Geology of Umnak and 

Bogoslof Islands 

Aleutian Islands 

Alaska

By F. M. BYERS, JR.

INVESTIGATIONS OF ^ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-L

Prepared in cooperation with the Office, 

Chief of Engineers, U.S. Army

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1959
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D.C.
PEEFACE

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. 

Field studies under general direction of G. D. Robinson, were begun 

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

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

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

1950 in Geological Survey Bulletin 974-B, "Volcanic Activity in the 

Aleutian Arc", by Robert R. Coats. The rest of the data has been 

included in Bulletin 1028.

The geologic investigations covered by this report were reconnaissance. The factual information presented is believed to be accurate, but many of the tentative interpretations and conclusions will 

be modified as the investigations continue and knowledge grows.

The investigations of 1946 were supported almost entirely by the 

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

Army. The Geological Survey is indebted to that Office for its early 

recognition of the value of geologic studies in the Aleutian region, 

which made this report possible, and for its continuing support.

in

CONTENTS

Page

Abstract__________._..!__'_____.--__-___________________ 267

Introduction. _________________ ___ _______________________________ 268

Fieldwork and acknowledgments____.-_________________---____ 269

History and previous geologic work____________________-___-_____ 270

Russian period, 1741-1867.-___---____--__.-_-_-_____ 270

United States Territorial period._______-_-_______._----__-- 271

Geography____________________________________________ 272

Location, settlements, and accessibility_-___-___-_---_-__--_-__- 272

Hydrography and topography___________---___-___-____--_-_-_ 272

Climate, vegetation, and animal life_______-_-_-_-_-_______-_-_ 275

General geologic setting________________________________________ 276

The eastern Aleutian arc-__-__-_-_----__--_-----_--_------_--_- 276

Area of this report________________._---____.____--._ 278

Rock names used in this report_______________________________ 279

Geology of Umnak______________________________________________ 285

General features______________________-_----__-_.___--_-_-- 285

Albitized sedimentary and igneous complex___-_______._-_____-_-_ 285

Distribution and thickness___---______-_-_--__--___---_--_ 285

Structure__--__________._____-_-_ --_._------- 286

Relation to younger rocks.-_-__--___-____-----_-_-_-------- 286

Petrography.._______________.._._.._.....-_.-----__ 286

Argillite__________._-_.___._._____... 286

Tuff.._____._..._._._.________________--_--_- 287

Albitized intrusive rock________________________________ 287

Keratophyre_____________-_- _-----------.-_ 288

Age and correlation..____.___--_-___________________---_--_ 289

Plutonic rocks____.._____._.___._.___.____.____'______'______ 289

, Geologic relations. .__.____-__-_-_______._._-____-_----__- 289

Petrography _________________._._..._..._._-_-___ 290

Diorite__________________..__-________._ 290

Quartz diorite______--_-.______-_-_----_______--__-__ 290

Quartz monzonite.-..-- -_-_-----_-----_------ ----_ 291

Granophyre___________-________._---_-__-.--_--__--_ 292

Altered contactrock__-____-__-_-__--_-_--_--_-_----___ 292

Volcanic rocks of central Umnak_________.__________.__----_._ 293

Geologic relations______--____-__-____--_-_-_--__-_--_-_- 293

Age and correlation..____._________________________._______' 294

Petrography and chemical composition.______________________ 295

Volcanic rocks of Mount RecheschnoL_____.__---__-----_-----_ 297

General geologic features..--_________________-____-----_-_ 297

. Hypersthene ahdesite______-_---__----__--_---------------- 297

Mafic phenocryst basalt of Kshaliuk Point________.-__-___-_ 298

Youngest hypersthene andesite flows.________________________ 298

Pyroclasticflow...__________________-____-_---_- 298

Lava flows______.-._-___---..________-_--_-.,-_ 299
VI CONTENTS

Geology of Umnak Continued

Volcanic rocks of Mount Recheschnoi Continued Page

Rhyolite domes.__________....---.______._-.____._____ 299

Quartz-olivine andesite flow____ ............................. 299

Age of lavas of Mount Recheschnoi.-____._.__.______-__ _ _ _ 299

Petrography ..................^.^......................... 299

Hypersthene andesite_-___-____..______________________ 300

Rhyolite__________.--_-____-_-___.____________ 301.

Quartz-bearing olivine andesite. ......................... 302

Volcanic rocks of Mount Vsevidof-----------__----_---__-__-____ . 304

General geologic features_____:_____._.___.__________________ 304

Age and correlation_---__--___-------_-__----__--_-__--- _ 304

Petrography. ............................................. 305

Basaltic andesite flows..:_---__.-------l--_-_----_-_-_- 305

Andesite and latite flows-__-_----_------_-_--__-------- -306

Andesitic and rhyodacitic pyroclastic beds.______-____-_-- 307

Volcanic rocks of Okmok volcano.._ ............................. 308

Rocks of precaldera age. ................................... 309

Ashishik basalt_________-----__---_-_-_----___---_-________ 309

Mafic phenocryst basalt.___'__-__ '.'...................... 309

Palagonitized pyroclastic rocks_____:____________________ 310

Aphyric and feldspathic bas'alt_l--_-l--_---_'____________ 311

Volcanic rocks of minor vents__----__-----___-__---_________ 312

Vitreous andesite____--_---_-____--_-_.________________ 312

Rhyolite--_______________________...__.._....__._ 312

; Plagioclase-olivine basalt of satellitic vents___1 __________ 312

Crater Creek basalt..______________________________________ 313

Age of precaldera rocks___________________________________ 314

Okmok volcanics of caldera age.. ........................... 314

Agglomerate deposits-----_-l_---_-----_----__-_-_-___- 315

Bedded ash_-__----____________-___-___---_-_____-_-_- 321

'; Origin and emplacement of Okmok volcanics--_____________ 321

; Age of the Okmok volcanics--------- -----__------__-__ 322

1 Basalt flows outside Okmok Caldera_'_____'__.______________ 323

; Mafic phenocryst basalt on caldera rim_________________ 323

Basalt flows on the arcuate ridges.___--_-------__---_-__ 323

Basalt flow oh Cape Aslik______-_l__-_---_---_-__--_.__ 324

Basalt flow near Cape Idak_______^_____________________ 324

Anorthite basalt flows near Inariudak Bay.__-____-_-___._ 324

Rocks of postcaldera age within Okmok Caldera_____________ 324

Rocks contemporaneous with caldera lake .1.............. 325

Basaltic flows later than caldera lake___________________ 327

Petrography of Okmok Volcaho_-_--_-----_--____-----_---__ 329

Olivine-rich basalt.-____________._________ 330

Plagioclase-olivine basalt_________-________________--__- 330

Plagioclase-augite-olivine basalt_____________--_-_---_-__ 330

Anorthite basalt.___'-__"- __'___'________--_____-___--_-- 332

Aphyric basalt and feldspathic basalt._.___----___-_--___ 334

Andesite and latite___________________________________ 335

Rhyolite_____-_::__-____----_._____-__------_-----_--- 336

Palagonitized pyroclastic rocks.__________-_-_____----_-- 336

Structure of Okmok Volcano..__________._- ____.___._..--- 337

Origin of Okmok Caldera.--.--------_________------------ 340
CONTENTS VII

Geology of Umnak Continued

Surficial deposits.__-_____-____-----_------_--__----- -------- 343

Till___________.______.__--_____-_--------_--------------_ 343

"Outwash__-__...___._.-.-_--_.-. -. _--- 344

Beach deposits.__________-_-___-_-_____--.___-_____-_--_-_ 344

Sand dunes----_.__-_-__---------------------------------- 346

Talus and other colluvium.--------------------------------- 346

Alluvium________________-_-_-_---___-___--_-__----------- 346

Glacier ice._--_-.__-______--___-_--_-__--_-_------_--__-_- 346

Erosional features.___________________-___-----_-_-_-_-_---__-- 347

Crater Creek valley_____________-_-_-_-______-_ ___----_ 347

Pedimentlike slopes on Okmok Volcano-_.___-___-._---___-_- 348

The Nikolski plain Evidence of an older glaciation.__________ 348

Geologic history____-_________------_---_------_-_--__--------- 349

Geology of Bogoslof_________,._-____-___-----_--___---_____ 353

Geology of Bogoslof in 1947_______.-._-_-_-__-_-_____--_____ 353

Vent agglomerate of 1796____-_____--_____-.._______--- 354

Hornblende andesite of Castle Rock.________________________ 354

Hornblende basalt of Fire Island.__--_--__-___--___-____-___ 355

Basaltic extrusive rocks.of 1926-27...-_______________________ 356

Beach deposits.__-_-_-_._---_--_--_----_--^-------__-__-_- 357

Structure_-___..____. _______________________________________ 357

Geologic history.___J___________-_____-.____--_-_______________ 357

Literature cited-_________.-__-_____-_-.-----___-.___--______-__-_. 361

Index. ______________________________.________.-.-_..____ 367

Page

PLATE 39. Generalized geologic map of Umnak, Bogoslof, and part of 

Unalaska Islands, showing adjacent submarine topoggraphy____________________________l_____________ In pocket

 ' 40. Geologic map and sections of southwestern Umnak Island. In pocket

41. Geologic map and sections of northeastern Umnak Island. In pocket

42. A, Photomicrograph of composite plagioclase crystal in albitized diorite. B, Photomicrograph of partly replaced 

albite phenocryst in hydrothermally altered keratbphyre_ Faces 290

43. Photomicrograph of plagioclase crystals veined and corroded

'by orthoclase in altered quartz diorite______________ Follows 290

44. Mount Vsevidof from the south____________________ Follows 290

45. View of east wall of Okmok Caldera.___________,_^ Faces 291

46. A, Nearly flat-lying, undisturbed flows of Crater Creek basalt 

exposed in the northwest wall of Crater Creek gorge. B] 

View northeast toward sea cliff northeast of Cape Chagak. Faces 306

47. A, Hand specimen of dark-gray welded andesitic agglomerate. 

B, Photomicrograph of small mass of basaltic andesite enclosed by rhyolite___-___________________________ Faces 307

48. Section through cones C and D_ ______________________ In pocket

49. Photomicrograph of anorthite phenocrysts in anorthite basalt

flow____.___________________._.____ Faces 322
VIII CONTENTS

Page 

PLATE 50. Perspective diagram of Okmok Volcano viewed from the

northeast-__--__------------- ------------------- Faces 323

51. Castle Rock, Bogoslof Island, from the southeast in 1947. Faces 354 

FIGURE 49. Index map of Alaska showing location of Umnak and Bogoslof

Islands________-_-------___:_______.-.-_ 269

50. Eastern part of Aleutian volcanic arc, showing positions of

volcanoes and Aleutian Trench_______________________ 277

51. Stereographic projection of bisectrices and optic normals of

anorthite pheriocrysts in anorthite basalt________________ 333

52. Profile through summit area of Okmok Volcano, showing

hypothetical maximum and minimum profiles.___________ 342

53. Geologic map of Bogoslof in 1947____________--_._ 353

54. Significant changes in Bogoslof Island between 1768 and 1947_ 358

TABLES

Page 

TABLE 1. Climatological data for two stations on Umnak Island, June

1942 to December 1948.________-___________ 275

2. Analyses of rocks from Umnak and Bogoslof Islands________ 280

3. Petrographic summary of chemically analyzed rocks from

Um'nak and Bogoslof Islands.-...____________________ In pocket

4. Petrographic analyses of plutonic rocks (modal analyses in

volume percent)______________________________________ 291

5. Partial analyses of hydrothermally altered (potassium feldspathized) volcanic rocks near Bottleneck Pass.____________ 296

6. Petrographic analyses of specimens of hyperthene andesite of

Mount Recheschnoi------------------------------------ 300

7. Petrographic analyses of three mutually perpendicular thin

sections of quartz-bearing olivine andesite_________________ 303

8. Comparative petrographic data: rhypdacite pumice of Mount 

Vsevidof and pumice fragments in ash beds of archeological 

profile, Nikolski______.___ _______._____ 305

9. Petrographic analyses of basaltic andesite, andesite, and latite

flows of Mount Vsevidof________---_-___--._________-___ 306

10. Petrographic analyses of rocks in dark andesitic scoria bed of

Mount Vsevidof cone.-___-_______-_-________________i__ 307

11. Petrographic analyses of agglomerate and ash marker beds of

the Okmpk volcanics--^--------.------------------.---- 318

12. Petrographic analyses of mafic phenocryst basalt, Okmok

Volcano_____________________________________________ 331

13. Optical data on anorthite phenocrysts in anorthite basalt near

Inanudak Bay____-_--------_-_------_----_---_-------- 334

14. Petrographic analyses of vitreous andesites and latite of Okmok

Volcano___.-___-____________.____..-_-____ 335

15. Petrpgraphic analyses of rhyolite specimens and xenolithic

inclusion, Okmok Volcano-----------------..------- __ 33fi
INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, 

ALEUTIAN ISLANDS, ALASKA

By F. M. BYEBS, JK.

, . . ABSTRACT

Umnak and Bogoslof Islands are in the eastern part of the Aleutian Island arc, 

a geanticlinal uplift paralleled on the southerly (convex) side by the Aleutian' 

Trench. Umnak Island, 675 square miles in area, is separated into northeastern 

and southwestern parts by a constriction in the central portion. The rocks 

consist of late Tertiary and Quaternary ^volcanic rocks that- rest on a basement - 

complex of probable early to middle Tertiary plutonic and low-grade meta-- 

morphic rocks! Bogoslof, 25 miles north of Umnak, is composed almost entirely 

of historic lavas. '.'"'"

The oldest geologic unit, an albitized sedimentary and igneous complex probably 

largely of early to middle Tertiary age, is exposed near sea level on southwestern - 

Umnak and includes folded argillite and tuff, keratophyre flows, and albitized 

intrusive rocks. Post-Oligocene plutonic rocks, largely quartz diorite, intrude 

the albitized sedimentary and igneous complex. Granophyre dikes cut the larger 

body of quartz diorite and are associated with a wide aureole of potassium feldspathized quartz diorite. The volcanic rocks of central Umnak include both 

potassium feldspathized volcanic rocks that are probably prepluton in age and 

unaltered lavas of probable late Tertiary to early Quaternary age. The unaltered' 

lavas rest unconformably on the albitized sedimentary and igneous complex. ' 

 Two Quaternary volcanoes, Mount Recheschnoi and Mount Vsevidof, overlie 

with an erosional unconformity the older rocks on southwestern Umnak. Mount 

Recheschnoi is a deeply dissected ridge remnant of a composite stratovolcanic 

cone composed largely of hypersthene andesite. Satellitic lavas of Mount 

Recheschnoi comprise mafic phenocryst basalt, extruded early in the history of 

the volcano, rhyolite domes, and a quartz-olivine basalt flow of early Recent age. 

Mount Vsevidof is composed of an older sequence of basaltic andesite and sparsely 

hypefsthene-bearing andesite flows, an intermediate sequence of pyroclastic 

rhyodacite pumice and andesite scoria beds of a culminating summit eruption, 

and younger andesite and basaltic andesite flows, including a historic(?) latite flow. 

: Okmok Volcano on northeastern Umnak, in contrast to the andesitic stratovolcanoes of southwestern Umnak, consists largely of basalt flows and has a 

central caldera, Okmok Caldera, 6 miles in diameter. The rocks of Okmok 

Volcano are conveniently divided into precaldera rocks, Okmok volcanics, the 

deposits associated with the caldera-forming eruption, and postcaldera rocks. 

The.precajdera rocks include the.Ashishik basalt of latest Tertiary and Quaternary age and volcanic rocks of minor vents, including vitreous andesite, rhyolite,

267
268 INVESTIGATIONS OF ALASKAN VOLCANOES

and plagioclase-olivine basalt of satellitic vents. The youngest precaldera unit 

is the Crater Creek basalt of latest Pleistocene or early Recent age. The Okmok 

volcanics of early Recent age are composed of a few very thick beds of unsorted 

andesitic agglomerate, welded in the middle parts of beds; and an overlying unit, 

consisting of sorted ash, bomb, and lapilli beds. Postcaldera rocks within Okmok 

Caldera include early postcaldera pyroclastic rocks, plagioclase basalt flows of 

cones C and D, and bedded volcanic sediments. Subaerial basaltic flows 

later than the caldera lake include those from several vents within the caldera, 

the latest flow having been extruded as recently as 1945. The undissected 

condition of several cones and flows outside Okmok Caldera suggests that they 

are also postcaldera in age.

The structure of Okmok Volcano is chiefly that of a low shield volcano with 

gentle outward dips. A fault is exposed for a short distance at the periphery of 

Okmok Caldera, and the measurable displacement is toward the caldera. The 

steep infacing cliff of Okmok Caldera is therefore inferred to be the scarp of a 

caldera ring fault. A few faults are radial to Okmok Caldera. The north and 

the east arcuate ridges may have originated in any one of three ways; they may 

be receded fault scarps of an older caldera ring fault, they may have formed 

concomitantly with the collapse of Okmok Caldera, though there are several 

objections to this hypothesis, or they may represent accumulations of volcanic 

debris erupted from vents localized along a fissure beneath the arcuate ridges themselves with little or no fault movement. The first hypothesis, .modified,by the 

third, seems most plausible.

Regardless of which of two extreme assumptions is made in regard to size and 

shape of the missing summit of Okmok Volcano, the total volume of lithic fragments in the Okmok volcanics is not nearly sufficient to account for the missing 

summit. Hence, the missing portion of the summit must have collapsed inward 

to form Okmok Caldera.

Surficial deposits of Umnak comprise older till, younger till, glacier ice, glacial 

outwash, alluvium, talus, beach deposits, and dune sand. Older till of latest 

Wisconsin or early Recent age forms a series of end moraines several miles in 

front of the present glaciers. Younger till is confined to moraines within a mile 

of the present glaciers and probably records the most recent glacial advance, 

100 to 300 years ago. Beach deposits are in as many as 20 concentric beach 

ridges parallel to the present beach. The older till is covered in part by these 

beach ridges, which are believed to date from the high, 3-meter stand of sea level, 

generally ascribed to the post-Wisconsin thermal maximum. 

. Hornblende andesite and basalt exposed in 1947 on Bogoslof Island are all of 

historic age. The oldest rock, representing the explosive phase of the 1796 eruption, is an andesitic vent agglomerate in fault contact with the hornblende andesite 

dome also extruded in 1796. The hornblende basalt dome .of Fire Island was 

extruded in 1883 following a violent eruption. Domes were erupted in 1906,1907, 

1910, but no specimens of these domes were available for study, owing to explosive disruption of the domes soon after they formed. Basaltic agglomeratic 

ash erupted in 1926 and a basalt dome extruded in early 1927 formed most of the 

area of the island in 1947. From 1927 to 1953 Bogoslof was steadily reduced in 

size by marine erosion.

INTRODUCTION

In June 1945, a spectacular eruption of Okmok Volcano on the 

northeast part of Umnak Island in the Aleutian Islands (fig. 49) 

threatened a military base on the eastern slope of the volcano. Al-
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 269

180° ^Q. ________-170°__________160"_________150°_________140°__________130°

160° 150° ' 140*

FIGURE 49. Index map of Alaska showing location of Umnak and Bogoslof Islands.

though this eruption did no damage, military authorities were alerted 

to the fact that personnel and installations in the Aleutian region are 

constantly exposed to possible danger and damage from its many active' 

volcanoes. In October 1945, the United States Geological Survey at 

the request of the Alaskan Defense Command, United States Army, 

began a program of systematic geological and geophysical investigation of the Aleutian Islands. As the site of a large base and of a currently erupting volcano, Umnak was one of the first islands investi^ 

gated. A restricted report to the Department of Defense in 1947 

emphasized the possibilities of damage by volcanic eruptions to the 

local military base. The present report deals primarily with the 

stratigraphy and petrography of rocks on Umnak and its tiny but 

famous neighbor, Bogoslof Island; the glaciers and glacial geology 

also are briefly discussed.

FIELDWORK AND ACKNOWLEDGMENTS

This report is based on fieldwork during the summers of 1946 and 

1947. The writer was assisted in 1946 by D. M. Hopkins and Bernard 

Fisher, geologists. Hopkins mapped most of the area within Okmok 

Caldera, (pi. 39) and part of the description and interpretation of the 

postcaldera deposits is based on his work. Fisher and two Army men
270 INVESTIGATION^. PE ALASKAN VOLCANOES . > were drowned in Umnak Pass on June 22, 1946, when they attempted 

to land their power dory on Ship Rock, a small island off the coast of 

Umnak. The loss of Fisher dealt a severe blow to the progress of the 

field work. In 1946 the geologists were aided by Kenneth L. Wier, 

geophysicist, and by field assistants W. E. Hassman, O. L. Smith, G. 

A. Hill, and J. M. Borglum. Fieldwork in 1946 was confined to 

northeastern Umnak. In 1947, the writer, accompanied by W. S. 

Ogburn, field assistant, completed mapping of northeastern Umnak, 

made a 6-week reconnaissance of southwestern Umnak, and spent 1 

day on Bogoslof. In 1948 the writer, accompanied by Howard A. 

Powers, supervising geologist, spent several days reexamining parts 

of northeastern Umnak.

Ray E. Wilcox, presently geologist of the Geological Survey, kindly 

made available specimens and information he had collected on Umnak 

during 1945, while serving in the Army.

Many officers and enlisted men, formerly stationed at Fort Glenn, 

cooperated fully in providing transportation and supplies. Maj. Jim 

McCali, commanding officer of Fort Glenn during the 1946 field season, 

Maj. Clarence J. Jackson, commanding officer during the 1947 field 

season, and Capt. Laverne E. Dye, adjutant during the 1947 season, 

were especially helpful. At Nikolski, H. Frank Barnett, and Harvey 

B. Bell gave material aid in August 1947, during two rainy periods. 

During 1947 and 1948, Harald Drewes gave valuable assistance in the 

office. The writer received many helpful suggestions in 1948 at the: 

University of Chicago from Tom. F. W. Barth on optical study and; 

chemical analysis of the lavas from Umnak. ;

HISTORY AND PREVIOUS GEOLOGIC WORK

RUSSIAN PERIOD, 1741-1867 !

Umnak Island was first sighted in 1741 by Capt. Alexei Chirikof of 

the St. Paul, a ship of Fleet-Captain Vitus Bering's expedition to 

North America. The name, Umnak, was the native name (Baker, 

1906, p. 650). No Russian investigations were made on Umnak during 

the latter part of the 18th century, although Stephen Glottof and other 

Russian fur traders visited Umnak during this period (Baker, 1906, : 

p. 36). William Coxe (1787, p. 141-142 and 173-181) gives interesting 

accounts of the early Russian traders and of the natives. The most 

complete descriptions of Umnak and Bogoslof during the Russian 

period are those of Veniaminof (1840) and Grewingk (1850). The first 

detailed map of Umnak appeared in the marine atlas of Capt. M. D. 

Tebenkoff (1852). . A rocky islet on the site of Bogoslof was first sighted in 1768 and 

named Sail Rock by the Russians; it was called Ship Rock by Captain 

Cook and other early navigators.',<.' The islet disappeared between
GEOLOGY OF UMNAK AND &OGOSLOF ISLANDS, ALEUTIAN ISLANDS 271

1884 and 1891. The name Io6nn Bogoslof (John the Apostle) was 

applied by the Russians to an island formed a little south of Ship 

,Rock by an eruption of viscous lava in 1796 (Merriam, 1902, p. 

291-294).

UNITED STATES TERRITORIAL PERIOD

In 1867, the United States purchased Alaska, but no survey or 

scientific study of Umnak by Americans was made until 1883, when 

the birth of a new volcanic dome, less than a mile north of Bogoslof 

Island, attracted interest. Ball (1884 and 1885) and Diller .(1884 and 

1885) used the names Grewingk and New Bogoslof, respectively, in 

describing the new dome, the remnant of which is now called Fire 

Island. Merrill (1885, p. 31-33) contributed petrographic descriptions and two analyses of specimens from Fire Island. Members of 

the Harriman Alaska Expedition stopped at Bogoslof in 1899 and 

collected specimens from Castle Rock. Merriam (1902), a member of 

that expedition, published the first complete paper in English on the 

history of Bogoslof during the period 1768-1900. Marcus Baker's 

Geographic Dictionary of Alaska (1906) is a valuable source of information, not only about origin of names but also about earlier workers 

-and their publications, especially maps and charts.

Volcanic activity at Bogoslof was renewed during 1906-1907. 

Jaggar (1908) published a history of Bogoslof and an account of his 

landing of August 7, 1907, Robert Dunn visited both Bogoslof and 

northeastern Umnak and was the first (Dunn 1908) to publish the' 

name Okmok, supposedly the native name for this volcano. Sidney 

Powers (1916) gave an account of the 1910 eruption of Bogoslof and 

the extrusion of the Taboma Peak dome and Jaggar (1930) gave an 

account of the 1926-27 Bogoslof activity. Lukens (1936) -gave a 

description of Bogoslof Island as it was in 1935. Paul A. Smith (1937) 

using Coast and Geodetic Survey data, described the submarine 

canyons near Bogoslof.

On Umnak, Waldemar Jochelson, an anthropologist, made excavations at ancient village sites in connection with his studies on the 

origin of the Aleut (Jochelson, 1925, 1933). Hrdli6ka (1945) also 

visited Umnak in connection with his anthropological studies of the 

Aleut. Umnak Island was briefly visited in 1931 by S. R. Capps 

(1934) of the U. S. Geological Survey, and later Hulten (1937) made 

botanical collections near Nikolski. An eruption in Okmok Caldera 

in 1931 was reported by Jaggar (1932).

During World War II, from 1942 to 1945, Okmok Caldera. became 

known to military personnel stationed at Fort Glenn. Dean Freiday 

.(1945, p. 453-454) described Okmok Caldera by means of photographs 

and speculated on its origin. Ray.E. Wilcox (1945 and 1946), while 

an officer at Fort Glenn, prepared file reports on.the 1945 activity.
272 .- INVESTIGATIONS OF ALASKAN VOLCANOES

G. D. Robinson and Howel Williams as consultants to the U.S. Army 

also observed the 1945 eruption and advised the Alaskan Defense 

Command that Umnak Airbase was not threatened. Byers, Hopkins, 

Wier, and Fisher (1947) prepared an administrative progress report 

on volcano investigations begun in 1946 on northeastern Umnak. 

Byers and Brannock (1949) sampled hot springs and fumaroles and 

described volcanic activity on Umnak Island during the period 1946-48.

GEOGRAPHY

LOCATION, SETTLEMENTS, AND ACCESSIBILITY

: Umnak and Bogoslof lie in the eastern Aleutian Islands, Alaska 

(fig. 49). Most of their area is enclosed by the meridians 168° and 

169° W. and parallels 53° and 54° N. Umnak Island is bounded on 

the north by the Bering Sea and on the south by the Pacific Ocean. 

Bogoslof Island is entirely within the Bering Sea, 25 miles north of 

Umnak and slightly off the main trend of the eastern Aleutians. 

Umnak is 75 miles long and has an area of about 675 square miles. 

Bogoslof was only slightly more than a mile long in 1947 and had 

an area of less than a quarter of a square mile.

The chief settlements are Cape Air Force Base (Fort Glenn prior 

to 1947) on northeastern Umnak Island and the Aleut village of 

Nikolski on the southwestern part (pi. 39). In 1947 a group of 

about 12 U. S. Coast Guardsmen operated the Cape Stair Loran 

Station, 4 miles west of Nikolski, a village with a population of several 

score Aleuts, a school teacher, and the manager and a few employees 

of a livestock company. No one has lived on Bogoslof since 1945, 

when a World War II outpost was abandoned. Umnak Island can 

be reached by Government vessel in about 1 week's time from Seward, 

Alaska. The dock of Cape Air Force Base can accommodate smallto medium-sized vessels, but at Nikolski all supplies and passengers 

must be lightered ashore. Bogoslof Island is accessible only by 

small boat in calm weather.

Roads, in bad repair, encircle northeast Umnak Island except on 

the northwest side. Southwest Umnak has only wagon roads and 

trails in the vicinity of Nikolski.

HYDROGRAPHY AND TOPOGRAPHY

Umnak, like nearly all the islands of the Aleutian arc, rests on 

a flat-topped submarine ridge, the Aleutian Ridge (Studds, 1950, 

p. 787). Bogoslof is the top of a submarine volcano rising from 

oceanic depths, 25 miles north of the Aleutian Ridge (pi. 39). The 

general submarine features of the Bering Sea, the Aleutian Ridge, 

and the parallel Aleutian Trench to the south have been described 

by Murray (1945).
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 273

The Aleutian Ridge near Umnak.Island reaches a maximum width 

of 35 miles along a line through northeastern Umnak but tapers to 

22 miles opposite the southwestern end of Umnak; its upper surface 

is almost entirely at depths of less than 100 fathoms (pi. 39). The 

break in slope between the ridge or shelf area and the north wall 

of the Aleutian Trench is at about 100 fathoms, 12 to. 30 miles south 

of the coast of Umnak. The north wall of the Aleutian Trench is 

dissected by short submarine canyons, which extend to depths ranging 

from 1,500 to 2,000 fathoms. The average gradient of the canyons 

is about 600 feet per mile. Below 2,000 fathoms the north wall of 

the Aleutian Trench has a more gradual slope. The bottom of the 

Aleutian Trench is 4,030 fathoms at about 100 miles south of Umnak 

(Murray, 1945, fig. 9) or about 50 miles south of the southern boundary 

of the area shown in plate 39.

Umnak Island is separated from Unalaska Island to the east by 

Umnak Pass, a strait 4 miles in minimum width and 26 fathoms in 

minimum depth. Samalga Pass, 20 miles wide and about 125 fathoms 

deep, separates Samalga Island off the southwestern tip of Umnak 

from Chuginadak, the nearest of the Islands of the Four Mountains, 

just west of the area shown in plate 39.

The submarine slope at a depth of about 100 fathoms (pi. 39) 

steepens off the northwest coast of Umnak within a few miles ol the 

shoreline at the widest parts of the island. The gradient is about 

600 feet per mile from 100 to about 600 fathoms and the submarine 

slope is incised by many steep V-shaped canyons as much as 1,000 

feet deep. Below about the 600-fathom contour the submarine slope 

becomes less steep and also less regular. Submarine canyons separated by spurs extend northwesterly for several tens of miles to join 

a much larger submarine canyon with a westerly gradient (extreme 

top part of pi. 39). This canyon, 6,000 to 9,000 feet deep, is the 

largest submarine canyon of the eastern part of the Bering Sea 

(Murray, 1945, pi. 1; Smith, 1937). The largely submerged volcanic 

cone of Bogoslof rests on one of the northwest-trending intercanyon 

spurs.

Umnak Island is divided into northeastern and southwestern parts 

at the narrow isthmus where Inanudak Bay makes a deep reentrant 

into the northwest coast. The two parts of Umnak have marked 

differences in topography. Southwestern Umnak is much more 

rugged, with two large volcanic mountains rising to nearly 7,000 feet. 

Northeastern Umnak Island is almost wholly occupied by Okmok 

Volcano, a low shieldlike volcano, less than 3,500 feet in altitude.

Southwestern Umnak Island contains the glaciated volcanic mountains, Re.cheschnoi and Vsevidof, in the northern part and the Nikolski 

plain in the southern part (pis. 39 and 40). Mount Recheschnoi
274 INVESTIGATIONS OF ALASKAN VOLCANOES

(altitude, 6,510 feet) is a deeply dissected ridge with radial flat-topped 

spurs separated by deep, glacier-filled valleys. Seven glaciers, 2 to 4 

miles long, extended in 1947 from near the summit of the ridge to an 

altitude of a few hundred feet. Outwash plains in the valleys extend 

seaward from the glacier fronts. Mount Vsevidof, west of Mount 

Kecheschnoi, is a symmetrical volcanic cone rising to about 7,000 feet 

above sea level. The crater of Mount Vsevidof, 0.8 mile across, is 

filled by a glacier with two distributaries, one north and the other 

east of the crater. The distributary glaciers have breached the crater 

wall and have incised gorges 200 to 400 feet deep. The larger, east 

distributary joins a glacier flowing southwest from Mount Recheschnoi. The composite glacier then flows southward 2 to 3 miles to a 

large end moraine at an altitude of about 1,000 feet. This glacier 

drains to the two forks of Black Creek, the largest stream on southwestern Umnak Island. The Nikolski plain southwest of Mount 

Recheschnoi and Mount Vsevidof is a rolling surface with a relief of 

.a few hundred feet and about 300 feet in average altitude. Small 

lakes, less than a mile long, abound on the surface of the plain. Small 

islands,, such as Ananiuliak, Samalga, and Vsevidof, off the coast of 

southwestern Umnak, are apparently remnants of the plain, which 

were isolated by marine erosion.

Okmok Volcano on northeastern Umnak is a shield-type volcano 

with a large central caldera, 6 miles in diameter. The run of the 

caldera ranges hi altitude from 1,600 to about 3,200 feet (pi. 39). 

Several cinder cones and craters, designated on plate 41 by letters A 

through H, are on the floor of Okmok Caldera. The caldera is 

(drained toward the northeast by Crater Creek through a gorge 600 

feet deep. North and east of Okmok Caldera, two arcuate ridges, 

roughly concentric to the walls of Okmok Caldera, are herein referred 

to as the north arcuate ridge and the east arcuate ridge, respectively. 

Radial spurs separated by broad short valleys are typical of the slopes 

of Okmok Volcano above altitudes of about 1,000 feet. At lower 

levels, slightly dissected plains slope 100 to 200 feet per mile seaward, except along the northwest coast between Fox and Aguliuk 

Points (pi. 39), where slopes are steeper and more dissected. The 

slopes on the lower flanks of Okmok Volcano have been incised with 

V-shaped gullies, as much as 200 feet deep. In exposed places where 

the plains intersect the coast, the sea cliffs are 100 to 250 feet high.

Northeast of Okmok Volcano, Mount Idak surmounts the southwestern end of a small lava plateau that slopes northward toward the 

Bering Sea. On the south flanks of Okmok Volcano are several 

parasitic cones in different stages of dissection. The largest of these, 

and also the highest peak on northeastern Umnak Island, is Mount 

Tulik, with an altitude of 4,111 feet. Low, rugged volcanic moun-
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 275

tains, rising about 2,500 feet above sea level, occupy the narrow 

.isthmus that joins the northeastern and southwestern parts of Umnak 

Island. This mountainous area is herein called the central Umnak

area.

CLIMATE, VEGETATION, AND ANIMAL LIFE

The climate (U. S. Coast Pilot, 1947, p. 36-43) is marine subarctic, 

'characterized by overcast skies, high winds, and small variations of 

diurnal temperature. Overcast skies and fog with considerable rain 

in the mountainous areas occur during the late spring and summer 

months, owing to warm southerly winds off the Pacific Ocean. Winds 

during the summer are usually accompanied by fog and drizzle and 

frequently exceed 30 miles per hour. Occasionally the winds reach 

velocities as high as 70 miles per hour, but winds of extreme velocity 

during the summer are probably williwaws, whose accelerated velocity 

is caused by local topographic conditions. Temperatures during the 

summer are cool, ranging from 46° to 68° F., on Umnak. After the 

September equinox, the average wind direction shifts from westerly 

to northwesterly, and winds of over 100 miles per hour are not uncommon. Fog and low overcast give way to broken overcast with 

occasional clear days. Drifting snow that blocks roads becomes a 

problem in whiter. Winter temperatures at sea level in the vicinity 

of Umnak and Bogoslof Islands average slightly above freezing and 

.the seas are ice-free throughout the year.

Table 1 summarizes climatological data at Cape Air Force Base 

and Nikolski from June 1942, to April 1948.

TABLE 1. Climatological data for two stations on Umnak Island, June 194® to

December 1948

[Compiled from llth Weather Squadron, 1950, Chart 19, and earlier flies of U. S. Air Force]

Temperature (degrees Fahrenheit):

Maximum annual .. ____ .. ___ . _ . ,.,.,.,..,.,. .... ...

Precipitation (Inches) :

Wind (miles per hour) :

Sky (condition in percent of hours) :

Cape Air 

Force Base

39.0

68.0

9.0

47.67

64.5

8.0

318

28.72

18.8

135.0

1.4

38.3

60.3

Nikolski »

39.2

64.0

11.0

41.19

49.9

13.0

307

13.0

19.4

72.0

1.7

34.5

63.8

i Data cover only a limited period from October 1942 to April 1945.

489292 5£
276 INVESTIGATIONS OF ALASKAN VOLCANOES : A general description of the vegetation of the Aleutian Islands is 

given, by Walker (1945, p. 63-71). Grasses, sedges, and flowering 

plants cover the slopes below an altitude of 1,000 feet on northeastern 

Umnak. On southwestern Umnak, grasses and flowering plants grow 

in protected places at altitudes up to nearly 2,000 feet, and several 

types of edible berries grow at altitudes below 1,000 feet on the southfacing slopes of Mounts Recheschnoi and Vsevidof. In 1947, Bogoslof 

had very little vegetation other than a few grasses.

Umnak and Bogoslof, like other Aleutian Islands, are characterized 

by a profusion of marine mammals and birds. (See Clark, 1946, p. 

31-61.) Fish and marine invertebrates likewise are abundant. By 

comparison the terrestrial fauna is insignificant. No reptiles or 

amphibians are known in the Aleutian Islands and the most common 

land mammal is the red fox. Reindeer, imported from Siberia, were 

stocked on Umnak about 1900 (Lantis, 1950, p. 28-29), and in 1946 

several hundred of these animals, then in the wild state, inhabited 

the northeastern part of the island. Sheep are raised on southwestern 

Umnak, where they produce a premium crop of wool.

GENERAL GEOLOGIC SETTING

Umnak and Bogoslof Islands are a small segment of the Aleutian 

Island arc, on the northern edge of the Pacific Ocean. Most Pacific 

island arcs (Umbgrove, 1947, p. 144-216; Hess, 1948, p. 417-445) are 

arcuate geanticlinal ridges with submarine trenches or downbuckles on 

their con vex oceanward sides. Menard and Dietz (1951) provide hydrographic evidence that the Aleutian Trench has sunk, probably in late 

Tertiary and Quaternary time.

The Aleutian arc is a single island arc at Umnak and adjacent is^ 

lands, but becomes a double arc to the northeast (Umbgrove, 1947, 

p. 189). In the-Umnak-portion of the arc, the geanticlinal ridge, 

marked by the line of Quaternary volcanoes, remains close to 100 

miles from the axis of the Aleutian Trench (fig. 50). Bogoslof and 

Amak Islands are partly submerged volcanoes 30 miles north of the 

main Aleutian geanticlinal ridge and hence are exceptions to this 

general relationship. Northeast from Umnak, the line of Quaternary 

volcanoes diverges from the axis of the Aleutian Trench and is 200 

miles away near Kodiak Island (fig. 50). The continental shelf south 

of the shoreline of the inner (northern) arc likewise widens from about 

20 miles, south of Umnak, to 150 miles, southeast of the Alaska 

Peninsula near the Katmai Volcano. Where the shelf widens, an 

outer (southern) chain of nonvolcanic islands begins with Sanak, and 

extends northeastward.
FIGUBE 50. Eastern part of Aleutian volcanic arc, showing positions of volcanoes and Aleutian Tren

Ridge and Aleutian Trench.
278 . INVESTIGATIONS OF ALASKAN VOLCANOES

The ge'ology of the outer, southeasterly islands of the double arc 

is known chiefly from the work of S. K. Capps (1937) on Kodiak \ 

Island. Capps (1937, pi. 2) has mapped sedimentary rocks of four; 

agesi an early Mesozoic and older, a late Mesozoic, an early Tertiary, 

and a late Tertiary. The rocks are progressively more deformed with j 

increasing age. In general the beds of Tertiary age crop out nearer [ 

the Aleutian Trench than the rocks of Mesozoic age. These sedi- ; 

mentary rocks strike northeast, parallel to the Aleutian Trench, and > 

are generally separated by great, northwesterly-dipping faults running' 

the entire length of Kodiak Island (Capps, 1937, p. 136).

The Alaska Peninsula, on the inner concave part of the Aleutian 

double arc, contains folded thick rocks of Mesozoic and Tertiary age 

(W. R. Smith, 1925, pi. 4; Atwood, 1911, p. 28; Knappen, 1929,: 

pi. 6), which extend as far southwest as Pavlof Bay (P. S. Smith, 

1939, pi. 1). Atwood (1911) mapped Jurassic to Tertiary sedimentary 

rocks, which have been bowed up into a broad anticlinorium. Thrust ; 

faults that strike northeast/arid dip northwest have been reported in : 

the Aniakchak district (Knappen, 1929, p. 207). ;

Southwest of Pavlof-Bay (fig. 50) the thick sedimentary formations 

of Miesozoic and Tertiar^ age are not exposed. Between Pavlof Bay 

and the end of the Alaska' Peninsula (fig. 50) only minor thicknesses 

of late Tertiary sedimentary rocks interbedded with basic tuffs crop 

out at lojw altitude beneath .-the Quaternary volcanoes (Kennedy and! 

Waldron^ 1955, p. 6-7;-H. H. Waldron, 1947, oral communication).. 

The 'Tertiary and olde? rocks probably are mostly concealed beneath j 

the Quaternary volcanoes/of Unimak, the easternmost island of the' 

single arc. The islands of Akun, Akutan, and Unalaska, west of: 

Unimak, are composed ajmost entirely of volcanic rocks of Cenozoic; 

age and their sedimentary derivatives. . On Unalaska, the next island ; 

east of Umnak, lower, Miocene tuffaceous sedimentary rocks and! 

associated volcanic rocks are cut by quartz dioritic plutonic rocks. :

AREA OF THIS REPORT f

Umnak and Bogoslof Islands are composed almost entirely of igneous ? 

or altered igneous rocks of Cenozoic age. The 2 islands are divisible i 

into 3 areas that with few exceptions differ significantly in geologic, < 

features and petrograptdc type. These areas are southwestern Umnak \. 

Island, northeastern .Umnak Island, and Bogoslof Island (pi. 39),, 

which are shown on separate geologic maps in plate 40, plate 41, and 

figure 53, respectively. Northeastern Umnak is arbitrarily separated 

from southwestern Umnak at the narrowest part of the island. South- ; 

western Umnak (pi. 40) contains the oldest rocks, a complex of 

albitized sedimentary and igneous rocks of possibly diverse geologic 

ages, mostly Tertiary, but possibly including still older rocks. Dioritic
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN 'ISLANDS 279

plutonic rocks, intrusive into this complex, were also found only on 

southwestern Umnak. Altered Tertiary volcanic rocks that crop out 

in the: central part of Umnak underlie Quaternary, volcanoes on both 

northeastern and southwestern Umnak. fTh6 petrography, icompositiori, and physiographic expression of the volcanic rocks, of Quaternary 

age in the northeastern part of Umnak idiff^r from those of southwestern Umnak; the volcanic rocks of Bogbslof Island, which are 

entirely of historic age, differ somewhat ;in jcomposition from those 

of either southwestern or northeastern Umjnak. :

Geologic dating of most of the rock units; is uncertain and based 

chiefly on inferences from contact relationships, degrees of alteration, 

and, to less extent, degree of dissection; of iconstructional i surfaces. 

Tertiary fossils recently discovered on near,by UnalaskaiIsland (pi. 39) 

have supplied some evidence, from which gejologic ages of the older 

rocks may be inferred. ! ; i

! : BOCK NAMES USED IN THISJ REPORT; I

'. ' , : ! ' 

Names of volcanic rocks in the suite bieing investigated are based

on the total content of silica in the rocls, following in general (and. 

extending to higher silica percentages), the system of Williams (1950 

p. 234-235). As defined in this report, Basailt contains less than 54 

percent silica, equivalent to "about 5 percent normative quartz; 

andesite contains more than 54 and less than 6J3 percent silica^ approximately equivalent to the range 5 to 14 percent of normative quartz; 

latite contains between 63 and 65 percent jof silica, ;appr6ximately 

equivalent to the range 14 to 17 percent of normative quartz; rhyodacite contains; about 65 percent of silica, equivalent to about 20 

percent of normative quartz; and rhyolite jcontains more! than 72 

percent of silica, or more than 25 percent of normative; quartz. (For 

chemical analyses of the rocks see table 2J) I ; :

Porphyritic ro.cks are named by applying 'the appropriate general 

rock name as determined by the silica content modified by mineral 

names, particularly those of mafic pheriocrystjs, to indicate the phenocrysts in the rock. The names applied to many of the porphyritic 

rocks on the maps and in stratigraphic ; descriptions are shortened 

versions of names based on chemical and petrographic analyses; that is, 

hypersthene andesite for hypersthene-bearing labradprite andesite. 

One term, mafic phenocryst basalt, found jusejful in the field mapping, 

is retained in the report as a map unit and general rock group. It includes porphyritic basalts that contain visible phenocrysts of olivine, 

augite, or both together in excess of aboui 2 percent by volume. 

Abundant plagioclase phenocrysts in association with mafic phenocrysts are not generally considered in naming the rock, especially if 

less than 10 percent, because plagioclase oheriocrysts are abundant in 

many of the rocks described.
TABLE 2. Analyses of rocks from Umnak and B

[Analyst Indicated by initials: S. M. Berthold (8MB); W. W. Brannock (W WB); R. L. Hill (RLH), and Le

and Brannock, 1952), U. S. Geological Survey; W. W. Hommel (WWH), of Ledbux and Co.; M. K. C

F. M. Byers, Jr. (FMB), of U. S. Geological Survey, on leave at University of Chicago laboratory; andPau

A. T. Myers (ATM), spectrographers, U. S. Geological Survey.] 

Northeastern Umnak

1

8MB 

RLH 

LMS

1A

EJT

2

WWH

3

.8MB 

RLH LMS

4

MKC

5

WWB

LMS

6

FMB 

WWH

7

FMB 

WWH

8

8MB 

RLH 

LMS

9

FMB 

WWH

IQ

WWH

Major oxides

SIOs-. .... ...

AljOj .... .

FeaOj... ... 

FeO .... .....

MgO. ... ....

CaO 

NazO-.. -. 

KjO-. ,... ...

HsO -'."'.'.".".'.

CO^.. . .........

TiOj..... 

PjOs 

MnO... .....

Total 

Powder sp. gr. .

45.7

12.9

3.6

5.6

18.2

9.6

1.7

.52

U-.16

.51

.10

.19

98.62 

3.02

48.07

15.61

2.34

6.98

12.68

10.76

1.86

.48

f .09 

\ .07 

I .01

.73

.12

.17

99.97

48.43

18.94

4.15

6.30

6.55

11. 04

2.51

.52

.12 

.11

1.20

.27

.17

100. 31 

2.97

48.7

14.9

4.7

3.4

4.2

7.8

2.1

.92

Ino.i

1.2

.22

.20

98.44 

2.49

48.97

16.27

2.83

6.24

9.62

12.86

2.00

.54

{

..09 

.04

-.72

.13

.18

100. 49 

«3.0

50.1 .

23.0

2.6

5.4

3.6

11.6

2.4

.44

['-.13

.74

,11

.13

100. 12 

2.80

50.52

19.71

1.67

6.82

4.64

11.96

12.71

1.58

f 1.08 

j i.Ol

.97

i.18

100.04 

2.86.

50.60

18.06

2.99

6.75

5.58

11.46

2.73

.53

.17 

.05

1.10

1.28

1.16

100. 46 

2.96

51.4

14.4

6.3

 1.3

1.5

8.4

4.4

1.1

1'6.3

.93

12.4

.23

98.66 

2.55

52.23

17. 04

3.22

6.60

5.54

10.39

2.97

.78

| lioe

1.24

1 . 25

100.63 

2.94

52.43

15.27

5.96

6.99

4.13

8.70

3.50

1.24

.20 

.11

1.48

.36

.17

100. 54 

2.89

Minor elements «

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

Ga_. ...........

La... ...........

Sc.... ..........

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

Yb_. ...........

B_ _ ________..

Be ...........

PRB

0.003

.002

AC

0.004

:0006

.004

.003

.0002

ATM

0.003

.0009

.002

KJM

0.005

.0007

.002

KJM

0.007

.0007

.005
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 281

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TABLE 2. Analyses of rocks from Umnak and Bogoslof 

[Analyst indicated by initials: S. M. Berthold (8MB); W. W. Brannock (WWB); R: L. Hill (RLH), and Leon

and Brannock, 1952), U. S. Geological Survey; W. W. Hommel (WWH), of Ledoux and Co.; M. K. Car

F. M. Byers, Jr., (FMB), of U. S. Geological Survey, on leave at University of Chicago laboratory; and Paul 

A. T. Myers (ATM), spectrographers, U. S. Geological Survey.] ' 

Northeastern 

Umnak Con.

19

FMB

20

MKC

Southwestern Umnak

21

WWB 

LMS

22

WWB

LMS

23

WWB

LMS

24

WWB 

LMS

25

WWB 

LMS

26

8MB . RLH 

LMS

27

SMB. 

RLH 

LMS

28

8MB 

RLH 

LMS

SiO2 - 

AhOs. ~ 

FeaOo.. ....... ...... 

FeO_. .................

MgO. _..._..._.._..,_.

CaO. ....... _ ......

Na2O. ........... ......

K3O. ..................

HS 0-K......_._ ....... .

H2 0-._ ...............

CO2 ... ................

TiOs._ ........ __ .....

PsOs-.... . ....... ...

MnO.. ....... .........

Total- __ . __

72.36 

13.75 

.59 

2.33 

.09 

1.53 

4.64 

4.16 

f .27 

| .03

.21 

.06 

.09

100.11 

«2.4

72.77 

13.29 

1.48 

1.55 

.08 

1.27 

4.72 

4.06 

.10 

.02

.21 

.06 

.10

99.71 

«2.4

54.3 

18.0 

1.7 

7.8 

3.8 

7.9 

3.4 

1.0

I 8 -. 20

1.2 

.32 

.18

99.60 

2.79

56.0 

15.6 

2.1 

4.8 

8.8 

7.7 

2.4 

1.6

'.03

.66 

.17 

.12

99.98 

2.88

57.4 

18.2 

2.0 

5.0 

4.0 

7.7 

2.9 

1.4

».20

.90 

.16 

.10

99.96 

2.81

57.5 

17.4 

2.2 

5.2 

3.8 

6.9 

3.6 

1.7

'.24

1.2 

.4 

.10

100.24 

2.84

60.6 

15.4 

2.7 

5.6 

2.0 

3.5 

5.4 

2.2 

f 1. 07 

\ .31 

1 .07 

1.0 

.43 

.15

100.43 

2.76

61.3 

16.3 

2.1 

6.0 

1.6 

5.6 

4.6 

1.7

i»-.51

1.0 

.42 

.20

100. 82 

2.59

61.4 

16.5 

2.6 

5.4 

2.2 

5.6 

4.3 

1.6

« -.29

1.1 

.18 

.17

101. 05 

2.67

64.4 

16.0 

2.1 

4.1 

1.2 

4.2 

4.5 

2.6

s-.l

.8

.2

.1

100.3

2.5

Minor elements * Continued

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

Ga....... ...... ........

La.....................

Sc.. ..................

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

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

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

Be...___,_.__, .......

KJM

0.02

.001

.01

.006

. .0005

ATM

0.007

.001

.004

. .002

ATM

0.005

.0008

.002

.002

AC

0.007

.002

.002

.004

.0003

..002

ATM

0.008

.001

.004
Sr.. ...................

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

Pb...... ....... .....

Mo 

Gu....... .. ....... . ,

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

Or.....................

Ni.. ................. .

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

Nb_. ..................

Total.. .......

.02

.09

.005

.004

.001

.0003

.1618

.05

.02

.009

.02

.0007

.001

.002

.1167

.03

.05

.006

.01

.007

f\f\A

.002

.1208

.06

no

.0006

.005

no

.01

rtrtO

003

.002

i4nQ

.03

.06

003

.02

.0003

001

.001

.1283

Norms Continued

C ... ..... .......

or ____________

plag{ab-:.....:...-

en..... ...... 

fa

hy{en.. .

ol/fo... ................

01 \fa 

11-.... .................

hm_. ______ __ ..

ap... .............

Total... .

25.2

24.6

39.3

4.4

1 9

1

0

2.4

.4

.1

100.0

27.0

24.5

39.8

2.8

1.2

.17

1.1

.03

.22

2.1

.4

.1

99.42

5.1

6.1

28.8

30.6

2.9

1.3

1.6

8.2

9.5

2.6

2.4

.7

99.8

6.0

& A

20.4

27.0

4.4

3.1

1C Q

5.3

3

f\

1

O

.3

99.9

n

o

8.3

24.6

32.3

O 0

1 9.

8 7

5.3

3.0

1.5

.3

99.6

9

0

10.0

26 4

2 1

1 O

ft 9.

5.0

3

0

2.3

1.0

100.0

9.5

19 8

45.6

11.4

1 ">

.5

.7

4.5

5.9

3 Q

2.0

1.0

99.0

10 a

10.0

OQ Q

1ft Q

2.6

g

1.7

3.1

6

0

3.6

2.0

1.0

100.8

14.0

9.5

36.2

21.1

2.4

1 9

I

n

4

0

5.0

3.7

2.1

.3

101.0

16.5

15.6

38.3

1.6

.7

g

2.3

3.8

3.0

1.5

.5

100.3

MggH values Continued

Si-... ..................

al... -....... ......

fm .. ................

alk.... _ . ...

k_ .... .... _

372

42

13

8

37

.37

.05

124

384

42

13

7

38

.37

.05

134

148

29

37

23

11

.17

.42

4

141

23

47^

21

QV£

.30

.70

7

167

31

34

24

11

.24

.51

23

169

30

35

22

13

.24

4A

17

207

31

33^

13

22)4

.21

.30

17

203

32

30

20

18

.20

.26

- 27

200

3H$

32

19H

17

.20

.34

32

240

35

26

17

22

.28

.26

50

See footnotes on page 284.
284 INVESTIGATIONS OF ALASKAN VOLCANOES

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GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 285

GEOLOGY OF UMNAK

GENERAL FEATURES

The oldest>rocks of the island,- exposed only in the southwest part 

'(pis. 39 and 40), include an albitized sedimentary and igneous complex 

intruded by plutonic rocks. These rocks have been deformed, and 

deeply eroded. The age of the sedimentary rocks of the complex is 

middle Tertiary or older; the plutonic rocks are post-Oligocene. Upon 

.a basement of these rocks, volcanic edifices have been built around four 

main centers of activity. The oldest of the centers is in the isthmus 

region of Umnak; the rocks are called the volcanic rocks of central 

Umnak. The initial forms of the volcanoes have been completely destroyed by erosion. The volcanic rocks of central Umnak probably 

include lavas that range in age from middle Tertiary to early Quaternary. The hypersthene andesite of Mount Recheschnoi, of probable 

'Quaternary age, overlies the. volcanic rocks of central Umnak on the 

southwest. The largely Recent symmetrical cone of Mount Vsevidof, 

consisting of basaltic andesite, andesite, and rhyodacite, buries the 

hypersthene andesite on the western slope of Mount R,echeschnoi. 

Mount Vsevidof has been active in historic tune. Okmok Volcano 

(pis. 39 and 41) lies northeast of the isthmus, and its_rocks make up 

almost all of northeastern Umnak. Most lavas of Okmok Volcano are 

porphyritic to aphyric feldspathic basalts. The .early activity of 

Okmok Volcano probably was simultaneous in part with that at 

Mount Recheschnoi but has persisted to the present.

ALBITIZED SEDIMENTARY AND IGNEOUS COMPLEX 

DISTRIBUTION AND THICKNESS

The oldest rocks of Umnak Island include,bedded argillite and tuff, 

keratophyre flows, albitized diabase sills, and irregular intrusive bodies 

exposed near sea level in the southwest tip and along the south-central 

coast of the island (pi. 40). In contrast to the younger rocks of 

the island, these rocks are deformed and characteristically show the 

effects of low grade metamorphic alteration. West and southwest of 

Kigul Island, the lake-studded rolling.plain of southernmost Umnak 

is underlain by gently dipping flows of keratophyre, subordinate beds 

of argillite and tuff, and albitized gabbro or diorite. Sections of bedded 

argillite and tuff are exposed along the east shore of Nikolski Bay, in 

the ndrth and east shore of Driftwood Bay, and in a small coastal outcrop 1% miles northeast of Lookout Point. Albitized intrusive rock is 

exposed in two small headlands at 2 and 3 miles southwest of Russian 

Bay, and bedded argillite and tuff with minor sills and dikes of albitized 

diabase crop out along the coast from the west shore of Russian Bay 

to the north shore of Partov Cove.
286 . : INVESTIGATIQNS OF ALASKAN VOLCANOES , The total thickness of the complex is unknown. About 5,000 feet 

of northeasterly dipping argillite, tuff, and albitized diabase sills are 

exposed along the shore northeast of Partov Cove, but parts of this 

sequence may be repeated by faulting.

!... STRUCTURE

The little that is known about the structure of the albitized sedimentary and igneous complex is included here with the stratigraphic 

descriptions, mainly because the structural features described below 

are confined to these oldest rocks and in part define them. The 

general strike of the sedimentary rocks is northwest in the largest 

area of their exposure near Russian Bay and Partov Cove (pi. 40), 

and dips range from 13° NE to vertical. Near Lookout Point, the 

sedimentary rocks dip 15° SE. Dips at the head of Driftwood Bay 

suggest an anticline plunging southeast. The sedimentary rocks 

north of Nikolski village, and the extensive keratophyre flows near 

Amos Bay, are nearly horizontal. No major faults were mapped in 

the field, but a few inferred faults in areas underlain by the albitized 

sedimentary and igneous complex have been mapped from well-defined 

alinements of swales, gullies, and lakes seen on the air photographs. 

The northwesterly strike of the sedimentary rocks suggests that axes 

of folds in these rocks cut across the northeasterly trend of the Aleutian 

arc, whereas the lineaments mapped from air photographs and the 

general elongation of the island parallel the trend of the arc.

RELATION TO YOUNGER ROCKS

In the few good exposures, the contact between albitized sedimentary 

and igneous complex and the other extrusive and sedimentary rocks 

of the island is one of .angular erosional unconformity. That this 

relationship is general is indicated by the fact that major deformation 

is confined to the older rocks, and that the replacement of primary 

intermediate plagioclase by albite is confined to these rocks.

 -.- PETROGRAPHY 

ABQHIITE

The argillite varies from massive dark-gray bands 1 to 3 inches 

thick, to laminated argillite made up of randomly alternating thin 

beds of light-gray siltstone and laminae of medium-dark-gray argillite 

as much as one-eighth of an inch thick. The massive bands of argillite 

have small joints that form a crude rectangular pattern, parallel and 

perpendicular to the major fold axes. Thin calcite veinlets as much as 

one-sixteenth of an inch thick are found along many of the joints. 

The laminated argillite has a shaly parting, along which many dark 

carbonaceous patches suggestive of leaf fragments are found. One
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS; ALEUTIAN ISLANDS 287

specimen of argillite collected southwest of Russian" Bay (pi. 40) 

contains a few small angular detrital fragments of plagioclase crystals, 

and rare fragments of quartz in a fine-grained groundmass of cherty 

and feldspathic material whose index of refraction is near that of 

albite. Patches and veinlets of calcite are common. The chemical 

analysis of this specimen is given in column 31; table 2.

TUFF . -. :..,

Greenish water-laid tuff interbedded with the argillite ranges from 

moderate yellow green to slightly different shades of greenish gray. 

The grain size ranges from fine to coarse with a few grains as much as 

1 millimeter. Three specimens from near Partov Cove are highly 

altered crystal tuffs in which fragments of the former pyroxene and 

plagioclase have been largely replaced by chlorite and albite respectively. A specimen from Driftwood Bay (pi. 40), however, contains 

fragments of unaltered pyroxene and labradorite, as well as fragments 

of quartz crystals. Calcite is common in scattered patches in all 

specimens. The groundmass is an aggregate of chlorite, calcite, 

albite, an unidentified zeolite mineral, and other unidentifiable lightcolored, cloudy material. Pyrite is rare in the specimens from near 

Partov Cove, but absent from the Driftwood Bay specimen.

ALBITTZED INTRUSIVE ROCK

Three varieties of albitized intrusive rock include albitized diorite 

or gabbro, an albitized porphyritic rock, and albitized diabase. 

The albitized diorite is exposed near the southwestern tip of Umnak, 

chiefly at Elbow Hill (pi. 40). The albitized porphyritic rock is from 

a thick sill or flow near Amos Bay, and the albitized diabase occurs 

as sills, 2 to 10 feet thick, in the sedimentary rocks at Russian Bay and 

Partov Cove. Each is altered to a different degree from a rock 

probably of original gabbroic or dioritic composition.

The albitized diorite near the southwest end of Umnak is the least 

altered of the three rocks. The plagioclase is mostly unzbned albite, 

but includes a few irregular-shaped relics 6f andesine (pi. 42A). 

Apatite needles are in both the andesine and the albite. The albite 

is clouded with aggregates of fibrous rosettes of sericite. A few small 

(less than 0.5 mm), clear crystals of plagioclase are zoned normally 

from a center of andesine-oligoclase to a rim of albite. Some of these 

are veined with albite, potassium feldspar, and a zeolite mineral. 

A few larger areas intersertal to the plagioclase contain cloudy 

potassium feldspar intergrown with quartz. The mafic minerals are 

largely intersertal to the feldspars. Anhedral patches of augite in 

places are intergrown with or replaced by green amphibole and 

greenish, biotite dusted with opaque oxides. Chemical and petro-
288 . ; INVESTIGATIONS OF ALASKAN VOLCANOES ;

graphic analyses of this rock are given in column 25, tables 2 (p. 282 

to 284) and 3 (in pocket), respectively. '

The albitized-pprphyritic rock ne.ar Amps-Bay contains albite that 

has entirely replaced the former plagioclase of both phenocrysts and 

groundmass. Patches of quartz and potassium feldspar are similar 

to the Elbow Hill rock. Many subhedral and anhedral pyroxene 

grains are unaltered, but some augite is intergrown with, or replaced 

by, greenish biotite and high-iron chlorite. Small albite laths, shreds 

of green biotite and chlorite, and anhedral quartz and. piotassium 

feldspar, all dusted with opaque oxides,.:make up the groundmass. 

Chemical and petrographic analyses of the rock are listed in column 

29, tables 2 and 3, respectively.

The diabasic sills at Partov Cove show evidence of both albitization 

and potassium feldspathization, as might be expected from their 

proximity to areas of intensely potassium feldspathized rock near the 

narrow part of Umnak. The feldspar in the most altered specimen is 

all clouded albite, in places cut by veinlets of chlorite and calcite. 

No pyroxene is present, but the space interstitial to the plagioclase is 

entirely occupied by calcite, chlorite, opaque oxides including some 

hematite, rare epidote, and pyrite. A few veinlets of quartz cut 

through all minerals. Optical data on the chlorite (alpha, 1.612, 

colorless; gamma, 1.618, light green) are indicative of high iron, 

suggesting prochlorite or ripidolite (Winchell, 1933, pt. 2, p. 284).

KERATOPHYRE

The keratpphyre.lava flows exposed between Amos and Driftwood 

Bays are greenish gray, specked with dark grains of opaque oxide, 

and more abundant, larger white phenocrysts of albite, 1 to 3 mm long. 

Some contain a few clear colorless calcite crystals of the same size and 

shape. Two specimens described in detail are representative of 

these flows.

A specimen of fine-grained, greenish-gray keratophyre near Black 

Cape (pi. 40), is specked with grains ofopaque oxide and mottled with 

white phenocrysts of albite and sparse clear, colorless crystals of calcite. 

In thin section, the phenocrysts are seen to be unzoned albite, flecked 

with wisps of sericite and minute rods of apatite. The groundmass is 

largely albite crystals (0.05 mm), patches and shreds of chlorite, 

opaque oxide euhedra, and rare anhedral potassium feldspar. Quartz 

and calcite are associated in patches and veinlets.

A specimen of hydro thermally altered (potassium feldspathized) 

keratophyre collected east of Partov Cove contains evidence of two 

periods of alteration. ..The. former, plagioclase phenocrysts are now 

albite remnants largely replaced by calcite, sericite, and quartz (pi. 

42 B}. The groundmass is an aggregate of chlorite, sericite, quartz,
adularia(?), a low index zeolite and an opaque white mineral. The 

later type of alteration that has occurred is completely different from 

the regional albitization and is typical of later> locally intense hydrothermal alteration of the volcanic rocks of central Umnak in the 

Inanudak Bay area (see p. 295 to 296).

AGE AND CORRELATION

Carbonized impressions suggestive of leaf fragments have been 

seen on bedding.planes of the argillite. Botanist Eric Hulten (1937, 

p. 22) also reports fragments of coal in what he calls slate, presumably, 

the bedded argillite of this investigation. Carbonaceous, coaly argillite or mudstone deposits that have yielded plant fossils elsewhere 

in the Aleutian Islands and the Alaska Peninsula, have generally, 

though not always, proved to be early Tertiary. Bedded argillite 

and tuffaceous graywacke, similar in degree of metamorphism and 

deformation to the rocks of Umnak, are late Paleozoic in age on Adak 

Island (Coats, 1956) and early Tertiary on the Alaska Peninsula 

(Knappen, 1929, p. 182-198), Kodiak Island, and the Kenai Peninsula (Capps, 1937, p. 138-153). A sequence of bedded argillite and 

tuff similar to that on Umnak has also been mapped by members of 

the Geological Survey party along the south coast of Unalaska about 

30 miles to the northeast; carbonized remains were likewise found in 

these rocks but were unidentifiable (G. D. Fraser, oral communication, 1955). Vertebrate-fossil remains, chiefly teeth, have been found 

in greenish volcanic graywacke exposed in a quarry on the north side 

of Unalaska. These teeth were identified by G. Edward Lewis 

(written communication, 1954) as those of Cornwallius sp., a marine 

mammal of early Miocene age. Both the fossiliferous graywacke and 

the argillite of Unalaska, like the argillite of Umnak, are intruded by 

plutonic rocks. The albitized sedimentary and igneous complex of 

Umnak is therefore probably early to middle Tertiary, but may 

possibly include older rocks.

PLUTONIC ROCKS

Coarse-grained dioritic rock is found in five small, widely separated 

areas around the margins of Mount Recheschnoi and Mount Vsevidof 

(pi. 40). The lava flows of these two volcanoes and the unaltered 

flows of the volcanic rocks of central Umnak rest on eroded surfaces 

in the dioritic rock. It intrudes the bedded argillite and tuff sequence 

and contains blocky xenoliths of these sediments in the area on the 

southeast slope of Mount Recheschnoi. Most of the rock is quartz 

diorite, but the composition ranges from diorite to quartz monzonite! 

Dikes and stringers of granophyre cut altered plutonic rock on the
INVESTIGATIONS OF ALASKAN VOLCANOES

west side of Cape Ilmalianuk. The coarse, even grain of the quartz 

diorite and its similarity to that of batholithic quartz diorite on 

Unalaska indicate a rather large plutonic body. The distribution of 

its few outcrops suggests that it may underlie much of the area of 

the two younger volcanoes. .

The plutonic rocks on Umnak and Unalaska are so similar petrographically and so closely parallel in range of composition and in 

geologic relations that they must certainly belong to the same intrusive episode. The plutonic rocks on Unalaska are intrusive into beds 

that, as noted above, are now known to be of early Miocene age. 

Thus the plutonic rocks can be no older than early Miocene. An 

upper age limit may be inferred from the fact that large areas of the 

plutonic rocks have been laid bare by erosion and subsequently 

covered by unaltered lava flows of central Umnak, which are probably early Pleistocene but may be as old as Pliocene. The erosion of 

the plutonic rock, prior to extrusion of the Pliocene and Pleistocene 

lavas, must have taken part of the Pliocene epoch so that the youngest 

possible age of the plutonic rocks would be mid-Pliocene. On the 

basis of present evidence, the plutonic rocks were most likely intruded 

sometime during middle to late Tertiary time.

PETROGRAPHY

DIORITE

A specimen of gray diorite from the small knob 4% miles south 

of Mount Vsevidof has an average grain size of about 1 mm. A few 

veinlets of light-gray granophyre cut the diorite, which is slightly 

altered adjacent to the veinlets. As seen in thin section the texture 

of the diorite (column 1, table 4) is hypidiomorphic granular except 

for a little interstitial micrographic intergrowth of quartz and orthoclase. A few plagioclase grains are veined with orthoclase and to less 

extent with biotite. Orthoclase appears as rare irregular wisps in the 

plagioclase. Dusty fragments of biotite and sericite are common, 

and radiating sheaflike aggregates with the birefringence and relief 

of prehnite are rare as inclusions in the feldspar. Augite remnants 

are enclosed by the hornblende and biotite, and hornblende remnants 

in turn are enclosed and apparently replaced by biotite. Apatite is 

found as sparse tiny needlelike laths, as larger euhedra up to 0.3 mm! 

long, and as anhedral grains 0.2 mm in diameter. )

QUARTZ DIORITE -

Fresh, granular quartz diorite is the most common plutonic rock, 

type on southwestern Umnak. It is similar to the quartz diorite that 

forms the large intrusive body on nearby Unalaska Island (Becker, 

1898, p. 42). The quartz diorite of southwestern Umnak has an
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 42

A. Photomicrograph of composite plagioclase crystal in albitized diorite (column 25, table 2). Discontinuous, 

low birefringent (darker in picture) "islands" are andesine remnants surrounded by albite that replaced 

original andesine of rock. Crossed nicols.

B. Photomicrograph of partly replaced albite phenocryst in hydrothermally altered (potassium feldspathized) 

keratophyre. Quartz (g) and calcite-service (light aggregates) have replaced earlier albite. Crossed 

nicols.
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 43

Photomicrograph of plagioclase crystals veined and corroded by 01 ilmrl,!^- in ;ilu>n>il quart/ diuritc (specimen 

no. 47ABy39) bordering granophyre dikes. Crossed nicola.
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 44

Mount Vsevidof from the south

Steep-sided pyroclastic cone and flows forming moderate slope at the base. Drained lava channels 

in center middle background apparently formed when lava flowed over a now-buried, eroded cliff of 

older rocks. Glacial-outwash plain in foreground.
GEOLOGICAL SURVEY 

View of cast wall of Okmok Caldera, showing prccaldera rocks, largely palagonitized 
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 291

TABLE 4. Petrographic analyses of plutonic rocks 

[Modal analyses in volume percent]

Mode In volume percent:

1

4.0

9.7

64.5

7.4

4.4

4.1

.0

.8

2.0

3.0

2.7

2

24

10.2

. 5.9

64.9

8.4

3.2

5.4

.2

.3

1.5 «.l

2.82

3

15.8

28.1

44.5

U.7

5.6

.4

.0

.1

.8

2.72

4

33

28.6

43.7

24.7

.3

.0

»2.6

.0

.01

.1 (J)

2.58

 Includes considerable chlorite, some epidote, and rare prehnite(7).

> Hock largely mlcrographlc Intergrowth of quartz and potassium feldspar; minerals listed separately 

above. 

»Epidote. 

«Myrmeklte.

1. Dlorlte from small knob surrounded by recent flow, 4# miles south of Mount Vsevldof.

2. Quartz diorite from sea cliff, 1H miles southeast of Cape Ilmallanuk.

3. Quartz monzonite from sea cliff, Cape Ilmallanuk.

4. Oranophyre, from sea cliff, three-fourths of a mile, southwest of Cape Ilmalianuk.

average grain size of about 2 mm and consists of dominant, mediumdark-gray plagioclase as much as 8 mm long, minor light-gray to 

light-pinkish-gray orthoclase, inconspicuous quartz, and mafic minerals, of which biotite is most abundant and easily recognizable 

(column 2, table 4). Quartz and orthoclase occur interstitially to the 

subhedral to euhedral plagioclase. Myrmekite occurs on lobate ends 

of several large plagioclase crystals and also on irregular borders 

along the 010 face, where islands of myrmekitic plagioclase surrounded by orthoclase extinguish in sequence with the nearby mainland plagioclase. The plagioclase of the myrmekite is calcic oligoclase. 

Augite and rare hypersthene occur as corroded remnants enclosed by 

hornblende and biotite. Biotite occurs as individual grains ranging 

down to submicroscopic size, as an interstitial mineral with quartz 

and orthoclase, as reaction runs around pyroxene and hornblende, and 

as veinlets cutting pyroxene. Opaque oxides are chiefly associated 

with biotite and in part appear to replace it. A minor amount of 

chlorite has replaced biotite, and sericite(?) is rare as tiny wisps in 

the feldspar. Apatite is scattered as rod-shaped inclusions in the 

other minerals. The chemical analysis of this specimen is given in 

table 2, column 24.

QUARTZ MONZONITE

The quartz monzonite is a facies of the quartz diorite and was 

observed only on Cape Ilmalianuk. In hand specimen the rock differs 

from the quartz diorite in the greater abundance of pink orthoclase 

and light-gray oligoclase. The hornblende and biotite are stained

489292 59 3
292 INVESTIGATIONS OF ALASKAN VOLCANOES

green with chlorite and epidote. The hand specimen appears distinctly 

granitoid, although quartz is not conspicuous. The quartz monzonite 

(column 3, table 4) as seen in thin section resembles the quartz diorite 

except for differing proportions of the minerals. Quartz and orthoclase 

appear interstitial, but show only a very slight tendency toward 

graphic intergrowth. Some oligoclase crystals are veined and contain 

irregular wisps of orthoclase as well as specks of calcite(?) and tiny 

flakes of sericite(?). A little of the orthoclase is perthitic. Augite 

remnants are sparse and contain reaction runs of hornblende and 

biotite. About half the biotite has altered to chlorite. Anhedral 

opaque oxides are associated with hornblende and biotite and appear 

to replace them. Minor epidote and rare prehnite(?) also appear as 

alteration products of the biotite.

GRANOPHYEE

Pinkish-gray granophyre (column 4, table 4), as dikes and narrow 

stringers, cuts the quartz diorite on the west side of Cape Ilmalianuk 

headland. The texture of the rock is largely micrographic superposed 

on an allotriomorphic texture. Each larger granular unit contains 

uniformly oriented quartz grains that differ in orientation from grains 

in a neighboring unit. A few larger grains of quartz, orthoclase and 

albite-oligoclase are not micrographically intergrown. Epidote is in 

aggregates interstitial to the other minerals. The biotite is largely 

altered to chlorite. Rare needlelike inclusions of apatite are in the 

quartz and feldspar. Actinolite also occurs as sparse aggregates. 

The chemical analysis of this specimen is listed in column 33, table 2.

ALTERED CONTACT ROCK

An altered monzonitic rock borders the granophyre dikes and extends for some distance on either side. The altered rock is invaded by 

smaller stringers of granophyre at closely spaced intervals. The contact with the granophyre is gradational, and the alteration within the 

monzonitic contact rock is variable. Streaks of pinkish-gray rock 

with spots of mafic minerals alternate with streaks of mottled lightgreenish-gray epidotized feldspar and pinkish-gray orthoclase. The 

streakiness in general parallels the approximate border of the granophyre dike. Dark-greenish actinolitic hornblende as much as 3 mm 

in length shows slight segregation in bands parallel to the feldspathic 

streaks but shows no visible lineation. As seen in thin section, the 

rock has a variable texture. An allotriomorphic texture involves 

chiefly quartz and orthoclase with minor albite-oligoclase and grades 

into a micrographic intergrowth of quartz and potassium feldspar and 

also into poikilitic intergrowths of small quartz grains in potassium 

feldspar. A few formerly euhedral andesine and calcic oligoclase crys-
 GEOLOGY OF XJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 293

tals have relict outlines and are veined and corroded by potassium 

feldspar and albite-oligoclase (pi. 43). Much of the plagioclase contains fine to coarse aggregates of epidote, probably derived from the 

decomposition of the more calcic plagioclase.

VOLCANIC BOCKS OF CENTRAL UMNAK 

GEOLOGIC RELATIONS

Lava flows, vent breccias and associated irregular shallow intrusive 

bodies crop out over a large area hi the narrow central part of Umnak 

Island. These rocks are herein grouped together as the volcanic 

rocks of central Umnak. The original constructional surface form of 

these volcanic rocks have been destroyed by erosion. Their original 

areal extent is not known because they are covered both to the northeast and to the southwest by Quaternary lavas of Okmok Volcano and 

Mount Recheschnoi, respectively. The volcanic rocks of central 

Umnak were in part erupted from several vents northeast of Hot 

Springs Cove (pi. 41).

These rocks include both hydro thermally altered and fresh unaltered 

volcanic rocks in each of the two map units: bedded volcanic rocks 

(QTv) and vent and intrusive complex (QTvp), as shown on plates 40 

and 41. No contacts between altered and fresh unaltered volcanic 

rocks within the units were seen. The intensely silicified, potassium 

feldspathized rocks included as map unit QTvq, (pis. 40 and 41), consists of rocks so completely replaced by silica, potassium feldspar, and 

in places tourmaline, that the original rock type cannot be identified 

in hand specimen or thin section, though field relations indicate that 

most grades into rock included in the map unit, QTv.

Altered lava flows and vent complexes make up most of the isthmus 

terrain east and north of Hot Springs Cove (pi. 41). Breccias and 

irregular intrusive rocks, interpreted as old vent complexes, are found 

at several places in the isthmus. Volcanic necks of hydro thermally 

altered basaltic porphyry at Thumb Point (pi. 40), and High Hill, 

2Y2 miles north of Nikolski, are included here. Areas of most intense 

alteration, mapped under the symbol, QTvq, are along the coastline 

just west of Hot Springs Cove and in the southwest wall of Bottleneck 

Pass (pi. 41). The base of the altered volcanic rocks has not been 

observed.

Unaltered hypersthene andesite flows considered part of this unit 

are found hi several localities south and west of Hot Springs Cove 

(pi. 40). South of Hot Springs Cove these unaltered flows dip about 5° 

SW. under the similarly appearing hypersthene andesite flows from 

Mount Recheschnoi, which dip northeast and lie on an erosion surface 

cut on the volcanic rocks of central Umnak. Similar relations between 

unaltered lava flows of central Umnak and those of Recheschnoi are
294 INVESTIGATIONS OP ALASKAN VOLCANOES

found at Broken Point, Geyser Bight, Partov Cove, Russian Bay, and 

in the west wall of the canyon 4 miles due south of the summit of 

Mount Recheschnoi. Along the Pacific coast at Partov Cove and 

Russian Bay, unaltered lava flows of this unit lie on an erosion surface 

cut on the albitized sedimentary rocks; on the Bering Sea coast at 

Broken Point and the west shore of Geyser Bight they lie on a plane 

erosion surface, possibly a raised marine bench, cut on the plutonic 

rocks; and west of Hot Springs Cove and along the south shore of 

Inanudak Bay they overlie hydrothermally altered lava flows and 

diorite in an irregular contact that was concealed or inaccessible for 

study in the field.

The nature of the contact between the altered and unaltered lavas 

of the volcanic rocks of central Umnak is unknown. Near the head 

of the easternmost fork of Geyser Creek (pi. 40), a patch of intensely 

altered rock, about 0.1 square mile in area, is just upstream from an 

area of borate-bearing hot springs, geysers, and fumaroles (Byers and 

Brannock, 1949, p. 728). This altered rock consists of a very finegrained aggregate of quartz, chlorite, pyrite, and colorless minerals, 

possibly including adularia. The altered rock may here be related to 

hot-spring activity in the recent geologic past. Elsewhere, however, 

as near the isthmus of Umnak, larger areas of tourmalinized, silicified, 

potassium-feldspathized rock are not associated with hot springs and 

are also somewhat coarser grained than the altered rocks on Geyser 

Creek. The relation between the unaltered and altered volcanic rocks 

may be gradational, inasmuch as gradation was observed in the field 

and confirmed by the laboratory study of specimens from the southeast 

wall of Bottleneck Pass (pi. 41). On the west side of Hot Springs 

Cove (pi. 40), however, intensely altered rock is overlain by fresh, 

unaltered hypersthene andesite flows. The overlying fresh lava could 

hardly have escaped some alteration during the intense silicification 

and potassium feldspatbization of the lower unit; hence, it is believed 

that a sharp contact, representing a significant period of geologic time, 

may be concealed between the altered and unaltered lavas at this 

place. Possibly the alteration is locally associated with buried 

plutonic rocks of the same age as those that now crop out at the 

surface.

AGE AND CORRELATION

The volcanic rocks of central Umnak, as shown on plates 40 and 41, 

include rocks that are probably older and unaltered lavas that are 

younger than the plutonic rocks. The altered volcanic rocks hi 

many places on Umnak closely resemble rocks on Urialaska known to 

be intruded by the post-Oligocene plutonic rocks. The relations between the altered volcanic rocks and the albitized sedimentary and
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 295

igneous complex are unknown, but from the petrographic evidence, 

the potash feldspathization of the altered volcanic rocks occurred 

later than the albitization of the complex; how much later remains an 

open question. The altered volcanic rocks therefore may range in 

age from early to middle Tertiary, though a still earlier age cannot be 

ruled out on present evidence. The unaltered lavas rest on the postOligocene plutonic rocks and are overlain by the Quaternary lavas of 

Recheschnoi and Okmok volcanoes; hence, the unaltered lavas of the 

volcanic rocks of central Umnak are late Tertiary or early Quaternary 

in age.

PETROGRAPHY AND CHEMICAL COMPOSITION

A volcanic neck at Thumb Point intrudes the albitized rock of the 

basement. Examination of a specimen from the basement rock 

adjacent to the neck indicates that it was first albitized and later 

potassium feldspathized (see p. 288). The rock of the neck, however, 

shows only potassium feldspathization. The rock is a dense, mediumdark-gray, sparsely porphyritic lava with greenish-gray spots resembling amygdules, but as seen in thin section former labradorite phenocrysts are much altered to chlorite and a zeolite. Former olivine 

phenocrysts are replaced by antigorite(?) and magnetite. The groundmass consists of andesine, magnetite, quartz veinlets (secondary), 

chlorite, and a cloudy white feldspar (index of about 1.52), probably 

adularia.

Fine-grained potassium feldspar was identified in several thin 

sections of the hydro thermally altered volcanic rocks of central Umnak. 

Partial chemical analyses were made of three specimens, progressively 

more altered, from the southwest wall of Bottleneck Pass (see pi. 41). 

The least altered specimen (column 12, tables 2 and 3) is representative of most of the hydro thermally altered lavas. Quartz, magnetite, 

chlorite, and serpentine in various proportions have replaced former 

olivine phenocrysts and portions of the groundmass glass. The 

chlorite is penninite. The serpentine mineral, more abundant than 

chlorite, is probably antigorite (a, colorless to faint yellowish green; 7 

pale green and parallel to fibers; birefringence, 0.015-0.020; indices 

variable, ranging from about 1.53 to 1.58). Zeolites, potassium 

feldspar, and calcite embay the primary plagioclase. The introduced 

zeolite minerals, identified on the basis of optical properties, are 

heulandite(?), chabazite(?), and natrolite. Amygdaloidal fillings of 

quartz and natrolite (?) are common.

A second specimen (12-A) collected about 0.7 mile southeast from 

the locality of the first specimen (column 12, table 2) and from apparently the same lavas, was originally a porphyritic lava. Secondary 

amphiboles, epidote, and sparse tourmaline have developed in addi-
296 INVESTIGATIONS OF ALASKAN VOLCANOES

tion to potassium feldspar, quartz, and zeolites. A fibrous actinolitic 

amphibole (uralite) like that in the granophyre replaces augite. A 

second amphibole fills druses with acicular crystals distinctly pleochroic from light green to bluish green. Epidote is common apparently 

in part as an alteration product of augite. Tourmaline, with strong 

greenish-blue absorption, has replaced plagioclase phenocrysts.

A third specimen (12-B) collected about 50 feet from the second, 

probably represents nearly the end stage of the potassium feldspathization. The original rock is no longer recognizable, but in the field it is 

gradational with rock whose relic texture indicates porphyritic lava. 

This rock is light colored, brown stained in weathered outcrop, and 

veined with quartz. Minute cubes of pyrite, 0.1-0.5 mm, are sparsely 

distributed. Small rosettes of black tourmaline, 1 to 2 mm long, 

appear to have preferentially replaced the former ferromagnesian 

minerals. All the secondary minerals observed in the less altered 

rocks are present, but quartz and potassium feldspar (adularia?) are 

much more abundant. Portions of the rock are completely replaced 

.by potassium feldspar and vuggy vein quartz. The more compact, 

massive portions of the rock consist of dense cryptocrystalline aggregates of quartz, potassium feldspar, and zeolites.

Partial chemical analyses of the three specimens (table 5) show that 

potash has greatly increased in the most altered rock at the expense of 

soda, iron oxide, magnesia and lime. The abundance of quartz 

observed in the completely altered rock indicates that silica was likewise introduced, despite the slight drop in bulk specific gravity with 

progressive alteration. Sulfur, boron, and carbon dioxide have been 

fixed in the hydrothermally altered rocks in pyrite, tourmaline, and 

calcite, respectively.

TABLE 5. Partial analyses of hydrothermally altered (potassium feldspathized) 

volcanic rocks near Bottleneck Pass

[Analyses of oxides In weight percent. For complete analysis of No. 12, see column 12, table 2. Alkalis of 

12A and 12B, analyzed by E. J. Benton, Petrographic Laboratory, U. S. Bureau of Eeclamation; total 

iron, lime, and magnesia of 12A and 12B, analyzed by Herbert M. Ochs, Denver, Colo.]

MgO... ...... .......................................... ...

CaO- .-_._- ......... .... ..........................

N&tO. ........................................................

K8O._ _ . -_ . 

Bulk specific gravity.. . __________

12

7.3

3.4

9.3

2.8

1.4

2.67

12A

5.5

3.0

5.0

2.3

2.9

2.43

12B

3.2

.31

.45

1.9

5.8

2.26

12. Slightly altered plagioclase basalt (47ABy44) collected just south of Bottleneck Pass (lat. 53°17.1' N.,

long 168°19.8' E). 

12A. Moderately altered porphyritic lava (47ABy43A) from south side of Bottleneck Pass (lat. 53°16.7' N,

long 168°19.3' E) about 0.7 mile southeast of No. 12. Outcrop areas occur as "islands" in 12B. 

12B. Completely altered rock (47ABy43B) collected about 50 feet from 12A. Predominant rock of locality.
GEOLOGY OF UMNAK AND BOGOSLQF ISLANDS, ALEUTIAN ISLANDS 297

VOLCANIC BOOKS OP MOUNT RECHESCHNOI 

GENERAL GEOLOGIC FEATURES

The volcanic rocks of Mount Recheschnoi occupy most of the width 

of the island west of the isthmus (pi. 40). The summit of Mount 

Recheschnoi is an east-west ridge of rugged aretes and divides, 

between cirques occupied by small glaciers. Most of the original 

surface of the summit area has been destroyed, but at lower altitudes 

large areas of original surface remain between the glaciated valleys. 

Projection of the slopes suggests that two major vents, separated 

by about 2 miles, were active in the construction of the volcano. 

The western part of the mountain is less cut up by glaciated valleys; 

possibly major activity at the western vent continued longer than 

at the eastern vent. Some glaciated valleys have been partly filled 

by lavas from the central vents, then reexcavated by continued glacial 

action. Products of a satellitic basaltic vent occupy about 6 square 

miles on the northwest flank of Recheschnoi. These lavas have been 

glaciated. Products of later volcanic vents, including cones, domes, 

and flows retain their constructional surfaces and rest on the glaciated 

surfaces of the older erupted rocks.

The volcanic rocks of Mount Recheschnoi are divided into five 

map units: hypersthene andesite, mafic phenocryst basalt of Kshaliuk 

Point, youngest hypersthene andesite flows, rhyolite domes, and 

quartz-olivine andesite flow (pi. 40). The hypersthene andesite that 

forms the main mass of the Mount Recheschnoi volcanic pile is the 

principal map unit. The mafic phenocryst basalt of Kshaliuk Point 

on the northwest slope is distinguished only by its petrographic 

character; its contact with the main body of hypersthene andesite 

is not exposed and the stratigraphic relations are not known. The 

youngest hypersthene andesite comprises flows, including a pyroclastic 

flow, separated from the main mass of hypersthene andesite because of 

their youthful land forms. The rhyolite domes and the quartz-olivine 

andesite flow are distinguished from the older hypersthene andesite by 

both land form and petrographic type.

HYPERSTHENE ANDESITE

The hypersthene andesite in the mam volcanic pile of Mount 

Recheschnoi occurs mostly in thick, widespread flows and interstratified pyroclastic material. Most of the rock is strongly porphyritic 

andesite, containing phenocrysts of plagioclase and pyroxene, both 

having two different ranges in composition. Some flows in the western 

part are less porphyritic. The volume of these porphyritic rocks is 

estimated at more than 16 cubic miles. The basal hypersthene 

andesite flows lie on surfaces eroded on the dioritic plutonic rocks,
298 INVESTIGATIONS OF ALASKAN VOLCANOES

the albitized sedimentary and igneous complex, and the volcanic rocks 

of central Umnak. The upper flows are overlapped by the volcanic 

rocks of Mount Vsevidof.

MAFIC PHENOCRYST BASALT OF KSHALH7K POINT

A vent complex and associated lava flows of basalt containing 

plagioclase and mafic minerals as phenocrysts underlies an area of 

about 6 square miles at Kshaliuk Point on the northwest slope of 

Mount Rechescbnoi (pi. 40). The area is surrounded by hypersthene 

andesite from Mount Recheschnoi, but no contacts were seen that 

expose the relations between the two rock types. Both terrains 

show similar erosion and have been glaciated. The basalt has not 

been altered nor deformed, so it must be considered younger than the 

basement rocks.

The basalt contains phenocrysts of gray, translucent bytownite (20 

percent; as much as 2 mm in size), green augite (4 percent; as much as 

8 mm in size), and lemon-yellow olivine (5 percent; as much as 3 mm 

in size) in a purplish-gray crystalline groundmass. The phenocrystic 

augite has optical constants that indicate an average composition 

(Hess, 1949, pi. 1) of Ca45Mg3sFe17 . Olivine phenocrysts range in 

composition from Fogg to Fo84- The average composition of the 

groundmass plagioclase is about An75 .

YOUNGEST HYPERSTHENE ANDESITE FLX>WS

PYKOCIASTTC FIOW

A flow of pyroclastic material fans out from one of the glaciated 

valleys on the east slope of Mount Recheschnoi. Its source was not 

seen, but is probably buried beneath the youngest hypersthene andesite 

lava that flowed down the same valley and overlies part of it. Older 

till of the earlier glaciation overlaps the pyroclastic flow along part of 

its western edge. The pyroclastic flow is made up of angular blocks 

embedded in a relatively small amount of matrix. Blocks range in 

size from 10 feet in diameter to fragments of microscopic size. Most 

of the rock fragments are hypersthene andesite similar to that of the 

lava flows, but a few represent altered sedimentary basement rocks. 

The matrix is light brownish to light purplish gray, dotted with 

grayish-black grains and intricately transected with thin (as much as 

1 mm thick) seams of clayey, grayish-orange-pink material. The 

matrix includes fragments of plagioclase, hypersthene, augite, and 

opaque oxide crystals, but is largely glass with an index of refraction 

of 1.542. Some of the glass is free of all fragments and appears to 

cut through the matrix as veinlets. The deposit is believed to be a 

mudflow of brecciated lava that was still somewhat hot, rather than 

any sort of ash flow associated with a nue"e ardente type of explosive 

eruption.
IAVA FLOWS

Three cinder cones and lava flows of the youngest hypersthene 

andesite of Mount Kecheschnoi are preserved essentially uneroded. 

Two of the lava flows, one of which flowed down a glacial valley almost 

to the Pacific Ocean, rest on the glaciated southeastern slope of Mount 

Recheschnoi (see pi. 40). The third cone and lava flow is in the saddle 

between Mounts Recheschnoi and Vsevidof. These flows are petrographically similar to the hypersthene andesite that forms the major 

part of Mount Recheschnoi.

RHYOIJTB DOMES

Three small domes of rhyolite west of the head of Russian Bay 

(pi. 40) are younger than the glaciated topography, and appear to 

have been formed as parts of a single episode of extrusion. The 

domes are partly buried in their own talus.

QUARTZ-OIJVENE ANDESITE FLOW

A quartz-olivine andesite flow lies on the glaciated surface of 

hypersthene andesite of Mount Recheschnoi just southwest of the 

isthmus (pi. 40) and covers somewhat more than 1 square mile. It is 

probably about 450 feet thick at the lava dome in the western part of 

the flow and averages perhaps 300 feet. The flow spreads in all 

directions from the dome, chiefly eastward to form a small plateau 

about 1,600 feet above the sea. The surface is a series of arcuate 

ridges about 50 feet high and 400 feet from crest to crest, eccentric to 

the dome.

AGE OF LAVAS OF MOUNT RECHESCHNOI

The major cone-building activity of Mount Recheschnoi may have 

begun as early as late Tertiary, but available evidence suggests that 

the main pile of hypersthene andesite is mostly if not entirely Pleistocene in age. The basal contact of the Mount Recheschnoi pile is well 

exposed and rests on a marine (?) erosion surface of low relief carved 

on a basement complex that includes fresh unaltered lavas that may 

be as young as Pleistocene. The exposed lavas of Mount Recheschnoi 

resting on this surface have been deeply dissected by glacial erosion 

probably during late Pleistocene and early Recent tune. The relations 

of the basalt at Kshaliuk Point to the hypersthene andesite are not 

known; hence this basalt may be as old as Pliocene. The essentially 

uneroded satellitic flows and domes, excepting possibly the pyroclastic 

flow beneath older till, are postglacial and therefore probably Recent 

in age.

PETROGRAPHY

All 12 specimens of volcanic rocks of Mount Recheschnoi examined 

microscopically are characterized by one or more minerals apparently
300 INVESTIGATIONS OF ALASKAN VOLCANOES

not in equilibrium with the groundmass minerals. The hypersthene 

andesite comprising the bulk of the volcano, though similar from 

outcrop to outcrop, appears to be a hybrid rock formed either by 

incomplete fusion and assimilation of country rock, by mixing of 

partly crystallized melts, or a combination of these processes.

HYPERSTHENE ANDESITE

The dominant rock type is hypersthene-bearing labradorite andesite 

with varying amounts of plagioclase, augite, hypersthene and olivine 

as phenocrysts. Petrographic analyses of several specimens of this 

rock type are given in table 6. In each rock, crystals of plagioclase, 

whose relations are not simple zoning, occur in two separate ranges 

of composition.

TABLE 6. Petrographic analyses of specimens of hypersthene andesite of Mount

Recheschnoi

Mode In volume percent: 

Phenocrysts >0.2 mm: 

Plagioclase _______ ... _____

Auglte. _- ______ ......

Hypersthene ____ .... ___ . ....

Opaque oxides ___ .... _ . ........

Composition of more calcic plagioclase >0.2 

mm. 

Composition of less calcic plagioclase >0.2 

mm. 

Approximate composition, groundmass plagioclase.

Composition of hypersthene crystals >0.2 mm.

la

33.1 

3.1 

2.1 

.1 

.1 

61.5

......

......

Ib

32.9 

17.7 

13.1 ».2 

.1 

56.0

An88-9J

AnooHis 

Ancj 

1.708

EneoHM

F068-78

Ic

28.9 

17.7 

>3.5 

».5 

.1 

59.3

2

31.3 

2.9 

1.8 

.2 

.2 

63.6

3

28.4 

2.9 

3.0 

.1 

.2 

65.4

4

1.9 

1.2 

.2

- .01

.2 

96.5

An78-88

Anjs-oa 

An«

F007-72

1.505

5

13.5 

1.2 

.6 

.4 

.1 

84.3 

Anso-s*

Anu-di 

Anso

1.707

Enj4-e>

F07(M»

tl-53

1 Mafic minerals somewhat higher than other sections owing to mafic inclusions.

la, Ib, Ic. Three mutually perpendicular thin sections, hypersthene-bearing labradorite andesite of hypersthene andesite of Mount Recheschnoi, altitude of 1,400 feet (lat 53°13.8' N., long 168°22.0' E.), 1.8 miles 

south of Hot Springs Cove (chemical analysis 23, table 2).

2. Hypersthene-bearlng labradorite andesite near base of hypersthene andeslte of Mount Recheschnoi at 

altitude of 850 feet on east side of valley (lat 53°12.5' N., long 168°25.2' E.) that drains into Geyser 

Bight.

3. Hypersthene-bearlng labradorite andesite near base of hypersthene andeslte of Mount Recheschnol 

collected at altitude of 200 feet in sea cliff Oat 53°10' N., long 168°20.7' E.) east of Russian Bay.

4. Hypersthene-bearing aphyrlc andesite of hypersthene andesite of Mount Recheschnoi, at altitude of 

about 500 feet Gat 53fQ5.3' N., long 168°28.5' E.)f 3.2 miles N. 60° E. of Amos Bay (chemical analysis 

27, table 2).

5. Hypersthene-bearing labradorite andeslte of youngest hypersthene andesite flow of Mount Recheschnoi, 

collected at lat 53°08.8' N., long 168°28' E., 7.3 miles N. 75° E. of Amos Bay.

i

After crushing and sieving 50 grams of the chemically analyzed 

specimen (columns la, Ib, and Ic), the ratio of anorthite-bytownite to 

calcic labradorite was determined to be 1:9 by counting grams in 

two size ranges: between U. S. Standard sieves no. 80 and no. 120 

and between sieves no. 150 and no. 200. The anorthite-bytownite is 

in separate crystals and irregularly shaped cores within wide rims of 

the calcic labradorite. There is no zone of the composition intermediate between calcic bytownite and calcic labradorite. The
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 301

more abundant phenocrysts of calcic labradorite commonly show a 

clear core and a surrounding zone of slightly more sodic labradorite 

clouded with dust inclusions. The borders of this clouded zone 

are rounded and enclosed in still another shell of clear, slightly 

more calcic labradorite with a composition of about An6S , which is 

the composition of the groundmass plagioclase. This type of plagioclase phenocryst is believed to be comparable to those described by 

Kuno (1950, p. 968) and suggests that the part of the crystal including 

the clear core and the dust-bearing zone was xenocrystic in the 

liquid and partly resorbed before the outer shell of plagioclase was 

precipitated.

In specimen 5 (table 6), from one of the recent flows, most 

of the plagioclase crystals contain a core of andesine, showing normal 

and slightly oscillatory zoning with dust inclusions in the outer 

zone. The inner core is separated by a rounded outline from an 

outermost shell of clear labradorite of composition An6o. A few 

phenocrysts have an inner core of bytownite, rounded and enclosed 

in a thick zone averaging about Anso . This zone is embayed and 

encased in an outermost rim of plagioclase of composition An60 , which 

is the same composition as the plagioclase microlites of the groundmass. Mafic phenocrysts include augite, hypersthene, and olivine. 

The rare olivine grains all show evidence of resorption, and are rimmed 

with monoclinic pyroxene. Hypersthene in euhedral crystals as 

much as 3 mm long is commonly rimmed with monoclinic pyroxene, 

and some hypersthene forms ragged cores in relatively thick aggregates of monoclinic pyroxene. Augite or ferroaugite phenocrysts 

reach a maximum length of 2 mm. The groundmass consists of 

fine microlites of plagioclase, clinopyroxene, and opaque oxides in a 

dusty, grayish-brown glass.

Inclusions composed of hypersthene, augite, and minor bytownitelabradorite are in part mutually intergrown, suggestive of an original 

gabbroic texture (specimen 1, table 6). Minor, opaque oxide-charged 

dark-brown glass occurs interstitially within them, suggesting their 

partial resorption. The plagioclase of the xenoliths is normally zoned 

with bytownite cores and a thick shell of clear labradorite. In one 

clump only augite crystals are clustered together. All the xenoliths 

are more mafic than the average of the rock, and they are abundant 

enough to make the rock somewhat heterogeneous on a small scale.

RHYOIITE

The rhyolite contains sparse, small euhedral phenocrysts of bluishgray quartz, feldspar, biotite, hornblende, and rare hypersthene in a 

light-purplish-gray felsitic groundmass. The rock weathers light 

brownish gray. Flow banding is conspicuous and the quartz is moder-
302 INVESTIGATIONS OF ALASKAN VOLCANOES

ately concentrated in certain thin bands. Hence, the thin-section 

analysis given in ta,ble 3 may be low in quartz phenocrysts.

A few plagioclase phenocrysts seen in thin section contain labradorite as small mottled cores inside a thick outer shell of sodic andesine. 

The other plagioclase phenocrysts and microphenocrysts down to 

0.05 mm are sodic andesine. All the plagioclase is normally zoned. 

Small inclusions of hornblende are associated with a few labradorite 

cores within andesine phenocrysts, and one xenolith consisting of hornblende and normally zoned labradorite-andesine was also observed.

The biotite phenocrysts show certain optical anomalies common in 

extrusive rhyolite. The optic angle of different biotite phenocrysts 

ranges from about 5° to 32°; the larger angles are in general measured 

on smaller phenocrysts. A few contain opaque oxides, probably 

magnetite, parallel to the lamellae. The deep color and high refractive 

index may be due to a high Fe2O3 : FeO ratio, which can be caused by 

heating at atmospheric pressure. A. Jean Hall (1941b, p. 35) has 

shown that the refractive indices are raised considerably with increase 

in the oxidation of the iron but that no effect of oxidation of iron on 

the color (Hall, 194la, p. 32) has been demonstrated.

The groundmass of the rhyolite consists of colorless glass, apatite 

needles, crystallites, and abundant spherulites amounting to .more 

than half the volume of the groundmass. In certain bands the glass 

is lightly charged with red hematite (?) dust. The spherulites are of 

two main types, clear radiating low birefringent laths (n. about 1.53) 

in nearly perfect sperulites reaching a maximum diameter of about 

0.1 mm, and white, cloudy, partly opaque, fine spherulitic aggregates 

that commonly contain a hollow cavity. These aggregates reach a 

maximum diameter of about 0.5 mm; and have an index considerably 

lower than balsam. A few vesicles are filled with a weakly birefringent 

zeolite with an index of 1.484.

QUARTZ-BEARING OLIVINE ANDESITE

This rock is unusual in that it contains phenocrysts of both quartz 

and olivine, which are incompatible under equilibrium conditions. 

Lemon-yellow olivine, pyroxene laths, dark-gray glassy andesine, 

minor amounts of white bytownite, and quartz occur as phenocrysts 

in a medium-light-gray to medium-gray groundmass that weathers 

light to medium olive gray on exposed surfaces. Petrographic analyses 

of three mutually perpendicular thin sections are given in table 7. 

The quartz occurs as rounded and embayed xenocrysts up to 0.5 mm, 

encased by a thin shell of radial fibers of clinopyroxene, probably 

augite. The abundant plagioclase phenocrysts consist of. normally 

zoned cores, ranging from sodic labradorite to sodic andesine. The 

outer zones contain a dust inclusion band that parallels the rounded
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 303

outlines of the phenocrysts rather than the pseudorectangular pattern 

of zoning. A relatively thin, outermost shell of about Aneo, essentially 

the same composition as the groundmass plagioclase, encloses the 

phenocrysts and follows the rounded edges into embayments as well. 

Normally zoned bytownite occurs in a few separate phenocrysts and 

anhedral inclusions in the andesine-labradorite phenocrysts.

TABLE 7. Petrographic analyses (modes in volume percent) of three mutually 

perpendicular thin sections of quartz-bearing olivine andesite

[Chemical analysis of the rock Is given in column 22, table 2]

Phenocrysts, microphenocrysts, and irregular masses > 0.2 mm:

Hornblende(?) "ghosts" (opaque oxide-clinopyroxene aggregate) ______

Groundmass (plagioclase- Anso, orthoclase(?), clinopyroxene, opaque oxides, and

1

0.7 17.0

'1.2

1.9

.1

8.2

.8

.2

79.9

2

1.6 19.9

U.7

3.1

.5

5.2

.9

.4

76.8

3

1 9

16.0

ll.l

3.1

.6

O 3

.4

.3

78.0

i Plagioclase ratios computed by crushing rock sample on sieves nos. 150/200, counting plagioclase grains 

In oil immersion, and then recomputing to total plagioclase percentage determined by Chayes point counter.

Both bytownite and andesine-labradorite also occur separately in 

xenoliths. The bytownite is associated with diopsidic augite and 

the andesine-labradorite with ferroaugite, some of which contains 

hypersthene cores. Brown glass containing apatite needles is interstitial to the larger (1 to 3 mm) plagioclase and pyroxene grains.

Mafic phenocrysts are more abundant than plagioclase. The ferroaugite generally contains tiny grains of opaque oxide and appears 

faintly pleochroic in brownish green. Its diagnostic optical properties (beta index, 1.710; 2V, 50°), though slightly variable, indicate 

a composition of about Ca38Mg28Fe34 (Hess, 1949, pi. 1). Sparse 

augite (beta index, 1.698; 2V, 53°), corresponding in composition to 

Ca43Mg35Fe22 also occurs. The hypersthene invariably occurs as 

deeply corroded, opaque oxide-charged remnants surrounded by ferroaugite. Hornblende (?) "ghosts" are a fine aggregate of opaque 

oxides and augite. Olivine occurs both as euhedral and deeply 

embayed crystals and is generally enclosed by a reaction rim of 

clinopyroxene. . .

The groundmass consists of a very fine (0.005-0.01 mm) microlitic aggregate of labradorite, clinopyroxene, opaque oxides, orthoclase, and sparse glass. The presence of orthoclase in the groundmass is not surprising in view of the moderate potash content (1.6 

percent) of the lava.
304 INVESTIGATIONS OF ALASKAN VOLCANOES 

VOLCANIC BOCKS OF MOUNT VSEVIDOP

GENERAL GEOLOGIC FEATURES

Mount Vsevidof is a single stratovolcano south of Mount Recheschnoi 

(pi. 40). A crater three quarters of a mile in diameter occupies the 

summit and is filled with snow and ice. The cone is steep above an 

altitude of 3,000 feet, approaching 30° near the 6,920-foot summit; 

below 3,000 feet the slope gradually decreases to about 15° at an 

altitude of 1,000 feet. A few narrow canyons as much as 400 feet deep 

have been incised by glaciers, but most of the surface of the mountain is uneroded and represents the constructional surface of the 

uppermost lava flow or pyroclastic bed (pi. 44). The drained lava 

channels illustrated on plate 44 resemble stream gullies, but their 

location on the higher spurs at the flanks of the volcano and the 

"natural levees" of lava on the sides of these features demonstrate 

that they formed at the time of extrusion and not as products of 

stream erosion.

The rocks exposed are mostly lava flows of aphyric basaltic andesite and andesite. Near the summit, the surface is blanketed with 

pyroclastic beds. A sequence exposed in an inacessible cliff in the 

east slope gives the impression of four distinct beds of pyroclastic 

rocks. Only two beds are exposed in accessible places: an underlying layer of consolidated grayish-orange rhyodacite pumice and a 

surface layer of dark-gray andesitic scoria. Three of the most recent 

lava flows, apparently younger than the pyroclastic beds, have been 

outlined on plate 40. The most recent (possibly historic) flow on 

the west flank of Mount Vsevidof has the composition of latite.

The andesitic scoria and rhyodacitic pumice thin markedly down 

the slope of the cone and essentially disappear between 1,000 to 2,500 

feet above sea level. In the north crater wall, the pumice and scoria 

beds are each about 50 feet thick. At an altitude of 4,000 feet on 

the north side of the cone an exposed thickness of 40 feet of pumice 

is overlain by 30 feet of scoria, whereas at 2,000 feet, 40 feet of pumice with scattered clots of scoria in the upper few feet rest on at least 

40 feet of mixed loose ash, older scoria fragments and talus blocks. 

Below an altitude of 1,000 feet, both the scoria and pumice are 

missing. The gullies on the north lower slopes below 1,000 feet 

contain abundant, rounded pumice fragments.

AGE AND CORRELATION

The lavas of Mount Vsevidof lie on glaciated hypersthene andesite 

of Pleistocene Mount Recheschnoi but the older till, of latest Wisconsin or early Recent age, rests on some of the oldest andesite flows 

of Mount Vsevidof; these flows are older than the pyroclastic beds of 

the summit cone. The geologic relations and the youthful state of
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 305

dissection of the volcano, indicate that the cone is latest Pleistocene 

and Recent in age; probably most of the cone is Recent, although the 

beginning of activity may date from late Pleistocene. The uppermost pyroclastic beds of the composite summit cone were probably 

deposited within the last few thousand years and possibly by one 

of the earlier historic eruptions. Mount Vsevidof was reported 

active in 1830, and a minor explosive eruption occurred in 1878 (Coats, 

1950, table 2). "Smoke" was also reported from the summit of 

Mount Vsevidof in 1784, 1790, and 1880. An eruption of steam 

and "smoke" (volcanic ash) from Mount Vsevidof occurred March 

11-12,, 1957, during earthquakes on southwestern Umnak. This 

eruption, probably from the west rift (pi. 40), was observed by local 

inhabitants at Nikolski, according to The Anchorage Times, Anchorage, Alaska.

Three beds of pumice ash are interbedded with windblown sand 

and silt in an archeological excavation near Nikolski. The middle 

bed, which contains charcoal, has a carbon-14 age of 3,018±230 

years (William S. Laughlin, written communication, 1951; Arnold 

and Libby, 1951, p. 118). Material in the three pumice beds is 

compared with dacite pumice from the north slope of Mount Vsevidof 

in table 8. The three beds appear to be made up of similar pumice, 

which differs petrographically from the sample from the slope of 

Mount Vsevidof. These ash beds were probably derived either from 

an older Vsevidof eruption or from one of the volcanoes in the Islands 

of the Four Mountains, 35 to 50 miles west of Nikolski (see fig. 50).

TABLE 8. Comparative petrographic data: rhyodacite pumice of Mount Vsevidof 

and pumice fragments in ash beds of archeological profile, Nikolski

[Samples of ash from Nikolski archeological profile were furnished by William S. Laughlin]

Locality

Approximate 

percentage 

ol pumice 

(and phenocrysts)

100

80

40

65

Refractive 

index of 

pumice glass

1.514

1. 520-1. 522

1.521-1.526

1. 520-1. 521

Composition 

of plagioclase

Composition 

of hypersthene

Index of 

auglte

1.705

1.705

1.704

mined

PETROGRAPHY

BASALTIC ANDESITE FLOWS

The basaltic andesite is a medium-dark-gray lava with sparse 

lath-shaped plagioclase phenocrysts about 2 mm in length. Thin 

section analyses of basaltic andesite flows are given in columns 

1 and 2 of table 9 (see also column 21, table 3). All the plagioclase 

phenocrysts are normally zoned. Subparallel plagioclase laths, whose
306 INVESTIGATIONS OF ALASKAN VOLCANOES

TABLE 9. Petrographic analyses of basaltic andesite, andesite, and latite flows of

Mount Vsevidof

Mode in volume percent: 

Phenocrysts>0.1 or 0.2 mm (see notes;: 

Plagioclase ___ . ____________________

1

1.2

.1

98.7

2

2.2

.2

.1

97.5

3

3.7

.3

.2

.5

95.3

4

3.8

.1

.5

.3

95.3

An<o(?)

1. Basaltic andesite flow at Twinlava Point older than pyroelastic beds. Chemical analysis, column 21, 

table 2. Crystals >0.2 mm are considered phenocrysts.

2. Youngest basaltic andesite flow on north slope of Mount Vsevidof; specimen from an altitude of 1,500 

feet. Crystals >0.2 mm are considered phenocrysts.

3. Youngest andesite flow on south slope of Mount Vsevidof; specimen from flow front on Black Creek 

outwash plain. Crystals >0.1 mm are considered phenocrysts.

4. Latite flow, possibly historic in age, on west slope of Mount Vsevidof; specimen collected from flow 

front at altitude of 650 feet. Chemical analysis, column 28 .table 2. Crystals >0.1 mm are considered 

phenocrysts.

average size is about 0.1 mm, comprise the bulk of the groundmass 

as defined in table 9, though a few, whose average size approaches 

0.2 mm, have lengths as much as 0.4 mm parallel to axis a. The 

tabular plagioclase crystals grade downward to tiny microlites, about 

0.02 mm long, that compare in size with microlites of clinopyroxene 

and opaque oxide. Olivine occurs as rare microphenbcrysts as much 

as 0.4 mm and as rare microlites grading downward to 0.05 mm. 

Euhedral olivine is enclosed by a thin shell of clinopyroxene and 

ranges hi forsterite content from Foes to Fo77 .

I ANDESITE AND LATITE FLOWS

The andesite flows and the historic (?) latite flow contain light-gray 

plagioclase phenocrysts in a dark-gray groundmass; the groundmass 

of the latite is somewhat more glassy and darker in hand specimen 

than those of the andesites. Thin section analyses are presented in 

columns 3 and 4, respectively, table 9 (see also column 28, table 3). 

A size cutoff of 0.1 mm between phenocrysts and groundmass was 

assumed, because microphenocrysts, ranging in size upward from 0.1 

mm, are set in a hyalopilitic groundmass. The microlites in the 

groundmass, with the exception of orthoclase, average from 0.002 to 

0.005 mm. Plagioclase phenocrysts are normally zoned through the 

slight range indicated in the andesite (column 3, table 9). The 

plagioclase of the latite has a slight tendency toward oscillatory zoning 

with a reverse zoning associated with glass hi the inner shells. A few 

of the plagioclase phenocrysts of the latite contain a small core of 

sodic andesine. Hypersthene occurs as small laths ranging from 0.2 

to 0.5 mm and has a thin shell of augite or ferroaugite. Augite occurs 

as microphenocrysts ranging from 0.1 to 0.3 mm in average dimension. 

Opaque oxide euhedra average slightly over 0.1 mm in diameter.
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 46

A. Nearly flat-lying undisturbed Crater Creek basalt flows exposed in the northwest wall of Crater Creek 

gorge. Uppermost thin resistant bed is lower welded agglomerate bed near base of Okmok volcanics. 

Arcuate ridge north of caldera is partly shrouded in cloud bank.

B. View northeast toward sea cliff northeast of Cape Chagak. About 250 feet of Okmok volcanics rests 

on precaldera feldspathic basalt flows of Ashishik basalt {vertical sea cliff and foreground). Note 

welded palagonitized agglomerate beds in lower third of exposure. Middle part of Okmok volcanics 

is eroded into grotesque forms.
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 47

A. Hand specimen of dark-gray welded andesitic agglomerate with {ragmen la of medium-gray aphyric anil 

plagioclase basalt. Collected from headwaters of New Jersey Creek (lat. 53°29', long. 1C8 0 05')-

B. Photomici-..., ., - ..,., ,,,,-- ... ,,.,-,,,,, ., , -.,. .,.,,,-,,, L., IN,., MI . i.olumn 19, table 2). Olivine 

(ol) , labradonte (t) , andrsitic: ^roLiiHlmass (gf), contains mirrolitcs of plagioclase, clinopyroxene 

and opaque oxides in glass. Note flow banding in rhyolite (r), Plane-polarized light.
The groundmass consists of very fine microlites of ferroaugite(?), 

andesine and sparse opaque oxide enclosed by glass and orthoclase. 

Orthoclase occurs in minor amounts in the groundmass of the andesite 

but is common in the latite groundmass. Part of the orthoclase in 

the latite occurs in euhedra, a few of which reach 0.2 mm in length 

but are considered part of the groundmass, because the fine microlites 

are poikilitically enclosed by the larger orthoclase euhedra. Orthoclase thus crystallized slightly later than the microlites or at about 

the same time as the associated interstitial liquid was cooled to a 

glass. The glass in the latite groundmass (column 28, table 3) has 

an index of refraction comparable to glass of the analyzed rhyolites 

from Umnak.

ANDESITTC AND RHYODACITTC PYROCLASTIC BEDS

The beds that form the uppermost part of the cone consist of an 

underlying solid, welded, grayish-orange rhyodacite pumice bed 

(column 30, table 3), gradational with and overlain by a streakily 

banded, medium to dark-gray scoria bed. This contains minor solid 

clots of black andesitic glass (column 26, table 3) gradational in 

texture with andesitic scoria. Both the pumice and the scoria contain 10 to 15 percent of accessory blocks of andesitic lava and dioritic 

intrusive rock.

Petrographic analyses of the nonvesicular andesite glass, the 

vesicular scoria, and a xenolith in the glass are presented in table 10. 

The plagioclase and augite of the limonite-stained xenolith average 

about 0.5 mm arid are mutually intergrown, as in an intrusive rock. 

Euhedral opaque oxide grains average slightly larger than 0.1 mm.

TABLE 10. Petrographic analyses of rocks in dark andesitic scoria bed of Mount

Vsevidof cone

Mode In volume percent (not determined in column 2) : 

Crystals larger than 0.1 mm: 

Orthoclase (?).. .... ___ . ....... ___ ... __ ... __ .

Eefractive index of groundmass glass . ___ . _______ .

1

<0.1

.3

.1

< .1

.0

99.5

.0

a 1. 710

EnH-{8

1.643

2

> 1. 710

1.545

3

(')

43

16

11

122

8

1.705

1 Cloudy orthoclase (?) is the dominant constituent of the mesostasls. 

a Minor augite with 0=1.703-05, probably from altered xenoliths?

1. Black andesitic glass clot (chemical analysis, column 26, table 2) witbin and gradational into dark andesitic scoria erupted after rhyodacitic pumice during last major eruption of Mount Vsevidof. Collected 

at 4,000 feet on N. Slope of cone (lat 53°07.7' N, long 168°41.3' E).

2. Dark andesitic scoria that forms nearly all of bed. Same location as 1.

3. Limonite-stained dioritic xenolith in .black andesitic glass. Same location as 1.

468292 59 4
308 INVESTIGATIONS OF ALASKAN VOLCANOES

The interstitial material is largely cloudy, limonite-stained potassium 

feldspar with abundant apatite needles. Sodic andesine occurs in 

the outermost shells of all the plagioclase of the xenolith, in some 

grains of the andesitic scoria and glass, and as mottled, irregular 

grains in all three rocks. The apatite-charged orthoclase(?) and 

mottled sodic andesine grains in the scoria and andesitic glass suggest 

that they are xenocrysts from the disaggregation of diorite xenoliths.

VOLCANIC BOCKS OF OKMOK VOLCANO

Erupted rocks of Okmok Volcano and satellitic vents occupy most 

of northeastern Umnak Island (pi. 41). The main cone of the central 

volcano is truncated by Okmok Caldera, an approximately circular 

depression 6 miles in diameter and 1,500 feet in maximum depth. 

Mount Tulik, Mount Idak, and Jag Peak are the most prominent 

secondary volcanic piles superimposed on the outer slopes of the main 

cone. The lava plateau northeast of Mount Idak and the ridge west 

of New Jersey Creek are made up of bedded volcanic rock with gentle 

dip away from the main center of Okmok Volcano and are probably 

faulted parts of the outer slopes.

The rocks are divided into three stratigraphic groupings: precaldera 

rocks erupted prior to the formation of the caldera, pyroclastic rocks 

associated with the caldera-forming eruption, and postcaldera rocks 

erupted after the caldera-forming eruption. The precaldera rocks 

are mostly of basaltic composition and are by far in greatest volume. 

They include lava flows and pyroclastic beds and range fromaphyric 

to porphyritic in texture. The pyroclastic rocks associated with the 

caldera-forming eruption consist of agglomerate, welded agglomerate, 

ash, and tuff-breccia. Basalt cones, plug-domes, and flows occur 

outside Okmok Caldera and cannot be dated precisely with respect to 

its formation. The known postcaldera deposits within Okmok 

Caldera include volcanic material erupted from different vents and 

also a sedimentary unit containing bedded volcanic material deposited 

in a caldera lake, which has now almost completely vanished.

Several formal stratigraphic names were proposed in the 1947 

administrative report describing Okmok Volcano (Byers, Hopkins, 

Wier, and Fisher, 1947, p. 19-53). These names in approximate order 

of age were: Ashishik basalt, Crater Creek basalt, Idak basalt, Tulik 

basalt, Tanak volcanics, and Okmok ash. The names, Ashishik 

basalt and Crater Creek basalt, are used but redefined in this report; 

Okmok ash is changed to Okmok volcanics and redefined in this report 

to include both the Tanak volcanics and Okmok ash, which are herewith abandoned. The names, Idak basalt and Tulik basalt are also 

abandoned and replaced by informal names.
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 309 

ROCKS OF PRECAUJERA AGE

The precaldera rocks are subdivided on plate 41 into three main 

categories based on geologic and geomorphic relations. These categories in order of relative age are the Ashishik basalt, volcanic rocks 

of minor vents, and the Crater Creek basalt. The three categories 

may in part overlap in time, but generally their ages are believed to 

be as indicated on the map explanation of plate 41.

ASHISHIK: BASALT

The oldest precaldera rock of Okmok Volcano is named the Ashishik 

basalt after exposures in the east-facing cliff, 2 to 3 miles south of 

Ashishik Point, at the northernmost part of Umnak Island (pi. 41). 

The formation as redefined in this report includes most of the bulk of 

Okmok Volcano and comprises the Ashishik basalt of the 1947 administrative report (Byers, Hopkins, Wier, and Fisher, 1947, p. 25), 

tuff-breccia and tuff, that part of the Crater Creek basalt on the outer 

flanks of Okmok Volcano, Idak basalt, excepting the mass of Mount 

Idak, and the "basalt flows of Hill 1200" (Byers and others, 1947, 

pi. 3). The Ashishik basalt is here subdivided into three lithologic 

units, mafic phenocryst basalt, consisting of porphyritic basalt with 

more than 2 percent of mafic megaphenocrysts, palagonitized pyroclastic rocks, and aphyric (nonporphyritic) and feldspathic basalt, 

made up mostly of aphyric basalt flows but with some porphyritic 

flows that contain phenocrysts of plagioclase and less than 2 percent 

of mafic phenocrysts. These units are listed hi general order of age, 

although they complexly intertongue.

The Ashishik basalt must rest on the volcanic rocks of central 

Umnak, although the contact is concealed by the extensive blanket 

of Okmok volcanics. The hydrothermal alteration, the topographically low position, and the great amount of dissection of the volcanic 

rocks of central Umnak, in contrast to the constructional surfaces yet 

remaining on the Ashishik basalt, leaves little room for doubt that 

the Ashishik basalt is a generally younger pile of lavas superposed on 

the volcanic rocks of central Umnak. The^volcanicjrocks of minor 

vents intrude or overlie the Ashishik basalt.

MAFIC PHENOCRYST BASALT

At the type locality of the Ashishik basalt in the prominent ridge 

west of New Jersey Creek (pi. 41), mafic phenocryst basalt flows dip 

gently northward. The sequence is several hundred feet thick, with 

the base not exposed, and is overlain by approximately conformable 

aphyric and feldspathic basalt flows. A thickness of 500 feet of these 

flows was measured on the cliff, IK miles upstream from the mouth 

of New Jersey Creek. Individual flows range from 20 to 60 feet in
310 INVESTIGATIONS OF ALASKAN VOLCANOES

thickness. Most are massive without flow banding, but horizontal 

sheeting possibly representing flow structure, is conspicuous in a few. 

The upper 2 to 5 feet of many flows is reddish oxidized flow-breccia. 

The rocks are chiefly light to dark purplish gray, and range from 

aphanitic to coarsely porphyritic. Phenocrysts are in order of 

abundance calcic plagioclase, olivine, and monoclinic pyroxene.

A group of six superimposed, nearly horizontal flows of porphyritic 

basalt with mafic phenocrysts is exposed in the base of the northeast 

wall of Okmok Caldera (pi. 41). The outcrop is about 1,000 feet long 

and 300 feet high. The lowest outcrops are about 1,100 feet above 

sea level, and do not expose the base. The rock is overlain by palagonitized pyroclastic rock, with intervening lenses of bedded conglomerate as much as 15 feet thick.

Mafic phenocryst basalt underlies or intertongues with aphyric and 

feldspathic basalt flows at several scattered places on the eastern 

outer slope of Okmok Volcano, but the base is nowhere exposed. 

Mafic phenocryst basalt flows, dipping gently northeast, are the 

oldest rocks exposed in the east face of the lava plateau east of Mount 

Idak. Similar flows crop out higher in the cliff section and intertongue with both feldspathic basalt flows and palagonitized pyroclastic 

rock. In part of the ridge, mafic phenocryst basalt is the uppermost 

rock in the section and directly underlies the Okmok volcanics.

PALAGOHITIZED PYEOCIASTIC ROCKS

The palagonitized pyroclastic rocks of the Ashishik basalt include 

bedded tuff, poorly bedded tuff-breccia, which crops out beneath the 

Okmok volcanics on the lower slopes of Okmok Volcano (pi. 41), and 

imbedded vent agglomerate exposed mainly in the walls of Okmok 

Caldera. These rocks were included in the tuff-breccia and tuff unit 

of the preliminary report (Byers and others, 1947, pi. 3). The color 

ranges from dull yellowish brown to bright yellowish orange; the 

breccias and agglomerates are mottled by darker lithic fragments 

against the bright-colored matrix of palagonitized tuff. Xenolithic 

fragments range in size from microscopic grains to 3-foot blocks, and 

are similar to the mafic phenocryst basalts and to the aphyric and 

feldspathic basalts exposed on the flanks of the volcano and in the 

caldera walls.

These rocks are most abundant near the caldera, particularly in 

the east wall (pi. 45), where yellowish-orange agglomerate is exposed 

through a vertical distance of 1,000 feet. Comparable agglomerates 

more than 800 feet thick appear to make up a large part of the high 

cliff that truncates the north arcuate ridge on the east. Many small 

dikes and irregular bodies of intrusive rock cut the agglomerate here. 

At the southwest end of this same ridge, the caldera fault scarp
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN:,ISLANDS 311

exposes 1,000 feet of palagonitized pyroclastic rocks cut by several 

vertical lava pipes or dikes as much as 300 feet wide. Many dikes 

are similarly associated with the palagonitized agglomerate elsewhere 

in the caldera wall.

Palagonitized pyroclastic rocks are exposed in many places on the 

flanks of Okmok Volcano. In some places, chaotic dips and associated irregular dikes suggest that they occupy old vents, as near the 

headwaters of Bering Creek east of the caldera. Most of the outcrops are of tuff-breccia or agglomerate with no well-defined bedding, 

but on the southwest flank of Okmok Volcano at the base of three 

of the satellitic mountains, bedded palagonitized tuff is exposed as 

part of the main mountain mass under the edges of plagioclase-olivine 

basalt flows of satellitic vents.

Beds of the palagonitized pyroclastic rocks are exposed in the 

southeast face of the plateau northeast of Mount Idak: one bed 

between the lower mafic phenocryst basalt flow and the overlying 

aphyric and feldspathic basalt flows, and the other between the 

aphyric and feldspathic basalt flows and the upper mafic phenocryst 

basalt flow that tops the plateau.

APHYRIC AND FELDSPATHIC BASALT

The aphyric and feldspathic basalt unit of the Ashishik basalt 

includes abundant flows of aphyric basalt and less common feldspathic 

basalt rich in phenocrysts of calcic plagioclase. Phenocrysts of mafic 

minerals are rare. Aphyric and feldspathic basalt flows form much 

of the constructional surface of the flanks of Okmok Volcano and 

are exposed hi the caldera walls, in the shallow stream cuts, and in 

much of the sea cliff around the periphery of Okmok. A thick unit 

of these rocks forms the middle of the sequence exposed in the lava 

plateau northeast of Mount Idak. These rocks cap the isolated ridge 

west of New Jersey Creek and form the bulk of Magazine Ridge 

south of Mount Idak. Except for those on the plateau northeast of 

Mount Idak, they were included in the Crater Creek basalt in the 

preliminary report (Byers, and others, 1947).

Individual flows of these rocks are from 10 to 70 feet thick, and 

tops and bottoms commonly are reddish oxidized breccia and scoria 

from 5 to 20 feet thick. Columnar jointing is not well developed. 

The aphyric rocks are generally gray to light gray and commonly 

contain closely spaced joints, about one-eighth of an inch apart, 

parallel to the top or bottom. Parallel orientation of minute plagioclase tablets gives a satinlike sheen to the joint surfaces. The 

porphyritic rocks are gray, and conspicuously speckled with light-gray 

or white plagioclase phenocrysts. Flows have an irregular jointing 

which yields blocks with partly conchoidal surfaces.
312 INVESTIGATIONS OF ALASKAN VOLCANOES 

VOLCANIC ROCKS OF MINOR VENTS

The volcanic rocks of minor vents are subdivided into three units: 

vitreous andesite in plugs, domes, and flows; rhyolite in the form of 

a dome, and the plagioclase-olivine basalt of satellitic vents. These 

rocks either rest on the Ashishik basalt or intrude it. They were 

emplaced after the major upbuilding of Okmok Volcano and yet were 

dissected prior to deposition of the Okmok volcanics. Because of 

similar geologic relations with the older Asbishik basalt, the three 

units are assumed to be about the same age, although nowhere are 

they in exposed contact. The mass of Mount Tulik may be slightly 

younger than the other plagioclase-olivine basalts, for it is little 

dissected.

VITREOUS ANDESITE

In the caldera wall, in Crater Creek gorge, and on the outer slopes 

of Okmok near the caldera (pi. 41) several pipes, plug-domes and 

necks, and a few thick stubby flows are of dense, vitreous andesitic 

rock. In the field, these rocks were distinguished from the more 

common aphyric basalts by vitreous lustre, darker color, and almost 

complete absence of vesicles. The largest exposed body is a neck in 

the northeast wall of Okmok Caldera about 500 feet across and exposed 

1,700 feet vertically. The total volume of these rocks is insignificant 

compared to that of the Ashishik basalt.

RHTOHTE

Khyolitic rocks crop out for a mile along the north side of the 

north arcuate ridge and also form a ridge that extends a mile northwest from the north arcuate ridge (pi. 41). The rhyolite of the ridge 

is made up of randomly alternating bands ranging from a fraction of 

an inch to several inches hi thickness and consisting of dense black 

obsidian, vesicular gray and black obsidian, and light-gray felsitic 

rhyolite. The banding strikes parallel to the ridge and dips inward 

from each margin, suggesting that the ridge is an elongated narrow 

extrusion or dome from a fracture rather than an elongated flow. 

The eastern part of the mass along the north base of the north arcuate 

ridge contains black obsidian but is made up of gray vesicular glass 

and light-gray felsitic rhyolite in crude bands that dip southward 

into the ridge. The rhyolite presumably rests on the Ashishik basalt, 

although the contact is covered by talus from the dome. It is overlain by the Okmok volcanics, and a Recent basaltic flow overlies it 

to the east.

PLAGIOCLASE-OLIVINE BASALT OF SATELLITIC VENTS

Flows of plagioclase-olivine basalt make up the bulk of satellitic 

piles on the outer slope of Okmok Volcano. These include volcanic 

rocks at the satellitic vents of Mount Idak on the east slope, Mount
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN. ISLANDS 313

Tulik on the southeast slope (pi. 41), and Jag Peak and three other 

volcanic mountains on the southwest slope. These satellitic mountains rise 1,000 to 2,000 feet above the adjacent slope of Okmok 

Volcano. The flows have quaquaversal dips as great as 30° away 

from their summits. Vent breccia or intrusive plug material is exposed at some of the summits. Thick ash and, in places, agglomerate 

of the Okmok volcanics blanket the eroded flanks of the satellitic 

mountains.

CRATER CREEK BASALT

The Crater Creek basalt, the youngest precaldera unit, was originally defined in the 1947 administrative report (Byers, Hopkins, Wier, 

and Fisher, 1947, p. 25-26) to include the uppermost aphyric basalt 

flows on the outer slopes of Okmok Volcano, but it is restricted 

herein to the sequence of basalt flows exposed in Crater Creek gorge 

and in the north wall of Okmok Caldera to the west (pi. 46.4). A 

fault separates the Crater Creek basalt in the north wall from the 

Ashishik basalt where the north arcuate ridge intersects Okmok 

Caldera. The Crater Creek basalt is separated from the Ashishik 

basalt by an erosional surface. The unit was mapped separately 

because it occupies a topographically low position between the arcuate ridges, composed largely of the older Ashishik basalt, and because 

it rests unconformably on the Ashishik basalt and vitreous andesite 

in the lower part of Crater Creek gorge (pi. 41); otherwise, the rocks 

are petrographically indistinguishable from the feldspathic basalts of 

the Ashishik basalt. The depression in -which flows of the Crater 

Creek basalt lie may have included part of an earlier caldera on the 

concave side of the arcuate ridges.

The Crater Creek basalt consists of at least 15 separate flows with 

a total exposed thickness of 500 feet. The north wall of Okmok 

Caldera exposes several with easterly dip components of 1° to 2° 

toward Crater Creek gorge. The base is not exposed, but the gorge 

of Crater Creek is cut through into older palagonitized pyroclastic 

rocks, vitreous andesite, and associated dikes. The dikes are truncated by an eroded surface upon which the basal basalt flows were 

extruded. The Crater Creek basalt is thickest in the northwest 

wall of Crater Creek gorge, appreciably thinner and less continuous 

in the southeast wall, and does not crop out at all in the east wall of 

the caldera south of the gorge.

The basalt rocks are dark to light gray, range from aphanitic to 

porphyritic, and are rich in plagioclase phenocrysts. One to 10 

feet of red ash or scoria separates several of the flows. Wherever 

this red scoria can be examined, it is seen to be the oxidized rubbly, 

scoriaceous surface facies of the underlying flow. Two specimens, 

whose chemical analyses are shown in columns 10 and 11, table 2,
314 INVESTIGATIONS OF ALASKAN VOLCANOES

were taken from flows 2 and 11 (numbered from base) of the 15-flow 

sequence exposed in the northwest wall of Crater Creek gorge. The 

chemical analyses of these two flows are as closely alike, except for 

oxidation of the iron, as two replicate analyses of the same specimen 

(Michael Fleischer, written communication, 1948).

AGE OF PRECAIiDERA ROCKS

The precaldera rocks are probably mostly of Quaternary age, 

though some of the lowermost flows may be late Tertiary in age. 

None of the precaldera rocks show general hydrothermal alteration. 

This lack of alteration suggests a latest Tertiary and Quaternary age. 

Erosional features on these rocks are also youthful. Several moderately developed cirques are high on Mount Idak, the satellitic mountains, and the outer slopes of Okmok Volcano. Glacial striae were 

found as much as 2 miles downslope from some of these depressions. 

Of equal importance, much of the major slope surface is clearly only 

slightly modified from the original surface of the uppermost lava 

flow. Construction of the main volcanic structures was apparently 

keeping ahead of the destructive processes of erosion at the time of 

the latest glaciation, and some of the uppermost flows of the Crater 

Creek basalt probably were later than the last major glaciation, 

presumably at the end of the Pleistocene epoch. The precaldera 

rocks of Okmok Volcano are therefore considered to be mainly 

Pleistocene to early Recent hi age though some of the oldest flows 

are possibly latest Tertiary.

OKMOK VOLCANICS OF CALDERA AGE

The thick deposit of pyroclastic material that covers much of the 

outer slopes of Okmok Volcano is herein named the Okmok volcanics. 

It includes the Tanak volcanics and the Okmok ash of the preliminary 

report (Byers and others, 1947), a subdivision abandoned herein, 

because it was based mainly on degree of sorting, consolidation and 

other field characteristics that are gradational both vertically and 

laterally and cannot be mapped consistently throughout the area.

The Okmok volcanics reach their maximum thickness of about 

400 feet at the type locality on the north run of Okmok Caldera and 

thins outward to as little as 20 feet. It has smoothed irregularities 

of the pre-Okmok surface, filling valleys to depths of several hundred 

feet and covering thinly most bedrock spurs between valleys. Erosion 

has cut many V-shaped gullies in the formation, and has removed it 

entirely from some of the ridge areas. The amount removed by 

erosion appears negligible compared to the probable original
amount of material not less than 7 cubic miles. An average 

thickness of 300 feet around the caldera rim and of 100 feet at the 

coast (an average of 6 miles distant from the rim) was used in this 

approximation.

The Okmok volcanics consist of thick deposits of unsorted agglomerate in the lower part and beds of moderately sorted bombs and well 

sorted ash in the upper part. A few thin lava flows, seen in transverse 

lenticular section, are interbedded in the agglomerate near the caldera 

rim. Bedding and sorting are irregular in detail. Generally speaking, 

however, unsorted agglomerate predominates in the lower part at the 

caldera rim and persists to the seacoast, whereas agglomeratic beds 

of sorted ash predominate in the upper part at the rim and grade 

outward into thinner, well-sorted ash at the seacoast. A measured 

geologic section is given below.

Measured section at type locality of Okmok volcanics in north wall, Okmok Caldera 

(lat 6S°S8.S'N., long 168°08.1'E.)

Okmok volcanics: Feet

Ash, loosely consolidated, bedded; contains spatter, teardrop bombs, 

dark scoria fragments, and very minor lithic fragments; beds appear 

nearly black from a distance_________-_-_-___-___--___--____ 120

Ash, loosely consolidated; contains scoria and bombs with many - 

large angular blocks of basalt as much as several feet in diameter; 

beds appear medium light gray from a distance; gradational 

contact with unit below____________________________________ 160

Agglomerate, dense, medium-gray, andesitic; streaky appearance; 

contains common angular basalt fragments; forms upper welded 

bed; gradational contact with unit below___.__________________ 20

Agglomerate, medium-dark-gray, moderately consolidated; consists of 

spatter, bombs, and basaltic blocks as much as several feet in 

diameter, all in fine friable ash matrix; gradational contact with unit 

below__..-__________--._______--___________-____-_____-____ 40

Agglomerate, andesitic, welded, moderate- to dark-red; appears 

streaky; contains common angular basalt fragments; forms lower 

welded bed; gradational contact with unit below__________________ 30

Agglomerate, moderately consolidated, medium-dark-gray; contains 

spatter, bombs, and large blocks; upper 10 feet oxidized to moderate 

red; unit rests disconformably on Crater Creek basalt filling Vshaped gullies.______________________________________________ 40

TotaL_____________________._..__..__.__'. 410

Crater Creek basalt_______ __ _________ _ ___ ___ ___ _ 270+

AGGLOMERATE DEPOSITS

The deposits of unsorted agglomerate in the lower part of the Okmok 

volcanics are in very thick beds as much as 100 feet in thickness. 

Two agglomerate beds are so completely indurated, or welded, that
316 INVESTIGATIONS OF ALASKAN VOLCANOES

they were first mistaken for lava flows. No sorting of fragments by 

size was seen either vertically or horizontally. Around the caldera 

rim the agglomerate is locally more than 200 feet thick but is missing 

from radial ridges. Near Cape Chagak the agglomerate deposits 

are more than 200 feet thick (pi. 46 B), but just east of Ashishik Point 

they are only 20 feet thick. A short distance eastward at Cape Tanak 

they pass below sea level with an unknown thickness greater than 20 

feet. Fragments of precaldera rock of all types commonly make up 

about 5 percent of the agglomerate but in places as much as 20 percent, and the agglomerate grades into tuff-breccia. Most of the 

material is juvenile lapilli and ash of very fine grained, partly glassy, 

erratically vesicular andesitic scoria. Fresh surfaces of the juvenile 

matrix are neutral gray, mottled with both lighter and darker colored 

lithic fragments, but at distances greater than about 2 miles from the 

caldera run the fine-grained juvenile matrix is brownish gray, approaching but not duplicating the yellowish brown to bright yellowish 

orange of the palagonitized pyroclastic rocks of the Ashishik basalt. 

In places a rhyodacite ash bed is interbedded within the agglomerate.

WELDED AGGLOMERATE BEDS

Near the run of the caldera, the agglomerate contains two welded 

agglomerate beds that are nonvesicular and as completely indurated 

as dense lava flows. These two beds are exposed in nearly continuous outcrop for 4 miles along the upper part of the north and east 

walls of the caldera (see pi. 45); one continues along the upper part 

of the walls of Crater Creek gorge. The two welded agglomerate 

beds range from 5 to 30 feet in thickness; the variation is apparently 

related to undulations of the underlying surface. These beds grade 

vertically into partly indurated agglomerate; one grades laterally into 

brownish-gray partly indurated (palagonitized) agglomerate, 2 to 3 

miles from the edge of Okmok Caldera.

The constituent juvenile lava particles of the welded beds were 

apparently welded at the time of their emplacement. Fine, streaky 

banding parallel to bedding, common in outcrops of the completely 

welded agglomerate, apparently is due to flattening of juvenile bombs 

that now differ in crystallinity. Dark streaks are extremely finegrained and are charged with opaque oxides; lighter streaks are composed of pyroxene, plagioclase, and opaque oxide grams, whose average size is about 0.01 mm. Aside from these differences, nothing in 

the groundmass of the completely indurated rock suggests boundaries 

between masses of the juvenile material. The xenolithic fragments are 

enclosed by the welded juvenile material (pi. 47A). A few coarser 

grained fragments grade into the predominantly fine-grained surround-
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 317

ing groundmass and some: patches appear to be ghosts of xenolithic 

fragments, whose ragged outlines suggest that the fragments have been 

partly assimilated into the matrix material. Though the microscopic texture of the completely indurated layers could be that of a 

rapidly cooled lava flow charged with xenoliths, the texture was more 

likely a result of complete welding of the juvenile lava masses, for 

these indurated layers grade vertically and laterally into unsorted 

agglomerates clearly composed of discrete bombs, lapilli, and xenolithic fragments. I

Two welded agglomerate beds are also exposed in less extensive 

outcrops in the north and east arcuate ridges. The following geologic 

section on the south slope of the north arcuate ridge includes these 

two welded agglomerate beds. This section differs in detail from 

the'one at the type locality (page 315), in that the upper welded bed 

is basaltic, rather than andesitic in composition.

Measured section of Okmok Volcanics on south slope, north arcuate ridge 

(lat63°29.S'N., long 168°06.3'E.)

[Measured with altimeter and tape] Feet

Vent^agglomerate, oxidized, medium-red, scoriaceous; contains many 

fragments of mafic phenocryst basalt; weathers grayish red purple; lower 

20 ft strongly indurated. 

Okmok volcanics:

Agglomerate, pale- to medium-yellowish-brown; contains reddish scoria 

fragments; possible vent agglomerate__________________________ 60

Agglomerate, basaltic, welded, medium-gray; contains many fragments of aphyric basalt; gradational with agglomerates above and 

below._____________________________________________________ 10

Agglomerate, dark-yellowish-brown; contains large blocks of vitreous 

andesite as much as 10 ft thick and smaller angular fragments of 

basalt. Partly covered__________-_-_-_--____________________ 100

Agglomerate, andesitic, welded, medium-gray; stained with mediumred iron oxide; has streaky (eutaxitic) texture; gradational with 

beds above and below______________________________________ 15

Ash, pumiceous lapilli, bedded, yellowish-gray; gradational with ash 

below____________________________________________________ 3

Ash, rhyodacitic, grayish-yellow; sharp contact with agglomerate 

below____________________________________________________ . 5

Agglomerate, dark-yellowish-brown, partly covered..--.----------- 40

Total-..._____________________________ 228. 5

Vitreous andesite flow___________________.______________ 100+

The welded agglomerate beds were intensively studied both petrographically and chemically in the hope that they could be identified 

elsewhere, and thus be used as marker beds in structural studies of 

Okmok Volcano. Petrographic analyses (table 11) and silica analyses
318 INVESTIGATIONS OF ALASKAN VOLCANOES

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GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 319

(see below) suggest that the lower welded bed of the type locality is 

correlative with similar beds exposed several miles distant on the north 

slope.

Silica analyses and bulk specific gravity determinations on welded agglomerate beds

of the Okmok volcanics

[Analysts: Leonard Shaplro, 3-8; author 1-2. Bulk specific gravity determinations by author]

SiOt Bulk 

(weight specific percent) gravity 

Okmok Caldera (lat 53°28.3' N., long 168°08.1' E.):

1. Lower welded agglomerate bed of type section (p. 315)

at north wall___^__________________________ 54.35 _______

2. Upper welded agglomerate bed of type section (p. 315) __ 57. 40 __ 

North arcuate ridge (lat 53°29.2' N., long 168°06.5' E.):

3. Lower welded agglomerate bed of section (p. 317) at

south slope_________ _____________ 56.7 _._.___

4. Upper welded agglomerate bed of section (p. 317) at

same locality as 3__________________________ n.d. 2.86

Crater Creek gorge (lat 53°28.0' N., long 168°5.2' E.):

5. Dense aphyric lava of welded bed near top of southeast

wall.______________________-_________--__-__-_ 54.0 2.80

6. Dark finely scoriaceous lava of welded bed, 2}_ mi.

northeast of 5, in upper part of southeast wall._____ 54. 0 _______

Crater Creek (lat 53°32.5' N., long 167°59.2' E.), a few hundred feet south of road bridge:

7. Black, scoria fragment in palagonite agglomerate bed, 

possibly equivalent to lower welded bed of type section._________________________ 53.7 ____ 

East arcuate ridge (lat 53°27.0' N., long 168°2.8' E.) at north 

end:

8. Welded, dense aphyric agglomerate bed at top of section_____________________________________ 52.6 2.84

The composition of the phenocrysts (column 4, table 11) and the 

bulk specific gravity of the upper welded bed exposed in the north 

arcuate ridge section indicate a basaltic composition. The 2 welded 

beds of the section at the type locality and the 2 in the north arcuate 

ridge section are not correlative from their petrographic and chemical 

character, although the upper welded bed (column 2) at the type 

locality may possibly correlate with the lower welded bed (column 3) 

of the north arcuate ridge section. The welded basaltic beds (Nos. 

4 and 8) are limited to the upper parts of the two stratigraphic sections 

at the north and east arcuate ridges. The present work does not 

demonstrate whether the section at the north arcuate ridge is partly 

correlative with that at the type locality. Possibly, the pyroclastic 

beds exposed in the north and east arcuate ridges were deposited by 

an earlier caldera-forming eruption related to the origin of the arcuate 

ridges.
320 INVESTIGATIONS OF ALASKAN VOLCANOES

PARTLY PALAGONITIZED AGGLOMERATIC TUFF-BRECCIA

No beds completely welded over a large area are found along the 

seacoast; there they vary from loosely consolidated to moderately indurated. On the plain south of Cape Tanak, the brownish-gray agglomeratic tuff-breccia lies in huge remnant blocks over 2 square miles. 

These blocks are associated with old channel scars of Crater Creek 

and appear to have been undermined by removal of a loosely consolidated agglomerate bed from beneath a thick bed of moderately 

indurated tuff-breccia.

This tuff-breccia south of Cape Tanak contains as much as 20 

percent of xenolithic fragments, which include medium- to dark-gray 

basalt, dark-gray to black vitreous andesite or latite, and yellowishbrown palagonite tuff. Abundant masses of juvenile grayish-black 

scoria are also enclosed in the palagonitic matrix which is light olive 

gray to dark yellowish brown. The vitreous latite (see column 17, 

table 2) is more siliceous than any known precaldera vitreous andesite 

and possibly represents chilled, unvesiculated juvenile lava of the 

Okmok volcanics. Less than 1 percent of euhedral to subhedral 

crystals of plagioclase, monoclinic pyroxene, and olivine are scattered 

through the grayish-black scoria clots and the matrix. The scoria 

clots include cryptocrystalline, nearly opaque amorphous material 

and clear, light-yellow to light-brown glass (see columns 5 and 6, 

table 11). The groundmass or matrix material enclosing the scoria 

clots consists mostly of a partly opaque,, cellular cryptocrystalline 

aggregate that ranges from light to dark brown, less commonly 

greenish, in transmitted light. The refractive index of the aggregate 

is mostly slightly lower than that of balsam.

Though this aggregate was not chemically analyzed it closely 

resembles the groundmass of the palagonite tuffs of the Ashishik 

basalt both in outcrop and in thin section and is therefore presumed 

to be hydrated basaltic glass or palagonite.

EHYODACITE PUMICE BEDS

Light-yellowish-gray, unconsolidated pumice and pumiceous ash 

is associated in three places with agglomerate of the Okmok volcanics. 

On the south in-facing slope of the north arcuate ridge, 3}£ feet of 

gray ash rests in sharp contact on 40 feet of partly indurated agglomerate, and is overlain by a 15-foot layer of welded agglomerate 

(see section, p. 317). In a sea cliff 0.6 mile southeast of Ashishik 

Point, 1% feet of pumice lapilli ash rests on Ashishik basalt and is 

overlain by 20 feet of partly indurated agglomerate. On the east 

slope at an altitude of 1,000 feet, 2% feet of gray pumice ash in the 

north wall of Camp Creek gully rests on partly indurated agglomerate 

and is overlain by loosely consolidated agglomerate. A few fragments
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 321

of gray pumice were found in the partly palagonitized agglomerate 

at an altitude of 100 feet along Crater Creek near Cape Tanak. Petrographic analyses of two rhyodacitic pumice specimens are shown in 

columns 7 and 8, table 11.

BEDDED ASH

Massive, poorly sorted ash and bedded, sorted, unconsolidated 

ash in general forms the upper part of the Okmok volcanics. In 

the caldera walls, the thickness ranges from a few feet to more than 

300 feet and averages about 170. (See page 315.) On the outer 

slopes the ash becomes distinctly sorted and thins rapidly with increasing distance from the caldera; less than 5 feet is exposed in the 

cliffs of the Bering Sea coast.

The bedded deposits consist mostly of 1 to 10 foot beds of wellsorted ash, lapilli, and fragments of accessory materials. A consolidated bomb bed, from 5 to 20 feet thick, is exposed near the 

bottom of the ash deposits in gullies on the upper slopes of the 

volcano and in places in the walls of the caldera. Abundant juvenile 

bombs, 1 to 2 feet long, consist of dark fine-grained aphyric lava, and 

xenolithic fragments of precaldera basaltic rocks locally as much as 

3 feet in median diameter. Angular blocks, 1 to 3 feet across, litter 

the surface of the ash near the caldera wall and consist of fragments 

of Ashishik basalt, including palagonite tuff. With increasing 

distance from the caldera rim the xenolithic fragments become progressively smaller, and at the seacoast they average only about 

1 inch in diameter.

ORIGIN AND EMPLACEMENT OF OKMOK VOLCANICS

The Okmok volcanics are thickest around the rim of Okmok 

Caldera, yet no recognizable deposits assignable to them are found 

within the caldera. The thick blanket of Okmok volcanics must 

therefore be related to the formation of the caldera itself, as Williams 

(1941) has demonstrated for many calderas throughout the world. 

The unsorted agglomerate and welded agglomerate beds of the lower 

part of the Okmok volcanics certainly must have been emplaced as 

a catastrophic eruption. The lack of bedding and poor sorting of the 

agglomerate clearly indicate that little if any winnowing of material 

took place. Furthermore, the presence of welded agglomerate beds 

also suggests that heat loss due to air transport was at a minimum. 

The deposits are therefore believed to have been emplaced as a redhot emulsion of volcanic gases, molten juvenile particles, and minor 

lithic fragments the glowing avalanche or nuee ardente (glowing 

cloud) type of eruption. The absence of the unsorted agglomerate 

from most of the higher ridges radial to Okmok Caldera further 

supports this hypothesis of origin.
322 INVESTIGATIONS OF ALASKAN VOLCANOES

The partly palagonitized agglomeratic tuff-breccia is not found 

within 2 to 3 miles of the rim of Okmok Caldera but occurs on the 

lower slopes of Okmok Volcano, where these partly palagonitized 

rocks occupy the same stratigraphic position as the nonpalagonitized 

agglomerate closer to the caldera. The change, moreover, from nonpalagonitized agglomerate to palagonitized agglomerate is gradational 

through a distance of about a mile. Xenolithic fragments are also more 

abundant in the partly palagonitized agglomerate, which with increasing abundance of xenolithic fragments grades into tuff-breccia. 

These features of the partly palagonitized deposits suggest that as the 

glowing avalanche reached the lower slopes, farther from the source 

vents, considerable water from melting snow and ice became admixed 

with the primary constituents of the glowing avalanche so that when 

the mass finally came to rest the juvenile particles were hydrated to 

palagonite. The texture of the partly palagonitized tuff-breccia in 

many exposures superficially resembles glacial till or, more likely, a 

mudflow deposit. This general impression suggests that some of the 

partly palagonitized deposits may have been emplaced as hot mudflows.

The greater abundance of xenolithic fragments in the partly palagonitized deposits may be due in part to the glowing avalanche picking up boulders enroute and in part to solidification of molten juvenile 

fragments with cooling. Part or all of the vitreous andesite-latite 

fragments in the tuff-breccia may actually represent chilled, unvesiculated juvenile lava. This same material closer to the source vents 

may have cooled more slowly into the finely microlitic lava of the 

welded agglomerate beds.

The bedded ash and lapilli in the upper part of the Okmok volcanics 

were emplaced as ash falls from explosive or vulcanian eruptions, in 

part associated with the caldera-forming eviscerating eruptions but 

in part subsequent to them. Generally, the bedded deposits are the 

product of the caldera-forming eruptions but include in the upper 

part an indeterminate amount of ash from postcaldera eruptions.

The part of the Okmok volcanics in the north and east arcuate 

ridges may possibly have been deposited as the result of an eruption 

earlier than that which deposited the pyroclastic material around the 

present caldera (seep. 319), but such deposits from an earlier calderaforming eruption have not been recognized during the present study.

AGE OF THE OKMOK VOLCANICS

The Okmok volcanics postdate the last major glaciation of Okmok 

Volcano as neither they nor the wall of Okmok Caldera have been 

significantly glaciated. However, the carving of Crater Creek gorge 

500 feet deep into basalt flows is all later than the Okmok volcanics.
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 49

Photomicrograph of anorthite phenocryats in anorthite basalt flow (column 5, table 2). Crossed nicols.
GEOLOGICAL SURVEY BULLETIN 1028 PLATE 50 

PEAK . 

SATELLITE 

FIELD NO. 1

' X, CAPE IOAK

Perspective diagram oL ukmuk Volcano viewed from the northeast.
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALETJTIAN ISLANDS 323

Hence, the Okmok volcanics were probably emplaced shortly after 

the waning of the last main glacial epoch on Umnak Island and are 

therefore considered early Kecent in age.

BASALT FLOWS OUTSIDE OKMOK CALDERA

Many slightly dissected flows, plugs, and cones of basaltic composition are outside Okmok Caldera and hence cannot be dated with 

respect to its formation. Vent lavas, such as plugs and cinder cone 

deposits, are discussed, where appropriate, with the flows with which 

the vent lavas are associated, though they are grouped together on 

plate 41 to reduce the number of color patterns. The relations of 

these youthful volcanic features to the Okmok volcanics' could not 

always be ascertained, because ash from postcaldera eruptions has 

obscured .critical contacts close to the caldera. At least one cone and 

its associated flow on the caldera rim are known to be precaldera in 

age. Some flows appear to be interbedded in the Okmok volcanics 

and are probably contemporaneous; two are shown on plate 41. One 

is in Missouri Creek about 2 miles north of Satellite field no. 2, and 

the other is near the mouth of Vermont Creek about 2 miles south of 

Satellite field no. 2. The unconsolidated pyroclastic debris of most 

cinder cones on the upper slopes of Okmok Volcano appears to grade 

laterally into.bedded ash in the upper part of the Okmok volcanics, 

whereas some of the lava flows associated with the same cones pass 

under the bedded ash of the Okmok volcanics. Some of the pones 

and flows may predate the Okmok volcanics, but if the bedded ash in 

the upper part of the Okmok volcanics is actually postcaldera in age, 

some of these cones and flows may likewise be postcaldera in age.

MAFIC PHENOCRYST BASALT ON CALDERA EIM

Mafic phenocryst basalt dikes are exposed in the wall of the caldera 

southeast of cone C. The dikes connect with a cinder cone and small 

lava flow on the caldera rim. The rough flow surface is barren of ash, 

suggesting that the flow may be younger than most ash eruptions in 

the vicinity, yet the dikes, cinder cone, and the upper edge of the 

flow are all truncated by the wall of Okmok Caldera, thereby proving 

that they are precaldera in age. The basalt is porphyritic with 

phenocrysts of calcic plagioclase, augite, and olivine.

BASALT FLOWS ON THE ARCUATE RIDGES

A number of small plug-domes, cinder cones, and associated small 

lava flows with surfaces barren of ash form part of the crest of the 

north arcuate ridge and the east arcuate ridge. A line of similar 

extrusive rocks branches from about the middle of the east arcuate 

ridge and extends southeast down the outer slope of the volcano.

489292 59 5 
324 INVESTIGATIONS OF ALASKAN VOLCANOES

The rock in all these cinder cones and lava flows is dense, mediumgray, aphyric basalt. The volume is estimated at less than 0.1 cubic 

mile.

BASALT FLOW ON CAPE ASLK

A large cinder cone, which has been slightly dissected by erosion, 

and an associated lava flow cover about 4 square miles of the lower 

western slope at Cape Aslik. The lava flow, which is 100 feet thick 

in the present sea cliff, extended a 3-mile section of the coastline 

nearly a mile seaward. There is no ash cover on most of the flow. 

The rock is aphyric basalt and its volume is estimated to be about 0.1 

cubic mile.

BASALT FLOW NEAR CAFE IDAK

Two small cinder cones and one very small lava flow, with a surfacb 

barren of ash, overlie the ash deposit of the Okmok volcanics, which 

mantles most of the plateau between Mount Idak and Cape Idak. 

The rock is dark-gray dense basalt with a few small phenocrysts of 

plagioclase. These volcanic rocks are almost certainly of postcaldera 

age, for they rest on the Okmok volcanics.

ANORTHTIE BASALT FLOWS NEAR INANUDAK BAY

Five cinder cones are alined easterly from Cinder Point hi the head 

of Inanudak Bay. Anorthite basalt flows are associated with four of 

the cones. For convenience, these lavas are considered with those of 

Okmok Volcano even though these anorthite basalt flows near 

Inanudak Bay came from a separate alinement of vents closer to 

much earlier vents that extruded the volcanic rocks of central Umnak. 

The basalt flow at Cinder Point forms the shoreline including Cinder 

Point, and covers an area of about 0.6 square mile. Two miles inland, 

a narrow flow extends \% miles northwest from the second cone. 

Lava from the third and fourth cones to the east floweql south and 

coalesced to form a lava field half a mile wide and over a. mile long. 

No lava flow was extruded at the fifth and eastermost cone. No ash 

covers the surface of the flows, and the cones and flows have not been 

modified by erosion, although the basal contact at the lower end of 

the eastermost flow is covered by alluvium. The rock in the cones 

and flows is porphyritic anorthite basalt with common to abundant 

phenocrysts of translucent sodic anorthite with a maximum length of 

6 mm. The total volume of anorthite basalt is estimated at 0.04 

cubic mile.

ROCKS OF POSTCALDERA AGE WITHIN OKMOK CAIJMERA

All the rocks within the caldera are obviously younger than the 

caldera. The most recent deposits are subaerial, and therefore 

younger than the shrinkage of a lake which temporarily filled the
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 325

caldera to a depth of 500 feet above the present low point of the floor. 

Other deposits, contemporaneous with or older than the lake, show evidence of subaqueous eruption and are overlapped by lake beds or show 

erosional features associated with the changes of level of the lake. 

Major relations between some of these early deposits are shown in the 

diagrammatic profile (pi. 48 in pocket) taken approximately north 

through cones C and D in the southeastern part of the caldera. The 

oldest material in the profile is a chaotic assemblage of pyroclastic 

material that shows no systematic structure; elsewhere in the caldera 

the oldest deposits are relatively deeply eroded pyroclastic cones. 

Prominent benches ringing cones C and D are underlain by subaerial 

flows, which are between 1,350 and 1,400 feet in altitude. Below 1,350 

feet they grade abruptly downward into brecciated pillow lavas, which 

were emplaced under water. The level of change at an altitude of 

1,350 feet is consistent around both cones and indicates the altitude of 

the lake surface at the tune of extrusion of the lava flows of cones C 

and D. The lake level rose subsequently to its highest altitude at 

1,560 feet, which is marked by wave-cut scars on cones C and D and 

by the top of sediments deposited at or near the shore of the lake. 

This altitude is also the top of hard Crater Creek basalt at the low 

point of the caldera rim, into which the gorge of Crater Creek has been 

entrenched. Temporary halts in the lowering of the lake level, as the 

outlet stream deepened its gorge, are indicated by wave-cut scars 

nicked in cone D at altitudes of 1,140, 1,120, and 1,100 feet and by a 

prominent graded surface at 1,120 feet on lake sediments just north of 

cone D. A small, shallow remnant of the lake remains north of cone 

D at an altitude of about 1,075 feet.

ROCKS CONTEMPORANEOUS WITH CAIDERA LAKE 

EARLY POSTCALDEHA PYKOCLASTIC ROCKS

This unit includes bedded deposits of cones G and H in the northwest 

floor of Okmok Caldera (pi. 41), the material that comprises an unnamed cone older than and due east of cone C in the southeastern part 

of the caldera floor, and deposits of pyroclastic material whose source 

vents could not be determined. The pyroclastic debris not identified 

with recognized source vents may be part of a breccia that presumably 

formed the original floor of Okmok Caldera when it first subsided. 

The cones have very wide low craters with narrow serrate rims. The 

outer slopes consist of many radial ridges, fluted by gullies, and have 

the general appearance of badlands. The pyroclastic material of the 

cones is well sorted, fine-bedded unconsolidated ash and lapilli, randomly interbedded with layers of consolidated palagonitized tuff. 

Many xenolithic fragments ranging from a small fraction of an inch to
326 INVESTIGATIONS OF ALASKAN VOLCANOES v;

i

blocks as much as 10 feet in diameter are randomly strewn through the 

sediments; most of them are of aphyric basalt but a few are of palagonitized tuff, similar to palagonitized tuffs of the Ashishik basalt. 

The juvenile material is fragmented cryptocrystalline scoria, shards 

of vesicular glass and palagonite, and broken crystals of plagioclase « 

and pyroxene. Some shards consist of a core of clear, unaltered glass 

surrounded by a green to yellow to dark-brown palagonitic aggregate 

of microcrystalline altered material; other shards are entirely the 

palagonitic aggregate.

The eroded, wide-mouthed cones, containing as they do, bedded, 

well-sorted pyro clastic debris with a large amount of accessory xeno- *. 

lithic fragments, resemble Diamond Head on Oahu, Hawaiian Islands 

(Wentworth, 1926; Stearns, 1935), and are believed to be the products v 

of phreatomagmatic eruptions caused when rising hot magma entered 

the water-saturated breccia beneath the caldera lake.

PLAGIOCLASE BASALT FLOWS OF CONES C AND D

Cones C and D are similar to each other but differ in form and rock 

type from all other cones in the caldera. Each consists of a cinder ^ 

cone surmounting a terrace. The terrace is underlain by a lava flow 

that has structures and textures normal hi subaerial flows in the upper 

part and pillow structures embedded in brecciated glass in the lower ; 

part (pi. 48). The physiographic form consisting of the cone, subaerial 

lava bench, and underlying pillow lava has been called a "tuya" by 

Mathews (1947) after Mount Tuya, a similar feature in northern * 

British Columbia. Cone D is symmetrical with a single crater at the t 

summit, exposing a vent plug of coarse crystalline basalt. Cone C is 

less symmetrical and near the summit has remnants of 5 different 

vents, one of which contains a plug of coarse-grained basalt. A line 

of active fumaroles crosses the lip of an asymmetric crater on the 

south flank, a hundred feet below the summit (Byers and Brannock, r 

.1949, p. 724). Bedded palagonite tuff is exposed locally at one of the , 

subsidiary vents. Most of the pyroclastic material of both cones C 

and D is dark ash and scoria, locally oxidized to brick red. The rock 

is porphyritic basalt, containing up to 25 percent of bytownite plagio- . 

clase phenocrysts, and about 1 percent each of olivine and monoclinic 

pyroxene phenocrysts, all less than 2 mm hi greatest dimension. The 

abundance of phenocrysts is about the same, whether in the chilled 

margin of the pillows with glass groundmass, in the subaerial flows 

with aphanitic but crystalline groundmass, or in the plugs with ganular, gabbroid groundmass. The volume of material extruded from 

cones C and D is estimated at about 0.1 cubic mile.
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 327'

BEDDED VOLCANIC SEDIMENTS

Bedded volcanic sediments are composed mainly of water-laid 

volcanic ash and silt and subordinate older dissected alluvium, 

graded to the surface of the caldera lake. They are scattered in 

small areas over all except the southernmost part of the caldera floor. 

The beds range in altitude from nearly 1,900 feet along the wall of 

the caldera to about 1,140 feet in the northeastern part, where Crater 

Creek leaves the caldera (pi. 41). The greatest exposed thickness is 

about 300 feet with the base not exposed. The older dissected alluvium 

stands highest west of cones G and H and is largely younger than these. 

The true lakebeds are of greatest extent in the center and southeastern 

part of the caldera floor below 1,560 feet, the altitude of a lakeshore 

scar on the flanks of cones C and D and the topmost horizontal beds 

in the lake terrace east of cone D. The lakebeds near the head of 

Crater Creek gorge are below about 1,120 feet, the altitude of a lakeshore scar in the pillow lavas of cone D.

The bedded volcanic sediments contain well-bedded silt, sand, fine 

gravel and lapilli; the silt beds are as thin as a fraction of an inch and 

the coarser material is hi beds as much as several feet in thickness. 

Light-gray to light-yellowish-gray silt beds form as much as 60 percent 

of the lakebed section in the northeastern part of the caldera, but 

coarser materials form most of the deposits, near cones G and H, 

which may have contributed Coarse debris to thjem. Blocks as much 

as several feet across constitute nearly 10 percent of the deposits near 

the caldera walls, but are less abundant away from the walls.

BASALTIC FLOWS LATER TEAK CALDERA LAKE 

BASALT FLOWS PBOM CONE F

Cone F, a symmetrical cinder cone, and its associated lava flow of 

aphyric basalt are southwest of cone C (pi. 41), almost against the 

south wall of the caldera. The lava flowed northward from the cinder 

cone, apparently following a shallow valley cut in lake beds and material from cone C after the lake shore had withdrawn to a lower level. 

A modern glacier encroaches on the south flank of the cinder cone, and 

the lower end of the lava flow has been covered by more recent 

alluvium.

BASALT FLOWS FROM CONE E

Cone E is a composite cone in the west-central part of the caldera 

floor. One of its youngest lava flows reaches over 2 miles eastward 

beyond the center of the caldera floor, beneath flows of cone A. The 

cone contains a pit crater, 3,000 feet in diameter at the rim. The 

lowest part is concealed by a crater lake 500 feet below the rim. Inter-
328 INVESTIGATIONS OF ALASKAN VOLCANOES

bedded pyroclastic rocks and lava flows are exposed in the crater walls 

down to lake level. Part of the lower outcrops consist of yellowish 

indurated palagonite tuffs, which may be older or contemporaneous 

with the caldera lake. The uppermost flows and pyroclastic material 

from cone E lie on a surface cut on lakebeds almost continuous with 

the present graded surface of the adjacent caldera floor. All the flows 

and most of the pyroclastic material from cone E are therefore considered younger than the lake of Okmok Caldera, although some 

pyroclastic material may be older. Pyroclastic material and the 

basalt flows are sparsely porphyritic, and their volume may be as 

much as 0.2 cubic mile.

WALL-FISSURE FLOWS

In the north wall of Okmok Caldera, just west of cone B, several 

steep-sided agglutinate cones are alined on a fissure nearly parallel 

to the caldera-wall scarp. Lava flows from two of the vents plaster 

the wall from just below the rim to the floor and more than a fifth 

of a square mile of floor. Small, thin flows from the other vents 

between altitudes of 1,400 and 1,950 feet cover part of the wall but 

do not reach the floor. The lava flow on the caldera floor has no 

alluvial cover but is partly covered by basaltic andesite flows from 

cone B. The rock is aphyric and very similar to the basaltic andesite 

from cone B. Comparable flow and vent material abuts against the 

southwest wall of the caldera west of cone A. The modern glacier 

partly covers these lava flows, and their extent on the caldera floor 

is obscured by erupted material from cone A.

BASALTIC ANDESITE FLOWS FROM CONE B

Two flows of aphyric basaltic andesite and the associated cinder 

cone form approximately 1% square miles of the surface of the northeastern floor of the caldera. The older lava flow, thinly mantled with 

red-stained ash from the cinder cone, extends no more than threequarters of a mile from the cone. The younger lava is devoid of ash 

cover and is made up of several parts: a few small flows lie on the 

ash-covered lava near the cone, but most of the younger lava extends 

southward beyond the southern margin of the ash-covered flow, 

apparently having moved laterally beneath the old lava and reached 

the surface at vents along its southern edge. The surface of both 

flows is rugged and broken; blocks of jagged lava are as large as a 

small house. Near the cone, blocks of tuff and agglutinate as much 

as 80 feet across have been rafted out on the surface of the flow. The 

volume of material at cone B is approximately 0.015 cubic mile.

The two lava flows appear similar, and their textures vary no more 

than between different specimens from the same flow. Both may be
products of one eruption, which might be one of those reported from 

northeast Umnak in the early nineteenth century (Grewingk, 1850, 

p. 128-133).

BASALT FLOWS FROM CONE A

Cone -A is a large compound cinder cone near the southwest wall 

of the caldera. Fumaroles were still emitting sulfur dioxide and other 

gases in 1948 from summit crater (Byers and Brannock, 1949, p. 722). 

At least five different lava flows extend from the cone and cover 2% 

square miles of the caldera floor. The oldest, a sparsely porphyritic 

basalt, extends north to cone E (pi. 41). Its surface is mantled with 

ash in contrast to the younger flows. Two ash-free aa flows extend 

from different flanks of the compound cinder cone and merge to form 

the lava field covering \% square miles of the floor to the northeast. 

A lava flow of June to August 1945, issued from the west flank of the 

cone, then moved south, east, and northeast along the southeastern 

margin of the large barren lava field. In December 1945, a second 

flow issued from the north flank of the cone and spread in a ramified 

pattern toward the north, covering parts of the older lava field in most 

of its course. Both flows of 1945 and the pre-1945 flows of the lava 

field northeast of cone A are of vesicular porphyritic basalt. The 

surface of all four flows is jagged aa, of the same color and "fresh" 

appearance; if the two flows of 1945 had not been observed during 

emplacement, it would have been impossible to distinguish their 

surfaces from those of the older aa flows. All activity at cone A may 

have occured since the Russians first sighted the island in 1741. The 

volume of lava erupted in 1945 is estimated at 0.005 cubic mile; that 

of all cone A material, at 0.02 cubic mile.

PETROGRAPHY OF OKMOK VOLCANO

The rocks of Okmok Volcano are predominantly f eldspathic basalts. 

Common phenocrysts are plagioclase, diopsidic augite and olivine, in 

varying proportions; hypersthene is extremely rare in contrast to 

the volcanic rocks of Mount Reschesnoi and Mount Vsevidof on 

southwestern Umnak. The lava flows of Okmok Volcano range in 

texture from aphyric to porphyritic with phenocrysts constituting 

over a third of the volume in many. In general, porphyritic rocks 

are more abundant among the older lavas, aphyric rocks among the 

younger lavas, though both textural types are found throughout. 

Porphyritic basalts containing magnesian olivine and diopsidic augite 

are most abundant among the older lavas, whereas lavas ranging in 

composition from andesite to rhyolite were erupted only during a 

late stage of construction of Okmok Volcano.
330 INVESTIGATIONS OF ALASKAN VOLCANOES

OUVINE-BICH BASALT

 Olivine-rich basalt is represented by 9 out of the 200 rock masses 

sampled and studied in detail, in the lower part of the Ashishik 

basalt near Cape Idak and the ridge west of New Jersey Creek. As 

much as 25 percent of the rock is magnesian olivine (Fo75_9 o) in euhedral 

to subhedral crystals (maximum size, 5 mm) in an intergranular 

groundmass of tabular plagioclase (An75), euhedral to subhedral 

monoclinic pyroxene and olivine, and opaque oxides. Chemical and 

petrographic analyses of olivine-rich basalt are given in columns 1 

and 1A, tables 2 and 3.

PLAGIOCLASE-OLIVINE BASALT

Porphyritic basalts containing phenocrysts of both plagioclase 

and olivine are much more abundant (48 sampjes) than those with 

only olivine. They are particularly common among the older lava 

flows of the plateau northeast of Mount Idak and of Mount Idak 

itself, and make up the bulk of the satellitic vents on the outer slopes 

of Okmok Volcano. A few are found in the main structure of Okmok 

Volcano. Euhedral to subhedral plagioclase crystals have the average 

composition of bytownite (labradorite in the analyzed sample from 

Mount Tulik), but the cores of most are anorthite. Magnesian 

olivine is in euhedral to subhedral phenocrysts, but some are embayed 

and have reaction rims of clinopyroxene. Augite is present as 

microphenocrysts as well as in euhedral to subhedral grains in the 

intergranular groundmass of tabular labradorite, clinopyroxene, 

olivine and opaque oxides. A chemical and petrographic analysis 

of a specimen from Mount Tulik is given in column 2, tables 2 and 

3, respectively. The percentages of the groundmass constituents 

are clinopyroxene, 24 percent; tabular labradorite, 26 percent; opaque 

oxides, 5 percent; and a mesostasis of alkali feldspar and brown glass, 

3 percent. Small grains of olivine are rare in the groundmass.

PLAGIOCLASE-AUGITE-OLIVOTE BASALT

Porphyritic mafic phenocryst basalt with conspicuous phenocrysts 

of both olivine and augite as well as plagioclase are most abundant 

(20 samples) among the mafic phenocryst basalt flows of the Ashishik 

basalt. The plagioclase basalt flows of cones C and D within the 

caldera are described here, though they are on the borderline between 

mafic phenocryst basalt and feldspathic basalt.

The upper 2,0 feet of a mafic phenocryst basalt flow on the southeast slope of Okmok Volcano is exposed 3 miles northwest of the 

hangar at Cape Air Force Base. Augite phenocrysts are much 

larger and more abundant in the lava 20 feet below the top of the 

flow than at the top. In the specimen collected near the top of the 

flow (columns 3 and 3a, table 12) diopsidic augite phenocrysts range
TABLE 12. Petrographic analyses of mafic phenocryst b

[All phenocrysts or crystals referred to 'are greater than 0.

Mode In volume percent: 

Phenocrysts or crystals:

Olivlne ____________________________

1

1

0.0

.0

24.2

.05

75.7

2 (55°)

FOBS

la

1

0.0

.0

20.2

.05

79.7

2

4

19.5

12.7

7.9

.0

59.9

1.695

54^°

FOM

2a

4

23.5

12.2

4.6

.0

59.7

i The smallest crystal is about 0.1 mm. 

»Data on groundmass augite.

1. Olivine basalt flow at Cape Idak (lat 53°31.4' N., long 167°47.8' E.).

la. Olivine basalt flow; same specimen as above but section cut perpendicular to No. 1.

2. Anorthite-augite-olivine basalt flow (lat 53°25.7' N., long 167°55.5' E.), 3 miles N. 

25° W. of Umnak Airfield. Specimen collected at nonvesicular base of 20-foot outcrop. 

2a. Anorthite-augite-olivine basalt flow; same specimen but different thin section cut 

perpendicular to No. 2. -

3. Anorthite-augite-olivine basalt flow, same locality as 2. Specimen collected 20 feet 

above No. 2 at vesicular top of flow. Note lower content of augite phenocrysts.

3a. Anorthite-augit

perpendicular 

4. Augite basalt f

long 168°05.5' 

5. Oh'vine-bearing 

yards north of 

6. Bytownite-augi

Oat 53°29.1' N

7. Olivine-bearing 

Umnak Pass 
332 ' INVESTIGATIONS OF ALASKAN VOLCANOES :

in size from 1 to 3 mm and make up about 4 percent of the rock. A 

specimen collected 20 feet below the surface of the flow (columns 2 

and 2a, table 12) contains 12 percent of diopsidic augite phenocrysts 

as much as 10 mm in maximum dimension. The greater concentration of large diopsidic augite crystals.below the top of the flow suggests 

gravity settling of diopsidic augite in this particular flow. Anorthite 

phenocrysts (An9 i) are in normally zoned, euhedral to subhedral 

crystals as much as 2 mm long; magnesian olivine phenocrysts (Fo 8 4) 

are in euhedral to subhedral crystals as much as 3 mm in diameter. 

The groundmass is intergranular (average grain size about 0.03), 

holocrystalline, and composed of tabular plagioclase, monoclinic 

pyroxene, and opaque oxides.

Augite basalt, exposed at the base of the east wall of the caldera, 

has some porphyritic facies grading into finely granular facies. The 

granular facies (column 4, table 12) contains strongly zoned, euhedral 

to subhedral plagioclase crystals which range in size from about 5 mm 

to 0.1 mm and in composition from An93 to An34 . The crystals of 

magnesian olivine are euhedral to anhedral and rimmed with pyroxene 

except on crystal faces protected by older crystals from reaction with 

the groundmass liquid. The augite crystals also range in size from 

about 3 mm to 0.1 mm. Some brown glass has a refractive index of 

about 1.50 about the same as that of the rhyolite obsidian.

A suite of samples from cone D and cone C shows textural gradation 

from the chilled vitrophyric margin of a lava pillow to the holocrystalline facies in the plug in the summit crater of cone C. The chilled 

vitrophyre contains almost 19 percent of plagiocase, 0.5 percent of 

augite, and 1.2 percent of olivine larger than 0.2 mm. Many smaller 

crystals of the same minerals range down to microlites in a groundmass 

of glass (refractive index, 1.592), sparsely dusted with opaque oxide. 

All crystals are euhedral to subhedral, including the olivine. In the 

holocrystalline facies, the mafic phenocrysts are slightly more abundant, and the olivine crystals are slightly embayed and surrounded by 

pyroxene grains. A chemical analysis of a specimen from cone D 

is given in column 7, table 2.

ANORTHITE BASALT

The anorthite basalt flows near Inanudak Bay and the 1945 

anorthite basalt flows from cone A are extremely rich in anorthite 

phenocrysts. These are set in a dark groundmass; that is slightly 

more mafic than most feldspathic basalts from Okmok Volcano. 

Inconspicuous mafic microphenocrysts comprise less than 2 percent in 

nearly all of the 10 specimens examined. Despite the slightly more 

mafic groundmasses and the sparse microphenocrysts, some of the
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 333

anorthite basalt flows nonetheless approach anorthositic gabbros in 

composition.

The high anorthite content, abundance, and size of the plagioclase 

phenocrysts in the lava flows near Inanudak Bay is unusual, even in 

basalt. The specimen from Cinder Point (column 5, tables 2 and 3) 

contains 30 percent of anorthite phenocrysts in a microlitic groundmass 

of labradorite, clinopyroxene, and opaque oxides, averaging about 

0.02 mm across. Most of the anorthite phenocrysts are approximately 

equidimensional euhedral crystals as much as 6 mm in greatest dimension, but some are subhedral and somewhat embayed by the groundmass. A few crystals having a pink tint were observed; All are. 

coated with a thin shell of calcic labradorite (about Anes). The large 

core of each is not conspicuously zoned, commonly shows Carlsbad, 

albite, and pericline twinning (pi. 49) and has a composition averaging 

Anfl3 (see fig. 51 and table 13). Determination of the alkalis suggests

FIGURE 51. Stereographic projection of bisectrices and optic normals of anorthite phenocrysts (nos. I to 

VI) in anorthite basalt projected on plane perpendicular to axis a. Open circles and curves after Duparc 

and Keinhard in Winchell (1933, p. 331).
334 INVESTIGATIONS OF ALASKAN VOLCANOES

TABLE 13. Optical data on anorthite phenocrysts in anorthite basalt near Inanudak

Bay

No. on fig. 51

I......... -- ._ .. 

II .. - . in....... 

IV..... __ ... _____ 

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

VI......_ --...- --- -- 

2V >

(2) 79°

80°

79°

(2) 78°

Pericline twin 

plane and 001

-10°

W

(2) -10°

(2)

(«

1 a is acute bisectrix. 

3 Unable to determine.

a composition of Anei . The same rock has some phenocrysts and 

microphenocrysts of olivine and a few of augite. The olivine crystals 

are all surrounded by a reaction rim of clinopyroxene, and those of 

augite by a reaction run of opaque oxide and cryptocrystalline material.

A sample from the similar flow, 2 miles east of Cinder Point contains 

less than 5 percent of anorthite phenocrysts similar in composition to 

those at Cinder Point, a few microphenocrysts ranging up to a composition of An85 , and microlites of labradorite in a groundmass of glass 

with an index of refraction of 1.545. A few phenocrysts of olivme 

and augite have reaction rims comparable to those of the flow at 

Cinder Point.

The 1945 lava from cone A contains nearly 30 percent of phenocrysts 

of anorthite, 0.5 to 2 mm in length, and about 1 percent each of 

inconspicuous augite and olivine microphenocrysts (column 6, tables 2 

and 3). The plagioclase crystals are slightly zoned from An93 to An89 . 

Olivine and augite are in euhedral to subhedral crystals; no reaction 

rim surrounds the olivine. The groundmass is microlitic, ranging 

from about 0.01 to 0.02 mm in average grain size, and contains 

plagioclase, clinopyroxene, and opaque oxides.

APHYRIC BASALT AND FELDSPATHIC BASALT

Four-fifths of the lava flows of Okmok Volcano consist of aphyric 

basalt and feldspathic basalt. The aphyric or nearly aphyric basalts 

consist almost entirely of a microlitic intergrowth of labradorite, 

clinopyroxene (pigeonitic?), and opaque oxides (see column 11, table 3), 

but all gradations may be seen between aphyric and f eldspathic basalts 

by increase of plagioclase phenocrysts. Most of both varieties have 

rare microphenocrysts of augite. A few relict microphenocrysts of 

olivine can usually be found. The groundmass in similar, but more 

coarsely crystalline rocks, is intergranular, and the plagioclase crystals 

grade in size from the small phenocrysts to those in the groundmass. 

The olivine is commonly embayed and surrounded by a reaction rim 

of clinopyroxene. One typical microgranular f eldspathic basalt contains about 17 percent of normally zoned phenocrysts of bytownite,
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 335

a rare microphenocryst of augite, and a few embayed crystals of 

olivine surrounded by clinopyroxene reaction rims. The intergranular groundmass is composed of 38 percent laboradorite, 35 

percent monoclmic pyroxene, 6 percent opaque oxides, and 4 percent 

interstitial light-brown glass (index of refraction, 1.498-1.503) and 

alkali feldspar.

ANDESITE AND IATITE

Rocks of andesitic to latitic composition form a few. dikes, plugs, 

and stubby lava flows of the volcanic rocks of minor vents near the 

walls of the caldera, and occur as fragments in the pyroclastic rocks of 

Okmok volcanics. They are dark-gray to black, dense to glassy, 

nonvesicular rocks, commonly aphyric, and many contain small 

xenoliths of basaltic rocks. Most specimens have hyalopilitic groundmasses with varying amounts of glass. Microphenocrysts of plagioclase (An37 to An66 ) are common to rare, and some rocks contain 

amhedral and embayed fragments of calcic bytownite (An80 to An88 ). 

Anhedral fragments of augite, also embayed and commonly bent, are 

less abundant than those of calcic bytownite. Grains of olivine from 

several specimens have indices of refraction corresponding to the high 

fayalite olivines, hyalosiderite to ferrohortonolite (Fo60 to Fo23) and 

contain no reaction rims of clinopyroxene. The index of refraction of 

the groundmass glass ranges from 1.545 in vitreous andesite to 1.511 

in vitreous latite. Petrographic analyses of 4 specimens are given 

in table 14.

TABLE 14. Petrographic analyses of vitreous andesites and latite of Okmok volcano 

[Phenocrysts are all greater than 0.2 millimeters]

Mode in volume percent: 

Phenocrysts:

Refractive index of groundmass glass _ . ________ ....

Bulk specific gravity of rock __________________

I

15

64.80

0.2

.1

<.01

.1

99.6

to

2.86

2

0.8

.1

<.01

.0

99.1

1.545

2.72

3

160.2

1.1

.2

< !

.2

 98.5

1.530

2.67

4

17

63.6

4.7

.3

< !

.2

94.7

1.511

2.60

1 SiOa determination by Leonard Shapiro, using rapid methods.

2 Specimen contains a very fine microlitic groundmass charged with opaque oxides and hence the glass, 

If present, could not be determined.

1. Top of andesite pipe at intersection of Crater Creek gorge and Okmok Caldera 

(lut 53°28' N., long 168°05.4' E.)

2. Andesite flow exposed at 2,400 feet on south side, north arcuate ridge (lat 

53°29.2' N., long 168°06.6' E.)

3. Top of small andesite plug or large block at 2,500 feet on north arcuate ridge 

(lat 53°29.3' N., long 168°06' E.)

4. Hand specimen of latite fragment in Okmok volcanics. Crater Creek canyon 

near Cape Tanak (lat 53°32.6' N., long 167°59.2' E.)
336

BHYOIFTE

One apparently elongated dome of rhyolite was found on the outer 

northwest slope of Okmok Volcano. The rock ranges from light-gray 

felsitic vesicular rhyolite to nonvesicular black obsidian. Xenoliths 

of basaltic andesite are sparse to common in the obsidian but lacking 

in the felsite. The xenoliths apparently were being resorbed (pi. 47.Z?) 

and fragments of laboradorite, augite and olivine in the rhyolite must 

be xenocrysts. Feldspar microlites in the glass have a composition of 

An25 , and the glass has an index of refraction of 1.497. Petrographic 

analyses are given in table 15.

TABLE 15. Petrographic analyses of rhyolite specimens and xenolithic inclusion,

Okmok Volcano

Mode in volume percent: 

Phenocrysts >0.2 mm: 

Plagioclase __________________

Olivine __________ . ________ ..

Eange in composition of plagioclase phenycrysts

Approximate composition, feldspar microlites in

groundmass.

Range in composition of olivine phenocrysts ____

1

19

0.5

.5

.1

98.9

1.712

1,497

2.40

2

0.2

.4

.1

99.3

Anas

1.497

2.41

3

9.4

.2

1.8

88.6

F074-83

4

4.0

.6

.1

95.3

Foes-?:

1.497

2.47

5

20

0.0

.0

.0

100.0

2 do

> Only one grain sampled.

1. Most vitreous obsidian band interlayered with less vitreous band and felsitic 

rhyolite near northwest end of northwestward extension of dome (lat 53 °29.5' 

N., long 168°10.3' E.).

2. Most vitreous obsidian band interlayered with less vitreous band and felsitic 

rhyolite on northeast side of northwestward extension of dome (lat 53°29.2' 

N., long 168°09.8' E.).

3. Purplish basaltic andesite xenoliths in obsidian, same locality as 2.

4. Less vitreous obsidian band interlayered with more vitreous obsidian and 

felsitic rhyolite, same locality as 2.

5. Light-gray felsitic rhyolite, east end of dome on north side of north arcuate 

ridge (lat 53°29.4' N., long 168°09' E.).

PAIAQONITIZED PYROCLASTIC ROCKS

Palagonitized pyroclastic rocks are most abundant and most completely altered in the agglomerate and tuff of the Ashishik basalt exposed in the walls on the outer slopes of the caldera. Partly palagonitized rocks are in the agglomerate of the Okmok volcanics more than 

2 miles from the caldera, and in some of the pyrcolastic cones within it. 

The palagonitized rocks of the Ashishik basalt contain fragments of 

vari-colored scoria, gray to black xenolithic fragments, and broken 

crystals of plagioclase in a yellowish-gray, earthy matrix. Fragments 

of light-brown scoria contain cores of unaltered clear light-yellow 

glass rimmed by palagonitized glass. In different specimens, the 

index of refraction of the unaltered glass ranges from 1.560 to 1.575.
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN:. ISLANDS 337

Fragments of yellow to orange-brown scoria consist of isotropic 

palagonite glass that ranges in refractive index from 1.54 to 1.57. 

The scoria fragments are slightly to moderately cellular with vesicles 

0.05 to 0.1 mm in diameter. The xenoliths probably are broken wallrock fragments of Okmok Volcano. Fragments of plagioclase, augite 

and olivine crystals may be in part xenocrystic as their abundance 

seems to vary directly with the abundance of xenolithic fragments. 

The matrix is a cryptocrystalline aggregate, apparently of palagonir 

tized glass shards, as shard shapes are discernible in parts of the mar 

terial. The cryptocrystalline palagonite is green, yellowish, to dark 

brown, as seen in plane-polarized light in thin section, and is made up 

of birefringent microlites in both felted and radial-fibrous texture.

Many vesicle fillings are compound and consist of a lining of 

concentric, moderately birefringent fibers, and a central filling of 

nonfibrous, low birefringent material with refractive indices 1.480 

and 1.484 (chabazite ?). Some vesicles contain a yellowish to reddishbrown mineral in crudely spherulitic aggregates of curved concentric 

fibers with positive elongation, birefringence about 0.01, and index of 

refraction about 1.56 to 1.57. Another cavity-filling zeolite, in 

sheaflike aggregates of radiating fibers, has positive elongation, 

parallel extinction, and indices 1.467 and 1.470.

Chemical analyses of two specimens of palagonitized rock are given 

in columns 3 and 8 in table 2.

The major structural features of Okmok Volcano the caldera ring 

faults and the probability of two calderas are mostly concealed by 

the Okmok volcanics and other pyroclastic debris. Therefore, their 

existence is in large part inferred from physiographic features and 

yent alinements, and by comparison with similar calderas elsewhere in 

the world. First, however, the known or observable structural 

features of Okmok Volcano are considered, followed by a discussion 

of the origin of the arcuate ridges.

Okmok Volcano is mainly composed of gently outward-dipping 

flows and interbedded pyroclastic rocks of the precaldera Ashishik 

basalt (see sections AAr and BBr , pi. 41). Dips greater than 5° are 

restricted to local vent masses of relatively small volume. The 

welded agglomerate beds of the east-central part of the north arcuate 

ridge actually dip gently southward toward Okmok Caldera. The 

low plateau between Mount Idak and Cape Idak is underlain by 

Ashishik basalt and minor pyroclastic interbeds that dip 2° to 3° 

northeastward, except in the eroded cone of Mount Idak. Inasmuch 

as the average outward dip of the layered volcanic rocks is less than 

the topographic slope, progressively younger flows are found as one
338 INVESTIGATIONS OF ALASKAN VOLCANOES

ascends Okmok Volcano. Most of the dips observed hi the Okmok 

volcanics are essentially parallel to the present topographic slope.

Okmok Caldera is in large part a feature of collapse and its infacing 

cliff walls are the slightly modified scarps of a ring fault. One fault, 

associated with the ring system, has been mapped. In the northeast 

wall of Okmok Caldera south of Crater Creek gorge (pi. 41), a fault 

that dips steeply southwest toward the caldera is exposed for about a

 mile. The southwestern block has subsided about 100 feet as shown

-by the displacement of a bed of welded agglomerate in the Okmok 

volcanics.

The bedded volcanic sediments in the caldera have been displaced 

in a few places by postcaldera movement, probably involving the 

caldera ring fault. Along the northwest wall, north of cone H 

(pi. 41), the bedded volcanic sediments dip as much as 30° toward the 

center of the caldera but decrease in dip at greater distance from the 

wall. Near the caldera wall south of the entrance to Crater Creek 

gorge, the lake beds dip away from the wall, with dips as much as 25° 

and open folds, too small to show, on plate 41, have produced local 

ridges and closed depressions with a relief of 20 to 40 feet. These 

features were formed by postcaldera movements, but their location 

with respect to the caldera wall suggests that the fault involved is the 

caldera ring fault.

A few faults are radial to Okmok Caldera. Two north-striking 

faults are marked by scarps extending inland from the mouth of 

New York Creek on the north coast. The east block has dropped 

about 20 feet on each of these faults. A radial fault is suggested by 

the line of recent vents extending southeastward from the middle 

portion of the east arcuate ridge. Faultline control is suggested by 

the straightness of the cliffs bounding Mount Idak on the south and 

the northwest and by the parallel sea cliff along the southeast margin 

of the Mount Idak highland. Two hypothetical radial faults are also 

suggested by the north-trending cliff at the east end of the north 

arcuate ridge and by the northeast-trending cliff at the north end of 

the east arcuate ridge.

Three hypotheses are considered relating to the origin of the arcuate 

ridges.

As one hypothesis, the north and the east arcuate ridges (pi. 41) 

are possibly receding fault scarps controlled by a concealed underlying 

ring fault, between the caldera and the ridges. In the northwest wall 

of Okmok Caldera, north of cone H, a normal fault dips about 50° SE., 

toward the caldera. This fault separates Crater Creek basalt on the 

down-thrown side from the older Ashishik basalt (pi. 41) and may be 

a faulted depositional contact. About 100 feet of vertical displacement has occurred along this fault during or following the collapse of
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 339

Okmok Caldera, for the fault displaces the Okmok volcanics and can 

be followed as a recent scarp decreasiog toward the northeast (pi. 41). 

This fault, however, may have been localized along an earlier caldera 

fault, whose scarp is now the southerly slope of the north arcuate 

ridge. No trace of this hypothetical ring fault was found in the gap 

underlain by Crater Creek basalt between the north and the east 

arcuate ridges. Exposures of the Crater Creek basalt flows are continuous on each side of the gorge of Crater Creek in the area where 

the ring fault should be found; furthermore, the Crater Creek basalt 

is not disturbed or even flexed in this area. (See pi. 46-4.) The exposure of older rocks of pre-Crater Creek age in the floor of the gorge 

is not continuous, so that this ring fault could be concealed. This 

fault or a similar fault intersects the east wall of Okmok Caldera 

northeast of cone D (pi. 41).

Two sets of ring faults are therefore suggested by the known faults, 

volcanic vents, and the topography: a younger ring fault bounding 

Okmok Caldera and truncating the Crater Creek basalt, and an 

earlier ring fault farther to the northeast, which bounds a possible 

older caldera whose rim includes the north and east arcuate ridges. 

The two Umnak calderas slightly separated ha space and time are not 

unlike two similarly related caldera subsidences at the island of Mull, 

Scotland (Richey, 1935, p. 61-73; 1937, p. 13-25). The arcuate 

ridges are in part analogous to the elliptical arrangement of vent 

debris around Medicine Lake caldera in California (Anderson, 1941).

As a second hypothesis, the two ring faults referred to above may 

possibly have formed at nearly the same time during a single calderaforming episode (Byers, Hopkins, Wier, and Fisher, 1947, p. 37). 

The present author no longer favors this hypothesis, because the 

slope of the arcuate ridges that faces the caldera has a subdued topography, blanketed by Okmok volcanics, in contrast to the youthful 

precipitous cliff of Okmok Caldera, and also because the thick sequence 

of many flows of the Crater Creek basalt at low altitude seemingly 

have been localized prior to Okmok Caldera in a gap between the 

arcuate ridges (see p. 313).

A third hypothesis is that the arcuate ridges are dominantly constructional. The vents that supplied the constructional debris were 

probably controlled by an underlying ring fissure (not shown on pi. 41), 

directly beneath the vents along the crest of the arcuate ridges. The 

chief evidence in support of this hypothesis is as follows: First, the 

many recent vents situated along the crest suggests that the arcuate 

ridges may have resulted from similar volcanic accumulations earlier 

in their history; second, the gentle southerly dip of the Ashishik 

basalt on the south side of the north arcuate ridge is away from the

489292 59 6
340 INVESTIGATIONS OF ALASKAN VOLCANOES

crest of the ridge and toward Okmok Caldera; and third, the frequency of volcanic pipes and dikes is greater where cross sections of 

the arcuate ridges are exposed in the walls of Okmok Caldera and in 

the high cliff at the east end of the north arcuate ridge (the many 

dikes are not shown separately on pi. 41). Under the third hypothesis 

the growth of the arcuate ridges took place largely prior to the extrusion of the Crater Creek basalt, and no faults of large displacement 

would be required to account for the existence of the arcuate ridges. 

However, their origin is perhaps best explained by the first with 

subsequent modification under the third hypothesis.

ORIGIN OF OKMOK CALDERA

In the following discussion of the origin of Okmok Caldera an 

attempt is made to compare the missing summit mass of Okmok 

Volcano with the amount of xenolithic fragments in the known volume 

of Okmok volcanics, after the: method used by Diller (1902) and 

Williams (1941 and 1942). Much and perhaps most of the original 

volume of the Okmok volcanics was deposited beyond the shores of 

Umnak, so the total volume of volcanic rocks cannot be calculated.

The total volume of rock missing from the summit of Okmok 

Volcano could easily be computed if the form and height of the missing 

portion on the .site of the present caldera were known. Two possibilities are considered. As one possibility the missing central portion 

might have included an earlier caldera with relatively low summit area. 

The infacing cliff of the north arcuate ridge and the topographically 

low position of the Crater Creek basalt in the gap between the north 

and east arcuate ridges suggest that an earlier caldera may have 

existed. The gentle, radially outward dips of the precaldera volcanic 

rocks likewise suggest a low shieldlike volcano. As the second 

possibility the central portion might have consisted of a siliceous steepsided pyroclastic cone surmounting the pile of Ashishik basalt. 

Although the vitreous andesitic lava fragments in the Okmok volcanics 

may represent chilled, juvenile lava, it is also possible that fragments 

of andesite, latite, and rhyodacite in the Okmok volcanics may have 

come from thick deposits of siliceous volcanic rocks closer to the 

central vent. An analogy to a possible form of Okmok Volcano prior 

to the caldera-forming eruption is afforded by the pyroclastic cone of 

Mount Vsevidof on southwestern Umnak. Were Mount Vsevidof to 

experience a caldera-forming eruption similar to that at Okmok, little 

evidence of the former presence of the cone would remain after caldera 

collapse except xenolithic andesitic scoria and dacitic pumice fragments 

of the cone in the pyroclastic ejecta beyond the limits of the caldera. 

Andesitic scoria fragments as lapilli and bombs in the Okmok volcanics 

are probably juvenile, though some of the broken fragments could
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN. ISLANDS 341

 conceivably have come from a precaldera cone. Hence, computations 

of the missing volume are made considering both possibilities as

 extremes.

Assuming an earlier caldera as one extreme, the average altitude of 

the summit area of Okmok Volcano prior to the caldera-forming

 eruption is estimated as 2,500 feet or about the same as the present 

average altitude of the caldera rim (fig. 52). The average altitude

 of the present caldera floor beneath the postcaldera deposits is estimated to be about 1,000 feet (fig. 52). The diameter of the summit 

.area is taken as 5% miles. The missing volume of rock calculated 

according to these assumptions would be about 7 cubic miles. As the

 other extreme, a pyroclastic cone is assumed with an average slope 

of 25° and a base, 5% miles in diameter, at 2,500 feet (fig. 52), on top 

of and in addition to the minimum portion already computed. Under 

these assumptions the missing central portion of Okmok Volcano 

would be about 16 cubic miles. The actual missing volume is probably between 7 and 16 cubic miles.

The xenolithic fragments in the Okmok volcanics are probably 

derived from the mechanical disintegration of the former summit 

portion of Okmok Volcano. The volume of xenolithic fragments 

observed in thin sections of the welded agglomerates averages less 

than one percent (see table 11), but the sections were taken from 

samples selected so as not to include the larger xenolithic fragments. 

The welded agglomerate beds as observed in outcrop were estimated 

to contain at least 10 percent of xenolithic fragments (see pi. 47-A). 

The two specimens of partly palagonitized agglomerates studied (see 

table 11) contain 15 to 20 percent of xenolithic fragments, and from a 

study of many outcrops 20 percent is believed to represent nearly the 

maximum content of xenolithic fragments. If we assume that an 

overall average of 15 percent of xenolithic fragments is in the 7 cubic 

miles of the Okmok volcanics deposited on Umnak (see p. 315) only 

about 1 cubic mile of the missing summit portion of Okmok Volcano 

is represented in the known portion of the Okmok volcanics. Even if 

the minimum figure of 7 cubic miles is assumed for the volume of the 

missing summit, at least 49 cubic miles of Okmok volcanics would have 

been deposited in order to contain the missing summit as xenolithic 

fragments, or 42 cubic miles of the pyroclastic rocks deposited beyond the present shoreline. Such a figure seems unreasonably large 

in view of the thinning of the Okmok volcanics from an average of 

about 300 feet at the caldera rim to an average of about 100 feet at 

the present coastline. Hence, most of the missing summit of Okmok 

Volcano is believed to have collapsed inward to form the present 

Okmok Caldera.
342

I a

"S 60
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 343

SUBFICIAL DEPOSITS

The surficial deposits on Umnak Island range in age from late 

Pleistocene to Recent and comprise till, glacial outwash, beach deposits, dune sand, talus and alluvium.

THJL

Two tills have been differentiated on the geologic map. The older 

till is restricted to the southwestern part of the island, where it occurs 

on the slopes of Mounts Vsevidof and Recheschnoi (pi. 40). Near the 

base of these mountains the older till occurs as isolated patches of 

end moraines as high as 100 feet. Upslope from these end moraines 

the older till forms a discontinuous mantle of ground moraine on some 

of the gentler surfaces. Downslope from the moraines on the piedmont surface south of Mount Vsevidof are a few patches of older till, 

which were impractical to map because of the thick tundra cover. It 

is not known whether these patches represent a farther advance of 

the same ice that deposited the end moraines or whether they date 

from an earlier and more extensive glaciation. (Geomorphic evidence for this earlier glaciation is discussed under the heading "Nikolski Plain.") No soil profiles were measured oq the older till but it 

has been exposed to weathering long enough so that it supports 

blueberry bushes.

The older till unconformably overlies the bedrock volcanic rocks, 

including the older lava flows of Mount Vsevidof. It is in turn overlain by the younger till, outwash, alluvium, and the oldest of the beach 

deposits. If, as is inferred below (see p. 345), the oldest beach ridges 

date from the post-Wisconsin thermal maximum of about 5,000 years 

ago, the till must antedate the thermal maximum and is therefore 

designated as Wisconsin (?) in this report. It may coincide with 

Sharp's (1951, p. 100) "later glaciation" in the Wolf Creek area of the 

St. Elias range, Yukon territory. Sharp dates this glaciation as latest 

Wisconsin and post(?)-Wisconsin.

The younger till occurs on the slopes of Mounts Vsevidof and 

Recheschnoi in the southwestern part of the island (pi. 40) and within 

Okmok Caldera (pi. 41) in the northeastern part. Its distribution 

is closely related to the pattern of existing glaciers, and it was evidently 

deposited in the very recent past when the glaciers were more extensive than they are today. In Okmok Caldera the younger till is 

represented by small patchy end moraines, many just below small 

rudimentary cirques hi the caldera wall. One approximately half 

way between cones G and Z? represents the maximum advance of a 

postcaldera glacier here. Another is at the base of the east wall 

1% miles southeast of cone D. Some small moraines of the younger 

till probably have been buried by the recent lavas of cone A. The
344 INVESTIGATIONS OF ALASKAN VOLCANOES

younger till consists entirely of unweathered bouldery debris. In 

contrast to the older till, which supports bushes, the exposed surfaces 

of the younger till are nearly bare. Some of the boulders hi the end 

moraines of the younger till on Mount Recheschnoi are brightly stained 

by iron oxide that may have been developed by weathering prior to 

glacial transport. The younger till rests unconformably on the 

older till, the bedrock lavas, and the early postcaldera pyroclastic 

rocks in Okmok Caldera and is overlain only by some of the most 

recent alluvial deposits.

The unweathered character of the younger till and its areal association with existing glaciers indicates that it was deposited in late 

Recent time, possibly during Matthes' "Little Ice Age," (Matthes, 

1941; Thorarinsson, 1939; Ahlmann, 1948; Lawrence, 1950; Sharp, 

1951; and Mathews, 1951).

OUTWASH

Outwash consisting chiefly of boulder gravel fills most of the 

valleys that lead from existing glaciers on southwestern Umnak 

(pi. 40). The outwash ranges in age from that deposited penecontemporaneously with the younger till to that which is being laid 

down today. No outwash correlative with the older till was identified, although some remnants of older glaciofluvial deposits are 

probably preserved locally.

BEACH DEPOSITS

Beach ridges are found along both the northeast and southwest 

coasts of Umnak Island and are best developed behind those broad 

bights where the longshore currents and waves are generously supplied 

with glaciofluvial sediments. In several areas the supply of sediment 

has been sufficient to permit the seaward extension of the coastline 

more than one-half mile. Stages in this extension are marked by 

12 or more concentric beach ridges subparallel to the present coastline. 

Along the Pacific side of Umnak, between Amos and Russian bays 

(pi. 40), 20 arcuate concentric beach ridges were deposited. No 

precise altitudes were measured, but the innermost or oldest ridge 

seemed to be roughly 10 feet higher than the outermost or youngest 

ridge.

The material of the beach ridges ranges in size from sand up through 

boulders. For convenience in mapping, those .beach deposits that 

consist primarily of sand and pebble gravel have been called sand 

beaches and those which consist of coarser material have been called 

boulder beaches. In general, the boulder beaches are small and are 

insignificant, and are found chiefly in the smaller coves exposed to
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 345

the open sea. Hypersthene andesite talus blocks from sea cliffs on 

some exposed headlands have been rounded by storm-wave action 

into nearly perfect spheres up to 6 feet in diameter. 

Concentric beach ridges subparallel with the present shoreline 

have been observed by the author on Unimak, Akun, Akutan, and 

Unalaska islands in the eastern Aleutians and on Attu in the western 

Aleutians. Bradley (1948, p. 226) describes concentric beach ridges 

on Adak and also describes a wave-cut terrace along Kulak Bay on 

Adak Island, about 10 feet above present sea level. Bradley ascribed 

the higher beach ridges and the wave-cut bench to tectonic uplift of 

Adak Island. However, the presence of beach ridges and wave-cut 

benches at about the same altitude above sea level on so many of 

the Aleutian Islands, suggests a small eustatic fall of the sea rather 

than, tectonic uplift.

. Beach ridges, wave-cut benches, wave-cut cliffs and other shoreline 

features subparallel to the present coastline, record a recent stand of 

the sea roughly 10 feet (3 meters) above present sea level in many 

parts of the world. Many investigators. have postulated that this 

higher stand of the sea coincided with the post-Wisconsin thermal 

maximum. (See Fan-bridge, 1948, p. 62-63; Idem, 19.50, p. 181-184; 

MacNeil, 1950, p. 1307-1308; Newell and others, 1951, p. 11; Ives, 

1951, p. 220-221; Upson, 1951, p. 425-427.) The writer suggests 

that the oldest beach ridges on Umnak Island were formed during 

the same high stand of the sea that was responsible for the 3-meter 

shoreline elsewhere in the world and, therefore, presumably date from 

the post-Wisconsin thermal maximum.

According to Flint and Deevey (1951, p. 275-278), the thermal 

maximum occurred 4,000 to 6,000 years ago, and a reasonable check 

on the correlation of the oldest beach ridges of Umnak with this 

period is provided by a carbon-14 age determination of charcoal from 

the Nikolski archeological site. The Nikolski site is on top of one 

of the older gravel beach ridges (not shown on pi. 40) along the 

southern shore of Mueller Cove in Nikolski Bay. The material at 

the site consists of windblown silt, volcanic ash, and human occupational debris (William S. Laughlin, written communication, 1951). 

A carbon-14 age determination of charcoal collected from a level 

about 150 centimeters above the top of the beach gravel gives an age 

of 3,018 ±230 years, (Arnold and Libby, 1951, p. 118). By extrapolating from the dated layer, Laughlin (written communication, 1951) 

estimates that the oldest deposits in this site are about 5,000 years old. 

Therefore, the beach ridge on which the site rests is probably older 

than 5,000 years, and may well have been formed during the thermal 

maximum.
346 INVESTIGATIONS OF ALASKAN VOLCANOES

SAND DUNES

Sand dunes derived principally from the beach deposits are encroaching on and burying their parent materials along several parts of the 

coastline, particularly on western and northern exposures, where 

on-shore winds frequently exceed 100 miles per hour during the 

winter months. The average height of the dunes in most areas is 

about 30 feet but some of the larger dunes at Geyser Bight, Russian 

Bay, and back of the bight south of Mount Vsevidof, are as high as 

75 feet. Most of the dunes were active in 1947; a few were stabilized 

and partly covered with grass.

TALUS AND OTHER COLLUVTUM

Talus and other colluvium in the form of cones and aprons are 

at the base of all, or nearly all the steep slopes on the island, but 

only locally are the deposits large enough to warrant separate mapping. 

In most places they have been included with the pyroclastic debris, 

and on the slopes of Mount Recheschnoi they have in places been 

included with the older till.

ALLUVIUM

Recent alluvium consisting chiefly of reworked pyroclastic material 

has been deposited as alluvial fans by intermittent streams, and as 

flood plain deposits by the larger streams that are not fed directly by 

glaciers. The larger areas of alluvium have accumulated at low 

altitudes just inland from the oldest of the beach ridges. The largest 

area in the southwestern part of the island underlies the flood plain 

of the stream draining into Geyser Bight (pi. 40). In the northeast 

part of the island are several large areas of alluvium. One area covers 

about 5 square miles just northwest of Mount Idak (pi. 41), and 

another area of nearly 7 square miles has accumulated behind the 

beach ridges north of Cape Kigunak. Still another area is found at 

Izhiga Cove, where no beach ridges are present and the alluvium has 

been subject to marine erosion. The alluvium rests on all bedrock 

and surficial deposits except some of the most recent glacial outwash 

and some of the most recent flows within Okmok Caldera. These 

outwash deposits and flows locally overlap alluvial deposits. Although 

alluvium is currently being deposited by many streams, at several 

localities, especially near the coast, erosion is actively gullying older 

alluvial deposits. The gullied deposits may well be graded to the 

higher beach deposits and therefore may be as old as 5,000 years.

GLACIER ICE

For convenience, glacier ice has been included under the category of 

surficial deposits. Except for an arcuate patch of ice, probably stagnant, along the south rim of Okmok Caldera (pi. 41), the existing gla-
GEOLOGY OF UMNAK.AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 347

ciers on Umnak are confined to the southwestern part of the island on 

the slopes of Mount Recheschnoi and Mount Vsevidof (pi. 40). Alto^ 

gether, there are 9 major valley glaciers on these 2 mountains, and in 

addition several small glaciers and perennial snowfields. Seven of the 

9 glaciers head from the central summit ridge of Mount Recheschnoi; 

4 drain to the north and are in general longer and narrower than the 3 

which drain to the south. The north-flowing glaciers extend down to 

altitudes of 200 to 300 feet, whereas the south-flowing glaciers, although bigger in volume, terminate at altitudes ranging from 500 to 

over 1,000 feet. Possibly the south slope of the mountain is both 

warmer and moister than the north slope; therefore, although the supply of ice may be greater, the wastage is also more rapid and the fronts 

terminate at higher altitudes than those on the north side.

Both glaciers on Mount Vsevidof head in the extinct summit crater. 

One drains to the north and divides into two branches about 1 mile 

from the crater. The other glacier flows east, then south, joining the 

westernmost glacier of Mount Recheschnoi. These two glaciers from 

Mount Recheschnoi and Mount Vsevidof are separated by a medial 

moraine.

All 9 valley glaciers have excavated their valleys to depths of about 

500 feet. The termini of 8 of the glaciers lie close to the end moraines 

of the younger till, the exception being the most westerly of the glaciers 

of the north slope of Mount Recheschnoi. The terminus of this 

glacier is more than 1 mile back of its recent end moraine, indicating 

that for some reason this glacier has retreated more rapidly than the 

others of Umnak Island.

EROSIONAL FEATURES

Three major physiographic features, not directly associated with 

either the volcanic deposits, or the surficial deposits, are described and 

interpreted in this section. These features are Crater Creek Valley, 

pedimentlike slopes on Okmok Volcano, and the Nikolski plain.

CRATER CREEK VALJLEY

Crater Creek heads hi Okmok Caldera, from which it flows northeast 

about 8 miles, entering the Bering Sea about 1% miles southeast of 

Cape Tanak (pi. 50). For a distance of about 2 miles from the rim of 

the volcano, Crater Creek flows in a deep gorge cut into the volcanic 

rocks of Okmok Volcano. The gorge is the result of downcutting of 

the outlet of the lake that once occupied Okmok Caldera. The gorge 

thus postdates the formation of the caldera and the 1,560-foot level of 

the caldera lake. Many small cascades 5 to 10 feet high, and one large 

waterfall 80 feet high indicate that rapid downcutting of the gorge is 

still in progress. Downstream from the gorge, Crater Creek crosses
348 INVESTIGATIONS OF ALASKAN VOLCANOES

a broad gently sloping plain south of Cape Tanak. Many strath 

terraces and abandoned channels subparallel to the present valley of 

Crater Creek indicate that the stream has wandered widely during its 

dissection of this plain. Two principal sets of terraces were noted; 

one about 200 feet above the present valley floor and the other about 

50 feet above the present stream bed (see pi. 50).

PBDLMENTUDKE SLOPES ON OKMOK VOLCANO

Most of the lower slopes of Okmok Volcano (pi. 50) are gentle, 

concave-upward erosional surfaces that in less humid climates would 

be called pediments. The surfaces start at altitudes ranging from 700 

to 900 feet with gradients of 200 feet per mile that gradually flatten 

seaward to about 100 feet per mile. The slope of the volcano just 

above the heads of the pediments is 250 to 500 feet per mile. Recent 

gullies of the perennial streams have been incised about 100 to 200 feet 

into the pedimentlike slopes (pi. 50). The bedding in the Okmok 

volcanics parallels the lower pedimentlike slopes, which are in many 

places constructional surfaces. Whether the pedimentlike surface on 

the Okmok volcanics reflects a buried erosion surface or is merely the 

result of smoothing a buried irregular topography could not be 

determined.

THE NIKOLSKI PLAIN EVIDENCE OF AN OLDER GLACIATION

The southwestern part of Umnak Island (pi. 40) consists of a 

gently rolling piedmont plain with a relief of less than 300 feet, 

herein called the Nikolski plain. Except for a few scattered patches of 

questionable till and some flanking beach deposits, the Nikolski plain 

is free of surficial deposits, but its surface is dotted with many undrained depressions, some of which serve as basins for small lakes. 

A few of the lakes near the coast have been drained by the cutting 

down of their outlets, and all of these former outlets are now youthful 

V-shaped gorges. The majority of the lakes have not been altered 

since they were formed.

A study of the air photographs of the islands south of Umnak 

suggests that they also have been glaciated. Kigul and Ananiuliak 

Islands (see pi. 39) consist of rolling surfaces with undrained depressions similar to the Nikolski plain, suggesting that the glacier extended 

a considerable distance south of the present shores of Umnak.

Comparison of the relatively flat level surface of the Nikolski plain 

with the flat submerged continental shelf area of the Aleutian Ridge 

(see pi. 39) suggests that the Nikolski plain is an uplifted portion of 

the shelf somewhat modified by extensive piedmont glaciation. 

Studies in the Near Islands of the western Aleutians (H. A. Powers, 

J. P. Shaefer, and others, oral communications, 1950) indicate similar
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 349

widespread glaciation of both the islands and the continental shelf 

areas.

The overall lack of drainage integration of the Nikolski plain 

suggests that the glaciation of the area was late Pleistocene, possibly 

Wisconsin in age. Capps (1937, p. 161-168) has described and illustrated similar features on Kodiak Island, and ascribes them to the 

latest Pleistocene or Wisconsin stage. Further work is necessary to 

substantiate this hypothesis. If the piedmont glacier that extended 

over the Nikolski plain and the island south of Umnak represented one 

of the late Wisconsin substages, it is possible that the older till of the 

end moraines (Qmo pi. 40) may represent some post-Wisconsin advance of the ice.

GEOLOGIC HISTORY

The earliest known geologic event was the deposition of a thick 

sequence of waterlaid bedded tuffs and volcanic muds, whose metamorphosed equivalents are now bedded tuff and argillite of the albitized 

sedimentary and igneous complex, exposed on southwestern Umnak 

Island. This event possibly took place as early as late Paleozoic time 

but more likely occurred sometime between the early and middle parts 

of the Tertiary period. Following deposition, the bedded tuff and 

argillite was folded along northwest axes, which are at a high angle to 

the present trend of the Aleutian arc. It is not known whether extrusion and intrusion of igneous rocks, now keratophyres and albitized 

dioritic rocks, occurred prior to or following the folding.

Volcanism probably continued intermittently until sometime in the 

middle to late part of the Tertiary period, when the plutonic rocks 

were emplaced. The plutonic mass was largely quartz diorite varying 

to diorite and quartz monzonite. In the latest stages of consolidation of the plutonic mass, granophyre dikes were injected and portions 

of the dioritic mass adjacent to the granophyre were altered to a rock 

richer in quartz, orthoclase, and hydrous ferromagnesian minerals. 

A related and possibly contemporaneous alteration locally affected 

masses of the Tertiary lavas to produce quartz, chlorite, serpentine, 

zeolites, adularia(?), epidote, green amphibole, tourmaline, and rare 

iron sulfides.

Following emplacement of the quartz diorite pluton, erosion 

exceeded volcanism during uplift in late Tertiary and early Quaternary 

time. Extensive erosion surfaces, at least in part marine, were cut 

on the plutonic and older rocks, although minor volcanism continued 

near the narrow part of Umnak. Volcanic activity possibly may have 

begun at the sites of Mount Recheschnoi and Okmok Volcano during 

the latter part of this erosional episode.

The building of the composite hypersthene andesite cone of
350 INVESTIGATIONS OF ALASKAN VOLCANOES . ' Mount Recheschnoi probably had begun by early Quaternary time. 

The mafic phenocryst basalt of Kshaliuk Point was probably extruded 

during the same interval, though it might possibly have been extruded in latest Tertiary time. During the Pleistocene epoch, the 

central vent of Mount Recheschnoi moved westward. During 

Pleistocene glacial maxima, Mount Recheschnoi was deeply dissected 

by mountain glaciers, but cone-building activity predominated 

over glacial erosion. In latest Pleistocene or early Recent time a, 

pyroclastic flow of hot wet mud and lava poured down a glacial valley 

2 miles southwest of Russian Bay, and basaltic andesite was extruded 

at the site of Mount Vsevidof.

Meanwhile on northeastern Umnak the upbuilding of Okmok 

Volcano had begun probably sometime during latest Tertiary or early 

Quaternary time. The earliest eruptions consisted largely of mafic 

phenocryst basalt flows of the Ashishik basalt. A few feldspathic 

basalt flows also were extruded in the early stages of the upbuilding 

of the volcano. As Okmok Volcano grew, explosive eruptions extruded gas-charged pyroclastic material that became palagonitized 

upon emplacement. As the extrusion of the basaltic rocks comprising 

the Ashishik basalt neared completion, feldspathic basalt flows formed 

the dominant extrusive product. During the latest stages of Ashishik 

activity and immediately following, domes and short viscous flows of 

vitreous andesite and subordinate rhyolite were extruded close to 

potential fissures along which subsidence later took place. Satellitic 

vents extruded viscous lavas charged with olivine and plagioclase 

crystals. These lavas accumulated into steep-sided piles, such as 

Mount Tulik.

An early caldera block may have subsided just inward from the 

present site of the arcuate ridges shortly after the extrusion of the 

andesite-rhyolite association. A wedge-shaped volcanic sector graben 

may likewise have subsided at the gap between the north and east 

arcuate ridges. Whatever the origin of the gap between the arcuate 

ridges, a topographically low, seaward-sloping surface probably 

existed prior to the outpouring of the Crater Creek basalt.

During the latter part of Pleistocene and early Recent time, flows 

of the Crater Creek basalt were extruded from vents situated somewhere within what is now Okmok Caldera. At least 15 flows poured 

northeastward through the gap between the arcuate ridges. Toward 

the end of the period of extrusion of the Crater Creek basalt, a few 

small cones and flows of basalt were extruded on the outer flanks of 

Okmok Volcano. The stage was now set for the catastrophic calderaforming eruption;
GEOLOGY OF TJMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 351

A catastrophic eruption at the summit of Okmok Volcano deposited 

the Okmok volcanics during early Recent time. The earlier phase 

of the eruption appears to have been peleean and consisted of gascharged glowing avalanches (nuees ardentes). As the eruption increased in violence, red-hot glowing avalanches of molten bombs 

were welded together when they stopped moving to form continuous 

beds of welded agglomerate close to the source vents. The glowing 

avalanches were gas-^charged and had great mobility as they moved 

swiftly down the slopes of Okmok Volcano. By the time they came 

to rest on the lower slopes, the avalanches had cooled and incorporated 

water from snow and ice so that they became partly palagonitized. 

The later phase of the eruption appears to have been violently vulcanian and consisted of showers of dark andesitic ash, bombs, lapilli, 

and large xenolithic blocks of earlier basalt and palagonitized rock 

to form a widespread blanket as much as 300 feet thick near the 

source vents.

Very soon after the catastrophic eruption, the central part of the 

caldera collapsed to form Okmok Caldera inside a new ring fissure 

eccentric and 1 mile south with respect to the hypothetical earlier 

ring fissure suggested by the arcuate ridges. Immediately after 

.collapse, the newly formed Okmok Caldera began to fill with water. 

Early postcaldera eruptions built bedded debris cones underwater. 

Basalt flows of cones C and D were extruded when the water level of 

the lake had reached an altitude of about 1,350 feet. The highest 

recorded level of 1,560 feet was reached when the lake spilled over 

the lowest point on the hard upper surface of Crater Creek basalt 

near the present Crater Creek gorge. Further eruptions furnished 

material that was sorted into beds of silt and sandy ash by. the lake. 

Foreset beds whose upper constructional surface was graded to the 

1,560-foot lake level were formed. As the lake outlet was cut into 

the Crater 'Creek basalt, Crater Creek gorge was formed, and wave 

scars on the lava flows of cone D were recorded at 1,140, 1,120, and 

1,100 feet.

On southwestern Umnak, late or post-Wisconsin glaciers deposited 

the belt of. older end moraines on the piedmont surface south of 

Mount Vsevidof and Mount Recheschnoi. The glaciers then receded 

to positions high in the mountains some may have disappeared 

completely. At the same time the sea level rose in response to the 

 world-wide deglaciation to a level 3 meters above present sea level, 

and Umnak enjoyed a slightly warmer climate during the postWisconsin thermal maximum than at present. Near the seacoast the 

older end moraines were partly buried by beach sand; older, now 

dead sea cliffs were cut by storm seas higher than present seas.
352 INVESTIGATIONS OF ALASKAN VOLCANOES

A succession of beach ridges was deposited in favorable bights as the 

sea withdrew from its 3-meter stand.

As the sea level continued to drop from the 3-meter level to its 

present stand, the glaciers on southwestern Umnak and those in 

Okmok Caldera advanced to the positions marked by the unweathered 

terminal moraines within less than 1 mile of the present glacier fronts 

or cirque heads. Northwesterly winds built parabolic sand dunes 

behind beaches with westerly and northerly exposures. The date 

of the most recent glacial maximum recorded on Umnak probably 

coincided with that of the worldwide glacial maximum, 100 to 300 

years ago. Following their maximum extent, the glaciers have 

receded. The greatest recession occurred in those glaciers that had 

previously been most successful in extending their fronts to a low 

altitude.

While the glaciomarine events were taking place during the early 

part of the Kecent epoch, the quartz-olivine andesite flow, the rhyolite 

domes, and the youngest hypersthene andesite flows of Mount 

Recheschnoi were extruded. Basaltic andesite flows continued 

to accumulate at the site of Mount Vsevidof. Toward the end of this 

period the cone of Mount Vsevidof attained its present height by a 

succession of frothy pumice and scoria eruptions. Lava flows, 

ranging in composition from basaltic andesite to latite poured out 

over the pumice and scoria on the north, south, and west sides of 

the volcano, probably beginning shortly after the culminating summit 

eruption and continuing into historic tune. A chain of cinder cones 

was built along a rift extending down the west slope. Several lava 

flows were extruded from this west rift and formed Cape Kigushimkada. The latest flow was latite from the west rift and probably was 

extruded during the recorded historic eruption of 1878. The most 

recent activity, probably on the west flank, was reported in 1957.

On northeastern Umnak the most recent volcanic activity occurred 

within Okmok Caldera, although outside the caldera a few piles such as 

the two cinder cones northeast of Mount Idak and anorthite basalt 

flows near Inanudak Bay may have been extruded during, the past 

few thousand years. In Okmok Caldera, cones E and F and their 

flows were extruded after the caldera lake had receded from the sites 

of their vents. The pit crater of cone E was formed soon after the 

extrusion of flows from cone E. Wall-fissure flows and.those from 

cone B were extruded on what is essentially the present alluvial surface. Finally, the lava flows of cone A were extruded probably 

since the first Russians sighted the island about 200 years ago. The 

most recent flows from cone A were extruded in 1945.
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 353

GEOLOGY OF BOGOSLOF 

GEOLOGY OF BOGOSLOF IN 1947

The rocks exposed on Bogoslof Island in 1947 (fig. 53) are all of 

historic age. Igneous rock units include: vent agglomerate of 1796 

in fault contact with the andesine-hornblende andesite of Castle Rock

168° 02'

Hornblende basa

of Fire Island

(1883)

500 500 1000 1500 2000 Feet

Contour interval 20 feet;

100 feet at steep cliffs

Datum is mean sea level

^

Fault showing dip;

dashed where inferred

u, uptfrrown side; D, downthrown side

2

Strike and dip of beds

© 

Horizontal beds

Basaltic ash and agglom--) 

erateof 192"

Hornblende andesite 

of Castle Rock (1796)111

Base from U. S. Coast and Geodetic Survey, 1935, slightly 

modified along coast to show island as of 1947

Geology by F. M. Byers, Jr. assisted by Harald Drewes"

FIGURE 53. Geologic map of Bogoslof In 1947.
354 INVESTIGATIONS OF ALASKAN VOLCANOES

extruded in 1796; hornblende basalt of 1883 on Fire Island; basaltic 

agglomeratic ash of 1926 and earlier explosive eruptions on Castle 

Rock and Fire Island; and bytownite-salite-hornblende -basalt extruded in 1927 on Bogoslof Island. Sand and boulder beaches form 

most of the shoreline of Bogoslof.

Two minor high angle faults are mapped; the attitudes of beds 

mapped in the agglomeratic ash are probably gentle initial dips.

VENT AGGLOMERATE OF 1796

The vent agglomerate is exposed in the southeast end of the sea 

cliff that rises to Castle Rock (fig. 53). It occurs in fault contact 

with the andesine-hornblende andesite of Castle Rock, and is overlain 

by the basaltic agglomeratic ash of 1926 and earlier explosions.

The vent agglomerate contains abundant angular fragments of 

limonite-stained lava in a grayish matrix of ash. The lava fragments 

are of olive-gray, dense andesite porphyry containing conspicuous 

phenocrysts of black hornblende, commonly 2 or 3 millimeters, but as 

much as a centimeter in length. As seen in thin section the rock 

contains phenocrysts of mbnoclinic pyroxene, hornblende, and sparse 

labradorite in a microlitic groundmass of andesine, clinopyroxene, 

opaque oxides, orthoclase, and a little antigorite(?). Remnants of 

the original hornblende are pleochroic (pale yellow to brownish green) 

and have a maximum extinction angle of 12°. The rims, cores, and 

in places entire crystals of the hornblende phenocrysts have been 

altered to an aggregate of magnetite, clinopyroxene, oligoclase(?), 

and antigorite(?). Calcite and chabazite occur in rare vesicles.

The vent agglomerate is exposed well within the area originally 

occupied by the large dome of Old Bogoslof extruded in 1796 but has 

a composition slightly different from the material of Castle Rock, 

the remnant of Old Bogoslof. Although there is no written record 

of its emplacement, the agglomerate must be an explosive phase of 

the 1796 eruption, for no volcanic mass occupied its site prior to 1796 

(see fig. 54).

HORNBLENDE ANDESITE OF CASTLE ROCK

The hornblende andesite of Castle Rock is an eroded remnant of 

the dome extruded in 1796. The twin-spired pinnacle of Castle Rock 

appears to have had approximately its present shape and size since 

about 1906 (Jaggar, 1931, p. 2-3). Chart 6433 of the U. S. Coast 

and Geodetic Survey, 1935, shows an altitude of 269 feet on the 

northwest pinnacle, and 333 feet on the southeast pinnacle. No 

change was observed in the shape and altitudes of the pinnacles in 

1947 (pi. 51).

Castle Rock is composed of a very light gray, fine-grained, holocrystalline andesine-hornblende andesite with abundant black horn-
GEOLOGICAL STJBVEY BULLETIN 1028 PLATE 61

Castle Rock, Bogoslof Island, from the southeast in 1947. Agglomerate ash beds of 1926 and older eruptions

overlie Casile Rock on inland slope.
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 355

blende laths ranging from a fraction of a millimeter to 3 millimeters 

in length. The abundant plagioclase phenocrysts about a millimeter 

in diameter are not conspicuous against the light-colored groundmass. 

Phenocrysts seen in thin section are plagioclase, oxyhornblende, 

ferrosalite and accessory minerals. Plagioclase phenocrysts are zoned 

andesine to calcic oligoclase (An25 ), but many contain a sharply 

defined core of anorthite-bytownite. The core boundary is generally 

sharp enough to- be conspicuous in plane-polarized light, but is gradational in some crystals. A few of the andesine phenocrysts are veined: 

with albite. Accessory microphenocrysts include magnetite (strongly 

magnetic grains), stubby prisms of apatite less than 0.3 millimeters 

in length, and sphene in both wedge-shaped and stubby prismatic 

forms. The groundmass is a fine-grained, xenomorphic aggregate of 

untwinned oligoclase-albite, orthoclase, analcite, and glass, with 

sparse subhedral microlites of clinopyroxene, apatite, and opaque 

oxides. The analcite is intergrown with other minerals in the groundmass, but also occurs as cavity fillings. It is isotropic to weakly 

birefringent (0.001-0.002) with mottled extinction and in a few grains 

resembles the polysynthetic twinning of albite. The identification 

as analcite was confirmed by X-ray photography by Richard C. Erd, 

of the Geological Survey. Chemical and quantitative analyses of 

the rock are given in column. 35, tables 2 and 3, respectively. The 

chemical analysis is similar to an earlier one of a specimen from 

Castle Rock (Emerson, 1904; C. N. Fenner 1926, p. 706-707).

HORNBLENDE BASAI/T OF FIRE ISLiAND

As seen from a short distance in 1947, Fire Island was a flat-topped 

stack resembling a lighthouse, less than one-half the size mapped 

and photographed in 1935 (Lukens, 1936, fig. 1). Two specimens 

of lava collected in 1884 by Lt. G. M. Stoney of the U.S. Revenue 

Cutter Corwin are described by G. P. Merrill (1885, p. 31-33). The 

petrographic description of one of them fits the lava of Castle Rock, 

but the other one is a dark-gray hornblende basalt, probably from 

Fire Island. Hornblende makes up about 10 percent of the rock, 

and plagioclase microlites are abundant in an indeterminant ground-, 

mass. This specimen was analyzed chemically by T. M. Chatard. 

Two specimens acquired by Diller (1885, p. 67) were from the lava 

of Fire Island and from the ash that fell at Unalaska village. Two 

determinations of silica by Chatard on the lava specimen were 51.54 

and 51.65 percent; the silica content of the ash was 52.48 percent, 

significantly higher than the two silica determinations on the lava 

specimen.

489292 59 7
356 INVESTIGATIONS OF ALASKAN VOLCANOES

Chemical analysis and norm of hornblende basalt from Fire Island dome (New

Bogoslof)

[T. M. Chatard, analyst; In Merrill, 1885, p. 33]

Analysis (weight-percent) Norm (weight-percent)

Si02 _.. 51.54 or.....______________ 14.46

A12O3 .- 20.31 ab__._-_____._____.__ 29.60

Fe2 03_ -_ _____ 4.64 an.___________________ 28.91

FeO_ _ 3.56 ne.........__________ 3.55

MgO ____ 3.16 di_________________ 12.01

CaO... _________ 9.55 ol____________.___ 3.54

Na2 0 _______ 4.29 mt___._._______....... 6.73

K2 q_---___ ________________ 2.47 il.________________________ 0.61

Ignition ____ 0.34 ap_____:.. __________ 1.34

Ti02 - ----._.__________ 0.32 

P2 05 .._-__._..._.__________ 0.57 Total__________._._._ 100.75

MnO_-..._______.___ 0.32

Total__________ 101.07

BASALTIC EXTRUSIVE ROCKS OF 1926-27

Basaltic ash and agglomerate of the explosions of 1926 are plastered 

against the steep north and east cliffs of Castle Rock (pi. 51) and 

iorm the extensive terrace that slopes away from Castle Rock to the 

northeast. None is on the dome-shaped western mass formed by the 

viscous extrusion of bytownite-salite-hornblende basalt in 1927. In 

an exposure at the north end of the east sea cliff the section of pyroclastic material contains two thin ash beds separated by about 8 feet 

of dark-colored agglomeratic ash. The lower bed of fine ash is 3 

inches thick and contains a grayish-yellow incrustation; the upper bed 

is 1 inch thick and contains a pale-yellowish-green incrustation, abundantly present as microbotryoidal coatings on the ash fragments. 

Gypsum, CaO-S03 -2H2O; paratacamite, CuCl2 -3Cu(OH) 2 ; botryogen, 

2MgO.Fe2O3 4S03 -15H20; and caledonite, 2(Pb,Cu)O-SO3 -H20 were 

identified in the incrustation from the upper ash bed by Richard C. 

Erd of the Geological Survey.

The lavas of both the dome and the larger fragments in the agglomeratic ash- are medium-dark-gray, fine-grained basalts with conspicuous phenocrysts of hornblende, and inconspicuous phenocrysts 

of plagioclase and clinopyroxene. Differences in mineralogy of the 

lava thrown out explosively in 1926 with the lava extruded as a large 

dome in 1927 can probably be correlated with the different cooling 

history of the two rock types. In the pyroclastic rock, phenocrysts 

of hornblende are unaltered, with extinction angles as much as 18°, 

whereas in the rock of the dome the hornblende has been partly to 

completely decomposed. The least altered hornblende has extinction 

angles as much as 14°, and is pleochroic in grayish yellow and brown. 

Clinopyroxene in clear, unaltered, subhedral to euhedral phenocrysts 

occurs in both rocks. In the rapidly chilled ejecta of the agglomerate, 

some of the larger crystals have grown around a core of unaltered 

hornblende. Both rocks contain inclusions composed of crystals of
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN ISLANDS 357

plagioclase, clinopyroxene, and unaltered hornblende like that in the 

phenocrysts. The groundmass of the chilled ejecta of the agglomerate 

is a fine-grained, dark aggregate of microlitic plagioclase, clinopyroxene, and opaque oxides in a matrix of dark glass. In addition to these 

constituents a feldspar of low index, probably anorthoclase, is in the 

groundmass of the dome. Nepheline was not identified, though it 

appears as a theoretical mineral in the norm (column 34, table 2).

BEACH DEPOSITS

Beaches composed of rounded cobbles and boulders as much as 

6 feet in diameter are found along three parts of the coast extending 

southeasterly from the rocky cliffs of Castle Rock, and both northeasterly and southeasterly from the rocky cliffs cut in the dome of 

1927 lava. A sand beach forms the east coast of the ash and agglomerate terrace. Sand and gravel with scattered cobbles and 

boulders form a large deposit around the saltwater lagoon.

STRUCTURE

Two faults were mapped on Bogoslof Island (fig. 53). The longer 

is poorly exposed hi the agglomeratic ash in the northeast sea cliff, 

but displaced beds exposed on opposite sides of the fault show that 

the northwest side has been dropped 5 feet relative to the southeast 

side. A prominent topographic scarp between altitudes of 60 and 80 

feet marks the probable trace of the fault to the southwest across the 

raised terrace almost to Castle Rock. This scarp is a prominent 

feature on air photographs taken in 1934. However, Jaggar states 

(1930, p. 3) that explosion debris enclosing the lagoon in July 1927, 

reached only about 10 feet above high tide. If his description applies 

to the terrain of the faulted terrace, the uplift of the terrace and the 

formation of the fault scarp must have taken place between 1927 and 

1934. The shorter southern fault separates the lava of Castle Rock 

from the vent agglomerate. The trace of the fault in the south sea 

cliff is nearly vertical. The direction of movement indicated on the 

map is inferred from the written record that the lava of Castle Rock 

is an extruded dome and presumably rose with respect to the vent 

agglomerate.

GEOLOGIC HISTORY

The first written record of the existence of Bogoslof is on the track 

chart of Krenitzin and Levashef, 1768-69 (Coxe, 1787, plate facing 

p. 205), on which a small unnamed island, later known as Ship Rock 

(Baker, 1906, p. 570), is shown on the approximate site of Bogoslof 

Island (see fig. 54A). Captain Cook placed Ship Rock 17 miles north 

of the north shore of Umnak on the chart of his voyage in 1778-79 

(Davidson, 1884, p. 282). A small island in the same position is also
358 INVESTIGATIONS OF ALASKAN VOLCANOES

SHIP ROCK

JS^i K^^

BOGOSLOF li

METCALF 

CONE

«$FIRE ISLAND

CASTLE * 

ROCK

PEAK

I Nautical mile

FlGTJKE 54. Significant changes In Bogoslof Island between 1768 and 1947 (diagrams A-a after Jaggar, 1907 

and 1931; diagram H after map In files of U.S. Coast and Geodetic Survey and after Lukens, 1936).

shown on Billings' track chart of 1802 (Baker, 1906, p. 570) and was 

named Ship Island. Some of the earliest works to which references 

are made were not accessible to the author, but those included within 

parentheses were consulted. 
GEOLOGY OF UMNAK AND BOGOSLOF ISLANDS, ALEUTIAN .ISLANDS 359

In May of 1796, eruption of explosive debris and the extrusion of a 

viscous lava dome built an island about a quarter of a mile south of 

Ship Rock (Grewingk, 1850, p. 136; DaU, 1870, p. 467). The new 

island was named Bogoslof by the Russians at Unalaska. The mass 

steamed and fumed as it continued to rise in altitude. The steaming 

had ceased by 1804, but in many places the surface was still uncomfortably hot. In 1806 lava flowed from the summit into the sea 

on the north side, and apparently the rising of the whole mass was 

renewed. This increase in size and altitude is said by Veniaminof 

(Grewingk, 1850, p. 140) to have ceased in 1823, when the dome was 

of pyramidal form (see fig. 545). Estimates of the height of the new 

island ranged from 350 to 2,500 feet (Grewingk, 1850, p. 140), but 

later, more careful measurements by DaU (1884, p. 32) indicate that 

the lower estimate was probably closer to the true height. The 

lava dome later became known as Old Bogoslof (Merriam, 1902, p. 

297), and the U. S. Coast and Geodetic Survey early in the present 

century first showed the official name, Castle Rock (Powers, 1916, 

p. 218), on the hydrographic charts.

No further volcanic activity occurred until 1882, when it was 

reported that residents of Unalaska noticed steam rising from the 

ocean somewhat north of Ship Rock (Merriam, 1902, p. 305). On 

September 27, 1883, Capt. Anderson of the schooner Mathew Turner 

first observed a new volcanic dome rising just to the north of Ship 

Rock (Davidson, 1884, p. 283: Merriam, 1902, p. 303). On October 20, 

1883, a violent eruption showered ash on the village of Unalaska, 60 

miles to the east. A week after the eruption the cone was seen to be 

still steaming by Capt. Hague of the schooner Dora. Capt. Hague 

suggested the name, New Bogoslof, for the new island; slightly later, 

Dall (1884, p. 92) proposed the name, Grewingk Island. The new 

island was visited several times during the folio whig year by the 

U. S. Revenue Cutter Corwin, and the three Bogoslof islands were 

charted by Lieut. Cantwell of the Corwin in 1884 (Healy, 1889, p. 39- 

41). .(See fig. 54(7.) The 3 Bogoslof islands were visited again by 

William C. Greenfield in 1887, at which time the 3 islands were connected as 1 by bars of volcanic debris and boulder and sand beaches. 

The new island to tbe north had a craggy profile, with pinnacles 

reaching to an estimated altitude of 500 feet. Old Bogoslof apparently 

had been greatly reduced hi area and altitude, its three main crags 

appearing somewhat lower than the summit of New Bogoslof. Ship 

Rock was still in existence in 1887, but had begun to disintegrate. 

In 1890, Capt. Z. L. Tanner of the U. S. Fish.Commission steamer 

Albatross reported that Ship Rock had fallen but that its original 

position was still marked by debris (Merriam, 1902, p. 312). In 

1891, Merriam (1902, p. 313) visited the Bogoslof Islands and found
360 INVESTIGATIONS OF ALASKAN VOLCANOES

steam and sulfur fumes escaping with a roaring noise from the principal 

fissure of New Bogoslof. An open channel separated Old and New 

Bogoslof. In 1895 (see fig. 54Z?) when Becker and Dall (Becker, 

1898, p. 26; Merriam, 1902, p. 317) visited the Bogoslof Islands, New 

Bogoslof was still steaming "vigorously, though not violently" and 

had also been changed to a flat-topped island about 300 feet in altitude. Later reports in 1897 and 1899 by passing mariners indicate 

that New Bogoslof or Grewingk Island had finally cooled (Merriam, 

1902, p. 319-320).

In 1906 another extrusion of viscous lava appeared in the water 

about midway between old and new Bogoslof (see fig. 54.E"). The new 

island, called Metcalf Cone, was conical, showing at the summit a 

broken spine similar to the famous Pelee spine on Martinique (Jaggar, 

1908, p. 388). An explosion early in 1907 destroyed the south half 

of Metcalf Cone, and another dome of lava grew into an island connecting Metcalf Cone with Old Bogoslof. This 1907 dome was named 

McCulloch Peak (see fig. 54.F)- A violent explosion on September 1, 

1907, only 23 days after the Massachusetts Institute of Technology 

expedition had left Bogoslof, showered the village of Unalaska with 

a quarter inch of volcanic ash. McCulloch Peak disappeared in the 

explosion, and the debris mantled the surfaces of the remaining island. 

In 1909-10, a new conical island, Tahoma Peak, was pushed up in 

the bay that was formed by the destruction of McCulloch Peak 

(Powers, 1916, p. 218-221). A deep crater in the summit of Tahoma 

Peak was formed by explosions on September 18 and 19, 1910.

Metcalf Cone and Tahoma Peak had a very short existence. By 

1922, explosion, collapse, and marine erosion had removed all traces 

of them (see fig. 54(7). Fire Island, a new name for New Bogoslof 

Island, was adopted by the Coast and Geodetic Survey on its published 

hydrographic charts. The island had been reduced further by erosion 

to a small islet. Castle Rock (Old Bogoslof) had been reduced to two 

rocky horns, and a big accumulation of sand and gravel had been 

heaped against them, particularly on the northern and eastern sides. 

A wide channel of open water separated Fire Island and Castle Rock 

(Jaggar, 1930, p. 3).

Submarine explosions between the two island remnants during the 

summer and fall of 1926 preceded the rise of another conical dome of 

lava during the winter of 1926-27. A ring of explosion debris, rising 

about 10 feet above high tide, surrounded the new dome and again 

completely connected Fire Island and Castle Rock into one elongate 

island. In July 1927, the new dome, about 200 feet high, was almost 

circular, a thousand feet in diameter, and surrounded by a shallow 

lagoon of warm water (Jaggar, 1930, p. 3). "Fire" was still observed
at the 1927 dome as late as October 31, 1931, and quantities of pumice 

floated in the water south of the island (Jaggar, 1932, p. 2).

In 1935, an open water channel again separated Fire Island from 

a larger southerly island that contained the 1927 dome and Castle 

Rock (see fig. 54ZJ).. Both islands were mapped by the Coast and 

Geodetic Survey at this time (Lukens, 1936, p. 205-206), and the 

name Bogoslof was officially applied to the larger island that included 

Castle Rock, pyroclastic rocks of the 1926 eruption, and the 1927 

dome.

The geology of Bogoslof Island was mapped in the summer of 1947 

by the present author. Between 1935 and 1947 the size of both Fire 

island and Bogoslof Island was appreciably reduced by erosion (see 

fig. 547). A Navy quonset hut erected sometime during World War 

II was partly undermined by retreat of the east-facing sea cliff. On 

September 21, 1951, a coastwise vessel ran through "muddy.water" 

for about 2 miles near Bogoslof; this may have been the result of a 

submarine eruption or landslide. Early in November, 1951, Bogoslof 

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communication, 1955), who reports further retreat southward and 

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area by about 25 percent. The Navy quonset hut was gone and must 

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INDEX

Page 

Aa ~ -............. ............ 329

Abstract . ._.__ 267-268 

Acknowledgments.....__..___.... . 269-270 

Actinolite. ................___......... 292

Adularia.......................... .. 289.295,296

Age and correlation, albitizcd sedimentary and

igneous complex 289

of rocks of central Umnak. ___. __ 294-295 

Agglomerate beds, welded.. ...... 316-317

Agglomerate deposits. .. ..... 315-316

Agglomeratlc tuff-breccia, partly palagonitized. 320 

Akun Island...................__........ 278,345

Akutan Island....______________ 278,345

Alaska Peninsula.._________....._.. 278

Albatross, U.S. Fish Commission steamer_. 359 

Albite .. ._..., 286,287,288,292,355 

Aleut, origin of.______ ._ ....... 271

Aleutian arc, eastern..!... 276-278 

Aleutian Ridge. . _ 272,273

Aleutian Trench....__ . 272,273,276

Amak Island......_ ...._........ 276

Amphibole ..........___- 287,295,296

Analcite................._ .... ... 355

Ananiuliak Island r ......... .... 274

Andesine ......__ 287,295,302,303,306,307,355

Andesite--.- - 297-302,312,335,354

quartz-bearing olivine._________ 302-303

vitreous. __ 312,335 

Andesite flows, basaltic, from cone B....... 328-329

quartz-oavrae.. 299,302-303

Andesite and latite flows........... 306-307,335

Animal life 275-276

Anorthite.-- ............ . 324,330,332,333,334

Anorthite basalt flow, near Inanudak Bay. 324 

Anorthoclase.___ .... 357

Antigorite.._......._..... ... 295,354

Apatite.., . 287,288,290,291,292,355 

Arcuate ridges, hypotheses for origin 338-340 

Area of report..................... ... 278-279

Argillite , 286-287,289

Ash, bedded. _............ 321

Ashishik basalt..... 308,309-311,312,313

Augite.... T ..,. .. ...... ..... 287,

290, 291, 298, 300, 301, 303, 306, 307, 310, 318, 

329, 330, 331, 332, 334, 335, 336, 337

Basalt, anorthite. _____.____ _ 332-333 

aphyricand feldspathic...____. 311-312,334 

Ashishik...................308,309-311,312,313

Crater Creek............................. 313

maficphenocryst.___ .._ . 309-310.323 

olivine-rlch............. ...... ......... 330

plagioclase-olivlne, of satellitic vents. 312-

313,330 

plagioclase-augite-olivine- __ 330-331

Basalt flows, from cone A..,-________ 329,332 

later than caldera Lake. 327 

near Capeldak.._._ .... ... 324 

on arcuate ridges.. ..... _ 323-324 

on Cape Aslik______________. 324 

outside Okmok caldera____ __. 323 

plagloclase of cones C'and D........ 326,330,332

Page

Basaltic andeslte flows..__________ 305,306 

Beach deposits.. T ____.___ _._ 344-345,357 

Beach ridges. ..__..__,____ 344,345,352 

Bering Sea. __..____. --.,, - 272 

Bering, Vltus, expedition of. . ______ 270 

Biotite................_.. 287,288,290,291,301,302

Black Creek ...... .... 274

Botryogen__.....___...________ 356

Bytownite ...... 298,300,302,303,330,334,335,355

Calcite . 286,287,288,292,295,296,354 

Caledonite.- . __________ 356 

Canyons, submarine.. ..__________ 273 

Cape Aslik, basalt flow on__________ 324 

Cape Tanak..__.___................. 320,321

Carbon-14..____ .-... .. 305,345 

Castle Rock.. _.._......... 271,354-355,357

Chabazite .__ _. ..... 295,337,354

Chirikof, Captain Alexel, cited... .. 270 

Chlorlte . __..... 287,288,291,292,294,295

Chuginadak__._____....____....... 273

Cinder Point...___.___.________ 324 

Climate..__..___.____1___'..... 275

Coal. __ ______________ 289 

Colluvium__.._______________ 346 

CornwaJlius sp _______________ 289 

Corwin, U.S. Revenue Cutter. .. 355,359 

Crater Creek basalt, use of name. ______ 308 

Crater Creek gorge... __..___ 312,338,351 

Crater Creek valley _____________ 347-348 

Contact rock, altered.. _________ 292-293

Diabase...__..___.___._______ 287,288 

Diorite __ ... 287,289,290 

Dora, Schooner. __________ 359 

Driftwood Bay _............... 285,286,287

Epidote....... ............. 288,292,293,295,296

Erosion, features of . :__ ___ 347-349 

Expedition, Bering .. __...... 270

Harrlman....__....__._______ 271

Faults ____ 286,338,339 

Ferrohortonollte....__..__________ 335

Ferrosalite_.________________ 355 

Fleldwork._________________ 269-270 

Fire Island . ... ................. 271,355

Fossils. ..____________._____ 279,289 

Fumaroles..._ T ______.____ 294,326,329

Gabbro... ... .............. 287

Geologic setting, general ...,.._.___ 276-279 

Geologic work, previous..._________ 270-272 

Geography. ....___ 272-276

Geyser Bight __1_.-....-.__._ 294,346

Geysers..___________________ 294 

Glaciers... ....__.................. 346-347,352

Granophyre. .... ...... 289,292

Graywacke___________________ 289 

Gypsum....... ______________ 356

Harriman, Alaska expedition of._______ 271

Hematite... _. __..______ 288

Heulandite.______.___________ 295

367
368 INDEX

Page

History, geologic. _.___ .. 349-352,357^361 

of report area 270-272 

Hornblende. ..__ 290,291,292,301,354,355,356,357 

Hornblende andesite, of Castle Rock.. 354-355 

Hornblende basalt, of Fire Island_.. 355-356 

Hot Springs._____.__ . 272,294 

Hot Springs Cove. ........ 293,294

Hyalosiderite._________ 335

Hyperthene . . 291,

298,300,301,303,306,307,318,329

Hypersthene andesite_____.. 297-298,300-301

Igneous complex 285-289 

Inanudak Bay.. . - 273 

Introduction.................- 268-269

Jag Peak...... . 308,313

Jochelson, Waldemar, anthropologist cited . 271

Keratophyre.__ - 285,288-289 

Klgul Island. . 285 

Kodiak Island.. . . 278

Labradorite__ : 287, 

300,301,302,303,330,333,335,336,354 

Latite.___.___.___ .: __. 306,335 

Lava flows... 298,299,304-306,323-329 

Lavas, age of, Mount Recheschnoi . . 299 

Literature cited___ .__ ___.... 361-365

McCulloch Peak_________.__ 360 

Mafic phenocryst basalt, of Kshaliuk Point.. 298 

Magnetite._.___________..__ 295,354,355 

Mathew Turner, Schooner.. 359 

Metcalf Cone...________________ 360 

Microlites.__.________......_ 306,307,336

Microphenocrysts. . 334,335 

Mt. Idak . . 308,330 

Mt. Recheschnoi-____ . 285,297-303 

Mt. Tulik.__ _____... .... ;._ 274,308,330

Mt. Vsevldof._____.____....... 274,304-308

Myrmeklte______'_____ _ 291

Natrolite ____...........__... ... 295

Nephelhie. . _ ________ 357 

Nikolski, Aleut village of _________ 272,305 

Nikolski plain...__. ......:..__ 274,348-349

Nomenclature, of rocks, used in report __ 279 

stratigraphic.... . ... __ __ 308,314

Nuee ardente-i_______________ 321,351

Obsidian._ . ..___..__^___ 312 

Okmok Caldera__________ 274,337-341,351 

Okmok volcanics, age..._:_______1.. 322-323

origin and emplacement. _ 321-322,351 

Okmok Volcano, petrography.... _.... 329-330

structure. _______:___ 337 

Ollgoclase...__..__ _. 291,292,293,354,355 

Olivine ................ 295,298,300,301,302,303,

306, 318,320, 329, 330, 331, 332, 334,335,336,337 

Orthoclase 290,291,292,303,306,307,308,354,355 

Outwash._____________;____ 344 

Oxyhornblende.;___.__.________ 355

Paratacamlte__________________ 356 

Partov Cove..-.. 285,286 

Penninite___._.____.________ 295

Page 

Petrography.______........ .....:~ 286-289,

290-293, 295-296, 299-304, 305-308, 329-330 

Phenocrysts___. ____ 297,298,302,303,305,306,

310, 311,319,330, 331, 332, 333,- 334, 354, 355, 356 

Plagioclase-- 287,290,

291, 300, 302, 303, 306, 311, 316, 318, 320, 323,

-326,329,330,331,333,334,335, 336, 355, 356, 357

Plutonic rocks.._______________ 289-293

Potassium feldspar 287,

288,291,292,293,295,296,308

Prehnite...-__ ____..._-_-__ 290,292 

Prochlorite.___.___.. . .- 288 

Pumice ___..__ 304,305,320-321,352,361 

Pyrite. __.___.........____- 287,296

Pyroxene 287,288,302,310,326,330,354,356,357

Quartz......_ 287,288,290,291,292,295,296,302,303

Quartz diorite.-.__._______... 289,290-291 

Quartz monzonite.. . 289,291-292

Reindeer- 276 

Rhyolite_:___.____....__ 301-302,312,336

Rhyolite domes__......_...... _ 299,312

Ripidolite..___... __.____ 288

Roads, on Umnak Island____.! 272

Rocks, contemporaneous with caldera lake.. 325-326

extrusive basaltic.__ . 356-357

of Central Umnak, chemical composition.. 295-

296

of postcaldera age, within Okmok caldera-. 324-

325

palagonitized pyroclastic.___ 310-311,336-337 

Precaldera, age of__...... 314

Russian Bay.. ....__.___ 285

Russian period, In history of report area.... 270-271

Sail Rock.............. ..... ......... 270

Salite.... .... 356

Samalga. Island .___ ... 273,274 

Samalga Pass... 273 

Sand dunes... . , 346 

Scoria.................... ......... 304,307,352

Sedimentary, complex.. ________.... 285-289

Se'diments, bedded volcanic_.. ...... 327

Sericite .__ __.." 287,'289)'290,291,292 

Serpentine- .... . * 295

Seward, Alaska... ______________ 272 

Ship Island..._________________. 358 

Ship Rock......-..........-. 270,359

Silica ____........____. ..... 279

Sills.-.--..--.-- 288

Sphene__.....________..... 355

Stratovolcano---.---______________ 304 

Structure.: 286,337-339,357 

Surficial deposits.. . 343-347

Tahoma Peak.______________.... 360

Talus... . 345,346

Temperature- . 275 

Thumb Point.- ...... 293,295

Till..... 343,344

Tourmaline_ _____________ 293,295,296 

> Tuff. ,. .. ... 287,289

Tuya .__............. ..._.. 326

Umnak Pass... ............... 270,273

Unalaska Island ... .. ..... 278,290,345
INDEX 369

Page 

Vegetation....__._.. .... 275-276

Volcanic rocks, of central Umnak... 293-296 

of minor vents__________ __ 312 

of Mount RecheschnoL...____ 297-303 

of Mount Vsevidof________._.. 304-308 

of Okmok volcano.._______...... 308-336

Volcanics, Okmok, of caldera age__ 314-315

Page

Vsevidof Island 274

Williwaws...... . 275

Xenocrysts_________________ 308,336 

Xenoliths.. - 289,303,308,316-317,335,336,337

Zeolite... ____._ 287,295,296,302,337

o