STICE, Gary Dennis, 1938- THE GEOLOGY AND PETROLOGY …the Manu'a Group, was simply called Manu'a. Perhaps Ta'u is a relatively new name for the island. Friedlander (1910) states,

This dissertation has been

microfilmed exactly as received 67-13,712

STICE, Gary Dennis, 1938-THE GEOLOGY AND PETROLOGY OF THE MANU'AISLANDS, AMERICAN SAMOA.

University of Hawaii, Ph.D., 1966Geology

University Microfilms, Inc., Ann Arbor, Michigan

THE GEOLOGY AND PETROLOGY OF THE

MANU'A ISLANDS, AMERICAN SAMOA

A DISSERTATION SUBMITTED TO THE GRADUATE SCHOOL OF THE

UNIVERSITY OF HAWAII IN PARTIAL FULFILLMENT OF THE

REQUIREMENTS FOR THE DEGREE OF DOCTOR OF

PHILOSOPHY IN GEOLOGICAL SCIENCES

SEPTEMBER 1966

By Gary Dennis Stice

Thesis Committee:

Gordon A. Macdonald, ChairmanDoak C. CoxKost A. PankiwskyjAgatin T. AbbottJohn J. Naughton

TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS. . • . . . . . • . . . . . . .. i

LIST OF TABLES.

ABSTRACT. . . .

INTRODUCTION

Background of the Project.Previous Investigations.Procedure of the Project .Acknowledgments .•..Geographic Description

Location and AreaHistory .•.Climate .••.Soils

GEOLOGY OF TA'U ISLAND

Nature and Distribution of Rock Types ..

ii

1

46

1011

13151926

27

Extra-caldera Volcanics . 30Intra-caldera Deposits of Ta'u. 35Pos t- ca lde ra De pos its . . • . 37Tunoa Shield Volcanics.. .....•.. 38Lua te le Shie ld Vo lcanics. . 41Post-erosional Deposits . .. .•.•. 43

Fa leas ao .Tuff Comp lex. . . . .. 43Fitiiuta Volcanics 46

Intrus ive Rocks . . . . 47Noncalcareous Sedimentary Deposits. 48Calcareous Sedimentary Deposits. • . 50

Ma j 0 r St r u c t u res . . • . •Geomorpho 10 gy. • •

5262

Features Caused by Volcanic Activity. 62Features Caused by Faulting . . • . . • .. 63Streams and Valleys. . . . . . 64Coastal Erosion 66Coastal Deposition. . . . 67

GEOLOGY OF OFU AND OLOSEGA ISLANDS

Nature and Distribution of Rock Types ...

Extra-caldera Volcanics .Intrusive Rocks . • . . .Intra-caldera Volcanics .•Post-erosional Volcanics ....•.Nonca lcareous Sed imen tary Depos its ..•Calcareous Sedimentary Deposits

Major Structures .Ge omorp ho 10 gy. .

Features Caused by Volcanic Activity ..Features Associated wEb Faulting.Streams and ValleysCoas ta 1 Eros ion . . .Coastal Deposition••.

PETROGRAPHY

Mic ros copy . . • • . . •• . . • . . • .

Page

73

808691949596

97100

100101102102104

107

Extra-caldera Rocks of Ta'u Shield. 107Intra-caldera Rocks of Ta'u Shield~ 112Tunoa Shie ld. . • . . 113Luatele Shield. • . 113Faleasao Tuff Compl~x .. .... . 114Fitiiuta Rocks. . . . . . • . 116Intrusive Rocks of Ta'u . . . . • . 116Extra-caldera Rocks of Ofu and Olosega. .. 117Intrusive Rocks of Ofu and Olosega. 118Intra-caldera Rocks of Ofu .....•.

Chemical Analyses

Theories on Oceanic Basalts • . . • . . •. 122Determinative Methods . . . . • . . . . .. 132Chemical Analyses of Rocks from Manu'a· 142

LIST OF ILLUSTRATIONS

Page

FIGURE 1. Location map of Samoan Archipelago. 14

FIGURE Z. Bouguer anomaly map of Ta'u Island. . . 29

FIGURE 3. Schematic diagram illustrating "gravitycollapse" and possible associated volcanicactivity. . • . . . • • . 57

FIGURE 4. Bouguer anomaly map for Ofu and 010segaIslands. . • .. .. • .. .... 78

FIGURE 5. Beach prof i 1e s on Ofu and Olos e ga Is lands. 104a

FIGURE 6. Stable joins of system Ne-Fo-SiOZ atlow and high pressures. System Ne-Fo-SiO Zat (a) 1050 0 C &nd 1 bar, and (b) 1250 0 Cand 33,000 bars . . . . . . . . . . . . . . 133

FIGURE 7. Alka1i:silica diagram of rocks from Manura 154

FIGURE 8. AFM diagram of rocks from Manu'a. . . . . . 156

PLATE 1. Geologic map of the Manu'a Islands ••••• backpocket

LIST DF TABLES

Page

TABLE 1. Percentage frequencies of w~nd.from

different directions by months atMulinu'u, Western Samoa. 20

TABLE 2. Stratigraphic section of Ta'u Island. 31

TABLE 3. Stratigraphic section of extra-calderavolcanics up Avatele Stream, northernTaln. . . . . . . . . . . . . . . . . . .. 36

TABLE 4. Stratigraphic section of Dfu and OlosegaIslands. • . . . 75

TABLE 5. Stratigraphic section of extra-calderavolcanics on Dfu . • . . . . . • • . . 83

TABLE 6. Stratigraphic section of extra-calderavolcanics on Dlosega . . . . . . . . . 87

TABLE 7. Stratigraphic section of intra-calderavolcanics on Dfu • . . . . • . • . 93

TABLE 8. Comparison of chemical analysis of thesame Hawaiian lavas. . . . . . 140

TABLE 9A. Chemical analyses and norms of rocks fromthe Manula Islands. . . . • . . . .. 143

TABLE 9B. Explanation of chemically analyzedspecimens from Manu'a .•..... 146

ABSTRACT

The three islands of the Manu1a Group--Ofu, 01osega,

and Ta'u--were built by volcanic activity along the crest

of the Manu1a Ridge, which trends approximately N 70° W. The

largest volcanic center has built up the southern part of

Ta'u with aa and pahoehoe flows of basalt, olivine basalt,

picrite-basa1t, and fe1dspar-phyric basalt to over 3000 feet

above sea level. These flows dip more steeply than those of

Hawaiian volcanoes, averaging about 20_25° and frequently

exceeding 30°. The total thickness of this section measured

from the floor of the ocean, about 9000 feet below sea level,

is over 12,000 feet. Summit collapse formed a caldera that

became partially filled with ponded lavas and pyroclastic

deposits which accumulated to a ~hickness of over 1000 feet.

Two smaller shields are located on the northeast and the

northwest rift zones. The latter rift zone coincides with

the regional rift of the Manu'a Ridge. Post-caldera parasitic

cones are approximately aligned along these rifts. Following

an extensive period of erosion, a tuff complex extended the

northwest corner of the island and buried a former sea cliff.

Lapi11i tuffs from this area contain large dunite xenoliths

and coral blocks. Several other post-erosional eruptions

occurred, some of which built out an area on the northeast

corner of the island, where the village of Fitiiuta now

stands. Dunite xenoliths are frequently found in the post­

erosional aa flows.

Ofu and Olosega consist of a complex of at least six

z.

composite cones approximately aligned along the regional rift.

Two of these were in the form of shields which gradually

coalesced and buried the older cones. The shields are com­

posed mainly of aa and pahoehoe flows of basalt, olivine

basalt and picrite-basalt, with average slopes of 10-ZO°.

Hawaiites occur in the uppermost part of the shield on

Olosega. The highest points on the two islands, Tumu (16Zl

feet) on Ofu and Piumafua (Z095 feet) on Olosega, represent

the approximate former summits after the collapse of these

two shields.

The depression on northern Ofu was partly filled by

ponding of olivine basalt, hawaiite, and ankaramite lavas.

The floor of the other depression lies offshore and may never

have been exposed above sea level. Following a period of

quiescence and erosion probably partly contemporaneous with

that on Ta'u, recent vulcanism built a lapilli tuff cone, the

remnants of which form Nu'utele and Nu'usilaelae Islets.

Chemical analyses of twenty widely selected samples show

the rocks of Manu'a to be high in titanium. All of these

rocks have NaZO + KZO : SiOZ ratios that place them in the

alkali basalt suite. Plots of the chemical analyses, includ­

ing rocks from the nearby island of Tutuila (Macdonald, 1944),

indicate a progressive enrichment in alkalis. The occurrence

of hawaiites and picrite-basalts indicates that a primitive

alkalic olivine basalt magma was just beginning to undergo

differentiation. The most important factor in this process

was crystal settling, especially of olivine. The dunite

3.

xenoliths in the late-stage rocks of Ta'u came from a residual

olivine layer, probably at the bottom of the magma chamber.

The magma did not become sufficiently acidic to produce

the trachytic end-members of the series, which would have

contained normative and possible some modal quartz. Iron and

titanium, in general, decrease slightly with increasing silica

content, but on Tutuila, the iron in the andesites and tra­

chytes begins to decrease rapidly with an increase in silica

and alkalis, perhaps due to an increase in the partial pres­

sure of oxygen within the magma chamber.

INTRODUCTION

Background of the Project

The volcanic islands of the Pacific Basin have received

little attention geologically, mainly due to their lack of

mineral resources, relative inaccessibility, and small popu-. .

lations. The major exception is the Hawaiian Islands, which

have been studied in detail by many geologists (Jaggar,

Stearns, Macdonald, Tilley, Powers, Kuno, and others), and

therefore are almost invariably regarded as the classic

example of a volcanic island group within the Pacific Basin.

In Hawaii, petrographic studies and chemical analyses have

shown that two fundamental trends of rocks are represented--

tholeiitic and alkalic. Moreover, Macdonald has worked out

the stratigraphic and structural relationships of the two

major trends (Macdonald and Katsura, 1964).

Generally, tholeiitic lavas are erupted predominantly as

thin pahoehoe flows with some interbedded aa flows during a

long shield-building stage of the volcano. Next, late in the

stage of caldera collapse and filling, tholeiitic basalts are

succeeded by more alkaline rocks; that is, the extrusions

become richer in alkalies and poorer in silica. Typically

there is a marked decrease in the frequency of lava flows on

the flanks of the volcano toward the end of the caldera-

filling stage, so that considerable erosion can take place.

A final stage is represented by differentiated volcanics (tra-

chytes, mugearites, and hawaiites commonly interbedded with

alkalic basalts), and by the still later nepheline basalts

5.

and related basanites and alkalic basalts.

Whether or not the generalizations made concerning the

Hawaiian volcanoes are applicable to other Pacific volcanic

islands can be determined only by extending similar studies

to the other islands. Many of them have been briefly des­

cribed geologically, a few have been mapped (more often than

not as a sketch map, due to the lack of good topographic base

maps), and isolated samples have been studied microscopically

and chemically analyzed (Washington, Barth, Lacroix, etc.).

But very few (except those of Hawaii) have been studied

systematically, with the proper sampling that requires a good

geologic map. The structural and stratigraphic relationships

of many of the rocks that have been studied and analyzed in

the past are not known. Before conclusions can be drawn

regarding the petrogenic relations of the rocks, their geo­

logical relationships must be known, and consequently the

area must be mapped geologically in the detail required.

The Manu'a Group of American Samoa was selected for this

project because it is one of the few unmapped volcanic Pacif~c

Island groups of the oceanic type for which there is a good

topographic base map. The only other likely choice would be

the Society Islands; permission was denied by the French

Government to undertake any investigations there. Previously,

the Government of American Samoa had requested the United

States Geological Survey to do detailed mapping of the islands

of eastern Samoa. Air photo surveys of the Manu'a Group was

undertaken in 1962 and again in 1964. From these air photos,

6.

the Topographic Division of the United States Geological

Survey has published a topographic map, released in 1965,

which was used as a base for the geologic map included in

this paper.

Previous Investigations

LaPerouse in 1798 seems to have been the first person to

note that the islands of Manu'a were of volcanic origin and

were surrounded by coral reefs. In 1842 Couthouy, a biologist

with the Wilkes E~pedition, reported large coral fragments 85

feet above sea level on the northwest end of Manu'a. During

the times of the early explorers Ta'u, the largest island of

the Manu'a Group, was simply called Manu'a. Perhaps Ta'u is

a relatively new name for the island. Friedlander (1910)

states, "Tau •.. ist die grosste und wird auch allein manchmal

Manua genannt." (There is some support for this also in that

the title, Tui Manu'a, meaning king of Manu'a, may be paral­

leled in rank today by the Tui Dlosega, thus .indicating that

the people of Olosega, and probably also Dfu, did not feel

they were subjects of the Tui Manu'a.)

Although a party from the Wilkes Expedition visited

Manu'a, the geologist of the party, J. D. Dana, did not accom­

pany them. He did go to Tutuila, Savai'i and Upolu, however,

and he did state that Dfu and Olosega were once united and

that "Manu'a (Ta'u) is described as having the form of a

regular dome" on the basis of the reports from other expedi­

tion members who had been there.

7 •

Friedlander (1910) was the first geologist to visit

Manu'a. He also thought that Ofu and Olosega were remnants

of a single volcano and that the embayments to the north and

south represented two central craters of collapse that nearly

coalesced. He pointed out that th9 lavas of Ta'u were rela­

tively recent in age and that the scarp on the south side of

Ta'u (he speaks of southeast, but this obviously is due to a

poorly o=iented and inaccurate map) was formed by caldera

collapse. Friedlander also thought that the present sea cliff

on the southern shoreline of Talu was the vestige of further

collapse. He described the high plateau of Ta'u and the cones

near the summit. He must have reached nearly the highest

point of the island and looked south into the caldera from

the rim--a view so spectacular that it seems rather peculiar

that he did not comment On it.

There is not much doubt that Friedlander did get to the

northern rim of the caldera, however, because it is marked

on his route map, and although the map is quite inaccurate,

it does show where he went in a general way. Judging from

his descriptions of the caldera, he must have had the good

fortune to have been there on a clear day. At that elevation

(about 3,000 feet), there are clouds covering the island most

of the time. The journey to the highlands must have taken

two or three days, unless there were trails at that time that

have since been obliterated by the dense vegetation.

Friedlander was told by an inhabitant of Olosega that in

1870 there was a submarine eruption between Olosega and Talu.

8.

He notes that the eruption was previously reported in the

literature as occur~ing in 1866 and correctly attributes the

contradiction to Polynesian custom. Since they have no sense

of time, the people are embarrassed by not knowing the exact

time. Even though they themselves are not certain, they feel

obligated to supply foreigners some kind of an answer to their

incessant questi0n, nWhen'i" According to one native, water

and dense smoke (not stated whether black or white) reached

high into the air. During the nights a distinct glow (from

the ash cloud?) was seen, and ash and pumice were thrown out.

Friedlander points out that the German sea charts show a

submarine volcano between Olosega and Ta'u only 46 meters

below sea level.

Daly (1924) spent a few days on a reconnaissance of the

Manu'a Group during the time of his more complete and exten­

sive study of Tutuila. From his study of Samoa, he made

valuable contributions regarding oceanic islands. Among his

observations in Manu'a are:

(1) The western slope of Ofu and the eastern slope of

Olosega largely preserve the constructional profiles of a

volcanic cone.

(2) Cliffs of approximately 100 meters have been cut

into the islands by the sea, whereas the much higher (500-600

m) curvilinear precipices On the north and south central shores

suggest an origin in a double evisceration through (a) vol­

canic explosion, (b) faulting into a sink analogous to Kilauea

or Mokuaweowec, and (c) large-scale landsliding due to

9.

foundering of large parts of the volcano. The first hypo­

thesis he considers improbable, but he could not decide

whether the foundering was due to double collapse or land

slip.

(3) The lavas of Ta'u are relatively fresh, whereas

deep weathering has laterized the flows on Ofu and Olosega.

Stearns (1944) made a one-day reconnaissance of Manu'a.

From his observations he produced a geologic sketch map. Con­

sidering the time allotted to his investigation, the sketch

map and conclusions are amazingly accurate. Stearns divided

the rock units into pre-caldera volcanics, dike complex and

post-caldera volcanics. In disagreeing with Daly that the

Ofu-Olosega volcano was a cone of the explosive type, Stearns

stated that pyroclastic beds are no thicker or more numerous

than around the main vents of many basaltic volcanoes. He

felt that Daly may have used "explosive type" loosely in the

sense that there were numerous fire fountains. (In the

opinion of the writer, Daly was referring to the large volume

of explosion breccia exposed on eastern Ofu and assumed most

of the island was built by such pyroclastic deposits.)

StearJS also stated that the steeply-dipping pre-caldera

lavas of the Ofu-Olosega cone indicate that they plunged into

deep water and mantled a steep-sided submarine cone probably

largely pyroclastic, calderas being formed in part by collapse

over a magma reservoir and in part by great landslides on the

steep underlying ash beds. He also suggested the possibility

that the 2000-foot cliff on the north side of Ta'u may be due

10.

to faulting in connection with another caldera offshore.

Macdonald (1944) made petrographic studies of the rocks

collected by Stearns. He describes three olivine basalts

and one picritic basalt collected from western Ta'u and th~~e

olivine basalts, one augite-rich picritic basalt (ankaramite),

and one basaltic andesite (hawaiite) collected from north­

western Ofu. He noted a higher Ti and Na content in the mafic

lavas of Samoa than those of Hawaii, which for the most part

are quite similar. Macdonald calculates the alkali-lime

index according to Peacock (1931) as 50.5, just within the

alkalic group. He concludes from his study, in concurrence

with Daly, that the parent magma of the Samoan province was

approximately equivalent to olivine basalt in chemical COm­

position and that all other rock types represented have been

derived from the parent magma by crystallization differentia­

tion. Macdonald attributes the mechanism of differentiation

largely to sinking of heavier crystals, with filter-pressing

and volatile transfer probably having minor influence.

Machesky (1965) conducted a gravity survey of the Manula

Islands. The results of this work showed a Bouguer anomaly of

up to +290 mi11iga1s over the center of the main caldera on

Ta'u (Figure 2). The highest Bouguer anomaly (+310 mi11iga1s)

on Ofu and Olosega was recorded at Sunulitao on eastern Ofu

(Figure 3). An intrusive plug occurs in this area.

Procedure of the Project

Field study, mainly involving the preparation of a geo-

11.

logic map and the collection of samples for petrographic

study in the laboratory, was undertaken during January 31-

April 26 and June IS-July 26, 1964. A final trip was made

during January 2-21, 19~6, with Dr. G. A. Macdonald, in order

to make a final check on the field data after the laboratory

work had been completed.

The laboratory work was undertaken at the University of

Hawaii during July 28, 1964-April 14, 1965 and at Tokyo Uni-

versity during April IS-July 14, 1965. The laboratory work

consisted mainly of a petrographic study of the rock samples

--thin section studies, interpretation of the chemical analy-

ses, electron microprobe analyses, etc.

Acknowledgments

All of the costs of field work and the laboratory work

at the University of Hawaii, chemical analyses, thin sections,

air photos, maps, etc., were defrayed by National Science

Foundation, Grant No. GP-2l96 (G. A. Macdonald, Principal

Investigator). The chemical analyses were made by the Japan

Analytical Chemistry Research Institute of Tokyo. The thin

sections were made by Mr. Fred Roberts. The air photos and

maps were obtained from the Topographic Division of the U. S.

Geological Survey. Mr. F. W. McCoy, Jr., ably assisted

during the field work. He presented a thesis on the "Geology

of Ofu and Olosega Islands" in partial fulfillment of the

requirements for the Master of Science degree at the Uni-

versity of Hawaii (February 1965). Dr. G. A. Macdonald has

12.

aided with many helpful comments both in the laboratory and

in the field. His suggestions have been invaluable and are

greatly appreciated.

Laboratory work involving thin section studies and elec­

tron microprobe analyses was done as a Research Fellow at the

Geological Institute of Tokyo University from April to July,

1965. Transportation and living costs were provided by an

East-West Center grant. Dr. H. Kuno was particularly helpful

with petrographic studies and in coordinating the research in

Japan. Mr. K. Nakamura devoted much of his time to assist in

the work with the electron microprobe analyzer. Dr. S.

Aramaki and many other geologists from Tokyo University,

Hokkaido University, Kyoto University and the Japan Geological

Survey graciously contributed much of their time in order to

give assistance.

The field work in Samoa was undertaken with the permis­

sion of the District Governor of Manu'a, High Chief Lefiti.

The Samoan people were extremely cooperative and their hospi­

tality was overwhelming. Especially appreciated was the

assistance of Ali'i Sili (High Chief) To'atolu Nua of Ta'u,

Faifeau (Reverend) Soloi of Fitiiuta, Ali'i (Chief) Milo and

Ai Fai'i of Olosega, Sione Malauulu, Tulafale Sili-iHigh

Talking Chief) Velega and Ali'i Sili (High Chief) Misa of Ofu.

Without the help of the Samoan people the field work would

have been much more difficult, if not impossible.

13.

Geographic Description

Location and Area. The Samoan archipelago is a chain of

South Pacific islands approximately 520 km (300 miles) long.

Samoa is about 2300 air miles southwest of Honolulu, 600 air

miles northeast of Auckland, 500 miles northeast of the

Colony of Fiji and only 100 miles north of the northernmost

portion of the oceanic deep alongside the Tongan Islands.

Politically, Samoa is divided into Western Samoa, which

includes the two main islands of Savai'i and Upolu, and

American or Eastern Samoa, which includes Tutuila, the islands

of the Manu'a Group, Swain's Island, and Rose Island (Figure 1).

The Manu'a Group is comprised of three small islands in

the eastern portion of the Samoan archipelago (lat 14-15 0 S,

long 169 0 W). Tutuila, the largest island in American Samoa,

lies about 60 miles west of the Manu'a Group. A small atoll,

Rose Island, is approximately 100 miles east of Manu'a.

Another atoll to the north, Swain's Island, is also under the

administration of the Government of American Samoa, but it

belongs to the Tokelau Island Group and both physically and

historically is not a part of the Samoan archipelago.

The islands of Samoa are located along the top of a sub­

marine ridge which extends over 300 miles from Savai'i to

Rose Atoll and has an approximate trend of S 75° E. However,

this feature is not continuous. That portion of the ridge

along which the volcanoes of Manu'a were built, hereafter

called the Manu'a Ridge, is offset to the north between

Tutui1a and the Manu'a Group, but has nearly the same

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direction, S 70° E, as the western portion of the ridge. It

seems most likely that this ridge at one time was continuous,

but it has been offset by later lateral displaceme~ts between

Tutuila and the Manu'a Group. Tutuila is surrounded by lIdeep

ocean" with depths greater than 3000 m between it and Upolu

to the west and the Manu'a Group to the east. Thus three

major volcanic piles are located along the archipelago:

(1) Savaii (700 square miles above sea level) and Upolu

(430 square miles above sea level), which were

probably two piles that gradually merged.

(2) Tutuila (52 square miles above sea level).

(3) Manu'a Group (22 square miles above sea level).

In addition, Rose Island (less than 1 square mile above sea

level) is the only remaining surface expression of a fourth

volcanic pile atop the Manu'a Ridge, approximately 100 miles

east of Manu1a. In a very general way then, volcanic activity

moved from east to west. There have been historic eruptions

in Savaii, Upolu, and a submarine eruption in Manu'a.

History. According to many anthropologists, the Samoan

archipelago is one of the first Pacific island groups to be in­

habited either entirely or predominantly by the Polynesians.

Archaeological and linguistic evidence indicates that these

people migrated from the Melanesian islands, possibly southern

Viti Levu in Fiji. According to most Samoan legends, these

people came from the heavens rather than "hava iki" or some

other place. The people of the Manu'a Group claim that.

Taungaloa, the "Adam" of Polynesia, ~ived in a place near

16.

Fitiiuta village-- to this day, there is a chief's title in

the village of Fitiiuta which still bears the name Taungaloa.

Even prior to the European discoverers, the Samoans had con-

siderable contact with other island groups, especially Fiji

and Tonga. In fact, Fitiiuta, which means "Fiji of the high-

lands," was supposed to have been named by the daughter of one

of the Kings of Manura (perhaps 400 or 500 years ago) who

married the King of Fiji and, upon her return to Manura, was

reminded of Fiji. The older name for Fitiiuta village was

Aga'i; this older name is still in wide use by the natNes

today. For approximately 400 years, from about 1300-1700, the

Tongans ruled all of Samoa with the exception of the Manura

Group. The King of Manu'a was one of the most respected titles\

in Polynesia. The first known Caucasians to discover\Manu'a

were members of a Dutch exploring party under the command of

Jacob Roggeren on June 14, 1722. They landed at Ta'u and

traded some goods. A few days later they also landed at Dfu

(Sharp, 1960). The first known Christian missionary to visit

the Manu'a Group was John Williams in 1832. He found that the

natives had already accepted Christianity through their con-

tact with Samoan missionaries from Upolu that he himself had

trained. In 1819, Arthur Young, the son of one of the British

missionaries, was born in Apia, Western Samoa. He declined to

follow in his parents' profession and became a trader instead.

Through his enterprise he met the daughter of the King of Manu'a

and married her around 1870. When his father-in-law died in

about 1890, Arthur Young's daughter, Margaret, became the regal

17.

authority in Manu'a. She died at an early age (22) in 1895.

Since there was no direct heir, the title was contested and

has been vacant since that time. The reason for the dis-

continuance of the title is not entirely clear, but it may

have been mostly because the Samoan people, being Christian­

ized, did not want to continue the practice of deifying one

of their own kind for fear of disrespect to God. The intro­

duction of Western influence and, indeed, European blood into

the title must have had considerable influence.

In 1838 the United States exploring expedition of the

Pacific, under the command of Lt. Charles Wilkes, U.S'.N:., made

a survey of the Samoan Islands. Included in this party was

the eminent geologist, James D. Dana. Thirty-four years

later, the chiefs of Tutuila agreed to give the U.S. Navy

exclusive rights to Pago Pago harbor, one of the finest

natural harbors in the world. During the latter part of the

19th century, Western powers were energetically competing for

territory in the Pacific. Consequently, in 1899, a conven­

tion among the United States, Great Britain, and Germany

signed an agreement whereby the United States "acquired"

Eastern Samoa and Germany "acquir~::1" Western Samoa. The high

chiefs of Tutuila "voluntarily ceded" their island to the

United States on April 17 of the following year, and the

islands of the Manu'a Group were ceded to the U.S. under simi­

lar conditions on July 16, 1904. After World War I, Western

Samoa became a trust territory of the League of Nations and

later of the United Nations, and was administered by New

18.

Zealand until 1962, when Western Samoa gained independence.

The Government of American Samoa was organized by the

Navy in 1900 and remained under its jurisdiction until 1951,

when the administrative responsibility was transferred to the

Department of the Interior. Up to this time, Samoa had under­

gone relatively little change other than a bank established in

1914 and a public school system founded in 1921; over 70 miles

of roads built during World War II, essentially all of them

on Tutui1a; and the deterioration of the people's customs and

culture probably more through new religious beliefs and lim-

ited contact with the outside world than through any efforts

of the Administration to "westernize" the territory. However,

since 1951, tens of millions of U.S. dollars have been pumped

into American Samoa to improve the livelihoods of the approxi­

mately 20,000 people while preserving as much of their culture

as possible, thereby making this territory "America's show­

place of the Pacific. 1I In spite of all the money invested in

a jet airport, educational television, paved roads, new school

buildings, etc., sufficient supplies of water and electrical

power, appropriate waste disposal systems, and adequate road

access are not available in most of American Samoa. Until

1965, all of the improvements were restricted to Tutui1a.

Although television is beamed over to Ta'u village in Manu'a,

there are no vehicles of any sort available to the Samoans.

There are not even any beasts of burden in the Manu'a Group.

Even now there are no villages in Manu'a with electricity or

adequate water supply systems (e.g., water-pumped wells or

19.

larger water storage tanks). The population of Manu'a,

especially young adults, is rapidly declining because of the

lack of modern conveniences and because of the people's desire

to improve their future. Unless something is done to check

this outward migration, within a very few years there will be

few people between the ages of 20 and 40 living in Manu'a.

The older people, being too old to support themselves by work­

ing the land, may be forced to move to Tutuila, Hawaii or

California, where they can be provided for by their children.

Climate. The climate of Samoa is tropical. Unfortu­

nately, no climatological data are available for the Manu'a

Group, because there are no weather stations on these islands.

However, the data for Tutuila and even Western Samoa should

be a close approximation to what would be expected in Manu'a.

The mean daily temperature in Pago Pago ranges from about 74­

89° F, the temperature at sea level being 81° F according to

the records of the U.s. Weather Bureau. The maximum recorded

temperature range for the period 1958-64 at Pago Pago was 60­

99° F. The Meteorological Office at Apia Observatory records

a range of only 1.2° F in mean monthly highs and a range of

2.7 0 F in mean monthly lows, thus indicating a very constant

year-round temperature. The average yearly relative humidity

at Apia is 83.6%, ranging from 80.4% in August to 84.8% in

February and March, which is the end of the "rainy season."

The trade winds in Samoa come from the east and south­

east. They are quite variable and, unlike Hawaii, Tahiti, or

20.

TABLE 1. PERCENTAGE FREQUENCIES OF WIND FROM

DIFFEREN~ DIRECTIONS BY MONTHS AT

MULINU'U, WESTERN SAMOA: 1946-1951

N NE E SE S SW W NW Calm (less than3 mph)

JAN 6 5 14 6 6 7 4 5 47

FEB 4 5 14 7 7 7 7 5 45

MAR 3 4 15 8 7 7 1 2 52

APR 1 1 22 11 8 4 1 2 50

MAY 1 1 23 17 6 5 2 2 43

JUN 0 1 31 22 7 5 0 1 33

JUL 0 1 28 21 10 7 0 1 31

AUG 1 1 34 25 9 3 0 1 26

SEPT 1 1 35 20 9 4 0 0 30

OCT 1 2 36 18 6 5 1 0 31

NOV 2 3 27 10 6 9 2 2 39

DEC 5 6 20 8 8 11 3 4 34

From Fox and Cumberland (l962), Table 2, p. 50.

21.

Tonga, which are at higher latitudes, are not predominant.

However, easterlies are the most frequent surface winds

classified to 8 compass directions in every month (Table 1).

Calms occur 25-50% of the times, with a general increase from

August to March. Usually the trade winds are predominant

during the months of June-October. After October, the winds

from the east and southeast gradually decline until, during

the "rainy season," from December to February, winds are

fairly equal from all directions. At this time, the north

winds, which generally bring rain, are the most frequent for

the year.

Rainfall is very high, averaging over 200 inches/year for

most areas. The average rainfall increases markedly with in-

creasing elevation. According to records of the u.s. Weather

Bureau, the average yearly rainfall in Pago Pago harbor

(Faga'alu) on Tutuila for the period 1958-64 is 186.49

inches/year at 80 feet above sea level, varying from a low of

144.54 inches/year to a high of 249.20 inches/year. For the

same locality, but at 820 feet in elevation, the average rain­

fall jumps up to 265.17 inches/year, varying from 248.99

inches/year to 314.23 inches/year. A 24-hour maximum of 15.87

inches was measured at the latter station.

The seasonal variation in rainfall is directly related

to the winds. Since the easterlies and southeasterlies bring

the least rain, the "dry season" is during the winter months,

June-October. The northerlies and westerlies are most fre­

quent during the summer months, December-March. In Western

22.

Samoa, the northwest coasts of Savai'i and Upolu have a lower

rainfall from May to October, when the east and southeast

trades are predominant. Although the islands of eastern

Samoa are quite small, there is some "shadow effect" on the

leeward sides of the island. For instance, a low average

monthly rainfall for January (13.36 inches) in Pago Pago is

not because of less rainfall on the island. Rather, it is

probably related to the fact that since Pago Pago is On the

south side of Tutuila, the winds were more directly from the

north than during December and February, when the average

monthly rainfall is higher (20.28 inches and 21~61 inches,

respectively). Also the southeast and southerly winds bring

more rain to Pago Pago during the summer months than that

which reaches the nOlthern coast of Tutuila. Therefore, these

two opposing effects make the rainy season less marked for

Pago Pago itself than for the northern and western sides of

the island. This same phenomenon was observed in Manu'a.

For example, the northeastern part of Ta'u, at Fitiiuta

village, experiences a less marked season than the north­

western end of the island, at Ta'u village.

Vegetation and Land Use. Due to the high rainfall and

temperature, the Samoan Islands are covered by dense tropical

jungle. This thick cover of vegetation greatly hampered the

field work. In Manu'a, rock exposures were rarely seen except

in the stream valleys or in the precipitous cliffs. On Ta'u,

where access to the interior of the island is poor, much time

23.

and effort was spent cutting pathways through the jungle.

Ofu and Olosega have much better trail access, and a much

larger percentage of the islands can be seen exposed in the

cliffs. However, even on these two islands, the dense vege-

tation made it difficult, indeed impossible, to solve some of

the field mapping problems.

The jungle area includes trees which are usually twenty

feet or so hig~ but occasionally grow to 70 or 80 feet.

Creepers mostly in the form of climbing vines are prolific in

the jungle. Fuesina occurs on the beaches, and fuesaina

(Mikania scandens) covers cleared areas. Several varieties

of ground ferns are common. Above 1500 feet elevation, large

~tree ferns occur. Mosses grow on trees and rocks where ~n-

light is restricted. Dense shrubs and trees of intermediate

height (10-20 feet) comprise most of the jungle. Numerous

varieties of ti plants are scattered throughout the forest.

Local patches of pandanus are especially common at higher

elevations, including the highest point in American Samoa on

Ta'u Island; this is probably because most of the lowlands

have been cleared of pandanus for plantations at one time or

another. The pandanus seems to grow just as well at lower

elevations due to the high rainfall even at low elevations.

Patches of grasses (pili grass is most common) are also often

found locally on the uplands of Ta'u. Among the trees observed

in Manu'a were few hibiscus, ala ala (Premna taitensis),

mulberry, several varieties of plumeria, coconut palm, the

24.

candlenut, and several other species that could not be identi-

fied. No mangroves occur in Manuta due to the lack of any

brackish water areas.

Much of the land in the lowlands and up to 1500 feet

elevation has been used for agricultural purposes. The coco-

nut palm is prolific but, of course, is rarely found at eleva-

tions higher than 600 feet or so. Taro, yams, tapioca,

breadfruit, several varieties of bananas, 'ava, cocoa, sugar

cane, lemons, oranges, and papaya are commonly cultivated. In

add i t ion, to ba cco, co f fee, an d grap e f r uitareo c cas i on a 11y

grown. Pineapples and sweet potatoes are found growing wild

in the uplands but are rarely cultivated.

Flies, ants, termites and mosquitoes are the insects in

-most noticeable evidence. Scorpions, centipedes, millipedes,

and gnats are also present. Black lizards up to 8 inches in

length are common; snakes of the constrictor type are quite

rare. Fresh water eels up to 6 feet long and several inches

in diameter are found in the upper parts of Laufuti Stream

which cut across the high benches within the caldera on the

southern side of the island. Bats are found in several areas,

particularly in a pahoehoe tube of the Lua Tele shield less

than one-half mile west of Fitiiuta. Indigenous birds include

lupe (pigeon), fu'ia (a brown bird about the size of a mynah

with a similar "whistling" call), and tatio (a white bird that

lives in the ground but is found only at high elevations in

the uplands). Coconut crabs (utu) are quite numerous. Pigs,

25 •

chickens, and dogs comprise the domestic animals.

With the steady decline of population in Manu'a, the

"plantations" are rapidly becoming overgrown. Except for

copra, little produce is exported, even to Tutuila. The copra

production is also falling off each year. According to the

1960 census, the population of Ta'u is 1661 people, but only

about one-half of this number can be considered as permanent

residents of the island. Similarly, the 1960 census showed

the population of Ofu as 605 and of Olosega as 429, but again

only about one-half of these people can be found living on the

islands at any given time.

Soils. The soils of Manu'a are predominantly stony

latosols, or lithosols. Curry (1955) describes the same soils

in Western Samoa as weakly acid, stony, clay loams. Latosols

are formed from the weathering of basic rocks in the humid

tropics. The process results in the loss of silica, alkalies,

and alkaline earth, and a proportionate increase in the resi­

dual material of the oxides of AI, Fe and Ti, as well as

primary minerals resistant to weathering such as magnetite

and ilmenite. In areas of steep slopes or young rocks, this

process of weathering is at an immature stage. Thus, much of

the soil contains fragments of relatively fresh rock. Such

soils are called stony latosols, or lithosols.

X-ray analyses of seven samples from Ta'u, five from Dfu,

and two from Dlosega indicate that very few clay minerals are

present. Nearly all of the samples contain halloysite, and

26.

over half contain gibbsite, but no kaolinite, dickite, or other

clay minerals were detected. The soils of Ofu and Olosega

have a larger portion of crystalline material than those of

Ta'u. Amorphous silica rather than halloysite occurs in the

samples taken from rngh altitudes on Ta'u, where there is more

rainfall. Hematite is also quite common, occurring in all of

the samples from Ofu and Olosega and in three from Talu.

On western Ofu and eastern Olosega lateritic soils have

developed due to deep weathering of thin-bedded lavas. These

more mature soils are probably not as fertile as the lithosols

because of leeching of potash and increasing acidity. Humus

produced from the tremendous supply of organic matter is

prevalent in all soils, especially in those of the level areas.

Fox (1962) states that lowland soils of Western Samoa compare

with the low-humic and humic latosols described from Hawaii

(Soil Survey of Hawaii, 1955), while soils developed from the

same rock types at higher elevations are similar to Hawaiian

hydro-humic la~osols.

GEOLOGY OF TA'U ISLAND

Nature and Distribution of Rock Types

Ta'u Island is composed of a major stratovolcano which was

built up on the crest of the Manu'a Ridge. The nature of the

lavas during the early shield-building stage is not known.

There may have been a long history of relatively quiet,

frequent thin lava flows from rift zones which steadily built

up a basal shield volcano, as is common in the Hawaiian

Islands. This early history is not revealed in the exposures

present on the island today. Collapse which may have been

the result of the extrusion of this large volume of lava

occurred near the summit of the volcano to form a caldera.

Following this, a period of somewhat more explosive eruptions

from cinder cones within the caldera and on its flanks con­

tinued to build up the island. However, the eruptions were

not as frequent during this stage, and erosion became more

intense. Even in the lower sections exposed on Ta'u, there

are numerous local erosional unconformities, indicating a

long history of intermittent lava flows even before the for­

mation of the caldera.

Two small shields built out the northeast and northwest

portions of the island. Tunoa shliad is located along the

regional rift zone. Luatele shield is part of a minor rift

zone extending down the northeastern slope of the main volcano.

These shields are composed mainly of thin-bedded olivine

basalt pahoehoe flows with dips of about 5°. The summits of

both of these shields collapsed to form central depressions

28·

that partially filled with subsequent volcanism. Small pit

craters are associated with both Tunoa and Luatele. After

the formation of these shields volcanism must have very

greatly or completely subsided, resulting in extensive

erosion. A great erosional unconformity resulted from

partial covering of this eroded surface by later flows. The

lava flows became so infrequent that an extensive sea cliff

formed around the island. Late stage lava flows sometimes

spilled over this cliff from cones on the flanks above. In

two areas, extensive late stage volcanism built large areas

of land in front of the old sea cliff. A tuff complex

approximately 1 square mile in area formed in this manner on

the northwest edge of the island. The historic village of

Fitiiuta is located on these late stage lavas, so that, even

though the lavas in this area look quite recent, they must be

at least 1500-2000 years old, i.e., prior to the settlement by

the Polynesians.

Thus, the rocks exposed On the island can be subdivided

into the following groups:

(1) Extra-caldera deposits, including:

a) pre-caldera volcanics, almost entirely lava flows,

erupted before the collapse of the main caldera

b) post-caldera deposits on the flanks of the

volcano during and after the formation of the

caldera

(2) Intra-caldera deposits, pyroclastics and lava flows

,::.71 .,,'\--

.

.....

- I

~e~i

29

~

s ,,-:-..... - .....

.­~.

30.

within the main ~dera

(3) Tunoa shield volcanics

(4) Luatele shield volcanics

(5) Post-erosional deposits, including Faleasao tuff

complex and Fitiiuta volcanics

(6) Intrusive rocks

(7) Recent sedimentary deposits (alluvium, talus,

beaches, etc.)

Extra-caldera Volcanics. The pre-caldera rocks are

exposed in the high cliffs of the southern part of the island.

The summit of the island, Lata Mountain, is 3185 feet above

sea level,and represents the edge of a vertical fault scarp

1400 feet high. Assuming that the pre-caldera flows have an

average slope of 25°, their thickness measured from sea level

is 3515 feet. The base of the volcano is about 9000 feet

below sea level, giving a total section about 12,000 feet

thick. On the flanks of the volcano, because of the dense

vegetation, it is impossible to distinguish pre-caldera lavas

from post-caldera lavas that have masked much of the island.

However, the rocks cut by deep valleys on the north shore are

certainly pre-caldera lavas, although late flows may have

filled the floors in a few of these valleys at lower eleva­

tions. Therefore pre-caldera lavas, since they cannot be

distinguished, are not mapped separately except in a few areas

where exposures are adequate. Otherwise the flanks of the

volcanoes are considered to be masked by post-caldera lavas,

TABLE 2. STRATIGRAPHIC SECTION OF TArU VOLCANO

Geologic Age

Recent

Formation

Sedimentarydeposits

Faleasao tuffcomplex

Fitiiutavolcanics

Thickness(in feet)

200±

400+, probab ly1100±

200+

General Description

Alluvium, beaches, and uncon­solidated deposits of talus atthe base of present-day cliffs.The only consolidated materialincludes deposits of beach rockand the coral reef offshore.

At least three cones composedalmost entirely of palagonitizedtuff. Black ash, explosionbreccia and thin flows are asso­ciated. Dunite nodules, magmaticbombs, coral fragments and lavablocks, some of which containdunite inclusions, are present.

Picritic basalt and olivine basaltflows commonly containing dunitexenoliths; local cinder andspatter associated with vents.

Great Erosional Unconformity-------------------Late Pleistocene(some flows may beRecent and post­erosional)

Tunoa volcanics 300±- Basalt and olivine basalt flows(aa and pahoehoe), relatively thin­bedded, but difficult to distin­guish from Taru volcanics. Gentlysloping shield with curvilineardepression associated with summitcollapse. Tumuli and recent sur­face flow features cover the floorof the depression; a few ash bedsare present locally. W

~.

Geologic Age

Late Pleistocene

Plio-pleistoceneto Recent

Formation

Luatelevolcanics

Extra-calderavolcanics

Thickness(in feet)

800.:!::

3515+

500±

General Description

Vesicular olivine basalts from asmall, gently-sloping shield, thesummit of which has collapsed toform a depression. A few pitcraters occur on the flank of thisshield.

Upper member composed of pic ritebasalts hawaiite and olivinebasalts which were extruded fromnumerous cinder cones located onthe flanks of the volcano. Gener­ally these lavas are thick aaflows, but often they are only1_2' thick. Some inter-beddedpahoehoe flows and ash beds occur.These post-caldera volcanics forma thin cap of 0-500' over the mainpart of the volcano and are partlyequivalent to Luatele and Tunoavolcanics. Some recent lavas havecascaded over the present sea clliIand therefore must also be partlyequivalent to the post-erosionalFitiiuta lavas and the Faleasaotuff complex.

The lower membe.r is also composedof inter-bedded aa and pahoehoeflows of basalt, olivine bas~ltand picrite basalt. These pre­caldera lavas cannot be distin­guished from the flows of the pos~

caldera cones, except in the deeplyVJ~.

Geologic Age

Middle Pleistoceneto Recent

Formation

Intra-calderavolcanics

Thickness(in feet)

600±-

General Description

eroded valleys of northern Ta'uand, of course, the caldera wallitself. Therefore these twomembers were mapped together,except in the one or two areaswhere pre-caldera lavas can berecognized with certainty.

Horizontal and nearly horizo~tal

ash beds cover most of the remain­ing caldera floor. The calderaapparently had not filled muchbecause some of the blocks down­dropped during the collapse arenot buried. There are at leasttwo distinct benches, the upperof which is almost, if notentirely, covered by thin-beddedash deposits. The lower benchcontains three large pit cratersand several cones, some of whichare post-erosional and probablyRecent, having flowed over the seacliff that has eroded into thecaldera itself.

ww

34.

since the latter probably do cover most of the island.

Even in the pre-caldera lavas numerous local erosional

unconformities were noted, particularly at higher elevations.

The lavas are dominantly olivine basalts with lesser amounts

of picrite-basalts and feldsophyric basalts at low elevations;

aa and pahoehoe flows are interbedded, with nearly exclusively

aa flows in the upper part of the section. The lavas at low

elevations have dips of about 15_20°, but the uppermost beds

have higher dips of 25_35°. These beds flatten out near the

summit, so that the same beds exposed in the high fault scarp

on southern Ta'u dip less than 15°. The be~s on the north side

of the caldera, then, have approximately the same attitude as

the present ground surface.

The olivine basalts generally contain phenocrysts of

olivine approximately 2-4 mm in diameter. About 1/3 of these

rocks contain plagioclase microphenocrysts 1 mm or less in

diameter. Most of the basalts contain phenocrysts or micro­

phenocrysts of plagioclase. A few flows exposed in the sea

cliff on the northern shore between 'Ao'auli and Lepula con­

tain plagioclase crystals up to 5 cm long. The massive

portions of these flows are only 1/2-2 feet thick, but are

separated by thick layers of clinker (up to 7 feet). They

are almost entirely plagioclase phenocrysts and therefore are

curiously friable. One 30-foot section is exposed at the b~e

of 'Ao'auli stream valley and another 60-foot section only

about 200 feet upstream. In several places higher in the

35.

section thin massive central portions of aa flows only 1-2

feet thick are associated with clinker beds up to 8 feet

thick; these flows generally have steep dips (30-35°), which

may explain the large amount of clinker. One tuff bed and a

soil horizon were found at about 1200 feet elevation in

'Ao'au1i stream valley. The section up Avate1e Stream

(Table 3) is characteristic of the extra-caldera volcanics.

Intra-caldera Deposits of Ta'u. Intra-caldera lavas

include picrite-basa1ts (both ankaramites and oceanites),

olivine basalts, hawaiites and possibly one or two flows of

mugearite. In addition, there are extensive deposits of ash

and lapi11i tuff. The flows vary from 5 feet to more than

30 feet in thickness. Horizontal and nearly horizontal ash

beds cover much of the remaining caldera floor.

The caldera was only partially filled. At least two

and possibly three normal faults formed major benches on the

caldera floor. The highest bench, at the base of the fault

scarp which rises vertically 1370 feet to the highest point

on the island, is composed of a series of approximately

horizontal pahoehoe flows. Intercalated with them is a bed

of ash 3-4 feet thick composed of individual layers less than

1 cm thick. The ash is basaltic glass. Both overlying and

underlying the ash, the flows are usually 1-4 feet thick and

are mostly vesicular olivine basalts with several oceanites.

The vesicles are filled with limonite or clay which may be

the result of alteration and deposition by rising gases and

36.

TABLE 3. STRATIGRAPHIC SECTION OF

EXTRA-CALDERA VOLCANICS UP AVATELE STREAM

Top Thickness(feet)

Nonporphyritic dense gray hawaiite dipping 28°N 25

Red vitric ash lying unconformably on an oldererosional surface and dipping 3l o N 1

A series of aa flows of olivine basalt and oceanite1-3 feet thick separated by clinker beds 2-6 feetthick, ropping approximately 15°N 25

No exposures 100

Aa flow of basalt dipping 55°N apparently pouredover fault scarp to form an angular unconformitywith underlying aa flows 5

A series of thin (1/2- 1 1/2 feet) aa flowscontaining abundant plagioclase laths up to 5 cmlong separated by clinker beds up to 7 feet thick(dip = 28°N) 50

A series of thin (1/2-11/2 feet) aa flows of basaltwith occasional olivine phenocrysts separated byclinker beds 1/2-4 feet thick (dip = 26°N) 20

Total thickness of section 226

37.

hot solutions in the vent area of th~'~ulcano, or may be

simply the result of ordinary weatheting.

Post-caldera Deposits. Post-caldera deposits cover most

of the island. The geologic map (Plate I) shows the location

of some of the vents; undoutiEdly there are many others that

were not discovered due to the dense jungle. For the same

reason, lava flows from these vents are extremely difficult

to delineate, but in many areas flows have spilled over the

old sea cliff, and apparently most of the flanks of the main

cone have been covered by these later deposits.

The contact between the post-caldera deposits and the

Tunoa shield is based on topography, since there is no petro­

graphic distinction between the rocks. The contact between

post-caldera deposits and lavas of the Luatele shield was

also based on topography. Even though the Luatele lavas are

quite characteristically vesicular olivine basalta, humus

soil cover and dense jungle obscure outcrops along most of

the contact.

Most of the lavas issuing from the post-caldera cones are

olivine basalts or picritic basalts. Less frequently vesicular

basalts and hawaiite occur; Often dunite inclusions are

found in some lavas approaching 5 cm in diameter. Rarely

some augite occurs along with the olivine in these inclusions.

Near the vents cinder and often welded spatter occur. The

olivine basalts typically have phenocrysts of olivine approxi­

mately 2-4 mm in diameter. Plagioclase microphenocrysts of

38.

approximately 1 mm in diameter are usually found in the

basalts, olivine basalts and hawaiites. The flows range from

2-3 feet up to 15 feet in thickness. Aa flows are greatly

predominant, although pahoehoe flows are found occasDna1ly.

The youthful appearance of the cinder cones and the fact

that flows are found spilling over the sea cliffs in many

places proves that the post-caldera deposits are in part con­

temporaneous with the post-erosional deposits. Possibly most

of the post-caldera eruptions occurred after the long period

of erosion, since the continuity of the great sea cliff around

the entire island could only have resulted from a long period

of erosion almost uninterrupted by lava flows. It seems more

likely, however, that there were two major stages of post­

caldera vo1canism--one before the formation of the sea cliff

and another afterwards, contemporaneous with the late-stage

lavas and related to them. Unfortunately no major erosional

unconformity like that represented by the lava-mantled sea

cliffs could be found in other areas.

Tunoa Shield Volcanics. Aligned with the submarine

volcano between Ta'u and 010sega and the caldera on the

southern part of Ta'u is a shield with a central depression.

The floor of this depression is covered by lava flows which

have a surprisingly recent appearance. This depression is

bounded on the eastern side by a curvilinear escarpment

approximately 200 feet high which was formed by normal fault­

ing. The western side of the shield has been eroded away, and

39.

the depression is now bisected by a sea cliff. At the

southern end the cliff disappears, gradually merging with the

slopes of the main cone. Post-caldera lavas have crossed this

area, but the disappearance of the cliff here is probably

mainly due to its intersection with the steeper slopes of the

main cone above. Just 0.3 miles southeast of the cliff is a

row of 3 small cones which are aligned parallel to the edge

of the cliff. These cones, plus two or more higher up on the

flanks of the main volcano, are approximately on the same line

with the submarine volcano and Tunoa shield.

On the northern edge of this fault scarp, near the old

sea cliff behind Si'ulagi Point, a small cinder cone has built

up. Half of the cone has either slumped down the cliff

during or after its formation, or else it has been eroded

back at about the same rate as the cliff. The former seems

more likely, because one would expect the cinder to e~ode

much more rapidly than the more resistant lava flows exposed

in the escarpment. At Fogapoa, I mile away on the southern

part of the cliff, is a pit crater. As with the cinder cone,

One side of the pit crater has either been eroded away, or

faulting during collapse of the shield has cut off one side

of the wall. Since the floor of the pit crater is at the

same elevation as the floor of the depression, it was probably

filled in by later lavas ponding within the collapsed area.

Therefore, it se-ems more likely that the northwest wall of the

pit crater was removed by faulting. Perhaps the pit crater

40.

was formed just prior to or during collapse of the shield.

Another less distinct indentation in the cliff approximately

1/4 mile north may represent the remnants of another shallower

pit crater (only 100 feet deep), or it could be only an ero­

sional irregularity in the escarpment. Located at the base

of the fault scarp 1/2 mile north of Fogapoa is a cinder cone

and its associated flow of olivine basalt.

The lavas which built up the Tunoa shield were predomi­

nantlyfuin-bedded pahoehoe flows with interbedded aa flows,

including a 10-foot thick flow of oceanite. The top of Tuno a

Ridge has a humic latosol cover approximately l-foot thick

with residual boulders of dense lava which was part of an aa

flow. Most of the lavas are probably aphanitic basalts with

some phenocrysts of olivine. Except for exposures at Tulatula

(the sea cliff southeast of Si'ulagi Point), which could only

be seen from a distance, outcrops which were definitely lavas

building the shield were not found. Tunoa Ridge and the fault

scarp are too deeply weathered to reveal much more than the

fact that the lavas appear to be thin-bedded with an occasional

dense flow.

The floor of the depression is covered by lava flows

which have a surprisingly recent appearance. Most of them

are pahoehoe flows with surface features and tumuli still fresh

in appearance. Some aa flows are also present. These flows

are mostly vesicular olivine basalts with feldspar micro­

phenocrysts of less than 1 mm. At least 5 vents were located

41.

on the floor of the depression; undoubtedly there are others,

but the dense vegetation makes their discovery more or less

a chance affair. The vents are low cinder cones about 200

feet in diameter. Some of the flows from these cinder COnes

contain dunite xenoliths up to 1/4 inch in diameter.

Two la-feet thick pahoehoe flows containing dunite

xenoliths are exposed in the sea cliff behind Ta'u village.

Underlying these is a palagonitized tuff bed of unknown

thickness. The same sequence was found exposed in the cliff

1/4 mile to the north. The increasing number of lapilli

indicates the source of the tuff was closer to the latter out­

crop. However, what is probably the same bed becomes finer­

grained again in the sea cliff near the contact with the

Faleasao tuff cone. Here the bed is only 2-3 feet thick and

is overlain and underlain by lava flows. Therefore, this

tuff, although fairly extensive, is much dder than that of

the Faleasao cone and predates the cutting of the sea cliff.

Luatele Shield Volcanics. The second group of pre­

erosional and post-caldera deposits are located on the north­

east side of the island. As at Tunoa, a secondary shield has

collapsed to form a depression, called Luatele. The topo­

graphic map published by the U.S. Geological Survey names

this place Judd's Crater. No one living on this island knows

where "Judd's Crater l! is. Therefore, this depression will be

referred to as Luatele, which is the place name used by the

Samoans.

42.

The collapsed area is only about 1/4 mile in diameter, but at

its deepest point it is 400 feet deep, twice the present

depth of the Tunoa depression. Both the depression and the

shield itself are only about 1/50 the area of the Tunoa

shie1d~ Less than 500 feet northeast of Luate1e is a smalll

pit crater, Lua1aitiiti, which is about 200 feet deep and

approximately 200 feet in diameter.

The shield is made up almost entirely of thin-bedded

pahoehoe flows of vesicular olivine basalt. The olivine

phenocrysts are up to 3-4 mm in diameter. Only one dunite

xenolith (1 1/2 em long) was found in the lavas. The flows

of this shield vary in thickness from less than one foot to

three or four feet, and the dip ranges from 3_4 0 near the sum-

mit to 6_8 0 farther down on the flanks.

The flows are exposed in the old sea cliff behind

Fitiiuta village. Unfortunately, due to poor exposures, the

contact between the Luate1e lavas and those from the main

volcano could not be located in this cliff. However, the

topography and characteristic lavas indicate the approximate

location of the contact. Talus boulders at the base of the

cliff also give an indication of the type of lavas present.

The western boundary of the Luatele lavas has been masked by

the eruption of a post-caldera cone, Sa'umane Crater. Ocean-

ite occurs at the vent, but the flow itself is a very oli~e-

rich vesicular basalt. Flows from this cone, as well as the

Luatele shield, all seem to have been erupted prior to the

43.

formation of the sea cliff. Along the southern boundary of

the Luatele lavas near Saua Stream, however, a few flows from

Recent extra-caldera cones have flowed over the sea cliff.

Post-erosional Deposits. The post-erosional or late stage

lavas are mapped separately in two areas where they can be

readily distinguished (Faleasao tuff complex and Fitiiuta

volcanics). Much, if not most, of the lavas from the post­

caldera cones are also post-erosional, as evidenced by

numerous flows spilling over the sea cliff. Since the flows

usually could not be traced to their source and since no major

erosional unconformity could be found to indicate post-eros~n~

flows, the post-caldera lavas on the flanks of the volcano

could not be separated into pre-erosional and post-erosional

members. Trerefore they had to be treated as a single unit.

Judging from the consistent height of the sea cliffs surround­

ing the island, there must have been a cessation of volcanism

after the formation of the Tunoa and Luatele shields.

Faleasao Tuff Complex. The two areas of post-erosional

deposits comprise the northeast and northwest portions of the

island. The area north of Taru village, including Faleasao

village and extending east beyond Sirulagi Point to Tulatula,

is a tuff complex made up of at least two and probably three

cones. One of these cones is centered at Faleasao, another

at Tora bay and probably a third smaller one at Farasemene

cove. Coral fragments included in the Faleasao and Tora cones

44.

prove that the eruptions built up through a contemporary or

relict reef and were therefore near sea level. The eruption

extended above sea level because it built up deposits which

covered the old sea cliff. Also, the occurrence of pisolites

on the surface of the beds en the southeast flank of the

Faleasao cone at the northern end of Ta'u village, indicating

that rain accompanied the eruption, are evidence for subaerial

deposition. From the offshore profiles, it appears that the

base of the cones lies at about 600 feet below sea level,

giving a total thickness of at least 1100 feet. Most of the

complex is a vitric-crystal lapilli tuff of basaltic composi­

tion which includes blocks and bombs. Most of the lapilli are

accidental, but some fragments of accessory basalt do occur.

There is a gradation from lapilli to blocks up to 2 feet in

diameter. Particularly in the area~ound Fa'asamene Cove,

the blocks increase in number and size. Along the northeast

part of the cove, oceanite boulders over 15 feet in diameter

are found. However, these larger blocks are probably remnants

of a small flow associated with the Faleasao tuff cone. Dunite

xenoliths up to 2 inches in diameter are found in the lava

blocks that are included in the breccia. Dunites are also

found as separate inclusions ranging from a few mm to over

15 cm in diameter. These nodules of dunite are extremely

abundant on the westernmost part of the cone which forms the

bay at Faleasao, Si'ua'i Point. In this same area a large

coral block approximately 10 cm across was found. The dunites

45.

are essentially 100% olivine; only two or three augite

crystals were found associated with the olivine. Although

magmatic bombs occur in many different areas, they were

particularly abundant in the cliff behind Faleasao.

Bombs up to 10 cm in diameter associated with bomb sags

were found in thin beds (1-30 cm thick), exposed in the south­

east portion of the inner crater wall. In the same area, the

pulsating activity which built the cone is recorded in rhyth­

mic graded beds, which are approximately 2 feet in thickness.

At least four or five of these beds are exposed; each unit

grades from lapilli tuff up into pure ash. Overlying this

series is a breccia bed approximately 3 feet thick.

Breccia occurs locally in other areas throughout the

complex. The largest concentration of breccias is in the area

extending from the northeast part of Faleasao to the south­

west part of Tola Cove, i.e., centered around Fa'asamene Cove.

The point between Falasamene and Tota contains an exposure of

the crest of a palagonitized tuff cone overlain by breccia

with an unpalagonitized, black ash matrix. The breccia is

mostly accidental basalt with some picritic basalt fragments.

The blocks usually range in size from about 2-10 cm, but some

are as large as 2 meters in d{ameter. Some of the blocks of

basalt contain dunite xenoliths; lapilli of dunite, coral, and

palagonitized tuff are included with the black ash matrix.

The whole area is mapped as a tuff complex, because there

is not enough evidence to determine the history of the area.

46.

It seems most likely that the Faleasao cone was formed near

enough to the base of the old sea cliff to erupt through what

apparently was a fringing coral reef. The eruption may have

occurred in wAter less than 100 feet deep. Where the eastern

part of To'a cone is in contact with the old sea cliff at

Tulatula, there is what appears to be a buried sea stack

which contains thin-bedded pahoehoe basalts unconformably

overlain by a thicker (approximately 10 feet) bed of oceanite.

The area between these beds and those of the sea cliff has

been filled in by palagonitized lapilli tuff.

Fitiiuta Volcanics. The northeastern corner of Ta'u

Island consists of post-erosional lavas which erupted from

at least two vents and extended the island beyond the old sea

cliff. This area is the site of Fitiiuta village. The fresh

appearance of the lava surfaces of numerous tumuli is clear

evidence for their Recent origin. The sea has already cut

into a lava cone now about 150 feet above sea level, revealing

its internal structure. This cone, Maluatia Hill, is composed

of cinder. Underneath the hill, extending from about 30-100

feet above sea level, is a sequence of thin-bedded pahoehoe

flows which includes thick lenses of red cinders~ These, in

turn, are underlain by a hexagonally jointed flow which is

30-40 feet thick. These lavas conform somewhat to the topo­

graphy of the hill, although they are of gentler dip. They

therefore probably came from the same vent, and may have been

a part of the same eruption as that of the overlying cinder.

47.

The whole platform seems to have been built out in front of

the sea cliff wfrh lavas issuing from only two or three erup­

tive centers. Deafening noises produced by air trying to

escape through cracks in the flow after being trapped and

compressed by waves entering eroded pahoehoe tubes can be

heard in several places along the shore north of Maluatia Hill.

Cinder and scoria reveal the proximity of another vent

inland to the southwest, near the base of the old sea cliff

behind the village. This vent, Maluatia Hill, the Lualaitiiti

pit crater and the central depression of the Luatele shield

all lie exactly on a line trending N 41° E.

The Fitiiuta lavas are almost entirely pahoehoe flows of

olivine basalt. Except for phenocrysts of olivine 1-4 mm in

diameter and plagioclase of less than 1 mm, these lavas are

aphanitic. The lavas contain abundant dunite xenoliths rang­

ing from a few mm to over 7 mm. Nearly all of the dunite

inclusions are composed entirely of olivine; only three or

four augite xenoliths or augite included in dunite xenoliths

were found.

Intrusive Rocks. The only exposure of a major dike com­

plex on Ta'u is located at Laufuti, which probably was near the

periphery of the summit aidera. A swarm of dikes and sills

crops out near the mouth of Laufuti stream, several dikes are

also found paralleling the cliff at Vailolo'atele. Only a few

widely scattered radial dikes were found in the Lata escarp­

ment. Except for occasional dikes, no other intrusive

bodies were found exposed on Ta'u. Most of the dikes are

48.

approximately I-foot thick; none were found to exceed 4 feet.

Many of them were magnetic enough to deflect a compass

needle.

The dikes associated with the complex at Laufuti are all

vertical and trend N 70-90 0 W. Numerous sills varying from

1/2-4 feet in thickness are found in this complex. They are

composed. of dense aphanitic basalt or olivine basalt. The

dikes are usually simple, but one or two multiple dikes were

found exposed in the steep sides of the narrow (approximately

200 feet in width) Laufuti stream valley about 100 yards from

its mouth. Some of the dikes are vesicular, particularly in

the outer portions just next to the chilled contact zone. The

chilled border is usually less than I-inch thick.

Noncalcareous Sedimentary Deposits. All exposed epi­

clastic sediments are Recent in age. Talus at the base of the

old sea cliff is the most prominent deposit of this group.

Much of the talus has been contributed by landslides, but

many boulders seem to have become dislodged one at a time,

rolling down to the base of the sea cliff. In most areas the

talus has been considerably weathered, mainly because its un­

consolidated state facilitates the circulation of air and

water. Indeed, many plantations are found on talus slopes

probably for this very reason. Talus occurs in varying

amounts at the bases of all the major cliffs on the island-­

the old sea cliff, the escarpments of the Tunoa and Luatele

shields, and the numerous escarpments associated with founder-

49.

ing on the southern part of Ta'u.

The alluvium of the streams is very similar to the talus,

except for the absence of soil and vegetative cover. In fact,

where streams run down a steep escarpment covered with talus,

it is sometimes difficult to locate the contact between the

two deposits.

Boulders up to 10 feet in diameter comprise the bulk of

the alluvial deposits. These boulders, especially the larger

ones, are usually aa lava. The majority of them are picrite

basalts. The vesicular pahoehoe lava is removed from the out­

crop by more extensive weathering in place, whereas non­

vesicular lavas fall down the slope as huge blocks. This

difference in weathering may be due to a combination of the

higher resistance to weathering of the denser flows, and the

relatively rapid removal of interlayered clinker beds,

leaving the central parts of the aa flows without support.

Perhaps a more important factor is that the vast majority of

the rocks exposed above sea level are aa rather than pahoehoe

flows, and in turn many of these are picritic basalts.

The cobbles and pebbles in the stream beds have been

formed mostly by chipping and breaking of the boulders. The

few pieces of pahoehoe lava found probably came relatively

unchanged from the outcrop, because these lavas, although

generally more weathered, are more elastic and therefore less

resistant to chipping and breaking while being carried down­

stream. Most of the granules occur as angular chips of

50.

non-vesicular or poorly vesicular flow rock. Less frequently

the granules are large phenocrysts of olivine, augite or

plagioclase. Parts of 'Ao'auli stream bed, for example, are

made up almost entirely of granules of plagioclase. These

granules have been derived from the flows exposed a few

hundred feet upstream which contain plagioclase phenocrysts

up to 5 cm long. These phenocrysts are so numerous that the

rock is quite friable and is easily eroded.

The stream beds on Ta'u are not of sufficient maturity or

development to contain material finer than medium-grained

sand. Some "powder" from breaking of boulders and soil from

rain wash do occur locally in pockets or pools, but not in

sufficient amounts to affect the general nature of the stream

deposits. Most such fine materials are immediately carried

down the steeply-sloping stream beds to the sea during the

first rainstorm that creates flowing water in the stream bed.

Calcareous Sedimentary Deposits. Most of the coastline

of Ta'u Island is fringed by long, narrow beaches that are

usually 40-100 feet wide at mean water leveL During high

tides some of the beaches nearly disappear, because the fore­

slope is usually only 10-12°. The beaches characteristically

are composed of only a foreslope with vegetation extending

inland from the berm crest.

Forty-two sand samples collected from various places

along the beaches and offshore were sieved on a Ro-Tap machine

using standard Tyler mesh screens. Three other samples were

51.

collected from stream beds on northern Talu. Three samples

were taken from the same point in several localities in order

to check if a representative sample was collected. These

multiple samples agreed within the statistical limits of error,

and it was therefore assumed that the method of sampling was

valid. The samples were also analyzed for weight percent of

insolubles. Frequency distributionrorves plotted for these

samples showed a variation in median grain size ranging from

0.29 mm to 3.50 mm for beach samples collected at sea level.

Samples from the beach and the reef flat at Talu village

are very well-sorted, possibly due to the strong periodic

currents of up to 2.8 feet/second that occur on this reef.

The insoluble material in these samples is usually lithic

fragments of lava rock with occasional mineral grains of

olivine, augite and magnetite. The soluble material is

mainly fragments of calcareous algae, foraminifera, coral,

mollusk shells and crustacean skeletons in approximate order

of abundance. Samples from Faga on northern Talu have the

highest percentage of insolubles. One of these samples con­

tained 32.3% insolubles. One reason fur the high content of

volcanic material in these samples is that several large

streams empty along this coast. Talus boulders cemented in

beachrock at Faga may also be a source for volcanic grains.

All other samples from Talu have less than 12% insolubles,

and all but three have less than 5% insolubles. Less than

1% insolubles was found in the samples collected from Tufu on

the southeastern tip of the island.

52.

Major Structures

Ta'u Island represents the remnant of a constructional

dome with two lesser shields located along northwest and

northeast-trending rift zones. The northwest rift zone,

along which lie the Tunoa shield and the Faleasao tuff complex,

extends seaward to Ofu and Olosega as the regional Manu'a

Ridge. According to bathymetric data (Plate 1), a dozen or

more volcanic cones are located along the crest of this ridge,

one of which erupted in about 1866. The water depth over the

ridge crest nowhere exceeds 750 feet, and in several places

it comes within 125 feet of the ocean surface. Midway between

Ta'u and Olosega, there also appears to be a rift zone that

trends approximately N-S, cutting across the Manu'a Ridge.

The northeast rift zone, along which lie the Luatele shield,

the Fitiiuta lavas, and a line of extra-caldera cones on the

flank of the volcano, also continues at le~ 3.6 miles off­

shore, where the soundings are not sufficient to draw contours.

The pre-caldera and the extra-caldera lavas have approxi­

mately the same dip as the present ground surface. On the

southern, eastern and western slopes, this averages about 15°.

The uplands of the island have very gentle slopes of only

2_3°; although no exposures were found, fue beds certainly

conform to this surface. The flows exposed in the main

caldera wall suggest a gentle dip of the summit lavas. On

the north side of the island the beds dip most steeply,

averaging about 30° at higher elevations, with the beds at

53.

lower elevations dipping about 15°. Many local erosional

unconformities were found in these upper pre-caldera lavas,

but no profound unconformity was noted. Therefore the change

to steeper dips apparently was a gradual one, accompanying a

decrease in volcanism and a corresponding increase in the rate

of erosion of the former slope. Beds of the Tunoa and Luatele

shields have average dips of less than 10°.

The caldera apparently was not formed at the exact summit

of the volcano but was located slightly to the south. The

beds on the southern side of the volcano have an average dip

of about 15°, conforming to the ground slope in that area.

This is also the approximate attitude on the east and west

flanks of the volcano. Within the caldera, two major benches

are present--one at Afuatai, and a lower one at 'Ele'elesa,

which contains three large pit craters and at least one cinder

cone (see Plate 1). However, no vents of any kind were found

on the upper bench. It is covered with thin-bedded, hori­

zontal flows of oceanite and olivine basalt with a few small

areas of a 3-4-foot layer of thin-bedded (less than 1 cm) ash.

Southeast along the bench towards Lilu, several fault scarps

bearing N 50_70° E with 5-10 feet of displacement are en­

countered. Also, the lava flows begin to have gentle dips of

about 5° S, as does the ground surface.

Southwest from Afuatai, a curved fault scarp 300-500 feet

high marks the northern boundary of the bench at IEle'elesa.

A little over 1/2 mil~ towards the southwest, at Leatutia, this

54.

bench is enclosed by another 300-foot fault scarp facing in

the opposite direction. At the top of this scarp is a gently

sloping area called Leavania. As its counterpart on the

eastern side of the caldera, Li'u, the thin-bedded pahoehoe

olivine basalts (1-5 feet thick) comprising that area have

dips of approximately 5_10° S which gradually increase to

17° S at Ta1i'i, about 1/2 mile downslope to the southwest.

The ground slope of this area is also the same as the dip of

the beds, varying from go at Leavania to 15° at Ta1i'i.

Again, as at Li'u, several faults are encountered on this

Leavania-Ta1i'i slope. One set trends N 50° E and forms

scarps 10-15 feet high which are approximately parallel to the

1000-foot high sea cliffs in this area. Also, a few faults

trending N 5_30° W, and forming scarps up to 10 feet high,

were observed. These faults cannot be traced for distances

greater than about 50 feet due to the dense undergrowth. The

sea cliff below Ta1i'i and the curvilinear fault scarp bound­

ing the north side of the depression merge and pinch out the

Leavania-Ta1i'i lava beds which, if extended, would continue

on out to sea.

It seems most likely that the bench at Afuatai with its

horizontal beds and the sloping area to the southwest

(Leavania and Ta1i'i) and southeast (Li'u) represent the

former summit of the volcano which has dropped vertically as

much as 1300 feet. The bench at 'E1e'e1esa and Leatutia,

which contains the pit craters and the cinder cone, probably

55.

represents the true caldera of the volcano. If this is 30,

the original caldera was approximately 0.9 mile in diameter and

was 300 feet deep. Later collapse has dropped down the nearly

horizontal summit area and its caldera to form the present

bench at Afuatai, as well as the peripheral slopes of the

Leavania-Tali'i and the Li'u areas. These areas have there­

fore tentatively been mapped as extra-caldera volcanics

(Plate 1).

The only major dike complex exposed on the island is at

the mouth of Laufuti Stream. Virtually all of the hundreds of

dikes in this area vary from N 75-90° Wand dip 80_90 0 S.

Most of them are only 2-3 feet thick. They are comprised of

dense, feldspar-phyric basalt, olivine basalt, and a few of

oceanite. Some of the dikes are vesicular, indicating that

they were formed near the surface. Olivine basalt sills were

found associated with the dike complex. The sills are usually

about 2 feet thick and have a maximum lateral extent of 20-30

feet.

The nature of the dike complex is further support for the

hypothesis of a caldera restricted to the 'Ele'elesa-Leatutia

bench. Generally a dike complex is located along a rift zone

just outside a related caldera. An extension of the proposed

caldera boundary would make the location and attitude of the

present dike complex approximately correct. It is also

striking that only 2 or 3 dikes were seen in the northern wall

of the 1300-foot fault scarp, and these were small (2-3 feet

56.

thick) radial dike~. The notable absence of associated con­

centric dikes paralleling this fault scarp supports the sug­

gestion that this cliff may not represent the true caldera

boundary formed by summit collapse of the shield.

A dozen or so thin (1-1/2-2 feet) dikes are exposed in

the cliff between Papaotoma and Si'ufa'alele Points, at the

southwest corner of the island. These dikes are parallel to

the cliff which merges with the sea cliff at Tali'i, 0.6 mile

to-the northeast. In the same area at the base of the cliff,

a Recent vent has extruded pahoehoe basalt flows which form

Latoaise Point. The horizontal flows of olivine-rich basalt

on Leatutoga Point only 0.1 mile north are probably from the

same source. Apparently the lava flowed out over the reef.

The offshore soundings, though very sparse, suggest the

possibility of large-scale foundering on the southern slopes

of Ta'u. The ridges on the east and west sides of the depres­

sion continue to slope about 15° southward on down to the sea

floor, more than 9000 feet below sea level. The caldera

apparently has no southern boundary, but continues seaward as

a huge depression or "valley" to at least 6000 feet, and

probably to 9000 feet below sea level. The numerous small

faults in the two large, seaward-sloping, down-faulted blocks

at each side of the caldera, could have resulted from tension

produced by large-scale foundering on the southern slope of

the volcano (Figure 3). Due to the lack of a better term for

this mechanism, it will be referred to as "gravity collapse."

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-~~ __..,..... ... - < ~_._......OV___..._;,·.~~• .:"'""... ··_-l'-·..~·.~·..-."'-~.-.: ..,•.• '":! ••~_,'":'••••-~.""':.....-__-~:>..•• -- ••~-_:..:!:.._=.:.=..,...:;~.:.:.::;..,...., ....;., •.. :_:•..,.~~ .•• :~..,._..._.-'...,,.-...... , .••.•"....-.._:"'., •• '"."."r.__...... .....,.,•.•·.. ~-.-....:.: ...-=_..=-.t; .._ .......:--"":-;-._.,=·.. :••",~~,"",,",_ •..;....,-.=.-...,..,...

58.

The 'Ele'elesa-Leatutia depression may, then, be the true

caldera while the larger Afuatai-Leavania-Tali'i depression

is the head of a large lands lipped area. If such a mechanism

was operative, it may have been associated with the collapse

of the summit of the dome which formed the caldera. A gravity

collapse of this nature would have probably also been associ&ed

with tectonic collapse. Perhaps it was not a simple landslide,

but was also related to normal faulting.

The sea cliff in this area may, then, be a tectonically­

controlled escarpment. The numerous faults parallel to the

coastline support this conclusion. Perhaps the suggested area

of gravity collapse contains a whole series of such faults

below sea level. Soundings were not adequate to determine

this. Also, at Laufuti, there are two small normal faults

which offset some of the dikes in the dike complex; these trend

N 60° E and dip 59° S with a 5-foot seaward downdrop.

It is possible that the Laufuti dike complex, the dikes

exposed in the cliff between Papaotoma and Si'ufa'alele Points

and the vent at Latoaise Point, are related to tectonic move­

ments along the sea cliff, rather than being associated with

minor rift zones and the caldera of the volcano. Gravity

collapse of such magnitude could be accompanied by volcanic

activity, if it were related to tectonic movements.

If this gravity collapse did occur, it probably was not

a single short event, but slowly took place over a long period

of time, perhaps even continuing at present. Except for the

59.

en-echelon faults perpendicular to the direction of this

proposed movement, the beds are little disturbe_d .for a dis­

placement of as much as 1400 feet. Some residents of Ta'u

claim they experience r'mafui' e," or earthquakes, once every

few years. It may be that no one has ever reported these

quakes for the simple reason that no "papalagi," or foreigner,

has lived on Ta'u for any extended period of time to confirm

or deny this statement. The gravity collapse may be associated

with a large-scale deflation of the Manu'a Ridge, particularly

in this area, where it is apparently sin~ing.

There may also be some effects from movements in the Tonga

Trench which trends N-S, its northern end reaching within 100

miles to the south of Manu'a. If extended north, it would run

between Manu'a and Tutuila. This is the same area where the

Manu'a Ridge is offset to the north from the regional arch

that forms the base for the rest of the Samoan Islands to the

west. Perhaps left-lateral faults are associated with the

movements in the Tonga deep, as right-lateral faults apparently

were in California, Alaska, and other continental borderlands

of the circum-Pacific region. There seems to be some similari­

ty in the depositional environment of the rocks in the Coast

Ranges during the Jurassic and that of the present continental

borderland west of the Tonga Trench.· Radiolarian cherts,

greenstone, spilites, and pods of limestone suggest that the

depositional environment for at least part of the Franciscan

formation was a deep trench with nearby island arcs.

60.

The crescent-shaped cliff bounding the Tunoa depression

suggests an original circular depression typical of calderas

formed by summit collapse. If the fault scarp is extended

seaward to form such a caldera its diameter would be approxi­

mately 1-1/4 miles. The escarpment varies from 200-300 feet

in height and has an average slope of about 34°. The presence

of one or possibly two pit craters, a cinder cone on the top

edge of the cliff, another cone at the base of the cliff, and

the alignment of 3 cones parallel to the southeast rim on the

flank of the shield are evidence that the summit collapse of

the shield was associated with volcanic activity. It is

possible that the depression was not bounded by cliffs on the

southwestern side, due to the intersection of the fault scarp

and the slope of the main volcano.

No exposures were found in the sea cliff south of Talu

village to indicate there had been a buried fault scarp going

out to sea where it might be suspected. The seaward trend

of the northern part of the fau~t scarp could not be checked,

because the sea cliff has been buried by subsequent eruptions

from the Tola tuff cone. Gravity measurements (Machesky,

1965) showed no high anomalies in this area. A high Bouguer

anomaly would be expected if this were a true caldera of

Hawaiian type, but of course, this is not conclusive

(Figure 2, p. 29).

The northeast rift zone of Talu, along which the Luatele

shield, Lualaitiiti pit crater, and at least two vents at

610

Fitiiuta are perfectly aligned, extends on out at least to

5000 feet below sea level. A line of four or five extra­

caldera cones just to the south also can be extended out to

sea along the same submarine ridge. The soundings are not in

enough detail to determine whether or not there are actually

two parallel submarine ridges. At any rate, subaerially at

least, the northeast rift zone seems to be comprised of two

nearly parallel rifts.

62.

Geomorphology

Features caused by Volcanic Activity. Ta'u Island was

originally a roughly circular dome which was only slightly

higher than the present summit at Lata Mountain (3185 feet).

Later volcanic activity b~i1t two shields, one each along the

northeast and northwest areas and extended the island in those

directions. Caldera collapse and possible gravity collapse

in its southern part have transformed the island to a butter­

fly outline.

Numerous cones dotted the flanks of the volcano and poured

lavas down its sides. These flank eruptions were not conti­

nuous in their activity, because local erosional unconformities

between the various flows are prevalent. Most of the flanks

of the volcano have uppermost beds with about fue same attitude

as the ground slope, indicating that activity of these cones

has been relatively continuous to the present, although no

historic eruptions have occurred on the island. Nevertheless,

probably in middle or late Pleistocene, there was a long

period of cessation in volcanic activity, during which an

extensive sea cliff was cut around the island.

Later flows from the cones on the flanks of the volcano

have spilled over the sea cliff, indicating a rejuvenation of

volcanic activity. Renewed activity continued to build out

the island along the northwest and northeast rifts. A tuff

complex has buried the old sea cliff in the northwest part of

the island, and two or three vents have extruded lava in front

63.

of the former sea cliff behind Fitiiuta, at the northeast

corner of the island.

Features caused by Faulting. The most striking feature

caused by faulting on Ta'u is the 1400-foot curvilinear

escarpment on the southern slope of the island. Within 150

feet from the rim there are elongated, trench-like features

which have their longest dimensions parallel to the escarp­

ment. A steady breeze comes "out of the ground" from these

holes. Actually, they are merely cracks developed near the

edge of the vertical and sometimes overhanging cliff. The

breeze is caused simply by the wind hitting the large face of

the cliff and coming through the cracks. Periodically, huge

blocks must falloff and crash to the ground 1400 feet below.

The escarpment is clearly a fault scarp.

Within the depression caused by the faulting there are

numerous topographic features, such as fault scarps, pit

craters, and cinder cones, which are discussed in detail

above (p. 53). There is a curvilinear fault scarp 300-500

feet high between Afuatai and the bench at 'E1e'e1esa. A

similar fault scarp of opposite displacement separates the

other side of the bench from Leavania 300 feet above. These

two faults were probably formed during collapse of the summit

of the volcano. Both of the two blocks which may have been

downdropped from the summit contain numerous en-echelon faults.

Most of the faults on the Li'u ridge trend N 50_70° E with

5- to 10-foot displacements. The majority of the faults on

64.

the Leavania-Tali'i ridge also trend N 50° E, approximately

parallel to the sea cliff. A few faults in the latter area

have a northwest trend and are 5 to 10 feet high. These

faults form "steps" down the ridges towards the sea.

The curvilinear escarpment forming Tunoa Ridge, the

Luatele depression, and the various pit craters described

above (pp. 60-61) are all features caused by minor collapse.

Streams and Valleys. The radial drainage pattern of the

original Ta'u dome is preserved today in a general way.

Recent eruptions from extra-caldera vents have buried many of

the older stream valleys. Daly (1924, p.132) noted, "the

deepest gorge observed is about 5 meters in depth." Except

for three stream valleys on the north and a very small part

of one on the south, this statement is nearly correct for the

. entire island. Ao'auli, Matautu'ao, and Avatele Streams have

cut canyons of over 300 feet in depth. Locally, Avatele

stream bed is over 600 feet lower than the adjacent ridges.

Apparently this portion of the northern coast has been left

relatively undisturbed by lava flows from post-caldera vents.

The beds exposed in these valleys even at the higher eleva­

tions are certainly pre-caldera lavas. Had there been no post­

caldera activity, the entire dome would probably have been

similarly dissected.

The only perennial stream, Laufuti, is located on the

southern side of the island where the dike complex is exposed.

The surface water travels in this stream less than 0.2 mile

65.

before reaching the sea. The stream continually pours out

thousands of gallons per minute from ground water reservoirs

formed by the dike complex, which strikes approximately per­

pendicular to the stream. These reservoirs are tapped by the

stream bed. The Laufuti stream drains much of the bench on

the eastern side of the down-faulted area, extending over

1-1/2 miles back into Afuatai and nearly to the base of the

precipice that rises vertically to the summit of the island

(Lata Mountain). Except for the lowermost portion that is

fed by springs at the base of the sea cliff, the stream flows

only after heavy rains. However, water stays ponded in some

of the uppermost parts of the stream, and numerous large fresh­

water eels (6-7 feet long, 4-5 inches in diameter) live there.

Several small streams drain the western portion of the down­

faulted area, the Leavania-Taliti ridge.

Among other important streams on Tatu not mentioned above

are Saua on the eastern side just south of Fitiiuta, Amouli

and Vaita on the western side, Faga and Patau on the northern

side, and Faleiulu on the northwestern side. The latter is

the longest stream on the island, extending over 2 miles, and

also drains the largest surface area. It begins at its source

as a radial stream on the main dome of Tatu, and then follows

down the northwestern rift zone. Upon encountering the back

slope of Tunoa shield, the stream turns to the north, running

alongside Tunoa Ridge until it reaches the sea.

None of the streams on the island are of sufficient

66.

maturity to produce alluvium finer than sand size, except for

minor amounts of soil removed as sheetwash. Most of the

stream beds are composed of large boulders 1-10 feet in

diameter. The steep gradient (usually about 25_30°) and the

large volume of water moving downstream after the frequent

heavy rains make movement of large boulders downstream

possible. The writer has witnessed an ordinary flash flood

at 'Ao'auli (all too nearly from the stream bottom itself),

during which cobbles and pebbles were moved downstream almost

as if they were made of cork. Boulders several feet in

diameter tumbled slowly seaward. Transport downstream of the

larger boulders probably is greatly facilitated by breaking

and chipping as they crash down the steep stream bed.

Coastal Erosion. During the long period of quiescence

sometime during the Pleistocene, an extensive sea cliff was

formed which can be traced completely around the island. The

height of this sea cliff is usually about 200 feet, except in

two areas. Along the northern shore the sea cliff increases

to a maximum height of 2400 feet. Offshore soundings of this

northern coast consist only of four lines running approximately

parallel to shore. The slope of the cliff in this area is

34_40°, whereas the beds dip 28_34°. The slope of the ocean

bottom offshore averages 22° down to at least 6000 feet.

Since the dips of the beds seem to be approximately the same

as the submarine slope, there is no reason to postulate land­

sliding on the order suggested for the south coast of the

67.

island. Nevertheless, landsliding on a smaller scale is an

important erosional agent in this area.

At the base of the steep slopes in this area are several

large landslides. At Faga a landslide has carried more than

8 million cubic yards of material down the steep slope. Sub­

sequently the talus boulders near sea level were cemented

into a beach conglomerate (boulders and cobb~s cemented in a

calcareous sand matrix). Much smaller landslides are found

at various other localities at the base of the sea cliff.

Landsliding and subsequent removal of the talus by wave

action is one of the important agents of erosion in Manu'a and

probably in most oceanic islands with high sea cliffs.

Notches at various localities in the tuff complex about

5 feet above high tide indicate that previously there was a

higher stand of the sea. This same 5-foot bench has been

noted on many islands throughout the Pacific Ocean (Stearns,

1941), and is therefore considered to be a eustatic change,

probably brought about largely by the formation of more ice

at the poles. This bench is noted only in the tuff complex

on the northwest side of the island. Apparently this stand

of the sea was of relatively short duration, so that only the

easily eroded tuff bears evidence of its occurrence. The

present sea level, of course, is eroding away this bench,

making it difficult to estimate the relative changes in sea

level.

Coastal Deposition. A terrace 13-15 feet above sea level

68.

occurs in several areas around the island. The most extensive

of these is the one on which Ta'u village i~ built. Other

examples of this l~ to IS-foot terrace are found at Faleasao,

Faga, Saua, Tufu, Amouli, and Si'ufa'alele. The terraces on

the southern part of the island are composed of sand and coral

shingle, whereas the others are entirely sand. The layers of

coral shingle are exposed in escarpments on the backbeach.

It is probable that these terraces were formed during a

higher stand of the sea perhaps related to the +12-foot stand

recognized in Hawaii (Ruhe, Williams, and Hill, 1965).

Nevertheless, during hurricanes, waves can and do come up over

the tops of these terraces. During the hurricane of 1959,

waves destroyed a trail more than 200 feet inland at the base

of the old sea cliff near Saua. However, the villages at Ta'u

and Faleasao have not been demolished by storm waves during

historic time. It is not likely that Faleasao ever has been

under heavy wave attack, because of good protection afforded

by the coral reef and the rim of the tuff cone which nearly

encircles the village.

Most of the coastline on Ta'u consists of narrow beaches

less than 100 feet wide between mean water level and the vege­

tation. During high tide the waves often wash up the foreslope

to the berm crest. Therefore, the vegetation usually does not

extend beyond the berm crest. The foreslope of the beaches is

getween 10 and 13°. Beachrock (cemented calcareous sand)

occurs in many places along these beaches. When present, it

69.

is near sea level or offshore. The beachrock in most areas

appears to be out of equilibrium with the present sea level

and is now being eroded. The slope of this beachrock is

usually somewhat less than the foreslope of the present beaches,

possibly indicating that it was formed during a higher +5-foot

(7) stand of the sea. The median grain size of the beach

material varies from medium-grained sand (0.29 mm) to gravel

(3.50 mm), whereas the beachrock is usually composed of fine

to medium-grained sand. In a few areas cobbles and boulders

are cemented in a matrix of medium to coarse sand (e.g. at

Faga and Ma'efu).

Around the island of Ta'u is a nearly continuous fringing

coral reef. Nowhere is the reef front more than 800 feet from

shore. Considering the age of the island, a more mature coral

reef would be expected. The island of Tutuila also has a

poorly-developed reef. However, soundings clearly indicate

that a barrier reef which extended over a mile offshore from

the present coast of Tutu~la has recently been submerged a

few hundred feet (see u.s. Coast and Geodetic Survey Chart

#4190). The only offshore soundings in Manu'a were completed

by the U.S. Navy in 1939(Plate 1). These soundings are

sparse, particularly for critical areas close to shore, and

their accuracy is questionable. (The islands themselves are

positioned 1.7 miles too far west on this chart.) There is

nO indication of a submerged reef around Ta'u, but the data

are too sparse and inaccurate to negate this hypothesis. The

70.

narrow reef suggests that the island has undergone some sink­

ing since Pleistocene time.

Usually a white sand beach is associated with the reef.

Of course, there is no lagoonal area. The reef flat contains

patches of sand, coral and coralline algae, whereas the fore­

reef is composed of prolific colonies of corals. At various

places along the reef there are channels about 15-25 feet wide

and 10-15 feet deep. The people living in the villages use

these channels as passes for their long-boats. Since there

is no anchorage in Manu'a, communication between sea-going

vessels and shore must be made by long-boat. Ta'u village,

for example, has two main channels, one at the southern end

of the village (Si'ufaga) and one at the northern end of the

village (Luma).

Water transported over the reef front by waves builds up

a hydrostatic head and runs back out to sea through these

channels. Current measurements were obtained by using a

tether ball and string. Dye

surface current measured was

was also employed. The maximum

2.8 feet/second near the main

channel at Luma during an ebbing tide. The maximum current

during flooding tide for the same area was 2.1 feet/second.

The difference is due to the greater hydrostatic head produced

at low tide between the water behind the reef and sea level.

Besides the main channels, there are numerous smaller tri­

butary channels running alongshore that join the main channel

in a manner similar to a subaerial stream drainage system.

71.

Another set of small channels cut perpendicularly across

the forereef and disappear at a depth of approximately 30

feet, where they merge with the slope of the forereef.

Usually the channels are spaced about 75 feet apart and are

4-8 feet wide. Rarely do they exceed 200 feet in length.

The heads of these channels terminate in the surf zone as

cirque-like features, some of which are 15 feet deep. The

maximum depth of the channel relative to the reef surface is

usually at the head of the channel, so that the gradient of

the channel bottom is considerably less than the slope of the

forereef. The bottoms of these channels usually are covered

with a thin veneer of sand, but often are paved with coral

boulders. Strong oscillatory currents dependent in part on

wave action occur in these channels. They are therefore

referred to as "surge channels." The sides overhang due to

the more rapid growth of coral at the reef surface. Inoome

channels the overhanging corals on opposite sides of the

channel have grown together to form natural bridges.

The species of coral identified on Ta'u Island are:

Maendraeuaensis (Hoffmeister)

Acropora hyacinthus (Dana)

Pocillopora eydonxi (Milne-Edwards and Haynes)

Psammocora contigua (Esper)

Goniopora muscosa (Wells)

Porites lobata (Dana)

Favia rotumana (Gardiner)

72.

Millepora tenera (Boschma)

Acropora formosa (Dana)

Pavona frondifera (Lamarck)

Pavona divaricata (Lamarck)

Favites magna (Hoffmeister)

Goniastrea retiformis (Lamarck)

In addition, Halimeda, Porolithon, Goniolithon and other un­

identifiable calcareous algae occur.

GEOLOGY OF OFU AND OLOSEGA ISLANDS

Nature and Distribution of Rock Types

Ofu and Olosega Islands are comprised of a complex

volcanic pile which was built up on the crest of the Manu'a

Ridge. At least seven Pliocene(?) cones exposed on these

islands are approximately aligned along the regional rift.

Two of these cones were built mainly by lava flows to form

coalescing shields. One is centered off the northwest coast

of Olosega near Sili, and the other is centered at Alofa on

the northern coast of Ofu. The older cones buried by lavas

from these shields include a small cinder cone at Tauga Point

on northwestern Ofu, a nearby tuff cone at the western end

of Samoli beach, a composite cone exposed in the cliff behind

Tolaga on southeastern Ofu, an explosion breccia cone with an

associated intrusive plug at Fatuaga Point on eastern Ofu,

and another tuff cone at Maga Point on the southern tip of

Olosega.

Ofu and 010sega Islands, plus the cones along the crest

of the Manu'a Ridge between Olosega and Ta'u, may have once

been physiographically similar to the present upland areas of

Upo1u and Savai'i, which consist of a double chain of cones

and craters that form the backbone of those islands (Kear and

Wood, 1959). Late stage cones along the central ridge on

Upo1u and Savai'i have poured lava over the older erosion

surfaces and across the reef to form extensive coastal areas

comprised of lowlands and swamps. The same general history

74.

applies to Tutuila, i.e., a line of cones along a regional

rift zone was modified by later volcanism (Stearns, 1944).

Apparently Ofu and Olosega have not been as active in late

stage volcanism, because there does not appear to be any ex­

tensive erosional surface buried by later lavas. Also, the

islands were probably lower than Savai'i and Upolu during

most of their history, as indicated by the presence of

palagonitized tuff cones, although no included coral fragments

were found in these deposits.

Most of the total volume of the islands above the present

sea level is composed of lava flows from the two shields, both

summits of w~ich collapsed. It could not be determined

whether these coalescing shields contained two separate

calderas or a single large caldera. Offshore soundings

(Plate 1) indicate the presence of a caldera northwest of

Sili. The only other evidences of the Sili caldera are the

numerous inward dipping dikes exposed in the high cliff

behind Sili and the attitudes of the lava beds on Olosega.

The depression at A'ofa On the north coast of Ofu is

clearly the result of summit collapse of the other shield.

Vertical dikes exposed in the sea cliff behind Samo'i parallel

the fault boundary of the depression and are thereby related

to summit collapse of the shield. The lava beds on south­

western Ofu dip gently away from this depression. These beds

are deeply lateritized, as are some of the beds on south-

eastern Olosega.

TABLE 4. STRATIGRAPHIC SECTION OF OFU AND OLOSEGA ISLANDS

Geologic Age

Recent

Formation

Sedimentarydeposits

Post-erosionalvolcanics

Thickness(Cn feet)

200±

255+

General Description

Unconsolidated deposits of talusat the base of present day seacliffs, alluvium, and beaches.Consolidated deposits of beach­rocks and offshore coralline reefu.

The remnant of a palagonitizedtuff cone forms Nulutele andNulusilaelae Islets off westernOfu. Two or three thick flows ofhawaiite and olivine basalt mayhave filled former deeply erodedvalleys.

___________________Gre at Eros iona 1 Unconformi ty _

Pleistocene

P lio- p Ie is tocene

Intra-calderavolcanics

Ext ra- ca Ide ravolcanics

400+

2175+

Thick flows of basalt, picritebasalt and olivine basalt wereponded within the Alofa caldera.One buried cinder cone is exposedin the sea cliff.

Upper member is composed of shiel~

building pahoehoe and aa flows ofpicrite-basalt, basalt and olivinebasalt-from the Sili shield onOlosega and from the Alofa shieldon Ofu. These two coalescingshields may have shared a singlecaldera. No extensive erosional

unconformity was recognized, but~Ln

Geologic Age Formation Thickness(in feet)

1000+

General Description

the flows seem to occur as thin­bedded pahoehoes in the lowestpart of the section and gradeupward into more frequent occur­rences of interbedded aa flows.Thick aa flows of hawaiite andpicrite-basalt culminate thisseries. These rocks were ex­truded prior to the formation ofthe caldera. The thin-beddedpahoehoe flows of southwesternOfu and southeastern Olosega aredeeply weathered and lateritized.

The lower member is composed ofa series of at least five cones,which are approximately alignedalong the regional rift. TheseCOnes were formed prior to thesummit collapse of the Sili andAlofa shields, perhaps even priorto the building of the shieldsthemselves.

-....J0'\

77 .

If the boundary of the depression at Alofa were extended

seaward as a crude circle, it would have a diameter of approxi­

mately 1 mile. This depression may not have been quite large

enough to be called a caldera, which by definition must have

a diameter of at least One mile, but the dimensions of its

seaward portion are not definitely known, and its average

diameter may have been considerably more than a mile. Since

this feature represents the collapse of the summit of a shieN,

it will be referred to as the A'ofa "caldera," and the shield

containing it will be called the Alofa shield. Judging from

the attitudes of lava beds and the character and position of

the dikes, the collapse of the shield centered northwest of

Sili was definitely large enough (about 1.7 miles in diameter)

to form a caldera; it will be referred to as the Sili caldera,

and the associated shield will be called the Sili shield.

By assuming that these two depressions were approximately

circular, it is estimated that they were about 0.7 mile apart.

However, no field evidence was found to confirm or deny this

supposition. No conclusive evidence for the presence of two

shields was obtained from the gravity data (Figure 4). It is

possible that, rather than two shields, there was only one

larger shield with a single elliptical caldera. The south­

eastern boundary of such a caldera would be the same as that

proposed for the Sili caldera; but the southwestern boundary

would include the Alofa depression in a relationship similar

to that of Kilauea Caldera and Kilauea Iki crater on Hawaii

J

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79.

(Stearns and Macdonald, 1946). The dimensions of this caldera

would be about 3 miles by 1-1/2 miles. The attitudes of the

beds in the cliff on northern Ofu and the absence of shield­

building lavas exposed in this area suggest that the presence

of two smaller eruptive centers is more likely. Tentatively,

then, the shield-building lavas on Olosega are mapped as part

of the Sili shield, and those on Ofu are mapped as part of the

Alofa shield.

The A'ofa caldera was partly filled with thick olivine

basalt, hawaiite, and ankaramite flows. One buried cinder

cone was found within the caldera at Sinapoto. There are

several parasitic cones along the flanks of the volcano. Some

of these cones, as well as some of the uppermost thick ankara­

mite flows on the islands, may also be post-caldera deposits.

Since these units could not be separated either in the field

or petrographically, however, they were mapped together with

the earlier rocks as extra-caldera deposits. Afterwards,

there was an extensive period of volcanic quiescence during

which deep valleys were carved into the slopes, and a sea

cliff 200-400 feet high was formed around the islands. There

may also have been large-scale foundering along the southern

coasts of the islands. Possibly this could have been ini­

tiated by the overburden of volcanic deposits on the earth's: crust.

Following this cessation in volcanic activity, it appears

that two or three thick hawaiite lavas flowed down a few of

the old valleys on the southwest side of Ofu at Tufu.

80.

Nu'ute1e and Nu'usi1ae1ae Islets, off the southwestern part

of Ofu, are the remnants of a Recent tuff cone.

The rocks exposed on Ofu and 010sega thus can be grouped

as follows:

(1) At least five pre-caldera cones approximately

aligned along the crest of the Manu'a Ridge

(2) Two (or possibly only one) later coalescing shields,

the summits of which collapsed

(3) Intrusive rocks

(4) Intra-caldera deposits of the Alofa caldera

(5) Post-erosional(1) flows and Nu'utele tuff cone

(6) Sedimentary deposits

Extra-caldera Volcanics. The To'agu composite cone and

the Fatuaga breccia cone are probably the oldest features

exposed on Ofu and 01osega. Unfortunately, they can be seen

only in the high cliffs on eastern Ofu. No outcrop could be

examined closely, but numerous talus blocks scattered along

the shore gave some indication of the rock types present in

the cliff above. The To'aga composite cone was built up in

large part by alternating sequences of aa flows, pahoehoe

flows, cinder and tuff. In the high vertical cliff the

exposures are disappointingly few, due to the dense vegeta­

tion, but it is apparent that this composite cone was buried

by subsequent flows from another source, probably the A'ofa

shield. An ash bed representing the former surface of the

cone forms an angular unconformity (on1ap) with the later lavas.

81.

West-dipping beds are found exposed at various places

in this cliff as far west as Va'oto. Exposed in the cliff

behind Va'oto is a bed of red ash up to 20 feet thick. It

extends up the slope of the ridge to Lepu1a. A lower

impermeable tuff bed provides a spring in the cliff behind

Va'oto that is utilized for drinking water in Ofu village.

If tm thick red ash bed in this area is the same uppermost

bed of the composite cone exposed in the cliff behind Tolaga,

then this cone must have extended at least to Valoto giving

a minimum diameter at sea level of 1-1/2 miles. The summit

of the cone was probably located about 0.4 mile due east of

Tumu Mountain at approximately 700 feet above sea level. It

is not possible to determine this accurately, because the

exposures are such that one is given little better than a

two-dimensional view.

There probably was another larger cone of at least the

same height that was centered offshore between Ofu and Olosega,

about 1500 feet due east of Fatuaga Point, which is formed by

a related intrusive plug. A tuff bed exposed in the cliff on

the north side of Olosega village is probably part of the

eastern slope of this same cone, giving it a minimum diameter

at sea level of 1.8 miles. This eastern portion of the cone

has been buried by subsequent aa flows from the Sili shield.

A large percentage of the total volume of this cone is

comprised of an explosion breccia. The breccia is almost

entirely made up of fist-sized blocks of at least six distinct

82.

kinds--one vesicular pahoehoe basalt, one ankaramite, one

dike rock, one olivine basalt, and two aphanitic basalts.

The matrix makes up less than 5% of the total rock. It is

composed of very well indurated vitric ash, containing

occasional crystal fragments of olivine 1-2 mm in diameter.

Near Tauga Point at the western end of Samoli beach, an

old tuff cone has been covered by a series of about 6 aa flows

which probably were erupted from the Alofa shield to the east.

The northern half of the cone has already been eroded away by

wave action, and a horizontal flow of basalt that was ponded

within this crater now forms a bench about 15 feet above sea

level. A 10-foot thick yellow lapilli tuff bed which dips

20° away from the cone and pinches out 500 feet to the east

is probably associated with this cone. This tuff overlies a

+SO-foot thick unstratified, palagonitized tuff that is

probably related to the deposits of the Alofa shield. Table 5

is a stratigraphic section at Samoli on northern Dfu.

An adjacent but slightly later small cinder cone at Tauga

Point has also been buried by later aa flows from the Alofa

shield. Numerous ribbon and spindle bombs occur in the ash

beds of this cone.

Thus, pahoehoe and aa flows from the Alofa shield over­

lapped, and eventually buried, the older coneso The pre­

caldera lavas of the shield are predominantly thin-bedded

pahoehoe flows with frequent interbedded aa flows and

occasional thin beds of ash and tuff. The lava flows are

83.

TABLE 5. STRATIGRAPHIC SECTION OF EXTRA-CALDERA

VOLCANICS ON OFU UP TIA RIDGE FROM THE

WESTERN END OF SAMO'I BEACH

Top(Estimated 220-foot elevation)

Nonporphyritic, deeply weathered vesicularflows dipping 15° W, mostly aa flows 2 to 5 feetthick, possibly some pahoehoe flows also

Gray nonporphyritic aa flows of hawaiite about8 feet thick with clinker beds 2 to 3 feet thick,dipping 20° SW, one ankaramite flow with thinveinlets of almost pure olivine and augiteconcentrations (groundmass less than 5%)

Yellow palagonitized vitric lapilli tuff withpumice lapilli and containing some thin ash beds,ribbon and cow dung bombs in ash bed, tuff pinchesout 200 feet to the west

Brown unstratified palagonitized vitric lapillituff, with pumice lapilli, and rare ash layers,dipping 14° W

Deeply weathered 2- to 3-foot thick pahoehoeflows, dipping 14° NW

Talus, containing much cinder as drift

Total thickness of section

Thickness(feet)

90+

30

10

40

20

20

210+

84.

basalt or olivine basalt except for a few flows of picrite­

basalt and hawaiite. Macdonald (1944) described a hawaiite

which was collected by Stearns from a talus block at the base

of the sea cliff near Tauga Point on Ofu. This talus block

must have come from the dense, thick aa flow near the top of

the sea cliff, which represents the upper portion of the

shield. The thick ankaramite and olivine basalt flows at

Tumu, the uppermost part of Ofu, are essentially horizontal

and probably represent nearly the original summit of the A'ofa

shield.

Pyroclastic pre-caldera deposits exposed on northern Ofu

include a +50-foot thick unstratified pa1agonitized tuff, a

few thin red ash beds, and a 3-foot thick breccia. Red cinder

found in the soil 700 feet due north of Tumu is probably from

an old post-caldera cinder COne in that area.

At Maga Point, on the southern tip of 01osega, is another

old tuff cone that has been buried by later flows. The northern

slope of this cone is 34°; its summit rose at least 250 feet

above the present sea level. The lower beds of this cone are

lapilli tuff with horizons locally rich in basalt blocks up

to 6 inches across. The upper 50 feet of this cone is comprised

of cinder and red ash. A 35-foot thick, dense flow of hawaiite

has ponded within the crater of Ehe"cone. A series of thin

pahoehoe flows has overridden the cone and forms Maga Point.

Interbedded aa and pahoehoe flows overlie this sequence. The

aa flows become increasingly dominant up-section. These flows

85.

are part of the main shield centered northwest of Sili.

The lowermost exposure of the pre-caldera lavas from the

Sili shield is found northeast of Sili, near Leaumasili Point.

A sequence of thin-bedded pahoehoe flows (1-2 feet thick)

with a few interbedded aa flows (up to 10 feet thick) are cut

by several thin dikes (varying from 1 to 4 feet in thickness)

that are parallel to and dip slightly towards the caldera

boundary to the west. The pahoehoe flows vary from 1 to 6

feet in thickness but are generally only 1-3 feet thick.

, These flows were erupted from vents near the center of the

shield before the formation of the caldera. They dip away

from the former summit .located northwest of Sili with atti­

tudes that are usually less than 10°.

Exposures in the steep cliffs behind Sili and Olosega

villages show that the interbedded aa flows generally increase

in number and become thicker up the section, ranging from 2

or 3 feet to over 20 feet. As on Ta'u, some of the steeply

dipping aa flows (locally up to 30°) have as much as 10 feet

of clinker associated with only 1 foot of the massive central

portion of the flow. Most of the lavas on Olosega are basalts

and olivine basalts. Usually the basalts are nonporphyritic,

but· often they contain microphenocrysts of plagioclase less

than 1 mm long.

Thick aa flows comprise most of the upper 800 feet or so

of the section, which extends to the summit at Pifumafua

Mountain. The dips of these later flows are relatively steep,

86.

ranging between 15° and 20°. A series of 3 or 4 flows of

hawaiite with a total thickness of over 75 feet are the

highest flows in the section that could be closely examined.

The rock is quite fresh except along joints where some manga-

nese deposition has occurred. A few exposures were found near

the summit, at Piumafua Mountain, but the lavas are too deeply

weathered to be identified. The only other outcrops found

stratigraphically above these are thick aa flows exposed in

the cliff behind Sili.

Thus, it is possible that all of the upper 600 feet of

the cone is capped with hawaiites. There seem to be few

picrite-basalts among the lavas of this cone, whereas both

ankaramites and oceanites are abundant on Ofu. A few oceanites

and ankaramites are exposed along the eastern coast of Olosega.

One of these ankaramite flows contains a concentration of

augite phenocrysts in the highly vesicular surface of the flow.

The frothy layer is 2-3 cm thick and contains 90% augite

phenocrysts up to I cm long. Table 6 is a stratigraphic sec­

tion of extra-caldera volcanics at Tafalau on eastern Olosega.

Intrusive Rocks. The intrusive rocks include numerous

dikes exposed in the cliffs of Ofu and Olosega, and one, or

possibly two, plugs on eastern Ofu at Fatuaga Point. Only one

dike was found intruding the intra-caldera volcanics. Nearly

all of the intrusive rocks are probably older than the intra­

caldera volcanics. The plug at Fatuaga Point may be part of

the core of the explosion breccia cone, which is probably the

oldest feature now exposed in Manu'a. There may be a smaller,

TABLE 6~ STRATIGRAPHIC SECTION OF EXTRA-CALDERA

VOLCANICS IN THE CLIFF BEHIND TAFALAU, OLOSEGA

87.

Top Thickness(feet)

Nonporphyritic, weathered vesicular pahoehoe 90flows 1 to 7 feet thick, dipping 12° SE

Olivine basalt, moderately vesicular aa flows 305 to 7 feet thick with clinker beds 3 to 5feet thick, dipping 24° E

Olivine basalt with feldspar microlites forming 15a dense, massive flow

Nonporphyritic, vesicular aa flows 1/2 to 3 feet 20thick with clinker beds 1/2 to 7 feet thick,dipping 24° E

Moderately vesicular aa flows of olivine basalt 354 to 7 feet thick, clinker beds 3 to 4 feetthick, with much red cinder and ash, dipping 24° E

Brown palagonitized vitric crystal tuff with 5olivine and augite crystals, lying unconformablyon lower flows, tuff dipping 24° E

Minor angular unconformity due to erosion

Nonporphyritic basalt with feldspar micro litesforming dense aa flows 3 to 5 feet thick,clinker beds I-foot thick, dipping 30° E

Nonporphyritic, vesicular pahoehoe flows, 1/2 to4 feet thick, dipping 30° E including:

ankaramite gradational to olivine-augitebas a 1t

olivine basalt with rare augite phenocrysts

basalt containing small laths of feldsparphenocrysts randomly oriented and rareolivine phenocrysts

Talus of blocks at base of cliff

Total thickness of section

30

40

10

10

65

350

88.

related plug about 1500 feet to the east. A concentric hill

there has the suspicious ovoid shape of an intrusive plug.

The rock cropping out on this small hill is aphanitic and

appears to be related to the dikes which form the razor-back

ridge on eastern Ofu. Perhaps the fine-grained borders of a

plug have net been eroded to reveal coarser-grained rock

comprising its center.

The Fatuaga plug represents a hypabyssal intrusion of

ankaramite magma. It was recognized and described by Daly

(1924, p. 134) as an elliptically-shaped plug with a maximum

diameter of 120 feet and a minimum diameter of 80 feet.

Actually it is considerably larger than this, probably at

least 500 feet by 300 feet. The highest Bouguer gravity

anomaly in Manu'a (more than +310 mi1liga1s) was recorded near

this plug (Machesky, 1964, see Figure 3). There is a gradation

in grain size from olivine-titanaugite gabbro in the central

portions of the plug to ankaramite near the peripheries. There

is also a gradation from a roughly ovoid plug near sea level

to a much more elongate ankaramite dike at higher elevations.

The general trend of the longest diameter of this feature

is approximately N 15° Wand vertical. At Sunu'itao, the

western edge of the plug cuts explosion breccia beds that trend

approximately N 85° Wand dip 12° toward the north. At sea

level near the top of the shark's-tooth peak at Vainu'u1ua,

this breccia appears to be trending N 30° E and dipping much

more steeply, about 30° towards the west. Some of the blocks

89.

included in the explosion breccia are identical to the in­

trusive olivine gabbro, except that they are slightly finer­

grained. Therefore, it seems likely that the intrusion

occurred nearly contemporaneously with the deposition of the

breccia. Included within the plug is a pod of brecciated

pahoehoe flows of vesicular basalt, which apparently was broken

off from the chamber wall and carried up in the magma chamber

during the intrusion. These flows, though brecciated, are not

the same as the heterogeneous explosion breccia which comprises

most of the associated cone.

Near sea level the plug is cut by numerous thin dikes that

strike approximately E-W and dip 55-85° toward the north.

Most of these dikes are dense and aphanitic, but some of the

thicker ones are vesicular, suggesting, along with the open

miarolitic texture of the olivine gabbro, proximity to the

surface at the time of intrusion. These dikes are also

parallel to the complex of dikes which form the backbone of

the ridge extending from Le'olo Ridge eastward to Olosega.

All of these E-W-trending dikes in turn are probably related

to the collapse that formed the Sili caldera.

The dikes forming the razor-back ridge of eastern Ofu

vary from I to 20 feet in thickness and are nearly vertical,

but some may dip steeply towards the north. The dike complex

in this area is about 400 feet wide. Most of the dikes are

dense basalts, although olivine basalts, ankaramites, and

feldspar-phyric basalts also occur. Large talus blocks of

90.

diabase lie along the shoreline north of Vainu'ulua and were

derived from the thick dikes ~t the top of the ridge.

The complex continues across Asaga Strait to the 2000-foot

cliff behind Sili village. Near Tamatupu Point, the western­

most tip of Olosega, thick dikes with dips as low as 50 0 N

occur. These may be slightly curved, but this was not

definitely established, because the individual dikes could

not be traced for great enough distances. Northeast of

Piumafua the number of dikes paralleling the face of the cliff

falls off sharply. North-dipping and vertical dikes related

to this complex can be observed in the cliff behind Olosega

village. The dikes decrease markedly in number both up-section

and away from the caldera. A few apparently related dikes

trending about E-W are found as far as 3000 feet south of the

cliff behind Sili. They cut steeply across flows dipping

10_20 0 away from the caldera.

At least one sill can be seen in the cliff at Faiava.

It is horizontal and appears to be about 400 feet long and 30

feet thick. A low-angle dike near the extreme eastern side

of the sill discordantly intrudes a series of pahoehoe flows

trending about N 50 E and dipping 15° E. It appears that

this dike is the feeder dike for the sill, but this relation­

ship cannot be seen clearly due to the dense vegetation.

Most of the other dikes in this portion of the cliff strike

about N 30 0 E and are vertical, indicating that the caldera

boundary swings from east-west towards the north in that area.

91.

The strike of the beds there seems to be about N 5° E, which

also indicates that the former summit of the shield was to

the west. A few of the thinner vertical dikes exposed in the

cliffs behind Si1i and Olosega villages are radial dikes which

are sometimes cut by later concentric dikes.

The A'ofa depression on northern Ofu is bounded on the

west side by a few vertical dikes that are nearly parallel to

the fault associated with the summit collapse of the shield.

Only about six of these dikes are exposed in the cliff behind

Samoli Beach; they vary from 1/2-6 feet in thickness. One of

these is a multiple dike approximately I-foot thick which is

intruded concordantly by several small dikes 3-6 inches thick.

A piece of dike rock containing dunite xenoliths was found on

the beach here. Several dikes occur in widely scattered places

along the cliffs of northern and southeastern Ofu. Usually

they are less than 4 feet thick, vertical, and approximately

parallel to the cliff face. An ankaramite dike more than 40

feet thick crops out at the top of the cliff at Mu1iolo and

Tumu, and is probably the source of nearly horizontal ankara­

mite flows on Tumu. It may be related to the collapse of the

Alofa caldera.

Intra-caldera Volcanics. Since the Si1i caldera lies

offshore, it is not known whether there were deposits that

filled it. The Alofa caldera on northern Ofu has been partly

filled with lava flows and pyroclastic rocks. The four lower­

most lava flows exposed are dense, nearly horizontal ponded

92.

flows of olivine basalt, three of which are 20-25 feet thick.

These are overlain by thinner (1-7 feet) interbedded aa and

pahoehoe flows of basalt and olivine basalt. One oc the upper­

most flows is a dense aa flow of hawaiite that is about 8 feet

thick and dips 25° towards the northwest. However, it rests

unconformably on another similar flow that is dipping about

6° towards the north. The upper lava apparently flowed over

a local irregularity, probably a fault scarp. Less than 300

feet away a small fault was found trending in a direction

parallel to the proposed fault. The downdropped sides of

both faults are on the north. Another thick flow lower in the

section plunged over a similar small fault scarp.

An ankaramite boulder was found on the beach 1000 feet

southeast of Tafe Stream. The block probably came from one

of the thick horizontal flows exposed near the top of the sea

cliff (about 400 feet high). This indicates that some of the

later intra-caldera flows were picrite-basalts as well as

hawaiites. Table 7 is a stratigraphic section of intra-caldera

volcanics at Sinapoto.

Just west of the mouth of Sinapoto Stream the lowermost

thick flows have ponded against a cinder cone, the highest

point of which is now about 60 feet above sea level. More

than half of the cinder cone has been eroded away by the sea,

however, and its summit was originally about 150 feet above

the present sea level. This cone must have been the source of

thin (4-8 feet thick) aa lavas of olivine basalt that flowed

93.

TABLE 7. STRATIGRAPHIC SECTION OF

INTRA-CALDERA VOLCANICS AT SINAPOTO, OFU

Top(Approximately 220-foot elevation)

Dense gray aa flow of hawaiite containing abundantmicrolites of feldspar and scattered microlitesof olivine, probably flowed over a fault scarp( dip = 25 0 NW)

Clinker

Dense medium-gray aa flow identical to that above,but dipping only 60 N

Clinker

Thickness(feet)

8

2

8

2

No outcrop~ covered due to thick soil and talus cover 15+

Nonporphyritic pahoehoe flows, 1-7 feet thick 30

Olivine basalt with olivine phenocrysts 2-3 mm in 25diameter occurring as approximately horizontalvesicular pahoehoe flows 1-15 feet thick; a seriesof thin-bedded pahoehoe flows cut by a lS-footthick flow that apparently plunged down a small faultscarp which had truncated the thinner pahoehoe flows

Nonporphyritic aa flow 4

Clinker 4

Olivine basalt occurring as vesicular pahoehoe flows 3

Clinker 4

Olivine basalt occurring as a dense horizontal flow 15

Do. 25

Clinker 1

Olivine basalt forming a dense horizontal flow, 25a small spring issues from its lower contact

Olivine basalt with abundant olivine phenocrysts 20forming a dense horizontal flow (exposed 0.3 mileeast of section)

Talus

Total thickness of section

30

221+

94~

down its northwestern flank to form the present Le1ua Point.

Post-erosional Volcanics. Nu'ute1e and Nu'usi1ae1ae

Islets, off the western coast of Dfu, are the erosional rem­

nants of the Nu'u 1api11i tuff cone. This cone was originally

about 4000 feet in diameter at the present sea level and was

approximately 300 feet high. The eruption must have occurred

near sea level, after an extensive period of erosion during

which a sea cliff was cut around Dfu and D1osega. The cone is

composed entirely of reddish-yellow pa1agonitized 1apilli

tuff with accidental blocks and lapi11i of basalt, plus a few

magmatic basalt 1api11i. Indiviuda1 beds vary from less than

1 inch to more than 5 feet in thickness. The slopes of the

original cone were about 30°. No coral fragments or any other

evidence of a submarine vent was found, but the eruption may

have been submarine in part.

Along the western coast of Dfu at least two flows of

aphanitic basalt over 35 feet thick in places have flowed

down the slopes of the A'ofa shield to form Nu'upu1e Rock,

just offshore from Dfu village. At sea level, just south of

Tufu Stream, one of these hawaiite flows is at least 25 feet

thick and is overlain by a 20-foot thick flow of olivine

basalt which contains a few small dunite xenoliths. It

appears that these flows poured down old, deeply eroded valleys

and possibly out over a reef. The source was probably a cinder

cone near the summit at Tumu which has since been obliterated

by erosion. Since it appears likely that these flows occurred

95.

after the formation of deep valleys and a sea cliff around

Ofu and Olosega, they are tentatively included among the post­

erosional volcanics.

Noncalcareous Sedimentary Deposits. The talus and

alluvial deposits are very similar to those described on Ta'u.

A recent-appearing landslide can be seen at Pouono, behind

the southern end of Olosega village. Several fan-shaped

slides can be seen along the base of the cliff along the coast

of southern Ofu, between Va'oto and To'aga. Talus boulders

have broken off the sea cliff and tumbled across the reef flat

in many places along the northern and southern coasts of Ofu

and Olosega. At Sili, large boulders on the reef, fresh scars

in the cliff above, and Samoan tales of people being crushed

beneath these boulders indicate that this cliff is actively

eroding at the present time.

Many of the streams On southwestern Ofu and southeastern

Olosega cut through deeply lateritized thin-bedded pahoehoe(?)

flows. Therefore, the stream beds are of a somewhat different

character in that few large boulders are found, whereas the

other streams in Manu'a are made up almost entirely of boulde~.

These stream beds instead are composed of a reddish-brown,

sticky silt. Parts of Tala'isina, Top~'a, Etemuli, and

Si'umalae Streams on southeastern Olosega and the upper por­

tions of Alei, Saumolia, Togalei, Malaeti'a, Tufu, and

Matasina Streams on southwestern Ofu have such stream beds.

The upper portions of Tafe and Sinapoto Streams on northern

96.

Ofu are also devoid of numerous large basalt boulders where

they traverse the relatively level floor of the caldera at

A'ofa.

Calcareous Sedimentary Deposits. Most of the beaches are

about 40 feet wide and rarely exceed 100 feet. They have an

even foreslope of 9_10° that extends right up to the vegeta­

tion. The bulk of the materials making up the beaches are

gravel, pebbles, and cobbles of coralline algae and coral.

On a few beaches some volcanic fragments of basalt, ankaramite,

tuff, olivine and augite occur. The median grain size is

usually coarse sand to gravel, as on Ta'u.

Beachrock is exposed in the intertidal zone of most

beaches on Ofu and Olosega. At Olosega village it is more

than 6 feet thick. The beachrock is composed of grains some­

what smaller than those making up most of the present beach,

usually medium to coarse-sized sand. Sometimes the percentage

of volcanic grains is slightly higher than that on the present

beach. Often the loose sand adjacent to the beachrock is about

the same size as the grains comprising the beachrock, being

finer grained than the rest of the beach. This is probably

because the finer-grained beachrock is being eroded (perhaps

with the aid of some process of de-cementation), and reworked

sand is deposited nearby. Also, the beachrock may act as an

obstruction to longshore currents carrying finer sand in

suspension, causing some of the load to be deposited. A few

beaches along the northern coast of Ofu, from Sinapoto to

97.

Oneonetele, and along the northeastern coast of Olosega con­

tain beachrock with subrounded basalt cobbles and occasionally

sub-angular boulders that are cemented in a calcareous sand

matrix.

Major Structures

The slopes of the explosion breccia cone centered off­

shore from the southeastern coast of Ofu were about 30°.

Interbedded aa and pahoehoe flows dip 20_25° from the center

of the composite cone located about 1.3 miles west-southwest

of Sunu'itao in the cliff behind Talaga. The tuff cone at

Maga Point on southern Olosega and a tuff cone and a cinder

cone at Tauga Point on northwestern Ofu also had slopes of

about 30°. These were all subsequently buried by aa and

pahoehoe flows dipping 10_20° away from the centers of the

two shields at Alofa and northwest of Sili. The later flows

on the highlands of Ofu are almost horizontal and must have

been near the summit of the Alofa shield.

A curious break in slope forms a bench-like feature at

Papausi on southeastern Olosega. Lateritized thin flows

exposed on the surface in this area dip 5_9° NE, somewhat

gentler than the present ground surface. Their approximate

strike (N 30_40° W) can be seen.w_ith the formation of ltsteps"

1-2 feet thick that presumably represent different lateritized

thin flows. A cross section drawn from the high cliff at 8ili

through Le'ala Point (cross section B-B', Plate 1) shows that

these beds most likely do not represent the natural slope of

98.

the Sili shield. There are no indications of any vents in

this area. These flows probably did come from near the summit

of the shield, but apparently their dips have been flattened

by encountering some kind of obstruction.

It seems most likely that another cone was located off­

shore from Le'ala Point, whkh lies exactly along the crest of

the Manu'a Ridge. Offshore soundings are sparse, but they do

suggest the possibility of a cone in this area. A thick aa

flow of picrite-basalt forms Le'ala Point. This flow dips

about 14° SE and probably also came from the Sili shield.

It suddenly increases in thickness downslope and seems to have

ponded against an obstruction, such as the slope of an older

cone just offshore.

Less than one-half of the A'ofa caldera is now present,

the rest having disappeared below sea level. Offshore sound­

ings are not in sufficient detail to determine whether the

northern half of this depression has simply been eroded away

or whether faulting or foundering was operative. The offshore

soundings suggest that at least part of the Sili caldera is

still present. Possibly erosion prior to regional sinking of

the islands and the Manu'a Ridge was largely responsible for

the disappearance of this feature, or perhaps the caldera

never did reach above present sea level. The collapse of the

Sili shield was associated with the later(7) intrusion of a

swarm of dikes approximately parallel to the caldera boundary.

The nature and distribution of this dike complex have been

99.

described above (pp. 89-91).

The Alofa caldera is bounded by a few thin dikes in the

cliff behind Samoli beach. The intrusion of dikes associated

with the summit collapse does not appear to have been nearly

as intense as those related to the formation of the Sili

caldera. However, the exposed dikes related to the Alofa cal­

dera are much nearer to the surface than those associated with

the Si1i caldera. Naturally, there would be a marked decrease

upwards in the number of dikes, as is seen in the cliffs

behind Olosega and Sili villages. Nevertheless, the estimated

dimensions of these depressions suggest that the Sili caldera

was considerably larger.

In the high cliff behind Tolaga, occasional dikes can be

seen trending approximately parallel to the cliff face.

Several large normal(?) faults can be seen in this cliff, and

also in the cliff along the northern coast of Ofu near One­

onete1e. The bedding in the cones has suffered large dis­

placements, but there is no surface expression of the faults

such that the direction or amount of their displacement could

be measured. These faults may have been related to the collapse

that formed the Alofa caldera.

Soundings are not in enough detail to indicate the nature

of the ocean bottom in the huge embayment between southeast

Ofu and southwest 01osega. As Daly (1924) suggested, some

type of foundering has occurred in this area, but there is no

evidence for another caldera. Perhaps gravity collapse,

100.

possibly associated with tectonic displacements at depth, is

responsible for the formation of these high cliffs. Support

for this hypothesis is much more easily obtained on Taru. If

Taru were submerged to the elevation of Afuatai (1700 feet

above sea level) all that would remain would be a parabolic

escarpment about 1400 feet high. It is curious that both of

these features occur on the southern coastlines and that in

both instances the highest gravity readings were obtained

near their central axes.

Geomorphology

Features caused by Volcanic Activity. At least seven

cones, two of which are shields, form the bulk of the volcanic

pile present above sea level today as the islands of Ofu and

Olosega. The highest point on the islands after summit

collapse of the two shields was probably only a few feet

above Piumafua, the present summit of Olosega with an eleva­

tion of 2095 feet above sea level. A concentration of pyro­

clastic deposits within a zone between sea level and an

elevation of about 500 feet suggest that the islands were

built at an elevation approximately the same as today.

Phreatomagmatic explosions would produce a concentration of

pyroclastic material near sea level. This implies that there

has been little sinking of these islands since their forma­

tion, because the pyroclastic deposits represent some of the

oldest rocks exposed on Ofu and Olosega.

If so, then the floor of the Sili caldera may never have

101.

reached above sea leve 1. Pe rhaps the northe rn boundary was

always much lower than the southern rim, so that the ocean

completely occupied the caldera, as it does the caldera of

Krakatoa Volcano in Indonesia. Wave action would then attack

only the southern rim of the caldera. Another possibility is

that only the northern portion of the caldera sank to a lower

level, leaving the southern rim exposed above sea level.

Local erosional unconformities, indicating periods of decline

in volcanic activity, were found stratigraphically low in the

pre-caldera section at Tafalau on eastern Olosega. Some of

the lavas in this area dip as steeply as 24° and lie unCOn­

formably over other lava flows. Nu'upule Rock and the dense

flows forming ridges behind Ofu village appear to be late­

stage flows which poured down old stream valleys that were cut

into the thin pre-caldera flows of the shield. The islets of

Nu'usilaelae and Nu'utele are remnants of a Recent tuff cone.

Features associated with Faulting. The boundaries of the

A'ofa caldera are the most obvious features caused by fault­

ing. The dike complex must have also been intruded into

fractures and faults bounding the Sili caldera. The cliff

behind Sili is therefore a fault-line scarp. The northern

portion of this cliff apparently has eroded back more rapidly

because of the absence or removal of the resistant dike com­

plex in that area. If the large parabolic embayment between

southeastern Ofu and southwestern Olosega was formed by

tectonic collapse, the cliffs bounding this area also are

102.

fault-line scarps.

Streams and Valleys. The stream valleys on southwestern

Ofu and southeastern Olosega extend away from the former sum­

mits of the shields in a radial drainage pattern. The thin

flows in much of these areas are highly lateritized, giving

a stream bed of sticky red-brown silt rather than large

boulders. The only other streams are those within the A'ofa

caldera, which drain the intra-caldera area and empty into

the sea along the cliffed north coast. Since these islands

are both lower in elevation and smaller in area than Ta'u,

there is considerably less rainfall and resultant runoff.

Therefore, the' streams are neither as large nor as numerous

as those on Talu. There are no perennial streams on Ofu and

Olosega. The streams flow only for a few minutes to a few

hours after a downpour. The stream valleys nowhere exceed

60 feet in depth, and usually are considerably less.

Coastal Erosion. After cessation of volcanic activity on

Ofu and Olosega an extensive sea cliff 200 to 400 feet high

was carved into the island by the sea. Behind Ofu village the

cliff is only about 80 feet high due to protection from wave

attack afforded by the Nu'u tuff cone offshore. The much

higher cliffs along the northern and southern coastlines

originated by faulting and/or foundering, but they have cer­

tainly been modified by marine erosion. The sea cliffs are

usually slightly higher than those on Ta'u. This may be

103.

partly due to the fact that there has been a shorter period

of time since frequent volcanic activity on Ta'u. However,

the larger amount of more easily eroded pyroclastic deposits

near sea level on Ofu and Olosega could also account for the

slightly higher cliffs.

Landsliding and subsequent removal of the talus by wave

action are important erosional processes, especially along the

high cliffs of the northern and southern coasts. The numerous

fan-shaped landslides and fresh scars in the cliffs above

indicate that this process is operative at the present time.

One of the landslides at Pouono, the southern part of Olosega

village, is so fresh in appearance that it probably occurred

since habitation by the Samoans. At the base of the cliffs

talus deposits are always present.

Regardless of whether or not there has been a·mndslide,

individual blocks work loose from the cliff face and fall

crashing to the bottom. The reef flats in front of areas

where cliffs are high enough for falling rocks to build up

sufficient momentum are littered with large talus blocks that

have rolled across the reef. A Samoan legend relates how a

young girl was killed by a large block that rolled across the

reef at Sili, where she was fishing.

On the seaward side of Nu'utele Islet a bench is present

at the same level as that which occurs in the tuff complex at

Faleasao on Ta'u. The bench has a maximum width of 30 feet

and was probably cut during a Recent +5-foot stand of the sea.

104.

As on Talu, this bench was not found developed on any hard

lava rock. The erosion of seaward-dipping beachrock on many

beaches around Ofu and 010sega may be evidence for a previous

slightly higher stand of the sea. The beachrock usually has

approximately the same fores1ope as the present beaches, about

10°. Everywhere it is exposed, the beachrock is out ofequi1~

brium with the present sea level and is being eroded away.

Coastal Deposition. The constructional bench 13-15 feet

above sea level that was found on Talu is also well developed

on Ofu and 01osega, but it is nearly 20 feet above sea level

in some places. The villages of Ofu, 01osega, and Si1i are

all built on these sandy areas. Other examples of this con­

structional bench are found at Valoto, Tolaga and Mafafa on

Ofu and at Oge on 01osega. The bench is about 900 feet wide

at Valoto and over 1000 feet wide at 010sega village, but the

average width is about 300 feet. Figure 5 shows profiles

made across these benches at various places around the islands.

Usually a 9_10° fores1ope rises steadily to the crest of the

berm 15-20 feet above sea level, but sometimes there is an

intermediate berm. At Valoto and 010sega swamps have formed

between the berm crest and the talus slope at the base of the

cliff. Although storm waves during hurricanes have obliterated

a trail across one of these benches on eastern Ta'u, many of

the areas with constructional benches probably are well-enough

protected that even storm waves can not come over them. None

of the villages in Manula has been known to have been comp1etay

.J,"II·

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t-'o~

III

105.

demolished by storm waves.or tsunamis. Therefore, these

benches were probably formed during some higher stand of the

sea. Perhaps storm waves built the benches during a higher

stand of the sea.

Like Ta'u, Ofu and Olosega are completely encircled by

a narrow fringing reef. The reef front is only 1 kilometer

offshore at its widest point, at Ofu village. The reef there

has more prolific corals than any other area in Manu'a. Most

of the species identified on Ta'u were also found on Ofu and

Olosega. In addition, Orbicella mansfieldi (Hoffmeister),

Millepora truncata (Dana), and Zoanthus vestitus (Verril)

were found only on Ofu. The latter is a soft coral that is

particularly abundant in the area between Alaufao and Nu'utele

Islet. A lithothamnium rldge occurs along the reef front near

Nu'usilaelae on the south and Tauga Point on the north.

Millepora tenera (Baschma), a stinging coral, was found only on

Ofu, but according to the natives it is present on Ta'u also.

At low tide most of the reef flat is exposed. A maximum

current velocity of 4.2 feet/second was measured 15 minutes

after low tide in the large channel between Alaufao and

Nu'utele Islet. The currents in that area had a curious

fluctuation from no current to maximum current which may have

been dependent on larger waves bringing water over the reef

front and into the backreef, where the water level builds up

until it can again run seaward out the channels. The period of fluc­

tuation varies considerably but averages about two minutes. Th~

10.6.

periodic fluctuation in current velocity was observed only in

the northern channel of the reef off Alaufao, at the northern

,end of Gfu village.

Most of the reef front encircling Gfu and Olosega has

surge channels cutting down the slope of the forereef iden­

tical to those described for Ta'u (pp. 70-71). In addition

to these small forereef channels, each reef system has at

least one, and usually two, larger channels that drain most

of the water from the backreef. The floors of these channels

are covered with poorly sorted coarse sand and coral rubble.

The ·depth of the channel is proportional to its cross-sectional

area. Numerous smaller channels join the main channel with

floors higher than the main channel bottom, giving an appear­

ance not unlike a hanging glacial valley.

Extensive areas of coarse sand lie in deeper water (30-50

feet) off the mouths of these channels. Sand carried off into

these areas by tidal currents in the large channels is removed

from the system. The sand in these "graveyards" is often

marked by large ripples (wave length = 21 inches), probably

formed by large storm waves that "feel bottom." Although the

sand may be moved occasionally in an oscillatory manner, it

probably undergoes little transport unless there is a change

in physical conditions, such as a lowering of sea level.

PETROGRAPHY

Microscopy

Over 200 rock samples from Manu'a were &udied in thin

section under the microscope. The rocks are predominantly

olivine basalt with lesser amounts of picrite-basalt (both of

the oceanite and ankaramite type), basalt, basaltic lapilli

tuffs and ash, olivine gabbro, and hawaiite. No rock more

acidic than hawaiite was found, although one specimen (#229)

from an intra-caldera flow on Ta'u approaches a mugearite.

It contains plagioclase micro lites with a subtrachytic texture

and an average composition of An32; a few small grains of

biotite and interstitial alkali feldspar are present. It is

quite possible that a few mugearites do occur in Manu'a, but

it is doubtful that anything as acidic as a trachyte is

present. Most of the later flows on Ta'u are picrite-basalts

and olivine basalts, some of which contain dunite xenoliths.

Probably, the differentiation by setting of olivine pheno­

crysts within the magma chamber was sufficient only to produce

rocks ranging from picrite-basalt to hawaiite or possibly

mugearite.

Extra-caldera Rocks of Ta'u Shield. The pre-caldera lavas

are composed of olivine basalt, feldspar-phyric basalt, and

hawaiites. No petrographic distinctions between pre-caldera

and post-caldera lavas caRd be established. With the excep­

tion of pre-caldera basalts that contain abundant large

labradorite phenocrysts, all of the rock types comprising the

108.

pre-caldera lavas are found among the post-caldera lavas and

vice versa. Therefore, they are herein described together as

extra-caldera lavas. Most of the olivine basalt flows are

porphyritic, but usually the olivine phenocrysts are only 1 mm

in diameter. Often they have coronas or golden outer rims

of "iddingsite," which is probably finely divided hematite or

goethite formed by weathering of the outer zone of the olivine.

Some of the phenocrysts are altered around the edges to form

a zone of true iddingsite which is in turn coated with a

border of fresh olivine from a later period of crystallization.

The formation of the iddingsite probably occurred during ex­

trusion of the lava, when the olivine was out of equilibrium

with the liquid. The outer rim of olivine probably crystal­

lized with the groundmass olivine after extrusion of the lava.

Often the olivine phenocrysts are elongated in the direction

of the b-axis. This grain shape is typical for olivines in

basaltic rocks.

The composition of the olivine phenocrysts is about

Mg80_85 (80-85% forsterite and 15-20% fayalite), judging from

interference figures and measurements obtained on the univers&

stage. The electron microprobe analyzer showed that the

olivine in the picrite-basalt and the dunite xenoliths is

Mg80_ 90 • These crystals are larger than the phenocrysts in

the olivine basalt lavas, approaching 1 cm· in diameter.

Some oceanites contain abundant olivine phenocrysts comprising

more than 65% of the rock. The similarity in olivine crystals

109.

from picrite-basa1ts and those from dunite xenoliths auggests

that the dunites are simply a concentration of olivine

crystals that have settled out from the same magma chamber

rather than coming from some deeper source. The groundmass

olivines are a little more Fe-rich, having a composition of

about Mg 75 according to measurements obtained on the electron

microprobe analyzer. Scanning various crystals with this

instrument showed that these smaller crystals are more strong~

zoned, but the large phenocrysts from picrite-basa1ts and

dunite xenoliths are remarkably homogeneous.

There is a mineralogical gradation of olivine basalt to

picrite-basa1t. No such gradation in olivine content was

found to occur stratigraphically, however. Picrite-basa1ts

are associated with fe1dspar-phyric basalts, olivine basalts,

and other picrite-basa1ts in an apparently random relation­

ship. The olivine phenocrysts in the picrite-basa1ts are

often quite large; some approach 1 cm in diameter.

No orthopyroxene was found in any of the rocks from

Manu'a, although half of them contained normative hypersthene.

C1inopyroxenes are quite abundant, both in the groundmass of

basalts and olivine basalts and as phenocrysts over 1 cm long

in ankaramites. Scanning these phenocrysts on the e1ctron

microprobe analyzer showed them to be very homogeneous augites,

whereas the groundmass pyroxenes show some zoning in composi­

tion. A very few black augite grains were f6~nd associated

with the dunite inclusions at Fitiiuta and Fa1easao, which

110.

essentially are 100% olivine. Usually when clinopyroxene is

prolific in the groundmass, the rock is a basalt. In about

1/3 of the thin sections containing clinopyroxenes, the

mineral has the characteristic purpe-brown shades of titan­

augite. The ZV for the titanaugite is about +35°. The other

augite grains have a +ZV of 50_60°.

Some of the light brown to colorless clinopyroxene pheno­

crysts have very low +ZV of 10-ZOo. Results of electron­

microprobe analyses, although only semi-quantitative, show

that these clinopyroxenes fall into the ferric sub-calcic

augite group. Therefore, some effect other than low Ca con­

tent is causing low ZV in the augite. The over-all composi­

tions of the analyzed rocks are very high in TiO Z ' varying

from Z.75% to 5.58%. Lacroix and Kuno (personal communica­

tion, 1965) have noted that titanium apparently lowers the

ZV in augite. Macdonald (personal communication, 1966) sug­

gested the possibility of ferric iron contributing to a lower

ZV. Macdonald (1944) also reported the occurrence of clino­

pyroxenes with low ZVls in the rocks collected from Samoa by

Stearns.

The feldspar-phyric basalts contain no phenocrysts, but

characteristically have microphenocrysts of plagioclase

(usually labradorite) approximately 1 mm long. The only known

exceptions in Manu'a are those pre-caldera flows exposed

along the north coast of Talu and at Samoli on the northern

coast of Ofu which contain laths of calcic labradorite up to

111.

5 cm long. About 65% of both rocks is composed of these

large phenocrysts. There are a few microphenocrysts of oli­

vine in these rocks, but in amounts of less than 1%. The

groundmass of the rock from Ta'u is crypto-crystalline

feldspar(?) and glass, the microlites being too small to

identify. The groundmass of the rock from Ofu is made up of

about equal amounts of poorly-twinned or untwinned oligoclase

and ilmenite. There are also a few grains of apatite and

interstitial alkali feldspar.

The groundmass minerals of the extra-caldera lavas of

the Ta'u shield in approximate order of abundance are: labra­

dorite or andesine laths, microlites ranging in composition

from labradorite to oligoclase, magnetite, ilmenite, apatite

needles, olivine, minoclinic pyroxene, iddingsite and inter­

stitial alkali feldspar. Biotite grains are sometimes present

in the hawaiites. The plagioclase composition is largely

dependent on grain size in a given rock, the larger grains

being more calcic. There is little apparent correlation

between rock types and groundmass texture. The most common

textures are intergranular, hyalophitic, and intersertal.

The intergranular texture is usually the result of an abun­

dance of magnetite and ilmenite, comprising as much as 30%

of the total rock in some samples. Less frequently olivine

and clinopyroxene contribute to this texture, especially in

the mafic olivine basalts and picrite-basalts. Crypto­

crystalline feldspar(?) and glass are prevalent in the rocks

112.

with hyalophitic and intersertal textures, and there is a

gradation between these two types. There may be a very slight

tendency for a higher percentage of glass in the picrite­

basalts, but many of these rocks are intergranular. Feldspar­

phyric basalts and hawaiites sometimes have a pilotaxitic

texture, although these rocks are more commonly int.~rgranular

due to the occurrence of magnetite and ilmenite. The sodic­

hawaiite from southern Ta'u (sample #229) has a nearly trachy­

tic texture due to sub-parallel alignment of andesine micro­

lites.

Intra-caldera Rocks of Ta'u Shield. The olivine ba~t

and picrite-basalt of the intra-caldera lavas of Ta'u are the

same as those of the extra-caldera lavas. There are only a

few feldspar-phyric basalt lavas present. The plagioclase is

usually andesine rather than labradorite, making the rocks

basaltic andesite or hawaiite instead. These rocks are quite

fine-grained; microphenocrysts are usually less than 2 mm

long. Some of the microphenocrysts may be sodic labradorite,

but the rest of the plagioclase is andesine. Conversely, in

a few rocks, there may be SOme oligoclase microlites present,

but the over-all average composition of the plagioclase is

andesine.

The ash beds in Afuatai are 85-90% palagonitized glass,

the rest of the rock being composed of magnetite grains vary­

ing from .01 mm to .5 mm in diameter. A few small grains of

olivine (approximately .05 mm) are sometimes present.

113.

Tunoa Shield. Nearly all of the lavas associated with

the Tunoa shield are basalt or olivine basalt which do not

differ in character from the pre-caldera flows. Picrite­

basalt lavas are not as common and no flows with abundant

large plagioclase phenocrysts, such as occur in SOme of the

rocks in the northern part of Ta'u, have been found. The

only picrite-basa1t associated with the Tunoa shield is

exposed near sea level in the sea cliff at Tu1atula, where a

la-foot thick oceanite flow occurs at the base of a sequence

of thin-bedded pahoehoe basalt flows. Part of this sea cliff

has subsequently been buried by the tuff deposits of the Tola

cone, thereby masking much of the section through the Tunoa

shield. One or two red-brown basaltic tuff beds are found

exposed in the sea cliff behind Ta'u village. This tuff

differs from that of the Faleasao and To'a cones in that the

glass is not palagonitized, there are few accidental basalt

lapi1li, magnetite crystals are present only in trace amounts,

and olivine crystals comprise more than 30% of the rock. The

reddish tint of this deposit is due to iron oxidation during

the eruption.

Luate1e Shield. The lavas of the Luatele shield are

quite homogeneous, being composed entirely of vesicular

olivine basalt that is gradational to picrite-basalt contain­

ing only about 5% olivine phenocrysts. Typically the rock is

composed of about 5% olivine phenocrysts 1-3 mm across in a

very fine-grained groundmass of plagioclase (labradorite?)

114.

micro1ites, olivine, augite, magnetite, and ilmenite. Some­

times the olivine and augite occur together in glomeroporphy­

ritic clusters. Usually the groundmass olivine granules are

zones with Fe-rich borders. No other rock types belonging to

this shield were found. Locally some of the lavas are quite

scoriaceous. Cinder occasionally occurs on the shield surface

as drift.

Fa1easao Tuff Complex. The post-erosional deposits com­

prising the tuff complex at the northwest corner of Ta'u

island are composed mainly of pa1agonitized vitric-crysta1

1api11i tuff of basaltic composition. More than 95% of the

rock is pa1agonitized glass, which occurs both as elongated

spheroids and as sharp angular fragments. The grain size

varies from less than .05 mm to 2 mm in diameter. The ash

fragments are moderately to poorly sorted. Grains of olivine

from .05-.5 mm make up about 1% of the rock. Local concentra­

tions of vesicles approximately .05 mm in diameter are evi­

dence of pockets of trapped magmatic gas and air. Magmatic

lapi11i of about 1-2 mm in diameter also represent pockets

or globules of partially crystallized material. Olivine and

feldspar micro1ites occur in a dark brown glass matrix.

Bombs up to 10 cm in diameter are of similar basaltic

composition. Accidental 1apil1i consist of fine-grained

basalt with micro-crystalline olivine and calcic labradorite

in a black glass groundmass. These fragments vary from

lapi11i a few millimeters in diameter to huge blocks several

115.

feet across. The large basalt blocks contain dunite xeno­

liths which are composed of olivine with a composition of

MggO • The 3-4-foot thick flow of basalt that apparently had

been ponded within the To'a cone contains microphenocrysts

of olivine up to 1 mm in diameter set in an intergranular

groundmass of olivine, magnetite and labradorite. The olivine

microphenocrysts have a thin rim of iddingsite, but otherwise

are unaltered.

The explosion breccia associated with the complex con­

tains a groundmass of black ash and crystals up to .25 mm in

diameter of olivine and magnetite. The lapilli are much

more abundant in this breccia than in the palagonitized

lapilli-tuff which makes up most of the tuff complex. Numer­

ous blocks of vesicular basalt, picrite-basalt, and aphanitic

basalt, some with dunite xenoliths up to 5 cm in diameter,

comprise most of the breccia. The dunite xenoliths, as at

Si'ua'i Point, are composed entirely of olivine grains

approximately 3 mm in diameter. Some of the blocks are more

than 4 feet across, but usually they are about 6 inches in

diameter. There are some palagonitized lapilli tuff blocks

and lapilli which have been carried up from the underlying

tuff deposit during the explosive eruptions. These fragments

of lapilli tuff are found only in the lowermost 1-2 feet of

the bed, showing a general decrease upwards in size.

The dunite lapilli and blocks, which are especially

116.

prolific on Si'ua'i Point at Faleasa~ consist almost entirely

of medium to coarse-grained olivine crystals. There are a

few scattered grains of augite, but these comprise much less

than 1% of the dunite. The olivines usually contain magnetite

crystals about .05 mm in diameter. A few olivine crystals

show what apparently was an old crystal boundary marked by an

alignment of magnetite grains.

Fitiiuta Rocks. The lava flows at Fitiiuta also contain

dunite xenoliths made up almost entirely of olivine. The

nature of these xenoliths seems to be identical to those at

Faleasao. The lava flows at Fitiiuta do not differ mineralogi­

cally from the pre-caldera olivine basalt lavas. The olivine

crystals are usually only 1 mm in diameter or less. The

microphenocrysts of olivine contain the typical fluid in­

clusions and occasional grains have glassy centers, because

of insufficient time for the complete formation of the crystal.

The olivines of these lavas have a composition of approximate­

ly Mg75

, whereas the composition of the olivine crystals in

the dunite xenoliths is about Mg gO •

Intrusive Rocks of Ta'u. The dikes on Ta'u are composed

of aphanitic basalt, olivine basalt, and picrite-basa1t.

Olivine phenocrysts if present are usually 1-4 mm. The ground­

mass texture of the dikes On Ta'u varies from intersertal to

a few specimens that are intergranular. Some dike rocks

contain microphenocrysts of olivine and/or augite, but many

117.

are composed entirely of groundmass augite or titanaugite,

labradorite, magnetite and dark brown or black glass. Some

are quite vesicular, indicating proximity to the surface at

the time of solidification.

Extra-caldera Rocks of Ofu and Olosega. The extra­

caldera lavas of Ofu consist of basalt, porphyritic and non­

porphyritic olivine basalt, picrite-basalt (both oceanite

and ankaramite), feldspar-phyric basalt, and hawaiite. These

rocks are identica~ to the ones described for Ta'u. A large

volume of explosion breccia occurs in eastern Ofu. This

breccia is composed of blocks of at least six distinguishable

rock types:

(1) An ankaramite composed of olivine and augite pheno­

crysts in anointersertal groundmass of labradorite

(An S4 ) with labradorite microphenocrysts (An64

).

(2) A vesicular basalt with a few augite phenocrysts in

an intersertal groundmass of augite, labradorite

(An 63 ), and magnetite.

(3) A medium-grained ankaramite that is transitional

to an olivine diabase composed of only 30% labra­

dorite (AuSS ) and subophitic augite (possibly

titanaugite) with larger augite and olivine pheno­

crysts and a few small calcite amygdules about

1 mm in diameter.

(4) A slightly vesicular basalt with only a few

olivine and augite phenocrysts in an intersertal

118.

groundmass of labradorite (AnS8

), ilmenite, black

glass and some augite.

(S) A vesicular augite basalt with augite micropheno­

crysts and a very few olivine microphenocrysts in

a subophitic groundmass of augite and labradorite

(AnSS )'

(5) A fine-grained dike rock of basalt composed of

iddingsitized olivine grains less than 0.2 mm in

diameter, labradorite microlites, magnetite, and

ilmenite in a brown glass matrix.

These angular blocks make up about 90% of the rock. The matrix

is a well indurated black an composed of glass and a few oli~

vine crystals. The other pre-caldera pyroclastic deposits on

Ofu and Olosega are composed entirely of ash-sized globules

and shards of basaltic glass, except in a few deposits where

olivine and magnetite crystals are present.

A palagonitized yellow ash collected at Samo'i on

northern Ofu contains a few olivine crystals; one unusually

large olivine was 2 mm in diameter. The uppermost ash bed of

the tuff cone at Maga on southern Olosega contains olivine

crystals up to 1 mm and a few tiny grains of magnetite (about

.02 mm across). Some lower beds of this cone contain abun­

dant accidental basalt lapilli. A ribbon bomb from the small

cinder cone at Tauga Point on northwestern Ofu is composed of

crypto-crystalline feldspar(?) and glass.

Intrusive Rocks of Ofu and Olosega. The plug at

119.

Vainu'u1ua on eastern Dfu is composed mainly of an olivine

gabbro or diabase that contains augite laths up to 8 mm long

and stubby olivine phenocrysts up to 6 mm in diameter.

Thirty per cent of the rock is composed of labradorite (An57

)

laths up to 5 mm long, most of which are Carlsbad twinned.

Magnetite crystals up to 1 mm across are scattered throughout,

the interstices. Apparently the augite started to crystallize

before the plagioclase, because some of the optically conti­

nuous augite laths are broken up and are crosscut by smaller

plagioclase crystals. Also the augite:plagioclase ratio is

greater than 2:1. Some of the augite phenocrysts show

oscillatory zoning. Towards the border of the plug there is

a gradation to a finer-grained olivine diabase with subophitic

augite and labradorite, but there is no apparent change in

mineralogical constituents.

The dike rocks are usually dense basalt, but olivine

basalt, ankaramite, and fe1dspar-phyric basalt dikes also

occur. These rocks are tre same as the dike rocks described

on Tafu (p. 116). A 40-foot thick, vesicular ankaramite dike

exposed at Tumu and Muliolo is composed of olivine pheno­

crysts up to 4 mm across and augite microphenocrysts about

.5 mm in diameter. The remaining 60% of the rock is a ground­

mass of labradorite microlites, magnetite, and black glass.

Vesicles up to 5 mm across, as well as its stratigraphic

position near the summit of Dfu, indicate that this portion

of the dike was consolidated near the surface and therefore

120.

was probably the source for the nearly horizontal ankaramite

flows on Tumu.

Intra-caldera Rocks. The olivine basalt flows and the

later hawaiite and ankaramite lavas that were ponded in Alofa

caldera are similar petrographically to the respective rock

types described for the extra-caldera lavas of Talu (pp. 107­

ll2). A light gray aphanitic hawaiite showing flow structure

was found as drift near the head of Sinapoto Stream. Alter­

nating light and dark gray bands less than!l cm thick are

caused by concentrations of magnetite. The darker bands con­

tain about 30% tiny magnetite grains less than .01 mm across,

whereas the light portions of the rock contain approximately

10% magnetite with a slightly larger average grain size.

Andesine micro lites form a hyalopilitic texture that is sub­

parallel to the flow direction indicated by the banding.

Since the lighter portions of the rock contain more clear

glass and less plagioclase, the preferred orientation is not

as well developed. A few small grains (less than .1 mm) of

augite, biotite, and olivine are also present in both portions

of the rock.

Post-erosional Deposits. Nu'utele and Nulusilaelae

Islets are the remnants of the Recent Nu'u tuff cone. This

lapilli tuff contains magmatic and accidental lapilli of

basalt. There is a gradational decrease in the size of the

vitric ash matrix from the bottom to the top of the cone.

121.

Except for an occasional euhedral magnetite grain about .01

mm across, the matrix is composed entirely of palagonitized

basaltic glass globules and shards. The thick, moderately

vesicular aphanitic flows at Ofu village which may have

flowed down old stream valleys during the post-erosional

stage are hawaiite lavas. Very small microlites of andesine

less than 0.1 mm long and magnetite grains about .02 mm across

are enclosed in a black glass.

122.

Chemical Analyses

Theories on Oceanic Basalts. N. L. Bowen, in his clas­

sical book on The Evolution of Igneous Rocks, has attempted

to show that all igneous rocks, even of the most diverse com­

positions, are all related to a single parent basalt magma.

Some of the most convincing evidence is found in the silica

variation diagrams. A series of rocks may be plotted on a

diagram with oxide percentages of the various elements vs.

per cent Si02

• These points may represent different stages

in the separation of a portion of liquid to form the rock.

Therefore, rocks lying on smooth curves represent a liquid

line of descent, whereas the scattered points represent rocks

affected by crystal accumulation or hybridism.

Bowen plotted Daly's averages of the composition of

igneous rocks on a silica variation diagram (Bowen, 1928,

Fig. 37, p. 123). On this diagram the smooth curves of

liquid descent begin at basalt. The basalt therefore repre­

sents the beginning composition of crystal fractionation.

If the parent magma were of some other composition, say

dioritic, then the diagram should show smooth liquid curves

descending from diorite to granite and basalt. If the magma

were more ultrabasic, say a peftdotite, the .smooth curves

of oxide composition should extend from peridotite, not

basalt. The scattering of the oxides of the ultrabasic rocks

can be readily explained by crystal settling and accumulation.

Aphanitic rocks give the most accurate liquid lines of

123.

descent. Phanerocrysta11ine rocks have naturally undergone

a greater amount of fractionation (crystal settling, zoning,

etc.) and therefore deviate more from the curves of liquid

descent. More emphasis should be placed on the glassy rocks.

The more coarsely crystalline the rock, the less reliable ~e

its oxide percentages as determinants of the line of liquid

descent. The crystalline rocks show a much wider range of

crystallization than the aphanitic rocks. There are no known

glassy equivalents of any ultrabasic rocks--peridotites,

pyroxenites, or even associated anorthosites. There also seems

to be a gradation in that the more u1trabasic the rocks, the

more coarsely crystalline is their occurrence. The lack of

glassy representatives of these rocks is probably due to the

lack of an original magma of such an u1trabasic composition.

The progressive increase in crystallinity with composition

among the u1trabasic rocks indicates the progressive importance

of crystal accumulation in their formation.

Bailey et ale (1924) recognized three major types of

basalt in Mu11--plateau (later called alkali basalt), central

porphyritic (high alumina), and central nonporphyritic

(tholeiite). The central porphyritic type is high in alumina

due to the concentration of plagioclase phenocrysts and is

closely related to the central nonporphyritic type. W. Q.

Kennedy (1933) called the plateau type olivine basalt, and the

central nonporphyritic type tholeiite. He also noted the

absence of the latter type in the oceanic provinces. However,

124.

Powers (1935) demonstrated that both silica saturated and

unsaturated basaltic rocks occur in the Hawaiian Islands.

Macdonald (1949) first noted that the silica saturated

primitive Hawaiian lavas are tholeiites. Tilley (1950)

recognized two distinct rock series in Hawaii, the tholeiites

and the alkali olivine basalts.

Bailey (1924) and Bowen (1928) felt that both the tholei­

ites (nonporphyritic central type) and the alkali basalts

(plateau type) stemmed from the alkali basalt magma by frac­

tional crystallization. Macdopa1d (1949, 1961), Tilley (1950~

and Powers (1955) present evidence in favor of tholeiite as

source magma of both trends. Kennedy and Anderson (1938),

Kuno et a1. (1959), and others feel that the two trends are

independent of one another. They suggest that two or more

primary magmas are independently derived from different levels

in the upper mantle.

Powers (1955), in his work with Hawaiian basalts, con­

cluded that the parent magma was formed due to fractional

melting of the peridotite layer in the mantle. He also felt

that each volcano, and possibly even several stages of a

single volcano, derived its lavas from different "batches rr of

magma. This primitive magma, comprising the bulk of the

Hawaiian volcanoes, is essentially a melt of pyroxene and

plagioclase. Olivine occurs abundantly in the mode, but is

usually less than 3% in the norm. The primary magma seems

to be nearly exactly silicasaturated. However, Powers (1955)

125.

claims that small differences in silica compositions of rocks

from Kilauea and Mauna Loa are evidence for different magma

sources.

During the end of the later caldera-filling stage, the

magma becomes silica undersaturated, and there is an increase

in alkali content (Macdonald and Katsura, 1964). These 1ate­

stage lavas also differ from the primitive lavas in that

augite phenocrysts, low-silica xenoliths (dunite, pyroxenite,

anorthosite, etc.), andesine plagioclase, and interstitial

alkali feldspars are found. According to Powers, the apparent

quiescence prior to this period indicates that the "batch" of

magma had been exhausted, leaving a dunite-rich residuum. The

fact that these lavas are richer in alkalies weakens the sup­

position that the decadent magma is undersaturated in silica

because it contains more olivine. Decrease in silica by

enrichment in olivine and enrichment in alkalis are not com­

patible within the same rock, according to the Bowen reaction

series. Powers feels that the only explanation for the com­

positions of these alkali basalts is fractional crystalliza­

tion of the decadent magma, which would also be rich in more

refractory components by the late stage of the volcano.

Perhaps enough time had elapsed for this mechanism to become

important, whereas the eruptions were too frequent during the

primitive stage.

Kuno (1959) claims that fractional crystallization of a

single parent magma to form alkali basalt and tholeiite suites

126.

does not occur. He feels that these two rock types originate

independently through the melting of the earth's mantre under

different physical conditions. As Powers, Kuno is of the

opinion that the presence of dunite xenoliths predominantly

in alkali and nepheline basalts indicates that these rocks

originated through partial melting of a more refractory peri­

dotite substratum.

However, Powers feels that the peridotite source is more

refractory because the components with lower melting tempera­

ture (plagioclase and pyroxene) have been depleted during the

tholeiitic shield-building eruptions. Further melting of

this same Ilbatch" after an extended period of time yields the

alkali basalt series. Kuno, on the other hand, feels that the

alkali basalt magmatic source is more refractory because it

originated at a greater depth. The two types of magmas differ

not because partial melting has exhausted the less refractory

components, but because the tholeiitic magma is derived by

complete melting of the basaltic layer above the Mohorovicic

discontinuity, whereas the alkali basalts originate through

partial melting of the peridotite substratum of the upper

mantle.

Since the basaltic layer is 12-15 km deep in Hawaii, it

is improbable that the temperature of melling for basalt (about

1200° C) could be attained within this shallow region. For

this reason, Tilley (1950) points out that basalt magma could

not be generated in the basalt layer, but is formed by partial

127.

melting of the upper mantle. Therefore, both magmas must

originate in the peridotite(?) substratum of the upper mantle.

Among the constituents of the upper mantle, enstatite

melts incongruently under low pressure. Partial melting of

peridotite under these conditions would result in a magma

oversaturated with Si02 , a tholeiitic magma. Pure silica

would melt first, followed by enstatite, and finally forster­

ite. Verhoogen in 1954 suggested that the melting re1ationsh~

between pyroxene and olivine changes with an increase in depth

and pressure. The incongruent melting of enstatite at low

pressure may disappear at depth because the rate of increase

of the melting point of enstatite with increase in pressure

is much greater than that of forsterite. Consequently, the

two minerals may melt simultaneously at higher pressures.

Kuno feels that partial melting of peridotite under these con­

ditions would result in an undersaturated or alkali basalt

magma.

Experiments by Boyd and England (1961) showed that

enstatite does melt congruently above about 6 kb (approxi­

mately 2-3 km) under anhydrous conditions. However, Ybder

and Tilley (1962) point out that with high water pressure

there is a tendency for melting coefficients to shift back

towards incongruent melting conditions. They also claim that

a thermal barrier, the Di-Ab join in the Ne-Di-Si02-An system,

prevents the derivation of a silica-deficient magma from one

that is silica-saturated or vice versa.

128.

Kuno supports his idea from studies of the Japanese

volcanic suites. The tholeiitic rocks occur predominantly

in the eastern part of Japan, where earthquakes occur at

shallow depths. The western part of Japan, however, contains

mostly volcanic rocks of the alkali basalt suite. Here earth­

quake foci are much deeper (greater than 200 km). Therefore

Kuno suggests that there are two parent magmas involved, one

produced by partial melting of the peridotite substratum at

depths shallower than 200 km; the other produced by partial

melting at depths greater than 200 km. Both types of parental

magma assimilate granitic material in the Cenozoic orogenic

belt of central Japan, forming volcanoes of the calc-alkali

series.

Later Kuno (1960) added a third, transitional type of

parent magma, the high-alumina basalt, to his list. This is

formed at depths of about 200 km.

Macdonald (1949), Tilley (1950), and Murata (1960) have

all shown by calculation that it is possible to produce an

alkali olivine basalt from a tholeiitic magma. Macdonald

calculated that the separation of a large amount of pyroxene

(both diopside and hypersthene) from either the tholeiitic

or alkali basalt magma could produce the alkalic rocks.

Tilley also derived alkali olivDe basalt from a tholeiitic

magma by separation of hypersthene. Murata (1960) calculated

that crystallization of clinopyroxene from an undersaturated

tholeiitic magma could result in an increase in silica

129.

undersaturation and hence the derivation of an alkali olivine

basalt.

Powers (1955) claimed that fractionation of phenocrysts

of olivine, hypersthene, augite, or plagioclase from either

suite could not produce rocks of the other suite, because

there is always a lack or excess of silica. He agreed that

removal of hypersthene can prevent an undersaturated magma

from becoming silica saturated, but it cannot produce de­

silication of the residual melt. Since desilication of a

magma requires removal of crystals richer in silica than the

original magma, and since such crystals can form only late in

the magmatic history of crystallization, it seems quite un­

likely that silica undersaturated rocks can be derived from

silica saturated rocks by crystal differentiation alone.

Therefore, Powers (1935), Macdonald (1949), and Tilley

(1950) have all suggested that separation of orthopyroxene from

a parent (tholeiitic) magma at higher pressure could yield a

residual melt critically undersaturated in silica. This magma

would then be the source of the late-stage alkali basalts and

their differentiates. Green and Ringwood (1964) ran a series

of high pressure experiments on an undersaturated tholeiitic

basalt (olivine tholeiite). As pressures were increased from

10 to 20 kilobars, olivine was replaced by orthopyroxene as

the primary liquidus phase. Electron microprobe analyses

showed that the orthopyroxene is higher in magnesium than the

ordinary enstatite formed at low pressures, indicating an

130.

iron-enrichment trend of the liquid even at pressures of 20

kilobars. Subtraction of 20% crystallized orthopyroxene

changes the liquid from 14% normative hypersthene to 1%

normative nepheline.

Yoder and Tilley (1962) investigated the melting relations

of some natural basalts and eclogites at the Geophysical

Laboratory of the Carnegie Institute in Washington, D.C.

These experiments involved pressures ranging from 1 to 40,000

atmospheres and water pressures of 1 to 10,000 atmospheres.

The melting relationships under high pressures indicate that

not only basalts but also eclogites are the partial melting

products of a common parent rock (e.g. a garnet peridotite).

The experiments on various types of basalts at low

pressure showed that, although olivine is usually the first

mineral to begin crystallizing from a basaltic melt, either

clinopyroxene or plagioclase may also appear as the primary

silicate phase. All three phases appear within the narrow

range of 80° C. The total range of crystallization for the

entire rock occurs within 135-195° C. The fact that all three

major phases crystallize at about the same temperature regard­

less of the bulk composition of the basalt proves that the

various basalts themselves must be derived from fractional

melting or cyrstallization from a COmmon source. If there

were more than one source, all three phases of such diverse

basalts would not be in equilibrium with the liquid at nearly

the same temperature. The chances would be almost nil that

131.

the major phase would just happen to be in equilibrium at

this same temperature for the two or more different sources.

Experiments performed by Yoder and Tilley (1962) also

show that up to a pressure of 14,000 atmospheres eclogite is

transformed into a gabbro (chemically equivalent to a basalt)

before any bf its phases begin to melt. All of the major

varieties of basalts were subjected to pressures of 20,000­

40,000 atmospheres under anhydrous conditions and, with the

exception of an olivine-nepheline basalt, yielded garnet plus• ...,r ....~.:::

clinopyroxene (apparen t ly lowe r in Na than omphaci te) ,i.e. eclo-

gite. The olivine nepheline basalt yielded clinopyroxene +

sphene + mica. Then, at least for every plagioclase-bearing

basalt, there is an equivalent eclogite of the same bulk com­

position at higher pressures. It was also found that above

19,000&mospheres the two major phases of eclogite, garnet

and clinopyroxene, occur together on the liquidus. Not only

do they begin melting at the same temperature, but these two

minerals coprecipitate over most of the range of crystalliza­

tion, which is only about 85° C. Therefore, the eclogite is

also only a partial melting product of a more primitive source

(e.g. a garnet peridotite composed of olivine + orthopyroxene

+ garnet + clinopyroxene).

At depths of about 60 km, where seismic evidence indicates

that magmas of the Hawaiian volcanoes may be generated, a

primitive source rock might undergo partial melting to yield

an eclogitic magma, the fractionation of which depends on the

132.

relative amounts of garnet and omphacite. A garnet-rich

magma at high pressure would give rise to a tholeiite-type

magma at low pressure; whereas an increase in the relative

amount of clinopyroxene at high pressure would yield an

alkali basalt-type magma at low pressure.

The same magma could produce tholeiite (silica-normative

trend) at low pressure and alkali basalt (nepheline-normative\

trend) at high pressure due to a shift in equilibrium thermal

divides at higher pressure (Figure 6). From the same source

rock, the latter would be expected to have been generated at

higher pressure and therefore greater depths than the former.

Determinative Methods. As originally defined by Kennedy

(1933), tholeiite is a basalt that is silica saturated, such

that olivine phenocrysts, which crystallize first from the

melt, show a reaction relationship with the melt to form

Ca-poor pyroxenes, usually hypersthene or pigeonite. The

Scottish plateau basalts, on the other hand, contain Ca-rich

clinopyroxene and olivine, which do not show any reaction

relationship because the olivine would naturally be in equi-

librium with the silica undersaturated melt. Since rocks

belonging to the latter group are characterized by high alkaa

content relative to the amount of silica they contain, Tilley

(1950) proposed that the term alkali basalt be used instead

of plateau basalt, which in most areas had been shown to be

silica saturated.

Kuno et al. (1957) state that the tholeiites of Japan

133 .

.·N~ptreline

/ A,/'"/.

//""-.

. / ./' /! .

~//

/ I ,// --//

. /L__==~_~I \Forsterite' -f . . ._.~ SilicaBnstatite

(a)

Nepheline

Forsterite

(b)

Jadeite

Silica

FIGUP£ 6--System Ne-Fo-Si02 at (a) 1050° C. and1 bar, and (b) 1250° C. and 33,000 bars.Heavy lines indicate stable joins. Liquidsin horizontally ruled area behave in anopposite way to those in cross-hatched area.Liquids in diagonally ruled area trend thesame way at both pressures. (from Yoder andTilley, 1962)

134.

contain modal hypersthene or its chemical equivalent, pigeon­

ite, in the groundmass, whereas the alkali basalts never con­

tain these minerals, either n"ormatively or modally. Later

Kuno (1959) noted the presence of interstitial alkali feldspar

in the alkali basalts and also claimed that the interstitial

glass of a tholeiitic rock should contain a silica mineral.

Macdonald and Katsura (1964) stated that in Hawaiian rocks

"the presence of titanian augite, of true groundmass olivine,

and of interstitial alkali feldspar appear to be positive

indications of the alkalic nature of the rock."

The reason for needing some basis of classification other

than a mineralogical one is that often there are not any

mineralogical criteria present in the rock, especially in

glassy volcanic rocks. The reaction relationship between

olivine and pigeonite or hypersthene is the best criterion,

but this relationship is rarely seen to occur in silica

saturated rocks of oceanic lavas. The presence of pigeonite

in the groundmass has been considered to be a criterion for

a tholeiitic basalt. However, as illustrated in the Samoan

lavas, the clinopyroxenes with a low 2V can actually be sub­

calcic augites, being high in titanium, ferric iron or some

other element that lower the 2V of the mineral. The only

apparently reliable mineralogical criteria are the presence

of titan-augite or interstitial alkali feldspar for the

determination of a rock belonging to the alkali basalt suite.

The majority of alkali basalts do not contain these criteria,

135.

at least not so that they can readily be recognized under the

petrographic microscope.

Therefore, many petrologists have hoped to find reliable

chemical criteria for separating these two suites. The strictly

chemical scheme of classification devised by Cross, Iddings,

Pirsson and Washington is accepted by some petrologists today

as the best criterion for determining whether a volcanic rock

belongs to the tholeiitic or the alkaline basalt suite. This

normative mineral classification involves computations of

simple theoretical minerals from standard oxide chemical

analyses. All rocks containing normative hypersthene, even

though it does not appear in the mode, are considered to be

silica-saturated and therefore are tholeiitic. Those rocks

containing normative nepheline are considered to be under­

saturated in silica and therefore belong to the alkali basalt

group.

Yoder and Tilley (1962) divided the basalts into five

groups according to their norm components:

(1) Tholeiite (oversaturated)--normative quartz and

hypersthene

(2) Tholeiite (saturated)--normative hypersthene

(3) Olivine tholeiite (undersaturated)--normative

hypersthene and olivine

(4) Olivine basalt--normative olivine

(5) Alkali basalt--normative nepheline and olivine

Macdonald and Katsura (1964) have pointed out the

136.

inadvisability of using the term olivine basalt in this new

classification. Olivine basalt is a widely used field term

that is applied to the majority of basaltic rocks. To re­

define it to cover such a limited scope of rocks based on a

chemical rather than a mineralogical definition would only

lead to hopeless confusion. Some other name should be

selected, preserving the term "olivine basalt" in its present

sense--a basalt containing more than 5% olivine phenocrysts

(Macdonald, 1949).

Another objection to using the normative method is that

a rock containing a high amount of ferric iron relative to

ferrous iron will often contain normative hypersthene, the

'most critical mineral for the determination of a tholeiite.

This is not necessarily because of any inherent characteristic

of the magma but may be only due to oxidation of a particular

flow. According to the rules of calculation, the amount of

ferric iron oxide is matched by an equal amount of ferrous

iron oxide to produce normative hematite. Therefore, the

amount of ferric iron oxide depletes the total iron oxide

content by double that value, leaving a much lesser amount

of iron oxide to be added with magnesia and silica to form

olivine. The excess silica must then be taken up by the

formation of hypersthene.

Results of chemical analyses have often been plotted on

a variation diagram, using per cent silica as abscissa and

per cent alkalis (Na2

0 + K2

0) as ordinate. For the Hawaiian

137.

lavas plotted on such a diagram it has been shown that the

tholeiitic rocks can be separated empirically from the alkali

rocks by drawing a diagonal line through an intermediate

region where the least number of analyses are plotted

(Macdonald and Katsura, 1964, pp. 86-87). For some reason,

possibly because it approximately represents the critical

plane of silica undersaturation (Yoder and Tilley, 1962), no

rocks that are of definitely tholeiitic characteristics have

plotted above this boundary line. Conversely, all rocks of

an alkalic nature fall above the line, with the occasional

exception of a few ankaramites, which may fall below the line,

presumably because of a depletion in the salic fraction due

to corresponding enrichment in olivine and augite by crystal

settling.

There are now at least three definitions for a tholeiite

--the original mineralogical definition, the definition based

on the presence of normative hypersthene, and the definition

based on the alkali:silica ratio as related to an empirical

boundary line. One analysis of a specimen collected in June

1960 from the liquid below the crust of the lava lake which

was formed during the eruption of November l4-December 20,

1959 in Kilauea Iki crater plots just above this boundary

line, in the alkali basalt field. The rock contains micro­

phenocrysts of augite with a large 2V, unresorbed olivine

phenocrysts and a high titania content. All other specimens

collected from the same lava lake appear to be tholeiites both

138.

mineralogically and chemically. However, this sample contains

one of the largest amounts of normative hypersthene (18.56%)

and one of the smallest amounts of normative olivine (6.16%)

of any of the analyzed Kilauea Iki specimens. The Fe 203

content in this sample is only 1.36% as compared with 12.99%

FeO. Therefore, the high value for normative hypersthene

cannot be attributed to a large amount of ferric iron.

Macdonald and Katsura (1961) suggest that, if this sample is

truly alkalic, alkali basalts in general can be derived from

undersaturated tholeiitic magmas. By the mineralogical

definition this rock is an alkali basalt; by the calculation

of normative minerals it is definitely tholeiitic; and accord­

ing to the a1ka1i:si1ica ratio it is alkalic, although tran­

sitiona1~ .

Powers (personal communication, 1965) points out that

there is considerable disagreement between the analyses of

Kilauea Iki lavas as reported by Macdonald and Katsura (1961)

and the analyses from the same eruption as reported by the

laboratory of the u.s. Geological Survey at Denver. So, not

only is there a discrepancy in the definition of the rock,

but the results of the chemical analyses themselves are in

doubt. Similarly, Macdonald and Katsura (1964) report that

32 analyzed Hawaiian rocks belong to the alkalic suite by

mineralogical definitions. Yet, only thirteen of these

samples do not contain normative hypersthene.

Recently eight samples representing the major types of

139.

volcanic rocks in Hawaii were analyzed by two major rock

analysis laboratories. Powdered samples from the same rock

were split and one was sent to each laboratory. Table 8

shows the results of these analyses. A glance at the figures

reveals large discrepancies in the results. The point is not

which laboratory is right or wrong, but it is a question of

how much emphasis should be placed on chemical analyses as a

method for determining whether a rock belongs to the tho1eiftic

or alkali basalt suite. The present methods of conducting

laboratory analyses simply are not consistent enough to regard

with any confidence, particularly when transitional rocks are

involved.

Actually the discrepancies in these analyses may not be

as critical~ they appear to be. Macdonald (personal com­

munication, 1966) calculated the normative minerals for both

sets of chemical analyses (Table 8). None of these rocks are

affected in their classification by even such considerable

differences in the chemical analyses. However, transitional

rocks could be affected by differences of this magnitude. A

change in Na 20 content from 2.00% to 1.82% in a sample (#156)

from Manu'a changes the rock from an alkali basalt (contain­

ing normative nepheline) to a tholeiite (containing normative

hypersthene).

It seems that the method of plotting alkalis vs. silica

used by Macdonald and Katsura (1964) is more reliable for

determining whether a rock is tholeiitic or alkalic than

140.

TABLE 8. COMPARISON OF CHEMICAL ANALYSESON THE SAME HAWAIIAN LAVAS

(Data supplied through courtesy of Dr. G. A. Macdonald)

SampleNo. HIGS 1 HIGS 2 HIGS 3 HIGS 4

.13 .37

.2 2 .26

.00

.00

.03

.11 .68

.02 .05

.01

.11

.14

.08 .31

.03 .12

.02

.01

.03

.12 .31

.02 .11

.01

.01

.03

La b...:...__U=....:S::....:G:..:S_..:..J:....:.A...::..CR:..;_I,-,--_U-,S-,G-,S_....:.J_A....:C:....:.R...::..I_--.:..U..:..S...::..G...::..S_J::....:A_C::....;.R....:.I__U_S....;.G_S_...::..J_A_C_RI

Si0 2 47.18 4659 5152 5129 62.23 6172 54.48 53.66

Ti0 2 2.04 2J.6 237 2.47 .37.41 1.65 171

A1 20 3 1051 10.10 14J.0 13.73 17.79 17.17 14;39 1338

Fe 2°3 1.62 2.15 1.7 0 2.39 134 2.07 2.60 3.69

FeO 10.17 10.12 9.28 9.29 2.85 2.92 7.38 7.47

MnO .18 .15 .17 .15 .33 .31 .15 .13

CaO 882 887 10.9210.89 .79 .85 8.28 8.40

MgO 17.18 _P~O 6 6.79 648 .41 .34 7:08 681

Na 20 1.74 2.05 234 2.75 836 8.27 3.03 3.40

K2 0 .4 0 .4 6 .42 .42 4.9 5 5.12 .42 .43

P 2 0 5 .20 .20 • 23 .27 .15 .16 .2 1 .27

H2 0+

H 0-2

CO2

C1

F

Totall0 0.2 3 10 03 3 100.01 10056 99.96 100.08 100.05 99.98

USGS - Laboratory of U. S. Geological Survey.

JACRI - Japan Analytical Chemistry Research Institute.

HIGS 1 - Olivine-rich pumice from 1959 eruption of Kilauea Iki.

HIGS 2 - Olivine-free basalt from prehistoric eruption ofKilauea.

HIGS 3 - Obsidian trachyte from Pu'u Wa'a (Hualalai Volcano).

HIGS 4 - Intrusive coarse-grained basalt from Pa1010 quarry(Koo1au Volcano).

TABLE 8. COMPARISON OF CHEMICAL ANALYSESON THE SAME HAWAIIAN LAVAS (Cont.)

141.

SampleNo. HIGS 5 HIGS 6 HIGS 7 HIGS 8

Lab. USGS JACRI USGS JACRI USGS JACRI USGS JACRI

Si02 36008 3548 50.29 4942 66J.9 65.77 47.60 47.00

Ti02 2.81 2.86 2.55 2.59 .58 0.61 2.95 3.07

A1 203

11J.1 11~ 1 1307 13J.4 15.:31 14E9 14.62 13.94

Fe 203

6JJ4 6.54 4.60 4.79 165 181 5J.5 5.96

FeO 10.26 10.27 7.04 7.10 1J.7 1.41 7.16 7.21

MnO .23 .21 .16 .14 .06 .04 .16 .14

CaO 12.99 13017 8.98 8.91 2.h 7 2.74 &73 &68

MgO 10.29 10.13 9.03 &65 170 161 7.52 7.36

Na 20 5.49 5.65 2.43 2B1 4.22 446 3.50 387

K20 192 2.04 .54 .53 3095 4;32 131 145

P205 1.12 1.0 3 .33 .36 .18 .20 .62 .64

H 0+ .60 .74 .57 121 2.03 2J.5 .29 .782

H 0- .45 .65 .37 .48 .08 41 .36 .482

CO 2 J.6 .00 .01 .02

C1 .12 D3 .03 .01

F J.O .04 .13 .06

Total 99.77 100.56 10U03 100.13 99.96 10043 10U06 lOM8

HIGS 5 - Coarse-grained nephc1ine-melilite basalt.

HIGS 6 - Tholeiitic pahoehoe flow from the Waianae dome.

HIGS 7 - Mica- hornblende trachyte from Mauna Kuwale.

HIGS 8 - Alkali basalt capping the Waianae dome.

142.

calculation of normative minerals using the CIPW system.

The latter method introduces possible sources of error, e.g.

the presence of hypersthene in the norm due to a high Fe+3 /

Fe+Z ratio. The former method relies only on the direct

results of the analyses. The real weakness in this method

rests with the accuracy of the analyses themselves.

Chemical Analyses of Rocks from Manu'a. Twenty samples

collected from Manu'a were analyzed chemically for 13 stan­

dard oxides (Table 9A). All of the analyses were done by the

Japan Analytical Chemistry Research Institute at Tokyo. An

attempt was made to select samples representative of the

entire suite of rocks in Manu'a. Since there was a possi­

bility that the older lavas might be tholeiitic basalts, it

was from these rocks that most samples were selected to be

chemically analyzed.

In spite of its proximity to Fiji and the Tonga Trench,

the Manu'a Group represents a true "oceanic" environment.

Chayes (1964) has shown statistically that over 90% of the

"circumoceanic" lavas contain less than 1.75% TiO Z' whereas

the "oceanic" lavas either contain normative nepheline or

more than 1.75% TiO Z ' All of the analyzed samples from Manu'a

fit into the latter category.

The analyzed specimens are very similar in chemical

composition to the Hawaiian rocks of the alkalic suite. The

only notable difference is that the rocks from Manu'a have a

very high TiOZ content, ranging from Z.75% to 5.58%, which is

TABLE 9A. CHEMICAL ANALYSES OF ROCKSFROM THE MANU'A ISLANDS

143.

Spec. no. 11 17 44 62 89 93 97

Si0246.13 44.85 46.59 46.09 44.35 46.88 49.44

Ti024.23 3.55 4.22 3.66 3.06 4.01 3.09

A1 203

14.56 11. 81 13.40 12.71 10.30 14.09 16.77

Fe 2035.79 6.39 3.08 5.16 2.87 2.19 2.87

FeO 7.30 6.88 8.94 7.69 10.97 9.26 7.48

MnO 0.17 0.19 0.16 0.18 0.18 0.16 0.14

CaO 9.83 9.94 11. 42 12.00 8.77 12.02 8.35

MgO 6.83 11. 35 7.26 7.94 16.19 6.87 5.26

Na 20 2.~0 2.76 2.61 2.32 1. 91 2.80 3.77

K20 0.95 1. 08 1. 07 0.94 0.84 1.14 1. 79

P2050.57 0.47 0.51 0.40 0.37 0.52 0.76

H2O (+) 0.24 0.75 0.54 0.52 0.33 0.29 0.22

H2O (- ) 0.42 0.46 0.57 0.27 0.33 0.20 0.45

Total 99.82 100.48 100.37 99.88 100.47 100.43 100.39

Norms (CIPW)

Q 0.18

or 5.56 6.12 6.67 5.56 5.00 6.67 10.56

ab 23.58 22.14 22 i• 01 19.39 15.98 20.70 31. 96

an 24.46 16.68 21.41 21.68 16.96 22.52 23.63

(wo 8.70 12.06 13.22 14.62 9.78 13.92 5.45di (en 6.90 10.10 8.40 10.80 6.90 8.60 3.40

(fs 0.79 0.40 3.96 2.38 2.24 4.49 1. 72

hy (en 10.20 0.50 4.50 2.00(fs 1. 06 1.85 0.92 0.92

01 (fo 12.81 5.81 3.15 3.24 6.02 5.39(fa 0.61 1.94 0.72 8.36 3.37 2.96

rot 8.35 9.28 4.41 7.42 4.18 3.25 4.18

i1 8.06 6.84 8.06 6.99 5.78 7. 60 5,.93

ap 1.34 1.34 1.34 1. 01 1. 01 1.34 1. 68

ne 0.71 0.28 1.': 6

TABLE 9A. CHEMICAL ANALYSES OF ROCKSFROM THE MANU'A ISLANDS

144.

Spec. No.107 110 117 129 131 136 156

Si02 46.56 48.45 45.'13 45.30 45.59 46.83 43.41

Ti02 4.36 2.88 3.42 3058 2.75 3.73 4.06

A12

03

14.54 18.90 12.74 12.67 10.23 12.74 10.52

Fe 2 0 3 3.74 5.7 8 2.12 2.39 3.08 3.30 6.30

FeO 9.37 5.21 10.12 10.61 9.68 8.60 8.21

MnO 0.17 0.16 0.17 0.18 0.18 0.16 0.17

CaO 10.13 7.98 11.51 10.51 10.86 12.17 10.52

MgO 6.10 2.97 9.53 10.36 14.23 7.63 12.78

Na2

0 2.89 4.10 2.28 2.34 1. 64 2.29 2.00

K20 1. 04 1.88 0.90 0.97 0.49 1. 51 0.97

P2

05

0.53 0.90 0.42 0.51 0.32 0.41 0.42

H2 O (+) 0.35 0.59 0.39 0.56 0.27 0.52 0.50

H2O (- ) 0.34 0.47 0.55 0.40 0.48 0.26 0.59

Total 100.12 100.27 99.88 100.38 99.80 100.15 100.45

Norms (CIPW)

Q

or 6.12 11.12 5.56 6.12 2. 78 8.90 6.12

ab 24.63 34.58 16.77 18.21 13.62 16.24

an 23.35 27.52 21.68 20.85 19.18 20.02 16.68

(wo 9.74 2.78 13.57 11. 60 13.69 15.66 13.69di (en 6.20 2.30 8.00 7.40 9.60 10.60 10.20

(fs 2.90 0.13 3.56 3 .l~3 2. 90 3.83 2.11

hy (en 4.60 3.80 6.30(fs 2.11 0.13 1.85

01 (fo 3.08 0.91 11. 06 12.95 13.79 5.95 15.26(fa 1. 63 0.00 6.12 6.53 5.00 2.45 1. 02

mt 5.34 8.35 3.02 3.48 4.41 4.87 9.05

il 8.36 5.47 6.54 6.84 5.17 7.14 7.75

ap 1.34 2.02 1. 01 L34 0.67 LOI LOI

ne * 1.42 0.92 0.57 0.28

145.

TABLE 9A. CHEMICAL ANALYSES OF ROCKSFROM THE MANU'A ISLANDS

Spec. No. 196 207 210 221-A 226 229

Si02

43.91 43.55 43.68 46.45 46.46 46.60

Ti02 5.54 3.00 5.58 3.26 4.32 4.22

A1 2 03

13.17 8.66 14.15 10.04 13.59 15.32

Fe2

03

4.75 5.22 6.76 2.92 2.67 4.94

FeO 9.29 8.93 8.63 10.10 9.87 7.89

MnO o. 19 0.17 0.18 0.16 0.17 0.17

CaO 11. 83 8.28 10.05 10.63 11.13 8.70

MgO 5.85 18.72 5.75 12.31 6.64 6.41

Na 20 2.65 1. 73 3.09 2.05 2.99 3.50

K2 0 1. 04 0.87 1. 37 0.81 1.12 1.41

P2 0 5 0.64 0.38 0.76 0.41 0.61 0.67

H2..~ e+) 0.58 0.43 0.11 0.40 0.50 0.20

H20 (-) 0.78 0.50 0.35 0.44 0.26 0.39

Total 100.22 100.44 100.46 99.98 100.33 100.42

Norms (CIPW)

Q

or 6.12 5.56 8.34 5.00 6.67 8.34

ab 21. 22 14.67 23.58 17.29 23.06 29.34

an 20.85 13.07 20.29 15.57 20.29 21. 96

(wo 13.80 10.56 10.44 14.38 13.11 6.96di (en 9.70 8.10 8.40 9.80 8.00 5.10

(fs 2. 90 1. 32 0.79 3.43 4.36 1.19

(fo 1.10 4.50 1. 50hy (fa 0.13 1. 58 0.40

01(en 3.43 26.32 4.20 11. 55 6.02 6.58(fs 1.12 4.49 0.51 4.59 3.57 1. 63

mt 6.96 7.66 9.74 4.18 3.94 7.19

i1 10.49 5.78 10.64 6.23 8.21 8.06

ap 1. 68 1. 01 1. 68 1. 01 1.34 1. 68

ne 0.71 oJ( 1. 42 1.14 ** Contains normative nepheline rather than normative hyper-sthene when FeO/Fe203 ratio adjusted to 3:1 keeping constantwt. % of total iron oxides.

146.

TABLE 9B. EXPLANATION OF CHEMICALLY

ANALYZED SPECIMENS FROM MANU'A

11 from Tunoa depression

A vesicular alkali olivine basalt with olivine pheno­crysts approximately .5 mm in diameter, some of which arealtered to iddingsite (finely-divided goethite?). Labrador­ite (An 60 ) microphenocrysts occur in an intergranular ground­mass of titanaugite, sub-calcic ferroaugite(?) (a few grainshave a +2V = 10°), olivine, iddingsite, andesine ( An 48),ilmenite, magnetite, apatite, and interstitial alkali feldspar.

17 from Fitiiuta, at Tiafala Point

Alkali nonporphyritic olivine basalt containing dunitexenoliths up to 2 cm long and a few microphenocrysts about2 mm across of olivine that has been altered to iddingsite.The intergranular groundmass is composed of labradorite (An57)with augite, olivine coated with iddingsite, magnetite, andinterstitial alkali feldspar. The sample was collected nearsea level, and the amygdules are composed of calcite thatprobably has been deposited in the vesicles after the lavasolidified.

44 from a post-caldera flow on the northwest flank of Ta'ushield, approximately 2500 feet south of Fogapoa

Vesicular alkali basalt, transitional to hawaiite, con­taining a few augite phenocrysts about 2 mm across. Micro­phenocrysts of olivine and augite up to .3 mm across occur,sometimes as glomeroporphyritic clots. The groundmass iscomposed of olivine, augite, magnetite, and ilmenite. Thetexture is intergranular.

62 from a post-caldera flow ponded within Lepu'e Crater

Vesicular hawaiite, transitional to alkali olivine basa~,

containing olivine phenocrysts about 1 mm in diameter. Thevery fine-grained groundmass is composed of andesine(?),olivine, augite, ilmenite, magnetite, and apatite. Theseminerals form an intergranular texture. A 1abradorite(?)xenolith about .5 mm across has been partly resorbed.

89 from intra-caldera flow on southern Ta'u

Vesicular alkali picrite-basa1t (oceanite type) contain­ing abundant olivine phenocrysts up to 6 mm in diameter anda few augite phenocrysts up to 5 mm across in an intersertal

147.

groundmass of labradorite, ilmenite, a few magnetite grains,apatite, cryptocrystalline material and glass.

93 from a dike at Laufuti, southern Ta'u

Alkali olivine basalt comprised mainly of labradorite(An55) and brown augite with some' olivine, ilmenite, andmagnetite to form an intergranular to diabasic texture.

97 from intra-caldera flow at Papa loa loa Point on southernTa'u

Nonporphyritic hawaiite transitional to alkali olivinebasalt which contains a few olivine microphenocrysts about.3 mm in diameter and labradorite microphenocrysts about1 mm long. A hyaloophitic texture is formed by labradorite(An

52) microlites that are approximately parallel, ilmenite,

magnetite, and black glass.

107 from pre-caldera flow exposed in Avatele stream bed onnorthern Ta'u

Hawaiite composed mainly of andesine (An4S) and augitein a ratio of about 3:1. A few microphenocrysts about .5 mmacross of olivine and augite occur. Usually the olivinegrains are partly altered to iddingsite. The groundmassconsists of feldspar, colorless clinopyroxene, olivine,abundant ilmenite, some magnetite, and brown glass to forman intergranu1ar to intersertal texture.

110 from a pre-caldera lava flow exposed in ~o'auli stream bed

Fe1dspar-phyric alkali basalt,transitional to hawaiite,composed of abundant laths of labrado~ite (An65) up to 3 cmlong in an intersertal groundmass composed of ilmenite,andesine(?) microlites, olivine, apatite, and brown glass.

117 from a pre-caldera (or possibly intra-caldera) flow atAfuatai, southern Ta'u

Vesicular alkali olivine basalt, approaching a picrite­basalt, which contains scattered olivine phenocrysts up to2 mm in diameter. A few glomeroporphyritic clots composedof olivine and augite grains up to 1 mm in diameter also occurin the intersertal groundmass of labradorite(?) micro1ites,abundant ilmenite, magnetite, apatite, and glass.

129 from Luatele shield

Vesicular picrite-basalt containing scattered olivinemicrophenocrysts up to 1 mm across. Some of these grainshave a birefringence in their outer zones, probably due to

148.

iron enrichment. A few glomerocrysts of olivine occur.The interserta1 groundmass is composed mainly of glass andcryptocrystalline material with labradorite(?) micro1itesand ilmenite.

131 from a post-caldera flow erupted from Salumaae Crater

Vesicular alkali picrite-basa1t (oceanite type) contain­ing olivine phenocrysts up to 3 mm across. A few augitemicrophenocrysts approximately .2 mm in diameter are present.The intergranu1ar groundmass is composed of labradoritemicro1ites, ilmenite, augite, cryptocrystalline material,and glass.

136 from post-erosional flow forming Moso Point on westernTalu

Alkali olivine basalt, approaching a hawaiite, composedmainly of labradorite (AnS2) microlites and cryptocrystallinematerial. Olivine grains approximately .3 mm across formglomerocrysts up to 2 mm in diameter. A few augite pheno­crysts up to 4 mm long occur. Sub-calcic augite micropheno­crysts about .3 mm across have a low +2V and probably arehigh in titanium, but show no purplish tint. Magnetite,ilmenite, olivine, augite, and apatite form an intergranu1argroundmass texture.

156 from a flow ponded within the A'ofa caldera on northernOfu

Alkali picrite-basa1t (ankaramite type) composed ofabundant olivine and augite phenocrysts up to 4 mm across.The former are thinly coated with rims of iddingsite about.02 mm wide. A glomeroporphyritic clot of labradorite micro­phenocrysts approximately 1 mm long is present. Abundantmagnetite, usually less than .005 mm, and some ilmenite occur.A few euhedra1 opaque grains of magnetite are up to 1 mm indiameter. The remainder of the groundmass is composed of1a~radorite (An60) micro1ites, augite, olivine, crypto­crystalline material and glass. The texture is intergranu1ar.

196 from a pre-caldera flow of the Si1i shield, just eastof Si1i

Vesicular nonporphyritic alkali olivine basalt contain­ing olivine microphenocrysts approximately 1 m~ long whichhave been altered to iddingsite in an intergranu1ar to hya10­ophitic groundmass composed mainly of labradorite (AnS4) andtitanaugite. Ilmenite, some magnetite, apatite, and blackglass also occur.

149.

207 from post-caldera(?) flow of the Sili shield at Le'alaPoint on southeastern Olosega

Alkali picrite-basalt (oceanite type) containing bothabundant olivine phenocrysts up to 4 mm in diameter andolivine in the groundmass. The olivine has been partlyaltered to iddingsite. The remaind~r of the rock containslabradorite microlites, ilmenite, magnetite (some grains upto .3 mm), cryptocrystalline material and glass to form anintersertal to intergranular texture.

210 from a thick flow ponded within Maga Crater on thesouthern tip of Olosega

Hawaiite, transitional to olivine basalt, that containstabular olivine microphenocrysts approximately .5 mm long,which are considerably altered to iddingsite. Plagioclasemicro lites range from An52 for larger grains to An46 forsmaller grains. The remainder of the groundmass is composedmainly of magnetite (grains up to .2 mm) and ilmenite witha few zoned olivine grains up to .3 mm apatite, and inter­stitial alkali feldspar. The texture is intersertal tointergranular.

22lA from pre-caldera flow exposed in 'Ao'auli stream bed

Vesicular olivine picrite-basalt containing olivinephenocrysts up to 3 mm across. A few labradorite (An63)phenocrysts up to 2 mm long and very few augite phenocrystsabout 2 mm long are present. The intersertal groundmasscontains abundant magnetite, fine-grained feldspar, crypto­crystalline material and dark glass.

226 from post-caldera flow on southeast flank of Ta'u shieldnear Lalomaota

Vesicular hawaiite transitional to alkali basalt composedmainly of labradorite (An50) micro lites with a sub-parallelorientation and a few larger laths of labradorite (An55).Olivine microphenocrysts about .5 mm across and micropheno­c~ysts of clinopyroxene (titanaugite?), some of which have alow +2V, are present. Abundant ilmenite, apatite, crypto­crystalline material, and brown glass form an intersertaltexture.

229 from intra-caldera flow on southern Ta'u

Hawaiite composed predominantly of andesine (An45) micro­lites, which have a sub-parallel orientation. A few olivinemicrophenocrysts (elongated parallel to the b-axis) are about.5 mm long and are partly altered to iddingsite. Abundant

,.u

150.

grains (approximately .01 mm across) of ilmenite and mag-

netite lie in the intergranu1ar groundmass, the remainder

of which is composed of very small amounts of biotite,

apatite, and interstitial alkali feldspar.

151.

1-2% higher than that of the corresponding Hawaiian lavas.

The rock with the lowest Ti02 content is the same one (#131)

that has the lowest alkali content. However, this sample

contains both normative olivine and abundant modal olivine

in the groundmass. Augite occurs only in the groundmass,

and the total feldspar comprises less than 30% of the rock.

It should therefore be defined as an alkali picrite-basalt.

Most of the titanium in the rocks of Manula apparently is

taken up by ilmenite and titanaugite. It is also likely

that titanium occurs in the sub-calcic augi~es with low +2V rs.

All of the analyzed spe6imens appear to belong. to the alkalic

suite according to their mineralogical characteristics, but

as discussed above (pp. 134-135) the reliable mineralogical

criter~. ~re very few, particularly when dealihg with fine­

grained transitional rocks.

Normative minerals were calculated for each of the

chemical analyses of the rocks of Manura. The standard

Cross-Iddings-Pirsson-Washington method as outlined by

Johannsen (1938, vol. 1) was used in these computations. The

value in using this scheme is that it facilitates comparison

with rocks from other areas (e.g. Hawaiian rocks).

All of the samples from Manula except one (#11) contain

normative olivine. Ten of the samples contain normative

nepheline, and the other ten contain normative hypersthene.

Only four contain more than 1% normative nepheline, the

largest amount being 1.56% (#93). No modal nepheline was

152.

found to occur.

Of the ten samples containing normative hypersthene,

only two had FeO/Fe2

03

ratios greater than 3:1. However,

only four of the samples containing normative nepheline had

FeO/Fe2

03

ratios greater than 3:1 eithert Nevertheless, the

FeO/Fe2

03

ratios for eight samples containing normative hypers­

thene was adjusted so that the FeO/Fe 2 03

ratio was 3:1, keep­

ing the weight per cent of the total iron oxides constant.

These samp1ffiwere then recalculated for normative mineral

content. Three of the samples (#110, #207, #229) then con­

tained normative nepheline rather than normative hypersthene,

leaving a total of seven samples with normative hypersthene

even though their FeO/Fe 20 3 ratios were all greater than 3:1.

These would be tholeiitic rocks according to the definition

of Yoder and Tilley (1962).

There is no correlation between the pre-caldera and

later-stage lavas in regard to their normative composition.

Some of the lavas containing normative hypersthene are post­

erosional lavas, whereas others with normative hypersthene

are among the oldest rocks exposed on the islands. Also,

there is no correlation between the Ofu-01osega volcanic pile

and that of Ta'u, except that all four samples from Ofu and

010sega contain normative nepheline and less silica than any

of the rocks from Ta'u. If the calculations are considered

to be valid, the only conclusions to be drawn are that the

rocks from Ta'u Island are made up of interbedded tholeiites

153.

and alkali basalts and that no tholeiites were found to occur

on Ofu and Olosega Islands. The inadequacies of calculating

normative minerals has already been pointed out. The alkali­

silica diagram with the empirical tholeiitic-alkalic boundary

line seems to be most reliable except for picrite-basalts.

What now appears to be the most serious problem is the relia­

bility of chemical analyses by the present methods. Until

chemical analyses can be reproduced within a reasonable amount

of accuracy by different laboratories, no acceptable method

is of much value, at least in classifying the transitional

rocks. Perhaps the real answer to this problem is that the

two suites are transitional in nature and therefore cannot be

separated by any method.

All but one of the Samoan specimens analyzed plot above

the empirical boundary line on the alkali:silica diagram

(Figure 7) and therefore belong to the alkali basalt group.

The single "tholeiitic" basalt was a Recent(?) post-caldera

flow of picrite-basalt from Sa'umane Crater (sample #131).

This picrite-basalt, which contains both normative hypers­

thene and olivine, is composed of olivine phenocrysts in a

groundmass of olivine, augite, and ilmenite. It is this

same sample that also has the lowest TiOZ

content. Because

the rock is undersaturated in silica, it is probably alkalic

rather than tholeiitic. It seems that the somewhat high

percentage of mafic minerals could have caused a decrease in

alkalies relative to silica. Therefore, all normative olivine

I ----------.------.

•

b'-

5

ol'l /", i_

~. "

i-

SRS 5 ,-~

~

2.

1~iI

..

....

.......- --- -

..

••~

.. .• ­. ......----......- __ ..... '- :----_X I

_- x.-------

.- - .....-

"

---­.-------......-

­.-----.....­--,-

FIGURE 7. Alkali: _silica diagram of rocks from Manu I a J _ sho''ling the .boundary (diagonaldashed line) between the tholeiitic and alkalic fields. The picrite-basalt from Ta'uwhich plots as tholeiite is shown by an X. The same rock after subtraction of allnormative olivine is shown by an XI.

L-..--L--....-----'-----..._ ...~_...__ ....'--. 1._•• __

:1-.s 44 45" ;16 470';iD

- -- - - . - -- \

S r48

I IJ~

--\ -

L19'-

50-_._. ---. \ -.- - ._.-. --

5"1-, ------5?_

t-'IJ1+:-

155.

was subtracted from the rock, and the recalculated figures were

plotted on the diagram (Figure 7). The sample still falls

below the boundary line by about the same amount, although,

of course, it was displaced to the right and upwards.

The samples were plotted on another type of variation

diagram, the AFM diagram (Figure 8), in order to determine

the nature of any trends in magmatic differentiation which

could have been important in producing the various rock types

found to occur in Manula. In this triangular plot the coord~

nates are the alkalies (NaZO + KZO), total iron (FeO + Fe Z0 3 )

expressed as FeO and magnesia (MgO). The diagram clearly

illustrates the importance of olivine in the differentiation

of basaltic rocks; this mineral falls directly on the main

trend of the lavas. The occurrence of hawaiite and picrite­

basalt indicates that a primitive olivine basalt magma was

just beginning to undergo differentiation by crystal settling

within the magma chamber. Apparently the magma did not bec~me

acidic enough to produce the trachytic end members of the

series, which most likely would have contained normative and

possibly some modal quartz.

The magma from which the rocks of nearby Tutuila were

derived was apparently similar to that of Manula. The mafic

rock samples fall in about the same area on the diagram as

the corresponding lavas from Manula. It seems probable that,

had there been sufficient time and/or other conditions to

__ allow complete differentiation within the magma chamber,

156.----------------_._--_._-_._---_._-----

\.

\I

oliviuG r;i7\'.

.::.L :..._.._._'I _.__ __ .\.1.-._. __.'.l\/

/ \,\\

~

\'\\

i\

FIGURE 8. A-F-M diagr28 shOWing the ~elative distri­Ducio:1 of hU\;!c1iite C-cI'i~~nglcs)~ picrito-bc:s:J.lt (opencirclos)? ~nd ~lkQli olivine o~sQlt (solid circ18s)';:''''-'0'(''' 1\Ii"'n'1l''''' fi. - ;-0·1-'11 "'11.~~1 i Co' (I·T .-, 0 .'. T~ 0) T;' - ;-0;-',..,1-- .:.u ......_.. J. ....... ljlo ... - - u v ...... _ <..;." _\.u__ .v -''--? . £-2 ~ - - u uc... _

i:c'on 3.8 FeO? and. IVl =1.'lgO. Also incIuded are 2..nal;y"s0sf'Y'O"71 . 1"'1':-'" i i r' (c''-'o C' ~,~ "" )\ 'C'V~'r'" r:;-.=. "-1" ':'Y1Q' 1 -i 'n r~ c o·f' u -.:1"1'" i l' ,.., y)__ ll ... .LL.:-LJ....c.___ _ -J;;) ...... ~ ., c:. c: f.. ... o ...... U t.::_ _-1-J.J.vv ..L. fi~\j ~_ t;..t.J.':'

tholeiite (solid line) and al.~~lic (dashed line) suites?" and typical olivine COL'lposition (sq~)are).

i ,L- . __ _._ .. ._ _ _ __.. . --1

157.

acidic rocks similar to those on Tutuila would have occurred

in Manu'a. Therefore these acidic rocks from Tutiila are

also included in the AFM diagram. This trend is very similar

to the trend for Hawaiian lavas of the alkali basalt series.

Iron and titanium, in general, decrease slightly with increas­

ing silica content for all types of basalts and hawaiites.

However, the iron in the mugearites and trachytes from

Tutuila~ as in those from Hawaii, begins to decrease rapidly

with an increase in silica and alkalies. Perhaps this

"turnabout" in the trend is caused by an increase in the

partial pressure of oxygen, which would yield silica-enriched

differentiates rather than iron-enriched differentiates

(Osborn, 1962). The fact that this reversal in trend always

occurs at about the same point in all suites, i.e. when

mugearites are beginning to form, weakens the latter sugges­

tion. It seems that the partial oxygen pressure of all magmas

yielding mugearite as differentiates is suddenly increased for

some unknown reason. Why should the partial oxygen pressure

just happen to increase at this particular point of magmatic

differentiation? Possibly some other physicochemical factors

related to the complex process of magmatic differentiation

(e.g. the effect of volatiles) are also important in this

change in differentiation trend.

REFERENCES CITED

BAILEY, K. B., CLOUGH, C~. TI., WRIGHT, loll. B., RICHEY, Jo E.,and WILSON, Gr. V~., 1924. Tertiary and post-tertiarygeology of Mull, Lock Aline, and Oban. Mem. Geo1.Survey Scotland, 445 pp.

BOWEN, N'. L., 1928. The Evolution of the Igneous Rocks.Princeton Univ. Press, Princeton, 322 pp.

BOYD, F. R'., and ENGLAND, J. V'., 1961. Melting of silicates.Carnegie Inst. Wash. Yrbk, 60, pp. 113-25.

CHAYES, FELIX, 1964. A petrographic distinction betweenCenozoic volcanics in and around the open ocean. J.Geoph. Res., vol. 69, pp. 1573-88.

DALY, R. A'., 1924. The geology of American Samoa. CarnegieInst. Wash. Pub1. no. 340, pp. 93-152.

DANA, Ji. D., 1849. Records of ~. Exploring Expeditionduring the years 1838-1842, under the command ~ CharlesWilkes, U.S'.NI;', v. 10 (Geology), National Archives,Washington, D. C., pp. 307-66.

FOX, J., and CUMBERLAND, r. B., 1962. Western Samoa.Whitcomb and Tombs, Ltd., Ghristchurch, 337 pp.

FRIEDLANDER, I., 1910. Beitrage zun der Samoa inse1in.K. Bayer. Akad. Wiss., Math-phys. K1., Munich Bd. 27,pp.358-69.

GREEN, D. Hio, and RINGWOOD, N. E., 1964. Fractionation ofbasalt magmas at high pressures. Nature, v. 201,pp • 127 6- 79 •

JOHANNSEN, ALBERT, 1938. ~ Descriptive Petrography of theIgneous Rocks. v. 1, Univ. of Chicago Press, Chicago,267 pp.

LEAR, D., and WOOD, B. L., 1959. The geology and hydrologyof West~rn Samoa. New Zealand Geo1. Survey, Bull.,no. 63, 92 pp.

KENNEDY, W. Q., 1933. Trends of differentiation in basalticmagmas. Amer. Jour. Sci., 5th series, v. 25, pp. 239-56.

KENNEDY, W. Q., and ANDERSON, E. M., 1938. Crustal layers andthe origin of magmas. Bull. Vo1cano1ogique, series II,tome III, pp. 23-82.

159.

KUNO, H., 1959. Origin of cenozoic petrographic provincesof Japan and surrounding areas. Bull. Vu1cano1ogique,series 2, v. 20, pp. 37-76.

KUNO, H., 1960. High alumina basalt. J. Pet., v. 1,pp. 121-45.

KUNO, H., YAMASAKI, K., SIDA, C., and NAGASHIMA, K., 1957.Differentiation of Hawaiian magmas. Japan J. Geol.Geog., v. 28, pp. 179-218.

MACDONALD, G., A., 1944. Petrography of the Samoan Islands.Bull. Geol. Soc. Amer., v. 55, pp. 1333-61.

MACDONALD, G. A., 1949. Hawaiian petrographic province.Bull. Geol. Soc. Amer., v. 60, pp. 1541-96.

MACDONALD, G. A., DAVIS, D. A., and COX, D. C., 1960. Geologyand groundwater resources of Kauai. Hawaii Div. Hydrog.,Bull. 13, 212 pp.

MACDONALD, G. A., and KATSURA, T., 1961. Variations in thelava of the 1959 eruption of Kilauea Iki. Pac. Sci.,vol. 15, pp. 358-69.

MACHESKY, L. F., 1965. Gravity relations in American Samoaand the Society Islands. Pac. Sci., vol. 19, pp. 367-73.

MURATA, K. J., 1960. A new method of plotting chemicalanalyses of basaltic rocks. Amer. J. Sci., v. 258-A,pp. 247-52.

OSBORN, E. F., 1962. Reaction series for subalkaline igneousrocks based on different oxygen pressure relationships.Ame r • Min., v. 47, p p • 2 11- 2 6 •

POWERS, H. A., 1935. Differentiation of Hawaiian lavas.Ame r. l. Sci., v. 255, Pp. 241- 53 •

POWERS, H. A., 1955. Composition and origin of basalticmagma of the Hawaiian Islands. Geoch. et Cosmoch. Acta.,v. 7, pp. 77-107.

RUHE, R. V., WILLIAM, J. M., and HILL, E. I., 1965. Shore­lines and submarine shelves, Oahu, Hawaii~ ~ Geo1.,v. 73, pp. 485-97.

SHARP, ANDREW, 1960. Discovery of the Pacific Islands. OxfordUniv. Press, 259 pp.

Soil Survey of Hawaii, 1955, ~. Dept. Agr. Soil Survey Bull.,no. 25 (1939 series), pp. 73-9.

160.

STEARNS, H. T., 1944. Geology of the Samoan Islands. Bull.Geol. Soc. Amer., v. 55, pp. 1279-1332.

STEARNS, H. T., and MACDONALD, G. A., 1946. Geology andgroundwater resources of Hawaii. Hawaii Div. Hydrog.,Bull. 7, 344 pp.

STEARNS, H. T., and MACDONALD, G. A., 1946. Geology andgroundwater resources of the island of Hawaii. HawaiiDiv. Hydrog., Bull. 9, 363 pp.

TILLEY, C. E., 1950. Some aspects of magmatic evolution.Quart. ~. Geo1. Soc. London, v. 106, pp. 37-61.

WILLIAMS, JOHN, '1907. Missionary Enterprises in the SouthPacific. Presbyterian board of publication and Sabbath­school work, Philadelphia. 416 pp.

YODER, H. S., Jr., and TILLEY, C. R., 1962. Origin of basaltmagmas. J. Pet., v. 3, pp. 342-532.

14 0

OS'S

1690 42' W

"'------- - - -- - --.,

-

10'

~." ..r·...'"'--

~--- ..~ ....,.- .,.""

/'/

//I{

\B

\Nu/us i Iaelae&\

Is land '\I

~\

\

---~140'~---~·

--- -- ---------_ .. -

---

----•

---- ----------

/.---._-~-

/ 1 ---~est ~

r,' <res I -----ougo ----

Po;n! ----Tete --t -----

\

-- - _. ------

-- -----::__:::~~ --1:'1Be-----..~8· 3000

t~ :.~>,",,----

-

".

-.1.-

p.

oLO SE:'.

/I

//

//

//

/~..--

--------

---

--

-

-

-

~ .~- - .~ ....... ' , ,

"~~.:~"'.~~.

14 0

oa's

.. ':,..

1690 42' W

10'

---.,

//

///(

\

\

- - ~-~--------

----/-----

--------..--- -- ------

/----------

\

--- - - - --- -- ----

Le'olo

OLOSEGA IS'--------

Leoumosili Pt.\ .

• Nu'ututol Rock

300 038'

----

34'

i:r6'oo

•

iA ISLAND

24 00

Ifooo

~FEET '"

3200r '"

:>'. -:"

. I

6000-

5400-

4200

~----------3600

Cindercone

•

' '___---- 4~

6000-----

5400 _____

----------

~.

~4200____ 3600

__---480 0

Caldera

--------------

Cinder cones

-- A'

n es

I lin t '" I '"

..

FEET

j3200

2400

Ia.n"

169 0 22'W

•

14°08'S

- LEG E N" D FO R T A I U ISLAND 10'

EJCALCAREOUS SEDIMENTS

Uodern beaches (:~b) composedo~· unconsolidated fragmen ts ofm~rine organisms. Beachrock isfr'~quently present.

t

'\- .:'.?:.~:

,":'.

"

.~...'

"

u

12'

BFEET2400

1600._, -.

800NU'ute I els I e tl

Reef

'14' 0

-800....

'\\

----- ,----..

Va i n u'u lUGI .

------~ ----- -.........

---

Le'o' 0

Ri d g e

I

\ 2. 00.

A'ofaCaldero

I

Ma koRidg e

\

.-.. ~~ .

'0.010.\ \ \

. OLOSEG~

Le'a la Pt.

c

Pt.

\SLAND24 00

,;

B' C-600~

/ .-20 o

.. ...

,1 '

FEET2,400':':' '.",.,' ",T"

~\600,- .

AFEET

3200

2400

1600 Cinder

800

Ol---------~

Faleosaotuff cone

I

/~oo

Si::ulaoi Pt.

Moega-a-uila

Fo'asamene Cove/A

7

~__--- ----- 3600----

~ --- 4800-4200

SECTION~A·LONG;

Ci ndercone

jer conesI I

------_0 ---

f800~

- -

- --- --

------ 54 0 0 ______

-------

---------- 4 20 0

3600

-----SEC T 10 N'----A·LO NG-'

4800-----

A~AJ

--

/Ieoo~

--- ---- - -

---

-- -- ----

---

--- A'

a

2400

800

1600

FEET3200

Formersea cl iTt M .

I aluatlo

", Qf I, I~Qfc

5

FITI

--- --....... ~.J(I.

/~

,OJ .... "...~ l.'.J, l~

/ t:-:ncb

""BLe;i

~

~~.~ ~ :'""\ ::~ [.--- ~----- ~ '\---~

~

"'"-- ---- --- ------- ~ "" FALF A~ '\ \ t

~ Fitiiuto Pt. A' ~ \ \ (Qftcrys

.>'" , \and

I ) f101lcone

Iand

Pt. _ .

I // I tAJ)R) EROS 10

Tiofolo Pt.

Fogomutie ! / II

~CALCAREOUS SEDIMENTS

Modern beaches (Qb) composedoi' unconsolidated fragments ofm3rine organisms. Beachrock isfrequently present.

NONCAlCAREOUS SEDIMENTSAlluvium eta), including

talus, landslide debris at thebase of cliffs, and stream de­posits. In areas behind con­structional benches marshesC~.Ir.) sorretimes occur.

Qf I

IUTA VOLCANICS

I­ZlJJUlJJ(k:

~·Jav~ rtD\,:S ~.,· .. .fl) of t:::s;jltcU'/1ne ~'~;,~lt J'::,rll;:rw :~he

cb ~t }~j.~j.i~ta. ~n as~:oci-

j :;in.Je':' ':l'W· C-(fi~) is :::.i[;f'!';j::~ [.~. t. 4 1,v ,

" .;~, :. .', -.. . ': ".".- ..:.",;,'..... ;. -.'

ASAOTUFF "COMPLEXUndifferentiated tuff complex

~ft) of palagonitized vitric­'ystal lapilli tuff, breccia,ld occasional horizontal lava.ows from at least three main,nes centered at Faleasao, Tota,Ld Fa' asemene Coves.,

ONAl UNCONFORMITY

lJJ:..~

\:.'. -;i

14'

"

L'EG END

800

o t-----_..J

r/

FOR' OFUAND OLOSEGA ISLANDS..

MAJOR EROSIONAL UNCONFORMITY

SEDIMENTS

.. ~

~:.;-: ,..•.~.~ . :,'::i~

;0:f;~~,.;\; ~",:::".;'r

Alluvium (~a), including~alus, landslide debris qt :~:o

,base of cliffs, and stream d~

'posits. In areas behind con­structional benches m~rshE~

(-im) sometin:es occ\" ., .

, PalagonitizeJ hpilli tuff (~,n)

of Nu'u tuff cone, whi~h form Nu'utele and Nu'usilaelae Islets onwest-ern O'fu. A few qe('ent(?) i.awai­ite flows (Qol) may h~ve poured downformer deep1.v eroded stream valleyson western Dfu.

OFU RECENT VOLCANICS

o'CALCAREOUS SEDIMENTS

Modern beaches ,(Qb) composedof unconsolidated fragments ofmarine organisms. Beachrock isfreq¥ently present.

i

NONCALCAREOUS

...... ' ~

::0rrJ0­rrJZ--I

,"-0"rrrJ

en--IoorrJZrrJ

.. " ,"

16'

Teol

lOS

Va in u'u luaI .

SECTION ALaN G

5400

6000-_~

u- D

EXPLANATIQN

Normal fault showing

u---D

•• JI •••

Eroded fault scarp s~

approxima te ly 1oca tel

Buried fault scarp,

Contact, definitely

Contact, approximate

Dip and strike of be<

HorizontAl hAna

G 8'

Pt.

Con e?

·1.,

c....~

S7Ql

oo"

QN 0 F SYM SOLS

wing downthrown CD) side.

rp showing downthrown (D) side,~a ted.

rp, approxim~tely located.

sly located.

nately located.

~ beds.

/

5400 __/

!

LISOO

7T 'r.,

1600f""" .

800 .

..

• i

----800 .

\

------10

\GEO. LOG I C MA

oF TH E ...

MANU'A. ISLAAMERICAN . SA~

SCALE.-_ 1:24,000

Moso

\\

Afu Ii

\\

Va iota

\\

\

\ Fogamala

\\

~

oo

Ma'afe'e"Island

. tUtumanu'a· Po In

MOA

~p

~NDS

N00

Lafogaufi Pt.

---........Qtel/

""-'"

Cove/

.\//

---/ ---/"// /

/ /

----Pt.

/// /

/ ,.~

/./ //

/ / // /

/./

Li'u

----- ~. ~-- - --........ "'",/' ~/-/

----- ~

/'..

'//

./

Pt.

//

Cove

//

//

//

//"

/

//

/Qtel /Papaotoma Pt.

IO'atele /

~oga Pt.

~ Pt. /

TL

andthewes'llhelapJ,YsitcryoliwhiAssarelevbassou

LUA

IVI M U VI\//II\

IIIj

J

J

I

I\\\\\,)

//

//

//

/.J

ITiafala Pt..

Fogomutie I

)

I

\)

I

I

J

//

Papaso'o Pt.

~ Q f I

)

bJ...J0.

16'

-

lJ.JZIJJo:0

c.ii. _

..JQ.

:-~

·lLI::·::·ZIJJUo.f- ..en

" ".

SERIES

Qtcv.

Qtel

VOLCANIC

Pre-caldera volcanics (Ttep)consi~t of lava flows of olivinebasalt, picrite-basalt, basalt,feldspar-phyric basalt, and ha­waiite with occasional beds oftuff. Most of these rocks havebeen mantled by post-calderaflows (Qtel) of olivine basalt,picrite-basalt, basalt, and ha­waiite which were erupted fromcones on the flanks of the shield.Nhere possible, late cinder cones(Qelc) and their associated lavaflows (~ell) have been mapped sep­arately. Usually the pre-calderalavas and the post-caldera lavascannot be separated and are mappedtogether as post-caldera volcanics(~tel) except in a few areas whereit appears likely that pre-calderavolcanics (Ttep) are exposed. Theintra-c~ljera volcanics (Qtcv)consist mainly of dense flows ofolivine basalt, picrite-nasalt,basalt~ and hawaiite with numerousthin beds of vitric-crystal ash.An intra-caldera cinder cone (Qtcc)and its associated lava flows (Qtcl)of basalt are mapped separately.Other Recent flows (Qtcl) of basaltoccur. A dike complex intrudes thepre-c81dera(?) (Ttep) volcanics.

TA'UVOLCANICS

Thin lava flows (Qle)· ofolivine basalt and picrite­basalt h3V~ built the .Luateleshield on the northeasternportion of the island.".The .depression w~s partly filledwith Donded olivine basaltlall8s' (Qlc).

TUNOA

Lava flows (Qnel) 0:' basaltand olivine basalt hav~ builtthe Tunoa shield on the north­western portion of the island.The depression formed by col­lapse of this shield was part­3y filled with volcanic depo­sits (Qncv) of red vitric­crystal ash~ lapilli tuff, andolivine basalt lavas, some ofwhich contain dunite xenoliths.Associated cinder cones (Qncc)are mapped where possible. Onelava flow (Qncl) of olivinebasalt plunged over the cliffsouth of Ta1u village.

Iv IV'" r-\L- 'VI"V"'I~' "'., ••••••

.UATELE

OFU RECENT VOLCANICS

..' ".

UNCONFORMITY

struc~lona~ oeocneB marlB~l~le~,,~---­

(Qm) sometimes occ~~.

EROSIONAL

. Palagonitizel lapilli tuff (Qn)of ~u'u tuff cone, which form Nu'utele dhd Nu'usilaelae Islets onw~stiern o"fu. A few 'iec.en t (?) hawai­ite flows (Qol) may have poured downformer deeply eroded stream valleys

~ on western Ofu.

'fTI

16''I:::0rr1O-mz-i

OFU - OLOSEGA VOLCANIC S.ERIES

-0r-

140 0IS'S ('")

rr1ZIT1

The lower member of the extra-caldera volcanicsconsists of at least five cones aligned along theregional rift zone--a cinder cone (Tetc) at TaugaPoint, a tuf! cone (Test) at the west end of Samo'iwith an associated lava flow (Tesl) ponded withinits crater, a composite cone (Tet) at Talaga withassociated red ash (Teta), a breccia cone (Tefb)with nn associated plug (Tefi) at Fatuaga ~oint,and a tuff cone at Maga Point. The upper memberof the extra-caldera volcanics consists of twocoalescinp; shields, the "A' ofa shield (Tea) andthe Sill shield (Tes), wh:ch are comprised mainlyof olivine basalt, basal4i, picri.te-basalt, andhawaiite with intercalated beds of ash, tuff, andbreccia. T~ese shields and the Fatuaga brecciacone are intruded by numerous dikes. The intra­caldera volcanics consist of thick ponded flows(Teal) of oliYine basalt, hawaiite, and ankara­'mite. A. buried cinder cone (Toac) is also present.

1690 421

W-, ....._---..----_......---------t "~

... '....10 ••',·' -,

~~?~.~~; , i- ..,/

--..

u- o

u---o

..", ...

.... . .

.1

EXPLANATION (

Normal fault showing de

Eroded fault scarp showapproximately located.

Buried fault scarp, apl

Contact, definitely lo(

Contact, approximately

Dip and strike of beds.

Horizontal beds.

Dike.

Intermittent stream.

Coral reef

\J OF SYMBOLS

ng downthrown CD) side.

showing downthrown (D) side,ted.

), approximately located •

.y located.

Ltely located.

beds.

m.

12°.- .. "

Il­e::oz

W::Je::I-

APPROXIMATE MEANDECLINATION, DECEMBER 1963.

____________________~i'

, \

• •• • • •

OF TH E

MANU 'A. ISLAN

AMERICAN SAMCseA L E"-. ,: ~4,0 0 0

I 'h 0

Er::a:::EI MIL EIS

1000 0 1000 3000 50007000~ I I I

FEET

CONTOUR INTERVAL 200 FEET.-

•DATUM IS MEAN SEA LEVEL

SUBMARINE CONTOUR INTERVAL 60

BASE FROM U.S. GEOLOGICAL SURVEY MAGEOLOGY BY G.D. STICE AND\F:W: :,McCOYBAT HYMET RYIN T E RPRE TED B't\ G. D. STIC ESOUNDINGS ON U.S. COAST ~N~ GEODETICBOAT SHEET * 23 (1939). "" \

~i.':···~."'·.·....".'..:C.""......-.. __ia••_1.34.~•.•." WDr.,__.~•. _

NOS

lOA

I

=3

ET

L

600 FEET

MAP (1963), .

~OY, JR ..CE FR OMTIC SURVEY

32'

\I-\TU I I

30'

/

6

--

Io

oroO

- --:-....

..

28'

//

/---~

,,/'/

//

//

//

//

.//

',./

\'2.00

//

/

/

-- -- -----

..

//

//

Pt. /

IO~O~~::::ma /t~toga Pt. ~

ise Pt. /ve,>-./

\

//\,

24'

/

/

intra-c~ldera vOlcan~c6 \~V~V)

consist mainly of dense flows ofolivine basalt, ~icrite-baBalt,basalt, and hawaiite with numerousthin beds of vitric-crystal ash.An intra-caldera cinder cone (Qtcc)and its associated lava flows (Qtcl)of basalt are mapped separate~y.O~herRecent flows (~tcl) or ba8a~~occur. A dike complex intrudes thepre-c~ldera(?) (Tte~) volcanics.

l,I

Q

____________.16.9.0

(22'W •,'..