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タイトル: Activities of the Gassan Volcano.
その他のタイトル: 月山火山の活動
著者: Ichimura, Takeshi
著者(別言語): 市村, 毅
発行日: 1955年12月10日
出版者: 東京大学地震研究所
掲載誌情報: 東京大學地震研究所彙報. 第33冊第3号, 1955.12.10, pp. 419-432
抄録: (i) The Gassan Volcano rises up on the boundary between W. Murayama-gun and E. Tagawa-gun or Mogami-gun of Yamagata Prefecture. It is approachable by several courses from various sides. (ii) It is composed of the Ishihane-gawa mud flow, Gassan lavas and ejecta and Sasa-gawa mud flow which successively erupted on the eroded surface of granite (granodiorite), quartz diorite, diorite, liparite, felsophyre, dacite, propylite, basalt and Upper Miocene sediments elevated frequently to 1300m.-1400m. above the sea-level. (iii) The lavas and ejecta including mud flows erupted successively from pre-existing crater and also from the explosion crater. Of these, the pre-existing crater is now obscure, being covered by lavas and ejecta which erupted subsequently, whereas the explosion crater opened northwestwards with a horse-shoe-shaped hollow just beneath the present summit. (iv) Most predominant are the Gassan lavas and ejecta which consist of olivine two-pyroxene andesite, olivine-bearing or olivine-horn-blende-bearing two-pyroxene andesite and their fragments. One of characteristic features indicated by these rocks is the frequent occurrence of olivine phenocryst. This mineral is sometimes found in association with oxyhornblende. (v) The activities of this area began with the eruption of the Ishihane-gawa mud flow and ended with the formation of a big explosion crater which poured forth a large amount of the Sasa-gawa mud flow. (vi) By this enormous explosion, the Gassan Volcano lost its upper half of konide(?) and left behind a quite different from of volcanic body. It has a very gentle slope on the east and is highly cliffed on the west. (vii) The first activity of the Gassan Volcano is supposed to have taken place in the beginning of the Quaternary, but it will be confirmed later in connection with the activities that happened at the Ha-yama volcanic area. In conclusion, the writer's thanks are due to Mr. N. Jimbo and Mr. T. Konda for the valuable suggestions which they offered during his field and laboratory works. The expence of this research was partly defrayed from the fund for the Scientific Research of the Educational Ministry.
URI: http://hdl.handle.net/2261/11823
ISSN: 00408972


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