Petrology of the

I Volcanic Rocks

of Guam

I GEOLOGICAL [SURREY PROFESSIONAL PAPER 403-C
Petrology of the 

Volcanic Rocks 

of Guam

By JOHN T. STARK

With a section on

TRACE ELEMENTS IN THE VOLCANIC ROCKS OF GUAM

By JOSHUA I. TRACEY, JR., and JOHN T. STARK

GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-C

A study of the basaltic and andesitic 

volcanic rocks of Guam and their relations 

to similar rocks of Saipan and Japan

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington, D.G. 20402
CONTENTS

Abstract _________ ____________ _______

Introduction _ _____ ________ _ _______

Acknowledgments _ _________ ____ ___

Outline of geology___ ___________ __ ___

Classification of the volcanic rocks__ _____ __

Mineralogy.- ______ _______ _________ __

Plagioclase feldspar _______ _____ ____

Alkali feldspar __ _____ ________ ____

Pyroxene.. _ _____________ _ _____ ___

Olivine_____________ _ ___ ___________

Silica minerals.. _____________________

Ironores___ _________ __________ __

Biotite- _____ ______ _________ _ __

Hornblende- _______ ____ ___ _______

Iddingsite.. _______ __________ _______

Serpentine ____ ___ ______ ________

Calcite... _ ________________________

Zeolites _ _ ___ __ _ _______________

Clay minerals- ________ _ ______________

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Rock type

Olivin

Basall

Hype]

Andes

Hypei

Pyroc

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Chemical

other ar

Petrogenes

Trace elen

I. Trace

References

Hypersthene-bearing basalt-___ 

Andesite________________ __

Hyperthene-bearing andesite.___ ___ 

clastics__ ___ _______ 

Water-laid tuffs and tuffaceous shales. __ __ 

Tuffaceous sandstones__ _ __ 

Pyroclastic conglomerate and breccia__________

Chemical composition and comparison with rocks of 

other areas_ ____ _____ __ 

Petrogenesis______________________ _____________

Trace elements in the volcanic rocks of Guam, by Joshua 

I. Tracey, Jr., and John T. Stark-_____ 

ILLUSTRATIONS

FIGURE 1. Index map of western north Pacific Islands______________________________-______________-______-___-___

2. Location of analyzed specimens________________________________________________________________

3. Average oxide composition of volcanic rocks of Guam by age__________________________________ _ 

4. Harker variation diagram of volcanic rocks of Guam______________________________________ __ 

5. Triangular ACF diagram of specimens of volcanic rocks of Guam_________________________________________

6. Triangular ACF diagram of average volcanic rocks of Guam, Saipan, the Izu Peninsula of Japan, and the Hawaiian Islands___________________________________________________________________________

7. Triangular ACF diagram of average volcanic rocks of Guam, Saipan, and Daly's average rock types____ 

8. Triangular SKM diagram of specimens of vocanic rocks of Guam_________________________________________

9. Triangular SKM diagram of average volcanic rocks of Guam, Saipan, the Izu Peninsula of Japan, and the Hawaiian Islands_-___________________________________________________________________________________-_

10. Triangular SKM diagram of average volcanic rocks of Guam and Saipan and of Daly's average rock types____

11. Triangular diagram of the norms of the andesites and basalts of Guam showing the ratios of or-ab-an and of 

femic mineral-quartz-feldspar._____________________________________________________________________

12. Composition of normative feldspar of average rocks of Guam and of Daly's average rock types_________ ___

13. Composition of normative pyroxene of average rocks of Guam and of Daly's average rock types._____________

14. Composition of normative feldspar of average volcanic rocks of Guam, Saipan, the Northern Mariana Islands, 

the Hawaiian Islands, and the Izu Peninsula of Japan______________________ _______________________

15. Composition of normative pyroxene of average volcanic rocks of Guam, Saipan, the Northern Mariana Islands, 

the Hawaiian Islands, and the Izu Peninsula of Japan_________________________________________________

16. Triangular FeO-MgO-alkali diagram of andesites and basalts of Guam, and Daly's average rock types______

17. Schematic diagram of Umatac formation______________________________________________________________

18. Relation of chromium, nickel, barium, and strontium to MgO in rocks of Guam._____________ ___________

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TABLES

Page 

TABLE 1. Description of principal volcanic rock types of Guam______________________________________________ C9

2. Chemical composition and norms of volcanic rock types of Guam_________________________________________ 13

3. Trace elements in lavas of Guam and the Hawaiian Islands...________________________________________ 28

4. Selected chemical and spectrographic analyses of volcanic rocks of Guam__________________________________ 32

in

GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

PETROLOGY OF THE VOLCANIC ROCKS1OF GUAM

By JOHN T. STARK

ABSTRACT

The volcanic rocks of Guam consist of basaltic and andesitic 

lava flows and dikes and pyroclastic deposits of similar composition. They are predominantly continental and belong to the 

circumpaciflc province; they show close affinities to the volcanic 

rocks of the nearby island of Saipan and to the Izu Peninsula 

of Japan. A classification after Kuno, according to color index 

of the groundmass, shows the Guam lavas to be about 45 percent 

mafic andesite, 10 percent medium andesite, and 45 percent 

basalt and olivine basalt. Both andesite and basalt have hypersthene-bearing varieties.

Chemical compositions and norms of major rock types are 

given and are plotted in variation diagrams to show the close 

affinity of the rocks of Guam with those of Saipan and the Izu 

Peninsula of Japan; they are also compared with those of the 

Hawaiian Islands and with Daly's average rock types. On a triangular MgO-FeO-alkali diagram, the variation trend for the 

rocks of Guam follows closely the variation trend for Kuno's 

hypersthenic rock series of Japan.

The Eocene and Oligocene, and the Miocene volcanic rocks of 

Guam originate from separate volcanic vents west and southwest of the island. Variations in the trace elements in these 

rocks suggest two differentiation series, one of Eocene and 

Oligocene age, and the other of Miocene age.

INTRODUCTION

This report on the petrology of the volcanic rocks of 

Guam is based on fieldwork which began in June 1952 

and concluded in October 1954, as a part of the Pacific 

Geological Mapping Program of the U.S. Geological 

Survey and Corps of Engineers of the U.S. Army.

The island of Guam is the southernmost of the 

Mariana Islands, which extend from lat 13° to about 

21° N. and between long 145° and 146° E. (fig. 1). 

Guam is 31 miles long and has an area of approximately 

212 square miles.

ACKNOWLEDGME3STTS

Thanks are expressed to Prof. Hisashi Kuno, of 

Tokyo University, for help in the petrographic examinations of thin sections and to Kiguma J. Murata, of 

the U.S. Geological Survey, for assistance in interpretating data on trace elements and for advice in preparing that section of the report.

OUTLINE OF GEOLOGY

The volcanic rocks of Guam (pi. 1, Tracey and 

others, 1963) are extensively exposed in the southern 

half of the island, where there are only subordinate outcrops of limestone. The volcanic rocks consist of 

Eocene, Oligocene, and Miocene flows, dikes, and pyroclastic deposits. The northern half of Guam is a plateau 

of Miocene to Recent limestone through which volcanic 

rocks are exposed in three small inliers Mount Santa 

Eosa, Mataguac Hill, and Palia Hill. No subaerial 

volcanic rocks have been recognized anywhere on the 

island; the flows and pyroclastic rocks appear to be 

wholly of submarine origin. The breccias and the tuff aceous shales and sandstones are generally well stratified, and blocks in some beds are rounded to cobbles and 

boulders by water action. Foraminif era and Eadiolaria 

are common in the tuffaceous shales and sandstones and 

in the fine-grained matrix of the pyroclastic breccias 

and conglomerates.

The stratigraphic sequence of the volcanic rocks on 

Guam is as follows:

Tertiary e (Miocene). 

Umatac Formation:

Dandan Flow Member

Bolanos Pyroclastic Member

Maemong Limestone Member

Facpi Volcanic Member 

Unconformity.

Tertiary & and c ( Eocene and Oligocene). 

Alutom Formation:

Mahlac Member

The Alutom Formation is dominantly tuffaceous 

shales and sandstones interbedded with mafic lava flows 

and many lensing conglomerate and breccia beds composed of cobbles and blocks of basalt and andesite. The 

Mahlac Member is a calcareous foraminiferal shale that 

has a maximum known thickness of 200 feet. The outcrop thickness of the Alutom Formation is estimated to 

be 2,000 to 3,000 feet.

Cl
C2 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

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FIGURE \. Index map of western north Pacific Islands.

The Umatac Formation consists of four distinct 

members. At the base is the Facpi Volcanic Member 

of pillow lavas, which are interbedded with increasing 

amounts of pyroclastics near the top. The Maemong 

Limestone Member, which is gradational into tuffaceous shales and sandstone, forms tongues within the 

Facpi. The Bolanos Pyroclastic Member is a waterlaid deposit of tuff breccia and conglomerate interbedded with a few lava flows. It is overlain by the 

Dandan Flow Member, composed of basalt and andesite flows, and beds of coarse water-laid pyroclastics. 

The Umatac Formation is about 2,200 feet thick.

A profound unconformity separates the Alutom 

Formation of Eocene and Oligocene age from the overlying volcanic rocks of Miocene age. Both series have 

been affected by gentle tilting and numerous faults.

The Eocene volcanic rocks have been locally intensively 

distorted by slumping and gravitational sliding. Some 

outcrops, ranging from a few yards to several acres in 

extent, are such a confused jumble of segments of pyroclastic beds and blocks of lava rock that subsidiary 

centers of explosive and extrusive volcanism are 

suggested.

CLASSIFICATION OF THE VOLCANIC ROCKS

Guam lies just west of the Andesite Line, which 

marks the transition zone, structural and physiographic, between the andesite-rhyolite kindred of the 

Pacific Margin (the circumpacific province) and the 

olivine basalt-trachyte association of the island groups 

within the Pacific Basin (the intrapacific province); 

(Turner and Verhoogen, 1951, p. 124). The volcanic
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM C3

rocks of Guam show some affinities with the rocks of 

both provinces, but they are predominantly continental 

and closely resemble the rocks of the Hakone Volcano 

and adjacent areas of Japan described by Kuno (1950).

A classification of the igneous rocks on the basis of 

the average composition of the modal feldspar, as used 

by Macdonald (1949, p. 154) for the rocks of the 

Hawaiian Islands would restrict all but a small percentage of the dikes and lava flows on Guam to basalt 

and olivine basalt. Less than 15 percent of the rocks 

now exposed have a sufficient amount of andesine to be 

called andesites. These few rocks would be called 

basaltic andesites, following! Macdonald, because of 

their resemblance in habit and mafic minerals to the 

basalts. In none of the basaltic rocks is the percentage 

of feldspar low enough to be equivalent to the picrite 

basalts of the Hawaiian petrographic province. Typical trachytes and feldspathoid-bearing rocks have not 

been found on Guam.

A classification after Kuno (1950, p. 958) according 

to the color of the groundmass, "namely the sum of the 

normative femic minerals Wo+En+Il+Hm as calculated from the groundmass composition," would include many of the rocks of Guam in mafic and medium 

andesite and fewer in basalt. About 45 percent of the 

specimens studied under this classification are mafic 

andesites, 10 percent are medium andesites, and 45 percent are basalts and olivine basalts. Silicic or sodic 

andesites are represented by only a few blocks in 

volcanic breccia.

Schmidt classified the igneous rocks of Saipan (1957, 

p. 131-132) "primarily on the basis of the sum of their 

modal and (or) normative femic minerals (pyroxene, 

hornblende, magnetite, ilmenite, and hematite)"; and 

secondarily on the average composition of the modal 

feldspar. Where the modal composition was not available but the chemical composition was known, the color 

index was taken as the sum of the normative femic 

constituents (Wo+En+Fs+Mt+Il+Hm). In the 

absence of chemical analyses, the rocks were classified 

on the basis of comparison to analyzed rocks. This, in 

general, accords with Kuno's classification. As the 

mafic rocks of Saipan appear to be closely related to 

the andesites and basalts of Guam, this classification 

has been generally followed. The Guam rocks are 

almost wholly mafic or calcic andesites, basalts, and 

olivine basalts.

The only rocks on Guam which approach the dacitic 

rocks of Saipan in modal composition are quartzbearing blocks from a volcanic conglomerate that crops

out on the southeast slope of Mount Santa Eosa. Similar quartz-bearing rocks were described by Eisaburo 

Tayama (1941) as occurring in gravels along the Fena 

Eiver in southern Guam. This area is now flooded, 

owing to impounding of river water by the Fena damThat such dacitic rocks did form a part of the volcanic 

pile on Guam and are now largely eroded is suggested 

by the quartz-bearing blocks in the conglomerate on 

Mount Santa Eosa. Kuno called them andesites with 

quartz xenocrysts (oral communication, 1954), but even 

though the quartz-bearing blocks are the result of contamination, the quartz xenocrysts must have been derived from a rock containing primary quartz.

In connection with the classification of the volcanic 

rocks of Guam, the chemical analyses (table 1) show 

an interesting contrast between the silica content of 

bedrock outcrops of flows and dikes and the silica content of residual boulders in highly weathered pyroclastic beds presumably derived from the same magmas 

as the mafic flows. (Locations of samples are shown in 

figure 2.) The bedrock outcrops average 51.19 percent, 

and the boulders average 60.56 percent silica. The 

number of analyses may be too few for this difference 

to have any significance. However, the question is 

raised as to whether there has been secondary addition 

of silica to the mafic boulders of the pyroclastic beds.

Veins of chalcedony and quartz are conspicuous in 

many areas, and open spaces are commonly lined with 

quartz crystals. The fine-grained volcanic tuffs in the 

Mount Tenjo and Mount Alutom areas have hardened 

in many places by silicification following brecciation, 

and in other outcrops hardening by silica occurs where 

no brecciation is evident. Analyses of the boulders 

were from hand specimens of seemingly fresh rock 

taken well below the altered periphery. Thin sections 

of the boulders show evidence of secondary silicification, and pore spaces are lined with chalcedony and 

quartz. In a classification of rocks based partly on 

their normative composition this possible contamination by secondary quartz is of considerable importance.

Macdonald (oral communication, 1957) suggests that 

the pyroclastics in general may represent the early, 

more explosive parts of eruptions, derived from the upper gas-rich and silica-rich portion of the magma column, whereas the flows probably came in large part 

from a lower portion of the magma column that was 

poorer in silica. This decrease in silica through the 

course of eruption is known in several instances for 

example, the 1948 eruption of Hekla and the 1955 eruption of Kilauea.
C4 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

3 MILES

FIGUEE 2. Location of analyzed specimens. Vertical letters and numbers refer to field locality numbers; inclined numbers refer to analyses

(tables 2-4).
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM C5

MINERALOGY

The essential minerals of the volcanic rocks of Guam 

are relatively few: plagioclase, pyroxene, olivine, and 

quartz. Primary accessory minerals are magnetite, 

hematite(?), hornblende, biotite, and an alkali feldspar. Silica minerals quartz, chalcedony, tridymite, 

cristobalite, and opal are conspicuous in cavities and 

interstitial in the fine-grain groundmass as late crystallizations and replacements. Secondary and alteration 

minerals are chlorite, serpentine, calcite, pyroxene, zeolites, quartz, chalcedony, iron oxides, and clay minerals.

PLAGIOCLASE FELDSPAR

The plagioclase feldspars are present in all the volcanic rocks as phenocrysts and as small grains in the 

groundmass. The proportion of phenocrysts ranges 

from a few widely scattered crystals to 50 percent of 

the rock. The total plagioclase averages between 50 

and 60 percent. Phenocrysts are less numerous in the 

very fine grained rocks. Most of the plagioclase is 

labradorite, averaging around An55 , but there is a range 

from bytownite (An75 ) in the youngest (Dandan) flows 

to andesine and oligoclase (An20 ) in the sodic andesite. 

Broad twinning bands, zoning, and dust inclusions are 

characteristic of both the phenocrysts and small grains 

of the groundmass. Zoning is especially well developed 

in the Dandan Flow Member of Miocene age, and in 

blocks of lava in volcanic water-laid breccias of the 

Alutom Formation of Eocene and Oligocene age. Centers of the zoned crystals are generally slightly more 

calcic than the peripheral zones. However, reversals 

were noted in which the composition changed outward 

from calcic labradorite to slightly more sodic labradorite, to an outer rim of calcic labradorite. In only a 

few crystals does bytownite form the central core.

Zones of small dustlike inclusions of pyroxene and 

iron ore parallel borders of the phenocrysts, forming 

clouded areas between clear centers and clear outer 

rims. In other crystals the centers are filled with small 

inclusions surrounded by clear labradorite. Some phenocrysts are clouded throughout by small dustlike inclusions. These inclusions appear similar in every way 

to inclusions in the plagioclase phenocrysts of the volcanic rocks of the Hakone Volcano (Kuno, 1950, p. 

967-968).

Kuno explains these finely divided particles of pyroxene and iron ore, with or without glass, as having

formed in plagioclase enclosed by a magma in equilibrium with 

more calcic plagioclase. The dust originated by partial crystallization of liquid particles formed by incipient melting of xenocrystic plagioclase along its cleavages. A part of the liquid 

particles was introduced from magma outside by diffusion.

677292 63 2

The extreme fracturing and fragmentation of many of 

the large dust-filled plagioclase crystals would appear 

to support the interpretation that they may be xenocrysts rather than phenocrysts. Such fracturing is 

strikingly well developed in the basalt boulders and 

blocks in the volcanic breccia of the Dandan Flow 

Member of the Umatac Formation. It should be noted, 

however, that the occurrence of inclusion-filled and 

"moth-eaten" feldspar is so common as to be characteristic of calc-alkaline andesite provinces (Macdonald, 

written communication, 1957").

The plagioclase crystals in the rocks of Guam show 

all degrees of alteration. Completely euhedral crystals 

are rare. The outer edges of many plagioclase grains 

are extremely irregular from resorption embayments 

and fracturing. Glomeroporphyritic clusters and fragments of crystals are extremely common. Calcite is 

conspicuous in the saussuritization of the plagioclase.

ALKALI FELDSPAR

The presence of alkali feldspar, probably anorthoclase, is suggested by the occurrence in the fine-grained 

groundmass of very small areas of clear to clouded 

feldspar with an index of refraction distinctly lower 

than that of adjacent plagioclase. The material is 

interstitial, irregular, or shows faint rectangular outlines. In a few slides examined by Kuno (oral communication, 1954) these were identified as probable 

anorthoclase. The amount is not large and it occurs 

more commonly in the andesites of medium to sodic 

composition, but it is present also in some basalts. A 

check on the presence of alkali feldspar was made on a 

number of the Guam rocks by staining thin sections 

and rock chips after the method described by Chayes 

(1952). Corroboration of alkali feldspar in some of 

the andesites was obtained, but the amount is extremely 

low, probably less than 0.01 percent.

PYROXENE

Monoclinic pyroxene is second only to plagioclase as a 

primary constituent of the volcanic rocks of Guam. It 

occurs abundantly as phenocrysts and small crystals 

interstitial to plagioclase in the fine-grained groundmass. It is generally present in all the igneous rocks 

and averages from 25 to 35 percent of all thin sections. 

The pyroxene is predominantly augite, occurs in euhedral crystals, and ranges from very fine grains in the 

groundmass to phenocrysts as large as 8 mm, although 

more commonly they are less than 2 mm. Elongation 

parallel to the c axis is general, although all variations occur from slender prisms to stout crystals. 

Glomeroporphyritic clusters of small phenocrysts and
C6 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

clusters of fragments of larger crystals that are especially prominent in many specimens perhaps suggest 

shattering of the large augite crystals when the lava 

was extruded and the floating together of the fragments before solidification. Twinning on the 100 plane 

and polysynthetic twinning on the 001 plane are fairly 

common. Some phenocrysts show a wavy extinction, 

and others have an hourglass structure. Maximum 

extinction angles are generally low, averaging around 

40°.

Hypersthene is associated with augite, and in places 

they are intergrown. The orthorhombic pyroxene is 

prominent in basalt lava flows of the Alutom Formation at Mount Santa Eosa and near Mount Alutom at 

Sasa, and is present in the basaltic flows of the Umatac 

Formation near Facpi Point. In a few flows of the 

Dandan Member it forms as much as 10 percent of 

the rock. The hypersthene occurs both as large platy 

phenocrysts and as small grains in the groundmass. 

Pleochroism is generally very faint in pale tints of pink 

and green. The negative optic sign and parallel extinction readily distinguish it from augite. Hypersthene 

also occurs as small crystals in reaction coronas surrounding fibrous pseudomorphs of serpentine and 

chlorite after olivine and monoclinic pyroxene. These 

small crystals are prismatic in habit, optically negative, 

and show parallel extinction.

OLIVINE

Fresh euhedral olivine occurs in only a few of the 

volcanic rocks of Guam, but its former presence is indicated in many of the basalts by fibrous pseudomorphs 

of serpentine which retain the outlines of original 

olivine crystals. In some thin sections small relict 

centers of fresh olivine are surrounded by fibrous areas 

of serpentine and associated chlorite, pyroxene, calcite, 

and magnetite, which fill in euhedral outlines of the 

original olivine phenocrysts. The serpentine is largely 

antigorite and is by far the most abundant alteration 

product. Calcite is rarely absent from the alteration 

areas. Other pseudomorphs after olivine consist of 

cores of serpentine and calcite and have peripheral 

zones of granular and vermicular magnetite surrounded by an outer zone of small randomly oriented 

crystals of augite and hypersthene. Films of chlorite 

and yellowish-green serpentine, due to reaction or 

weathering, fill joint fractures and form zones around 

the pseudomorphs. In many rocks the olivine crystals 

are stained by dark brownish-red iron oxide, and the 

crystal borders are altered to iddingsite.

Beach sands which border cliffs of basaltic lava in 

southwest Guam contain numerous fresh olivine grains 

that apparently are residual from weathering of the 

lava flows. However, specimens from nearby flows

show only extremely small amounts of fresh olivine. 

Olivine is most common in the olivine basalt of the 

Facpi Volcanic Member of the Umatac Formation, but 

it is represented by serpentine pseudomorphs in both 

the earlier Alutom Formation and the later flows of 

the Dandan Member.

SILICA MINERALS

Quartz grains, resembling phenocrysts in hand specimens, occur in andesitic blocks and boulders of waterlaid pyroclastic conglomerate and breccia of the 

Alutom Formation on the southeast slopes of Mount 

Santa Eosa and in the Nimitz Hill area. The quartz 

crystals range in diameter from a fraction of a millimeter to 3 mm. Under the microscope they show 

rounded resorption edges and embayments. They are 

associated with highly shattered dust-filled fragments 

of plagioclase. Professor Kuno examined thin sections 

of the quartz-bearing rocks and interpreted the quartz 

grains as xenocrysts in a groundmass of andesite. The 

total pyroxene content was too high for the rock to be 

called dacite under his classification (Kuno, 1950, p. 

958).

The quartz grains are more abundant in the vicinity 

of Mount Santa Eosa than at Nimitz Hill, but at both 

places they occur in sodic andesite boulders. Near 

Mount Santa Eosa a few small doubly terminated 

quartz prisms, from 2 to 3 mm long, were found in a 

sandy tuffaceous bed. Similar crystals were collected 

from tuffaceous sandstone talus about 1 mile east of 

Nimitz Hill. Under the microscope the quartz prisms 

are relatively free from inclusions and appear to be 

erosion relicts from a rock containing true phenocrysts 

of primary quartz. It is possible that they originated 

from dustfalls or represent erosion relicts from a dacite 

porphyry similar to those on Saipan described by 

Schmidt (1957, p. 139-141). The quartz-bearing blocks 

and boulders of Mount Santa Eosa and Nimitz Hill 

may have been contaminated by such relict quartz 

crystals.

Chalcedony, tridymite, and cristobalite occur in the 

groundmass of many of the basalts and andesites. Only 

chalcedony is abundant, and much of it is clearly late 

fillings of small pores, vesicles, and fractures. Cristobalite, commonly as slender rods, and tridymite, rudely 

wedge-shaped in outline, occur interstitiaUy in the finegrained groundmass and probably were formed during 

a late stage of crystallization rather than being introduced by pneumatolytic or hydrothermal processes, as 

Kuno has suggested for volcanic rocks of the Hakone 

area (1950, p. 969). Veins of chalcedony and quartz 

commonly fill fractures and vesicles in flows and dikes 

and are probably partly of hydrothermal origin. Veins 

of these silica minerals, ranging in width from a frac-
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM 07

tion of an inch to 5 inches, transect the pyroclastic tuffs, 

sandstones, and conglomerates. They commonly withstand erosion and form tabular ridges a few inches high 

and several yards in length. Chalcedony is particularly abundant in the groundmass of the fine-grained 

and glassy flows.

Volcanic tuffs near Tenjo and Alutom peaks have 

been hardened in many places by silicification, following brecciation. Shear zones thus cemented by chalcedony and quartz are easily recognized by the greenish 

color that commonly accompanies the silicification. 

Wider areas of light green tuffs where silicification has 

occurred show no evidence of having been brecciated 

following deposition. Small amounts of opal are associated with secondary quartz and chalcedony in the 

weathered zones of these light-green tuffs.

IRON ORES

Magnetite is an accessory mineral in all the volcanic 

rocks of Guam. The amount ranges from a few extremely small widely scattered grains in the groundmass of sodic andesite blocks of Eocene age on Mount 

Santa Rosa to as much as 6 percent of the Dandan flows 

of Miocene age. The magnetite occurs in small irregular grains interstitial in the groundmass, as small 

octahedra in the groundmass, as poikilitic inclusions in 

larger crystals of plagioclase and pyroxene, and less 

commonly as euhedral grains one-half millimeter in 

diameter in the groundmass. Small grains, associated 

with other reaction minerals, also occur in peripheral 

zones around fibrous pseudomorphs after olivine and 

pyroxene. In a few thin sections, rods and ladderlike 

growths of magnetite appear to have developed later 

than the groundmass grains.

Hematite in some basalt specimens appears to be 

primary, but generally it is an alteration product after 

other iron-bearing minerals. It commonly forms rims 

around magnetite. Ilmenite was not recognized as a 

separate mineral. The presence of titanium in very 

small amount is shown in the chemical analyses, and it 

is probably incorporated in the magnetite.

BIOTITE

Rods and flakes of biotite, rarely more than a quarter 

of a millimeter in length, occur in fine-grained foraminiferal tuffs of the Alutom Formation. The biotite is 

associated with hornblende in some of the tuffaceous 

beds, which are now largely altered to clay. The small 

biotite rods and flakes show pleochroism, parallel extinction, and negative optic sign typical of biotite. Biotite was recognized only in one or two thin sections from 

the dikes and flows.

HORNBLENDE

A few widely scattered grains of green hornblende 

are associated with abundant calcite and a few rods of 

biotite in foraminiferal tuffs of Eocene age in the vicinity of Mount Alutom. The grains are generally fragments of larger crystals and appear to be derived from 

the explosive brecciation of a rock containing hornblende phenocrysts. A few euhedral basal sections are 

diamond shaped and show the characteristic intersecting prismatic cleavages. Elongated fragments parallel to c show a faint pleochroism and maximum extinction angles between 12° and 25°.

The only other hornblende observed was in quartzbearing blocks of sodic andesite in volcanic conglomerate on Mount Santa Rosa. The hornblende there is 

similar to that in the Alutom Formation. The total 

amount of hornblende in the specimens studied is extremely small, probably less than 0.01 percent of the 

rocks now exposed.

APATITE

Apatite occurs in minor amounts as small acicular 

crystals in olivine basalts.

IDDINGSITE

Iddingsite occurs as alteration rims around the 

pseudomorphs of serpentine from primary olivine 

crystals.

SERPENTINE

Few of the volcanic rocks of Guam are free from 

weathering, and serpentine is generally present as an 

alteration product of olivine and pyroxene. In many 

places the original crystals are completely changed to 

pseudomorphs of fibrous antigorite. In other specimens 

greenish-yellow serpentine forms alteration borders 

around olivine and pyroxene grains. Incipient alteration penetrates along fractures, and all degrees of serpentinization occur. Commonly, small centers of original pyroxene and olivine grains are all that remain 

unaltered in thin sections.

CALCITE

Calcite is present in most of the volcanic rocks of 

Guam. It occurs as vesicle and fracture fillings, as an 

alteration product of olivine and pyroxene in association with serpentine, and as a product of the saussuritization of plagioclase. Dark-green extremely fine 

grained tuffaceous material cemented by calcite commonly fills spaces between ellipsoids in pillow lavas. 

Many surface outcrops of flows and dikes are intimately 

traversed by a stockwork of calcite veinlets.
C8 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

ZEOLITES

Zeolites are abundant as vesicle and fracture fillings 

and as replacement veinlets in many exposures of flows 

and dikes. They are especially conspicuous in the 

Facpi Volcanic Member, which forms the sea cliffs between Umatac and Facpi Point. In places, zeolites 

constitute about 5 percent of the outcrop. Zeolite 

amygdules, ranging from extremely small pellets to 

grains one-half inch in diameter give a white spotted 

appearance to many exposures. Radiating clusters as 

large as 2 inches in diameter and intricate stockworks 

of small veinlets are characteristic. The most abundant 

zeolites are analcite, natrolite, chabyzite, and heulandite. 

All are closely associated with calcite.

CLAY MINERALS

Deep weathering and the formation of clay materials 

characterizes most exposures of the volcanic rocks 

(Carroll and Hathaway, 1963). Halloysite, montmorillonite, and gibbsite are commonly present and are intimately associated with hematite and goethite.

ROCK TYPES

The volcanic rocks of Guam consist of lava flows, 

dikes, and pyroclastic beds formed of the following 

types: olivine basalt, basalt, hypersthene-bearing basalt, sodic to calcic andesite, and hypersthene-bearing 

andesite. In two small areas, Mount Santa Rosa and 

Nimitz Hill, blocks in the pyroclastic beds have a silica 

content which places them within a type between sodic 

andesite and dacite. However, because the quartz 

grains in these blocks are believed to be xenocrysts 

rather than phenocrysts, the rocks are classed as sodic 

andesites.

Rocks with labradorite and augite as essential minerals are by far the most abundant on the island. The 

weathering and alteration of the exposed rocks are 

generally so intense that many flows, now called basalts 

because no olivine is recognizable, originally may have 

been olivine basalts. Other flows contained olivine as 

a primary constitutent, as indicated by small relict 

grains of fresh olivine and fibrous pseudomorphs of 

serpentine, chlorite, and calcite which show orthorhombic crystal outlines. Many flows are so completely 

weathered that no euhedral outlines of olivine (if ever 

present) are preserved. The abundance of olivine 

grains in sands bordering cliffs of highly weathered 

basaltic lavas also suggests that olivine basalts are important on the island.

The ratio of basaltic to andesitic rocks, based on 

specimens studied, is approximately 45:55. Calcic 

andesite predominates greatly over sodic andesite. The 

tuffaceous shales and sandstones and the coarser pyroclastics are of the same general composition as the flows 

and dikes. A summary of the principal rock types is 

given in table 1.

OLIVINE BASALT

Olivine basalts are found in both the Alutom and 

Umatac Formations, especially in the Facpi Volcanic 

Member of the Umatac. Textures of the olivine basalts 

are intergranular to intersertal, generally porphyritic, 

with traces of ophitic and subophitic textures commonly preserved. Glomerporphyritic clusters of crystals and fragments of crystals are conspicuous in many 

flows. The olivine basalts range in color from black 

when fresh to shades of gray, brown, and red when 

weathered. Many flows weathered to soft claylike material show a characteristic light mauve tint. Fresh 

olivine is rare and is preserved only in relic grains less 

than 0.02 mm in diameter, surrounded by fibrous serpentine (largely antigorite), chlorite, and calcite. In 

some flows the relict olivine grains are enclosed in 

opaque areas of hematite and have thin rims of iddingsite. Pseudomorphs of serpentine and chlorite as much 

as 5 mm in length have been observed. Phenocrysts 

of plagioclase (Ango-7o) are much more abundant than 

the olivine, form as much as 40 percent of some specimens, and range in length from 0.5 to 2 mm. They are 

commonly fractured, altered, and show deep resorption 

embayments. Many show zoning, with cores of slightly 

more calcic composition than the peripheral rims.

Pyroxene in the olivine basalts is chiefly augite, 

although small amounts of hypersthene are closely 

associated with the augite. The pyroxene phenocrysts 

are subordinate to plagioclase and, like them, show 

fracture, resorption embayments, and are for the most 

part altered. They are generally smaller than the plagioclase, and form glomeroporphyritic clusters as well 

as individual phenocrysts as much as 1.5 mm in length. 

Magnetite is always present and averages between 2 

and 3 percent of the rock. It occurs as small octahedra, 

elongated rods, and irregular grains, and is commonly 

oxidized to hematite and limonite. Small amounts of 

apatite in acicular needles are enclosed in other 

minerals.

In general, the groundmass is fine grained and contains as much as 20 percent interstitial glass. The intergranular texture of the groundmass gives way in a 

number of flows to ophitic and subophitic texture. 

Hyalopilitic texture occurs near edges of dikes and 

flows. Glassy rims are prominent around ellipsoidal 

structures of the pillow lavas. The plagioclase of the 

groundmass is generally labradorite (Ango-es), less calcic than the plagioclase phenocrysts. Monoclinic pyroxene forms 25 to 40 percent of the groundmass.
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM 

TABLE 1. Description of principal volcanic rock types of Guam,

C9

Rock types

Composition

Phenocrysts Groundmass

Olivine basalt. Olivine, 0-15 percent, 1-5 mm long, partly altered

to serpentine and chlorite with only small relict

grains of fresh olivine. 

Plagioclase, 0-40 percent, 0.5-2 mm long, zoned

An50-7o, much fractured and altered, glomeroporphyritic clusters. 

Augite, 0-25 percent, 0.5-1.5 mm long, partly

altered.

Plagioclase (An50-65), 30-55 percent.

Olivine, 1-13 percent, partly altered to serpentine and

chlorite.

Augite, 25-40 percent, altered. 

Magnetite, 1-4 percent, oxidation rims to hematite and

limonite.

Apatite, acicular needles enclosed in other minerals. 

Glass, interstitial as much as 20 percent, selvages around

pillows and chilled borders of dikes.

Basalt. Olivine, 0^-5 percent, 1-4 mm long, pseudomorphs of 

serpentine and chlorite show euhedral outlines of 

primary olivine.

Plagioclase, 0-35 percent, 1-5 mm long, partly 

altered, deep resorption embayments.

Augite, 0-20 percent, 1-3 mm long, glomeroporphyritic clusters, partly altered, resorption embayments.

Plagioclase (Anso-ro) 35-60 percent.

Olivine, 0-5 percent, almost completely altered to

serpentine and chlorite; Iddingsite rims. 

Augite, 25-40 percent. 

Quartz xenocrysts, rare. 

Biotite and Hornblende, rare. 

Hypersthene, 0-5 percent. 

Magnetite, 1-12 percent. 

Alkali feldspar, small amount.

Glass, interstitial as much as 30 percent, chilled contacts.

Hypersthenebearing basalt.

Olivine, 0-12 percent, partly altered to serpentine

and chlorite.

Plagioclase, 0-50 percent, 0.5-5 mm long. 

Augite, 0-20 percent, 1-4 mm long, for the most

part altered. 

Hypersthene, 0-5 percent, 0.5-5 mm long, broken,

altered, resorption embayments, glomeroporphyritic clusters.

Plagioclase (Anso-To), 35-45 percent.

Olivine, 0-3 percent, almost completely altered to

serpentine and chlorite.

Augite, 10-15 percent, for the most part altered. 

Hypersthene, 5-20 percent, fresh flakes to altered. 

Magnetite, 1-9 percent, small octahedra, irregular

grains, and elongated rods. 

Glass, 1-20 percent, interstitial.

Andesite. Olivine, rare, almost completely altered. 

Plagioclase, 0-20 percent, 1-8 mm long. 

Augite, 10-20 percent, 1-5 mm long. 

Hypersthene, scarce. 

Hornblende, rare <0.1 percent.

Plagioclase (An25-5o), 40-60 percent.

Olivine, traces.

Augite, 15-35 percent.

Hypersthene, 0-5 percent, associated with augite and

in alteration rims of olivine. 

Alkali feldspar, small amount. 

Magnetite, 1-8 percent. 

Hornblende, rare. 

Glass, 0-15 percent, interstitial.

Hypersthenebearing andesite.

Olivine, rare, almost completely altered to serpentine.

Plagioclase, 0-40 percent, 1-6 mm long. 

Augite, 5-20 percent, mostly altered. 

Hypersthene, 0-5 percent, 1-5 mm long.

Plagioclase (Anso-ss).

Olivine, traces.

Alkali feldspar, small amount, interstitial.

Augite, 25-30 percent, 1-4 mm long.

Hypersthene, 5-10 percent.

Magnetite, 2-6 percent.

Pigeonite, common in the olivine basalts of Hawaii 

(Macdonald, 1949, p. 1545) was looked for, but it could 

not be identified conclusively, as the optic angles measured were too high. Norms of the rocks of Guam show 

a range of about En59-to-En65 (fig. 15), and with this 

composition it is doubtful that any of the augite is 

pigeonite. A few flakes of hypersthene occur in the 

groundmass.

BASALT

The basalts of Guam differ from the olivine basalts 

chiefly in having less than 5 percent recognizable 

olivine. Basalts are found in flows and relict boulders 

of the Alutom and Umatac Formations. The textures 

of the basalts range from porphyritic to nonporphyritic and commonly show traces of ophitic and subophitic patterns. Extremely small vesicules characterize many of the fine-grained basalts and extend 

throughout some of the dikes and flows. Interstitial 

glass is abundant in most flows and ranges from 0 to 

30 percent. The phenocrysts are dominantly of labradorite (An^-io), range from 0 to 35 percent of the 

rock, and show much fracturing, resorption embayments, and saussuritization. Labradorite forms 35 to 

60 percent of the groundmass. The abundance of serpentine and chlorite in the groundmass suggests that 

olivine may have been present in greater amount, but 

pseudomorphs with euhedral outlines are missing. 

Small amounts of interstitial alkali feldspar (anorthoclase?) are present. Monoclinic pyroxene, predominantly augite, forms as much as 20 percent of the
CIO GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

phenocrysts and 40 percent of the groundmass. Hypersthene forms 0 to 5 percent of the groundmass and is 

generally closely associated with augite. Xenocrysts 

of quartz occur rarely in basalt blocks in the coarser 

pyroclastics. Both hornblende and biotite are extremely rare. In a few thin sections small flakes and 

narrow rods of brown biotite were observed. In one 

thin section small prismatic and basal flakes of green 

hornblende showed characteristic pleochroism and 

cleavage. Similar flakes of hornblende occur in some 

of the tuffaceous shales and sandstones.

HYPERSTHENE-BEARING BASALT

Hypersthene-bearing basalts are found in flows and 

relict boulders of the Alutom and Umatac Formations. 

Many of the basalt flows contain more than 5 percent 

hypersthene. Phenocrysts of the orthorhombic pyroxene range in length from 0.5 to 5 mm long and show 

faint pleochroism, parallel extinction, and negative 

optic character of hypersthene. Hypersthene is intimately associated with augite and commonly occurs 

with it in glomeroporphyritic clusters. Hypersthene 

ranges from 5 to 20 percent in the groundmass. Augite 

is always present and is generally more abundant than 

the hypersthene. Labradorite (An50_7o) averages between 35 and 45 percent of the groundmass. Except for 

the greater amount of hypersthene, these rocks are 

generally similar in texture and composition to the 

other basalts of Guam.

ANDESITE

The andesites of Guam, found in flows and relict 

boulders of the Alutom and Umatac Formations, range 

from calcic in which plagioclase is close to An50, to 

sodic andesite in which the plagioclase is close to the 

oligoclase boundary. The calcic andesites predominate 

and grade into the basalts. In general, the andesites 

are less porphyritic than the basalts, although a few 

flows contain phenocrysts, as much as 8 mm. in length, 

that form as much as 40 percent of the rock. Some 

flows have subtrachytic textures. The pyroxene is 

chiefly augite, which forms 10 to 20 percent of the 

phenocrysts and 15 to 35 percent of the groundmass. 

Small irregular grains of alkali feldspar are interstitial in the groundmass. Orthorhombic pyroxene is 

associated with the augite in amounts less than 5 percent of the groundmass. Magnetite is generally more 

abundant than in the basalts, ranging from 1 to 8 percent of the groundmass and averaging about 5 percent. 

The presence of olivine is indicated by traces of relict 

grains in masses of fibrous serpentine and chlorite. 

Green hornblende is present in extremely small 

amounts in a few andesite flows.

HYPERSTHENE-BEARING ANDESITE

In some andesite, found in the flows and boulders 

of the Dandan Flow Member of the Umatac Formation, the amount of orthorhombic pyroxene averages 

between 5 and 10 percent of the groundmass; but it is 

never more than the augite, which ranges from 25 to 

30 percent of the groundmass, and forms from 5 to 20 

percent of the phenocrysts. Except for the increase in 

orthorhombic pyroxene, hypersthene-bearing andesites 

are similar in texture and composition to the other 

andesites.

PYROCLASTICS

The pyroclastic sedimentary rocks of Guam are composed of fragments of rock and minerals and their 

weathered products that are similar in texture and 

composition to the flows and dikes. All these volcanic 

rocks were deposited under subaqueous conditions. 

Sedimentary structures and marine organisms in most 

outcrops leave no doubt as to their submarine origins. 

In some exposures the field evidence is not conclusive^ 

but in no place has clear evidence been found for 

subaerial deposition.

WATER-LAID TUFFS AN1> TmPFACEOTJS SHALES

The water-laid tuffs are fine-grained friable to wellindurated pyroclastic deposits of dust-sized particles, 

presumably derived from submarine explosions. They 

grade horizontally and vertically into tuffaceous shales 

and sandstones. Tuffs and shales are predominantly 

light in color in most outcrops and range from dead 

white through grays to dark gray. The darker colors 

are due to slightly coarser grain size and to an increasing amount of dark mineral and rock particles. 

A light-green color characterizes the tuffs where silica, 

chalcedony, and quartz have been deposited by 

circulating waters.

Under the microscope the tuffs and shales show 

sharply angular particles of volcanic glass, crystal, and 

rock fragments, which average from 0.2 to 0.5 mm in 

length. In a few sections some rounding of particles 

is evident, but angularity is the dominant characteristic. The larger particles are embedded in a matrix of 

extremely fine grained material that is isotropic or 

weakly birefringent, according to the degree of 

alteration and weathering.

Most of the fine-grained pyroclastics are vitric tuffs, 

which average less than 25 percent crystal fragments 

(PettiJohn, 1943, fig. 73). These grade into vitric 

crystal tuffs. Rarely is the original glassy material less 

than 50 percent, and commonly slides show more than 

90 percent of isotropic and opaque groundmass. A 

few beds, composed almost entirely of rock fragments,
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM Gil

are lithic tuffs. None of the outcrops are wholly fresh, 

and nearly all thin sections show white chalky clouding 

due to secondary development of clay minerals.

The clastic glassy matrix ranges from almost completely isotropic through stages of alteration, where 

shards and rock fragments are outlined by thin lines 

of brightly polarizing material, to completely devitrified glass. The devitrified glass has a greenish appearance suggestive of chlorite. Patchy areas of chalcedonic silica and probably opal are abundant, in both 

granular "pepper and salt" aggregates and radiating 

fibrous wedges with gradational boundaries. In other 

sections the chalcedony forms areas of smaller circular 

rosettes; these are especially abundant in the glass of 

rock fragments and as vesicle fillings. Amygdules are 

commonly banded chalcedony. Fragments of glass 

show varying degrees of alteration to clay minerals, 

and chloritic stainings are accompanied by an increase 

of weakly polarizing lines of birefringence. The 

formation of chlorite appears intimately connected with 

the alteration to chalcedony and opaline silica. The 

zones most highly silicified in outcrop are characteristically green to yellowish green and greenish brown.

In general, the tuffs are too fine grained to show 

recognizable fossils megascopically, but under the microscope abundant Radiolaria, globigerinid Foraminifera, 

and algae were observed. In some of the finest 

grained tuffs they make up as much as 50 percent of 

thin sections, and they are distributed throughout the 

tuffs in the Ten jo area. Careful examination of sections from most outcrops of any extent reveal organisms which indicate the marine environment in which 

the tuffs were deposited.

The crystal fragments of the tuffs are, in general, 

too small for identification, but the coarser grains are 

predominantly plagioclase, monoclinic pyroxene, and 

magnetite. Extremely small amounts of green hornblende and biotite were observed in a few thin sections. 

Clastic quartz grains form a small portion (less than 

0.01 percent) of some tuffs. Quartz amygdules are 

fairly common. Calcite is present in all the tuffs containing organisms, and in a few outcrops it is abundant 

and forms calcareous tuffs which grade into limestone 

beds.

TUFFACEOTJS SANDSTONES

The tuffaceous sandstones differ chiefly from the tuffs 

and tuffaceous shales in being coarser grained and 

darker in color. They grade laterally and vertically 

into one another. With an increasing amount of black 

crystal and rock fragments, the beds become black and 

cindery and resemble mafic igneous rocks in massive 

field outcrops. The degree of induration ranges from 

beds which are easily crumbled in the hand to thoroughly cemented sandstones. As with the tuffs, the

more thoroughly silicified sandy beds are green and 

range from lighter shades where silicification is slight 

to dark-green flintlike beds.

. Most of the pyroclastic sandstones contain less than 

25 percent of crystal fragments and are classified as 

vitric. There is, however, a greater amount of vitriccrystal (25 to 50 percent crystal fragments) and lithic 

(more than 33 percent of rock fragments) material 

in the tuffaceous sandstones than in the tuffs and tuffaceous shales. The crystal and rock fragments in the 

sandstones average between 0.5 and 2.0 mm in length, 

and they are embedded in a fine-grained clastic matrix 

of isotropic and opaque devitrified glass and dust particles. All degrees of alteration occur but, in general, 

the larger fragments are fresher and more easily identified than the commonly highly altered interstitial 

material.

Many thin sections show the microbreccia pattern 

of graywacke. In decreasing order of abundance the 

crystal fragments are plagioclase, pyroxene, and magnetite. Extremely small amounts of biotite and hornblende were noted in a few specimens. Angular 

fragments and shattered quartz grains are conspicuous 

in zones of brecciation and silicification. These zones 

are cut by numerous quartz grains and spotted with 

small nuclei of quartz. Some of the quartz may be from 

quartz amygdules of earlier flows involved in explosive 

action that formed the pyroclastic rock. The prominence of crystal quartz fragments in the silicified and 

brecciated zones suggests an origin associated with 

quartz veining. Repeated silicification and brecciation 

is clearly indicated in outcrops. This is supported by 

the extreme fracturing and separation of quartz and 

feldspar fragments in thin sections, showing that the 

grains were shattered after being incorporated as constituents of the bedded pyroclastics. Silica in the form 

of chalcedony, granular quartz, and quartz veins is 

abundant in brecciated zones.

A few small doubly terminated crystals of quartz 

were collected from loose debris resting on tuffaceous 

sandstone. The crystals average from 2 to 3 mm in 

diameter, are clear, and show slight rounding of the 

edges. Presumably they are weathered from the underlying pyroclastic rocks.

Calcite is present in all the pyroclastic sandstones 

that are rich in organic remains. It occurs as replacement of tests of Foraminifera, and Radiolaria, in irregular granular masses, and in extremely small grains 

scattered throughout the rocks.

The lithic fragments show the textures and mineral 

composition of the volcanic rock types. The interstitial material of the groundmass of the fragments 

ranges from clear small isotropic grains and shards to
C12 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

opaque mixtures of dusty material and devitrified glass 

particles. During devitrification the rock particles 

become greenish with the development of chlorite and 

chalcedony, and red and brown from oxidation of iron 

minerals to hematite and limonite. Most thin sections 

are clouded owing various degrees of alteration to clay 

minerals.

PYROCL.ASTIC CONGLOMERATE AND BRECCIA

Pyroclastic conglomerates and breccias are composed 

of lapilli, cobbles, blocks, and boulders which range in 

size from a fraction of an inch to several feet in diameter. The fine material of the matrix consists of tuffaceous shale and standstone. The coarse fragments are 

similar in texture and composition to the volcanic rock 

types previously described.

CHEMICAL COMPOSITION AND COMPARISON WITH 

ROCKS OF OTHER AREAS

Chemical composition and norms of the volcanic 

rocks of Guam are given in table 2. The rocks include 

5 andesites and 4 basalts from the Alutom Formation, 

3 andesites and 6 basalts from the Facpi Volcanic 

Member of the Umatac Formation, and 2 andesites and 

1 basalt from the Dandan Flow Member at the top of 

the Umatac Formation.

The average composition of each group is shown in 

figure 3. The oldest (Alutom) and youngest (Dandan) 

andesites have a higher average silica content than the 

andesites of the Facpi. However, this range of silica 

content may be related in part to the alteration of the 

rocks. There is a strong possibility that secondary 

silica has been introduced into some of the rocks, and 

that a moderate amount of the more soluble oxides has 

been leached. It was noted earlier that relict boulders 

from the Dandan flows show a slightly higher silica content than do massive outcrops, and also that the quartz 

grains in some of the mafic flows may be xenocrysts 

rather than phenocrysts.

The average composition of seven andesites from 

Saipan is also shown in figure 3, and it approximates 

closely the composition of the average andesite of the 

Dandan Flow Member of the Umatac Formation of 

Guam.

According to Cloud, Schmidt, and Burke (1956, p. 

40) "Risaburo Tayama (written communication to 

Cloud, July 13,1949) reports a small inclusion of dacite 

in an andesitic conglomerate in Talofofo Valley on 

Guam, and the writers collected a few cobbles of porphyritic quartz dacite in a bed of andesite conglomerate 

at Mount Santa Rosa." A chemical analysis of the 

quartz-bearing blocks from Mount Santa Rosa is given 

in table 2, specimen 20 (Ss 9-1). The high silica content (68.89) is due to the abundance of quartz grains 

which are thought to be xenocrysts rather than phenocrysts. According to Kuno (oral communication, January 1954), the color index (11.5) and the pyroxene 

content are too high for the rock to be classified as a 

dacite. That dacites comparable to those of Saipan are 

present on Guam seems likely, however, for quartz-bearing rocks must have supplied the grains of quartz that 

occur as xenocrysts in the andesite and basalt flows 

and the small quartz crystals found in some of the sandy 

tuffs; but no rocks of dacitic composition, in either flow 

outcrop or pyroclastic beds, have been found on Guam.

As the analyses in table 2 are of porphyritic rocks (an 

attempt was made to select specimens relatively free of 

phenocrysts), the sum of the normative femic minerals 

(color index) in table 2 is probably slightly lower and 

the silica content slightly higher than that of the 

groundmass alone. It is probable that some secondary 

silica was added, as all the rocks show some alteration 

and weathering. Throughout the groundmass many 

flows and dikes have extremely small vesicles that contain small amounts of secondary minerals.

Figure 4 is a Harker variation diagram in which the 

weight percent of various oxides are plotted as ordinates against the weight percent of silica along the 

abscissa. Rocks with a composition intermediate between the basalts and andesites show anomalous ratios 

of color index and silica content. This transitional 

group is well illustrated on the diagram. Specimen 3 

(Go 7-1) from the bottom of a flow has a color index of 

36.3 (CIPW) or 38.3 (Niggli), and a silica content of 

48.58 percent, which places it with the basalts. Specimen 10 (Go 7-1-c) from the top of the same flow shows 

a color index of 34.2 (CIPW) or 35.8 (Niggli), and 

a silica content of 53.50 percent. Both specimens appear fresh in outcrop, but under the microscope alteration in the larger crystals and in the groundmass is 

evident. Specimen 10 is called basalt in spite of the 

low color index because of its probable greater alteration and lesser bulk due to weathering. Similar anomalies occur in specimen 7 (Ee 4-2-c) and 8 (Ee 4-2), 

which are also from the middle and bottom of a single 

flow. The color index of specimen 7 (Ee 4-2-c) is 

36.4, which is slightly less than the required 37 for 

basalt. This low index is believed to be due to alteration and weathering, and the rock is classed as a basalt.

Schmidt (1957, p. 153-163) has made a careful comparison between the volcanic rocks of Saipan and the 

rocks of Guam, the Palau Islands, the Bonin Islands, 

the northern Mariana Islands, the Volcano Islands, the 

Izu Peninsula of Japan, and the Hawaiian Islands. As 

the rocks of Guam appear to be closely related to those 

of Saipan, diagrams similar to those in Schmidt's report (1957) have been prepared for purposes of comparison. A detailed description of these diagrams is 

given in Schmidt's report.
TABLE 2. Chemical composition and norms of volcanic rock types of Guam

[Samples 1, 7,10,12,13,14 by rapid methods]

SiOj.... ...

AhOsFejOs........

FeO -----

MgO __---

CaO........

NasO....- 

K,0 

HjO+

TiOj.-

COa. .....

MnO_. .._-_.

Total 

1

Go 7-ld

2

FolO-1

3

Go 7-1

4

Df2-l

5

Ie3-l

6

Ch4-3

7

Ee4-2c

8

Ee4-2

Basalt

47.7 

15.6 

4.6 

3.8 

9.0 

10.3 

2.0 

.36

} 5.8

.54 

.05

.08 

.13

100

48.48 

15.99 

3.69 

4.64 

8.93 

10.90 

1.80 

.14 

/ 2.83 

X 1.91 

.57

.07 

.14

100.09

48.58 

16.53 

3.39 

3.74 

8.55 

9.88 

2.01 

.45 

3.95 

1.93 

.62

.09 

.13

99.85

50.85 

13.55 

2.24 

5.43 

10.01 

9.55 

1.58 

.16 

4.13 

1.91 

.34 

.07 

.03 

.14

99.99

51.06 

18.05 

3.63 

6.08 

4.56 

10.09 

2.45 

1.02 

.81 

.71 

.73 

.15 

.22 

.20

99.76

51.63 

14.10 

2.27 

5.93 

9.93 

9.47 

2.21 

.52 

1.84 

1.42 

.43 

.07 

.05 

.15

100.02

51.7 

14.6 

4.5 

7.9 

4.7 

9.1 

2.6 

.70

} 3.0 

.92 

<.05 

.25 

.20

100

52.09 

15.81 

3.71 

7.81 

4.29 

9.44 

2.48 

.76 

/ 1.18 

X 1.12 

.97 

.01 

.24 

.20

100.11

9

Fd2-l

Andesite

53.19 

17.51 

1.61 

7.16 

3.97 

9.20 

2.71 

.94 

.85 

1.25 

1.04 

.01 

.26 

.19

99.89

10

Go 7-lC

Basalt

53.5 

14.9 

3.7 

3.2 

7.0 

7.8 

2.9 

.62

.50 

.50 

.10 

.12

100

11

Ch4-4

12

Fd2-lb

Andesite

53.53 

16.28 

2.53 

5.24 

5.41 

5.59 

5.14 

1.77 

/ 1.09 

X 2.45 

.60 

.11 

.08 

.12

99.94

54.1 

14.8 

4.7 

6.1 

3.7 

8.0 

2.8 

.82

} 3.8

1.00 

.05 

.26 

.15

100

13

Dh6-lc

14

Dh6-lb

Basalt

54.2 

14.6 

5.1 

3.4 

8.0 

7.9 

2.4 

1.4

2.9

.36 

.05 

.08 

.12

100

55.0 

13.8 

3.8 

4.0 

9.4 

7.9 

2.3 

.88

2.6

.34 

.05 

.07 

.11

100

15

Ie2-l

16

Gm25-l

17

If 6-1G

And

55.73 

19.50 

2.56 

4.59 

1.98 

8.03 

3.24 

2.14 

/ .26 

1 .75 

.67 

.02 

.36 

.15

99.98

58.43 

14.80 

1.82 

5.10 

6.58 

7.77 

2.57 

.54 

.73 

.93 

.50 

.02 

.08 

.14

100.01

60.02 

16.74 

3.50 

3.31 

2.86 

6.67 

3.35 

1.52 

.65 

.30 

.56 

.01 

.19 

.13

99.81

Norms, in weight percent

Q --

ab ... 

di._. .........

hy._. -

11 -. -_-

Ol .......

Total ...

2.00

2.40

18.80

34.40

15.80

20.70

5.10

.80

100.00

2.73

.58

16.87

36.94

15.82

22.22

4.01

.81

99.98

2.99

2.93

18.78

36.67

11.27

22.41

3.70

.94

.29

99.98

5.70

1.18

14.70

31.16

29.04

2 4.7

.47

100. 01

4.47

6.30

22.91

36.08

10.54

14.09

3.95

1.03

.63

100.00

2.85

20.54

27.67

16.43

27.50

2.40

.57

100.01

7.40

4.10

24.60

27.50

14.40

15.20

4.90

1.30

.60

100.00

7.43

14.64

23.22

31.05

12.77

14.86

4.00

1.39

.64

100.00

6.26

5.74

25.26

33.87

8.96

16.07

1.72

1.49

.63

100.00

9.60

3.60

27.40

27.20

8.40

17.70

4.10

1.30

.70

100.00

10.75

46.95

16.40

8.82

2.83

2.71

.91

.28

10.35

100.00

12.60

5.30

26.50

26.80

10.60

10.90

5.10

.70

100.00

7.90

8.60

22.20

25.40

U QA

ift ftft

5.50

60

100.00

8.10

5.70

24.90

19 ftft

4.10

40

100.00

7.81

13.02

9Q 4*

32.84

3.85

8.49

2.72

.91

.91

100.00

13.43

3.40

23.23

27.76

8.61

20.74

1.87

RS

100.00

15.85

9.09

30.68

26.70

5.00

7 84.

3.75

.80

.29

100.00

Normative plagioclase, in mol percent

ab...-. ....

4.2

33.9

61.9

1.1

29.8

69.1

5.1

30.9

64.0

2.5

30.0

67.5

9

0

33.8

56.4

5.7

38.8

55.5

7.2

43.8

49.0

8.3

38.0

53.9

9.1

37.5

53.4

6.0

47.3

46.7

15.0

62.0

23.0

9

Q

43.6

47.1

15.2

39.6

45.2

11.0

40.6

48.4

17.7

37.7

44.6

6.4

41.3

52.3

14.1

44 7

41.2

Normative pyroxene, in mol percent

fs

21.6

72.3

6.1

20.8

68.2

11.0

16.7

74.6

8.7

17.2

67.1

15.6

21.4

53.0

25.6

18.7

fi4 4

16.9

24.4

45.6

29.1

23.1

31.9

17.9

45.4

36.7

15.6

77.7

6.7

37.9

44.6

17.5

24.7

50.6

24.7

19.9

76.3

3.8

16.8

74 4

8.8

15.6

45.9

38.5

14.7

63.7

21.6

19.5

63.7

16.8

Alutom formation:

1. Go 7-l-d Basalt flow, near bottom, Spruance Drive. Harry F. Phillips, Paul L. D. Elmore, 

Katherine E. White, analysts.

2. Fo 10-1 Basalt flow, Spruance Drive roadcut. Edwin J. Tomasi, analyst.

3. Go 7-1 Basalt flow, Spruance Drive roadcut. Edwin J. Tomasi, analyst. 

Umatac formation:

4. Df 2-1 Lower flows in Facpi Volcanic Member. Olivine basalt flow, old road to Umatac. 

Edwin J. Tomasi, analyst.

5. le 3-1 Dandan Flow Member. Basalt boulder, Martinez Pasture, Dandan area. Edwin J. 

Tomasi, analyst.

6. Ch 4-3 Lower part of Facpi Volcanic Member. Basalt dike, 3,600 ft northeast of Facpi Point. 

Edwin J. Tomasi, analyst.

7. Ee 4-2-c Upper flows in Facpi Volcanic Member. Basalt flow (near middle) above Umatac 

Springs. Harry F. Phillips, Katherine E. White, analysts.

8. Ee 4-2 Upper flows in Facpi Volcanic Member. Basalt flow (near bottom) above Umatac 

Springs. Edwin J. Tomasi, analyst.

9. Fd 2-1 Upper flows in Facpi Volcanic Member. Sodic andesite, 520 ft upstream from road

above Merizo Dam. Edwin J. Tomasi, analyst. 

Alutom formation:

10. Go 7-1-c Basalt flow, near top, Spruance Drive. Harry F. Phillips, Paul L. D. Elmore, Katherine

E. White, analysts. 

Umatac formation:

11. Ch 4-4 Lower flows in Faopi Volcanic Member. Sodic andesite dike, 3,600 ft northeast of Facpi 

Point. Edwin J. Tomasi, analyst.

12. Fd 2-1-b Upper flows in Facpi Volcanic Member. Calcic 

Dam. Harry F. Phillips, Paul L. D. Elmore, Ka

13. Dh-6-l-c Upper flows in Facpi Volcanic Member. Basalt 

Harry F. Phillips, Paul L. D. Elmore, Katherine 

14. Dh 6-1-b Lower flows in Facpi Volcanic Member. Basalt flo

Harry F. Phillips, Paul L. D. Elmore, Katherin

15. le 2-1 Dandan Flow Member. Andesite bouldr-r, Martin

J. Tomasi, analyst. 

Alutom formation:

16. Gm 25-1 Calcic andesite boulder, east of Mount Tenjo. Ed

Umatac formation:

17. If 6-1 Dandan Flow Member. Sodic andesite, Martinez 

Tomasi, analyst. 

Alutom formation:

18. Gm 1-2 Sodic andesite boulder, east of Mount Alutom. Ed

19. Go 10-2 Sodic andesite boulder in pyroclastic conglomerate, 

analyst.

20. Ss 9-1 Sodic andesite boulder in pyroclastic conglomerate, 

L. O. Trumbull, analyst.

21. Go 6-1 Andesite boulder, pyroclastic conglomerate, Spr

analyst.
C14 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

Basalt »t*- Andeslte

zu

5 15

10

ro 

OCM 

Q) c U_ 5

O

0> 

U_

0 

10

1- z.

UJ

O

^ * UJ O 

Q.

1

0 

12

O

(0

O

6

0

CM 

«J z.

0 

5

OCM

0

1 ' ' ' 

_ A _

~ A ~

- o -

_ * A A _

- n - 

o -

I i I I

i 1 i i

~ A

 A

 + -

n 

I i i i

I I I 1

- A 

~~ A ~

_ O

I *BA D + 0 ~_

1 1 1 1

1 1 ' '

- -

 ,'*', ° - ,

45 50 55 60 65

EXPLANATION

Average andesite of Saipan 

Schmidt (1957, p. 164, table 9)

Basalt, Dandan Flow Member of

Umatac Formation (Miocene)

Analysis 5

D

Andesite, Dandan Flow Member of 

Umatac Formation (Miocene) 

Average of analyses 15 and 17

Basalt, Facpi Volcanic Member of

Umatac Formation (Miocene) 

Average of analyses 4, 6, 7, 8, 13, and 14

Andesite, Facpi Volcanic Member of

Umatac Formation (Miocene)

Average of analyses 9, II, and 12

Basalt, Alutom Formation

(Eocene and Oligocene)

Average of analyses I, 2, 3, and 10

Andesite, Alutom Formation

(Eocene and Oligocene) 

Average of analyses 16, 18, 19,20, and 21

PERCENT Si02

FIGURE 3. Average oxide composition of volcanic rocks of Guam by age. Analyses referred to are given in table 2.
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM C15

'55 a '« "w *; *;

« <o ® « 15 15

 o w tJ -a in M

c « c c to to

< CO < < m m

Basalts 91011121314_______ Andesites 

Specimen No.' l 23 *4 5 67 T\\ I///' 15 16 17 18 19

45 50 55 60

Si02, IN WEIGHT PERCENT

FIGUBE 4. Harker variation diagram of volcanic rocks of Guam. Numbers refer to analyses and corresponding rock specimens in table 2.
C16 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

In figure 4 as in a similar variation diagram for the 

volcanic rocks of Saipan, there is a characteristic difference in the silica content of the andesites and the 

dacites. The silica content of the analyzed dacites of 

Saipan exceeds 76 percent. The highest silica content 

of the analyzed rocks of Guam is 69.04 percent (specimen 21, Go 6-1). The quartz grains in this rock are 

believed to be xenocrysts, and secondary silica is probably also in the fine-grained groundmass. In the Harker variation diagram of the rocks of Saipan, the alkalilime index is about 65, whereas that of the rocks of 

Guam is about 70. Here, as on Saipan, the exact index 

is uncertain, but it would appear that the index of the 

rocks of Guam is slightly more calcic.

Figure 5 is a standard ACF diagram which shows 

the composition of the analyzed rocks of Guam in terms 

of three components, where A, C, and F are the molecular amounts: A=A12O3 K2O Na2O, C=CaO, and 

F=MgO+FeO-Fe2O3 -TiO2 . On this diagram the 

Guam andesites fall within the triangle anorthitediopside-hypersthene. The andesites of Saipan 

(Schmidt, 1957, fig. 17) also lie within this triangle 

(with two exceptions), but, in general, they are closer 

to the anorthite apex than are the rocks of Guam. Five 

of the andesites from Guam fall in the region of the 

basalts, but they are closer to the diopside-hypersthene 

join and indicate the transitional character of the rocks. 

The 11 basalts from Guam all fall closer to the diopsideA

Sillimanite 

AI203-K20-N320

80

EXPLANATION

A

Andesites

O 

Basalts

60 40

Anorthite

40

Cordierite

60

80

CaO 

Wollastonite

60 Diop'side 40 

MOLECULAR PERCENT

20

MgO+FeO F6203 Ti02 

Hypersthene

FIGTJEE 5. Triangular ACF diagram of specimens of volcanic rocks of Guam. Numbers refer to analyses and corresponding rock specimens in

table 2.
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM C17

hypersthene join than do any of the andesites from 

Saipan.

Figure 6 is an ACF diagram showing the average 

andesite and basalt of Guam, average dacite and andesite of Saipan, average basalt of the Izu Peninsula, and 

average olivine basalt of the Hawaiian Islands. Part

(1957). The average andesite of Guam falls close to 

the average basalt of Izu, and the average basalt of 

Guam lies between the average basalt of Izu and the 

average olivine basalt of the Hawaiian Islands.

Figure 7 is an ACF diagram in which the rocks of 

Guam and Saipan are compared with Daly's average

of the data is taken from figures 20 and 21 of Schmidt rock types (Daly, 1933). The average basalt of Guam

A

Sillimanite 

AI203-K20-

80

Q Olivine basalt of 

Hawaiian Islands

Anorthite

40

Cordierite

60

80

/ V

C

CaO 

Wollastonite

V V

80

V V

60 Oiopside

V

40

V V

20

MOLECULAR PERCENT

V \\

F

MgO+FeO Fe20sHypersthene

FIGURE 6. Triangular ACF diagram of average volcanic rocks of Guam, Saipan, the Izu Peninsula of Japaoi, and the Hawaiian Islands. Data

for rocks of Saipan, Izu, and Hawaii from Schmidt (1957, figs. 20, 21).
CIS GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

lies close to Daly's average basalt, but the average andesite of Guam is considerably more mafic than Daly's 

average for all andesites.

Figures 8-10 are SKM diagrams in which the composition of the analyzed rocks is shown in terms of 

three components, where $, K, and M are the molecular 

amounts: £=SiO2 -2CaO, ^=6(Na2O+K2O), and 

,¥=MgO+FeO - Fe2O3 - TiO2 - CaO - Na2O - K2O+ 

A12O3 .

In figure 8 the andesites and basalts of Guam, with 

a single exception, lie above the silica saturation line

(the horizontal line at 50) in the triangle quartz-alkali 

feldspar-hypersthene. The exception, specimen 11 (Ch 

4-4), is from a dike that cuts basaltic rocks of the Facpi 

Volcanic Member on a point of land just north of Facpi 

Point. In thin section the rocks shows a devitrified 

groundmass and considerable alteration of the feldspar 

phenocrysts. The pyroxene phenocrysts are augite and 

hypersthene, and both appear fresh in contrast to the 

altered feldspar grains. In the SKM diagram given 

by Schmidt (1957, fig. 17), all the dacites and andesites 

of Saipan fall in the triangle quartz-alkali feldsparA 

Sillimanite

Anorthite

40,

EXPLANATION

 

Guam rocks

Data from this report 

A

Saipan rocks

Data from Schmidt (1957, fig. 21) 

D

Daly's rock types

Data from Schmidt (1957, fig. 21)

.Gordierite

.60

Andesite of Saipan AnsAllandesite

j-j Hypersthene andesite 

 Andesite of Guam

Andesite groundmass A 

of Saipan"

20

Basalt of Guam « 

All basalt Q

Quartz basalt

Plateau basalt

V

80

V V V_

C

CaO 

Wollastonite

80 60 Diopside 40 

MOLECULAR PERCENT

20

MgO+FeO-Fe203-Ti02 

Hypersthene

FIQUEH 7. Triangular ACF diagram of average volcanic rocks of Guam, Saipan, and Daly's average rock types.
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM C19

hypersthene, the dacites lying above a horizontal line 

at 70 and the andesites above a horizontal line at 60. 

The basalts of Guam (fig. 8) fall below the 70 percent 

line and are scattered between the horizontal lines at 

52 and 70. The andesites of Guam tend to be higher in 

silica and to lie toward the alkali feldspar side of the 

diagram, and the basalts tend to be lower in silica and 

to lie toward the hypersthene side of the diagram. Between these rocks is a group transitional to the andesites 

and basalts.

Figures 9 and 10 show a close correspondence in composition between the average andesites of Guam and

Saipan and between the average basalts of Guam and 

Izu.

Figure 11 is a triangular diagram in which the 

normative feldspar composition and the normative femic 

mineral-quartz-feldspar ratios of the volcanic rocks of 

Guam are plotted according to the method suggested by 

Larsen (1938). The two values for each rock are connected by lines, and the general increase in quartz and 

feldspar with increase of albite content of the feldspar 

is shown by the changing slopes of the lines. The single 

undersaturated andesite of Guam, specimen 11 (Ch 

4-4.), because of its approximate 10 percent of normative

Quartz 

Si02-2CaO

80 20

EXPLANATION

A

Andesites

O 

Basalts

Alkali feldspar

Leucite 40

Hypersthene

Olivine

Nepheline 

20 80

K 

6 (Na20+K20)

80 60 40 

MOLECULAR PERCENT

20 M 

MgO+FeO-Fe203-Ti02-CaO-Na20-K20+Al2(>3

FIODBB 8. Triangular SKM diagram of specimens of volcanic rocks of Guam. Numbers refer to analyses and corresponding rock specimens in

table 2.,
C20 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

olivine, stands out as the only rock with a f emic to f elsic 

ratio greater than anorthite to albite plus orthoclase 

ratio.

Figures 12 and 13 are triangular diagrams which 

show, respectively, the composition of normative feldspar and normative pyroxene of the average andesite 

and basalt of Guam and also the average normative 

feldspar and pyroxene of Daly's average rock types. 

The normative feldspar of the average andesite of Guam

(fig. 12) is very close to that of Daly's average basalt. 

The normative feldspar of the average basalt of Guam 

contains slightly more normative anorthite than that of 

the average world basalt. The normative pyroxene of 

the average andesite of Guam (fig. 13) is close to that 

of the average world basalt. The normative pyroxene 

of the average basalt of Guam contains more normative 

enstatite and slightly less normative wollastonite than 

the normative pyroxene of Daly's average basalt.

Quartz 

Si02 2CaO

80 20

60 40

Alkali feldspar Hypersthene

A Andesite of Saipan 

 

Andesite of Guam

40

Olivine

Nepheline 

20 80

60 40 

MOLECULAR PERCENT

20 M 

MgO+FeO-Fe203-Ti02-CaO-Na20-K20+Al20s

FIGURE 9. Triangular SKM diagram of average volcanic rocks of Guam, Saipan, the Izu Peninsula of Japan, and the Hawaiian Islands. Data

for rocks of Saipan, Izu, and Hawaii from Schmidt (1957, figs. 20, 21).
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM C21

Quartz 

SiC-2-2CaO

80

60

EXPLANATION

 

Guam rocks

Data from this report 

A

Saipan rocks

Data from Schmidt (1957, fig. 22) 

D

Daly's rock types

Data from Schmidt (1957, fig. 22)

/Dacite groundmass of Saipan 

ff-Dacite of Saipan

A Andesite groundmass of Saipan 

Rhyolite

Andesite of Saipan D

Dacite A 

D« Andesite of Guam

D

All andesite 

Quartz basalt n

Alkali feldspar Hypersthene

60

Oiivine

Nepheline 

20 80

K 80 60 40 20 M 

6 (N820+K20) MOLECULAR PERCENT MgO+FeO-Fe203-Ti02-CaO-Na20-K20+Al203

FIGURE 10. Triangular SKM diagram of average volcanic rocks of Guam and Saipan and of Daly's average rock types.
C22 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

EXPLANATION

 

Or-Ab-An

x 

Femic mineral - quartz - feldspar

Feldspar 60 40 

MOLECULAR PERCENT

... Femic mineral

FIGURE 11. Triangular diagram of the norms of the andesites and basalts of Guam showing the ratios of Or-Ab-An, and of femic mineralquartz-feldspar.
PETROLOGY OP THE VOLCANIC ROCKS OP GUAM C23

EXPLANATION

Andesite of Guam

Basalt of Guam

D 

Daly's rock types

60 40

(Ab

Andesite of Guam * Quartz basalt

Plateau basalt D Basalt of Guam

60

An 80 60 40 20

MOLECULAR PERCENT

FIQOEB 12. Composition of normative feldspar of average rocks of Guam and of Daly's average rock types.
C24 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

Wo

40

.EXPLANATION

A

Andesite of Guam

 

Basalt of Guam

a 

Daly's average rock types

60 40

Quartz basalt D

Q All basalt

... Plateau basalt

Andesite of Guam

Hypersthene andesite

V

All andesite

V V V

80

En 80 60 40 20

MOLECULAR PERCENT

FIGURE 13. Composition of normative pyroxene of average rocks of Guam and of Daly's average rock types.

Fs

Figures 14 and 15 are triangular diagrams which 

show, respectively, the composition of normative feldspar and the composition of normative pyroxene of 

average rocks from Guam, Saipan, the Izu Peninsula 

of Japan, the northern Mariana Islands, and the 

Hawaiian Islands. The normative feldspar of the andesites of Guam (fig. 14) contains slightly more orthoclase than the normative feldspar of the andesites from 

the other areas. The normative plagioclase of the andesites and basalts of Guam averages about An50 ; the 

rocks from the other areas generally contain more calcic plagioclase and range from AIL^ to An75 . The nor-
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM C25

mative pyroxene of the average andesites and basalts of 

Guam (fig. 15) contains approximately the same 

amount of normative wollastonite as the average andesite of Saipan, but it generally contains more normative enstatite (En59-En6g ) than the normative pyroxene 

of the rocks of other areas, which range from En^ to 

En54.

The andesites and dacites of Saipan and the andesites and basalts of Guam are close in composition to 

the volcanic rocks of the Hakone region of Japan.

Kuno (1950, p. 993; 1953, p. 268) divides the Hakone 

rocks into (1) a pigeonitic rock series which he believes 

was formed by fractional crystallization of parent olivine basalt, and (2) a hypersthenic rock series which 

he believes originated through contamination of the 

parent olivine basalt by granitic material. Schmidt 

(1957, p. 159) points out the close correspondence between the rocks of Saipan and the hypersthenic rock 

series of the Hakone region. A similar correspondence 

is apparent for the andesites and basalts of Guam.

80 20

40

40 60

20 80

Andesite of Guam A Olivine basalt of 

Hawaiian Islands

O

V V v

Andesite of Saipan OQ 

Basalt of northern Mariana Islands

V______V______V

Basalt of Guam

o

Basalt of Izu Peninsula 

V______V _V

Ab 80 60 40 

MOLECULAR PERCENT

20 An

FIGUEB 14. Composition of normative feldspar of average volcanic rocks of Guam, Saipan, the northern Mariana Islands, the Hawaiian Islands, 

and the Izu Peninsula of Japan. Data for rocks of Saipan, northern Mariana Islands, Hawaii, and Izu from Schmidt (1957, fig. 23).
C26 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

This relationship is further illustrated in figure 16, in 

which the rocks of Guam are plotted on a triangular 

MgO-FeO-alkali diagram. A comparison of figure 16 

with a similar diagram for the nonporphyritic rocks 

of the Izu and Hakone regions (fig. 16, inset) shows 

that the trend of the variation of rocks of Guam follows 

closely that of the hypersthenic rock series of Japan.

PETROGENESIS

The volcanic rocks of Guam are generally similar in 

composition to the rocks of Saipan and to the rocks of

the island groups scattered along the western margin 

of the Pacific Basin, all of which belong to the circumpacific petrographic province. The volcanic rocks of 

this province are decidedly less alkalic than the volcanic 

rocks of those islands lying within the more orogenically stable Pacific Basin, which form the intrapacific 

province. The writer believes that the volcanic rocks of 

Guam originated through the same processes that were 

responsible for the volcanic rocks of Saipan.

Schmidt (1957, p. 170) considers the high silica content and the peraluminous character of the dacites of

80

60

60

Andesite of Guam

Olivine basalt of 

Hawaiian Islands

Basalt of northern Mariana Islands O

O Andesite of Saipan

° Basalt of Izu Peninsula

V

80

V

En 80 60 40 

MOLECULAR PERCENT

20 Fs

FIGDEB 15. Composition of normative pyroxene of average volcanic rocks of Guam, Saipan, the northern Mariana Islands, the Hawaiian Islands, 

and the Izu Peninsula of Japan. Data for rocks of Saipan and northern Mariana Islands selected from Schmidt (1957, tables 5, 6), 

Hawaii from Macdonald (1949, table 5), and Izu from Kuno (1950, tables 12, 14).
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM C27

Total iron as FeO

EXPLANATION

Andesite of Guam

Basalt of Guam

Daly's average rock types

NONPORPHYRITIC ROCKS 

OF IZU AND HAKONE

Pigeonitic rock series

o 

Hypersthene rock series

Trends of variation of the nonporphyritic rocks of Izu and Hakone, the 

Skaergaard liquids, and Oaly's (1933) averages of basalt, andesite, dacite, 

and rhyolite (Kuno, 1953, fig. 1)

Alkalies 80 60 40 

MOLECULAR PERCENT

20 MgO

FIGDEB 16. Triangular PeO-MgO-alkali diagram of andesites and basalts of Guam, and Daly's average rock types. Numbers refer to analyses

and corresponding rock specimens in table 2.

Saipan difficult to reconcile with any hypothesis of 

simple fractional crystallization, and he believes it is 

"necessary to assume some special process such as perhaps extreme fractionization of an andesitic magma 

coupled with assimilation of significant amounts of 

siliceous and aluminous crustal material" to account 

for their composition. If beneath the rocks now exposed on Saipan there are basaltic flows comparable to 

the basalts of Guam, and if rocks of dacitic composition were once on Guam and have since been removed 

by erosion, as is suggested by the occurrence of quartz 

crystals in the sandy tuffs and quartz xenocrysts in 

some of the basic flows, it would seem probable that the 

rocks of both islands have had a similar mode of origin 

and have been derived from the same type of parent 

magma.

TRACE ELEMENTS IN THE VOLCANIC ROCKS OF GUAM

By JOSHUA I. TBACEY, JR., and JOHN T. STAKE

The results of spectrographic determinations of trace 

elements and the chemical analyses of 6 basalts and 5 

andesites of Guam are presented in table 3. Also given 

are chemical analyses and trace elements of basalts and 

andesites of the Hawaiian Islands, quoted from Wager 

and Mitchell (1953, p. 218, table 1). The samples from 

Guam are arranged stratigraphically, rather than according to their silica content as in table 2, in order to 

show possible variations with time. Locations, field 

numbers, and descriptions of the samples are given in 

table 2, and localities are shown in figure 2. Samples 

within each stratigraphic subdivision are arranged 

from left to right in order of decreasing MgO content.
TABLE 3. Trace elements of lavas of Guam and the Hawaiian Islands

[Field numbers and descriptions given in table 2]

Guam

(SpectrograpMc analyses by Paul R. Barnett. Chemical analyses from table 2)

Alutom Formation

2 3

Flow

16 18 21

Boulders

Umatac Formation

Facpi Volcanic Member

Lower

4

Flow

6

Dike

Upper

8 9

Flow

Dandan Flow 

Member

5 15

Boulders

Chemical analyses, in percent

SiO2 -----

AUOs - -_-

FeO.____-__

MgO.___-_-

CaO .....

NajO.......

K2 0...._...

HiO- .

HiO+......

TiOi. .

COj. 

P»0,_. _ MnO ....

48.48 

15.99 

3.69 

4.64 

8.93 

10.90 

1.80 

.14 

2.83 

1.91 

.57

.07 

.14

48.58 

16.53 

3.39 

3.74 

8.55 

9.88 

2.01 

.45 

3.95 

1.93 

.62

.09 

.13

58.43 

14.80 

1.82 

5.10 

6.58 

7.77 

2.57 

.54 

.73 

.93 

.50 

.02 

.08 

.14

61.49 

14.01 

1.62 

2.19 

3.23 

7.73 

2.65 

.80 

.86 

.69 

.39 

.94 

.08 

.08

69.04 

12.71 

3.20 

1.03 

1.96 

5.79 

2.50 

1.00 

1.23 

.81 

.36 

.09 

.05 

.05

50.85 

13.55 

2.24 

5.43 

10.01 

9.55 

1.58 

.16 

4.13 

1.91 

.34 

.07 

.03 

.14

51.63 

14.10 

2.27 

5.93 

9.93 

9.47 

2.21 

.52 

1.84 

1.42 

.43 

.07 

.05 

.15

52.09 

15.81 

3.71 

7.81 

4.29 

9.44 

2.48 

.76 

1.18 

1.12 

.97 

.01 

.24 

.20

53.19 

17.51 

1.61 

7.16 

3.97 

9.20 

2.71 

.94 

.85 

1.25 

1.04 

.01 

.26 

.19

51.06 

18.05 

3.63 

6.08 

4.56 

10.09 

2.45 

1.02 

.81 

.71 

.73 

.15 

.22 

.20

55.73 

19.50 

2.56 

4.59 

1.98 

8.03 

3.24 

2.14 

.26 

.75 

.67 

.02 

.36 

.15

Trace constituents, in parts per million

Atr

Bg;.::::::::

Ba_. .......

Co .

Or .......

Cu... ...._.

Oa.. .......

NL...._ 

Pb.. .......

Sc

Sr.____ _ V.

Y..

Yb. ... .

Zr.. ........

0

0 

20 

200 

500 

70 

7 

100 

0 

40 

200 

50 

30 

2 

10

0 

0 

20 

200 

500 

80 

8 

100 

0 

50 

300 

40 

30 

2 

20

0 

20 

60 

100 

500 

60 

6 

200 

0 

30 

200 

40 

30 

2 

40

0

20 

80 

80 

400 

10 

6 

90 

0 

20 

200 

40 

50 

3 

40

0 

20 

60 

50 

300 

200 

7 

40 

0 

20 

200 

20 

20 

1 

40

0 

0 

20 

200 

900 

70 

7 

300 

0 

40 

200 

30 

30 

2 

10

2 

0 

70 

200 

700 

100 

10 

200 

50 

40 

300 

40 

30 

3 

10

0 

0 

200 

200 

10 

200 

10 

10 

0 

40 

800 

60 

50 

4 

30

0 

0 

200 

200 

9 

200 

10 

10 

0 

40 

900 

60 

50 

5 

40

0 

20 

400 

200 

10 

200 

8 

100 

0 

30 

1,000 

50 

30 

3 

30

0 

30 

500 

70 

2 

100 

9 

2 

0 

20 

1,000 

30 

30 

3 

80

NOTE. Looked for but not found: As, Au, Be, Bi, Cd, Ge, In, La, Mo, Nb, Pt, Sb, Sn, Ta, Tn, Tl, U, W, and Zn.

Hawaiian Islands

(Chemical and spectrographic analyses from Wager and M

Picrite

basalt

Picrite 

basalt

Olivine 

basalt Basalt

Andesine 

andesite

Andesi

andesi

Chemical analyses, in percent

SiO3 ...

A12O3 _ -

FeaOs.- 

FeO ....

MgO ...

CaO-.._-

Naa O-__._

KsO 

HjO-_ _ H,0+-.-._

TiOj......

CO2 ..

PsOs 

MnO _ ..

47.25 

9.07 

1.45 

10.41 

19.96 

7.88 

1.38 

.35

1.61

.21 

.13

49.27 

9.38 

1.28 

10.31 

17.74 

7.46 

1.80 

.42

2.58

.26 

.09

50.52 

13.85 

0.98 

9.77 

7.07 

11.33 

1.51 

.47

3.63

.22 

.14

52.28 

10.83 

1.90 

9.30 

7.69 

10.03 

2.29 

.51

4.43

.32 

.12

51.55 

13.59 

2.33 

9.04 

8.02 

10.31 

2.43 

.27

1.98

.24 

.12

50.6

16.4

5.7

6.2

4.2

6.4

4.7

2.1

2.6

.1

.2

Trace constituents, in parts per million

A a

BS::::::::

Ba.. ......

Co .

Or. 

Cu... 

Qa........

Ni........

Pb___.____

Sc

Sr 

V.. .----.

Y

Yb-. 

Zr .....

Be.. ...

MOLi .... 

La 

Kb. 

150 

70 

1,000 

150 

15 

400

400 

300 

20

100

1 

1

70 

80 

2,500 

150 

20 

1,500

200 

200 

20

50

2 

1

150 

30 

400 

200 

25 

80

10 

800 

400 

20

100

1

100 

40 

500 

100 

25 

100

10 

600 

250 

25

50

1 

1

100 

30 

500 

200 

20 

70

i,66o

200 

20

150

3

600 

12

<10 

20 

6

2,000 

70 

60

300

20 

30 

40

60

<1

2

3,00

7

5

40

2

3
PETROLOGY OF THE VOLCANIC ROCKS OF GUAM C29

Rocks from the Alutom Formation shown in table 3 

range from 48.48 to 69.04 percent SiO2 and from 8.93 

to 1.96 percent MgO. Samples comparatively low in 

silica are from lava flows, whereas those comparatively 

high in silica are from boulders in pyroclastic breccias 

or conglomerates. The stratigraphic position of the 

samples relative to each other within the formation is 

not certainly known; therefore all samples are arranged 

in order of increasing MgO content.

Rocks from the Umatac Formation shown in table 3 

range from 50.85 to 55.73 percent SiO2 and from 10.01 

to 1.98 percent MgO. These samples come from known 

stratigraphic positions relative to each other within 

the formation (fig. 17), which is more than 2,000 feet 

thick. Samples analyzed for trace elements from the 

thick Facpi Volcanic Member represent lower pillow 

basalt flows near the coast (sample 4, 13, 14), dikes 

that cut these flows (sample 6) several hundred feet 

below the tongues of the Maemong Limestone Member, and upper basalt and andesite flows above the 

tongues of the Maemong Limestone Member close to 

the top of the Facpi Volcanic Member (samples 8, 9). 

The remaining samples are from large residual boulders 

from the Dandan Flow Member at the top of the 

Umatac Formation (samples 5, 15). A general 

decrease in MgO is evident from bottom to top of the 

formation.

Ranges for each minor element, relative to decreasing MgO, are considered below for each of the two 

formations.

Boron was not detected in any flow samples, but it 

is present (20 ppm; parts per million) in boulders 

from pyroclastic breccias from the Alutom, and (20

to 30 ppm) in residual boulders from the Dandan 

Flow Member.

Barium increases in the Alutom from 20 to 80 ppm 

and in the Umatac from 20 to 500 ppm. Distinct 

increases are evident between lower and upper flows 

of the Facpi and between the Facpi and the Dandan.

Cobalt decreases in the Alutom from 200 to 50 ppm 

In the Umatac it remains constant at 200 ppm, except 

for sample 15, which contained 70 ppm.

Chromium decreases from 500 to 300 ppm in the 

Alutom. In the Umatac it decreases from 900 to 2 ppm 

and shows a very sharp break between lower and upper 

flows of the Facpi.

Copper ranges from 10 to 200 ppm throughout, but 

shows no significant trend. It is generally higher in 

samples from the Umatac than in those from the 

Alutom.

Gallium ranges from 6 to 10 ppm in all samples, but 

shows no consistent trend.

Nickel ranges from 200 to 40 ppm in the Alutom and 

from 300 to 2 ppm in the Umatac. It decreases in a 

general way within each formation, and, like chromium, 

shows the greatest break between samples from the 

lower and upper flows of the Facpi Volcanic Member.

Lead was found only in sample 6.

Scandium generally decreases in each formation and 

ranges in all samples from 50 to 20 ppm.

Strontium remains nearly constant in the Alutom, 

ranging from 200 to 300 ppm. In the Umatac Formation strontium ranges from 200 to 300 ppm in the lower 

flows of the Facpi to 800 to 900 ppm in the upper 

flows, and to 1,000 ppm in the Dandan.

Maemong Limestone Member 

14,

15, 17

6, 11

SEA LEVEL

FIGURE 17. Schematic diagram of the Umatae Formation. Relative stratigraphic positions of volcanic rock samples from the Faepi Volcanic 

Member and the Dandan Flow Member are shown. Sample descriptions are given in table 2, and locations are shown in figure 2.
C30 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS

Vanadium ranges from 20 to 60 ppm, but shows no 

notably consistent trend.

Yttrium ranges from 20 to 50 ppm without any significant trend.

Ytterbium ranges from 1 to 5 ppm and generally 

fluctuates with yttrium.

Zirconium ranges from 10 to 80 ppm and consistently 

increases with decreasing MgO in each formation. 

The amount of zirconium is extremely low in all samples, relative to most igneous rock provinces.

As is evident from table 3, several of the minor constituents show systematic variations within each formation and suggest the possibility that the volcanic rocks 

of Guam can be divided into two differentiation series. 

The first, consisting of the rocks of the Alutom Formation, of Tertiary b and c age (late Eocene and early 

Oligocene), shows a greater range in silica content and 

lesser ranges in some of the minor elements. The 

second, consisting of the volcanic rocks of the Umatac 

Formation of Tertiary e age (early Miocene), shows a 

smaller range in silica content, but greater ranges in 

minor elements.

Each of these series of rocks shows, with decreasing 

MgO, a general increase in barium, strontium, and zirconium, and a general decrease in cobalt, chromium, 

and nickel, similar to variations shown in the rocks of 

the Hawaiian Islands given in table 3. A quantitative 

appraisal will not be attempted because of the differences in average amounts of minor elements in each 

suite of rocks, and because the rocks of Guam do not 

include rock types at either end of the Hawaiian suite, 

nor are individual types common to both places closely 

similar, as is explained earlier in this chapter. Wager 

and Mitchell (1953, p. 222) concluded that the "close 

qualitative, and in many cases quantitative similarity 

in trace element variation of the Hawaiian and Skaergaard series" supported a view that both resulted from 

the same process, which on other grounds is considered 

to be fractional crystallization. The general correspondence in variation of some of the trace elements 

from Guam with those from the Hawaian Islands suggests that the same process has operated for the volcanic 

rocks from Guam, with the additional factor of incorporation of silicic rocks in the formation of the types 

containing quartz xenocrysts.

Systematic variations in trace elements are evident 

when the rocks of Guam are considered as two stratigraphic suites. Variations are not nearly so systematic 

if the rocks are combined to form one petrographic 

suite ranging from olivine basalt to sodic andesite; or 

if all the rocks are arranged as a series with respect 

to increasing silica (as in table 1) or decreasing magnesia. The difference between the two stratigraphic 

suites is shown in figure 18, in which chromium, nickel,

barium, and strontium are plotted against the MgO 

content for samples from the Alutom Formation (triangles) and from the Umatac Formation (squares). 

For each of these minor elements, the Alutom and the 

Umatac series slope in the same direction; but for each 

the slope of the Alutom is gentle, whereas that of the 

Umatac is steep.

A further confirmation of the stratigraphic nature 

of the variations in the minor elements is obtained 

by rearranging stratigraphically (table 4) the samples 

analyzed in table 1. Chemical analyses are given for 

SiO2, MgO, K2O, CaO, and P2O5 in percent. Spectrographic results for chromium, nickel, barium, and 

strontium in parts per million are given for 11 of the 

samples. Within the Alutom, samples are arranged 

generally in order of decreasing MgO from bottom 

to top of the group. Samples 1, 3, and 10 are from the 

bottom, middle, and top of one flow; sample 2 is from 

a second flow a little lower stratigraphically. Within 

the Umatac all samples are arranged in stratigraphic 

order (fig. 17), and within each subdivision they are 

arranged in order of decreasing magnesia content 

upward.

In the Alutom series, CaO decreases with decreasing 

MgO in most samples. In the Umatac series, CaO decreases with decreasing MgO in the lower flows of the 

Facpi; but CaO increases again in the upper part and 

in the Dandan, although MgO continues to decrease. 

In each series the K2O generally increases with decreasing MgO, but in the series from the Umatac Formation a break occurs between the lower and the upper 

parts of the Facpi member. P2O5 ranges irregularly 

from 0.03 to 0.11 percent in the Alutom and in the 

lower part of the Facpi. Samples from the upper 

part of the Facpi and the Dandan, however, contain 

from 0.19 to 0.36 percent P2O5 . The significance of 

this jump in phosphate content is not known, but it 

happens at the same stratigraphic break that separates 

large changes in chromium, nickel, barium and strontium. Wager and Mitchell (1951, p. 178) report that 

apatite appears in a flood in late stages of the Skaergaard series, but in the Guam rocks apatite was noted 

only as rare inclusions in olivine basalt.

The available chemical and trace element data, 

therefore, suggest that the volcanic rocks of Guam belong to two differentiation series. The first formed 

the Alution Formation of late Eocene and early Oligocene age, and the second formed the Umatac Formation 

of early Miocene age. Rocks from these two series 

cannot be distinguished megascopically or petrographically except in a general way. Further reason 

for a twofold division of the volcanic rocks is seen in 

the geologic and bathymetric evidence presented in 

the section on structure in Tracey and others (1963),
1UU\J

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PETROLOGY OF THE VOLCANIC ROCKS OF GUAM

500

 31

Si 300

CO

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200

UJ *:

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100

4 6

MgO, IN PERCENT

A

10 4 6

MgO, IN PERCENT

B

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4 6 

MgO, IN PERCENT

c

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MgO, IN PERCENT

D

10

FIGURE 18. Relation of chromium, nickel, barium, and strontium to MgO in rocks of Guam. Samples from the Alutom Formation are shown

by triangles; samples from the Umatac Formation are shown by squares.

which indicates that the volcanic rocks of the Alutom 

and the Umatac Formations originated from two 

separate volcanic centers west of the present island.

A considerable break in the trend of a number of the 

constituents, notably CaO, K2O, P2O5 , Cr, Ni, and Sr, 

occurs within the Facpi Volcanic Member of the Umatac Formation. This break is marked geologically by

lenses of the Maemong Limestone Member of the Umatac Formation and at two localities by an unconformity 

(fig. 17) associated with the Maemong Limestone 

Member. It is possible, therefore, that the break between the lower and upper parts of the Umatac Formation might be emphasized by contamination by limy 

rocks. No detailed systematic collections were made,
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ALUTOM FORMATION

Basalt flows

Flow, near top _ .Middle.... _ ..._Bottom. ______ Flow.. _________

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UMATAC FORMATION

Facpi Volcanic Member

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