Interpretations for Magmatic Process and Eruptive Phenomena by Way of Volcanic Gas Studies(＜Special Section＞The 60th Anniversary of VSJ “Latest Advances and Future Perspectives in Volcanology”)

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  • KAZAHAYA Ryunosuke
    Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology
  • MORI Toshiya
    Geochemical Research Center, Graduate School of Science, The University of Tokyo

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Other Title
  • 火山ガス観測研究から見る地下のマグマ挙動および噴火現象の解釈(＜特集＞日本火山学会60周年「火山学の最新動向と今後の展望」)
  • 火山ガス観測研究から見る地下のマグマ挙動および噴火現象の解釈
  • カザン ガス カンソク ケンキュウ カラ ミル チカ ノ マグマ キョドウ オヨビ フンカ ゲンショウ ノ カイシャク

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Abstract

Volcanic gas plays a crucial role in the dynamics of eruption and ascent of magma. Volatiles degassed from magma are emitted to the surface as high temperature volcanic gases. Lately observation techniques to measure volcanic gas have been developed, allowing us to monitor volcanic activities and to compare the volcanic gas data with geophysical data. The volcanic gas composition and emission rates have been measured so as to elucidate the magma plumbing system. The volcanic gas composition gives us the information of degassing pressure and temperature of the volcanic gas within the volcano. The volcanic gas emission rates reflect the production rates of the degassed magma within the volcano. In many cases, the amount of volcanic gas observed exceeds the gas amount which can be degassed from the erupted magma. At some active volcanoes, a significant amount of volcanic gas is emitted not only during eruptive periods but even during quiescently degassing periods. These results suggest that only a portion of magma is erupted yet the rest is degassed at a depth without discharge. These observational results are known as “excess degassing”. To explain this, degassing models (the permeable flow, magma convective degassing, and gas percolation models) were proposed. Recent studies suggest that the condition of bubble segregation from the magma is a key parameter for the magma degassing process, which controls if an eruption becomes explosive or not. If the bubble separation from the magma does not occur during the magma ascent, the gas volume fraction of the magma increases monotonously, leading to the fragmentation of magma. The transition of the closed- to open-degassing within the conduit was proposed so as to explain the significant volcanic gas emission without eruptions. Recent developments of volcanic gas observation techniques have opened up the possibility to reveal the linkages between degassing and geophysical (seismic or geodetic) phenomena. The relationship between very-long-period seismic events and volcanic gas exhalation was found by multi-disciplinary observations. From the viewpoint of the geodesy, the volcanic gas emission could cause deflation of the volcanic body. The examination of the magmatic and volcanic processes from both viewpoints of geochemical and geophysical studies is important.

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