10:45 AM - 12:15 PM
[SVC29-P05] Oscillatory rheology measurements of bubble- and particle-bearing basaltic andesite magma
Keywords:rheology, suspension, magma
The rheology of magma governs the mobility of magma, the possibility of fragmentation, and the characteristic of seismic wave propagation. Therefore, the rheology of magma is one of the most important physical properties for eruption dynamics and the monitoring of underground magma. Although there are many viscosity measurements of molten magma, rheology measurements are limited. In the 1990s, Bagdassarov et al. measured the rheology of magma at relatively low temperatures (<1050 oC) and reported behavior close to that of Maxwell fluids. In contrast, magma is a complex fluid bearing bubbles and crystals, similar to colloids and suspensions, but the structure-dependent properties created by particles have not been directly measured due to the technical difficulties of rheology measurements at high temperatures.
We here developed a new system to measure the rheology at high temperatures using a commercial rheometer and a high-temperature furnace and measured a partially molten basaltic andesite magma. For a sample, we used volcanic ash from Aso Volcano. The measurements were performed at 1080-1180 oC. This temperature is between solidus and liquidus, where the ash melts and crystals float in the melt. In addition, voids between the ash before melting become bubbles. Two different methods were used for the measurements. In the first type, the angular frequency was changed while the strain amplitude was kept constant to measure the frequency dependence. In the second type, the stain amplitude was changed while the angular frequency was kept constant to measure the strain dependence. After the measurements, we observed the texture of solidified samples to estimate the amounts of bubbles and crystals during the measurements.
The sequence of solid-liquid transition was observed in this temperature range. Above 1130 oC, the sample was liquid-like; at 1105 oC, it was liquid- or solid-like depending on the deformation rate and strain; and at 1080 ℃, it was solid-like. Moreover, the rigidity at 1080 oC, which behaves like a solid, is about 107 Pa, much lower than the value usually used. The colder samples show a stronger frequency and strain dependence in the complex viscosity. The relative viscosity to the melt phase calculated by the MELTS program (Gualda & Ghiorso, 2015) became larger than 10 times at all temperatures.
Our rheology measurements suggest that the temperature decrease not only increases the viscosity of melt but also increases the strain dependence of the viscosity of the bubble- and crystal-bearing magma. The strain-dependent viscosity suggests a large amplitude shaking may become a rejuvenation mechanism of a solidifying magma chamber. The low rigidity implies the slow propagation of the shear wave, possibly slow as a velocity of 40 m/s.
We here developed a new system to measure the rheology at high temperatures using a commercial rheometer and a high-temperature furnace and measured a partially molten basaltic andesite magma. For a sample, we used volcanic ash from Aso Volcano. The measurements were performed at 1080-1180 oC. This temperature is between solidus and liquidus, where the ash melts and crystals float in the melt. In addition, voids between the ash before melting become bubbles. Two different methods were used for the measurements. In the first type, the angular frequency was changed while the strain amplitude was kept constant to measure the frequency dependence. In the second type, the stain amplitude was changed while the angular frequency was kept constant to measure the strain dependence. After the measurements, we observed the texture of solidified samples to estimate the amounts of bubbles and crystals during the measurements.
The sequence of solid-liquid transition was observed in this temperature range. Above 1130 oC, the sample was liquid-like; at 1105 oC, it was liquid- or solid-like depending on the deformation rate and strain; and at 1080 ℃, it was solid-like. Moreover, the rigidity at 1080 oC, which behaves like a solid, is about 107 Pa, much lower than the value usually used. The colder samples show a stronger frequency and strain dependence in the complex viscosity. The relative viscosity to the melt phase calculated by the MELTS program (Gualda & Ghiorso, 2015) became larger than 10 times at all temperatures.
Our rheology measurements suggest that the temperature decrease not only increases the viscosity of melt but also increases the strain dependence of the viscosity of the bubble- and crystal-bearing magma. The strain-dependent viscosity suggests a large amplitude shaking may become a rejuvenation mechanism of a solidifying magma chamber. The low rigidity implies the slow propagation of the shear wave, possibly slow as a velocity of 40 m/s.