Viscosity of bubbly magmas from torsional experiments on pumice

Abstract

Bubbles in magma affect its viscosity, one of the most important properties for modeling volcanic eruptions. We performed new viscosity measurements on rhyolitic magma with bubble volume fractions, $\varphi$, between 0.15 and 0.80. Pumice samples from Medicine Lake Volcano, California, were deformed in torsion-compression experiments at a temperature of $975{}^{°}C$, and strains up to ∼3. Capillary numbers during the experiments were large and viscosity, $\eta$, decreased with increasing $\varphi$. The experiments have coherent trends in $\eta$ vs. $\varphi$ with little scatter. We define a new constitutive relation for the relative viscosity of bubbly rhyolitic melt, ${\eta }_r=exp\left(5.5\varphi /\left(2-\varphi \right)\right),$ and for bubbly suspensions at high Capillary numbers in general, reducing the uncertainties associated with scatter among the the body of prior experiments. Our results are useful for more robust modeling of volcanic eruptions.