Fall-units and Refractive Index Fluctuations of Eniwa-a Tephra Glass, Hokkaido, Japan

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  • Nakamura Yugo
    the Japan Society for the Promotion of Science Graduate School of Environmental Earth Science, Hokkaido University
  • Hirakawa Kazuomi
    Graduate School of Environmental Earth Science, Hokkaido University

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  • 恵庭ａテフラのフォールユニットと火山ガラス屈折率
  • エニワ aテフラ ノ フォールユニット ト カザン ガラス クッセツリツ

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Abstract

Eniwa-a tephra (En-a), which erupted from Eniwa volcano, western Hokkaido, at 17ka BP, is composed of more than three fall-units in the southern part of the Ishikari Lowlands. The refractive indices of En-a volcanic glass are significantly different among fall-units. The refractive index is relatively high (n=1.510-1.514) in the upper fall-unit and low (n=1.505-1.510) in the middle and lower fall-units. It is generally accepted that the petrographic properties of some tephras differ considerably between fall-units. However, this has not always been considered in collecting samples and measuring petrographic properties of marker tephras. In order to obtain petrographic properties for tephra identification, we should take samples from every fall-or flow-unit at an outcrop near a source vent. The refractive indices of En-a volcanic glass from the Hidaka Range tend to have remarkably low values and large variances due to incomplete hydration in volcanic glasses.

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