Flow and Depositional Mechanisms of Debris Avalanche : A case study of Iwasegawa debris avalanche deposit, Tashirodake Volcano, northern Japan

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  • TAKARADA Shinji
    The Graduate School of Science and Technology, Kobe University:(Present address)Geological Survey of Japan

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Other Title
  • 岩屑流の流動・堆積機構 : 田代岳火山起源の岩瀬川岩屑流の研究
  • ガンセツリュウ ノ リュウドウ タイセキ キコウ タシロダケ カザン キゲン

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Description

Iwasegawa debris avalanche deposit is distributed along river channels in the southeastern foot of Tashirodake Volcano, northern Japan. The volume of the deposit is estimated to be 0.1 km3. Debris avalanche block (DB) in Iwasegawa debris avalanche deposit is composed of materials, e. g. altered tuff breccia, lava with jigsaw cracks, derived from the source area ; and materials captured during flowage from the basement formations and river deposits. DB and debris avalanche matrix (DM) show lateral variation, where the maximum size of DB decreases, and sorting of DM becomes better from the source area towards the distal end. Volcanic clasts within DM show normal grading. However, wood fragments within DM show reverse grading. The result of preferred orientation measurements on 273 pieces of wood fragments coincide with the local flow direction. Plug flow model is used to explain the flow mechanism for the Iwasegawa debris avalanche, wherein a rigid plug and a laminar boundary layer exist as distinct parts of the flow. As the Iwasegawa debris avalanche flowed down the steep slope at high speed, clasts of basement formations were eroded and incorporated into the laminar boundary layer under a strong shear stress field. DM was produced by shear stress either in between two DB or DB and basement formations. Fragile DB were carried within the rigid plug without any major ruptures. The size of DB decreases down stream due to the progressive collision of each DB. Collision between DB and basement formations permitted additional fracturing during flowage. Wood fragments included within the DM in the laminar boundary layer were rotated and aligned themselves parallel to the flow direction. Lithic clasts having higher densities than those around DM tend to settle toward the bottom, whereas wood fragments of lower density tend to float toward the top of the flow. When the shear stress in the laminar boundary layer became smaller than the yield strength of the flow due to deceleration, Iwasegawa debris avalanche was freezed and stopped thixotropically.

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