A study on the mechanism of shallow-level degassing of conduit magmas as a controlling factor of eruption styles

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We examined the crystal size distribution (CSD) of ground crystals in the erupted materials of the 2011 Shinmoe-dake volcano. We found that magma residence time in the shallow conduit in the explosive subplinian phase is shorter than in the less-explosive vulcanian and lava dome phases. We also conducted textural relaxation experiments of andesitic pumices in shallow conduit conditions and clarified the vapor pressure dependence of the relaxation and bubble coagulation. We conclude that the textural relaxation enhances formation of “gas pockets" via compaction of shallow melt foam.