; Geology of Northern 

«« Kanaga Island 

: Alaska

By ROBERT R. COATS

 *

* INVESTIGATIONS OF ALASKAN VOLCANOES

rr

' GEOLOGICAL SURVEY BULLETIN 1028-D

Prepared in cooperation with the Office, 

Chief of Engineers, U. S. Army

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956
UNITED STATES DEPARTMENT OF THE INTERIOR 

Fred A. Seaton, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D. C. - Price 75 cents (paper cover)
PREFACE

In October 1945 the War Department (now Department of the 

.Army) requested the Geological Survey to undertake a program of 

volcano investigations in the Aleutian Islands-Alaska Peninsula area. 

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

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

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

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

1950 in Geological Survey Bulletin 974-B, Volcanic activity in the 

Aleutian arc, by Robert R. Coats. The remainder of the data has 

been revised for publication in Bulletin 1028.

The geologic and geophysical investigations covered by this report 

were reconnaissance. The factual information presented is believed 

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

grows.

The investigations of 1946 were supported almost entirely by the 

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

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

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

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

continuing support.

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CONTENTS

Page

Preface __--___.__-_---__--________-__._-----___.---_- _________ in

Abstract_______ __________________---________-__-_-__-__--__ __- 69

Introduction- _ ______________________-__-_-_-__---_--__---_-------- 69

Geography_ _ _ __________-_________-_-___-_-_________------_------ 69

Geology______--_-__-.___________________-_------________ 71

Igneous rocks__________________________-_-__-____-_--_-_--_--- 71

Rocks extruded from vent near north coast ___________________ 71

Rocks of ancient Mount Kanaton___________________________ 72

Andesite tuff____________________________.._.. 73

Rocks of Kanaga Volcano_____________-___-___---_-____-__- 73

Recent pyroclastic deposits_________________________________ 74

Recent flows of Kanaga Volcano_______________-____-_______ 75

Sedimentary rocks Quaternary alluvium______ ____________________ 76

Structure..-_--______________________.-__-__-__-. 76

Faults in old rocks near north coast___ _____________________ 76

Kanaton caldera and related faults_______-___-_-_____.____ 76

Age of the rocks_____________--____-_-_____-__-_---_-_-_-_-__ 78

Geologic history__________________-____--_____---___.___-__ 78

Historic and current volcanic activity._______________________________ 79

Future volcanic activity_______________________________-_.__________ 80

Literature cited.__________________________________________________ 80

--____---___.__________-____________-___--__-__---._ 81

ILLUSTKATIONS

Page 

PLATE 15. Geologic map of northern Kanaga Island, Alaska_____.__ In pocket

16. Aerial view of Kanaga Volcano from the east.__________ Faces 70

FIGURE 5. Map of the Alaska Peninsula and Aleutian Islands. _ _. _ _ 70

v
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INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGY OF NORTHERN KANAGA ISLAND, ALASKA

BY ROBERT R. COATS

ABSTRACT

The northern part of Kanaga Island, one of the central Aleutian Islands, was 

examined briefly in July 1946. The oldest rocks recognized comprise a sequence 

of basalt flows and tuff beds, remnants of a cone that was formed near the present 

north coast. These rocks are believed to be late Tertiary in age. After the cone 

was partly destroyed by erosion, a basaltic shield volcano called Mount Kanaton 

was built nearby, and on its flanks at least two composite basalt cones were 

formed. Near the end of Pleistocene time, following an eruption of andesitic 

ash, the central part of the shield volcano and the cones on its flanks collapsed and 

disappeared as a result of downfaulting, leaving a great caldera. Renewed 

eruptions from the floor of the caldera built up the present cone of Kanaga Volcano, which has been intermittently active during historic time. All of the 

caldera rim except the southeastern part has been removed either by downfaulting 

or marine erosion. Some recent explosive eruptions have blanketed the island 

with basaltic and andesitic pumice, and have apparently flung blocks of lava a 

distance of as much as 30 miles. Basaltic lava was erupted from the sides of 

Kanaga Volcano as recently as 1904 and 1906. Present activity is mild and 

fumarolic, but minor, infrequent explosive eruptions and quieter extrusions of 

lava can be expected.

INTRODUCTION

The purpose of this report is to describe briefly the geology, history 

of volcanic activity, and nature of future volcanic activity of the 

extreme northern part of Kanaga Island.

Field work was carried out between July 12 and July 22, 1946, by 

Robert R. Coats, geologist, assisted by Will F. Thompson, Jr., 

recorder. Performance of the work would have been impossible 

without the cooperation of officers and men of the U. S. Army, 

especially of Lt. Col. R. E. Ware, post engineer, Adak, and Lt. Col. 

C. E. Johnson, port commander, Adak.

GEOGRAPHY

Kanaga Island is one of the Andreanof group of the Aleutian 

Islands. It lies between the parallels of 51°40' N. and 51°57' N., 

and between the meridians of 177°03' W. and 177°42' W., and is one

69
70 INVESTIGATIONS OF ALASKAN VOLCANOES
GEOLOGICAL SURVEY 

AERIAL VIEW OK KANAGA VOLCANO FROM THE EAST

The rim of an ancient caldera is at the left of the symmetrical cone. The high cliff nearest the camera is that of 

northern Tanaga Island are visible in the distance to the left. Photograph by 
'GEOLOGY OF NORTHERN KANAGA ISLAND 71

of the most southerly of the Aleutian Islands (fig. 5). The area 

herein called northern Kanaga Island is made up of the part of the 

island north of latitude 51°52' N.

The outline of Kanaga Island on the map may be likened to that of 

a pistol, the barrel pointing west, and the butt extending north. The 

landscape of the southwestern part of the island the part coinciding 

with the barrel and breech of the pistol consists of successive 

marine terraces, the highest of which is 600 feet in altitude. The 

terraces are dotted with lakes and partly dissected by streams. On 

northern Kanaga Island an arcuate ridge, which trends northeastward 

and is convex to the southeast, rises to an altitude of 2,600 feet 

(pi. 16). Many small ridges extend to the south and east from the 

arcuate ridge, and a few variously trending minor "splinter" ridges 

(Gilbert 1928, p. 80-81) project from its steep inner scarp. This 

scarp faces, across a large depression, the simple cone of Kanaga 

Volcano, 4,305 feet in altitude, the most conspicuous feature of the 

island. In the extreme southeastern part of ,the depression lies an 

unnamed lake, nearly 1 mile long, which has no surface outlet. The 

cliffs of Round Head, the eastern extremity of the island, expose a 

section through the southwestern flank of a minor cone, the site of 

whose vent is now beneath the Bering Sea.

GEOLOGY

The geologic history of northern Kanaga Island is essentially the 

history of the development of an ancient shield volcano (herein called

**- Mount Kanaton after an obsolete name for the island), its decapitation by eruption and collapse, and the building of Kanaga Volcano 

within its broken rim. The rocks belonging to the several stages are

 f-l discussed in chronologic order of formation, and they are shown on 

the geologic map (pi. 15). The petrography of the rocks has been

discussed in more detail in a previous report (Coats, 1952).

t

IGNEOUS BOCKS 

ROCKS EXTRUDED FROM VENT NEAR NORTH COAST

The rocks of a deeply dissected small basaltic cone, cropping out

-1 on the north shore, are apparently the oldest rocks exposed on north- 

^ ern Kanaga Island. They are made up largely of lava flows, but include a few tuff beds. The lava flows are composed mostly of fine-

*- grained, dense, dull greenish gray basalt. The basalt contains

^ abundant labradorite phenocrysts, less conspicuous augite phenocrysts, and accessory hypersthene, olivine, hornblende, and magnetite

 y grains in a groundmass of plagioclase laths, magnetite dust, and 

orthoclase(?). These are the only rocks on northern Kanaga Island

877558 56 2
72 INVESTIGATIONS OF ALASKAN VOLCANOES ^

showing evidence of widespread hydrothermal alteration. The dips *" 

of the flows indicate that their vent was about 2 miles east of the ^ 

present summit of Kanaga Volcano. The remnants of the cone are 

overlapped by the younger basalt flows of ancient Mount Kanaton. y ?"

ROCKS OF ANCIENT MOUNT KANATON ^

..A

A thick sequence of basalt andv andesite lava flows is exposed on the 

inner face of the arcuate ridge of Mount Kanaton and on the radial -< 

ridges diverging from it. The flows, averaging about 20 feet in 

thickness, have an aggregate thickness of more than 1,500 feet. Low 

outward dips of 5°-14° indicate that the flows issued from a vent >». 

near the present vent of Kanaga Volcano. The distribution and dips 

of the lava flows indicate that they once formed a broad domelike * 

shield volcano, much more extensive but lower than the present 

cone.

Weathered surfaces of the rocks are dark gray, but fresh surfaces 

are riiedium-to-light gray. The rocks consist of phenocrysts of ^ 

labradprite, olivine, hypersthene, and augite, and accessory magnetite, in a groundmass of plagioclase laths, augite rods, magnetite 

granules, and interstitial glass or orthoclase(?). A few of the lava 

flows contain hornblende and apatite.

In general the proportion of hypersthene to olivine increases from 

the bottom to the top of the sequence of flows. Olivine basalt is -f,^ 

common in the lowermost flows, olivine-hypersthene basalt in the 

intermediate flows, and hypersthene basalt and andesite in the ^' 

highest ones. +

Thin basalt flows interbedded with tuff-breccia the remnants of a

composite cone built on the flanks of Mount Kanaton crop out near '

the west coast. The beds dip nearly 30° away from the center, which \,- f^

was about half a mile south of the present summit of Kanaga Volcano. The rocks of these flows differ little from those of the earlier ~ 

flows of Mount Kanaton, except that they contain a higher propor- ^ 

tion of phenocrysts. Most of the flows consist of hypersthene basalt 

and olivine-hypersthene basalt, but a few consist of hornblende /*" 

andesite. The fragments in the tuff-breccia are similar to the basalt _ u .. 

flows in composition.

Many thick, low-dipping, olivine basalt flows, exposed in the 

vicinity of Round Head, appear to have issued from a vent on the a. 

northeastern slopes of Mount Kanaton. The flows are typically light

pinkish gray or pale gray, and contain conspicuous large pnenocrysts '*

of dark-green augite and smaller phenocrysts of olivine and plagio- ^ 

clase in a medium-grained groundmass. On the north, the lava flows 

apparently banked against a sea cliff cut in early flows from Mount V* 

Kanaton. On the south, the latest flow from this northeastern vent .
GEOLOGY OF iSTORTHERN KANAGA ISLAND 73

overlaps a flow from Mount Kanaton. The outer slopes of the cone 

built of these flows are somewhat less dissected than are those of the 

cone near the west coast, indicating that the cone near Round Head 

may be somewhat younger.

ANDESITE TUFF

Andesite tuff forms a thin blanket, widespread over northern 

Kanaga Island south and east of the cone of Kanaga Volcano and 

the curving ridge bordering it, except on the summits of the high 

ridges and on the steepest slopes. The tuff blanket commonly is only 

a few feet thick, but locally is as much as 30 feet thick. The tuff is 

well exposed in a small gully about 3,000 feet south of the southeast 

corner of the large lake and in a stream-cut bank about a mile southeast of the same corner of the lake. The tuff is a rather uniform, 

fine-grained rock, only slightly stratified. Where unaltered, it is 

light gray; where weathered, it is pale buff. Near the stream-cut 

bank, the tuff is yellowish gray and contains fragments of pumice as 

much as a quarter of an inch hi diameter. The pumice fragments are 

rather fragile; after shipment only 5 percent of a sample of the coarser 

material was retained on a 10-mesh sieve. Of the part thus retained, 

about half was made up of fragments of accidental and accessory 

rocks, the rest of essential and cognate material, largely pumice and 

cognate crystalline rocks.

The accidental material includes quartz-oligoclase-biotite hornfels, 

amphibolite, anthophyllite-feldspar schist, and biotite schist. It is 

inferred that the walls of the magma chamber from which this tuff 

was erupted extended down to the sialic crustal rocks of continental 

type, not now exposed anywhere on Kanaga or adjacent islands, so 

far as is known.

The principal essential and accessory constituents are labradorite, 

green hornblende, brown basaltic hornblende, augite, hypersthene, 

magnetite, apatite, and glass. All these constituents except the green 

hornblende are present in older volcanic rocks of the island and could 

have been derived from them; the older volcanic rocks probably were 

fragmented by volcanic explosions. Green hornblende, however, is 

very rare in extrusive rocks (the color of the hornblende typical of 

most extrusive rocks is reddish brown). Its presence suggests that 

the tuff was for the most part the product of a* body of deep-seated 

magma that was erupted so rapidly the hornblende did not completely 

oxidize.

ROCKS OF KANAGA VOLCANO

The cone of Kanaga Volcano consists of interbedded hypersthene 

basalt and hypersthene andesite flows, scoria layers,, and beds of
74

tuff-breccia. Most of the rock exposures on the mountain are of lava 

flows, but a few cliff exposures indicate that the interior of the cone 

probably contains a high proportion of fragmental material. Around 

the base of the steeper part of the cone are wide block-strewn areas 

that have the characteristics of mudflows.

The rocks are dark gray to black, generally vesicular, and are 

characterized by labradorite phenocrysts, 2-4 millimeters long, and 

smaller augite and hypersthene phenocrysts set in a groundmass of 

plagioclase microlites, augite needles, and brown glass.

RECENT PYROCLASTTC DEPOSITS

Beyond the immediate slopes of the present cone, turf and underlying peat rest on a mantle of volcanic ash and pumice, which has a 

maximum measured thickness of about 7 feet. Most of the ash and 

pumice was presumably erupted from Kanaga Volcano while the 

present cone was being built, and some may have been derived from 

explosive eruptions on other islands. This mantle has not been 

shown on the geologic mapj because it is so widely distributed it 

would obscure the details of the bedrock geology. The ash is generally firm and impermeable, but it contains a few beds of pale 

pumice that are permeable and that serve as aquifers. The removal 

of these permeable layers by headward erosion of small streams has 

resulted in the development of miniature caves in the ash.

The large lake between the cone 'and the arcuate ridge southeast of 

it has often been showered with ash and pumice. Much of the 

coarser pumice was light enough to float, and has been concentrated 

on the east and west shores of the small island in the lake, and in a 

large bar of water-sorted stratified pumice rising about 30 feet above 

water level in the extreme southeast corner of the lake. This material was not separately mapped but was included with the alluvium.

Following the deposition of the pumice, two more eruptions of 

pyroclastic material from Kanaga Volcano occurred. These are of 

special interest because they provide clues to the kind of explosive 

activity that can be expected from the volcano. Their products,

no-wever, are insignificant in volume.

During the first of these eruptions, pumice was sprinkled over the 

slopes of the volcano in two layers, the earlier one consisting of 

pale-gray andesitic pumice, and the later one, of dark pumice derived 

from a basaltic magma that appears to have remelted and partly 

digested the andesitic rock.

During the second eruption, blocks of previously consolidated 

pale-gray compact basalt were flung over the landscape, producing 

numerous imposing impact craters where they fell on the lower slopes 

of the volcano and producing other imposing but widely separated
GEOLOGY OF NORTHERN KANAGA ISLAND 75

craters at least as > far as 2% miles from the summit. The largest 

impact crater observed ,has major and minor diameters of 20.and 15 

,feetj and is 5 feet deep. Some craters that are 5 feet or less in diameter 

are as much as 3-4 feet deep. The basalt blocks contain labradorite 

and augite phenocrysts and accessory magnetite in a groundmass of 

equant feldspar grains and pale glass. Fragments apparently of the 

same rock, weighing only a few pounds, are present on the slopes of 

Mount Moffett on Adak Island and near Hot Springs Bay on Tanaga 

Island (Coats, 1956).

RECENT FLOWS OF KANAGA VOLCANO

The youngest rocks associated with Kanaga Volcano comprise 

four basalt flows emitted from the cone during historic or late prehistoric time. The oldest of these flows, referred to as the early 

southwestern flow, appears to have issued from a fissure on the 

southwest side of the cone at an altitude of about 3,900 feet. It is 

now covered by a younger flow, called the late southwestern flow, 

at an altitude of about 500 feet.

The next younger flow the southern flow apparently issued 

from the same vent as that from which the early southwestern flow 

issued. It was ponded against the base of the arcuate ridge to a 

depth of at least 100 feet. This flow is perhaps the one emitted in 

1904 (p. 79).

A flow that also issued from a fissure high on the southwestern 

flank of the cone the late southwestern flow covers the lower end 

of the early southwestern flow and extends past the caldera wall to 

the sea. Another flow the northeastern flow moved northeastward from the crater. Its terminus is marked by a flow scarp about 

 100 feet high on a grassy flat about 600 feet above sea level.

The late-southwestern flow and the northeastern flow were probably 

erupted in 1906 (p. 79). All but the oldest of the four flows were 

"extruded after the two explosive eruptions described in the section 

"Recent pyroclastic deposits."

The flows resemble one another closely. Their thickness on the 

steep slopes of the cone does not exceed 20 feet. (On the geologic 

sections of plate 15 their thickness has been exaggerated for clarity.) 

The southern and northeastern flows contain labradorite phenocrysts,. 

ranging in length from 0.1-3 millimeters, smaller augite, hypersthene, 

and magnetite-phenocrysts, and a few large olivine phenocrysts, as 

much as 6 millimeters across, in a groundmass of plagioclase laths; 

augite rods, magnetite granules, and brownish glass. . The two 

southwestern flows are similar in composition to the southern-and 

northeastern flows> except that-they do not contain olivine. .. ; :.
76

SEDIMENT ART BOCKS QUATERNARY ALLUVIUM

A large area of alluvium occurs on the east shore of the island south 

of Eound Head. Its surface forms a plain half a mile wide, rising 

from a few feet above sea level at the outer edge to about 40 feet 

above sea level at the inner edge. Where exposed by marine erosion 

the alluvium consists of intermingled ash, wind-blown sand, and peat. 

The plain probably is underlain in part by stream-transported ash 

and pumice from the outer slopes of former Mount Kanaton.

Narrow beaches covered by rounded boulders several feet across 

fringe the island. The highest parts of the beaches are as much as 

20 feet above normal high water.

STRUCTURE

The structure of northern Kanaga Island is diagramatically illustrated by the geologic section of plate 15. As nearly as can be determined, the rock units shown on plate 15 have retained their initial 

dips, but have been displaced relative to one another by numerous 

faults. Some of the faults are probably older than the formation of the 

caldera, and some came into existence at the time of formation of the 

caldera. Northern Kanaga Island appears to have been a relatively 

stable platform for a long time. After the volcanic structures were 

built, the platform as a whole was subjected to vertical movements, 

and parts were subjected to downfaulting, which caused fragmentation. 

Deformation by compressive stress has not occurred.

FAULTS IN OLD ROCKS NEAR NORTH COAST

A group of faults cuts the rocks older than those of Mount Kanaton 

near the north coast. The trends of these faults range from N. 47° W. 

to N. 30° W. All the faults are vertical, or very nearly so. Data are 

not available for determining the direction of movement. The wall 

rocks of some of the faults have been hydrothermally altered for considerable distances from the faults, but the nature of the alteration 

has not been determined.

KANATCwsr cAl,r>ERA AND RELATED FAULTS

The arcuate ridge, its steep inner face, and its radially disposed outer 

ridges constitute the most unusual feature on Kanaga Island, The 

outward dips of the flows composing the ridge indicate a common 

source from a Center near the present summit of Kanaga Volcano, and 

it is partly on this basis that Mount Kanaton is assumed to have 

existed. The ridge is clearly a remnant of a former large volcanic 

mountain. Several explanations for the disappearance of most of the
77

upper parts of Mount Kanaton and the formation of the ridge are 

considered.

The ridge could conceivably be a cliff produced by marine erosion, 

except that sea cliffs developed in relatively simple structures commonly are straight or convex, rather than concave. It might be 

explained as the result of a gigantic explosion that destroyed the entire 

summit area. Such an explosion, however, would be expected to leave 

enormous quantities of coarse, angular basaltic rock debris volcanic 

breccia on the flanks of the volcano. No breccia deposits are present; 

instead, the pyroclastic eruption preceding the formation of the ridge 

deposited very fine andesitic tuff in a thick bed on the outer slopes of 

Mount Kanaton.

The most tenable explanation for the destruction of the summit area 

of ancient Mount Kanaton seems to be that of collapse by downfaulting of the central part, forming a caldera of which the arcuate 

ridge is a remnant. How the space for the collapsed superstructure of 

the volcano was formed remains an unsolved problem. The total 

volume of the blanket of ejected andesitic tuff, even assuming an average thickness of about 30 feet, which is the maximum measured thickness, is inadequate to account for the space into which the upper part 

of the cone collapsed. Additional space to accommodate the collapsed, 

roof was presumably provided by withdrawal of lava through submarine extrusion or intrusion at depth.

There is considerable evidence to support the conclusion that the 

arcuate ridge was formed by downfaulting of the central part of 

Mount Kanaton along lines tangential to the contours of the mountain. 

The simple outlines and undissected form of the inner face suggest 

that it is a fault scarp. On the south side of the ridge, south of the 

highest point, a recent fault parallels the crest of the ridge; the 

north side of the fault is downthrown a few feet, as shown by the 

displacement of the surface where the fault crosses ridges. This 

fault is considered to be a branch of one of the faults along which 

subsidence took place. The major caldera-forming faults are not 

exposed. In a few places the scarp may nearly coincide with the 

position of a principal caldera-forming fault. A zone of rock alteration in the wall of the caldera, just north of the northeast corner of 

the lake, suggests the presence of one of the major caldera-forming 

faults. The conspicuous short ridge at the west end of the lake and 

the northward-trending ridge east of the lake are both believed to be 

down-dropped blocks that have remained at a relatively high 

elevation. The arcuate ridge may be the remnant of a rim that 

originally encircled the caldera, but which was destroyed by subsequent marine erosion or downfaulting, or the caldera may never 

have been a completely enclosed depression.
78 INVESTIGATIONS OFu ALASKANu VOLCAIfOES

AGE OF THE, ROCKS . r;

The relative age of the rocks of northern Kanaga can be deter* 

mined fairly 'accurately from superposition and the relative degree 

of dissection by erosion, but the actual geologic age of the rocks can 

be determined only with difficulty, as none are fossiliferous. ' :

The shield volcano of Mount Kanaton does not appear to have 

been glaciated at the level of the present caldera rim, although a 

volcano on Tanaga Island, a few miles to the west, shows much 

evidence of vigorous glaciation at corresponding altitudes and some 

evidence of glaciation down to sea level. Moreover, the caldera 

wall itself is unglaciated, although ridges of similar altitude on southern 

Adak Island have been trimmed and sharpened by glacial erosion. 

On the other hand, Mount Adagdak, an isolated mountain on nearby 

Adak Island, is only 500 feet lower than the arcuate ridge, but has 

not been glacially dissected though it is of preglacial age. It is 

inferred that glaciers will form on a mountain whose altitude is 

about the same as that of the lower limit for the accumulation of 

glacial ice if the mountain is one of a group of peaks of about the same 

height, but not if it is an isolated peak. The lack of evidence for 

development of glacial cirques on the caldera rim does not, therefore, 

indicate that the caldera was formed after the close of the glacial 

epoch in this area.

The absence of signs of glaciation seems best explained by assuming 

that the construction of Mount Kanaton and the composite volcano 

on its southwest flank was relatively rapid, beginning in late Tertiary

time and continuing into Quaternary time. Therefore, glaciers had 

little opportunity to develop on Mount Kanaton; those that may 

have existed were probably small and confined to'the slopes of the 

volcano above the level of the present caldera rim.

On. the basis of the foregoing interpretation of the age of Mount

Kanaton, the lava flows extruded from the vent on the north COast are 

assumed to be late Tertiary in age, the flows of the composite cone 

built on the flanks of Mount Kanaton are believed to be late Tertiary 

and Quaternary in age, and the present cone of Kanaga Volcano is

'believed to be Quaternary in age, as are the pyroclastic and alluvial

deposits lying on bedrock. The collapse that created the caldera 

probably occurred toward the close of the Pleistocene epoch.

GEOLOGIC HISTORY

The known geologic history of northern Kanaga Island began with 

the opening of a vent about 2 miles east of the present summit of 

Kanaga Volcano and the construction of a small cone. The cone' 

'which was made up largely of basalt flows, was partly destroyed, bymarine and subaerial erosion. A broad shield .volcano1 ancient
'GEOLOGY OF.1 NORTHERN KANAGA ISLAND 79

Mount Kanaton was then built of basalt and andesite flows that 

were extruded from a vent near the present vent of Kanaga Volcano. 

After a brief erosional interval, vents on the flanks of Mount Kanaton 

were opened. A cone made up of basaltic flows and scoria was built 

on the southwestern, flank, and a "similar cone, consisting largely 

of thick lava flows, was built seaward from Round Head, which is on 

the present northeastern, shore. After a period of quiescence and 

erosion, Mount Kanaton erupted violently, covering all but the steepest slopes of northern Kanaga Island with a thick blanket of andesitic 

tuff. During or soon after this eruption the'upper part of the cone 

collapsed, forming a caldera. The northern and western threequarters of the caldera rim subsequently disappeared beneath the sea, 

either by downfaulting or as a result of marine erosion. After a 

brief interval during which the blanket of andesitic tuff was partly 

eroded, the eruptions that produced the present cone of Kanaga 

Volcano began and continued-sporadically to the present time.

HISTORIC AND CURRENT VOLCANIC ACTIVITY

1 The earliest volcanic activity on Kanaga Island was described as 

"smoke" rising above Kanaga Volcano in 1790 and 1791 (Becker, 

1898). Information given by Grewingk (1850) suggests that the 

"smoke" was merely steam from hot springs around the base of the 

cone. Grewingk gives Luetke as a reference for the reported emission 

of smoke in 1827. The volcano was described as having been very 

active in 1904 (Jaggar, 1927). An eruption in May 1906 was reported 

in a letter to the writer from Simeon Oliver, teacher at Atka, who 

obtained his information from William Dirks of Atka. According to 

Dirks, who was trapping near Kanaga Volcano at the time, earthquakes occurred and lava poured' down the east and west sides of 

the cone to the sea. It is possible that these lava flows are the ones 

now .present on the southwest and northeast sides of the cone; however, 

only one of them reached the sea.

 The existence of fumaroles on Kanaga Island was reported at a 

very-early date. According to Grewingk (1850, p. 208) the occurrence 

of fumaroles and the collection of sulfur were reported by Tolstykh, 

the discoverer of the Andreanof group.. The sulfur was probably 

collected by natives for use as tinder (Collins, Clark, and Walker, 

1945, p. 26). ' 

,. Hot springs at the foot of the mountain, mentioned in many of the 

early records, were not found in 1946. At present, the most conspicuous fumaroles are those near the summit of the mountain on 

the southeast, south, and southwest-.sides. The most spectacular of 

them emits so much vapor that the resulting cloud- can be seen on a 

clear day from Adak. This fumarole was not visible in the fog pre-
80 INVESTIGATIONS OF ALASKAN VOLCANOES

vailing when we climbed Kanaga volcano, but several others, both 

within the crater and a short distance outside, were examined: 

Within the crater, which has a maximum depth of about 200 feet, 

several fumaroles were found in an area about 200 feet square, mostly 

floored with white or brightly colored mud, presumably opaline 

products of rock disintegration. Crusts of sulfur are common in 

fissures through which the hot gases ascend. The maximum tempert 

ature measured was 104°C, and a faint odor of hydrogen sulfide was 

detected. On the southeast side of the volcano, several fumaroles 

emitted odorless water vapor in small quantities. Some of these 

fumaroles are as much as 200 feet below the crater rim. The maximum temperature measured was 102°C. ;

FUTURE VOLCANIC ACTIVITY

On the basis of the historic and geologic records, Kanaga Volcanq 

must be regarded as an active volcano. Analogy of Kanaton caldera 

with calderas of other volcanic centers indicates that major eruptions 

such as the one that produced Kanaton caldera are not likely to occur 

in the near future, but minor infrequent eruptions of ash and coarser 

fragments, sometimes accompanied by the emission of lava, are to 

be expected. Nearby islands such as Adak and Tanaga may receive 

falls of ash during these eruptions, but, judging from the thicknesses 

of ash beds that were probably derived in part from similar eruptions, 

the ash deposited on the neighboring islands from any one eruption 

on Kanaga Island should not exceed a few inches in thickness.

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

some notes on the general geology: U. S. Geol. Survey 18th Ann. Rept., 

pt. 3, p. 16-17. ' j_

Coats, R. R.., 1952, Magmatic differentiation in Tertiary and Quaternary volcanic 

rocks from Adak and Kanaga Islands, Aleutian Islands, Alaska: Geol. Sod. 

America Bull., v. 63, p. 485-514.

 1956, Reconnaissance geology of some western Aleutian islands: U. S..

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

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

Their people and natural history: Smithsonian Insi, Washington, War (

Background Studies, no. 21. 

Gilbert, G. K., 1928, Studies Of Basin Range structure: U. S. Geol. Survey Prof..

Paper 153, p. 80-81. 

Grewingk, Cpnstantin,. 1850, Beitrag zur Kenntniss der orographlschen und

geognostischen Beschaffenheit der Nordwest Kuste Amerikas, mit den

anliegenden Inseln: Russ. K. min. Gesell. Verh., St. Petersburg, 1848-

1849, p. 208.

r T. \,» 1927. Volcano Letter? no. 116, Hawaiian Volcano Observatory.. *'

Honolulu, T. H.
INDEX

Page 

Alluvium.-.. ............................. 76,78

Andesite tuff............................ .. 73

Ash....................... ................ 74

Basalt, hydrothermally altered._______ 71-72 

Beaches.................._...._.......... 76

Caldera, formation of....._..........._ 77,78,79

Composition of rocks, alluvium.._ _____ 76 

cone of Kanaga Volcano _________ 73-74 

flows of Kanaga Volcano...______ 75 

flows of Mt. Kanaton.-__________ 71-72 

tuff.. . 73

Description of islandFaults-.- .__.. . . ._ 

Fumaroles__________________... 79,

Geologic history________________ 71

Geologic structure_______________ 76

Glaciation......_.......................... 78

Hornblende, green, in tuff ___________ 73

Hot springs...._______________ 79

Hot Springs Bay _______________ 75

Hydrothermal alteration of rocks. __ _ __ 71-72

Impact craters, recent. ____________ 74-75 Xenoliths, metamorphic

Page

Kanaga Volcano, age of cone ..--.-.. 78 

description.__________ __ 71 

history.___________________ 79-80 

lava flows......._.._... . 75.

Lava flows, Kanaga Volcano. __ - _____ 75. 

Mt. Kanaton................... .... 71-72,78.

Mt. Adagdak .............. .. 78.

Mt. Kanaton, destruction.________ 76-77,78

history._.. . - _........ 7Q.

lavaflows....... -.. 71,72,78.

Mt. Moflett ............. .. 75,

Mudflows______________ _... 74

Pumice. ...--.-------...---..-------._.. 73,74

Pyroclastic deposits..._______.___ 74-75,78.

Round Head. ____.__.____._._. 71 

Sulfur......_ . . -. - 79-80.

Terraces, marine._____------ .. 71

Tuff beds.... . .. . 71,73.

Volcanic activity, future. 80.

81

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