有珠山2000年噴火における地球化学的研究 : 火山灰付着水溶性成分の変動と火山活動(<特集>2000年有珠山噴火 (2))

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Usu volcano located in southwestern Hokkaido is one of the most active volcanoes in Japan. On March 31, 2000, it erupted at the western foot of Nishiyama with successive formation of new craters after 23 years of dormancy. On the following day, another eruption newly forming craters occurred at the western foot of Konpirayama located I km to the east of Nishiyama new craters. Monitoring of volcanic gases discharged from active craters will provide us with authentic information on the volcanic activity, however, due to intermittently violent explosions, vicinity of the new craters were not accessible. Analyses of water-soluble components adhering to volcanic ash were available for estimating composition of volcanic gases and monitoring eruption of Usu volcano. Volcanic ash samples were collected around the volcano for about eight months. All the ash was relatively depleted in SiO_2 and enriched in FeO in comparison with essential ejecta of the historic eruptions of Usu volcano. Smectite was a dominant clay mineral and kaolin was a minor constituent. All the ash samples were hydrated considerably. Temporal variation in the constituent minerals and chemical composition was not recognized. This result designated that variation of the water-soluble components adhering to the ash was not due to change of nature of volcanic ash but due to change of flux and chemical composition of volcanic gases.
2002-09-17