COUPLING OF VISCOUS AND DIFFUSIVE CONTROLS ON BUBBLE GROWTH DURING EXPLOSIVE VOLCANIC ERUPTIONS

Abstract

The coupling of viscosity and diffusivity during explosive volcanic degassing is investigated using a numerical model of bubble growth in rhyolitic melts. The model allows melt viscosity and water diffusivity to vary spatially and temporally with water content. We find that the system is highly sensitive to the distribution of volatiles around the bubble, primarily as a consequence of the great sensitivity of melt viscosity to water content at low water concentrations. The dehydrated portion of the magma near the bubble exerts the major control on the effective viscosity of the melt; however the values of effective viscosity are lower than previously reported values calculated using an approximated concentration profile. Degassing is found to be highly sensitive to the choice of the solubility law, which controls the volatile concentration near the bubble, but insensitive to the equation of state. The form of the concentration profile in the case of concentration-dependent diffusivity is such that the effective viscosity is substantially lower (by as much as an order of magnitude) than for constant diffusivities of similar magnitude. This leads to unexpectedly high bubble growth rates, particularly for cases of highly non-equilibrium degassing. This is because low values of diffusivity are more than compensated by low melt viscosity due to higher volatile concentrations. This complex interplay between viscosity and diffusivity means that models of bubble growth must take into account the concentration-dependent nature of both these parameters; approximating either of these with a constant value will lead to significant errors in estimations of bubble growth rates.