Potassium-Argon Ages from Lavas of the Hawaiian Islands

Geological Society of America Bulletin

doi: 10.1130/0016-7606(1964)75[107:PAFLOT]2.0.CO;2 1964;75, no. 2;107-128Geological Society of America Bulletin

IAN McDOUGALL Potassium-Argon Ages from Lavas of the Hawaiian Islands

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IAN McDOUGALL Dept. Geophysics, Australian National University, Canberra, A.C.T., Australia

Potassium-Argon Ages

from Lavas of the Hawaiian Islands

Abstract: Thirty six samples of lava from seven ofthe Hawaiian volcanoes have been dated by theK-Ar method. The results show that the order ofextinction of the volcanoes, excluding minoractivity after the main shield-building phase wascompleted, occurred along the island chain fromnorthwest to southeast. All the lavas are latePliocene and Pleistocene, excepting the MaunaKuwale trachyte, which is early to middle Pliocene.

The exposed parts of the volcanoes dated werebuilt in less than 0.5 m.y., with the exception of theWaianae volcano of West Oahu, which was activeover more than 0.7 m.y., and Kauai, which wasactive over more than 1.8 m.y. The thin veneerof alkali lavas of the main shield-building phasegenerally is less than 0.2 m.y. younger than theunderlying "primitive" basaltic lavas, of which theHawaiian volcanoes are mainlv built.

CONTENTS

Introduction 107General 107Geological time scale 109

Acknowledgments 109Experimental methods 109

Sample selection 109Sampling and sample preparation 109Argon extraction and measurement 110Potassium analysis 110Precision and accuracy 110

Kauai IllOahu 113Molokai 116Maui 117Hawaii 118Migration of volcanic activity in the Hawaiian

island chain 119

Petrological considerations 121Summary and conclusions 122References cited 123Appendix 125

Figure1. Map of the Hawaiian Islands 108Table1. Simplified stratigraphic sequence on Kauai . . I l l2. Potassium-argon results on lavas from Kauai . 1123. Age results on lavas from Oahu 1144. Age results on lavas from Molokai 1165. Age results on lavas from Maui 1186. Order of extinction of Hawaiian volcanoes, ex-

cluding minor activity after main shield-building phase 120

INTRODUCTION

General

The Hawaiian Islands are at the southeasternend of a chain of volcanic islands that extendsover 1500 miles across the Pacific Ocean toMidway and Kure. Geological evidence sug-gests that volcanic activity moved progressivelytoward the southeast along a major rift zone.Mauna Loa and Kilauea, at the extreme south-east limit of the chain (Fig. 1), are activevolcanoes, whereas the other volcanoes con-stituting the Hawaiian Islands are extinct andgenerally are more deeply eroded toward thenorthwest. Little is known concerning the ageof the lavas or the time involved in buildingthese large shield volcanoes, which rise 16,000-30.000 feet above the surrounding ocean floor,

although estimates based on superposition andgeomorphic development have been made pre-viously by a number of workers.

Results are given here of ages determined bythe K-Ar method, mainly on whole-rock sam-ples of lava flows from several of the Hawaiianvolcanoes. Ages measured on rocks from theWaianae Range, West Oahu, were presentedearlier (McDougall, 1963a) and are includedhere for completeness. It was hoped that bydating lavas from the Hawaiian volcanoes themovement of volcanic activity along the islandchain could be demonstrated, and that in-formation as to the time involved in buildingthese volcanoes would be obtained. Many ofthe Hawaiian volcanoes are deeply eroded, sothat radiometric ages may provide the basisfor more precise estimates as to the rates of

Geological Society of America Bulletin, v. 75, p. 107-128, 1 fig., February 1964

107

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Figure 1. Map of the Hawaiian Islands



INTRODUCTION 109

erosion, a semiquantitative study of which wasmade by Wentworth (1927). Recent work hasshown that some Hawaiian lavas are normallymagnetized and others reversely magnetized(Tarling, 1962). Using the K-Ar data it ispossible to place age limits on the periods ofnormal and reversed magnetization (McDougalland Tarling, 1963).

The Hawaiian shield volcanoes are builtmainly of olivine basalt lavas ("primitive"basalts), which are thought to have a com-position approximating that of the undif-ierentiated parent magma of the Hawaiianpetrographic province (Macdonald, 1949).These lavas are considered to be mainly tho-leiitic (Tilley, 1950; Macdonald and Katsura,1961). Lavas of the alkali olivine series eruptedduring the declining stages of activity com-monly veneer the main basaltic shield (Mac-donald, 1949). Many of these late-stage lavashave been called andesine andesite or oligoclaseandesite; they, however, are quite distinct fromthe typical calc-alkaline andesites of continentalregions (Tilley, 1950; Macdonald, 1960). Inorder to minimize confusion Macdonald (1960)proposed the term hawaiite for the andesineandesitesand recommended the well-establishedterm mugearite for the oligoclase andesites.This terminology is followed in the presentpaper.

Because most of the lavas used in thisstudy are very fine-grained (<0.2 mm), theserocks may be expected to lose radiogenic argonby diffusion. However, the fact that the lavashave never been deeply buried or folded de-creases the possibility that argon loss hasoccurred. It will be shown that, with few ex-ceptions, the ages obtained are consistent withthe stratigraphic sequence. This suggests, butdoes not prove, that the lavas generally haveretained radiogenic argon quantitatively sub-sequent to their eruption and crystallization.

Geological Time Scale

For the purpose of comparison with previouswork it is necessary to refer to the geologicaltime scale. There is general agreement that theMiocene-Pliocene boundary is about 12-13m.y. (Evernden and others, 1961; Kulp, 1961).Because of difficulties in defining the Pliocene-Pleistocene boundary, no consensus as to itsage has been reached. Kulp (1961, p. 1109)assumes an age for this boundary of 1 m.y.,".. . with the understanding that this figuremay be subject to considerable revision."Evernden and others (1964) place an age ofabout 3 m.y. on the Pliocene-Pleistocene

boundary, as defined by the 18th InternationalGeological Congress (1950), on the basis ofpossible time equivalence of Villafranchianmammals with those of late Blancan age inNorth America. Owing to the uncertainty inthe age of the Pliocene-Pleistocene boundary,no clear statement can be made as to whetherrocks with an age between about 1 and 3 m.y.are Pliocene or Pleistocene. As the Pliocene-Pleistocene boundary probably is little morethan an arbitrary point in the geological historyof the Hawaiian Islands, the inability to stateprecisely which division a given rock orsequence belongs is not of great importance.

ACKNOWLEDGMENTSThe assistance given by Dr. D. H. Richter

and D. A. Davis of the U. S. Geological Surveywhile the writer was in Hawaii is gratefullyacknowledged. Dr. R. R. Doell first interestedthe author in attempting to date Hawaiianlavas. D. H. Tarling kindly provided the sam-ples from the Koolau volcanic series and severalsamples from the Waianae volcanic series.The assistance of R. T. Pidgeon in the statis-tical analysis of the results is acknowledged.Thanks are due to J. A. Cooper for his patienceand skill in carrying out the potassium analyses,and to Dr. D. H. Green for critically readingthe paper.

EXPERIMENTAL METHODS

Sample Selection

Specimens generally were collected from themassive lower parts of aa flows. A thin sectionof each rock was examined; any specimen thatshowed evidence of alteration other thanalteration of olivine (usually to iddingsite)was rejected. A negligible amount of potassiumoccurs in olivine, and the alteration to idding-site probably takes place during and immedi-ately after eruption. Wherever possibleamygdaloidal lavas were avoided, partlybecause of uncertainty as to the time of fillingof the vesicles, and partly because of samplingdifficulties for argon analysis. All the ages weremeasured on whole-rock samples, except forthe Mauna Kuwale trachyte, from whichbiotite was separated. Petrographic descrip-tions and the locations of the specimens aregiven in the appendix.

Sampling and Sample PreparationIn measuring K-Ar ages on young rocks it is

imperative to reduce argon contaminationoriginating from air to the lowest possible level



110 IAN McDOUGALL—K-AR AGES FROM LAVAS OF HAWAIIAN ISLANDS

in order to detect radiogenic argon. Althougha sample is heated under vacuum prior to theextraction of argon, not all the air is removed.Experience has shown that the air contamina-tion is dependent largely on the surface area;the smaller the pieces of lava used in the argonextraction the greater the surface area and thegreater the air correction. Hence, it is desirableto use as large fragments as possible. However,it is equally important to crush the specimento a size which allows a representative sampleto be obtained for argon and potassiumanalysis. The grain size of the individual min-erals in the Hawaiian volcanic rocks (exceptingthe small proportion present as phenocrysts)generally is less than 0.2 mm. In the majorityof cases the results show that an adequatesample could be obtained for argon extractionsby breaking the specimen into fragments about1 cm in diameter. After crushing to this size thesample was quartered; one quarter was groundto —100 mesh for potassium analysis, and theremainder was kept for argon extractions.From 10 to 15 g of the 1-cm fragments wasused for each argon run; sampling was byquartering.

Argon Extraction and Measurement

Argon extractions were made in a pyrex highvacuum line, which was baked overnight at250°-290°C. No evidence was found for loss ofradiogenic argon from the sample at thesetemperatures as replicate determinations atbakeout temperatures of 250° and 290°Cyielded results agreeing to better than 2 percent. Each sample was fused at about 1350°Cfor 15 minutes by induction heating. A tracerof Ar38 (—5 X 10-6 ccSTP) was added duringfusion, and purification of the evolved gaseswas made over hot CuO and Ca or Ti. Theargon was measured isotopically with a Rey-nolds mass spectrometer by the static method(Reynolds, 1956). The ratios Ar40/Ar38 andAr38/Ar36 at zero time were found by plottingthe ratios against time. Unless the plots werestraight lines and the uncertainty in the zerotime ratios was less than one half per cent a runwas rejected, as the majority of runs had a highair correction. A 1 per cent error in the correc-tion for atmospheric argon at the 80 per centair argon contamination level results in a 5 percent error in the calculated age, and at the90 per cent contamination level it results in a10 per cent error. Mass discrimination, de-termined by analyzing air argon in the spec-trometer, and orifice corrections were applied

to the zero time ratios. The correction for airargon was made by assuming the isotopiccomposition for air argon given by Nier (1950).Argon was extracted from two basalts of thePololu volcanic series of Kohala, Hawaii. Thepotassium content for each rock was about0.12 per cent. In both cases the argon wasfound to have a Ar40/Ar36 ratio within 1 percent of the value given by Nier for air argon,and the measured quantities of air argon were7.21 and 9.42 X 1Q-8 cc STP/g. These data,together with the consistency of the age re-sults, suggest that volcanic rocks contain noexcess radiogenic argon (Hart and Dodd, 1962)and that the air contamination is that ofpresent-day air adsorbed onto the surface of thesample. Even if some of the air was adsorbed atthe time of eruption of the lavas, or shortlyafter, the rocks are so young that the composi-tion of the argon would differ little from that ofpresent-day air argon. Calibration of the Ar38

tracer was made against air, and secondarystandards, including Massachusetts Institute ofTechnology biotite B3203.

The constants used in the calculations are:\f} = 4.72 X 10-10 yr^1; Xk = 0.584 X 1Q-10

yr-1; and K40 = 1.19 X 10~2 atom per cent.

Potassium Analysis

Potassium was determined flame photo-metrically by the method described by Cooper(1963). Each sample was measured in duplicate.Dissolutions were made with HF and H2SO4and diluted to a standard volume after addi-tion of Li (200 ppm) as an internal standard,and 500 ppm Na. The Na acts as a buffer andreduces interference effects of Al, Fe, Mg, andCa to a low level. The samples were comparedwith standards, which were buffered in thesame manner, on a Perkin Elmer flamephotometer using a propane-air flame.

Precision and AccuracyFor each of the 36 specimens dated potassium

analyses were made in duplicate, and argonanalyses were made in duplicate (and rarely intriplicate) on 26 of the samples. The argonmeasurements usually are reproducible tobetter than 2.5 per cent (See Tables 2-5), evenin those cases were the air correction is as highas 90 per cent. Replicates disagree by more than5 per cent in only two cases; this is probablythe result of inadequate sampling, particularlyfor GA 560, which is an amygdaloidal lava.Statistical analysis of argon measurements madein duplicate or triplicate show that the 95 per



EXPERIMENTAL METHODS 111

cent confidence level is + 2.6 per cent, andif uncertainties in the tracer calibrations areincluded, this figure is about +2.9 per cent.In these calculations the analyses of GA 560were omitted, owing to the unusually largediscrepancy between duplicates.

In all cases replicate potassium determina-tions agree to within 1 per cent; the 95 per centconfidence interval is +0.6 per cent. Fromprevious comparisons with isotope dilutionmeasurements (McDougall 1963b; Cooper,1963) the accuracy is probably better than+ 2 per cent.

all the rocks of the major shield-building phaseof Kauai. It is composed mainly of tholeiiticbasalts with minor hawaiite toward the top ofthe sequence. The Napali formation of theWaimea Canyon series constitutes the majorpart of the shield and consists of thin basaltflows that dip outward from the summit at lowangles. A large caldera formed at the summit ofthe Kauai shield volcano, and this was filled bymassive, thick flows of the Olokele formation.On the southwest side of the volcano a largegraben, which developed in the Napali forma-tion, was filled by lavas of the Makaweli for-

TABLE 1. SIMPLIFIED STRATIGRAPHIC SEQUENCE ON KAUAI(AFTER MACDONALD AND OTHERS 1960)

ThicknessUnits (feet)

Koloa volcanic seriesMAJOR EROSIONAL UNCONFORMITY

Waimea Canyon volcanic Makaweli formationseries Haupu formation

Olokele formationNapali formation

2000+

1500+1800+2600+2700+

Using the 95 per cent confidence levels forargon and potassium found on the whole arrayof measurements, and an approximation afterTaylor's Theorem to calculate the probableerror in the ratio radiogenic argon/potassium40, the 95 per cent confidence level for an agedetermined in duplicate is ±2.6 per cent. Inthese calculations uncertainties in the valuesof the decay constants are not taken into ac-count. In the text errors usually are not quoted,but the uncertainty of about + 3 per centshould be borne in mind. For those samples inwhich the air correction is greater than 85 percent a more realistic figure for the error proba-bly is in the range ±5 to + 10 per cent.

KAUAIThe geology of Kauai was described by

Macdonald and others (1960); the synopsis ofthe geology given herein is taken primarilyfrom this study.

Kauai consists of a single large shield volcanobuilt mainly of thin basaltic flows. The volumeof lava is estimated at about 1000 cubic miles.A simplified stratigraphic sequence, afterMacdonald and others (1960), is given inTable 1.

The Waimea Canyon volcanic series includes

mation. These lavas are thought to have formedduring the later part of the eruption of theOlokele formation by overflow from the cal-dera. The Haupu formation comprises thoserocks occurring in a small caldera in HaupuRidge, but as no specimens were collected fromthis formation, it will not be discussed further.

The Koloa volcanic series was erupted into alarge depression on the eastern side of theKauai shield. These lavas are separated fromthose of the Waimea Canyon series by a pro-found erosional unconformity, marked bycanyons up to 2500 feet deep. In contrast withthe underlying lavas, the Koloa volcanic seriesconsists of undersaturated alkalic lavas repre-sented by olivine basalt, nepheline basalt, mel-ilite-nepheline basalt, and basanite.

The K-Ar ages measured on lavas from Kauaiare given in Table 2. Rocks of the Napali for-mation yield ages ranging from 5.62 to 4.5 m.y.Sample GA 564, from which the older age wasobtained, was collected at an altitude of about250 feet on the south side of the shield andappears to be considerably lower stratigraph-ically than the other two samples, which werecollected high on the western rim of WaimeaCanyon. The age results support this view andsuggest that lavas of the Napali formation were



112 IAN McDOUGALL—K.-AR AGES FROM LAVAS OF HAWAIIAN ISLANDS

erupted over a period of at least 1 m.y.Macdonald and others (1960, p. 27) found noevidence for a major break in the sequence, andthey concluded that the lavas were eruptedrapidly. Hence, the relatively long period oferuption indicated by the K-Ar measurementsseems excessive, particularly as results fromother Hawaiian volcanoes show that generally

and demonstrate that it is younger than theNapali formation, in agreement with thegeological evidence. The results show that aperiod of 0.5 m.y. or less separates the Napaliand Makaweli formations. Specimen GA 565gives a reproducible age of 3.5 m.y., and GA563 and GA 650 yield ages agreeing at 3.95 +0.1 m.y. These three samples were collected in

TABLE 2. POTASSIUM-ARGON" AGE RESULTS ON LAVAS FROM KAUAI

Specimennumber

GA 642 (1)(2)

GA 566 (1)(2)(3)

GA 565 (1)(2)(3)

GA 650 (1)(2)

GA 563 (1)(2)

GA 561 (1)(2)

GA 649 (1)(2)(3)

GA 564 (1)(2)(3)

Potassiumanalyses

(wt.per cent)

0.517, 0.5210.517, 0.5211.805, 1.8051.805, 1.8051.805, 1.8050.320, 0.3210.320, 0.3210.320, 0.3210.211, 0.2120.211, 0.2120.189, 0.1900.189, 0.1900.131, 0.1320.131, 0.1320.138, 0.1380.138, 0.1380.138, 0.1380.140, 0.1400.140, 0.1400.140, 0.140

Averagepotassium

(wt.per cent)

0.5190.5191.8051.8051.8050.3200.3200.3200.2110.2110.1890.1890.1310.1310.1380.1380.1380.1400.1400.140

A40*/K40

8.34 x 10~5

8.422.22 x 10~4

2.272.242.052.042.072.302.262.362.372.662.622.772.622.603.303.243.34

Aircorrection(per cent)

8687545958899090908855477067838389878686

Calculatedage (m.y.)

1.411.433.803.883.843.503.483.533.933.864.044.054.554.474.724.484.445.625.525.72

Altitude Stratigraphic(feet) position

Koloa volcanic series

1400

550

450

400

2600

2400

Makaweliformation

WiameaCanyonvolcanicSeries

Napali |f formation '.

'250

J

*—radiogenic argon

the main shield-building phase occupied timesmuch less than 1 m.y. The possibility of loss ofradiogenic argon from GA 561 and GA 649relative to GA 564 therefore must be con-sidered. The fact that samples GA 561 andGA 649 give ages agreeing at 4.5 + 0.2 m.y.suggests that argon loss is unlikely, as the tworocks may be expected to lose argon at differentrates, owing to differences in composition andgrain size. Clearly more data are required toresolve this question, but the present limitedevidence favors the interpretation that noargon loss has occurred, and that the Napaliformation was erupted over a time intervalexceeding 1 m.y.

The measured ages from the Makaweli for-mation range from 3.5 to 4.0 m.y. (Table 2)

Waimea Canyon at Stratigraphic horizonsdiffering by no more than 150 feet. The resultsindicate that GA 565 has lost radiogenic argonrelative to GA 563 and GA 650. This conclu-sion is supported by the age of 3.84 m.y. forGA 566, which occurs about 850 feet higher inthe sequence than GA 565. Specimen GA 565in thin section is not altered, and glass is absent;hence, there is no obvious reason why this rockhas lost radiogenic argon. This is the only sam-ple found in the present study for which argonloss can be demonstrated. The small differencesin age between the several specimens (ex-cluding GA 565 from consideration) suggeststhat the lavas of the Makaweli formationwere erupted rapidly. In addition the dataindicate that the change in composition of the



KAL'AI 113

lavas from tholeiitic (GA 563, GA 650) toalkalic (GA 565) occurred without any majortime break.

Only one specimen from the Koloa volcanicseries was dated (GA 642); this gives an age of1.4 m.y. (Table 2). The maximum time in-terval between the eruption of the Makaweliand Koloa lavas is, therefore, about 2 m.y. TheKoloa lavas overlie the Makaweli formationonly locally, and in one place they occur about500 feet lower than adjacent Makaweli rocks(Macdonald and others, 1960, p. 55).

Most observers have regarded Kauai as theoldest of the Hawaiian shield volcanoes, andseveral estimates of its age have been made.Wentworth (1927, p. 132) calculated that thelatest lavas of the main shield-building phaseof Kauai are about 2 m.y. old, on the basis ofthe average rate of reduction of the land surfaceof Lanai by fluvial erosion. Hinds (1931, p. 200)pointed out that this age should be reduced as1 he rate of erosion on Kauai, where the rainfallis high, is much greater than that on Lanai, adry island. However, Macdonald and others(1960, p. 22, 23) showed that Wentworthunderestimated the amount of erosion onKauai, and by assuming Wentworth's figurefor the rate of erosion, they calculated that thelatest lavas of the Waimea Canyon volcanicseries are about 4 m.y. old. The youngestK-Ar age measured from the Waimea Canyonseries is 3.8 m.y. (excluding the result fromGA 565), which confirms the estimate based onimperfectly known rates and depth of erosionon Kauai.

Macdonald and others (1960, p. 23) showedthat the Kauai shield could have been builtin the very short time of 136,000 years, usingthe rate of historic eruption of lava on MaunaLoa and neglecting isostatic sinking. Allowingfor some isostatic compensation, some longerperiods of quiescence, and a somewhat lowerrate of eruption, they suggested that the build-ing of the Kauai shield probably took not morethan 2-4 m.y. The K-Ar data show that theminimum time interval for the eruption ofthe lavas of the main shield-building phasewas 1.8 m.y. This interval would be increasedconsiderably by the time required for buildingthe volcano from the sea floor to sea level, sothat the estimates of Macdonald and others arenot unreasonable. It will be shown that themain shield-building phase of the otherHawaiian volcanoes examined in this studyappears to be much shorter than that forKauai. In this regard it is worthy of note that

Macdonald and others (1960, p. 1) stated thatKauai is structurally the most complicated ofthe Hawaiian shield volcanoes. In accord withthe geological evidence the K-Ar data indicatean appreciable hiatus between the main shield-building phase and the eruption of the lavasof the Koloa volcanic series.

The lavas of the Waimea Canyon volcanicseries are late Pliocene, and those of the Koloavolcanic series are latest Pliocene to Pleisto-cene, in agreement with the views of Mac-donald and others (1960, p. 23).

OAHUThe synopsis of the geology of Oahu given

here was taken mainly from Stearns andVaksvik (1935) and Stearns (1939; 1940).Oahu consists of two large shield volcanoes;western Oahu was built by the Waianaevolcano and eastern Oahu by the Koolauvolcano. Both volcanoes are deeply eroded.Wide, flat-floored, amphitheater-headed val-leys up to 3000 feet deep, separated by steepridges, are particularly well developed on theeast and west coasts.

The lavas comprising the Waianae Range aredivided into the lower, middle, and uppermembers of the Waianae volcanic series. Eachmember is about 2000 feet thick. The lowermember consists mainly of thin tholeiiticbasalts, and in most places is separated from themiddle member by an angular unconformityand talus breccia. Lavas of the middle memberare thought to have accumulated in a caldera,and it is likely that the lower and middle mem-bers are, in part, contemporaneous (G. A.Macdonald, personal communication). Thelavas of the middle member are similar incomposition to those of the lower member, butthe flows are generally more massive andthicker. The middle and upper members of theWaianae series are conformable and are grada-tional one into the other. The upper member ismade up predominantly of thick, massive,alkali basalt and hawaiite flows. The MaunaKuwale trachyte, which is about 400 feetthick, was included in the Waianae series byStearns and Vaksvik (1935, p. 68). Petrolog-ically it appears to be related to the upper mem-ber of the Waianae series, although it lies farbelow the base of the upper member and isoverlain, to the west, by basalts mapped as be-longing to the lower or middle member of theWaianae series (Macdonald, 1940, p. 81).

Two groups of lavas are recognized in theKoolau volcano of eastern Oahu. The Kailua




volcanic series occurs only locally and probablyaccumulated in a caldera; the bulk of theKoolau Range was built by lavas of the Koolauvolcanic series. This latter series consistsmainly of tholeiitic basalts of fairly uniformcomposition (Wentworth and Winchell, 1947,

ness the results are repeated here, togetherwith results from three additional specimens(Table 3). Five ages measured on rocks fromthe Koolau volcanic series also are reported(Table 3). No measurements were made onrocks of the Kailua or Honolulu volcanic series.

TABLE 3. AGE RESULTS ON LAVAS FROM OAHU

SpecimenNumber

GA 643GA 644GA 645 (1)

(2)(3)

GA 646GA 647GA 553 (1)

(2)GA 556 (1)

(2)GA 809 (1)

(2)(3)

GA 554 (1)(2)(3)(4)

GA 560 (1)(2)

GA 557 (1)(2)(3)

GA 810 (1)(2)

GA 397 (1)(2)

Potassiumanalyses

(wt, per cent)

0.334,0.391,0.424,0.424,0.424,0.320,0.410,

.217,

.217,

.446,

.446,

.394,

.394,

.394,0.638,0.638,0.638,0.638,0.479,0.479,0.394,0.394,0.394,1.137,1.137,6.13,6.13,

0.3360.3930.4260.4260.4260.3200.4061.2191.2191.4481.4481.3941.3941.3940.6380.6380.6380.6380.4830.4830.3940.3940.3941.1381.1386.196.19

Averagepotassium

(wt. per cent)

0.3350.3920.4250.4250.4250.3200.4081.2181.2181.4471.4471 .3941.3941.3940.6380.6380.6380.6380.4810.4810.3940.3940.3941.1371.1376.166.16

Ar40*/K40

1.30 x 10-4

1.351.301.251.251.481.501.621.611.611.601.681.641.651.831.721.811.741.922.141.711.751.721.901.944.844.95


919185898791915856515643474176706664898569778775934147

Calculatedage (m.y.) Stratigraphic position

2.222.312.232.142.152.52

Koolau volcanic scries,East Oahu

2.56 j2.772.752.752.732.862.802.82

Uppermember

3.13]2.93 1 WaianaeT no volcanic3-08 1 Middle } .^- member i ... ' ~ ,3.273.652.922.982.943.24

I Wesr Uahu

Lowermember

3.318.26 \ Mauna Kuwale8.46 / trachyte

Ar40*—radiogenic argon N.B. GA 397 date is on biotite

p. 50), and the exposed thickness exceeds 3000feet. Lavas of the Koolau volcanic series inthe Schofield Plateau area overlap lavas of theWaianae volcanic series. After a long period oferosion lavas of the Honolulu volcanic serieswere erupted locally at the southern end of theKoolau Range. The volume of these lavas isextremely small in comparison with the volumeof the Koolau lavas.

A number of K-Ar ages measured on lavas ofthe Waiane volcanic series were publishedpreviously (McDougall, 1963a); for complete-

Lavas of the Waianae volcanic series yieldages ranging from 3.5 to 2.75 m.y. (Table 3).The large uncertainty in the measurementsof GA 560 is probably the result of samplingdifficulties, as the rock is amygdaloidal. SampleGA 557, a lava from the lower member of theWaianae series, gives a reproducible age of 2.95m.y.; this result is not significantly differentfrom the measured age of 3.0 m.y. for GA 554,a lava of the middle member. However, themeasured age of GA 557 is lower than that ofGA 560 (3.5 + 0.3 m.y.), which is from the



OAHU 115

middle member of the Waianae series. As thetwo members may be partly synchronous (G.A. Macdonald, personal communication) theages are not necessarily in disagreement withthe geological observations, but the possibilitythat some loss of radiogenic argon has occurredfrom GA 557 cannot be excluded. SpecimenGA 810 was collected at the southern end ofthe Waianae Range from the lower member ofthe Waianae series, adjacent to the mappedboundary between the lower and upper mem-bers. The rock is an alkali basalt, which suggeststhat it may be a lava of the upper member,but the measured age of 3.27 m.y. is consistentwith the view that it is from the lower member.Specimen GA 809 was sampled from a flow ofthe upper member, about 100 feet stratigraph-ically higher than GA 810, and yields an ageof 2.83 m.y. The other two samples from theupper member of the Waianae volcanic series(GA 556, GA 553) give ages of 2.75 m.y.

The age data suggest that the lavas of thelower and middle members of the Waianaevolcanic series are essentially contemporaneousand that the lavas of the upper member aresomewhat younger. The change in compositionfrom the dominantly tholeiitic basalts of thelower and middle members to the alkali lavasof the upper member apparently occurred inabout 0.2 m.y. The results show that theWaianae shield volcano was active over atleast 0.7 m.y.

As noted previously (McDougall, 1963a)the Mauna Kuwale trachyte (GA 397) givesan age of 8.4 + 0.2 m.y. (Table 3), which ismuch older than the lavas of the Waianaevolcanic series. It is apparent that there was aprolonged hiatus between the eruption of thetrachyte and the outpouring of the lavas ofthe Waianae series and the date supports thesuggestion of Stearns and Vaksvik (1935,p. 181) that the Mauna Kuwale trachyte isrelated to an older volcano buried by theWaianae lavas. This trachyte is the oldest rockfound in the present study of the age of thelavas of the Hawaiian Islands. According to thetime scale of Kulp (1961) the Mauna Kuwaletrachyte is early to middle Pliocene. RecentlyMenard and others (1962) reported a probableMiocene marine fauna in material dredged fromA depth of about 500 m, 10 km southwest ofHonolulu. Reef corals indicate that the depositwas formed in shallow water. This importantdiscovery, together with the old age of theMauna Kuwale trachyte, shows that a ridgehigh above the ocean floor existed at the pres-

ent site of Oahu during Miocene-Pliocenetimes.

The lavas of the Koolau volcanic series thathave been dated (Table 3) give ages rangingfrom 2.15 to 2.55 m.y.; hence, the Koolau lavasare younger than the Waianae lavas, in agree-ment with the geological evidence. It shouldbe noted that for all the Koolau lavas datedthe air correction is in the range 85 to 91 percent, and although an age is reproducible toabout 4 per cent (see GA 645), the error foreach may be as great as + 10 per cent. Speci-mens GA 643, 644, and 645 are from lavas onthe southeastern slopes of the Koolau Rangeadjacent to Pearl Harbor; their ages agree at2.25 + 0.1 m.y. Specimens GA 646 (southernslopes of the Koolau Range) and GA 647(Waimea Bay, northwest coast of eastern Oahu)appear to be slightly older than the lavas fromthe Pearl Harbor area. Palmer (1955) suggestedthat the Koolau series lavas were erupted inthree distinct and separate episodes on thebasis of differences in geomorphological de-velopment. The present limited data may beinterpreted as being in agreement with thishypothesis but show that probably no morethan 0.1 m.y. separated each episode. This timeinterval appears to be much too short for thecontrasts in geomorphological development tobe formed, particularly as more than 2 m.y.has elapsed since the outpouring of the lavas,during which time any differences in degree oferosion because of age would probably beobliterated. It seems more likely that the dif-ferences were brought about by climaticfactors.

The large amphitheater-headed valleys,which are so well developed on the west andeast coasts of Oahu, were eroded in less than3 m.y. in western Oahu and less than 2.2 m.y. ineastern Oahu. Hitchcock (1900, p. 24) pointedout that the great valleys on the west side ofthe Waianae Range are unlikely to have beeneroded under the present climatic conditions,as the rainfall is now relatively low. He sug-gested, and was supported by Hinds (1931,p. 185) and Stearns and Vaksvik (1935, p. 32),that the Waianae Range probably receivedmuch heavier rainfall prior to the building ofthe Koolau Range to a height sufficient tointercept the northeasterly trade winds. If thishypothesis is correct then the age data suggestthat the erosion of the valleys on the west sideof the Waianae Range mainly occurred in therelatively short period of about 0.5 m.y.

The Waianae and Koolau series lavas are late




Pliocene to possibly early Pleistocene, de-pending on the actual age of the Pliocene-Pleistocene boundary. The Mauna Kuwaletrachyte is early to middle Pliocene. Stearnsand Vaksvik (1935, p. 178) and Stearns (1946,p. 77) suggested that the Waianae and Koolauvolcanoes were built during the Tertiary andthat the Waianae volcano became extinct inthe late Tertiary and the Koolau volcano be-came extinct in the early Pleistocene. Theseestimates, based mainly on geomorphologicalarguments, are qualitatively correct.

trachyte. The two members are conformableand in most places are separated by only a fewinches of soil. Owing to the high rainfall onEast Molokai, deep canyons and amphitheater-headed valleys have been eroded, particularlyon the windward side. That at least some of theEast Molokai lavas are later than those ofWest Molokai is demonstrated by an erosionalunconformity between the two series inWaiahewahewa Gulch.

The K-Ar results on rocks from Molokai aregiven in Table 4. The only specimen dated

TABLE 4. AGE RESULTS ON LAVAS FROM MOLOKAI

SpecimenNumber

GA 573GA 567GA 571 (1)

(2)GA 570GA 569 (1)

(2)GA 572 (1)

(2)

Potassiumanalyses

(wt. per cent)

1.527, 1.5292.033, 2.0421.788, 1.8011.788, 1.8010.750, 0.7520.740, 0.7430.740, 0.7430.883, 0.8870.883, 0.887

Averagepotassium

(wt. per cent)

1.5282.0381.7951.7950.7510.7410.7410.8850.885

Ar40*/K40

7.74 x 10-5

7.998.528.618.748.658.811.08 x 10-4

1.08


536972728279707973

Calculatedage (tn.y.) Stratigraphic position

1.311.351.441.461.481.471.49

Upper 1member

East Molokaivolcanic scries

Lower,member j

1.84 1 West Molokai1.84 / volcanic series

Ar40*—radiogenic argon

MOLOKAIMolokai, like Oahu, consists of two shield

volcanoes, the geology of which were describedby Stearns and Macdonald (1947). EastMolokai attains a height of 4970 feet, and WestMolokai rises to 1380 feet altitude. The WestMolokai volcanic series comprises the lavas ofthe West Molokai shield. These lavas aremainly thin tholeiitic basalts that dip gentlyaway from the major rift zones from whencethey were erupted. No later alkalic lavas occuron the West Molokai shield. Except for highsea cliffs on the windward side the shield isbut little modified by erosion, because of thelow rainfall.

The East Molokai volcanic series includesall the lavas constituting the East Molokaishield volcano and is divided into a lower andupper member. The lower member is composedpredominantly of olivine basalts and is at least5000 feet thick, whereas the upper member,which forms a relatively thin veneer about 500feet thick, consists mainly of mugearite and

from the West Molokai volcanic series wasobtained from near the summit of the shieldand yields an age of 1.84 m.y. This age mustcorrespond closely to that of the last eruptionson West Molokai.

The rocks dated from the East Molokaivolcanic series were collected mainly from thewestern part of the shield and give ages rangingfrom 1.48 to 1.31 m.y.; hence, they are signif-icantly younger than the West Molokai series.The results show that the West Molokaivolcano probably became extinct before thewestern part of the East Molokai shield wasbuilt above sea level. However, it is possiblethat the central part of East Molokai, about 10miles east of the area where most of the sampleswere obtained, was above sea level while WestMolokai was still active, as suggested byStearns and Macdonald (1947, p. 15). In anycase the age data suggest that the time intervalbetween the cessation of eruption on WestMolokai and the appearance of East Molokaiabove sea level cannot be greater than 0.3 m.y.

Specimens GA 569, 570, and 571 were col-



MOLOKAI 117

lectecl from lavas in the well-exposed sectionthrough the East Molokai volcanic series on theKalae to Kalaupapa trail. Samples G A 569 and570 are from successive flows of the lowermember, and although the sequence consistsclominantly of "little-differentiated" basalticlavas (Stearns and Macdonald, 1947, p. 92),which presumably are mainly tholeiitic (Tilley,1950), these two lavas appear to be alkalibasalts. Their ages agree at 1.48 m.y. SampleGA 571, a mugearite of the upper member,occurs about 350 feet above GA 569 and 570and yields an age of 1.45 m.y., only slightlyyounger than the lavas of the lower member.If the lower member is composed dominantly oftholeiitic basalts then the transition to mainlyalkali series lavas occurred rapidly, as in theWaianae Range of Oahu. In any case the changefrom eruption of mainly basalt to mainlymugearite took place over a short period of time.The other two results from rocks of the uppermember of the East Molokai series (Table4) give younger ages of 1.35 and 1.31 m.y.Stratigraphically these two rocks must lieabove GA 571; hence, the ages are consistentwith the geology and suggest that the timeinterval between successive flows was increasingin the declining stages of activity.

The age data indicate that the immenseamphitheater-headed valleys, up to 2000 feetdeep, which occur on the windward (northern)side of East Molokai were eroded in less than1.5 m.y. Although East Molokai is deeplyeroded, many of the interfluves in the uppermember on the western slopes of the shield areessentially planar and slope away from themain rift zones of the volcano. These possiblyrepresent the original upper surfaces of flows;almost certainly they are parallel to flow sur-faces.

All the lavas exposed on Molokai appear tobe of latest Pliocene to Pleistocene age. Stearnsand Macdonald (1947, p. 31) suggested thatthe Molokai volcanoes were built during theTertiary, and probably in the Pliocene.

MAUIMaui is similar to Molokai and Oahu in that

it was built by two large shield volcanoes. Thegeology of Maui was described by Stearns andMacdonald (1942); the following summary istaken mainly from this work.

West Maui, a deeply eroded volcano, risesto an altitude of 5788 feet. East Maui, orHaleakala volcano, attains a height of 10,025feet, and although markedly dissected by ero-

sion, preserves much of its original form. Theflat area between the two volcanoes consists oflavas from Haleakala that banked up againstWest Maui, indicating that West Maui becameextinct before East Maui.

West Maui volcano is built mainly ofolivine basalt lavas, averaging about 15 feetthick, which are grouped together in theWailuku volcanic series. The lavas dip at an-gles of 5°-20° away from the two major riftzones and the summit vent from which theywere erupted. The exposed thickness exceeds5500 feet. Overlying the Wailuku series is adiscontinuous veneer, 50-500 feet thick, ofmassive mugearites and trachytes known as theHonolua volcanic series. Between the twoseries a thin layer of soil is commonly found.Minor volcanic activity occurred at a laterstage (Lahaina volcanic series).

The bulk of the shield of East Maui is builtof basalts erupted from three rift zones andprobably from a summit vent also. Theselavas, in excess of 8500 feet thick, are called theHonomanu volcanic series. As with otherHawaiian volcanoes the shield is veneered bymassive, more silicic lavas of the alkali suite,which are known as the Kula volcanic series.The Honomanu and Kula series are conforma-ble in most places and locally are separatedby a thin red soil. Following a period ofquiescence, lavas of the Hana volcanic serieswere erupted; this activity continued untilvery recent times (circa 1750). A profound ero-sional unconformity separates the Hana seriesfrom the older lavas in most places.

The K-Ar ages obtained on rocks from Mauiare listed in Table 5; most of the results are onrocks from West Maui. Three specimens ofbasalt from the Wailuku series of West Maui,collected in road cuttings on the north coast,yield ages agreeing at about 1.29 m.y. Theselavas occur at the margin of the shield and areprobably among the latest flows of the Wailukuseries. The Wailuku volcanic series, whichconstitutes the bulk of the West Maui shield,consists mainly of primitive, undifferentiatedbasalts (Stearns and Macdonald, 1942, p. 8,313). These basalts probably are mainly tholei-itic (Tilley, 1950), but the lavas dated appearto be alkali basalts. The four samples of theHonolua volcanic series are from widespreadlocalities in West Maui, and the measured agesagree well at 1.16 m.y. The short period of timeof about 0.13 m.y. between the Wailuku andHonolua series again shows that the changefrom eruption of dominantly basaltic lavas, of




which the bulk of the shield volcano is built, tolavas that are more silicic and belong to thealkali series, occurred rapidly.

Two specimens from the Kula volcanicseries of East Maui were the only samples datedfrom this volcano. Sample GA 574 is from theKula series immediately overlying the typesection of the Honomanu series in a road cut-ting on the west side of Honomanu Gulch(Stearns and Macdonald, 1942, p. 69). This

number of unconformities and interstratifiedsoil beds shows that the Kula lavas accumulatedin the waning phase of a volcano when thetime interval between flows became progres-sively longer." The age of 0.45 m.y. for GA 577suggests that the Hana volcanic series lavas areall younger than this.

All the lavas dated from Maui are latestPliocene to Pleistocene. Stearns and Macdonald(1942, p. 62, 157) suggested that both shield

TABLE 5. AGE RESULTS ON LAVAS FROM MAUI

SpecimenNumber

GA 577 (1)(2)

GA 574 (1)(2)

GA 640 (1)(2)

GA 638GA 637 (1)

(2)GA 568GA 579 (1)

(2)GA 578GA 580 (1)

(2)

Potassium Averageanalyses potassium

(wt. per cent) (wt. per cent) Ar40*/K40

1.359,1.359,1.333,1.333,2.237,2.237,2.946,2.211,2.211,2.703,0.812,0.812,1.047,0.701,0.701,

1.3611.3611.3341.3342.2452.2452.9542.2252.2252.7230.8130.8131.0510.7020.702

1.3601.3601.3331 .3332.2412.2412.9502.2182.2182.7130.8130.8131.0490.7020.702

2.71 x 10-52.614.865.116.936.806.906.926.866.777.557.797.507.527.68


747981694539354443496573727380

Calculatedage (m.y.) Stratigraphic position

0.460.440.820.861.171.151.171.171.161.151.281.321.271.271.30

Kula volcanic series,East Maui

Honolua volcanic series,West Main

Wailuku volcanic series,West Maui

Ar40*—radiogenic argon

rock gives an age of 0.84 m.y., which probablydates the beginning of the Kula eruptions.The dated flow is separated from the under-lying Honomanu series lavas by a single flowabout 25 feet thick and about 6 inches of soil,suggesting only a short time break between thetwo series. Also it was shown that the alkaliclavas overlying the major dome-buildingbasalts of West Maui, East Molokai, and WestOahu are only slightly younger than the under-lying basalts. Hence, the age of 0.84 m.y. forthis hawaiite of the Kula series probably ap-proximates closely to the age of the lastHonomanu eruptions. The other Kula serieshawaiite dated (GA 577) gives a much youngerage of 0.45 m.y. This specimen was collectedfrom the south side of the island at KalepaPoint. The measured age is 0.4 m.y. youngerthan GA 574, but this appears to be consistentwith the geological data. Thus Stearns andMacdonald (1942, p. 76) stated, "The great

volcanoes probably were built above sea level inPliocene times; the present data are consistentwith this view provided that the Pliocene-Pleistocene boundary proves to be about 1m.y. The age results substantiate the opinion ofStearns and Macdonald (1942, p. 62) that theKula volcanic rocks are Pleistocene. The Hanavolcanic series appears to be younger than0.45 m.y., in agreement with the estimate ofStearns and Macdonald that it is Pleistoceneto Recent.

The results suggest that the steep-walledcanyons of West Maui were eroded in only alittle over 1 million years. The large canyonson the slopes of Haleakala were eroded over aperiod of less than 1 million years.

HAWAII

Hawaii, the largest island of the Hawaiiangroup, was built by five volcanoes, which,excepting Kohala, are but little modified by



HAWAII 119

erosion. Mauna Loa and Kilauea are still active,Hualalei last erupted in 1801, and Mauna Keaprobably erupted in Recent times. KohalaMountain constitutes the northern end of theisland and is the oldest volcano of Hawaii(Stearns and Macdonald, 1946). The westslope of Kohala is barely modified by erosion,whereas the east (windward) side has canyonscut into it up to 2000 feet deep.

Kohala Mountain, like the other volcanoesof the Hawaiian Islands, is built mainly of thin-bedded tholeiitic basalts, known as the Pololuvolcanic series. Later mugearites and trachytesof the Hawi volcanic series thinly veneer thePololu volcanic series. An attempt was made todate two specimens of tholeiitic basalt of thePololu volcanic series from Waipio Canyon. Inboth cases no radiogenic argon was detected.Although the amount of potassium in the tworocks is low (0.11 and 0.12 per cent), an olderlimit for the age of the Pololu volcanic seriesis estimated at not greater than 800,000 years,and probably considerably less than this. Doelland Cox (1962) showed that the lavas of thePololu volcanic series have a normal directionof magnetization; this also supports the viewthat these rocks are young and probably lessthan 1 m.y. old. The balance of evidence favorsa Pleistocene age for Kohala rather than aPliocene age as suggested by Stearns andMacdonald (1946, p. 183).

MIGRATION OF VOLCANICACTIVITY IN THE HAWAIIANISLAND CHAIN

It was noted previously that the HawaiianIslands are at the southeastern extremity of achain of volcanic islands extending over 1500miles across the Pacific to Midway and Kure,and that geological evidence suggests thatvolcanic activity moved progressively towardthe southeast along a major structure in theoceanic crust.

Dana (1849) first suggested that the volca-noes of the Hawaiian Islands became extinctfrom northwest to southeast, on the basis ofthe degree of modification of the shield vol-canoes by erosion. According to Bryan (1915,p. 90) this idea already was expressed in legendsof the Hawaiian people. Pele, the powerfulmythical god of fire, was supposed to have re-posed at first on Kauai, and then moved alongthe island chain, finally to settle in Kilauea.Dana (1890, p. 259) gave an order of extinctiondetermined from geomorphological evidence(Table 6). Additional support for this hy-

pothesis was given by Hillebrand (1888, p.XVII-XXII) from studies of the flora of theHawaiian Islands. He found that the number ofendemic species and the degree of specializationwas greatest on Kauai, and that there was aprogressive decrease in number of species andspecialization along the island chain, withMauna Loa having the poorest and most uni-form flora. This pattern he attributed to theprogressive decrease in age of the volcanoesfrom northwest to southeast; the order found isgiven in Table 6.

The much greater apparent age of Kauaicompared with the other islands, because of itshigh degree of dissection, was commented onby many workers (Dutton, 1884, p. 83;Hitchcock, 1909, p. 14; Bryan, 1915, p. 103;Martin and Pierce, 1915, p. 37; Cross, 1915,p. 9). Wentworth (1927) made semiquantita-tive estimates of fluvial erosion of the HawaiianIslands, and from these studies he suggested asomewhat different order of extinction thanthat given by Dana or Hillebrand (Table 6).Hinds (1931) pointed out that Wentworthfailed to take into account the marked clima-tological variations found over the Hawaiianvolcanoes, which results in very different ratesof fluvial erosion on the several shields anddifferent parts of any one shield. Taking theseobservations into consideration Hinds (1931,p. 202-205) suggested an order of extinction(Table 6) that differs from the previous es-timates mainly in that he considered bothshield volcanoes of Oahu to be older than thatof Kauai. Stearns (1946, p. 95), on the basis ofhis classic geological investigations in theHawaiian Islands, proposed that the order ofextinction of the main shield-building phaseof the volcanoes proceeded from northwest tosoutheast strictly in that order (Table 6), incontrast to previous workers.

The K-Ar dates obtained in the present studyamply confirm the order of extinction sug-gested by Stearns (Table 6), provided thatthe Mauna Kuwale trachyte be omitted fromconsideration. If this is included in the se-quence then two volcanoes would appear in thelist for West Oahu; the Mauna Kuwale volcanowould precede Kauai, and the Waianae volcanowould follow Kauai. The age results suggestthat the main shield-building phase of avolcano essentially was complete before thenext youngest volcano erupted lavas above sealevel. However, as many of the rocks datedprobably were collected from the younger lavasin any given volcano, further work may show




that there was some overlap in time of eruption only to estimate the order of cessation of thein adjacent volcanoes. The present data enable major activity from the degree of dissection ofan estimate to be made of the maximum time the shields, and that no definite conclusionsinterval between eruption of lavas above sea may be drawn as to the order of commencementlevel for successive volcanoes; this ranges from of the building of the volcanoes. The K-Ar0.7 to 0.0 m.y. (Table 6). ages similarly only provide information as to

The preceding discussion concerning the the order of extinction. However, the K-Arorder of extinction of the Hawaiian volcanoes data indicate that, with the exception of

TABLE 6. ORDER OF EXTINCTION OF HAWAIIAN VOLCANOES, EXCLUDING MINOR ACTIVITYAFTER MAIN SHIELD-BUILDING PHASE

VOLCANOES NOT MENTIONED WERE NOT STUDIED.

1.

2

3.

4.5.

6.

7.

Dana(1890)

Kauai

West Oahu

West Maui

KohalaEast Oahu

East Maui

Mauna Kea

Hillebrand(1888)

f West Oahu \\ Kauai /f East Oahu \\ Molokai /f West Maui \\ Kohala J

Mauna KeaEast Main

Hualalei

I Mauna Laoj Kilauea

Wentworth(1927)

West Oahu

Kauai

East Molokai <

West MauiEast Oahu

Lanai <

West MolokaiNiihau

Hinds(1931)

West Oahu

East Oahu

Kauai \Niihau /West MolokaiEast MolokaiWest MauiLanaiEast Maui 1Kohala fKahoolawe J

Stearns(1946)

Kauai

West Oahu

East Oahu

West MolokaiEast Molokai

West Maut \Lanai /

Kahoolawe

Range of K-ArThis paper ages (m.y.)

Kauai

West Oahu*

East Oahu

West MolokaiEast Molokai

West Maui

East Maui

5.6—3.8

3.4—2.7

2.2—2.5

1.81.5—1.3

1.3—1.15

0.8—

8. Hualalei Kahoolawe Mauna Kea East Maui \ Island ofKohala / Hawaii <1

9. j Mauna Loa 1\ Kilauea /

10.

H.

Kohala

Rest ofIsland ofHawaii

Hualalei

Mauna LoaKilauea

{

Mauna Kea

Hualalei

Mauna LaoKilauea

*Mauna Kuwale trachyte result excluded

applies to the main shield-building phase; itis clear that on Kauai and on East Oahu, in par-ticular, there was considerable later activity,which, however, was of very small volume incomparison with the main shield-buildingphase. Hence, on Kauai where the shield build-ing phase was completed about 3.8 m.y. ago,activity occurred as recently as 1.4 m.y.(Koloa series), and probably even much morerecently than this (Macdonald and others,1960). Similarly, on East Oahu the Honoluluseries erupted probably during the late Pleis-tocene glaciation (Stearns and Vaksvik, 1935,p. 179).

Most workers clearly noted that it is possible

Kauai, eruption of the lavas at present exposedin the volcanoes examined occurred veryrapidly, so that it may be supposed that theorder of commencement of eruption probablyis reflected in the order of extinction; Chubb(1957) accepted this view, and many otherworkers implied acceptance of this view. Op-posing this idea is the fact that the oldestrock found in this study is the Mauna Kuwaletrachyte of West Oahu. Its age of 8.4 m.y. ismore than 2 m.y. older than the greatest agefound on Kauai, and 5 m.y. older than theother rocks dated from West Oahu. Not-withstanding, the general pattern may still becorrect to a first approximation.



MIGRATION OF VOLCANIC ACTIVITY IN THE HAWAIIAN ISLAND CHAIN 121

That this migration of volcanic activity is afeature common to many island chains in thePacific Ocean was pointed out by Chubb(1957) and Wilson (1936a).

It was suggested by several workers that thevolcanoes of the Hawaiian island chain haveformed along a major fracture in the oceaniccrust (Betz and Hess, 1942; Stearns, 1946;Eaton and Murata, 1960). However, Wilson(1963a; 1963b; 1963c) concluded that evidencewas lacking for this fault zone, and he putforward the idea that convection in the mantleplays an important part in the developmentof island chains. From the suggestion of Menard(1960) that a convection current ascends be-neath the East Pacific rise and flows laterallyaway from it, Wilson postulated that part ofthis convection system flows approximatelyhorizontally in a northwesterly direction underthe Hawaiian island chain. Wilson reasonedthat if the upper parts of such a convection cellmove more rapidly than the deeper centralparts then a source of lava within the slowermoving core may give rise to a linear chain ofvolcanoes of progressively older age in thedirection of convectional flow.

Calculations as to the approximate rate ofmovement implied by this hypothesis, in theHawaiian Islands, can be made from the agedata given here. Using the distance of eachvolcano from the still active Mauna Loa, themeasured K-Ar ages for the main shield-building phase of each volcano, and assuming ahxed source for the lavas, calculations show thatthe rate of movement would have to be 10-15cm/year, excepting Mauna Kuwale, in whichcase a rate of movement of about 4 cm/yearis indicated. These velocities must be regardedas a minimum owing to the possibility that thesource also is moving in the same direction asthe main convection cell. By the use of earthsatellites it should be possible in the nearfuture to determine whether movement at therate of at least 10 cm/year is occurring in theHawaiian Islands.

The calculated rates of movement average afactor of about four greater than the 3 cm/yearsuggested by many workers (e.g., Runcorn,1962; Wilson, 1963c, p. 868) as a likely upperlimit to the velocity of convection currents,should they occur, in the mantle. Hence, thedata given here suggest that either the velocityof the convection has been considerably un-derestimated, or that the hypothesis advancedby Wilson needs modification.

It should be noted that the age data indicate

that two volcanoes, which differ in age by about5 m.y., probably were active in West Oahu:the Waianae volcano, and that with which theMauna Kuwale trachyte is associated. To takethis factor into account Wilson's hypothesiswould have to be modified to provide forwidely separated sources, instead of a singlesource, for the lavas that built these twovolcanoes. Because each of the Hawaiian vol-canoes appears to have passed through a similarcycle of eruption of large volumes of tholeiiticbasalt followed by a much smaller volume ofalkali lava, a separate source for the lava ofeach volcano seems more likely, rather than asingle source, as advocated by Wilson.

The present results are not at variance withthe hypothesis of a major fracture in the oceanfloor being the controlling structure in the de-velopment of the Hawaiian island chain.

PETROLOGICAL CONSIDERATIONSThe thin veneer of alkali lavas of the main

shield building phase generally is less than0.2 m.y. younger than the underlying "primi-tive" basaltic lavas of which the bulk of theHawaiian volcanoes are built. These "primi-tive" basalts are considered to be dominantlytholeiitic (Tilley, 1950; Macdonald and Kat-sura, 1961); the proportion of alkali basalt isthought to be small. The present study appearsto support this conclusion,except in the case ofEast Molokai and West Maul, where alkalibasalts seem to be moderately abundant in thelower, mainly basaltic, part of the shields. It isclear from the age and field data that the tholei-itic and alkali lava series are closely related intime, with the eruption of tholeiitic lavas pre-ceding the eruption of much smaller volumes ofalkali lavas. The two lava series also may berelated as to their source. Any hypothesis ad-vanced to explain the relationship between thetwo series must be consistent with the shorttime interval between them.

Yoder and Tilley (1962, p. 507) presentedarguments in favor of a single primary sourcefor all basalt magmas. Eaton and Murata(1960, p. 930) showed that beneath Kilaueamajor earthquakes occur at depths of 45-60km; this is thought to be the zone in whichmagma is collecting. Yoder and Tilley sug-gested that at depths of this order a sourcerock, such as garnet peridotite, by partialfusion could give rise to tholeiitic magmas atlower pressure (i.e., shallower depths), and toalkali magmas at higher pressures (i.e., greaterdepths). Possibly beneath each of the Ha-




waiian volcanoes the source region in the man-tle was depleted by its basaltic fraction at thedepth of generation of tholeiitic magmas, andfurther magma formation occurred at greaterdepths, giving rise to alkali magmas (cf. Kunoand others, 1957, p. 216). Yoder and Tilley(1962, p. 507) also demonstrated that it ispossible, in principle, to derive tholeiitic andalkali lavas from the same source rock at con-stant high pressure by effective removal ofomphacite and garnet respectively from aliquid which approximates to the compositionof eclogite. The short time interval betweenthe two series in many of the Hawaiian volca-noes is in accord with these views. It should benoted, however, that the last mechanism ofderiving tholeiitic and alkali basalt from thesame source rock does not provide an obviousreason why tholeiites appear to be dominantin the major shield-building phase of the vol-canoes, to be followed by a much smallervolume of alkali lavas. The age data also areconsistent with a hypothesis of derivation ofalkali magma from tholeiitic magma by frac-tionation, if this can occur in times of the orderof 0.2 m.y. or less.

Although alkali basalt was erupted duringthe latter part of the main shield-buildingphase of many of the Hawaiian volcanoes,hawaiite, mugearite, and trachyte bulk large involume relative to alkali basalt. These moresalic and alkalic lavas are considered to havebeen derived from an alkali basalt parentmagma (cf. Muir and Tilley, 1961). Fractiona-tion in the alkali series was more marked thanin the tholeiite series in Hawaii, both in thevolume of these derivative lavas and theircomposition. These observations indicate afundamental difference in behavior in the twoseries. The relatively salic and alkalic composi-tion of the derivatives of the alkali basaltrenders it unlikely that they were formed atthe depths of generation of the parent magma;it seems probable that these derivative lavasoriginated from alkali basalt at considerablyshallower depths. The age data show thatthese more salic and alkalic lavas probablyformed in times somewhat less than 0.2 m.y.

Chayes (1963) noted that in the ocean basinslavas with compositions between mugeariteand trachyte are relatively rare and are ap-parently of small volume in comparison withthe amount of trachyte. This may indicatethat crystal fractionation is not the mechanismby which the trachytes are derived from alkali

basalt. On West Maui, where mugearite andtrachyte occur together in the Honolua series,the age data (Table 5) show that both types oflava are of the same age, and therefore theyare almost certainly closely related genetically.However, no new evidence as to their genesiscan be advanced from the age results.

An interesting pattern emerges from thepotassium analyses on tholeiites. In those caseswhere more than two rocks have been analyzedfrom the same sequence of tholeiites thepotassium content lies between narrow limits.The three rocks analyzed from the Napaliformation, Kauai, have a potassium contentbetween 0.13 and 0.14 per cent (Table 2). Inthe Makaweli formation the spread is from0.19 to 0.32 per cent. Five rocks that wereanalyzed from the Koolau volcanic series,Oahu, range from 0.32 to 0.42 per cent in theirpotassium content (Table 3). These limiteddata suggest that each sequence of tholeiiteshas a characteristic potassium content, and thatthe lavas of any one formation probably werederived from similar parent material undervery similar physical conditions.

SUMMARY AND CONCLUSIONS(1) Using whole-rock samples it is possible to

obtain reliable K-Ar dates from lavas of latePliocene to Pleistocene age, even in those caseswhere the potassium content is as low as 0.2per cent, and the air correction in the argonruns is as high as 90 per cent. Most of the lavasused in this study were very fine-grained andhence may be expected to have lost radiogenicargon by diffusion. However, the consistencyof the results suggests that, in the majority ofcases, the lavas measured have retained radio-genic argon quantitatively.

(2) The order of extinction of the Hawaiianvolcanoes with respect to the main shield-building phase occurred along the island chainfrom northwest to southeast. Oahu, Molokai,and Maui consist of two shield volcanoes, andon each island the western volcano is the olderby about 0.3 m.y.

(3) Considering the main shield-buildingphase only, the results suggest that the exposedparts of the Hawaiian volcanoes were built inless than 0.5 m.y., with the exception of theWaianae volcano, West Oahu, which wasactive over at least 0.7 m.y., and Kauai, whichwas active over more than 1.9 m.y.

(4) The relatively old age of the MaunaKuwale trachyte of West Oahu and the much



SUMMARY AND CONCLUSIONS 123

younger age of the overlying Waianae volcanic (7) The thin veneer of alkali basalt, hawaiite,series suggests that the Mauna Kuwale rocks mugearite, and trachyte of the main shield-are associated with an earlier volcano. building phase generally is less than 0.2 m.y.

(5) Except for the Mauna Kuwale trachyte, younger than the underlying "primitive"which is early to middle Pliocene, all the lavas basaltic lavas of which the bulk of the Hawaiiandated from the Hawaiian Islands are late volcanoes are built.Pliocene to Pleistocene. Should the Pliocene- (8) On the basis of the age measurements thePleistocene boundary be as old as 3 m.y., as hypothesis advanced by Wilson (1963c) for thetentatively suggested by Evernden and others origin of the Hawaiian island chain implies(1964), then most of the lavas are Pleistocene, that the minimum velocity of convection cur-with the exception of the Waimea Canyon rents in the mantle is of the order of 10-15volcanic series of Kauai and the older lavas of cm/year.the Waianae volcanic series of West Oahu, (9) The immense canyons developed on thewhich would remain in the late Pliocene. Hawaiian volcanoes were eroded in times of the

(6) The lavas of the main island of Hawaii order of a few million years, and in many casesare very young; those of the Pololu volcanic in appreciably less than 2 m.y. Using theseseries of Kohala Mountain, regarded as the K-Ar ages it should be possible to make quan-oldest exposed rocks on Hawaii, probably are titative estimates of the rates of erosion in theless than 800,000 years old. Hawaiian Islands.

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MANUSCRIPT RECEIVED BY THE SOCIETY, JUNE 20, 1963



APPENDIX 125

APPENDIXMany of the lavas used in this study are petro-

graphically similar to one another; hence, the tho-] elites, hawaiites, and mugearites are describedpreceding the list of localities and details of theindividual specimens. Lavas that do not belongto one of these three rock types are described sep-arately. Comprehensive descriptions of the rocktypes found in the Hawaiian Islands are given byMacdonald (1949) and in the several Bulletins ofthe Hawaii Division of Hydrography. Followingdescriptions of the three rock types mentioned, thespecimens are listed in the order they appear in theTables, from oldest to youngest.THOLEIITIC BASALT: Tholeiitic basalts used in thisstudy were massive to slightly vesicular. Pheno-crysts of olivine almost ubiquitous, constituting1-5 per cent by volume. Commonly show evidenceof partial resorbtion, alteration to iddingsitefrequently observed. Plagioclase and orthopyroxeneoccur as phenocrysts more rarely. Orthopyroxeneoccurs in several of tholeiites from Koolau volcanicseries (cf. Wentworth and Winchell, 1947). Pheno-crysts range up to 5 mm in size, occur in an inter-granular groundmass of plagioclase (35-60 per cent),clinopyroxene (35-55 per cent) and iron ore (2-10per cent); rarely a small amount of glass is present.Plagioclase, mainly labradorite, forms elongatelaths; in some cases crystals aligned to producefluidal texture. Grain size of groundmass averagesbetween 0.02 and 0.1 mm.

HAWAIITE: Massive, light- to dark-gray rocks,commonly with well-developed platy jointingparallel to flow planes. Generally nonprophyriticand composed of olivine (0-15 per cent), plagio-clase (50-70 per cent), clinopyroxene (10-25 percent), iron ore (10-15 per cent), accessory biotiteand apatite. Olivine usually altered to iddingsite.In the two rocks from the Kula series (GA 574,GA 577) olivine is essentially absent. Plagioclaseoccurs as laths, mainly of andesine, although smallamount occurring intersertally may be potassic(Macdonald, 1949, p. 1549). Texture intergranularand fluidal; average grain size 0.05-0.2 mm.

MUGEARITE: Light-gray, massive, platy rocksconsisting of olivine (5-10 per cent), partiallyaltered to iddingsite, clinopyroxene (10-15 percc;nt), iron ore (10-15 per cent), and abundantplagioclase, which is predominantly oligoclase withsome intersertal potassium-rich feldspar. Biotiteand apatite occur as accessory minerals. Texturefluidal, and grain size averages 0.1-0.2 mm.

KauaiGA 564: Tholeiitic basalt of Napali formation.

21°59'11"N, 159°42'55"W. Road cuttingof road to Waimea Canyon Lookout, IJ^Jmiles northwest of Kekaha. Altitude 250feet

GA 649: Tholeiitic basalt of Napali formation.22°03'14", 159°39'32". About 600 feet

below western rim of Waimea Canyonon Kukui Trail. Altitude about 2400feet. Flow 20 feet thick

GA 561: Tholeiitic basalt of Napali formation.22°03'08", 159°39'36". About 400 feetbelow rim of Waimea Canyon on KukuiTrail. Altitude about 2600'feet

GA 563: Tholeiitic basalt of Makaweli formation.22°01'53", 159°38'58". Confluence ofWaimea and Omao rivers, Waimea Can-yon. Altitude about 400 feet. Specimenfrom center of massive flow 20 feet thick.A medium-grained, holocrystalline basaltcomposed of olivine (phenocrysts), labra-dorite, pigeonite, augite, and iron ore.Grain size averages about 0.3 mm. Tex-ture subophitic

GA 650: Tholeiitic olivine basalt from flow 30 feetthick of Makaweli formation. 22°02'06",159°38'46". Altitude about 450 feet.About 600 yards north of confluence ofWaimea and Omao rivers, Waimea Can-yon

GA 565: Tholeiitic basalt of Makaweli formation.22°2'57", 159°38'31". Altitude about550 feet. Waimea Canyon, immediatelynorth of ditch intake and power stationon Waimea River. Slightly vesicular,massive basalt with sporadic olivinephenocrysts embedded in groundmass ofplagioclase, clinopyroxene, and iron ore.Average grain size about 0.1 mm. Plagio-clase laths aligned to give the rock flowtexture. Rock fresh, except for slightalteration of olivine to iddingsite

GA 566: Hawaiite of Makaweli formation.22°00'2S", 159°36'32"; altitude about1400 feet. From flow about 100 feetthick on Olokele ditch track, 1.2 milesN. 63°E of Camp Nine (Macdonald andothers, 1960, p. 46)

GA 642: Nepheline basalt of Koloa volcanic series.21°53'41", 159°30'26". Flow about 20feet thick in road cutting on west side ofLawaii Bay, south coast of Kauai, andabout 50 feet above stream level (Mac-donald and others, 1960, p. 61). Abun-dant phenocrysts of olivine occur inintergranular groundmass of clino-pyroxene, nepheline, and iron ore.Olivine partly iddingsitized. Olivine upto 3 mm ranging down to grain size ofgroundmass, which is about 0.3 mm (cf.Macdonald and others 1960, p. 103-107)

OahuGA 397: Hornblende-biotite-trachyte from flow

400 feet thick at Mauna Kuwale, WestOahu. 21°27'44", 158°09'3<5"; altitudeabout 300 feet. To the southwest trachyteis overlain, apparently conformably, bybasalts mapped as lower or middle Wai-




anae volcanic series lavas (Macdonald,1940, p. 81). Dark-gray, massive rockwith phenocrysts of oligoclase, horn-blende, and biotite in feldspathic micro-crystalline groundmass, which has fluidaltexture. This lava, which contains nearly20 per cent normative quartz (Mac-donald, 1940, p. 84), should be classifiedas rhyodacite, and appears to be relatedto the tholeiitic suite (Macdonald, 1963)

GA 810: Alkali basalt believed to be from lowermember of Waianae volcanic series. Inroad cut at southwest end of WaianaeRange, 21°20'51", 158°07'23"; altitude40 feet. Vesicular rock with commonphenocrysts of olivine altered to idding-site occurring in groundmass of olivine,plagioclase, clinopyroxene. and iron ore.Texture porphyritic and intergranular;grain size averages about 0.3 mm

GA 557: Tholeiitic olivine basalt of lower memberof Waianae volcanic series. 21°02'42",158°14'44"; altitude 30 feet. Road cut onFarrington Highway, 1 mile northwestof Makua, West Oahu

GA 560: Tholeiitic basalt of middle member ofWaianae volcanic series. 21°28'02",158°07'17"; altitude about 950 feet, onKolekole Pass road. Massive, slightlyamygdaloidal rock

GA 554: Tholeiitic olivine basalt of middle mem-ber of Waianae volcanic series. 21°29'07",158°13'35"; altitude about 400 feet onwestern cliff of Keaau Ridge

GA 809: Alkali olivine basalt of upper member ofWaianae volcanic series. 21°20'41",158°06'42"; altitude 150 feet. From roadcut at southern end of Waianae Range.Light-gray, massive rock composed ofolivine altered to iddingsite, plagioclase,clinopyroxene, and iron ore. Olivineoccurs rarely as phenocrysts. Intersertallysome poorly crystallized feldspar (K-richoligoclase?) occurs. Grain size averages0.2 mm. Rock has well-developed fluidaltexture.

GA 556: Hawaiite of upper member of Waianaevolcanic series. 21 "21'21", 158°06'38";altitude about 550 feet. From flow about20 feet thick, 1.4 miles WNW. of PuuPalailai, south end of Waianae Range(Macdonald, 1940, p. 80)

GA 553: Alkali olivine basalt of upper member ofWaianae volcanic series. 21°33'51",158°15'08"; altitude about 1120 feet, 2.3miles southeast of Kaena Point. Massive,slightly vesicular, intergranular basaltcomposed of plagioclase, olivine alteredto iddingsite, titanaugite, iron ore, andabout 10 per cent intersertal oligoclase

GA 647: Tholeiitic basalt of Koolau volcanicseries, East Oahu. 21°38'28", 158°3'57";altitude about 20 feet. From road cut,

south side of Wiamea Bay, northwestcoast of East Oahu

GA 646: Tholeiitic basalt of Koolau volcanicseries. 21°17'54", 157°48'51"; altitudeabout 160 feet. Road cut in Honoluluadjacent to University of Hawaii

GA 645: Tholeiitic basalt of Koolau volcanicseries. 21°23'22", 157°57'01"; altitude60 feet. Road cut 0.3 mile north ofKamehameha Highway, 0.8 mile north-west of McGrew Point, Pearl Harbor

GA 644: Tholeiitic basalt of Koolau volcanicseries. 21°25'45", 158° 00'42"; altitude320 feet. Road cut, Kamehameha High-way, 2.7 miles north of Waipahu

GA 643: Tholeiitic basalt of Koolau volcanicseries. 21°22'46", 157°54'32"; altitudeabout 250 feet. From quarry 1.5 milesnorth of north edge of Salt Lake, about4 miles northwest of Honolulu

MolokaiGA 572: Basalt from flow about 5 feet thick of

West Molokai volcanic series. 21°08'54",157°10'35"; altitude about 1080 feet.Cutting on road from Molokai Airportto Mauna Loa, 1 mile west of Puu Nana.Massive, dark-gray, basalt with sporadicelongate vesicles. Sporadic phenocrystsof clinopyroxene and plagioclase em-bedded in groundmass of plagioclase,clinopyroxene, and iron ore. Pale-brown,isotropic glass forms about 10 per centof rock. No olivine present. Grain sizeaverages about 0.05 mm.

GA 569: Olivine basalt of lower member of EastMolokai volcanic series. 21°10'45", 157°OO'OO". About 450 feet below top ofKalae-Kalaupapa Trail at altitude ofabout 1150 feet. Approximately 260feet below base of upper member (Stearnsand Macdonald, 1947, p. 93). Flowabout 3 feet thick. Moderately vesicularnonporphyritic basalt consisting of oli-vine altered to iddingsite (10 per cent),clinopyroxene (40 per cent), plagioclase(35 per cent) and iron ore (15 per cent).Texture intergranular; grain size averages0.1 mm.

GA 570: Olivine basalt of lower member of EastMolokai volcanic series. From flowabout 6 feet thick, overlying flow fromwhich GA 569 obtained. Similar to GA569, except that grain size averages about0.2 mm. Petrographically both GA 569and GA 570 appear to be alkali basalts.

GA 571: Mugearite of upper member of EastMolokai volcanic series. 21°10'42", 157°00'2". From basal flow of upper member,about 30 feet thick, exposed on theKalae-Kalaupapa Trail (Stearns andMacdonald, 1947, p. 93). Altitude about1500 feet



APPENDIX 127

GA 567: Mugearite of upper member of EastMolokai volcanic series. 21°7'49", 157°3'27"; altitude about 350 feet. Collectedin road cutting on road from Kaunakakaito airport, 3J-^ miles northwest ofKaunakakai. Possibly from same flow asanalyzed specimen of Stearns and Mac-donald, (1947, p. 102)

GA 573: Hawaiite or mugearite of upper memberof East Molokai volcanic series. 21°4'34",156°58'57"; altitude about 20 feet. Roadcutting at Alii Fishpond, 3 miles east ofKaunakakai. Light-gray, massive rockwith sporadic small vesicles. Phenocrystsof plagioclase and olivme set in ground-mass of plagioclase, clinopyroxene, ironore, and olivme. Plagioclase is oligoclase-andesine.

MauiGA 580: Olivme basalt of Wailuku volcanic series,

West Maui. 21°1'13", 156°35'6"; altitudeabout 200 feet. Road cut at B.M. 216,between Papanalahoa Point and Kea-walua, north coast of West Maui.Sparingly vesicular nonporphyritic rockcomposed of clinopyroxene (30 per cent),plagioclase (40 per cent), olivine (15 percent), and iron ore (15 per cent). Textureintergranular; grain size averages about0.1 mm.

GA 578: Olivine basalt of Wailuku volcanic series.21°00'5", 156°33'15". Road cut atKahakuloa, north coast of West Maui.Altitude about 130 feet. Sample fromslightly vesicular flow about 6 feet thick.Sporadic phenocrysts of labradorite occurin holocrystalline, intergranular ground-mass of plagioclase (45 per cent), clino-pyroxene (40 per cent), iron ore (10 percent), and olivine (5 per cent). Grainsize about 0.1 mm

GA 579: Olivine basalt of Wailuku volcanic series.Same locality as GA 578, and from flow6 feet thick about 20 feet above GA 578.Similar to GA 578 except that rock haspoorly developed flow texture. SamplesGA 578, GA 579, and GA 580 appear tobe alkali basalt.

GA 568: Trachyte of Honolua volcanic series,West Maui. 21°00'10", 156°32'49"; alti-tude about 250 feet. From Puu Koaebulbous dome intrusive into and over-lying the Wailuku basalts (Stearns andMacdonald, 1942, p. 178, 325). NearKahakuloa. Pale-gray, massive rock withwell-developed trachytic texture. Pheno-crysts of oligoclase occur in groundmassmainly of albite with lesser amounts ofhornblende, aegerine augite, and ironore. Grain size variable but averagesabout 0.2 mm

GA 637: Mugearite of Honolua volcanic series.21°1'8", 156°38'26"; altitude about 150feet. From flow about 50 feet thick, 0.2mile northwest of Honolua (Stearns andMacdonald, 1942, p. 322)

GA 638: Trachyte of Honolua volcanic series20°50'30", 156°38'30"; altitude about350 feet. Specimen from Launiupokobulbous dome, Puu Launiupoko. Light-gray, massive rock with well-developedflow banding. Phenocrysts of oligoclaseand hornblende occur in trachyticgroundmass of albite laths, and minorhornblende, aegerine augite, and ironore (Stearns and Macdonald, 1942, p. 22,175, 324, 334). Grain size averages 0.1mm.

GA 640: Mugearite of Honolua volcanic series.20°46'53", 156°31'33"; altitude about 70feet. Road cut on south coast of WestMaui at McGregor Point (Stearns andMacdonald, 1942, p. 322)

GA 574: Hawaiite of Kula volcanic series, EastMaui. 20°52'22", 156°10'13"; altitudeabout 300 feet. Road cut on eastern sideof Honomanu Bay, north coast of EastMaui. Flow about 20 feet thick, separatedfrom underlying Honomanu volcanicseries by a single flow 25 feet thick(Stearns and Macdonald, 1942, p. 69)

GA 577: Hawaiite of Kula volcanic series. 20°39'-13", 156°05'04"; altitude about 50 feet.Road cut on south east coast of EastMaui, between Kalepa Point and Kaa-pahu



Documents

Potassium-Argon Ages from Lavas of the Hawaiian Islands