Butte in northwest corner of map. Phenocrysts (25%): 

pl (≤3 mm, euhedral, commonly in stellate clusters), 

aug (≤4 mm, commonly oikocrystic), opx (≤4 mm), 

and Fe-Ti oxides (≤0.2 mm) in a fine intergranular 

groundmass. Rare ol (≤2 mm, resorbed) also may be 

present. Common aggregates (≤5 mm) contain pl ± 

aug + opx + oxide ± resorbed ol ± melt. Joint pattern 

suggests source was to southeast and fault block has 

been tilted gently to south. Overlies unit bow; overlain by bo (K-Ar age of 201±13 ka). Undated

bow Basaltic andesite west of Oasis Butte (middle? Pleistocene)—Medium-gray porphyritic basaltic andesite 

lava near northwest corner of map. Phenocrysts (~2%): 

conspicuous ol (≤5 mm) and equant pl (≤1 mm) with 

or without uncommon aug (<1 mm, rarely to 3 mm) in 

an intergranular groundmass; ol more abundant than 

pl. Microxenoliths (≤4 mm) of pl ± ol may be present. 

Undated. Overlain by units ao and bo

bbf

bbc

Basaltic andesite of Bert Creek (early Pleistocene)—

Dark-gray finely porphyritic basaltic andesite lava 

flows (bbc) and weathered monolithologic mudflow 

deposit (bbf) with clasts as large as 40+ cm underlying glacial till on high ground in northwest corner of 

map. Distinguished from underlying basalt flows by 

abundant conspicuous pl. Clast from bbf has ~40% 

phenocrysts of pl (≤2 mm; seriate; finely sieved), 

ol (typically ≤1.5 mm; rarely to 8 mm), aug (≤1 

mm), and opx (≤1 mm) in an intersertal groundmass. Aggregates (≤3 mm) pl ± ol + aug ± opx 

common. Overlies unit bck (K-Ar age: 1,879±22 

ka). Undated

bck Basaltic andesite north of Crater Creek (early Pleistocene)—Medium-gray sparsely porphyritic basaltic andesite flows underlying the bulk of the high 

area in the northwest corner of the map between 

the drainages of Crater Creek and National Creek 

and bounded on east by fault-line scarp. Unit may 

contain flows of differing ages and source vents. 

One vent may have been located ~1 km northeast 

of Spruce Lake. Isolated ol basaltic andesite outcrop (bck?) 4 km east of Spruce Lake provisionally 

included in this unit. Phenocrysts (typically <5%; 

may have abundant seriate pl microphenocrysts): pl 

(≤2 mm) and ol (≤2 mm; commonly <1 mm) in an 

intergranular to intersertal groundmass. Interiors of 

thick flows verge on diabasic texture. Overlain by 

units bbc, bbf, and bo. K-Ar age: 1,879±22 ka

Regional volcanism, southwest

bc

bcp

Basalt of Castle Point (early Holocene or latest Pleistocene; postglacial, <14–16 ka)—Medium-gray variably porphyritic basalt (HAOT; 48–50% SiO2

) lava 

(bc) and grayish-red cinders (bcp) at Castle Point, 

southwest of the caldera. This is the youngest precaldera volcanic unit that is not rhyodacite. Lava issued 

from three vents: (vent 1) phenocryst-poor primitive HAOT from a small shield at the west base of 

Castle Point, (vent 2) porphyritic more-differentiated 

HAOT from a prominent vent marked by cinders on 

Castle Point itself, and (vent 3) a less conspicuous 

vent at the west base of the Castle Point ridge, southeast of vent 1. Thin HAOT flows locally exposed in 

Castle Creek near Little Castle Creek belong to an 

older unit, bcc, based on their paleomagnetic orientation (D.E. Champion, written commun., 2007). 

Phenocryst content variable. Chemically primitive 

HAOT at vent 1 has <1% ol (≤3 mm, rarely to 7 mm; 

brown spinel inclusions common). More differentiated lava may be porphyritic, as at Castle Point, containing a total of 10–20 percent pl (≤3 mm; coarsely 

sieved; abundant) and ol (≤3 mm) with or without 

minor aug (≤1.5 mm). Abundant microxenoliths 

(≤1 cm) of pl + ol ± aug are source of many phenocrysts in porphyritic lavas. Groundmass texture is 

typically intergranular; more primitive lavas may be 

ophitic. Note that lavas of unit bc are distinct from 

HAOT flows west of map area along Rogue River 

near Farewell Bend, which may be about 1 Ma in age 

(K-Ar age 1.25±0.11 Ma near Prospect; Fiebelkorn 

and others, 1982); middle or late Pleistocene HAOT 

occurs east of Bald Crater, north of map area (Bacon 

and others, 1994). Holocene age of unit bc is evidenced by little-modified volcanic landforms, including spatter ramparts and variably collapsed lava tubes 

near Castle Point, and by relations with latest Pleistocene deposits. Overlies unit g lateral moraines of 

last glacial maximum, west and north of Castle Point, 

and apparently overlies talus at vent 3. Overlain by 

cf, climactic ignimbrite

blcp Basaltic andesite north of Little Castle Creek (late 

Pleistocene)—Dark-gray porphyritic basaltic andesite (52.5% SiO2

) bombs and grayish-red cinders of 

breached cone and adjacent tephra mantle on north 

wall of valley of Castle Creek near confluence with 

Little Castle Creek, southwest of the caldera. Phenocrysts (~20%): pl (≤4 mm; finely sieved), ol (≤4 
18

mm), and aug (≤1 mm) in a fine-grained groundmass 

bearing common pl microphenocrysts. Abundant 

microxenoliths (≤6 mm) of pl ± ol ± aug are source of 

many phenocrysts. Undated. Preservation of cone and 

tephra at valley margin suggests late Pleistocene age

bsc

bscp

Basaltic andesite of Scoria Cone (late Pleistocene)—

Medium-gray porphyritic basaltic andesite (54.5–

55% SiO2) lava flows (bsc) and cinders (bscp) 

erupted from Scoria Cone (just south of map) 

and associated vents in 1-km-long north-northeast alignment between Pumice Flat and Annie 

Creek. Lava is offset down-to-the-east by the 

Sevenmile Creek fault, part of the West Klamath 

Lake fault zone (Bacon and others, 1997b, 1999). 

Recognized in hand specimen by abundant blockyappearing ~1 mm pl and typically smaller ol, 

commonly accompanied by conspicuous microxenoliths (to 1 cm) of coarse blocky aug ± small ol 

± pl. Phenocrysts (30–40%; greater if all microphenocrysts included): pl (≤2 mm, most ≤1 mm; seriate; stellate clusters common), ol (typically ≤1 mm, 

but may be up to 2 mm), and aug (≤3 mm, derived 

from microxenoliths; oikocrystic) in a fine-grained 

intergranular groundmass. Outcrop at Annie Falls 

has particularly large augite-rich microxenoliths. 

Overlain by unit abt. 40Ar/39Ar plateau age of bsc: 

53±4 ka

bf

bfi

bfp

Basaltic andesite northwest of Pumice Flat (late Pleistocene)—Medium-gray finely porphyritic basaltic 

andesite (55–55.5% SiO2) lava flows (bf), plugs 

and dikes (bfi), and grayish-red cinders (bfp) of Hill 

6926 and chain of smaller vents forming north-south 

ridge composed of glaciated remnants of eruptive 

fissure system bounding Pumice Flat on west, southcentral part of map. Recognized in field by common 

occurrence of microxenoliths (≤7 mm) of ol + pl. 

Phenocrysts (~2–7%, many derived from xenoliths): 

ol (≤2 mm) and pl (≤1 mm) in a pilotaxitic or intergranular groundmass. Overlies units bx, baw, and 

bu. K-Ar age: 116±24 ka

bu

bui

bup

Basaltic andesite of Union Peak (middle Pleistocene)—

Light- to medium-light-gray porphyritic basaltic 

andesite (54–56.5% SiO2

) lava flows (bu), grayish-red and yellowish-orange (palagonitic) cinders 

(bup), and very light gray to medium-light-gray 

intrusive rock (bui) comprising the Union Peak 

shield volcano in southwestern part of map. Units 

bup and bui restricted to eroded core preserved as 

Union Peak itself and in remnants within glacial 

valley to north. Distinguished in hand specimen by 

presence of a few percent ol and lack of aug phenocrysts, with or without visible pl. Phenocrysts (typically ~2% ol, but variable; up to ~40%, including 

seriate microphenocrysts, in pl-phyric examples): 

ol (≤2 mm, rarely to 3 mm; commonly to 5 mm in 

intrusive rocks) with or without pl (≤1 mm, rarely 

to 3 mm; larger crystals finely sieved). Groundmass 

of lava specimens is intergranular to pilotaxitic or 

trachytic, intergranular to subophitic in intrusive 

rocks. Microxenoliths or aggregates (≤3 mm) of ol 

± pl common but not abundant. Overlies units buc, 

bac, and bx; overlain by bf. K-Ar age: 164±11 ka

bx

bxi

bxp

Basaltic andesite of Whitehorse Bluff (middle Pleistocene)—Medium-gray finely porphyritic basaltic 

andesite (55–56.5% SiO2) lava flows (bx), grayishred cinders (bxp), and medium-gray plugs (bxi). 

Named for lava forming tuya at Whitehorse Bluff 

southwest of the caldera. Unit includes lavas from 

four vents in a north-south array extending from 

Hill 6428, 2 km west of Mazama Campground, 6 

km south to Hill 6189, 1 km west of Pumice Flat. 

Also includes flows from vent at Hill 6252, ~1 km 

south of Whitehorse Bluff. Distinguished by relatively high concentrations of incompatible trace 

elements (for example, 1.3–1.4% K2O, 800–1,100 

ppm Sr) and low phenocryst content. Phenocrysts 

(typically 1–3%, rarely to ~7%): ol (≤1.5 mm) with 

or without pl (≤1 mm); minor aug present in flows 

west of northwest corner of Pumice Flat, which also 

may contain microxenoliths (≤4 mm) of ol ± pl ± 

aug. Groundmass typically pilotaxitic to trachytic, 

but also may be finely intergranular. Overlies units 

bxn, bwcp, and bcw; overlain by units dm, aa, bu

and bf; apparently overlain by compositionally more 

extreme and coarser-grained unit baw. K-Ar age: 

217±16 ka (west of Pumice Flat). If accurate, date 

indicates southern flows of this unit are younger than 

northern flows that are overlain by unit dm

at

atg

ati

Andesite of Arant Point (middle Pleistocene)—Sparsely 

porphyritic to virtually aphyric andesite (57–58% 

SiO2

) comprising the hill in the south-central part of 

the map known as Arant Point and the mesa to the 

east interpreted as a tuya. Arant Point is the heavily 

glaciated light-gray intrusive core (ati) of a cinder 

cone that marks the vent for light- to medium-gray 

lava of unit at. Cinders are locally preserved in the 

area mapped as at on the hill of Arant Point. Unit atg

is blocky to finely columnar jointed grayish-black 

vitric andesite of the chilled margin of the intrusion 

and carapace of the tuya where lava contacted glacial ice. Arant Point is cut by the Annie Springs fault 

(Bacon and others, 1997b, 1999), which displaces 

unit at a maximum of 160 m down to the east (Bacon 

and others, 1997b, table 4). Andesite of Arant Point 

is distinctive in having very few visible phenocrysts 

in hand specimen; may have a strong preferred orientation of microphenocrystic and groundmass pl 

that gives samples from thick outcrops of units at
19

or ati a sheen on broken surfaces and a light-gray 

color. In thin section, all subunits have a pilotaxitic 

to trachytic texture dominated by seriate pl laths 

<0.5 mm long, although unit ati may have a more 

intergranular- to diktytaxitic-textured groundmass 

locally where preferred orientation of pl laths is 

muted. Phenocrysts (1–3%, excluding microphenocrysts): pl (<0.5 mm; rare sieved xenocrysts 1–2 

mm) and ol (≤0.5 mm, rarely to 1 mm). Aggregates 

(<2 mm) of pl + ol are rare. Microphenocrysts of 

opx and opx rims on ol suggest pl + ol phenocrysts 

reflect high concentration of dissolved H2O in the 

melt prior to eruption. Appears to have flowed along 

and over scarp where baw is displaced along Annie 

Spring fault; overlain by dg. K-Ar age of 297±12 

ka is consistent with eruption during glacial interval 

(MIS 8; see discussion)

bxn

bxni

Basalt northwest of Whitehorse Bluff (middle? Pleistocene)—Medium-dark-gray porphyritic basalt and 

basaltic andesite (51–54% SiO2) lava flows (bxn) and 

light-gray diabase plug (bxni) 0.5 km north-northwest of Whitehorse Bluff, southwest of the caldera. 

Intrusive core and flows on top of small cinder cone 

remnant are coarsely porphyritic basalt with ~25 

percent phenocrysts of ol (≤4 mm; some skeletal), pl 

(≤2 mm; large crystals have finely sieved cores), and 

aug (≤3 mm) in an intergranular groundmass rich in 

aug; common microxenoliths (≤7 mm) of ol + pl ± 

aug are source of many phenocrysts. Other flows are 

finely porphyritic basaltic andesite with ~3 percent 

phenocrysts of ol (≤0.7 mm), pl (≤1 mm), and aug 

(≤0.5 mm) in an intersertal to intergranular groundmass; abundant pl microphenocrysts give a seriate 

texture. Basaltic andesite contains variable amounts 

of crystals found in basalt, and some hand specimens consist of both rock types. Possibly younger 

than unit bwc; overlain by unit bx. Undated

baw Basaltic andesite west of Arant Point (middle Pleistocene)—Medium-dark-gray sparsely porphyritic 

basaltic andesite (55.5–56% SiO2

) extending ~1 

km west of Arant Point in south-central part of map 

where it forms low hill marking probable source. 

Also exposed locally south and north of Arant Point 

and in cut along Hwy 62 in scarp of Annie Springs 

fault near park entrance station where baw flow 

unconformably overlies bcw lavas. Flows of unit 

baw have high incompatible trace element concentrations (for example, 1.6% K2

O, 1,400 ppm Sr) and 

are compositionally similar to rocks of unit bx but 

have higher Sr content and notably coarser-grained 

groundmass texture. Phenocrysts (≤3%; 40–50% 

seriate microphenocrysts): pl (≤1 mm) and ol (≤1 

mm). Preferred orientation of seriate plagioclase 

laths results in pilotaxitic to trachytic texture. Overlies units bbi, bcw and apparently bx; overlain by 

units at, dm, and bf. Undated. Probably about 200 

ka

bbi

bbp

Basaltic andesite west of Bear Bluff (middle? Pleistocene)—Medium-gray porphyritic olivine basaltic 

andesite (53–54% SiO2

) necks and minor lava (bbi) 

and locally adhering grayish-red cinders (bbp) forming two low hills west of Bear Bluff in south-central 

part of map. Phenocrysts (3%): ol (≤1–4 mm, rarely 

to 1 cm) with or without pl (≤2 mm; finely sieved) in 

a groundmass of seriate pl laths displaying preferred 

orientation and intersertal to intergranular texture. 

Aggregates or microxenoliths of ol + pl common 

(typically ≤3 mm, rarely to 3 cm). Overlain by units 

baw and bf. Undated

bcw Basaltic andesite west of Mazama Campground 

(middle? Pleistocene)—Medium-gray porphyritic 

basaltic andesite (55.5% SiO2) lava; minor andesite 

(60% SiO2) lava included in unit at northern locality 

may be genetically unrelated. Named for exposures 

south-southeast of the caldera along east-facing 

scarp of Annie Springs fault south of Hwy 62 and 

provisionally correlated outcrops north of where 

Hwy 62 descends into drainage of Castle Creek. 

Phenocrysts in basaltic andesite (~3%; pl microphenocrysts may be abundant): pl (≤1 mm) and ol (≤2 

mm) in a pilotaxitic groundmass. Microxenoliths (≤1 

cm) of pl + ol common in northern locality. Andesite 

has coarsely sieved pl (≤3 mm), ol (≤0.5 mm), and 

minor aug (≤1 mm) in an intergranular groundmass 

with common microxenoliths of pl + ol ± aug (≤3 

mm). Undated. Overlain by units baw, bx, aa, and 

dg. Eroded surface of bcw lava with paleosol, where 

overlain by baw in road cut where Hwy 62 crosses 

Annie Springs fault scarp, suggests early or middle 

Pleistocene age

bwc

bwci

bwcp

Basaltic andesite of Whitehorse Creek (middle? Pleistocene)—Medium-gray finely porphyritic 

basaltic andesite (53.5–54.5% SiO2

) lava flows 

(bwc), palagonitic tuff (bwcp), and light-gray dike 

(bwci) north of Whitehorse Bluff and ~0.7 km east 

of Whitehorse Creek, southwest of the caldera. 

Dike forms 800-m-long north-south ridge of Hill 

6082. Adjacent pyroclastic deposit consists of variably palagonitic bedded lapilli tuff containing darkgray bombs at least 20 cm across. Rock textures 

vary widely. A bomb has ~7 percent phenocrysts of 

pl (≤1 mm, rarely to 3 mm; some crystals highly 

sieved) and ol (≤2 mm) accompanied by microxenoliths (≤4 mm) of ol + pl or aug + pl in an intersertal groundmass rich in equant pl microphenocrysts. 

Samples from the dike have intergranular texture, 

typically with trachytic pl preferred orientation, but 

vary widely in grain size. Coarser-grained samples 
20

preserve ol only in uncommon microxenoliths and 

consist of pl + opx + aug + Fe-Ti oxide, all with 

an average grain size of ~0.5 mm. Finer-grained 

samples are faintly porphyritic, commonly showing pl in ~1 mm clusters. Smaller average grain 

size appears to be due in part to shearing near the 

margin of the dike, as in the quarry exposures at the 

north end of the outcrop. The dike outcrops are light 

gray because of the relatively large and abundant 

plagioclase. Lava flows above ~6,100 ft elev on the 

southern part of the ridge of Castle Point ~3.5 km 

west-southwest of the dike provisionally included 

in unit bwc; here bwc overlies unit bac and is overlain by unit bcp (postglacial). Lava and palagonitic 

tuff south of dike overlain by unit bx (K-Ar age: 

217±16 ka). K-Ar age of bwci sample of 1,379±22 

ka appears to be inconsistent with field relations

acc

accp

Andesite north of Castle Creek (middle Pleistocene)—

Sparsely porphyritic medium-gray to medium-darkgray andesite (58.5% SiO2

) lava flows (acc) on the 

ridge north of Little Castle and Castle Creeks and 

descending to Castle Creek near the western boundary of the park; eroded cinder cone (accp) forms 

highest point of the ridge. Phenocrysts (5%): pl 

(≤3.5 mm, most <2 mm; resorbed and finely sieved), 

aug (≤1.0 mm), and opx (≤1.6 mm) in a fine-grained, 

pilotaxitic groundmass. Gabbroic microxenoliths 

(<5 mm, most ≤2 mm; pl + aug + opx) common. 

Overlies unit bcn; overlain by dcn and blcp. K-Ar 

age: 587±18 ka

b

bi

Basaltic andesite, undivided (Pleistocene or Pliocene)—

Light- to dark-gray variably porphyritic basaltic 

andesite lava flows (b) and intrusive rocks (bi). Phenocrysts: ol and pl with or without aug

bcc Basalt of Castle Creek (middle or early Pleistocene)—

Medium-gray porphyritic basalt (HAOT; 47.5% 

SiO2) lava flows exposed in Castle Creek near confluence with Little Castle Creek. Phenocrysts: ≤5% ol 

(≤5 mm). Based on paleomagnetic orientation (D.E. 

Champion, written commun., 2007) and chemical 

composition (Bacon, 1990), lava of unit bcc is provisionally correlated with HAOT flows west of map 

area along Rogue River near Farewell Bend, which 

may be ~1 Ma in age (K-Ar age 1.25±0.11 Ma near 

Prospect; Fiebelkorn and others, 1982). Undated

bln Basaltic andesite northwest of Little Castle Creek 

(early? Pleistocene)—Medium-gray porphyritic 

basaltic andesite (54.5–55.5% SiO2

) lava on north 

wall of valley of Castle Creek west of confluence 

with Little Castle Creek southwest of the caldera. 

Phenocrysts (5–10%): pl (≤1 mm) and ol (≤2 mm) in 

a pilotaxitic groundmass. Microxenoliths (≤4 mm) 

of pl ± ol uncommon. Undated. Overlies unit bac; 

overlain by dcn and blcp

bcn Basalt north of Castle Creek (early? Pleistocene)—

Medium-light-gray porphyritic basalt flows north 

of Castle Creek near west boundary of map. Phenocrysts (~15%): pl (≤1 mm, rarely 2 mm; most 

in aggregates), ol (<3 mm; abundant), and aug (≤1 

mm; minor) in an intergranular groundmass. Aggregates or microxenoliths (≤3 mm, rarely to 7 mm) of 

pl ± ol ± aug abundant. Overlain by unit acc (K-Ar 

age: 587±18 ka). Undated

bac Basaltic andesite of Castle Point (early? Pleistocene)—

Medium-gray sparsely porphyritic basaltic andesite 

(52.5–54% SiO2

) forming glaciated ridge of Castle 

Point (below ~6,100 ft elev) southwest of caldera 

and nearby lower part of canyon wall north of Castle 

Creek (~5,200 ft elev). Source probably was north 

of Castle Point. Phenocrysts (≤3%; 30–40% seriate microphenocrysts): pl (≤0.5–1 mm; rare sieved 

xenocrysts to 2.5 mm) and ol (≤1.5 mm) in a pilotaxitic to intergranular groundmass. Ubiquitous aggregates or microxenoliths (≤5 mm; rarely to 2 cm) of 

ol + pl ± Fe-Ti oxide appear to be source of many 

ol phenocrysts. Overlain by units bln, bwc, bu, and 

bc. Undated 

buc

buci

Basaltic andesite of Union Creek (Pleistocene or Pliocene)—Medium-gray to brownish-gray porphyritic 

basaltic andesite (52–55% SiO2) lava flows (buc; 

locally includes cinders north of Rocktop Butte) 

and intrusive rock (buci) near the headwaters of 

Union Creek in southwest corner of map. Unit consists mainly of lavas and intrusive core of dissected 

Rocktop Butte volcano but also includes related(?) 

intrusion of south-facing slopes northwest of Varmint Creek, dike near Hill 5811 1.1 km southeast 

of Varmint Camp, and patch of diktytaxitic basalt at 

5,400 ft elev 1.0 km south of Varmint Camp. Phenocrysts (≤5%, excluding abundant seriate pl microphenocrysts): ol (≤2 mm) with or without pl (≤1 

mm, rarely to 2 mm). Augite phenocrysts (≤2 mm) 

present in a few samples probably derived from aug 

± ol microxenoliths (to 7 mm; abundant in flow at 

bend in road 1.1 km north of Rocktop Butte). Overlain by unit bu. Undated

Mount Mazama

r

rb

Rhyodacite of the postcaldera dome (Holocene)—

Finely porphyritic medium-gray rhyodacite (71.5% 

SiO2) lava (r) and breccia (rb) of small dome reaching within 27 m of the lake surface east-northeast 

of Wizard Island (Bacon and others, 2002). Phenocrysts (~15%): euhedral pl (≤3 mm, most ≤1.6 

mm; rarely sieved or resorbed), opx (≤1.0 mm), hbl 

(≤2.5 mm, most ≤0.7 mm), Fe-Ti oxides (≤0.2 mm), 
21

and rare biotite (≤0.3 mm) in a glassy groundmass. 

Aggregates (≤2 mm) of pl ± opx ± hbl ± oxides 

sparse. Nearly all phenocrysts appear to have been 

in equilibrium with melt. This is the youngest, most 

differentiated lava in the map area and the only one 

in which biotite has been identified. Overlies units 

apc and aw. Correlative ash in sediment core from 

top of central platform has a 14C age of 4,240±290 

radiocarbon yr B.P. (Nelson and others, 1994) or 

~4,800 calendar yr B.P. (Stuiver and others, 1998)

aw

awb

awp

Andesite of Wizard Island (Holocene)—Porphyritic, 

commonly seriate textured, dark-gray to grayishblack blocky andesite (58.5–60% SiO2

) lava flows 

(aw) and breccia (awb). Lava forming Wizard 

Island was extruded from vents at base of cinder 

cone (awp) and flowed into lake, forming pillowed 

flows and glassy breccia mantling the slopes below 

the lake’s surface (Bacon and others, 2002). Prominent increase in slope below depth of ~75 m indicates 

lake level at time of final eruptions. Edifice of Wizard 

Island rises ~550 m above the lake floor, the cinder 

cone adding another ~200 m. Phenocrysts (~20%; 

~50% when microphenocrysts are included): pl (≤4 

mm; commonly sieved or resorbed), aug (≤1.2 mm), 

opx (≤1.0 mm), Fe-Ti oxides (≤0.3 mm), and sparse 

ol (≤0.5 mm, rarely to 2 mm; typically with pl) in 

very fine grained or glassy groundmass rich in ≤0.4 

mm pl crystals. Some samples contain rare resorbed 

hbl (≤5 mm). Aggregates (up to at least 5 cm) of pl 

± ol + aug + opx ± oxides + melt common locally, as 

are medium-gray subangular xenloliths (to at least 

10 cm) of fine-grained pl-rich igneous rocks. The 

xenoliths may be partially melted and commonly 

have hornfelsic textures. Partially melted quartzbearing granitic xenoliths present locally. Samples 

from presently exposed Wizard Island tend to have 

coarser microphenocrysts and more fully crystallized groundmasses than samples from the slopes 

below 75 m depth. Composition of most SiO2

-rich 

samples of aw is virtually identical to those of amc

and some apc. Overlain by unit r. Age constrained 

by inference that drowned subaerial lava flows were 

erupted when lake level was ~75 m lower than 

today, which implies eruption within at most ~500 

years of caldera collapse, or ~7,200 yr B.P. (Bacon 

and others, 2002). This age is consistent with paleomagnetic measurements and secular variation history and with the ~4,800 yr B.P. age of unit r

amc

amcb

Andesite of Merriam Cone (Holocene)—Porphyritic 

medium- to dark-gray blocky andesite (60.5% SiO2

) 

lava (amc) and breccia (amcb) of 430-m-high subaqueous cone south of Pumice Point (Bacon and 

others, 2002). Phenocrysts (~25%): pl (≤3 mm, commonly sieved or moderately resorbed), aug (≤1.3 

mm), opx (≤1.5 mm), Fe-Ti oxides (≤0.3 mm), and 

rare ol (≤0.4 mm, commonly with pl) in a glassy 

groundmass rich in pl crystals ≤0.2 mm long. Small 

aggregates (~1 mm) of pl + aug + opx ± oxides ± melt 

± resorbed ol common. Fracture patterns and lack of 

oxidization of blocks suggest eruption that built Merriam Cone was subaqueous. Compositional similarity to lavas of units apc and aw suggests Merriam 

Cone was active during the same interval. Relations 

between probable water depth and likely rate of filling of lake also suggest that Merriam Cone is only a 

few hundred years younger than the caldera

apc

apcb

Andesite of the central platform (Holocene)—Porphyritic andesite (57.5–62.5% SiO2

) lava (apc) and 

breccia (apcb) of 300-m-high flat-topped mound 

beneath west-central part of lake and lava-flow fields 

to north and east (Bacon and others, 2002). Central 

platform lavas are known from limited sampling by 

submersible and dredging. Four lava samples from 

the north flank of the platform are medium-gray 

silicic andesites (~62.5% SiO2) with ~20% phenocrysts: pl (≤4 mm, rarely 6 mm; rounded, some 

sieved), ol (≤1.5 mm), aug (≤1.3 mm), opx (≤1.6 

mm), and Fe-Ti oxides (≤0.5 mm) in a fine-grained 

pilotaxitic groundmass. Aggregates (≤6 mm) of pl 

± ol + aug + opx + oxide ± melt common. Andesitic 

enclaves (≤1.5 cm) may be present. Samples from 

>350 m depth altered medium light to light greenish gray. Three samples from the southeast slope 

range from mafic andesite to andesite (57.5–61.5% 

SiO2). The medium-gray mafic andesite contains 

~7 percent phenocrysts: pl (≤3 mm, finely sieved), 

ol (≤1.8 mm), and sparse aug (≤2 mm) in an intergranular groundmass. Andesites of the southeast 

flank contain 15–30 percent phenocrysts: pl (3.5 

mm, many coarsely sieved), aug (≤2.0 mm), opx 

(1.8 mm), and Fe-Ti oxides (≤0.3 mm) in a finegrained groundmass. Rare ol (≤1.0 mm) occurs in 

the less silicic sample (60.5% SiO2), which has pl 

with seriate texture and resembles lavas of unit aw. 

A single large dredge sample from the south flank 

is markedly crystal-rich medium-gray andesite 

(60.5% SiO2) with ~50% phenocrysts: pl (≤3 mm, 

rarely to 6 mm; blocky), aug (≤1.6 mm, rarely to 5 

mm), opx (≤1.4 mm), hbl (≤4 mm, resorbed and oxidized), and Fe-Ti oxides (≤0.3 mm) in a heterogeneous glassy groundmass rich in ≤0.4 mm crystals. 

Common enclaves (≤2 cm) are rich in ≤0.4-mmlong pl laths similar to those locally abundant in lava 

groundmass. Aggregates or microxenoliths (≤1 cm) 

of pl + aug + opx + oxides ± glass abundant. Finegrained pl-rich xenoliths similar to those in unit aw

present in south and some southeast flank samples. 

Moderately porphyritic seriate-textured dark-gray 
22

andesite (60% SiO2) sample from northern central 

platform deep flow field has ~15 percent phenocrysts (~40% including microphenocrysts): pl (≤3 

mm; many sieved or resorbed), ol (≤1.5 mm), aug 

(≤1.3 mm), opx (≤1.1 mm), and Fe-Ti oxides (≤0.2 

mm; associated with pyroxenes) in a fine-grained 

groundmass containing abundant ≤0.4 mm microphenocrysts. Contains aggregates (≤7 mm) of pl + 

aug + opx ± oxides ± melt and basaltic andesite(?) 

enclaves (≤5 mm). Fossil shorelines, identified 

by slope breaks between subaerially erupted lava 

flows and subaqueous breccia slopes, indicate that 

the central platform volcano was active during the 

filling of Crater Lake, concurrently with the early 

eruptions of the Wizard Island volcano (Bacon and 

others, 2002). The north and east flow fields below 

the slopes of the central platform are subaqueous lava flows that were fed by lava issuing from 

a vent at the west end of the platform, flowing in 

prominent channels or tubes, and cascading down 

the slopes to the deep caldera floor. Models for filling Crater Lake suggest that eruption of the central 

platform volcano ceased within at most ~200 yr 

after collapse of the caldera

ae Andesite of the east basin (Holocene)—Probable lava 

flows extending 1.7 km south of east basin in Crater 

Lake. Likely vent obscured by unit ls south of 

mapped extent of ae. Inferred to be andesite lava 

by Bacon and others (2002) on the basis of bathymetry and acoustic backscatter intensity. Undated, but 

must be younger than caldera collapse. Appears to 

be overlain by unit apc

Deposits of the climactic eruption of Mount Mazama 

(Holocene)—Pumice, scoria, and lithic blocks, 

lapilli, and ash deposited by pyroclastic falls and 

flows during the caldera-forming, climactic eruption 

of Mount Mazama (Williams, 1942; Bacon, 1983). 

Explosive eruption of ~50 km3 of magma, mainly 

compositionally uniform rhyodacite, from a shallow 

reservoir over perhaps a few days was accompanied by collapse of Crater Lake caldera. Ash from 

this eruption fell throughout much of the Pacific 

Northwest and well into southern Canada. 

cu Fine-grained lithic- and crystal-rich ignimbrite—

Covers large areas of the slopes of Mount Mazama, 

pre-Mazama rhyodacite lavas and domes, and the 

Union Peak volcano and vicinity. Most cu consists 

of either a few meters of late-deposited ignimbrite 

veneer (Druitt and Bacon, 1986) or thicker deposits 

of fine-grained ignimbrite near the heads of valleys; 

unit also includes areas of fine-grained lithic breccia, pumiceous or scoriaceous ignimbrite, and fall 

deposits that are too small or poorly exposed to map 

separately. Where exposed in section, the primary 

deposit is gray or reddish-brown, fine-grained crystal-rich nonwelded or, rarely, indurated ignimbrite 

that either overlies cb on slopes and interfluves or 

grades laterally into cb or cf in valleys. 

cd Undivided climactic unit—Shown only on the 

Bedrock Map (sheet 3); includes all deposit types. 

On the 1:24,000-scale geologic map (sheet 1) climactic deposits are divided into five mapped units that 

document the change in eruptive and depositional 

modes during the course of the event (Bacon, 1983): 

(1) extensive Plinian pumice-fall deposit (cp) from a 

towering column rising from a single vent, followed 

by (2) welded ignimbrite (cw, Wineglass Welded 

Tuff) from pyroclastic flows recording the collapse 

of the Plinian column, and finally, (3) voluminous 

compositionally zoned, valley-filling ring-vent-phase 

ignimbrite (cf), fine-grained lithic- and crystal-rich 

ignimbrite (cu), and lithic breccia (cb) from pyroclastic flows fed by a ring-fracture vent system as the caldera collapsed. Where shown on map, deposits of each 

unit are exposed or are just beneath reworked surficial 

material; true extents of units cb and cw likely to be 

significantly greater than suggested by mapped exposures because of cover by units cf and cu.

 Juvenile magmatic clasts (Bacon and Druitt, 

1988; Druitt and Bacon, 1989) and rare variably 

fused granitoid fragments (0–50% melt; Bacon, 

1992) are present in all units with the exception that 

the most mafic scoriae are absent from cp and cw. 

Many juvenile clasts are compositionally banded 

(more correctly, layered). All pumices with between 

61 and 70 percent SiO2

 contain streaks or blebs of 

scoria; many scoriae consist of two or more scoria 

lithologies interlayered, with or without streaks of 

rhyodacitic pumice. Most banding is due to incomplete syneruptive mixing, but some banding in scoriae reflects primary modal layering in cumulate 

mush. Rhyodacitic pumice of the climactic eruption (~70.5% SiO2

) is effectively homogeneous and 

identical in mineralogy and composition to rhyodacite of the Cleetwood flow. Phenocrysts (~10–15%): 

pl (≤1.5 mm, rarely to 3 mm), opx (≤1 mm, rarely 2 

mm), aug (≤1 mm), hbl (≤0.5 mm, less commonly to 

1 mm), and Fe-Ti oxides (≤0.4 mm) in vesiculated 

glass (typically ~72–72.5% SiO2). Crystal aggregates (≤3 mm) of pl + opx ± aug ± oxides common. 

Scoriae of the climactic eruption vary substantially 

in chemical and modal composition. Two types are 

distinguished on the basis of concentrations of trace 

elements incompatible in mantle minerals, here 

characterized by Sr. The Sr content of groundmass 

glass reflects the high- or low-Sr affinity of the bulk 

rock. A third type is recognized by abundant ol and 

aug. All types have aggregates or microxenoliths 

 
23

of the same phases present as discrete phenocrysts, 

although these are less abundant in high-Sr scoriae 

than in other types. High-Sr scoria (53–61% SiO2) 

is present in all eruptive units, abundant in mixed 

cf ignimbrite, and the most voluminous of the three 

scoria types. Homogeneous clasts contain 1,400–

1,800 ppm Sr at 54 percent SiO2 and 1,000–1,400 

ppm Sr at 60 percent SiO2. There appear to be at 

least two subtypes that can be distinguished by their 

incompatible element concentrations. Phenocrysts 

(28–51%): (1) abundant pl (≤2 mm) and prismatic 

hbl (≤4 mm, commonly smaller) and (2) uncommon 

to rare aug (≤2 mm), opx (≤1 mm), and Fe-Ti oxide 

(≤0.3 mm) in vesiculated glass (61–71% SiO2). LowSr scoria (51–60% SiO2

) is known to be present 

in mafic cf ignimbrite, cu, and cb. Homogeneous 

clasts contain ~800 ppm Sr at 54 percent SiO2 and 

~600 ppm Sr at 60 percent SiO2. Phenocrysts (50–

66%): pl (≤2 mm; commonly to 3 mm), aug (≤1.5 

mm; commonly to 2.5 mm), opx (≤1.5 mm), and FeTi oxides (≤0.5 mm) in vesiculated glass (63–72% 

SiO2); blocky, commonly oikocrystic hbl (≤4 mm) 

may be present. Rare low-Sr scoriae lack pyroxene 

and have pl, prismatic hbl, and Fe-Ti oxide phenocrysts. Olivine + pyroxene-rich scoria (47–53.5% 

SiO2, up to 25% MgO) has modal ol and compositions that reflect its mafic, cumulate nature. Modal 

layering commonly is visible. Phenocrysts (~60%): 

ol (≤3 mm) and aug (≤3 mm; rarely to 1 cm) with or 

without some or all of hbl (≤6 mm; rarely to ~2 cm; 

commonly oikocrystic), pl (≤2 mm; rarely to 5 mm), 

opx (≤2 mm), or Fe-Ti oxide (≤0.3 mm; uncommon). Ol-bearing microxenoliths or aggregates (≤6 

mm) ubiquitous. Both high-Sr and low-Sr affinities 

have been identified on the basis of glass analyses. 

 Many radiocarbon ages have been published 

that purport to date the climactic eruption of Mount 

Mazama. Because of the importance of Mazama 

ash as a stratigraphic marker, the age of this event 

is likely to be revised in the future. For this reason, 

this publication quotes the weighted mean age of 

charcoal twigs from units cp and cf given in Bacon 

(1983): 6,845±50 yr B.P., from which a calibrated 

age of ~7,700 calendar yr B.P. can be obtained 

(Stuiver and others, 1998)

cb Lithic breccia—Proximal facies of climactic ignimbrite is widely exposed at the caldera rim, on slopes 

of Mount Mazama <12–15°, and in the heads of deep 

valleys south and east of the caldera (Bacon, 1983; 

Druitt and Bacon, 1986). It also may be found as far 

as at least 18 km from the caldera rim on hills and 

interfluves but grades abruptly into cf in valleys 2–11 

km from the rim. The base is only exposed at the caldera rim, where cb is as thick as 20 m. Lithic breccia 

locally fills channels cut into cp (Cleetwood Cove) 

or may grade vertically into cw (Pumice Point) or 

nonwelded pumiceous ignimbrite (Llao Bay); also 

forms irregular concentrations in cu and at the 

margins of valleys containing cf (Goodbye Creek). 

Semicircular, arcuate, and linear bedforms of 1–10 m 

amplitude are preserved near topographic obstacles, 

abrupt changes in slope, and valley margins (crests 

of larger bedforms indicated on map). The typically 

massive, clast-supported deposit consists of a variety of types of lithic fragments within a sand-sized 

matrix of lithic fragments and crystals that is poor 

in vitric ash. Many blocks are cracked or shattered, 

yet coherent. Maximum clast size is ~3 m high on 

the slopes of Mount Mazama but several clasts measuring 5–7 m are known. Clast lithologies correlate 

with geographic position around the caldera (Bacon, 

1983; Suzuki-Kamata and others, 1993), demonstrating that pyroclastic flows were derived from eruption columns from a ring-fracture vent system. Some 

clasts are locally derived, many can be assigned to 

units exposed in the caldera walls, and still others 

are from deeper levels, including granodiorite and 

related rocks from the apparent walls of the magma 

chamber (Bacon, 1992; Bacon and others, 2000; 

Bacon and Lowenstern, 2005). The corners of many 

clasts are rounded owing to thermal spalling, clasts 

retain the climactic eruption thermoremanent magnetization direction, and localized fumarolic alteration 

of clasts and matrix is present even where the deposit 

rests on glaciated lava. The matrix is compositionally 

zoned in juvenile lapilli and blocks in the same vertical and caldera-radial, rhyodacite-andesite-mafic 

sense as unit cf; common at the top of the deposit 

within a few kilometers of the caldera rim are accumulations of dense, mafic, crystal-rich scoria blocks. 

Although a complete section of cb lying on cw, 

cp, and Cleetwood and Llao Rock pumice falls is 

exposed at the caldera rim above Skell Head (Druitt 

and Bacon, 1986, fig. 5), good examples of cb can be 

visited with comparative safety in roadcuts near Park 

Headquarters. 

cf Ring-vent-phase ignimbrite—Partially fills valleys 

and depressions on the flanks of Mount Mazama and 

is present well beyond the map area, extending up to 

~70 km from the caldera (Williams, 1942; Bacon, 

1983; Druitt and Bacon, 1986; see fig. 2 for distribution map). The deposit consists of poorly sorted 

rhyodacitic pumice (70.5% SiO2) and crystal-rich 

andesitic to mafic-cumulate scoria (61–47% SiO2; 

up to 25% MgO) clasts, vitric ash, crystals, and 

lithic fragments. Within map area, unit is mainly 

medial facies (Bacon, 1983; as opposed to proximal 

facies, unit cb), comprising silicic ignimbrite (>80% 
24

rhyodacitic clasts) grading up into mixed ignimbrite to mafic ignimbrite (<20% rhyodacitic clasts) 

(Druitt and Bacon, 1986). The zonation in wellexposed sections appears smooth and well-defined 

flow unit boundaries are lacking, suggesting continuous aggradation from a pyroclastic-flow stream. 

Well-exposed medial-facies sections in canyons 

range from nonwelded, buff silicic through darkgray mafic tuff to partly welded light-gray silicic 

through dark-gray mafic tuff. Partly welded tuff 

has pumice flattening ratios of 1:2.5–4 and widely 

spaced columnar joints that cut juvenile clasts. The 

upward color transition of the tuff may be sharp or 

gradational and does not appear to be associated 

with a compositional or depositional break but is 

the product of increasing emplacement temperature 

of the matrix of successively deposited ignimbrite. 

The best zoned sections (as much as 110 m thick) 

are in Castle, Annie, Sun, and Sand Creeks ~5–20 

km from the caldera rim. The amount of mixed and 

mafic ignimbrite decreases with distance from the 

caldera and towards valley margins. Buff to white 

distal-facies ignimbrite, present mainly beyond the 

map area, is nonwelded and nonrhyodacitic juvenile 

clasts are rare or absent. Distinct flow unit boundaries are uncommon; carbonized wood is abundant 

at sufficient distance from source that emplacement 

temperature was low. Fumarolic pipes or sheets are 

abundant in medial-facies deposits and in distal 

facies where wet ground was overridden. Secondary 

crystallization of medial-facies tuff has created erosion-resistant curtains and pinnacles in many canyons; a white, bleached zone of fumarolic alteration 

occurs at some localities beneath capping fine ash. 

Secondary explosion craters and related surge and 

fall deposits are present in nonwelded tuff that was 

deposited in wet meadows or river valleys.

cw Wineglass Welded Tuff of Williams (1942)—Present 

in topographic depressions at the caldera rim from 

Pumice Point clockwise to Skell Head and locally 

south of Pumice Castle and west of Llao Rock 

(Bacon, 1983; Kamata and others, 1993). The tuff 

is locally exposed in valleys to the north and east 

of Crater Lake to approximately the limits of the 

map area. The pinkish-orange to light-brown, partly 

welded to densely welded ignimbrite is as much as 10 

m thick in paleovalleys at the caldera rim and is thin 

to absent on topographic highs. It consists of up to 

four flow units that form a single cooling unit. Juvenile clasts are rhyodacite (>99%) and rare andesite. 

Lithic components suggest Wineglass pyroclastic 

flows came from a collapsed column that issued 

from an enlarged, single vent that had produced the 

Plinian phase of the eruption. The distribution of unit 

cw indicates that the vent was north of the summit 

of Mount Mazama, in the northeast quadrant of the 

area of caldera collapse. Wineglass pyroclastic flows 

eroded and locally disrupted underlying unit cp.

cp Plinian and other Holocene pumice-fall deposits—

Thickest to the north and east of the inferred vent 

location northeast of the caldera center. As described 

by Young (1990), the deposit consists of a basal phreatomagmatic ash bed (≤15 cm thick), lower pumice 

subunit with early dispersal to the east and later to the 

north, divider ash bed (<10 cm), and reversely graded 

upper pumice subunit; the divider ash and upper 

pumice subunit have northeast dispersal axes. Juvenile clasts in the lower and upper pumice subunits are 

>99 percent very pale orange rhyodacite; medium- to 

dark-gray andesitic scoria (59–61% SiO2) is uncommon to rare. The two subunits are each typically a 

few meters thick near their dispersal axes in the map 

area. Proximal exposures of both subunits contain 

numerous nonwelded pyroclastic-flow beds that 

result in local thickness of either subunit of as much 

as 20 m; at many localities unknown thicknesses of 

unit cp have been removed by later pyroclastic flows. 

The divider ash bed of medial localities is believed 

to be correlative with proximal pyroclastic-flow 

deposits at the top of the lower pumice subunit. A 

consistent population of lithic fragments through the 

entire thickness of pumice fall indicates a single vent 

(Suzuki-Kamata and others, 1993). Upward coarsening of pumice clasts in the upper subunit suggests 

this phase of the eruption ended when the Plinian 

column was no longer stable and collapsed owing 

to increasing eruption rate (Bacon, 1983; Young, 

1990). On the geologic map, unit cp includes other, 

less voluminous Holocene pumice fall deposits (Llao 

Rock and Cleetwood) that are impractical to separate 

at map scale.

rh

rhv

rhc

rhp

Holocene preclimactic rhyodacite (Holocene)—Porphyritic rhyodacite of the Llao Rock and Cleetwood 

flows of Williams (1942) and related pyroclastic 

deposits and dikes in the north caldera wall (see 

panoramas, sheet 4). Lava outcrops of medium-gray 

pumiceous carapace (rhc), dark-gray to black obsidian (vitrophyre; rhv), and medium-light-gray felsite 

(rh) mapped separately where possible; one of the 

two dikes (rh) below Llao Rock is shown on panoramas (second is hidden by promontory); pumicefall deposits (rhp) are shown where sufficiently 

thick. Eruptions at both vents began with Plinian 

explosions that left coarse, poorly-sorted, lithic-rich 

deposits near vent and extensive well-sorted pumicefall deposits for great distances downwind to the 

southeast and ended with extrusion of thick lava 

flows. The Llao Rock pumice fall is lighter gray and 
25

pumice clasts tend to be denser than those of the typically coarser Cleetwood pumice fall; clasts in both 

units are denser than those of the overlying climactic 

pumice fall. Llao Rock and Cleetwood pumice-fall 

deposits are included with climactic pumice fall on 

map. Further information on the stratigraphy, distribution, and character of the fall deposits may be 

found in Young (1990). The Llao Rock fall is correlative with the Tsoyowata Bed of Davis (1978, 1985) 

in Nevada and eastern California. The Cleetwood 

pumice-fall deposit is not readily distinguished from 

the compositionally identical climactic pumice fall 

in distal exposures. Good exposures of Llao Rock 

and Cleetwood pumice falls may be visited on the 

trail to the boat landing where ~1 m of Llao Rock 

pumice fall rests on glacial till and is overlain by 

~1 m of Cleetwood pumice fall containing abundant, large lithic blocks, overlain in turn by a much 

greater thickness of climactic pumice fall. Llao Rock 

lava flow fills the explosion crater marking its vent. 

Pumice-fall deposit lining crater is increasingly compacted and fused upward against base of lava flow. 

A great wedge of Llao Rock pumice fall underlies 

the east wing of the Llao Rock lava flow in the caldera wall. Small exposures of Cleetwood pumice 

fall occur beneath the Cleetwood lava flow on either 

side of the “backflow” above Cleetwood Cove. First 

described by Diller (Diller and Patton, 1902), the 

“backflow” is a tongue of lava remobilized from the 

thick interior of the Cleetwood flow that oozed down 

the caldera wall immediately following caldera collapse (Bacon, 1983; Kamata and others, 1993); far 

greater masses of Cleetwood lava slid to the northeast during collapse of the caldera, presumably due to 

vigorous and prolonged seismic shaking, during the 

climactic eruption. Vent for Cleetwood eruption was 

a few hundred meters northeast of the caldera rim 

at Cleetwood Cove. Products of the Llao Rock and 

Cleetwood vents are described separately because 

they are chemically and petrographically distinct 

(Bacon and Druitt, 1988; Druitt and Bacon, 1989). 

 Llao Rock deposits—Rhyodacite of the Llao 

Rock flow, pumice, and dikes ranges from ~70.5 

to 72 percent SiO2

. The entire range appears to be 

represented by the pumice fall, the earliest-erupted 

pumice being the most silicic; marginal vitrophyres 

of the two dikes in the caldera wall are comparable 

to early-erupted pumice; the lava flow typically has 

~70.5 percent SiO2. The rhyodacite contains ~7 percent phenocrysts of pl (≤1.5 mm, rarely to 3 mm), 

hbl (≤1 mm), opx (≤1 mm), Fe-Ti oxides (≤0.4 mm), 

and uncommon aug; hbl is more abundant (hbl≥opx) 

and phenocrysts are relatively small in the dikes and 

early-erupted pumice. Groundmasses range from 

vitric (pumice, vitrophyre) to finely crystalline 

(felsite). Crystal aggregates (≤3 mm) of pl + hbl ± 

oxides or pl + opx ± oxides common. Light-brownishgray andesitic enclaves (≤3 cm; rarely to 10 cm) are 

ubiquitous and abundant (~1%) in the lava flow; they 

commonly are present in the pumice fall as mediumgray pumiceous blobs in pumice or as separate clasts. 

Most enclaves have a total of 1–10 percent hbl (≤1 

cm) and pl ± ol ± aug ± opx phenocrysts (all generally 

≤4 mm); rare enclaves have up to ~25 percent of the 

same phases. Overlies glacial till and many Pleistocene units of Mount Mazama; overlain by Cleetwood 

pumice fall. Charcoal from beneath the Llao Rock 

pumice fall yielded a 14C age of 7,015±45 radiocarbon 

yr B.P. (Bacon, 1983) or 7,800–7,900 calendar yr B.P. 

(Stuiver and others, 1998).

 Cleetwood deposits—Rhyodacite of the 

Cleetwood flow and pumice is homogeneous and 

identical in mineralogy and composition to rhyodacite of the climactic eruption (~70.5% SiO2

). Phenocrysts (~10%): pl (≤1.5 mm, rarely to 3 mm), opx 

(≤1 mm, rarely 2 mm), aug (≤1 mm), hbl (≤0.5 mm, 

less commonly to 1 mm), and Fe-Ti oxides (≤0.4 

mm). Crystal aggregates (≤3 mm) of pl + opx ± aug 

± oxides common. Medium-gray andesitic enclaves 

(≤3 cm, rarely to ~10 cm) extremely rare. Enclaves 

have phenocrysts of pl (≤5 mm), hbl (≤3 mm), aug 

(≤1 mm), and opx (≤1 mm). Groundmasses range 

from vitric (pumice, vitrophyre) to finely crystalline 

(felsite). Cleetwood pumice fall overlies Llao Rock 

pumice fall; overlain by climactic pumice fall (cp). 

Obsidian of Cleetwood flow oxidized reddish brown 

where overlain by climactic pumice fall. Because 

Cleetwood flow was hot and interior was capable 

of flow at time of climactic eruption (Bacon, 1983; 

Kamata and others, 1993), age is within uncertainty 

of 6,845±50 yr B.P. radiocarbon date for climactic 

eruption (Bacon, 1983; 7,675 +20/-60 calendar yr 

B.P.; Stuiver and others, 1998)

rs Rhyodacite of Sharp Peak (late Pleistocene)—Porphyritic medium-gray rhyodacite (70.5% SiO2

) lava 

composing 12 small domes northeast of the caldera in 

two ~N.-35°-E.-trending en echelon arrays, 4–7 km 

northeast of Wineglass. Phenocrysts (~10%): pl (≤3 

mm, most ≤2 mm), opx (≤1 mm, rarely 2 mm), aug 

(≤1 mm, rarely 2 mm), Fe-Ti oxides (≤0.4 mm), and 

hbl (≤1 mm, uncommon) in a microvesicular pilotaxitic groundmass rich in ~0.1 mm pl microphenocrysts. 

Aggregates (≤4 mm) of pl ± opx ± aug ± oxides and 

andesitic enclaves (≤1 cm, rarely to 3 cm) common. 

Compositionally identical to climactic rhyodacite. 

Microphenocrysts probably formed during transport 

in dike(s). Lightly glaciated during last Pleistocene 

ice advance. 40Ar/39Ar isochron age 18±4 ka
26

ab

abp

abt

Andesite south of Bear Bluff (late Pleistocene)—Porphyritic medium-dark-gray andesite (58–59.5% SiO2

) 

capping lava (ab), oxidized cinders and bombs (abp) 

of subaerial vent at southeast top of tuya, and palagonitic tuff (abt) containing bombs as large as ~60 

cm across and underlying lava of tuya south of Bear 

Bluff, south-central part of map. Low hills to east 

and southeast of tuya are blocks of abt that appear 

to have slid away from tuya, perhaps when adjacent 

glacial ice receded. Jointing in glassy lava outcrops 

on southwest slope of tuya suggests chilling against 

ice or meltwater. Phenocrysts (~7%; also contains 

moderately abundant pl microphenocrysts): pl (≤4 

mm; commonly sieved), ol (≤0.5 mm, rarely to 1.5 

mm), opx (≤1 mm), aug (≤0.5 mm, uncommon), and 

Fe-Ti oxide (≤0.5 mm, uncommon) in a pilotaxitic 

groundmass. Andesite bombs streaked with rhyodacitic pumice and rare rhyodacite pumice blocks in 

unit abt near southeast corner of tuya indicate coeruption of two magmas. Similarly, lava outcrops 

(ab) commonly exhibit layers of coarsely porphyritic, vitric rhyodacitic or hybrid lava ~1 cm to ~1 m 

thick. Composition of andesite is similar to those of 

enclaves in unit re. Overlies unit bsc. Coeval with 

unit rbb (40Ar/39Ar isochron age 24±3 ka). Erupted 

during last glacial maximum

rbb Rhyodacite of Bear Bluff (late Pleistocene)—Mediumlight-gray porphyritic rhyodacite (69.5% SiO2

) lava 

dome forming glaciated hill known as Bear Bluff at 

the north edge of tuya of unit ab in south-central part 

of map. Grayish-black vitrophyre present locally 

along south margin. Composition similar to, but 

slightly less differentiated than, rhyodacite of climactic eruption. Phenocrysts (~20%): pl (≤3 mm, 

few to 5 mm), opx (≤1 mm), subordinate aug (≤1.2 

mm), Fe-Ti oxides (≤0.3 mm), and rare, resorbed 

brown hbl (0.1–0.5 mm) in a microlite-bearing 

glassy or devitrified groundmass. Abundant aggregates (≤6 mm) of pl + opx ± aug ± oxides ± brown 

glass ± rare hbl are source of many phenocrysts. 

Rhyodacitic pumice from unit abt in south slope of 

tuya contains identical phenocrysts and aggregates. 

Coarsely porphyritic rhyodacitic or hybrid layers 

in lava of unit ab appear to be from same magma 

as rbb lava dome. Composition of rhyodacite can 

be explained by admixture of a small amount of ab

magma to rhyodacitic magma similar to unit re. Unit 

rbb has 40Ar/39Ar isochron age of 24±3 ka. Emplaced 

during last glacial maximum

re

rev

rec

rep

Evolved Pleistocene preclimactic rhyodacite (late 

Pleistocene)—Porphyritic rhyodacites (70–72% 

SiO2) of Redcloud and Grouse Hill flows of Williams 

(1942), dome at north caldera rim above Steel Bay, 

and related pyroclastic deposits that are the most 

chemically evolved (differentiated) of the Pleistocene 

preclimactic rhyodacites (Bacon and Druitt, 1988). 

Lava outcrops of medium-gray pumiceous carapace (rec), dark-gray obsidian (vitrophyre; rev), 

and medium-light-gray felsite (re; Panoramas E, I; 

sheet 4) mapped separately where possible. Pumicefall deposit (rep), locally exposed beneath Redcloud 

flow in caldera wall and on road to Redcloud Cliff 

viewpoint, probably is correlative with the Trego 

Hot Springs Bed of Davis (1978) (Rieck and others, 

1992). Phenocrysts (~15%): pl (≤3 mm, rarely to 5 

mm), hbl (≤2 mm, rarely to 5 mm), opx (≤1.5 mm), 

aug (≤1.5 mm), Fe-Ti oxides (≤0.5 mm), and traces 

of zircon (≤0.2 mm). Microxenoliths (≤6 mm) of pl ± 

hbl ± opx ± aug ± oxides common. Many phenocrysts 

are similar to crystals in microxenoliths. Partially 

resorbed rims (≤0.2 mm thick) on plagioclase and 

tiny rods of Fe-oxide in plagioclase cores are characteristic of this unit. Zircon and most phenocrysts 

derived by disaggregation of late Pleistocene granodiorite (Bacon and Lowenstern, 2005). Groundmass 

textures range from vitric (pumice, vitrophyre) to 

finely crystalline (felsite). Andesitic enclaves (2–25 

cm; uncommon) present in Steel Bay and Grouse 

Hill domes. Vents located beneath Grouse Hill and 

Steel Bay domes and Hill 7960 east of Redcloud 

Cliff. Overlies units aa, dc, and asb; overlain by 

rhp. Many exposures of unit rev on the flanks of the 

Grouse Hill and Redcloud flows and of the latter in 

the caldera wall are jointed with 10–15-cm-diameter 

subhorizontal columns indicative of chilling by contact with ice and meltwater; spectacular examples 

may be seen at the east end of the Redcloud flow. 

All three extrusions have been glaciated. Paleomagnetic orientations (D.E. Champion, written commun., 

1983) and unique petrographic features indicate the 

three lava flows were erupted in a short time interval, 

most likely during the Mono Lake paleomagnetic 

excursion ~30 ka. A radiocarbon date on organic 

carbon beneath Trego Hot Springs Bed (Davis, 

1978), correlated with pumice-fall deposit beneath 

and related to Redcloud flow (Rieck and others, 

1992), is 23,200±300 yr B.P. or a reservoir-corrected 

calibrated age of ~27,000 calendar yr B.P. (Benson 

and others, 1997)

mw Mingled lava of Williams Crater (late Pleistocene)—

Coarsely porphyritic light-gray dacite interlayered 

(mingled) with dark-gray hybrid andesite (60.5–67% 

SiO2

) forming small dome immediately west of Rim 

Drive, two small lava flows at the south and east summits of Williams Crater cinder cone, and a lava flow 

extending 1.2 km west from near the west base of the 

cone. Assigned to Mount Mazama because dacitic 

magma of unit was derived from that source. The 
27

sharply bounded layers range in thickness from <1 

mm to 3+ cm. Vents for the Williams Crater complex form a N. 75° W. trend, radial to the caldera. 

Phenocrysts (~30%) in dacite end-member: pl (≤5 

mm), opx (≤1.5 mm), aug (≤1 mm; less abundant 

than opx), hbl (≤0.6 mm; sparse), and Fe-Ti oxides 

(≤0.5 mm) in a fine-grained groundmass. Ol, pl, and 

(or) aug xenocrysts derived from basaltic andesite 

commonly present. Aggregates (≤1 cm) of pl + opx ± 

aug + oxide ± ap common. Gabbroic microxenoliths 

or aggregates are similar to those in bw lava, dense 

enclaves of gabbro-contaminated basaltic andesite 

(magma of unit bw; to 2 m) are present in all mw

lavas, and pl microphenocrysts increase in abundance with decreasing bulk SiO2

 content. All but 

most silicic layers in mw lavas are hybrid mixtures 

of dacite and gabbro-contaminated basaltic andesite 

end-members. Proportion of dacite in mingled lava 

appears to be greatest in dome from vent nearest 

caldera. Superposition of unit mw on bw, bwp, and 

bombs cored with angular blocks of mw indicate the 

following sequence of events (Bacon, 1990), which 

took place in a short interval of time constrained by 

identical paleomagnetic directions of both units and 

by petrologic data (McKnight and Bacon, 1992): (1) 

entrainment of gabbroic crystal mush within basaltic andesite magma (bw), (2) intersection of basaltic 

andesite dike with dacitic magma at west margin of 

shallow magma chamber and dispersal of fragments 

(enclaves) of gabbro-contaminated basaltic andesitic magma within dacite, followed by incomplete 

mixing to form layers (bands) of enclave-bearing 

dacite and hybrid andesite, (3) eruption of gabbrocontaminated basaltic andesite to form bulk of 

Williams Crater cinder cone (bwp) and lava flow to 

west (bw), and (4) eruption of viscous, mingled and 

hybrid lavas of unit mw. Overlies units bh, ah, bw, 

and dvb. Undated. Moderately glaciated. Dacite end 

member is compositionally similar to most silicic 

blocks in unit dvb and rare andesitic enclaves are 

similar to those in dv and dvb. These observations 

and smooth contact of bwp tephra on dvb at caldera 

wall south of Devils Backbone suggest similar age 

for unit mw, or about 35 ka

dvb

dv

Dacite of Munson Valley (late Pleistocene)—Unconsolidated fragmental deposits of porphyritic dacite (juvenile clasts contain 63.5–69.5% SiO2

; most <65%) 

mainly on the southwest flank of Mount Mazama. 

Unit dv is monolithologic breccia with mediumgray to grayish-black dense to pumiceous dacite 

clasts, is locally oxidized reddish brown or bleached, 

and occurs in the head of Munson Valley and on the 

northwest flank of Munson Ridge. Prismatically 

jointed clasts in this unit reach a length of 5 m near 

the headwaters of Castle Creek; fragments of such 

clasts typify other localities. Unit dv is interpreted to 

be an avalanche deposit formed by collapse of a lava 

dome high on the southwest flank of Mount Mazama. 

Emplacement temperature was high enough to cause 

oxidization of clasts and matrix but below Curie 

point of dacite. The apparently younger unit dvb is 

characterized by medium-light- to medium-darkgray, dense, intact prismatically jointed blocks ≤1 m 

across (rarely to 3 m), commonly is heterolithologic 

containing a variety of accidental lithic blocks, and 

occurs locally from the west slope of Garfield Peak 

(near 7,700 ft elev) to Devils Backbone. Unit dvb is 

thought to have been deposited by pyroclastic flows 

originating at the same source as and virtually contemporaneous with the monolithologic avalanches; 

prismatically jointed blocks in dvb were deposited 

hot, at temperatures above their Curie point. Both 

units can be seen in west-southwest to east-northeast road cut at 6,900 ft elev on road between Park 

Headquarters and Rim Village. Undivided deposits 

shown as unit dv on Bedrock Map (sheet 3). Phenocrysts (~15%, ~30% including crystals <0.4 mm) in 

dacite clasts: pl (≤5 mm, commonly resorbed with 

finely sieved zone and clear overgrowth), aug (≤1.5 

mm), opx (≤1.5 mm, more abundant than aug), and 

Fe-Ti oxides (≤0.3 mm); many samples contain 

minor hbl (≤1.0 mm). Very fine grained to glassy 

groundmass in all but the most silicic clasts is characterized by abundant 0.1–0.4 mm pl laths. Ubiquitous aggregates (≤1 cm) of pl + aug + opx + oxides 

+ glass are probably source of most phenocrysts. 

Smaller crystals may be derived from groundmass 

of common andesitic enclaves (≤20 cm; 60% SiO2

), 

which themselves contain resorbed aggregates identical to those in host dacite. Phenocrysts in the most 

silicic clasts are not resorbed and lack sieved zones. 

Textures and bulk compositions indicate that typical dacite of Munson Valley is a mixture of andesitic 

enclave magma and dacitic end member containing crystal aggregates. Overlies units db, aa, aww, 

ah, atw, and als; overlain by bwp south of Devils 

Backbone. Preservation commonly restricted to the 

lees of topographic highs, and rare striae on large 

clasts indicate units were glaciated. Although units 

are undated, prismatically jointed blocks in dvb

record an unusual paleomagnetic pole position similar to that of br, which has K-Ar and 40Ar/39Ar ages 

of 36±12 ka and 35±4 ka, respectively

als Andesite of Lightning Spring (late Pleistocene)—Porphyritic medium- to dark-gray andesite (61.5–65.5% 

SiO2; most <64%) lava flows at the caldera rim from 

west of Rim Village northwest to the Lightning 

Spring trailhead, where unit is well exposed in road 
28

cuts, and on the southwest flank of Mount Mazama 

as far as 4 km from the caldera in the drainage of 

Bybee Creek. Pumiceous vitric pillow-like blocks 

(to ~1.5 m) form surface overlain by till 1.3 km 

west of Discovery Point parking area as though 

lava flowed in meltwater channel beneath ice; alternatively, blocks are cauliflower bombs in a volcanic flowage deposit. Dike near lake level below 

Discovery Point mapped as unit als (Panorama 

A, sheet 4). Phenocrysts (20–30%): pl (≤5 mm but 

variable in maximum size so that ≤2 mm in many 

samples; some sieved or resorbed, some euhedral, 

many appear to be crystal fragments; seriate, ~0.1 

mm and larger), aug (typically ≤1.5 mm, rarely to 

4 mm), opx (typically ≤1 mm, rarely to 3 mm), and 

Fe-Ti oxides (≤0.5 mm) in a glassy or very fine 

grained groundmass. Ol crystals (≤0.5 mm, rarely 

to 3.5 mm) present in many flows may be euhedral 

or skeletal and have adhering quenched mafic melt 

containing pl laths or may be resorbed. Ubiquitous 

aggregates and microxenoliths (≤5 mm, rarely to 

1.2 cm) consist of pl + aug + opx ± oxides ± melt 

and rarely contain resorbed ol mantled by pyroxene; many phenocrysts appear to be derived from 

these materials. Rare hornfels xenoliths (~1–2 cm, 

rarely to 20 cm) present in a few samples. Enclaves 

(≤6 cm, rarely to 40 cm) rare. Unit includes youngest lava on the southwest side of Mount Mazama; 

overlain by unit dvb; overlies units aa, dcn, dwv, 

and atw; dwp intercalated with lowest als flows in 

caldera wall. K-Ar age: 47±8 ka. Flows of als that 

overlie unit atw are probably ~50 ka

asb Andesite of Steel Bay (late Pleistocene)—Porphyritic 

medium- to dark-gray andesite (59–61% SiO2

) lava 

flows high on the caldera wall between Llao Bay and 

Steel Bay (Panoramas D, E, F; sheet 4). Exposed 

locally north of caldera rim and along Rim Drive at 

west end of eastern-most turnout above Steel Bay. 

Phenocrysts (~25%): pl (≤3 mm, rarely to 5 mm; 

typically sharply euhedral, commonly with zone of 

coarse melt inclusions near rim; finely sieved pl also 

may be present), aug (typically <2 mm; larger crystals are oikocrysts up to 3 mm), opx (≤1.5 mm), and 

Fe-Ti oxides (≤0.4 mm; sparse to virtually absent) in 

a pilotaxitic groundmass. Rare ol (≤0.5 mm) present 

in some flows. Aggregates and microxenoliths (≤7 

mm) of pl + aug + opx ± oxide common. Uncommon 

small enclaves (≤2 cm) present in some exposures. 

Youngest andesite on north flank of Mount Mazama. 

Overlies units bs, bsp, apw, aww, dcp, dlp, and 

apu; overlain by re, rhp, and rh. K-Ar ages: 42±6 

ka and 43±6 ka

apu Andesite of Pumice Point (late Pleistocene)—Sparsely 

porphyritic medium-gray andesite (59.5–61% SiO2

) 

present locally above Steel Bay and fills paleovalley 

in unit dcp at Pumice Point in the north caldera wall 

where apu is highest lava (Panoramas F, G; sheet 

4). Phenocrysts (~3–7%): pl (≤3 mm, most clear 

and euhedral but some have finely sieved cores with 

clear overgrowths), ol (≤2 mm), aug (≤2 mm), opx 

(≤1 mm; rare), and Fe-Ti oxides (≤0.2 mm, rare) 

in a pilotaxitic groundmass. Aggregates and gabbroic microxenoliths (<6 mm) of pl ± ol + aug + 

opx ± oxides ± melt present but not abundant. Comparatively low phenocryst content and distinctive 

groundmass texture distinguish this unit from others 

nearby. Overlies units dlp, dc, and dcp; overlain by 

asb. K-Ar age: 47±20 ka at Pumice Point

ad Andesite of Devils Backbone (late Pleistocene)—Moderately porphyritic dark- to light-gray andesite 

(57.5–61% SiO2; most 60–61) lava flows, emanating from a vent fed by the Devils Backbone dike 

on west caldera wall and since removed by glaciation. Unit ad extends 15 km down the west flank 

of Mount Mazama where a thick flow reaches 2 km 

west of the Crater Lake National Park boundary 

in the drainage of Copeland Creek. Identical rock, 

mapped as unit ad, forms the Devils Backbone dike 

(Panoramas B, C; sheet 4). Phenocrysts (10–15%): 

pl (≤3 mm, rarely to 5 mm), aug (≤2 mm, rarely to 

3 mm), opx (≤0.5 mm), and Fe-Ti oxide (≤0.2 mm). 

Characteristic blocky pl and pyroxene ~0.1–0.4 mm 

across in very fine grained groundmass give rock a 

seriate appearance and a total crystal content of ~30 

percent. Ubiquitous enclave fragments as much as 5 

mm across are rich in or consist largely of ol (~1–3 

mm) so that rock appears olivine-phyric. Mediumgrained gabbro and, less commonly, troctolite xenoliths as large as 1 cm across are common. Despite 

these contaminants, unit is remarkably homogeneous in composition. Cuts units all, am, aww, and 

atw at Devils Backbone; overlies aww, bh, and ah; 

overlain by bwn and dvb. K-Ar age: 75±6 ka, ~4.5 

km northwest of Devils Backbone dike, ~6,200 ft 

elevation. Relation with other units suggests K-Ar 

age is too old, possibly owing to abundant augitebearing gabbro microxenoliths, and eruption age is 

about 50–40 ka. Alternatively, correlation of dated 

lava with Devils Backbone dike and lava outcrops 

near caldera rim may be incorrect

atw Andesite south of The Watchman (late Pleistocene)—

Coarsely porphyritic medium-light-gray silicic 

andesite (62.5–63.5% SiO2

) forming thick lava flows 

on west caldera rim at Hill 7319 ~1.5 km south of 

The Watchman (see Panoramas A, B; sheet 4), up to 

2 km west of there, and immediately north of Devils 

Backbone at Hill 7478. Phenocrysts (15–20%): pl 

(≤4 mm; variably sieved), ol (≤2 mm; not present in 
29

all outcrops), aug (≤3 mm, rarely to 4 mm), opx (≤2 

mm), and Fe-Ti oxides (≤0.4 mm; rarely to 1 mm) 

in a pilotaxitic groundmass. Aggregates (typically ~3 

mm but commonly to 1 cm) of cumulate mush consisting of pl ± ol + aug ± opx ± oxides + glass (devitrified) are abundant and ubiquitous. Large phenocrysts 

clearly derived from aggregates. Basaltic(?) andesitic 

enclaves (typically <10 cm, rarely to 40 cm; ol phenocrysts common) may be present but not abundant. 

Texture suggests magma consisted of silicic melt and 

fragments of incompletely solidified gabbro. Glassy 

columns at some exposures suggest chilling against 

ice (such as 2 km southwest of The Watchman). 

Overlies units aww, dwp, dwv, and dwf; overlain 

by als and dvb; cut by ad (Devils Backbone dike). 

K-Ar ages: 62±7 ka, north of Devils Backbone; 55±6 

ka, west of Lightning Spring; 52±4 ka, south of The 

Watchman; weighted mean 55±3 ka 

dwf

dwv

dwp

Dacite of The Watchman (late Pleistocene)—Porphyritic dacite (65.5–68% SiO2

; most ≥67%) forming 

The Watchman flow (Williams, 1942), its feeder 

dike, and pumiceous pyroclastic-flow deposits in 

the west caldera wall and locally preserved on ridge 

top ~4 km west of Rim Village (Panoramas A, B; 

sheet 4). Medium-gray pumiceous carapace (upper 

surface of The Watchman flow) and grayish-black 

to black dense vitrophyre (dwv), medium-lightgray crystalline lava and dike (dwf), and nonwelded 

to densely welded vitric pyroclastic-flow deposits 

(dwp; grayish black, pale yellowish brown, grayish orange; pumiceous clasts to 60 cm but most 

≤20 cm, lithics to 3 cm). The high point of the 2-

km-long Watchman flow forms a glaciated horn on 

the flank of Mount Mazama. Earlier lava flowed an 

equal distance to the south and may have vented at a 

somewhat higher elevation. Pyroclastic-flow deposits (dwp) probably were extensive in the drainages 

of Bybee and, possibly, Castle Creek prior to glacial erosion. Pumiceous clasts in the caldera wall 

above unit dlp at Palisade Point may correlate with 

dwp. North-south normal fault offsets base of platyjointed zone in dwf a minimum of ~15 m, down to 

the east, 750 m west of Discovery Point. All units 

shown as undivided dwf on bedrock map. Phenocrysts (25%): pl (≤4 mm, commonly in clusters; 

many have melt inclusions ≤0.2 mm in cores), aug 

(≤3 mm), opx (≤2 mm), Fe-Ti oxides (≤0.8 mm) 

in a glassy or fine-grained groundmass. Hbl phenocrysts (≤0.7 mm) rare. Aggregates (≤4 mm) of 

0.5–1 mm pl + aug + opx + oxides with intergranular glass common and probably represent source of 

many phenocrysts. Also common are aggregates 

(≤1 cm) of euhedral pl (≤0.5 mm) + aug + opx + 

oxides forming a mesh with a higher percentage of 

intergranular glass, and these are source of many 

of the smaller crystals in the dacite. The latter are 

particularly abundant in the dike selvage. Andesitic 

enclaves (61–61.5% SiO2

; ≤15 cm), also containing 

the first type of aggregates, are rare. Unit overlies 

units aa, dcn, aww, and ah; overlain by atw and 

als. K-Ar age: 50±3 ka

ah Andesite of Hillman Peak (late Pleistocene)—Markedly porphyritic dark- to medium-light-gray andesite 

(57.5–59% SiO2) lava flows forming summit of 

Hillman Peak and exposed in west caldera walls and 

on west flank of Mount Mazama. Vent was east of 

Hillman Peak. Phenocrysts (25–35%): euhedral pl 

(typically ≤1.5 mm, rarely to 3 mm; commonly in 

stellate clusters), abundant aug (typically ≤1.2 mm 

but commonly to 3 or 4 mm), opx (≤1.2 mm, rarely 

to 2 mm; less abundant than aug), and Fe-Ti oxide 

(≤0.3 mm) in a fine-grained or pilotaxitic groundmass. Relatively small (≤4 mm, rarely to 7 mm) 

aggregates of pl + aug ± opx ± oxides moderately 

common. Rock has distinctive texture of abundant 

euhedral pl ~1 mm across and relatively large, conspicuous aug phenocrysts. Unit has unusually high 

concentrations of incompatible elements for andesite 

of Mount Mazama (Bacon and others, 1994). Overlies units aww and bh; overlain by dwf, dwp, ad, 

bwnp, bw, bwp, mw, and dvb. K-Ar age: 61±8 ka

dlp Dacite below Llao Rock (late Pleistocene)—Porphyritic pinkish- to brownish-gray and medium-darkgray dacite pumice (66–67.5% SiO2

) occurring as 

fall and pyroclastic-flow deposits in northwest and 

north caldera walls from north of Devils Backbone 

to Palisade Point (see panoramas, sheet 4). Welded 

fall deposit dipping into caldera wall above Steel 

Bay indicates proximity to vent to south, in area of 

present caldera. Correlated by Davis (1985) with 

Summer Lake (Oreg.) tephra bed 8 and with ash at 

Lake Malheur, Oreg., on the Snake River in western 

Idaho, and in drill core from Tulelake, Calif. (A.M. 

Sarna-Wojcicki, written commun., 2002). Phenocrysts (~15%): pl (≤2.5 mm, rarely to 4.5 mm, commonly sieved); aug (≤2 mm; opx (≤1.5 mm); Fe-Ti 

oxide (≤0.2 mm). Glassy groundmass speckled with 

a few percent ~0.1 mm pl crystals visible in poorly 

vesiculated clasts. Gabbroic aggregates and microxenoliths as large as ~2 mm common. Distinguished 

from dcp by generally smaller pl and aug, subequal 

proportions of aug and opx, and, especially, less 

differentiated glass composition. Undated. Overlies units dc (K-Ar ages: 71±6 and 72±7 ka), aww, 

bh, and am; intercalated within agc at Cleetwood 

Cove (at Grotto Cove, agc K-Ar ages are 68±13 and 

74±10 ka); overlain by apu (K-Ar age: 47±20 ka) 

and asb (K-Ar ages: 42±6 and 43±6 ka)
30

agc

bh

bhi

bhp

Andesite of Grotto Cove (late Pleistocene)—Porphyritic 

medium-light- to medium-dark-gray andesite 

(61.5–63% SiO2

) lava flows in the caldera wall 

from Pumice Point (north wall) to Cleetwood Cove. 

Forms 100-m-high cliffs at caldera rim at Grotto 

Cove (northeast wall); includes related block-andash flow deposit between flows at Grotto Cove. The 

two thick flows at Grotto Cove extend up to 3 km 

from the caldera rim; joint patterns near their mutual 

contact at Grotto Cove suggest interaction with ice 

on caldera side and that vents for these flows were 

near the southern limit of each, outboard of the caldera rim. Phenocrysts (20–30%): pl (≤5 mm, commonly coarsely sieved), aug (≤3.5 mm), opx (≤3.0 

mm; more abundant than aug), and Fe-Ti oxides 

(≤1.0 mm) in a glassy to finely crystalline groundmass. Aggregates (≤5 mm), which vary in grain size 

and amount of intergranular melt, contain pl + aug + 

opx ± Fe-Ti oxide ± ol and are particularly abundant 

in flows between Pumice Point and Cleetwood Cove. 

Ol (≤2.5 mm) in aggregates may be conspicuous in 

hand specimen. Andesitic enclaves, locally common 

between Pumice Point and Cleetwood Cove (≤40 cm; 

58.5% SiO2

), contain crystal aggregates and phenocrysts of same phases as host rock. Similar but less 

porphyritic andesitic enclaves abundant (5–10%) 

near tops of flows at Grotto Cove. Here, the larger 

enclaves (12–100 cm) have porous diktytaxitic 

cores and 2–6-cm-thick denser rinds. Bacon (1986) 

described how intergranular melt was expelled from 

enclave cores at Grotto Cove by gas-driven filter 

pressing that caused enclaves of initially andesitic 

composition (60% SiO2) to develop relatively mafic 

cores (≥55.5% SiO2). Discrete phenocrysts in host 

lavas appear to have been derived from crystal mush, 

represented by aggregates, and from enclaves. Order 

of eruption appears to have been from most to least 

silicic lavas, starting at Cleetwood Cove (most abundant aggregates) and ending with the southern flow 

at Grotto Cove (most abundant enclaves). Flows at 

Cleetwood Cove overlie units dpt, dpe, abl, and 

dcp; pumice fall dlp is intercalated with lava flows 

near top of agc above east shore of Cleetwood Cove; 

agc overlain by rh; stratigraphic relations suggest 

an age of ~70 ka. Flows at Grotto Cove overlie unit 

aa. K-Ar ages of flows at Grotto Cove: 74±10 ka, 

northern flow; 68±13 ka, southern, stratigraphically 

higher flow; weighted mean 71±5 ka. Paleomagnetic 

data suggest the flows at Grotto Cove erupted over a 

period of ~100–200 years (D.E. Champion, unpub. 

data, 1984)

Basaltic andesite of Hillman Peak (late Pleistocene)—

Medium-gray porphyritic hornblende basaltic andesite and andesite (55.5–59% SiO2

) lava flows (bh), 

intrusions (bhi), and fall deposits (bhp) of west caldera wall and flank of Mount Mazama. Distinguished 

by ubiquitous hornblende needles that appear black 

in hand specimen. Samples of this unit have highest 

incompatible element concentrations (for example, 

2.0% K2

O, 2,000 ppm Sr) and most radiogenic Sr 

(Bacon and others, 1994) of any from the Mazama 

edifice. Flows of bh reach as low as 5,600 ft elev in 

North Fork of Copeland Creek. Small outcrops may 

be readily visited at the caldera rim and west of Rim 

Drive south of junction with north park entrance 

road. Unit bhp forms a symmetrical triangular pile 

high in the west caldera wall below Hillman Peak. 

Source vent was immediately east of crags of vaporphase indurated rock (bhp) of this deposit. Phenocrysts (20–30%): pl (<2 mm, rarely to 4 mm), brown 

hbl (≤4 mm, rarely to 1 cm), and aug (≤3 mm), with 

or without opx (≤1 mm) or Fe-Ti oxide (≤0.2 mm) 

in a pilotaxitic to trachytic groundmass. Rare ol (≤3 

mm) present in some samples; intergrown with hbl 

in one example. Aggregates and microxenoliths 

(≤4 mm, rarely to 1 cm) of pl + aug ± opx ± hbl ± 

oxide common; hbl aggregate (1 cm) present in one 

sample. Overlies units aww and dcp; overlain by 

dlp, ah, ad, and bw. K-Ar age: 73±6 ka

dc

dcp

Dacite of Pumice Castle (late Pleistocene)—Porphyritic 

light- to dark-medium-gray dacite lava flows (dc; 

66–67% SiO2

) in caldera wall from north of Devils 

Backbone east to Cloudcap Bay. Similar dc lava 

forms Cloudcap and Scott Bluffs. Related dikes 

(67.5–68% SiO2) occur in the caldera wall at Cloudcap Bay (Panorama I, sheet 4); dike sampled by submersible off Llao Rock provisionally identified as dc

cutting unit all lava flows. Extensive Plinian fall of 

pinkish-gray to light-brown dacite pumice (dcp; 66–

68% SiO2) forms ~75-m-thick deposit of alternating 

welded and nonwelded layers at Pumice Castle and 

nonwelded fall, pyroclastic-flow, or reworked deposits exposed locally from north of Devils Backbone to 

Palisade Point, above Cloudcap Road, and in small 

exposure in roadcut near intersection of Rim Drive 

with Dutton Ridge; underlies dc. Correlated by Davis 

(1985) with Summer Lake (Oreg.) tephra bed 6 and 

also found in drill core from Tulelake, Calif. (A.M. 

Sarna-Wojcicki, written commun., 2002), indicating extensive downwind deposition. Unit represents 

most voluminous silicic eruptions between ~400 ka 

and early Holocene. Vented beneath Cloudcap and 

immediately north of Pumice Castle; other vents for 

lava flows possibly to northwest of these within what 

is now the caldera. Phenocrysts (15–25%): pl (most 

0.1–3 mm but commonly to 6 mm), aug (≤1.5 mm, 

rarely to 6 mm), opx (≤1.2 mm), and Fe-Ti oxides 

(≤0.3 mm) in a glassy or devitrified groundmass. 
31

Characterized by large, blocky aug, which is more 

abundant than the generally smaller opx, and relict 

perlitic texture of groundmass. Grayish-black vitrophyres commonly have iridescent perlitic fracture 

surfaces. Ubiquitous gabbroic and diabasic crystal 

aggregates, typically ≤8 mm; rare ol gabbro microxenoliths ≤5 mm. Overlies units aww, bsp (inferred), 

apw, dsb, alu, aa, and ac; overlain by bh, dlp, asb, 

apu, agc, and re; dikes cut ak, dr, ac, and all. K-Ar 

ages of dc: 71±6 ka, flow above Pumice Castle; 72±7 

ka, Steel Bay; weighted mean 71±5 ka

bsp

bs

Basaltic andesite of Steel Bay (late Pleistocene)—

Medium-dark-gray porphyritic basaltic andesite 

(53% SiO2) lava flows (bs) and pyroclastic deposit 

(bsp) exposed in north caldera wall above Steel Bay 

(see Panorama E, sheet 4). Pyroclastic layers consist 

of bedded palagonitic lapilli tuff containing bombs 

to at least 1.5 m. Unit is the most mafic lava known 

in the caldera walls. Phenocrysts (~7%; commonly 

in clusters): ol (≤2 mm, rarely to 5 mm), pl (≤4 mm; 

cores finely sieved), and minor aug (≤1.3 mm) in an 

intergranular groundmass. Overlies units aww and 

dsb; overlain by units dcp and asb. Undated. Evidence for explosive interaction with water suggests 

eruption during wet period immediately following emplacement of units directly beneath bs. Age 

bracketed by K-Ar dates for dsb (116±5 and 116±9 

ka) and dc (71±6 and 72±7 ka); provisionally considered to be about 115 ka

apw Andesite west of Pumice Point (late Pleistocene)—Porphyritic dark-gray andesite lava flows (61% SiO2

) 

midway up the north caldera wall and feeder dike 

(59.5% SiO2) at Steel Bay (Panoramas E, F; sheet 4). 

Dike cuts at least the lower of two lava flows, which 

rest on >50-m-thick alu flow. Phenocrysts (~40%): 

pl (≤4 mm; some finely sieved), ol (≤1 mm; rare), 

aug (≤2.5 mm), opx (≤1 mm), and Fe-Ti oxides 

(≤0.7 mm) in fine-grained groundmass. Aggregates 

and microxenoliths (≤6 mm) of coarse pl + opx ± ol 

± aug ± oxide abundant. Some ol-bearing aggregates 

have subophitic texture. Overlies unit alu; overlain 

by dcp and asb. Undated; eroded top of lava suggests closer to alu in age, perhaps about 110 ka

aww Andesite of the west wall (late Pleistocene)—Finely 

porphyritic seriate-textured medium- to dark-gray 

andesite (57–61% SiO2

) lava flows exposed in west 

caldera wall from Discovery Point to Llao Rock (see 

panoramas, sheet 4), on the west flank of Mount 

Mazama, and as much as 5–7 km west of the caldera 

in drainages of Copeland and Bybee Creeks, respectively. East of Llao Rock, unit includes two thick lava 

flows petrographically and compositionally similar 

to (older) unit ar, overlain by a thinner flow of typical aww lava. Principal vent presumed to have been 

east of Hillman Peak; also flank vent (see below). 

Phenocrysts (15–40%; mostly ~30%): pl (≤3 mm, 

rarely 4 mm; most euhedral, some may be in stellate 

clusters, some sieved or rounded; seriate), aug (≤2 

mm, rarely to 3.5 mm; not evident in all samples), 

opx (≤1 mm, rarely to 2 mm), and Fe-Ti oxides (≤0.5 

mm; varies from rare to common) in a fine-grained 

to pilotaxitic groundmass. Aggregates (≤1 cm) of 

relatively coarse pl + aug + opx ± oxides + melt 

moderately common. Diabase microxenoliths(?) (≤5 

mm) commonly contain ol (≤2 mm, rarely 3 mm); 

common ol xenocrysts in lava resorbed or armored 

with pl + pyroxene probably are from same source. 

Distinguishing petrographic characteristics of unit 

are ubiquitous seriate texture and, in most samples, 

relatively small pyroxene phenocrysts; many have 

visible ol. Caldera wall exposures altered below 

The Watchman and Hillman Peak. Similar in age, 

texture, and composition to unit arw; mapped separately because of geographic separation of likely 

source vents (see arw) and because flows of arw are 

overlain by finer-grained and more ol-rich flows of 

aww ~2 km southeast of Crater Springs. The latter 

aww flows apparently originated at a flank vent, ~2.5 

km southeast of Crater Springs, which presumably 

was fed by a radial dike and subsequently buried by 

ad flows. Generally less differentiated composition 

than texturally similar unit als. Overlies units af, aa, 

dcn, am, and dsb (separated from older units am

and dsb by paleosol or fragmental deposits, respectively); overlain by ad, bh, dcp, dc, dlp, asb, dwp, 

atw, als, and dvb. K-Ar age: 70±4 ka

arw Andesite west of Red Cone (late Pleistocene)—Finely 

porphyritic seriate-textured medium-gray andesite 

(59% SiO2

) lava flows exposed west of Red Cone 

Springs fault as far west as Oasis Butte and ~1 km 

south of Crater Springs in northwest part of map. 

Phenocrysts (~30–40%, including microphenocrysts): pl (≤3 mm, rarely 4 mm; seriate; may show 

strong preferred orientation), aug (≤1.5 mm, rarely 

2.5 mm), opx (≤1.5 mm), and Fe-Ti oxides (≤0.4 

mm; relatively low abundance) in a fine-grained 

intergranular or intersertal groundmass. Ol (≤0.5 

mm) commonly present; ol may be in local (≤3 

mm) irregular blebs of basaltic-appearing texture; 

ol also in rare ol + pl aggregates (to 1 cm). Aggregates (≤6 mm) of pl ± aug + opx ± oxide ± glass 

common. Similar in age, texture, and composition 

to unit aww; distinguished as a separate unit on 

basis of likely source vent ~2 km west of Red Cone 

that is ~7 km north-northwest of inferred source of 

aww. Vent for arw probably fed by dike from same 

magma reservoir beneath Mount Mazama that produced aww and that intersected Red Cone Springs 
32

fault. Flows of arw are overlain by finer-grained 

and more ol-rich flows of aww ~2 km southeast of 

Crater Springs. Overlies unit bo; overlain by ad and 

br. K-Ar age: 84±13 ka

asl Andesite east of Spruce Lake (late or middle Pleistocene)—Porphyritic medium-gray silicic andesite 

poorly exposed in south fork of Crater Creek ~1.5 

and ~2 km east of Spruce Lake, near northwest edge 

of map. Phenocrysts (~20%): pl (≤3 mm), aug (≤2 

mm), opx (≤1.5 mm), and Fe-Ti oxides (≤0.5 mm) in 

a pilotaxitic groundmass. Aggregates (≤5 mm) of pl + 

aug + opx ± oxides abundant. Considered to be from 

Mount Mazama (strongly resembles units such as 

atw). Overlies unit bo (201±13 ka). Undated. Probably older than units aww and ad (about 50–80 ka)

abl Andesite of the boat landing (late Pleistocene)—Porphyritic medium- to dark-gray andesite (63–63.5% 

SiO2) lava flows and underlying related fragmental deposit between Pumice Point and Cleetwood 

Cove. Trail to boat landing descends along the base 

of the lower flow of this unit where it lies on probable till (see Panorama G, sheet 4). Phenocrysts 

(30%): pl (≤5 mm; many crystals finely sieved), 

aug (≤2.5 mm), opx (≤2.0 mm; less abundant and 

generally smaller than aug), and Fe-Ti oxides (≤0.3 

but rarely to 0.8 mm) in a fine-grained to glassy 

groundmass. Ol xenocrysts (≤1 mm) derived from 

enclaves common in some specimens. Abundant 

aggregates (≤8 mm) of relatively coarse pl + aug + 

opx + oxide ± ol with intergranular melt are probably source of many phenocrysts. Pl laths (≤0.4 

mm) abundant. Andesitic enclaves (≤30 cm; 57.5% 

SiO2), very common in upper flow and present in 

lower, contain ol phenocrysts (≤2.5 mm). Overlies 

units ags, dpt, g, and dsb; overlain by dcp, dlp, 

and agc. K-Ar age: 102±10 ka, flow above trail to 

boat landing

dpe Dacite east of Palisade Point (late Pleistocene)—Porphyritic medium-light-gray dacite (65% SiO2

) lava flow 

(Palisade flow of Williams, 1942) ~150 m thick, 

forming northeast caldera wall midway between 

Palisade Point and Roundtop (Panoramas G, H; sheet 

4). Phenocrysts (10%): pl (≤3 mm), aug (≤2 mm), 

opx (≤1 mm, rarely to 3 mm), and Fe-Ti oxides (≤0.5 

mm) in a glassy to devitrified groundmass containing 

minor 0.1–0.3 mm pl laths. Aggregates (≤6 mm) of 

pl + aug + opx + oxides + glass common. Enclaves 

(≤30 cm) relatively abundant. Overlies unit dpt, 

apparently banked against ar; overlain by agc. Lies 

on probable till; also overlain by till. Bacon (1983, p. 

82) suggested 26–22 ka age on basis of radiocarbon 

date on underlying paleosol (evidently contaminated 

with modern carbon) and incorrect provisional identification of underlying tephra. K-Ar age: 111±9 ka

dsb

dsbp

Dacite of Steel Bay (late Pleistocene)—Porphyritic 

medium-light- to medium-gray dacite (64.5–66.5% 

SiO2) lava and dikes (dsb) in north caldera wall from 

Llao Rock to Pumice Point (Panoramas D, E, F, G; 

sheet 4) and pyroclastic-flow deposit (dsbp) below 

Hillman Peak (Panorama C, sheet 4) carrying mediumdark-gray dacite (64.5% SiO2

) prismatically jointed 

blocks. Two prominent dikes feed a small dome and a 

lava flow at Steel Bay. Phenocrysts (15–30%): pl (≤4 

mm, rarely to 7 mm; some finely sieved; resorbed and 

coarsely sieved in small dome east of Llao Rock), aug 

(≤2 mm), opx (≤3 mm), and Fe-Ti oxide (≤0.4 mm) in 

very fine grained, glassy, or devitrified groundmass. 

Several percent 0.1–0.3 mm pl laths common. Unit 

dsbp also contains hbl (≤1.3 mm). Aggregates (≤1 

cm) of pl + pyroxenes + oxides common. Flow at 

Pumice Point contains angular xenoliths (to 2 cm) of 

metamorphosed(?) igneous rocks. Andesitic enclaves 

(≤1 m; 57.5–59% SiO2) uncommon except in eastern 

dike at Steel Bay and near Pumice Point. Overlies 

units alu, am, and dpt; overlain by bs, aww, abl, and 

dcp. K-Ar ages: 116±9 ka, below Llao Rock; 116±5 

ka, Pumice Point

am Andesite of Merriam Point (late Pleistocene)—Porphyritic light-gray to medium-dark-gray andesite 

(60.5–61.5% SiO2) lava flows, domes, and breccia 

in the northwest caldera wall from below Hillman 

Peak to below Llao Rock (Panoramas C, D; sheet 

4). Thickest (~200 m) near feeder, which displaced 

and tilted unit all, southwest of Merriam Point. 

Phenocrysts (25–30%; feeder contains ~50%): distinctive blocky pl (≤3 mm, most ≤2 mm, sharply 

euhedral, commonly in stellate clusters as large 

as 3 mm across; may be finely sieved), aug (≤2.5 

mm), and opx (≤2.0 mm) in a fine-grained groundmass. Iddingsitized euhedral to skeletal ol (≤4 mm) 

noted in highest flow west of Llao Rock. Common 

small (≤3 mm, rarely to 7 mm) aggregates of pl + 

aug + opx ± oxide contain most of the larger pyroxene crystals. Rock composition is relatively poor 

in incompatible elements (for example, 0.8–1.2% 

K2O). Overlies units all and alu; overlain by dsbp, 

dsb, aww, dcp, dc, and bh; cut by ad. Separated 

from underlying alu by clastic deposit and from 

overlying dsb by indurated bedded mudflow(?) 

deposits (unit s); glaciated where overlain by dc. 

K-Ar age: 131±18 ka

alu Andesite of Llao Bay, upper unit (late Pleistocene)—

Variably porphyritic medium- to dark-gray basaltic 

andesite and andesite (54.5–61% SiO2) lava flows in 

northwest caldera wall from Merriam Point to east of 

Pumice Point (see panoramas, sheet 4). Andesite lava 

flow forms prominent cliff midway up wall at Steel 

Bay. Subtle basaltic andesite dike mapped as alu cuts 
33

flows of unit all below Llao Rock (Panoramas D, E). 

Together with unit all, forms broad shield making 

up lower part of northwest caldera wall. Phenocrysts 

(25–40%; as low as 10% in higher, pilotaxitic flows 

west of Steel Bay): pl (≤5 mm, most ≤2 mm; some 

pl sieved, especially in pilotaxitic, ol-bearing flows), 

aug (≤2.5 mm), opx (≤1.5 mm), and Fe-Ti oxide 

(≤0.5 mm) in fine-grained to pilotaxitic groundmass. 

Abundance of pl 0.1–0.3 mm in length varies markedly from a few equant crystals to numerous laths 

in pilotaxitic rocks. Ol pseudomorphs (≤1 mm) rare 

in most samples but common in upper pilotaxitic 

flows. Aggregates (typically ≤5 mm but rarely to 2 

cm) of pl + aug + opx ± hbl + oxide ± melt common. 

Enclaves rare and small (≤1 cm, rarely to 6 cm). 

Altered to varying degrees west of Steel Bay, giving 

flows an olive-gray color in extreme cases. Overlies 

units all and dpt; overlain by am, dsb, apw, and 

dcp. Lava at Pumice Point has been dated by K-Ar 

at 106±7 ka and by 40Ar/39Ar at 117±3 ka. The latter 

age is consistent with K-Ar ages of contiguous units. 

The dated flow is believed to have interacted with 

ice because it has closely spaced (<20 cm) columnar 

joints that grade into vitric breccia at Pumice Point, 

where it lies on glaciated flows of unit all that have 

striae trending N. 20° E.

dpt Dacite of Palisade Point (late or middle Pleistocene)—

Porphyritic medium-gray dacite lava flow on north 

shore of Crater Lake (Panorama G, sheet 4) at 

Palisade Point and east of Pumice Point (66% SiO2). 

Phenocrysts (~20%): pl (≤3 mm, rarely to 6 mm), 

aug (≤1.3 mm, rarely to 4 mm), opx (≤1.1 mm), 

and Fe-Ti oxides (≤0.5 mm) in a very fine grained 

or devitrified groundmass. Gabbro microxenoliths 

(<1 cm) abundant; enclaves to 30 cm common 

in some outcrops. Undated: overlies units ags

(

40Ar/39Ar plateau age: 189±3 ka), all (local K-Ar 

age 122±20 ka, east of Pumice Point), and g; overlain by alu (40Ar/39Ar plateau age: 117±3 ka), dsb, 

g, dpe, abl, and dcp

all Andesite of Llao Bay, lower unit (middle Pleistocene)—

Variably porphyritic medium- to dark-gray andesite 

(57–60.5% SiO2) lava flows forming the lowest unit 

in the northwest caldera wall from south of Devils 

Backbone to east of Pumice Point (see panoramas, sheet 4). Together with unit alu, forms broad 

shield making up lower part of northwest caldera 

wall. Tilted by intrusive feeder of unit am north of 

Devils Backbone. Two petrographic types are recognized, coarsely and finely porphyritic. Coarsely 

porphyritic lavas are present below Llao Rock, 

where samples collected by manned submersible 

indicate the finely porphyritic lavas are lower in 

the section. Phenocrysts in coarsely porphyritic 

lavas (15%): pl (≤4 mm, euhedral, rarely resorbed; 

some finely sieved), sparse ol (≤1.3 mm, euhedral, 

commonly altered), aug (≤2 mm), and opx (<2 

mm) in a pilotaxitic groundmass rich in 0.1–0.4 

mm pl laths. Aggregates of coarse pl and pyroxene common (to 1 cm); Fe-Ti oxides present only 

in association with pyroxene. Phenocrysts in finely 

porphyritic lavas (20–30%): pl (≤1.5 mm, rarely to 

2.5 mm, euhedral, commonly in clusters; may be 

sieved), rare ol (≤1 mm, commonly altered), aug 

(≤1.5 mm), and opx (≤1 mm) in a fine-grained 

groundmass. Fe-Ti oxides present only with silicate phenocrysts. Pyroxenes commonly intergrown 

with pl. Enclaves (≤3 cm) rare or absent. Both lava 

types may show patchy alteration of groundmass to 

clay minerals, light olive brown in hand specimen, 

and amygdules containing clay ± carbonate minerals. Overlain by units dpt, alu, and am; cut by 

ad. K-Ar ages: 172±11 ka (below Llao Rock, west 

side), 153±12 ka (below Llao Rock, east side, 300 

ft depth), 150±9 ka (Devils Backbone), 137±17 ka 

(below Llao Rock, east side, 193 ft depth), 122±20 

ka (Pumice Point, west side), 109±19 ka (below 

Llao Rock, east side; altered)

ar Andesite of Roundtop (middle Pleistocene)—Porphyritic medium-light-gray andesite (62% SiO2

) forming cliff in northeast caldera wall at Roundtop and 

extending 2.5 km to northeast of caldera rim. Joint 

pattern in cliffs suggests vent beneath Roundtop. 

Resting on 115 m of glacial deposits, ar lava is probably an ice-bounded flow (Lescinsky and Sisson, 

1998). Phenocrysts (~15%): pl (≤4 mm; typically 

has coarsely resorbed zone overgrown by clear rim), 

ol (≤2.5 mm), aug (≤3 mm), opx (≤1.5 mm), and 

Fe-Ti oxide (≤0.4 mm) in fine-grained groundmass 

with abundant plagioclase microphenocrysts. Aggregates of relatively coarse pl + opx ± aug ± oxide with 

intergranular patches of glass abundant. Light-gray 

basaltic andesite enclaves common (typically <2–3 

cm but may be up to 20 cm). At Roundtop, unit dpe

appears to be banked against flow of ar. K-Ar age: 

159±13 ka (MIS 6)

amv Andesite east of Munson Valley (middle Pleistocene)—

Porphyritic medium-gray andesite (57% SiO2

) lava 

flows capping ridge between Middle and East Forks 

of Annie Creek. Vent must have been ~500 m north 

of Rim Drive. Phenocrysts (25%): pl (≤4 mm, most 

≤1.5 mm; finely sieved), ol (≤2 mm, most ≤0.5 mm; 

rounded and resorbed), aug (≤1 mm), and opx (≤0.5 

mm) in a fine-grained groundmass. Aggregates of pl 

+ aug + opx (≤4 mm) uncommon; most pyroxene 

phenocrysts are associated with pl. Abundant seriate ol distinguishes this unit in thin section. Overlies 

units db, aa, dg, and ag. K-Ar age: 172±15 ka
34

ags Andesite of the gaging station (middle Pleistocene)—

Porphyritic medium-light-gray andesite (60% 

SiO2) lava flow at gaging station west of boat landing, north shore of Crater Lake (see Panorama G, 

sheet 4). Phenocrysts (~20%, including abundant 

0.1–0.3 mm pl microphenocrysts): pl (≤2.5 mm), ol 

(≤1.5 mm), aug (≤1 mm; rarely to 3 mm), opx (≤1.5 

mm), and Fe-Ti oxide (≤0.2 mm) in a pilotaxitic 

groundmass. Microxenoliths (≤5 mm) of pl ± aug 

± opx + oxide common. Compositionally and petrographically resembles some lavas of unit aa, but 

is ~20,000 yr younger than highest unit aa on south 

side of caldera. Forms glaciated platy-jointed promontory overlain by indurated till at lake gauge and 

small outcrops at lake level to west. Overlain by 

units dpt and g. 40Ar/39Ar plateau age at gauging station 189±3 ka

dcn

dcnp

Dacite north of Castle Creek (middle Pleistocene)—

Finely porphyritic medium-light- to medium-darkgray dacite lava (dcn; 64.5–66.5% SiO2

) present 

southwest of the caldera from ~1 km west of Rim 

Village to drainages of Copeland and Bybee Creeks. 

Flow forming divide between Bybee and Castle 

Creeks reached at least 10 km west of vent marked 

by agglutinate (dcnp). Dikes (65–66% SiO2) in caldera wall (Panorama A, sheet 4) match compositions 

and petrography of specific flows, the more northerly 

dikes corresponding to the relatively crystal-rich, 

northern flows. Phenocrysts (10–25%): pl (most ≤2 

mm but rarely to 4 mm), aug (≤1 mm), opx (≤1 mm), 

and Fe-Ti oxides (<0.2 mm) in a glassy to devitrified 

groundmass carrying variable but minor amounts of 

<0.1 mm pl laths. Many phenocrysts appear to have 

been derived from cumulate mush now represented 

by abundant crystal aggregates (typically 1–2 mm 

but rarely to 8 mm). Although range in phenocryst 

content is large, all samples are characterized by 

unusually high TiO2 content for dacite from Mount 

Mazama (0.92–0.97%). Outcrops typically have 

closely spaced platy joints and relict perlitic texture. 

Overlies units bln, acc, and aa; overlain by aww, 

als, and dwp; dikes in caldera wall cut arv, af, and 

aa but not aww or als. K-Ar age: 216±4 ka, Bybee 

Creek ~500 m north of Hill 5604

ag Andesite of Garfield Peak (middle Pleistocene)—Moderately porphyritic medium-gray andesite (60.5–

61.5% SiO2) lava flows and breccia at Garfield 

Peak, on slope to south, and on the west side of 

Munson Valley. Dike (ag?) in caldera wall east of 

Garfield Peak provisionally assigned to unit (Panorama K, sheet 4). Phenocrysts (15–20%): pl (≤3.5 

mm, rarely to 6 mm; many finely sieved, with clear 

overgrowths), aug (≤2.5 mm), opx (≤0.7 mm), hbl 

(≤3 mm), and Fe-Ti oxide. Pl laths abundant in seriate groundmass. Medium-grained pl + pyroxene ± 

ol aggregates as much as a few millimeters across 

present in some flows; small enclaves rare (containing hbl as large as 1 cm). Fresh ag has sharp basal 

contact with underlying, altered unit aa south of 

Garfield Peak; also overlies db and dg, abuts dm. 

K-Ar age: 224±9 ka, highest flow on Garfield Peak

dg Dacite south of Garfield Peak (middle Pleistocene)—

Porphyritic medium-gray dacite lava (64.5–65.5% 

SiO2) traceable south of the caldera from 1 km 

south of Garfield Peak for nearly 6 km to vicinity of Pole Bridge Creek ~2 km southeast of Arant 

Point; medium-dark-gray to black vitrophyre locally 

preserved at top of fault scarp north of Arant Point. 

Phenocrysts (~30%): blocky pl (0.5–5 mm), aug (≤3 

mm), opx (≤3 mm), and Fe-Ti oxide (<0.4 mm) in a 

glassy to devitrified groundmass. Abundant pl laths 

(0.1–0.5 mm) apparently are derived from enclaves. 

Enclaves as large as 50 cm near source, a few centimeters distally; enclave fragments containing ol 

(≤1 mm) and gabbro fragments (≤1 cm) typically 

present. Much phenocrystic pl probably derived 

from gabbro. Characteristic marginal resorption 

of pl phenocrysts distinguishes dg from relatively 

silicic flows of consanguineous unit aa. Overlies 

bcw, baw, at, db, and aa; overlain by ag and amv. 

Undated: younger than unit aa below Garfield Peak 

(K-Ar age: 269±12 ka), older than ag (K-Ar age: 

224±9 ka)

aa

aai

Andesite of Applegate Peak (middle Pleistocene)—

Light- to dark-gray porphyritic andesite and 

dacite (57–66.5% SiO2

) flow-banded lava flows, 

agglutinated bombs, and near-vent fall deposits. 

Unit consists of multiple lava flows believed to have 

come from summit vent of Mount Mazama or possibly satellite vents fed by same magma system. 

The most extensive unit of Mount Mazama, unit 

aa is exposed in the caldera wall from Wineglass 

to Discovery Point, as far as 9 km from the caldera 

east rim near Scout Hill, and 8 km from the west 

rim in Bybee Creek. The upper south slopes of 

Mount Mazama and high cliffs at Applegate Peak 

and Dutton Cliff are composed of this unit. Small 

andesite exposure (ags) at lake level on north side 

of caldera may be from same source vent as unit 

aa. The most silicic flows are west of Kerr Notch 

and below Pumice Castle; the most mafic, immediately east of Garfield Peak, lie on apparently glaciated, earlier unit aa flows. Related dacite lava with 

distinctive texture mapped as unit dg. Andesite of 

Applegate Peak forms dikes (aa) in the caldera 

wall at Grotto Cove, Cloudcap Bay, Sun Notch, and 

Chaski Bay and on the Garfield Peak trail at 7,650 ft 

elev; correlated with a large intrusive body (aai) in 
35

caldera wall below Applegate Peak (see Panorama 

K, sheet 4). Good exposures of unit aa are in road 

cuts at the Crater Peak trail head (columnar-based 

flow) and on the west flank of Dutton Ridge. Fresh 

in most exposures; increasingly altered west of 

Applegate Peak to Garfield Peak. Phenocrysts (typically 20–30%): pl (most ≤3 mm, rarely to 6 mm; 

commonly two populations, one coarsely sieved and 

resorbed, the less-abundant other finely sieved and 

having clear overgrowths), aug (≤3 mm but rarely to 

5 mm), opx (≤2 mm), and Fe-Ti oxide. Characteristic pl laths (≤0.3 mm, ≤15%) contrast with very fine 

grained or glassy groundmass in thin section. Rare 

hbl microphenocrysts (≤0.3 mm; <1%) present in 

some flows along Garfield Peak trail and on Munson 

Ridge. Ol xenocrysts (≤2 mm) not abundant but generally visible in hand specimen; in thin section, ol 

typically rimmed by basaltic andesitic groundmass. 

The most mafic flows have as little as 7 percent total 

phenocrysts of pl, aug, and ol in a groundmass rich 

in pl laths. Enclave fragments and medium-grained 

crystal aggregates (to ~1 cm) common, giving similar appearance to unit db, but large enclave (greater 

than a few centimeters) abundance typically lower. 

Many phenocrysts derived from crystal aggregates; 

pl laths <1 mm long may originate in enclaves. 

Varied composition results from mixing dacitic 

magma with basaltic andesitic magma (for example, thin flows east of Garfield Peak); at least two 

basaltic andesite components indicated by range in 

incompatible element concentrations. Some flows 

(for example, Grotto Cove; east of Trapper Creek) 

mapped as unit aa lack ol xenocrysts and characteristic pl laths <1 mm long. Altered rocks of caldera 

wall have groundmass locally stained moderate 

reddish brown by Fe-oxide in fractures or oxidized 

pale or grayish red; pl typically colorless or light 

brownish gray when extensively sieved, but may 

be altered to dusky yellow or oxidized dark reddish brown; joint surfaces may be grayish-red to 

dusky yellow. Intensely altered lava at Chaski Bay 

contains disseminated pyrite, has greenish-gray 

to grayish-red-purple groundmass, and weathers 

yellow brown; there, flow-top breccia is light bluish 

gray weathering to grayish orange. Medium-lightto medium-dark-gray enclaves (55–58% SiO2

) as 

much as 20 cm across present in many flows; may 

be oxidized grayish red or altered dusky yellow. 

Common, variably oriented, small glassy columns 

and related breccia at flow bases and margins suggest chilling against ice (for example, Wineglass, 

Dutton Ridge, west of Maklaks Pass, and along Rim 

Drive southeast of Kerr Notch; MIS 8). Top of unit 

aa flows removed by glaciation before being overridden by lava of slightly younger units ag south of 

Garfield Peak and dcn north of Little Castle Creek. 

Overlies units asw, dw, rpb, dpn, db, ak, dr, dm, 

ac, af, awe, and arv; overlain by dg, dcn, ag, agc, 

aww, dcp, dc, als, dv, and re. K-Ar ages: 269±12 

ka, highest flow below ag south of Garfield Peak; 

258±7 ka, west of Dutton Creek; 258±8 ka, top flow 

west of Rim Village; 245±10 ka, top flow, Sentinel 

Rock; 244±4 ka, top of Dutton Cliff; 231±10 ka, 

first flow above dr at Sentinel Rock; 211±16 ka, top 

flow ~300 m south of Applegate Peak

awe Andesite east of Wineglass (middle Pleistocene)—

Strongly porphyritic medium-dark-gray andesite (60–60.5% SiO2

) lava flows locally exposed 

low in northeast caldera wall for ~0.5 km east of 

Wineglass (see Panorama H, sheet 4). Phenocrysts 

(~40%, including pl microphenocrysts): pl (≤3 

mm, rarely to 6 mm), ol (≤3 mm), aug (≤3 mm), 

opx (≤2.5 mm), and Fe-Ti oxides (≤0.2 mm; few) 

in a fine-grained groundmass. Ol-gabbro microxenoliths (≤7 mm) containing coarse pl + ol + aug + 

opx abundant. Overlies units ac and g; overlain by 

unit aa, cut by dike of aa. Undated. Probably about 

220–240 ka

ac Andesite of Cloudcap Bay (middle Pleistocene)—Moderately porphyritic medium-light- to dark-gray 

andesite (55.5–60% SiO2; most >57) sheet-like lava 

flows midway up the caldera wall between Grotto 

Cove and Cloudcap Bay and east-northeast-trending 

andesite (57.5–61% SiO2) dikes south of lava outcrops (Panoramas H, I; sheet 4). Presence of dikes 

and relative abundance of oxidized reddish-brown 

scoriaceous material indicate source vent was above 

Cloudcap Bay. Phenocrysts (10–20%): pl (≤6 mm, 

most <2 mm; some crystals may be resorbed and 

finely sieved), ol (<3 mm, most <1 mm, commonly 

euhedral; not observed in all flows), aug (≤3 mm, 

most <0.5 mm), opx (≤1.5 mm, most <0.5 mm; 

more abundant than aug), and Fe-Ti oxide (≤0.4 

mm, generally in or with pyroxenes) in a pilotaxitic groundmass charged with 0.05–0.4 mm pl laths. 

Unit is characterized petrographically by groundmass texture, small size of mafic silicates (typically 

≤0.5 mm), opx more abundant than aug, paucity of 

discrete Fe-Ti oxide microphenocrysts, and abundant aggregates (typically ≤0.3 mm, rarely ≤12 mm) 

of pl + aug + opx ± oxide ± ol ± melt (vary in grain 

size from ≤0.5 to ≤3 mm; some are subophitic). 

Rare enclaves (≤3 cm) contain phenocrysts similar 

to those of host lavas. Overlies unit dr; overlain by 

units aa, dcp, dc, and re. K-Ar ages: 231±6 ka and 

288±13 ka; older age probably more accurate

dm Dacite of Munson Ridge (middle Pleistocene)—Porphyritic medium-light- to medium-dark-gray dacite lava 
36

(64.5–65% SiO2) forming crest of Munson Ridge 

from 1 km south of Rim Drive to Munson Point. 

Vitric breccia and columns on upper surface, particularly well developed on south-southeast flank of 

Munson Point, indicate that lava flowed beneath glacial ice. Joint and flow-banding patterns, and relatively abundant enclaves there, suggest vent beneath 

Munson Point; other vents or eruptive fissures may 

be present beneath Munson Ridge. Phenocrysts 

(~30%): pl (typically ≤3 mm but some to 5 mm), 

aug (≤3 mm), opx (≤2.5 mm), and Fe-Ti oxide (<1 

mm) in glassy to devitrified groundmass. Phenocrysts apparently derived from aggregates composed of coarse-grained ol gabbro microxenoliths 

(≤7 mm) and crystal mush consisting of euhedral pl 

(0.2–1.0 mm) + aug + opx + Fe-Ti oxides in glass. 

Most aggregates <1 cm, rarely to 15 cm (56.5% 

SiO2). Pl phenocrysts commonly resorbed near margins. Andesitic enclaves (58.5–59.5% SiO2

) to 15 

cm uncommon, may contain rare 1 mm ol. Unit is 

cut by north-south normal fault northwest of Annie 

Spring; overlies units baw, bx, and db; overlain by 

unit aa; units aa, dg, and ag bank against dm. K-Ar 

age: 276±11 ka (MIS 8)

af Andesite west of Fumarole Bay (middle Pleistocene)—

Finely porphyritic medium-light-gray to mediumgray andesite (61.5–62% SiO2

) lava flows low in the 

west caldera wall between Discovery Point and The 

Watchman (also see Panorama A, sheet 4). Phenocrysts (40%): sharply euhedral pl (≤1 mm, rarely to 

3 mm; seriate), aug (≤0.3 mm, rarely to 0.6 mm), 

and opx (≤1.7 mm, most ≤1 mm; much more abundant than aug) in a fine-grained groundmass. Pl 

phenocrysts commonly in stellate clusters (≤2 mm); 

aggregates (≤2.5 mm) of pl + opx ± aug also may be 

present. Rare xenocrysts of pl (≤3 mm, resorbed and 

sieved) or hbl (≤6 mm; larger are oikocrysts) present 

in a few samples. High Al2O3 and Sr and low incompatible element concentrations suggest pl accumulation. Overlies unit arv; overlain by units aa and 

aww. K-Ar age: 276±8 ka

arv Andesite below Rim Village (middle Pleistocene)—Porphyritic medium-gray andesite (59–62% SiO2

) forming lower part of southwest caldera wall between 

Rim Village and Discovery Point (Panorama A, sheet 

4). Phenocrysts (~20% phenocrysts; ~40–50% total, 

including microphenocrysts): pl (to 5 mm but typically ≤3 mm), aug (≤3 mm but typically ≤1.5 mm), 

opx (≤1.5 mm), and Fe-Ti oxide (≤0.5 mm) in a finegrained groundmass. Small hbl phenocrysts (≤0.3 

mm, rarely to 0.8 mm; <1%) common but not ubiquitous. Aggregates and microxenoliths of pl + aug + 

opx + oxide as much as ~6 mm across are common. 

Distinctive texture displaying large phenocrysts and 

apparently separate population of microphenocrysts 

(≤0.3 mm) in fine-grained groundmass and common 

occurrence of hornblende (not visible in hand 

specimen) characterize most flows mapped as arv. 

Locally altered dark gray with light-olive-brown 

pl; reddish- to grayish-brown Fe-oxide staining on 

joint surfaces and locally within rock. Overlies unit 

db; overlain by af and aa. K-Ar age near lake level 

below Discovery Point: 302±10 ka. Radiating small 

glassy columns in basal outcrop near lake level suggest contact with ice, consistent with 302 ka age near 

onset of MIS 8

dr Dacite of Sentinel Rock (middle Pleistocene)—Porphyritic medium-light- to medium-dark-gray dacite 

lava flows (63–65.5% SiO2) in southeast caldera 

wall between Skell Head and Kerr Notch (Panoramas I, J; sheet 4); related fragmental deposit forms 

lower half of unit ~500 m south of Skell Head. 

Phenocrysts (15–25%): blocky pl (≤5 mm; most 

1–2 mm; moderately resorbed and sparsely sieved 

with coarse melt inclusions), aug (≤2.5 mm), opx 

(≤1.3 mm), and Fe-Ti oxides (≤0.5 mm) in very fine 

grained or devitrified groundmass. Pl and pyroxenes 

~0.1–0.4 mm, apparently derived from enclaves, 

vary in abundance but are generally <5%. Larger 

phenocrysts appear to have originated as gabbroic 

crystal mush represented by common fragments 

(≤7 mm). Enclave fragments ubiquitous except in 

dike (66% SiO2) and lowest flow on north side of 

Sentinel Rock, which have relatively few large pl 

phenocrysts and appear to lack crystals derived from 

enclaves. Basaltic andesitic enclaves (55.5–56.5% 

SiO2) carry gabbroic aggregates and microxenoliths 

that commonly contain ol (≤2 mm). Exposures of dr

immediately north of Kerr Notch altered. Overlies 

and cuts glaciated unit ak; overlain by ac and aa. 

K-Ar ages: 306±5 ka, lowest flow south of Sentinel 

Rock; 336±6 ka, Cloudcap Bay

ak Andesite of Kerr Notch (middle Pleistocene)—Variably 

porphyritic andesite and dacite (59–66% SiO2; most 

60–63%) lava flows exposed in caldera wall from 

Skell Head (east) to Chaski Bay (south) and in the 

valleys of Sun and Sand Creeks up to 7 km southeast 

of the caldera. Phenocrysts in andesites (5–20%): pl 

(most <4 mm, rarely to 6 mm), aug (≤1.6 mm), opx 

(≤1 mm), and Fe-Ti oxide; oriented pl laths ≤0.4 

mm common to abundant in some flows, resulting 

in total crystal contents of ~30%. Phenocrysts in 

dacite (20–25%): pl (≤3 mm), aug (≤1.5 mm), opx 

(≤1 mm), and Fe-Ti oxide. Enclaves as much as 15 

cm across, enclave fragments, and medium-grained 

pl + pyroxene aggregates as large as ~5 mm present in some andesite and dacite flows. Fresh rock 

has medium- to dark-gray groundmass, which is 
37

variably altered dark reddish brown in some specimens; alteration, restricted to vicinity of caldera, less 

intense than in underlying units; pl generally colorless but may be dusky yellow or dark reddish brown; 

reddish-brown Fe-oxide stain marking healed 

fractures common; below Applegate Peak joint surfaces altered yellowish gray to moderate reddish 

brown. Overlies units db, dpn, apn, rpb, and as; 

overlain by dr, aa, and dc. Distinguished from contiguous db and aa by smaller and fewer phenocrysts 

and presence of andesites with abundant pl ≤0.4 

mm; where well exposed in caldera wall or road 

cuts, separated from these units by erosion surfaces 

and sedimentary deposits or paleosol. K-Ar ages: 

341±8 ka and 340±6 ka, Cloudcap Bay; 315±8 ka, 

east of Sand Creek; 307±12 ka, Sun Valley; 306±7 

ka (40Ar/39Ar plateau age 303±3 ka), lithic block in 

ascp southeast of Mount Scott

db Dacite of Chaski Bay (middle Pleistocene)—Porphyritic 

andesite and dacite (59.5–67% SiO2; predominantly 

low-silica dacite) lava flows and breccia exposed 

low in the south caldera wall from Kerr Notch to 

Rim Village (Panoramas A, J, K; sheet 4), in Kerr 

and Munson Valleys, and near the headwaters of the 

middle and east forks of Annie Creek. Outcrops of 

unit are accessible along the abandoned road on the 

northeast side of Kerr Valley 0.7–1.5 km south of 

the Kerr Notch overlook and on the Garfield Peak 

trail below 7,550 ft elev. Phenocrysts (25–30%): 

pl (≤5 mm, commonly coarsely sieved), aug (≤1.5 

mm), opx (≤1 mm, commonly altered), and Fe-Ti 

oxide; ol ≤2.5 mm occurs in a few flows. Most flows 

characterized by several percent of pl laths ≤0.2 mm 

in fine-grained granular groundmass, common aggregates of pl + aug + opx crystals similar to phenocrysts, 

and enclave fragments as large as several millimeters 

(source of ≤0.2 mm pl laths?). Andesitic enclaves as 

large as 50 cm common to abundant. Near caldera, 

groundmass altered brownish, light brownish, greenish, or light bluish gray in hand specimen; pl colorless or light olive brown; reddish- to grayish-brown 

Fe-oxide staining on joint surfaces and locally within 

rock; enclaves greenish gray to pale olive; relatively 

porous breccias and lava flow tops more conspicuously altered than dense flow interiors. Unit may 

contain lavas from more than one eruptive center. 

Overlies units apn and rpb; overlain by ak, arv, aa, 

dg, ag, amv, dm, dv and dvb. Prominent grayishyellow-green, pumiceous mudflow(?) and overlying 

sand and gravel rest on eroded db lava beneath unit 

ak in caldera wall at Sun Notch. K-Ar ages: 382±8 

ka, east fork of Annie Creek; 351±12 ka, below Rim 

Village; 40Ar/39Ar plateau age 354±4 ka (333±6 ka by 

K-Ar), below Garfield Peak

ams Andesite south of Mount Scott (middle Pleistocene)—

Porphyritic medium-gray silicic andesite lava (63% 

SiO2

) forming small dome and flow remnants on 

south flank of Mount Scott. Phenocrysts (30%): 

pl (≤3.5 mm, rounded and resorbed, some finely 

sieved), aug (≤1.0 mm), opx (≤1.0 mm), and Fe-Ti 

oxides (≤0.4 mm) in a fine-grained groundmass. 

Microxenoliths (<1 cm) of pl + aug + opx and basaltic andesite (56% SiO2

) enclaves (≤15 cm) common. 

Overlies unit rsc; appears to be banked against ds. 

Undated. Relations with these units and degree of 

erosion suggest an age of ~300–400 ka

dpn Dacite of phantom cone (middle Pleistocene)—Porphyritic medium-gray dacite forming lowest exposed 

lava flow below Rim Drive ~1 km south of Kerr 

Notch southeast of the caldera. Correlated with 

identical intrusive dacite (66% SiO2) feeding lava 

at the top of phantom cone in the southeast caldera 

wall (Panoramas J, K; sheet 4). Ubiquitous (but 

<1%) resorbed quartz xenocrysts (≤1 mm) visible 

in thin section are unique to this unit. Phenocrysts 

(~25%); pl (≤5 mm, rounded; some finely sieved; 

many in aggregates), aug (≤2.5 mm, rarely to 5 

mm), opx (≤1 mm), and Fe-Ti oxide (≤0.3 mm) 

in a very fine grained groundmass. Enclaves to 1 

m abundant (~10%) in all outcrops; enclave fragments abundant in thin section, accompanied by 

less-common gabbro microxenoliths (typically ≤5 

mm). Rock is altered, showing oxidized patches of 

dark reddish brown in Kerr Valley; more intense 

alteration of caldera wall exposures adds patches of 

yellowish-brown clay, most noticeable in enclaves. 

Overlies and cuts unit apn; overlain by ak and aa. 

Undated: probably virtually same age as youngest 

apn (K-Ar age: 346±20 ka)

ds

dsf

dsp

Dacite of Mount Scott (middle Pleistocene)—Porphyritic 

medium-gray dacite (63.5–67% SiO2

; most ≥65%) 

erupted from Mount Scott and related vents to east. 

Unit grades from pyroclastic breccia and agglutinate 

sheets high on Mount Scott itself to massive lava (ds) 

distally and is everywhere characterized by basaltic andesitic enclaves (≤50 cm; 53.5–56.5% SiO2

), 

which are abundant in some exposures. Greatest 

concentration of enclaves in outcrops on southeast 

ridge of Mount Scott, in which enclaves are virtually touching, suggests ejection as rigid clasts in 

eruptive fountains and accumulation in near-vent 

agglutinate, followed by flowage. Related mudflow 

breccias (dsf) are locally preserved high on the north 

side of Mount Scott; oxidized indurated cinder and 

agglutinate (dsp) marks a flank vent on the northeast. Like those of many other andesites and dacites 

of Mount Mazama, ds textures result from mixtures 

of varying proportions of silicic melt, undercooled 
38

enclave material, cumulate crystal mush, and gabbroic microxenoliths. Phenocrysts (25–30%): pl 

(≤5 mm; commonly coarsely sieved, blocky crystals and crystal fragments), aug (≤1.2 mm, rarely 

to 5 mm), opx (≤1.2 mm), and Fe-Ti oxides (≤0.2 

mm) in a glassy to very fine grained groundmass. 

Least silicic flows contain ~10 percent phenocrysts 

and ~20 percent 0.1–0.4 mm pl laths and pyroxene 

crystals, apparently derived from enclaves. Enclave 

fragments are ubiquitous, commonly accounting for 

several percent of a cut surface. Many of the abundant gabbroic and diabasic microxenoliths and crystal aggregates (≤8 mm) contain ol (typically ≤1.5 

mm, rarely to 4 mm). Intensely altered to residual 

silica + minor specular hematite locally on Mount 

Scott; clinopyroxene + K-feldspar present in vugs 

and on fracture surfaces and replaces groundmass of 

rock in lowest exposures in cirque. Lava flows of ds

extend 8 km east of Mount Scott, over rsc and rpb. 

Lowest flows exposed below Anderson Bluffs, from 

a source to the north, are provisionally mapped as 

ds? and are overlain by ak. Elsewhere, ds is overlain by dc and asc. K-Ar ages: 422±10 ka, west side 

of cirque; 416±7 ka, east flank; 355±8 ka, ~5,650 ft 

elev east of Mount Scott

dcc Dacite of Cavern Creek (middle? Pleistocene)—Finely 

porphyritic medium-gray dacite (64.5% SiO2

) lava 

exposed south-southeast of Mount Scott where 

Cavern Creek enters Pinnacle Valley and ~1 km to 

west. Phenocrysts: (~15%): pl (≤2 mm), aug (≤1 

mm), opx (≤0.8 mm), and Fe-Ti oxide (≤0.2 mm) in 

a glassy to devitrified groundmass containing abundant <0.1 mm pl laths. Aggregates ≤3 mm abundant. 

Apparently issued from vent to north and flowed 

south and west around hill composed of unit rcc. 

Assigned to Mount Mazama on basis of texture and 

composition. Undated: believed to be younger than 

rcc (K-Ar age 724±5 ka)

apn

apni

Andesite of phantom cone (middle Pleistocene)—Moderately porphyritic silicic andesite (60.5–61.5, 

rarely 63.5% SiO2) indurated near-vent fall deposits and lava flows (apn) and dikes and larger intrusions (apni) of phantom cone (Williams, 1942) 

form approximately the lower half of the southeast 

subaerial caldera wall adjacent to Phantom Ship 

(See Panoramas J, K; sheet 4). Phantom Ship island 

is composed of altered apn lava flows. Phenocrysts 

(10–20%): pl (≤3.5 mm, rarely to 6 mm; most <2 

mm), aug (≤1 mm), opx (≤1.3 mm), and Fe-Ti oxide; 

abundant pl microphenocrysts, commonly oriented, 

give rock a total crystal content of ~30 percent. 

Medium-grained pl + aug + opx ± ol aggregates 

common. Basaltic andesitic enclaves to 1 m present 

in intrusive rock, smaller in lava flows, generally 

altered dusky yellow. Medium- to dark-gray groundmass of dense rocks may be altered dark bluish 

gray or greenish black in hand specimen; pl colorless or light olive brown; reddish- to grayish-brown 

Fe-oxide staining on joint surfaces and within rock; 

relatively porous pyroclastic breccias and lava flow 

tops conspicuously altered to pale-olive, grayishyellow-green, dusky yellow, or light-olive-gray 

colors. Overlain by units aa, ak, db, and dpn. K-Ar 

ages: 403±12 ka at lake level southeast of Phantom 

Ship; 346±20 ka at lake level at Kerr Notch 

su Submerged outcrops of the caldera walls, undivided 

(Pleistocene)—Bedrock outcrops revealed by bathymetric survey in 2000 of Crater Lake (Bacon and 

others, 2002) that have not been correlated with 

exposed map units. Some of the deeper outcrop 

areas may be large slump blocks

Regional volcanism, east

asc

ascp

Andesite of Scott Creek (late? Pleistocene)—Porphyritic medium-gray ol andesite (57.5–59% SiO2

) lava 

flow (asc) and source vent marked by eroded cinder 

cone (ascp) of Hill 6587 southeast of caldera. Lava 

crops out locally south of Scott Creek above ~5,350 

ft and forms incised intracanyon flow below ~5,220 

ft elev. Phenocrysts (~15%): pl (≤4 mm), ol (≤3 

mm; abundant, sharply euhedral to skeletal crystals), aug (typically ≤1.5 mm, rarely to 3 mm), opx 

(≤2 mm), and Fe-Ti oxide (≤0.2 mm, uncommon to 

rare) in a pilotaxitic groundmass. Microxenoliths 

(≤1 cm) common but not abundant, typically coarse 

granular but may be diabasic; pl ± ol ± aug + opx + 

oxide. Overlies units rsc, rpb, and ds; overlain by 

g. Degree of erosion of intracanyon portion of flow 

and presence of glacial till on lava indicate eruption before last glacial maximum, consistent with 

40Ar/39Ar isochron age of 87±15 ka

acr

acrp

Andesite of Crater Peak (late Pleistocene)—Moderately 

porphyritic medium-gray to medium-dark-gray 

andesite lava flows (acr) and Crater Peak cinder 

cone (acrp) marking vent south of the caldera. 

Lava flowed in topographic depressions between 

flows and domes of three map units of pre-Mazama 

rhyodacite and thence into drainages of Annie and 

Sun Creeks. Exposed in Annie Creek upstream 

from confluence with East Fork Annie Creek and 

west of Sun Creek as far south as park boundary, ~8 

km from vent. Phenocrysts (15–20%): pl (≤5 mm, 

some resorbed and finely sieved), ol (≤2 mm, abundant; commonly resorbed or hopper shaped), aug 

(≤2 mm, rarely to 4 mm), opx (≤1.6 mm), and Fe-Ti 

oxide (≤0.3 mm; uncommon; typically associated 
39

with pyroxene) in a pilotaxitic groundmass. Rare 

amphibole pseudomorphs (≤0.5 mm) present in one 

sample. Aggregates (≤5 mm, rarely 10 mm) of pl 

+ aug + opx ± ol ± oxide ubiquitous. In the field, 

unit is characterized by large pl, conspicuous aggregates, and abundant ol in fine-grained groundmass. 

Rock and crystal compositions imply that lavas are 

mixtures of basaltic or basaltic andesitic magma 

and dacitic magma ± plutonic rock (Bacon, 1990). 

Units bcnp and bcsp may represent the mafic end 

member of this mixture. All four units would have 

been erupted during a short period of time along 

the same north-south fissure system, with the relatively voluminous unit acr venting from a longerlived, central conduit. Crater Peak may be a satellitic 

vent of Mount Mazama rather than a monogenetic 

regional volcano. Overlies units rsc, rcs, and rpb. 

K-Ar ages: 98±8 ka and 136±8 ka

bcsp Basaltic andesite south of Crater Peak (late Pleistocene)—Medium-dark-gray sparsely porphyritic 

basaltic andesite (53.5% SiO2

) bombs and lava of 

small exposure that probably mark vent ~1 km south 

of Crater Peak south of the caldera. Phenocrysts 

(7%): pl (≤3.5 mm, resorbed and finely sieved) and 

abundant ol (≤3.5 mm, resorbed and hopper shaped) 

in a pilotaxitic groundmass. Contains aggregates 

(≤5 mm) of pl + aug and fused xenoliths (≤1.5 cm) 

of porphyritic rhyodacite. Believed to approximate 

mafic end member in hybrid lavas of unit acr. 

Compositionally similar to unit bcnp. Overlies unit 

rcs. K-Ar age: 118±48 ka (indistinguishable from 

that of acr)

bcnp Basaltic andesite north of Crater Peak (late? Pleistocene)—Grayish-red oxidized basaltic andesite 

(53.5% SiO2) cinders and agglutinate of poorly 

exposed cinder cone banked against east wall of 

canyon of East Fork of Annie Creek ~2 km north 

of Crater Peak, south of the caldera. Phenocrysts 

(3%): rare pl (≤2 mm) and abundant ol (≤3 mm, 

resorbed and hopper shaped) in a pilotaxitic groundmass. Fused rhyodacite xenoliths (≤2 cm) common. 

Thought to be contemporaneous with compositionally similar unit bcsp which is believed to approximate mafic end member in hybrid lavas of unit acr. 

Overlies unit rpb. K-Ar age: 207±53 ka (believed to 

be too old owing to excess 40Ar derived from rhyodacite xenoliths) 

alb

albp

Andesite south of Lookout Butte (middle Pleistocene)—

Sparsely porphyritic medium-dark- to dark-gray 

andesite (59% SiO2) lava (alb) and cinder cone 

(albp) marking vent near northeast map boundary. 

Phenocrysts (10%): pl (≤3 mm, resorbed and sieved; 

commonly in clusters), ol (≤1.6 mm, euhedral to 

hopper shaped; clusters to 3 mm), aug (≤2 mm), and 

opx (≤1.5 mm, rounded, commonly rimmed by aug) in 

a pilotaxitic groundmass rich in 0.1–0.3 mm pl laths. 

Microxenoliths (≤4 mm) of pl + aug ± opx ± Fe-Ti 

oxide ubiquitous. Disequilibrium phenocryst assemblage indicates this andesite is a hybrid or contaminated lava. Overlies unit rpb. K-Ar age: 155±6 ka

bsw Basalt west of Sun Creek (middle Pleistocene)—

Medium-gray coarsely porphyritic basalt (51% 

SiO2

) lava west of Sun Creek near south boundary of map. Apparently from subtle vent at ~5,100 

ft elev, 0.7 km south-southeast of Hill 5765. Phenocrysts (~10%): ol (<2.5 mm, rarely to 6 mm), 

pl (≤4 mm; commonly sieved), and aug (≤3 mm; 

commonly has resorbed opx core) in an intergranular groundmass. Many phenocrysts are from same 

source as abundant microxenoliths (≤1 cm) of ol + pl 

(uncommonly + aug). Overlies unit rsc (K-Ar age: 

415±11 ka). Undated. Degree of preservation suggests late middle Pleistocene age

bmc

bmcp

Basaltic andesite of Maklaks Crater (middle Pleistocene)—Grayish-red cinders (bmcp) of Maklaks 

Crater cone and associated medium-dark-gray finely 

porphyritic basaltic andesite (53.5% SiO2) lava 

(bmc) southeast of the caldera. Phenocrysts (~10%, 

exclusive of abundant pl microphenocrysts): pl (≤1 

mm) and ol (≤0.6 mm) in a pilotaxitic groundmass. 

Microxenoliths (≤4 mm) of pl + ol common. Overlies units rsc and rpb. K-Ar age: 220±67 ka

bsr

bsrp

Basalt of Sand Ridge (middle Pleistocene)—Brownishgray porphyritic basalt (52% SiO2

) lava (bsr) and 

cinders (bsrp) of eroded cone at southeast boundary of map. Phenocrysts (~10%): pl (≤1.5 mm; commonly in clusters) and ol (≤0.5 mm, rarely to 1 mm; 

oxidized) in a fine-grained pilotaxitic groundmass. 

Appears to be younger than unit rpb, which makes 

up the north end of adjacent Sand Ridge. Undated

bt

btp

Basaltic andesite northeast of Boundary Butte 

(middle Pleistocene?)—Medium-dark-gray porphyritic basaltic andesite (54.5–56.5% SiO2

) lava 

(bt) and grayish-red cinders (btp) of cone 3 km 

east-northeast of Boundary Butte and low hill 1 km 

farther north, southeast corner of map. Phenocryst 

texture and incompatible element abundances suggest affinity with unit bap. Characterized in hand 

specimen by relatively abundant and large pl phenocrysts. Phenocrysts (5–10%): pl (≤3 mm, rarely 

to 5 mm; finely sieved) and ol (≤0.5 mm, rarely to 1 

mm; commonly skeletal) in a pilotaxitic or intersertal groundmass. Microxenoliths (≤2 mm) of pl + 

aug present. Undated. Morphology suggests middle 

Pleistocene age

rpb

rpbv

Rhyodacite of Pothole Butte (middle Pleistocene)—Porphyritic light- to medium-gray and light-brownishgray felsite (rpb) and local dark-gray vitrophyre 
40

(largest exposure, 1 km north-northwest of Pothole 

Butte, mapped as rpbv) (71.5–73% SiO2) lava 

domes and thick lava flows east and south of Mount 

Mazama. Several prominent domes northeast of 

Mount Mazama are composed of unit rpb, which 

also is exposed south of Scott Creek, on Grayback 

and Sand Ridges, between Sand and Cavern Creeks, 

west of Sun and Sand Creeks, and in the valley of 

the east fork of Annie Creek. Northernmost exposure west of Sand Creek hydrothermally altered. 

Phenocryst content, chemical composition, and 

paleomagnetic pole position distinguish unit from 

other pre-Mazama rhyodacites (Nakada and others, 

1994). Phenocrysts (10–20%): pl (≤2 mm, rarely to 

5 mm), opx (≤1 mm, rarely to 2.5 mm), aug (≤1 mm, 

rarely to 3 mm), and Fe-Ti oxides (≤0.2 mm, rarely 

to 0.5 mm) in a vitric to finely crystalline groundmass. Common aggregates or microxenoliths (typically ≤4 mm but to 7 mm) of pl ± opx ± aug ± oxides 

are source of many phenocrysts. Andesitic enclaves 

(≤10 cm; 57.5–60% SiO2

) rare. Pleistocene glaciation has removed virtually all pumiceous carapace so 

that exposures are typically felsite characterized by 

centimeter-scale platy joints, subhorizontal in flow 

interiors and steeply inclined at flow tops and margins. Vitrophyre is locally exposed near flow bases 

and in rarely preserved upper vitric zones. Overlies 

units dw, asw, blp, and rcs; overlain by ds, ak, aa, 

bsr, alb, acr, and asc. Age relation to rsc south of 

Scott Creek ambiguous; elsewhere, appears to be 

younger than rsc. K-Ar ages: 468±9 ka (west of Sun 

Creek) and 448±8 ka (Pothole Butte)

rcs Rhyodacite south of Crater Peak (middle Pleistocene)—Porphyritic light- to medium-gray and lightbrownish-gray felsite and local dark-gray vitrophyre 

(71–72% SiO2) lava flows south of the caldera, west 

of Sun Creek, and south of Crater Peak. Phenocryst 

content and chemical composition distinguish rcs

from other pre-Mazama rhyodacites (Nakada and 

others, 1994). Phenocrysts (20–25%): pl (typically 

≤1 mm, but up to 6 mm), opx (≤1.5 mm), aug (≤0.8 

mm, typically less abundant than opx), Fe-Ti oxides 

(≤0.3 mm, but up to 0.5 mm), and, rarely, hbl (≤0.8 

mm) in a vitric to finely crystalline groundmass. 

Common aggregates or microxenoliths (≤5 mm) of 

pl ± opx ± aug ± oxides are source of many phenocrysts. Small andesitic enclaves (≤3 mm) common; 

rare examples are up to 15 cm. Pumiceous carapace 

removed by Pleistocene glaciation, except upper 

vitrophyre at top of flow west of Sun Creek; typical felsite exposures characterized by centimeterscale platy joints, subhorizontal in flow interiors 

and steeply inclined at flow tops and margins. Basal 

vitrophyre locally exposed east of Annie Creek. 

Overlies units baf, aaf, and rsc; overlain by rpb, 

bcsp, and acr. Undated. K-Ar ages of contiguous 

units suggest age of rcs is about 450 ka. Glass is 

compositionally similar to that from tephra layers 

above the Rockland ash bed and below the Bend 

Pumice in sediment cores from Buck Lake and 

Wocus Marsh, ~75 km south of Crater Lake, and 

from Mohawk Lake in the northern Sierra Nevada, 

(A.M. Sarna-Wojcicki, written commun., 2002), 

consistent with an age of about 450–550 ka

rsc Rhyodacite of Scott Creek (middle Pleistocene)—Porphyritic light- to medium-gray and light-brownishgray felsite and local medium-dark-gray vitrophyre 

(68–71.5% SiO2

) lava domes and lava flows east 

and south of Mount Mazama. Forms several prominent domes and thick flows north and south of 

Scott Creek. Also present locally from west of Bear 

Butte to southeast of Pothole Butte, north and west 

of Maklaks Crater, and south of Crater Peak; with 

further study, some of these occurrences might be 

assigned to additional units. Sample collected with 

submersible at 456 m depth in Crater Lake (possibly derived from outcrops at ≤330 m depth) ~500 m 

southwest of Palisade Point provisionally identified 

as rsc. Chemical composition, phenocryst content, 

and paleomagnetic pole position distinguish unit 

from other pre-Mazama rhyodacites (Nakada and 

others, 1994). Phenocrysts (typically 25–35%, but 

may be as little as ~10%): pl (≤2 mm, rarely to 5 

mm), opx (≤2 mm), aug (≤1 mm, rarely to 3 mm), 

and Fe-Ti oxides (≤0.2 mm, rarely to 0.5 mm) in a 

vitric to finely crystalline groundmass; hbl (≤1 mm) 

present in rare specimens. Common aggregates or 

microxenoliths (typically ≤5 mm, rarely to 20 cm) 

of pl ± opx ± aug ± oxides ± hbl (rare) are source 

of many phenocrysts; relatively fine grained aggregates (pl ~0.1–1 mm) also present in many specimens. Andesitic enclaves (≤30 cm, rarely to 80 cm; 

54–59% SiO2

) rare except east of Cavern Creek in 

two domes 0.3 km south and 1 km northwest of Hill 

6358. Pleistocene glaciation has removed pumiceous 

carapace; exposures are typically felsite characterized by centimeter-scale platy joints, subhorizontal 

in flow interiors and steeply inclined at flow tops and 

margins. Rare vitrophyre exposures are upper vitric 

zone. Flowed against dome of unit dd; overlain by 

units rcs, ds, ams, bep, bmc, bsw, and asc. Age 

relation to rpb south of Scott Creek ambiguous; 

elsewhere, appears to be older than rpb. K-Ar age 

415±11 ka (flow north of Dry Butte; inconsistent 

with apparent age relation elsewhere with unit rpb). 

Glass is compositionally similar to that from tephra 

layers that occur above the Rockland ash bed and 

below the Bend Pumice in sediment cores from Buck 
41

Lake and Wocus Marsh ~75 km south of Crater Lake 

(A.M. Sarna-Wojcicki, written commun., 2002); 

consistent with an age of about 450–550 ka

aq Andesite of Sand Creek quarry (middle? Pleistocene)—

Porphyritic medium-gray olivine andesite (59% 

SiO2) lava exposed at top of north-northeast-trending down-to-the-east normal fault scarp from vicinity of Sand Creek to north flank of low hill (elev 

~5,420 ft) ~2.5 km south of Sand Creek Pinnacles; 

forms upper lava flow at Sand Creek quarry; also 

exposed in cut on Hwy 232 at 5,100 ft elev, southeast corner of map. Phenocrysts (~20%): pl (≤2.5 

mm; many are coarsely sieved), ol (≤1.5 mm), aug 

(≤1.3 mm), and Fe-Ti oxides (≤0.3 mm; few) in an 

intergranular groundmass. Aggregates (≤2 mm) of 

pl + aug ± oxide and pl + ol moderately common. 

Overlies probable unit acs? at Sand Creek quarry; 

if this correlation is correct, lava of unit aq evidently 

flowed around north side of hill ~5,420 ft marking 

vent for acs. Undated 

aaf Andesite northeast of Annie Falls (middle Pleistocene)—Dark-gray porphyritic andesite (57.5% SiO2

) 

flow exposed at base of cliffs 1 km north-northeast 

of Annie Falls, south-central part of map. Joint pattern suggests source north-northeast of outcrops. 

Phenocrysts (15%): pl (0.1–2 mm, commonly sieved 

and resorbed), ol (≤2 mm, commonly in ≤4 mm clots 

with pl), aug (≤1 mm, commonly in ≤2 m clots with 

pl), and Fe-Ti oxides (≤0.1 mm; many opaques are 

oxidized ol) in a fine-grained pilotaxitic groundmass. 

Overlain by unit rcs. K-Ar age: 517±15 ka

bep Basaltic andesite east of Cavern Creek (middle? Pleistocene)—Grayish-red basaltic andesite (52.5% SiO2

) 

cinders and bombs locally exposed on eroded cinder 

cone east of Cavern Creek southeast of Mount Scott. 

Phenocrysts (~25%, excluding microphenocrysts): 

pl (≤2.5 mm; coarsely sieved), ol (≤1 mm; oxidized), 

and aug (≤2 mm; resorbed) in a vesicular, finegrained groundmass rich in pl microphenocrysts. 

Appears to be older than unit rsc (K-Ar age: 415±11 

ka). Undated

acs

acsp

Andesite south of Sand Creek (middle? Pleistocene)—

Sparsely porphyritic medium-gray hornblende andesite (61.5–62.5% SiO2

) lava (acs) and cinders (acsp) 

poorly exposed on low hill (elev ~5,420 ft) marking 

vent ~2.5 km south of Sand Creek Pinnacles, southeastern part of map. Phenocrysts (~3%): pl (≤3 mm), 

ol (≤0.5 mm; present in quarry sample), hbl (≤0.6 

mm; as opaque pseudomorphs), aug (≤0.8 mm), opx 

(≤0.8 mm), and Fe-Ti oxide (≤0.1 mm) in a finegrained pilotaxitic (vent hill) to trachytic (quarry) 

groundmass. Welded spatter at vent charged with ≤1 

cm angular olivine basaltic andesite lithic fragments. 

Probable acs? lava at bottom of Sand Creek quarry 

overlain by unit aq. Cut by down-to-the-east normal 

fault. Undated

bdp Basaltic andesite east of Dry Butte (middle? Pleistocene)—Medium-dark-gray porphyritic basaltic 

andesite (56% SiO2

) bombs and grayish-red cinders 

locally exposed on small cinder cone ~4 km east of 

Dry Butte near the southeast map boundary. Phenocrysts (10%): pl (≤2 mm, rarely to 3 mm; coarsely 

sieved) and ol (≤2 mm; commonly resorbed) in an 

intersertal groundmass. Microxenoliths (≤2 mm) of 

pl ± ol ± aug ± opx common. Undated. Morphology 

suggests middle Pleistocene age

bap Basaltic andesite of Boundary Butte (middle? Pleistocene)—Medium-dark-gray porphyritic basaltic 

andesite (53% SiO2) agglutinate and grayish-red cinders exposed in upper part of Boundary Butte cinder 

cone in southeast corner of map. Phenocryst texture and incompatible element abundances suggest 

affinity with units bt and btp. Characterized in hand 

specimen by abundant and relatively large ol phenocrysts. Phenocrysts (5%): pl (≤1.5 mm, rarely to 2.5 

mm; finely sieved) and ol (≤2 mm, rarely to 3 mm) 

in an intergranular groundmass. Microxenoliths (≤2 

mm) of pl ± ol ± aug present. Undated. Morphology 

suggests middle Pleistocene age

as Andesite of Sun Creek (middle Pleistocene)—Porphyritic medium-gray andesite (62.5% SiO2

) lava cropping out at northeast end of valley of Sun Creek 

(elev 6,200 ft), southeast of the caldera. Phenocrysts 

(~25%): pl (≤2.5 mm, rarely to 5 mm), ol (≤0.4 mm; 

sparse), aug (≤1.5 mm, rarely to 3 mm), opx (≤1.5 

mm), and Fe-Ti oxides (≤0.3 mm) in a pilotaxitic 

groundmass. Microxenoliths (≤7 mm) of pl + aug 

+ opx + oxides common. Overlain by unit ak. K-Ar 

age: 623±16 ka

dw Dacite west of The Pinnacles (middle Pleistocene)—

Coarsely porphyritic medium-gray dacite (64–65% 

SiO2) lava domes west of The Pinnacles and at the 

west base of Maklaks Pass southeast of the caldera (Nakada and others, 1994). Phenocrysts (20–

30%): pl (≤6 mm, rounded, commonly in clusters), 

aug (≤3 mm), opx (≤2 mm), hbl (≤0.5 mm), and 

Fe-Ti oxides (≤0.3 mm) in a fine-grained groundmass with common 0.1–0.4 mm pl laths. Common 

ol (≤2 mm) xenocrysts are encased in fine-grained 

mafic groundmass. Aggregates (typically ≤5 mm 

but rarely to 3 cm) of pl + aug + oxide ± opx ± ol 

gabbro abundant; similar pl + ol + aug + oxide crystal mush also forms streaks ~1 cm wide and many 

centimeters long in outcrops west of Pinnacles road. 

Unit is characterized by unusually abundant basaltic 

andesitic enclaves (55.5–56.5% SiO2; ≤30 cm; up to 

~20% of rock; commonly oxidized pale red), which 

vary in grain size and phenocryst content, display ol 
42

phenocrysts (≤2 mm) along with pl ± aug ± opx, and 

commonly have groundmass hbl. Enclave magma 

appears to have been the source of ol xenocrysts 

in dacite whereas many phenocrysts are derived 

from aggregates. Although of similar SiO2 content, 

enclaves vary widely in incompatible trace element 

concentrations (Nakada and others, 1994). Unit is an 

excellent example of mingling (incomplete physical 

mixing) of silicic magma (the end member composition is uncertain because analyzed samples invariably contain some xenocrysts), gabbroic crystal 

mush, and undercooled basaltic andesitic magma 

(enclaves) where complex textures can be seen in 

outcrop. Unit is overlain by units aa and rpb. K-Ar 

age: 612±8 ka below Maklaks Pass

blp Basaltic andesite north of Lookout Butte (middle Pleistocene)—Medium-gray porphyritic basaltic andesite (53% SiO2

) lava and grayish-red cinders (blp) 

exposed on small cinder cone ~1 km north-northwest of Lookout Butte in northeast corner of map. 

Phenocrysts (5%; abundant seriate pl microphenocrysts): pl (≤1.5 mm; finely sieved) and ol (≤2 mm) 

in a fine-grained groundmass rich in pl microphenocrysts. Microxenoliths (≤3 mm) of pl + ol common. 

Overlain by unit rpb. K-Ar age: 605±25 ka

asw Andesite west of Sand Creek (middle Pleistocene)—

Sparsely porphyritic medium-dark-gray silicic 

andesite or low-silica dacite (62.5% SiO2) lava cropping out in two places at west edge of valley of Sand 

Creek (~6,100 ft and ~6,300 ft elev), southeast of the 

caldera. Phenocrysts (~5%): pl (≤3 mm), aug (≤1.5 

mm), opx (≤1 mm), and Fe-Ti oxide (≤0.3 mm) in 

a pilotaxitic to trachytic groundmass. May contain 

rare hbl phenocrysts (≤0.6 mm). Small aggregates 

(≤3 mm) of pl + aug + opx + oxide common. Platy 

joints are finely spaced and fracturing is pervasive. 

Hydrothermally altered, as evidenced by amygdules 

(typically ~3 mm) partially lined with tridymite, thin 

felsic veins, and rusty fracture coatings, accompanied in some specimens by replacement of opx with 

clay minerals. Overlain by units rpb and aa. K-Ar 

age: 670±12 ka 

baf Basaltic andesite east of Annie Falls (middle Pleistocene)—Medium-gray porphyritic basaltic andesite lava flow(s) (52.5% SiO2

) forming canyon wall 

east of Annie Falls, south-central part of map. Phenocrysts (2–3%): ol (≤3 mm, most ≤1 mm; some 

crystals skeletal or hopper shaped) in a fine-grained 

intergranular groundmass. Joint pattern locally suggests source to northwest. Overlain by unit rcs. K-Ar 

age: 651±28 ka

rcc Rhyodacite west of Cavern Creek (middle Pleistocene)—Light-gray felsite and rare black obsidian 

of deeply eroded, poorly exposed lava dome of Hill 

6610 (73% SiO2) and altered porphyritic light-gray 

felsite of eastern Anderson Bluffs (72.5% SiO2) 

south of Mount Scott. Phenocrysts (~2–10%): pl (≤1 

mm, rarely to 3 mm), opx (≤0.5 mm, to 1 mm in 

porphyritic rocks), aug (≤0.5 mm), hbl (≤1.5 mm), 

and Fe-Ti oxides (≤0.2 mm) in a glassy or devitrified 

groundmass; hbl replaced by Fe-oxide + silicates and 

pyroxenes, largely altered in porphyritic rocks. Phenocrysts commonly occur in aggregates (≤3 mm). 

Dome of Hill 6610 overlain by unit ak and, apparently, dcc; Anderson Bluffs exposures overlain by 

ds and ams. K-Ar age of dome 724±5 ka

dsc Dacite of Sand Creek (early Pleistocene)—Porphyritic medium-light- to medium-dark-gray dacite 

(65.5–68% SiO2) domes forming low hills between 

Sand Creek and Dry Butte (Nakada and others, 

1994) near southeast map boundary. Phenocrysts 

(20–25%): pl (≤2.5 mm), aug (≤1.0 mm), opx (≤1.5 

mm), and Fe-Ti oxides (≤0.2 mm) in a fine-grained 

groundmass. Abundant aggregates (≤1.2 cm) contain relatively coarse pl + aug + opx + oxides, 

rarely hbl; finer grained aggregates consist of pl + 

ol + aug. The less silicic samples appear to have 

been contaminated with debris from the ol-bearing 

aggregates. Basaltic andesitic enclaves (57% SiO2; 

to 30 cm), containing same phases as in aggregates, 

uncommon. K-Ar age: 1,058±16 ka. Field relations, 

K-Ar ages, and composition of enclave suggest dsc

may be coeval with adjacent andesite plug adsi (KAr age: 1,088±13 ka)

adsi Andesite south of Dry Butte (early Pleistocene)—

Medium-gray porphyritic andesite (57% SiO2) of 

plug and minor flows exposed on northeast flank of 

hill immediately south of Dry Butte, southeast part 

of map. Phenocrysts (~3%): pl (≤2 mm, rarely to 5 

mm) and minor opx (≤1 mm) in a pilotaxitic groundmass containing abundant oxidized hornblende 

crystals (0.1–0.3 mm). Microxenoliths (≤6 mm) of 

pl ± opx ± aug ± Fe-Ti oxide uncommon. Relation to 

adjacent unit dsc uncertain but similar composition 

of andesitic enclave in dsc lava suggests dsc and 

adsi are coeval (Nakada and others, 1994). K-Ar 

age 1,088±13 ka.

dd Dacite of Dry Butte (early Pleistocene)—Porphyritic 

light-greenish-gray dacite (63% SiO2

) of poorly 

exposed domes near southeast map boundary that 

form Dry Butte. Phenocrysts (~30%): pl (≤2 mm), 

hbl (≤3 mm), opx (≤0.5 mm), and Fe-Ti oxide (≤0.1 

mm) in a pilotaxitic groundmass. Oldest unit in eastern part of map area. K-Ar age: 1,275±14 ka
43

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