Geology of Great

Sitkin Island

Alaska

By FRANK S. SIMONS and DONALD E. MATHEWSON

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-B

. Prepared in cooperation with'the 

Office, Chief of Engineers, U.& ^Ar

J( UNITED.?STATES GOVERNMENT PRINTING jQFFIGiE, WASHINGTON : 1955
UNITED STATES DEPARTMENT OF THE INTERIOR

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. G. - $1 (paper cover)
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 

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 geologic and geophysical investigations covered by this report 

were reconnaissance. The factual information presented is believed 

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

grows.

The investigations of 1946 were supported almost entirely by the 

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

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

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

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

continuing support.

in
Jt- 
CONTENTS

Page

Abstract____._____________________._______._____ 21

Introduction__.___________________________________________________ 22

History. _________________________________________ 22

Geography___________________________________________________ 23

Location.____________________________________________________ 23

Topography. _____ _________________________ _________ 23

Climate.____________________________________ 26

Vegetation__________________________________________________ 26

Animal life______________________________________________ 26

^ Geology___________________________________ 27

Igneous rocks.______-___--__-_______-___-__-__-_-__--____-____ 27

* Finger Bay volcanics_______________________________________ 27

Sand Bay volcanics________________________________________ 28

Great Sitkin volcanics...____________________________________ 29

. Sedimentary rocks _____________________________________________ 32

Hot springs and fumaroles____________________________ _____ 34

Structure___________________________________________________ 35

Geomorphology _______________________________________________ 36

t Geologic history_______________________________________________ 38

Future volcanic activity. _________________________________________ 40

Literature cited_____________________________________________ 41

Index_________________________________________ 43

[Plates 6-8 follow page 30]

PLATE 5. Geologic map and sections, Great Sitkin Island, Alaska. _ _ _ _ In pocket

6. Crater and 1945 crater dome of Great Sitkin Volcano.

7. A, Sand Bay volcanics north of Zaliva Point. B, Alluvial fan 

north of Cape Akuyan.

8. Marine terrace and pond between Cape Kiugilak and Turmath

Creek. 

FIGURE 2. Map of the Alaska Peninsula and Aleutian Islands __________ 24

3. Isometric drawing of Great Sitkin Island, looking north ____ 25

TABLE

Analyses of samples of fumarole gases. 35
V
INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGY OF GREAT SITKIN ISLAND, ALASKA

By FRANK S. SIMONS and DONALD E. MATHEWSON

ABSTRACT

Great Sitkin Island lies near the middle of the Aleutians, about 25 miles 

northeast of the Army-Navy base on Adak Island. It is about 11 miles in maximum diameter, and its highest altitude is 5,740 feet, the summit of Great Sitkin 

Volcano. The island is very rugged an active volcano makes.up most of the 

northern half, and the deeply dissected remnant of an older volcano makes up 

most of the southern half.

The oldest rocks exposed are the Finger Bay volcanics, which form the rugged 

headlands of the south and southeast coasts. These rocks, believed to be of 

late Paleozoic age, are highly altered and deformed lavas, volcanic breccia, and 

tuff of andesitic and basaltic composition, cut by numerous dikes. The source 

of the rocks is unknown.

The Sand Bay volcanics, which form most of the southern half of the island, 

comprise a lower pyroclastic sequence, consisting mainly of agglomerate, and 

an upper sequence of andesite and basalt flows. The age of these rocks is believed to be late Tertiary. They are essentially undeformed, and their radial 

dips suggest a source near the present cone.

The rocks of the main cone that form the northern half of the island, are 

designated Great Sitkin volcanics and are of late Tertiary and Quaternary age. 

The uppermost rocks of the cone are lava flows, similar in appearance to the lava 

flows of the Sand Bay volcanics, but the steep slopes of the cone suggest that 

its core is made up of pyroclastic material. The crater of Great Sitkin Volcano 

lies on the west slope of the cone at an altitude of about 4,000 feet; it is about 

three-quarters of a mile long and half a mile wide. The eruption that produced the crater blanketed the island with a layer of pumice and rock fragments 

ranging in thickness from a few inches to more than 20 feet. A recently formed 

volcanic dome occupies the center of the crater. Five plugs have been intruded 

into the northwest flank of the cone. A recent deposit of ash, apparently erupted 

from a vent a short distance northwest of the crater, blankets the northwest 

slope of the cone. The floor of one creek valley is covered by a fairly recent 

mudflow.

The cone supports 5 small glaciers, the 2 largest of which have built large 

moraines. More extensive glaciation occurred in the past, but the only glacial 

deposits found at lower altitudes, other than those of the present glacier fronts, 

consist of erratic boulders on the floor of the valley of Big Fox Creek.

Entrenched streams and low marine terraces suggest that the island has 

been uplifted recently.

The geologic history of Great Sitkin Island comprises at least four stages of

21
22 INVESTIGATIONS OF ALASKAN VOLCANOES

volcanism designated as Finger Bay, Sand Bay, peak volcano, and crater volcano 

stages. An angular unconformity separates deformed Finger Bay volcanics 

from deposits of the Sand Bay volcano. After prolonged erosion the center of 

volcanic activity shifted north and formed the peak volcano, and then shifted 

west to form the crater volcano. Each, stage ended with the partial destruction 

of its volcano. The crater volcano was modified by erosion and decapitated by 

an eruption during which the pumice blanket was deposited. Great Sitkin 

Volcano, the ^present double cone, is a composite of the ancestral peak and crater 

volcanoes.

Minor volcanic activity was reported in 1792, 1829, 1904, and 1933; a crater 

dome, which was formed probably in March 1945, was emitting much steam and 

some acid gas in September 1946. . . A survey of geologic literature on volcanic domes similar to that of Great 

Sitkin Volcano indicates that the course of their evolution cannot be predicted 

with certainty; some have been destroyed or partly destroyed by violent explosions, others have not. The 194{> dome of Great Sitkin Volcano may remain 

quiescent indefinitely, or it may become violently active. .

INTRODUCTION

Great Sitkin Island, one of the Andreanof islands, is near the 

middle of the Aleutians about 25 miles northeast of the large ArmyNavy base on Adak Island. An active volcano occupies most of the 

northern half of the island. The possible source of danger of the 

volcano to the installations on Adak and to the naval station led to a 

geologic investigation of the island during the summer of 1946. 

Field work was carried out from August 16 to October 2 by a party 

including Frank S. Simons, geologist-in-charge, Donald E. Mathewson, geologist, and Joseph C. Roberts and Thomas E. Turner, 

recorders.

Acknowledgment is made of the cooperation of Commander S. O. 

Cole, chief staff officer of the Adak Sector, and Lt. J. R. Geis, officerin-charge of the naval fueling station at Sand Bay, in providing transportation, quarters, and mess facilities.

HISTORY

Great Sitkin island was probably first sighted on September 8 or 9, 

1741, by Chirikof, of the Bering Expedition, who visited Adak Island

on September 9 on his return voyage from southeastern Alaska, to

Kamchatka. Bering, in another ship, sighted the island on September 25 (Collins, Clark, and Walker, 1945) .

Andreanof Islands were explored in 1760-1764, and were

fOr Andrean TolStykh, one of the explorers. ^ report by a

of the party, Peter Vasyutinsky, mentions Great Sitkin 

Island in a more detailed description of AcLak; "The tiura isiana,

Chetkin, lying to the east, across the strait, at a distance of about 40 

versts [26.4 miles] has a circumference of about 80 versts [52.8 miles]. 

There are many mountain ridges, one mountain of which is called
GEOLOGY OF GREAT SITKEST ISLAND 23

Belaya [white]. In the valleys are hot springs. No rivers are found 

on the island. It is inhabited by about 400 families." (Jochelson, 

1933.) Although Vasyutinsky calls the island Chetkin, its location 

and size and the hot springs he ascribes to it coincide with those of 

Great Sitkin Island.

Little is known of the subsequent history of the island until 1943, 

when the naval fueling station was established at Sand Bay. No evidence of permanent inhabitation by Aleuts was found by the geologic 

party; apparently they left the island many years ago. However, 

the island was used as a fox farm before World War II.

GEOGRAPHY

LOCATION

Great Sitkin Island, together with Adak Island and the chain of 

intervening smaller islands, forms the middle part of the Andreanof 

Islands, which lie near the middle of the Aleutian arc (fig. 2). The 

island is about 600 miles west-southwest of the tip of the Alaska 

Peninsula and 480 miles east of Attu Island, which is at the west end 

of the chain. Its geodetic position is 52° N., 176"06' W. (Sand Bay).

TOPOGRAPHY

Great Sitkin is a deeply dissected, extremely rugged island about 

11 miles in maximum diameter (fig. 3). Most of the northern half 

is occupied by Great Sitkin Volcano, whose altitude is 5,740 feet at 

a distance of only 2i£ miles from the north coast. A crater that 

measures about half a mile by three-quarters of a mile, and whose 

walls rise to a maximum height of 600 feet, lies on the west flank 

of the cone at an altitude of about 4,000 feet. Many steep-walled 

valleys radiate from the summit. The original constructional surface of the cone has been almost completely destroyed. This asymmetrical cone offers a striking contrast to the perfect cone of Kanaga 

Volcano, 40 miles west-southwest.

The southern half of the island is separated from the cone by the deep 

valleys of Sitkin Creek on the east and Akuyan Creek on the west. 

It consists of the dissected remnant of a former shield volcano upon 

which the present cone has been built.

U-shaped valleys radiating from the center of the island have been 

cut 1,000-1.500 feet below the intervening ridges and are headed by 

cirquelike amphitheaters. The ridges are generally very narrow, and 

many are knife edged, although in a few places the original surface 

is preserved in broad rolling uplands. Several large streams and 

many small creeks drain the island; in the main valleys the streams 

occupy meandering courses incised 5-15 feet below the valley levels.

34764,3 55 2
24 INVESTIGATIONS, OF ALASKAN VOLCANOES

'o
A- T.

Cape 

Akuyan

Cape 

Kiugilak

Zaliva 

Point

2 Mile

Approximate scale 

FIGURE 3. Isometric drawing of Great Sitkin Island, lookin
26 INVESTIGATIONS OF ALASKAN VOLCANOES

The coastline is rocky and irregular. Sea cliffs as much as 1,000 

feet high alternate with narrow sandy beaches at the mouths of valJeys. Along the south coast are a feAv terraces. Natural harbors are 

nonexistent and sheltered anchorages are few; Sand Bay and Yoke 

Bay are fairly well sheltered.

Several small lakes fill depressions on or near beaches. Two rockbasin lakes are known, one on Bugle Point and the other on Sulphur 

Point.

Five small glaciers occupy valleys on the upper slopes of the cone. 

The crater itself is partly filled with ice, which moves out through 

saddles in the north and south walls to form two glaciers; the largest 

of these glaciers, 1 mile long and nearly 1 mile wide, is very active 

and has built up a large moraine in the valley of Glacier Creek.

CLIMATE

The climate of Great Sitkin, like that of the other Aleutian Islands, 

is cool and wet. Prevailing winds are from the north in winter and 

from the southwest in summer. High ridges and peaks near Sand 

Bay cause the wind to blow in strong gusts and to make rapid variations in direction in that area. During much of the summer, fog 

blankets the island from sea level to altitudes of 3,000 or 4,000 feet. 

The upper slopes of the volcano are usually cloud covered, but the 

top of the cone is visible enough of the time to be useful as a landmark for navigation.

VEGETATION

The lower parts of the island support a heavy growth of grass, 

which reaches a height of from 4 to 5 feet in late summer. Interspersed, 

with the grass are patches of moss and lichen, which give the landscape a blotched appearance from the air. Most of the small gullies 

are choked with ferns, and at places some are completely bridged by

heavy mats of grass, moss, and lichen. Many plants are in flower during the summer, the most conspicuous of which are monkshood and 

lupine. Vegetation is heaviest at sea level; above an altitude of about

1,500 feet there is practically no plant life. Trees do not grow on the 

island.

ANIMAL LIFE

The only land animals seen were blue foxes, which presumably

were introoluceeL Marine life, however, is abuncla.ivt; tKe kicla pools

and rocky basins along the coast abound in sea urchins, red and 

orange sea anemones, chitons, and mussels. The sea urchins, which 

form a green mantle on many submerged rocks, are particularly 

abundant. Numerous salmon were seen in Turmath, Big Fox, and 

South Yoke Creeks, and many Dolly Yarden trout in Big Fox Creek.
GEOLOGY OP GREAT SITKIN ISLAND 27 

GEOLOGY

, The geology of Great Sitkin Island is shown on plate 5. The island can be divided both topographically and geologically into two

* principal units: the nearly flat-lying remnants of an old shield volcano, which form most of the southern part of the island, and a young, 

steep-sided cone and its related plugs and parasitic cone, which form

<> most of the northern part of the island. The shield volcano is fringed

*> on the south and east by small areas of much older volcanic rocks.

A recent explosive eruption blanketed the island with a mixture of

'* pumice and rock fragments to depths ranging from a few inches to 

more than 20 feet. The crater produced by the explosion lies west of 

the summit of Great Sitkin Volcano and about 1,700 feet below it;

* the crater is filled with ice. Into it a basalt dome was recently inj, truded, apparently in March 1945. By September 1946, considerable

ice had melted from the crest of the dome, which occupied about half

* . of the crater, and steam was rising to an altitude of about 3,000 feet 

above the center of the dome (pi. 6).

ft. IGNEOUS ROCKS 

" *S, FINGER BAY VOLCANICS

The oldest rocks exposed are those forming the rugged headlands 

of the south and southeast coasts. These rocks have been correlated

*^ by Coats (1955a) with rocks exposed at Finger Bay on Adak

Island, which have been named by him the Finger Bay volcanics.

The chain of islands between Great Sitkin and Adak seems to be comV posed largely, if not entirely, of Finger Bay volcanics. These islands

were seen only from a boat, however, and may consist in part of

younger rocks.

~ The Finger Bay volcanics are predominantly andesitic and basaltic 

^ greenstone, but include coarse- to fine-grained porphyritic flows,

amygdaloidal flows, coarse flow-breccia, pillow lava, dense fine- 

"* grained rocks that are probably flows, and coarse- to fine-grained tuff. 

v The rocks are cut by numerous dikes, as much as 3 feet wide, and by a

few thin quartz veins. All the rocks are altered, the dikes commonly 

~^ less so than the flows and tuffs. Plagioclase phenocrysts are milky 

A. and ferromagnesian minerals are generally altered to a fine-grained

green aggregate. Calcite, epidote, pyrite, and prehnite are the prin- 

~r cipal secondary minerals: calcite occurs in veins and irregular patches; 

_j epidote in veins, irregular masses, and vesicles; and pyrite in veins

and tiny scattered grains. 

i The source of the Finger Bay volcanics is not known and probably

cannot be determined from the sparse exposures on Great Sitkin

Island.
28 INVESTIGATIONS OF ALASKAN VOLCANOES

The rocks are intensely deformed and commonly have dips ranging 

from 60° to vertical. No well-defined structures were found, but detailed structural analysis was not attempted. In general, the rocks 

are very resistant to erosion; the topography that has developed on 

them is characteristically hummocky, and sea cliffs that have formed 

in the rocks are as much as 800 feet high.

The age of the Finger Bay volcanics is not definitely known, but 

is believed to be late Paleozoic, which is the age tentatively assigned 

to similar rocks on Adak Island (Coats, 1955a).

SAN1> BAY VOLCANICS

The gently dipping volcanic rocks that form the southern half of 

the island are well exposed in the vicinity of Sand Bay and are designated Sand Bay volcanics (pi. YA). They overlie the Finger Bay 

volcanics unconformably. The Sand Bay volcanics comprise a lower 

pyroclastic sequence consisting of agglomerate and a few thin flows, 

and an upper sequence composed of andesitic and basaltic lavas.

The source of the Sand Bay volcanics is not exposed, but dips of 

the lava flows are roughly radial and suggest a central vent near the 

present cone. The age of these rocks is not accurately known. They 

are older than the Great Sitkin volcanics, which have been glaciated, 

and of which at least part are probably of Pleistocene age. In the 

absence of diagnostic fossils, the age of the rocks can only be surmised 

by analogy with rocks of better known areas. The present cycle of 

volcanism is believed to have begun on the Alaska Peninsula in 

the early Tertiary (Capps, 1934; Knappen, 1929), and Tertiary

fossils have been found in tuffs on Akutan, Unalaska, Umnak, and

Atka Islands (Capps, 1934). The Sand Bay volcanics are therefore 

tentatively assigned a Tertiary age. Many volcanic rocks of the

western Aleutians were beveled by a marine abrasion plane, presumably in the middle Tertiary (Coats, 1955b). Because the

Sand Bay volcanics do not show evidence of this beveling, they are 

believed to be of late Tertiary age.

Agglomerate. The lowest part of the exposed section of the Sand 

Bay volcanics is at the base of the south end of the Akuyan cliffs on

the west side of the island, At the base of the cliffs, coarse wellbedded tuff that dips north at small angles is overlain by 1,000 to 1,500

feet of coarse agglomerate. Numerous dikes are exposed in the cliffs.

Similar agglomerate forms the lower slopes of the southern half of

the island. It is well indurated and in many places forms cliffs several hundred feet high. At Sand Bay the agglomerate is fairly well

bedded but poorly sorted; it is composed of subangular to rounded 

boulders, as much as 3 feet across, in a tuff matrix. The boulders are 

mainly of two rock types: a black, glassy, highly vesicular lava, and 

a reddish vesicular porphyry containing phenocrysts of plagioclase
GEOLOGY OF GREAT SITKIN ISLAND 29

feldspar, pyroxene, and olivine. Farther inland, toward the cone, 

the agglomerate contains interbedded thin lava flows and is cut by 

dikes. The ridge west of the valley of Big Fox Creek and the one 

overlooking Cape Kiugilak are capped by lava flows interbedded 

with agglomerate.

Lava -flows. The main ridges in the southern part of the island 

are capped by andesite and basalt flows, which reach an aggregate 

thickness of 400 feet on Triple Divide Peak at the head of the valley 

: of Middle Yoke Creek.

The lava flows are porphyritic and include andesite, olivine 

andesite, andesite flow-breccia, and basalt. Their colors are black, 

purplish gray, reddish gray, and light gray. Most of the flows are 

medium grained but a few are very fine grained. Platy lavas .are 

rare, and most of the rocks show little or no flow structure. Columnar 

jointing is common. The most conspicuous lava flow .on the island 

caps the ridge west of Sand Bay, forming the top of a 300-foot cliff. 

It consist of olivine andesite, is about 40 feet thick, and exhibits excellent columnar jointing (pi. 7A). "" . . The lava flows are notably fresher in appearance than those of the 

Finger Bay volcanics. Plagioclase, pyroxene, olivine, and magnetite 

can be distinguished in hand specimens. Most-of the flows appear 

to contain little or no glass, and highly glassy lava seems to be absent. 

Vesicular flows are rare. The lava flows dip south or southeast at 

small angles, and they thicken toward the center of the island. 

Nowhere do they appear to be folded or faulted.

GREAT SITKIN VOLCANICS

The,rocks of the north half of the island include the rocks of the 

main cone of Great Sitkin Volcano, a newly formed crater dome and

. five older plugs, a basalt flow near the head of Sitkin Creek, and a 

parasitic tuff cone on the northwest flank of the main cone.

The main cone is composite, built up of lava flows and tuff beds; 

the flows predominate, at least in the upper part of the cone. The 

flow rocks, which consist mostly of porphyritic andesite and basalt, 

are similar to those of the Sand Bay volcanics. On the lower slopes 

o'f the cone the rocks are fresh, but the rocks forming the summit peak 

are altered, discolored, and riddled with tiny, irregular cavities lined 

with tridymite crystals. The lava flows are clearly exposed in the 

walls of the crater, but are difficult of access because the walls are steep 

and high. Most of the rocks of the main cone are covered by a thin 

blanket of pumice and rock fragments.

. The only mineralized rocks seen on the cone are near the foot of 

the large glacier on the southeast slope. Here a greenish. breccia

, containing angular to rounded fragments as much ^as 8 inches across
30 INVESTIGATIONS OF ALASKAN VOLCANOES

has been thoroughly impregnated with pyrite, which locally forms 

as much as 10 percent of the rock. The breccia is highly altered and 

contains much calcite and chlorite. The feldspar phenocrysts of the 

breccia fragments are dull and milky. The altered rocks crop out 

in an area 75 by 200 feet, probably the site of former solfataric 

activity.

The crater is occupied by a dome of black glassy porphyritic basalt. 

A sample of the basalt, collected by K. E. Coats, was analyzed by 

W. W. Hommel, Ledoux, and Company. The analysis is given below; 

the normative values and the Niggli values were calculated by E. E. 

Coats.

166

29.2

35.9

22.5

14.4

. 186

.386

8.2

Chemical analysis 

Si02 __ ___ 57. 42

A1.0aFe2O3 _ ___ _

FeO

MgO___-__-

CaO_. ___ .

Na2 O3 _______

H O+

H 0-

Ti02 ___, .

17 12

4. 36

4. 47

3. 03

7. 23

4. 17

1.45

.13

.03

.84

.24

Norm

ab

wo ___________

fs

mt_ ___________

il _ _ __

ap

Symbol II, (4)5,3 

Andose

9. 13 

8.57 

35.28 

23. 70

4.41 

7.55

3. 19 

6. 32 

2.00 

.57

,4"

Niggli vah

al

fm

alk

k

mg_.

qz__.

The dome is roughly oval, about one-quarter of a mile wide by 

three-eighths of a mile long. Its top surface is blocky and rather 

flat, and its sides are steep. It resembles the 1909 dome of Tarumai 

Volcano on Hokkaido Island, Japan (Powers, 1916), and the 1912 

dome of Novarupta in the Katmai region, Alaska (Fenner, 1923). 

It differs from the latter in that the lava contains abundant phenocrysts instead of being largely glassy, Presumably in March 1945,

th© dome intruded a thick icefield, which filled the crater, doming 

the ice and melting much of it. On September 12, 1846, the dome

resembled a gigantic aster, the petals outlined by crevasses in the

rim Of the breached ice (pi. 6). Steam was escaping from a number

of vents around the edge of the dome, but the main steam column

rose from near the center to a height of about 3,000 feet above the

crater. The steam was under little or no pressure, and probably

was being produced largely by vaporization of melt water from the

surrounding ice. Occasionally, acrid gas was detected; and the rime

ice on the peak east of the crater had a sour taste, indicating the 

emission of acid gas.

Three plugs on the slope of the cone, probably intruded along a 

fissure,, are aligned in a northwest direction from the crater. The 

plug nearest the crater rim is believed to have been the source of the

-A 

4,

-v 

r'

V
GEOLOGICAL SURVEY 

CRATER AND 1945 CRATER DOME OF GREAT SITKI.X VOLCAN
GEOLOGICAL SURVKY BULLETIN 102S PLA.TE 7

A, SAND BAY VOLCANICS NORTH OF 2ALIVA POINT.

The cliff, 300-350 feet high, is composed of well-bedded agglomerate, capped by an olivine andesite flow 40 

feet thick. The flow shows excellent columnar jointing. Photograph by U. S. Navy.

B, ALLUVIAL FAN NORTH OF CAPE AKUYAN.

Sea cliff in foreground is about 30 feet high. The narrow boulder beach is typical of the west and north 

coasts of the island. Photograph by U. S. Navy.
< 4 < > 

GEOLOGICAI, SURVEY 

MARINE TERRAU. V.ND POND BETWEEN CAPE KIUGILAK AND TUR

The terrace was cut on landslide material. Sand Bay volcanics and valley of Turmath Creek are in 
GEOLOGY OF GREAT SITKIN ISLAND 31

recent ash that blankets the northwest slopes of the cone. Two 

other plugs, one of which forms Swallow Head, the northernmost 

point of the island, are aligned in a north-northwest direction. The 

plugs range from 1,000-3,000 feet in diameter and exhibit welldeveloped columnar jointing. They were not examined and their 

composition is not known.

A thick basalt flow near the head of Sitkin Creek, the only fairly 

young flow that was found, was extruded from the flank of the 

cone. It is 100-125 feet thick, and its columnar jointing is conspicuous. The flow is at least half a mile long; its lower end has 

been destroyed by glacial erosion, so its original length is unknown. 

The source is buried by pumice.

The Great Sitkin volcanics unconformably overlie the Sand Bay 

volcanics. The upper parts of the cone of Great Sitkin Volcano 

have been dissected by glaciers that have largely disappeared. Therefore, the building of the present cone probably began in late Tertiary 

or early Quaternary time and was largely completed before the end 

of Pleistocene glaciation.

Pumice. Pumice, mixed with scoria and rock fragments, blankets 

the island to depths ranging from a few inches to more than 20 feet. 

The pumice blanket is not shown on the geologic map because it would 

obscure the bedrock geology.

The pumice is buff to light brownish gray in color, and is composed 

of fragments as much as 4 inches across. Several miles south of the 

crater the pumice is extremely light and frothy and will float on water, 

but it is progressively less vesicular in the direction of the crater, 

and on the slopes of the cone itself much of the rock is more properly 

termed scoria. Crystals of transparent plagioclase, pyroxene, and 

amphibole, set in a matrix of pale glass, can be easily recognized with 

a hand lens; some of the pyroxene crystals are as much as one-quarter 

of an inch across.

Mixed with the pumice are angular fragments of dark-gray porphyritic basalt as much as 3 inches across. From 10-15 percent of 

the pumice deposit near the crater is composed of these fragments; 

at increased distances from the crater the number of fragments decreases, and at Sand Bay practically none are present. These fragments are thought to have been torn from the throat of the volcano 

and from the summit of its cone at the time of the pumice eruption.

A minor but striking constituent of the pumice blanket is a very 

coarse-grained gabbrolike rock consisting of transparent plagioclase 

and amphibole, and small amounts of divine, pyroxene, and magnetite. Only a few pieces of the rock were seen, the largest being 

4 inches across. Amphibole crystals 1 inch long and plagioclase 

tablets half an inch across are common. The rock having alternate

847643 55 3
32 INVESTIGATIONS OF ALASKAN VOLCANOES

coarse- and fine-grained layers is prominently banded; the coarsegrained layers are themselves separated into feldspar-rich and amphibole-rich bands. The pumice of some areas, particularly on the.ridge 

south of Sitkin Creek, contains many single crystals that evidently 

were derived from the gabbrolike rock by fragmentation.

The thickest pumice deposit was seen near the divide between Sitkin 

Creek and the west fork of Big Fox Creek. Here, gullies 20 feet 

deep have been cut in the pumice without exposing the base of the 

deposit. Thick deposits also occur in the upper valley of Sitkin 

Creek and as a major constituent of the moraine at the head of Glacier 

Creek. At other places reworked pumice forms alluvial-fan deposits 

8-10 feet thick; these fans are especially well developed west of 

Big Fox Creek, where the original pumice deposit, overlain by 6-8 

feet of alluvial pumice, is exposed in gullies. A thin, discontinuous 

pumice bed resting on sand dunes is exposed in several road cuts at 

the mouth of the valley of Little Fox Creek, and between Zaliva 

Point and Rip Point, in the area most remote from the crater.

Presumably, the great eruption that produced the pumice occurred 

long before the discovery of the Aleutian Islands by white men. The 

freshness of the pumice, the very small amount of soil developed 

on it, and its presence in considerable quantity on divides such as 

those between Sitkin Creek, Big Fox Creek, and Glacier Creek, indicate that it is not more than few thousand years old.

A.sh deposit of northwest side of Great Sitkin Volcano. The northwest slopes of the main cone are covered with a very recent deposit 

of ash, apparently ejected from the flank vent immediately northwest 

of the crater. This deposit was seen only from the sea, but its distribution is readily determined from aerial photographs. The ash 

is light colored and appears to be several feet thick in places. It 

overlies the pumice deposit and is at most only a few hundred years 

old.

SEDIMENTARY BOCKS

Alluvium and beach deposits. Most of the valleys in the southern 

half of the island are floored with alluvium. In the lower course of

Sitkin Creek well-bedded fanglomerate about 100 feet thick is exposed

in the walls of gullies cut by recently rejuvenated streams. Elsewhere

the exposed thickness of alluvium is less; a minimum of 30 feet of

.aluvium is exposed in the cliff at the lower edge of the large alluvial 

fan north of Cape Akuyan (pi. 7B). In the valleys of Big Fox and

Little Fox Creeks the most recent alluvium is largely reworked pumice.

Narrow sandy beaches lie along the island's coasts, except OH the 

rugged north and northwest sides where there are only a few small 

boulder beaches. Sand dunes are present on all beaches along the
GEOLOGY OF GREAT SITKIN ISLAND 33

south and east coasts from the narrow beach at the mouth of Turmath 

Creek to the beach, iy2 miles long, between Sulphur and Bugle Points. 

At Bugle Point the dunes are roughly elliptical in plan, their long axes 

trending north. Most of the dunes are grass covered and appear to be 

relatively stable; those at the mouth of Little Fox Creek are mantled 

with pumice and soil. At present, dunes do not appear to be forming.

Mudflow. The floor of the lower part of the valley of Glacier Creek 

is made up of what is believed to be a fairly recent mudflow. It was 

seen only from the sea at a distance of several hundred yards. The 

valley averages 2,000 feet in width for a distance of about 1^ miles 

from its mouth, and its floor is slightly convex upward in transverse 

cross section. Along each side of the valley is a steep-walled canyon 

 incised well below the present valley surface; Glacier Creek occupies 

the larger of the two canyons, the one along the southeast side. The 

mudflow is thought to have had its source in the extensive moraine at 

the head of Glacier Creek, and was perhaps activated by heating and 

consequent melting of snow and ice around the moraine during some 

phase of the volcano's activity.

Exposures along the low cliff at the end of the mudflow suggest that 

two mudflows rather than one may lie in the valley an older mudflow, 

made up largely of pumice, along the southeast side of the valley; and 

a younger and more extensive flow, made up of coarser debris, along 

the northwest side.

Glacial deposits. Two of the island's five small glaciers have 

produced fairly large moraines.

The larger of the two glaciers, whicK is south of the crater at the 

head of Glacier Creek, is about 1 mile wide and half a mile long. Its 

moraine extends for at least half a mile down the valley; the upper 

part is made up largely of pumice, which has been built up into a 

hummocky terrain.

The smaller of the two glaciers occupies a valley on the southeast 

slope of the cone. It has built a blocky moraine that extends to the 

bottom of the valley of Sitkin Creek, a distance of about half a mile. 

The higher part of the moraine is actually a rock glacier. A medial 

moraine, between the main moraine and a small tongue of ice to the 

cast, forms a ridge 10-15 feet high. Deep striations on outcrops 

hundreds of feet below the present terminus of the glacier indicate 

that formerly the glacier was of greater extent.

About three-quarters of a mile north of the mouth of Big Fox Creek 

many angular boulders are strewn across the valley floor. The largest 

is about 8 feet across. They are composed of well-indurated agglomerate. The blocks may have slumped from the canyon walls, 

but in view of the fact that some of them lie several hundred yards 

from the nearest local source, it seems more likely that they are glacial
34 INVESTIGATIONS OF ALASKAN VOLCANOES

H'

erratics. The valley of Big Fox Creek is believed to have been 

glaciated.

Landslides. One landslide and two other features believed to 

be landslides are shown on the geologic map. The lower course 

of Middle Yoke Creek has been blocked by a large landslide from the 

steep valley wall to the northeast, and landslide material extends V; 

across the beach to the sea. Nearly 1 square mile is covered by thi& 

landslide debris, forming a typical hummocky surface. A small pond 

probably occupied the valley above the slide for a short time. /» 

The rounded low headland between Cape Kiugilak and the valley 

of Turmath Creek is probably an ancient landslide; the cirquelike 

indentation above, the irregular topography of the lower slopes, and ?* 

the large blocks of rock near the beach, all suggest a landslide origin. f 

Another landslide may have occurred near the north bight of Yoke . Bay, where a block 4,000 feet long and 400 feet high appears to have V 

slid 50-100 feet down the steep slope forming the central part of the 

northwest shoreline.

HOT SPRINGS AND FUMAROLES 4>

A large group of hot springs, mud pots, and fumaroles occurs at 

an .altitude of about 2,000 feet near the head of the west fork of 

Big Fox Creek. On clear days a conspicuous cloud of condensed 

steam hangs over the largest of the springs. An area about 1,000 feet 

long and 400 feet wide is underlain by almost completely decomposed 

rocks that are bright red, pink, and yellow in color. Most of the rock <rwhose texture has been preserved is a breccia of porphyritic volcanic 

rocks; it probably fills an old vent. The ground in the area is warm: 

grass and moss grow profusely, whereas the surrounding terrain is- p 

barren, and dark-green gelatinous algae clog the overflow channels of 

the springs.

Several of the low mounds in the fumaroUe area are hollow shells, *--* 

from beneath which can be heard the sound of boiling water. The 

largest mound is about 25 feet long, 20 feet wide, and 6~8 feet high. 

The shell of the largest mound is about 2 feet thick, and the -water i» 

level seems to be several feet below the roof. The roofs of some of the

mounds are not strong, and may break under a man's weight. Steam

vents are especially abundant on all the mounds, t-\

The largest vent contains a pool of hot muddy -water 6 8 feet across

through which steam was bubbling with considerable violence, Little

Water was being discharged at the outlet, indicating that the boilin-g- r~

was due largely to passage of steam and not to the influx of boilingwater. Most of the springs in the area discharge yerj little water,' 

they appear to be fumarole craterlets in which surface water has col- * 

lected. Many of the springs are muddy, and several are mud pots
GEOLOGY OF GREAT SITKIN ISLAND 35

 containing viscous gray mud. Pyrite was being deposited in one of 

the mud pots. Sublimates are rare; the bright color of the rocks is

 due mainly to rock alteration.

A gas sample was collected from each of the four largest fumaroles 

in a heat-resistant glass pipette (volume, 25 cubic centimeters) by 

inserting the pipette in the f umarole and allowing it to be filled by the 

gas pressure of the f umarole; because much steam was being emitted 

from each f umarole, it was easy to ascertain when gas was passing 

through the pipette. The pipette was sealed off with an oxybutane 

torch. Kesults of the gas analyses are given in the following table:

Analyses, in volume percent, exclusive of water, of samples of f umarole gases

from Great Sitkin Island

[By mass spectrometer, National Bureau of Standards. After Byers and Brannock, 1949, p. 732]

Sample no.

1. . 

2_. .....................................

3 _ ... ...... ...................

-4:....l..... ............................

H

0.2

N

'69.2

>78.2

72.5

16.7

0

17.8

20.9

17.9

.8

A

0.8

.9

.9

.2

C08

12.1 

.02

8.5

181.5

SOs

0.1

0.8

Total

100,0

100. 02

100.0

100.0

i Fraction is uncertain.

Sample 1: Fumarole 1, on low rounded knoll beneath which is hollow shell and bubbling water.

Sample 2: Fumarole 2, six to eight feet above bottom of gully.

Sample 3: Fumarole 3, at bottom of gully.

Sample 4: Fumarole 4, at bottom of gully, within 50 feet of fumarole 3.

The water from 12 hot springs was tested for temperature, pH, and 

Cl~(with AgNO3 ), and SO4~~ (with BaCl2 ) ions. The range in temperature was from 88°-98°C, and in pH, from 2-7; only one spring 

had a pH as low as 2. The water of 7 springs had a strong reaction 

to the test for SO4~~, the water of 4 had a weak reaction, and the water 

of 1 was too murky to test. All but one sample had a positive reaction 

to the test for Cl".

STRUCTURE

The structure of Great Sitkin Island is diagramed in the geologic 

sections of plate 5.

The oldest rocks, the Finger Bay volcanics, are intensely deformed 

and are overlain with marked structural and erosional unconformity 

by the nearly flat lying Sand Bay volcanics. These, in turn, are overlain unconformably in the northern part of the island by the present 

cone of Great Sitkin Volcano.

The slope of the cone is considerably steeper on the north, northwest, 

and west sides than elsewhere; moreover, the crater is located somewhat west of the center of the cone. A two-stage development of the 

cone is suggested to account for these anomalous features. The first 

may be called the peak volcano stage, and the second, the crater vol-
36 INVESTIGATIONS OF ALASKAN VOLCANOES

cano stage (see p. 39-40). The center volcano is thought to have been 

built after the destruction of part of the peak volcano, and is the site 

of the present crater.

The present crater is believed to have been formed by both explosion and subsidence. The minimum volume of the cavity, before the 

cavity was partly filled with snow and ice and before the emplacement of the crater dome, is estimated to have been 2.5 XlO8 cubic 

yards. A comparable volume of rock was probably present in the 

part of the cone that formerly extended above the level of the crater 

rim. However, the amount of cone rock that occurs as fragments in 

the pumice deposit seems to be less than the amount of rock removed 

from the cone.

A rough calculation has been made that is believed to give the 

maximum amount of fragmental rock ejected. Inasmuch as few 

rock fragments occur in the pumice at Sand Bay, the distance of Sand 

Bay from the crater (10,000 yards) was taken as the radius of a 

circular area tover which rock fragments probably were deposited. 

The greatest observed depth of pumice was 20 feet, although this 

depth may be exceeded locally. Kock fragments from the cone are. 

estimated not to exceed 10 percent of the total volume. Assuming 

an average initial depth of 5 yards of pumice, having an average 

rock-fragment content of 10 percent, the total volume of ejecta (pumice and rock fragments combined) would be 1.57XlO9 cubic yards;, 

the volume of the rock fragments alone would be 1.57XlO8 cubic 

yards. Only about 32 percent of the rock removed is accounted for on 

the basis of these generous assumptions of average thickness of pumice (in which allowance is not made for the high porosity of pumice) 

and proportion of rock fragments.

It is concluded, admittedly on inadequate information, that not 

more than 25 percent of the rock removed can be accounted for by

explosion. Probably the remainder of the cavity was formed by subsidence of part of the cone during or after eruption of the pumicer 

when abrupt evacuation of the gas-filled lava in the form of pumice, 

perhaps accompanied by intrusion of dikes or sills at depth, removed 

Support from the upper part Of the cone. The sequence of events 

wKicK seems to Kave produced Great Sitkin crater approximately 

parallels that which Williams (1942) proposes for the formation of

ike crater of Crater Lake, Oregon,

GEOMORPHOLOGY

Aside from Great Sitkin Volcano and its crater, the most striking 

features of the island are the deep, steep-walled valleys radiating 

from near the center of the island, and the sea cliffs, which reach their
GEOLOGY OF GREAT SITKIN ISLAND 37

greatest development in the towering Akuyan cliffs on the west side 

of the island south of Cape Akuyan.

The valleys of Akuyan, Glacier, Turmath, Big Fox, Little Fox, 

South Yoke, Middle Yoke, and Sitkin Creeks have a number of features in common. They have wide, more or less flat bottoms, and 

steep, in places precipitous, walls that merge smoothly into the floors 

to give the valleys roughly U-shaped cross sections. The steepness 

of many slopes is reduced by accumulation of alluvium in broad fans. 

Alluvial fans are best developed along the north side of Sitkin Creek, 

where they form a piedmont fan nearly 3 miles long.

That glaciation was once more widespread than at present is inferred largely from topographic evidence, such as U-shaped valleys 

and the knife-edge ridges at the heads of Glacier, Big Fox, and Middle 

Yoke Creeks. Eock-basin lakes of unquestionable glacial origin doriot occur on the island. The glacial striae seen were above an altitude 

of 1,500 feet and cannot with certainty be attributed to other than 

recent glaciation. Moraines that might have been left by more extensive glaciers are now covered by alluvium and pumice. The large 

erratics in the valley of Big Fox Creek make up the only glacial 

deposits seen at low altitude. Although glaciation was clearly more 

extensive in the past, there were probably many separate centers of 

ice accumulation rather than one large ice cap. For example, there 

is no evidence that glaciers heading on Great Sitkin Volcano ever 

occupied the valleys to the south, for all these valleys have cirquelike 

heads rather than notches leading to the valley of Sitkin Creek. 

That the valley of Sitkin Creek had been at least partly eroded 

before the last extensive glaciation is shown by the glacially eroded 

nose of the basalt flow on the floor of the valley.

A rather curious geomorphic feature is the neve (granular ice) 

ridges in the pumice field in the upper part of the valley of Sitkin 

Creek at an altitude of about 2,500 feet. It had been noticed on 

August 16 that some of the purnice at altitudes of 3,500-4,000 feet on 

the lower slopes of the cone was underlain by coarsely crystalline 

neve, exposed in the bottoms of several gullies cut through 8-10 feet 

of pumice. At that time no neve ridges were .present, but on August 

26 several had formed. In cross section they resembled miniature 

fault-block mountains; the steep fronts exposed stratified neve and 

the gently inclined back slopes were capped by pumice. Invariably 

the ridges formed on slopes and were elongated parallel to the direction of maximum local slope. Most of them formed along the sides 

of snow- and ice-filled gullies. The longest ridge was several hundred 

feet long and exposed 4-5 feet of neve in its steep front. No ridges 

formed in flat terrain.

A tent was pitched at the lower end of one of the ridges, near the 

foot of the steep neve front. During the 5 days that the tent was
38 INVESTIGATIONS OF ALASKAN VOLCANOES

so situated, its foundation rose several inches and titlted 3°-4° away 

from the newly forming ridge; a dull crunching sound was heard 

at irregular intervals.

The ridges are clearly the result of faulting of a neve layer underlying the pumice, probably induced by differential downhill sliding 

of some part of the neve-pumice mantle. Inasmuch as .the ridges 

were not seen by R. R. Coats, who crossed the same area in June 

(personal communication), nor by the writers earlier in August, 

it appears that a long period of relatively warm weather is a prerequisite for their formation. Perhaps water from melting snow 

percolates to the base of the neve and acts as a lubricant, which makes 

sliding possible.

The island has been uplifted recently, or the level of the sea has 

fallen, as indicated by terraces and entrenched streams. The landslide just east of Cape Kiugilak has a wave-cut terrace at its-tip, 6-8 

feet above sea level (pi. 8). There are numerous small terraces elsewhere at about the same altitude. Rejuvenated streams also suggest 

recent uplift; all the main creeks are incised in their valley fill, and 

Big Fox and Sitkin Creeks have incised meanders.

The great cliff south of Cape Akuyan is the most prominent coastal 

feature of the island. It is nearly vertical at its base and very steep 

to a height of more than 1,000 feet; its maximum height is 2,400 feet. 

Although the rocks forming the lower 1,000 feet of the cliff are part of 

the Sand Bay volcanics, those of the upper part are thought to be 

the remnant of a cone whose summit was a short distance west of the 

cliffs. This cone, now almost destroyed, probably reached an altitude 

of about 3,000 feet. The method of destruction of the cone is unknown ; it may have been by explosion, downf aulting, marine erosion, 

or by some combination of these factors.

GEOLOGIC HISTORY

The formation of Great Sitkin Island appears to have occurred

 during at least four stages of volcanic activity, which, from oldest to 

youngest, are in this report designated the Finger Bay, Sand Bay, 

peak volcano, and crater volcano stages.

The geologic record of the island probably begins in late Paleozoic 

time with the deposition of the Finger Bay volcanics. Events of the 

Finger Bay stage are practically unknown, as outcrops of the rocks

 of this stage are limited to a few headlands. It is apparent, however, that these rocks were intensely deformed and altered, and were

 considerably dissected by erosion before deposition of the Sand Bay 

volcanics, which are essentially undeformed and unaltered. These"

 conditions indicate that a long time interval occurred between the two 

stages. Apparently the main land mass of Finger Bay and early
, GEOLOGY OF GREAT SITKENT ISLAND 39

Sand Bay time lay to the south, as Finger Bay volcanics were not 

found on the north coast of the island.

Beginning in late Tertiary time, Sand Bay volcanics were extruded 

from a vent or group of vents, probably near the present cone. Early 

Sand Bay activity was explosive; coarse tuff was deposited first, then 

agglomerate. Explosive activity was followed by the quiet effusion 

of andesite and basalt lava. The Sand Bay volcano at the height 

.of its development had much gentler slopes than Great Sitkin Volcano, was probably considerably larger in area, and late in its history 

may have resembled a shield volcano.

A long period of erosion ensued, during which the volcano was dissected and its summit lowered. The writers believe that the valley 

of Sitkin Creek was formed and eroded to considerable depth shortly 

after Sand Bay time, and that the course of Sitkin Creek, which is. 

broadly convex to the south, has been shifted southward by accumulation of volcanic material forming the cone.

Toward the end of Tertiary time or in early Quaternary time a vent 

became active on the dissected north slope of the Sand Bay volcano, 

and formation of the present cone began. This vent probably was 

intermittently active from late Tertiary to Recent time. Most of the 

cone probably was constructed during the Pleistocene, as the cone 

appears to have been subjected to prolonged erosion since the last significant eruption of ash and lava. Most of the rocks exposed on the 

cone are lavas, but the steep slopes indicate that the core of the volcano is probably made up of pyroclastic material. While the cone 

was being built, the plugs of the northwest slope were intruded.

The building of the cone may have taken place in two stages, a somewhat hypothetical peak volcano stage, and a crater volcano stage. 

During the peak volcano stage a cone reaching an altitude of perhaps 6,500 feet was built and partly destroyed. The cone of the 

crater volcano stage was then built a short distance west of the former 

cone and may have reached nearly the same altitude. The peak volcano stage may have terminated when a crater east of the present 

crater was formed, but little evidence has been obtained to indicate 

that this crater existed. However, the highest point on the island is 

so much higher than any other point on the crater rim and so far from 

the present crater that it appears unlikely that the peak is actually 

a part of the rim; the peak may be part of an older crater rim, produced when the peak volcano was partly destroyed. Possibly, the peak 

volcano vent was sealed after formation of a crater, and the vent relocated farther west, in its present position.

If the cone of the peak volcano was decapitated by explosive eruption before the construction of the crater volcano cone, the products 

of the eruption have been eroded away or covered by the products of
40 INVESTIGATIONS OF ALASKAN VOLCANOES

later eruptions, as no deposits containing f ragmental material of appropriate composition appear to be exposed.

A basalt flow was extruded in the upper part of the valley of Sitkin 

Creek after the crater volcano cone had been built but before the 

last extensive glaciation.

During the Pleistocene several small glaciers probably covered 

parts of the southern half of the island, and almost certainly a small 

ice cap lay on the cone. The glaciers modified the valleys cut in the 

Sand Bay volcano and may have reached the sea, although the island 

does not have a fiord coastline such as that of Adak Island.

A glacier descended the southeast slope of the cone at least as far as 

the floor of the valley of Sitkin Creek where it removed the lower 

part of the young basalt flow. The glacier still exists but has retreated 

more than a mile from Sitkin Creek.

Perhaps a few hundred years ago, after the withdrawal of the ice 

to about its present position, the summit of the crater volcano cone 

was destroyed by a great eruption that covered the island with pumice 

and rock fragments, and the present crater was formed. The hot 

springs and fumaroles are known to have been active for nearly 200 

years and may have been formed at the time of the pumice eruption. 

After the pumice eruption, a vent was opened on the northwest slope 

of the cone, and a small amount of ash was erupted from it.

After its formation the crater was filled with snow and ice, and 

from time to time minor eruptions occurred. Smoke was seen in 

1829 and 1904, and minor explosive eruptions were reported in 1792 

&nd 1933 (Coats, 1950). The ash of the northwest slope may have 

been deposited during one of these eruptions. Other eruptions may

not have been detected because Great Sitkin Island is remote from 

the principal shipping lanes.

Extrusion of the crater dome marks the latest phase of activity. The 

dome is thought to have formed in March 1945, at which time a

.glow was visible at night from Adak Island, and Army aviators

noticed clouds of Steam rising from the crater. During that month

a, strong- earthquake -was felt at Sand Bay. Activity Since TOrmatlOn

of the dome has been limited to emission of steam and occasionally 

smoke; a small smoke cloud was visible from Adak Island on August

14, 1946.

FUTURE VOLCANIC ACTIVITY

The likelihood of dangerous eruptions in the future by Great Sitkin 

Volcano can be estimated only by comparison with volcanoes of other 

:areas of similar geologic setting for which more detailed histories 

.are available. Several of these volcanoes, which are discussed in a 

.general summary of volcanic domes by Williams (1932a), are referred to on page 41. The histories of volcanic domes of andesite,
GEOLOGY OF GREAT SITKIN ISLAND 41

basalt, and similar rocks, are different; some domes have been destroyed 

or partly destroyed by violent and destructive explosions, and others 

have not. Among those that have not been destroyed are the 1866 

domes, Georgios Kameni, Aphroessa, and Keka, on Santorin in the 

Grecian Archipelago; the 1909 Tarumai. dome..on Hokkaido Island, 

Japan (Powers, 1916, p. 261-263); and the Ousu dome of Volcano 

Usu, also on Hokkaido Island (Powers, 1916, p. 263-267). Those 

domes whose emplacement was followed by violent eruption include 

two domes in the crater of Merapi Volcano in Java, which were destroyed in the late 19th century; the 1914 dome of Ruang Volcano 

in the Sangi Islands, East Indies; the 1906 and 1907 domes of Bogoslof Island in the Aleutians; the 1922 dome of Santa Maria Volcano 

in Guatemala, which erupted nuees ardentes 7 years after its first 

appearance; and the two domes of the Chaos Crags in Lassen Volcanic National Park, California (Williams, 1932b).

It is evident that the evolution of volcanic domes follows no single 

course, and that the future of a dome cannot be predicted with certainty on the basis of present knowledge. Powers (1916, p. 272) 

has remarked that the formation of a dome often terminates the 

volcanic activity at a vent, and Williams' summary contains a number 

of such examples. That the termination may not be permanent was 

demonstrated at Mount Pelee on Martinique Island in the West Indies, 

Avhere the dome that was formed after the catastrophic eruption of 

1902 was partly destroyed and replaced by a new dome following 

the eruptions of 1929 and 1930 (Perret, 1937). The surface of the 

dome of Great Sitkin Volcano is blocky and broken by many fissures, 

a condition which Williams believes has a tendency to minimize the 

danger of explosive decapitation by offering easy escape to the volatiles thought to be the motivating agents in dome formation and 

destruction.

LITERATURE CITED

Byers, F. M., Jr., and Brannock, W. W., 1949, Volcanic activity on Umnak and

Great Sitkin Islands, 1946-1948: Am. Geophys. Union Trans., v. 30,

p. 719-734. 

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

Islands: U. S. Geol. Survey Bull. 857-D, p. 143-144.; 

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

974-B, p. 35-47. 

 1955a, Geology of northern Adak Island: U. S. Geol. Survey Bull. 1028-C.

In press.

1955b, Reconnaissance geology of some western Aleutian Islands: U. S.

Geol. Survey Bull. 1028-E. In press. 

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

their people and natural history: Smithsonian Inst, Washington, War Background Studies no. 21, p. 8-10.
42 INVESTIGATIONS OF ALASKAN VOLCANOES r*

 Fenner, G. N., 1923, The origin and .mode of emplacement of - the great tuff der

posit of the Valley of Ten Thousand Smokes:.Nat. Geog. Soc., Contr. Tech.

Papers, Katmai ser., no. 1, p. 14, 44, and 51-61. ^ 

Jochelson, Waldemar, 1933, History, ethnology, and anthropology of the Aleut:

Carnegie Inst. Washington Pub. 432, p. 7. ^ 

Knappen, R. S., 1929, Geology and mineral resources of the Aniakchak district:

U. S. Geol. Survey Bull. 797, p. 195. *- 

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

776, p. 61. * 

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

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

Ferret, F. A., 1937, The eruption of Mount Pel<§e, 1929-1932: Carnegie Inst ^

Washington Pub. 458, p. 84-105. 

Powers, Sidney, 1916, Volcanic domes in the Pacific: Am. Jour. Sci., ser. 4, /* 

v. 42, p. 261-274. 

Williams, Howel, 1932a, The history and character of volcanic domes: Call- * 

fornia Univ., Dept. Geol. Sci. Bull., v. 21, no. 5, p. 51-146.

 1932b, Geology of the Lassen Volcanic National Park, California: Call- v 

fornia Univ., Dept. Geol. Sci. Bull., v. 21, no. 8, p. 354-356.

 1942, The geology of Crater Lake National Park, Oregon: Carnegie

Inst. Washington Pub. 540, p. 99-107. v
INDEX

Page

Acknowledgment_.__.________.__ 22 

Adak Island... ..... ...... . 23, 27, 28

Agglomerate....--.--- ._ ._...- 28-29, 39

Aleutian Islands___.__. ___...-__ 24 

Aleuts -_ .._ --__ -___.-__ 23 

Alluvium........__.___.___..__... 32, 37

Amphlbole crystals in pumice.... _. 31

Analyses, basalt_______________ 30

fumarole gasses. . . 35

Andesite -- . ........... . 28, 29, 39

Andreanof Islands..... _ __ _. 23

Anlmal.life.. 26

Ash deposits... ..... ... _. 32

Attu Island........ ....... -....... .. 23

Basalt..... . .. 27, 28, 29, 31, 39

Beaches, deposits________.______ 32-33

features of. .. . . .. 32

Bering Expedition__..._..._______ 22

Breccia........ . ... 27,29-30,34

Bugle Point 25, 26

Cape Akuyan.. ...............__..._ 25, 32

Cape Kiugilak ........ . ...... 25

Crater, dome. ..... . .... 27, 30, 40, 41 

features of... . 23, 27, 35, 36, 39,40

Description of island..... . 22, 23,25,26 

Dikes. . . 27, 28

Eruptions..._...._______- 27, 32, 36, 39, 40-41

Finger Bay volcanics_.____...... 27-28, 35, 38

Flows, lava 27, 28, 29, 31, 39, 40 

mud.._.. . . . .... 33

Fumaroles..________________ 34-35,40

Gas... .........._..................... 30,35

Glaciers, deposits_-....__...______ 33

features..-.-.-..... ___.____ 26, 37, 40

Great Sitkin volcanics_____________ 29-32 

Great Sitkin Volcano, described....... 23, 25,29-30

future activity._ ... _......... 40-41

stages of development . ... 35-36, 39-40

Page 

History 22

Hot springs............................_ 34-35,40

IcefieldLakes__.___. 

Landslides..._.. 

Location of island.

30

Mud pots_____________________ 34-35. 

N6v6 ridges........................._...... 37-38

Parasitic cone..- -- .- _.. .. 27 

Plants, distribution........_________ 26

Plugs................i .......... 29, 30-31, 39

Pumice ...-_..___._-_ 27, 31-32, 37, 40

Quartz veins. 

Kip Point .

Sand Bay, features of........_ ... .. 25, 26

volcanics.... .. _... .. 28-29, 35, 39

volcano.. 39 

Sand dunes.._ ___ _________ 32-33 

Sea clifls........... .......... .. 26,28, 36, 38

Sulphur Point.....________ ___.... 25, 26

Terraces..._.. ... ..__ __. .. 26, 38 

Tufl...__ _.. .. ...... 27, 28, 29, 39

Uplift of island. 38

Valleys 37

Vasyutlnsky, Peter, quoted. 22-23 

Volcanic domes, evolution._ _ .... 41 

Yoke Bay... . ... .-.... 25,26

Zaliva Point. 29

o

43