北海道駒ケ岳, 2000年の小噴火とその意義 : 噴出物と火山灰付着性成分の時間変化から見たマグマ活動活発化の証拠

概要

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After 54 years' dormancy since the last eruption in 1942, volcanic activity of Hokhaido-Komagatake volcano has obviously increased. Small phreatic eruptions sporadically occurred in 1996 and 1998, and several small phreatic eruptions intermittently continued from September to November in 2000. Eruptions on September 4 and October 28 were relatively large in the 2000 eruption, with a total eruptive mass of 10^5 and 3×10^4 tons, respectively. Volcanic ashes of the 1996, 1998, and 2000 eruptions contain fragments constituting the shallow levels of the volcanic edifice. No juvenile materials have been identified in the eruption products. However, ashes of the two major eruptions in 2000 contain fragments of hornblende and mafic andesite characteristic of the rocks seated at deeper levels of the edifice. In addition, as determined the water-soluble components adhered on the ashes, the Cl/S molar ratio increases up to 0.2 to 0.4 in 2000 eruptions while it remains<0.1 in 1996 and 1998 eruptions. The depth of explosion has been getting deeper and the effect of magmatic gas has increased in 2000 eruptions than in 1996 and 1998 eruptions. We thus suggest that the vent system has been developed well toward a deeper magma storage system during the 2000 activity.
2002-01-15