著者

高田 亮 大島 治 荒牧 重雄 小野 晃司 吉田 克史 梶間 和彦

出版者

The Volcanological Society of Japan

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.37, no.5, pp.233-250, 1992-11-15 (Released:2017-03-20)

参考文献数

34

被引用文献数

1

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The subaerial history of Aogashima volcano (about 3 km3 in volume) was studied. The sequence of volcanic activity is summarized as follows: the growth of Kurosaki volcano (0.3 km3 in volume; basalt>andesite) in the northwestern area of Aogashima island; the construction of the main edifice of Main stratocone (basalt≫andesite) in the southeastern area; the fissure eruptions of Aphyric basalts (<0.1km3 in volume) on the northwestern flank; a surge activity (Ojiroike surge deposits) (basalt>ndesite) at about 3,000 y.B.P.; the eruptions of Kintagaura lavas (0.15 km3 in volume) filing the Southeasern basin, and airfalls (Yasundogo airfalls tephras) (0.4 km3 in volume; basalt>andesite) on the east and north flanks 3,000-2,400 y.B.P.; the occurrence of a debris avalanche (Nagashizaka debris avalanche deposits) associated with the formation of the Ikenosawa crater (1.7 km×1.5 km insize); the Tenmei (A.D. 1781-1785) eruption (0.08km3 in volume; andesite). Based on the historical records and the geological data obtained in this study, the sequence of the Tenmei eruption is restored as follows: according to historical records, a small ash eruption occurred in 1781; in 1783, the Tenmei eruption began with an explosive scoria effusion (Tenmei airfall tephras 1) associated with cone building (Maruyama pyroclastic cone); in 1785, ash fall continued intermittently for more than one month (Tenmei airfall tephras 2); finally, the Ikenosawa crater was filled with lava flows (Tenmei lavas 1 and 2). During the development of Aogashima volcano, magma paths were shifted over a distance of about 4 km. Some geological units derived from different magma paths have different, parallel trends of chemical composition in SiO2 vs. oxide diagrams. The chemical composition of magma changed with the shift in magma path. Each trend consists of a combination of plagioclase accumulation and crystallization. The magma-supply rate of Aogashima volcano was fluctuating in time and space with the growth of each geological unit, which may have led to the generation of andesite magma.