有珠火山2000年噴火におけるマグマ水蒸気 : 水蒸気爆発による破砕深度とその時間的変化(＜特集＞2000年有珠山噴火 (2))

抄録

The phreatic eruptions continued at the Nishiyamanishi and Kompirayamanishi craters, northwest flank of Usu volcano, in the southwestern part of Hokkaido, Japan, followed the initial phreatomagmatic eruption on March 31, 2000. The phreatomagmatic- and phreatic ejecta is characterized by abundantly containing accidental and crystal fragrnents, and clay minerals originated in the Neogene and Plio-Pleistocene altered volcanic rocks. The fragments in the ash were correlated with subsurface geology; Miocene altered dacitic and andesitic rocks (Osarugawa and Sohshunaigawa Formations), Plio-Pleistocene argillic tuff (Yanagihara Formation), altered andesite (Lower Pleistocene Andesitic Rocks), and non-altered andesite and basaltic andesite (Usu Somma Lava). Alteration zones in this area were classified into seven zones (I-VII) by their mineral assemblages. They are Plio-Pleistocene alteration zones (e.g., I-IV) and Miocene alteration zones (e.g., V-VII), and the depth of the latter is greater than several hundreds meters. In zone IV, kaolinite is distinctively abundant, so that kaolinite/smectite ratio is an useful parameter to evaluate the degree of contribution of the zone as an origin of fragments in the ash. In the Nishiyamanishi craters area, there is another kaolinite zone (IV) in shallower level (＜ 200 m below sea level), which was resulted from diffusion of sulfuric acidic hydrothermal water related to Pleistocene FeS and S mineralization (Abuta mine).