09:15 〜 09:30
[SVC28-20] 鬼界カルデラ・アカホヤ噴火による水中火砕堆積物の堆積様式
キーワード：カルデラ、火砕流、地震探査、堆積様式
Pyroclastic density current entering into water is not well understood on its emplacement mechanism. The emplacement mechanism in marine environment is subjects to location of vent, and temperature, velocity, density and angle of entrance of flow with water (Cas&Wright, 1991). When subaerial pyroclastic flow encounters water, it is subdivided into subaerial and subaqueous parts: subaerial flow interacts with water, making steam explosions, accelerating the flow, and generating co-ignimbrite ash cloud; subaqueous pyroclastics can be mixed with water, making ash plumes and seafloor hugging density current, or displace the water in shallow water. Many experimental studies and numerical modeling have documented for a variety of emplacement mechanisms (e.g., Freundt, 2003). However, a few attempts to explain the subaqueous emplacement with actual seafloor deposits on modern eruptions have been still controversial.
We have conducted high resolution seismic reflection studies around Kikai caldera located to the south of Kyusyu Island, Japan. Our seismic studies, together with chemical analyses of volcanic glasses, revealed that the uppermost unit distributed in an area of >4,500 km2 with a volume of >71 km3 is composed of pyroclastic deposits of 7.3 ka Akahoya eruption. This study is aiming to fill the gap between subaerial pyroclastic-flow deposits on proximal islands and distal islands where open water dominates the area surrounding the caldera in terms of volume and emplacement mechanism in the marine setting.
The distribution of the pyroclastic unit lying on the seafloor around Kikai caldera is confined by the basin topography. Although some areas of the unit were reworked by the mass movements observed as debris flow, slumping, and debris avalanche from the caldera rims to the basin, the east side of caldera has no evidence of such mass transports, possibly capturing the syn-eruptive deposition. This east area exhibits smooth seafloor on almost flat to slightly upward to the shelf of Tanegashima. The unit forms exponential thinning from the caldera wall to the shelf, filling the basin, depressions and fault steps. This deposition contrasts with shallow-marine deposits of Krakatau 1883 eruption that does not form systematic thinning but mottle-like thickening distribution, and bypass the annular moats, interpreted as that initially subaerial flow sank through the water column as increasing the density by gravity segregation (Mandeville et al. 1996; Legros&Druitt, 2000). Subaqueous fall-out deposition was also demonstrated by Freundt (2003); it occurs entirely in the water only on shallow-tank setting. In other deep-tank settings, either hot or cool-ash flow, generate seafloor-hugging currents, forming exponentially thinning extensive deposits. The exponential thinning of pyroclastic flow deposition was introduced by Wilson (1991), and the rate of the decay is decided by volume flux and settlement velocity of particles (Woods et al.,1998). This rate indicated in our pyroclastic unit is comparable to the values of on-land super-eruption ignimbrites that is much bigger than Kikai 7.3 ka eruption. This is possibly explained by not large mass flux as such super-eruptions generate, but slower settlement due to the subaqueous flow. The volume flux caused in the water is expected to be lower than that in the air because a large fraction of pyroclastics is elutriated from the flow into the subaerial flow over water and co-ignimbrite ash. On the other hand, the subaqueous settlement speed is supposed to be slower by reduced gravity than that in hot air entrained in subaerial flow. On a given mass flux, the submarine pyroclastic current can make longer run-out deposition than that made by subaqueous current. From the distribution and morphology of the unit, we can conclude that the pyroclastic density current of 7.3 ka eruption made the submarine deposit predominantly emplaced by seafloor-hugging current characterized by long run-out distance.
We have conducted high resolution seismic reflection studies around Kikai caldera located to the south of Kyusyu Island, Japan. Our seismic studies, together with chemical analyses of volcanic glasses, revealed that the uppermost unit distributed in an area of >4,500 km2 with a volume of >71 km3 is composed of pyroclastic deposits of 7.3 ka Akahoya eruption. This study is aiming to fill the gap between subaerial pyroclastic-flow deposits on proximal islands and distal islands where open water dominates the area surrounding the caldera in terms of volume and emplacement mechanism in the marine setting.
The distribution of the pyroclastic unit lying on the seafloor around Kikai caldera is confined by the basin topography. Although some areas of the unit were reworked by the mass movements observed as debris flow, slumping, and debris avalanche from the caldera rims to the basin, the east side of caldera has no evidence of such mass transports, possibly capturing the syn-eruptive deposition. This east area exhibits smooth seafloor on almost flat to slightly upward to the shelf of Tanegashima. The unit forms exponential thinning from the caldera wall to the shelf, filling the basin, depressions and fault steps. This deposition contrasts with shallow-marine deposits of Krakatau 1883 eruption that does not form systematic thinning but mottle-like thickening distribution, and bypass the annular moats, interpreted as that initially subaerial flow sank through the water column as increasing the density by gravity segregation (Mandeville et al. 1996; Legros&Druitt, 2000). Subaqueous fall-out deposition was also demonstrated by Freundt (2003); it occurs entirely in the water only on shallow-tank setting. In other deep-tank settings, either hot or cool-ash flow, generate seafloor-hugging currents, forming exponentially thinning extensive deposits. The exponential thinning of pyroclastic flow deposition was introduced by Wilson (1991), and the rate of the decay is decided by volume flux and settlement velocity of particles (Woods et al.,1998). This rate indicated in our pyroclastic unit is comparable to the values of on-land super-eruption ignimbrites that is much bigger than Kikai 7.3 ka eruption. This is possibly explained by not large mass flux as such super-eruptions generate, but slower settlement due to the subaqueous flow. The volume flux caused in the water is expected to be lower than that in the air because a large fraction of pyroclastics is elutriated from the flow into the subaerial flow over water and co-ignimbrite ash. On the other hand, the subaqueous settlement speed is supposed to be slower by reduced gravity than that in hot air entrained in subaerial flow. On a given mass flux, the submarine pyroclastic current can make longer run-out deposition than that made by subaqueous current. From the distribution and morphology of the unit, we can conclude that the pyroclastic density current of 7.3 ka eruption made the submarine deposit predominantly emplaced by seafloor-hugging current characterized by long run-out distance.