Processes Controlling Evolution of Volcanic Activity Deduced from Volcanic Gas Observations

  • SHINOHARA Hiroshi
    Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology

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  • 火山ガス観測から推定される火山活動推移過程
  • カザン ガス カンソク カラ スイテイ サレル カザン カツドウ スイイ カテイ

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<p>Volcanic gases are derived from a magma or a hydrothermal system, which are sources of magmatic or phreatic eruptions, respectively. Changes in a magma or a hydrothermal system, leading to eruptions, likely cause changes in volcanic gas emission rate and compositions. SO2 emission rates correlate well with volcanic activities and precursory changes of the emission rates are also common. The SO2 emission rate also tell us a volume and a degassing rate of the SO2 sources, that can constrain processes of magma ascent or crystallization. CO2/SO2 ratios increases are observed prior to major eruptions of volcanoes with Strombolian activity by automatic monitoring of volcanic gas compositions, and are attributed to accumulation of CO2-rich bubbles in a magma chamber. Common occurrence of excess degassing by explosive eruptions implies an eruption of a bubble-rich magma, suggesting that a bubble accumulation is a necessary condition of an eruption. Various precursory signals are observed prior to phreatic eruptions but quantitative modeling of the processes is not simple because the volcanic gas compositions are largely affected by various reactions at a shallow hydrothermal system. Combination of volcanic gas emission rate and composition monitoring data may help us for the quantitative evaluation of the volcanic gas changes associated with phreatic eruptions.</p>


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