Classification and idealized limit-equilibrium analyses of dome collapses at Soufrière Hills volcano, Montserrat, during growth of the first lava dome: November 1995-March 1998

Styles of dome collapse at Soufrière Hills volcano (SHV; November 1995-March 1998) are classified by relations between extrusion rate prior to collapse and collapse volume. Four separate modes of collapse behavior are apparent. Notably, moderate rates of extrusion are shown to result in two disparate modes of collapse: small-to-large collapses on steeply inclined failure planes that switch to collapse volumes an order of magnitude larger that cut deeply into the dome core. For constant effusion rates, this bifurcation in behavior is explained by the monotonic growth of a soft core that ultimately promotes the development of a deep-seated failure over previously favored shallow failure modes. Models are developed to test this hypothesis that first constrain magnitudes of cohesive and frictional strength with observed dome collapse morphologies and volumes. Evaluations of dome strengths confirm the important role of a soft core in promoting deep failure. A nested model representing a cohesive dome core, surrounded by a frictional rind, with constant rate of magma input, confirms the observed bifurcation in behavior, and for invariant effusive activity. Importantly, failure volumes are shown to increase by close to an order of magnitude for a few percent change in the proportion of dome core comprising cohesive material. This model is capable of replicating, a posteriori, the approximate timing of failure for both small (250 m) and large (325 m) domes. The timing and style of the 17 September 1996 and June to November 1997 collapses are honored.