GeoJourna! 28.2 109-121 © 1992 (Oct) by Kluwer Academic Publishers

109

Krakatau Revisited: The Course of Events and Interpretation of the 1883 Eruption

Self, Stephen, Prof. Dr., University of Hawaii at Manoa, Dept. of Geology and Geophysics, School of Ocean and Earth Science and Technology (SOEST), Honolulu, Hawaii 96822, USA

ABSTRACT: Magma chamber over-pressuring by volatile saturation and/or a magma mixing event may have triggered the 1883 eruption of Krakatau. From the beginning of activity on 20 May to the onset of the 22-24 hour-long climactic phase on 26-27 August, Krakatau produced a discontinuous series of vulcanian to sub-plinian eruptions. Based on contemporary descriptions, the intensity of these phases may previously have been underestimated. The most realistic estimate of eruptive volume (magnitude) is about 10 km 3 of dacitic magma. The climax of the eruption began at 1:00 pm on 26 August with a plinian phase which led into a 5- hour-long ignimbrite-producing phase. Caldera collapse most probably occurred near the end of the eruption on 27 August, precluding large scale magma-seawater interaction as a major influence on the eruption column and characteristics of the pyroclastic deposits. Very rapid displacement of the sea by pyroclastic flows remains the best explanation for the series of catastrophic sea waves that devastated the shores of the Sunda Straits, with the last and largest tsunami coinciding with the slumping of half of Rakata cone into the actively forming caldera, perhaps during a period of great pyroclastic flow production. The large audible explosions recorded on 27 August may have been the rapid ejection of large pulses of magma that collapsed to form pyroclastic flows in the ignimbrite-forming phase. Co-ignimbrite ash columns rising in the atmosphere immediately after the generation of each major pyroclastic flow may have contributed to the magnitude of the air waves. A reappraisal of the eruption in the light of this, in conjunction with the pressure (air wave) and tide gauge (tsunami) records from Jakarta, suggests that the relationship between the latter two has been oversimplified in previous studies. Tsunami travel times from Krakatau to Jakarta probably varied more than hitherto thought and there need not be a simple correlation between the times of the explosions and the initiation of the tsunamis. However, tsunamis in the Sunda Straits and vicinity probably were not caused or influenced by coupling with the air waves. Various hypotheses about the cause of the tsunamis and explosions are reviewed and it is concluded that the cause of both is most likely related to the sudden emission of large pulses of magma that led to formation of the Krakatau ignimbrite.

Introduction

Krakatau remains one of the most famous and enigmatic eruptions of the recent historic past. It was the first major erupt ion to occur after the deve lopment of a

worldwide communica t ion network, and, as such, it made a vivid impression over much of the world almost immediately. It also occurred at a t ime of great scientific curiosity and the story of the eruption was told in all manner of writ ten media, f rom the learned Royal Society o f London Report (Symons 1888) to popular children's

Submitted to the Conference Proceedings of the symposium "The Krakatau Islands - a case study of natural change in biodiversity" (27th Pacific Science Congress, Honolulu, 27 May - 2 June 1991)

comics. News of the great eruption provided the world with an awareness of the lethal nature o f volcanic eruptions, as about 35,000 people lost their lives, almost all in the tsunamis. Fur thermore , the atmospheric effects of

the eruption, seen throughout the nor thern hemisphere , were widely publicized.

This paper reviews the eruption sequence and reexamines some of the more problematic and challenging volcanological aspects o f the Krakatau event. Work on the Krakatau eruption completed since the author 's earlier papers on the topic (Self and Rampino 1981; Francis and Self 1983 a) is also discussed. The paper is arranged more or less chronologically, following the eruption sequence. Topics discussed are the nature of the opening phase and cause o f the eruption, events that occurred during the

110 GeoJournal 2 8 . 2 / 1 9 9 2

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paroxysmal phase, the timing of caldera collapse, and the relationship between the generation of pyroclastic flows and the cause of the great explosions and tsunamis, with evaluation of previous hypotheses regarding these phenomena. The book Krakatau 1883 (Simkin and Fiske 1983) contains summaries of most of the available historic and recent literature, including translations of R. D. M. Verbeeck's classic accounts and interpretations of the eruption.

Before the 1883 eruption, Krakatau consisted of three islands and some islets, parts of a prehistoric caldera of unknown age (perhaps about 60,000 years old; Ninkovich 1979) situated in the Sunda Straits (Fig 1). The largest island (Krakatau) possessed a moderate-size composite cone (Rakata) and several other smaller cones and craters apparently aligned on a NNW trending fissure (Fig 2). These volcanic edifices had developed since the previous caldera collapse event and had an eruption in 1680 (Simkin and Fiske 1983 p. 283-91). The 1883 eruption lasted four months altogether but the climactic phases were confined to one 22 to 24-hr period on 26-27 August (Fig 3), during which over 90% of the erupted material was released.

The Opening Phase (20 May to 25 August) and Eruption Trigger

Krakatau became active on 20 May 1883 after a repose period of 203 years. The eruption began with emission of fine-ash and pumice-lapilli in small- to moderate-size vulcanian to sub-plinian activity from a vent on Perbuwatan, one of the smaller cones (Fig 4). In the first week, fall deposits accumulated on the island of Krakatau to a depth of 0.5 to 1 m (Verbeeck 1885), and extensive floating pumice was reported from the Sunda Straits.

Previous studies have often overlooked details of the early phases of the eruption in deference to the more spectacular later events, but these early phases were probably more intense than has generally been credited although the deposits are not well exposed for study. In some accounts, these initial phases, largely due to their fine-grained nature, are described as phreatomagmatic, ie, involving explosive mixing of magma and water. However, fine-grained ash falls are also typical of vulcanian activity involving viscous and often crystal-rich magma, as shown by recent studies of the 1759-71 Jorullo eruption, Mexico (Rowland et al. 1991). Contemporary reports of Krakatau's activity show that both extensive lapilli and ash falls were experienced, that conditions for ships in the Sunda Straits were sometimes very trying due to ash fall which occurred at least up to 375 km away, and that solar haloes and blue moons (indicative of ash and/or volcanogenic aerosols in the upper troposphere and stratosphere) were reported in July from almost 3000 km away. These factors suggest that eruption columns attained about 20 km height, indicating that they were quite powerful. Mass eruption rates would have been 107-108 kg/s if the columns were maintained or individual clouds would have had masses of 101°-10 ]2 kg if they were discrete thermals, estimated from the model of Woods and Kienle (in press). Another eruption could conceivably have caused the atmospheric effects, and other Indonesian volcanoes did erupt between May and July 1883 (Simkin and Fiske 1983 p. 26-31), but available records do not indicate any suitably sized events in this period (Simkin et al. 1981). The atmospheric dispersal of material from the eruptions of E1 Chich6n, 1982, and Pinatubo, 1991, have shown that volcanic aerosol clouds can encircle the global stratosphere in two to three weeks, so widespread dispersal of the early Krakatau ashes and aerosols by July 1883 is feasible. Moreover, the extensive pumice rafts

G e o J o u r n a l 2 8 . 2 / 1 9 9 2 111

occurring in the Sunda Straits and vicinity during this same period could be an indication that pumiceous pyroclastic flows had entered the sea around Krakatau. Pumice rafts were also reported from almost 2000 km away in July, but it is uncertain whether these can be ascribed to Krakatau. In summary, the early intermittent Krakatau activity may have been quite substantial, with moderately high eruption columns, possible pyroclastic flows (for instance the ash that fell on to ships up to 3000 km away in the Indian Ocean), and widespread ash fallout.

Throughout the intermittent activity of June and July 1883, the island of Krakatau remained intact and apparently suffered no major topographic changes until early August, when vents opened on Danan cone. Explosions became more sustained towards late August, and after a new vent opened on 12 August, little is known of the structural state of the island.

Deposits of the early phase contain evidence for a possible triggering mechanism of the event. Verbeeck (1985) and co-workers sampled and described the first pyroclastic deposits from the Perbuwatan vent to accumulate in the period 20-26 May, a mafic, grey, ash fall layer overlying a light-colored pumice fall bed. Both layers

were collected on 27 May 1883 at the foot of Perbuwatan cone and were later chemically analyzed (Tab 1, analyses 1 and 2). These analyses (given by Stehn 1929) unequivocally show that Krakatau erupted two distinct compositions, light-colored dacite and grey high-alumina basalt, in the first few days, similar to the recent 1991 eruption of Mt. Pinatubo, Philippines (Pallister et al. 1992). The early dacite pumice is similar in composition to but slightly less chemically evolved than the main August 1883 dacite (Tab 1, analyses 3-7), but the high-alumina basalt has distinctly lower SiO2 and higher A1203, MgO and CaO. This led Francis and Self (1983 a) and Self and Wohletz (1983) to propose that a magma-mixing event triggered the 1983 eruption by injecting hotter fresh, volatile-rich basalt magma into the subjacent dacitic magma body which promoted convective overturn and volatile concentration towards the top of the chamber (Sparks et al. 1977). Volatile concentration may have then led to over-pressuring which fractured the cap rocks over the magma chamber, allowing small volumes of magma to erupt, initially the dacite at the top of the chamber, followed by a small burst of the basalt. This hypothesis, supported by the petrologic work of Camus et al. (1987), deserves further investigation.

Fig 2 Simplified maps of Krakatau volcano and bathymetry of the local area in Sunda Straits (a) before and (b) after the 1883 eruption. From (a) Dutch surveys conducted in the Straits (Verbeek 1885; Simkin and Fiske 1983); (b) Sigurdsson et al. (1991). Submarine contours in meters

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GeoJournal 28.2/1992 113

Fig 4 Vigorous vulcanian eruption column from Perbuwatan vent on 27 May 1883 (Fig 2 from Krakatau 1883, by T. Simkin and R. S. Fiske, Smithsonian Instituion, 1983). This activity is similar to, but much more intense than, the vulcanian activity witnessed on Anak Krakatau in 1979 by the author. The phase of the eruption photographed probably deposited the grey basaltic ash layer that was collected and analyzed (see text).

114 GeoJournal 28.2/1992

Tab 1 Selected chemical analy- ses of products of the 1883 Krakatau eruption

Also suggestive of the contrasting magma compositions are the grey and white banded pumices which occur in the deposits of the main phase of the 1883 eruption (Fig 5). Microprobe analyses of the glasses in these bands show that both consist of very similar evolved dacite (S. Self, unpublished data), but the dark bands contain less plagioglase and more pyroxene phenocrysts than the light bands, perhaps caused by mechanical disruption of the mafic magma during mixing in the dacite magma body or conduit (Blake and Campbell 1986).

Other possible triggering mechanisms for the climactic phase of the Krakatau eruption include increasing volatile concentration and over-saturation during crystallization of the dacite magma body (Williams 1941; Blake 1984), and hydrofracturing due to ingress of seawater. Although a popular early theory for the Krakatau eruption, a lack of evidence for large scale magma-water interaction in the Krakatau pyroclastic products, such as very fine grained, accretionarys lapilli-bearing fall and flow deposits (Self 1983), right up to the time of the climactic phases may indicate that hydrofracturing was not an important eruption-triggering mechanism at Krakatau.

The Climax I: Main Plinian Phase (10 am 26 August to 5 am 27 August)

After activity of gradually increasing intensity during August, Krakatau apparently underwent a short repose from 19-22 August. During this same period, unusual

atmospheric colors at low sun angles were noted in South Africa. Explosive activity then intensified and ash fall began in earnest around the Sunda Straits. Krakatau entered the first stage of its climactic activity at about 10:00 am on 26 August (local time is used throughout) extending to 5:00 am on 27 August (Fig 3). The stratigraphy of the deposits of this phase is displayed by the sequences found on Rakata, Sertung, and Panjang (Fig 6). By 2:00 pm on 26 August a major plinian eruption was underway from a vent in the Danan-Perbuwatan area, with eruption column heights of up to 26 km measured from the ship Medea, some 120 km to the NE. This phase produced a 2 to 3-m- thick series of bedded plinian pumice fall and pyroclastic flow deposits on the island of Krakatau; some flows were expanded and produced cross-bedded deposits (Fig 7a), while most were not. These pyroclastic flows (also called ash flows or nudes ardentes) must have been generated by partial collapse of a fluctuating but maintained plinian eruption column, in contrast to the apparently much larger pyroclastic flows of the climactic phase. Pumice composition is dacitic (Tab 1), as is the main 1983 ignimbrite, and occasional juvenile (magmatic) vitrophyre clasts, much more common in the ignimbrite, have the same composition (Tab 1, analyses 6 and 7).

The volume of fall deposits from this phase is not well determined because most of the material was deposited at sea and most of the distal fallout was probably co- ignimbrite ash of the next phase in the eruption. A first- order volume estimate for the main plinian phase can be obtained by combining available field data from the islands

GeoJournal 28.2/1992 115

Fig 5 Banded 1883 pumice, the result of mixing and co-mingling in the volcanic conduit. The sample comes from Krakatau ignimbrite on Rakata island. Scale bar is 5 cm long

with the 1 m of pyroclastics (including hot pumice) that Units

was reported to fall on the ship Berbice in about 8 hours up to 2:00 on 27 August, 96 km WSW of Krakatau (probably downwind, as with the overall dispersal pattern documented by Verbeeck 1888). Assuming an exponential decrease in thickness and using the method of Fierstein = and Nathanson (1992) yields a bulk volume of about 12 km 3. This compares with the 8.5 km3estimated by Self and g- Rampino (1981) based on distal isopachs derived from Verbeek's data, but Verbeek's information was limited to =E areas where ships made reports of ash fall. The larger

"6 figure, recalculated to 4-5 km 3 of dense magma, may be more realistic.

It is noteworthy that the first widespread tsunami ~- occurred during this phase o f the eruption, between 5:00- 5:30 pm on 26 August, at about the same time as the first widely audible explosions, heard "all over Java" (Verbeek 1888). This was followed by smaller tsunamis through the night of the 26 August. These tsunamis occurred well _ before the great tsunamis of 27 August. The occurrence of ~=° some, albeit smaller, sea waves during the plinian phase, = o before major collapse of the edifice could have occurred, ~ O-E suggests that at least some of the Krakatau tsunamis were ~5 related to pyroclastic flows entering the sea, as discussed below. ~re-~sa3

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The Climax II: Paroxysmal Ignimbrite-Forming Phase (5 am to 11 am 27 August)

As best as can be determined, the main ignimbrite- forming phase began in the early morning of 27 August. Due perhaps to a sudden vent-widening episode, supported by the presence of a lithic-rich layer in the early

Fig 6 Composite stratigraphic section of the 1883 products compiled from several locations on Panjang, Sertung, and Rakata islands (modified from Self and Rampino 1981)

major ignimbrite units, most of the material leaving the vent collapsed to form pyroclastic flows. This was also when the first of the series of very large explosions was heard, and there was also perhaps an escalation in eruption column height as large co-ignimbrite ash columns (Woods

116 GeoJourna! 28.2/1992

Fig 7 (a) Bedded pumice falls and thin cross-stratified pyroclastic flow deposits of the climactic plinian phase (I) on Sertung island. Bars on scale are 10 cm. (b) Detail of 1883 ignimbrite on Rakata island showing vitrophyric bomb with vesiculated centre and several other dense vitrophyre clasts in a matrix of pumice and ash. Bars on scale are 1 cm.

and Wohletz 1991) would have risen above the pyroclastic flows as they crossed the sea. An increase in column height from the plinian to the ignimbrite-forming phase would imply a considerable increase in mass eruption rate, at least during the times ofpyroclastic flow production. More than half the total erupted volume was emitted in the next 7 to 8 hours as large pyroclastic flows, probably in several large volume pulses. These formed 4 to 7 main depositional (or flow) units of non-welded ignimbrite on land (Fig 6), and deposits that extend about 15 km from Krakatau, mainly to the N and W (Self and Rampino 1981). The ignimbrite on the islands reaches a maximum thickness of about 60-70 m (older reports give up to 100 m). To the W of the island of Sertung, where it apparently filled a pre-existing submarine basin, the ignimbrite is about 80 m thick (Sigurdsson et al. 1991). To the N, the ignimbrite was deposited on to a shallow platform, forming shoals and two islands (Calmeyer and Steers) that persisted above sea level for a few months.

The 1883 ignimbrite on the Krakatau islands is pumice and ash-rich, and the ash has a shard structure (eg Heiken and Wohletz 1985, p. 156-161) and grain size characteristics of normal ignimbrite, as compared to phreatomagmatic ignimbrite (Self 1983). Vitrophyre clasts are common, representing partially to non-vesiculated dacite magma (Fig 7 b). G. P. L. Walker (personal communication; 1981) noted that large pumice clasts in the ignimbrite exposed along the coast of the islands have a pink color due to oxidation and have crudely prismatic joints indicating that they were hot when emplaced but that the enclosing matrix was cool. Cooling could have taken place by emplacement into the sea. Sigurdsson et al. (1991) examined the submarine ignimbrite and sampled it using a short hand-held coring device. The characteristics of the submerged ignimbrite are apparently similar to those of the ignimbrite on the islands.

An important incident occurred during the ignimbrite- forming phase, but it is difficult to place it accurately in the eruption chronology. One or more of the pyroclastic flows travelled across 40 km of open water and reached the S shore of Sumatra, killing 2,000 people in the region around Lampong Bay and Raja Basa volcano (Fig 1), and the flow deposits have been located there (T. Simkin and R. S. Fiske, personal communication). On the way, this pyroclastic flow would have crossed two small islands in the straits (Sebesi and Sebuku); about 1 m of normal ignimbrite deposit has been found on Sebesi (G. P. L. Walker, personal communication 1981; Sigurdsson et al. 1991). This incident demonstrates that pyroclastic flows were capable of travelling across the sea surface, as known from other volcanoes such as Koya, Japan (Walker 1983), as well as being deposited subaqueously around Krakatau (Fig 8).

A moat or an annular zone of little or no ignimbrite deposition, especially prominent on the N side between Steers and Calmeyer and Krakatau, surrounds the archipelago (Francis and Self 1983b; Sigurdsson et al. 1991). One possible explanation of this moat is that the pyroclastic flows could have travelled across the sea on a raft of floating pumice, depositing only after the edge was reached. Sigurdsson et al. (1991) suggested that the pyroclastic flows were for a time buoyant and became submerged only as basal particle concentrations and density increased at some distance from Krakatau, accounting for the moat. This idea is appealing because in the deflation zone (Druitt and Sparks 1984; see Fig 8), the pyroclastic flows would be turbulent and have particle concentrations too low to deposit significant amounts of ignimbrite; however, as they flowed away, particle concentration and flow density would increase and ignimbrite would be deposited. There was, of course, much

6eodournal 28.2/1992 117

Fig 8 Representation of pyroclastic tlows from Krakatau undergoing density segregation and density filtering by the sea. Low concentration flows Co-ignimbrite ash cl travel over the sea surface (modified ~ ~ " . - ~ from Francis and Self, 1983 a). ~ i i i ! ~ ~

c ~ . ( :/::.:~ e r u p r ~ ~ ~

. . f ~ " l Tu br ulent, less dense

deposition on the islands, so only some of the Krakatau pyroclastic flows would have behaved in this manner; there must have been deposition from high concentration pyroclastic flows even close to vent. About 14-22 km 3 bulk volume of ignimbrite was produced, most of it now residing beneath the Sunda Straits (Self and Rampino 1981, Fig 6).

The precise timing of caldera collapse remains uncertain, and it may have taken place in increments over the period from 5:00 to 10:00 am. Self and Rampino (1981) thought that a lithic-rich deposit occurring near the top of the ignimbrite sequence marked the onset of caldera collapse. A coarse, late lithic-rich ignimbrite unit on Panjang (Sigurdsson et al. 1991) could have formed at the time of maximum discharge and may be related to caldera collapse. Less and less constraint exists on the timing and sequence of events as activity escalated in the last hours, but caldera collapse most probably occurred late in the eruption between 8 and 10 am, presumably after most of the ignimbrite had been erupted. Significant explosive activity occurred until noon on the 27th and some of this may have been within the newly formed caldera, including late pyroclastic flow generation, but little contemporary or present geological information exists to provide details of these last hours. This essentially ended the eruptive activity, except for extensive mud rains, most probably caused by secondary explosions in the ignimbrite deposited in the Sunda Straits.

The Great Explosions of 27 August and Generation of Tsunamis

The Great Explosions

Previously, many different origins have been proposed for the series of eleven major explosions that took place

between 5:00 am and 1:15 am on 27 August, eg, ingress of water into the evacuated magma chamber, explosions due to the eruption of pyroclastic flows, or blowing apart of the volcano, as previously discussed by Francis and Self, 1983 a; Camus and Vincent (1987) more recently attributed the explosions to magma-seawater interaction. However, the generation of the sound waves has received scant attention but it has been related to the origin of the devastating tsunamis of 27 August. Evidence for violent phreatomagmatic activity in the early part of the ignimbrite-forming phase (Camus and Vincent 1983) comes from mud rains which fell on ships in the Sunda Straits at about 5:30 am on 27 August, but these could have been caused by secondary phreatic (littoral) explosions in the fresh ignimbrite. Another possible indication of water- magma interaction is the presence of poorly to non- vesiculated blocks of dacite vitrophyre, common in the ignimbrite but more scarce in the fall deposits below. Arrested vesiculation could have been caused by seawater quenching the magma in the vent but, alternatively, partly degassed batches of magma may have existed near the upper wall and ceiling of the chamber before its failure. All gradations from fully vesiculated to non-vesicular juvenile dacite vitrophyric clasts occur, and clasts with more vesiculated interiors are common (Fig 7 b), indicating that there was sufficient magmatic gas to cause exsolution. Thus, magma was in variable states of vesiculation upon ejection. In my opinion, the evidence available to date suggests that little magma-water interaction occurred at the vent while the main ignimbrite was being produced and the origin of the series of great explosions on 27 August may not be due to hydrovolcanic activity.

Identification of likely mechanisms for the explosions is particularly important, however, as the origin of the tsunamis in part depends on this. Verbeek (1885) posed the all important question "... did these two events [generation of explosions and tsunamis] occur entirely

118 GeoJournal 28.2/1992

together or not" (quoted in Simkin and Fiske 1983). The record of the Krakatau explosions comes from the gasometer at Jakarta (then Batavia) some 165 km away, which acted as a barometer as the large pressure waves moved through the atmosphere.

Explosions and other audible phenomena occur due to the passage of atmospheric shock waves at supersonic velocities, and infrasonic sound waves, generated by the forcible injection into the atmosphere of material which undergoes a sudden increase in volume. In a study of acoustic noise from volcanoes, Woulff and McGetchin (1978) classify this type of noise as a monopole radiator. The Krakatau sound waves were the greatest ever recorded in terms of audible range and estimated energy of the air waves (about 1024 ergs; Gorshkov 1959; Harkrider and Press 1967). The accompanying air waves that reverberated around the atmosphere had very large amplitudes (Strachey 1888), exceeding those from the most explosive nuclear bomb tests (Reed 1987). In eruptions, the instantaneous pressure change can produce a shock wave in the immediate vicinity of the volcano, such as when pulses of material are suddenly injected into the air above the vent at supersonic velocity (eg, Nairn 1976). At Mount St. Helens in 1980, the opening blast event and ensuing pyroclastic flow or surge caused pressure waves that were recorded on barographs but not generally heard within about 100 km of the volcano, and which were reported as explosions and boomings at greater distances (Banister 1984). Although detailed timing cannot be resolved from the available records, the blast and related phenomena (see below) probably caused an infrasonic wave with a slow compression that travelled to the upper stratosphere where at the low ambient pressures the wave formed a shock front which was then refracted back to the surface and was heard as explosions at considerable distances from the volcano (Kieffer and Sturtevant 1984; Reed 1987).

Other cases of distant infrasonic (acoustic-gravity) wave detection of volcanic eruptions occurred as a result of E1 Chich6n (1982), where several very violent pyroclastic flows were emplaced, and at Gunung Agung, Indonesia, in 1963, where vulcanian to sub-plinian and pyroclastic flow- forming activity occurred (Mauk 1983; Goerke et al. 1965). In the above studies of volcanogenic sound waves, the exact initiation mechanism of the waves is not discussed. The following argument is speculative and requires proper substantiation, but I here propose a mechanism that appears to explain several disparate factors about the Krakatau explosions.

A New Proposed Mechanism

As noted above, major pyroclastic eruptions often proceed in comparative silence in the near field, as witnessed recently at Mount Pinatubo, 1991 (R. Hoblitt, personal communication), but explosions and rumblings

are audible in the far field, as was also the case at Krakatau. At Mount St. Helens, the most widely heard explosion was generated between about 8:45 and 9:00 am PST on May 18, 1980, before the main plinian eruption, but at the time when the giant umbrella cloud was generated above the blast flow (Sparks et al. 1986). Further, the lack of audible sound near erupting volcanoes can be explained by both the slow nature of sound wave propagation in the unusually dense, ash-laden atmosphere (Kieffer 1981), although this may be a local effect, and the manner in which infrasonic wave fronts become steeper and are refracted offlayers in the upper atmosphere (Reed 1987). A possibility is that the sound waves at Krakatau may have originated as very large air compression waves at subsonic speeds, and since they developed into long-travelled pressure waves and shocks with very large amplitudes, they must have been caused by very large displacements of air. Another important fact is that the widely audible sound waves occurred mainly during the ignimbrite-producing phase of the eruption. Apparently, the more maintained plinian eruption column did not produce strong shock waves but the ejection of large pulses of material that collapsed to form pyroclastic flows and led to discrete co- ignimbrite clouds, as recently modelled by Woods and Caulfield (1992), was associated with sound and shock wave phenomena. The rapid rise of a co-blast (-ignimbrite) ash column in the atmosphere at Mount St. Helens, and of the much larger co-ignimbrite ash clouds at Krakatau, may have contributed to the generation of air waves and audible explosions.

Other facts of possible signifcance to the origin of the Krakatau explosions include: (1) The emplacement of pyroclastic flows and the rise of the co-ignimbrite eruption column takes on the order of several minutes, therefore the generation of sound waves may occur at the initial ejection of the pulse and at any time during the period of flow. (2) The amplitude and wavelength of the air waves and the intensity of the shock waves that develop from them will be complexly linked to the amount of material released and the size of the eruption column. In this regard, the nature of the compression wave recorded near Mount St. Helens indicates that some phenomenon other than a single point explosion was involved, that the signal was caused by a large displacement of air, and that there was an associated inrushing wind (Banister 1984; Reed 1987).

At Krakatau, the rapid ejection of pulses of magma as dense erupted mixtures that collapsed to form pyroclastic flows and the ensuing rise and expansion of co-ignimbrite ash clouds can best account for the long series of explosions on the 26 and 27 August, with the great explosions occurring at the times of major ignimbrite production, especially the 9:30 am and 10:00 am mega- explosions, which represent very large displacements and pressure waves. That these sound waves were the biggest in terms of pressure wave amplitude may indicate that these pulses emitted the greatest volume of magma and, incidentally, may help to pinpoint the time of major caldera

GeoJournal 28.2/1992 119

collapse which would follow the largest eviscerations of the magma chamber. At 10:45 and 11:15 am, two additional large explosions were probably caused by late ignimbrite- generating pulses, perhaps accompanied by further collapse of the island.

Generation of Tsunamis

More has been written about the cause of the devastating tsunamis of 27 August than any other aspect of the Krakatau eruption. Previous studies make a firm case for relating the origin of the tsunamis to the great explosions and, as pointed out above, tsunamis and explosions occurred throughout the eruption. If this relationship existed, and the cause of the air waves (explosions) is related to ignimbrite-forming pulses in the eruption, then these should also cause the tsunamis.

Francis (1985) re-evaluated the record of anomalous air and sea waves at Jakarta and critically reviewed previous ideas on the origin of the tsunamis. His study showed that some previous interpretations do not stand up to detailed analysis. The main points and conclusions of Francis (1985), with additions based on the author's recent work, are as follows.

(1) The 150 to 151 minute travel time for sea- propagated tsunamis from Krakatau to Jakarta's harbor (Verbeek 1885; Yokoyama 1981, 1987) cannot be substantiated due to considerable uncertainties in the values of some parameters in the Airy equation, used to estimate wave velocity (v):

v = [g(h+a)(2h+a)/2h] 1/2

where g = acceleration due to gravity, a=wave amplitude, and h = water depth. The water depths near Krakatau were changing during the eruption by amounts similar to the estimated amplitude of the largest waves (up to 30 m; Yokoyama 1981), so h cannot be calculated accurately, and estimates of a are uncertain. Thus, calculated travel times are not accurate enough to rigorously determine the exact synchronicity of tsunamis and air wave origins, nor to show that some of the tsunamis began at points nearer to Jakarta than Krakatau (Latter 1981; see discussion in Francis' paper).

(2) Many of the sea waves recorded at Jakarta, and by inference along the Sunda Straits, were air-sea coupled waves, not tsunamis deriving from the vicinity of Krakatau itself. The coupling mechanism refers to the proposal of Harkrider and Press (1967) that the distant, small amplitude anomalous sea waves were produced by coupling of the air waves travelling around the earth from Krakatau with the sea surface, with resulting amplification of sea-surface movement. However, it is doubtful whether such a mechanism could operate locally along the Sunda Straits or the coast of Java as distances involved and the estimated speed of the air/sea waves, about 30 m/s in the

Straits, would be too slow to produce resonant coupling (Press and Harkrider 1967). Therefore the sea wave record from Jakarta's harbor probably did not result from a complex interplay of air-sea coupled waves but represents actual tsunamis generated by pyroclastic flows around Krakatau.

(3) Yokoyama's (1981, 1987) suggestion that shallow submarine explosions in the new Krakatau caldera caused all the tsunamis is untenable. Although it is feasible to generate water domes by such explosions, leading to propagated sea waves, this could only account for the last tsunamis after caldera collapse, which most workers from Verbeek onwards place at about 10:00 am on 27 August.

(4) The largest tsunamis all originated at Krakatau and the one great sea wave generated at about 10:00 am may have been propagated before the largest airwave occured. This could be the case if the tsunami was produced by a very large volume pyroclastic flow impacting the sea, whereas the air wave was produced by the expanding co- ignimbrite cloud minutes later.

(5) Matching the biggest sea wave at Jakarta with the biggest explosion suggests that some large air waves (explosions) were generated without tsunamis occurring, eg, the one at about 08:20 am. Again, this can be ex- plained if the sound waves were due to an eruptive pulse that formed a pyroclastic flow but did not, for some reason, displace the sea surface sufficiently to cause a tsunami.

A simple, common origin for the large explosions and the tsunamis is therefore unlikely (Francis 1985). The first tsunamis, which occurred on 26 August may have been produced by a pyroclastic flow generated during the plinian phase. A pyroclastic flow will induce a wave even if it does not sink. Thus, flows that crossed the Sunda Straits on the sea surface may well have caused a wave by displacing water, as could any that entered the sea and deposited material on the sea floor. Moreover, sea-borne pyroclastic flows may well have generated sea waves up to 10-15 km away from the Krakatau vents, but this cannot by substantiated by the available chronology. A similar suggestion was made by Latter (1981), but such details will be difficult to show satisfactorily. Other facts consistent with a pyroclastic flow origin for the tsunamis are that the long (60 m) wavelengths of the far-travelled sea waves suggest a very large volume displacement at the origin. Voluminous pyroclastic flows entering the sea over a region of some 15-20 km diameter could account for this. Further, the first onset of the waves at many places along the shores of straits was positive, (ie, onshore), indicating an origin by displacement of water by addition of material.

Another way to explain the occurrence of the largest wave at 10:00 am is by the collapse of Rakata cone into the newly formed and immediately flooded caldera, as concluded by Francis (1985) and others dating back to Verbeeck (1884). Alternatively, this mechanism could have reinforced a wave caused by a pyroclastic flow. Further

120 GeoJournal 28.2/1992

collapse of the volcanic superstructure may have played a part in generating those waves afterwards, as discussed in the previous section. As the sea to the N of Krakatau was relatively full of ignimbrite by that stage of the eruption, propagation of the sea waves should have been damped in that direction, and, in fact, the last waves are described as having a greater effect on the S (Java) coast of the Sunda Straits (Simkin and Fiske 1983). However, this could also be because there was little left to damage, or few persons left living, along the N shore of the Straits to report destruction by the late waves.

Comments on some other Conclusions about the Eruption

Several hypotheses concerning various aspects of the Krakatau eruption, other than those discussed above that I believe cannot be substantiated, have been proposed in the past and have already been countered by Self and Rampino (1983), Francis (1985), and Sigurdsson et al. (1991). These are briefly mentioned here for the sake of completeness.

Yokoyama (1981) suggested that the caldera was formed by the ejection of lithic material (the N part of Krakatau island) into the Sunda Straits, and that this also produced tsunamis by displacing water. Most others who have worked at Krakatau have mentioned that the ignimbrites are comparatively lithic-poor (except in restricted lithic- enriched lag breccia horizons), and most now agree that the caldera formed largely by collapse of the N part of Krakatau into the evacuated magma body near the end of the eruption.

Camus and Vincent (1983; see also Vincent and Camus 1986, and this volume) proposed a "Mount St. Helens" scenario for the eruption, in which most of the events occurred at or about 10:00 am, with the occurrence of a large lateral blast, collapse of Krakatau/Rakata to form a debris avalanche to the N contributing to Steers and Calmeyer islands, and eruption of "wet" ignimbrite, all causing tsunami generation. This scenario can be criticized on several grounds (Francis 1985) and, in general, does not fit with the accepted chronology of events. Nor does it explain phenomena such as the early (26 August) pyroclastic flows and tsunamis.

A new and critical observation that contradicts both the Yokoyama (1981, 1987) and Camus and Vincent (1983) proposals is the finding by Sigurdsson et al. (1991) that the submarine desposits in the Sunda Straits are pumiceous Krakatau ignimbrite, and that these are thickest to the west, rather than the north, of the caldera.

Conclusions

The 1883 eruption of Krakatau may have been triggered by the injection of basaltic magma into a dacite magma

body under Krakatau. The event went through several stages, including a lengthy, intermittent vulcanian to subplinian phase from May to August 25, a 13 to 14 hour plinian phase on 26-27 August, and an 8 hour-long ignimbrite producing phase on August 27. The total volume of ejecta is difficult to estimate, but at least 16 km 3 (bulk volume) of ignimbrite was produced; if the slim evidence for fallout thickness at a considerable distance can be substantiated, about 12 km 3 bulk volume of pumice and ash fall may have accumulated in the plinian phase. The total bulk volume of erupted material was thus over 20 km 3, as suggested by Self and Rampino (1981), making this eruption about as voluminous as the biggest twentieth century eruption so far (Katmai-Novarupta, 1912, 28-30 km 3 bulk volume; Fierstein and Hildreth 1992), but not as big as Tambora 1815 (over 100 km 3 bulk volume; Self et al. 1984). Another way to constrain the total volume erupted at Krakatau is to consider it equal to the volume of the 1883 caldera, about 9.0 km 3 (Sigurdsson et al. 1991). This is the dense rock equivalent of about 18-25 km 3 bulk volume of pyroclastic deposits.

The idea that the great explosions were rapidly ejected pulses of material in the ignimbrite-forming stage, perhaps augmented by air waves due to rapidly rising and laterally spreading co-ignimbrite ash clouds in the atmosphere is tentative, but does reconcile many details of the timing of explosions and tsunami generation. A simple link between tsunami generation and the explosions cannot be substantiated from the air-shock wave record, as previously suggested by Francis (1985). These records have been over-interpreted and many complex phenomena were operating at about the same time. Generation of some tsunamis and sea waves in the Sunda Straits by air wave-sea surface coupling does not appear to be viable due to the inferred low wave velocity. Pyroclastic flows entering and rapidly displacing sea water is the most logical cause of the majority of the devastating tsunamis. The final great waves in the period from 9:20 to 11:15 am on 27 August may have been accentuated by the slumping of Krakatau island, accompanying formation of the caldera, as originally suggested by Verbeek (1884, 1885). In fact, as more recent interpretations have come and gone, many of Verbeek's initial ideas have survived the test of time. Despite the fact that Verbeek's remarkable treatise on Krakatau was his only volcanological work, it must be considered one of the most significant contributions to volcanology.

Acknowledgements

I thank Peter W. Francis for discussions on the Krakatau eruption and a review of the manuscript, Susan W. Kieffer for discussions on volcanic explosions, and Michael R. Rampino, George R L. Walker, Andrew W. Woods, Martha L. Sykes and Ian W. B. Thornton for reviews and comments. This is SOEST Contribution No. 2943.

GeoJournal 28.2/1992 121

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