Geology of Segula, Davidof 

and Khvostof Islands 

Alaska

By W. H. NELSON

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-K

Prepared in cooperation with the 

Departments of the Army, Navy, 

and Air Force

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.
PKEFACE

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 field studies were made during the years 1946-1954. 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 Buildir Island, Aleutian Islands, Alaska," both by Robert R. Coats. 

Unpublished results of the early work and all of the later studies are 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 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.

m

483663-59

CONTENTS

	Page 

Abstract___------_--_--_-___.-______.___.---_-----_-.-. 257

Introduction. _ ____________________________________________________ 257

Geography__ ____________________________________________________ 257

Topography _.._..._.........._.__.._...-._...-_-_--_.-.---_ 257

Segula Island....____.____--_.--_.-._---_._--------.------ 257

Khvostof and Davidof Islands.____.___.___._-_--.__-___-___ 259

Submarine topography_____________________________________ 259

Climate..______.._..__.._______._______-_-..-._____ 259

Vegetation .------..-........------------.-...-------------- 260

Geology.__.__.._......_..___._____._._._._..._._.__-.___.__ 260

General features..___-__.___.___________-.__.__._----_---____-_ 260

Composite volcanic material-_--__--__--_-__--_-_--------------_ 261

Surface pyroclastic layers...____________________________________ 261

Surface lava flows.--_.__-__________--.-_--_-___----_---------. 262

Unaltered flows.---.---_.___._._...............___... 262

Altered volcanic rocks___________._._____-_-----_-___-___- 264

Sedimentary deposits on Segula Island__________-___-----_-__---_ 264

Structure......--_-__--_-_____-__-_-_---_----..----------____ 265

Segula Island........_________.._..._.......__ 265

Davidof Island and Khvostof Island._______-_-_-_-_-_-_-__-- 265

Geologic history.____._..__________.__._____._._-._--_._-_--___ 266

Future volcanic activity..__________-_-____-____--_-_---_------. 266

References cited.__--_____________________________--------_-_-_____ 266

ILLUSTKAT10NS

[Plates are in pocket]

Page

PLATE 36. Geologic map of Davidof and Khvostof Islands and vicinity,

Aleutian Islands, Alaska.____________________________ In pocket

37. Geologic map of Segula Island, Aleutian Islands, Alaska. _ _ In pocket

38. Segula Island, aerial view..___________________________ Facing 260

FIGURE 48. Location of Khvostof, Davidof, and Segula Islands____... 258

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGY OF SEGULA, DAVIDOF, AND KHVOSTOF 

ISLANDS, ALASKA

By W. H. NELSON

ABSTRACT

Segula, Davidof, and Khvostof are volcanic islands that rise from an exten-- 

sive submarine platform about 350 feet below sea level. The formation of this 

platform by erosion is the oldest geologic event recorded here. The subsequent 

history is largely a record of eruptions of andesitic lava and pyroclastic material 

which formed Segula Volcano and a hypothetical volcanic mountain immediately 

north of Davidof Island. A catastrophic eruption, perhaps during late Tertiary 

time, destroyed this mountain, formed the caldera north of Davidof Island and 

left Davidof and Khvostof Islands as remnants. The most recently erupted 

lava and pyroclastic materials on Segula Island are young enough to have been 

virtually untouched by erosion. Deposits of reworked volcanic debris underlie 

small areas of these islands.

INTRODUCTION

Segula, Davidof, and Khvostof Islands are part of the Kat Islands^ 

near the center of the western half of the Aleutian Islands chain (fig. 

48), and lie between 51°57' and 52°03' north latitude and 178°04' and 

178°31' east longitude.

Keconnaissance, upon which this report is based, was done during 

3 days in August 1951. Landings on Segula Island and Davidof 

Island were supplemented by observations from shipboard. Kichard 

Q. Lewis, Dennis P. Cox, and Edward C. Stoever participated in the 

fieldwork.

GEOGRAPHY

TOPOGBAPHY 

SEGULA ISLAND

Segula Island is formed by a single conical mountain 3,800 feet high 

and about 4 miles in diameter. The slopes curve relatively smoothly 

from the summit to sea level. A cinder cone forms the highest point 

on the island. It covers part of the rim of a poorly defined crater, 

about one-half mile across, on the south; younger lava flows have 

poured over and partly obscured the crater on the north.

257
258 INVESTIGATION OF ALASKAN VOLCANOES

so.

Kiska Island! / Segula Isl

BERING SEA

Area of 

plate 37

Area of

plate 36 Semisopoch'hoi Island

51° 30'"

K Khvosto f Islan fo.i

-> 3avTdof TTT-I blaQi LittleSitkin 

| s,and

.Amchitka Island

RAT ISLANDS

PACIFIC 0 C BAN

FIQURB 48. Location of Khvostof, Davidof, and Segula Islands, Alaska.
GEOLOGY OF SEGULA, DAVIDOF, AND KHVOSTOF ISLANDS, ALASKA 259

An extensive lava field covers the northern slope, and a layer of 

pyroclastic material the southern and eastern slopes. Steep cliffs 

along much of the western side expose the layered internal structure of 

the mountain. A deep gully extends from the summit down the south 

slope of the mountain. A small lava field extends to the coast from 

its place of origin at a small subsidiary cone and crater about a mile 

southeast of the summit.

The entrance of a single narrow cove on the north coast of the island 

is nearly closed by shallow reefs. Landings can be made in favorable 

weather on narrow beaches around most of the island, but steep sea 

cliffs almost everywhere back of the beaches make inland access 

difficult.

Segula has no perennial streams; although the turf blanket is commonly saturated with moisture, the underlying volcanic material is 

very permeable and allows much of the surface water to seep downward.

KHVOSTOF ISLAND AND DAVIDOF ISLAND

Khvostof has a rudely rectangular outline, and consists of a gently 

sloping central upland that rises westward to the high point of the 

island, about 850 feet above sea level. There are several small lakes 

and streams on the island.

Davidof is very irregular in outline and has a maximum elevation 

of about 1,050 feet. Most of its surface is steep, and it is completely 

bounded by very steep sea cliffs. There are no lakes or streams, but 

small quantities of water probably could be obtained from seepage out 

of the water-soaked turf blanket.

SUBMARINE TOPOGRAPHY

Davidof and Khvostof Islands are the highest parts of two broad, 

submerged, coalesced cones that rise from a platform about 350 feet 

below sea level (pi. 36). Davidof, Pyramid, and Lopy Islands are 

parts of the rim of a submerged caldera on the eastern cone. Khvostof 

is the subaerial part of the western cone. About 2 miles south of 

Khvostof, a flat-topped submarine knob, about 2Y5 feet below sea level, 

may be another small volcanic cone.

Besides the caldera, several other submarine depressions are shown 

on plate 36. One, about a mile west of Khvostof Island, is probably 

surrounded by lava flows. Depressions northeast of Davidof Island 

are probably of tectonic origin.

CLIMATE

The Rat Islands have a cool, wet, windy, marine climate, as does 

the entire Aleutian region. Temperatures recorded at Amchitka from 

February 1943 through April 1949 (Arctic Weather Central, 1950)
260 INVESTIGATION OF ALASKAN VOLCANOES

ranged between 65°F and 15°F and averaged 39°F. August, the 

warmest month, had a mean temperature of 48°F, and January, the 

coldest, a mean temperature of 31 °F. The annual precipitation averaged 34.6 inches and included 70.6 inches of snow. Precipitation 

during August, the wettest month, averaged 4.32 inches, and during 

February, the driest month, 2.76 inches. Snow can be expected from 

October through May, the greatest amount falling in February, when 

snowfall averages 15.5 inches. Snow accumulates only at higher 

elevations because winter temperatures close to sea level allow the 

snow to melt. Fog and cloud are nearly always to be reckoned with. 

The Rat Islands are in the path of major storms that frequently 

bring winds in excess of 100 miles per hour. Winds exceeding 75 

mph occur on all the islands from 2 to 15 times a year, and winds 

exceeding 32 mph occur about 80 times a year. Over 90 percent of the 

strong winds arrive during the 6-month period, October through 

March. The maximum wind velocity recorded at Amchitka is 115 

mph. Average wind velocities range between 31 mph in December and 

19 mph in June.

VEGETATION

Stable slopes below about 1,500 feet are covered with a deep, watersoaked carpet of lush vegetation underlain by a thick mat of peaty plant 

remains. No trees or shrubs grow in these islands. The vegetation 

is chiefly composed of moss, lichens, and plants of the heath family. 

 Grasses and sedges abound, especially along the beaches, where thick 

stands of wild rye or strand wheat attain heights of 4 or 5 feet. Fungi, 

liverworts, horsetails, clubmosses, ferns and a wide variety of herbaceous flowering plants add to the variety of the vegetation. Prominent among the flowering plants are the white-flowered narcissus 

anemone, lupines, and several species of orchids. Luxuriant growths 

of marine algae or seaweed are abundant along the shores and on rocky 

reefs .between high tide and about 240 feet below sea level.

Segula Volcano and the caldera north of Davidof Island are two 

 of a row of volcanoes that extends eastward from Kiska Volcano at 

least to Semisopochnoi Island, and possibly as far as the Andreanof 

Islands (fig. 48), and that probably marks an extended zone of weakness in the ocean floor. Segula Island is essentially a single composite 

volcano composed of many superimposed and interbedded layers of 

lava and pyroclastic rocks all dipping radially outward from the
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 38

SEGULA ISLAND 

Aerial view. Photograph by Air Force Photographic and Charting Service (MATS), United States Air Force.
GEOLOGY OF SEGULA, DAVIDOF, AND KHVOSTOF ISLANDS, ALASKA 261

center of the island (pis. 37 and 38). Davidof, Khvostof, Pyramid 

and Lopy Islands are made up of interbedded lava and pyroclastic 

deposits in every way similar to the rocks of Segula Island.

There is no fossil evidence on which to assign an age to these rocks. 

It is inferred that the Segula Volcano is of late Quaternary age from 

the absence of any evidence of glaciation and the small amount of 

destruction of the volcanic cone by marine erosion. The rocks of 

Khvostof and Davidof are somewhat older than the youngest lava 

flows on Segula, but probably are about the same age as the main body 

of Segula.

The submerged caldera north of Davidof and the linear topographic 

feature that crosses Segula Island in a north-northwesterward direction are structural features formed by volcanism. Except for these, 

there has been little structural disruption and most of the rocks probably have their original depositional attitudes.

COMPOSITE VOLCANIC MATEBIAL

Interbedded lava and pyroclastic material can be seen wherever 

erosion has exposed thick sections, especially in the sea cliffs that 

bound all the islands and in the riftlike gully on the south side of 

Segula Volcano. The lavas accumulated as many small, lenticular 

tongues and sheets from a few to about 100 feet thick. The pyroclastic 

layers are comparable in thickness to the lava flows but individual 

units cover much wider areas.

SURFACE PYROCLASTIC LAYERS

A pyroclastic mantle, lithologically similar to the older pyroclastic 

deposits, forms constructional surfaces on much of Segula Island, 

and possibly Davidof and Khvostof Islands. On Segula the pyroclastic mantle is largely covered at lower elevations by vegetation, 

but light-gray pyroclastic debris at least 150 feet thick is well exposed 

at Gray Point on the east side of Davidof Island. The ash mantle 

subdues or obscures older topography, and can be recognized by its 

relatively smooth surf ace on aerial photographs, even though much 

of it is covered by vegetation.

The pyroclastic fragments range in size from fine sand to large 

blocks of lava several feet across, but fragments between one-eighth 

inch and 2 or 3 inches predominate. The fragments vary from pumiceous and scoriaceous to massive andesite and glass, and may be red, 

brown, gray, tan, or black. The ejecta of individual eruptions usually 

abound in fragments of one color and, less consistently, of one texture, 

at any particular point of observation.
262 INVESTIGATION OF ALASKAN VOLCANOES

SURFACE LAVA FLOWS 

UNALTERKD FLOWS

Eecent lava flows analogous to the older lavas cover about onequarter of the surface of Segula Island. On the north slope of Segula. 

Volcano, lava apparently issued from several openings within thesummit crater and from a fissure that extends northward for about 

one third of a mile from a place 3,200 feet due north of the summit. 

This field of lava is formed of many separate flow tongues. Smaller 

lava flows that reach the sea along the southeast shore of Segula 

Island issued from vents in two small coalesced subsidiary conesabout 1,200 feet above sea level. At Chugul Point two recent flowslie beneath a thin cover of volcanic ash. Parts of Davidof and 

Khvostof appear to have constructional topography although none of 

the volcanic material there is recent enough to be free of somevegetation.

Many of the flows are covered by vegetation, but can be identified 

on the aerial photographs by their lobate form, natural dikes or 

levees, and flow-ribbed surfaces.

Basal parts of many flows consist of brick-red breccia. The flowsare generally dark gray, glassy or fine-grained, compact or diktytaxitic. Phenocrysts of feldspar, pyroxene, and olivine are common. 

The unaltered, relatively young lava flows are petrographically similar. Seven specimens from Segula Island and one from Davidof 

Island were studied petrographically and two were chemically 

analyzed. All are porphyritic and feldspathic. Phenocrysts of bytownite, clinopyroxene (augite), olivine, and in some specimens,, 

orthopyroxene, are in a groundmass of labradorite, pyroxene, olivine,. 

magnetite, and cryptocrystalline material. Maximum refractive indices of bytownite phenocrysts are between 1.576 and 1.578. Labradorite phenocrysts in a specimen from Segula Island were found tohave nZ=1.564.

Most of the plagioclase phenocrysts have normal zoning. Small 

poikilitic inclusions of mafic minerals and amorphous material are 

common, generally in only one part of the phenocryst, either the core 

or one of the zones. All the specimens contain phenocrysts of monoclinic pyroxene. In one specimen the monoclinic pyroxene has a 2V 

of 54° and a nZt of about 1.695, indicating that this pyroxene is 

augite. The phenocrysts are euhedral to subhedral, usually twinned, 

and range from 0.2 to 1.5 millimeters long. They make up about 5 

to 15 percent of the volume of the rock.

In four of the specimens some of the orthohombic pyroxene phenocrysts are rimmed by monoclinic pyroxene, although many are not.
GEOLOGY OF SEGULA, DAVIDOF, AND KHVOSTOF ISLANDS, ALASKA 263

Analyses of porphyritic feldspathic lavas of Segula volcano 

[Chemical an&lyses by L. Kebl; spectrographic analyses by P. R. Barnett, June 1964]

Chemical analyses: 

Si02 __ .......

A12 03 ------

Fe203 _. ------

FeO.. .......

MgO__ . --.--. 

CaO_. .......

Na2 0. .--.-..

H2O-.

TiO2 

C02 _ 

P20,_

MnO__ 

BaO_. -

62. 64

16. 22

2.29

4.33

1.72

4.62

4.86

1.86

.02

.00

.88

.01

.34

.04

. 18

.04

Total..-----------------------____ 100. 05

less O.._._______________________ .02

Spectographic ^analyses: 

B_ ..----------

Co_

Cr_

Cu.

Ga_

Li_

Ni_

Rb_

So_

Sr_

V..

Yb 

Zr_

Norm:

or. 

ab_

fs._ 

en. 

mt.

ap.

100. 03

0.005

.06

.001

.0003

.003

.001

.001

.0

.008

.002

.04

.008

.008

, 0005

.02

14.58

11. 12

40. 87

16.96

1.28

4.88

4.30

3.25

1.67

1.01

C. I. P. W. classification.....-.___.._._._____ IL(4)5-(2)3-4

andose 

Rittmann classification.._:____________ dacite

Older flow of

divine-bearing

basalt *

51.21

18.91

2.70

6.21

5.81

11.24

2.57

.66

.00

.00

.60

.01

. 14

.01

. 17

.02

100. 26

.00

100. 26

0.0

.03

.004

.008

.02

.0009

.0002

.003

.0

.004

.08

.03

.006

.0004

.005

1.32

3.89

21.48

38.09

16. 82

8.32

14.50

3.94

1.22

.34

hessase

labradoriteandesite

i 52°02'25" N. lat., 178°07'23" E. long, 

l" N. lat., 178°07'04" E. long.
264 INVESTIGATION OF ALASKAN VOLCANOES

These phenocrysts are euhedral to subhedral, range from 0.2 to 1.5 

mm in length, and make up a small percentage of the rock;.

Olivine phenocrysts were seen in six of the specimens studied. They 

are anhedral, vary from 0.5 by 2 mm in length, and make up 1 to 5 

percent of the rock volume.

The groundmass of the lava is composed of plagioclase, pyroxene,, 

magnetite, cryptocrystalline material, and olivine in some flows. .The 

plagioclase commonly forms fine lathlike crystals, with maximum 

extinction angles of from 28° to 32°, indicating compositions of about 

A.n55. Most of these plagioclase crystals have narrow normally zoned 

rims. The other groundmass minerals occur as small euhedral 

grains.

ALTERED VOLCANIC ROCKS

Much of the rock on Lopy Island and on the north end of Davidof 

Island has been intensely altered to bright yellow, orange, and brown,, 

probably by hydrothermal solutions. The most highly altered rock 

is mapped on plate 36. In a specimen from an altered flow on Davidof 

Island the only minerals that retain their original appearance are the. 

plagioclase phenocrysts; the mafic phenocrysts have all been altered 

to chlorite, or chlorite and magnetite, and the groundmass to chloritecryptocrystalline quartz, and magnetite. The quartz, which .now 

makes up 10 to 15 percent of the rock volume, was probably introduced, 

during the alteration.

SEDIMENTARY DEPOSITS ON SEGULA ISLAND

Deposits composed of more or less reworked volcanic debris underliesmall areas on Segula Island.

Unconsolidated rudely sorted, moderately well bedded volcanic 

detritus (mostly pyroclastic) at least 500 feet thick crops out around 

the cove on the north coast. The fragments resemble those elsewhere 

interbedded with flows and range in size from fine sand to blocks of" 

lava several feet across. Sizes between one-eighth inch and 3 inches, 

predominate. Although the material is unconsolidated it stands in 

steep slopes where eroded at sea cliffs. The stratigraphic position of 

this deposit is uncertain, but its poor state of consolidation, as well as, 

the fact that it is composed mostly of pyroclastic material that could, 

have been derived from the volcano above suggests that it is younger 

than much of the composite cone material.

Large amounts of rock dislodged from the composite volcanic 

material on the west side of the island form landslide cones along the 

coast.

Poorly sorted volcanic debris fills a channel at the mouth of the: 

large gully on the south shore.
GEOLOGY OF SEGULA, DAVIDOF, AND KHVOSTOF ISLANDS, ALASKA 265

Wave-rounded boulders mixed with minor amounts of fine gravel 

form beaches along parts of the coast. The beach at the back of the 

cove on the north coast is the only one that contains appreciable 

quantities of sand.

STRUCTURE 

SEGULA ISLAND

Evidence of tilting or warping of rocks on Segula Island is lacking, 

but some topographic features suggest volcanic rifts. The deep gully 

on the southeast slope of the volcano, the summit crater, and the cove 

on the north coast form a linear topographic depression that transects 

the island. A recent flow apparently fills parts of this depression on 

the north slope of Segula Volcano. Kecent constructional land surfaces on either side of the rift do not appear to be displaced, and the 

cuspate shape of the sides of the gully suggests that a series of explosions or collapses took place at several places along the rift, possibly 

as a result of the stoping action of molten lavas within or below the 

cone.

Such volcanic rifts, without displacement, are common in the 

Aleutian Islands. The summit of Kana-ga Volcano is cut by a linear 

series of depressions associated with active fumaroles; these depressions occur within the crater and on the outer flanks of the cone but 

the crater rim itself is not broken (Snyder, G. L., oral communication). A curvilinear series of thirteen separate craters and four 

small lava flows occur on the southeast flank of Gareloi Volcano as a 

result of an eruption in 1929. (Coats, in preparation, 1956.) Several 

series of alined explosion craters and small pyroclastic cones occur on 

the northwest flank of Makushin Volcano (Fraser, G. D. oral 

communication).

Another possible fissure or rift extends northward from a point 

3,200 feet north of the summit of Segula Volcano, and appears to have 

provided the conduits which feed some of the recent flows. Another 

fissure or rift is suggested by a topographic high near the source of the 

recent lava flows on the north flank of Segula Volcano.

DAVIDOF ISLAND AND KHVOSTOF ISLAND

Attitudes of some lava flows and pyroclastic layers on Davidof 

Island and Khvostof Island are shown on plate 36. The dips are 

mostly gentle or moderate and generally to the southeast and east.

A large submarine caldera lies between the two islands. A lava 

flow that forms a dip slope extending southward from the highest 

point on Davidof Island may have originated from an undiscovered 

vent near the present crest of the island; its appearance suggests,
266 INVESTIGATION OF ALASKAN VOLCANOES

however, that it flowed down the slopes of a volcanic mountain that 

no longer exists. Such a mountain probably was present immediately 

north of Davidof Island prior to the eruption that formed the 

caldera there.

The two submarine depressions northeast of Davidof Island are believed to be at least partly due to faulting. Steep interior north rims 

suggest fault scarps.

GEOLOGIC HISTORY

The oldest recorded geologic event in the Segula, Davidof, Khvostof 

area was the development of an extensive flat platform, now about 350 

feet below sea level; probably the platform was formed by erosion in 

middle Tertiary time. Subsequently, volcanism built the islands and 

the submerged knobs and depressions on the submarine platform. 

One of the later events in the Davidof-Khvostof area was the catastrophic eruption that resulted in the formation of the caldera north of 

Davidof Island. This eruption probably occurred during later Tertiary time. The caldera, which is northwest of Davidof Island, appears to have been little modified since its formation.

FUTURE VOLCANIC ACTIVITY

Although Segula Volcano and the caldera north of Davidof Island 

are now quiescent, a major eruption of lava, ash, or both can occur at 

any time. Segula Volcano has erupted intermittently until recent 

times as proven by the presence of flows and pyroclastic materials, 

which can not be more than a few hundreds of years.

REFERENCES CITED

Arctic Weather Central, 1950, Climate, weather and flying conditions of Alaska 

and eastern Siberia: Arctic Weather Central, llth Weather Squadron, Elmendorf Air Force Base, Alaska.

Coats, B. R., 1959, A geologic reconnaissance of Gareloi Island, Alaska: U. S. 

Geol. Survey Bull., 1028-J, p. 249-256.

a. S. GOVERNMENT PRINTING OFFICE, I9B9