Geology of 

Little Sitkin Island

By G. L. SNYDER

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-H

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

The U. S. Geological Survey Library has cataloged this publication as follows :

Snyder, George Leonard, 1927-

Geology of Little Sitkin Island, Alaska. Washington, 

U. S. Govt. Print. Off., 1958. -

vi, 169-210 p. illus., 2 maps (1 fold. col. in pocket) tables. 24 cm. 

(U. S. Geological Survey. Bulletin 1028-H. Investigations of Alaskan volcanoes)

Prepared in cooperation with the Departments of the Army, Navy, 

and Air Force.

"References cited" : p. 206-207.

1. Rocks, Igneous. 2. Petrology Aleutian Islands. 3. Little Sitkin 

Island. I. Title. (Series: U. S. Geological Survey. Bulletin 

1028-H. Series: U. S. Geological Survey. Investigations of Alaskau 

volcanoes)

557.98

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 Buldir Island, Aleutian Islands, Alaska, both by Robert R. Coats. 

Unpublished results of the early work and all the later studies are 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

CONTENTS

	Page

Abstract _.___...^-. .-^_.__.._-.-.----. . ..-._________ 169

Introduction. ____________ _________ _________ ___________ 169

Climate.______^_^______^_..___ ....................... 170

Vegetation, wild life, and human habitation__-____________________ 170

Physiography. _________ ______________________________ 172

Fieldwork and acknowledgments._ __-_-_--_-___-_.____________ 173

Description of rock units.___.i_-_____________--_--___-__-____-____-- 173

Williwaw Cove formation _ __________ 173

Sitkin Point formation______-__^__________-____________________ 175

East Point formation... -____.____- 177

Double Point dacite__._____-_______ .____________ 177

Patterson Point formation ___ __ .___ ________ 180

Little Sitkin dacite__-_---------_----_-----------------_--____- 181

Surficial deposits.___--_-_-_-_____-_---__----___-_-----_____-_- 1 183

Structural geology.________________________________________________ 184

Petrography._____________----_-_____-___--_-----__--__-___-____-_ 186

Geochemistry-_________-__------_-__---__-__----_---_-_-_-______-- 193

Geologic history..-_______-__---_--__-__--_-----_-_-_-----__-_-___- 202

Economic geology_______--_-_-_-_-__-____--_--_-___-__--___-__-___ 205

References cited.____-_-_-__---_---_--_-__-__--------------_---__-- 206

______-__---_--_----------------_---_--------------------- 209

ILLUSTRATIONS

Page 

PLATE 23. Geologic map and sections of Little Sitkin Island, Alaska.. In pocket

24. Vertical aerial photograph of Little Sitkin Island. ____ facing 170

25. Andesite inclusion in dacite lava of Double Point dacite_ facing 171

26. Graph showing time, composition, and volume of rock formations of Little Sitkin Island...____________ facing 188 

FIGURE 39. Index map of Little Sitkin Island area_____________ 171

40. Drawing of cliff exposure of tuff-breccia in Sitkin Point formation.._ ___.______________________ 176

41. Diagrammatic view of unconformity on Sitkin Point____ 177

42. Variation diagram of rocks of Little Sitkin Island_________ 197

43. Diagrammatic review of geologic history of Little Sitkin Island. 203
VI CONTENTS

TABLES

Page 

TABLE 1. Summary of petrographic characteristics of volcanic rocks on

Little Sitkin Island------ .__-__..____.________ 187

2. Chemical composition, normative values, and Rittmann nomenclature for volcanic rocks on Little Sitkin Island-___________ 194

3. Volume, weight, and percent weight for volcanic rocks of Little

Sitkin Island,______.______________________ 199

4. Weighted chemical compositions for volcanic rocks of Little

Sitkin Island...___.. ______________._ _.-__ 200
INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA

By G. L. SNYDER

ABSTRACT

Little Sitkin Island, in the Bat Islands group of the Aleutian Islands, is 

composed of extrusive igneous rocks ranging in age from late Tertiary (?) to 

Eecent. The youngest rocks are about half a century old. The island is in a 

calcic petrographic province and has an alkali-lime index of 63.5. Composition 

of lavas and pyroclastic debris ranges from basalt to rhyodacite, but most rocks 

are andesite, bandaite, or dacite. Three main periods of volcanism were separated by periods of caldera formation. Lavas and pyroclastic debris of the 

first volcanic period were dominantly andesite and originally constituted 78 

percent by weight of all extrusive rocks on Little Sitkin. Lavas of the second 

period were high-silica dacite containing many andesite inclusions and originally 

constituted 17 percent by weight of all extrusive rocks on Little Sitkin. Lavas 

of the last, and presently active, period are low-silica dacite that now constitutes 4 percent by weight of all extrusive rocks on Little Sitkin. Minor 

amounts of dacitic and andesitic pyroclastic debris were ejected during or 

following each caldera-forming episode.

Considerable heterogeneity of magma type characterizes each extrusive 

epoch. Lavas from the same or adjacent vents differ markedly in composition. 

In one case rhyodacite and low-silica dacite were extruded simultaneously as a 

lava flow from the same vent. Similar phenomena are recorded in layered inhomogeneous volcanic bombs. The lack of local compositional pattern contrasts markedly with the overall chemical interrelationships of the suite of 

rocks on Little Sitkin Island. All oxide constituents of 12 of 13 chemical 

analyses have a remarkably rectilinear relationship with each other when 

plotted against SiOa on a variation diagram. The best explanation of the 

volumetric, temporal, and compositional relationships of lavas on Little Sitkin is 

believed to be as follows: tectonic kneading in the Aleutian welt has mechanically mixed portions of continental and oceanic rock; subsequently magmatic 

inelts, which are local in both space and time, have developed in this mixed 

zone and penetrated to the surface.

INTRODUCTION

Little Sitkin Island is in the Rat Islands group of the Aleutian 

Islands (fig. 39) approximately 48 statute miles northwest of Con169
170 INVESTIGATIONS OF ALASKAN VOLCANOES

stantine Harbor on Amchitka Island and approximately 36 statute 

miles east of Kiska Harbor. The Rat Islands have no permanent 

inhabitants; the nearest human habitations are 200 miles away on 

Shemya Island in the Near Islands group and on Adak Island in the 

Andreanof Islands.

The rocks of Little Sitkin Island are all subaerial lavas and pyroclastic deposits or shallow-water sedimentary deposits derived from 

nearby volcanic rocks. Eight bedrock units have been distinguished 

(pi. 23). The chemical, temporal, and volumetric interrelationships 

of these eight units pose complex problems in petrogenesis.

CLIMATE

Little Sitkin Island has a maritime climate characterized by relatively high humidity and small range in annual temperature. Like 

other Aleutian Islands, Little Sitkin lies along the North Pacific belt 

of storms and thereby receives abundant wind, rain, and fog. Owing 

to its strong relief (3,897 feet) and rugged topography the orographic 

effect is an important factor in local weather conditions.

Short-term weather records from nearby Kiska and Amchitka 

Islands indicate a maximum annual temperature range of only 50° 

F. The mean July temperature falls in the range 44°-49° F. for 

the recorded years, and the mean January temperature falls in the 

range 28°-33° F. for the recorded years. The lowest winter temperature to be expected is 15° F. The annual precipitation ranges from 

30 to 35 inches and the mean annual snowfall is about 70 inches. 

Cloud ceilings average less than 1,000 feet, and visibility is less than 

3 miles during 60 percent of the year and during 83 percent of the 

summer. Winds average 20 to 25 miles per hour, but gales of 70 

to 80 miles per hour are common; gusts as much as 150 miles per hour 

occur (Arctic Weather Central, 1950, p. 1-52; U. S. Coast and Geodetic Survey, 1947, p. 1-147; Weather Central Alaska, 1945, p. 1-59).

A dangerous Aleutian wind phenomenon common on Little Sitkin 

is the "williwaw", a type of downslope wind that results from the 

damming of air on windward slopes followed by a very gusty overflow of air down the leeward slopes. The unfavorable weather impedes field work considerably and is a major logistic and economic 

factor influencing activity on the island.

VEGETATION, WILD LIFE, AND HUMAN HABITATION

Low leafy plants, moss, grass, and sedge grow profusely on Little 

Sitkin below 1,500 feet altitude, annually adding organic debris to 

the blanket of peat on which they grow. The few woody plants are
CKOI.OGICAI, SURVEY BULLETIN 1028 PLATE 21

VERTICAL AERIAL PHOTOGRAPH OF THE CENTRAL PART OF LITTLF, SITKIN ISLAND 

Taken from an altitude of 18,000 feet.
dKnl.oclCAT, SURVEY BULLETIN 102 S PLATE 2»

INCLUSION OF MIAHOr.ITIC ANDES1TE IN 1IIGII-SI1.IC \ 1JACITK LAVA OF DOUBLE POINT DACITE 

Airnws show location of "prppsure-slia<l(ju" cavities. About twice natural size.
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 171

178° 179° 180°

BERING SEA

. 52° Kiska Island

ocy

{*~N Segula Island

Khvostof Island &.j> 

Davidof Island Little Sitkin Island 

Area of plate 23

52° 

00'

Semisopochnoi Island

AMCHITKA ISLAND

51° 30''

Constantine Harbor

177°

PACIFIC

..128°

OCEAN

179° 180°

FIGURE 39. Index map of the Aleutian Islands and land bordering the Bering Sea, showing location of the Rat Islands, and index map of the Rat Islands, showing location of 

Little Sitkin Island.

466927 59 2
172 INVESTIGATIONS OF ALASKAN VOLCANOES

low, creeping types. The vegetal mantle thins at highest elevations, 

and exposed ridges are completely barren.

The animal life consists of the blue fox, a very few sea birds, a few 

insects, and, the marine invertebrate assemblage of the littoral zone. 

In 1930 an estimated 250 blue foxes, kept to this level by native trappers, lived on the island. In 1951 there were considerably fewer 

foxes. The foxes live along the coast and subsist mostly on echinoderms, snails, and sand "fleas". Neither trout nor salmon are 

known to run in the streams. Sea otters, seals, and sea lions are 

sometimes seen offshore but no rookeries are known on the island.

Apparently the native Aleuts who formerly inhabited the Rat 

Islands found Little Sitkin inhospitable, for there are no village 

sites and kitchen middens such as are quite common on the other 

Rat Islands. The Aleuts may have avoided Little Sitkin because of 

its rugged topography, or, more probably, because of its volcanic activity. The Aleut word "sitkin" connotes "excretion", indicating that 

volcanic vents of the island were probably active when the island 

was named.

PHYSIOGRAPHY

The general shape of Little Sitkin is a quadrilateral with rounded 

corners extending toward each of the main compass points. The 

northwest side is fretted by small bays and sharp headlands, but elsewhere the coast is remarkably smooth. The coastline is about 27 

miles long and encloses an area of about 24 square miles. The longest dimension, almost 7 miles, trends northwest.

The land surface is rugged. Six mountains rise more than 1,300 

feet from sea level and the highest reaches 3,897 feet. Along a northward-trending caldera scarp in the center of the island, the slopes are 

very steep or vertical; slopes are also steep at higher elevations on a 

cone of Recent age, and along the sea cliffs that completely surround 

the island. The sea cliffs along the northeastern coast rise precipitously for 2,000 feet or so. Somewhat more than 60 percent of the island has a surface slope exceeding 30 percent, but level land is found 

near the mouths of alluviated valleys, atop some grass-covered lava 

flows, and along benches above sea cliffs.

The topography of Little Sitkin is dominated by constructional 

land forms, such as volcanic cones, lava flows, and blankets of ash. 

Erosive forces are also important in the development of the landscape. 

Running water, wind, waves, frost, and mass wasting have all been 

operative on Little Sitkin. The amount of erosion of a constructional 

form can locally be useful for estimating relative age. In many 

places on Little Sitkin comparison of the dissection of two or more 

volcanic landforms affords an estimate of their relative ages.
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 173 

FIELD WORK "AND ACKNOWLEDGMENTS

A topographic map at & scale of 1:25,000 was published by the 

U. S. Army Corps of Engineers in 1943. The U. S. Coast and Geodetic Survey began work on a 1:20,000 scale topographic map in 

1950. An advance copy of this map was used as the base for the 

present geologic map (pi. 23).

Geologic field work was completed during the summer of 1951 by 

Richard A. Robie and the author. The equivalent of 21 working days 

was spent mapping the island. Carl Vevelstad, and Charles Best 

provided the logistic support necessary for many small boat landings. 

Walter H. Newhouse, Hans Ramberg and Tom F. W. Earth contributed many ideas to an early form of the manuscript which was 

submitted in partial fulfillment of the requirements for the degree of 

Master of Science at the University of Chicago.

DESCRIPTION OP ROCK UNITS

WILLIWAW COVE FORMATION

The formation, here named the Williwaw Cove formation from 

exposures in sea cliffs on the west side of Williwaw Cove, includes the 

oldest rocks of Little Sitkin Island: a thick sequence of dacite, andesite, and basalt flows and associated pyroclastic rocks, widely exposed 

along the north side of the island (see pi. 23). The Williwaw Cove 

formation extends beneath the island, cropping out elsewhere around 

the coast, and forms the base upon which all later volcanic deposits 

accumulated.

Between Finger Point and Williwaw Cove the rocks are light-gray 

to purplish-gray porphyry with phenocrysts of feldspar and 

pyroxene. Vertical sea-cliff exposures, as much as 550 feet high, appear to represent a single flow. A wide bench extending southeastward from the sea cliff suggests that this flow continues inland beneath the 1,303-foot mountain. On the upper part of the mountain 

thinner flows of similar feldspar porphyry are exposed.

Near Patterson Point the lavas vary from light-gray pyroxene 

porphyry to dark-bluish-gray andesite containing sporadic large 

phenocrysts of plagioclase. From Patterson Point along the coast 

to East Point, light- to dark-gray andesite flows contain phenocrysts 

of augite, plagioclase, and sparse hornblende. Half a mile west of 

East Point an outcrop of chloritized andesite tuff-conglomerate is possibly the oldest rock exposed on Little Sitkin Island.

Massive purplish-gray to light-bluish-gray andesite of the Williwaw Cove formation is exposed in a steep 850-foot cliff southeast of 

East Point. It is strikingly like the flow from Williwaw Cove to
174 INVESTIGATIONS OF ALASKAN VOLCANOES

William Cove; possibly the two flows were extruded during a single 

voluminous eruption.

Southwest of Pratt Point, rocks of the Williwaw Cove formation 

vary from light-gray feldspar porphyry to dark-gray pyroxene 

porphyry. Olivine phenocrysts are rare. Local flows, dikes, and 

pyroclastic deposits of basalt contain 30 to 50 percent augite. They 

are the most basic rocks on the island and have the largest content of 

basic megaphenocrysts. The flows capping the 1,405-foot mountain 

and the southern end of the 540-foot hill southwest of it are, however, 

silicic.

The Williwaw Cove formation in the southern part of the island 

contains many varieties of andesitic and basaltic flows and pyroclastic deposits. The flows near the summit of the 1,960-foot mountain are black glassy feldspar andesite, greenish-gray cryptocrystalline andesite, or gray to purplish-gray olivine basalt; near the coast 

they are nonporous, dark-gray plagioclase porphyry, augite basalt, 

or andesite. On Sitkin Point, flows in the Williwaw Cove formation 

have been highly altered by fumarolic activity, locally to a multicolored soft clay. The andesite of the Williwaw Cove formation adjacent to the West Cove member of the Little Sitkin dacite is similarly altered. Pods of chloritized material are preserved locally 

within the f umarolically altered areas.

In summary, the flows of the Williwaw Cove formatioruare andesite 

with minor amounts of basalt and dacite. With these flows, except 

in the northwestern part of the island, are moderate to abundant 

amounts of interbedded pyroclastic debris. The pyroclastic deposits 

average 10 to 50 percent coarse lapilli, bombs, and lithic debris in a 

finer ash matrix and occur in beds, 1 to 40 feet thick, interbedded 

with the flows. The relative proportion of pyroclastic debris to 

flows is much greater in the Williwaw Cove formation than in all 

the succeeding formations. The contact of the Williwaw Cove formation with younger rocks is very irregular; locally it has a relief of 

several thousand feet. The Williwaw Cove rocks are probably late 

Tertiary or early Quaternary judging from their advanced dissection. 

No fossils were found on Little Sitkin Island.

The dikes in the Williwaw Cove formation are here considered a 

part of the formation. Most of them are nearly vertical. Dike 

swarms occur on the north side of the 1,960-foot mountain and along 

the sea cliffs of the southeast coast. The dikes north of the mountain 

radiate crudely from the center of the island. The other dikes are 

not systematically oriented. Isolated dikes crop out east of Prokhoda Point and at two localities on the north coast.
GEOLOGY OP LITTLE SITKIN ISLAND, ALASKA 175

Dikes on the southeast coast are composed of augite basalt, and 

apparently were feeders for augite basalt flows in this area. The 

other dikes are andesite, mostly vesicular, many with the vesicles 

elongated in layers parallel to the walls. Dikes of vesicular porphyritic andesite and massive nonporphyritic andesite crop out side 

by side on the northeast spur of the 1,960-foot mountain and a dike 

of vesicular andesite cuts the volcanic sequence in the west spur. 

Massive, slightly chloritized andesite dikes with zeolite amygdules 

occur on the north coast. Several dikes have crude columnar joints 

normal to the walls and sheeting parallel to the walls.

SITKIN POINT FORMATION

The Sitkin Point formation is here named for a sequence of waterlaid pyroclastic deposits more than 450 feet thick exposed at Sitkin 

Point. The formation is -also exposed along the low saddle on the 

caldera rim north of the 1,960-foot mountain.

On the tip of Sitkin Point the formation consists of semiconsolidated volcanic wacke or graywacke (Williams, Turner, and Gilbert, 

1954, p. 303) containing sparse boulders of gray dacite and sparse 

smaller fragments of white glassy pumice in beds 2 to 20 feet thick 

and averaging about 4 feet thick. Individual beds are massive and 

have well-developed vertical parting or jointing.

The boulders and rock fragments in the graywacke become more 

abundant toward the east. In the upper part of the cliff at the rightangle bend of the coast north of Double Point, 10 to 15 percent of 

the deposit is boulders, and round fragments of black pumice make 

up another 10 to 15 percent. These boulders have no preferred 

orientation.

In the Sitkin Point formation north of the 1,960-foot mountain 

many massive beds of dacite boulder tuff-breccia are exposed in the 

caldera scarp. The rock contains as much as 50 percent -angular 

blocks of massive to pumiceous dacite. The sorting is very poor, but 

the boulders lie parallel to the bedding in some of the upper layers 

(see fig. 40). This preferred orientation is probably due to current 

action in a shallow body of water.

As shown in figure 41, the beds on Sitkin Point lie on inclined subaerial lavas of the Williwaw Cove formation. The beds of tuffaceous 

partly consolidated sedimentary rock rise slightly and thin toward 

the inclined contact. The basal contact is also exposed on the south 

side of West Cove where the contact and beds rise eastward. Similarly the beds in the central area are believed to overlie the Williwaw 

Cove formation unconformably. The deposit is interpreted as subaqueous debris close to the volcanic source. The Sitkin Point for-
176 INVESTIGATIONS OF ALASKAN VOLCANOES

 ^ '. riSV J'^-^^-'-tif-yS_ >< (&} ' -^r-VTcSv ' ' : Vg? 

" ' )^^^^^^^)^^^&ff "-^^S^^^^^ct^ -:r^S^5.^::

*

WOP

40. Cliff exposure of tuff-breccia in the Sitkin Point formation as exposed in vertical face of "Caldera Two" scarp north of the 1,900-foot mountain. The boulders are 

.alined parallel to bedding. Drawn from a photograph.
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 177

FIGDBB 41. Diagrammatic view of unconformity on Sitkin Point looking northwest. 

Drawn from a photograph. A, ash and turf mantle;, B, boulder beach; 0, massive and 

iron stained breccia layers of subaerlal flows of Williwaw Cove formation; Dt horizontally bedded, vertically jointed tuffaceous graywacke of Sitkin Point formation.

mation is probably early Quaternary because it overlies the rocks 

of the Williwaw Cove formation.

EAST POINT FORMATION

The East Point formation is here named for an uninterrupted sequence of andesitic lava flows, over 500 feet thick, exposed at sea 

level on either side of East Point. Two dozen separate flow units 

can be distinguished in the 700-foot sea cliff on the southeast side of 

East Point. The extent of this formation is shown by Collins, Clark 

and Walker (1945, pi. 7) in an excellent aerial oblique photograph.

Lavas of the East Point formation are light- to dark-gray andesite and basalt locally containing plagioclase, pyroxene, and olivine 

phenocrysts. Massive phases predominate, but clinkery flow tops are 

vesicular. At one place near the base the flows are interbedded with 

rhyolite(?) pumice that is probably slopewash deposited between 

flow gushes rather than dissimilar eruptive material contemporaneous 

with the flows.

The East Point formation rests on a very rugged terrain, apparently in a large valley. The basal contact of this formation rises 

750 feet in a half mile on both sides of East Point.

The mutual stratigraphic relations of the East Point formation 

and the Sitkin Point formation are in doubt, but the topography 

of their upper and lower contacts perhaps suggests clues as to their 

relative age. The topography beneath the East Point formation is 

more deeply dissected than that beneath the Sitkin Point formation. The overlying Double Point dacite is unconformable on the 

Sitkin Point formation, but apparently conformable on the East Point 

formation. These relations perhaps suggest that the East Point formation is the younger.

DOUBLE POINT DACITE

The Double Point dacite is the name here given to the dacite flows 

exposed north of Double Point on the southwest coast of Little
178 INVESTIGATIONS OF ALASKAN VOLCANOES

Sitkin Island. These flows form a volcanic pile 3,000 feet high. Although as shown by their chemical composition (p. 194) the rocks are 

classified as high-silica dacite (Eittmann, 1952, p. 93-102), the composition ranges into rhyodacite. The Double Point dacite is composed 

of light- to dark-gray, glassy to lithoidal rocks containing many small 

plagioclase phenocrysts ordinarily visible in hand specimen. The 

rocks commonly are irregularly vesicular, the vesicles commonly being 

lined with sugary-textured altered glass, tridymite, or both.

The largest outcrop area extends from Double Point northeastward to the drainage of the Williwaw Cove stream and includes 

the present caldera scarp. Most of the rock is a light-gray highsilica dacite but ranges, very locally, from low-silica dacite, 1 mile 

southeast of the 1,303-foot mountain, to rhyodacite north of Double 

Point. Topographic remnants of the outlines of individual lava 

flows are apparent throughout this 'area, obvious in the southwest 

half, but less so along the caldera scarp and in the north (see pi. 24). 

Outlines of flows indicated on plate 23 show where the general dip 

of the formation parallels the general slope. At the mouth of the 

stream entering the angle of the coast north of Double Point, lava 

flows of the Double Point dacite are bordered by a deposit of glassy, 

spherulitic, flow-layered and irregularly vesicular (some semipumiceous) dacite boulders in a finer, unconsolidated ash matrix. Possibly this debris resulted from marginal explosions induced by surface water at the edge of a flow (as described by Wentworth and 

MacDonald, 1953, p. 28). If so, this deposit would indicate that sea 

level during Double Point time was within 200 feet of present sea 

level.

The second largest area of Double Point dacite, southwest and west 

of East Point, occupies the whole northern flank of the Little Sitkin 

cone. As elsewhere, light- to dark-gray high-silica dacite is predominant. At least a dozen thin flow units are exposed in cross section 

on the partly dissected steep slope northwest of Little Sitkin volcano, 

but the sharp spurs radiating to the north, northeast, and west, appear to be part of a single thick flow. Smooth flow layers are prominent on the northeast spur. From a distance the steep northern sea 

cliff appears to be made of thick flows. Landslides at the base contain gigantic blocks of massive dacite. At an 1,825-foot elevation 

due north of the summit a thin layer of tuff-breccia is interbedded 

with the flows.

Two inliers of Double Point dacite are surrounded by ash of the 

Patterson Point formation on the northwest flank of the Little Sitkin 

cone. These rocks are presumed to be high points on a caldera rim* 

which formed in post-Double Point time.
GEOLOGY OF LITTLE SITKEST ISLAND, ALASKA 179

An outlier of the Double Point dacite on the east flank of the 

1,960-foot mountain is bounded on the east by younger lava flows of 

the Little Sitkin dacite and on the west by a normal fault. In places, 

a whitish-gray frothy phase is intimately associated with the massive 

lava flows. A few lapilli-ash beds crop out in the southern quarter. 

Another outlier, a single flow thick, at the base of the south spur of 

the 1,303-foot mountain is a normal feldspar dacite. Several small 

dikelets of dacite richer in feldspar, tridymite, and vesicles than the 

more usual rock, cut the outlier. Near this outlier several float blocks 

of bluish-black glass rich in spherulites, resembling blocks in the 

debris at Double Point, are believed to be part of the Double Point 

dacite.

Almost every outcrop of Double Point dacite contains many round 

inclusions of miarolitic andesite; tiny crystals of feldspar, pyroxene, 

and hornblende can be seen with a hand lens projecting into spherical 

or irregularly shaped vesicles. The crystals are characteristically 

long and needlelike. All the inclusions have a distinct intersertal 

rock texture markedly different from the texture of any of the subaerial lava flows. The inclusions probably comprise only 5 percent of 

the Double Point dacite but locally, as on the 850-foot knoll due east 

of the head of West Cove, they make up 30 percent of the volume. 

They range in size from microscopic to 4 feet in diameter and average 

1 to 3 inches. Stream or flow lines ordinarily surround the inclusions and pressure-shadow cavities occur at both ends (see pi. 25). 

The presence of these unusual pressure-shadow cavities attests to the 

relative solidity of the glassy inclusions when the lava flows of the 

.Double Point dacite were erupted and to the viscosity of the dacite 

flows. The inclusions are surprisingly well-rounded, possibly owing 

to a combination of mechanical attrition and chemical solution. Xenoliths like this occur in other formations but not as abundantly. Andesitic rocks of the East Point or Williwaw Cove formation, or their 

intrusive equivalents are probable sources of the inclusions.

The basal contact of the Double Point dacite is well exposed in 

only two places: on the sea cliff south of West Cove and on the sea 

cliff on the west side of Double Point. Neither contact was visited, 

but, from a small boat directly offshore, the relations appeared clear. 

On the south side of West Cove tuffs beneath the Double Point dacite 

are chaotically jumbled. On the north side of the island the Double 

Point dacite terminates at the base of a prominent south-facing scarp 

in the Williwaw Cove formation which is interpreted as an eroded 

remnant of an older caldera fault scarp. West of East Point two 

streams, emerging from the steep northern sea cliff at an altitude of 

600 feet, probably mark the contact. North of the 1,960-foot mountain

466927 59 3
180 INVESTIGATIONS OF ALASKAN VOLCANOES

the basal contact is indicated by changes in lithologic character of the 

rocks and topography. The edge of the formation west of the mountain has not been visited but is drawn at the prominent break in 

slope. Fumarolic alteration has made it difficult to determine the 

boundary of the formation in the northwestern part of the island.

The age of the Double Point dacite is judged to be middle Quaternary because of its partly dissected lava-flow topography, and because of its overlap relationship with the Williwaw Cove formation, 

the Sitkin Point formation, and the East Point formation.

PATTERSON POINT FORMATION

A deposit of ash several hundred feet thick, extending 2 miles 

northwestward from Little Sitkin volcano toward Patterson Point, 

is here named the Patterson Point formation after the feature of that 

name. The type section is designated as the exposure in the stream 

cut north of the 1,980-foot mountain at latitude 51°57'40" N., longitude 178°30'50" E. Because the ash is only semiconsolidated it forms 

few outcrops, but V-cut stream valleys dissect it and mark its extent 

(see pi. 24). The best exposures are in two streams in the west central 

part of the formation where from 20 to 40 feet of the end of the 

deposit is well exposed and two lithologic varieties can be distinguished. An upper unit 10 to 15 feet thick consists of light-grayishwhite to bluish-gray, firmly consolidated ash and pumiceous lapilli 

and contains about 1 to 2 percent angular to subangular lava blocks 

as much as 2 feet in diameter. The blocks range from hornblende 

andesite and feldspar basalt to dacite and their long dimensions lie 

parallel to the slope. A lower unit as much as 25 feet thick consists 

of salmon-pink to grayish-white (depending on the relative abundance 

of ferric oxide), very firmly consolidated or welded dacitic glass and 

pumice containing from 5 to 10 percent rounded to subrounded 

bombs. The bombs differ from their dacitic matrix and consist of 

conspicuously contrasted layers of black and white glass. The black 

glass is dominant, averaging 70 percent of the bombs; the white glass 

commonly is isolated as layers, blebs and stringers. The composition 

of the two types of glass was not determined, but the general lithologic character of the black phase suggests a composition different 

from the dacitic matrix.

The salmon-pink oxidation color of the matrix of the lower unit 

probably indicates that the ash contained enough heat to oxidize its 

iron when deposited. Most of the ash is massive or irregularly vesicular, but some contains very "hairy" or "stringy" pumice resembling 

boiled sugar and water at the "string" stage. The "hairs" are 

elongate parallel to the underlying slope showing that the ash was
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 181

plastic and evolving gas when deposited, and was stretched by further 

creep downslope.

The Patterson Point .formation is massive except for oriented glass 

strings and foreign blocks. The upper and lower units intergrade 

through a zone about 6 inches thick. The basal contact of the lower 

unit is poorly exposed in a gulch at one place but lithologic details 

are obscured.

Other less complete exposures of the Patterson Point formation 

are higher on the volcano. At 3,500 feet elevation southwest of the 

summit 15 to 30 feet of ash is exposed. The lower 5 to 10 feet consists of dark-grayish-black pumice in a lighter ash matrix; the upper 

10 to 20 feet consists of white to pinkish pumice and less-vesicular 

material in a reddish "sand" matrix. ' .,..;', 

. Northeast of the Little Sitkin summit partly indurated ash, pumicej 

and lithic debris, several hundred feet thick, are exposed. The pumice consists of black glass, black and white layered glass, and reddish 

glass. . . ' ::;

Ash and pumice of the Patterson Point formation on the northwest side of the 1,980-foot mountain are poorly exposed, but slope 

wash of black pumice, black and white layered pumice, reddish pumice 

and hornblende andesite rock debris, which resembles that in the, 

main body of the formation, are distributed over a wide area.

Areal distribution of the Patterson Point formation and its internal, 

structure and lithology suggests its mode of origin. Eruption of 

pyroclastic material and layered bombs was succeeded by a gasgenerating "glowing cloud", probably like the "St. Vincent vertical" 

type of nuee ardente (MacGregor, 1952, table 2), which rushed down 

the northwest slope of Little Sitkin volcano. A rain of ash, pumice, 

and rock debris ended the eruption. The Patterson Point formation 

blankets a caldera fault scarp cutting the Double Point dacite (Galr 

dera Two, see p. 185). The eruption producing the deposit may have 

been trigged by the collapse of the caldera.

LITTLE SITKIN DACITE

The name Little Sitkin dacite is here introduced for widespread 

lava on the east and south sides of Little Sitkin volcano. The type 

section is designated as the exposure in the crater rim of Little Sitkin 

volcano at latitude 51°57'2" N., longitude 178°32'57" E. Although 

these lavas are shown by analysis to range in composition from 

andesite to rhyodacite, 90 percent by volume are estimated to be lowsilica dacite (see table 4). At least 300 feet of Little Sitkin dacite 

is exposed in cliffs on Pratt Point and near the crater, but the formation is doubtless even thicker because it fills two valleys and .part of
182 INVESTIGATIONS OF ALASKAN VOLCANOES

a caldera depression. The estimated maximum thickness shown on 

sections A-A' and B-B', plate 23, is about 1,000 feet. Constructional 

flow forms are generally recognizable everywhere on the surface of 

the Little Sitkin dacite (see pi. 24). The outlines of individual flow 

units, as indicated by the topography, are shown on plate 23.

Two members have been mapped separately from the bulk of the 

Little Sitkin dacite. They are here named the Pratt Point member 

and West Cove member. The type locality of the Pratt Point member is the exposure on the north side of Pratt Point along the coast. 

The type locality for the West Cove member is the exposure along 

the coast on the north side of West Cove. The Pratt Point member 

of the Little Sitkin dacite is a long narrow body of obsidian (chemically a rhyodacite) from 200 to 700 feet wide 'and about 2 miles long 

that extends from the crater rim to the coast north of Pratt Point. 

The rhyodacite ranges from light-gray to white glass, has many 

plagioclase and a few pyroxene and hornblende phenocrysts, ranges 

from slightly to very vesicular and, locally, contains small pink 

devitrification spherulites.

The contact of rhyodacite of the Pratt Point member with surrounding low-silica dacite of the Little Sitkin dacite is extremely 

complex. Rhyodacite and dacite interfinger in complex swirls and 

loops. The zone of interfingering is from several yards to several 

tens of yards wide. Fresh and oxidized phases of each kind of rock 

are randomly interlayered. In several areas the dacite contains sharp 

angular fragments of rhyodacite, but on a smaller scale dacite is also 

included in the rhyodacite. Apparently the rhyodacite and dacite 

were extruded contemporaneously from the same vent (a vent that 

later supplied the homogeneous "crater flow").

The West Cove member consists of two aa flows, one at West Cove 

(cove flow) the other due south of the present crater (crater flow). 

"Push moraines" of blocky surface debris and "lateral moraines" or 

"levees" are distinct surface features on both flows. The crater flow 

breached the south side of the crater and flowed down to an altitude 

of 500 feet. The cove flow rose from a fissure (probably on an old 

caldera fault zone) and flowed westward to a point 150 feet below 

sea level in West Cove (see pi. 23, section A-A' and Gates and Gibson, 

1956, fig. 9). The rocks of both flows are black glassy dacite containing abundant small plagioclase phenocrysts and shown by analyses to 

be low in silica for dacite. Judging from amounts of vegetation on 

each flow at the 700-foot altitude, the crater flow is the younger. A 

similar comparison with vegetation on the northeast Recent flow 

of Kanaga Volcano on Kanaga Island which erupted in 1906 (Coats, 

1952, p. 492; 1956, p. 79) suggests that the "crater flow" on Little
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 183

Sitkin is not older than the eruption on Kanaga. The fragmentary 

historical records give no reliable date for the eruption on Little Sitkin (Coats, 1950, p. 37, 39; table 2). Probably the crater flow and 

cove flow are nearly contemporaneous and were extruded about 50 

years ago.

Most rocks of the Little Sitkin dacite are black, glassy to cryptocrystalline dacite shown chemically to be low in silica. A few massive 

light-gray nonporphyritic andesites occur interbedded with the 

other lava, for example on the east side of Sealy Point and west of 

the center of the crater flow. A few areas of small miarolitic, basic 

inclusions have been noted but nowhere in the abundance and persistence of the inclusions in the Double Point dacite. Where the lava 

is well exposed, irregularly conchoidal joints are prominent. Locally 

these joints group themselves in vertical sets that roughly resemble 

columnar joints. The upper portions of flows, beneath the surface 

aa blocks, locally have contorted flow layers especially on the east 

flank of the volcano.

The northwest edge of the Little Sitkin dacite is easily mapped. A 

marked lithologic contrast with the Double Point dacite and a promk 

nent erosional scarp mark the boundary. The topographic contrast 

is reversed along the west margin where flows of the Little Sitkin 

dacite lap up against the caldera scarp of the Double Point dacite: 

Where the Little Sitkin dacite is in contact with the rocks of the 

Williwaw Cove formation along the east and south boundaries of 

the mass, lithologic distinctions are more difficult. Except for the 

surficial deposits the Little 'Sitkin dacite overlies all other rocks.

SUKFICIAL DEPOSITS

The unconsolidated surficial deposits on Little Sitkin have 'been 

grouped into four categories: alluvium, beach deposits, colluvium, 

and eolian deposits. Although glacial deposits are not definitely 

recognized, debris left by former glaciers may lie somewhere beyond 

the shore. In addition to the deposits mapped, a mantle of surficial 

ash, peat, and turf is widespread and boulder and block beaches are 

everywhere along the coast except at very exposed headlands.

Eight mappable deposits of alluvium are scattered around the 

island. Three are at valley mouths and two on flats below masses of 

fumarolic clay. The alluvium in the valley between Sealy and Prokhoda Points, and that at Williwaw Cove are composed of gravel 

and sand. The alluvium at William Cove and north of the source of 

the cove flow is composed of sand, silt, clay, and organic muck. Four 

deposits are on the gentler slopes near the base of the cone and consist 

mostly of thin deposits of very coarse bouldery debris moved down-
184 INVESTIGATIONS OF ALASKAN VOLCANOES

slope during the 'Spring runoff. Many other local alluvial deposits 

are too small to map.

Beach deposits on Little Sitkin Island are of two 'ages: those now 

forming, and those that record a higher stand of the sea. Older 

 beach deposits are preserved at two places on the east coast. One, 

half a mile north of Pratt Point, consists of well-rounded boulders, 

gravel and sand atop a 50-foot high bedrock terrace backed by a 

cliff. Thick peat covers the deposit. The other, 2 miles southwest of 

Pratt Point, is a semiconical mound of partially indurated conglomerate at the mouth of the stream draining the volcano crater. The 

top of the mound is about 50 feet above sea level. The present stream 

has cut a channel through the deposits. This mound may have been 

deposited at a relatively higher stand of the sea.

The beach between Prokhoda and Sealy Points is composed of 

boulders and gravel. The deposit in William Cove is a shingle beach 

of small flattened pebbles. The beach in Williwaw Cove is sand.

Colluvial deposits on Little Sitkin Island are varied. Materials 

mapped as colluvium include: talus inside the crater and along the 

west side of the crater flow; a long probably active miidflow above the 

head of West Cove; and several landslides. A land slide at the head 

of West Cove consisting of slumped rotated lava blocks, may still 

be moving slowly. Two landslides west of East Point, which may 

have occurred at about the same time, contain disoriented blocks of 

lava tens of feet in diameter. A deposit of unsorted material east of 

the 1,980-foot mountain is regarded as colluvium; one of unconsolidated surficial debris exposed in the sea cliffs southeast of Finger Point 

is possibly glacial till.

A large area of partially grass-covered sand dunes lies back of the 

Williwaw Cove beach. A blowout on Prokhoda Point exposes an 

area of windblown sand too small to map.

STRUCTURAL GEOLOGY

; Eelict topography gives important clues to the structural relations 

on Little Sitkin Island. Four peaks, 1,303, .1,405, 1,960, and 2,095 

feet high, within the Williwaw Cove formation around the margin 

of .Little Sitkin Island, have steep inner slopes and lie along the circumference of a circle 3.8 miles in diameter. Between the 1,303-foot 

mountain and the 2,095-foot mountain is a low curvilinear scarp that 

faces south. These mountains are apparently greatly dissected 

remnants of a caldera rim. The inference of a caldera from the 

topography is supported by the attitude of the bedding, which dips 

generally outward from the center of the island. The beds were 

probably all laid down on a single large volcanic cone rather than
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 185

in separate smaller volcanic piles. The postulated caldera, here called 

Caldera One, has been almost completely filled by younger deposits 

since its formation.

Topographic relations in the Double Point dacite indicate a second 

caldera, here called Caldera Two. A steep, northward-trending 

scarp, that traverses the central part of the island culminating in the 

1,980-foot mountain, is interpreted as a fault scarp. On the north 

in line with the scarp is a wide horizontal bench in an otherwise steep 

slope. The inner boundary of this bench is regarded as the fault 

trace and its marked change in surface expression is probably due 

to a change in attitude of the fault plane. The fault is probably 

nearly vertical along the western scarp; in the northern area the fault 

plane is probably nearly horizontal. Direct evidence of the caldera 

scarp is buried by younger formations on the east, south, and northwest. Little Sitkin volcano, a mountain 3,900 feet high, is a large 

block of the Double Point dacite that did not collapse far into Caldera 

Two. Probably the block tilted toward the center of the caldera, as 

indicated by nearly horizontal lava flows on the northern side of the 

summit. By analogy with normal lava flow attitudes near the summit of a volcano, these beds have been tilted at least 30°.

Caldera One, as postulated, is roughly circular and about 3 miles 

in diameter. Caldera Two is an elliptical structure measuring 1.7 

by 2.5 miles. Coats (1950, p. 43) lists and Gibson and Nichols (1953, 

p. 1184) repeat a diameter for a Little Sitkin caldera of 4.5 km (=2.8 

miles). This probably represents the long dimension of Caldera Two. 

The two calderas may have utilized the same zone of weakness on their 

eastern and southern margins. Several faults branch off from the 

southern part of the Caldera Two fault and are evident topographically at the northeast base of the 1,960-foot mountain. They extend 

southeastward beneath the Little Sitkin dacite to the sea, whence their 

submarine extension is expressed by a topographic scarp, the Prokhoda Scarp, that can be traced for 35 miles between Little Sitkin 

Island, and Semisopochnoi Island (Snyder, 1957, pi. 22). Movements 

along this larger structural feature may have triggered the collapse of 

one or both of the Little Sitkin calderas.

The magma of the cove flow of the West Cove member has probably risen along the old Caldera One zone of weakness (see section 

A-A') pi. 23). The cove flow issues from beneath the Double Point 

dacite near where the dacite covers the old fault trace.

In the northwest part of the island a relatively undissected scarp as 

much as 50 feet high outlines three sides of a downthrown block (see 

pis. 23 and 24). The volumes of material displaced by the dropped 

block (more than 1 billion cubic feet) is roughly equivalent to the vol-
186 INVESTIGATIONS OF ALASKAN VOLCANOES

ume of the cove flow (less than 1% billion cubic feet). This plus the 

roughly equivalent age of each suggests a causal relationship between 

the two.

A structurally weak zone is suggested by the west northwest alinement of three major areas of fumarolic activity across Little Sitkin 

Island passing through the present crater. The alinement roughly 

parallels the trend of the volcanic centers of Davidof, Segula, and 

Little Sitkin Islands and is expressed on the ocean floor by parallel 

linear ridges and troughs west northwest of Finger Point (Snyder, 

1957, pi. 22).

Most joints on Little Sitkin Island are confined to individual flow 

units; though few are columnar, they probably formed during cool-, 

ing. A well-developed set of shear joints on Finger Point probably 

formed at the margin of a flow of the Williwaw Cove formation, but 

may be related to tectonic movements. Prominent linear depressions 

in the crater flow of the West Cove member (see pi. 24) are believed 

to be shear zones formed when this flow was nearly solidified; the confinement of these depressions to this flow suggests a volcanic rather 

than a tectonic origin.

PETROGRAPHY

Petrographic characteristics of the volcanic rocks on Little Sitkin 

Island are summarized in table 1. Compositions of plagioclase feldspar were determined principally from extinction angle curves of 

high-temperature plagioclases as given by Troger (1952, fig. 234). 

Although the feldspar megaphenocrysts range in composition from 

calcic oligoclase (An24 ) to sodic bytownite (An76 ), the great majority 

range between andesine (An40 ) and labradorite (An60 ). This range 

occurs within single oscillatory zoned crystals as well as between different crystals. The microphenocrysts are generally somewhat more 

sodic than the megaphenocrysts, as are the normative feldspars. Apparently the phenocryst and groundmass are in disequilibrium as is 

normally true of glassy lava. Because most of the rocks on Little 

Sitkin contain more cryptocrystalline or glassy groundmass than 

phenocrysts, nomenclature based on the mode as indicated by the 

phenocrysts might give an inaccurate root name. Accordingly the 

chemical classification of Rittmann (1952, p. 93-102) which has been 

proposed for use in the International Catalogue of Volcanoes, is used 

in this report. The Rittmann names sometimes contain mineralogical 

modifiers that are confusing if these minerals are not present in the 

actual rock. Only Rittmann's root names have been retained in this 

report.

Individual formations mapped on Little Sitkin Island are statistically uniform petrographically. The rocks of the Williwaw Cove
TABLE 1. Summary of petrographic characteristics of volcanic rock

LittleSitkindacite.. ..- 

Plagloclase feldspar

Megaphenocrysts

Average 

composition

An«

An«

Anjj 

An«

Anti 

An«s

Anas 

An«9

Range

AlUo-79

Anj3-78 

AJ135-4J

An«-7« 

Anio-eo

Anz4-jo 

An<o-w

Mlcrophenocrysts

Average 

composition

Anso 

An«

An«

An43

An<o(?) 

An<o

Average 

normative*

Anso

Anas

Ansg 

An«

Ann 

An«

Occu

Free silica

None _ _ 

Tridymite common.

Trldymite very 

rare.

O

Rare to 

Rare _

Very ra

Rare-. 

Rare _

i Calculated from data in table 2; includes normative orthoclase as alblte.

' Hornblende rare in mapped areas of these rocks, abundant In intersertal inclusions in the rocks of the Double P

waw Cove formation.
188 INVESTIGATIONS OF ALASKAN VOLCANOES

formation including the feeder dikes (pi. 23) contain plagioclase feldspar with an average composition of An55 and a range in composition 

of AiLto to An75 . Plagioclase microlites have an average composition 

of An50 . Olivine is generally present, sometimes even visible in hand 

specimen, and there is no free silica. Oxyhornblende is rare in most 

rocks but abundant in the miarolitic inclusions contained in the lava 

of the Double Point dacite. In general, all rocks of the Williwaw Cove 

formation contain much more clinopyroxene than orthopyroxene. The 

index of refraction of the groundmass glass is only determinable in 

inclusions of the Double Point dacite where it ranges between N=1.498 

and N=1.514(1).

Petrographic knowledge of the Sitkin Point formation is limited 

to the study of several thin sections of lava blocks as shown in figure 

40. The unconsolidated matrix has not been studied petrographically. 

Plagioclase phenocrysts have an average composition of AiLts 5 microlites, An43 . Olivine and hornblende are present but uncommon and 

free silica is completely absent. Clinopyroxene and orthopyroxene 

are present in varying proportions.

The uniform flow units of the East Point formation contain plagioclase feldspar with an average composition of An59 and range 

in composition of An53 to An76. Olivine is present and free silica and 

hornblende are absent. Clinopyroxene is much more abundant than 

orthopyroxene.

The high-silica dacite of the Double Point dacite contains plagioclase 

feldspar with an average composition of An42 and range in composition 

of An35 to An52 . Groundmass microlites have about the same composition as the large plagioclase phenocrysts. Tridymite is common 

as a groundmass mineral in many rocks; rare olivine and oxyhornblende have been observed in some others. In general the amount of 

clinopyroxene is less than or equal to the amount of orthopyroxene. 

Crystallites (longulites), skeletal crystals, and spherulites are common 

in a fluidal glassy groundmass which has an index of refraction from 

N=1.490toN=1.504.

The main juvenile eruptive rocks in the Patterson Point formation 

are characterized by plagioclase feldspar with an average composition of An5 i and range in composition of An48 to An76 . No free silica, 

olivine, or hornblende are known to occur. Clinopyroxene is present in 

amounts greater than or equal to the amount of orthopyroxene. Two 

determinations of the index of refraction of the glassy groundmass 

give N=1.504 and N=1.506.

The low-silica dacite of the Little Sitkin dacite contains plagioclase 

feldspar with an average composition of An48 and a range in composition of An40 to An60. Plagioclase microphenocrysts have an aver-
 > £ '

C <

3 § 5 O- 3°" 1-3 >-i

I a-

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JL o a 

^ «' I

w S 2. =

a o

Q- 2. " 

S: E. >

TERTIARY OR QUATERNARY QUATERNARY

Hill ^

~ 2 !T 2S -nTJ

m

HIGH-SILICA DACITE

RHYODACITE
GEOLOGY OF LITTLE = SJLTKIN ISLAND, ALASKA 189

age composition of Ar^a . Olivine is rare and tridymite was observed 

in only one specimen. No hornblende was found. In general, there 

is more clinopyroxene than.orthopyroxene in the Little Sitkin dacite.

The Pratt Point member of the Little Sitkin dacite contains plagioclase feldspar with an average composition of An38 and a range in 

composition of An24 to An50. The groundmass feldspar have about 

the same average composition as the phenocrysts. Oxyhornblende 

and corroded quartz are common and olivine is never present. In 

general, clinopyroxene is much less abundant than orthopyroxene. 

Crystallites (longulites), skeletal crystals, and spherulites are common 

in the rhyodacitic glass which has an index of refraction of N=1.488 

to N=1.491.

The West Cove member of the Little Sitkin dacite has practically 

the same petrographic characteristics as the main body of the formation. The average plagioclase megaphenocryst has a composition 

of Aii49 and the range in composition is f rom An40 to An56. Plagioclase microphenocrysts have an average composition of AiLio. There 

is no free silica or hornblende. The few grains of olivine are surrounded by reaction rims of orthopyroxene and magnetite. Clinopyroxene is more abundant than orthopyroxene.

Comparison of table 1 with table 4 will show that, in general, the 

rocks low in silica contain plagioclase relatively high in anorthite 

content, olivine, and much more clinopyroxene than orthopyroxene, 

and that rocks high in silica contain plagioclase relatively low in 

anorthite (iontent, quartz, and much more orthopyroxene than clinopyroxene. Intermediate rocks are intermediate in these respects. 

Oxyhornblende occurs abundantly in the most silicic and least silicic 

rocks and is rare or absent in the intermediate rocks. The index of 

refraction of the glassy groundmass also varies systematically from 

N=1.488 in the most silicic to N= 1.514 (?) in the least silicic rocks. 

The more silicic rocks generally contain more glass and many skeletal 

crystals, crystallites, and spherulites.

Plagioclase is by far the most common phenocryst-forming mineral 

in nearly all of the rocks on Little Sitkin Island. Plagioclase phenocrysts constitute from 10 to 60 percent of the rocks; 20 to 30 percent 

is an average amount. 'Strong oscillatory zoning of plagioclase is 

universal and a few negative crystals in the feldspar contain two 

fluids. Albite and carlsbad twins are very common and pericline and 

baveno twins are present.

Pyroxene is the next most common mineral. Typically the maximum amount of pyroxene phenocrysts is 15 percent, but specimen 1 

from the Williwaw Cove formation contains 43 percent augite megaphenocrysts. Hypersthene is. the common orthopyroxene. Pleo-
190 INVESTIGATIONS OF; ALASKAN VOLCANOES

chroism in pale tints of pink and green characterizes the hypersthene 

and some of the clinopyroxene. No pigeonite was found.

Olivine constitutes as much as 5 percent of many rocks. Although 

this mineral is not in equilibrium with the rock as a whole (see norms, 

table 2), reaction rims are very rare. Euhedral outlines of some 

crystals suggest that the olivine was growing when the rock solidified. 

Several dikes cutting the Williwaw Cove formation contain masses 

of serpentine(?), probably altered from olivine. 

, Euhedral basaltic hornblende is common in the Pratt Point member 

of the Little 'Sitkin dacite, in the miarolitic inclusions of the Double 

Point dacite, and in the lithic blocks contained in the Patterson Point 

formation. Elsewhere hornblende usually occurs us anhedral remnants with reaction rims of hematite or magnetite. Strong pleochroism ranging from olive green and golden yellow to scarlet and 

deep brown is common.

Quartz makes up as much as 4 percent of the rhyodacitic Pratt 

Point rocks. Magnetite and ilmenite are present in amounts from 

a fraction of a percent to 5 percent. Tridymite lines cavities in many 

rocks of the Double Point dacite. Other accessory minerals include 

apatite and titanite(?). Alteration minerals include kaolm(?), 

leucoxene, serpentine(?), chlorite, and hematite. Introduced minerals include pyrite, chalcedony^ zeolites, calcite, and sulfur. The 

secondary minerals are rare except in altered zones or active fumarolic areas (see pi. 23) where all but serpentine are abundant.

The following petrographic descriptions of the chemically analyzed 

rocks are based on studies of a single thin section of the particular 

rock, and of crushed fragments of the groundmass glass in immersion 

oils. Mineral percentages are based in part on Rosiwal analyses 

and in part on visual estimates. Bubbles or vesicles smaller than 

about 0.03 mm are included in the groundmass percentages. The 

color names and numerical designations used are based on the Munsell 

color system as shown in the rock-color chart printed in 1948 by the 

Rock-color Chart Committee of the National Research Council. The 

field location is given in general terms in the following descriptions 

and is also located exactly on plate 23.

Petrographic descriptions of chemically analysed rocks

1. Basalt from Williwaw Cove formation on southeast coast 1 mile southwest 

of Pratt Point. (Chemical data given in column 1, table 2.) Color: light 

reddish gray (5R 6/1). Texture: holocrystalline, porphyritic, with large, 

diameter 15 mm phenocrysts of greenish black augite (5 QY 2/1). Plagioclase 

phenocrysts, 10-15 percent, range from An^.to An<0 in individual and separate 

crystals and average An67. Augite (nZ: 1.705, nX: 1.676) occurs as remarkably 

inclusion-free megaphenocrysts and as groundmass dust, making 40 to 45 percent
GEOLOGY OF LITTLE SITKEST.!ISLAND, ALASKA 191

of 'the rock. Subhedral olivine, 2, percent, and magnetite-ilmenite, 1 percent, are 

accessory minerals. The groundmass is a microcrystalline aggregate of all 

the above minerals.

2. Andesite from typical inclusion in lavas of Double Point dacite. (Chemical data given in column.2, table 2.) . Collected from small inlier of Double 

Point dacite about 1 mile west-northwestward of summit of Little Sitkin volcano. 

Similar inclusion from same area shown in plate 25. Color: dark gray (N 3). 

Texture: miarolitic intersertal. Pore space, miarolitic cavities: 20-30 percent. 

Plagioclase lathes, 45-55 percent, average AnM but range from An*, to An»; 

megaphenocrysts, 3 percent, show fritted texture. Oxyhornblende, 14 percent, 

is pleochroic in deep brown and olive brown, occurs in needles with a lengthwidth ratio of 15:1; many hornblende crystals have hollow centers. OrthOr 

pyroxene, 6 percent, occurs in euhedral needles with a length-width ratio as 

much as 30:1. Clinopyroxene, 4 percent, is present as stubby subhedral crystals. 

Magnetite, 4 percent, anhedrons have a length-width ratio as much as 10:1. 

Clear'brown glass, 14 percent, in groundmass has N=1.498.

3. Andesite of Little Sitkin dacite from coast at stream mouth 0.8 mile south 

of terminus of crater flow of West Cove member. (Chemical data given in 

column 3, table 2.) Color: medium gray (N 5). Texture: hypocrystalline, porphyritic with feldspar phenocrysts as much as 4 mm in diameter, and pyroxene 

phenocrysts as much as 3 mm long. Plagioclase megaphenocrysts, 20-30 percent, 

range from AnM to Anw and average An«; complex zoning and twinning 

common. Clinopyroxene, 8 percent, contains rare centers of orthopyroxene, 1 

percent. Both unreacted olivine, 5 percent, and pyroxene are stained amber 

along joints. Magnetite, 2 percent, as anhedrons. Average composition of 

feldspar microlites in trachytic groundmass is An«.

4. Andesite of Williwaw Cove formation from coast in Williwaw Cove near 

"Shack" (pi. 1). Color: medium light gray (N 6). Texture: holocrystalline, 

porphyritic with both feldspar and pyroxene phenocrysts as much as 4 mm 

long. Plagioclase, 45-60 percent, ranges in composition from An« to An60 and 

averages AnBi; complex oscillatory zoning and graphic fritting common within 

individual plagioclase grains. Subhedral Clinopyroxene, 10 percent; euhedral 

orthopyroxene, 10 percent; and anhedral olivine, 1 percent, characterized by 

brown stains on some fractures. Anhedral magnetite, 5 percent, particularly 

abundant. Groundmass consists.of mesh of feldspar and pyroxene microlites 

and magnetite dust. Groundmass plagioclase averages An«.

5. Oxidized low-silica dacite of Little Sitkin dacite from thin boudinaged 

seam at contact of the rhyodacite in the Pratt Point member on coast half 

a mile north of Pratt Point. (Chemical data given in column 5, table 2.) Color: 

pale red (5R 5/2). Texture: porphyritic with plagioclase phenocrysts as 

much as 4 mm long. Plagioclase, 25-30 percent, ranges in separate and individually zoned crystals from An29(?> to An^ with an average composition of about 

An50 ; many of the larger feldspars form a graphic "intergrowth" with dusty 

glass; some have a rim of clear feldspar in optical continuity with the fritted 

cores; two-fluid inclusions and small needles (0.002 mm by 0.04 mm) of apatite (?) or pyroxene (?) occur in feldspar. Orthopyroxene, 6 percent, occasionally 

cores Clinopyroxene, 9 percent, crystals. Accessory minerals: magnetite, 3 percent, and tridymite, 1 percent. Fluidal groundmass highly charged with 

hematite dust.

6. Low-silica dacite of Little Sitkin dacite from interfingered contact zone on 

north side of rhyodacite of Pratt Point member about 0.9 mile south of East 

Point. (Chemical data given in column 6, table 2.) Color: medium gray
192 INVESTIGATIONS OF ALASKAN VOLCANOES

(N 5). Texture: hypocrystalline, massive. Plagioclase, 20 to 25 percent, ranges 

in composition from Auss to Ann', a few have fritted centers. Orthopyroxene, 

2-3 percent, and clinopyroxene, 2-3 percent, both distinctly pleochroic in light 

pink and light green; a few orthopyroxene and elinopyroxene centers in differently oriented clinopyroxene crystals. One relict olivine grain, with a thick 

reaction rim of pyroxene and magnetite; No oxyhornblende xenocrysts although 

this mineral is plentiful in nearby thin laminae of rhyodacite. Groundmass 

brownish glass filled with magnetite dust and crystallites.

7. Low-silica dacite of Little Sitkin dacite from coast half a mile north of 

Pratt Point. (Chemical data given in column 7, table 2.) Specimen taken 

from within 6 inches of contact with rhyodacite of Pratt Point member. : Color: 

grayish black (N 2). Texture: hypocrystalline, porphyritic with plagioclase 

as much as 5 mm in length. Most plagioclase megaphenocryst and microphenocfysts, 25-50 percent, group around AnM but range from An25 to An ; complex 

oscillatory zoning common; many feldspars contain vermicular areas of dusty 

brown glass. Orthopyroxene, 8-10 percent, occurs as individual euhedrons and 

as cores in many clinopyroxene, 6 percent, crystals. Accessory minerals: 

magnetite, 2 percent, and oxyhornblende, 1 percent (xenocrysts?) Fluidal 

glassy groundmass contains angular crystal fragments, 3 percent, and euhedral 

rods of pyroxene(?). ' " ' 

8. Low-silica dacite of Little Sitkin dacite from crater flow of West Cove 

member'1 mile south of crater lake. Color: dark gray (#3). Texture: hypocrystalline, porphyritic with prominent feldspar phenocrysts as much as 6 mm 

long. Plagioclase, 20-25 percent, ranges from Anse to An62 in individual zoned 

phenocrysts and between different crystals; average composition is about AnM ; 

graphic fritting texture and two-fluid inclusions present in centers or selected 

zones of some feldspar crystals. Clinopyroxene, 6 percent, appears as reaction 

rims on some orthopyroxene, 5 percent, and olivine, 1 percent. Accessory: 

magnetite, 1-2 .percent. Groundmass is clear brown glass choked with plagioclase (average composition, An«) and acicular orthopyroxene (?) microlites. 

Index of refraction of an artificial glass of the powdered rock is greater than 

1.540.

9. Low-silica dacite of the Patterson Point formation from stream cut 0.9 mile 

north of the summit of the 1,980-foot mountain. (Chemical data given in column 

9, table 2.) Color: light brownish gray (5 YR 6/1). Texture: hypocrystalline, 

vesicular with "hairy" pumice developed locally, porphyritic with pyroxene 

and feldspar phenocrysts as much as 3 mm long. Plagioclase, 25-30 percent, 

varies from An« to An7o and has an average composition of An5ij subgraphie 

areas of brownish glass and two-fluid negative crystals are included within the 

plagioclase. Clinopyroxene, 5 percent, and orthopyroxene, 4 percent, in part 

form agglomerations with magnetite-ilmenite, 1 percent. Groundmass consists 

of clear pink to brown glass (N=1.506) containing many small gas bubbles.

10. High-silica dacite of Double Point dacite from same locality as analysis 

2. This rock is groundmass of plate 25. (Chemical data given in column 10, 

table 2.) Color: medium dark gray (N 4). Texture: hypocrystalline, porphyritic with blocky feldspar phenocrysts with maximum diameter of 5 mm. 

Eleven sections of the plagioclase, 25-30 percent, perpendicular to both albite 

twinning and basal cleavage give anorthite contents that appear to be distributed 

evenly from An^ to An0sj complex zones and intergrowths common; groundmass microlites may group around AnaT but cannot be determined accurately; twofluid inclusions from 0.004-0.02 mm in diameter occupy negative crystals In 

certain zones of the feldspar. Stubby euhedral hypersthene, 5-10 percent, is
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 193

pleochroic in light pink and light green. Accessories are clinopyroxene, 1 percent ; magnetite, 2 percent; oxyhornblende, 1 percent; and tridymite, 1 percent. 

Fluidal groundmass glass has N=1.504( ?).

11. High-silica dacite of the Double Point dacite located half a mile northeast 

of the summit of Little Sitkin volcano. (Chemical data given in column 11, 

table 2.) Color: brownish gray (5 YR 5/1). Texture: hypocrystalline, porphyritic with feldspar phenocrysts as much as 3 mm long. Zoned and twinned 

plagioclase, 25-30 percent, varies in composition from An«i to An« and averages 

An«. Accessory minerals are euhedral orthopyroxene, 2 percent; subhedral 

clinopyroxene, 2 percent; magnetite, 1 percent; and tridymite, 1 percent. The 

fluidal glassy to cryptocrystalline groundmass carries pyroxene, magnetite and 

plagioclase microlites. The plagioclase microlites have an average composition 

of An«.

12. Rhyodacite of Pratt Point member of Little Sitkin dacite from central 

part of mapped area (pi. 23) about 0.9 mile south of East Point. (Chemical data 

given in column 12, table 2.) Color: yellowish gray (5 Y 7/1) to medium light 

gray (N 6). Texture: hypocrystalline, vesicular with "hairy" pumice developed 

locally, porphyritic with hornblende phenocrysts as much as 7 mm long and feldspar phenocrysts as much as 4 mm long. Plagioclase, 15-20 percent, ranges in 

composition from An^ to AnB<> and averages An*,; sometimes poikilitic with inclusions of hornblende. Accessory minerals: oxyhornblende, 3^4 percent; hypersthene, 2-3 percent; quartz, 2-3 percent; magnetite, 1 percent. The quartz 

is characterized by conchoidal fractures and is very susceptible to plucking during grinding. The groundmass is about half small bubbles and half clear pinkish glass (N=1.490). The few plagioclase microlites average Ans»(?).

13. Rhyodacite lava from Pratt Point member of Little Sitkin dacite. Collected from within 6 inches of contact with rock represented by analysis 7. 

(Chemical data given in column 13, table 2.) Color: yellowish gray (5 Y 7/1). 

Texture: hypocrystalline, vesicular, porphyritic with plagioclase phenocrysts 

as much as 5 mm and hornblende phenocrysts as much as 3 mm long. Plagioclase megaphenocrysts, 15-20 percent, are complexly zoned from cores of An5M<> 

to rims of Anss^i; several feldspars poikilitic with small hornblende inclusions, 

0.09 by 0.2 mm; minute clinopyroxene rods, 0.01 by 0.05 mm; and two-fluid inclusions in cavities with a maximum diameter of 0.04 mm. Hornblende, 3-5 

percent, is pleochroic in shades of olive green, reddish brown, and black; several 

crystals have deeper pleochroism at borders than at center. Hypersthene, 1-2 

percent, is the only pyroxene inegaphenocryst; two crystals have a core of oxyhornblende. Quartz, 1 percent, occurs in large phenocrysts with resorbed outlines and conchoidal fractures, which tend to aid plucking of this mineral during 

grinding of the thin section. Anhedral magnetite forms 1 percent of the rock. 

The groundmass consists of clear pinkish glass (N=1.491) containing numerous 

small vesicles.

GEOCHEMISTRY

Thirteen chemical analyses of representative rocks from Little Sitkin Island are presented in table 2 together with their CIPW norms 

and Rittmann values. Five spectrographic analyses of minor constituents of rocks representative of the chemical spread of the suite are 

also presented. The province is a silicic one; all rocks have normative quartz. Modal quartz is found only in the rhyodacite (tridymite
TABLE 2. Chemical composition, normative values, and Rittmann nomenclature for 

11 2' 31 49 5' 61 7» 8» 

Chemical analyses for major constituents

Name used in 

this report

SiOa.. ........

AlaO3 ._ .......

FeO-_ ....

MgO. ........

CaO. .........

NajO 

K2O_.........

H2 0.........

HsO+ ~ 

TiOt. ........

COs.... 

P2 0s.._..-. 

MnO... 

TYifrnl

Basalt 

52.74

11.99

1.90

6.72

11.02

12.49

1.78

.64

.01

.09

.57

.00

.11

.17

100. 23

Andesite 

55.02

17.91

4.45

4.23

3.92

8.58

3.11

1.09

.05

.41

.85

.00

.15

.15

99.92

Andesite 

55.35

17.72

3.04

5.45

4.03

8.77

3.28

1.00

.07

.15

.89

.00

.17

.17

100. 09

Andesite 

55.37

18.02

2.91

5.53

4.24

8.70

3.03

.91

.06

.05

.87

.01

.16

.16

100. 02

Lowsilica 

dacite

58.47

16.91

5.71

1.89

3.37

7.32

3.40

1.23

.17

.37

.77

.01

.19

.15

99.96

Lowsilica 

dacite

58.63

17.04

2.60

4.50

3.19

7.15

3.37

1.33

.23

.71

.75

.01

.15

.15

99.81

Lowsilica 

dacite

59.21

16.85

2.98

4.09

3.25

7.25

3.49

1.31

.05

.43

.72

.01

.15

.15

99.94

Lowsilica 

dacite

59.94

16.79

2.25

4.77

3.07

6.89

3.49

1.46

.05

.17

.77

.00

.15

.15

99.95

Lo

s

d

Spectrochemical analyses 8 for minor constituents

[A. A. Chodos, analyst]

Ni....... -

Co...........

Cr-...... .

Cu... ........

V... ..........

na

Sc. ... -

Ba.. ... ......

Sr.. ..........

Mo.... ...... .

Y.. . .........

Yb.. ...... ...

Zr............

B. ...........

0.02

.005

.07

f!9

.03

.0008

.006

.03

.1

.003

.0003

.008

.001

0.001

.003

.002

.006

.03

.001

.002 .

.03

.07

.0002

.002

.0002

.008

.001

0. 0008

.003

.002

.008

.02

.001

.002

.02

.06

.002

.0003

.007

.001

0.0006

.002

.002

.008

.02

.001

.002

.05

.08

.0002

.002

.0003

.008

.002
Norms

o

ab..______ ....

is... ..........

is.............

a.............

H.O+........

Total. 

Symbol «. __

0.84 

3.34 

15. 20 

23.07 

15.89 

10.70 

3.96 

16.90 

6.07' 

2.78 

1.06 

.34 

.09

100.24 

III".5.4.4. 

Auvergnose

9.66 

6.67 

26.20 

31.41 

4.41 

3.10 

0.92 

6.70 

1.98 

6.50 

1.67 

.34 

.41

99.97 

n.4(5).(3)4.4". 

Bandose

7.62 

6.12 

27.77 

30.68 

4.99 

2.90 

1.85 

7.20 

4.49 

4.41 

1.67 

.34 

.15

100. 09 

II.(4)5.(3)4.4". 

Hessose

8.64 

6.56 

25.68 

32.63 

4.06 

2.40 

1.45 

8.20 

5.15 

4.18 

L67 

.34 

.05

99.91 

II.(4)5."4.4". 

Hessose

15.48 

7.23 

28.82 

27.24 

3.36 

2.90

6.50

4.18 

1.52 

.34 

.37

4 99. 82 

H.4.3".4. 

Tonalose

13.56 

7.78 

28.30 

27.52 

3.02 

1.70 

1.19 

6.30 

3.96 

3.71 

1.52 

.34 

.71

99.61 

n.4".3".4. 

Tonalose

13.92 

7.78 

29.34 

26.69 

3.60 

2.20 

1.19 

5.90 

2.90 

4.41 

1.37 

.34 

.43

100.07 

n.4".3".4. 

Tonalose

13.98 

8.90 

29.34 

25.85 

3.13 

1.70 

1.32 

6.00 

4.49 

3.25 

1.52 

.34 

.17

99.99 

II.4".3".4. 

Tonalose

16

8

31

25. 

1

1

.

6

2

3.

1

.

1

99

"II.4.3

Tonalo

Rittmann nomenclature

SiO2 ____......

Al.__ .. 

Alk. __ ......

CaO.... ...

FM.. .....

k...__ __ 

an. ___________

ca" ___________

Rittmann «

name.

Occurs In:.....

52.74

10.79

3.31

12.49

31.52

.19

.63

8.00

Basalt

Williwaw

Cove

formation

55.02

16.12

5.76

8.58

13.19

.19

.47

2.36

Labradorite

andeslte or

labradorite

dacite

(bandaite)

Double point

dacite (inclusion)

55.35

15.95

5.92

8.77

17.28

.17

.46

2.75

Pigeonite

andesite or

pigeonite

labradorite

andesite

Little Sitkln

dacite

65.37

16.22

5.46

8.70

17.65

.17

.50

2.24

Labradorite

dacite

(bandaite)

or pigeonite

labradorite

andesite

Williwaw

Cove formation

58.47

15.22

6.33

7.32

14.69

.19

.41

1.99

Dacite

Little

Sitkin

dacite

58.63

15.34

6.39

7.15

14.10

.21

.41

1.78

Dacite

Little

Sitkin

dacite

59.21

15.16

6.55

7.25

14.14

.20

.40

2.08

Dacite

Little

Sitkin

dacite

59.94

15.11

6.70

6.89

13.80

.22

.39

1.84

Dacite

West

Cove

member.

60.

15

7.

6.

11.

.

.

1.

Dacit

Patter

son

poin

form

tion

i Edythe E. Engleman, analyst. . . * Harry M. Hyman, analyst.

»Looked for but not found: Pb, Be,"Ag,'Au, Pt, W,IQe,rSn,-As,'Sb, Bi,£Cd, Tl, IB, La.JNb, Ta, Th, XI.

* Includes 2.88 percent hematite. 

«See Washington, 1917. 

«See Rittmann, 1952.
196 INVESTIGATIONS OF ALASKAN VOLCANOES

in the high-silica dacite). Olivine, which constitutes as much as 5 

percent of some of the modes is not represented in the norms.

Because the phenocrysts and groundmass minerals differ chemically, 

Rittmann names (based on bulk chemical analyses) are used in the 

discussion that follows. The most basic member is basalt; the most 

acid one, rhyodacite. Both rocks occur in minor amounts. The great 

mass of rock material is andesite, bandaite, or dacite. A further subdivision of Rittmann's names seems desirable within the dacite range. 

One group of rocks, referred to as "low-silica dacite", contains about 

60 percent SiO2 ; and another, referred to as "high-silica dacite", contains about 65 percent Si02 . The terms "high-silica" and "low-silica" 

have no relation to Rittmann's adjectives "light" and "dark", whiph 

describe the quantity of iron and magnesium.

The oxide constituents of the chemical analyses are plotted on a 

standard silica-variation diagram, figure 42, which shows that the 

suite from Little Sitkin has an alkali-lime index of 63.5 that is, the 

silica percentage at which the sum of the alkalies equals the lime 

(Peacock, 1931, p. 54-67). The alkali-lime index is comparable to 

that of the Katmai province, 63.8 (Peacock, p. 62); to the AdakKanaga province, 63.0 (Coats, 1952, p. 493); and to the Buldir province, 64.5, extrapolated (Coats, 1953, p. 18).

Except for specimen 1, the variations shown by the chemical analyses plotted .on figure 42 are linear. Rectilinear variation diagrams 

of this sort have been discussed before (Fenner, 1926, p. 704r-772, 

especially p. 760-771; 1938, p. 1467-1481; Wilcox, 1944, p. 1067-1073; 

Earth, 1952, p. 167, 168; Reynolds, 1954, p. 582) and they present 

a petrogenic problem. If all the trends are linear .and successively 

younger rocks become progressively more siliceous, then there must 

be very stringent requirements for their mode of derivation. If the 

younger rocks become more siliceous owing to removal of basic 

minerals from a magma, then the composition of these minerals must 

have remained constant or changed systematically. These minerals 

must be removed from beginning to end of the series and no other 

minerals can be removed at any intermediate point in the series or 

the trends on the variation diagram would not be linear. Because 

these linear trends are inconsistent with the Bowen reaction series 

these rocks probably did not evolve by crystal differentiation.

With this particular series the hypothesis that the more acid members are derived from the more basic members is also refuted by the 

actual time relationships of the members involved. Although the 

rocks of Little Sitkin became, on the whole, more siliceous with 

succeeding eruptions, considerable fluctuations in composition occurred (fig. 43).
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 197

I I 1 . . I I I I I I I I I 1 I I I I

WEIGHT PERCENT Si02

FIQORH 42. SHica-rarlation diagram of rocks of Little Sitkln Island. Lines drawn by 

inspection. Analyses numbers correspond with column numbers In table 2.
198 INVESTIGATIONS OF ALASKAN VOLCANOES

Kectilinear variation diagrams can also be explained as the mixing 

of two end members, namely: the mixing of acid and basic magmas; 

the incorporation and dissolution of basic inclusions in an acid 

magma; the incorporation and dissolution of acid inclusions in a basic 

magma; or the formation of discrete magmatic melts in various parts 

of a zone where there has previously been mixing of rock by other 

geologic processes.

An accurate knowledge of the volume and composition of the various rocks on Little Sitkin Island would be helpful in evaluating their 

mode of origin. Table 3 is an attempt to calculate the volume. 

Column 1 of table 3 gives the calculated volume of all the mapped 

formations above sea level at the present time. These figures were 

determined by the combined use of the topographic and geologic 

map. The contour lines of a "restored topography" were drawn from 

the margin of a particular formation beneath it to the corresponding 

contour line on the opposite margin. Control in the placement of 

these contour lines was provided by the relict constructional topography of both the younger and older land surfaces; the internal structure, that is, bedding of the formation involved; the external structure, for example, steep caldera faults, which control one or more 

margins of the formation. It is obvious that such control is best for 

the youngest formations and progressively less reliable for successively older sequences of rock. After drawing the contours, a 1,000- 

foot square grid was placed over the map. At each grid intersection 

the thickess of the formation was noted by obtaining the difference 

in the elevations shown by the two sets of contours at that point. If 

each grid square is regarded as the approximation of a prism in three 

dimensions, then the volume of this prism can be estimated by multiplying the area of the square by the average of the thickness values 

at each corner. Summing the volumes of the prisms gives the volume 

of the formation.

Volume calculated by the above method is less than the original 

volume of the extrusive material for three reasons: since extrusion 

the rocks have been partially eroded; since extrusion some of the 

rocks have been downfaulted during episodes of caldera formation, 

and during extrusion the lavas involved may have flowed into the 

sea. But losses due to erosion can be minimized by utilizing the 

relict constructional topography to reconstruct the original constructional topography, the precaldera structure can be reconstructed to 

compensate for losses due to faulting, and the amount of lava that 

flowed into the sea may possibly be calculated by a study of the submarine topography (see pi. 22, Snyder, 1957). The volcanic structure 

of Little Sitkin appears to have been built on a wide flat platform at
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA

about 300 feet below sea level, the "Ridge Shelf" of Gates and Gibson, 

{1956, p. 134). The steepening of slope above this level in some areas, 

notably off Prokhoda Point, appears to be more or less continuous 

with the relict constructional slopes of the basement rocks. From 

this evidence it is postulated that the volcanic rocks of Little Sitkin 

Island have been deposited above a 300-foot platform. This 

is used as the base for the estimated original volume which is 

presented in column 2 of table 3. The volume should be considered 

only as an estimate, for large errors are possible in some of the figures. 

Despite this uncertainty the data may be useful, at least in a scrutiny 

of orders of magnitude. The estimated original volume of the Williwaw Cove formation is the approximate volume of a single volcanic 

mountain whose outer limits are the present 300-foot contour and 

whose slopes are comparable to those of the single volcanic mountain 

of Segula Island.

TABLE 3. Volume, weight, and percent weight for volcanic rocks of Little Sitkin

Island

t>

West Cove member.... ...

Pratt Point member.. ....

Little Sitkin dacite--- .

Patterson Point formation ......... ...

Double Point dacite ...

East Point formation .....

Sitkin Point formation ....

Willlwaw Cove formation.

1

Calculated 

volume above 

sea level at 

present time ' 

(cu ftXlO")

3

.5

onav

10

260

5

10

205

2

Estimated

original volume above 

 300 ft before 

erosion and 

caldera 

faulting *

(cuftXIOO)

3

.5 inn i\j\i

12

600

10

.20

2000

3

Average 

, rock 

specific 

gravity

2 40 IV

2.18

2.60

1.93

2.50

2.91

2.60

2.79

4

Estimated 

original 

weight 

(tonsx 10»)

234

34

7,800

717

39,000

905

1,620

174,000

6

Estimated

original

weight percent of rocks 

younger 

than Willlwaw Cove 

formation

(percent)

0.48

.07

15.6

1.4

77.5

1.8

3.2

6

Estimated

original 

weight percent of all 

rocks of 

Little Sitkin 

Island

(percent)

0.10

.016

3.6

.32

17.4

.40

.72

77.6

i Total calculated present volume of all formations=588.5 by 109 cu ft=approximately 4 cu miles. 

»Total estimated original volume of all formations=»2,645.5 by 10» cu ft=approximately 18 cu miles.

Specific gravity listed in column 3 of table 3 is determined with a 

Jolly balance using small hand specimens that included vesicles, miarolitic cavities, and other pore space. Column 4 is computed from 

the product of columns 2 and 3, multiplied by the weight of a cubic 

foot of water. Columns 5 and 6 present the relative weight of each 

formation given as percent of the total weight. From these figures 

it can be seen that the Little Sitkin dacite, the Double Point dacite 

and the Williwaw Cove formation include more than 98 percent 

of the rocks.

Table 4 lists the weighted chemical composition of each of the rock 

sequences on Little Sitkin Island. These data are used with that in
200

columns 5 and 6, table 3 to calculate the average composition of all 

rocks younger than the Williwaw Cove formation, and the average 

composition of all rocks extruded on the present minus 300-foot bench. 

Composition of the minor amounts of the East Point and Sitkin Point 

formations, for which chemical analyses are lacking, are assumed from 

analyses of similar rocks. Plate 26 combines graphically the calculations given in tables 3 and 4 with the relative position of each rock 

sequence in time.

TABLE 4. Weighted chemical compositions, for volcanic rocks of Little Sitkin Island

SlOz.... ..

AljOs. 

F62O3- 

FeO.... ... ..

MgO.. .... ..

CaO.. - ... .

NasO... ....

KiO.... ..

HsO+._. ... .

TiOj ... .... .

PjOs ~ 

MnO_. .- 

Total.....

West 

Cove 

member i

60.00

16.81

2 91

4.77

3.07

6.90

 * 40

1.46

0.17

0.77

0.15

0.15

99.09

Pratt 

Point 

member 2

69.23

15.12 i 43

2.13

1.05

3 44

4 1Q

2 fW

0.58

0.36

0.12

100.00

Little 

Sitkin 

dacite 8

68.89

17 04

o Ra

4.67

3.31

7.33

3.43

1.30

0.39

0.76

0.16

0.15

99.99

Patterson 

Point formation*

60.63

17.00

2.64

3.44

2.71

6.11

3.74

1.43

1.40

0.72

0.16

0.13

100,01

Double 

Point 

dacite 

64.40

16 44

n 17

2.85

2.11

6.27

4

AK

1.67

0.21

0.55

0.14

0 11

99.99

East 

Point formation'

54.38

1 *\ O7

' q f\n

6.49

6.39

9 09

2 (J4

O QQ

0.18

0.76

0.14

0.16

100.00

Sitkin 

Point formation'

68.89

17 04

9 fifi

4.67

3.31

7.33

3.43

1.30

0.39

0.76

0.16

0.15

99.99

Williwaw 

Cove formation'

64.98

17 Qft

3:50

6.06

4.77

9.04

2 94

0.96

0.22

0.83

0.15

0.16

99.99

Average 

of volcanic rocks 

younger 

than Williwaw 

Cove formation «

63.1

16.6

2

O

3.2

2.4

6.8

3 Q

1.6

0.3

0.6

0.1

0.1

100.0

Average 

volcanic 

rock of 

Little 

Sitkin 

Island w

56.8

17.2

3

0

4.7

4.3

8.3

3

1

1.1

0.2

0.8

0.2

0.2

100.1

«Analysis 8.

> Average of analyses 12 and 13.

* Average of analyses 5 (except on FejOa and FeO), 6, 7, and 8, recomputed to contain 10 percent of 

analysis 3.

* Analysis 9.

* Average of analyses 10 and 11, recomputed to contain 6 percent of analysis 2. 

«Average of analyses 1, 2 and 4.

7 Average of analyses 5 (except on FeiOj and FeO), 6, 7 and 8, recomputed to contain 10 percent of 

analysis 3.

8 Average of analyses 2 and 4, recomputed to contain 10 percent of analysis 1.

* Computed from data in this table and in column 5, table 3. 

i° Computed from data in this table and in column 6, table 3.

Can information of the sort presented in plate 26 be used to 

decide the relative importance of the various end-member-mixing 

hypotheses which were formulated to explain the rectilinearity of 

the variation diagram? I think so. Mixing of acid and basic magmas can be ruled out because the quantity of rhyodacite or more acid 

material necessary to form the abundant dacite by admixture with 

the andesite is insufficient. For example, to form a magma of the 

composition of the Double Point dacite by 'admixture of magmas of 

the composition of the Williwaw Cove formation and the Pratt 

Point member would require over 750 times more rhyodacite than 

is known to exist. Incorporation and dissolution of basic inclusions 

in an acid magma can be excluded for the province as a whole for the 

same reason, but may have caused internal variations in a formation. 

For example some low-silica dacite in the Little Sitkin dacite may
201

have been derived by mobilization of basic inclusions in high-silica 

dacite magma of the composition of the Double Point dacite. To 

form all the rocks of the Little Sitkin dacite in this way would 

require an inclusion-magma ratio of 3:2; the observed ratio in the 

Double Point dacite is only 1:20, which suggests that only onethirtieth of the low-silica dacite could have formed by dissolving inclusions. The third mechanism, incorporation and dissolution 

of acid inclusions in a basic magma, is not supported by field evidence. No acid inclusions have been found in the rocks of Little 

Sitkin Island. The fourth mechanism, magmatic melting in a zone 

where the rocks are mixed, for example, by tectonic processes, is 

permissible on the basis of known evidence. Such a mechanism 

reconciles the chemical linearity with the volumetric relationships 

and is supported by the small scale inhomogenieties observed in the 

field (layered bombs, composite flow).

This belief in local crustal melting is supported by the current 

work of Powers (1955, p. 77-107) who has recently made an exhaustive survey of the chemistry of Hawaiian rock, in which he concludes 

that locally derived magmas are responsible for building up the primitive volcanic shields of the islands. The source of energy for the 

melting process, according to Powers, is friction produced by a cyclic 

kneading of the equatorial bulge of the earth's crust. This would 

only be applicable for regions within the earth's equatorial bulge 

but, for the Little Sitkin area, a tectonic kneading (not necessarily 

cyclic) along the Aleutian welt might be an even more efficient source 

of energy. Powers' remarks (1955 p. 101) in this connection are 

pertinent:

The proposed process of volcanism, if valid, should be applicable in part 

only to volcanoes situated along the equatorial bulge. However, its operation 

probably would be completely obscured by the processes of volcanism associated with great structural deformation in regions where the equatorial bulge 

is coincident with zones of Cenozoic mountain-building. Also different lava 

types would be expected where the bulge crosses the great blocks of sial. . . .

Other authors are concerned with magma mixing or sialic contamination in the origin of the intermediate lavas in orogenic areas. 

Because of their simultaneous extrusion with floods of uniform basalt 

on either side, Waters (1955, p. 712, 713) theorizes that andesites 

from the Cascade Kange of Oregon and Washington have been formed 

by the mixing of geosynclinal sediments with tholeiitic magma in 

tectogene roots. From an extensive study of numerous xenocrysts 

and from the close field association of many lava types Larsen and 

others (Larsen, Irving, Gonyer, and Larsen, 1938, p. 429) have concluded that the lavas of the San Juan volcanic field have been derived 

from intimately mixed magmas of diverse original composition.
202 INVESTIGATIONS OF ALASKAN VOLCANOES

Tilley (1950, p. 37, 58, 59) has tentatively concluded from a review 

of the world literature that andesitic rocks of the erogenic zones evolve 

from basaltic magma modified by sialic contamination.

In this report the author accepts the hypothesis that tectonic kneading may have been the cause of local crustal melting. Possibly this 

same tectonic kneading may have been the cause of the mixing of a 

crustal sialic component and an oceanic, or subcrustal, periodotic component which, when melted in magmas, could give rise to lava compositions that would produce rectilinear variation diagrams. Incomplete local tectonic mixing and the random migration of the loci 

of magma formation would explain both the random variation of the 

composition of rock formations through time and the second and 

third order inhomogeneities. All this would be dependent upon the 

constant composition of the contributed sialic and oceanic material, 

a condition that might be met only rarely. This would be consistent 

with the rarity of rectilinear variation diagrams.

GEOLOGIC HISTORY

The geologic history of Little Sitkin Island begins with probable 

middle Tertiary uplift of the Aleutian Islands arc (Gates, Fraser 

and Snyder, 1954; Gates and Gibson, 1956, p. 130) when Amchitka, 

Rat, and Kiska Islands were uplifted, and the general shape of the 

crest of the Aleutian Ridge and Bowers Bank were determined. Possibly the platform beneath Segula and Little Sitkin Islands was also 

formed. By this time also the great fissures that later became magma 

channels for the eruptions that produced Little Sitkin Island had 

been formed. At a depth of several miles in the earth andesitic magma 

began to flux, stope, or force its way to the surface during latest 

Tertiary or earliest Quaternary time. This magma, like the others 

to follow, was quite inhomogeneous. Parts of it were as basic as augite 

basalt; parts were as acid as dacite; most of it was close to the border 

between andesite and bandaite. With the first outpourings of this 

magma on the Segula-Sitkin platform the record decipherable from 

the rocks begins. Figure 43 gives a diagrammatic account of the 

geologic history.

The magma that formed rocks of the Williwaw Cove formation 

must have been rich in volatile material. Explosive eruptions were 

common and much ash and pyroclastic debris were buried with the 

massive flows. Although some fissures fed flows on the flanks of the 

growing composite cone, most of the eruptive products were distributed from a central vent. A volcano, perhaps 7,000 or 8,000 feet 

high, was built. A long period of erosion followed this eruptive activity. Probably mid-Pleistocene glaciers blanketed the slopes and
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 203

FIGURE 43. Diagrammatic review of geologic history of Little Sltkln Island. (1) Erection 

of andesitic composite cone on platform 300 feet below sea level In late Tertiary or 

early Quaternary time. (2) Formation of Caldera One after long period of erosion. 

(3) Deposition of Sitkin Point formation in offshore basin and in caldera lake and 

extrusion of flow sequence of East Point formation. These flows probably represent the 

last extrusion of the andesitic magma. (4) Erection of high-silica daclte lava-flow 

cone of Double Point dacite in late Pleistocene or early Recent time. All lava of this 

magma contains inclusions of solidified andesitic conduit rock. (5) Formation of Caldera Two with attendant nue"e ardente eruptions. A large block of the Double Point 

daclte (size exaggerated) has dropped only a small amount Into the caldera. (6) Extruslon of low-silica dacite lava continuing to the present. The Little Sitkin dacite 

here represented contains a composite dacite-rhyodacite flow.
204 INVESTIGATIONS OF ALASKAN VOLCANOES

accelerated the disintegration of the land, although most if not all 

of the deposits of this ice are now beyond the island margins. While 

the land was being denuded pressures were gradually building up 

inside the volcano. Finally a gigantic eruption occurred and a large 

part of the mountain collapsed. Possibly Prokhoda Scarp, between 

Little Sitkin Island and Semisopochnoi Island, was formed at this 

time. The remaining mountain was lowered relative to sea level; 

a caldera lake was formed; and the margins of the mountain were 

inundated. Explosive eruptions continufd for some time and tuffs 

and breccias, partly andesitic and partly dacitic, were deposited in 

offshore basins and in the caldera lake.

After the deposition of these tuffs and breccias (the Sitkin Point 

formation), the island was uplifted relative to sea level and the tuffaceous graywacke and tuff-breccia deposits were subjected to subaerial erosion. At about the same time a new vent was formed on 

the northeast side of the island and a thick series of fluid basalt and 

andesite (the East Point formation) was extruded. The volume 

of these post caldera pyroclastic deposits and flows was small compared to the huge mass of rock in the original mountain. Probably 

these deposits represent the last extrusion of the andesite magma.

Probably in latest Pleistocene or earliest Kecent time, immediately 

following extrusion of the East Point formation, a new magma was 

formed. This magma, the source of the Double Point dacite, was 

more silicic than the previous magma and formed high-silica dacite 

with minor amounts of more basic and more .acidic material. One 

distinctive characteristic of the magma was the presence of unique 

andesitic inclusions. Texturally these inclusions resemble none of 

the earlier rocks, but they ure chemically similar to the original 

magma of the present Williwaw Cove formation and were probably 

torn from the conduit walls at shallow depth by the silicic magma. 

The magma of the present Double Point dacite was relatively poor 

in volatile materials compared to the previous magma. Pyroclastic 

materials were subordinate and most eruptions were thick massive 

flows. A large central cone was constructed from these flows within 

the remaining shell of Caldera One. The lavas from this cone reached 

the sea on the southwest and northeast coasts of the island.

The volcanic cone that formed in Double Point time was then 

somewhat eroded. During the erosion period gas pressure was again 

building up and a new magma was forming, possibly partly by 

mixture with the inclusion-containing magma of the Double Point 

dacite. Movements on the Prokhoda Scarp fault zone may have triggered a cataclysmic eruption that resulted in the formation of another 

caldera. The part of the volcano formed during Double Point time 

that was nearest the extension of the Prokhoda Scarp fault was
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 205

shattered and sank in the abyss of Caldera Two. The northern third 

of the volcano, however, was only tilted slightly south into the caldera. Meanwhile voluminous nuee ardente eruptions from the new 

magma blanketed the island with low-silica dacite ash, and a thick 

deposit of welded ash 'accumulated on the northwest side of the island. 

Minor inhomogeneities in the magma formed "marble cake" bombs 

that were incorporated in the welded deposit. The initial nuee 

ardente phase was soon succeeded by a vulcanian phase in which 

relatively cold ash and a minor amount of angular lithic debris was 

erupted. Some lava flows, the lower part of the Little Sitkin dacite 

were possibly extruded at this time.

After the Caldera Two eruption, the magma that formed the 

Little Sitkin dacite approached closely to the surface in a number 

of places along a zone trending N. 67° W. across the island. Rocks 

in this zone were altered by fumarolic activity. Lava eruptions 

occurred slightly east of the center of the island on the south flank 

of the remainder of the cone that formed in Double Point time. 

Volatile content in the magma was low so that only lava flows were 

extruded. These lavas were predominantly low-silica dacite but 

several andesitic flows were extruded and, in the Pratt Point member, 

a considerable quantity of mixed low-silica dacite and rhyodacite 

were extruded from the same vent. About a half centry ago, two aa 

flows were extruded, one from the breached central crater and one 

from a fissure of Caldera One in the west central part of the island. 

Some faulting was probably associated with the eruptions. Both flows 

have been completely altered to clay in areas where they have crossed 

active fumarolic centers. A small amount of native sulfur was deposited in the fumarolically altered area of the crater flow. Two small 

avalanche deposits on the north coast may have been instigated by 

earthquakes 'associated with this last eruption of Little Sitkin 

volcano.

ECONOMIC GEOLOGY

Sulfur is the only economic mineral available in potentially 

commercial quantities on Little Sitkin Island. A large deposit of 

sulfur riddled with active fumaroles occupies an area of about 10 

acres on the south side of the present breached crater where it replaces 

low-silica dacite of the crater flow. Traces of native sulfur have 

been observed around the fumarolic area south of the 1,303-foot 

mountain. In the crater area all rocks are bleached and altered to 

white kaolinitic( ?) clay. The sulfur occurs as veins and vug linings 

in this clay. Usually the entire surface deposit is composed of massive, mammillary aggregates of sulfur crystals without much rock 

clay.
206 .INVESTIGATIONS OF ALASKAN VOLCANOES

Sulfur replacement presumably decreases markedly with, depth. 

Small amounts of H2S gas are always present in the abundant 

quantities of water vapor.being emitted. Sulfur is probably being 

deposited primarily as the result of near-surface partial oxidation of 

this gas and secondarily <as a sublimate.

A sample collected in the central part of the deposit contained 

95.8 percent sulfur by weight. Assuming that the deposit averages 

this grade throughout a surface zone 10 feet thick, 200,000 tons of 

sulfur is present. Probably, this estimate, is too large. Estimates of 

the amount of sulfur in the fumarolic deposit atop Makushin Yolcano 

on Unalaska Island (roughly the same size or slightly smaller than 

the deposits on Little Sitkin Island) range from 33,500 to more than 

77,000 tons of recoverable sulfur (Maddren, 1919, p. 291,292; Sweeny 

and Myers x ). :

Kecovery of the sulfur on Little Sitkin Island is favored by a 

source of heat and energy at the fumarole, and by a source of pure 

snow water at the crater lake. Other factors hinder recovery: great 

shipping distance from the world market, absence of a suitable harbor 

for ships, rugged terrain between coast and crater deposit, generally 

inclement weather. In the face of these difficulties it does not seem 

probable that the sulfur deposit on Little Sitkin Island will be exploited in the near future. In the Andes, however, similar volcanic 

deposits subject to rigorous climate and altitude conditions are being 

commercially utilized at the present time (Cooke, 1954, p. 44r48, 71).

REFERENCES CITED

Arctic Weather Central, llth Weather Squadron, 1950, Climate, weather and

flying conditions of Alaska and eastern Siberia: Elmendorf Air Force Base,

Alaska, p. 1-52. 

Earth, T. F. W., 1952, Theoretical petrology: New York, John Wiley & Sons,

Inc., 387 p.; 

Coats, R. R., 1950, Volcanic activity in the Aleutian Arc: U. S. Geol. Survey Bull.

974-B, p. 35-49.

 1952, Magmatic differentiation in Tertiary and Quaternary volcanic rocks 

from Adak and Kanaga Islands, Aleutian Islands, Alaska: Geol. Soc. 

America Bull., v. 63, no. 5, p. 485-514.

 1953, Geology of Buldir Island, Aleutian Islands, Alaska: U. S. Geol. 

Survey Bull. 989-A, p. 1-26. 

-1956, Geology of northern Eanaga Island, Alaska: U. S. Geol. Survey

Bull. 1028-D, p. 69-81. 

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

Their people and natural history: Smithsonian Inst., Washington, War

Background Studies, no. 21, p. 1-131. 

Cooke, H. R., Jr., 1954, How high can mining go?: World Mining, v. 7, no. 9,

p. 44-48, 71.

1 Sweeny, H. P., and Myers, D. B., 1908, The sulfur deposit of Mount Makushin, 

alaska: unpublished undergraduate thesis, Massachusetts Inst. Technology, p. 14-17.
GEOLOGY OF LITTLE SITKIN ISLAND, ALASKA 207

Fenner, C. N., 1926, The Katmai magmatic province: Jour. Geology, v. 35, no. 7,

pt. 2, p. 673-772. 

 1938, Contact relations between rhyolite and basalt on Gardiner River,

Tellowstone Park: Geol. Soc. America Bull., v. 49, p. 1441-1483. 

Gates, Olcott, Fraser, G. D., and Snyder, G. L., 1954, Preliminary report on

the geology of the Aleutian Islands: Science, v. 119, no. 3092, p. 446-447. 

Gates, Olcott and Gibson, W. M., 1956, Interpretation of the configuration of

the Aleutian Ridge: Geol. Soc. America Bull., v. 67, no. 2, p. 127-146. 

Qibson, W. M., and Nichols, Haven, 1953, Configuration of the Aleution Ridge,

Rat Islands Semisopochnoi I. to west of Buldir I.: Geol. Soc. America Bull.,

v. 64, no. 10, p. 1173-1187. 

Larsen, E. S., Irving, John, Gonyer, F. A., and Larsen, E. S. 3d, 1938, Petrologic

results of a study of the minerals from the Tertiary volcanic rocks of the

San Juan region, Colorado: Am. Mineralogist, v. 23, no.. 7, p. 417-429. 

MacGregor, A. G., 1952, Eruptive mechanisms Mt. Pelee, the Soufriere of St.

Vincent [West Indies] and the Valley of Ten Thousand Smokes [Alaska]:

Bull. Volcanologique, ser. 2, v. 12, p. 49-74. 

Maddren, A. G., 1919, Sulphur on Unalaska and Akun Islands and near Stepovak

Bay, Alaska: U. S. Geol. Survey Bull., 692-E, p. 283-298. 

Peacock, M. A., 1931, Classification of igneous rock series: Jour. Geology, v. 39, no.

1, p. 54-67. 

Powers, H. A., 1955, Composition and origin of basaltic magma at Hawaii:

Geochiinica et Cosmochimica Acta, v. 7, p. 77-107. 

Reynolds, D. L., 1954, Fluidization as a geological process, and its bearing

on the problem of intrusive granites: Am. Jour. Sci., v. 252, no. 10, p.

577-613. 

Rittmann, A., 1952, Nomenclature of volcanic rocks: Bull. Volcanologique, ser. 2,

v. 12, p. 75-102. 

Snyder, G. L., 1957, Ocean floor structures, northeastern Rat Islands, Alaska:

U. S. Geol. Survey Bull. 1028-G, p. 161-167. 

Tilley, C. E., 1950, Some aspects of magmatic evolution: Quart. Jour. GeoL

Soc. of London, v. 56, part 1, p. 37-61. 

Troger, W. E., 1952,' Tabellen zur optischen Bestirnmung der gesteinsbildenden

Minerale: 147 p., Stuttgart, E. Schweizerbart'sche Verlagsbuchhandlung

(ErwinNagele) p. 113. 

U. S. Coast and Geodetic Survey, 1947, United States coast pilot, serial no. 680,

Alaska, Pt. 2, Yakutat Bay to Arctic Ocean, p. 1-659; or serial no. 659,

Alaska, Aleutian Islands, p. 1-147. 

Waters, A. C., 1955, Volcanic rocks and the tectonic cycle: Geol. Soc. America

Special Paper 62, p. 703-722. 

Washington, H. S., 1917,, Chemical analyses of igneous rocks: U. S. Geol. Survey

Prof. Paper 99. 

Weather Central, Alaska, llth Weather region, A. A. F., 1945, The climate,

weather behavior, and topography of selected Alaskan and Aleutian stations:

p. 1-59. 

Wentworth, C. K., and Macdonald, G. A., 1953, Structures and forms of basaltic

rocks in Hawaii: U. S. Geol. Survey Bull. 994, 98 p. 

Wilcox, R. E., 1944, Rhyolite-basalt complex on Gardiner River, Yellowstone

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Williams, Howel, Turner, F. J., and Gilbert, C. M., 1954, Petrography an introduction to the study of rocks in thin sections: San Francisco, W. H. Freeman

and Co., 406 p.

INDEX

Page

Abstract..._____..__.________ 169 

Acknowledgments-______________ 173 

Alluvium deposits.......................... 183-184

Altitude .. .._....... .....____. 172

Andesite, analyses of..___________ 194-195

petrographlc character of.._______ 191

Animal life. .... . _. ...... 172

Basalt, analyses of __ _ 194-195

petrographlc charactei of. _ _ 190-191

Beach deposits 184

Oaldera One ___ ._ .. 184-185,203,204 

Caldera Two. . 185,203,204-205 

Chemical analyses of rocks.. . __ _ 194-195 

Chemical composition of rocks, weighted... 199-200 

Chodos, A. A., analyses by_________ 194 

Climate..____....____..... ...__ 170 

Colluvlal deposits.. ... _ _.. 184

Daclte, hlgh-slllca, analyses of._______ 194-195

high-silica, petrographic description of 192-193

low-silica, analyses of_________ 194-195

petrographic description ot. 191-192

Dikes, In Williwaw Cove formation; 174-175,187-188

Double Point dacite, age of ___......... 180

basal contact... .. . 179-180 

chemical analyses..___.___-__ 194-195 

chemical composition of _________ 200 

description...___....._._.___ 177-179

extrusion of.___________.____ 203,204 

outcrops. __ ..._ __ . 177-179 

petrographic character of..-.._ _ 187,188 

petrographlc description of andesite from. 191 

petrographic description of high-silica

dacito from.........._ _ 192-193

volume___________________ 199 

See alto Specific gravity and Weight of 

volcanic rocks.

East Point formation, chemical composition.. 200 

description 177 

petrographic character of._______ 187,188 

volume. ___.___..___.___. 199 

&ee also Specific gravity and Weight of

volcanic rocks.

Engleman, Edythe E., analyses by..._ 194,195 

Economic geology______.___ _ 205-206

Fieldwork.................................... 173

Oeochemlcal studies.____..___ _ 193-202 

Geologic history.......... ............. 202-205

Hornblende......_.. ................... 190

Hyman, Harry M., analyses by. __.__ 194,195

Introduction.

Page 

169-173

Joints _ _ -._ ___ 186

Little Sitkin dacite, chemical analyses. 194-195 

chemical composition of. ... . 200 

exposures of.__ .._ __.__ 181-182 

lithologlc character..__._______ 183 

petrographic character of. 187,188-189 

petrograpbic description of low-silica

dacite from . 191-192

petrographic description of andesite from. 191

Pratt Point Member..._ _...._ 182,

187,189,193,194-195,199, 200

West Cove member . 182-183,

185,187,189,194-195,199, 200

volume of-- _ __ ______ 199

See also Specific gravity and Weight of

volcanic rocks. 

Location.______.........___... 169-170,191

Normative values . _ 195

Olivlne ____....__.__.__ 190,193-196

Origin of volcanic rocks. . 196-202

Patterson Point formation, chemical analyses. 194-

195

chemical composition of _ ....___ 200 

exposures of---__ . . 180-181 

Tithologic character . 180-181 

mode of origin. _ . .. _..... _ 181 

petrographic character of.. ___-_ 187,188 

petrographic description of low-silica dacite

from. . 192 

volume. 199 

See alto Specific gravity and Weight of

volcanic rocks.

Petrographic character of the rocks 186-193 

Physiography____..... . . 172 

Plagioclase__ 189 

Powers, H. A., quoted 201 

Pratt Point member of Little Sitkin dacite,

chemical analyses .__ . 194-195 

chemical composition of... _ . 200 

description... 182 

petrographic character of. 187,189 

petrographic description of rhyodacite

from __1 193 

volume of. 199 

See also Specific gravity and Weight of

volcanic rocks. 

Pyroxene-___-___ - 189-190

Quartz.- __.... 190,193-199

209
210 INDEX

Page

References cited ____ ______ 206-207

Rhyodacite, analyses of__________., 194-195

petrographlc description of _ ____ . 193

Rittmann nomenclature ___ 195,196

Sand dunes___.____._______-... . 184

Silica-variation diagrams. ___._.. 196-198

Sitkin, meaning of._____-_____-,.. 172

Sitkin Point formation, age of.... ... 175,177

chemical composition of..._____.... 200

deposition of.... ... . ... 203,204

description....____._______.... 175

exposures of..._ ... ____.. . 175

petrographlc character of...._ 187,188

volume_________._____---- 199

See also Specific gravity and Weight of

volcanic rocks.

Specific gravity of rocks of Little Sitkin Island. 199 

Spectrochemical analyses___________ 194 

Structural geology.___.___.____.. 184-186 

Surflcial deposits_________.___... 183-184 

Sulfur...................................... 205-206

Topography . . ......... 172

Vegetation....-.-......... ............. 170,172

Volcanic rocks, mode of derivation....__.. 196-202

methods of computing volume.____ 198-199 

volume of.. ___._._____.... 199

Page

Volcanic rocks Continued

weighted chemical composition of 199-200 

weight of._... .. . 199

Volume, method of computing._ ___ 198-199 

of volcanic rocks _.__.. ...__ 199

Weight of volcanic rocks.__.. .. 199

West Cove member of Little Sitkin dacite,

chemical analyses . ..... 194-195

chemical composition of- 200 

description________ ..___ 182-183 

petrographic character of.. 185,187,189 

volume of_______...... ___.. 199

See ako Specific gravity and Weight of volcanic rocks.

Williwaw Cove formation, age of. 174 

analyses of basalt from_ . . _ 194-195 

chemical composition_ . .. ... 200 

description._____. . . 173-174 

dikes in...__..... .. ..-. 174-175 

eruption and deposition of. 202-204 

outcrops________ . 173 174 

petrographic character of. - 186-188 

petrographic description of andesite from. 191 

petrographic description of basalt from. 190-191 

volume____________ ___. 199 

See also Specific gravity and Weight of volcanic rocks.

Williwaw winds.___....__. ........ 170

o