妙高火山のPyroclastic flow

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The history of activity of Myoko, an extinct double volcano in central Honshu, Japan, may be divided into the following three stages: 1a Formation of a stratovolcano. 1b Formation of an explosion caldera accompanied by descent of a voluminous mud flow. - erosion interval, enlargement of the caldera - 2 Eruption of Tsubame andesite lava and pyroclastics, and formation of a flat cone within the caldera. - erosion interval - 3 Eruption of central dome lava and pyroclastic flows. The last stage of activity was caused by a hypersthene-hornblende andesite magma. As in the case of Mt. Pelee, the protrusion of the central dome lava was accompanied by the eruption of pyroclastic flows, and the activity seems to have continued for months or years. All pyroclastic flows, except one, which flowed down southward over the caldera rim, advanced eastward through the canyon of the Otagiri river, that cuts the eastern caldera wall, and spread over the eastern foot. The total volume of the pyroclastic flow deposits, except the southern one, whose distribution is not certain is estimated to be 0.6〜0.7km^3. The pyroclastic flow deposits consist of ashy matrix and angular compact blocks variable in size. Accidental blocks are rare in the deposits. Most of the blocks, matrix material and the dome lava are petrographically similar to one another. Scattered orientations of magnetic dipoles measured from these blocks suggest temperatures of the blocks at the time of deposition were lower than the Curie points of ferromagnetic minerals in the blocks.
特定非営利活動法人日本火山学会の論文
1961-06-30

妙高火山のPyroclastic flow

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