Abstract

We present new theoretical results into the mechanical behaviour of fluidized suspensions that may have bearing on the dynamical evolution of crystallising subvolcanic magma chambers and mode of ignimbrite eruption. A key part of our analysis relates to the mechanical role of a dissolved gas phase, in particular the expansion and contraction of small bubbles during fluctuating external pressures. For cases where the bubbles fail to collapse completely due to insufficient magma layer depth, associated micro currents may cause significant particulate motion of surrounding crystals, resulting in a downwards propagating liquifraction front. Depending on the rate of porpagation and thickness, this instability will lead to potentially significant erosion within the crystal pile at the chamber base. We identify the critical gradient for fluidization, and its associated pore pressure drop. This physical effect may lead to sudden changes in the composition of the magma and we speculate that some types of chemical zonation and resorbtion seen in plagioclase grains in acidic volcanic rocks may be due to the local flow of melt into the expanding pore space of the crystal mush, as opposed to larger, chamber-wide processes. For the extreme case of near instantaneous shear of chambered magma arising from earthquakes, interstitial melt flow rates of up to 1 m/s are predicted. These high flow rates will result in large-scale disruption of the solid mush, and extremely rapid segregation of evolved interstitial fluid to produce a gas-rich, high melt fraction layer, the bulk composition of which may resemble continental rhyolites.