rb

Qty

ah

Qty

ap

Qto

Qty

b44

Qou

av

ar

ad2

bsm

Qty

at

Qty

Qty

mrd

am

db

Qty

amh

mhr

Qty

drp

ac

awp

Tlv

ap

arp

Qto

Qf

dr

mhl

mrm

Qto

am

rk

ad2

pr

Qwb

pc

Tdb

atd

ag

brk

adc

at

Taw

Tt

am

adc

rmz

bhc

Qty

bsh

dt

mgp

bl22

a74

aa

amc

ad2

Qsj

rsm

drr

rsf

Qoy

atb

bbz

abm

rlm

arp

Qou

bwv

Qtr

Qty

Qoy

dp

Qty

Qto

amc

Tac

bs2

bcc

ab

arr

amd

Tam

Qty

Qc

Qto

bgv

rsm

Qto

Tad

b11

acs

Qto

Tbm

am

Qty

mcm

Tad

bdt

Tdm

Qsh

Qf

btb

at

amr

asp

mhr

Qor

am

bs3

Qto

mcf

abm

am

bec

Qtb

Qf

bic

Tam

Qty

Qta

Qf

att

Qto

Qf

Tab

mbg

acc

btb

bec

Qty

br80

Qta

Qta

abm

mrr

Qto

pr

br2

Qty

amz

mfp

pr

Qta

bl21

Tah

am

pw2

Qtr

Qty

abf

Qtr

Qc

amc

bcs

am

mrd

ar

pr

Qto

arr

Qty

Tad

acl

bsm

alg

pw2

ao

ad2

mp2

dmm

ahc

Qf

Qou

cc

Qf

dch

w2

Qwb

mj

dhm

rd

Qty

adb

Qto

Qty

bbh

Tah

adm

Qc

mdb1

Qto

re

mce

mtm

dpg

bs4

bs1

bsh

mdp

rcb

Qc

abm

dbl

ad2

bp6

bb5

Qoy

Qc

bwv

Qta

Tml

bii

Qf

Qty

rr

Qoy

msg

Tdb

Qto

Qc

bie

Tdm

Qoy

Qf

bsm

Qtr

mt5

Tdm

pe

amz

bc1

Qto

ml18

Tad

dc

mdb1

Qto

Qta

mf1

Qtr

bnt

Tdm

ahc

agr

Tlv

Tbm

Qor

Qou

ds

ads

btb

mrc

dfr

pw2

Tam

bp3

mc8

mrg1

Qc

bk2

Qoy

psf

Qta

rlp

Qta

Qto

Qf

Tam

Qty

Tdb

bec

bs5

brt1

mt67

mf2

amc

Qto

Qsl

f9

Qf

mf1

Qc

mrg3

Qoo

Qty

mcj

Qf Qor

Qoy

mee1

Qty

dl

bk1

Qty

Tam

Qto

bbp

bg3

bs5

pw2

brk

pc

psf

Qf

Qf

mcr

Qty Tad

br25

Qc

Qc

Qc

att

brm

Qcl

db

Qtr

Tbm

Qf

Qf

ad2

Qf

Qoa

dr

bec

Qtr

pw2

pe

bb10

btb

bl30

bb3

Qoy

bc2

Qf

mm

bp4

bsb

cc

Qta

ad2

Qtr

mhl

Qou

b18

brt2

Qty

br2

dv

msl

dt

pdh

bt2

pr

Qty

Qou

Qto

Qf

mm

rd3

msm

msn

bb4

Qc

bg4

Qc

mtb

att

Qf

Tbm

brc

Qc

rlp

Qty

pw2

md36

mcf

Qf

Qty

bpc1

Qsh

mcn4

boc

dh

bt3

bsh

Qoy

Qwb

Qf

pc

Qtr

adc

Qsh

Qwb

Qty

mte

bk2

Qoy

bct

rb

Qf

mrg3

Qty

rmc

amz

Qwb

bb8

bbb

mb

Qty

dsc

Qtb

sw9

bec

mt54

rb

h

w2

mm

meh

Tt

bmc

mtt

ms47 Qf

ar

Qta

msg

Qf

Qf

Qcl

msn

btb

Qc

dcy

mbx

bb1

bb6

Qoy

bk2

mt61

am

Qoy

mc8

bo

Qto

Tmsr

mgp

Qf

mch

amc

rmp

Qto

am

Qc

mrr

pc

br80

bb9

ad

bt4

mp1

bp1

Qc

mcn5

pcd bra

bb2

Qc

Tbm

bl2

brk

Qty

Qc

Qf

bgv

Qcl

dhm

Qf

Qc

Qc

Qc

dbc

bii

mc11

mtt

Qc

bp3

amz

Qor

asp

Tad

bgv

pce

Qty

ar

rce

Qc

msw

am

bbf

Qty

am

Qs82

Taw

Qou

Qtr

mp1

bsl

av

Qoy

amd

am

Qc

mtu

dhm

Qto

bcp

f9

amz

Qty

dpl

Qf

am

pr

bsh

Tad

rcd

Qta

mhl

Qoa

mcn3

mc71

rcf

Qc

Tdm

Qf

Tbm

pc

rlm

dr

dt

w2

rmc

Qc

rsm

Qc

Qc

pr

Qc

Qty

alg

bb11

mrg2

bb12

arp

Qf

boc

amc

ad2

dr

Qta

av

mt5

mrg3

Qf

a16

mhr

Qoy

msn

br22

Qf

cc

Qf

ar

Qty

Qf

drp

rb

Qf

Tlv

Qf

bbf

mc10

rb

arr

bt1

Qf

Qty

Qto

pk

pr

Qf

mee2

amc

Qty

Qc

Qc

Qto

bsb

mrr

mt61

boc

Qc

bie

rg

rmp

bt2

Tlv

rmc

Qf

Qf

Qor

Qta

bsh

Qc

adc

Qtr

mbc

arr

mrm

Qtal

Qtb

dmz

Qc

Qc

amd

amz

Qcl

Qf

Qc

Qty

Qty

bg2

drp

brc

Qwb

Qc

bcw

Qc

msn

mc6

mcn1

bie

Qty

Qtr

Qf

bbf

Tdb

bpc2

Qto

bsb

rd3

Qta

bg4

Qc

Qc

ar

Qc

Qtb

Qf

bhc

Qty

mb

Qoy

Qto

mb

bpc3

ad2

rd3

Qf

Qty

bsh

Taw

Qoy

Qc

mbc

Qf

bhr

Qto

Qto

bbp

brw

Qc

am

Qcl

bs2

Tam

pe

Qwh

mm

Qs82

Tdm

pe

bl35

brc

Qc

ar

bsb

dsc

rsm

Tam

Qf

mp2

ads

Qty

Qta

mtt

ass

bp2

amc

rd

Qf

btb

mo

rd

abf

bec

rmp

Qoy

Qoy

bec

ap

btb

mch

Qc

dl

Qf

Qwb

pw2

Qc

bsm

Qty

Qty

Qty

Qf

Qty

abm

bbf

pfl

mm

abm Qsl Qc

rb

dl

Qf

bcc

mb Qc

rkr

br1

sw9

bnt

Qtrl

dfr

Qto

Qtal

mc4

Tml

Tt

bsm

bec

mc1

Qc

amc

mt20

arp

mp2

Qc

pe

Qor

boc

ar

h

mb

bmc

p2

bnt

adc

pk

rcc

d9

pc

Qc

adc

Qty

mb

Qoy

Qf

Qf

mc2

Qoa

Qta

pk

Qoy

Qor

bsd

sw9

p17

mch

Qf

amc

Qty

bs3

Qf

mc8

pr

Qty

mtr

Tmc

ad2

Qf

Qta

Qf

Qf

Qc

bcp

Qcl

mte

bn

Qf

Qoy

bss2

Qf

Qc

mrg2

Qf

Qc

Qta

a22

Qf

bss1

bcb

Qc

Tad

r27

mbc

Qty

dv

Qc

bie

acl

mrb

Qc

Qta

bnt

Qc

Qs82

Qty

pc

pc

Qty

adc

aso

mch

boc

Tdm

Qtr

abl

Taw

mc3

pw2

Qc

Qty

pc

rb

pfl

Qty

Qty

amz

Qf

Qty

bsh

bec

ad2

bhc

br80

ad

bci

pc

rmc

mc9

Qtr

Qc

ad

boc

Qc

Qc

Qtr

Qc

Qf

Qc

Qc

Qoo

mcn8

Qc

Qc

Qtal

mc1

Qtal

bk2

bnt

bnt

Qf

a22

amd

Qs82

rg

Qcl

Tam

bn

Qtr

ar

boc

dt

Qty

mbg

am pe

mc1

Qf

amc

Qta

Qto

Qoy

Qc

beb3

pc

Qc

dl

Qtal

mt20

Tdm

Qty

Qd

Qto

w2

Qc

mcm

Qty

Qoa

ad

boc

h

Qty

Qwb

Qty

Qsh

bec

Qf

rmp

a22

Qf

ad2

Qf

msg

ar

Qc

Qtr

Qc

pw2

rsm

Qty

bsb

rcc

bcw

Qoy

Tdm

mhr

amc

Qc

Qf

adc

bcw

abm

Qsl

Qf

arp

Qf

mp1

Qf

Tdm

mc7

mrr

Qc

mcn2

mc9

Qta

bsd

bgv

Qta

boc

sw9

Qs82

rmp

Qc

Qc

mrr

Qsl

Qc

br80

mb

awp

Qty

Qsl

Tbm

ab

Qf

Qf

Qc

mf1

pe

pc

bie

Qty

msn

Qf

boc

Qf

dl

bsl

Qc

Qf

Qor

rmp

Qc

boc

Qc

Qf

ar

Qf

Qsl

Qc

Qta

pe

Qty

mrg3

Qor

Qoy

Qf

Qf

brt1

Qty

Qor

Tad

bn

Qf

mee2

Qtal

mc4

Qsl

mp2

boc

Qty

ae

p9

mee2

bre

Qf

Qf

boc

abm

rlm

rkr

pe

dt

cc

mt5

Qc

arp

ad

Qc

Qty

d4

Qc

Qty

bsh

Qth

w2

bcw

Qtr

mrg3

Qty

boc

ap

pe

bre

mrb

mb

msl

abf

am

ad2

Tad

Qf

brw

boc

ht

Qoy

Taw

mcm

am

bwv

Qf

mcn2

rcd

Qsh

bwv

Qoa

rmc

boc

ad

boc

Qf

arp

sw9

arr

Qoy

mch

ap

beb3

boc

Qc

mp1

am

Qc

boc

awp

bcb

rca

boc

mb

bcb

Qf

Qf

mhl

Qwh

boc

bcw

mgp

Qc

Tt

adc

mp1

Tdm

mb

Qc

Qta

Qd

boc

Tam

Qc

dl

am

Qf

r27

boc

pk

aw

bii

Taw

Qwh

mb

Tbm

pr

Qf

mr

h

boc

brw

boc

mb

ar

Qf

Qty

ad2

boc

boc

Tam

bie

boc

Qc

Qty

mb

mb

boc

arp

amh

Qc

ass

Qsl

mch

bsb

brw

pfl

bsh

mc5

bbh

Tmsr

brw

Qc

Tam

mdb1

br22

mb

Qc

btb

Qf

Qta

Taw

mch

sw9

mcn7

Qtr

boc

ab

p2

Qf

at

bcw

brw

brc

dl

bsb

Qf

boc

bp1

Tmsr

Qc

amc

boc

bpa

boc

ad2

boc

bsb

bcc

mc10

Qty

bcb

a22

Qoy

mes

bcb

bnt

pr

b44

mfp

bec

mrg1

d82

ad2

Qf

Qf

mm

br80

mb

am

mch

Qd

amz

boc

bnt

mdb1

mp2

mc1

b44

Qf

d82

mee2

boc

brk

mch

pk

bcb

bcc

av

d82

r27

pc

alg

bcs

bnt

amc

boc

boc

amc

ad2

bsb

brk

duh

bbf

pc

ad2

bie

bnt

ad2

bs5

bnt

bsb

bcb

rd

boc

rmc

bdt

bsb

pr

rb

bsb

boc

Tdm

boc

mee1

asp

brm

mc1

dsc

rd3

brk

brm

boc

bie

Tad

mee2

Tbm

brk

bnt

ad

amz

awp

Qd

Qf

mfp

amz

Qoy

boc

Qs82

Qto

a22

br80

a22

mp1

ap

boc

bcb

ht

Qs82

mp2

asp

mp1

mf2

mp2

Qth

rkr

dc

Qf

bnt

dl

ad

Tam

bbh

bsb

mrg1

ad2

a22

bnt

mfp

bs5

Qty

bss1

aso

bcr

bp5

bg1

bg1

bcg

adb

m20

bl20

mlc

bsl

bss2 mlc

mlc

bcw

mdb2

mdb1

bs72

bre

mfp

mf2

amz

pk

db

db

w2 p2

p2

w2

mrb

mcn6

beb3

beb2

beb1

mc4

bs4

40°20’

40°40’

121°37’30”

121°07’30”

40°40’

121°07’30”

121°37’30” 40°20’

121°30’ 121°22’30”

121°22’30”

121°15’

121°15’

40°37’30”

40°37’30”

40°30’

40°30’

40°22’30”

40°22’30”

121°30’

200

100

50

25

10

5

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government.

For sale by U.S. Geological Survey, Information Services, Box 25286, 

Federal Center, Denver, CO, 80225, 1–888–ASK–USGS

Digital files available at http://pubs.usgs.gov/sim/2899/

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 2899

Sheet 1 of 3

Pamphlet and CD-ROM accompany map

Base from U.S. Geological Survey, 1:24,000, Lambert Conformal Conic projection: 

Chester, 1979; Childs Meadows, 1985; Grays Peak, 1985; Lassen Peak, 1985; 

Lyonsville, 1985; Manzanita Lake, 1985; Mineral, 1986; Mount Harkness, 1985; Old 

Station, 1985; Prospect Peak, 1985; Reading Peak, 1985; Red Cinder, 1979; Stover 

Mountain, 1985; Swains Hole, 1985; Thousand Lakes Valley, 1985; Viola, 1986; 

West Prospect Peak, 1985. 

Base from U.S. Geological Survey, 1:24,000, Universal Transverse Mercator 

projection: Bogard Buttes, 1983; Poison Lake, 1983;

Geology mapped by M.A. Clynne (1979-1992) and L.J.P. Muffler (1980, 

1987-1992), assisted by Deborah Trimble (1980-1985), Marianne Guffanti 

(1980), Lars Borg (1986), Miranda Fram (1987), Gordon Keating (1988), and 

Charles Melancon (1989)

Digital database by L.J.P. Muffler, J.E. Robinson, T.J Felger, D. Dutton, and 

M.A. Clynne, (with contributions from D. Ramsey and E. Lougee)

Edited by J.L. Zigler

Digital cartographic production by K.P. Nimz

Manuscript approved for publication September 12, 2005

CALIF.

APPROXIMATE MEAN MAP LOCATION DECLINATION, 2008

14 / ° 1 2

MAGNETIC NORTH

TRUE NORTH

Geologic Map of Lassen Volcanic National Park and Vicinity, California

By

Michael A. Clynne and L.J. Patrick Muffler

2010

3000 0 3000 6000 9000 12000 15000 18000 21000 FEET

SCALE 1:50 000

1 1/2 0 1 2 3 4 MILES

1 .5 0 1 2 3 4 5 KILOMETERS

CONTOUR INTERVALS 5, 10, 20, AND 40 FEET

SUPPLEMENTARY CONTOUR INTERVALS 5, 10, AND 20 FEET

LIST OF MAP UNITS

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

that are not labeled, because they are too small for color identification 

and (or) labeling will obscure data, see database (accompanying 

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

well as unit name, page number of unit description in pamphlet, 

radiometric age, location by heading in Correlation of Map Units, and 

quadrangle location in database. See Methodology section for detailed 

information on methods used to describe and name map units. Unit 

labels are abbreviations for age (all units without capital are Quaternary), rock type, and unit name: unit bb10 represents Quaternary 

basalts and basaltic andesites of Bogard Buttes, unit 10. Rock type 

abbreviations (volcanic units): b, basalt (<53% SiO2); m, basaltic 

andesite (53%–57% SiO2); a, andesite (57–63% SiO2); d, dacite 

(63–68% SiO2); r, rhyodacite (68–72% SiO2) and rhyolite (>72% SiO2); 

p, pyroclastic flow and fall deposits. Rock type abbreviations (glacial 

and sedimentary units): c, colluvium, including talus; d, diatomite; f, 

fluvial deposits, including all alluvium; h, hydrothermally altered rocks 

and travertine deposits; o, glacial outwash deposits; s, landslide and 

avalanche deposits; t, till; w, mudflow and other debris-flow deposits. 

Common mineral abbreviations: pl, plagioclase; ol, olivine; aug, 

augite; hyp, hypersthene; pyr, pyroxene; hbl, hornblende; bt, biotite; 

and qtz, quartz. Abbreviations in brackets at end of unit description 

represent 1:24,000-scale quadrangle(s) where unit is mapped (see fig. 3)]

GLACIAL, SEDIMENTARY, AND HYDROTHERMAL 

DEPOSITS

NONVOLCANIC SEDIMENTARY DEPOSITS

Alluvium and Colluvium

Alluvium (Holocene and Pleistocene)—[BB, CH, CM, 

GP, LP, MH, MN, MZ, OS, PL, PP, RC, RP, ST, SW, 

TH, VI, WP]

Colluvium and talus (Holocene and Pleistocene)—[BB, 

CH, LP, LY, MH, MN, MZ, OS, PL, PP, RC, RP, ST, 

SW, WP]

Talus on Lassen Peak (late Pleistocene)—[LP, MZ, RP]

Mass-Wasting Deposits

Landslide deposits in hydrothermally altered core of 

Brokeoff Volcano (Holocene)—[LP, RP]

Avalanche deposits of Chaos Jumbles (Holocene)—[MZ]

Debris-flow deposits from the northeast side of Lassen 

Peak (Holocene)—[LP, MZ, RP, WP]

Avalanche debris from Lassen Peak spread across 

glacial ice (late Pleistocene)—[WP]

Avalanche deposit from dacite of hill 8283 (middle 

Pleistocene)—[MZ, VI]

Debris-flow deposit from Brokeoff Volcano (middle 

Pleistocene)—[GP]

Hydrothermal Deposits

Hydrothermally altered rocks in active thermal areas 

(Holocene)—[LP, RP]

Travertine (Holocene)—[LP]

Glacial Deposits

Till

Till or protalus-rampart debris (Holocene)—[RP]

Till, younger glaciations (late Pleistocene)—[BB, CH, 

CM, LP, MH, MZ, PP, RC, RP, ST, TH, VI, WP]

Late till of Anklin Meadows (Holocene to late 

Pleistocene)—[LP, MZ, RP, WP]

Till of Anklin Meadows (late Pleistocene)—[GP, LP, MZ, 

RP, WP]

Post-maximum till of Raker Peak consisting of Lassen 

Peak avalanche debris (late Pleistocene)—[WP]

Till of Raker Peak (late Pleistocene)—[GP, MZ, WP] 

Till, older glaciations (late Pleistocene)—[BB, CH, CM, 

GP, LP, LY, MH, MN, MZ, PP, RC, RP, ST, VI]

Till of Badger Mountain (late Pleistocene)—[MZ, WP]

Outwash

Outwash gravel, younger glaciations (late Pleistocene)— 

[BB, CH, CM, LP, MH, MN, MZ, OS, PP, RC, RP, ST, 

TH, VI, WP]

Outwash gravel, undivided (late Pleistocene)—[GP, LY, 

MN, PP, SW, VI]

Outwash gravel of Anklin Meadows (late Pleistocene)— 

[MZ, OS, WP]

Outwash gravel of Raker Peak (late Pleistocene)—[MZ, 

OS, WP]

Outwash gravel, older glaciations (late Pleistocene)— 

[CM, MN]

Sedimentary Deposits

Diatomite (Holocene)—[PP]

VOLCANIC CENTERS OF SOUTHERNMOST CASCADES

LASSEN VOLCANIC CENTER

Lassen domefield

Twin Lakes sequence, younger

Deposits of 1914–1917 eruption of Lassen Peak 

(Holocene)—[LP, MZ, OS, RP, WP]

1917 C.E.

Phreatic deposit, May–June—[LP]

1915 C.E.

Viscous debris-flow deposits, May 22—[LP, MZ, RP, 

WP]

Pumice-fall deposit, May 22—[LP, RP, WP]

Pyroclastic-flow and associated fluid debris-flow 

deposits, May 22—[LP, MZ, OS, RP, WP]

Dacite flow, May 19–20—[LP]

Flood deposits, May 19–20—[WP]

Avalanche and debris-flow deposits, May 19–20—

[WP]

Pyroclastic deposit, May 19—[LP]

Dacite dome, May 14–19—[LP]

Basaltic andesites of Cinder Cone (Holocene)—[PP]

Fantastic Lava Beds

Flow 2—[PP]

Flow 1—[PP]

Painted Dunes

Flow 2—[PP]

Flow 1—[PP]

Old Bench flow—[PP]

Andesite of hill 7416 (late Pleistocene)—[OS, PP, SW]

Andesite of Hat Mountain (late Pleistocene)—[MH, PP, 

RP, WP]

Andesite of Eagle Peak (late Pleistocene)—[LP]

Basaltic andesite of Fairfield Peak (late Pleistocene)— 

[PP]

Andesite of Crater Butte (late Pleistocene)—[MH, PP]

Twin Lakes sequence, older

Andesite and basaltic andesite of Cluster Lakes (middle 

Pleistocene)—[PP, WP]

Andesite of Raker Peak (middle Pleistocene)—[MZ, WP]

Andesite of Viola (middle Pleistocene)—[LP, MZ, VI]

Eagle Peak sequence

Rhyodacite of Chaos Crags (Holocene)—[MZ, OS, VI, 

WP]

Talus, emplaced hot from domes B–F—[MZ]

Dome F—[MZ]

Lithic pyroclastic-flow deposit from partial collapse of 

dome E—[MZ, WP]

Dome E—[MZ]

Lithic pyroclastic-flow deposit from partial collapse of 

dome D—[MZ]

Dome D—[MZ]

Dome C—[MZ]

Dome B—[MZ]

Pumiceous pyroclastic-flow and fall deposits—[MZ, 

OS, VI, WP]

Dome A—[MZ]

Dacite of Lassen Peak (late Pleistocene)—[LP, MZ, RP, 

WP]

Lithic pyroclastic-flow deposit from partial collapse of 

dome—[WP]

Dome—[LP, MZ, RP]

Rhyodacite of Kings Creek (late Pleistocene)—[LP, RP, 

WP]

Flows—[LP, RP]

Pumiceous pyroclastic-flow deposits—[LP, RP, WP] 

Rhyodacite of Sunflower Flat (late Pleistocene)—[MZ, 

WP]

Domes—[MZ, WP]

Pumiceous pyroclastic-flow deposits—[MZ]

Rhyodacite of Krummholz (late Pleistocene)—[MZ, WP]

Rhyodacite of section 27 (late Pleistocene)—[MZ, WP]

Rhyodacite of Eagle Peak (late Pleistocene)—[GP, LP, 

MZ]

Dome and flow—[LP]

Pumiceous pyroclastic-flow deposits—[GP, LP, MZ]

Bumpass sequence

Rhyodacite of Dersch Meadows (middle Pleistocene)— 

[RP, WP]

Dacite of Reading Peak (middle Pleistocene)—[RP]

Dacite of Bumpass Mountain (middle Pleistocene)—[LP, 

RP]

Dacite of Crescent Crater (middle Pleistocene)—[MZ, 

WP]

Dacite of Ski Heil Peak (middle Pleistocene)—[LP]

Dacite of Mount Helen (middle Pleistocene)—[LP, RP]

Dacite under Mount Helen (middle Pleistocene)—[RP]

Dacite of Summit Creek (middle Pleistocene)—[RP]

Dacite of Lassen Peak parking lot (middle Pleistocene)— 

[LP]

Dacite of Vulcans Castle (middle Pleistocene)—[LP]

Dacite of hill 8283 (middle Pleistocene)—[MZ, WP]

Dacite of upper Manzanita Creek (middle Pleistocene)— 

[MZ]

Rhyodacite of Manzanita Chute (middle Pleistocene)— 

[LP, MZ]

Rhyodacite of Mount Conard (middle Pleistocene)—[LP, 

RP]

Rhyodacite of Loomis Peak (middle Pleistocene)—[LP, 

MZ]

Brokeoff Volcano

Diller sequence

Andesite of Mount Diller (middle Pleistocene)—[LP, RP]

Andesites of Diller sequence, undivided (middle 

Pleistocene)—[GP, MZ, VI, WP]

Andesite of Digger Creek (middle Pleistocene)—[GP, LP, 

MZ, VI]

Andesite of Manzanita Creek (middle Pleistocene)— 

[MZ, VI]

Andesite of Glassburner Meadows (middle 

Pleistocene)— [LP]

Andesite of Bluff Falls quarry (middle Pleistocene)—[LP]

Andesite of Rice Creek (middle Pleistocene)—[CM, LP, 

RP]

Mill Canyon sequence

Dacite of Twin Meadows (middle Pleistocene)—[LP, RP]

Andesites of Mill Canyon (middle Pleistocene)—[LP, RP, 

WP]

Rockland caldera complex

Rockland tephra (middle Pleistocene)—[GP, LY, VI]

Rhyolite of Raker Peak (middle Pleistocene)—[WP]

Dacite of Red Rock Mountain (middle Pleistocene)— 

[GP, LP]

Dacite of Bench Lake (middle Pleistocene)—[RP]

Dacite of Bailey Creek (middle Pleistocene)—[LP]

Dacite of Rocky Peak (early Pleistocene)—[GP, LP]

Dacite of Flatiron Ridge (early Pleistocene)—[MH, RP]

Dacite of Panther Creek (early Pleistocene)—[RP]

Dacite of Plantation Gulch (early Pleistocene)—[LP, 

MN]

Dacite of Christie Hill (early Pleistocene)—[LP, MN]

Dacite of Morgan Mountain (early Pleistocene)—[LP, 

MN]

Dacite of Canyon Creek (early Pleistocene)—[LP, RP]

MAIDU VOLCANIC CENTER

Stage 3

Rhyolite of Blue Ridge (early Pleistocene)—[GP, LY]

Rhyolite of Mill Creek Plateau (early Pleistocene)— 

[CM, LP, MN] 

Rhyolite of Lost Creek Plateau (early Pleistocene)— 

[CM]

Rhyolite of North Stover Mountain (early Pleistocene)— 

[CM, ST]

Rhyodacite of Gurnsey Creek (early Pleistocene)—[CM, 

LP]

Stage 2

Andesites of Hampton Butte (late Pliocene)—[GP, LY]

Dacites of Maidu Volcanic Center, Stage 2, undivided 

(late Pliocene)—[GP, LY, MN]

Stage 1

Andesites of Maidu Volcanic Center, Stage 1, undivided 

(late Pliocene)— [LP, LY, MN]

Andesite of South Fork Battle Creek (late Pliocene)— 

[GP, LY]

Basaltic andesite of Lassen Lodge (late Pliocene)—[LY]

DITTMAR VOLCANIC CENTER

Stage 3

Rhyolites of Dittmar Volcanic Center, Stage 3 (early 

Pleistocene)—[MH, RP]

Stage 2

Andesites of Dittmar Volcanic Center, Stage 2, undivided 

(early Pleistocene)—[MH, PP, RP, ST]

Andesite of Doe Mountain (early Pleistocene)—[CM, 

MH]

Andesite of Domingo Spring (early Pleistocene)—[MH, ST]

Andesites of Benner Creek (early Pleistocene)—[CH, 

ST]

Stage 1

Andesites of Dittmar Volcanic Center, Stage 1, undivided 

(late Pliocene to early Pleistocene)—[MH, PP, RP, ST]

Andesites of Wild Cattle Mountain (late Pliocene)— 

[CM, LP, MN, RP]

LATOUR VOLCANIC CENTER

Rocks of Latour Volcanic Center (late Pliocene)—[VI]

TUSCAN FORMATION

Tuscan Formation (late Pliocene)—[LY]

REGIONAL VOLCANIC ROCKS

NORTH AND WEST OF LASSEN VOLCANIC CENTER

Late Pleistocene volcanic rocks

Hat Creek Basalt (late Pleistocene)—[OS]

Tholeiitic basalts of Big Lake (late Pleistocene)—[MZ, 

TH, VI]

Unit 2—[MZ, TH, VI]

Unit 1—[MZ, VI]

Basaltic andesite of Eskimo Hill (late Pleistocene)—[MZ]

Basaltic andesite and andesite of Red Lake Mountain 

(late Pleistocene)—[MZ, VI]

Basaltic andesite of Red Mountain (late 

Pleistocene)—[MZ]

Tumble Buttes chain

Andesite of Devils Rock Garden (late Pleistocene)—[TH]

Andesite of Bear Wallow Butte (late Pleistocene)—[MZ, 

OS, TH, WP]

Basaltic andesite of hill 6770 (late Pleistocene)—[TH]

Basaltic andesite of Tumble Buttes (late Pleistocene)— 

[TH]

Basaltic andesite of hill 5410 (late Pleistocene)—[TH]

Basaltic andesite of Bear Wallow Butte (late 

Pleistocene)—[TH]

Basaltic andesite of hill 6138 (late Pleistocene)—[TH]

Basaltic andesite of section 5 (late Pleistocene)—[TH]

Andesite of Tumble Buttes (late Pleistocene)—[MZ, OS, 

TH, WP]

Basaltic andesite of Mud Lake (late Pleistocene)—[TH]

Sugarloaf chain

Andesite of Sugarloaf Peak (late Pleistocene)—[OS]

Basaltic andesite of Little Potato Butte (late 

Pleistocene)—[OS]

Andesite of Potato Butte (late Pleistocene)—[OS]

Basaltic andesite of hill 4709 (late Pleistocene)—[OS]

Andesites of Old Station (late Pleistocene)—[OS]

Middle Pleistocene and older volcanic rocks

Basaltic andesite of Little Bunchgrass Meadow (middle 

Pleistocene)—[WP]

Basaltic andesite of Box Canyon (middle Pleistocene)— 

[WP]

Tholeiitic basalt of Calif. Hwy 44 (middle Pleistocene)— 

[OS, PP, SW]

Tholeiitic basalt of hill 1879 (middle Pleistocene)—[BB, 

PL, PP, SW]

Tholeiitic basalt of Eagle Canyon (middle Pleistocene)— 

[GP, MZ, VI]

Basaltic andesites of Magee Volcano (middle 

Pleistocene)—[TH]

Tholeiitic basalt of Nobles Trail (middle Pleistocene)— 

[WP]

Andesite and basaltic andesite of Prospect Peak (middle 

Pleistocene)—[PP, SW, WP]

Lithic tephra from Deep Hole (middle Pleistocene)— 

[MZ]

Basalt of Cherry Thicket (middle Pleistocene)—[VI] 

Basalt of Onion Springs (middle Pleistocene)—[LP, MZ]

Andesites of West Prospect Peak (middle Pleistocene)— 

[OS, PP, SW, WP]

Basalt of Badger Flat (middle Pleistocene)—[OS, WP]

Basaltic andesite of Hootman Ranch (middle 

Pleistocene)—[GP, VI]

Andesite of Wilcox Peak (middle Pleistocene)—[OS]

Andesite of Logan Mountain (middle Pleistocene)—[TH]

Andesite of Grayback Ridge (middle Pleistocene)—[MZ]

Andesite of Cabin Spring (middle Pleistocene)—[GP]

Basaltic andesite of Rock Spring (middle Pleistocene)— 

[GP]

Andesite of Onion Creek (middle Pleistocene)—[GP]

Andesite of Table Mountain (middle Pleistocene)—[MZ]

Andesites of Badger Mountain (middle Pleistocene)— 

[OS, WP]

Tholeiitic basalt of Twin Bridges (middle Pleistocene)— 

[OS, WP]

Andesite of Ashpan Butte (early Pleistocene)—[MZ]

Basaltic andesite of Signal Butte (early Pleistocene)— 

[OS, SW]

Andesites, basaltic andesites, and basalts of Hat Creek 

Rim (early Pleistocene)—[OS, PP, SW]

Andesite of Butte Lake (early Pleistocene)—[PP]

Andesite of section 22 (early Pleistocene)—[PP]

Dacite of Huckleberry Mountain (early Pleistocene)— 

[TH, VI]

Older regional andesites and basaltic andesites, 

undivided (early Pleistocene)—[GP, MZ, OS, TH, VI]

Basaltic andesites of Camp Forward (early 

Pleistocene)—[GP]

SOUTH AND EAST OF LASSEN VOLCANIC CENTER

Tholeiitic basalts of Buzzard Springs (late Pleistocene)— 

[CM, RP, ST]

Tholeiitic basalt of Ice Cave Mountain (late 

Pleistocene)—[CM]

Basalts of Cold Creek Butte (late Pleistocene)—[LY, MN]

Tholeiitic basalts of Mill Creek Plateau (middle to late 

Pleistocene)—[CM, MN]

Basalts and basaltic andesites of Sifford Mountain 

(middle Pleistocene)—[CM, MH, RP, ST]

Basaltic andesite of Small Butte (middle Pleistocene)— 

[CM, RP]

Andesite and basalt of Mount Harkness (middle 

Pleistocene)—[MH]

Basaltic andesite of Huckleberry Lake (middle 

Pleistocene)—[LP, MN]

Tholeiitic basalt of Warner Valley (middle Pleistocene)— 

[CH, MH, ST]

Basaltic andesite of Grays Peak (early Pleistocene)— 

[GP, LY]

Basalt of Panther Creek (early Pleistocene)—[LY]

Andesite of Martin Creek (early Pleistocene)—[LP, MN]

Andesites of Cowslip Campground (early Pleistocene)— 

[GP, LP, LY, MN]

Basaltic andesites of South Fork Battle Creek (early 

Pleistocene)—[LY]

REGIONAL VOLCANIC ROCKS OF THE CARIBOU AREA

Caribou Volcanic Field

Tuya chains

Basaltic andesite of Turnaround Lake (late 

Pleistocene)—[BB, RC]

Basaltic andesites of Evelyn Lake (late Pleistocene)— 

[RC]

Red Cinder chain

Basaltic andesite of Red Cinder Cone (late 

Pleistocene)—[MH, PP]

Basalt of Red Cinder Cone (late Pleistocene)—[MH, PP]

Basaltic andesite of Red Cinder (late Pleistocene)—[BB, 

MH, PP, RC]

Basalt of hill 8030 (late Pleistocene)—[MH, RC]

Basalt of Cameron Meadow (late Pleistocene)—[MH]

Basalt of Ash Butte (late Pleistocene)—[PP]

Basalt of hill 2283 (late Pleistocene)—[BB, PP]

Basalt of section 25 (late Pleistocene)—[PP]

Andesite of Red Cinder (late Pleistocene)—[BB, MH, PP, 

RC]

Basalt east of Ash Butte (late Pleistocene)—[PP]

Basalt of Widow Lake (late Pleistocene)—[BB, PP]

Basaltic andesites of Long Lake (late Pleistocene)—[MH, 

RC]

Unit 3—[MH, RC]

Unit 2—[MH, RC]

Unit 1—[RC]

Basaltic andesite of Caribou Wilderness (late 

Pleistocene)—[BB]

Basalts of Triangle Lake (late Pleistocene)—[BB]

Unit 2—[BB]

Unit 1—[BB]

Bidwell Spring chain

Andesite of Bidwell Spring (late Pleistocene)—[PP, SW]

Basaltic andesite of Pole Spring Road (late 

Pleistocene)—[PP]

Basaltic andesite of section 36 (late Pleistocene)—[BB, 

PP]

Basalt of Twin Buttes (late Pleistocene)—[BB, PL, PP]

Basaltic andesites of Black Butte (late Pleistocene)— 

[BB, PP]

Unit 2—[BB]

Unit 1—[BB, PP]

Poison Lake chain

Basalts of Cone Lake Road (late Pleistocene)—[BB]

Unit 2—[BB]

Unit 1—[BB]

Basalts of old railroad grade (late Pleistocene)—[BB, PL, 

SW]

Unit 4—[PL]

Unit 3—[PL, SW]

Unit 2—[PL]

Unit 1—[BB, PL]

Basalts of Poison Butte (late Pleistocene)—[PL]

Unit 6—[PL]

Unit 5—[PL]

Unit 4—[PL]

Unit 3—[PL]

Unit 2—[PL]

Unit 1—[PL]

Basalts and basaltic andesites of Bogard Buttes (late 

Pleistocene)—[BB, PL]

Unit 12—[BB]

Unit 11—[BB]

Unit 10—[BB]

Unit 9—[BB]

Unit 8—[BB] 

Unit 6—[BB]

Unit 5—[BB, PL]

Unit 4—[BB]

Unit 3—[BB]

Unit 2—[BB]

Unit 1—[BB]

Basalts of Pine Creek (late Pleistocene)—[BB]

Unit 3—[BB]

Unit 2—[BB]

Unit 1—[BB]

Basalts of Pittville Road (late Pleistocene)—[BB, PL]

Unit 4—[BB, PL]

Unit 3—[BB]

Unit 2—[BB, PL]

Unit 1—[PL]

Basaltic andesite west of hill 2078 (late Pleistocene)— 

[BB]

Basalts of Stephens Campground (late Pleistocene)— 

[BB, PL]

Unit 5—[BB, PL]

Unit 4—[BB]

Unit 3—[BB]

Unit 2—[BB]

Unit 1—[BB]

Basalts of Robbers Spring (middle Pleistocene)—[BB, 

PL]

Unit 2—[BB]

Unit 1—[BB]

Cone Lake chain

Basalt of section 35 (middle Pleistocene)—[BB]

Basalt of section 2 (middle Pleistocene)—[BB]

Basalt of hill 2088 (middle Pleistocene)—[BB]

Basalt of section 30 (middle Pleistocene)—[BB]

Basalt of hill 2232 (middle Pleistocene)—[BB]

Basalt of hill 2109 (middle Pleistocene)—[BB, PP]

Basaltic andesites of Cone Lake (middle Pleistocene)— 

[BB]

Basaltic andesite of hill 1868 (middle Pleistocene)—[BB, 

PL]

Caribou chain

Triangle Lake sequence

Basaltic andesite of Triangle Lake (middle 

Pleistocene)—[BB]

Basaltic andesites of section 20 (middle Pleistocene)— 

[BB]

Echo Lake sequence

Basalts of Bond Valley (middle Pleistocene)—[RC]

Unit 3—[RC]

Unit 2—[RC]

Unit 1—[RC]

Basaltic andesites of Echo Lake (middle Pleistocene)— 

[RC]

Unit 2—[RC]

Unit 1—[RC]

Basaltic andesite of Star Butte (middle Pleistocene)— 

[RC]

Caribou sequence

Basaltic andesite of Eleanor Lake (middle Pleistocene)— 

[BB]

Basaltic andesite of Jewel Lake (middle Pleistocene)— 

[BB, RC]

Basaltic andesite of Rim Lake (middle Pleistocene)— 

[RC]

Basaltic andesites of North Caribou (middle 

Pleistocene)—[RC]

Unit 8—[RC]

Unit 7—[RC]

Unit 6—[RC]

Unit 5—[RC]

Unit 4—[RC]

Unit 3—[RC]

Unit 2—[RC]

Unit 1—[RC]

Basalt of South Caribou (middle Pleistocene)—[RC]

Basaltic andesites of Middle Caribou (middle Pleistocene)— 

[RC]

Basaltic andesite of Hay Meadow (middle Pleistocene)—[RC]

Basaltic andesite of hill 7114 (middle Pleistocene)—[RC]

Basalt of Indian Meadow (middle Pleistocene)—[RC]

Basaltic andesites of Silver Lake (middle Pleistocene)—[BB, 

RC]

Unit 11—[RC]

Unit 10—[RC]

Unit 9—[BB, RC]

Unit 8—[BB, RC]

Unit 7—[RC]

Unit 6—[RC]

Unit 5—[RC]

Unit 4—[BB]

Unit 3—[RC]

Unit 2—[RC]

Unit 1—[RC]

Basalt of Caribou Wilderness (middle Pleistocene)—[BB]

Basalt of Caribou Peak (middle Pleistocene)—[BB]

Basalt of Cowboy Lake (middle Pleistocene)—[BB, RC]

Sunrise Peak sequence

Basalt of Lost Spring (middle Pleistocene)—[PP]

Basalt of Duck Lake (middle Pleistocene)—[PP]

Basalt of hill 7243 (middle Pleistocene)—[PP]

Basalts of Sunrise Peak (middle Pleistocene)—[PP]

Unit 2—[PP]

Unit 1—[PP]

Basalt of Bathtub Lake (middle Pleistocene)—[PP]

Island Lake sequence

Basalt of Island Lake (middle Pleistocene)—[MH]

Basalt of East Lake (middle Pleistocene)—[MH, RC]

Beauty Lake sequence

Basalt of Beauty Lake (middle Pleistocene)—[RC]

Basalt of Posey Lake (middle Pleistocene)—[RC]

Basalt of Hidden Lakes (middle Pleistocene)—[RC]

Older volcanic rocks

Tholeiitic basalt of Grays Valley (middle Pleistocene)—[BB, 

PL]

Basalts and basaltic andesites of Swains Hole (middle 

Pleistocene)—[BB, PL, PP, SW]

Older basalts and basaltic andesites south of Caribou 

Volcanic Field (middle Pleistocene)—[BB, CH, MH, RC]

Basaltic plug near Heckle Ranch (middle Pleistocene)— 

[RC]

Basaltic andesite of Jakey Lake (middle Pleistocene)—[MH]

Basalt of Butte Creek Campground (middle Pleistocene)— 

[PP]

Basaltic andesite of Black Cinder Rock (middle 

Pleistocene)—[MH, RC]

Basaltic andesites and basalt of Snag Lake (middle 

Pleistocene)—[MH, PP]

Basalt of Bonte Peak (middle Pleistocene)—[MH, RC]

Basalt of Butte Creek Ranch (early Pleistocene)—[SW]

Andesite of section 16 (early Pleistocene)—[BB]

Tholeiitic basalt of Rock Creek (early Pleistocene)—[CH, 

RC]

Basaltic andesites of Swain Mountain (early Pleistocene)— 

[RC]

Basalt of section 11 (early Pleistocene)—[BB]

Basalts and basaltic andesites of Mud Creek Rim (late 

Pliocene)—[CH, RC]

Dacite of Bogard Buttes (late Pliocene)—[BB]

Andesite of Cal Mountain (late Pliocene)—[PL]

Basaltic andesite of Cone Mountain (late Pliocene)—[PL]

Basaltic andesites of Susan River (late Pliocene)—[BB]

EXPLANATION OF MAP SYMBOLS

Contact—Dashed where approximately located, dotted where 

concealed

Normal fault—Dotted where concealed. Ball and bar on 

downthrown block

Volcanic vent—Known and inferred

Cinder and agglutinate cones

Ash from Cinder Cone—Isopach contours show thicknesses of 

5, 10, 25, 50, 100, and 200 cm

Phreatic eruption deposit—Dacite of Bumpass Mountain

Fissure

Crest line of moraine

Lassen Volcanic National Park boundary

Qf

Qc

Qcl

Qsh

Qsj

Qwh

Qsl

Qs82

Qwb

Qth

Qty

Qtal

Qta

Qtrl

Qtr

Qto

Qtb

Qoy

Qou

Qoa

Qor

Qoo

p17

w2

p2

pw2

d9

f9

sw9

p9

d4

mf2

mf1

mp2

mp1

mo

a74

ah

ae

mfp

ac

acl

arp

av

cc

rcf

pce

rce

pcd

rcd

rcc

rcb

pc

rca

pfl

dl

rk

pk

rsf

psf

rkr

r27

rd

dr

db

dc

ds

dh

duh

dsc

dpl

dv

d82

dmz

rmz

rmc

rlm

pr

rr

drr

dbl

dbc

drp

dfr

dp

dpg

dch

dmm

dcy

rb

rmp

rlp

rsm

ad2

adm

ads

adb

meh

mrd

mrm

Tad

Taw

bk2

bk1

mt67

mtu

mt54

atd

atb

mt5

att

mtm

mtb

mt61

asp

ms47

aso

ass

msl

b44

b18

bec

mm

bn

ap

mbg

mbx

bo

awp

bbf

pdh

bct

alg

agr

acs

mr

ao

at

mhr

aw

abm

btb

ahc

abl

a22

dhm

am

mcf

aa

msg

bbz

bic

bcc

bmc

bsm

msm

beb3

beb2

beb1

mee2

mee1

mes

mce

mcj

mcr

mcn8

mcn7

mcn6

mcn5

mcn4

mcn3

mcn2

mcn1

bcs

mcm

mch

mc71

bci

mc11

mc10

mc9

mc8

mc7

mc6

mc5

mc4

mc3

mc2

mc1

bcw

bcp

bcb

bsl

bsd

bs72

bss2

bss1

bsb

bii

bie

bbp

bbh

bbb

bsh

boc

bgv

mj

bcg

mb

bhr

bnt

bcr

msn

brk

msw

b11

a16

Tbm

Tac

Tmc

Tmsr

Tdb

mtr

mt20

Tlv

Tt

bhc

rg

Tah

Tdm

Tam

Tab

Tml

rd3

ad

adc

amz

ag

abf

ar

dt

amc

re

pe

Qd

h

ht

amr

acc

mbc

mtt

mte

mrc

brc

mrr

bra

br22

br25

arr

bre

brw

br80

brm

adb

mdp

md36

bdt

mdb2

mdb1

mrg3

mrg2

mrg1

mrb

brt2

brt1

bb12

bb11

bb10

bb6

bb5

bb4

bb3

bb2

bb1

bpc3

bpc2

bpc1

bg4

bg3

bg2

bg1

bp6

bp5

bp4

bp3

bp2

bp1

bt4

bt3

bt2

bt1

m20

bs5

bs4

bs3

bs2

bs1

bl35

bl2

bl20

br2

br1

bl30

bl22

bl21

mlc

ml18

bc2

bc1

bb9

bb8

amd

bwv

mgp

bpa

amh

mhl

25

Rocky

Peak

schematic original form

of Brokeoff Volcano

Brokeoff

Mountain Eagle

Peak

Bumpass

Mountain

Mt. Helen Mount

Diller

Pilot

Pinnacle

May 19–20,

1915 flow

Lassen

Peak

Mount

Conard

Reading

Peak

dacite flow from

Bumpass Mountain andesite of

Rice Creek

andesite of

Mount Diller

eroded core of

Brokeoff Volcano

upper part of

Sulphur Works

Mineral

dacite of

Twin Meadows

Little Hot

Springs Valley

andesite of

Glassburner

Meadows

dacite of

Plantation Gulch

dacites of Maidu

Volcanic Center, Stage 2

andesites of

Mill Canyon

dacite of

Rocky Peak

Christie Hill

Martin

Creek

andesites of Maidu

Volcanic Center, Stage 1

andesites of Maidu

Volcanic Center, Stage 1

andesite of

Martin Creek

andesite of

Martin Creek

basaltic andesite of

Huckleberry Lake

Nanny

Creek

Battle Creek Meadows

Morgan

Mountain

Lassen and Maidu Volcanic Centers

View looking north from Turner Mountain (Mineral quadrangle) towards Lassen 

Volcanic Center. Battle Creek Meadows, now filled with glacial outwash (unit 

Qou) marks the approximate core of the Maidu Volcanic Center. The forested hill 

in the right foreground consists of andesites of the Maidu Volcanic Center, 

Stage 1, undivided (unit ~2,150–2,375 ka), overlain to the east by dacites of the 

Maidu Volcanic Center, Stage 2, undivided (unit Tdm, 2,120±46 ka). The hills 

just beyond the town of Mineral are also andesites of the Maidu Volcanic Center, 

Stage 1, undivided, overlain by the andesite of Martin Creek (unit amr, ~1,100– 

1,300 ka), a regional calc-alkaline unit. The dacite domes of Plantation Gulch 

(unit dpg), Christie Hill (unit dch) and Morgan Mountain (unit dmm) are ~800– 

830 ka and are part of the Rockland caldera complex of the Lassen Volcanic 

Center. The dacite of Rocky Peak (unit drp, 803±27 ka, also part of the Rockland 

caldera complex) makes up Rocky Peak and a prominent outcrop in the left 

center of the photo. The skyline comprises Brokeoff Volcano and the Lassen 

Domefield. The eroded core of Brokeoff Volcano is composed of the andesites of 

Mill Canyon (unit amc, 590–470 ka). Mount Conard (also unit amc) marks the 

east rim of the erosional amphitheater in Brokeoff Volcano. The southwest rim is 

marked by Brokeoff Mountain, composed primarily of the dacite of Twin Meadows (unit dt, 470±10 ka, the uppermost unit of the Mill Canyon sequence) but 

capped on the west by the andesite of Mount Diller (unit amd, 387±10 ka, part 

of the Diller sequence). The northwest rim is marked by Mount Diller and Pilot 

Pinnacle, both composed of andesite of Mount Diller. More distal units of the 

Diller sequence include the andesite of Glassburner Meadows (unit ag, ~450 ka) 

at the left center and the andesite of Rice Creek (unit ar, 480±5 ka) at the far 

right. The basaltic andesite of Huckleberry Lake (unit mhl, ~300 ka) is a regional 

calc-alkaline unit that is younger than the Brokeoff Volcano. Mt. Helen (unit dh, 

249±12 ka), Reading Peak (unit dr, 212±5 ka) and Bumpass Mountain (unit db, 

232±8 ka) are domes of the Bumpass sequence of the Lassen domefield. Eagle 

Peak (unit re, 66±4 ka) and Lassen Peak (unit dl, 27±1 ka) are domes of the Eagle 

Peak sequence of the Lassen domefield. The dark, triangular mass on the west 

side of Lassen Peak is the west lobe of the dacite flow of May 19–20, 1915. 

Photograph by Michael A. Clynne.

Eagle Peak

tephra from Eagle Peak

Ski Heil

Peak

andesite of Mount Diller

hill 8740

May 19–20, 1915 flow Lassen Peak

andesites of Mill Canyon

1915 flow on Lassen Peak

Telephoto view northeast from summit of Brokeoff 

Mountain (Lassen Peak quadrangle). The west lobe 

of the dacite flow of May 19–20, 1915 (unit d9) is 

on the west side of Lassen Peak (unit dl, 27±1 ka). 

Eagle Peak (unit re, 66±4 ka) is part of the Eagle 

Peak sequence. Ski Heil Peak (unit ds, 244±10 ka, 

part of the Bumpass sequence) is mantled by tephra 

(unit pe) erupted from Eagle Peak. Ridge below 

and to the left of Ski Heil Peak is composed of the 

hydrothermally altered andesite of Mount Diller 

(unit amd, 387±10 ka). Gray slopes in the 

foreground are the andesites of Mill Canyon (unit 

amc, 590–470 ka).

Photograph by Patrick Muffler.