有珠火山2000年3月31日噴火の噴出物とマグマプロセス(<特集>2000年有珠山噴火 (3))

概要

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The March 31 eruption was the first and largest event in the 2000 eruption of Usu volcano. The eruptive products (ash and pumice) contained considerable amount of accidental or accessory fragments, however, we identified the essential materials among the products. Firstly, we concluded that the pumice (Us-2000pm) is essential, rather than reworked material of the past pumices. It is because Us-2000pm contains magnetite phenocrysts whose Mg/Mn ratio and Al component are different from those in any other historic pumices of Usu volcano. Secondly, we found microlite-rich vesiculated fresh glass (Us-2000g) in the ash and concluded that it is also essential. It is because magnetite in Us-2000g is compositionally identical to that in Us-2000pm. Therefore, the March 31 eruption was phreatomagmatic, rather than phreatic. The 2000 eruptive product (Us-2000pm) has similar petrological feature to the historical ejecta, especially the 1977 eruptive product. Zoning patterns of plagioclase phenocrysts in Us-2000pm represent crystal growth since the 1663 eruption, as well as in other historical ejecta. This means that the 2000 magma was derived from the same magma-feeding system of the historical eruptions, and that the magma-feeding system has existed since the 1663 eruption.
2002-11-29