Geologic Reconnaissance 

of Kiska Island 

Aleutian Islands, Alaska

By ROBERT R. COATS, WILLIS H. NELSON, RICHARD Q. LEWIS, and HOWARD A. 

POWERS

INVESTIGATIONS OF ALASKAN VOLCANOES

' X

GEOLOGICAL SURVEY BULLETIN 1028-R

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 i-.y the Superintendent of Documents, U.S. Government Printing Office 

Washington 25, D.C.
PREFACE

In October 1945 the War Department (now Department of the 

Army) requested the Geological Survey to undertake a program of 

volcano investigations in the Aleutian Islands-Alaska Peninsula area. 

The first field studies, under general direction of G. D. Robinson, were 

begun as soon as weather permitted in the spring of 1946. The results 

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

as two administrative reports. Part of the data was published in 1950 

in Geological Survey Bulletin 974-B, "Volcanic Activity in the Aleutian Arc," by Robert R. Coats. The remainder of the data has been 

revised for publication in Bulletin 1028.

The 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.

iii

CONTENTS

Page

Abstract____.___.____.______.____________________ 563

Introduction_________________ _______________ ___ ____ 563

 * Regional setting of Kiska Island__-___--__--_-__-__________________ 564

Local topography. __________ ___. _ ___ _ 566

' Submarine topography______________________________...__ 566

Shoreline and raised marine erosional features_______________ 566

Island topography______-____-_-__-____________________ 567

Geology_____________________________________ 568

General features___-_.___._____-____ ____________________ 568

Vega Bay formation.__________-__--___-____---_____--___-_____ 569

Gabbro__________________-________________ 570

 * Kiska Harbor formation________________-_________________-.____ 571

Rocks of the Kiska Volcano______________________ 573

Surficial deposits-_-____-__-_---_-----_---___--_-_-----_. 574

Dune sand____________________________________ _______ 574

Alluvium--_____----_-__---_--------_--_--_--_---___--_-__ 574

, Beach deposits-_--____--_--_-----_--_--__---__------_--___ 574

Volcanic ash--____---__-_--_-----_--_-_------__-_- 575

J Structure_________.___----- __._-___ _ 576

Geologic history_____________________________________ _. _ 577

References_-__________-__-___-_-_--_-__------_--____-----______- 581
VI CONTENTS

ILLUSTRATIONS

Page 

PLATE 71. Geologic reconnaissance map of Kiska Island, Aleutian Islands

Alaska___________-_______-______________________ In pocket

72. A, Southern part of Kiska Island; B, Kiska Volcano and part

of plateau region_______________________________ Facing 564

73. A, Volcanic bomb; B, Breccia in Kiska Harbor formation._______________________________________ Facing 565'

74. Crossbedding in pumiceous phase of Kiska Harbor

formation_________________________________ Facing 572

FIGURE 80. Location of the Aleutian Islands and_Kiska Island on the western Aleutian Ridge___ - _________________ 565

TABLES

TABLE 1. Chemical and spectrographic analyses and norms of igneous

rocks from Kiska and Little Kiska Islands._______________ 578
INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGIC RECONNAISSANCE OF KISiKA ISLAND, 

ALEUTIAN ISLANDS, ALASKA

By ROBERT R. COATS, WILLIS H. NELSON, RICHARD Q. LEWIS, and

HOWARD A. POWERS

ABSTRACT

Kiska Island, in the Rat Island group of the Aleutian Islands, is composed of 

two major geologic elements. The south half of the island is part of a submarine ridge, and the north half is a volcano. Kiska Volcano is a young composite andesitic volcano. It is underlain and flanked on the south by the Kiska 

Harbor formation, which represents remnants of an older composite volcano. 

The Kiska Harbor formation is unconformable with the Vega Bay formation 

to the south. The Vega Bay formation is a mass of deformed submarine volcanic 

rocks.

Both Kiska Volcano and the Kiska Harbor formation are composed of interbedded andesitic to basaltic pyroclastic rocks, lava flows, and sedimentary rocks 

formed of volcanic debris. The rocks of the Vega Bay formation are older and 

more deformed than the other rocks of the island; they are composed of moderately well indurated pyroclastic rocks and substantial amounts of dark submarine basalt flows and minor amounts of sandstone and conglomerate formed 

of volcanic debris.

The island was sculptured first by marine and fluvial erosion, and the southern 

part of the island was later modified by glaciation. The northern part of the 

island shows no signs of glaciation. The island is thinly veneered by volcanic 

ash derived largely from volcanoes on nearby islands.

Kiska Island and its neighbor Little Kiska are the westernmost 

islands in the Rat Island group of the Aleutian Islands. Kiska has 

an area of 110 square miles, and Little Kiska has an area of 3 square 

miles. Kiska Island was one of the islands sighted by Bering on his 

voyage of discovery in 1741. At the time of its discovery it had a 

substantial Aleut population which thereafter rapidly declined and 

disappeared during the 19th century. During the latter part of the 

19th century natives of Atka, with their families, were brought to 

Kiska each year to hunt sea otters. With the decline in numbers of 

sea otters and subsequent restrictions placed on hunting them, the 

annual visits continued, on a smaller scale, for fox trapping.

563
564 INVESTIGATIONS OF ALASKAN VOLCANOES

Kiska Harbor is one of the few good harbors in the Aleutians. A 

detailed survey of the harbor was made in 1904, but was never published. Additional surveys were made in 1934 and 1935 after which 

Kiska was closed to the public.

Kiska was occupied in 1942 by Japanese armed forces, and later in 

1943 by American and Canadian armed forces. Since 1947 the island 

has been uninhabited.

This report is based on reconnaissance mapping by two parties of 

U.S. Geological Survey personnel. In 194T Robert R. Coats, geologist, 

and Will F. Thompson, Jr., field assistant, were on Kiska for about 3 

weeks; and during 1951 a party of 6 geologists, Dennis P. Cox, Joseph 

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

and Edward C. Stover, Jr., spent about 3 weeks on the islands. During 

1947, transportation was provided by the U.S. Coast and Geodetic 

Survey and by the U.S. Army. The help of the officers and men of 

these services is gratefully acknowledged. During 1951 transportation 

was by the U.S. Geological Survey's motorship Eider operated under 

the direction of Capt. Carl Vevelstad.

The parts of this report dealing with areal geology are the joint 

responsibility of all the authors; the interpretations of submarine 

topography have been made by Powers from detailed boat sheets, 

kindly furnished by the U.S. Coast and Geodetic Survey.

REGIONAL SETTING OF KISKA ISLAND

Kiska Island rises from the Aleutian Ridge in an area where the 

ridge appears to consist of two major linear elements (fig. 80). The 

larger and older element is the line of three approximately rectangular 

segments, partly separated from each other by the Heck and the 

Murray Seavalleys. The western segment is represented above sea 

level by the Near Islands; the eastern segment, by the southern part 

of Kiska, Little Kiska, Rat, and Amchitka Islands; the middle segment is entirely submerged. The rocks of these islands (and, by 

inference, of these segments of the Aleutian Ridge) are mostly 

older than late Tertiary, and the landforms of the islands were 

produced by faulting and erosion (Gates, Fraser, and Snyder, 1954; 

Gates and Gibson, 1956; Coats, 1956; Powers, Coats, and Nelson, 

1960). The smaller and younger linear element is a line of active or 

dormant volcanoes Buldir, Kiska, Segula, Khvostof, Davidof, Little 

Sitkin, and Semisopochnoi the Buldir Depression with its associated 

seamounts, and numerous other submarine features between Kiska 

and Semisopochnoi Islands that are inferred to be of volcanic origin 

(Coats, 1953, 1956, 1959; Nelson, 1959; Snyder, 1957, 1959). Some 

parts of the composite volcanoes of this linear element are of Recent
GEOLOGICAL SCKVKY Hdl.LETIN 1II2S PLATE 72

A. SOUTHERN PART OF KISKA ISLAND

View is southwest from above center of island. Note steep west slope (right) fronting the submarine 

Murray Sp a valley am! gentle east slope (left center) fronting Vega Bay. Photograph by U.S. Navy.

li. KISKA VOLCANO AND PART OF PLATEAU REGION 

View is north from Kiska Harbor (foreground). Photograph by U.S. Navy,
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 73

A. VOLCANIC BOMB 

Bomb has weathered out of the Vega Bay formation about one-half mile northeast of Dark Cove.

U. BRECCIA I.N KISKA HARBOR FORMATION

Block is from the cliffs at North Head on the north side of Kiska Harbor. Shows poor sorting and lack

of bedding.
172 « 176

53'

52'

51

,

\/-4

- V

OCEAN

/ x.ri^^^cf'

172" 174° 176" 

DEPTH CURVE INTERVAL 6000 FEET 

DOTTED SUPPLEMENTAL DEPTH CURVE 600 FEET

FIGURE 80. Location of the Aleutian Islands and Kiska Island on the western Aleutian 

the southern linear element of the ridge; solid pattern shows active and dormant 
566

age; the oldest parts are believed to be no older than late Tertiary 

because the degree of destruction of their volcanic forms by erosion is 

similar to that of the Kanaton volcanic rocks on Kanaga Island that 

contain marine fossils of Pliocene age (Fraser and Barnett, 1959).

Kiska Island includes parts of both these major linear elements; 

the north half of the island has topographic and geologic features 

characteristic of the younger northern linear element, and Little Kiska 

and the south half of Kiska have characteristics similar to the islands 

of the older southern linear element.

LOCAL. TOPOGRAPHY

SUBMARINE TOPOGRAPHY

The general aspect of the submarine topography near the shore of 

the southern half of Kiska Island differs from that near the north 

half.

Along the south and southeast coast there is a broad and fairly continuous bench between 300 and 350 feet in depth on which a number 

of knobs are indicated by closed 300-foot-depth contour lines. Closer 

inshore, the shape of the 100- and 200-foot-depth contours suggests 

ridge and valley topography. Along the west coast of southern Kiska, 

adjacent to Murray Sea valley, there is no broad bench at depths of 

300 feet, and the suggestion of ridge and valley topography is expressed only by the 100-f oot-depth contour.

Offshore to the east and the west of the north half of the island, the 

submarine topography is characterized by seamounts with relief of 

several hundred feet, both circular and irregular in shape, and by 

closed depressions with irregular, angular shapes and depths of as 

much as several hundred feet below their rims. North of the cone of 

Kiska Volcano, the submarine slope below a depth of 200 feet appears 

to be a continuation of the subaerial slope of the cone.

SHORELINE AND RAISED MARINE EROSIONAL FEATURES

The sea cliffs around Little Kiska and Kiska, except those around 

Kiska Volcano, are mostly not subjected to marine erosion at-the present time. Colluvium, talus, or beach deposits, all covered with wellestablished vegetation, lies at the base of the sea cliffs out of reach of 

present storm waves. The turf-covered beach deposits are behind and 

several feet above the modern storm beach deposits, and they commonly 

are the site of early Aleut dwellings and kitchen midden. Similar 

shore features are described from other Aleutian islands (Byers, 1959; 

Powers, Coats, and Nelson, 1960) and have been interpreted as the 

result of a prehistoric lowering of sea level by a few feet.
GEOLOGIC RECONNAISSANCE OF KISKA ISLAND 567

The coastline of Kiska Volcano, in contrast, is continuous cliff that 

ranges in height from a few tens of feet to over 1,500 feet, most of 

which is being subjected to present marine erosion. In a few places 

massive rock faces extend below sea level, but at the base of much of 

the cliff is talus and boulder beach material that is being worked by 

storm waves.

A few terraces at altitudes up to 220 feet appear to have been formed 

by marine erosion. The terrace inland from Vega Point at an altitude 

of 140-160 feet truncates both bedded clastic rock that has an apparent 

dip of as much as 20° SE., and lava (in Vega Point) that is jointed in 

vertical columns. The terrace is almost undissected. A bench on both 

sides of Dark Cove at an altitude of 220 feet truncates bedded tuffaceous sandstone that dips about 13° SW. Bounded boulders as much 

as a foot in diameter, now pitted and broken, are found on the surface 

of this bench east of the cove. A lower terrace east of Dark Cove 

truncates this same sandstone at an altitude of 65 feet.

ISLAND TOPOGRAPHY

The island comprises three regions with conspicuously different 

topography; the south half, a region of strong dissection dominated 

by a sharp, sinuous drainage divide; the north-central part, a region 

of isolated plateaus; and the northern tip, which is the symmetrical 

cone of Kiska Volcano. The landforms in the first two regions are 

the result of destructional processes, faulting and erosion, whereas the 

form of Kiska Volcano is principally constructional, slightly modified 

by erosion. Much of the surface of the island is covered by a poorly 

drained blanket of tundra vegetation; there are no trees on the island. 

Between the tops of the sea cliff and an altitude of 500 feet the vegetative cover is nearly complete and outcrops are very scarce, but small, 

shallow, peat-walled lakes are common.

The south half of the island contains the highest destructional landforms ; several of the peaks exceed 1,400 feet altitude and the highest 

peak is somewhat over 1,800 feet (pi. 72A). The drainage divide is 

much closer to the northwest coast. Streams on the northwest slope 

occupy short, deep amphitheatrical valleys, whereas the valleys of the 

southeast slope are longer and less deeply entrenched. The longer, 

streams are characterized by straight stretches and abrupt, angular 

changes of direction; their courses appear- to. be fracture controlled. 

Most of the north west-trending valleys, and the more northerly of 

the southeast-trending valleys head in well-developed glacial cirques in. 

which some cirque lakes still remain. Other glacial landf orms such 

as roches.moutonnees and irregular closed bedrock depressions, .now 

occupied by ponds, are abundant both in valleys and on ridge surfaces,
568

but no striated and polished rock surfaces were seen. Marginal or 

terminal moraines are not present, but thin deposits of till are exposed 

in a few artificial cuts.

In the north-central region of nearly or completely isolated plateaus, 

the maximum altitude of about 1,200 feet is a prominence on the gently 

undulating surface of the largest plateau (pi. 725). Over much of 

this region the plateau surfaces truncate the layered structure of the 

bedrock, but locally, as in the flat area at an altitude of 900 feet to 

the southwest of Sredni Point, the surface conforms to the attitude 

of the uppermost layer of country rock. Several small ponds at different altitudes occupy depressions in bedrock. Most of the high 

surface is mantled with a colluvium of angular fragments broken 

from local bedrock mixed with small amounts of volcanic ash.

Most of the surface of Kiska Volcano is the constructional surface 

of block lava flows modified slightly by stream erosion. Locally, the 

gullied surface is underlain by bedded pyroclastic material. In general, the local relief on the surface of the cone is due to steep flow 

margins, collapsed lava channels, arcuate flow ridges, and similar 

features related to the emplacement of viscous lava. None of the 

surface of the cone appears to have been glaciated.

GEOLOGY

GENERAL FEATURES

As noted previously, Kiska Island lies at the junction of, and is composed of parts of, two major geologic elements that diverge to the 

east (fig. 80). The southern part of Kiska and Little Kiska are 

part of a submarine ridge that extends southeastward and includes 

Eat Island and Amchitka Island. The northern part of Kiska is 

formed by Kiska Volcano, which is one in a line of volcanoes that 

includes Buldir, Segula, Khvostof Davidof, Little Sitkin, and the volcanoes on Semisopochnoi Island.

The southern part of Kiska is composed of interbedded pyroclastic 

rocks and laval flows, mostly of basaltic composition, with minor 

amount of intrusive rocks. These rocks, which are here given thename Vega Bay formation, are more deformed and altered than the 

younger rocks to the north. The source or sources of these rocks has 

not been recognized.

After their deposition, the rocks of the Vega Bay formation were 

intruded by a gabbro mass now exposed east of Gertrude Cove. The 

rocks of the Vega Bay formation are altered for a distance of about 

one-third mile on all sides by this gabbro mass, and alteration elsewhere in the same region suggests that the gabbro may be more extensive at depth.
GEOLOGIC RECONNAISSANCE OF KISKA ISLAND 569

The northern part of Kiska, from Kiska Harbor northward, is made 

up chiefly of pyroclastic rocks and flows that form an erosional remnant of an older volcano, surmounted by the slightly dissected cone of 

Kiska Volcano, which has been active in relatively recent time.

Much of Kiska Island was probably glaciated during Pleistocene 

time. Subsequent to the glaciation of blanket of fine stratified basaltic 

ash was laid down over most of the land surface, north of Gertrude 

Cove by several successive eruptions. Subsequently the ash has been 

stripped by the wind from more exposed localities and redeposited 

on more sheltered sites as fine, nearly structureless deposits, closely 

resembling loess and locally as much as 6 feet thick. The distribution 

of volcanic ash is not shown on the geologic map (pi. Yl).

VEGA BAY FORMATION

The rocks that underly the south half of Kiska and most of Little 

Kiska Island are here named the Vega Bay formation. The formation is composed predominantly of pyroclastic rocks interbedded with 

substantial amounts of flow rocks and minor amounts of sandstone 

and conglomerate, which consist of reworked volcanic material.

The pyroclastic rocks are all rather drab colored and range from thin 

layers of well sorted, fine tuff to thick layers of unsorted tuffaceous 

volcanic breccia. They are composed of fragments that range from 

fine dust to blocks several feet in diameter. The small fragments are 

glass shards, scoria, and broken crystals of pyroxene and plagioclase; 

the larger fragments are volcanic bombs and broken pieces of volcanic rock (pi. 73^1). Many of the fragments, especially the smaller 

ones, are dull in luster owing to the development of secondary minerals 

and mineraloids, principally palagonite, nontronite, and saponite, 

probably formed by alteration of volcanic glass. Most of the material in these rocks is probably basaltic in composition.

Most of the flows are compact dark-gray predominantly crystalline 

rocks, but locally they are vitrophyric. Most of the flows are probably basaltic. The chemical composition of a sample of a flow from 

the ridge summit 4^ miles northwest of Bukhti Point is given in 

column 1, table 1.

Pillow structure is common, notably on the west side of the small 

peninsula at the head of Sargeant Cove. The pillows are composed 

of fine-grained dark-gray basalt with a darker glassy selvage about 

1 inch thick forming the outermost part of the pillows. The interstices between pillows are filled in part with greenish earthy material, apparently a hydration product of basaltic glass, and in part 

with pale-grayish-green limestone, probably a consolidated limy mud 

into which the basalt flowed.
570 ' INVESTIGATIONS;.OF ALA&KAN VOLCANOES

The sedimentary rocks consist of fragments of volcanic rock that 

range from.silt to cobbles. The fragments are subangular to subrounded and consist largely of porphyritic basaltic lava; a few are 

chalcedony and red and green jasper.

In general, the rocks of the Vega Bay formation south of latitude 

51°55' are relatively unaltered except for the change of olivine to 

iddingsite or bowlingite, and of glass to palagonite, nontronite, 01 

saponite. However, north of this latitude, in the central part of the 

island and including Little Kiska, alteration of many rocks is conspicuous, and in some it has obliterated the original minerals. The 

assemblage of secondary minerals, includes albite, chlorite, epidote, 

calcite, titanite, and quartz. The alteration of these rocks may be related to the gabbro, which is exposed east of Gertrude Cove, and 

which may extend under the altered area.

Mode of origin. The Vega Bay formation was largely the product 

of submarine deposition of basalt as indicated by pillow lava with 

interstitial limestone; however, conglomerate and sandstone, made up 

of fragments of the volcanic rocks of the formation, suggest that some 

parts of the formation may have been exposed to subaerial or marine 

erosion during its accumulation.

Thickness. No complete sequence of the Vega Bay formation is 

exposed. Along the coast of Vega Bay between long. 177°22' and 

11T°25/ E., west of Gertrude Cove, which is the type locality, the rocks 

all dip to the west, and the section apparently is uninterrupted. Here 

there is about 2,000 feet of the Vega Bay formation, a minimum value 

for the thickness of the formation.

Age. As noted above the parts of Kiska and Little Kiska Islands 

that are underlain by the Vega Bay formation, are part of a submarine 

ridge that is also surmounted by Amchitka and Rat Islands. The 

composition, degree of deformation, and alteration of constituents 

of the rocks of the Vega Bay formation are similar to that of rocks 

on Amchitka and Rat Islands that contain fossils of late Oligocene or 

early Miocene age, and on this basis the Vega Bay formation is considered to be of middle Tertiary age.

GABBRO

About 1 square mile of gabbro is exposed along the south coast, east 

of Gertrude Cove. A body of similar rock (whose boundaries could 

not be mapped in the dense fog that prevailed during the available 

tune) crops out in a saddle on the summit ridge (in the vicinity from 

which sample 51.N23 was collected). These rocks contain grains rangr 

ing from 1 to 3 mm in. size, and are composed of euhedral to subhedral tabular plagioclase, subhedral tabular clinopyroxene, with an
GEOLOGIC .RECONNAISSANCE OF KISKA ISLAND 571

interstitial matrix of chlorite, some saponite, a little alkalic feldspar, 

and small crystals of apatite and opaque oxides. A few patches of 

bowlingite (?) may be pseudomorphic after olivine, and a few 

patches of chlorite may be pseudomorphic after orthopyroxene with 

bronzite structure. The plagioclase is labradorite, some of which is 

progressively zoned to an outer rim of oligoclase. An analysis of this 

gabbro is given in column 2, table 1. On the whole, the gabbro is 

little altered in contrast to the altered rock of the Vega Bay formation into which it is intruded. The gabbro is probably of middle 

Tertiary age.

KISKA HABBOE FOBMATION

The rocks which occupy about 30 square miles in the north-central 

part of the island are here named the Kiska Harbor formation. The 

rocks are unconformable on the Vega Bay formation on the south, 

and to the north are covered by the rocks of Kiska Volcano and 

alluvium. On Little Kiska, water-laid rocks of the Kiska Harbor 

formation lie in a depositional contact unconformably on a northsloping surf ace'eroded on steeply dipping beds of Vega Bay formation. On Kiska Island, the contact between these two formations is 

probably a fault.

The Kiska Harbor formation consists of subaerial lava flows, autoclastic breccias, pyroclastic rocks, and water-laid pumiceous sand and 

conglomerate. Lava flows predominate in the north and waterlaid beds in the south. The type section of the pyroclastic facies of 

the formation is the bluff north of Trout Lagoon at the head of Kiska 

Harbor; that of the lava flow facies is in Sredni Point. Between these 

two extremes the flows and sedimentary rocks interfinger; the flows 

thin southward and grade into autoclastic breccias, and the sedimentary layers thin northward. Pyroclastic layers are interbedded 

with the flows and water-laid sedimentary rocks, and some of the 

coarser pyroclastic layers resemble the autoclastic parts of flows. One 

discontinuous lava flow and one discontinuous volcanic breccia layer 

are mapped as members of the formation. This volcanic breccia layer 

is believed to be of autoclastic origin.

The water-laid pumiceous sand and conglomerate are composed 

exclusively of volcanic debris. Most of this material is crossbedded; 

in some places the crossbeds are inclined dominantly to the east and 

southeast, in other places they are inclined to the south and southwest 

(pi. 74). The finer fragments are shards .and mineral crystals; the 

larger fragments are pieces of volcanic rock. Some of the fragments 

are light- to medium-gray or yellowish-gray finely vesicular rocks; 

others are somewhat darker and more compact. Both kinds are 

slightly porphyritic. The spiecific gravity of the vesicular rocks
572 INVESTIGATIONS OF ALASKAN VOLCANOES

ranges from 1.5 to 1.95, and the specific gravity of the darker rocks 

from 1.75 to 2.61. The lighter colored fragments consist of plagioclase phenocrysts that have a median composition of An66, green 

hornblende, and augite, all embedded in a hypohyaline groundmass 

composed of plagioclase microlite in clear glass. The glass makes up 

about 60 percent of the groundmass and has an index of refraction 

of 1.50. The lighter rock includes some microxenoliths of magnetiterich lava that includes oxyhornblende phenocrysts. The lighter rock 

is probably dacite. The principal phenocrysts in the darker fragments are zoned plagioclase with a median composition of An62, and 

augite. The groundmass consists of plagioclase laths and microlite, 

hypersthene and pyroxene needles, magnetite grains, and about 30 

percent of brown glass with a mean index of refraction of 1.525. 

The composition of the darker rock is andesitic to basaltic.

The lava flows of this formation vary in thickness from a few to 

several hundred feet. Many flows exhibit columnar jointing and 

many are vesicular. The lack of pillow structures suggests that these 

are subaerial flows. The rocks in these flows are all vitrophyric with 

numerous phenocrysts of plagioclase (bytownite-labradorite) as much 

as 4 mm. across in either a dark vitric groundmass, or dark-gray somewhat diktytaxitic groundmass. Smaller phenocrysts of hypersthene 

are the most abundant (as much as 10 percent) dark minerals, and 

clinopyroxene is always present. The chemical analysis of a sample 

of one of these flows from Little Kiska Head is given in column 4, 

table 1.

The volcanic breccia is made up of coarse angular to subrounded 

basalt fragments in a matrix of dark-brownish-gray fine-grained volcanic debris (pi. 73#). The basaltic fragments are as much as 2 feet 

in diameter. Locally the proportion of rock fragments to matrix is 

low, and the rock is a finely divided yellowish-brown material that resembles pyroclastic tuff with a few scattered angular rock fragments. 

More commonly rock fragments predominate. The fragments are 

chiefly medium-gray finely crystalline, slightly porphyritic basalt, 

with a small volume of irregular vugs. Phenocrysts are plagioclase, 

olivine, and augite as much as 2 mm across; their abundance varies 

from sample to sample, the feldspar from 1 to 25 percent, and olivine 

and augite from less than 1 to 5 percent. The chemical composition 

of a specimen of this rock is given in column 3, table 1.

Locally this breccia grades into massive flow basalt. This fact 

rules out a pyroclastic origin for the unit in spite of the pyroclastic 

appearance of its matrix. Possibly the fragmentation of this breccia 

is the result of a part of the flow entering water. In addition some 

of the fragmentation may be normal autobrecciation due to flowage 

of a cooling lava.
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 74

CROSSBEDDING IN PUMICEOUS PHASE OF KISKA HARBOR FORMATION

Section is on north side of Kiska Harbor. Shows poor sorting, lenticularity of beds, concave bedding, and 

persistence in vertical cliffs. View shows about 8 feet of section.
GEOLOGIC RECONNAISSANCE! OF KISKA ISLAND 573

In the vicinity of Haycock Rock the Kiska Harbor formation is 

more altered than it is elsewhere. This alteration is probably the 

result of fumarolic action either associated with the extrusion of the 

rocks of the Kiska Harbor formation, or the extrusion of the rocks 

of Kiska Volcano.

Mode of origin. The dominance of reworked volcanic debris in 

the south and the gradual northward increase in the abundance of 

flows suggest an eruptive source to the north. It seems likely that 

the source of the rocks that make up the Kiska Harbor formation was 

in about the same locality as the present Kiska Volcano, and that the 

formation is all that is left of an older volcano that occupied approximately the same site as the present-day volcano.

Age. The Kiska Harbor formation overlies, and is thus younger 

than, the Vega Bay formation; and it is in turn overlain by the rocks 

of Kiska Volcano, which are at least as old as Pleistocene. The 

rocks of the Kiska Harbor formation resemble the rocks of dissected 

composite volcanoes on the north end of Tanaga and Kanaga Islands 

that contain fossils of Pliocene age. The Kiska Harbor formation is 

thus probably late Tertiary or early Pleistocene in age.

ROCKS OF KISKA VOLCANO

The rocks of Kiska Volcano are interbedded flows and pyroclastic 

rock that form the composite volcano at the north end of Kiska Island. 

The flows of this unit range from a few feet to more than 100 feet 

in thickness. Locally they have red top and bottom breccias. They 

are gray fine-grained porphyritic andesites. Some of them are hypersthene andesite, in places with a trace of olivine; and some are 

two-pyroxene andesite. Table 1 gives analyses of specimens from 

three flows. The debris, which makes up the pyroclastic rocks, ranges 

in size from fine volcanic ash to large volcanic bombs. The fragments 

range from vesicular to massive and from glassy to fine grained. Individual ash layers are generally much more extensive than individual 

flows.

On the west coast, 2 miles south of Vulcan Point, a minor subsidiary 

cone has been truncated by marine erosion. At least one thin lava 

flow with a coarse breccia top of hypersthene-olivine andesite, extensive exposures of tuff-breccia, and one dike or plug of hyperstheneaugite-hornblende andesite are exposed in the shore cliffs.

Five of the most recent flows on Kiska Volcano are mapped separately on the basis of the degree of preservation of original surface 

details, and on the sparsity of vegetation or pyroclastic cover. The 

young lava flows are uniformly block lava. Where they have accumulated on relatively gentle slopes, as at Sirius Point and on the southeast side of the volcano, they have come to rest with very steep flow
574 INVESTIGATIONS OF ALASKAN VOLCANOES

fronts as much as 100 feet high. Most of the lava flows weather 

nearly black, but are medium gray to light brownish gray on fresh 

surfaces. They are augite-hypersthene andesites, locally with a little 

olivine; the plagioclase is labradorite. Some of the vesicular lapilli 

collected near the summit crater are richer in glass and darker in 

color than most of the flows, and they contain traces of brownishgreen hornblende. Chemical analysis of the lava from Sirius Point 

is given in column 9, table 1.

Age. The surface of Kiska Volcano has not been glaciated, so the 

surface flows must be younger than the time of last major glaciation, 

though the interior flows of the cone might be as old as late Pleistocene. The most recent flows still show original cooled surfaces, and 

these flows cannot be many hundreds of years old. At any given a]titude the most recent flows of Kiska Volcano are more heavily vegetated than are flows erupted during the 20th century on other Aleutian 

Islands, which suggests that the most recent flows on Kiska are one 

to several centuries old.

STJBFICIAL DEPOSITS 

DUNE SAND

The principal areas of sand dunes are at the mouth of Salmon 

Lagoon, at the head of Kiska Harbor, and at the heads of Beach and 

Bluff Coves. The highest dunes, which attain a height of 125 feet, 

are south of the entrance to Salmon Lagoon. All the dunes are 

close to beaches from which the sand was derived. The dunes form 

only in areas underlain by the Kiska Harbor formation, apparently 

because that is the only formation which provides sufficient finegrained material.

ALLUVIUM

The alluvium on Kiska Island has been deposited in a depression 

that resulted from a lava flow blocking the lower end of a broad swale 

northwest of Haycock Kock, in the low area south of Kiska Volcano 

which is closed off from the sea by bars, and in some of the larger valleys. In part these deposits are deltaic. In most places these deposits are still accumulating, but locally they are slightly dissected, 

such as the small body southwest of Eeynard Cove. Most, of the 

alluvium consists of silt, sand, and gravel; near the foot of Kiska 

Volcano it includes poorly sorted f anglomerate.

BEACH DEPOSITS

Most of the beach deposits are composed, of coarse material that 

ranges from pebbles to boulders 2 feet and locally 3 feet in diameter.' 

Sand beaches are rare, and are confined to some of the more sheltered 

bays; elsewhere the coasts are exposed to intense wave action, and
GEOLOGIC RECONNAISSANCE, OF KISKA ISLAND 575

fine-grained material is speedily transported to deeper water. The 

most conspicuous beach deposits form the bars west of West Kiska and 

Christine Lakes; these deposits are largely gravel and cobble material. 

The altitude of the highest berm of these exposed beaches is as much 

as 20 feet.

VOLCANIC ASH

Volcanic ash forms a thin discontinuous blanket not shown on the 

geologic map. The ash is commonly bedded and is made up of angular 

fragments that range in size from fine dust to pieces 2 mm across. A 

composite sample of volcanic ash contains palagonitized pumice 

fragments, fragments of clear glass with an index of 1.495, plagioclase 

grains (chiefly sodic labradorite), hornblende, augite, olivine, 

hypersthene, and magnetite. Many of the mineral grains have sharp 

unworn faces that show they have undergone little reworking. This, 

and a predominance of unbroken crystals, suggest that the ash is 

crystal-vitric tuff.

The following table presents a measured section of Kecent volcanic 

ash about 2,300 feet northwest of the northwest corner of Kiska 

Harbor.

Material

Top of section :

Ash...........

Do____-__.

Do_____-_.

Do__ _..._.

Do........

Do._.__._.

Disintegrated bedrock.

Grain size

Fine..... _

.....do ...

..--.do......

.....do......

Medium. __ .

.....do....--

Fine to

medium. 1

Color (Goddard and others, 1948)

Moderate-yellowish-brown

(10YR 4/3). 

Light-yellowish-brown (1 0 YR

5/2) grading upward to medium 

gray. 

Light-yellowish-brown (10 YR

5/2). 

Light-yellowish-brown (10 YR

5/2) grading upward to medium 

gray. 

Dusky-yellowish-brown (10 YR

3/2) grading upward to medium 

gray. 

Moderate-yellowish-brown

(IQYR 5/4). 

Dusky-yellow-green (5GY 5/2). _.

Total thickness __ ____.

Thickness 

(inches)

7

1. 4

2,

2

3.4

n ^

2.5

18.8

1 Contains two layers, 2 mm thick of coarser fragments.

Little evidence is available to permit determination of how much of 

the finer ash is derived from Kiska Volcano, and how much from 

Little Sitkin or Segula volcanoes to the east, both of which appear to 

have been more active than Kiska Volcano in recent time; the lack 

of any rapid changes in thickness or grain size with distance from
576 INVESTIGATIONS OF ALASKAN VOLCANOES

Kiska Volcano suggest the derivation of most of it from Segula or 

Little Sitkin.

In many places, the bedded ash is overlain by brown, nearly structureless loesslike pyroclastic material, which may be as much as 6 feet 

thick in sheltered spots. This loesslike ash is probably derived from 

the bedded ash, inasmuch as the bedded ash is missing from places exposed to wind erosion, and the massive ash is found largely in sheltered 

locations at lower altitudes. It is overlain by peat of variable thickness, so it is not being formed at the.present time, and is exposed 

mostly in artifical cuts.

STRUCTURE

Kiska and Little Kiska Islands, taken together, are composed of 

three structural units, which correspond with the three major rock 

units of the islands. The parts of the islands underlain by the Vega 

Bay formation are the oldest and structurally most complex parts; 

the parts underlain by the Kiska Harbor formation are younger and 

structurally less complex; and Kiska Volcano is the youngest part 

and has the least structural complexity.

Linear features of two dominant trends are recognized on aerial 

photographs. These features trend about N. 30° E., and about N. 60° 

W. A few of them are continuations of normal faults recognized in 

the field. Haycock Depression, the deep submarine trench east of 

the island, and a smaller trench west of Kiska Volcano may be related 

to the system of northwestward-trending faults. Other submarine 

features, such as Murray Seavalley and Stephen Eidge, shown by 

Gibson and Nichols (1953, pi. 1), appear to be controlled by northeastward-trending faults.

The Vega Bay formation is the most deformed. Most of the beds 

dip rather steeply and appear to be folded. Southwest of Gertrude 

Cove most of the rocks dip westward, which suggests that the dominant structure there is the limb of a large fold. North of Gertrude 

Cove the rocks of the Vega Bay formation seem to dip mostly away 

from the gabbro intrusive mass, which suggests that the dominant 

local structure in that area is a large dome.

Most dips in the rocks of the Kiska Harbor formation are less 

than 15°, and the dominant structure there seems to have resulted 

from gentle tilting of blocks between normal faults. The lowlands 

south of Kiska Volcano probably are downdropped on the north 

side of at least two faults that trend parallel to the N. 60° W. group 

of lineaments. The lava flow member of the Kiska Harbor formation, mapped in this area, is the key bed that demonstrates this downdropping. Some of the bedded rocks of the formation under the 

lava flow member have dips several degrees steeper than the base of 

the flow. Part of this discordance may be due to discordance of
GEOLOGIC RECONNAISSANCE OF KISKA ISLAND 577

initial dips, but some of it seems to represent intermittent structural 

dislocation during the accumulation of the rocks of the Kiska Harbor 

formation. The large submarine Haycock Depression to the east, 

and a smaller submarine depression due west of Kiska Volcano, are 

alined with this downdropped zone. These submarine depressions 

are inferred to be younger than the Kiska Harbor formation; if 

older than, or coincident with, the deposition of these rocks, the depressions would probably have been filled.

All the structure that has been observed in the rocks of Kiska 

Volcano can be attributed to original dip of the flows and pyroclastic 

layers that make up the volcano.

GEOLOGIC HISTORY

The known geologic history of Kiska Island begins with the eruption of basaltic pyroclastic material and flows probably from a number 

of subaerial and submarine vents. This material accumulated to an 

unkown thickness; parts of it were exposed to subaerial and marine 

erosion from time to time and redeposited as conglomerate and volcanic graywacke. These rocks, represented by the Vega Bay formation, were disturbed by block faulting and intruded by small bodies 

of gabbro. Subsequently, a composite volcano grew above sea level 

north of an island composed of these older rocks, and erupted large 

quantities of fine dacite pumice, which accumulated in stream valleys 

and sheltered embayments. Continuing eruptions deposited breccias 

and lava flows of olivine basalt and pyroxene basaltic andesite. These 

rocks make up the Kiska Harbor formation. After a long period 

of erosion the island was uplifted, perhaps by block faulting, during 

which time several terraces, now above sea level, may have been cut 

by marine planation. During the Pleistocene, ice probably covered 

much of Kiska Island perhaps more than once. Localized volcanic 

activity began to build Kiska Volcano, perhaps during part of the 

epoch of glaciation, but eruptions continued until very recent times. 

Minor amounts of volcanic ash, mostly from volcanoes on other islands, 

represent some of the most recent deposits on Kiska and Little Kiska 

Island.

Evidence of historic activity of Kiska Volcano is scanty. The 

earliest report (Egbert, 1905) states that solfataric activity was noticeable in the crater. Egbert's description of the form of the crater 

corresponds closely to that seen in 1947; this suggests that the activity 

in the crater during the intervening years was probably no more than 

solfataric. Newspaper reports in June 1943, shortly before the island 

was occupied by American and Canadian forces, tell of an ash eruption accompanied by earthquakes, but in 1947, when the mountain was 

climbed, no evidence of any recent ash eruption, and no solfataric or 

fumarolic activity were noted.
578 INVESTIGATIONS OF ALASKAN VOLCANOES

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TABLE 1. Chemical and specirographic analyses and norms of igneous rocks from 

1 2 3 4 6 

Modified CIPW norm

Quartz_______-__ __._.__

Orthoclase _ _______ ___

Albite. __ _______ .

Anorthite _ _____________

Wollastonite. ____________

Ferrosilite _ __ ___ _____

Enstatite_____ _________

Fayalite _ . -.__ _______

Forsterite_ ______________

Magnetite. ______________

Ilmenite __ _ ____________

Apatite. _________ ____ _

H2 0____________________

CIPW symbol. .___ _ _.

Rittmann symbol. _ ____

2. 5

22. 0

29. 7

7.8

5.2

14. 3

3.5

8.8

3.4

1.5

. 3

1. 2

III.5.3.

(4)5 

B8

2.5

2.5

21.0

42. 5

6. 1

4. 6

10.8

6.7

1. 5

. 2

1. 6

II (3) 4. (3)

4. (4) 5 

B8

2.8

5.3

26. 7

37.4

3.5

6.2

10.6

4.4

1.7

.3

1. 2

II.(3)4.

3.4" 

B8

11.0

8. 1

30.9

29.5

2.6

3. 7

7.8

4.9

1.3

. 4

. 2

11.4(5).

"3.4

A7

8. 3

8.9

36.9

23.4

2.7

5.9

6. 8

3.6

2. 1

.5

1.0

II. (4) 5.

2".4 

B7

1. Olivlne basalt, Vega Bay formation: phenocrysts olivlne (altered to bowlinglte) and 

clinopyroxene. Specimen 51P65. Lat 51°57'52" N.; long 177°25'24" E.

2. Oabbro, Vega Bay formation: plagioclase zoned from labradorite to oligoclase, 

clinopyroxene and orthopyroxene. Specimen 61N23. Lat Bl°66'50" N.; long 

177°22'12" E.

3. Olivlne basalt breccia, Kiska Harbor formation: rich in phenocrysts of bytownitelabradorite, olivine (altered to bowlingite), and clinopyroxene. Specimen 51P54. 

Lat 51°59'38" N.; long 177°31'37" E.

4. Porphyritic andesite, Kiska Harbor formation: rich In phenocrysts of labradorite, 

orthopyroxene, and clinopyroxene. Specimen 51N6. Lat 51°58'15" N.; long 

177°38'17" E.

5. Fine-grained dike, intrusive rocks in Kiska Harbor formation: few microphenocrysts 

of labradorite, very few of clinopyroxene. Specimen 61L10. Lat 62°00'23" N.; 

long 177°35'22" E. -

6. Porphyritic andesite, 

orthopyroxene, clin

opaque dust. Spec

7. Porphyritic andesite, f

clinopyroxene, orth

52°07'13" N.; long 17

8. Porphyritic andesite, 

pyroxene, and clin

177°40'00" E.

9. Porphyritic andesite, f

clinopyroxene, and 

177°36'18" E.
GEOLOGIC RECONNAISSANCE, OF KISKA ISLAND 581 

REFERENCES

Byers, F. M., Jr., 1959, Geology of Umnak and Bogoslof Islands, Aleutian

Islands, Alaska: U.S. Geol. Survey Bull. 1028-L, p. 267-369. 

Coats, R. R., 1950, Volcanic activity in the Aleutian arc: U.S. Geol. Survey BulL 

974-B, p. 35-49.

 1953, Geology of Buldir Island, Aleutian Islands, Alaska: U.S. Geol. Survey Bull. 989-A, p. 1-26.

 1956, Reconnaissance geology of some western Aleutian Islands, Alaska: 

U. S. Geol. Survey Bull. 1028-E, p. 83-100. 

 1959, Geologic Reconnaissance of Semisopochnoi Island, Western Aleutian Islands, Alaska: U.S. Geol. Survey Bull. 1028-O, p. 477-519. 

Dietz, R. S., and Menard, H. W., Jr., 1951, Origin of abrupt change in slope at

continental shelf margin: Am. Assoc. Petroleum Geologists Bull., v. 35, no9, p. 1994-2016. 

Egbert, J. H., 1905, Report on the natural history of Kiska Island: Forest and

Stream, p. 333. 

Fraser, G. D., and Barnett, H. F., 1959, Geology of the Delarof and westernmost

Andreanof Islands, Aleutian Islands, Alaska: U.S. Geol. Survey Bull. 1028-

I, p. 211-248.

Fuller, R. E., 1931, The geomorphology and volcanic sequence of Steens Mountain in southeastern Oregon: Washington Univ. Pub. in Geology, v. 3, no. 1,

p. 1-130. 

Gates, Olcott, and Gibson, W. M., 1956, Interpretation of the configuration of

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

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

geology of the Aleutian Islands: Science, v. 119, no. 3092, p. 446-447. 

Gibson, W. M., and Nichols, Haven, 1953, Configuration of the Aleutian Ridge,.

Rat Islands Semisopochnoi I. to west of Buldir I.: Geol. Soc. America Bull.,

v. 64, no. 10, p. 1173-1187. 

Gilbert, C. M., 1947, Cleaning mineral grains for petrographic study: Jour. Sed.

Petrology, v. 17, p. 83-85. 

Goddard, E. N., and others, 1948, Rock-color chart: Washington, D.C. NatL

Research Council. 

Melton, F. A., 1940, A tentative classification of sand dunes: Jour. Geology,

v. 48, no. 2,113-174. 

Nelson, W. H., 1959, Geology of Segula, Davidof, and Khvostof Islands, Alaska i

U.S. Geol. Survey Bull. 1028-K, p. 257-266. 

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

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

p. 521-554. 

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

U.S. Geol. Survey Bull. 1028-G., p. 161-167. 

 1959, Geology of Little Sitkin Island, Alaska: U.S. Geol. Survey BulU

1028-H., p. 169-210.

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