Measurement and implication of "effective" viscosity for rhyolite flow emplacement

Abstract

Hazardous explosive activity may sporadically accompany the extrusion of silicic lava domes. Model- ling of the emplacement of silicic domes is therefore an important task for volcanic hazard assessment. Such modelling has been hampered by a lack of a sufficiently accurate rheological database for silicic lavas with crys- tals and vesicles. In the present study, the parallel-plate viscometry method was applied to determine the shear viscosity of five natural rhyolitic samples from a vertical section through the Ben Lomond lava dome, Taupo Volcanic Centre, New Zealand. Rheological measure- ments were performed at volcanologically relevant tem- peratures (780-950°C) and strain rates (10 -5 -10 -7 s -1 ). Although these samples are in the metastable state, vis- cosity determinations, melt composition, as well as water and crystal contents of samples were demonstrably sta- ble during experiments. For samples containing up to 5 vol.% microlites, the composition of the melt, rather than the physical effect of suspended crystals, had great- er influence on the effective viscosity of the silicic mag- ma. Samples with 10 vol.% microlites and containing a flow banding defined by microlites show no significant orientational effects on apparent viscosity. The rheologi- cal measurements were used together with a simple cool- ing model to construct thermal and viscosity profiles re- vealing conditions during the emplacement of the Ben Lomond lava dome.