1. 溶岩の記載とその成因(VI. 秋田駒ケ岳 1970-71 年噴火の噴出物)

概要

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The new lava erupted in the 1970 activity of Akita-Komagatake comprises volcanic bombs, volcanic blocks and lava flows of hypersthene-augite andesite (V_<d→c> type). The mode comprises phenocrysts of plagioclase, augite, hypersthene and magnetite in a groundmass of plagioclase, clinopyroxene, hypersthene, magnetite and brown glass. Rarely anhydrite-bearing xenoliths, consisting of tridymite, anhydrite, augite and glass, are found. These may have been formed by pyrometasomatism of basement rock rich in opaline silica. Petrochemical features of the new lava are discussed by means of various diagrams. It noted that K_2O is very low, though total alkalis are nearly the same as in the older lavas. The new lava plots near the boundary line between the tholeiitic series and calc-alkalic rock series, whereas the older ones fall always in the central part of the tholeiitic series in the MgO-(FeO+Fe_2O_3)-(Na_2O+K_2O) diagram. When plotted in the (FeO+Fe_2O_3)/(FeO+Fe_2O_3+MgO)-SiO_2 diagram, the new lava shows a trend different from that of the older ones. This change may have been brought about by increase of pO_2 as the result of increasing water content in the magma reservoir. If this trend goes further, a pyroxene andesite of Vd type with about 60% SiO_2 might be erupted by this volcano in the future.
特定非営利活動法人日本火山学会の論文
1971-12-01