Weathering environments based on rock color changes with depth and the probability of recent sector collapses in the Shirataka Volcano area, Yamagata, Japan.
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Rock color values a* and b* (CIE L*a*b* color measurement system) of borehole cores from the caldera of the Quaternary Shirataka Volcano, Yamagata, Japan, were measured with a colorimeter, and their changes with depth and lithofacies were analyzed to understand weathering environments in volcanic areas. In general, the weathering environment is reflected in oxidative changes to iron minerals in rocks. The correspondence of a* and b* color values to the colors of goethite, which is brownish as a result of low-temperature oxidation, and hematite, which is reddish brown, reflecting high-temperature oxidation, makes it possible to distinguish the spatial distribution of these iron minerals by examining color distribution.<BR>The distribution of a* and b* values in rocks of the caldera indicates that a goethite zone of low-temperature oxidation extends widely, but hematite zones of high-temperature oxidation are localized. b*-values, indicating goethite, were higher at shallow depths and decreased with depth, reaching a constant low value at 50 to 150m depth. Low-temperature (i. e., normal atmospheric temperature) oxidation has thus prevailed in this volcanic region for a long time period, as indicated by the thick weathered zone represented by high b* values.<BR>The thickness of the high-b*-value zone seems to be independent of lithofacies or geologic structures instead, it is almost concordant with the present-day topographic relief in the caldera. The continuity of the weathered zone suggests that the caldera has not been exposed to recent large-scale surface sedimentation or erosion. Therefore, recent large-scale avalanches or sector collapses, which would have caused large-scale sedimentation, probably have not occurred in this area.
- 公益社団法人 日本地すべり学会の論文
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