著者

上野 龍之

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.61, no.3, pp.533-544, 2016-09-30 (Released:2016-11-08)

参考文献数

26

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The Tsumaya pyroclastic flow deposit is one of the main units of the Aira pyroclastic eruption, which produced the Aira caldera in Southern Kyushu, Japan, 30,000 years ago. The Tsumaya deposit overlies the main plinian unit, the Osumi pumice fall deposit, and is covered by the large-volume pyroclastic flow unit, the Ito pyroclastic flow deposit. The Tsumaya deposit consists of massive facies associated with smaller volume of stratified facies. The total eruption mass is 2.8×1013 kg (estimated by the crystal method), of which approximately 48 % was elutriated to a co-ignimbrite ash fall. The upper part of the Osumi pumice fall deposit is intercalated with the stratified facies of the Tsumaya pyroclastic flow deposit, indicating that the Tsumaya eruption began during the final phase of the Osumi eruption. The Tarumizu pyroclastic flow and the Osumi pumice fall were produced from the same vent in the southern part of the caldera. The Tsumaya pyroclastic flow deposit has been considered to be the same stratigraphic unit as the Tarumizu deposit;however, the two deposits have contrasting origins and different contents of lithic fragments, indicating they were erupted from different vents. Lateral variations in the altitudes of the depositional surface of the Tsumaya deposit indicate that the Tsumaya pyroclastic flow was erupted from the northeastern part of the Aira caldera.