Dynamics of magma ascent and lava extrusion at Soufrière Hills Volcano, Montserrat

Abstract

Growth of the andesitic lava dome during eruption of Soufrière Hills Volcano, Montserrat involved general increases in discharge rate and dome height with superposed fluctuations on timescales from hours to months. We have modelled magma conduit flow incorporating viscosity variations caused by degassing and crystallization. Gas loss is modelled by permeable flow, with variations of permeability constrained by measurements. Crystallization kinetics is modelled by an Avrami law. Observations and petrological studies constrain conduit diameter (c. 30 m), magma chamber crystal content (c. 60–65%), melt water content (c. 5%), magma temperature (c. 850°C) and chamber depth (c. 5 km). Gas loss and rheological stiffening cause a maximum in magma overpressure (magma pressure minus lithostatic pressure) at depths of a few hundred metres below the dome. This maximum can explain ground deformation patterns, shallow seismicity, and short-lived Vulcanian explosions. Crystallization kinetics causes a strong feedback mechanism and multiple steady solutions for discharge rate. Small changes in chamber pressure, magma viscosity and conduit diameter strongly amplify discharge rate. Multiple solutions allow cyclic variation of discharge rate. Escalation of activity is attributed principally to influx of hot mafic magma into the chamber, resulting in chamber pressure increasing with time. Dome growth can be intrinsically unpredictable due to non-linear effects.