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.4, pp.593-629, 1956-03-30

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(i) The South Zao is constructed by such two volcanoes as Byobu and Aoso. Of these, the Byobu Volcano occupies the western half of this area and includes Nonomori, Byobu-dake, Fubo-zan, Ushiro-eboshi-dake, Mae-eboshi-dake, Sugiga-mine, Nyudo-zan, and Manokami-dake, rising to 1817.3meters above the sea-level at the highest point. They are now deeply dissected and thickly forested as compared with the Zao Proper. (ii) The Byobu Volcano rests upon the eroded surface of such base rocks as Upper Miocene sediments, granite and other intrusives or extrusives. It is built up of various volcanic products represented by the Nonomori agglomerate, Sumi-kawa lava and agglomerate, Byobu-dake lava, Fubo-zan lava and ejecta, Sugiga-mine lava and agglomerate, Ushiro-eboshi-dake lava and ejecta, Kara-sawa mud flow, Nyudo-san lava and agglomerate, Manokami-dake lava and Nanokahara mud flow. (iii) The largest source of these lavas and ejecta is the pre-existing crater or explosion crater which opened at the central part of this volcanic area. It is now surrounded by Byobu-dake, Fubo-zan and Ushiro-eboshi-dake, but is partly deformed and obscured by the explosion and erosion as well as by the formation of the central cones. Other craters and explosion craters ejected the Nonomori agglomerate, Sugiga-mine lava and agglomerate and Nanokahara mud flow. The most characteristic explosion crater is preserved on the southern flank of Mae-eboshi-dake. (iv) The beginning of activities repeated here is shown by the eruption of the Nonomori agglomerate, which was followed by the eruption of the Sumi-kawa lava and agglomerate. The latter is the biggest eruption of the Byobu Volcano, and a large amount of lava flowed down northwards beyond the Nigori-gawa and Sumi-kawa at that time. Such activities had happened several times until the Nanoka-hara mud flow poured out finally from the explosion crater near Numa, but there has been no activity in the historic time. (v) Two central cones were succesively formed within the preexisting crater or explosion crater mentioned above. These cones were formed at the last stage of several activities, being composed of two-pyroxene andesite and its agglomerate. They are characterized by the absence of such dacite as is exposed on the central cones of the neighbouring volcano. (vi) The geological age of the first eruption here may be the Pleistocene. It is inferred from the fact that the lavas and ejecta of the Byobu and Aoso Volcanoes erupted together, and the oldest mud flow of the latter is unconformably underlain by alternate beds of clay, sand and gravel assigned to Pleistocene. In conclusion, the writer wishes to offer his hearty thanks to Dr. N. Nasu and Dr. H. Tsuya who helped him to continue his work even after he had been transferred from the Earthquake Research Institute of Tokyo University to Yamagata University. The writer's sincere thanks are also due to Mr. F. Hori for his co-operation in the petro-graphical investigations of lavas and ejecta obtained from this volcano. A part of this investigation was carried out with the aid of the Education Ministry Grant for Natural Science.遠刈田温泉に近い青麻火山の西方に,大きく根を張る屏風火山は,屏風岳,不忘岳,杉ヶ峰,後烏帽子岳,前烏帽子岳,入道山,馬神岳,ノノ茶などよりなり,蔵王火山群の重要なる部分を占めるものである.この火山は,花崗岩(花崗閃緑岩),新第三系,これ等を貫き,或は被覆する各種火成岩を基盤として,その侵蝕面上に盛上り,火山体を構成する多量の橄欖石複輝石安山岩,含橄欖石複輝石安山岩,複輝石安山岩,砕屑岩は,極めて複雑なる分布を見せている.現在不忘山,屏風岳,後烏帽子岳により包囲され,入道山,馬神岳の両中央火口丘が存在する地域は,屏風火山活動の主要な中心と考えられ,又これに接近してノノ森噴火口,沼爆裂火口,その他からも,熔岩,集塊岩,泥流などが噴出しているのを見ることが出来る.この火山地域の活動は,ノノ森噴火口の生成に始まり,七日原泥流の噴出を以て,終りを告げたものと見做し得可く,青麻火山や蔵王火山群の他の火山と相呼応し,恐らく第四紀に入つてから活動が開始されたものと想像される.ただしこの火山では,青麻火山と同じ様に,有史以後の活動が知られていない.