« * Geology o»/ of Northern

: Adak Island, Alaska

By ROBERT R. COATS

 «- INVESTIGATIONS OF ALASKAN VOLCANOES

 Of __________________________________________________________________________________________

_, GEOLOGICAL SURVEY BULLETIN 1028-C

Prepared in cooperation with the Office, 

Chief of Engineers, U. S. Army

v

Y

s

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

Fred A. Seaton, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

-

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

Washington 25, D. C. . v
PREFACE

In October 1945, the War Department (now Department of the 

Army) requested the Geological Survey to undertake a program of 

volcano investigations in the Aleutian Islands-Alaska Peninsula, 

area. The first field studies, under general direction of G. D. Robinson, were begun as soon as weather permitted in the spring of 1946, 

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

assembled as two administrative reports. Part of the data was published in 1950 in Geological Survey Bulletin 974-B, Volcanic activity 

in the Aleutian arc, by Robert R. Coats. The remainder of the data 

has been revised for publication in Bulletin 1028.

The geologic and geophysical investigations covered by this report 

were reconnaissance. The factual information presented is believed 

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

grows.

The investigations of 1946 were supported almost entirely by the 

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

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

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

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

continuing support.

in

CONTENTS

Page

Preface._----_____--_-__-_-__________------_-__._------________ 111

Abstract_______________________________________________________ 45

Geography.____-________--___.___-__--_-__-___---_-__-____________ 46

Location. ._ __________________________________________________ 46

Physiography.________________________________________________ 46

Drainage and water power_____--_-_----_---_____---__-_-----___ 48

Geology._________________________________________________________ 48

Rocks of Paleozoic(?) age-______---_-----_-_--__---_-_-_-----__- 48

Finger Bay volcanics___________________________'___.______ 48

Gabbro_,_..___-_---------------------_----_-------------- 50

Rocks of Cenozoic age.__..___---__-__________________-___-____ 50

Andesite porphyry domes____._______________________________ 51

Northeastern area.____________---_-__-_-________-___._____ 52

Rocks of Andrew Bay volcano.___-_____-___-_-_---_-_-_ 52

Boulder conglomerate.--------------.---...-------....- 53

Fossiliferous marine sandstone._________________________ 53

Rocks of Mount Adagdak._____________________________ 54

Southwestern area.________________________________________ 56

Rocks of Mount Moffett______._____._.__-_-__-__._____ 56

Parasitic cone of Mount Moffett_______________________ 59

Glacial drift_____...__..____.______^_.____._..___....._. 60

Marine terrace boulder gravel______--_-_____-_____--_.______ 61

Recent unconsolidated deposits _____________________________ 61

Structure. ____________________________________________________ 62

Geologic history.______________________________________________ 63

Future volcanic activity_____.__-._--_--__--____-__-__________-.-_-- 66

Literature cited___________________________________________________ 66

Index.___________________________________________________________ 67

ILLUSTKATIONS

[Plate 9 in pocket; plates 10-14 follow page 54]

V Page 

PLATE 9. Geologic map of northern Adak Island, Alaska.

-f 10. A, Hills southwest of Sweeper Cove. From hill one-half mile 

west of head of cove. B, Dome of columnar andesite

-^ porphyry one mile west of Gannet Rocks. C, Cape

Adagdak. 

"*JI' 11. Aerial view of Mount Moffett from the north.

12. Aerial view of Mount Moffett from the south.

13. Aerial view of Cape Adagdak from the north.

^ 14. A, View on east side of Mount Adagdak. B, View eastward along 

a recent fault on the north face of Mount Adagdak. C, WellX stratified postglacial volcanic ash in road cut near Finger 

Bay.

-**i FIGURE 4. Map of the Alaska Peninsula and Aleutian Islands. ______.___» 47

-r

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGY OF NORTHERN ADAK ISLAND, ALASKA

By ROBERT R. COATS

ABSTRACT

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 

PaleozoicC?) age, intruded 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 the three recognized 

volcanic centers of the northern area. Minor amounts of sedimentary rocks 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.

Volcanic activity probably will not be resumed on Adak Island in the foreseeable future. Frequent earthquakes however, indicate that the earth's crust 

is unstable in that area; whether this instability signifies the beginning of a new 

volcanic cycle remains to be determined by further investigation.

INTRODUCTION

Northern Adak Island was mapped during the summer of 1946 as 

part of a geologic reconnaissance of the western Aleutian Islands. 

Because no reports of volcanic activity on the island have been made 

by visitors to the area, and because aerial photographs indicate deep 

erosion since the last important activity, it has been assumed that the 

volcanoes present no immediate threat of eruption. Geologic mapping of parts of the island was undertaken to determine the local 

geologic history, and on this basis to test the foregoing assumption.

The report is the result of 25 days of field work by Robert R. Coats, 

geologist, assisted by Will F. Thompson, Jr., recorder.

Acknowledgment is made of the cooperation of many officers and 

men of the U. S. Army stationed at Adak, especially Lt. Col. R. E. 

Ware, post engineer, and Lt. Col. C. E. Johnson, port commander.

45
46 INVESTIGATIONS OF ALASKAN VOLCANOES

GEOGRAPHY 

LOCATION

Adak Island is the largest of the Andreanof group of the Aleutian 

Islands. It lies between the parallels of 51°35' N. and 52° N., and 

between the meridians of 176°25/ "W. and 177° W., and has a total area 

of more than 280 square miles (fig. 4). An area of about 73 square 

miles, herein called northern Adak Island (pi. 9), was mapped in 1946.

PHYSIOGRAPHY

All but the northernmost part of Adak Island is characterized by 

short, diversely trending ridges, numerous rock-basin lakes, and a 

coastline deeply indented by fiords. This topography has been 

developed largely by intense glaciation of the Finger Bay volcanics 

(pi. 10.4), a sequence of relatively resistant rocks. The locations of 

stream valleys, lakes, and fiordlike inlets such as Finger Bay, have 

been determined principally by faults that have cut the Finger Bay 

rocks. Only the northern part of this region is included in the area ' * - 

mapped. Results of a reconnaissance study of Adak Island as a 

whole are given by Bradley (1948).

The physiography of northernmost Adak Island has been largely -^ 

controlled by volcanic activity originating at three centers Mount 

Moffett, Andrew Bay volcano, and Mount Adagdak, named in order 

from west to east. The highest point on the eroded cone of Mount ^ 

Moffett is 3,876 feet in altitude. The remnant of a parasitic cone, 

3,250 feet in altitude, stands on Mount Moffett's northeastern flank ^ 

(pi. 11). The central vent of the Andrew Bay volcano was located * 

west of the present eastern shore Of Andrew Bay, where steep cliffs 

now expose a section through its remnants. Most of the remainder of

the peninsula northeast of Andrew Lagoon and north of Clam Lagoon > 

(pi. 9) constitutes a single volcanic structure Mount Adagdak 

the culminating point of which is 2,072 feet in altitude. '*"'" 

Mount Moffett has been deeply dissected, except on its southeast v 

slope (pi. 12). Numerous valleys descending from open, steep-walled 

cirques narrow downstream into deep V-shaped canyons cut by glacial 

melt water and by later stream erosion. Similar evidences of glaci- > 

ation are absent on Mount Adagdak, probably because its isolated 

position and low altitude were unfavorable for the accumulation of 

glacial ice. ^

Vigorous wave erosion has created imposing cliffs as much as 2,500 

feet in altitude on the north sides of Mount Moffett and Mount ^ 

Adagdak (pis. 11 and 13). Remnants of marine terraces, partly ,.< 

destroyed by marine erosion, are visible in the cliffs.
170° 180° 170°

FIGURE 4. Map of the Alaska Peninsula and Aleutian Is
48 INVESTIGATIONS OF ALASKAN VOLCANOES 

DRAINAGE AND WATER POWER

In the northernmost part of the island, stream courses are relatively 

short and steep, and most drainage basins are less than 1 square mile 

in area. The larger streams of this area, which drain Mount Moff ett r 

and Mount Adagdak, have deeply incised canyons and a fairly uniform flow, maintained in late summer by snow banks at high altitudes, 

and in part by percolation of ground water from permeable lava flows - 

on the slopes of the mountains. Ample water supply for the military 

base is available. The topographic setting is, in general, favorable to 

the development of water power. Streamflow is not uniform, how- ^ 

ever, because of the steep slopes and lack of soil cover over much of .^ 

the area. Therefore, expensive water-storage facilities would be required if development were undertaken. ^

The drainage basins in the southern part of the mapped area are ^ 

more irregular in shape and are larger than those in the northern part. 

Slopes are steeper, and much more bedrock is exposed. The bedrock /,' 

is denser and less pervious than that in the northern area. Valleys 

are predominantly U-shaped in cross section, rather than V-shaped, 

and the streams have less regular profiles, because of the lakes and « 

alluviated areas along their courses. The size of the drainage basins 

and the presence of numerous lakes are favorable to the development 

of surface-water supply, while the scantiness of soil cover and the 

imperviousness of the bedrock are unfavorable to the development of 

ground-water supply.

GEOLOGY ^

The rock units of northern Adak Island, shown on plate 9, comprise * 

volcanic and intrusive rocks of Paleozoic(?) age in the southern part of

the area; Tertiary or Quaternary volcanic rocks, most of which make ' 

up the cones in the northern part of the area; and Quaternary rocks, 

largely glacial drift and other unconsolidated materials.

r

B-OCBTS OP PALEOZOIC(P) AGE

y

FINGER BAY VOLCANICS

Rocks believed to be of Paleozoic age include the Finger Bay "''

volcanics and a gabbro that intrudes them.

The southern part of northern Adak Island is underlain by a sequence of volcanic rocks consisting of black, dark-gray, purplish-gray,

greenish-gray, and grayish-yellow-green basaltic tuff, basalt flows,

basaltic tuff-breccia, agglomerate, and dikes, and a few small rhyolitic

flows and dikes. These rocks are well exposed in Finger Bay and are

here designated the Finger Bay volcanics (pi. 10A). Some of the 

rocks are conspicuously porphyritic, containing abundant phenocrysts 

of hornblende and plagioclase. The hornblende ranges in color from
v GEOLOGY OF NORTHERN ADAK ISLAND 49

 *- black to dark green. It commonly lacks the thick resorption border

of magnetite that characterizes the hornblende of extrusive rocks.

The unaltered plagioclase ranges from calcic bytownite to labradorite,- 

«r but much of the plagioclase has been albitized and caleitized. In

general these rocks have been intensely altered and locally recrystallized near intrusions.

* The degree of deformation of the rocks is not everywhere the same. 

In the area west and south of Kuluk Bay dips are steep and the attitudes irregular; in the area north of Kuluk Bay, which is isolated from

-< the other area, the attitudes of the rocks are more regular and the dips 

less steep. The southern part of this northern area consists largely of 

basaltic flows and sills; the northernmost outcrops, just south of the 

abandoned runway of Mitchell Field (not shown on the map), reveal 

rocks consisting largely of sandstone and tuffaceous sandstone interbedded with basaltic flows and nearly contemporaneous sills. In 1948 

a few impressions of leaf rosettes were collected from a bed of coarse 

sandstone, ranging in color from pale yellow to weak yellow-greenyellow (Munsell system), exposed on the south side of the west end of 

the airstrip. These leaf impressions have been identified by R. W. 

Brown as Annularia stellata (Schlotheim) Wood, of Pennsylyanian or 

Permian age.

Because of the lack of recognizable key horizons and because of the 

complicated deformation, the thickness of the Finger Bay volcanics 

has not been determined. Continuous exposures without apparent 

repetition of beds indicate a minimum thickness of 2,000 feet. The 

total thickness may be as much as 15,000 feet.

The age of the Finger Bay volcanics is not known with certainty. 

In 1946 the writer was inclined to correlate these rocks, on lithologic 

grounds, with rocks of probable Triassic age in the Iliamna region 

(Martin and Katz, 1912) and at Alinchak Bay on the Alaska 

Peninsula (Martin, 1926). The well-preserved and certainly identified 

leaf impressions (Annularia stellata} should, however, be given more 

weight than this long-distance lithologic correlation. For the present, 

the writer is content to regard the .Finger Bay volcanics as Paleozoic(?) 

in age. It must be recognized, however, that the isolation of the sequence in which the fossil was found from the type area about Finger 

Bay and Sweeper Cove introduces the possibility that rocks of more 

than one age are included in the mapped unit; if so, the rocks in the 

type area are probably older than the fossiliferous rocks. In the type 

area abrupt changes in the degree of deformation of rocks of a given 

age occur within a short span. Therefore, these rocks are not necessarily older, because they are more deformed, than the rocks north of 

Kuluk Bay.
50 INVESTIGATIONS OF ALASKAN VOLCANOES

GABBRO

The Finger Bay volcanics are intruded by a large mass of gabbro, ^ 

which crops out south of Finger Bay. The gabbro is a dark-gray rock 

whose nearly equal amounts of dark augite and white plagioclase of "*" 

uniform grain size give it a salt-and-pepper appearance. Close to , _ 

the margin of the mass almost all the augite has been altered to a 

pale-green amphibole. Magnetite is the chief accessory; apatite, "* 

only a small "amount of which is present; is also an accessory. ^

About a mile-from Finger Bay, close to the fault that forms much 

of the southwestern margin of the gabbro mass, is a body of augitequartz syenite of undetermined size and shape that is thought to be 

a differentiate of the gabbro. It is a dense, pinkish-gray rock containing white flecks that resemble phenocrysts, and wisps of darkgreen augite. The white flecks are even-grained aggregates of microperthite that include minor amounts of interstitial micropegmatite. 

The augite is locally replaced by serpentine and epidote.

The original shape of the gabbro mass was not determined. The 

gabbro cuts the Finger Bay volcanics, but none of the younger rocks. 

It is as much faulted as the Finger Bay volcanics,; and it resembles 

them in degree of alteration. The gabbro seems more closely related 

in age to the Finger Bay volcanics than to the younger rocks and is 

therefore tentatively assigned to the Paleozoic era.

In the quarry on the hill about half a mile southwest of the head 

of Sweeper Cove a small irregular intrusive mass, which probably is 

made up of diorite, is exposed. It has not been separately mapped. 

Much pyritization of the intruded rock and the intrusive mass obscures the contact. Assays of the pyritic rock showed negligible 

quantities of gold and silver.

BOCKS OF CENOZOIC AGE

The mountainous northern portion of Adak Island consists largely 

of volcanic rocks associated with three main eruptive centers of 

Tertiary or Quaternary age. The petrography and chemistry of these 

rocks are discussed in another report (Coats, 1952). Bouldery conglomerate and marine sandstone, some of which is fossiliferous, are 

interbedded with volcanic rocks from one of these centers. Five small

domes of andesite porphyry in the vicinity of Kuluk Bay are also

believed to be of Tertiary age. These volcanic rocks and associated

sedimentary rocks are found in two areas separated by Andrew Bay 

and the lagoon at its head. The southwestern area is occupied by

Mount Moffett; the northeastern area includes Mount Adagdak and

remnants of an older volcano, herein called the Andrew Bay volcano. 

These rocks have been assigned a Tertiary or Quaternary age because 

they overlie or intrude the Finger Bay volcanics, and they have been
GEOLOGY OF NORTHERN ADAK ISLAND 51

extensively glaciated. Fossils from, ike marine sandstone are probably 

Tertiary species that have a wide time range.

Other Quaternary rocks of northern Adak Island occur as glacial 

drift, including ground moraines, lateral moraines, and fluvioglacial 

.deposits;.marine-terrace boulder gravel; and Recent unconsolidated 

beach, delta, lagoon, alluvial, and aeolian deposits that have been 

accumulating since the retreat of the glaciers. The results of a study 

of the relative ages of some of these deposits have been given by 

Judson (1946).

ANDESITE PORPHYRY DOMES

Five domical masses of andesite porphyry lie close to the margin 

of Kuluk Bay, two north of the entrance to Sweeper Cove and three 

at, and north of, Zeto Point. The largest of the masses, about 230 

feet thick, lies north of the entrance to Sweeper Cove (pi. 105). This 

body has been extensively quarried for road material and riprap. 

Comparison of the present shape of these domes with the shapes of 

similar domes in volcanic areas elsewhere suggests that they are preserved in essentially their original thicknesses. The masses have all 

been glaciated; excellent striae are preserved on the dome that forms 

what is locally called 90-mm Hill, 1 about a mile north of the entrance 

to Sweeper Cove (pi. 105).

All the rocks have abundant calcic plagioclase phenocrysts as much 

as 1 centimeter in length, numerous smaller needles of hornblende, 

and accessory magnetite in a medium-gray to light-gray groundmass. 

The rocks locally contain phenocrysts of quartz, augite, and biotite. 

The groundmass consists of blocks of prismatic plagioclase and 

magnetite granules and interstitial orthoclase(?).

The andesite porphyry domes intrude the Finger Bay volcahics, 

but show none of the deformation or alteration associated with 

them. On the other hand, they have been glaciated and on this 

basis are older than latest Pleistocene. They are therefore presumably Tertiary in age, but their relation to other Tertiary rocks 

on the island is not known. They are considered older than the 

sequences of Tertiary rocks of the three principal volcanic centers, 

because their present topographic form is erosional rather than 

constructional. Furthermore, they differ petrographically from 

rocks of the other Tertiary sequences, and show no geographic relationship to the later volcanic centers. For these reasons they are 

considered to be" the only remaining exposed representatives on 

northern Adak Island of an earlier sequence of Tertiary volcanic 

rocks.

' Place name not shown on standard maps but used for convenience in this report.
52 INVESTIGATIONS OF ALASKAN VOLCANOES

NORTHEASTERN AREA

The northeastern area comprises the peninsula north of Clam 

Lagoon and east of Andrew Bay. The rocks of the northeastern 

area include the volcanic rocks erupted from the -Andrew Bay volcano 

and Mount Adagdak and some sedimentary rocks that accumulated 

before the development of Mount Adagdak. The rocks pf Andrew 

Bay volcano make up the ridge that rises steeply from the water's 

edge on the east shore of Andrew Bay; the rocks of Mount Adagdak 

constitute most of the peninsula; and the sedimentary rocks crop 

out along the eastern margin of the peninsula.

BOOKS OF ANDREW BAY VOLCANO

A thick accumulation of breccia, tuff-breccia, and lava'flows crops 

out on the east side of Andrew Bay. These rocks are the products of an 

ancient volcano :the Andrew Bay Volcano the vent of which was 

apparently west of the present eastern shore of Andrew Bay.

The cone of the ancient volcano was composed of interbedded 

tuff-breccia and lava flows. Exposed in the base of the steep cliff 

that forms the eastern face of Hill 1045 2 is about 500 feet of tuffbreccia, which probably was formed as a series of mudflows, dipping 

about 5° S. 30° E. Some of the fragments are as much as 6 feet long; 

90 percent are more than half an inch long. These mudflows include 

fragments of olivine-hypersthene basalt, hypersthene andesite, and 

hornblende-hypersthene andesite, ranging in texture from compact 

to porous, and in color from dark gray to medium gray or pinkish 

gray. Overlying the mudflows is a flow of pale-gray compact hornblende-hypersthene andesite, 300 feet thick, characterized by reddishbrown hornblende, augite, hypersthene, and plagioclase phenocrysts. 

The dip of the flow is about 16° S. 30° E.

At the north end of the area are high cliffs of breccia composed of 

blocks of hornblende basalt as much as 4 feet across that contain 

phenocrysts of labradorite, brownish-green hornblende, and augite,

and grains of magnetite in a fine-grained groundmass, This breccia

appears to make up the rocks offshore from the cliffs. Small masses 

of horizontally bedded, fine-grained, silicified tuff crop out sporadically on the sea cliff near the southern edge of the area of breccia. 

The tuff and breccia are underlain by a large irregular mass of darkgray to medium-gray hornblende andesite porphyry, in which the

phenocrysts are sodic labradorite and hornblende, calcitized and

chloritized. The accessory minerals are magnetite and apatite. 

The groundmass is a mosaic of quartz, feldspar, and calcite. The

breccia, the hornblende andesite porphyry, and the silicified tuff are 

all regarded as parts of the filling of a lateral vent on the s

* Place name not shown on standard maps but used for convenience in this report.
GEOLOGY OF NORTHERN ADAK' ISLAND 53

Bay volcano. The maximum thickness .of the rocks that now remain 

of the main cone of the Andrew Bay volcano is about 1,000 feet, 

but the original thickness was much greater.

A few small outcrops of poorly indurated sandstone and conglomerate dipping about 20° N., occur south of the rocks of Andrew 

Bay volcano, along the shore about a mile southeast of the north end 

of Andrew Lagoon. These beds are composed of angular to rounded 

fragments derived from the Finger Bay volcanics. They are believed 

to represent stream deposits laid down during an erosional interval 

before the deposition of the rocks of Andrew Bay volcano, but for 

convenience they have been included with the rocks of Andrew Bay 

volcano on the geologic map.

BOULDER CONGLOMERATE

A thickness of a few hundred feet of poorly bedded, bouldery conglomerate is exposed in the cliffs on the coast east of Mount Adagdak, 

beginning about \}( miles north of the northeast corner of Clam 

Lagoon and extending northward about 3,000 feet. The beds observed by the writer dipped steeply eastward. The boulders range 

in length from a few inches to 10 feet; the larger boulders are somewhat angular; therefore, some of the beds are classed as breccia. 

The material consists of volcanic rocks, some of which resemble the 

rocks of Mount Adagdak (see below), and some, which are lightcolored, are apparently andesitic rocks. No rocks known to be older 

than the conglomerate are in contact with it.

These rocks are probably marine talus deposits that accumulated 

against a steep coast. They may have been deposited at the same 

time or before the rocks of Andrew Bay volcano were deposited.

FOSSILIFEROUS MARINE SANDSTONE

A fossiliferous marine sandstone is exposed west of the southern 

part of the bouldery conglomerate and overlies the conglomerate 

unconformably. Because of its softness this formation is poorly 

exposed, but it is probably several hundred feet thick. On the steep 

cliffs just above the conglomerate outcrops, the fossiliferous rocks 

strike N. 30° W. and dip 80° W.

The rocks are predominantly soft yellow-to-gray sandstone, but 

contain lenses of dark pebble conglomerate. The sandstone is crossbedded and contains lenses rich in fragments of marine fossils. The. 

fossils are not diagnostic species, and most of them appear to be of 

genera represented on the present beaches of the Aleutians. It is inferred from the characteristics of the fossils and the degree of deformation of the rocks that the sandstone is probably of late Tertiary age. 

Its relationship to the interbedded basalt flows and tuffaceous sand-
54 INVESTIGATIONS OF ALASKAN VOLCANOES

stone of Mount Adagdak is obscure; the contact is probably along a 

fault or is unconformable.

ROCKS OF MOUNT ADAGDAK

Interbedded basalt flows and tuffaceous sandstone. :Interbedded basalt 

flows and tuffaceous sandstone make up most of the southern half of 

Mount Adagdak. They are best exposed on the sea cliffs on the east 

side of Mount Adagdak, from half a mile to 1% miles north of the northeast corner of Clam Lagoon (pi. 14A). The flows of this sequence form 

the main structure of a shield volcano, of which only the southern half 

remains. Their total thickness probably exceeds 1,000 feet. Horizontal beds of fine-grained tuffaceous sandstone in the lower part of 

the section appear to have accumulated in quiet, shallow water far 

from the volcanic center. They probably form the foundation on 

which this part of the shield volcano was built. The earliest lava flows 

probably did not extend far from the vent, but later flows lapped 

farther and farther out on the horizontal upper surface of the sandstone 

part of the sequence. The lava flow in immediate contact with the 

upper surface of the sandstone also is horizontal at that point, but 

presumably is steeper toward the core of the cone. Following is a 

section on the face of the sea cliff on the east side of Mount Adagdak.

Section on cliff on east side of Mount Adagdak

[Thickness estimated from barometric altitudes] Feet

Erosion surface.

Olivine basalt-_____ -__ __ - - - -_ _-.--- -_- - 40

Light-colored hornblende andesite pumice, and dark basaltic boulders-___ 70 

Olivine basalt__-__-_--------------------------------------------- 100

Reddish-brown tuffaceous sandstone, poorly indurated-_______ _ __ 25

Black sandstone, rich in basaltic debris, poorly indurated--_____________ 15

Brown and gray sandstone, poorly indurated._____._-__.____--_------- 10

Dun-colored sandstone, poorly indurated; some beds rich in breadcrust

bombs of basalt as much as 1J_ feet long; fills channels in underlying

..' bed-_________________________________________________________ 0-15

Nearly massive, coarse, volcanic sandstone, containing a few well-rounded 

black basaltic pebbles and grains, and some scattered angular fragments as much as L foot long; upper surface channeled; base not exposed..__-______________-_-________-__-___.____-_--_------__ 10-25

285

The two lava flows in this section are similar. The lower flow is 

pinkish gray, containing conspicuous phenocrysts of dark-green augite,

as much as 4 millimeters across, and of light-brown olivine, as much as 

1 millimeter across; the upper flow is medium gray, and contains more 

olivine and less augite. In addition to augite and olivine, the rocks 

contain basic bytownite phenocrysts and accessor}7 magnetite. The
GEOLOGICAL SURVEY BULLETIN' 1028 PLATE 10

A. HILLS SOUTHWEST OF SWEEPER COVE.

From hill one-half mile west of head of cove. Shows the rough topography characteristic of areas underlain 

by Finger Bay volcanics and later overridden by glacial ice. Smoother slopes in foreground are blanketed 

by postglacial ash.

T

/?, DOME OF COLUMNAR ANDESITE IM3RPHYKY,

One mile west of Gannet Rorks. Contact with Finger Bay volcanics is near right edge of picture, but is 

obscured by glacial drift. Glaciated surface of porphyry is overlain by a mantle of windblown sand and 

volcanic ash.

C, CAPE ADAGDAK.

The marine terrace, covered by boulder gravel, is built on mudflow.
GEOLOGICAL SURVEY 

AKIUM, VIEW OF MOUNT MOKFKTT FROM THE NOR

The highest clifl's iran.sect the |uiriiKiti<- cone of Mount Moffc-tt. F'hotouraph 
OKOLOCK'AL SURVEY

AKKIAL VIEW OF MOl NT MOI I-'K'I"I' VliOM THE SOU

Gentle constructional slopes of the composite cone are smoothed by mudflow material from the volcano. Ice-cont

ible below the snowline. Photograph by U. S. Navy.
GEOLOGICAL sriiVKY 

AERIAL VIEW OF CAPE ADAODAK FROM THE N

Keren I faull.s in the older composite cone of Mount Adapdak are outlined by snow. In ihe distance is the 

constructional slopes of Mount Model 1. Photograph by U 

Mil
 pr i m

GEOLOGICAL SURVEY 

VIEW ON EAST SIDE OF 

MOUNT ADAGDAK.

Olivine basalt flow overlies 

poorly consolidated tuffaceous 

sandstone with apparent conformity.

VIEW EASTWARD ALONG 

RECENT FAULT.

On the north face of Mount 

Adagdak, showing sag pond.
GEOLOGY OF NORTHERN ADAK ISLAND 55

groun'dmass, which forms only about 2(> percent of the-rock, is made 

up of blocky plagioclase, augite grains, and magnetite granules in a 

mosaic of quartz and orthoclase.

This sequence of flows accumulated against a sea cliff cut in the lavas 

of the Andrew Bay volcano, and is therefore younger than that sequence. It is undeformed and therefore inferred to be younger than 

the fossiliferous marine sandstone, which is folded. The older composite cone of Mount Adagdak rests upon it and is the next younger 

phase in the development of the volcano.

Older composite cone. Viewed from the south, the long, gentle 

grass-covered slopes of the shield volcano that forms the substructure 

of Mount Adagdak are seen to be crowned by the steep, relatively 

smooth, bare slopes of a wide, low cone. The break in slope occurs at 

altitudes ranging from 800 to 1,000 feet; the smooth slopes of the upper 

cone are localty broken by crags developed on small lava flows. This 

cone represents an older composite cone of Mount Adagdak. It consists largely of lapilli-tuff beds, but includes subordinate lava flows 

and necks of hornblende andesite. This material accumulated first 

as a cone about 800 feet high, the sides of which are as steep as 26°, 

on the top and the deeply eroded northern flank of the shield volcano. 

A later explosive episode, centering about a vent northwest of the 

earlier one, produced an inner, eccentrically nested crater, the summit 

of which was as much as 1,920 feet in altitude, about 200 feet higher 

than the outer rim. The time interval between the formation of the 

two craters was probably very short. The fragmental material ejected 

from the craters consists of pale-gray to dark-gray, porous-to-compact 

hornblende andesite, and pinkish-gray porous hornblende andesite, 

containing conspicuous feldspar and hornblende phenocrysts. Some 

of the fragments are angular blocks of glass}^ lava, which apparently 

cooled under reducing conditions. The flows are made up of reddishgray hornblende.basalt. All the rocks are characterized by labradorite 

phenocrysts, and augite and magnetite are generally present. The 

groundmass consists of plagioclase microlites, magnetite granules, and 

interstitial orthoclase.

Younger composite cone. A sequence of rriudflows consisting largely 

of fragments of hornblende andesite, capped by two thin hornblende 

andesite lava flows, makes up the western flanks of Mount Adagdak. 

These rocks represent a younger cone built after the sea had partly 

eroded the older composite cone. The mudflows are composed mainly 

of fragments of medium-gray porous rock that contain phenocrysts of 

white plagioclase and black hornblende, as much as 2 millimeters long, 

and smaller phenooysts of brown augite. Magnetite is the common 

accessory mineral, and the groundmass consists of plagioclase laths

377559 56 3
56 INVESTIGATIONS OF ALASKAN VOLCANOES

and magnetite granules in a cryptocrystalline mosaic, apparently of 

quartz and feldspar. The matrix of the mudflows is fragmental 

material of finer size and lighter color than the fragments but of 

similar composition. The two thin flows of reddish-gray hornblende 

andesite that cap the mudflows are similar in composition to the 

fragments in the mudflows.

Basalt and andesite domes. Two hornblende andesite domes occur in 

the crater of the younger composite cone, and a basalt dome occurs low 

on the southern flank of the older one. The northern and larger of the 

two crater domes shows planar banding, which trends roughly parallel 

to the contact and dips steeply. The rock is pale gray and contains 

conspicuous phenocrysts of labradorite and greenish-brown hornblende. The southern and smaller crater dome is light pinkish gray, 

and its hornblende grains are reddish brown. The groundmass of 

both rocks consists of plagioclase microlites but includes interstitial 

orthoclase. Apparently the domes arose toward the end of the period 

of eruptions during which the younger composite cone was built.

Correlation of the two crater domes with the dome on the southern 

flank of Mount Adagdak is based primarily on similarity of topographic form rather than similarity of composition. The rock of the 

flank dome is reddish gray and contains phenocrysts of dark-green 

augite, black 'hornblende, and white calcic bytownite. Magnetite 

is an abundant accessory mineral. The groundmass consists of plagioclase laths, augite rods, magnetite grains, and interstitial glass.

SOUTHWESTERN AREA

Volcanic rocks erupted from Mount Moffett and from a parasitic 

cone on its northeastern flank make up the southwestern area.

BOCKS OF MOUNT MOFFETT

The composite cone of Mount Moffett and the parasitic cone on its 

northeastern flank form a mountain mass that dominates the entire 

northwestern part of Adak Island. Small domes of basalt have been 

extruded on the flanks of both cones. Late in the history of the main 

volcano, after the volcano had been considerably eroded, a summit 

cone of coarse tuff-breccia was formed. An andesitic plug that rose in 

the crater of this cone now forms the summit of Mount Moffett.

Composite cone. Rocks of a composite cone exposed on the sea cliff 

forming the north face of Mount Moffett for a distance of several

miles west from Andrew Lagoon are the oldest rocks attributed to the 

Mount Moffett eruptive center. The cone consists chiefly of layers of 

tuff-breccia and breccia, whose angular fragments are as much as 5 feet 

long,- but includes -a .few lava flows near its top. Tilfi breccia fragments consist of pale-gray hornblende basalt, red hornblende andesite
GEOLOGY OF'NORTHERN ADAK ISLAND 57

that is partly scoriaceous, and dark-gray to black basalt that contains 

conspicuous feldspar phenocrysts.

Interbedded with the pyroclastic rocks are two thin beds of rounded 

boulders, 40 and 60 feet above sea level, respectively. These beds, 

 each about 2-4 feet thick, strike parallel to the cliffs and dip seaward. 

They are probably of marine origin, and the tuff-breccia interbedded 

with them is interpreted as being the product of mudflows deposited in 

part under water. The sequence of tuff-breccia beds is at least 400 

feet thick and is overlain by massive lava flows and by lapilli-tuff similar in lithology to the fragments in the tuff-breccia.

All of the earlier lava flows are gray, but locally they have been 

reddened near the base, presumably by steam rising from the surface 

over which they flowed. The conspicuous*crystals consist of plagiocla.se, dark-green augite, brown olivine, and in some flows, deeply 

resorbed black hornblende. The plagioclase is zoned from sodic 

bytownite to sodic labradorite. Magnetite is a common accessory 

mineral. The groundmass contains plagioclase, magnetite, augite, 

and interstitial material that is probably orthoclase.

The later lava flows of Mount Moffett include dark-gray compactto-porous lava; light-gray porous lava; and pale pinkish gray lava. 

Most of the rocks are made up of hornblende basalt that contains 

phenocrysts of labradorite and prismatic black hornblende, as 

much as 3 millimeters long, and subordinate augite and magnetite. 

The groundmass consists of plagioclase and magnetite granules and 

interstitial orthoclase(?). Some of the rocks contain hypersthene and 

olivine phenocrysts and are classed as olivine-hypersthene basalt.

Basalt domes. Five basalt domes occur on the slopes of Mount 

Moffett. Two are high on the southeast flank, one is near sea level on 

the southeast flank, and two are on the north flank. These bodies 

were examined only briefly. Some may prove to be the termini of 

thick flows, or abnormally thick dikes, rather than domes. The three 

on the southeast flank are described in the following three paragraphs.

Hill 2416,3 1% miles southeast of the summit of Mount Moffett, is 

composed of pale-gray hornblende basalt that contains inconspicuous 

glassy labradorite and conspicuous black hornblende phenocrysts as 

much as 2 millimeters long. Augite and magnetite are abundant. 

The groundmass consists of augite rods, plagioclase laths, and magnetite granules in a mosaic of quartz (?) and orthoclase (?).

Hill 1981,3 about 2% miles southeast of the summit of Mount 

Moffett, is made up of a compact medium-gray rock that contains 

plagioclase crystals as much as 2 millimeters long and augite grains as 

much as 1 millimeter long. Magnetite is abundant, and hypersthene, 

olivine, and basaltic hornblende- are -common but irregularly dis8 Place names not shown on standard maps but used for convenience in this report.
58

tributed. Plagioclase laths, augite grains, and interstitial orthoclase 

make up the groundmass.

The hill known locally as Red Bluff,4 on the north side of the eastwest runway of the airbase, 2% miles south of the southern extremity 

of Andrew Lagoon, is made up of fine-grained, tough,.slightly-,;porous, 

medium-gray basalt. The basalt contains scattered small phenocrysts of augite and plagioclase, as much as 1 millimeter across, in a 

groundmass of bytownite laths and interstitial magnetite, augite, and 

glass.

Tuff-breccia cone and its andesitic vent filling. Remnants of a small 

cone of coarse, poorly stratified tuff-breccia, erupted from a vent 

about on the site of the present summit of Mount Moffett, rest 

unconformably on the lava beds of the composite cone of Mount 

Moffett. The tuff-breccia cone was built after deep erosion of the 

composite cone. The crater of the tuff-breccia cone is filled with 

porous, light pinkish-gray hornblende-hypersthene andesite. Most .of 

the phenocrysts consist of zoned labradorite and augite, but some 

consist of basaltic hornblende and hypersthene. Magnetite and 

apatite are common accessory minerals. The groundmass consists of 

plagioclase, augite, and magnetite in a cryptocrystalline mosaic. 

Secondary tridymite lines many vesicles.

The tuff-breccia cone has largely been destroyed by glacial erosion; 

presumably its age is early Quaternary or pre-Quaternary. It is 

younger than the composite cone and the basalt domes of Mount 

 Moffett.

Basalt jlo-w. A small basalt flow crops oxit 1% miles west of the 

head of Sweeper Cove. The lava apparently issued quietly from a 

small vent and flowed southward down the hill for about 2,000 feet. 

It has been cut through by a small stream near its terminus, where a

maximum thickness of about 20 feet is exposed.

The basalt is a fine-grained, medium-gray rock, containing many 

irregular, flattened vesicles commonly lined with tridymite. Sparse

small plagioclase phenocrysts arid, considerable accessory magnetite

are set in a groundmass of blocky plagioclase, interstitial augite 

grains, magnetite, and brown glass.

The flow unconformably overlies the Finger Bay basalt. No 

glacial striae were observed, but the presence of small rock basins on

the surface and the absence of coarsely vesicular rocks that were

expected near the original upper surface of the flow suggest that it is

preglacial in age. The basalt flow appears to be one of the youngest

extrusive rocks attributed to the Mount Moffett center. It is probably late Tertiary in age, but may be early Quaternary.

« Since this report was written the hill has been removed.
GEOLOGY OF NORTHERN AQAK - ISLAND 59 

PARASITIC CONE OF MOUNT MOFFETT

On the northeast flank of Mount Moffett is a composite parasitic 

cone, 3,250 feet in altitude, which was formed rather early in the life 

of the host volcano. Its vent is filled by gabbro and basaltic agglomerate. On its southeastern flank is an eruptive basaltic dome. Judging from the amount of erosion, activitj7" at the cone ended before the 

latest activity at Mount Moffett proper.

The cone occupies approximately the northeast quarter of the 

area underlain by Mount Moffett rocks! On the northern sea cliff, 

the lowest beds of lava of the parasitic cone overlap the beds of lava 

and the tuff-breccia of Mount Moffett. Some of the rocks of the 

parasitic cone probably interfinger with late flows from the main 

vent of Mount Moffett.

The parasitic cone is composed of rather uniform lava flows, 5-10 

feet thick, interbedded with coarse basaltic lapilli-tuff containing 

scoriaceous fragments as much as 6 inches long. A typical flow 

consists of purplish-gray porous olivine basalt, displaying dark-green 

augite phenocr37sts as much as 4 millimeters long, yellowish-brown 

olivine phenocrysts as much as 2 millimeters across, labradorite 

phenocrysts from 0-1.5 millimeters ;long,an'd accessory magnetite. 

The grounclmass is made up of plagioclase, augite, magnetite, and 

interstitial orthoclase.

Fine-grained olivine gabbro plug and associated basaltic vent agglomerate. Light-colored rocks that form the summit of the composite cone 

can, even at a distance, be readily distinguished from the dark interstratified lava flows and beds of lapilli-tuff forming the dissected outer 

slopes of the cone. This central light-colored material constitutes a 

plug that filled the former vent of the cone. Its main constituent 

is pale purplish gray fine-grained olivine gabbro. Intricately mixed 

with the gabbro are masses of basaltic vent agglomerate, in places 

hydro thermally altered.

Basaltic dome. Hill 1848,5 about a mile west of Andrew Lagoon, 

is a basaltic composite dome. It is made up of one of the youngest 

volcanic rocks on northern Adak Island. The dome is tripartite, the 

two outer parts being the remnants of what is assumed to have been 

a continuous ring that has been trimmed away by erosion on the 

north and south. The rocks of the central, highest part represent a 

slightly later intrusion, which broke through and forced apart the 

rocks of the original ring. The outer parts of the dome consist of a 

medium-gray porous lava, containing very irregular vesicles into 

which crystals of the lava project. The phenocrysts are made up of 

plagioclase and green augite in a groundmass of plagioclase laths, 

magnetite, augite, and interstitial brown glass.

* Place name not shown on standard maps but used for convenience in this report.
60 INVESTIGATIONS OF ALASKAN VOLCANOES

The central part of the composite dome co'nsists of rock similar to 

the gray rock that makes up the outer part of the dome, but it has 

been reddened and contains xenoliths of the gray. rock. ..

GLACIAL DRIFT

Most of northern Adak Island was covered by glacial ice during; 

part of Pleistocene time. The chief deposit of this ice is a ground 

moraine left by an ice sheet that moved into the mapped area from 

highlands to the south. It covered most of the southern part of the 

area and lapped up on the southern and southeastern slopes of Mount 

Moffett and Mount Adagdak. The ground moraine is as much as 

12 feet thick, and contains numerous subangular and rounded boulders, 

many of them faceted, set in a tough and clayey matrix. The surface 

of the ground moraine is comparatively smooth, and the till has been 

oxidized to a depth of only a fraction of an inch. A thin blanket of 

volcanic ash and, adjacent to the coast, a blanket of windblown sand 

rest unconformably on the ground moraine.

Near the base of Mount Moffett much of the material carried by 

the ice sheet was deposited as irregularly stratified beds of sand and 

fine gravel. These beds are intermingled with beds of unsorted 

boulder gravel, some of which may have been brought down the slopes 

of Mount Moffett. In places, where more material was brought down 

the mountain slopes, relatively flat terraces were formed, bounded on 

their down-mountain side by steep ice-contact slopes. At other places 

swales, bounded on the downr-moimtain side by low ridges of moraine, 

mark successive positions of the ice front during its retreat.

Ground moraine, swales, and terraces similar to those described are 

present on the southern slopes of Mount Adagdak, where they reach 

an altitude of nearly 500 feet. Sediment brought by glacial Streams 

from the Mount Adagdak area and other areas was ponded in a pocket

between arms of the ice sheet, forming an extensive terrace north of

the east end of Andrew Lagoon.

Valley glaciers, which headed in cirques on the north and northeast 

sides of Mount Moffett, built prominent lateral moraines as well as 

some ground moraines. Locally, a kfge Olltei lateral moraine was 

Succeeded by a smaller inner one. One of these outer moraines, exposed in the sea cliff on the northeast face of the parasitic COne Of

Mount Moffett, is about 400 feet thick and a little more than 4,000

feet long. Immediately south of this moraine is a younger moraine 

that is only about half as long and kalf as thick. Other great moraines 

form much of the higher parts of the sea cliffs on the northwest side 

of Mount Moffett.
GEOLOGY OF NORTHERN ADAK ISLAND 61 

MARINE TERRACE BOULDER GRAVEL,

Marine terraces veneered with boulder gravel a few feet thick occur 

in 3 areas; 2 large ones on the shores north of Mount Moffett and 

Mount Adagdak (pi. 10(7), respectively, and 1 small one on the east 

side of Mount Adagdak. The terraces on which the boulder gravel 

lies range in height from 20-100 feet above sea level. The terrace on 

the north side of Mount Adagdak slopes both eastward and westward 

from a maximum altitude of 100 feet to a minimum of 20 feet. Individual boulders on the outer edges of the terraces are as much as 3 

feet across. On their inland margins these deposits grade into talus 

and alluvial fans. The gravel-covered terraces have been cut back by 

wave action but have not been dissected by streams. They are postglacial in age.

RECENT UNCONSOLODATED DEPOSITS

\

Patches of Recent unconsolidated beach, delta, lagoon, alluvial, and 

aeolian deposits are scattered throughout the area. Because of the 

intensity of the wave attack on exposed coasts during the violent 

storms of the Aleutians, the present beach deposits consist mostly of 

coarse materials, ranging in size from that of gravel to that of boulder 

gravel. Boulders as much as 2 feet across are shifted appreciably by 

storm waves.

Streams emptying into protected bays have built deltas of mappable 

size. The material of the deltas is subangular to subrounded; some 

of the fragments are of cobble size. The delta deposits consist largely 

of reworked pyroclastic and glacial deposits, as few streams have been 

able to excavate deeply in the hard bedrock since glacial time. Similar 

alluvial deposits form small patches along some of the streams, particularly where these streams empty into the numerous lakes in the 

glaciated areas.

Aeolian deposits include sand dunes and deposits of ash; the ash was 

carried by winds from eruptions on other islands. Large areas of sand 

dunes occur on both sides of the entrance to Clam Lagoon. The sand 

supplied by longshore currents is concentrated on tide flats, from which 

it is picked up by the wind during low-water periods. Smaller patches 

of sand dunes are found on the spits of Shagak Bay and on the bar 

enclosing Andrew Lagoon. Sand dunes were present also in the area 

north of Sweeper Cove before extensive construction work began 

there. The maximum height of the dunes is about 100 feet. All appear to be stable, except those whose cover of native vegetation has 

been recently disturbed.

Most of the lower slopes of northern Adak Island are covered by a 

volcanic-ash layer of strikingly uniform thickness (pi. 14(7). The ash 

layer has been omitted from the geologic map in order to show the
62 INVESTIGATIONS OF ALASKAN VOLCANOES.

bedrock geology. The ash is composed largely of fine fragments of 

brown-to-black" basalt, but contains layers of coarser, lighter colored 

pumice which may be andesitic. Near Finger Bay it is 5.8 feet thick, 

and on the west side of Clam Lagoon, 5:3 feet. In a quarry 3 miles 

east of the head of North Spit, on the southern slope of Mount Moffett 

at an altitude of 400 feet, the ash is 7.5 feet thick.

The lower 3.4 feet of ash exposed in the quariy is preserved only 

in the small valley in which the quarry is located. It is overlain unconformably by a younger layer of ash. The .deposit,has-a greater, 

total thickness in this valley than in the other areas where it was 

measured, probably because, the valley was not covered by glacial ice 

at the tune the ash was deposited, whereas the other areas were covered. One of the striking features of the ash blanket at lower altitudes 

is its uniform thickness, which is almost independent of steepness of 

slope. The general presence of stem and root casts suggests that this 

uniformity is due at least in part to the material having been held in > 

place by vegetation. If part of the ash had been derived from one of 

the volcanic centers on Adak Island, the coarseness and thickness of 

some layers should vary inversely with distance from one of those cen- ^ 

ters. This variation-has not been observed, hence it is concludecLtha:b 

all of the deposit has been derived from explosive eruptions of volcanoes on other islands. It is further concluded that the deposit must ' 

be largely postglacial, as glacial erosion would have removed preglacial ash from most of the area. ~*

 4

STRUCTURE

The structure of northern Adak Island is diagrammatically shown 

on the geologic sections of plate 9. The internal structure of the -* 

Finger Bay volcanics is not shown by the sections. In response both 

to deformation and to erosion this sequence has acted as an essentially 

homogeneous unit. It formed the irregular surface upon which the >-' 

three younger volcanoes were erected. The volcanic structures of

Tertiary ancl Quaternary age are relatively simple, and largely reflect

the processes of construction, rather than of tectonic deformation. y

The present crustal instability of the area is indicated by the numerous 

earthquakes felt at Adak. None is believed to have been due to 

movement along any of the faults that have been mapped. <~ 

Many persistent and well-defined faults occur in the southern part

of the area. Most of them appear to be steeply dipping normal faults 

of small displacement. The predominant trends of these faults 

range from N. 60° E. to N. 60° W. and from N. 20° E. to N. 10° W., 

but faults of intermediate trend are present, and some faults curve 

through a wide range of trends. The age of these faults is not V 

definitely determinable. They do not appear to cut any of the
GEOLOGY OF NORTHERN ADAK ISLAND 63

Tertiary rocks of the island, but they do cut the Finger Bay volcanics 

and the gabbro that intrudes the Finger Bay volcanics. Some of the 

faults shown on the geologic map extend beyond the borders of the 

mapped area; those whose trends average about N. 65° E. are apparently part of a system present on other islands of the Anclreanof 

group lying to the east of Adak.

The rocks near the summit of Mount Adagdak are cut by Recent 

faults (pis. 13 and 145). The fault shown in plate 145 is the upper 

fork on the right side of the fault most conspicuously outlined by 

snow in plate 13. The apparent throw of the upper fork is about 4 

feet and the throw of the lower fork of the same fault is about 8 feet. 

The downthrown sides of both forks are those nearest the center of 

the mountain, as are those of most other Recent faults in the area. 

Only the more active talus cones have overridden the sags formed 

where the faults cross the bottoms of gullies. The faults a,re at most 

only a few thousand years old.

GEOLOGIC HISTORY

The geologic history of northern Adak Island is essentially the 

history of the formation and destruction of the series of cones that 

are represented today by the rugged mountains of Mount Moffett 

and Mount Adagdak.

The geologic record begins in the pre-Mesozoic with the accumula-, 

tion of a thick sequence of basaltic extrusive rocks and dikes the 

Finger Bay volcanics. These rocks were folded, intruded by bodies 

of gabbro, and attacked by hot solutions. Afterward, numerous 

faults developed, then a long interval of erosion occurred, unmarked 

by notable volcanic action.

In Tertiary time volcanic activity was resumed. Five masses of 

andesite porphyry were formed as steep-sided viscous protrusions near 

the-present east coast. A central vent, west of the present eastern 

shore of Andrew Bay, became active, building a cone largely of 

andesitic lava flows and mudflows the Andrew Bay volcano.

Coincident with or before the accumulation of the rocks of Andrew 

Bay volcano, several hundred feet of extremely coarse conglomerate 

accumulated along what is now the eastern side of the Mount Adagdak 

peninsula. The coarse conglomerate was tilted and eroded, and 

fossiliferous sandstone, containing lenses of pebble conglomerate, 

was deposited.

After a period of volcanic quiescence, in which the sedimentary 

rocks were again tilted'and the Andrew Bay volcano was almost 

completely destroyed by marine erosion, volcanic activity recurred 

at a new center near the present Mount Adagdak volcano. The 

eruption at first was explosive and perhaps submarine, and tuffaceous
64

sands were deposited around the base of the new volcano. Later, 

thick flows of basalt and olivine basalt accumulated to form a broad 

low dome, attaining an elevation of perhaps 1,500 feet and a diameter of more than 2 miles. This shield volcano was attacked by 

marine erosion, particularly on the north, creating a cliff which 

gradually was cut back almost to the volcano's center. Activity was 

then renewed, and a composite cone, made up primarily of lapilli-tuff 

and andesite flows, but containing small andesitic intrusions, was 

formed around the same vent. After a brief cessation of activity, a 

nested crater was constructed, the center of which was about half a 

mile to the northwest of that of the older and larger crater. A long 

period Of quiescence ensued, permitting marine erosion to cut away 

the northern half of the composite cone. The accumulation of a 

series of mudflows, succeeded by the extrusion of hornblende andesite 

as two flows from and two domes in the crater, completed the volcanic 

history of Mount Adagdak. An olivine basalt dome on the southeastern flank of the mountain may date from this final period of 

extrusion.

The rocks of Mount Moffett and those of Mount Adagdak are 

nowhere in contact, thus making the relative dating of geologic events 

in their histories practically impossible; however, it appears likely 

that the two were at least in part contemporaneous. Early eruptions 

from a vent south of the present summit of Mount Moffett produced 

tuff-breccia and flows of olivine and hypersthene basalt; later, hornblende basalt was extruded as flows. Late in the history of Mount 

Moffett, steep-sided viscous domes of basalt formed on the flanks of 

the COne. A period Of erosion followed, during which the main cone 

of Mount Moffett was considerably dissected. By a series of explosive

eruptions, a steep cone of coarse basaltic tuff-breccia was constructed, 

centered about a vent which occupied approximately the position of 

the present summit of Mount Moffett. The vent was later filled 

by an intrusion of andesite. Basalt that forms a flow 1% miles west 

of Sweeper Cove was erupted at about this time.

A steep-sided parasitic cone on the northeast flank of Mount 

Moffett was built contemporaneously with the tuff-breccia summit 

cone. This parasitic cone consisted largely of a succession of thin

flows of olivine basalt, but included a few beds of basaltic scoria. 

After formation of the cone a viscous, partly cooled mass of finegrained olivine gabbro rose into the vent, dragging up the flows 

adjacent to its margin. Subsequently, a minor explosive eruption 

blasted a small vent through the central plug; the vent was later 

filled with basaltic vent agglomerate. Apparently the most recent
GEOLOGY OF NORTHERN ADAS ISLAND 65

event in the history of th& parasitic cone was the extrusion of a 

basaltic dome about 2 miles west of the center of Andrew Lagoon.

During early Quaternary time much of Adak Island was covered 

by glacial ice. The principal collecting ground was the highland 

lying south of the latitude of Sweeper Cove. From this area the ice 

moved as a sheet northward to Mount Moffett, where it divided. 

Part of the ice flowed westward into Shagak Bay and into the area 

now occupied by Adak Strait. The other part advanced northward 

and eastward to the slopes of Mount Adagdak, where it again divided. One arm, at its greatest extent, reached the sea along a front 

which probably lay in what is now Andrew Bay. The other arm 

reached the sea along a front which lay somewhere on Kuluk Bay, 

and which may have extended eastward as far as Great Sitkin Island. 

The retreat of the ice front was gradual, interrupted by numerous 

halts. During each of these halts fragmental material, carried down 

from the easily erodible tuff-breccia deposits higher on the mountain 

slopes, was deposited against the ice margin to form long, narrow 

terraces. Several flights of terraces were thus built, each with a 

gently sloping upper surface and a steep face. The terraces are 

comprised of both glacial and mudflow material.

Similar but less conspicuous terraces were formed on the southern 

flank of Mount Adagdak. Most of the material for these terraces 

was supplied by glacial erosion of rocks to the south.

Five small glaciers appear to have occupied valleys on the east, 

northeast, north, northwest, and west sides of Mount Moffett, but 

all disappeared thousands of years ago. At some stages the valley 

glaciers on the east and northeast sides may have descended the 

mountain slopes far enough to join the ice sheet. Altitudes on the 

island are not great enough to permit the formation of glaciers under 

present climatic conditions.

After the withdrawal of the glacial ice from the shoreline, large 

areas of poorly consolidated Tertiary volcanic rocks and unconsolidated 

glacial deposits were exposed to vigorous marine erosion. A wide 

wave-cut bench was carved along much of the northern coast. Longshore currents are now constructing imposing bars and spits that 

shut off Andrew Lagoon, Clam Lagoon, and Shagak Bay from the 

sea. Fine material exposed above water level has been transported 

inland by the wind to form a blanket that mingles imperceptibly on 

the landward side with the blanket of volcanic ash laid down in postglacial time by eruptions from the volcanoes on nearby islands. 

Where the supply of sand has been unusually abundant, sand dunes, 

now mostly stabilized by vegetation, have formed. In relatively
{56 INVESTIGATIONS OF ALASKAN VOLCANOES

recent times the remnants of the wave-cu* bench were uplifted to form 

terraces a few tens of feet above sea level. Perhaps related to this 

uplift was the development of peripheral normal faults on Mount 

Adagdak, by which the central part of the mountain subsided a few 

feet relative to the outer slopes.

The volcanoes of northern Adak Island apparently have not been 

active since early Quaternary time. The postglacial volcanic ash 

on the island is believed to have been derived from explosive eruptions K 

of volcanoes on nearby islands, chiefly Kanaga and Great Sitkin. , On this basis, it appears that the volcanoes, if not extinct, can 

reasonably be expected to remain inactive in the foreseeable future. "I

On the other hand, there is abundant evidence that volcanic activit}^ 

has been renewed several times in the past after long periods of quiescence. Judging by the amount of marine erosion before each renewal, - 

each has occurred after a period of quiescence that was longer than ^ 

the present quiet period. The length of time that has passed since 

the building of the tuff-breccia cone on Mount Moffett and the ^ 

extrusion of the basalt that formed the dome on its parasitic cone ^ 

the latest episodes in the volcanic history of Mount Moffett is 

probably about the same as that of some of the previous periods of 

quiescence in the history of these cones, perhaps several thousand ^ 

years. The possibility of a new c}7 cle of volcanism on northern Adak 

Island cannot, therefore, be dismissed. < <

Frequent earthquakes felt at Adak indicate that the earth's crust 

in that area is unstable. It remains to be determined by investigation 

whether this instability is directly related to local volcanic activity, -* 

and may thus herald a new volcanic cycle on Adak, or whether it is 

related only in a general Avay to volcanic activity in the Aleutian arc.

LITERATURE CITED

\

Bradley, C. C., 1948, Geologic notes on Adak Island: Am. Jour. Sci., v. 246,

no. 4, p. 214-24O. ^

Coats, R. R., 1952, Magmatic differentiation in Tertiary and Quaternary rocks

from Adak and Kanaga Islands, Aleutian Islands, Alaska: Geol. Soc. "<

America Bull., v. 63, p. 485-514.

Judson, Sheldon, 1946, Late-glacial and postglacial chronology on Adak: Jour. '?- 

Geol., v. 54, no. 6, p. 376-385. .<

Martin, G. C., 1926, Mesozoic stratigraphy of Alaska: TJ. S. Geol. Survey Bull.

776, p. 61.

Martin, G. C., and Katz, F. S., 1912, A geologic reconnaissance of the Iliamna

region, Alaska: U. S. Geol. Survey Bull. 485, p. 40. ;
INDEX

	Page 

Acknowledgment._. . . 45

Adak Strait, glaciation....................... 65

Aeolian deposits.__-.-...-_...--. ---.. 51,61

Alluvial deposits...__-_._.__--------- 51

Andesite porphyry domes.....____-....- 51

Andrew Bay, glaciation..._.__......__ 65

Andrew Bay volcano, features..--.----------- 46

Geologic history.__._....___.__ 63

Andrew Lagoon, bars and spits_.-._--.-.... 65

Sand dunes_..----.-.__ 61

Annularia stellata............................. 49

Ash.......-..........-.--....-.....-. .-. 61-62

Bars --- _-------- ----- 65

Beach deposits-- _..-.-..-._. 51,61

Boulder gravel..._.. ___ __ 61

Clam Lagoon, bars and spits. 

sand dunes.___.--......

Coastline_____ ..........

Conglomerate, boulder.......

Deformation of Finger Bay volcanics......... 49

Delta deposits-.....-.---.......-----....-. 51,61

Diorite. - . 50

Drainage.._..._-----_-----___.. .. 48

Dunes - - . 61,65

Earthquakes-.....-...--..-...-.-..-.... 62,66 

Erosion . . ....-.-. 45,64,65

Faults ----------.----...---46,50,62-63,66

Field work.._.___...___...___... 45 

Finger Bay..________.__________ 46 

Finger Bay volcanics, faults._.............. 63

features..__.____________46,48-49 

geologic history....._._____..__. 63

Fossils. 49, 51, 53-54

Gabbro 50,63

Geologic history..---..-.------.-..----. 63-66

Glacial deposits..._._.___....___.. 51,60

Glaciation, extent..__._._.___ 60,65 

Finger Bay volcanics________...... 46

rocks of Cenozoic age...__-_...._ 50-51

Gold- . 50

Ground water___.___.__.______. 48

Hill 1848 59

Hill 1981 . - 57-58

Hill 2416.

Lagoon deposits. 

Lakes. . 

Page 

57

51

46

Mitchell Field. . 49

Mt. Adagdak, faults 63

features.. ___-__.__......__ 46

geologic history______-----___ 63-64

glaciation_-----------___------------- 60

terraces...____________-__.... 46,65

Mt. Moffett, features 46 

future volcanic activity................... 66

geologic history...._____________ 64

glaciation.. - 60,65

terraces.__----------------_-..._ 46

Mudnows...... ... 55-56

Red Bluff. 58

Sandstone-____-_--------__._------- 53-54

Sea cliffs 46

Section, Mt. Adagdak..................._.. 54

Sedimentary rocks..--------...............b^, 53,57

ShagakBay.. 61,65

Silver..-. 50

Sine of area inapped. 

Spits.__--.___. 

stellata, Aimularia.. 

S treamflow-.........

 .... . 46

 65

 ....... 49

 48

Structure. - - 62-63 

Syenite .._._._-----__...___._ 50

Terraces, marine.--____------_ 46,51,61,65

Topography..____----------------------- 46

Unconsolidated deposits.

Volcanic activity, centers. 

future. ___ 

Volcanic rocks, Andrew Bay volcano...._ 52-53

Finger Bay volcanics...__.....__._ 48-49

Mt. Adagdak.. .........-.-...52,54-56

Mt. Moflctt . - 56-60

Water power. 

Zeto Point .

67

U. S. GOVERNMENT PRINTING OFFICE: I95G