著者

福島 大輔 小林 哲夫

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.45, no.4, pp.225-240, 2000

参考文献数

38

被引用文献数

2

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The Tarumizu pyroclastic flow deposit with intercalation of Osumi plinian fall deposit of 25 ka eruption is distributed in Tarumizu area, southeast of Aira caldera. The deposit is composed of many flow units and the source is estimated to be at the same position as the vent of plinian eruption based on grain-size variation and depositional structure. From these facts, the Tarumizu pyroclastic flow is considered to be a type of intra-plinian fiow generated by successive partial collapse of the sustained plinian eruption column. Based on the stratigraphic suecessions, the generation of this flow is estimated to be a continuous process from the early to the terminal stage of the plinian eruption. The pyroclastic flow distribution, which is in the same direction with the dispersal axis of plinian fall deposit, may suggests that the wind influenced the direction of partial column collapse. Two lithofacies were identified in the Tarumizu pyroclastic flow deposit: the massive facies and the stratified facies. The former is a 'normal' pyroclastic flow, while the latter is a pile-up of many thin layers of pyroclastic flow and pumice fall. The massive facies are mainly distributed on the lower plain near the coast and differ from the stratified facies that accumulated at a bit higher level, usually along the foot of the mountain. These facts suggest that the main pyroclastic flow accumulated on the lower level and that the corresponding marginal part of the flow eventually reached the higher level exhibiting characteristic thin stratified structure.