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    Volcanoes of Alaska

    Encyclopedia Arctica Volume 1: Geology and Allied Subjects

    Unpaginated      |      Vol_I-0321                                                                                                                  
    EA-I. (Robert R. Coats)




    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



    Fig. 1 Volcanoes of the Aleutian arc 1-a

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    EA-I. Coats: Volcanoes of Alaska



            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.

    001      |      Vol_I-0324                                                                                                                  
    E.A-I. [Robert R. Coats]





            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




            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 I. Volcanoes of the Aleutian Arc .

    (Arranged in geographic order, from west to east)
    Name Geographic

    Latitude Longitude Altitude,

    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.

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    Table I. Volcanoes of the Aleutian Arc (Continued) .
    Name Geographic

    Latitude Longitude Altitude,

    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

    Latitude Longitude Altitude,

    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

    Latitude Longitude Altitude,

    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


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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


            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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    EA-I. Coats: Volcanoes

            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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    EA-I. Coats: Volcanoes

            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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    EA-I. Coats: Volcanoes


    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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    EA-I. Coats: Volcanoes


    1. Allen, E.T, and Zies, E.G. A Chemical Study of Fumaroles of the Katmai

    Region . Wash.,D.C., Nat.Geogr.Soc., 1923. The Society. Contributed

    Tech.Pap., Katmai Ser . No.2.

    2. Capps, S.R. “An ancient eruption in the upper Yukon Basin.” U.S.Geol.Surv.,

    Prof.Pap . 95. Wash.,D.C., G.P.O., 1916, pp.59-64.

    3. Cloos, Ernst. “‘Feather joints’ as indicators of the direction of movements

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

    Wash., Proc . Vol.18, pp.387-95, 1932.

    4. Coats, Roberts. “Geology of northern Adak island, [?] U.S.Geol.Surv. Alaskan

    Volcano Investigations. Report No.2. Wash.,D.C., G.P.O., 1947, pt.5,


    5. ----, “Geology of northern Kanaga island,” U.S.Geol.Surv. Alaskan Volcano

    Investigations. Report No.2. Wash.,D.C., G.P.O., 1947, pt.6.

    6. Dunn, Robert. “On the chase for volcanoes,” Outing , vol.51, pp.540-50, 1908.

    7. Finch, R.H. “Akutan volcano,” Zeitschrift für Vulkanol . B.16, H.3, pp.155-60,


    8. Grewingk, Constantin. “Beitrag zur Kenntniss der orographischen und geognos–

    tischen beschaffenheit der nordwest Kueste Amerikas, mit den anliegenden

    Inseln,” Vserossiiskoe Min. Obahch., Leningr., Zapiski , 1849/50.

    9. Griggs, R.F. “The K [?] a tmai National Monument and Valley of Ten Thousand

    Smokes,” Science , n.s., vol.49, pp.236-37, 1921.

    10. Gutenberg, Beno, and Richter, C.F. Seismicity of the Earth . N.Y., 1941.

    Geol.Soc.Amer. Spec.Pap . no.34.

    11. ----, ----. “Seismicity of the earth,” Geol.Soc.Amer. Bull . Vol.56, pp.603-

    58, 1945.

    12. Knappen, R.S. “Geology and mineral resources of the Aniakchak district,”

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

    13. Knopf, Adolf. The Sitka Mining District, Alaska . Wash.,D.C., G.P.O., 1912.

    U.S.Geol.Surv. Bull . 504.

    14. Maddren, A.G. “Sulphur on Unalaska and Akun Islands and near Stepovak Bay,”

    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.

    16. Mertie, Jr., J.B. A Geologic Reconnaissance of the Dennison Fork District,

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

    17. Murray, H.W. “Profiles of the Aleutian Trench,” Geol.Soc.Amer. Bull . Vol.56,

    pp.757-81, 1945.

    18. Powers, Sidney. “Volcanic domes in the Pacific,” Amer.J.Sci . ser.4, vol.42,

    pp.261-74, 1916.

    19. Simons, F.S., and Mat h t hewson, D.E. “Geology of Great Sitkin Island,” U.S.

    Geol.Surv. Alaskan Volcano Investigations, Report No.2. Wash.,D.C.,

    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 .

    1932. Wash.,D.C., G.P.O., 1925. pp.139-49.

    22. U.S. Coast and Geodetic Survey. United States Coast Pilot. Alaska. Part II.

    Yakutat Bay to Arctic Ocean
    . 5th (1947) ed. Wash.,D.C., G.P.O., 1947.

    23. Williams, Howel. “Calderas and their origin,” Calif.Univ. Publ.Geol.Sci .

    vol.25, no.6, pp.239-346, 1941.

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

    pp.51-146, 1932.


    Robert R. Coats

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