Considerations for the Mechanisms of the Formation of Pyroclastic Flow Deposits on the Basis of Grain-size Distribution

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  • FUKUDA Toshiro
    Institute of Earth Sciences, Faculty of Science, Kagoshima University

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  • 粒度分布からみた火砕流の形成機構について
  • リュウド ブンプ カラ ミタ カサイリュウ ノ ケイセイ キコウ ニ ツイテ

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Abstract

Mechanisms of the formation of pyroclastic flow deposits of Japan have been investigated on the basis of their grain-size dize distribution. The pyroclastic flow deposits of Japan can be classified into A-, B- and C-type on the basis of differences of their grain-size distributions, and the pyroclastic flow deposits of these types correspond to Murai's (1961) nuee ardente deposits and intermediate-type pyroclastic flow deposits; St. Vincent-type ash flow deposits; and Krakatoa-type ash flow deposits respectively. 0n a Rosin’s law paper, cumulative curves of the A-type pyroclastic flow deposits resemble those of ideal pyroclastic flow deposits which are composed of particles concentrated in the lower part of the eruptive column. The pyroclastic flow deposits of this type, there-fore, was probably formed when the particles of the lower part of the eruptive column dashed down the mountain slope. It can be considered that the B-type pyroclastic flow deposits were formed in the same way as the A-type deposits were formed, because the grain-size distribution of this type on a probability scale is very similar to that of the A-type. Whereas, on the probability scale, the grain-size distribution of the C-type pyroclastic flow deposits does not resemble that of the A-and B-type deposits, but resemble that of the turbidity current deposits stated by Visher(1969). It seems that the pyroclastic flow of C-type traveled as the turbidity current did.

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