Geology of Rat Island 

Aleutian Islands 

Alaska

By RICHARD Q. LEWIS, WILLIS H. NELSON, and HOWARD A. POWERS

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-Q

Prepared in cooperation with the Office, 

Chief of Engineers, U.S. Army

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1960
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D.C. - Price 35 cents (paper cover)
PREFACE

In October 1945 the War Department (now Department of the 

Army) requested the Geological Survey to undertake a program of 

volcano investigations in the Aleutian Islands Alaska Peninsula 

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

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

were assembled hastily as two administrative reports. Part of the data 

was published in 1950 in Geological Survey Bulletin 974^-B, "Volcanic 

Activity in the Aleutian Arc," by Robert R. Coats. The remainder of 

the data has been revised for publication in Bulletin 1028.

The investigations of 1946 were supported almost entirely by the 

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

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

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

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

m

529705 60

CONTENTS

	Page 

Abstract- _____________________________________________________ 555

Introduction._____________________________________________________ 555

Physiography.____________________________________________________ 557

Geology________________-______________________ 558

Rat formation______________________________________________ 558

Gunners Cove formation.______________________________________ 560

Structure. ________________________________________________________ 561

Geologic history.____-_-___-_-_--__----_-_-_-__-_--_-_-________---- 561

References cited.__________---__--_--_--_-_______--________-___--__ 562

ILLUSTRATIONS

PLATE 70. Geology and submarine topography of Rat Island__._ In pocket 

FIGURE 79. Location of Aleutian Islands and Rat Island ______________ 556

_x

INVESTIGATIONS OF ALASKAN VOLCANOES

By 

EICHARD Q. LEWIS, WILLIS H. NELSON, and HOWARD A. POWERS

ABSTRACT

Rat Island is formed of volcanic rocks, conglomerate, and sandstone that 

are divided into two stratigraphic units. The older, Rat formation, consists of 

porphyritic hornblende andesite lava flows and breccias, and conglomerate derived from them. The younger, Gunners Cove formation, contains tuffs and lavas 

of basalt, sandstone and conglomerate composed largely of basaltic fragments, 

and some hornblende andesite similar to that of the Rat formation. Marine 

conglomerate interbedded in the basalts contains a small fauna of middle Tertiary age. The older andesitic rocks, and, to a lesser extent the basaltic rocks, 

have been chemically altered and pyritized along west-northwest-trending fracture zones. Normal high-angle faults are abundant and strike generally westnorthwest or within a few degrees of north; the few dikes, all of basalt, strike 

north. Zones of altered and mineralized rock seem to trend west-northwest.

The physiographic forms are all those of erosion marine platforms and cliffs 

that are greatly <to slightly dissected by stream erosion, and are modified by 

glaciation. An extensive emerged marine platform, slightly dissected and glaciated, rises to an inland altitude of about 180 feet. Much of the sea cliff is not 

attacked by present wave action, being separated from the active shoreline by 

emerged boulder beaches and rock benches.

INTRODUCTION

Rat Island is near the center of a segment of the Aleutian Ridge 

on which also are the islands of Amchitka to the southeast and the 

southern part of Kiska to the northwest (fig. 79). Rat Island is between lat 51°45'50" and 51°50'15" N. and long l78°ll/32// and 

178°23'10" E. About half the island is a partly dissected plateau 

ranging from 125 to 200 feet in altitude. A strongly dissected highland ridge in the east half of the island rises above the plateau to a 

maximum altitude of 1,127 feet. The surface is almost completely 

mantled with tundra-type vegetation.

The island is uninhabited and undeveloped, although the remains 

of many Aleut dwellings and kitchen midden indicate a former Aleut 

population.

555
556 GEOLOGY OF RAT ISLAND, ALEUTIAN ISLANDS

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INVESTIGATIONS OF ALASKAN VOLCANOES 557

The climate is typical of the western Aleutians. Earn, fog, and 

overcast predominate during the summer; strong winds, heavy snow, 

and violent storms are common in the winter. The marine influence 

tempers the climate; the summers are cool and the winters seldom have 

temperatures as low as 0° F. Snow and ice accumulate only on the 

higher slopes and prolonged freezes are rare.

This report is based on about 3 days of fieldwork in August 1951, 

by a U.S. Geological Survey party made up of Dennis P. Cox, Joseph 

P. Dobell, Richard Q. Lewis, Willis H. Nelson, Howard A. Powers, 

and Edward C. Stover, Jr. The party was supported logistically 

by the U.S. Geological Survey MS Eider, Carl Vevelstad, captain.

PHYSIOGRAPHY

The island is about 6 miles long and 2 miles wide. About half of 

the island is a partly dissected terrace ranging from 125 to 200 feet in 

altitude. One small isolated knob near the west end of the island rises 

above the terrace to an altitude of 404 feet. A strongly dissected ridge 

rises above the bench in the east half of the island to a maximum altitude of 1,127 feet. The crest of the western part of the ridge is narrow 

and sinuous, and the east end is a broad summit of low relief.

Locally the higher slopes show physiographic features typical of 

glaciated areas cirque-shaped valley heads, smoothed rock knobs, 

and undrained rock depressions. These features are not noticeable on 

the lower slopes or on the partly dissected terrace; however, the absence of any moraines indicates that ice must have covered the entire 

island.

Shoreline features such as wave-cut benches 6 to 8 feet above sea 

level and at mean sea level, pebble and boulder benches caused by 

present wave activity, and slightly higher and turf-covered sea cliffs 

resemble those of Amchitka Island (Powers and others, 1960).

Rat Island is on the midpart of that segment of the Aleutian Ridge 

which includes Amchitka and southern .Kiska.Islands (fig. 79). Some 

features of the submarine physiography around Rat Island resemble 

those around each of the other two islands. A wide bench at a depth 

of about 300 feet is southwest, and perhaps northwest of the island. 

To the northeast, this bench is missing and is replaced, at that depth, 

by a steep slope which resembles both the slope into Murray Seavalley 

(Gibson and Nichols, 1953) west of southern Kiska Island and the 

slope north of the east end of Amchitka Island. A second, more 

steeply sloping bench terminates shoreward in submerged cliffs and 

more rugged topography. This bench is well formed at a depth of 

about 165 feet along the south, but poorly formed along the north side 

of the island (pi. 70). A third bench, between the 50-foot submarine
558 GEOLOGY OF RAT? ISLAND, ALEUTIAN ISLANDS

contour and the present shoreline, is more than half a mile wide 

around the island except in a very few places.

GEOLOGY

Volcanic rocks of subaerial and submarine emplacement make up 

Kat Island. The rocks are mapped as two formations, one limited to 

the southeastern part of the island and probably older, and the other 

is in the northwestern two-thirds of the island. The rocks in the southeast area, here named the Eat formation, are lava flows and breccias 

of porphyritic hornblende andesite and conglomerate derived from 

them. The rocks forming the larger northwest area, here called the 

Gunners Cove formation, from the small embayment of that name on 

the north side of the island, are tuffs and lava flows of basaltic composition, and sandstone and conglomerate made up largely of basaltic 

material, but including some hornblende andesite possibly from the 

Rat formation.

The rocks of the Rat formation are lithologically similar to parts of 

the Amchitka formation; those of the Gunners Cove formation are 

similar in lithology and type of occurrence to the Banjo Point of 

Amchitka Island (Powers and others, 1960). The same fossil pecten 

has been collected from the Gunners Cove and the Banjo Point rocks.

RAT FORMATION

The Rat formation contains only andesitic rock and conglomerate 

derived from it. The most abundant andesite is porphyritic and has 

many conspicuous phenocrysts of zoned plagioclase and elongate 

prisms of hornblende. The rock occurs as thick, columnar-jointed 

lava flows and massive flow breccia. It is the only constituent of the 

pebbles and cobbles of a few layers of conglomerate within the formation. Phenocrysts make up about a third of the rock: zoned labradorite-andesine crystals as large as 1 cm are abundant but not conspicuous 

because they are unaltered, and prisms of green hornblende from 1 

to 5 mm long are conspicuous but less abundant (from 10 to 15 percent) . There are also a few phenocrysts of augitic clinopyroxene and 

apatite. The groundmass is dominantly stubby crystals of sodic 

plagioclase in intergranular texture, containing a very small amount 

of interstitial cryptocrystalline material having a low index of refraction. Commonly the rock is altered so that the plagioclase crystals 

appear a chalky gray to white, and the groundmass is speckled gray 

and greenish gray. At the head of the cove about 2i/£ miles southeast 

of Gunners Cove, the rock is intensely altered and pyrite is abundant. 

One specimen of slightly altered material was collected for analysis
INVESTIGATIONS OF ALASKAN VOLCANOES 559

from the small isolated area of the Rat formation a mile north of 

Gunners Cove, though even here most outcrops are red from oxidized 

iron. The sample is from a thick flow that dips about 20° N. judging 

from the inclination of well-developed columns. The results of the 

analysis are given below:

Analysis of porphyritic hornblende andesite from the north coast of Rat Island at 

lat N. 61°49'15", long E. 178° 18'41", laboratory No. 64-444, Denver rock 

analysis laboratory, U.S. Geological Survey

Analysis

Chemical 

(analyst, L. Kehl)

Si02 . _____ .

A]2 03 -------

Fe2 03 --__

PpO

MgO -------

CaO - _-

Na2 0 ... .--

K2 0 -------

E2 0- -----

H2 0 + 

Ti02 ---_ .-.

r*n

P2 06 -

F_____________

MnO__________

57.81 

18.34 

3.80 

1.80 

3.81 

6.49 

3.61 

1.02 

1. 17 

1.21 

. 52 

.01 

. 15 

.02 

. 12

99. 88 

.01

99.87

Spectrographic 

(analyst, P. E. Barnett)

B .__----

Ba___ __ _

Co.. -__.

Cr... _____ _

Cu._______

Ga___ _____

Li. ..--__. _

Ni ______

Rb-.._..__

Sc--._ .

Sr_-_______

V. _-_-.

Y--______-

Yb_____-_

Zr-_______

<0. 001 

.04 

.002 

.004 

.005 

. 001 

.001 

.002 

.001 

.002 

.07 

.02 

.004 

. 0002 

. 006

Norm

Q _._ _

or. ________

wo __ _____

en______ ___

mt___ __ ____

il__ __ _ _

hm_ _______

ap__ _______

H2 0.__---_

13. 9 

6. 1 

30. 4 

30,7 

. 2 

9.5 

4.6 

. 9 

.6 

.3 

2. 4

Symbol "II. 4". "3. 4"

Less abundant in the Rat formation is an andesite containing many 

amphibole phenocrysts, a few biotite phenocrysts, and no augite crystals. The amphibole in this rock is also a hornblende, in yellow, green, 

and brownish green, but the colors are fainter than in the hornblende 

of the more common rock. The texture and other constituents are 

similar in both rocks.

No fossils were found in rocks of the Rat formation, and the contact with the Gunners Cove formation is not exposed. The trace of 

the contact trends north-south across the high crest of the island and 

is probably a high-angle normal fault. The small area of Rat formation north of Gunners Cove is probably a window through overlying
GEOLOGY OF RAT ISLAND, ALEUTIAN ISLANDS

beds of the Gunners Cove formation (pi. 70). Some cobbles of porphyritic andesite similar to the rocks of the Rat formation are in the 

conglomerate of the Gunners Cove formation; it is inferred that the 

Rat formation is older than the Gunners Cove formation, and probably, is of Tertiary or older age.

GUNNERS COVE FORMATION

The Gunners Cove formation includes, in approximate order of decreasing abundance, tuffaceous conglomerate and sandstone, crystalvitric basaltic tuff, and thin flows of basaltic lava. These rocks make 

up two-thirds of the island; they are exposed from sea level to an 

altitude of about 1,050 feet. Beds dip from 5° to 35° in diverse directions. Because structural details were not determined, no estimate of 

total thickness can be made.

The sandstone and conglomerate is made of moderate- to wellrounded fragments, mostly of basalt but some of porphyritic hornblende andesite, in a matrix containing less well-rounded fragments 

of basaltic glass and commonly fragments of marine shells. Many of 

the beds were deposited in the littoral zone, and one cobble deposit 

now exposed on the north shore of Gunners Cove is especially rich 

in fragments of barnacles, echinoids, crinoids, and pectens.

The crinoid and the pectens were identified in 1954 by F. Stearns 

MacNeil of the U.S. Geological Survey who reports:

The crinoid, Isocrimts aff. /. oregonensis (Moore and Yokes) and the pectens, 

Chlamys aff. C. washbumei Arnold, are considered to be of probable middle 

Tertiary age (Oligocene or early Miocene). These two species are associated 

in rocks in the sea cliff north of the mouth of the Yachats River, Oregon, which 

have been assigned variously to the Pliocene (Arnold), Middle Miocene? 

(Weaver), and to the early Oligocene (Yokes, written communication 1954). 

According to Parke D. Snavely (oral communication 1959) the locality near 

Yachats is equivalent to some part of the Keasey formation.

Basaltic lava occurs as thin flows, some with pillow structure and 

some with poorly formed columnar jointing, in thick crudely bedded 

partly glassy scoria intruded by irregular-shaped bodies, and in 

swarms of thin dikes. One complex of scoria, irregular intrusive 

material and thin flows, probably a local vent deposit, forms the 

headland of the north shore of Gunners Cove, and a second similar 

complex forms a promontory on the Pacific coast due west of Gunners 

Cove. The basalt is porphyritic to microporphyritic containing 

phenocrysts of augite and olivine, and a few phenocrysts of calcic 

plagioclase. The groundmass ranges from glassy to fine intergranular 

in texture, and is composed of euhedral to subhedral crystals of 

plagioclase and clinopyroxene.
INVESTIGATIONS OF ALASKAN VOLCANOES 561 

STRUCTURE

The only obvious structures on Kat Island are high-angle normal 

faults. Two directions of faults seem dominant, one strikes about 

N. 60° W., parallel with the alinement of the Aleutian Kidge, and 

the other, nearly at right angles, strikes approximately north. 

Several high-angle faults appear in sea-cliff exposures and others 

have been inferred from linear elements of the topography.

One fault near the west end of the island is nearly vertical, strikes 

N. 10° E., and has about 50 feet of displacement. This fault is nearly 

parallel to the north-trending contact between the Rat and the 

Gunners Cove formations south of Gunners Cove. This contact is 

exposed as a fault in the sea cliff at Gunners Cove. The high-angle 

contact observable on the ridge crest south of Gunners Cove suggests 

that the fault is continuous across the island to the south coast.

A shear zone representative of the faults striking N. 60° W. is 

exposed in the west shore cliff of the small cove 21/£ miles southeast 

of Gunners Cove. The fault dips about 70° NE. Many linear 

elements to the topography and the apparent fault scarps in the 

submarine topography suggest the existence of many additional faults 

in the area.

Fieldwork was inadequate to determine the presence of any significant folds or structures in the rocks.

GEOLOGIC HISTORY

The first geologic event recorded on Rat Island is the eruption 

of porphyritic andesite lava flows and flow breccias, probably subaerial. Land areas probably were small, as marine conglomerate derived from these lava flows are locally interbedded with them. The 

age of this activity is known only to be before the emplacement of 

the Rat formation.

The rocks were faulted and chemically altered, and some pyrite was 

introduced along the fracture zones. By analogy with the history of 

Amchitka Island, this occurred perhaps in more than one episode.

Erosion of these rocks preceded and accompanied middle Tertiary 

volcanic activity during which rocks of basaltic composition were 

erupted. Much of this activity was submarine, but locally shoals 

and many small islands were formed. Regional subsidence during 

this time is inferred from accumulation of these shoal deposits to 

thicknesses of several hundred feet in a region including the Rat, 

Amchitka, and southern Kiska Islands.

Before the last major glaciation, the island was uplifted, perhaps 

in more than one episode, and the topography shaped by both sub-
562 GEOLOGY OF RAT ISLAND, ALEUTIAN ISLANDS

aerial and marine erosion. Possibly the ridge crest at an altitude of 

350 to 400 feet and almost certainly the terrace at about 180 feet, 

are wave-cut platforms. The 180-foot terrace * may be 'a platform 

cut at the sea level of the last interglacial sea.

1 The broad bench submerged about 300 feet * may be the wave-cut 

bench of a low sea level during a glacial maximum. A shallower 

bench that terminates in an inferred shoreline at a depth of 165 feet 1 

may be related to the last glaciation.

After the last glaciation, a shoreline was formed whose cliffs are 

beyond present wave attack and whose boulder beaches were the site 

of many Aleut dwellings.

Six to eight feet below these boulder beaches are the tops of the 

storm beaches, which result from present wave activity.

REFERENCES CITED

Gibson, William, and Nichols, Haven, 1953, Configuration of the Aleutian Ridge,

Rjit Islands Semisopochnoi Island to west of Buldir Island: Geol. Soc.

America Bull., v. 64, no. 10, p. 1173-87. 

Powers, H. A., Coats, Robert R., Nelson, Willis H., 1960, Geology and submarine

physiography of Amchitka Island, Alaska: U.S. Geol. Surv. Bull. 1028P,

p. 521-554.

1 These figures of altitude and depth cannot be used to measure eustatic shift of sea level 

because they have been affected by unmeasured amounts of fault movement.