著者

柳澤 妙佳 小畑 大樹 杉田 匠平 藤縄 明彦

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.66, no.2, pp.83-100, 2021-06-30 (Released:2021-07-27)

参考文献数

17

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Akita-Komagatake volcano, located at 30 km west of the volcanic front in the Northeast Japan arc, has been active in the recent 100,000 years with caldera-forming eruptions occurred around 13,000 years ago. The formation history of the pre-caldera stratocone has not been fully established, though it is inevitable to grasp the whole development scenario of the volcano, and also to mitigate potential volcanic hazards in the future. We reconstruct the stratigraphy of the lavas and pyroclastics that erupted during the stratocone building stage, by combining the new field and geomorphological observations with petrographic, lithologic and geochemical data. Geomorphology involves preservation degrees of original micro-geomorphic features on their surfaces, such as lava levees and lava wrinkles. We identify 38 eruptive units that made up the stratocone, including 31 units of low-K tholeiitic (TH) as the dominant magma series, with 4 units of calc-alkaline (CA) series, and additionally 3 units of MD (medium) series that show intermediate characteristics between TH and CA. The volcanic activity of the stratocone is divided into two stages on the basis of the distinctive eruption centers and their resultant contrastive edifices. The latter stage (stage 2) can be further divided into two substages, 2-1 and 2-2, respectively, because of contrastive preservations of micro-geomorphologic features on the lava surfaces. In stage 1, fluidal lava flows, mainly basalt to basaltic andesite in compositions, were effused from the southern crater to form the southern stratocone showing a shield-like gentle slope. There are several observations that suggest dormancy and/or erosion interval might be present between the stages 1 and 2; epiclastic deposits are characteristically recognized immediately below the lavas of the stage 2, and one of the deposits directly overlies a lava flow of the stage 1. The crater moved northward and commenced discharge magmas considerably silica-rich compositions compared with those erupted in the stage 1, and built up another steeper stratocone (northern edifices). Although, the northern edifices ware mainly developed in the stage 2-1, three lava flow units display distinctively better preservation of micro-geomorphic features on their surfaces. The freshness of these topography together with some tephrochronologic data suggest that the final stage (stage 2-2) must have lasted immediately before the caldera collapse occurred ca. 13,000 years ago.