2012

U.S. Department of the Interior

U.S. Geological Survey

Geologic Map of Mount Gareloi, Gareloi Island, Alaska

By Michelle L. Coombs, Robert G. McGimsey, and Brandon L. Browne

Pamphlet to accompany

Scientific Investigations Map 3145

Mount Gareloi’s south summit, looking north. Black sinuous lava flows, erupted sometime between 

1950 and 1980, pour out of the crater floor. The volcano’s south flank is covered in pyroclastic 

deposits from the 1929 eruption, and older lava flows that are vegetated are visible in the 

foreground. (Photograph by R. McGimsey, August 30, 2003).
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i

Contents

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

Previous work.................................................................................................................................................1

Methodology...................................................................................................................................................1

Geologic Setting.............................................................................................................................................3

Geology of Mount Gareloi.............................................................................................................................3

Submarine geology...............................................................................................................................3

Historical Activity...........................................................................................................................................6

1929 eruption..........................................................................................................................................6

Recent observations ...................................................................................................................7

Lava flows ............................................................................................................................7

Pyroclastic deposits...........................................................................................................7

Overall eruptive sequence ................................................................................................7

1950–1980 South Peak effusive eruption ..........................................................................................9

1987 flow(?) ..........................................................................................................................................10

Recent ash eruptions .........................................................................................................................10

Fumarolic activity................................................................................................................................10

Composition of erupted products..............................................................................................................10

Acknowledgments.......................................................................................................................................11

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

Surficial deposits ................................................................................................................................14

Holocene volcanic rocks—south peak...........................................................................................14

Holocene volcanic rocks—north peak ...........................................................................................16

Pleistocene volcanic rocks...............................................................................................................17

References Cited..........................................................................................................................................18

Appendix 1. Whole-rock compositions of rocks from Mount Gareloi.................................................18

Figures

1. A, Location of Mount Gareloi on Gareloi Island; B, Geographic setting of Mount Gareloi

....................................................................................................................................................2 

2. View, looking east, of Gareloi Island .............................................................................................4

3. View, looking northwest, of north peak.........................................................................................4

4. Map showing shaded-relief topography and bathymetry for Gareloi Island and the 

surrounding seafloor ..............................................................................................................5

5. A, Photograph, looking north, of 1929 craters, lava flows, and pyroclastic deposits on 

southeast flank of south peak; B, Explanation of geologic features seen in A.............6 

6. Stratigraphic sections showing deposits from the 1929 eruption of Mount Gareloi.............8

7. Diagram showing correlations between pyroclastic deposits, lava flows, and debris-flow 

deposits formed during 1929 eruption .................................................................................9

8. Photograph of debris-avalanche deposit (unit d29) ..................................................................9

9. View of lava flow(?) formed during 1987 eruption.....................................................................10

10. Variation diagrams showing compositions of Mount Gareloi rocks compared to other 

Aleutian lavas; A–D..........................................................................................................12,13

Tables

1. 40Ar/39Ar ages for Mount Gareloi lavas, Gareloi Island, Alaska.................................................3

2. 14C age on soil beneath tephra section on north flank of Mount Gareloi, Gareloi Island, 

Alaska........................................................................................................................................4
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1

Introduction

Gareloi Island (lat 51.7° N., long 178.8° W.) is located in 

the Delarof Islands group of the Aleutian Islands, approximately 

2,000 km west-southwest of Anchorage and 150 km west of 

Adak, the westernmost town in Alaska (fig. 1). This small (~8- 

x 10-km diam), uninhabited island is constructed exclusively of 

eruptive products from Mount Gareloi volcano, a Pleistocene 

and Holocene, steep-sided, composite stratocone that rises 1,573 

m (5,161 ft) above sea level (asl). Mount Gareloi has been one 

of the most active Aleutian volcanoes since its discovery by 

the Bering expedition in the 1740s (Miller and others, 1998), 

though, because of its remote location, detailed observations of 

eruptive activity have been scant.

As part of an effort to both monitor and study all historically active volcanoes in Alaska, the Alaska Volcano 

Observatory (AVO) undertook a field program at Mount Gareloi 

in the summer of 2003. During a month-long period, seismic 

networks were installed at Mount Gareloi and the neighboring 

Tanaga volcanic cluster. During this time, we undertook the first 

geologic field study of the volcano since Robert Coats visited 

Gareloi Island for four days in 1946. Understanding the geology of this relatively small island is important from a hazards 

perspective, because Mount Gareloi lies beneath a heavily 

trafficked air route between North America and Asia and has 

frequently erupted airborne ash since 1760. At least two landslides from the island have deposited debris on the sea floor; 

thus, landslide-generated tsunamis are also a potential hazard 

(Coombs and others, 2007). Since seismic instruments were 

installed in 2003, they have detected small but consistent seismic signals from beneath Mount Gareloi’s edifice, suggesting 

an active hydrothermal system (Caplan-Auerbach and Prejean, 

2005). Mount Gareloi is also important from the standpoint of 

understanding subduction-related volcanism, because it lies in 

the western portion of the volcanically active arc, where subduction is oblique to the arc front. Understanding the compositional evolution of Mount Gareloi fills a spatial gap in along-arc 

studies (for example, Kelemen and others, 2003; Kay and Kay, 

1994; Singer and others, 2007).

Previous Work

Prior to our field work in 2003, only one other geologic 

party had visited Gareloi Island: Robert Coats and his field 

assistant Will Thompson spent four days there in 1946. Their 

brief field work, conducted on foot, led to a preliminary geologic map and a general outline of the eruptive history of Mount 

Gareloi (Coats, 1959). His field work on Gareloi and neighboring islands is also briefly described elsewhere (Coats, 1947, 

1950, 1956).

Coats described two main phases of volcanic activity on 

Gareloi Island, separated by a considerable period of volcanic quiescence. The older of the two units was thought to be 

Pleistocene in age, because of the presence of U-shaped valleys 

probably eroded by ice, and consists of intercalated lava flows 

and pyroclastic deposits. These older rocks are displayed in 

particularly prominent cliffs on the island’s southwest coast. 

Holocene eruptive products are primarily lava flows. Coats 

also mapped the products of a 1929 fissure eruption on the 

volcano’s south flank. This eruption, observed by a resident of 

Atka (Finch, 1931), produced lava flows, a series of craters, 

and a pyroclastic apron that covers the south half of the island. 

Coats obtained no geochemical analyses but thought the eruptive products to be basaltic and andesitic. The 1929 eruption of 

Mount Gareloi was noted by Nakamura (1977) as an example 

of a fissure that is aligned with plate convergence and, thus, 

indicative of tectonic stress orientation.

In 2005, a research cruise in the western Aleutian Islands 

dredged samples from seafloor volcanoes and collected multibeam bathymetry and sidescan data of the seafloor. During the 

cruise, two dredges were conducted on satellite vents offshore 

of Gareloi Island, and the seafloor to the north and east was 

mapped. This provided the first detailed information about 

the submarine portion of Mount Gareloi (Coombs and others, 

2007).

On the basis of 2003 field work, Coombs and others (2008) 

described the volcanic hazards associated with eruptions of 

Mount Gareloi and concluded that the primary hazard is airborne ash from relatively frequent explosive eruptions, although 

much less frequent edifice collapses also send debris into the 

sea and produce at least localized tsunamis.

Methodology

Field work for this geologic study was conducted in 

August and September of 2003, in conjunction with the installation of a seismic network on Mount Gareloi. Access to the 

island was aboard the 86-ft Maritime Maid, a helicopter-supporting vessel contracted from Maritime Helicopters. Using the 

boat and helicopter allowed us to cover much of the island in 

six and a half days of field work. A helicopter overflight in 2005 

allowed observations of summit features, but no landings were 

made.

The Gareloi Island Army Map Service topographic map 

sheet, at 1:25,000 scale, was used as a field mapping base. 

Field mapping was supplemented by analysis of aerial photographs taken in August 2002, as well as by imagery provided 

by the U.S. Geological Survey (USGS). A digital geologic map 

was created through a combination of digitizing scanned field 

mapping and mapping directly on digital imagery. All digital 

mapping was performed using ESRI ArcGIS 8.3–9.3 software.

Rock samples were prepared for geochemical analysis, 40Ar/39Ar geochronology, and thin section preparation upon 

returning from the field in the fall of 2003. Major and trace element whole-rock analyses (appendix 1, available at http://pubs.

usgs.gov/sim/3145) were performed using XRF and ICP-MS 

techniques at the GeoAnalytical Laboratory at Washington 

State University. 40Ar/39Ar dating (table 1) was performed by A. 

Calvert at the USGS in Menlo Park, CA. One soil sample was 

dated using traditional 14C techniques at the Geochronological 

Laboratory in Cambridge, MA.
2

PACIFIC

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artin

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Buldir

Island

Historically active

volcanoes

Gareloi

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Figure 1. A, Location of Mount Gareloi on Gareloi Island in relation to other historically active volcanoes of Alaska (as listed in Miller and 

others, 1998). B, Geographic setting of Mount Gareloi, Gareloi Island, and nearby islands, Alaska. Bathymetric contour interval, 400 m.
3

Geologic Setting

Mount Gareloi is located in the westernmost volcanically 

active portion of the Aleutian Arc (fig. 1), which is formed by 

subduction of the Pacific plate beneath the Bering Sea portion 

of the North American plate. Whereas subduction is normal to 

the arc farther east, it becomes oblique to the west due to the 

curvature of the arc (Minster and Jordan, 1978). Subduction of 

Paleocene oceanic crust in the western arc occurs obliquely at a 

rate of approximately 8 cm/yr (4 cm/yr orthogonal component; 

Engebretson and others, 1985).

The Aleutian Ridge, which forms the platform upon which 

the Quaternary stratovolcanoes sit, is built of predominantly 

Eocene igneous and sedimentary rocks that were formed during 

the first, highly active phase of Aleutian Arc growth (Scholl and 

others, 1987). The ridge has been part of an active volcanic arc 

since at least 46 m.y. (Jicha and others, 2006), and much of the 

mass of the ridge had been formed by 37 Ma (Scholl and others, 

1987). The current tectonic configuration and active arc have 

existed since about 5 Ma, when an increase in the subduction 

rate led to increased magma genesis and volcanism (Scholl and 

others, 1987).

Whereas Mount Gareloi is aligned with neighboring 

Quaternary stratovolcanoes to the east and west, the Aleutian 

Ridge is irregularly shaped in this area, placing Mount Gareloi 

slightly north of the main ridge platform (fig. 1B). Geist and 

others (1988) proposed that the Aleutian Ridge comprises a 

series of rotated blocks. Mount Gareloi sits north of the Delarof 

Block, which is bounded by Amchitka Pass to the west and 

informally named Adak canyon to the east. Because of this configuration, it sits on a pedestal presumably created by its own 

constructional vigor that rises from at least 2,400 m below sea 

level (bsl) on the west, north, and east.

Geology of Mount Gareloi

On the basis of appearance and correlation with neighboring stratovolcanoes, the oldest rocks on Gareloi Island are 

presumably Pleistocene, though no rocks from what appear 

to be the oldest outcrops along the southwest coast have been 

sampled. These older rocks crop out as deeply dissected stacks 

of lavas and pyroclastic deposits that form wedge-shaped map 

sectors on the southwest, southeast, and northeast flanks (Coats, 

1959). Holocene lavas, most that are little modified by erosion, 

have filled low topography between these older sectors and, 

in some places, spill over older erosional surfaces (fig. 2). The 

northwestern and eastern portions of the older edifice appear 

to have been incised by two sector collapses, on the basis of 

their subaerial geomorphology and offshore debris-avalanche 

deposits (Coombs and others, 2007). The eastern scar is about 

2 km across, reaches from north peak’s summit to the coastline, 

and is currently 100 m deep relative to the unaffected surfaces 

to either side (fig. 3). The northwest sector scar is broader, 5 

km wide at its base, and currently as deep as 120 m. Both were 

likely deeper prior to infilling by younger lavas.

Most recognized Holocene eruptions from Mount Gareloi 

appear to have occurred from one of two summits, north peak 

and south peak, which are separated by a 500-m-wide saddle 

at 4,650 ft asl. North peak is the island’s true topographic high 

point and possesses an enclosed crater at least 70 m deep. North 

peak’s intracrater eruptive stratigraphy is abruptly interrupted 

by near-vertical local unconformities on the northwest wall, 

which represent a possible collapse scar that has since been 

filled. South peak, 4,995 ft asl, has a south-facing, 300-m-wide 

crater formed by the partial collapse of its crater wall (likely 

during the 1929 eruption) and contains several active fumaroles. 

The island has been significantly modified by recent eruptive activity, most notably the 1929 eruption and a lava flowproducing eruption that occurred sometime between 1950 and 

1980. The products of these eruptions are discussed in more 

detail in the Historical Activity section. In addition to these specific events, numerous recent eruptions have produced a blanket 

of coarse tephra, included in map units pnp and psp, that 

covers the summit areas. Some unmapped tephras are locally 

preserved on the volcano’s lower flanks, but they are poorly 

exposed. A soil near the base of a 120-cm-thick, tephra-soil 

section on the north flank yielded a 14C date of 1,090±70 yr B.P. 

(table 2). This section contains nine tephra layers ranging from 

thin, fine ash layers to a 10- to 16-cm-thick scoria lapilli bed.

Submarine Geology

Unlike most other Aleutian volcanoes, Mount Gareloi is 

located slightly north of the summit plateau of the Aleutian 

Ridge and sits upon its own constructional pedestal that rises 

Table 1. 40Ar/39Ar ages for Mount Gareloi lavas, Gareloi Island, Alaska.

[Samples irradiated at USGS TRIGA reactor using 27.87 Ma Taylor Creek sanidine as a neutron flux monitor. Preferred ages in 

bold. MSWD, mean square of weighted deviates]

40Ar/39Ar Weighted mean plateau age 40Ar/39Ar isotope correlation (isochron) age 40Ar/39Ar total gas

Sample No. Material Age (ka) %39Ar[steps, °C] MSWD Age (ka) %39Ar[steps, °C] MSWD 40Ar/36Ari Age (ka)

03GRBB32 Basalt -10.3 ± 3.6 92.8 [550-990] 2.87 1.8 ± 4.5 92.8[550-990] 2.07 292.4 ± 2.0 -17.5 ± 3.3

03GRBB38 Basalt 6.1 ± 1.0 97.3 [600-1100] 0.5 7.0 ± 1.8 97.3[600-1100] 0.51 293.1 ± 1.9 3.2 ± 1.3

03GRGM20 Basalt 0.3 ± 1.4 100 [550-1150] 0.94 1.6 ± 2.4 100 [550-1150] 1.27 295.0 ± 1.7 -0.9 ± 1.6
4

south 

peak

north 

peak

Lava flows and pyroclastic deposits of the Pleistocene edifice

Holocene lava flows and 

pyroclastic deposits 

 Figure 2. View, looking east, of Gareloi Island. Photograph by M.L. Coombs, USGS, October 2005.

pnp

lnpu

lnpu, pef

pnp

l87

see

see see

north peak

lnpu+sef

northern margin of 

east collapse scar

southern margin of 

east collapse scar

Figure 3. View, looking northwest, of north peak. Lavas and pyroclastic-flow deposits of units lnpu

and pef are delineated in the foreground. North peak is built in a collapse scar incised into rocks of 

unit see. Photograph by M.L. Coombs, USGS, August 2003.

Table 2. 14C age on soil beneath tephra section on north flank of Mount Gareloi, Gareloi Island, Alaska.

[Analysis performed at Geochron Laboratories, Cambridge, MA. Age referenced to year A.D. 1950]

Sample No. Age, in 14C yr B.P. Material

03GRMV12E 1,090±70 Soil immediately below 118-cm-thick tephra-soil sequence with ~8 

tephra-fall layers, on east side of gully wall
5

from 2,600 m bsl on the north and east (fig. 4). Submarine 

debris fields are present east and north-northwest of Gareloi 

Island (Coombs and others, 2007). Apparent in GLORIA 

seafloor sidescan imagery, debris-avalanche deposits form two 

long lobes north-northwest of the island, and a stubbier, rougher 

debris lobe to the northwest partially overlaps these. The debrisavalanche deposit to the east of the island is smaller, widening 

seaward from a narrow zone near the shoreline to about 13 km 

across at its distal end, about 13 km from shoreline. The area is 

just downslope of the east scar in the subaerial edifice. The age 

of this deposit is unknown but is likely middle or late Holocene, 

based on the lack of sediment cover and the sharp morphology 

of the subaerial collapse scar.

The submarine flank to the northeast, which lies immediately off an inactive sector of the subaerial volcano, is characterized by closely spaced canyons that form smooth, uniform 

ramparts. This morphology is consistent with being downslope 

from a relatively inactive portion of the subaerial volcano. A 

shallow shelf connects the island to the main Aleutian Ridge to 

the south, but high resolution bathymetry is lacking for this area. 

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Figure 4. Map showing shaded-relief topography (in gray) and bathymetry (color-scaled for depth) for Gareloi Island and the 

surrounding seafloor. Bathymetric contour interval is 200 m. Bathymetry dataset has been merged from 1:400,000-scale map 

(Nichols and Perry, 1966) and multibeam data (Coombs and others, 2007). Smoother areas are typically lacking in the higher 

resolution multibeam data. Subaerial topography is from the Shuttle Radar Topography Mission (Gesch and others, 2006). Star 

marks the location of a submarine flank cone that was dredged in 2005 (G. Yogodzinski, personal commun., 2005).
6

A conical feature off the island’s northeast flank is apparently 

a constructional cone on the flank of Mount Gareloi and was 

dredged in 2005 (G. Yogodzinski, personal commun., 2005).

Historical Activity

Mount Gareloi has been relatively active during historical 

times, with 19 reports of eruptive activity since 1760 (Miller 

and others, 1998). Of these, four may be considered questionable, and some can be lumped into periods of continuous activity. Below, we describe the recent eruptions that have resulted in 

mappable deposits on the island. For a full account of historical 

activity, see Coombs and others (2008).

1929 Eruption

The largest historical eruption at Mount Gareloi occurred 

in 1929. The eruption is notable for its size, its unusual eruptive 

style, and the composition of the erupted material. This explosive eruption formed a 4.3-km-long, south-southeast-trending 

fissure of 13 aligned craters, reaching from the summit of 

south peak down to the southeast shoreline (fig. 5). The craters 

individually range from 80 to 1,600 m in diameter. The eruption 

also produced a substantial blanket of pyroclastic deposits on 

the volcano’s south flank (unit p29).

The details of the eruptive sequence were first described by 

Coats (1959, p. 252), who summarized the first-hand observations of a resident of Atka, William H. Dirks, Sr.: “In April of 

1929, there was an eruption. A terrible quake... split the mountain from its crater right to the beach. Ashes fell, completely 

covering their cabins and boats...” Coats further elaborated on 

the series of events from observations of the deposits, which 

were 17 years old when he visited the island and were still 

unmodified by either vegetation or further eruptive activity. 

According to his chronology, the eruption caused the excavation of a series of explosion craters along a the south-southeasttrending fissure (fig. 5). The eruption began with formation 

of the south peak crater, followed by excavation of the lower 

craters, explosive eruption of tephra, and effusion of lava 

flows from the lower craters. Coats (1959) notes that, in 1946, 

crater 10 was filled with a milky blue-green lake. Coats (1956) 

describes a blanket of ash on Ogliuga Island, 18 km southsoutheast of Gareloi Island, as 5.5 ft thick, and attributes it to 

Mount Gareloi on the basis of its “freshness.”

1

2

3

4

7

5

6

8 9

10 ssf

p29

p29

l29

l29

l29

ssp

t29

A B

Figure 5. A, Photograph, looking north, of 1929 craters, lava flows, and pyroclastic deposits on southeast flank of south peak. Photo by R.G. 

McGimsey, USGS, August 2003. B, Explanation of geologic features seen in A. Numbers refer to craters formed during 1929 eruption, and 

units are described in the text.
7

Recent Observations

By the time of our field work in 2003, surprisingly thick vegetation had covered the 1929 lava flows and pyroclastic deposits 

below approximately 1,300 ft asl. In our limited time on the 

island, we were able to visit several exposures of 1929 eruptive 

products (fig. 6). Good exposures in beach cliffs along the south 

coast and in gully walls, not yet examined, would allow for substantial additional refinement of the stratigraphy presented here.

In addition, we modified the crater numbering scheme 

of Coats (1959) upon discovery of several additional features. 

We mapped 16 numbered craters, starting with crater 1 near 

the coast and crater 16 at the summit of south peak (figs. 5, 6). 

These form a nearly linear southeast alignment. The craters 

vary in size and morphology, and some have been completely 

obscured by 1929 lava (for example, crater 4) or lava from a 

later eruption (craters 11 and 12 are completely covered by 

lavas of unit ssp). Some craters (1, 2) have substantial tephra 

cones indicating that they were vents, whereas others appear to 

be collapse features (craters 3, 5, 6).

Lava Flows

Of the visible craters, three have lava flows emanating 

from them, another lava flow extends from beneath 1950–1980 

lava flows between craters 9 and 10, and crater 9 has 1929 lava 

exposed within its walls. The flow from crater 1 fans out from 

the vent and forms a lava delta at the coastline; it is underlain 

by unit p29 (see following) but is not overlain by fall deposits 

(fig. 6). The flow from crater 4 is the most voluminous of the 

eruption, wholly filling the crater and spreading to the east. No 

obvious fall deposits were observed on the flow’s incredibly 

rugged, vegetated surface. A lava flow effused from crater 8, 

filling a subtle depression before finally spilling into crater 7. A 

fourth lava flow issued from a now-covered vent between craters 9 and 10. Informally termed the “wishbone flow” because 

of its characteristic shape, it splits into two fingers, one of which 

spills into crater 9. All 1929 lavas have rubbly tops and are as 

much as several meters thick.

Chemical analysis (appendix 1) shows that lavas from 

craters 1, 4, and 8 are all compositionally similar latites, with 

57.8 to 58.7 wt. percent SiO2

, and have been assigned to unit 

l29. Lava flows exposed in the wall of crater 9 and the wishbone flow are trachytes with 60.5 to 61 wt. percent SiO2

 and are 

labeled unit t29. Both latites and trachytes have similar mineralogy, including plagioclase, clinopyroxene, opaques, and olivine 

in decreasing order of abundance; amphibole was found in trace 

amounts. Samples of both rock types typically contain about 10 

volume percent phenocrysts on a void-free basis. The volume of 

effused lava is roughly 1.7 M m3, assuming an average lavaflow thickness of 4 m.

Pyroclastic Deposits

Fall deposits from the 1929 eruption cover a sector of the 

southeast flank, smoothing underlying lava flows. No tephras that 

appear to correlate with those on the southeast flank were found 

on other flanks of the volcano. Whereas the surface of the deposit 

is mostly covered with vegetation, erosion has cut some severalmeter-deep gullies, exposing stratigraphy (see cover photograph).

Two dominant clast types were present in 1929 pyroclastic 

deposits: buff- to pinkish-colored pumice and dark-gray scoria. 

Banded juvenile clasts and dense lava-like lithics were minor 

lithologies. Chemical analysis of several clasts from the fall 

deposits indicate that the pumice is trachytic, with 60.1 to 60.7 

wt. percent SiO2

 and 8.7 to 9.3 wt. percent total alkalis. Scoria 

clasts are latites, with 58.3 to 58.8 wt. percent SiO2 and 8.1 to 

8.5 wt. percent total alkalis. The pumice and scoria essentially 

overlap compositionally and mineralogically with trachyte and 

latite lavas of units t29 and l29, respectively.

At site 03GRBB16, the 1929 pyroclastic deposits are at 

least 7 m thick; our measured section did not reach the base (fig. 

6). Pyroclastic deposits are dominated by clast-supported fall 

deposits that are generally pumice rich at the base and scoria 

rich at the top and that are interbedded with minor matrix-supported pyroclastic-flow deposits. The eruption’s total volume is 

unknown, because much of the pyroclastic material was deposited offshore. The volume of subaerial pyroclastic deposits is 

approximately 16 M m3, assuming an average thickness of 3 m. 

Combined with lava volume, the subaerial volume of eruptive 

products is approximately 18 M m3.

Overall Eruptive Sequence

Based on both stratigraphic sections (fig. 6) and compositional correlation between lava flows and pyroclastic deposits, 

the general emplacement sequence for the 1929 eruption follows (fig. 7): (1) a possible pyroclastic surge, forming a deposit 

consisting of pumice blocks as much as 70 cm in diameter in 

a sandy pink-brown matrix; (2a) tephra fall of mostly trachyte 

pumice with minor latite scoria; (2b) thin pyroclastic flows 

interbedded within 2a; (3) effusion of trachyte lava from (at 

least) crater 10; (4) tephra fall of roughly equal proportions 

latite scoria and trachyte pumice; (5) latite-scoria-rich tephra 

fall; (6) latite lava flows interbedded with latite-scoria tephra 

fall; and (7) debris flows (fig. 8). We do not know the order in 

which the craters were formed, but, because latite lava flows 

are thought to have effused later in the eruption and they are 

from the lowermost craters, it seems plausible that the eruption 

commenced from the summit and crater formation propagated 

downslope to the coast.

The general sequence of events during the 1929 eruption 

was similar to that seen at flank-fissure eruptions elsewhere. 

Some points of similarity, summarized from Nakamura and 

others (1977) follow: (1) the flank-fissure eruption was immediately preceded by (or accompanied by) a summit eruption; (2) 

the eruptive fissure grew downslope and the eruptive activity 

persisted longer downslope, resulting in lava flows; and (3) the 

erupted magma had a relatively low viscosity.

In a model for flank-fissure eruptions from composite volcanoes with stable central pipe-like conduits, magma ascends 

the central conduit, if the magmatic pressure is not sufficiently 

relieved from a summit eruption, and a radial vertical fracture develops laterally and is filled with the source magma 
8

??

??

??

1 m

??

BB16G

60.1 wt% SiO2

BB16F

BB16D

BB16E

BB16C

BB16B

BB16A

58.7 wt% SiO2

03GRBB16 03GRBB09 03GRBB56

15-20% lithics; 2-3 % scoria at base and 10% at top.

Lava flow from vent 8

Bombs to 80 cm

20% lithics 

<10% scoria plus lithics

GM06

GM16A

GM16B

BB09

l29

03GRGM

l29

03GRMC07

d29

BB56 lsp

Pumice- and scoria-bearing soil Scoria = pumice

5-10% lithics, <5% scoria

<1% scoria, <5% banded 

Pink-brown silt-sand matrix with subangular scoria and pumice blocks to 70 cm 

Pumice-rich fall deposit Soil

Scoria-rich fall deposit Surge(?) deposit

Pyroclastic-flow deposit

!(

!(

!(

8

9

10

15

16

14

13

p29

lsp

ssp

lnpu

see

psp

psp

d

dsf

d29

dsf

l29

asp

d

t29

B

BB09

MC07

BB16

178°48'W 178°47'W 

51°47'N51°46'N51°45'N

0 

!( 

EXPLANATION

Figure 6. Stratigraphic sections showing deposits from the 1929 eruption of Mount Gareloi. Question marks at the base of a section indicate 

1929 deposit was not identified. Blue, green, and yellow fields connect units that potentially correlate between sections. Abbreviated sample red; preface “03GR” has been removed from sample and site numbers for brevity. Index map shows locations of stratigraphic sections (abbrsoutheast flank of Mount Gareloi.
9

(Nakamura and others, 1977). This model is consistent with the 

summit-then-flank pattern observed at Mount Gareloi.

Fall deposits

Based on stratigraphy Based on chemistry

local pfs

crater 9 

rim lava

wishbone flow

 (near crater 10)

crater 4

crater 8 lava flow

lava flow

crater 1 

lava flow

west 

debris flow

east 

debris flow

?

?

?

?

? ?

?

silt-sand layer

(surge?)

90:10

scoria:pumice fall

10:90

scoria:pumice fall

50:50

scoria:pumice fall

syn-fall post-fall

base

mafic silicic

Fall (variable composition)

Trachyte lava (60.5 to 61 wt% SiO2) 

Latite lava (57.8 to 58.7 wt% SiO2)

Pyroclastic-flow deposits

scoria = latite

pumice = trachyte

Figure 7. Diagram showing correlations between pyroclastic deposits, lava flows, and debris-flow deposits formed during 1929 

eruption. Fall and pyroclastic-flow deposits compose unit p29; debris flows are unit d29; trachyte lava flows are unit t29; and latite 

lava flows are unit l29.

Figure 8. Photograph of debris-avalanche deposit (unit d29). 

Shovel is 1 m long. Photo by M.L. Coombs, USGS, August 2003.

1950–1980 South Peak Effusive Eruption

One of the most prominent features on Mount Gareloi’s 

south flank is a field of dark, sinuous, overlapping aa lava 

flows that emanate from the floor of the south peak crater. 

Unfortunately, no record exists to indicate exactly when these 

flows were emplaced. They are not depicted on an Army Map 

Service topographic map that was field checked in June 1950 

but are visible in images from August 1980. The only reports of 

activity during this interval were during 1950 and 1952, when 

minor ash and fumes were observed (Jones, 1951, 1952).

The eruption produced an elaborate blocky lava flow 

(unit ssp) that extends down the southeast flank to 1,200 ft asl. 

This lava flow is basaltic trachyandesite and contains abundant 
10

coarsely sieved plagioclase phenocrysts with minor clinopyroxene and olivine. The upper reaches of the flow, as well as 

the summit region, are covered by a layer of black scoria possibly erupted at the same time as the lava. The eruption likely 

occurred when the volcano was snow covered, because two 

scoria-rich debris-flow deposits on the south flank (unit dsf) 

probably formed during the eruption as hot scoria fell on snow.

1987 Flow(?)

On September 4, 1987, a commercial pilot observed a 

narrow flow-like feature on the east flank of Mount Gareloi that 

extended from the north crater rim at 4,900 ft altitude down to 

at least 3,200 ft, below which it was obscured by clouds. Steam 

rose 100 m above the deposit and the crater was vigorously 

steaming (Smithsonian, 1987). The deposit, identified from the 

air in 2003 (fig. 9) but not sampled, is mapped as unit l87.

Figure 9. View of lava flow(?) formed during 1987 eruption. Photo 

by M.L. Coombs, USGS, August 2003.

Recent Ash Eruptions

After several decades of only fumarolic activity, the largest 

eruption since 1929 took place in August and September 1980. 

On August 8, the crew of a Northwest Orient Airlines plane 

spotted a plume, apparently ash, that rose to 35,000–40,000 ft 

asl (Smithsonian, 1980). David Evans of the USGS reported 

that a seismic network on Adak recorded events from the west 

on August 8 and 9, though the exact location is not precise 

(Associated Press, 1980). No ash was reported to have fallen in 

Adak. Cloud cover obscured further observations after August 

10. Evans said, “As far as we know, there was no lava eruption” 

(Associated Press, 1980). On August 10, airborne stratospheric 

sampling detected silicate ash and elevated sulfate concentrations at lat 54°–60° N., long 150° W., at an altitude of 19.2 km 

(Sedlacek and others, 1981). This plume was plausibly a result 

of the August 8 eruption, based on wind direction and NOAA-6 

satellite imagery (Sedlacek and others, 1981). The Smithsonian 

Institution (Smithsonian, 1980) reported renewed ash emissions 

as high as 6 km asl in September 1980. North peak’s crater 

appeared to be the source of the 1980 eruption sequence, which 

is classified as Volcanic Explosivity Index (VEI) 3 based on 

plume height (Smithsonian, 1980).

On January 14, 1982, a magnitude 3.2–3.3 earthquake was 

recorded in the vicinity of Gareloi Island. On the following day, 

January 15, a 7- to 9-km-high eruption cloud was observed on 

satellite imagery (Miller, 1998).

Evan Klett, of the U.S. Fish and Wildlife Service, was traveling aboard a Reeve Aleutian Airways Inc. 727 Boeing Jet at 

14:30 AKDT on August 17, 1989, flying from Shemya Island to 

Adak Island, when he observed a grayish to black tephra plume 

that was about 700 m above north peak’s crater (Reeder, 1992). 

The plume covered the top of the north peak crater, which prevented Klett from seeing down into it. No other observations of 

this event exist.

On September 27, 1996, the Aviation Weather Unit in 

Alaska received a pilot report of a minor eruption of ash and 

steam at Mount Gareloi (Smithsonian, 1996). The pilot reported 

that the plume rose to 1,500 m above the summit. It was not 

visible on infrared satellite imagery, suggesting that it may not 

have ascended that high. As late as December 5 of that year, 

AVO had learned of no further sightings of activity at Mount 

Gareloi.

Fumarolic Activity

Both the south peak and north peak craters of Mount Gareloi 

have been sites of hydrothermal activity. Photos from 1942 and 

1986 show billowing white steam emanating from the north 

crater, but since 2003 this crater has been snow filled and cold. 

Observations since 2003 have shown an area of warm ground, 

sulfur deposition, and diffuse steaming on the west rim of the 

south peak crater between approximately 4,200 and 4,600 ft asl.

Composition of Erupted Products

Prior to this study, no analyses of Mount Gareloi lavas 

existed, though Coats suggested that most eruptive products 

were andesitic to basaltic in composition. We have analyzed 74 
11

lava and scoria samples collected in 2003, including samples 

from most of the mapped units, for major and trace elements 

(appendix 1, file at http://pubs.usgs.gov/sim/3145). These 

analyses were made for the purpose of understanding magmatic 

diversity at the volcano, correlating syn-eruptive map units, and 

looking for evolutionary trends.

Mount Gareloi lavas range from 51.3 to 62.7 wt. percent 

SiO2. Silica-variation diagrams (fig. 10) clearly reveal that most 

analyzed lavas from Mount Gareloi fall outside of the normal 

range of compositions for Aleutian lavas. On a total alkalis 

versus silica diagram, most Mount Gareloi rocks are shoshonites and latites as defined by LeMaitre (2002). These lavas are 

high-K tholeiites, which have high values of large-ion lithophile 

elements and high field-strength elements and low MgO for 

a given silica content, and include rocks from the oldest and 

youngest units on the island and lavas from both north and 

south peaks. Products of the 1929 eruption form the most differentiated end of this enriched trend. Phenocryst phases in all 

lavas are plagioclase and clinopyroxene, with minor olivine and 

rare hornblende.

An exception to this enriched trend are lavas from unit 

asp—andesite flows from south peak. Several lava samples 

from this unit are calc-alkine andesites with approximately 

62 wt. percent SiO2 and major- and trace-element abundances 

similar to or lower than other Aleutian andesites. They contain a 

phase assemblage of plagioclase, clinopyroxene, orthopyroxene, 

and oxides.

A single gabbroic bomb, sample 03GRBB22, has 45.2 wt. 

percent SiO2 and low values of incompatible elements. It contains plagioclase, clinopyroxene, olivine, and biotite.

The extreme enrichment in incompatible elements in many 

of the Mount Garleoi lavas can be explained by slight enrichment in the most mafic lavas followed by fractional crystallization. Initial enrichment in the more mafic erupted magmas 

could have been caused by additions of fluid, sediment, or sediment melt in the magmatic source region of the mantle wedge. 

The two very different compositional trends, interleaved in time 

and not spatially distinct, suggest that two or more sources fed 

magmas at this small volcano.

Acknowledgments

We thank Maritime Helicopters and the crew of the 

Maritime Maid (Bill Choate, Joe Christman, Jeannie Duran, 

and Dave Throop), who made our boat-based helicopter work 

such a success in 2003. We especially thank helicopter pilot Bill 

Springer, without whom most of the ground studied to produce 

this report would still be terra incognita. Andy Calvert’s ages 

were critical in understanding Mount Gareloi’s geology. Cheryl 

Cameron compiled the extremely useful historical eruption 

database for the Aleutian Arc. Evan Thoms provided essential 

GIS assistance. John Paskievitch expertly arranged many of the 

logistics for the 2003 field season. Reviews by Julie DonnellyNolan, and, in particular, Mike Clynne greatly improved the 

map.
12

45 50 55 60 65 70

0

1

2

3

4

45 50 55 60 65 70

2

4

6

8

10

medium-K

high-K

A

B

SiO2, IN WEIGHT PERCENT

Na

2O + K

2O, IN WEIGHT PERCENT

K

2O, IN WEIGHT PERCENT

SiO2, IN WEIGHT PERCENT

basaltic

andesite basalt andesite dacite

trachybasalt

shoshonite

latite trachyte l29

p29

t29

ssp

asp

lsp

sef

lnpu

snw

anf

ssf

see

scoria

lnp

South Peak

North Peak

Pleistocene Holocene

Undifferentiated

gabbroic bomb

Figure 10. Variation diagrams showing compositions of Mount Gareloi rocks compared to other Aleutian lavas (in gray). 

A, Total alkalis versus SiO2

. Classification scheme from LeMaitre and others (2002). Mount Gareloi lavas that fall in 

the basaltic trachyandesite and trachyandesite fields can be further classified as shoshonite and latite, respectively, 

because they contain Na2

O – 2.0 < K2O. B, K2O versus SiO2. C, FeO*/MgO versus SiO2. Tholeiitic-calcalkaline boundary from 

Miyashiro (1974). D, Zr (in ppm) versus SiO2

. Other Aleutian data from Alaska Volcano Observatory database (Chris Nye, 

personal commun., 2008).
13

45 50 55 60 65 70

0

1

2

3

4

5

SiO2, IN WEIGHT PERCENT

FeO*/MgO, BY WEIGHT

calc-alkaline

tholeiitic

Zr, IN PPM

SiO2, IN WEIGHT PERCENT

45 50 55 60 65 70

100

200

300

400

C

D
14

DESCRIPTION OF MAP UNITS

[All units on Gareloi Island are Quaternary. Map units have been organized according to age, but also 

according to eruptive vent—either south peak or north peak. The approximate time equivalence of units 

from the two vents is portrayed in the Correlation of Map Units on the map sheet. Units for which chemical analyses are available have been assigned rock names based on the scheme of LeMaitre (2002), with 

appropriate mineral modifiers. Where available, we included ranges of SiO2 and total alkalis (K2O+Na2O) 

for each unit, in wt. percent. Descriptions of phenocrysts, which are defined as crystals ≥0.5 mm in length, 

were determined by observations of both hand specimens and thin sections. In addition, a subset of thin sections has been point counted]

SURFICIAL DEPOSITS

d Debris-flow deposits (historic)—Recent, thin (<<1 m) deposits of scoria-rich debris. Three 

exposures are on south flank at roughly 1,200 and 4,000 ft asl and on north flank at 2,300 

ft asl. May have been emplaced at same time as dsf. All deposits very young; uppermost 

deposit on south flank overlies ssp

b Beach deposits (Holocene)—Cobble pocket beaches on east and south coastline of island

dv Viscous debris-flow deposits (late Holocene)—Poorly sorted, heterolithologic, massive deposits are several meters thick and consist of sandy matrix with dense lava clasts as large as 

boulder size. Two exposures are at 900 and 100 ft asl on south flank. Eastern of two exposures underlies d29 and overlies lsp

HOLOCENE VOLCANIC ROCKS—SOUTH PEAK

csp Altered crater rocks of south peak (Holocene)—Altered, layered lavas and pyroclastic rocks 

exposed on steep west crater wall of south peak. This alteration zone coincides with a fumarole field that steamed vigorously during visits in 2003, 2005, and 2010. Rocks in this area 

are yellow, green, white, and light gray and have apparently undergone extensive hydrothermal alteration, which is strongest within fragmental beds. Unaltered equivalents are units 

lsp, asp, and psp

psp Pyroclastic rocks of south peak (Holocene)—Scoria, sand, and gravel deposits on uppermost 

reaches of south peak. Much of surface of and around south peak consists of an ~1-m-thick 

unit of loosely consolidated black scoria fall, with lapilli to 40 cm in diameter. Beds of variably altered fragmental breccias and fall deposits are exposed in uppermost walls of south 

peak crater. Likely late Holocene to historic in age. Overlies units asp and lsp; overlain by 

unit ssp

ssp Shoshonite lava-flow field of south peak crater (historic)—Composition: 53.4–53.5% SiO2; 

6.3–6.4% total alkalis (number of analyses, n=4). A field of dark-gray to black porphyritic 

augite-olivine shoshonite aa lava flows emanating from central floor of south peak crater, 

reaching 2.5 km down flank of volcano; they cover a total area of 1.63 km2. Field consists of 

numerous overlapping, sinuous flow fingers, most with well-developed flow levees. Some 

flow fingers follow gullies that have been cut into the 1929 pyroclastic apron. Height of flow 

levees ranges from 4 to 10 m. Total volume of flow field is estimated to be 0.01–0.02 km3

. 

Flows completely cover craters 11 and 12 from the 1929 eruption and partially cover craters 

10, 13, and 14. Phenocrysts: 20% plagioclase, 3 mm across; 3% augite, 1 mm across; 2% 

olivine, ≤1 mm across. Date of their eruption is unknown, though it must have postdated 

1950 and predated 1980, on the basis of bracketing field work and photographs. Upper 

reaches of the flow are covered by dark scoria, but this fall layer is not extensive. Overlies 

units p29, l29, d29, and dsf

dsf Debris-flow deposits of south flank (historic)—Two localized, thin debris-flow deposits, 

likely emplaced during early phase of eruption that produced unit ssp. One southwest of 

ssp flowed down west flank of south summit, consists of scoria (both brick-red oxidized 

and black), and forms a braided pattern on the underlying 1929 pyroclastic apron. A second, 

smaller deposit occurs just downslope of westernmost fingers of ssp. Both deposits are thin 

and probably ephemeral, preserved only because of their relative youth. They suggest that 

shoshonite flows of unit ssp, and accompanying tephra, were emplaced while snow was 

present in south crater
15

d29 Debris-flow deposits (1929)—Two small, poorly sorted fragmental deposits associated with 

1929 eruption. Westernmost is a localized heterolithologic debris-flow deposit, maximum 

thickness unknown, that thins to 1 m thick at its margins. On its western edge, it overlies 

scoria fall of p29, indicating that it was emplaced late in 1929 sequence. Matrix is sand 

and gravel and contains clasts as large as 5 m in diameter. Many blocks are hydrothermally 

altered clastic rocks (breccias or some layered rocks) that are derived from units csp and 

psp. The most altered blocks are crumbling into rubble mounds. Smaller clasts in deposit 

are variably oxidized dense lavas and scoria, but none appear to be juvenile pyroclasts. 

Largest blocks extend downslope beyond visibly gray matrix of flow. An eastern deposit is 

smaller, nearly monolithologic, and spreads out from a channel at the northwestern point of 

unit l29 that erupted from crater 4. A section exposed in a gully wall shows that unit d29 is 

as thick as 3 m, poorly sorted, and has a sandy matrix, with some discontinuous pumice-rich 

layers near its base. Top half of unit is especially rich in angular, buff-colored pumice bombs 

to 70 cm and is likely derived from unit p29. No hydrothermally altered blocks were seen in 

this deposit. Both deposits overlie unit p29

p29 Pyroclastic deposits (1929)—Composition: 58.7–60.6% SiO2; 8.1–9.3% total alkalis (n=5). 

Porphyritic augite-olivine trachyte and latite fall, surge, and pyroclastic-flow deposits from 

1929 eruption. Strongly directed to south-southeast, thinning over short distances in other 

directions. Mapped contacts are approximate and show extent of deposit that thickly mantles 

and obscures underlying morphology. A significant portion of tephra was deposited offshore. 

No 1929 deposits were identified on north flank of island. Cut by multiple, several-meterdeep gullies that expose its internal stratigraphy. Fall deposits, at least 7 m thick and 2.5 

km from the south peak crater, consist of buff to gray trachyte pumice, capped by dark gray 

latite scoria. Within 1929 craters, interbedded with units t29 and l29. Overlies unit lsp; 

overlain by units d29 and ssp

t29 Trachyte lava flows (1929)—Composition: 60.5–61.0% SiO2, 9.2% total alkalis (n=4). 

Porphyritic augite-olivine trachyte aa lava found as flow effused from crater 7 and within 

walls of crater 6 of 1929 eruption. Phenocrysts: 10% plagioclase, 1–3 mm across; 3–5% 

augite, generally 1 mm across; 1–3% olivine, generally 1 mm across. Clots that include all 

phenocryst phases range to 5 mm. Emplaced during first half of 1929 eruption on basis of 

compositional similarity to earliest fall deposits of unit p29

l29 Latite lava flows (1929)—Compositon: 57.8–58.3% SiO2, 8.1–8.4% total alkalis (n=8). 

Porphyritic augite-olivine latite. Small-volume, thin blocky lava flows that effused from 

lower craters (1, 4, 5) of the 1929 fissure eruption. Lower two flows are already heavily 

vegetated. Largest flow, from crater 4, exhibits flow ridges that range to several meters in 

thickness. Internal stratigraphy, exposed only in lowest flow that has been eroded at the 

coastline, consists of a massive interior above a vesicular rubbly breccia and below a thicker 

upper flow breccia made of blocks as large as 1 m in diameter. Flow interior is light gray and 

breccias are dark gray and variably reddish. Phenocrysts: 7% plagioclase, generally 1–2 mm 

across, rarely to 3 mm; 3% augite, 1–3 mm across; 1% olivine, ≤1 mm across. Emplaced 

during second half of 1929 eruption on basis of compositional similarity to latest fall deposits of unit p29

asp Andesite of south peak (late Holocene(?))—Composition: 61.7–62.7% SiO2

, 5.7–5.8% total 

alkalis (n=6). Augite-hypersthene andesite lava that partially caps south peak crater rim and 

crops out due west and southeast of south peak. Lava is dark gray and glassy with conchoidal fracture but lighter gray on weathered surface than other young flows. These flows are 

unusual for Mount Gareloi because they follow a medium-K, calc-alkaline trend. They have 

highest silica content of any analyzed Mount Gareloi rocks. Phenocrysts: 20–25% plagioclase, generally as large as 1 mm (rarely to 3 mm); 2% augite, as large as 1 mm across; 

2% hypersthene, as large as 0.5 mm across. One of the few Mount Gareloi units to contain 

orthopyroxene. Flows have well-developed, ~15-m-high levees; interior of flows is ~10 m 

thick. West flow is morphologically young and is not thickly covered by pyroclastic debris. 

Lowest kilometer of length is covered in vegetation similar to that seen on lower-elevation 

1929 lava flows. Tight compositional clustering suggests emplacement during a single eruption. Overlies unit lsp; overlain by unit psp

lsp Lavas of south peak (Holocene)—Composition: 52.1–56.1% SiO2

; 5.2–7.5% total alkalis 

(n=10). Nearly aphyric to porphyritic augite-olivine shoshonite to latite. Multiple young 
16

undivided lava flows erupted from south peak. Lavas contain as much as 25% plagioclase 

phenocrysts and as much as several percent augite plus olivine. Where possible on map, 

internal flow boundaries divide individual flow lobes. These lavas occur over a sector of 

the island from the northwest to the south. On island’s southwest flank, they clearly flow 

unconformably over deeply eroded shoshonite lavas of the early edifice. On west, they cover 

a broad expanse and merge to northwest with young lavas from north summit. To south, they 

probably underlie most deposits from 1929 eruption. On the basis of intact flow morphology, they are likely Holocene in age. Two lava samples from unit lsp that crop out beneath 

unit dv on the southeast coast yield 40Ar/39Ar ages of 0.3±1.4 and 1.8±4.5 ka, suggesting 

they are both latest Holocene in age. Overlies units lnpu and see; overlain by units asp, 

psp, and p29

HOLOCENE VOLCANIC ROCKS—NORTH PEAK

l87 Lava flow (1987)—Thin finger of lava(?) that reaches from east rim of north peak crater down 

to 3,200 ft asl; consistent with description of flow observed on September 4, 1987, by a 

commercial pilot, who described a narrow flow-like feature on east flank that extended from 

north crater rim at 5,000 ft asl down to at least 3,500 ft, where it became obscured by clouds. 

Steam rose hundreds of feet above flow and crater was vigorously steaming (Smithsonian, 

1987)

pnp Agglutinate, spatter, and pyroclastic deposits of north peak (Holocene; historic)—Primary 

pyroclastic deposits that cover upper flanks of north peak. From summit to approximately 

100 ft below, this unit consists of fine gray mud, probably phreatic in origin, scattered with 

prismatically jointed bombs as large as 1 m in diameter, some hydrothermally altered. Small 

scarps expose layered lapilli and ash-fall deposits beneath this mantle. At slightly lower 

elevations on north flank of north peak, this unit appears to consist of variably oxidized fall 

and agglutinate deposits, though steep terrain prevented direct observation. Also includes 

interbedded, variably altered lava flows and pyroclastic deposits exposed in walls of north 

peak crater and along a scarp high on north peak’s west flank. Lowest extent on northwest 

flank consists of poorly sorted sand and gravel deposits

lnp Sheet lava field of north peak (Holocene)—Composition: 52% SiO2; 5.6% total alkalis 

(n=1). Augite-olivine shoshonite with seriate texture of continuously varying crystal sizes. 

Morphologically smooth flow field of gray lavas that are 1–2 m thick, have smooth surfaces, 

and are filled with vuggy vesicles to 4 mm in diameter. Phenocrysts: 20% plagioclase, as 

large as 2 mm across; 4% augite, as large as 2 mm across; 3% olivine, as large as 1 mm 

across. Lavas in this unit bank up against, and are younger than, lavas of unit snw and 

occupy a discrete region on upper northwest flank below north peak’s summit. Overlies unit 

lnpu; overlain by 1-m-thick section of soil and tephra

pef Lithic pyroclastic-flow deposits of east flank (Holocene)—Package of thin (<2 m(?)), lithicrich pyroclastic-flow and interbedded surge deposits on east flank of north peak. Forms distinctive, smooth, pavement-like surface on top of unit lnpu lava flows. Contains dense but 

likely primary cauliflower-shaped bombs of plagioclase-pyroxene-olivine basalt to 1.3 m 

and scoria clasts commonly to 25 cm. May be more extensive than mapped. Overlies units 

lnpu and see. Age relative to units sef, snw, and lnp unknown

snw Shoshonite lavas of northwest flank (late Holocene)—Composition: 53.4–53.5% SiO2

; 

1.95–2.0% K2O (n=5). Two prominent porphyritic olivine-augite shoshonite lava flows 

that erupted from north peak. Lava flows are medium gray, massive, and >4 m thick and 

have prominent levees. They are likely coeval, because they are compositionally similar. 

Flows fall just below main Mount Gareloi trend on silica-K2O variation diagram (fig. 10B). 

Phenocrysts: 20–25% plagioclase, 1–3 mm across; 5% augite, 1–2 mm across; 2% olivine, 

as large as 1 mm across. Clots of olivine and augite are common and as much as several millimeters in size. Top of northernmost flow is covered by 1 m of scoria-rich sand and gravel. 

One bomb on flow is 2 m in length. Both flows are topped by pods of red-brown soil as 

thick as 15 cm. In one locality, south flow is covered by 90 cm of interlayered soil and ash. 

Upper reaches of the two flows are covered by unit pnp; they overlie unit lnpu

sef Shoshonite lavas of east flank (late Holocene)—Composition: 52.8–52.9% SiO2

; 5.8–5.9% 

total alkalis (n=3). Porphyritic augite-olivine shoshonite. Two leveed lava flows reach from 
17

north peak’s summit to coastline, forming prominent lava deltas. Phenocrysts: 12–15% 

plagioclase, generally to 3 mm across; 1–5% augite, to 2 mm across; ~1% olivine, to 1 mm 

across. On the basis of morphology and composition, flows appear to be similar in age, 

though northern of two is slightly richer in augite phenocrysts. Flows overlie Holocene lavas 

of unit lnpu that fill sector-collapse scarp on east flank

lnpu Lavas of north peak, undivided (Holocene)—Composition: 52–54.4% SiO2; 5.0–6.7% total 

alkalis (n=11). Porphyritic augite-olivine shoshonites. Lavas fall into two groups: some are 

plagioclase rich and contain as much as 25% plagioclase and only small (~1%) amounts of 

augite and olivine; others, also with abundant plagioclase, have as much as several percent augite plus olivine. Contacts between individual flows (internal flow boundaries) are 

mapped where possible. Flows are extensive on northwest flank, cover an apparent sector 

collapse scar on east flank, and flowed over shoshonite of the early edifice on volcano’s 

southeast and northeast flanks. Northeast flow field of unit lnpu appears to have erupted 

from a flank vent at approximately 2,600 ft asl. One arm of flow field reached the coastline, 

forming a lava delta 850 m wide and <30 m high. Lava flow field is covered by >1 m of 

soil, and its morphology is subdued accordingly. Older than units snw, sef, and lnp, with 

the possible exception of the northeast flow field. All lavas of unit lnpu are unmodified by 

glaciation or extensive erosion. One flow on east flank yielded an 40Ar/39Ar age of 6.1±1.0 

ka. Overlain by some lavas of unit lsp

PLEISTOCENE VOLCANIC ROCKS

see Shoshonite of early edifice (Pleistocene(?))—Composition: 51.9–54.4% SiO2; 4.9–6.2% total 

alkalis (n=9). Porphyritic augite-olivine shoshonites and basaltic andesites. Eroded to deeply 

eroded lava flows. Limited sampling coverage means that unit is likely more compositionally or geologically varied than described here. Most lavas from this unit lie along a high-K 

trend and fall squarely in shoshonite field (fig. 10). Three lavas on the low-K trend were 

analyzed: 03GRBB52, 03GRBB57, and 03GRGM14 (appendix 1). First two are on southwest flank, third is on north flank. They are light-gray, blocky to massive lavas that compositionally lie on line between basaltic andesite and shoshonite (fig. 10A). Except for units 

ssf and anf, this unit forms most of undivided Pleistocene edifice. Steep, 650-m-tall cliff 

on the southwest coast strikes N. 45° W., parallel to plate motion, and may be normal fault. 

Unit is undated and assumed to be Pleistocene on the basis of the high degree of erosion and 

unconformity between it and overlying units lsp and lnpu. A single 14C date of a soil horizon 

at least 3 m above bedrock surface yields 1,090±70 yr B.P.

ssf Shoshonite dome of south flank (Pleistocene)—Composition: 51.1% SiO2, 5.3% total alkalis 

(n=1). Augite-olivine shoshonite. Ash-fall-covered, rounded lava mound on south flank at 

1,675 ft asl. Compositionally similar to other shoshonites of the early edifice but distinctive 

in its highly vesicular texture, with large, wormy vesicles to 1 cm in diameter and light gray 

color. Some outcrops are columnar jointed. Phenocrysts: 15% plagioclase, 1–2 mm across; 

5% augite, 1–3 mm across; 3% olivine, to 1 mm across. It is older than and surrounded by 

lavas of unit lsp

anf Andesite of north flank (Pleistocene)—Composition: 61.2% SiO2

, 6.3% total alkalis (n=1). 

Porphyritic hornblende-augite-hypersthene andesite. Eroded lava dome or flow remnant at 

700 ft asl on north flank. Outcrop is knob 200 m in diameter. Phenocrysts: 25% plagioclase, 

as large as 3 mm across; 20% augite, as large as 1 mm across; 5% hornblende, as large as 

2 mm across; 2% hypersthene, as large as 1 mm across; trace amounts of olivine. Lava is 

distinguished from other rocks of the older edifice by its lower-K composition, by its high 

phenocryst content (~50%), and by its phase assemblage of plagioclase, augite, amphibole, 

hypersthene, and olivine, in decreasing abundance. Only amphibole-bearing lava sampled 

from older edifice. Exposures are deuterically altered, with some mineralization in vugs. 

Alteration and subdued surface morphology suggest a Pleistocene age
18

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Appendix 1. Whole-rock Compositions 

of Rocks from Mount Gareloi

The whole-rock data, with accompanying sample locations 

and descriptions, is tabulated in an Excel spreadsheet file at 

http://pubs.usgs.gov/sim/3145 and is also included as a table in 

the ArcGIS geodatabase file.