. Ocean Floor Structures 

* Northeastern Rat Islands 

Alaska

* By GEORGE L. SNYDER

INVESTIGATIONS OF ALASKAN VOLCANOES

* GEOLOGICAL SURVEY BULLETIN 1028-G

*> Prepared in cooperation with the 

, Departments of the Army, Navy, and 

Air Force

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957
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 55 cents (paper cover)

4* 

4,
PREFACE

The U. S. Geological Survey, in response to the October 1945 request of the War Department (now Department of the Army), made 

a reconnaissance during 1946-54 of volcanic activity in the Aleutian 

Islands-Alaska Peninsula area. Results of the first year's research, 

field, and laboratory work were hastily assembled as two administrative reports to the War Department. Some of the early findings, 

as recorded by Robert R. Coats, were published in Bulletin 974-B

(1950), Volcanic activity in the Aleutian arc, and in Bulletin 989-A

(1951), Geology of Buldir Island, Aleutian Islands, Alaska.

Unpublished results of the early work and all data gathered in 

later studies are being published as separate chapters of Bulletin 

1028.

The investigations of 1946 were financed principally 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 jointly furnished financial and logistic assistance.

nx

423218 57

CONTENTS

Page 

Abstract....___.________________________________ 161

Introduction. _______________________________________________ 161

Classification of submarine topography_____________________ 164

Literature cited._________________________________________ 167

ILLUSTKATIONS

Page 

PLATE 22. Topography of the ocean floor near the northeastern Rat

Islands, Alaska_________________________ In pocket

FIGURE 37. Index map of Rat Islands, Alaska. ______________ 162 

38. Sounding-density control for submarine topography shown

on plate 22. _________________________ 163
4 f
INVESTIGATIONS OF ALASKAN VOLCANOES

OCEAN FLOOR STRUCTURES, NORTHEASTERN RAT 

ISLANDS, ALASKA

By GEORGE L. SNYDER

ABSTRACT

Configuration of the ocean floor near the northeastern Rat Islands, presented 

here by depth contours at 100-foot intervals, reveals many volcanic, sedimentary, and tectonic structures; and erosional features. The most striking 

! submarine feature is a 35-mile long scarp between .'Little Sitkin and Semisopochnoi Islands which can be traced into a subaerial caldera fault system on 

Little Sitkin Island. ' -. . ' ' :

INTRODUCTION ;,

The northeastern1 Rat Islands are in the Aleutian chain immediately 

west of the 180th meridian (fig. 37). Six large islands, Segula, 

,Khvostof, Davidof, Little Sitkin, Semisopochnoi, and Rat, and^five 

small islands, Pyramid, Lopy, Tanadak, McArthur Reef and Sea 

Lion Rock, are within the area studied. Prewar native fox trappers 

and wartime military, sentries were, the last temporary inhabitants 

of the northeastern Rat Islands and, today, the nearest permanent 

human habitations are more than 200 miles away on the Near and 

Andreanof Islands. ' "' ' .,. :

During preparation of the Little Sitkin Island geologic map (Snyder, in preparation) it was-observed that a prominent scarp on the 

ocean floor is correlative with several branches of a subaerial caldera 

fault. This scarp was traced further in the submarine topography. 

All submarine topography .of the northeastern Rat Islands was then 

compiled on one map (pi. 22)';' the1" description and interpretation of 

this map is the. substance of this report.

The data upon which plate 22 is based were taken from published 

topographic maps of the Corps of-Engineers, from one topographic 

manuscript, and from 18 smooth sheets (sounding charts) of the 

Coast and Geodetic Survey.. The original scales of the maps.and 

smooth sheets ranged from 1:20,000 to 1:100,000. A key to density 

of the sounding data (number of soundings per unit area) is provided by figure 38.

161
162 INVESTIGATIONS OF ALASKAN VOLCANOES

177° 178° 

BERING

179° 

S E A

180°

Area of this report

.52* Kiska Island

t^^^^<SS^SS>^^WSSJ^^

p^ Segula Island

\£ Khvostof Island *>.^ 

Oavidol Island

1

Little Sitkin Island

 Semisopochnoi Island

^<SS^^^^y^^s>^^^

AMCHITKA ISLAND

Constantine Harbor

PACIFIC

128°

i_______ -

OCEAN

179°

37. index map of the Aleutian islands, including tbe Kat Islands, and land 

bordering the Bering Sea: index map of the Rat Islands showing areas mapped <m 

plate 22 and figure 38.
EXPLANATION

NUMBER OF SOUNDINGS PER SQUARE STATUTE MILE

15 to 50 ?0 to 100 100 to 150 150 to 200 200 to 300

FIGUBE 38. Sounding-density control for submarine topography shown on plate 22.

8 

i

COIo

a) te)§

H

aa t>

aGO

8

05 

CO
164 INVESTIGATIONS OF ALASKAN VOLCANOES i;1 

CLASSIFICATION OF SUBMARINE TOPOGRAPHY

Unlike some other volcanic structures fringing the Pacific Ocean, 

those of. the Aleutian Islands are hidden beneath the sea. Structural inferences, howeveiymay be drawn from the configuration of 

the sea floor. An excellent small-scale map (approximately 

1:440,000) of the entire Eat Island Group (Gibson and Nichols, 

1953, pi. 1) and available seismic data proved the existence of numerous fault traces on the sea floor of the greater Eat Island area. 

Several of these faults have been plotted on a smaller scale map 

(approximately 1:3,960,000) by Gates and Gibson (1956, fig. 12).

This report is an interpretation of a map (pi. 22) of the submarine 

topography of a small area in the northeastern Eat Islands that 

shows several features overlooked by Gibson and Nichols. Topographic features shown on plate 22 are classified as:

1. Linear troughs and scarps.

2. Meandering or linear canyons.

3. Large, subconical hills.

4. Broad, gently rounded banks.

5. Broad, flat plains.

6. Small, elongate or irregular hills.

7. Irregularly fluted ridge crests.

Linear troughs and scarps. A prominent topographic feature 

shown on plate 22 is an east-west scarp, here named the Prokhoda 

scarp, which extends 35 statute miles eastward from Prokhoda Point 

(51°54' N., 178°31' E.) on Little Sitkin Island to Semisopochnoi 

Island. The faults which branch from the southern part of the 

Caldera no. 2 fault on Little Sitkin Island trend toward the Prokhoda 

scarp (Snyder, in preparation).

The Prokhoda, scarp faces on the north an enormous flat-floored 

depression, here named the Pratt depression, and is backed by a 

curvilinear parallel ridge. The maximum relief is more than 1,250 

feet. The average slope of the scarp in the steeper central part is 

about 15°. The ridge and scarp are broken in the center into two 

segments so that the west half was apparently displaced a half mile 

south. Displacement was probably along a tear fault that trends 

about N. 20° E. This is the only place where the submarine contours 

Seem to express horizontal displacement. Elongate depressions or 

scarps at several places on the south side of the ridge suggest parallel 

faults downthrown on the south. The medial ridge appears to be 

a horst about 11,4 miles wide, The surface of the block south of 

the horst slopes gently south toward the Oglala trough and this 

block has been downdropped to a lesser extent than the block forming the Pratt depression. The extremities of the Prokhoda scarp
OCEAN FLOOR STRUCTURES, NORTHEASTERN RAT ISLANDS, ALASKA 165

disappear in shallow water, but on the western end/there appears 

to tie a landward extension.

' A scarp similar to the Prokhoda scarp extends from a depression 

shown in the .northwest corner of the map, here named the Haycock 

depression, to Oglala Trough, southeast of Little Sitkin Island. 

Kiska' Volcano is alined with this scarp. Off the East Cape of 

Amchitka maximum relief of the scarp is more than 4,650 feet, but 

in "the area shown on figure 38 the relief is only 2,000 .feet. Dissection of this scarp east of Eat Island suggests that it is older than 

the Prokhoda scarp.

Many other small troughs and ridges revealed by the submarine 

contours trend parallel to the Aleutian Crest. West of Little Sitkin 

Island the volcanic centers are. alined about N." 65°-80° W. A possible fault in line'with!this trend is estimated by Gates and Gibson 

:(1956, figs. 12. and 13) to dip about 60° north. A scarp that trends 

N. 65: ° W. off Finger Point is alined with fumarolic centers on Little 

Sitkin Island (Snyder, in preparation).

1 A .fissure. trending N. 20° W. bisects Segula Island, but is not 

expressed in the submarine topography (Nelson, in preparation). 

A sca'rp that begins at the western end of Oglala Trough, trending 

NV 13° W., apparently represents a fault that curves through Little 

Sitkin'Pass into the upper reaches of Sitkin Canyon and down another canyon on the north slope which trends N. 14° E. Another 

long, curving structure that begins with a south-facing scarp that 

itrends'N. 65° W. north of Gunners Cove, Eat Island, and continues 

eastward1 along the north edge of Oglala Trough is represented southeast;^ fcit.tle Sitk^ by, a curving (N. 70 6 E. to;N. 50° E.): 

string of five closed depressions and other topographic irregularities 

which may represent a fault dipping steeply under. Little Sitkin Island.' OtHer, smaller faults of diverse orientation probably exist 

in the complicated topography north of Oglala Trough. 

! Most of the submarine'ridges and troughs around Semisopochnoi 

Island radiate from the island. Ashore, numerous volcanic vents 

are grouped along strong iones of crustal weakness thai; trend almost 

perpendicular to Petrel Bank to the northeast (Coats, .E. E., written 

communication). A bank northwest of Semisopochnoi Island, here 

named Tuman Bank (52°03' N., 179°33' E.), is bounded on the northfeast ity; a' series of structural lineaments trending about. N. 30° E.; 

tile /ijiprthwest margin is marked by a series of'troughs and ridges 

trending;1 N. 10° E. wtiich resemble hogbacks with!'the steep slope 

fading east,andthe/gentleslope facing.west. '/ : , . 

\ fyetinderihg or linear canyons. Canyons such as iSegiila Canyon, 

SK, -. -i .. ^rollg}j- jgjtkin Canyon, the small canyons on the scarp east
166 INVESTIGATIONS OF ALASKAN VOLCANOES

' ' ' - . of Rat Island, and those radiating from the southern flank of Semisopochnoi probably have a complex origin. There are many suggestions of structural control, which have already been cited, but 

there are also suggestions of submarine erosion, probably by turbidity currents. Although Sitkin Canyon lacks large branches suggestive of tributaries (Gibson and Nichols, 1953, p. 1183), its 

resemblance to an erosional canyon is quite striking. Slip-off slopes 

and undercut banks are expressed well by the contours. Here and 

elsewhere in the area the flat floors and unbroken longitudinal profiles 

suggest the channeling of sediment-laden submarine currents. Probably fault scarps or fault zones in many of the canyons control the 

course of submarine "drainage" in a manner similar to that of their 

subaerial counterparts.

Large, subconical hills. Large subconical mounds from several 

hundred to several thousand feet high probably represent composite 

cones built by volcanic extrusions. Kay Sea Cone, Williams Crater, 

McArthur Reef, the two cones in Segula Canyon (51°57' N., 178°07' 

E., and 52°00' N., 177°55' E.), and cones southeast of Semisopochnoi 

(51°51' N., 179°50' E.; 51°48' N., 179°46' E.; and 51°50' K, 179°40' 

E.) are examples of smaller piles. The low mound south of Khvostof 

Island (51°56' .N., 178°17' E.) may also be a small truncated cone 

(Nelson, in preparation). The platform-type structures supporting Segula, Davidof, Khvostof, Little Sitkin, Kiska to the west of 

the area, and. Semispochnoi, are examples of larger piles.

Broad, gently rounded banks. Two large banks shown on plate 

22 are the Aleutian Crest and Petrel Bank. The Aleutian Crest is 

a long, broad ridge extending from Rat Island to Tanadak Island 

and beyond. Petrel Bank is part of Bowers Bank which is a long 

curving shallow platform that extends northeastward from Semisopochnoi Island; it was charted first by the Coast Guard cutter 

Chelan in 193'5 > (Hutchins, 1937, p. 543). These banks are regarded 

as "tectonic constructions" that is, crustal blocks which were lifted 

to their present positions by tectonic forces. This belief is based 

on the presence of submarine lavas and tuffs on Rat Island, and on 

Amchltka Island to the south of this area, above 1,000 feet altitude 

(Powers, H. A., written communication, and Gates and Gibson, 1956, 

p. 132). ..,.

Broad) ftafplffiiris' The flat floors of canyons or basins-shown-on 

figure 38, Segula Canyon, Sitkin Canyon, Oglala Trough, Pochnoi 

Sea Valley, Pratt depression and the basin beneath Little Sitkin 

Pass, probably are the result of sedimentary fill. The mechanism 

of transportation and deposition of this fill could be that, ot-turbidity currents carrying material supplied partly by erosion and
OCEAN FLOOR STRUCTURES, NORTHEASTERN RAT ISLANDS, ALASKA 167

partly by volcanic eruption. The large, nearly flat area in the sector 

northeast of Kay Sea Cone contains five small conical hills, each 

paired with a neighboring depression probably created by compaction of bottom muds by the weight of the nearby volcanic pile.

Small, elongate or irregular hills. The elongate hills shown on 

figure 38, such as the hills between Williams Crater and Kay Sea 

Cone, are believed to be lava flows or shallow laccoliths that bowed 

up small amounts of overlying mud. (A few may be asymmetric 

cones.) The irregular topography southeast of Little Sitkin probably is due partly to submarine lava flows. The elongate hills are 

from 1 to 2 miles long and a- maximum of 500 feet high. Igneous 

bodies of similar dimensions are found in sea-cliff exposures along 

the southern coast of Unalaska Island. Because these bodies, which 

are associated with pillow lavas, are intrusions in argillaceous rocks, 

it is believed that they were originally injected into unconsolidated 

bottom muds. On Unalaska these sills or laccoliths have demonstrably 

bowed up the overlying strata. It is possible that originally they 

resembled the mounds shown on figure 38. The elongate hills are 

found in areas susceptible to fissuring, such as southeast of Little 

Sitkin Island, and north of Kay Sea Cone along the east-west zone 

which is the hinge line of the downdropped Pratt depression block.

Irregularly fluted ridge crests, Complex fluted topography is 

characteristic of the inshore areas of all islands and,also is found 

on the summits of the Aleutian Crest, Tuman Bank, and Petrel Bank. 

This complex topography probably was formed by submarine erosional forces such as waves and powerful tidal currents, or by subaerial erosional forces during times of lowered sea level at glacial 

maxima. Because a greater number of soundings per unit area was 

taken here than in the offshore areas it cannot be stated definitely, 

as yet, to what extent the submarine inshore topography is more 

complex than that of the offshore areas.

LITERATURE CITED

Gates, Olcott, and Gibson, William, 1956, Interpretation of the configuration 

of the Aleutian Ridge: Geol. Soc. America Bull., v. 67, figs. 12, 13, p. 132.

Gibson, W. M., and Nichols, Haven, 1953, Configuration of the Aleutian Ridge 

Rat Islands-Semisopochnoi Island to west of Buldir Island: Geol. Soc. 

America Bull., v. 64, pp. 1173-1188, pis. 1, 2.

Hutchins. I. W., 1937, Discovery of a new reef near Attu Island: Geog. Jour. 

(London), v. 90, p. 543.

Nelson, W. H., in preparation, Geology of Segula, Davidof and Khvostof Islands, Alaska : U. S. Geol. Survey Bull. 1028.

Snyder, G. L., in preparation, Geology of Little Sitkin Island, Alaska: U. S. 

Geol. Survey Bull. 1028.

U. S. GOVERNMENT PRINTING OFFICE: 1987