VOLCANISM IN HAWAIIChapter 42

DYNAMICS OF HAWAIIAN VOLCANOES: AN OVERVIEW

By Robert W. Decker

ABSTRACT

UtOil and Murata', crou ledioo of Kilauea Volcano, pub­lished ill t960 (Science." 132), ......u.. the buie model of bowHawaUon ...1......... wodt. Macma forms by partial mehinc ofmantle rock at depths bet..- 60 and 170 Ion ill a .....m. ....of mantle known .. the Hawaiian bot spot. The mck accumu­lates by mi.,..tiDa throulh .mall fractures, or by .bearcoaIeaccnae. into yolumel t....e enouah to ueencl. The ucen-­,ift pl"Uaw-e is caused by the greala" density of the rodusurroundiDc and ...erlyinc the maema. Aaeent throucb thelithosphere Gceun in diKOOtinUOUl cooduib that are ceneratedby mapnafractin,. Apparently there are enouch of these ucen­,we conduit. so that maema i. supplied to • shallow reservoir.ystem 3-7 Ian beneath the summit calder. at rates that vary byIe•• than an order of ma...itude over time acaIea of month. todecades. When the .hallow mapna reservoir 6I.l, to about1itho.tatic pre..ure, magmafractin, emplace. dike. upward orlaterally into the rift zonel; thole dike. that reach the surfaceerupt. Eruptiona at hiah.volume rate, rapidly reduce the pre_.awe i.D the ,hallow mapna reservoir and are of brief duration.Lonl-li. eruptions occur at low-volume rate. and are lUI­

taiDed by the resupply of maema from depth. La.. fOUllWns areproduced by the rapid czpanlion of mapnatic Cueto-H20,CO2, and 502-aI near-aurface pre..ure•. M.;or caldera col­lapses, sometime, associated with explosive eruptions, occurrepeatedly at intervals of • few thouaand years. One probablecause of these collapses is inb'uaion or eruption on the sub-.marine rifts which produce major draindown of the shallowreservoir system. The newly formed wdcanic islands subside atexponentially cleueuillc ..... u the Pac:m. plate mcnea slowlynorthwestward put the Hawaiian hot spot.

INTRODUCfION

The literature on Hawaiian volcanism contains many descrip­tions of eruptions, bot fewer intetpretatioos of the dynamic processesthai form magma and tbat cause it to ascend and erupt at thesurface. Present studies. however. are concentrating on obta.inin8 theneces.sary surface and subsurface data to compare with inferencesfrom dynamic models of how Hawaiian volcanoes work. 1hosemodels, coostrained by geolosical, geophysical, and geochemicaJdata, are heginning to give a glimpse of the compb. hiddenprocesses that occur beneath Kil..... and Mauna Loa Volcanoes.The purpose of this chapter is to synthesize these studies into acomprehensive modd cl the dynamics of Hawaiian volcanoes,particularly duriR8 thOr stage of vigorous growth.

The earliest recorded speculations on the subsurface structure

and dynamics of Hawaiian volcanoes appear in the journal ofWilliam Ellis, a Christian missionary frOOl England, who walked

around the Island J Hawaii in 1823. While visiting Kilaueacaldera, Ellis (1827, p. 171) noted tbat his Hawaiian guides toldhim ahout many eruptions 00 the flanks of Kilauea ·00 whichoccasioos they supposed Pele went by a road tmder ground from herhouse in the crater to the shore." Present-day interpretations of rih­zone eruptions on the flanks of Kilauea are considerably moredetailed, but in essence are the same as this "road Wider ground" ofthe native Hawaiians.

Ellis goes 00 to speculate about the dynamic processes beneaththe active lava lakes that he saw in Kilauea caldera (p. 181):"Perforated with innumerable apertures in the shape of craters. theisland forms a hollow cone over one vast furnace. Of This concept hasnot survived as well as Pde's road.

J.D. Dana (1890, p. 174-17)~ geologist witb tbe U.S.Exploring Expedition to Hawaii in 1840, was lhe tirst to distmguishclearly the more or less steady ascensive force of magma at depthfrom the rapid fountaming of erupting; lava that is caused by thesudden boilmg off of gases at near-surface pressures. Dana (1895,p. 303) attributed the ascensive force in the volcano to ·1) theexpansive action of moisture from the deep-seated source of thelavas; and 2) the gravitational pressure of the contracting; crust of thegIohe, forcing up the lavas* * *." He noted that the top of theKilauea matllIla column bad risen 360 feet in 6 years. Both ofDana's interpretations ar~ close to, but not quite the same as, presentmterpretations. These two causes of ascensive force could now ~reworded as follows: (I) The "",ansi", actioo of fluids, particularlyCO2, from the deep-seated source of the lava, forming separalephases; and (2) the gravitational pressure of the overlying, heaviercrust and upper mantle of the globe, forcing up the lighter lava.

Eaton and Murata (1960) were the first to draw a dynamiccross section of a typical Hawaiian volcano (fig. 42.1} In theirmodel, magma lDOYCS upward by density contrast, mor~ or lesscootinuous!y, through conduits from the top of a source region at 60km depth to a storage reservoir aI a depth of 3-7 km. Intermittenteruptions from the shallow magma reservoir to the surface occurthrough Il<WIy formed fractures breaking upward to the caldera orlaterally into the rift 2ODCS. The eruption rales from the sha1Iowmapta reservoir to the surface are often much~ than the rate ofresupply frOOl depth. and thus the eruptions lend to he self-limitingand intermittent. Somet:imes low-volume-rate eruptions fonn activelava lakes in the caldera, presumably sustained by the resupply rate.Such more or less steady-state eruptions can last for months or years.This overall modd fits well with the seismic data and obsenedground~surfacedeformation on Kilauea Volcano, and it is still the

997

U.S. Geololk.l SUI'YeJ Profe..ional Paper 1350

998 VOLCANISM IN HAWAII

FtGURE 42.1.-Sc:bem.tic CTOM Iedion throuP idealized Hawaiian volcano.Maema from iI. source about 60 km deep atreams up through conduits and coUec::tsin WJlow reservoir beneath caldera. Oceuional di.charse of lava from shallowreservoir through dike. dw split to surface col'llblutc ernptions. Note sligLldepresAon of MoOorov;cic diKontinuity beneath wkano. Vp i. the ~Iocity ofseWmc P-wa'l/'t:$. Modhd from Uton and M\D'ata (1960).

TIMETIME

N

B

cR

FIGURE 42.2.-EIecb1caJ anaIot: of dynamics of IGI.uea VokiilllO. Electricalcircuit is • battery (8) dtarsini • condenser (C) through • resistor (R). Whenvo.hagc auosl condenser reaches fimg voItqe of neon bulb (N), bulb winks on

until voltqe drops bdow -tainins level. In Ka.uea deep UCCDSive force ofmasma reprelellts battery, conduits from deep source to shallow masma mervoirform resiator. and ah.alkww mqma reservoir is condenser. Dike breUins: to surfaceor intrucin&. rih zone reduces masma pressure (vokase) and is equivalent to neonbulb. Variation of stram in volcanic edi6ce is sUnilar to variation of voltqe incircuit. but leu regular .. its cyc5es.

the various components are computed or assumed, then their quan­titalive interrdation can be calculated using well-established elec­trical or hydraulic equalions.

Realistic models of the structure and dynamics of Hawaiianvolcanoes are strongly dependent on the physical properties ofmagma and its host rocks. Some of these properties vary greally,aod the numerical values « others are only poorly known. Re6nedmodels are therefore dependen' on improved knowledge « thesephysical properties.

Atteml'linl! to understand the hidden dynamics « an acti..volcano calls upon the most eclectic abilities of volcanologists. Avalid dynamic model must ctplain the geologic and eruptive historyof the volcano, as well as the seismic, deformational, and elec­tromagnetic changes that occur before, during, and after eruptions.In addition it must aplain the composition and changes in composi­tion of the solid, liquid, and gaseous products emitted from thevolcano. Besides all this, the modd must be consistent with the lawsof physics and chemistry and, if possible. satisfy the philosophy ofOckham's Razor-that is, not to be unnecessarily complicated.Such a task clearly requires a team effort. Ths article is therefore aselected synthesis of the dala and interpretations of many students ofHawaiian volcanism.

u160 180 200 ~

~

~>

8.25 Mantle 7.9 3.27

DeMily-2.77g/em3

60 80 100 120 140[MSTANCE. IN KILOMETERS

4020

Mohof"ovicic diacontinuity/

o

60

SEA"LEVEL

""' 10"~ 152 209

"~ 30

>i~ 40

~~ 50~

basic concept of how Hawaiian vokanoes work.Building 011 Eaton and Murata's model, Decker (1968) and

Shimozuru (1981) ha.. noted that the 8uid and fracture dynamics«this model are analogous to the storage and discharge of electricity ina resistanee-capacitance (Re) circuit (fig. 42.2} The ascensivepressure is analogous to a battery, the resistance to Ruid Row throughthe system is analogous to a network of electrical resistors-withinthe battery, helw<m the battery and the condenser, aod between thecondenser and the swface-and the shaDow magma reservoir isanalogous to a condenser. If the resistance between the condenserand the surface operates in two modes-almost infinite resistance orvery low resistance, as in a neon bulb-then intenniuent dischargesof the condenser in the electrical model wiD be analogous toeruptions.

Although this dectrical analog model is greatly ...rsimplified,it has the advantage of allowing thought aperiments ahout whathappens when changes occur in the ascensive pressure, the resistanceto Row, or the volume of the shallow magma reservoir, eilherseparately or in combination. For cumple, if the ascensive pressure...... doubled. the repose time hetween eruptions woold he halvedand long-term lava output would double, but the individual erup­tions would have the same volume as before. Increasing the capacityof the shallow magma reservoir would increase the repose timebetween eruptions, but the eruptions would be of proportionatelygreater volume. Changing the breakdown strength of the rockshelw<m the shallow magma chamber and the surface-analogousto changing neon bulbs-could change both the repose time Ibetween eruptions and the volume of the eruptions. If the values of

42. DYNAMICS OF HAWAIIAN VOl£ANOES, AN OVERVIEW 999

ACKNOWUDGMENTS

JerTY Eaton. Howard Powen. and Cordon Eatoo were mykind hosts and pabmtteachen while I was a guest investigator atth.Hawaiian Volcano Observatory (HVO) in !he 1960'. and 1970'•.Robert TdIing offered me !he opportunity of relut'ninlto HVO a.Scientist in Charg•• and my line colleagues at HVO cootinued my

education during my tenur.ther. from 1979 to 1984. Jerry Eaton.Dan Dzurisin. and Barbara Deck.r gently butthorougbly ....iewedearlier versions of this chapter. My wannest thanks to all these feUowstudents of volcanoes.

MAGMA FORMATION

When. wber•• and how does magma Iorm} Ther. are no clearanswers to these questions. but there is no lack of Irypotbeses. Sinc.volcanism bas hem a persistent~c process for billions of years.

it seems reasonable to assurm that magma has continued to formduring moot of !he Earth·. history. A coun.... argument could be !hatmallJll'l only formed in !he early history of !he Earth. has alway.hem present within !he Earth. and bas hem r.leased slowly """"Ilhto stretch out volcanism for billions cl years. This ahemative has notbeen proven wrong. but since most geologic processes work at levelsdose to equilibrium. it is difficult to accept a process that could berestrained for such enormous periods of time. For this reason it isa5Sumed that magma is fonning within the Earth at the present time.

There is reasonably 800<1 agreement about where basalticmagma is Iormed. Yoder (1976) car.fuIly reviewed th. geophy.icaland geochemical evidenc. and concloded that basaltic m"llJll'l maybe generated in broad regions g....rally at depths of 50-170 km; inmore restricted environments the 50lUCe of basaltic magma may be asdeep a. 300-400 km. Eaton and Murata (1960) concluded on !hebasis of depths of earthquake sources that the magma source region

beneath Hawaii has its top at a depth of 60 km. but !hey were not

able to infer how far !he sourc. region may ..tend bdow this Ievd.~t·. mudd (1984) for !he oriiin of Hawaiian tholeiit. proposes

a depth of fonnation r""llins from 60 to 90 km. R.cent studies byAnderson and Dziewonski (1984) using sei.mic 'tOlllOllTaphy indi­cat. an anomalously slow (hot) zone in !he Earth·. mantle at a depthof 350 km OYer a broad ar.a south of Hawaii. Mantle Row in !hisarea is apparently upward. At shallower depths in the uppermantle, the anisotropy of Rayleish wave velocity sunests horizontalflow in a southeastward direction beneath Hawaii. As a workinghypothesis. it is assumed that magma is currently being formedben.ath Hawaii at some depth interval between 60 and 170 km.

The question of how magma forms is even more complex thanwhen or where it forms. From what rock material is it meked. andwhat is tbe sourc. of beat} Yoder (1976) think. that !he bestcandidate for the parent rock. c:i basaltic liquids is garnet peridotite.a crystalline __esat. of olivine. orthopyrox.... clinopyrox.... andgarnet. H. accepts thi. sourc.-tentativdy-and ~.ts tbe followinsfeatures of garnet peridotite as .upportive evidence (Yoder. 1976. p.

42):(1) Its occurrences are appropriate to deep-seated environ

ments of origin.

(2) It. close compositional rdation to meteont.. support. aderivation from material accumulat.d in !he primordial Earth.

(3) Partial mdt. of natural samples at high pr....r. havebasaltic compositions.

(4) The mineral a...b1ag. was found ...,.nm.ntally to bestable at high pressure and temperature.

(5) It has appropriate density and seismic velocity,Wright (1984) prefers a source rock. for Hawaiian tholeiite

magma composed of lherzolite. a crystalline aggregate cl olivine,orthopyroxene. and clinopyroxene. with some other constituents(amphibole. nephelinite. ihnenite, apatite, and ferrous iron oxide)added from depth.

The source of beat to mdt a basaltic ~quid from a cry.tallinerock agIVesale must be .ither internal or ..ternal to !he rock beingmelted. If it is memal, some thermal process such as conduction.convection, or radiation must transport heat into the source rqion. Ifit is internal. the heat must be generated by some process such asradioactiv. decay or mechanical friction. unI... it is beat that i.already present in the rock. In ,he latter case, some other processmust be called upon to lower !he mdtins temperatur. of !he basalticliquids, such as lowering the confining pressure or introducing ftuxinsdements or compounds into the source region.

The problem is not a lack c:l possibilities. but too manypermutations and combinations to choose among. Yoder (1976. p.60-61) lists 21 proposed mechanisms to melt rock into magma.Several c:l these are not tenable for Hawaii; for example. thosemechanisms that require subsidence of continental crustal layers ofth. Earth by 5 km or more (H.... 1960~ Amoos !he morereasonable mechanisms for generating Hawaiian magmas are thefollowing (in alphabetical order by author):

(I) Melting caused by !he friction of shear strain r.1eased fromstored da.tic ...ain energy along a propagating crack (CrisP andBaker. 1969~

(2) In....nal radinactiv. beat production (Joly. 1909~

(3) Conductive beat trapping caused by chanaes in !hennalcooductivity with temperatur. and pressure (McBimey. 1963~

(4) Rise of !he sourc. material as a buoyant diapir. thu.reducint conlining pressure and melting temperature (Ramberg,1972~

(5) M.lting caused by tbe friction of viscous ...ain. fn thisthermal feedback or nmaway modd. constant shear stress on acrystalline rock at its melting temperature causes smaU viscous strainsthat increase the temperature. This temperature increase lowers theviscosity. increases the strain rate, and thus further increases thetemperatur. (Shaw. 1973~

(6) Melting caused by !he friction of .hear strains ....aatedwith tidal ....rgy dissipated in !he solid Earth (Shaw. 1970~

(7) Rise of !he sourc. region by solid-stat. coo..ction. th..reducing tbe Coofining pressure and mdtins temperature (Ver­boosm. 1954~

(8) RedUCtiOll in mdtins temperatur. of !he basaltic ~quids byaddition of volatiles (HzO. CO,) to !he parent-rock source region(Yoder and Tilley. 1962~

Radioactive uranium. potassiwn. and thorium isotopes are

1000 VOLCANISM IN HAWAII

apparently ubiquitous in ~per-mantle rocks but low in amount.Hawaiian lava exhibits no abnormal content cl these radioactiYeisotopes. so radioactive beating (mechanism 2) does 001 ..... to be apriocipal cause cE the hillh rale cE Hawaiian masma productioo.HoweYe!'. on a global basis and ...... tens to hundreds cE mil~.... ofyean. radioactive heat production probably cootribut.. signi6cantlyto heating the rocks cE the upper mantle to near melting tem­

peratures.ChoosioS among the 7 remaining mechaniuns to aplain

Hawaiian magma generation depends in part on tectonic prejudices.Those who favor the hot-spot hypoth..is (chapter I. part I) willprobably favor the diapiric rise (4) and (or) volatile transfer (8)mechanisms. while those who fa.... a prop;ogating fradure mayprefer the stored dutic eoergy rde... (1) and (or) the co.ducti..heat trap (3) mechanisms. ~t's model (1984) combines athermal plume to drive up the added chemicaJ demeots from dePthand thermal feedback to provide the principal heat of melting. It isprobably best to say that the melting problem is 001 resolved at thistime. Detailed seismic tomography studi.. may soon provide moreddioite Mdence on this important but diIlicult subject cE the heatsource of Hawaiian magma.

[. all the proposed mechani.... for meltiog crystaUioe rock toform magma. the eothalpy of meking is ahout 4.2-5.2 X 10' 111<s(I 00-125 caVg~ Ths beat cE £USio. has a major bufferios effect 00

the temperature and rate of meltin&: or freezing in the source region,implying that basakic rnagmcu of similar composition at similardepths of formatioo are oearly isothermal. At 6O-km depth,Hawaiian tholeiite magma has a calculated formation temperatW"e of1.350-1.400 -C (T.L. ~t, oral commun.• 1985~ Measuredtemperatures cl erupting Hawaiian lava are in the range of1,100-1,200 -C.

MAGMA ACCUMULATION AND ASCENT

Melting a garnet peridotite or lherzolite mantle involYeS a

10-15 perCellt volume incr.... from the crystal qgregate to thebualtic fluid. A change from solid to 30 perceot partial melt woWdthus bring about a 3-5 percmt incr.... in the bulk vol..". cE therock-liquid mixture.

I. experimeotal studi.. on crystalline silicate "ll8"ll..... thefirst melt forms fi.hns between mineral grains that then unite into a~quid-bonded a_ate-a ..twork cE melt coating the mi.eralgrains with a tlu. 6lm of Ouid (Yoder, 1976. p. 164-165~ Thevolume cE these mohe. films can reach 10 perceot cE the mineral­liquid mixture. As the volume of melt increases, deformation frominternal volume expansion and. (or) interaction with enemal streSlelltakes on an increasingly important role.

Exteosion fractures perpeodicular to the directioo cE the leastcompr..si.. stress can form eYeD at mantle depths if the pore-Ouidpressure exceeds the tensile streosth cE the rock. Shaw (1980) foundthat th... cooditions are probable in the thermal and stress domaiosestimated to occur in the astheoosphere beneath Hawaii. His moddcE melt aceumulatioo aDd &SCmt is shown in ligure 42.3; in it theaccumulatioo fractur.. are subborizootal (sills) if the least com-

pressive stress is ~rticaI, or they are nearly ""tical fractures (dikes)if the iotermediale compressive stress is ""tical. Because cE smalltransient changes in the shear stresses. pore pressures, or gravita­tional load no this deep r<sioo where the three major compressivestr..... are nearly but 001 quite equal. Shaw thinks both cE the abovecooditions will occur and will _rale a set of planar melt lensesoriented at 50°-90" to one another, with intenections parallel to theseneral direction of maximum compressive stress. These intersectionsare showo scheruatically i. ~re 42.3 by the broken horizo.taIlmesqments within the asthenosphere. The inferred lenses of meltaccumulation would tend to lie both (vertically) in the plane cE thecross section and (horizontally) perpeodicular to it. Since there is no

seismic evideuce for major brittle fracturing io the astbeuospberebeneath Hawaii, these proposed cracks woWd ha~ to be small orform slowly.

Au alternative way cE expressing tbe fracture-accumulationhypothesis is shearing cE the 6Ims cE melt into progressively 1"'lIefpockets cE molten rock. Shaw (1969. p. 533) ref..-s to the meltfradioo as being "kueaded from the crystaUioe source"; Weertman(l972~ on the basis cE observed bubble coalesce... in deformingglacial ice, bdiews a simiIar process may occur during shear creep inpartially meked rock. Spera (1980) points out that the rale cE meksegregation is governed by the rheology cE the rock-liquid mush andbY the balance &IllODtl bonYODCY. pressure, and viscous forces. Healso notes that he can estimate buoyancy reasonably well and stressgradtents to within an order of magnitude, but viscous forces andpermeabilities only poorly. Nevertheless, Spera, on the basis ofcorrelation to rigid porous-media systems, estimates that the segrq:a­tioo rale is hillhIy depeodeot on the melt fradioo; in dimellSiooIessterms it woWd be 6.500 times Iarser for a 3O-perceot partial meltthan for a 2-perceot partial melt.

Whatever the mechanism cE melt 5ellfel!ation is beoeathHawaii, it must be rapid enough throughout the source volume toprovide about 0.1 km'/yr of magma to Kilauea Volcano for timeperiods \onge.- tbao a few decades (Swanson, 1972; Dzurisio andotben, 1984) and a roushly similar but less well known amouot toM....a Loa Volcano. Assuming that the source regioo is approx­imately cylindrical, as wide as the island (I 00 km~ and 30 km thick,the source volume woWd be approximately 2.5 X 10' km'. Using apartial melt cE 30-40 perceot~t and others. 1979; Wright,1984~ the calculated average fradio. cE the total mek ~vered tosbailow levels i. Hawaii on a yearly basis is only 2-3 X 10-6.

About 2.5 X10" 1 (6x 10'6 cal) of heat of fusion are needed bythis slowly evolving 5QUrce region on a yearly basis to maintain asteady-state supply rate cE 0.2 km'/yr. UDder the above assump­tions. this translat.. into about 3.2 X 10-' llm'ls (7.7 X 10- 12

caVcm'/s~ an arDount cE heat nearly 10' times great..- than the heatproduction from radioactivity in an aYerage garnet peridotite. These.....ben assume that the melting and segregation processes arespread eveoly tbrooghout the source regioo; it is Iikdy that theseprocesses in fact become more CODCeIItrated near the apex cE thesource replII.

The mechanism cE the asceot of magma through the more rigidrocks of the ~thospbere must be both qualitatively and quantitatively

42. DYNAMICS OF HAWAIIAN VOLCANOES, AN OVERVIEW IOOJ

Volcanic propagation •

lITHOSPHERE

-',-

••• ? ..----_ ' ----------: . +-- (Motion vector)

Zone of isostatic stress relaxation .·{I r'!,,0-3 normal to•. ' FUnne]" section

.. '.' .''; ,(I II! I i'1(. ,, S2 L S_ _ _n.LrLu iJJ !J ill! !JIlli.l.J: i :u.y.u.:,'_I_.....- - - - ~ .. ,

Shear." .. " - Diking in plane of section -".',.

melting " - - - - - -" ASTHENOSPHERE"': 0-~0-30r~ 0-2:::~'\

OJ /V 0-2 ,N 0-3 + K ' - :Zone of melt accumulation:-_-_-and.'-. Zone of residual :.

crystal accumulates:

t t tFIGURE 42.3.-Schel:uit Iooaitudinal teetioo sbowiaaltress domains bmea.hactivc Haw.n.n vok:.moes. S. loUI DOI'ID&I.rreu acting. a pWrt mrod,:a, effective normal

.tress actiDa in rock c:ontaiUJ& lID~ IIuid pt.ae. SIIbIcripu I, 2. and 3 releT to muimta. io~.lIndrainiaJIw priocip.al 1tresIes. repec:tiYdy. K.te.ae strqth. In zaoe c:l mdt acanuI.Iboo, fractures waWd be eidler nearly wrtital ill PlMe ti tedioo. or- Dearly horizoruI ED pboe perpmdicular to section. I..uceol: fwmd. frac:twa -Mel be Dearly Yatic.al in pbae of Jediocl. ModiiecI from SMw (1980).

different from that of its segregation and ascent in the source region.The tensile strength of the lithospheric host rocks is greater than thatin the asthenosphere, aIthoush it may become weakened by heatingfrom ascending m'llJll". D.vialor1c stresses in the lithosphere may bemueb greater than in the astbmospb<re, and beneath Hawaii thelithosphere is seismically acti.. whereas the astbmospb<re is appar­ently not.

M'llJlI" is pushed upward into and through the lithosphere bythe pressure gradient caused by gravity actins 00 the densitydifference between lighter magma and heavier host roclts. Thisexplanatioo of the aseensi.. force was lint dearly proposed by Daly(1914, p. 183) who sugg<sted"that magmatic eruptioo is, in the lintinstance, a hydrostatic pbeoomenoo." Daly later became -. morecoovioced. His second edition (1933, p. 247) states "The leadingcause fw the asceot of primary 1llatlJII&, assumed to moYe up abyssallWures, is naturally found in the ...isht of the adjacent crust."Almost no ooe would now disagree with this ""planalioo of theascensive force; the origin of the abyssal fissures, bowever. is muchmore debatable.

Does Hawaiian IllatlJII& passively rise through conduits in thelitbospben: g<s>er&ted by ""ternal stresses, or does it activdr geuer-

ate or help to generate these cooduits? Turcotte and Oxbursh (1978)propose that defonnation of the Paci6c plate in adjusbnent to thermaland Eartb-eurvature changes may cause a major propagabntl: intra­plate fracture. They suggest that magroa leaking up this propagatingfracture bas progressively funned the Hawaiian volcanic chain.Since Wilsoo (1963) proposed the bot-spot oritPn for the HawaiianIslands, most other researcben believe the melting anomaly is fixedin position and tblIt plate molioo ..... the bot spot causes the ageprogression of the Hawaiian chain (Dalrymple and ..hen, 1973,McDougaU, 1979~ Implicit in the bot-spot hypothesis is the ideathat magma in sufficient quantities can somehow force itself upwardsthroogb the lithosphere. Possibly the stresses proposed by Turcotteand Oxbursh (1978) may assist in this magmafractins process.

lbe coocepl that magroa can actively fracture its way upwardis strengthened by the distriootioo of earthquake hypoceotenbeneath Kilauea Volcano (see chapter 43} (),.,. periods of years

this earthquake distribution outlines a zone ci active fracturing in thelithosphere that is shaped like a steeply dipping inverted funnel",aching generally to a depth of 45 km and occasiooally to nearly 60km (Eatoo, 1962} Close associatioo of the earthquakes with theinferred conduit path of the rising 1llatlJII&, rather than with the site of

1002 VOlJ:ANISM IN HAWAII

FICURE "2.4.-~ CI'OSI sectioa of Ii.thospher-e bene.lh Kilauea VolcanoaDowina aae- ascent funnel ... recion of eate...toaal n.c:tureI aDd mqma blll:d­....---. upward from asthenosphere. Inlel tboW5~ p/an of rift zones.StreM orien~ and dike orientations ba~ different dinctiODS in Iitboapbcre andia edifice r:I Yowo. Symbols have lame meaninc as in _e 42.3. Modi6edfromShaw (I980}

order to estimate the volwne and magma fraction of the activeintrusion zone in the fithospher. located betwem the deep sourceI'<llion in the asthenosphere and the shallow magma-reservoir systembmeath Kil..... Volcano. His calculations _t that this zone ha3• volume of.bout 100!un' and a melt fraction betwem 10-' and10- 4 (thai is, 0.01-0.1 !un' of_~ For continuity, the 0.1­!un' yearly magma supply to Kil..... would thereby ba.. an .....08<..,ide3lc. time of only • month to • year while rising thrnosh thiszone. M"8JIlO ascent m.y appear rdati..Jy continuous over timespans of months to y<an, but can be quite vari.ble at time scales ofminu.es to d.ys. Se..ral batch.. of Iav. m.y be moving upward inindependent spurts. Some batch.. may rise 30 !un in • few weeks,while other batch.. may ...... reach shallow bois. "The varying rat.of inlIation of the summit rqion of Kilauea, as measured bycontinuously recordins tiItmet...., support> this modd of sanicon­tinuous rise of magma into the shallow reservoir system.

MAGMA STORAGE

Batch.. of Hawaii.. magma risins thrnosh the fithosphereapparently take various paths and various amounts of time in theirascent, but most of the m"8JllO supplying Kil..... Vokano duringthe past sevual decades has paused in a shallow magma storagesystem before erupting to the surface.

Mogi (1958X usins data from Wilson (1935X 6n. identifiedthe presence and depth of this shaUow magma ....rvoir by analysisof SUlface uplift and subsidence of the summit and upper Ranks ofKil.uea. Usins elastic-deformation theory he identi6ed two m.jorsites of pressure and (or) volume change in the region beneathKil.... c.Id....: one wdI-de6ned site at 4 !un depth and another I."wdI ddined sit. at 25 !un depth. Eaton (1962) rejected the deepersite as an apparent artifact of leveling-rod error in the long surveyIra..... from Hilo to the summit of Kil..... Usi"ll .dditionall.....and tib data, Eaton con6nned .he shallow r.......... system andshowed it on his schematic cross section (6ll. 42. I) as an irregular_tical cylinder about 4 !un wid. and at 3-7 !un depth bmeath thesummit of Kil......

Patterns of continuously recorded summit lib over the past two

dec.de> (6ll. 42.5) indicare that this shallow magma-......oirsystem slowly inftat., over periods of mon.hs to y<an during theintervals betw... eruptions and then rapidly dd1_ over periods ofd.ys to weeks durinIl8an1: eruptions or lateral intrusions into the riftzones. Expansions and contractions of the calder. di........ by asmuch as • few ........ acCUDpany these inftations and ddIations.

On many of the inftation cYCles shown in fisure 42. 5, the rise ofthe summit shows • high rate of inlIation imnsediatdy aft.... dd1atioo...... This is followed by .. "'POlleDtiaIly decreasins rale ofinlIation, probably caused by the decreasins p=ur. ll'adien.betwem the shallow JDalllll'-reservoir system and the sourc., ofJDalllll' at ll'eater depths and (or) by the increased Ieakag. ofmagma into parts of the shallow magma reservoir away from thesummit as the pressure in the shallow resemor system increases.

Coosiderabk r.....-ch ha3 been done to attempt '0 defi.. thedepth, sh.pe, and dynamic nature of the shallow magma-reservoir

lithosphere

a; CT.V'

A'

Composition pt."

r----7A'---, EdifK:8

A Wo rN.:.E__--"....r-_.:O.:C=EA.::N.:..:.:Fl::O::O::;R~ S::;W:;.

1,1;1III

ll,l \~I ~\\I~+ \1 t ~\

\1:t~1 1\'11 ~ \\~~l+\\ ~\ 1+ \ 0;

\ t 'I I\~o;~ 10 to 20 km~ a:.Source funnel-Cross section I--401tm

the postulated intraplate fractw"e. indicales that the magma plays anactive role in the formation of the coodui13.

Sh.w (1980) propo'" th.t m"lllDO i, injected in.o thelithosphere by • proc." analog0U3 to hydraulic fr.cturinll of the wallrock> in • borehole by incr..,;"g IIuid preuur.. "The normalcompressive stresses in the buried bast rod. can be balanced orexceeded by the magma pressure to produce zero or even negativedff:d:i...e stresses. When these effective stresses equal or exceed thetema. ,tn3l11h of the rock, ..t<miooal fr.ct..... form perpmdicularto the direction of lea>t compres3i.. ,tr." (fisure 42.4~ Yoder(1976, p. 179) ha3 called this proc." ~actintl. In this viewmost of the earthquak., bmeath Hawaii are the effect, not thecause, of the transport of Jna1llDO.

A. the _icaI dik., of magma shown in 6gur. 42.4 intermit­tendy forc. their way upward by brittl. fr.ctur., they must beresupplied with ..".. magma from the sourc. rqion, or the trailingedse of the rising dike will dose off because of the n:<XM:ringcomp....ive str..... (Pollard, 1976~ HiD (1977) shows that shearstresses will cause the acb...e dike system to become a complicatednetwork of dik...liIt. I..... connected by f...lts. Mov.....t of magmawithin 0.. of the dikes will change the local _ of deviatoric andeffective stresses. and this may trigger new fractures or mowmentson the .,ostintl fracture network. HiD ..... this modd to _lainearthquake swarms in Hawaii, and Shaw calls socb resultins swarmsfailure cascade>.

Shaw (1980) then inv...13 the seismic d... from H.waii in

42. DYNAMICS OF HAWAIIAN VOLCANOES, AN OVERVIEW 1003

I.'" ,-l----'L-.L....L-'_L---'..----'._L-.L....L-'_L---'..----'._L-.L....L-'_L---'..----'._L--L....L-'_.L....L---'.-',

'-r-'1980

'-r-'1970

YEAR

FIGURE 042.5. -Radial tilt lit H~aiian VokaDo Ob.ervatory (HVO) on hOfthwe.l rim J KiI..ea caldera. Increasing tilt (r.malty outward) indicatet "'ion J IllImIitrqion; decreuinl; tilt iDdiates det.tion. ODe Iuded micror.diant (.-r&d) of tilt II. HVO Ddicaks upiift ~ apex ci aunni.t buJce by about 50 an. Rapid ddIatioDmarked by .hup drop. in tilt rq>raUll volume chanse of buJac cl.bout 3.3 X IOSI...,ad and prellUre chp in shaBow maama raemMr .ylkm clahout 90 kPaf....,ad.Mqma is added dowly to shdow reservoir system from depth betwem eruption. aDd i. rapidly TCmCI¥edd~ Bank eruptions. Verticallmes mdtcal:e eruption.; S•....... crupbont; F, bnk eruptions. Offld in ti&: rec:ord in Nowmber 1983 wu clUH'd by M..fJ.6 earthquake (lee Buchanan·Banks, chapter 44).

tilZ<~ 1,500

'"lil~::E~

~;::

.y._. Geologic evidence frOO1 the location and form of Kilaueacaldera and the pit craters along the upper east rift zone gives someindication of the .ubsurface locaIioo and lateral ettent of the reservoir.ys_. The caldera and pit cralen ha.. formed by collapse andthus identify the .ubsurface regions &001 wbieb magma has beenr...."..J in such 1"'3" quantities that the deflatioo cannot beaccommodated by dutic or plastic surface deformatioo. Presumablythese large removal. occur during major eruptions from the distantsubmarine portions of the east rift UlDC. Interpretation of thismorphological evidence indicat.. that the main magma reservoirbeneatb Kilauea caldera is (or was) rooabIy elliptical in pIan-4 kInNE.-SW. by 3 kIn NW.-SE.-and that about 3 kIn' of magma

had. been ra:now:d from the shallow subsurface reservoir to accom­modate the vol.... of the caldera as it appeared in 1823.

The Iocatioo of the pit craters is ovidence that the lJlalPIlastorage .ystem ..tend. (or ..tended) southeast and then northeastbeneatb the upper east rift zone at least a. far as Napau Crater. 15km east of the swmtit caldera. The pit craten range in diameter andvolume from I kIn and 77 X 10" m' (Makaopuhi before 1965) to 25m and 90 X 10' m' (Devil'. Throat); these ligures put some limits onthe width and volume of pt"evious magma-.torage regions beneaththe east rift UlDC. There are only two small but deep pit crat.,.,along the southwest rift zone, indicating its subordinate role as amasma-.torage resion. Duflidd and others (1982) conclude that the

.outhwest rift UlDC breaks primarily because of accumulated m0ve­

ments cl. the east rift zone and to a lesser degree because rl magmaticpressure from the summit reservoir system.

Geodetic data on the changing dcvatioo. and di.p1acements ofthe ddonnintl ground .urface abo.. the ,haUOW magma-reservoirsy.tem ha.. provided the best evidence on the dynamic nature ofthese lJlalPIla .torage chambers (Ryan and oIbers. 1983~ Detailedbding of the .ummit area of Kilauea during an infIatioo period byFiske and Kinoshita (1969) shows that the apcI( of infIatioo migrat..."... distanc.. of 1-3 kIn within or near the caldera (figure 42.6~

This shift ci the center of inRation indicates tbat the summit reservoiris a plexus c:i interconnected storage zones. It can be compared to aMichy Mouse balloon whose head infIat.. separaldy from the can.Recordintl tiltmeters near HVO,,- tbat this migratioo of the apcI(

of infIatioo or ddIatioo during the l"'llel cycl.. of masma influx anddiscb"'3" has a repetiti.. pattern. After a major .ummit deflation.reinflatioo besim beneatb the northern part of the caldera and thenIJlOVOS southward; during the "",t major ddlatioo. the northern partof the caldera also .ubsides fint and is quiddy followed by the southend .ubsidintl. l1sis implies that the main input and withdrawalconduit. for the summit reservoir pi.... are on the nortb side of thegeneral storage zone. which h.. mainly beneatb the soutb end of thecaldera.

Dvorak and other. (1983) analyzed the .urface deformation

FtcURE "2.6.-Laaeral shift cl apex 01 uplift of infialina bulae at .ummit clKiI_a Volcano, modibed from FISke and KinoIbita (1969~ lcYeI. Slln'q'S

<kmon.trak dUh'" ""'" 01 upI;ft, (I) )anowy-Joly 1966; (2) Joly-Octoo.,1966, (3) Octo"', 1966-}anoMy 1967; (4) )anuary-F.lxuMY 1967; (5) daringFebruary 1967; (6) February-May 1967; (7) May-June 1967; (8) June-July1967, (9) Joly-Saplambao- 1967, (10) s."....bao--O<tob<, 1967. Poe';'" 01Hawaiian Volcano Observatory and caldera and aaler walls (heavy lines) shown(or I'dtUnCe. Total uplift of the IO·km-wide bulge at its apcL: from january 1966to October 1967 was approximalely 70 an; swnmil enIpbOIl oi Kilauea began inNO'o'eRlber 1967.

data 00 Kilauea for !he period 1966-1970 by .imolta..... inver­sion of the level, tik, and horizontal-distance measurements. Theydetennined 32 centers of inflation and deflation, which clustered intotwo groups. one at the south end of Kilauea caldera at depths from2-7 kin and the other neaf Makaopuhi Crater OIl the east rift zoneat depth. of 3-8 km. 1beir results show that akhoush the generaldefonnalion pattern is apparently elastic, there are departures fromideal dastic behavior. The differences between measured. and calcu­lated horizontal displacements indicates that a gradual north-southex:tension across the summit caldera occurred during 1966-1970.

Changes in gravity as much as a few tenths of a milligalaccompany the inflation and deflation c:i the summit area of Kilauea

Uolmsoo. chapter 47; Dzurisin aod others. 1980; Jachen. aodEa'on. 1980~ These changes are more complex than those expectedfrom simple changes in devation, and they indicate that mass that isDOl proportional to the volwne changes can be both added to andremoved. from the summit region.

1004

HawaillanVolcano

ObMrvatory

•

o

Kilauea caldera

1 KILOMETERI

VOLCANISM IN HAWAII

The volume of the shaDow magma-reservoir system beneath

Kilauea can be estimated by assuming that it is roughly sphericalwith a diameter similar to the 3. 5-km diameter of the caldera.Because the magma-reservoir system is probably a network ofmagma bodies separated by screens ri solid rock. ooIy part of this22-km3 volume is estimated to be molten rock. A net volwne of IIkm3 would be the same as the estimate of magma-reservoir volumemade by 'Migh, (1984. p. 3238) on ,he ba.is ri hi••ummary

analysis of the geodetic data.Earthquake hypocenters also provide important and independ­

ent evidence on the shallow magma-reservoir system. Koyanagi and

olbers (1976~ on the basis of an aseismic zone largdy surrounded byan envelope of seismic sources beneath Kilauea caldera, estimatedthat the zone of primary SWIUIlit magma storage has a mean diameter

of 3 km and extend. from a depth ri 3 km to 6 km beneath !hesurface. Their interprdation is based on the concept that the maplareservoir is too ductile to produce earthquake-generating fractures,

whereas the rocks above and around the magma storage zonerespond to inftation and deflation of the magma reservoir by brittlefracturing.

Ryan aod others (1981) expanded this approach and pro­duced a detailed modd of Kilauea's magma-reservoir system from

the .orfate to a depth ri 15 km (fig. 42.n 1heir model has aprimary conduit otending vertically beneath the south end of thecaldera, an upper-east-rift-zone pipe extending verticaUy through

dep'h. of 2-6 km beneath the Koae faolt zone I km southwest riKokoolau Crater, two horizontal ducts connecting the primary

conduit and !he pipe at depths ri aboot 2 km and 6 km. and anextension of lbe upper horizontal duct beneath the east rift zone tothe Mauna Ulu eruption site.

The main shallow magma-storage reservoir beneath Kilauea is

apparently not completely aseismic. During times of rapid summitcoUapse and initial reinflation, swanns of peculiar microearthquakeshave their apparent source within or very near the margins of thesummit magma reservoir. These earthquakes generally have magni­

tudes less than I and have a low-frequency, emergent signal. Theyare difficull to locate because they are only recorded on a few

seismographs near the swnmit of Kilauea and their onset signals are

not clear enoU3h to establish accurate arrival times. 1hese swanns oflong-period earthquakes are thought to be relaled to rapid readjust­ments among magma pockds within the plexus of the main magma

reservoir.The question of a deeper ,Iorage reservoir about 25 km

beneath Kilauea remains unresolved. At present, deformation data

are not accurate enough 10 detect small inRations and deflations of azone that deep. The region at that depth is rdativdy aseismic. bul noenvelope of earthqoake hypocenter••orround. it. 'Might (1971)proposed a region of tonporary accumulation of magma at a depth

of 20-30 km beneath Kilauea to allow for rellloval ri olivine andpyroxene (fig. 42.8) He called this regioo ,he intermediate .taging

area.

The dynamic nature ri the well-established shallow magma­

reservoir system benearh Kilauea is currendy being investigated fromseveral directions. Epp and others (1983) have shown that there i. a

42. DYNAMICS OF HAWAIIAN VOLCANOES, AN OVERVIEW

KILAUEA CALDERA

1005

2

3

<: 5'1.l' 6

't 7

'%,8~ 9

10111213

1415 "'£:,lX'i,X'16 "1718

58o l~m

".

6

7

8OCEANIC CRUST 9

10

"'··X:.i) ':lilY/if 11...• 1213

14

3

2

o

FIGURE 42.7.-Penpectivc vM:w northward and downward into interior of Ka-.. Volcano, from Ryan and otben (1981} Mqma conduits are defined by ZODei ofearthquake hypoccnkn that surround them ow:r periods of ~aI yean. Numben 011 mqma conduits rcp~nt horizontal seebons at depths corTeSpondina to leveI­number leale on left lide.

strong linear correlation between the amount of summit deflation andthe devation of a flank eruption of short duration (6g. 42.9~ Summiteruptions involve little or no deflation. whereas low-elevation flankeruptions are accompanied by larse deflations. A simple explanationof this relation is that the magma pressure in the reservoir drops untilit is balanced by the back pressure from the column r:J magma in theconduit from the reservoir to the surface. If this is the case, themagma pressure in the reservoir at the end of an eruption is simplythe product of the average density of the magma colwnn multipliedby the elevation difference and the gravitational constant. For

elUlDlple. assuming the average density of the magma column is 2.7g/cm3 , the pressure in the magma res~rvoir 3 Ian below sea levelbeneath Kilauea following the eruptioo at Kapoho (30-m elevation)in 1960 would ha.. been 83 MPa (830 ban); .he lowest pressure inthe magma reservoir during the past few decades. Sinc~ th~re are nomajor deflations associated with summit eruptions, there must belittle or no pressure drop in th~ magma reservoir during theseeruptions. 1ne pressure of a magma column from the present floor ofKilauea caldera to a depth of 3 km below sea level would be 109MPa. This difference of 26 MPa (109 minus 83) is the approxi-

1006 VOLCANISM IN HAWAII

110 '----------'----'--_---'_-i._-l

1100,----,--,---,---,----,--,----,

8000

'"wI;; 700

"<;;i 600Q

Ii:~ 500'"w~

0Z 400Q>-<> 300w~

w

200

100

0

0

niches in the reservoir system and never erupt, while other magmabatches, such as some rJ that which supplied the Kilauea Iki eruptionin 1959, may spend only a small fraction of the average residencetime in the magma reservoir system.

Another dynamic qua~ty of shallow Hawaiian magma-reser­voir systems on a vastly different time scale is their apparent abilityto migrate upward as the volcano they supply grows in height. Thesubmarine volcano Loihi is assumed to be in a youthful stage ofgrowth, yet already a caldera and rift zones have apparentlydeveloped upon it (Klein, 1982a; Malahoff and others, 1982~ Thisimp~es that it has probably devdoped a shallow m"llllla-reservoirsystem. Tbe depth to Lolhi's magma reservoir is not known, but thesunuuit area of Loihi is ahout 1 km below sea level. Mauna LoaVolcano, at the other ",treme, has reached a height of 4, 169 m, andrecent studies by Decker and others (1983) indteate that it has ashallow magma-reservoir system similar to that beneath Kilauea.The source of inftation shown by deformation measuranents beforethe 1984 eruption of Mauna Loa was at a depth of only 3 kmbeneath the summit. Thus, the top cl the magma-reservoir systembeneath Mauna Loa is at nearly the same elevation as the ground

50 100 150 200 250 300CHANGE IN SUMMIT TILT. IN MICRORAOIANS

FIGURE 42.9.-Ddlabon meas.-ed by change in radial sunnit tik .. KillIIleaVolcano __ elevation ci UIOC::i8ted. _tsd~ eruption. frua 1955-1979.Bars in&ate tbIII eruptions O«'UITed from 6.surel widJ consida-able ranee inelevation. Uneu rdlll:ion ci ddlacion and ¥all deYalion indicates that preuuredrop mswamit IMIIM racrvoir is closely related to hydraulic bead of magma ateruptinc ¥alt. Modi6ed from Epp and othen (1983)

2.5

REGION OFCOLLECTIONOf MAGMA

MAUNA LOA

MANTlE

30

20

040'"w>-w

" 500~

;;;<; 60

:i>-~ 70w0 ..

90

100 0,

mate maximum range of pressure change in the shallow magma­reservoir system beneath Kilauea during the past few decades. Inaddition, the pressure chanse related to the amount of rapiddeflation can be estimated from the data of Epp and others as 0.17MPa (1.7 bars) per centimeter of SUIIlIIUt deflation. Davis andothers (1973, 1974, 1979) coocluded fr... the absence of anymarked piezomagnetic effects related to tilt chaJl8eS at the summit ofKilauea that the volcano can support no large-scale pattern of shear

stresses. Th~ suggested that the rocks surrounding the shallowmagma reservoir are of low shear strength and. may behave in a

plastic manner. Because the summit region of Kilauea probably doesnot behave in a perfectly elastic manner, particularly oYer timeperiods of months to years, slow inflations may represent volumechanges more closely than pressure changes in the magma reservoir.

M"lllIlafracling of the ho.t rocks surrounding the m"lllll"reservoir appears to take place when the magma pressure reaches orslightly exceeds lithostatic pressure. This indicates low bulk. tensilestrength in the surrounding rocks.

The residence time for magma storage in the shallow reservoirsystem beneath Kilauea is easily estimated from the annual supplyrate (0.11 km'/yr, Swanoon 1972; 0.086 km'/yr, Dzumin andothers, 1984) and the volume of the reservoir system (11 lan3,Wright, 1984~ The result, 100-120 yr. should be regarded as anaverage residence time. Some magma may crystallize in remote

FIGURE 042.8.-smm..tic eros. section cl KiIa.ea Volcano by \tTisbt (1971)outliniaa: rqion. of meltins. transport, and storage of maema. Fracbooation cJ.DlapIa ocaan mainly in intumcdilte stlllPnl area and sba)ow ruervoir. but abodlfttc its ucent fn:n 60 to 30 bu.

42. DYNAMICS OF HAWAIIAN VOLCANOES, AN OVERVIEW 1007

wrface at the smunit c:1 Kilauea. The long-term evolution ofHawaiian volcanoes apparently involves progressive upward remdt­

ing, sloping. and (or) shoving aside of the rocks overlying theshallow magma-reservoir systems.

SHALLOW INTRUSIONS

As the shallow magma-reservoir system slowly inflates withmagma added from depth, the increasing pressure and volume puts

new stresses on the surrounding rocks. Sometimes the summitdeflates slowly or remains level as magma moves almost aseismically

into the open magma conduit rlthe upper and middle east rift zoneof Kilauea.~ slow intrusions or readjustmenls of the aistingshallow magma-resen'Oir system involve volume flow rates of 1-10m'/s. When sh"ess<s reach the breaking strength r:i /he hos' rockssurrounding the magma- resavoir system, fractures occur and a dikeor sm is mtnKled upward or laterally into the shallow, brittle rocks ofthe volcanic edifice. These rapid intrusions are common al theswnmit and along the rift zones of Hawaiian vokanoes, and t~occur at volume flow rates of IOZ-IO' m'/s. During 1959-1980.2 r raptd intrusions without eruptions occurred at Kilauea Volcanoand 2S other rapid intrusions led to eruptions (Klein. 1982b~

The orientation of rapid. shallow intrusions is controlled by the

strength of the rocks and by ambient stresses generated by bothinternal magma pressure and external gravitational forces. Fiske and

Jackson (1972) have shown that the topography of the earliervolcanoes in Hawaii affects the orientation of the rift zones of laterfonning, adjacent volcanoes (see Peterson and Moore, chapter 7.fig. 7. 7} Rift zones fonn perpendicular to the direction of leastcompressive stress; once a topographic ridge has formed fromnumerous dike-fed eruptions along a rift moe, the gravitational

slwnping of this ridge reinforces the least compressive stress in adirection perpendicular to the ridge. lOe name Mauna Loa meanslong mountain. and this great shield volcano is markedly dongatedby the gently-sloping ridges along its northeast and southwest riftzones. KilaJ.rea Volcano is younger than Mauna Loa Vokano and

inherited the gravitational Str6S system on the southeast Rank ofMauna Loa. Kilauea thus developed a principal east rift zone andsubsidiary southwest rift zone.

A typical rapid intrusion into a rift zone of Kilauea Volcano ischaracterized by a swarm of small earthquakes occurring at rates of

2-4 events per minute that lasts for a few hours to a few days. The

sources of these earthquakes are concentrated at 2- to 4-km depthsbeneath a segment of the rift zone several kilometers long. The

hypocenters of the swarm outline a tabular shape a few kilometershigh and about I km wide. The hypocenters also migrate pro­gressively in time and space, both vertically and along the rift zone,

indicating that the intruding fracture is propagating at a velocity of afew hundred to a few thousand meters per hour. The linear trace ofintrusions that reach the surface as eruptions, and the erCK!ed riftzones of older Hawaiian volcanoes, indicate that the rapid intrusionsare steeply dipping dikes that parallel the rift zones. Ho....-ever. theshapes of the rapidly intruded bodtes and the more open magma

conduit along the upper and middle east rih zone of Kilauea below a

few kilometen depth ha,~ not been established. The concentration clearthquake hypocenters at depths of 2-4 km in the seismic swarmsassociated with rapid intrusions suggests that the intrusion has SOIlle

sort of axis at thai depth.

In addition to their associated earthquakes, rapid intrusions are

generally characterized by rapid deformation of the swnmit area andground deformation above the intruded region. Volcanic tremor. new

gas emissions and electromagnette field changes also accompanysome intrusions.

Rapid intrusions into the summit region of Kilauea Volcanoare accompanied by rapid summit inflations. whereas rapid flank

intrusions into the rift zones remove magma from the summit regionand are accompanied by rapid subsidence of the slITlmit area.

Rapid intrusions into the rift zones of Kilauea have typical magmavolumes r:i 1()6-IO' m' and they occur at volume rates r:i IOZ-IO'm3/s. These typical volumes are associated with subsidence of the

summi. by 1-15 cm and pressure drops of 0.3-3 MPa in /heshallow magma reservoir. An approximate rate of magma supply to

a dike intruding the rift zone5 can be estimated from the tilt record.The radial tilt at HVO on !.he northwest side ol Kilauea caldera has

been calibrated as representing about 3.3 x 10) m3 of subsided

volume for each microradian (~rad) of inward tilt (Dzurisin andothers, 1984} This is a minimum value for the amount of magmaremoved because of the unknown values of strength in the rocks

surrounding the summit magma reservior and of the bulk com­pressibility of the reservoir. In addition. one microradian of inward

radial tilt at HVO also indicates an approximate vertical subsidenceof 5 mm at the apot. of the deformation bulge and. an approximatepressure drop in !.he summit magma reservoir of 90 kPa (0.9 bars}

Ground deformation in the area above a rapid intrusion cause5rapid occursions and sometimes reversals of tilt vectors on nearby

recording tiltmeters. Ground cracking is also common with the larger

and shallower intrusions, and new or renf!'\\'td volcanic-gasfumaroles are sometimes formed along these swface fractures.

When the volume rate of magma intrusion occeeds about 0.5

~rad r:i summit subsidence per hour (about 50 m'/s), shallowvolcanic tremor~ to appear as a background vibration betweenindividual events in the earthquake swarm. Shallow vokanic tremoris a continuous ground vibration wlth a predominant frequency ol2-5 Hz, and it is associated with shallow subsurface movement anderuption of magma.

Increases in the electrical self-potential anomalies associatedwith active rift zones and changes in the electrical conductivity of the

summit region of Kilauea also occur in association with the largerintrusions. The absence of observable magnetic anomalies associated

with rapid subsidence of the swnmit area of Kilauea has led Davisand oihe" (1974) to postula'e a plastic e",~lope r:i low-r;gidity rockmaterial surrounding the swrunit magma reservoir.

During 1980 there were 6ve rapid intrusions without eruptions

into /he eas' rif' zooe r:i Kilauea Volcano (table 42.1) During 1981the intrusions shifted into Kilauea's southwest rift zone and. on

Augus' 10-12 a large intrusioo affected an Il>-km-Iong segment r:ithat rift. Excellent observations .....ere made on this rapid intrusion,

and they are summarized as follows:

1008 VOLCANISM IN HAWAII

TABLE 42.1.-RopfJ intrusions wilJtooI eruptions into the «Ul rift zone ofK~ Volcano d:aing /980

[ •• possible eruption ci a few cubic mctcn of lava; EM. ekctromagnetic; n.d., not determined]

,-.h HeightMinimum

"""~PropagMion EMD." Loonion """h ~, 0-

(km) (km) (km) (mlxIO') (mIh) anomalies

Mauh 2 Hiiaka 2 2 I •• n.d. no noMarch 10-12* Mauna Ulu 5 3 .5 5.3 200-500 '" 00hugusl 27-28 Purumau 4 l .5 2.3 1,000 '" >"October 22 Moun. Ulu 2 2 2 •• n.d. '" >"N<wcmbu 2 ""'"""""' 2 2 I 2.• 100 no no

The Augus' 10-12, 1981, even, involved at leas' 4 X 10' m'of magma and was the largest rapid intrusion without an eruption of

Kilauea Volcano since increased. seismic and deformation monitoringbegan in Ihe late 1950's.

l'bousands of small earthquakes occurred in a swarm at ratesas high as 3-4 per minute and althQUih most were microearth­quakes, a few reached nJal!lliludes of 3.5-3.9. The h}'l>Ottlllers ofthe earthquakes in the region of the intrusion formed a zone '8 kmlong, 1-2 km wide, and exleuding from ,he surface 10 5-km deplhbenealh lhe "",lhwes' rilt zone (see Klein, chapler 43, fig. 43. 9O~Downrift migration of the earthquake locations indicates an intrusionpropagalion rate of 2 kmlh slowing '0 0.5 kmIh (fig. 43.90~

Shallow volcanic tremor and rapid summit subsidence at about10 f.Lradlh indicate a high-volume rate of intrusion, as much as 103

m3/s. The tilt at HVO totaled 107 v-rad of deflalion, and an elasticmodel of the summit level changes indicates 4 X 107 m3 of subsidence

volume. The ralio of the volume rale to lhe downrift migration rategives an estimate of the cross sectional area of the intrusion. This

area is 2 x 103m2 , and if !he intrusion is a dike 3 km high, ,hen ilswidth i.5 about 0.7 m.

large horizontal and w:rtieal displacemenls occurred along thesouthwest rift zone, indicating major extension across the rift on theorder of 1 meter; and a horst-and-graben pattern developed that

indicates a shallow dike intrusion parallel to the rift and dippingsteeply southeast. New surface cracks parallel to the rift follow thetrend of the 1974 eruption vents and continue southwest for several

kilometen (fig. 42.IO} Much of the new surface cracking involvesreactivation of 1974 or older fractures.

Renewed. gas emissions and increased temperatures were meas­ured at a fumarok on a newly activated surface crack. Self-potmtialanomaly changes were mea..!ured in the Koae fauh system 5 km east

of the trend cl the seismic swann; excq>t near the renewed gas vent.no significant self-potential changes occurred over the seismic zone.

During 1982, rapid intrusions causing eruptions occurredtwice in the summit area of Kilauea, in April and September. InJune 1982 there was another large rapid intrusion without eruptioninto the southwest rift zone. Activity then switched back to the eastrift zone and a major rapid intrusion and eruption occurred there

during January 2-3, 1983. That. event fanned the magma conduit

that has erupled through 47 episodes so far (.. " June 1986) during!he long-I..ting 1983-1986 (and continuing) eruplion of KaaueaVolcano.

The dike swarms formed by many rapid intrusions over theyears tend to wedge apart the volcanic edifice and lead to long-tenn

dynamic evolution of the rlft zones.

RIFT ZONES

The rift zones cl Hawaiian volcanoes are "Pele's under groundroads" referred 10 by Ellis' Hawaiian guides in 1823 (Ellis, 1827~

They are persistent zones of weakness in the volcanoes' flanks.resultins from thousands of rapid lateral intrusions. Many of theserapid intrusions also cause eruptions.

The rift zones are not primary conduits that penetrate to

magmatic source depths; rather they are secondary features that fonnfrom radial pressures generated in the summit reservoirs at depths of

FIGUHE 42.1O.-Aerial photOjraph of sroun<! a-Adls fonned durins: AU8U51 10,1981. inlnuion benealh liOUlhwesl rift zone cI Kilauca Vokano. Area here coymby barren lava Bows and by rewort.~ volcanic ash from 1790 eruption. Cr-ack.s areas mUt:h as I m wide. and a shallow gr.lben about 20 m wide has formed betweenprincipal fractures.. Some rJ these a-ach cxisled~ !he AlIffU5l 10. 1961intrusion and wen ructi~ by the t¥tnl. U.S. Cco&oP:aI.~ photographby Norman Banks.

42. DYNAMICS OF HAWAIIAN VOI£ANOES, AN OVERVIEW 1009

FIGURE 42. II.-Schematic block diagr8lDl ~ the east riftzoneu Kilauea VobooIookilll downrift. from MOOR and Krivoy (1%4~ In this interprctalioa stressescausing rift are luidY gravitational. and rift zone dips .e_ard at depth. Pitallen (cross-hatched example) form by collapse into voids wittm rift if mqma isdrdled far downrift.

a few kilometers. Earthquake locations along the rift, of KilaueaVolcano indicate that the main dynamic axes of the rifts are about 3Iun beneath the surface and that the overaU rift zones o.tend from thesurface to a depth of about 8 Un below sea level. Because of isostatic,ubsidence of the [,land of Hawaii, 6 km i, about the depth of theold Cretaceous sea Roor on which the Hawaiian volcanoes funned.It appears that the rift zones penetrate virtuaUy the entire volcanic

pile but do not extend down into the underlying oceanic crust.Fiske and Jackson', (1972) analy,i, of the evolutioo and

orientation of Hawaiian rift zones is also consistent wilh the conceptthat the rift zones are fairly shallow and secondary features com­pared to the deep magma conduits that supply the summit magma

reservoirs. In their view, the rifts are fed by lateral intrusions fr<.m

the sunum' magma reservoirs, and. their orientations are controlledby the gravitational stresses of the surface topography.

1he north-south elongation of the submarine topography and,ei,micity of Loihi Seamount (Klein, 19620) indicates that rift zone,are generated in the early stages of life of Hawaiian vokanoes. and

the major rift systems still active on Hualalai and HaleakalaVolcanoes indicate that rift zones persist into the waning stages.

Thus. ahhough the rifts are secondary features, they playa majorand long-lilted role in the structure and dynamics of Hawaiianvolcanism.

"The rift zones of Kilauea Volcano have been very adive during

the past few decades. and their dynamic behavior is therefore fairlyweU known. From 1959 through 1964 there were 20 rapid intru­sions into the east rift zone without eruptions and 19 rapid intrusions

that resuked in eruptions. During the same period there were 8 rapidintrusions into the southwest rift zone without eruptions and 2 rapid

N

SEA lEVEl

s

intrusions that resulted in eruptions. Swnrnit activity over this same

period consisted of 4 rapid intrusions without eruptions and 12 with

eruptions. Eleven of these intrusions beneath the summit area wereemplaced between 1966 and 1964.

Since each rapid intrusion and eruption involves a dike beinginjected into the carapace or flank of the volcano, the volcano mustwiden to accommodate these multiple intrusions. Using an estimate

of O. 5 m for an average dike width, we can infer that the summit andupper rift zones of Kilauea Volcano should have widened by 5.5 mover this 19-year period. The actual measured widening of Kilaueacaldera, between HVO on the northwest rim and Ahua on thesoutheast is 4.2 m from 1966 to 1984. There are various possiblereasons why the estimate and the corrected measurement differ: theaverage dike width may he les, than 0.5 m; each dike prohablyextends only part way across the summit area; and there probably is

some elastic compression stored in the rocks adjacent to the intrudeddikes. Even allowing for these considerations, it is dear that majorwidening of the summit of Kilauea Volcano perpendicular to its rift

zones has occurred in the past few decades.The widening of the rift zones becomes less downrift because

fewer dikes reach long distances from the summit. The fad that somesummit eruptions are foUowed by progressive downrift migration of

rapid intrusions and flank eruptions can be explained by thisprogressive summit wedging effect Cltending down the rift zones.

Eaton and Murata (1960) 'Iltlgested thai a core of magma inthe east rift zone of Kilauea remains molten between some eruptions,and Swanson and others (1976b) discussed this concept more

thoroughly on the basis of seismic evidence from 1968 and 1969eruprion, on the east rift zone. Hardee (1962) ha, ,hown bytheoretical heat-flow calculations that if a sufficient number andvolume of intrusions are injected into a region, part of theseintrusions wiU remain moken. He calculated (written commun.•

1983) that the recorded intrusions into the east rift zone of KilaueaVolcano should result in a molten conduit from the sununit to about30-40 km downrift. n.e number and volume of intrusions inlo the

southwest rift zone, however. have not been. sufficient to maintain amolten conduit. n.e evidence for slow intrusions into the east riftzone provided by slow aseismic deflations of the summit andmeasurable inflation along the east rift zone as far as 32 km from the,ummit (Dieterich and Declter, 1975; Swan,on and others, 1976a)give strong support to Hardee's condusion.

Moore and Krivoy (1964) were among the first to consider the

dynamics of the east rift zone of Kilauea Volcano. They suggestedthat gravilational slumping was the major factor causing the rift to

widen and that magma moved rather passively into the fraduresfonned in the head of the ,lumping block (fig. 42.11} Swanson and",he... (1976a) took the oppo,ite posirioo that magmatic pressurepushed the rift apart and that slumping was the result rather than thecause of the rifting process (fig. 42.12} In a sen,e they are aUcorrect; the gravitational and magmatic stresses are additive, and

together they control the evolution of the rift zone.

Swanson and others (1976a) measured the progressive short­

ening of survey lines across the Hilina fault system on the south flankof Kilauea Volcano south of the east rift zone. They recognized that

1010

lJlII:w...w::; Sea 00 level..J

IIZ ·5

z0

·10j:::«>w

·15..Jw

VOLCANISM IN HAWAII

East rift zone

~

Active Inactivepart part

~

~........~~~~.... ".";'. + + + + ++.+++++++ ~ ....::::'

.:;.:.::"

Pre·Mauna Loa oceanic crust

Mantle

oIo 5

5

I10

10

i I

15

15 MILES

20 KILOMETERS

I:~ ~IIGtauea Mauna Loa

EXPLANATION

Clastic rocks Kilauea Mauna Loa

Subaerial shield Pillow lava-Direction and relative amount of displace-ment caused by forceful intrusion ofmagma as dikes. Length proportionalto amount of displacement

FIGURE 42. 12.-Schemabe CI'OM Kdion. tnIe KAle ri east rift ZOfte of KiIaaea Volcaao lookinc upriit. from Swanson and otlaers(I976} In .. interpretation 'bales cauain& rifl ariK from forceful injection of III....., and dikes are nearly m-tica.I.

the intrusions and. eruptions ocamlng on the east rift zone in the late1960's and early 1970's _e wediing the rift apart but that much ofthe wideni"1l was being accommodated by compression and uplift ofthe flanks of the rift zone. They roaIized that this aeeumulatins strainmiPt be released by sudden failure of the Hilina fank system, andthey wrote in 1974 (Swansoo and others, 19760, p. 35): ·Some ofthe dala s_t that the Hilina fank system is poised for anotherepisode of subsidence, with only minor additionallll'O"fld displace­ment needed to trigger it. Most cl the strain acquired since 1965remains. howevtt. and we anticipate a subsidence (strain-release)event of unknown magnitude in the not too distant future."

Fifteen months later, but before their work was formallypublished, a JDatlIlilude-7.2 earthquake released the pent-up a>m­

pressn. strain in the south flank of Kilauea. Portions of the southcoast moved seaward ......al meters and subsided as much as 3.5 m(Tilli"1l and othen, 1976; Lipman and othen, 1965) Swanson and

others (19760) had deady forecast the nature and imminence of thatmajor earthquake.

Analysis of the source mechanism and aftenhock pattern forthis earthquake (Ando, 1979; FlII'WIIOIo and Kovach, 1979)indicates that the strain was released by rupture on a nearlyhorizontal fault about 8-10 Jun beneath the south fIaaIt of Kilaueaalong 50 Jun of the coastline and across a region 10-20 Jun wide,bounded on the north by the east and the southwest rift UlIlCS. The,6p on the fauk was &eYeraJ meters in a seaward direction away fromthe rift zones (fit!. 42.13) The net long-term result of dikcs wedlPnginto a rift zone therefore appears to be widenina of the rift zonedown to a depth of about 8 Jun, and this wideni"1l is accommodatedby intermittent ..".",...t of the seaward fIaaIt of the volcano along anearly horizontal slip surface dose to the old sea floor on which thevolcano grew.

More than 3 m of otension occurred on survey lines on the

42. DYNAMICS OF HAWAIIAN VOLCANOES, AN OVERVIEW 1011

KiiMJe. etst rift zone

~ K.I.p.n.w

i SEA LEVEL ~=_=_=_c:_=_c:_i~~·~",,~'...~m~~:-:_=_c:_:-:_=_c:_:-:_=::>c~IL ----------------------

; ~~~~~~---------~ 5 r-~=~~=~T:::::::::::::::::'::::;::;;::::::;;;11:::::=:==_D~.~.~,:o:m:..:.:.---~o::::::::::::::::::=~~~~~-.... "bUN Lo.~ eWrihwM

..... Feun or rupture Ancient ••• tkKlr

15 10 5 0 5 10 15 20DISTANCE FROM KAlAPANA, IN KILOMETERS

FIGURE "2.13.--Sc:Mm.tic cross scctioo cl east rift zone of Kilauea Vokano lookina: uprift and showina: inferred mec:IwUsm «1975 earthquMc(M.7.2) and ttunami. Dikeintrusions (Ill: Wt side cJ. dike c:ompks) comprcll adjaccot rocks 0Ya' time. When streu ruches brewD@; point, llI'Ibuttrased lank cl rift zone suddeDly fails. ModifiedI""" F..-o ood ""'ad> (1979~

Lava Rows fed by the erupting vents move down the .Iopes ofthe volcano or pond in topographic depressions. The formation ofvarious lava textures and structures such as aa, pahoehoe, and lavatubes depend. upon the complex interaction of the rheologicalproperties of the lava, the volume rate and continuity of the eruption,and the environment and topography OYer which the flaws aremoving (SW...... 1973; Holcomb and others. 1974; Peterson andSwanson. 1974; Peterson and Tilling. 1980~ Lava commonlydrains back down old surface cracks or even dawn the active fissuresas the eruption wanes.

sooth IIanIt of Kilauea Volcano during the period of the maiorearthquake; figure 42.14 shows changes from 197010 1984 in the8.6-km-long line th.. crtend. from the north side of the Hilina faultsystem to the south coast.

It appears that Hawaiian volcanoes do not only grow upwardfrom successive lava Rows piled one upon another. but they also grow

sideways by successive intrusions into the sunvnits and rift zones andby intenniUenl failure of the seaward Ranks of the rift zones toaccommodate these intrusions. The east rih zone of Kilauea Volcano•Iopes down gently for 50 km from the summit at 1.200 m 10 sealevel. It continues for another 70 km eastward as a s1igbtly .leeper

submarine ridge to depths of nearly 5.000 m below sea 1"",1(Moore. 197n Since Epp and other. (1983) h... sbown th..eruptions at lower elevations on the rift tend to be more voluminousthan .ingle short-hved eruptions high on the rift. it is clear th..eruptions must become progressi...dy much rarer at large distancesfrom the summit. Thi. decrea.. i. probably caused by .. least two

facton: the widening rI. the rih zone and its wedging effectapparently decrease with distance from the summit; and the moltencore in the rift zone also apparently decreases in size and finallydisappears with di.tance from the summit. 'The ..ry even slope ofthe ridge along the axi. of the eas' rift zone indic.... th.. thefrequency and volumes « eruptions maintain a steady equilibriumover long time spans.

'"ffi.t;;

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00 ;: .. ., .. '" .. ~ .. 0> Ii ;;; .. .,

~~ ~ ~ ~ ~ ~ ~ ~ ~ .. ..0> ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

DATE

FIGURE 42.14.-MoItip~....,.. (unsII Crcks) of8.fHm-1ooo Ime"" oouth_r:l upper east rift zone cl Kilauea Volcano iodK:ale sraduat contradm from 1970to 197;, auddc:a IlSttoUoo rel.l:ed to tIte M-7.2 earthquake of November 1975.aDd reaewed sradual contraction si.cc 1975.~ Iioe nlftS (rom Cod toApua Point.

ERUPTIONSw

'"z«"Most vokanic eruptions in Hawaii are outpourings «mcan- U

descent lava from linear or point-source vents. Lava fountains fromlinear vents are called curtains of fire and reach heights up to about200 m. Lava foWltainS from point-source vents reach heights up toabout 600 m.

A reduction In the vohme rate of eruption or in the gas contentreduces the vigor of lava fountaining until incandescent lava simplypours out from the linear or point-source vent. Vents in the bottomsor sides of craters feed active lava lakes that, at low-volume rates oferuption. can remain in eruption for yean.

1012 VOLCANISM IN HAWAII

Data on the eruptions of K~auea and Mauna Loa Vokanoessince 1823 are compiled in Peterson and Moore (chapter 7. tables7.3, 7.4~ The long-lived, low-volume-rate eruptions of active lavalakes in the caldera of Kilauea Volcano that continued with a fewinterruptions from their first-recorded observation in 1823 until1924 are nol included in table 7.4.

The quality of these data on eruptions becomes, in general.better for more recent limes. Continuous scientific observation beganin 1912. and aerial observation and helicopter access to remoteeruption sites has increased progressively during the past fewdecades.

Several major changes in the character of eruptions of Kilaueaare evident in table 7.3. Between 1823 aud 1919, duri"ll tbecentury of active lava lakes and slow 6.lling of Kilauea caldera. therewas only one confirmed east-rift-zone eruption (in 1840) and. onesouthwest-rift-zone eruption (1868~ Beginning in 1919 andculminating with a probable east-rift submarine eruption in 1924,

three rift eruptions marked the end of a long period of predominanceof summit activity. The 1924 swnmit eruption, associated withsubsidence of more than 400 m in the magma column feeding the

active lava lake in Halemaumau Crater, has been the only ex:plosiveeruption in Hawaii since wriuen records began in 1823.

From 1924 to 1934, seven small eruptions filled tbe boltom ofthe deep Halemaumau Crater fonned by subsidence in 1924; from1934 until 1952 there were no eruptions of Kilauea. This hiatus wasended by two eruptions in Halemaumau Crater in 1952 and 1954,

and then a major eruption on the lower east rift zone in 1955 began astill-continuing period (1955-1986) nf major eruptive aclivily onthe east rift zone (18 eruptions~ tbe summit (12 eruptions~ and tbesouthwest rift zone (2 eruptions~

What constitutes a separate eruption of Kilauea is not alwaysclear. The breakout of an eruption in an entirely new location-notjU5t an extension of a fracture associated. with an ongoing rapid

intrusion-is one criterion. Another criterion is the occurrence of aseismic swann at the beginning of an eruption in a familiar location,

like Halemaumau Crater. This indicates thai: a new subsurfacefracture has been opened to feed the surface eruption. In long-livederuptions such as the Mauna Ulu eruption on the upper east riftzone from 1969 to 1974 and the current Puu 00 eruption on themiddle east rift zone from 1983 to 1986 (and still continuing), therehave been periods of repose between episodes of high lava fountain­

ing: or other changes in the character of the eruption phases.However. the same general conduit system appears to be involvedthroughout these continuing eruptions, and on this basis these

distinct episodes or phases are not considered separate eruptions.Under these limits of de6nition, the eruption statistics for

Kilauea from 1918 through 1984 are .. follows: 1be average timefrom the beginning of one eruption to the next is 494 days (range is 2

to 6,504 days~ The average duration of an eruption is 86 days(range is less than 1 to 1,884 days~ The average volwne of aneruption is 2.7 X 107 m3 (range is less than 105 to over 4 X 1()8 m3 ~

The average area coverered by an eruption is 4.4 km2 (range is 0.1to 55.7 1tm2)

Dzurisin (1980) has compared tidal stresses to the onset times

of eruptions of Kilauea and concludes that more eruptions begin nearmaximum fortnightly tidal stress (new moon and fun moon) thanwould be expected by random chance at the 90·percent con6dencelevel. He ascribes this to the triggering effect of tidal stresses on asystem that has almost reached the point of eruption.

Klein (I982b) bas in_tigated the statistical pattern nf theeruptions of Kilauea since 1918 and concludes that the overallpattern is largely random, but that some non-random featuresappear. He finds that summit eruptions tend to clU5ter, that large­volume eruptions are usually followed by repose times that are longerthan average. and. that long periods of increased activity of Kila­uea-for example during 1960-1979, when the average reposeperiod was only 261 days-are associated with periods of lessvigorous activity of Mauna Loa Volcano.

A complete list of known historical Mauna Loa eruplions ispresented in lable 7.4. Mauna Loa has nol had any sustained lava­lake activity during this time period. There is less apparent variationin the pattern of Mauna Loa eruptions than in that of Kilaueaeruptions, but the elght swnmit eruptions of Mauna Loa thatoccurred between 1870 aud 1876 show an unusual clustering nflocations and shorter-than-average repose times. In addition, thecomplete lade. of eruptions between 1950 and 1975 was an unusuany

long repose period for Mauna Loa.Statistics for Mauna Loa eruptions are as follows: The average

lime period from the beginning: of one eruption to the ned is 1.459

days (range is 104 to 9,165 days~ The average duration of aneruption is 69 days (range is less than 1 to 547 days~ The average

volume of an eruption is 1.88 X lOS m3 (range is 2.7 X 107 to6.88 X lOS m3~ The average area covered above sea level by an

eruption is 34 lan2 (r"lllle is 5 to 91 1tm2)

All historical eruptioos nf Kilauea aud Mauna Loa Volcanoesappear to have started with a rapid intrusion injecting a dike

upwards or sideways from some part of the shallow magma reservoirinto the summit or Rank of the volcano. These rapid intrusionsprecede the eruptions by minutes to days, and provide important

warnings of both the probable imminent occurrence and the proba­ble location of an eruption.

The character of an eruption depends on several factors and is

best explained by examples:The long-lived eruptions of active lava lakes in Kilauea caldera

begin with a dike injection into the summit area. Pressure in theshallow magma reservoir is high enough to push magma to thesurface, and the resupply rate from depth is sufficient to match thelow-volume rate of eruption, which is typicany about 3 m3/s. Therift zones are slrong enough to withstand. magmafracting:. lbeseeruptions have become less conunon as Kilauea caldera has re61ledbecause the higher the magma coluDUI. the more pressure there is tofracture the rift zones and thereby lower the magma column. Thelatest long-lived eruption of an active lava lake occurred inHalemaurnau Crater in 1967 and lasted 251 days (Kinoshita andothers, 1969)

Short-lived eruptions at the swmnits and. along the rift zones ofKilauea and Mauna Loa Volcanoes are the most nwnerous. Thevolume rales of these eruptions are typically 10-200 m3/s; since this

42. DYNAMICS OF HAWAIIAN VOI.£ANOE'5, AN OVERVIEW 1013

ex:ceeds the resupply rate from depth. pressure rapidly drops in theshallow magma reservoirs. The eruptions stop when the reservoirpressure can no longer push magma to the surface. The latesteruption of this type on Kilauea Volcano occurred in the caldera inSeptember 1982 and lasted 16 hou... (ScientiJic E..." AlertNetwork. 1982} The latest one ci this type on Mauna Loa Volcanooccurred along the northeast rif, zone in March-April 1964 andlasted 21 days (Lockwood and others. chapler 19}

Although long-lived Rank eruptions are more unusual. twohave occurred on Kilauea Volcano since 1969: the Mauna Ulueruption on the upper east rift zone from 1969 to 1974 (Tilling andothe.... chap'er 16; Peterson and othe.... 1976; Swanson and othe....1979~ and the Puu 00 eruption on the middle east rift zone from1983 to 1986 and s,ill continuing (Wolfe and othe.... chapter 17}These eruptions consist of many episodes from essentially the samevent area. The subsurface magma conduits apparently remain open

between eruptive events because the onsets of new episodes are notpreceded or accompanied by earthquake swann,. Individual epi­sodes of lava fountaining involve volume Row rates of 50-400 m3/s.bu' they generally last only abou' 10-20 hou... and produce lavavolumes of about 5x )06-15x 106 m3 • The longer term volumeRow rates through several fountaining episodes or during sustainedlow-vohune-rate phases average about 3 m3/s, which is approx­imately the resupply rate of Kilauea Volcano for the past twodecades.

The spectacular lava foWltains and curtains of fire that charac­terize many Hawaiian eruptions are caused almost entirely by therapid effervescense of gases boiling out of the magma at near-sunacepressures and by the geometry of the vents. High-volume-rateeruptions of gas-rich magma in constricted vents produce the highestfoun,ains. Gerlach and Graeber (1985~ and Gerlach (writtencommun., 1985) have determined that magmatic gases of the volumeand composition normally occurring in flank eruptions of KilaueaVolcano begin to boil rapidly out of magma at pressures equivalentto depths ci only 40-100 m. In addition. as Paul Greenland basnoted (written commun., 1984). near-surface pressures will movedownward into an open magma column as rapid lava fountainingprogressively m10ads the upper part of the column, and the actualdepth at whicb sus,ained effervescense is formed will be deeper than40-100 m below the surface.

Most Hawaiian eruptions begin with linear vents (curtains of6re} After a few houn or days the fountaining becomes restricted toone or a few principal point-source vents. Apparently zones alongthe erupting 6ssure that were wider than average at the start of theeruption become enlarged by melting and erosion, while the nar­rower zones along the original 6ssure tend to freeze shut (Delaneyand Pollard. 1982}

Once the magma flow supplying a fountaining vent is no longerreplenished from depth, the fountaining will cease, and degassedsurface lavas ponded near the vent will drain back into the conduit.Drainback in an active lava lake may also occur if the feeder dike isextended or some newly formed dike is generated in the subsurface.Magma in hydraulic communication will move toward the lowestpressure region in the system.

CALDERA COLLAPSE AND EXPWSIVEERUPTIONS

Hawaiian volcanoes have a reputation for producing numerousbut relatively harmless eruptions r:l fluid lava. Explosive eruptions.similar to those at Surtsey Volcano in Iceland in 1963 (Thor­arinsson, 1965~ were probably common during the transition fromsubmarine to subaerial eruptions in the Hawaiian Islands, but theseusually cease after the summit of a volcano grows above sea level.Less than I percmt of Hawaiian volcanic products above sea levelare pyroclastic (Macdonald. 1972. p. 355} and most ci these ....tephra from high lava fountains. Nevertheless, major c:xplosiveeruptions of hydromagmatic and hydrothermal character haveoccurred in Hawaii. Their occurrence appears to be closely relatedto episodes of major caldera collapse.

The last major collapse and explosive eruption of Kilaueacaldera occurred in 1790 (Swanson and Christiansen. 1973; Chris­tiansen, 1979; Decker and Cbristi...... 1984} The caldera was

more than twice its present depth wben visited bY Ellis in 1823(Ellis. 182n bu' its depth and configuration prior to 1790 are notknown.

Hokomb's paleomagnetic data (1980) indicate that lava over­Rows were occurring from the summit region of Kilauea during theperiod ci about 1-0.3 ka. and Ellis' Hawaiian guides ,old him that"in earlier ages it used to boil up, overflow its banks, and inundatethe adjacent country" (Ellis. 1827. p. 171}

A much smaller collapse accompanied by small explosiveeruptions occurred a' Kilauea in 1924 Oa88Or and Finch. 1924} Anearly full lava lake in Halemaumau Crater drained. down more than400 m as a major intrusion was injected into the lower east rift zoneof Kilauea near Kapoho. This intrusion possibly fed an eruption onthe submarine ex:tension of the east rift zone, but there was no directevidence ci this.

As the magma colunm in Halemaumau Crater drained belowzones of subsurface water beneath the summit of Kilauea Volcano,the equilibrium between the magma column and subsurface waterwas destroyed and steam ex:p1osions of hydromagmatic and hydro­thennal origin issued from tbe crater (fig. 42. 15}

The subsidence in 1924 occurred during February througbApril, and the multiple steam explosions occurred. for 18 days inMay. Since magma moves out of the summit reservoir throughconduits in the rift zones, the volume rate of magma removal islimited and the process of caldera collapse occurs over periods ofweeks to months rather than minutes to hours. In theory there shouldbe time for people to evacuate the summit region of Kilauea in case offuture caldera collapses.

Overflows of lava that were erupted from the summit region ofMauna Loa Volcano as recmtly as 590 years ago as determined byradiocarbon dating O.P Lockwood. oral commun., 1985) arebeheaded by Mauna Loa caldera. Wben firs' mapped in 1641(Wilkes, 1845~ the caldera was 50-100 m deeper than the presentsurface of ponded. lavas. Thus it appears that the present caldera onMauna Loa is similar in many ways to Kilauea caldera; it formedonly a few hundred ye.... ago anil bas been rapidly refilled witb

1014 VOLCANISM IN HAWAII

MAY 10 to 27

JANUARY 1

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, MOI.TEN /lAVA

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960 m

JUNE 1

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--II--I II I

TA8l.EWATER•••••• •••••••J l .

II I \. I I •

(11 ~ "11.' I I •/ ,

.,' 121 I I 121 .......I I •

--II--SEA LEVEL I I

I I

FICURE 4Z.IS.-SchaD.bc CI"llIS secticIas~ CIlXIditioD. It H......···, Cr... belone. cbiaa. aod alkr'19Z4~~. On Ja-y I. one zone ofaobuf...."", (I);' <ooIa- tLoo boaO.c _ ...d__ (2);' .. "'"'"" _ boW ......, duI.-dot ... ....k. -...-... boooodwy. DoriaoM.y 10-21. as mqma cob-. JUb.ides b8uw WIta' tab&e, eqailaDriua bdMaI .m.unac:e waRr aDd IDIIIIUa~. Mel wakt from lODe I. r.Les into lwokm hotroW~ sabIidibc IUI'U c:oIumo Mel 8aaht. iDto Rum. Wiler from zooe 2 alto Bathe. into AeIn .. hydro.uIic JlftS'G'U an tueIcIcNy~.~

are adipIe bec-e of~~ of the ...... c:ob.; they stop in '* May with cD-.bon of d.maaI fDft'I)' abO'l'e the AJbt,ided ....... c:oIumo.~ a<011......~ F.... o.a.. ud a.n.u...a. (I984}

ponded lavas. There are scattered blocks of explosive ejecta on therim cl Mama Loa caldera, but any explosive eruptions that mayhave accompanied the caldera c:oUapse were minor compared to the1790 eruption of Kilauea. This difference in explosive habits ofKilauea and Mauna Loa Volcanoes is probably related 10 thegreater depth from the surface to zones of subsurface water beneathMauna Loa.

Major caldera collapses <i. Kil....a Volcano It... occurredrepeatedly, with an average recurrence time cl about 2.000 years(Decker and Christiansen, 1984; Dzurisin and Casad..aIl, 1986~

Refilliq the caldera with ponded lavas appar<ntly takes ......alhundred yean. Since hcth the _ submarine volcano Loihi andKilauea. older neighbor volcano Mauna Loa have calderas. ohiorepeated proc... <i. caldera collapse and rdillins apparently occunduring most of a Hawaiian volcano's active life span-roughly0.5-1 m.y. If the caldera recurrence interval <i. 2.000 yean aI

Kil....a is typical, ohio indica... that 200-500 gmerations <i. majorcakiera collapse and re6lling occur during the evolution of aHawaiian volcano.

This concept of repealed caldera coDapse and rdilling. com­bined with the evidence of progressively upward migration of theshaDow mCllma reservoir S)'1tems of Hawaiian volcanoes (fig.42.16), SUlll!es.. thai some lavas ponded in early calderas may bepartly reassimilated into the suanit maama-reservoir S)'1~.

SUBSIDENCE OF THE ISLANDS

Hawaiian volcanoes form a massive load on the lithosphere ofthe Pacific pi.... lhe great positive gravity anomalies <i. _Hawaiian volcanoes-as much as 330 mGaI Bouguer (Kinoshita,1965)-indicale that they arc 001 compensated by local roots <i.l...d..... rock. A. Hawaiian voIcaoocs fonn, therefore, they .... begin

42. DYNAMICS Of HAWAIIAN VOU::ANOES, AN OVERVIEW 1015

FICURE 42.16.-ScJ:.:s..bc aoa Kdion .. true Kale <:l iafcned JIloaIIQA·racrvoirt)'Item bmeath ..... of Malft& Loa. Vokano. Zt::-:. A is more .clift r" <I......a additioa .d ranovaI. Znoe B is DIan: p.usive; it an_ IMlPU andaUows mapBa to IDO'I'e throach it. lu docs not Dreue or decrease ill YOlume udocs %ODe A. Zone A mip"et upward as the vokaDo paws in bciBbI. Pba.dsquares repraeaI: wel-locaIcd hypocCllkn tl earthqu....cs (M-Z or lfU!Ier) ....occllfl'ed frcmJanUoUY 1962 tl.-ouab May 1963 within 2.5 Un ti tDeO'ON-Kdioopi.... from o.w, ...d oU.n (1983}

to .ink, and this "'OC... has been larsely completed when thevolcano becomes Clbnct.

Moore (chapter 2) and Moore and I'omari (1964) havedetermined from tide-galll!e and drowned-reef data that the averagerate ci subsidence of the west shore of the Island of Hawaii iI about2 mrnIyr over the past 150.000 yr and that the rate of sub.idence hasbeen generally increaslng during this time period. The subsidence isapparently more rapid near the center of the island than at theshoreline. E..tatic sea-level changes are superimposed on thisgeneral.ub.ideoce and c...plicate the actual change of sealevel. Forexample, the worldwide rise in sea level over the past century addedto the subsidence of the Island cl Hawaii has caused a rise in sealevel at Hila of about 4 mrnIyr during the pasl r.w decades. 1ide­gauge data at Honolulu re6ect a eustatic: rise in sea bel but do not

inelicate any measurable sub.idence of the Island of Oahu.Very high sea-level .,ands reported by Steams (1978), which

imply periods of major uplift followins the .ub.idenu of .....Hawaiian island" have recently been interpreted by Moore andMoore (1964) as deposits frcxn giant waves. Present opinion favongeneral subsidence of the Hawaiian islands rather than 50me com­plex oscillating teclonic movements.

Teclonic subsidence p<ohably results from at least four factors:(I) removal of magma frcxn beneath the active volcanoes; (2) elasticccxnpression of the~~ nearly as fast as load is added; (3)plastic Oow in the lower ~thosphen: and upper mantle as a result ofvolcanic load over periods of IOS-IQ6 yr; and (4) the .Iowthickening of the oceanic: Iitbospbere and its .ubsidence as it losesheat over period. of 10' to 10" yr (Yoshii and othen. 1976~ uuand Kosioff (1978) calculated factors (2) and (3) using an elastidplastic plate-bending model with laboratory data on rock deforma­tion.

This concept of general .ubsidence of the Hawaiian 1.land. hassome inleresling geolOllic implications. If any mineral deposits wereever formed by interaction of magma and subsurface water, these

o '0 20 km

-5

-10 km

deposits would p<obably not be ""I""ed above sea level by latererosion. In addition. subaerial erosion would in general not expose

any Hawaiian vokanic rods or structures formed below sea level.

FUTURE RESEARCH DIRECTIONS

As in most geoIOllic ",oblcms. our understanding of howHawaiian volcanoes work decre.... with depth below the surface.Neverthel.... then: are ..... resear<h paths that look ",omising overthe ...t decade or two for providing new insights into the .tructureand dynamics of Hawaiian volcanoes.

More detailed information on the variations in temperature andkinetics of the Earth·. mantle should be forthcoming frcxn worldwideseismic tomOlll"aphy. "The deep anomalies beneath the centralPacilic:. emerging in the presenl work of Andersen and Dziewonslti(1964), should heccxne clearer if a worldwide ..t of ditlital seis-mometers is deployed. particularly at si on the ocean Boor.

D.p\oyment of a r.w ocean-bolt seismometers around theIsland of Hawaii would aIsu greatly imp<ove the capability of thepresent seismometer network ci the Hawaiian Vokano Observatory.Potential .pecific bendits from this type of research effort includebelter location of earthquake source. beneath Loihi Seamount, theabi~ty 10 determine the source functions of deeper earthquakesbeneath Hawaii. and the ability to gain more depth and detail forlhe seismic velocity structure beneath Hawaii.

Vokanic tremor speaks in a language we do not yel clearlyundersland. but the technology probably ",isl. to translate thisbabble into meaninsful worcls and sentences. Array. of portable~ilal broad-band seismometers are needed to record volcanictremor in suf6.cienl detail 10 interpret ils source functions. With suchdata in hand. we may be able 10 resolve some of lhe ambiguity inpresent models of how volcanic tremor is generated.

Recent developments in satellite geodesy suggesl that a revolu­tion in the technology ci measuring the surface deformation on activeand p«mtially active voIeanoes is only a r.w yean away. "The abi~ty

10 measure distance changes between points that are not intervisible.and the potentially great consequent im",ove"""ts in the quality offar-field data, would make observatinns possible that cannot oow bemade with conventional sunoying devices. 1hese new observationsare .....tial for testing more campi", theoretical model. of the siu,.hape. and depth of subsurface magma bodies. Add 10 this the greatp«mtial savings in sunoying time and rnanpowor. and the attrac­tiveness of satellite global positioning .y...... bec..... clear.

"The recent .acc..... in observing detailed gravity changesrelated to activity of Kilauea and M....a Loa Volcanoes (Johnsoo.chapter 47; Dzurisin and othen. 1980; jach... and Eaton. 1980),and the fact that these new data indicaI:e important mass changes inthe shallow magma-.lorage sy.tems not quantitatively shown by.urface deformation measurements. clearly denonstrate the impor­tance of Ions-term measurement of gravity changes on the 1UIIlIRi1.and rifl zones of Kilauea and Mauna Loa Volcanoes. Recordinggravity meters in conjunction with recording tiltmelers would providethe most definitive data on the ma.. and volwne changes in the,hallow mqrna reservoir systems.

1016 VOLCANISM IN HAWAII

Significant changes in the electrical, magnetic, and electromag­netic fields associated with the activity of Kilauea Volcano (Davisand othen. 1973; Zablocki. 1976; Dzurisin and others. 1980;Jackson. 1983) indicate that th... methods of geophysical researchshould continue to be pursued vigorously at HVo. Needed most inthis research are a comprehensive analysis of the available data andthe generation of theoretical moods to try to explain the data. Thistype of effort would deline more dearly what types of field observa­tions are most important.

Great progress has been made in the past few years indetermining the composition of gases in Hawaiian magma, and inunde!"$tanding the complex. manner in which some of these gases",solve from the meh and escape to the atmosphere (chapten27- 36; Gerlach and Graeher. 1985) Th... studi.. have set thestage for determining the role of volcanic gases in the dynamics of theascent, the shaUow storage and intrusion, and the eruption ofmagma. Carbon dioxide apparently forms a separate but dense fluidphase at depths .. great .. 40 !un heneath the swnmi.. of Hawaiianvolcanoes. Studi.. of the thermodynamic and mechanical behavior ofoxygen, carbon. hydrogen, and sulfur in the generation, ascent,shaDow storage and intrusion, and eruption of Hawaiian Jna.gma$ isa research opportunity of great imporlance.

The destruction of homes and property in the 1983-1985eruption of Kilauea Volcano, and the threat to Hilo from fl_ ofthe 1984 Mauna Loa eruption. malte it dear that we need to knowmore about the mechanical processes of Hawaiian lava Rows in orderto predict their behavior better. The complex interaction of thevolume rate of eruption. the rheological properti.. of lava, and thetopography over which the flows are moving combine to make achallenging research area that needs more .tudy. Careful plans mUlltbe made for future experiments. When an eruption begins it isusually too late to start planning.

As a parting note, one area of the dynamics of Hawaiianvolcanism that has been almost entirdy overlooked is the study of theinteraction of magma with subsurface water beneath the surrmits and

rift zones of the active volcanoes. Few direct data are available: onlyone 1.260-m drill bole into the summit of Kilauea (Kell.r andothers. 1979~ and a few geothermal wells on the ...t rift zone ofKilauea whose data are still largely proprietary. Some resistivitydata on the depth to the water table at various places are available.The potOltial for important new insights into the interaction ofmagma and ground water awaits a creative hydrogeologist whowants to venture into volcanology.

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