著者

八木 健三

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:04534360)

巻号頁・発行日

vol.16, no.1, pp.28-35, 1971

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Calderas are classified into salic and mafic types according to their association with either salic pyroclastic flows or mafic lava flows. The salic type corresponds to the low gravity anomaly type, and the mafic type to the high gravity anomaly type of YOKOYAMA. When the longer diameters of the two types of calderas in Japan and the world are plotted against the SiO₂ contents of the pyroclastic flows, or lava flows, they are distributed in diverging U-shaped areas, i.e., the size of salic type calderas becomes larger with increasing SiO₂ content, and that of mafic type calderas increases with decreasing SiO₂ content. The colossal amounts of salic magmas and their high explosivity explain the larger size of the more salic type calderas, while the lower viscosity of the more mafic magmas explains the larger size of the more mafic type calderas. The relation between the diameters and the depths of the salic type calderas of Japan is examined. For smaller calderas, less than 7 km in diameter, there is some linear relation between the two parameters, but for the larger ones the depths are nearly constant at 500~700 m, irrespective of their diameters. Comparison is made with artificial explosion and lunar craters, and their genesis is discussed.