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The 1883 Eruption of Krakatau
The appearance of Krakatau prior to the 1883 eruption is well documented. A relatively detailed map was made only 2 weeks prior to the catastrophic eruption by Captain Ferzenaar and correlated to previous sketches (M. Neumann van Padang "A Swedish Sketch of the Island of Krakatau in 1748"). The Island consisted of three adjoining cones, Rakata (800m), Danan (450m) and Perboewatan (120m) which had been constructed by a series of eruptions, the last recorded ones being in 1680-1681 and 1684, on the floor of the caldera formed during the last catastrophic eruption.
The chronology of the 1883 eruption is based on eyewitness accounts from nearby coastal areas or ships, barometric data, tide gauge data and numerous subsequent geological studies. None of the eyewitnesses was close enough to directly observe the sequence of events at Krakatau during the final catastrophic phase of the eruption. Simkin and Fiske provide a detailed account of the climactic eruption ("Krakatau 1883: The Volcanic Eruption and its Effects", Smithsonian Inst. Press (1983)).
On the 20th of May 1883 the awakening of Krakatau were first noted. Activity centered around Perboewatan cone and involved the explosive generation of several km high ash clouds. This activity lasted for about a week after which it died down somewhat. On the 20th of July, violent explosive activity recommenced from new vents in the area between Perboewatan and Danan. The eruption gradually gained in intensity and on 11th of August eruptions were reported from at least 14 distinct vents, of which 3 were distinctly most active. The volcano entered into the penultimate Plinian eruption phase at 12:53 on 26. August. An incredibly powerful explosion marked the onset of this phase and created a cloud containing pumice and ash that reached an estimated height of 25km. The eruption gradually intensified and the ash column is thought to have been about 35km high by nightfall. At the same time, a transition from ash to pumice fall was observed in Telok Betong, 85km N of Krakatau. Nevertheless, little ash and pumice fall on land can be attributed to the eruption, possibly as the prevailing winds would have carried much of this material westwards out to sea. Tsunamis were recorded in Telok Betong harbour and hindered loading and unloading of ships. Localized pyroclastic flow activity due to intermittent partial column collapses can be attributed to this phase based on analysis of deposits on Rakata, Panjang and Sertung. The ultimate and most destructive part of the eruption involved a series of four tremendous explosions on the morning of 27. August. These occurred at approx. 5:30, 6:42, 8:20 and 10:02, with the last explosion having the highest intensity and propelling ash and pumice to an estimated height of over 40km. Whilst these explosions represented peaks in the eruption, the eruption was sustained by near-continuous explosive activity in the intervening periods. A fifth large explosion was reported at 10:52 and part of the wall of Rakata is thought to have subsided at 16:38. The largest explosions could be heard over 4500km away and thus probably represent the loudest sounds in historical times. The trigger mechanism for the eruption and in particular its most explosive phase may have involved the influx of fresh hot basaltic magma into the magma chamber. This could account for the rise in pressure therein. However, whilst small amounts of darker basaltic material can be seen in some of the erupted pumice, the levels observed are well below 5%. On the other hand, the suggestion that the eruptions were phreatomagmatic is not consistent with the fact that the subaerial eruption deposits lack extensive fragmentation and accretionary lapilli which would both be characteristic of phreatomagmatic activity (Self and Rampino, 1981. Nature 294, p.699-704). Further, sulfur and oxygen isotopic studies on tephra from the eruption suggest that degassing was open-system and occurred without significant involvement of seawater (Mandeville et al., 1998. Earth Planetary Sci. Lett. 160, p.709-722)
Each of the massive eruptions was accompanied by large tsunamis. Whilst the final one was by far the largest, most coastal communities had already been severely damaged long before its arrival. The main tsunami reached Sebesi island (15km to the NW) first with an estimated height of 30m. Sebuku island, 30km to the NW was hit shortly afterwards, as was Lagoendi island, 40km to the N. The coastlines of Sumatra and Java adjoining the Sunda Strait at distances of at least 40 and 50km from Krakatau, respectively, were hit by tsunamis having estimated crest heights ranging from 15-35m and reaching run-up heights of about 15m on average. Observational inaccuracies and varying coastal topographies can account for the huge differences in reported wave heights. Analysis of the tsunamis is provided in e.g. Pararas-Carayannis 2003, Intl. J. Tsunami Soc. 21(4), p.191-221. The official death toll was over 36000 (including the whole population of 3000 on Sebesi and all inhabitants on the sparsely populated islands of Sebuku and Lagoendi), although many more may have perished.
The 1883 Eruption of Krakatau
The appearance of Krakatau prior to the 1883 eruption is well documented. A relatively detailed map was made only 2 weeks prior to the catastrophic eruption by Captain Ferzenaar and correlated to previous sketches (M. Neumann van Padang "A Swedish Sketch of the Island of Krakatau in 1748"). The Island consisted of three adjoining cones, Rakata (800m), Danan (450m) and Perboewatan (120m) which had been constructed by a series of eruptions, the last recorded ones being in 1680-1681 and 1684, on the floor of the caldera formed during the last catastrophic eruption.
The chronology of the 1883 eruption is based on eyewitness accounts from nearby coastal areas or ships, barometric data, tide gauge data and numerous subsequent geological studies. None of the eyewitnesses was close enough to directly observe the sequence of events at Krakatau during the final catastrophic phase of the eruption. Simkin and Fiske provide a detailed account of the climactic eruption ("Krakatau 1883: The Volcanic Eruption and its Effects", Smithsonian Inst. Press (1983)).
On the 20th of May 1883 the awakening of Krakatau were first noted. Activity centered around Perboewatan cone and involved the explosive generation of several km high ash clouds. This activity lasted for about a week after which it died down somewhat. On the 20th of July, violent explosive activity recommenced from new vents in the area between Perboewatan and Danan. The eruption gradually gained in intensity and on 11th of August eruptions were reported from at least 14 distinct vents, of which 3 were distinctly most active. The volcano entered into the penultimate Plinian eruption phase at 12:53 on 26. August. An incredibly powerful explosion marked the onset of this phase and created a cloud containing pumice and ash that reached an estimated height of 25km. The eruption gradually intensified and the ash column is thought to have been about 35km high by nightfall. At the same time, a transition from ash to pumice fall was observed in Telok Betong, 85km N of Krakatau. Nevertheless, little ash and pumice fall on land can be attributed to the eruption, possibly as the prevailing winds would have carried much of this material westwards out to sea. Tsunamis were recorded in Telok Betong harbour and hindered loading and unloading of ships. Localized pyroclastic flow activity due to intermittent partial column collapses can be attributed to this phase based on analysis of deposits on Rakata, Panjang and Sertung. The ultimate and most destructive part of the eruption involved a series of four tremendous explosions on the morning of 27. August. These occurred at approx. 5:30, 6:42, 8:20 and 10:02, with the last explosion having the highest intensity and propelling ash and pumice to an estimated height of over 40km. Whilst these explosions represented peaks in the eruption, the eruption was sustained by near-continuous explosive activity in the intervening periods. A fifth large explosion was reported at 10:52 and part of the wall of Rakata is thought to have subsided at 16:38. The largest explosions could be heard over 4500km away and thus probably represent the loudest sounds in historical times. The trigger mechanism for the eruption and in particular its most explosive phase may have involved the influx of fresh hot basaltic magma into the magma chamber. This could account for the rise in pressure therein. However, whilst small amounts of darker basaltic material can be seen in some of the erupted pumice, the levels observed are well below 5%. On the other hand, the suggestion that the eruptions were phreatomagmatic is not consistent with the fact that the subaerial eruption deposits lack extensive fragmentation and accretionary lapilli which would both be characteristic of phreatomagmatic activity (Self and Rampino, 1981. Nature 294, p.699-704). Further, sulfur and oxygen isotopic studies on tephra from the eruption suggest that degassing was open-system and occurred without significant involvement of seawater (Mandeville et al., 1998. Earth Planetary Sci. Lett. 160, p.709-722)
Each of the massive eruptions was accompanied by large tsunamis. Whilst the final one was by far the largest, most coastal communities had already been severely damaged long before its arrival. The main tsunami reached Sebesi island (15km to the NW) first with an estimated height of 30m. Sebuku island, 30km to the NW was hit shortly afterwards, as was Lagoendi island, 40km to the N. The coastlines of Sumatra and Java adjoining the Sunda Strait at distances of at least 40 and 50km from Krakatau, respectively, were hit by tsunamis having estimated crest heights ranging from 15-35m and reaching run-up heights of about 15m on average. Observational inaccuracies and varying coastal topographies can account for the huge differences in reported wave heights. Analysis of the tsunamis is provided in e.g. Pararas-Carayannis 2003, Intl. J. Tsunami Soc. 21(4), p.191-221. The official death toll was over 36000 (including the whole population of 3000 on Sebesi and all inhabitants on the sparsely populated islands of Sebuku and Lagoendi), although many more may have perished.
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