THE VOLCANIC ORIGIN OF THE HAWAIIAN ISLANDS The volcanic origin of Hnrvni'i guarantees fln ongoin cycle of destructi011 and creation, to whrch all I@ on these is nnds must adapf.

f

Christina Heliker

The Hawaiian Islands are the hest-known Dart of a 1.500-mi (2.400-krill chain of volcanoes that has hccn active fix ov& 70 nli l l ion years. 'The origi;i of this chain can be expl:~inetl by rhe theorv of plute tectonics, which holds that the earth's crust is dwided lnro a nomher of rigid ylatcs that move ;~bot~t on the semi-liauid laver (mantle) beneath. Most d th'e carth's seismic and &&;lnic nctiv& occ&% tne pl&e boundaries as adjoining platescollide or move past nne anolher. In contrast. the Hawa'lhn isla?ds were formed in the middle of the Pacific Plate. and the source of' the volcanic activirv that huil t rhese Islands is :I f:wd hot spot in the carth's mantle. The hot spot can he visualized as a ris1r.g plume of molten rnatcnal, called magma. driven by convection currents w t h ~ n the mantle. A< thc Pacfic Plate drifts toward the northwest, each volcano in turn is rafted away from the hot spot w d dies. Through the proceses of erosion and subsidence, the oldcr islands eventuslly disap ear hcneath the ocean surFace.

&e islandof iIitwai;i, the youngest in the chain, is currently located over the hot spot. The Island is composed of five volcilnoes, which are progres- sively youngcr toivwd thc sootbeast. Koh:tlo, the Island's oldest volcano, has been inactivc for ahout 60.000 years, and Mauna Kea last erupted spprori- matcly 3,00u years ago. The othcr thrcc volcanoes have all been uctive within the past 200 years. Hualalai last cnqted in 1800-1801 and Mauna Lua in 1984; Kilauea's latcst cruption, which hcgnn in 1983, continues as of this writinr (Fehruarv 1988). In addition to the active volcanoes of the Bie - . ~ ~~

Is!und. a jt~hmnr;nc vo.cano. Ln'ihi seamomt, is forming off the routFern coast of Hawai.~. 3,300 f t (1.000 m) helow sen level. Lo'ihi is ludged rn have been active in recent vears. hoth hecause it is the source olfieaient earthauake swarms (clo5e iri timh and loc~tion) and hecause samples'of lava that'are no more than a few centuries ok! have been obtained from its r~mmit hy d r e d ~ n g and by submersil~le vessel.

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CHARACTERISllICS OF HAWAIIAN VOLCANOES

t1awaii:ln vnlcmocs have long artractcd intcnsivc study, in large art he Cause of the non-cxolosive 11:1ture of most of their erurrtwns. This mifd-

- mannered hehavio;can be nrtribu~ed to the chernica.;o~?-~os~~~on of the magma, most of which is classified :IS basalt. Rehtive to other types of

Vdcanic Origin \Hclikcr 12

magma, basalr is less sticky, which allows much of thc gas contained within it to streamupward through the magma and escape p;nsively to the earth's surface. When bilsaltic magma erupts, it forms long, fluid l a w flows. In contrast, the stickier composition ol the magma at volcanucs such as Mount St. Helens in the stnte of Wiishinpton results in ex~losive eruntions hecause the gates are trapped r l ~ d c r grc~t-preswre within the rising niaglna. When the niagm;! reaches the surcace o f the earth, the pressure is suddenly released, and the kas e x ~ ~ n d s viole~rtlv. fraarnct~ti~w the lava and fornlinu a hoilittr - cloixl o f h e &h rather t h a i a la& flow.

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The hasallic cc~mposition of Hxwaiian lava is alco responsilie for the sham of Hawi ian volcanoes. whicn are called shield vo!cnnocs because of their lone. nentlv-slooinc orofiles. Mauna Los. a volcano still in its vrime. typifies tk;form, \ d i c l i ~ ; the result of fluid l a k flows that travel f i r manv kilometers bcvond the erupt~ve vent During the later ctagcs in the life hisrorv of a lla\wiibn volcano. stickier lavas mav 6e oroduccd. r&ultine in rxplo&e erup:iuns and the f h n ~ a t i o n of smal l ,~ tee~-s ided co'ncs s u c h i s those on top of Maunn Kex. However. rlwse late eruptions represent only a snid1 Derccntarc of the total ouroourinnuf 1;n.a durin?! thc vol.mo'n lifetime.

E i . ~ ~ t t o t ~ s ;f Hawaiian volc&toej Gay occur either at their sutnniitc or on t!icir ilands 31011g rifl zones, jvstems of fractures wfhin the volc:tno. Rift zoncs tmicnllv cxrend from the G n m i t of the volcano to the ocean, and sometihies miny kilonieters beyond. Young I-hwaiim vo:c~noes are usually topped hy calderas, large craters that form oy collapse when m n p a d r a i n s away from he~ie;rrh the vent area after an eruption is over.

HISTORIC ERUITONS OF MAUNA LOA AND KILAUEA

Written accounts of Hawaiian eruptions exist from the early IROOs, ill- though rhe record remain$ spotty we!! onto the last part of rhe ceutury. Thro::rhnut this interval, nearlv all of Mnuna Loa's e n ~ ~ t i o n s h e w n at the summl't caldera: during half ofihese the aclivitv suhseoi~entlv s h h e d toone n~ the two rift z h e s ( r h h e a s t and southwest): Since ~ 9 0 0 , ' ~ a u n a L n a h k eruptcd IS times, ufith eruptions lasting from one to 145 davs. Historic nctiv- itv i t Kilauea has becn mi re localized~oarlicularlv durinr! <he 18% and the early part of thic century, \<hen a hv3 like was a c h e a l m k t con~~nual lp in the sunimlt cadera . In 1924, the lava lake abruptly drained away. and ground water within the caldera came info contact with'the hot rocks suiroundTnx the magma conduit, creating a series of steam exdocions that threw large h!&ks out onto the caldera flonr. The sudtlen w~thdrawal of llte magma produced the deeo co l l a~se nit of the vreient dav Halcma'uma'u Crater. Sooradic -

cruptio;ls c o n h u f d a t ~ale'ms'urnau'until I W . \<hen all activiti ceased for 18yar s . In 1'452, Kilauea renwoke with another enlntion in Ha!emaCuma'u. Foi the vast three decades. most of the activitv has been concentrated on the volcano'stwo r ~ f ! zones, p&ticularly the east r;h zone. Until the omet of the current eruption, Kikuea'h most voluminous ril: eruption in this century occurred from 1969-1971, at the Mauna L'lu vent on the upper east rift znne.

THE CURRENT ERUPTION OF KIJAUEA TI,- n w r ~ n t emotion series at Kilauea heean in Januaw 1983. InitiaIlv. ...- -~~ , .

the shifted hack and forth ?Ion$ eruptke fissure5 eiendlng from N . ~ H crater to Kalalua cone, 5 ml (8 kln) downrift (F~E. 1). In June of 1983. :~~~&i% hecame localized at the Pu'u '0'0 vent. wh?ch'lies on the bound- . ~ ~~ ~~

i&ii%&&i Volcanoes National Park. Ovcr the k t three ycars, the activ- ;W at PI^ '0'0 estublished a regular cycle of episodic eruptions. each epi- :'L~;-~&oin averaee of 13 h i m and senaraied hv rendse neriods avirae- ." ....-. = - . ~ ~ - ing 26 days. 7he erupli~e episodes ycrc characreriled by spictncular lavaY fountains. some of wl.~ch reachcd helohts of 1,509 it (460 m). Tnese fountainr ., hllitt cinder and soatter conc that r&dlv hecame the mosi orominent land- " -. . . - -. . . - - - - - mark on the east rih zonc as it grew td a height of 850 ft (2.551~1).

Lava flows from Pu'u '0'0 repeatedly invaded Royal Gardens subdivision dmina 1983 and 1984. destrovlnR 16 houses. Because of the short duration of thpb:tntive enisoder h o w e k i h e t e flows nevcr reached the ocean. ...--.-r.... - . .~

In Julv 1986, aitG47 erup&v episodes, tl!c natore of tse eruption ahmpt- lvchaneed. The vertiwl maenla cond~lit beneath Pu'u '0'0 ruptured nnd new ~pxmd~issures began fountGning, first nprift, tl~eti downrilt, dl the massive cone. Within two days, the eruption h :d settled in at a new site near Kalalua cone, 2 mi (3 km) downrift of Pu'u 'Do. From July 20, 1986, through the time of this writine the new vent (~reviouslv referred to as the C-vent I n ~ t recenllv &&d ~ u ~ a y a n a h a ) has beeii continu6uslv erupting.

The shlt? in :he eruption site was accomp;lnied by ;I significant clim Y i[ts the style of emotion. Rather t l iun the episodic. hich-vulume eruptions o Pu u '0'0. Kuoaiana'ha nroduces a srcadv. low-volumc Fffusion of lavi. An active ~iva~on&scion f? over the nek Lent, and frequent overflows from the pon bulk a lava qh~eld. a low mound of lava approrim3tely 200 f t (60 m) high

1 mi (1.5 km) in diameter. A lava tube sysiirn slowly developed within - the shield. as channelled Ixva flows eraduallv crusted over. The main tube . ~~

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leads from the edge of the toward the ocean, 7.5 mi (12 km) to thesouth- eat. The course of the tubes can he traced ahove ground both by fuming areas, where gases given off by the lava escape to ihe surface, and by sky: lkht6where the roof of the tuhe has cnllanserl in nrovide awindow into the

~~~~ ~ -. .. ~ r . . . . . r . ~~~~ .~~ iicandesant river of lava below. Insulated within the tubes, law flows cnn travel much farther hefore thev cool and stncnate.

BY November 1986. the laka tuhe svstemextended to the ton of the mli ~ . ~

(c1iH)abovo the Kalapina coastline. ~Iowx quicklv>dvanced d6wn the :lope to the co-unlt) of Kapa'aiu, destroying hnrnc\, cutting the coastal highuay. andcnterlng the ocean tor t'le first time slnce the Mauna Ulu eruption. After a h m f respite rn Deccmher, flows ag:~in reached the coastal arm. this lime fanher to the east, where 17 homes were destroyed within two days' time in the Kalapana Gardens subdivi~lon. l'he rest of the subdivihion was spared when the tuhe system became hlocked near the shield, and the flows 5elou. the pali stagnated. In March 1987, fl~~i\,s again crossed the highwuy at Kapa'ahu on their way !o the sea. Since then, flow activity at the court has been almost continuous. Sixty-two homes have been destro)ed since J3nual-y 1983. and 2 mi (2.6 krr) of the highway linking the tou.n of Pahoa and the Chain cdCraters road in H?.waii Volcanoes Nat~onal Park have heen covered. A total of 67 acres (27 ha) ofnew land has been formed by the lava flows entering the ocean.

MWNA KEA

0 5 10 20 MILES

Figure 1. 1Iawai'i island, showing five vnleanoer and key place names.

~ l t h o u g h the current eruption of Kilaueu is the most voluminous and lonYst-lived rift eruptiqn of the last two centuries, geologists know from stu( vmg Kllsuea's preh~etoric lava flows that such long-lived activity is not

i? the 11fe hlstory of the volcano. Not e n o ~ g h is known about the averare duraiior. of st.ch eruptions, however, to allow us to forecast the conc l&m of thjs one. As yei, there is no indication that the current eruption is n e m n g :in end.

VOLCANOES AND LIFE

Volcanic activity creates unique conditions for life o n the Ilawaiian Islands. Lava flows destroy large tracts of native forest and shrubland o n active volcanoes. Appruximately 95% of Kilauea's surface is covered by flows less than 1,000 years old, attesting to the frequency with which any given area on the volcano IS covcrcd h new lava The lava flows may isolate patches of vegetation by creating kipuEa, islandsof older land surrounded by lava. l l i e flows also create new environments for plant and animal life, such as lava tubes. which tnav form moist cnvcs after a flow has cooled. - - ~ ~~, ~~- ~

In nreas of h igh rainfall, cuch oc Kllaucn'> e w t r ~ f t zone, revegetation of the new Iiwa cJn begin l e s tIi:111 a !.ear after thc floms itre ernplxed. The fertile w:1c fh3t hwimrt II~wal'i". ~wicul ture form bv the breakdown of lava . . - over periods raliglng from ten\ 10 rhwsands or year>, depei~tlii~g mainly on :he amount of r:iii11~11 (sce Strcet. t h e volume).

Ornanisms liviiie on an nctivc volcano muit also adant to the eas emis- sions f 6 n l both erupting and inactive vents. The main c;rnponenk of these emission?, are wxer v;qlor, cnrhon diox~de, and sulfur dioriue. Sulfuric acid forms when ruin mixes with the sullur dioxide in thc volcanic plume. The resulting acid rain c:~n significantly retard the growth of vcgetatiun in the area downwind c ~ f a ven: t ! ~ degases over n long period of time; the Ka'ti Desert e~~urhweat of fl:~leni:i'tima'u crater on Kilnuea's summit is the moct pronounced example uf this phenomenon.

Lava flows cntering the ocean slowly add new land area to the Island. Fishes that vcnturc too close to the advancing flow front mav he killed by the elevated tclnncraturc of thc watcr. hut the increased sediment load of the

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wuter near an i ~ r i i w flou prohahl" ha; a much greater cffcrt on occan life thali the heat. Newlv h r ~ ~ l e t l black sand niay be carried for several ~nilcs hv the longshore currerits, cresting new he:ichei where previously only sea cliffs had existed

Thc &~rrent eruption of Kilouea is typical of the volcanic activity that has formed the Hawaiian Islands over the v ~ s t 70 million vears. The volcanic orlgln of 1 lawal'n guermtees an o~ ip in 'g cycle of de&ctlon and creation, lo which ill1 life on these l s l ; i~~Js must adapt.

Important References

Deekcr.R., and B. Deckrr. 1981. Vdrmmr. W.H. Frccman an3 Cumpmv, San Franciwn m t r , R.W.. T.L Wright, and P.11. Staullcr (cds.). 1987. K,lcu,ris,rt iu Huwuii. US.

Geological S u n y Profcsional Papcr 139. US. Govi. Prin~ing Otke. Wishinylon, D.C.

Hclihr, C., J.D. Griggs, T.J. Takabashi, and T.L. Wright. 1986. Volwno munitnring at the U.S. Geological Suwey'r Hawaiian Volwno Obsfnatury. E u ~ h q w k e r vrtd Volcunes lR(l)l-72.

Macdonald, G.A.. A.T. Ablmft, and EL. Pclcrron. 1983, Vo!=o~rm.c in tlre Sw: The Gedogp nfHawoii. 2nd edition. Univ. Ehvaii Press, Honoluln.

Slrarns, H.T. 1985. Cieolufl oJflw Sfnfe uJHowaii. 2nd cdilion. Pacific Books Publishers, Pillo Alto, California.

Srrccl, J.R. [this uolume] Soils in Ilawai'i. Tilling, R.I. 1964. Enrptions oJMa181t Sf. H'le~ts: Post, Present, and Ftisrre. US.

Geological Survey serics nf bcncrrl-intcrcst publications. US. Go*. Priming OIfice, Wshington, D.C.

Tilling, R.I., C. Helikcr, and T.L. Wriphl. I'J87. Enrplionr of Hrrwaiiun VoIcmoc~; Post, Prmc~tf, andFamrrc. U.S. Gcologisnl Suncyrerics ofgmncral-inbxert publicalions U S . Gorl. Printing Office, Wnshington, D.C.