Geology of Southern

Adak Island and

Kagalaska Island, Alaska

GEOLOGICAL SURVEY BULLETIN 1 028 -M

Prepared in cooperation with the 

Departments of the Army, Navy, and 

Air Force

Geology of Southern 

Adak Island and 

Kagalaska Island, Alaska

By GEORGE D. FRASER and GEORGE L. SNYDER

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-M

Prepared in cooperation with the 

Departments of the Army, Navy, and 

Air Force

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1959
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 the general direction of G. D. Robinson, 

were made during the years 1946-48. The results of the first year's 

field, laboratory, and library work were hastily assembled as two administrative reports, and most of these data have been revised for 

publication in Geological Survey Bulletin 1028. Part of the early 

work was published in 1950 in Bulletin 974-B, Volcanic activity in 

the Aleutian arc, and in 1951 in Bulletin 989-A, Geology of Buldir 

Island, Aleutian Islands, Alaska, both by Robert R. Coats. During 

the years 1949-54 additional fieldwork was done under the direction 

of H. A. Powers. Unpublished results of the early work and all the 

later studies are being incorporated as parts of Bulletin 1028. The 

geologic 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. From 1947 until 1955 the Departments of the Army, Navy, 

and Air Force joined to furnish financial and logistic assistance. 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, and to the several military departments for their support.

in

CONTENTS

Abstract__.________....___.______._._____.____________.-----.-__ 371

Introduction._____________________________________________________ 372

History.__ _._..______...__..____..__..__.____.___._...__.__. 372

Geography..----------------____-_-_-------------------------- 375

Summary of geology of northern Adak Island-________________________ 376

Petrology.....----..-._--_---_--------------_-.-------_---_------- 377

Finger Bay volcanics..----------------------------------------- 377

Lavas-______'____----_-____.----_-_____-_---.____--------- 378

Pyroclastic rocks._________________________________________ 379

Sedimentary rocks-________-.-_-________-_----_------------ 380

Alteration andmetamorphism.______________________________ 381

Regional relationships and age_ ----------------------------- 383

Granodiorite and related rocks.___________-_______-_____-_--_-.- 384

Petrography __._____.._._.._._____....._...._.___.____.._. 384

Granodiorite and quartz diorite. ------------------------ 384

Diorite...........__._____._._____._._...._.___-___... 386

Gabbro -----------.__---_-_---_-------------------- 387

Feldspathic granodiorite porphyry.___-____.__-_--____.__ 388

Petrogenesis-_______-__________________-_------.-------.-- 388

Hornfels and flow structure.____._.____.______._._._... . 388

Mafic border rocks____-_____-_______-__________----_-_. 388

Migration of felsic material.____________________________ 389

Summary and age. _____.___________-__._--___-____.--- 390

Andesitic dikes and sills. _--______._-__--_--_----_-------__----- 391

Dike rocks in the Finger Bay volcanics._______--_--_-_-__--__ 391

Dike rocks in the granodiorite-____-___________-____-__-_---- 391

Summary and age_-__----____-_____--_.-_-.______--_--_--- 392

Surficial deposits._____-_-_-___-___-_______.-___________----___ 392

Glacial deposits.__-_-_--____----_-..---_---___---_--_--_-.- 392

Volcanic ash and soil_._____-_.......____..._.__._..._.____ 393

Age of ash-soil mantle_____-__-_-_-_____---____------------- 394

Beach and alluvial deposits. -------------------------------- 394

Structure. _ _.__.._._.__._.....__......__...._._._____....._...__.. 395

Regional setting-______________________________________________ 395

Fracture pattern___.._-_-_...--.___-._.-___._^-.._....._..____. 396

Folds-._.._._.__......._..............._................._.._. 398

Geomorphology _ _____--_.____--_-_...-_._-.-_..___.-_...___.--_---_. 399

Marine planation... _....__..--__.-__-__..________._...__._---..__ 400

Glaciation ______......._--_____...--.-._...._..__.._..___.. .__._._. 401

Geologic history.-___.._.._..._..__.. . ___. ._.._.______..-._...____._._ 402

Economic geology. _.-_______.__.... ._._..._____.__..___. __..._.__.___ 404

Volcano and earthquake ha/ard_ _-..._-..-.......____......._.-_._.__..__ 404

Literature cited.____-_-..____......._-_-..-_..___.....__._.-..._._. 405

Index.. ________._._-.-___......__...__-_--__-._._---__:____-- 407
VI CONTENTS

ILLUSTRATIONS

[Plates arc In pocket]

PLATE 52. Reconnaissance geologic map, southern Adak and Kagalaska

Islands, Aleutian Islands, Alaska. 

53. Structural interpretation map, Adak and Kagalaska Islands,

Aleutian Islands, Alaska.

FIGURE 55. Map of Alaska Peninsula and Aleutian Islands showing loca- Page 

tion of index map__________-______--__-----------_--. 373

56. Index map for areas covered by U.S. Geological Survey reports in the western Andreanof and Delarof Islands.-,---. 374

57. Relacement breccia at granodiorite contact, southeast shore

of Blind Cove, Adak Island__._____-_---_--_--._---_-__ 389

58. Aerial view north-westward from first bay north of Beyer

Bay, Adak Island.___._._..__._____..._'.-__.._.-.__._ 396

59. Aerial view of south-central Adak Island. View northwestward from Beyer Bay_-_______-_____---_--___-__--__._ 397

60. Aerial view northward along Yakak peninsula.............. 400

61. Topographic profiles of Yakak peninsula, Adak Island- . _ _ _ _ 401

TABLE

Page 

TABLE 1. Chemical composition of rocks from Adak Island, Alaska____ 385
INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGY OF SOUTHERN ADAK ISLAND 

AND KAGALASKA ISLAND, ALASKA

By GEORGE D. FRASER and GEORGE L. SNYDER

ABSTRACT

Southern Aclak and Kagalaska Islands are composed of Finger Bay volcanics of probable Tertiary (?) age, an altered andesitic and basaltic sequence 

of marine pyroclastic deposits and lava flows with minor argillite and graywacke

beds, intruded by composite grauodiorite, quartz-diorite, diorite, and gabbro 

plutons of probable middle to late Tertiary age; many aphanitic dikes and sills, 

generally altered, which cut plutonic rocks and Finger Bay volcauics; and surlicial deposits, mostly volcanic [ash and soil, which obscure much of the bedrock 

below elevations of 1,500 feet.

The Finger Bay volcanics and many of the dikes are altered to greenstone 

and keratophyre with crystallization 'Of albite, epidote, and chlorite. Some of 

the alteration preceded the pluton,s, and some either accompanied or followed 

them. A narrow zone of pyroxene-labradorite hornfels is discontinuou'sly formed 

along major intrusive contacts. Elsewhere late aplitic magmas or solutions 

appear to replace areas of country rock. A dark border zone is present in the 

granodiorite plutons in many areas. Most diorite and gabbro are found in this 

dark border zone or in apophyses located in what is interpreted as a shattered 

roof zone over the main plutons. In some areas the evidence for assimilation 

is compelling, and here the mafic borders are interpreted as hybrid rocks. Elsewhere recrystallization. in place of mafic country rock and other processes appear 

to have produced the gabbro and diorite.

Adak Island and Kagalaska Island may be considered as a portion of the crest 

of a 1,900-mile mountain range that <rises, in this area, 28,000 feet above the 

foredeep and 13,000 feet above the backdeep. Quaternary volcani'sm is limited to a belt along tire north edge of the islands -outside the map area of this 

report. The Finger Bay volcanics are conspicuously fractured and mildly folded, 

but no dynamic metainorphism is apparent. Major intrusive activity, dated as 

middle to late Tertiary elsewhere in the chain, apparently preceded deformation 

and uplift.

Remnants of a composite marine platform are found on southwest Adak 

Island and perhaps elsewhere. This -surface formed prior 'to the latest extensive Pleistocene glaciation, which has gouged out bold mountains in the higher 

central portion of the area and scoured the peripheral lowlands. Most of the 

glacial debris has been dumped in the sea.

A surficial ash and soil mantle accumulated 'by -small increments through a 

long epoch of Recent volcanism. The presumed sources for the ash component 

are volcanoes within the Andreanof Islands, but probably neither Mount Moffett 

nor Mount Adagdak, both of which became extinct before or very shortly after 

glaciation ceased.

371
372 INVESTIGATION OF ALASKAN VOLCANOES

INTRODUCTION

Southern Adak Island and Kagalaska Island were studied in 1952 

and 1953 as part of a larger program of volcano investigations in the 

Aleutian Islands. Previously northern Adak Island and several other 

areas in the Andreanof Islands had been studied. Figure 55 is a regional index map and figure 56 a larger scale index map showing areas 

covered by various Survey reports on the Andreanof-Delarof Islands 

area.

Fieldwork on southern Adak Island was done by two-man reconnaissance teams from 12 separate base camps established in various 

bays and inlets. A total of about 40 such traverses were made and 

200 square miles were mapped during the summer of 1952 by R. E. 

Wilcox, geologist-in-charge, H. F. Barnett, B. H. Bieler, G. D. Fraser, E. H. Meitzner, W. H. Nelson, R. A. Robie, and G. L. Snyder. 

Fieldwork was supplemented by frequent observations from the Survey vessel Eider and by study of aerial photographs.

Kagalaska Island, 46 square miles, was mapped in 1953 from five 

separate base camps. A total of 20 traverses were made by G. D. 

Fraser, geologist-in-charge, V. E. Ames, H. F. Barnett, E. H. Meitzner, R. P. Platt, H. B. Smith, G. L. Snyder, and L. D. Taylor.

Captain Carl Vevelstad and Engineer Charles Best served as key 

men on the Eider during all Aleutian investigations since 1949. Their 

experienced help was essential to the amphibious operations.

Mapping on both Adak and Kagalaska Islands was done on Army 

Map Service topographic sheets, scale 1: 25,000; but detail commensurate with map scale was seldom attempted. Physical limitations imposed by amphibious beach operations, adverse weather conditions, 

and extensive surficial cover emphasize that the information gathered 

and conclusions reached cannot be considered detailed or final. Key 

beds for deciphering structure were not found during the 

reconnaissance.

Kagalaska Island is uninhabited and completely undeveloped. 

Most of Adak Island is similarly primitive, but there are military 

installations in the Kuluk Bay area and elsewhere. Air and sea access 

to the Adak base is possible with military permission. Kuluk Bay 

is the only all-weather harbor for large vessels, but there are many 

anchorages on both islands for small vessels and partial protection 

from weather in many bays and inlets.

HISTORY

Adak Island was discovered on September 9,1741, when the Russian 

vessel St. Paul, under command of Captain Alexei Chirikof, anchored 

in a small bight on the south coast. At that time 20,000 to 25,000
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 373

504449- 59-
 CO CH O 

-.005 

'

03 P 

^3'

Op

S?B

Island Attu to west extends Reconnaissance 

O

JLO
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 375

Aleuts, an Eskimauan people, are estimated to have lived in the hundreds of villages from Attu Island to the Alaska Peninsula. Nearly 

every island, including Adak, was inhabited (Collins, and others, 

1945, p. 2-10). Today there are only about 1,000 Aleuts and four 

villages remaining in the islands all east of Adak Island. Prior to 

World War II foxes were introduced and trapped on Adak Island 

and many of the other islands, but trapping has not been resumed 

since the war. None of the trappers' cabins are used today, and many 

of them are in poor repair.

GEOGEAPHY

Adak Island is located close to the middle of the Aleutian chain, 

in the Andreanof Islands (figs. 55 and 56). It is about 8 miles east 

of Kanaga Island and one-fourth mile west of Kagalaska Island. 

The geodetic position for the head of Kuluk Bay is 51°53/ N., 

176°39' W. Adak Island is 32 miles long and 21 miles wide, but very 

irregular in shape. It is the largest island (289 square miles) and 

strategically the most important in the central Aleutian Islands. 

Kagalaska, Island, separated from Adak Island by narrow Kagalaska 

Strait, is 10 miles long (north) and 6.5 miles Avide with an area of 

46 square miles.

Adak and Kagalaska Islands are characterized by four types of 

topography listed in order of decreasing areal importance: rugged 

mountain areas, broad rolling lowland areas, modified volcanic cones 

(northern Adak Island only), and narrow beaches backed by sea 

cliffs. The rugged mountain areas, unrelated to constructional volcanic landforms, result from Pleistocene glacial erosion and stream 

erosion of altered volcanic deposits and large plutonic masses (fig. 59). 

Parts of southern Adak and Kagalaska Islands, here included with 

the mountain areas because of rugged topography, probably contain 

glacially dissected marine platforms that, genetically, should be 

grouped with the lowland areas (fig. 58). The broad lowland areas, 

of greatest extent on Yakak peninsula, probably result from preglacial 

wave planation modified by stream and glacial erosion (fig. 60.) The 

volcanic landforms on northern Adak Island are outside the area of 

this report and have been discussed elsewhere (Coats, 1056a). The 

shoreline, except where straightened by fault scarps, is very irregular, 

a result of glaciation and further crenulation by vigorous wave action. 

Sea stacks, rugged offshore rocks and islands, and precipitous sea 

cliffs, generally about 200 feet high, are characteristic (fig. 60). 

Northern Adak Island includes local areas of lagoons, tidal flats, 

and sand dunes.
376 INVESTIGATION OF ALASKAN VOLCANOES

The Aleutian Islands are well known for their foggy, wet, and 

stormy climate. From short-term weather observations a few 

general statements can be made concerning the climate of Adak 

Island. Local shifts of wind and rapid changes of velocity are common so that figures for average wind velocity are of limited value 

in understanding actual conditions. For example, average wind 

velocity at the Adak base is 15 miles per hour, but extreme wind in 

winter may exceed 100 miles per hour, and summertime gusts frequently exceed 50 miles per hour. Total annual precipitation is only 

47 inches, but during summer months the sky is usually overcast and 

fog, drizzle, or rain is very common. Average annual snowfall is 

71 inches, limited largely to winter months. Little snow accumulates 

except in topographically favorable upland areas, and there are no 

glaciers at the present time. Mean annual temperature is 40° F with 

seasonal variations of about 10° F and correspondingly small diurnal 

changes (U.S. Air Force, 1950, Chart 20).

The lower parts of Adak and Kagalaska Islands support heavy 

vegetation with grass and flowers attaining heights of 5 or 6 feet late 

in the summer. Patches of moss and lichen become increasingly dominant with increasing height above sea level. At altitudes above 1,500 

feet, there is virtually no grass and many ridges are bare rock. There 

are no native trees, but there are many flowering plants, ferns, grasses, 

and mosses. A tough mat of vegetation frequently forms bridges over 

gullies and between talus blocks and dams small lakes.

The only land mammals are blue fox, rats, and mice, all introduced in 

recent years. Birds, trout, and salmon are common and the sea 

abounds with shallow-water invertebrate species. Sea otter were rare 

in 1952 but appear to be expanding eastward from the Amchitka area. 

Seals, and to a lesser extent sea lions, are common along the coastline.

SUMMARY OF GEOLOGY OF NORTHERN ADAK ISLAND

Coats has described the geology of northern Adak Island (1956a). 

Part of his abstract is quoted here to give a background for the geology 

of southern Adak and Kagalaska Islands.

The geology of the northern part of Adak Island, largest of the Andreanof 

group of the Aleutian Islands, was mapped in the summer of 1946. The part of 

Adak Island mapped comprises two physiographic and geologic divisions: A 

deeply glaciated southern area of folded, faulted, and intensely altered volcanic 

rocks of Paleozoic (?) age, introuded by gabbro and rocks of intermediate composition ; and a mountainous northern area comprising remnants of three distinct basaltic volcanoes of Tertiary or Quaternary age. In the northern part 

of the southern area there are five volcanic domes of light-colored andesite 

porphyry, probably of early Tertiary age; they cannot be correlated with any of
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 377

the three recognized volcanic centers of the northern area. Minor amounts of 

sedimentary rock are associated with the volcanoes. The volcanoes have been 

trimmed by marine erosion, and locally dissected by subaerial and glacial erosion. The glaciers have disappeared. A blanket of volcanic ash from volcanoes 

on nearby islands covers most of the lowland area.

Constructional volcanic landforms and volcanic rocks of late Tertiary and Quaternary age are limited to northern Adak Island. Upper 

Tertiary fossiliferous marine sandstone crops out in a small area on 

Cape Adagdak (Coats, 1956a, p. 53, 54). The relation of these beds to 

the later Tertiary and Quaternary volcanic rocks and to the Finger 

Bay volcanics is uncertain, but Coats has grouped them with the 

younger volcanic rocks. The age of the Finger Bay volcanics is discussed in Coats' report (1956a, p. 49) and on pages 383 and 384 of this 

report.

PETROLOGY

The rocks on southern Adak and Kagalaska Islands may be divided 

into four units: Finger Bay volcanics, the most abundant rocks in the 

area; granodiorite, quartz-diorite, diorite, gabbro, and associated 

rocks in large intrusive masses; aphanitic and porphyritic-aphanitic 

rocks, mostly andesite, in small intrusive bodies; and Pleistocene and 

Recent surficial deposits including an extensive ash-soil blanket over 

much of the area. These units are listed and described in order of 

decreasing geologic age, except in the case of the third unit, the aphanitic andesite, which probably embraces rocks of several different ages, 

both younger and older than the granodiorite plutons.

FINGEB BAY VOLCANICS

The name "Finger Bay volcanics" was applied to rocks near Finger 

Bay, northern Adak Island, by Coats (1947, p. 75). Minor dikes, 

which Coats included with the Finger Bay volcanics, will be discussed 

in a separate section because some of them appear to be more closely 

related to the major plutons, and some of the dikes may belong to an 

independent volcanic episode.

The Finger Bay volcanics (pi. 52) cannot be subdivided by reconnaissance methods. They are dominantly pyroclastic deposits and 

lava flows andesitic and basaltic rocks rendered more or less homogeneous in outward appearance by pervasive alteration and an intricate 

joint pattern. The color is generally a low chroma variation of 

medium-gray or medium dark gray, roughly reflecting the initial color 

index. Greenish hues resulting from the presence of chlorite, epidote, 

and nontronite are most common. Hues grading through yellow, red, 

purple, and brown are also common and they reflect various types and 

amounts of iron oxides.
378 INVESTIGATION OF ALASKAN VOLCANOES

Lava, pillow lava, agglomerate, tuff breccia, autoclastic breccia, 

lapilli tuff, tuff, volcanic wacke, argillite, and minor porphyritic masses 

have been recognized.1 Rhyolite mentioned by Coats (1956a, p. 48, 

pi. 9) has not been found on southern Adak or Kagalaska Islands. 

Many gradations exist between the various rock types, and in the field 

it is difficult to determine the precise character and origin of a particular rock. Perhaps 70 percent of the rocks are pyroclastic, 20 percent 

lava flows (including probable sills whose relationships are obscure), 

and a scant 10 percent truly sedimentary deposits.

Total thickness is unknown, but a very approximate value obtained 

from the geologic map (pi. 52) for the area from Hatchet Lake to Bay 

of Islands is 8,000 feet. This value is probably less than the true 

thickness, because neither top nor bottom is exposed.

LAVAS

In a group of 30 thin sections of lavas from the Finger Bay volcanics, 13 appear to be andesite, 6 basalts, and 11 are so altered or 

thermally metamorphosed that the original character cannot be estimated. Most of the lava sections probably are pyroxene andesite or 

f eldspathic basalt, but extreme alteration and an average of 80 percent 

aphanitic groundmass make even this general statement uncertain. 

The most common lava flows contain 10-20 percent phenocrystic 

plagioclase and 1-5 percent phenocrystic augite; rocks with more 

augite phenocrysts than plagioclase are less abundant. Greenstone 

and keratophyre are useful group names for the suite, but absence of 

general alteration of the pyroxene and irregular development of 

probably secondary keratophyre make either term somewhat 

ambiguous.

Distinctly basaltic rocks, some containing pseudomorphs after 

olivine, were sampled in the following areas: North Arm of Three 

Arm Bay, south of Gannet Cove, southwest coast of Boot Bay, and 

southeastern Adak Island along Kagalaska Strait; but andesitic rocks 

were also found in or near the same areas, and no large area is known 

to be dominantly basalt.

Rocks that originally were hornblende andesite or dacite and are 

now largely keratophyre or quartz keratophyre have been found 4 

miles south of Finger Bay, between Bay of Islands and Three Arm 

Bay, and 011 Yakak peninsula. Olivine-bearing basalt and dacite 

thus mark the probable original compositional limits for the bulk of 

lavas from the Finger Bay volcanics.

Specimen 6 (table p. 385) is the only analysed rock from the Finger 

Bay volcanics. It is porphyritic, brownish-gray albitized pillow lava

1 Names for pyroclastic rocks follow Wentworth and Williams (1932). Volcanic ivacke 

is defined by Gilbert (Williams, Turner, and Gilbert, 1954, p. 303) as "composed principally 

of debris eroded from volcanic terranes * * *."
379

from the southwest shore of Boot Bay, Adak Island. About 15 percent of the rock is tabular cloudy albite crystals (4 nun) altering to 

clay minerals and containing small patches of epidote; 5 percent of the 

rock is fresh, euhedral to subheclral augite crystals (1.1 mm). Some 

of these phenocrysts are clustered together in subophitic patches. 

About 10 percent of the rock is subspherical, chertlike greenish masses 

(1-4 mm), either A^esicle fillings or foreign pellets, containing chlorite, 

stilpnomelane, iron-rich mineraloids, and central masses of quartz. 

Some of the quartz is intergrown with orthoclase. About 70 percent 

of the rock is a cloudy fine-grained groundmass containing devitrified 

glass (chlorite, nontronite, epidote), altered albite microlites and 

skeletal crystals (0.3 mm), and elongate clinopyroxene microlites 

(0.3 mm), commonly in splayed aggregates. The groundmass is 

crowded with needlelike skeletal crystals of opaque oxide. The original composition of this rock is unknown; the altered rock might be 

called albitized trachyandesite. Albite, quartz, and orthoclase may 

all be secondary.

Most of Mount Vincennes is a mass of porphyritic pyroxene anclesite, and some parts of the mass are less altered than others. One 

small area about 150 feet below the summit is dark, surprisingly fresh, 

olivine-bearing andesite. Further mapping may show that these 

rocks are not properly part of the Finger Bay volcanics.

PYROCIjASTIC ROCKS

Large areas of Adak and Kagalaska Islands are underlain by 

altered tuff breccia and lapilli tuff. Altered tuff, flow breccia, and 

lava commonly are intercalated with, the tuff breccias and lapilli 

tuffs. The coarser pyroclastic rocks (tuff breccia and lapilli tuff) 

probably make up more than half of the Finger Bay volcanics. Most 

of them show little or no bedding and cannot be traced by reconnaissance mapping. The largest area known to be dominantly coarsely 

clastic rocks is the area from the Bay of Islands to Three Arm Bay. 

The southeast p'art of Kagalaska Island and large areas in southern 

and northern Adak Island also are dominantly coarse pyroclastic 

rocks.

In the field the nature of f ragmental rocks was in doubt in many instances. Many rocks, whose matrix megascopically appears to be 

igneous, have a tuff matrix rich in broken crystals, and most of the 

rocks contain enough angular unsorted pyroclastic material to rule out 

origin by conventional sedimentary processes. Most of the rocks, 

consequently, are classified as tuff breccia or lapilli tuff, and are believed to represent pyroclastic deposits or mud flows. Criteria for 

identfying deposits that have moved en masse as suibaerial or submarine mudflows were not found, so that the importance of mass 

movement cannot be assessed. ! "
380 INVESTIGATION OF ALASKAN VOLCANOES

Compositions, as far as can be determined in the altered pyroclastic 

rocks, were originally those of andesite and feldspathic basalt. In a 

group of 20 rocks, 11 are classified as andesitic, 3 as basaltic, and 6 

unknown. Fourteen of the rocks are conspicuously albitized, and all 

contain epidoate-clinozoisite or chlorite or both.

Many of the pyroclastic deposits contain coarse angular fragments 

as much as several centimeters in diameter, and a few rounded bombs 

or lapilli. Some contain, abundant bombs and can be classified as 

agglomerate, but no vent areas were recognized. Lapilli tuff with a 

coarse fraction of mostly angular crystalline rock fragments between 

1 and 8 mm is common. Coarser lapilli tuff and tuff breccia are also 

common, but proportions of the various grade sizes are unknown. 

Rock fragments are altered hyalopilitic or intersertal, porphyritic 

andesite and basalt -and less commonly devitrified basic glass. One 

coarse-grained diorite or gabbro fragment (5 cm) was found in a tuff 

breccia from the south shore of Quail Bay, Kagalaska Island. This 

is the only plutonic rock fragment found. Broken crystals of altered 

plagioclase (4mm) and generally fresh pyroxene (1-2 mm) are common. The coarse fragments are set in a tuffaceous matrix of similar 

composition.

SEDIMENTARY ROCKS

There are few sedimentary rocks in the Finger Bay volcanics on 

southern Adak and Kagalaska. Most of them have a direct pyroclastic component; and probably all are ultimately derived from volcanic rocks. Greenish-gray, banded, siliceous and probably tuffaceous argillite and medium- to coarse-grained volcanic wacke are both 

sparsely represented in the dominantly igneous and pyroclastic 

sequence.

There are several thin argillite beds (± 20 feet ) in the Gannet Cove 

area. Similar argillite and siltstone beds, some with structures connoting penecontemporaneous deformation, crop out in the Wedge 

Point area. Pebbles of a chertlike, finely banded, and minutely 

faulted argillite were found on the west beach at Blind CoAre. No 

argillites were found on Kagalaska.

Well-bedded pyroclastic rocks or volcanic wacke are common in 

the following locations and sparse elsewhere; Ring-gold Island, around 

the head of Bay of Islands, in the Boot Bay area (some show graded 

bedding), on Boot Point, and on Yakak peninsula especially in the 

Wedge Point area. These rocks are intercalated and gradational with 

poorly bedded pyroclastic deposits. The poorly stratified rocks in 

most cases are demonstrably pyroclastic, and many of the stratified 

rocks may be more pyroclastic than epiclastic.

On the northwest shore of Bay of Waterfalls highly altered lapilli 

tuff contains angular accidental fragments. The rock is very calcare-
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND '381

ous and a few of the fragments appear to be limestone. This suggests 

that limestone may be present in the Finger Bay volcanics or immediately below them.

Further mapping may reveal separable parts of the Finger Bay 

volcanics, such as the sequence of rocks containing fairly well bedded 

clastic rocks that appear to be crudely peripheral to Adak Island on 

the west, southwest, and south sides, and local members will probably 

be found within the sequence. Present information indicates that all 

these rocks are highly altered and precede the granodiorite in age.

ALTERATION AND METAMORPHISM

Alteration of rocks of the Finger Bay volcanics is pervasive. Nearly 

all of the rocks, regardless of original lithologic character and proximity to plutons, have been subjected to chloritization, epidotization, 

albitization, silicification, and other diverse low-intensity alterations. 

Locally albitization, silicification, epidotization, and potash metasomatism have been inferred within the plutons and in immediately 

adjacent country rock, but not all the alteration can be related directly to late stage solutions from the plutons. Narrow zones at the 

contact with plutonic rocks have been thermally metamorphosed, but 

no dynamic metamorphism has been noted. No tight folding or regional shearing is apparent.

The alteration products and secondary minerals observed are: 

chlorite, epidote, clinozoisite, albite, quartz, chalcedony, nontronite 

a,nd other clay minerals, sericite, calcite, prehnite, zeolites, iron-rich 

rnineraloids and iron oxides. Most pyroxenes are surprisingly fresh, 

but a few have been altered to uralite, bastite, chlorite, epidote, and 

other minerals. Bowlingite-antigorite locally is pseudomorphous 

after olivine. Disseminated pyrite and potash feldspar are common 

near plutonic borders and sparse elsewhere. Spessartite (?) was 

observed in one slide. This mineral is not out of equilibrium in a mineralogical environment of this type (Williams, Turner, and Gilbert, 

1954, p. 222) and may be sparsely represented elsewhere in the 

sequence.

Nearly all rocks contain epidote and chlorite, especially in plagioclase and devitrified parts of the rock. About 60 percent of the rocks 

examined are conspicuously albitized, and many contain albite as the 

sole plagioclase. All gradations from unaltered calcic or intermediate 

feldspar relicts through clinozoisite and plagioclase, veined, rimmed, 

and partly albitized plagioclase, chessboard albite, and cloudy or even 

clear albite are visible. Fewer lavas than pyroclastic rocks appear to 

be completely albitized. In a few rocks a radiating aggregate, texturally like some zeolites, is actually albite, suggesting that albitization 

may have accompanied or followed zeolitization.

504449 59 3
382 INVESTIGATION OF ALASKAN VOLCANOES

Many of the crystalline rock fragments in pyroclastic rocks, and 

even the entire rock, may be chertlike and brittle. Argillite also is 

chertlike and commonly breaks with conchoidal fracture. These features and the abundant silica veinlets in many areas suggest that pervasive silicification may be more important than is apparent in the 

generally enigmatic thin sections.

In many areas, especially near Bay of Waterfalls and in northeastern Adak Island, a narrow hornfels zone occurs in the Finger Bay 

volcanics at the contract with plutons. The hornfels texture generally 

obscures or obliterates original texture, but phenocrysts and breccia 

fragments may, locally, be identified. Labradorite and clinopyroxene 

is perhaps the most common mineral association; but andesine, orthopyroxene, hornblende, tremolite or actinolite, and biotite are also 

common. Magnetite is a conspicuous accessory mineral. Finegrained amphibolite without hornfels texture forms the septa of country rock in the Blind Cove area where granodiorite selectively invades 

the Finger Bay volcanics. Some of the fine-grained amphibolite may 

be metasediments.

In general the homf els plagioclase is unzoned, but relict zoning has 

been observed in a few grains. In some slides poikiloblastic but otherwise homogeneous labradorite crystals (recrystallized phenocrysts) 

contain pyroxene arranged in zones which strongly suggest clinozoisite alteration patterns found elsewhere in lavas of the Finger Bay volcanics. The general absence of albitization in the hornfels rocks and 

occurrence of the zonally arranged pyroxene suggests that these rocks 

were hydrothermally altered prior to intrusion and thermal 

metamorphism.

In some areas a third episode of change is also apparent: albite, 

epidote, quartz, and potassium feldspar derived from the plutons have 

modified the plutonic borders and invaded the hornfels zone. This 

last stage is similar to the earlier pervasive alteration, but includes 

some potash metasomatism and is usually limited to veins and small 

areas near the plutoii.

The source for the pervasive solutions that altered the Finger 

Bay volcanics before plutonic invasion is not known; several possibilities to be considered are: sea water, deuteric solutions from the 

Finger Bay magmas, activated connate water, and fluids emitted from 

the granodiorite plutons before or during intrusion. Abundant petrographic evidence for secondary changes eliminates the possibility of 

initial crystallization of the Finger Bay volcanics in their present 

state.

Marine volcanic rocks of very similar original lithologic character 

are not altered on most of Kanaga Island, Tanaga Island, and the cen-
383

tral Delarof Islands (Fraser and Barnett, 1959, p. 217). This forces 

the conclusion that sea water and deuteric solutions from the volcanic 

emissions themselves are probably inadequate to cause the type of 

alteration common in the Finger Bay volcanics. Part of the Finger 

Bay alteration may have been diagenetic or deuteric, but general 

alteration has exceeded any early palagonitization or extremely low 

intensity changes that may have resulted from the nearly syngenetic 

processes. Apparently a period after the deuteric and preceding the 

granodiorite epoch of hydrothermal activity is required. This requirement is met by activated connate waters or juvenile fluids from 

the plutons. The altering wave of solutions, regardless of source, 

may have been driven upward by the advancing plutons.

These conclusions are in general agreement with those presented in 

an extensive review of the keratophyre-spilite problem (Gilluly, 1935) 

and are similar to views recently expressed by Waters (1955, p. 710) 

for altered andesitic rocks in the Cascade Mountains. Waters considers alterations in the Cascade Mountains as essentially independent 

of the later plutons. We agree that alteration precedes intrusion, but 

believe that there may be a relationship between the two events on 

Adak.

There is a further possibility that the Finger Bay volcanics are 

themselves a very early product of the same magmatic cycle, so that 

a single process culminating in major intrusive activity may control 

most of the seemingly separate events recorded on southern Adak and 

Kagalaska Islands.

REGIONAL RELATIONSHIPS AND AGE

Coats (1956a, p. 49) included a small body of fossiliferous yellow 

sandstone of volcanic composition, basaltic flows, and sills that crop 

out between Clam Lagoon and Andrew Lake in northern Adak Island 

in his Finger Bay volcanics as mapped. The rocks are of Pemisylvanian or Permian age as indicated by impressions of Annulana stellata 

(Schlotheim) Wood, identified by R. W. Brown, U.S. Geological Survey. The relations of these fossiliferous rocks to the rocks of Finger 

Bay, which are continuous with those mapped as Finger Bay volcanics 

in southern Adak Island (and identical with those of Kagalaska Island) are uncertain.

A sequence of volcanic and sedimentary rocks containing some 

pillow lava deposits occurs on Kanaga Island, 10 miles west of Adak 

Island (Fraser and Barnett, 1959, p. 217). From Kanaga Bay north 

for several miles these rocks have been epidotized. Miocene (?) marine fossils have been found in this locally altered sequence. These 

rocks are certainly as much like the deposits on Yakak peninsula as 

are the Paleozoic sandstone beds, 25 miles north on Adak Island, and
384 INVESTIGATION OF ALASKAN VOLCANOES

a correlation in one direction is as reasonable as a correlation in the 

other.

We therefore hesitate to adopt the age of the volcanic sandstone 

beds of northern Adak Island as fixing that of the Finger Bay volcanics of the type area. They are doubtless terrestrial sandstone beds 

as suggested by the leaves, whereas we interpret the Finger Bay volcanics of both our area and the type area as marine. Although the 

age assignment for these rocks on northern Adak Island has been 

Paleozoic (?), the authors believe that a Tertiary (?) age is more probable for the Finger Bay volcanics elsewhere on the island and on 

Kagalaska Island.

GRANODIORITE AND BELATED ROCKS

Plutons, composed largely of granodiorite, but varying locally to 

quartz monzonite, quartz diorite, diorite, and gabbro invade the Finger Bay volcanics in several areas (pi. 52). The largest of these 

masses, located in south-central Adak Island, covers an area of about 

40 square miles. Other plutons occur in central, north-central, and 

northeastern Adak Island and in Kagalaska Island. Five chemical 

analyses of these rocks are given in the following table. The several 

varieties are not separated on the map, but most of the rocks examined 

petrographically have enough potash feldspar to be classified as 

granodiorite according to Lindgren's definition (Lindgren, 1900).

The plutonic rocks, characteristically, are medium gray and of 

medium-sized grain, granitoid in appearance, with biotite and hornblende as the dark minerals. The color index (percent total mafic 

minerals) is generally less than 20 and may be as low as 8. Small 

aplite and pegmatite dikes are common locally, especially near the 

contact in both wall rock and pluton. Near some of the contacts and 

in areas of many inclusions, the color index may rise to 40, pyroxene 

is an important constituent, the feldspars are more calcic, and the 

rocks are diorite and gabbro.

PETROGRAPHY

GRANODIORITE AND QUARTZ DIORITE

111 the common granodiorite and quartz diorite zoned andesineoligoclase is the main feldspar, usually making up 55-60 percent of 

the rock. Other minerals are: quartz 10-25 percent; orthoclase and 

perthite 7-15; hornblende 2-15; biotite 1-8; clinopyroxene, orthopyroxene (rare), magnetite, epidote, apatite, and sphene usually total 

less than 5 percent. In some specimens, near the border zones of 

the plutons, albite is conspicuous in perthite, rims on other plagioclase, 

irregular masses, and as crystals containing a chessboard pattern of 

twins. Irregular masses and rims of orthoclase and also antiperthite 

have been noted, but euhedral orthoclase was not found. Patches
GEOLOGY OF SOUTHERN ADAK ISLAND AND.KAGALASKA ISLAND 385

TABLE 1. Chemical composition of rocks from Adak Island, Alaska 

[E. J. Tomasi, analyst, U.S. Geological Survey]

Si02 . ------ -

A12O.1. ------

T?Oi>O->

FcO .........

MgO. ........

CaO ...... -

KsO.. 

HzO-. 

HzO+- ...

TiO2 ........

CO,. .........

P2 0s .........

01- 

F-. 

S.. ------

MnO -

Total. ........

Powder density. . 

Name and occurrence.

Specimen No.

1

02. 02 

16.94 

2. 6.1 

2.77 

2.47 

5.39 

4.17 

J.81 

.08 

.58 

.08 

.02 

.20 

.05 

.03 

.00 

.10

99.98 

.02

99. 90

Bay of Waterfalls, 

southwest 

Adak Island.

Biotite 

quartz diorite, 

pluton.

2

03. 20 

17.21 

1.90 

2.33 

2.48 

5.39 

4.11 

2.15 

.04 

.45 

.40 

.00 

.18 

.02 

.04 

.01 

.08

100. 05 

.02

100. 03

.Kagalaska 

Strait, 

northeast 

Adak Island.

Oranodiorite, 

pluton.

3

52. 80 

18.00 

4.24 

4.78 

4.53 

8.09 

3.30 

1.12 

.07 

.79 

1.03 

.03 

.31 

.00 

.05 

.01 

.16

100. 09 

.03

100.00

Beyer Bay, 

south 

Adak 

Island.

Dioritc, 

pluton 

border 

zone.

4

45. 08 

18. 77 

5.64 

5.13 

0.74 

12.93 

2.02 

.11 

.22 

1.37 

.74 

.35 

.05 

.01 

.02 

.00 

.14

99. 92 

.01

99. 91

2. GO 

Chapel Cove 

(Bay of 

Waterfalls) 

southwest 

Adak Island. 

Olivino gabbro, sill.

5

58.32 

18.75 

3.23 

2.70 

1.81 

6.16 

4.56 

2.52 

.02 

.38 

.92 

.02 

.40 

.07 

.07 

.00 

.12

100.11 

.05

100. 00

2.72 

Bay of Waterfalls, 

southwest 

Adak Island.

Feldspathic 

granodiorite 

porphyry, 

pluton border zone.

0

50. 06 

15.46 

3.94 

4.69 

3.09 

5.60 

4.86 

2. 19 

.23 

1.59 

.93 

.05 

.38 

.02

; .06

.00 

.14

99. 89 

.03

99. 86

West shore of 

Boot Bay, 

south Adak 

Island.

Albitized pillow lava 

(Finger Bay 

volcanics) .

of quartz monzonite are found in orthoclase-ricli areas, usually in 

association with gabbroic rocks, which may also have been albitized 

and veined with orthoclase.

Analyzed specimen no. 1 (see table) is representative of the quartz 

diorite. It is a gray, medium-grained biotite quartz diorite with a 

color index of 16. Volume percentages are: zoned and imzoned 

andesine-oligoclase, 56; orthoclase, 7.2; quartz, 21; biotite, 8.0; hornblende, 3.6; clinopyroxene, 2.4; opaque oxide, 1.0; chlorite, 0.4; apatite, 

0.3; sphene and epidote, trace. This rock, from the east shore of 

Bay of Waterfalls, is located near the edge of the largest pluton. 

Coarser and darker border phases with septa of recrystallized basaltic 

rocks occur just a few yeads from this rock, but the main contact is 

underwater in Bay of Waterfalls (pi. 52). Characteristically, rocks 

this close to the border are more mafic than the main mass, and appear 

blotchy from irregular concentrations of dark minerals.

The rocks 1,500 yards south of Blind Cove have a color index near 

10 and are more felsic than most of the granodiorite. These rocks 

contain a few dark rounded, fine-grained patches as much as several

504449 59 3
INVESTIGATION OF ALASKAN VOLCANOES

centimeters in diameter that appear to be recrystallized and partly 

assimilated wall rock. A marked preferred orientation of biotite and 

hornblende was noted in some outcrops. A peculiar texture, suggestive of mortar structure, is visible in thin section. This texture 

consists of large (3 mm) zoned plagioclase crystals with narrow 

interstitial bands of granular quartz (0.4 mm) and orthoclase 

(0.2 mm). The quartz and orthoclase crystallized later than the 

plagioclase; possibly magma motion helped to produce the texture. 

A modal analysis of one of these rocks (analyzed specimen no. 2) 

gives the following volume percentages: zoned andesine-oligoclase, 

57; orthoclase, 11; quartz, 21; biotite, 4.7; hornblende, 4.5 (some of 

these in blocky crystals with a maximum diameter of 8 mm); opaque 

oxides, 0.9; chlorite (mostly after biotite), 0.8; sphene, apatite, and 

zircon, total, 0.1.

Many variations of the main granodiorite exist along the borders 

and in small outliers. Near the contact of a small pluton on Kagalaska Strait (Campers Point) a dike of quartz diorite shows conspicuous clusters of hornblende needles as much as 2 cm long. On 

Kagalaska near Chaika Point the contact is rich in hornblende 

here apparently recrystallized Finger Bay volcanics. In many areas 

dark-colored phases are developed at or near the border.

DIORITE

Diorite is common near the borders of plutons, but is volumetrically insignificant. Analyzed specimen no. 3 is medium-dark-gray, 

medium-grained diorite from the west shore of Beyer Bay. It came 

from, an outcrop containing abundant xenoliths and large tabular 

septa of basaltic country rock. The xenoliths and septa have a conspicuous hornf els texture. Clinopyroxene, amphibole, orthopyroxene, 

biotite, magnetite, and sodic labradorite are common in the recrystal 

lized rock. Large biotite flakes rim the xenoliths, and magnetite 

crystals are visible in them. Under the microscope a few large crystals of plagioclase- can be seen in the xenoliths. These crystals are 

not zoned, but they show interior and zonal patterns of minute inclusions suggesting that the original rock contained zoned feldspar and 

was porphyritic. Xenoliths occur in all stages of fragmentation and 

change toward diorite, and those that have been greatly made over 

appear as vague dark-colored clots in the diorite.

The pluton here is low-silica diorite whose clotted amphibole and 

poikilitic biotite suggest contamination of an originally granodioritic 

magma. The estimated mode of the diorite (specimen no. 3) exclusive of recognizable xenoliths is as follows: zoned plagioclase 

(An65_45 ), 60 percent; hornblende, generally in clusters around a 

pyroxene nucleus, 15 percent; biotite, many with sieve textures, 10
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 387

percent; clinopyroxene, altering to hornblende, 5 percent; magnetite, 

5 percent; quartz, 5 percent.

GABBRO

Gabbro occurs in several border areas and as a few isolated small 

intrusive deposits. The border zone gabbro appears to be directly 

related to the granodiorite, but the relationships of the other masses 

are less clear. The detached masses could be genetically related intrusive rocks injected into an altered and shattered roof zone in 

advance of the main plutons, or .they could belong to an earlier epoch 

of more mafic intrusions.

Analyzed specimen no. 4 is coarse-grained gabbro from a sill(?) 

500 yards south of the head of Chapel Cove, Bay of Waterfalls. 

Similar gabbro and diorite appear in small patches nearby (pi. 52). 

The irregularly intruded area south, east, and northeast of Chapel 

Cove is interpreted as a roof zone over a large pluton. The area

south of Chapel Cove contains at least three sill,-like masses of 

gabbroic rock that intrude altered tuff breccia and lapilli tuff of the 

Finger Bay volcanics. The gabbro in this area is extremely variable 

with a variable color ratio and local porphyritic facies. Specimen 

no. 4 is the most mafic rock analyzed or examined. It is a bytownitelabradorite olivine gabbro with abundant magnetite. Small chips of 

the rock can be picked up with a pocket magnet. Dark minerals 

occur in single crystals or as clusters as great as 1 cm in diameter, 

but usually less than 5 mm. These dark constituents are set in a 

finer (less than 3 mm) matrix of feldspar laths. Feldspars are subhedral and the clinopyroxene generally anhedral. An estimated 

mode gives the following volume percents: zoned bytownitelabradorite, 65 percent; clinopyroxene, 15 percent; magnetite, 7 

percent; olivine pseudomorphs (antigorite?), 8 percent; interstitial 

and pseudomorphic penninite and carbonate, 5 percent.

Some of the dark border rocks, interpreted in the field as gabbro, 

have been so .changed by sodic, potassic, and silicic solutions that their 

original characters are uncertain. Some dark quartz-diorite and granodiorite have probably been mistaken for gabbro or diorite in the 

border zones. The essential fact is that the border facies commonly 

is conspicuous for dark minerals, and locally intense deuteric alteration that produced light-colored (aplitic) masses as well.

Coats (1956a, p. 50) described a gabbro pluton containing quartzrich phases in north-central Adak Island near Finger Bay. This 

mass is probably related genetically to the other plutons, and thus 

the exposed rocks may be largely peripheral or roof rocks on a more 

silicic pluton.
388 INVESTIGATION OF ALASKAN VOLCANOES 

FELDSPATHIC GRANODIORITE PORPHYRY

Near the contact with Finger Bay volcanics, a dark-gray porphyritic feldspathic granodiorite, color index commonly 10 to 15, is a 

conspicuous and characteristic rock. Analyzed specimen no. 5, from 

the contact at the north end of Bay of Waterfalls, is one of these. 

About one-third of the rock is large (±7 mm) crystals of cloudy, 

dark-gray zoned plagioclase that give it the appearance of anorthosite. A preferred orientation of these crystals was observed in several outcrops. The large plagioclase crystals are set in a fine, subhedral-granular matrix. The mode is: plagioclase, 65.5 percent; alkali feldspar and orthoclase, 13 percent; quartz, 8.2 percent; hornblende, 7.6 percent; clinopyroxene, 1.6 percent; opaque oxide, 3.0 percent; biotite, 0.4 percent; chlorite, 0.3 percent; apatite, 0.3 percent, 

and sphene, 0.1 percent.

This rock was never found in the interiors of plutons; its appearance, on a traverse through granodiorite, was used to help locate a 

contact with Finger Bay volcanics. Rocks of this type are usually 

associated Avith gabbro and diorite, and by increase in mafic constituents and calcic feldspar grade into them. Differential development of 

albite and orthoclase, as overgrowths on previously formed plagioclase, obscure the original nature of the .feldspathic granodiorite.

PETROGBJfESIS

HORNFELS AND FLOW STRUCTURE

The hornfels zone estimated to be betAveen 1 and 50 feet thick on 

clean steep contacts and possibly much thicker in roof zones, surrounded septa, and intricately injected areas such as the mixed zone 

on Kagalaska Island (pi. 52) demonstrates a steep thermal gradient 

across many of the plutonic borders. This zone is interpreted as evidence for hot magma injection.

Mineral orientation was not mapped but was noted in widely separated areas. This suggests that parts of the .magma were moving 

during crystallization. A dimensional orientation of large tabular 

inclusions or septa (pi. 52) may be interpreted as relict orientation, 

reflecting joints or bedding in rocks of the Finger Bay volcanics, and 

cannot be used as evidence for movement without more field work.

MAFIC BORDER ROCKS

Most mafic rocks are limited to narrow border, zones on larger, more 

felsic plutons. Wide roof .zones of mixed hornfels and basic plutonic 

rocks have been mapped on Kagalaska Island. These zones usually 

contain abundant dark-colored recrystallized and. rounded xenoliths 

of basaltic or andesitic country rock. Clinopyroxene and subordinate 

orthopyroxene are abundant in hornfels and xenoliths, less abundant
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 389

in the dark igneous rocks, and rare in the interiors of plutons. Sieve 

texture in biotite, pyroxene, and amphibole is common in xenoliths 

and in immediately adjacent diorite and gabbro but is uncommon in 

the interior of plutons.

In several areas a fine-grained, porphyritic, quartz diorite was noted 

in small separate plutons. These suggest that when a pluton was small 

enough it was chilled and the chilled part is more silicic than the border rocks of large plutons. Thus basic border rocks are not merely 

chill zones, especially as most are coarse grained; they must result, 

at least in part, from reactive assimilation or recrystallization of dark 

wall rocks. Xenoliths and xenocrysts by reaction would precipitate 

minerals in equilibrium with the early hot magma, and a hybrid mafic 

rock would result (Bowen, 1928, p. 197 ff). Some mafic rocks, however, appear not to have moved at all they have a very coarse grained 

metamorphic fabric and are simply recrystallized basalt and pyroxene 

andesite; they could be partly metasomatic. Gabbro away from the 

border zone may represent early-phase basic magmas or peripheral 

hybrid rocks injected into the shattered roof zone. Their sill and dikelike character argues against a purely metasomatic emplacement.

MIGRATION OF FELSIC MATERIAL

There is abundant evidence of migration of aplitic and pegmatitic 

material at a late stage within the plutons and also in the adjacent wall

FIGURE 57. Replacement breccia at granodiorite contact, southeast shore of Blind Cove, 

Adak Island. Dark rock is recrystallized Finger Bay volcanies ; light-colored material 

is granodiorite (nearly aplite) with seams of epidote along fractures and at contact between light-colored and dark rock. Photograph by B. II. Bieler.
390 INVESTIGATION OF ALASKAN VOLCANOES

rocks. Dikes, small masses of quartz monzonite, and quartz veinlets 

are conspicuous. In addition, a more delicate but commonly fracturecontrolled silicic infiltration is apparent under the microscope. Albitization, potash metasomatism, epidotization, and silicification within 

the pluton and in closely adjacent areas are probably directly related 

to the granodiorite magma, but widespread pervasive alteration 

(p. 381) may have occurred earlier.

Figure 57 is one type of contact between granodiorite and country 

rock. Here the wall rock was intricately fractured and then infiltrated 

with aplitic magmas or solutions; no strong motion is apparent, but 

some of the structures suggest dilation as well as replacement. Epidote is abundant as veins in the central fractures in dikes, along contacts with recrystallized Finger Bay volcanics, and as disseminations 

and clusters especially in the light-colored rocks. The wall rock here 

is a medium- to fine-grained biotite-plagioclase-quartz-epidote rock. 

The biotite is green aggregate of fine grains and flakes and may be 

transitional between chlorite and hornblende.

The light-colored aplitic rock is extremely variable. One rock has 

a color index of 25 with abundant pleochroic green to brown hornblende and mostly unzoned sodic andesine feldspar. This same rock 

contains 10 to 15 percent quartz. Another specimen from the same 

outcrop shows a sharp contact with wall rock and contains no hornblende, sparse biotite "masses" detached from the wall rock, and 

abundant quartz. This rock is about 60 percent albite, and epidote 

clusters are conspicuous. The rock types in the replacing dikes vary 

from syenodiorite to light-colored albite granite. Apparently the 

light-colored phases can be differentially contaminated by dark wall 

rock.

Replacement, evident at such contacts, is locally important, especially as at late-stage phenomenon associated with aplitic rest magmas. 

That entire plutons were so emplaced seems improbable in light of 

evidence for magmatic emplacement.

SUMMARY AND AGE

The processes discussed above all tend to produce contrasting rock 

types close to the borders and roof zones. In general these peripheral 

rocks are mafic, but late-stage felsic magmas and solutions have modified some areas. Moreover, the central zone of the pluton would be 

the locus for strongest vertical movements and latest surges of the 

presumably more felsic magmas; but the mafic sides would be retarded 

by friction and early crystallization. Generally the hybrid rocks of 

the roof zone were removed by erosion, exposing a pluton with a clean 

felsic center and a contaminated mafic border. Probably neither 

extreme represents the source magma which may have been diorite or 

quartz diorite a logical partial melt from a mafic basement.
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 391

Age of the main plutons is unknown. All dated plutons within the 

Aleutian Islands are Tertiary, but several cannot be dated. A mid-to 

late-Teritary intrusive episode, followed by uplift and erosion, has 

been postulated for the Aleutian arc (Gates, Fraser, and Snyder, 1954) 

and probably the plutons-on Adak Island are part of this episode,

ANDESITIC DIKES AND SILLS

Small, generally porphyritic dikes and sills of andesite, dacite, rhyodacite, and keratophyre cut Finger Bay volcanics and granodiorite on 

southern Adak and Kagalaska Islands. Sixteen of these small intrusive bodies were sampled and studied petrographically.

DIKE BOCKS IN THE FINGER BAY VOLCANICS

The Finger Bay volcanics are cut by many small dikes and a few sills. 

A few of the larger dikes and sills have been mapped (pi. 52) but many 

undoubtedly were missed because of poor exposure, similarity with the 

wall rock, and incomplete coverage. These intrusive rocks probably 

range from basalt to rhyodacite, though no basalt was detected among 

those studied in thin section. They are dominantly andesitic and are 

known to range from pyroxene andesite to rhyodacite. Two of seven 

studied rocks are keratophyre and one is quartz keratophyre. Typically the dikes are porphyritic or microporphyritic with plagioclase, 

hornblende, augite, and altered orthopyroxene (bastite?) as common 

phenocrysts. The groundmass is generally a felted or trachytic mass 

of feldspar microlites, clinopyroxene grains, and opaque oxides; some 

rocks show a protoclastic structure. One dike contains abundant 

interstitial chlorite, believed to be devitrified glass.

All dikes and sills examined in the Finger Bay volcanics have been 

altered like the host rock epidote and chlorite are in every slide. In 

some albite is the sole plagioclase. Vein albite, clear patches of albite 

in masses of cloudy albite, and associated chlorite, epidote, and calcite 

suggest that these rocks are albitized andesite and dacite or secondary 

keratophyre (Gilluly, 1935; Williams, and others, 1954). If the 

original magma had been as sodic as the present rock, the amphibole 

and pyroxene (mostly unaltered) should likewise be sodic, but they 

are not.

DIKE ROCKS IN THE GRANODIORITE

Dark dikes are more conspicuous in the light-colored plutonic rocks 

than in the dark Finger Bay volcanics, but are probably less abundant. 

Near the pluton margins tabular septa of hornfelsed Finger Bay 

volcanics with the same general dimensions as the average intrusive 

dike are easily mistaken for dikes, and some may have been so mapped.

Nine dikes cutting granodiorite were sampled. They are mainly 

andesitic, but range in composition from dark-colored hornblende
392 INVESTIGATION OF ALASKAN VOLCANOES

andesite to dacite. Two are unaltered, but the others are chloritized, 

and most contain epidote and sericite. Abundant carbonate was 

found in three slides. One quartz keratophyre and one hornblende 

keratophyre contain albite as the sole plagioclase; these are believed to 

be albitized dacite and andesite. One trachyandesite is the only 

sampled dike rock with appreciable potash feldspar.

SUMMARY AND AGE

The abundant small intrusives in the rocks of southern Adak and 

Kagalaska Islands are dominantly andesitic more silicic than the 

andesite and basalt of the Finger Bay volcanics. Many of the dikes, 

especially the sporadic orthoclase-bearing ones, are probably late 

apophyses from the deeper, uncrystallized granodiorite magma, but 

some of those that cut only the Finger Bay volcanics are probably 

feeders and associates of the country rock.

Alteration of the dikes that intrude the plutons does not prove that 

all the alteration of the Finger Bay volcanics and dikes postdates the 

emplacement of the plutons. There is evidence for at least two stages 

of differential albitization: one widespread epoch preceding major 

plutonic invasion, and one localized epoch accompanying and closely 

following plutonic invasion (see p. 382). The earlier alteration epoch 

may still be related to the plutons as indeed some lavas of the Finger 

Bay volcanics themselves may be related in an indirect, preliminary 

way. Possibly some lavas, of the Finger Bay volcanics, early dikes, 

and altering solutions were precursors of the grandiorite just as some 

later dikes and alterations may be after effects of the intrusion.

Coats (1956a, p. 51) reports five andesite porphyry domes along the 

northwest coast of Kuluk Bay and typically developed at Zeto Point. 

These domes are outside the area of this report and were assigned by 

Coats to an independent sequence of Tertiary volcanic rocks, younger 

than the larger intrusive masses but older than the volcanic rocks at 

Mount Moffett and Mount Adagdak. The relationship of these domes 

to the dike rocks just discussed is unknown.

SURFICIAL DEPOSITS 

GLACIAL DEPOSITS

Visible glacial deposits are extremely rare on southern Adak and 

Kagalaska Islands because most of the glacial debris was dumped in 

the sea, and the remainder has been covered by subsequent ash layers. 

Nevertheless scattered small deposits of till and occasional erratics 

have been found on southern Adak Island (Bradley, 1948, fig. 11), 

and more extensive deposits have been mapped on the northern part 

of the island (Coats, 1956, pi. 9).
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 393 

VOLCANIC ASH AND SOIL.

Recent ash layers, soil, and vegetation cover much of the bedrock 

below 1,500 feet. These deposits are nearly continuous at lower elevations and have been omitted from the map. A typical surficial 

mantle on southern Ada.k and Kagalaska Islands would consist of 

about 5 feet of gray to brown volcanic ash, banded with soil and 

reddish-brown layers, and capped with a still-forming, dark-brown 

soil-A7 egetation layer as much as 3 feet thick. The bedded ash is unstable on slopes, commonly showing distortion of layers by soMuction. 

It is alAvays saturated Avith water, but so impermeable that small 

ponds form AA7 herever vegetation has been broken by slump or wind 

scour. Despite the tendency for mass movement, the streams cutting 

the mantle remain clear throughout the year (Bradley, 1948). Bradley states further that the ash mantle probably accumulated above the 

level of streams, so that streams, which are apparently incised, formed 

by removing ash as fast as it settled much as a stream removes 

falling snow.

Kellogg and Nygard (1951, p. 39) have measured and described the 

present soil on Adak Island, Avhich they classify as tundra without 

permafrost, and in another place as aHuvial-tundra soil. They have 

demonstrated an incomplete tundra zonation in the measured profile 

(30 inches) of tundra without permafrost. Bradley (1948) records 

4 or 5 buried soil horizons in addition to this most recent laj^er. Elsewhere in the Andreanof Islands, Anderson and Bank (1952, p. 84) 

and Bank (1953, p. 497) haA^e measured profiles through the entire 

sequence.

In the Adak Island to Great Sitkin Island area (Bradley, 1948) and 

on nearby Kanaga Island (Coats, 1956b; Fraser and Barnett, 

1959) a coarse pumiceous layer commonly lies at the base of the ashsoil ma.ntle. This coarse layer is extremely widespread-and may haA-e 

originated during an episode of catastrophic eruption related to 

caldera formation on Tanaga, Kanaga, or Great Sitkin Volcanoes.

Judson (1946) has emphasized the layering in stabilized dunes on 

northern Ada.k Island, but his study is of an uncommon type of surficial deposit Avhose relations AA^ith the more general ash-soil mantle are 

not clea-r. Apparently three late episodes of dune formation occurred 

on northern Adak Island, and these wind-deposited beach sands mask 

and perhaps intertongus with the ash-soil mantle. The thickness 

of the dunes reported by Judson (as much as TO feet) is far in excess 

of the thickness in the uniform mantle OA'er most of Adak, Kagalaska, 

and Kanaga Islands.

Coats (1956a) and Bradley (1948) have remarked on the uniform 

thickness of the ash-soil mantle on Adak Island, and have used this 

uniformity as evidence for a distant rather than local source for the
394 INVESTIGATION OF ALASKAN VOLCANOES

ash beds. It must be emphasized, however, that the mantle is a composite of many fine ash layers, possibly from many sources, and a few 

thin soil or humic zones which may not be of islandwide significance. 

The uniform thickness suggests that the mantle is approximately correlative throughout the area, but does not demonstrate equivalence of 

the various layers in widely separated areas. The thickness is maintained over most of the area considered in this report, where, for example, on southern Kagalaska the mantle aggregates 3 to 5 feet. Local 

variations are believed to result largely from an interplay of topographic factors, wind, and subsequent erosion. Bradley pointed out 

that volcanic activity on Adak Island was mostly preglacial and ash 

deposition postglacial; this also indicates a distant source. The presumed sources are volcanoes of the Andreanof Islands.

AGE OF THE ASH-SOIL, MANTLE

Deposition over glacially striated surfaces and local till dates the 

surficial mantle as younger than the latest extensive glaciation and 

suggests a Recent (post-Wisconsin) age for most of it. Radiocarbon 

dates for modern beach shells and for shells and peat from an Aleut 

midden on Adak Island (all above the bedded ash-soil mantle) are 

anomalous. Three dates have been reported (Kulp and others, 1952, 

p. 412). These dates are about 1,900 years for shells, which contained 

living organisms when collected; 4,600 years for Aleut midden shells; 

and 3,300 years for buried peat stratigraphically below the midden 

shells. The peat has artifacts strewn on its upper surface and possibly 

within it. All these dates come from samples collected by H. A. Powers in the Clam Lagoon area. The midden shells and peat come from 

a measured section above 4 feet of stratified ash. Buried beach or 

tsunami deposits are interstratified with the midden material.

As the radiocarbon dates are undependable, it is only safe to call all 

the ash mantle postglacial (Recent). The earliest Aleut habitation is 

younger than most of the ash mantle. Thin soil or humic zones may 

lack the age significance of soil zones developed elsewhere because here 

volcanic ash may bury and separate easily established vegetation layers 

that elsewhere would appear as a single unit.

BEACH AND ALLUVIAL DEPOSITS

Raised beach deposits on northern Adak Island, 20-100 feet above 

present sea level, have been noted by Coats (1956a) ; another beach 

deposit, 10 feet above present sea level and below stratified dune deposits, was described by Judson (1946). Three beach or tsunami deposits, slightly above present sea level, are reported by H. A. Powers 

(oral communication) : two below and one above an Aleut midden at 

Clam Lagoon, and all above a still older deposit of artifacts, peat, and
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 395

bedded volcanic ash. Similar raised beach deposits probably exist on 

southern Adak and Kagalaska Islands, but none have been found. 

Precise ages for the various raised beaches, and particularly the highest levels, cannot be established. The lower levels are definitely 

postglacial.

Only a few of the more recent surficial deposits on southern Adak 

and Kagalaska Islands are mappable on the scale of plate 52. These 

include stream valley deposits and alluvium filling interior drainage 

basins and lakes; the present beach deposits, commonly composed of 

talus blocks and boulders, are all too small to include on plate 52.

STRUCTURE

REGIONAL SETTING

The largest and most obvious structural element, of which Adak 

and Kagalaska Islands are a part, is the island arc extending about 

1,300 miles from the Komandorskiye Ostrova to Unimak Island with 

a 600 mile extension on the Alaska Peninsula (fig. 55). At the northeastward extremity of this belt active volcanism decreases and the 

arc trend becomes part of a more complex zone of mountain ranges 

that curve sharply to the southeast (see U.S. Geol. Survey, Alaska 

Map E, shaded relief, 1954).

South of Unimak Island and increasingly well developed through 

the Shumagin Islands, Kodiak Island, and Kenai Peninsula a well 

defined outer nonvolcanic arc parallels the inner arc; the Aleutian 

Trench, foredeep for the arc system, swings out to the seaward side 

of the double arc portion (Murray, 1945, pi. 1). The outer arc, if 

present at all in the Aleutians proper, is only crudely formed beneath 

the sea in a narrow zone between the crest of the Aleutian Eidge and 

the Aleutian Trench (the "Aleutian Bench" of Gates and Gibson, 1956, 

p. 143).

The axis of the Aleutian Trench is about 70 statute miles south of 

Adak Island, where the trench is 24,000 feet deep. North of Adak 

Island the bottom drops abruptly to about 9,000 feet in 10 miles and 

then gradually to about 12,000 feet. The 1,900-mile mountain range 

thus rises almost 28,000 feet above the foredeep and 13,000 feet above 

the backdeep in this area.

Abundant pre-Tertiary and regionally metamorphosed rocks are in 

the double arc segment east of Unimak Island, but at only one locality 

are the rocks older than Late Cretaceous, and no regional deform a- i . o ' .

tional metamorphism is known in the Aleutian Islands proper. 

Apparently the outer arc of the peninsular segment is older than the 

leas deformed island segment, though the oldest rocks known from the
396 INVESTIGATION OF ALASKAN VOLCANOES

Aleutian arc proper lie on the northern side of Adak Island toward 

the inner side of the arc.

The Adak-Kagalaska portion of the arc is typical of a large segment of the Aleutians in that central-type, shallow-marine, and subaerial volcanism of Quaternary age and all constructional volcanic 

landforms are limited to the north edge of land areas. Capps (1934) 

remarked on the linear arrangement of Aleutian volcanoes and suggested a major fault control. Gates and Gibson (1956), Fraser and 

Barnett (1959), and Snyder (1957) have also favored a major fault 

or faults for localizing volcanoes along the north edge of the arc. In 

the Adak Island area this fault, if present, is masked by the Bering 

Sea and volcanic debris. A major change in the trend of volcanoes 

occurs over Knluk Bay. From Tanaga Island to Adak Island the 

volcanoes are on a straight line and from Great Sitkin Island to Atka 

Island there is a similar linearity on an offset and slightly different 

trend.

FKACTURE PATTERN

In the map area of this report (pis. 52 and 53) similar fracture patterns are developed in the granodiorite and Finger Bay volcanics, and 

the resistance to erosion is about, the same for both rock types. Consequently, it is often impossible to tell from aerial photographs or from 

shipboard and mountain-top observation whether a distant rock mass

FIGURE 58. Aerial view north-northwestward from first bay north of Beyer Bay, Adak 

Island. Note the conspicuous fracture pattern in the granodiorite pluton. In the distant highlands, parts of the same pattern can be detected in Finger Bay volcanics (contact hidden). Photograph by Air Photographic and Charting Service (MATS', U.S. 

Air Force.
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 397

is granodiorite (fig. 58). This fact, coupled with extensive surficial 

cover, makes reconnaissance geology less productive than in areas of 

strong physical contrast between major rock units. Curiously, the 

surficial cover, quite effective as a mask for some of the geology, fails 

to conceal the intricate pattern of linear features (best seen from the 

air) that have been interpreted by Bradley (1948, fig. 2) as joints and 

faults and are similarly interpreted here.

Three factors may have been instrumental in projecting the linear 

pattern through a thick ash and vegetation mantle: glacial scouring 

accentuated the fracture pattern prior to deposition of the surficial 

cover; gradual accumulation of thin ash layers may have effectively 

"painted'' over the irregularities and preserved minute differences in 

relief; and Recent movement on some fractures is possible in this 

seismically active area.

The fracture pattern is deduced from hundreds of straight lines, 

many several miles long, which are visible on topographic maps and 

aerial photographs (pi. 53, figs. 58 and 59). Topographic lineation 

is conspicuous even on 1: 250,000 maps and particularly on the shaded 

relief map (U.S. Geol. Survey, Alaska reconnaissance topographic 

series, Adak, Alaska, 1951). Bradley (1948, fig. 2) has plotted the 

more conspicuous lines: those on the north half of the Yakak peniiiFIOURE 50. Aerial view of south-central Adak Island. View is northwestward from 

Beyer Bay. Conspicuous linear features on the mountain in the foreground are joints 

or small faults in granodiorite. Note that a linear projection of the lake shore in foreground reflects a conspicuous fracture zone. Photograph by Air Photographic and 

Charting Service (MATS), U.S. Air Force.
398 INVESTIGATION OF ALASKAN VOLCANOES

sula are known to be bedding traces, but nearly all of the remaining 

lines represent probable fractures. From Bradley's plot, and from 

study of annotated maps and photographs, many of the linears appear 

to be grouped in sets; the northeast direction is strongest, but northwestward-trending linears are also common. Regional abundance 

and grouping of linears, and their presence in granodiorite as well as 

Finger Bay volcanics, suggests that they are of tectonic origin and 

mostly postdate the granodiorite in age. Ten linears coincide partly 

with faults mapped in the field, but most of them, on field evidence, 

can be interpreted as joints or groups of joints accentuated by glacial 

erosion.

Several field observations show that most joints trend northeastward 

and are vertical or dip steeply to the northwest. The topographic 

lineation produced by joints is especially conspicuous near the southcentral coast of Adak Island (pi. 52). Many lakes are alined or have 

angular shapes that reflect this joint pattern.

In addition to the fine-grained linear pattern certain major topographic alinements (lineaments), both subaerial and submarine, are 

suggestive of much larger structures structures that may have been 

instrumental in blocking out or segmenting the islands. ' Among these 

are the Finger Bay and Adak faults of Bradley (1948, fig. 2) and the 

Adak Strait, Kagalaska Strait, and Little Tanaga Strait lineaments. 

The Adak fault of Bradley shows topographic discordance on a major 

scale; comparable displacements cannot be demonstrated elseAvhere, 

though depth contours suggest that the east side of Adak Strait has 

been uplifted 3,000-5,000 feet south of Cape Yakak, and irregularities 

in depth curves suggest that the lineaments of Kagalaska Strait and 

Little Tanaga Strait extend for many miles south of the islands (U.S. 

Coast and Geodetic Survey chart 8863, reproduced in part by Bradley, 

1948, p.219).

Faults mapped by both Coats (1956a, pi. 9) and by us all appear 

to have minor offsets. Joints and small normal faults appear to 

characterize the structure within the island blocks. This fact plus 

the existence of abundant dikes and the absence of tight folding suggest that significant compression was not operative at the level now 

exposed by erosion. The rocks reacted as a brittle mass in a nearsurface environment, but the nature and origin of the deforming 

stress remain obscure. Gates and Gibson (1956) interpret the arc 

as an arched and faulted, asymmetrical fault wedge, and this explanation seems to fit the facts as far as they are known.

FOLDS

Random dip observations on southern Adak Island suggest a 

major east-northeastward-trending arch through north-central Adak
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 399

Island. Apparently there is a general dip reversal near Bay of 

Islands, and layered rocks south of this latitude commonly show southerly dip components (pi. 52). Bradley suggested the presence of a 

volcano remnant in the Bay of Islands area (1948, fig. 10); but its 

presence was not confirmed by our fielchvork. We interpret the dip 

reversal in this area as an elongate arch rather than a domelike structure. No volcanic landforms and no structural evidence for ancient 

central-type volcanoes were found in the area.

A similar, very approximately located arch was postulated for the 

area just west of Adak Island (Fraser and Barnett, 1959, pi. 27), 

and a projection of its axis falls close to the area of reversal near 

Bay of Islands. Probably this arch trend lies close to the crest of the 

much larger and more complex Aleutian arch, which can be inferred 

from regional subaerial a.nd submarine topography.

The syncline on Yakak peninsula reverses the general southerly dip 

on the south flank of this arch, and the presence of this syncline near 

Boot Bay on the east side of the main pluton (pi. 52) is strongly indicated. Thus the main pluton in south-central Adak Island appears 

to occupy a syncline, although this structure may be one of many 

broad flexures within the regional arch. The fold on Yakak peninsula was found by plotting linear bedding traces on aerial photographs ; reexamination of field data confirmed the presence of the 

fold and demonstrated its synclinal nature. Southwest of the mouth 

of Bay of Islands the presence of a series of curved linears indicates 

that there is another fold in the Finger Bay volcanics, but, because 

bedding attitudes noted in the field do not confirm this no fold is 

plotted.

Local strike and dip variations are common, and recorded observations are not available in most areas so that open folds of the Yakak 

type may have been missed. Erratic dips ranging from 5° to 45° 

may indicate mild folding, doming by subjacent plutons, tilted fault 

blocks, or drag along faults. Steeply dipping platy fractures were 

observed near a few plutonic contacts; the best explanation for them 

is that they are sheared areas related to forcible intrusion.

GEOMORPHOLOGY

An excellent reconnaissance report of the geomorphology of Adak 

Island is already available (Bradley, 1948), and little can be added 

here. Nearly all physiographic features, large as well as small, reveal an angular fabric controlled by fractures. This controlling 

fabric is so conspicuous that many nonstructural factors affecting 

the surface features have here been slighted.
400 INVESTIGATION OF ALASKAN VOLCANOES

MARINE PLAWATION

Inroad rolling lowlands border or transect part of the area and are 

particularly well developed between Mount Moffett and the central 

highland, on southwestern Adak Island, and on central Kagalaska 

Island. The peripheral lowland is much dissected and very irregular 

in south-central Adak Island (fig. 58) ; but here, as in other locations, the mountains seem to begin at about 700 feet, and below that 

level the area gives the impression that it had once been a surface of 

low relief (Bradley, 1948).

The largest lowland area is the gently undulating surface of the 

Yakak peninsula (figs. 60 and 61). It is possible that this represents

FIGURE 60. Aerial view northward along Yakak peninsula. Mount Moffett is visible in 

background. Note the lake-dotted surface of low relief believed to result from several 

stages of marine planation followed by glaciation. Photograph by Air Photographic and 

Charting Service (MATS), U.S. Air Force.

a series of wave-cut benches later smoothed by glacial erosion, as 

suggested by Bradley. The surfaces of planation grade into each 

other by gentle slopes and thus, if marine, are either tilted or modified later. The subsequent glaciation has reworked any stacks, beach 

gravels, or other unequivocal evidence of marine origin of these surfaces; indeed they may be products of subaerial erosion by streams 

and solifluction. If they are marine they imply uplift of about 

670 feet, presumably in Pleistocene time.

Similar surfaces have been found on many islands west of Adak 

Island, Kanaga Island, Tanaga Island, the central Delarof Islands,
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 4Q1

600 -

200' -

Sea- level

r-600

-200

Sea, level

Vertical scale 10X

YAKAK PENINSULA 

Showing location of topographic profiles 

1 0 12 3 4 Miles

FIGURE 61. Topographic profiles of Yakak peninsula, Adak Island. Profiles were prepared from 1: 25,000 topographic maps ; contour interval is 20 feet. The presence of 

incomplete wave-cut surfaces are suggested at levels indicated by horizontal broken 

lines.

Amchitka Island, and the Near Islands (Fraser and Barnett, 1959; 

Powers, Coats, and Nelson, in press). These surfaces, generally interpreted as marine platforms, may indicate a regional uplift during 

Quaternary time. Where data are available, planation and uplift 

occurred during preglacial or interglacial time.

Besides these extensive older surfaces, minor benches and raised 

beach deposits on many islands indicate uplift in the Recent or a 

change in sea level of 6-10 feet. In general the latest deposits record 

relatively stable conditions; but local structural disturbance of the 

youngest levels has been noted on Attu Island (Gates and others AVI itten communication) and on northern Adak Island (see Coats, 1956a, 

p. 63, pis. 13 and 14B, and p. 394 this report).

GLACIATION

The^latest major agents of erosion that sculptured southern Adak 

and Kagalaska Islands Avere glaciers. Glacial striae are common on
402 INVESTIGATION OF ALASKAN VOLCANOES

most recently exposed surfaces; roches moutonnees, cirques, and fiords 

are abundant; and the scoured appearance of glaciated terrain is 

immediately apparent from aerial photographs. The cirques and 

fiord pattern indicates an east-northeast highland through central 

Adak Island from which glaciers moved to the north and south. 

Prior to valley glaciation, which has left its characteristic topography, 

an ice cap probably covered nearly all of the island (Bradley, 1948). 

Kagalaska Strait, a strong linear tectonic feature, was established 

in preglacial time, for glacial valleys empty into it from Adak and 

Kagalaska Islands.

Mount Moffett, now highly dissected, was the center of glacial 

activity on northern Adak Island. Coats (1956a) and Bradley (1948) 

report till on the north end of the island and between Mount Moffett 

and the mountainous interior. Elsewhere on the island glacial deposits are rare (Bradley, 1948, fig. 11). Preliminary contouring of 

Kuluk Bay from detailed soundings shows a hummocky surface, probably in part the result of glacial deposition. Similar deposits may 

occur south of Adak Island, but detailed soundings are not available 

for that area.

The. south boundary of Kuluk Bay (the northern coastline of eastern Adak and Kagalaska Islands) is a straight-line feature that 

abruptly truncates narrow fiords and headlands. The peripheral 

lowland is absent, and the offshore area is remarkably free of islands. 

On southern Adak and Kagalaska Islands, however, there is a dissected 

lowland and the shoreline is characterized by broad bays and many 

islands. The Adak fault (p. 398) may be the reason for this contrast. Some movement on this fault probably occurred after wave 

planation, but presence of hanging valleys on northern Adak Island 

cannot be used as positive evidence for postglacial faulting (Bradley, 

1948, p. 230).

GEOLOGIC HISTORY

Earliest known events in the geologic histor}7 are recorded in the 

Paleozoic volcanic-derived sandstone beds of northern Adak Island 

and in the Finger Bay volcanics of probable Tertiary age. This 

complex sequence of pyroclastic deposits, lava flows, and sedimentary 

rocks started to accumulate in and near the sea in late Paleozoic time. 

A period representing most of Mesozoic time is probably missing from 

the record. A deposition basin, known to extend from Attu Island 

to the Alaska Peninsula, received some of its load in late Mesozoic 

time, but most of the deposition probably occurred during early 

and mid-Tertiary time. There is no evidence for assuming' a 

major stage of deformation between the beginnings of Paleozoic 

deposition (known only from northern Adak Island) and the middle
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 403

to late Tertiary intrusion and uplift, which marked a major change 

in conditions.

The mass of rocks deposited in this geosyncline are andesitic and 

basaltic. Albitized, chloritized, and epidotized pyroclastic deposits, 

massive and pillow lavas, argillite, and graywacke are characteristic. 

The deposits were buried, regionally altered by hydro thermal solutions, and then invaded by granodiorite plutons, which thermally 

metamorphosed small border zones and locally caused additional 

hydrothermal alteration in and near the plutonic borders. The major 

intrusiA^e episode, probably middle to late Tertiary, was not accompanied by dynamic metamorphism at the relatively shallow level now 

exposed by erosion. Here the old volcanic rocks Avere gently folded; 

and a well-defined fracture pattern formed in the plutons and wall 

rock alike. This fracture pattern developed after intrusion of the 

granodiorite and before deposition of late Tertiary and Quaternary 

volcanic rocks. The presence of abundant dikes and conspicuous 

joints, normal faults, and the absence of positive eAddence for compression indicate that extension, probably on an arch or arched wedge, 

may have been a controlling factor in the deformation.

During undated epochs, before and after the major intrusions, small, 

andesitics dikes and sills penetrated the Finger Bay volcanics. Similar dikes penetrated the plutons, possibly after some uplift and 

erosion.

A period of uplift and erosion, which removed an unknown thickness of Finger Bay volcanics and exposed portions of the plutons, preceded the subaerial volcanism recorded in the composite cones on northern Adak Island. Contributions to this later volcanic sequence came 

from three distinct centers. These deposits thus contrast sharply with 

the Finger Bay volcanics, which are believed to be largely submarine 

without discernible centers of origin. This last volcanic episode began 

in latest Tertiary time and ended on Adak Island before completion 

of the last major glaciation. A few small hot springs are active on the 

east side of Andrew Bay, and volcanoes are still active on adjacent 

Ivanaga and Great Sitkin Islands.

A peripheral surface of low relief, probably a series of shore platforms, Avas developed in preglacial or interglacial time. Glaciers, 

moving outAvard from a central highland, cut through and overrode 

this surf ace.

Subsequent to the latest major glaciation a surficial blanket of volcanic ash Avas deposited. Ash layers are separated at intervals by 

humic zones, and peat, Avith a questionable radiocarbon date of about 

3,300 years, has been found near the top of the series of ash-soil layers, 

indicating that most of the surficial blanket accumulated before this 

date.
404 INVESTIGATION OF ALASKAN VOLCANOES

The shape of Adak and Kagalaska Islands is still being modified by 

erosion and possibly by tectonic movement. Current seismicity may 

indicate movement along some of the old fractures in response to isostatic adjustment, or renewed pulses in the tectonic-magmatic continuum. Islands have existed intermittently since Paleozoic time, but 

the present island arc and series of volcanoes appear to have been 

established in late Tertiary or Quaternary time.

ECONOMIC GEOLOGY

The Aleutian Islands have proved remarkably barren of economic 

mineral deposits. Brecciated fault zones as much as 50 yards wide 

containing disseminated pyrite and quartz were discovered in 'the 

Blind Cove area; a similar zone 100 yards wide was found on the west 

shore of Beyer Bay; and secondary copper minerals were found associated with a quartz-pyrite mineralized zone upstream from the head 

of Bay of Islands. Small iron-stained fault zones occur in many 

places on Adak Island and in a few places on Kagalaska Island; disseminated pyrite is common in the Finger Bay volcanics, especially 

near the contacts with plutons. No metallic deposits of 'economic 

grade have been found on Adak and Kagalaska Islands or any of the 

other Aleutian Islands.

VOLCANO AND EARTHQUAKE HAZARD

The volcanoes on northern Adak Island have been inactive since 

prehistoric time, and the possibility of reactivation seems remote. 

However, three volcanoes in the same chain (Mount Katmai, 1912; 

Trident, 1953; and Mount Spurr, 1953) have erupted from vents regarded as extinct. Great Sitkin Volcano, 27 miles northeast of the 

base at Adak Island; and Kanaga Volcano, 22 miles northwest, are 

quite active, and their explosive products could reach Adak Island. 

Seismographs 011 Adak Island have recorded many earthquakes originating within a few miles of the island. Volcanic and seismic activity 

in the Andreanof Islands should continue for some time. Though 

there is little probability that catastrophic eruptions will endanger 

the base at Adak Island in the near future, some earthquake damage 

is possible.

Tsunamis associated with submarine earthquakes and volcanic activity are a potential hazard at Adak Island. Fortunately Kuluk 

Bay is on the north side of the Aleutian chain and is therefore less 

vulnerable to the rather common tsunamis originating south "of the 

islands in the Aleutian Trench. The tsunami of April 1, 1946, which 

destroyed the Scotch Cap lighthouse on Unimak Island and caused 

the water there to rise 115 feet, did not affect Kuluk Bay (Shepard 

and others, 1950, p. 443). Kuluk Bay was likewise in the lee from an-
GEOLOGY OF SOUTHERN ADAK ISLAND AND KAGALASKA ISLAND 405

other tsunami that originated off Kamchatka Peninsula on November 

4, 1952. Several waves did cause the water to rise about 8 feet, 

but no damage resulted.

Conceivably, a major eruption of Great Si.tk.in Volcano could originate a seiche or tsunami in Kuluk Bay which might damage harbor 

installations and, especially in the case of a tsunami, the Adak base 

as well. Some evidence that two such waves inundated an Aleut 

village site about 3,000 years ago is found on the north end of the tombolo bounding Clam Lagoon (H. A. Powers, oral communication). 

More recent tsunamis, if they occurred, have left no record in the 

usually uninhabited Kuluk Bay area. Tsunamis cannot be predicted 

nor prevented, but installations should be located as high as possible, 

especially on the south sides of islands. The seismic warning net, now 

in operation, should prove of great benefit in saving life and property 

if the tsunami originates far enough away to allow adequate Avarning 

time (Zerbe, 1953, p. 9).

LITERATURE CITED

Anderson, S. T. and Bank, T. P., II, 1952, Pollen and radiocarbon studies of

Aleutian soil profiles: Science, v. 116, no. 3004, p. 84-86. 

Bank, T. P., II, 1953, Biological succession in the Aleutians: Pacific Science, v.

7, no. 4, p. 493-503. 

Bowen, N. L., 1928, Evolution of the igneous rocks: Princeton Univ. Press,

p. 1-332. 

Bradley, O. O., 1948, Geologic notes on Adak Island and the Aleutian chain,

Alaska : Am. Jour. Sci., v. 246, p. 214-240. 

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

Islands: U.S. Geol. Survey Bull. S57-D, p. 141-153. 

Coats, R. R., 1947, Geology of northern Adak Island, pt 5, in Alaskau volcano

investigations, Kept. no. 2, Progress of investigations in 1946; Prepared by

U.S. Geol. Survey for Chief of Engineers, U.S. Army, limited distribution,

p. 71-85.

 1950, Volcanic activity in the Aleutian arc : U.S. Geol. Survey Bull. 974-B, 

p. 35-49.

 1956a, Geology of northern Adak Island: U.S. Geol. Survey Bull. 1028-C, 

p. 45-67.

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

1028-D, p. 69-81.

 195Cc, Reconnaissance geology of some western Aletuian Islands: U.S. 

Geol. Survey Bull. 102S-E, p. 83-100.

 1959, A geologic reconnaissance of Gareloi Island, Alaska: U.S. Geol.

Survey Bull. 1028-J, p. 249-256.

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

their people and natural history : Smithsonian Inst, War Background Studies 

no. 21, p. 1-131.

Fraser, G. D., and Barnett, H. F., (1959), Geology of the Dalarof and westernmost Andreauof Islands, Aleutian Islands, Alaska: U.S. Geol. Survey Bull. 

1028-1, p. 211-248.

Gates, Olcott, Fraser, G. D., and Snyder, G. L., 1954, Preliminary report on the 

geology of the Aleutian Islands : Science, v. 119, p. 446.
406 INVESTIGATION OF ALASKAN VOLCANOES

Gates, Olcott, and Gibson, William, 1956, Interpretation of the configuration of the

Aleutian Ridge: Geol. Soc. American Bull., v. 67, no. 2, p. 127-146. 

Gilluly, James, 1935, Keratophyres of eastern Oregon and the spilite problem :

Am. Jour. Sci., v. 29, p. 225-252 and 336-352. 

J'udson, Sheldon, 1946, Late-glacial and post-glacial chronology of Adak: Jour.

Geology, v. 54, no. 6, p. 376-385. 

Kellogg, C. E., and Nygard, I. J., 1951, Exploratory study of the principal soil

groups of Alaska: U.S. Dept. of Agriculture Mon. no. 7, p. 1-13S. 

Kulp, J. L., Tryon, L. E., Eckelmau, W. R, and Small, W. A., 1952, Lament natural

radiocarbon measurements, II: Science, v. 116, no. 3016, p. 409-414. 

Lindgren, Waldemar, 1900, Granodiorite and other intermediate rocks : Am. Jour.

Sci., 4th ser., v. 9, p. 269-282. 

Murray, H. W., 1945, Profiles of the Aleutian Trench: Geol. Soc. American Bull.,

v. 56, no. 7, p. 757-782.

Powers, H. A., Coats, R. R.. and Nelson, W. H., .in press, Geology and submarine physiography of Amchitka Island, Alaska: U.S. Geol. Survey Bull.

1028-p. 

Shepard, F. P., Macdonald, G. A., and Cox, D. C., 1950, The tsunami of April 1,

1946: Calif. Univ., Scripps Inst. Oceanography Bull., v. 5, no. 6, p. 391-528. 

Snyder, G. L., 1957, Ocean floor structures, northeastern Rat Islands, Alaska:

U.S. Geol. Survey Bull. 102S-G, p. 161-167. 

U.S. Air Force, llth Weather Squadron, Elmendorf Air Force Base, Alaska, 1950,

Climate, weather and flying conditions of Alaska and eastern Siberia,

chart 20. 

Waters, A. C., Volcanic rocks and the tectonic cycle: Geol. Soc. America Special

Paper 62, p. 703-722. 

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

of the pyroclastic rocks: Natl. Research Council Bull., 89, p. 19-53. 

Williams, Howel, Turner, F. J., Gilbert, C. M., 1954, Petrography: San Francisco, W. H. Freeman and Co.. p. 1-406. 

Zerbe, W. B., 1953, The tsunami of November 4, 1952, as recorded at tide stations :

U.S. Dept. of Commerce, Coast and Geodetic Survey, spec. pub. no. 300,

p. 1-62.
INDEX

Page 

Abstract.- . . - 371

Acknowledgments _. . . 372 

Aclak fault................................. 398,402

Age, ash-soil mantle........................ 394,403

dikes.---.---.-....--.-.---.-..------- 392,403

plutons....................-.-.-.-.......- 391

Aleutian Trench...._.. ... .._...... 395-396

Aleut middens..---.....--.-...-..--.-.... 394-395

Alluvium._ - - 395

Alteration products.-.-----------------.-. 381-382

Analyses, chemical..._....._............. 385

Andesitic dikes. See Dikes.

Andrew Bay, hot springs ------------------- 403

Animal life.. ..-.-..-...-.------..-...-.- 376

Argillitc ...... ......... ...- . 380,382

Ash. See Volcanic ash.

Bay of Islands, mineralized zone near...___ 404 

rocks.--..... _-_ - -- - 404

Bay of Waterfalls, rocks . 380-381,382,385,387,388 

Beach deposits...-...-..--.-.----.-.. 394-395,405

Bedding traces.-.---.--.--------.-------- 397-398

Beyer Bay, mineral deposits.- .___ 404 

rocks near........... .... 385,386-387,396,397

Blind Cove area, mineral deposits............ 404

rocks 382,385-386,389

Boot Bay area, rocks from.-.---..-. 378-379,380,385 

Boot Point, rocks..-.....-----...---..--- 380

Chapel Cove area, gabbro.------_........ 385,387

Climate...-.--.---.-.---.--..--.--- 376 

Coats, E. R., quoted . - --- 376-377

Dacite - 391

Dikes, age.--..---...-.--------.---.-. 392,403

in Finger Bay volcanics.... 377,391

in granodiorite. ....._...... 377,384,391-392

Diorite, chemical composition.--------------- 385

petrography..--..---...--.-....-. 386-387

Earthquake hazard 404-405

Economic geology 404

Faults 398

Feldspathic granodiorite, chemical composition..----........------ 385

Felsic material, migration of .. ._ 389-390 

Fieldwork . 372

Finger Bay volcanics, age....._ .. 383-384,403

alteration and metamorphism.......... 381-383

argillite 380

color....- .__.____.---.-_____ 377

dikes and sills...------------- 391

fold.-... 399

lapilli tuff... . . . . 379-380

Finger Bay volcanics Continued Page 

lava -.-. 378-379

limestone- 380-381

lithologic character ......._..__.... 378

regional relationships. 383-384

thickness...___._ 378

tuff breccia.-.-..-..--.-.-..----.. 379-380

Flow structure....-.---.--.---..----.-- 388

Folds-.-- 398-399

Fracture pattern.. 396-398,403

Gabbro, chemical composition............._ 385

petrography ____ ...... 387

Oannet Cove area, rocks................... 378,380

Geography. . 375-376

Geologic history-------.---------.--- 402-404

Geology of northern Adak Island, summary. 376-377 

Geomorphology __.___._... 399-402 

Glacial deposits....________.__.__ 392

Glaciation 401-402,403

Granodiorite, chemical composition......._ 385

dike rocks in. 377,384,391-392 

petrography-.-..-..---.------- 384-386

Granodiorite porphyry, feldspathlc. __ 385,388

History.- 372,375

Hornfels zone..-.._ ...______...__ 388 

Hot springs..-.-.________..._____ 403

Introduction ____________,..,._ 372,376 

Joints..___._____...___... ___ 398

Kagalaska Strait, rocks near...----.-- 378,385,386 

Keratophyre.-.-.._ .- ... 378,391-392 

Kuluk Bay seiche-..-.-.--. -._-------__ 405

Lapilli tuff. ... .. 379-380

Lava, chemical composition....__._..._ 385

Finger Bay volcanics, description-.-... 378-379 

Limestone.____________...__._ 380-381 

Literature cited.------.----...---.-.-. 405-406

Location.... _ _-_.____.___._._._ 375

Mafic rocks in border zones..._... 388-389

Mapping and maps.--.__.---.--....--... 372-374

Marine planation.. ........................ 400-401

Mineral deposits - --_ 404

Minerals, secondary.-.. ___._.. ... 381-382 

Moffett, Mount, glaciation...-.......... 400,402

North Arm of Three Arm Bay, rocks near. 378

Petrogenesis...__ 388-391 

Petrography, granodiorite and related rocks. 384-388 

Petrology.. 377-392

407
408 INDEX

Page 

Pillowlava 378,335,403

Phanerite rock fragments In tuff....-_..---_-- 380

Plutons..-_.-.....-.... 384,385,386-387,388-391,403

Population..-..--.--..-..---.-.--..--.-.-.- 372,375 

Pyroclastic rocks, composition.--..---------. 380

description._..-..----.----.-.---....... 379

relation to sedimentary deposit-_--.-. 380-381

Quail Bay, rocks near....-------------------- 380

Quartz diorite, chemical composition.-------. 385

petrography......--..-...--.-.---.-.-.. 384-385

Quartz monzonite__ ---------. - - 384,385

RadiDcarbon dates for soil mantle..._..-. 394,403 

Rhyodacite...----..---.------...-----------. 391

Rhyolite 378

Ringgold Island, rocks....-.....-----.--..-.. 380

Seiche, Kuluk Bay... ---.---..-.-..--.-.-- 405

Shape of the islands, modification..---..---.- 404 

Sills ------------------------------------- 391,403

Size-....--.----....--..----.-----..-------... 375

Soil, age....-------------.-----------.--.--- 394,403

classification............................. 393

thickness.----------------------------. 393-394

Page 

Structure, arch through north-central Adak

Island , 398-399

fracture pattern____________ 396-398,403 

linear arrangement of volcanoes._.-.._ 396 

syncline on Yakak peninsula.-----.---.-- 399

volcanic and nonvolcanic arcs...----.-- 395-396

Surflcial deposits. - 392-395

Tomasi, E. J., analyses by.. . 385 

Topography. __.__.........._....... 375

Tsunami deposits. See Beach deposits. 

Tsunamis, potential hazard....__-..._ 404-405 

Tuff breccia.............. ... 379-380

Vegetation...-.-------.---.--......----.--. 376,393

Vincennes, Mount, rocks on.....----.--..__ 379

Volcanic activity__.----------.---.--------- 404

Volcanic ash.-----------------------.-. 393-394,403

Volcanic\vacke.--.-_.--..-...--.....-...-. 378,380

Wedge Point area, rocks.....--..._........ 380

Xenoliths 388-389

Yakak peninsula, rocks from............... 378,380

surfaces of planation_ ... ...... 400,401

syncline on_----------_-- ------.-. 399

o