Volcanic Activity 

in the 

Aleutian Arc

By ROBERT R. COATS

CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

GEOLOGICAL SURVEY BULLETIN 974-B

Including a list of all. known volcanoes 

and a summary of activity between 

1760 and 1948

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1950
UNITED STATES DEPARTMENT OF THE INTERIOR 

Oscar L. Chapman, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. C. - -Price 40 cents (paper cover) .'.'.'. ' ".:.'?
CONTENTS

Page

Abstract________________ __________..___-_____.._____________ 35

Introduction____________________________________________________ 35

Distribution of volcanoes__-__________-_-___-________.______________ 37

Dates of historic eruptions________________________________________ 40

Periodicity of eruptions_______________________________.____________ 41

Nature of volcanic activity-___-___----_-___---_------_--_-__--_--_- 43

Calderas_____________-_______--____--____-_______-__.-__--_ 43

Volcanic domes___________________________________________ 44

Petrographic character of the v<)lcanic rocks______________________ 44

Relation of volcanism to structure___________-_-______-___-__-_--_-__ 45

References cited_________________________________________________ 46

Index _. _ 49

ILLUSTRATIONS

PLATE 1. Map of the Aleutian Islands and the Alaska Peninsula, showing

the location of the principal volcanoes.______________ In pocket

FIGUBE 16. Five-year moving averages of numbers of volcanoes in eruption in the Aleutian arc________________-_--_-___--_-_- 42

TABLES

TABLE 1. Volcanoes of the Aleutian arc__________-____-_--_____------ 38

2. Summary of volcanic activity in the Aleutian arc, 1760-1948. In pocket

3. Calderas of the Aleutian «rc_________ ___________________ 43

889513 50 m

VOLCANIC ACTIVITY IN THE ALEUTIAN ARC

By ROBERT R. COATS

ABSTRACT

At least 76 major volcanoes, active and extinct, have been recognized in the 

Aleutian arc, extending from Buldir ; island on the west to Mount Spurr on the 

east. Of these, 36 are indicated as haying been active since 1760. When plotted 

against time, the activity, of the arc shows some signs of periodicity, the intervals 

between maxima being multiples of 20 years. In this report the geographic 

positions and altitudes of all the known volcanoes are tabulated, together with 

all known eruptive activity between :1760 and 1948. The kind of activity that 

has characterized the arc is described briefly; a list of 17 calderas, 9 of them 

hitherto unmentioned in the literature, is given; and the relation of volcanism 

to the structure of the arc is reviewed.

INTRODUCTION

This report is intended to present a brief summary of the present 

state of geographic and volcar. ologic information concerning the 

volcanoes of the Aleutian arc. It deals with the place, time, and 

nature of the activity of those volcanoes whose foci of eruption still 

are easily discernible and whose products form conspicuous, individ-' 

ualized landscape features at thu present day. The term "Aleutian 

arc" is here used to include the entire arc of the Aleutian Islands and 

its structural extensions, the Alaska Peninsula and the Aleutian 

Range.

Of the volcanic arcs that fringe the Pacific Ocean, that which 

stretches from Buldir Island to Mount Spurr is one of the least known. 

Remoteness from centers of population and the disagreeable climate 

have combined to discourage gee legists from undertaking the study 

of this chain of volcanoes. The energies of Government geologists 

have been largely focused on problems that appeared to be of more 

immediate economic interest, and from this standpoint there was little 

justification for the study of these volcanoes. In the eastern part of 

the arc, studies have been made of mineral deposits and of the occurrence of petroleum in the older rocks that form the foundation on 

which the volcanoes sit, but in the western part of the arc, in the 

Aleutian Islands proper, there s.ppeared to be little to beckon the 

prospector or to justify geologic {studies intended to assist him.

Until the middle of the nineteenth century, most of the information 

on the volcanoes of the arc had been derived from the more or less

35
36 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

casual observations of geologists or other scientists attached to several 

exploring expeditions; this information was summarized by Grewingk 

(1850), whose compilation has been the most useful ami accessible 

source of information on the Aleutian arc to that date. Since the 

publication of Grewingk's volume, the principal sources of information have been the reports of E>all( 1918),.of Becker (1898), and of 

several Survey geologists, who, while engaged in petroleum investigations on the Alaska Peninsula, made incidental observations on 

the volcanoes. Especially worthy of mention are W. W. Atwood, 

W. R. Smith, A. A. Baker, R. S. Knappen, and F. H. Mbffit. The 

famous eruption of Katmai in 1912 led to much careful work in that 

areabyGriggs (1921), Fenner, and their associates. ;

The events of the recent war have magnified the importance of the 

Aleutian arc in the geography of war and peace. Enormous sums of 

money have been spent in the development of army and naval bases 

and of airfields adequate for heavy bombers arid transport planes. 

The chain of bases extends the entire length of the Alaska Peninsula 

and the Aleutian Islands. In these circumstances, the temporal and 

spatial distribution and the nature, of .the volcanic activity along the 

arc become of considerable importance to those who .are responsible 

for the installations and for the personnel in this.area, whicn during 

the war was, in all probability, more densely populated than at any 

time in its previous history.

The importance of a knowledge of the volcanoes of the arc was 

emphasized dramatically by the eruption, beginning in June 1948', 

of a small cone within the great caldera of Okmqk on Umnak Island.

As n. consequence of this eruption, which was reported by Robinson

: (1948), the United States Geological Survey undertook a program

of volcanologic studies in the Aleutian Islands and the Alaska Peninsula. This program began in the fall of 1945. The assembling of 

all available data on the earlier volcanic activity was considered a

necessary preliminary to field work, and this report represents that

compilation, modified, to some extent by the results of field, work dojie

in the summers of 1946,1947, and 1948, *

The writer's field work has been limited to the western Ale.utians 

' ' .'"'.,''/ therefore, statements concerning other parts of the arc are. based on 

published information, on a study of aerial photographs, or on information contributed by colleagues in the Geological Survey. The

author is particularly indebted to Frank S. Simons, who worked on

Great Sitkin; David M. Hqpkihs, who worked on Umnak; Frank M. 

Byers, Jr., who has mapped extensively on tJmnak% TJnalaska, Akun,

and Akutan; and George C. Kennedy and H. H. Waldron, who protided information concerning the activity of Frosty and the volcanoes

of the Pavlof area. ; .',... :.. ... :
VOLCANIC "ACTIVITY IN THE ALEUTIAN ARC 37

DISTRIBUTION OF VOLCANOES

The kno.Fn.geologic.record in nhe Aleutian arc extends, with great 

gaps, as far back as the late Paleozoic; it is dominated throughout 

by volcanic, rocks., and. sediments derived from them. The gaps are 

due in part to conditions Unfavorable for the preservation of fossils, 

in part to a paucity of exposures,.and in part to incomplete geologic 

studies. : Because of limited exposures, little can;be said about the 

geographic distribution of the older volcanic activity, but the youngest 

cones, as! has been previously not id by many observers (Capps, 1934, 

p. i46),!are restricted in.their spatial distribution with respect to 

the axis .of the arc (pi. 1)." lixthe western part pf the arc, as far 

east as Akutan,, these volcanoes 1;end to lie to the north of the older 

volcanic rocks of the Aleutian" Islands. Along Unimak Island and 

the'Alaska Peninsula, they lie in general along the axis' of the 

 peninsula as far north west .as Mount Peulik, beyond which the main 

axis of the peninsula is northwest of the volcanic arc.

About:76 major volcanoes are spread in the broad arc that extends 

about 1,700 miles from Buldir Island, eastward through the Aleutian 

Islands, 'along the Alaska Peninsula and the mainland of Alaska to 

Mount Spurr. Both active arid extinct volcanoes are included, but 

all form independent structures of considerable size and either are 

known to have been active in historic time or show features indicating 

that they were probably active in. Pleistocene or Recent time. Most 

of them are shown on the. accompanying map (pi. 1), and all are 

listed with their heights and positions in the following table. The 

latitude and longitude given refer in some instances to the highest 

point on the caldera rim; in others, to the highest cone occupying 

the caldera. The 36 volcanoes marked with an asterisk (*) are believed to have been active in .historic time rthat is,, since 1760.

The names of numerous supposed volcanoes that have appeared in 

the literature have been omitted ::rom the foregoing list; sufficient information is now at hand to discredit many reported discoveries. 

Some of them were probably due. to the-inaccurate location of actual 

volcanoes, others to a confusion.in.names, and still others to the mistaken identification of fog streamers and cloud caps as eruption 

clouds. In many- earlier lists, toe, actual volcanic centers in f umarolic 

stages have been reported as active volcanoes because .of the confusion 

of f umarolic steam with an eruption cloud. For this reason, some 

volcanoes for which eruptions have been reported-are here removed 

from the.class of active volcanoes.

Sitignak Island, listed by Sapper (1927, p. 338) on Grewingk's authority as the .site of an eruptio:.i in 1776, does not correspond with 

any existing island,, as. Sapper, hi Qtiself.remarks.;, nor .does Rat Island, 

which Sapper suggests as the most probable site of the reported erup-
38 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

TABLE 1. Volcanoes of the Aleutian arc 

[Arranged in geographic order from west to east]

Name

 Little Sitkin... ___ .......

Bobrof.. . ....... ......

 Great Sitkin.- __ __ 

 Koniuji.. _ . .... _ ......

Kliuchef ........................

 Sarichef _____________

Chagulak ____________

 Carlisle. . .___.. ______.

 Cleveland ____________

Uliaga. . ....... ___ . __ .....

"Tana" _________ . ........

 Kagamil. .......................

 Vsevidof ....... ... .... .

 Okmok --. 

Tulik... ____ ..... _ .

 Bogoslof .._... ...............

 Makushin ____________

Akun.. .........................

 Pogromni. ___ .. ... ........

Westdahl ___ . __ ......

 Fisher. _____________

 Shishaldin... _

 Isanotski _____________

Roundtop _______________

Emmons _____________

Hague. . ......... ____ ....

"Double" Crater _______

 Pavlof . . . .. __ ....... __ . ...

 Pavlof Sister. _ . _ . __ ....

Dana. . .....................

Kupreanof . _______

 Veniaminof _________ ..

Purple __ Aniakchak ________ 'Chiginagak....

 Peuh'k.... ... Martin ____________ _

 Mageik.. ........................

Knife Peak ________ _ .

Trident ____________ .

 Katmai. ........................

Denison .........................

Steller...........................

Kukak . __ .......... .

Unnamed.___ _

 Iliamna...... _ ......... _ ...

 Redoubt 

Black.... ___ ........

Geographic locality

Buldir Island.. ________

Little Sitkin Island... ..........

-- do-.... . -. ___ 

Bobrof Island. .. ....

Kanaga Island. __ ... ..

 do 

Qreat.Sitkin Island... ........

Kasatochi Island __ ..........

Koniuji Island. _________

Atka Island ___________

 do .. __ ...... __ . ___ .

 do... .

. .do __... __ .

Seguam Island _ ........

Chagulak Island. ________

Yunaska Island _________

Herbert Island .. ........

Carlisle Island.. _ __.

Chuginadak Island ._...........

Uliaga Island.. ..........

Chuginadak Island _______

Kagamil Island. . ....._..

Umnak Island. _________

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

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

 do .... .............,

Bogoslof Island _________

Unalaska Island ................

Akun Island. _..___.

Unimak Island _________

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

 do. .

 do-. ... ____ .

___ do ______ . .___ .

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

..... do.. .... __.

.....do 

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

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

.. do ... _____ 

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

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

 do .. __ .... __ . ___ .

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

 do 

__ do _ _ .

_ do ____ _________

__ do ___________________

__ do _______________

_ do^ _ _ __ . _ ....

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

 do .. 

__ do. _______________ .

Aleutian Range.................

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

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

 do .

Latitude

52°23' N.

52°06' N.

52°01' N.

51°58' N.

51°57' N.

51°56' N.

51°64' N.

61°48' N.

51°53' N.

51°52' N.

51°55' N.

51°65' N.

51°56' N.

51°59' N.

52°04' N.

52°11' N.

52°13' N.

52°19' N.

.52°23' N.

52°19' N.

52°19' N.

52°19' N.

52830' N.

52°35' N.

52°39' N.

52°45' N.

52°54' N.

52°49' N.

63°04' N.

52°50' N.

52°58' N.

63°08' N.

53°09' N.

63°25' N.

53°23' N.

53°56" N.

63°52' N.

54°08' N.

54°16' N.

54°34' N.

54°31' N.

64°38' N.

54°45' N.

54°45' N.

54°48' N.

55°04' N.

55°20' N.

65°22' N,

55°23' N.

55°25' N.

55°27' N.

55°37' N. 56°01' N

56°10' N.

57°08' N.

57°45' N.

58°12'N.

58°26' N.

58°26' N.

S8°27' N.

58°37' N.

59°22' N.

60°02' N.

60°28''N.

60°44' N.

60°51' N.

Longitude

176°01' E.

177°36' E.

178°08' E.

178°20' E.

178°32' E.

179°35' E.

179°38' E.

178°48' W.

178°07' W.

178°00/ W.

177°27' W.

177°10' W.

176°45' W.

176°36' W.

176°07' W.

175°30' W.

175°08' W.

174°23' W.

174°10' W.

174°09' W.

174°03' W.

172°23' W.

171°16' W.

171°09' W.

170°39' W.

170°07' W.

170°04' W.

169°58' W.

169°47' W.

169°46' W.

169°44' W.

168°42' W.

168°33'W.

168°03' W.

168°03' W.

168002' W.

168°56' W.

166°00' W.

166°39' W.

164°42' W.

164°39' W.

164°25' W.

163°58' W.

163°44' W.

163°36' W.

162°04' W.

161°89' W,

161°57' W.

161°54' W.

161°51' W.

161°12' W.

1 1O° A. ft' W

159°23' W.

157°00' W.

156°21'W.

155°15' W.

155°08' W.

154°59' W.

154°27' W,

154°24' W.

1 <U°91 ' W

154°05' W.

153°25' W.

153°06' W.

1S2°4S' W.

152°35' W.

i52°25' W.

Altitude 

(meters)

1,220

1,159

330

1

1QC

775

01:7

1*640

793

1,348

1

1QA

631

1,750

310

9fia

560+ i Aftfi

01 g L

610+

1,050

1,058

1,142

600

1,290

1.610

1,730

887

1,170

809

2,110

1 Qdfl

1,072

1

OKO

50+

2,036

1 293

819

2,286

1,532

1,074

2,858

O AOn

1,777

1,326

1,4.00+

1 4(104.

2,715

9 13^-L.

i oon i

1,525+

o gftn.L

1

OXQ

2,420

1,525

2,210

2,326

2,272

0 04A

915+

2,105

1,210

3,073

3,110

2,160

1,083
VOLCANIC ACTIVITY IN THE ALEUTIAN ARC 39

tion, appear to have been the sits of any geologically recent volcanic 

activity. The eruption reported, by Grewingk probably should have 

been, and is here, attributed to Little Sitkin.

Adak Island (Sapper, 1927, p. 338), given in some compilations 

as the location of a so-called White Mountain or White Volcano active 

in 1760 and 1784, has certainly not been the site of any eruption in 

geologically recent time. The ascription of the White (Belaya) 

Volcano to Adak is probably due to a misreading by some geologist of 

an early report by Vasyutinsky. This interpretation is suggested by 

the wording of the translation oi his report by Jochelson (1933, p. 7), 

in which it is not made quite clear whether the mention of the White 

Mountain refers to Adak Island or Great Sitkin Island. The Great 

Sitkin volcano is much higher than any mountain on Adak and, 

with its permanent glaciers and large snow fields, merits the adjective 

"white."

Mount Dutton (Medvenikovsky), at latitude 55°08' N., longitude 

162°04' W., has been described as a volcano (Sapper, 1927, p. 341), but 

according to G. C. Kennedy and H. H. Waldron, who mapped that 

area in 1946, it is not volcanic. Moreover, several volcanoes are mistakenly indicated on a map accompanying a report by P. S. Smith 

(1939, pi. 16); Kagalaska Island is certainly not the site of any volcano, Conical Mountain on Atkca Island is not a volcano, and Alai 

Mountain according to Jacob Freedman, who worked, in the area 

is not volcanic. Walrus Peak (Morshowsky), at latitude 54°59' N., 

longitude 162°51' W., has been reported (Becker, 1898) as active in 

1768 but also is nonvolcanic.

The geodetic positions of the summits of the volcanoes have been 

scaled from Aleutian Islands Flight Charts 1,2, and 3 of the Army Air 

Force's 1944 editions. In a few instances, more accurate data were 

obtained from later maps, principally detailed topographic maps 

prepared by the Army engineers. The flight charts also are sources 

for most of the heights given on plate 1, but more recent information 

from the Coast and Geodetic Survey has introduced a few differences 

between the figures on the map and those in the table, which embodies 

changes made since the drafting' of the map. Where summit elevations were determined by the highest closed contour on the map, the 

value of this contour is used for the height, with a plus sign (+) 

to indicate the approximation. As the contour heights on existing 

maps are given in feet, the figures in meters are not in even hundreds 

or thousands.

An attempt was made to determine whether there is any regularity 

or system to the spacing of active and probably active volcanoes along 

the principal arc. The distances separating successive volcanoes

889513 50 2
40 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

shown on plate 1 were tabulated by class intervals of 5 kilometers. 

No well-defined mode is present. Many of the distances fall in class 

intervals of 10 kilometers or under; these distances are appropriate 

for subsidiary and parasitic centers about a main focus of eruption. 

Other distances fall in class intervals up to 130 kilometers. Certain 

differences are more apparent when the arc is divided into three segments one from Buldir to Seguam, one from Seguam to Dana, and 

one from Dana to Redoubt. In the first segment the class intervals 

between 56 and 75 kilometers are best filled; in the third segment 

those from 46 to 75 are best filled. On the average, the volcanoes 

are more closely spaced in the central part of the arc; insofar as 

closeness of spacing of active centers is a measure of the degree of 

activity, this part of the arc may be considered the most active 

segment. As will be shown later, this segment also contains the volcanoes for which the historic record of activity is most voluminous.

DATES OF HISTORIC ERUPTIONS

The dates of the known eruptions in the Aleutian arc from 1760 

to 1948 are shown in the "Summary of volcanic activity in the Aleutian 

arc" that constitutes table 2. Data concerning the past activity 

of the active volcanoes listed on page 38 were largely derived from 

the references given at the end of the report and from notes in three 

periodicals the United States Weather Bureau's Climatological Data, 

Alaska Section, volumes 1-34 (1914r48); Volcano Letters 1-502 

(1925-48) of the Hawaiian Volcano Research Association; and the 

Zeitschrift fiir Vulkanologie, Bande 1-17 (1914-38), published in Berlin. 'The primary sources of information were published records of explorations containing eyewitness accounts of eruptions, newspaper 

reports quoting accounts obtained from eyewitnesses, official weather

reports and other scientific findings, and first-hand manuscript reports 

from observers. The principal secondary sources were summaries,

obtained chiefly from the technical press. 

Numerous gaps and inaccuracies are undoubtedly present in the

information gathered from so many and such diverse sources. The

literate population of the Aleutian Islands, until the beginning of

the recent emergency, has never been large, and most of it has been

concentrated in a few villages for the greater part of the year. Moreover, the weather is not favorable for observations. The same conditions exist in lesser degree throughout the Alaska Peninsula, although 

there the activity of the salmon fishermen has provided opportunity 

for observations that otherwise would not have found their way into

tKe literature. No newspapers of general circulation are published in

that part of Alaska, and published local records are practically non-
VOLCANIC ACTIVITY ."N THE ALEUTIAN ARC 41

existent. Many of the earlier descriptions of volcanoes are in Kussian 

and have been accessible to the author only in translation.

One probable source of much confusion in the record is the popular 

tendency to call the cloud that results from the condensation of water 

vapor "smoke." When the accompany ing summary of volcanic activity was compiled, the extent of this confusion was not realized. 

Because of it, an undetermined pa.rt of all records that do not specify 

the nature of the activity, as w<ill as all the records that indicate 

"smoke," should be regarded with considerable suspicion.

Some volcanoes that hitherto have been listed as historically active 

are therefore relegated to the solfataric class. Akun, for example, 

has been reported active by Grewingk and Sapper, but F. M. Byers, 

Jr., told the author in 1948 that i; has almost certainly been in a solfataric stage during all of historic time. Kukak (Kutlyukhat, Kugak, 

Kugat) also has been reported in a, solfataric stage, and one report of 

ash eruption (Doroshin, 1870) was doubtfully attributed to this peak; 

but as Kukak is deeply glaciated, :he report is perhaps erroneous. i

Still other volcanoes that havd been listed as active may be con-- 

sidered suspect. The depth of erosion on Koniuji suggests that the 

report of its activity may be wrong, and the ascription of an eruption1 

to Fisher Caldera may not be justified. This ascription is based on 

a statement by Grewingk (1850, pp. 113-114) that is very indefinite as 

to location; however, Grewingk distinguishes all the other well-defined, 

volcanoes on Unimak Island, and his mention of white ash suggeststhat this may have been the caldera-forming eruption. Great thicknesses of light-colored ash are now present on the slopes of the 

mountain around the caldera; presumably they accumulated at the 

time the caldera was formed.

On the basis of the type of in: formation available, discrimination 

between rnajo? and minor explosive eruptions is very difficult. Probably many inaccuracies that may be attributed to this difficulty are 

present in the summary as given.

PERIODICITY OF ERUPTIONS

Figure 16 shows 5-year moving r,verages of the numbers of volcanoes 

in eruption in the Aleutian arc. Successive points were plotted at 

1-year intervals on the time scale, each point representing the average 

number of volcanoes in eruption for the year plotted and the two preceding and two succeeding years. The points have been connected by 

a solid line.

Inspection of the graph indicates fairly well defined maxima in the 

years 1768,1790,1828,. 1908, and 1929. The corresponding successive 

intervals are 22, 38,80, and 21 year.3, respectively. The curve suggests,
42 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

also, that the higher the peak of activity, the longer the period of 

relative repose before the next maximum, but several objections may 

be raised against the unequivocal acceptance of the apparent demonstration, of periodicity.

1. For the period of the Kussian occupation of Alaska, most of the 

recorded information concerning volcanic eruptions is in the scientific 

publications resulting from a very small number of exploring expeditions. Two of these expeditions, those of Billings and Liitke, 

coincide approximately with two maxima indicated on figure 16. The 

times of other expeditions, however, show no discernible relationship 

to maxima.

2. The long and nearly complete hiatus in the activity of the western volcanoes suggested by the blanks in the summary (table 2) 

from about 1830 to the first decade of this century may be merely a

o

<J>

CO CO CD CD CD CD <£ _ O> O» O>

DATE

FIGURE 16. Five-year moving averages of numbers of volcanoes in eruption in the

Aleutian arc.

gap in reporting, due to the relative infrequency of travel in the western part of the arc during that period.

3. The well-known high frequency of activity on the part of 

Makushin, Shishaldin, and Pavlof is obvious from an inspection of 

the summary. Grewingk recognized that the mountains toward the

eastern end of the arc are, on the average, higher than those toward 

the west. For mountains in the same stage of erosion, this may indicate more frequent activity, a longer history of activity, or more 

material erupted per unit of time. . The apparent periodicity indicated by figure 16 is the periodicity 

of the activity of the arc as a whole and not of any individual YOlcano*

inasmuch as the volcanoes whose activities are represented by one

maximum on the chart are not identical with 'those represented by 

other maxima.
VOLCANIC ACTIVITY IN THE ALEUTIAN ARC

NATURE OP VOLCANIC ACTIVITY

Although the study of the volcanoes of the Aleutian arc is far from 

completed (many of them have not yet been examined even in a reconnaissance way), certain generalizations based on the work done to date 

may be offered with considerable confidence that they will not be invalidated by further work.

The older volcanoes of the arc, seem to include both shield volcanoes, 

characterized by many relatively thin flows, with a small proportion 

of fragmental material, accumulated on slopes of low declivity, and 

stratovolcanoes or composite cones, made up both of flows and fragmental material, the slopes of which approach the angle of repose 

of the fragmental material. The major active volcanoes of the arc 

are without exception composite cones.

CALDERAS

A striking feature of the Aleutian arc is the presence of many 

calderas, some of great size. For the purpose of this report, the definition of Williams (1941, p. 242) will be adopted: "Calderas are large 

volcanic depressions, more or less circular * * * in form * * *." 

The lower limit to the diameter suggested by Williams is 1 mile. The 

list that follows includes all calderas known to the writer at the date 

of writing (May 1949), but others doubtless will be discovered, especially where erosion has altered the characteristic topographic form 

sufficiently to disguise its origin. Many of the calderas in the list have 

been seen only from the air or in aerial photographs, but the characteristic form and relation to tie structure exposed in the rimming 

cliffs are seldom ambiguous.

TABLE 3 Calderaa of the Aleutian arc

Name

Little Sitkin ________

Semlsopochnoi > *. ____ 

Tft.Tmtrn J

Kliuchef ... . ___ -

Yunaska . . ...

Okmok ' ____ . ____ 

Fisher. . _________ 

Frosty (Morzhovoi) J ........

Purple. .. .__. 

Anlakchak . .... 

Katmai _ .... ___ ...

Geographic loaJity

Semisopochnoi Islsnd.. ...

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

Atka Island ___ ........

Umnak Island..... .......

Unimak Island _ . .......

Alaska Peninsula. . ___ 

__ .do __ ___ . ....

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

.....dO .._.-..--....-.;__

... ..do... ...i...... ........

Size,i in 

kilometers

2.4.... ....

4.5........

6.1 by 7.5-

11± ....

3.2.. ......

4 ..........

4.5± - ...

2.9 by 2.4-

12... __ ..

2.4 by 3.2-

2 _____

10 by 11 ...

5.6........ 

8 4

2.7 by 2.6- 

9.7 by 8.4- 

2.1 by 2...

Source of information

U. 8. Coast Pilot, Alaska, pt. 2,

1947, p. 430. 

Dunn, Robert, 1908, pp. 540-550.

Finch, R. H., 1935, pp. 155-160.

Waldron, H. H., personal communication, 1948.

Do. 

Smith, W. R., 1925, pp. 139-149. 

Qriggs, R. F., 1921.

' One figure indicates diameter; where two figures are given, caldera is notably elliptical.

«Examined in the field since 1946 by the writer or his colleagues of the U. 8. Geological Survey.

' Cerberus Volcano (p. 38) is a younger central <:one in Semisopochnoi Caldera; Sugarloaf lies outside it.

4 Eanaga Volcano (p. 38) is a younger central ccne in Kanaton Caldera (Coats, 1947, p. 89).
44 CONTRIBUTIONS- TO GENERAL GEOLOGY, 1950

The recognition of most of these calderas cannot be regarded as a 

very difficult achievement, given the availability of detailed topographic maps or of- aerial photographs. Where no source of information is given, the writer is the responsible source; otherwise, 

recognition of the caldera is credited to the first known observer describing it as such or, in the case of discoveries yet unpublished, to 

the first observer known to the author to have recognized its .origin, 

regardless of whether -he referred to it as a caldera or a crater. .

It should be .noted that the dimensions given are minima, scaled to 

the base of the lowest accessible part of the bounding cliff. This 

accounts for the fact that many of the dimensions are smaller than 

those in the original reports referred to.

VOLCANIC DOMES

The term "volcanic dome" is used to mean a steep-sided viscous protrusion of lava forming a more or less dome-shaped mass around its 

vent. (See Williams, 1932, p. 54.) Such protrusions are common, as 

Powers (1916) has shown, in the vicinity of many volcanoes of the 

rcircum-Pacific zone; commonly they occupy the summit crater and 

p°ut a temporary or permanent halt to the activity of the cone. Others 

form on the flanks of the volcano. The volcanic domes of Novarupta, 

near Katmai, and of Bogoslof are well known; others have been described from Adak. Island (Coats, 1947, pp. 81-82) and from Great 

Sitkin. Island (Simons and Mathewson, 1947, p.. 63). F. M. Byers, 

Jr., according to a.personal communication, has.recognized an obsidian 

dome on Umnak and W. F. Thompson, Jr. (1949, p. 460), a dome in 

the crater of Mount Augustine. Domes are known to be present on 

Buldir Island, on Bobrof Island, in Kliuchef Volcano, and at the 

summit of Roundtop on Unimak. Many "plugs" mapped by Knap- 

..pen on the flanks of Veniaminof and Aniakchak are probably domes. 

On the whole, however, domes are .rare in the Aleutian arc compared, 

..for example, with the southern part of the Cascade Range in Oregon 

California. ....

PETROGRAPHIC CHARACTER OF THE VOLCANIC ROCKS 

The volcanic rocks of the Aleutian arc range from olivine basalt to

rhyolite. They include basalts characterized by olivine and andesites

'without olivine, in both of "which hornblende and Lypersthene occur 

separately or together. Relatively high percentages of conspicuous

calcic plagioclase crystals and usually less conspicuous green augite

characterize mqst of the rocks. Those that are comparatively rich 

in silica, such as dacites and rhyolites, are -much less plentiful; most

of them are present; either, as small bodies i of highly glassy lava or as

blankets of light-colored pumice.
VOLCANIC ACTTVirr IN THE ALEUTIAN ARC 45

RELATION OF VOLCANISM TO STRUCTURE

The Jmost conspicuous poshive element in the structure of the 

Aleutian arc is the great curvilinear ridge extending southward and 

westward to Attu from the Akskan mainland, where it merges at the 

northeastern end with the Alas ka Kange. The ridge is notably wider 

at the eastern end, than at the western. From it a branch about 300 

miles long extends northward and : "westward from Semisopochnoi 

Island, forming a submarine mountain ridge rising in places 12,000 

feet above the sea*floor. ' " ;

.The .main r.idge is bounded on the north by the lowlands o;f'the 

Alaska Peninsula and by the-Bering Sea. The eastern part of the 

Bering Sea, as far west as a line extending northwestward from Unimak Pass, is very shallow.' The western part of the Bering Sea is 

deep, and large areas of the bottom form extensive plains at depths 

close to 2,000 fathoms. (3,700 rt.eters): On the south, the ridge of the 

'Aleutian Islands is bordered by the Aleutian Trench (Murray, 1945)-, 

a typical marginal deep extending to depths of about 4,200 fathoms 

*(7,700 meters). , ^ ' ' " 

In general, the volcanoes are superficial structures, built upon a basement of Tertiary and older rocks that is exposed at intervals throughout the length of the arc. The nature of the structures that have 

determined the position of the eruptive centers can be determined in 

few places. 1 Some writers have thought that the line of volcanoes, 

because of its narrowness, represents the trace of a great thrust plane 

or fault, movement on which is thought to be responsible for the fre- 

'quent earthquakes. In detail, the volcanic line'does not form a perfectly simple arc, but consists of segments of different lengths; the 

included angles between adjacent segments may be as little as 140°. 

Certain volcanoes, like Bogos lof and Amak, lie some distance away 

.from the main, line, .on; the concave side of the arc. In the Aniakchak 

region, Kriappen mapped a tension fault with an east-west trend, along 

or close to which several volcandc structures are alined; he considered 

that, the site of the eruptive ceiter was determined by the existence of 

the fault. It is probable : that similar relationships exist elsewhere in 

.the arc and that most of the volcanoes have had their sites determined 

by minor tensional fractures-striking at an angle to. the major overthrust zones. The distance o:: a volcano from the major active zone 

of movement is probably dependent; upon, the deptn at which such a 

tensional fracture, originating; in and .limited to an overthrust'block, 

taps eruptible magma. The m inor tensional 'faults, .by this; hypothesis, 

fcear'a spatial-relationship to the hypothetical'major'active zone^similar to that of the "feather fractures"-of Cloos (1932, pp. 387-395). 

' The distribution of earthquake foci (Gutenberg and Richter, 1941, 

p. ; 25; .1945, pp. .654-655-) is such .that,the! epicenters -of shallow earthr
46 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

quakes tend to be south of the chain; those of intermediate-depth earthquakes (deeper than 60 kilometers) are in the islands or north of the 

chain. It seems probable that the general structural picture of the 

Aleutian Islands, when more information is available, will resemble 

that presented by Gutenberg and Bichter for the structurally similar 

Japanese arc (1941, p. 110). The distribution of deep and intermediate earthquake foci will probably fall along an active zone or surface, 

which will be shown to reach the surface of the crust along the northern slope of the Aleutian Trench and to dip northward at a moderate 

angle.

The older rocks of the Aleutian arc, in some places dating back to the 

Paleozoic, are both folded and faulted. The folding in general is 

relatively open, and dips are moderate. Major faults are reverse 

faults, trending nearly parallel to the trend of the arc; the north side 

has generally moved south and up with respect to the south side. In 

the Aniakchak area (Knappen, 1929, p. 207), the northwestern overthrust block has generally moved northeast and up with respect to the 

southeastern block. Tensional faults trending at an angle to the trend 

of the arc also have been noted.

REFERENCES CITED

Becker, G. F., 1898, Reconnaissance of the gold fields of southern Alaska, with

some notes on general geology: U. S. Geol. Survey 18th Ann. Kept., pt. 3»

pp. 1-86. 

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

Islands: U. S. Geol. Survey Bull. 857-D, pp. 141-153. 

Cloos, Ernst, 1932, "Feather joints" as indicators of the direction of movements

on faults, thrusts, joints, and magmatic contacts: Nat. Acad. Sci. Proc.,

vol. 18, pp. 387-395.

Coats, Robert, 1947a, Geology of northern Adak Island: U. S. Geol. Survey Alaskan Volcano Investigations Kept. 2, pt. 5, pp. 72-85 (limited distribution).

 , 1947b, Geology of northern Kanaga Island: U. S. Geol. Survey Alaskan 

Volcano Investigations Kept. 2, pt. 6, pp. 89-94 (limited distribution).

Dall, W. H., 1918, Reminiscences of Alaskan volcanoes: Sci. Monthly, vol. 7, pp. 

80-90.

Doroshin, p, f>,, 18T9, 0 niefctoryfcti Yulkanakh; Buss, K, min, Gesell, Yerli., 2d

ser., vol. 25, p. 44, St. Petersburg < trans. by M. M. Elins).

Dunn, Robert, 1908, On the chase for volcanoes: Outing Mag., vol. 51, p. 546. 

Finch, R. H., 1935, Akutan volcano: Zeitschr. Vulkanologie, Band 16, Heft 3,

pp. 155-180.

Grewingk, Constantin, 1850, Beitrag zur Kenntniss der orographischen und geognostlschen Beschaffenheit der Nordwestkiiste Amerikas. mit den anliegenden Inseln, St. Petersburg, K. Kray (previously published in Buss.

K, min, Gesell, Verb,, 1818-19),

Grlggs, R. F., 1921, The Katmai National Monument and the Valley of Ten 

Thousand Smokes: Science, vol. 49, pp. 236-237. Beno, ana mcnter, G, F,, isn, seismicity or tne eartu; Geol, soc>

America Special Paper 34.

, 1945, Seismicity of the earth: Geol. Soc. America Bull., vol, 56, pp. 603-658.
VOLCANIC ACTIVITY IN THE ALEUTIAN ARC 47

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-F, pp. 161-223. 

Maddren, A. G., 1919, Sulphur on Unalaska and Akun Islands and near Stepovak

Bay: U. S. Geol. Survey Bull. 692-E, pp. 283-298. 

Murray, H. W., 1945, Profiles of the Aleutian Trench: Geol. Soc. America Bull., vol.

56, pp. 757-781. 

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

vol. 42, pp. 261-274. 

Robinson, G. D., 1948, Exploring Aleutian volcanoes: Nat. Geog. Mag., vol. 94,

no. 4, pp. 509-522.

Sapper, Karl, 1927, Vulkankunde, pp. 338-342, Stuttgart. 

Smith, P. S., 1939, Areal geology of Alaska: U. S. Geol. Survey Prof. Paper 192,

p. 80. 

Smith, W. R., 1925, Aniakchak Crater, Alaska Peninsula: U. S. Geol. Survey Prof.

Paper 132-J, pp. 139-149. 

Thompson, W. F., Jr., 1949, Observations in Kamishak, Alaska: Geog. Rev.,

pp. 450-471.

United States Coast and Geodetic Survey, 1947, U. S. Coast Pilot, Alaska, part 2. 

Williams, Howel, 1932, The history and character of volcanic domes: Calif. Univ.,

Dept. Geol. Sci., Bull., vol. 21, no. 5, pp. 51-146. 

 , 1941, Calderas and their origin: Calif. Univ., Dept. Geol. Sci., Bull., vol.

25, no. 6, pp. 239-346.

INDEX

Page 

Abstract_____ _ ........... 35

Acknowledgments .. . 36 

Altitude of volcanoes... * 38; pi. 1

Oalderas ,..... .......................... 43-44

Dates of eruptions __ _ - 40-41; table2 

Distribution of volcanoes. . . 37-40; pi. 1 

Domes, volcanic.. 44

Earthquakes.. . 45-46

Eruptions, dates of 40-41; table 2

periodicity of.. ......... .. .. 41-42

products of... .. .... 43-44

Identification of volcanoes, difficulties of.. . 37,39 

Inaccuracies in record. . 37,40-41 

Information, sources of.__.__ 35-36,40-41,46-47 

Investigations in area, history of.. _ . 35-36

Page 

Location of volcanoes.... 38; pi. 1

Periodicity of eruptions 41-42 

Petrography 44

References cited____ _ 46-47 

Rocks, volcanic__... 44

Structure, relation of volcanism to 45-46 

Summit positions, how determined ____ 39

Volcanoes, altitude of... ... 38; pi. 1 

dates when active___...__... 40-41; table 2 

difficulties of identifying .. 37,39 

distribution of..._ . W-^Q; pi. 1 

list of. . _ ... 38 

location of..____ . . 38; pi. 1 

nature of activity of__..__... 43-44; table 2 

types represented_ . 43

49

O