Geology and 

Petrology of the 

Pribilof Islands 

Alaska

By TOM F. VV. EARTH

INVESTIGATION OF ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-F

Prepared in cooperation with the 

Department of Defense

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956
UNITED STATES DEPARTMENT OF THE INTERIOR 

Fred A. Seaton, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D. C. - Price 60 cents (paper cover)
CONTENTS

Page

Abstract..__.__.___._..______.__.______.-___.__._.._-__.-.._._..__ 101

Introduction._____________________________________________________ 102

Location and physical geography._______________________________ 102

Field work and acknowledgements-__-__-_________--__-___--___._ 103

St. Paul Island -.___.___._-____-.-__-_--_-__---____----------- 103

Lava flows and basic dikes_______---__---__----_-_-_-----_------ 103

Pyroclastic deposits.___________________________________________ 104

Pleistocene sediments____________-_-___-___-_-___-__----------- 106

Alluvial sediments-______-__-_-_--_-_-______-_-_-__-_--__----__ 106

Otter Island.________-_____.._-..__.-_-__-_-_---__-_-_-_.-._--._.- 106

St. George Isla.nd-----_-___-__-__--------_-------.----------------- 110

Peridotite______--__________-_-_-_-__--___--__-_-_-_---_---_-- 110

Aplite ___----_-____________-____--__--_----____-_-------_ 112

Lava flows and basic dikes___---__----_-----_-------------~---_- 112

Pyroclastic deposits.___________________--____-_______--..__-_-_- 115

Fossiliferous sediments _________________________________________ 117

Fossils._____--__----_-_--_-____---_---_-._-_-__-__--_-----_-- 118

Petrography.___-____-____--________-____-______-__--_-__-----__-- 119

Peridotite.. __----------___--__-- _----_--.----_.--_-_----_----_ 119

Aplite -.---.__--_-------_--___-----.----_--.-_------.---_-_ 121

Lava_______________________________________________________ 124

General features.__________________________________________ 124

Mineral components._______________-__---_______-_--_---__ 125

Analyses of lava___--_-___-__----_--___---__---__---_-----_ 126

Analyses of basic dikes-________________________________________ 130

Pyroclastic deposits____-_____-___-_________--________-__-_---__ 136

Some petrogenetic problems____-__-____---_----__---_---__--------__ 139

Tuflfs_ .- _._- --_- _-_.-_- .- --.._--__ ----------- 139

Limburgite _____-_______________--_______-._____--_-_-____--_-_ 141

Olivine nodules_____--________-__-_________________-_--___--___ 144

Petrochemistry___ _ ________________________________________________ 146

Analyses of the rocks__ ________________________________________ 146

Origin of the alkalic rock suite. _________________________________ 150

Geologic analysis._________-_______.----__---__--___-__-_--_-_-__._ 152

Crustal movements ____________________________________________ 153

Glaciation _________-________-___-.-_-_____,____-_-__---___-_--_ 153

Literature cited.__________________________________________________ 155

Index.-__._-__.._________________________________________________ 159

ill
IV CONTENTS

ILLUSTRATIONS

[Plates in pocket]

PLATE 20. Geologic map of St. Paul and St. George Islands, Pribilof Islands.

21. Map of Bering Sea region showing distribution of Tertiary and 

Recent volcanic rocks.

Page 

FIGURE 17. Section one-third mile south of the village, St. Paul Island___ 104

18. View from North Hill looking eastward, St. Paul Island_____ 105

19. Lake Hill, a tuff ring 280 feet high, St. Paul Island.______ 107

20. Crater Hill, St. Paul Island.-----_____-_____________ 108

21. Black Bluffs, east of the village, St. Paul Island.___________ 109

22. View of western tip of Otter Island. _______ _______________ 110

23. Section of Otter Island_-_---_-_-__--_-_______________. Ill

24. Vertical section through upper part of the peridotite northeast of Garden Cove_-___-_-_-_-_-___-_-_____--_______ 112

25. Sea cliffs north of Garden Cove__ ________________________ 113

26. Section two-thirds mile south of Tolstoi Point, St. George

Island. __________-_--_______-_-_--__-_-____-_________ 114

27. Tolstoi Point, St. George Island__-----__________-_____ 116

28. Minerals in lava__-_---__---___-_--__-__-_____________" 120

29. Map of aplite intrusion, north of Garden Cove, St. George

Island______-------__---_____--_____-____-_____-__ 122

30. Section through Einahnuhto Bluffs, west coast of St. Paul

Island,____________________________________________ 131

31. Section a\, the seashore one-third mile east of the village, St.

George IslaHd__-----_----_--------_-----_-_---_----__ 133

32. Section at the seashore one mile south of Garden Cove, St.

George Mand__----_-------____--_-_--_--_----____--_ 134

33. Pribilof rocks projected on the ternary diagram anorthitenepheline-quartz _____________________________________ 136

34. The tuff conduit at Esogemunga, St. George Island, coinpared with tuff necks from Kimberley, South Africa, and 

Schwaben, Germany._________________________________ 140

35. Diagram illustrating the crystallization of a polynary melt_ _ _ _ 143

36. Diagram showing the variation in alkali and lime with increasing silica for Aleutian lavas and for the Trans Japanese 

alkalic rocks.________________________________________ 149

TABLES

Page

TABLE 1. Comparison of standard cells_______-___--__-__-_--___-_-_ 123

2. Computation of mode, rock sample 13___----------_------ 130

3. Chemical data on tuff, 1 mile east of Suskaralogh__-_----__ 139

4. Limburgites_____--------------_-------_--------------_ 144

5. Analyses of rocks from the Pribilof Islands._____--__-_____ 147

6. Analyses of lava flows from the Pribilof Islands. ___________ 148

7. Analyses of dikes from the Pribilof Islands.______________ 148
INVESTIGATION OF ALASKAN VOLCANOES

GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS,

ALASKA

By TOM F. W. BAKTH

ABSTRACT

The Pribilof Islands, situated between 56°35' 57°II' north latitude and 

Ifi9°35' T7p°24/ west .longitude, are dominantly made up of lava flows and sills 

of basaltic habit (mainly olivine hyalobasanite), with minor amounts of pyroclastic tuffaceous material and glacial sediments. In parts of St. George Island 

there is a basement of periodotite. Basic dikes are common, and a unique 

occurrence of a composite aplite dike was noted from the southeast coast of St. 

George.

No trace of glaciation is seen on the surface of the islands, but the top of the 

basement peridotite, which is practically horizontal, is obviously glaciated and 

fossil shells are cemented directly on this glaciated surface. The fossils range in 

age from Pleistocene to Recent. Glacial sediments, commonly intercalated with 

basaltoid flows and sills occur in many localities on St. George Island, St. Paul 

Island, and Otter Island. The basal conglomerate has boulders of peridotite 

in many places. The sediments are coarse grained, polymict, and many are 

crossbedded; locally, the thickness is at least 325 feet.

By large fissure eruptions the Pribilof area was built up during the late Pleistocene. The area may have maintained a high average elevation in early Pleistocene. But later a general foundering of the area, in combination with fissuring, 

faulting, and outpouring of lava, reduced the area to a low position, oscillating 

around sea level, where it remained during the subsequent volcanic history.

The tuffaceous material contains fragments of peridotite, clear olivine, blackgreen hornblende, and cleavage flakes of feldspar of a rather unusual composition: 

about Or^ Abs2.

The pyroclastic rocks are basaltic and chemically identical to the lavas; but a 

very small amount of a pumiceous glass of rhyolitic composition was found at 

Polovina Hill on St. Paul Island.

All dikes, flows, and sills are undersaturated in silica and correspond to olivine 

basalt-basanite; the average of seven flows shows 5 percent nepheline in the norm. 

One flow is chemically an olivine basalt, but usual in its mineralogic relations, 

possessing olivine and plagioclase as the only crystalline silicates; the groundmass 

is glassy. This rock is called glassy olivine basalt, Pribilof type in this report.

The dikes are petrographically olivine basalts, basanites, limburgites. When 

a glass base is present in dikes or flows, it tends toward a phonolitic composition. 

The glassy rhyolite does not represent a residual liquid derived from the alkalic 

magma of the lava flows.

101
102 INVESTIGATIONS OF ALASKAN VOLCANOES

The alkalic character of the Pribilof magma is assumed to reflect the tectonic 

environment great crustal instability, vertical faulting, and block elevation in 

the hinterland of the orogenic island arc of the Aleutian chain.

INTRODUCTION 

LOCATION AND PHYSICAL GEOGRAPHY

1 The Pribilof Islands are situated in the Bering Sea about 250 

miles north-northwest of Dutch Harbor in Unalaska. The existence 

of these islands seems to have been known to the native Aleuts long 

before the first white men, piloted by the Russian hunter and explorer 

Gavrilo Pribilof, landed in 1786 on the southernmost island and 

named it St. George. In the following year the northern island, which 

was called "Peter and Paul", was descried and visited (Elliott, 1895; 

Grewingk, 1850, p. 185-191; Veniaminov, 1840).

The name "Pribilof Islands" is now generally applied to the whole 

group of islands. In the Aleut language they were called "Amig". 

Other names that have been applied to them arc the "New Islands", 

the "Northern Isles", the "Seal Islands" as well as the Russian 

designations: "Lebedev Ostrova" and "Zubov". The largest island, 

originally named "Peter and Paul" is now usually referred to as St. 

Paul Island. The group consists of five islands for which geographical 

data are given below.

The Pribilof Islands

St. Paul Island.........................................

Geographical position

Latitude

N.

57°10'

50°35' 

57°03' 

57°11' 

57°06'

Longitude 

VV.

170°15'

1G9°35' 

170°24' 

169°56' 

170°18'

Area 

(sq. miles)

44* 

35.5 

.77 

.02 

.003

Highest 

elevation 

(feet,)

GG2. 5 

994

285

Walrus Island and Sea Lion Rock (being translations of the Russian 

Morzhovyy and Sivuchy respectively) are very small, and in stormy 

weather the breakers wash over them.

All these islands are made up of lava flows of basaltic habit and 

alkaline character (basalts, olivine basanite) with minor amounts of 

pyroclastic tuffaceous material and intercalated sedimentary beds. 

Excellent exposures are seen in the high sea cliffs (attaining in St. 

George Island an altitude of 984 feet). The outcrops are particularly 

good where the waves have cleaned and polished the rock surfaces; 

in other places the movement of the seals has made the ground bare 

and, indeed, polished the rocks (Jordan, 1898); in many places in 

the interior, soil and plant cover is so sparse that the hard rocks are 

excellently exposed.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 103 

FIELD WORK AND ACKNOWLEDGMENTS

The expedition to the Pr.ibi.lof Islands took place in July and August 

1948. The planning of the trips inside the Aleutian area, the establishment of the necessary local contacts, and in short, the burden 

of the arrangements for the expedition was borne by F. M. Byers 

to whom I wish to extend my hearty thanks.

In the field I enjoyed the pleasant company and interesting discussions of Mr. G. R. Arnett. Valuable information and kind assistance 

was always freely offered by the officials of the sealing operations, 

directed by the Fish and Wildlife Service; in particular my obligation to Dr. V. B. Scheffer, Branch of Wildlife Research, is gratefully 

acknowledged.

The material collected was in part worked up in the Department of 

Geology, University of Chicago. To the chairman of the department, 

Dr. W. H. Newhouse, I wish to express my gratitude for stimulating 

discussions and for free use of the facilities of the department; to 

Mrs. Ursula Chaisson I am greatly indebted for microscopic determinations of a great number of rocks.

ST. PAUL ISLAND

LAVA FLOWS AND BASIC DIKES

Lava Hows and sills of basaltic habit make up the bulk of St. Paul 

Island. Some individual flows seem to merge into each other, but 

in many places they can be distinguished, and thicknesses from 10 

inches to 7 yards were measured; still thicker flows probably exist. 

Some flows are vesicular, some are dense. In places the top part of a 

flow may be highly vesicular with a scoriaceous surface whereas the 

bottom part is dense. Figure 17 shows a peculiar difference in the 

orientation of the vesicles of two successive lava flows.

On the surface of the island no trace of glaciation is noticeable. 

Some of the lava flows have an even surface and are clothed with a 

thin layer of moss and grass. Others are very sparsely overgrown and 

may have a very uneven surface of lava "waves", spongy scoria, and 

lava foam, as if they were made yesterday. Between Fox Hill and the 

west coast is a rather extensive lava field with a magnificent display of 

miniature volcanic forms: a multitude of hornitoes, chimneys with 

foam and splatters, blowholes, diatremes, and miniature craters. 

The somewhat older lava flows surrounding North Hill are pitted with 

potholes and rutted with furrows, which are now covered by vegetation . In all probability they represent forms analogous to the hornitoes, craters, and other forms of the Fox Hill flow. They suggest 

multiple-vent eruptions arising from the confluence of lava flows 

from a large number of small and closely spaced orifices (aerial eruptions, according to H. Reck). However, the vents feeding the several
104 INVESTIGATIONS OF ALASKAN VOLCANOES

orifices are highly hypothetical; in conformity to observable facts 

and in analogy with the explanation advanced by Thorarinsson (1951) 

for similar formations in the lava flows of Myvatn, Iceland, the miniature chimneys and blowholes of the Fox Hill flow are better explained 

as pseudocraters formed by steam from water trapped in hollows 

under the lava flow.

The sources of the many lava flows are not obvious. In the island 

itself the following observations are pertinent: The central portion 

rises to a height of 587 feet culminating in an old crater on Bogoslof

o <=>

FUTURE 17. -Section one-third mile south of the village, St. Paul island, through two flows, separated by 

a scoriaceous layer. In the top flow the vesicles are alined vertically; in the bottom flow they are arranged 

horizontally. Total thickness, Wi feet.

Hill, out of which lava has flowed, as observed by Stanley-Brown 

(1892). A broad flow, easily discernible, extends eastward from the 

southern top and straight toward Lake Hill. At the southern edge of 

this flow is an extensive fissure trending about east-northeast and 

partly followed by a dike of vesicular, glassy chrysophyric hyalobasanite. Approximately 1% mile south of Bogoslof Hill, at a place called 

Kaminista, additional lava flows of olivine-rich basanites apparently 

originated in a system of fissures trending northeastward and southwestward.

Most of the St. Paul lava flow seem to have issued from fissures, of 

which only traces can now be seen, but the existence of such fissures, 

presumably in large numbers, at the sea floor of the Pribilof mound is 

probable (see p. 153).

PYROCLASTIC DEPOSITS

Bogoslof Hill, a volcanic cone, and a large number of explosion 

craters are prominent landmarks on the island (fig. 18). The explosion
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 105

377(>34 5
106 INVESTIGATIONS OF ALASKAN VOLCANOES

craters (tuff rings) cannot be regarded as potential sources of any 

extensive lava flow, for their products, in addition to the original vapor 

and gases, are pyroclastic material of various kinds now conspicuous in 

the crater ruins or deposited on the slopes of the cones and on the surrounding lava fields. (See figs. 19 and 20.)

Crater Hill (515 feet high) as an explosion crater whose semicircular 

southern wall is perhaps a ring dike; the other walls are of pyroclastic 

rocks. All the other craters, as marked on the map, are made up of 

pyroclastic and tuffaceous material and no traces of lava produced by 

these craters can be seen.

PLEISTOCENE SEDIMENTS

Pleistocene sediments occupy an intercalary position between the 

lava flows and are exposed in vertical sections along the steep sea 

cliffs. But, as they rarely appear on horizontal surfaces, they cannot 

be shown on the map. The most conspicuous deposit is at the extreme 

southeastern shore, straight east of the village on St. Paul Island, in a 

placed called Black Bluffs (see fig. 21), which contains water-laid 

poorly consolidated agglomerates, probably dumped by ice because of 

the polymict character man-sized boulders mixed with pebbles, sand, 

and even clayey particles. Crossbedding is common, and in places 

conspicuous. Most of the material is derived from basaltic rocks. 

The graveyard of the village is on top of these beds.

In many other places in the sea cliffs intercalated sedimentary beds 

may be seen, for example between Village Cove and Tolstoi Point, 

at Zapadni Point, along the precipitous west coast (Rush Hill, 655 feet 

high, and Einahnuhto Bluffs).

The sediments contain fossils that range in age from Pleistocene to 

Recent. A list of fossils from both St. Paul Island and St. George 

Island is given on page 118.

ALLUVIAL SEDIMENTS

Recently formed alluvial sediments, mostly sand, cover about oneseventh of the area of the island. In most places the sand is loose and 

moves with the wind. Along the north shore, particularly west of 

Big Lake, impressive dunes are formed.

OTTER ISLAND

The Russian name for Otter Island is Bobroviy Ostrov. The island 

is situated 5 nautical miles south of St. Paul Island. Its western tip 

(fig. 22) rises to a height of 285 feet, displaying a continuous section 

of sedimentary layers dipping about 30° SW (fig. 23). The true 

thickness of the sediments here is at least 325 feet, indicating that a 

rather high land mass must have existed in the vicinity.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 107

f!

 a
108 INVESTIGATIONS OF ALASKAN VOLCANOES
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 109
110 INVESTIGATIONS OF ALASKAN VOLCANOES

FIGURE 22. View of western tip of Otter Island, looking south.

Most of the island is made up of lava flows of hyalobasanitic composition. At the eastern tip the flows were disrupted by a pyroclastic 

explosion.

ST. GEORGE ISLAND

PEBIDOTITE

Peridotite is perhaps the most interesting rock in St. George Island 

(pi. 20). Its existence was noted by Stanley-Brown (1892) who says 

(erroneously) that it dips northeastward at about 45°. It is exposed 

in sections in the sea cliffs from Cascade Point in the south to within 

3,300 feet of Tolstoi Point in the northwest. It is overlain by sediments and by several lava sills and flows. The upper surface of the 

peridotite is worthy of special description. It is obviously a surface
FIGURE 23. Section of Otter Island. From east to west are pyroclastic rocks
112 INVESTIGATIONS OF ALASKAN VOLCANOES

eroded by glaciation; it is nicely polished by the ice, and furrows and 

striae are conspicuous in places. In other places a brownish-black 

weathering varnish can be seen, and weathered boulders of brecciated 

peridotite are truncated by the surface (fig. 24). The surface is 

almost exactly horizontal and in vertical section the peridotite may 

easily be mistaken for a flow. Over this horizontal surface the 

younger lavas have flowed, but in many places they are separated 

from the peridotite by a sedimentary layer (figs. 25, 26).

Serpentine Talc 7

1 Meter

FIGURE 24. Vertical section through upper part of the pcridotite, northeast of Garden Cove, showing 

weathered boulders, with serpentinizcd olivine in the joints, in a talc matrix. (See p. 119.)

The best exposures are at Garden Cove. The erosion surface of 

the peridotite there is about 30 feet above sea level. The peridotite 

itself is massive; on large clean surfaces, polished by the sea, no 

structure can be seen. The joints, in places very conspicuous, have 

no system except that they do not seem to be flat lying. All dip 

angles are steeper than 45°.

APLITE

An aplite granite intrudes the peridotite just south of Sea Lion 

Point. Elliott (1887) called it a "large dike of bluish or greenish-grey 

phonolite, in which numerous small crystals of spinel [sic] are found". 

But Stanle}r-Brown, who looked for this occurrence was unable to 

rediscover it. It represents a dike or neck, approximately 325 feet 

wide with fine-grained chilled margins and a coarse-grained core, the 

upper portion being truncated by the erosion surface of the peridotite. 

The aplite is, therefore, younger than the peridotite and older than 

the overlying lava flows and sedimentary layers.

LAVA FLOWS AND BASIC DIKES

Lava flows of basaltic habit make up the bulk of the island. The 

flows are analogous to those found in St. Paul Isla.nd. Many flows 

show the similar differences in vesiculation from top to bottom as
O 

K 

O 

f 

O OKj

FIGURE 25. Sea cliffs north of Garden Cove showing flows, sill and intercalated sediments (.4) and overlying peridotite basement (B). Photograph by U. S. Navy.

O tr1O OW

K

O

CO

IO COen W
114 INVESTIGATIONS OF ALASKAN VOLCANOES

,B o

o> o

O p,

O 1=1
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 115

those described on p. 104. Thus a flow extending from Tolstoi Point 

and half a mile southwestward along the coast has a dense, granular 

hyalobasanitic bottom part (pyroxene, plagioclase, and glass, with 

only 2 percent olivine) merging upward into a holocrystalline, highly 

vesicular chrysophyric basalt containing at least 20 percent strongly 

corroded olivine phenocrysts (see fig. 28E}. Both in mineralogy 

and texture the top and bottom part could scarcely be more dissimilar. In figure 26 this flow is marked as 23.

At Tolstoi Point (fig. 27) it seems that seven flows, all horizontal, 

make up the whole cliff; that makes an average flow thickness of 

more than 25 feet (cliff is 200 feet high). But in other places most 

flows are thinner; they are usually separated by scoriaceous layers. 

North Sea Lion Hill and South Sea Lion Hill are exogenous lava 

domes. Ulakiya Hill is partly lava, partly loose, pyroclastic material. 

Similarly constructed are also Maynard Hill and Gull Hill. There 

is no doubt that these rather high hills (Ulakiya Hill reaches an altitude of 932 feet) represent loci of lava outpour. But lava flows have 

also issued from fissures. A vertical fault is seen in the sea cliff 

about two-thirds mile south of Tolstoi Point, trending straight westward for about 2% miles, then turning toward southwest. It is, at 

least in parts, filled by a semicrystalline porphyritic dike of olivine 

hyalobasanite; lava flows have issued from it around Ulakiya. Another 

fault, trending west-southwestward, forms an impressive scarp (called 

Esogemunga) across the western tip of the island. The south wall 

represents an elevated ridge, Samlalogh Ridge, rising higher in the 

east than in the west in relation to the north wall, the maximum 

displacement being 490 feet. West of this fault line, the entire western 

tip of the island is mainly pyroclastic material. Fox Castle is simply 

a volcanic neck that penetrated the pyroclastic deposits. It is 

believed that most of the lava flows of this island, as was the case in 

St. Paul Island, came from fissures that are now under the sea.

PYROCLASTIC DEPOSITS

Pyroclastic materials have a wide distribution in the island. All 

the volcanic cones evidently showed explosive activity with lava 

flows alternating with pyroclastic cjectementa. In addition large 

areas are covered with very thick pyroclastic deposits.

Tuffaceous materials in some places are mixed with other pyroclastic deposits. Along the north shore between Myak and Suskaralogh Point, and just west of the Staraya Artil Rookery, tuffaceous 

deposits attain a minimum thickness of 836 feet. At Myak the island 

plateau plunges precipitously into the sea, forming a shear cliff, 820 

feet high, made of scoria, lava foam, and pother pyroclastic materials, tuff, and sediments, intercalated by flat-lying sills of basalt.
116 INVESTIGATIONS OF ALASKAN VOLCANOES
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 117

Somewhat farther west' the tuffaceous mass has been dissected by 

wave erosion and shaped into a most impressive landscape of deep 

vertical gullies and high weird pinnacles, all composed of loose 

whitish or yellow tuffs, with numerous small brown dots that may 

represent altered pieces of basalt. The mineral composition has not 

been completely determined. Close to Staraya Artil Rookery the 

tuff contains fragments of peridotite and odd fragments of large 

mineral crystals: clear green, divine, large cleavage flakes (2 cm long) 

of black-green hornblende, and of milky-white feldspar. Only in 

one place (top of deposit west of Myak) was it possible to observe 

any bedding (the dip is about 45° S.). In all other places the tuff 

is not bedded.

FOSSILIFEROUS SEDIMENTS

Fossiliferous sediments crop out in sections in the sea cliffs at 

several places: Right at the village shores is a sedimentary layer, 10 

feet thick, the bottom of which is at high tide mark. It extends eastward for about \% miles then wedges out. It is composed of sand and 

yellow tuff with rounded pebbles of basalt (fig. 30). Another thin 

sedimentary layer, about 20 inches thick, follows the sea cliffs west 

of Zapadni Bay. An almost-continuous section is exposed over a 

distance of about 5 miles along the east coast. The character of the 

sediment differs considerably and changes abruptly from place to 

place: clay, boulder clay, sand, till, tuff, pyroclastic deposits. Figure 

26 shows the section at the sea cliff, two-thirds of a mile south of 

Tolstoi Point.

The base is of peridotite showing no change from bottom to.top. 

The upper surface of the peridotite is an erosion surface. It is 

horizontal, which, at the first glance, makes the peridotite look like a 

flow. The surface has ice striae and appears polished and coated 

with a brownish-black weathering varnish; weathered boulders are 

truncated by this surface (fig. 24).

The basal conglomerate on top of the peridotite has boulders of 

peridotite in places, but contains predominantly rounded pebbles of 

basalt. Fossil shells are cemented to the glaciated surface. The 

conglomerate is well consolidated and about 8 inches thick. It 

is overlain by a layer of sand, about 6% feet thick, rather unconsolidated, containing casts of fossil shells which can be seen on freshly 

opened surfaces, but which disappear as phantoms when touched: 

all CaC03 is gone. It is overlain by alternating beds of clay, sand, and 

pyroclastic deposits, each bed from 2 to 9 inches thick; the total 

thickness is about 13 feet. On top of the sedimentary secjuence is a 

columnar dense basalt flow.

Toward the north the section is terminated by an olivine basanite 

dike about 300 feet wide. On the north side of the dike both perido-
118 INVESTIGATIONS OF ALASKAN VOLCANOES

tite and sediments have disappeared; there is nothing but lava flows 

(fig. 26). This dike seems to follow a fault which according to the 

topography may extend inland toward Zapadni Bay. (See pi. 20.)

A remarkable sediment is deposited two-thirds of a mile south of 

Garden Cove, directly on the glaciated surface of the peridotite. 

Almost unconsolidatecl, it is made up of well-rounded pebbles, 

ranging from 2 inches to 6 feet in diameter, inbedded in sand (fluvioglacial moraine?). Farther to the southwest the moraine gives place 

to other types of sediment: clay and sand, usually crossbedded. 

Southward, these sediments become very thick, and are intercalated 

with basaltic flows. In places they are highly metamorphosed. 

Still farther south two distinct beds of sediment, each about 60 feet 

thick, separated, by a thick basalt flow or sill, can be seen in the 

bluffs. The total height of the cliff is approximately 200 feet. At 

the top of the lower sediment bed, the overflowing lava has picked 

up and incorporated big chunks of sediment (fig. 30).

Finally it should be emphasized that my investigation of these 

sediments has been strictly of a reconnaissance nature. The short time 

spent on the islands was mostly used in obtaining petrographic and 

volcanologic data. Much more work should be done in sedimentology 

and paleontology; careful studies of the various profiles are necessary, 

and an identification of the source material of the various sedimentaiy 

beds should be attempted.

Data from such studies may become of great importance to the 

study of Pleistocene geology. They may unravel details of glaciation 

and land elevation, in particular they may serve to correlate in space 

and time the lava outpours with epirogenic movements or changes in

sea level.

FOSSILS

Fossil shells from the Pribilof Islands were collected by Ilia 

Wossnessensky more than one hundred years ago (Grewingk, 1850) 

and by Elliott in 1872-74 (1875, 1887, 1895). Later investigators 

include Stanley-Brown (1892), Dawson (1894), Ball (1892, 1896, 

1899, 1919), Gilmore (1908) and Hanna (1919).

A few shells collected during the summer of 1948 were studied by 

Dr. Charles E. Weaver who decided that the fossils were probably of 

Pleistocene age. Subsequently, my collection was compared with 

the material from the region in the Alaskan collection of the Geological Survey. Mr. F. S. MacNeil prepared the following report:

St. Paul:

Black Bluffs, east of the village. 

Pelecypoda:

Cardita (Cyclocardia) sp., a completely decorticated specimen. 

Astarte sp. cf. A. rollandi Bernardi
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 119

St. Paul Continued

Einahnuhto Bluffs, west coast under Rush Hill. 

Gastropoda: Colus sp. 

Pelecypoda:

Astarte sp. cf. A. rollandi Bernardi

Panomya! fragments of a heavy large pelecypod with an irregular

surface. 

St. George:

East coast at Sea Lion Point.

Gastropoda: Natica sp. cf. (Tectonatica) clausa Broderip and Sowerby 

internal molds. 

Pelecypoda:

Laevicardium (Cerasloderma) sp. cf. L. californiense (Deshayes),

internal molds.

Serripes sp. cf. S. gronlandicus (Bruguiere), internal molds. 

Mya sp. cf. M. arenaria Linnaeus var.? internal mold. 

Saxicava sp. cf. S. arctica Linnaeus, internal mold. 

East coast at Garden Cove. 

Gastropoda:

Colus sp., internal mold.

Tricholropis bicarinata Sowerby, internal mold. 

Pelecypoda:

Astarte? sp., poor internal mold.

Cardila (Cyclocardia) sp. cf. C. crebricosluia (Krause), fragments.

Laevicardium (Cerasloderma) sp. cf. L. californiense (Deshayes),

internal molds and fragments.

Serripessp. cf. S. gronlandicus (Bruguiere), internal and external molds. 

Macoma sp. cf. M. calcarera (Gmelin) 

Mya sp. cf. M. arenaria Linnaeus 

Saxicava sp. cf. S. arctica Linnaeus, internal molds.

All collections are roughly contemporaneous and probably of early Pleistocene 

or, at the earliest, late Pliocene age. The Hanna collection (1918) from near 

Tolstoi Point, St. George Island, is from rock like that at Sea Lion Point and 

Garden Cove and contains several forms which are believed to be extinct.

Paleon tological evidence of a more uncertain character is connected 

with scattered remains of mammoth in the Pribilof Islands. It is 

said that in 1836 a tooth was found on St. George Island (Dall, 1892, 

1896) and a tusk in the sands of Northeast Point on St. Paul Island. 

Dawson (1894) concluded that this indicates a former connection 

with the mainland. In 1897, two teeth of a mammoth and. bones of 

a bear, apparently distinct from the polar bear were obtained from 

a lava cave at crater on Bogoslof Hill, St. Paul Island (Maddren, 

1915). Preble and McAtee (1923) think that the remains were 

transported to the island by ice and that their occurrence has.no 

special geologic significance.

PETROGBAPHY 

PERIDOTITE

The distribution of peridotite, shown on plate 20, commands special interest in that it seems rather unrelated to any other rock type
120 INVESTIGATIONS OF ALASKAN VOLCANOES

in the area. It is massive; the joints, which in places are very conspicuous, have no system. The original rock was made up of anhedral, randomly oriented crystals of olivine; now the crystals have 

separated along cleavage planes and grain boundaries where they are 

altered to serpentine (fig. 24) with a very small amount of magnetite 

(fig. 28A). Serpentine occurs partly in irregular bands dissecting 

the rock in all directions, partly as fine veinlets growing into the 

individual olivine crystals. Olivine shows /?= 1.665, 7=1.687, corresponding to Fo9 2, Fa8 ; the serpentine is magnesian without any 

iron.

Chemically and mineralogically (see following table) this rock 

seems unrelated to any other rock type in the Pribilof area. Gen0.2

.010

0.2

D

FIGURE 28. Minerals iu lavas. A, Scrpeutinizod olivino from peridotitc, Garden Cove, rock sample 1- 

Olivine, serpentine, and a small amount of magnetite. B, Olivine crystal in a state of resorption from 

plagioclase-hematite rook sample IS, top of road between village and Zapadni Bay, St. George Island. 

Olivine, plagioclase and hematite. C, Olivine phenocrysts and plagioclase laths illustrating preferred 

orientation in feldspathic basanite from rock sample 9 northeast shore of Otter Island. D, Hollow phenocrysts of olivine, plagioclase laths, and black pigments of magnetite in glass groundmass. Basanite rock 

sample 17 intrusive into Hush Hill, St. Paul Island. F., Resorbcd olivine phenocryst in fluidal glass 

ground-mass). Basanite flow, Tolstoi Point, St. Paul Island. Scales injnillimctcrs.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 121

erally, dunites are found in the folded mountains. Here, on the concave side of the Aleutian arc, the peridotite may represent an upwelling 

of the substratum. Olivine nodules found in basanites on both St. 

George Island and St. Paul Island (p. 144) may represent fragments 

of this substratum.

Chemical and mineralogical composition of peridotite, Garden Cove, St. George Island

[Analyst, 13. Brumi|

Si02 ------------ 39. 83 Norm ' (without water)

TiOa------------ tr. Ab_.___._______ 1,2 Serpentine.----. 70

Al2 03-_-------_-- 1.54 An.------------ .6 01ivine____--___ 26

FeaOs----------- 3.70 C_.__.._._-----_- 1.1 Magnetite.-.--- 3

FeO ---------- 3.77 Hy_______-__-__ 23.5

MnO__-___.-__- .12 OL------------- 70.0

MgO___-_-______ 41.95 Mt_..--_-------- 3.7

CaO ---------- . 10 

Na_0_ ---------- . 12 100. 1

KoO-_---_--_--- .0.1.

Pa06 _------ -.-- .04

H2O+--_----_-_ 8. 34

H2 0 ---------- . 62

COa ---------- tr.

100. 14

1 Throughout this paper the molecular norm and mode are used, correspondong to the "aquivalenten 

Molekularprozente" of Niggli (1936).

APLITE

Aplite is another rock that seems rather unrelated to any other 

rock type in the area. The mode of occurrence of this stock is shown 

in figure 29. It is intrusive into the peridotite and composite in 

character, has very fine-grained chilled margins, and a mediumgrained, granular central portion. There is an appreciable difference 

in composition between the rock of the chilled margin and the central 

portion rock. The transition is gradual but seems to take place over 

a small distance (less than one yard); the width of the fine-grained 

margin is about 30 feet, the width of the total intrusion about 1,300 

feet. In table 5, analysis 3 and 4, the chemical composition, norm, 

and mode of the two portions of the dike are listed.

The central portion is a holocrystalline, granular rock essentially 

consisting of plagioclase (strongly zoned oligoclase n^ 1.543), in tabular subhedral crystals forming a sort of blocky mesh, with clear 

quartz filling the interstices between feldspars. Fresh red garnet 

is present in very small amounts (?i=1.790). Sericite has replaced 

a good deal of the plagioclase, in many cases beginning with the inner 

zone leaving the rim relatively fresh. Chlorite occurs in elongate, 

unoriented shreds and in scaly masses between feldspars.

377634 50 .
122 INVESTIGATIONS OF ALASKAN VOLCANOES

y-Aplite with chilled margins

Peridotite

A

Moraine -p. a- ->: ^.' ;°;°^^---r-z-z^:-£:-:L-^.-JE^j- Clay

500i_, 1500 Feet

Contour interval 20 feet

Datum is sea level

FIGURE 29. Map of aplite intrusion, north of Garden Cove, St. George Island.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 123

The marginal portions are made up of a very fine grained, almost 

dense rock with small, poorly defined phenocrysts of an albite-rich, 

nonperthitic alkali feldspar partly altered to sericite and quartz. 

The groundmass consists essentially of countless small, needlelike 

laths of albite-rich feldspar displaying a very definite subparallel 

orientation; they are altered and intergrow.ii with quartz, sericite, 

chlorite, and a nameless yellowish powdery product probably iron 

staining. A very small amount of garnet is present.

A discussion of the mode of formation of this composite dike has 

been given in a previous paper in which the author (Barth, 1952) 

concluded that a difference in composition between the outer and 

inner portions of the dike could best be explained by a process of 

thermal diffusion as defined by Wahl (1946).

If the original magma was homogeneous the necessary diffusion to 

produce the two fractions is shown in table 1. The figures demonstrate that the necessary migration is, indeed, on a modest scale: 

in a reference volume containing more than 2,600 atoms a net diffusion of only 7 atoms each wa}^ suffices to bring about the observed 

compositional differences between the chilled-margin rock and the 

central rock.

The calculated composition of the original homogeneous magma 

as given in column 3 of table 1 corresponds to 1 part of the chilledmargin phase and 10 parts of the central-portion phase. It is not 

possible to determine the volume relations with certainty. The intrusion is about 1,300 feet wide, the marginal parts are about 33 feet 

wide. If the intrusion is circular the proportion of margin to core 

is 1: 10, but less if the intrusion has the shape of a plate. However, 

the accurate figure is of no consequence for the following discussion, 

and is therefore, somewhat arbitrarily, taken as 1: 10.

TABLE 1. Comparison of standard cells

Si.... ....... .............

TL._...........__...._._

Al._. __--.-_. _____...._

Fe '....-.- ...............

Mg......................

Ca._ ............ .........

1

02.0

15.8

1.3

.0

2

50.0

18.4

3.2

2.0

3

57.1

.1

18.2

3.0

1.2

2.5

Na. .....................

K...... ..................

P

H.............. ..........

0........ ................

1

8.0

3.5

.1

7.2

100. 0

2

7.1

2.7

.4

11.2

3

7.3

2.8

.4

10.8

100. 0

i Ferrous iron, ferric iron, and manganese arc added together.

1. Chilled-margin rock; recalculated from column 4, table 5.

2. Central-portion rock; recalculated from column 3, table 5.

3. Calculated composition of the original homogeneous magma.
124 INVESTIGATIONS OF ALASKAN VOLCANOES

If calculation of the cations is referred to a unit of 1,600 oxygens (a 

unit of 10 standard cells) the following cation migration must have 

taken place into the marginal part (round numbers):

5 Si

1 Na

1 K

7 cations

and into the central part:

2 Al 

2 F

1 Mg

2 Ca 

4 H

7 cations+ 4. protons

Still unsolved is the problem of the relation of the original dike 

magma to the other rock magmas present in the Pribilof area. The 

dike is older than the lava flows and cannot therefore be derived from 

the lava. Nor can its chemical composition, according to the present 

theories, be derived from a magma similar to that of the Pribilof 

lava. A possible connection between this dike and the glassy material found at the Polovina Hill, St. Paul Island, is discussed on 

page 137.

LAVA

GENERAL FEATURES

Basanite is basaltic lava containing an appreciable quantity of 

nepheline. It is rather common in these rocks that nepheline and 

(or) plagioclase are not expressed as actual minerals but are hidden in 

the glass groundmass; following the usage of Washington (1930) we 

shall distinguish rocks with hidden nepheline in the glass as hyalobasanites. Lacroix (1928) has used the special names, basanitoid if 

nepheline is hidden, mandchourite if plagioclase is hidden, and limburgite if both plagioclase and nepheline are absent mineralogically. 

Pribilof type is here used if augite is hidden in the glass groundmass, 

whereas olivine and plagioclase represent the only minerals actually 

present in the rock.

The lava flows that cover the largest parts of the Pribilof area 

issued from fissures and are alkali subbasaltic in habit and composition. The four main types of lava show gradual transitions into each 

other.

1. Chrysophyric basalts: olivine and subordinate amounts of augite 

as phenocrysts, groundmass of olivine, augite, plagioclase, and magnetite. Very small amounts of glass are usually present.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 125

2. Aphyric, well crystallized olivine basalts: olivine, augite, plagioclase, and magnetite.

3. Chrysophyric hyalobasanites: olivine and augite in subordinate 

amounts as phenocrysts, groundmass of olivine, augite, plagioclase, 

magnetite, and glass. This type is dominant on the islands.

4. Aphyric, olivine hyalobasanites: plagioclase, olivine, augite, and 

magnetite of fine grains in a glassy groundmass.

On St. Paul Islands all flows and sills examined are olivino hyalobasanite. The majority are porphyritic with much olivine and little 

augite as phenocrysts and a very fine grained groundmass of augite, 

plagioclase, olivine, magnetite, and glass. They contain about 10 

percent normative nepheline. Another large group of lavas is represented by specimens in which olivine likewise shows a tendency to 

porphyroid texture, but the groundmass is so coarse as to obscure the 

porphyritic structure, that is, the grains of olivine, augite, and plagioclase are nearly equal in size. Only a little glass is present, and only 

small amounts of nepheline appear in the norm. A big lava flow 

between Bogoslof Hill and Lake Hill contains plagioclase as well as 

olivine and augite as phenocrysts.

On St. George Island again the majority of the lava flows and sills 

are olivine hyalobasanites containing predominant olivine and more 

sparsely augite phenocrysts in a groundmass of augite, plagioclase, 

olivine, magnetite, and glass. Normative nepheline attains 15 percent. Another group consists of aphyric, rather coarse-grained lavas 

in which the three chief minerals, olivine, augite, plagioclase, assume 

nearly equal dimensions, although the olivine still has a tendency to 

appear as phenocrysts. Glass is sparsely present, and the amount of 

normative nepheline is small. All these rocks are rather ferric, and 

have a color index higher than 50. One flow (top of the ridge northwest of Garden Cove) contains both plagioclase and olivine as phenocrysts, the groundmass is of the usual kind, and the rock is still rather 

mafic. One aphyric highly feldspathic, very fine grained flow (from 

Esogemunga) contains small grains of olivine and magnetite but no 

augite, in addition to plagioclase. Chemically it does not differ much 

from an ordinary olivine basalt. A related lava flow was observed at 

the northeast shore of Otter Island. Finally there is one flow of a 

rather ordinary olivine-free basalt on Gull Hill. It shows ophitic 

structure with plagioclase laths and grains of a pigeonitic augite.

MINERAL, COMPONENTS

Olivine crystallized first in most magmas, forming conspicuous 

phenocrysts, in many places several millimeters long. The range of 

composition, determined by measuring the axial angle and the indices 

of refraction, is rather small: Fo9oFai0 to Fo75Fa25. Zoning is common.
126 INVESTIGATIONS OF ALASKAN VOLCANOES

In some rocks, olivine is thoroughly stained with red or "rusty" 

material, and in some fumed rocks, for example, sample 18, hematite 

has grown from olivine, which has become almost pure forsterite. 

Olivine phenocrysts are usually euhedral but show signs of strong 

resorption, pitting, and parting along cleavage planes. But conversion to pyroxene does not seem to take place. Probably olivine'has 

no reaction relation to the pyroxene of the present magma. Peculiar 

"hollow" crystals have a tendency to form in many rocks (see fig. 28).

Pyroxene occurs in the groundmass of all lavas. Phenocrysts likewise occur, but in most rocks they are completely outnumbered by 

olivine phenocrysts. The composition corresponds to a titaniferous 

augite. The crystals in thin section are light-reddish brown with 

absorption alpha greater than gamma. The optical axes are highly 

dispersed, r^>v, the angle being about (+) 2 V=QO°, but may drop to 

50°, extinction varies somewhat but is close to 7Ac^45°, an average 

index of refraction is 0=1.70. Zoning is conspicuous in most augites, 

hourglass structure is common. Pigeonitic pyroxenes have not been 

observed; the course of crystallization is from diopsidic to hedenbergitic, that is, all pyroxenes are calcic and therefore show no reaction 

relation to olivine. During most of the crystallization period, olivine 

and pyroxene separated together.

Plagioclase is rarely observed as phenocrysts. All over the Pribilof 

area the lava flows apparently were so mafic that olivine and part of 

the augite separated before any feldspar could crystallize. In the 

groundmass of most lava flows, plagioclase is present. It is always 

zoned, with composition ranging from An80 to An35 . More sodic 

plagioclase has not been observed.

Glass is present in many lava flows; it is colorless in thin sections. 

The refractive index is close to 1.54. Most lava is rather low in 

silica and high in alkalies, leading to the presence in the norm of an 

appreciable amount of nepheline and orthoclase; neither mineral is 

present in the glass base.

ANALYSES OF LAVAS

Sample 6. Columnar, dense flow of chrysophyric hyalobasanite, 

two-thirds of a mile south of Tolstoi Point, St. George Island (fig. 26). 

The phenocrysts are fresh, euhedral olivine crystals. The groundmass 

consists of a brownish glass in a state of devitrification containing 

small grains of olivine, augite, plagioclase, and magnetite. Many 

secondary growths of chalcedony and zeolite occur throughout the 

glass.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 127

Mode Composition of the (/lass 

Olivine-________-_____-_ 25 Or_______________________} fi

Ores._-_---_-.____________

Apatite. ____________________

Plagioclase (An 70 )____________

Glass.______________________

Chalcedony, zeolites and alteration products.____________

100

Sample 7. Large pebble of hyalobasanite from St. Paul Island, 

collected by Mr. Ranch and analyzed by M. G. Keyes (1930): Phenocrysts are yellow olivine (Fa20) making up about 5 percent of the rock 

with an occasional phenocryst of black augite. The groundmass consists 

of thin laths of labradorite, irregular grains of brownish augite, and 

small rounded grains and some skeletons of ore. Considerable colorless 

glass is present which has a refractive index of n= 1.53-1.54.

Sample 8. Dense flow of chrysophyric hyalobasanite, cropping out 

over a distance of several miles along the west shore (and under 

Einahnuhto Bluffs), St. Paul Island. The rock is gray with finely 

crystalline groundmass studded with clear, yellow phenocrysts of 

olivine from 1 mm to 10 mm across, some zoned, with a refractive 

index 0^1.690, 2 y^90°, corresponding to about Fa20 . Very few 

phenocrysts are clinopyroxene; they are always zoned and usually 

pitted and interg.rown with minerals of the groundmass. Positive 2 V 

= 60°; 7Ac=44°, 0=1.700, corresponding to He40 . The glassy 

groundmass includes small grains of olivine and pyroxene. Small 

plagioclase laths form a mesh; indices of refraction lie in the range 

1.555-1.565, indicating about An55 . The glass shows incipient 

crystallization, index 71=1.53-1.54.

Mode Composition of the glass

1 Phenocrysts ----------------i ,_ ^

i .. 

__--_.__ __________ 4 J Ab

Olivine________ ___________ 5

Augite_______ ____________ 19

Ores, apatite______________ 4

Plagioclase (Aii50)____ ______ 23

-----_-___________ 20

Au_ ______________ .3.5

Groundinass Ne________________ 8. 0

Hy______ _________ 1.5

20. 0
128 INVESTIGATIONS OF ALASKAN VOLCANOES

Sample 9. Holocrystalline flow of aphyric basanite (pacificite 

defined as "feldspathic basalts, without nepheline and usually with 

olivine the analyses of which show the presence of ... nepheline 

in the norm, although none of this appears in the rocks" by Barth in 

1931, p. 401-402), northeast shore of Otter Island. The rock is very 

fine grained; rare microphenocrysts of plagioclase and pyroxene are 

seen. The groundmass shows sharp, small euhedral crystals of 

olivine, and very small crystals of pyroxene. The plagioclase laths 

average 0.02X0.0015 mm and show a vague preferred orientation. 

(See fig. 28.)

Mode

Olivine___ ______

Diopsidic augite. 

Ores, apatite___- 

Zoned feldspars.

21

10

5

64

Or, 

Ab.

Composition of the feldspars

100

9. 5

26. 0

23. 5

5.0

64. 0

The observed range of the feldspars is from An55 to An35 . Average 

composition is An40 . Nepheline could not be identified with the 

microscope.

Sample 10. Dense flow of phyric hyalobasanite, southeast point of 

Otter Island. Olivine augite, and plagioclase occur as phenocrysts. 

Augite is commonly twinned, 7Ac=43°, (+) 2F=50° 55°, r>_> 

distinct. The plagioclase crystals are also twinned, extinction angles 

correspond to An60 . In the groundmass the plagioclase ranges from 

An60 to An35 .

Mode 

Olivine. _________

Augite_ ___---_-_

Plagioclase (Aiieo)- 

Olivine___ _______

Augite_ _________

Ore, apatite______

Plagioclase (Atu.). 

Glass_.._.- ---

Phenocrysts

.Groundmass

Composition of the glass 

°t........... 6

An_ 

Ne_ 

Hy

18

Sample 11. Vesicular gray flow of olivine basalt, pacificite, northeast 

point of St. Paul Island. The rock is well crystallized with olivine 

and augite ranging in grain size from 0.1 to 1 mm, the lengths of 

the plagioclase laths range from one-third to 3 mm. Nepheline could 

not be identified with the microscope.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 129

Mode Calculated composition of the feldspars

01ivine_____-_-__-____------_ 15 Or_____________________ 6.5

Augite._______.---___---..__ 21 Ab-_-_--__-------.._-._- 28.0

Ore, apatite_______-___--_-_ 6 An_____.________________ 21.0

Zoned feldspar (ranging from Ne_ _____________________ 2.5

ANeo to An 30)-------------- 58 

 58.0

100

Sample 12. Vesicular, columnar flow or sill or aphyric olivine 

basalt, between Zapadni Bay and Red Bluffs, St. George Island. The 

rock consists of large, randomly oriented laths of plagioclase forming 

an irregular meshwork filled with olivine, augite, apatite, and magnetite, the latter being in large measure a reaction product of olivine; 

indeed, the olivine has the properties of pure forsterite, indicating that 

the fayalite component has been used up in the reaction.

Mode 

Olivine (Fom)___----------------------.-_----------_------------_-_ 9

Augite ___________________________________________________________ 27

Ore, apatite, and decomposition products of olivine____________________ 12

Plagioclase (An_e) _ - - --------------------------------------------- 52

100

Sample 13. Glassy olivine basalt, Pribilof type, a vesicular gray 

flow or sill of a glassy basaltoid rock from Esogemunga escarpment, 

St. George Island. The only crystalline silicates are olivine in small 

grains measuring about 0.05 mm across, and plagioclase in laths 

about 0.07 x 0.01 mm. They lie in a glassy, inhomogeneous matrix 

containing ore and apatite in distinct crystals, and various unidentified 

products of incipient crystallization. Such lavas are unusual, but a 

very similar variety has been described by Powers (1932, p. 275, 286, 

290). The Pribilof type basaltoid lava flows have olivine, plagioclase, 

and glass as the only essential phases.

Mode Cow/position of the glass

Olivine Fa 24---_------------ 21 Percent

Plagioclase (An45)___________ 30 Q._____--____-_____ 1.8 5

Ore, apatite, etc.:___________ 8 Or___---------------- 4. 0

Glass.--------.----------- 35 Ab----_---_--------- 12.0

 An_.._--.-__.- ------ 7.0 20

100 Di__-----_-_--_------ 10.8 30

35. 6 100

In table 2 the cation percentages of the rock are listed to show the 

method by which the mode has been computed.
130 INVESTIGATIONS OF ALASKAN VOLCANOES

TABLE 2. Computation of mode, rock sample 13

Computed mode

Modal mineral

Total-.. .. . ..

Percent

21.3

4.3 

2.4 

30.5

35.5

100.0

Cation percentages

Si

7.1

.8 

18. 0

19.5

46.0

Ti

0.5

.8

.1

1.4

Al

10.6

5.9

16.5

Fe

2.5

2.5

Fe 

Mn

3.4

.5

2.5

6.4

Mg

10.8

.2

11.0

Ca

0.5

.8

3.3

4.1

8.7

Na

3.8

2.4

6.2

K

0.2

.8

1.0

P

0.3

0.3

A comparison of this mode with the norm (table 6) brings out the 

following facts. The norm has a composition not uncommon to an 

ordinary olivine basalt with much more pyroxene than olivine. In 

the actual rock the observed amount of olivine is almost three times 

as large as that appearing in the norm; obviously the olivine has crystallized out in excess of its stoichiometric proportion; it has taken 

away from the melt a good deal of the ferromagnesian cations and has 

later failed to react with the melt which, therefore, has retained its 

silica to such an extent that quartz appears in the norm of the residual 

glass. The residual glass is also interesting in that in its composition 

it comes close to the artificial haplodioritic melts studied by Bowen 

(1915). If the normative potash and soda feldspar of the glass are 

added together and called albite, the projection point of the glass'falls 

approximately on the cotectic curve of the haplodioritic system albiteanorthite-diopside, as was to be expected of a glass representing a 

residual melt.

Clearly the formation of a basalt of Pribilof type depends upon 

special physicochemical conditions during the rise and the effusion of 

the lava; in the usual cases the same lava would have produced a 

rather ordinary olivine basalt.

ANALYSES OF BASIC DIKES

The lava flows of the islands at many places are transsected by dikes 

with similar composition to that of the flows (see fig. 26 and analyses 

nos. 14-19, table 7). Sills are in most places indistinguishable from 

flows. But sills do exist (see fig. 31), and indeed, many of the apparent 

"flows" overlain by sejdiments may actually represent sills. A dike 

intrusive into a pyroclastic cone is pictured in figure 32 (p. 134).

Sample 14. Apophysis of limburgite at the seashore (fig. 30) 1 mile 

south of Garden Cove, St. George Island. On top of the basal peridotite is a thin, discontinuous layer of clay with boulders of peridotite. 

Right on top of it is a sheet of a semicrystalline, porphyritic hyalobasanite about 3 feet thick. It may be a flow, but the following observations make it reasonable to regard it as a sill: Figure 30 shows that
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 131

m

PIGUIIE 30. Section at the seashore 1 mile south of Garden Cove, St. George Island. 

A, pcridotite, B, thin layer of clay with boulders of pcridotite; C, sill of hyalobasanite sending an apophysis down into the peridotite; at 14 the basanite has become 

limburgitic (analysis 14); D, sediments, 59 feet thick, consisting of clay, sand, and 

tuff with fragments of peridotite; crossbedcling is conspicuous; E, lava flow with 

incorporated sedimentary material. At the top of this lava another series of sediment is deposited and again overridden by lava flows. Total height of the cliffl is 

about 200 feet.

the basanite sends apophyses into the peridotite, indicating that the 

basanite is no flow but rather a sill intruded between the peridotite 

and the overlying sediment, and penetrating into fractures or cracks 

in the surface of the peridotite. In the apophyses the basanite becomes very dense and black but still remains slightly porphyritic. 

The thin sections reveal that it corresponds to a limburgite containing 

olivine, augite, and magnetite but practically no feldspar, as. primary 

phases. Olivine shows 2 y=k90°, but is often distinctly zoned. 

Augite is reddish in color, shows hourglass structure and is highly
132 ' INVESTIGATIONS OF ALASKAN VOLCANOES

zoned. Positive 2 V is around 60° with strong dispersion r^>v; the 

directions of the extinctions are also highly dispersed, 7Ac=45°. It is 

a titaniferous augite. Zeolites are seen in cavities. Chemical analyses, norm and mode, are shown in table 4. A special description of 

the mechanism of crystallization in this dike is given on pages 141-143. 

Sample 15. Dense dike of chrysophyric hyalobasanite, following a 

fault zone extending from the seashore 1 mile south of Tolstoi Point, 

southwestward toward Zapadni Bay, St. George Island (see fig. 26). 

In thin sections this dike rock is essentially similar to sections of lava 

flow sample 6, except that no secondary growths of zeolite or chalcedony 'were observed.

Mode Composition of the glass

Or, Ab________. ___________ 7

Augite- - __ An _ _____________________ 11

Plagioclase (An60) .

Glass. _ ______ __

_____ 20

_____ 5

 - 2

16

----- 32

Di________________________ 2.6

01_'__. ____________________ 0.3

32. 0

99

Sample 16. Phyric hyalobasanite forming a dike spreading out to 

a sill (fig. 31), one-third of a mile, east of village, St. George. Phenocrysts of olivine and augite make up" about 30 percent of the rock. 

The groundmass is fine grained with very small crystals of pyroxene, 

olivine, and magnetite in a meshwork of plagioclase laths and glass.

Composition of the glass

Or, Ab__-_--__-__-_ 5

An.________________ 5

Mode

Augite _ _ __

Olivine. _______ __

Plagioclase (Ar>65)___ _

12"

 17

10

6

97

22

'phenocrysts 

 groundmass

Di_

22

100

Sample 17. Hyalobasanite injected under observation house at 

Einahnuhto Bluffs, St. Paul Island. (See fig. 32.) The rock has 

fluidal structure, is black and gray with small phenocrysts of plagioclase, about 0.5 x 0.01 mm, and peculiar "hollow" phenocrysts of 

olivine, from 0.1 to 0.5 mm across (see fig. 28). The groundmass is 

glassy with opaque pigments of magnetite arnd (or) ilmenite, and small 

grains of augite 0.01-0.02 mm in diameter and laths of plagioclase, 

about 0.5 mm long and 0.01 mm thick.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 133

Mode

Olivine (Fego) ___ 

Plagioclase (Auyo)

Augite. ____-----_--__---- 25

Ore, apatite _ --__-_______

Plagioclase (Aruo)- --------- 25

-_-_-__----------_ 33

Phenocrysts

Groundmass

Composition of the glass 

Or, Ab.--_ __.-_ 12 

An.__--__--__--._ 6 

Ne._.__---. __. 13

H_-__-___----___ 2

33

100

Sample 18. Ore basalt, forming a sill or dike on top of road between the village and Zapadni Bay, St. George Island. To the eye 

and under a low power lens the rock appears dark and almost dense 

because of its unusually high percentage of ore. It is very fine grained 

with some odd, small phenocrysts of plagioclase (optical constants 

corresponding to An6o), and a very few, small phenocrysts of olivine 

(fig. 285) that are almost wholly resorbed and often obscured by tiny

30ft

FJGURE 31. Pike spreading out to form a sill. Section at the seashore one-thjrd. mije east of th0 village,

gf. George Island,
134 INVESTIGATIONS OF ALASKAN VOLCANOES

& 3
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 135

grains of ore (hematite?) which have grown along closely spaced 

cleavage planes. Some olivines have been effectively replaced by ore. 

The profusion of ore gives the rock its distinctive character.

Mode 

Olivine (pure forsterite)____________________________________________ 3. 0

Augite (diopsidic)_________________________________________________ 17. 4

Ore (2.0 ilmenite + 7.6 hematite)._-__._______-_________-___--____._ 9. 6

Apatite_______-_-___-___--______-----_-_---__---__-___-_--__--__ 1. 0

Plagioclase (An..)-------------------------------------------------- 69.0

100.0

It is reasonable to assume that the iron necessary for the production of the ore minerals was in part taken from the original olivine 

and augite during the stewing of the rock in hot vapors and other 

volcanic gases, deuteric or otherwise. The separation of finely granular ore is, according to Macdonald (1944), common in reheated (?) 

olivine phenocrysts in blocks thrown out of Halemaumau crater, 

Kilauea.

Dr. H. S. Yoder of the Geophysical Laboratory has informed me 

that L. 0. Nicolaysen made some hydrothermal tests on fayalite under 

oxidizing conditions. He found that fayalite is destroyed at all temperatures investigated (250°-970° C and 100 atmospheres). Above 

approximately 350° C he gets magnetite plus quartz, and below 

hematite plus quartz. This gives an idea about the temperature at 

which the alteration of the present rock has taken place.

Sample 19. Olivine basalt, at Fox Castle,, St. George Island. 

Fox Castle is a huge pile of loose lava blocks forming a "castle" which 

seems to represent the top of a dike or neck that has penetrated the 

tuffs in the Esogemunga fissure. The rock is well crystallized with 

conspicuously zoned laths of plagioclase, 1-2 mm long. According 

to their extinction angle the average composition is around An55 . 

Iritersertally are olivine, pyroxene, and some ore in grains of 0.1 to 

0.5 mm.

Mode Average modal composition of the 

01ivine___-_-_____-_-_------ 11 plagiodases

Pyroxene..-..______________ 10 Or____-_-____-___-___-_-___ 12.5

Ilmenite, hematite (?)______.. 5 Ab_____..__________._______ 38.4

Sphene (?)_______-___---_--_ 3 An_________________________ 16.8

Apatite... __________________ 2 Ne_--__-_-_-_---_--________ 1.3

Plagioclase____----__________ 69 

____ 69.0

100
136 INVESTIGATIONS OF ALASKAN VOLCANOES

PYBOCLASTIC DEPOSITS

Most of the pyroclastic material is basaltic. It is usually found as 

loose pebbles forming mounds or filling the interspace between two 

lava flows. The sedimentary beds likewise contain much pyroclastic 

matter. There is always much glass in this material, and an occasional pebble is made up entirely of glass. It is a black basaltic glass, 

highly vesicular, almost spongy, in thin sections brownish and transparent. Index of refraction is n=1.66, but it is inhomogeneous. 

This index is similar to that of -the sideromelane of Iceland (olivine 

basaltic glass). The crystalline phases are olivine, which evidently 

always was the first to crystallize, and a fine-grained mixture of 

plagioclase, pyroxene, and magnetite. The rather predominant role 

of magnetite in these rocks is remarkable. It seems to have been 

introduced by pneumatolytic (or hydrothermal) activity.

There is no doubt that gas action represents one of the major 

factors in the formation of the pyroclastic material: Explosion pipes,

Ne(Kph) Ab (Or)

1'iGUKE 33. Pribilof rocks projected on the ternary diagram anorthite-nepheline-quartz. Numbers correspond to the rocks in tables 5-7, and to descriptions in the text. Points representing the position of glassbearing rocks are connected with corresponding glasses by lines. Cotectic lines (so far unpublished) were 

determined by Dr. J. F. Schairer of the Geophysical Laboratory.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 137

chimneys through which vapor and gases found egress, also served as 

conduits for glassy and solid ejectamenta. The mineral composition 

indicates that these ejectamenta were derived from subbasaltic 

magma and subbasaltic rocks. The explosion craters now visible are 

evidently younger than the lava flows. But this does not mean that 

the explosive activity was limited to the last phase of the building 

up of the islands. The existence of layers of pyroclastic material 

between the lava flows, and the presence of unmistakable pyroclastic 

material in the sedimentary beds testify to the importance of this 

kind of explosive activity throughout the volcanic history of the area. 

If ever a residual liquid was formed from the magma of this area, 

one would expect it to be associated with the gases and volatile constituents. Vestiges of such a liquid should therefore be found among 

the pyroclastic deposits. A thorough search was made for many days 

with negative results: most of the large pyroclastic cones seem to be 

built up entirely of subbasaltic material. But, eventually, high on the 

east slope of Polovina Hill, St. Paul Island, a small amount of whitish, 

pumiceous material was found and collected. Under the microscope it 

was shown to be completely glassy with an index 71=1.54. A chemical 

analysis for it is listed in the following table. The rock is, therefore,

Rhyolitic glass, Polivina Hill, St. Paul Island

[n=1.54]

SKV.---- 1 -------..---.----- 69.09 Norm

Ti02 ._ _.._._:_..__.__._-____ .43 Q___. ....__ ________________ 25.8

Ala03__---_--__---_---_---_- 14.90 Or___. _____________________ 13.8

Fe203 ______. ___-___.__._-__- 1.53 Ab__. ______________________ 36.7

_______. _---_____--__ 1.69 An____. _______-____-__._ ___ 12.0

--__-_. ----_---_---__ . 07 , Q 1.00 --------------------------

CaO_______ _________________ 2.97 Hy___________. _.___________ 3.6

Na_0______-______--___-__-_ 4.18 Mt-_____---_____-_--____-._ 1.5

K_0___---_---_---_--_---__- 2.37 II-------------------------- .8

P2 0, ._._ __________________ .33 Ap_______. ___.____ _________ .8

H.O _____________________ 1.31 H2 0__._____-_______________ 4.0

H.O- ------------------- .23 

C02 ________. ________________ . 00

100. 10

a rhyolitic glass (a glassy rock of similar chemical composition has 

been described under the name of cantalite in 1921 by von Leonhard) 

which, indeed, can be looked upon as.a residual liquid but not residual in relation to the common basanitic magma of this area, that 

is, it cannot be derived from a basanitic magma by simple fractional crystallization; it seems to belong to an entirely different 

magma suite. It shows a striking chemical similarity to, the magma 

of the aplite stock of St. George Island (fig. 33). However, the aplite
138 INVESTIGATIONS OF ALASKAN VOLCANOES

is older than the lava flows, and the rhyolitic glass is younger. It 

occurs in negligible quantity as compared with the other rocks making 

up less than 10~ 12 part of the lavas of St. Paul. The pyroclastic pipes 

are therefore chimneys of egress for gaseous, liquid, and solid material 

from the practically pure basanitic sublava. The tuff vent at Esogemunga did eject feldspar crystals, however, whose composition relates 

them to the rhyolitic glass (see p. 140).

In one place, at Einahnuhto Bluffs on the west coast of St. Paul 

Island an imposing sea cliff, 379 feet high, plunges precipitously into 

the sea, thus cutting a vertical section through the peripheral parts of 

the Rush Hill cone of pyroclastic material. The following sequence of 

events can be read out of the sketch in figure 32: Lava flows were 

extruded, one on top of the other, interrupted by periods of sedimentation; during this time the area was close to sea level. Towards 

the end of the eruptive period, incarcerated volcanic gases built up a 

tremendous pressure underneath. The roof of solid lava was lifted 

approximately 100 meters, and the adjacent flows tilted up for a 

distance of about 1% miles from the central bulge. Then, with rupture 

of the restraint imposed, paroxysmal explosion of the gases occurred. 

The liquid lava in the pipe, highly charged with gases that through the 

release of pressure suddenly came out of solution, was inflated and 

became spongy; then it quickly congealed as it was flung out of the 

throat.

Thus was formed the bulk of the pyroclastic material that broke 

up the overlying solid lava, penetrated laterally and disrupted the 

adjacent lava flows and sedimentary beds, and eventually was deposited in great amounts around the orifice of the pipe, forming 

an. impressive cone of cinders and pyroclastic material, attaining an 

altitude of 650 feet on top of the tilted lava flows. Following the 

explosive discharge of gases and pyroclastic material, normal lava 

from below entered the lower end of the pipe and was forced upward. 

This lava was liquid except for intratelluric crystals of olivine, which 

had a peculiar hollow habit (fig. 28). The lava flowed upward as a 

viscous liquid insinuating itself between fragments of solid lava, 

sediments, and pyroclastic material. It is now seen as an intrusive 

dike, markedly fluidal in structure, that seems to merge with the older, 

disrupted lava flows at the top end of the dike. PetrograpIdeally the 

dike is a chrysophyric hyalobasanite, and chemically it is identical 

to the average olivine-rich basanite of the area. See analysis 17, 

table 7, and the petrographic description, page 132.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 139

SOME PETROGENETIC PROBLEMS

TUFFS

The tuff deposits at St. George Island are rather unique. The 

sources of the huge accumulation of the material seem to be close at 

hand: as seen from figure 34, the Esogemunga escarpment, is interpreted 

as the rim of a large fissure or vent filled with tuffs composed of compacted pyroclastic fragments and indurated volcanic material. A 

chemical analysis is entered in table 3, and analyses of large cleavage 

pieces of homogeneous feldspar contained in the tuff are given in 

the following table.

TABLE 3. Chemical data on tuff, 1 mile east of Suskaralogh Point, St. George Inland

] Analyst, B. Bruunl

Cation percentages

A.verfige 

Norm. lava. Tuff

Si02 ________ 35.47 Q.___.______ 1.4 Si_________ 42.8 30.5

Ti02 -_---__- 2/52 Or__________ 9.0 Ti.________ 1.5 2.1

AJ 2 Os_-_____- 15.12 Ab_-.______ 4.0 Al_________ .1.0.7 >I9. 8

Fe2 (V__--_- 7.61 An_________ 27.5 Fe________- 9.0 9.4

FeO____--_- 2.98 C__-__---__- 6.2 Mg________ 12.5 19.3

MnO-.__--__ .21 En______.__ 38.6 Ca________. 9.4 > 6. 6

MgO_.______ 11.67 Mt_________ 2.7 Na____i___ 6.2 > .8

CaO_______. 5.57 Il___________ 4.2 K__.______ 1.5 1.8

Na2 0__.____ .37 Hm_________ 4.6 P__________ .4 .7

K2 0__..___- 1.27 Ap__-_.____ 1.8 

Pj08 ..--_.__ .78 100.0 100.0

H2 0+-----_- 7.84

H20-____.__ 8.71 ' +HJO_ .-_ 1.5 29. J

C02 ----__ .00

CL.._..__.- .00

100. 12

The association of alkali feldspars with basalts is unusual, but 

similar feldspar fragments have been found by Mahony (1926) at or 

near volcanic vents belonging to the so-called Newer and Older 

Basalt of Victoria, Australia. Homogeneous alkali feldspar with 

high soda content has been described from Pantelleria by Forstner 

(1884) who showed it to be polymorpheous, changing from triclinic 

to monoclinic symmetry on heating. Apparently no one checked his 

observations, and they have been neglected in the interpretation of 

the alkali feldspar relations until Laves (1952) showed that the St. 

George feldspar pieces behaved similarly, possessing triclinic symmetry 

at ordinary temperature, inverting to monoclinic at around 500° 0.
140

Analyses of crystals of feldspars from tuff

[Corresponding to 0m, Abso, and Ans. Crystal 1 and 2 analyzed spectrographically by 0. Joensuu, crystal 

3 analyzed chemically by B. Bruun]

CaO_ ___________________.__---_-__- 

Na2 0 - _________

K2 O

Crystal 1

0. 9

8. 6 

4. 1

Crystal 2

Rim

0.43

8.9 

3.0

Core

0. 45 

9. 1 

2. 8

Crystals

9. 00 

2. 66

FIOURE 34. The tuff conduit at Esogemunga, St. George Island (diagonally hatched) compared with tuff 

necks from Kimberley, South Africa (1-8) and from Schwaben, Germany (9-15). 1, Bulfojtein; 2, Wesselton; 3, Kaffyfontein; 4, Ottos-Kopje; 5, Du Toitspan; 6, Premier; 7, Jagersfontein; 8, De Beers; 9, Orafenberg; 10, Florian; 11, Metzinger-Weinberg; 12, Neuhauser; 13, Jusi; 14, Limburg; 15, Randecker Maar. 

(1-15 after H. Cloos, 1941).

It is possible that the tuffaceous material has been somewhat altered 

by weathering and leaching. This is best seen in the norm: there is an 

excess of alumina and ferric iron, and a corresponding deficiency in 

lime and soda. Table 3 gives a direct comparison between the cation 

content of the tuff and that of an average lava flow (from last column 

of table 6), and brings out rather clearly these chemical relations.

Hans Cloos (1941) in his masterly treatise on the origin of tuff vents 

has demonstrated that volcanic tuffs form in consequence of an amalgamation of volcanic material from below and crustal material from 

above. This idea has been applied by Lehmann (1952) to explain 

the origin of the keratophyre tuffs of Sauerland. He introduces the 

word mictite to designate a tuff-magma-migmatite. The beginning of 

the process is a tectonic fissuring of the crust extending down to "mag-
GEOLOGY AND PE'EROLOGY OF THE PRIBILOF ISLANDS, ALASKA 141

/

matic" regions. Using the initial fissures, gas and tuffaceous material 

flow upward, sometimes resulting in an explosive blowout, sometimes 

effecting a quiet "tuffization" of the surrounding rocks.

In the Pribilof area such tectonic fissures evidently reached down 

into a region of olivine basanitic magma: first the fissures gave vent 

to the volatile material of "tuffization" followed by the ascending 

magma itself which in subsequent eruptions flooded the surroundings 

of the fissures, thus concealing the vestiges of the initial tuffaceous 

discharges. Only the Esogemunga fissure of St. George Island escaped 

destruction arid remained in testimony of an early stage in the history of 

development of those great fissure eruptions so characteristic of many 

basaltic provinces (the plateau type, Hawaiian type, and others). 

Worthy of special notice is the fact that the Esogemunga fissure apparently represents the biggest tuff conduit on record.

Tuffs are, of course, widely distributed in many volcanic areas (Ross, 

1951), but tuff conduits of this size have never been described before. 

The tuff pipes at Kimberley, Scwaben, and other places, are associated 

with volcanoes of the central vent type, and therefore of much smaller 

dimensions than the Esogemunga fissure (fig. 34).

LIMBUBQITE

The limburgite dike (sample 14, p. 130) presents an interesting petrological problem.

The chemical data, as given in table 4, in combination with the study 

of thin sections suggest the following history of crystallization.

Let us assume an original melt of the composition of the rock. If 

ore and apatite are neglected, and potash is added to sodium, we obtain 

the following simplified composition of the original melt:

Ab._.------ 16

An _-___- 20

Ne__._ ----- 14

Di-_-____- 27

01.....____ 23

50 percent 

50 percent

Studies of thin sections show that from this melt the ferromagnesian 

minerals (olivine and augite) crystallized first, and, indeed, continued 

to crystallize until the mother liquor became very feldspathic, the end 

product being a rock with olivine and augite as sole crystalline phases, 

and a feldspathic glass. This means that the Bowen reaction series 

of the ferromagnesian minerals was well advanced before the feldspathic-feldspathoidal series started. It is well known that rock melts 

rarely show eutectic crystallization. Rather there are two or more 

leading mineral reaction series, each crystallizing out semi-independently of the other. It is possible, therefore, in order to survey the
142 INVESTIGATIONS OF ALASKAN VOLCANOES

crystallization processes of the present melt, to discuss separately the 

ferromagnesian types of mineral series (olivine and diopside), and the 

feldspathic mineral series (anorthite, albite, and nepheline). Bowen 

has pointed out that between the two types of series there is a suggestion of a eutectic relation, for members of one series lower the 

"melting" temperature of the members of the other series. Let us 

therefore, look at the original melt as consisting of a neutral solvent 

of feldspathic composition from which, the ferromagnesian minerals 

precipitated.

The composition of the ferromagnesian fraction is 46 percent 

olivine and 54 percent diopside. In a dry melt of this composition 

(Bowen, 1914) crystallization will start with separation of olivine 

(at around 1,650° C.), and olivine will continue to separate until the 

melt becomes eutectic in composition at 13 percent olivine and 87 

percent diopside at 1,387° C. These conditions will be notified by 

the feldspathic solvent and by the presence of volatiles, mostly H2O; 

the content of H2 0 will be taken, rather arbitrarily, as 4 percent, and 

the corresponding depression of the temperature of crystallization 

will bo about 200° C.

The diagram in figure 35, helps to describe the crystallization process: one side of the diagram represents olivine and diopside together 

(melting at 1,650° C), the other side represents a mixture of albite, 

anorthite, and nepheline, the melting point of which can be estimated 

to be about 1,200° C. Between them, and close to the feldspathic 

side, is a (pseudo) eutectic, the temperature of which is about 50° 

lower. Referred to the actual rock melt containing 4 percent H2O 

and other foreign ions the whole diagram will be displaced about 200° 

lower than that referring to dry melts. In the diagram, x represents 

the composition of the rock. Under intratelluric conditions and slow 

cooling, olivine started to separate at 1,175° C. At 1,025° C both 

olivine and diopside together began to crystallize out of the melt. 

But approximately at this stage the intrusion took place causing a 

rapid cooling with consequent rapid precipitation of a mixture of 

diopside and olivine (as now shown in the groundmass of the rock). 

Thus the pseudoeutectic point at 950° C was approached. But here 

the melt became very viscous and the cooling became very rapid because the magma penetrated into the cracks of the cold peridotite, so 

the actual pseudoeutectic crystallization never materialized; instead 

the anchieutectic liquid congealed as glass. .'

A feature of special interest of the present study is the demonstration of the pseudoeutectic point (actually a cotectic field) between 

olivine and diopside on the one side and undersaturated feldspathic 

mixtures on the other being located close to the feldspathic compo-
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 143

V

1650 

1600-

Glass + (Diopside. olivine)

900

Ab,An,Ne ' Di.OI

35. Diagram illustrating the crystallization of a polynary melt containing diopsicle and olivine 

as ferromagnesian constituents, and albite, anorthite, and nepheline as "feldspathic" constituents.

nents. Experiments at the Geophysical Laboratory have demonstrated that artificial feldspathic liquids show this relation.

It is interesting to see, from table 7 and figure 33, that the limburgites differ very little, chemically, from the basanites. Consequently 

it is the cooling history, rather than the chemical composition of the 

magma that produces a limburgite.
144 INVESTIGATIONS OP ALASKAN VOLCANOES

TABLE 4. Comparison of three samples of limburgite

Si02 -. 

Ti02 -. 

A12 O3 . 

Fe2O3 - 

FeO_- 

MnO.

CaO_ 

Na2O. 

K2 0_.

H2 0±

H

40. 30

3. 25

13. 14

5. 34

7. 62

. 20

11. 24

10. 33

3.40

.80

. 86

1. 68

2.24

none

B

43. 46

2. 21

15. 34

1. 46

8. 1.7

. 16

12. 60

11. 39

2. 61

. 69

. 48

. 59

. 41

C

44. 32

1. 18

12. 51

3. 68

5. 27

,20

13. 98

10. 82

3. 15

.80

. 57

2. 71

1. 04

100. 40 99. 63 100. 23

Norm

Or

Ab

An- ______________________

Di

Mt

An

H2O________________________

Mode of sample 14 

Phenocrysts:

Augite______ __ _ ___

Crystalline groundmass: 

Olivine_ _ _ _ _ _______

Augite___ ___ _ _____

Ore__ ___ _ _ ______ -_

Serpentine, chlorite, iron 

hydroxides-.. _- _____

Glass (including zeolite)-..

_-___-___-___-____ 4.5

--_----___________ 10.0

_____ _ ___________ 17.8

__---____-___-____ 12.0

21. 2

17. 9

__-___-_____--__-- 5.4

_ _ ______-___-_-_ 4.4

-_____-____---___- 1.8

5. 0

10 

2

4 

25 

5 

2

6 

46

100

Composition ofgla. 

Or _____

^^6

01

Di

H2 0

Zeolite .-______ .

4. 0 

8. 5 

28. 0 

9. 0 

20. 0 

24. 0 

1. 5 

3. 2 

1. 0

C)

s for sample 14

5. 0 

13. 0 

17. 5 

9. 0 

26. 0 

22. 5 

3. 9 

1. 6 

1. 3 

0)

4. 5 

8. 5 

12. 0 

12. 0 

3.0 

1.0 

4. 0

45. 0 

1.0

46. 0

1 The norms of samples B and C have been calculated to a water-free base.

14. Limburgite, 1 mile south of Garden Cove, St. George Island, Pribilof Islands. Analyses. B. Bruun.

B. Limburgite, Nonopapa Landing, The Leeward Islands, Washington and Keyes (1926).

C. Limburgite, Ryudo, Meisen district, Korea, Lacroix (1928).

OLIVINE NODULES

Several olivine nodules about an inch in diameter were observed in 

basanite lava flows of St. George Island and a larger nodule, 3 inches 

across, was observed in pyroclastic material forming a layer in an 

intercalated sedimentary bed at Tolstoi Point, St. Paul Island. For 

almost a hundred years, olivine-rich inclusions in basaltic rocks have 

attracted the attention of petrologists. In his famous monograph on 

the inclusions in volcanic rocks, Lacroix (1893, p. 484) writes:

"Ces nodules sont plus abondants parmi les produite de projection que dans les 

roches massives . . . ils out ete si souvent decrits qu'il reste pen de chose a en 

dire au point de vue mineralogique."
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 145

Since Lacroix's time the nodules again have been described; among 

others Chudoba and Frechen (1941) have studied the mineralogy, 

Ernst (1935) and Turner (1942) have made a special study of the 

petrofabric, and recently Ross, Foster and Myers (1954) have given 

complete chemical and mineralogical data on the various minerals. 

All have wanted to find out if the nodules represent mineral segregates 

from the magma, or foreign inclusions.

The present investigation brings little that is new, but the following 

data are offered for consideration.

In the present nodules, olivine dominates (about 70 percent) with 

enstatite second (about 30 percent). Less than one percent spinel is 

present.

Olivine shows (+) 2 "7=89°, 0=1.682, corresponding to FaK .

Enstatite shows (+) 2 "7=85°, 0=1.672, corresponding to Fs]2.

Spinel shows index of refraction, rc.= 1.762; edge of the unit cube, 

a0 =8.12 A, corresponding to 70 MgAl204 , 26 FeAl204 ,4 Fe3 04 . The 

distribution of iron among these minerals is interesting. In harmony 

with the observations of Ramberg and de Vore (1951), iron enters into 

olivine and enstatite in approximately equal proportions. Iron usually 

prefers pyroxene to olivine, but observations show that iron nevertheless distributes itself with approximately equal coefficients between 

the magnesian olivines and orthopyroxenes. In spinel the Fe:Mg 

proportion is much higher than in the silicates, in harmony with, the 

fact that oxygen ions in most oxides are more readily polarized than 

in the silicates. The distribution of iron in spinel, olivine, and enstatite supports the a priori conclusion that all three minerals were in 

equilibrium with a common magma. But this conclusion makes it 

hard to see, generally speaking, how the distribution relations of the 

chemical elements may contribute anything to the genetic history 

of the rock. If equilibrium was attained it means that all elements 

will distribute themselves in the "right" proportions within olivine, 

enstatite, spinel, and magma, regardless of the provenance and 

original composition of the mineral phases. We may go further along 

this line of thought: spinel phenocrysts (not inclusions) have been 

observed in some of the lavas. They are particularly conspicuous 

in an olivine-rich, mafic basanitic lava (at top 570, one-half mile 

west of Staraya Artil Rookery, St. George Island). The lack of any 

evidence of assimilation of aluminous material by any of the lavas 

makes it reasonable to assume that spinel here developed as a primary 

phase during the crystallization. Olaf Andersen (1915) has shown 

that in certain parts of forsterite-anorthite-enstatite melts, spinel 

will appear as a primary phase on crystallization. Osborn and Tait 

(1952) have extended the studies to include diopside and finds in the 

ternary system forsterite-anorthite-diopside a rather large field of
146 INVESTIGATIONS OF ALASKAN VOLCANOES

spinel. The olivino-plagioclase-rich lava may come sufficiently 

close to this composition to yield primary spinel. This spinel, a 

phase of crystallization formed in equilibrium with the magma, shows 

index of refraction, n= 1.761, and the length of the edge of the unit 

cube, a0 =S, 12 A. These constants are practically the same as those 

in the spinel of the nodule.

The chemistry of spinels in basic lavas is not well known. Thomas 

(1922) has published a description with analysis of a spinel derived 

from assimilation of aluminous sediments by Tertiary lavas of Mull. 

The chemical composition (in percent) is MgAl204 ,40; FeAl2O4 ,50; 

Fe3O4 ,4; A12O3 ,6. The high iron content reflects the fact that the 

lava of Mull (a tholeiite) is more ferriferous than the basanite of 

Pribilof. It indicates that equilibrium was reached with the lava 

(so much iron could scarcely reside in the clay sediment) and supports the view that the composition is governed by reactions with 

the magma.

The general conclusion of the study of the spinels from the Pribilof 

Islands is that the composition of the mineral phase (in the olivinerich inclusions) and the distribution of the elements in them will not 

contribute to the genetic history of the nodules.

According to a suggestion by Tomkeieff (1952), phenocrysts of 

olivine, pyroxene, and spinel will sink easier in the relatively fluid 

basanite magma than in ordinary basaltic magma, and at some depth 

consolidate in the form of peridotite, fragments of which become 

olivine nodules. This explains why nodules are more frequent in 

basanites than in common basalts.

PETROCHEMISTRY

ANALYSES OF THE BOCKS

All analyses of the rocks from the Pribilof area are listed in tables 

5, 6, and 7. Unique in its relation is the peridotite (sample 1, p. 119) 

The aplite stock also stands apart (samples 3 and 4), as does the 

rhyolitic glass (sample 5) which, however, seems to represent a 

residual liquid, perhaps related to some of the mineral fragments of 

alkali feldspar (analyzed on p. 140) found in the tuff (sample 2).

Analyses of eight of the extensive lava flows, and of six of the 

numerous basic dikes in St. Paul, St. George, and Otter Islands are 

compiled in tables 6 and 7. The dikes are by and large very similar, 

chemically, to the flows; together they form a typical assemblage 

of olivine basaltic-basanitic rocks, clearly unusual in this part of 

the world.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 147

The Pribilof Islands are made up almost wholly of lava flows. 

Consequently the average composition of the rocks of the islands 

would be represented approximately by the average composition of 

all lava flows. Table 6 shows that rock sample 7 has an unusually 

high content of titania; also, alumina is rather low as compared to the 

high alkali content, thereby anorthite becomes low in the norm. For 

these reasons and because the sample may not represent a flow, this 

analysis has been omitted from the average. The last column of 

table 6 shows the average composition of the lava flows as computed 

from samples 6, 8, 9, 10, 11, 12, and 13. In the norm system the lava 

obtains the symbol III, 6, 2, 4 and the name monchiquose (but not 

far from limburgose).

The Niggli values are as follows:

fm_

C- _

alk.

27. 3

29. 4

30. 7

12. 6

si= 140 

C|z=-I0 

k = 0.20 

ing= 0. 68

The lava flows belong to the essexite gabbroid group of magmas 

(Niggli, 1936).

TABLE 5. Analyses of rocks from the Pribilof Islands 

[All analyses by B. Brunn]

Si02 ---- -------

Ti02 _. -----------------------------

AhOj .. . ... .... ....... -.-..- . 

FejOs

FcO

MnO -----.------_ -----------

MgO.-------..-- -- --. - -

CaO

"NatO...... ........................................ ..

KtO............. ................ -------------

P2 06 . - - -. . .. - - .

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

H2O-_

' 1

39.83

trace

1. 54

3. 70

.12

41.95

.10

.12

.01

.04

8.34

.62

100. 14

2

35. 47

2.52

15. 12

7.01

2.98

.21

11.07

.37

1.27

.78

7.84

8.71

100. 12

3

04. 35

.23

17.70

50

.15

.90

2.79

4.18

2.39

.50

1.86

.41

100. 05

4

71. 89

15. 04

7ft

.03

.24

(19

5. 10

.08

.39

100. 40

5

69.09

.43

14. 90

1 09

.07

1.00

2.97

4.18

1.31

.23

100. 10

1. Poriclotite, St. George Island, see p. 119.

2. Tuff, St. George Island, see p. 139.

3. Core of aplitc, St. George Island, see p. 121.

4. Margin of aplite. St. George Island, soo p. 121.

5. Rhyolitic glass, St. Paul Island, sec p. 137.
148 INVESTIGATIONS OF ALASKAN VOLCANOES

TABLE 6. Analyses of lava flows of the Pribilof Islands

[The numbers correspond to those in the description in the text, p. 126-130. Analyses 6 and 8 by B. 

Bruun; 7, by Kcyes (1930); 9-13 by rapid methods, S. M. Berthold]

Si02 - ---

TiO2 ----------------

AbOs- ----------

FezOs 

FeO

MnO.-- --------

MgO. -------------------

CaO-. . ---------------

Na2O----. -------------- -

K2O-. ------------------

P2 05 -----------------

H2 0+. ----------------

'HtO-. ...................

6

4.1. 70

1.25

14.68

3.38

8.97

.18

12. 62

9.93

2.95

1.22

.(17

1.15

99. 80

7 '

44.21

4.78

2.35

9.91

.13

7.34

8.92

4.70

.79

. 19

JOO. 13

8

44. 37

2.19

12.74

2.15

9.42

.18

14.71

9.30

3. 00

.71

.20

100.14

9

47.0

2.8

4.3

7.0

.2

5.1

9.8

4.0

.51 }.......

99. 5

10

47.0

2.8

3.9

7.4

.2

4.9

9.9

4.1

. 50

98. 9

11

47.2

2.3

4.8

6.8

9

8.1

9.8

3.8

.38

99. 9

12

46.7

2.1

7.6

4.8

.2

10.1

8.9

2.8

99. 5

13

49.4

2.0

3.5

7.9

7.9

8.7

3.4

y

99. 3

Average 

of 7 

flows '

46. 12

2.21

4.22

7.46

.19

9.04

9.46

3.43

.49

f .51

\ .39

100.00

Nor

Or---. ------------------

Ab ----------

A ll

Ne

Hy__ .....................

Di 

01 

Mt. ----------------------

11 

Hm .. ..----- -

A r\

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

12 y

17.6

26.0

9 n

1.2

3.5

22.6

11.7

9.1

2.0

18.8

29.1

3.8

1.4

.6

9. 5

94 '-I

17.6

7.4

4.0

1.0

22.5

19.6

6.8

4.0

1.0

25. 5

22.0

19.2

12.5

3 9

.8

25. 0

25. 5

9.6

13.6

9.0

3 A

23. 3

10.8

8.1

4.2

2.8

.8

22. 5

16.8

3.0

1 Analysis for sample 7 omitted from average because the sample may not be representative.

TABLE 7. Analyses of dikes from the Pribilof Islands

[The numbers correspond to those, in the descriptions in the text, p. 130-135. Analyses 14 and 15 by 

Bruun, 16-19 by rapid methods, S. M. Berthold]

Si02-- -

TIOz ----------------------------------

Ml03-. -- --------------------------------

FeO... -----

MnO . 

MgO. ----------------------------------

CaO .

Na2 0 

K2 O_. ------------- .._--.-..-.

P2 05 - 

H2 0+ ... 

H2O-_ ---------------------------------

14

40.30

3.25

13.14

5.34

7.62

.20

10.33

3.40

.80

.86

1.68

100. 40

15

42.44

1.70

14.78

3.08

9.40

.20

10.15

3.01

.77

.79

.39

100. 02

16

44.1

14.0

3.4

8.0

.2

9

Q

10.8 3.7'

fid

>

98.3

17

4K (!

2.8

16.4

3.4

7.7

.2

6. 6

9.4

64

99.1

18

48.5

17.3

10.8

1.1

4.4

8.5

4.2

Cfl

99.8

19

49.fi

15.5

4.9

6.9

3.5

7.8

4.4

9 n

.78

98.4

Norm

Or _

Ab ------- _

An-.... 

Ne ------------------------------------

Di 

01. -------------------------------------

Mt._ ---------------------------

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

Hm 

A n

HjO -... - -------------------

Ti . 

4.5

10.0

17.8

12.0

21.2

17.9

5.4

4.4

1.8

5.0

8 fi

6.0

22.0

17.6

24.3

3.1

3.0

1.6

2.4

7.5

11.0

17.7

13.2

15.8

4.0

3.4

1.3

12.0

15.5

19.0

14.4

19.2

11.0

3.6

4.0

1.3

10.0

23.5

.9

9.6

5.7

2.0

7.6

1.0

9 7

12. 5

40.5

16.7

14.4

5.1

5.2

4.0

1.6
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 149

Figure 36 is an ordinary variation diagram showing CaO and 

(Na2O+K2O) plotted against silica; the alkalic character of the present 

rock suite is obvious, but the alkali-lime index cannot be determined 

accurately, as only members of low silica content are known. For 

the same reason the trend of the differentiation cannot be defined 

adequately by a mere tabulation of the various rock analyses. But 

there is another way in which the trend of the fractional crystallizao 5-

UJ

O

01

nese

40 50 60

PERCENT OF Si02

70 80

FIGURE 36. Diagram showing the variation in alkali and lime with increasing silica for Aleutian lavas 

and for the Trans-.iapancsc alkalic rocks. The projection points of 13 Pribilof lava flows are plotted.

tion is revealed: in each rock certain minerals (mainly olivine, some 

diopside-augite, and calcic plagioclase) have crystallised first, leaving 

a residual liquid that in many rocks has congealed as glass. From 

each rock analysis in combination with optical determinations of 

the mineral phases it is possible to calculate the composition of this 

residue (see p. 127). The results of these calculations show that 

if soda and potash are combined the composition of the residual 

glass can be expressed, approximately, in terms of quartz, anorthite, 

and nepheline. Figure 17 contains the projection points of the 

rocks and their constituent glasses. It demonstrates that the trend 

of differentiation was in all rocks "down" the fusion surface. The 

differentiation trends toward the cotectic curve separating the field 

of the feldspars from the field of the nephelines. The glass fraction
150 INVESTIGATIONS OF ALASKAN VOLCANOES

of the rocks thus shows the composition of a phonolite, and if this 

fraction had had the chance to be separated from the antecedent 

minerals, it would have formed lava of the composition of a phonolite. 

In spite of a careful search for such lava in the islands, none were 

found; the only indication of a residual lava is a rhyolitic glass occurring in a few pieces of white pumice at Polovina Hill, St. Paul Island 

(fig. 33 sample 5, see also p. 137). As seen from figure 33 the rhyolite 

is quite unrelated, chemically, to the expected but still hidden phonolites of this province.

ORIGIN OF THE ALKALIC ROCK SUITE

The effusive rocks in the Alaskan-Aleutian range of Tertiary to 

Recent age are in the main andesitic in composition, although basaltic 

phases on the one hand and rhyolitic on the other are also represented: 

the rock suite is typically calc-alkalic; (Byers; 1 Coats, 1950; 1952; 

Snyder 2 ). Inferior analyses have been cited as evidence that the 

rocks of Bogoslof Island (straight west of Unalaska) are alkalic. 

A modern restudy of these rocks by F. M. Byers has shown that 

such is not the case.

St. Matthew, Hall, and Pinnacle Islands lie about 250 miles north 

of the Pribilof area. According to P. S. Smith (1939) they are composed almost exclusively of Tertiary to Recent volcanic rocks of 

andesite character. Again on Nunivak and Nelson Islands much 

of the bedrock is basaltic lava which seems indistinguishable from the 

basaltic rocks along the lower Yukon, at St. Michael and in Seward 

Peninsula (pi. 2).

But on the St. Lawrence Island, far north in the Bering Sea, about 

150 miles southwest of Nome, volcanic rocks of post-Cretaceous age 

and of alkalic character form a considerable part of the island. These 

rocks have been sampled by the Geological Survey, and the following 

description of them has been sent to me by Dr. Louis L. Ray:

The alkaline nature of the rocks is indicated by:

a) Recognition of analcime, nepheline, or sodalite in more than 25 percent 

of the basalts collected. For specimens from the central part of the island 

(Kookoolgit Mountains) the proportion is considerably higher with 

slightly less than half the specimens being somewhat alkalic and with 

information on the remainder being inadequate for conclusive determination.

b) Although megascopic determination of the feldspars cannot be definite, 

specimens from the Ongovewhat River area probably contain 47 percent 

orthoclase and 47 percent albite.

c) Inclusions and injections within the basalts were described as syenite 

with minor nepheline content.

1 Byers, F. M., 1955, The Petrology of Umnak and Bogoslof Islands: Ph. D. thesis, Univ. Chicago (in 

preparation).

2 Snyder, G. L., 1952, Some considerations of Aleutian lava with special emphasis on those of Little Sitkin Island: mimeographed and on file in the library of the University of Chicago.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 151

Thus we see that in the northern part of the Bering Sea on St. 

Lawrence Island and in the southern part on the Pribilof Islands 

alkalic rock suites occur; and, according to Washington, who also 

quotes Zygmuntonska, soda rhyplite occurs on the Komandorskiye 

Ostrova, off the coast of Kamchatka; nepheline syenite and comendite 

have been found at East Cape, Siberia, where monzonite and rhyolite also occur (Washington, 1902).

Isostatic compensation prevails in most parts of the earth, but 

several very interesting exceptions have been demonstrated by 

gravity measurements conducted at sea by Vening-Meinesz (1934): a 

mass deficiency prevails along the foredeeps of the arcuate erogenic 

island chains (that is, parallel to the convex side of the island arcs of, 

for example, the Malay Archipelago, the arcs of the Antilles, of the 

Aleutians, and other arcs). There is a relation therefore, between 

island festoons, oceanic deeps, and mass deficiency. This deficiency 

attains a magnitude of more than 200 milligals. A rock blanket 5,600 

feet thick, of density 2.7 and with indefinite horizontal extension 

would be required to compensate for this deficiency; and if concentrated along a band, the thickness must be still greater. At the same 

time a mass excess builds up on the inner (concave) side of the arc. 

Obviously strong tangential forces are thus produced, releasing themselves in horizontal mass movements of large dimensions. Great 

crustal instability will ensue, and spread far into the hinterlands of the 

orogenic areas.

The tectonic environment is now recognized as an active factor in 

the evolution of magmas. Vertical faulting of nonorogenic areas are 

the accepted prerequisites for the formation of alkalic rock suites. 

Tomkeieff (1949) has pointed out that igneous rocks with marked, or 

even strong alkalic character, have developed in the hinterland of 

many circum-Pacific arcs: for example, the Australian Cainozoic 

igneous rocks in the hinterland of the New Guinea-New Zealand arc; 

the rocks of the inner side of the Java-Sumatra arc; the "TransJapanese alkaline province" comprising the Sea of Japan, Korea, 

eastern China and Mongolia. In all these places the alkalic rocks are 

also associated with vertical faulting.

Consequently, it seems to be a rule that alkalic rock suites develop 

in the hinterlands of the folded arcuate mountains, probably always 

associated with vertical movements. The alkalic rocks of the Pribilof 

mound were, therefore, to be expected, and find their counterparts in 

other regions of similar geologic relations.

The visible amount of alkalic rocks on the inner (concave) side of the 

Alaskan-Aleutian arc is small because the area is mostly under the sea. 

But it is quite possible that the whole Pribilof area between (3,000 and 

4,000 square miles), being a region of crustal instability, has produced
152 INVESTIGATIONS OF ALASKAN VOLCANOES

extensive flows of markedly alkalic lavas now at the floor of the shallow 

Bering Sea.

To test this we should try to find out what is underneath the Pribilof 

Islands. The geographic position on the continental shelf indicates a 

continental basement. However, no obvious traces of continental 

rocks exist, and no interaction (assimilation relations) between lava 

and sialic rocks can be inferred. Quite to the contrary, the alkalic, 

strongly basic character of the lavas is indicative of primary basaltic 

substratum that reached the surface without any contamination 

(Barth, 1940). The existence of a peridotite basement in St. George 

Island, although quite possibly of local extent, is a similar indication. 

The only evidence in favor of "continental" rocks underneath is an 

aplite stock of granitic composition in St. George Island (p. 121) and the 

presence of a small amount of rhyolitic glass in St. Paul Island (p. 137). 

Also, large loose crystals of anorthoclase in the tuff deposits west of 

Staraya Artil in St. George Island similarly indicative of a continental 

basement (p. 139). Whence came these crystals? They are volcanic 

ejectamenta, but certainly not derived from any of the lavas known 

in the Pribilof area. The only magma compatible with the composition of the anorthoclase would be syenitic or granitic (granodioritic) 

in composition. They may have the same origin as the rhyolite glass.

GEOLOGIC ANALYSIS

The Bering Sea is a triangular basin between Alaska and Siberia 

bounded to the south by the Aleutian Island arc (pi. 21). The 

greater depths are in the southwestern part, where a maximum of 

2,240 fathoms (4,070 m) is attained 43 miles northeast of Attu. The 

northeastern part of the basin is conspicuously flat and shallow. It is 

commonly referred to as the Bering Sea shelf and is nowhere deeper 

than 100 fathoms. Say Buffington, Carsola and Dietz (1950):

Straight line traverse over distances as great as 5° of latitude (343 nautical 

miles) have resulted in traces which are almost completely smooth at relatively 

constant depth. Slight roughnesses which occur occasionally in an otherwise 

perfectly smooth trace are attributed to locally stony bottom. Over long distances the bottom gradients are typically less than 1 foot per mile (a slope angle of 

considerably less than 1 minute). The straight line traverse between the Pribilot 

Islands and St. Lawrence Island showed a net rise of 25 fathoms in. 343 nautical 

miles. This gradient of 0.43 feet per mile is not to be found on any land surface 

of comparable extent and is more nearly approached by the gradients of very old 

and slow-flowing rivers. The break-in-slope at the southern edge of the Bering 

Sea shelf occurs at a depth of about 85 fathoms, a depth which is constant within 

a few fathoms at least 150 miles of the length of the break. This depth is about 

20 fathoms greater than the 65-fathom depth typical of continental shelf breaks 

throughout the world. It is believed that the Bering Sea break, like other 

breaks on continental margins, marks an old shore-line of a lowered sea-level or, 

more exactly, that it is a wave-abraded feature cut within the breaker zone of the 

lowered sea level.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 153

CBTTSTAL MOVEMENTS

Close to the edge of this remarkably flat and monotonous shelf lie 

the Pribilof Islands (pi. 21). St. Paul Island rests on a broad submarine mound; St. George Island rises more abruptly from the sea, 

but east and northeast of it is another broad platform with broken 

water shoaling from 3 to 8 fathoms. It is believed that these conspicuous variances in the monotony of the shelf call for a special 

explanation. They represent accompaniments to the igneous activity 

that is manifest in the extensive fragments of Kecent lava flows now 

dominating the bulk of the islands. Consequently, these marginal 

parts of the shelf, covering an area of almost 4,000 square miles, 

must have been subject to crustal movements, faulting, and block 

elevation attendant on the Recent lava effusions. The oldest lava 

flowed over a glaciated surface on which marine shells are found in 

their original position (Saxicava, Mya, Astarte, and others, see p. 117) 

thus proving that the area was close to sea level at the beginning of 

the volcanic activity. During the later stages of the lava formation 

the intercalated fossiliferous sedimentary beds, in places conspicuously crossbedded, indicate shallow water. There can be no doubt, 

therefore, that during all this time the area oscillated around sea 

level, and that the crater fissures as well as the extensive, flat-lying, 

lava flows were formed close to sea level. On the other hand, the 

great thickness, of some of the sediments (p. 118) strongly indicates 

high local relief probably caused by faulting and block elevation.

This area of crustal instability will be referred to as the Pribilof 

mound. It is now mostly submerged, only the islands themselves, 

or 2 percent, being above sea level.

Most geologists agree that a eustatic lowering of sea level occurred 

during the Pleistocene. The drop is variously estimated from 300 

to 392 feet (50 to 66 fathoms). This range is supposed to be related 

to the present depth of the break in slope between the continental 

shelf and the continental slope, which in the Bering Sea happens to 

be 85 fathoms. In any event; a drop of only 50 fathoms would be 

enough to make dry land of most of the Bering shelf, with the Pribilof 

area emerging as an elevated plateau. (See pi. 21.)

The building up of the Pribilof area can certainly be dated as 

Pleistocene. It developed as a raised tableland with the highest 

parts attaining, and, at that time locally exceeding an altitude of 

1,300 feet, abruptly plunging into the ocean to the southwest, gently 

sloping into the remarkably flat and low-lying hinterland of the north 

and east.

QLACIATION

IF No traces of glaciation are evident on the present surface, but, as 

stated in the description of St. George Island (p. 110), the[basement
154 INVESTIGATIONS OF ALASKAN VOLCANOES

periodotite was eroded and leveled by glaciation before the effusion 

of the basaltic lavas. Various types of glacial sediments were also 

deposited, partly directly on top of the peridotite, partly in alternation 

with, lavas. These sediments are very thick. The general impression 

is that the glaciation was extensive in both space and time and a ready 

inference is that it corresponds to a continental glaciation in Alaska. 

However, according to the best information (A. L. Horberg, personal 

communication, and Capps, 1931), it is hardly possible that Alaska 

at any time during the Pleistocene was glaciated to such an extent 

that the ice could spread westward over the present Bering Sea and 

reach the Pribilof Islands. Rather, it seems that in the western 

parts of Alaska only the higher mountain ranges were glaciated. In 

the lowlands no permanent cover of snow or ice existed. Or, is it 

possible that a cover of ice rested motionless over the Bering shelf 

and eventually melted without leaving any vestiges?

How is it possible, then, to account for the glaciation of the peridotite of St. George Island? High elevation might seem to be the 

answer. This is contradicted, however, by the fact that fossil marine 

shells lie directly on the glaciated surface; the first lava outpours 

evidently rode over lowlands and the shallow sea floor. The presence 

of similar fossil shells in the intercalated sediments higher up in the 

basalt profile indicates that the area was close to sea level during 

most of the subsequent volcanic activity. This conclusion needs 

some modification: parts of the area must have reached higher 

altitudes, for only in this way is it possible to explain the great thickness of some of the sediments, for example at Otter Island. Consequently, high local relief existed, which in all probability was produced by faulting and block elevation.

This supports the view that the Pribilof area during the Pleistocene 

was subject to large vertical movements. It may have maintained a 

high, average elevation in early Pleistocene for example in Giinz and 

Mindel, subdivisions in European usage and during this time it became heavily glaciated. Thereafter a general foundering of the area 

took place in combination with nssuring and outpouring of lava. 

During all the subsequent volcanic history it remained in a low 

position, oscillating around sea level. 3

The only other way to account for the glaciation of the peridotite 

would be by pack ice. This means that the glaciation took place in 

late Pleistocene, after the eustatically lowered sea level again had risen 

and inundated the shallow Bering Sea on which the pack ice would 

have to float. The still younger lava sequence and the deposition of 

thick sediments between the lava flows would thus be just a few

3 A submarine eruption in this area is recorded as having taken place in 1815 (Ross, 1951) and Landgrcbe 

(1855) states that flames have been seen to rise from the sea northeast of the Pribilof Islands. However, in 

spite of these assertions any present volcanic activity must be regarded as doubtful.
GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 155

thousand years old. In view of the strong erosion of the peridot!te 

surface and the great thicknesses of some of the subsequent sediments 

including glacial till, indicating local high reliefs in very recent time, 

this explanation does not seem probable.

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156 INVESTIGATIONS OF ALASKAN VOLCANOES

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GEOLOGY AND PETROLOGY OF THE PRIBILOF ISLANDS, ALASKA 157

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INDEX

Page 

Abstract. _ . _ _ -.............. 101-102

Acknowledgments..--....-----_---...-...-- 103

Alkalic rock suite, origin of__-.....-..-... 150-152

Alluvial sediments, on St. Paul Island........ 10(5

Aplitc, chemical composition of dike of.. 123-124, 147

description.___.____._____... 121-124

v on St. George Island --...__....... 112. 147

Analyses, aplitc-.-.....-----.---.-------...-- 147

basalt-..---.-----...------.------.-...- 129, 13.5

hasanito__.__...__......_--....- 12S

basic dikes...--......-...........---... 130-135

chemical, rhyolitic glass.-.. _....--- 137,147

(likes................................. 147, 148

feldspar crystals..-...--....-...-.-..--... 140

hyalobasanite._..._.-..-..-.... 127, 128, 132

lava flows.--------.----------._---------- 148

limburgite__............................ 144

pcridotite -. - - 121, 147 

tuff..................................... 139, 147

Basalt.... ------- 124-125,128-130,133,135

Basanitc. ----------------- 124, 128, 130-132, 134

Bertholcl, S. M., analyses by...--------- 147, 148

Big Lake, dunes near_...................... 100

Black Bluffs, fossils from..................... 118

Pleistocene sediments at_---...-...... 100,109

Bogoslof Hill-.--...--..-.--. 104-10(5, 119, 125 

Bruun, B., analysis by........ 121, 139, 140, 144, 148

Bufiington, E. C., Carsola, A. J., and Dietz, 

Jl. S.. Quoted. --._---_--.----_--- 152

Carsola, A. 3., with Bufiington, E. C., and

Dietz, 11. S., quoted. . .- - 152 

Cascade Point____.. ._......-------... 110

CratcrHill .--.-- .- 100,108

Crustal movements, geologic analysis of...... 353

Crystals, in peridotitc..-..- -.... 119-120

in tuff.....-.---. ....... 117, 139-140, 152

Dietz, K. S., with Bufiington, E. C., and 

Carsola, A. ,T., quoted--- .. ........ 152

Dikes, on St. George Island----.-------- 115,

117, 130-132, 133-135, .140, 148 

See also Limburgite.

on St. Paul Island--.--... 104, 132-133, 140, 148 

Dunes...----. ----------------- .- ---.- 100

Einahnuhto Bluffs.. . 106, 119, 127, .132, 134, 138 

Elliott, H. W., quoted........................ 112

Esogemunga escarpment...-------- 115, 129, 139,140

Esogemunga fissure____.- - . 141

Faults, on St. George Island ----------- 115

Feldspar in tuff.....----.----.-------- 117, 139-140

Field work............ ................. 103

Page 

Fossils ----------------- 117, 118-119

Fossiliferous sediments, St. George Island... 117-118 

Fox Castle.-...-------..----.-.-----.-- 115, 135

Fox Hill...... .......... .............. 103-104

Garden Cove.. 112,113,118,119,121,122,125,130-132 

Geography, physical...------------_------- 102

Glaciation, evidences of.___ 110,112,118, 153-155 

Glass.--.--..-..----.--..-...-.-.- 126, 130-138 

See also Analyses of basanitc, basalt, 

hyalobasunitc, and limburgitc.

GullHill ----- - . .- 115, 125 

Hyalobasanite... ------- 125, 120-127, 132-133

Introduction................ ............ 102-103

Isostatic compensation, a factor in evolution of 

magmas-.----.--------.--------.--_------ 151

Jocnsuu, 0., analysis by_------------------- 140

Kaminista. 104

Lacroix, A., quoted__...................... 144

LakcHill - 104, 105, 107, 125

Lava, analyses of.-__..._ __...._ 126-130

mineral components of._------------- 125-126

Pribiloftypeof 124

types of-.---------------._-..---...---- 124-125

See also names of lavas. 

Lava domes....-.---.--....-.---...-......... 115

Lava flows, on St. George Island...-- ...... 112,

114-115, 140-148

on St. Paul Island- -,--. 103-104, 125, 146-148 

Limburgite ---___........... 130-132, 141-144

Literature cited--. - .. .155-157

Locat ion------------------------------------- 102

MacNeil, F. S., identification of fossils by.- 118-119 

Magmas, factors in evolution of.............. 151

Mammoth, remains of__........_,___...... 119

Maynard Hill- ..--.-.-,, .-.--..--.. ... 115 

Mode, computation of... - .._ ._.___ 132 

Myak, St. George Island...-.----..-....... 115,117

Niggli values, quoted...........--..-.-.-.-... 147

Northeast Point, St. Paul Island........... 105,119

North Sea Lion Hill .... -.. ._ 115

Olivene.- ....... ....... 120,125-120

See also Basalt and Hyalobasanite. 

Olivene nodules............ ---------------- 144-146

Otter Island, description..................... 100

lava flow.....-.......--......---.-- 125,128,146

location................................. 102,100

sections on............................. 110,1 a

Paciflcite.-----.-----.--..--- .-.... 128-129

159
160 INDEX

Page

Peridotite, description---.-..--__-__.-.-- 119-121 

deposits on St. George Island--.----.-. 110,

112,113,117, 121,122,152 

distribution of .-.-------.-.--..--- 122,1)1.20

mineralogic composition of...------------ 121

chemical composition of ____._.__._---.-.. 121

Petrography.. -.----..--..-...---.-.-.--- 119-136

Place names, variants of _.. _----_-.--- 102,106

sources of._----.-..---.--..-..--..-.--.. 102

Plagioclase.-.------..------.-----------.--- . 12G

Pleistocene sedimentary deposits, on St. Paul

Island---..._-----.---.-.-----.- 106

Polovina Hill, rhyolitic glass on............ 137,150

Pyroclastic deposits, description_......... 130-138

on St. George Island -.----.----.----- 115,117

on St. Paul Island ----------- 104,100,137-138

Pyroxene...-..-....---.-.---.------.-.-----.- 120

Hay, L. L., quoted......--..--....-.-.-.-.-.. 150

Red Bluffs, St. George Island------------.--. 129

RushHill. -. - - 100,138 

llhyolitic glass, chemical analysis --------- 137

St. George Island, evidences of glaciation..-.. 110,

112,118 

fossils. _---.--------------.--------------- 119

lava Hows on----..----.-----... 112,115,140-148

location.-..-----..-------------....----.- 102

section......-.-...-....-..-.-......-.-.-. 114

tuff deposits on..._ __._..____.___ 139-141,152

St. Paul Island, fossils from..-............. 118-119

Page 

lava flow on................ 103-104,125,146-148

See also Lavas, analyses of. 

location-__-------_...._.__.__ 102

miniature volcanic forms..._....---.. 103-104

pyroclastic deposits oil......... 104,106,137-138

section.................--......-.--....... 115

sedimentary deposits......-------.---...- 106

sources of lava (lows...-.-.....---........ 104

Samlalogh Ridge.............._.........__ 115

Sea Lion Point....................._-.... 112,119

Sea Lion Rock, location.-...-...--.----...... 102

Serpentine. __________________________________ 120

Sills, analyses of............................ 130-132

South Sea Lion Hill.......................... 115

Spinel.-.--- -----__---------.-.--.--.._ 145-146

Staraya Artil Rookery...................... 115,117

Suskaralogh Point. .___...........__ 115,139

Tectonic environment, as a factor in evolution

of magmas__................... 151

Tolstoi Point, St. George Island.... 110,115,116,126

St. Paul Island------..---------------- 106,144

Tuff, analysis of.-... ...........-.-...-...-. 139

crystals in.-----.................... 117,139-140

Ulakiya Hill.- ............................ 115

Walrus Island, location__-.-...-.-..-....... 102

Zapadni Bay......_.-........-. 117-118,129,132,133

Zapadni Point, sedimentary beds._--------- 106

Zeolite crystals.. ----------------------------- 132

o