大山火山東麓小鴨川流域の軽石流と火砕流の地形
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概要
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The purpose of the present study is to make clear the topographical characteristics of pumice flow and pyroclastic flow in the dissected valley of the Daisen Volcano. Along the Ogamo River, one of the dissected valley of the Daisen Volcano, the terrace-like topography consisting of tuff breccias is laid from the upper stream to the mid-stream. The cross section of the depositional surface of tuff breccias is convex. In the lower part of both sides, the dissected valleys are developed. The relative height above the river is about 150m at the upper stream and about 30m at the mid-stream. In the longitudinal profile of the depositional surface, there is a break of the slope at Myoko, which it divides into two parts that show straight profiles. The mean inclination is 3.7°in the upper part and 1.7°in the lower part. The deposits consist of poor particle-size sorting, unwelding andestic materials and contain some carbonized wood materials. From such facies and distribution of the deposits, it is concerned that they flowed in a state of glowing cloud. The writer concludes that, from the facts that the lithic fragments of the deposits in the upper part of the slope are andesitic blocks, they are deposited by a pyroclastic flow. As the deposits of the lower part contain some pumice particles, they were formed as the result of a pumice flow. Based upon his observation on the distribution and the stratigraphical correlation of the deposits, existence of pumice, petrological characteristics of the lithic fragments and stratigraphy of the volcanic ash beds, and his analysis of the matrix particle-size distribution of the deposits in the laboratory work, he concludes that the tuff breccias developing from Myoko to the upper part, corresponds to the new pyroclastic flow deposits in the history of the volcanic activities of Mt. Daisen and the tuff breccias developing from Hori to the lower part, correspond to the pumice flow deposits. At the time when the pumice flow or the pyroclastic flow flowed through the dissected valley of the Daisen Volcano, the pumice flow was laid from the middle to the lower stream and formed the gentle slope topograhy, while the pyroclastic flow was laid in the upper stream and formed the steep slope topography. The longitudinal profiles of both depositional surfaces are straight.
- 1970-03-31