[S-VC31] 火山噴火のダイナミクスと素過程

[SVC31-P04] 爆発的噴火の軽石粒子の見かけ密度を支配している要因

*寅丸 敦志¹、緒方 美季¹、伊野 遥¹、スヘンドロ インドラノバ¹、ニーン ガブリエラ¹ (1.九州大学大学院理学研究院地球惑星科学部門)

キーワード：軽石、見かけ密度、発泡度、斑晶、斑泡、マトリックス気泡

The bulk density of pumice clasts produced by explosive eruptions has thought to have the significant information on vesiculation processes that the magmas experience in the volcanic conduits and the magma chambers. As known well, there are the wide variations of bulk density even in the single eruption deposits. However, it is unclear that what causes the variation in bulk density, e.g., vesicularity or crystallinity. To reveal factors controlling the bulk density variations, we examine the correlation between the bulk density and other textural properties such as vesicularity, phenocryst-crystallinity, pheno-bubble abundance, and matrix bubble abundance for three eruptions; Aira caldera forming eruption, Japan, Mount St. Helens 1980 eruption, USA, and Manijau caldera-forming eruption, Indonesia. We determine the bulk density of pumice clasts (mainly 8 – 16 mm) from the volume by 3D scanner and the weight by electric balance, and conduct the textural analysis for vesicularity (pheno-bubble (PB) (>0.1 mm) and matrix-bubble (MB) (< 0.1 mm)), phenocryst-crystallinity by the 2D image analysis for the SEM microphotographs. In the case of the Aira caldera-forming eruption, both PB and MB abundance have negative correlations with the bulk density and bulk vesicularity, the crystallinity has no correlation with bulk density. In the case of the Mount St. Helens 1980 plinian eruption, PB abundance has no correlation with bulk density, whereas MB abundance has a negative correlation and crystallinity has a positive correlation with bulk density. In the case of Maninjau caldera-forming eruption, both PB and MB abundance have negative correlations with bulk densities, whereas the crystallinity has no correlation with bulk density. Thus, in two large-volume caldera-forming eruptions among these three eruptions, only vesicularity controls the bulk density. In smaller plinian eruption of Mount St. Helens 1980 eruption, in addition to vesicularity, the crystallinity controls the bulk density. In the presentation, data for other eruptions such as Toba YTT caldera forming eruption will be shown to discuss the processes to make such PB, MB and crystallinity.