月山の残雪凹地における礫の風化皮膜の厚さの空間変化

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タイトル別名
  • Spatial Variations of Weathering Rind Thickness at a Snow Accumulation Hollow in Mt. Gassan, Northern Japan
  • ガッサン ノ ザンセツ オウチ ニ オケル レキ ノ フウカ ヒマク ノ アツ

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Recently, several geomorphologists have applied a relative dating method using weathering rind thickness to date glaciations or periglaciations during the late Quaternary in Japan. However, some problems remain to be solved. The most important one is to evaluate the field environments factors (e. g., total amount of snow melt water supply) affecting rind growth over time. In this study the following investigations were carried out at a partly vegetated snow accumulation hollow in Mount Gassan, northern Japan.<br> First, weathering rind thickness on surface cobbles were measured. The weathering rind criterion is the mean maximum rind thickness for each plot. Second, ages of slope stabilizations in the study site were determined by means of radiocarbon dating and tephrochronology. Third, retreat of the snow patch margin was repeatedly observed. Based on the above results, the relations between spatial varia-tions of weathering rind thickness and ages of slope stabilizations are discussed. The fundamental prob-lems of this method are also pointed out. The principal results are summarized as follows:<br> (1) Spatial variations of rind thickness do not always correspond to the duration time of slope stabih ity. Slopes with the maximum value of 5.5 mm in rind thickness are considered to have been stable since the mid or late Holocene. However, the oldest stage of slope stabilization in the study site could be the Late Glacial or early Holocene.<br> (2) The mean value of rind thickness varies by a factor of about two within the confines of slopes that are considered to have been stable or to be unstable. For example, the value changes from 1.3 to 3.3 mm within the snow patch bare ground. This slope is regarded as unstable, because active niveo-fluvial transportation and physical weathering occur there.<br> (3) Within the snow patch bare ground the average value of rind thickness never shows 0 mm. This fact indicates that weathering rind thickness as a rule of time is not always set at zero when a slope is stabilized. The assumption that weathering rind thickness was 0 mm at the slope stabiliza-tion should be re-examined. Researchers who attempt to use this method in snowy environments like those in the Japanese high mountains should carefully collect abundant data.

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