Radial variations of melt viscosity around growing bubbles and gas overpressure in vesiculating magmas

The viscosity of silicic melts depends strongly on their water content. As bubbles grow in a supersaturated melt, water evaporates from the bubble-mel t interface. A diffusive profile develops and leads to steep viscosity grad ients across the melt shell. Here we investigate the effects of radial visc osity profiles on the dynamics of bubble growth. We fmd that the effective melt viscosity resisting gas overpressure in the bubbles is close to the vi scosity at the dehydrated rind, and may be higher than that of the surround ing melt by more than an order of magnitude. As a result, bubbles may retai n pressures that are higher than ambient pressure for longer times, magma d egassing is delayed to shallower depth, and fragmentation of magma due to g as overpressure may occur over a wider range of conditions. Measured water content in eruption products yields information on the average melt viscosi ty, however additional information about the concentration profiles is need ed for estimating the effective viscosity that controlled the evolution of bubble growth in the ejecta. (C) 2001 Elsevier Science B.V. All rights rese rved.