Water-soluble Components on Volcanic Ashes from Tokachi-dake Volcano, Hokkaido, Japan during the 1988-1989 Eruption

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  • 十勝岳1988-1989年噴火で放出された火山灰の付着水溶性成分
  • トカチダケ 1988-1989ネン フンカ デ ホウシュツサレタ カザンバイ

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Tokachi-dake volcano in Central Hokkaido, Japan, began to erupt in Dec. 1988 from the 62-II crater after dormancy for 26 years, and the intermittent eruptions occurred 23 times until Mar. 1989. Volcanic ashes were ejected 14 times during the 1988-1989 eruption and water-soluble components on the ashes were analyzed. The ashes were rich in the water-soluble components and the great part of the volcanic ejecta was altered stale rocks in the vent. This feature suggest that the water-soluble components were derived not only from volcanic sublimates that formed during the 1988-1989 eruption but also from the sublimates that have formed in the vents since the last eruption in 1962. Cl/S molar ratio in the ash leachates came to be high as the volcanism became active. Although accumulated volcanic sublimates before eruption made the Cl/S ratio of the ash leachates different from those of eruptive gases, change of the ratio of the leachates agreed well with the observed change of the volcanic activity. It was shown that analysis of water leachates of volcanic ashes was a safety and effective method for estimation of volcanic activity. Volcanic sublimates ; halite, sylvite, thenardite, gypsum, alunogen, halotrichite, epsomite and melanterite were assumed to be sources of the water-soluble components and the normative mineral compositions of the samples were calculated. Proportion of halite + sylvite was changed corresponding to the eruption scale. The change of the mineral composition suggested that temperature of volcanic gases discharged went up in the middle of January, 1989 and down early in March, 1989.

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