The origin of the 1883 Krakatau tsunamis, by P.W. Francis

Journal o f Volcanology and Geothermal Research, 30 (1986) 169--177

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Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE ORIGIN OF THE 1883 K R A K A T A U TSUNAMIS, by P.W. Francis DISCUSSION

PIERRE M. VINCENT and GUY CAMUS

Centre de Recherches Volcanologiques, Universit~ de Clermont-Ferrand H & C.N.R.S., 5 Rue Kessler, 63038 Clermont-Ferrand Cedex, France (Received November 25, 1985; accepted for publication April 8, 1986)

INTRODUCTION

At the end of his paper on the tsunamis o f Krakatau, Francis says "The most plausible mechanism for the eruption is likely to be a Mount St. Helens scenario, in which collapse of part of the original volcanic edifice propagated a major explosion". We have developed this interpretation for several years, so we are delighted to see that Francis shares our views! However, even though he does n o t deny the priority of our explanation based on a Mount St. Helens type of mechanism, he asserts that several predecessors have proposed very similar models (Verbeek, 1884; Self and Rampino, 1981), and that the author himself has proposed a "collapse hypothesis" (Francis and Self, 1983}. Such assertions require c o m m e n t . But, most important, we would like to show that acceptance of the Mount St. Helens model as an explanation for the major tsunami, means giving up the timing of the events recently proposed by various authors, including Francis, because this timing is inconsistent with the available geological data. CHRONOLOGY OF THE EVENTS: PLINIAN ERUPTION - - T S U N A M I -IGNIMBRITIC ERUPTION

Francis and Self (1983), as well as Self and Rampino (1981), state that the emplacement o f ignimbrites began at about 5.30 a.m. on Aug. 27th., and continued after the large tsunami was triggered a b o u t 10.00a.m. According to this timing, the large tsunami would n o t represent a major break in the progress of the eruption and could be related to large pyroclastic flow plunging into the sea. This is the view of Latter (1981), and of Self and Rampino (1981); it was also that of Francis and Self (1983) who state: "Given that many cubic kilometers of materials entered the sea in the form of pyroclastic flows, this possibility seems to us the likeliest o n e " (p. 156). On the other hand, Verbeek, who also mentioned such a mechanism involving fall ejecta, applied it only to the minor tsunamis that occurred

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before 10.00a.m. The field data prove without ambiguity that the succession of the events was: (a) plinian eruption; (b) major tsunami a b o u t 10.00 a.m.; (c) ignimbritic eruption. The plinian deposits associated with pyroclastic surges are subaerial and were emplaced at high temperature as testified by the incipient welding of their upper levels (Self and Rampino, 1981). A major erosional unconformity separates them from the ignimbrites (Fig. 1) which lie on the former wherever they have been preserved by their welded tops (northwest of Lang Island). Elsewhere, including the western part of Lang Island, the ignimbrites lie directly on old lavas from which the pre1883 soil and their superficial weathered parts had been swept away (Vincent et al., 1984). The boundary is very irregular. We interpret this major unconformity formed during the eruption as evidence that the large 10.00 a.m. tsunami passed over the lower parts of the islands. This fact was correctly interpreted by Stehn, 1929 (in Simkin and Fiske, 1983, p. 323). The same point of view was also shared by Williams (1941) who, speaking of the bedded pumices, wrote: "Though the lower contact of the smoke grey pumice (i.e. the welded layer of Self and Rampino) is almost horizontal, the upper surface is deeply channeled, probably, as Stehn believes, owing to the inrush of the great tidal wave of 10.00 a.m." (in Simkin and Fiske, 1983, p. 345). The subsequent massive pumice deposits which represent more than 90% of the whole deposit were emplaced in a few hours by a succession of pyroclastic flows as established for the first time by Williams, 1941. Thus, we have to admit thai the large 10.00 a.m. tsunami represents a major break in the progress of the eruption and that its cause must also explain the dramatic change in the discharge of magma and in its eruptive style. #';

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171 THE ERUPTION BECOMES SUBMARINE AFTER THE MAJOR TSUNAMI

According to the Mount St. Helens model that we have proposed, and the timing inferred from the geological data, the drastic change from plinian to ignimbritic activity is explained b y the sudden release o f pressure due to the flank-failure of the volcano. This explanation is also in agreement with the large-scale flooding of sea-water which made the ignimbritic eruption submarine and therefore produced low-temperature deposits, as shown previously (Camus and Vincent, 1983; Vincent and Camus, 1983). In our opinion the ignimbritic deposits are related to the nearly steady rate of flow of a shallow submarine eruption; the grain-size characteristics of the deposit were acquired before contact with the water. Since water did not play an important role in this stage of the eruption, it cannot be considered a true hydromagmatic eruption just because the ignimbrites came into contact with the sea. This sub-stationary submarine phase, which allowed the w e t products to collapse "en masse as an avalanche" {Williams, 1941) was preceded b y a transitory phase, also submarine b u t probably short. This assertion rests on the discovery, at the southeastern part of Sertung (Verlaten) of an interesting deposit t o p p e d by the ignimbrite, and lying unconformably on the earlier lavas cleared of their pre-1883 soft. It is made up of pumice fall deposits alternating with pyroclastic surge deposits containing many accretionary lapilli, armoured lapilli coated with mud, and uncarbonized w o o d . We interpret these layers as base-surge deposits (Camus and Vincent, 1983). They should n o t be confused with the layered pumice deposits emitted during the plinian phase (as they were before), because of their different character and their emplacement above the erosional surface attributed to the 10.00 a.m. tsunami. These deposits were greatly influenced by sea water and must be related to the famous m u d rain which fell within a 100 km radius of Krakatau, just after the big explosion and the "burning ashes of Ketimbang" (Verbeek, 1886; Furneaux, 1964). This mud rain, which was a unique event in the history of the eruption, was emphasized by Verbeek who looked u p o n this phase as corresponding unequivocally to the beginning o f the sub-marine eruption. It was essentially to account for this rain that Verbeek p u t forward his hypothesis of the early collapse of what we now must call the caldera. T H E M O U N T ST. H E L E N S M O D E L A N D T H E E X C E P T I O N A L E V E N T S A R O U N D 1 0 . 0 0 A.M. ON A U G . 2 7 T H

According to this timing, the events that occurred around 10.00 a . m . which were an exceptional stage of the eruption are readily explained: (a) The crumbling of the northern flank of the volcano created an avalanche crater {North cliff of Rakata Island) and, as we shall explain later, this was partly under water. (b) The tsunami is related to the immersion of the debris-flow. (c) The tremendous explosion at the origin of the lateral blast

172 occurred shortly afterwards. The blast reached the southern coast of Sumatra, almost 4 0 k m to the north, as is attested by the witnesses and victims of the "burning ashes of Ketimbang" at Rajah Bassah (Verbeek, 1886; Furneaux, 1964). We can suppose that the strength of the explosion was related to the combined effects of (a) the vaporization of the water content of the volcano and of sea-water reaching the avalanche crater and (b), the increased expansion of magmatic gases, when the fragmentation level had abruptly deepened by the sudden unloading. The magmatic column above this new fragmentation level quickly became potentially explosive through a vertical distance several hundred meters, according to the model of Wilson (1980). Since blast deposits have not yet been identified, it is not possible to know whether a cryptodome played a part in this eruption, as it did at Mount St. Helens. The expected abundance of the superficial water and the mass of magmatic gases available can explain the exceptional character of the explosion which had an energy one order of magnitude greater than that of the Bezymianny volcano (Yokoyama, 1981). THE ORIGIN OF THE LARGE TSUNAMI For several years, we have supported the view that the tsunami was related to the immersion of a debris-flow. The arguments to account for it have been laid out in several papers (Vincent and Camus, 1983; Vincent et al., 1984); the best arguments are (a) the h u m m o c k y surface of the bottom of Sebesi channel; (b) the discovery on the western shore o f Lang Island of a polygenic breccia with blocks of hydrothermally altered andesite, resting on a pre1883 lava flow and covered by the ignimbrite. Such altered andesites are known elsewhere only as accidental xenoliths, so we consider that such a view is more than a "superficial hypothesis". Francis mentions only our preliminary paper (Camus and Vincent, 1983) in which we consider in fact two possibilities for triggering the tsunami, a debris-flow and blast. The 10.00a.m. explosion which was partly underwater could itself in theory have triggered the tsunami as in the model proposed by Yokoyama (1981). We emphasized, however, that both possible causes could act in the same way. In the following papers we mention only the immersion of the debris-flow, because this immersion, which was enough to generate the wave, occurred before the blast. Nevertheless, we think that when the blast overtook the tsunami, it was able to modify the character of the wave. It is unlikely that the velocity (controlled by the depth of the sea bottom) would have increased, but the amplitude could have been magnified. The association of the two phenomena could have produced the largest known tsunami of volcanic origin. According to Latter (1981), the major tsunami left Krakatau 15 minutes before the "big bang". Even if Francis is critical of these computations, we note that the chronology of the two phenomena agrees with the Mount St.

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Helens model. In no way do any other explanations proposed up to-date agree with a sequence in which the tsunami precedes the explosion. T H E M O U N T ST. H E L E N S M O D E L A N D E A R L I E R VIEWS

Verbeek: In order to explain the major tsunami, Verbeek had to take into account~his own certainties, i.e.: (a) The eruption became submarine after the large tsunami and formed a chasm at the northern end of Rakata (b) The slight volume of old material in the pumice (less than 5%) excludes the hypothesis of an explosive origin of the depression (c) An important volume of products was emitted after the tsunami (d) The tsunami cannot be related to pyroclastic falls. " T h e origin of the major wave must not be attributed to the submarine ejaculation of the mass of lavas, and, even less to the falling into the sea of the products emitted in this way because these events occurred shortly after (underlined by Verbeek) the starting of the tidal wave" (Verbeek, 1886, p. 407). We wholly agree with Verbeek. As for the volume of pumice emitted after the tsunami, we have now more evidence than Verbeek, who observed Krakatau before erosion of the coastal cliffs. Following Stehn, one can assert that the massive pumice was emplaced after the tsunami and that it represents the greatest volume of erupted products -- more than 90% according to Williams. The apparent contradiction between these facts was an obstacle to a satisfactory solution of the problem, owing to the limited understanding of volcanic mechanisms at the end of the XIXth century. The main problem was the 'disappearance' of older materials from Krakatau at this early stage of the eruption. In the Mount St. Helens model, the contradiction disappears if one takes into account debris flow moving far towards the north into the Sebesi Channel. The explanation arrived at by Verbeek is quite different and is only briefly p u t forward in his thick memoir; it mainly requires internal melting of the volcano by an ascending magma until such a thin 'shell' of old material remained t h a t its disappearance was n o t a real problem. Later, when the sea-water entered the crater and the remaining part of the volcano collapsed " . . . penetration in the crater of sea-water from above, and, very probably, simultaneous cave in of Krakatau must have happened before the materials were ejected . . . " . This material was " . . . a tremendous q u a n t i t y of mud, formed from a mixture of ejected ashes with sea-water . . . " but it is n o t clear whether this material represents for Verbeek all the "massive pumice", i.e. the ignimbrite. On the other hand, one can imagine that in the mind of the writer, the " e f f o n d r e m e n t " (French edition of 1886), "collapse", "cave-in", or "subsidence" responsible for the major wave extended straight down into the magma and ended in formation of a depression between the islands, i.e. morphology to-day named a "caldera". This was

174 also the meaning understood by later authors, including Williams (1941) who quoted Verbeek's view in his chapter "Origin o f the caldera". We must emphasize that Verbeek's concern for a quantitative approach to volcanic phenomena makes him a pioneer of modern volcanology. His hypothesis for the internal melting of the volcano is one of the rare parts of his work that remind the reader that Verbeek was an author of the XIXth century. This idea did n o t seem outlandish at that time; we need only recall that Judd, an advocate of explosion calderas, thought that the internal melting of the volcano played a major part, because it accounted for the origin of the pumice (Judd, in Symmons, 1888). Unlike Francis, we consider that it is putting it a bit strongly to include Verbeek's interpretation among " t h e Mount St. Helens m o d e l " {Francis, 1985). On the other hand, we have shown that all the constraints emphasized by Verbeek must still be taken into account in any a t t e m p t for reconstruct the eruptive phenomena; it is in this view that Verbeek is a precursor of the model, being the scientist who had formulated the problem in the best terms. But we cannot underestimate the contribution of his successors, namely Stehn and Williams among those who have passed away. Williams made almost all volcanologists recognize the necessity and reality of the posteruptive caldera formed on Aug. 28th. We have shown furthermore that this caldera cuts another structure interpreted by us as the avalanche crater (Vincent et al., 1984). Self and Rampino: The Mount St. Helens model, as we have proposed it, accounts for triggering o f the large tsunami. Self and Rampino {1981) " . . . suggest that the major tsunamis that accompanied the eruption were produced when the pyroclastic flows entered the sea . . . " . It appears that they apply this point of view to the 10.00 a.m. wave. On the other hand these authors also consider another possibility " . . . The slumping of large parts of the volcano, for example the segment of Krakatau cone that probably slid into the caldera, could have been a factor in generating tsunamis late in the eruption": this type of slumping may have triggered late waves that were post-caldera (in the sense of Williams), but it is inconsistent with the earlier major tsunami. However, triggering of minor tsunamis related to the crumbling of the crater (or caldera) wall is plausible, in view of according the weakness of recent walls. Examples were seen at Fernandina Island in the Galapagos and at Mount St. Helens. Francis and Sell: Their interpretation has already been discussed above. Even though the authors do not state that a tsunami could be related to such crumbling, their scheme on p. 150 is very clear; one can see the crumbled material from the northern face of Rakata Island in a restricted area, in the southern part of the caldera. It appears to be a late and minor event, compared with the previous one. To quote this paper, as does Francis {1985), in reference to a "collapse hypothesis" appears excessive. Francis ends his paper saying " . . . none o f his (Verbeek) successors has

175 been able to improve substantially on his approach, and some have succombed to rather superficial hypothesising". We do not share this opinion. REFERENCES Camus, G. and Vincent, P.-M., 1983. Discussion of a new hypothesis for the Krakatau volcanic eruption in 1883. J. Volcanol. Geotherm. Res., 19: 167--173. Francis, P.W., 1985. The origin of the 1883 Krakatau tsunamis. J. Volcanol. Geotherm. Res., 25: 349--364. Francis, P.W., Self S., 1983. The eruption of Krakatau. Sci. Am., 249: 172--187. Furneaux, R., 1964. Krakatau. Prentice Hall, New York, N.Y., 207 pp. Latter, J.H., 1981. Tsunamis of volcanic origin: Summary of causes, with particular reference to Krakatoa, 1883. Bull. Volcanol., 44 (3): 467--490. Self, S. and Rampino, N.R., 1981. The 1883 eruption of Krakatau. Nature, 294: 699-704. Simkin, T. and Fiske, R., 1983. Krakatau 1883. Smithsonian Inst. Press, Washington D.C., 464 pp. Stehn, C.E., 1929. The geology and volcanism of the Krakatau group. 4th Pac. Sci. Congr., Batavia, Guide Book, part 1, pp. 1--55. Symmons, G. J., 1888. The eruption of Krakatau and subsequent ph0nomena. Report of the Krakatau committee of the Royal Society, London. Verbeek, R.D.M., 1884. The Krakatoa Eruption. Nature, 30: 10--15. Verbeek, R.D.M., 1886. Krakatau (French ed.). Imprimerie de l'~tat, Batavia, 567 pp. Vincent, P.-M. and Camus, G., 1983. The 1883 Krakatau eruption initiated by a Mount St. Helens t y p e event? A.G.U. Fall meeting, San Francisco. EOS, 64, 45, 872, Abstr. VII A02. Vincent, P.-M., Camus, G., and Larue, 1984. Origine du grand tsunami du Krakatoa (27 Aofit 1883) par immersion d'une coulee de d~bris. Bull. P.I.R.P.S.E.V. (C.N.R.S.I.N.A.G.), 89, 18 pp. Williams, H., 1941. Calderas and their origin. Bull. Dep. Geol. Sci. Univ. Calif., 25: 238-346. Wilson, L., 1980. Relationships between pressure, volatile content and ejecta velocity in three types of volcanic explosion. J. Volcanol. Geotherm. Res., 8: 297--313. Yokoyama, I., 1981. A geophysical interpretation of the 1883 Krakatau eruption. J. Volcanol. Geotherm. Res., 9: 359--378.

REPLY

P.W. FRANCIS Department o f Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6 A A Great Britain

I t is n o t c l e a r w h a t p o i n t V i n c e n t a n d C a m u s a r e a t t e m p t i n g t o m a k e i n their lengthy contribution. My paper was a detailed analysis of the timings o f t h e g r e a t a i r a n d s e a w a v e s o f t h e K r a k a t a u e r u p t i o n s , as r e c o r d e d o n t h e contemporary pressure and tide gauges. Vincent and Camus' comments