Viscous flow of magmas from Unzen volcano, Japan - implications for magma mixing and ascent

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In the proposed project the viscosity of magmas of the Unzen volcano in Japan (compositions: rhyodacite to andesite) will be determined at conditions relevant to the local geological situation. Effects of pressure, water content, redox state of iron and degree of crystallization on rheological properties of the magma are of special importance in our studies. At present the influence of these parameters on viscosity of rhyodacitic to andesitic melts can not be modeled due to a lack of reliable experimental data. In the high-viscosity range nearby the glass transition, viscosities will be measured under pressure using a parallel-plate viscometer developed in our institute. Previous measurements were performed only at ambient pressure in this viscosity range. In the low-viscosity range (stable melt), the fallingsphere method will be applied. Combining both methods, the temperature dependence of viscosity can be defined in a large T range. Based on our viscosity measurements and on data from literature we want to develop a viscosity model applicable to melts with rhyolitic to andesitic composition. Using the viscosity data and results on phase equilibria and volatile solubility (project Ho1337/3+7), an attempt will be made to reconstruct the evolution of Unzen magmas from pre-eruptive conditions to the beginning of the eruption.