Petrology of the Basalt 

Cores From Midway Atoll

GEOLOGICAL SURVEY PROFESSIONAL PAPER 680-B
Petrology of the Basalt 

Cores From Midway Atoll

By GORDON A. MACDONALD

GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 680-B

Descriptions of altered basalt lava flows 

from beneath the reef cap

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

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

Washington, D.C. 20402 - Price 25 cents (paper cover)
CONTENTS

Abstract. __________

Introduction. ______

Megascopic featuresMicroscopic features. 

Chemical analyses _ _ 

Conclusions ________

References cited_.__

Page 

Bl

1

2

3

6

7

10

ILLUSTRATIONS

FIGXJKE 1. Index map of the Hawaiian chain____-_--____________________________----__-_--_---_-------_____-----

2. Map of Midway atoll, showing location of drill holes.____________________________________________________

3. Lithologic logs of Midway drill holes._______-_-___________________________--_____-___-_-----___--_-__--

4. Variation diagram showing abundance of elements in Midway basalts and basalts from the major Hawaiian 

Islands.________________________________________________________________________________________

Page 

B2

2

3

TABLES

TABLE 1. Logs of basalt cores from the two drill holes on Midway _

2. Chemical analyses and CIPW norms of Midway basalts.

3. Abundance of minor elements in Midway basalts. ______

Page 

B4

6

7

GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

PETROLOGY OF THE BASALT CORES FROM MIDWAY ATOLL

By GOKDON A. MACDONALD

ABSTRACT

Below 1,261 feet in the Reef hole and 516 feet in the Sand 

Island hole the Midway cores are entirely of greenish-gray 

altered basalt lava flows that contain a few reddish- or yellowish-brown weathered zones. Both aa and pahoehoe flow types 

are present, and the vesicularity and other structures are the 

same as those in Hawaiian subaerial flows. No pillow lavas, 

hyaloclastite, or tuff are present. Vesicles commonly are filled 

with clay, chlorite, and serpentine, and less commonly with 

calcite and zeolite, but many remain open. Soil zones occur 

between lavas at depths of 1,316 and 1,334 feet in the Beef hole. 

Olivine is altered to iddingsite and serpentine. Feldspar ranges 

from labradorite to andesine in composition and is partly altered 

to clay minerals, particularly in the upper part of the section. 

Pyroxene is generally fresh, but may be altered to serpentine; 

it is predominantly augite, but some grains have a small optic 

axial angle. The lava flows are believed to have been subaerial, 

but the alteration is believed to be the result of long exposure 

to sea water. The volcanic cone must have sunk more than 1,260 

feet relative to sea level since the flows were formed. Two 

analyzed specimens are tholeiitic basalt, but the original nature 

of most of the altered lava flows in the cores is uncertain. However, pebbles from the gravels that overlie the lava flows include alkalic basalt and mugearite, indicating that the Midway 

volcano had entered the late, alkalic stage.

INTRODUCTION

For many years, geologists have postulated that the 

low limestone islands of the mid-Pacific rest on submerged volcanic peaks. Two decades ago, seismological 

studies began to confirm this hypothesis. Finally, it was 

incontrovertibly proven, for one island at least, when a 

drill hole on Eniwetok Atoll penetrated the sedimentary 

cap and entered basalt beneath (Ladd and others, 1953). 

During the summer of 1965 two holes drilled on Midway 

atoll also penetrated the sedimentary rocks and entered 

underlying basalts (Ladd and others, 1967), thus providing a second definitely proven example. The drilling 

was carried out by the Hawaii Institute of Geophysics, 

University of Hawaii, supported by the National Science Foundation (grant No. GP4728), in cooperation 

with the U.S. Geological Survey and the Office of Naval 

Research.

Midway is one of the Leeward Islands of the Hawaiian 

chain. Its location is shown in figure 1. The locations of 

the two drill holes are described by Ladd, Tracey, and 

Gross (1969) and are shown in figure 2. The hole drilled 

on the reef is known as the Reef hole, and that on Sand 

Island as the Sand Island hole.

Below 1,261 feet in the Reef hole and 516 feet in the 

Sand Island hole, the Midway cores are entirely of 

altered basalt lava flows. The basalts are overlain by 

detrital sedimentary rocks that contain much fragmental volcanic material. In the Reef hole, about 1 foot 

of basalt conglomerate rests on the upper surface of the 

lava flows; in the Sand Island hole 9 feet of material 

that directly overlies the lava flows consists of loose 

fragments of highly weathered basalt and is apparently 

regolith.

The lava cores have been examined both megascopically and microscopically in the laboratories of the Institute of Geophysics, University of Hawaii. Through 

the kindness of D. M. McGuire, of the Institute of 

Geophysics, I have also been able to examine thin sections of the volcanic pebbles from the sediments above 

the lavas. Only a few brief comments on the pebbles are 

given here because the detrital sediments are being 

studied in detail by McGuire and Ralph Moberly, Jr., 

University of Hawaii.

In studying the lava cores, two main concerns have 

been whether the flows were subaerial or submarine and 

whether the rocks are tholeiitic or alkalic. If the flows 

were erupted subaerially or in very shallow water their 

present position indicates that the volcanic mass has 

sunk relative to sea level. This confirms the general hypothesis of subsidence of the volcanic mountains in the 

mid-Pacific and adds to the evidence of subsidence found 

in the sedimentary rocks.

In the major islands of the Hawaiian chain, the 

greater part of the shield volcanoes consists of tholeiitic 

rocks, which are overlain by relatively small volumes of 

alkalic rocks (Powers, 1955; Kuno and others, 1957; 

Macdonald and Katsura, 1964). The same sequence is

Bl
B2

30°

180°

GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

175° 170° 165° 160° 155°

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100 300 400 500 MILES

FIGURE 1. The Hawaiian chain, showing the three major physiographic subdivisions (bounded by dashed lines). From

Ladd, Tracey, and Gross (1969).

28°177°31' 28 21 ' "

19' 177"13'

5 KILOMETERS

CONTOURS IN FATHOMS

FIGURE 2. Midway atoll, showing the location of the Reef (R) 

and Sand Island (S) drill holes.

presumed to be present in other mid-Pacific islands, but 

this has not actually been demonstrated. In the Galaposos Islands, the active shield volcanoes are erupting 

tholeiitic lavas, whereas the older volcanoes consist of 

alkalic rocks (Williams, 1965), but tholeiitic rocks have

not yet been found underlying alkalic rocks in the same 

volcano. Tholeiitic rocks have not been identified in any 

other volcanic island of the Pacific, though they may be 

present in the submerged parts of the volcanoes (Macdonald, 1968a), and they seem to be widespread on the 

deep ocean floor (Engel and others, 1965). In the Indian 

Ocean, rocks transitional between tholeiitic and alkalic 

types have been found beneath typical alkalic rocks in 

the dissected volcano, Piton des Neiges, on Reunion Island (Upton and Wads worth, 1966). In the Atlantic 

Ocean, tholeiitic rocks have been dredged from the lower 

slopes of the Mid-Atlantic Ridge (Engel and Engel, 

1964), though lavas in the parts of the ridge that is above 

sea level are alkalic. The finding of tholeiitic basalts 

overlain by alkalic rocks in Midway extends the known 

distribution of tholeiitic rocks in the Hawaiian Ridge, 

in both time and space, and strengthens the general 

petrologic hypothesis stated above.

MEGASCOPIC FEATURES

The basaltic lava flows in the Midway cores range 

from medium to dark greenish gray and contain a few 

reddish- or yellowish-brown zones. Altered phenocrysts 

of olivine are recognizable in most samples, and in some 

they composed as much as 20 percent of the rock. Megascopically, most of the rocks more closely resemble 

typical tholeiitic lavas than alkalic lavas of the major 

Hawaiian Islands.
PETROLOGY OF BASALT CORES, MIDWAY ATOLL B3

Both pahoehoe and aa flows are present. The central 

parts of aa flows are massive and dense, and core recovery from them was nearly 100 percent. Core recovery 

from clinker parts of aa flows was much less. The clinker 

fragments are generally more highly altered than the 

massive parts of the flows and commonly are soft enough 

to be crushed between one's fingers. Spaces between the 

fragments that were originally open have been partly 

or completely filled with secondary material clay, 

chlorite, serpentine, and, less commonly, calcite and 

zeolite.

Pahoehoe flows range from moderately to highly vesicular. The vesicles usually are partly or wholly filled 

with serpentine and chlorite or, more rarely, with calcite 

and zeolite, but in some zones even at the deepest levels 

the vesicles remain mostly open.

Zones of brown and red soil are present in the Eeef 

hole between 1,316 and 1,318 feet and between 1,334 and 

1,336 feet. Between 1,309 and 1,311 feet, the vesicular 

basalt and clinker are stained by red oxides, which 

probably formed during subaerial weathering. Between 

1,633.0 and 1,633.6 feet the basalt is altered to red clay, 

in which slickensided shear surfaces bear flecks of metallic copper. The underlying 1.5 feet of basalt is somewhat reddened.

Detailed logs of the cores are given in table 1, and the 

general relationships of the rocks are shown in figure 3.

No pillow structures were found in the Midway cores; 

there was neither any hyaloclastite, resulting from 

granulation of the hot lava in contact with water, nor 

tuff, resulting from explosions caused by hot lava coming in contact with water. The vesicularity of the rocks 

covers the same range as that found in lavas erupted 

above sea level in the Hawaiian and Samoan Islands. 

As at Sylvania Seamount and Eniwetok (Macdonald, 

1954, 1963), the lavas probably were not erupted far 

below sea level, and they very likely were erupted above 

sea level. This conclusion is supported by Moore's (1965) 

findings on the submerged slopest)f Hawaiian volcanoes, 

where the abundance and size of vesicles in the lavas 

decrease with increasing depth of water until at depths 

greater than 12,000 feet the rocks are essentially devoid 

of visible vesicles. The theory that the Midway flows 

are of subaerial origin is supported by the presence 

of soil zones and reddened oxidized zones that could 

not have been formed below sea level. The conclusion is 

inescapable that the lavas now at depths as great as 

1,654 feet below sea level at Midway were erupted near 

or above sea level.

MICROSCOPIC FEATURES

Thin sections were cut from all the fresher appearing 

parts of the lava flows in the cores. The friable highly

DEPTH _ . . , 

METERS FEET Reef hole 

-T SEA LEVEL -j-[

Sand Island hole

-- 500

300-

400-

NC

NC

/ EXPLANATION

/

No core

FIGURE 3. Logs of the two Midway drill holes, showing 

the basaltic parts of the cores and zones of soil and regolith in relation to the sedimentary rocks. Parts 

marked no core are those in which a rock bit was used 

instead of a core bit.

decomposed parts were not sectioned. Twenty-six thin 

sections of lava flows from the Eeef hole and four from 

the Sand Island hole were examined under the microscope. In addition, about 100 thin sections of pebbles 

from the sediments above the lava flows were scanned, 

and 18 studied in detail.

Feldspar compositions were determined mostly by 

extinction-angle methods, these were checked occasionally by optic axial angle and sign, and, in a few samples, by refractive index determined in immersion 

liquids. The optic axial angle in pyroxenes and feldspars was estimated from the optic axis or nearly centered acute bisectrix interference figures.
B4 GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

TABLE 1. Logs of basalt cores from the two drill holes on Midway

Depth (ft) Description

Beef hole

0-1096...._ Reef limestones. 

1096-1260. _. Sedimentary materials.

1260-1261.. _ Basalt conglomerate.

1261-1266..... Partly altered, moderately vesicular to dense, olivine basalt.

1266-1270__. Similar to above but more altered; contains clinkery zones at 1266-1267 

and 1269.7-1270 ft. At 1267.5-1268 ft, a former cavity is filled with 

brown clay.

1270-1271.4.... Clinkery olivine basalt, like above.

1271.4-1273.3.. Moderately vesicular, partly altered basalt.

1273.3-1275.... Clinkery basalt, altered.

1275-1277_.. Same as above; cavities partly filled with clay.

1277-1281.2.... Moderately vesicular pahoehoe basalt, partly altered.

1281.2-1285.... Clinkery basalt; cavities partly filled with calcite and zeolite.

1285-1286.5.... Moderately vesicular basalt, much altered.

1286.5-1290.... Clinkery basalt, much altered; bottom 2 in. reddened.

1290-1293.. _ Much altered clinkery basalt.

1293-1295..... Moderately to highly vesicular pahoehoe basalt, partly altered.

1295-1295.7.... Sparingly vesicular, partly decomposed basalt.

1295.7-1299_ Moderately vesicular basalt that contains scattered phenocrysts of 

olivine, highly altered.

1299-1302._. Sparingly vesicular basalt; less altered than above; many vesicles 

filled with calcite and zeolite (?).

1302-1304..... Moderately vesicular altered basalt.

1304-1308.5.... Sparingly vesicular altered basalt.

1308.5-1309.5.. No core.

1309.5-1310_ Sparingly to moderately vesicular altered basalt, slightly reddened.

1310-1310.7.... Clinkery basalt, reddened and much decomposed.

1310.7-1313.5.. Clinkery basalt, decomposed, reddened at top.

1313.5-1315.7.. Moderately vesicular olivine basalt, decomposed.

1315.7-1316.3.. No core.

1316.3-1317.7.. Brown soil.

1317.7-1318.1.. Red soil.

1318.1-1321.3.. Much decomposed, gray to brown or red, basalt clinker(?).

1321.3-1326.3.. Same as above; resembles subsoil.

1326.3-1334.4.. Same as above.

1334.4-1336.3.. Basaltic soil, reddened at top.

1336.3-1398.5.. No core.

1398.5-1413.5.. Moderately to sparingly vesicular olivine basalt, moderately decomposed.

1413.5-1418.5.. Same as above, but middle 2 ft somewhat clinkery.

1418.5-1591-.. No core.

1591-1597.2.... Moderately to sparingly vesicular basalt, moderately decomposed.

1597.2-1599.3.. Clinkery basalt.

1599.3-1606.... Moderately vesicular basalt, slightly decomposed.

1606-1611..... Alternating dense and vesicular basalt, moderately decomposed.

1611-1616..... No core.

1616-1618..... Clinkery basalt, reddened.

1618-1633..... Sparingly vesicular to dense basalt, somewhat decomposed.

1633-1633.6_ Basalt altered to red clay; flee s of metallic copper on slickensided 

shear surfaces.

1633.6-1635.... Somewhat reddened altered basalt.

1635-1636..... Sparingly vesicular basalt that contains abundant altered phenocrysts 

of olivine.

1636-1641..... Moderately to highly vesicular basalt pahoehoe; many altered phenocrysts of olivine.

1641-1646..... Same as above, but a zone from 1641.7 to 1642.8 ft contains abundant 

vesicles filled with secondary minerals, whereas in the rest of this 

interval the vesicles are mostly open.

1646-1649.5.... Sparingly vesicular basalt, locally clinkery; contains only scattered 

phenocrysts of altered olivine.

1649.5-1652.... Reddened altered basalt.

1652-1654.. Sparingly vesicular basalt, locally clinkery.

Sand Island hole

0-501.........

501-503.2....

503.2-504.5.. 

504.5-506.8. .

506.8-507.... 

507-516_...

Reef limestones.

Brownish-green clay, calcareous; contains shell fragments.

Like above, but darker brown and without shell fragments.

Like above, but greener and contains fragments o. dark-brown rotted

vesicular basalt.

One fragment of partly altered dense basalt. 

Loose fragments of much weathered basalt.

TABLE 1. Logs of basalt cores from the two drill holes on Midway 

Continued

Depth (ft) Description

Sand Island hole Continued

516-520.3..... Core mostly lost. About 0.8 ft of material like above.

520.3-525..... Much altered, moderately vesicular olivine basalt; vesicles are largely

filled with secondary miner ils. 

525-530. -----. Much altered olivine basalt containing altered phenocrysts of olivine;

patches of bright green secondary material in a dark clay matrix

between 529 and 530 ft. 

530-534.4..... Moderately vesicular basalt, thoroughly altered, though somewhat

less so than the rock between 525 and 530 ft; vesicles are partly filled

with yellowish-green clay. 

534.4-538...- No core. 

538-541.6..... Altered olivine basalt; the bottom 6 in. less altered than the rest;

resembles the rock between 530 and 534.4 ft.

541.6-558-.... A few fragments of altered vesicular basalt, from core catcher. 

558-564...--.- Moderately vesicular basalt; vesicles are filled with secondary chlorite,

and abundant olivine phenocrysts are altered to serpentine; some

vesicles and fractures are filled with white calcite and zeolite(?). 

564-567. ----- Moderately vesicular, slightly porphyritic, altered basalt. 

567-568. ___ Dense, somewhat altered basalt.

All samples of the lava flows examined under the 

microscope are very similar. All are olivine basalts, and 

the chief variation is in the abundance of olivine phenocrysts. The phenocrysts are as much as 5 mm long, but 

most are less than 1 mm long; they grade in size into 

the groundmass. Many are rounded and embayed as a 

result of resorption, but others remain sharply euhedral. 

Skeletal crystallization is fairly. common. The larger 

phenocrysts are altered around the edges to iddingsite, 

or less commonly to goethite(?), and in the center to 

serpentine (antigorite?). The microphenocrysts and 

olivine grains of the groundmass commonly are altered 

throughout to iddingsite. Thin rims of serpentine surround the iddingsite on a few of the phenocrysts and 

apparently represent a second generation of olivine deposited after the alteration of the outer part of the 

phenocryst to iddingsite. Similar rims of late-formed 

olivine are common in Hawaiian olivine basalts. In 

those rocks the iddingsite is believed to have formed 

just before and during eruption, when gases were concentrated in the magma, and, additional olivine was precipitated during crystallization of the groundmass after 

eruption (Macdonald, 1949, p. 1545). Olivine in the Midway lava flows is completely altered, but in some pebbles 

in the overlying gravels part of the olivine remains unaltered. In one such pebble, the texture resembles that 

of the lava, flows, and the olivine has +2V *» 85°.

Small phenocrysts and microphenocrysts of augite 

(+ 2V=55°-60°) are present in a few rocks but are not 

common; feldspar phenocrysts more than 0.3 mm long 

are even rarer. Both grade in size into the groundmass. 

The groundmass is generally intersertal, less commonly 

intergranular, and is composed of plagioclase, altered 

olivine, monoclinic pyroxene, magnetite, ilmenite, and
PETROLOGY OF BASALT CORES, MIDWAY ATOLL B5

usually interstitial serpentine. The serpentine probably 

is an alteration product of original interstitial glass. 

Plagioclase ranges from intermediate labradorite to 

calcic or intermediate andesine. Most grains are subhedral, and many show normal zoning. Some anhedral 

interstitial grains are present, and these commonly are 

andesine. No alkalic feldspar or silica minerals were 

recognized.

In some rocks, particularly in the upper part of the 

section, the feldspar is partly altered to clay minerals. 

The groundmass olivine may be completely altered to a 

reddish-brown substance that resembles iddingsite but 

is probably a mixture of goethite and clay minerals or is 

altered partly to this mixture and partly to serpentine. 

Rarely, the olivine has altered completely to serpentine. 

No fresh olivine or fresh glass was found in any of the 

lava flows. Both the serpentine which replaces olivine 

and the interstitial serpentine appear to be antigorite.

Most large grains of groundmass pyroxene are augite 

(+2V=50°-60°), but in a few specimens some grains 

have an optic axial angle near 0°. In other rocks some 

small grains also appear to have a small 2V, but good 

interference figures could not be obtained on them. 

Whether these small grains are augite or pigeonite remains in doubt. No orthorhombic pyroxene was found. 

The pyroxene usually appears to be completely fresh, 

but in a few slides it is slightly altered to what is apparently very fine grained serpentine.

The vesicles are partly or completely filled with 

fibrous secondary minerals, which commonly show concentric banding. In some vesicles the material seems to 

be antigoritic serpentine throughout, but more commonly a band of serpentine (y a « 0.005) is succeeded 

inward by a mineral that has a higher birefringence 

(« 0.009), perhaps a chlorite. Fibers of both minerals 

show positive elongation. Some vesicles have a very thin 

outer band of isotropic serpophite, within which are 

the bands of serpentine and chlorite (?). No quartz and 

very little zeolite and calcite were recognized in the 

vesicles of the lava flows, though zeolite and calcite are 

commonly present in pebbles in the overlying gravels.

Skeletal development of olivine phenocrysts and continuation of crystallization of olivine into the groundmass generation are characteristic of alkalic olivine 

basalts; however, though far less common, these features 

are also found in tholeiitic olivine basalts. Resorption of 

olivine phenocrysts is characteristic of tholeiitic basalts, 

though it has also been found in some alkalic basalts 

(Macdonald and Katsura, 1964, p. 91). Groundmass 

pigeonite is characteristic of tholeiitic basalts, but augite 

also is common, particularly among the large grains.

However, clinopyroxene that has a small 2V also occurs 

in alkalic basalts (though it may be an augite rich in 

ferric iron rather than a true pigeonite), so that the 

occurrence of pyroxene with small 2V is not in itself 

wholly diagnostic of tholeiitic basalt (Macdonald and 

Katsura, 1964, p. 90). Interstitial feldspar more sodic 

than labradorite is characteristic of alkalic basalts but 

is found also in tholeiitic basalts (Wentworth and 

Winchell, 1947, p. 66). Thus, the microscopic criteria are 

not wholly adequate to identify the lava flows in the 

Midway cores as definitely tholeiitic or alkalic.

Among the pebbles in the overlying sedimentary section, several are composed largely of tiny acicular microlites of oligoclase or andesine arranged in the very 

pronounced flow pattern that is characteristic of 

mugearites and hawaiites. Two pebbles contain large 

flakes of brown biotite, and one contains large grains of 

dark apatite similar to those observed in several samples 

of mugearite from the major Hawaiian Islands and in 

similar lavas of Mount Etna, Sicily. These pebbles are 

unmistakably mugearites. The texture of other pebbles 

that contain abundant tiny grains of olivine in the 

groundmass very closely resembles that of typical alkalic 

olivine basalts. The presence of the mugearites definitely 

indicates that the upper part of the Midway volcano 

contained alkalic lavas.

Except in the soil zones and reddened oxidized zones 

that probably resulted from normal subaerial weathering, the alteration of the Midway rocks differs from any 

alteration found in rocks of the major Hawaiian Islands. Serpentinization, even of olivine, is very rare in 

weathered subaerial lavas in Hawaii, where the serpentinization of glass and filling of vesicles with serpentine 

are unknown. Alteration of lavas in the caldera areas of 

the Koolau volcano, Oahu, and the East Molokai volcano 

superficially resembles the alteration of the Midway 

rocks but differs markedly from it in detail. Whereas the 

pyroxene in the Midway rocks is almost unaltered, that 

in the caldera rocks, altered by rising volcanic gases, 

has been almost wholly converted to chlorite. Olivine in 

the caldera rocks has been altered to chlorite, not to 

serpentine as at Midway. Epidote is fairly common in 

the caldera rocks, and silica released by the alteration of 

pyroxene to chlorite has been redeposited as quartz 

and chalcedony in veinlets and amygdules; but no 

epidote and free silica have been found in the Midway 

rocks. The alteration of the Midway rocks apparently 

took place under conditions different from those affecting the subaerial lavas of the major Hawaiian Islands 

and probably resulted from long saturation of the rocks 

with sea water.

351-473 O 69-
B6 GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

CHEMICAL ANALYSES

Most Midway lavas are so altered that chemical 

analyses are of little value in determining the original 

composition of the rocks. Analyses of two of the least 

altered parts of the cores, from depths of 1,401.3 and 

1,600.2 feet in the Keef hole, r, are given in tables 2 and 

3. It is immediately obvious from the large water content 

and the high degree of iron oxidation that even these 

samples are considerably altered; the sample from 1,401 

feet is more altered than the deeper sample. CIPW 

norms calculated from the analyses also are given in 

table 2. Both norms contain excess silica, which suggests 

that the rocks are tholeiitic. The high degree of iron 

oxidation greatly increases the amount of free silica 

in the norm, however, and even markedly alkalic rocks, 

if they are highly oxidized, may show quartz in the 

norm.

To minimize the effects of alteration on the norm, the 

analyses have been recalculated (Ib and 2b, table 2) 

water free and with the Fe2O3 /FeO ratio reduced to 0.2, 

which is the average ratio found in fresh Hawaiian 

tholeiitic basalts. The average Fe2O3 /FeO ratio is higher 

in alkalic rocks than in tholeiitic rocks, but assumption 

of the lower ratio has the greatest possible effect in reducing the amount of quartz in the norm and, in this 

respect, reducing the tholeiitic aspect of the norm. Note 

that even with this extreme assumption the recalculated 

norms (table 2) still contain quartz and abundant hypersthene. Thus the norms of the recalculated analyses 

are also decidedly tholeiitic.

Some leaching of the alkalies, particularly sodium, 

probably occurred during alteration of the rocks, but 

there does not seem to be an adequate way of estimating 

the exact amount. Although potassium also has no doubt 

been liberated during alteration of the feldspars, most 

has probably been retained in the secondary alteration 

minerals, and the amount of potassium in the total rock 

may remain little changed. The potassium content of 

the Midway basalts is within the normal range for Hawaiian tholeiites, but it is decidedly lower than that for 

Hawaiian alkalic rocks (fig. 4). In the less altered specimen, from the 1,600-foot depth, the sodium content is 

within the normal range for Hawaiian tholeiites but 

much below that for the alkalic rocks (fig. 4).

Titanium oxide content is somewhat higher than that 

in most Hawaiian tholeiites but falls within the range 

found in those rocks (fig. 4); it is near the average for 

alkalic basalts (Macdonald, 1968b). Titania content of 

tholeiitic basalts varies considerably from one volcano

to another, and that of the fresher Midway sample (Ib, 

table 2) is nearly the same as the average for Kilauea 

volcano (Macdonald and Katsura, 1964, table 9, p. 124) . 

Thus, although the titania content in the Midway 

samples favors an alkalic interpretation for these basalts, it is indeterminate.

TABLE 2. Chemical analyses and CIPW norms of Midway 

basalts 

[Results in weight percent. Analyst: Vertie C. Smith]

la 2a Ib 2b

Major oxides

SiO2 ___ _-_________--__

AUOa ---_---_._----

Fe2O3 _ ___--._-_-----

FeO_________ -_______-

MgO___ ..._________..

CaO__-------------

Na8O_ ____-__-----_

K2O____ ._________..__

H20+_ ------------

H2O--_____. ___._---.

TiOi---_____-__-_ 

P_0,__-__________ _

MnO ___-___.__-_-_

C02-- ---------

Cl___ _____________

F___-___________-----

Less O_ --------

Total __ -------

-------- 46. 21

--_.____ 14. 67

--_.__-_ 7.36

----.-__ 5.04

-------- 5.96

_.___.__ 9.52

__._.___ 2.38

___-_-_- .36

________ 1.52

-_-___-_ 3.39

-__-___. 2.78

-------- .26

---.____ . 18

-__---._ .04

-------- .24

-------- .03

_-___._- 99. 94

-_-.___- .07

________ 99.87

41.79 

12.78 

10.72 

2.32 

8. 98 

6. 63 

1. 29 

. 25 

3. 83 

7.78 

2. 63 

.23 

. 16 

.05 

. 15 

.02

99. 61 

. 04

99. 57

48.9 

15. 6 

2. 1 

10. 4 

6.3 

10. 1 

2.5 

.4

2. 9 

.3 

. 2

.3

48.0 

14. 8 

2. 3 

11. 5 

10.3 

7.6 

1.5 

. 3

3. 0 

.3

. 2

. 2

100. 0 100. 0

CIPW norms

Q.._. ________________

or ___ _ __________ __

ab___ ____ _ __ _____

an___ __ __ _ ____ __

di:

pit

fB_ _ ______- 

hy: 

en__ _____ ___

fS--_--__-___--_-

mt_ ____ ______ _ _

il_-_____._-----_-_-_-

-____.__ 6.24

-------- 2.22

______-- 19.91

________ 28.36

-_--_-_- 7.08

.___--._ 6.10

--._.__. 8.80

________ 8.82

________ 1.28

_____--- 5.32

-.___--_ .67

6.96 

1.67 

11.00 

28.08

1. 16 

1. 00

21.40

. 23 

10. 56 

5.02 

.67

.8 

2. 2 

21. 0 

30.3

7.4 

4. 0 

3.2

11.8 

9.8 

3.0

5. 5

.7

la. Basalt from 1,600.2 ft in the Reef hole. 

2a. Basalt from 1,401.3 ft in the Reef hole. 

Ib. Sample la recalculated to 100 percent, water free and with Fe2Os/FeO 

reduced to 0.2. 

2b. Sample 2a recalculated in same way as sample Ib.

1. 3 

1.7 

12. 6 

32.8

1.3

.8 

. 4

25. 0 

14.3 

3.2

5.8 

.7

ratio
PETROLOGY OF BASALT CORES, MIDWAY ATOLL B7

TABLE 3. Abundance of minor elements in Midway basalts

[Sample 1 is from 1,600.2-foot depth, and sample 2 is from 1,401.3-foot depth, in the 

Reef hole-^-the same as those in table 2. Results are in parts per million. Analyst: 

R. H. Heidel. Elements looked for but not detected: Ag, As, Au, Be, Bi, Cd, Ce, 

Ge, Hf, Hg, In, La, Li, Mo, Pb, Pd, Pt, Re, Sb, Sn, Ta, Te, Th, Tl, U, W, Zn. Nb is 

present, but below the limit of measurement]

B______________ (i) 50 Sc-___-____--_. 30 30

Ba_________ 70 20 Sr___________ 300 100

Co_____________ 70 70 ¥._____________ 500 200

Cr____.________ 150 700 ¥______________ 50 50

Cu_-__-___-_.__ 150 150 Yb_________.___ <5 <3

Ga-_______-____ <30 <30 Zr_____________ 150 100

Ni_____________ 100 300

1 Looked for but not detected.

The strontium and zirconium contents of the Midway rocks are much lower than the normal range for 

alkalic basalts but are well within the range observed for 

tholeiitic basalts (fig. 4). Indeed, the zirconium content 

is low even for the tholeiitic rocks; in Hawaiian soils 

produced by subaerial weathering, however, sufficient 

zirconium is usually retained to serve as a good indication that the' soils were derived from alkalic basalt 

(Kimura and Swindale, 1967). The abundance of copper is similar to that in Hawaiian tholeiitic basalts but

much greater than that normally found in alkalic basalts 

or related alkalic rocks (fig. 4).

Thus, most data from the chemical analyses indicate 

quite definitely that the samples analyzed are 

tholeiitic.

CONCLUSIONS

In the cores from Midway atoll, altered basalt lava 

flows occur beneath the sedimentary caprock. The lower 

part of the basalt section contains tholeiitic olivine basalt. Alkalic olivine basalts may be associated with the 

tholeiites higher in the basalt section, and mugearites 

definitely are present, as indicated by pebbles of this 

composition in the overlying sedimentary rocks. The 

lava flows are ordinary pahoehoe and aa, have the normal vesicularity of subaerial flows, and were erupted 

above sea level or in very shallow water. If the lavas 

were erupted in shallow water, they were at times elevated above sea level for periods long enough to permit 

the formation of soil on them by subaerial weathering. 

The present position of the lavas must have resulted 

from subsidence of the volcanic mountain. The pervasive 

alteration of the rocks is the result of long saturation by 

sea water.
B8 GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

500

300

100

1100

900

700

500

300

100

200

100

0 -

44 45

X X 

X

XX

x x

X X

X X

X X

*

X v xx x

46 47 48 49 

PERCENT SiO2

50 51 52

FIGURE 4. The abundance of alkalies, titania, copper, strontium, and zirconium in relation to 

silica in Hawaiian tholeiitic and alkalic rocks and in the analyzed samples from the Midway 

basalts. (Data on alkalies and titania in Hawaiian rocks from Macdonald and Katsura, 

1964, and Macdonald, 1968b; on strontium and zirconium from Hubbard, 1967, and Macdonald and Eaton, 1964; on copper from Herlicska, 1967, and Macdonald and Baton, 1964, p. 87.) 

 , Hawaiian tholeiitic basalts; x, Hawaiian alkalic basalts and hawaiites; A> Midway 

samples.
PERCENT

*'

CJ1

ft

^

Tl

m

70

o m .i*

z oo

H

£fi

O

10

S

CJl

CJ1

K2O Na 20 

3 !- IS3IS3 W *> 

| , , , |

X

-

X

X

X X

x x

 x

 ^ ^ x x x

 x x

, * * x x x

1 X

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.. ?" 

^

» .% 

i

-"... ^

X

* 

..* *

 X

! 

l*".". 

^

 

, " , , 1 , , , 1

1 ' 1 ' ' ' 1 ' ' ' 1 ' '

X

X

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X ^

x x

X

X

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" .. X x ^ x

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 x 

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X

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1 1 1 , , , 1 I , , 1
BIO GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

REFERENCES CITED

Engel, A. E. J., and Engel, C. G., 1964, Compositions of basalts 

from the Mid-Atlantic Ridge: Science, v. 144, p. 1330-1333.

Engel, A. E. J., Engel, C. G., and Havens, R. G., 1965, Chemical 

characteristics of oceanic basalts and the upper mantle: 

Geol. Soc. America Bull., v. 76, p. 719-734.

Herlicska, Edward, 1967, Selected trace elements in Hawaiian 

lavas by atomic absorption spectrophotometry: Honolulu, 

Hawaii Univ. unpub. Ph. D. thesis, 253 p.

Hubbard, N. D., 1967, Some trace elements in Hawaiian lavas: 

Honolulu, Hawaii Univ. unpub. Ph. D. thesis, 123 p.

Kimura, H. S., and Swindale, L. S., 1967, A discriminant function using zirconium and nickel for parent rocks of strongly 

weathered Hawaiian soils: Soil Sci., v. 104, p. 69-76.

Kuno, Hisashi, Yamasaki, Kazuo, lida, Chuzo, and Nagashima, 

Kozo, 1957, Differentiation of Hawaiian magmas: Japanese 

Jour. Geology and Geography, v. 28, p. 179-218.

Ladd, H. S., Ingerson, Earl, Townsend, R. C., Russell, Martin, 

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Ladd, H. S., Tracey, J. I., Jr., and Gross, M. G., 1967, Drilling on 

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1094.

 1969, Deep drilling on Midway atoll: U.S. Geol. Survey

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 1954, Igneous rocks [of Sylvania Guyot], in Emery, K. 

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p. 120-124.

 1963, Petrography of the basalt beneath the limestones, 

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U.S. Geol. Survey Prof. Paper 260-BB, p. 1038-1048.

 1968a, A contribution to the petrology of Tutuila, American Samoa: Geol. Rundschau, v. 57, p. 821-837.

 1968b, Composition and origin of Hawaiian lavas: Geol.

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Macdonald, G. A., and Eaton, J. P., 1964, Hawaiian volcanoes

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Macdonald, G. A., and Katsura, Takashi, 1964, Chemical composition of Hawaiian lavas: Jour. Petrology, v. 5, p. 82-133. 

McBirney, A. R., and Williams, Howel, 1966, Volcanic rocks of

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Moore, J. G., 1965, Petrology of deep-sea basalt near Hawaii:

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Powers, H. A., 1955, Composition and origin of basaltic magma

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Upton, B. G. J., and Wadsworth, W. J., 1966, The basalts of R6-

union Island, Indian Ocean: Bull. Volcanol., ser. 2, v. 29,

p. 7-23. 

Wentworth, C. K., and Winchell, Horace, 1947, Koolau basalt

series, Oahu, Hawaii: Geol. Soc. America Bull., v. 58 p.

49-78. 

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188-189.

US. GOVERNMENT PRINTING OFFICE: 1969 O 351-473