Reconnaissance Geology 

of Some Western Aleutian 

Islands, Alaska

^ By ROBERT R. COATS

GEOLOGICAL SURVEY BULLETIN 1028-E

Prepared in cooperation with the Office, 

Chief of Engineers, U. S. Army

'--A

-V

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956
UNITED STATES DEPARTMENT OF THE INTERIOR 

Fred A. Seaton, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D. C.
PREFACE

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. 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 Robert R. Coats. The remainder of the data has 

been revised for publication in Bulletin 1028.

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

-i

CONTENTS

, Page

"*" Preface__--__--_-_______-____________--_-__-_____-_-----_-___-- in

Abstract__ _ ____________________________________________________ 83

Introduction. _____________________________________________________ 83

^ Geography_ _____________________________________________________ 84

Location and size of area___________________-_______--___---____ 84

 >*-. Terrain types_____________________________________-_--________ 86

Low-lying islands________________________________________ 86

-t~ v Mountainous islands_-__-----_-_---_--_--_---------------__ 87

Vegetation__ ____________________________________-__^_-______ 88

* Geology______--_--__________-__-____-__-__-__-__-_-__---___-__-._ 88

n Granitic and metamorphic rock_____________.._-__-_____--__----_ 88

Granite (?)_____________________________-_-__..______-_-_--__-__ 89

^ Basaltic rocks, argillite, and graywacke-_________________________ 89

Finger Bay volcanics_____________________________-__-_-----._ 89

~_,-' Graywacke and graywacke breccia_____________________________ 90

Early Tertiary basaltic rocks__________________________________ 90

"^ Late Tertiary and Quaternary basaltic rocks._.________-____-__--_ 91

Tertiary and Quaternary sedimentary rocks_____________________ 92

"~* Structure.--_-__-__-_____-___-______--_-_--._-_____-_-_---____ 93

Pre-Tertiary_____________________________ 93

Tertiary and Quaternary,._________________________________ 94

Geologic history.______________________________________________ 95

Future volcanic activity_______-___________-_-__-__--________--_--_- 96

 -} Selected bibliography........______________________________________ 97

Index._____________________________-__-_____-_____-___.____ 99

Jk

,y ILLUSTRATIONS

Page 

v ( PLATE 17. Geologic reconnaissance map of some western Aleutian Islands,

Alaska _ _________________________________________ In pocket

 \ 18. A, Ilak Island from the northeast; B, Angular granitic boulders

on Amchitka Island __ ____-_______-_______--_-- __ _ Faces 92

19. Gareloi Island from the southwest. _ __________________ _ Faces 93

FIGURE 6. Map of the Alaska Peninsula and Aleutian Islands. 85
yic

-V V
INVESTIGATIONS OF ALASKAN VOLCANOES

RECONNAISSANCE GEOLOGY OF SOME WESTERN 

ALEUTIAN ISLANDS, ALASKA

By ROBERT R. COATS

ABSTRACT

The general features of the western Aleutian Islands, from Kasatochi to Attu, 

were studied in 1946 in connection with investigations of northern Adak Island, 

northern Kanaga Island, and other Aleutian islands.

The geology of the western Aleutian Islands is complex. Its deducible history 

begins in the Paleozoic (?) era, when a land made up of metamorphic rocks, intruded by granite, occupied the region. Successive additions of basaltic volcanic rocks and sediments of volcanic derivation were made during Paleozoic(?) 

and early Tertiary time, producing a highlands south of the present islands from 

about the longitude of western Tanaga Island to that of eastern Amchitka Island. Submergence of most of the land resulted in the carving of wide wave-cut 

benches that later emerged from the water. In late Tertiary time, glaciers 

accumulated in the highland and moved northward over the uplifted benches.

In very late Tertiary and in Quaternary time many basaltic shield volcanoes 

and composite cones were built on the wave-cut and locally glaciated remnants of 

the older volcanoes. The new volcanoes were alined on a great arc along the 

northern margin of the present belt of islands. The eruptions that produced 

many of these volcanoes began at a time when sea level was higher, relative to 

the land, than it is now; marine sediments were deposited offshore and are now 

exposed above sea level. A period of downfaulting followed, and most of the 

ancient highlands in the southern part of the belt disappeared beneath the Pacific 

Ocean. Many of the islands were again beveled by marine erosion. Later, the 

islands reemerged in about their present positions. In Quaternary time, new 

glaciers developed in the higher parts of the islands, except on the very active 

volcanoes and on some isolated peaks. Some of the volcanoes became extinct 

before and during the most recent period of glaciation, but many are still active.

Mild volcanic activity, such as eruptions of ash, can be expected to occur 

frequently in the northern part of the island belt; violent activity can be expected 

to occur infrequently.

INTRODUCTION

In the summer of 1946 several of the Aleutian islands west of 

Atka Island were mapped geologically. In addition to areas mapped 

in detail on Adak and Kanaga Islands, a number of the islands from 

Great Sitkin to eastern Attu were examined briefly on the ground, 

and others from Kasatochi westward were studied from the air. The 

purpose of this reconnaissance investigation was to choose areas for 

later, more detailed work, and to acquire general knowledge of the

83
84 INVESTIGATIONS OF ALASKAN VOLCANOES

place of the western Aleutian Islands in the volcanic history of the 

Aleutian arc.

In order to present a more nearly complete picture, this report 

summarizes not only the reconnaissance investigations made from 

Tanaga Island to Attu Island, but also the more detailed work done 

on Adak and Kanaga Islands. The map accompanying this report 

(pi. 17) is therefore generalized, even in the areas that have been 

covered by more detailed maps. The geologic units used in the 

mapping and description are also broad and generalized.

This report is based almost entirely on field work done during the 

summer of 1946 by Robert -R; Coats, assisted during most of the 

season by Will F. Thompson, Jr. Eighteen days were spent on 

Tanaga Island, 4 days on Gareloi Island, half a day each on Ogliuga, 

Attu, and Amatignak Islands, 4 days on Amchitka Island, and 2 

days each on Shemya and Great Sitkin Islands, in addition to time 

spent on Adak and Kanaga Islands. The Semichi Islands,-Buldir 

Island, and parts of Attu, Kiska, Rat, Amchitka, and the Delarof 

Islands were observed from the air. Most of the islands from Adak 

to Amchitka were also observed from shipboard. Aerial photographs 

of many of the islands, taken by both the U. S. Navy and U..S. Army, 

materially aided the study.

Acknowledgment is made of the cooperation of officers and men 

of the Army and Navy, especially Lt. Col. R. E. Ware, post engineer, 

Adak; Lt. Col. C. E. Johnson, port commander, Adak; Commander

S. 0. Cole, chief staff officer, Adak Sector; Lt. Col. Leonard Cox,

acting post commander, Shemya; Lt. Col. Ivan Cunning-ham, post 

engineer, Shemya; and Lt. John Rogers, Attu.

GEOGRAPHY

LOCATION AND SIZE OF AREA

The area discussed herein includes that part of the Aleutian chain 

from Kasatochi Island westward to Attu Island, and is referred to 

as the western Aleutian Islands (fig. 6). It extends from longitude

175°30' W. to 173° E., and from latitude 51°15' N. to slightly more

tKan 53° N. TKe length, of tliis part of the cKain is about 500 statute

miles, and the greatest width, which is in the vicinity of Semisopochnoi 

Island, is about 50 miles. The largest islands are Adak, 284 square

miles, and Attu, 318.

The areas of the 10 largest of the western Aleutian Islands (west 

of Atka Island) are given in the table on page 86. The measurements 

were made by Harold Drewes, using a planimeter on maps of a scale 

of 1:250,000.
RECONNAISSANCE 'GEOLOGY, WESTERN ALEUTIAN ISLANDS 85

377557 56 2
86 INVESTIGATIONS OP ALASKAN VOLCANOES

Square Square Square 

Island miles Island miles Island miles

Attu___________ 317.8 Amchitka. _______ 114.1 Agattu__1__ __ 77.5

Adak._______ 284.1 Kiska_________ 109.8 Great Sitkin____ 61.6

Tanaga__-_-_____ 184. 8 Semisopochnoi____ 78. 3 Kagalaska______ 45. 4

Kanaga____._ 134. 6

TERRAIN TYPES

Great topographic diversity exists among the western Aleutian 

Islands as a result of numerous differences in their geologic histories. 

Broadly, the islands are of two terrain types low-lying islands and 

mountainous islands. The low-lying islands are wave-cut platforms 

whose surfaces are nearly featureless. The relief of the mountainous 

islands is constructional, because of volcanism, or destructional, 

because of dissection of an upland surface, or a combination of the 

two. The topography of many areas of each terrain type has been 

modified by glaciation. Both terrain types are present on some of 

the larger islands.

LOW-LYINQ ISLANDS

The elevated wave-cut platforms that constitute the low-lying 

islands and parts of some of the mountainous islands are of three 

types. The platforms that emerged most recently retain the initial 

surface form, practically unmodified by fluvial erosion. Some of 

these surfaces are covered by thin layers of boulder gravel, perhaps 

of glacial derivation, that were reworked by marine abrasion before 

the emergence of the platforms. The second type of platform 

emerged earlier, and the topography has been modified to a greater

eXteilt by SUbaerial erOSion; distinctions may be made within this 

group on the basis of the relative age. The third type of platform haS 

been eroded by ice sheets, formed by the confluence of valley glaciers

tUt originated in nearby highlands, since the latest emergence of 

the island.

The platforms that emerged most recently retain the almost featureSurfaces produced by marine abrasion, and are either horizontal

or are gently tilted. A few StonlS head near the crenulated margins 

of the platforms, which they breach and descend in short, steep canyons. The steep margins of the platforms are fringed at their bottoms

by IiarrOW boulder beaches. A few large, round or elliptical lakes are

present on the platform surfaces. Islands of tklS type hlClUdG OgllUga,

ilak (pi. 18 A), Skagul, Kavalga, and Unalga in the Delarof Islands,

and Shemya and Oubeloi in the Semichi Islands.

The second type of platform emerged earlier than the erst and has

a more matUTe tOpOgraphy. Several successive marine benches are

commonly present, and the surface of'the highest one generally lOMS 

the principal interstream divide. The bench surfaces are poorly 

drained and many have lakes of different size and irregular shape.
-S RECONNAISSANCE GEOLOGY, WESTERN ALEUTIAN ISLANDS 87

Stream valleys are longer and drainage patterns better developed

-^ than on the platforms that emerged most recently. Drainage patterns 

are poorly adjusted to bedrock structure. An example of this type of

"*"*" physiography is eastern Amchitka Island, where glacial boulders occur

>r» beneath marine gravel at altitudes of more than 100 feet (pi. 18.6).

On the oldest platforms the interstream divides are narrow, and

lakes are few, large, and of simple outline, except where the encroachK ment of vegetation has made them irregular. Narrow alluvial plains 

are present along the larger streams. With increasing altitude the

~^* ancient sea cliffs between successive levels of marine abrasion have

£- progressively more gentle slopes. The wave-cut platforms of southwestern Kanaga Island and southern Tanaga Island are of this type.

^ Wave-cut platforms on several islands were glaciated during the

^ Pleistocene epoch. The effect of glaciation has been to emphasize 

the bedrock structure. Streams and chains of lakes lie along fault

~*~ r zones and weak beds, preferentially excavated by glacial erosion. 

The topography in general is minutely irregular or hummocky, and 

numerous bedrock knobs, small depressions, ponds, and lakes are 

present. Glacial erosion has generally beveled off the abrupt shoulders 

between uplands and sea cliffs. In places cliffs have been, cut in the 

slopes by postglacial marine erosion, but these cliffs are generally low. 

The platform of Ulak Island is of this type as is that of the extreme 

southwestern part of Adak Island and the adjacent part of Kanaga 

Island. The sources of the eroding glaciers were the highlands of 

Amatignak and Adak Islands, respectively.

MOUNTAINOUS ISLANDS

Two types of mountainous or hilly islands have been distinguished. 

The relief of one type, herein referred to as an oldland, is destructional, 

being due to differences in the resistance of the rocks to erosion, 

although uplift of the entire landmass was probably an accessory 

factor in producing the topography. The relief of the other type is 

constructional, owing its height to upbuilding by volcanic activity.

The oldland islands of the Aleutian chain comprise three groups: 

those that have been dissected by normal subaerial erosion but not by 

glaciers, those that formerly had small glaciers near the higher summits, and those that were extensively glaciated. The highlands of 

Alaid Island seem not to have been glaciated. A few relatively small 

glaciers were present in the highlands of western Amchitka Island. 

Islands extensively covered by glaciers include Attu, Kiska, Agattu 

(Sharp, 1946), Amatignak, Adak, and the southern Andreanof Islands 

east of Adak. These islands are characterized by knife-edge ridges 

at high altitudes, glacially striated and rounded ridges at lower 

altitudes, numerous rock-basin lakes, flat-floored passes, and wide, 

U-shaped valleys.
88 INVESTIGATIONS OR ALASKAN VOLCANOES

The highlands built by volcanic activity include those that have. 

been dissected by streams and. glaciers, and those, either lower in, 

altitude or of relatively recent construction, that have undergone 

little or no dissection and still retain their original conelike forms. 

The volcanic cones on Tanaga Island, except the high triple peak of 

Tanaga Volcano, have been dissected by glaciation; and all but 

small remnants of the southern flanks of the lower peaks along, the 

eastern part of the north coast have been removed by vigorous marine 

erosion. The two volcanoes on Adak Island Mount Moffett and 

Mount Adagdak have been somewhat dissected by subaerial erosion, 

and by marine truncation on the sides exposed to the sea; Mount 

Moffett has been locally glaciated. The low cone on Bobrof Island 

has been deeply dissected and large parts have been removed by 

marine erosion.

The cone on Gareloi Island (pi. 19) has been only slightly dissected, 

even though it supports two glaciers. The large island of Semisopochnoi consists of a great postglacial caldera and several young, 

undissected volcanic cones. The symmetrical cones on Segula and 

Kiska and Buldir Islands have not been deeply glaciated or dissected 

by streams.

VEGETATION

The low-lying areas of the western Aleutian Islands support a 

dense growth of grass and moss during the late summer. Narcissus, 

anemone, and lupine are abundant, and dense stands of wild rye occur 

near beaches. Heath plants associated with a very thick, spongy 

carpet of moss and lichens are common on poorly drained ridgetops. 

A species of prostrate willow grows on some exposed slopes. The 

principal plant associations have been described by Collins, Clark, 

and Walker (1945, p. 64-68).

GEOLOGY

Eight geologic units have been distinguished in the western Aleutian

Islands. Six of these have been mapped, and their distribution is

shown on plate 17. The units and their distribution are discussed 

below, but no attempt is made to describe separatel}* the geology of 

each island.

GRANITIC AND METAMORPHJC ROCK

§0fll6 SlUOOOlllJ rounded

platform of western Tanaga Island and on Ogllllga Island a^Ped!1 lO

be derived from the oldest POCks 111 the WBStem Aleutians. The 

contrast between these bOUldfiTS and tll6 roilgt angular boulders 

derived from the exposed bedrock of these islands is marked. The 

rounded boulders are of several rock types, including hornfels,
"»-* f

^ RECONNAISSANCE GEOLOGY, WESTERN ALEUTIAN ISLANDS 89

""* hornblende gneiss, slate, schist, granulite, granodiorite, and several

j varieties of granite, including biotite granite and hornblende granite;

The bedrock source of the boulders was not seen and may not be

~~*-*- exposed. Therefore, the relative ages of the granitic and metamorphic

^ rocks composing the boulders could not be definitely determined.

The granitic rocks are believed to be younger, however, because

 "* most of them lack directional textures, indicating that they have 

^ not been metamorphosed, although some are slightly gneissic. The 

boulders may represent rocks of many ages and more than one interval 

"^ :* of-erosion. The boulders of metamorphic rock have undergone 

more thorough recrystallization than is known in any rocks now 

exposed in the western Aleutians, suggesting that the parent rocks 

are older than the Finger Bay volcanics (p. 89), and that they may 

be of Paleozoic age. The presence of crystalline rocks clearly of 

continental type indicates that a large land mass must have existed

 ^ in the vicinity in Mesozoic or Paleozoic time.

T GRANITE (P)

"^ The U. S. Coast and Geodetic Survey (1944, p. 120), reports that 

Ilak Island is composed of granite. [In the summer of 195.2 landings 

were made on the island and the rock was found to be a light-colored 

granular rock, resembling the quartz diorite that forms intrusions

__j in many of the islands.]

^ BASALTIC ROCKS, ARGILLITE, AND GRAYWACKE

A sequence of dark-green to black basalt flows and basaltic tuff, 

gray hard argillite, arid gray-green coarse graywacke, containing small

-V lenses of brown chert and brown limestone, was seen on Attu and 

__^ Shemya Islands. The rocks on Attu, described by Capps (1934, 

p. 151), evidently are part of this sequence. Most of the rocks have 

A been intensely sheared and fractured.

, The age of the rocks is not known. They have been found in

contact only with early Tertiary volcanic rocks which intrude them.

\ Because of their greater degree of shearing and deformation, they

are tentatively considered older than the Finger Bay volcanics and

^ the graywacke and graywacke breccia sequence of Amatignak Island

-4 (see b.elow).

j FINGER BAY VOLCANICS

A sequence of black, dark-gray, purplish-gray, greenish-gray, and 

^ grayish yellow green volcanic rocks crops out along the north shore 

i^ of Finger Bay on Adak Island and has been named Finger Bay volcanics. These rocks, which underlie most of Adak Island, consist of 

f basaltic flows, tuff-breccia, agglomerate, and tuff, intruded by large 

> masses of gabbro and small masses of rhyolite. The volcanic rocks
90 INVESTIGATIONS OF ALASKAN VOLCANOES

and many of the intrusive masses included with them have been 

greatly deformed and hydrothermally altered. Similar rocks are 

present on southeastern Great Sitkin Island, and they probably compose most of the Andreanof Islands east of Adak. The rocks have 

been studied on Great Sitkin and Adak Islands and are discussed by 

Simons and Mathewson (1955) and Coats (1956a).

The deformed and altered Finger Bay volcanics were correlated, 

on lithologic grounds, with volcanic rocks of Mesozoic age on .the ^ 

Alaska Peninsula (Simons and Mathewson, 1955). Because a late 

Paleozoic plant has been identified in the Finger Bay volcanics '*" ~ 

exposed in an isolated outcrop on Adak Island, these rocks are now -v 

regarded as Paleozoic(?) in age (Coats, 1956a).

GRAYWACKE AND QRAYWACKE BRECCIA

Beds of grayish-green graywacke and graywacke breccia, containing

subordinate mudstone, were seen on Amatignak Island. They are '* -* 

exposed in sea cliffs along a small bay on the northeast side of the 

island, and between the bay and the highest point of the island. The 

rocks are composed of fragments of chloritized, albitized, and epi- t 

dotized basalt. The rocks are well bedded and well indurated. They 

are intruded by sills of medium-grained granophyric, partly uralitized 

gabbro, as much as several hundred feet thick.

The relative age of this rock sequence has not been definitely determined, as its contact with another sequence has not been seen. All 

fragments appear to be those of basaltic rocks; no metamorphic or *-. 

granitic fragments are present. The sequence could be the sedimentary equivalent of the Finger Bay volcanics, or it might be regarded as consisting of material derived from Finger Bay volcanics r.

that were eroded and deposited in shallow water. Sediments that

are somewhat similar to these, except that they contain a great

proportion of chert, occur on Agattu Island (Sharp, 1946, p. 193-194). *

EARLY TERTIARY BASALTIC ROCKS >

Volcanic rocks of probable early Tertiary age have been examined 

on Tanaga, Ogliuga, Amchitka, and Shemya Islands. Rock types

include basalt and hornblende toasalc tuff, tyuffaceous aggiomei-sxtt;, -^=,--

tuff-breccia, agglomerate tuff, flows, and dikes. Many of thfi frag- ^ 

ments in the tuff-breccia are large, some as much as i feet in length.

Tla-<3 flows o/ro, gonorn-lly tWi 11 t».n cl Jir. r>la.Gos a/l-o TT1 tcrkecl fled. With SCOriR- - On eastern Amchitka Island these volcanic rocks are intruded by a ,;.

stock of quartz diorite.

The early Tertiary volcanic rocks include five masses of andesite V' 

porphyry near Zeto Point on the east coast of northern Adak Island w 

(Coats, 1956a), and a similar mass of hornblende andesite porphyry on s 

Shemya Island. \
^ RECONNAISSANCE GEOLOGY, WESTERN ALEUTIAN ISLANDS 91

Evidence for age determination is inconclusive. Because the rocks

, are much less altered than the Finger Bay volcanics, are but slightly

deformed the greatest measured secondary dips being only 10° to

-*- >- 15° and have been glaciated, they are believed to be of Tertiary age. 

They have been beveled during one or more stages of wave cutting, 

and are therefore assigned an early Tertiary age to distinguish them

-^ from other glaciated volcanic rocks that have not been similarly 

^ beveled and that are assigned a late Tertiary or Quaternary age.

^-v> LATE TERTIARY AND QUATERNARY BASALTIC ROCKS

*^ The northern parts of Great Sitkin, Adak, Kanaga, and Kiska

Islands, and most of Kasatochi, Bobrof, Gareloi, Semisopochnoi,

Little Sitkin, Segula, and Buldir Islands are composed of practically

^ undissected volcanoes. All the volcanoes seem to be stratovolcanoes

(composite cones) made up of interbedded basaltic and andesitic

*"" P37roclastic rocks and lava flows, and of mudflows and volcanic sedi-

-^ mentary rocks on their lower slopes. The oldest of these rocks appear 

to rest unconformably upon a plane of marine abrasion that bevels 

the early Tertiary volcanic rocks. Assuming the period of marine

.4 erosion to be middle Tertiary, the extrusion of these rocks is believed 

to have begun during the late Tertiary and to have continued intermittently to the present.

 < On many of the islands, the oldest rocks of this sequence are 

well-bedded volcanic san'dstone and conglomerate, interbedded with 

or grading laterally into tuff-breccia and overlain by thick flows of, 

basalt that retain low initial outward dips. Around Kanaga and 

Tanaga Volcanoes and on northern and western Semisopochnoi

* Island, the low-dipping flows and pyroclastic rocks form the radial

 <; ridges and arcuate rims of large calderas. Within these calderas, single

or multiple composite cones have been built. The cones are made up

"^ of lava and scoria whose initial dips are as much as 30°. The extrusion

 / of black to dark-gray vesicular-to-compact lava has been occasionally 

interrupted by the explosive ejection of much fine basaltic and 

A andesitic lapilli and ash, and smaller quantities of pumice.

-v, A blanket of fine ash covers large areas on Kanaga, Tanaga, Ogliuga, 

and other islands at least as far west as Amchitka. The thickness of 

this blanket varies considerably, depending on the length of time

J since the land surface was last scoured by sea or glaciers, the distance 

from active volcanoes, the direction of prevailing winds, and the rate 

of removal of the ash after it was deposited. The maximum thickness 

is'more than 5K feet on Adak Island, more than 7% feet on northern 

Kanaga Island, and more than 19 feet on northern Tanaga Island. 

The ash on each island probably accumulated to its present depth 

over a period of several thousand years and as a result of the eruptions
92 INVESTIGATIONS OF'ALASKAN VOLCANOES''

of various volcanoes. The ash on Ogliuga Island is about 5}£ feet 

deep, but its freshness suggests that almost all of it may'have been 

erupted recently, perhaps by Mount Gareloi.

TERTIARY AND QUATERNARY SEDIMENTARY ROCKS

Tertiary and Quaternary sedimentary rocks occur on most of 

the islands in minor quantity. The outcrop areas are too small to 

be conveniently indicated on plate 17.

The only fossiliferous Tertiary or Quaternary sedimentary, rocks 

recognized are exposed in small areas on Adak Island near the base 

of the east side of Mount Adagdak, and on Amchitka Island at the 

head of a small bay 2.6 miles west of East Cape. Other areas have 

been mentioned by Dall (1896). The rocks consist of poorly consolidated fossiliferous sandstone and conglomerate. The precise age 

of the fossils is not known, but they are Tertiary in aspect. On Adak 

Island these rocks overlie beds of boulder conglomerate. On 

Amchitka Island, west of the head of Constantine Harbor, a sequence 

of regularly interbedded, unfossiliferous, yellow-gray, poorly consolidated sandstone and siltstone, about 60 feet thick, is exposed in 

borrow pits. The sandstone and siltstone are overlain by till and 

unconformably underlain by early Tertiary volcanic rocks.

On Adak Island, glacial drift forms a tattered mantle covering 

much of the northern part of the island. It consists chiefly of ground 

moraine, but includes subordinate amounts of lateral moraine: and 

fluvioglacial deposits. Glacial deposits are less conspicuous on other 

islands; small deposits were observed on Tanaga, Amatignak, 

Amchitka (pi. 185), and Attu Islands. On Amchitka Island, on the 

north shore of Constantine Harbor, three successive layers of till and

interbedded fluvioglacial sediments are exposed.

Wave-cut terraces, covered by a thin veneer of marine boulder 

gravel, are present at several places along the shores of the higher

islands. On northern Adak Island are remnants of three terraces, 

the highest about 100 feet above sea level. Most of southern Tanaga

Island is mantled by marine gravel, resting on a wave-cut platform. 

On Ogliuga Island coarse boulder gravel veneers a wave-cut platfoi-m

and is OVerlain by COarse volcanic cinders. Unconsolidated sand and

gravel as much as 10 feet thick rests on a similar wave-cut platform

on SHemya, Island. Small Jjeaclies are present on most of the islands j

most of the material composing them ranges in size from that of 

 cobbles to that of boulders.

Streams emptying into bays protected by bay-mouth bars have 

built deltas, a few tens or hundreds of acres in extent. Most of the 

delta sediments are reworked pyroclastic or glacial material. Similar
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 18

A, ILAK ISLAND FROM THE NORTHEAST.

A remnant of a marine abrasion platform, uplifted and tilted to the west. Photograph by U. S.

Navy.

11, ANGULAR GRANITIC BOULDERS ON AMCHITKA ISLAND.

Origin, probably glacial. Overlain by marine gravels.
GEOLOGICAL SURVEY 

GARELOI ISLAND FROM THE SOUTHWEST. 

A relatively simple cone of basalt and basaltic pyroclaslic rock*. I'liotograpli 
RECONNAISSANCE GEOLOGY, WESTERN ALEUTIAN ISLANDS 93

material forms small patches of alluvium along some of the streams, 

particularly where streams empty into glacial lakes.

On Adak Island, a few acres are occupied by sand dunes on both 

sides of the entrance to Clam Lagoon. Dune areas are also present 

on Tanaga Island. The maximum height of the dunes is about 100 

feet. All appear to have been stabilized, except those from which 

the cover of vegetation has been recently removed. On Ogliuga 

Island, isolated hillocks, rising a few feet above the featureless plain 

of volcanic ash, are apparently composed of wind-blown ash.

STRUCTURE

Knowledge of the structure of the western Aleutian Islands is 

incomplete because of the absence of information about many of the 

islands, concealment of many critical areas beneath the sea, lack of 

fossils that indicate the age of the rocks, and lack of key horizons 

that reveal structural details.

As now conceived, the structural history of the western Aleutians 

begins with an oldland, made up of rnetamorphic rocks intruded by 

granite. The structural trends in this oldland cannot be inferred, but 

the younger graywacke of Amatignak Island and, locally, the Finger 

Bay volcanics on Adak Island have fold axes that trend northward. 

Locally, the Finger Bay volcanics and the early Tertiary rocks have 

been folded along axes trending eastward. All the rocks formed before 

late Tertiary time are cut by faults that trend roughly parallel to the 

trend of the Aleutian arc. The late Tertiary and Quaternary volcanoes are evidently related to a fault or fracture zone extending the 

length of the Aleutian arc. Too little is known about the distribution 

of the early Tertiary and older rocks to permit a statement of their 

relation to the present arc; it is inferred that the Aleutian Islands 

were established in about their present form and distribution late in 

Tertiary time.

PRE-TERTIARY

The; sequence of older basaltic volcanic rocks, argillite, and graywacke of Attu and Shemya Islands is intensely sheared and faulted. 

Determining the age of the faults is difficult because no rocks of definitely known age are involved. In a quarry 2% miles north of the 

Attu airbase, many faults are exposed which strike N. 10°-30° W., 

anol dip 65°-68 ° SW. It is not known whether the faults were formed 

earlier than a vertical fault that trends west-northwestward and cuts 

early Tertiary rocks on Shemya Island, or were produced simultaneously as complementary shears.

The attitudes of the Finger Bay volcanics are diverse. On the 

southern shore of Adak Island, rocks that from the boat were judged
94 INVESTIGATIONS OF ALASKAN VOLCANOES

to be part of the Finger Bay volcanics occur as a great open syncline 

trending westward; the dips on the flanks of this fold approach 20°. 

Comparable open folding and trends are suggested by the low dips 

in the series of Finger Bay volcanics between Andrew and Clam Lagoons, on Adak Island. Farther south, between Kuluk Bay and 

Shagak Bay, and between Finger Bay and Sweeper Cove, steeper 

dips and northward trending folds are present. This discordance in 

trends on Adak Island may be due to the presence of two unconformable sequences in the rocks mapped as Finger Bay volcanics.

Many faults, which appear to be steeply dipping normal faults, 

occur in the Finger Bay volcanics on Adak Island. The predominant 

trends of these faults range from N. 60° E. to N. 60° W. and from N. 

20° E. to N. 10° W., but faults of intermediate trend are present, and 

some faults curve through a wide range of trends. Their age is not 

definitely determinable. They do not appear to cut any of the Tertiary rocks of the island, but they cut gabbro, which intrudes the 

Finger Bay volcanics and is probably the youngest pre-Tertiary rock.

Beds of graywacke and graywacke breccia on Amatignak Island 

have been tilted and now trend about N. 8° W., and dip 30° E. 

These beds and the gabbro sills that intrude them have been faulted; 

the faults, apparently reverse faults, trend N. 35°-50° E. and dip 

50°-70° SE. The age of these faults is uncertain, as no younger 

Tertiary sedimentary or volcanic rocks appear to have been cut by

them.

TERTIARY AND QUATERNARY

in addition to the cones built of Tertiary and Quaternary volcanic 

rocks, Tertiary and Quaternary structures of tectonic origin occur on

some western Aleutian Islands.

Bally Tertiary YOlCttniC rocks On Amchitka Island, and the sedimentary rocKs associated witK tKom, O.-L-& gently folded or tilted.

The beds trend westward and dip from 10°-27° S.

Layers of Pleistocene glacial till on Amcllitka Island are CUt by

normal faults, ranging in trend from N. 80° E. to N. 45° E. Some Of

these faults outline clownfa-ulted blocks in -which the younger sediments are preserved; one such block forms Constant Harbor, the

floor of which, has been dropped about 130 feet on its north side.

Some of the numerous faults cutting the basaltic volcanic rocks,

argUllCe, ana graf) r«M>J*« -o^o^oc, ~~ si_myQ. _;id _A.fctu u^d* seem

to be related to a fault which cuts early Tertiary volcanic rocks on

Shemya ISland. ThiS fault), exposed on the north shore of Shemya 

JSlS/nd. Ql few hundred feet from, the eastern end, cuts a tufFaceOUS

basaltic conglomerate, evidently part of the early lerilal'y' WJlffllC 

rocks. The fault is vertical and strikes N. 60° W. A mylonitized zone 

is developed along it. Faults on Attu Island that are possibly re-
~ ^

RECONNAISSANCE GEOLOGY, WESTERN . ALEUTIAN ISLANDS Q5

i lated to this fault trend N. 85° W. and dip from 50° S. to vertical. 

The trend of the fault striations is northeast, and the south sides of

-y the fault appear to have moved up and eastward relative to the north

_^ , sides.

The youngest faults found are those on the flanks of Mount Adagdak

x on Adak Island. They are probably Recent in age, and they appear 

to be normal, the downthrowu side being toward the mountain summit. They were probably caused by subsidence which occurred when

->- magma was withdrawn from beneath Mount Adagdak.

^V GEOLOGIC HISTORY

 The earliest event that can be inferred in the geologic history of the

^- western Aleutian Islands is the deposition, perhaps in Paleozoic tune,

of sedimentary and volcanic rocks in the vicinity of what are now

** Tanaga, Ogliuga, and Adak Islands. These rocks were deformed and 

^v metamorphosed, then intruded by granite. After a long interval of 

erosion, volcanism was resumed in Mesozoic(?) tune.. Stratovolcanoes 

"^ were built on Shemya and Attu and perhaps oh other islands. Around 

^ the margins of these volcanoes, marine deposits of argillite and graywacke were made between intervals of extrusive activity. Graywacke 

^ and graywacke breccia were deposited in quiet water on what is now 

Amatignak Island, probably either at the same time or slightly after 

the Finger Bay volcanics were extruded.

""* The Finger Bay volcanics and the older rocks were compressed, in 

w part by forces acting nearly normal to the present trend of the chain, 

and extensively faulted. Deformation was accompanied by hydrothermal alteration. Gabbro intruded the rocks, and some of the intrusive masses were faulted and" hydro thermally altered along with their 

host rocks. After a period of deformation and erosion, basaltic and 

" ^ andesitic volcanic rocks were erupted in early Tertiary time to form 

_4 new sequences on the dissected foundation of older volcanic and intrusive rocks.

A The early Tertiary volcanoes were beveled by marine erosion at a

^ time when sea level was as much as 150 feet higher, relative to the

land, than it is at present. It is not known whether this difference

^ indicates that the; level of the sea was actually higher or that a later

^ upward movement of the land occurred. The terraces produced by

this marine beveling are not of uniform height, and some are tilted.

Volcanic activity .was resumed in late Tertiary time. The new

vents were, in general, north of the Mesozoic and early Tertiary vents.

The early flows from some of the late Tertiary vents rest upon sedi-

~+ ments that apparently accumulated under marine conditions, suggest-

, ing that when the building of the late Tertiary cones began the level

of the sea was again relatively higher than at present; fluctuations of
96 INVESTIGATIONS OF ALASKAN VOLCANOES ;

relative sea level apparently occurred several times during later 

Tertiary time. Fine-grained marine sediments, locally fossiliferous, 

which accumulated during periods when sea level was relatively high, 

are now exposed.

During a period of relatively low sea level, perhaps in early Pleistocene, glaciers advanced northward from a highland, presumably developed on the older volcanic, metamorphic, and intrusive rocks. This 

highland is inferred to have lain south of the present islands and to 

have extended from about the longitude of western Tanaga Island to 

that of eastern Amchitka Island. Glaciers moved northward across 

the emergent platforms, then retreated. Subsequent submergence 

permitted waves to work the glacial tills on Ogliuga, Tanaga, and 

perhaps other islands. The disappearance of most of the highland 

is thought to be due chiefly to doMoifaulting below sea level.

In Quaternary time, glaciers born in the highlands of many of the 

present islands greatly modified the topography of the upper mountain slopes before retreating. Narrow marine terraces, now covered 

with thin layers of boulders, were cut during one or more interglacial 

stages. Since the most recent glaciation, the rocks on some islands 

have been cut by steep-dipping faults of small displacement. Several 

cones, such as Mount Moffett and Mount Adagadak on Adak Island, 

became inactive before or during the last interval of glaciation, but 

volcanoes on most of the islands from Kasatochi to Buldir have been

active since then.

FUTURE VOLCANIC ACTIVITY

Many of the western Aleutian Islands have active volcanoes. The

volcanic cones on other islands are deeply eroded, and early renewal of

activity is unlikely. Volcanoes believed or known to be active include 

those of Kasatochi, Great Sitkin, Kanaga, Tanaga, Gareloi, Semisopochnoi, Little Sitkin, Segula, and Kiska Islands. Inactive volcanoes

include that on Bobrof Island, Mount Adagak and Mount Moffett on '

activity-years 1 for Kiska, Little Sitkin, Semisopochnoi, Gareloi,

Tanaga, Kanaga, and Great Sitkin Islands total 30 during the 185

years since 1760. None of the recorded eruptions on.n to classed o.s w

major. Many other eruptions have undoubtedly occurred but have L

not been recorded. On the basis of the fragmentary records, a mild

eruption can be expected, on the average, once every 6 years in the

western. Aleutian Islands. > One great caldera-forming eruption has occurred on these islands in

postglacial time, but such an eruption, analogous to the one that *

' Activity-year is defined herein as a calendar year during which one volcano is active. For example, if 

three volcanoes are active during a single calendar year, then three activity-years are recorded.
RECONNAISSANCE GEOLOGY, WESTERN ALEUTIAN ISLANDS 97

created Okmok Caldera on Umnak Island, could occur on any of the 

active volcanic islands. Caldera-forming eruptions are least to be 

expected on the islands on which calderas have already been formed 

Kanaga, Tanaga, and Semisopochnoi. No evidence has been found 

suggesting that such violent activity is in prospect for the near 

future, except possibly at Great Sitkin Island (Simons and Mathewson, 

1955, p. 40-41).

In general, the zone of greatest volcanic activity has moved northward in the western Aleutian Islands. The most recently active 

and the least eroded volcanoes are on the northern parts of the large 

islands or on the more northerly small islands. Activity in the near 

future can be expected to be limited to the northern part of the 

western Aleutian arc.

SELECTED BIBLIOGRAPHY

Capps, S. R., 1934, Notes on the geology of the Alaska Peninsula and Aleutian

Islands: U. S. Geol. Survey Bull. 857-D, p. 141-153. 

Coats, R. R., 1956a, Geology of northern Adak Island, Alaska: U. S. Geol.

Survey Bull. 1028-C. 

 1956b, Geology of northern Kanaga Island, Alaska: U. S. Geol. Survey

Bull. 1028-D. 

Collins, H. B., Jr., Clark, A. H., and Walker, E. H., 1945, The Aleutian Islands:

Their people and natural history: Smithsonian Inst. Washington, War

Background Studies, no. 21. 

Dall, W. H., 1896, Report on the coal and lignite of Alaska: U. S. Geol. Survey,

17th Ann. Rept., 1895-96, pt. 1, p. 811-814. 

Grewingk, Constantin, 1850, Beitrag zur Kentniss der orographischen und

geognostischen Beschaffenheit der Nordwest Kiiste Amerikas, mit den

anliegenden Inseln: Russ. K. min. Gesell. Verh., St. Petersburg, 1848-49,

p. 123-278. 

Sharp, R. P., 1946, Note on the geology of Agattu, an Aleutian Island: Jour.

Geology, v. 54, p. 193-200. 

Simons, F. S., and Mathewson, D. E., 1955, Geology of Great Sitkin Island,

Alaska: U. S. Geol. Survey Bull. 1028-B. 

U. S. Coast and Geodetic Survey, 1944, U. S. Coast Pilot, Alaska, v. 3.

INDEX

Page 

Abrasion, marine........ _.....__ . 86

Acknowledgement. .._.._.--...-_.. 84

Adak Island, ash_____________. __ 91

dunes._________________ 93

early geologic history.__.___....__ 95

faulting . 94,95

Finger Bay volcanics-..---.__. 89-90,93-94 

folding_ -. .- 93-94 

fossiliferous rock.___._______.. 92 

glaeiation__________________ 87,92 

marine terraces_______..___._ 92 

rock types.____ ..._._____-__ 90 

size-___---. - --.----_.- 86

type. .. ----- --. --. - 87

volcanoes-___._-____.....__-_ 91,96

Agattu Island, glaeiation-.........___... 87

rock types______...________ 90 

size --. -_-. .. ------ 86

Amatignak Island, graywaeke breccia...__. 90 

faulting _ ... ._-. ........ 94

folding,- _.. . .- 93-94 

glacial deposits._____..________ 92 

physiography . 87 

rock types .. ........ . 89

Ainchitka Island, fossiliferous rock-.....___ 92

glaeiation....-__ ................__ 87,92

rock types.__. __________ .. _ 90 

size______________________ 86 

type -- ----- ..... ..... 87

Andreanof Islands, Finger Bay volcanics..... 90

glaeiation......_______________ 87

Areas of islands_______________.. 86

Argillite_____._ .____-___.__.- 89

Ash, postglacial.___._________._ 91-92

Attu Island, early geologic history______ 95

faulting......._...................... 93,94-95

glaeiation._________________ 87,92 

rock types.__..__.....___.__ 89

size....__...._________ -.___ 86

Basaltic rocks..-.._______._______ 89 

Beaches.- -.-___._.__________ 86,92 

Bobrof Island, marme erosion. __._ 88

volcanoes__ _._..._.._ 91,96

Boulder gravel, marine-_________.._-. 92 

Boulders, exotic..-. _____________ 88 

Breccia, graywacke_------__-._... .. 90

Buldir Island, dissection._...._.......... 88

volcanoes___.._._..............__ 91

Calderas 91,96-97

Constantine Harbor..., ...-. ....... 94

^ Page

Delarof Islands/type- - -- 86 

Deltas... 92-93

Dunes...-.-.-_ 93

Erosion....._._.............- ...... 88,91,95

Faulting . ..- 93-95

Field-work. . 84

Finger Bay......_... .._._.__ .. 89

Finger Bay volcanics, faulting..._ ....... 94

features... - 89-90

folding.-......--.... ................... 93-94

geologic history . _ .. 95 

Folding.... . 93-94

Qareloi Island, glaciation.__ 88

volcanoes_._._.....___._ 91,96

Geologic history_____ ..._ . 95-96 

Glaeiation-......--.. .-..-_-... 86,87-88,91,96

Granite--...-.-.. ...-._._... 88-89

Graywacke- -- _____ 89,90

Great Sitkin Island, Finger Bay volcanics__ 90

future volcanic activity..------ 97

size . . 86

volcanoes.__....___ 91,96

Ilak Island, granite.. . 89 

type 86

Island types.. 

Kagalaska Island, size__.. . 86

Kanaga Island, ash.. . 91

caldera 91,96-97

size.. __....... 86

type 87

volcanoes____._____ .. 91,96 

Kasatochi Island, volcanoes. 91,96 

Kavalga Island, type._.. . 86 

Kiska Island, size-- - 86

volcano _...... ..... 88,91,96

Lakes.. . _ 86,87

Little Sitkin Island, volcano-. .. 91,96

Metamorphic rocks ..

Mt. Adagdak, erosionfaulting 

fossiliferous rock .. 

volcanic activity.. _

Mt. Gareloi..-.--.--...

Mt. Moffett, galciation. 

volcanic activity...

95

92

96

92

99
INDEX

Page 

Ogliuga Island, ash..........___.____.. 91,92

dunes-............_..___......__._.. 93

early geologic history--______.--._-_.. 95

roek types.__._._________ 88-89,90,92

type.-..-.__._.-_____________ 86

Okmok Caldera._.__.________.... 97

Oubeloi Island, type...--__._____-_.... 86

Platforms, wave-cut. 

Rat Island.......... 84

Scope of report..--.______._._-_._.. 83-84 

Segula Island, volcano.___._______ 88,91,96 

Semichi Island..___-____....._____ 84

Semisopochnoi Island, size___._____.. 86

volcanoes_______-________ 88,91,96

Shemya Island, early geologic history..___ 95

faulting. __-..-__._________ 93,94'

rock types.........._.................... 89,92

type_________________-___ 86 

Skagul Island, type..____ -________ 86 

Structural history.,__..____.._.... 93

Page

Tanaga Island, asb. _______________ 91 

caldera.---_____...-___.__.. 97 

dunes-..____--..-.. ...________ 93 

early geologic history.__...._____. 95^

glaciation.....__....................... 88,92

rock types_______________ 88-89,90,92 

size-_____________________ 86- 

type --------_.........____....... 87

volcanoes___.__.._________.- 96- 

Tanaga Volcano_______.______._ 89,91 

Topography.................................. 86-88

Ulak Island, type..______________.. 87 

Unalga Island, type..____________ 86 

U. S. Coast and Geodetic Survey, landings on

Ilak Island...__...___...... 89

Vegetation..--_........_..-... ......... 88

Volcanic activity, future____----------_ 96-97

Zeto Point-

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