霧島火山群，大幡山及び大幡池の噴火史

抄録

Ohatayama and Ohataike are adjacent volcanoes aligned in NE-SW direction at the northeastern portion of Kirishima Volcano Group, Kyushu, Japan. Ohatayama volcano has three small craters around the summit area, and Ohataike has a summit crater lake with a diameter of 400 m. Both volcanoes are composed mainly of tephra fall deposits by plinian or sub-plinian eruptions, with intercalations of pyroclastic flow deposits. The main volcanic edifice of the highest point of Ohatayama volcano was formed at least 22 cal ka BP. Tsukiyama were formed earlier than the highest point, and there is a possibility that they can be separated from the highest point. The triangulation point of Ohatayama volcano started its activities on the highest point somewhere around 17-11 cal ka BP, and forming craters A and B. Ohataike volcano was formed by erupting again around 17-11 cal ka BP, after the main volcanic edifice was formed during around 22-17 cal ka BP on the northeast side of the triangulation point. This was followed by four different modes of eruption at Ohatayama C crater in the following order: a phreatic eruption (Oy-4) in 7.6 cal ka BP, Ohatayama lava emission, magmatic and phreatic eruptions (Oy-3) in 7.1 cal ka BP, magmatic and phreatic eruptions (Oy-2) in 6.8 cal ka BP, and lastly, phreatic eruption (Oy-1) in 6.5 cal ka BP. Although no volcanic activity occurred from Ohataike volcano since 11 cal ka BP, foamy volcanic gas is currently being detected on the surface of the crater lake.