The transition from endogenous to exogenous growth of lava domes with the development of shear bands

Abstract

The transition from an endogenous to an exogenous regime of lava dome growth must be achieved by the formation of discontinuities within the dome. Such transitions (and vice versa) are an important characteristic of most long-lived lava domes and often coincide with significant changes in the dynamics of magma supply and lava dome collapse events. For the purpose of this paper, following recent experimental and observational evidence, we assume that such a transition occurs when shear bands are generated. A model for the formation of shear bands, and therefore the growth transition within a dome and coupled conduit domain is presented. Shear bands are most likely to initiate at the junction of the conduit and base of the dome, where the shear stress experienced between new lava entering the dome and existing lava is greatest. Stress accumulation within shear bands is likely to lead to brittle shear, resulting in the formation of fractures. Finite element modelling of lava flow shows that such shear bands only develop for certain extrusion rates and lava viscosities. Similarly, the growth regime of the lava dome will depend upon the extrusion rate and viscosity within the conduit, which is largely controlled by volatile loss and the growth of crystals in the upper part of the conduit. We consider a simplified rheology during lava dome growth considering isothermal conditions with crystal growth. The development of shear bands in the conduit is explored with a numerical model parameterized with values appropriate for Soufrière Hills Volcano, Montserrat. During October to December 1996 this lava dome forming eruption experienced a transition from endogenous to exogenous growth as it grew in height by about 90 m. Modelling indicates that the observed fall in magma extrusion rate from about 2.0 m 3 s − 1 to 0.5 m 3 s − 1 , as a result of the increased pressure head from the dome and the evolution in viscosity, could have subsequently changed the dome growth regime due to the development of shear bands. Our models provide insight into the shear stress fields possible within the conduit and the shear stresses required for shear band development.