Geological Study on the Mechanism of Landslides in Owakudani, Kanagawa Prefecture (I)

概要

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Landslide in volcanic altered area is studied at the Owakudani area, Hakone, kanagawa Prefecture. Geologic structure and alteration of rocks in the area of landslide are checked by means of many boring holes, for the purpose of making clear the basic causes of landslide in volcanic altered area. The zones of alteration seem to be developed owing to actions of volcanic gases and secondary weathering by ground water. Development of a weak clay zone is mainly influenced by the weathering. The zones of alteration may be divided as follows: (1) the zone of clay, which mainly consists of clay, (2) the zone of strongly altered andesite, which consists of clay and altered rock, (3) the zone of altered andesite, which mainly consists of rocks, but they are changed into clay more or less along the crack. Another classification of the altered zones by component mineralsis as follows: (1) montmorillonite-calcite zone (lower part), (2) montmorillonite-gypsum zone (middle part), (3) kaoline-opal zone (upper part), (4) kaoline-alunite zone (nearly surface), (5) mixed-layer clay minerals-montmorillonite zone (environs). Ground water and springs in the altered area are characterized by pH of 2.0-4.0 and low Eh. Sulphates with Ca2+, Mg2+, Al3+ and sometimes Fe2+ etc. are plentifully dissolved in the ground water, making it active in weathering.
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