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The source mechanism for the tsunamis is subject to debate. It is today widely considered that the emplacement of pyroclastic flows in the sea around Krakatau was intricately linked to the generation of the tsunamis. Pyroclastic flows and surges clearly played a major role in the eruption. Over 1000 lives were lost in the town of Ketimbang and nearby communities on the S coast of Sumatra (NW of Krakatau) as a result of burn injuries. Many more were injured. Accounts by Controller Beyerinck and his wife clearly describe how a hot pyroclastic surge inundated their hut on the slopes of Radja Bassa Volcano 400m above the town of Ketimbang, causing severe burns to those inside. This means that a pyroclastic surge travelled 40km across the sea in a predominantly NW direction whilst maintaining a high enough temperature to cause fatalities. The total extent of the surge was even greater, reaching a run-out distance of at least 80km over the sea. This can be deduced by reports from the ships Loudon, Charles Bal and W.H. Besse which were located 65km NW, 65km NE and 80km NE of Krakatau, respectively. Each of these ships reported being showered with ash and later mud rain by sudden hurricane-force winds. The winds were strong enough to strip the Loudon of its sails and rigging. The surge was however no longer hot enough at these distances to cause fatalities. It is possible that hot parts of the surge also reached the coastline of Java. Both surge deposits and reports of burnt vegetation support this assumption.
The pyroclastic surge and its deposits on the islands NW of Krakatau and the mainland is discussed extensively in Carey et al., 1996 (Bull. Volc. 57, p.493-511). The high energy of the flows was probably associated with the collapse from a high eruption column. In its final stages, the eruption appears to have shifted from one with a convecting eruption column which occasionally collapsed to a dominantly collapsing column. This can be deduced the layers of deposits left by the eruption on the Krakatau islands, which include up to 6m thick pyroclastic flow deposit units from the latter stages of the eruption (Self and Rampino, 1981. Nature 294, p.699-704). The largest explosion possibly propelled more material out of the volcano than any of the preceeding ones, maybe as it occurred at the time of onset of caldera formation. Peak magma discharge is likely to be associated with caldera formation as the collapsing roof of the caldera may squeeze more magma out of the underlying chambers. The pyroclastic flow/surge speeds are estimated at anything from 130-320 km/h near source based on ability to mount topographical barriers, visual observations and arrival times at different points. Most of the trees on the islands of Sebesi and Sebuku were uprooted or snapped by the surge, including many on the 800m high summit. The ability of the surge to travel large distances over water is proposed to be the result of the production of a steam cushion by interaction of hot basal parts of the flow with the sea-water. This cushion would have not only reduced friction but also produced a barrier inhibiting the sedimentation of particles in the flow. The extent of the flows is not exactly known since no observations were reported W of the volcano. A minimum estimate of the area inundated by them is about 4000 square kilometers.
The pyroclastic surges only represent the top less dense sections of pyroclastic flows that were emplaced in the sea during the climactic phase of the eruption. The emplacement of pyroclastic flow units in the sea around Krakatau has been analysed by correlating changes in bathymetry around Krakatau following the eruption with analysis of the composition of the deposits following the extraction of short cores therefrom (Mandeville et al., 1996. Bull. Volcanol. 57, p. 512-519). Interestingly, the pyroclastic flow deposits under water are similar in morphology to those on land. Mandeville had already shown that the submarine deposits were emplaced at temperatures of 475-550´C. This strongly suggests that little interaction between the core of the flow units and sea-water had taken place. Further, it is noted that the highest level of deposition was not immediately surrounding the vents but actually starts at a distance of a couple of kilometers therefrom. In fact, two new islands (named Stears and Calmeyer) were formed by deposits from the eruption several km NNW of Krakatau. These were however rapidly eroded by the sea and had disappeared 3 years later. Interestingly, one study calculated that the source of the tsunamis was near this position of high deposition, rather than at Krakatau itself (Latter 1981. Bull. Volcanologique 44(3), 468-490). It is thus concluded that the flows had sufficient energy to travel for significant distances across the sea surface before deceleration and increasing density at their base, due to gravitational segregation, resulted in a sinking of a high density component of the flow to the sea floor, whilst a lower density surge continued to flow across the sea surface. The sinking of the pyroclastic flows would have rapidly displaced a huge amount of water, and is today considered to be the most likely cause of the main tsunami(s). The PF deposits around Krakatau are extensive and occurred relatively concentrically around Krakatau, which suggests that the main eruption is unlikely to have been directed in a single direction. The total amount of ignimbritic (PF) deposits around Krakatau is estimated at over 20 cubic kilometers, most of which is found in a 15km radius of the vents. Taking this volume of deposits one can also see how much water must have been displaced by pyroclastic flows entering into the sea.
Other theories have been proposed for the generation of the main tsunami. These are summarized in De Lange et al., 2001 (Natural Hazards 24, p.251-266). The suggestion that a large scale phreatomagmatic explosion, or indeed a series of such explosions in relatively short succession, caused the main tsunami has been put forward by Yokoyama, largely based on analysis of pressure wave and tsunami arrival times (1981. J. Volc. Geotherm. Res. 34, p.123-132 and 1987. J. Volc. Geotherm. Res. 34, p.123-132). However, as already noted earlier, studies on the products of the eruption provide little evidence for extensive phreatomagmatic activity. It has also been suggested that the large-scale caldera collapse may have been the source mechanism for the tsunami. However, a subsidence of the sea floor should initially cause a drop in sea level. Nevertheless, the caldera collapse may have indirectly contributed to the tsunami by increasing the rate of magma output (Sigurdsson et al., 1991. Natl. Geogr. Res. Explor. 7, p.310-327) as has already been discussed in the main model. A further theory is that the tsunami was caused by a NW-directed lateral blast (Camus et al., 1992. Geo Journal 28, p.123-128). The Mt St Helens eruption of 1980 showed that a lateral blast and associated debris flow could displace a significant body of water i.e. spirit lake. Further, the changed bathymetry to the NW was considered reminiscent of the hummocky deposits at Mt St Helens. However, as already discussed, significant deposits are not only found to the NW and the deposits do not appear to contain many lithic fragments (i.e. fragments of the edifice) as would be expected in the case of a lateral blast. Further, one may expect a debris flow to be most dense near source, which is not the case at Krakatau. Nevertheless, it is conceivable that a highly energetic debris avalanche may be able to "skim across" the sea surface for some distance. Certainly the eruptions were powerful enough to destroy significant parts of the edifice, since Perboewatan crater which lay N of the 1883 caldera perimeter must have been removed by explosive activity. Nevertheless it appears that there is presently little evidence which coroborates the lateral blast theory.
The eruption subsided rapidly after the main explosive event. Most reports of activity in the following weeks and months could be discounted. After the eruption, a sizeable portion of Rakata appears to have remained at the SE rim of the caldera. Large rockfalls were reported 2 weeks after the eruption, 9 months later and again in 1896 and 1897 (Simkin and Fiske, 1983 "Krakatau 1883: The Volcanic Eruption and its Effects", Smithsonian Inst. Press (1983)) and are likely to have further contributed to the present shape of Rakata island with its NW oriented cliff-face, from which eruptions of Anak Krakatau can today often be heard echoing. The eruption produced copious amounts of pumice which was deposited on the sea surface in large rafts. These were meter-thick in places and hindered shipping for the next weeks. Most of Krakatau had been removed and the remaining islands were covered in a series of 2-6m thick pyroclastic deposit layers, reaching a total thickness of up to 60m (Mandeville et al., 1996. Bull. Volc. Vol.57, p512-529). The total erupted volume is estimated at 10-15 cubic km of dense rock equivalent (DRE). The eruption belched so much ash and sulfur dioxide into the atmosphere that it had a global climatic impact, resulting for example in a small drop in average temperature in the Northern Hemisphere the following year.
The source mechanism for the tsunamis is subject to debate. It is today widely considered that the emplacement of pyroclastic flows in the sea around Krakatau was intricately linked to the generation of the tsunamis. Pyroclastic flows and surges clearly played a major role in the eruption. Over 1000 lives were lost in the town of Ketimbang and nearby communities on the S coast of Sumatra (NW of Krakatau) as a result of burn injuries. Many more were injured. Accounts by Controller Beyerinck and his wife clearly describe how a hot pyroclastic surge inundated their hut on the slopes of Radja Bassa Volcano 400m above the town of Ketimbang, causing severe burns to those inside. This means that a pyroclastic surge travelled 40km across the sea in a predominantly NW direction whilst maintaining a high enough temperature to cause fatalities. The total extent of the surge was even greater, reaching a run-out distance of at least 80km over the sea. This can be deduced by reports from the ships Loudon, Charles Bal and W.H. Besse which were located 65km NW, 65km NE and 80km NE of Krakatau, respectively. Each of these ships reported being showered with ash and later mud rain by sudden hurricane-force winds. The winds were strong enough to strip the Loudon of its sails and rigging. The surge was however no longer hot enough at these distances to cause fatalities. It is possible that hot parts of the surge also reached the coastline of Java. Both surge deposits and reports of burnt vegetation support this assumption.
The pyroclastic surge and its deposits on the islands NW of Krakatau and the mainland is discussed extensively in Carey et al., 1996 (Bull. Volc. 57, p.493-511). The high energy of the flows was probably associated with the collapse from a high eruption column. In its final stages, the eruption appears to have shifted from one with a convecting eruption column which occasionally collapsed to a dominantly collapsing column. This can be deduced the layers of deposits left by the eruption on the Krakatau islands, which include up to 6m thick pyroclastic flow deposit units from the latter stages of the eruption (Self and Rampino, 1981. Nature 294, p.699-704). The largest explosion possibly propelled more material out of the volcano than any of the preceeding ones, maybe as it occurred at the time of onset of caldera formation. Peak magma discharge is likely to be associated with caldera formation as the collapsing roof of the caldera may squeeze more magma out of the underlying chambers. The pyroclastic flow/surge speeds are estimated at anything from 130-320 km/h near source based on ability to mount topographical barriers, visual observations and arrival times at different points. Most of the trees on the islands of Sebesi and Sebuku were uprooted or snapped by the surge, including many on the 800m high summit. The ability of the surge to travel large distances over water is proposed to be the result of the production of a steam cushion by interaction of hot basal parts of the flow with the sea-water. This cushion would have not only reduced friction but also produced a barrier inhibiting the sedimentation of particles in the flow. The extent of the flows is not exactly known since no observations were reported W of the volcano. A minimum estimate of the area inundated by them is about 4000 square kilometers.
The pyroclastic surges only represent the top less dense sections of pyroclastic flows that were emplaced in the sea during the climactic phase of the eruption. The emplacement of pyroclastic flow units in the sea around Krakatau has been analysed by correlating changes in bathymetry around Krakatau following the eruption with analysis of the composition of the deposits following the extraction of short cores therefrom (Mandeville et al., 1996. Bull. Volcanol. 57, p. 512-519). Interestingly, the pyroclastic flow deposits under water are similar in morphology to those on land. Mandeville had already shown that the submarine deposits were emplaced at temperatures of 475-550´C. This strongly suggests that little interaction between the core of the flow units and sea-water had taken place. Further, it is noted that the highest level of deposition was not immediately surrounding the vents but actually starts at a distance of a couple of kilometers therefrom. In fact, two new islands (named Stears and Calmeyer) were formed by deposits from the eruption several km NNW of Krakatau. These were however rapidly eroded by the sea and had disappeared 3 years later. Interestingly, one study calculated that the source of the tsunamis was near this position of high deposition, rather than at Krakatau itself (Latter 1981. Bull. Volcanologique 44(3), 468-490). It is thus concluded that the flows had sufficient energy to travel for significant distances across the sea surface before deceleration and increasing density at their base, due to gravitational segregation, resulted in a sinking of a high density component of the flow to the sea floor, whilst a lower density surge continued to flow across the sea surface. The sinking of the pyroclastic flows would have rapidly displaced a huge amount of water, and is today considered to be the most likely cause of the main tsunami(s). The PF deposits around Krakatau are extensive and occurred relatively concentrically around Krakatau, which suggests that the main eruption is unlikely to have been directed in a single direction. The total amount of ignimbritic (PF) deposits around Krakatau is estimated at over 20 cubic kilometers, most of which is found in a 15km radius of the vents. Taking this volume of deposits one can also see how much water must have been displaced by pyroclastic flows entering into the sea.
Other theories have been proposed for the generation of the main tsunami. These are summarized in De Lange et al., 2001 (Natural Hazards 24, p.251-266). The suggestion that a large scale phreatomagmatic explosion, or indeed a series of such explosions in relatively short succession, caused the main tsunami has been put forward by Yokoyama, largely based on analysis of pressure wave and tsunami arrival times (1981. J. Volc. Geotherm. Res. 34, p.123-132 and 1987. J. Volc. Geotherm. Res. 34, p.123-132). However, as already noted earlier, studies on the products of the eruption provide little evidence for extensive phreatomagmatic activity. It has also been suggested that the large-scale caldera collapse may have been the source mechanism for the tsunami. However, a subsidence of the sea floor should initially cause a drop in sea level. Nevertheless, the caldera collapse may have indirectly contributed to the tsunami by increasing the rate of magma output (Sigurdsson et al., 1991. Natl. Geogr. Res. Explor. 7, p.310-327) as has already been discussed in the main model. A further theory is that the tsunami was caused by a NW-directed lateral blast (Camus et al., 1992. Geo Journal 28, p.123-128). The Mt St Helens eruption of 1980 showed that a lateral blast and associated debris flow could displace a significant body of water i.e. spirit lake. Further, the changed bathymetry to the NW was considered reminiscent of the hummocky deposits at Mt St Helens. However, as already discussed, significant deposits are not only found to the NW and the deposits do not appear to contain many lithic fragments (i.e. fragments of the edifice) as would be expected in the case of a lateral blast. Further, one may expect a debris flow to be most dense near source, which is not the case at Krakatau. Nevertheless, it is conceivable that a highly energetic debris avalanche may be able to "skim across" the sea surface for some distance. Certainly the eruptions were powerful enough to destroy significant parts of the edifice, since Perboewatan crater which lay N of the 1883 caldera perimeter must have been removed by explosive activity. Nevertheless it appears that there is presently little evidence which coroborates the lateral blast theory.
The eruption subsided rapidly after the main explosive event. Most reports of activity in the following weeks and months could be discounted. After the eruption, a sizeable portion of Rakata appears to have remained at the SE rim of the caldera. Large rockfalls were reported 2 weeks after the eruption, 9 months later and again in 1896 and 1897 (Simkin and Fiske, 1983 "Krakatau 1883: The Volcanic Eruption and its Effects", Smithsonian Inst. Press (1983)) and are likely to have further contributed to the present shape of Rakata island with its NW oriented cliff-face, from which eruptions of Anak Krakatau can today often be heard echoing. The eruption produced copious amounts of pumice which was deposited on the sea surface in large rafts. These were meter-thick in places and hindered shipping for the next weeks. Most of Krakatau had been removed and the remaining islands were covered in a series of 2-6m thick pyroclastic deposit layers, reaching a total thickness of up to 60m (Mandeville et al., 1996. Bull. Volc. Vol.57, p512-529). The total erupted volume is estimated at 10-15 cubic km of dense rock equivalent (DRE). The eruption belched so much ash and sulfur dioxide into the atmosphere that it had a global climatic impact, resulting for example in a small drop in average temperature in the Northern Hemisphere the following year.
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