浅草火山の地質

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タイトル別名
  • Geology of Asakusa Volcano
  • アサクサ カザン ノ チシツ

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Geological study on Asakusa volcano located on the southern part of Moriyoshi volcanic zone in the Northeast Japan arc has been carried out to construct the volcanic history. The volcano had been active in the middle Pleistocene age arid ejected about 6km^3 of the volcanic materials. Volcariism of this volcano began with eruption of minor amounts of lava flows (early stage lava flows), followed by development of the principal constituents of the volcano. The volcanic history of its principal constituents is summarized as follows. Stage 1: Major lava flows and associated pyroclastic flows erupted from a central vent to form a cone-shaped stratovolcano. The estimated volume of the volcanic product is about 2.5km^3. The rocks are composed mainly of andesite and basalt, with augite, hypersthene and sometimes olivine phenocrysts, which is accompanied by a small amount of dacite with phenocrysts of hornblende, augite and hypersthene. Stage II: Central eruption of large amounts of pyroclastic flows and subordinate lava flows also occurred and the stratovolcano built by the first stage volcanism was further grown. The volume of the volcanic edifice is estimated to be about 3 km^3. Andesite with phenocrysts of hornblende, hypersthene and augite was erupted in the early period of this stage, which was followed by eruptions of pyroxene-olivine basalt and pyroxene andesite. The central vents were situated in the uppermost stream of the Tadamizawa Valley throughout the first to second stage. Stage III: After completion of the stratovolcano, small eruptions of lavas composed largely of olivine bearing pyroxene basalt and andesite, and pyroxene andesite occurred from several vents and these lava flows covered the northern and southern flanks of the volcano. Stage IV: Small eruptions of lava flows and pyroclastic flows occurred and their products mainly covered the northern flank. The erupted materials are basalt and andesite containing phenocryst minerals same as those of the rocks in Stage III. Total volume of the volcanic edifice in Stages III and IV amounted roughly to 0.5km^3.

収録刊行物

  • 地球科学

    地球科学 45 (2), 101-112, 1991

    地学団体研究会

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