著者

早津 賢二

出版者

地学団体研究会

雑誌

地球科学 (ISSN:03666611)

巻号頁・発行日

vol.26, no.3, pp.91-103, 1972

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The Myoko Volcano Group, consisting of Yakeyama, Myoko, Kurohime, lizuna, Madarao and Sadoyama Volcanoes, is situated in the boundary area between Nagano and Niigata Prefectures, Central Japan. Most of the volcanoes are distributed along the anticlinal or synclinal axes of basal Neogene strata. Except for Sado-yama and Madarao Volcanoes, the volcanic cones are arranged in a north-south trend with each nearly equal interval (about 8 km), and generally the age of each volcano tends to become younger northward. The rocks of this Volcano Group, characterized by hornblende andesite, are in striking contrast to those of sothern part of Fuji Volcanic Zone. They also differ from the basement volcanic rocks with biotite phenocrysts. Both Myoko and Kurohime Volcanoes are double stratovolcanoes of the latest stage of the Quaternary. Their activities may be divided into four stages; the older stratovolcano, the younger stratovocano, the caldera and the central cone stages. Myoko Volcano is constructed by a large amount of lava flow, pyroclastic flow, pyroclastic fall and mud flow. The rocks are characterized by common occurrence of hornblende and/or olivine phenocrysts, and by larger petrographical variation. On the other hand, in Kurohime Volcano, lava flows are predominant overwhelmingly, and pyroclastics are subordinate. Most of the rocks are augitehypersthene andesites, and the petrographical nature is rather uniform.