Formation of a Pyroclastic Cone in the 1783 Plinian Eruptions of Asama Volcano

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  • 浅間火山1783年のプリニー式噴火における火砕丘の形成
  • アサマ カザン 1783ネン ノ プリニーシキ フンカ ニ オケル カサイキュウ ノ ケイセイ

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Description

The edifice named “Kama yama” on the summit of Asama Volcano is a well-proportioned cone, truncated by an U-shaped, small depression on its northern outer-slope. Thick welded pyroclastic fall deposits are exposed on the crater-wall of Kama yama and the top surface of the welded deposits is topographically traced to the top surface of the lava flow of the “Tenmei” (1783 A. D.) eruption (“he Onioshidashi lava flow”). Welded pyroclastic materials correlative with the welded deposits of Kama yama crater-wall also locally crop out on the outer surface of Kama yama just below the deposits derived by the recent vulcanian-type explosions. These pyroclastic materials are petrologically identical to the juvenile materials of the 1783 eruption, and the welded stratigraphic succession of Kama yama is remarkably similar to that of the 1783 pumice fall deposits in the distal area. Kama yama was thus most likely built-up during the 1783 eruption, which was mainly plinian-type. Coarse pyroclastic materials likely accumulated near the vent to form Kama yama cone during the climactic phase of the plinian eruptions. The depression of the northern part of the cone and topographical continuity between the cone and the Onioshidashi lava flow indicate that the effusion of the Onioshidashi lava flow was accompanied with partial collapse of the northern steep slope of Kama yama. Old documents and old pictures also support this scenario of the growth of Kama yama during the climactic phase of the 1783 eruption. Formation of a pyroclastic cone being accompanied with plinian activity is one of the common features of explosive eruptions in many andesitic volcanoes. Geology of such cones may provide key information for understanding of the eruption dynamics of intermediate magma compositions.

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