THE MORPHOLOGICAL DEVELOPMENT OF KRAKATAU VOLCANO, SUNDA STRAIT, INDONESIA BY ADJAT SUDRl,'DJA T '3 0 p.,UG 198(1 THE MORPHOLOGICAL DEVELOPMENT OF KRAKATAU VOLCANO, SUNDA STRAIT, INDONESIA BY 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 volca- nic 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. - 2 - 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. 2 ,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. \ - 3 - 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.