Shape Characteristics of Tafoni Developing on Sandstone Cliff, and the Processes from the Enlargement of Tafoni to the Occurrences of Rock Fall in the Oshihara Gorge, Shimane, Japan

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  • 島根県忍原峡の砂岩急崖に発達するタフォニ群の形状特性とそれに基づいたタフォニ拡大から岩盤崩落への過程
  • シマネケン ニン ゲンキョウ ノ サガン キュウガイ ニ ハッタツ スル タフォニグン ノ ケイジョウ トクセイ ト ソレニ モトズイタ タフォニ カクダイ カラ ガンバン ホウラク エ ノ カテイ

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 If convex-concave structure of rock surface grows on steep slopes, convex portions will become unstable and will finally collapse. To understand these processes and mechanisms, the authors have surveyed the shape characteristics of tafoni developing on steep rock slopes in the Oshihara Gorge, Shimane, Japan, and have studied their processes and geologic factors controlling them. <br> Rock slopes where tafoni developed are composed of coarse-grained pebbly arenite sandstone. Remarkable precipitation of gypsum and softening of rocks along upper inner wall of tafoni show that infiltrated water from the slope surface has brought about the enlargement of tafoni through salt weathering. Inner dip structure of strata influence on the process.<br> Although the diameter of mouth of tafoni ranges from a few centimeters to a several meters, the ratio of height/width tends to decrease roughly with the increase of its width. This means that a small ellipsoid vertically elongated changes to a large horizontally elongated ellipsoid. As a tafoni develops, upper convex portion enlarges and finally will become unstable. Arch action force supporting upper blocks will decrease with the horizontal elongation of the mouth. Consequently, the instability more increase, and finally rock fall will occur. These developments may influence the slope hazards by the occurrence of rock falls in such slopes with joint planes perpendicular to bedding planes, which dip steeply outer ward.

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