神津島南部の火砕堆積物

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Kozushima consists of rhyolitic monogenetic volcanoes (lava domes and thick lava flows), and is located on Zenisu Ridge, northern part of Izu-Bonin Arc. Activities of the momogenetic volcanoes were accompanied with pyroclastic flows and surges. But their succession and stratigraphy are not well known. We tried to discuss them based on observation in construction field of Kozushima Airport and its neighborhood in southern part of the island. Three pyroclastic surge deposits were confirmed on thick lava flow which was fomed tens of thousands years ago. They are, Chichibuyama pyroclastic-surge deposit-B which was older than 22,000 y.B.P., Chichibuyama pyroclaslic-surge deposit-A which erupted 21,000-19,000y.B.P. and Tenjosan pyroclastic-surge deposit which was fomed 838 A.D. in ascending order. Two regional ash-fall deposits which originated from gigantic eruptions in Southern Kyushu and several ash-fall deposits from another islands were interbeded with them. Thus, only three pyroclastic surge deposits have come to deposit in southern part of Kozushima while tens of thousands years. It does not mean that only three eruptions occurred in Kozushima the while, but suggests that most eruptions could not bring pyroclasic materials on the thick lava flow whose height is 100-300 m in southern part of Kozushima except above mentioned three eruptions. Most pyroclastic flows and surges were strongly controled by topography in Kozushima, but some were not. That might be caused by the difference of eruption energy or eruption types.
特定非営利活動法人日本火山学会の論文
1992-06-01

神津島南部の火砕堆積物

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