著者

Ichimura Takeshi

出版者

東京大学地震研究所

雑誌

東京大學地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.33, no.3, pp.419-432, 1955-12-10

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(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.月山は,山形県の東田川郡,最上郡及西村山郡に跨り,鳥海火山帯の中でも特異な形態を有する火山である.その東側は,アスピーテを思わせる緩傾斜をみせ,西側は,大きな爆裂火口のために物凄い断崖をなしている.この火山は,花崗岩(花崗閃緑岩)石英閃緑岩,閃緑岩,石英粗面岩,珪長斑岩,石英安山岩,変朽安山岩,玄武岩,新第三紀(上部中新世)堆積岩などよりなる基盤岩の侵蝕面上に盛上り,その活動は,石跳川泥流,月山熔岩及砕屑岩,笹川泥流,順次に噴出せしめている.大爆裂火口は,月山熔岩及砕屑岩の噴出後に生じ,これがために旧月山火山の上半部が噴き飛ばされるに至つた.月山の現形態は,この際に生じたものであり,笹川泥流は,爆発に伴つて噴出したものと思われる.月山火山の活動は,恐らく第四紀の初期に始まり,大爆裂火口の生成と笹川泥流の噴出を以て,その終りを告げている.