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EA-I. (Robert R. Coats)

VOLCANOES OF ALASKA

       

CONTENTS

	Page
Ages of Volcanism	1
Types of Volcanic Activity	2
Relationship of Volcanism to Structure	4
Volcanic Areas	4
The Aleutian Arc	4
Nature of Volcanic Activity	9
Petrographic and Chemical Character of Volcanic Rocks	12
Relation of Volcanism to Structure	12
The Wrangell Mountains	15
Geographic Distribution	15
Nature of Volcanic Activity	16
Petrographic Character of Volcanic Rocks	17
Bering Sea-Seward Peninsula	17
Geographic Distribution	17
Types of Volcanic Activity	17
Petrographic Character of Rocks	18
Southeastern Alaska	18
Geographic Distribution	18
Types of Volcanic Activity	18
Petrographic Character of Rocks	18
Yukon-Tanana Region	19
Bibliography	20

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EA-I. Coats: Volcanoes of Alaska

       

LIST OF FIGURES

	Page
Fig. 1 Volcanoes of the Aleutian arc	1-a

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EA-I. Coats: Volcanoes of Alaska

       

PHOTOGRAPHIC ILLUSTRATIONS

        With the manuscript of this article, the author submitted 4

photographs for possible use as illustrations. Because of the high

cost of reproducing them as halftones in the printed volume, only a small

proportion of the photographs submitted by contributors to Volume I,

Encyclopedia Arctica , can be used, at most one or two with each paper;

in some cases none. The number and selection must be determined later

by the publisher and editors of Encyclopedia Arctica . Meantime all

photographs are being held at The Stefansson Library.

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E.A-I. [Robert R. Coats]

       

VOLCANOES OF ALASKA

       

AGE OF VOLCANISM

        Alaska has been the scene of volcani s c activity on a grand scale many —

times in the geologic past. In its broadest sense, volcanism is considered

by some geologists to include not only the eruption of solid, liquid, and

gaseous matter at the surface of the earth, but also the emplacement of

masses of igneous rock at great depths, and even the formation of ore deposits

as a result of those intrusions. The volcanic activity to be considered in

this article, however, will be that which has taken place in the latest

episodes of the earth’s history, comprised within the Quaternary period,

which embraces the Pleistocene (or glacial) epoch and Recent epoch.

        In Alaska, it is seldom possible to separate the products of these two

epochs, or to draw a line between them, because the Recent is also a time of

extensive glaciations, and large areas in the mountainous parts of Alaska are

still occupied by glaciers, many of which are surviving remnants of the

vastly larger glaciers of the Pleistocene. This is particularly true of the

larger volcanic mountains, and thus we find volcanoes in all stages of glacial

attack, from those still active, like Shishaldin (Fig.1) and Pavlof, which are

sheathed in their [ ?] upper parts by a thin but nearly continuous mantle of ice,

to those like Mount Iliamna (Fig.2), on whose flanks the work of ice and frost

has carved gigantic cirques, with walls many thousands of feet in height.

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        Although the epoch of decline of volcani s c activity thus ranges, at many —

of the older centers now recognizable, from the Pleistocene to the Recent, the

time of beginning of the activity is much less easily ascertainable. In the

Wrangell Mountains, and in the vicinity of Mount Spurr, west of Anchorage, there

is evidence that volcanic activity began as far back as the beginning of the

Tertiary period, but it is by no means certain that the same centers have been

active throughout the long interval since that time.

        Types of Volcanic Activity . The commonest type of volcanic activity, the

products of which bulk the largest, is the persistent eruption from a long–

lived central focus of lava and fragmental material, or pyroclastics. It is

this central type which is responsible for the beautifully symmetrical cone,

rising to a summit crater of relatively small dimensions. Fujiyama, in Japan,

is perhaps the best-known example of this type, but many Alaskan volcanoes,

such as Shishaldin and Mount Cleveland (Fig.3), are scarcely inferior to it

in perfection of form.

        The most spectacular central volcanic eruptions, such as that of Katmai,

in 1912, have sometimes resulted in the collapse of the summit regions of the

volcanoes, leaving in their stead great circular or elliptical depressions

called calderas. Many of these, where favorably situated, have been filled with

ice, through which the feebly reviving volcanic activity has in some places

erupted cones of black cinders (Fig.4) or protruded massive eruptions of lava,

now encumbered by carapaces of great blocks, piled in the wildest confusion.

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        Elsewhere, the volcanic activity has been shorter-lived and less strictly

confined to a few centers and has, therefore, resulted not in symmetrical

mountains or in profound calderas, but in inconsiderable hills or minor cones

of cinders and scoriae, rising but a few hundred or a thousand feet above

their bases. Some of these are surrounded by extensive plains composed of

several flows of funereal basalts, their surfaces still betraying the recency

of their formation by festoons or arcuate ridges of cinders and blocks, with

intervening moatlike depressions — the wrinkles on the surface of the flowing

lava. At other places the chance guidance of topography has confined the

superfluent lava to a few narrow channels, where the reduced rate of dissi–

pation of heat may permit it to reach much greater distances from the source.

        All of these expressions of volcanism with which we shall deal have, as

a common consequence of their youth, an influence upon the landscape dispro–

portionate to their total bulk, and one which is far more expressive of their

volcanic origin than of the effects of the subaerial agents of denudation.

For this and other reasons, this article will be confined to these latest

manifestations of volcanic activity.

        The Quaternary volcanic activity in Alaska thus falls naturally into two

major types: ( 1 ) central eruptions, leading to the erection of isolated vol–

canic mountains or groups of mountains or to the formation of calderas; and ( 2 )

cinder cone and plateau eruptions, resulting in thin, though perhaps widespread,

basaltic flows and generally minor and inconspicuous hills. The locations of the

principle volcanic mountains are fairly well known, and they can be delineated

upon a map. The areas of the other type of eruption are not as well known, and

in many places the information required to separate them from accompanying flows

of greater age is not available. Therefore, the places where these lavas occur

will be described in general, but no attempt will be made to delineate them on a map.

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        Relationship of Volcanism to Structure . The two types of volcanisc acti–

vity distinguished above are also associated with two different types of

structure. The linear chains or groups of central volcanoes are closely re–

lated geographically to the axes of Quaternary and recent mountain-building

movements in Alaska. The close association of the Aleutian volcanic chain

with the profound foredeep of the Aleutian Trench has been remarked upon by

many. Compressional deformation with open folding and th ur ru sting is character–

istic. The basalts that form the minor centers characterize areas of much

greater recent stability, where the Quaternary and recent movements are largely

epirogenic, characterized by broad warpings and minor faults, generally

tensional.

       

VOLCANIC AREAS

        The Quaternary volcanism of Alaska can be considered, for convenience, as

confined to five principal regions. These are, in order of diminishing impor–

tance: the Aleutian arc, including the Aleutian Islands and Alaska Pen i nsula;

the Wrangell Mountains; the Bering Sea-Seward Peninsula region; southeastern

Alaksa; and the Yukon-Tanana region.

       

The Aleutian Arc

        About 74 mountains known to be volcanoes are spread in a broad arc extending

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

along the Alaska Peninsula and the mainland of Alaska, to Mount Spurr. These

are listed with their heights and positions in Table I.

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        The 74 volcanoes listed include only those that form independent structures

of considerable size, and which are known to have been active in historic time,

or which show features indicating that they have probably been active in

Quarternary or recent time. Thirty-six volcanoes are known to have been active

in historic time (since 1760); their names are indicated by asterisks in this

table.

Table I. Volcanoes of the Aleutian Arc . —

(Arranged in geographic order, from west to east)

Name	Geographic Subdivision	Latitude	Longitude	Altitude, meters
Buldir	Buldir I.	52°32′N.	176°01′E.	620
Kiska*	Kiska I.	52°06′N.	177°36′E.	1,220
Segula	Segula I.	52°01′N.	178°08′E.	1,159
Little Sitkin *	Little Sitkin I.	51°57′N.	178°32′E.	1,195 —
Cerberus*	Semisopochnoi I.	51°56′N.	179°35′E.	775
Sugarloaf	Semisopochnoi I.	51°54′N.	179°38′E.	857
Gareloi*	Gareloi I.	51°48′N.	178°48′W.	1,627
Tanaga*	Tanaga I.	51°53′N.	178°07′W.	2,125
Takawangha	Tanaga I.	51°52′N.	178°00′W.	1,540
Bobrof	Bobrof I.	51°55′N.	177°27′W.	793
Kanaga*	Kanaga I.	51°55′N.	177°10′W.	1,348
Moffet	Adak I.	51°56′N.	176°45′W.	1,180
Adagdak	Adak I.	51°59′N.	176°36′W.	631
Great Sitkin*	Great Sitkin I.	52°04′N.	176°07′W.	1,750
Kasat o chi	Kasatochi I.	52°11′N.	17 6 5 °30′W.	310 —

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Table I. Volcanoes of the Aleutian Arc (Continued) . —

Name	Geographic Subdivision	Latitude	Longitude	Altitude, meters
Koniuji*	Koniuji I.	52°13′N.	175°08′W.	268
Sergief	Atka I.	52°19′N.	174°23′W.	560 ± plus or minus —
Korovin*	Atka I.	52°23′N.	174°10′W.	1,480
Kliuchef	Atka I.	52°19′N.	174°09′W.	915 +
Sarichef*	Atka I.	52°19′N.	174°03′W.	610 +
Seguam*	Seguam I.	52°19′N.	172°23′W.	1,052
Amukta *	Amukta I.	52°30′N.	171°16′W.	1,058 —
Chagulak	Chagulak I.	52°35′N.	171°09′W.	1,142
Yunaska*	Yunaska I.	52°39′N.	170°39′W.	600
Herbert	Herbert I.	52°45′N.	170°07′W.	1,290
Carlisle*	Carlisle I.	52°54′N.	170°04′W.	1,610
Cleveland*	Chuginadak I	52°49′N.	169°58′W.	730
Uliaga	Uliaga I.	53°04′N.	169°47′W.	887
Tana	Chuginadak I.	52°50′N.	169°46′W.	1,170
Kagamil*	Kagamil I.	52°58′N.	169°44′W.	892
Vsevidof*	Umnak I.	53°08′N.	168°42′W.	2,110
Recheschnoi	Umnak I.	53°09′N.	168°33′W.	1,940
Okmok*	Umnak I.	53°25′N.	168°03′W.	1,072
Tulik	Umnak I.	53°23′N.	168°03′W.	1,252
Bogoslof*	Bogoslof I.	53°56′N.	168 _ °02′W.	50 +
Makushin*	Unalaska I.	53°52′N.	166°56′W.	2,036
Akutan*	Akutan I.	54°08′N.	166°00′W.	1,293

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Table I. Volcanoes of the Aleutian Arc (Continued) . —

Name	Geographic Subdivision	Latitude	Longitude	Altitude, meters
Akun	Akun I.	54°16′N.	165°39′W.	819
Pogromni*	Unimak I.	54°34′N.	164°42′W.	1,987
Fisher*	Unimak I.	54°38′N.	164°25′W.	1,074
Shishaldin*	Unimak I.	54°45′N.	163°58′W.	2,858
Isanotski*	Unimak I.	54°45′N.	163°44′W.	2,480
Roundtop	Unimak I.	54°48′N.	163°36′W.	1,872
Amak	Amak I.	55°25′N.	163°09′W.	513
Frosty	Alaska Pen.	55°04′N.	162°51′W.	1,777
Emmons	Alaska Pen.	55°20′N.	162°04′W.	1,326 [?] —
Hague	Alaska Pen.	55°22′N.	161°59′W.	1,400 +
“Double” Crater	Alaska Pen.	55°23′N.	161°57′W.	1,400 +
Pavlof*	Alaska Pen.	55°25′N.	161°54′W.	2,715
Palvof Sister*	Alaska Pen.	55°27′N.	161°51′W.	2,135 +
Dana	Alaska Pen.	55°37′N.	161°12′W.	1,280 +
Kupreanof	Alaska Pen.	56°01′N.	159°48′W.	1,525 +
Veniaminof*	Alaska Pen.	56°10′N.	159°48′W.	2,560 +
Purple	Alaska Pen.	56°32′N.	158°37′W.	955 +
Aniakchak*	Alaska Pen.	56°53′ , N.	158°10′W.	1,348
Chiginagak*	Alaska Pen.	57°08′N.	157°00′W.	2,420
Peulik*	Alaska Pen.	57°45′N.	156°21′W.	1,525
Martin	Alaska Pen.	58°09′N.	155°24′W.	1,830 +

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Table I. Volcanoes of the Aleutian Arc (Continued) Concluded. —

Name	Geographic Subdivision	Latitude	Longitude	Altitude, meters
Mageik*	Alaska Pen.	58°12′N. .	155°15′W.	2,210
Knife Peak	Alaska Pen.	58°20′N.	155°08′W.	2,313
Trident	Alaska Pen.	58°14′N.	155°07′W .	2,070
Katmai*	Alaska Pen.	58°16′N.	154°59′W.	2,285
Denison	Alaska Pen.	58°25′N.	154°27′W.	2,326
Steller	Alaska Pen.	58°26′N.	154°24′W.	2,272
Kukak	Alaska Pen.	58°27′N.	154°21′W.	2,046
Unnamed	Alaska Pen.	58°37′N.	154°05′W.	915 +
Fourpeaked	Alaska Pen.	58°47′N.	153°42′W.	2,105
Douglas	Alaska Pen.	58° 4 5 2′N.	153°33′W.	2,328 —
Augustine*	Augustine I.	59°22′N.	153°25′W.	1,210
Iliamna*	Aleutian Range	60°02′N.	153°06′W.	3,073
Redoubt*	Aleutian Range	60°28′N.	152°45′W.	3,110
Double	Aleutian Range	60°44′N.	152°35′W.	2,160
Black	Aleutian Range	60°51′N.	152°25′W.	1,983
Spurr	Aleutian Range	61°18′N.	152°15′W.	3,375

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        The geodetic positions of the summits of the volcanoes have been scaled

from the Aleutian Islands Flight Charts Nos. 1, 2, and 3, of the U.S. Army

Air Forces, 1944 editions. In a few instances, more accurate data were obtained

from other maps, principally detailed topographic maps prepared by the Army

Engineers. The flight charts are also the sources for most of the heights

given in the table. Where summit elevations are not precisely known, the

plus sign is used to indicate the approximation.

        Nature of Volcanic Activity . Although the study of the volcanoes of the

Aleutian arc is far from completed (many of them having 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 in–

validated by further work.

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

characterized by many relatively thin flows accumulated on slopes of low de–

clivity, with a small proportion of fragmental material, and strato-volcanoes

(or composite cones) made up of both 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.

        A striking feature of the arc is the presence on it of large numbers of

calderas, many of great size. For the purpose of this article, the definition

of Williams (23, p. 242) will be adopted: “Calderas are large volcanic de–

pressions, more or less circular … in form …” The lower limit to the

diameter suggested by Williams is one mile. Table II includes.

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        Table II includes all calderas known to the writer in May 1949. Others

will be discovered, especially where erosion has altered the characteristic

topographic form sufficiently to disguise their origin. Many of those listed

have been seen only from the air or in aerial photographs, but the characteris–

tic form and relation to the structure exposed in the rimming cliffs are seldom

ambiguous. Where no source is given, the observations of the writer are the

source. Recognition of the caldera is credited to the first-known observer

who recognized it as such.

Table II. Calderas of the Aleutian Arc.

Name	Size, miles	Source	Name	Size, miles	Source
Devidof	1.5		Makushin	1.5 × 2	(14)
Little Sitkin	2.8		Akutan	1.25	(7)
Semisopochnoi	3.8 × 4.7		Fisher	6.2 × 6.8
Tanaga	7 ±		Morzhovoi (Frosty)	3.5
Takawangha	2		Veniaminof	5.2	(12)
Kenaton (Kanaga I.)	2.5	(5)	Purple	1.7 × 1.6	(12)
Kliuchef	2.8 ±		Aniakchak	6 × 5.2	(21)
Yunaska	1.8 × 1.5	(22)	Katmai	1.3 × 1.2	(9)
Okmok	7.5	(6)

        It should be noted that the dimensions given in Table II are minima, being

scaled to the base of the lowest accessible part of the bounding cliff; this

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

in the original reports. Where two figures are given, the caldera is notably

elliptical.

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        The term “volcanic dome” is used in the sense of Williams (24, p. 54) to

mean a steep-sided viscous protrusion of lava forming a more or less dome–

shaped mass around its vent. Such protrusions are common, as Powers (18)

has shown, in the vicinity of many volcanoes of the circumpacific zone;

commonly they form the final [ ?] phase of activity of the expiring cone. Many

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

(4, ppl 81-82) and from Great Sitkin Island. (19, p. 63). F. M. Byers (personal —

communication, 1948) has recognized an obsidian dome on Umnak, and J. C. Reed

(personal communication, 1948) a dome in the crater of Mount Augustine. Others

are known to be present on Buldir Island, Bobrof Island, L Kliuchef Volcano, —

and at the summit of Roundtop on Unimak. Many “plugs” mapped by Knappen on the

flanks of Veniaminof and Aniakchak are probably domes. On the whole, however,

they are rare in the Aleutian arc compared with the southern part of the

Cascade Range in Oregon and California.

        Byers (unpublished manuscript) has recently discussed the hot springs

and fumaroles of the Aleutian Islands and prepared the following classification [?] listed in Table III.

listed in Table III.

Table III. Fumaroles and Hot Springs of

the Aleutian Arc.

double space Type	Temperature range, °C.
Acid fumaroles	200-800
Sulfur dioxide solfataras	100-200
Hydrogen sulfide solfataras and springs	5-100
Mofettes and derived springs	5-100
Boroniferous springs and soffioni	5-102

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        Most of our information concerning the acid fumaroles is due to Allen

and Zies (1) who worked in the Katmai National Monument. These fumaroles are

characterized by halogen acid gases. The changes that take place with falling

temperature are in part brought about by reaction of the bases with atmospheric

oxygen, with each other, and with the wall rocks. The boroniferous emanations

are generally found in older terranes and are believed by Byers to be derived

from cooling granitic intrusives. The boroniferous springs are characterized

by high content of boron, sodium chloride, and total solids, ranging from

1,000 to 2,500 parts per million.

        Petrographic and Chemical Character of Volcanic Rocks. The volcanic rocks

of the Aleutian arc range from olivine basalt to rhyolite. They include basalts

characterized by olivine, with hornblende, hypersthene, or both. Relatively

high percentages of conspicuous crystals of calcic plagioclase and usually less

conspicuous green augite characterize most of the rocks. The more silica-rich

rocks, such as dacites and rhyolites, are less plentiful; most are present

either as small bodies of highly - glassy lava or as blankets of light-colored —

pumice.

        Relation of Volcanism to Structure . The most conspicuous positive element

in the structure of the Aleutian arc is the great curvilinear ridge, extending

southward and westward to Attu from the Alaska mainland, where it merges at its

northeastern end with the Alaska Range. The ridge is notably wider at the

eastern end than at the western end. From it, one 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.

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        The main ridge is bounded on the north by the lowlands of 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 meters). On the south,

the ridge of the Aleutian Islands is bordered by the Aleutian Trench, a typical

marginal deep, extending to depths of about 4,200 fathoms (7,700 meters). (17).

        The distribution of earthquake foci (10, p.25; 11, pp. 654-55) is such that

the epicenters of shallow earthquakes tend to be south of the chain; those of

intermediate depth (more than 60 km.) earthquakes are in the islands or north

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

Aleutian Islands will, when more information is available, resemble that pre–

sented by Gutenberg and Richter for the structurally similar Japanese arc. (10,

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 north–

ward at a moderate angle.

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

Paleozoic, are involved in both folding and faulting. The folding is, in general,

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

ing 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 , (12, p. 207),

the northwestern overthrust block has generally moved northeast and up with re–

spect to the southeastern block. Tensional faults, trending at an angle to the

trend of the arc, have also been noted.

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        In general, the volcanoes are superficial structures, built upon a base–

ment of Tertiary and older rocks that is exposed at intervals throughout the

length of the arc. The nature of the structures which have determined the

positions of the eruptive centers can be determined in few places. Some have

thought that the belt of volcanoes, because of its narrowness, coincides with

the surface trace of a great thrust plane or fault, movement on which is thought

to be responsible for the frequent earthquakes. In detail, the volcanic belt

does not form a simple arc, but consists of segments of differing length; the

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

Aniakchak region, Knappen has mapped a tension fault with an east-west trend,

along or close to which several volcanic structures are alined; he considered

that the site of the eruptive center has been 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 ten–

sional fractures striking at an angle to the major thrust zones. The distance

of the volcano from the major active zone of movement is probably dependent

upon the depth at which such a tensional fracture, originating in and limited

to an overthrust block, taps eruptable magma. The minor tensional faults, by

this hypothesis, bear a spatial relationship to the hypothetical major active

zone similar to that of the “feather fractures” of Cloos . (3, pp. 387-95).

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Wrangell Mountains

        Geographic Distribution . The Wrangell Mountains form an isolated group of

high peaks about 150 miles north of the northernmost part of the Gulf of Alaska.

They form the drainage divide between the waters of the Tanana, the Copper, and

the Yukon rivers.

Table IV. Volcanic Centers in the Wrangell Mountains.

Name	Latitude	Longitude	Altitude, meters
Sanford	62°13′N.	144°08′W.	4,938
Wrangell	62°01′N.	144°04′W.	4,285
Drum	62°07′N.	144°39′W.	3,658
Jarvis	62°02′N.	143°36′W.	3,749
Blackburn	61°44′N.	143°24′W.	4,919
Regal	61°44′N.	142°52′W.	4,084

        The principal volcanic centers in the Wrangell Mountains are listed in

Table IV. Mount Wrangell (Chehitno, Chitina) is the sole active volcano in

this group. It is reported (8) , somewhat doubtfully, to have been in eruption

in 1760 and 1784, as well as in 1884; according to F. H. Moffit (personal com–

munication), 1899 was marked by considerable explosive [ ?] activity. In recent

years, steam condensation clouds have been observed about the summit on many

occasions; photographs taken in June 1948 showed a thin deposit of cinders on

the snow near the present vent. All recorded activity has been mildly explosive.

        It is probable that volcanism in this area commenced in the Eocene; it is

not certain that so great an age can be ascribed to any center now recognizable

as a volcano.

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        Nature of Volcanic Activity . Both explosions and quiet extravasation of

lava have had a part in building up the Wrangell Mountains, as shown by the

character of the material exposed in the slopes, which includes tuffs, flows,

and bombs. The relatively low dips recorded from many areas suggest that some

of these mountains may be shield volcanoes, constructed largely of numerous

flows of rather fluid lava. Doubtless a substantial part of the structures

consists of tuff-breccias, the result of extravasation of lavas beneath the

glaciers, in a manner analogous to the subglacial eruptions of Iceland. The

descriptions in some of the older reports suggest that volcanic domes may also

be present . (15, p. 62).

        In all probability the most spectacular volcanic eruption of postglacial

time in this area was that responsible for the blanket of light-colored volcanic

ash that covers a total area of more than 140,000 sq.mi. in eastern Alaska and

southwestern Yukon Territory. Capps (2) computed the volume of the deposit

at 10 cu.mi., and thought the most probable source was a small vent, reported

by Thomas Riggs, Jr., in the area just northeast of Mount Natazhat. Mount

Natazhat itself is not a volcano, but is made up of Carboniferous lava flows

and sediments. Examination of aerial photographs of the St. Elias Range, taken

since the war, shows a large, circular ice-filled basin, about 3.7 miles in

diameter, at about 61°24′N. and 142°03′W. The precipitous walls attain an alti–

tude estimated at about 10,000 feet. It seems probable that this caldera is

the source of the recent ash of the upper Yukon and White rivers.

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        Petrographic Character of Volcanic Rocks . The Wrangell lavas (15, pp. 54-62)

range from black aphanitic and glassy rocks to holocrystalline, porphyritic,

light-gray varities. Others are brick-red, pink, pale lavender, brown, or

greenish. The common phenocrysts are plagioclase of intermedia r t e to calcic —

composition, brown hornblende, hypersthene, augite, biotite, and olivine.

The commonest rocks are hypersthene or hornblende andesites, but basalts and

dacites are also present. Sufficient information is not available to permit

any conclusions as to petrographic differences between the Wrangell lavas and

those of the Aleutian Islands.

       

Bering Sea-Seward Peninsula

—

        Geographic Distribution . Within the Bering Sea-Seward Peninsula region,

the principal areas of Quaternary volcanism are the following (20, pp. 84-86) :

the Pribilof Islands; the north-central part of St. Lawrence Island; an area

south and southwest of Imuruk Lake in the central part, and north of the Koyuk

River in the eastern part of the Seward Peninsula; areas in the vicinity of St.

Michael, east of the mouth of the [ ?] Yukon River; Nunivak and Nelson Islands;

and Hagemeister Island and an area in the vicinity of the Togiak River.

        Types of Volcanic Activity . All of the areas, wherever [ ?] studied, are charac–

terized by flows of lava with ropy, aa, or blocky surfaces, generally, and, because

of their recency, quite undissected. Some have extensive lava tubes, which, to–

gether with the low gradients on which the lavas flowed, indicate their fluidity.

Fragmental material is much less important, but cinder cones mark some sites of

eruption. On St. Lawrence Island in the Kookooligit Mountains, numerous cinder cones,

composite cones, and plugs have been mapped (H. B. Allen, personal communication, 1949).

Some of these have a random distribution, others are aligned on zones trending east

and west. The cones crown a plateau of basalt of slightly greater age, and are sur–

rounded by widespread flows and intercalated pyroclastics, the flows ranging in

thickness from 20 to 100 feet.

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        Petrographic Character of Rocks . Little information concerning the charac–

ter of the lavas is available. Those of the Seward Peninsula and St. Lawrence

Island are said to be chiefly olivine basalts; according to T. F. W. Barth (un–

published manuscript), the lavas of the Pribilof Islands belong to the plateau

basalt type.

       

Southeastern Alaska

        Geographic Distribution . The principal center of Quaternary volcanic

activity in southeastern Alaska is Mount Edgecumbe, 3,467 ft. (1,057 m.) high,

57°03′N. 135°46′W., just west of Sitka. The other area of Quaternary volcanic

rocks includes patches near Unuk River, and also on the southern part of

Revillagigedo Island and the nearby mainland.

        Types of Volcanic Activity . The activity of Mount Edgecumbe, which is

doubtfully reported to have continued to 1796, has resulted in the construction

of a composite cone of flows and pyroclastic rocks, about the base of which

the flows have spread rather widely . (13, pp. 14-15). Ash eruptions, presumably

from Mount Edgecumbe, attain depths of 10 feet or more in the vicinity of Sitka;

even in the Chichagof mining district, 45 miles to the northwest, it averages

perhaps 6 inches in thickness.

        The flows of the Revillagigedo Island area are flat lying, and generally

confined to the present lowlands. The nature of the source vents is not known.

        Petrographic Character of Rocks . The lavas in the vicinity of Revillagigedo

Island are reported by L.G. Westgate (unpublished manuscript) to be hornblende-bearing —

augite andesites and basalt. The lavas of Mount Edgecumbe are reported by Knopf

(13, p. 14) to be basic andesite and basalt, generally olivine-bearing.

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Yukon-Tanana Region

        Quaternary volcanic rocks are unimportant in the Yukon-Tanana region.

A small cinder cone of basalt has been described by Mertie . (16, p. 39) . Its

complete lack of dissection suggests that it is relatively youthful. In a

few other places in the Yukon-Tanana area, minor basalt flows have been found,

which generally occupy the present valleys, and are also somewhat dissected

by the steams in them, suggesting a somewhat greater age.

       

Robert R. Coats

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---

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U.S.Geol.Surv. Bull . 797. Wash.,D.C., G.P.O., 1929, pp.161-227.

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U.S.Geol.Surv. Bull . 692. Wash.,D.C., G.P.O., 1919, pp.283-98.

021      |      Vol_I-0344                                                                                                                  
EA-I. Coats: Volcanoes

15. ✓ Mendenhall, W.C. Geology of the Central Copper River Region, Ala k s .

Wash.,D.C., G.P.O., 1905. U.S.Geol.Surv. Prof.Pap . 41.

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Alaska . Wash.,D.C., G.P.O., 1931. U.S.Geol.Surv. Bull . 827.

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pp.757-81, 1945.

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pp.261-74, 1916.

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G.P.O., 1947. Pt.4, pp.55-70.

20. Smith, P.S. Areal Geology of Alaska . Wash.,D.C., G.P.O., 1939. U.S.Geol.

Surv. Prof.Pap . 192.

21. ✓ Smith, W.R. “Aniakchak crater, Alaska Peninsula,” U.S.Geol.Surv. Prof.Pap .

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24. ✓ ----. “The history and charac t t er of volcanic domes,” Ibid . vol.21, no.5,

pp.51-146, 1932.

       

Robert R. Coats