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We have installed a new furnace equipped with a rotational viscometer for use in viscometry at high temperature (one atmosphere) under controlled oxygen fugacities. This furnace allows sampling at the time of the viscosity measurement. Utilizing this furnace, we examined the relationship between textural parameters (crystallinity and the apparent aspect ratio of crystals) and viscosity during cooling of a basaltic magma. The 1707 basalt from Fuji volcano was used for the experiment and the result shows a drastic increase in viscosity from the liquidus temperature (1230°C; η = 52 Pa s), through 1170°C (crystallinity = 11%; η = 350 Pa s), to 1130°C (crystallinity = 23%; η = 1950 Pa s). The increase in viscosity during cooling is affected by both the increase of the crystallinity and the change of melt composition as a result of crystallization. For the basalt from Fuji volcano, crystallization of thin tabular plagioclase enhanced the strong effect of crystal entanglement, whereas the changes of melt compositions did not affect the viscosity because of the increase of ferromagnesian components and the decrease of feldspathic component in the residual liquid during crystallization. The effect of the presence of crystals on the viscosity is much larger than that expected from the Einstein Roscoe equation with Marsh's constant (η = η0 (1 − ϕ/0.6) 2.5 ), where ϕ denotes the crystal fraction. The additional correction of Simha on the solid shape parameter slightly overestimates the increase in relative viscosity in the presence of thin platy crystals. A shear rate dependence of the viscosity was observed at lower temperature crystal rich conditions.

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