The Morphological Development of Krakatau Volcano, Sunda Strait, Indonesia

THE MORPHOLOGICAL DEVELOPMENT OF KRAKATAU VOLCANO, SUNDA STRAIT, INDONESIA

ADJAT SUDRl,'DJA T '3 0 p.,UG 198(1

THE MORPHOLOGICAL DEVELOPMENT OF KRAKATAU VOLCANO, SUNDA STRAIT, INDONESIA

ADJAT SUD!UJ'DJAT

r , BJDANG BmLJOGRAFI

DAN DEPOSIT

Tanggal : . Group I \

THE MORPHOLOGICAL DEVELOP1~ENT OF KRAKATAU VOLCANO 1 SUNDA STRAIT, INDONESIA )

by 2) Adj at Sudradj at

ABSTRACT

A morphologic development of Anak Krakatau after the 1883 Plinian eruption was studied based on the available bathymetric charts (Verbeek, 1885; Symo ns, 1888; Escher, 1919) and terres trial surveys 1arried o ut by the Vo lcanological Survey of Indo nesia. Additio jal information ",· a5 also gathered from t.ne .. ae..x-~ .~'l. ~ .phO~

. t 03r aphic interpretation of th ~ a rea. Within the first 30 ye a r . (1888-1919) after the eruption no sig nificant changes of. the s eafloor configuration have beentdQ.ticed . around Krakatau isl and complex. The submarin'e crater's bottom

see mina1y sho wed a deepening o f a ~pr o ximately 20 meters perhaps

clo sely relatedwiththe cODpacti ~ n and bedding · in of the volcanic deposits filled in the crater. The volcanic depvsits prcdu- ced by the newly-bo rn Anak Kray. atau volcano filled in particular ly the northe ast e rn upper inner slope of the craterwiti1a rate next o f 1 t o 3.5 0 a nnua lly d urin:; tho ;'l0 year period (1-9J.9··1940). A study on the morphologic development of Anak Krakatnu since i t s birth p h a se .(1927-19 30 ) t o February 1981 revealed the· _ chang es of elevatio n and size of Anak Krakatau Island. Three stages were no ted namely; (a) t he birth phase (1927-1930); (b) the developcent nnd destruction of the outer rine (1930-1950); and (c) the buildin6-up of inner-cone. At present the highest nhove s c a-l.c vel.. elevation exceeding 199.29 --in/is - at:tained -b), the summit of the ~bove s ~a-lev~l. inner-co ne while the out.r r1ng is 151.66 c/ The development of the the new· cone has been chnrecterized by/lava outpourinB since 1960-1963 period conc1udin6 each activity episode.

1). PrG: pared for tha"Syaposium on lOOth Year Dovelopment of Krakatau and its Surroundings·' , Jakarta, 23-27 August, 1983. 2). Volcanological Survey of Indonesia. Bandung.

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The shifting of . activity center of Anak Krakatau which towards SW,/perhaps related closely to the pred~inant structural settina of the area was also no ted . It is speculated that the activity will possibly continue shifting in to thi~'dt~'~~ion until it reaches the forDer Perbuatan-Danan-Rakata line. \ THE MORPHOLOGI CAL DEVELOPH ENT OF KRAKA'fAU VOLCANO SUNDA STRA IT, INDONES I A 1)

b:r ADJAT SUDRADJ AT 2)

I NTRODUCTION

Kr aka t au volcano l ocated i n Sunda Strait between Sumatr a ' and Ja\Y'a i s Horld", i de knot-in because of t he de s tructi ve Pl ini an eruption that t ook pl a ce in 1813. The eruption effect was fel t all over the world s eawavas t r avelled as far as 20 , OOOkm reachinB the South Ame rican Pas i fic Coas t and I ndi an coast of Afri ca \·:hile volcanic ashes \-Iere ejected up to the hei gh t of 90 km that t r avelled ar ound the globe .

The adjacent Sumatr a and J ~ ""a beaches '-Ier e compl etel y destroyed b:r t he t sunam i es r eaching a height of 30 me ters and ki lled approxi matel:r 36 , 000 peopl e . The booming sounds \·,er e heard a t Jalcar ta and Bogor situ ated nearl y; 150 km dist ant from Kr akat au . The pr i ncipal expl osion:> occur r ed at 05 :30 , 06 :41f , 10 :02 and 10 :52 on Au gust 27 , 1883. These explosions had caus ed t he vo l cano di sappear ed under t he s e a . Sc vcr &l fie ld "/or ks had been cl one i mm ediatel y af ter the eruption ( Verbeck , 1835 , Symo ns , 18B8) und nuccass i ve i nvestiGati ons on t he s ub mar ine volcano \'le l' e l a t e r done unt il 1927 wher e f or t he first time the

nC\-I vent of l u~akat au cill e r gcd ~b o v e the Gcal c ve l . Th e nC\'/l y born vol cano called Anak Krakatau ( t he Child of Krakatau), s i nce then was frequently s urveyed . Hany r epor ts concerning Kr akatau have, been publ ished , among other s b:r Ve r beek (1885) , Escher (1919 ) and Stehn (1929) . The most r ecent investigat i ons 1,oler e carri e d out by the Vo l canol ogical Sur vey of Indonesia part icularly as concerned \d't h t he a cti vi t i es o c curred since 1950.

,The pr esent paper co~p iles the mor phol ogical chan ~es caused by t he deposition of volcanic mater ial s newl y pr oduced by Anak Kr akatau based on the - ter res tri al surveys. Th e background infor mation on t he sea- f l oor

• conf l. tSUr ation '-lClS compile d from the available sources , mainly from Symons (1888)· KHAKATAU VOLCANO I SLAND COHPLEX OF THE PRE-1883 };RUPTION

Bef ore the des t ruc t ive eruption of 1883 Kr akat au volcano i s l an d com pl ex comprised pr imar ily of 3 ma i n islands , nam ely Rakata ( Kr akatau) , Ser t ung (Verla ten) and Panjang (Lang ) i s l ands. The bi ggest i sland was Rakata trendi'ng SSE- NNII \ii th l~ km long and appr oximately 5 km wide at the m. ximum. Thi s i slend " as bu ilt essentially by 3 '101canic bodies llaka to (the highest, 8000) , Danan(456m) and Per bua t an(120m). Du ring the eruption the t\iO l att er vol canoes Here comp l e t e ly destroyed \'lhile Hakata di sapJ?eared part ly. The sea bottom confi[;ur ati on showed that the ialand compl ex or igi nally consi s t ed of an oval shaped volcanic r ui n tha t partly covered by Sea\'/at er with t he depth of 50 rn. The activities presumabl y concentra ted in the twin volcano of Danan and Per buatan ( Neumann van Pad ang , 1951 ) . The mos t r e cent ~ c t i 'Ii ties before the H183 eruption most pr obabl y t ook " l ace in 1680 "nd 1681 ( Ve r beek , 1881).

333Cd on t he volcanic dcposts invest igati on i n the vo lC ~lJlO complex,

hypo t heses have bee n dr .:nm 4'e 3e r~. i ng the dcvelo?nJcnt of the Krakat au vol

ceno among other G the famous K r 3 1 ~ atau grO\'.,th by :;,sclH: r (1919) . It \'/ a s as numed that in t !1.e very ear l y s t a f) ,:) a 3i f,3.nt i c s trato volcano of a!1:.'roxi

matel y 10 - 12 knl <.l i a!iletcr and 2000 :':1 high ros e in the Bunda str ait \-! a ter s .

The volcano p r e ~, u r~ab 1 y had a majestic ancl. per fect syr.ll.1e trical s l opes of a ver y t yyi c Cl l l at e l') ha so build- up s t age. Til l S vas called by Escher (191 9 )

as t he Ancie nt I~ r a k a t au vol c,",no ( l!' i g I - a ) . A plini an t ype erupt i on took pl ace concluding t he buil d-up stulle of the majestic volcano. This gi 6G'nt ic er upt ion had destroyed the volcano itself l eaving t he r ui n of 3 island s: HE/k a t a , Sertung and Panjang (!i'ig l-b). A s ubma rine crater 7 let'.! diame t er cam e int o be ing , ,oti t h its orif i ce ,.,es . r epr esented by t hos e thre e islands . The depth of the crater' s base was estimated to be 50 t o ·100 me t ers under seal evel. \

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The volcanic acti vities presumably of vulcanian type ~, as then started at the Ra.lwta v.o lcano island. The activities had built up the isl and \iith a typical strata deposits consisting of tephra, lava flows and dyke instrusions unti l it e xceeded 'approximatel y 800 m height above ·' sealevel. It pr oduced a volcano island of approximately 5 km in diameter wi th ",as called Rakuta Island of the pre-1883 erup tion (Fig I-c). Follow i ng the dor ma:1cy of Hakata volcano the activit i es shifted nort hw cstwards to the central of the crater 'concurrently ''lith the

predominant gen ~ r al s tructural trend of Sumatra. The a ctivities produced b/o volcanoes name l y Danan and' Perbuatan that presumably gre\'1 s imultanoustly• . Th e tHin volca noes as called by Ne umann VDn PDdang ( 1 951) e xceeded the summ its- h e i ghts of r espectivoly 456 and 120 meter s above seal e ve l. Those three acti vi ty center s produced cocl bincd volcanic bodies that represented an i s l and some times referred to as Krakatau of the prc-1883 eruption (Fig I - d) •

TI!E DEVEI.OPHB!';T OF SUBNAilINE KRAK ATAU VOLCANO , :\fter a gi gantic e ruption of 1 383 s e veral investigations hnd been

done. 'rhe first surveys wa G c ~r:::'i eu out imm e di atel y afte r the e :~:"u p tion ceased (Verbeck , 1385 and Symons , 18G8 ) that recorded the nelV' configur ati ons of the i s landG of Krnko t ou' Cor.lplc~ ~ ( Fi£\ 2) . Volcanic ~cposi t s ma inly

cons isting ,of pumi c e exist ed s ur round ,i~g the surviving islCl nds. Bxccpt the three volcanic bodies of Ral\.ata , Donan and l.)erbuatan t hat destr oyed by the eruption , the other i s l a nds grc\f up and became lare;e r. Serttlne; Island

l octlte:'!d NU of ilakata had s rown \!. lmo~) t three titHes as large as it '

r~veal ed the i nner geoloBical for mation of the dor mant Raka ta vol-cano. This n a tural section runs almost across the middl e part of the:'! volcanic body. Escher (1919) a nd l ater van Demmelen ( 1934) studi ed the sect ion and yie lded a petrographic c lues as to the prediction to the coming eruption. \

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A typical strato depos its intruded by dykes lias r eadily obser vable across the liall. Es cher (1919 ) noted ,",ore t han 25 dykes '·/e r e cDunted. The ba thymetric char t \-l as first pr epared by Symons (1888) and then by Escher (1919) . It appear s that a caldera \-l i th a depth of more t han

250 me t er s occurred ~1\·1 of Rakata Island , el ongated approxi matel y E- II . Th e aver age s ize of the summarine crater a t t he depth of 100 m bel o\of seal evel me asured appr oxi mately 9- 10 km l ength and about 2 km wi dt h . Volcano clastic deposits were found scatter ed around Sertung and Panjang { i s l ands , much pr obably cons isted mainly of volcanic sands or base s urge deposi ts. The bQ t hymetri c survey by Escher ( 1919 ) i ndicated morphol ogy of a parasitic cone t hat gre\-, f r om crater ' s floor , however an accur a t e posi tion of t he Anak Krakatau done by Ne umman van Padang (J. 933) reveal ed the cent r al activity location north of the submar i ne r idge supposedly the embryo of the vo l cano as based on s ubmar i ne mor phological account . Appa r ently t he s ubmarine Kraka t au Vo l cano had graHn up at the ori f i ce or the i nner NE s l ope of the 1833 crater , or sli (\h tly nor t h of· the line connecting previous Danan Rne! f'erbuatan central vents. The und crH D. tcr shocsli}:.e ridge wi th a small r ock i s l and s t i cking out was cncotmt ered b y l S19 s ur vey in the l ocati on S\" of Panj ang I s land , nam e ly the Boatsmans Ro ck. This tiny island pe:chaps r epr esent ed t he Nl~ or ifice of the crater as par t of the sur vivi ng f or me r Kr akatau I s l and of t he DEman volcanic body.

Based on bathymetric cha:rt~ l're:). r ~d ':l y S Ylil 011S (1888) , Es cher (1919 ) and VolcanoloGica l :Jurve;y of l.\(~oT!. (~s i:.t (1~40 ) B. de vel opment of the s ub ma rine crater could be studied . t.t first , immedi atel:, after t he eruption an e l onguted E- vl submarine cra t er 'vIas formed N\v of nak ·:1 t u Island . Th e maxi mum crater ' s bottom wa s e stimated more than 200 m und er seal e vel. It seems t hat t he crater had become deeper as it i s compar ed \'l i t h t he 1919 chart pr epared by Hl1S "van Goe;h" ( Escher, 1919) . This per haps very closel y rel a t ed with the s ubsi dence of t he cra t er' s floor due to the compaction and beddi nG- in of the cr.:lter.

· -

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The initial activity of Anak I< raka tau had not been noticed in the surveys. There are no under\'fater to po~r aphic signatures as to mark .the initia l development of tho s ubma rine Krakatau vol~ano , befor e the eruption was noticed in the sur face . Apparent ly the activity had not taken place in the ridge of the und er\-/ater crater' s bottom a.s it \'1a5 originally thought before the birth of Anak Krakatau , ins tead it grew from the orifice or the upper inner s lope of t he crater .

The a ctivity 1-/aS first noticed in December 1927 until J anuary 1928. It mainly produced s team that rose sever al of 10 me ters high. The first a ir ~ e conn a issanc e \'183 carried out i n J anuary 1929 by Nash ( 1929) to locate the acti vity. A new i nl a nd was observed in the location of the activity f or ming a sand dune - l i k e s hape \li th the concaVQ s t e e per slope facing southv/es t . Th.: s ize \'H1S approxima t e ly 8- 10 m. The survey carried out later measured the h e i ght of 3. 93 m of the crest above sealevel. 'rIds \'las the birth of Child of Kra l(atau that \."a s Hi tnessed. 'rhe development ",as then studied through u series of .topographical surveys carried out r egularly by Nou mman van Padang (1933 . 1935 . 1936) a nd then by von Bemm el en (1941) and the Volca nologica l Sur vey of I nd one sia s ince i 950. Th e l ocati on of Anak

Krakatrl u Volca no a n d th ~ ba t h ym<.:. .' t ri c chart of 1919 s urve y fl r e s ho,,,n in Fig 3. f],lh e d evc lop ri1cnt of t h e cra t cr" conf i Guration "f/as fairly r api d a fter the er uption \·,a ::; no t i ced . Vo lccnic product .."a s d ~ po G i t ed mainly in the S\of s lop<.. or t he NE i nne r s l o:!l ~ 0 :(' the under\'/a t er crate r be cause of b"o r easons a . Th e s t ee p e r s l ope faced to \-! ~:::'c ' ct S' J b. Th e ven t ,,,a s l oca t ed i n S'd - c rn pa r t of thc volcano s urround ing by a dune- like sha pe i s land in the NE pre venting the product to deposit in the NE part of the volcano.

Th0 most deeper s u bm a rine cra ter I s floor had become shallO\'/er from

279 m to appr oximatel y 250 !u , \'lhile the ridge loco.ted at the f l oor \."as found to be betvJe en 200- 250 r.i bclo\1 seale vel. In general the crater had been filled ,'lith approxima t e ly 20-30 m of volcanic ma terials during the period of 1919 to 1940 (ba thymetric chart by the Volcanological Survey of

Indonesia) 1 or even in the NE part of the cra ter ,."h ere it is closer to the activity it exceeded 20-70 m. ( 6 -

The rate of depositi onal changes, therefore, is estimated to be approximately bet"leen 1 to 3.5 m annua;ty. The very r ecent bathymetric chart has not yet b een obtained in orde r to compare the rate for the last 40 years . HO>lever from the per iodicity and magnitude of the e ruption of Anak Krakatau it is ve ry like ly t hat the rate has remained stable, during these periods (Fig 4).

TOPOGRAPH I C DEVELOPMENT OF ANAl( KRAKATAU

The nc,.,ly born Anak Kr a katau ,-ras intensively surveyed in 1930's f or t he fear of the recurr ence of t he hazardous 1883 eruption. An observa tion post was set U j? in Panjans I s land to observe the volca no , and regular visi ts ,.., er e carried out uy volcanologists and obs ervers.

The accurate to po~r aphica l chan f~eB therefore \

chang:es ''lere also accompanied by th e'! d~ f f cronce of the i s l o.nd s i ze i t80lf ( Fi g 5) . rraol e 1 8110\'IS t :1C topo::raphic d 01.; clopmc nt of Anak f(rak~ ta.u island from 1930 t o 1901 . ;icC!surcmOllb.: havr.,: boon riw.de on the devc lop:oent of the

N\'! - SE and S\J ... HE {~ i ;"ti1e ter of the i :;).,: nd t o record the devel opmen t of the

coa.s tline . The tH'l - SE diameter ::; hm'.'8 a continous deve lopment of the island. A minor i:>r.:ak ....las encountered b.) blC'Cl1 the ,el':i.od of 1950 and 1960 , perhaps

r e l a t ed to the buil

the crater' s floor as suggest ed by I~eumann van Padang (1936)

.. ..

The volcanic activity controlled very clos ely the development of t he cOD.ntl i nes and the elevations of the i s land as 2. result of the piling up of volcanic proQucts. The r e gular increase on the elevation height in the first s tage of t he Anak Kr akatau volcano was accompanied by the increase of the diameter of the i s l and . A collapse occurred in 1934 both r eflected in S\'I - NE di meter and i n the el evation hei ght (point A of Fig 6) . A similar s i t uati on occurred i n 1935 ( point B). whereas in 1950 (point C) collapse influenced the wi dth of the i sland bu t not much on the elevation itself. The el evati on de ve l opment of the isl and ( OCR, elevation of Old Crater ' s Bi ni ) decreased in early 1960's ( point E) perhaps after it exceeded the culmi na t i on po i nt , due to the development of ne\'1 cone that s t art ed i n 1 950 ( NVS , Ne"1 Vo l cani c Summi t ). Her e 'the devel opment has been progr essi nz very smooth s i nce its birt h (point D) within the last 30 year period. The pr esent elevation of the Anak Krakat«u rate lias estimated at . approxime:lte ly 4- 4. 25 m a year. Based on the i s l and outl i ne map and elevation chart a r oue;h calcu l ati on "las compiled t o estif:'\3te the volcanic deposit tha t built up the island . I Gnor ing the ma t erial s deponi ted on th E:' seabed and Hashed a\'1ay f r ow. the vol canic body , and a130 the ma t erial s carried out by \·/ind , the volcanic m:) teria1 pr oduced i n t he l ast 50 years \'las esti mated to be 26 .2. 6 6 10 m3 or II rate of 1 . 3 . 10 m3 a year . The devel opment of 11e\i cone s ince 6 6 1950 had produced 1+2 . 6 . 10 m3 material s or approxima tely 1./,. 10 m3 a year . I t scems t he devel opment "/as s t ~,b l ~ \·ti th a r a te of 1.3 to l. l ~ • .$ 10 m3 a yCi.1.r.

DISCUSSI ON ON TH E !-10RPHOL OGIC DEVELOPMENT OF ANAK KRAKATAU

Th e submarine crat er t hat exiGted after the destructive eruption of 1883 vias formed in the cra t er of the anci ent Krakatau. A hypothetical e;i ganti c vol cano as s ugges t ed by :':s eher (1919) des.tr<:>y.s "" itself several centuries bcfol'e t he bvin vol cano of Danan and Perbuatan came i nto being. /\. speculation envisaged by de Neve (orDl communication, 1981) suspected thc ancient caldera forming perhaps took place at the third century AD bas ed on the ava i l able history document and the sea floor volcanic deposit found in tho Java trench.

If the assumption Has true, t he development of Raka ta, Danan and Perbuatan had taken place around 15 centuries until they destroyed themselves and later l\nak Krakat au volcano calile into exis tence. The 1883 crater or caldera formed \'I i t hin the ancient one '-las , somewhat non-concentric in SW direction. / I The cal d er a for mi ng had ~ecn discussed by sever a l authors. Stehn (1939) assumed the cal dera as a r es ult of volca.'rlo colla pse due to the space left by the ga s .'3:nd !uagma. The colla pse perha ps had gcner a t 'cd t s unamies that

S~Tcpt J a\'rGl a nd Suma tra coast s , \·,hile he also ga ve other a r guments such a s

the en-a r gy r elease during the eruption or perhaps the submarine l ~nds lidi-ng. Yokoya ma ,(1980) B~ v e a ccount on the basi s of geophysical r esult of gravi metric s urve y in the a rea. H\? concluded thD.t the cD.ldera forming \'1as not caused by the volc Cl nic collapse ba sed on his co.lculation of the s pecific gr a vity of the vol ca n ic product th(' t filled the s pace l e ft by the ejected materials .. Th c:: r efore he sugges t ed t hat the tsuna mies \'las generated by the

energy r t:: l ~ as e \-,hcn the volcanic t!w t e ria l totally amounted to 18 km3 \o1C'.. S ejec t ed. His a na lys i s \'I ns 41 150 supported by the s i Gna tures . of the \'lave records e Vi.l i l able i n placGs , ui1 ;)!'e the first ''l uve arrival sugcrest ed the upHa;)' movement.

~L'h o loca tion of .'\n a k Kr· .. ;; kc:. t a u '-t as at the orifice or the upper s l ope

of the s u bmarine cr a te r ~ Th i s h el d influe nced the d e vel opli1Cnt of Annk Kra- oi loco tir):1 .k .?l. t z.u 8nc. perhaps t he s hifting /of a ct.i vi ty. Basod on t he d u vcloj;)J"~nt of the

i t::il<.:md' i t ma y be noticed tha t ther e ",D S a n a pll':.lr cnt s hiftine: to S'" of the c cnt ~ r of e ctivitics s i n ce 1930 t o 1931 (Fir; 5). The pr eviou,? a ctivities of Rcka t a - ) o.n £-.n - Pe rbu o. t .:m locotr:d a l onE t h e lin.zoir zone trending N\'I - S :J:'~ , whildt t h e 1883 erup tion s upposcdly ~ ls o occurred u t this lineament (Yokoyc.'ma , 1980 , Self , J.979 ). It i s sl)c ct! l ated that the ucti vi ty cent er ,-Till tlossibly continue • - t M t shifting to\-TCl r ds R<"lkDto- D£ma n-Per buntmi lillO o The fact w .:.~s lthe d~velopment

of the Anak ~~ r

outpourings tha t occurred s ever a l times i n the l ast 20 ye ars. This m~y also have been the r e s ult of the s e afloor configurc"l tion. The l and sliding e vi d0ncc noted by Neumann van Padang (1936) i n the early stages of Anal<. KrD

k at':'lu cont ri bu t 0~ to the idea of the s0Cl floor influence to the developlilcnt of Ana l-;. Kra ka t a u island.

- 9 - llEFERENCES

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· Sudr adjat , Adjat , 1979 , ~2~p~_~~~~E_~E_p i n~6 ke Kr akat au t a nGg~ 14 Agu stus , Vo l ,cllno l . Surv. of Indon. , uupubl . re po r t, 7 p.

Suma ilani 1 Rochanlln 1 1981 , J..apora.ll .~enguku~_n r O p o ~ l' a!i G. Anak Krakil tau,

~ , Volcano l . Surv. of Indon . unpub l . report , 11 p. van Bemme l en , R. H. , 1941, Kr alGl.tau no . 18 , .?ull. of East I ndj.an Volcanol. ~ . n. 95 -98 , Vo l canol. Su rv. of Indon., p 53- 60.

Ve rbeek , R . D . ~', . , 1885 , Kr akat au,1 v . 1- 2 , J33 t avi a , 1~59 p. Zen , H. T. , 1 9 61 ~ , Recent Chenl)es i n the An:il.: I\ r a~mtau V o 1c ~n o t B ull.,~ ~o l ~-,- 'fome 27 , P 259- 268 •

. ------1970 t Gro\ith. and State of Ponco.!t Kr d.kr t an i n September 19GB,

Bul l . Volcanol., t Tome :)I.~ , p .. 205- 215 .

·\

PEnp USTAKAA ~ NA S lO :-; ~

· Table 1. Topographic development of Analt Krokatau (1930 - 1981)

Island size Highest Elevation above sea level (m