Geologic Reconnaissance 

of Semisopochnoi Island 

'Western Aleutian Islands 

-Alaska

i

By ROBERT R. COATS

..INVESTIGATIONS OF ALASKAN VOLCANOES 

GEOLOGICAL SURVEY BULLETIN 1028-O

> Prepared in cooperation with the Office, 

t Chief of Engineers, U. S. Army

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1959
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 $1.25 (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 the general direction of 

G. D. Robinson, were made during the years 1946-48. The results of the first year's field, laboratory, and library work were 

hastily assembled as two administrative reports, and most of 

these data have been revised for publication in Geological Survey Bulletin 1028. Part of the early work was published in 1950 

in Bulletin 974-B, "Volcanic Activity in the Aleutian Arc," and 

in 1951 in Bulletin 989-A, "Geology of Buldir Island, Aleutian 

Islands, Alaska," both by Robert R. Coats. During the years 

1949-54 additional fieldwork was carried out under the direction 

of H. A. Powers. Unpublished results of the early work and all 

of the later studies are being incorporated as parts of Bulletin 

1028.

The geological 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. From 1947 until 1955 the Departments of 

the Army, Navy, and Air Force joined to furnish financial and 

logistic assistance. The Geological Survey is indebted to the 

Office, Chief of Engineers, for its early recognition of the value 

of geologic studies in the Aleutian region, and to the several 

military departments for their support.

in

CONTENTS

* Page

Preface. ........._ ............_........ .._.................._........ ......................_. in

Abstract _ -................. ..................._........_. .............____ ..._ ._____..._.__.. 477

Introduction ......_..._........_ ...... ..._..._.. ..... ..............._._.._ ....._____.___. . 478

Geography -- ..... ..__-_.._-. ._-......._-. -. _. _ __.-._.._.__ ............. _.__ _._. 478

Location and size......... .............. ..............._.... .. .................................. 478

Accessibility .............-.... _ ._. ... ..._.-........ . ......_ ......... ....... 480

I Topography and drainage........_. ..._. .... .......... .. 480

Geology .---..._.. ...... .... ... ... .......__ ...... .. _........... .._... _-....... 481

Old volcano Pochnoi volcanics..... ........... ........ _........... . 481

Early subsidiary cones. .. ..... .. .................. ._. . 483

Late Pleistocene volcanoes........... .............. ... ......_......................... 486

Glaciation ...-...-._............ . ......... . . .................. ...................... . 488

Pumice eruption and the formation of the caldera.... ............... . 489

j Volume and mode of eruption........................................................... 490

Early postcaldera eruptions............... ....................._........_. .. . 494

Recent cones .. .. ......................... _......... ............... ................-. 495

Mount Cerberus ~ .................. 496

Lakeshore Cone .............-...... .............-............. ._.........-............... 498

Sugarloaf Peak ..._........................_........-....................................... 498

Unconsolidated deposits ....._....... ................. .............._... 500

* Alluvium, eluvium, and talus........... . ... 500

Sand dunes .... ....... ...... .... ....................... 500

Geologic history -i...__......_.___...._.._..___........_________ 501

Historic volcanic activity. ... . ... 502

Future volcanic activity.. . .. . ..... - 502

Petrography . .. . ........ ._........ .. ........... ....... ............. 503

Pochnoi volcanics .. ......... .... ..... ..................... 503

*i Early subsidiary cones................... _ ..... .. . ..... 504

Late Pleistocene volcanoes. ..__. _._. . . .. _. _ 504

4 Pumice eruption.... ... .. ...-.. . ...... . ......... 505

Recent cones ....... ......_ .... . ... ........... 505

Chemistry ................................................._......._.. . ...... 507

Differentiation at SemisopochnoL ............._. _...._.. .............. 507

Rarer constituents ........ ...... ...... ...._.......... ...... 512

3, Literature cited ..... ... . . -..... _ . . 516

Index ...........................................................-..........- .. .....-.. 519
VI CONTENTS

ILLUSTRATIONS

Page

PLATE 59. Geologic map and section of Semisopochnoi Island, Aleutian

Islands ........._.._. ....._ .-_ .-... .. . In pocket

60. A. Gentle southward-sloping remnant of the old Pochnoi

cone. B. Dissected cone of Ragged Top..._._..._. ...Facing 482 w

61. Aerial view of the southeast coast of Semisopochnoi

Island . ----.. - -.-.-. --- ----- Facing 483 V

62. A. View of flat-lying lava flows. B. Roche moutonnee on 

east side of valley between Anvil Peak and Perret 

Ridge .---......... - .--. . ...Facing 488

63. A. Gully near Threequarter Cone. B. Dacitic pumice..Facing 489

64. Aerial view of the southwest coast of Semisopochnoi

Island -....- .-.. __. Facing 492

65. View looking southward from the west side of Ragged

Top _ .._. .._ . .....Facing 493

66. Panorama of Mount Cerberus from the north and northeast ~ _._ . Facing 496

67. A. Lava flow from Mount Cerberus. B. Lava toes. Facing 497

68. A. Agglutinate. B. View on south side of Sugarloaf

Peak . .._.......... _ ......... _.._.Facing 498

FIGURE 73. Index map of central Aleutian Islands, showing position of

Semisopochnoi Island .... ___ . ..... . ..._ .__ ... . 479

74. Variation diagram for volcanic rocks from Semisopochnoi

Island . ...... __ . ...._ _ ..._..... 508

75. Relation of magnesia (MgO), iron (as FeO), and alkalies 

(Na20 + K«0) in volcanic rocks from Semisopochnoi 

Island . . . 510

76. Relations of al, alk and c 4- fm in volcanic rocks from

Semisopochnoi Island, plotted in 2 sections of the Niggli ^ 

tetrahedron that include the alk-al edge... - 513

TABLES

TABLE 1. Major and minor constituents of 15 volcanic rocks from 

Semisopochnoi Island, with norms and Niggli numbersPage

514
INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGIC RECONNAISSANCE OF SEMISOPOCHNOI 

ISLAND, WESTERN ALEUTIAN ISLANDS, ALASKA

By ROBERT R. COATS

ABSTRACT

Semisopochnoi Island, the largest of the young volcanic islands of the 

western Aleutians, is the easternmost of the Rat Island group, and is near 

the southern extremity of the great Aleutian arc. The island consists entirely 

of volcanic rocks, and sediments derived from them, of probable Pleistocene 

and Recent age. It is a large shield-shaped basaltic volcano, with parasitic 

cones of andesite and basalt. The higher, central part, which once supported 

extensive glaciers, subsequently collapsed to form an elliptical caldera almost 

5 miles in greatest width. The collapse was attended and, at least in part, 

caused by the extrusion in the form of glowing clouds of large volumes of 

pale-gray dacitic pumice and ash. In most of the present valleys there are 

thick deposits of this pumice and ash, but in the parts of the island that were 

sheltered from this flood of hot, gas-charged pumic and ash there are only 

thin layers. After the collapse, eruptions of basalt and basaltic pyroclastic 

material took place in the caldera. Recent composite cones of basaltic composition, 2 large and 1 small, have erupted intermittently and have built up 

domes of lava, the largest of which is nearly 3,000 feet high. The lava flows 

emitted from these cones have nearly filled the caldera floor, and cover a 

considerable area in the southeastern quarter of the island.

The rocks are generally poor in phenocrysts; the principal minerals are 

intermediate to calcic plagioclase, olivine, hypersthene, clinopyroxene, usually 

augite in the phenocrysts and augite or pigeonite in the groundmass, magnetite 

or ilmenite, apatite, quartz, and orthoclase. Hornblende is present only in the 

dacitic tuff erupted at the time of the formation of the caldera. The analyses 

indicate that the alkali-lime index is about 59, hence the province is calcalkaline, thus differing perceptibly in this respect from other Aleutian volcanoes 

from which extensive rock series have been analyzed. The relations of the 

oxides make it evident that the rocks of Semisopochnoi are somewhat enriched 

in iron in the intermediate composition range, thus departing slightly in this 

regard from the normal basalt-andesite-dacite sequence. The content of a few 

of the minor constituents in the rocks of Semisopochnoi differs significantly 

from that of the rocks of other western Aleutian islands.

477
478

Semisopochnoi Island, the largest of the young volcanic islands 

of the western Aleutians, is the easternmost of the Rat Island 

group, and is about 40 statute miles north-northwest of Constantine Harbor on the east end of Amchitka Island. Its proximity to the harbor and to Amchitka Air Force Base, which were 

being used in 1947, made the island easy to reach, and the volcanic activity that had recently occurred on it invited early 

study.

No geologic work seems to have been done on Semisopochnoi 

Island prior to the study made by me. My field assistant, Will F. 

Thompson, Jr., and I were landed on a small beach, just west of 

the south point of the island, on August 22, 1947, and re-embarked from the same beach on September 11. Much additional 

information about areas not reached during our study has been 

furnished by Howard A. Powers, who was stationed on board 

the USCGS ship Pioneer for one week in May 1949, and had 

several opportunities to land on the northeast, north, and west 

sides of the island.

The work would not have been possible without the friendly 

assistance received from Maj. John Davis, CAC, port commander, 

Adak, from John Mack, master of the FS 244, on which we were 

transported from Adak to Semisopochnoi, and from Dr. H. D. 

Gray, of the Fish and Wildlife Service ship Brown Bear, which 

took us from Semisopochnoi to Amchitka. The cooperation of Lt. 

Col. R. E. Ware, Corps of Engineers, post engineer, Adak, and of 

Commdr. J. M. Oseth, commander Naval Operating Base, Adak, 

is also gratefully acknowledged. We are also indebted to Commdr. 

B. Moore, Jr., commander Naval Air Facility, Adak, for arranging for aerial reconnaissance of the island, and to Capt. H. L. 

Underwood, Signal Corps, for assistance in obtaining radiocommunication facilities.

GEOGRAPHY

LOCATION AND SIZE

Semisopochnoi Island (fig. 73) lies near the southernmost point 

of the Aleutian arc of active volcanoes, between long 179° 28' 

and 179° 45' E., and between lat 52°02' and 51°53' N. It is 

12.4 miles long by 10.6 miles wide and, according to a planimetric measurement by Harold Drewes, has an area of 78.3 square 

miles.
177° 178° 179° 180° 

52°

1

RAT

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Kiska \( /

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1

1

j ________ "__

B E

1 S L

pvSegula 1

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Amchitka 1^

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AND

Semisopochnoi 1 O

X

A/^\ Constantine Harbor

^,CJ>East Cape

 l^~-^~^

1C O

E A

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^

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177"

178° 179° 180° 

10 0 10 20 30 40 50 Statute 

FIGURE 73. Index map of the central Aleutian Islands, Alaska, showing position o
480 INVESTIGATIONS OF ALASKAN VOLCANOES

ACCESSIBILITY

Most of the island is bordered by high rocky cliffs and many 

offshore reefs. Two broad sandy beaches on the southeast and 

two on the southwest sides of the island offer easy landing 

places when the surf is not too high. The island has a number of 

narrower, steeper, and more bouldery beaches, and even these 

may be used under favorable circumstances. Several trappers' 

cabins and the remains of a U.S. Army weather station offer 

shelter of a kind.

TOPOGRAPHY AND DRAINAGE

The topography of Semisopochnoi Island (pi. 59) has a greater 

degree of symmetry and simplicity than that of most Aleutian 

Islands of its size. The outer slopes consist of radially disposed, 

maturely dissected ridges and intervening valleys that are partly 

steep walled. Some of the valley floors are flattened by alluviation, some are covered with exceedingly rough-textured flows of 

block lava, and some are partly filled with a youthfully dissected 

mass of soft volcanic material. The radial ridges (pi. 60A) 

terminate inland in a steep scarp that is 200 to 1,000 feet high 

and that faces a partly obliterated elliptical central depression. 

This depression is about 3.8 by 4.7 miles in diameter and its long 

axis trends northeastward. Such central depressions, more or 

less circular in form, are common in volcanic regions; the example on Semisopochnoi will hereafter be referred to as the 

caldera, since it conforms closely to the definition of the term 

"caldera" as given by Williams (1941, p. 242). It is one of those 

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

the diameters of which are many times greater than those of 

the included vent or vents." Above the general surface of the 

peripheral ridges and valleys rise several composite volcanic 

cones in various stages of dissection.

The highest of these cones is Anvil Peak, which attains an altitude of 3,867 feet. Sugarloaf Peak, a conspicuous slightly dissected cone, 2,808 feet high, and the parasitic cone on its 

south flank form a well-known landmark near the southeast 

coast. Farther to the north, a steep and deeply dissected cone, 

Ragged Top, is also a conspicuous topographic feature. The central caldera has been partly filled, and its walls partly concealed, 

by the products of later eruptions, which have resulted in the 

erection of several volcanic cones, the largest of which is Mount 

Cerberus. As Mount Cerberus is southwest of the center of the
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 481

caldera, the lowest point of the depression lies close to the frowning cliffs on the north side. This part of the depression is occupied by a lake, Fenner Lake, about 2 miles long- and somewhat 

less than half a mile wide at the widest. The outlet of the lake, 

which is at an altitude of 250 feet, is a substantial stream of 

clear water, that flows southward between alluvial banks for 

about li/£ miles, and then enters a small pond. This pond, about 

300 yards long, has been formed by the damming of the stream 

behind one of the latest lava flows from Mount Cerberus. The 

stream, confined between the flow and the northeast wall of the 

valley, plunges in several small falls down to the alluvial floor of 

the valley near the snout of the lava flow and then meanders 

toward the sea, which it enters about 2 miles east of the summit 

of Sugarloaf.

Fenner Creek, the outlet stream just described, is the largest 

stream on the island and the most constant in its flow, being 

stabilized not only by Fenner Lake, but by several large springs 

that feed its upper part. Its total flow, at a point just above the 

mouth, was estimated in August 1947 to be 250 cubic feet per 

second. Other streams, which occupy alluviated valleys near sea 

level, carry substantial amounts of water at relatively steady 

rates of flow, but the streams that cross highly permeable lava 

fluctuate widely, and often run dry except when supplied by rain 

or melting snow.

GEOLOGY

OLD VOLCANO POCHNOI VOLCANICS

The oldest rocks of Semisopochnoi Island are a sequence of 

tuff-breccia, lava, and agglomerate, and a minor amount of sandstone, herein called the Pochnoi volcanics, because the best exposures appear to be west and north of Pochnoi Point. These 

rocks were apparently erupted by a volcano, which had approximately the same dimensions as the present island, whose center 

of eruption lay somewhere within the area of the present central 

depression.

In deduced form the old volcano corresponds fairly well with 

that of typical Hawaiian shield volcanoes. This resemblance, 

however, is chiefly in outward form and in the low dip of the 

materials that make up the structure, rather than in the type of 

materials, which on Semisopochnoi seem to be largely basaltic 

pyroclastic material and minor andesitic flows, rather than the
482 INVESTIGATIONS OF ALASKAN VOLCANOES

basaltic flows typical of Hawaii. The gentle dips of the pyroclastic rocks of the Pochnoi volcanics can perhaps be attributed to 

submarine accumulation. Alternatively, and perhaps more accurately, the fundamental volcanic structure of the island might 

be referred to as a submarine tuff cone, the word "tuff" being 

used in its most general sense.

The rocks of this unit are now chiefly exposed in the northeast, south, and southwest parts of the island, outside of the 

central lowland; elsewhere they are covered by the products of 

later volcanic eruptions. Many places mapped as Pochnoi volcanics are covered by a relatively thin, uneven layer of volcanic 

ash; this ash is either the light-gray dacitic ash of the calderaforming eruption or the darker colored basaltic ash erupted from 

the Recent volcanoes, and has not been separately mapped. In 

the walls of the caldera in the northwest and northeast sectors, 

the Pochnoi volcanics extend to heights of 1,700 feet.

The oldest material recognized is a poorly sorted tuff-breccia 

that dips gently seaward. The fragments are mostly angular, 

and the largest are several feet in diameter. The fragments 

identified range in composition from olivine basalt through olivine-hypersthene basalt to hypersthene basalt. Yellow and 

brown stains, due to the development of limonitic and palagonitic 

alteration products, are common.

At Pochnoi Point a widespread flow of olivine basalt is prominently exposed. Along the north coast of Pochnoi Point, about 

11/2 miles south of Northeast Point, the lowest rock exposed in 

the sea cliff is basaltic lava. Its top is an eroded plane surface, 

possibly a wave-cut bench, on which is horizontally bedded lenticular indurated sandstone. Further west, in the eastward facing 

cliff between 2,000 and 1,000 yards south of Northeast Point, 

the sandstone is covered by relatively unconsolidated scoria agglomerate that underlies the surface for some distance inland. 

A line of springs marks the contact of the agglomerate on the 

indurated sandstone in the sea cliff. The sandstone is composed 

chiefly of black basaltic and yellow palagonitized glassy fragments, but includes some short threads of palagonite that are 

believed to represent lava fountain debris. The sand is medium 

olive brown (5Y 4/4) l and ranges in grain size from mediumsized sand to coarse granules. According to Powers (oral communication, 1949) these rocks were apparently deposited in

1 Symbols in parentheses after color names refer to Munsell system. Comparison has been 

made with the Rock-Color Chart (National Research Council, 1948).
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 60

A. GENTLE SOUTHWARD-SLOPING REMNANT OF THE OLD POCHNOI CONE 

Exposed rock is principally tuff-breccia.

B. DISSECTED CONE OF RAGGED TOP

View is from a point south of Anvil Peak. Fenner Lake (center) is retained on the left by caldera wall, and 

on the right by lava flows and alluvial fans.
GEOLOGICAL SURVEY 

AERIAL VIEW OF THE SOUTHEAST COAST OF SEMISOP

Shows Sugarloaf Peak (center) with its double parasitic cone (foreground) that was the source of one of th

structional surface on its seaward face. Photograph by 
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 483

partly sheltered embayments on the margin of the island during 

the time of eruption of the Pochnoi volcanics and during a stand 

of the sea higher than the present one.

In the sector of the old Pochnoi cone about a mile southwest 

of Mount Cerberus (pi. 60A), the predominant types of lava 

are platy to vesicular partly glassy augite-hypersthene andesite 

that is from medium light gray (N 6) to medium dark gray 

(N 4). Similar platy lava flows crop out between Sugarloaf Peak 

and the flood of younger lava from Cerberus that reaches the 

sea west of Sugarloaf. The headland about 2 miles south of 

Tuman Head seems to have been an isolated center of eruption, 

somewhat younger than the main cone, judging from the steep 

outward dips of the flows and pyroclastic rocks that rest upon the 

gently dipping older Pochnoi volcanics.

The rocks referred to in this unit may include unrecognized 

parasitic eruptive centers on the flank of the major volcano, 

similar to those separately described under the next two 

headings.

EARLY SUBSIDIARY CONES

The older subsidiary cones that once diversified the slopes of 

Pochnoi Volcano have been deeply dissected, and are now mere 

ruins of their former selves some of them very picturesque 

ruins. The most spectacular of these old cones is Ragged Top 

(pi. 60Z?), whose altitude, 2,965 feet, is probably not far from the 

maximum original height. As shown in plate 61, in which 

Ragged Top is the peak on the right-hand side, parts of what 

seem to be the original constructional surface of the cone are 

preserved. Such topographic features have been called planezes 

by Cotton (1944, p. 365), following de Martonne. The exposed 

core of the volcanic plug forms what appears in plates 605 and 

61 to be the highest peak of the mountain.

What may be the oldest of the observed flows from Ragged 

Top is a flow 20 feet thick of medium-gray olivine basalt that 

forms the point just east of the outlet of Fenner Creek. The 

lower surface of the flow is slightly slaggy and scoriaceous, and 

the tuflfaceous agglomerate beneath has been reddened for a 

thickness of as much as 8 feet; this agglomerate, of which about 

80 feet is exposed, is apparently conformable with the flow. About 

90 percent of the total volume consists of fragments larger than 

one-fourth inch, and 50 percent of the fragments range in greatest dimensions from 1*4 inches to 6 feet. Most of the fragments
484 INVESTIGATIONS OF ALASKAN VOLCANOES

are red to black lapilli of scoriaceous basalt, with subordinate 

subrounded fragments of accessory compact basalt. A foot-long 

fragment of a bipolar bomb was seen.

Another flow from Ragged Top, exposed in the caldera wall on 

the shoulder overlooking the central lowland, is a medium-gray 

(N 5) porphyritic augite-hypersthene andesite with a streaky 

texture.

To the north of the main peak, a number of bombs of varying 

composition and texture have been brought down to the floor of 

a cirque by disintegration of the weak pyroclastic rocks in which 

they were embedded. Some of the bombs and compact breadcrusted reheated blocks are as much as 3 feet in diameter. The 

bombs have a scoriaceous and irregular exterior and a compact 

interior, locally with cores of gabbroic texture. Nearly all these 

bombs are largely or partly glassy augite-hypersthene andesite 

and olivine-hypersthene-augite andesite. Some of the bombs and 

blocks are bleached and solfatarized, but it does not seem necessary to infer that they were erupted after their alteration, as the 

material has been reworked by glaciers since its eruption.

Another of the older cones, at an altitude of 1,405 feet, lies 

southwest of Fenner Creek valley and north of the summit of 

Sugarloaf Peak. The material that crystallized in the throat of 

the old volcano now stands in steep cliffs, which face the outlet 

valley and the snout of one of the latest flows from Mount Cerberus. This is a light-gray (N 7) fine-grained phanerocrystalline 

olivine microgabbro. Slight deuteric alteration has produced 

some albite and nontronite, which line cavities and replace bytownite and olivine respectively. A finer grained phase of the 

rock in the neck is a medium-gray (N 5) compact porphyritic 

olivine basalt, which perhaps represents a later diking of the 

coarser gabbro described above.

The material that makes up the main structure of this cone 

north of the summit of Sugarloaf is partly tuffaceous agglomerate, that contains bombs and accessory fragments, mainly of 

black vesicular basalt, as much as 10 inches in diameter. These 

pyroclastic rocks dip outward from the neck at angles that locally exceed 30°. Elsewhere the pyroclastic material may be 

considered a brecciaceous tuff that contains a few angular compact to scoriaceous basaltic fragments as much as 8 inches in 

diameter in a matrix of sand and silt grade. The beds of pyroclastic rock are as much as 20 feet thick. Lava flows make up only a 

small part of the cone; typical of these is a flow 4 feet thick of 

medium-brownish-gray (5R 5/1) aphanitic olivine basalt that
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 485

contains a few small phenocrysts of plagioclase and many flattened vesicles.

Tuman Head seems to be made up of flows and pyroclastic material erupted from two distinct centers not far apart; the older 

center is nearer the coast. Each of the cones is about nine-tenths 

pyroclastic material that contains many bombs which generally 

are 1 or 2 inches, rarely as much as several feet in diameter; the 

larger ones are generally broken. Interbedded with the pyroclastic rocks are a few stringy flows; most of these are 10 to 20 feet 

thick, but the lowest one exposed is about 30 feet thick. The 

flows generally consist of compact medium-gray to medium-lightgray (N 5 to N 6) seriate olivine basalt and olivine-hypersthene 

basalt; some contain numerous phenocrysts of plagioclase, and 

less numerous ones of olivine and augite. Another center of 

eruption, active somewhat later than the one that liberated 

most of the rocks of the northern part of Tuman Head, is represented by a peak 1,180 feet high about 1.6 miles east of South 

Tuman Point. The nature of the eruptions and the types of 

rocks appear to be similar to those of the older cone.

A peak 1,389 feet high about 3 miles east-southeast of South 

Tuman Point, is a remnant of another center, the crater of which 

has been completely destroyed by erosion.

What seems to be the remnants of two separate basaltic cones, 

rather similar in eruptive products to those of Tuman Point, 

form the 1,412-foot summit south of North Head, and the 1,193- 

foot summit three-fourths of a mile west of Northeast Point. 

Subsequent erosion has enlarged the constructional valley between the two cones and the valley is now partly filled with later 

rocks.

Ferret Ridge, the rugged comb ridge that forms the high 

mountain wall northeast of Fenner Lake, is probably, judging 

from the dips of its flows, the remnant of a steep cone that once 

surmounted the main shield volcano which forms the basic structure of the island. What is interpreted as a minor parasitic 

cone was apparently formed at about the same time as the cone 

of Perret Ridge over a vent lying just north of the west end of 

Fenner Lake; it is now exposed as a massive buttress in the 

steeply cliffed walls of the caldera. The only sample examined 

in thin section is a medium-gray vesicular olivine basalt that 

contains scarce hypersthene. The pyroxene of the groundmass is 

pigeonitic.
486 INVESTIGATIONS OF ALASKAN VOLCANOES

The only evidence of the age of the Pochnoi volcanics, which 

are unf ossilif erous, and the material of the associated subsidiary 

cones is the relation of these rocks to glacial erosion (see p. 488). 

Inasmuch as these rocks have been glacially eroded, and the volcanic structures have been dissected by cirques and U-shaped 

valleys, it is inferred that neither series can be younger than the 

latest glaciation, and the possibility of late Tertiary age cannot 

be excluded.

LATE PLEISTOCENE VOLCANOES

Three cones classed as late Pleistocene are distinguished from 

the earlier ones by their being less eroded. There is evidence that 

the period of formation of some of them overlapped that of the 

caldera. Two of the cones, altitude 914 and 1,705 feet, are cinder 

cones, or scoria rings, of basaltic material, and lie on a line that 

trends southeastward from the northern point of Tuman Head 

to the westernmost cone of Mount Cerberus. The third cone is 

Anvil Peak, 3,867 feet high, the highest point on the island; there 

is a parasitic cone, 3,230 feet high, southwest of the main summit.

Anvil Peak, partly because of its greater height and partly 

because of the smaller permeability of its rocks, has been eroded 

more deeply than the two smaller cones. Its more northerly and 

higher summit is bordered on all sides by wide, steep-floored 

barrancos in which the snow lies late in the year, and the 

effects of nivation and frostwork are evident. The highest point 

of the mountain seems to be a part of the old rim of Anvil Peak 

crater, somewhat diminished in height by erosion. North of 

this point is a nearly flat surface (pi. 62A) that is roughly 

circular in outline and about 1,200 feet in diameter. This surface 

is underlain by six flat-lying lava flows that represent the filling 

of the old crater of Anvil Peak; each flow is about 20 feet thick, 

and they are exposed in the cliff that bounds this high plateau 

on all sides but one.

The flows rest upon yellow and red palagonitic tuffs that are 

traversed by many irregular dikes. Overlying these thick lightcolored flows in the old crater is a flow 4 feet thick of highly 

vesticular, almost pumiceous, olivine basalt. The lighter color of 

the less vesicular flows beneath is apparently due to incipient 

fumarolic action on exposed joint faces, as these rocks also are 

olivine basalt, with relatively few large vesicles and conspicuous crystals of bytownite, augite, and olivine. The surface of the 

flat area is covered with loose material, much of which obviously 

is pyroclastic, including black basaltic lapilli and a few cored
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND

bombs. The core of one of the bombs apparently was originally 

sandstone, and is now converted to an aggregate of tridymite 

and pale-brown glass that is cut by veinlets of black glass. In 

thin section this glass appears darker brown and is seen to contain small well-formed prisms of pyroxene that range from augite 

to pigeonite. The glass also encloses a few corroded relicts of 

quartz. The rind of the bomb is glassy basalt and has phenocrysts 

of bytownite. Such accidental specimens of the underlying basement are seldom brought up by the Aleutian volcanoes.

A small tuff ring about 21/2 miles east of South Tuman Point 

is 914 feet high and appears to be late Pleistocene. This structure has an external diameter of more than four-tenths of a mile, 

and the crater floor is about half that width. The crater rim is 

notably asymmetrical; its lowest point is only about 90 feet 

above the bottom of the crater, and its highest point is 384 feet 

above the bottom.

The mound of scoria, 1,705 feet high, about l1/^ miles northwest of the nearest cone of Mount Cerberus is also strongly 

asymmetrical and is called Threequarter Cone. One of its latest 

eruptive phases was the extrusion of a small lava flow that barely 

overtopped the low northwestern side of the crater. The almost 

wholly pyroclastic material erupted from this center ranges in 

structure from scoriaceous to compact. It consists of dark-gray 

or black to dusky-red glassy to cryptocrystalline augite andesite, 

containing sporadic hypersthene and olivine.

The asymmetry of both these cones probably was caused by 

strong winds blowing constantly from a single direction during 

a single brief eruptive episode.

Determining the relation of these late Pleistocene cones to the 

formation of the caldera is somewhat difficult. Both Anvil Peak 

and Threequarter Cone are transected by the walls of the caldera, whose collapse was apparently due to the extravasation of 

a great volume of the dacitic pumice described on page 489; yet the 

material of Threequarter Cone definitely, and that of Anvil Peak 

possibly, overlies this pumice. In Threequarter Cone, the relations are so well exposed on the wall of the caldera that there 

can be no doubt of the relative age of the two formations. The 

material from Anvil Peak, on the other hand, that was seen to 

overlie the dacitic pumice may not have arrived at that position 

directly by eruption; a great mass of it may have slyd down the 

slope of the volcano, which is considerably dissected. It is also 

possible, as will be shown later, that the caldera was enlarged 

by subsidence after it was first formed, and that the walls now

5105ZZ O -60 -3
488 INVESTIGATIONS OF ALASKAN VOLCANOES

enclose a greater area than they did when the dacite pumice was 

being erupted, or when the caldera first collapsed, or at the time 

of the last eruptions of the late Pleistocene.

GLACIATION

Evidence for the glaciation of Semisopochnoi Island is rather 

scanty. Aside from physiographic evidence, such as cirque basins 

and U-shaped valleys, the exposure of glacially shaped and striated bedrock found on the rim of the caldera (pi. Q2B) is the 

most substantial bit of evidence. No moraines or glacial tills were 

found in the course of the mapping, but any that may have 

existed must have been largely concealed beneath later volcanic 

and solifluctional deposits.

Characteristic cirquelike valley heads may be found on several of the higher peaks, particularly on Ragged Top (2,965 feet) 

and Ferret Ridge (2,916 feet), though even on these the form of 

the cirques is relatively immature, lacking the flat or retrograde 

floor, and with headwall slopes of at most 60°. A few steepwalled, flat-floored valleys, remarkably cirquelike in form, have 

been found at low altitudes on Tuman Head, where the valley 

floors have altitudes of 300 to 500 feet above sea level, and on the 

headland just south of Tuman Head. Similar-shaped consequent 

valleys have been formed by stream erosion in the Hawaiian 

Islands (Hinds, 1925); possibly some of the low-level valleys on 

Semisopochnoi are similar in origin to the Hawaiian valleys. 

Similar valley forms, at similar altitudes, also have been formed, 

however, in northward-facing valley heads on Kiska Island; 

here the details of the glacial modeling are still visible, because 

there was no later volcanic activity in that part of the island.

Yet the much higher summit of Anvil Peak (3,867 feet) 

shows the effects of nivation rather than cirque erosion. The 

valley profiles differ but little from the average profile of the 

surface of a young volcanic cone. The lower slopes on the west 

side are modified by festoons of arcuate ridges; these ridges have 

not been examined in the field, but they closely resemble the 

solifluctional terraces common in periglacial areas.

This contrast between the degree of glaciation on the lower 

peaks and that on the highest peak of the island suggests strongly 

that Anvil Peak erupted periodically during the period of glaciation, and thus maintained, through continual rejuvenation by 

fresh accretions of volcanic material, a youthful volcanic profile.
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 62

A. VIEW OF FLAT-LYING LAVA FLOWS 

Flows are left after erosion of the crater they once filled on the north flank of Anvil Peak.

&&t%mm& .?- '&&%

B. ROCHE MOUTONNEE ON EAST SIDE OF VALLEY BETWEEN ANVIL PEAK AND FERRET RIDGE 

View is a few hundred feet north of the caldera wall. Stoss (left) side faces toward the caldera. The trend

of the striations is N. 20° E.
GEOLOGICAL SURVEY 

A. GULLY NEAR THREEQUARTER CONE

Cully is about 2 miles south of Thteequarter Cone. Shows 150 feet of dacitic pumice overlain by 

2 feet of basaltic scoriaceous ash. In background, flow erupted from Mount Cerberus rests on the 

even surface of the ash.
489

PUMICE ERUPTION AND THE FORMATION OF THE CALDERA

The most cataclysmic events in the history of Semisopochnoi 

Island were the eruption of tremendous volumes of light-gray 

rather fine-grained dacitic pumice, followed by the subsidence of 

the great central caldera. The dacitic pumice is well exposed in 

a gully about 2 miles south of Threequarter Cone. It is overlain 

by 2 feet of basaltic cinders, including scoriae as much as 10 

inches in diameter, which in turn is irregularly capped by several 

flows from Mount Cerberus. The total thickness of the dacitic 

pumice here exposed is 150 feet, but the base is hidden. The 

pumice seems (pi. 63A) almost structureless and uniform from 

top to lowest exposed part, but certain minor differences can be 

found on examination. It is slightly indurated, having about 

the same resistence to a pick as a stiif till. The largest fragments of pumice seen here are about 6 inches in diameter and 

contain irregular cavities; they are commonest in the highest 

part of the pumice bed, which also contains some pumice fragments with elongate streaks and strands of dark-gray material 

(pi. 635). There are also some smaller fragments of dark 

obsidian, which grade, by increase in amount of elongate vesicles, 

into a light-gray pumice with silky texture. The uppermost 6 feet 

of the deposit includes many accidental fragments, as much as 

3 feet wide, of medium-gray phenocryst-rich basaltic andesite, 

typical of the material making up the old Pochnoi cone.

Some of the pale-gray (N 7) pumice is inflated to the point 

where some fragments will float on water at least 16 hours. Undoubtedly more would do so if the vesicles were isolated. Most 

of the material is sand to silt in size, and some will remain in 

suspension in water for at least 16 hours. As shown by the 

transition in character, the pumice and the obsidian are very 

similar; both consist of glass, are colorless in thin section, 

range in refractive index from 1.512 to 1.517, and contain sparse 

phenocrysts of labradorite (An53 ), augite, and brownish-green 

hornblende wholly free from any resorption border. A little 

magnetite and some clear needles of apatite are present in specimens from some localities. The index of refraction suggests 

(George, 1924) a silica percentage of about 65 and a probable 

dacitic composition. The dark streaks are largely composed of 

brown glass with an index of 1.524, suggesting a silica percentage of about 62. The darker streaks are rich also in augite and 

plagioclase, and enclose nodules of andesite. Other inclusions 

contain olivine, as well as augite arid plagioclase.
490 INVESTIGATIONS OF ALASKAN VOLCANOES

It seems that a part of the superstructure of the volcano has 

been stoped into the magma chamber underneath, and that some 

of this more mafic rock of the old cone has been wholly or partly 

remelted and in part incorporated into the eruptible magma. 

Evidences of contamination are found in only a few of the fragments of pumice. This fact suggests that the erupted material 

contained a much smaller proportion of material assimilated 

from the walls of the magma chamber than Fenner (1920, p. 

593-596) thought to be the case at Katmai. Apparently the tearing away and erosion of the walls of the conduit by the gas 

stream and the ejection of fragments of the old rock were important only toward the close of the cataclysm. The denser 

fragments of dacitic glass suggest that a part of the magma was 

chilled by contact with the atmosphere or with cold rock, an 

event that must have preceded the catastrophic emptying-out of 

the magma chamber. Perhaps relatively stiff, viscous dacitic 

glass was extruded as a tholoid, or dome, into the crater of the 

volcano, just before the eruption. The elongate tubular shape of 

the vesicles in the denser glass contrasts strongly with the more 

nearly spherical, irregular vesicles of the larger pumice fragments, and suggests viscous flow rather than the turbulent expansion of a hot, volatile-rich glass from which the pressure has 

been suddenly released. The presence of the green hornblende 

completely free from any sign of oxidation or resorption indicates that the glass could not have been exposed, for any appreciable period, to conditions of high temperature and low pressure, which have been shown by Barnes (1930) to result in 

oxidation of the hornblende. Kozu and his colleagues (1927) 

found that the oxidation of green hornblende can be completed 

over a narrow temperature range near 750° C. That the temperature of the eruption must have been relatively low is shown 

by the lack of welding of the tuff.

VOLUME AND MODE OF ERUPTIOX

The total volume of material erupted will probably never be 

known with any degree of accuracy, as great thicknesses are 

now exposed by the sea cliff in some valleys, indicating that much 

material was carried beyond the shoreline. On land, the greatest 

observed thickness of the dacitic pumice is 160 feet, where it is 

overlain by the scoriae of Threequarter Cone. It is 140 feet 

thick in the wall of the caldera just south of Anvil Peak. The 

thickness ranges widely, however, and in many places is only 1 or 

0 feet. In other places the pumice either was not deposited or
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND

has been removed by erosion. It is noticeable that the thinner 

deposits are in places that were topographically sheltered from 

the central crater of the mountain.

On the northwest shore, 3% miles east-northeast of Tuman 

Point, Powers found only 2 feet of siliceous pumice, underlying 

25 feet of loose basaltic ash and fine loesslike ash. About 1% 

miles northwest of South Point and one-fourth mile from the 

sea, along the west wall of a small gully, 2% feet of coarse 

yellowish-gray (57 8/1) pumice, in fragments as much as 3 

inches in size, directly overlies a 1-inch layer of pale-gray ash of 

medium-grained sand size, which in turn rests on the disintegrated andesitic bedrock. Along the sector of the old Pochnoi 

rocks to the west of this station, stripped surfaces show no recognizable remnants of the dacitic ash, until the high cliff 2% miles 

northwest of South Point is reached. Exposures in the face of 

this cliff show about 80 feet of light-colored material, presumably 

the dacitic ash, near the top of the cliff. Oblique aerial photographs (see pi. 64) show about 80 feet of light-colored ash near 

the east flank of Southwest Knob. What the depth is near the 

axis of the valley cannot be determined, but it may well be somewhat greater. The thickness in the valley to the west of Southwest Knob is considerably less, judging from aerial photographs, 

perhaps because the ash on this side of the knob had traveled 

farther from the crater. The apparent thickness at a point on 

the sea cliff just south of the 1,437-foot peak is about 15 feet.

The dacitic tuff obviously was not deposited by an ash shower 

that-fell vertically from the sky, instead it flowed down the valleys like a heavy fluid; it was, in short, a glowing-cloud deposit, 

though one of relatively low temperature. It is possible that some 

of the pumice, particularly the coarser fragments, may have 

been ejected at a relatively high angle from the crater, and that 

this more nearly vulcanian mode of projection may account for 

the coarser average size of the fragments in the places where the 

deposit is thin. Whether the coarser fragments represent any 

particular phase of the eruption it is now difficult to say, but in 

the thick exposure of ash in the valley northeast of Southwest 

Knob the coarse fragments of pumice are commoner near the

An estimate of the volume of dacitic ash, between the caldera 

walls and the present shoreline of the island, made by counting 

1,000-yard squares and assuming that the thickness diminishes 

about 20 feet in each thousand yards, indicates that the total
492 INVESTIGATIONS OF ALASKAN VOLCANOES

volume has an order of magnitude of 1.5 X 10 6 cubic yards. As 

the present area of the caldera is about 51.5 X 10 6 square 

yards, the ash, not allowing for the considerable degree of porosity,2 would fill the caldera to a height of only 26 yards, or about 

75 feet. Even the most generous estimates of material dispersed 

beyond the present shores of the island will not account for 

enough material to come near filling the great caldera in its 

primal state, for it occupies the heart of a mountain that once 

must have risen to a height of 5,000 feet or more.

This problem is the same as that faced by Williams (1941) at 

Crater Lake, by Byers and others (1947, p. 36) at Umnak Island, 

by Simons and Mathewson (1955, p. 36) at Great Sitkin Island, 

and by Coats (1956, p. 77) at Kanaga Island. In many parts of 

the world then, the volumes of material extravasated by calderaforming eruptions are inadequate to account for the void into 

which the superstructure of the volcano has collapsed, and this 

fact must be something more than coincidental. Williams (1942, 

p. 105) has suggested that this material may be disposed of by 

intrusion into radial fractures opening at the time of maximum 

magmatic pressure, which as he has pointed out, is precisely the 

moment preceding the first explosion. It seems to me, however, 

that there are two objections to this hypothesis. There is insufficient surface extrusion dating from the time of caldera formation to account for any sizeable fraction of the surplus hot, 

gas-charged silicic magma that must have been in the magma 

chamber before the formation of the caldera. It is scarcely conceivable that doming and stretching of the rock of the magma 

chamber adequate to produce fissures with so large a total volume (10.5 cubic miles, in the case of Crater Lake) should have 

produced only blind fissures, none of which reached the surface. 

This objection becomes especially telling when the mechanics of 

the doming and stretching are considered; for the steeply dipping 

radial cracks formed by stretching the solid cover of the magmatic blister would tend to be widest at the surface and grow 

narrower downwards surely an unfavorable circumstance for 

the production of "headed dikes" that terminate without reaching the surface.

Two clues to a much more feasible method of disposing of the 

excess magma may be found. One of these is aiforded by studies 

of ground tilt at Kilauea, which have shown that the surface

2 The bulk specific gravity as measured on an 1,128-gram sample of the ash, including accidental fragments, and after excavation and transportation, was 1.3. The probable specific gravity 

before excavation is 1.5. I am indebted to W. G. Schlecht for assistance in making this measurement.
510522 O-60 (Face p.492)

GEOLOGICAL SUEVEY 

AERIAL VIEW OF THE SOUTHWEST COAST OF SEMISOPO

Shows the irregular surface of a Recent flow (right) that is somewhat modified by ash deposits. Sea cliffs at le

Anvil Peak is in background. Photograph by U.S. N
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 65

1 

J

VIEW LOOKING SOUTHWARD FROM THE WEST SIDE OF RAGGED TOP

Shows the gentle dip slope (left) and steep scarp face (right) of the 705-foot remnant of the early postcaldera

cone.
493

here is affected by the rise of magma at Mauna Loa, and thus 

presumably by magrnatic tension in the Mauna Loa magma chamber. I would infer a magma chamber that is not steep roofed, 

but has a considerable lateral extension; in other words it is 

blisterlike, or laccolithic in shape, in the Gilbertian sense. Given 

such a shape, space for the excess magmatic volume will much 

more readily be made by sheetlike or laccolithic intrusion than 

by diking of the superficial crust. As Hunt (1942, p. 198) has 

shown for the Henry Mountains, a considerable degree of stretching of the superjacent cover above an intrusive mass is possible. 

Here also, although dikes are present, they are negligible in volume by comparison with the laccoliths, which depart widely 

from the classic shape deduced by Gilbert. 3

The most telling objection to this explanation of the possible 

fate of the missing magmatic material is the lack of physiographic evidence of the doming that it would involve. If the injection were relatively sheetlike, little tilting of the superjacent 

cover would be expected; but if any considerable quantity of the 

missing magmatic material has been stowed away beyond the 

line of the caldera walls, there should be evidences of uplift, in 

the form of marine terraces about the shores of Semisopochnoi. 

If such terraces were ever present, they have been obscured by 

subsequent rise of the sea level, or isostatic sinking of the island 

in response to the added weight of the volcanic cone on the 

earth's crust.

A still greater difficulty is that the necessary intrusion can be 

accomplished only by a magma under considerable pressure, and 

the development of this pressure before the formation of the 

caldera hardly seems compatible with the creation of a substantial void into which the superstructure of the mountain can 

collapse. It would seem probable that the intrusion could be 

accomplished only by a magma in which the pressures at a given 

depth were higher than at any previous time. But then it becomes difficult to see why the caldera-forming eruption should 

take place just after the intrusions, which have presumably gotten rid not only of the surplus of material, but also of the excess 

pressure.

Another possibility to which relatively little attention has been 

given is that a considerable degree of vesiculation had already 

taken place in the magma over a long period before eruption, so 

that a larger part of the bulk volume of the erupted pumice may

3 G. A. Macdonald (oral communication, 1956) has pointed out that there is no direct evidence, 

in older dissected Hawaiian volcanoes, for a magma chamber of this shape.
494 INVESTIGATIONS OF ALASKAN VOLCANOES

be credited to the cavity into which the mountain has collapsed. 

Williams (1942, p. 98) has suggested the possibility that there 

was some degree of vesicularity in the magma before eruption; 

but even if the volume of the vesicles in the pumice is included 

the calculated volume of the erupted material is less than the 

volume of the cavity. It seems certain thr\t much of the material 

must have been disposed of elsewhere, either by widespread sheetlike intrusions, or, in the case of Semisopochnoi, by flows of 

lava into the ocean.

Precise dating of the eruption of the dacitic pumice is difficult. 

The physiographic evidence of the truncated glacial valleys, now 

partly filled with ash, indicates that the caldera was formed 

after the latest glaciation. Glacial striae found on the saddle in 

the caldera wall.north of the west end of Fenner Lake also indicates that caldera collapse was preceded by some glacial erosion. 

One outcrop (plate 62#) indicates that the glacial ice here moved 

northward nearly horizontally from left to right. This outcrop 

is about 60 feet higher than the axis of the valley, and a few 

hundred feet north of the edge of the caldera wall. There must 

have been a glacial gathering ground of some extent on the high 

ground that once occupied the place of the caldera.

EARLY POSTCALDERA ERUPTIONS

The dating of the various volcanic materials erupted since the 

caldera collapse cannot be precise, because in many places the 

earlier materials are completely buried beneath the later ones, 

and the products of the different vents cannot always be distinguished from one another. Dating must depend therefore on 

the degree of destruction or dissection, which is influenced not 

only by the nature of the material and the differing tectonic and 

erosional accidents that have befallen it but also by the dates of 

cessation of eruptive activity, rather than of its commencement.

Of all the postcaldera structures, the one that has been most 

nearly destroyed is the one represented by a remnant that lies 

west of Ragged Top and north of Sugarloaf. The highest remaining part of this cone reaches an altitude of 705 feet. As shown in 

plate 65, the upper surface is a dip slope, slightly modified, and 

declining eastward. Some of the material erupted by this cone 

extends down the present valley of Fenner Creek almost to the 

seashore, and the constructional upper surfaces, though separated by later alluvium and lava flows, can readily be connected 

by projection across the intervening gaps.
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 495

The gradient of the surface averages about 5 percent, but is 

about 2 percent near the mouth of the valley, where the upper 

surface, close to the present shoreline, is 160 to 200 feet above sea 

level. Such a gradient, projected, would bring the upper surface 

down to the present sea level at a distance of 8,000 to 10,000 

feet beyond the present shoreline. If the rate of removal of the 

material by marine erosion was as rapid as the fragmental character of the material would suggest, it does not necessarily follow 

that the original upper surface was ever tangential to the surface 

of the sea, or that a primary constructional slope extended out to 

sea anything like the distance suggested by these computations. 

The gentle dip slope is terminated abruptly on the west by a low 

but steep scarp. At the base of the scarp is a constructional surface of lava flows and detrital material that slopes gently northward from Mount Cerberus. The origin of the scarp is not certain, but its orientation parallel with the caldera rim, its straightness, and its lack of dissection, suggest that it is a fault scarp, 

caused by renewed subsidence along faults within the original 

caldera-collapse faults, which lie nearly a mile farther south, and 

roughly parallel to them.

The material making up the cone is almost wholly pyroclastic. 

Just east of the scarp the material is largely tuff-breccia that 

contains angular fragments of medium-gray olivine basalt. A 

few thin flows of basalt are also exposed near the base of the 

scarp. On the south side of the valley of Fenner Creek at a 

point about a mile from the coast, the material may be better 

described as brecciaceous tuff, containing a few angular, compact to scoriaceous basaltic fragments as much as 8 inches in 

diameter, though more than 5 percent of the fragments are less 

than H4 inches in diameter. Exposures are sparse, but at least 

one of the beds is as much as 20 feet thick. Elsewhere along the 

valley walls the material ranges from tuff-breccia to coarse duncolored tuff.

RECENT CONES

Three Recent cones are distinguished on the map, together 

with the lava flows erupted from each: Mount Cerberus and 

Lakeshore Cone (pi. 66) within the caldera, and Sugarloaf Peak, 

which lies outside the caldera near the southern point of the 

island. There is not enough evidence to warrant any definite 

statement as to which is the oldest of these three. It is quite 

possible that renewed eruptions may take place at any of them.

510522 O -60 -4
496 INVESTIGATIONS OP ALASKAN VOLCANOES

MOUNT CMBBHHffiUS

The name Mount Cerberus has been given to a compound composite volcano, which has three summits of nearly equal height, 

and which occupies most of the southern part of the old caldera. 

The three craters lie at the apices of a scalene triangle. They are 

not of equal size. The crater on the easternmost of the three 

peaks is smaller than the other two, and has to the northwest a 

large platform of irregular surface apparently built in a period 

of mild eruption during which the vent was shifting position 

slightly. About a mile to the east of the easternmost major summit is a small cone, now partly destroyed, that attains an altitude 

of about 860 feet.

Six lava flows from Cerberus have been distinguished on the 

map, and also an area underlain by a number of lava flows that 

were not mapped separately. The distinction between the several flows rests mainly on the basis of age, and the principal 

criteria of age are the relative degrees to which the flows are 

buried with ash and covered with vegetation. Obviously these 

criteria must be used with caution, because the amount of ash 

on top of a flow depends not only on its age, but also on its position with respect to the principal sources of ash, and on the unknown wind directions prevailing at the time of the ash eruptions; the amount of vegetation, also, depends on the amount of 

ash, and on the altitude as well. These two criteria are fairly 

reliable, however, if used in discriminating between adjacent or 

nearly adjacent flows.

The numbering used on plate 59 reflects an endeavor to assign 

relative ages on the basis discussed above, the lowest number 

being assigned to the earliest flow. What is regarded as the 

youngest flow from Cerberus is one about 2,000 feet long, which 

was emitted near the base of the north side of the western cone. 

It is visible in the center of plate 66 as a small dark spot near 

the base of the cone. A somewhat older flow, about 2^ miles 

long, was emitted from a vent at an altitude of about 1,120 feet 

on the slopes of the easternmost cone. For most of its length it 

flowed at the base of the caldera wall, spreading out to a width 

of half a mile on the undissected detrital surface at the base of 

the cone but narrowing to a width of less than 300 feet where it 

emptied through a steep subsequent valley separating two 

remnants of the early postcaldera cone. Just below the narrows it entered the valley of Fenner Creek, where it spread upstream for a few hundred yards and downstream for about 0.8 

mile. In the lower part of its course two separate pulses of ex-
510522 O-60 (Face p.496)

GEOLOGICAL SURVEY 

PANORAMA OF MOUNT CERBERUS FROM THE NORTH AND 

On the right is Lakeshore Cone lying near the shores of Fenner Lake; on the extreme righ
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 67

A. LAVA FLOW FROM MOUNT CERBERUS

View is up the west margin of one of the latest flows from Mount Cerberus. Double levees formed in a constriction where block lava poured down a steep valley floor, leaving the central part of the flow lower than 

the levee margins.

B. LAVA TOES

Extruded from the margin of a late lava flow from Mount Cerben
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND

trusion can be recognized, the later less powerful than the first, 

as is shown by the smaller extent of the later part of the flow 

and the presence of a double set of levees at the narrows, where 

the middle part of the flow is now notably lower than the double 

marginal levee (pi. 67A). This phenomenon is a physiographic 

expression of the continuation, after extrusion has stopped, or 

nearly stopped, of movement in the lava already extruded, causing the central, still fluid part of the flow to drain out, leaving 

the already solid marginal portions standing above the center. 

If an early interruption of the supply is followed by a later renewal, but at a lower rate, the second pulse will form an inner set 

of bilateral marginal levees, within an outer set. Each set of 

levees consists of two, one on each margin of the flow.

Many of the older flows on the northern flanks of Cerberus 

have been almost completely buried beneath the loose detrital 

material washed from the flanks of the cone. Presumably many 

other flows are completely buried. The flows on the south flank 

appear to be somewhat older than those to the north. About a 

mile northwest of South Point the later flows evidently poured 

over sea cliffs about 180 feet high. Veneering of cliffs of this 

height appears to be unusual in the Recent lava flows of 

Semisopochnoi, as compared for instance with those of Hawaii 

(Sterns and Macdonald, 1946, p. 69).

An interesting demonstration of the low strength of the hot 

solid lava is afforded by the chatter-marked lava toes shown in 

plate 67S. These structures, which range in width from 1 to 5 

feet, and in thickness from a few inches up to about l1/^ feet, 

show a crude longitudinal grooving, and, on a smaller scale, 

transverse circumferential grooves, at nearly uniform intervals 

corresponding to the regularly spaced cracks on the stretched 

side of a thick metal turning. Apparently the lava toes were 

extruded from cracks in the front of the flow when the lava had 

about the consistency of a very stiff paste, and the stretching of 

the convex side produced the transverse markings shown in the 

view. Genetically, these structures are the equivalent, for block 

lava, of the toes described by Jaggar (1931, p. 1-3) in a pahoehoe 

flow. Similar tongues of lava, formed on the front of the 1948 

lava flow from Hekla, in Iceland, have been described by Einarsson (1949, p. 31) under the name of "toothpaste tongues." 

Einarsson observed the tongues in the process of formation and 

found that the motion is extremely slow, so that it can be detected only by comparing observations made at intervals of 

several hours. He believed that the tongues contained fluid lava
498

"which is crystallizing outwardly, not because of cooling but 

because of shearing movement."

The toothpaste tongues or lava toes described above were observed at the front of a flow opposite the north point of the 

southerly sector remnant of the old volcano, 1.2 miles southsouthwest of the summit of the southwestern peak of Mount 

Cerberus. A few feet away what appears to be a rootless scoria 

mound rises from a cinder-floored depression to a height of about 

30 feet. It is composed principally of black and red scoriae, some 

of them as much as 5 feet across. This mound may have been 

produced by the advance of an older, now buried flow across a 

small body of water or water-soaked alluvium that then lay at the 

base of the old caldera scarp. It is possible, however, that it 

was produced by magma ascending along the marginal fault of 

the caldera; the presence of agglutinate (pi. 68A) suggests a 

rather high temperature for the ejecta, and may be taken as 

evidence in favor of such an origin.

Most of the lava flows from Cerberus appear to have been 

emitted either near the base of the cones or from the flanks at 

altitudes of not more than 1,600 feet. They consist of porphyritic 

basalt, which contains hypersthene accompanied in places by 

olivine. The fragmental material making up the cones consists 

mostly of fragments less than 1 foot in diameter; about 95 percent of the material is made up of angular pieces of accessory 

lithic basalt, and about 5 percent of the material is highly scoriaceous black to dark-gray essential lapilli. Near the rim of the 

southeastern crater, a few partly solfatarized blocks as much as 

5 feet across give evidence of minor phreatic explosions that 

have dislodged altered material from the crater floors.

LAKESHORE CONE

Lakeshore Cone is a small scoria mound, with a maximum altitude of 585 feet, on the western shore of Fenner Lake. From 

the northwest side of its base two flows have escaped. The 

younger one has crossed the older one and the irregularities of 

its front now make minor coves and peninsulas near the northern 

extremity of Fenner Lake. The cone was not examined at close 

range and nothing is known of the composition of its lava flows.

SUGARLOAF PEAK

To the mariner, one of the most conspicuous landmarks on 

the island of Semisopochnoi is the cone called Sugarloaf Peak,
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 68

A. AGGLUTINATE

On top of small cone at south base of Mount Cerberus and near foot of inner scarp of the south sector of the 

shield volcano. Camera case gives scale.

B. VIEW ON SOUTH SIDE OF SUGARLOAF PEAK 

Shows thin Java flows and beds of pyroclastic material.
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 499

which is 2,808 feet high. This is the most southerly of the higher 

cones of the island, and it still retains its original form, though 

nothing remains of the old crater but a rather shallow saucer, 

opening toward the northeast, with a cliffy escarpment a few 

feet high on some sides, suggesting that the whole surface of 

the mountain has been lowered somewhat by erosion. Sugarloaf 

is chiefly composed of loose materials, though thin lava flows, as 

much as 6 feet thick, make up a respectable proportion of its bulk. 

Many of the larger fragments seen in plate 6SB are fragments of 

these flows, detached by gravity when the flow became too viscous 

to move farther as a fluid.

On the southern flank of the main cone is a double parasitic 

cone. The higher part, 1,620 feet high, has a double crater; the 

more southerly and lower crater has on the south a narrow rim 

that rises 10 feet above the bottom of the crater, and 20 feet, in 

a steep cliff, above the gravity slope of cinders on the outer face 

of the cone. It is evident that gravity, wash, and the effects of the 

strong winds have lowered the whole surface of the cone about 

30 feet since the latest eruption. Yet some traces of volcanic heat 

still remain, for lukewarm steam escapes feebly from the interstices of a bed of reddened tuff-breccia and agglutinate cakes at 

the rim separating the two compartments of the double crater. 

The tips of the projecting filaments, wisps, and mammillae of 

glass along the steam-producing fissure are coated with opaline 

silica, which does not extend into the less accessible crevices of 

the fragments. It appears that the steam has transported and 

deposited the silica to form the opaline coating.

About 2,000 feet west of the double crater, and forming the 

other part of the parasitic cone, is a half-cone of scoriae about 

1,050 feet high. One of the most recent flows escaped from the 

southern end of the gap between the two structures; it apparently 

was emitted at a steep tumulus, made up of broken fragments of 

the flow crust 3 or 4 feet thick standing at steep angles. About 

90 percent of the 1,050-foot cone is composed of angular fragments of basalt, the remainder being scoriaceous lapilli. A few 

larger blocks, about 10 percent of which are compact and the 

rest-scoriaceous, were also noted. Some of these blocks are breadcrusted, suggesting reheating, or disruption of a not quite solid 

lava. A few very small flow tongues, about 1 foot thick and 4 feet 

wide, are exposed on the outside of the cone. The faint, nearly 

horizontal circumferential banding seen on some photographs of 

the cone is due to the fact that the erosional surface is slightly 

steeper than the original surface of the cone.
INVESTIGATIONS OF ALASKAN VOLCANOES

The lava flows of Sugarloaf and its parasite, including the 

pyroclastic fragments, are olivine basalt and picritic olivine 

basalt, apparently without hypersthene. In lacking this mineral 

they differ notably from, those of Mount Cerberus which, with 

one doubtful exception, uniformly carry that mineral. As the 

two cones were active at nearly the same time, it may be that 

they tap different magma chambers, or different parts of the 

same chamber.

UNCONSOLIDATED DEPOSITS 

ALLUVIUM, ELUVIUM, AND TALUS

The pyroclastic deposits of the various volcanic cones, particularly those consisting largely of fine ash, are readily set in 

motion when saturated with water, and move down the steeper 

slopes to form wide aprons of eluvium, deposited in part by mass 

movement of saturated material, in part by slope wash and 

stream transport. Most of the deposits thus formed have not 

been mapped, as they have not obscured the underlying contacts; but extensive aprons have been formed on both sides of 

the lower part of Fenner Creek valley. The lower margins of 

these rest upon the terraces of tuff-breccia from the early postcaldera cone, which have protected them from removal by Fenner 

Creek.

Much larger bodies of alluvial material have formed in the 

bottoms of the wider stream valleys, particularly in those to the 

north of Ragged Top, and south of Tuman Head, and in that of 

Fenner Creek. Much of the material of the alluvium is reworked 

ash, some of it from the dacitic pumice eruption and some of it 

from the later basaltic eruptions. The cross profiles of these 

valleys in their alluviated reaches suggest that the bedrock floors 

lie some distance below present sea level.

SAND DUNES

Where the wider flat-floored valleys reach the sea the wind 

has erected a single or multiple dune ridge, parallel to the shoreline. None of these ridges is more than 40 feet high. Whether a 

ridge is stationary or is migrating landward or seaward depends 

chiefly on the ratio of the supply of material by the local stream 

to the effectiveness of marine erosion in removing it. Where the 

rate of supply exceeds the rate of removal, the shoreline tends to 

advance seaward, and the drier material exposed at low tide is 

available to the wind for the building up of the dune ridge.
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 501

Where the supply is insufficient, wave erosion removes the ridge 

and attacks the alluvium on the landward side of the ridge, unless the rate of eolian transport is greater than the rate of 

marine erosion. The presence of dune ridges at the mouths of 

all the alluviated valleys suggests that the supply of material that 

filled the valleys has not yet failed. As the beach progrades 

beyond the protection of the adjacent headlands, a kind of equilibrium is probably established, governed chiefly by the variation 

inliffectiveness of wave attack with variation in exposure.

GEOLOGIC HISTORY

The oldest episode in the history of Semisopochnoi of which 

there is any record begins with the eruption of a series of basaltic 

tuff-breccia, followed by flows of mafic lava, building up a volcano having nearly the dimensions of the present island and 

perhaps as much as 3,000 feet high. Upon the broad low dome 

thus erected several smaller and steeper cones of andesitic lava 

and pyroclastic material were superposed, including probably a 

central cone that raised the total height of the mountain to 5,000 

or 6,000 feet. Small cones of basaltic material, consisting chiefly 

of cinders and bombs but including some lava flows, were 

constructed near the north and west coasts of the island. Much 

of this eruptive activity was preglacial, or at least precedes the 

latest glaciation, as shown by the fact that the cones produced 

have been dissected by glacial cirques and transected by glacial 

valleys. The activity was probably preceded by a time when the 

sea stood at least 80 feet higher than it does now, and the debris 

eroded from the volcanoes was locally deposited about the shores 

of the island as volcanic sand.

Near the end of the glacial period, the volcano produced a totally new type of product. It had apparently been quiescent for 

some time, and the nature of the magma had changed by differentiation during the interval, becoming much more silicic and 

rich in volatiles. Great volumes of hot, highly inflated, pale-gray 

dacitic pumice and dust were spewed out in the form of dense 

clouds, which rushed like torrents down the accessible valleys, 

overtopping low divides and filling up the lowest parts of the 

valleys to a depth of as much as 160 feet. The removal of the 

great volume of magma from the chamber by this eruption, and 

presumably also by unexposed intrusions extending laterally 

from it, prepared the way for the collapse of the top of the mountain to form an elliptical caldera, almost 5 miles in greater diameter. After a period of quiescence, eruptive activity was
502 INVESTIGATIONS OF ALASKAN VOLCANOES

renewed, but once again basaltic tuff-breccia predominated. The 

new eruptions built up a low cone on the southeastern part of 

the caldera, from which some of the erupted material escaped 

as mud flows over the rim of the caldera and filled the outlet 

valley to an unknown depth, forming a broad gentle slope that 

probably extended some distance out to sea beyond the present 

shore. Somewhat later, several cones on the northwest side of 

the volcano, the largest of which may have begun to erupt before 

the caldera collapse, again emitted basaltic and andesitic lava. 

Renewed downfaulting, on faults more or less concentric with 

the earlier ones that bounded the caldera, destroyed part of 

the tuff-breccia cone in the southeastern part of the caldera, and 

enlarged the caldera on the north and west. Subsequent eruptions have built up one major composite basaltic cone and a 

minor cone within the caldera, and another major cone outside 

the caldera near the southeast shore of the island.

HISTORIC VOLCANIC ACTIVITY

Records of eruptions on Semisopochnoi Island are very scant. 

Grewingk (1850, p. 232, 281) found four reports of "smoke" in 

the records of early explorers, for the years 1772, 1790, 1792, 

and 1830. Becker (1898, p. 17) reports that a volcano on the 

island was active in 1873. Since one report specified that the 

activity was in the interior of the island, and since the cone of 

Mount Cerberus is less dissected than that of Sugarloaf Peak, this 

part of the island was probably the source of the dated eruptions. 

It cannot be determined whether any of the lava flows distinguished on the map occurred in historic time, but one of the 

late flows from Cerberus may be less than 100 years old, for its 

vegetation is little heavier than that on flows 40 years old that 

may be seen at a slightly higher altitude on Kanaga Island.

FUTURE VOLCANIC ACTIVITY

The nature of the material erupted during the periodic minor 

eruptions of the smaller composite cones suggests that no greater 

Plinian eruption is to be expected on Semisopochnoi within the 

foreseeable future; this volcano has had its fling, and settled 

down to a humdrum existence. It is probable, however, that relatively minor infrequent eruptions may occur from time to time, 

contributing new lava flows and ash layers, the latter measurable 

in inches, to the upbuilding- of the island surface. The effects OB 

neighboring islands will be negligible.
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 503

PETROGRAPHY 

POCHNOI VOLCANICS

The most mafic of the lava flows attributed to the Pochnoi 

volcanics is an olivine basalt, but the series includes rocks as 

silicic as hypersthene andesite vitrophyre. As the analyzed rocks 

were collected from widely separated areas, the age relations are 

not completely known, and it is possible that some of the rocks 

may have been erupted by some of the earlier subsidiary cones, 

perhaps even by cones centered within the area of the present 

caldera depression.

A sample of olivine basalt (P49Se4) collected by Powers 1*4 

miles S. 32i/£° E. of Northeast Point, is a porphyritic dense 

medium-gray (N 5) slightly vesicular rock, with intergranular 

texture. The phenocrysts consist of plagioclase, which averages 

about An85 (as determined by the average index of refraction); 

olivine (Fa32 as determined by the mean index of refraction), 

about 6 percent of the rock and ranging in length from 0.1 to 1 

mm; and sparse augite, ranging from 0.1 to 0.25 mm. The 

groundmass is chiefly plagioclase, averaging An57 ; augite granules that are apparently pigeonitic, as shown by a 2V of 35°; 

magnetite; and intersertal brown glass, charged with minute 

magnetite granules.

One-tenth of a mile northeast of the 1,405-foot peak (120,500' 

E., 288,200' N.) is a lava flow consisting of a medium-gray (N 5) 

sparsely porphyritic and vesicular augite andesite that contains 

very sparse macrophenocrysts (1 to 3 mm in size) of plagioclase 

(An62 ) and augite, which is dark green in hand specimen and 

very pale green in thin section. About 10 percent of the rock is 

microphenocrysts of plagioclase (0.4 mm long), ranging in composition from An48 to An55 ; about 1 percent of the rock is 

microphenocrysts of augite. The groundmass consists of plagioclase laths, augite granules, and magnetite granules (0.004 mm), 

with interstitial quartz and orthoclase, and a little cristobalite in 

the vesicles.

The most silicic rock attributed to this sequence is a hypersthene dacite vitrophyre, at the 800-foot level on the ridge 244 

miles northwest of South Point (126,300'E., 275,100'N). The 

flow from which the specimen came is not well exposed, but is 

certainly no more than 20 feet thick, and shows intricately contorted flow parting, parallel to the layering, which is an expression of the textural range from lithic to partly glassy. The part 

sectioned and analyzed was from the glassier phase, and is a 

medium-dark-gray (N 4) aphanitic dense lava, with planar struc-
504 INVESTIGATIONS OF ALASKAN VOLCANOES

ture and waxy luster, having about 10 percent of well-oriented 

microphenocrysts, principally plagioclase of composition An44, 

hypersthene (En80 ), augite, and a trace of magnetite and pseudodichroic apatite. The groundmass consists of plagioclase, augite, and magnetite in pale glass. Vesicles are filled with cristobalite, locally replaced by tridymite.

EARLY SUBSIDIARY CONES

Specimen P49Sel6 is from the next to the lowest flow exposed 

on Tuman Head, (104,475'E., 3,450'N.). According to Powers, 

who collected this specimen, the section here, is largely pyroclastic and rich in bombs and scoria. The flow from which the specimen was collected ranges in thickness from 10 to 20 feet. The 

rock is dense medium-gray (N 5) olivine-hypersthene basalt, 

with seriate texture. The plagioclase, in crystals as much as 3 

mm in size, averages An77, judging from the average refractive 

index. The olivine (Fa20 ) is in grains as much as one-half mm 

in diameter, and is pale yellowish green in hand specimen. The 

augite grains, which are as much as 1 mm in diameter, are yellowish green in thin section. Hypersthene prisms are much 

smaller, on the average. Magnetite and apatite are recognizable 

accessories.

Specimen 47AC98 is a medium-brownish-gray (5R 5/1) dense 

lava, with planar vesicles, from a flow 4 feet thick on the flank of 

the denuded cone just south of Fenner Creek and north of Sugarloaf (141,450'E., 280,500' N.). The texture is seriate, and 

intersertal. The constituents are plagioclase (0.01 to 1 mm in 

diameter) averaging about An77, augite (0.01 to 0.2 mm in diameter), olivine (Fa30, determined from 2V), and magnetite, 

with intersertal glass.

Specimen 47AC99 is olivine basalt from one of the latest flows 

from Ragged Top (148,800' E., 276,000' N.). The flow is about 

20 feet thick, consists of medium-gray (Af5) holocrystallineporphyrytic dosemic basalt, with.pilotaxitic texture. The phenocrysts consist of plagioclase (2 mm in diameter), averaging about 

An82, olivine (0.5 mm in diameter), augite (2 mm in diameter) 

that is dark green in hand specimen, and magnetite (0.5 mm 

in diameter). The groundmass consists of magnetite, augite, 

plagioclase, and interstitial orthoclase( ?).

LATE PLEISTOCENE VOLCANOES

Two analyses were made of the rocks from the cones that are 

relatively young, but older than part of the caldera collapse.
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 505

One of these specimens, P49Sel4, (117,375 E., 10,750 N.) is 

from a flow, apparently derived from Anvil Peak, that predates 

the eruption of the dacitic tuff, according to Powers. This is a 

medium-brownish-gray (SYR 5/1) porphyritic, slightly vesicular 

basaltic andesite. The phenocrysts (3 mm in size) include bytownite (An82 ), olivine (Fai5 ), and sparse augite that is green 

in hand specimen. The groundmass is an hyalopilitic web of 

plagioclase microlites, augite, and magnetite.

Another fragment (47AC116) from an agglomerate from 

Threequarter Cone (121,700' E., 291,900' N.) is an andesite 

vitrophyre. This is a dark-gray rock, aphanitic, with small tabular plagioclase phenocrysts showing a parallel arrangement. 

About 5 percent of the rock is plagioclase phenocrysts (1 mm in 

diameter), having an average composition of An58 . A few phenocrysts are hypersthene and olivine. The groundmass is mostly 

plagioclase microlites (0.05 mm long), minute granules of augite (?), and magnetite dust, in brown glass, having an index of 

1.546.

PUMICE ERUPTION

Three of the samples of the dacitic pumice eruption were analyzed. One sample (47AC113), is a composite representing the 

full thickness of the tuff, as exposed in a gully at 122,600' E., 

284,250' N. No section was cut as it was not certain that one section could properly represent the composite. Specimen 47AC90A 

is a fragment of pumice from the relatively thin layer that accumulated at 128,300'E., 273,500'N., in a protected sector of the 

old cone. This is largely a pumiceous clear glass of index 1.515, 

containing small shreds of dark-green glass of index 1.554. The 

clear glass has crystals of labradorite (An57 ), hypersthene, 

augite, green hornblende, and apatite. Another specimen is a 

fragment from the pumice, exposed in the caldera wall at 

130,000'E., 297,000'N. This fragment consists of brown glass, 

of index 1.534 with elongate vesicles, sparse augite, plagioclase, 

and olivine. The high index of the glass and character of the 

minerals suggest a composition in the andesite range; the lower 

index of specimen 47AC90A suggests a silica percentage of about 

66. It is probable that all the composite samples are contaminated dacite.

RECENT CONES

Late-central cones. Two analyzed specimens are available 

from Mount Cerberus, specimen 47AC88 (130,550' E., 270,000' 

N.) is from one of the longer flows that reached the coast, and
506 INVESTIGATIONS OF ALASKAN VOLCANOES

specimen 47AC110 (131,000' E., 282,250' N.) is from a relatively 

short flow, which was sampled on the southeast side of the southeast peak of Mount Cerberus. The first is a medium-light-gray 

(N 6) porphyritic basaltic andesite; most of the visible phenocrysts are plagioclase, averaging An82, but there are also olivine, 

hypersthene, augite, and magnetite crystals. The olivine and 

augite are commoner and in larger crystals (0.5 mm in diameter), than the hypersthene. The groundmass consists of plagioclase microlites with augite and magnetite and a mesostasis of 

glass.

The other flow (47AC110) is a dark-gray (N 3) dense vitrophyre, with a seriate texture. About 25 percent of the rock is 

plagioclase (1 mm in size), averaging about An45 ; the other 

phenocrysts are augite, sparse hypersthene (En65 ) in slender 

needles, apatite as pseudodichroic prisms, and magnetite. The 

groundmass consists of plagioclase microlites, augite, and magnetite in pale-brown glass.

Late-lateral cone. The only Recent cone that has grown outside the caldera is Sugarloaf and the small parasitic cone on its 

south flank. Two samples were analyzed.

One of these (P49Se6) was from a very recent flow on the 

eastern shore (142,200' E., 269,550' N.), which Powers considered 

younger than the most recent eustatic bench, and which is unmantled by ash. Powers estimated the thickness at more than 

20 feet, and notes that there was much copper staining. It is a 

dense medium-gray rock with conspicuous green augite phenocrysts. It is seriate, with an intergranular texture. The principal constituents are bytownite (An72 ), augite (Di7oHe38 ), and 

olivine (Fa35 ) as much as 1 mm in diameter, in part forming 

clots of small prisms, and magnetite. The groundmass augite is 

pleochroic from pale brown to pale green, has an 2V of 20°, and 

hence is rather pigeonitic. The augite phenocrysts also have thin 

pigeonitic outer zones.

Specimen 47AC80 (137,150' E., 271,850' N.) is from a compact 

block on the rim of the crater of the westernmost part of the 

small parasitic cone of Sugarloaf. It also is an olivine basalt, 

with plagioclase averaging An64 , olivine (Fa30 ) as much as 0.3 

mm in diameter, augite less than 0.5 mm and green in hand 

specimen, and magnetite. The groundmass has an intergranular 

texture, with prisms of clinopyroxene and microlites of plagioclase (0.02 mm in diameter).
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 507

CHEMISTRY

The analyses of 15 specimens from Semisopochnoi Island are 

shown in table 1, together with the spectrographic analyses for 

minor constituents, and the Niggli numbers. The rocks of Semisopochnoi differ somewhat from those of other western Aleutian 

volcanoes. The total range of variation in silica content is greater 

than in other islands. Tlie rocks are slightly more alkalic, as indicated by the fact that the alkali-lime index is 59, hence the 

province is calcalkaline (Peacock, 1931, p. 54). This compares 

with an index of 63 for Adak and Kanaga, 64.5 for Buldir, 65.5 

for the common Japanese volcanic rocks (Tsuboi, 1934, p. 2271), 

and 63.8 for Katmai. On the other hand, the index for Semisopochnoi is comparable with that of the Newberry volcano (Williams, 1935, p. 297), and contrasts with calcic High Cascade 

volcanoes. The differences between the Newberry volcano and 

the High Cascade volcanoes seem to be correctable with geographic position. No such difference can be seen here the Semisopochnoi structure lies on the same line as the other recently 

active volcanoes of this area. The only structural feature of the 

Aleutians that seems to distinguish Semisopochnoi Island from 

the other islands is the occurrence of the great curving submarine feature of Petrel Bank, which extends, with a trend changing 

gradually from the northeastward through north to west, for a distance of about 240 miles. The geologic significance of this 

feature is unknown. Nor is there any difference in time both 

Semisopochnoi and Kanaga have been active in Recent time.

DIFFERENTIATION AT SEMISOPOCHNOI

In the differentiation diagram (fig. 74), three separate sets of 

trend lines are indicated for the major constituents A1203, FeO 

(including 0.9 Fe2 03 ), CaO, and MgO. These are indicated as 

Early, Late central, and Late lateral. The Early trend includes 

the rocks of the main cone and of minor subsidiary cones, formed 

before the first collapse of the caldera, and the pumice associated 

with the formation of the caldera. The Late-central trend includes the rocks of Anvil Peak and Threequarter Cone, which 

are later than the pumice eruption and probably earlier than the 

latest caldera collapse, and the rocks of Mount Cerberus, essentially Recent. The Late-lateral rocks are those of Sugarloaf Peak 

and its parasitic cone. The Early sequences are indicated by solid 

lines, the Late-central sequences by dashed lines, and the Late-lateral sequences by dotted lines. For NajjO and K2 0, a single solid line 

represents the trend of each constituent. The differences between
508 INVESTIGATIONS OF ALASKAN VOLCANOES

50 55 60 

WEIGHT PERCENT. Si02

65

FIGURE 74. Variation diagram for volcanic rocks from Semisopochnoi Island. 

(See explanation on facing page.)
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 509

the Early and Late trend lines are thought to be due to mixing of 

magmas or assimilation of material. For most constituents, the 

position of the Late-central lines can be explained by assuming 

a differentiation following a trend like that of the Early rocks, 

and beginning with an initial material consisting of a mixture 

of a primitive magma like the least silicic rock shown, with 

dacitic material closely resembling that erupted at the time of 

the formation of the caldera in other words, a resurgence of 

primitive magma into the basal portion of the old magma chamber, still occupied by dacitic material, and the mixing of the two. 

The Late-lateral lines, comparatively short in trend, seem to be 

explainable as the product of hybridization of a similar primitive 

magma with material lying on the earlier course of magmatic 

evolution, at about 54 or 55 percent silica, and the evolution of 

this magma. This hypothesis is consistent with the idea that the 

evolution of the most silicic rocks has been confined to the central 

magma chamber, and that such rocks were available for purposes 

of hybridization only to magma rising in, or near the borders of, 

the old caldera.

A comparison of the differentiation curves for the analyzed 

Semisopochnoi rocks with those of Adak and Kanaga Island 

(Coats, 1952, p. 494) shows differences that are in part reflected 

by the difference in alkali-lime index. In general, it may be said 

that the alumina curve is higher, and declines more steeply in 

the Semisopochnoi rocks than in those from Adak and Kanaga. 

While the CaO line declines more steeply toward the silica-rich 

end, the iron, taken as FeO, shows no significant differences. The 

MgO line is consistently lower, and falls more rapidly toward 

the silicic side ; the Na2 0 and K20 lines both climb more steeply 

in the same direction. These trends are consistent with a difExplanation for figure 74.

f 

Shield volcano and subsidiary cones 

Early <

I X

Caldera-forming pumice

, Late Pleistocene basaltic cones 

Late central J

Mount Cerberus

Late lateral J 

] Sugarloaf Peak
510 INVESTIGATIONS OF ALASKAN VOLCANOES

ferentiation that differs from that at Adak and Kanaga in the 

more effective removal of ferromagnesians, at first olivine, and 

later pyroxene. What causes could bring about such a trend is 

uncertain. Assimilation of a certain amount of aluminous sediments may have contributed to the effect.

By using the magnesia-iron oxide-alkali-metal oxide diagram 

that was used by Poldervaart (1949, p. 177-188), modified by 

combining 0.9 of the Fe203 with the FeO, the contrasts between Semisopochnoi rocks and those from some of the other 

Aleutian volcanoes are easily seen (fig. 75). For comparison, 

the positions of the 4 points representing the averages computed 

by Daly (1933, p. 9-17) for basalt, andesite, dacite, and rhyolite, 

are plotted; also the positions representing the analyses of 5 

rocks from Buldir Island. The points representing the Daly averages fall on a smooth curve that is convex toward the iron 

vertex, and trends toward a point on the iron-alkali side about 

15 percent of the way from the alkali vertex to the iron vertex. 

The analyses of the Buldir rocks fall below the calcalkaline 

curve; those of the Semisopochnoi rocks fall somewhat above it,

FeO+0.9 Fe203

EXPLANATION

Semisopochnoi Island

Averages (Daly, 1933) for basalt, andesite, 

dacite, and rhyolite, in order from upper 

right to lower left

MgO

FIGURE 75. Relation of magnesia (MgO), iron (as FeO), and alkalies (Na20 + K20) in volcanic rocks from Semisopochnoi Island.
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 511

on the average. The Adak and Kanaga rocks fall on, or equally 

to either side of, the curve (Coats, 1952, p. 500). These relations 

suggest that there has been a slight degree of iron enrichment in 

the intermediate rocks from Semisopochnoi Island.

Bowen (1947, p. 273) has suggested that the differences reflected by these differing trends are due to the retention of volatiles, consequent upon a less pervious roof, in the mass that 

follows the "normal" differentiation trend, and that iron enrichment is due to substantially dry differentiation, in a mass with a 

thin and porous roof. The petrography of the several Aleutian 

Islands thus far studied is consistent with this explanation: 

the rocks of Adak and Kanaga commonly carry some hornblende; 

in those of Buldir hornblende, or pseudomorphs after hornblende, 

is very common, whereas not a single .rock from Semisopochnoi 

shows hornblende, except the dacitic pumice associated with the 

caldera-forming eruption. On the other hand Kennedy (1948, p. 

529-549) has shown by experiment the influence of the partial 

pressure of oxygen in controlling the ferrous-ferric iron ratio 

and hence the early appearing phases. When oxygen pressure is 

modest, early crystallization of magnesio-spinel removes much of 

the iron and conditions a crystallization that will follow the 

"normal trend." If partial pressure of oxygen is very low, differentiation toward an iron-rich residue will occur. None of 

Kennedy's experiments resulted in the production of hydroxylbearing ferromagnesians such as hornblende or biotite, that are 

commonly associated with the "normal" calcalkaline differentiation trend. It seems likely that the addition of water under high 

pressure conditions would result in the crystallization of hornblende or biotite rather than pyroxene, that the hydrogen 

released by the formation of hydroxyl-containing minerals would 

suffice to keep the partial pressure of oxygen relatively low. If 

the magma is extruded, or intruded under shallow cover, the 

hornblende or biotite can and does react with the magma to 

form oxyhornblende or oxybiotite, or, under some conditions, to 

a mass of pyroxene plus magnetite, suggesting that the hornblende is more iron rich than the pyroxene. It is also possible 

that the basaltic magma has been contaminated by selective solution of interstitial material, approaching granite in composition, in wallrocks and xenoliths, that has been carried into the 

magma along with the interstitial water (Kennedy, 1955, p. 492). 

Is it possible that the tectonic position of Semisopochnoi, at the 

intersection of the major Aleutian arc with the minor arc of 

the Petrel Bank, has resulted in more frequent dislocation of the
512 INVESTIGATIONS OF ALASKAN VOLCANOES

crust, easier ascent of the magma, with less assimilation of the 

wallrock, to relatively higher positions in the crust, and in easier 

loss of volatiles, thus resulting in a magmatic evolution that is 

characterized by a slightly greater early enrichment in iron than 

in the "normal" basalt-andesite-dacite-rhyolite sequence?

Some of the differences and resemblances between the rocks of 

Semisopochnoi and those of other Aleutian volcanoes are illuminated by the Niggli plot of al, alk, and c+fm (fig. 76). The dashed 

line in the diagram encloses the positions of points representing 

all analyzed rocks from Buldir, Adak, and Kanaga Islands. The 

much greater spread of compositions on Semisopochnoi is evident. It is also evident that the chief differences are in the direction of increasing al plus alk in the rocks in which the ratio of 

c to fm is low, while alk is low where the ratio of c to fm is high. 

The diagram of figure 76 is a double triangle, corresponding to 

cross sections IV and V of Niggli's system, whereas the range of 

ratios for c to fm in the rocks of Adak, Kanaga, and Buldir is 

small enough so that the parameters could be adequately represented on a single section.

RARER CONSTITUENTS

The results of spectrographic analyses of the analyzed rocks 

are shown on table 1, together with a list of the elements looked 

for by the spectrographer, but not found. In the table, a dash 

for a particular constituent indicates that it was not found in 

that determination. The limits of sensitivity of the spectrographic method used, for some of the elements shown as "not 

found," is as follows (K. J. Murata, written communication 1950).

Lanthanum .. . .. ... _ . . 0.002

Boron ._ __ _ _ ___________ .001

Lead __.__ ___________.__ _. . _ .001

Beryllium ...--.. . . _ _-. .0001

Nickel .... _.........-_______ 0.0007

Chromium _...._._______ .0002

Cobalt _._._ _.___.__.____--__ .001

There are insufficient data from which to compute precise ratios (Coats, 1952, p. 511). In a general way, however, certain 

trends are evident. The ratio of cobalt to MgO seems to increase 

with increasing silica percentage, whereas the ratio of nickel to 

MgO show considerable scatter. The ratio of zirconium to silica 

increases with increasing silica percentage. The ratio of barium 

to K20, when spectrographic values are used for barium content, 

shows considerable scatter; if the few chemical determinations 

for barium are used, however, the ratio of barium to K20 is 

about the same as that (0.024) found in the rocks of Adak and 

Kanaga, from which the barium was determined spectrographi-
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 513

, c+fm (c/fm=0.43-0.67)

A/VVVVVVVVV

\AAAA/\A/VV\/V

c+fm (c/fm=0.67-1.0)

FIGURE 76. Relations of al, alk, and c+fm in volcanic rocks from Semisopochnoi Island, 

plotted in 2 sections of the Niggli tetrahedron that include the al-alk edge. Dashed 

line encircles positions of points representing all analyzed rocks from Adak, Kanaga, 

and Buldir Islands.
TABLE 1. Major and minor constituents of 15 volcanic rocks from Semisopochn

[Analyses for major constituents by Wilbur J. Blake, Jr. (WJB) ; Robert N. Eccher (RNE) ; and Ray T. 

Hufschmidt]

Major constituents

Field No..... . 

Lab. No. .....

SiO2 .........

AlaOs....... .

FeO. ........

MgO........ .

CaO. ........

Na2O. .......

KzO.........

H2O+. ......

HzO........

TiO2 ........ .

CO2.. .......

PzOs........ .

Cl...........

F........... . MnO. .......

BaO........ .

Total ....

Total ....

P49Sel6 

ID-20750

WJB

49.70

18.81

3.12

6.61

5.63

10.45

2.98

.99

.09

.04

1.14

.01

.32

.18

100.07

47AC98 

ID-19750

RTT

50.40

19.92

3.28

6.32

9.71

2.98

.87

.50

.18

.87

.00

.26

.16

99.94

P49Se6 

ID-20550

WJB

51.62

19.09

3.34

6.44

4,. 17

9.28

3.35

v 1-. 14

..06

.01

1.02

.01

.28

.20

100.01

47AC99 

ID-19850

RTT

52.06

3.21

4.68

2 8Q

9.87

3.32

1.09

.11

.06

.87

.00

.16

.19

99.84

47AC80 

ID-19250

RNE

52.35

19.26

2.18

7.34

3.65

8.41

3.64

1.24

.39

.09

1.04

.02

.31

.23

100.15

P49Sel4 

ID-20650

WJB

53.36

18.39

2.24

6.49

4.41

9.00

3.22

1.18

.34

.06

.90

.00

.22

.18

99.99

P49Se4 

ID-20450

WJB

55.38

18.52

2.50

5.93

2.80

7.95

3.85

1.02

.32

.11

1.02

.00

.24

.19

99.83

47AC88 

ID-19350

RTT

55.88

19.55

2.43

4.83

2.25

8.08

3.75

1.63

.01

.00

.79

.02

.33

.20

99.75

47 AC 116 

ID-20250

RTT

58.55

16.94

2.09

5.85

2.62

6.21

4.33

1.79

.08

.02

1.02

.01

.38

.20

99.09

47 A

ID-

R

59

16

3.

3.

2.

4.

4.

2.

99.

Minor constituents 

[Be, Pb, La, were looked for but not found]

Lab. No. .....

Cu. .........

Co.......... .

Ni.......... .

50-1847

SC

0.02

.004

.003

Kft_1 QJQ

SC

0.02

.003

.002

50-1856

SC

0.02

.003

.001

50-1846

SC

0.01

.002

.0007

50-1857

SC

0.01

.003

en i QAQ

sc

0.01

.003

.002

en_i Q1O

SC

0.006

ni9

50-1854

SC

0.007

no9

50-1850

SC

0.001

.002

50-

S

0.

.
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 515

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516 INVESTIGATIONS OF ALASKAN VOLCANOES

cally by K. J. Murata (Coats, 1952, p. 509). In the rocks of 

Buldir, Adak, and Kanaga, the ratio of strontium to barium 

seems independent of silica percentage, and dependent largely on 

provenance (Coats, 1953, p. 21); in the rocks from Semisopochnoi 

the ratio of strontium to barium shows a good deal of scatter.

Some average ratios in volcanic rocks from four Aleutian 

Islands are given below.

Island

Adak ............

Buldir. ..........

Cobalt 

to 

MgO

4v in <

4

3

7

Nickel 

to 

MgO

3X10- <

3

10

Chromium 

to 

MgO

Q v in '

3

3

Ratio of 

Strontium 

to 

Barium

0.7

3

2

.6

Barium 

to 

K2O

7y If) 2

3

2

6

Vanadium 

to 

FeO

3X10-'

2

2

3

Zirconium 

to 

Si02

ixio-<

7X10- 5

8X10- 5

20X10- 5

The ratios for nickel and chromium to MgO have been 

omitted for Semisopochnoi Island in the above compilation because many of the values were below the limit of detectability. In 

all probability differences of one in the first significant figure are 

not statistically significant; differences that are severalfold are 

believed to be genuine and significant.

LITERATURE CITED

Barnes, V. E., 1930, Changes in hornblende at about 800° C.: Am. Mineralogist, v. 15, p. 393-417. 

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

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

p. 1-86.

Bowen, N. L., 1947, Magmas: Geol. Soc. America Bull., v. 58, p. 263-280. 

Byers, F. M., Jr., Hopkins, D. M., Wier, K. L., and Fisher, Bernard, 1947,

Volcano investigations on Umnak Island, 1946: U.S. Geol. Survey Alaskan

Volcano Inv., Rept. no. 2, pt. 3. 

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

volcanic rocks from Adak and Kanaga Islands, Aleutian Islands, Alaska:

Geol. Soc. America Bull.', v. 63, p. 485-514.

 1953, Geology of Buldir Island, Aleutian Islands, Alaska: U.S. Geol. 

Survey Bull. 989-A, p. 1-24.

 1956, Geology of northern Kanaga Island, Alaska: U.S. Geol. Survey

Bull. 1028-D, p. 69-80. 

Cotton, C. A., 1944, Volcanoes as landscape forms: Christchurch, New Zealand,

Wellington, Whitcombe and Toombs, Ltd., 416 p. 

Daly, R. A., 1933, Igneous rocks and the depths of the earth: New York,

McGraw-Hill. 

Einarsson, Trausti, 1949, The flowing lava; studies of its main physical and

chemical properties, V. 4, Pt. 3 of The eruption of Hekla, 1947-48:

Reykjavik, Visindafelag Islendinga [Soc. Sci. Islandica], 70 p.
GEOLOGIC RECONNAISSANCE, SEMISOPOCHNOI ISLAND 517

Fenner, C. N., 1920, The Katmai region, Alaska, and the great eruption of

1912: Jour. Geology, v. 28, no. 7, p. 569-606. 

George, W. 0., 1924, The relation of the physical properties of natural glasses

to their composition: Jour. Geology, v. 32, p. 353-372. 

Grewingk, Constantin, 1850, Beitrag zur Kenntniss der orographischen und

geognostischen Beschaffenheit der Nordwest Kiiste Amerikas, mit den

anliegenden Inseln: Russ. K. min. Gesell. Verh., St. Petersburg, 1848-49. 

Hinds, N. E. A., 1925, Amphitheater valley heads: Jour. Geology, v. 33,

p. 816-818. 

Hunt, C. B., 1942, New interpretation of some laccolithic mountains and its

possible bearing on structural traps for oil and gas: Am. Assoc. Petroleum

Geologists Bull., v. 26, no. 2, p. 197-203. 

Jaggar, T. A., 1931, Lava stalactites, stalagmite, toes, and "squeeze-ups":

Volcano Letter no. 345, p. 1-3. 

Kennedy, G. C., 1948, Equilibrium between volatiles and iron oxides in igneous

rocks: Am. Jour. Sci., 5th ser., v. 246, p. 529-549.

 1955, Some aspects of the role of water in rock melts in Poldervaart, 

A., ed., Crust of the earth a symposium: Geol. Soc. America Spec. Paper 

62, p. 489-504.

Kozu, Shukusuke, Yoshiki, Bunpei, and Kani, Koichi, 1927, Note on the study 

of the transformation of common hornblende to basaltic hornblende at 

750° C.; Tohoku Imp. Univ. Sci. Repts., ser. 3, v. 3, nos 2, 3.

Peacock, M. A., 1931, Classification of igneous rock series: Jour. Geology, 

v. 39, p. 54-67.

Poldervaart, Arie, 1949, Three methods of graphic representation of chemical 

analyses of igneous rocks: Royal Soc. South Africa Trans., v. 32, p. 

177-188.

Simons, F. S., and Matthewson, D. E., 1955, Geology of Great Sitkin Island, 

Alaska: U.S. Geol. Survey Bull. 1028-B, p., 21-43.

Stearns, H. T., and Macdonald, G. A., 1946, Geology and ground-water resources of the Island of Hawaii: Hawaii Div. Hydrography Bull. 9.

Tsuboi, Seitaro, 1934, Some remarks on the petrology of the common volcanic 

rocks of Japan: Pacific Sci. Cong., 5th, Canada 1933, Proc., v. 3, p. 

2271-2273.

Williams, Howel, 1935, Newberry volcano of central Oregon: Geol. Soc. 

America Bull., v. 46, p. 253-304.

 1941, Calderas and their origin: California Univ., Dept. Geol. Sci. 

Bull., v. 25, no. 6, p. 239-346.

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

Inst. Washington Pub. 540.

INDEX

Page 

Acknowledgments __........_.....___._... 478

Age, criteria .__.......__.__.___..__... 496

Agglutinate ..................__.___ 498, 499; pi. 68A

Aleutian arc . ._.......___.____..._._ 478

Alkali-lime index ________....__ 507, 509

Alluvium ____________.__..____ 500

Analyses, chemical _________.._._ 514

spectrographic ._______.___ 512, 515

Andesite, augite ...__________..... 487, 503

augite-hypersthene ______._ 483, 484 

basaltic ......__________ 489, 505, 506

olivine-hypersthene-augite _.___. 484 

Andesite vitrophyre .__________._ 505 

Anvil Peak ____ 480, 486, 487, 488, 507; pi. 59 

Ash, dacitic, volume of _...______. 491, 492 

Assimilation of wallrock_ 490, 509, 510, 511, 512

Barium ___.__.____.____._. 512, 516

Basalt, hypersthene __.____.__..__.. 482

picritic olivine ...__.__.__....._.... 500

olivine ......__... 482, 483, 484, 485, 486, 495,

500, 503, 504, 506 

olivine-hypersthene .................. 482, 485, 504

Beaches ..___.___..___......____........ 480

Blake, W. J., analyses by .......................... 514

Block lava ..._...____.._____.............. 480

Bombs ._...___......._________ 484, 485, 487

Breadcrusted blocks __._____.......... 484, 499

Brecciaceous tuff __..._..........._..__ 484, 495

Caldera, definition _.._.._.......___......... 480

Chemical analyses ____________._. 514

Cobalt ....____.........___.....__.____ 512, 516

Drainage _......

Dune ridges _...

.... 481 

500, 501

Eccher, R. N. analyses by ____._......... 514

Einarsson, T., quoted ......_..___...._. 497, 498

Eluvium _.____.__.._......._._..____ 500

Fenner Creek .......................................... 481; pi. 59

Fenner Lake ........___............ 481; pis. 59, 60B

Glaciation ........_..__.

Glowing-cloud deposit ...

488

491

History, geologic __................_.....__ 501, 502

volcanic ____.._.........__.____ 502

Hornblende, green ..._._...........__.._..... 490

Hufschmidt, E. L., analyses by .___...... 515

Hypersthene andesite vitrophyre ....__... 503

Hypersthene dacite vitrophyre _........... 503-504

Page 

Lakcshore Cone ._._ .._..... 495, 498; pis. 59, 66

Lava fountain debris___._..........____. 482

Lava toes .....__.....___._...... 497, 498; pi. 67A

Location of area ...._____..__..__... 478, 479

Magma, excess ....................__._........... 492, 493

vesicularity ___.__..________ 494 

Marginal levee ...._........._...._._... 487, pi. 67A

Microgabbro, olivine ..._._....______... 484

Mount Cerberus ....._-.... 480, 481, 496-497, 498,

502, 507; pis. 59. 66, 67

Nickel ............................__....................... 512, 516

Niggli plot ..._.........._.._............................. 512, 513

Obsidian __.........__......__.................._... 489

Oxygen, partial pressure of ............. .... . 511

Palagonite .._...........-----__.........._...... 482

Perret Ridge .__....___....._. 485, 488; pi. 59

Petrel Bank .........._. .._..................... 507, 511

Phreatic explosions ............... .......... ..... 498

Planezes .._.___......................................... 483

Pochnoi volcanics ......... 481, 482, 48'3, 486, 503

Pochnoi Volcano .....~_........___481, 483; pi. 60A

Pumice, dacitic _._.._.._.. 489, 490, 491, 494, 501,

505; pi. 63B 

Pumice eruption ........ ......... .... .. 489-494, 505

Ragged Top ....480, 483, 484, 488; pis. 59, 60B, 61

Sand dunes _...._................___..._._ 500. 501

Sandstone __.....................___....... 481. 482, 487

Size of island ...........~--.~~--.-...~~~~.-~- 478

Spectrographic analyses ............... ...... 512, 515

Strontium ......_...... ._... - - 516

Submarine tuff cone .... ............ ~ ... 482

Sugarloaf Peak ............_.- 480, 496, 498, 499, 500;

pis. 59, 61, 68B

Talus ...................--... ---- -----. 500

Telfer, R. T., analyses by ......... ............ 514

Threequarter Cone ........ 486, 487, 490, 505; pi. 59

Toothpaste tongues .... _. ............ 497, 498

Topography ........ ...... ........ .. 480

Tuff-breccia ._._.._.. - 481, 482, 495, 499, 500

Vanadium - . . ....

Volatile transport of silica

516

499

Williams, H., quoted ........_..._... .......... 480

Zirconium .__....................... . ....... 512, 516

519

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