姶良Ｔｎ火山灰の風化

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タイトル別名
  • Weathering of Aira-Tn Ash.
  • アイラ Tn カザンバイ ノ フウカ

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抄録

Weathered acidic volcanic glass shows features such as pitted textures. These microstructures were considered to be related to chemical weathering, but little have been studied about their formation. In order to clarify this process, we analyzed the Aira-Tn Ash which was erupted about 25, 000 years ago from the Aira Caldera in southern Kyusyu, and covers almost the whole area of the Japanese Islands.The glassy ash has been weathered under various subsurface environments, and many shallow, rounded pits are formed on the glass surfaces. The fresh glass, which is interbedded in clayey deep-sea sediments in the Sea of Japan, has no such pits but has smooth surface with elongated sharp holes like gashes. On the other hand, at Chigaki, Toyama Prefecture, we examined the ash interbedded in the terrace deposits, in which a successive change of the microstructure on the glass surface from well pitted to almost non-pitted was observed. Detailed analysis of this ash layer clarified that along with the progress of weathering, the size of the pits increases, the thickness of the glass decreases, and refractive index of the glass decreases.Etching by HF solutions shows that the small holes on the fresh glass have extended gradually and changed into shallow rounded pits similar to those on the weathered glass surface. The refractive index of heavily etched glass decreases to the same value as that of the weathered glass.From these observation, the volcanic glass in the Aira-Tn Ash is considered to be composed of the outer layer of 3 to 4μm thick where the refractive index is high and inner layer whose refractive index is low. Weathering dissolves the volcanic glass and removes the outer layers. This process may form the shallow rounded pits on the surface and decrease the refractive index.

収録刊行物

  • 地質学雑誌

    地質学雑誌 105 (6), 410-420, 1999

    一般社団法人 日本地質学会

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