八丈島東山火山の最近22,000年間の噴火史

抄録

Hachijo island, located 300 km south of Tokyo, consists of two Quaternary volcanoes; Higashiyama in the southeast, and Nishiyama in the northwest. The eruptive history of the Higashiyama volcano was investigated. The volcanic edifice of Higashiyama is composed of Early stratovolcanoes I,II, Middle stratovolcano, and Late volcano. A series of dacitic pumice fall and flow eruptions took place ca. 22,000 yBP, at the end of the Middle stratovolcano, Sueyoshi stage. Evacuation of a large amount of magma in this stage caused formation of Mihara caldera of 1.4 km×0.9 km. The history of the Late volcano can be further subdivided into the Nakanogo and Mitsune stages. In the former stage, basaltic scoria-fallout eruptions frequently took place inside the Mihara caldera and east to south flank of the volcano. While in the latter stage, eruptive vents were restricted on the flank of the volcano. The major erupted magma type changed to differentiated andesite and dacite. The last eruption of Higashiyama which covered the whole volcano was estimated to be 5,000-4,000 yBP. Beside tephras from Higashiyama, at least seven hydromagmatic-eruption deposits from northwest of Higashiyama volcano are detectable. They are most likely the products resulted from the submarine to the earliest subaerial eruptions of the Nishiyama volcano, in 10,000-8,000 yBP.