Kozushima is located on the Zenisu Ridge which is one of across-arc NE-SW oriented echelon ridges in Northern Izu Arc. The island consists of rhyolitic lava domes, thick lava flows and pyroclastic materials. The eruptive history of the island is not well known.
Takodoyama Volcano is mainly composed of lava dome and is arranged on NW-SE line with Osawa Volcano and Matsuyamahana Volcano. Previous studies showed that these three volcanoes have been formed by the same eruption of 22,000-19,000 yrs BP (about 26-22 cal ka BP) and have deposited Chichibuyama pyroclastic surge deposit A (Cb-A) on Membo Volcano in southern Kozushima. However, this latter clarifies that the pyroclastic flow and surge deposits which appear to form the pyroclastic cone of the Takodoyama Volcano are overlain by AT ash (25,000 yrs BP; about 29 cal ka BP) and are correlated with Chichibuyama pyroclastic surge deposit B (Cb-B) on Membo Volcano. Chemical compositions of the pumice lapilli and essential block in the pyroclastic deposits are the same as those of rhyolitic lavas from Takodoyama Volcano. These data show the pyroclastic deposits and Cb-B on Membo Volcano are derived from Takodoyama Volcano, and Takodoyama Volcano was formed before the deposition of AT ash and is older than Osawa Volcano and Matsuyamahana Volcano.
Keywords:Kozushima, AT ash, pyroclastic deposit, rhyolite