Hawaiian Calderas!GORDON A. MACDONALD

ABSTRACT: Hawaiian calderas form by collapse during the last stages of growthof shield volcanoes built by frequent eruptions of tholeiitic basalt. They range from2-12 miles across, have sunk several thousand feet, and in part have grown piece­meal by coalescence of smaller collapse craters. They may never have formed onsome volcanoes, and all are partly or wholly filled by continued eruption. Towardthe end of the filling activity slows, and alkalic lavas complete the filling and builda thin cap over the caldera.

Gravity studies reveal masses of ultra-dense rocks only 1-2 km below the sur­face of several of the volcanoes-perhaps olivine-rich cumulates in the feeding pipeof the volcano, or perhaps protrusions of the mantle. The idea that these may haveled to formation of the calderas by isostatic sinking of a heavy column culminatingin the caldera appears to be negated by the facts that some calderas show no associ­ated gravity high, in some the high is offset to one side of the caldera, and somehighs are not associated with any apparent caldera collapse.

Caldera formation probably depends on the formation of a magma reservoirwithin the mass of the shield volcano, with its top within a few kilometers of thesummit of the shield. The Glen Coe mechanism of caldera formation seems to beruled out by the lack of upward displacement of magma around the sinking block.

-_. Caldera collapse is -probably -the .resulr of. sinking -of .a -block.bounded.by~ inward-_

dipping conical fractures, permitted by distension of the top of the volcano andremoval of support due to drainage of magma into the rift zones, with or withoutflank eruption. The distension of both the summit region and the rift zones mayresult from a lateral spreading of the lower part of the ultra-dense core of thevolcano.

THE ORIGIN OF HAWAIIAN CALDERAS has beendiscussed several times before (Stearns andClark, 1930; Stearns and Macdonald, 1946;Macdonald, 1956), but with the increase inknowledge of the physical properties of Ha­waiian rocks and magmas and the geophysics ofHawaiian volcanoes during the last decade it isdesirable to review the former conclusions, tosee whether they need modification. It may beassumed as a starting point that the calderasare formed by collapse. They obviously are notformed by erosion, and the very small amountof pyroclastic debris at and near the summits ofthe shields clearly eliminates any possibility oftheir having been blasted out by explosion.There is general agreement that they have been

1 Hawaii Institute of Geophysics Contribution No.92.

formed by the sinking in of the summit regionsof the shield volcanoes. The term "volcanicsink" used for them by R. A. Daly (1914 :144­147) is an appropriate one, though it has notbeen widely used, at least partly because of po­tential confusion with other types of geologic"sinks."

Though the generality of the collapse originof Hawaiian calderas is unquestioned, the causeof the sinking is still uncertain. The purpose ofthis paper is to describe briefly the calderas, tosketch their spatial and time relationships tothe volcanic structures and history , to reexam­ine the suggested causes of collapse, and tosuggest a new mechanism that may be of im­portance in bringing about the distension of themountain that is indicated both by the sinkingof the caldera and by the rift zones.

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Hawaiian Calderas-MACDONALD

MORPHOLOGY OFTHECALDERAS

The summits of the active shield volcanoes,Kilauea and Mauna Loa, are indented by ovalcalderas. That of Kilauea is4.4 km long and3 km wide. Mokuaweoweo caldera, at the sum­mit of Mauna Loa, is 4.5 km long and 2.6 kmwide. Both are slightly excentric with respectto the precise summits of the mountains. Ki­lauea caldera is 150 m deep at its western edge,beneath the highest point of the mountain, butits walls decrease in height essentially to zeroat its south side. It is separated from the adja­cent pit crater of Kilauea Iki by only a low,narrow ridge (the Byron Ledge). Mokuaweo­weo is 180 m deep at its western edge, 130 mdeep at its eastern edge. At the south it mergeswith the pit crater, South Pit, and at the northits floor is continuous into the pit crater knownas North Bay, which is bounded at the northby a wall only about 5 m high. At its northeastedge Mokuaweoweo also merg es with the very

_smallpit-crater, East Bay; and-farther -north-thepit crater Lua Poholu lies within the outermostfaults of the caldera. It is generally agreed thatthe pit craters, like the calderas, have formedby collapse.

Extending outward from the calderas are therift zones of the volcanoes (zones of fracturingaveraging about 3 km wide ) that have served asthe feeding conduits for most flank eruptions ofthe volcanoes. On the southwest rift zone ofMauna Loa, within 3 km of Moku aweoweo, liethree pit craters, the central one of which hasbeen formed since 1840. Along the east riftzone of Kilauea within 20 km of the calderalies a whole series of pit craters (Stearns andMacdonald, 1946: PI. 1).

In most places the boundaries of the calderasare steep cliffs, with an average slope of about75 0

• Layers of lava in the cliffs slope outwardaway from the summit of the mountain andtheir truncated edges project upward into spacein the present area of the depression . The ventsthat fed them must have been located at ahigher level, and must have dropped Out ofsight at the time of origin of the caldera. Inother words, the caldera cannot have been pres­ent in anything approaching its present dimen­sions until the shield had reached essentially its

321

present size. The cliffs bounding the calderasare fault scarps. Along most of the boundarythe cliff is simple , but in places it consists of aseries of step-fault blocks. The fault planes sep­arating the step blocks appear to dip towardthe center of the caldera at about the sameangle as the rest of the boundary scarp. Locally,the faults pass into monoclines (Macdonald,1957).

The main calderas are partly surrounded bybenches as much as 3 km wide that have alsobeen dropped down on faults, but not as far asthe central caldera. Viewed from an airplane ashan distance away, the summit areas of bothKilauea and Mauna Loa are seen to have saggeddownward over a subcircular area 6-8 kmacross, with the sharply defined downfaultedcalderas at the center.

The floor of Kilauea caldera is a very gentlysloping dome or cone rising to an apex at Hale­maumau crater, in the southwestern part ofthe ca!~~r~. .TII.e cone was formed by repeated _overflows from Halemaumau during the longperiod of lava lake activity before 1924, withthe minor addition of the lava flow of the 1954eruption (Macdonald and Eaton, 1957). Thefloor of Mokuaweoweo.ralso slopes upwardtoward the cones of the 1940 and 1949 erup­tions. During the last 150 years the history ofthe calderas has been one of repeated collapseof the floor and refilling by eruptions on thefloors. In 1825 the center of Kilauea calderawas a pit some 260 m deep, surrounded by anarrow "black ledge" 30 m or so below thepresent floor level. This central depression ispresumed to have formed by collapse at thetime of the flank eruption in 1823. By 1832 thecentral pit had been filled to overflowing, butin that year it was reestablished in much itsformer condition by another collapse. Again itwas refilled, only to be reformed by collapseaccompanying the eruption of 1840. Still againit was refilled, only to sag down in a less ex­tensive depression at the time of the 1868 erup­tions, and so on. From 1840 onward each col­lapse was smaller than the one that preceded it,until in the 1920's two small collapses werefollowed by a large one (Table 1) . It shouldbe noted here that the collapses of the 19thcentury listed in the table represent the volume

322

19°--+-----'-

PACIFIC SCIENCE, Vol. XIX, July 1965

FIG. 1. Map of the island of H awaii , showing the lo cations of the major volcano es, the calderas (solid blackspots ), and the approximate centers of the gravity highs (bla ck triangles ) as located by Ki noshita et al. (1963 ) .

of the sharply defined central depression only,whereas those of 1924 and 1955 include thevolume of a gentle sinking of 'the mountaintopthat could not have been detected without sensi­tive tiltm eters or precise surveying, so thatactually the early collapses must have beensomewhat larger in relation to the later onesthan is indicated by the table.

The history of Mokuaweoweo is known infar less detail. A large collapse of the calderafloor in the middle part of the 19th centurymay have been associated with one, or possiblymore than one, of the flank eruptions of MaunaLoa. Since then the depression has been grad­ually refilled, until in 1949 lava pour ed Out ofthe caldera into South Pit , filled it to over-

Hawaiian Calderas-MACDONALD

flowing, and cont inued for several kilometersdown the mountainside. Until 1948 a crescenticbench ( the South Lunate Platform) remainedaround the south side of the central pit, butit has been largely bur ied by the lavas andcones of the 1949 eruption.

The original morphological forms of thecalderas of the older Hawaiian volcanoes havebeen wholly destroyed by erosion, but thecalderas are neverth eless clearly delineated bymapping. The lavas that filled the calderasare thick-bedded, generally moderately to verydense, and essentially horizontal, in contrast tothe t hin-bedded moderately to highly vesicularflows that accumulated with slopes of 2-10 0

on the flanks of the volcano outside the caldera.The thickness and horizontality of the caldera­filling flows resulted from their being confinedwithin the caldera depression, and their dense-

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ness resulted from the greater proportion ofgas that was able to bubbl e OUt of the thickerflows before they solidified. The contrast ingeneral aspect and attitude of the two groupsof flows is usually sufficient to indicate theformer position of the edge of the caldera. Inaddi tion, banks of talus commonl y formedagainst the foot of the caldera-boundary cliffs,as they are doing in Kilauea and Mauna Loacalderas today. The lavas accumulating withinthe caldera buried these taluses, and later ero­sion has exposed ' them as prisms of brecciabetween the caldera-filling and extra-calderalavas. Careful mapping by H . T. Stearns hasdelineated the calderas of the Koolau and Waia­nae volcanoes on Oahu, and those on east Molo­kai, W est Maui, and the exposed end of thatof Kaho olawe. The caldera of the Koolau vol­cano was about 8 km long and 5 km wide, cen-

TABLE 1

VOLUMES OF CALDERA COLLAPSE COMPARED WITH VOLUMES OF FLANK LAVA FLOWS .- ..AT ·K II.AUEA' V OL<;AN O D UIl.INGHISTORICAL TIMES

VOLUME OF FLANK LAVA FLOWS ( rn")

VOLUME OF COLLAPSE Reduced to approximateYEAR ( rn") Actual volume in magma chamber

-

1823 539,500,000 14,000,000' 11,200 ,000

1932 580,600,000 ? .... . ... . .

1840 219,500,000 214,800,000· 171,800 ,000

1868 188,300,000 <5,000,000 < 4,000,000

1886 39,600,000 ? . .. . . . . . ..

1891 34,000,000 ? . __ . . . . .. .

1894 8,500,000 ? ... .......

1916 8,300,000 0 0

1919 10,000,000 73,800,000c 59,000,000c

1922 21,200 ,000 8,700,000d 7,000,000

1924 201,600,000 0 0

1950 0 0

1955 200,000,000 141,000,000 113,000,0 00

• Includes an estima ted 2,000,000 m" th at poured int o th e ocean .b Includes an est imated 150,000,000 m" that flowed into the ocean.c Includes the volum e of the flow in th e calde ra in 1919 a nd the flow on the southwest rift zone in 1919 an d 1920.d Includ es the vol umes of flows on the caldera floor in 1921 and on the east rift zone in 1922.

324 PACIFIC SCIENCE, Vo!. XIX, July 1965

cent rift fractures or elsewhere , to make roomfor the sinking block.

rering at the Kawainui Swamp near Kailua(Stearns, 1939: PI. 1). That of east Molokaiwas about 7.2 km long and 5 km wide (Stearns,and Macdonald, 1947: 19, P!. 1). The larg- RELATION OF CALDERA FORMATION TO

est of the Hawaiian calderas was that of VOLCANIC AND MAGMATIC HISTORY

Kauai, some 16 by 20 km across (Stearns, As a result of his years of study of the areal1946: Fig. 22; Macdonald, Davis, and Cox, geology of the Hawaiian Islands, H. T. Stearns1960: P!. 1). In and close to the east Molokai was able to distinguish a series of stages in theand Koolau calderas, and locally in that of West history of the volcanoes (Stearns, 1940; 1946 :Maui and near the boundary of that of Kauai, 17-19). These may. be summarized briefly asrising gases brought about chloritization of the follows, with some additions and modificationsrocks and deposited secondary minerals, includ- from Stearns' original statements:ing quartz and chalcedony. The boundary cliffs 1. A youthful shield-building stage, duringof the calderas, as delineated by mapping, sloped which frequent eruptions of very fluid basalticinward at angles of 50° to nearly 90° ,averaging lava build a shield volcano composed almostabout 75 0 . The slope of the boundary cliffs is wholly of thin extensive lava flows. Eruptionsnot, of course, necessarily the same as that of come so frequently that there is not time forthe faults that bound the sunken block at any appreciable amount of weathering or ero­depth. Outward-dipping faults would produce sion between successive flows. Pyroclastic rna­surficial scarps that are unstable, and slumping terial forms probably less than 1% of the partwould quickly form inward-sloping fault-line of the shield above sea level. Hydromagmatic

. _.s~~~ps. However, other evidence discussed on a . explosions may have produced a much largerlater-page--iilso -suggesES --d lil f the - subsurface- _·prop or tion of pyrod ascics ·iii· -tfie- zone withinfaults dip inward. a few hundred feet below sea level, but at

Nowhere has the bottom of the caldera- greater depths the pressure of overlying waterfilling mass been reached by erosion. The sink- probably effectively restrained both explosioning of the original tops of the east Molokai and vesiculation, and the lava flows were prob­and Kauai shields exceeded 1,400 m. ably much denser than those later poured out

There is considerable evidence that Mokua- above sea level, as are those formed in deepweoweo caldera grew in part by the repeated water on the east rift zone of Kilauea (Moore,inclusion of marginal pit craters (Stearns and 1965 ).Clark, 1930:49; Stearns and Macdonald, 1946 : 2. Late in the period of shield building the29). On the floor of Kilauea caldera also, the summit of the shield collapsed to form thedistribution of areas of alteration of the rocks caldera. There followed the so-called caldera­by rising gases indicates the presence of a filling period, during which thick massive lavaseries of pits buried by the lavas erupted within flows accumulated within the caldera, graduallythe last 150 years (Macdonald, 1955a). The filling it. It should be noted, however, thatKilauea Iki and Keanakakoi pit craters nearly eruption is not restricted to the caldera, butcoalesce with the inner caldera of Kilauea, and continues to take place on the flanks of thethe outermost caldera faults are beyond them, volcano also, and the shield continues to build.so that they are actually within the major Volcanism continues vigorous and eruptionssunken area. The pit craters probably have frequent; the activity remains almost purelyformed by sinking of a roof block into an effusive and pyroclastic material still is formedunderlying magma chamber eaten upward into in only very small amount.the mass of the volcano by stoping and melting 3. Eventually the caldera becomes filled andof the lavas above the magma body (Macdon- the volcano enters on the post-caldera, or old­aId, 1956 :281), but since the sinking is not age, stage. Eruptions become less frequent, andaccompanied by a rise of magma along the many of them are more explosive, partly be­bounding ring fractures into the crater the cause of greater viscosity of the erupting magmamagma must have been withdrawn, into adja- but partly also because of a greater abundance

Hawaiian Calderas-MACDONALD

of gas. Local erosional unconformities, streamgravels, and soil beds are found between suc­cessive flows, and the proportion of pyroclasticmaterial increases. The spatter cones built byHawaiian-type eruptions characteristic of theshield-building stage are largely replaced bycinder cones built by strombolian-type erup­tions. A relatively thin cap, a few tens to severalhundreds of meters thick , is built over the topof the shield and the filled caldera.

4. Extinction of the major volcano is followedby a long period of volcanic quiescence, withdeep erosion and weathering. Sea cliffs severalhundred meters high and canyons several hun­dred meters deep are cut into the volcanicmountain.

5. At some volcanoes (whether eventuallyat all is unknown) volcanic activity returns.In this rejuvenated, or post-erosional, stage neweruptions take place from rift zones that havelittle or no relationship in. position or directionto those of the earlier stages. Lava flows partlyfill valleys and relati"eli smallIava,-CInd er,.~~rtuff cones are built against the dissected sur­face of the old mountain, separated from it bya profound erosional unconformity and some­times by intervening sedimentary deposits suchas coral reefs. .

It should be noted that not all Hawaiianvolcanoes have passed through all of the abovestages. Lanai appears to have stopped activityat about the end of the caldera-filling stage.Kilauea and Mauna Loa are still in the caldera­filling stage and Hualalai appears to have barelyentered the old-age stage. Neither Hualalai norwest Molokai now has any caldera, nor do theyshow any indication of ever having had one,though on both a caldera may have been hiddenby later flows. On Mauna Kea the suggestion ofa former caldera is very tenuous, consisting onlyin an arcuate arrangement of some of the cin­der cones in the summit region that may reflectcaldera-related ring fractures at depth (Mac­donald , 1945). These volcanoes may haveskipped the caldera-forming and caldera-fillingstage.

Recent petrographic studies (Macdonald andKatsura, 1964) have demonstrated the relation­ship of rock types to the erup tive history OUt­lined above. The lavas of the shield-building

325·

and most of the caldera-filling stages are tholei­itic basalt (including tholeiite, olivine tholeiite,and oceanire ). In the upper part of the caldera­filling sequence, and at a ' corresponding strati­graphic level outside the caldera, there occursa change to alkalic lavas (alkalic basalt, olivine'basalt, ankaramite, hawaiite, mugearite, andtrachyte ), and these rock types persist throughthe old-age stage. The rocks of the post-erosionalstage are characteristically nephelinites andmelilite-nephelinites, with associated undersat­urated alkalic olivine basalts.

Thus the formation of the calderas, thoughit may vary somewhat from one volcano to an­other and may be lacking from some, takes:place during the end of the period of shieldbuilding, while volcanism is still vigorous, andsomewhat before the slowing down of volcan­ism that accompanies the change of petro­graphic types from tholeiitic to alkalic.

One important bit of evidence that shouldbe considered in relatio~to. tl1~ .origin .oL the.Hawaiian- csJctera;; ls- that the caldera floorsometimes moves upward as well as downward.Not only does the floor swell up as a broad flatdome during tumescence of the volcano (Jag­gar and Finch, 1929) , but it ....r.n~y. move enmasse in the manner of a huge piston. Thegreat collapse of Kilauea caldera in 1840 pro­duced a central pit 1.5 km wide, 3 km long,and about 150 m deep. About 1845 the floorof the pit started to rise bodily, and by 1850it had risen about 150 m and was approximatelylevel with the "black ledge" that surrounded it.The taluses that had accumulated on it at thefoot of the bounding cliffs had been pushed upuntil they stood as arcuate ridges of angularrock fragments that projected as much as 45 mabove the surrounding caldera floor. Althoughit is possible that this elevation of the calderafloor could have resulted from shallow intru­sion of magma beneath it, as did the elevationof the floor of Halemaumau crater in 1952(Macdonald, 195Sa ) , it appears ' more probablethat it was caused by inflation of the underlyingmain magma chamber of the volcano.

INTERNAL STRUCTURE OF THEVOLCANOES

In none of the Hawaiian volcanoes has ero­sion cut deeply enough to expose the congealed

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magma body that fed the surface eruptions.Knowledge of the position and nature of themagma chamber is almost wholly implied fromseismic evidence and the pattern of swellingand shrinking of the volcano before and aftereruptions. Earthquakes originating from adepth of about 60 km below sea level, withinthe upper part of the earth 's mantle, are ac­companied by "harmonic" tremor that appearsto be the same as the tremor that is known toaccompany movement of magma in the volcanicconduits close to the surface. From this it isimplied that the magma that feeds the volcanois formed at a depth of about 60 km (Eatonand Murata, 1960). It has long been knownthat Kilauea volcano gradually tumesces overperiods ranging from a few weeks to severalyears before eruptions (Jaggar and Finch,1929), presumably because of inflation of anunderlying magma reservoir, and detumescesduring and after the release of magma by erup­

-tion. :Recent-analyses -of the -patterns -of _groundtilting resulting from the tumescence and de­tumescence, coupled with the depth distributionof the shallow-seated earthquakes that precedeeruptions, have led to the conclusion that thetop of the magma body lies at a depth of onlyabout 2 km below the summit of Kilauea vol­cano ( Eaton and Murata, 1960). It should beemphasized that this is well above the level ofthe surrounding ocean floor and consequentlywell within the mass of the volcanic mountainitself. Room for the magma body must havebeen obtained either by lateral displacement ofits margins or by melting of the rocks that for­merly constituted the core of the volcano.

The lateral extent of the magma body is lesscertain, and indeed it appears doubtful (frominductive reasoning) that it has any sharplydefined margin. More likely there is a gradualpassage from freely fluid magma to somewhatplastic but essentially solid material, and thenceto the truly solid rocks of the volcano's exterior.There can be no question that the magmaticcore, or at least material in a very mobile con­dition, extends outward from beneath the sum­mit of the mountain for many kilometers intothe rift zones. Thus, the 1955 eruption of Ki­lauea from the east rift zone 30 km east of the

PACIFIC SCIENCE, Vol. XIX, July 1965

caldera was preceded by a marked swelling ofthe rift zone, with lifting of the ground surfacealong the crest of the rift-zone arch in theeruption area of more than a foot (Macdonaldand Eaton, 1964: 105).

There is no real evidence of magma cham­bers at shallow depths beneath the other Ha­waiian volcanoes. Even at Mauna Loa the meas­urements of tilting are far from sufficient todemonstrate such a chamber. By analogy withKilauea, however, it may be assumed that suchmagma chambers probably exist, or did existwhen the volcano was active. The collapse ofthe shield to form the caldera is probably de­pendent on the existence of this magma body,and the fact that caldera formation does notoccur until near the end of the shield-buildingstage is probably because sufficient remeltingof the core of the shield to form the chamb erhas not taken place until that stage.

During recent years' gravity studies in theHawaiian Islands have indicated very high val­ues ifi:-and n:ear -tne -areasof -some-or tnecalderas (Woollard, 1951; Woollard et aI., 1964).Strange , Woollard, and Rose (p. 381 in thisissue ) reports that whereas the average Bou­guer gravity anomaly value along the Ha­waiian ridge is about +200 mgal, the in­tensity in the central parts of the Koolau andWaianae calderas, Oahu, is in excess of 310mgal, and "the maximum Bouguer anomalyvalues over most of the volcanic centers rangebetween +285 and +325 rngal." He pointsout, as had Woollard (1951), that to explainthe anomalies it is necessary to assume a veryhigh density for the material in the volcanic"pipes." Similarly, Adams and Furumoro (p.296 in this issue) and Furumoto, Thompson,and Woollard (p. 306 in this issue) findseismic velocities in the Koolau "plug" greaterthan 7 krn/sec, as compared with 4.6 krrr/secin surrounding material. Adams estimates theplug to be about 6 km across, with its topabout 1.6 km below sea level. Similar highseismic velocities were found by Shor ( 1960)approaching close to the surface in the north­western part of the Hawaiian ridge, near theGardner Pinnacles. To explain the observedpattern of high gravity values combined withhigh seismic velocities, one is forced to assume

Hawaiian Calderas-MACDONALD

rocks beneath the Oahu calderas with densitiesapproximating 3.2 grn/cc extending to at leastthe depth of the ocean floor (5.5 km). Verydense caldera-filling tholeiitic basalts may havedensities up to a little more than 3, and ocean­ires have measured densities ranging up toabout 3.2; but of Hawaiian rocks only theperidotites found as inclusions in flows haveconsistently a density greater than 3.2.

The dense high-velocity rock has generallybeen considered to be part of the earth's man­tle injected to high levels in the crust. How­ever, it conceivably could be cumulate materialsettled from the overlying magma in the con­duit and core of the volcano or lagging behindas the more fluid portion of the magma rosearound it. Fragments of dunire and wehrlitebrought up by lavas of the old-age stage ofthe volcanoes, such as the 1801 eruption ofHualalai (Macdonald, 1949:76; Richter andMurata, 1961) have textures resembling thoseof the cumulate rocks of layered intrusives(Wager, Brown, and Wadsworth, 1960) ' andprobably represent fragments of cumulate rockbrought up from relatively shallow depths be­neath the volcano. The mineral assemblages arenot particularly indicative of high-pressureequilibria. On the other hand, the garnet pyrox­enite ("eclogite") found as inclusions at SaltLake Crater on Oahu . does represent a high­pressure equilibrium assemblage, and quiteprobably represents material brought up fromthe mantle. In chemical composition it is veryclose to tholeiitic oceanite, though somewhatricher in silica and poorer in alkalies (Mac­donald and Katsura, 1964 : Table 8, col. 13),and may represent an oceanitic intrusive masscrystallized under high pressure in the uppermantle. Its density (2.71-2.81) and seismicvelocity (V, = 5.52-6.06 ) as determined byManghnani and Woollard (p. 291 in thisissue) are too low to account for the materialin the primary volcanic "pipes," which is char­acterized by high gravity values. Moreover,there is no significant gravity "high" associatedwith Salt Lake Crater.

Brief mention of the rift zones of Hawaiianshield volcanoes has already been made. Therift zones generally radiate outward from thesummit of the shield-that is, from the caldera.

327

Usually there are three distinct rift zones, withangles of roughly 120 0 between them, andwith one rift zone less well developed than theothers. In addition to the lines of spatter conesand cinder cones resulting from eruption, therift zones are marked by pit craters, many openfissures, and by long narrow grabens. The depthof the grabens is generally unknown, becausethey have been partly filled with later lava.At depth in the older, dissected volcanoes, therift zones are marked by thousands of thin dikes.Sections across them yield counts of more than600 dikes per mile. Although a few instances ofstrike-slip displacement on rift-zone fissures areknown (Macdonald, 1956:278), the configura­tion of the walls of the dikes generally indicateshorizontal opening without any appreciable dis­placement parallel to the fissure. There can beno question that the rift zones represent a veryconsiderable distension of the visible part ofthe shield volcano, a distension on the orderof 0.75-1 km.

The Hawaiian rift zones have recently beenexplained by J. G. Moore (at a lecture beforethe Peninsula Geological Society, Stanford Uni­versity, January 7, 1965) as the result of land­sliding on a gigantic scale. Specifically, he be­lieves that the southern slope of Kilauea issliding seaward, the fractures on which themovement is taking place steepening to nearverticality to form the east rift zone, withgraben collapse along the upper edge of thesliding block. The distension in the rift zonehe attributes to the southward movement ofthe block to the south . He supposes that magmamakes its way surfaceward along the plane ofsliding. Essentially the same suggestion wasmade for the origin of the southwest rift zoneof Kilauea by Stearns and Clark (1930). With­out at present entering into any debate onwhether or not there is large-scale landslidinggoing on along the south flank of Kilauea, itappears very unlikely to me that the east riftzone (or any other) can have the origin sug­gested by Moore. The essentially vertical atti­tude of the dikes in the rift zones down to thedeepest level of exposure on the deeply erodedislands of Oahu and Kauai, a level equal tomore than half of the probable depth to themagma chamber at Kilauea, is inconsistent with

328

such an origin. So also is the fact that the grav­ity highs that extend laterally as bulges fromthe highs beneath the calderas usually coincideclosely with the surface trace of the rift zones(Kinoshita er aI., 1963 : Fig. 1; Strange etal., Fig. 1, p. 382 in this issue ) . Whetherthe high value of gravity over the rift zonesis due to exceptionally dense material at depthDr whether it is wholly the result of a largenumber of dense dikes, if the path of theri sing magma were inclined markedly to oneside at shallow depth the gravity high shouldn ot only lie to one side of the rift zone,but should be only a fraction of the magnitudeobserved and of much greater width. The closecoincid ence of the "high" and the surface riftzone, therefore, strongly suggests continuationof the ri ft zone essentially vert ically at depth.Furthermore, before any landslide or landslide­induced rift could exist, there must have beena volcanic mass for the slide to form upon. Theeas t rift zone of Kil auea occupies the top ofa --gentle --construcrional -arch - that extends allthe way to the sea floor, more than 100 km eastof the summit of the shield. The arch has ob­viously been built by eruptions from the rift,and the shape of the shield is fundamentallygoverned by the position of the rift zone. Thesame is true of the other H awaiian shields. Theshields are generally not round or oval, butlobate, resembling thre e-pointed stars in groundplan (Wentworth and Ma cdonald, 1953), as aresult of building by erup tions pr edominantlyalong the three rift zones. The close dependencein shape of the major shields upon the positionof the rift zones indicates that the rifts areearlier-formed and are more fundamental struc­ture s than any produced by landsl iding.

Displacement of the ground surface duringthe 1955 erup tion on the east rift zone of Ki­lauea (Macdonald and Eaton, 1964 ) involvedslight elevation of the surface adjacent to theeruptive fissures, sinking of a graben alongthe rift, and lateral displacement of the groundoutside the graben by an amount up to a littlemore than 1.5 m in a direction essentially nor­mal to the rift. There was no detectable sink­ing of the surface outside the graben on eitherside of the rift in relation to the other side.

Th e predominance of earthquake foci southof the rift zone noted by Moore is probably

PACIFIC SCIENCE, Vol. XIX, July 1965

related to movement on faults of the Hilinasystem, which lies along the south slope ofKilauea and converges eastward with the riftzone (Stearns and Macdonald, 1946 : PI. 1),rather than to movement on the rift zone itself.Commonly, group s of earthquakes on the riftzone are quite distinct from those originatingfarther south, as is clearly shown on the mappublished by Koyanagi (1964 ) .

The rift zone pattern is closely similar tothat resulting from distension of the surface ofdomes pushed up over intrusions (Cloos, 1955);and the most probabl e cause of Hawaiian riftzones still appears to be inflation of the volcanoby intrusion of magma within it.

CAUSES O F CALDERA COLLAPSE

Two principal hypotheses have been ad­vanced to account for the sinki ng of the sum­mits of the Hawaiian shields to form calderas.Both depend upon the presence of a magma

--body of ar.leasr.moderare size at a shallow.dep th _beneath the summi t area-a once-hypotheticalmagma body the actual existence of which nowappears to be demonstrated. Williams (1941:246, 286- 292 ) took the Hawaiian calderas asexamples of his calderas of "Kilauean type,"which he believed resulted from removal ofsupport caused by drainage of magma frombeneath them, caused in turn by rapid effusionon the flanks of the volcano or intrusion asdikes or sills. Stearns and Macdonald (1946: 33)suggested as an alterna tive explanati on that thecalderas resulted from weakening of the summitarea by large-scale magmatic stoping and caul­dron subsidence as in the Scottish ring com­plexes-calderas classified by Williams ( 1941:246) as the "Glen Coe type." The mechanismof thei r formation was believed to be the up ­ward enlargement of the magma body by stop­ing and cauldron subsidence until the overlyingcrust became too thin and weak to supportitself, when ring fractures developed and thesurficial block sank into the und erlying magmabecause of its greater density. The same mech­anism should operate if the enlargement of theunderlying magma chamber was largely bymelting of the enclosing rocks instead of bysroping.

Hawaiian Calderas-MACDONALD

Reynolds ' (1956) suggestion that the Scot­tish cauldron subsidences and the calderas thatpresumably lay above them were formed bythe gas-coring ·mechanism suggested by Escher( 1929), with "fluidization," intrusion, and ejec­tion of an ignimbritic solid-gas emulsion, canhave no bearing on Hawaiian calderas becauseof the complete absence in Hawaii of ignim­britic material and the extreme paucity of frag­mental explosive material of any sort.

Before considering further these older hy­po theses, let us look briefly at a new one. Thevery high density of the material in pipe-likemasses beneath the summit areas of Hawaiianvolcanoes has led to the suggestion that theformation of a caldera might be the result ofthe isostatic sinking of the heavy column, car­rying the overlying mountain top down with it.In this connection it is necessary to considerthe gravity field -found by Kinoshita and others(1963) on the island of Hawaii. Their Figure1 shows that the gravity high .for Maunaloa(a Bouguer anomaly reaching between - 330and 340 mgal) is offset several kilometers tothe southeast of Mokuaweoweo caldera, fartherto one side than is the presumed top of thedense material below the surface. There is nosign whatever of sinking of the mountain sur­face above the center of the gravity high. Thehigh for Kilauea (reaching about 315 mgal)also is excentric to the caldera, the center ofthe high lying some 2 km or more to the south­west of the center of the caldera. Thus, it ap­pears unlikely that the caldera formation canhave resulted simply from isostatic sinking,unless the improbable mechanism of a highlyoblique subsidence is invoked. Furthermore, nodiscernible sinking has disrupted the POSt­caldera cap on volcanoes such as Mauna Kea,beneath which a markedly high gravity anom­

aly still exists (Kinoshita er aI., 1963). Iso­static sinking in the ordinary sense, therefore,appears improbable as an explanation for theHawaiian calderas.

The high-density column beneath the vol­canoes may have another effect, however. Ris­ing, as it apparently does, some 5 km or moreabove the base of the volcanic mountain, thebase of the mass must have a considerabletendency to spread, and must exert a consid-

329

erable lateral thrust on the lighter materialadjacent to it. This must be particularly truewhen the mass is still somewhat mushy. Doesthe tendency for the heavy mass to spread resultin spasmodic lateral movements of its lowerpart into the proximal ends of the rift zones,causing a wedging open of the rift zone and adistension of the volcanic edifice?

The principal reason given by Stearns andMacdonald (1946:29-33) for the rejection ofWilliams' proposed mechanism for formationof calderas of Kilauean type was the fact thatflank eruptions with voluminous drainage ofmagma are frequent throughout the period ofbuilding of the shield, whereas the formationof the caldera takes place only near the end ofit. Actually, however, if the formation of thecaldera depends on the existence of a magmachamber in the core of the volcano, the absenceof such a chamber in the earlier stages wouldaccount for the absence of a caldera. Consider­able time must benecessary for enlargement of ..

-the cha;;b~r to the point where its roof is toobroad and thin to support itself.

A more conclusive argument can be madeagainst the application of the Glen Coe mech­anism to Hawaiian calderas : namely, the factthat the sinking caldera block must displace anequal volume of magma. Where does thismagma go? Does it rise into the ring fracturesaround the sinking block, as in the classicalinterpretation of the cauldron subsidence ofGlen Coe (Clough, Maufe, and Bailey, 1909 :Fig. 14)? There is no evidence to suggest it.Dikes are nearly absent along the calderaboundaries at levels exposed by erosion. Erup­tive vents on the caldera-boundary fractures arevery rare, and the few that are found appear tohave no fundamental relationship to the frac­ture. Thus, although the main cone of the 1949eruption of Mauna loa lies on the calderaboundary, the eruptive fissure was not thecaldera-boundary fissure, but one that extendedacross the center of the sunken caldera block,up over the caldera wall, and several kilometersdown the flank of the mountain. But if the dis­placed magma does not rise around the sinkingblock, where does it go? Out into the rift zones?This brings us right back to Williams' Kilau­ean mechanism!

330

The failure of magma to rise along thecaldera-boundary fractures itself calls for an ex­planation. The erupting basaltic magma has aspecific gravity of about 2.7, and it appearslikely that the magma even at the depth ofthe magma chamber has a density of only about2.73 (Macdonald, 1963: 1076). Compared withthis, the gross density of the caldera-filling rocksis at least 2.8, and probably is between 2.9 and3.0. If a mass of this density is underlain by amagma body of equal or greater horizontal di­mensions and lower density, why does not thecaldera block sink completely into the magma?The answer probably lies in the wedge formof the sinking block, bounded by fractures thatdip inward instead of outward. Reynolds(1956) has pointed Out that a downward con­vergence of the boundary fractures is impliedby the up-bending of the edges of the lava bedsfilling many cauldron subsidences, includingthat of Glen Coe. The margins of the older,eroded Hawaiian calderas are seldom well~~o~gh- exp()sed to revealwheiner··or .not -theedges of the beds are bent upward. However,Stearns (1940 : Fig. 7) has described a basin­ing of the lavas in the Koolau caldera, Oahu,and at least inone .sector the beds filling theKauai caldera are dragged slightly upwardagainst the caldera boundary (Macdonald, Davis,and Cox, 1960 :36) . There is a definite implica­tion that the boundary faults converge down­ward. Since sinking of a wedge-shaped blockwould tend to keep the boundary fracturestightly closed, this would also help to explainthe failure of magma to rise to the surfacealong them and the lack of eruptive vents onthe caldera boundary.

Why do the fractures converge downward,instead of diverging in the ring-dike mannerdeduced mathematically by Anderson (1936)?Perhaps the answer lies in the fact that thefractures were first established as a result ofupthrust of magma beneath a relatively smallportion of the mountain top, resulting in up­ward-divergent fractures of cone-sheet type,and that once established these fractures servedas the surfaces on which the caldera block latersank. The tumescence frequently observed atKilauea shows that magmatic pressure is greatenough to push up the tOp of the mountain,

PAOFIC SOENCE, Vol. XIX, July 1965

and the piston-like rise of the caldera floor inthe 1850's shows that at times the elevationtakes place by displacement of a fault-boundedblock rather than by quasi -plastic arching.Where upward pressure continues long enoughcone sheets may form, like the numerous con­centric inward-dipping dikes that surround thecaldera of the Ofu-Olosega volcano in Samoa(McCoy, 1965); and concentric lines of spat­ter and cinder cones may form by surficial erup­tion on these fractures, as on some of the vol­canoes of the Galapagos Islands (H. Williams,personal communication, 1964). In Hawaii,however, only a few dikes with the attitudesof cone sheets have been found, on Oahu andKauai. Even fewer examples are known of erup­tion on concentric fractures, but one such lineof cones lies JUSt southwest of·~ilauea caldera.For some reason, in Hawaii magmatic pressurehas usually resulted in distension of the volcanicstructure by upward bending of the summitfollowed by splitting open of the rift zones

- instead ·of lifting-of the apex of the volcano oninward-dipping conical fractures.

A factor that must be explained before wecan accept Williams' Kilauean mechanism isthe very considerable discrepancy that existsbetween the volumes of some of the subsidencesin the caldera during historic times and thoseof the simultaneously-erupted lava flows. Thevolumes of historic subsiden ces are listed inTable 1, the figures being taken from papersby Finch (1940,1941) except the one for 1955,which is from Macdonald and Eaton (1964).The volumes for 1924 and 1955 include bothmarked collapses at Hal emaumau and a generalsinking of the whole mountaintop over a radiusof 15 km or more, but this was undetectablewithout instrumental measurements in the ear­lier episodes of collapse. In these the figuregiven is only for the conspicuous sinking thatrook place in the caldera. Undoubtedly, how­ever, a wider-spread general subsidence, likethose in 1924 and 1955, also took place duringeach of the earlier episodes of collapse, andthe volumes of those were accordingly greaterthan shown. The volumes of the lava flows aretaken from Stearns and Macdonald ( 1946),again except for that of 1955. They includeestimates of the volume of subaerial flows that

Hawaiian Calderas-MACDONALD

entered the ocean; but the volumes, and eventhe existence, of any eruptions that may havetaken place below sea level are wholly unknown.

A mere glance at Table 1 shows that thereare large discrepancies between the volumes ofseveral of the collapses and those of the accom­panying lava flows. The smaller of the discrep­ancies, such as those for 1840 and 1955, canreasonably be attributed to intrusion of magmaas dikes in the rift zones. The volume of extru­sion in 1868 can only be guessed at, becausewe do not know the depth of the fill in KilaueaIki crater . However, the eruption on the south­west rift zone had a volume of less than 200,000rn", and that in Kilauea Iki cannot be reason­ably assumed to have been much more than5,000,000 m" and may well have been a gooddeal less than that, so that we are left with adiscrepancy of something more than 180,000,000m", This also could be accounted for by theintrusion of an unusually large dike, averag­ing about 2 m thick and 2 km high above thefop of -the magma chamber, and-extending -45km across the top of the volcano. (Both riftzones opened during the eruption.) The vol­ume of lava erupted above sea level in 1832also is not accurately known, but it was verysmall. Furthermore, the only known vents wereabove the level of the caldera floor, so thateruption from them can hardly be considereda simple draining of magma from beneath thecaldera block, allowing the latter to sink. Onthe other hand, the sinking of the block mayhave helped squeeze some magma to the sur­face. In 1924 there was no eruption of lavawhate ver above sea level. The discrepanciesbetween the subsidences of 1823, 1832, and1924 and the volume of known contemporane­ous lava flows above sea level are, respectively,more than 525 million, approximately 580 mil­lion, and 200 million m", During the 1924 sub­sidence the east rift zone opened for a distanceof more than 45 km, to and beyond the eastcape of the island, and Jaggar (1934) believedthat a submarine eruption occurred. It is cer­tainly a distinct possibility, and the same couldhave happened during any of the other epi­sodes of subsidence. An eruption in deep waterwould quite likely have escaped detection. Thevolume discrepancies can reasonably be attrib-

331

ured to rift-zone intrusion and/or submarineeruption.

Thus, the facts that must be taken into con­sideration in a theory explaining caldera forma­tion appear to be: Caldera collapse takes placegenerally, if not always, following a period oftumescence of the volcano, accompanying or di­rectly following an opening of one or both ofthe main rift zones, commonly accompanied bysubaerial flank eruption and/or possible subma­rine eruption; and the sinking takes place onfractures that converge downward, which in turnmeans that sufficient outward movement of thecircumferential mass must take place to makeroom for the increasing diameter of the sinkingwedge at any given level. The distension of thesummit region is very probably a part of, and iscaused by the same mechanism as, the simul­taneous distension of the rift-zone regions.Although eruptions are nearly always accom­panied by a general sinking of the mountain­top, _cal~er.acollapsemay nor .rake place. .Erup­tion may be confined to the caldera or theimmediate summit region without any apparentopening of the rift zones, or opening of therifts only close to the caldera.

The following possible interpretation of theabove facts, together with the other facts asto the general structure of the volcano pre­sented earlier, is suggested. Magma very richin olivine crystals rises from the mantle intothe volcano, adding to the bulk of the ultra­dense core; and part'-of. rhe liquid portion ofthe magma rises on upward, leaving behindmost of the load of solid crystals, to form apool of fluid tholeiitic basalt in a high-levelmagma reservoir at the top of the ultra-densecore or, in some volcanoes, somewhat to' oneside of the top of the core, beneath the sum­mit of the shield and extending outward be­neath the near-summit portions of the riftzones. The volcano swells in response to theaddition of magma. Local cupolas on the magmabody may rise high enough to perforate theroof, allowing long-continued open-vent activ­ity such as the lava lake that existed in Hale­maumau crater most of the time through the19th and early part of the 20th centuries, orthe eruption in Mokuaweoweo during most of1873 and 1874. Occasional withdrawals of

332

magma from the cup olas may allow the collapseof p it craters in and near the caldera and alongthe rift zones . Eventually th e top of the moun­tain splits open and some of the fluid tholeiiticmagma rises to the surface, allowing the volcanoto deturnesce. The force that drives the magmato the surface is primarily the hydrostatic pres­sure on the magma body resulting from theweight of the overlying rocks, but as the magmagets very close to the surface there is added toth is the expansive forc e of the gas that isseparating from solution. Eruptions of thistype are confined to the summit region.

As the mass of ultra-dense material in thecore of the volcano grows, from time to timeits weight becomes sufficient to cause a slightpushing apart of its confining walls in thelower part of the volcanic structure. This resultsin a splitting open of the volcano as a whole,including one or both of the rift zones andthe summit region. Magma drains ourward intothe rift .zones, ~ and cOIl1IIlQI!ly flank eruptionresults. The volcano derumesces, al;XiTdisren­sian of the summit region has been sufficientto allow sinking of the wedge-shaped calderablock true caldera collapse ensues . At othertim es, without any appreciable downfaulting ofthe caldera block, the detumescence may besimply a gentle over -all sinking detect able onlyby instrumental . methods (as it commonly isin the case of summit eruptions ); or it may beaccompanied by marked collapse only at pointswhere the underlying magma body approachesor reaches the surface, such as, the collaps e ofH alemaumau crater during the subsidence of1924 or the basining of the caldera floor in1868 and 1894. Sinking of the caldera blockdepends on distension of the summit more thanon draining away of the underlying magma,since the heavy rock of the caldera fill can sinkindefinitely into the less dense magma. H ow­ever, sinking of the caldera is generally accom­panied by lateral movement of the underlyingmagma into the rift zones because the samelateral displacement that stretches the summitregion enough to allow sinking of the calderablock also opens the rift zones, and room forthe sinking is largely provided by drainage ofmagma into the rift zones. Some magma issqueezed our in the summit region by sinking

PAOFIC SOENCE, Vol. XIX, July 1965

of the overlying rocks, bur the dens eness ofthe caldera-filling rocks and the wedge shapeof the sinking block keeps the underlying coni­cal subsidence fractures tightly closed andlargely prevents the rise of magma throughthem. Discrepancies between the volume ofsummit sinking, including caldera collapse, andthe volume of lava extruded in subaerial erup­tions ( both flank and summit) are accountedfor partly by intrusion in ro the rift zones, prob­ably partly by submarine erup tion, at timespartly by squeezing of magma upward into fis­sures in overlying rocks, and partly by spaceprovided by the slight sinking of the top of theul tra-dense core as a result of spreading of itslower portion.

The eruption is brought to an end by drain­age of the easily-eruptible magma down to thelevel of the opening of th e fissures, but theerup tion may be prolonged by rise of additionalmagma from deep levels, as appears probablyto have been the caseduring the 1959 eruption

. in "Ki lauea Iki (Richter and E atoil ; 1960) .Afterward the volcanic structure is sealed bypartial or complete congealing of magma inthe fissures, and the whole cycle repeats itse lfas more ultra-dense magma rises from the man­tle and more tholeiitic magma accumulates inthe shallow magma reservoir.

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