著者

宮縁 育夫 渡辺 一徳

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.1, pp.25-32, 2000-03-10

被引用文献数

3

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Jigoku spa is located west of Ikenokubo basin of Yomineyama volcano, southwestern part of the central cones of Aso volcano. In this basin, "Ikenokubo tuff ring" and two smaller maars were developed. Two layers of ejecta, about 10^5 m^3 in bulk volume, formed by phreatic explosions were found in this area. They are very poorly sorted and mainly composed of altered fine-grained ash including lithic fragments. Kikai Akahoya ash erupted about 6,300 years BP (^<14>C) is intercalated between two layers of phreatic ejecta in thick ash layers from Nakadake, the only active central cone. Based on ^<14>C ages of buried Andisols and the succession of the deposits, the eruption ages of the two ejecta are estimated at about 10,000 years BP for older deposit and at about 4,500 years BP for younger one. Distributions of thickness and maximum grain size of the lithic fragments suggest that the both of two phreatic ejecta were exploded not from "Ikenokubo tuff ring" and two maars but from the Jigoku explosion crater in the western area. Ejecta correlative to the "Ikenokubo tuff ring" Iies under the older of the phreatic ejecta layers. Consequently, the eruption age of "Ikenokubo tuff ring" is probably older than 10,000 years BP. Frequency of phreatic explosions, Iarger than 10^5 m^3 in volume, is considered to be once in about 5,000 years. This fact is noteworthy for volcanic hazards assessment in the surrounding area of Jigoku spa and adjacent Tarutama spa.