Geology of Unalaska 

Island and Adjacent 

Insular Shelf, Aleutian 

Islands, Alaska

\

By HARALD DREWES, G. D. FRASER, G. L. SNYDER, and H. F. BARNETT, JR.

INVESTIGATIONS OF ALASKAN VOLCANOES

.

GEOLOGICAL SURVEY BULLETIN 1028-S

' Prepared in cooperation with the 

Office, Chief of Engineers, U.S. Army

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1961
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D.C. L
PEEFACE

In October 1945, the War Department requested the U.S. Geological Survey to undertake a program of volcano investigations in 

the Aleutian Islands-Alaska Peninsula area. The resulting program 

was under the general direction of G. D. Robinson during 1946^48, 

and of H. A. Powers during 1949-54. Some of the results have been 

published in two administrative reports which, now revised, are 

largely incorporated in bulletins describing specific islands or groups 

of islands. Two bulletins (Coats 1950; 1951) and parts of Bulletin 

1028 are published; other parts of Bulletin 1028 are in various stages 

of completion.

The Army, Navy, and Air Force jointly furnished financial and 

logistic assistance, and the Geological Survey is indebted to these 

departments for their support.

in

CONTENTS

""' Page

Preface __ _________________________________________ ______________ in

Abstract._____________________________________________ 583

/ Introduction ______________________________________________________ 584

Geography__ _ _____-_--____--_---_--_-____--_______-____________ 587

Igneous and sedimentary rocks._____________________________________ 590

Unalaska formation__________________________________________ 590

V Sedimentary rocks___----_--_--_-_-_-_--____-______________ 591

Flows, sills, pillows, and bulbous intrusive bodies______________ 593

Dikes...__________________.______________ 599

Petrography of the Unalaska. formation ______________________ 600

Specific data for analyzed rocks.________________________ 601

Alteration. __________________._________ 603

Fauna and age______----_--_-_-_-_______-_________________ 606

T\ Environment and origin.___________________________________ 607

Granodiorite batholith.________________________________________ 610

Field relations___.-._____________._______.______...__.__ 610

Petrography of the batholith______________________________ 612

Shalerpluton____________________________ 619

Beaver pluton_______________________________________ 619

Captains pluton_______________________________________ 620

^ Aplites and pegmatites ________________________________ 620

Small holocrystalline intrusive bodies ____________________ 620

Recrystallized wallrocks.___--_-_-___--_-__-___________- 622

Comparative mineralogy__________________-____________ 624

Specific data for analyzed rocks..__-___-___-_-__________ 626

t Alteration____________._______________________________ 630

Plutonic rocks: summary and interpretation__________________ 631

Makushin volcanics.___________________________________________ 634

Unconformity beneath the Makushin volcanics.____-_-___---__ 634

\- Petrography of the Makushin volcanics______________________ 637

Specific data for analyzed rocks..___--_____---_____-_-_- 640

Age of the Makushin volcanics-________________---_-_-_----- 641

Eider Point basalt --__------_--_---__ ________ .___ _ 641

Makushin Volcano; fumaroles and hot springs______-_-_-____ 642

Pakushin Cone____-__--__--______-__-__-_____----- 642

Point Kadin vents-_--_---_-_-_----_-_------__---_--------- 643

, Bishop Point volcanic mudflow_______-_______---___-_-__-_ 643

Sugarloaf Cone and lava flows_________________-----_----_- 644

Cone and lava flows of Tabletop Mountain___________________ 644

Wide Bay cone__________________________________________ 645

j Other possibly young flows_______________________________ 645

Petrography of the Eider Point basalt____.____---_-___----_ 645

Specific data for analyzed rocks-________________________ 645

Surficial deposits and geomorphology________________-__--_-_-----_- 646

Glacial landforms and deposits._________________________________ 646

Stream erosion and alluvium.________________________^_-_-_---_- 648

Colluvium and eolian deposits____________-_______------------- 649

Soil-ash profiles._-___-_--____-___-___---------------_----- 650

Eolian features..__________________________________________ 652

Marine erosion and deposits._________________.!____-_-___-_----- 652
VI CONTENTS

Page

Structural geology _---______-____---_-_-____-__-----_-_____--_____ 653

Faults_________________.___________________- 654

Volcanic structures -________---_____-__-_--_--_-_____-__-______ 655

Linear features.________--_-_-_-____--___-_-______-.___________ 655

Economic geology.___-___________--_--___--_-____-__--_-_______-__ 656

Sulfur_._.........._._._._._________.____._._.._..______._.._. 657

Gold............... ......._..._._._.___.._.._..____..._..__ 657

Zinc___________.--_..._._._______________.._ 657

Other mineralization ________-_--_-_____--_-__-_-_i_-__________ 658

Potentialities of economic deposits_-____-----_-_-__----_-___---_- 658

Submarine geology_______________________________________________ 659

Physiography.___________________-___-______---_-__-_--__-____ 660

Insular shelf..__________________________________________ 660

Lower insular shelf.._______________________________________ 661

South insular slope_________-__---__-______-__---_-___--___ 661

North insular slope__________--___-_-_-__--_----_--____-__- 661

Bering Basin and Continental Slope_______-___--_--________ 661

Wave and current activity___-______-___-___---_------_----_-___ 662

Glacial features__-__-__-____--_-_________--_--___---_-------__ 663

Submarine structure..-___-_-_-___-_________--_______--_______- 665

Geologic history.___________________________________________________ 666

Literature cited_..________________________________________________ 669

Index._______..__..______.......___._._.__.__._..._..________..__ 763

ILLUSTRATIONS

[Plates are In pocket]

PLATE 75. Geologic map of Unalaska Island, Alaska, and adjacent submarine 

area.

76. Percent frequency distributions of mineral amounts and plagioclase 

composition parameters in holocrystalline rocks of Unalaska Island.

77. Analysis of linear features in the rocks of Unalaska Island.

78. Map and fence diagram showing geographic distribution of holocrystalline igneous rock types in the Skan Bay-Naginak Cove area 

of the Shaler pluton.

FIGURE 81. Map of southwestern Alaska_____.______-___-_______..-_- 585

82. General view of a part of Unalaska Island-_-________---___ 588

83. Homogeneous tuff breccia from vertical sea cliff on north

shore of Boulder Bay.___-___--__--____-_--_______---_-_ 594

84. Large pillows, layered argillites, and sills in a sea cliff about 

900 feet high on the east side of Reef Point, southwestern 

Unalaska__--.-_-___-_____________________-_-________ 596

85. Partially brecciated pillows from near the top of small intrusive 

body exposed in the bench on the south side of the north arm 

of Kismaliuk Bay----_--_---_--_----------------------- 597

86. Disruption of sills, Tower Point, southwestern Unalaska . 598

87. Horizontal section of a layered dome on a tidal bench along

the west side of Alimuda Bay, western Unalaska Island. ____ 599
CONTENTS VH

Page 

-< FIGTJBE 88. Silica variation diagram of igneous rocks of Unalaska Island.. 604

89. Alteration and mineralization on Unalaska Island._________ 609

90. View south toward 3600 Mountain roof pendant, showing 

y Shaler pluton and Unalaska formation.___________________ 611

1 91. Joint pattern in homogeneous central granodiorite of Shaler

pluton, looking south from south shoulder of 3600 Mountain.. 613

92. Joint pattern in inhomogeneous border rocks of Shaler pluton 

^ on Kof Point._.__________________________ 614

93. Percent-frequency distributions of the average percent anorthite in plagioclase crystals of typical holocrystalline rocks of 

Unalaska Island______________________________________ 615

94. Typical phaneritic rocks from the Shaler pluton of Unalaska

Island_____________________________________--. 618

95. "Plum-pudding" contact rock from large polished beach 

y boulder at head of north arm of Skan Bay_______________ 621

96. Recrystallized inclusions of layered argillite in gabbro from

small stock on north side of Humpback Bay _______________ 623

97. Combined distribution of total mafic mineral content and 

f anorthite content of average plagioclase for holocrystalline

rocks of Unalaska Island________________________________ 625

98. Concentric mineral zones within Shaler pluton____._-__-__- 632

99. Makushin Volcano, looking southeast from an airplane over 

1 Point Kadin____________________________ 635

100. Isometric diagram of Makushin Volcano area, Unalaska

Island________________________________ 636

101. Map of northern Unalaska Island._-_-----_-_-----____-__ 638

102. Soil-ash profiles.._________________________ 651

103. Block diagram showing relation between shelf ice and a pseudo

end moraine.__________________________________________ 664

4

TABLES

Page 

TABLE 1. Chemical composition, normative values, CIPW nomenclature,

and Rittmann nomenclature for Unalaska Island rocks.._ In pocket

2. Incidence of holocrystalline rock types on Unalaska Island-__ 617

3. Unalaska Island faults and their displacement.______________ 655

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGY OF UNALASKA ISLAND AND ADJACENT 

INSULAR SHELF, ALEUTIAN ISLANDS, ALASKA

By HARALD DREWES, G. D. FRASER, G. L. SNYDER, 

and H. F. BARNETT, JR.

ABSTRACT

Unalaska Island is the second largest island west of the Alaska Peninsula 

and includes about 1,200 square miles. Most of the island is ruggedly mountainous and the coastline is deeply indented by fiords, but the southwestern end 

is hilly and is less deeply indented by bays. Makushin Volcano is still active 

and forms a broad, glacier-capped, truncated cone rising 6,680 feet above the 

sea on the northern bulge of the island.

The oldest rocks on the island, the Unalaska formation, consist of altered 

andesitic intrusive and extrusive rocks, and sedimentary rocks derived from 

similar igneous rocks. Conglomerates and coarse breccias are the dominant 

sedimentary rocks in the northern and eastern part of the island; finer epiclastic 

and pyroclastic rocks, particularly argillite, are dominant in the south. Fragments of a desmostylid, distant relative of the modern sea cow, date some of 

the formation as early Miocene. The coarse facies is probably a nearshore 

marine deposit; the finer facies suggest deposition in deeper basins. Conspicuous, bulbous igneous masses whose diameters range from a few feet to 

several hundred feet were intruded into and possibly extruded upon muds and 

are associated with pillow lavas.

The batholiths are granodiorite; the heterogeneous border phases are as 

mafic as gabbro against hornfelsed argillites as mafic as, and equivalent in composition to, melagabbro. Belts of hydrothermal wallrock alteration surround 

the batholiths, which were themselves slightly altered by deuteric solutions, 

and several areas of wallrocks were dynamically metamorphosed when the 

adjacent batholiths were intruded. Assimilation, stoping, and forceful intrusion were all active mechanisms of emplacement. The core of one batholith is 

relatively rich in quartz, potassic feldspar, allanite, and amphibole, indicating 

either hydrothermal introduction of these materials or only slight contamination of the core by assimilated basic material.

Basalt and andesite flows and pyroclastic rocks of the Makushin volcanics 

unconformably cap the Unalaska formation and plutonic rocks and form most 

of Makushin Volcano. The surface on which they rest has a relief of about 

3,000 feet, comparable with the relief of the present topography; and the major 

valleys on the unconformity lie close to the present drainage systems. Most of 

the rocks are porphyritic and trachytic, containing as essential minerals andesine 

or laboradorite, augite, hypersthene, and olivine. The bulk of the Makushin 

volcanics is probably of late Pleistocene age. Practically all rocks exposed were 

deposited subaerially.

580537 O 61 2 583
584 INVESTIGATIONS OF ALASKAN VOLCANOES

Bocks of scattered cinder cones, composite cones, and lava flows with young 

constructional forms make up the Eider Point basalt, most of which is late 

Wisconsin to Recent in age, and which includes the recent products of stillactive Makushin Volcano. These volcanic rocks rest unconformably on 

glaciated rocks of the Makushin volcanics; some are in turn slightly glaciated. 

Most of the Eider Point basalt is porphyritic and trachytic and many specimens 

contain up to 2 percent partly resorbed xenocrysts. Bytownite or labradorite, 

augitic pyroxene, olivine, and hypersthene are the essential minerals. The explosion craters at Point Kadin are associated with a very small amount of 

andesite vitrophyre. The summits of Makushin Volcano and Pakushin Cone 

contain breached craters.

Thirty-four new chemical analyses, and spectrographic analyses of trace elements of many of these rocks, are tabulated. The rocks belong to the calcalkaline suite common to the Aleutian island arc.

Much of the island is discontinuously veneered by a thin mantle of till, 

volcanic ash, humus, and soil. Moraines are restricted to the vicinity of the 

present glaciers. Beaches raised in Recent time are local and poorly preserved.

The Unalaska formation is faulted, tilted, and warped; the structure of the 

island is poorly understood because few stratigraphic horizons are mapped. 

A statistical analysis of linear topographic features from aerial photographs 

shows a dominantly northwest trend in the more altered rocks near and in the 

batholiths, and a strong rectilinear pattern of north- and east-trending sets of 

linear features in the less altered rocks away from the batholiths.

Sulfur, gold, zinc, and copper occur in small deposits on the island, but under 

the present market conditions the economic potentials of these prospects are 

unfavorable.

Inferences regarding glacial and structural geology are extended by interpretation of a submarine contour map covering 12,000 square miles of the 

surrounding area. The island is the subaerial part of the wave-beveled platform of the Aleutian Ridge, here about 40 miles wide, that extends to a depth 

of about 500 feet, with incised slopes to deep water beyond. The amount and 

rate of marine erosion suggest that the insular shelf is no older than middle 

Pleistocene. Most of the Makushin volcanics and all of the glaciation of the 

island postdate the marine beveling of the platform. *

INTRODUCTION

The geological reconnaissance of Unalaska Island, near the eastern 

end of the Aleutian Islands (fig. 81), was made during the summers 

of 1953 and 1954. The geology was mapped mostly on six topographic 

maps, scale 1: 62,500, prepared by the U.S. Army Engineers in 1940 

and 1941. Most of these maps have a contour interval of 100 feet 

and are generally accurate, but the southwestern end of the island is 

shown only by form lines, which locally are highly inaccurate. In 

the few unsurveyed portions of the maps, which were cloud covered 

on the compilation photographs, and in parts of the area of form-line 

maps, the geology was mapped on more recent aerial photographs. 

Submarine topography was compiled from smooth sheets (sounding 

charts) available from the U.S. Coast and Geodetic Survey.

Logistic problems involved transportation, inclement weather, great 

relief, and heavy upland snow cover. The motorship Eider, then of
S I B E R I A

170° 180' 170" 

FIGURE 81. Map of southwestern Alaska showing locations of Un
586 INVESTIGATIONS OF ALASKAN VOLCANOES

the Geological Survey, provided transportation to and around the 

island and served as supply base for the shore parties. In places 

short traverses were made in a powered skiff, and near the village 

of Unalaska trucks occasionally were used, but most of the island was 

mapped on foot by four two-man parties operating independently. 

The high terrain around Makushin volcano was reached from 

temporary advanced camps.

The geology was mapped in 1953 by G. D. Fraser, G. L. Snyder, 

H. F. Barnett, Jr., and E. H. Meitzner, assisted by V. E. Ames, R. P. 

Platt, H. B. Smith, and L. D. Taylor. The following year the geology was mapped by Fraser, Snyder, Barnett, and Harald Drewes, 

assisted by Platt, Smith, W. B. Bryan, and C. E. Chapin. In both 

years the parties were led by Fraser, Carl Velvelstad, captain, Charles 

Best, engineer, and the crew of the Eider provided constant and 

capable service.

The cooperation of the commanding officers and other military 

personnel at Kodiak Naval Base, and of the men of the Marine 

detachment at Dutch Harbor, is sincerely acknowledged. The 

friendly help of many of the inhabitants of Unalaska village, and 

particularly that of Walter Dyakanoff, mayor and manager of the 

Northern Commercial Co. store, is greatly appreciated.

The first Europeans to visit Unalaska Island were Russians under 

the command of Korovin on August 15, 1763, according to Coxe 

(1787, p. 103), though other accounts by him and by Grewingk 

(1850, p. 314) suggest that other fur hunters may have preceded 

Korovin shortly. In following years many others arrived, and Captains Bay soon became a Russian outpost. In 1778 James Cook 

stopped at Unalaska (Collins and others, 1945, p. 12). Accounts 

of the early history of the Aleutians and of the natives and their 

customs .are presented by Jochelson (1933), Hrdlicka (1945), Collins, 

Clark, and Walker (1945), and by Laughlin and Marsh (1951).

The geologic map and description by Grewingk (1850) probably is 

the earliest geologic work done on Unalaska Island. In 1907, T. A. 

Jagger, A. S. Eakle, and others of the Technology Expedition (Massachusetts Institute of Technology) spent several weeks on Unalaska 

Island to study the geology and botany. Only a diary was published 

by them (Jagger, 1908, p. 1037), but most of the notes, manuscripts, 

and geologic collections were available for the present report and 

several thin sections were cut from Jaggar's specimens. During 

the early part of this century, several short papers were written on 

the gold, zinc, and sulfur prospects; these are discussed in the section 

on mineral deposits. S. R. Capps (1934, p. 147-149) briefly reconnoitered the Unalaska Bay and Chernofski Harbor areas. In 1948 

F. M. Byers, H. H. Waldron, and others made a preliminary geologic
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 587

study of parts of the island for the U.S. Geological Survey, and their 

maps, notes, and collections were available in the preparation of 

this report. The geologic map of Alaska (Dutro and Payne, 1957) 

shows the early ideas of several of the authors of this report.

GEOGRAPHY

Unalaska Island, second largest in the arcuate chain of Aleutian 

Islands extending from the Alaska Peninsula to Kamchatka Peninsula, is located between latitudes 53°15' and 54° N., and between 

longitudes 166° and 168° W., 130 miles southwest of the Alaska Peninsula. East of Unalaska Island are the smaller islands of Unalga, 

Akutan, Akun, and the Krenitzin Group, and the largest Aleutian 

island of Unimak. Umnak Island, smaller than Unalaska Island, lies 

to the west across a strait 4 miles wide. Sedanka, Amaknak, and Hog 

Islands are separated from Unalaska Island by straits less than onefourth mile wide and, unless specifically omitted, these three adjacent 

islands and numerous islets will hereafter be included with Unalaska 

Island in all general description. Amaknak Island is the site of 

Dutch Harbor Naval Base.

Unalaska Island is elongate at about N. 60° E., the trend of this 

segment of the Aleutian arc. It is 85 miles long and 39 miles wide 

and has an area of about 1,200 square miles. The active Makushin 

Volcano (fig. 99) is north of the main elongation of the island and 

in line with volcanos of the adjacent islands. Numerous photographs 

of the volcano and other features illustrate an article by Freiday 

(1945 p. 444-455).

Many fiords embay the coast (fig. 82; pi. 75) : Beaver Inlet, 

which is more than 20 miles long, splits the eastern end of the island; 

Unalaska and Makushin Bays are each more than 10 miles long. 

Smaller bays and fiords are abundant everywhere except around the 

north shore of the northern bulge and on the southwest end of the 

island. Much of the coast is fringed with narrow, steep boulder 

beaches and rock benches, bare at low tide and interrupted in many 

places by cliffs. Beaches and tidal benches (fig. 87) 'are narrowest 

around protected bays; broad sand and gravel beaches commonly lie 

at the heads of shallow bays exposed to strong wave action. Further 

details on beach and bar physiography are discussed with the deposits 

that form them.

Makushin Volcano, the most conspicuous landform on the island, 

is 6,680 feet high. Its broad, domical summit is capped by a glacier 

with tongues that descend the larger flanking valleys to elevations as 

low as 1,000 feet. Several symmetrical conical hills and craters are 

on the flanks of Makushin Volcano.
588 INVESTIGATIONS OF ALASKAN VOLCANOES

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GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 589

South and east of Makushin Volcano the terrain is rugged. Sharp 

ridges and peaks commonly reach 1,800 feet in elevation on the 

easternmost part of the island and are progressively higher toward 

the center of the island, where they exceed 3,500 feet. Small glaciers 

remain in some of the highest valleys. West of Kashega the peaks 

are subdued, and around Chernofski Harbor only a few rolling hills 

are more than 1,200 feet high. Peaks in the rugged part of the island appear on distant view to have accordant summits (figs. 82, 99) 

but they range 1,500 feet in elevation, so that the accordance is more 

an illusion than a reality. There is a crude terrace at about 600 feet 

elevation west of Kashega.

The character of valleys varies with their size, the ruggedness of 

the surrounding land, and the presence or absence of glaciers in the 

drainage basins. In the more rugged areas, the larger valleys are 

narrow, with discontinuous broad and flat segments. Outwash-filled 

valleys are exceptionally broad and flat. Lakes are most numerous 

in flat terrain; lakes on the western end of the island and near English Bay on the eastern end are small and shallow, and lakes in the 

upland or valley basins of the rugged central area are deep. Lakes 

and closed lagoons are conspicuous in the lower reaches of many 

broad valleys; some are several miles long.

Wind, rain, fog, and overcast make the weather of the Aleutian 

Islands unpleasant and navigation hazardous. The mean annual 

precipitation is 58 inches at Dutch Harbor and 52 inches at Chernofski Harbor. Most of the precipitation falls as snow during the winter 

and as drizzle at other times, and on an average of 221 days per year 

exceeds 0.01 inch. Fog is most common during the summer, and 

gales and williwaws (local, very strong, gusty winds) are most frequent during the fall and early winter. Mean summer temperatures 

are about 52° F; mean winter temperatures are about 30° F. At low 

elevations the snow accumulation is small because of the relatively 

warm maritime climate, but above 2,000 feet the accumulation is 

large and much snow persists into late summer. Further details of 

weather conditions are available from the "Climatic Atlas for 

Alaska" (U.S. Army Air Forces, 1943), which contains a record 

covering 16 to 26 years at Dutch Harbor and 4 to 8 years at Chernofski Harbor.

Trees are virtually lacking on the island. Several groves and a 

few isolated trees of Sitka spruce were.planted by the Kussians in 

1840 (Jaggar, 1908, p. 9) around Unalaska Bay, and of these only 

the grove on Hog Island appears to be spreading. Willow thickets 

grow at low elevations in the more protected valleys; blueberry shrubs 

and salmonberry and crowberry plants are common. In the flat lowland valleys tall grass and herbaceous plants are abundant, and the
590 INVESTIGATIONS OF ALASKAN VOLCANOES

uplands are largely covered with heath and tundra vegetation. During the summer the meadows abound with a large variety of flowers.

The fauna of the island is small, consisting of indigenous foxes, 

lemming, and mice, and introduced ground squirrels, sheep, and rabbits (on Hog Island only). The sea supports abundant bird life: 

ducks, geese, cormorants, puffins, murres, gulls, and a host of other 

birds. Seals and sea lions are found along the shore, and fish and 

invertebrates are plentiful.

The village of Unalaska, the nearby naval base at Dutch Harbor, 

and a sheep ranch at Chernofski Harbor were the only permanently 

inhabited places on the island in 1953 and 1954. The abandoned villages of Biorka, Makushin, and Kashega, seasonal fishing camps, and 

the extensive abandoned military bases of Fort Hears and part of 

Fort Glenn are located on many bays. A few wharfs and many miles 

of roads were built at Chernofski Harbor and Unalaska Bay. The 

airport at Dutch Harbor is still in use.

IGNEOUS AND SEDIMENTARY ROCKS

UNALASKA FORMATION

The oldest rocks on Unalaska Island, here named the Unalaska formation, are exposed over two-thirds of the island. It is a thick sequence of coarse and fine sedimentary and pyroclastic rocks intercalated with dacitic, andesitic, and basaltic flows and sills, cut by 

numerous dikes and small plutons and unconformably covered by 

basalt and andesite of the Makushin volcanics. The rocks of the 

Unalaska formation are typically exposed along the upper reaches of 

the Nateekin River and in the sea cliffs at Hive Bay.

High ridges of the Unalaska formation are irregularly jagged in 

appearance and are without the massive bosses of the plutonic rocks or 

the well-developed layered habit of the younger volcanic rocks. Steep 

slopes at lower elevations are smoother and are covered with relatively 

fine rubble and surficial deposits through which scattered outcrops 

project. Some of these steep slopes show crude, gently dipping, massive outcrop bands encircling hills. Grassier slopes usually at even 

lower elevations have fewer exposed rocks, which are irregularly 

distributed except where they occur as parts of linear outcrops commonly associated with terrace or bench topography. The rocks of the 

Unalaska formation are somber brownish gray and greenish gray, 

with scattered yellowish-gray to reddish-brown patches as large as a 

few thousand feet across. The somber appearance of the rocks probably is intensified by constant wetness. Sea cliffs along the south, 

shore are banded in contrasting very light and dark grays. On southwestern Unalaska rounded igneous masses break the layered continuity and locally form the entire sea cliff.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 591

Extensive surficial cover, the massive nature of the important 

igneous and pyroclastic members, and the lack of key beds prohibit 

subdivision of the unit; these factors also make difficult a close estimate of the total thickness. Topographic relief indicates a minimum 

thickness of 3,000 feet; a maximum thickness of 55,000 feet is calculated from the regional dip of the rocks of 15° to the north-northwest 

on the eastern part of the island, but duplication by faults and flexures 

is likely. The formation is probably a few tens of thousands of feet 

thick.

SEDIMENTARY ROCKS

The clastic components of the Unalaska formation are mostly argillites south of Reaver Inlet, graywacke and conglomerate in the 

northern bulge area and south of Makushin Bay, and coarse pyroclasr 

tic deposits in northeastern Unalaska Island, separated by wide transition zones. Graywackes, conglomerates, and pyroclastic rocks also 

form minor components of the rocks in other areas. Along the south 

coast of Unalaska Island the dominant sedimentary rock is layered 

argillite, well exposed in sea cliffs and headlands, but mostly covered 

in the interior of the island. Argillite is found as far north as 1 mile 

north of Portage Bay, where it is intercalated with coarse graywacke. 

Coarse graywacke is conspicuous south of Portage Bay for at least 1 

mile and occurs as a minor constituent in the argillite sequence on the 

south coast of the island. There is thus a lateral gradation by intertonguing between argillite and graywacke across the neck between 

northern and southern Unalaska Island, but the northerly dips of 

that region indicate that exposed coarse beds may be partly younger 

than the fine beds.

The argillite layers are generally darker in the grossly banded sea 

cliffs than the albitized sills with which they occur; argillite and sills 

make up about equal portions of the stratigraphic section. The rock 

colors are light or dark gray, with low-chroma greenish and brownish 

hues most common. Thinner argillite units appear on figures 84 and 

86, and some argillite is characterized by varvelike lamination. Some 

rocks split along laminae into thin, brittle chips, but more commonly 

the}' break across bedding with a conchoidal fracture.

Southwest of Cape Prominence much of the argillite was intruded 

by penecontemporaneous andesitic sills, domical masses, detached 

igneous pods, and pillow lavas. The unusual "intrusive flows," not 

recognized in the coarser clastic rocks farther north, are discussed 

below. Many andesitic dikes, sills, and irregular intrusive masses of 

more conventional habit occur within the Unalaska formation, but 

true flows are rare or absent in the argillite section. Chalcedonic and 

cherty masses, as well as crystalline quartz masses, are common in 

some areas of pillow lavas.

580537 0 61 3
592 INVESTIGATIONS OF ALASKAN VOLCANOES

Typical section of part of the Unalaska formation, from the east shore of the

bay west of Hive Bay; thicknesses are estimated r

Thickness

(feet) V 

Top of section. 

Upper porphyry:

Andesite sill, light-gray; weathers grayish yellow; feldspar porphyry, altered; cut by many dark-gray andesite dikes; lower contact irregular_________________________________ 300 X

Upper argillite:

Argillite, light- to dark-gray, laminated; weathers grayish brown; < 

gross black-and-white layered appearance; contains minor crystal 

tuff, graywacke, and p6p6rite(?) layers; cleavage locally developed, some beds crenulated; cut by andesite dikes _ 200

Argillite, greenish-gray, laminated; contains altered, undeformed

layers, very siliceous layers, dark chert layers, and sheeted l^

peperite(?) layers 6 in. to 3 ft thick__________________ 100

Argillite, brown___ ____ _ _ 10 wMiddle porphyry:

Andesite sill, grayish-red; altered feldspar porphyry; highly irregxilar top of sill penetrates overlying argillite. Base of sill exposed

at head of bay______________________________ 95

Lower argillite:

Argillite, brown and light-gray, laminated________________ 15

Argillite, contains zone of crenulated beds________________ 30 v

Argillite and tuff, brown, laminated__________________ 180

Breccia, green, laminated tuffaceous__________________ 5

Argillite, laminated, brown____..____________________ 7

Siltstone, pale-yellowish brown; beds 1 to 2 in. thick_________ 12

Argillite, brown, layered, slaty________________________ 15

Tuff or graywacke, brown- to grayish-green______________ 3

Argillite, gray to brown, laminated; contains coarse nonlaminated >_

layers 2 or 3 in. thick___ __ ______ 2

Tuff or graywacke, dark-brown to green, well indurated______ 5 'r

Argillite, brown, laminated________________________ 7

Siltstone, pale-yellowish-brown; beds 2 in. to 1 ft thick_______ 12 /

Argillite, brown, laminated, slaty______________________ 15

Tuff or graywacke, brown to grayish-green______________ 3

Argillite, grayish-brown, laminated ; contains nonlaminated layers v

2 to 3 in. thick_______________________________ 2

Tuff or graywacke, dark-brown to green, well indurated_______ 5 *

Argillite, brown to black, laminated___________________ 6

Siltstone, yellow________________________________ 3

Siltstone, gray_________________________________ 3

Argillite, gray, weathers dark brown, laminated, slaty________ 12

Lower porphyry: ( 

Andesite sill, grayish-red; altered feldspar porphyry. Best exposed

at the mouth of the bay__________________________ 200 ,

Approximate total____________________________ 1, 200
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 593

Coarse sedimentary and pyroclastic rocks of the Unalaska formation include much graywacke and tuffaceous sandstone, and mudstone. 

Conglomerate, agglomerate, and coarse breccias are common on northcentral Unalaska Island, and tuff breccia on the northeastern end. 

Bedding in the coarser sediments is characteristically crude and lithologic units are massive. One prominent zone of thick-bedded coarse 

conglomerate along the upper part of the Nateekin Eiver is shown 

schematically on figure 100. A horizontal unit of coarse bluish-green 

tuff more than 300 feet thick caps the highest hills between Lion Bight 

and Alimuda Bay south and southwest of Kashega.

Most of the fragments in the coarser elastics consist of assorted 

angular to subrounded andesite or basalt common in the Unalaska 

formation (fig. 83). Some graywacke and tuffaceous sandstone contains boulders in an unsorted matrix. A few tuffaceous rocks contain 

pumice shards and obsidian fragments with relict perlitic structure, 

and other tuffs contain scattered angular fragments of pyroxene and 

plagioclase. In the vicinity of Unalaska village the sandstone is relatively clean, moderately well sorted, and contains the few fossils found 

on the island.

FLOWS, SILLS, PILLOWS, AND BULBOUS INTRUSIVE BODIES

In parts of northern and northeastern Unalaska Island, lava flows 

occur with coarse pyroclastic, sedimentary, and intrusive rocks. Lofty 

Mountain and adjacent hills between Unalaska village and English 

Bay, for example, are uncpnformably capped by about 800 feet of 

andesite or basalt flows which weather to smoother topographic form 

than do the clastic rocks below them. The uncomformity beneath 

these flows is the most conspicuous of several within the Unalaska 

formation so much more conspicuous that an alternate interpretation of placing the flows with younger volcanic rocks is possible. The 

flows are generally porphyritic, nonvesicular, irregularly fractured, 

and in places have a reddish oxidation color.

In southern Unalaska Island most of the igneous rocks within the 

Unalaska formation are sills or peculiar modifications of sills, whose 

structures are suggestive of both intrusive and extrusive rocks. Adjacent to some of these sills are concordant layers of rock a few feet 

to a few tens of feet thick that look like decomposed concrete and 

contain coarse, oriented greenish-gray chips of laminated argillite in 

a light-gray matrix of highly altered and brecciated igneous material. 

Bleached plagioclase crystals are visible in many specimens.

Unequivocal sills are especially conspicuous in the argillites along 

the south coast from Sedanka Island to Usof Bay, where they range 

from a few feet to more than 300 feet in thickness and make up about 

half of the section. Apophyses intrude overlying argillites, and in 

one area a large necklike mass projects from a sill through a thick
594 INVESTIGATIONS OF ALASKAN VOLCANOES

FIGURE 83. Homogeneous tuff breccia from vertical sea cliff on north shore of Boulder1 

Bay. Rocks of this type, characteristic of much of the Unalaska formation, have been 

formed variously as accumulations of nearshore submarine pyroclastic ejecta, mudflow 

deposits, autoclastic flow breccia, and intrusive breccia.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 595

overlying sequence of argillite. A few of the sills are partly discordant and cut the argillite layers at a slight angle. Most sills are massive and some have columnar structure and horizontal layering. Colors 

range from very light gray to grayish purple.

In addition to the massive sills, and occurring with them in a section several thousand feet thick stretching along 80 miles of coastline on the southwestern portion of southern TJnalaska, is an irregularly stratiform mixture of argillite and igneous rock (figs. 84-87) 

that includes modified sills or "intrusive flows" with many unusual 

structural features. These features range from small glassy nodules 

to pillows and small laccoliths. In general, altered mud forms a 

matrix for the pillows; this mixture of mud and pillows surrounds, 

in turn, larger pods, domes, or laccoliths several hundred feet high. 

The smallest structures, examined in detail 1 mile northeast of Cape 

Aiak, for example, are glassy nodules or crudely rounded pods, about 

fist size, set in a matrix of much altered greenish sedimentary material. Here the nodules constitute less than one-fourth of the whole 

unit, which is massive and at least several tens of feet thick. Along 

the contact of this unit with a bulbous aphanitic intrusive mass that 

truncates it is a zone of breccia mixed with tongues and pods of the 

aphanite.

Irregular large pillows up to 40 feet long, but commonly 5 to 10 

feet long, are abundant (fig. 84). Some are elongate and are variously oriented within the same layer. Tabular chunks of banded 

argillite, and tabular or irregular masses of green cherty material 

(believed to be altered siliceous mud) occur between the pillows 

throughout units as thick as several hundred feet. Light-gray chalcedonic and cherty deposits with typical crustification banding line 

cavities and fractures. Most of the pillows are unzoned but some 

have firm reddish rims and friable greenish-gray cores, and others 

have glassy borders and altered cores. Glass or mudstone occurs 

inside some deformed pillows. Sheets of pillows grade into bulbous 

sills by way of larger, irregular pillows. Pillows also appear as buds 

forming at the apex of a small, deltoid-shaped laccolith. In rare 

cases septa of overlying argillite project into pillow layers, and at 

Raven Bay delicate branching andesite tongues penetrate into the 

overlying argillite.

Pillows only a few feet in maximum diameter, featured by chilled 

outer margins, vesicles, radial and concentric fractures, slightly 

dimpled or depressed undersides where in contact with underlying 

pillows, and including a sedimentary filling between pillows, occur 

in a few places. As shown in figure 85, they are much smaller than 

the large irregular pillows discussed above and obviously are less 

deformed.
596 INVESTIGATIONS OF ALASKAN VOLCANOES

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598 INVESTIGATIONS OF ALASKAN VOLCANOES

Laccolithic pods and domes, a few demonstrably attached to sills, 

are conspicuous large members of the family of structures formed on 

southwestern Unalaska Island. Segmentation of sills into major and 

minor structures is illustrated in figure 86. One layer shows a large 

sill separated into a banded dome and two pods, all surrounded by 

imperfectly formed pillows.

A. 600-foot horizontal sheet of concentrically banded, deltoid-shaped, 

domes, which are completely surrounded in cross section by large 

pillows, is present in the sea cliffs at Cape Prominence. Elsewhere on 

the south coast, bulges like the deltoid-shaped domes occur on the 

terminal or medial parts of sills. Concentric bands about 1 foot thick 

are present in most laccolithic masses, but banding is not always 

visibly concentric (figs. 84, 86, 87). The banding may appear lobate 

in plan or horizontal in vertical section, and it is not limited to detached or even to deformed portions of the sills. Columnar fan 

joints either replace or obscure the concentric banding in some structures, particularly the domed sills. The banding consists of alterFIGDRE 86. Disruption of sills, Tower Point, southwestern Unalaska Island. Cliff is 

1,200 feet high. The albitized (andesite?) dome and pods with strong concentric layering lie beneath partly fragmented layered argillites and an unbroken sill. Coarsebedded . 

tuff caps cliff.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 599

nating reddish and greenish layers, of which the reddish layers are 

more resistant to erosion and project above the other layers on tidal 

flats (fig. 87).

DIKES

Dikes are common throughout the Unalaska formation as probable 

feeders of flows and intrusive pillow layers. In the northeastern part 

of the island, where they are unusually abundant, they form two 

sets. Closely spaced dikes, or multiple dikes, strike N. 70° W. and 

dip steeply southwest on the rounded ridge between Summer and 

Agamgik Bays and on the northern end of the ridge at the head of 

Kalekta Bay. The second set of dikes, striking N. 45° W. and dipping 

steeply southwest, are most closely spaced southwest of Lofty Mountain. In these areas dikes commonly form 75 percent of the rock, 

and locally, as at the end of the ridge extending into the head of 

Kalekta Bay, they form all of the rock. Isolated dikes of both sets 

are scattered throughout adjacent areas, and in the few places where 

dikes of the two sets occur together, those trending N. 45° W. cut the 

others and are intrusive into strongly sheared, discontinuous sheets 

and septa of tuff breccia of the Unalaska formation. Similar dike 

complexes in the Hawaiian Islands mark the location of volcanic 

rift zones (Wentworth and Macdonald, 1953, p. 89-91, fig. 45).

The rocks of both dike sets are greenish-gray to dark-bluish-gray 

andesite porphyry. Some dikes are bordered by flow breccias; the

FIGURE 87. Horizontal section of a layered dome on a tidal bench along the west side of 

Alimuda Bay, western Unalaska Island.
600 INVESTIGATIONS OF ALASKAN VOLCANOES

northeast boundary of the Kalekta Bay dike swarm is such a zone, 

300 feet wide. Many dikes contain intensely fractured zones parallel 

with the walls, and rounded fragments of the parent andesite are 

imbedded in a dense matrix of the same composition.

PETROGRAPHY OF THE TTNAX.ASKA FORMATION

The igneous rocks in the Unalaska formation are albitized andesite 

(spilite) and more rarely, albitized basalt and dacite. The dominant 

minerals are plagioclase and clinopyroxene, which generally occur as 

phenocrysts as well as in the groundmass. Some rocks also contain 

hypersthene and others common hornblende. Accessory minerals are 

magnetite, apatite, and rare sphene, zircon, and primary quartz. 

Pyrite euhedra of possible primary origin occur in some vitrophyres.

Felty texture is ubiquitous and the long feldspar laths in many 

rocks are alined, forming a flow texture. Phenocrysts rarely exceed 

about 15 percent of the volume of the rock and average only about 

5 to 10 percent, but in a few dikes the phenocrysts make up as much 

as 50 to 75 percent. Glass is a minor constituent in a few rocks, and 

rarely it is dominant. In many thin sections, especially of sedimentary rocks, original textures and compositions are barely discernible because the rocks are so altered. Completely recrystallized igneous 

and sedimentary rocks of the Unalaska formation are described on 

pages 622-624.

The term "basalt" is used in this report for an igneous rock whose 

microphenocryst and groundmass plagioclase composition is greater 

than 50 percent anorthite. In the field the color of the rock was an 

approximate guide; if the large feldspar crystals were chalky, albitizatkm was suspected and the distinction between andesite and basalt 

was not made. The composition of the groundmass and microphenocryst plagioclase was determined from extinction angles in the [100] 

plane, and the high temperature curve given by Troger (1952, p. 113). 

The average plagioclase in the groundmass from the Unalaska formation and from other volcanic rocks on the island contains about 5 to 

10 percent less anorthite than the plagioclase phenocrysts (see p. 624); 

hence, a rock that has a very fine groundmass or is much altered is 

considered a basalt if the plagioclase phenocrysts are at least as calcic 

as An60, especially if they are accompanied by abundant pyroxene. 

Names of glassy rocks are obtained by comparing their chemical analyses with analyses of similar but coarser rocks reported by Washington 

(1917). The original composition of some albitized rocks is estimated 

from unaltered cores of crystals. Compositions of most albitized 

rocks, however, are unknown or are estimated by comparison with the 

composition of nearby rocks. Petrographic descriptions are based on 

a study of about 220 thin sections, mostly of the igneous rocks.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 601

Plagioclase crystals are generally lath shaped. Stubby crystals are 

more common as pheriocrysts than in the groundmass, are more common in basalt than in andesite, and are more common in dikes than in 

other igneous bodies. Normal, oscillatory, and possible reverse zoning 

are common, and in many crystals the zones or cores contain dusty 

inclusions. The crystals with dusty inclusions and possible reverse 

zoning do not show clear twinning, and their composition cannot be 

determined in. this section. The effect of their omission from the composition averages is unknown. Plagioclase composition ranges from 

al bite to bytownite, but generally is andesine or labradorite. The 

plagioclase of dikes is labradorite to anorthite. The clinopyroxene is 

either augite or diopside; those grains with pleochroism or color are 

recorded as augite (?) and those without are listed as augite or diopside. Hypersthene and common hornblende and magnetite appear as 

phenocrysts in some dikes. In one dike, quartz, biotite, and plagioclase phenocrysts occur together. Olivine occurs in the volcanic pipe 

on Eider Point.

SPECIFIC DATA FOR ANALYZED ROCKS

Chemical and spectrographic analyses of nine igneous rocks from 

the Unalaska formation are listed in table 1. General locations from 

which the rocks were collected are given on the following pages and 

detailed locations are indicated on the map.

Specimen 1 is feldspathic basalt porphyry from a dike in the Unalaska formation near Summer Bay in Unalaska Bay. It is dark bluish gray and contains 

phenocrysts as large as 1 millimeter. Phenocrysts compose 10 percent of the 

rock: 9 percent calcic labradorite, which fonnis embayed crystals, euhedral 

laths, and grains with inconspicuous normal zoning; 1 percent augite(?), which 

forms altered euhedra and is replaced by penninite(?) ; and a trace of pyrite. 

The groundmass has a felty texture and consists of abundant euhedral and subhedral laths of calcic andesine and scattered granules of a black opaque mineral. 

Alteration is moderate, producing disseminated calcite and penninite and traces 

of uralite and chlorite.

Specimen 9 is feldspathic basalt porphyry from the Unalaska formation 1^ 

miles northwest of Humpback Bay in Makushin Bay. It is greenish gray to 

grayish green and contains phenocrysts 1 to 2 millimeters long. Phenocrysts 

compose 15 percent of the rock: 12 percent sodic labradorite, which forms embayed crystals, euhedral laths, and irregular grains and which contains normal 

and oscillatory zones and zones of dusty inclusions; 3 percent augite or diopside, 

which form euhedral to anhedral crystals and crystal clusters and which alter 

to chlorite and uralite (?) pseudomorphs; and a trace of olivine(?), which appears as chlorite and uralite (?) pseudomorphs. The groundmass has a felty to 

granular texture and consists of plagioclase, altered pyroxene(?), and a black 

opaque mineral. Alteration is moderate; chlorite and uralite (?) replace pyroxene arid olivine(?).

Specimen 14 is andesite porphyry from the Unalaska formation along the 

shore halfway between Humpback Bay and the mouth of Glacier River in Makushin Bay. It is medium gray to medium bluish gray and contains phenocrysts

580537 O 61 4
602 INVESTIGATIONS OF ALASKAN VOLCANOES

2 to 4 millimeters long. Phenocrysts compose 10 percent of the rock; 8 percent 

sodic andesine, which forms embayed crystals and subhedral to anhedral laths 

and which contains normal and oscillatory zones and zones of dusty inclusions; 

2 percent augite( ?), which forms euhedral to subhedral crystals, crystal clusters, 

and pseudomorphs of penninite (?) ; a trace of ilmenitic magnetite, which forms 

subhedral to anhedral crystals and embayed crystals. The groundmass has a 

felty to flow texture and consists of abundant subhedral laths of sodic andesine, 

much pyroxene, and traces of a granular black opaque mineral. Alteration is 

slight; there are traces of disseminated penninite( ?) and uralite( ?) and veinlets 

of epido'te and quartz.

Specimen 15 is andesite porphyry from a dike in the Unalaska formation 

along the south shore of Kalekta Bay. It is light bluish gray to medium gray 

and has phenocrysts 1 to 3 millimeters long. Phenocrysts compose 20 percent 

of the rock: 19 percent sodic labradorite to sodic andesine, which forms embayed crystals and subhedral laths and which is zoned normally; a trace of 

clinopyroxene, which alters to uralite(?) and chlorite; and a trace of pyrite, 

which forms subhedra associated with clinopyroxene. The groundmass, which 

has a felty and intersertal texture, consists of abundant subhedral laths of sodic 

andesine, and also a trace of subhedral clinopyroxene and a granular black 

opaque mineral. Alteration is moderate: chlorite and uralite(?) are common, 

and calcite forms disseminated crystals and veinlets.

Specimen 19 is pyrite-bearing dacite vitrophyre from the Unalaska formation 

1 mile east of Chernofski Harbor. It is black and contains plagioclase phenocrysts 1 to 4 millimeters long. Phenocrysts compose 5 percent of the rock: 2 

percent calcic labradorite, which form embayed crystals, euhedral laths and 

stubby crystals, and fragments, and which have oscillatory zones and apatite 

inclusions; 1 percent euhedral augite(?) ; 1 percent euhedral hypersthene; and 

a trace of euhedral to subhedral ilmenitic magnetite. Microlites of plagioclase 

are common and of apatite rare. The groundmass is a perlitic glass, which contains a few small chlorite ( ?) spherulites.

Specimen 20 is dacite vitrophyre from the Unalaska formation at Buttress 

Point, 10 miles southwest of Kuliliak Bay. It is black and has plagioclase 

phenocrysts as large as 2.5 millimeters, and amygdules with a diameter of 1 to 4 

millimeters. Phenocrysts compose 5 percent of the rock: 4 percent sodic andesine, which forms embayed crystals and euhedral to subhedral laths and 

stubby crystals, and which has normal zones and apatite inclusions; a trace of 

subhedral hypersthene; a trace of subhedral augite(?) ; and a trace of subhedral to euhedral ilmenitic magnetite. Microlites of plagioclase are common, 

and of apatite rare. The groundmass is glass, which is slightly devitrified and 

altered to chlorite (?) and contains I percent zeolite (?)-filled amygdules.

Specimen 22 is albitized andesite (?) porphyry from the Unalaska formation, 

1% miles south of Hive Bay and about halfway between Staraya Bay and 

Udagak Strait. It is medium-light gray to medium gray and has chalky plagiociase phenocrysts 1 to 5 millimeters long. Phenocrysts compose 5 percent of 

the rock: 4 percent albite, which forms euhedral to subhedral laths and stubby 

crystals having apatite inclusions; and 1 percent euhedral to anhedral magnetite. The groundmass is mottled and indistinct. Alteration is intense: alhite and kaolin are pervasive; calcite is common and forms irregular patches 

near albite clusters; uralite(?) is present; and quartz is rare and forms scattered grains on crudely alined pods.

Specimen 23 is albitized dacite (?) porphyry from the Unalaska formation 

at Hive Bay halfway between Staraya Bay and Udagak Strait. It is light gray
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 603

and contains chalky plagioclase and ferromagnesian phenocrysts 1 to 2 millimeters long. Phenocrysts compose 1 percent of the rock, consisting of subhedral 

laths of albite with apatite inclusions. The groundmass has an indistinct intersertal texture and consists of abundant subhedral laths of albite, a trace of 

a granular black opaque mineral, and a trace of quartz that may be primary. 

Alteration is intense, albite and kaolin are pervasive and abundant, quartz is 

common and intersertal; calcite is common and forms scattered patches, especially near altered ferromagnesian minerals; and chlorite and uralite (?) 

are common alteration products of the ferromagnesian minerals.

Specimen 24 is albitized dacite porphyry from one-half mile west of Buttress 

Point and lO1/^ miles southwest of Kuliliak Bay. It is brownish gray and has 

chalky plagioclase and ferromagnesian phenocrysts 1 to 2 millimeters long. 

Phenocrysts compose 1 percent of the rock, consisting of euhedral to subhedral 

laths and stubby crystals of albite with apatite inclusions and a trace of subhedral magnetite. The groundmass has an in distinct intersertal texture, consisting of abundant subhedral laths of albite and much interstitial altered glass, 

as well as traces of anhedral clinopyroxene, a granular black opaque mineral, 

and interstitial quartz that may be primary and that has apatite inclusions. 

Alteration is intense; albite and kaolin are pervasive and abundant; quartz is 

common and interstitial; and there are traces of chlorite and uralite ( ?) altering 

clinopyroxene.

The major oxides of the analyzed rocks are plotted against silica in the variation diagram of figure 88. In general, these rocks are among the more silicic 

and alkalic of the suite from Unalaska Island. Specimens 22, 23, and 24 are 

completely albitized and cannot be compared directly with the unaltered rocks, 

though they are from sills and deltoid-shaped domes of the Unalaska formation. 

Specimen 1, which contains the least silica of the entire suite, is from a dike in 

the Unalaska formation; it is highly aluminous (note the normative corundum, 

table 1) and comparatively low in ferromagnesian minerals. It may represent 

an igneous episode unrelated to that which produced the rocks of the Unalaska 

formation. In the volcanic rocks of Unalaska Island the average content of 

boron, barium, scandium, and strontium is higher, and the average content 

of chromium and copper is lower than in similar rocks reported by Goldschmidt 

(1954, respectively, p. 281-282, 252, 305, 245, 548, and 179).

ALTERATION

Rocks of the Unalaska formation have been altered during possibly 

two episodes. The earlier episode, perhaps penecontemporaneous with 

deposition, has resulted in the development of abundant spilitic rocks. 

Superposed upon this sodium enrichment, or occurring with it, is a 

hydrothermal alteration most probably associated with the introduction of granodiorite plutons.

Spilites are common in all parts of the Unalaska formation except 

between Kalekta Bay and Beaver Inlet in the northeastern part of the 

island. The "fresher" appearance, poorer induration, and the absence 

of pale colors associated with albitization of rocks in this northeastern 

area suggests that these rocks may be younger than those of the rest 

of the formation, but the altered and unaltered types apparently are 

.laterally gradational and probably are different alteration facies.
604 INVESTIGATIONS OF ALASKAN VOLCANOES

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GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 605

About one-half of the thin sections of the Unalaska formation show 

albite with kaolinitic alteration. These rocks are lighter colored than 

their nonspilitic counterparts, and the plagioclase phenocrysts have 

a chalky appearance. Several thin sections contain both albitized and 

relict calcic plagioclase. Some of the phenocrysts contain clear 

patches of andesine or labradorite in an irregular network of kaolinitic 

and more sodic plagioclase, showing incomplete albitization.

Hydrothermal alteration, which is responsible for the widespread 

distribution of chlorite, uralite, and epidote (fig. 89) in the rocks of 

the Unalaska formation, is far more conspicuous. Some of the 

granodioritic rocks show similar alteration, but the Makushin volcanics and the younger volcanic rocks are notably free of these secondary minerals.

Chlorite is usually pseudomorphous after amphiboles or pyroxenes 

and commonly includes several different composition,! varieties. As 

many as three varieties of chlorite occur in some thin sections. Altered 

glass generally contains abundant chlorite, some of which occurs as 

veins and cavity fillings or is interstitial. Most of the chlorites show 

low birefringence, but some have anomalous blue, violet, or bluishgreen interference colors. Uralite is also a common alteration product 

of f erromagnesian minerals and is altered in turn to chlorite. Epidote 

is more restricted and was found onljy within 5 miles of the granodiorite plutons, except for a single occurrence at the southwest end 

of the island (fig. 89). Generally epidote is granular; it is scattered 

widely, but is subordinate to chlorite except near pluton contacts.

Other alteration products include sericite, kaolinite(?), pyrite, 

quartz, calcite, and less common secondary minerals such as leucoxene, 

hematite, limonite, and possibly some sphene. Kaolinite (?) is a more 

common alteration product of feldspar than is sericite. Some of the 

pyrite disseminated in igneous rocks and that which occurs as phenocrysts in glassy pillow lavas is possibly primary, but the pyrite that 

forms pods or veins is clearly secondary. About half of the thin 

sections examined show secondary quartz as veinlets, vug fillings, or as 

a replacement. Material between pillows is commonly rich in secondary crystalline and cryptocrjystalline quartz. The reported frequency 

of primary quartz may be underestimated because of the difficulty in 

recognizing it when secondary quartz is abundant. The amount of 

secondary silica in the extremely fine, chertlike argillites may have 

been underestimated because the determination of minerals in thin 

sections is doubtful. Calcite is less abundant than quartz and occurs 

as veinlets and pockets, and rarely as pseudomorphic replacements. 

Hematite and limonite are generally found where the pyrite content 

is high, and many of the bright oxidation colors along sea cliffs mark
606 INVESTIGATIONS OF ALASKAN VOLCANOES

pyrite-rich rock, which is commonly confined to zones suggestive of 

veins or faults.

Locally along the contacts of the large intrusives with the Unalaska 

formation, there is a zone of biotite-hornblende-pyroxene hornfels, 

some of which is sheared. Patches of these minerals, and in places 

also epidote, cut hybrid rocks containing relict volcanic texture. 

Within and near the roof of the largest pluton a few veins of tourmaline were noted.

Some of the common hjydrothermal alteration minerals are distributed in concentric belts around the batholiths, as shown in figure 89. 

Extreme alteration of the host rocks may be represented by mobilized 

border rocks discussed below. Near the contact with the batholith, 

scattered tourmaline crystals and the biotite-hornblende-pyroxene 

hornfels mark a high- to medium-temperature zone which commonly 

contains more than 20 percent secondary minerals. Farther outward, 

but within 5 miles of the nearest batholith, is the medium- to lowtemperature epidote zone in which the rocks contain about 10 percent 

of these alteration minerals. Beyond this distance the rocks rarely 

contain more than 5 percent of combined chlorite and uralite.

FAUNA AND AGE

Fossils were found in road and building cuts immediately east of 

Unalaska Lake, 1 mile from the dock area in the village. Pelecypods 

were identified by F. S. MacNeil as My a cf. M. truncata, which has a 

wide range in the Tertiary. Bones and teeth were identified by G. E. 

Lewis as parts of a desmostylid sirenian a relative of the sea cows, 

 which probably differed from them in having legs instead of flippers. 

The most significant specimen is a low-crowned cheek-tooth fragment 

with typical desmostylid cusps. The fragments probably are referable to a single animal of the genus Comioallius, which is generally 

indicative of an early Miocene age, although identification as Desmostylus is possible. * * * "Cornwallius lived in shallow coastal littoral, 

deltaic, lagoonal or estuarine waters, and fed on shellfish," according 

to G. E. Lewis. F. S. MacNeil states that the present-day representatives of the clam "Mya cf. M. truncata live in intertidal depths for a 

few fathoms below low tide." Perhaps Cornwallius found Mya a succulent morsel. Mya indicates a cold-water environment, probably 

very much as the Unalaska Island waters are today.

Comwallius is known in the Sooke formation of the Puget Sound 

area and in the San Gregorio formation of Baja California. These 

formations, or others correlated with them, are considered to be lower 

Miocene by some (Lewis, written communication, Oct. 14, 1954) but 

upper Oligocene according to others (Durham, 1950, p. 26, 34-36; 

Clark and Arnold, 1923, p. 125). Desmostylus is generally considered
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 607

to be indicative of Miocene age, but in Japan it may occur in the upper 

Oligocene as well (Onodera, S., 1957, p. 238-253; Minato, M., Matsui, 

M. Ishhii, J., 1957, p. 308-316).

F. M. Byers reports (oral communication) pelecypods which have 

not been identified from a locality about ll/2 miles south of Pyramid 

Peak near Unalaska village.

In the vicinity of the village the Unalaska formation is of probable 

early Miocene age; elsewhere on the island the nonfossiliferous rocks 

of the formation are possibly middle or early Tertiary; the older age 

is included because the fossiliferous rocks are possibly high in the 

Unalaska formation.

On the geologic map of Alaska, Dutro and Payne (1957) show the 

rocks herein called Unalaska formation as part of a generalized unit 

which is of questionable Mesozoic age in this area. This age was based 

on information available before the discovery of the desmostylid teeth. 

The Mesozoic and Tertiary volcanic rocks shown north of Beaver Inlet on this map are now known to be merely unaltered representatives 

of the Unalaska formation (see fig. 89).

ENVIRONMENT AND ORIGIN

The finely laminated argillaceous and siliceous rocks probably 

accumulated in deep basins, for the great uniformity and continuity 

of the laminae suggest quiet conditions of sediment accumulation. A 

similar environment is proposed by Bramlette (1946, p. 34-35) for 

the layered siliceous rocks of the Monterey formation of California, 

which resemble in many respects the finely layered rocks of the Unalaska formation. Klienpell (1938, p. 17) further suggests that 

portions of the Monterey formation were deposited at depths as great 

as 3,000 feet.

The abundance of coarse clastic rocks on northern and northeastern 

Unalaska Island, and several fossils which are shallow-water forms, 

probably indicate a near-source environment. The persistence and 

thickness of the conglomerate zone along the upper reaches of the 

Shaisnikof River suggest a beach deposit or a combination beach 

and submarine talus deposit.

Physical conditions during formation of the pillow structures and 

deltoid-shaped domes are uncertain, but mixing of sills with wet 

muds probably played a major role. Normal-sized pillows and larger 

pods or pillows of magma were kneaded and folded as the whole mass 

moved; during such movement mud was folded into them and argillite was dragged into the pillow layers. The breccia with fist-size 

pods at Cape Aiak may be a submarine flow breccia, perhaps a special form of the more common peperite (Macdonald, 1939, p. 329) 

that retained its mobility until after the bulbous mass was injected.

586532" O 61 5
608 INVESTIGATIONS OF ALASKAN VOLCANOES

Magmas moving on a sheet of submarine muds might be too heavy 

to be supported as flows and would sink, forming pods of all sizes 

by fragmentation of the viscous material. Or the magmas could 

spread laterally before reaching the mud-sea interface, and form 

tongues and pods in the mud. Budding above sill-like bodies formed 

some pillow structures and possibly some detached domes. The immiscibility of magma and mud, with the early development of a skinlike cooling surface on the budding magma, may help to explain the 

tendency for detachment and isolation. Deltoid-shaped domes may 

be laccolithic intrusives, or sills completely detached from the parent 

mass. Motion of the magma, indicated by brecciated contacts, 

peperites, and deformed pillows, was a constant and perhaps necessary element in the formation of the pillows and other bulbous structures.

The regular distribution of oxides with respect to silica in the variation diagram, figure 88, suggests that one parent magma was the 

source of all volcanic and plutonic rocks on the island.

The origin of spilites on Unalaska Island may be twofold: some 

spilites probably are hydrothermal; others may have been altered 

during deposition. That the spilites originated from a sodic magma 

is doubtful, for nonsodic ferromagnesian minerals occur with albitized plagioclase. Those dikes which are albitized must have been 

altered much later than any diagenetic alteration of the rest of the 

Unalaska formation. As the degree of hydrothermal alteration (chloritization and uralitization) increases near some dikes, some albite 

was probably introduced hydrothermally also. For these reasons, 

too, a hydrothermal origin of most or all spilites on the island seems 

slightly more probable than does a diagenetic one, hence albite alteration is shown with other hydrothermal minerals on figure 89. However, the widespread, somewhat spotty distribution of spilites on the 

southwestern end of the island, away from the largest pluton, and 

their close association with pillow structures and related submarine 

volcanic forms still leave room for an explanation considering diagenetic spilitization. No diagnostic criteria are available to determine the relative importance of these two origins of sodium enrichment ; we cannot even ]be certain that both were operative on Unalaska 

Island.

It is further possible that spilitization was due to other processes. 

For instance, the sodium-rich fluid which effected spilitization may 

have been derived from connate water trapped in submarine sediments 

and volcanic rocks. Later plutonic invasions may have mobilized 

this fluid and enabled the cold feldspars to become albitized.

The chlorite-uralite-epidote alteration of the Unalaska formation is 

interpreted as a widespread hydrothermal reaction to both connate
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 609

EXPLANATION

INTENSITY AND DOMINANT 

MINERALOGY OF ALTERATION

No albite, chlorite, uralite, epidote, or pyroxene alteration

Rock slightly altered; 

sericite, epidote, chlorite, uralite

« = « to c J2 CSV

H g

Rock severely altered; 

hornfels-biotite, amphibole, pyroxene

Thickness and continuity exaggerated

MINERALIZATION 

Au Gold

Cu Copper sulfides 

Fe Iron sulfide 

S Sulfur ' 

Zn Zinc sulfide DECLINATION, 1960

Rock moderately altered; epidote, albite, 

chlorite, uralite

Fault

Dashed where approximately 

located, dotted where concealed

30 MILES

30' 167° '

FIGURE 89. Alteration and mineralization on Unalasfca Island.

fluids and fluids introduced by the granodiorite batholiths prior to 

the last stage of their emplacement. Alteration zones, marked by 

secondary mineral assemblages as well as by secondary mineral abundance, are symmetrical about the batholiths and progressively of higher 

grade and intensity toward them. The maximum hydrothermal alteration thus coincides with the source of maximum available energy. 

Eocks of the batholith were deuterically altered, and perhaps fluids 

involved in this late alteration also escaped into the host rocks.
610 INVESTIGATIONS OF ALASKAN VOLCANOES

GRANODIORITE BATHOLITH 

FIELD RELATIONS

Three small batholiths and 25 smaller plutons ranging in size 

from sills and dikes 10 feet wide to stocks 4 miles wide have been 

studied. The three small batholiths are: Shaler pluton, centered on 

the Shaler Mountains in the south-central part of the island; Beaver 

pluton, south of Beaver Inlet; and Captains pluton, between Captains 

Bay and Erskine Bay. The northern part of the Shaler pluton between Skan Bay and Anderson Bay is typical of the plutonic suite 

and was studied in detail; about half of the 263 thin sections of 

plutonic rocks are from this area.

The topographic forms developed on the plutonic rocks are similar

to those on the Unalaska formation which they intrude, but the 

volcanic rocks of the Unalaska formation generally produce jagged 

high ridges, whereas the batholithic rocks erode to rounded buttresses and more smoothly textured landscapes (fig. 90). Where 

homogeneous granodiorite is exposed at the shoreline, layers of characteristic dark- and light-weathering coloration are produced at 

water level. The volcanic rocks and the less homogeneous plutonic 

rocks show no distinctive shoreline weathering.

The contacts of the large plutons are generally very steep to vertical. In the Skan Bay-Naginak Cove area, however, many contacts 

are horizontal or gently dipping. The general attitude of this contact, despite all the irregularities, is nearly horizontal, and this contact is the roof of the northern part of the pluton.

Schistose or gneissic rocks form a narrow vertical zone along 

the southern margin of the Shaler pluton =at least from Johnson 

Cove to Raven Bay and possibly to Pumicestone Bay and in the 

eastern half of the 3600 Mountain pendant. At Johnson Cove the 

schistose zone is about 100 yards wide, strikes N. 70° E. and is nearly 

vertical; the schistosity within the zone strikes N". 50° E. and dips 

65° SE. If magma motion produced the dynamic metamorphism 

at this portion of the contact, then this combination of attitudes 

suggests that in this area the magma moved upward and from west 

to east. In an outcrop about one-half mile southwest of the summit 

of 3600 Mountain, a recrystallized volcanic agglomerate has inclusions flattened in the plane of the nearly vertical schistosity and 

elongated downdip; this lineation, presumably an "a'.' lineation, suggests that magma movement was nearly vertical. Forceful intrusion 

of the magma must- be inferred locally from these relationships, but 

most other information suggests passive emplacement: The contacts 

of the plutons, for example, though noticeably discordant, are seldom 

marked by drag of bedding in adjacent wall rocks. This abrupt
O 

OsO

f 

I

O§O§t>

O

> 

td

H f

FIGURE 90. A view south toward 3600 Mountain roof pendant showing Shaler pluton (below broken line) and 

Unalaska formation (above line). Topographic development is characteristic of: A, plutonic rocks; B, sedimentary and volcanic rocks of Unalaska formation ; and O, schist and gneiss. Circle indicates outcrop where pluton border rocks are quartz gabbro and pendant border rocks are pyroxene hornfels (of composition equivalent to melagabbro)
612 INVESTIGATIONS OF ALASKAN VOLCANOES

discordance is especially well exposed between Usof Bay and Raven 

Bay, on the eastern side of Three Island Bay, and in the southern 

half of Udagak Strait. In the Skan Bay-Naginak Cove area the 

Unalaska formation dips gently northwestward both near Cape 

Starichkof and in the unmetamorphosed portions of the 3600 Mountain pendant. It is nearly horizontal in the ridge between Cape 

Starichkof and Anderson Bay. The roof pendants are not infolded 

remnants; the randomly undulating contact owes its shape to joint 

control of stoping or assimilation rather than to folding of the overlying rocks.

In the interiors of the three small batholiths, widely spaced parallel 

joints are separated by massive unjointed rock (fig. 91). Some large 

joints or faults extend beyond the plutonic rocks (p. 656, pi. 77). The 

heterogeneous border rocks are characterized by very irregular fracture patterns, with near-random joints and joint sets, down to handspecimen size without large intervening areas of un jointed rock 

(fig. 92). Probably the relative homogeneity of composition controlled the mode of fracture.

The age of all large coarse-grained plutons and most small coarsegrained plutons is probably younger than earljy Miocene and older 

than middle Pleistocene, as rocks of the Captains pluton crop out 

iy2 miles upstream from the altered rocks at the desmostylid locality 

and the Makushin volcanics uncomformably overlie quartz diorite 

stocks in the Makushin massif area.

Most of the plutonic rocks of Unalaska Island have hypidiomorphicgranular textures, but a few are porphyritic. The wallrocks bordering the large plutons are schistose or hornfelsic. A summary of the 

mineralogy of all holocrystalline rocks of Unalaska Island is given in 

plate 76. Johannsen's (1950, v. 1, p. 141-161) terminology for igneous 

rocks is followed throughout, even for the recrystallized wallrocks, to 

facilitate direct comparison of rock compositions except that Johannsen's tonalites are called quartz diorites, and his adamellites, 

quartz monzonites. Mineral percentages given are visual estimates; 

they are believed to be accurate within 1 to 5 percent of any one 

constituent.

The principal felsic minerals of the holocrystalline rocks of Unalaska Island are plagioclase, potassic feldspar, and quartz. The 

plagioclase ranges in composition from albite to anorthite; most is 

unzoned, but much exhibits progressive oscillatory zoning from wide 

calcic cores to thin sodic rims. The range of average composition 

of individual plagioclase crystals can be large within a single rock
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 613

FIGUKE 01. Joint pattern in homogeneous central granodiorite of Shaler pluton, looking 

south from south shoulder of 3600 Mountain. Belief is 2,600 feet. Joints are 50 feet 

to several hundred feet apart; the massive rock between is unjointed except for irregular 

face fractures of outcrops.

(fig. 93), and for this reason the plagioclase composition is given 

in plate 76 only for specimens for which determinations on six or 

more grains have been averaged. Composition of plagioclase in the 

plutonic rocks was determined by measurement of extinction angles 

of albite twin lamellae in MP sections j_ to 010 (Troger, 1952, p. 113, 

upper diagram, low temperature curve). Practically all the potassic 

feldspar is orthoclase, with large optic angle and no cross-hatch twin-
614 INVESTIGATIONS OF ALASKAN VOLCANOES

ning. All potassic feldspar is interstitial and crystals are usually 

anhedral to subhedral; the common appearance in thin section is that 

of an irregularly branching mass with concave outward surfaces. 

Some orthoclase crystals are developed in former miarolitic cavities, 

which were subsequently filled with quartz. Micropegmatitic intergrowths with quartz are common; feathery perthite is commonly developed. Orthoclase or potassium-rich alkalic feldspar is commonly

FIGCRB 92. Joint pattern in inhomogeneous border rocks of 'Shaler plnton on Kof Point. 

Rocks are intimate mixtures of gabbro, granite, quartz diorite, quartz monzonite, and 

granodiorite, broken by discontinuous closely spaced joints of many orientations.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 615

20 30 40 50 60 70 

PERCENT An

80 20 30 40 50 60 70 80 90 100

FIGURE 93. Percent-frequency distributions of the average percent anorthite in plagioclase 

crystals of typical holocrystalline rocks of Unalaska Island. X is arithmetic mean for 

each rock ; S« is standard deviation for each rock.

optically continuous with the albite rims of plagioclase. Except in 

a few quartz porphyries having subhedral to enhedral quartz phenocrysts, most of the quartz is late and occurs as anhedral crystals with 

a mammillary form. Primary muscovite was observed in three rocks, 

and zeolites, probably thomsonite and stilbite, form up to 5 percent 

of the groundmass of the sills on the southwest end of the island.

In figure 93 the plagioclase composition is shown diagrammatically 

for the following rocks from the Shaler pluton, as represented by 

designated chemical analyses: 1, leuco albite quartz monzonite from 

marginal aplitic body near the south-central contact of the 3600 

Mountain pendant, analysis 34; 2, leuco granite from a marginal 

aplitic body at the north contact of the 3600 Mountain pendant, near 

the contact indentation southwest of the head of Naginak Cove; 3, 

granodiorite from northeast of the head of Raven Bay, analysis 29; 

4, granodiorite from the south shore of large lake north of 3600 

Mountain pendant, analysis 33; 5, granodiorite from shore bench 

south of mouth of stream emptying into north side of Skan Bay, 

analysis 26; 6, granodiorite from sea cliff outcrop 2 miles west of 

Tarasof Point, which intrudes gabbro represented by specimen 11 

(below); 7, granodiorite from head of north arm of Skan Bay; 9,
616 INVESTIGATIONS OF ALASKAN VOLCANOES

quartz diorite from near contact 3 miles southwest of Tarasof Point; 

10, quartz gabbro from near contact 2% miles southwest of Tarasof 

Point, analysis 25; 11, gabbro from sea cliff outcrop 2 miles west of 

Tarasof Point, which is intruded by granodiorite represented in rock 

6 (above). Rock 8 is a granodiorite from the Captains pluton, west 

of the stream 1 mile southwest of the head of Captains Bay, and rock 

12 is a gabbro from the Cape Aiak sill 3 miles north-northeast of 

southern tip of Cape Aiak, analysis 2. In plate 76, aplites, large 

plutons, small- intrusive bodies, and recrystallized wallrocks are represented, respectively, by approximately 17, 163, 35, and 43 specimens, except in distributions dealing with plagioclase composition 

where they are represented, respectively, by approximately 13, 158, 

29, and 56 specimens. In plate 76, amphibole percentages include 

amphibole altered from pyroxene.

The principal mafic minerals of the holocrystalline rocks of Unalaska Island are orthopyroxene, clinopyroxene, amphibole, biotite, and 

magnetite-ilmenite. Olivine is present (to 8 percent) in only a few 

rocks. The orthopyroxene, hypersthene where detenninable, is invariably euhedral and acicular, and may be partially or completely replaced by bastite. The clinopyroxene, generally augite, is anhedral 

to subhedral (rarely euhedral and stubby) and poikilitic, and is generally partly replaced by amphibole. Secondary amphibole (uralite) 

may be hornblende or hornbleride-actinolite or actinolite-tremolite 

(pleochroism in light greenish tints or colorless). Primary amphibole 

ranges from anhedral to euhedral and from massive to fibrous, an<J 

is usually hornblende or hornblende-actinolite pleochroic in green, 

bluish green, olive green, brownish green, or brown. Riebeckite( ?) 

occurs in one hornfels in vuglike spaces also containing sodic oligoclase and pyrite. Biotite is usually subhedral to anhedral and frequently is associated with magnetite-ilmenite. Magnetite-ilmenite is 

universally present and is always anhedral. Leucoxene alteration is 

developed sparingly, which suggests that magnetite is dominant over 

ilmenite.

Accessory constituents of plutonic rocks of Unalaska Island are: 

anhedral titanite (sometimes veinlike); euhedral apatite (rarely anhedral) ; anhedral allanite (commonly in veins or interstitial and always metamict); euhedral tourmaline; euhedral zircon; anhedral pyrite (commonly in veins); anhedral chlorite (penninite); anhedral to 

subhedral epidote-clinozoisite; hematite; rutile, as submicroscopic 

needles in feldspar and stubby euhedra in titanite; anhedral calcite; 

and anhedral chalcopyrite (?). The occurrence of a dusky green spinel 

in one border hornfels from Blueberry Bay has been confirmed by 

X-ray powder studies. A mineral believed to be prehnite is present
GEOLOGY OP UNALASKA ISLAND AND ADJACENT INSULAR SHELF 617

in several rocks as elongate, radiating aggregates which dilate biotite 

cleavages, an unusual occurrence similar to that described by Wells 

and Bishop (1955, p. 154, figs. 3, 5) in diorite from the Channel 

Islands.

Probably 60 to 80 percent of the coarse-grained rock at the ground 

surface of Unalaska Island is granodiorite (pi. 75, fig. 86), and even 

this figure probably increases rapidly with depth. Only 38 percent 

of the 263 rocks studied fall in the granodiorite category (see table 2), 

but the volumetrically minor aplites, small intrusive bodies, and hornfelses bulk disproportionately large in this table. However, many 

rocks besides granodiorite are associated with the Unalaska coarsegrained intrusive complex (fig. 94).

Holocrystalline rocks occur in varieties as felsic as leucogranite 

and kalialaskite and as mafic as melagabbro and hornblendite; there 

are also many intermediate types. Except for the tabular bodies of 

aplite and pegmatite, these rocks grade imperceptibly into each

TABLE 2. Incidence of studied holocrystalline rock types on Unalaska Island

(Johannsen nomenclature)

[Johannsen (1950) igneous rock names applied to recrystallized wallrocks to facilitate direct comparison

igneous rock compositions]

Rock name

Leuco albite quartz monzoPercent of grand total _

Shaler 

pluton

87

8

9

12

7

6

1

5

1

3

1

1

1

142

54

Beaver 

pluton

2

1

1

3

4

2

1

1

1

16

6

Captains 

pluton

3

1

2

1

7

3

Aplite and 

pegmatite 

dikes 

cutting 

Plutonic 

rocks

1

9

9

1

2

1

1

18

7

Small 

intrusive 

bodies 

cutting 

volcanic 

and sedimentary 

rocks

4

5

3

3

1

8

3

6

1

1

1

36

14

Recrystallized 

vplcanic 

and sedimentary 

rocks, and 

inclusions 

in plutonic 

rocks '

4

5

1

7

4

5

1

2

2

1

1

1

1

6

1

1

1

44

17

1 Includes 30 border hornfels, 6 border schists and gneisses, and 8 inclusions.
618 INVESTIGATIONS OF ALASKAN VOLCANOES

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GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 619

other. * A granodiorite that grades in one direction into a syenodiorite may grade in other directions into quartz monzonite and quartz 

gabbro, and all three of these rocks may grade into one another. Apparently nearly all chemical constituents act as independent variables. The following descriptions are concerned with the distribution 

of this multiplicity of igneous rock types within six natural groupings of holocrystalline rocks.

SHALER PLTTTON

The Shaler pluton is exposed in the Shaler Mountains of southern 

Unalaska Island. The rocks are best exposed above altitude 1,500 

feet in the glaciated, mountainous interior (figs. 90, 91), and along 

fiords (fig. 92). Most of the granodiorite is light colored, equigranular, and medium to coarse grained. White andesine-oligoclase, pink 

orthoclase, clear quartz, green hornblende, and brown biotite are 

usually recognizable with a hand lens. A preferred orientation of 

stubby subhedral or euhedral plagioclase crystals is evident in many 

thin sections. The contact with the surrounding Unalaska formation is marked by a strong topographic depression for 20 miles, in 

three segments: From north of the head of Pumicestone Bay eastward to the east side of Usof Bay; from the "thumb" of Three Island Bay northward along a stream valley; and westward from south 

of Tarasof Point.

The large majority of the rocks studied from the Shaler pluton 

are granodiorites. Fewer than a third are less silicic: quartz diorite, 

syenodiorite, diorite, gabbro, quartz gabbro, syenogabbro, or calcic 

quartz monzonite, in order of observed abundance; gabbroic rocks 

total only 10 percent. A small number are more silicic or felsic than 

granodiorite: quartz monzonite, granite, albite granite, or monzonite, in order of abundance (table 2). All of these rocks those that 

are less silicic and those that are more silicic than granodiorite are 

associated with roof pendants or with portions of the main contact 

(fig. 90, pi. 78) (Jagger, 1908, p. 19, 32). Locally, swarms of small 

inclusions of wallrocks are also present, as in the mafic rocks 4% 

miles south-southwest of Tarasof Point, or in the area of small roof 

pendants northeast of Skan Bay.

BEAVEE PLUTON

The Beaver pluton is exposed on the headlands south of Beaver 

Inlet. The contact is marked topographically by the east side of

1 In a few outcrops sharp contacts between coarse-grained rocks have been observed. In 

these places the felsic rocks always dilate the mafic rocks; elsewhere these same rocks are 

gradational into each other. Sharp internal contacts are an extreme rarity within plutons 

of Unalaska Island, though sharp external contacts are quite common.
620 INVESTIGATIONS OF ALASKAN VOLCANOES

Udamat Bay, and perhaps by the trough line of Beaver Inlet on the 

north side of the mass. Most of the specimens studied from the Beaver pluton are gabbroic; by comparison, only 10 percent of the Shaler 

pluton rocks studied are gabbroic. The more mafic nature of the 

Beaver pluton rocks may be due to assimilation of the more mafic 

wallrocks surrounding this pluton; or a disproportionate number of 

specimens were collected from the border zone of the pluton, where 

mafic rocks are more common.

CAPTAINS PLUTON

The Captains pluton is exposed at the head of Captains Bay. Of 

the 7 thin sections studied, 3 are from granodiorite; 2, quartz diorite; 

1, quartz monzonite; and 1, diorite

APLITES AND PEGMATITES

Thin dikes of aplite and pegmatite, dominantly of leucogranite 

composition, occur throughout the large plutons and in closely adjacent wallrocks and have sharp contacts. Included with this category 

are anastomosing masses found only near the borders of the large 

plutons, which have indistinct contacts against surrounding and included rocks (fig. 95). The small anastomosing masses grade into 

the larger felsic bodies at the border of the batholiths. Inclusionsrich "plum-pudding" rocks, like that in figure 95, are well developed 

on the north side of Skan Bay, especially on Kof Point (fig. 92).

SMALL HOLOCRYSTALLINE INTRUSIVE BODIES

About one-half of the small holocrystalline intrusive bodies have 

gabbroic affinities and one-half have dioritic affinities (table 2). These 

bodies include 4 small stocks north of Humpback Bay, 1 small stock 

north of Staraya Bay, 10 unmapped dikelike apophyses near batholithic contacts, and 11 isolated intrusive masses between Riding Cove 

and Konets Head (pi. 75). Eight thin sections were made of specimens from the large sill exposed in several isolated masses near Cape 

Aiak. Elsewhere 1 or 2 thin sections were studied from each small 

holocrystalline intrusive body. Probably many aphanitic dikes and 

sills are also related to the batholithic intrusion, but as these cannot be 

distinguished from the dikes feeding sills in the Unalaska formation 

they are all discussed together (p. 599-600).

The Cape Aiak gabbro sill has a maximum thickness of about 350 

feet, wedges out eastward and westward, and covers at. least 30 square 

miles. At its easternmost exposure, the sheet dips 40° to the south and 

perhaps there becomes a dike. The sill locally truncates the bedding 

of the host rocks and sends small stringers into them. Two probable 

feeder dikes of this sill appear beneath it northeast of Cape Aiak.
Q 

H 

O 

f O O

«l

O 

^i

d

O O

O

H

I

H

FIGURE 95. "Plum-pudding" contact rock from large polished beach boulder at head of north arm of 

Skan Bay. Inhomogeneous matrix of granite replaces recrystallized wallrock (inclusions) of syenodioritic composition.

O5 to
622 INVESTIGATIONS OF ALASKAN VOLCANOES

The rocks of the Cape Aiak sill and associated bodies are generally 

moderately coarse, but fine-grained border phases and less common 

pegmatitic varieties also occur. Columnar joints are conspicuous in 

all the sills exposed in coastal or inland cliffs. The unaltered rock 

is medium to dark gray; where the plagioclase has been albitized the 

rock is mottled dark gray and very light gray.

A noteworthy diabase sill occurs at Boulder Bay near the southwestern end of the island. The wallrocks there contain 15 to 20 percent of 

epidote and other secondary minerals. This local and more intense 

hydrothermally altered area apparently was heated by the magna of 

the nearby pegmatitic diabase sill. The sill contains numerous phenocrysts of feldspar and pyroxene up to 5 inches long which are intermeshed in such intricate fashion that they must have grown in place 

after intrusion.

RECRYSTALLIZED WALLROCKS

The overall composition of the recrystallized wallrocks tends to be 

more mafic but also more heterogeneous than any other holocrystalline 

group (table 2). Most representatives would be compositional 

equivalents of igneous rocks in the range from granodiorite to 

melagabbro with the volcanic rocks clustered toward granodiorite. 

The composition of the recrystallized fine-grained sediments tends to 

cluster around a composition equivalent to melagabbro. Rocks studied 

under this category include 30 border hornf elses, 6 border schists and 

gneisses, and 8 inclusions in plutonic rocks.

Holocrystalline wallrocks are limited to a narrow discontinuous 

zone (exaggerated in fig. 89) around the borders of the batholiths and 

larger stocks. The best exposures occur in the east half of the 3600 

Mountain roof pendant southwest of Naginak Cove (fig. 90) and in 

the prominent valleys between Raven Bay and Johnson Cove.

The border hornfelses generally are dark-gray or greenish-gray 

fine-grained rocks (often mistaken for an igneous chill zone in the 

field; for a similar account see Cady and others, 1955, p. 87, 88) with 

sporadic porphyroblasts of biotite, hornblende, or pyroxene. Volcanic structures such as agglomerate fragments, pillows, and bombs 

are generally obliterated, but the fine bedding characteristic of the 

sedimentary argillites is commonly preserved, even in small inclusions 

away from the main contact (fig. 96). The argillites are converted 

to hornblende-bytownite rocks with alternate layers richer in one or 

the other of these minerals; long dimensions of crystals are commonly perpendicular to this relict layering.

The foliation of the border schists and gneisses is produced by a 

preferred mineral orientation and by a parallel arrangement of lenticular rock fragments. These foliated rocks are generally somewhat
GEOLOGY OP UNALASKA ISLAND AND ADJACENT INSULAR SHELF 623

FintiEB 96. Reerystallized inclusions of layered argillite in gabbro from small stock on 

north side of Humpback Bay. Average composition of inclusions is equivalent to that of 

a melagabbro. Long dimensions of many hornblende and plagioclase crystals in coarse 

layers are perpendicular to laverini?-

580537 O -61 - 7_
624 INVESTIGATIONS OF ALASKAN VOLCANOES

coarser grained than the structureless hornfelses. The hornfelses 

and gneisses are miner alogically similar. The usual orthopyroxene 

and clinopyroxene, green or brown amphibole, biotite, plagioclase, 

quartz, orthoclase, and magnetite are present. Spinel-bearing amphibolite hornfels from Blueberry Bay also contains olivine porphyroblasts. Textures of the holocrystalline border rocks range from 

schistose to porphyroblastic granulitic; many porphyroblasts of clinopyroxene, hornblende, or biotite are extremely poikiloblastic. Inclusions near contacts have textures resembling those in nearby wallrocks, 

whereas inclusions far from contacts have textures resembling those 

in the surrounding igneous rock. Boundaries of some inclusions are 

very diffuse. Some shadowy remnants of plagiclose phenocrysts in 

former lavas are completely replaced by granular aggregates of quartz 

and alkalic feldspar; in grains where the replacement has not gone 

to completion and where the surrounding matrix is recrystallized, the 

matrix plagioclase is 5 to 10 percent more sodic than the phenocryst 

plagioclase, indicating the amount of disequilibrium between the 

phenocrysts and groundmass in the original rock.

Border hornfelses are rare in the Aleutians, but have been recognized on Adak Island (Fraser and Snyder, 1959, p. 381, 382, 388); 

border schists and gneisses are even rarer than hornfelses. A few 

boulders of coarse gneiss have been found in surficial deposits on 

Amchitka Island (Powers and others, 1960, p. 542-545; Coats, 1956, 

pi. 18B), on the shores of Kanaga Island (Fraser and Barnett, 1959, 

p. 231), and on western Tanaga and Ogliuga Islands (Coats, 1956, 

p. 88, 89), but the sources of these boulders are not known. The 

exposures on Unalaska Island are the only known outcrops of dynamically metamorphosed rocks in the Aleutian Islands.

COMPARATIVE MINERALOGY

The compositions of the average plagioclase of the holocrystalline 

rocks 2 are An39 in the Shaler pluton; An57 in the Beaver pluton; An44 

in the Captains pluton; An24 in the aplites cutting plutonic rocks; 

Anr>1 in the small intrusive masses cutting volcanics and sediments, 

and An47 in the recrystallized wallrocks. The average plagioclase 

for all holocrystalline rocks of Unalaska Island is An42 (pi. 76).

Figure 97 relates the anorthite content of the average plagioclase 

and the total percent of mafic minerals for four groups of holocrystalline rocks on Unalaska Island and illustrates the intermediate compositional position of the plutonic border rocks and apophyses between 

the relatively uncontaminated central plutonic rocks and the mafic 

wallrocks.

2 Determined by taking the arithmetic mean of the anorthite content of 6 to 40 individual plagioclase crystals per specimen, and then taking the arithmetic mean of all the 

average anorthite contents for each specimen within a particular group.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 

lOOi I 

625

50

50100 0 

PERCENT An IN AVERAGE PLAGIOCLASE 

A Plutonic rocks (Shaler, Beaver, and 

Captains plutons) greater than onehalf mile from a mapped contact: 45 

specimens

50 100 

PERCENT An IN AVERAGE PLAGIOCLASE

8. Plutonic rocks (Shaler, Beaver, and 

Captains plutons) less than one-half 

mile from a mapped contact: 115 

specimens

100

50

50 100 £T

PERCENT An IN AVERAGE PLAGIOCLASE PERCENT An IN AVERAGE PLAGIOCLASE

C. Small intrusive bodies (Stocks, sills, D Recrystallized wallrocks (contact hornapophyses): 30 specimens fels and schist and inclusions in 

plutonic rocks): 36 specimens

EXPLANATION

Contours showing percentage of specimens 

per 1-percent area of graph

5-10 10-15 15-20 More than 20

FIGUUK 97. Graph showing distribution of total mafic mineral content and anorthite 

content of average plagioclase for holocrystalline rocks of Unalaska Island; illustrates 

increasing mafic content, increasing calcic content, and decreasing homogeneity In the 

series A, B, C, D.
626 INVESTIGATIONS OF ALASKAN VOLCANOES 

SPECIFIC DATA FOR ANALYZED ROCKS

Chemical analyses of 13 specimens of plutonic rocks of Unalaska 

Island and 10 wallrocks of the Unalaska formation, which are reported 

in table 1 and plotted on figure 88, support the indirect conclusions 

about the compositions derived from the mineralogical data. Some 

of the analyses are of rocks of rare occurrence and are included to 

illustrate the range of composition of the suite. Both plutonic rocks 

and wallrocks have representatives throughout the range of chemical 

composition. Practically all the specimens analyzed are magmatic 

types, and the fine-grained sediments of the Unalaska formation are 

not represented.

Analysis 17 is of a composite sample of 32 small hand specimens 

of the Unalaska formation from the Skan Bay-Naginak Cove area. 

In this sample, lava makes up 51 percent of the weight, tuff 34 percent, 

and clastic sediments the remaining 15 percent; these proportions are 

crudely representative of the formation in that area near the north 

margin of the Shaler pluton. Specimen 33 is most nearly representative of the north part of the pluton and is less femic in all respects 

than specimen 17.

The estimated modes agree reasonably well with the appropriate 

normative minerals. The difference between normative and modal 

quartz is 3 percent or less for all but two of the analyzed rocks. The 

difference between normative and modal orthoclase is 5 percent or less 

for all but four of the analyzed rocks. The anorthite composition of 

the normative plagioclase and the average modal plagioclase agree 

within 5 percent for all but three analyzed rocks. The normative 

nepheline in analysis 2 and the normative corundum in analysis 34 

have no observed counterparts in thin section. Specific agreements 

and discrepancies will be discussed under the detailed descriptions 

which follow.

Specimen 2 is a gabbro from the sea cliff 3 miles north-northeast of the 

southern tip of Cape Aiak. The texture is porphyritic and hypidiomorphic, 

and the mafic minerals are locally aggregated. Felsic minerals range in abundance from 50 to 70 percent, of which about 1 percent is orthoclase and.the rest 

is plagioclase replaced by 10-15 percent albite and. minor sericite. The unaltered plagioclase composition, by extinction angle, is An78 [mean of 26 

crystals, standard deviation is 11 percent An, average amount of zoning per 

individual plagioclase crystal is 3 percent An per crystal (fig. 93, no. 12)] ; by 

index oils, is An08 . Mafic minerals make up 30-50 percent, of which 25-30 percent 

is stubby euhedral phenocrysts of clinopyroxene, 2-3 percent clinopyroxene or 

uralite needles (length-to-width ratio is 20:1) grown on the margins of the 

large pyroxenes rather than altered from them, 0-1 percent biotite(?) now 

completely altered to antigorite, 8-10 percent magnetite, <1 percent is apatite, 

and <1 percent serpentine.

Discrepancy in plagioclase compositions (above) is probably due to two 

causes. (1) Low temperature curves were used; using the same angles, high
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 627

temperature curves give average plagioclase as An07. (2) Alteration to albite 

lowers normative plagioclase. Presence of 7.10 percent olivine and 0.85 percent 

nepheline in the norm is unexplained, but other specimens from the Cape Aiak

,T' sill contain abundant zeolites through out the the groundmass, and if the analyzed 

sample contained a pocket of these the total rock might be undersaturated in 

silica.

Specimen 13 is a recrystallized wallrock, equivalent to syenodiorite in composition, from a beach boulder at the southeast end of the north arm of Skan

~ Bay; it occurs as an inclusion in granite like that represented by analysis 32. 

The texture is hypidiomorphic-granular. Felsic minerals compose 75-80 percent

"' of the rock: 2-3 percent quartz, 5-10 percent orthoclase partially altered to 

kaolin (1-2 percent), and 65-70 percent plagioclase partially altered to sericite 

(2-3 percent) and clinozoisite (0.1 percent). The plagioclase composition, by 

extinction angle, is An35 (mean of 10 crystals, standard deviation is 6 percent 

An, average amount of zoning per individual plagioclase crystal is 11 percent An 

per crystal). Mafic minerals compose 20-25 percent: 10-14 percent hornblende 

(much is interstitial), 5-10 percent biotite partially altered to epidote (0.1-0.5

^ percent) and penninite (0.1-1 percent). 1-3 percent magnetite, 0.1-1 percent 

sphene (associated with the magnetite) partially altered to leucoxene (0.1-0.5

T percent), one large grain of tourmaline (about 0.3 percent), and <0.01 percent 

hematite.

Specimen 25 is a quartz gabbro from elevation 1,000 feet on the west-facing 

hill slope 2% miles southwest of Tarasof Point. The texture is hypidiomorphicgranular. Felsic .minerals compose 75-80 percent of the rock: 4-6 percent

 X quartz, <0.1 'percent orthoclase, and 70-75 percent plagioclase slightly altered 

to sericite «0.1 percent). The plagioclase composition, by extinction 

angle, is AnB4 [mean of 36 crystals, standard deviation is 7 percent An, average 

amount of zoning per individual plagioclase crystal is 9 percent An per crystal 

(fig. 93, no. 10)]. Mafic minerals compose 20-25 percent: 0.5-1.5 percent clinopyroxene (remnants in hornblende and euhedra in feldspar), 8-10 percent hornblende (an indeterminate amount derived from clinopyroxene), 6-8 percent 

biotite slightly altered to penninite «0.1 percent) and cliozoisite «0.1 percent), 2-5 percent magnetite, <0.1 percent hematite, 0.5-1.5 percent apatite, 

4» 0.5-1 percent tourmaline, and 0.1-1 percent penninite (interstitial).

- 1 ' Potassium oxide apparently is contained in biotite, sericite, and plagioclase. 

Thin sections stained for K2O failed to reveal any more than the amount of 

orthoclase shown.

Specimen 26 is a granodiorite from the rock bench at the tip of the headland 

south of the largest stream on the north side of Skan Bay. The texture is 

hypidiomorphic-granular. The rock contains 70-75 percent felsic minerals: 

^ 7-10 percent quartz, 15-18 percent orthoclase partially altered to kaolin (1-3 

percent), and 45-50 percent plagioclase preferentially altered in centers to 

sericite (3-5 percent). The plagioclase composition, by extinction angle, is An40 

[mean of 35 crystals, standard deviation is 9 percent An, average amount of 

zoning per individual plagioclase crystal is 0 percent An per crystal (fig. 93,

j no. 5) ] ; by index oils, is An3o. Mafic minerals make up 25-30 percent; 5-10 percent clinopyroxene slightly altered to hornblende f 0.3-0.5 percent), 3.7 percent

_v hornblende partially altered to antigorite (1-2 percent) and minor magnetite, 

10-15 percent biotite slightly altered to penninite «0.01 percent), 2-3 percent 

magnetite, <1 percent sphene slightly altered to leucoxene «0.01 percent), and 

0.5-1.5 percent apatite.

Specimen 27 is a granogabbro occurring one-third mile from the Beaver pluton 

contact on the east side of Final Bay. The texture is hypidiomorphic and the
628 INVESTIGATIONS OF ALASKAN VOLCANOES

groundmass is myrmekitic. Felsic minerals compose 80-85 percent of the rock:

7-10 percent quartz, 15-25 percent alkali feldspar partially altered to kaolin

(2-3 percent), and 50-58 percent plagioclase partly altered to sericite (about 

5 percent) and epidote-clinozoisite (about 1 percent). The plagioclase composi- ^

tion, by extinction angle, is Anc2 [mean of 28 crystals, standard deviation is

10 percent An, average amount of zoning per individual plagioclase crystal is

2 percent An per crystal], by index oils, is AnB4. Mafic minerals compose 15-20

percent: 10-13 percent uralitic hornblende (probably 85 percent of this after

pyroxene, 15 percent after biotite) partly altered to> penninite (0.5-1.5 percent).

0.5-1.5 percent sphene partly altered to leucoxene (0.1 percent), 4-6 percent jmagnetite, and <1 percent apatite.

The discrepancy in plagioclase composition is probably due to sodium in the 

alkalic feldspar, and potassium in the plagioclase(?).

Specimen 28 is a granodiorite from the Shaler pluton near the contact on 

the ridge between Blueberry Bay and Three Island Bay. The texture is hypidiomorphic-granular. Felsic minerals compose 70-80 percent of the rock: 10-15 ^ 

percent quartz with undulatory extinction, 5-10 percent orthoclase partly altered _ 

to kaolin (<1 percent), and 50-60 percent plagioclase partly altered to sericite ' » 

(<5 percent). The plagioclase composition, by extinction angle, is An43 [mean 

of 22 crystals, standard deviation is 12 percent An, average amount of zoning 

per individual plagioclase crystal is 4 percent An per crystal]; by index oils, 

is An^. Mafic minerals compose 20-30 percent: <1 percent clinopyroxene (remnants), 9-14 percent hornblende (an indeterminate amount after pyroxene) 

partially altered to magnetite (<1 percent) and hematite (<1 percent), 9-14 , 

percent biotite partly altered to penninite (2-3 percent) and sphene (0.1-0.5 

percent), 0.5-1.5 percent of apatite, 0.5-1.5 percent magnetite-ilmenite, <1 percent sphene, <0.01 percent zircon (two small crystals, one with a pleochroic halo 

in biotite, the other with a pleochroic halo in hornblende). The rock is cut by 

a thin vein of orthoclase.

Specimen 29 is a granodiorite from 1 mile northeast of the head of Raven 

Bay. The texture is hypidiomorphic-granular. Felsic minerals compose 75-80 ^ 

percent of the rock: 10-15 percent quartz, 10-15 percent orthoclase partly altered 

to kaolin (<1 percent), and 50-55 percent plagioclase partly altered to sericite ^ 

(<1 percent). The plagioclase composition, by extinction angle, is An3« [mean T 

of 21 crystals, standard deviation is 3 percent An, average amount of zoning 

per individual plagioclase crystal is 3 percent An per crystal (fig. 93, no. 3)], 

by index oils, is Anas. Mafic minerals compose 20-25 percent: 4-6 percent clinopyroxene partly altered to hematite (<0.01 percent) and hornblende (3-4 percent, occurs as reaction zones between pyroxene and biotite; pyroxene adjacent 

to feldspar and quartz is not altered to hornblende), 13-17 percent biotite slightly . 

altered to hematite (<0.01 percent), 1-3 percent magnetite-ilmenite, <1 percent 

sphene (this includes <0.1 percent sphene which occurs as reaction rims around 

some magnetite-ilmenite in biotite), and 0.5-1.5 percent apatite. Parallel 

oriented euhedral inclusions of pyroxene, biotite, magnetite, and apatite occur 

in some of the larger plagioclase crystals. ^

Specimen 30 is a granodiorite from the Captains pluton 1 mile southwest 

of the head of Captains Bay. The texture is hypidiomorphic-granular to 

porphyritic. Felsic minerals compose 75-85 percent of the rock: 10-15 percent quartz, 13-17 percent orthoclase partly altered to kaolin (2-4 percent), 

and 50-55 percent plagioclase partially altered to sericite (0.5-1.5 percent) 

and clinozoisite (<0.01 percent). The plagioclase composition, by extinction 

angle, is An« [mean of 24 crystals, standard deviation is 14 percent An, aver-
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 629

age amount of zoning per individual plagioclase crystal is 16 percent An per 

crystal (fig. 93, no. 8)], by index oils, is An44. Mafic minerals compose 15-25 

percent: 1 percent pyroxene (all as small inclusions in one large plagioclase 

crystal), 12-15 percent hornblende (50 percent of this is massive and poikilitic 

with magnetite, 50 percent of this is uralitic fibers), 3-5 percent biotite partially altered to penninite (1-2 percent) and ilmenite and sphene (0.5-1.5 

percent) which in turn are altered to leucoxene (<1 percent), 2.4 percent 

magnetite, and <1 percent apatite (in feldspar).

Specimen 31 is a granite, the matrix of a "plum pudding" rock like that of 

figure 95, from the shore bench one-half mile southeast of the tip of Kof 

Point (near fig. 92). The texture is hypidiomorphic, micrographic; the original rock contains many inclusions but this analysis is of the matrix only. 

Felsic minerals compose 75-80 percent of the rock: 5-7 percent quartz occurring in myrmekitic to micrographic intergrowths with much alkalic feldspar, 

45-55 percent alkalic feldspar partly altered to kaolin (3-7 percent), and 

15-25 percent plagioclase partly altered to sericite (2-A percent), kaolin (2-5 

percent), albite (lumped with alkalic feldspar) and epidote (1-2 percent). 

The plagioclase composition, by extinction angle, iis An4i [mean of 10 crystals, 

standard deviation is 16 percent An, average amount of zoning per individual 

plagioclase crystal is 13 percent An per crystal], by index oils, is An37 . 

Mafic minerals compose 20-25 percent: 12-15 percent clinopyroxene 

mostly replaced by actinolitic hornblende (10-13 percent), sphene (< 1 percent), and magnetite « 1 percent), 5-7 percent biotite completely replaced in 

most areas by epidote « 1 percent), penninite (3-4 percent), titanite (0.5-1.5 

percent) and magnetite-ilmenite (0.5-1.5 percent), 2-4 percent magnetiteilmenite, < 1 percent apatite, < 0.1 percent epidote, sphene, penninite, and a 

fibrous amphibole that fills tiny.holes in the groundmass.

The discrepancy in the plagioclase composition between the mode and the 

norm is probably due to the presence of secondary albite in the rock. Also, 

the alkalic feldspar probably contains large quantities of both orthoclase and 

albite molecules.

Specimen 32 is a granite, the matrix of a "plum pudding" rock like that of 

figure 95, from a beach boulder near the head of the north arm of Skan Bay. 

The texture is hypidiomorphic-granular; the original rock contains many inclusions of the type represented by analysis 13 but this analysis is of matrix 

only. Felsic minerals compose 93-97 percent of the rock: 10-20 percent quartz, 

50-70 percent alkalic feldspar heavily altered to kaolin (10-20 percent), and 

15-25 percent plagioclase partly altered to epidote (0.1-0.5 percent), sericite 

(0.1-1 percent), and kaolin (lumped with above). The plagioclase composition, by extinction angle, is Ani8 [mean of 5 crystals, standard deviation is 9 

percent An, average amount of zoning per individual plagioclase crystal is 0 

percent An per crystal]. Mafic minerals compose 3-7' percent: 2-3 percent 

hornblende and 0.1-0.5 percent is biotite both partly altered to epidote (0.1-1 

percent) and penninite (0.1-0.3 percent), 1-1.5 percent is magnetite, 0.1-1 

percent is sphene slightly altered to lucoxene, 0.1-0.5 percent is rutile occurring as 

stubby yellow rods in sphene, and 0.1-0.3 percent is penninite (interstitial).

Comparison of the mode and norm for this rock indicates that the alkalic 

feldspar contains large quantities of both orthoclase and albite.

Specimen 33 is a granodiorite from north of the 3600 Mountain roof pendant, 

midway between Skan Bay and Naginak Cove. The texture is hypidiomorphicgranular. Felsic minerals compose 80-85 percent of the rock: 15-20 percent 

quartz, 10-15 percent orthoclase slightly altered to kaolin (<1 percent), and
630 INVESTIGATIONS OF ALASKAN VOLCANOES

50-55 percent plagioclase partially altered to sericite (1-2 percent). The 

plagioclase composition, by extinction angle, is An39 [mean of 40 crystals, 

standard deviation is 13 percent An, average amount of zoning per individual 

plagiclase crystal is 8 percent An per crystal (fig. 93, no. 4)], by index oils, is 

An4 o. Mafic minerals compose 15-20 percent: 5-8 percent actinolitic hornblende as long as 10 mm, 8-12 percent biotite up to 7 mm in diameter and partially altered to penninite (1-2 percent) and sphene (<1 percent, itself slightly 

altered to leucoxene <0.01 percent), 1-2 percent magnetite, and <0.01 percent 

apatite (in plagioclase).

 Specimen 34 is a leuco albite quartz monzonite from an irregular aplitic body 

at the margin of the 3600 Mountain roof pendant, 0.7 mile southwest from the 

summit of the 3,600-foot peak for which the pendant is named. The texture 

is allotriomorphic-granular, locally subgraphic, locally pseudoperthitic. The 

specimen contains 94-98 percent felsic minerals: 20-24 percent quartz, 25-34 

percent orthoclase slightly altered to kaolin, and 40-49 percent plagio'dase (only 

one-sixth of which is twinned) slightly altered to sericite. The plagioclase 

composition, by extinction angle, is An7 [mean of 12 crystals, standard deviation 

is 3 percent An, average amount of zoning per individual plagioclase crystal 

is 0 percent An per crystal (fig. 93, no. 1)]. There are 2-6 percent mafic minerals of which 2-6 percent is biotite (?) completely altered to magnetite (0.5-1.5 

percent), penninite (2-3 percent), epidote (<0.1 percent), and sphene (0.1-1 

percent), and <0.3 percent is allanite associated with minor epidote.

The discrepancy in the amounts of normative and modal quartz may be due 

to unrecognized fine-grained quartz in the groundmass, or perhaps some of 

the quartz was mistakenly identified with the abundant untwinned plagioclase.

Specimen 35 is a granodiorite from near the head of Captains Bay. No 

hand specimen or thin section is available.

ALTERATION

Almost none of the plutonic rocks studied in thin section contain 

more than 10 percent of mineral alteration products (fig. 89). The 

hornfelses, schists, and gneisses are completely recrystallized but contain virtually no later alteration; they are only rocks of the Unalaska 

formation that are completely recrystallized.

Alteration minerals in the plutonic rocks include: sericite (from 

alteration of plagioclase feldspar, often associated with late orthoclase feldspar, usually present in amounts of 0.5-3 percent); kaolin 

(from alteration of potassic and occasionally plagioclase feldspar, 

amount difficult to estimate but usually less than 5 percent) ; epidoteclinozoisite (epidote is commonly an alteration product of biotite, 

and clinozoisite of plagioclase, in few places present in amounts greater 

than 1 percent); tremolite-actinolite (common after hornblende, rare 

after plagioclase); zeolites (includes analcite, thomsonite, and stilbite, 

present to 10 percent in some of the western Unalaska sills); prehnite 

(associated with zeolites, rare); calcite (present in only one-tenth 

of the rocks as alteration of either felsic or mafic minerals, generally 

1-2 percent where present, a few concentrations greater than 10 percent known); sphene (commonly present as small parallel blebs or
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 631

groups of blebs reflecting former biotite cleavage, usually less than 

1 percent); leucoxene (nearly all specimens present as very minor 

amounts on sphene or ilmenite); hematite (pseudomorphous after 

pyrite or as small flakes in mafic minerals, nearly always less than 

0.5 percent); serpentine (alteration of olivine (?) in sills of western 

Unalaska, present to 5-10 percent in some of these rocks, absent elsewhere) ; vermiculite (present as alteration of biotite in one pegmatite) ; magnetite (rare as alteration product); limonite (rare); rutile 

(occurs as stubby yellow rods in alterated (?) sphene, rare); and 

chlprite (includes bastite after orthopyroxene, fine-grained antigorite 

after hornblende and some biotite, and penninite after biotite and 

some hornblende; bastite rare, antigorite uncommon, penninite commonly present in amounts of 1-4 percent). There is a suggestion 

that the penninitic chlorite possesses different optical properties (abnormal interference colors) in different parts of the Shaler pluton 

in the Skan Bay-Naginak Cove area.

Of the plutonic rocks studied in thin section, 22 percent contain 

dilation veins, though many veins are too thin to be seen in hand specimen. Most of these rocks (86 percent) were collected within 0.5 mile 

of a known plutonic contact, more than one-half were collected within 

0.1 mile of a known contact. Chlorite and orthoclase are the commonest fillings; all other major primary and secondary minerals except 

olivine, calcic plagioclase, biotite, orthopyroxene, zircon, and apatite 

are also represented.

PLUTONIC ROCKS: SUMMARY AND INTERPRETATION

The plutonic rocks of Unalaska Island are the products of crystallization of a granodioritic magma that invaded the rocks of the Unalaska formation by assimilation, stoping, and forceful intrusion, and 

possibly by melting. During late crystallization of at least one of the 

batholiths the core became enriched in alkalies, silica, and rare earths. 

Late solutions developed a uniform kaolin-chlorite-sericite alteration 

throughout the plutons.

The typical plutonic rock is equigranular granodiorite with hypidiomorphic texture. Practically all changes in composition of plutonic 

rocks are gradational. The most mafic though compositionally variable igneous rocks are in the small intrusive bodies or are near the 

borders, especially the roof zones of the large intrusive bodies, and the 

wallrocks are still more mafic and inhomogeneous. Locally, near the 

borders of the largest plutons, porphyroblastic hornfelses and schists 

have been formed from wallrocks. In at least the Shaler pluton, 

broad composition zones (ignoring areas of roof rocks) are concentric 

with the outer walls of the pluton and are increasingly felsic toward 

the center (fig. 98).

580537 O 61 6
632 INVESTIGATIONS OF ALASKAN VOLCANOES

EXPLANATION

Mapped outer contact

Postulated outer contact

20 percent quartz contour

Quartz increases inward

20 percent potassic feldspar contoui

Potassic feldspar increases inward

iiiiiiiiHmraiiiiiimiiimiMitiuiiiiin

Limit of allanite occurrence

Up to I percent allanite occurs

within this contour

Limit of pyroxene occurrence

No pyroxene occurs within this 

contour

10 MILES

FIGURE 98. Concentric mineral zones within Shaler pluton, with roof rocks and plutonic

border phases removed.

The increase in both mafic content and inhomogeneity from batholith center to and including wallrocks indicates that the magma bodies 

have been modified in composition near their borders, especially their 

roofs, by a wallrock complex of heterogeneous volcanic rocks and 

basic sediments. Because of these compositional characteristics, some 

form of assimilation in place or melting of wallrock must be the requisite explanation. Similar conclusions have recently been drawn for 

other Aleutian areas (Fraser and Snyder, 1959, p. 389) and for the 

much larger California batholiths (Hamilton, 1956a, p. 1596-159Y; 

1956b, p. 1795; McCulloh, 1954, p. 21).

A possible mechanism of emplacement for the Unalaska batholiths 

is suggested: Melting of the Unalaska formation or older formations 

begins and proceeds irregularly outward until a large molten mass 

has been formed within the crust of the earth. Motion caused by tectonic stresses or convection currents produces dynamically metamorphosed wallrock in a few places, results in small apophyses of basic 

border rocks intruding upward into the Unalaska formation, and 

orients stubby plagioclase crystals. Locally the more basic, more 

viscous border phases of the pluton are displaced by the more homo-
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 633

geneous, less viscous central phases of granodiorite magma and very 

locally sharp internal contacts are produced. The final position of 

the batholith may be slightly above the position where it was originally formed or may be very far above this position, depending upon 

the quantity and duration of the available supply of heat and upon 

the relative efficacy of the stoping, fluxing, and forceful intrusive 

mechanisms. As the heat supply dwindles, crystallization begins at 

the outer walls and proceeds irregularly inward until the mass is 

completely solidified, a process possibly lasting more than a million 

years (Lovering, 1955, p. 271, 277). If upward migration of the molten magma bodies was significant, it is possible that the many finegrained sills and lava flows in the Unalaska formation were derived 

from this rising series of batholiths. Later in their history the plutons would have engulfed and altered their own earlier extrusive 

products.

Other causes of diverse plutonic compositional suites are not applicable here. Successive intrusion of different plutons of decreasing basicity is ruled out, first by the extreme coincidence required to 

explain the compositional progression into the wallrock, and second 

by the gradational nature of the great majority of the contacts between'the different igneous rock types. Separation of immiscible 

fractions from a single parent magma body is refuted for the. same 

reasons. The Unalaska situation is the reverse of that normally 

prescribed for crystal differentiation. The basic border rocks are 

largely at the roof of the pluton, opposite to what would be expected 

if gravitative settling had played a large part in their disposition. 

Filter pressing of a partly crystalline mass does not satisfactorily 

explain the inhomogeneity of the border areas.

The late history of the Shaler pluton is partly revealed by the concentric mineral zones within it beneath the roof areas (fig. 98). All 

four of these zones can be shown texturally to have developed late 

in the history of the rock. The limit of pyroxene occurrence represents the point at which all the pyroxene has been converted to amphibole or where amphibole has crystallized instead of pyroxene 

events late in the reaction series. Quartz, potassic feldspar, and allanite show by their typical shapes in thin section (p. 615-616) that 

they formed last in the history of the rock and their concentration in 

the central area suggests that this area was subjected longest to crystallization processes. All of these mineral zones may be the result of 

late crystallization of uncontaminated central portions of the magma, 

but at least part of them probably were formed from hydrothermal 

solutions which utilized the core of the crystallizing magma column 

as a flue until it was completely solid. If so, the original magma 

may have been a granodiorite lower in quartz and potassic feldspar
634 INVESTIGATIONS OF ALASKAN VOLCANOES

than the rock exposed in this central zone. Late solutions rich in 

alkalies and silica probably congregated at many local points beneath the roof to give rise to the contact "plum pudding" rocks and 

many irregular aplites, A similar interpretation has previously 

been advocated for the Adak batholith (Fraser and Snyder, 1959, 

p. 389, 390). During this late hydrothermal period a uniform kaolinchlorite-sericite alteration was developed throughout the plutons, and 

small joints in the more brittle rocks near the contacts were filled with 

all the late minerals. The hornfels zone, with its complete lack of 

hydrothermal alteration, apparently was impervious to, or too distant 

from, the late alteration solutions. Indirectly, this lack of alteration in the hornfels zone points to the regional alteration of the Unalaska formation as a prebatholith phenomenon or a phenomenon 

related to the early stage of batholith emplacement.

MAKTJSHIN VOLCANICS

A thick pile of unaltered and little-deformed lava and pyroclastic 

rocks overlies the Unalaska formation and granodiorite plutons with 

slight angular unconformity and forms Makushin Volcano, a broad 

volcanic dome more than 6,000 feet high and 10 miles wide (figs. 99 

and 100).

These rocks, which consist of basalt and subordinate andesite, are 

here designated the Makushin volcanics; they are now restricted to 

the northern part of the northern bulge of the island, though they 

probably once extended farther south. Locally they are unconformably capped by basaltic flows and pyroclastic rocks that retain their 

constructional forms. The thickness of the Makushin volcanics is 

highly varied but probably does not exceed several thousand feet. 

The formation is of Pliocene(?) and Pleistocene age. The bulk of 

the rocks is probably late Pleistocene. The rocks of the formation 

are typically exposed in the sea cliffs and ridges near Reese Bay.

UNCONFORMITY BENEATH THE MAKUSHIN VOLCANICS

The lower surface of the volcanic pile is irregular, with marked 

local relief in many places, and a general northward slope so that the 

pile caps ridges on the south and reaches below sea level on the north. 

The basal unconformity is marked by structural discordance, by the 

upward limit of strong alteration, by the truncation of many dikes, 

and by a discontinuous basal conglomerate locally containing cobbles 

of granodiorite and rocks of the Unalaska formation. A deep zone of 

intense oxidation and clay formation in the older rocks is particularly 

prominent on ridges adjacent to Glacier Valley and in places beneath 

the Makushin volcanics, and resembles the remnants of an exhumed 

weathered surface.
GEOLOGY OF TJNALASKA ISLAND AND ADJACENT INSULAR SHELF 635

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INVESTIGATIONS OF ALASKAN VOLCANOES
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 637

The configuration of the unconformity beneath the Makushin volcanics is shown on the structure contour map, figure 101. The contours 

are most accurately located near this unconformity; minimum elevations are shown beyond the limits of the Makushin volcanics, and 

maximum elevations in areas covered by that formation. As a result 

of these limitations, the reconstructed relief of the unconformity is 

subdued, and the results are wholly qualitative rather than quantitative ; yet this relief is similar to that of the present land in this area.

The major ridges and valleys before deposition of the Makushin 

volcanics were approximately in the position of similar topographic 

features today. Driftwood and Glacier Valleys in particular are 

reflected in the drainage on the unconformity. A high area on that 

surface underlying Makushin Volcano may represent an original, 

drainage divide on the old land surface, or it may be the result of 

later volcanic doming of the unconformity. Using the present stand 

of the sea as a datum, one can say that the extrusion of Makushin 

volcanics has built the island several miles northward and westward.

The Makushin volcanics consist of about four-fifths basalt and 

andesite lava flows and one-fifth intercalated agglomerate, tuff breccia, 

and flow breccia. Flows 10 to 50 feet thick form small steplike cliffs 

on steep slopes, alternating with benches underlain by the fragmental 

rocks. In most places platy fractures are parallel to the flow structures, emphasizing the gentle dips, but locally these structures are 

erratic. The flow rocks are grayish purple to brownish gray, and the 

more fragmented and vesicular rocks are pale red or reddish gray. 

Thin ash beds and flow breccia dominate the fragmental deposits. 

Thicker pyroclastic and mixed pyroclastic and epiclastic units are 

prominent in several areas; these might serve as key beds in a detailed 

study. Most of the pyroclastic units are massive and unsorted; a few 

are well sorted and well bedded, even crossbedded or graded bedded, 

and contain rounded debris, induing scattered volcanic bombs and 

lapilli.

Dips of flows radiate from Makushin Volcano and suggest this site 

as the principal vent area. The summit is an ice-filled caldera almost 

2 miles in diameter, which is probably the remnant of one of the 

youngest vents of the Makushin volcanics. The roots of two vents and 

associated intrusive bodies in the Unalaska Bay area are described 

separately. A few dikes and, more rarely, sills intrude this basalt and 

andesite sequence. An unusually thick sill-like body is exposed along 

the coast northwest of Mount Marshal Reese.

PETROGRAPHY OF THE MAKUSHIN VOLCANICS

The Makushin volcanics are broadly uniform in texture and composition except in the few places where xenocrysts or xenoliths ap-
638 INVESTIGATIONS OF ALASKAN VOLCANOES
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 639

pear. Representative rocks contain between 10 and 30 percent phenocrysts, largely plagioclase but including augitic pyroxene, hypersthene, and olivine, and a few rocks contain xenocrysts or xenoliths. 

Felty or flow textures characterize the groundmass, the plagioclase is 

typically in long laths but less commonly stubby, with glassy or finely 

granular interstitial material. Augitic pyroxene, hereafter called 

augite although it may include diopside, commonly encloses hypersthene and less commonly encloses olivine in typical reaction-rim 

relationship.

The essential minerals are plagioclase, generally labradorite (most 

common), augite (abundant), hypersthene (scarce), and olivine 

(generally scarce, rarely abundant). Accessory minerals are magnetite, apatite, zircon, and, very rarely, amphibole and cristobalite. 

Secondary minerals are largely pervasive hematite and limonite that 

obscure the groundmass of most of the fragmental and highly vesicular rocks. A few uncontaminated basalts also contain a little chlorite 

and zeolite.

A few of the smaller intrusive bodies associated with the Makushin volcanics have anomalous textures and compositions. The basalt 

intrusive complex 2 miles west of Nateekin Bay (pi. 75) contains 

cigar-shaped augite crystals with the e-axes normal to the direction 

of crystal elongation, which suggests strong resorption of xenocrysts. 

Contamination of the magma is possible, but the resorbed augite 

crystals are not likely to have been derived from the Unalaska formation host rock; rather they may have come from an earlier crystallized phase of Makushin volcanics.

In the cirque immediately south of this complex and on the spur a 

mile and a half southwest of Pakushin Cone there are small intrusive 

bodies of dacite and andesite. Amphibole and markedly embayed 

quartz and biotite crystals are present, and chlorite is abundant. Some 

of the large plagioclase crystals are calcic oligoclase and are mottled; 

the groundmass plagioclase, however, is clear sodic labradorite. The 

ubiquitous pyroxene and olivine of the basalt and andesite of the 

Makushin volcanics are absent. Presence of a basaltic magma contaminated with granodiorite could indicate the origin of these bodies, 

for the amphibole, quartz, biotite, and oligoclase are all common only 

in the plutons. Two other small intrusive bodies one a mile northwest of Pakushin Cone and the other a mile northwest of Wide Bay 

Cone are noteworthy for their relatively coarse texture; they are 

best described as fine-grained diorites, younger than the major plutonic masses elsewhere on the island.
640 INVESTIGATIONS OF ALASKAN VOLCANOES 

SPECIFIC DATA FOR ANALYZFJ) ROCKS

The seven analyzed rock specimens of the Makushin volcanics are 

. described below. The analyses show that, as a group, the Makushin 

volcanics have silica and alkali contents between those of the rocks 

of the Unalaska formation and those of the JEider Point basalt; however, there is considerable overlap in them. Chemical and spectrographic 'analyses appear on table 1 and additional data appear on 

the silica variation diagram, figure 88.

Specimen 5 is feldspathic basalt porphyry from the Makushin volcanics 13,4 

miles west of Wide Bay Cone. It is light gray to medium light gray and 

contains pyroxene phenocrysts one-third millimeter long. Phenocrysts compose 

5 percent of the rock and include augite or diopside, which forms euhedral to 

anhedral crystals and crystal clusters. The groundmass has a felty to coarsely 

crystalline texture and includes abundant subhedral laths of sodic to calcic 

labradorite with normal zones and faint oscillatory zones, much granular and 

anhedral augite(?), and subhedral and anhedral olivine and magnetite.

Specimen 6 is olivine basalt porphyry from the Makushin volcanics 4*4 miles 

south of Tabletop Mountain and 4 miles east of Sugarloaf Cone. It is medium 

dark gray to grayish purple and contains plagioclase, pyroxene, and olivine 

phenocrysts 1-4 millimeters long. Phenocrysts compose 30 percent of the rock: 

15 percent labradorite(?), which forms euhedral laths to anhedral fragments, 

and stubby crystals, and which contains oscillatory and normal zones, and 

zones of dusty inclusions; 10 percent augite or diopside, which form faintly 

zoned euhedral grains and fragments; 5 percent subhedral to anhedral olivine. 

The groundmass has a granular to flow texture and consists of abundant plagioclase, much clinopyroxene and a black opaque mineral, and traces of olivine(?) 

and cristobalite. Alteration is sparse; chlorite(?) replaces olivine.

Specimen 8 is olivine-bearing feldspathic basalt porphyry from the Makushin 

volcanics 3^4 miles north-northeast of Sugarloaf Cone. It is dark gray to grayish 

black and contains plagioclase and pyroxene phenocrysts 1-3 millimeters long. 

Phenocrysts compose 25 percent of the rock: 15 percent sodic bytownite to sodic 

labradorite, which forms stubby euhedral crystals and fragments having faint 

oscillatory zones, narrow rims of normal zones, vermicular zones, zones of 

dusty inclusions, and apatite inclusions; 5 percent euhedral to subhedral 

augite (?) ; 4 percent subhedral olivine with pseudomorphs of serpentine(?) ; 1 

percent hypersthene with augite reaction rims; and a trace of anhedral magnetite. The groundmass has intergranular texture and includes abundant subhedral laths and stubby crystals of sodic bytownite to sodic labradorite, and 

much granular augite (?) and a black opaque mineral. Alteration is sparse; 

3 percent of the rock is chlorite(?), which replaces olivine.

Specimen 10 is olivine-bearing feldspathic basalt porphyry from Makushin 

volcanics 1 mile northeast of Pakushin Cone. It is pale purple to grayish 

purple and contains plagioclase, pyroxene, and olivine phenocrysts 1^ millimeters long. Phenocrysts compose 25 percent of the rock: 15 percent sodic 

labradorite, which forms euhedral stubby crystals and fragments, and which 

contains faint oscillatory and normal zones, zones of dusty inclusions, and 

apatite inclusions; 6 percent euhedral to subhedral augite (?) ; 2 percent hypersthene with thin augite (?) reaction rims; 2 percent euhedral to anhedral 

olivine; and 1 percent subhedral magnetite(?), with some myrmekitic intergrowth in augite(?). The groundmass has a felty to intergranular texture
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 641

and consists of sodic labradorite, abundant subhedral laths and stubby crystals of much granular augite(?), a black opaque mineral, and a trace of granular olivine.

Specimen 11 is olivine-bearing basalt porphyry from Makushin basalt and 

andesite 21/£ miles southwest of Pakushin Cone. It is pale purple to medium 

light gray and contains plagioclase, pyroxene, and olivine phenocrysts 1-2 millimeters long. Phenocrysts compose 15 percent of the rock: 9 percent labradorite, 

which forms euhedral to subhedral stubby crystals having normal and oscillatory 

zones; 2 percent subhedral augite( ?) ; 3 percent anhedral olivine with augite( ?) 

reaction rims; 1 percent subhedral to anhedral and embayed magnetite (?). The 

groundmass has crudely trachytic to intergranular texture and includes abundant 

subhedral laths and stubby crystals of calcic labradorite, much granular 

augite(?) ; a trace of a granular opaque mineral, and a trace of cristobalite(?). 

Alteration is slight; olivine and magnetite (?) alter to hematite and limonite.

Specimen 16 is olivine-bearing andesite porphyry from the Makushin volcanics 

one-half mile south of the vent to Makushin Volcano. It is dark gray and vuggy 

and contains plagioclase and pyroxene phenocrysts 2-3 millimeters long. Phenocrysts compose 25 percent of the rock: 14 percent sodic labradorite, which forms 

euhedral to anhedral grains, fragments, and stubby crystals, and which has 

normal and oscillatory zones, zones of dusty inclusions, and apatite inclusions; 

5 percent euhedral crystals and fragments of augite(?) ; 2 percent subhedral 

to anhedral hypersthene with augite(?) reaction rims; 3 percent anhedral 

olivine; and 1 percent subhedral to anhedral magnetite. The groundmass has 

an intergranular to obscured texture and includes abundant laths and stubby 

crystals of calcic andesine, much glass and granular augite(?), and traces of 

hypersthene, olivine, and a granular black opaque mineral.

Specimen 18 is andesite porphyry from the Makushin volcanics 3 miles southeast of Sugarloaf Cone. It is medium gray to purplish gray and contains 

plagioclase and pyroxene phenocrysts 1-4 millimeters long. Phenocrysts compose 20 percent of the rock: 12 percent andesine to bytownite, which forms 

euhedral laths and fragments, and which has normal and oscillatory zones, zones 

of dusty inclusions, and apatite inclusions; 5 percent augite(?), which forms 

euhedral to anhedral grains, crystal clusters, and reaction rims around hypersthene ; 2 percent subhedral hypersthene with reaction rims; a trace of anhedral 

olivine(?) ; a trace of subhedral to anhedral magnetite; and a trace of euhedral 

apatite. The groundmass has a felty to flow texture and includes abundant subhedral andesine laths, much augite(?), and a trace of a granular black opaque 

mineral.

AGE OF THE MAKUSHIN VODCANICS

Most of the Makushin volcanics are believed to be middle to late 

Pleistocene in age, if the method of dating the insular shelf, presented 

below, is accepted. If the dating of the insular shelf proves unsatisfactory, then the lower age limit of the Makushin volcanics is set by 

the late Miocene or Pliocene batholith which intruded the Unalaska 

formation. The basalt and andesite is extensively glaciated and must 

precede at least part of late Pleistocene time.

EIDER POINT BASALT

Numerous small volcanic cones, associated volcanic mudflows, and 

lava flows that largely retain details of their constructional forms
642 INVESTIGATIONS OF ALASKAN VOLCANOES

are scattered about the northern bulge of Unalaska Island. One of 

these cones is still active. These young rocks lie unconformably on 

the deeply dissected surfaces of Makushin volcanics or older rocks. 

The rocks from these vents are not contemporaneous, but a chronology 

is not available. These rocks are here named the Eider Point basalt, 

for the rocks of Wide Bay Cone above Eider Point are typical of the 

unit.

Rocks of the Eider Point basalt resemble those of the Makushin 

volcanics; flows of both units are grayish purple to brownish gray and 

relatively thin, but the younger flows are less continuous. The constructional forms and, in places, conspicuous basal unconformities are 

the chief distinguishing features of the Eider Point basalt. Pyroclastic deposits, usually separate cones, are far more conspicuous in 

the younger rocks than in the Makushin volcanics. Phenocrysts are 

ubiquitous and compose on an average 20 percent of the volume. 

Felty or flow textures are most common, and vuggy and vesicular 

structures abundant. Sodic bytownite or labradorite plagioclase, 

augite, olivine, and in some rocks hypersthene are the essential minerals ; magnetite and apatite are the chief accessories.

MAKUSHIN VOLCANO; FUMAROLES AND HOT SPRINGS

A small cinder cone and remnants of other probable cones project 

through the ice field within the summit caldera of Makushin Volcano 

(fig. 99). Scoriaceous basalt from this cone is typical of the Eider 

Point basalt. Photographs of the summit of Makushin Volcano 

showing Makushin cone and fumaroles are presented by Freiday 

(1945, p. 448^149).

Scattered about the main vent are several fumaroles and much 

sulfur and sulfur-impregnated clay. This sulfur is discussed under 

the heading "Economic geology." Gases from the vent area are 

largely water vapor; some H2S and probably SO2 are emitted. Occasional minor amounts of ash accompany the gases and discolor the 

snow about the vent, which has been mildly active since 1826 (Coats, 

1950, table 2).

A large fumarole area is active at the head of Glacier Valley about 

3 miles southeast of the summit vent, and a smaller fumarole area lies 

2 miles south of the summit. Hot springs are found in these areas, 

and also on the east side of the valley bottom 1% miles from the 

shore of Summer Bay, some 20 miles from Makushin Volcano.

PAKTJSHIN CONE

A basalt composite cone lies midway between Makushin Volcano 

and Makushin Bay. The base of this cone has an elevation of about 

2,000 feet, and its multiple-cratered summit reaches an elevation of
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 643

3,395 feet. Ash and cinders from this cone are draped over bordering knobs of older rocks and fill adjacent valleys. Low on the northeastern flank some of the lava flows from Pakushin Cone are slightly 

glaciated, probably in Recent time during a period of expansion of the 

icecap. The vitric groundmass contains an unusually large percentage of phenocrysts, of which the plagioclase phenocrysts are nearly all 

equidimensional. Olivine exceeds 15 percent of the rock volume.

POINT KADIN VENTS

Ten small cones and explosion craters lie on a rift zone trending 

N. 75° W. northwest of Makushin Volcano (fig. 99). Several of 

these consist of multiple craters; others form elongate fissures. Remnants of the original rim of the largest of these craters are marked 

by alined ledges of agglutinated lava. Fragments of Makushin volcanics, in this area lighter in color and conspicuously porphyritic, 

appear among the ropy and scoriaceous ejecta on the low cones.

North of this line of vents as far as Koriga Point the flows, agglomerate, and most of the ejecta are dark-gray, vitreous to cryptocrystalline olivine-free rocks. All of these flows have well-developed 

flow structures and some contain inclusions with porphyritic feldspar. 

The composition of these rocks cannot be estimated from petrographic 

study, but a chemical analysis of one vitrophyre, analysis 21, shows 

that the specimen is andesite with some affinities toward dacite.

The very fresh constructional forms of these vents and the lack of 

glacial erosion of those away from the present ice limit indicate that 

they are very young features and certainly postglacial. This age is 

verified by comparison of the offshore profile here, which has only a 

suggestion of a submarine bench, with that of Makushin Bay, which 

has a wider postglacial submarine bench. They need be no older 

than a few thousand years.

BISHOP POINT VOLCANIC MUDPLOW

The valley south of Bishop Point contains a terrace about 100 feet 

above the dissecting streams, the surface of which slopes seaward unscarred by streams. The agglomerate forming this terrace is massive 

and internally structureless, contains abundant cinders, and overlies 

the basalt and andesite flows of the Makushin volcanics. It is probably a volcanic mudflow formed at a relatively recent time. The 

mantle of soil and volcanic ash and cinders contain only the upper 

two of the three distinctive ash-cinder layers characteristic of this 

area, and it is conceivable that the mudflow was contemporaneous with 

the lowest of the three layers. A heavy ash-cinder fall blanketing the 

icefields at the head of the valley south of Bishop Point could readily
644 INVESTIGATIONS OF ALASKAN VOLCANOES

have become mobilized and could have incorporated much glacial till 

enroute to the lower part of the valley.

SUGARLOAF CONE AND LAVA FLOWS

The upper reaches of Driftwood and Makushin Valleys contain unaltered lava flows and pyroclastics. Along the .drainage divide between these valleys is a small cinder cone called Sugarloaf Cone by 

Jaggar (1908, p. 11). The flows, which are older than the cinder cone, 

dip away from this divide, which may be their vent area.

Sugarloaf Cone consists of steeply outward dipping, crudely bedded 

pyroclastics unconformably capped by well-bedded, gently inward 

dipping clastic beds. This unconformable cap suggests the remnant 

of a crater fill with well-reworked pyroclastics, which may have been 

deposited in part in a pond. The remnant of the cone also shows that 

a considerable outer portion of the cinder cone has been removed without destroying its conical form, for the crater rim that contained the 

pond is missing.

The composition of the vuggy flows and pyroclastics associated with 

Sugarloaf Cone is characteristic of the younger basalt except that' s 

olivine, hypersthene, and augite form reaction rims in almost every 

combination about each other. Keaction rims in accord with Bowen's 

reaction series as well as those abnormal to it occur in the same thin 

section. One clinopyroxene grain is surrounded by grains of olivine, 

and outside of these is another thin rim of clinopyroxene. Several 

varieties of plagioclase phenocrysts occur in the same thin section, 

some clear and others with clouded cores or zones.

Prior to extrusion of the flows around Sugarloaf Cone the upper 

basin of Makushin Valley probably drained into Driftwood Valley, 

for the unconformity at the base of these flows conforms to that valley. 

This old drainage also explains the disproportionate width of upper 

Driftwood Valley in relation to its present drainage area. These 

lavas were overriden by ice they bear glacial striae and roches 

moutonees during a late glacial advance, after which Sugarloaf Cone 

was formed.

CONE AND LAVA FLOWS OF TABLETOP MOUNTAIN

Tabletop Mountain is a pyroclastic cone surrounded by flows originating from small flank domes, and like Sugarloaf Cone is capped by 

well-bedded pyroclastics and reworked poorly indurated sandstones 

dipping gently inward. The truncated outer edge of this crater fill 

and the several dikes projecting from the slopes of the cone indicate 

that it is considerably eroded. Coalescent summit depressions may 

indicate volcanic subsidence at the end of its explosive history. The 

flows are vuggy and contain a little more olivine and augite than the
645

average Eider Point basalt. These flows have also been slightly glaciated and presumably are of late Pleistocene age.

WIDE BAY CONE

A small, symmetrical composite cone overlies rocks of the Unalaska 

formation and Makushin volcanics north of Wide Bay. Its summit 

crater is essentially unfilled and the outer slopes of the cone are largely 

intact, for individual flows may be traced from the crater edge to the 

base of the cone. The flows west of the cone cover the floor of a cirque, 

but their toes are slightly abraded by glacier movement. The flows 

resemble those of Tabletop Mountain: they are vuggy and contain 

prominent olivine and augite phenocrysts, which are more abundant 

than in the average Eider Point basalt.

OTHER POSSIBLY YOUNG FLOWS

On the south wall of Nateekin Valley, 2*£ miles from the head of 

the bay, is a possibly young basalt flow with what appear to be push 

moraines and lateral levees. Although containing chlorite and uralite, 

it is much less altered than the adjacent rocks of the Unalaska formation. The flow contains aplitic inclusions, rich in quartz and biotite, 

and contains amphibolite xenocrysts. Possibly the amphibole and 

even the uralite and chlorite are derived from inclusions. Contamination of the basaltic magma by quartz-rich granodiorite, such as occurs 

at nearby Captains Bay, ma.y explain these xenoliths and xenocrysts.

On the bluff above the mouth of McLees Lake is a small dome and 

lava tongue that suggests a young flow. In the possible vent area 

scoriaceous slabs dip outward. The rock is petrographically similar 

to the adjacent Makushin volcanics.

PETROGRAPHY OF THE EIDHR POINT BASALT

SPECIFIC DATA FOR ANALYZED ROCKS

Chemical and spectrographic analyses of five rocks of the Eider 

Point basalt appear on table 1.

Specimen 3 is olivine basalt porphyry from a flow of Eider Point basalt 1 mile 

northeast of Tabletop Mountain. It is grayish red purple to medium gray and 

contains plagioclase phenocrysts as long as 1 mm. Phenocrysts compose 20 percent of the rock: 10 percent bytownite, which forms euhedral laths, stubby 

crystals, and fragments, and which has normal zones, apatite inclusions, and 

rare vermicular inclusions; 5 percent augite(?), which forms euhedral to subhedhral crystals and crystal clusters, and which has apatite inclusions; 5 percent 

subhedral to anhedral arid embayed olivine. The groundmass has an intergranular texture, with stubby crystals and 25 percent vugs, and consists of 

abundant euhedral to subhedral plagioclase, much subhedral to granular 

augite(?) and a black opaque mineral, and a trace of olivine. Alteration is 

sparse; olivine and the black opaque mineral alter to hematite and limonite.

Specimen 4 is olivine basalt porphyry from an Eider Point basalt flow threefourths mile southwest of Wide Bay Cone. It is medium gray and vuggy, and
646 INVESTIGATIONS OF ALASKAN VOLCANOES

contains pyroxene and olivine phenocrysts 1-4 millimeters long. Phenocrysts 

compose 20 percent of the rock : 10 percent sodic bytownite, which forms euhedral 

laths, stubby crystals, and fragments, and which has normal zones, rare oscillatory zones, zones of dusty inclusions, and apatite inclusions; 5 percent augite(?), 

which forms euhedral to anhedral grains and embayed crystals, and which is 

faintly zoned; and 5 percent olivine, which forms subhedral to anhedral crystals 

and embayed crystals. The groundmass has an intergranular texture and consists of abundant stubby subhedral calcic andesine, much granular clinopyroxene 

and a black opaque mineral, and traces of olivine and tridymite.

Specimen 7 is olivine-bearing feldspathic basalt from the Eider Point basalt 

1 mile southwest of Pakushin Cone. It is medium dark gray and contains plagioclase phenocrysts shorter than 1 mm. Phenocrysts compose 50 percent of the 

rock: 35 percent sodic labradorite, which forms subhedral crystals and stubby 

crystals, and which has normal zones, rare oscillatory zones, and some vermicular zones of intergrown groundmass; 15 percent olivine, which forms euhedral 

to subhedral crystals and embayed crystals. The groundmass has a granular 

and glassy texture and consists of much euhedral stubby calcic andesine, much 

subhedral to anhedral olivine, much euhedral to granular augite( ?), and a trace 

of cristobalite(?).

Specimen 12 is olivine-bearing andesite porphyry from a flow in the Eider 

Point basalt % mile southeast of Sugarloaf Cone. It is medium gray and 

contains plagioclase, olivine, and pyroxene phenocrysts 1-3 millimeters long. 

Phenocrysts compose 30 percent of the rock: 15 percent calcic andesine to sodic 

labradorite, which forms embayed crystals, euhedral laths, and fragments, and 

which has normal and oscillatory zones, zones of dusty inclusions, and apatite 

inclusions; 10 percent euhedral to anhedral augite(?), of which one crystal 

has a reaction rim of granular olivine, one crystal has an inner rim of granular 

olivine and an outer rim of clinopyroxene, and one crystal has a clinopyroxene 

rim; 3 percent subhedral hypersthene, which has augite(?) reaction rims, 

and one crystal of which has an olivine (?) and myrmekitic magnetite (?) rim; 

and 2 percent anhedral olivine with clinopyroxene reaction rims. The groundmass has a felty to granular texture and consists of abundant subhedral labradorite, much clinopyroxene and granular olivine(?), and traces of a granular 

black opaque mineral and cristobalite.

Specimen 21 is andesite vitrophyre from a flow of Eider Point basalt 4 miles 

northwest of Makushin Volcano. It is grayish black and contains plagioclase 

phenocrysts as large as 1 millimeter in sheeted and finely vesicular glass. 

Phenocrysts compose 1 percent of the rock and include euhedral to fragmental 

labradorite and traces of clinopyroxene and magnetite. The glassy groundmass has alined microlites, dusty inclusions, and strong flow bands of light 

and dark streaks, and contains some euhedral cristobalite on vesicule walls.

SURFICIAL DEPOSITS AND GEOMORPHOLOGY

GLACIAL LANDFORMS AND DEPOSITS

Landforms showing glacial erosion are very prominent on 

Unalaska Island. In the mountainous parts of the island there are 

U-shaped valleys and passes, cirques and aretes, and ice-scoured features of every size. Benches that slope gently seaward are perched 

at various elevations on the walls of many of the glaciated valleys; 

these may be zones of structural weakness accentuated by glacial
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 647

scour. The benches along the east wall of Anderson Bay disregard 

the observable structure; they have been ascribed by Jaggar (1908, 

p. 32, 33, pi. facing p. 35; written communication, 1927) to marine 

erosion but are probably best explained by glacial scour. Many cirques 

hang above the main valley floors; some contain rock-basin lakes, 

and some high valleys contain several levels of cirques, which are 

separated steplike by steep walls. The erosional forms of this glaciated topography are little altered by postglacial erosion and probably reflect a late stage of Pleistocene glaciation.

Small glaciers fill the basins in the high peaks of the Shaler Mountains; an ice field of 15 square miles caps Makushin Volcano. The 

glacier covering y2 square mile, 3 miles, 3 miles south of Naginak 

Cove of Anderson Bay descends to about 2,000 feet. In the valley 

west of Makushin Volcano a glacier reaches (1954) as low as 700 

feet; it is till veneered below 1,200 feet and bare ice above. 

Smaller glacial tongues northwest of the volcano showed bare ice at 

about 3,000 feet in 1954, with buried ice below 2,000 feet. Crevassed 

surfaces are common to most of the tongues about the ice cap and to 

the other small glaciers. Semipermanent snow fields are common 

above 2,500 feet.

The snowline during maximum Pleistocene ice advance is estimated 

to have been at least as low as 1,200 feet in elevation; possibly it was 

as low as 400 feet. On the rugged part of the island, few cirques occur 

below 1,200 feet, but on the extreme eastern and western ends of the 

island, where the topography is more subdued, poorly developed cirques are found as low as 400 feet. Small cirquelike features at 500 

feet on the north side of the island may owe their form to artesian 

sapping in flat-lying young volcanics. If one considers the lowest 

cirque floors as indicating a Pleistocene snowline (Sharp, 1938, p. 

312), the relations indicate a minimum elevation of that snowline of 

between 400 and 1,200 feet. A similar estimate was made by J. P. 

Schaefer (written communication, 1955) for Attu Island.

The present snowline lies near 3,000 or 4,000 feet, the estimate varying with allowances for past climate and annual variations in weather. 

The photographs of the ice cap on Makushin Volcano taken by Freiday (1945, p. 448 and 450) in September 1944, a particularly warm 

summer, provide the best basis for such an estimate and show the upper limit of abundant bare ice to be about 3,000 feet. Where abundant 

ice patches appear through the neve, ablation exceeds accumulation, 

and the upper limit of this defines the snowline. During still warmer 

summers, bare ice may appear as much as 1,000 feet higher. A rise of 

the snowline since the maximum Pleistocene glaciation of 2,000 to 

3,000 feet is indicated.
648 INVESTIGATIONS OF ALASKAN VOLCANOES

At the lower ends of the glaciers northwest of Makushin Volcano, 

two minor moraines reflect recent fluctuations of the ice position. In 

the valley south-southwest of Koriga Point, for example, the highest 

and youngest of these moraines forms an unbroken, lobate ridge of 

coarse rubble, 5 to 10 feet high. No vegetation grows on this moraine, 

and ice patches appear nearby beneath the till mantle. A few hundred yards farther down the valleys, a second, more subdued moraine 

of youthful appearance parallels the first, and is covered sparsely by 

mosses and flowering plants. Both moraines rest on the Point Kadin 

flow, which follows this valley. Above the lateral moraines associated 

with these terminal moraines there is more till, which extends farther 

down the valley. The extremely fresh appearance of these moraines 

and their close proximity to existing glaciers indicates that they represent small advances or pauses during recession, perhaps a few hundred 

years old. Comparable fluctuations of glacier margins are recorded 

on Umnak (Byers and others, 1947, p. 35, 38), Great Sitkin (Simons 

and Mathewson, 1947, p. 65, pi. 4; 1955, p. 33, pi. 5), and Attu 

(Schaefer, written communication, 1955).

Many Cirque bottoms contain irregular mounds of till consisting 

of coarse blocky debris without appreciable interstitial fine material. 

This young glacial debris is perhaps another product of Recent glaciation. Other till, particularly in the lower cirques, probably dates 

back to the latest Wisconsin ice advance. A thin veneer of ground 

moraine covers parts of the island but is absent in the rugged interior. Till is well exposed in road and building cuts in the vicinity 

of Unalaska village and the military bases, is stony, has a matrix of 

clay and silt, and is plastered against depressions in the bedrock surface. Elsewhere exposures of such till are few. The reconnaissance 

nature of the mapping prohibited the general mapping of the till, 

but most of the till was deposited below sea level.

STBEAM EROSION AND ALLUVIUM

Postglacial stream erosion has produced abundant steep-walled, 

narrow-bottomed ravines and gorges. The most characteristic feature of the large valleys, exclusive of those draining the large existing glaciers, is their steplike long profile, broken by rock spurs. Some 

rock-basin lakes have been alluviated. Alluviation is conspicuous and 

erosion of bedrock valley bottoms less important in valleys below 

existing large glaciers.

Alluvial fans deposited by tributaries on the bottoms of the main 

valleys are conspicuous, especially where the main streams are not 

burdened with outwash. Alluvial deposits grade upward into colluvium, talus debris, and, in places, till. Downstream, alluvium
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 649

grades into beach deposits and, in some valleys, into lagoonal or 

deltaic deposits.

The rivers draining Makushin Volcano are burdened with glacial 

outwash; their valleys are wide and flat bottomed, with markedly 

braided channels and distributaries where outwash is being deposited. 

Vegetation is scarce and young in areas of outwash, and the frequently shifting channels keep resurfacing the valley floor. In the 

lower half of the wide part of Makushin Valley a thick sod cover and 

tall over 6 feet willows are common; distributaries are few and 

the channel not braided. Willows grow on the streamside levees, but 

the flats away from the channel are grass covered and swampy. Overbank floods add silt and mud to these swamps, but floods affect the 

channel position more rarely in the lower part of the valley than 

in the outwash-choked segment. The upper reaches of many of the 

larger valleys are flanked by terrace remnants of outwash gravels. 

Till is exposed at the base of such an alluvium-capped terrace at the 

head of the broad-valley segment of Makushin Valley.

COLLUVIUM AND EOLIAN DEPOSITS

Surficial deposits in the alluvial or till-filled valleys consist of an 

assortment of material derived largely from mechanical disintegration of rock, with admixtures of ground moraine and layers of ash, 

lapilli, and cinders. Ash probably is intercalated in soils of every 

valley on the island; Recent pyroclastics are most conspicuous in the 

northern bulge of the island and to a lesser extent on the extreme 

western end of the island. This distribution clearly reflects the proximity of a center of explosive volcanic activity on Unalaska Island 

and one on eastern Umnak Island. Kellogg and Nygard (1951, p. 

45-46, map) describe one soil section at Dutch Harbor as consisting 

of dominantly reddish-brown and yellowish-brown silt loam and 

loam. They classify the soils as tundra and bog soils without permafrost at lower elevations, and as lithosols on rock and above about 

1,000 feet.

Creeping and sliding mantle is extensive because of abundant moisture and.steep slopes. Solifluction terraces and, in flat areas with 

little vegetation, stone stripes are widespread indicators of gradual 

downhill movement of the mantle. More rapid movement is indicated by the turf-covered lobate mudflows that in places exceed 1,000 

feet in width. Flows and landslide scars are particularly common 

on glacially steepened valley walls. Mounds of landslide debris and 

ponds filling depressions produced by impact of landslides on boggy 

ground occur along the edges of some of the alluvial valleys, as for 

example along the north wall of Makushin Valley. Along steep
650 INVESTIGATIONS OF ALASKAN VOLCANOES

slopes, particularly sea cliffs and cirque headwalls, talus cones and 

aprons are prominent. The above-mentioned surficial deposits are 

not mapped separately.

SOILrASH PROFIUBS

Soil-ash profiles are helpful in reconstructing some of the postglacial, prehistoric eruptive history of the volcanoes. Most of the 

sporadic volcanic eruptions are probably older than the arrival of 

pre-Aleut people over 4,000 years ago (Laughlin and Marsh, 1951, p. 

82), for only a few thin ash layers are found in the midden sites.

Undisturbed and complete soil-ash profiles are most readily found 

along small streams incised into the floor of cirques at low elevations. 

A few sections are exposed by marine and eolian action. Major 

valley bottoms contain too thick a cover of alluvium and muck to 

provide such sections. In thin ash layers the thickness of the layer 

should be useful for correlations, for the ash would not bury the 

vegetation and drift freely, but in thick ash layers the fragment size 

may be a better correlating criterion than the thickness of the layer.

All profiles are from near Unalaska Bay and details of color and 

grain size of the soil and ash layers are not presented (fig. 102). Soil 

profiles in widely separated localities are grossly similar, but only 

those clustered together can be correlated with even fair reliability. 

In general the soil is progressively looser and more humic upward 

and contains more clay downward. Clay layers range in color from 

yellowish gray to orange, reddish brown, olive gray, and bluish gray. 

Bluish clays are common with till; reddish ones may be associated 

with limonite-rich bog deposits. The ash and cinder layers are generally medium gray or yellowish gray to orange, and perhaps some 

yellow clays are decomposed ash. Bombs and lapilli are scoriaceous, 

and some layers contain pumiceous lapill^ The bombs from the 

lowest pyroclastic layer in section C contain a few phenocrysts of 

bytownite, hypersthene, and other ferromagnesian minerals, and are 

similar to the pyroclastics of the Eider Point basalt.

Since the last deposition of glacial till at altitudes below most small 

cirques, several major episodes of explosive activity are recorded in 

each section, as indicated by the thick layers of coarse bombs and 

lapilli. The upward decrease in grain size of pyroclastics points to a 

gradual decrease in violence and frequency of eruptions. The areal 

changes of size of pyroclastics and the abundance of pyroclastic layers 

indicates the source of some or most of this material lies in the 

western part of the northern bulge of the island, but some of the 

fine ash layers might have had a more distant source than Unalaska 

Island.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 651

-w^Hi- 40 -w4Mk. 3 J^^ - " "- -i- -. 

HH^ miles -_±-E feet E.--SE feet r_ -." miles - :"feet -=^=

G FEET 

rO

-5

L10

EXPLANATION 

Unalaska Bay

/I Soil, humus-rich Ash Till

UNALASKA

Soil, clay-rich Lapilli Bedrock

Bombs Colluvium

H I J K L M

Is^- 0-3 -jli-JL- 0.3_jfe^u_ 5.2 _jifc.ji.L. 2.5 J^^L. 1.6 r^*^j 

^g= mile ^^= mile %=H ""miles"" -i^i^. ""miles'* rrC= "miles'* i:=:H

FEET 

rO

L.10

FIGURE 102. Soil-ash profiles.
652 INVESTIGATIONS OF ALASKAN VOLCANOES

EOL.IAN FEATURES

Depressions produced by deflation are prominent on the lower, 

western end of Unalaska Island, around the northern flank of 

Makushin Volcano, and on exposed ridges that have some vegetation 

cover. The bottoms of some of the larger pits are partly covered 

by pebble pavement; where coarse pyroclastics appear in the mantle 

these form lag deposits.

Dunes have formed behind a few large sand beaches, their apparent 

sources. The largest dune areas are in Surveyor Bay, the bay east of 

Cape Aiak, and Volcano Bay. The innermost area of dunes at Surveyor Bay has an unbroken cover of vegetation and is probably stable. 

The seaward edges of dunes nearest this bay and those east of Cape 

Aiak are partly destroyed by marine attack; grass-capped hummocks, 

10 or more feet high, form isolated residual dunes.

MARINE EROSION AND DEPOSITS

Cliffs surround most of Unalaska Island, indicating the importance of marine erosion. Hanging valleys abound along most 

headlands exposed to direct wave attack for in such settings stream 

erosion is subordinate to marine erosion. Cliffs that are continuous 

with sea cliffs border the edges of some large fiords for a considerable 

distance inland, suggesting that prograding is changing the shoreline 

position in places. Wave-cut rock benches, emergent during low tide, 

are conspicuously present along moderately protected shores (fig. 8Y) 

but absent from shores completely protected from waves.

Beach deposits, ranging from boulders to sand, are prominent in the 

heads of all but the most protected bays, and thin strips of beach 

deposits commonly separate tidal benches from the sea cliffs. The 

latter deposit is undoubtedly of transient debris, locally derived. 

Beach berms at various low altitudes and variously prominent are 

common to those bays which are open to wave attack and not hampered by excessive alluviation or outwash deposition.

Bars and spits are built across many of the shallow bays. Cuspate 

spits along the sides of Pumicestone and Kuliliak Bays and at Eider 

Point represent the earliest stage. The spit at Dutch Harbor, representing a later stage, is almost iy2 miles long and reaches the middle 

of Iliuliuk Bay. The bar across Staraya Bay may just have been 

completed and is subject to spillover by storm waves, for during the 

last 1Y years, as indicated by two sets of aerial photographs, part 

of the lagoon side of the bar has been built higher and wider. This 

lagoon is still brackish and its level fluctuates with the tide. The 

bar across Reese Bay also appears very youthful, for at its western 

end the bay and lagoon are separated by only a narrow boulder bar-
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 653

rier. The asymmetry of the plan of the spits that have joined at this 

narrow section of the bar indicates that the strong longshore current 

is westward. It is also noteworthy that during at least one storm in 

the last 15 years, waves spilled over the bar into the lagoon without 

breaching the bar; for large sections of a floating steel dock, part of 

an army installation, were tossed to the top of the bar and across its 

narrow section. This lake was connected to the sea, according ,to the 

inhabitants of the place as reported by Grewingk (1850, p. 121), and 

is thus about 150 years old. Completed stable bars include those at 

Volcano, Summer, and Hive Bays. Continued prograding of bars, as 

at Kalekta Bay (Freiday, 1945, p. 446), Staraya Bay, and Protection 

Bay, produces a wide, stable bar area, and eventually the lagoons are 

filled by alluviation. In other valleys, alluvium and beach deposits 

gradually fill the bay without development of a bar or lagoon. Amaknak Island consists of three high rocky masses joined by bars, and 

Cape Kalekta and a small island at Kashega are joined to Unalaska 

Island by narrow bars.

Evidence of sea-level change is scanty. There are a few suggestions of a marine terrace between elevations of 350 and 400 feet. 

A bench at Cape Kovrizhka is 1 mile wide and contains closed depressions that might represent irregularities in a wave-cut platform. 

Possible additional remnants of a terrace occur in the English Bay 

area and on Unalga Island a few miles east of Unalaska Island. This 

may be a single surface which is tilted gently eastward; it is about 

200 feet in elevation on the east side of Unalga Island and 1,200 

feet east of Agamgik Bay. The relatively flat-capped promontories 

at, and west of, Alimuda Bay (fig. 82) may reflect a much-dissected 

marine terrace.

STRUCTURAL GEOLOGY

The Aleutian Eidge, marked above sea level by the Aleutian island 

arc, is bordered on the south by the Aleutian Trench, which lies on 

an average 28,000 feet below the island peaks and 1,000 to 10,000 

feet below the adjacent floor of the Pacific Ocean, and on the north by 

the Bering Basin, more than 6,000 feet deep.

On Unalaska Island, bedding dips are generally less than 30° and 

average 15°. The rocks are nearly horizontal in the western and 

northeastern ends of the island. East of Kashega, northward dips 

are dominant in the exposed older rocks. Along the northern shore 

near Alimuda Bay and Station Bay, the rocks dip northward 10° 

to 15° on the north side of a gentle anticline which parallels the coast. 

Local flexures are known in the Unalaska formation, but the abundance of fractures and probable faults and poor stratigraphic con-
654 INVESTIGATIONS OF ALASKAN VOLCANOES

trol make interpretations of folds suspect. For example, along the 

south shore of Captains Bay the rocks dip gently southward for the 

first iy2 miles from the head of the bay; beyond a covered interval 

y2 mile wide the rocks dip gently northward as far as Unalaska 

village. This structure may be an anticline, or may indicate tilted 

fault blocks.

Within a mile-wide zone along the southeastern contact of the Shaler 

batholith, the Unalaska formation dips variably and more steeply 

than elsewhere. Some of the rocks are sharply truncated by the batholith. Strata more than ,a mile from this section of the contact have a 

persistent gentle southward dip. Along the northeastern and southwestern borders of the batholith, dips are away from the batholith. 

Along the irregular northwest border, the regional northward dip 

prevails. The variable dips around the Shaler batholith may have 

originated by forceful intrusion of magma, with some outward and 

upward displacement of contact rocks, or may represent a terrain disrupted by fault jostling into which the magma passively stoped or 

fluxed its way. The Shaler batholith and, to a lesser extent, the other 

plutons control the overall shape of the island at the places where they 

occur. The Shaler pluton is protected from the sea by a carapace of 

Unalaska formation rocks about 5 miles wide, except for the narrow 

bays which dissect it to its core. This may reflect the original shape 

of the island when it was initially uplifted above sea level or may be 

the result of differential erosion of a much larger uplifted island.

FAULTS

Faults, joints, and related linear features are abundant (pi. 77), 

but the length, direction, and amount of displacement are known for 

only a few of them. Most of the faults are nearly vertical; low-angle 

faults have not been recognized.

The faults range from tight fractures to those with strongly sheared 

zones or minor amounts of breccia. Some faults are mineralized 

slightly with some combination of quartz, pyrite, and calcite.

The strong topographic alinement of Beaver Inlet, Erskine, Portage, and Makushin Bays bisects the island and suggests a major fault. 

If the Beaver and Captains plutons are parts of a single small batholith, more contaminated by mafic argillites along the southern border 

and separated by roof rocks near Uniktali Bay, then possible horizontal displacement on the fault is north side 6 miles west. This is 

supported by the alteration patterns shown in figure 89. Positive evidence for this fault has not been discovered in the valley between 

Portage Bay and Erskine Bay but exposures are poor in this area.

There is little regional consistency to the known faults, listed on 

table 3, except that they are steep normal faults.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 655 

TABLE 3. Unalaska Island faults and their displacements

Location

Cape Aiak, north fault .._ ____ _.__

Cape Aiak, south fault--._-_______.

Mount Kimble, southeast of Chernofski Harbor (2 faults) ._.__._..____

Ridge north of Makushin Valley (6 

faults). _____________________ __

North side Nateekin Bay .__---..__.

South side Nateekin Bay (may be 

continuous with fault on north side 

of bay) __________ __-____________

East of Pyramid Peak _ ___________

\y-i miles east of KOF Point (5 faults) _ 

Headland west of Udagak Strait. _ __

Northeast of Staraya Bay __ _-...-_

Hive Bay _. .. _ __________ __ __

Bay north of Staraya Bay _______ _ _

Strike

ENE.

ENE.

NE.

N.65°W.

WNW.

WNW.

NW.

NW.

E.

N.

E

NW.

NW.

Amount 

vertical 

displacement 

(feet)

40

600

Small

Small

Small

20-30 

each

1,500

1,500

Downthrown 

side

N.

N.

NW.

S.

S.

S.

S.

NE.

N. 

E.

S.

Age

Recent.

Recent.

Re bent.

Recent.

VOLCANIC STRUCTURES

The caldera at Makushin Volcano and the craters at the summit of 

Pakushin Cone are rimmed by faults. The caldera on Makushin 

Volcano is about l1/^ miles in diameter, and the peripheral fault is 

buried by the glacier cap. There are two Pakushin craters a small 

one eccentrically within an older and a recent cone within the smaller 

crater. The persistently gentle dips in the Makushin Volcano area, 

the thin flows, and the relatively large proportion of flows to pyroclastics present a shieldlike form on which the younger, and in part 

still active, pyroclastic and composite cones are set. The Point Kadin 

explosion pits are along an eastward-trending fissure that was probably 

formed above molten lava at shallow depth. Similar features have 

been reported from the top of Kanaga Volcano (Fraser and Barnett, 

1959, p. 234), and Segula Volcano (Nelson, 1959, p. 265).

LINEAR FEATURES

Aerial photographs of Unalaska Island show linear features that 

commonly make regular patterns. Many are marked on the ground 

by slight depressions, knobs, alined drainage, or swamps; glacial 

erosion emphasizes many of these depressions. Others are marked 

o'nly by changes in vegetation. A few correspond with known faults, 

as at Cape Aiak. Most linear features in the Shaler batholith cor-
656 INVESTIGATIONS OF ALASKAN VOLCANOES

respond to joints and the sheared margin of the batholith and are 

generally shorter than those in the Unalaska formation. Presumably 

the large remainder represent unexposed joints and faults. Most 

are undeflected by topography and hence define essentially vertical 

planes; but a few on Sedanka Island and elsewhere are deflected by 

topography and thus indicate less steeply inclined planes.

A statistical count of more than 5,000 linear features in the Unalaska formation and the plutonic rocks is shown on the rose diagrams of plate 77, together with a representative selection of features 

on the map. The variable density of the counted linear features is 

due partly to the varying quality of aerial photographs. The northern bulge of the island has particularly poor photographic coverage 

and, hence, few mapped linear features.

The 10 areas for which rose diagrams are given were chosen to 

determine whether the linear pattern within the Shaler batholith is 

distinctive from patterns of adjacent areas and whether there is any 

islandwide Change in trend. The linear patterns are independent of 

the contacts of the Shaler pluton, and many cross the contacts. TKe 

rectilinear pattern of Konets Head is strikingly different from the 

persistent but more disperse northwest-trending patterns found in 

most of the island, and it is probably significant that the Konets Head 

pattern is developed on the least altered terrane (compare fig. 89, 

pi. 77).

The pattern of Konets Head might have been formed tectonically 

before the batholiths were intruded and the country rock altered, and 

this rectilinear pattern is now concealed elsewhere on the island by 

that pattern impressed on both plutonic and wall rocks during a late 

stage of intrusion. On the other hand, most of the linear features 

might have been formed after solidification of the batholiths, by orogenic forces perhaps extending the length of the Aleutian arc, and 

the difference in linear patterns is due either to the different physical 

characteristics (state of alteration or recrystallization) of the rocks 

involved, or both the alteration and linear patterns were controlled 

by the same mechanism. The linear features, in any case, were largely 

formed after the intrusion of the batholiths, but before the eruptions 

of Makushin Volcano.

ECONOMIC GEOLOGY

Parts of Unalaska Island have been prospected for sulfur, gold, 

and zinc, and there are some deposits of pyrite and chalcopyrite, but 

almost no production or development work. Mineralized areas are 

located on figure 89.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 657

SULFUR

Sulfur occurs at fumaroles near the vent of Makushin Volcano, 

where the heat of escaping gases keeps part of the caldera free of ice. 

Most of the exposed pyroclastics at the solfatara are impregnated 

with sulfur sublimates. Maddren (1919, p. 292) found five samples, 

taken at depths down to 16 feet, to have an average sulfur content of 

23 percent by weight. He measured (p. 288) fumes as hot as 310° F. 

The account by Maddren (p. 283-292) is the most complete yet made 

of the geology and estimate of reserves of the sulfur deposits. Lesser 

accounts were presented by Roth well (1899, p. 643), Pratt, W. A. 

(written communication,'1901), Sweeny and Myers (unpublished 

undergraduate thesis at Massachusetts Inst. Technology, 1908), and 

Smith (1936, p. 83). The glacier-covered volcano summit is accessible only with difficulty, and the prevailing weather is adverse.

GOLD

The island contains a few poorly auriferous quartz veins (Jaggar, 

1908, p. 9; and A. J. Collier, 1905, p. 102-103, who is quoted by W. W. 

Atwood, 1909, p. 151-152; 1911, p. 127-128). Notable among these 

are those on the northwest slope of Pyramid Peak, 1% miles south of 

Unalaska village, where gold prospectors, probably delayed enroute 

to or from Nome about 1900, opened a small drift and erected a 3- 

stamp mill; the deposit was soon abandoned.

Another auriferous quartz vein on Amaknak Island, between Dutch 

Harbor and Unalaska village, was mentioned by Eakle (written communication, 1907 ?); more veins were reported to him by the villagers 

but none was verified. Two tributaries on the north side of Makushin River near the head of the broad, flat valley that were panned 

gave, according to Eakle, a few minute grains of gold. Above these 

two streams is a conspicuous gossan formed of severely altered and 

pyritized rocks of the Unalaska formation near a contact with granodiorite.

ZINC

A zinc deposit on Sedanka Island is about % mile from the southeast cove of Udamat Bay and about 2% miles south of the abandoned 

village of Biorka. Sphalerite is the prominent ore mineral; pyrite is 

abundant, and some chalcopyrite and galena occur; the gangue is 

quartz and ankerite. During the month of exploration of this deposit, 

Webber, Moss, and Rutledge (1946) stripped the overburden adjacent 

to Lode Creek, mapped the area, and took abundant samples, which 

were assayed and given preliminary beneficiation studies. The potential ore is readily concentrated by selective flotation. The zinc 

ore contains minor quantities of copper, lead, silver, and gold. The
658 INVESTIGATIONS OF ALASKAN VOLCANOES

average zinc content of 29 samples taken along a narrow mineralized 

zone 240 feet long is reported as 6.8 percent; of these, 19 consecutive 

samples averaged 9.1 percent zinc.

The mineralization is described by Webber, Moss, and Rutledge 

(19-46, p. 4) as "confined to the hanging-wall section of a fault striking 

N. 83° E. across Sedanka Island and dipping approximately 55° 

south * * * Mineral deposition has been controlled structually by a 

series of nearly vertical fractures (80°-90° SW.) in the diorite forming the hanging wall of the fault. The fractures have an average 

strike of S. 60° E. and are more closely spaced and mineralized as they 

approach the fault plane * * * The footwall is an unmineralized finegrained greenstone." The diorite of the hanging wall of the fault 

is a basic variety of the plutonic rocks, and the greenstone is part of 

the Unalaska formation.

Kullerud (1953, p. 130) has deduced from considerations of the 

FeS-ZnS equilibrium relations that the Sedanka ores were deposited 

at a temperature below 138°C.

OTHER MINERALIZATION

Disseminated pyrite and, less commonly, chalcopyrite occur in many 

scattered localities on the island. Some of these mineralized acreas, 

as those on Devilfish Point and on the north side of Broad Bay, are 

associated with andesite dikes. Other mineralized areas, as those near 

the head of the Broad Bay Valley, the walls of Anderson Bay, Cape 

Starichkof, and Broad Bay suggest that much of this enrichment is 

associated with the granodiorite batholith.

POTENTIALITIES OF ECONOMIC DEPOSITS

The economic possibilities of mineral deposits on Unalaska Island 

were not evaluated during this reconnaissance. No economic deposits 

have yet been found in the Aleutian Islands, which may in part reflect 

the inaccessibility of the area. Some islands south of the Alaskan 

Peninsula, notably Unga and Kodiak^Islands, contain small economic 

gold deposits, and one of the Komandorskiye Ostrova, the westernmost, Russian-owned Aleutian Island group, contained, in beach 

gravels, native copper that was utilized in the 18th century (Coxe, 

1787, p. 206).

On the basis of such comparisons it seems possible that the Aleutian 

Islands may contain a few small deposits of copper, zinc, or gold; 

Unalaska Island is probably one of the more favorable islands for 

such mineralization because of the abundance of intrusive granodiorites. Prospecting the border and roof areas of the granodiorite 

may prove fruitful. The possibility for producing nonmetallic materials, other than sulfur, from Unalaska Island appears remote.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 659 

SUBMARINE GEOLOGY

Only a small part of the crest of the Aleutian Ridge is exposed on 

Unalaska Island, although the island covers half its width. Submarine contours are shown in plate 75, providing the basis for additional geological observations'and inferences. The contoured area 

covers about 12,000 square miles.

In the reports on several other Aleutian Islands, the submarine 

topography and geology adjacent to the islands are also incorporated. 

The submarine studies of P. A. Smith (1937), H. W. Murray (1945), 

and Gibson and Nichols (1953) are unaccompanied by subaerial geologic mapping.

The contours were constructed from U.S. Coast and Geodetic Survey smooth sheets (hydrographic surveys) dated 1934 through 1939, 

on scales ranging from 1:5,000 in harbors to 1:20,000 along most 

coasts, to 1:80,000 and 1:160,000 beyond about 10 miles offshore. 

Nearshore soundings were located by visual fixes on triangulation 

stations; offshore soundings, by the radio-acoustic ranging method 

described by P. A. Smith (1935, p. 10-12). The accuracy of a visual 

fix is usually one part in 500 to 1,000, and that of a fix obtained by 

radioacoustic ranging is about one part in 200 to 500 (Smith, 1935, 

p. 13). Depths were obtained by echo soundings, which vary in 

abundance from about 850 soundings per square mile on the 1:5,000 

smooth sheets to about 2 soundings per square mile on the 1:80,000 

and 1:160,000 sheets. The limitations of echo sounding and the corrections used are described by Gibson and Nichols (1953, p. 1177- 

1178). ]

Above a depth of about 400 fefet, the level of the insular shelf, the 

submarine topography was first contoured with an interval of 25 feet; 

intermediate contours have been omitted from plate 75, but they 

increase the precision of the remaining contours. To obtain a valley 

configuration, independent evidence of a deflection in a contour was 

obtained from many contours. In areas of few soundings, valleys and 

ridges were contoured in the most generalized way as rounded forms, 

unless specific evidence gave contrary indications. These procedures 

of depth contouring apparently differ from those used by Smith 

(1937) to compile his map covering a portion of the Bering Sea near 

Unalaska Island; he shows more detail than the present map, although 

the same smooth sheets were used. Depth contours are calculated 

from mean lower low water, and subaerial contours from mean high 

water, a difference of about 3 feet (U.S. Coast and Geodetic Survey, 

1947,p.383).
660 INVESTIGATIONS OF ALASKAN VOLCANOES

PHYSIOGRAPHY 

INSULAR SHELF

The submarine landforms of the insular shelf, the prominent platform capping the Aleutian Ridge, are contoured in greater detail and 

therefore appear more varied than those of the bordering slopes. The 

abrupt change of slope at the outer margin of the insular shelf, or 

shelf break (Dietz and Menard, 1951, p. 1996), is marked approximately by the 400-foot depth contour, and is sharper north of the 

island than south of it. The active volcanoes of Unalaska Island 

and adjacent islands are situated near the northern edge of the shelf; 

elsewhere the shelf is about 10 to 15 miles wide.

Elongate troughs, as much as 700 feet deeper than the insular shelf, 

occupy Beaver Inlet, Makushin Bay, Usof Bay, and other fiords. 

The sides of the troughs near the heads of the fiords are continuous 

with the valley walls above sea level, but near the open sea the valley 

walls and trough walls are separated by a bench submerged 200 to 

300 feet and continuous with the insular shelf at the mouth of the 

fiord. In the deepest parts of. the troughs, as shown by Beaver Inlet 

and Makushin Bay, the bottom surfaces are relatively flat and abut 

into the adjacent walls. Gentle transverse ridges or lips divide the 

trough bottoms into smaller basins.

Closed basins located off the mouths of most fiords differ from fiord 

troughs in that they are elliptical in plan and have more gently sloping walls. Few are more than 100 feet deeper than the adjacent insular shelf. On the seaward side of most of these basins are arcuate 

rims, concave landward, and south of Usof Bay and Akutan Island 

there are incomplete arcuate rims at depths as great as 600 feet beyond the better defined inner rims. Knobby topography is found 

inshore from the arcuate rims; in some places mounds and depressions are elongate parallel to the fiords, in others they are irregular or 

circular.

In Umnak, Unalga, and Akutan Passes submarine topographic 

features parallel the trend of the passes. The most prominent scarp 

in Umnak Pass lies 4 miles north of Boulder Bay, faces northwest, 

and is about 200 feet high. Unalga Pass contains many small ridges, 

depressions, and small scarps.

The topography of the remainder of the insular shelf is comparatively featureless. Near shore the bottom drops about 100 feet in 

the first mile and gradually flattens to about 20 feet per mile. Near 

headlands the submarine topography is irregular; between headlands 

it is relatively smooth.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 661

V

LOWER INSULAR SHELF

The lower insular shelf, between the insular shelf and the south 

insular slope, forms a plane sloping seaward 100 feet per mile, and 

lies parallel to, and from 7 to 15 miles south of, Unalaska Island. 

Its average width is 6 miles, but it is nearly absent opposite the ends 

of the island. Most of the upper margin of the slope coincides with 

. the 400-foot depth contour; however, near the incomplete arcuate 

rims off Usof Bay and south of Akutan Island the top of the lower 

insular shelf is at 600 feet. The lower margin of the lower insular 

shelf is irregular in plan and depth, for the submarine canyons on 

the south insular slope head at various depths and distances from the 

island; maximum depth to the edge of the shelf is 2,400 feet.

SOUTH INSULAR SLOPE

' South of the insular shelf and lower insular shelf the insular slope 

drops about 1,000 feet per mile to the Aleutian bench (Murray, 1945, 

p. 775) and beyond that into the Aleutian Trench, which is deeper 

than 21,000 feet in this area. Only the upper part of this slope is 

shown on plate 75; it is broken by numerous submarine canyons, of 

which some are oblique, rather than parallel, to the general bottom 

slope.

NORTH INSULAR SLOPE

The north insular slope descends from a depth at the shelf break 

of about 400 feet to the Bering Basin, which is over 7,000 feet deep in

 * the western part of the area. Near the shelf break the slope drops 

1,000 feet per mile, but the gradient is gentler at depth. Submarine 

canyons make the north insular slope irregular in detail. In the 

steep upper part of the slope, canyons are numerous and intricate. 

A few of the canyons appear to be flat floored, and some near the top 

of the north insular slope have a V-shaped cross profile. Most can-

~s yon bottoms are shown as rounded because the data are too scanty 

to define their shapes.

A prominent ridge trends westward from Point Kadin on the north 

shore of Unalaska Island to Bogoslof Island, just west of the margin 

of the map.

BERING BASIN AND CONTINENTAL SLOPE

Little information is obtained from the portion of the broad-floored

J Bering Basin and Continental Slope in the map area, for the sound-

^ ings are few. The Bering Basin is wedge shaped in plan, and its

floor slopes gently south westward from its apex, 1,800 feet deep north

of Unalaska Bay, to a depth of 7,000 feet.

North of the Bering Basin the Continental Slope rises to about 

2,400 feet. A conspicuous canyon more than 15 miles long, the mouth
662 INVESTIGATIONS OF ALASKAN VOLCANOES

of which lies due north of Point Kadin at the edge of the map, trends 

southeast across this slope and forms a large reentrant in the edge of 

the continental platform.

WAVE AND CURRENT ACTIVITY

The dominant geomorphic agent now active on the shallow parts 

of the insular shelf is wave action, which is impressively effective along 

the shore. Hanging cirques, hanging stream courses, cliffed headlands, foreshore rock benches, spits, and bars are very common. The 

broad and almost featureless insular shelf north and south of Chernofski Harbor is best explained by wave erosion and deposition. The 

simplicity of these slopes is broken only near headlands, and the 

irregular submarine contours probably show submerged extensions of 

the headlands. Depth of wave action may be as much as 250 to 350 

feet, tKe depth of the flattest portion of the insular shelf, and the depth 

above which glacial features are absent. Kuenen (1950, p. 228) presents evidence supporting transportation of debris by wave turbulence 

at this depth; it is likely, however, that the bulk of erosion by waves 

is restricted to shallower depths.

Along the southern shore of Makushin Bay a submerged bench 

separates the relatively steep fiord wall below it from an even steeper 

wall above the sea. This bench is widest, about 3,300 feet, at the 

mouth of the bay, narrows toward Tarasof Point, and is absent from 

there to the head of the bay. There are similar narrow benches near 

the mouths of Usof Bay, Beaver Inlet, and near Eider Point in Unalaska Bay, but only the one flanking Makushin Bay faces the open 

sea. These benches are believed to be cut by wave action since the 

ice vacated the fiord. This is suggested by their gradual narrowing 

toward the head of the bay and away from the direct attack of 

strong waves, by their continuity with the insular shelf and by the 

absence of valley-in-valley topography on the island. Had the cutting 

of the bench preceded the gouging of the fiord, the glacier would undoubtedly have rounded the edges of the bench.

Clear evidence of marine deposition on the insular shelf around 

Unalaska Island is scarce, but it is widely suggested. The seaward 

end of the trough in Makushin Bay is flanked by low ridges, the seaward ends of which are slightly recurved away from the intervening 

trough, as shown by the 200-foot depth contour north of the trough 

and by the 300-foot contour south of it. They have the plan of 

spits and are located precisely where strong currents leaving the 

trough might be expected to "spill over" its edge, lose their energy on 

the open shelf, and deposit their load. These recurved ridges are 

probably submarine spits. Another suggestion of marine deposition 

occurs in the flat bottoms of some of the troughs. The glacial origin
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 663

of these troughs is discussed below, but the areas of flat bottoms are 

believed to be underlain by marine sediments. The deep closed 

troughs should trap much sediment from the adjacent land. Probably 

much of the outer part of the shelf has a thick veneer of sediments 

derived from the retreating shore, but there is no direct evidence to 

support this idea.

GLACIAL FEATURES

Submarine glaciated landforms include fiords, elongate knobs and 

depressions, and possibly the basins on the insular shelf. The first 

two of these features have subaerial counterparts and, in places, are 

continuous with them.

Submarine glacial deposits include moraines, which form relatively 

narrow arcs, and possible till, which forms a broad zone of nonelongate knobs and depressions. The arcuate ridge at the junction of 

Skan Bay and Makushin Bay troughs, which is over 50 feet high, 

and the arcuate ridge near the mouth of Usof Bay are interpreted as 

moraines. The arcuate ridges near the shelf break off Makushin and 

Unalaska Bays and Beaver Inlet are probably also glacial depositional features, and the other irregular ridges on the shelf off the 

eastern two-thirds and mountainous portion of the island are possible 

glacial deposits. A line connecting the more conspicuous arcuate 

ridges is roughly parallel to, and inside of, a line connecting the less 

conspicuous ridges. These probably represent two glacial episodes; 

the moraines off Skan Bay and the mouth of Usof Bay may represent a third episode, possibly a pause during retreat of the glaciers. 

The zone of knobby topography without an elongate pattern in 

Unalaska Bay, which is conspicuous where contoured using a 25-foot 

contour interval, is constructional; it has not been ice scoured or 

modified by currents. Such topography could be the expression of 

landslides, volcanic eruption (both postglacial), or ground moraine; 

of these the last seems most reasonable.

The arcuate ridges near the shelf break need not be end moraines 

or recessional moraines, for the submarine environment permits 

another variation of moraine with similar topographic features to 

form. Conceivably, a ridge resembling a moraine could be formed 

along the line at which shelf ice becomes free floating. Along this 

line of contact with salt water the ice sheet probably thins relatively 

abruptly, and the load, which is generally greater near the 

bottom of a glacier, would be dropped to form a ridge approximately parallel to the floating ice front and hence concave landward 

(fig. 103). This ridge would not be a true end moraine, for it does 

not form at the toe of the shelf ice, and a ground moraine sheet would 

lie on both sides of this pseudo end moraine. With a fluctuation of
664 INVESTIGATIONS OF ALASKAN VOLCANOES

FIGURE 103. Block diagram showing relations between shelf ice and pseudo end moraine.

sea level or volume of ice the pseudo end moraine would be widely 

distributed on the flat insular shelf as a true ground moraine. However, where the bottom of the sea drops more steeply from the land, 

the chance of obtaining a pseudo end moraine is greater, for changes 

in the sea level or ice thickness will shift relatively little the line 

along which the ice sheet becomes free floating. Perhaps, then, the 

arcuate ridges of glaciated deposits along the outer margin of the 

insular shelf off Makushin Bay and Unalaska Bay were formed at 

the line along which the ice last rested on the insular shelf for a relatively long time; they may be pseudo end moraines. The relatively 

smooth upper part of the north insular slope adjacent to the pseudo 

end moraine may be ground moraine dumped seaward of the pseudo 

end moraine as shown in figure 103.

In summation, then, probably two and possibly even three episodes 

of glacial advances or stands are recorded in the submarine physiography. The oldest and most extensive of these, the one least substantiated, passed beyond the outer edge of the insular shelf and 

onto the lower insular shelf. A second one is recorded by the more 

distinct basins and their arcuate ridges. During these episodes the 

higher mountains of Unalaska Island were fringed with shelf ice, but 

the stand of the sea was probably much lower than it is today. A 

third episode is marked by terminal or recessional moraines near the 

mouths of the larger fiords; the mountain glaciers no longer merged 

on the insular shelf. The cirque-head and terminal moraines near 

existing glaciers represent the latest advances of the glaciers.

The shoreline was probably free of shelf ice some time after the 

end of the latest Wisconsin advance and before the climatic optimum, 

or an estimated 8,000 years ago. The climatic optimum is dated 

4,000 to 6,000 years ago by Flint (1947, p. 487) and the retreat from 

the Mankato maximum is dated by Flint (1955, p. 253) at about
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 665

V

10,800 years, based on carbon-14 determinations. These dates are 

roughly verified by ionium-method dates from deep sea cores (Hough, 

1953, fig. 2), but may be almost doubled by Antevs (1957, table 2); 

for the present we shall accept the younger age. It is reasonable to 

suppose that the ice retreat of the last glacial substage was delayed 

perhaps as much as 1,500 to 2,000 years by the Aleutian climate, so 

the shoreline of the island was probably ice free between 9,300 and 

6,000 years ago perhaps about 8,000 years ago.

SUBMARINE STRUCTURE

Geologic structures interpreted from submarine topography include 

faults and a tilted marine bench. Scarps and alined topography are 

most pronounced in Umnak and Akutan Passes, where they trend 

parallel to these passes. Some of these features may be faults, but 

most are analogous to the linear features described on the island and 

are structural breaks of unknown origin. The restricted distribution 

of submarine faults and linear features may indicate that the location 

of the passes was controlled by structurally weak zones, or that submarine topographic detail is least obscured by marine deposition in 

passes where currents are strong, or both. From the abundance of 

linear features on the island the second view is most reasonable.

Submarine valleys that run obliquely down an insular slope appear 

to be structurally controlled. Several of the large submarine canyons 

south of Usof Bay fit this criterion, and the canyons immediately 

north of Cape Idak on Umnak Island may also be structurally controlled, for not only is their trend oblique to the north insular slope, 

but they also trend parallel to the linear features in Umnak Pass, 7 

to 12 miles to the south.

The lower insular shelf may be a marine erosion surface tilted 

about 1° southward. This tilting would predate the cutting of the 

present insular shelf.

The rate of postglacial cutting of the insular shelf-at the mouth 

of Makushin Bay is 3,300 feet in 8,000 years (see p. 664 for determination of time shore has been ice free), or an average of about 0.4 feet a 

year. The annual erosion of sea cliffs between 21 and 54 feet high at 

La Jolla, Calif., during 12 years is 0.9 to 1.7 feet (Vaughan, 1932, p. 

250) and the average rate of retreat of a shore in glacial till in Yorkshire, England, during the last 1,800 years, as well as the present rate, 

is about 5.5 feet a year (Wheeler, 1902, p. 222). Coleman (1914, p. 

437) estimated the annual rate of retreat of cliffs 100 to 350 feet high 

at Toronto, Canada, as 1.62 feet. These values are calculated for 

shorelines of differing heights, cut in various materials, and subject to 

attack of storms of various severity, but they are of the same order of
666 INVESTIGATIONS OF ALASKAN VOLCANOES

f

magnitude as the estimated annual rate of 0.4 feet on Unalaska Island. 

Thus far the calculations are within reason.

The width of the insular shelf off the south shore of Makushin Bay > 

is about 12 miles. Assuming, as a first approximation, that marine 

erosion averaged 0.4 feet per year, this shelf would have been cut during about 160,000 years. But marine erosion should decelerate as it x 

advances, so the rate of erosion used in these calculations is probably 

substantially smaller than the average rate, and the estimate of 160,000 

years needed to cut the shelf is too large. Let us take this age as a y 

conservative one, and pursue the analysis further. During the last 

160,000 years the shore was protected by shelf ice during times of maximum glacial advance, and probably was protected intermittently by V 

pack ice. Today the winter pack ice generally reaches the Pribilof 

Islands and the Bering Sea coast of the western end of the Aleutian 

Peninsula; twice it was reported to have reached Unalaska Bay (U.S. 

Coast and Geodetic Survey, 1947, p. 379). During glacial minima the 

sea level may have been too high to cut much of the shelf; conditions 

were probably most favorable for this cutting between glacial maxima '- 

and minima. Granting the uncertainties, this estimate nevertheless 

suggests that most of the insular shelf probably was cut during late 

Pleistocene time.

The relative youthfulness of the insular shelf is also suggested by 

structural and glacial evidence. The Aleutian arc is an unstable crustal area, and geomorphic surfaces are not apt to be preserved very long 

under these conditions. The simple form of the valleys and the ab- » 

sence of valley-in-valley topography on Makushin Volcano suggest 

that only one major cycle of stream and glacial erosion, probably the 

latest major glaciation, is necessary to explain the present topography. 

The absence of abrupt changes in depth of the outer edge of shelf in 

an unstable crustal area also suggests that the shelf is young.

GEOLOGIC HISTORY r

The geologic record of the island began with the deposition of the ' 

Unalaska formation in the middle Tertiary; it consists of andesite v 

and basalt and of abundant epiclastic and pyroclastic sediments, 

deposited partly in an offshore trough or troughs, and partly nearshore. Offshore deposits are represented by argillites, pillow lavas, .x. 

sills, and tuffs of the south shore and west end of the island; the v 

abundance and thickness of well-bedded argillite indicates deposition 

undisturbed by waves and currents. Deposition was perhaps in a h 

linear depression, for lava and shallow intrusives are abundant 

throughout but are nowhere distributed in the manner of a central 

volcano. The emission of andesite and basalt lavas into muds de- vrived from similar lavas produced many unusual local structures
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 667

%

ranging from those wholly extrusive to those fully intrusive: flows,

pillows, bulbous masses, and sills. Albitization of many of these

< rocks may have occurred soon after deposition, but at least some of

the albitization is a feature of later alteration.

Nearshore deposits are represented by the abundant, coarse, angu-

* lar clastic rocks common to the Unalaska formation on the northern 

and eastern parts of the island. There the conglomerates and sedimentary breccias, many with a tuffaceous matrix, form thick, poorly

^ bedded deposits, of which some local lenses or wedges may be submarine talus deposits. Submarine flows are less common than in the 

offshore deposits.. The fossils Cornwallius and Mya cf. M. truncata

^ also indicate a shallow-water environment, and Mya indicates a coldwater environment.

Two or three batholiths and numerous smaller bodies of granodiorite intruded the Unalaska formation at some time between early 

Miocene and late Pleistocene, As postbatholith history is complex, 

perhaps the batholithic emplacement was early in this interval, perhaps in late Miocene time. The original batholithic magma may have 

been formed by local melting of the earth's crust, and emplacement 

of the batholiths probably took place by a combination of reactive 

assimilation and melting of roof rocks, stoping, and forceful intrusion. Evidence of assimilation is best preserved near the very irregular roof areas, and the roofs owe their great irregularity to random 

fluxing or stoping rather than to infolding of country rocks. Some

*. of the argillites are very mafic, and these contributed abundant material to the magma; the effects of the other rocks of the Unalaska 

formation, felsic to mafic volcanic rocks and minor siliceous argillites, are less noticeable. Assimilation produced widely varied mafic 

plutonic rocks as gradationally bounded variants along and near 

wallrock contacts. Contact metamorphism by the granodiorites proi' duced narrow zones of pyroxene, hornblende, and biotite hornfelses, 

which locally were made schistose by upward magma movements. 

The amount and intensity of alteration of the Unalaska formation 

decrease away from the batholiths, which were probably the energy 

source for most of that alteration. 

Late hydrothermal solutions and the relative absence of contam-

*J inants combined to leave the core of the Shaler pluton uniformly 

i richer in quartz, potassic feldspar, allanite, and amphibole with respect to pyroxene. Late hydrothermal fluids replaced small amounts 

of border rocks and filled small and large joints. Warping, faulting, 

uplift, and erosion of the rocks of the region followed, and probably 

in part accompanied, the intrusion of the batholiths. The batholiths 

^ with their carapaces of altered country rocks became central structures in the emerging island framework.
668 INVESTIGATIONS OF ALASKAN VOLCANOES

A time of deep erosion of rocks of the Unalaska formation and 

of the batholiths is recorded by the unconformity at the base of the 

Makushin volcanics. The unconformity is possibly Pliocene, although it could be as old as Miocene or as young as Pleistocene. The 

configuration of the unconformity beneath the Makushin volcanics 

resembles the present-day erosion surface in relief and position of 

large valleys and ridges. The island, then, was a moderately rugged 

landmass during Pliocene(?) time, and the gross drainage features 

have been inherited from that time. Perhaps the lower insular shelf 

south of Unalaska Island records an early period of marine erosion, 

or else it is the submarine surface equivalent to the major unconformity. If the age of the insular shelf is accepted as late Pleistocene, 

then the lower insular shelf, truncated by the upper insular shelf, is 

early Pleistocene or earlier.

On the northern part of the island, the extrusion of basalt and 

andesite of the Makushin volcanics perhaps began in late Tertiary or 

early Pleistocene time, but the bulk of the lavas and pyroclastics of 

the Makushin Volcano, across which but little insular shelf is cut, 

are late Pleistocene. Before the close of the Pleistocene, volcanism 

ceased or diminished considerably, and the volcano was deeply eroded 

by streams and glaciers. The summit of Makushin Volcano collapsed 

to form a caldera.

During the two older glaciations most of the high country south 

of Makushin Bay and around Makushin Volcano was capped by ice 

which debouched into the sea through fiords and shelf ice. Smaller 

glaciers and ice fields capped lower parts of the island. The extensive ice sheets and winter pack ice retarded or stopped marine erosion. 

The last glaciation, during which glaciers reached the sea, was much 

more restricted than the preceding ones.

Before the last major glaciation, eruption of the Eider Point basalt 

as flows and pyroclastics began from several vents, including the postcaldera vent at Makushin Volcano. Some of the vents continued active until after the last major glaciations; probably the last of these 

eruptions produced the lava near Point Kadin.

Sporadic late eruptions from these vents and possibly from volcanoes on adjacent islands added much ash and lapillae to the mantle. 

Most of the ash eruptions occurred before the arrival of the pre-Aleut 

people (about 4,000 years ago), but since the glaciation in Skan Bay 

(about 9,000 years ago). Minor eruptions from the young cone in 

the caldera of Makushin Volcano, and the emission of gases at fumaroles and of hot water at some springs are the only signs of volcanic 

activity during historic times. The toes of the residual glaciers probably fluctuated twice within the last few centuries.
GEOLOGY OF UNALASKA ISLAND AND ADJACENT INSULAR SHELF 669

LITERATURE CITED

Antevs, Ernst, 1957, Geological tests of the varve and radiocarbon chronologies:

Jour. Geology, v. 65, no. 2 p. 129-148. 

Atwood, W. W., 1909, Mineral resources of southwestern Alaska: U.S. Geol.

Survey Bull. 379, p. 108-152.

 1911, Geology and mineral resources of parts of the Alaska Peninsula:

U.S. Geol. Survey Bull. 467, 137 p. 

Bramlette, M. N., 1946, The Monterey formation of California and the origin

of its siliceous rocks: U.S. Geol. Survey Prof. Paper 212, 57 p. 

Byers, F. M., Jr., and others, 1947, Volcano investigations on Umnak Island,

1946, in Alaskan Volcano Investigations Report 2, pt. 3, 105 p.: U.S. Geol.

Survey special pub. (limited disirib.). 

Cady, W. M., and others, 1955, The central Kuskokwim region, Alaska: U.S.

Geol. Survey Prof. Paper 268, 132 p. 

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

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

Clark, B. L., and Arnold, Ralph, 1923, Fauna of the Sooke formation, Vancouver

Island: California Univ. Dept Geol. Sci. Bull., v. 14, no. 5, p. 123-234. 

Clarke, F. W., and Hillebrand, W. F., 1897, Analyses of rocks, with a chapter

on analytical methods, laboratory of the U.S. Geological Survey, 1880 to

1896: U.S. Geol. Survey Bull. 148, 306 p. 

Coats, R. R., 1950, Volcanic activity in the Aleutian arc: U.S. Geol. Survey

Bull. 974-B, p. 35-49

 1951, Geology of Buldir Island, Aleutian Islands, Alaska: U.S. Geol. 

Survey Bull. 989-A, p. 1-26.

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INDEX

Acknowledgments.._____________ 586 

Aerial photographs, cartography from____ 584 

linear features._. .. -_______ 655 

Akutan Pass___._.___.._____ 660,664 

Albitized rocks__.___________ __ 600-601 

Aleutian Arc._________________ 666 

Islands of._.____.__.____ 586-587 

Aleutian Eldge_...__._...___ 585,653,659 

Aleutian Trench_______________ 653,661 

Alteration, in plutonlc rocks....______ 630-631

Alluviation..__.____._.___.__ 648

Analyzed rocks_______________ table 1

andesite porphyry___._______ 602,641

altered............................. 601-602

olivine-bearlng___ ____ 641,645

andesite (?) porphyry, albitized...___ 602

andesite vitrophyr____________ 646

basalt porphyry, feldspathic._____ 601,640

feldspathic, altered__._.__.__ 601

oil vine-bearing. _._______ 640,645

olivlne_____....________ 640,645

ollvme-bearing____________ 641

daclte (?) porphyry, albitized..-.___ 602-603

dacite vitrophyr______________ 602

pyrite-bearing_____________ 602

gabbro. .................__..... 616,620,626

granite______.___..._.__... 619,629 

granodlorite________________ 615, 

616,619,620,627,628,629,630 

granogabbro_______._____... 627-628 

quartz gabbro-____....____... 616,627

quartz monzonlte, leuco albite_____ 615,630

syenodiorite, recrystallized wallrock.. 619,626-627

Andesite porphyry, Makushin volcanics .... 641

olivlne-bearing, Eider Point basalt..__ 646 

Andesite vitrophyr, Eider Point basalt. __ 646 

Aplites and pegmatites, petrography.... ~ 620

Arcuate ridge, submarine__________ 663-664 

Argillite, Unalaska formation....... 591-592,593,595

B

Basalt porphyry, olivlne .....____ .. 645

olivlne-bearing feldspathic.. ___ ____ 645

Bays............ .......................... 587

Beaches, physiography____________ 587 

Beaver Inlet, argUlite....._______...... 591

troughs.---...----.....-. .. ..-.-..- 660 

Beaver pluton, petrography____.__... 619-620

relation to topography..___ ... 654 

Bench, physiography.._________ 587,646 

Bering Basin_____________... 653,661 

Bishop Point, volcanic mudflow_______ 643 

Blueberry Bay, granodiorite_________ 628

linear features In rocks.....-.---____ pi. 77 

Border rocks, composition________ 622

position In pluton..----.---____ 633

Page

Boulder Bay, tuff breccia___________ 594

Broad Bay, olivlne basalt porphyry.____ 640

Buttress Point, albitized dacite porphyry__ 603

dacite vitrophyr............__.____ 602

Caldera.. .----..--..-----...--._-... 654

Cape Aiak area, dunes . ..__...:... 652 

gabbro.__ __._ _.____. 626 

Igneous stuctures. .. . ........ 607-608

Cape Aiak area, mixed rocks......._ .. 595

Cape Starlchkof, quartz gabbro___..._. 627 

Captains Bay, granodiorite . ..... 628,630

young flows.._______________ 645 

Captains pluton, petrography.______... 620 

relation to topography..________... 654 

Chernofski Harbor, pyrite-bearing dacite

vitrophyr...._....__.. .. 602

Cirques.-- .. . 646-647

Climate...___.....___________.- 589

Colluvium........... ....... ....... 649

Conglomerate, Unalaska formation __ .. 591 

Continental Slope.. ....................... 661

Deflation 650,652

Diabase. . 622

Dilation veins, in altered plutonic rocks 631 

Diortte ..... .... ... ..... 620

Dips... . - 653-654

Driftwood Valley, Eider Point basalt.. .. 644 

Dunes.______-____________ 650,652 

Dynamically metamorphosed rock, outcrops. 624

E

Economic geology 656-658 

Eider, motorshlp.. 584,586

Eider Point basalt, analyses .. table 1 

analyzed rock, specific data.. 645-646 

Bishop Point mudflow 643 

constructional forms.... . 643

glaciation......... 642,643

Makushin Volcano, andesite vitrophyre.. 646

cones, fumaroles, and hot springs 642

name and character... 641,642

Pakushin Cone, composition of rocks 642

ollvine-bearing feldspathic basalt 646

petrography.. 645-646

Point Kadin vents, composition and

forms_____ . 643

pyroclastic deposits . 642

Sugarloaf Cone and lava flows.. - 643-644

oil vine-bearing andesite porphyry. 646

olivlne-bearing feldspathic basalt

porphyry... 640

673
674 INDEX

Page 

Eider Point basalt Continued

Tabletop Mtn., cone and flows... __ __ 644

olivlne basalt porphyries_______ 645

Wide Bay cone.. __..._.__... 644-645

Eolian features_ _______________ 652

Faults.._--...-....____.......__... 654-655

submarine._________________ 665 

Final Bay, granogabbro.___________ 627-628 

Fiords....................................... 587

Fossils..................................... 607,667

Fumaroles______________.. 642,657

Oabbro, Cape Alak..._____....____ 626

Geography of area______________ 587-590 

Geologic history___.____.____.... 666-668

Geologic reconnaissance_________... 584-585 

Glacial features, submarine_________ 663-664 

Glacial land forms and deposits..._____ 646-648 

Glacial outwash and vegetation....____ 648-649

Glaciation of lava flows_ __________ 644 

Glaciers..... ._...---....--.-_........ 647

Glacier Valley.......................... 634,638,642

Gold......................................... 657

Granite.........____.___......._ 619,629

leuco......____....._....._ 615,619,690

Granodiorite, Captains pluton_______ 628 

Final Bay.............................. 627-628

Shaler pluton......_.__.__._._ 628

Skan Bay.......................... 627,629-630

Qrandiorite and related rocks, age.. ____. 612 

aplite and pegmatite...._............ 610,620

Beaver pluton_._.__.....__ 610,619-620

Captains pluton.................... 610,612,620

emplacements.___________._ 610,612 

Field relations............ ........- 610-612

jointing................................... 612

lineatlon along batholith border........ 610-611

pendants_________________ 610,611 

recrystallized wallrock.......___ 622-624,627

shaler pluton....___...____... 610,614,619

small intrusive bodies...__._.._.. 610,620

topographic forms.-..__________ 610

Graywacke, Unalaska formation______ 591,593

H

Hanging valleys________________ 652 

History of area, ________________ 586 

Hive Bay.................................... 590

albitized andesite (?) porphyry......_ 602-603

Holocrystalline rocks, fence diagram of distribution______._____.. pi. 78 

percent frequency of mineral amounts

and plagioclase composition.. __ pi. 76 

Hot springs..._...........___....._.... 642

Humpback Bay, altered andesite porphyry. 601,602

Igneous and sedimentary rocks ______. 590-646 

Intrusive bodies in batholith............... 620-622

Insular shelf, age. ______________ 668 

erosion _ __________ 666-666

Page

Insular shelf, lower______________ 661,668

physiography_ _____________ 659-660

Insular slopes..--...-.________._..... 661

Intrusive flows, Unalaska formation____ 590,595

K

Kalekta B ay, andesite porphyry______.. 602 

Kashaga Bay, linear features in rocks... __.. pi. 77 

Kismaliuk Bay, pillow structure.......__.. 597

Kof Point, granite.______________ 629 

Konetz Head, linear features in rocks____ pi. 77 

Koriga Point, constructional forms, Eider

Point basalt...... ............. 643

moraines.._________________ 648

Linear features._____.---....655-656,665; pi. 77 

Location and extent of area______ 585,586-587 

Lofty Mountain, lava flows,...______.. 593 

Logistic problems..._____________ 584,586

M

McLees Lakes, basalt porphyry.._____ 640,644

young flows_______ _.._.._ 645

Makushin Bay, argillite.__._...____ 591

troughs....-.........----.-.-......- 660

Makushin Valley, Eider Point basalt....__. 644

Makushin volcanics, analyses of volcanic

rocks. _____.__.____..__.. table 1 

analyzed rocks, specific data.. _____.. 641. 

basal unconformity..... . .. 634-638,668

general description of rocks. __. _..... 638

glacial outwash..-.._._.-,-..--.-..-- 649 

name and character__.....____._ 634

petrography of rocks . .. 638-639 

Pleistocene age...___ 641 

topographic effect....__.____..... 638

vents 638

Makushin Volcano, caldera. ...._ 642,654

Eider Point basalt....... .. 642,643,646

erosion of 666

fumaroles and hot springs..____ 642 

geologic relations..... 634-638

glacial outwash and vegetation.... 648

glaciers near.._____...___ 647 

petrography............ ....-.----. 637-641

physiography......... __ 587,634

sulfur....-. 656

oil vine-bearing andesite porphyry..... 641

Marine bench, tilting_.._..._-.,.. 665 

Marine deposits. . 652-653

basin-__-_._ ___ -- 607

near-shore..______ 606 

Marine erosion.... ... 652,665-666

Melagabboric composition, recrystallized sediments.._____-__--- 622 

Mineral amounts, percent frequency in holocrystalline rocks.. Pi. 76 

Mineralization...... 608-609,658

Moraines 647-648

N

Naginak Cove, geographic distribution of holocrystalline rocks ....-.-.. .. pi. 78 

Nateekln Bay area, basalt complex. . .. 639
INDEX 675

Page 

Nateekln River.__......................... 590

Nateekin Valley, young flows....______ 645

Pack Ice.....-.--.--...........-.--...._.... 666

Pakushin Cone, craters at__________ 654 

Eider Point basalt.............. 640-641,642,646

intrusive dacite and andesite..._____ 639 

Petrography of the batholith, accessory minerals........ .................... 616

analyses of holocrystalline rocks...___ table 1 

analyzed rocks, specific data.....___ 624-630

border rocks, composition.... ______ 622

comparative mineralogy_._______ 624 

felsic minerals...._...-.-..-.-..._.. 612-616

general..........__...........__... 612-619

mafic minerals, description..______ 616 

distribution_....................... 624

Plutonic rocks, alteration in....____ 630-631

roof pendants_______________ 619

terminology_______________ 612

textures...________________ 612

wallrock inclusions in plutonic rocks.._ 622

wallrocks, description...__.______ 612

recrystallization.. _______ 620,622-624

Physiography.____..____..___._ 660

Pillow structure, origin___________ 607-608

Unalaska formation____________ 595

Plagioclase composition, percent frequency in

holocrystalline rocks....____ pi. 76

Plutonic rocks, assimilation of wallrock__ 631-632 

ground surface __.. ___.. 616 

compositional zones____._._____ 631 

magma...........__......---..-...-.. 631,633

mechanism of emplacement.._____ 632-633

summary and interpretation_ ____ 631-634

mineralogy of____________ 612, pi. 76

Plutons, geomorphic control__________ 654

Point Kadin, Eider Point basalt.....-..--... 643

submarine geomorphology___ _ ___ 661 

Point Kadin flow, moraine at__ _____. 648 

Portage Bay, argillite_____________ 591 

Postglacial explosive activity. ________ 650 

Pyramid Peak, auriferous quartz. ______ 657 

Pyroclastic rocks, Eider Point basalt. ____ 642 

Unalaska formation...........___.. 591

Q

Quartz diorite---.-.._...._...-.---.--.- 619,620

Quartz gabbro, Tarasof Point... ______ 616,627

Quartz monzonite.._____________ 619,620

leucoalbite, 3600 Mountain............. 615,630

R

Raven Bay, granodiorite__________ 628

Recrystallized wallrocks..__________ 622

Reef Point, igneous structure..________ 596

mixed rocks_____________. __ 596

Sea-level change_______________ 652 

"Sedanka Island, zinc____________ 657-658 

Settlement_............._......._._ 590

Page

Shatshnikof River, linear features in rocks_ pL 77 

Shaler pluton, geographic distribution of holocrystalline rocks____ pi. 78 

petrography_____._____ 619 

relation to topography 654 

Shelfice - . . - 666

Skan Bay area, geographic distribution of holocrystalline rocks.____ pi. 78 

granite--.-.-__................... 629

granodiorite 627,629,630

linear features in rocks____.. pi. 77

syenodioritic wallrock..____ 627

Snowflelds and icefields_______ . .. 647

Snowline, Pleistocene and Recent.... 646,647

Soil-ash profiles........................ -- 650,651

Staraya Bay, linear features in rocks_.... pi. 77

Structural geology.....-.--..-.---..------.- 653-656

Submarine canyons, structural control. .... 665

Submarine geology....--..--....-... 659-666; pi. 75

structure_ _______________ 665-666 

Submarine topograph y, compilation of.... _ 584

Sugarloaf Cone area, andesite porphyry__.. 641 

Eider Point basalt 643-644,646

feldspathic basalt porphyry_. _____ 640 

Sulfur.-.. . 657

Summer Bay, feldspathic basalt porphyry_ 601 

Surflcial deposits and geomorphology-.._. 646-653 

Surveyor Bay, dunes...______... - 662

Tabletop Mountain, cone and flows.. .. 645

olivine basablt porphyry...___... 640

Tarasof Point, quartz gabbro... ___ 627

Terrain... - 587-589

3600 Mountain, granite- ___________ 629

leuco albite quartz monzonite . 630

Till............... 648

Topography of unconformity beneath Makushin volcanics.-____ 638

U

Udagak Strait, linear features in rocks. pi. 77 

Udamak Cove, linear features in rocks. pi. 77 

Unalaska, linear features in rocks pi. 77 

Umnak Pass.-.. 660,664

Unalaska Bay, soil profiles near...... 660,661

Unalaska formation, alteration. .. 603-606,608-609 

analyzed rocks, data.....___...... 601-603

andesite, albitized 600-601

basalt and dacite, albitized-____ 600 

bedding, sedimentary rocks. . -- 693 

bulbous intrusive bodies............ 595,598,599

clastic components_... ___j.. 693 

depositional environment-___ 607 

dikes..... ............... . 599-600

fauna.-............................. 606-607

general.....___.___.. 590

hornfels.. ............. ......,. 606-607

igneous structures description.. 693,596,598-599 

origin... - 607-608

intrusive flows_ ________ 691,695 

intrusive rocks--._____._ 595-598 

lava flows_ __________ 593
676 INDEX

Page

Unalaska formation Continued

mineralization. ..___________ 608-609 

mioceneage. - _________.. 607 

name and character.________ __ 590-591 

origin. 607-608

percent composition of composite sample. 626;

pi. 1 

pillows--.___--...------......__..... 595

petrography...______________ 600-606 

sedimentary rocks____________ 590-593 

silica variation-_____________.. 604 

sills. . - 593-595

spilitization 608

stratigraphic section. .I 592 

thickness......_- 590-591

topographic forms, volcanic rocks___. - 610 

unconformity within_ ___ _____ 593 

Unalaska village, cooperation of inhabitants.. 586 

Unconformity, at base of Pakushin volcanlcs- 668 

Unalga Pass _ . __________ 660 

U.S. Coast and Geodetic Survey, cooperation. 584

Page

U.S. naval bases________________ 586

Usof Bay, linear features in rocks.__.__... pi. 77

submarine glacial deposits .._~__. 663-665

troughs__-....--__... .- -.-- 660

Vegetation..... - . 589-590 

Volcanic structures.-.-.__ 654-655 

Volcano Bay, dunes____-- -. .. . 652

W

Wallrock, syenodlorite, Skan Bay..____ 626-627

Wave and current activity. ... . 662

Wave-cut benches_____-. .. . . 652

Wide Bay Cone, feldspathic basalt porphyry. 640

olivlne basalt porphyry_________ 640,645

form and flows_______________ 645

Zinc 657-658

U. S. GOVERNMENT PRINTING OFFICE : 1961 O -580537