1:45 PM - 2:00 PM
[MIS08-13] The significance of the surface excavation in the Kikai caldera, southern Kyushu
Keywords:Kikai caldera, Koya ignimbrite, low aspect ratio ignimbrite
The 7.3-ka Koya ignimbrite, is widely distributed over the proximal/adjacent islands and the mainland of southern Kyushu. It has been suggested that this ignimbrite shows a rather low aspect ratio with rather thin (<2m) and wide (~100km) deposition (Ui, 1973), which provided a volume estimate for this ignimbrite (<50km3). However, our recent on-land survey revealed that both the ignimbrite thickness and the maximum pumice size tend to decrease with increasing the distance from the caldera. The thickness of ignimbirite is thinner in the area deposited crossing the sea longer than the area near the source area. This may lead to a hypothesis that a significant amount of pyroclastics including large pumices and ashes was lost during traveling over the sea and deposited on the sea floor. Revealing the mass of Koya-ignimbrite-derived pyroclastics on the sea floor should be thus a key to estimating the total volume of this ignimbrite.
Koya ignimbrite contains essential pumices and volcanic glasses, both exhibiting a bimodal compositional distribution, e.g., ~60 and ~70 wt.% SiO2, and showing a chemical trend different from those of post-caldera volcanics such as stratovolcanoes and lava domes of Kikai supervolcano. These chemical characteristics of Koya ignimbrite should provide a better chance for discrimination of this ignimbrite from other ejecta.
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