火山麓扇状地と成層凝灰亜角礫層

概要

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Considerable parts of the foots of stratovolcanoes are occupied with volcanic fans composed of stratified tuff breccias. A volcanic fan has smooth and gentle slopes which are 3-16km in length, 300-700m in height, 2.5-7 degree in gradient, 20-30km in area and linear in profile. In many volcanoes some volcanic fans connect with one another and build confluent volcanic fans surrounding the stratovolcanoes. A volcanic fan is composed of stratified tuff breccias which are chaotic mixture of volcanic sub-angular breccias and their fine materials. As a distance from the center of a volcano increases, roundness of breccias, sorting of the deposit and the fine materials in the deposit increase. The stratified tuff breccia may be a deposit composed of volcanic detritus which flows down along valleys as a density flow caused by heavy rain-falls. Volcanic fan may have been progressively made through all the time of building of the volcano. And it may have been actively made in the stage when pyroclastic materials were mainly produced. But climatic change may have exerted upon development of the volcanic fan.
駒澤大学の論文