In this study, a procedure to measure the viscosity of multi-phase magma at high temperatures (>1000°C) was developed by using a simple apparatus comprising a commercially available desktop furnace and viscometer. In particular, the use of a disposable container enabled observations of the microstructure of an entire sample. The procedure was applied to basaltic andesite magma of the 1986 Izu–Oshima fissure eruption, Japan. The results show that reliable data, consistent with previous studies, were obtained and that the magma rheology became non-Newtonian with decreasing temperature, showing clear shear-thinning behavior. The rheological properties of the magma at 1180°C are quantitatively described as a function of shear rate based on three simple non-Newtonian fluid models. Sectional views of the sample confirm that plagioclase and Fe–Ti oxide crystals were nearly uniformly dispersed in the sample. The mean crystal volume fraction of 0.14 enabled crystal interactions inducing changes in crystal arrangement, affecting the rheology.

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