Geologic Reconnaissance 

of Gareloi Island 

Aleutian Islands, Alaska

By ROBERT R. COATS

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-J

Prepared in cooperation with the Office, 

Chief of Engineers, U. S. Army

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1959
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

The U. S. Geological Survey Library has cataloged this publication as follows:

Coats, Robert Roy, 1910-

Geologic reconnaissance of Gareloi Island, Aleutian 

Islands, Alaska. Washington, U. S. Govt. Print. Off., 1959.

v, 249-256 p. illus., 1 fold. col. map (in pocket) 25 cm. (U. S. 

Geological Survey. Bulletin 1028 J. Investigations of Alaskan volcanoes)

"Prepared in cooperation with the Office, Chief of Engineers, U. S. 

Army." Bibliography: p. 256.

1. Geology Aleutian Islands. 2. Volcanoes Aleutian Islands. i. 

Title: Gareloi Island, Aleutian Islands, Alaska. (Series: U. S. 

Geological Survey. Bulletin 1028-J. Series: U. S. Geological Survey. Investigations of Alaskan volcanoes)

557.984

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C.
PEEFACE

In October 1945 the War Department (now Department of the 

Army) requested the Geological Survey to undertake a program of 

volcano investigations in the Aleutian Islands-Alaska Peninsula area. 

The first field studies, under general direction of G. D. Robinson, were 

begun as soon as weather permitted in the spring of 1946. Part of 

the results of the first year's field, laboratory, and library work were 

assembled as two administrative reports. Part of the data was published in 1950 in Geological Survey Bulletin 974-B, "Volcanic Activity in the Aleutian Arc," by Kobert R. Coats. The remainder of the 

data has been revised for publication in Bulletin 1028, which also 

includes material not included in the earlier administrative reports.

The geologic and geophysical investigations covered by this report 

were reconnaissance. The factual information presented is believed 

to be accurate, but many of the tentative interpretations and conclusions will be modified as the investigations continue and knowledge 

grows.

The investigations of 1946 were supported almost entirely by the 

Military Intelligence Division of the Office, Chief of Engineers, U. S. 

Army. The Geological Survey is indebted to the Office, Chief of 

Engineers, for its early recognition of the value of geologic studies 

in the Aleutian region, which made this report possible, and for its 

continuing support.

in

CONTENTS

	Page 

Preface__ _ ______________________________________________________ in

Abstract____________________________________ 249

Introduction ____ ______________ 249

Geology______________________________________ 250

Older volcanic rocks______-__--__---___-__-___-___-_-_____---__ 250

Younger volcanic rocks_______________________________________ 251

Historic activity_________________________________________________ 251

Early activity_______________________________________________ 251

Eruption in 1929. ______________ ____________ 251

Literature cited-____________________________________________ 256

ILLUSTRATIONS

Pag* 

PLATE 33. Preliminary geologic map of Gareloi Island____ _._ In pocket

34. Aerial view of Gareloi Island from the southeast, _______ Facing 250

35. Aerial photograph showing dense cloud of acid steam from

fumaroles in active crater of Mount Gareloi__-___-___ Facing 251

INVESTIGATIONS OF ALASKAN VOLCANOES

By ROBERT R. COATS

ABSTRACT

Gareloi Island is the northernmost island and Mount Gareloi is the only active 

volcano in the Delarof group, which is part of the Western Aleutian Islands. It 

is a relatively simple composite active volcano 5,160 feet high, and with minimum 

and maximum diameters of 5 and 6 miles, respectively: The volcanic activity on 

Gareloi Island occurred in two periods, separated hy a long period of erosion 

and apparent volcanic quiescence. The younger ro'cks are substantially the 

same in composition as the older. The older rocks are a sequence of olivine 

basalt flows and scoria; the cone constructed of these rocks was deeply eroded 

and the ridges truncated by high sea cliffs. The younger lava flows are olivine 

basalt of variable texture, but generally porphyritic. Reports of activity date 

from 1760; the most violent eruption of which there is record occurred in 1929. 

The eruption was initially phreatic; a number of small craters were formed and 

from some of them glassy pumiceous andesite tuff was erupted, followed by 

blocky, highly viscous andesite lava flows. A small elongate fln of lava was extruded along the feeding fissure in one place. Small amounts of fumarolic material deposited along the fissure Include atacamite, paratacamite, and.hematite. Active emission of sulfur dioxide continues in the northern summit crater^

INTRODUCTION

Gareloi Island, one of the smaller of the western Aleutian Islands, 

is topographically a relatively simple composite cone. The island is 

roughly circular in outline, averaging 5-6 miles in diameter; the 

highest point is at lat. 51°7i/2 ' N., long. lY8°7i/2 ' W. A slight ellipticity reflects the presence of two summits of nearly the same 

height (pi. 34). The more northerly summit is on the rim of the 

active crater, which is usually filled with a cloud of fumarolic steam 

heavily charged with sulfur dioxide. Gareloi Island is remarkable 

chiefly 'as a volcano in which activity was resumed at a single center 

after a long period of erosion and apparent quiescence, without substantial change in composition of the lava. It is also of some interest as the site of one of the most recent recorded eruptions in the 

western Aleutians.

249
'250 INVESTIGATIONS OF ALASKAN VOLCANOES

The investigations, the results of which are contained in this report were made between the 12th and 16th of August 1946, by the 

writer, assisted by Will F. Thompson, Jr. The Geological Survey 

party was transported to and from the island by the Army Transport 

Service ship BSP 788, then commanded by Robert Seifert. We are 

indebted to him, and to numerous officers and men of the U. S. Army, 

Navy, and Army Air Force, for much 'assistance and many courtesies 

received during our work in the Aleutians.

GEOLOGY

OLDER VOLCANIC ROCKS

The older volcanic rocks of Gareloi Island comprise a sequence of 

olivine basalt flows and beds of scoria, exposed in three sectors of the 

mountain: the southern, the eastern, and the northeastern. These 

sectors are interpreted as erosional remnants, or planezes, of an 

earlier composite cone, as the contact between the older volcanic rocks 

and the younger series appears to be an unconformity. Sector faulting, as the principal cause of the present distribution of the two series 

of volcanic rocks is, however, a possibility.

The older volcanic rocks differ in average thickness and composition of the flows. In the northeastern sector the flows exposed in the 

high cliffs that bound the planeze laterally are relatively thin (4-8 

feet) and are interbedded with rubble and coarse scoriae, both reddened. In the eastern sector the oldest flows those exposed, in the 

sea cliffs appear to be relatively thick (10-20 feet) light-colored 

holocrystalline basalt, in part rich in augite phenocrysts. The difference in thickness may be more apparent than real: parts of flows 

on steeper slopes may be thinner and less crystalline than parts on 

less steep slopes. Some flows seem to be flank, rather than terminal 

eruptions. Whether there is a corresponding difference in composition between the rocks of the eastern and northeastern areas is not 

known, because the northeast sector has not been sampled. The dips 

of the older lava flows are not much different from those of the 

younger lava flows.

The duration of the erosion interval that followed the extrusion of 

the older volcanic rocks is unknown; no fossils were found. The 

apparently U-shaped cross sections of the upper parts of some valleys 

in the older volcanic rocks may hint that Gareloi was quiescent during 

at least part of the Pleistocene, to permit the growth of glaciers capable 

of carving U-shaped canyons of appreciable size. No glacial striations 

were observed, but, as shown on the map, there are small- masses of 

glacial ice on either side of the saddle between the northern and southern cones of the present volcano.
AERIAL VIEW OF GARELOI ISLAND

Low-oblique aerial view of Gareloi Island from the southeast, showing the fissure and craters of the eruption of 19

lava flows. Photograph by U. S. Navy. 
GEOI.OC.ICAr, SVKVF.Y 

ACTIVE CRATER OF MOUNT GAREEOI

Aerial photograph showing dense cloud of acid steam from fumarolcs in active crater of Mount Gareloi. Amoun

Photograph by U. S. Army Air Force, 1942.
GEOLOGIC RECONNAISSANCE OF GARELOI ISLAND 251 

YOUNGER VOLCANIC ROCKS

The younger volcanic rocks are not extensively dissected by erosion. 

The sea cliffs at their margins are nowhere more than about 100 feet 

high, in contrast to the higher cliffs formed on the older volcanic 

rocks in many places. Locally, sea cliffs in the older volcanic rocks 

are veneered and protected by aprons of younger lava flows; such relations may be seen near the northwestern edge of the northeastern 

planeze.

In many places the exact boundary between the older and younger 

volcanic sequences has been obscured by cinders and scoria the primary deposits from Recent eruptions and the reworked pyroclastic 

materials of Recent and older eruptions, mingled with debris from 

 eroded flows.

Like the older flows, the younger flows are olivine basalt, differing 

from place to place in content of augite (generally pale green in thin 

section) and of olivine. Some rocks are dominantly fine grained and 

have sparse small phenocrysts; others are seriate to porphyritic, with 

as much as 90 percent of the total rock volume consisting of megascopically recognizable grains. It is not known whether these differences represent differences in rates of cooling in a uniform liquid or 

enrichment of some parts of the magma body in early-formed crystals 

of augite, olivine, and plagioclase, such as has been demonstrated by 

Macdonald (1944) for one of the eruptions of Kilauea. In any event, 

the amount of such enrichment has not been great enough to permit 

the formation of new mineral phases, and the total range of variation 

in the rocks collected probably is no greater than that from basalt to 

andesite. There was little or no variety in the mode of eruption; the 

cones of Gareloi, both old and new, are typical composite cones and all 

the rocks are andesitic in habit.

HISTORIC ACTIVITY 

EARLY ACTIVITY

The accounts of the activity of Mount Gareloi are contained in four 

reports (Grewingk, 1850; Beeker, 1898; Finch, 1931; and T. A. Jaggar, 

written communication, 1927). Mount Gareloi is said to have been 

"active" in 1760, "spitting fire" in 1790 and 1791, smoking and emitting 

lava in 1792, smoking in 1829, "active" in 1873, "showed fire" in 1922, 

and was fuming in 1927. '

ERUPTION IN 1929

The most violent eruption of recent time apparently was that of 

1929 (Finch says 1930; possibly the activity continued into that year).
252 INVESTIGATIONS OF ALASKAN VOLCANOES

Concerning this the writer has been fortunate to receive a report from 

Simeon Oliver, former Alaska Native Service teacher at Atka, who 

obtained his information from the oldest resident of Atka, William 

H. Dirks, Sr. Mr. Oliver writes: "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 which to this day still remain covered." Mr. Oliver inserted 

in his letter a note to the effect that the eruption took place on the 

northeast side, but, as will be seen later, this is obviously in error the 

only apparent error in an otherwise accurate description, as far as 

recent observations are able to check this account.

The succession of events in that eruption is fairly clear from observations in the field, and may, in large part, be read from the geologic 

map (pi. 33). The recency of the eruption permits a more elaborate 

account than can be given for earlier eruptions in the history of the 

island.

The earliest event was the blasting of a series of explosion craters 

alined along a fissure, or possibly several linked fissures, the surface 

trace of which trends nearly south near the summit of the southern 

peak of Gareloi and more southeasterly at lower altitudes. The 

reason for the change in azimuth of the trace of the fissure is not 

known; the lack of information about the dip of the fissure makes 

it difficult to substantiate any hypothesis of the dynamics of the formation of the fracture. Ode (1957, p. 567-576) has offered an explanation of a dike swarm (that about West Spanish Peak, Colorado) 

that show marked departures from radial directions. Ode calculates 

that if the dikes were formed in a homogeneous medium, the presence 

of a regional-stress field superposed on the local-stress field would 

account for the divergencies in strike of the dikes. The Spanish Peak 

dikes extend over much greater lengths than the fissure on Gareloi, 

and are correspondingly more likely to be independent of local variations in load due to topographic irregularities. The radial arrangement, as pointed out by Ode, is the commonest; this generalization 

probably reflects the common production of fissures by pressure from 

a magma chamber beneath the center of the cone. Intuitively, it 

seems probable that dikes originating in a magma chamber at a relatively high level, but ;not centrally located, would not be arranged in 

radial fashion. Further speculation seems pointless.

Plate 33 shows 13 craters, which have been distinguished by rom.au 

numerals, beginning with Crater I, nearest the beach, and ending with 

Crater XIII, the highest, largest, and oldest. The map shows the 

approximate limit of the pyroclastic debris derived from this eruption; 

the area included is easily recognizable by the light color and lack of
GEOLOGIC RECONNAISSANCE OF GARELOI ISLAND 253

vegetation. The distribution of the ash blanket is symmetrical with 

respect to the higher craters, suggesting that the debris is largely 

from them; the lowermost craters are near the border of the ash 

blanket. The milky blue-green water of the small lake in Crater X 

suggests continuing activity of acid fumaroles in this crater.

Some evidence is available concerning the relative periods at which 

the several craters ceased to be active. Essential scoria ejected from 

Crater VII appears to be overlain by accidental material from Crater 

VIII.1 The original pit of Crater VII seems to have been higher up 

the slope than the present, slightly younger pit, which is at the base 

of a flow of extremely rough block lava 250 feet wide and 50 feet thick. 

The lava apparently rose along a part of the fissure that connects the 

craters and cuts the lowest point on the Crater VII rim; it was too 

viscous to fill the crater. Although this flow extends down the mountain for some distance, the feeding fissure may have been almost 

equally long.

The walls of Crater VI are made up of black to dark reddish brown 

basaltic lava and scoria. Just above and east of the crater, a steepsided, rough-surfaced ridge over 40 feet high (indicated by a small 

closed contour line on the map) was formed by the extrusion of black,, 

nearly solid lava through the flow discussed in the last paragraph. 

The steeper side of the ridge is on the eastern, or downhill, side,; 

on the west, slabby spalls are partly detached and crumbling away. 

Part of the lava emitted above Crater VI flowed downhill into this, 

steep-walled crater. The lower side of the cone surrounding Crater 

VI has slumped as a result of the extrusion of a flow of reddened aa 

lava from the base of the cone; some of the lava flowed into Crater Vr 

and is now partly buried by material washed into the crater.

The lowest visible part of the walls of Crater V is made up of lightgray lava and layers of reddened scoria (probably belonging to the: 

sequence of younger volcanic rocks) earlier than the material erupted 

in 1929; these rocks are overlain by a cone of light-buff material 

probably essential glassy pumiceous material of the 1929 eruption., 

This pumiceous material is overlain by coarser blocks apparently derived from the light-colored lava flows beneath. The lower side of the 

pyroclastic cone of Crater V was undermined by the extrusion of lava; 

the present rim of the crater is undeformed, and the highest on the 

downhill side, suggesting that repair was effected, by a final pyroclastic 

eruption. The rim consists of reddened and black essential scoria^, 

with some blocks.

1 Essen tal and accidental are here used in the sense of Wentworth and Williams (1982, 

p. 47).
254 INVESTIGATIONS OF ALASKAN VOLCANOES

Between Crater V and Crater IV, a few transverse fissures still 

emit some odorless steam, which has a maximum temperature of 

144°F.

The cone of Crater IV was largely destroyed by slumping resulting 

from the undermining effect of the extrusion of the largest of the four 

lava flows produced by the 1929 eruption.

Some f umarolic products were collected along the line of the eruptive fissure of the 1929 eruption, and analyzed in the laboratories of 

the Geological Survey.

A sample of green copper sublimate from the scoriaceous surface of 

the flow southeast of Crater IV was determined by R. L. Smith to be 

a mixture of atacamite and paratacamite; the identification of the 

minerals was confirmed by J. .M. Axelrod by examination of the 

X-ray patterns and by comparison with patterns of other volcanic 

sublimate copper chlorides. Semiquantitative spectrographic analysis, 

of the same material, by K. J. Murata, gave the following results:

Percent

Pb__________________________________________ O.X 

V, Mg, Mn, Fe_____________________________________ .OX 

Mo, Or, Ba_j__________________________________ .OCX

Elements not found: Ag, Au, Pt, Bi, Zn, Cd> W, In, Ge, Ga, Y, Yb, 

La, B, Be, Zr, Nb, Ta, Sr, U, Th, Tl.

A sample of fine-grained specular hematite lining a fissure between 

Craters II and III gave the following spectrographic results (determined by K. J. Murata):

Percent 

Ti_________________________________________ O.X

Mg, Sn, Mn, Cr, Cu______________________________ .OX 

Ba, V, Co, Ni___________________________________ .OOX

Elements looked for, but not found, were similar to those looked for, 

but not found, in the copper chloride sublimate.

A well crystallized white alteration coating in the wall of a fissure 

in Crater II was reported by R. L. Smith to be a mixture of gypsum 

and an unidentified clay mineral, perhaps allophane.

Crater III is a small explosion pit.

Crater II is somewhat larger than Crater III, and apparently 

erupted both essential and accidental material. The essential material 

of the 1929 eruption is highly scoriaceous or vesicular, dark colored 

where fresh and generally partly glassy. The material erupted shows 

a considerable range of mineral composition: some of the lava flows 

contain sparse hypersthene phenocrysts; others have occasional small 

phenocrysts of brown basaltic hornblende; the buff to tan pumiceous 

tuff is largely glass with an index of refraction of 1.522-1.525, corresponding to a composition of andesite. The composition of the
GEOLOGIC RECONNAISSANCE OF GARELOI ISLAND 255

plagioclase phenocrysts, estimated from the average index, is about 

An48. The principal ferromagnesian mineral is green augite. The 

older lava flows, where exposed, are generally more compact, perhaps 

because they were more fluid than the extremely viscous lava of the 

1929 eruption. Blocks of gray columnar holocrystalline lava, on the 

rim and outer slopes of Crater II, resemble flows both in the older and 

younger sequences, but are much more compact than the material of 

any of the 1929 flows. A thin rind of bleached and reddened rock 

found on both columnar and cross joints indicates a period of fumarolic alteration after consolidation that is incompatible with the interpretation of an essential origin for this material.

Crater'1, the nearest to the shoreline, is now horseshoe shaped, the 

lower side having been carried away completely by a lava flow which 

must have nearly reached the sea. The flow is now exposed in a sea 

cliff, overlying a bed of coarse lapilli that may be the product of the 

interaction between the flow and the sea water. The lava flow was 

sufficiently fluid to subside, in the upper part of its course, below the 

highest level reached during the eruption, leaving levees banked 

against the walls formed by the breaching of the cone.

The buff essential pumice lapilli are the earliest and most silicic 

phase of the 1929 eruption. The lava extruded from the fissure at 

Crater VII had fewer phenocrysts of all kinds, especially augite and 

olivine, and was less crystalline than lava collected from the end of 

the stubby flow from Crater IV, which was erupted at an altitude 

about 1,200 feet lower. The obviously high viscosity of lava at both 

places, suggested by the minor topographic features of the flows, forbids the assumption that the differences between the two lava flows 

are caused by crystal settling in the lava after eruption. The lower 

flow appears not only to have more phenocrysts but also to have had, 

if anything, a somewhat lower viscosity, judging from the distance 

traveled on a substantially lower gradient. Had the observed difference between the rocks of the two flows been entirely due to crystallization of a uniform liquid, the more crystalline lower flow would also 

have been cooler and more viscous, and it would have flowed less 

readily. The evidence suggests, however, that the lava flows differed 

in crystal content before eruption, and that the difference was due to 

accumulation of crystals in the part of the magma body from which 

the lower flow was erupted. The small volumes of the flows suggest 

that they came from a relatively small body of eruptible magma; it 

seems probable that this was the volcanic conduit, at a relatively high 

level, and that the lower lava flow came from the lower part of the 

erupted portion of the magma body. The differentiation recalls that 

described by Macdonald (1944) in the 1840 eruption of Kilauea.
256 INVESTIGATIONS OF ALASKAN VOLCANOES

Sequence of events in the eruption of 1929. The sequence of events 

may be summarized as follows, recognizing that the evidence for relative ages, especially of the upper craters (seen only from a distance), 

is not very firm:

1. Phreatic eruption from Crater XIII. No definitely juvenile 

material in the early deposits in the western part of the debris-covered 

sector.

2. Further explosions opening Craters I-XII, probably nearly 

simultaneously, except that Crater VIII may be slightly older.

3. Explosive eruption of pumice lapilli from the lower craters.

4. Extrusion of very viscous andesitic magma, probably beginning 

.at Crater VII and progressively extending farther downslope. Eruption of bombs from Crater V continued while the lava flow southeastward of it was being extruded.

5. Fumarolic action, emission of sulphur dioxide at northern summit 

crater (still continuing; see pi. 35) and of gases depositing metallic 

oxides and halides at Crater IV.

6. Fumarolic action along line of fissure declines to emission of 

warm steam.

LITERATURE CITED

Becker, G. F., 1898, Reconaissance of the gold fields of southern Alaska, with 

some notes on general geology: U. S. Geol. Survey 18th An. Kept., pt. 3, p. 

1-86.

Finch, R. H., 1931, Notes from, the Aleutian Islands: Volcano Letter No. 357.

Grewingk, Constantin, 1850, Beitrag zur Kenntniss der orographischen und geognostischen Beschaffenheit der Nordwest Kiiste Amerikas, mit den anliegenden Inseln: Russ. K. min. Gesell Verh., St. Petersburg, 1848-49, p. 76-342.

Macdonald, G. A., 1944. The 1840 eruption and crystal differentiation in the 

. Kilauean magma column [Hawaii] : Am. Jour. Sci., v. 242, no. 4, p. 177-189.

Od6, Heliner, 1957, Mechanical analysis of the dike pattern of the Spanish Peaks 

area, Colorado: Geol. Soc. America Bull. v. 68, p. 567-576.

Wentworth, C. K. and Williams, Howel, 1932, The classification and terminology 

of the pyroclastic rocks: Natl. Research Council Bull. 89, p. 45, 47.