Eruption mechanisms and mineral textures of the ejecta

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(1) Comparative petrological study of volcanic ejecta of Fuji volcano: We examined petrographic features of the 3000ybp Omuroyama flank eruption, AD.864 Jyogan eruption, and AD. 1707 Hoei eruption to understand the relations between the mode of eruptions and crystal textures of the ejecta at Fuji volcano. Compositional zoning of plagioclase phenocryst and microphenocryst tells us the processes of degassing and recharge of hydrous aphyric magmas during the magma uprise and eruptions. The 3000 yr by Omuroyama eruption had subplinian pyroclastic fall deposit which contains limited amount of plagioclase microphenocryst, suggesting only limited time for crystallization of magmas due to degassing of water just before the eruption. The AD.864 eruption products 20-30% of plagioclase phenocryst often with reverse zoning near the rim, indicating magma replenishment of degassed shallow magma by hydrous aphyric magmas just before the effusive eruptions.(2) Analogue experiments of the effect of mag … More ma pocket on the mixing of magmas: This experiment intends to examine the mixing processes of two magmas at low Reynolds number. Previous experiments (e g. Blake and Campbell, 1986) only examined the mixing conditions at high Reynolds number. Presence of a magma pocket increases the instability of laminar annular flow because of the increase of gravitational force against viscous force. It is shown that flow instability occurs when the dimensionless parameter I (=viscous force/gravitational force) is less than 0.1. Analyses of the condition of end member magmas indicate that flow instability occurs in a magma pocket beneath Unzen volcano.(3) Water content and hydration of volcanic ejecta: We examined andesitic-rhyolitic glasses of the AD.1783, AD. 1108, and 13ka bp eruption products of Mama volcano. Water contents are 03-0.7 wt % for AD.1783 and AD.1108 ejecta, whereas they are 15-4 wt% for 13ka ejecta. In back-scattered electron images of 13ka volcanic glasses, we observed clear hydration front in the core of the vesiculated bubble wall glass, suggesting that high water content of the old deposits are affected by secondary hydration of the glass.(4) Viscosity measurement of subliquidus magmas:A Kilauea thoeiite: Crystal phases are mainly augite and plagioclase with minor amount of olivine. The relative viscosity of the crystal-bearing magma is almost well explained by the Einstein-Roscoe-Marsh equation indicating that the effect of anisotropic crystal morphology is counterbalanced by the effect of crystal size dispersion.B: MORB tholeiite: An ODP Leg 206 MORB sample is used for the viscosity measurement. Crystal phases also consist of augite and plagioclase with mirror amount of olivine. The relative viscosity of the crystal-bearing magma is larger than the values of Einstein-Roscoe-Marsh equation by a factor of 3-5, which may be due to the higher aspect ratio of the plagioclase in the MORB charge. Less