火砕流を伴う火山活動と断裂

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The author tried to show close relationship between the distribution of fractures and location of large-volume pyroclastic flows since the Pliocene, using information from Landsat image together with geologic data. The following conclusions are reached : Centers of caldera volcanism in Kyushu Island are situated at intersections of three fractures. One of them is the Kagoshima graben and its northward and southward extension, and the others are tectonic lines which run across the Kyushu Island in NW or NE directions. Northeast Japan is cut by main three fractures which are distributed in northern, middle and southern parts and have NE, NW and NW directions respectively. Large scale pyroclastic flows had been erupted in these fractured zones. In southwest Hokkaido many fractures of NW to WNW directions controled the distribution and forms of calderas which were erupted in the latest Pleistocene. The Late-Pleistocene calderas are nearly square but not circular in shape. The rims of calderas are usually in parallel with the extention of the fractures. The pyroclastic flows erupted at the broken portions in the large scale structural zone such as grabens. The same phenomena indicate that the fractures have been helped the generation of the magma resulted from the melting of shallow zone material and its ascent with contamination by sial materials to produce voluminous acidic magma.
特定非営利活動法人日本火山学会の論文
1984-04-01