Rheological behavior of crystal-bearing magma is a controlling factor of magma flow dynamics and hence the style of volcanic eruptions. In this study, we performed torsional deformation experiments to determine the viscosity of crystal-bearing rhyolitic and dacitic magmas. The experiments were conducted at a temperature of 800 °C and under confining and pore fluid pressures of 100 and 80 MPa, respectively, by using an internally heated gas-medium deformation apparatus. To simulate the rheology of natural magma, we deformed volcanic rocks with 16 and 45 vol% crystallinities and < 4 vol% gas bubbles. These rocks have different crystal and bubble shape and size, which appear to influence magma rheology. By using the mechanical data obtained, we calculated flow indices and viscosities for these magmas. For magma with 16 vol% crystallinity, the flow index showed good agreement with previous data obtained from experiments on synthetic magma analogues and model predictions. In contrast, the flow index was smaller than those obtained from previous experiments and model predictions at 45 vol% crystallinity, which may be explained by considering that natural magma contains crystals with different shapes and sizes. We also found that the apparent viscosity of the magma increased when sample-scale fractures were observed in run products. This means that the heterogeneity of natural magma causes locally stiff regions within the sample owing to crystal interaction. Our data indicate that the viscosity of crystal-rich magma is strongly dependent on the strain rate. This implies that magma ascending in a volcanic conduit shows a plug-type flow because the viscosity decreases near the conduit rim where the strain rate is high. Additionally, if shear localises near the rim, a currently unrecognised mechanism may contribute to outgassing from the central portion of the conduit because outgassing is difficult to facilitate without shear deformation.
- Bubble- and crystal-bearing magma
- High PT deformation experiment
- Magma rheology
ASJC Scopus subject areas
- Geochemistry and Petrology