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Geochemical Study on the Volcanic Activity of Kiso-Ontake Volcano, after the 1979 Eruption

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  • 木曾御岳火山 1979 年噴火後の活動状況と地球化学的研究
  • 木曽御岳火山1979年噴火後の活動状況と地球化学的研究
  • キソ オンタケ カザン 1979ネン フンカゴ ノ カツドウ ジョウキョウ ト

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Kiso-Ontake Volcano erupted suddenly on the 28th of October, 1979. The eruption started forming ten new craters on the southern flank of the volcano’s summit. This paper deals with geochemical study on its volcanic activity after the eruption. The ratio of Cl to S was found to be high in the water-soluble components of the volcanic ash. The content of SO2 was larger than that of H2S in the volcanic gas. According to these observations and considering the sulfur isotopic ratio, the under ground temperature was estimated to be higher than 250℃, while that at the orifice was measured to be as low as 90℃. These facts and seismic observation indicate that magma was not elavated to a sarrow part, but that only high temperature-gas, which was separated from the magma, came up rapidly along the crack, and then rushed into the mud reservoir near the surface, where a large amount of water was evaporated, following the eruption caused by prompt increase of pressure. At an early stage of the eruption, the content of Cl- was larger than that of SO24 in spring and pond waters, which were in contact with the fumarolic gases at the summit, but its relation was reversed later. This has been explained by the absorption of HCl, which had been contained in the volcanic gas at the early stage, into the water phase near the summit. From 1980 to 1982, the outlet temperature of fumarole increased to 108-145℃. However, the ratio of SO2 to H2S reduced rapidly, which indicates the decrease of temperature at depth. Therefore, the increase of the temperature of the fumarole is thought to be caused due to the decrease of cooling effect by ground water, because water was evaporated by the continuation of the eruptive activity. Recently no significant changes have been observed in temperature and chemical compositions of volcanic gas, suggesting that the activity will remain to be low for the time being.


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