2012

U.S. Department of the Interior

U.S. Geological Survey

Aerial view northward along glaciated summits of South Sister, Middle Sister, and North Sister volcanoes. Snow and icefilled South Sister crater (rim at 10,358 ft) was created between 30 and 22 ka, during most recent of several explosive summit 

eruptions; thin oxidized agglutinate that mantles current crater rim protects 150-m-thick pyroclastic sequence that helped 

fill much larger crater. Middle Sister (10,047 ft) is capped by thick stack of radially dipping, dark-gray, thin mafic lava flows; 

asymmetrically glaciated, its nearly intact west flank contrasts sharply with its steep east face. Blue lake (near far right edge) 

is impounded by sharp-crested Neoglacial moraine. North Sister (10,085 ft) is glacially ravaged stratocone that consists of 

hundreds of thin rubbly lava flows and intercalated falls that dip radially and steeply; remnants of two thick lava flows cap 

summit. Broad mafic shield beyond North Sister is Black Crater; distant peak on horizon is Mount Jefferson; and Mount Hood is in 

dim distance. Photograph by John Scurlock, 2007.

Geologic Map of Three Sisters Volcanic Cluster, Cascade 

Range, Oregon

By Wes Hildreth, Judy Fierstein, and Andrew T. Calvert

Pamphlet to accompany

Scientific Investigations Map 3186
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i

Contents

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

Physiography and Access...................................................................................................................1

Previous Work .......................................................................................................................................1

Methods..................................................................................................................................................4

Geologic Setting.............................................................................................................................................4

Intra-Arc Extension...............................................................................................................................4

Cross-Oregon Crustal-Melting Anomaly...........................................................................................5

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

The Quaternary Volcanoes...........................................................................................................................9

The Mafic Periphery.............................................................................................................................9

Broken Top and the Tumalo Volcanic Field.....................................................................................10

North Sister..........................................................................................................................................10

South Sister..........................................................................................................................................11

Middle Sister........................................................................................................................................11

Rhyolite Anomaly ................................................................................................................................12

Composition of Eruptive Products.............................................................................................................12

Postglacial Eruptions...................................................................................................................................13

Volcanic Hazards .........................................................................................................................................15

Acknowledgments.......................................................................................................................................18

Introduction to Description of Map Units ................................................................................................18

Description of Map Units............................................................................................................................19

Surficial Deposits................................................................................................................................19

Volcanic Rocks....................................................................................................................................20

References Cited..........................................................................................................................................59

Figures

1. Regional location map of western Oregon, showing principal physiographic provinces, 

selected rivers and cities, and major Quaternary volcanic edifices of Cascade 

Range, including Three Sisters volcanic cluster. ..............................................................2

2. Map of Three Sisters region, showing locations of many geographic features mentioned 

in text. .......................................................................................................................................3

3. Tectonic setting of Quaternary Cascade arc. ............................................................................6

4. Rhyolite vents of Three Sisters and rear-arc Newberry caldera region. ..............................8

5. Chemical variation diagrams for more than 730 samples of Quaternary eruptive products 

of Three Sisters volcanic cluster........................................................................................14

6. Plots showing Zr, MgO, TiO2Sr, FeO*, and Al2O3 versus SiO2 contents for eruptive 

products of Three Sisters volcanic cluster. .....................................................................16 

Tables

1. Chemical data for Three Sisters volcanic cluster....................................................................64 

2. 40Ar/39Ar ages for Three Sisters volcanic cluster....................................................................104

3. K-Ar ages for Three Sisters volcanic cluster..........................................................................107

Sheets

1. Geologic Map

2. Correlation of Map Units and List of Map Units
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1

Introduction

The cluster of glaciated stratovolcanoes called the Three 

Sisters forms a spectacular 20-km-long reach along the crest 

of the Cascade Range in Oregon, 35 km west of Bend and 100 

km east of Eugene (fig. 1). As observed by trailblazing volcanologist, Howel Williams (1944), “For magnificence of glacial 

scenery, for wealth of recent lavas, and for graphic examples 

of dissected volcanoes, no part of this range surpasses the 

area embracing the Sisters and McKenzie Pass.” The area we 

have now mapped in detail consists exclusively of Quaternary 

volcanic rocks and derivative surficial deposits. Although most 

of the area has been modified by glaciation, the volcanoes are 

young enough that the landforms remain largely constructional. 

Locations of many of the geographic features mentioned in this 

report are shown in figures 1 and 2.

Scientific and journalistic interest in the Three Sisters 

volcanic cluster was aroused a few years ago when ongoing 

uplift centered about 5 km west of South Sister was identified, 

first recognized by satellite imagery (interferometric synthetic 

aperture radar, InSAR) in 2001 (Wicks and others, 2002). 

Subsequent geodetic measurements and continuing InSAR 

analysis confirmed 3 to 4 cm/yr uplift during the interval from 

1997 to 2004; the uplift has been modelled as inflation thought 

to be caused by an intracrustal intrusion (Dzurisin and others, 

2006, 2009), largely aseismic and plausibly involving mafic 

magma like that which was last erupted 11 km to the north 

about 1,500 years ago.

Twelve of the 145 eruptive units described below are 

postglacial, younger than the deglaciation that was underway 

by about 17 ka. The most recent eruptions were of rhyolite near 

South Sister, about 2,000 years ago, and of mafic magma near 

McKenzie Pass, about 1,500 years ago. Despite the recency of 

eruptive activity, no fumaroles or hot springs are recognized 

in the area. West of South Sister, however, several springs 

(some of which are geochemically anomalous) that have orifice 

temperatures in the range of 2° to 12°C are present (Evans and 

others, 2004). 

The three eponymous stratocones, though contiguous and 

conventionally lumped sororally, could hardly display less 

family resemblance. North Sister is a monotonously mafic 

edifice at least as old as 120 ka; Middle Sister, an andesitebasalt-dacite cone built between 48 and 14 ka; and South Sister, 

a bimodal rhyolitic-intermediate edifice that was constructed 

between 50 ka and 2 ka. For each of the three, eruptive volume 

is likely to have been in the range of 15 to 25 km3

, but such 

estimates are fairly uncertain, owing to glacial erosion.

Physiography and Access

Nearly all of the map area lies within the Three Sisters 

Wilderness, which is roadless, is free of buildings and shelters, 

and is protected by the Deschutes and Willamette National 

Forests. Seasonal access on foot is nonetheless simple, as 

numerous trails (many maintained and others informal) crisscross the wilderness from trailheads near the McKenzie Pass 

Highway (Hwy 242) to the north (fig. 2), the Cascade Lakes 

Highway (Hwy 46) to the south, and several Forest Service 

roads to the east. Most trails are accessible from July through 

October, but for the rest of the year, owing to heavy snowfall, 

few people (other than cross-country skiers) enter the wilderness. Average annual precipitation on the crest of the Cascade 

Range, measured at Santiam Pass (elevation 4,817 ft [1,468 

m], about 28 km north of North Sister), is 217 cm (85.3 in), 

and average snowfall is 1,145 cm (451 in) [Western Regional 

Climate Center (http://www.wrcc.dri.edu/index.html)]. Only 

8 percent of the average annual precipitation (and very little 

snow) falls there in the July-to-September quarter of the year. 

Precipitation falls off sharply east of the crest, to annual averages of about 34 cm at Sisters and 30 cm at Bend (fig. 1). Like 

the Sierra Nevada of California, the Three Sisters is a “gentle 

wilderness:” its summer climate is usually moderate, and its 

relief, distances, and wildlife hazards are modest, yet it remains 

spectacular and largely unspoiled.

The three principal volcanoes are similar in elevation: 

North Sister, 10,085 ft (3,074 m); Middle Sister, 10,047 ft 

(3,062 m); and South Sister, 10,358 ft (3,157 m). Their surrounding lava-flow aprons descend southward to about 5,440 

ft (1,658 m) near Sparks Lake, northeastward to below 4,400 ft 

(1,340 m) at Trout Creek Swamp and along Whychus Creek (on 

topographic maps, shown as “Squaw Creek”), and westward to 

about 5,000 ft (1,525 m) near Indian Holes and as low as 3,500 

ft (1,065 m) at Linton Lake. To its southeast and northwest, 

however, lavas of South Sister were buttressed by older edifices 

and, thus, bank as high as 7,000 ft (2,135 m) against Broken 

Top and up to 6,100 ft (1,860 m) against The Husband. Middle 

Sister, although lowest of the three main summits, nevertheless 

has the greatest relief of about 6,550 ft (2,000 m) because its 

western apron extends all the way down to Linton Lake.

The glacial ice that formerly mantled virtually the entire 

mapped area has diminished to 16 small glaciers, largely confined to cirques on the Three Sisters edifices, as shown on 1997 

USGS topographic maps, which are based on aerial photographs 

taken as recently as 1982. Of the seven named glaciers on South 

Sister, however, Skinner and Carver Glaciers have since shrunk 

into stagnant snowfields, as has Irving Glacier on Middle 

Sister. Thayer and Villard Glaciers on North Sister and Renfrew 

Glacier on Middle Sister are small and close to extinction. As 

the climate changes, more and more rubble is exposed at the 

expense of ice.

Previous Work

The first geological overview of the Three Sisters region 

was presented in a book by Hodge (1925), which (although volcanologically wanting) remains worth reading for its historical, 

geographic, and natural history contributions. A volcanological 

reconnaissance by Williams (1944), which was based on only 

six weeks fieldwork, nicely established the main outlines of 

the Three Sisters and of several peripheral mafic volcanoes. 

Lacking chemical data, however, Williams called all the rhyolite 

lavas “dacite” (although he speculated that some would eventually be proven rhyolitic), and he also reckoned the ages of all 

the glaciated edifices to be far greater than now established 
2

Figure 1. Regional location map of western Oregon, showing principal physiographic provinces, selected rivers and cities, and major 

Quaternary volcanic edifices of Cascade Range, including Three Sisters volcanic cluster. Solid black lines indicate main faults; ball and 

bar indicates downthrown side of normal faults. Dashed line through Three Sisters and nearby mountains and passes shows trace of 

Cascade Range crest, which is main drainage divide and approximate axis of Quaternary volcanic chain.

0

0 50 MILES

100 KILOMETERS

Columbia River

Jo

h

n

Day River

Willa

m

ette

River

Deschutes

Riv

e

r

PA

C I F I

C

O

C

E A

N

C

O

A

S

T

R

A N

G

E

Portland

Hood River

Mount

Hood

Mount

Jefferson

Diamond

Peak

Cappy

Mountain

Mount

Thielsen

Mount

McLoughlin

Crater Lake

caldera

Newberry

caldera

Klamath

Graben

Klamath Falls

Medford

Eugene Bend

Sisters

Corvallis

Salem

THREE

SISTERS

Santiam

Pass McKenzie

Pass

KLAMATH MOUNTAINS

HIGH

LAVA

PLAINS

C

A

S

C

A

D

E

R

A

N

G E C

R E S

T

OREGON

CALIFORNIA

WASHINGTON

OREGON

BASIN AND RANGE PROVINCE

46°

45°

44°

43°

42°

124° 122° 120°
3

Figure 2. Map of Three Sisters region, showing locations of many geographic features mentioned in text.

EXPLANATION

Major Quaternary 

volcanic edifice

Volcanic crater

Silicic volcanic vent

Mafic volcanic vent

Fissure vent

Holocene rhyolite lava 

flow

Ridge

Cliff—Hachures point 

downhill

Spruce

Spring 

Trout Creek

Swamp

Hinton Creek

Soda Creek

Goose

Creek

Linton Creek

Linton

Lake

Three

Creek

Lake

Linton

Spring

Carver

Lake Golden

Lake

Green

Lakes

Moraine

Lake

Sisters Mirror

Lake

Todd Lake

Sparks

Lake

Demaris

Lake

Yapoah

Lake

Matthieu

Lakes

Chambers

Lakes

Camp

Lake

Separation Creek

James Creek

Mesa Creek

White Branch

Trout Creek

Alder Creek

Brush Draw

Pole Creek

Teardrop

 Pool

Devils

Lake

Upper

Chush

Falls

Soap Creek

North Fork

West Fork

Park Creek

East Fork

Snow Creek

Whychus Creek

Sink Creek

Fall Creek

Obsidian Creek

“ D e v i l s c h a i n ”

44°15’

121°50’ 121°45’

44°

0 5 KILOMETERS

Kokostick

Butte

Kaleetan

Butte

Devils

Hill 

Rock

Mesa

Newberry

flow

Tam McArthur Rim

Park Meadow

Red Meadow

Lane

Plateau 

NORTH

SISTER

MIDDLE

SISTER

SOUTH

SISTER

MOUNT

BACHELOR

Obsidian

Cliffs 

Wickiup Plain

HWY 242

McKenzie

Pass

Scott Pass

"Black Hump"

“Racetrack

 Meadow”

Indian

Holes

Linton 

Meadows

Hodge

Crest

Dome

5803

CASCADE LAKES HIGHWAY

HWY 46

M C K E N Z I E

PA S S

V O L C A N I C

F I E L D

BROKEN

TOP

Ball

Butte

Black

Crater

Millican

Crater

Yapoah

Crater

Four in

One Cone

Collier 

Cone

Little

Brother

Trout Creek

Butte

The Husband

Substitute

Point

Sphinx Butte

The Wife

The House

Rock

Sims Butte

Le Conte

Crater

Talapus

Butte

Katsuk

Butte 

Condon

Butte

“Egan Cone”

Tumalo

Mountain

Cayuse

Crater

Koosah

Mountain

South Fork
4

by modern geochronology. Progressively more complete 

reconnaissance mapping and compilations subsequently were 

produced by Taylor and others (1987) and Sherrod and others 

(2004). Williams (1957) also published a reconnaissance geologic map of the entire Cascade Range of central Oregon from 

Crater Lake to Mount Jefferson (fig. 1). More detailed geologic 

maps of two quadrangles that overlap the eastern part of our 

study area were published by Taylor (1978, 1987). The most 

detailed geologic investigations here were by Scott (1987), on 

late Holocene rhyolites peripheral to South Sister, and a map 

by Scott and Gardner (1992) that covers the southeast flank 

of South Sister and the entire Mount Bachelor chain farther 

south. A geochemical study of mafic lavas in the region that 

includes the Three Sisters was published by Hughes and Taylor 

(1986) and Hughes (1990). Topical thesis studies emphasizing 

chemistry and petrography of particular areas were completed 

by Wozniak (1982), Clark (1983), Hill (1991), Webster (1992), 

Schick (1994), Gardner (1994), and Schmidt (2005). Mercer 

and Johnston (2008) and Schmidt and Grunder (2009) investigated the eruptive history and petrology of North Sister. A 

summary of the Sisters reach of the Quaternary Cascade arc—a 

90-km-long subsegment centered on, but extending well north 

and south of, our Three Sisters map area—was included in an 

arc-wide overview by Hildreth (2007).

Methods

We spent about one month each summer from 2000 to 

2009 mapping the volcanic field on foot, generally late in the 

season when snowmelt was most advanced. About 200 days 

were devoted to productive fieldwork, excluding stormy days, 

rest days, and travel to and from our California office. Because 

most of the area is glaciated and till strewn, aerial photographs 

were of only minor utility, and our detailed mapping was done 

directly on USGS 1:24,000-scale topographic quadrangle maps 

(South Sister, North Sister, Linton Lake, Trout Creek Butte, and 

Broken Top). A hand-held GPS device commonly was useful in 

the woods. Our goal was to study in detail the Three Sisters and 

the eruptive products of contemporaneous vents on their aprons. 

Accordingly, no effort was made to complete the quadrangles 

nor to undertake detailed mapping of the older (mostly middle 

Pleistocene) mafic centers that surround the Three Sisters cluster. The Three Sisters map area is bordered on its northwest side 

by the postglacial McKenzie Pass volcanic field (Taylor, 1968; 

Sherrod and others, 2004), which extends 25 km farther northwestward; we chose to show its southernmost component (lava 

flows from Collier Cone) and its easternmost component (the 

eastern lava flow from Yapoah Crater) as convenient margins 

for the Three Sisters map area. 

One or several thin sections for each of the 145 volcanic 

units mapped were examined microscopically, and the observations are summarized in the unit descriptions. Most samples 

were analyzed chemically, as described and listed in table 1. 

For a large fraction of the 130-odd pre-Holocene units 

herein defined, radioisotopic ages were determined in the U.S. 

Geological Survey geochronology laboratory in Menlo Park, 

California (supervised by A.T. Calvert). Most samples in this 

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

(2006) and Hildreth and others (2007), and the results are 

listed in table 2. Samples were irradiated at the U.S. Geological 

Survey reactor in Denver, Colorado, using 27.87-Ma Taylor 

Creek sanidine as a neutron-flux monitor. In table 2, weightedmean plateau ages, isotope-correlation (isochron) ages, and 

total-gas ages are reported, and our interpreted eruption ages are 

highlighted in bold font. Plateau ages generally are interpreted 

as the most reliable if the 40Ar/36Ar intercept is within analytical 

error of atmospheric argon (40Ar/36Ar = 295.5). Isochron ages 

are interpreted as the most reliable if the 40Ar/36Ar intercept is 

outside analytical error of atmospheric argon. Total-gas ages are 

interpreted as the most reliable on fine-textured samples that 

show evidence for significant 39Ar recoil. In several cases, we 

tested 39Ar-recoil effects by conducting K-Ar analyses on the 

same rocks (table 3), following methods described in Hildreth 

and Lanphere (1994). Results from K-Ar determinations were 

concordant with the 40Ar/39Ar results, and, in one sample (TS94, unit rkb), K-Ar analysis yielded a reliable age that we were 

unable to obtain with 40Ar/39Ar methods.

Geologic Setting

For a convergent-margin volcanic belt, the density of 

Quaternary volcanoes in the Cascade Range of Oregon (commonly called the Oregon Cascades) is extraordinary (about 1,050 

vents). The only comparably extensive areas that have such 

dense concentrations of Quaternary-arc volcanoes are in southern Washington, northern California, Michoacán (Mexico), and a 

few short reaches of the Chilean Andes (Hildreth, 2007).

No well-defined volcanic front exists in Oregon (Hildreth, 

2007). The crest of the Cascade Range, site of the Three Sisters, 

lies about 270 km east of the submarine deformation front that 

marks the site of a sediment-filled trench. The crust beneath 

the Cascade crest in Oregon is thought to be about 44 km thick 

(Leaver and others, 1984; Mooney and Weaver, 1989). The 

subducting slab beneath Oregon is virtually aseismic, and, 

because tomographic models for its dip and depth beneath the 

Oregon arc and forearc vary widely (Michaelson and Weaver, 

1986; Rasmussen and Humphreys, 1988; Harris and others, 

1991; Bostock and others, 2002; McCrory and others, 2004), its 

configuration remains uncertain.

Intra-Arc Extension

The plate-convergence rate at the latitude of the central 

Oregon Cascades is only about 3.5 cm/yr, increasing slightly 

northward (McCaffrey and others, 2007). The azimuth at which 

the Juan de Fuca plate underthrusts the Oregon plate margin is 

about N. 45° E., oblique to the north-trending plate boundary.

In addition to the basal traction that probably is contributed by the oblique plate convergence, tectonic deformation 

of Cascadia and adjacent areas is driven fundamentally by the 

dextral shear between the Pacific and North American plates 

(Atwater, 1970) and by the extension promoted by buoyant 
5

mantle beneath the western Basin and Range Province (Jones 

and others, 1996). Opposite the Cascade arc, most of the relative motion between the two megaplates is accommodated by 

spreading on the Juan de Fuca Ridge and subduction beneath 

Cascadia, but some 25 percent of the relative motion is distributed far inboard as block rotations and translations (Pezzopane 

and Weldon, 1993; Walcott, 1993; Wells and others, 1998; 

McCaffrey and others, 2007).

In response to the dextral shear couple, and in concert both 

with northwestward motion of the nearly rigid Sierra Nevada–

Great Valley microplate (Dixon and others, 2000; Hammond 

and Thatcher, 2004) and with Basin and Range extension 

inboard (fig. 3), the Oregon forearc has undergone clockwise 

rotation since at least the middle Miocene (Wells and Heller, 

1988; Wells, 1990; Wells and others, 1998). Along its trailing 

margin, rotation of the forearc block apparently contributes 

to the intra-arc and rear-arc east-west extension significant 

along the Oregon Cascades, from the Klamath Graben to the 

Columbia River (figs. 1, 3). On the decadal timescale of GPS 

measurements, however, intra-arc extension here is currently 

limited to about 1 mm/yr or less (McCaffrey and others, 2007). 

Oblique subduction of the Juan de Fuca plate probably also contributes by basal traction to northward motion of the Cascadia 

forearc. Northwestward translation of the Sierra Nevada–Great 

Valley microplate involves a broad dextral inboard shear zone 

(Walker Lane) that extends from southern Nevada to northeastern California, where the northwest-trending strike-slip belt 

merges diffusely into the array of north-striking normal faults 

that reach northward through the Klamath Graben, Mount 

Mazama, and the Cascade Range of central Oregon (fig. 3). 

The transition from predominantly dextral shear in California 

to block rotation in Oregon lies between the Lassen and Shasta 

segments of the arc, coinciding with the transition offshore from 

a strike-slip to a subducting plate boundary (fig. 3).

Farther north, the Brothers Fault Zone (fig. 3), a belt of 

weaker northwest-trending dextral shear, provides a diffuse 

northern margin for the Basin and Range extended region 

(Lawrence, 1976). The Brothers Fault Zone likewise terminates 

diffusely against the Cascade arc, first stepping left (west) 

to form the northwest-trending Sisters (Tumalo) Fault Zone, 

which, in turn, merges with the north-striking High Cascades 

Graben (fig. 3; see also Taylor, 1981, 1990; Smith and Taylor, 

1983; Smith and others, 1987; Sherrod and others, 2004). 

Elements of this intra-arc belt of modest east-west extension, 

structurally active at least since 5 Ma, may or may not be 

continuous along the Cascade Range axis from Green Ridge 

(44.5° N.) northward through the Mount Jefferson reach to join 

the Hood River Fault (fig. 3) near the Columbia River (Walker 

and MacLeod, 1991; Conrey and others, 1997, 2002), where 

discernible extension apparently peters out.

Although the Juan de Fuca plate subducts northeastward 

beneath the North American plate, the maximum principal 

horizontal-stress direction in the upper plate is margin parallel, 

not in the convergence direction (Spence, 1989; Zoback, 1992). 

Presumably owing to the combination of oblique convergence 

and the northward push of the Sierra Nevada–Great Valley 

microplate through the Klamath Block (fig. 3), the direction of 

compression is north-south in Washington and Oregon and as 

far south as Mount Shasta, within the arc, forearc, and parts of 

the rear arc such as the Yakima Foldbelt. This stress orientation 

is reflected in north-south vent alignments from Mount Shasta 

to Mount Adams, but nowhere in the Cascade Range are vent 

alignments more conspicuous than here in the reach containing 

the Three Sisters, where lines of mafic volcanoes (and even a 

chain of Holocene rhyolite vents) form north-striking arrays 

(Bacon, 1985; Hughes and Taylor, 1986; Scott, 1987; Scott 

and Gardner, 1992). Without specific evidence, however, such 

vent alignments should not be assumed to have been supplied 

contemporaneously by a common dike system or to reflect 

a fissure-fed eruption. Many such alignments include varied 

magmas that erupted thousands of years apart, and parallelism 

of the dikes that presumably underlie them results from the 

enduring stress field, not necessarily from a common magma 

reservoir.

Differential extension and block rotation of the lithosphere 

may well play a more important role than subduction geometry 

in controlling segmentation of the volcanic belt, in defining its 

several distinguishable reaches in Oregon, and in promoting 

its exceptional width in California and southern Washington 

(Hildreth, 2007). The Cascade arc has been a well-defined, 

continuous belt since the Eocene, but dextral shear, block rotations, and Basin and Range extension were not imposed on 

the arc until the middle Miocene. Had none of these external 

tectonic processes been superimposed on the Cascade Range, a 

Cascade arc would, nonetheless, still exist today, owing to the 

slab-derived contribution to mantle-wedge melt production. In 

all likelihood, however, there would be no rear-arc center at 

Newberry Volcano, none of the H2O-poor, low-K, high-alumina 

olivine tholeiite so abundant in central Oregon, and no true 

rhyolite in the Three Sisters reach of the arc.

Cross-Oregon Crustal-Melting Anomaly

The Three Sisters lie at the leading end of a crustal-melting 

anomaly that has propagated across Oregon (fig. 3) since the 

middle Miocene (MacLeod and others, 1975; Jordan and others, 

2004). The anomaly manifested in the eruption of numerous 

units of true rhyolite (72–77% SiO2), which is rare elsewhere 

along the Cascade arc (although common at rear-arc centers and 

in the extensional continental interior). Initiation of rhyolitic volcanism started in southeastern Oregon, advancing across Harney 

Basin by 9 Ma and Fort Rock Valley by 5 Ma. After this the rate 

of westward propagation slowed markedly (Jordan and others, 

2004), approaching the later site of Newberry Volcano in the 

early Quaternary. Several middle and late Pleistocene rhyolites 

subsequently erupted near Newberry Volcano and in the Tumalo 

volcanic field (east of Broken Top; Hildreth, 2007), followed 

by rhyolites still farther northwest that range in age from 50 ka 

to 2 ka at Middle Sister and South Sister (fig. 4). Progression 

of the Oregon melting anomaly is sometimes said to mirror 

the comparable northeastward propagation of rhyolite initiation along the Snake River Plain as far as Yellowstone over the 

same time interval (Christiansen and McKee, 1978; Christiansen 

and others, 2002; Jordan and others, 2004), but the volume of 
6

0 50 100 KILOMETERS

San Andreas Fault (50 m

m/yr)

50

40

30

60

HCG

PL

WV

HRF

GRF

SFZ

LG

WR

HCF

Yakima Foldbelt

SILETZIA

TERRANE

KLAMATH BLOCK

COAST PLUTONIC COMPLEX

NORTH

CASCADES

OLYMPIC

PENINSULA

12-13 mm/yr

SIERRA NEVADA

GREAT VALLEY

WALKER LANE (6 mm/yr)

COAST RANGES

ML

M

N

T

J

H

SH SM

A

R

GP

B

G

MZ

DP

MS

SS

S

LVC

MVF

HLF

WSF

Lake

Tahoe

Pyramid

Lake

Strait of Georgia

Vancouver CANADA 

UNITED STATES

Seattle

Portland

Eugene

Medford

Redding

Susanville

Klamath

Falls

Bend

Burns

Alturas

Cedarville

BFZ

2

4

6

8 10

PACIFIC OCEAN

WA

OR

OR

CA

CA

NV

49°

122° 120° 118° 126° 124° 50°

48°

47°

46°

45°

44°

43°

42°

41°

40°

39°

 EXPLANATION

Selected large volcano

Caldera

Simcoe Mountains volcanic 

field

Area of deep graben fill

Approximate outline of 

major valley

Fault—Arrows show relative 

movement

Normal fault—Tick on 

downthrown side

Anticlinal axis

Direction and rate of 

movement of Sierra 

Nevada–Great Valley 

microplate

Direction of Cascadia 

rotation

Structure contour, showing 

depth (in km) of 

subducting slab

Isochron, showing age (in 

Ma) of inception of 

rhyolitic volcanism

10

50

OR

NV
7

rhyolite erupted along the Oregon trend is only about 1 percent 

of that along the Snake River Plain. Along both trends, eruption 

of considerable amounts of basalt and some rhyolite continued 

long after passage of the rhyolite-initiating front.

Because true rhyolites are so rare elsewhere within the 

Quaternary Cascade arc, as opposed to the rhyodacites (68–72% 

SiO2) so abundant at Mount Jefferson and Crater Lake, it is 

likely that the rhyolite-generating crustal-melting anomaly 

beneath the Tumalo and Three Sisters areas is principally attributable to whatever combination of mantle flow and decompression has been responsible for the 16-m.y.-long westward 

propagation of intracrustal melting and rhyolitic volcanism 

across the High Lava Plains. Now that the leading edge has 

penetrated the Cascade arc, rhyolite production may recently be 

enhanced in the Sisters reach by the presence of preheated arc 

crust.

Local Basement Rocks

Although crystalline basement rocks of Mesozoic and 

greater age underlie the northern and southern parts of the 

Cascade arc, from near Mount Rainier southward 500 km to the 

Oregon-California border, basement rocks are wholly concealed 

(in the so-called “Columbia Embayment”) beneath Cascade-arc 

rocks of Eocene to Holocene age. A large window of basement 

rocks 20 km southeast of Mount Rainier (Miller, 1989) has age 

and lithologic affinities to the thrust sheets and mélange belts 

of both the Klamath Mountains and the Northwest Cascades. 

Consisting of tectonic slices and accretionary mélange of 

submarine-fan deposits, hemipelagic seafloor deposits, pillowed MORB greenstones, and tonalitic and mafic metaigneous K-poor arc rocks, all of Mesozoic age, the assemblage 

may represent the thick crust concealed beneath much of the 

“embayment.” Although it has been speculated that Mesozoic or 

Tertiary seafloor may underlie the “embayment,” a thick stack 

of varied accretionary rocks, which were thrust or underplated 

inboard and translated dextrally northward along the Mesozoic 

continental margin, is more consonant with the 44-km-thick 

sub-arc crust and with the absence of any along-arc discontinuity in the seismic-velocity structure along a refraction profile up 

the axis of the Oregon Cascades (Leaver and others, 1984).

Older rocks that are exposed adjacent to the Quaternary 

volcanic belt in central Oregon include the following three 

suites: 

Suite 1—Cascade-arc assemblages of late Eocene to late 

Miocene age form a 70-km-wide terrain that crops out from 

the Willamette Valley to the western margin of the Quaternary 

volcanic belt and dips gently eastward beneath it. The maturely 

dissected terrain consists of varied sequences of tuffs, lavas, 

and lahar deposits, roughly 5 km thick, that are deeply eroded 

and regionally affected by low-grade alteration, although only 

modestly deformed (Peck and others, 1964; Sherrod and Smith, 

2000). Pyroclastic and derivative sedimentary rocks dominate 

over lavas in the lower three-quarters of the pile, but andesitic 

Figure 3. Tectonic setting of Quaternary Cascade arc. For complete Quaternary vent distribution and offshore plate system, see Hildreth 

(2007). Abbreviations for volcanoes, calderas, and volcanic fields: A, Mount Adams; B, Mount Baker; DP, Diamond Peak; G, Mount 

Garibaldi; GP, Glacier Peak; H, Mount Hood; J, Mount Jefferson; LVC, Lassen volcanic center; M, Mount McLoughlin; ML, Medicine Lake 

volcano; MS, Middle Sister; MZ, Mount Mazama (Crater Lake); N, Newberry Volcano; R, Mount Rainier; S, Mount Shasta; SH, Mount St. 

Helens; SM, Simcoe Mountains; SS, South Sister; T, Mount Thielsen. Abbreviations for faults, as well as for geomorphic and geographic 

features: BFZ, Brothers Fault Zone; GRF, Green Ridge Fault; HCF, Hat Creek Fault; HCG, High Cascades Graben; HLF, Honey Lake Fault; 

HRF, Hood River Fault; LG, La Pine Graben; MVF, Mohawk Valley Fault; PL, Puget Lowland; SFZ, Sisters (Tumalo) Fault Zone; WR, Walker 

Rim; WSF, Warm Springs Fault; WV, Willamette Valley. Worth noting are the following features: (1) Extensional region has expanded 

westward since late Miocene to overlap subduction-induced magmatic arc in California and Oregon. (2) Plate-boundary shear traction 

principally drives northwestward translation of rigid Sierra Nevada–Great Valley microplate, accounting for about 25 percent of relative 

motion between Pacific plate and stable North America plate (Dixon and others, 2000), most of which takes place along San Andreas 

Fault. Dextral shear (6 mm/yr) on Walker Lane (belt of rheologically weak lithosphere) accounts for about half of microplate translation 

and diffusely merges northwest into region of intra-arc normal faulting. (3) Microplate compression through Klamath Block helps (along 

with oblique plate convergence) to drive northward translation and clockwise rotation of Oregon forearc, contributing to intra-arc 

extension along its trailing edge. Oregon forearc block dominantly consists of 30-km-thick accreted oceanic crust of Siletzia terrane; its 

northward motion compresses accretionary Washington forearc against Canadian batholithic buttress (Wells and others, 1998). North 

Cascades and Coast Plutonic Complex predominantly consist of crystalline basement terranes of Mesozoic and older metamorphic and 

plutonic rocks. Directions of Cascadia rotation generalized from hundreds of GPS sites referenced to stable North America (McCaffrey 

and others, 2007). (4) Seaward-concave plate margin promotes slab arch, axis of which plunges toward 48o N.; slab depth inferred 

from seismicity and tomography by Crosson and Owens (1987). (5) Proximal forearc depression, 50 to 100 km seaward of active arc, 

is occupied by Strait of Georgia, Puget Lowland, and Willamette Valley and may reflect densification of slab at eclogite transition; 

its absence south of Eugene may reflect upper-plate deformation by northward-translating Klamath Block. (6) Formation of Yakima 

Foldbelt, which principally deforms lavas of the Columbia River Basalt Group, began about 15 Ma during their main eruptive interval and 

continues today; Mount Adams, Mount Hood, and Simcoe Mountains volcanic field overlie Yakima folds. (7) In central Oregon, isochrons 

depict westward progression of inception of rhyolitic volcanism across High Lava Plains (MacLeod and others, 1975; Jordan and others, 

2004); in the Quaternary, eruptions of true rhyolite (rare elsewhere in Cascade arc) have further advanced through Newberry Volcano 

and as far northwest as South and Middle Sisters (see fig. 4).
8

 EXPLANATION

Selected major stratovolcano

Selected major shield volcano

Quaternary rhyolite vent—Also includes a 

few rhyodacite (68–72% SiO2) vents. 

Map-unit labels are shown for vents 

within map area; ages (in Ma) are shown 

for selected vents outside map area

Alignment of multiple, near contiguous 

rhyolite vents

Outline of Newberry caldera—Hachures 

point inward

Normal fault—Ball and bar on downthrown 

side. Dashed line indicates general trace 

of Tumalo Fault Zone

Approximate outline of surficial deposits of 

Deschutes River lowland

La Pine

Bend

Santiam Pass

Sisters

McKenzie Pass

TUMALO

VOLCANIC

FIELD

0 50 KILOMETERS

TUMALO

FAULT ZONE

BW

MeB

3CB

TMR

(0.2)

roc

ruc rsf

rpc

rpg rsg

rgl

rct

rsw

rrm

rkb rdh

rdc

rse rmc

McB

(0.6)

PP

(0.08)

CH

(0.8) EB

(0.9) QM

(1.1)

r58

rsc

BF

MxB

SaM

Bk

BC

TCB

ScM

TH

SP

SB

TW

KM

HM

TM

PM

WM

IM

CM

CB

TT

LM

SM

MP

MM

RM

HB

RB

DM

PB

CrB

GB

TrH

BM

OB

LB

TFJ

BB

MW

NS

SS

BT

MB

NC

MS

(continues to southeast)

HLP

44°

43°30’

44°30’

122° 121°
9

(and lesser amounts of basaltic and silicic) lavas dominate in the 

top kilometer or so. The western Cascade-arc suite is weakly 

folded here (characterized by dips typically 5° to 15°), although 

it is affected by somewhat stronger folding and faulting farther 

north. Many plugs and a few modest granitoid plutons have 

been unroofed locally.

Suite 2—Rear-arc volcanic rocks and derivative sedimentary rocks, also of late Eocene to early Miocene age, that are 

generally attributed to the John Day Formation crop out only to 

the east of the Quaternary volcanic belt but probably also extend 

beneath it, presumably interlayered there with rocks of suite 

1. The volcanic rocks are predominantly ignimbrites, ash-fall 

deposits, and lava flows and domes, mostly of rhyolitic composition, accompanied by varied tuffaceous sedimentary rocks and 

subordinate amounts of mafic alkalic lavas. In western exposures, however, ash-fall and tuffaceous sedimentary deposits 

of andesite-dacite composition were proposed by Robinson 

and others (1984) to represent the interfingering of distal-arc 

facies of suite 1. The John Day Formation assemblage (suite 

2) likewise is faulted, tilted, or weakly folded but not strongly 

deformed (Peck, 1964; Smith and others, 1998; Sherrod and 

others, 2004).

Suite 3—Virtually undeformed and unaltered arc volcanic 

rocks, mostly 7.5 to 3 Ma, crop out directly beneath both sides 

of the Quaternary volcanic belt and interfinger eastward with 

fluvial sedimentary deposits that filled the Deschutes Basin just 

east of the arc. Dominant within the range are lava flows (mafic 

through silicic) and, in the basin to the east, ignimbrites and 

fluvial sediments. Ignimbrites were common before 5 Ma but 

not thereafter (Smith, 1987; Smith and others, 1987). Suite 3 is 

cut and partly contained by a Pliocene graben just north of our 

map area (Smith and Taylor, 1983). Numerous scoria cones and 

small shields interfinger with and overlie the sedimentary basin 

deposits as far as 50 km east of the present Cascade Range crest 

(Sherrod and others, 2004), suggesting that the vent belt was at 

least as wide in the Pliocene as during the Quaternary. Basaltic 

andesites dominate this suite, but eruptive products range from 

basalt to rhyodacite.

The Quaternary Volcanoes

The Sisters reach of the Cascade arc, which extends about 

40 km north and south of the Three Sisters, contains at least 

466 Quaternary volcanoes (Sherrod and Smith, 2000; Hildreth, 

2007). Most are monogenetic, but scores of mafic shields 

and several stratocones are also present. At the latitude of 

the Three Sisters, the vent belt is about 35 km wide, extending about 12 km west and 22 km east of the Cascade crest. In 

addition, at least 450 separate vents are present on the great 

rear-arc Newberry Volcano edifice, adjacent to the east. Along 

the arc itself (fig. 1), the nearest substantial evolved (nonmafic) centers are Mount Jefferson (60 km north) and Cappy 

Mountain (90 km south). Large mafic edifices, however, are 

especially common along this reach of the arc (fig. 4); these 

include Mount Washington, Three Fingered Jack, and Black 

Butte to the north and Mount Bachelor and many more to the 

south (Williams, 1957; Sherrod and Smith, 2000; Hildreth, 

2007). 

The Mafic Periphery

As emphasized throughout a century of geologic investigations in the Three Sisters region, a central area of more 

evolved (andesitic to rhyolitic) and relatively young volcanic 

rocks, focussed on and around South Sister and Middle Sister, is 

almost completely surrounded by mafic eruptive centers (figs. 

2, 4; see also Hodge, 1925; Williams, 1944; Taylor, 1990). Most 

of the peripheral mafic centers are middle and late Pleistocene, 

older than the evolved central complex (largely 50 ka and 

younger), although the distributed mafic volcanic fields around 

McKenzie Pass to the north and Mount Bachelor to the southeast are both younger.

One of the oldest mafic volcanoes in the near periphery 

is The Wife (~375 ka), a large glaciated shield centered 6 km 

southwest of South Sister. Proceeding clockwise around the 

periphery (fig. 2), The Wife is adjoined by the deeply incised 

Figure 4. Rhyolite vents of Three Sisters and rear-arc Newberry caldera (NC) region. Vent locations are from MacLeod and others 

(1995), Sherrod and Smith (2000), Sherrod and others (2004), and this study. All vents are Quaternary; thus, they extend the 

northwestward rhyolite-inception age progression across High Lava Plains (HLP; see fig. 3). Rhyolites are as old as 1.1 to 0.8 Ma 

just east of Newberry caldera, and they are middle Miocene to Pliocene on High Lava Plains farther southeast. Surficial deposits of 

Deschutes River lowland near La Pine may cover additional rhyolites. True rhyolites (72–77% SiO2) are rare in Quaternary Cascade 

arc and are almost absent along arc everywhere north of Lassen segment (Hildreth, 2007), except within Three Sisters volcanic 

cluster. Rhyolites on Newberry Volcano edifice, within Three Sisters cluster, and in between are all of middle or late Pleistocene or 

Holocene age. Trend of Cascade arc here is north-south, as depicted by belt of selected major shield volcanoes and stratovolcanoes. 

Abbreviations for shield volcanoes and stratovolcanoes: BB, Black Butte; BC, Black Crater; Bk, Belknap Crater; BM, Browns Mountain; 

BT, Broken Top; CB, Charlton Butte; CrB, Crescent Butte; CM, Cultus Mountain; DM, Davis Mountain; GB, Gilchrist Butte; HB, Hamner 

Butte; HM, Horse Mountain; IM, Irish Mountain; KM, Koosah Mountain; LB, Little Brother; LM, Lookout Mountain; MB, Mount Bachelor; 

MxB, Maxwell Butte; MM, Maklaks Mountain; MP, Maiden Peak; MS, Middle Sister; MW, Mount Washington; NS, North Sister; 

OB, Odell Butte; PB, Pringle Butte; PM, Packsaddle Mountain; RB, Ringo Butte; RM, Royce Mountain; SB, Sphinx Butte; SaM, Sand 

Mountain; ScM, Scott Mountain; SM, Sheridan Mountain; SP, Substitute Point; SS, South Sister; TCB, Trout Creek Butte; TFJ, Three 

Fingered Jack; TH, The Husband; TrH, Triangle Hill; TM, Tumalo Mountain; TT, The Twins; TW, The Wife; WM, Williamson Mountain. 

Abbreviations for rhyolites outside map area: 3CB, Three Creek Butte; BF, Benham Falls; BW, Bearwallow Butte; CH, China Hat; EB, East 

Butte; McB, McKay Butte; MeB, Melvin Butte; PP, Paulina Peak; QM, Quartz Mountain; TMR, Tam McArthur Rim.
10

Sphinx Butte shield (~280 ka), just north of which lies the large 

multivent shield called The Husband (150 ka or older), centered 

6 to 7 km west of South and Middle Sisters. The northwest and 

northeast sectors of the mafic periphery are occupied, respectively, by the dissected Little Brother shield (~48 ka) and the 

towering ice-ravaged cone of long-lived North Sister (~120 to 

45 ka). Just north of this pair spreads the postglacial McKenzie 

Pass volcanic field (fig. 2), marked by numerous mafic vents 

(mostly 4 ka to 1.5 ka; Sherrod and others, 2004). Trout Creek 

Butte, a large mafic shield and the oldest eruptive unit in the 

map area (~530 ka), lies 13 km northeast of Middle Sister, 

somewhat farther from the compositionally evolved focus than 

the younger mafic edifices mentioned. The eastern sector of 

the mafic periphery lacks a substantial edifice, but, instead, it 

is sprinkled with mafic scoria cones and silicic domes thought 

mostly to be of middle Pleistocene age (Taylor, 1987; Taylor 

and Ferns, 1995). To the southeast lies glacially gutted Broken 

Top (~300 to 150 ka), an extensive mafic shield surmounted 

by a modest stratocone, centered only 6 km from South Sister. 

To the south, the mafic peripheral ring is completed by several 

small shields and lava fans (fig. 2) that include The House 

Rock, Koosah Mountain, Le Conte Crater, and Talapus Butte, 

and Katsuk Butte (all late or latest Pleistocene), as well as 

several postglacial mafic vents on the apron of Mount Bachelor 

(Scott and Gardner, 1992). 

Despite an abundance of silicic andesite, dacite, and 

rhyolite at Middle and South Sisters, such evolved products 

are nearly absent at most of the vents composing the mafic 

periphery just outlined. A striking exception is Broken Top and 

the upland of middle Pleistocene volcanic rocks (the Tumalo 

volcanic field) that extends east of it toward Newberry Volcano 

(fig. 4).

Broken Top and the Tumalo Volcanic Field

A highland area of numerous distributed silicic to mafic 

vents east of Broken Top was called the “silicic highland” by 

Taylor (1978, 1987), the “Tumalo volcanic center” by Hill and 

Taylor (1990), and the “Tumalo volcanic field” by Hildreth 

(2007). The 25-km-wide vent field includes andesitic and mafic 

scoria cones, as well as numerous rhyolitic and rhyodacitic lava 

flows and domes. The Tumalo volcanic field is thought to be the 

source of several rhyolitic to andesitic ignimbrites and plinianfall deposits exposed near the towns of Tumalo and Bend 

(Taylor, 1981; Hill and Taylor, 1990; Mimura, 1992; Taylor and 

Ferns, 1994) that erupted between about 650 ka and about 200 

ka (Sarna-Wojcicki and others, 1989; Gardner and others, 1992; 

Lanphere and others, 1999; Sherrod and others, 2004). Owing 

to extensive cover by younger mafic lavas and glacial deposits, 

neither its integrity as an eruptive center nor its western limit 

is well defined. Because silicic lavas near Todd Lake and Tam 

McArthur Rim (fig. 2), each adjoining the Broken Top edifice 

(Williams, 1944; Taylor, 1978), erupted during the same time 

interval as those of the Tumalo volcanic field (Hill, 1991), 

they could equally well be considered parts of an extensive 

middle Pleistocene silicic volcanic field. Of the four nearby 

stratovolcanoes, South, Middle, and North Sisters are wholly 

younger than the distributed Tumalo activity, but the eruptive 

lifetime of Broken Top overlapped with it.

Volumes of three extensive ignimbrites, patchily but 

widely preserved in the periphery of the highland, are poorly 

known but are likely to be in the range of 5 to 10 km3

 each. 

Because such volumes are near the usual threshold for caldera 

formation, the Tumalo volcanic field might or might not contain 

an unrecognized filled caldera that is obscured by products of 

younger eruptions. Following Hill and Taylor (1990), Sherrod 

and others (2004) suggested that a 5-km-wide buried source 

area may be centered on Triangle Hill (fig. 4), whereas Williams 

(1957) and Conrey and others (2002) suggested locations 5 to 8 

km farther west (near Tam McArthur Rim) as possible sources 

for the youngest of them. Pinning down the source vents for 

the three Tumalo-field ignimbrites (the Desert Spring, Tumalo, 

and Shevlin Park Tuffs) remains one of the more challenging 

problems in Oregon Cascades volcanology.

Broken Top (9,175 ft; 2,796 m) was a modest volcano 

(eruptive volume, 7–10 km3), active in the middle Pleistocene 

between about 300 and 150 ka but long extinct. Its mafic apron 

lavas bank against the dacitic Todd Lake volcanic pile (460±30 

ka, according to Hill, 1991; see also Taylor, 1978); they also 

sandwich thick rhyodacite lavas at Tam McArthur Rim (213±9 

ka; Hill, 1991) and near Upper Chush Falls (on topographic 

maps, shown as “Squaw Creek Falls”) (169±2 ka; table 2). 

Although the basal shield, stratocone, and intrusive core of 

Broken Top are dominantly of basaltic andesite composition 

(Webster, 1992), subordinate eruptive products range from 

andesite to rhyodacite. Internal structure of its ice-ravaged edifice (Taylor, 1978; Grubensky and others, 1998) is well exposed 

on cirque headwalls, on radial ridges, and in a large southeastfacing amphitheatre of complex but uncertain origin. Cayuse 

Crater (fig. 2) is an unrelated postglacial basaltic (50–52% SiO2) 

complex of lava flows and scoria cones (unit bcc) that erupted 

about 11±1 ka on the southwest apron of the long-inactive 

Broken Top edifice. 

North Sister

North Sister (10,085 ft; 3,074 m) is a glacially dissected 

stratocone of fairly uniform basaltic andesite (52.5–55.1% 

SiO2; table 1; see also Taylor, 1987; Schmidt, 2005; Schmidt 

and Grunder, 2009), sufficiently eroded to expose hundreds of 

dikes and sills internally. Many dikes are zigzag or irregularly 

oriented within the pile of thin rubbly lava flows, but Schmidt 

and Grunder (2009) reported a long-term shift from edificeinfluenced radial diking toward later dominance of north-south 

dikes controlled by the regional stressfield. Oldest and longest 

active volcano of the Three Sisters, its period of construction 

(about 120 ka to about 45 ka), nonetheless, appears to have 

entirely postdated that of Broken Top. Distal lavas from North 

Sister lie stratigraphically above the northwesternmost remnant 

of the Shevlin Park Tuff (unit spt), youngest ignimbrite of the 

Tumalo volcanic field. Although a few lavas at North Sister 

may be older still, the oldest reliably dated lava flow yielded 

an age of 119±6 ka (unit mns; table 2). Younger parts of the 
11

multistage edifice (which has a few severe internal unconformities) are as young as 46±6 ka, and the summit crag yields an 

age of 55±5 (all part of unit mns; table 2). It is remarkable that 

so mafic an edifice remained relatively monotonous compositionally during such an extended eruptive history (Schmidt, 

2005; Mercer and Johnston, 2008). North Sister is part of the 

mafic periphery and is almost entirely older than eruptive products of the silicic focus represented by the edifices of South 

and Middle Sisters.

Also younger than North Sister is a late Pleistocene northtrending chain of mafic to intermediate scoria cones, lava flows, 

and fissure-fed agglutinates (Sherrod and others, 2004; Schmidt 

and Grunder, 2009), some components of which erupted through 

and then draped the (by then, severely eroded) North Sister edifice. Components of the chain are distributed for 11 km from the 

northwest ridge of North Sister northward past Matthieu Lakes 

to the shoulder of Black Crater. Another north-striking fissure 

vent, about 500 m long, unzipped down the south slope of the 

North Sister edifice. Although eruptive products of the several 

vents range in SiO2 content from 53.2 to 58.7 percent, they hold 

in common higher FeO contents (8.1–9.5%) than central-vent 

products of the North Sister edifice (6.95–8.1%). Because the 

components are of varied composition and of several different 

eruptive ages (as old as 59±4 ka and as young as 20±5 ka; table 

2), and because the vents are both fissural and point sourced, we 

prefer to call the chain the “Matthieu Lakes vent alignment.” 

The alignment reflects the enduring influence of the region’s 

extensional stress field, but its persistence sustains no inference 

of either contemporaneous magma storage or close magmatic 

affinity of the several batches erupted. The youngest eruptive 

units along the alignment (units mps and mpn) together form a 

high but narrow divide that extends from the base of North Sister 

to the northern edge of the map area. The slender north-trending 

ridge formed by these strips of mafic agglutinate and scoria are 

an expression of their fissural eruption but may also reflect glacial confinement, owing to eruption of some about 20 ka, during 

the Last Glacial Maximum.

South Sister

South Sister (10,358 ft; 3,157 m), once considered the 

youngest stratocone in the cluster (Wozniak, 1982; Clark, 

1983), is largely contemporaneous with Middle Sister. During 

the interval from 50 to 30 ka, South Sister was a dominantly 

rhyolitic volcano, consisting of numerous and varied (aphyric to 

phenocryst-rich) rhyolite (72–74% SiO2) lava flows and domes 

that crop out radially on the cone up to elevations as high as 

8,560 ft, within 1,800 ft of the present summit. Around the base 

of the edifice, two large rhyolite domes (Kaleetan Butte and 

Devils Hill; units rkb and rdh) at the south toe of South Sister, 

as well as one near Green Lakes at the east toe (unit rgl) and a 

coulee along Whychus Creek northeast of the cone (unit rsf), 

erupted during this same time interval; all four have 74 percent 

SiO2. Intermediate magmatism began with the radial outflow 

of several dacite and rhyodacite lavas in the interval from 38 

to 32 ka, alternating with the rhyolite eruptions, followed by 

construction of a broad cone of andesitic (62–64% SiO2) lavas, 

(62–64% SiO2), and culminating at 27+3 ka in growth of a 

more steeply dipping summit cone of agglutinate-dominated 

andesite (56–60.5% SiO2). Associated emplacement of a multiphase andesitic intrusive complex culminated in formation of 

a summit crater 700 m wide, one rim of which is exposed just 

inboard of Hodge Crest. Subsequent filling and overtopping of 

the large crater by a thick dacite lava flow (unit dlg) and more 

than 150 m of dacitic pyroclastic ejecta (unit des), followed by 

draping of the summit by a final thin sheet of mafic agglutinate 

(unit mtp; 54–56% SiO2) that erupted from the much smaller 

present-day summit crater, together constitute a sequence that 

lasted until the latest Pleistocene.

After at least 15,000 years of inactivity, the South Sister 

locus underwent a compositional reversal, yielding two complex but separate rhyolitic eruptions at about 2.2 and 2.0 ka 

(Taylor, 1978; Scott, 1987). The first episode produced the 

0.5-km3 Rock Mesa coulee and subordinate tephra and satellite 

domelets (unit rrm; altogether about 0.53 km3; all ~73.5% SiO2) 

at the southwest toe of the South Sister edifice. The second 

episode produced the dike-fed, 5-km-long “Devils chain” of 16 

vents (unit rdc) that cuts both the 35-ka Devils Hill dome (unit 

rdh) and the southeast flank of the South Sister edifice itself. 

North of a 3-km-long gap that bypasses the summit, another 

1.2-km-long parallel chain of minor vents runs down the north 

slope of the edifice and may have been fed concurrently by 

the same dike system (Scott, 1987). All vents of the younger 

episode (altogether, about 0.32 km3) produced rhyolite lava or 

tephra that contains 72.5 percent SiO2, clearly distinguishable 

compositionally from rhyolite of the Rock Mesa episode. The 

northernmost vent of the “Devils chain” proper lies at 7,880 ft 

(2,400 m) elevation, well up the South Sister edifice and close 

to vents for several of the late Pleistocene rhyolites. That some 

60 percent of the magma released during the younger episode 

issued from this uppermost vent (the Newberry flow) supports 

the likelihood that the rhyolitic magma reservoir lies under the 

South Sister edifice itself (Bacon, 1985; Scott, 1987).

Middle Sister

Middle Sister (10,047 ft; 3,062 m) is the youngest cone of 

the three, the present edifice having been built during the interval from 48 to 14 ka but mostly between 25 and 18 ka (table 

2), thus overlapping in time with construction of South Sister. 

Glaciation (persistent from the late Pleistocene to the present) 

has already removed much of the east half of the cone, including most of the summit-vent complex, and has sharply steepened its east face. The young cone has issued a range of mafic, 

andesitic, and dacitic (52–65% SiO2) lavas from its central-vent 

area, as well as dacites from six flank vents (three of them high 

on the edifice) and andesites from three more. The largest dacite 

flank vent, which fills the saddle between North Sister and 

Middle Sister, built a substantial pile (informally called “Prouty 

Point,” “Black Hump,” or “Step Sister”) of at least five thick 

flows (64% SiO2) that have been dated from 27 to 18 ka. Distal 

mafic lavas that erupted from an older buried edifice extend 

eastward from beneath the limit of lavas of the modern cone 

and yielded ages of 180 to 160 ka. A western apron of young 
12

andesite-dacite lavas (48–19 ka) that erupted at Middle Sister 

banks against glaciated mafic centers of middle Pleistocene 

age. Thus, there was an extended period (more than 100,000 yr 

long) that had little or no activity on the site prior to initiation 

of modern cone growth. Eruption of the rhyolite of Obsidian 

Cliffs (unit roc; 38±2 ka) at the northwest toe of the subsequent 

Middle Sister edifice was one of its earliest events.

Rhyolite Anomaly

True rhyolite (72–77% SiO2) is rare along the Quaternary 

Cascade arc (Hildreth, 2007), except in the extensional Lassen 

segment, at the extensional rear-arc centers (Simcoe Mountains, 

Newberry, and Medicine Lake volcanic fields), and here in 

the Tumalo–Three Sisters region at the leading edge of the 

cross-Oregon age-progressive rhyolite trend discussed earlier. 

No fewer than 15 middle Pleistocene rhyolites (including the 

Tumalo Tuff, not mapped herein) are exposed in the greater 

Tumalo volcanic field (Hill, 1991; Sherrod and Smith, 2000) 

between Newberry Volcano and the Three Sisters (fig. 4). In this 

report, we describe 14 discrete eruptive units of true rhyolite 

(50 to 2 ka) at South Sister and the Obsidian Cliffs, a highsilica rhyolite coulee (unit roc; 38 ka) northwest of Middle 

Sister. The northwesternmost rhyolite (unit r58) crops out as 

glaciated Dome 5803, 4 km north of the Obsidian Cliffs (Hill, 

1991; Sherrod and others, 2004). This remarkable clustering 

of Quaternary rhyolites contrasts with the presence of a single 

rhyolite dome in the Cascade Range of British Columbia, none 

at all along the arc proper in Washington or northern Oregon, 

and only one or two along the arc between the Three Sisters and 

the Lassen region (Hildreth, 2007). 

Some rhyolitic (and rhyodacitic or dacitic) magmas can 

be produced by direct partial melting of crustal rocks, with or 

without subsequent crystal fractionation or mixing with mafic 

magmas that helped heat their source. In this sense, some 

silicic crustal melts can be at least as primitive (near-primary) 

as the basaltic melts that result from the conjunction of slaband wedge-derived contributions and that undergo prolonged 

melt-matrix reaction during many tens of kilometers of ascent. 

There is broad agreement that silicic crustal melts provide 

persistent, varied, and substantial contributions to intermediatecomposition arc magmas that range from basaltic andesite to 

dacite (Smith and Leeman, 1987, 1993; Hildreth and Moorbath, 

1988; Bullen and Clynne, 1990; Hill, 1991; Green, 1994; Borg 

and Clynne, 1998; Conrey and others, 2001b), but the petrogenetic challenge has always been to identify silicic eruptive 

products that approach near-primary crustal melts and to infer 

their source materials. For the rhyolites at Three Sisters, Tumalo 

volcanic field, and Broken Top, Hill (1991) showed geochemically that none could have evolved by crystal fractionation from 

the broadly contemporaneous dacitic or rhyodacitic magmas. 

Instead, Hill proposed that the Three Sisters rhyolites resulted 

from 20 to 30 percent dehydration-melting of basaltic amphibolite and that the Broken Top–Tumalo (slightly more sodic) 

rhyolites originated by 30 to 50 percent melting of deep-crustal 

Cascadian tonalite. Both rhyolitic suites have 87Sr/86Sr ratios 

in the range of 0.7036 to 0.7037, favoring sources in Cenozoic 

Cascadian-arc crust. Because both suites also have relatively 

undepleted and flat heavy-rare-earth-element patterns, residual 

garnet appears to be excluded, suggesting that melt extraction 

took place in the middle crust rather than in the deepest arc 

crust, which is here as thick as 44 km. 

As widely noted for “bimodal” extensional volcanic fields 

elsewhere, the elevated proportions of true rhyolite and true 

basalt in the Sisters reach (compared to the basaltic andesite 

to dacite suites that dominate adjacent reaches of the arc in 

Oregon) may reflect impingement on the Cascade arc by whatever combination of mantle flow and lithospheric extension has 

guided the progression of rhyolitic volcanism westward across 

the High Lava Plains and the Newberry Volcano region since 

the middle Miocene (MacLeod and others, 1975; Jordan and 

others, 2004).

Composition of Eruptive Products

Chemical data for 788 samples that represent all eruptive 

units defined herein are tabulated in table 1 and summarized in 

figures 5 and 6. Figure 5A shows the Three Sisters suite to be 

calc-alkalic, having an alkali-lime index of 59, as defined by 

Peacock (1931). Most samples are subalkaline, as defined by 

LeBas and others (1986), although some intermediate products 

of South Sister, Middle Sister, and Broken Top overlap marginally into the trachydacite field (fig. 5A). The suite straddles 

broadly the tholeiitic–calc-alkaline boundary (fig. 5B) defined 

by Miyashiro (1974), and it largely falls within the medium-Fe 

field defined by Arculus (2003), although many North Sister 

samples and a few other units plot in the low-Fe field (fig. 

5B). Nearly all samples plot in the medium-K field (fig. 5C). 

Some of the South Sister silicic samples in the high-K field 

(fig. 5C) may have been K-enriched by alkali exchange during 

posteruptive vapor streaming or secular hydration; the four 

high-K samples from Middle Sister are pristine-looking summit 

dacites (unit dms). The two lowest-K basalts (fig. 5C) erupted 

from postglacial vents at the north end of the Mount Bachelor 

chain (units bes and bsl); the two other basalts that have less 

than 0.45% K2O (unit bnf) are probably plagioclase accumulative (about 22% Al2O3) and stand out (at 51% SiO2) on several 

panels in figure 6.

Chemical-variation diagrams (fig. 6) illustrate compositional features that highlight a variety of affinities and differences among the many volcanoes in the Three Sisters cluster, 

some of which are summarized below. 

(1) North Sister basaltic andesites (unit mns) extend to 

higher contents of Al2O3 (18–20%) and to lower contents of Fe, Ti, and P than do most other mafic products 

of the volcanic field, including those of nearby Little 

Brother, Trout Creek Butte, the Matthieu Lakes vent 

alignment, and the thick summit-forming pile of Middle 

Sister (unit mms). Products of The Wife (unit mtw) and 

The House Rock (unit mhr), however, compositionally 

resemble those of North Sister. Previous studies have 

suggested that basaltic andesites of the central Oregon 
13

Cascades can be assigned to two compositional types 

(or end members), one relatively richer and the other 

poorer in K, Fe, Ti, P, rare-earth elements, and highfield-strength trace elements; the abundant data of figure 

6 illustrate that such rocks (52–57% SiO2) in the Three 

Sisters cluster form a compositional continuum.

(2) Nothing that erupted from the three main stratocones 

(the Three Sisters) is primitive. MgO contents of 135 

samples of North Sister basaltic andesite (table 1; see 

also Schmidt, 2005) range only from 6.5 to 4.0 percent; 

mafic products of Middle Sister have less than 6.0 

percent MgO, and those of South Sister, less than 4.6 

percent. Ni and Cr contents are correspondingly modest: 

North Sister has 20 to 150 ppm Ni and 40 to 150 ppm 

Cr; mafic products of Middle Sister have 25 to 80 ppm 

Ni and 45 to 105 ppm Cr; and the most mafic products 

of South Sister have only 12 to 30 ppm Ni and 10 to 65 

ppm Cr (although its postglacial satellite cone, unit mlc, 

carries 100 to 130 ppm Ni and 150 to 205 ppm Cr).

(3) Twenty-five of the 788 samples plotted in figure 6 

contain 7 to 9 percent MgO; they range in SiO2 content 

from 49 to 52 percent, and all erupted from peripheral 

vents. The 25 samples represent 12 map units: bbr, bes, 

bsl, and btk, in the south near Sparks Lake; bjc and bsb, 

to the southwest of the Three Sisters; bwm, to the north; 

bac, bsq, and bss, to the northeast; bsc, to the east; and 

bcc, to the southeast. Most of the 25 have 115 to 160 

ppm Ni and 140 to 290 ppm Cr (table 1); only two units 

are still richer in these compatible elements, bcc (440 

ppm Cr) and bwm (as much as 213 ppm Ni and 355 

ppm Cr).

 (4) Considering all vents in the map area, no andesite 

(57–63% SiO2) has more than 4 percent MgO (except 

unit aoc); dacites have less than 2 percent, and rhyolites, 

only 0.6 to 0.1 percent. Most silica-variation diagrams 

(fig. 6) converge from broad fields of varied mafic compositions into narrowly coherent arrays that extend from 

61 to 70 percent SiO2. This suggests routine mixing 

of the various fractionating mafic magmas to produce 

compositionally condensed parental lines of descent for 

the fractionating andesite-dacite arrays (which generally 

blend concurrently with silicic melts of crustal origin). 

Andesites and dacites of South Sister nonetheless tend to 

be slightly richer in Mg and poorer in Fe than most (but 

not all) of those from Middle Sister. The rarity of rocks 

having 70 to 72 percent SiO2 (fig. 6) suggests a role for 

additional processes in generating the rhyolitic magmas, 

probably including large contributions of intracrustal 

partial melts (Hill, 1991) and extraction of silicicmelt batches directly from intermediate crystal mush 

(Hildreth, 2004). 

(5) The Sr, Zr, and TiO2 panels of figure 6 illustrate 

the wide ranges of large-ion-lithophile elements and 

high-field-strength trace elements among the varied 

mafic magmas that erupted in the volcanic field. Zr 

remains incompatible until the silicic-dacite range of the 

array, where zircon fractionation first becomes apparent and then dominates the Zr-depletion trend among 

rhyolites.

(6) Most of the 70 rhyolite samples analyzed range in 

SiO2 content from 72 to 74.4 percent, and only unit roc

(76.4–76.9% SiO2) is a high-silica rhyolite. Rhyolites in 

the Three Sisters area are ordinary continental-margin 

arc rhyolites that have low TiO2 (0.34–0.09%), low 

FeO* (2.2–1.0%), and moderate Al2O3 (14.6–13.0%) 

contents, each of which varies inversely with SiO2 content (fig. 6). All are medium- to high-K and subalkaline, 

and all have low abundances but wide ranges of P2O5

(0.18–0.02%), MgO (0.65–0.11%), and CaO (1.96–

0.86%) contents. Like most arc rhyolites, those of the 

Three Sisters are not as highly evolved in trace-element 

signature as intracontinental rhyolites of various distinctive types: Cl-rich peralkaline rhyolites; F-rich topazbearing rhyolites; high-temperature, Fe-rich anorogenic 

rhyolites; or allanite-monazite-bearing, light-rare-earthelement-depleted, subalkaline, high-silica rhyolites. 

Among true rhyolites of continental margins, when 

compared globally, those of the Three Sisters have moderate abundances (neither strikingly high nor low) of Sr 

(104–234 ppm), Ba (800–1,070 ppm), Zn (25–45 ppm), 

Ce (33–58 ppm), Y (13–22 ppm), and Zr (90–300 ppm). 

Their slightly low concentrations of Rb (58–80 ppm), 

Nb (8–13 ppm), and Th (3–8 ppm) are typical of mature 

arcs that lack mature continental basement (Macdonald 

and others, 1992).

Postglacial Eruptions

Postglacial mafic eruptions are more common in the 

Sisters reach than anywhere else in the Cascade Range 

(Hildreth, 2007). In the area of McKenzie and Santiam Passes 

(fig. 1), more than a dozen separate mafic eruptions are radiocarbon-bracketed between 4.5 and 1.2 ka (Sherrod and others, 

2004), representing a 15-km3, distributed mafic eruptive pulse 

in the late Holocene (Taylor, 1968). In the immediate periphery of the Three Sisters, several additional mafic vents, which 

include Sims Butte, Cayuse Crater, Le Conte Crater, the Mount 

Bachelor chain, the “Egan Cone” cluster, and the “KatsukTalapus chain” may all have erupted in the interval 17 to 8 

ka, during deglaciation or in early postglacial time. Just north 

of North Sister, Yapoah Crater (54.5–57% SiO2) produced a 

lava-flow fan about 2.5 ka; Four in One Cone (56–59% SiO2), 

another such fan at about 1.9 ka; and Collier Cone, at about 1.5 

ka, an apron as long as 13 km of lava flows that range in composition from 56 to 65 percent SiO2 (Schick, 1994).

Mount Bachelor, a mafic shield volcano capped by a steep 

mafic summit cone, lies 15 km southeast of South Sister. It is 
14

the largest volcanic center along a 25-km-long chain of 50 vents 

that produced 40±5 km3 of basalt and basaltic andesite (49–57% 

SiO2) in a few major eruptive episodes during the interval from 

17 to 8 ka (Scott and others, 1989; Scott and Gardner, 1992; 

Gardner, 1994). Most vents lie along a north-south alignment 

marked by numerous scoria cones, but most of the magma 

Figure 5. Chemical variation diagrams for more than 730 samples of Quaternary eruptive products of Three Sisters volcanic cluster, 

subdivided as in explanation. All data are tabulated by map unit in table 1. All data are normalized to 99.6 weight percent, volatile free 

(leaving 0.4% for trace oxides and halogens). A, Total alkalies (Na2O+K2O) and CaO versus SiO2. Intersection of arrays gives alkalilime index of 59, calc-alkalic suite as defined by Peacock (1931). Boundary between subalkaline and alkaline suites from LeBas and 

others (1986). B, FeO*/MgO versus SiO2. FeO* is total Fe, calculated as FeO. TH/CA boundary separates tholeiitic and calc-alkaline 

suites, as defined by Miyashiro (1974). Strongly Fe-enriching Thingmuli trend from Carmichael (1964). High-, medium-, and low-Fe suite 

boundaries from Arculus (2003). Data for the andesite of Obsidian Creek (unit aoc) are labelled. C, K2O versus SiO2. High-, medium-, and 

low-K suite boundaries extended from Gill (1981). Figure 5 continued on next page.

Thingmuli

alkaline

subalkaline

Na2O+K2O, in weight percent

SiO2, in weight percent

CaO, in weight percent

FeO*/MgO

50

6

8

4

2

0

6

8

10

12

4

2

0 0

6

10

8

12

2

4

54 58 62 66 70 74 78

TH

CA high-Fe

medium-Fe

low-Fe

aoc

A

B

 EXPLANATION

South Sister and vicinity

Middle Sister and vicinity

Broken Top and vicinity

North Sister edifice 

North Sister vicinity

Peripheral centers and 

other lavas
15

issued effusively to build shields and lava fields. Five main eruptive episodes have been defined (stratigraphically and paleomagnetically) and related to late-glacial features, suggesting that 

most of the material erupted between 15 and 12 ka and that as 

much as 25 km3 of it may have erupted in less than 1,500 years. 

Comparably large effusion rates for basalt and basaltic 

andesite marked the late Holocene just north of the Three 

Sisters, in the McKenzie Pass volcanic field (fig. 2), where the 

6 to 9 km3, multivent Belknap shield was constructed in less 

than 1,500 years (Taylor, 1965, 1968). Only slightly older than 

Belknap, a nearby chain of scoria cones that includes Nash 

Crater and Sand Mountain produced 2 to 3 km3 of Holocene 

mafic lavas and ejecta in about 1,000 years (Taylor, 1968; 

Sherrod and others, 2004). 

From the Three Sisters proper, no postglacial mafic eruptions, nor any of andesite or dacite, have occurred. The only 

postglacial eruptions within the central volcanic focus were the 

late Holocene rhyolite episodes at Rock Mesa and the “Devils 

chain,” both described in detail by Scott (1987). The stark 

contrast between the relative inactivity within the focal region 

(just two rhyolitic leaks in postglacial time) and the repeatedly 

voluminous postglacial mafic activity in adjacent areas both 

north and south plausibly attests to Holocene growth of substantial silicic-magma reservoirs that intercept ascending batches of 

mafic magma beneath South Sister and Middle Sister (Bacon, 

1985; Scott, 1987).

Volcanic Hazards

Hazards associated with potential eruptions and slope failures in the Three Sisters region, as well as annual probabilities 

of several kinds of events, were summarized by Scott and others 

(2001). The abundance of small mafic volcanoes, many of them 

postglacial, just north and south of the Three Sisters cluster is 

strongly suggestive of the style most likely for the next eruption: local scoria accumulation, ash fall as far as ten kilometers 

or more away from the vent, and slow-moving lava flows that 

might close a road or descend along a valley floor.

Because summit vents of the stratovolcanoes have been 

inactive throughout the Holocene, revival of activity high on 

the main cones is unexpected. Such a revival might represent 

either a new episode of intermediate-magma ascent from great 

depth or the reactivation of a shallow rhyolitic reservoir. The 

two important rhyolitic eruptive episodes on the apron of South 

Sister about 2,000 years ago were comparable in extent and 

volume to 10 other rhyolite eruptions that took place on the 

slopes of Middle Sister and South Sister in the late Pleistocene 

(50–25 ka). For these 10, the tephra record has been removed 

or concealed, but Scott’s (1987) investigation of the two late 

Holocene eruptive episodes gave an indication of what might 

be anticipated in the event of another rhyolitic eruption from 

the reservoir that potentially still underlies South Sister. The 

multivent, dike-fed eruptions of Rock Mesa and the “Devils 

chain” both opened explosively, depositing extensive blankets 

of subplinian tephra fallout, as well as thin pyroclastic flows 

and surges that extended about 1 km from their vents. The 

10-cm isopach for pumiceous fallout from the Rock Mesa episode extends 17 km east and 13 km south; that for the “Devils 

chain” extends 11 km east and 6 km south. Ash layers about 1 

cm thick are preserved locally as far as 20 km from the vents, 

and a trace of fine ash is detectable as far east as 30 km, almost 

to Bend (figs. 1, 4). The threat of ash clouds to the engines of 

aircraft is well known, and even millimeters of ash fall can foul 

machinery, vehicles, water supplies, and the respiration of most 

living things.

K2O, in weight percent

SiO2, in weight percent

50

3.0

4.0

2.0

1.0

0 54 58 62 66 70 74 78

high-K

medium-K

low-K

C

 EXPLANATION

South Sister and vicinity

Middle Sister and vicinity

Broken Top and vicinity

North Sister edifice 

North Sister vicinity

Peripheral centers and 

other lavas

Figure 5.—Continued
16

100

50

150

200

250

300

350

2

0

4

6

8

10

0.5

0

1.0

1.5

2.0

2.5

50 52 54 56 58 60 62 64 66 68 70 72 74 76 78

bnf

bnf

bnf

bsf

mlb

aoc

aoc

roc

ruc

rgl

mth mpm

bwm

bwm

bwm

aoc

aoc

r58

SiO2, in weight percent

Zr, in parts per million MgO, in weight percent TiO2, in weight percent

 EXPLANATION

South Sister and vicinity

Middle Sister and vicinity

North Sister edifice 

North Sister and vicinity

Broken Top and vicinity 

Peripheral centers and 

other lavas 

Figure 6. Plots showing Zr, MgO, 

TiO2, Sr, FeO*, and Al2O3 versus SiO2 

contents for eruptive products of 

Three Sisters volcanic cluster. FeO* is 

total Fe, calculated as FeO. Data are 

divided into five volcanic groups, as 

in Correlation of Map Units and List of 

Map Units, as well as one additional 

grouping for North Sister edifice 

alone. All data are tabulated by map 

unit in table 1. Selected extreme or 

deviant units are identified by mapunit label. For example, the andesite 

of Obsidian Creek (unit aoc) stands 

out on several panels as low-Ti, 

high-Mg suite that is chemically 

collinear with North Sister and several 

mafic peripheral centers. All or most 

samples that have more than 19.5% 

Al2O3 are plagioclase rich and are 

probably accumulative.
17

150

50

250

350

450

550

650

750

850

950

12

14

16

18

20

22

24

2

0

4

6

8

10

12

50 52 54 56 58 60 62 64 66 68 70 72 74 76 78

bnf

bnf

bbd

mth mth

dse

r58

dtl

mbc

roc

SiO2, in weight percent

Sr, in parts per million Al2O3, in weight perccent FeO*, in weight percent

bnf

 EXPLANATION

South Sister and vicinity

Middle Sister and vicinity

North Sister edifice 

North Sister and vicinity

Broken Top and vicinity

Peripheral centers and 

other lavas 

Figure 6. —Continued.
18

The only large Quaternary pyroclastic-flow deposits 

nearby are the three middling ignimbrites mentioned above, 

thought to have erupted in the Tumalo volcanic field in the 

middle Pleistocene (at roughly 650, 400, and 200 ka). No 

evidence exists for any such extensively devastating eruption 

during growth of the Three Sisters.

Lahars, which can result from eruptions that melt snow and 

ice, from breaching of moraine-dammed lakes, or from voluminous avalanches from steep edifices into stream valleys, have 

the capacity, if large enough, to be destructive far downstream 

(as discussed extensively for this area by Scott and others, 

2001). Within our map area, stream valleys susceptible to such 

devastation include the many forks of Whychus Creek (which 

drains toward the town of Sisters), as well as White Branch and 

forks of Separation and Linton Creeks (all of which drain to the 

McKenzie River). Lakes susceptible to rapid breakout include 

moraine-dammed Carver Lake (O’Connor and others, 2001) 

and lava-dammed Sparks, Linton, and the Green Lakes, which 

would require influx of a major debris load to overtop their 

outlets.

Acknowledgments

We are grateful to Dave Tucker, George Cagwin, and Dean 

Miller for field assistance; to Ed Taylor for decades of field 

mapping and generous transfer of unpublished information; 

to Willie Scott for cogent field and logistical advice; to Julie 

Donnelly-Nolan for helpful project oversight and for encouraging us to undertake it in the first place; to Mariek Schmidt for 

her study of North Sister; and to James Saburomaru and Dean 

Miller for essential laboratory work. Helpful reviews of the 

map and manuscript were provided by Scott and by L.J. Patrick 

Muffler.

Introduction to Description of Map Units

The geologic map of the Three Sisters volcanic cluster represents part of a late Quaternary volcanic field within 

which scores of eruptions have taken place over the last 50,000 

years, some as recently as about 1,500 years ago. No rocks of 

early Pleistocene (or older) age crop out within the map area, 

although volcanic and derivative sedimentary rocks of Miocene 

and Pliocene age are widespread to the east and west and are 

certainly buried beneath the younger volcanic field. Of the 

145 volcanic map units described below, only 22 are certainly 

older than late Pleistocene (older than 126 ka), and 12 are 

postglacial (younger than about 15 ka). The oldest unit identified yielded an age of 532±7 ka, and the second oldest, 374±6 

ka. Compositionally, 10 percent of the units are true basalt; 36 

percent, basaltic andesite; 20 percent, andesite; 21.5 percent, 

dacite; and only 12.5 percent, rhyodacite or rhyolite.

Volcanic-rock nomenclature, which is kept simple here, is 

based principally on SiO2 contents. Basalt has 47–52% SiO2; 

basaltic andesite, 52–57%; andesite, 57–63%; dacite, 63–68%; 

rhyodacite, 68–72%; and rhyolite, more than 72%. For reasons 

of convenience or uncertainty, we sometimes use the terms 

“mafic” to lump basalt with basaltic andesite, “intermediate” 

for andesite-dacite, and “silicic” to cover the range of rhyodacite and rhyolite. In unit descriptions, we sometimes state the 

number of samples analyzed; for example, “n=10” indicates that 

ten samples were analyzed from that unit.

In unit descriptions, we abbreviate names of some common 

volcanic phenocrysts as follows: clinopyroxene, cpx; orthopyroxene, opx; plagioclase feldspar, plag. Phenocrysts are defined 

here as 0.5 mm and larger; microphenocrysts (mph), 0.1 to 0.4 

mm; and microlites, smaller still. The modifier “phenocrystrich” means that a rock has 12% or more crystals larger than 0.4 

mm; “phenocryst-poor” signifies 5% or less; “aphyric” means 

what it says—none. Rocks containing 6 to 12% phenocrysts we 

typically characterize as moderately porphyritic or of moderate phenocryst content. Because crystal sizes vary widely for 

each species within most thin sections (and more so within 

most map units), we give estimates of combined abundance 

of phenocrysts and microphenocrysts for each species in each 

unit, followed parenthetically by the size range measured in 

thin section for each, from microphenocrysts (>0.1 mm) to the 

largest crystal observed: for example, 7–10% plag (mph to 4 

mm); 2–3% pyroxenes (mph to 1.5 mm); and sparse oxide mph. 

No systematic attempt was made here to estimate separately the 

proportions of opx and cpx nor to estimate quantitatively the 

proportions of oxide microphenocrysts present (titanomagnetite, 

ilmenite, and their oxyexsolved daughters, which are rarely 

larger than 0.3 mm and seldom more abundant than 0.1–0.3%).

Most of the 145 volcanic map units described herein are 

newly defined, although equivalents of several were described 

by Taylor (1978, 1987), Scott (1987), and Scott and Gardner 

(1992). Each is an eruptive unit derived from a single vent 

or fissure. Some are simple flow units, but many are shields, 

cones, or stacks of several lava flows that have chemical and 

mineralogical coherence. Each unit has been delineated by field 

mapping on foot, its integrity having been confirmed, challenged, or revised by chemical and microscopic work in the 

laboratory. Definition of a few units required iterative acquisition of field and lab data over a period of years, providing a 

firm basis for subdividing, lumping, or correlating slightly 

heterogeneous sequences of lavas. Most units have narrow 

compositional ranges, but some show zoning or heterogeneity 

spanning ranges of a few percent SiO2.

SiO2 contents reported in the volcanic unit descriptions 

are based on major-element analyses (table 1) normalized on an 

anhydrous basis, as determined by x-ray fluorescence methods, 

in either the U.S. Geological Survey laboratory at Lakewood, 

Colorado (supervised by J.E. Taggart), or in the Washington 

State University GeoAnalytical Laboratory (supervised by J.A. 

Wolff).

Ages are estimated for all units, on the basis of their 

mutual stratigraphic positions and their relations to glacial 

deposits and erosion, as well as numerous new 40Ar/39Ar age 

determinations (table 2), and a few new K-Ar ages (table 3). 

All units mapped are Quaternary. Boundaries between early, 

middle, and late Pleistocene are widely agreed to be about 780 

ka and about 126 ka (Gradstein and others, 2004). The formally 
19

defined boundary between the Pleistocene and Holocene is 

not very useful here, even if modified to 11.7 ka (Walker and 

others, 2009); accordingly, we refer to some units as postglacial 

or “latest Pleistocene” if they erupted subsequent to the widespread recession of glacial ice that took place in the Oregon 

Cascade Range during the interval from 18 to 15 ka. The radioisotopic age determinations were made in the U.S. Geological 

Survey geochronology laboratory at Menlo Park, California 

(supervised by Calvert), following methods described in Calvert 

and Lanphere (2006) and Hildreth and others (2007).

Locations of most geographic features cited in the text 

are shown in figures 1 and 2. Elevations are given in feet (1 

ft = 0.3048 m; 1 m = 3.2808 ft) because the 1:24,000-scale 

topographic maps upon which the geologic map is based are 

available in feet only. All other measurements are metric. Grid 

references to site locations mentioned in the text are given to 

the nearest 100 m using the Universal Transverse Mercator 

(UTM) grid (1927 North American datum, zone 10), which 

is shown on U.S. Geological Survey topographic maps of the 

area. The first three digits are easting, and the second three are 

northing: for example, the summit of Middle Sister is approximated as 973/889 (97.3 km east, 88.9 km north). Occasionally, 

a fourth digit is added for precision to 10 m. At the end of 

each unit description, the name of the U.S. Geological Survey 

7.5' quadrangle(s) in which the unit crops out is indicated in 

brackets, using the following abbreviations: BT, Broken Top; 

LL, Linton Lake; NS, North Sister; SS, South Sister; and TCB, 

Trout Creek Butte.

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

i Glacial ice (late Holocene)—Present-day (2009) glaciers. Because all are currently shrinking, 

contacts with ice-cored moraines are actively shifting and, thus, are only approximate. Area 

and volume estimates (as of 1981) for all 16 glaciers on Three Sisters, including measured 

ice-radar thicknesses for six of them, were given by Driedger and Kennard (1986) [NS, SS]

p Pumice deposits (late Holocene)—Pumice-fall deposits, local proximal pyroclastic-flow deposits, and reworked secondary deposits resulting from eruptions of units rrm and rdc. Shown 

only where thick enough to obscure underlying rock units sufficiently to prevent inference of 

their extent. See units rrm and rdc for discussion of regional extent of tephra fall, as mapped 

by Scott (1987) [BT, SS]

pdf Pumiceous debris-flow deposits (late Holocene)—Local tongues of rhyolitic pumice and ash, 

a few meters thick, that overlie parts of southwestern lava-flow lobe of Newberry flow (unit 

rdc), as discussed by Scott (1987). Deposits were remobilized (over snow?) from syneruptive pumice-fall deposits that accompanied extended extrusion of compound flow [BT, SS]

al Alluvium (Holocene)—Unconsolidated, water-transported mud, sand, gravel, and coarser debris 

deposited in or adjacent to present-day (2009) streams, lakes, and swamps. Includes a few 

pond and lake deposits and debris-flow levees along streams. Many small deposits have 

been omitted, particularly along narrow streambeds. Includes some debris-flow deposits that 

resulted from lake breakouts related to failure of moraine dams, as mapped by O’Connor 

and others (2001) [BT, LL, NS, SS, TCB]

g Glacial deposits, undivided (Holocene and late Pleistocene)—Till and minor amounts of 

associated glacial-outwash gravels. Till makes up many well-formed moraines, as well 

as extensive or patchy deposits that form irregular topography; consists of poorly sorted, 

boulder-rich, gravelly, silty sand. Moraine crests of four age groups are indicated by different styles of lines: Little Ice Age (late Neoglacial); early Neoglacial (about 5 to 2.5 ka); 

latest Pleistocene readvance (roughly equivalent to Younger Dryas of Europe, about 13 to 

12 ka); and late Pleistocene (roughly 25 to 17 ka, including Last Glacial Maximum). Age 

assignments, in part, follow O’Connor and others (2001) and Marcott and others (2009). 

Little Ice Age moraines high on Three Sisters edifices are steep and sharp crested, and they 

commonly rise more than 100 m above adjacent terrain; they were deposited during last 

few centuries (1500–1900 C.E.) and remain barren. Early Neoglacial moraines were largely 

buried or removed by Little Ice Age advance, but a few surviving remnants are mapped 

below Prouty, Hayden, and Diller Glaciers. Modest readvance of latest Pleistocene glaciers, 

as documented elsewhere in Oregon Cascade Range by Scott (1977), left moraines typically 

1 to 3 km downslope from Neoglacial moraines. Constructional moraines of late Pleistocene 

age are well represented in northeastern part of our map but generally lie outside map area 
20

elsewhere. No still-older glacial deposits have been confirmed here, with exception of single 

exposure at 6,060 ft elevation along North Fork Whychus Creek (UTM grid 018/889), where 

window of till is overlain by unit adl (24±1 ka) [BT, NS, SS, TCB]

s Surficial deposits, undivided (Holocene and late Pleistocene)—Generally a combination 

of glacial, fluvial, pond, and reworked pumice-fall deposits or accumulations of till, talus, 

protalus, or other colluvium. Widely omitted; mapped where contacts of underlying units are 

badly obscured. Large area between Devils Hill and Goose Creek consists of poorly exposed 

Pleistocene moraines mantled by late Holocene rhyolitic-pumice deposits many meters thick 

but variably reworked [BT, LL, NS, SS, TCB]

av Debris-avalanche deposit (Holocene or latest Pleistocene)—Mapped only at east edge of 

map area as swath more than 7 km long and 1 to 2 km wide, extending from south of Park 

Meadow to near Upper Chush Falls (on topographic maps, shown as “Squaw Creek Falls”). 

Abundant hummocks and crags are 10 to 50 m across, 1 to 8 m high, and internally shattered 

or disrupted; they consist of phenocryst-poor and phenocryst-rich andesite (57–58% SiO2) 

lavas, stratified scoria, and tack-welded to dense agglutinate, mostly oxidized brick red. 

Avalanche descended East Fork Park Creek, evidently derived from failure of north slope of 

Broken Top edifice, possibly during deglaciation; it then crossed what is now Park Meadow, 

deposited blocks of agglutinate as big as 4 m as high as 60 m up west wall above modern 

meadow surface, and continued at least 6 km farther downvalley. Exposures of nonlithified diamict deposit are more than 10 m thick along Park Creek gorge just north of Park 

Meadow and more than 15 m along East Fork Park Creek gorge south of meadow; hummocks rise additional 1 to 6 m above general deposit surface. Craggy block of red blobby 

agglutinate beside trail on west side of Park Meadow (UTM grid 0355/8525) is 5 m high, 4 

m thick, and 8 to 10 m long, rising sharply above 20-m-wide apron of its own disintegrating 

rubble; within this agglutinate, a homogenized layer, 5 to 40 cm thick, of platy gray andesite (58% SiO2) lava stands vertical, showing that whole block has rotated about 90°. Such 

fragile blocks and numerous hummocks have not been overrun by ice, and it is unlikely that 

they were transported far on glacier surface after first coming to rest. If avalanche overran 

stagnant and wasting valley glacier at close of Pleistocene glaciation, it might help account 

for excess elevation and better preservation of megablocks and hummocks on valley walls 

but generally smaller blocks and more subdued hummocks protruding from thick avalanche 

deposit along lowland axial meadows. Overlies late Pleistocene glacial deposits and units 

awf, drm, mpm, and ruc [BT, TCB]

ls Landslide deposit (Holocene or latest Pleistocene)—Mapped only as single complex slump 

deposit from steep west side of Katsuk Butte (unit btk), probably deposited soon after deglaciation, as discussed by Scott and Gardner (1992). Includes rotated megablocks and chaotic 

rubble. Overlain by unit mlc [SS]

VOLCANIC ROCKS

[Listed alphabetically by 3-letter map-unit label]

abt Andesite and dacite of Broken Top (middle Pleistocene)—Varied apron of glaciated andesitic 

and dacitic (59.5–68.6% SiO2) lava flows on southwest slope of Broken Top volcano, shown 

only west and southwest of Cayuse Crater. Interstratified with more mafic lavas (see unit 

mbt) that make up much of Broken Top edifice (which we reconnoitered only as limit to our 

mapped area). Includes varied andesite flows containing 5–25% plag (1–3 mm), cpx, opx, 

and olivine, as well as at least three dacite flows that contain 1–10% plag and sparse pyroxenes. Underlain and overlain by mafic lava flows from Broken Top (unit mbt) and overlain 

by lavas and ejecta from Cayuse Crater (unit bcc). Eruptive ages probably mostly 300 to 

150 ka; dacite lava at 5,820 ft elevation on west bank of Fall Creek yielded 40Ar/39Ar age of 

178±1 ka [BT]

acc Andesite of Collier Cone (Holocene)—Andesitic scoria cone (stippled on map), 160 m high and 

700 m in diameter, centered 2.5 km northwest of summit of North Sister, and western lavaflow apron that divides into northwestern tongue that extends 4 km from vent and narrow 

western tongue that extends 13 km. Both tongues have several levees and remain coarsely 

blocky, scoriaceous, and little eroded. Flow sequence was subdivided into five distinguishable subunits (not shown separately here) by Schick (1994); first four range in SiO2 content 
21

from 55 to 61%, whereas fifth is small proximal flow only 500 m long that has 65% SiO2. 

Compositional range was attributed by Schick to crystal fractionation, to mixing with plutonic xenoliths (69–74% SiO2) found partially melted and disaggregating in all subunits, and 

to entrainment of olivine and plag cumulates. Short fountain-fed lava flow (57.5% SiO2) just 

north of cone is compositionally similar to andesitic parts of main apron. Crystal contents 

highly varied: 5–25% plag (<1–8 mm); 1–5% olivine (<1–4 mm); trace mph of cpx and 

Fe-Ti oxides. Younger than all volcanic units nearby. Little Ice Age till banks against south 

slope of scoria cone. 14C age: about 1,500 calibrated yr B.P. (Taylor, 1968, 1981; Sherrod 

and others, 2004) [LL, NS]

acp Andesite of Pole Creek scoria cone (late or middle Pleistocene)—Andesitic (56.4, 57.7% 

SiO2) scoria cone (stippled on map) on south bank of Pole Creek (south of Trout Creek 

Butte) and lava-flow apron 2 km farther east, just outside limit of late Pleistocene moraine 

deposits. Lava apron is scoriaceous, rubbly, and widely oxidized on its surface, massive and 

block jointed on streambank scarps, and virtually aphyric. Scoria cone covers area of 400 

by 600 m and has about 110 m of relief; it mainly consists of loose oxidized scoria lapilli 

and bombs, but ejecta on steep north slope are widely agglutinated. Nearly aphyric scoriae 

contain only trace amounts of plag phenocrysts (rarely as big as 0.5–2 mm) and rare olivine 

and cpx mph. Groundmass has scattered plag-lath mph, seriate to microlites. Cone is not 

glacially eroded but is located right along distal limit of late Pleistocene ice advance, its 

southern and eastern margins abutted by outer edges of moraine deposits. West toe of cone 

is wrapped by younger lava flows of unit bss. Lava-flow apron overlies units mwh (195±5 

ka) and bwl and extends unknown distance beyond east edge of map area. Cone is directly 

overlain by cream to buff pumice-fall deposit as thick as 60 cm, of unknown source; its 

pumice lapilli have 67.3% SiO2, contain trace amounts of plag and cpx (<1% phenocrysts), 

and are predominantly 2–3 cm but as coarse as 8 cm; its largest lithic fragments are 2–3 mm. 

Undated [TCB] 

adh Andesite dome of Lewis Glacier headwall (late Pleistocene)—Andesitic (62.6% SiO2) lava 

dome and major apron of derivative talus that together make up two-thirds of steep wall 

above northeast edge of Lewis Glacier on South Sister. Most of glaciated dome remnant is 

blocky and strongly fractured glassy lava that envelops 25-m-thick, massive, more coherent 

interior. Total thickness is now about 100 m, but unit must have stood at least 40 m higher 

to have shed nearly 50-m-thick talus apron exposed on its east flank. Talus is monolithologic, matrix poor, and clast supported, consisting of subangular to subrounded glassy clasts, 

mostly 5 to 50 cm across and finely vesicular; uncommon blocks are prismatically jointed, 

suggesting that part of talus could be proximal lags of block-and-ash flows from dome. 

Best outcrops (through surficial scree) are block-rich ledges, typically about 3 m thick, that 

dip 30° to 35° southeast. Vague layering in deposit reflects crude size segregation, having 

somewhat larger matrix fractions in horizons that contain fewer large clasts; a few laterally 

persistent, matrix richer intervals and at least five coarse ledges indicate several emplacement pulses. Subordinate sandy to gravelly matrix is made up of material comminuted from 

accompanying monolithologic blocks. Dome and talus are cut by three or four dikes (unit 

ahi), same dikes that also cut subjacent unit alh. Phenocrysts: 10% plag (mph to 2 mm); 1% 

pyroxenes (mph to 0.5 mm), commonly in clots with plag. Northwest side of unit is truncated (together with underlying stack of andesite lava flows of unit alh) by northwest-facing 

wall of a paleocrater. Thick lava of unit dlg (25.3±1.4 ka, 22.3±1.6 ka) and fragmental unit 

dbl later filled paleocrater, banking against and overlying dacite dome of unit. Thick talus 

from dome directly underlies unit aeg (here, 27±2 ka; 28.5±2.1 ka, nearby). Undated [SS]

adl Andesite of Demaris Lake (late Pleistocene)—Andesite (61.1–62.9% SiO2) lava, one or more 

flows that make up much of interfluve between North and South Forks of Whychus Creek. 

Proximal lava bench about 100 m thick, centered on Demaris Lake, lies 5 km from South 

Sister summit and 4 km from Middle Sister summit. Source vent is uncertain, as it is covered by younger lavas, but windows along North Fork Whychus Creek (through unit mcl), 

as far as 1.3 km northwest of Demaris Lake and as high as 6,620 ft elevation, favor source 

at or near Middle Sister. Everywhere glaciated and widely till mantled, unit extends 5 km 

northeastward from Demaris Lake, cropping out extensively north of South Fork Whychus 

Creek canyon. Locally vitrophyric, including many ice-contact exposures that have chunky 

or slender subhorizontal columns; however, glacial and fluvial erosion has widely exposed 
22

block-jointed to slabby or platy, variably devitrified interior facies. Phenocrysts: 15–20% 

plag (mph to as big as 4 mm); 3% pyroxenes (mph to 1 mm, rarely to 2 mm); sparse oxide 

mph; and common oxide-pyroxene-plag clots. Overlies units bwy, mdl (181±21 ka), mws

(175±3 ka), msf (166±16 ka), mns (here, 119±6 ka), mnf (48±10 ka), and drm (29±1 ka), as 

well as undated till at 6,060 ft elevation along North Fork Whychus Creek. Overlain by unit 

mcl (16±10 ka, 23±16 ka) and by till of last Pleistocene glaciation. 40Ar/39Ar age: 24±1 ka 

[TCB]

aef Andesite tephra fall at base of east face of Middle Sister (late Pleistocene)—Stratified proximal ejecta at base of headwall of Hayden and Diller Glaciers, representing stratigraphically 

lowest internal exposures of Middle Sister edifice. Two sequences of similar bedded ejecta 

(not mapped separately) are separated by smoothly bevelled unconformity; base of lower 

15-m-thick sequence is concealed by ice; upper sequence, about 30 m thick, is overlain by 

stack of ochre-weathering lava flows (unit mhd) that forms middle part of east face of Middle 

Sister. Both sequences dip 15° to 20° west beneath Middle Sister. Layers are mostly 5 to 20 

cm thick, defined by fluctuations in sizes of coarse clasts in poorly sorted matrix that consists 

of very coarse ash and small lapilli. Coarse clasts are dominantly 2 to 5 cm (as large as 15 

cm), range in texture from massive to finely vesicular, and appear to be compositionally uniform andesite (62.6–63.1% SiO2). Phenocrysts: 5% plag (mph to 2 mm); about 1% pyroxenes 

(mph to 1.5 mm); sparse oxide mph; also contains common crystal clots of plag, pyroxenes, 

and oxides. Base of unit not exposed. Overlain by lavas of unit mhd (37±9 ka), which is, in 

turn, directly overlain by unit mms, which makes up upper half of east face. Stratified ejecta 

are cut by several dikes, which may have fed both overlying units. Undated [NS]

aeg Andesite of Eugene Glacier (late Pleistocene)—Cone-forming, red and gray stacks of numerous thin andesitic (56.0–60.4% SiO2; n=32) lava flows, agglutinate sheets, and intercalated 

scoria falls that make up much of upper cone of South Sister between elevations of 8,200 

and 10,000 ft (but not its summit). Glacially eroded apron extends radially downslope to 

8,200 to 8,400 ft elevation on Hodge Crest (to east) and around Clark Glacier (to south and 

southwest), to 7,700 ft elevation near Eugene and Lost Creek Glaciers (to northwest), and 

to 7,600 ft elevation north and east of Carver Lake (to northeast); remnants are as low as 

6,700 ft elevation in upper Separation Creek (to northwest). Irregular upper limit of radially 

dipping stacks defines ragged rim of late Pleistocene crater that truncates unit; Hodge Crest, 

now isolated by headward erosion of cirques of Prouty and Lewis Glaciers, forms integral 

part of unit’s slope. Paleocrater, which originally was about 700 m in diameter, has been 

wholly filled by units dlg and des. Where exposed on glacier headwalls, individual stacks 

consist of 4 to 15 massive gray layers, each layer typically being 1 to 5 m thick (rarely, 10 

m) and representing a discrete fountain-fed eruptive pulse. Many such (rheomorphic) massive flow units grade up, down, and proximally into agglutinate and, further, into red fragmental interlayers (also 1–5 m thick) of mixed origin: partly scoriaceous fallout (tack welded 

to nonwelded; rarely unoxidized) and partly scoriaceous flow-top rubble. Dense zones are 

texturally varied, from partly glassy to devitrified and either massively homogenized or 

retaining streaky to blobby ghost-agglutinate texture. Where steeply dipping (20° to 35° 

radially) on upper slopes, most flows are thin (and, locally, even convolutely foliated). A few 

northern flows that reached break in slope lower on edifice ponded to 15 to 25 m, and some 

of these exhibit glassy to aphanitic, chunk-jointed features suggestive of ice contact; where 

incised, such thickened flows are commonly block jointed or slabby internally. On headwall 

of Lost Creek Glacier, stratified pile of brick-red scoria more than 100 m thick (stippled on 

map) is thought to be early phase of unit. Considerable ranges in phenocryst contents among 

numerous sheets that make up unit: 10–20% plag (mph to 3 mm); 1–5% cpx (mph to 1 mm, 

rarely to 2 mm); varied amounts of subordinate opx and olivine (each largely mph but as big 

as 1.5 mm); plag+pyroxene±olivine±oxide clots range from sparse to abundant, as do oxide 

mph; a few, to as many as half of, plag phenocrysts are sieved internally but overgrown by 

clear rims; a modest fraction of plag phenocrysts are broken. Overlies units alg, dcg, dgl, 

drm, dwl, rct, and rsw. Overlain by units awf, dcl, dlg, des, mtp, and rsc. 40Ar/39Ar ages: 

27.0±3.2 ka (north planèze), 26.7±1.7 ka (Hodge Crest; south wall, Prouty Glacier), and 

28.5±2.1 ka (north wall, Lewis Glacier) [BT, SS]

ahc Andesite tephra fall of Hodge Crest (late Pleistocene)—Variably stratified, coarse andesitic 

(61.6–61.9% SiO2) proximal fallout that extends across 700 m of Prouty Glacier headwall 
23

(South Sister). Unit is host to andesite intrusive complex of Hodge Crest (unit ahi) near 

southeast corner of headwall, where unit is thickest; although base there is covered by ice, 

fallout is more than 200 m thick. Lowest 60 m is unstratified to vaguely so, having layering that is subhorizontal to slightly outboard dipping. Upper part, which is coarser and well 

stratified, has layers and lenses (variably, centimeters to meters thick) that dip 10° to 20° 

inboard and that overlie unconformity that smoothly bevels outward-dipping strata below. 

Fallout thins northward to only 60 m at distance of 100 m from intrusive complex of unit 

ahi and remains well stratified throughout, having layers that dip 20° to 25° outboard in all 

but top 10 m or so, which dip inboard 10° to 20°. Unconformity defined by outboard- to 

inboard-dipping strata may mark transient rim of paleocrater subsequently filled by overlying lava of unit dlg. Fallout was baked beneath unit dlg and is oxidized to several meters 

below contact. In addition, intruding dikes of unit ahi promoted hydrothermal alteration of 

host fallout. Coarsest part of fallout is inboard dipping and contains subrounded to angular clasts as big as 30 cm, all dense or slightly vesicular and lithologically similar; some 

blocks are prismatically jointed or breadcrusted. Matrix is coarse ash, seriate to smaller 

clasts. Phenocrysts: 15–20% plag (0.5 to 3 mm); ~1% pyroxenes (mph to 0.5 mm), mostly 

in plag+pyroxene clots. Overlies unit dpg, which in turn rests on units rpg (33±1.5 ka) and 

aph′. Overlain by units aeg (27±3.2 ka, 28.5±2.1 ka, 26.7±1.7 ka) and dlg (25.3±1.4 ka, 

22.3±1.6 ka). Intruded by unit ahi. Undated [SS]

ahi Andesite intrusive complex of Hodge Crest (late Pleistocene)—Family of dikes and irregular intrusions as thick as 20 m on both sides of saddle between Hodge Crest and South 

Sister summit; well exposed on headwall of Prouty Glacier and on slope northeast of Lewis 

Glacier. Dikes on Prouty Glacier headwall are chemically and lithologically varied. One dike 

that reaches rim has 60.5% SiO2 and carries 10–20% plag (mph to 2 mm) and ~1% pyroxenes (mostly mph but also sparse crystals as long as 2 mm), as well as common pyroxeneplag clots and oxide mph; dike resembles lava of unit awf. Another dike at base of Prouty 

Glacier headwall has 62.0% SiO2 and is lithologically similar to lava of unit aph. These 

dikes intrude gray fragmental strata of unit ahc on Prouty Glacier headwall. On slope northeast of Lewis Glacier, three or four dikes cut stack of four lava flows of unit alh and also 

thick overlying unit adh; one of these dikes has 56.6% SiO2, resembles unit aeg, and carries 

10–15% plag phenocrysts and ~1% olivine and pyroxene mph. Host rocks and some dikes 

are truncated by west-facing wall of large paleocrater, which also truncates overlying coneforming stacks of unit aeg (27±3 ka) and was subsequently filled by units dbl, dlg (here, 

25±1 ka), and des. Dikes are thought to have fed part of cone-forming unit aeg, and others 

may have fed lavas of units aph and awf (24±1 ka). Intrusive complex may thus represent 

events spread over interval a few thousand years long. Additional dikes (not sampled) that 

cut unit aeg on Lost Creek Glacier headwall (west of South Sister summit) also are truncated by paleocrater wall and probably represent same time interval. 40Ar/39Ar age: 23.3±6.3 

ka, for thick dike that cuts ridgeline saddle at Hodge Crest [SS]

ahl Andesite of headwaters of Linton Creek (late Pleistocene)—Thick andesitic (57.1–57.9% 

SiO2) lava flow, distinguished by consistently small phenocrysts; crops out for about 1 km 

between headwaters forks of Linton Creek at elevations of 6,550 to 6,900 ft at west toe of 

Middle Sister. Glacially eroded unit is as thick as 70 m; a few oxidized flow-breccia lenses 

appear to reflect internal shear rather than separate vent-derived flows. Most outcrops are 

flow foliated and at least partly glassy, although a few devitrified exposures are massive and 

block jointed or slabby. Widespread remnants of glassy chunk-jointed exterior of flow suggest ice contact. Separate exposure in small window about 1 km north (UTM grid 951/891) 

is largely devitrified, massive, and block jointed, and it is marked by pale- to dark-gray 

mottling also characteristic of main outcrops. Phenocrysts: 2–3% plag; 1–2% olivine; both 

plag and olivine largely mph, but a few of each as large as 0.5–1.3 mm; also contains sparse 

oxide mph and sparse plag-olivine-oxide clots. Overlies units alc (27±1 ka), dlc, and mhl. 

Overlain by unit mms and probably by unit dlp (21.4±2 ka). 40Ar/39Ar total-gas age: 21±3 ka 

(well bracketed by ages of enclosing units) [NS]

alc Andesite of Linton Creek (late Pleistocene)—West apron of distal andesite (61.6–63.0% SiO2) 

lava flows that extends from toe of Middle Sister edifice (where concealed more proximally 

by major unit mms) for 7 km farther west-northwest, as far as Linton Lake. Medial thickness 

is at least 120 m; distal relief in cliffs above Linton Lake is about 450 m, but much of this 
24

owes to flow having draped stairstep topography of previously glaciated valley. Both brickred flow-breccia zone along steep margin of unit adjacent to Lane Plateau and scoriaceous 

flow-breccia zone along Pacific Crest Trail at 6,400 ft elevation (UTM grid 943/880) suggest two flow units, but such breaks are not exposed elsewhere. Most exposures are glassy 

or partly so, and many have glassy chunk jointing or inclined and subhorizontal slender 

columns suggestive of ice contact. Devitrified internal exposures are common where deeply 

incised, and they are block jointed or slabby to platy (locally convolute). Sparse pale-gray, 

phenocryst-poor, relatively mafic enclaves are rarely larger than 1 cm. Highest exposure 

(at 6,880 ft) lies just 2 km west of Middle Sister summit, its presumed source. Phenocrysts: 

10–20% plag (mph to 5 mm, mostly ~1 mm); 2–3% pyroxenes (mph to 1.5 mm), mostly in 

abundant clots with plag and oxides; sparse oxide mph. Overlies units mth, dlc, and mlb

(~48 ka); banks against unit aoc (49±4 ka). Overlain by units dlp (21.4±1.9 ka), mms, mmt, 

and ahl (21±3 ka). 40Ar/39Ar ages: 26.8±1.2 ka (distal), 27.2±1 ka (proximal); same age as 

compositionally similar lavas of unit alg that erupted at South Sister [LL, NS]

alg Andesite west of Lost Creek Glacier (late Pleistocene)—West-northwest apron of fairly 

uniform phenocryst-rich andesite (61.5–63.5% SiO2) lava flows from South Sister, cropping 

out from Neoglacial moraines of Lost Creek Glacier down to Separation and Hinton Creeks 

(from 7,800 to 6,400 ft elevation), 1 to 3 km from summit of South Sister. Stacks of flows 

are glacially eroded into stairstep benches. Flows are mostly 2 to 10 m thick upslope and 

commonly separated by oxidized flow breccia, whereas toward lowland extremity of unit, 

several flows thicken to as much as 15 to 20 m. Sets of several shingled flows are glacially 

scoured into radial ridges that diverge downslope. Massive vitrophyre zones are preserved 

locally, but glacial erosion has been severe enough in this sector that most exposures are 

devitrified and platy, slabby, or block jointed. Flow foliation, locally convolute or ramped, 

is common, especially on higher, steeper slopes. Phenocrysts: 10–15% plag (mph to 2 mm), 

some of larger ones sieved; 2–3% pyroxenes (mph to 1.5 mm); sparse olivine and oxide 

mph; most pyroxenes are in clots with plag and oxides. Base not exposed. Overlain by units 

aeg and dwl directly and by units mms and rsc peripherally; lavas of unit msc (21.4±5.5 

ka) appear to bank against toe of unit’s apron. 40Ar/39Ar age: 27.1±1.1 ka (essentially same 

age as compositionally similar lavas of unit alc that erupted from Middle Sister) [SS]

alh Andesite of Lewis Glacier headwall (late Pleistocene)—Stack of four andesitic (62.0–62.9% 

SiO2) lava flows separated by oxidized flow breccias in steep wall above northeast edge of 

Lewis Glacier on South Sister. Massive zones stand out as 2- to 12-m-high ledges on steep 

slope. Massive interior of fourth flow thins eastward from 6 to 2 m, draping three underlying 

flows. Lowest flow is largely devitrified, platy, and oxidized pink, its base covered at glacier 

margin; second and third flows are dark gray or oxidized red-brown, mostly massive and 

partly glassy; and fourth flow is black, glassy, and vesicular. All four are cut by three or four 

dikes that are assigned to unit ahi. Phenocrysts: 10–15% plag (mph to 2 mm), somewhat less 

(~10%) in lowest flow; 1–2% pyroxenes (mph to 1 mm), commonly in clusters of mph or 

in clots with plag; more pyroxenes (2–3%) in lowest flow; also contains abundant oxidepyroxene-plag clots. Overlain conformably by unit adh. At its west end, stack is truncated 

by wall of paleocrater that was filled by units dbl and dlg (25.3±1.4 ka, 22.3±1.6 ka), both 

of which bank against this unit. Glaciated stack of flows of unit alg (27±1 ka) over on northwest flank of South Sister is chemically and petrographically similar to lowest flow in stack 

here, which yielded a 40Ar/39Ar age of 30.4±1.3 ka [SS] 

als Andesite south of Lewis Glacier (late Pleistocene)—Plagioclase-rich andesitic (62.4% SiO2) 

lava flow at 8,600 ft elevation on South Sister that forms glaciated cliff and crags (UTM 

grid 990/823) beneath southeast edge of Little Ice Age moraine of Lewis Glacier; only 400 

m from current ice front. Steep, bluff-forming lava is as thick as 60 m and exposes both 

dark-gray, partly glassy zones and pale-gray, devitrified platy zones. Lava is compositionally distinct from other nearby plagioclase-rich units (aeg, dmn, and dgl) but petrographically and chemically similar (not identical) to unit alg on west flank of South Sister edifice. 

Phenocrysts: 10–15% plag (mph to 3.5 mm); 1–2% pyroxenes (mph to 1 mm), mostly in 

clots with plag and oxides; free oxide mph are sparse. Overlies units dmn (here, 27.7±1.0 

ka) and rct (24±5 ka, 30±8 ka). Undated [SS]

anh Andesite agglutinate north of Hayden Glacier (late Pleistocene)—Fissure-fed andesitic 

(57.9–58.0% SiO2) scoria, spatter, and fountain-fed lava flow that drape lower south face 
25

of North Sister. Upper end of 500-m-long fissure lies at about 9,100 ft elevation; lower end 

at about 8,200 ft; and rheomorphic lava drapes additional 500 m south, then southeast, to 

glacially eroded terminus at 7,800 ft elevation. Eroded remnants along fissure are only 50 

to 100 m wide, but lava tongue broadens downslope over terrain less steep to about 200 

m. Agglutinate and scoria along fissure are brick red and tack welded to fairly dense but 

blobby; lava flow also is largely oxidized and has blobby exterior but becomes denser and 

more homogenized (though still streaky) distally. Terminal lobe is 25 m thick and contains 

patchy exposures of gray, massive interior; it probably ponded against ice and was subsequently overridden and surficially stripped by it. Phenocrysts: ~1% plag (mostly mph, but 

rare crystals 0.5–1.5 mm in plag-pyroxene-oxide clots); rare cpx (≤0.5 mm), largely in clots; 

sparse oxide mph. Unit erupted through and drapes far older stack of previously eroded thin 

lavas of North Sister (unit mns). Overlain by Middle Sister lavas of unit mms. 40Ar/39Ar 

age: 21.0±6.3 ka [NS]

aoc Andesite of Obsidian Creek (late Pleistocene)—Extensive apron of phenocryst-poor andesite 

(59.9–63.2% SiO2) lava flows that extends just beyond Linton Lake, 7 km from northwest 

toe of Middle Sister, its presumed source vent. Its most proximal exposures, where buried 

by younger lavas of Middle Sister, lie 3 km from summit of that edifice. As thick as 120 m 

at medial exposures, glacially scoured unit ponded (possibly against ice) near its terminus, 

where it now has 285 m of relief (although true thickness is less, owing to steepness of 

paleovalley floor it buried). Massive to columnar, black glassy base is exposed near 4,900 ft 

elevation along Obsidian Creek, but most exposures have been glacially sculpted into ridges 

and benches of devitrified, nonvesicular, fine-grained andesite that ranges from block jointed 

to thinly platy. Most commonly pale gray and mottled on joint planes but locally dark gray 

where still partly glassy. In otherwise massive devitrified rock, pale-gray films commonly 

accentuate pervasive hairline cracks spaced 2 to 5 mm apart that represent flow foliation. At 

snout of flow northwest of Linton Lake, thick, glassy, chunk-jointed carapace is preserved. 

Phenocrysts: virtually aphyric, but locally contains rare plag (0.5–1.5 mm), commonly 

rounded; rarer olivine (≤0.5 mm), typically partly resorbed and has reaction rims. Overlies 

unit mlb (~48 ka). Overlain by units acc, alc, dlp, mmm, mms, mrg, and roc. 40Ar/39Ar 

ages: 49±4 ka (proximally), 44.6±1.8 ka (at distal snout). Eroded nose assigned to this unit, 

located about 500 m northwest of Montague Memorial Plaque (UTM grid 940/912), is lithologically identical but has only 58.6% SiO2; apparently a discrete but related flow, it yielded 40Ar/39Ar age of 47.9±2.2 ka [LL, NS]

apc Andesite of Park Creek (late Pleistocene)—Phenocryst-rich andesite (61.0–61.6% SiO2) lava 

flow exposed discontinuously through till along West Fork Park Creek, 0.7 km to 2 km 

downstream from Red Meadow. Glassy zones preserved only locally in severely glaciated lava; mostly devitrified, nonvesicular, and block jointed to platy. Strikingly platy 

along gorge near waterfall at 5,820 ft elevation. Phenocrysts: 10–15% plag (mph to 4 mm); 

2–3% pyroxenes (mph to 1.5 mm), mainly in plag-pyroxene-oxide clots; sparse oxide mph. 

Overlies unit ruc (169±2 ka). Overlain by unit drm (29±1 ka). 40Ar/39Ar age: 31±9 ka [TCB]

aph Andesite of Prouty Glacier headwall (late Pleistocene)—Phenocryst-poor andesitic 

(61.4–61.6% SiO2) lava flow that crops out as ledge at ice level for about 250 m along 

northwest base of Prouty Glacier headwall on South Sister. Most of 7-m-thick exposure is 

gray and partly glassy, weathered pink to buff on blocky joint surfaces; base is covered by 

glacier. At both ends of exposure, flow margins are enveloped in crudely stratified, oxidized 

tephra-fall deposit that has agglutinated lenses. Midway along ledge, exposure is conspicuous 12-m-wide hemispherical devitrified zone that has distinctively concentric platy 

jointing. Phenocrysts: ~1% plag (mph to 2 mm); sparse pyroxenes (mph to 0.5 mm); sparser 

Fe-Ti oxides; most pyroxenes in oxide-pyroxene-plag clots. Directly overlain by stratified 

sequence of agglutinate and breccia (unit aph′), which is, in turn, capped by unit rpg (33±2 

ka). Undated. One isolated outcrop in ice, just below base of headwall and 300 m along 

strike southeast of section just described, is 4-m-thick andesitic (61.6% SiO2) lava ledge, 

apparently also of this unit; lava is enveloped in ochre breccia that contains vesicular, black 

clasts of glassy lava (59.3% SiO2) that probably is coarse proximal fallout more than 5 m 

thick, its base covered by ice; lithologies are similar to unit aph′ [SS]

aph′ Andesite tephra fall near base of Prouty Glacier headwall (late Pleistocene)—Stratified 

tephra-fall deposit about 5 m thick that contains thin lenses and layers of massive 
26

andesitic (59.3–63.7% SiO2) agglutinate, atop ledge of unit aph along northwest base of 

Prouty Glacier headwall on South Sister. Massive intervals are black, glassy, and finely 

vesicular, and they grade into rubbly zones above and below. Coarse nonwelded intervals 

contain moderately vesicular, glassy juvenile clasts as big as 20 cm (similar in lithology to 

massive layers), as well as abundant dense, angular, nonjuvenile lithic fragments; subordinate ochre coarse-ash matrix mostly consists of comminuted juvenile material. Individual 

nonwelded layers range laterally in thickness from 20 cm to 2 m and in color from ochre 

to brick red. Phenocrysts: ~2% plag (mph to 1.5 mm, rare to 3 mm); sparse pyroxene 

(mph to 0.5 mm). Rests on unit aph and is overlain by units rpg (32.8±1.5 ka) and dpg. 

Undated [SS]

asn Andesite south of Newberry flow (late Pleistocene)—Glaciated plagioclase-rich andesite 

(61.7% SiO2) lava flow exposed only locally beneath snout of late Holocene Newberry flow 

(unit rdc), 4 km southeast of South Sister summit. Deeply covered by proximal rdc ejecta, 

unit is exposed principally as glacially eroded bench of block-jointed massive lava at 6,400 

ft elevation. Apparently also overlain by unit rse (34±1 ka), extensive ridge of rhyolitic lava 

that stands between unit and its presumed source, South Sister edifice. Phenocrysts: 7–10% 

plag (mph to 2 mm); 3% pyroxenes (mph to 0.7 mm); sparse Fe-Ti oxides; contains abundant oxide-pyroxene-plag clots. Base not exposed. Undated [BT]

asw Andesite of southwest slope of Middle Sister (late Pleistocene)—Small window (100 by 300 

m) of phenocryst-poor andesite (61.5% SiO2) lava flow near southwest toe of Middle Sister 

cone. First draped by extensive lava apron of unit mms and then jointly glaciated, exposure is, in part, devitrified, pale gray, and platy or block jointed and, in part, black, blocky, 

and glassy to aphanitic. Locally, unit exhibits ramped flow foliation, oxidized joint films, 

and scoriaceous crusts along shear planes. Phenocrysts: <1% plag (mph to 1.2 mm); sparse 

pyroxenes (mostly mph, but very rare crystals 0.5–1 mm); sparse oxide mph. Base not 

exposed. Overlain and largely concealed by unit mms. 40Ar/39Ar age: 25±4 ka [NS]

asy Hornblende-bearing andesite southeast of Yapoah Crater (late Pleistocene)—Block-jointed 

bench of glaciated phenocryst-rich andesitic (58.7% SiO2) lava that protrudes eastward 

from east-facing cliff of unit mwa, about 1.5 km southeast of Yapoah Crater. Bench is only 

about 100 m long, 60 m wide, and 30 m high, but its scarp sheds coarse talus that covers its 

base downslope. Lithologically unique in local area, it is either plug or remnant of unit that 

is elsewhere wholly concealed. Phenocrysts: 7–10% plag (0.5–4 mm); 3–5% hornblende 

prisms (0.5–15 mm long), mostly opacitized; rare cpx mph (0.3–0.4 mm); common 

hornblende-plag clots and clusters of plag mph. Groundmass rich in plag mph, seriate to 

microlites. Apparently intrudes unit mwa (59±4 ka), but contact relations are obscured by 

surficial deposits. Undated [NS]

awa Andesite west of upper Alder Creek (late Pleistocene)—Phenocryst-poor andesitic (55–58% 

SiO2) lava flows and subordinate agglutinate that form three separate cliffy exposures about 

1 km west of upper Alder Creek. East-facing wall and buttress (UTM grid 984/936) that protrudes from it (1 km east of Collier Cone) are 40 to 90 m high and consist of three fountainfed lava flows that are separated by thin sheets of red agglutinate at western exposures but 

merge eastward into single cliff that has vertical jointing and no surviving partings. Another 

prominent buttress, isolated by scree just east of that wall (UTM grid 987/933) is single 

lava flow more than 30 m thick that grades down into stratified, variably agglutinated, red 

scoria-fall deposit more than 25 m thick; lava is pale gray, block jointed to slabby, and flow 

foliated, and it dips about 20° northeast; fall deposit mostly consists of lapilli but also has 

bombs and dense blocks as big as 30 cm and numerous lenses of dense agglutinate. Third 

exposure, about 300 m northeast of first, is slender ridge (UTM grid 988/942), 600 m long, 

100 m wide, and 50 m high, that trends N. 10° E. and consists of single ice-sculpted lava 

flow; lava is massive and block jointed, and its basal oxidized breccia rests on lava flow of 

unit mns (xenoliths of which, 1–5 cm across, are common in overlying unit). Phenocrysts: 

<1% plag (0.5–2.5 mm); rare cpx (0.5–1 mm); rare olivine mph. Overlies unit mns. 

Overlain by pyroclastic unit mps, which buried vent for this unit; these two phenocryst-poor 

units may have erupted successively from common fissure-vent system. Distinguished from 

unit mwa, which crops out along same wall just north, by its much lower phenocryst content, lower Al2O3 content (<17%), and higher FeO content (>8%). 40Ar/39Ar age: 25.2±2.2 ka, 

for lava of northeast ridge [NS]
27

awc Andesite west of Collier Glacier (late Pleistocene)—Glaciated fan (2 km long, 1 km wide) of 

fairly uniform andesite (61.6–62.3% SiO2; n=13) lava flows that extends north-northwest 

from Middle Sister along west side of Collier Glacier. Among numerous shingled flows, 

sequences of five are exposed locally along glacier-marginal cliffs. Till-strewn surface of 

unit is glacially eroded into radial ridges and stairstep benches cut on stack of flows 5 to 

25 m thick, all dipping moderately northwest. Uppermost flow of unit adjacent to Collier 

Glacier is more than 70 m thick, probably ponded originally against ice. Glassy chunkjointed zones, mostly at bases of flows, crop out widely, but most rock exposed is devitrified 

and block jointed or platy. Phenocrysts: 5–10% plag (mph to 2.5 mm); 1% pyroxenes (mph 

to 1.5 mm), many in common plag-pyroxene-oxide clots; sparse oxide mph. Overlies units 

dss (25±3 ka) and mlb (~48 ka), which here, in turn, overlies unit mns. Overlain by units 

mrg (20±6 ka), mms, and dbh (18±2 ka). 40Ar/39Ar ages: 21.9±1.7 ka, 24.5±1.5 ka [NS]

awf Andesite of West Fork Park Creek (late Pleistocene)—Glaciated set of andesite (59.2–60.7% 

SiO2) lava flows from South Sister, east of Prouty Glacier, that descends 3 km eastward from 

Little Ice Age moraines to lowland apron north of Golden Lake. One thick flow banked 

against northwest ridge of Broken Top and was diverted southward to Green Lakes. Most 

exposures on apron are devitrified and block jointed to slabby or platy, but proximal outcrops through ice and till are at least partly glassy, as are sparse flow-base exposures, even 

on lower apron. Green Lakes lobe is largely glassy, perhaps reflecting contact with valleyfilling ice. Phenocrysts: 15–20% plag (mph to 2.5 mm); 3% pyroxenes (mph to 1.5 mm); 

minor olivine (mph to 1.5 mm); sparse oxide mph; also contains common plag-pyroxeneoxide clots. Overlies units mpm and aeg (27±2 ka); banks against units mbt, rpc, and drm

(29±1 ka). Compositionally similar to dike of unit ahi on rim above Prouty Glacier. 40Ar/39Ar 

age: 23.7±1.3 ka [BT, SS]

awm Andesite west of Middle Sister (late Pleistocene)—Fan of ruggedly glaciated, moderately 

porphyritic andesite (59.1–60.6% SiO2) lava flows on west slope of Middle Sister (7,150 

to 8,100 ft elevation), cropping out in seven or more separate windows through till, scree, 

and unit mms. Lithologically similar to nearby unit awr (just north), but slightly richer in 

Ti, Fe, Mn, Mg, and Ca and slightly poorer in K and P. Several outcrops each expose from 

one to six flows, most of which retain some vitrophyre facies, but thick flows are dominated 

by variably devitrified internal facies, ranging from block jointed to platy. Scoriaceous and 

oxidized flow-breccia zones crop out widely, separating massive zones of stacked flow units. 

Flow thicknesses vary widely, between 2 and 50 m. Phenocrysts: 7–10% plag (mph to 2.5 

mm); 2% pyroxenes (mph to 2.2 mm), mostly in common plag-pyroxene-oxide clots; sparse 

oxide mph. Base not exposed. Overlain by units mms and mms′; cut by dikes of unit mms

and intruded and overlain by unit dlp (21±2 ka). Undated [NS]

awr Andesite west of Renfrew Glacier (late Pleistocene)—Fan of several (compositionally similar) 

andesite (60.8–61.6% SiO2) lava flows that forms northwest medial slope of Middle Sister 

at elevations between 6,900 and 7,900 ft. Stacks of as many as six moderately porphyritic 

flows are glacially eroded into several west-trending buttresses, overlapped and surrounded 

by till and derivative coarse alluvium. Lithologically similar to unit awm (just south) but 

slightly richer in K and P and slightly poorer in Ti, Fe, Mn, Mg, and Ca; likewise similar to 

unit awc (just north) but richer in Ti, Fe, and Mg and slightly poorer in Si, Na, and K. Some 

flows are only 2 to 5 m thick and largely vitrophyric, having oxidized flow-breccia zones 

separating them; most are 15 to 40 m thick, extensively devitrified, and block jointed to 

platy. Where flows are steep, flow foliation commonly is convolute. Phenocrysts: 5–7% plag 

(mph to 2.2 mm); 1–2% pyroxenes (mph to 1.2 mm); sparse oxide mph; also contains abundant plag-pyroxene-oxide clots. Base not exposed. Overlain by unit mrg (20±6 ka), (inferentially) by mms and dbh (18±2 ka), and extensively by glacial deposits (unit g). Undated 

[NS]

awy Andesite of Whychus Creek (middle Pleistocene)—Phenocryst-rich andesite (61.3–62.0% 

SiO2) lava flow exposed along gorge floor of Whychus Creek from near confluence of 

its north and south forks downstream for 2 km; remnant also banked against west wall of 

valley about 1 km still farther downstream. As thick as 25 m along walls of gorge, where 

base is not exposed and outcrops are devitrified and slabby to platy. Exposure through 

thick till on hill east of gorge rim and 500 m northeast of confluence is block-jointed black 

vitrophyre. Where banked against west sidewall downstream, columnar basal vitrophyre 
28

forms 6-m-thick ledge overlain by about 20 m of platy devitrified andesite (mantled by till). 

Phenocrysts: 10–15% plag (mph to 4.5 mm); 2–3% pyroxenes (mph to 1.5 mm), mostly in 

common plag-pyroxene-oxide clots; sparse oxide mph. Base not exposed along river, but 

flow banks against unit mbw on west sidewall. Overlain by units bwy and mwh (195±5 ka). 

Source vent unknown; most likely Broken Top. 40Ar/39Ar age: 298.7±1.4 ka [TCB] 

bac Basalt of Alder Creek (late or middle Pleistocene)—Nearly aphyric basaltic (51.2–53.1% 

SiO2) lava that forms prominent ridge at 5,800 to 5,950 ft elevation on north wall of Alder 

Creek, 6 km northeast of North Sister summit. Steep narrow ridge, about 400 m long, is 

nose of former glacially sculpted cleaver that separated ice streams. Rock is more pervasively altered than any recognized in map area other than in near-vent regions affected by 

hydrothermal or fumarolic processes. Where least altered (or merely silicified), rock is 

dark gray to greenish gray, massive, aphanitic, and almost aphyric. Somewhat more altered 

domains have medium- to coarse-grained groundmass and are diktytaxitic or exhibit angular 

vesicles. Most widespread lithology has coarse groundmass and extensive porosity, is 

locally brick red interspersed with green and yellow patches, weathers tan to yellow brown, 

and disintegrates to grus. Phenocrysts: 2–3% olivine (mostly mph, but rarely 0.5–1.2 mm). 

Groundmass, which contains abundant tiny olivine, varies from finely felty, rich in plag 

microlites, to dense tangle of plag-lath mph that are seriate from 0.1 to 0.5 mm and include 

rare laths as long as 1.5 mm. Base not exposed. Overlain by units mey and mns. Unique 

exposure is stratigraphically oldest unit in its area. 40Ar/39Ar age: 128±7 ka [TCB]

bbd Basalt of Brush Draw (late or middle Pleistocene)—Moderately porphyritic basaltic 

(51.1–51.4% SiO2) lava flow that crops out south and east of Brush Draw at southeast toe of 

Trout Creek Butte. Exposed only beyond glacial limit, its surface is scoriaceous to rubbly. 

Source unknown, as proximal extent is wholly covered by younger lavas. Phenocrysts: 10% 

plag (mph to 3.5 mm); ~1% olivine (0.2–1.5 mm); rare cpx (0.5–1 mm); some plag crystals are 

sieved, and some are composite. Base not exposed here, but flow extends well northeast of map 

area. Banks around base of unit mtb shield (532±7 ka). Overlain by unit bss. Undated [TCB]

bbr Basalt of Sparks Lake boat ramp (early Holocene or latest Pleistocene)—Sparsely to 

moderately porphyritic basaltic (50.8–51.5% SiO2) lava flows at shoreline of large eastern 

embayment of Sparks Lake and extending 1 to 2 km east and south thereof. Source vents 

are north-south chain of three basaltic scoria cones (stippled on map) about 3 km east of 

lakeshore and surrounded by apron of younger lava flows from “Egan Cone” (unit mec). 

Northern cone is 600 m wide, having 115 m of relief on its west side, whereas middle and 

southern cones are 200 m wide and only about 30 m high. All three cones have small craters: middle crater contains pond, and southern crater is breached southward. All cones are 

dominantly lapilli and are widely oxidized, and each has scattered scoria bombs as big as 1 

m. Lava-flow surfaces are rubbly to blocky, uneroded, and pervasively vesicular. Because 

unit is not extensively rifted like underlying lavas of units bsl and bes, more massive 

interior exposures are rare. Unit forms main peninsula near Sparks Lake boat ramp and most 

offshore islands. Phenocrysts: ~3–8% olivine (0.2–2 mm); sparse plag laths (≤1 mm, mostly 

mph); sparse to abundant plag-olivine clots and intergrowths are diagnostic of unit, as are 

olivine clusters; cpx-plag clots are very rare. Groundmass is rich in plag-lath mph, seriate to 

microlites. Overlies units bsl and bes. Overlain by unit mec and local remnants of Mazama 

ash (7.7 ka; not mapped separately). Equivalent to parts of units “mb4” and “mb4b” of Scott 

and Gardner (1992). Undated [BT]

bcc Basalt of Cayuse Crater (early Holocene or latest Pleistocene)—Postglacial basaltic 

(49.7–51.9% SiO2) scoria cones (stippled on map) and lava-flow apron (49.7–51.9% SiO2) 

on southwest slope of middle Pleistocene Broken Top edifice. Fissure-fed alignment, striking northwest, built three small mounds and one substantial breached cone (having 200 m 

of downslope relief) of loose scoria and variably agglutinated spatter, widely oxidized. Two 

thin fountain-fed lava flows extend about 1 km southwest from northwestern vents. Main 

leveed flow fan from large breached cone divides downslope into one tongue along Fall 

Creek and another that extends 4 km to Soda Creek and alluvial floodplain at Sparks Lake. 

Scoriaceous rubbly flow surfaces are marked by several blocky levees, some having 10 m 

of local relief. Phenocrysts: ~7% olivine (mph to 1.5 mm); plag absent or limited to mph 

laths in groundmass. Overlies several glaciated lavas from Broken Top volcano and banks 

against Todd Lake volcanic edifice. Overlain by remnants of Mazama ash (7.7 ka) and late 
29

Holocene pumice-fall deposits of units rrm and rdc (not mapped separately). On nearby 

Broken Top edifice, scoria fall from Cayuse Crater overlies till thought to represent 13–12 

ka [12.5–11 ka uncalibrated] glacial readvance of Scott and Gardner (1992); elsewhere, 

organic-rich sediment beneath fallout gave age of 9,520±100 14C yr B.P. (considered minimum age by Scott, 1987) [BT] 

bes Basalt east of Sparks Lake (early Holocene or latest Pleistocene)—Phenocryst-poor basaltic (49.3–49.8% SiO2, at 0.35–0.41% K2O) lava flows just east of (but only locally along) 

east shore of south arm of Sparks Lake. Flow surfaces are rugged, scoriaceous, and rubbly, 

virtually uneroded. Like underlying unit bsl, flows are cut by several linear rifts 1 to 15 m 

wide and as deep as 5 m that expose prismatically jointed or block-jointed interior facies. 

Crude columns 0.3 to 1 m thick and 1 to 5 m long exposed on rift walls are widely marked 

by subhorizontal vugs, which are typically 3 to 20 cm long, 0.5 to 5 cm thick, and irregular 

in shape. Phenocrysts: almost aphyric, except for very rare plag megacrysts, which are typically shattered and as big as 9 mm, and 3–5% tiny olivine mph (0.1–0.3 mm). Groundmass 

contains conspicuous plag microlites. Overlies unit bsl. Overlain by units bbr and mec and 

by local remnants of Mazama ash (7.7 ka). Undated [BT]

bhs Basalt south of The Husband (middle Pleistocene)—Assemblage of thin to thick basaltic 

(51.1–53.0% SiO2) lava flows that make up glacially eroded floor and walls of Indian Holes 

valley at south base of The Husband. Less silicic than most lavas of The Husband (unit mth: 

52–57% SiO2), these basalts may represent early stage of The Husband, overlain by its main 

(less mafic) apron lavas. Alternatively, they may have issued from unrelated vents that later 

were buried, or some may have erupted at Sphinx Butte. Mapping in this area is inadequate 

to identify location(s) of source vent(s). Knob 6760, 3 km north of Indian Holes, on southeast apron of The Husband, is glaciated flank vent for lavas of unit mth′; its 40Ar/39Ar age, 

149±5 ka, provides minimum age for The Husband edifice itself (unit mth) and for subjacent 

lavas of this unit. Basalts of this unit are all phenocryst poor but include varieties that range 

from plagioclase free, having 3–5% olivine (mph to 1 mm), to flows that have as much as 

2% plag and 3% olivine (each 0.5–1 mm), with or without clots of olivine-plag mph. All or 

most flows carry sparse oxide mph, as well as sparse plag xenocrysts, sieved and rounded. 

Felty holocrystalline groundmass is dominated by plag laths, which are seriate from mph 

to microlites. Base of basalt pile is exposed downstream to west but not within map area. 

Overlain by units msc, ddl, and mth. Undated [SS]

bjc Basalt of James Creek (middle Pleistocene)—Vent cone and apron of phenocryst-poor olivinebasalt (51.5–51.8% SiO2) lavas that extends 2 km west along James Creek and 2 km south 

to Mesa Creek. Glaciated Knob 6482 has 120 m of relief and consists of lava flows, agglutinate, sparse loose scoria, and 30-m-high vertically jointed lava wall on its north side that 

appears to partially expose a plug. Benches of dacite lava (unit ddl; 32±2 ka) that banked 

against interior facies of knob indicate that basalt was glaciated at least once prior to dacite 

eruption. Ice-scoured apron lavas are mostly massive and widely till strewn, showing little 

preservation of vesicular facies. Phenocrysts: 3–5% olivine (mph to 1 mm), some in clusters 

and many carrying spinel inclusions; no truly phenocrystic plag, but felty holocrystalline 

groundmass contains plag, seriate from microlites to microphenocrystic laths, a few of 

which are as long as 0.5–0.7 mm; also contains rare plag xenocrysts (1–1.5 mm), sieved and 

rounded. Overlies unit dmc. Overlain by unit ddl. 40Ar/39Ar age: 148±4 ka [SS]

bnf Basalt of North Fork Whychus Creek (middle Pleistocene)—Four mutually isolated windows 

of phenocryst-rich basaltic (50.9–52.5% SiO2) lava that crop out (progressively downstream) 

on floor of North Fork Whychus Creek, at 5,320 ft elevation, and on west rim of Whychus 

Creek valley, at 5,180 ft, 5,160 ft, and 5,000 ft elevations. Correlated, in part, on basis of 

unusually high Al2O3 contents (20.2–22.0%). At streambed location, flow forms massive 

ledge 10 m thick that supports cascade; sandwiched by units mws below and msp above, 

its outcrop pinches out laterally within 30 m. Middle two outcrops are glacially scoured 

ledges of massive, block-jointed to slabby basalt mantled with till, one of them overlain by 

unit msp. Downstream outcrop is lithologically similar ledge, more than 25 m thick, that 

overlies units mwh and mws and is overlain by thick till. Phenocrysts: 20–35% plag (mph 

to 4.5 mm), blocky or elongate; 0.5–2% olivine (0.5–3 mm); rare cpx (1–2 mm); many large 

olivine crystals are partly resorbed and unusually elongate; tiny plag, olivine, and oxide 

crystals of felty groundmass are not seriate with phenocrysts. Overlies units mwh (195±5 
30

ka) and mws (175±3 ka). Overlain by unit msp, which is in turn overlain by lava flow of 

unit mns (which here gave 40Ar/39Ar age of 119±6 ka). Buried source vent could be far to 

west or southwest. Undated [TCB]

bsb Basalt of Sphinx Butte (middle Pleistocene)—Deeply eroded basaltic (51.3–52.7% SiO2) 

edifice exposed on south wall of Mesa Creek (and possibly farther northwest at western 

limit of map area). Glacially eroded butte has 550 m of relief on its steep north face, which 

exposes massive plug (unit bsb′) 300 m wide and retains thick deposits of stratified scoria 

and agglutinate (stippled on map) adjacent to its summit. Along Mesa Creek, apron of lava 

flows, which are largely massive, slabby, block jointed, or chunk jointed, supports stairstep 

set of small waterfalls. Phenocrysts: 2–5% olivine (8–10% in plug rock, unit bsb′); no truly 

phenocrystic plag, but felty groundmass is rich in plag-lath mph, a few of which are as long 

as 0.7 mm; also contains sparse sieved plag xenocrysts as big as 1 mm and sparse oxide 

mph. Investigation of edifice only cursory; contact where unit banks against lavas of The 

Wife (unit mtw; 374±6 ka) was mapped by Wozniak (1982). Overlain by unit ddl (32±2 ka). 40Ar/39Ar age: 279±6 ka [SS]

bsl Basalt southeast of Sparks Lake (early Holocene or latest Pleistocene)—Phenocryst-poor 

basaltic (51.1% SiO2, at 0.56% K2O) lava flows exposed along east shore of south arm 

of Sparks Lake; lowest of four distinguishable sets of lava flows that make up extensive 

postglacial apron east of lake. Source vent unknown, apparently buried by younger mafic 

lavas. Flow surfaces are rugged, scoriaceous, and rubbly. Block-jointed or columnar interior 

is widely exposed where large cracks have opened near lakeshore flow margin, creating several rifts 1 to 5 m deep and 1 to 15 m wide. Phenocrysts: 3–5% olivine (0.5–2 mm); sparse 

plag (mostly mph, but rare crystals to 2 mm). Groundmass commonly finely diktytaxitic and 

rich in plag-lath mph. Base not exposed. Overlain by associated postglacial units bes and 

bbr and by local remnants of Mazama ash (7.7 ka). Equivalent to part of unit “mb4” of Scott 

and Gardner (1992) and Gardner (1994). Undated [BT]

bss Basalt of Spruce Spring (late Pleistocene)—Apron of thin basaltic (50.0–50.4% SiO2, at 

0.6–0.7% K2O) lava flows just southeast of Trout Creek Butte that emerges from beneath 

distal limit of late Pleistocene moraines and extends 4 km farther east (between Pole Creek 

and Brush Draw) to terminus just beyond edge of map area. Unglaciated surface is scoriaceous, rubbly, and locally block jointed, especially on levees. Phenocrysts: 2–3% olivine 

(0.5–1 mm, very rarely to 4 mm); at least 5% olivine mph (0.2–0.4 mm); ~1% plag phenocrysts (0.5–2 mm), but ~30% plag mph in holocrystalline plag-lath groundmass. Coarsegrained groundmass has abundant plag mph and microlites, as well as diktytaxitic porosity 

that has scattered 1-mm-wide equant vesicles superimposed. Normal magnetic polarity 

(Taylor, 1987). Overlies units bbd, acp, and mwh (195±5 ka), wraps around scoria cone 

of unit acp, and banks against base of unit mtb shield (532±7 ka). Overlain and proximally 

concealed by late Pleistocene till (unit g); source vent probably far to west. Undated [TCB]

btk Basalt and basaltic andesite of Talapus and Katsuk Buttes (latest Pleistocene)—Compound 

edifice of basaltic (51.7–53.4% SiO2) lavas, hyaloclastite, and scoria cones that form west 

wall of Sparks Lake. Elongate edifice, only 1.5 to 2.5 km wide, extends 6 km north-south 

and has 225 m of relief. Chain of vents near north end, probably dike fed, includes two 

substantial subaerial scoria cones (stippled on map). In its northern part, lower 140 m of 

edifice is interpreted as hyaloclastite tuff by Scott and Gardner (1992) that may have erupted 

through its own meltwater pond within latest Pleistocene recessional ice. Plateau above is 

capped by stacks of thin subaerial lava flows from several vents. Basal contact of subaerial 

deposits descends southward, almost to base of edifice at its south end (possibly reflecting 

southward slope of ice surface at time of eruption). Fragmental deposits are locally palagonitized. Slumps of more coherent rimrock lavas include rotated megablocks 10 to 200 m 

long (Scott and Gardner, 1992), as well as chaotic rubble. Large area of slumps west of 

Katsuk Butte was overrun by lava flows of unit mlc, itself of latest Pleistocene age, suggesting that slumping took place soon after deglaciation. Phenocrysts: ~5% olivine (0.2–2 mm, 

mostly 0.5–1 mm); abundant plag consists mostly of microphenocrystic laths (or clusters 

thereof), but some lavas contain sparse plag laths 0.5–1.2 mm long, and olivine-plag clots 

are common. Southwest edge of Katsuk Butte overlies unit dse, and stratified ejecta of 

Talapus Butte (stippled on map) overlie unit rdh (34.8±1.5 ka) on slope north of Devils 

Lake. Overlain by units mlc and rdc. Undated [BT, SS]
31

bwl Basalt of lower Whychus Creek (late or middle Pleistocene)—Phenocryst-poor basaltic 

(50.8% SiO2) lava flow exposed at 4,150 to 4,220 ft elevation on north wall of Whychus 

Creek near east edge of map area. Deeply eroded flow is block jointed but locally still scoriaceous and rubbly on its surface, which lies barely outside belt of moraine deposits. Source 

vent unknown, presumably far upstream. Downstream extent beyond map area not determined. Phenocrysts: sparse plag (0.5–2.5 mm), but abundant groundmass plag-lath mph, 

seriate to microlites; 5–7% olivine (mph to 1.3 mm), seriate to groundmass olivine. Overlies 

unit mwh (195±5 ka). Overlain by unit acp. Undated [TCB]

bwm Basalt west of Millican Crater (late Pleistocene)—Glaciated aprons of olivine-rich basaltic 

(48.6–50.6% SiO2) lava flows that crop out widely on both sides of north-trending fissurefed ridge of unit mpn, which probably conceals their vent. East apron slopes gently southeast for 2 km, overlapping west toe of Millican Crater edifice (unit mmc). West apron 

slopes northwest for more than 2 km, extending at least as far as Lava Camp Lake (1.2 km 

north of edge of map area). Most exposures are slightly vesicular but otherwise are massive 

and block jointed, forming numerous ledges (1–3 m high), knolls, and ice-scoured swells. 

Window through moraine just south of east apron exposes two ledge-forming lava flows for 

about 100 m (UTM grid 992/988). Another window, through unit myl about 1.5 km farther 

southeast, is block-jointed ledge only about 1 m high and 50 m across (UTM grid 010/981). 

Phenocrysts: moderate variations in plag and olivine abundances among lava flows; 3–10% 

olivine (mph to 1.5 mm, rarely 2.5 mm); 0–5% plag (0.5–2 mm, rarely as long as 3.5 mm), 

many subhedral; clots of olivine+plag mph and clusters of olivine mph are as big as 10 mm 

and generally are sparse but in some flows are abundant. Groundmass is rich in plag-lath 

mph and microlites. Overlies unit mmc. Overlain by units mpn, myc, myl, and (inferentially) mnt (20±5 ka). Undated [NS, TCB]

bwy Basalt of Whychus Creek (middle Pleistocene)—Two windows of phenocryst-poor basalt 

(51.1–51.8% SiO2): one is at 5,120 to 5,160 ft elevation just downstream from Upper Chush 

Falls; other is 2 km north at about 4,800 ft on lowermost North Fork Whychus Creek. 

Although slightly different chemically, both are nearly aphyric and unlike any other rare 

basalts in this area. Exposures about 500 m downstream from Upper Chush Falls include 

10-m-high ledge (slabby to block jointed), which supports a small waterfall at 5,140 ft elevation on Whychus Creek, and knife-edge point at nearby confluence of Park Creek. On North 

Fork Whychus Creek, coarsely block jointed rimrock ledge more than 15 m thick supports 

cascades; its basal flow breccia and massive lava flow drape andesite of unit awy and old 

till sandwiched between them on right-bank wall. Phenocrysts: almost none; holocrystalline 

groundmass microlites are seriate to ~3% olivine mph (0.2–0.4 mm) and <1% plag laths 

(0.2–0.7 mm long); most mph are in olivine-plag-oxide clots. Overlies units mbt and awy

(299±1 ka). Overlain by units adl, drm, mwh (195±5 ka), and ruc (169±2 ka). Undated 

[TCB] 

dbh Dacite of “Black Hump” (late Pleistocene)—Glaciated pile of effusive dacite (64.0–64.5% 

SiO2) lava flows that erupted at vent in saddle between North Sister and Middle Sister. 

Compact ice-sculpted edifice, variously called “Black Hump” (Hodge, 1925), “Prouty 

Point,” or “Step Sister,” is about 1 km across, consists of at least five flow units, and has 325 

m of relief, including several cliffy faces. In addition, two of these thick flows extend separately for about 1 km west and northwest of proximal pile as narrow arêtes. Another flow 

crops out atop medial cleaver in Collier Glacier. Individual flows range in thickness from 25 

to 90 m. Lithologies vary from black vitrophyre, locally columnar, to pale-gray devitrified 

felsite, block jointed or slabby to platy, commonly having oxidized scoriaceous rubbly zones 

between flows. Phenocrysts: 7–10% plag (mph to 4 mm, rarely larger); 1–2% opx (mph to 

2 mm, rarely to 3.5 mm); sparse oxide mph; contains common plag-pyroxene-oxide clots, 

which account for most pyroxene crystals. Overlies units awc (22±2 ka), dss (25±3 ka), and 

mms. Relations with unit dhr at col are obscured by scree. 40Ar/39Ar ages: 17.9±2.2 ka (for 

western lava flow), 23±7 ka (for base of pile north of Hayden Glacier) [NS]

dbl Dacite breccia of Lewis Glacier headwall (late Pleistocene)—Crudely stratified, pale-gray- to 

tan-weathering, fragmental dacite (62.6–62.8% SiO2) deposit as thick as 50 m that banks 

against paleocrater wall above Lewis Glacier and directly underlies unit dlg. Clasts are 

monolithologic, glassy, dense or weakly vesicular, subangular to subrounded, and as big as 

20 cm throughout most of section but as large as 1 m near top; matrix is seriate from ash to 
32

lapilli-sized clasts. Clasts are slightly less silicic than overlying lava flow of unit dlg and 

contain twice as many plag phenocrysts. Strata are subhorizontal to slightly inboard dipping 

in headwall exposures, but craggy remnants preserved outboard of wall (atop eroded rim of 

wall-forming unit adh dome) dip steeply outboard. Phenocrysts: 15% plag (mph to 3 mm); 

<1% pyroxenes (mostly mph), largely in clots with plag. Top 2 m of unit is baked brick red, 

contrasting starkly with black, glassy horizontal columns of overlying lava flow of unit dlg

(25.3±1.4 ka, 22.3±1.6 ka). Undated [SS]

dcg Dacite southwest of Clark Glacier (late Pleistocene)—Southwest apron of South Sister 

edifice, consisting of many compositionally uniform dacite (63.0–63.5% SiO2; n=26) lava 

flows. Stairstep stack of flows, each 15 to 100 m thick, forms glaciated 3 by 3 km surface 

that extends from Clark Glacier to toe of edifice at Hinton and Mesa Creeks. Lithologies 

vary from black vitrophyre, commonly chunk jointed or columnar, to devitrified and block 

jointed or slabby to platy. Flow foliation is commonly conspicuous and locally convolute 

in thickest flows. Scoriaceous rubbly oxidized zones are common between flows. High 

on apron, steep and thick eruptive masses have internal shear zones of oxidized breccia 

intimately alternating with vitrophyric and platy domains, perhaps reflecting influence of 

snow and ice. Phenocrysts: 10–20% plag (mph to 4 mm); 1–3% pyroxenes (mostly mph, but 

as big as 1 mm); sparse oxide mph; abundant plag-pyroxene-oxide clots; most pyroxene and 

half of plag crystals reside in clots. Overlies units bjc, ddl (32±2 ka), rmc (47±8 ka), and 

rsw (51±10 ka). Overlain by unit aeg (here, 27±2 ka) and also by units rct (24±5 ka, 30±8 

ka) and rrm (~2.1 ka), which bank against it. 40Ar/39Ar age: 30.7±2.6 ka [SS]

dcl Dacite of Chambers Lakes (late Pleistocene)—Agglutinated tephra fallout and rheomorphic 

dacite (64.8–65.9% SiO2; n=13) lava flows, widely exposed in saddle between Middle 

Sister and South Sister. Principal outcrop forms glacially eroded ridge about 1 km wide that 

separates two clusters of Chambers Lakes. Relief exposed exceeds 110 m, but true thickness 

is less if ridge-forming agglutinate drapes unexposed units. Deposit is crudely stratified or 

lensoid, eroded into ledges; variable dips as steep as 25° suggest buried paleotopography. 

Main lithology is orange and black streaky vitrophyre that grades from densely eutaxitic to 

tack welded and, locally, to loose pumice and scoriae (as big as 40 cm), which can be black, 

dark brown, or tan but mostly are oxidized orange. Fiamme vary from tiny wisps to as long 

as 50 cm and as thick as 15 cm. Lithic clasts include phenocryst-poor mafic lavas but are 

predominantly pyroxene andesite-dacite, mostly smaller than 10 cm but some as large as 

50 cm. Source probably is glacially excavated bowl now occupied by largest lake, adjacent 

to which juvenile and lithic clasts are both largest. On ridgecrest, least viscous agglutinate sections spread southward and eastward as rheomorphic flows that range in texture 

from blobby to massive and virtually homogenized; they grade up and down into eutaxite. 

Where glacially beveled, such flows range internally from densely glassy to devitrified and 

platy. Phenocrysts: 3–7% plag, varying widely in clasts of similar composition; most plag 

ranges in size from mph to 2.5 mm, rarely 3–6 mm; 0.5–2% pyroxenes (mostly mph, but 

sparse grains 0.5–1.5 mm); common oxide mph; some but not most plag crystals are sieved 

internally; also common are plag-pyroxene-oxide clots, which account for most pyroxene 

crystals. Overlies unit aeg (27±3 ka), but base is rarely exposed. Overlain by units mcl, 

mms, and dig (14±3 ka), as well as pumice-fall remnants of unit rdc. 40Ar/39Ar age: 26±2 ka 

[NS, SS]

dcn Dacite north of Carver Lake (late Pleistocene)—Sequence of two or three glacially scoured 

dacite (66.9–67.4% SiO2) lava flows exposed in windows through Neoglacial till between 

Carver Lake and Chambers Lakes; all dip northward from buried vent on South Sister edifice, summit of which lies only 2 km south. Lavas are vesicular and glassy or partly devitrified, as well as block jointed and having densely vitrophyric bases that locally are chunk 

jointed. Phenocrysts: ~10% plag (mph to 4 mm, rarely 6 mm); 1–2% pyroxenes (mph to 2 

mm); sparse oxide mph. Base not exposed. Apparently (not certainly) overlain by unit aeg

(here, 27±2 ka). Undated [SS]

ddl Dacite of Dew Lake (late Pleistocene)—Thick, glacially scoured, phenocryst-poor dacitic 

(64.1–64.4% SiO2) lava flows that extend 4 km westward between Separation and Mesa 

Creeks from beneath southwest apron (unit dcg) of South Sister edifice. Flows tightly wrap 

vent knob of unit bjc, around which they divide into two tongues. Proximal area exposes as 

much as 50 m of local relief, and distal cliffs are as high as 80 m in both tongues. Lithology 
33

varies from massive black vitrophyre, commonly chunk jointed or columnar, to pale gray, 

devitrified, and slabby or platy. Phenocrysts: 2–3% plag (mph to 1.5 mm, rarely 3 mm); 

0.5–1% pyroxenes (nearly all mph, but rarely as large as 0.8 mm); sparse oxide mph; 

also contains common plag-pyroxene-oxide clots. Overlies units mtw, bsb, bhs, and bjc. 

Overlain by units dcg (30.7±2.6 ka) and msc (21±6 ka). 40Ar/39Ar age: 32.3±1.8 ka [SS]

des Dacite ejecta of South Sister summit (late Pleistocene)—Stratified dacitic (62.8–64.2% SiO2; 

n=14) fall and surge deposit, ultraproximal, largely phreatomagmatic, that makes up much 

of uppermost 120 to 150 m of summit cone of South Sister. Coarse fragmental deposit crops 

out best above Prouty Glacier headwall east and northeast of summit but also is exposed 

above Lewis Glacier headwall to southeast and on crater rim west of Teardrop Pool; it is 

overlain only by dacitic lava flows locally along north rim of summit (units dnt, dnt′) and by 

thin andesitic agglutinate (unit mtp, topmost eruptive unit that extensively drapes summit). 

Much of unit banks inside ragged rim of (locally breached) outer paleocrater wall that 

consists of unit aeg; it rests on intracrater lava dlg and fills paleocrater to unknown depth. 

Thickest exposed section, on steep slope above Prouty Glacier headwall consists conspicuously of four subunits (not mapped separately): 

1. Lowest 10 m is coarsely lithic rich, unstratified to crudely stratified section, poor 

in juvenile clasts, that dips 10° to 25° outboard; its poorly exposed base rests on unit dlg. 

Lithic clasts, mostly 1–40 cm across, rarely as big as 100 cm, are angular fragments of 

underlying unit dlg, most of them hydrothermally altered. Subordinate juvenile dacite material is limited largely to lapilli and finer fractions. 

2. Overlying subunit 1, about 30 m of section contrasts in being predominantly juvenile, lithic poor, and well stratified. Its basal 50 cm is sharply bounded, pale-gray interval 

dominated by juvenile dacite that is glassy, angular, slightly to moderately vesicular, and 

seriate from ash to 20 cm blocks; this layer banks against and drapes a 5-m-long block of 

prismatically jointed dacite that rests atop subunit 1 and may represent initiation of dome 

disruption that followed episode of vent excavation. Nearly all of this subunit consists of 

thin-bedded to laminated, lithic-poor strata, typically in coherent bedsets 10 to 30 cm thick, 

that also dip 10° to 25° outboard, like section beneath. Far more thinly stratified than other 

subunits, its layering is defined by coarse-to-fine alternations on subcentimeter scale. Most 

clasts are only 0.5 to 2 cm across, although sparse clasts are as big as 20 cm; all are glassy 

and angular, some are prismatically jointed, and vesicularity varies from fine to poor. 

3. Middle 25 to 30 m is coarsest part of section, marked by three very coarse layers, 

each 4 to 10 m thick, separated by wavy-bedded intervals 1 to 3 m thick; strata dip 15° to 

20° inboard and overlie unconformity that smoothly bevels underlying outward-dipping 

strata. Clasts are massive or poorly vesicular, glassy, angular, matrix-supported, and monolithologic crystal-rich dacite; many are 5 to 50 cm across, and some are prismatically 

jointed; matrix is crystal-rich coarse ash, seriate to small lapilli. Basal few meters of bedded 

middle section is disrupted by several small faults. 

4. Upper 75 m is stratified only crudely and dips gently inboard, conformable with 

coarse middle part of section. As in subunit 3 beneath it, clasts are massive or poorly vesicular, glassy, angular, matrix supported, and monolithologic; most clasts are only 0.5 to 2 cm 

across, although scattered coarser (5 to 15 cm) clasts in lenses or vague horizons help define 

crude layering. Matrix consists of crystal-rich coarse ash, seriate to small lapilli. Within 

continuous 145-m-thick section of four subunits, no sedimentary intervals are observed that 

might represent prolonged breaks in eruption. Wavy and contorted beds and bomb sags are 

common. Accidental lithic fragments are sparse except in basal 10 m, where angular and 

altered dacite lithic fragments (0.5–40 cm across, some to 100 cm) are abundant, and all 

or most are derived from underlying unit dlg. Phenocrysts in juvenile clasts: 15–20% plag 

(mph to 3 mm); 2–3% pyroxenes (mph to 1 mm); sparse oxide mph; also contains common 

plag-pyroxene-oxide clots as big as 5 mm, which account for most pyroxene crystals. 

Overlies units aeg (here, 27±3 ka) and dlg (25.3±1.4 ka, 22.3±1.6 ka). Overlain by units 

mtp, dnt (23.5±1.1 ka), and dnt′. Undated [SS]

dgc Dacite of Goose Creek (late Pleistocene)—Glaciated dacite (64.8–65.3% SiO2) lava flow 

that extends 3.5 km southward from southeast toe of South Sister to alluvial plain north of 

Sparks Lake. Maximum relief about 75 m proximally and medially, but only 35 m at distal 

terminus. Source vent buried by younger units. Ranges in lithology from black vitrophyre 
34

to devitrified, block jointed or slabby to platy. Locally preserved are glassy margins that 

have vertical, horizontal, and inclined slender columns, suggestive of ice contact. Normal 

magnetic polarity (Taylor, 1978). Phenocrysts: 5–7% plag (mph to 3 mm); 1–2% pyroxenes (mostly mph, rarely as big as 1 mm); sparse oxide mph; also contains common plagpyroxene-oxide clots. Intruded and overlain by unit rdc; appears to underlie eroded ridge 

(UTM grid 004/786) of unit dml (33.6±1.2 ka). Overlies unit abt (178±1 ka, at Fall Creek). 40Ar/39Ar age: 31.6±2.0 ka [BT]

dgl Dacite of Green Lakes (late Pleistocene)—East-southeast apron of South Sister edifice, consisting of many phenocryst-rich dacite (62.9–64.2% SiO2) lava flows. Glacially scoured unit 

forms most of edifice flank on north side of trough draining Lewis Glacier, extending from 

beneath Hodge Crest planèze (unit aeg) down to Green Lakes basin. Total elevation difference exposed within unit is 600 m, but greatest local relief (and, thus, maximum thickness 

exposed) is only 130 m (on scarp just west of Green Lakes). Consists of 12 or more flows 

that range in thickness from about 15 m to 60 to 80 m, thicker ones being strongly flow 

foliated. Flows range in lithology from dark-gray vitrophyre to devitrified, block jointed, or 

slabby to platy, weathering tan or pale gray. Phenocrysts: 5–10% plag (mph to 3 mm); 1–3% 

pyroxenes (0.2–1 mm); sparse oxide mph; also contains common plag-pyroxene-oxide clots, 

which account for most pyroxene crystals. Overlies unit rgl (32±1 ka). Overlain by units 

dmn (28±1 ka, 25±1 ka) and aeg (here, 27±2 ka). Undated [BT, SS]

dhr Dacite of Hayden Glacier–Renfrew Glacier col (late Pleistocene)—Single dacite (63.6–64.3% 

SiO2) lava flow, 15 to 20 m thick, that drapes north ridge of Middle Sister cone from 9,600 

ft elevation down to 9,200-ft-high saddle between Hayden and Renfrew Glaciers. Vent is 

presumed to have been slightly east on part of edifice since removed by Hayden Glacier. 

Surface of flow is extensively vesicular, oxidized pink, or laced with secondary minerals 

that lend outcrops an orange cast. Exposures of flow interior are flow foliated, partly glassy, 

massive or vesicular, block jointed, or locally devitrified and platy. Phenocrysts: ~10% 

plag (mph to 5 mm); ~2% pyroxenes (mph to 2 mm, rarely 3 mm); sparse oxide mph; also 

contains abundant plag-pyroxene-oxide clots. Rests unconformably directly on smoothly 

beveled stack of flows of unit mms. Relations with adjacent unit dbh obscured by scree that 

fills col. 40Ar/39Ar age: 19.1±1.7 ka [NS] 

dig Dacite of Irving Glacier (latest Pleistocene)—Dacite (64.1–64.2% SiO2) coulee from inconspicuous vent at 8,500 ft elevation on south slope of Middle Sister. Proximal to medial reach is 

leveed, unglaciated, and 150 to 400 m wide. Distally, it bifurcates around rhyolite lava dome 

of unit rsc; southwest arm ends 3 km from vent in 100-m-high, glacially modified cliff above 

Separation Creek; southeast arm ends 2 km from vent in 135-m-high cliff, where flow banked 

against ice that had ponded in basin now occupied by Chambers Lakes, as shown by glassy 

subhorizontal columns widely preserved on flow front. Latest Pleistocene ice encountered by 

both arms probably still flowed down to saddle from high on South Sister when south slope 

of Middle Sister was ice free. Thick vitrophyre zones form upper and lower parts of flows, 

whereas central parts of terminal cliffs include devitrified internal zones exposed by glacial 

erosion. Vent area is 50-m-wide depression enclosed by blocky sidewall levees 10 to 15 m 

high and upslope headwall consisting of stack of thin mafic lava flows of older unit mms, 

which was oxidized brownish orange by dacite eruption. Airborne ejecta are sparse; glassy 

angular blocks, some prismatically jointed, are strewn about depression, but most tumbled 

in from levees. Inner east wall of depression is tack-welded to densely agglutinated spatter. 

Levees adjacent to vent are capped by oxidized flow breccia and locally by agglutinate, but 

elsewhere levees are composed of black glassy blocks of unoxidized dacite, mostly massive 

but some having scoriaceous surfaces. Films of secondary minerals coat joints in near-vent 

dacite and, more conspicuously, in mafic rubble of headwall. Phenocrysts: 5–10% plag (mph 

to 3.5 mm, rarely to 8 mm); 1–2% pyroxenes (mph to 1.5 mm); sparse oxide mph; also contains common plag-pyroxene-oxide clots; limited thin-section data suggest greater abundance 

of plag and pyroxene mph in proximal levees than in distal lava lobes. Overlies units mms, 

rsc (25±1 ka), dcl (26±2 ka), and dsn (19±4 ka). 40Ar/39Ar age: 14.4±2.7 ka [NS, SS]

dkb Dacite of Kokostick Butte (late Pleistocene)—Phenocryst-rich dacite (62.5–63.1% SiO2) coulee 

and chain of small satellite domes 2 to 5 km south of Rock Mesa. Coulee is 2 km long and 

1 km wide, having maximum relief of 100 to 150 m at its southern flow front, from which 

springs emerge as source of Sink Creek. Chain of five satellite domelets extends 1.8 km 
35

south-southwest from main coulee, to which they are compositionally identical; domelets 

range in height from 30 to 90 m and in width from 130 to 300 m. Coulee and domelets are 

glacially scoured but have little till on top; all are widely glassy and retain extensive outcrops of slender exterior columns, which are commonly inclined or subhorizontal, suggestive of ice-contact effusion. Where most deeply eroded by ice, steep faces of coulee and two 

largest satellites expose partly devitrified, vertically jointed to slabby interiors. Phenocrysts: 

15–20% plag (mph to 3 mm); 2–3% pyroxenes (mostly mph, but as big as 1 mm); sparse 

oxide mph; also contains abundant plag-pyroxene-oxide clots, which account for most 

pyroxene crystals. Southern domelets intrude and overlie apron of mafic lava flows (unit 

mkm) from Koosah Mountain. Mafic lava flows of postglacial unit mlc wrap coulee and 

two domelets. 40Ar/39Ar age of coulee: 27.3±2.6 ka [SS]

dlc Dacite of Linton Creek (late Pleistocene)—Window of glaciated dacite (66.8% SiO2) lava 

that has 70 m of steep relief, exposed only in 100 by 300 m area at about 5,800 ft elevation in headwaters of middle fork of Linton Creek, just above Pacific Crest Trail (UTM 

grid 946/876). Glassy, finely vesicular, flow-foliated, chunk-jointed zone below is overlain 

by platy devitrified zone near top of exposure. Phenocrysts: 5–8% plag (mostly mph, but 

as long as 1.7 mm); ~1% pyroxenes (mph to 1.5 mm); sparse oxide mph; also contains 

common plag-pyroxene-oxide clots, which account for most pyroxene crystals. Base not 

exposed. Overlain by units ahl, alc (27±1 ka), mhl, and mms. Undated [NS]

dlg Dacite of Lewis Glacier headwall (late Pleistocene)—Thick dacite (63.5–63.8% SiO2) lava 

flow near summit of South Sister that crops out as bold cliffs high on headwalls above 

Lewis and Prouty Glaciers. Convolutely foliated flow is 75 to 100 m thick and banks against 

ragged wall of paleocrater that consists of units aeg, ahc, adh, and alh, as exposed on 

both headwalls. Vitrophyre and partly devitrified upper zone overlie cliff-forming devitrified internal zones, block jointed or slabby, exposed above glaciers. Basal zones exposed 

in both headwalls exhibit subhorizontal columns as thin as 8 cm; vertical fractures cutting 

columns impart hackly appearance. Overlies fragmental deposit of unit dbl in Lewis Glacier 

headwall and another older fragmental deposit (unit ahc) in Prouty Glacier headwall; lava of 

this unit baked both underlying units. Overlain by 150-m-thick section of phenocryst richer 

fragmental dacite strata (unit des). Another glaciated dacite (63.7–64.4% SiO2) lava flow 

as thick as 60 m, which crops out 2 km north of summit (and 600 m lower), is correlated 

chemically and petrographically and here also assigned to this unit. Flow is widely devitrified and platy, although locally glassy, block jointed, or brecciated; it weathers pale gray or 

tan, and its exposure extends about 1 km northward along west side of steep gulch draining 

into eastern cluster of Chambers Lakes. Base of flow is not exposed; flow is directly overlain by rhyolitic lava flow of late Holocene unit rdc and is apparently intruded by its feeder 

dike. Phenocrysts: 3–5% plag (mph to 2 mm); 1–2% pyroxenes (mostly mph, but including 

rare crystals 1–2.5 mm); sparse oxide mph; also contains common plag-pyroxene-oxide 

clots, which account for most pyroxene crystals. Overlies or banks against units aeg (here, 

27±3 ka), adh, ahc, alh (30±1 ka), and dbl. Overlain by units des and rdc. 40Ar/39Ar ages: 

25.3±1.4 ka, 22.3±1.6 ka [SS]

dlp Dacite of Lane Plateau (late Pleistocene)—Sandal-shaped dacite (64.2–65.3% SiO2) coulee 

and glacially eroded remnant of narrow lava stream that fed it from vent at 7,800 ft elevation on west slope of Middle Sister. Coulee, 2 by 1 km in area and having 90 m of relief 

distally, forms steep-sided ice-scoured plateau that slopes gently westward between Linton 

and Obsidian Creeks. Remnant of feeding flow forms 800-m-long steep ridge about 150 

m wide that has as much as 40 m of local relief; segments formerly connecting it to both 

vent and coulee have been eroded away. Extrusive vent mass, 2.2 km east of main coulee 

and 425 m higher in elevation, is unusual protrusion 150 m long and 20 to 50 m thick that 

stands as wall-like monument about 25 m high. Steep downhill face of wall is largely glassy 

and widely retains horizontal polygonal joints, including long slender radial and inclined 

columns at its upper edge, suggesting that extrusion took place into ice. Near-vertical joints 

crop out at base of wall where dacite welled out over (and baked) andesite lava of unit awm. 

Both coulee and feeder ridge are lithologically varied, widely exhibiting vitrophyric exterior 

facies (locally columnar) but slabby to platy devitrified interior facies where more deeply 

eroded. Phenocrysts: 5–7% plag (mph to 3 mm, rarely to 8 mm); 2–3% pyroxenes (mph to 1 

mm, rarely to 2 mm); sparse oxide mph; also contains common plag-pyroxene-oxide clots; 
36

limited thin-section data suggest that phenocryst abundance of near-vent lava is roughly half 

that of medial and distal lava. Overlies units aoc, alc (here, 27±1 ka), ahl (21±3 ka), and 

awm. Younger units mms and mms′ bank against uphill wall of vent mass. 40Ar/39Ar age: 

21.4±1.9 ka [NS]

dmc Dacite of Mesa Creek (middle Pleistocene)—Poorly exposed window of glaciated dacite 

(65.1% SiO2) lava that forms north slope of Mesa Creek, 0.5 to 1.5 km west of Rock Mesa. 

South-sloping surface has about 60 m of total relief; pink and gray exposures exhibit strong 

flow foliation, locally platy. Source vent, concealed by younger units, probably is buried 

beneath much younger South Sister edifice. Phenocrysts: ~7% plag (mph to 1.3 mm); ~2% 

pyroxenes (nearly all mph, rarely as long as 1 mm); sparse oxide mph; contains common 

plag-pyroxene-oxide clots. Base not exposed. Overlain by units mlc and bjc (148±4 ka). 

Undated [SS]

dml Dacite southeast of Moraine Lake (late Pleistocene)—Glaciated stack of phenocryst-rich 

dacite (64.0–65.7% SiO2) lava flows north of Devils Hill that extends 2 km southeast from 

Moraine Lake to Goose Creek. One flow extends 1 km southwest, where its snout emerges 

from beneath younger coulee of unit dmw. Main pile south and east of Moraine Lake has as 

much as 140 m of relief and consists of at least four flows, all heavily mantled by proximal 

pumice-fall deposits of units rrm and rdc. Moraine Lake has no moraine; valley-crossing 

ridge above its south shore is spur of glaciated lava of this unit, thickly mantled by pumice 

and discontinuous subordinate veneer of glacial debris. Distal crags, severely ice sculpted at 

Goose Creek, have about 100 m of relief. External vitrophyre facies are widely preserved on 

stairstep ledges, whereas shelves and plateaus, more deeply scoured, tend to expose internal 

devitrified zones, commonly platy. On steep ledge banked against Devils Hill and on westfacing scarp south of Moraine Lake, slender glassy columns (commonly inclined or subhorizontal) are widespread, suggesting ice-contact emplacement. Compositionally similar 

to underlying unit dwp and overlying unit dmw. Phenocrysts: 5–10% plag (mph to 3 mm); 

~1% pyroxenes (nearly all mph, rarely as big as 0.8 mm); sparse oxide mph; also contains 

common plag-pyroxene-oxide clots, which account for most pyroxene crystals. Overlies 

unit dwp (34±3 ka); appears to overlie unit dgc (32±2 ka), but relation is uncertain. Banks 

against units rkb and rdh (35±2 ka). Underlies units dmn (28±1 ka), dmw, rse (34±1 ka), 

and rdc. 40Ar/39Ar age: 33.6±1.2 ka [BT, SS]

dmn Dacite north of Moraine Lake (late Pleistocene)—Glacially scoured dacite (65.1–66.3% SiO2) 

lava flows on south slope of South Sister; exposed as high as 8,840 ft elevation at snout 

of Lewis Glacier, extending 3 km south, nearly to Moraine Lake, in two (compositionally 

identical) tongues, each a single flow. Thickness as much as 80 m, proximally, medially, and 

distally. Source vent probably central on South Sister, now covered by unit aeg and other 

younger units. Vitrophyre facies are patchily preserved and typically block jointed, but most 

exposures are devitrified (or partly so) and either block jointed or platy. Phenocrysts: 5–7% 

plag (mph to 3 mm); 1–2% pyroxenes (mostly mph, sparsely 0.5–1.5 mm); sparse oxide 

and olivine mph; clots of plag+pyroxenes+oxides are common, accounting for most pyroxene crystals. Overlies units dgl, dml (33.6±1.2 ka), dmw, and rse (34±1 ka). Overlain by 

units aeg (here, 27±2 ka), als, and rct (24±5 ka). 40Ar/39Ar ages: 27.7±1.0 ka (west tongue), 

25.2±0.8 ka (east tongue) [SS]

dms Dacite of Middle Sister summit (late Pleistocene)—Glassy, phenocryst-poor dacite 

(63.9–66.7% SiO2) lava flow that drapes upper southeast ridge of Middle Sister, extending from within 30 m (south) of true summit (10,047 ft elevation) downslope for about 350 

m to 9,420 ft elevation. Small outlier 80 m farther down ridgecrest and another just north 

of summit suggest that much more was eroded away. Main remnant is 100 to 200 m wide 

across ridgeline and 10 to 20 m thick; its ragged southwest contact reflects instability of 

subjacent thin rubbly flows of unit mms on steep slope. Blocky surface is frost riven but 

was not overridden by glacial ice. Eroded mass of possible vent plug cuts unit mms below 

east rim in precipitous chute about 100 m southeast of true summit. Black glassy blocks are 

generally massive, rarely vesicular or partly devitrified. Greenish-black, finely vesicular, 

phenocryst-poor pumice lapilli sparsely scattered among blocks on ridgecrest are compositionally similar. Phenocrysts: 2–3% plag (mph to 1.5 mm); ~1% pyroxenes (nearly all mph, 

rarely as big as 1 mm); sparse oxide mph. Rests directly on unweathered surface of unit 

mms. Unit mss banks against northern remnant. Undated [NS]
37

dmw Dacite coulee west of Moraine Lake (late Pleistocene)—Glacially scoured, steep-sided dacite 

(64.6–65.5% SiO2) lava flow at south toe of South Sister. Flow emerges from beneath 

younger lavas on lower slope of edifice and extends 3 km southward where it banks against 

unit dml at north side of Kaleetan Butte. Flow is 0.5 to 1 km wide, about 60 m thick distally, 

and 90 to 120 m thick medially, adjacent to Moraine Lake. Phenocryst-rich dacite is widely 

flow foliated. Devitrified interior, block jointed to platy, is widely exposed, but vitrophyric 

margins (dense or locally vesicular) are also extensively preserved, including widespread 

chunk jointing and slender columns, inclined or subhorizontal, indicative of ice-contact 

emplacement. Rusty films common on joint surfaces. Phenocrysts: 10–15% plag (mph to 3 

mm); 1–2% pyroxenes (mostly mph, but as large as 0.8 mm); also contains abundant plagpyroxene-oxide clots, which account for nearly all oxide mph present. Proximal part of flow 

also has trace hornblende (mph to 1 mm long), all grains of which are marginally or fully 

opacitized. Overlies units dml (33.6±1.2 ka) and dwp (34±3 ka). Overlain by lithologically 

similar unit dmn (28±1 ka) and by unit rct (24±5 ka). Undated [SS]

dnt Dacite north of Teardrop Pool (late Pleistocene)—Block-jointed and fractured, glassy, phenocryst-rich dacite (62.7–63.1 % SiO2) lavas that directly cap compositionally similar fragmental unit des for about 200 m along northwest rim of South Sister summit. Sandwiched 

between unit des and summit-mantling agglutinate unit mtp, total lava thickness is 8 to 

10 m near rim. Outboard of rim, lavas extend about 100 m downslope in three overlapping 

fingerlike lobes that drape steep slope of fragmental unit des; blocky lobes are marked by 

joint-coating films of white and orange precipitates. Phenocrysts: 10% plag; 2–3% pyroxenes; also contains common plag-pyroxene clots. 40Ar/39Ar age: 23.5±1.1 ka [SS]

dnt′ Silicic dacite north of Teardrop Pool (late Pleistocene)—Massive glassy ledge of phenocrystrich dacite (67.1% SiO2) on north rim of South Sister summit; petrographically similar to, 

but far more silicic than, dacite lavas of unit dnt, only about 100 m west. Ledge is only 30 to 

50 cm thick and crops out laterally for only about 15 m. Films of white and orange precipitates are present on smooth joints and on its upper surface. Phenocrysts: 7–10% plag (mph 

to 3 mm); 2–3% pyroxenes (mph to 0.7 mm); sparse oxide mph; also contains common 

plag-pyroxene-oxide clots. Sandwiched between top of unit des and summit agglutinate unit 

mtp, ledge is in same stratigraphic position as unit dnt but represents anomalously silicic 

effusion, presumably of small volume. No evidence is preserved in outcrop or thin section 

for welding or pyroclastic texture, but origin as rheomorphically homogenized agglutinate 

seems likely. Undated but presumed to be virtually contemporaneous with unit dnt [SS]

dpg Dacite pumice fall of Prouty Glacier headwall (late Pleistocene)—Dacite (64.6% SiO2) 

scoria-fall deposit, pale gray to buff weathering, 5 to 20 m thick, exposed on steep headwall 

above Prouty Glacier. Crudely stratified in lower half, moderately so in upper half, having 

nearly horizontal layering. Unit consists predominantly of phenocryst-poor scoria lapilli, 

mostly 1 to 2 cm but as big as 20 cm, and it lacks fine-grained matrix. Pale-gray lithic 

ejecta as big as 10 cm, principally of intermediate lava, are common near base. Most small 

scoriae are buff, although some are dark brown or mixed; larger scoriae are dark brown 

or gray internally but typically retain buff exterior; all are glassy and finely to coarsely 

vesicular. Stratification in basal several meters at south end of exposure is accentuated by 

alignment of elongate (slightly flattened), large dark clasts. Phenocrysts: ~1% plag (mph to 

1-mm-long laths); <1% pyroxenes (all mph); sparse oxide mph; sparse fine-grained clots of 

plag+pyroxenes+oxides. Rests directly upon lava of unit rpg (32.8±1.5 ka) and unit aph′. 

Overlain unconformably by tephra-fall deposit of unit ahc and by layered scoria, agglutinate, and fountain-fed lavas of cone-building andesite unit aeg (here, 27±3 ka). Undated but 

compositionally and petrographically similar to lava of unit ddl (32±2 ka) on opposite side 

of South Sister [SS]

drm Dacite of Red Meadow (late Pleistocene)—Extensive set of glacially scoured dacite 

(62.6–66.3% SiO2; n=20) lava flows on northeast apron of South Sister. Emerging from 

beneath Neoglacial moraines of Prouty Glacier, ledgy stack of several 10- to 20-m-thick 

flows condenses medially to two thick flows, one of which continues 6 km northeast, forming broad divide between Whychus and Park Creeks. Where two flows are exposed locally 

(see internal contact near UTM grid 024/877), unit is as wide as 2 km medially, and surface 

is extensively mantled by Pleistocene till. Thickness is as great as 150 m medially and at 

least 60 m near distal terminus. Despite having been everywhere overrun by Pleistocene ice, 
38

unit widely retains vitrophyric external facies, including flow-margin carapace of slender 

black inclined columns facing Red Meadow, suggesting emplacement against valley glacier. 

In many patches, however, erosion has exposed devitrified internal facies, partly to fully 

devitrified and commonly slabby to platy. Phenocrysts: 10–15% plag (mph to 3 mm); 1–2% 

pyroxenes (mostly mph, but as large as 1.5 mm); sparse oxide mph; also contains abundant plag-pyroxene-oxide clots, which account for most pyroxene crystals. Nearly all plag 

is clean and unsieved; rare sieved plag crystal observed is apparent xenocryst attached to 

1.5 mm olivine that has reaction rim. Overlies units apc (31±9 ka), rpc, rpg (32.8±1.5 ka), 

and bwy; wraps around snout of unit rsf coulee (36±2 ka). Overlain by units adl (24±1 ka), 

awf (24±1 ka), and, proximally, aeg (here, 27±3 ka). 40Ar/39Ar age: 28.7±0.8 ka [BT, SS, 

TCB]

dse Dacite east of Sink Creek (late or middle Pleistocene)—Phenocryst-poor dacite (66.2% 

SiO2) lava flow forming 200-m-wide glaciated shelf that crops out for about 800 m beneath 

southwestern margin of Katsuk Butte. Ledge front has about 25 m of relief. Mantled by 

surficial deposits, unit is poorly exposed, consisting mostly of angular blocks of massive 

flow-foliated dacite. Phenocrysts: almost none; very rare plag (0.5–2 mm); very rare cpx 

mph prisms. Groundmass contains sparse quench needles of flow-aligned oxides and plag 

(mostly microlites, but rarely as big as 0.1 by 1 mm). Base not exposed. Overlain by unit 

btk. Postglacial lava flows of unit mlc bank against unit. Undated [SS]

dsn Dacite north of Separation Creek (late Pleistocene)—Glacially scoured flow of moderately 

porphyritic dacite (64.3–64.4% SiO2) lava between headwaters forks of Separation and 

Linton Creeks. Forming barren plateau (0.5 by 1 km) of extraordinarily platy lava at about 

6,850 ft elevation, flow has maximum exposed relief of about 40 m near its western limit. 

Source vent buried beneath younger units of Middle Sister edifice; composition is remarkably similar to that of overlying coulee of unit dig, which also erupted on Middle Sister. 

Thick flow is almost horizontal, having probably ponded at lowland toe of edifice. Massive 

vitrophyric lower zone is exposed along marginal drainages, but most of extensive outcrop 

is devitrified and platy, commonly splitting into plates only 0.5 to 1 cm thick. Phenocrysts: 

2–5% plag (mph to 3 mm); <1% pyroxenes (nearly all mph, but rarely 0.5–1 mm); sparse 

oxide mph; also contains common plag-pyroxene-oxide clots, which account for most 

pyroxene crystals. Base not exposed. Overlain by units mms and dig (14±3 ka). 40Ar/39Ar 

age: 19±4 ka [NS, SS]

dss Dacite of Sister Spring (late Pleistocene)—Glacially scoured dacite (63.6–63.7% SiO2) lava 

flow at northwest toe of Middle Sister edifice, directly overlying coulee of the rhyolite of 

Obsidian Cliffs (unit roc). Depressions on barren plateau surface contain several ponds, 

including “Arrowhead Lake.” Thickest exposure is northwest scarp, 60 to 80 m high, from 

base of which Sister Spring emerges. Most outcrops are devitrified and very platy, though 

patchy remnants of vitrophyre remain on plateau surface. Phenocrysts: 5% plag (mph to 

2.5 mm); ~1% pyroxenes (mostly mph, rarely as big as 0.5 mm); sparse oxide mph; also 

contains common plag-pyroxene-oxide clots. Overlies unit mlb (~48 ka); also overlies unit 

roc (38±2 ka), draping several lithologic zones of rhyolite. Overlain by units awc (22±2 ka), 

mms, and mrg (20±6 ka). 40Ar/39Ar age: 25±3 ka [NS]

dsw Dacite domes southwest of Middle Sister (late Pleistocene)—Three small dacite (65.4–65.5% 

SiO2) lava domes, identical in composition, low on southwest slope of Middle Sister. Two 

western domes, which are contiguous, are 500 m and 400 m in diameter, and each has about 

90 m of maximum relief. Smaller dome, 500 m east, has only 50 m of relief and is 150 m 

wide. Line transecting three vents strikes N. 70° E., unlike any other such alignment in 

region. Although all were lightly scoured by ice, glassy carapace is widely preserved, varying from densely vitrophyric to micropumiceous. Outward-directed polygonal jointing and 

slender columns (locally horizontal) are retained in glassy exterior facies of all three domes, 

suggesting possibility of extrusion through ice. Limited exposures of devitrified, blockjointed to slabby felsite were found atop eastern and central domes. Phenocrysts: 7–10% 

plag (mph to 4.5 mm); ~1% pyroxenes (mostly mph, but sparsely 0.5–1.7 mm); sparse oxide 

mph; also contains common plag-pyroxene-oxide clots. Extruded through and onto Middle 

Sister apron of unit mms lavas. 40Ar/39Ar age: 17±15 ka [NS]

dtl Dacite of Todd Lake (middle Pleistocene)—Multiflow edifice of glaciated dacitic (62.6–65.0% 

SiO2) lavas at southeastern margin of map area, between Sparks and Todd Lakes, at south 
39

toe of Broken Top volcano. Edifice is elongate northeast, is 3.5 km long, covers about 4 km2, 

and has 475 m of relief on its southwest slope. Extensively till-mantled, eroded lavas flows 

are block jointed or slabby to platy. All or most flows are phenocryst rich. Phenocrysts: 

7–10% plag (0.5–3 mm); 1–2% each of opx, cpx, and Fe-Ti oxides. Petrographic summary 

given by Taylor (1978), who also determined normal paleomagnetic polarity for several lava 

flows and described plug just north of summit. Overlies unit mbl. Holocene unit mec banks 

against west toe of edifice, and (according to Taylor, 1978) middle Pleistocene mafic and 

andesitic lavas from Broken Top bank against north toe. Whole-rock K-Ar age of 460±30 ka 

was given by Hill (1991) [BT]

dwl Dacite west of Lost Creek Glacier (late Pleistocene)—Stairstep pair of thick dacite (63.1, 

64.6% SiO2) lava flows that form steep glaciated buttresses on west slope of South Sister, 

0.8 to 1.1 km west of upper Lost Creek Glacier. Lower flow forms dark-gray-weathering 

cliff, 40 m high, that has extensive exposures of flow-foliated devitrified lava that splits 

platy. Upper flow, more silicic, is as thick as 75 m and generally is glassier and irregularly 

jointed or block jointed, although devitrified slabby zones also are exposed. Both cliffforming flows have low-relief benches on top. Phenocrysts: 10–15% plag (mph to 4 mm); 

1–3% pyroxenes (mph to 1.5 mm); sparse oxide mph; both flows also contain abundant 

plag-pyroxene-oxide clots, which account for most pyroxene crystals; upper flow is rich in 

pale-gray, finely vesicular, fine-grained enclaves, mostly 1–2 cm, that contain mph of plag 

and pyroxene. Overlies unit alg (27±1 ka). Overlain by stack of numerous thin lavas of 

cone-building unit aeg (here, 27±3 ka). Undated [SS]

dwp Dacite of Wickiup Plain (late Pleistocene)—Glacially scoured dacite (64.0–64.5% SiO2) lava 

flow beneath southeastern margin of Rock Mesa (unit rrm). Exposed in several windows 

through thick pumice-fall deposit of unit rrm. More than 50 m of relief is exposed distally; 

35-m-high crag along flow margin at Wickiup Plain retains slender glassy columns, horizontal and inclined, that suggest ice-contact emplacement. Sparse scattered exposures along 

surface of largely pumice-buried flow are either vitrophyric and polygonally jointed or 

devitrified and slabby to platy. Flow continues south beneath unit rkb and crops out further 

at south toe of Kaleetan Butte, where it is likewise variously glassy or devitrified and locally 

oxidized. Phenocrysts: 10–15% plag (mph to 4.5 mm); 1–2% pyroxenes (mostly mph, rarely 

0.5–1 mm); sparse oxide mph; also contains common plag-pyroxene-oxide clots. Base not 

exposed. Overlain by units rct (24±5 ka), rkb, rrm, dml (33.6±1.2 ka), and dmw, and its 

snout is wrapped by postglacial unit mlc. Like adjacent dacite lavas, flow probably vented 

well up on South Sister edifice; if so, it would also extend north beneath unit dcg (31±3 ka), 

but lowland trough between them is covered entirely by Rock Mesa pumice deposits (unit 

rrm). 40Ar/39Ar age: 34±3 ka [SS]

mbc Basaltic andesite of Black Crater (late or middle Pleistocene)—Large mafic (55–57% SiO2) 

lava and scoria cone, situated mostly north of map area (fig. 2). Cone is 3 km in diameter, 

has 900 m of relief, and produced lava-flow apron that extends 4 to 5 km north and northeast 

of summit. Stratified proximal agglutinate, scoria, and fountain-fed lava flows dip radially 

away from central microdiorite plug. Lower slopes were glacially scoured, and two supposed “craters” on north and east are actually glacial cirques. Lavas exposed at north edge of 

map area are massive, less commonly vesicular, and glacially eroded into series of blockjointed or irregularly jointed ledges. Phenocrysts: 0.5–3% plag (0.5–1 mm); ~1% olivine 

(mph to 1 mm, rarely to 2.5 mm, locally in clusters); trace amounts of cpx. Overlain by unit 

mmc. Normal paleomagnetic polarity (Taylor, 1987). Undated [TCB]

mbl Basaltic andesite of Bare Lake (middle Pleistocene)—Sparsely to moderately porphyritic 

mafic (54.8–57.2% SiO2) lava flows and ejecta exposed in two areas beneath south slopes 

of Todd Lake dacitic edifice (unit dtl). Western exposure forms steep glaciated nose that 

has 140 m of relief; eastern exposure has 250 m of relief, much of it cliff forming. Consists 

of several ledgy flows, block jointed or slabby to thinly platy; thickest flow, which forms 

80-m-high southeast cliff above “Todd Creek,” has steep curviplanar joint sheets that 

disintegrate into slabby talus. Fissure vent, which is marked by stratified scoria and agglutinate, trends north for 400 m along ridgecrest northwest of Bare Lake. Roadcuts in western 

outcrop expose brick-red scoria-fall deposit, loose to variably agglutinated and 3 to 10 m 

thick, underlying lava ledge into which agglutinate grades; fallout consists mostly of lapilli 

but also includes bombs as big as 25 cm, suggesting likelihood of second vent (concealed) 
40

closer than fissure vent 2 km east. Phenocrysts: most flows have 1–2% plag (0.5–1 mm) and 

sparse olivine and cpx (each 0.1–0.4 mm); cliffy flow facing “Todd Creek” contains ~8% 

plag (1–4 mm), common 1 mm olivine, and sparse cpx. Lavas also carry abundant finegrained inclusions, 0.5–6 cm across, that contain opx, cpx, and plag phenocrysts and, thus, 

may be dacitic xenoliths. Tiny glaciated nubbin beside highway 100 m southeast of “Todd 

Creek” (UTM grid 051/744) is outlier of phenocryst-poor cpx-olivine-plag lava that carries same inclusions. Lavas bank against dome of pyroxene-plag rhyodacite (unit rtl; 69.5% 

SiO2) southwest of Todd Lake, according to Taylor (1978), who also reported that unit mbl

has normal paleomagnetic polarity. Overlain by unit dtl (460±30 ka; Hill, 1991); Holocene 

unit mec banks against toes of both exposures. Undated [BT]

mbt Basaltic andesite of Broken Top (middle Pleistocene)—Catch-all assemblage of varied and 

undifferentiated mafic lava flows, all thought to have erupted at or near Broken Top 

volcano. Generally mapped only around fringes of Broken Top apron, essentially to define 

boundary of present map area. Broken Top volcano has not yet been studied in detail, but 

its apron lavas, crater walls, and central intrusion are predominantly basaltic andesite. Our 

reconnaissance and that of Taylor (1978) and Webster (1992) indicate that andesite, dacite, 

and rhyodacite are subordinate and that edifice was built between roughly 300 ka and 

150 ka. Lavas lumped in this map unit range in texture from phenocryst poor to phenocryst rich and in composition from 52.5% to 57.8% SiO2 (n=28); all contain plag (seldom 

bigger than 3 mm) in excess of olivine (mph to 1 mm, rarely to 2 mm) and typically subordinate cpx (0.5–1 mm); sparse oxide mph are present in some samples, but not all. Older 

than most contiguous units described herein except for a few intercalated Broken Top 

andesite-dacite lavas east of Fall Creek that are assigned to comparable catch-all andesitic 

unit abt [BT, TCB]

mbw Basaltic andesite of Whychus Creek bridge (middle Pleistocene)—Four limited exposures 

of phenocryst-rich mafic (52.9% SiO2) lava flows, which crop out as windows through till 

in bed of Whychus Creek near bridge at 4,400 ft elevation on U.S. Forest Service Road 

1514 and on noses of ridges just southwest and northeast of there. Glaciated ledges are 

block jointed and massive or sparsely vesicular. Phenocrysts: ~20% plag (mph to 4 mm); 

3–5% olivine (mph to 2 mm); rare cpx. May have erupted at or near Broken Top. Base not 

exposed. Overlain by units awy (299±1 ka) and mwh (195±5 ka). Undated [TCB]

mcl Basaltic andesite of Camp Lake (late Pleistocene)—Apron of phenocryst-poor mafic 

(56.3–56.8% SiO2) lava flows that extends from its glacially eroded vent complex at about 

7,700 ft elevation on southeast flank of Middle Sister for 4 km eastward between North and 

South Forks, Whychus Creek. Vent area is ice-ravaged scoria-cone remnant (stippled on 

map) exposed in 600-m-wide window through lavas of unit mms. Scoria deposit is crudely 

stratified, mostly coarse ejecta but locally is well-bedded ash and lapilli. Bombs are as big 

as 1 m, and deformed spatter blobs are as long as 2 m and as thick as 30 cm. Deposit varies 

from loose to moderately agglutinated and is mostly brick red but locally black, and its 

exposed thickness is as great as 25 m; base is not exposed. Where capped by coeruptive lava 

flows or cut by dikes of unit mms or overlain by lavas of unit mms, scoria deposit is further 

oxidized and fused near contacts. Central lava mass, 60 by 100 m wide and 15 to 20 m thick, 

is massive, largely devitrified, and block jointed, except that its basal 2 to 3 m is platy. This 

platy jointing dips radially inward, as does bedding in subjacent scoria. If it is manifestation 

of plug, effusing lava must have flared radially outward, such that all its contacts dip moderately, and no steep contacts or steep flow foliation are exposed. Outflow lavas are eroded 

into stairstep benches, cliffs, and pond-filled hollows. Many exposures are 60 to 90 m thick, 

and distal cliff at South Fork Whychus Creek (UTM grid 022/882) is 120 m high; unit consists of several flows, individually 10 to 30 m thick. Black, glassy, chunk-jointed zones are 

exposed locally, but most outcrops are devitrified and block jointed to platy. Phenocrysts: 

1–3% plag (mph to 1 mm); ≤1% olivine (mph, rarely 0.5–1 mm); very sparse oxide mph; 

many plag phenocrysts are in clusters, which less commonly also include olivine and oxide 

mph. Groundmass typically is rich in plag microlites, which are in some samples seriate 

to plag mph. Overlies units adl (24±1 ka), mdl, mnf, and msf; fills relief cut into unit dcl

(26±2 ka) just northeast of largest of Chambers Lakes (UTM grid 983/868); banks against 

toe of unit rsf coulee (35.8±2 ka). Overlain by units mms′ and mms; cut by dikes of unit 

mms. 40Ar/39Ar ages: 16±10 ka, 23±16 ka [NS, TCB]
41

mdl Basaltic andesite north and south of Demaris Lake (middle Pleistocene)—Glacially eroded, 

cpx-bearing mafic (53.0–53.6% SiO2) lava flow exposed in two windows through younger 

lavas, each about 500 m away (to south and to north) from Demaris Lake. Compositionally 

similar but not identical, they may be different flows. Southern outcrop (53.0% SiO2) is 

single flow more than 60 m thick, block jointed to slabby, that forms east-facing cliff. 

Northern outcrop, which is more subdued, has ice-scoured, block-jointed ribs of modest 

relief. Phenocrysts: 7–10% plag (mph to 2.5 mm); ~1% olivine (mph to 1.5 mm); sparse 

1 mm cpx, found principally within three-phase clots; clusters of plag crystals also 

common. Base not exposed but probably lies atop nearby unit msf, which is here extensively till covered. Overlain by units mcl, adl, and rsf. 40Ar/39Ar age: 181±21 ka (southern 

window) [TCB]

mdw Basaltic andesite west of Dutchman Flat (late or middle Pleistocene)—Glaciated mafic 

(56.8% SiO2) scoria cone, forming Hill 6485 at north edge of Mount Bachelor Ski Area carpark. Cone is 75 m high and 350 by 500 m wide, and it consists of stratified brick-red scoria 

and agglutinate, mantled with glacial deposits. Phenocrysts: common plag; sparse olivine. 

Wrapped on west by early Holocene lava flows of unit mec and on east by postglacial lavas 

of Mount Bachelor (Scott and Gardner, 1992). Undated [BT]

mec Basaltic andesite and basalt of “Egan Cone” (early Holocene)—Postglacial basaltic 

(50.7–52.2% SiO2) scoria cone (stippled on map) at southeast edge of map area (at north toe 

of Mount Bachelor) and extensive apron of mafic (51.2–54.1% SiO2) lava flows that extends 

2 km north and 4.5 km west and northwest to Sparks Lake (which it helped to impound). 

Flows are rugged, scoriaceous, rubbly, commonly leveed, and little eroded. Breached scoria 

cone, which is 800 m wide and 200 m high, consists of brick-red scoria lapilli and bombs. 

Phenocrysts: almost none; common olivine and plag mph (0.1–0.5 mm); uncommon mph 

clusters of olivine or olivine+plag; rare plag megacrysts (as big as 6 mm and typically shattered). Groundmass rich in plag-lath mph (rarely as long as 0.7 mm), seriate to microlites; 

plag microlites appear to be increasingly abundant distally. Overlies unit bbr and postglacial 

lavas of Mount Bachelor; banks against units dtl, mbl, mdw, mts, and rtl. Primary scoriafall deposit just east of “Egan Cone” is virtually unweathered where overlain directly by 

Mazama ash (7.7 ka), suggesting that unit is only slightly older (Scott and Gardner, 1992). 

Also overlain by late Holocene pumice falls of units rrm and rdc. Equivalent to unit “mb6” 

of Scott and Gardner (1992), youngest unit of entire 25-km-long Mount Bachelor volcanic 

chain. Undated [BT]

mey Basaltic andesite east of Yapoah Lake (late Pleistocene)—Apron of mafic (52.8–53.2% SiO2, 

at 1.3% TiO2 and 0.85% K2O) lava flows on north-northeast flank of North Sister that carry 

conspicuous clusters of olivine and plag mph. Glaciated flows crop out widely between 

6,000 and 5,400 ft elevation between Yapoah Lake and Alder Creek. Source is probably 

North Sister or unidentified vent on its north flank. Ice-scoured ledges, ribs, and knobs 

are mostly devitrified and block jointed or slabby, but zones that remain partly glassy crop 

out locally. Phenocrysts: true phenocrysts are sparse, but clusters of olivine mph and clots 

of intergrown plag and olivine mph are abundant and diagnostic of unit; most clots are 

0.5–1.5 mm across and rarely as big as 5 mm; altogether contains 3–5% olivine (0.2–2 mm) 

and <1% plag phenocrysts (0.5–1 mm), but abundant plag mph are seriate to groundmass 

microlites. Overlies units myl and bac (128±7 ka) and either overlies or intercalates with 

some clot-poor older North Sister lavas (unit mns). Overlain by units mfy, mwy, mys, and 

younger lavas of unit mns. Unit appears to be sequence of North Sister flows that represent subtly distinguishable variant intercalated within dominant sequences of unit mns

lavas (which are chemically slightly different at 53–55% SiO2, 1.0% TiO2, and 0.7% K2O). 

Undated [NS, TCB]

mfy Basaltic andesite of fissure vent south of Yapoah Lake (late Pleistocene)—Glaciated ridge of 

fissure-fed, brick-red mafic (56.2–57.6% SiO2) scoria lapilli and bombs, just north of Knob 

6435 and about 1 km south of Yapoah Lake. Scoria ridge is 400 m long, trends N. 10° to 

20° E., has 40 to 60 m of relief, is strewn with varied erratics, and is surrounded by glacially 

eroded remnants of apron of comagmatic fountain-fed lavas. Scoria exposures generally are 

nonindurated, not agglutinated; lavas are massive, block jointed, and eroded into bold ledges 

that shed coarse talus. Phenocrysts: almost none; rare plag (0.5–1.5 mm); rarer olivine (<1 

mm). Overlies units mey, mns, and myl. Overlain by unit mwa. Undated [NS]
42

mhd Basaltic andesite of Hayden Glacier–Diller Glacier cleaver (late Pleistocene)—Stack of 

several mafic (54.5–56.2% SiO2) lava flows and intercalated coarse scoria falls that make 

up prowlike cleaver between Diller and Hayden Glaciers below east face of Middle Sister. 

About seven flows, ranging in thickness from 5 to 30 m, form cliffy, 150-m-high section at 

head of cleaver. Scoria-fall intervals as thick as 10 m include spindle bombs. Lower flows 

dip west (as if they originally banked against ice or were later rotated?), whereas upper flows 

dip 15° east and project across ice-filled saddle into stack of about 15 ochre-weathering 

lava flows, which make up middle of three main sequences that constitute steep east face of 

Middle Sister. Lithology varies widely from vitrophyric and vesicular to massive devitrified 

interior facies, commonly slabby or block jointed. Phenocryst contents vary widely among 

flows: 7–15% plag (mph to 1.5 mm, rarely as big as 3 mm); 1–3% olivine (mph to 1 mm, 

rarely to 2 mm), some having spinel inclusions; rare to common oxide mph; most flows also 

contain sparse cpx (mph to as big as 0.7 mm). Cleaver is surrounded by till and ice, so base 

of stack is not exposed. On face of Middle Sister, stack of flows overlies stratified tephra fall 

of unit aef and underlies cone-forming stack of mms lavas. 40Ar/39Ar age: 37±9 ka, for thick 

flow at east end of cleaver [NS]

mhl Basaltic andesite of headwaters of Linton Creek (late Pleistocene)—Small (300 by 200 m) 

window of glacially scoured, moderately porphyritic mafic (54.6, 55.7% SiO2) lava flows at 

head of middle fork of Linton Creek (UTM grid 949/876). Ledgy exposure at 6,550 to 6,750 

ft elevation on west slope of Middle Sister is set of four flows, each 2 to 6 m thick, that have 

normal lithologic zoning, from rubbly bases to massive devitrified block-jointed interiors 

sandwiched by vitrophyric (commonly vesicular) tops and bottoms. Phenocrysts: ~5% plag 

(mph to 1.5 mm); <1% cpx (mostly mph, rarely as large as 0.7 mm); traces of olivine and 

oxide mph; also contains common crystal clots, either all plag, plag-cpx, or plag-cpx-oxide. 

Overlies unit dlc. Draped by unit ahl (21±3 ka) and by great apron of unit mms that largely 

conceals it. Chemically identical to lavas of unit mhd, which likewise is directly overlain by 

unit mms lavas on opposite side of Middle Sister edifice. Undated but probably older than 

unit alc (27.2±1 ka) [NS]

mhr Basaltic andesite of The House Rock (late Pleistocene)—Small, asymmetrical mafic 

(53.3–53.5% SiO2) shield west of Wickiup Plain, built on south flank of much older larger 

shield, The Wife. Glaciated remnants of scoria cone and small plug (not mapped) crop out 

atop shield; lava-flow apron extends 2 km southward. Phenocrysts: 5–7% plag (0.5–2 mm); 

3–5% olivine (mostly mph, rarely as big as 1.5 mm); sparse oxide mph. Groundmass rich in 

plag-lath mph. Northwest contact only approximate where lavas overlie unit mtw; adjacent 

lava-flow apron of glaciated Koosah Mountain shield (unit mkm) appears to be younger; 

postglacial lava flows of unit mlc bank against The House Rock. 40Ar/39Ar age: 67±4 ka 

[SS]

mkm Basaltic andesite of Koosah Mountain (late Pleistocene)—Small mafic (52.6–53.3% SiO2) 

shield volcano centered 2 km west of Kokostick Butte, at southwest edge of map area. From 

vent complex marked by scoria cone (stippled on map) atop Koosah Mountain, apron lavas 

extend at least 1.5 km northward and at least 4 km southward beyond edge of map area. 

Everywhere glaciated, lava flows crop out as ice-smoothed ribs and steps, widely massive, 

block jointed, and well exposed. Unit forms undulating bedrock surface of ice-scoured basin 

that contains 25 or more lakes of Sisters Mirror Lake cluster. Scoria cone that caps shield 

is about 150 m high and 1.2 km long, elongate north-south, and is marked by fissure vent 

along its crest; ejecta near summit are generally oxidized and widely agglutinated, ranging from tack-welded through densely blobby to fully homogenized sheets of lava. Bombs 

along crest are commonly as big as 50 cm, rarely bigger than 1 m, and mostly brick red. 

Phenocrysts: 3–5% olivine (0.2–2 mm), commonly in clusters; plag sparse (0.5–1 mm). 

Groundmass is choked with plag laths, seriate from microlites to mph, rarely as long as 0.5 

mm. Apron lavas appear to bank against unit mhr (67±4 ka); they are intruded by domes 

of unit dkb (27±3 ka) and overlain by lava apron of postglacial unit mlc. Distal lava flows 

near and southeast of highway are likewise glaciated but slightly more evolved, containing 

54–55% SiO2, less olivine, and slightly more plag than most of unit; although here assigned 

to this unit, they could represent a sequence of older flows. Undated [SS]

mlb Basaltic andesite of Little Brother (late Pleistocene)—Glacially sculpted mafic (52.2–53.1% 

SiO2; n=10) edifice, about 2 km in diameter, at northwest toe of North Sister, as well as 
43

apron of numerous lava flows that extends 6 km west to Obsidian Creek and 6 km northwest 

to Frog Camp Creek (outside mapped area). Steep edifice has more than 300 m of proximal 

relief and consists of agglutinate, scoria-fall layers, and hundreds of thin lava flows, many 

of them fountain fed and only 1 to 5 m thick. On west ridge of edifice, as many as 30 such 

flows are exposed in given sections. A few late flows are 5 to 15 m thick. Near summit, 

many fragmental deposits, both loose and agglutinated, are oxidized brick red. On slopes, 

many scoria layers are black, although colorfully palagonitized locally. Several comagmatic 

dikes and small plug cut edifice strata, principally on north and northeast sides. Apron lavas 

tend to be thicker, 5 to 15 m, but are poorly exposed, glaciated, and till strewn; they commonly are glassy, or partly so, and vesicular, although massive devitrified interior zones 

are exposed in thicker flows. Window at 4,600 to 4,800 ft elevation along lower canyon of 

Obsidian Creek is mostly devitrified, massive, and slabby. Phenocrysts: 1–5% plag (0.5–1.5 

mm), seriate to as much as 15% plag-lath mph in some samples; 3–5% olivine (mph to 1 

mm, rarely as big as 2.5 mm); most samples have abundant olivine-plag clots and fewer 

all-plag clots, both types limited largely to mph. Schmidt (2005) listed chemical data for 

30 additional samples, which range in SiO2 content from 52.6 to 53.9%. Appears to overlie youngest (valley-floor) flows of unit mns near snout of Collier Glacier (UTM grid 

966/9175; there, 46.2±5.6 ka). Overlain by units acc, alc, awc, mmm, msb, dss, roc (38±2 

ka), and aoc (45–49 ka). Preserved locally atop west ridge of Little Brother edifice are remnants of two sets of stratified fallout, each about 50 cm thick, that are predominantly pumice 

lapilli and lithic fragments (both of aphyric rhyolite) probably related to unit roc (Sherrod 

and others, 2004). 40Ar/39Ar ages: 47.2± 5.8 ka (medial), 48.4±3.6 ka (distal) [LL, NS]

mlc Basaltic andesite of Le Conte Crater (early Holocene or latest Pleistocene)—Postglacial 

scoria cone (stippled on map) at southern margin of Rock Mesa and associated aprons of 

mafic (54.5–55.9% SiO2) lava flows that extend 3 to 4 km northwest to Mesa Creek and 6 

to 7 km south in tongues that bifurcate around Kokostick Butte (unit dkb). Cone is 120 m 

high, is 600 m in diameter, and has shallow 200-m-wide circular summit crater. Cone has no 

exposure, as it is heavily mantled with late Holocene pumice-fall deposit of unit rrm; where 

exposed through fallout, lava-flow surfaces are rubbly, scoriaceous, and virtually uneroded, 

having many ridges, hillocks, and blocky levees. Flows are not incised but ubiquitously 

have 2 to 10 m of local relief. Southeast tongue impounds Devils Lake. Phenocrysts: 3–5% 

olivine (mph to 1 mm, very rarely to 3 mm), some in clusters; virtually no truly phenocrystic plag. Groundmass commonly is rich in plag-lath mph (<0.5 mm long); plag antecrysts 

(sieved, partly resorbed, and 0.5–1.8 mm) are sparse but widespread. Clark (1983) reported 

resorbed quartz xenocrysts. Overlain by unit rrm and Mazama ash (7.7 ka). Younger than all 

other contiguous units, including slump mass from Katsuk Butte. Undated [SS]

mls Basaltic andesite of Linton Spring (late or middle Pleistocene)—Cliff-forming, phenocrystpoor mafic (55.1% SiO2) lava flow adjacent to Linton Meadows, 4 km southwest of Middle 

Sister summit. Exposed as west-facing scarp for about 700 m, glaciated flow is more than 

50 m thick. Voluminous Linton Spring (discharge roughly 2,000 l/s) gushes from numerous 

orifices in flow-breccia zone that makes up lower 20 m of outcrop. Most of unit is dark gray, 

chunk jointed, and glassy, aphanitic, or finely crystalline. Thickness, jointing, and texture 

suggest ice-contact emplacement. Phenocrysts: 1–2% plag (0.5–3 mm, mostly ~1 mm); 

<<1% olivine (mostly mph, rarely 1–2 mm). Base not exposed, but unit probably overlies 

unit mth. Overlain by unit mms. Undated [NS]

mmc Basaltic andesite of Millican Crater (late Pleistocene)—Mafic (53–54% SiO2) scoria cone and 

associated lava-flow apron at north-central edge of map area. Fragmental cone has 250 m of 

relief, is 1,200 m wide, and banks against Black Crater edifice; its 2 by 3 km apron extends 

mostly to south and east. Glaciated apron consists of numerous block-jointed lava flows 

that are meagerly exposed but support several peripheral ledges 5 to 12 m high. Upper part 

of cone consists of radially dipping stratified scoria, extensively oxidized brick red, locally 

agglutinated, but only modestly eroded, probably having stood higher than regional glacial 

surface. Subhorizontal layers of scoria and agglutinate near summit are fallout that filled 

transient crater. Phenocrysts: sparse plag (0.5–1 mm); only tiny olivine (≤0.5 mm); sparse 

oxide mph; sparse olivine-plag clots; some lava flows have no phenocrysts (≥0.5 mm). 

Overlies unit mbc. Overlain by units bwm and mnt (20±5 ka). Normal paleomagnetic polarity (Taylor, 1987). Undated [NS, TCB]
44

mmi Basaltic andesite intrusion of Middle Sister (late Pleistocene)—Vertical mafic (52.8, 53.3% 

SiO2) intrusive mass exposed on northeast face of Middle Sister for about 200 m from 

Hayden Glacier nearly to ridgecrest just north of summit. Flares upward from thick dike 

below to as wide as 40 m at top. Probably one major conduit for later parts of 225-m-thick 

pile of (compositionally similar) thin rubbly flows (unit mms) that built top part of Middle 

Sister edifice. Porphyritic, having medium-grained groundmass. Phenocrysts: 30–40% plag 

(mph to 4 mm); 5% olivine (mph to 1.5 mm, rarely 3 mm), much of it altered; abundant 

oxides (0.1–0.2 mm) limited to groundmass. Cuts stacks of lava flows of units mms and 

mhd, and probably underlying tephra-fall deposit of unit aef as well. 40Ar/39Ar age: 18±4 ka 

[NS]

mml Basaltic andesite of South Matthieu Lake (late Pleistocene)—North-trending set of three separate glaciated windows of moderately porphyritic mafic (54.4–55.6% SiO2) lava exposed 

along contacts between units mpn, mps, and postglacial lava flows from Yapoah Crater 

(unit myc). Ice-scoured ledges and knobs are massive, slabby to block jointed, or locally 

partly glassy and chunk jointed. Phenocrysts: 2–3% plag (0.5–1.5 mm, rarely to 3 mm); <1% 

olivine (0.2–1 mm); rare cpx mph; olivine commonly in clusters; olivine-plag intergrowths 

probably antecrystic. Overlies unit mps. Overlain by units mmn, mpn, and myc. Undated 

[NS]

mmm Basaltic andesite of Montague Memorial (late Pleistocene)—Single thick phenocryst-rich 

mafic (56.3–57.3% SiO2) lava flow that extends about 2.5 km northwest from toe of Middle 

Sister edifice, as narrow tongue that is banked against southern margin of Obsidian Cliffs 

coulee (unit roc). Makes up narrow strip of glaciated knobs that may have been emplaced 

along margin of glacier during last major glacial episode. In periphery of southeastern 

knob (most proximal remnant exposed), unit was wholly removed by glacial erosion. Vent 

apparently was farther southeast, higher on Middle Sister, and is now concealed by younger 

units. Internal zones are devitrified and block jointed or slabby. Black vitrophyre and partly 

glassy zones are locally exposed, and scoriaceous oxidized rubbly base (atop unit aoc) crops 

out in several places. Small remnant of second flow unit caps largest (central) Knob 6543. 

Phenocrysts: 15–20% plag (mph to 2.5 mm, rarely as big as 4 mm); 3–5% pyroxenes (mph 

to 2 mm, rarely as long as 3.5 mm); sparse oxide mph. Overlies units aoc, mlb, and roc

(37.8±1.8 ka). Overlain only by till, locally. 40Ar/39Ar age: 19±7 ka [NS]

mmn Basaltic andesite of North Matthieu Lake (late Pleistocene)—Phenocryst-poor mafic (54.6% 

SiO2) lava flow that forms north-facing scarp 75 m high and ice-scoured bench 600 m wide 

that supports North Matthieu Lake. Knobby bench lava is massive, locally vesicular, and 

block jointed to slabby. Phenocrysts: 1–2% plag (0.5–1.5 mm, rarely to 3 mm); sparser 

olivine (mostly mph). Overlies unit mml. Overlain by unit mpn and by postglacial lava flow 

from Yapoah Crater (unit myc). Undated [NS]

mms Basaltic andesite of Middle Sister (late Pleistocene)—Shingled stacks of thin phenocryst-rich 

mafic lava flows and intercalated blocky rubble that constitute most of upper half of Middle 

Sister cone and mantle much of its south and west apron. Most flows contain 52.2–52.9% 

SiO2 (n=25), but some late flows and dikes contain 53.3–54.1% SiO2. Unit is 225 m thick on 

east face of edifice. Apron extends continuously 4 km south and west, and patchy remnants 

survive as far as 2 km north, 3 km northwest, and 5 km east of summit. Altogether, unit consists of hundreds of flows, many only 1 to 2 m thick; a few medial sections expose as many 

as 40. East face and lower parts of aprons are glacially excavated, but steep upper slopes of 

cone on south and west are not. Least modified surfaces remain scoriaceous and rubbly to 

blocky. Eroded flows form stairstep ledges that dip radially and tend to thicken downslope. 

Where 1 to 2 m thick, most flows remain partly glassy; where 3 to 7 m thick, most have 

devitrified internal zones (1–5 m thick) that are typically massive or slightly vesicular and 

block jointed. Oxidized, scoriaceous rubble zones are widely exposed between flows. A few 

remnants survive of distal flows that ponded as thickly as 10 to 15 m, perhaps along glacier 

margins. Several dikes, 1 to 2 m thick, are exposed on west slope and east face of Middle 

Sister; compositionally similar to lavas they cut, they supplied later increments to accumulating pile of flows; similar dikes also cut unit mcl on southeast slope. Summit area consists predominantly of ejecta, poorly or crudely stratified and having variably agglutinated 

stringers that grade outward into thin, ledge-forming, fountain-fed lava flows. Summit ridge 

is mantled locally by units dhr and dms and by one tiny remnant of unit mss (UTM grid 
45

9736/8893; about 80 m north of high point at 10,047 ft elevation). Summit ridge is remnant 

of west rim of former vent complex sited slightly eastward that was subsequently removed 

by glacial excavation of east face of edifice; upward-flaring intrusive unit mmi is remnant or 

offshoot of feeder system. Fumarolic alteration of fragmental summit material, now orange, 

yellow, ochre, or brick red, and locally coated by white films on joint faces, extends about 

200 m north and south of true summit and to depth of at least 30 m as exposed on east face. 

Phenocrysts: 25–35% plag (mph to 4 mm, equant or elongate); 5–10% olivine (mph to 2.5 

mm), commonly in clusters and some with spinel inclusions. Few plag crystals are sieved 

except in near-vent samples from summit. Overlies units adl, ahl, alc, alg, anh (21±6 ka), 

aoc, asw (24.5±4.2 ka), awc, awm, dcl (26±2 ka), dlp, dsn (19±4 ka), dss (24.5±3.1 ka), 

mcl, mhl, mls, mnf, mrg (19.6±6.1 ka), msc (21.4±5.5 ka), and mth. On west slope of 

Middle Sister, several flows bank against previously eroded vent-filling extrusion of unit 

dlp (21.4±1.9 ka). Overlain by units dbh (18±2 ka), dhr (19±2 ka), dig (14±3 ka), dms, 

and mss; intruded and overlain by domes of unit dsw (17±15 ka). On medial cleaver of 

Collier Glacier, stack of five thin flows is sandwiched between lavas of unit awc (22±2 ka) 

below and dbh (18±2 ka) above. 40Ar/39Ar age: 16.4±11.7 ka, for sample from true summit 

of Middle Sister (0.79% K2O). Ages of so many enclosing units bracket eruptive interval to 

about 20 ka; statistical propagation of ages of bracketing units yields 20.3±2.5 ka [NS, SS, 

TCB] 

mms' Pyroxene-bearing variant of the basaltic andesite of Middle Sister (late Pleistocene)—

Moderately porphyritic lava flows at or near top of sequence of flows of unit mms, distinguished by much lower total phenocryst content, presence of sparse cpx, and slightly more 

evolved bulk composition (54.5–54.9% SiO2). Directly atop of, or intercalated near top with, 

stack of lavas of unit mms, this variant is presumed to be comagmatic with unit mms and 

to have been more extensive prior to glacial erosion. Recognized in two areas on opposite 

sides of Middle Sister: (1) banked against and lapped onto vent intrusion of unit dlp at 7,850 

to 8,200 ft elevation on west slope, and (2) on east apron at 6,600 to 6,800 ft elevation, close 

to North Fork Whychus Creek. Steep western flows are 2 to 4 m thick, vesicular, rubbly, 

and partly glassy; on more gently sloping eastern apron, flows are 5 to 8 m thick, glacially 

scoured, massive, devitrified, and block jointed to slabby. Phenocrysts: 5–10% plag (mph to 

3 mm); ~1% olivine (mph to 2 mm); sparse cpx (mostly mph, rarely 0.5–2 mm); trace oxide 

mph. Erupted at or near end of eruptive sequence of unit mms. Overlies units awm, dlp, and 

mcl. Undated, but same age as unit mms (about 20 ka) [NS, TCB]

mmt Basaltic andesite tuff of Middle Sister (late Pleistocene)—Discontinuous patches of poorly 

sorted mafic (51.7–55.2% SiO2; n=3) tuff (samples of bulk tuff were picked free of lithic 

fragments), exposed in hollows on glacially scoured surface of multiflow lava apron of unit 

mms along west toe of Middle Sister. Originally intercalated between thin lava flows that 

were variably stripped by glacial erosion, abundant remnants of tuff suggest formerly continuous flow fan that extended from Separation Creek northward for 3.5 km to Lane Plateau. 

Exposures are limited to apron of modest relief below break in slope of steep cone of Middle 

Sister, ranging in elevation from 6,850 ft down to 6,000 ft near Linton Meadows. Flows may 

have originally extended farther down Linton Creek, as sandy postglacial alluvium there 

appears to be in considerable part derived from this tuff. Exposures are on walls of drainages 

or form broad ice-scoured whalebacks; tuff is moderately to poorly indurated, medium gray 

(weathering pale gray or tan when dry), and either massive or crudely stratified. Locally 

developed internal shear and crude size sorting produced vague partings that result in slabby 

splitting and ledgy outcrops that erode into ribs 5 to 50 cm thick. Base is rarely exposed, 

but observed thicknesses of remnants range from 4 to 25 m. Juvenile clasts are phenocrystrich black scoriae, poorly to strongly vesicular, mostly 1 to 10 cm (but as big as 20 cm), and 

identical to material in lavas of enclosing unit mms. Matrix is predominantly crystal-rich 

coarse ash, which supports most clasts and commonly is vesicular. Accidental lithic clasts, 

rare or absent in higher exposures, are common distally, apparently entrained downslope; 

most are andesite-dacite lavas, and fewer are basalts; most clasts are smaller than 5 cm, but 

some are as large as 35 cm. Absence of welding makes mafic pyroclastic-flow emplacement 

process unlikely. Mud-lined vesicles in gritty matrix suggest dewatering of granular material 

and transport of sparse fines. Lack of wood fragments in deposit probably reflects barren 

edifice in late Pleistocene; upper slopes of Middle Sister remain barren today. Deposit can 
46

be interpreted as product of one or many primary lahars, presumably initiated by modest 

scoria showers or surges in summit region during eruptive interval that also produced lava 

flows of unit mms. Falling on snow or ice, hot pyroclasts would mix with meltwater and 

pour rapidly downslope, probably bulking by incorporation of loose ash and scoria during 

transit (but picking up accidental lithic fragments only distally, where units older than mms

were exposed). Still today, upper slopes of Middle Sister remain snow covered for 9 to 10 

months a year. Tuff has not been observed east of edifice, where apron is deeply eroded 

and extensively till covered. Phenocrysts: same as unit mms. In thin section, indurated tuff 

matrix (exclusive of basaltic lapilli) typically consists of ~25% glass shards (0.2–2 mm), 

5–10% plag (mph to 2 mm), and 3–5% olivine (mph to 1 mm) in finely comminuted matrix 

of same constituents. Overlies units alc and msc. Undated (but same as unit mms, about 20 

ka) [NS, SS]

mnf Basaltic andesite of North Fork Whychus Creek (late Pleistocene)—Glaciated, moderately 

porphyritic, mafic (53.6–54.4% SiO2) lava flows exposed through till in several windows 

along, and north of, North Fork Whychus Creek between 6,200 and 7,200 ft elevation. 

Crops out along stream gorge, where set of four flows, each 1 to 3 m thick, is well exposed 

(UTM grid 012/8885). Conformably beneath them, fifth flow is compositionally similar but 

phenocryst richer and more than 10 m thick, its base not exposed. For 400 m north of North 

Fork gorge, unit crops out through till in isolated patches as ledges and gully walls, mostly 

devitrified. Largest exposure is 50-m-thick stack of 10 flows (each 2 to 7 m thick) that forms 

steep till-enveloped buttress (UTM grid 995/883) at 7,000 to 7,200 ft elevation near head of 

North Fork. Most lavas there are partly glassy and vesicular, but thicker ones expose devitrified massive block-jointed interiors. Phenocrysts: 5–10% plag (0.5–2.5 mm); 1–3% olivine 

(mph to 1.5 mm); ±sparse cpx (≤1 mm). In some coarsely crystalline samples, plag mph 

(0.1–0.4 mm) make up 30–40% and olivine mph as much as 10% of thin sections. Probably 

erupted at Middle Sister (prior to eruption of major summit-capping unit mms). Base not 

exposed. Overlain by units adl (24±1 ka), mcl, and mms. Chemically identical to lavas of 

unit mnh (2 km northwest). 40Ar/39Ar age: 48±10 ka [NS, TCB]

mnh Basaltic andesite north of Hayden Glacier (late Pleistocene)—Stack of thin, moderately porphyritic mafic lava flows that crops out through till as two ledges, each less than 100 m long, 

along north edge of Hayden Glacier. Upper ledge at 8,450 to 8,500 ft elevation exposes two 

flows, each partly glassy and 1 to 2 m thick (UTM grid 9793/8978). Lower ledge at 8,300 

to 8,350 ft elevation exposes five flows (UTM grid 9806/8963), among which first four are 

similarly thin and compositionally like those of upper ledge (all 53.4–54.4% SiO2); all have 

massive devitrified interior zones (1 m thick or less) enclosed by glassier, more vesicular 

tops and bottoms. Top flow of lower ledge is different, but it is not possible to represent it 

separately at scale of map; although conformable with those below, it is 7 to 10 m thick, has 

57.1% SiO2, and (in common with fourth flow) contains fewer crystals. Phenocrysts (most 

flows): 7–12% plag (mph to 2 mm); 1–2% olivine (mph to 1 mm); ±subordinate cpx. Top 

two flows of lower ledge contain only 2–3% plag (mph to 1 mm); ≤1% olivine (mph to 1 

mm); ~1% cpx (0.5–1 mm); in other flows, cpx is sparse or absent. All flows contain sparse 

oxide mph and clots of all minerals, in varied combinations. Although contacts are everywhere obscured by till, unit probably underlies nearby units mms and dbh; it certainly is 

younger than nearby unit mns. All flows but top one are chemically similar to flows of unit 

mnf, about 2 km southeast. 40Ar/39Ar age: 21±6 ka (for top flow of lower ledge) [NS]

mns Basaltic andesite of North Sister (late and middle Pleistocene)—Glacially ravaged mafic stratocone (52.5–55.1% SiO2; n=140; see also Schmidt, 2005) on Cascade Range axis just north 

of Middle Sister and 10 km south of McKenzie Pass. Steep, asymmetrically preserved edifice, which is 2 by 3 km across and has about 1,000 m of relief, consists of hundreds of thin 

rubbly lava flows that dip radially and steeply, are intercalated with proximal agglutinate and 

scoria falls, and are cut by 200 or more thin dikes (Schmidt and Grunder, 2009). As many as 

70 such lava flows are exposed in individual sections on walls of several ridges and cirques. 

Gently dipping aprons of thicker lava flows extend from edifice as far as 7 km east and 9 km 

northeast of summit. South and west of cone, any such apron is concealed by younger units. 

Half or more of cone volume has been glacially excavated and redeposited as till, largely 

to east and northeast. Most flows on edifice are fountain fed, 1 to 5 m thick, extensively 

vesicular (though many have massive internal zones), and typically separated by 1 to 5 m 
47

of oxidized rubbly flow breccia, scoria-fall deposits, and lag breccias left by block-and-ash 

flows. Exposed on steep faces within stacks of such thin rubbly flows are a few massive 

lavas 10 to 20 m thick that may be either extrusive flows or thick sills. Northwest ridge is 

capped by a 200-m-thick stack of flows that have massive zones 10 to 20 m thick, thicker 

on average than elsewhere on cone. Remnants of two massive lavas (each ~54% SiO2) cap 

summit and help retard edifice destruction: (1) Southern one (“Prouty Pinnacle”) forms 

north-elongate wall 200 m long and 60 m high that appears to be plug that flares out radially over brick-red scoriaceous rubble; saddle on its crest separates two prongs, northern of 

which is true summit. (2) Other massive lava (“Glisan Pinnacle”) lies 100 m northeast across 

deeper saddle and consists of 100-m-wide remnant of single flow about 35 m thick that dips 

to northeast away from plug, forms vertically jointed cliffs, and is capped by 10-m-thick 

agglutinate remnant.

North Sister was subdivided into several unconformable sets of flows by Schmidt 

(2005), who listed chemical data for 114 samples (including 38 dikes). Transition from 

myriad thin rubbly flows below to generally thicker and fewer flows higher on edifice is at 

elevations of about 9,300 ft on north face and about 9,200 ft on east face. Modest unconformities between sets of thin flows are common everywhere on edifice, but one profound 

unconformity crops out at about 8,750 ft elevation on east ridge, separating east-dipping 

flows below from west-dipping ones above; this suggests either shift in vent location, rim 

of filled paleocrater, or interval of severe erosion not observed elsewhere. Northeast face 

exposes higher proportion of intercalated brick-red scoria falls than elsewhere on edifice. 

West face is almost dipslope, slightly bevelling steeply dipping stack of thin brecciated 

flows and layers of brick-red scoria, all cut by numerous dikes. 

Two parts of edifice (stippled on map) are pervasively altered to yellow-orange 

palagonite: entire east buttress, and 150-m-thick section low on north face. Both sections 

are riddled by numerous dikes (not shown on map). Buttress section, 250 m thick, consists 

dominantly of ash-rich, poorly sorted, fragmental-flow deposits, in thin to thick layers that 

dip 25° to 30° E. and display numerous scoured, modestly unconformable contacts. Northface section consists of ash-rich phreatomagmatic fallout, in layers 5 to 200 cm thick, some 

sloughed but mostly primary, many well sorted but others less so. In both sections, altered 

ashy matrix contains unaltered black or brick-red scoriae (mostly lapilli and small bombs) 

and angular fragments of dense gray lava 5 to 30 cm across. Lack of palagonitization of 

coarse clasts implies grain-size control on alteration, largely restricted to glassy ash-grade 

material; same can be inferred from unaltered condition of brick-red, ash-poor layers of 

scoria lapilli that intimately underlie, overlie, and interfinger with yellow palagonitized 

sections. Elsewhere on North Sister edifice, isolated ashy yellow layers within thick stratified sections of gray lava and red scoria confirm that alteration was controlled principally by 

grain size rather than depositional environment. Regularity of stratification in palagonitized 

sections and absence of convolution and slumping weighs against ice-contact emplacement.

Off cone proper, northeast planèze exposes about 10 lava flows, which have massive 

zones 2 to 20 m thick, are separated by thin rubbly layers, and are cut proximally by several 

dikes. Outflow lavas are, on average, thicker than most on edifice, commonly as thick as 

10 to 20 m and typically are devitrified and block jointed or platy; they are discontinuously 

exposed through glacial deposits on forested peripheral apron. North of Pole Creek Spring, 

2 km2

 driftless window was not overrun by ice and uniquely retains near-primary blocky 

scoriaceous surface. 

Although numerous flows examined vary significantly in texture and in phenocryst 

content, nearly all are weakly porphyritic, carrying small but varied proportions of olivine 

and plag phenocrysts (and, in a few flows, rare cpx). Most common are lavas that have 

0.5–3% plag (0.5–1 mm, rarely as large as 2.5 mm) and 1–5% olivine (mph to 1 mm, rarely 

as big as 3 mm), some in clusters. A few flows are nearly aphyric, and a few have as many as 

8% plag phenocrysts. In samples that have coarse-grained groundmass, however, plag laths, 

seriate from 0.1 to 1 mm, can constitute 10–30% of rock. Many of largest plag (1.5–4 mm) 

are actually composite intergrowths of several crystals. Oxides typically are absent or limited 

to groundmass and seldom >0.1 mm. Plag-olivine clots are sparse, in contrast to associated 

north-distal set of clot-rich flows mapped as unit mey. Flows exposed in windows through 

till in east-distal part of apron (for 6 km along north rim of Whychus Creek and its north fork) 
48

are petrographically typical North Sister lavas, but they are slightly richer than most in Ti, 

K, and P, as well as relatively deficient in Al (shown as unit mnsʹ in table 1 but not mapped 

separately). East-distal apron overlies units msp and mws. Northeast-distal apron overlies 

unit bac, and its flows intercalate with distinctive flows assigned to unit mey. Base of unit 

is not exposed to south or west. North Sister lavas are overlain by units anh, awa, dbh, mfy, 

mlb, mms, mnh, mps, mst, and mwa. 40Ar/39Ar ages: 119±6 ka, for flow at east-distal edge 

of apron on rim above confluence of Soap and Whychus Creeks; 83±25 ka, for flow on north 

rim of northeast planèze at 7,300 ft elevation; 71±3 ka, for flow at 8,600 ft elevation on crest 

of southeast ridge of edifice; 55±5 ka, for lava that forms true summit (“Prouty Pinnacle”) 

of North Sister; 46±6 ka, for valley-floor flow near snout of Collier Glacier. Older ages for 

North Sister, which were reported by Hildreth (2007) and Schmidt and Grunder (2009), we 

now consider unreliable [NS, TCB]

mnt Basaltic andesite north of Trout Creek (late Pleistocene)—Major phenocryst-poor mafic 

(54.9–56.9% SiO2) lava flow about 1 km wide, which extends 6.5 km east from beneath 

base of 150-m-high scoria cone (Cone 6302) across northern part of map area to 60-m-high 

eroded scarp at Trout Creek Swamp. Taylor (1987) correlated it with flow that extends to Cold 

Spring, additional 9 km northeast of map area. Within map area, flow is everywhere glaciated; 

not so farther northeast. Scoria cone (Cone 6302; 54.6% SiO2; mapped as unit mpn) is widely 

oxidized, agglutinated, and draped by thin fountain-fed lavas; although comparably phenocryst 

poor, cone products are not chemically identical to lava flow of this unit and, thus, are not 

likely to represent same eruptive episode; cone buries source vent for lava flow. Exposures of 

main ice-scoured lava tongue typically are massive, devitrified, and block jointed, although 

flow-foliated and glassy chunk-jointed facies crop out locally (more extensively in proximal 

areas). Phenocrysts: distally, almost none; rare plag (1.5–3 mm); even rarer 1-mm olivine and 

cpx, all probably antecrysts. Medially, similar to distally, except that olivine mph (all <0.4 

mm) are abundant; sparse plag (1–2.5 mm, sieved and partly resorbed). Proximally, sparse plag 

(0.5–1 mm) and cpx mph. Oxides scattered throughout, all ≤0.1 mm. Overlies units mmc and 

myl. Overlain by units mpn and mps. 40Ar/39Ar age: 20±5 ka [NS, TCB]

mpm Basaltic andesite of Park Meadow (late or middle Pleistocene)—Glaciated swath, about 200 

m wide, of moderately porphyritic, oxidized agglutinate and fountain-fed mafic (55.5–56.0% 

SiO2) lava flows that extends from eroded spatter cone (stippled on map) at about 6,600 ft 

elevation (UTM grid 030/844) for about 500 m northeast toward Park Meadow. Phenocrysts: 

cone has 7–10% plag (mph to 1.5 mm, rarely to 3 mm); ~3% olivine (mph to 1 mm); sparse 

cpx (mph to 1 mm); abundant plag-pyroxene clots (3–10 mm); sparse oxides, rarely >0.1 

mm. Probably overlies unit mbt. Overlain by unit awf (24±1 ka). Undated [BT]

mpn Basaltic andesite north of Scott Pass (late Pleistocene)—North-trending ridge of fissure-fed, 

phenocryst-poor mafic (54.6–55.2% SiO2; 1.48–1.54% TiO2) scoria and agglutinate that 

extends 2.7 km northward from Scott Pass to pass on Millican Crater trail just north of 

map area (UTM grid 085/986). Glaciated ridge is 0.5 to 1 km wide and steep on both sides, 

having as much as 150 m of relief; its crest is marked by several hills that apparently were 

foci of enhanced ejection of pyroclasts, but only one shallow crater is preserved at Cone 

6302. Exposures are dominated by loose lapilli and abundant bombs (many larger than 1 m) 

that range from black to predominantly brick red and from highly vesicular to fairly dense. 

Tack-welded layers are present locally, but exposures of denser cliff-forming agglutinate 

are few; this contrasts with adjacent fissure-fed alignment segment that continues south of 

Scott Pass (unit mps), which (although 2–3% more silicic) is dominated by thick sections 

of densely welded agglutinate. Vent alignment also continues for 2 km north of map area; 

however, northernmost 1-km-long reach (on west shoulder of Black Crater) produced mafic 

(54.4–54.5% SiO2) scoria and agglutinate that is far richer in plag and olivine and poorer in 

Ti and Fe than this unit and unit mps. Phenocrysts: <1% plag (mph to 1.5 mm); sparse cpx 

mph (0.1–0.4 mm). Overlies units mml, mmn, mnt (20.3±4.6 ka), mps, and bwm. Steep 

narrow topography may, in part, reflect lateral confinement by glaciers owing to eruption 

during Last Glacial Maximum. Overlain by postglacial unit myc. Undated [NS]

mps Basaltic andesite south of Scott Pass (late Pleistocene)—South-trending ridge of fissure-fed, 

phenocryst-poor mafic (56.0–58.7% SiO2; 1.25–1.64% TiO2; n=18) scoria and agglutinate 

that extends about 6 km southward from Scott Pass; also includes intercalated fountain-fed 

lava flows. Glaciated ridge is as narrow as 100 m east of Yapoah Crater, but it is about 1 km 
49

wide near Collier Cone, and 600 m wide at Cone 6315, which has shallow degraded crater 

on top and 150 m of relief on its steep east side. Ejecta exhibit gradations from loose scoria 

that has spindle bombs as big as 1 m through tack-welded and blobby, rheomorphic (locally 

convolute) sheets to massive, homogenized lava flows that have hackly, slabby, or blocky 

jointing. Areas of weakly indurated, yellowish-orange-brown, ash-dominant lapilli tuff 

(stippled on map) were probably mildly palagonitized by fallout over snow or ice. Oxidation 

of ejecta likewise varies widely from black to gray to ochre to brick red. Exposed stacks of 

glaciated agglutinate as thick as 70 m form cliffs and steep ledgy slopes, although ridgecrest 

generally is strewn with loose or weakly welded ejecta. Phenocrysts: <1% plag (0.5–2 mm); 

rare olivine (mph only); very rare cpx mph, generally only in cpx-plag clots; polycrystalline plag intergrowths probably antecrystic. Overlies units awa (25±2 ka), mns, mnt (20±5 

ka), mwa (59±4 ka), mwy, and mys. Steep narrow topography may, in part, reflect lateral 

confinement by glaciers owing to eruption during Last Glacial Maximum. Overlain by postglacial units acc and myc and by glaciated units mml and mpn. Along much of its western 

margin, it also overlies agglutinate and scoria of Ahalapam Cinder Field (not mapped), 

likewise glaciated and phenocryst poor, that erupted from different vent chain and differs 

chemically (55.5–56.5% SiO2; 1.10–1.30% TiO2; n=6). Undated [NS]

mrg Basaltic andesite of Renfrew Glacier (late Pleistocene)—Fan of moderately porphyritic mafic 

(54.6–54.8% SiO2) lava flows about 1 km wide, in three lobes, on northwest flank of Middle 

Sister. Erupted high on edifice, but vent is covered by younger units. Highest exposures are 

windows, at 8,300 ft elevation, through Little Ice Age moraines of Renfrew Glacier; most 

distal exposure lies at 6,600 ft elevation near Pacific Crest Trail. Lavas are everywhere 

ice scoured but nowhere deeply incised; greatest thickness exposed is about 35 m. Flows 

are 3 to 20 m thick, separated by a few meters of oxidized flow breccia and rubble. Unit is 

eroded into knobs, ledges, and smooth ridges and generally is massive, devitrified, and block 

jointed or slabby. Flows drape previously eroded steep slopes cut on unit awr. Phenocrysts: 

7–10% plag (0.5–2 mm), seriate to ~25% plag mph; trace to as much as 1% cpx (mph to 1 

mm); <1% olivine (all mph); sparse tiny oxide mph. Overlies units dss (25±3 ka), awc, and 

awr (although extensively stripped off unit awr); banks against unit roc. Overlain by unit 

mms. 40Ar/39Ar age: 20±6 ka [NS] 

msb Basaltic andesite of Sims Butte (latest Pleistocene)—Postglacial scoria cone near west edge 

of map area and associated intracanyon tongue of mafic (52.3–56.6% SiO2) lava flows 

that extends about 15 km downcanyon to west (Sherrod and others, 2004). Cone (stippled 

on map) has 200 m of relief on east, 300 m on west. Lapilli-fall layer extends about 1 km 

beyond base of cone; ash fall, unmapped distance farther. Proximal scoriae commonly 

weather yellow to ochre. Thin proximal lavas are fountain fed, rubbly, and scoriaceous. 

Medial to distal stacks consist of numerous rubbly flows, each 0.3 to 5 m thick and finely to 

coarsely vesicular. Thick piles of oxidized rubble accumulated locally where flows broke up 

over steep pitches. Phenocrysts: ≤1% olivine (seriate 0.1–1 mm, but a few exceed 0.4 mm); 

plag phenocrysts (0.5–1.5 mm) rare; groundmass plag is seriate from microlites to abundant 

mph laths, small fraction of which are as long as 0.5 mm; sparse plag antecrysts (0.5–2 mm, 

sieved and resorbed). Analyses of 17 samples (table 1; see also Conrey and others, 2002) 

show no systematic compositional progression downcanyon; scoria cone, however, is least 

evolved, and levee at northwestern margin of lava-flow apron provides most evolved material sampled. Overlies unit mlb (47–49 ka). Overlain by unit acc and Mazama ash (7.7 ka). 

May be relatively early postglacial, in light of only modest weathering of late Pleistocene till 

beneath Sims Butte fall deposits (Sherrod and others, 2004). Undated [LL, NS]

msc Basaltic andesite of Separation Creek (late Pleistocene)—Glacially scoured 4 km2 apron of 

phenocryst-rich mafic (56.7–57.3% SiO2) lava flows at west-northwest foot of South Sister, 

between Hinton Creek and Linton Meadows, extending into west-trending lobe south of 

Separation Creek. Vent is marked by small scoria-spatter cone (stippled on map; 56.9–57.3% 

SiO2) only 300 m wide and 60 m high, just north of Separation Creek. Greatest thicknesses 

exposed on lava apron are about 60 m, near southern limit of unit and at terminus of western 

tongue. Scoria and agglutinate of eroded cone are locally black, mostly oxidized brick red. 

Lavas are locally vitrophyric and chunk jointed (as near “Racetrack Meadow”) but mostly 

ice scoured and plucked into ledges, knobs, and pavements that are massive, devitrified, and 

block jointed or slabby. Phenocrysts: 15–25% plag (mph to 3.5 mm); 3–5% pyroxenes (mph 
50

to 1.5 mm); sparse oxide and olivine mph; abundant plag-pyroxene-oxide clots. Overlies 

units bhs, mth, and ddl (32±2 ka); banks against toe of unit alg (27±1 ka). Overlain by units 

mms and mmt. 40Ar/39Ar age: 21.4±5.5 ka [NS, SS]

msf Basaltic andesite of South Fork Whychus Creek (middle Pleistocene)—Phenocryst-poor 

mafic (53.7–54.0% SiO2) lava flows and tuff exposed only near floor of South Fork 

Whychus Creek, at 5,800 to 6,000 ft elevation, 300 to 500 m east and southeast of Demaris 

Lake. Unit is at least 25 m thick, but base is nowhere exposed. Tuff is nonindurated scoriaflow deposit exposed only on right-bank wall of stream gorge at about 5,920 ft elevation; at 

least 12 m thick and overlain by lava flow of identical composition. Black, crystal-poor scoriae are as large as 8 cm, enclosed in poorly sorted coarse-ash matrix, which weathers orange 

and includes varied accidental lithic fragments (1–5 cm). Lavas are exposed as glaciated 

and water-worn ledges and gully walls; massive, devitrified, and block jointed. Phenocrysts: 

1–2% plag (mph to 2 mm, rarely to 4 mm); sparse olivine mph (rarely bigger than 0.2 mm). 

Groundmass rich in plag-lath mph, rarely as long as 0.5 mm. Overlain by units adl, mcl, and 

mdl. 40Ar/39Ar age: 166±16 ka [TCB] 

msp Basaltic andesite of Soap Creek–North Fork Whychus Creek confluence (late or middle 

Pleistocene)—Small, glacially eroded mafic (54.2% SiO2) scoria cone at 6,200 ft elevation 

between Soap Creek and North Fork Whychus Creek, as well as derivative phenocryst-poor 

mafic (54.6, 54.8% SiO2) lava flows 2 to 4 km downstream, near their confluence. Cone 

(stippled on map) is 350 m wide and displays ice-scoured, ledgy slopes that reflect layering and varied degrees of agglutination (tack welded to moderately dense) in brick-red 

scoria-fall deposit; densest agglutination is on steep east face, which has 70 m of relief. 

Although dominantly lapilli, ejecta are as big as 50 cm. Deposit is locally palagonitized 

yellow-orange. Lava flows are exposed along stream gorge for about 600 m near confluence 

and extend additional 1.2 km northeast along left-bank rim of North Fork Whychus Creek 

canyon. Lavas crop out as massive, devitrified, block-jointed or slabby ledges as thick as 

20 m. Oxidized rubbly zone separates two 10-m-thick flows, each supporting a waterfall at 

elevations 5,370 and 5,340 ft along North Fork gorge. Phenocrysts: in cone, <1% plag (mph 

to rarely as big as 1 mm), mostly sieved or partly resorbed; 2–3% olivine (mph to 0.7 mm), 

some rimmed by iddingsite. In lavas, almost none; groundmass is rich in plag-lath mph, 

seriate to a few as long as 0.7 mm; 2–3% olivine mph (0.1–0.4 mm), some in clusters; very 

rare olivine grains as large as 0.5–1 mm. Base of cone not exposed. Lavas overlie unit bnf

(at 5,320 ft elevation on gorge floor and at 5,180 ft on canyon wall); also overlie unit mws

(175±3 ka), which in turn rests on ignimbrite of unit spt. Both cone and lavas are widely 

overlain by till and by distal lava of North Sister (unit mns, dated nearby at 119±6 ka, where 

it rests directly upon this unit). 40Ar/39Ar age: 129±6 ka (scoria cone) [TCB]

mss Basaltic andesite of Middle Sister summit (late Pleistocene)—Sequence of thin, phenocrystrich mafic (56.1% SiO2) lava flows that caps ridgecrest 100 to 300 m north of true summit 

of Middle Sister, between 9,800 and 10,000 ft elevation; also exposed on sheer east face of 

ridge. All exposures along summit ridge consist of partly glassy, coarsely vesicular, loose 

blocks as big as 2 m that have yellow, white, and ochre alteration minerals and sublimates on 

most joint faces. Phenocrysts: 15–20% plag (mph to 4.5 mm); <1% olivine (mph to 1 mm). 

Overlies unit mms; banks against northern remnant of unit dms. Undated [NS]

mst Basaltic andesite south of Trout Creek (late or middle Pleistocene)—Isolated 1 km2 window 

of moderately porphyritic mafic (53.3% SiO2) lava along and south of headwaters of Trout 

Creek, at north edge of apron of North Sister lavas. May be single flow, cropping out best 

as glacially eroded, southeast-facing scarp 6 to 10 m high, crossed by Scott Pass trail. 

Exposures are massive, block jointed or slabby, and mostly devitrified. Phenocrysts: ~10% 

plag (mph to 3 mm); 2% olivine (0.5–2 mm); sparse cpx (~1 mm) and oxide mph. Overlies 

older set of distal lavas of unit mns. Overlain by unit myl and, although contact is covered 

by till, probably also by nearby unit mey. Undated [TCB]

mtb Basaltic andesite and basalt of Trout Creek Butte (middle Pleistocene)—Steep mafic 

(51.8–52.7% SiO2) shield in northeast corner of map area, 4 km wide and having about 400 

m of relief, as well as apron of lava flows that extends as far as 5 km northeast of shield’s 

summit (Sherrod and others, 2004). Scores of flows are present; many thin proximal ones 

are fountain fed; flows are thicker on apron. Slopes of shield are mostly nonglaciated 

blocky colluvium that has few true outcrops; sparse exposures are block jointed and finely 
51

to coarsely vesicular. Two summit knolls consist of cliffy stratified agglutinate, widely 

oxidized, having layers that vary from tack welded through blobby to densely welded and 

nearly homogenized. Late Pleistocene moraines bank against west and southwest base of 

shield, but most of edifice is unglaciated. Associated ash-fall deposit is said to be preserved 

in nonglaciated area northeast of shield (Taylor, 1987). Phenocrysts: 1–3% plag (0.5–1 mm), 

most plag anhedral; 1–3% olivine (mostly mph, rarely 0.5–1 mm); sparse cpx in some flows; 

mph clots of plag or olivine+plag are ubiquitous and conspicuous. Groundmass typically 

choked with plag-lath mph, seriate to microlites. Older than units bss and bbd that bank 

against base of shield. 40Ar/39Ar age: 532±7 ka. Oldest unit identified in map area [TCB]

mth Basaltic andesite of The Husband (middle Pleistocene)—Glacially ravaged, polyintrusive, 

multivent mafic (52–57% SiO2; n=22) stratovolcano centered 8 km northwest of South 

Sister summit. Edifice is 5 by 8 km (elongate northwest) and retains about 1,225 m of relief 

despite severe erosion. Not mapped in detail by us. Extent of lava apron and distribution 

of vent-related intrusions was shown by Sherrod and others (2004); numerous dikes were 

shown schematically by Taylor and others (1987). Lithology varies widely, from glassy and 

vesicular to massive, devitrified, and block jointed to slabby. Plugs tend to be microgranular, having abundant olivine and plag mph. Lavas are phenocryst poor, many containing 

only ~1% olivine and 1–2% plag (both ranging from mph to as big as 1.5 mm). Overlies 

unit bhs, which may, in part, be an early product of The Husband center. Overlain by units 

acc, alc, mls, mms, and msc. Knob 6760, 3 km north of Indian Holes on southeast apron 

of The Husband, is glaciated flank vent (unit mth′; 52.8% SiO2) that intruded and overlies 

less mafic apron of this unit. 40Ar/39Ar age for lava of Knob 6760 (149±5 ka), thus, provides 

minimum age for The Husband edifice and for subjacent lavas of unit bhs [LL, NS, SS]

mtp Basaltic andesite of Teardrop Pool (late Pleistocene)—Mafic (53.8–56.1% SiO2) agglutinate 

and scoria-fall deposit that caps summit of South Sister. Crudely stratified deposit mantles 

350-m-wide summit crater (from which it erupted), which includes ice-ringed pond called 

Teardrop Pool. Ice-radar measurements show crater to be flat-floored, funnel-shaped depression filled to rim with ice as thick as 60 m at crater center (Driedger and Kennard, 1986). 

Deposit also mantles outer slopes of cone on south and west, thinning but extending 200 

to 400 m outboard from crater rim. On north and east, however, outboard part of sheet has 

been eroded away, leaving edge of deposit as steep rim scarp 2 to 20 m high. Sheet grades 

up, down, and outboard from densely agglutinated (even rheomorphic) to tack welded, then 

to loose scoria; locally black but widely oxidized brick red; gray in 10-m-thick densely massive core of sheet exposed above headwall of Lewis Glacier. Easily confused with red and 

gray agglutinates and fountain-fed lavas of cone-forming unit aeg, which it drapes in places. 

Blocky surface of unit in summit bowl is glassy and scoriaceous, having many scoria bombs 

variably deformed in nondense parts of agglutinate. Just west of Teardrop Pool, stratified 

agglutinate (10 m thick) dips steeply (45°–65°) both inboard and outboard, draping rim of 

inner crater blasted through outward-dipping strata of fragmental unit des. Around Teardrop 

Pool, surface of sheet dips radially inward, probably reflecting differential compaction 

(welding) of thicker central accumulation where infilling its own crater. This inner crater is 

not to be confused with outer crater (700 m wide at its rim) that was earlier filled by units 

dlg and des. Phenocrysts: 7–15% plag (mph to 5 mm); 1–3% cpx (mph to 1.5 mm); ≤1% 

olivine mph. Overlies units aeg, dlg, des, dnt, and dnt′. Youngest unit erupted from South 

Sister summit vents. 40Ar/39Ar age: 22±13 ka [SS]

mts Basaltic andesite southeast of “Todd Creek” (middle Pleistocene)—Small glaciated mafic 

(56.1–56.8% SiO2) spatter and scoria cone about 1 km due south of Todd Lake and 100 m 

northeast of Cascade Lakes Highway (UTM grid 0540/7435). Only about 25 m high and 120 

m in diameter, this degraded knoll largely consists of oxidized agglutinate that crops out as 

blobby to densely homogenized ledges on most slopes; north slope is strewn with loose red 

scoria. Phenocrysts rare: trace amounts of plag (0.5–3 mm) and mph of olivine and pyroxene. Knoll is wrapped by postglacial lava flows of unit mec. North end of cone is in contact 

with glaciated rhyodacite lava (not mapped by us), which may be younger, as its surface is 

devoid of scoria lapilli from this contiguous cone. Undated [BT]

mtw Basaltic andesite of The Wife (middle Pleistocene)—Deeply eroded mafic (52.1–53.6% SiO2) 

shield west of Wickiup Plain and Rock Mesa. Massive summit plug, steeply jointed, intrudes 

stratified, radially dipping, scoria-fall deposits and agglutinate. Several vertical dikes, 2 to 
52

5 m thick, cut proximal strata. Apron of glacially eroded lava flows extends 2 km north and 

south and several kilometers west beyond map area. Scoria falls exposed about 500 m southeast and 170 m lower than shield summit are intercalated with lava flows and are palagonitized yellow orange, unlike red and black scoriae near summit. Lavas exposed largely are 

massive and devitrified. Phenocrysts: 2–5% plag (0.5–1.5 mm long), seriate to felty groundmass rich in plag mph; 2–5% olivine (mph to 0.7 mm); oxide mph abundant in plug, sparse 

to absent in lavas. Some proximal scoria bombs appear to contain more plag phenocrysts 

than apron lava flows. Base not recognized in map area. Overlain by units bsb (279±6 ka), 

mhr, bjc (148±4 ka), ddl, mlc, and rrm. 40Ar/39Ar age: 374±6 ka, for massive lava on east 

face of summit. Second oldest unit identified in map area [SS]

mwa Basaltic andesite west of upper Alder Creek (late Pleistocene)—Moderately porphyritic 

mafic (53.2–56.1% SiO2) lava flows exposed as glacially sculpted ridges and cliffs, 3 to 5 km 

north of North Sister summit. Sets of two or three flows form east-facing cliffs that extend 

2 km north-south, directly beneath ridge of phenocryst-poor pyroclastic unit mps; branching from that cliff (at UTM 985/945), second sharp ridge, about 250 m wide and as high as 

100 m, extends 1.2 km northeast to bank against Knob 6435 (unit myl). Individual flows are 

25 to 60 m thick, and stacks of two or three are as thick as 100 m; lavas are massive, block 

jointed or slabby, pale to medium gray, and everywhere ice scoured. Phenocryst contents vary 

widely among flows: 5–15% plag (0.5–2 mm, rarely to 4 mm); 1–3% olivine (mph to 2 mm); 

sparse to common cpx (0.5–1.5 mm); stellate clusters of 10 to 25 crisscrossed plag laths (each 

0.2–1 mm long) are common. Overlies units mns; banks against units mfy, myl, and (probably) mys. Overlain by unit mps and intruded by thick aphyric mps dike; also intruded by 

apparent plug of unit asy. May have vented from same fissure-vent system that later produced pyroclastic unit mps. Distinguished from unit awa, which crops out along same wall 

just south, by much lower phenocryst content, lower Al2O3 content (<17%), and higher FeO 

content (>8%) of unit awa. 40Ar/39Ar age: 59±4 ka, for basal flow of northeast ridge [NS]

mwh Basaltic andesite of Whychus Creek (middle Pleistocene)—Seven mutually isolated exposures 

of moderately porphyritic mafic (53.2–55.2% SiO2) lava flows on upstream floor and downstream rims of Whychus Creek. Five are scattered for 4 km, from 4,900 ft elevation in bed of 

North Fork Whychus Creek to 4,500 ft elevation in right-bank roadcut near confluence with 

Snow Creek; exposures are correlated with fair confidence on basis of petrographic similarity and narrow compositional arrays on chemical variation diagrams. These five crop out as 

ice-scoured, till-strewn knobs or ledges, massive, devitrified, and block jointed or slabby; 

exposures are less than 10 m thick except 70-m-thick knobby outcrop on left-bank rim 

(UTM grid 063/920); nowhere has base been observed. Two additional exposures lie along 

Whychus and Pole Creeks near and just beyond limits of late Pleistocene moraines, near east 

edge of map area. Blocky, scoriaceous levee is exposed along north bank of Pole Creek at 

about 4,200 ft elevation, and sets of slabby, water-worn ledges on both walls of Whychus 

Creek at 4,100 to 4,200 ft elevation provide 15 to 20 m of vertical exposure. Phenocrysts: 

5–10% plag (1–3 mm and abundant mph); ≤1% olivine (seriate from mph to 1 mm); rare cpx 

(≤1 mm, mostly in sparse clots with olivine and plag); sparse oxide mph. Overlies units awy

(299±1 ka), mbw, and bwy. Overlain by units acp, bnf, bwl, bss, and mws (175±3 ka); 

appears to lie closely beneath ignimbrite of unit spt in North Fork Whychus Creek gorge. 40Ar/39Ar age: 195±5 ka [TCB]

mws Basaltic andesite west of Snow Creek (middle Pleistocene)—Phenocryst-poor mafic 

(53.6–55.0% SiO2) lava flows recognized in three mutually separated locations along forks 

of Whychus Creek; correlated on basis of unusually high Ti and Fe contents. Upstreammost 

exposure is 600 m east of Upper Chush Falls, suggesting derivation from Broken Top volcano. Medial exposure along lower North Fork Whychus Creek rests directly atop ignimbrite of unit spt. Distal exposure, more than 80 m thick, forms west-bank wall for 800 m 

downstream from confluence of North and South Forks, Whychus Creek. All three are glacially eroded, predominantly devitrified, massive, and block jointed or slabby. Phenocrysts: 

~1% plag (mph to 1.5 mm laths); sparse olivine and cpx (both 0.5–1 mm); rare oxide mph. 

Overlies units spt and mwh (195±5 ka). Overlain by units adl (24±1 ka), msp, bnf, and ruc

(169±2 ka). 40Ar/39Ar age: 175±3 ka [TCB]

mwy Basaltic andesite scoria cone west of Yapoah Lake (late Pleistocene)—Small mafic 

(56.4–56.6% SiO2) cone, 600 to 800 m across, just northwest of Yapoah Lake. Built on 
53

east-facing paleoslope, cone has maximum relief of about 90 m on its east side but only 

10 m on its west side. Crater on top is 300 m wide and has 75 m of relief on its inner west 

wall. Rimrock is largely agglutinate, showing gradation from loose oxidized scoria through 

tack-welded scoria and knobbly dense agglutinate to streaky or homogenized lava, which 

is locally block jointed and slabby. On 5,800-ft-high nose just south of crater, its densitygraded agglutinate mantles glaciated ridge of unit mys and small cliffy window of unit 

myw. Cone is degraded but only modestly eroded; may have stood above or aside main late 

Pleistocene ice streams. Phenocrysts: almost aphyric; groundmass plag includes rare laths 

as long as 0.8 mm; <<1% olivine (0.2–0.4 mm); very rare oxide mph. Overlies units myl, 

mys, and myw. Probably overlain marginally by unit mps, but surficial deposits render that 

conclusion uncertain. May be related to nearby chain of scoria cones and ridges that runs 

north-south past Matthieu Lakes, but cone (this unit) lies east of that alignment. Undated [NS]

myc Basaltic andesite of Yapoah Crater (Holocene)—Moderately porphyritic mafic (54.5–57.0% 

SiO2) lava flows that extend as far as 12 km northward from an uneroded scoria cone 

(stippled on map), 160 m high and about 500 m in diameter, located about 5 km north of 

North Sister. Easternmost eruptive unit of postglacial McKenzie Pass volcanic field (fig. 

2); only eastern of several flows from Yapoah Crater is mapped here, essentially as border 

for our map area. Full extent of Yapoah lava apron has been depicted by Taylor (1968) and 

Sherrod and others (2004). Flows are blocky to rubbly, vesicular, little eroded, and extensively leveed. Phenocrysts: varied amounts of plag and olivine; sparse cpx. Undated, but age 

is bracketed by enclosing units to roughly 2.5 ka (Licciardi and others, 1999; Sherrod and 

others, 2004) [NS]

myl Basaltic andesite of Yapoah Lake (late Pleistocene)—Glacially eroded, phenocryst-poor mafic 

(53.4–54.2% SiO2) lava flows that form walls enclosing Yapoah Lake and extend about 3 km 

east-northeast and about 1 km south from lake. Stack is more than 70 m thick on scarp south 

of lake, but base is not exposed there. Bold ice-sculpted outcrops form knolls, ledges, ridges, 

and cliffs that are generally massive, devitrified, and block jointed or slabby. Vent plug is 

preserved as stout Knob 6435, about 1.5 km south of Yapoah Lake; steep-walled, glaciated 

knob is 75 m high, 200 by 250 m wide, and consists of massive fine-grained lava that has 

widely spaced, irregular joints and veneers of brick-red blobby agglutinate fused to its 

north and east faces. Phenocrysts: few crystals >0.5 mm; rare equant plag (1–2 mm) may be 

antecrystic; groundmass plag laths are seriate to as many as 20% mph, including rare laths 

as long as 1.5 mm; 1–3% olivine mph (0.2–0.4 mm, rarely to 0.8 mm); distal flows have 

abundant oxides (rarely >0.1 mm). Overlies units mst and bwm. Overlain by units mey, 

mfy, mnt (20±5 ka), and mwa (59±4 ka), as well as by scoria cone of unit mwy. Undated 

[NS, TCB] 

mys Basaltic andesite southwest of Yapoah Lake (late Pleistocene)—Moderately porphyritic mafic 

(52.5–53.2% SiO2) lava flow that forms east-facing cliff as high as 50 m and ice-scoured 

bench at 6,050 to 6,100 ft elevation. Bench extends 1.5 km northward from prominent crag 

at 6,400 ft elevation (UTM grid 982/959) as far as scoria cone of unit mwy west of Yapoah 

Lake. Fine exposures are massive and slabby or block jointed. Phenocrysts: 3–5% plag 

(0.5–1.5 mm, rarely 4 mm); 5–8% olivine (0.5–1 mm, commonly in clusters; rarely as big 

as 3 mm). Steeply jointed crag is vent plug, intruding and fusing its own oxidized scoria 

deposit and displaying 60-m-high scarp on its east face. North of scoria cone of unit mwy, 

600 m along strike from principal cliffy exposures, massive angular blocks of this lava crop 

out poorly through till and colluvium along Scott Pass trail at 5,640 ft elevation (UTM grid 

9905/9765). Overlies units mey and, inferentially, myw. Overlain by scoria cone of unit 

mwy and by variably agglutinated facies of phenocryst-poor pyroclastic unit mps, which 

banks against west wall of vent crag. Unit mwa apparently banks against south wall of vent 

crag, but surficial deposits obscure contact relations. Undated [NS]

myw Basaltic andesite window west of Yapoah Lake (late or middle Pleistocene)—Moderately 

porphyritic mafic (53.0% SiO2) lava flow that crops out only as ledgy window (UTM grid 

989/971), less than 100 m long, at 5,720 to 5,760 ft elevation on northeast-facing scarp 

above crater wall of scoria cone of unit mwy. Steep outcrops are massive and block jointed. 

Unit is chemically similar to predominant lavas of unit mns but differs in being plagdominant and richer in total phenocrysts. Phenocrysts: 7–10% plag (mph to 3 mm); 2–3% 

olivine (mph to 2 mm); some composite intergrowths. Base not exposed. Draped and largely 
54

concealed by agglutinate of unit mwy. Inferred also to underlie nearby unit mys. Undated 

[NS]

rct Rhyolite of South Sister climbers trail (late Pleistocene)—Phenocryst-rich pyroxene-rhyolite 

(72.7–73.4% SiO2) lava flows that emerge from beneath Lewis Glacier moraines high on 

south slope of South Sister. Main flow apron divides into two lobes that extend 2 km and 

3 km downslope, respectively, latter of which reaches lowland toe of edifice. Smaller third 

lobe about 200 m east of upper end is largely covered by surficial deposits. All exposures are 

glacially eroded, strongly flow foliated, and widely spherulitic or vitrophyric. Phenocrysts: 

7–10% plag (mph to 3 mm); ~1% opx (mph to 0.8 mm, plus rare quench needles 1–2 mm 

long); sparse oxide mph. Also contains sparse clots: either all plag, pyroxene-oxide, or plagpyroxene-oxide. Maximum thickness exposed is about 75 m proximally and about 60 m at 

southwestern lowland flow front. Medial exposures are eroded into cliffy ridges where foliation exhibits sweeping ramps and folds. Lowland lobe is heavily mantled by late Holocene 

pumice-fall deposits associated with unit rrm and is intruded by fissure-aligned minidomes 

of unit rrm. Overlies units dcg (31±3 ka), dmn, dmw, dwp, and rse (34±1 ka). Overlain 

(inferentially) by unit aeg. 40Ar/39Ar ages: 30±8 ka, 24±5 ka [SS]

rdc Rhyolite of “Devils chain” (late Holocene)—Chain of nearly contiguous, virtually uneroded, 

rhyolite lava domes (72.3–72.8% SiO2; n=31; see also Scott, 1987), 5 km long and aligned 

north-south on southeast apron of South Sister. Chain consists of seven main lavas and several minidomes (Scott, 1987). Three domes issued short coulees; largest coulee is 2-km-long 

Newberry flow, which accounts for 60 percent of total eruptive volume and, only 2.5 km 

from summit of South Sister, is northernmost vent along principal chain. Dike-fed alignment 

is expressed between domes by fissures and graben cut in 10-m-thick proximal subplinian 

pumice-fall deposits that preceded effusion of most lavas. Only exception to generality of 

lavas following fallout is southwest lobe (unit rdc′) of Newberry flow, much of which is 

tephra mantled (white stipple on map). Ejecta ring (black stipple on map) wraps west end 

of Newberry flow near its vent; ridges of thick ejecta likewise flank chain of four small 

domes about 1 km south of Newberry vent, heavily mantling and locally concealing subjacent unit rse. For composite (multivent) subplinian-fall deposit, 10-cm isopach extends 11 

km east and 6 km south of vent chain, as mapped by Scott (1987), who calculated eruptive 

(magma) volumes of 0.3 km3 for lavas and 0.02 km3 for tephras (mostly fallout but including thin proximal pyroclastic flows). Unit has sometimes been called “Devils Hill chain,” 

but its name is here shortened to avoid confusion with the (subjacent) Pleistocene rhyolite 

of Devils Hill (unit rdh). Spring-fed Hell Creek emerges from beneath unit rdh; Satan 

Creek from beneath this unit. Beyond 3.5-km-long gap north of Newberry flow, second 

north-south fissure alignment near Carver Lake on north-northeast slope of South Sister is 

marked by 1.2-km-long chain of minor vents that released very small volumes of lava and 

pumice (Scott, 1987); these, however, are high enough in elevation to have been scoured 

by Neoglacial ice. Nonetheless, because products of all 20 or so vents (along two chains) 

are compositionally identical, parallel en echelon chains probably erupted contemporaneously. Thickest exposures are about 120 m (north side of Newberry flow) and about 50 m for 

several southern domes and for small lava flow northwest of Carver Lake. Lavas are either 

coarsely blocky, dense black vitrophyre or pale gray and vesicular; strongly to weakly flow 

foliated. Spherulitic layers are rarely exposed, but oxidized rehealed flow-breccia layers 

are common. Slabby, tan, partly devitrified interior facies is exposed locally on ice-scoured 

lavas near Carver Lake. For interpretation of brecciated obsidian-bearing margin (largely 

tephra free) of otherwise tephra-mantled southwest lobe of Newberry flow (unit rdc′), see 

Scott (1987, his fig. 9). Phenocrysts: 5–7% plag (mph to 3.5 mm); <1% opx (mph to 0.7 

mm); sparse oxide mph; many plag crystals carry prominent melt inclusions, and others are 

coarsely sieved; unit also contains common plag-pyroxene-oxide clots. 14C age: about 2,000 

yr B.P. (Scott, 1987; Sherrod and others, 2004). Unit is younger than all others nearby except 

Little Ice Age till. Intervening weak soil suggests that pumice fall of this unit postdates that 

from Rock Mesa (unit rrm) by at least a century (Scott, 1987) [BT, SS]

rdh Rhyolite of Devils Hill (late Pleistocene)—Steep rhyolite (73.4–74.1% SiO2) lava dome that 

has sprawling flow lobes to south and east; altogether, 2 km across, having nearly 500 m of 

total relief. Dome stands 6 km south of South Sister summit and is almost everywhere glacially scoured. Lithologically varied: widely vitrophyric, partly glassy or spherulitic; block 
55

jointed and only locally vesicular or vuggy but pervasively flow foliated; gray to white, only 

locally oxidized. Little pumiceous carapace preserved except near its summit. Phenocrysts: 

3–5% plag (mph to as big as 3 mm, but few >1 mm); 1–2% pyroxenes (mostly mph, rarely 

as long as 1.2 mm); sparse oxide mph; sparse clots, either all plag or plag-pyroxene. May be 

roughly contemporaneous with chemically identical (but phenocryst poorer) adjacent dome 

of unit rkb. Unit dml (34±1 ka) banks against and wraps around dome (this unit). Intruded, 

and overlain by, late Holocene domes of unit rdc. Overlain by fallout associated with units 

rdc, rrm, and btk. 40Ar/39Ar age: 34.8±1.5 ka [BT, SS]

rgl Rhyolite of Green Lakes (late Pleistocene)—Rhyolite (72.2–72.9% SiO2) lava dome partly 

buried by dacite lava flows (unit dgl) at east toe of South Sister edifice, just west of Green 

Lakes. Exposure is 700 m long and 150 m high. Glacially eroded, flow-foliated lava varies 

from dark-gray vitrophyre to pale-gray felsite, partly devitrified and locally spherulitic; 

widely vesicular or vuggy. Jointing causes rock to break into chunks or blocks. Phenocrysts: 

5–7% plag (mph to as big as 3 mm); ~1% pyroxenes (mph to 0.5 mm, plus rare rods to 1 

mm long); sparse oxide mph; also contains common plag-pyroxene-oxide clots. Base not 

exposed. Overlain by South Sister lavas of unit dgl. 40Ar/39Ar age: 31.9±0.7 ka [BT]

rkb Rhyolite of Kaleetan Butte (late Pleistocene)—Rhyolite (73.5–73.6% SiO2) dome-flow complex adjacent to west side of Devils Hill dome (unit rdh) and 6 km south of South Sister 

summit. Complex is 2 km wide and has 320 m of relief on its steep south side. East and west 

lobes are compositionally identical but may have extruded independently; also is chemically similar to adjacent Devils Hill dome (unit rdh) but carries fewer phenocrysts. Glacially 

scoured surface consists largely of flow-foliated vitrophyre, although partially devitrified 

layers locally alternate with obsidian and spherulitic lenses; commonly is finely vesicular 

but only locally is oxidized streaky pink. Deeper erosion has exposed ledges of thinly platy 

felsitic interior on steep southeast face. Phenocrysts: 2–3% plag (mph to 1 mm); <1% opx 

(all mph); sparse plag-opx clots. Abuts the (chemically identical) rhyolite of Devils Hill 

(unit rdh); units may be roughly contemporaneous, but age relation is not clear at contact. 

Overlies unit dwp (34±3 ka). Unit dml (34±1 ka) banks against northeast margin of dome; 

west side of dome is wrapped by postglacial mafic lava flows from Le Conte Crater (unit 

mlc). K-Ar age: 30±5 ka [SS]

rmc Rhyolite of Mesa Creek (late Pleistocene)—Moderately phenocryst-rich rhyolite (72.3–72.6% 

SiO2) lava flow or lobe of partly buried lava dome exposed for 800 m along uppermost 

Mesa Creek at southwest toe of South Sister. Flow direction was toward southwest, down 

steep slope; exposed relief is about 340 m, but true thickness probably is only about 150 m. 

Glaciated south cliffs are pervasively flow foliated, exposing black vitrophyre, vesicular 

and oxidized layers, and zones of platy felsite. Ice-scoured lava benches above are variably 

devitrified and mostly medium gray, weathering cream to pale gray. Phenocrysts: 5–7% 

plag (mph to 2 mm), some slightly rounded; <<1% opx (mph, rarely as big as 0.6 mm, plus 

rare rods 0.2 by 3 mm); still sparser hornblende (mph to 0.7 mm, along with tiny needles), 

mostly opacitized; sparse oxide mph. Normal magnetic polarity (laboratory determination 

by D.E. Champion, 2006). Overlain, and extensively buried, by South Sister lava flows of 

unit dcg (31±3 ka); coulee of postglacial unit rrm banks against it. 40Ar/39Ar ages: 47±8 ka, 

45±15 ka [SS]

roc Rhyolite of Obsidian Cliffs (late Pleistocene)—High-silica rhyolite (76.4–76.7% SiO2) coulee 

that emerges from beneath younger lavas of Middle Sister edifice about 3 km northwest of 

its summit and extends 3 km farther northwest. Everywhere glacially scoured, flow is 1 km 

wide and as thick as 120 m near Obsidian Falls and 135 m at its distal flow front. Aphyric 

lava is strongly flow foliated and ranges from black obsidian to white felsite, some layers 

being vesicular or oxidized. Unit is most silicic Quaternary lava in Oregon Cascade Range. 

Phenocrysts: strictly aphyric. Overlies units mlb (47–49 ka) and aoc (~48 ka). Syneruptive 

lapilli-fall deposit (white stipple on map) containing clasts of aphyric pumice, obsidian, and 

felsite is preserved atop west ridge of Little Brother. Abutted by unit mrg (20±6 ka) and 

wrapped by Holocene lava flows from Collier Cone (unit acc). Overlain near Sister Spring 

by thick lava of unit dss (25±3 ka), which drapes down across successive lithologic zones in 

rhyolite of this unit. 40Ar/39Ar age: 37.8±1.8 ka [NS] 

rpc Rhyolite of Park Creek (late Pleistocene)—Moderately porphyritic, glaciated rhyolitic (72.5% 

SiO2) lava flow that crops out as single 30-m-high bench (less than 100 by 350 m) on north 
56

wall of West Fork Park Creek at about 7,000 ft elevation (UTM grid 014/852). Flowfoliated lava is dominantly black vitrophyre but includes layers and lenses of gray or 

pink felsite, some strongly spherulitic, and thin shear zones of pink to brick-red breccia. 

Phenocrysts: 5–7% plag (0.5–2 mm); <1% opx (0.1–0.5 mm); sparse oxide mph. Base not 

exposed. Overlain by unit drm (29±1 ka); unit awf (24±1 ka) banks against base of unit 

[BT]

rpg Rhyodacite of Prouty Glacier (late Pleistocene)—Rhyodacitic (68.1–68.4% SiO2) lava flow 

high on South Sister edifice, cropping out principally as prominent ledge that wraps northwest corner of headwall of Prouty Glacier, 0 to 20 m above ice surface. At 9,300 ft elevation, 

this is highest exposure on South Sister edifice of any lava flow that is more silicic than 

dacite. Glacially eroded, slabby lava is exposed laterally for about 500 m; lava generally is 

about 10 m thick, thickening northward where it disappears beneath younger units. Locally 

vesicular to vuggy and has pink oxidized films along joint planes. Second outcrop, compositionally identical, is exposed through Little Ice Age moraines about 500 m east of terminus 

of Prouty Glacier as narrow, 300-m-long, ice-sculpted, northeast-trending rib of massive to 

vuggy gray lava (at about 7,900 ft elevation, 500 m southeast of Carver Lake; UTM grid 

001/846). Phenocrysts: ~5% plag (mph to 2 mm, plus rare antecrysts to 8 mm long); <1% 

pyroxenes (mph to 0.7 mm, nearly all in clots with plag and oxides); <1% hornblende (0.1–1 

mm long, mostly opacitized); sparse oxide mph; also contains sparse plag-pyroxene-oxide 

clots, plag-bearing mafic blebs (as big as 5 mm), and olivine xenocrysts (0.1–0.5 mm). 

Underlain on Prouty Glacier headwall by units aph and aph′; overlain there by unit dpg, 

which is, in turn, overlain by units ahc and aeg (27±2 ka). Second outcrop (southeast of 

Carver Lake) is overlain by unit drm (29±1 ka). 40Ar/39Ar age: 32.8±1.5 ka [BT, SS]

rrm Rhyolite of Rock Mesa (late Holocene)—Pancake-shaped rhyolite (73.3–73.6% SiO2; n=9; 

see also Scott, 1987) coulee, associated subplinian pumice-fall deposits, and nearby chain 

of satellitic minidomes. Virtually uneroded coulee is 2 by 3 km in diameter, 50 m thick at 

most margins, and as thick as 85 m at northwest terminus where descent into Mesa Creek 

stalled. Small early lava lobe spread northeast, but main outflow was south, southwest, and 

northwest from vent. Broad swell over vent is 320 m higher than northwest terminus of 

coulee. Lava is strongly blocky and has steep flow fronts, consisting principally of dense to 

pumiceous, flow-foliated vitrophyre. Stratified pumice-fall deposit is more than 10 m thick 

proximally and includes agglutinate locally exposed along gullies. Its 10-cm isopach extends 

17 km east and 13 km south of vent, as mapped by Scott (1987), who calculated eruptive 

(magma) volumes of 0.5 km3 for lava and 0.03 km3 for tephra (mostly fallout but also 

includes 1-m-thick pyroclastic flows that extend as far as 1 km); traces of ash are recognized 

as far as 30 km downwind. About 1 km east of principal vent, a fissure vent aligned N. 5° 

to 10° E. (“Rock Mesa ENE” assemblage of Scott, 1987) produced 200-m-wide dome, as 

well as several tiny extrusions only meters across, modest ejecta rings, and small volumes 

of pumiceous fallout and pyroclastic flows. Eruptive reach of fissure system is about 500 m 

long, but fractures and graben continue for another 700 m northward across previously glaciated surface of unit dcg. Main ejecta ring around fissure-vent system heavily blankets and 

widely obscures subjacent unit rct. Lavas of fissure-vent system are uneroded, flow foliated, 

and coarsely blocky, having coarsely to finely pumiceous surfaces. Products of coulee and 

fissure are compositionally identical. Phenocrysts: 5–7% plag (mph to 2 mm), many slightly 

rounded; <1% opx (mph only); trace amounts of hornblende and oxide mph; also contains 

sparse plag-opx-oxide clots. 14C age: 2,150±150 yr B.P. (preferred age of Scott, 1987). 

Younger than all nearby units except rdc, fallout of which overlies weak soil developed atop 

fallout of this unit, which may, thus, be a century or more older (Scott, 1987) [SS]

rsc Rhyolite of Separation Creek (late Pleistocene)—Rhyolite (69.6–73.9% SiO2) of Dome 7930 

just west of Chambers Lakes in saddle between Middle Sister and South Sister, about 2.5 km 

from summit of each. Steep, compact, glacially scoured dome is 1 km in diameter and has 

350 m of relief. Most exposures are flow-foliated, dark- or medium-gray vitrophyre, dense 

or pumiceous, and eroded into hackly jointed knobs and ledges. Some layers are vuggy or 

oxidized, and many joints have pink oxidation films. Pale-gray, devitrified, slabby interior 

crops out locally. Apparent chemical heterogeneity may reflect disaggregation and dispersal 

of material from relatively mafic, phenocryst-poor enclaves, 1 to 20 cm across and ovoid 

or crenulate. Phenocrysts: 2–3% plag (mph to 2.5 mm); <1% pyroxenes (mph to 0.7 mm, 
57

mostly in clots with plag and oxides); trace amounts of hornblende (0.1–0.7 mm), rims 

opacitized; sparse oxide mph. At southwest toe of dome, what appears to be early lobe of 

this unit (subunit rsc′; 72.3% SiO2) differs in that it contains larger feldspar phenocrysts, 

some as big as 8 mm. Overlies distal flows of units aeg (here, 27±3 ka) and alg (27±1 ka) at 

Separation Creek. Dome is wrapped by two tongues of dacite coulee of unit dig (14±3 ka). 40Ar/39Ar age: 25.4±1.3 ka [NS, SS] 

rse Rhyolite southeast of Lewis Glacier (late Pleistocene)—Phenocryst-poor rhyolite (73.6–74.0% 

SiO2) lava flows on south slope of South Sister. Flows extend 2.5 km southward from 

exposures as high as 8,050 ft elevation along recently deglaciated trough of Lewis Glacier to 

eroded flow fronts as low as 6,500 ft (northeast of Moraine Lake) and 6,400 ft (just south of 

Newberry flow, unit rdc). All exposures are glacially scoured, and some are deeply incised. 

Two flow units are exposed on glaciated cliff just north of vent for Newberry flow, which 

overran their southeast apron. Nearly all exposures are flow foliated, whether glassy or 

devitrified. Unit is lithologically varied, from massive, block-jointed or platy, white felsite to 

pale-gray, micropumiceous or dark-gray, dense vitrophyre; oxidized layers and breccia zones 

are not uncommon. Fused rheomorphic flow breccia crops out near base of unit both north 

and south of Newberry flow. Phenocrysts: 1–2% plag (mph, rarely as big as 1.5 mm); <<1% 

opx (all mph); <<1% hornblende (mph to as long as 1 mm); sparse oxide mph (0.1–0.2 

mm). Overlies units asn and dml (34±1 ka). Overlain by units dmn (28±1 ka, 25±1 ka) and 

rct (30±8 ka, 24±5 ka); intruded, and overlain, by several domes of unit rdc and heavily 

mantled and widely concealed by proximal pumice falls that preceded extrusion of those 

domes. 40Ar/39Ar ages: 35.0±2.7 ka, 33.6±0.5 ka [BT, SS]

rsf Rhyolite of South Fork Whychus Creek (late Pleistocene)—Large rhyolite (73.8–74.4% 

SiO2) coulee on lower northeast flank of South Sister that extends from just north of Carver 

Lake for 3 km east along Whychus Creek drainage. Glacially scoured coulee is about 1 

km wide and as thick as 300 m proximally, where three (compositionally identical) flow 

units are locally discernible; unit is 150 m thick at its northeast lowland terminus. Vertical 

jointing prominent in high vent region; flow foliated elsewhere, commonly convolutely. 

Lithologically varied, from dense vitrophyre or obsidian to platy, slabby, or block-jointed 

felsite, in addition to local spherulitic zones and oxidized flow-breccia zones. Extensive 

vitrophyre preserved atop flow where only lightly scoured by ice. Phenocrysts: widely 

aphyric, but locally has <1% plag (generally ≤1 mm) and rare opx mph; in thin section, rare 

rounded or partly resorbed plag grains (0.2–1 mm) are probably antecrysts. Overlies unit 

mdl (181±21 ka). Overlain by unit aeg (here, 27±2 ka) and is wrapped by units dcn, drm

(29±1 ka), and mcl. 40Ar/39Ar age: 35.8±2.0 ka [BT, NS, SS, TCB]

rsg Rhyodacite southwest of Golden Lake (late or middle Pleistocene)—Rhyodacite (70.1% 

SiO2) plug that intrudes brick-red mafic (56.5% SiO2) scoria cone of unit mbt at elevation 

of 7,150 ft on distal nose of northwest ridge of Broken Top volcano, 1 km southwest of 

Golden Lake. Glaciated plug is only 70 m in diameter; its finely crystalline interior is block 

jointed or slabby to platy, is variably mottled in shades of gray, and has sparse irregular 

vugs. Locally black, glassy or aphanitic at its margins, plug intrudes and fuses adjacent red 

scoriae. Phenocrysts: 3–5% plag (mph to 1 mm, rarely as big as 4 mm); <1% pyroxenes 

(0.2–0.4 mm, rarely as large as 0.8 mm); sparse oxide mph; contains sparse clots, either all 

plag or plag-pyroxene-oxide. No associated lava flows are preserved. Taylor (1978) suggested that thick stratified pumice-fall deposit preserved atop same ridgecrest about 1 km 

southeast is correlative with compositionally similar plug; if so, plug may be late Pleistocene 

and, thus, related magmatically to nearby South Sister rather than to much older Broken Top. 

Ridge hosting scoria cone and rhyodacite plug consists of middle Pleistocene stack of mafic 

lava flows that erupted at Broken Top. Undated [BT] 

rsw Rhyolite southwest of Lewis Glacier (late Pleistocene)—Phenocryst-poor rhyolite (73.3, 

73.6% SiO2) lava exposed only in two small windows on south flank of South Sister. Upper 

one is cliffy knob that has 150 m of relief, its top at elevation 8,550 ft on south end of 

cleaver between Clark and Lewis Glaciers; remnant of possible vent dome, it sheds large 

talus. Lower exposure about 1 km southwest is 150-m-long window incised through apron 

of unit dcg lavas by uppermost tributary of Mesa Creek. Phenocrysts: 1–2% plag (mph to 

1 mm); <<1% opx (all mph). Contrasts with nearby crystal-rich rhyolite units rct and rmc, 

just east and west. Glacially eroded, flow foliated, varying from coarsely block jointed and 
58

massive to slightly vesicular; mostly gray or grayish brown, weathering pale gray or purplish 

brown. Obsidian widespread in lower window. Base not exposed. Underlies units dcg (31±3 

ka) and aeg. 40Ar/39Ar age: 51±10 ka [SS] 

rtl Rhyodacite southwest of Todd Lake (middle Pleistocene)—Glaciated rhyodacite lava dome 

(69.4% SiO2; see Taylor, 1978) above southwest shoreline of Todd Lake. Dome is 500 m 

wide, 100 m high, and part of cluster of such silicic lavas that extends east and northeast of 

lake (Taylor, 1978). Scattered outcrops through extensive colluvium are mostly gray or pink 

felsite and local remnants of black vitrophyre. Phenocrysts: 5–10% plag (0.5–3 mm); ~1% 

opx and <1% cpx (each mph to 1 mm). Abutted by younger lavas of unit mbl. Undated [BT]

ruc Rhyodacite of Upper Chush Falls (middle Pleistocene)—Rhyodacite (69.1–69.7% SiO2) 

coulee as thick as 110 m and as wide as 1.5 km that crops out 5 to 7 km north of Broken Top 

volcano. Supports spectacular waterfalls on forks of both Park and Whychus Creeks, along 

which strongly flow-foliated felsites form gorge walls. Rocks are partly glassy or devitrified and block jointed or slabby to platy; foliation is locally convolute. Rarely exposed base 

of glaciated flow consists of massive or slabby black glass and flow breccia. Phenocrysts: 

2–3% plag (mph to 1 mm, rarely 2 mm); <1% pyroxenes (all mph); sparse oxide mph. 

Overlies unit bwy. Overlain by avalanche and glacial deposits and mafic lava flows, all 

derived from Broken Top, and by unit apc (31±9 ka) and probably by unit drm (29±1 ka). 

Coulee may represent part of silicic eruptive interval that produced similar pyroxene rhyodacites of Tam McArthur Rim, about 6 km southeast. 40Ar/39Ar age: 169.3±2.1 ka [TCB]

r58 Rhyolite of Dome 5803 (late Pleistocene)—Aphyric rhyolite (74.0% SiO2) lava dome centered 

7.5 km northwest of North Sister and 700 m southeast of Condon Butte. Compact dome is 

300 m in diameter, 60 m high, glacially stripped of its glassy vesicular carapace, and strewn 

with erratics of mafic lavas. Consists of dense, fine-grained felsite, pale gray to cream white, 

that splits platy, chunky, or into small blocks. Obsidian and flow-foliated glassy blocks 

were found only in colluvium on slopes of dome. Overlies apron of uncorrelated glaciated 

mafic lavas. Abutted by lava flow from andesitic Four in One Cone (about 2 ka; Sherrod 

and others, 2004). Located 4 km north of Obsidian Cliffs (unit roc; 38±2 ka), Dome 5803 

is northwesternmost unit in belt of rhyolites for which age of onset has propagated northwestward across Oregon since middle Miocene (MacLeod and others, 1975). 40Ar/39Ar age: 

40.4±1.4 ka [NS]

spt Shevlin Park Tuff (middle Pleistocene)—Compositionally heterogeneous andesite-dacite 

ignimbrite that (within map area) crops out only along 800-m-long reach of North Fork 

Whychus Creek (UTM grid 050/902), ending about 1 km upstream from confluence with 

South Fork Whychus Creek. Outcrop is westernmost exposure of widespread ash-flow sheet 

that extends about 30 km southeast to city of Bend and as far as toe of Newberry Volcano. 

Regionally, multiflow, simple cooling unit typically is dark gray, nonwelded to modestly sintered but locally is densely welded; tuff is seriate from gritty matrix to large scoria blocks. 

Unit fills dissected paleotopography, ranging in thickness from a few meters on plateaus to 

as much as 45 m along paleovalleys (Taylor, 1981; Mimura, 1984, 1992; Sherrod and others, 

2004). Juvenile scoriae predominantly are black, range from 55 to 62% SiO2, and contain 

only about 1% plag and traces of opx, cpx, oxides, and olivine; sparse white to buff pumices range in composition from 64 to 68.5% SiO2 and contain same phenocryst suite minus 

olivine (Conrey and others, 2001a). Maximum thickness along North Fork Whychus Creek 

is 18 m. Except for basal 1 m, unit is unstratified, nongraded, fines poor, rich in scoria lapilli 

and blocks, and indurated though nonwelded; basal 1 m is sintered and locally eutaxitic, 

having black fiamme. Largest black scoriae here are 20 to 35 cm, and largest white pumice 

clasts are about 5 cm; latter make up only 1 to 2% of juvenile lapilli and are virtually absent 

in lower third of deposit. Accidental lithic fragments are nearly all sparsely or moderately 

porphyritic, mafic and intermediate lavas, mostly 1 to 4 cm and as big as 6 cm; lithic fragments are sparse in upper parts of deposit but are as abundant as 5% in its basal few meters. 

Upper half of deposit is dark gray, but much of lower half is oxidized brick red. Northward 

direction of flow at this site was suggested by Mimura (1984) on basis of weak imbrication 

of clasts. Overlain directly by lava flow of unit mws (175±3 ka). Base of ignimbrite rests on 

fine-grained paleosol, dark gray to reddish brown and 6 to 20 cm thick. Beneath paleosol, in 

descending sequence, are following subunits (not possible to represent separately at scale of 

map): 
59

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63

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64

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Andesite and dacite of Broken Top 

TS-324 abt BT 63.60 1.20 15.57 6.03 0.13 1.61 3.87 5.38 1.84 0.38 0.02 99.13 

TS-325 abt BT 59.91 1.22 16.96 6.67 0.13 2.42 5.86 4.70 1.39 0.33 <0.01 99.25 

TS-326 abt BT 64.95 0.99 15.58 5.26 0.11 1.46 3.68 4.87 2.35 0.35 0.21 99.06 

TS-331 abt BT 59.52 1.26 16.74 6.99 0.14 2.71 5.95 4.61 1.35 0.33 0.02 99.39 

TS-332 abt BT 59.95 1.21 16.81 6.72 0.13 2.54 5.84 4.68 1.38 0.33 <0.01 99.52 

TS-335 abt BT 63.03 1.24 15.78 5.94 0.14 1.73 4.15 5.40 1.81 0.39 na 99.17 

TS-343 abt BT 68.64 0.63 15.29 3.67 0.11 0.69 2.18 5.95 2.23 0.22 0.04 99.69 

TS-344 abt BT 63.58 1.21 15.62 6.01 0.13 1.60 3.95 5.28 1.80 0.41 0.09 99.47 

TS-716 abt BT 63.62 1.20 15.88 5.79 0.14 1.51 3.78 5.45 1.90 0.34 na 99.41 

Andesite of Pole Creek scoria cone 

TS-250 acp P 57.73 1.60 16.57 8.60 0.15 3.17 6.53 3.78 1.03 0.44 -0.03 99.20 

TS-614 acp P 56.38 1.64 16.81 8.54 0.16 3.35 6.70 4.57 1.05 0.41 na 99.67 

Andesite dome of Lewis Glacier headwall 

TS-690 adh SS 62.57 1.09 16.23 5.78 0.12 2.00 4.62 5.06 1.86 0.27 na 99.83 

Andesite of Demaris Lake 

TS-187 adl MS 62.86 1.03 16.52 5.49 0.10 1.89 4.81 4.65 1.97 0.27 0.19 98.87 

TS-447 adl MS 61.07 1.16 16.99 6.27 0.11 2.16 5.40 4.36 1.75 0.33 0.09 99.65 

TS-462 adl MS 62.26 1.11 16.81 5.83 0.11 2.01 5.11 4.17 1.85 0.34 0.15 100.14 

TS-562 adl MS 61.30 1.16 16.94 6.05 0.11 2.09 5.32 4.52 1.85 0.27 <0.01 98.79 

TS-679 adl MS 61.99 1.10 16.82 5.78 0.11 1.98 4.99 4.70 1.88 0.26 na 100.28 

TS-681 adl MS 61.95 1.09 16.77 5.72 0.11 1.97 5.14 4.72 1.87 0.26 na 99.68 

Andesite tephra fall at base of east face of Middle Sister 

TS-475A aef MS 62.58 1.25 16.25 6.37 0.14 1.83 4.41 4.75 1.58 0.44 0.83 99.32 

TS-475B aef MS 62.66 1.25 16.22 6.36 0.14 1.78 4.41 4.77 1.57 0.43 0.94 98.87 

TS-476A aef MS 62.57 1.25 16.35 6.51 0.14 1.68 4.40 4.78 1.48 0.43 1.25 98.69 

TS-476B aef MS 63.07 1.26 16.40 6.07 0.13 1.55 4.31 4.64 1.73 0.44 1.76 98.38 

Andesite of Eugene Glacier 

TS-45A aeg SS 60.28 1.21 17.24 6.23 0.11 2.63 5.93 4.38 1.30 0.29 0.41 98.80 

TS-45B aeg SS 56.61 1.16 17.45 8.74 0.11 3.49 6.71 4.08 0.93 0.33 1.31 98.18 

TS-46 aeg SS 58.24 1.17 17.18 7.07 0.13 3.45 6.30 4.44 1.31 0.32 <0.01 99.70 

TS-49 aeg SS 60.43 1.11 16.66 6.83 0.11 2.65 5.38 4.45 1.70 0.28 0.85 99.22 

TS-195 aeg SS 57.09 1.24 17.36 7.59 0.13 3.68 6.77 4.30 1.13 0.29 -0.09 99.26 

TS-196 aeg SS 57.16 1.24 17.38 7.58 0.13 3.64 6.77 4.28 1.15 0.28 -0.05 99.15 

TS-197 aeg SS 56.55 1.25 17.38 7.70 0.13 3.96 7.11 4.19 1.05 0.29 0.03 99.17 

TS-198 aeg SS 57.41 1.24 17.12 7.53 0.13 3.66 6.75 4.29 1.19 0.28 0.27 98.89 

TS-200 aeg SS 57.56 1.27 17.18 7.54 0.13 3.44 6.70 4.32 1.19 0.29 0.16 99.16 

TS-202 aeg SS 56.52 1.25 17.46 7.77 0.13 3.86 7.10 4.18 1.05 0.29 -0.05 98.68 

TS-203 aeg SS 57.54 1.26 17.09 7.59 0.13 3.51 6.67 4.31 1.19 0.29 0.40 98.49 

TS-229 aeg SS 56.73 1.26 17.45 7.74 0.13 3.79 6.94 4.22 1.07 0.29 -0.21 99.90 

TS-230 aeg SS 56.49 1.27 17.47 7.76 0.14 3.90 7.02 4.21 1.05 0.30 -0.17 99.80 

TS-232 aeg SS 56.39 1.32 17.10 8.85 0.14 3.35 6.88 4.30 1.00 0.28 -0.19 99.62 

TS-259 aeg SS 56.65 1.25 17.48 7.64 0.13 3.87 7.03 4.20 1.08 0.28 0.28 99.16 

Table 1. Chemical data for Three Sisters volcanic cluster.

[Data are listed numerically by sample number within map units, which are listed alphabetically by unit label: initial letter a indicates andesite; b, basalt; d, 

dacite; m, (mafic) basaltic andesite; r, rhyolite or rhyodacite. Volcanic group refers to groupings of volcanic rocks in Correlation of Map Units and in List of 

Map Units: BT, Broken Top and vicinity; MS, Middle Sister and vicinity; NS, North Sister and vicinity; P, peripheral centers and other lavas; s of BT, surficial 

deposits of Broken Top; SS, South Sister and vicinity. Other abbreviations: LOI, weight loss on ignition at 900°C; na, not analyzed. Whole-rock major-element 

analyses are given in weight percent; trace elements, in ppm. Major elements normalized to volatile-free total of 99.6% (leaving 0.4% for trace oxides and halogens). Original total is volatile-free weight before normalization. FeO* is total Fe calculated as FeO. Determinations by x-ray fluorescence spectrometry at U.S. 

Geological Survey laboratory in Lakewood, Colo., supervised by J.E. Taggart (Taggart and others, 1987), or at GeoAnalytical Lab, Washington State University, 

Pullman, Wash., supervised by J.A. Wolff (www.sees.wsu.edu/Geolab/note/xrf.html)]
65

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Andesite and dacite of Broken Top

564 41 na 23 25 18 14 24 na 38 355 114 50 82 276

468 34 6 41 15 8 8 15 3 26 427 139 27 72 170

750 55 <5 14 16 33 13 47 3 53 292 76 40 55 293

457 34 6 37 16 11 8 13 3 24 423 152 26 71 170

465 33 <5 34 17 10 8 <10 3 26 426 137 27 64 171

525 42 0 19 20 20 10 26 0 34 366 79 37 90 202

712 49 <5 <2 16 22 10 30 3 39 241 8 39 62 257

581 41 <5 19 15 18 11 43 3 36 347 80 39 78 222

545 40 1 30 19 20 11 26 0 38 330 76 37 88 219

Andesite of Pole Creek scoria cone

334 26 6 33 20 12 7 13 14 17 517 216 28 93 128

358 29 7 67 21 11 5 20 3 16 531 226 27 88 125

Andesite dome of Lewis Glacier headwall

552 38 7 39 19 19 8 22 1 33 403 123 27 78 197

Andesite of Demaris Lake

588 33 9 26 20 19 12 26 10 41 388 107 27 69 212

570 36 <5 25 16 13 8 30 3 32 421 142 22 68 195

605 38 <5 23 15 17 8 <10 3 36 406 127 22 63 202

597 34 <5 25 16 14 8 20 8 32 420 127 23 58 193

568 38 12 26 18 19 8 22 7 35 400 125 25 73 196

552 33 12 30 19 17 9 17 8 36 406 130 25 69 191

Andesite tephra fall at base of east face of Middle Sister

627 45 <5 12 16 15 8 42 3 25 415 93 29 105 211

637 43 <5 22 16 13 9 29 3 25 419 90 30 116 212

646 42 <5 2 16 18 8 29 3 27 418 84 29 79 212

620 40 39 4 17 13 9 35 3 27 415 102 31 89 213

Andesite of Eugene Glacier

456 29 25 25 21 12 9 11 9 22 469 144 23 68 173

370 26 60 28 22 12 7 17 34 15 478 159 23 65 151

417 27 34 42 21 14 8 17 18 21 466 151 23 79 168

511 30 13 37 22 16 8 8 17 31 423 144 29 72 180

378 23 38 57 21 11 8 <10 23 18 497 168 24 79 154

381 24 33 66 21 10 7 <10 24 19 500 175 23 77 153

361 30 46 65 20 12 7 16 24 16 506 174 22 80 147

383 27 36 54 20 13 8 17 22 20 487 170 23 78 156

390 29 31 61 20 15 8 <10 21 21 485 183 24 83 156

353 29 39 50 19 12 7 15 26 16 513 168 23 76 146

387 28 27 68 20 12 8 22 18 21 487 177 24 84 155

367 30 37 64 19 15 7 17 24 17 510 174 23 85 151

362 29 41 75 20 14 7 12 25 16 513 171 22 85 145

381 30 9 94 19 14 8 17 16 16 556 234 23 87 134

370 27 47 68 22 11 9 16 30 18 505 172 24 81 153
66

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Andesite of Eugene Glacier—Continued 

TS-274 aeg SS 59.81 1.21 16.69 6.87 0.12 2.84 5.77 4.53 1.48 0.29 0.13 99.09 

TS-275 aeg SS 58.12 1.26 16.97 7.42 0.13 3.33 6.37 4.44 1.26 0.29 0.08 99.22 

TS-276 aeg SS 58.27 1.27 16.94 7.40 0.13 3.23 6.34 4.46 1.28 0.29 0.09 98.80 

TS-277 aeg SS 58.27 1.26 16.91 7.39 0.13 3.26 6.41 4.41 1.27 0.29 -0.23 98.96 

TS-278 aeg SS 57.29 1.25 17.25 7.63 0.13 3.58 6.77 4.29 1.14 0.28 -0.07 98.75 

TS-279 aeg SS 56.84 1.24 17.34 7.62 0.13 3.84 6.97 4.23 1.10 0.28 0.04 99.35 

TS-485 aeg SS 57.55 1.18 17.45 7.31 0.13 3.86 6.80 3.84 1.16 0.32 <0.01 99.86 

TS-513 aeg SS 57.44 1.22 17.20 7.59 0.13 3.70 6.87 4.00 1.13 0.32 <0.01 99.02 

TS-546 aeg SS 56.33 1.28 17.53 7.78 0.14 4.00 7.08 4.16 1.03 0.26 <0.01 98.84 

TS-547 aeg SS 56.04 1.32 17.61 7.85 0.15 3.96 7.10 4.28 1.02 0.26 <0.01 99.53 

TS-573 aeg SS 56.39 1.29 17.59 7.75 0.13 3.88 7.00 4.25 1.06 0.26 <0.01 99.08 

TS-577 aeg SS 59.15 1.19 16.83 6.99 0.13 3.07 6.01 4.49 1.40 0.34 <0.01 100.03 

TS-585 aeg SS 57.07 1.17 17.48 7.33 0.13 3.89 6.85 4.21 1.14 0.33 0.36 99.13 

TS-586 aeg SS 56.21 1.19 17.73 7.58 0.13 4.15 7.28 4.02 0.99 0.31 0.31 99.40 

TS-583 aeg SS 57.03 1.17 17.44 7.35 0.13 3.89 6.87 4.23 1.15 0.34 0.17 99.37 

TS-584 aeg SS 56.69 1.18 17.50 7.43 0.13 4.06 7.02 4.18 1.07 0.33 <0.01 100.14 

TS-721 aeg SS 57.11 1.25 17.42 7.38 0.13 3.68 6.82 4.40 1.15 0.27 na 99.60 

Andesite tephra fall of Hodge Crest 

TS-713A ahc SS 61.85 1.18 16.39 6.05 0.12 2.06 4.89 4.98 1.79 0.28 na 99.18 

TS-713D ahc SS 61.68 1.20 16.31 6.06 0.13 2.20 4.95 4.70 2.09 0.28 na 98.57 

TS-713E ahc SS 61.65 1.19 16.40 6.08 0.12 2.25 5.06 4.67 1.90 0.28 na 98.77 

Andesite intrusive complex of Hodge Crest 

TS-44 ahi SS 60.52 1.19 16.64 6.76 0.11 2.42 5.35 4.65 1.62 0.33 0.82 98.74 

TS-689 ahi SS 56.56 1.19 17.62 7.24 0.13 4.03 7.24 4.24 1.10 0.25 na 99.24 

TS-714 ahi SS 61.95 1.30 15.96 6.39 0.11 2.21 4.50 4.94 1.89 0.34 na 98.92 

Andesite of headwaters of Linton Creek 

TS-269 ahl MS 57.92 1.33 16.85 8.20 0.14 2.90 6.12 4.60 1.22 0.31 0.37 98.71 

TS-281 ahl MS 57.76 1.33 16.98 8.25 0.15 2.90 6.11 4.64 1.17 0.31 0.03 99.14 

TS-282 ahl MS 57.73 1.33 16.96 8.25 0.14 2.90 6.16 4.63 1.18 0.32 0.39 98.69 

TS-283 ahl MS 57.86 1.32 16.85 8.25 0.14 2.88 6.11 4.68 1.18 0.32 -0.24 99.32 

TS-285 ahl MS 57.14 1.33 17.01 8.49 0.14 3.08 6.42 4.56 1.12 0.30 -0.26 99.53 

Andesite of Linton Creek 

TS-267 alc MS 61.77 1.22 16.39 6.35 0.13 1.98 4.65 5.06 1.72 0.34 0.01 99.65 

TS-284 alc MS 61.60 1.21 16.56 6.37 0.13 2.00 4.63 5.08 1.69 0.33 0.64 98.63 

TS-286 alc MS 61.74 1.22 16.34 6.39 0.13 1.98 4.61 5.12 1.73 0.33 -0.10 99.38 

TS-293 alc MS 61.84 1.23 16.19 6.39 0.13 1.99 4.67 5.05 1.78 0.32 0.84 98.41 

TS-298A alc MS 61.90 1.22 16.35 6.37 0.13 1.91 4.62 5.02 1.75 0.33 0.00 99.28 

TS-298B alc MS 61.86 1.22 16.29 6.39 0.13 2.01 4.64 4.99 1.75 0.32 -0.03 99.02 

TS-299 alc MS 61.89 1.21 16.35 6.38 0.13 2.01 4.62 4.95 1.74 0.32 -0.04 99.30 

TS-300 alc MS 62.16 1.20 16.34 6.28 0.13 1.89 4.50 4.98 1.78 0.32 0.06 99.35 

TS-301 alc MS 62.08 1.20 16.20 6.30 0.13 1.97 4.62 5.00 1.78 0.32 0.42 98.99 

TS-548 alc MS 61.73 1.26 16.31 6.42 0.13 2.03 4.70 4.95 1.78 0.29 <0.01 99.55 

TS-549 alc MS 62.99 1.14 16.15 5.84 0.12 1.88 4.40 4.89 1.94 0.26 0.86 98.67 

TS-557 alc MS 61.88 1.24 16.43 6.34 0.14 1.99 4.67 4.89 1.74 0.29 0.05 98.83 

Andesite west of Lost Creek Glacier 

TS-256 alg SS 62.20 1.17 16.40 6.06 0.12 2.01 4.62 4.84 1.85 0.33 -0.03 99.60 

TS-258 alg SS 62.28 1.16 16.32 6.04 0.12 2.00 4.64 4.88 1.84 0.32 -0.09 99.47 

TS-260 alg SS 61.67 1.18 16.36 6.25 0.12 2.18 4.91 4.86 1.76 0.32 -0.18 99.82 

TS-261 alg SS 61.45 1.19 16.41 6.37 0.12 2.26 4.97 4.78 1.72 0.31 -0.08 99.52 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
67

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Andesite of Eugene Glacier—Continued

459 31 19 43 20 15 9 15 12 26 448 152 29 79 180

402 31 25 53 20 11 8 14 17 22 477 172 24 80 164

408 29 23 54 20 10 7 15 15 22 475 172 24 80 166

412 30 24 56 23 17 8 <10 17 22 478 170 23 77 162

379 22 33 62 23 9 8 14 23 19 495 170 24 83 154

363 29 49 63 20 11 9 20 27 18 506 175 24 80 150

413 27 38 47 16 10 6 <10 24 18 501 177 21 72 157

na na na na na na na na na na na na na na na

389 24 28 45 14 6 5 <10 22 17 520 194 19 69 139

382 27 31 32 16 7 5 15 21 14 521 182 19 66 138

391 28 28 53 14 9 7 <10 19 14 502 211 23 68 171

476 34 13 33 16 11 6 <10 5 23 464 143 23 67 173

410 27 30 50 15 11 5 <10 20 20 498 170 20 70 154

371 29 36 58 16 10 6 14 21 17 527 179 17 71 139

410 29 31 51 16 8 5 <10 23 19 501 170 21 67 155

391 28 32 45 15 8 5 <10 21 17 514 158 19 65 150

385 33 41 53 20 10 7 19 20 19 499 179 23 83 150

Andesite tephra fall of Hodge Crest

533 37 4 27 19 19 8 23 0 33 420 137 27 86 188

521 37 4 27 19 19 7 23 2 34 417 139 28 82 185

518 40 4 27 20 20 8 22 2 33 422 141 26 83 187

Andesite intrusive complex of Hodge Crest

504 28 22 54 20 17 8 12 11 29 428 131 29 73 185

364 26 50 62 19 13 6 14 24 18 513 176 21 82 138

565 45 3 21 20 19 10 24 0 33 393 268 30 74 213

Andesite of headwaters of Linton Creek

439 31 <5 38 22 12 9 16 7 19 534 191 25 97 158

450 34 <5 52 19 17 8 16 4 18 538 178 24 88 159

440 37 <5 40 23 13 9 13 6 19 540 188 25 89 160

442 35 <5 39 23 16 9 18 6 19 538 185 25 88 158

423 30 <5 56 21 14 8 20 7 17 549 211 24 81 149

Andesite of Linton Creek

543 39 <5 14 22 18 10 19 9 31 434 121 33 85 208

571 34 <5 15 22 15 10 21 7 30 427 120 29 86 212

545 37 <5 11 21 19 10 20 6 30 429 111 32 79 205

542 34 <5 16 20 15 10 21 4 30 423 109 33 81 205

550 31 <5 12 20 14 9 20 7 31 429 119 33 75 206

540 35 <5 24 21 18 10 23 6 31 426 114 32 83 205

551 33 <5 12 22 15 10 22 5 30 429 112 33 79 207

557 35 <5 17 20 12 10 20 4 32 418 110 33 86 208

541 36 <5 16 21 16 10 18 3 31 422 123 33 85 210

603 39 <5 6 15 13 8 18 3 29 427 114 23 64 195

644 37 <5 8 13 13 8 20 3 35 403 97 22 63 200

595 37 <5 3 16 15 8 26 3 29 427 109 24 62 196

Andesite west of Lost Creek Glacier

563 38 7 34 20 18 10 29 12 34 400 116 37 72 223

563 34 <5 28 21 17 9 23 7 32 400 113 35 70 218

533 32 10 36 19 18 9 21 12 32 411 117 33 78 213

527 36 9 32 20 18 9 25 9 31 417 128 33 68 208
68

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Andesite west of Lost Creek Glacier—Continued 

TS-262 alg SS 62.11 1.17 16.40 6.12 0.12 2.00 4.63 4.90 1.83 0.32 -0.04 99.59 

TS-263 alg SS 62.16 1.16 16.39 6.05 0.12 2.04 4.66 4.87 1.84 0.32 -0.10 99.66 

TS-264 alg SS 62.29 1.17 16.30 6.06 0.12 1.98 4.61 4.89 1.86 0.32 -0.14 99.61 

TS-265 alg SS 62.05 1.15 16.44 6.10 0.12 2.05 4.68 4.86 1.82 0.32 -0.06 99.36 

TS-272 alg SS 62.08 1.17 16.12 6.41 0.12 2.11 4.79 4.70 1.81 0.28 0.05 99.47 

TS-312 alg SS 62.25 1.16 16.37 6.03 0.11 1.96 4.61 4.94 1.86 0.31 -0.10 99.20 

TS-313 alg SS 62.12 1.16 16.36 6.10 0.12 2.04 4.64 4.91 1.84 0.31 -0.16 99.24 

TS-314 alg SS 62.11 1.17 16.23 6.14 0.12 2.04 4.71 4.94 1.82 0.32 -0.06 99.43 

TS-316 alg SS 62.01 1.17 16.36 6.19 0.12 2.05 4.68 4.90 1.81 0.32 -0.06 99.26 

TS-317 alg SS 62.07 1.17 16.42 6.10 0.12 1.99 4.65 4.94 1.83 0.32 -0.09 99.49 

TS-318 alg SS 61.90 1.17 16.40 6.17 0.12 2.07 4.65 4.97 1.82 0.33 -0.03 99.60 

TS-319 alg SS 62.19 1.17 16.32 6.07 0.12 2.02 4.55 4.99 1.84 0.32 -0.04 99.45 

TS-273 alg SS 63.46 1.04 16.17 5.62 0.11 1.75 4.29 4.94 1.96 0.27 0.15 99.20 

Andesite of Lewis Glacier headwall 

TS-580 alh SS 62.03 1.13 16.20 6.10 0.12 1.98 4.65 5.20 1.82 0.36 0.16 100.19 

TS-687 alh SS 62.93 1.10 16.17 5.79 0.11 1.87 4.44 4.99 1.93 0.27 na 99.80 

TS-688 alh SS 62.61 1.10 16.19 5.76 0.12 1.99 4.65 5.05 1.86 0.27 na 99.32 

TS-688R alh SS 62.58 1.10 16.17 5.81 0.12 2.00 4.64 5.05 1.86 0.27 na 99.57 

Andesite south of Lewis Glacier 

TS-479 als SS 62.35 1.04 16.18 6.11 0.11 2.48 5.21 4.08 1.76 0.28 <0.01 100.33 

Andesite agglutinate north of Hayden Glacier 

TS-182 anh NS 57.86 1.42 16.85 8.11 0.14 2.86 6.04 4.73 1.23 0.36 -0.24 99.32 

TS-472 anh NS 58.04 1.42 16.90 8.20 0.15 2.93 6.13 4.23 1.20 0.39 <0.01 100.21 

Andesite of Obsidian Creek 

TS-104 aoc MS 61.53 0.77 16.40 5.52 0.10 3.89 5.73 3.89 1.60 0.19 0.09 100.20 

TS-294 aoc MS 59.91 0.83 16.60 6.02 0.11 4.36 6.38 3.83 1.41 0.14 0.10 99.58 

TS-295 aoc MS 60.04 0.82 16.61 5.95 0.11 4.32 6.31 3.87 1.42 0.14 0.05 99.54 

TS-296 aoc MS 62.18 0.74 16.07 5.37 0.10 3.70 5.72 3.93 1.66 0.14 0.39 99.16 

TS-386 aoc MS 58.60 0.89 16.71 6.38 0.11 4.73 6.95 3.81 1.25 0.17 <0.01 98.92 

TS-426 aoc MS 61.78 0.75 16.17 5.48 0.10 3.82 5.82 3.92 1.61 0.15 0.14 99.14 

TS-427 aoc MS 61.73 0.79 16.41 5.54 0.10 3.74 5.90 3.60 1.60 0.19 0.79 98.91 

TS-553 aoc MS 60.81 0.79 16.46 5.75 0.11 4.12 6.08 3.84 1.52 0.11 0.10 99.25 

TS-559 aoc MS 60.12 0.83 16.61 5.95 0.12 4.32 6.35 3.75 1.43 0.11 <0.01 99.56 

TS-560 aoc MS 62.39 0.74 16.00 5.29 0.12 3.68 5.62 3.96 1.67 0.13 0.21 99.61 

TS-610 aoc MS 63.15 0.71 15.91 4.92 0.10 3.51 5.40 4.05 1.75 0.11 na 100.64 

Andesite of Park Creek 

TS-498 apc SS 61.44 1.20 16.26 6.68 0.12 2.45 5.33 4.08 1.73 0.32 0.02 99.86 

TS-621 apc SS 60.96 1.20 16.20 6.57 0.12 2.49 5.39 4.67 1.74 0.27 na 97.43 

TS-622 apc SS 61.60 1.13 16.16 6.27 0.12 2.39 5.20 4.68 1.79 0.25 na 98.58 

Andesite of Prouty Glacier headwall 

TS-708 aph SS 61.40 1.28 16.17 6.42 0.13 2.23 5.08 4.81 1.75 0.32 na 99.09 

TS-712 aph SS 61.61 1.28 16.11 6.31 0.09 2.31 4.90 4.82 1.84 0.32 na 98.26 

Andesite tephra fall near base of Prouty Glacier headwall 

TS-709 aph′ SS 59.29 1.31 16.66 7.05 0.13 2.81 6.05 4.62 1.39 0.29 na 98.52 

TS-710 aph′ SS 61.81 1.28 16.08 6.49 0.11 1.84 5.04 4.83 1.80 0.32 na 98.83 

 [lithic]

TS-711 aph′ SS 63.74 1.10 15.95 5.50 0.12 1.66 4.21 5.12 1.89 0.30 na 99.10 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
69

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Andesite west of Lost Creek Glacier—Continued

565 36 6 36 21 19 10 24 8 33 400 111 36 74 222

558 38 7 31 22 21 11 21 11 34 401 104 37 77 222

556 34 6 35 21 16 10 32 9 34 401 109 35 71 222

556 36 7 31 21 19 10 17 8 32 398 114 32 74 219

548 37 <5 30 20 16 11 28 7 34 409 145 25 78 201

562 37 <5 34 22 15 10 23 8 33 395 110 34 74 221

565 36 7 33 20 17 9 23 7 33 399 111 34 69 221

559 37 <5 27 22 19 10 17 8 33 399 118 32 74 216

551 33 6 35 22 14 10 24 11 31 397 122 32 72 216

559 34 8 31 20 16 10 19 9 33 397 108 34 74 218

557 42 6 34 21 18 9 31 7 33 398 109 33 78 220

567 35 11 34 20 20 9 22 9 33 394 117 30 75 218

589 36 6 18 20 17 10 <10 7 38 391 101 26 72 213

Andesite of Lewis Glacier headwall

606 37 <5 16 16 9 8 28 3 31 415 123 23 62 201

574 36 4 23 20 20 9 19 0 36 399 120 26 81 194

552 39 5 24 21 19 8 22 0 34 404 121 26 81 190

556 36 6 23 20 20 8 19 0 34 407 120 27 80 192

Andesite south of Lewis Glacier

566 30 <5 20 13 10 8 <10 3 31 404 147 21 59 187

Andesite agglutinate north of Hayden Glacier

434 34 <5 45 23 15 9 22 9 18 516 194 27 89 177

516 64 <5 42 22 72 15 44 25 25 540 282 43 114 198

Andesite of Obsidian Creek

483 25 63 33 18 10 5 10 49 33 352 107 16 60 111

422 20 75 47 17 9 5 15 48 28 384 125 17 64 106

436 23 77 34 18 11 6 12 53 29 381 128 17 56 107

487 21 69 35 16 11 6 15 40 34 351 116 17 56 111

378 23 81 33 19 11 7 12 na 26 436 161 23 62 130

486 23 67 43 22 13 8 12 na 35 374 149 22 63 137

520 27 56 30 12 11 3 <10 27 31 357 131 15 52 107

518 22 54 32 11 11 4 18 44 28 369 133 15 48 104

486 21 59 20 13 8 4 <10 48 27 383 130 15 48 103

557 26 51 28 11 10 5 <10 39 33 348 118 14 48 109

514 23 63 34 16 12 5 12 39 37 349 115 18 55 110

Andesite of Park Creek

573 37 <5 34 15 12 8 15 3 29 406 147 20 59 189

518 35 14 36 18 16 8 19 8 32 404 165 24 75 183

522 34 14 36 19 17 8 19 8 34 399 158 24 70 185

Andesite of Prouty Glacier headwall

543 40 8 27 21 19 9 23 1 32 412 139 29 87 200

537 39 5 23 19 18 9 23 1 32 397 133 29 88 204

Andesite tephra fall near base of Prouty Glacier headwall

469 36 21 42 20 19 8 21 12 28 451 175 26 84 176

538 38 8 25 21 17 10 21 3 30 408 154 28 81 199

615 42 3 21 19 19 10 23 0 38 371 139 29 80 218
70

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Andesite south of Newberry flow 

TS-589 asn SS 61.72 1.25 16.51 6.27 0.12 1.83 4.72 4.89 1.85 0.44 0.25 99.57 

Andesite of southwest slope of Middle Sister 

TS-225 asw MS 61.53 1.13 16.99 6.22 0.14 1.93 4.61 5.24 1.45 0.36 -0.01 99.06 

TS-280 asw MS 61.56 1.14 16.84 6.18 0.14 1.93 4.65 5.33 1.45 0.36 -0.11 99.33 

Hornblende-bearing andesite southeast of Yapoah Crater 

TS-764 asy NS 58.68 1.07 17.96 6.82 0.15 2.50 5.75 5.16 1.14 0.38 na 99.62 

Avalanche blocks (in the debris-avalanche deposit unit) 

TS-517 av s of BT 57.99 1.05 17.36 7.15 0.11 3.80 6.87 3.78 1.20 0.29 1.75 97.56 

TS-618 av s of BT 57.24 1.09 17.62 7.15 0.12 3.70 6.94 4.28 1.21 0.24 na 97.63 

Andesite west of upper Alder Creek 

TS-655 awa NS 57.53 1.53 16.80 8.14 0.14 2.97 6.15 4.76 1.26 0.33 na 99.26 

TS-754 awa NS 57.60 1.53 16.73 8.21 0.14 2.96 6.12 4.73 1.25 0.33 na 99.23 

Andesite west of Collier Glacier 

TS-367 awc MS 62.17 1.27 16.12 6.45 0.14 1.78 4.46 5.18 1.58 0.45 <0.01 99.48 

TS-369 awc MS 62.07 1.26 16.09 6.47 0.14 1.81 4.50 5.24 1.58 0.44 <0.01 99.64 

TS-371 awc MS 62.00 1.26 16.20 6.43 0.14 1.81 4.41 5.31 1.60 0.43 <0.01 99.59 

TS-372 awc MS 62.06 1.25 16.17 6.43 0.14 1.80 4.42 5.30 1.61 0.42 <0.01 99.18 

TS-374 awc MS 62.25 1.26 16.01 6.42 0.14 1.81 4.43 5.22 1.59 0.45 <0.01 99.51 

TS-376 awc MS 62.11 1.27 16.08 6.44 0.14 1.80 4.49 5.23 1.58 0.46 <0.01 99.74 

TS-377 awc MS 61.63 1.33 16.16 6.56 0.14 1.90 4.51 5.30 1.62 0.45 <0.01 98.59 

TS-380 awc MS 62.25 1.21 16.24 6.31 0.14 1.77 4.38 5.27 1.57 0.44 <0.01 99.35 

TS-392 awc MS 62.12 1.25 16.10 6.41 0.14 1.79 4.44 5.32 1.59 0.44 <0.01 99.58 

TS-393 awc MS 62.19 1.27 16.00 6.44 0.14 1.81 4.45 5.26 1.60 0.45 0.15 99.62 

TS-394 awc MS 62.18 1.27 16.07 6.42 0.14 1.80 4.43 5.24 1.62 0.44 0.55 99.16 

TS-396 awc MS 62.18 1.27 16.02 6.43 0.14 1.80 4.44 5.24 1.60 0.46 0.10 98.83 

TS-399 awc MS 62.00 1.26 16.08 6.44 0.14 1.85 4.48 5.35 1.59 0.43 <0.01 99.13 

Andesite of West Fork Park Creek 

TS-506 awf SS 59.58 1.22 16.84 6.98 0.13 3.05 5.94 4.11 1.42 0.33 <0.01 99.97 

TS-510 awf SS 59.96 1.21 16.42 7.06 0.13 2.86 5.99 4.13 1.52 0.32 <0.01 99.50 

TS-518 awf SS 59.66 1.23 16.67 7.06 0.12 2.76 6.21 4.10 1.48 0.32 <0.01 99.17 

TS-574 awf SS 60.71 1.30 16.28 7.00 0.14 2.33 5.32 4.63 1.60 0.29 <0.01 99.09 

TS-576 awf SS 60.26 1.19 16.39 6.81 0.12 2.76 5.75 4.39 1.61 0.33 <0.01 99.66 

TS-718 awf SS 59.20 1.24 16.79 6.96 0.13 2.83 6.12 4.55 1.50 0.27 na 99.82 

Andesite west of Middle Sister 

TS-302 awm MS 59.72 1.47 16.30 7.58 0.15 2.40 5.23 5.00 1.37 0.38 -0.15 99.57 

TS-303 awm MS 60.21 1.44 16.36 7.23 0.15 2.25 5.02 5.14 1.42 0.39 -0.19 99.25 

TS-305 awm MS 60.17 1.44 16.29 7.29 0.15 2.24 5.07 5.14 1.42 0.39 -0.10 99.65 

TS-306 awm MS 60.28 1.43 16.27 7.22 0.15 2.27 5.05 5.14 1.42 0.39 -0.16 99.80 

TS-307 awm MS 59.11 1.50 16.52 7.76 0.15 2.46 5.46 4.94 1.31 0.39 0.58 98.90 

TS-416 awm MS 59.23 1.52 16.49 7.89 0.15 2.51 5.52 4.57 1.32 0.41 0.40 99.05 

TS-418 awm MS 60.62 1.42 16.33 7.21 0.15 2.21 4.97 4.81 1.45 0.42 <0.01 99.40 

TS-420 awm MS 60.54 1.43 16.39 7.18 0.15 2.21 5.05 4.81 1.43 0.42 1.49 99.05 

TS-425 awm MS 59.76 1.48 16.40 7.58 0.15 2.42 5.33 4.71 1.36 0.41 <0.01 99.01 

Andesite west of Renfrew Glacier 

TS-400 awr MS 60.94 1.37 16.13 6.99 0.14 2.12 4.92 5.09 1.45 0.44 <0.01 100.02 

TS-401 awr MS 60.83 1.41 16.16 7.04 0.14 2.13 4.98 5.04 1.45 0.43 <0.01 99.87 

TS-408 awr MS 61.48 1.33 16.37 6.73 0.14 2.00 4.70 4.89 1.53 0.44 <0.01 99.79 

TS-413 awr MS 61.39 1.32 16.40 6.78 0.14 2.03 4.69 4.90 1.52 0.44 <0.01 99.62 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
71

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Andesite south of Newberry flow

630 42 <5 17 15 16 10 27 3 33 399 119 28 68 226

Andesite of southwest slope of Middle Sister

548 37 <5 19 21 15 10 23 6 22 488 77 32 90 191

538 36 <5 14 20 16 11 27 5 23 497 80 33 87 194

Hornblende-bearing andesite southeast of Yapoah Crater

431 38 1 22 21 17 10 22 5 13 557 110 28 96 175

Avalanche blocks (in the debris-avalanche deposit unit)

na na na na na na na na na na na na na na na

400 34 37 77 20 17 7 19 20 22 577 184 24 79 143

Andesite west of upper Alder Creek

442 32 4 54 21 16 8 21 5 19 465 222 28 90 180

430 37 3 51 21 15 9 21 9 21 467 218 29 90 179

Andesite west of Collier Glacier

614 43 <5 9 15 16 8 19 3 26 422 <5 29 82 211

603 41 <5 9 16 16 9 21 3 27 429 96 30 83 213

630 62 na 22 32 35 13 34 na 31 481 119 40 120 279

577 39 na 19 24 16 13 28 na 27 450 131 45 100 271

623 42 <5 8 17 17 9 30 3 25 419 97 31 84 214

610 41 <5 3 17 14 8 37 3 26 434 96 30 81 209

574 39 na 18 23 20 13 22 na 29 446 129 45 99 271

617 42 <5 6 16 14 9 23 3 25 432 77 29 86 206

575 39 na 20 24 22 13 25 na 27 453 124 45 89 268

621 41 <5 9 16 15 9 37 3 26 423 79 30 84 212

616 42 <5 9 16 17 9 27 3 27 416 92 30 89 211

615 44 na 12 16 15 9 42 3 26 421 inf 30 86 210

574 38 na 23 26 16 13 27 na 28 453 121 44 97 268

Andesite of West Fork Park Creek

495 33 12 32 16 9 6 21 4 25 461 143 23 66 175

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

542 36 <5 3 14 12 11 13 3 28 432 185 28 64 227

542 35 <5 32 15 11 8 <10 4 27 427 153 21 66 186

476 30 21 37 20 18 8 18 13 26 448 179 26 78 173

Andesite west of Middle Sister

495 33 <5 30 22 15 10 24 8 23 453 143 35 88 191

511 38 <5 24 22 17 9 23 7 23 446 120 36 92 199

506 36 <5 24 22 15 10 26 8 23 448 130 35 89 192

512 36 <5 29 23 16 11 14 7 23 453 131 35 93 196

477 36 <5 44 22 16 10 23 6 21 456 154 33 95 189

512 39 <5 30 17 12 8 <10 3 21 459 156 28 83 187

562 41 <5 11 17 12 8 22 3 25 450 126 29 82 200

565 42 <5 17 17 15 8 17 3 23 451 128 29 79 199

546 39 <5 25 17 12 7 29 3 23 451 149 27 81 187

Andesite west of Renfrew Glacier

572 43 <5 15 16 15 8 <10 3 24 439 123 29 81 197

567 38 <5 11 16 14 8 31 3 24 449 119 28 80 197

595 40 <5 11 16 13 8 29 3 24 433 100 31 82 206

589 39 <5 15 17 15 8 31 3 25 436 105 30 81 206
72

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Andesite west of Renfrew Glacier—Continued 

TS-414 awr MS 61.58 1.33 16.37 6.70 0.14 1.98 4.61 4.92 1.55 0.42 0.06 99.15 

TS-415 awr MS 61.27 1.34 16.39 6.86 0.15 2.02 4.68 4.93 1.53 0.44 <0.01 99.65 

Andesite of Whychus Creek 

TS-439 awy BT 61.68 1.11 16.81 6.01 0.09 2.13 5.29 4.27 1.79 0.43 0.03 11.67 

TS-452 awy BT 62.03 1.11 16.69 5.94 0.11 2.08 5.15 4.30 1.87 0.32 0.34 99.06 

TS-453 awy BT 61.30 1.08 17.13 6.04 0.13 1.97 4.98 5.06 1.56 0.34 0.17 99.43 

Basalt of Alder Creek 

TS-601 bac NS 51.23 1.44 17.23 9.13 0.16 7.49 8.39 3.61 0.61 0.30 na 98.38 

TS-652 bac NS 53.11 1.32 18.09 8.13 0.15 5.28 8.58 3.78 0.81 0.35 na 97.56 

Basalt of Brush Draw 

TS-528 bbd NS 51.06 1.34 18.73 8.11 0.14 5.48 10.37 3.36 0.65 0.34 0.20 98.90 

TS-615 bbd NS 51.44 1.29 18.39 7.94 0.14 5.53 10.39 3.38 0.76 0.34 na 100.16 

TS-615R bbd NS 51.41 1.30 18.40 7.89 0.14 5.58 10.40 3.37 0.76 0.34 na 99.66 

Basalt of Sparks Lake boat ramp 

TS-672 bbr P 50.75 1.36 17.52 8.64 0.15 7.93 9.07 3.31 0.61 0.26 na 99.50 

TS-673 bbr P 51.02 1.30 17.24 8.38 0.16 8.06 9.17 3.34 0.65 0.27 na 100.34 

TS-674 bbr P 51.46 1.21 17.47 8.19 0.15 7.45 9.43 3.34 0.64 0.25 na 99.40 

TS-771 bbr P 51.20 1.31 17.03 8.50 0.15 7.98 9.10 3.39 0.66 0.27 na 99.76 

TS-772 bbr P 51.49 1.22 17.52 8.30 0.15 7.30 9.45 3.32 0.59 0.25 na 99.81 

TS-773 bbr P 51.39 1.28 17.27 8.33 0.15 7.68 9.23 3.36 0.65 0.27 na 99.53 

Basalt of Cayuse Crater 

TS-625 bcc P 51.72 1.07 16.57 8.42 0.15 8.65 9.07 3.11 0.61 0.22 na 99.99 

TS-626 bcc P 51.79 1.06 16.47 8.36 0.15 8.69 9.11 3.12 0.64 0.21 na 100.21 

TS-627 bcc P 51.90 1.05 16.40 8.39 0.15 8.66 9.06 3.12 0.65 0.21 na 100.63 

CR-42 bcc P 51.48 1.04 16.26 8.70 0.15 8.74 9.40 2.97 0.65 0.21 na 99.83 

Basalt east of Sparks Lake 

TS-671 bes P 49.77 1.49 17.89 9.58 0.16 7.32 9.17 3.63 0.38 0.22 na 100.17 

TS-770 bes P 49.72 1.50 17.50 9.70 0.16 7.75 9.12 3.49 0.41 0.24 na 100.28 

CR-40 bes P 49.33 1.48 17.74 10.16 0.16 7.40 9.30 3.46 0.35 0.22 na 99.95 

Basalt south of The Husband 

TS-121 bhs P 52.97 1.40 17.76 8.34 0.14 5.02 8.57 3.87 0.99 0.54 -0.26 99.29 

TS-122 bhs P 51.11 1.39 17.20 8.85 0.15 6.91 9.52 3.37 0.72 0.37 -0.26 99.57 

TS-123 bhs P 52.89 1.20 18.00 8.18 0.14 5.80 8.62 3.53 0.89 0.35 0.00 99.06 

TS-124 bhs P 51.38 1.50 17.53 8.56 0.15 6.61 9.34 3.39 0.80 0.34 -0.04 99.44 

TS-126 bhs P 52.50 1.40 17.33 8.62 0.15 5.99 8.50 3.67 0.98 0.47 -0.35 99.99 

TS-152 bhs P 52.49 1.22 18.06 8.21 0.14 6.08 8.72 3.47 0.84 0.35 0.20 99.25 

TS-153 bhs P 52.68 1.41 17.93 8.40 0.14 5.14 8.60 3.83 0.94 0.53 0.01 99.45 

Basalt of James Creek 

TS-110 bjc P 51.78 1.37 17.36 8.50 0.15 6.81 9.15 3.40 0.75 0.32 -0.21 99.26 

TS-116 bjc P 51.54 1.28 17.45 8.70 0.15 7.14 8.93 3.41 0.68 0.32 -0.14 99.34 

TS-119 bjc P 51.62 1.42 17.47 8.52 0.15 6.79 9.26 3.32 0.73 0.32 0.07 99.18 

Basalt of North Fork Whychus Creek 

TS-434 bnf P 51.09 0.98 21.99 6.67 0.11 4.35 10.80 2.98 0.41 0.22 0.06 99.62 

TS-503 bnf P 52.51 1.25 20.15 7.71 0.13 4.15 9.69 3.17 0.57 0.28 0.01 100.34 

TS-526 bnf P 50.92 1.03 21.80 6.86 0.13 4.29 10.65 3.30 0.44 0.17 0.01 99.16 

Basalt of Sphinx Butte 

TS-130 bsb P 52.68 1.10 17.33 7.73 0.13 7.11 8.65 3.53 0.99 0.35 -0.21 99.45 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
73

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Andesite west of Renfrew Glacier—Continued

596 40 <5 13 16 18 9 <10 3 23 427 <5 29 82 208

593 41 <5 11 17 17 9 20 3 25 437 113 31 83 208

Andesite of Whychus Creek

590 38 <5 21 16 12 7 29 3 34 414 136 22 62 200

607 34 <5 20 16 13 8 31 3 35 404 117 23 63 200

567 40 <5 12 17 13 8 19 3 26 529 107 27 60 194

Basalt of Alder Creek

265 26 256 60 18 8 6 17 151 8 476 200 24 85 117

323 28 84 65 19 14 9 20 57 11 504 192 25 84 150

Basalt of Brush Draw

402 39 96 78 15 10 7 20 38 3 726 212 19 63 123

363 37 126 86 18 15 8 21 41 10 720 216 23 74 124

366 39 126 84 18 17 8 23 41 9 714 213 23 75 123

Basalt of Sparks Lake boat ramp

215 27 269 67 17 9 9 16 155 10 450 190 24 78 128

222 24 292 67 17 10 7 16 159 11 427 200 25 78 131

225 27 221 74 17 11 7 17 127 11 419 202 25 77 121

220 24 281 70 17 11 8 17 159 12 434 198 26 77 130

237 31 216 73 18 12 8 17 125 12 433 195 27 74 122

227 26 248 74 18 13 9 17 145 12 438 201 27 74 126

Basalt of Cayuse Crater

230 23 434 65 17 10 6 14 158 10 454 199 20 79 101

231 23 445 65 18 14 6 12 157 11 454 200 20 77 99

225 23 447 62 16 11 6 14 155 12 451 200 19 76 99

na na na na na na na na na na na na na na na

Basalt east of Sparks Lake

152 18 152 70 18 8 3 14 117 5 413 203 24 85 106

158 26 207 65 17 7 7 16 140 6 417 205 27 80 113

na na na na na na na na na na na na na na na

Basalt south of The Husband

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

Basalt of James Creek

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

Basalt of North Fork Whychus Creek

187 12 74 35 15 7 3 <10 13 3 642 157 12 51 70

226 17 34 46 15 9 5 <10 11 7 581 174 18 58 91

187 14 72 42 15 5 3 <10 28 4 627 161 12 47 71

Basalt of Sphinx Butte

na na na na na na na na na na na na na na na
74

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Basalt of Sphinx Butte—Continued 

TS-131 bsb P 51.52 1.10 16.94 8.22 0.14 8.02 9.27 3.32 0.77 0.30 0.43 98.80 

TS-132 bsb P 51.25 1.43 17.15 8.90 0.15 6.71 9.45 3.42 0.76 0.38 0.23 98.74 

Basalt southeast of Sparks Lake 

TS-769 bsl P 51.10 1.21 17.74 8.28 0.14 7.88 9.09 3.38 0.56 0.22 na 99.98 

Basalt of Spruce Spring 

CR-116 bss P 50.11 1.56 17.34 9.33 0.15 7.41 9.24 3.39 0.68 0.40 na 99.97 

TS-527 bss P 50.35 1.64 17.45 9.26 0.17 7.15 9.07 3.39 0.72 0.40 <0.01 99.31 

TS-592 bss P 50.00 1.49 17.77 8.89 0.16 7.75 9.08 3.45 0.62 0.37 na 98.29 

TS-592R bss P 50.07 1.49 17.77 8.87 0.16 7.75 9.06 3.45 0.62 0.37 na 98.89 

Basalt and basaltic andesite of Talapus and Katsuk Buttes 

TS-346 btk P 53.44 1.31 17.81 8.94 0.15 5.10 8.05 3.74 0.69 0.36 <0.01 99.53 

TS-347 btk P 51.66 1.31 16.85 8.75 0.15 7.42 9.10 3.30 0.66 0.38 <0.01 99.28 

TS-642 btk P 51.66 1.37 16.93 8.64 0.16 7.38 9.02 3.43 0.68 0.33 na 99.85 

Basalt of lower Whychus Creek 

TS-617 bwl P 50.77 1.49 17.44 8.99 0.16 7.32 8.85 3.56 0.65 0.37 na 100.65 

Basalt west of Millican Crater 

TS-727 bwm P 50.56 1.67 17.70 8.97 0.16 6.44 9.43 3.62 0.71 0.35 na 100.37 

TS-729 bwm P 49.84 1.55 16.81 9.28 0.16 8.63 8.98 3.37 0.65 0.33 na 100.55 

TS-730 bwm P 49.53 1.53 16.72 9.43 0.16 9.13 8.80 3.35 0.63 0.33 na 100.08 

TS-734 bwm P 50.26 1.58 17.17 9.20 0.16 7.64 9.11 3.50 0.64 0.34 na 99.72 

TS-743 bwm P 50.13 1.56 17.14 9.27 0.16 7.84 9.14 3.42 0.60 0.34 na 99.88 

TS-744 bwm P 50.52 1.64 17.61 9.13 0.16 6.55 9.37 3.60 0.67 0.35 na 100.21 

NS-07-198 bwm P 49.82 1.55 16.57 9.40 0.16 8.92 8.84 3.38 0.63 0.33 na 100.69 

NS-07-206 bwm P 49.48 1.49 16.21 9.54 0.16 9.85 8.63 3.29 0.62 0.32 na 100.64 

Basalt of Whychus Creek 

TS-458 bwy BT 51.11 1.23 17.14 8.93 0.15 7.92 9.18 2.94 0.70 0.31 <0.01 99.39 

TS-500 bwy BT 51.71 1.35 18.63 8.99 0.15 5.95 8.72 3.31 0.49 0.31 <0.01 99.97 

TS-676 bwy BT 51.83 1.39 18.56 8.65 0.15 5.76 8.60 3.86 0.54 0.26 na 100.19 

Dacite of "Black Hump" 

TS-100 dbh MS 64.04 0.91 16.41 5.44 0.14 1.23 3.42 5.93 1.70 0.38 <0.01 99.54 

TS-215 dbh MS 64.47 0.93 16.04 5.51 0.14 1.27 3.35 5.79 1.73 0.36 -0.11 99.34 

TS-215A dbh MS 64.24 0.95 16.09 5.52 0.14 1.32 3.41 5.84 1.74 0.36 0.43 99.07 

TS-216 dbh MS 64.35 0.92 16.16 5.50 0.14 1.29 3.40 5.75 1.72 0.36 -0.13 99.21 

TS-368 dbh MS 64.47 0.93 15.94 5.54 0.14 1.23 3.38 5.83 1.73 0.40 0.21 99.33 

TS-370 dbh MS 64.36 0.95 15.97 5.56 0.14 1.29 3.44 5.76 1.72 0.42 0.07 99.81 

TS-397 dbh MS 64.38 0.93 15.94 5.52 0.14 1.27 3.45 5.84 1.71 0.41 0.26 99.33 

TS-405 dbh MS 64.05 0.98 16.31 5.62 0.14 1.38 3.63 5.40 1.67 0.40 <0.01 99.52 

Dacite breccia of Lewis Glacier headwall 

TS-582 dbl SS 62.83 1.09 16.39 5.92 0.12 2.00 4.58 4.30 1.99 0.36 2.06 98.44 

TS-582-2 dbl SS 62.62 1.08 16.25 5.88 0.12 2.05 4.63 4.62 2.08 0.27 n.d. 97.39 

TS-691 dbl SS 62.60 1.10 16.22 5.86 0.12 2.07 4.69 4.64 2.01 0.27 na 97.80 

Dacite southwest of Clark Glacier 

TS-71 dcg SS 63.23 1.08 16.33 5.57 0.11 1.71 4.30 4.94 1.98 0.36 <0.01 100.03 

TS-117 dcg SS 63.31 1.10 16.18 5.66 0.11 1.72 4.24 4.94 2.00 0.34 -0.01 99.12 

TS-120 dcg SS 63.36 1.11 16.09 5.61 0.11 1.74 4.29 4.96 1.98 0.34 -0.06 99.03 

TS-133 dcg SS 63.19 1.09 16.17 5.64 0.11 1.80 4.31 4.96 1.98 0.34 -0.56 99.15 

TS-134 dcg SS 63.22 1.10 16.18 5.64 0.11 1.76 4.30 4.96 1.99 0.34 0.06 99.09 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
75

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Basalt of Sphinx Butte—Continued

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

Basalt southeast of Sparks Lake

201 22 251 63 18 7 8 14 160 9 480 193 22 74 102

Basalt of Spruce Spring

na na na na na na na na na na na na na na na

306 34 162 58 13 12 11 20 122 8 496 203 23 67 151

268 30 210 50 18 13 12 18 141 8 496 193 24 80 139

265 30 214 52 18 16 11 19 143 8 497 194 24 79 141

Basalt and basaltic andesite of Talapus and Katsuk Buttes

330 25 77 56 16 9 5 <10 69 8 605 223 18 88 97

267 28 286 54 16 11 8 <10 115 9 497 188 22 72 127

274 25 285 68 18 13 8 18 127 10 478 202 25 80 127

Basalt of lower Whychus Creek

288 31 214 62 19 14 9 20 153 8 505 203 26 84 136

Basalt west of Millican Crater

246 28 142 65 17 14 12 18 84 9 468 214 25 79 145

225 27 310 70 16 12 11 19 190 9 445 197 23 82 133

222 23 356 70 16 12 12 18 213 8 437 197 23 81 131

246 25 238 75 17 10 10 15 145 7 454 210 24 82 136

242 28 253 73 17 13 10 17 153 8 456 205 24 82 135

242 33 147 72 18 15 14 20 91 8 481 203 27 79 142

232 23 343 74 16 13 12 16 204 8 453 202 24 79 134

218 25 425 71 17 11 14 18 266 8 441 198 25 82 130

Basalt of Whychus Creek

266 19 267 58 13 8 8 <10 129 8 476 184 21 65 120

252 13 57 51 15 10 4 <10 78 3 560 251 18 69 99

237 16 85 77 18 9 5 13 97 6 553 198 22 85 104

Dacite of “Black Hump”

593 38 <3 3 24 17 12 30 3 25 363 33 41 90 250

603 46 <5 5 23 23 11 34 6 25 341 43 42 98 254

586 43 <5 6 22 17 11 24 6 27 351 41 44 96 255

588 43 <5 <2 21 20 11 34 7 25 350 44 43 87 251

635 47 <5 2 17 16 10 43 3 26 343 29 35 85 254

637 42 <5 2 19 15 10 43 3 24 349 31 34 85 248

642 47 <5 2 17 21 10 38 3 25 354 17 34 89 249

627 44 <5 2 17 16 10 39 3 24 370 19 33 81 243

Dacite breccia of Lewis Glacier headwall

627 35 <5 17 15 9 8 31 3 36 396 103 24 65 208

556 36 8 20 17 19 8 21 0 41 393 117 27 76 193

557 33 7 17 20 20 8 21 1 42 396 118 27 80 195

Dacite southwest of Clark Glacier

616 40 <3 18 21 21 10 33 8 38 410 96 28 74 217

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na
76

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Dacite southwest of Clark Glacier—Continued 

TS-139 dcg SS 63.21 1.10 16.15 5.66 0.11 1.80 4.30 4.95 1.99 0.33 -0.18 99.27 

TS-140 dcg SS 63.20 1.10 16.35 5.63 0.11 1.73 4.24 4.92 1.99 0.34 0.01 99.29 

TS-142 dcg SS 63.24 1.11 16.16 5.69 0.11 1.78 4.27 4.91 2.00 0.34 0.11 99.22 

TS-143 dcg SS 63.16 1.10 16.17 5.65 0.11 1.80 4.32 4.97 1.99 0.33 0.01 99.19 

TS-145 dcg SS 63.24 1.09 16.14 5.67 0.11 1.78 4.31 4.94 1.97 0.34 0.11 99.37 

TS-146 dcg SS 63.12 1.11 16.18 5.69 0.11 1.80 4.34 4.93 1.98 0.34 0.26 99.09 

TS-154 dcg SS 63.14 1.11 16.24 5.67 0.11 1.80 4.34 4.90 1.96 0.33 -0.11 99.38 

TS-155 dcg SS 63.11 1.11 16.28 5.65 0.11 1.77 4.30 4.94 2.00 0.33 -0.19 99.12 

TS-158 dcg SS 63.14 1.11 16.24 5.70 0.11 1.77 4.29 4.91 1.98 0.34 -0.08 99.38 

TS-530 dcg SS 63.08 1.13 16.27 5.66 0.13 1.76 4.31 4.96 2.00 0.30 0.13 99.16 

TS-531 dcg SS 63.28 1.12 16.29 5.64 0.12 1.76 4.30 4.78 2.01 0.30 0.09 99.64 

TS-532 dcg SS 63.08 1.12 16.27 5.66 0.13 1.78 4.31 4.98 2.01 0.27 0.02 99.79 

TS-533 dcg SS 63.05 1.11 16.34 5.66 0.11 1.76 4.35 4.99 1.90 0.31 0.01 99.36 

TS-534 dcg SS 63.00 1.12 16.40 5.68 0.11 1.80 4.29 4.92 2.00 0.28 <0.01 99.60 

TS-535 dcg SS 62.95 1.12 16.27 5.72 0.13 1.81 4.37 4.91 2.01 0.30 0.60 98.56 

TS-538 dcg SS 63.16 1.12 16.49 5.62 0.12 1.73 4.28 4.86 1.94 0.29 0.01 99.67 

TS-539 dcg SS 63.07 1.12 16.50 5.66 0.11 1.75 4.25 4.92 1.93 0.29 <0.01 99.02 

TS-540 dcg SS 63.13 1.12 16.26 5.70 0.13 1.79 4.27 4.93 1.98 0.30 0.03 99.87 

TS-541 dcg SS 63.20 1.12 16.23 5.63 0.12 1.71 4.36 4.93 2.00 0.30 0.01 99.44 

TS-543 dcg SS 63.46 1.13 16.22 5.58 0.11 1.67 4.21 4.92 1.98 0.31 <0.01 98.87 

TS-544 dcg SS 63.15 1.12 16.36 5.58 0.11 1.72 4.31 4.97 1.99 0.29 <0.01 99.21 

Dacite of Chambers Lakes 

TS-6 dcl SS 65.22 0.95 15.85 4.75 0.10 1.39 3.63 5.04 2.31 0.35 0.06 99.27 

TS-7 dcl SS 65.01 0.97 16.23 4.82 0.10 1.38 3.63 4.88 2.24 0.35 1.15 98.82 

TS-19 dcl SS 65.41 0.95 15.80 4.71 0.10 1.35 3.56 5.09 2.30 0.33 0.11 98.98 

TS-20 dcl SS 65.70 0.94 15.70 4.65 0.10 1.36 3.58 4.96 2.28 0.32 0.06 99.59 

TS-21 dcl SS 65.65 0.94 15.74 4.68 0.10 1.35 3.56 4.98 2.28 0.32 0.29 99.37 

TS-170 dcl SS 65.93 0.92 15.73 4.62 0.10 1.31 3.50 4.88 2.32 0.29 0.00 99.41 

TS-171 dcl SS 65.36 0.94 15.66 5.05 0.10 1.52 3.78 4.71 2.23 0.25 -0.04 99.20 

TS-208 dcl SS 65.44 0.97 15.83 4.80 0.10 1.39 3.65 4.88 2.25 0.29 0.32 98.78 

TS-211 dcl SS 65.43 0.96 15.76 4.80 0.10 1.38 3.67 4.93 2.27 0.29 0.10 99.24 

TS-212 dcl SS 65.18 0.99 15.87 4.92 0.10 1.40 3.73 4.89 2.24 0.29 0.04 99.17 

TS-213 dcl SS 65.36 0.96 15.84 4.79 0.10 1.44 3.68 4.88 2.26 0.29 -0.02 99.36 

TS-214 dcl SS 65.74 0.93 15.78 4.63 0.10 1.32 3.58 4.92 2.31 0.29 0.11 99.09 

TS-239 dcl SS 64.78 1.01 15.92 5.07 0.10 1.50 3.89 4.87 2.15 0.31 0.20 99.47 

Dacite north of Carver Lake 

TS-10 dcn SS 66.89 0.70 15.65 3.98 0.08 1.45 3.63 4.54 2.45 0.24 0.35 99.92 

TS-11 dcn SS 67.39 0.64 15.57 3.78 0.08 1.43 3.54 4.44 2.50 0.23 0.21 99.76 

TS-12 dcn SS 67.12 0.67 15.56 3.94 0.08 1.49 3.60 4.46 2.46 0.23 0.36 99.87 

Dacite of Dew Lake 

TS-68 ddl SS 64.10 1.06 16.00 5.34 0.12 1.48 3.82 5.21 2.08 0.39 <0.01 99.60 

TS-115 ddl SS 64.26 1.04 16.09 5.30 0.11 1.43 3.69 5.19 2.11 0.38 0.08 99.04 

TS-118 ddl SS 64.43 1.04 15.98 5.26 0.11 1.46 3.80 5.08 2.10 0.35 0.14 99.09 

TS-151 ddl SS 64.12 1.07 15.88 5.39 0.12 1.56 3.86 5.18 2.06 0.37 0.07 99.11 

Dacite ejecta of South Sister summit 

TS-480 des SS 62.87 0.98 16.22 5.80 0.11 2.43 5.02 3.69 2.19 0.27 1.86 98.22 

TS-481A des SS 62.82 0.98 16.13 5.80 0.11 2.40 4.97 3.24 2.88 0.27 1.75 98.78 

TS-481B des SS 62.76 0.98 16.32 5.72 0.11 2.31 4.92 3.51 2.72 0.26 1.36 98.86 

TS-484 des SS 63.86 0.96 15.99 5.60 0.11 2.15 4.60 3.97 2.08 0.27 1.79 98.41 

TS-630B-1 des SS 64.08 0.97 16.44 5.43 0.09 1.72 4.27 4.15 2.26 0.19 na 96.25 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
77

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Dacite southwest of Clark Glacier—Continued

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

682 39 <5 14 14 18 9 34 3 36 405 87 25 59 214

671 41 <5 10 16 13 8 32 3 36 400 81 24 65 211

661 37 <5 13 16 15 9 26 3 36 393 95 23 63 215

668 35 <5 9 14 16 10 32 3 35 399 94 24 59 211

676 42 <5 16 15 22 9 25 3 35 400 93 24 61 214

673 42 <5 12 15 14 9 35 4 35 398 85 24 64 213

668 40 <5 15 13 13 9 31 3 39 402 101 26 62 217

690 43 <5 13 15 20 9 30 3 36 395 90 26 58 215

688 42 <5 14 14 19 9 23 3 37 403 98 26 64 215

679 45 <5 16 14 19 9 26 3 37 398 95 24 64 213

679 43 <5 15 16 18 9 27 3 37 386 95 24 60 214

689 42 <5 11 14 19 9 27 3 37 395 88 26 57 215

Dacite of Chambers Lakes

667 40 <3 10 22 19 12 26 4 45 351 73 27 73 240

671 43 <3 19 20 22 11 22 8 43 337 71 26 72 237

689 39 <3 22 19 20 10 20 5 43 332 67 24 70 236

678 40 4 11 20 21 11 22 5 46 335 70 25 64 236

680 40 4 12 19 21 11 23 4 45 332 68 26 73 234

692 41 <5 22 17 19 11 23 6 46 332 78 27 65 234

670 36 <5 17 18 21 12 23 8 44 338 86 25 67 226

668 40 <5 19 20 19 12 26 6 46 352 79 27 73 242

674 40 <5 12 21 19 12 20 3 45 341 84 27 67 237

672 39 <5 18 19 18 11 20 6 43 349 72 26 65 234

680 42 <5 16 20 24 11 25 3 43 342 75 26 65 233

678 41 <5 17 18 21 11 29 6 46 340 73 27 68 237

655 43 <5 18 19 23 11 25 6 42 358 85 27 70 232

Dacite north of Carver Lake

699 32 12 22 19 20 9 20 15 51 309 60 19 56 207

707 39 14 20 17 23 9 25 13 55 307 57 17 52 206

698 41 14 30 19 25 9 29 14 55 308 65 18 51 205

Dacite of Dew Lake

629 41 4 8 22 21 13 24 4 39 371 62 31 82 246

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

Dacite ejecta of South Sister summit

616 34 <5 44 14 11 6 <10 3 43 388 124 21 59 187

964 32 <5 51 14 13 7 <10 3 44 379 118 22 60 187

845 33 6 45 16 13 7 12 3 42 383 126 22 60 189

642 36 <5 19 15 13 8 <10 3 38 368 109 22 56 201

573 34 13 11 19 16 8 19 3 44 375 121 24 59 188
78

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Dacite ejecta of South Sister summit—Continued 

TS-630B-2 des SS 64.20 0.98 16.20 5.57 0.09 1.75 4.26 4.05 2.31 0.20 na 97.56 

TS-632A des SS 63.98 0.96 15.69 5.48 0.16 2.13 4.52 4.28 2.19 0.22 na 96.66 

TS-632B des SS 63.47 0.94 16.12 5.40 0.10 2.08 4.61 4.46 2.19 0.21 na 97.71 

TS-632C des SS 63.12 1.00 16.14 5.62 0.11 2.20 4.69 4.35 2.15 0.22 na 96.14 

TS-695 des SS 63.84 1.10 16.26 5.05 0.07 1.66 4.19 5.02 2.10 0.31 na 98.72 

 [lithic]

TS-696A des SS 63.52 1.10 16.16 5.51 0.12 1.80 4.40 4.55 2.14 0.30 na 96.86 

TS-696B des SS 63.73 1.10 16.26 5.48 0.12 1.99 4.42 4.04 2.17 0.30 na 95.85 

TS-696D des SS 63.41 1.07 16.47 5.55 0.12 2.03 4.48 3.99 2.17 0.30 na 95.55 

TS-696DR des SS 63.41 1.08 16.42 5.56 0.12 2.08 4.48 3.99 2.17 0.30 na 95.79 

Dacite of Goose Creek 

TS-92 dgc SS 64.81 0.99 16.03 4.94 0.10 1.46 3.84 4.88 2.19 0.36 0.02 100.04 

TS-323 dgc SS 65.13 0.99 15.94 4.76 0.10 1.43 3.78 4.95 2.23 0.28 na 99.68 

TS-336 dgc SS 65.28 0.99 15.84 4.86 0.10 1.34 3.76 4.83 2.24 0.35 <0.01 99.33 

TS-340 dgc SS 65.15 0.99 15.73 4.89 0.10 1.45 3.83 4.86 2.24 0.35 0.32 98.76 

TS-706 dgc SS 65.17 0.96 16.09 4.72 0.10 1.35 3.66 5.01 2.27 0.27 na 99.25 

Dacite of Green Lakes 

TS-508 dgl SS 64.17 1.10 15.89 5.63 0.11 1.67 4.17 4.42 2.08 0.36 0.50 99.65 

TS-509 dgl SS 64.19 1.09 15.77 5.65 0.11 1.65 4.17 4.55 2.07 0.34 <0.01 99.15 

TS-519 dgl SS 64.22 1.07 15.83 5.60 0.11 1.63 4.25 4.51 2.04 0.34 <0.01 99.42 

TS-520 dgl SS 64.20 1.09 15.77 5.63 0.11 1.67 4.26 4.51 2.03 0.34 <0.01 99.14 

TS-578 dgl SS 62.85 1.11 16.13 5.92 0.12 1.87 4.49 4.91 1.86 0.34 <0.01 100.00 

TS-579 dgl SS 63.56 1.01 15.99 5.60 0.11 1.89 4.30 4.87 1.97 0.31 <0.01 100.29 

Dacite of Hayden Glacier–Renfrew Glacier col 

TS-410 dhr MS 64.05 0.95 16.19 5.42 0.15 1.32 3.55 5.86 1.75 0.35 na 99.31 

TS-411 dhr MS 64.31 0.93 16.35 5.60 0.14 1.27 3.45 5.49 1.67 0.39 <0.01 99.28 

TS-412 dhr MS 63.58 1.01 15.94 5.91 0.14 1.51 3.71 5.71 1.66 0.42 <0.01 99.33 

Dacite of Irving Glacier 

TS-5 dig MS 64.23 0.92 16.16 5.53 0.14 1.26 3.33 5.90 1.73 0.39 <0.01 99.24 

TS-5A dig MS 64.11 0.92 16.23 5.56 0.14 1.27 3.31 5.92 1.73 0.41 0.05 100.05 

TS-238 dig MS 64.21 0.91 16.23 5.50 0.14 1.27 3.42 5.85 1.70 0.37 0.22 99.43 

Dacite of Kokostick Butte 

TS-63 dkb SS 63.05 0.99 16.29 5.57 0.11 2.03 4.62 4.77 1.91 0.27 0.10 99.04 

TS-246 dkb SS 62.54 1.02 16.46 5.83 0.11 2.07 4.85 4.63 1.84 0.26 0.01 99.22 

TS-348 dkb SS 62.45 1.04 16.37 5.79 0.11 2.05 5.01 4.64 1.84 0.30 0.96 98.57 

TS-349 dkb SS 62.80 1.04 16.30 5.70 0.11 1.97 4.67 4.81 1.91 0.28 0.08 98.96 

TS-563 dkb SS 62.49 1.05 16.41 5.76 0.12 2.07 4.81 4.69 1.94 0.25 0.97 98.34 

Dacite of Linton Creek 

TS-268 dlc MS 66.82 0.81 15.54 4.37 0.09 1.32 3.38 4.73 2.32 0.21 0.55 98.68 

Dacite of Lewis Glacier headwall 

TS-14 dlg SS 64.35 1.01 15.79 5.33 0.10 1.65 4.09 4.86 2.12 0.32 <0.01 99.68 

TS-43 dlg SS 63.58 1.09 16.07 5.53 0.11 1.70 4.22 4.94 2.00 0.35 0.77 98.53 

TS-205 dlg SS 63.65 1.10 15.91 5.68 0.11 1.70 4.24 4.85 2.04 0.31 -0.14 98.90 

TS-581 dlg SS 63.55 1.09 16.11 5.52 0.11 1.70 4.25 4.85 2.03 0.38 0.44 99.53 

TS-631 dlg SS 63.75 1.09 16.00 5.46 0.11 1.64 4.23 4.97 2.03 0.30 na 99.38 

TS-692 dlg SS 63.67 1.09 16.10 5.35 0.11 1.68 4.24 5.02 2.04 0.30 na 98.63 

TS-693 dlg SS 63.82 1.09 16.12 5.13 0.11 1.69 4.22 5.08 2.04 0.31 na 99.58 

TS-694 dlg SS 63.61 1.09 16.06 5.38 0.12 1.73 4.22 5.04 2.03 0.30 na 98.86 
79

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Dacite ejecta of South Sister summit—Continued

586 35 14 12 20 19 8 19 4 45 369 126 26 61 194

678 34 12 36 17 16 8 21 5 40 359 121 24 66 181

572 38 12 27 17 20 7 20 4 41 378 117 24 63 187

560 38 12 26 18 19 7 17 6 40 371 129 24 69 182

613 44 1 18 19 20 9 25 0 38 394 96 28 80 212

594 41 1 18 18 23 10 22 0 38 392 93 26 77 206

585 35 1 20 19 21 8 19 0 37 393 96 28 79 203

572 40 3 17 18 20 9 24 2 36 395 95 28 75 200

574 42 3 17 19 21 10 24 3 35 398 91 27 76 201

Dacite of Goose Creek

659 41 <3 22 20 19 11 30 9 44 363 75 27 69 235

661 42 4 15 19 17 9 23 0 43 357 80 26 66 220

722 40 <5 14 15 16 9 25 3 42 348 76 23 53 232

706 38 <5 14 13 13 9 37 3 41 346 76 24 60 232

659 44 2 12 19 21 9 22 1 43 344 76 27 65 221

Dacite of Green Lakes

668 41 <5 24 15 15 9 25 3 39 360 93 24 63 224

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

624 36 <5 14 16 14 8 28 3 34 403 115 23 63 205

654 37 <5 5 15 14 8 13 2 35 375 95 22 54 206

Dacite of Hayden Glacier–Renfrew Glacier col

570 47 2 5 22 18 10 28 0 26 360 27 39 101 235

642 47 <5 2 17 15 9 35 3 23 352 25 34 84 249

563 38 na 8 26 18 14 33 na 25 386 77 50 94 302

Dacite of Irving Glacier

598 46 <3 5 24 22 12 28 5 24 341 33 41 96 254

595 41 <3 4 25 18 11 28 10 25 336 36 42 98 254

588 44 <5 7 22 20 12 32 5 24 348 32 42 95 256

Dacite of Kokostick Butte

573 36 6 12 18 19 10 15 3 36 404 107 21 64 191

560 32 11 16 23 18 9 18 12 35 425 113 23 66 190

584 37 <5 23 15 13 9 28 3 34 432 136 22 57 187

576 34 5 15 23 18 12 16 na 37 441 141 37 66 247

613 35 <5 6 14 13 8 29 3 34 413 120 21 54 186

Dacite of Linton Creek

697 34 <5 10 20 19 8 16 8 46 332 72 20 62 217

Dacite of Lewis Glacier headwall

628 37 5 22 19 20 10 23 3 40 355 93 24 68 216

618 39 <3 20 20 20 11 24 4 39 392 95 27 78 227

614 36 <5 21 23 17 11 24 6 39 369 91 27 71 226

685 38 <5 10 14 16 9 33 3 37 385 99 26 62 218

603 43 1 20 18 18 9 23 0 39 398 94 27 74 211

606 44 1 20 19 20 10 22 0 39 390 94 28 77 209

609 45 2 19 19 21 9 25 0 38 389 92 28 77 211

603 42 1 20 20 21 10 22 0 38 390 94 27 78 211
80

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Dacite of Lewis Glacier headwall—Continued 

TS-697 dlg SS 63.73 1.09 16.08 5.37 0.11 1.68 4.24 4.93 2.06 0.31 na 98.66 

Dacite of Lane Plateau 

TS-103 dlp MS 65.31 0.73 16.18 5.22 0.14 0.97 2.85 6.07 1.83 0.30 <0.01 99.13 

TS-287 dlp MS 64.22 0.92 16.18 5.51 0.14 1.25 3.36 5.93 1.73 0.37 -0.08 99.11 

TS-308 dlp MS 64.21 0.91 16.20 5.48 0.14 1.27 3.40 5.92 1.71 0.35 0.09 99.58 

TS-419 dlp MS 64.35 0.91 16.04 5.48 0.14 1.25 3.35 5.95 1.76 0.37 <0.01 98.74 

Dacite of Mesa Creek 

TS-111 dmc P 65.08 0.87 16.52 4.74 0.10 1.37 3.82 5.00 1.87 0.23 0.36 98.87 

Dacite southeast of Moraine Lake 

TS-337 dml SS 64.43 0.99 16.01 5.07 0.11 1.57 4.11 4.92 2.05 0.33 0.22 99.55 

TS-353 dml SS 64.64 1.00 15.88 5.05 0.10 1.51 3.96 4.93 2.19 0.34 0.44 98.47 

TS-354 dml SS 65.02 1.01 15.76 4.96 0.10 1.44 3.89 4.88 2.20 0.35 0.27 99.87 

TS-355 dml SS 65.17 0.99 15.77 4.88 0.10 1.42 3.85 4.86 2.22 0.35 0.24 99.80 

TS-357 dml SS 64.43 1.00 16.13 5.10 0.11 1.55 4.03 4.85 2.05 0.34 0.15 100.02 

TS-359 dml SS 64.39 1.00 16.05 5.09 0.11 1.58 4.11 4.89 2.05 0.32 0.21 99.30 

TS-361 dml SS 65.14 1.01 15.79 5.00 0.10 1.41 3.75 4.83 2.22 0.36 <0.01 99.69 

TS-362 dml SS 65.72 0.95 15.68 4.72 0.10 1.34 3.63 4.84 2.29 0.33 0.24 99.72 

TS-440 dml SS 64.30 1.00 16.38 5.07 0.11 1.58 4.12 4.68 2.04 0.32 0.31 99.13 

TS-441 dml SS 64.57 1.00 16.30 5.07 0.11 1.54 3.94 4.67 2.10 0.31 0.63 98.41 

TS-442 dml SS 64.27 1.01 16.34 5.10 0.11 1.58 4.11 4.69 2.03 0.34 0.31 99.33 

TS-704 dml SS 64.05 0.99 16.44 5.04 0.11 1.54 4.06 5.06 2.06 0.26 na 99.34 

TS-705 dml SS 64.01 1.00 16.59 5.07 0.11 1.48 3.97 5.04 2.07 0.26 na 99.66 

Dacite north of Moraine Lake 

TS-161 dmn SS 65.14 0.93 15.98 4.79 0.10 1.44 3.78 5.02 2.13 0.28 -0.06 99.07 

TS-363 dmn SS 65.37 0.96 15.79 4.83 0.10 1.40 3.67 4.91 2.26 0.32 0.13 99.04 

TS-363 dmn SS 65.38 0.97 15.79 4.82 0.10 1.41 3.68 4.88 2.26 0.31 0.14 99.02 

TS-478 dmn SS 66.25 0.94 15.74 4.62 0.10 1.33 3.57 4.44 2.30 0.32 <0.01 99.98 

TS-488 dmn SS 66.32 0.92 15.66 4.60 0.10 1.32 3.57 4.47 2.33 0.32 0.14 99.88 

TS-491 dmn SS 66.14 0.93 15.88 4.61 0.10 1.29 3.57 4.43 2.33 0.33 0.06 99.70 

TS-492 dmn SS 65.67 0.92 16.04 4.75 0.10 1.47 3.76 4.45 2.15 0.29 0.09 99.95 

TS-495 dmn SS 65.76 0.96 15.89 4.78 0.10 1.36 3.71 4.44 2.27 0.34 <0.01 99.66 

TS-496 dmn SS 66.13 0.87 15.93 4.53 0.10 1.38 3.71 4.48 2.18 0.28 0.27 99.40 

TS-588 dmn SS 66.06 0.86 15.54 4.64 0.09 1.50 3.63 4.69 2.31 0.29 0.22 99.37 

Dacite of Middle Sister summit 

TS-233 dms MS 66.73 0.77 15.39 4.27 0.08 1.48 3.43 4.28 2.94 0.22 1.04 98.36 

TS-236 dms MS 65.92 0.84 15.42 4.57 0.08 1.64 3.64 4.39 2.85 0.24 0.78 98.81 

TS-237A dms MS 63.88 0.99 15.95 5.31 0.09 1.93 4.24 4.40 2.52 0.29 0.34 99.31 

TS-237B dms MS 65.70 0.83 15.77 4.57 0.08 1.63 3.64 4.35 2.78 0.24 0.61 99.14 

Dacite coulee west of Moraine Lake 

TS-72 dmw SS 64.63 0.97 16.16 4.96 0.11 1.52 3.85 5.01 2.07 0.33 0.09 99.87 

TS-162 dmw SS 65.45 0.90 16.01 4.84 0.10 1.44 3.58 4.85 2.17 0.25 0.30 98.92 

TS-163 dmw SS 64.92 0.96 15.98 4.91 0.11 1.53 3.87 4.94 2.10 0.28 0.19 99.12 

TS-356 dmw SS 65.11 0.95 15.93 4.87 0.11 1.46 3.81 4.91 2.12 0.33 <0.01 99.44 

Dacite north of Teardrop Pool 

TS-629A dnt SS 63.10 0.95 16.11 5.55 0.11 2.20 4.61 4.75 2.00 0.21 na 98.00 

TS-629B dnt SS 62.74 0.96 16.29 5.55 0.11 2.30 4.78 4.73 1.94 0.21 na 96.88 

TS-698 dnt SS 63.12 0.95 16.35 5.41 0.10 2.18 4.71 4.44 2.10 0.23 na 98.24 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
81

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Dacite of Lewis Glacier headwall—Continued

608 49 0 19 20 22 9 24 0 38 391 93 28 77 210

Dacite of Lane Plateau

662 49 4 5 24 29 11 36 2 27 297 23 51 96 274

604 43 <5 6 22 22 12 29 7 25 349 37 47 100 254

590 45 <5 4 22 19 13 28 4 26 351 38 45 102 257

608 43 na 8 27 20 14 34 na 26 370 67 53 100 319

Dacite of Mesa Creek

na na na na na na na na na na na na na na na

Dacite southeast of Moraine Lake

653 36 <5 5 14 16 10 24 3 38 390 92 22 59 216

666 38 na 12 22 18 13 24 na 44 383 116 32 71 292

690 36 <5 10 14 13 10 35 3 42 360 73 25 59 234

700 42 <5 10 15 16 9 33 3 42 356 77 24 63 234

671 41 <5 3 14 19 9 28 3 37 394 87 23 60 218

667 39 <5 4 16 18 10 24 3 37 395 90 24 67 219

720 40 <5 10 15 19 9 30 3 41 351 75 25 70 234

738 42 <5 7 13 18 9 33 3 43 343 60 24 57 234

675 42 <5 2 14 13 10 <10 3 37 393 89 22 61 214

698 38 <5 2 13 16 10 24 3 37 383 69 23 56 222

664 39 <5 7 15 15 10 36 3 36 397 85 24 62 217

612 42 3 11 19 18 10 22 0 37 396 90 26 67 207

616 44 3 9 20 20 10 20 0 38 398 91 25 69 209

Dacite north of Moraine Lake

na na na na na na na na na na na na na na na

680 37 na 23 22 21 14 20 na 49 370 108 38 70 296

na na na na na na na na na na na na na na na

753 42 <5 10 15 18 10 31 3 45 339 58 25 57 237

737 43 <5 6 13 17 9 25 3 45 336 68 23 58 231

753 40 <5 15 14 15 9 25 3 44 333 62 23 56 231

698 38 <5 <2 13 14 10 41 3 38 364 73 21 52 221

738 44 <5 10 14 19 9 30 3 44 347 60 25 51 232

711 42 <5 2 13 15 9 24 3 41 364 75 20 55 215

742 36 <5 7 14 13 9 21 3 45 323 80 20 52 224

Dacite of Middle Sister summit

717 45 12 30 19 20 12 22 12 65 291 67 28 54 275

696 45 14 36 19 22 12 26 14 62 307 76 30 55 273

654 47 15 46 20 22 12 27 18 54 339 107 31 64 265

696 45 12 32 19 20 12 30 13 60 312 77 28 57 272

Dacite coulee west of Moraine Lake

632 37 <3 9 21 17 12 18 7 40 383 83 25 70 226

na na na na na na na na na na na na na na na

637 40 6 7 19 20 12 24 7 40 382 81 24 75 226

687 37 <5 2 15 15 10 32 3 39 380 73 22 60 225

Dacite north of Teardrop Pool

565 33 16 47 19 18 7 20 7 40 373 120 24 67 186

552 32 15 72 19 15 6 17 7 38 378 122 24 68 180

567 31 16 64 19 18 7 16 8 41 378 136 24 78 185
82

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Silicic dacite north of Teardrop Pool 

TS-483 dnt′ SS 67.08 0.70 15.34 4.18 0.09 1.71 3.63 3.61 3.03 0.22 1.26 99.33 

Dacite pumice fall of Prouty Glacier headwall 

TS-204 dpg SS 64.56 1.01 15.99 5.27 0.11 1.44 3.72 4.99 2.15 0.36 2.60 96.58 

Dacite of Red Meadow 

TS-189 drm SS 63.86 1.03 16.12 5.40 0.11 1.69 4.17 4.90 2.03 0.29 0.76 98.26 

TS-431 drm SS 63.73 1.04 16.38 5.42 0.11 1.78 4.21 4.62 1.98 0.32 0.41 99.09 

TS-432 drm SS 65.30 0.86 16.23 4.72 0.09 1.60 4.05 4.21 2.28 0.26 0.24 98.83 

TS-460 drm SS 63.57 1.05 16.30 5.52 0.11 1.82 4.34 4.55 2.01 0.32 1.05 98.39 

TS-461 drm SS 63.97 1.04 16.32 5.44 0.11 1.72 4.22 4.45 2.00 0.33 <0.01 100.11 

TS-497 drm SS 64.49 1.01 16.32 5.21 0.11 1.60 4.09 4.46 1.98 0.33 0.40 99.46 

TS-511 drm SS 64.30 1.01 16.05 5.40 0.11 1.64 4.21 4.55 2.01 0.32 0.30 98.67 

TS-570 drm SS 63.70 1.05 16.18 5.34 0.11 1.65 4.20 5.08 2.03 0.26 0.23 98.50 

TS-571 drm SS 63.04 1.11 16.41 5.61 0.12 1.80 4.39 4.91 1.91 0.29 0.23 99.54 

TS-575 drm SS 64.25 1.02 16.16 5.18 0.11 1.59 4.04 4.91 2.07 0.28 0.18 99.83 

TS-619 drm SS 63.40 1.06 16.21 5.45 0.12 1.77 4.28 5.04 1.99 0.27 na 97.48 

TS-620 drm SS 63.87 1.05 16.06 5.35 0.12 1.70 4.12 5.06 1.99 0.28 na 99.38 

TS-623 drm SS 64.27 1.00 16.10 5.10 0.12 1.61 4.07 5.05 2.03 0.26 na 99.90 

TS-635 drm SS 64.05 1.01 16.08 5.24 0.11 1.66 4.15 5.00 2.04 0.26 na 98.90 

TS-677 drm SS 64.08 1.01 16.16 5.12 0.11 1.62 4.10 5.10 2.04 0.26 na 98.73 

TS-678 drm SS 63.92 1.02 16.27 5.21 0.11 1.63 4.11 5.04 2.02 0.27 na 99.23 

TS-680 drm SS 66.34 0.75 15.80 4.13 0.08 1.47 3.80 4.66 2.40 0.18 na 98.23 

TS-719 drm SS 63.77 1.04 16.24 5.22 0.12 1.64 4.17 5.11 2.00 0.27 na 99.47 

TS-720 drm SS 63.08 1.09 16.31 5.61 0.12 1.76 4.44 5.00 1.92 0.27 na 99.04 

TS-723 drm SS 62.64 1.12 16.22 5.73 0.12 1.97 4.58 4.98 1.94 0.30 na 100.28 

Dacite east of Sink Creek 

TS-641 dse P 66.20 0.63 16.47 4.02 0.09 1.24 3.65 5.04 2.05 0.20 na 97.32 

Dacite north of Separation Creek 

TS-228 dsn MS 64.31 0.97 16.05 5.65 0.14 1.32 3.46 5.53 1.80 0.37 -0.02 99.28 

TS-257 dsn MS 64.40 0.97 16.00 5.64 0.14 1.28 3.41 5.59 1.81 0.37 0.06 99.61 

Dacite of Sister Spring 

TS-101 dss MS 63.58 1.01 16.32 5.69 0.14 1.43 3.71 5.56 1.69 0.46 <0.01 100.10 

TS-387 dss MS 63.74 1.02 15.99 5.73 0.14 1.49 3.80 5.58 1.70 0.42 <0.01 99.69 

Dacite domes southwest of Middle Sister 

TS-222 dsw MS 65.45 0.72 16.19 5.15 0.14 0.93 2.79 6.13 1.85 0.26 -0.10 99.67 

TS-222A dsw MS 65.47 0.71 16.14 5.18 0.14 0.92 2.83 6.12 1.83 0.26 -0.01 99.34 

TS-223 dsw MS 65.42 0.70 16.33 5.14 0.14 0.88 2.79 6.10 1.84 0.26 -0.08 99.42 

TS-224 dsw MS 65.48 0.71 16.12 5.22 0.14 0.95 2.77 6.10 1.84 0.27 0.08 99.48 

Dacite of Todd Lake 

TS-700 dtl P 62.62 1.08 16.50 5.80 0.12 1.83 4.33 4.85 2.17 0.31 na 98.65 

TS-782 dtl P 63.49 0.99 16.10 5.64 0.11 1.83 4.25 4.68 2.22 0.28 na 99.64 

TS-783 dtl P 62.75 1.15 15.83 6.26 0.14 1.84 4.33 4.89 2.07 0.35 na 99.92 

Dacite west of Lost Creek Glacier 

TS-315 dwl SS 63.10 1.07 16.15 5.78 0.12 1.82 4.34 5.03 1.90 0.29 0.01 99.29 

TS-545 dwl SS 64.60 0.92 15.85 5.19 0.10 1.85 3.99 4.68 2.21 0.21 0.14 99.29 

Dacite of Wickiup Plain 

TS-64 dwp SS 64.05 0.99 16.29 5.06 0.11 1.61 4.08 5.06 2.04 0.32 0.06 99.67 

TS-135 dwp SS 64.16 0.99 16.27 5.08 0.11 1.58 4.08 4.98 2.05 0.30 0.00 99.19 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
83

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Silicic dacite north of Teardrop Pool

724 33 5 16 13 12 7 26 3 52 298 81 19 43 178

Dacite pumice fall of Prouty Glacier headwall

634 41 <5 11 18 21 12 27 5 37 345 69 30 79 241

Dacite of Red Meadow

608 37 <5 8 21 17 11 19 7 38 388 96 25 74 218

676 40 <5 3 14 17 10 22 3 35 404 87 24 61 225

721 33 <5 22 14 14 8 26 3 46 342 91 19 51 213

649 37 <5 4 15 13 10 24 3 36 399 93 23 60 211

674 40 <5 5 14 15 10 23 3 36 402 93 23 64 218

695 42 <5 4 13 20 10 30 3 32 391 83 25 63 222

na na na na na na na na na na na na na na na

684 37 <5 2 14 16 12 30 3 35 387 102 27 53 259

643 40 <5 4 15 16 13 23 3 33 400 113 28 59 253

659 37 <5 <2 15 13 12 32 3 37 388 121 27 52 256

589 41 2 12 20 17 10 22 0 36 398 105 26 73 202

600 40 2 9 18 19 10 21 0 37 398 96 26 73 207

623 39 1 9 19 22 11 21 0 38 401 95 26 72 211

602 36 2 10 18 18 11 18 0 38 401 96 26 72 206

592 42 2 8 18 19 11 20 0 37 390 89 25 74 207

621 34 3 8 19 20 10 17 0 36 394 96 25 76 209

649 35 12 20 17 17 8 16 5 50 319 76 20 56 195

600 44 3 7 19 20 11 22 0 37 401 95 27 75 208

567 39 2 11 18 18 10 22 0 34 412 111 26 73 200

610 42 6 11 19 23 12 24 2 36 444 111 28 80 209

Dacite east of Sink Creek

617 33 1 8 20 15 8 17 0 38 401 53 19 58 181

Dacite north of Separation Creek

660 49 <5 19 21 22 12 25 7 30 382 47 43 91 238

664 47 5 24 22 21 11 26 8 29 373 50 37 88 237

Dacite of Sister Spring

636 44 <3 9 22 21 11 27 9 26 405 53 36 91 222

626 42 na 16 26 20 12 26 na 28 433 84 46 99 275

Dacite domes southwest of Middle Sister

638 41 <5 6 24 18 12 31 8 27 293 21 41 95 281

644 40 <5 4 23 17 11 24 7 26 297 25 37 93 271

645 41 <5 7 24 16 12 29 9 26 297 21 37 96 275

639 38 6 6 23 17 11 26 8 27 292 19 40 93 277

Dacite of Todd Lake

682 55 5 25 20 26 14 28 2 44 329 106 38 85 276

633 55 10 29 19 26 13 31 7 58 360 113 38 76 257

683 66 7 29 21 26 14 34 6 47 344 121 44 91 267

Dacite west of Lost Creek Glacier

592 34 <5 22 21 17 10 19 7 37 394 96 28 69 214

na na na na na na na na na na na na na na na

Dacite of Wickiup Plain

612 37 <3 12 20 18 12 22 <2 39 401 91 25 71 219

na na na na na na na na na na na na na na na
84

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Dacite of Wickiup Plain—Continued 

TS-136 dwp SS 64.00 1.00 16.48 5.12 0.11 1.58 4.01 4.96 2.04 0.30 0.26 99.14 

TS-137 dwp SS 64.21 0.99 16.25 5.02 0.11 1.58 4.10 5.00 2.04 0.30 0.16 98.66 

TS-345 dwp SS 64.49 0.98 16.17 5.03 0.11 1.50 3.84 5.06 2.10 0.32 0.11 99.16 

Basaltic andesite of Black Crater 

TS-746 mbc P 56.22 1.02 17.67 7.06 0.13 4.57 7.72 3.75 1.12 0.34 na 98.93 

NS-07-197 mbc P 55.11 1.00 18.50 6.98 0.12 4.52 8.27 3.85 0.96 0.27 na 100.03 

NS-07-199 mbc P 55.85 1.33 17.27 8.22 0.15 3.93 7.29 4.35 0.95 0.28 na 100.19 

Basaltic andesite of Bare Lake 

TS-785 mbl P 55.79 1.67 16.22 9.53 0.16 3.32 7.01 4.50 1.02 0.36 na 99.89 

Basaltic andesite of Broken Top 

TS-165 mbt BT 54.15 0.98 18.25 7.28 0.12 5.77 8.41 3.49 0.86 0.29 1.55 97.67 

TS-322 mbt BT 55.98 1.47 16.68 9.07 0.15 3.39 7.24 4.45 0.91 0.25 <0.01 99.10 

TS-327 mbt BT 54.69 0.97 18.70 6.93 0.12 4.99 8.42 3.82 0.73 0.23 0.30 99.07 

TS-328 mbt BT 56.02 1.53 16.40 9.37 0.15 3.37 7.15 4.43 0.90 0.28 <0.01 99.57 

TS-329 mbt BT 56.08 1.46 16.77 8.89 0.15 3.36 7.15 4.38 0.98 0.37 <0.01 98.57 

TS-330 mbt BT 56.71 1.19 18.20 6.91 0.13 3.30 7.58 4.24 0.99 0.34 <0.01 99.58 

TS-333 mbt BT 55.68 1.78 16.02 9.56 0.16 3.43 7.05 4.57 0.97 0.37 <0.01 99.45 

TS-334 mbt BT 56.30 1.44 16.95 8.00 0.15 3.50 7.61 4.42 0.96 0.27 na 100.09 

TS-334R mbt BT 56.22 1.44 16.95 8.14 0.15 3.51 7.58 4.39 0.96 0.27 na 100.29 

TS-341 mbt BT 52.83 1.32 19.66 7.84 0.13 3.47 9.74 3.66 0.67 0.29 0.45 98.80 

TS-342 mbt BT 55.77 1.57 17.16 8.69 0.14 3.00 7.55 4.34 1.03 0.34 <0.01 99.82 

TS-450 mbt BT 52.54 1.16 20.51 7.40 0.12 3.62 9.98 3.39 0.62 0.25 0.06 99.53 

TS-451 mbt BT 57.79 1.68 16.03 8.75 0.16 2.87 6.06 4.56 1.29 0.41 <0.01 99.44 

TS-454 mbt BT 54.24 1.16 18.58 8.27 0.14 4.38 8.21 3.61 0.75 0.26 0.36 99.15 

TS-501 mbt BT 56.07 1.50 17.17 8.47 0.15 3.44 7.25 4.05 1.10 0.40 <0.01 100.37 

TS-515 mbt BT 56.49 1.42 16.85 9.12 0.15 3.39 7.01 4.03 0.90 0.24 0.32 98.74 

TS-524 mbt BT 54.11 1.31 18.47 8.64 0.14 4.33 7.80 3.84 0.67 0.29 0.14 98.67 

TS-529 mbt BT 57.28 1.80 15.92 8.91 0.18 2.95 6.19 4.80 1.24 0.33 <0.01 99.47 

TS-568 mbt BT 55.89 1.56 16.53 9.24 0.16 3.42 6.99 4.39 1.10 0.32 <0.01 99.43 

TS-675 mbt BT 55.50 1.51 17.42 8.26 0.15 3.44 7.39 4.53 1.07 0.34 na 99.25 

TS-682 mbt BT 55.82 1.74 16.40 9.06 0.16 3.40 6.94 4.51 1.13 0.44 na 100.20 

TS-683 mbt BT 55.72 1.74 16.51 9.10 0.16 3.41 6.96 4.44 1.12 0.44 na 100.20 

TS-683R mbt BT 55.72 1.73 16.51 9.10 0.16 3.41 6.93 4.46 1.13 0.44 na 99.73 

TS-684 mbt BT 57.70 1.68 16.16 8.45 0.17 2.81 6.05 4.93 1.30 0.36 na 99.62 

TS-685 mbt BT 54.61 1.92 16.22 9.73 0.17 3.59 7.46 4.53 0.98 0.36 na 100.38 

TS-686 mbt BT 54.80 1.65 17.16 9.35 0.17 3.33 7.35 4.60 0.92 0.28 na 99.24 

TS-715 mbt BT 55.39 1.80 16.43 9.26 0.16 3.37 7.23 4.68 0.96 0.33 na 99.63 

TS-717 mbt BT 53.68 1.53 17.65 8.78 0.15 3.88 8.44 4.19 0.90 0.38 na 100.07 

Basaltic andesite of Whychus Creek bridge 

TS-448 mbw BT 52.90 1.32 18.00 8.62 0.14 5.34 8.68 3.51 0.78 0.31 0.11 99.60 

TS-456 mbw BT 52.93 1.32 18.14 8.49 0.14 5.28 8.74 3.43 0.80 0.32 0.07 99.36 

TS-659 mbw BT 52.91 1.35 17.99 8.17 0.15 5.39 8.72 3.85 0.80 0.27 na 99.96 

Basaltic andesite of Camp Lake 

TS-1 mcl MS 56.31 1.33 17.00 8.86 0.14 3.37 6.80 4.46 1.01 0.31 <0.01 99.58 

TS-2 mcl MS 56.78 1.32 17.06 8.60 0.14 3.24 6.63 4.48 1.04 0.31 <0.01 99.81 

TS-22 mcl MS 56.45 1.33 17.09 8.83 0.14 3.33 6.81 4.33 0.99 0.31 <0.01 99.68 

TS-169 mcl MS 56.34 1.34 17.10 8.90 0.14 3.33 6.88 4.27 1.02 0.28 0.01 98.99 

TS-186 mcl MS 56.79 1.31 16.99 8.64 0.14 3.28 6.79 4.36 1.04 0.27 0.12 99.09 

TS-446 mcl MS 56.59 1.32 17.14 8.73 0.14 3.42 6.79 4.14 1.03 0.31 <0.01 99.97 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
85

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Dacite of Wickiup Plain—Continued

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

650 39 na 9 23 18 15 22 na 40 406 112 38 76 286

Basaltic andesite of Black Crater

492 49 84 50 20 23 12 28 62 18 533 163 26 84 192

416 36 59 54 20 18 6 20 41 14 853 184 17 80 120

373 30 16 72 19 13 6 20 8 14 570 217 23 85 137

Basaltic andesite of Bare Lake

407 32 4 62 21 13 10 22 6 18 498 282 30 96 144

Basaltic andesite of Broken Top

289 28 111 91 21 13 7 17 100 11 735 155 18 73 123

318 29 na 71 23 10 8 13 na 16 527 321 31 85 150

288 22 63 68 15 8 4 <10 42 9 686 221 14 65 94

339 27 <5 77 17 8 5 <10 3 12 489 297 22 77 118

415 35 <5 74 16 11 7 15 3 14 530 228 24 81 140

357 25 25 49 16 7 6 22 3 17 510 198 23 66 130

325 30 na 76 22 11 12 19 na 15 486 325 37 96 187

337 25 38 64 19 14 7 19 6 16 472 239 26 84 135

333 29 38 63 20 14 6 17 5 16 473 239 27 86 136

263 22 19 54 16 9 6 <10 3 8 626 275 20 62 104

352 30 <5 41 16 12 6 <10 3 18 534 279 25 80 136

242 12 6 53 15 10 3 <10 3 8 745 204 13 49 76

437 40 <5 20 16 13 12 23 3 22 394 208 35 83 218

285 23 50 71 15 9 5 10 23 15 568 207 17 65 109

418 42 <5 22 17 12 9 11 3 14 583 196 26 74 163

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

427 40 <5 19 15 11 11 13 10 21 399 217 35 84 212

412 30 <5 38 15 8 7 <10 3 18 519 309 27 71 165

391 35 23 71 20 17 9 22 9 14 577 221 28 92 156

397 37 10 64 19 17 7 23 5 21 509 261 29 96 148

408 37 9 49 20 18 8 23 4 21 513 266 29 98 148

406 38 7 49 20 18 8 25 6 21 510 262 29 95 147

431 46 9 25 20 20 12 29 1 21 384 222 38 97 216

343 32 16 42 21 14 10 22 5 15 430 314 33 100 171

354 26 2 50 21 13 7 19 4 15 487 266 27 99 127

337 30 5 51 19 14 8 20 4 15 458 290 30 97 145

361 37 19 41 20 16 8 23 11 13 636 257 25 87 135

Basaltic andesite of Whychus Creek bridge

315 25 42 71 16 8 5 <10 62 13 549 195 21 67 117

321 25 41 79 16 10 6 <10 53 13 549 203 21 72 120

313 27 69 80 18 14 6 19 78 14 532 215 24 88 120

Basaltic andesite of Camp Lake

383 29 5 81 22 13 8 11 9 15 546 253 21 93 135

401 27 4 86 22 12 8 <7 6 16 540 220 22 88 139

386 28 <3 66 20 13 8 12 2 15 552 231 21 87 136

388 28 <5 89 20 11 8 <10 13 15 557 242 22 87 139

391 29 <5 89 19 14 8 11 6 17 550 223 22 88 137

419 35 <5 78 17 8 6 <10 3 15 558 226 20 76 134
86

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Basaltic andesite north and south of Demaris Lake 

TS-188 mdl P 52.96 1.30 18.56 8.07 0.14 4.30 9.45 3.68 0.79 0.34 0.04 98.73 

TS-463 mdl P 53.63 1.19 17.61 8.02 0.14 6.26 8.43 3.11 0.88 0.33 <0.01 100.10 

Basaltic andesite and basalt of "Egan Cone" 

TS-350 mec P 54.08 1.31 17.98 8.62 0.15 4.63 7.94 3.83 0.74 0.31 na 99.54 

TS-699 mec P 53.58 1.36 18.11 8.75 0.16 4.77 8.06 3.75 0.72 0.35 na 99.59 

TS-707 mec P 52.91 1.51 17.83 9.18 0.16 4.93 8.09 3.81 0.73 0.45 na 99.76 

TS-778 mec P 51.94 1.72 17.37 10.11 0.17 5.15 8.17 3.68 0.72 0.57 na 99.59 

TS-779 mec P 51.59 1.70 17.45 10.04 0.17 5.39 8.32 3.72 0.68 0.53 na 100.25 

TS-780 mec P 52.17 1.70 17.49 9.99 0.18 5.11 8.03 3.66 0.71 0.56 na 99.38 

Basaltic andesite east of Yapoah Lake 

TS-593 mey NS 52.80 1.28 17.61 7.98 0.14 6.41 8.58 3.65 0.85 0.30 na 98.70 

TS-599 mey NS 52.94 1.28 17.67 7.80 0.14 6.40 8.54 3.67 0.86 0.30 na 98.37 

NS-02-61 mey NS 53.06 1.30 17.57 8.19 0.14 6.22 8.74 3.66 0.82 0.30 na 97.60 

NS-02-96 mey NS 53.21 1.30 17.91 7.96 0.14 5.84 8.81 3.68 0.85 0.30 na 98.96 

Basaltic andesite of fissure vent south of Yapoah Lake 

TS-649 mfy NS 57.49 1.55 16.68 8.20 0.14 3.03 6.21 4.71 1.25 0.32 na 99.96 

TS-758 mfy NS 56.62 1.48 16.93 8.70 0.15 3.20 6.50 4.59 1.12 0.32 na 98.61 

Basaltic andesite of Hayden Glacier–Diller Glacier cleaver 

TS-178 mhd MS 54.77 1.73 16.52 9.84 0.16 3.69 7.46 4.24 0.86 0.32 -0.35 99.47 

TS-179 mhd MS 54.66 1.73 16.58 9.87 0.16 3.73 7.45 4.23 0.87 0.32 -0.11 99.13 

TS-180 mhd MS 54.53 1.74 16.60 9.89 0.16 3.77 7.50 4.24 0.86 0.32 0.27 99.00 

TS-477 mhd MS 56.24 1.63 16.71 8.80 0.15 3.58 7.06 3.77 1.25 0.40 0.12 99.54 

Basaltic andesite of headwaters of Linton Creek 

TS-270 mhl MS 54.55 1.72 16.62 9.84 0.16 3.77 7.54 4.23 0.84 0.33 -0.21 99.51 

TS-271 mhl MS 55.68 1.71 16.46 9.53 0.16 3.36 6.99 4.39 0.98 0.34 -0.31 99.82 

Basaltic andesite of The House Rock 

TS-150 mhr P 53.36 1.08 18.52 7.81 0.13 5.57 8.45 3.66 0.72 0.30 -0.13 99.50 

TS-702 mhr P 53.44 1.10 18.52 7.62 0.13 5.50 8.57 3.66 0.77 0.27 na 100.05 

TS-703 mhr P 53.37 1.09 18.71 7.56 0.14 5.43 8.57 3.75 0.72 0.26 na 100.09 

Basaltic andesite of Koosah Mountain 

TS-351 mkm P 54.75 1.64 17.10 8.77 0.16 4.01 7.55 4.31 0.98 0.35 na 99.80 

TS-352 mkm P 53.86 1.23 17.87 7.88 0.14 5.17 8.24 3.85 0.94 0.43 na 100.13 

TS-564 mkm P 52.57 1.26 17.73 8.28 0.14 6.36 8.62 3.48 0.83 0.33 0.07 98.89 

TS-701 mkm P 52.80 1.22 17.53 8.06 0.14 6.51 8.63 3.53 0.85 0.32 na 99.79 

TS-774 mkm P 53.33 1.23 17.69 8.07 0.14 6.36 8.12 3.53 0.80 0.32 na 98.44 

TS-775 mkm P 53.08 1.19 17.35 7.95 0.14 6.75 8.54 3.45 0.83 0.31 na 99.48 

Basaltic andesite of Little Brother 

TS-79 mlb P 52.63 1.41 17.74 8.52 0.15 5.38 8.56 3.71 0.92 0.56 <0.01 99.35 

TS-80 mlb P 52.48 1.42 17.56 8.64 0.15 5.63 8.53 3.66 0.98 0.53 0.15 99.25 

TS-81 mlb P 52.89 1.41 17.56 8.49 0.15 5.26 8.48 3.77 1.01 0.57 <0.01 99.80 

TS-383 mlb P 53.05 1.41 17.48 8.45 0.15 5.09 8.59 3.82 1.00 0.55 <0.01 99.13 

TS-385 mlb P 52.74 1.41 17.45 8.62 0.15 5.49 8.56 3.71 0.95 0.51 0.08 99.33 

TS-395 mlb P 52.97 1.40 17.46 8.45 0.15 5.23 8.61 3.77 1.01 0.54 <0.01 99.28 

TS-554 mlb P 52.60 1.43 17.10 8.63 0.15 6.15 8.49 3.63 0.99 0.43 <0.01 99.60 

TS-555 mlb P 52.24 1.46 17.38 8.77 0.16 6.03 8.54 3.60 0.94 0.48 <0.01 99.14 

TS-611 mlb P 52.30 1.42 17.86 8.50 0.15 5.66 8.55 3.76 0.92 0.48 na 98.11 

TS-612 mlb P 52.89 1.42 17.52 8.39 0.15 5.48 8.54 3.79 0.96 0.47 na 100.42 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
87

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Basaltic andesite north and south of Demaris Lake

322 36 36 130 19 16 9 <10 21 12 671 208 24 74 127

350 35 122 59 15 12 8 <10 109 11 614 180 19 67 135

Basaltic andesite and basalt of “Egan Cone”

346 30 41 53 20 15 6 20 48 11 537 183 25 95 121

341 31 47 51 20 13 6 20 52 10 535 180 26 99 124

353 33 54 51 19 14 8 20 56 11 527 187 28 101 133

359 43 58 48 20 12 10 26 58 11 520 194 34 108 144

338 38 59 47 20 17 10 25 62 10 505 199 34 104 141

358 40 59 49 20 15 10 25 57 10 515 189 34 106 147

Basaltic andesite east of Yapoah Lake

329 32 151 58 19 16 8 20 121 11 546 188 22 80 136

330 30 151 64 19 13 9 18 120 11 541 190 22 79 137

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

Basaltic andesite of fissure vent south of Yapoah Lake

436 41 3 50 20 15 8 24 5 20 465 224 27 93 181

411 39 1 47 21 15 8 21 5 18 536 233 27 93 154

Basaltic andesite of Hayden Glacier–Diller Glacier cleaver

326 26 5 171 21 12 8 20 13 14 513 289 25 93 132

319 24 8 104 19 9 8 20 16 14 512 289 28 97 137

339 33 6 107 22 13 7 <10 16 13 516 282 26 93 132

429 34 26 89 17 9 8 <10 16 22 490 222 27 74 185

Basaltic andesite of headwaters of Linton Creek

321 25 13 108 23 9 7 11 22 12 516 288 25 94 135

357 28 <5 71 22 13 8 19 20 16 503 260 27 91 146

Basaltic andesite of The House Rock

na na na na na na na na na na na na na na na

321 26 94 48 18 12 7 18 93 12 597 186 20 80 118

323 29 94 72 18 12 7 18 87 11 598 193 20 82 118

Basaltic andesite of Koosah Mountain

365 31 42 89 21 16 8 21 23 13 511 233 27 90 140

400 48 98 72 19 22 12 26 68 11 764 168 24 88 179

368 33 152 59 13 10 9 26 99 12 529 197 20 70 146

336 33 206 63 18 15 11 20 121 12 518 195 23 81 143

343 34 204 66 18 12 11 19 120 12 507 168 25 84 141

339 34 234 62 19 17 11 20 136 12 526 193 25 81 137

Basaltic andesite of Little Brother

414 40 99 49 20 16 16 25 63 13 676 199 28 93 191

405 37 122 58 21 16 15 19 83 14 664 208 26 99 178

413 45 97 98 20 20 15 19 62 14 662 203 27 98 190

420 43 86 58 22 18 20 34 na 15 711 242 35 98 239

408 40 89 51 25 20 19 24 na 15 711 217 33 94 224

411 42 86 72 24 17 19 27 na 16 706 232 33 98 236

na na na na na na na na na na na na na na na

457 45 100 44 16 14 11 15 84 11 671 192 22 79 164

443 44 96 63 18 23 12 27 66 12 686 194 26 91 170

434 46 96 50 19 22 12 27 65 13 662 194 27 91 170
88

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Basaltic andesite of Le Conte Crater 

TS-67 mlc P 54.46 1.32 16.69 8.06 0.14 6.30 7.48 3.59 1.17 0.40 0.31 99.67 

TS-247 mlc P 54.50 1.31 16.46 8.12 0.14 6.39 7.53 3.61 1.17 0.36 0.17 99.24 

TS-565 mlc P 55.85 1.30 16.29 7.69 0.14 5.79 7.15 3.70 1.39 0.30 0.44 98.44 

TS-628 mlc P 54.48 1.33 16.51 7.96 0.14 6.40 7.55 3.74 1.16 0.33 na 100.50 

Basaltic andesite of Linton Spring 

TS-292 mls MS 55.09 1.43 17.02 9.23 0.15 3.81 7.34 4.25 0.98 0.29 0.24 98.89 

Basaltic andesite of Millican Crater 

TS-745 mmc P 53.59 1.10 18.52 7.64 0.13 5.40 8.50 3.73 0.73 0.24 na 99.60 

TS-745R mmc P 53.54 1.10 18.51 7.76 0.13 5.40 8.47 3.72 0.74 0.24 na 100.60 

Basaltic andesite intrusion of Middle Sister 

TS-181 mmi MS 52.79 1.52 17.33 9.30 0.15 5.42 8.38 3.60 0.79 0.32 0.81 98.30 

TS-474 mmi MS 53.25 1.40 17.88 8.82 0.16 5.20 8.59 3.28 0.71 0.32 <0.01 99.70 

Basaltic andesite of South Matthieu Lake 

TS-638 mml NS 54.82 1.51 17.09 9.31 0.16 3.84 7.39 4.22 0.97 0.28 na 99.81 

TS-731 mml NS 54.42 1.48 17.26 9.26 0.15 3.96 7.68 4.14 0.97 0.27 na 99.48 

TS-737 mml NS 55.64 1.24 17.53 8.14 0.15 3.87 7.63 4.21 0.96 0.25 na 99.53 

Basaltic andesite of Montague Memorial 

TS-106 mmm MS 56.28 1.32 17.77 7.74 0.13 3.57 7.12 4.22 1.12 0.33 0.05 100.34 

TS-384 mmm MS 56.43 1.32 17.60 7.78 0.13 3.53 7.23 4.14 1.12 0.32 0.44 98.49 

TS-428 mmm MS 57.27 1.32 17.73 7.61 0.13 3.10 6.79 4.07 1.26 0.32 <0.01 100.01 

TS-429 mmm MS 56.62 1.34 17.43 7.79 0.13 3.42 7.08 4.33 1.16 0.30 0.11 98.86 

Basaltic andesite of North Matthieu Lake 

TS-724 mmn NS 54.58 1.51 17.08 9.33 0.15 3.89 7.59 4.25 0.94 0.28 na 100.34 

TS-724R mmn NS 54.60 1.51 17.10 9.29 0.15 3.90 7.57 4.26 0.93 0.27 na 100.10 

Basaltic andesite of Middle Sister 

TS-3 mms MS 52.47 1.31 18.39 8.60 0.14 5.48 8.57 3.73 0.65 0.27 0.03 99.66 

TS-4 mms MS 52.16 1.35 18.42 8.79 0.14 5.52 8.69 3.59 0.62 0.31 0.09 100.05 

TS-8 mms MS 52.40 1.34 18.33 8.79 0.14 5.32 8.58 3.70 0.70 0.31 <0.01 99.99 

TS-172 mms MS 52.54 1.30 18.15 8.74 0.14 5.66 8.66 3.56 0.62 0.23 -0.25 99.34 

TS-173 mms MS 52.49 1.32 18.20 8.79 0.14 5.66 8.64 3.48 0.63 0.24 -0.04 99.04 

TS-174 mms MS 52.91 1.37 18.01 8.75 0.14 5.28 8.44 3.70 0.72 0.28 -0.23 99.02 

TS-175 mms MS 52.70 1.37 17.87 8.87 0.14 5.56 8.53 3.59 0.68 0.28 -0.05 99.21 

TS-183 mms MS 52.65 1.31 18.05 8.76 0.14 5.74 8.54 3.51 0.66 0.24 0.09 98.76 

TS-184 mms MS 52.62 1.32 18.01 8.83 0.14 5.65 8.60 3.55 0.64 0.24 -0.35 99.57 

TS-192 mms MS 52.76 1.37 18.42 8.57 0.14 4.92 8.77 3.67 0.70 0.28 -0.15 99.50 

TS-218 mms MS 52.50 1.33 18.06 8.83 0.14 5.68 8.61 3.57 0.64 0.24 -0.33 99.79 

TS-226 mms MS 52.52 1.38 18.14 8.79 0.14 5.41 8.69 3.58 0.68 0.27 0.03 99.38 

TS-227 mms MS 52.62 1.34 18.34 8.62 0.14 5.31 8.69 3.57 0.70 0.26 -0.04 99.37 

TS-235 mms MS 53.44 1.42 18.41 8.40 0.14 4.26 8.73 3.73 0.79 0.27 -0.11 99.53 

TS-297 mms MS 52.23 1.41 17.84 9.16 0.15 5.73 8.58 3.58 0.64 0.26 -0.04 99.35 

TS-304B mms MS 52.56 1.88 16.85 10.32 0.17 4.61 8.07 3.84 0.89 0.40 0.03 99.30 

TS-375 mms MS 52.75 1.30 17.95 8.75 0.14 5.67 8.59 3.54 0.64 0.28 <0.01 99.89 

TS-378 mms MS 52.55 1.55 17.48 9.35 0.16 5.35 8.39 3.67 0.75 0.35 0.04 99.70 

TS-382 mms MS 52.64 1.54 17.28 9.35 0.15 5.44 8.45 3.68 0.75 0.33 <0.01 99.72 

TS-398 mms MS 52.75 1.29 17.95 8.69 0.14 5.73 8.62 3.52 0.63 0.27 <0.01 99.31 

TS-403 mms MS 52.60 1.28 18.20 8.46 0.14 5.50 8.92 3.58 0.62 0.29 <0.01 99.59 

TS-404 mms MS 52.41 1.27 18.37 8.52 0.14 5.76 8.90 3.34 0.61 0.28 <0.01 99.77 

TS-406 mms MS 52.51 1.28 18.40 8.52 0.14 5.66 8.87 3.32 0.62 0.28 0.16 99.59 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
89

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Basaltic andesite of Le Conte Crater

375 33 204 55 17 16 14 15 128 20 444 180 24 77 165

374 34 193 56 17 16 13 16 110 21 448 171 23 70 165

441 33 150 43 13 11 11 28 102 26 417 163 22 62 165

389 30 205 63 16 18 11 19 124 21 453 172 24 74 159

Basaltic andesite of Linton Spring

357 29 20 71 20 11 8 19 11 15 552 247 22 94 137

Basaltic andesite of Millican Crater

312 23 85 65 17 12 7 15 90 12 599 184 20 82 108

323 26 87 65 19 11 7 14 91 13 605 188 20 80 109

Basaltic andesite intrusion of Middle Sister

279 27 88 93 19 10 7 <10 73 12 515 216 25 82 131

292 26 78 36 16 13 4 <10 55 10 547 210 19 69 110

Basaltic andesite of South Matthieu Lake

368 32 16 57 21 18 7 19 7 14 547 281 23 91 141

352 29 23 64 19 11 7 19 11 14 544 280 22 92 134

362 30 25 102 21 13 6 18 12 14 571 225 22 88 124

Basaltic andesite of Montague Memorial

379 31 27 72 20 19 8 18 24 19 492 188 23 73 158

399 31 15 70 15 13 7 <10 13 17 496 206 22 69 158

452 34 8 65 17 13 7 11 6 21 492 194 22 66 167

383 29 22 78 24 12 11 18 na 20 521 252 30 76 203

Basaltic andesite of North Matthieu Lake

358 33 17 60 21 16 8 21 9 14 548 293 22 92 138

352 30 18 60 20 13 7 18 9 13 539 290 22 93 137

Basaltic andesite of Middle Sister

242 20 91 85 21 10 6 <7 72 10 550 208 21 83 100

248 21 100 85 19 10 6 8 76 10 552 208 23 84 110

252 23 93 87 20 7 6 12 70 10 544 204 22 81 111

234 19 98 97 19 6 5 11 80 10 577 203 20 77 94

244 17 103 96 19 7 5 13 82 9 566 208 20 82 97

266 20 77 88 19 7 7 <10 66 11 557 187 22 74 116

253 21 96 88 19 9 8 <10 75 12 548 214 24 82 112

245 21 100 95 21 10 6 <10 76 10 564 207 21 77 97

239 18 92 98 19 5 5 13 81 9 572 203 20 81 96

263 26 81 86 20 11 7 <10 59 11 552 196 22 73 114

237 25 95 102 22 8 5 <10 79 9 572 205 20 79 96

242 21 92 88 19 6 7 <10 69 11 546 194 23 76 114

243 22 90 73 19 7 7 <10 69 11 551 193 22 73 110

275 25 81 99 20 10 7 <10 40 13 551 217 24 73 119

244 21 84 120 20 9 6 11 77 10 565 212 22 77 105

338 29 67 115 21 12 10 21 42 14 489 277 34 103 173

250 22 86 78 13 7 4 <10 69 9 566 237 17 70 95

284 24 71 82 14 8 5 <10 53 13 526 242 22 74 135

289 25 71 91 16 10 5 <10 57 11 522 234 23 76 132

255 13 93 87 17 6 3 12 75 9 576 224 18 122 92

245 19 89 76 15 9 3 <10 58 10 574 217 17 73 94

242 13 81 76 16 <5 4 <10 62 8 578 207 17 61 94

249 23 90 75 14 10 4 <10 64 10 571 215 17 68 95
90

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Basaltic andesite of Middle Sister—Continued 

TS-407 mms MS 54.10 1.40 18.17 8.34 0.14 4.23 8.49 3.55 0.87 0.31 <0.01 99.23 

TS-409 mms MS 53.59 1.42 18.30 8.48 0.14 4.45 8.65 3.45 0.81 0.31 0.73 98.51 

TS-422 mms MS 52.76 1.73 17.25 9.76 0.16 4.81 8.32 3.54 0.86 0.40 <0.01 99.30 

TS-465 mms MS 52.50 1.33 18.24 8.94 0.14 5.79 8.69 3.12 0.58 0.27 <0.01 99.41 

TS-469 mms MS 52.77 1.32 18.09 8.85 0.14 5.70 8.64 3.18 0.64 0.28 <0.01 99.66 

TS-551 mms MS 52.30 1.46 17.37 8.73 0.16 5.96 8.50 3.72 0.95 0.45 <0.01 99.23 

TS-304A mms MS 53.34 1.39 18.64 8.23 0.14 4.26 8.64 3.88 0.81 0.28 -0.09 99.90 

 [dike]

Pyroxene-bearing variant of the basaltic andesite of Middle Sister 

TS-193 mms′ MS 54.53 1.76 16.63 9.91 0.16 3.71 7.51 4.19 0.88 0.31 0.19 98.81 

TS-421 mms′ MS 54.56 1.73 16.51 9.89 0.16 3.73 7.50 4.32 0.85 0.34 <0.01 98.94 

TS-423 mms′ MS 54.78 1.75 16.52 9.93 0.16 3.71 7.45 4.08 0.87 0.35 0.10 99.46 

TS-424 mms′ MS 54.68 1.72 16.49 9.88 0.16 3.67 7.49 4.30 0.87 0.34 <0.01 99.09 

TS-464 mms′ MS 54.91 1.74 16.63 9.94 0.16 3.73 7.41 3.85 0.87 0.35 <0.01 99.40 

Basaltic andesite tuff of Middle Sister 

TS-266 mmt MS 51.74 1.33 19.80 8.69 0.14 5.30 8.39 3.40 0.56 0.26 2.26 97.60 

TS-288 mmt MS 51.98 1.49 18.93 9.26 0.15 5.07 8.21 3.55 0.68 0.29 1.71 97.35 

TS-289 mmt MS 55.21 1.20 18.44 7.79 0.14 4.20 7.30 4.15 0.91 0.26 1.26 97.78 

Basaltic andesite of North Fork Whychus Creek 

TS-185 mnf MS 53.64 1.04 18.48 7.64 0.13 5.44 8.75 3.60 0.67 0.21 -0.13 99.16 

TS-194 mnf MS 54.35 1.03 19.09 7.23 0.12 4.62 8.52 3.73 0.70 0.21 0.23 99.14 

TS-444 mnf MS 54.23 1.07 18.58 7.45 0.13 5.03 8.58 3.48 0.76 0.29 0.17 99.72 

TS-445 mnf MS 53.78 1.10 18.26 7.64 0.13 5.56 8.71 3.35 0.76 0.30 0.11 99.26 

TS-473 mnf MS 54.38 0.99 19.15 7.18 0.12 5.03 8.61 3.20 0.68 0.26 0.01 100.37 

Basaltic andesite north of Hayden Glacier 

TS-217 mnh MS 57.13 1.30 17.02 8.60 0.14 3.06 6.51 4.49 1.07 0.28 -0.30 100.07 

TS-470 mnh MS 54.41 1.08 18.37 7.58 0.13 5.03 8.69 3.28 0.76 0.27 0.11 99.77 

TS-471 mnh MS 53.44 1.06 18.84 7.62 0.13 5.37 8.98 3.27 0.63 0.25 0.06 99.90 

Basaltic andesite of North Sister 

TS-28A mns NS 54.25 1.07 18.52 7.40 0.13 4.90 8.49 3.77 0.77 0.29 0.36 99.51 

TS-28B mns NS 53.46 1.06 18.75 7.43 0.13 5.01 8.87 3.86 0.77 0.27 0.10 99.86 

TS-29 mns NS 53.54 1.04 18.55 7.48 0.13 5.48 8.80 3.66 0.68 0.24 <0.01 99.89 

TS-219 mns NS 54.32 1.04 18.85 7.26 0.12 4.77 8.65 3.64 0.74 0.23 0.57 98.83 

TS-220 mns NS 53.42 1.08 18.04 7.74 0.13 6.39 8.71 3.32 0.51 0.25 1.54 97.70 

TS-248 mns NS 54.06 1.06 18.22 7.42 0.13 5.39 8.50 3.60 0.94 0.28 0.49 98.93 

TS-249 mns NS 54.00 1.05 18.37 7.42 0.13 5.41 8.55 3.55 0.83 0.28 0.29 99.24 

TS-373 mns NS 53.89 1.16 19.33 7.45 0.13 3.89 8.89 3.92 0.67 0.27 <0.01 99.42 

TS-388 mns NS 53.77 1.10 18.22 7.68 0.13 5.25 8.69 3.82 0.70 0.24 <0.01 98.92 

TS-389 mns NS 54.00 1.16 18.58 7.46 0.13 4.64 8.71 3.98 0.73 0.21 na 99.92 

TS-390 mns NS 53.61 1.11 19.00 7.38 0.13 4.72 8.94 3.85 0.67 0.19 na 100.55 

TS-391 mns NS 53.92 1.16 19.34 7.48 0.12 3.86 8.89 3.90 0.66 0.26 0.49 99.37 

TS-594 mns NS 53.61 1.11 18.56 7.70 0.13 5.17 8.52 3.86 0.72 0.23 na 98.59 

TS-595 mns NS 54.13 1.08 18.26 7.28 0.13 5.36 8.66 3.71 0.76 0.23 na 99.94 

TS-596 mns NS 53.41 1.03 18.58 7.27 0.13 5.83 8.87 3.64 0.66 0.19 na 100.35 

TS-600 mns NS 53.52 1.04 18.60 7.30 0.13 5.60 8.85 3.69 0.68 0.19 na 100.04 

TS-602 mns NS 52.51 1.21 18.44 7.96 0.14 5.78 8.85 3.81 0.65 0.26 na 100.15 

TS-653 mns NS 54.63 0.99 18.51 7.06 0.12 5.39 8.21 3.73 0.75 0.20 na 100.37 

TS-654 mns NS 54.39 0.98 18.36 7.19 0.12 5.69 8.19 3.74 0.74 0.21 na 99.00 

TS-656 mns NS 54.47 1.01 18.67 7.04 0.12 5.11 8.33 3.84 0.78 0.21 na 98.41 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
91

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Basaltic andesite of Middle Sister—Continued

312 27 62 68 16 12 5 <10 26 14 545 228 20 67 122

289 24 <5 73 15 8 4 <10 31 12 547 <5 20 67 116

335 31 46 91 16 11 8 <10 35 13 510 264 26 89 154

264 17 71 82 14 6 4 <10 75 6 580 210 17 67 93

254 25 85 86 16 6 4 <10 77 9 571 229 17 78 90

447 44 104 51 14 11 11 26 74 12 679 179 22 73 163

286 25 82 92 21 12 6 17 41 13 555 211 24 82 126

Pyroxene-bearing variant of the basaltic andesite of Middle Sister

329 30 6 103 20 10 7 13 16 13 511 287 25 98 134

344 29 na 108 23 9 10 21 na 13 544 357 31 96 170

345 25 <5 86 17 9 6 <10 3 15 517 5 9 88 9

332 27 <5 104 19 9 8 18 na 14 540 361 32 107 171

356 29 <5 148 15 12 4 11 4 13 519 44 22 82 128

Basaltic andesite tuff of Middle Sister

243 21 98 95 19 8 6 17 70 8 550 192 20 75 99

404 25 97 92 18 10 8 19 58 11 515 210 24 85 126

390 25 77 71 17 10 8 12 49 14 506 155 24 86 142

Basaltic andesite of North Fork Whychus Creek

270 25 75 78 20 8 6 15 65 8 578 153 18 68 101

267 27 45 70 20 11 7 13 60 9 603 147 19 75 114

309 28 57 67 14 7 5 <10 45 9 582 192 15 64 117

329 29 73 64 14 7 6 12 58 11 568 179 17 66 124

280 18 35 52 15 7 4 <10 81 9 614 173 15 59 104

Basaltic andesite north of Hayden Glacier

408 32 10 65 22 13 8 19 10 17 549 209 23 92 144

325 27 60 64 14 11 5 19 41 9 566 185 17 62 112

288 26 33 88 21 30 9 17 94 11 616 228 27 75 106

Basaltic andesite of North Sister

287 29 78 70 20 13 8 10 62 10 570 175 18 79 120

295 26 79 66 20 12 7 <7 54 9 588 176 19 74 119

268 22 91 76 20 10 6 7 67 8 577 176 18 72 104

274 25 63 74 20 10 7 <10 64 10 595 159 19 71 117

289 30 152 55 19 11 8 16 148 6 537 134 20 74 123

342 31 90 80 19 11 8 17 71 13 614 178 20 76 134

333 29 89 73 19 11 8 16 80 12 620 171 21 76 135

262 24 7 68 14 8 4 <10 22 6 623 213 17 62 103

268 24 69 94 21 7 8 <10 na 9 618 237 24 82 138

280 23 38 89 20 11 6 15 48 9 592 210 20 78 106

256 19 43 87 19 11 4 13 53 8 609 207 19 74 98

269 24 5 81 17 8 4 <10 23 6 624 209 16 67 99

319 21 42 68 19 11 5 16 74 11 595 190 19 78 99

321 31 98 55 17 13 6 16 66 11 567 190 20 75 119

274 19 104 70 19 13 5 13 97 8 580 192 18 72 99

280 22 97 79 19 10 5 17 85 8 583 194 19 73 101

285 26 46 84 20 12 6 17 92 8 594 199 20 77 117

299 25 100 33 19 15 6 16 95 9 618 176 18 73 111

288 32 122 62 18 13 5 17 102 9 607 170 17 70 107

299 25 68 59 18 13 4 14 83 10 617 176 17 73 109
92

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Basaltic andesite of North Sister—Continued 

TS-657 mns NS 54.86 1.02 18.47 7.04 0.12 4.98 8.29 3.82 0.79 0.22 na 100.42 

TS-658 mns NS 54.17 1.03 18.49 7.14 0.13 5.42 8.54 3.63 0.81 0.24 na 99.68 

TS-752 mns NS 54.44 0.99 18.58 7.09 0.12 5.28 8.40 3.76 0.73 0.20 na 99.29 

TS-753 mns NS 54.51 1.01 18.68 7.09 0.12 5.05 8.40 3.78 0.74 0.21 na 98.89 

TS-755 mns NS 54.51 0.97 18.34 7.22 0.12 5.65 8.20 3.65 0.73 0.20 na 99.06 

TS-761 mns NS 54.46 1.00 18.56 7.16 0.12 5.24 8.36 3.75 0.74 0.21 na 98.91 

TS-768 mns NS 54.45 1.01 18.46 7.06 0.12 5.35 8.40 3.71 0.80 0.22 na 99.68 

TS-768R mns NS 54.35 1.00 18.40 7.29 0.12 5.32 8.38 3.71 0.80 0.22 na 100.59 

NS-07-201 mns NS 54.10 1.10 18.36 7.51 0.13 5.35 8.27 3.72 0.80 0.25 na 99.97 

NS-07-203 mns NS 54.67 1.00 18.55 7.02 0.12 5.12 8.37 3.78 0.76 0.20 na 100.38 

NS-07-204 mns NS 53.73 1.11 18.53 7.69 0.13 5.16 8.50 3.84 0.68 0.22 na 100.71 

TS-433 mns′ NS 54.75 1.31 17.48 7.52 0.13 5.45 8.05 3.48 1.07 0.36 0.26 99.15 

TS-435 mns′ NS 54.40 1.27 17.73 7.64 0.13 5.37 8.26 3.49 0.99 0.33 0.05 99.97 

TS-436 mns′ NS 54.61 1.32 17.30 7.56 0.13 5.45 8.32 3.50 1.06 0.35 <0.01 99.59 

TS-437 mns′ NS 54.53 1.32 17.47 7.55 0.13 5.55 8.19 3.43 1.06 0.36 0.14 99.19 

TS-443 mns′ NS 53.07 1.20 17.72 8.11 0.14 6.48 8.48 3.25 0.80 0.33 <0.01 99.46 

TS-467 mns NS 53.31 1.00 18.83 7.49 0.13 5.83 9.01 3.16 0.61 0.24 <0.01 99.96 

TS-468 mns NS 53.35 1.01 18.81 7.52 0.13 5.76 8.98 3.15 0.64 0.26 <0.01 100.08 

TS-505 mns′ NS 55.09 1.31 17.14 7.46 0.13 5.50 8.20 3.31 1.10 0.37 <0.01 100.53 

TS-561 mns′ NS 55.02 1.32 17.04 7.46 0.14 5.33 7.99 3.83 1.15 0.31 0.02 99.37 

Basaltic andesite north of Trout Creek 

TS-637 mnt NS 56.85 1.59 16.63 8.53 0.15 3.23 6.43 4.59 1.27 0.33 na 98.43 

TS-663 mnt NS 55.51 1.43 17.19 8.40 0.15 3.97 7.36 4.19 1.11 0.28 na 98.97 

TS-664 mnt NS 55.66 1.42 17.13 8.26 0.15 3.98 7.37 4.32 1.02 0.28 na 99.10 

TS-670 mnt NS 55.31 1.37 17.28 8.17 0.15 4.26 7.63 4.26 0.91 0.27 na 100.66 

NS-07-200 mnt NS 54.94 1.41 17.43 8.42 0.15 4.26 7.61 4.21 0.88 0.28 na 98.19 

Basaltic andesite of Park Meadow 

TS-516 mpm P 55.90 0.93 17.93 7.21 0.12 4.93 8.02 3.41 0.89 0.26 0.28 99.42 

EMT-363 mpm P 56.00 0.90 17.60 7.20 na 4.70 7.90 4.20 0.80 na na 99.30 

EMT-364 mpm P 55.50 0.95 17.30 7.30 na 5.70 8.40 3.80 0.80 na na 99.75 

Basaltic andesite north of Scott Pass 

TS-639 mpn NS 54.56 1.48 17.18 9.25 0.15 3.98 7.66 4.16 0.91 0.27 na 99.84 

TS-725 mpn NS 54.80 1.52 17.13 9.32 0.16 3.80 7.34 4.27 0.98 0.28 na 99.51 

TS-726 mpn NS 55.22 1.54 17.07 9.20 0.16 3.63 7.15 4.32 1.00 0.30 na 99.57 

TS-728 mpn NS 55.08 1.54 16.93 9.42 0.16 3.71 7.22 4.27 1.00 0.29 na 99.19 

Basaltic andesite south of Scott Pass 

TS-82 mps NS 55.97 1.25 17.29 8.35 0.14 3.76 7.33 4.24 0.96 0.33 <0.01 99.66 

TS-83 mps NS 57.59 1.42 17.03 8.08 0.14 2.94 6.05 4.71 1.23 0.41 0.10 99.44 

TS-84A mps NS 58.69 1.37 16.91 7.56 0.14 2.68 5.64 4.86 1.32 0.42 <0.01 100.12 

TS-84B mps NS 57.65 1.42 16.92 8.14 0.15 2.95 6.08 4.66 1.23 0.40 <0.01 99.51 

TS-85 mps NS 56.57 1.46 16.89 8.83 0.15 3.22 6.42 4.52 1.16 0.39 0.10 99.65 

TS-86 mps NS 56.14 1.49 16.98 9.01 0.15 3.33 6.59 4.43 1.10 0.38 <0.01 99.70 

TS-87 mps NS 56.44 1.47 16.95 8.87 0.15 3.25 6.50 4.49 1.11 0.38 <0.01 99.89 

TS-88 mps NS 56.37 1.48 16.99 8.89 0.15 3.26 6.47 4.50 1.11 0.38 <0.01 99.65 

TS-640 mps NS 56.25 1.50 16.85 8.88 0.15 3.34 6.69 4.54 1.09 0.31 na 99.83 

TS-648 mps NS 57.41 1.63 16.39 8.56 0.15 2.96 6.13 4.67 1.36 0.34 na 99.84 

TS-732 mps NS 56.06 1.49 16.99 8.87 0.15 3.34 6.73 4.58 1.09 0.30 na 99.71 

TS-733 mps NS 55.95 1.49 17.05 8.94 0.15 3.34 6.79 4.50 1.08 0.30 na 99.10 

TS-738 mps NS 56.29 1.54 16.88 8.59 0.15 3.43 6.74 4.51 1.16 0.32 na 99.84 

TS-740 mps NS 57.13 1.63 16.58 8.62 0.15 2.99 6.10 4.71 1.34 0.34 na 99.78 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
93

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Basaltic andesite of North Sister—Continued

311 26 67 46 18 13 5 16 80 11 627 179 17 75 115

344 30 94 63 18 14 6 19 75 11 619 175 20 76 126

289 27 81 63 18 13 6 15 99 11 627 172 17 74 106

304 30 71 63 20 14 6 16 82 11 631 173 18 73 111

283 29 123 43 18 12 6 16 106 9 613 169 18 74 107

290 24 78 63 19 11 6 13 96 9 625 171 19 73 107

310 33 80 70 20 11 7 17 102 11 632 176 19 74 112

311 31 79 69 20 13 7 17 101 12 634 176 19 76 114

342 25 93 50 17 15 7 15 84 14 578 185 19 80 121

305 28 72 41 17 14 6 16 85 9 631 177 17 74 112

313 20 41 66 20 10 5 13 76 11 608 191 19 79 101

423 38 104 59 15 9 10 17 40 15 688 180 19 63 151

387 32 83 47 16 14 9 <10 43 14 679 179 17 63 127

414 33 86 26 15 10 10 22 37 16 684 187 17 59 135

419 36 97 42 14 12 10 <10 45 15 685 179 18 65 146

341 30 116 58 14 9 6 <10 107 10 604 169 17 65 128

271 22 74 65 15 7 3 <10 89 5 607 163 14 57 90

269 25 79 64 16 8 4 <10 90 7 598 195 14 62 92

408 40 92 44 14 16 11 <10 44 15 684 172 18 63 146

422 38 89 47 15 15 11 10 51 18 663 179 17 64 142

Basaltic andesite north of Trout Creek

421 34 4 54 21 18 8 22 8 21 447 232 28 93 181

345 35 17 68 20 14 8 20 14 15 486 219 25 89 150

340 28 17 70 19 15 7 17 14 15 487 220 25 87 149

326 33 28 68 19 12 7 20 18 13 504 220 23 85 142

345 26 21 80 20 14 8 15 16 12 505 220 25 84 144

Basaltic andesite of Park Meadow

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

Basaltic andesite north of Scott Pass

350 31 23 64 20 13 7 21 8 14 554 287 22 90 133

370 30 16 59 19 16 8 21 8 14 536 284 24 93 140

389 36 11 46 20 14 8 21 4 15 529 258 24 97 149

375 35 11 50 20 12 7 19 5 14 528 258 23 98 145

Basaltic andesite south of Scott Pass

367 25 21 88 19 11 7 15 18 15 569 223 21 88 134

435 34 <3 33 18 15 8 17 11 19 527 191 26 98 178

456 33 <3 32 21 15 10 23 10 20 507 149 28 92 193

430 37 <3 36 25 17 9 22 12 18 527 184 26 97 179

406 33 <3 53 21 13 9 23 12 17 538 216 26 97 166

401 34 <3 68 22 16 9 17 12 17 544 232 25 95 159

399 30 <3 50 21 14 9 13 12 17 541 233 26 99 162

403 33 <3 52 24 14 9 20 12 17 538 241 26 93 159

415 34 2 56 21 17 7 19 2 16 539 261 24 91 150

457 40 1 54 21 18 9 24 5 23 440 230 30 94 195

402 29 2 54 20 14 7 19 3 15 533 257 24 94 150

401 39 3 61 20 15 7 22 4 16 533 259 24 93 148

399 35 7 52 20 16 9 22 12 18 465 228 28 93 174

450 38 1 49 21 18 10 25 8 21 431 227 29 101 196
94

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Basaltic andesite south of Scott Pass—Continued 

TS-742 mps NS 56.15 1.49 16.98 8.80 0.15 3.30 6.67 4.65 1.11 0.31 na 100.77 

TS-762 mps NS 57.34 1.64 16.55 8.50 0.15 2.97 6.11 4.67 1.32 0.35 na 99.35 

Basaltic andesite of Renfrew Glacier 

TS-105 mrg MS 54.55 1.73 16.69 9.82 0.16 3.75 7.40 4.27 0.86 0.38 <0.01 99.69 

TS-379 mrg MS 54.56 1.77 16.58 9.97 0.16 3.70 7.53 4.17 0.81 0.36 0.10 99.13 

TS-381 mrg MS 54.64 1.73 16.47 9.87 0.16 3.75 7.45 4.32 0.87 0.34 <0.01 99.17 

TS-402 mrg MS 54.71 1.72 16.36 9.86 0.16 3.83 7.49 4.24 0.87 0.36 <0.01 99.23 

TS-417 mrg MS 54.83 1.73 16.61 9.92 0.17 3.61 7.37 4.11 0.88 0.35 <0.01 99.54 

Basaltic andesite of Sims Butte 

TS-634 msb P 52.80 1.30 17.29 8.29 0.14 6.87 8.33 3.54 0.78 0.26 na 99.94 

TS-667 msb P 54.66 1.20 17.23 7.54 0.14 6.13 7.71 3.73 1.03 0.23 na 99.06 

TS-747 msb P 56.59 1.09 16.83 7.14 0.13 5.52 7.11 3.69 1.29 0.21 na 98.72 

TS-748 msb P 54.28 1.21 17.08 7.88 0.14 6.43 7.76 3.59 0.97 0.25 na 98.70 

TS-749 msb P 54.24 1.24 17.06 8.06 0.14 6.40 7.69 3.57 0.96 0.24 na 98.84 

TS-750 msb P 52.74 1.30 17.26 8.37 0.14 6.87 8.28 3.60 0.79 0.26 na 99.10 

TS-751 msb P 52.35 1.31 17.52 8.41 0.15 6.98 8.44 3.48 0.70 0.26 na 98.35 

CR-111 msb P 54.02 1.22 16.81 8.19 0.13 6.51 7.93 3.58 0.96 0.25 0.00 100.12 

Basaltic andesite of Separation Creek 

TS-113 msc P 57.10 1.22 17.36 7.47 0.13 3.80 6.73 4.34 1.15 0.29 -0.16 99.25 

TS-114 msc P 56.76 1.22 17.48 7.52 0.13 3.91 6.86 4.31 1.12 0.30 -0.17 99.15 

TS-127 msc P 57.05 1.26 17.23 7.58 0.13 3.78 6.82 4.30 1.16 0.30 0.33 98.29 

TS-255 msc P 57.27 1.23 17.58 7.50 0.13 3.66 6.68 4.17 1.10 0.28 0.41 99.12 

TS-290 msc P 56.92 1.22 17.43 7.55 0.13 3.81 6.78 4.35 1.14 0.28 0.47 98.87 

TS-309 msc P 57.16 1.21 17.39 7.44 0.13 3.73 6.77 4.35 1.15 0.28 -0.11 99.66 

TS-310 msc P 56.81 1.19 17.57 7.49 0.13 3.92 6.87 4.25 1.09 0.28 -0.07 99.76 

TS-311 msc P 56.74 1.19 17.61 7.48 0.13 3.96 6.89 4.23 1.09 0.28 0.56 98.99 

TS-320 msc P 56.91 1.22 17.53 7.60 0.13 3.74 6.78 4.27 1.12 0.29 0.20 99.40 

TS-537 msc P 57.09 1.23 17.38 7.48 0.13 3.76 6.82 4.26 1.17 0.27 0.42 98.58 

Basaltic andesite of South Fork Whychus Creek 

TS-190 msf P 53.69 0.99 18.74 7.69 0.13 5.39 8.57 3.67 0.58 0.15 0.45 98.88 

TS-191 msf P 53.97 1.02 18.63 7.66 0.13 5.10 8.56 3.78 0.60 0.15 -0.03 98.91 

TS-459 msf P 53.94 1.01 18.65 7.70 0.13 5.28 8.57 3.52 0.60 0.20 0.08 99.35 

Basaltic andesite of Soap Creek–North Fork Whychus Creek confluence 

TS-466 msp P 54.18 1.32 17.59 7.58 0.13 5.60 8.69 3.15 1.03 0.33 <0.01 99.09 

TS-502 msp P 54.77 1.26 17.60 7.56 0.13 5.30 8.29 3.37 0.99 0.33 <0.01 100.19 

TS-504 msp P 54.56 1.30 17.46 7.58 0.13 5.44 8.46 3.34 1.00 0.34 <0.01 99.85 

Basaltic andesite of Middle Sister summit 

TS-234 mss MS 56.11 1.32 17.60 7.76 0.13 3.75 7.60 3.73 1.33 0.27 0.44 99.04 

Basaltic andesite south of Trout Creek 

NS-07-202 mst NS 53.34 1.16 19.86 7.49 0.13 3.99 8.59 4.09 0.72 0.23 na 99.19 

Basaltic andesite and basalt of Trout Creek Butte 

TS-660 mtb P 51.94 1.14 17.65 8.21 0.15 6.01 10.57 3.15 0.57 0.21 na 100.09 

TS-660R mtb P 51.89 1.14 17.65 8.30 0.15 5.99 10.55 3.15 0.56 0.21 na 100.38 

TS-661 mtb P 51.82 1.23 17.75 8.45 0.16 5.84 10.47 3.18 0.47 0.23 na 99.33 

TS-662 mtb P 52.14 1.29 17.51 8.45 0.16 5.65 10.32 3.27 0.57 0.25 na 100.46 

CR-115 mtb P 52.72 1.31 17.61 8.29 0.14 5.91 8.77 3.67 0.85 0.32 < 0.01 99.56 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
95

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Basaltic andesite south of Scott Pass—Continued

413 35 3 59 20 15 7 21 4 16 540 256 25 94 152

450 46 3 52 21 19 11 25 11 22 443 228 32 96 196

Basaltic andesite of Renfrew Glacier

324 28 7 87 23 13 8 19 19 13 515 257 26 90 133

363 26 <5 98 18 7 5 <10 3 11 526 303 23 86 136

324 28 na 91 20 13 11 12 na 15 545 371 35 97 173

345 29 <5 88 17 10 5 19 4 13 518 309 23 89 129

356 26 <5 97 17 11 6 <10 3 12 513 296 23 84 132

Basaltic andesite of Sims Butte

272 29 215 59 18 16 6 16 121 14 459 188 23 76 128

315 26 156 45 16 14 6 15 106 19 423 168 22 75 131

351 29 131 44 17 12 6 16 95 27 403 152 23 67 128

305 28 190 52 18 10 8 16 120 18 435 170 24 75 131

307 23 188 55 17 13 8 13 121 18 430 170 24 74 132

268 26 209 64 18 10 7 15 128 13 456 191 23 78 124

261 29 212 59 16 8 8 17 130 11 462 189 24 78 125

na na na na na na na na na na na na na na na

Basaltic andesite of Separation Creek

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

397 29 42 63 22 12 7 24 31 16 491 164 23 79 162

375 26 43 58 20 11 7 17 27 19 498 165 23 76 155

376 26 39 62 19 10 7 17 23 19 501 163 21 79 153

369 27 47 62 20 15 8 18 24 18 503 160 22 79 146

359 28 43 42 21 11 7 16 31 18 508 163 21 75 145

372 27 42 64 22 11 7 20 27 19 504 171 22 86 153

422 26 27 52 14 13 6 <10 23 18 498 179 20 71 149

Basaltic andesite of South Fork Whychus Creek

217 18 76 89 18 6 4 <10 40 8 589 176 16 75 73

209 16 68 79 17 <5 5 16 30 7 582 178 18 69 76

225 11 53 76 15 6 <2 <10 27 5 592 170 13 57 69

Basaltic andesite of Soap Creek–North Fork Whychus Creek confluence

362 36 92 5 14 14 10 <10 44 17 702 196 17 65 128

382 33 82 40 14 13 7 <10 41 14 666 170 15 65 123

372 35 82 43 15 12 9 <10 38 15 687 190 16 61 128

Basaltic andesite of Middle Sister summit

356 29 67 64 21 12 8 18 41 24 496 197 25 77 151

Basaltic andesite south of Trout Creek

271 20 15 44 20 11 4 15 41 8 641 190 19 76 105

Basaltic andesite and basalt of Trout Creek Butte

231 28 152 44 19 11 5 19 28 6 594 215 22 79 108

234 24 151 43 18 9 4 16 29 6 598 219 23 78 108

234 26 146 46 19 13 5 18 28 4 570 212 24 80 118

247 31 137 40 18 13 6 20 28 5 595 227 24 84 129

na na na na na na na na na na na na na na na
96

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Basaltic andesite of The Husband 

TS-112 mth P 52.77 1.18 17.82 8.21 0.14 6.20 8.66 3.69 0.67 0.25 -0.25 99.47 

TS-125 mth P 55.87 0.95 17.72 7.04 0.12 5.09 7.75 3.59 1.14 0.33 0.51 98.94 

TS-291 mth P 55.77 0.90 18.19 6.79 0.12 4.87 8.06 3.71 0.94 0.26 -0.08 99.65 

TS-536 mth P 56.15 0.91 17.95 6.72 0.11 5.17 7.86 3.50 0.99 0.23 0.35 99.33 

TS-550 mth P 55.41 1.05 17.53 7.43 0.14 5.14 7.60 3.71 1.22 0.37 0.39 98.86 

TS-558 mth P 56.99 0.92 15.88 6.43 0.13 5.52 8.33 3.31 1.71 0.38 0.12 99.10 

TS-633 mth P 52.03 1.70 16.93 9.55 0.17 5.52 8.25 3.95 0.99 0.51 na 100.41 

Basaltic andesite of Teardrop Pool 

TS-47 mtp SS 53.82 1.33 18.08 8.65 0.14 4.56 8.13 3.97 0.67 0.25 0.70 98.64 

TS-48 mtp SS 56.13 1.28 17.31 8.02 0.14 3.90 7.24 4.26 1.04 0.26 <0.01 99.54 

TS-50 mtp SS 55.83 1.20 17.84 7.61 0.13 4.20 7.29 4.20 0.99 0.31 <0.01 99.37 

TS-587 mtp SS 54.23 1.32 17.94 8.45 0.14 4.35 8.14 3.98 0.78 0.26 0.40 98.81 

Basaltic andesite southeast of "Todd Creek" 

TS-784 mts P 56.10 1.27 17.32 8.37 0.15 3.60 7.04 4.47 0.99 0.29 na 99.26 

Basaltic andesite of The Wife 

TS-66 mtw P 53.03 1.09 18.28 7.81 0.13 5.87 8.76 3.53 0.77 0.33 0.06 99.72 

TS-109 mtw P 52.96 1.19 17.89 8.15 0.14 5.86 8.63 3.56 0.86 0.36 0.02 99.11 

TS-128 mtw P 52.42 1.21 18.04 8.22 0.14 6.12 8.85 3.49 0.77 0.34 0.24 98.81 

TS-129 mtw P 52.08 1.24 17.86 8.37 0.14 6.32 9.00 3.48 0.75 0.34 0.04 99.25 

TS-147 mtw P 53.59 1.07 18.20 7.60 0.13 5.46 8.83 3.53 0.92 0.28 0.34 99.06 

TS-148 mtw P 52.82 1.10 18.21 7.90 0.13 6.06 8.84 3.44 0.79 0.30 0.09 99.56 

TS-149 mtw P 53.46 1.09 18.35 7.89 0.13 5.55 8.43 3.63 0.75 0.31 0.19 99.31 

Basaltic andesite west of upper Alder Creek 

TS-756 mwa NS 53.25 1.14 19.99 7.46 0.13 4.13 8.60 3.99 0.69 0.22 na 99.03 

TS-759 mwa NS 53.60 1.38 18.34 7.70 0.14 4.59 8.85 3.83 0.86 0.30 na 98.00 

TS-760 mwa NS 54.96 1.54 17.70 8.71 0.15 3.51 7.31 4.61 0.84 0.27 na 99.34 

TS-763 mwa NS 55.42 1.29 18.43 7.88 0.14 3.67 6.92 4.63 0.91 0.32 na 99.90 

TS-765 mwa NS 54.21 1.18 19.21 7.32 0.13 3.95 8.60 4.05 0.73 0.22 na 100.06 

TS-766 mwa NS 56.12 1.24 18.37 7.60 0.14 3.36 6.72 4.76 0.96 0.33 na 100.26 

Basaltic andesite of Whychus Creek 

TS-252 mwh P 53.16 1.52 18.72 8.79 0.14 3.57 8.61 4.02 0.73 0.32 -0.12 99.48 

TS-438 mwh P 54.89 1.64 17.13 9.66 0.16 3.40 7.26 4.20 0.92 0.34 <0.01 99.43 

TS-449 mwh P 55.10 1.62 17.10 9.45 0.16 3.39 7.29 4.21 0.94 0.34 <0.01 99.61 

TS-455 mwh P 53.38 1.63 18.33 9.24 0.15 3.55 8.26 3.92 0.78 0.37 0.09 99.45 

TS-457 mwh P 53.73 1.65 17.91 9.37 0.16 3.54 8.05 3.98 0.83 0.38 <0.01 99.55 

TS-613 mwh P 55.08 1.63 17.06 9.31 0.17 3.28 7.31 4.56 0.92 0.28 na 100.05 

TS-616 mwh P 55.17 1.62 16.95 9.27 0.16 3.30 7.32 4.58 0.94 0.28 na 99.86 

Basaltic andesite west of Snow Creek 

TS-254 mws BT 54.61 1.93 16.06 10.14 0.17 3.63 7.36 4.35 0.95 0.39 -0.15 99.21 

TS-525 mws BT 53.74 1.92 16.11 10.39 0.18 3.90 7.90 4.25 0.88 0.33 <0.01 99.53 

TS-567 mws BT 53.56 1.99 16.25 10.41 0.17 3.97 7.84 4.25 0.81 0.34 <0.01 99.30 

TS-669 mws BT 54.97 1.89 16.17 9.60 0.17 3.54 7.34 4.55 1.02 0.36 na 99.57 

Basaltic andesite scoria cone west of Yapoah Lake 

TS-643 mwy NS 56.44 1.58 16.95 8.48 0.15 3.31 6.47 4.71 1.17 0.34 na 98.52 

TS-643R mwy NS 56.48 1.58 16.98 8.44 0.15 3.29 6.49 4.69 1.16 0.34 na 98.75 

TS-645 mwy NS 56.56 1.59 16.90 8.49 0.15 3.29 6.46 4.65 1.16 0.33 na 99.75 

Basaltic andesite of Yapoah Lake 

TS-597 myl NS 54.16 1.10 18.29 7.51 0.13 5.34 8.25 3.76 0.80 0.25 na 100.67 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
97

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Basaltic andesite of The Husband

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

382 34 83 58 20 12 6 20 62 15 686 129 17 63 137

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

774 62 117 39 13 24 8 35 50 41 849 152 20 66 153

481 51 98 64 20 25 12 28 66 14 799 230 28 105 177

Basaltic andesite of Teardrop Pool

290 21 65 55 21 8 6 <7 24 9 518 212 21 84 109

347 24 50 57 20 10 6 17 17 18 476 192 22 82 127

337 27 53 59 19 12 8 18 27 15 522 170 21 79 139

310 25 43 54 16 8 5 <10 12 12 525 213 19 73 106

Basaltic andesite southeast of "Todd Creek"

400 37 30 67 21 10 7 22 17 16 599 230 24 94 133

Basaltic andesite of The Wife

352 27 108 65 21 16 8 14 75 11 645 168 21 71 123

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

Basaltic andesite west of upper Alder Creek

261 23 13 59 20 9 7 17 54 8 649 187 20 77 102

338 30 86 67 17 14 11 21 54 10 570 191 25 78 146

319 33 1 62 23 10 7 20 8 9 572 265 26 93 125

347 34 5 62 21 12 8 21 30 10 585 175 26 92 143

271 27 19 79 20 12 6 16 37 9 620 197 22 79 106

359 31 3 29 20 13 8 19 26 10 584 162 26 88 148

Basaltic andesite of Whychus Creek

299 26 13 64 22 9 9 17 22 10 567 213 25 87 117

373 32 <5 35 18 11 6 <10 8 14 499 238 25 78 127

369 29 <5 44 18 7 6 <10 3 16 501 235 25 83 132

350 30 <5 46 18 8 7 <10 4 9 550 213 23 75 122

352 32 <5 53 18 10 6 <10 3 12 537 216 22 71 122

362 23 4 51 20 14 7 19 3 15 495 268 28 100 131

362 30 2 46 20 18 7 20 3 15 491 272 26 98 128

Basaltic andesite west of Snow Creek

321 31 13 46 22 13 11 20 10 15 435 297 36 94 176

318 32 14 29 18 11 8 <10 27 13 462 300 28 72 147

308 30 6 44 16 12 10 <10 4 9 447 359 31 77 170

340 32 14 48 21 15 11 23 5 16 423 281 33 102 174

Basaltic andesite scoria cone west of Yapoah Lake

418 36 6 58 21 16 8 24 10 17 473 210 28 94 172

416 34 6 58 20 15 8 21 11 17 475 215 28 93 173

426 34 5 63 20 18 7 22 11 18 480 217 28 90 173

Basaltic andesite of Yapoah Lake

346 27 94 69 19 12 6 16 83 13 579 186 19 78 122
98

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Basaltic andesite of Yapoah Lake—Continued 

TS-598 myl NS 53.48 1.11 18.58 7.75 0.13 5.19 8.54 3.87 0.71 0.23 na 98.54 

TS-636 myl NS 53.42 1.10 18.70 7.72 0.13 5.27 8.51 3.86 0.67 0.22 na 98.82 

TS-650 myl NS 53.62 1.09 18.68 7.66 0.13 5.20 8.44 3.86 0.69 0.22 na 98.85 

TS-651 myl NS 53.56 1.10 18.63 7.71 0.13 5.20 8.49 3.86 0.69 0.22 na 99.04 

TS-665 myl NS 53.49 1.10 18.72 7.55 0.14 5.35 8.53 3.73 0.75 0.24 na 99.93 

TS-666 myl NS 53.65 1.11 18.68 7.58 0.13 5.12 8.54 3.86 0.71 0.23 na 100.28 

TS-757 myl NS 53.73 1.09 18.54 7.62 0.13 5.24 8.51 3.84 0.67 0.22 na 99.51 

TS-767 myl NS 53.61 1.10 18.64 7.77 0.13 5.18 8.45 3.82 0.67 0.22 na 99.85 

NS-07-205 myl NS 53.78 1.10 18.56 7.70 0.13 5.42 8.22 3.68 0.76 0.24 na 99.78 

Basaltic andesite southwest of Yapoah Lake 

TS-646 mys NS 53.08 1.49 18.07 9.35 0.16 4.50 7.60 4.33 0.76 0.26 na 100.70 

TS-647 mys NS 52.50 1.43 17.88 9.57 0.16 5.37 7.57 4.17 0.70 0.25 na 100.48 

TS-647R mys NS 52.49 1.42 17.86 9.56 0.16 5.37 7.60 4.18 0.70 0.25 na 100.36 

TS-739 mys NS 53.21 1.48 18.19 9.09 0.16 4.40 7.68 4.39 0.76 0.26 na 100.00 

TS-741 mys NS 53.11 1.48 18.23 9.27 0.16 4.34 7.64 4.36 0.77 0.26 na 100.11 

TS-776 mys NS 53.13 1.47 18.06 9.19 0.15 4.60 7.66 4.33 0.74 0.26 na 99.71 

Basaltic andesite window west of Yapoah Lake 

TS-644 myw NS 53.04 1.03 19.21 7.25 0.13 5.37 9.07 3.69 0.63 0.19 na 100.11 

Rhyolite of South Sister climbers trail 

TS-51 rct SS 72.87 0.31 14.33 2.03 0.06 0.41 1.59 4.74 3.14 0.12 0.21 99.37 

TS-52 rct SS 72.94 0.30 14.29 2.00 0.05 0.40 1.60 4.75 3.15 0.13 0.36 99.00 

TS-157 rct SS 72.79 0.31 14.38 1.99 0.06 0.42 1.63 4.79 3.14 0.10 0.14 99.06 

TS-157A rct SS 72.70 0.30 14.40 1.97 0.05 0.41 1.62 4.43 3.61 0.10 0.46 98.92 

TS-164 rct SS 73.05 0.30 14.27 1.98 0.06 0.40 1.63 4.67 3.15 0.09 0.44 98.45 

TS-489 rct SS 73.41 0.31 14.22 2.01 0.06 0.40 1.66 4.24 3.15 0.14 0.60 100.13 

Rhyolite of "Devils chain" 

TS-13 rdc SS 72.46 0.33 14.31 2.13 0.05 0.58 1.91 4.52 3.17 0.13 0.24 99.52 

TS-95 rdc SS 72.43 0.34 14.41 2.12 0.05 0.57 1.93 4.45 3.14 0.16 0.46 99.56 

TS-199 rdc SS 72.56 0.34 14.35 2.14 0.05 0.56 1.94 4.40 3.16 0.10 0.50 98.55 

TS-206 rdc SS 72.57 0.34 14.31 2.16 0.06 0.56 1.93 4.40 3.16 0.10 0.61 98.82 

TS-207 rdc SS 72.63 0.33 14.32 2.14 0.05 0.55 1.94 4.37 3.16 0.10 0.82 98.73 

TS-240 rdc SS 72.61 0.34 14.36 2.13 0.05 0.53 1.95 4.36 3.15 0.11 0.35 99.17 

TS-241 rdc SS 72.55 0.33 14.31 2.14 0.05 0.59 1.93 4.40 3.17 0.11 0.53 98.84 

TS-245 rdc SS 72.57 0.34 14.35 2.16 0.05 0.57 1.95 4.36 3.14 0.10 0.74 98.54 

TS-358 rdc SS 72.63 0.33 14.27 2.08 0.05 0.54 1.95 4.53 3.14 0.08 na 99.44 

TS-365 rdc SS 72.65 0.33 14.19 2.12 0.05 0.53 1.92 4.50 3.19 0.12 0.32 98.98 

TS-591 rdc SS 72.54 0.34 14.31 2.13 0.05 0.52 1.94 4.44 3.14 0.18 0.35 99.54 

TS-590 rdc SS 72.49 0.34 14.34 2.14 0.06 0.52 1.96 4.46 3.12 0.18 0.24 100.03 

Rhyolite of Devils Hill 

TS-93 rdh SS 73.35 0.28 14.21 1.86 0.05 0.38 1.49 4.66 3.17 0.14 0.46 99.53 

TS-242 rdh SS 73.66 0.27 14.05 1.85 0.05 0.37 1.48 4.62 3.18 0.08 0.51 99.25 

TS-243 rdh SS 73.69 0.28 14.04 1.84 0.05 0.36 1.46 4.63 3.17 0.08 0.13 99.35 

TS-244 rdh SS 73.67 0.27 14.03 1.85 0.05 0.37 1.46 4.63 3.18 0.08 0.25 98.70 

TS-338 rdh SS 74.05 0.26 13.85 1.73 0.05 0.30 1.40 4.58 3.24 0.14 0.40 99.26 

TS-339 rdh SS 74.01 0.26 13.91 1.74 0.05 0.31 1.36 4.61 3.26 0.09 0.26 98.78 

Rhyolite of Green Lakes 

TS-507 rgl SS 72.92 0.33 14.46 2.09 0.06 0.44 1.73 4.31 3.11 0.15 0.79 99.85 

TS-569 rgl SS 72.18 0.33 14.54 2.12 0.06 0.47 1.73 4.72 3.38 0.09 0.41 97.98 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
99

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Basaltic andesite of Yapoah Lake—Continued

303 18 39 72 20 11 5 13 74 10 595 192 19 78 99

306 22 41 59 18 11 5 15 76 10 593 190 18 78 96

311 26 39 60 20 11 5 15 74 9 592 183 18 77 100

309 16 40 71 19 13 5 13 74 10 596 186 18 79 98

324 28 86 60 19 15 5 15 86 11 588 187 19 82 112

308 24 41 78 18 10 5 14 77 11 596 187 19 80 102

301 23 39 71 20 10 6 13 85 10 604 188 20 77 97

305 25 41 48 19 8 6 15 81 10 603 191 20 79 97

338 27 95 70 19 12 6 17 84 12 573 176 20 80 121

Basaltic andesite southwest of Yapoah Lake

335 26 3 56 20 14 5 16 42 8 608 232 21 96 102

312 25 4 39 20 12 5 18 72 7 601 221 20 95 95

311 25 5 40 19 14 4 17 72 6 602 220 21 94 94

324 23 3 56 20 11 5 15 42 7 594 227 22 94 102

335 29 4 63 21 13 5 20 40 8 591 220 22 96 103

321 27 5 67 20 10 6 17 50 7 607 227 24 95 99

Basaltic andesite window west of Yapoah Lake

267 21 51 79 19 10 4 16 94 6 614 190 18 72 96

Rhyolite of South Sister climbers trail

901 45 5 6 18 28 10 24 3 66 188 16 15 35 236

913 45 6 3 17 30 10 20 3 68 184 17 16 40 237

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

915 50 7 5 17 29 10 30 5 67 188 19 16 45 238

977 40 <5 <2 11 20 9 11 2 65 189 6 14 31 233

Rhyolite of "Devils chain"

855 46 5 6 17 33 11 27 4 73 221 21 16 39 194

847 42 5 7 17 26 10 27 9 75 225 23 16 37 198

853 44 7 9 17 28 10 23 3 74 226 26 16 40 196

851 44 6 9 17 28 10 31 <2 73 222 22 16 39 195

852 45 6 6 17 29 9 29 3 73 221 23 16 39 195

864 42 8 8 17 27 11 21 8 74 225 21 17 40 196

861 47 8 9 17 28 10 25 9 74 226 24 17 39 196

850 45 7 7 17 28 10 25 7 74 225 25 16 38 197

794 40 3 5 16 23 8 16 0 72 233 20 18 40 181

852 43 <5 9 19 24 13 26 na 79 234 31 22 41 247

938 36 <5 <2 10 16 9 11 2 71 223 6 15 31 196

926 37 <5 <2 10 18 8 28 2 68 226 6 14 30 194

Rhyolite of Devils Hill

900 51 6 5 16 33 10 18 7 69 174 16 15 42 229

899 45 7 5 16 28 9 25 6 67 171 16 16 37 225

912 48 7 6 17 30 10 24 6 68 173 13 15 39 225

905 43 7 6 16 27 11 17 5 71 172 17 15 40 228

976 39 <5 <2 10 20 8 14 2 67 156 6 13 29 212

913 39 7 5 19 25 13 18 na 73 164 22 18 38 264

Rhyolite of Green Lakes

960 39 <5 <2 12 19 9 19 2 62 191 6 17 34 252

939 39 <5 <2 11 19 13 14 2 60 193 7 20 27 301
100

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Rhyolite of Kaleetan Butte 

TS-73A rkb SS 73.53 0.28 14.04 1.89 0.05 0.36 1.49 4.59 3.24 0.12 0.39 99.29 

TS-94 rkb SS 73.60 0.28 14.06 1.89 0.05 0.37 1.50 4.63 3.10 0.13 0.30 99.87 

TS-138 rkb SS 73.52 0.27 14.12 1.83 0.05 0.38 1.46 4.68 3.20 0.09 1.27 98.08 

Rhyolite of Mesa Creek 

TS-69 rmc SS 72.32 0.33 14.56 2.13 0.06 0.44 1.71 4.84 3.06 0.14 0.40 99.16 

TS-70 rmc SS 72.38 0.33 14.64 2.09 0.06 0.43 1.68 4.77 3.08 0.14 0.25 99.36 

TS-141 rmc SS 72.59 0.32 14.46 2.05 0.06 0.43 1.67 4.80 3.10 0.10 0.91 98.51 

Rhyolite of Obsidian Cliffs 

TS-102A roc MS 76.54 0.11 13.04 1.00 0.04 0.15 0.86 4.34 3.42 0.11 0.38 100.07 

TS-107 roc MS 76.37 0.13 13.21 1.06 0.04 0.16 0.87 4.26 3.40 0.11 0.25 100.30 

TS-430 roc MS 76.69 0.11 13.00 1.02 0.04 0.11 0.91 4.10 3.52 0.09 0.46 96.50 

Rhyolite of Park Creek 

TS-777 rpc SS 72.54 0.32 14.44 2.07 0.06 0.42 1.69 4.86 3.13 0.07 na 99.13 

EMT-209 rpc SS 72.50 0.35 14.60 2.20 <0.1 0.30 1.50 5.10 2.95 ~0.1 na 99.50 

Rhyodacite of Prouty Glacier 

TS-201 rpg SS 68.44 0.67 15.22 3.72 0.07 1.08 3.04 4.62 2.57 0.18 0.18 98.82 

TS-572 rpg SS 68.11 0.65 15.17 3.85 0.08 1.35 3.16 4.66 2.43 0.14 0.25 99.15 

Rhyolite of Rock Mesa 

TS-65 rrm SS 73.47 0.30 13.98 1.88 0.05 0.48 1.68 4.26 3.35 0.15 0.44 99.77 

TS-159 rrm SS 73.43 0.30 14.02 1.87 0.05 0.48 1.70 4.27 3.37 0.10 0.55 98.74 

TS-160 rrm SS 73.53 0.29 13.94 1.85 0.05 0.47 1.70 4.27 3.39 0.10 0.65 98.62 

Rhyolite of Separation Creek 

TS-209 rsc SS 73.85 0.26 13.87 1.76 0.05 0.48 1.51 4.40 3.36 0.07 0.41 99.13 

TS-210 rsc SS 69.63 0.49 15.15 3.05 0.07 1.07 2.60 4.71 2.69 0.14 0.14 99.27 

TS-321 rsc SS 73.33 0.29 14.04 1.89 0.05 0.53 1.60 4.53 3.26 0.07 0.30 99.28 

TS-231 rsc′ SS 72.30 0.34 14.48 2.19 0.06 0.65 1.86 4.51 3.12 0.09 0.20 99.74 

Rhyolite southeast of Lewis Glacier 

TS-364 rse SS 73.81 0.28 13.90 1.90 0.05 0.33 1.54 4.53 3.11 0.14 0.14 99.58 

TS-366 rse SS 73.68 0.28 14.01 1.91 0.05 0.36 1.49 4.55 3.17 0.10 0.29 98.81 

TS-486 rse SS 73.98 0.30 14.04 1.95 0.05 0.35 1.56 4.10 3.13 0.15 0.16 100.04 

TS-487 rse SS 73.92 0.28 14.07 1.92 0.05 0.34 1.56 4.19 3.12 0.15 0.04 99.84 

TS-490 rse SS 73.69 0.29 14.30 1.95 0.06 0.36 1.58 4.14 3.08 0.16 0.66 99.62 

TS-521 rse SS 74.02 0.28 13.94 1.91 0.05 0.37 1.55 4.22 3.13 0.12 0.34 99.30 

TS-522A rse SS 73.59 0.28 13.99 1.88 0.05 0.38 1.57 4.69 3.10 0.07 na 98.45 

TS-522B rse SS 73.64 0.28 13.98 1.89 0.05 0.37 1.54 4.68 3.10 0.06 na 99.81 

TS-522C rse SS 73.98 0.29 13.91 1.93 0.05 0.36 1.57 4.24 3.12 0.13 0.95 98.82 

TS-523 rse SS 74.01 0.29 13.90 1.93 0.05 0.35 1.54 4.26 3.14 0.13 0.14 99.59 

Rhyolite of South Fork Whychus Creek 

TS-9 rsf SS 73.97 0.26 13.83 1.77 0.05 0.34 1.42 4.65 3.18 0.12 0.15 99.37 

TS-15 rsf SS 73.94 0.26 13.89 1.77 0.05 0.35 1.42 4.65 3.16 0.12 0.16 99.68 

TS-16 rsf SS 73.85 0.26 13.92 1.85 0.06 0.36 1.43 4.58 3.14 0.15 0.24 100.20 

TS-17 rsf SS 73.84 0.26 14.05 1.79 0.05 0.35 1.40 4.56 3.16 0.14 0.25 99.95 

TS-18 rsf SS 74.00 0.25 13.92 1.79 0.05 0.32 1.41 4.60 3.15 0.11 0.16 99.47 

TS-166 rsf SS 74.03 0.26 13.94 1.79 0.05 0.33 1.43 4.50 3.17 0.08 0.38 99.29 

TS-167 rsf SS 74.00 0.26 14.03 1.79 0.05 0.33 1.41 4.46 3.18 0.08 0.26 98.66 

TS-168 rsf SS 74.26 0.25 13.79 1.76 0.05 0.31 1.42 4.50 3.19 0.07 0.19 98.98 

TS-512 rsf SS 74.42 0.27 13.82 1.79 0.05 0.32 1.45 4.18 3.17 0.13 0.19 99.44 

TS-722 rsf SS 73.95 0.25 14.00 1.75 0.05 0.32 1.42 4.63 3.17 0.06 na 99.61 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
101

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Rhyolite of Kaleetan Butte

902 42 4 4 16 28 10 18 <2 67 171 17 16 44 194

891 47 5 5 17 30 11 30 8 68 174 19 17 41 201

na na na na na na na na na na na na na na na

Rhyolite of Mesa Creek

901 50 6 4 17 29 12 26 3 63 195 17 18 40 253

909 46 6 7 18 26 10 18 9 62 201 16 17 38 258

na na na na na na na na na na na na na na na

Rhyolite of Obsidian Cliffs

987 46 6 2 15 31 8 12 7 76 104 8 13 33 92

1000 59 6 3 15 41 9 51 7 73 107 9 13 31 96

1070 33 <5 <2 10 24 8 12 2 75 104 6 13 25 94

Rhyolite of Park Creek

835 42 2 5 17 22 10 16 2 63 205 15 19 43 251

na na na na na na na na na na na na na na na

Rhyodacite of Prouty Glacier

744 38 6 11 18 20 9 26 4 53 287 60 19 48 227

758 32 <5 10 10 10 10 22 2 46 281 71 20 41 269

Rhyolite of Rock Mesa

888 43 6 7 16 28 10 21 7 80 190 19 16 35 183

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

Rhyolite of Separation Creek

846 49 7 10 17 28 9 29 2 74 162 21 15 37 156

760 47 12 17 18 33 9 38 9 56 261 43 20 44 216

849 43 9 7 17 28 9 18 4 71 167 23 16 31 157

832 48 9 12 17 31 9 23 8 66 197 29 17 41 177

Rhyolite southeast of Lewis Glacier

898 36 6 7 20 25 13 18 na 69 184 28 19 39 247

927 35 <5 9 19 22 12 20 na 73 186 32 16 39 250

981 44 <5 3 12 24 9 22 2 63 177 7 17 30 199

981 37 <5 <2 11 18 9 10 2 64 176 6 16 31 196

994 38 <5 <2 11 20 9 15 2 58 181 6 17 34 197

na na na na na na na na na na na na na na na

815 40 2 5 16 20 9 16 0 66 183 11 18 39 181

826 37 3 5 16 20 9 15 0 65 183 10 18 38 182

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

Rhyolite of South Fork Whychus Creek

903 42 6 3 16 27 11 29 4 69 165 14 16 40 190

907 49 7 7 17 32 11 32 4 68 164 14 16 41 189

901 41 6 5 15 28 10 27 8 67 166 16 16 40 191

926 44 7 3 17 27 10 21 5 67 164 15 14 38 188

915 41 6 4 17 27 10 25 <2 66 165 13 16 38 190

911 45 7 7 17 31 11 33 4 68 167 14 16 38 193

922 45 6 7 16 32 11 32 3 68 168 18 16 40 193

908 45 8 7 16 30 10 26 4 70 166 16 16 41 194

na na na na na na na na na na na na na na na

827 39 1 2 15 21 9 14 0 67 171 9 17 36 177
102

Sample Unit

label

Volcanic

group

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI Original

total 

Rhyodacite southwest of Golden Lake 

TS-514 rsg BT 70.07 0.64 14.90 3.19 0.09 0.60 1.86 5.47 2.58 0.19 0.13 98.93 

Rhyolite southwest of Lewis Glacier 

TS-144 rsw SS 73.64 0.28 13.96 1.91 0.05 0.38 1.52 4.61 3.13 0.11 0.29 99.14 

TS-156 rsw SS 73.30 0.29 14.11 1.98 0.05 0.43 1.59 4.62 3.13 0.10 0.55 98.11 

Rhyodacite of Upper Chush Falls 

TS-91 ruc BT 69.07 0.50 15.39 3.52 0.10 0.55 1.94 6.05 2.30 0.18 0.06 99.64 

TS-499 ruc BT 69.68 0.50 15.52 3.42 0.09 0.46 1.90 5.54 2.31 0.18 0.19 99.48 

TS-624 ruc BT 69.21 0.49 15.41 3.49 0.09 0.49 1.88 6.12 2.31 0.10 na 99.44 

Rhyolite of Dome 5803 

TS-735 r58 P 73.96 0.20 14.21 1.64 0.05 0.29 1.44 4.67 3.08 0.06 na 99.63 

Table 1. Chemical data for Three Sisters volcanic cluster.—Continued
103

Ba Ce Cr Cu Ga La Nb Nd Ni Rb Sr V Y Zn Zr

Rhyodacite southwest of Golden Lake

na na na na na na na na na na na na na na na

Rhyolite southwest of Lewis Glacier

na na na na na na na na na na na na na na na

na na na na na na na na na na na na na na na

Rhyodacite of Upper Chush Falls

669 41 5 7 22 22 12 25 10 45 219 18 38 79 271

729 40 <5 <2 15 18 10 22 2 42 217 7 27 62 265

639 37 0 4 21 20 10 23 0 43 221 7 35 75 253

Rhyolite of Dome 5803

844 37 3 3 17 20 8 13 0 65 186 7 16 42 160
104

Sample Unit label Whole-rock SiO2 Material 40Ar/39Ar weighted-mean plateau age 40Ar/39Ar isotope-correlation (isochron) age 

Age (ka) %39Ar[steps, °C] Age (ka) %39Ar[steps, °C] MSWD 40Ar/36A

TS-324 abt 63.6 GM 177.9±1.1 95[600–1100] 176.9±2.1 95[600–1100] 0.44 296.6±

TS-562 adl 61.3 GM 24.2±1.1 83[600–800] 29.8±6.0 83[600–800] 1.48 290.4±1

TS-202 aeg 56.5 GM 27.0±3.2 77[650–925] 24.4±4.8 95[600–1100] 0.79 296.9±

TS-577 aeg 59.2 GM 26.7±1.7 73[600–750] 28.5±3.6 73[600–750] 2.16 295.2±

TS-584 aeg 56.7 GM 28.5±2.1 77[640–850] 29.0±6.3 77[640–850] 0.42 295.3±

TS-44 ahi 60.5 GM 16.8±2.1 78[640–940] 23.3±6.3 78[640–940] 0.62 291.5±

TS-283 ahl 57.9 GM 44.5±3.6 58[550–740] 33.7±10.5 58[550–740] 0.15 298.3±

TS-298A alc 61.9 GM 27.2±1.0 96[600–1100] 25.5±1.9 96[600–1100] 1.24 297.4±TS-557 alc 61.9 GM 26.8±1.2 74[550–750] 30.9±7.0 74[550–750] 0.91 292.0±1

TS-256 alg 62.2 GM 27.1±1.1 100[550–1150] 25.4±2.2 100[550–1150] 1.02 296.5±

TS-580 alh 62.0 GM 30.4±1.3 89[550–850] 29.5±5.6 89[550–850] 2.64 296.0±

TS-472 anh 58.0 GM 17.9±2.5 93[550–925] 21.0±6.3 93[550–925] 1.44 294.6±

TS-386 aoc 58.6 GM 47.9±2.2 67[550–700] 52.0±11.9 67[550–700] 0.29 293.2±TS-426 aoc 61.8 GM 29.2±2.3 74[650–830] 48.9±3.7 74[650–1050] 1.14 285.0±

TS-559 aoc 60.1 GM 44.6±1.8 74[600–750] 45.2±12.7 74[600–750] 0.15 295.1±

TS-622 apc 61.6 GM 8.4±3.3 48[650–725] 31.4±8.7 82[600–850] 0.30 290.2±

TS-225 asw 61.5 GM 24.5±4.2 58[600–800] 36.7±15.4 58[600–800] 0.38 292.6±

TS-655 awa 57.5 GM 25.2±2.2 83[550–750] 18.1±8.5 83[550–750] 0.52 298.8±

TS-372 awc 62.1 GM 21.9±1.7 90[600–1250] 24.8±9.6 90[600–1250] 2.20 293.9±

TS-392 awc 62.1 GM 24.5±1.5 92[600–1100] 27.4±4.4 92[600–1100] 2.20 294.0±

TS-510 awf 60.0 GM 23.7±1.3 87[600–850] 24.9±4.0 87[600–850] 0.30 294.6±

TS-453 awy 61.3 GM 298.7±1.4 79[650–830] 292.7±8.1 79[650–830] 0.46 302.5±1

TS-652A bac 53.1 GM 127.9±6.6 73[780–1150] 94.6±21.5 73[780–1150] 0.80 300.7±

TS-119 bjc 51.6 GM 147.9±4.3 88[650–975] 140.9±8.6 88[650–975] 1.03 297.8±

TS-131 bsb 51.5 GM 278.9±5.5 88[700–1100] 284.9±5.4 88[700–1100] 0.31 295.2±

TS-100 dbh 64.0 GM 17.9±2.2 99[650–1100] 20.1±6.8 99[650–1100] 1.50 294.5±

TS-216 dbh 64.4 GM 27.1±1.8 92[550–925] 23.0±6.8 100[550–1200] 0.32 298.2±TS-117 dcg 63.3 GM 25.9±1.2 69[650–800] 30.7±2.6 69[650–800] 0.57 290.5±

Table 2. 40Ar/39Ar ages for Three Sisters volcanic cluster.[Samples irradiated at USGS reactor, using 27.87-Ma Taylor Creek sanidine as neutron flux monitor. Abbreviations: GM, groundmass concentrate; PL, plagioclase; MSWD, mdetermined. Whole-rock weight-percent SiO2 values are from table 1. 40Ar/ 39Ar ages determined by incremental-heating analysis; weighted-mean plateau ages calculated from 

within analytical error; samples without listed plateau ages yielded discordant 40Ar/ 39Ar age spectra, but acceptable isochron or total-gas ages. Interpreted eruption ages are hig
105

Sample Unit label Whole-rock SiO2 Material 40Ar/39Ar weighted-mean plateau age 40Ar/39Ar isotope-correlation (isochron) age 

Age (ka) %39Ar[steps, °C] Age (ka) %39Ar[steps, °C] 

 MSWD 40Ar/36A

TS-19 dcl 65.4 GM 19.8±1.6 87[600–850] 26.0±2.1 100[550–1250] 0.47 290.3±

TS-115 ddl 64.3 GM 32.3±1.8 99[600–1100] 34.1±2.7 99[600–1100] 1.16 294.7±TS-706 dgc 65.2 GM 31.6±2.0 100[550–1100] 31.1±4.4 100[550–1100] 1.15 295.8±

TS-412 dhr 63.6 GM 19.1±1.7 76[630–900] 18.8±3.8 76[630–900] 0.75 295.6±

TS-5 dig 64.2 GM 14.4±2.7 77[700–1000] 22.9±11 77[700–1000] 0.91 296.6±

TSAC1 dkb ~62.5 GM 27.3±2.6 85[600–950] 26.5±4.5 85[600–950] 1.99 295.8±

TS-581 dlg 63.6 GM 25.3±1.4 70[620–780] 26.4±4.4 70[620–780] 3.50 291.4±

TS-631 dlg 63.8 GM 22.3±1.6 78[600–850] 23.2±5.0 78[600–850] 0.14 295.0±

TS-103 dlp 65.3 GM 21.4±1.9 81[700–1250] 20.4±7.5 81[700–1250] 2.80 294.2±

TS-705 dml 64.0 GM 33.6±1.2 79[550–750] 37.5±6.2 79[550–750] 0.95 291.8±1

TS-492 dmn 65.7 GM 27.7±1.0 82[600–1015] 27.8±2.5 82[600–1015] 1.70 294.6±

TS-491 dmn 66.1 GM 25.2±0.8 95[590–1000] 27.7±2.4 95[590–1000] 0.97 292.9±

TS-629A dnt 63.1 GM 23.5±1.1 83[550–850] 27.9±6.3 83[550–850] 0.39 291.3±

TS-511 drm 64.3 GM 28.7±0.8 78[700–1250] 28.9±1.8 78[700–1250] 0.89 295.1±

TS-257 dsn 64.4 GM 26.0±1.5 93[630–1150] 19.1±3.7 93[630–1150] 0.67 299.3±

TS-387 dss 63.7 GM 18.5±1.5 94[600–1000] 24.5±3.1 100[550–1150] 1.20 292.4±

TS-223 dsw 65.4 GM 28.2±2.4 77[650–925] 16.8±15.3 71[650–850] 0.72 300.2±

TS-345 dwp 64.5 GM 25.6±1.7 72[550–770] 33.6±3.3 82[550–875] 0.31 290.2±

TS-22 mcl 56.5 GM 16.1±10.4 73[640–850] 65.9±14.7 100[550–1250] 8.28 291.5±

TS-169 mcl 56.4 GM 35.1±9.0 63[650–1000] 23.3±15.8 63[650–1000] 0.36 297.6±

TS-188 mdl 53.0 GM 205.5±7.1 79[550–850] 180.8±20.7 79[550–850] 0.86 299.6±

TS-178 mhd 54.8 GM 36.8±8.9 92[550–925] 44.0±18.0 92[550–925] 0.52 293.8±

TS-150 mhr 53.4 GM 67.0±4.1 100[600–1250] 67.3±1.2 100[600–1250] 0.16 295.4±

TS-383 mlb 53.1 GM 47.2±5.8 50[700–750] 3.8±44.2 50[700–750] 0.33 302.3±TS-554 mlb 52.6 GM 48.4±3.6 74[550–800] 43.0±16.8 74[550–800] 0.18 296.7±

TS-474 mmi 53.3 GM -- -- 18.1±3.9 88[550–925] 0.98 303.9±

TS-429 mmm 56.6 GM 19.2±6.9 50[650–790] 19.6±5.4 100[550–1075] 1.40 293.8±

TS-235 mms 53.4 GM 16.4±11.7 83[580–1250] 34.6±29.5 83[580–1250] 0.96 294.2±

TS-444 mnf 54.2 GM 47.7±9.5 99[550–1075] 92±43 99[550–1075] 1.80 291.6±

TS-217 mnh 57.1 GM 20.9±5.5 86[550–900] 16.2±17.2 86[550–900] 0.84 296.6±

TS-505 mns 55.1 GM 119.3±5.6 76[600–900] 136.0±9.4 100[550–1150] 2.70 292.9±

Table 2. 40Ar/39Ar ages for Three Sisters volcanic cluster.—Continued
106

Sample Unit label Whole-rock SiO2 Material 40Ar/39Ar weighted-mean plateau age 40Ar/39Ar isotope-correlation (isochron) age 

Age (ka) %39Ar[steps, °C] Age (ka) %39Ar[steps, °C] 

 MSWD 40Ar/36A

NS-02-79 mns 53.9 GM -- -- 83.1±24.5 100[550–1150] 2.99 294.4±

NS-02-160 mns 54.8 GM 70.5±3.2 80[550–800] 73.7±15.3 80[550–800] 0.81 294.1±1

NS-02-183 mns ~54 GM 55.0±5.3 60[600–750] 37.6±47.0 60[600–750] 0.23 299.7±1

TS-388 mns 53.8 GM 46.2±5.6 91[600–1250] 52.8±12.0 100[550–1250] 0.77 294.2±

TS-670 mnt 55.3 GM 20.3±4.6 79[600–900] -1.5±18.4 79[600–900] 0.47 299.4±

TS-381 mrg 54.6 GM -- -- 19.6±6.1 83[650–1060] 1.17 289.4±

TS-114 msc 56.8 GM 17.6±2.4 100[550–1150] 21.4±5.5 89[550–975] 0.58 294.3±

TS-191 msf 54.0 GM 166.2±16.0 94[600–1100] 162.6±43.0 94[600–1100] 0.13 295.8±

TS-466 msp 54.2 GM 116.3±2.9 96[550–1000] 129.1±5.8 96[550–1000] 0.57 291.8±

TS-662 mtb 52.1 GM 531.7±7.3 63[550–725] 539.5±17.3 63[550–725] 0.62 294.1±

TS-112 mth′ 52.8 GM 148.9±4.8 76[600–850] 157.7±9.6 100[600–1150] 1.44 294.3±

TS-47 mtp 53.8 GM 22.3±12.9 100[550–1150] 18.1±32.5 100[550–1150] 0.84 295.7±

TS-147 mtw 53.6 GM 373.7±6.0 82[550–850] 361.5±10.0 82[550–850] 1.30 299.6±

NS-02-93 mwa 54.9 GM 58.8±3.6 63[550–700] 57.1±18.5 63[550–700] 0.96 295.9±1

TS-252 mwh 53.2 GM 194.6±5.0 69[600–750] 206.6±19.2 69[600–750] 0.63 293.2±

TS-254 mws 54.6 GM 174.5±2.9 80[550–800] 182.0±12.3 80[550–800] 0.51 292.0±

TS-51 rct 72.9 PL 24.0±5.4 55[900–1050] 29.7±14.5 55[900–1050] 4.05 295.3±

TS-157 rct 72.8 PL 30.4±7.7 56[900–1075] 69.6±6.1 100[550–1300] 7.23 286.4±

TS-339-1 rdh 74.0 GM 27.2±1.1 76[550–680] 34.8±1.5 76[550–680] 0.76 285.2±

TS-569 rgl 72.2 GM 31.9±0.7 79[600–820] 33.7±2.6 79[600–820] 1.30 292.3±

TS-70 rmc 72.4 PL 47.4±8.2 88[700–1150] 40.9±21.0 88[700–1150] 2.09 296.7±

TS-141 rmc 72.6 GM -39.7±5.1 76[690–1150] 45.2±15.0 91[580–975] 1.84 288.9±

TS-102 roc 76.5 GM 37.8±1.8 59[650–800] 37.6±17.9 100[500–1200] 5.70 295.4±1

TS-201 rpg 68.4 GM 32.8±1.5 88[550–800] 30.6±2.6 100[550–1200] 0.99 297.4±

TS-321 rsc 73.3 GM 25.4±1.3 91[580–1100] 23.7±3.6 91[580–1100] 0.28 297.1±

TS-364 rse 73.8 GM 31.4±1.1 53[720–800] 35.0±2.7 97[600–1150] 2.70 286.1±

TS-487 rse 73.9 GM 33.6±0.5 100[550–1100] 34.0±1.6 100[550–1100] 0.98 295.0±

TS-18 rsf 74.0 GM 35.8±2.0 76[700–890] 46.5±6.2 97[600–1020] 1.76 282.6±

TS-156 rsw 73.3 PL 50.5±9.5 77[850–1150] 55.3±20.7 77[850–1150] 2.21 293.7±1

TS-91 ruc 69.1 GM 169.3±2.1 87[600–850] 172.6±6.0 87[600–850] 1.20 295.1±

TS-735 r58 74.0 GM 40.4±1.4 93[650–1150] 43.1±2.9 93[650–1150] 1.02 292.9±

Table 2. 40Ar/39Ar ages for Three Sisters volcanic cluster.—Continued
107

Sample Unit 

label

Whole-rock 

SiO2

Material K2O 40Ar* 

(moles/g)

Radiogenic 

Ar (%) K-Ar age (ka)

TS-100 dbh 64.0 GM 1.731±0.005 6.04E-14 1.27 24.1±11.3

TS-19 dcl 65.4 GM 2.339±0.002 1.08E-13 3.01 31.9±8.5

8.12E-14 1.52 23.9±7.3

(wt.av.) 27.3±5.5

TS-5 dig 64.2 GM 1.770±0.000 5.25E-14 0.768 20.4±12.2

TS-94 rkb 73.6 GM 3.187±0.055 1.39E-13 5.825 30.3±5.3

TS-18 rsf 74.0 GM 3.115±0.003 1.42E-13 5.896 31.5±5.1

TS-91 ruc 69.1 GM 2.304±0.006 6.25E-13 13.8 188.3±6.7

Table 3. K-Ar ages for Three Sisters volcanic cluster.

[Abbreviations: GM, groundmass concentrate; 40Ar*, radiogenic argon; wt.av., weighted average of two K-Ar experiments. 

Whole-rock weight-percent SiO2 values from table 1; weight percent K2O values by flame photometry at U.S. Geological 

Survey laboratory in Lakewood, Colo., Dave Siems analyst. Interpreted eruption age is highlighted in bold]