Modelling of conduit flow dynamics during explosive activity at Soufrière Hills Volcano, Montserrat

Abstract

Abstract Magmatic explosive activity at Soufrière Hills Volcano involved a sub-Plinian eruption, on 17 September 1996 and two series of repetitive short-lived (c. 1 min) Vulcanian explosions in 1997. Explosive activity followed major collapses of the dome. We have modelled unsteady conduit flow in explosive eruptions after unloading. Two cases are investigated: (i) equilibrium between gas dissolved in the melt and bubbles for sustained sub-Plinian eruption; and (ii) no mass transfer between pre-existing gas bubbles and melt for Vulcanian explosions. The models for Vulcanian explosions agree with observations of erupted volumes, eruption durations (tens of seconds), typical drawdown depths (a few hundred metres to c. 2 km), exit velocities and discharge rates. Explosive mixtures are predicted to have high densities consistent with the occurrence of fountain collapse. The models for sub-Plinian eruption show good agreement with observed erupted volumes and drawdown depths (c. 4 km). Three fragmentation criteria were studied, namely fragmentation at fixed porosity, at a critical gas overpressure, and at a critical elongation strain rate. Results are similar for the three cases, but the critical overpressure and critical strain-rate criteria both predict strong pulsations, whereas the fixed-porosity criterion predicts continuous fragmentation. Pulsations are caused by feedback, with the threshold conditions for magma fragmentation being repeatedly crossed. Pulsations are indicated from seismic and video observations.