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Chapter 6 Dome growth and valley fill during Phase 5 (8 October 2009–11 February 2010) at the Soufrière Hills Volcano, Montserrat

Abstract Extrusion during Phase 5 (8 October 2009–11 February 2010) produced significant volumetric and geomorphic changes to the lava dome and surrounding valleys at the Soufrière Hills Volcano, Montserrat. Approximately 74×106 m3 of lava was extruded at an average rate of 7 m3 s−1 during the short period of activity. Addition of lava to the pre-existing dome resulted in a net volumetric increase of up to 38×106 m3. Pyroclastic density current (PDC) and ashfall deposits accounted for the remaining 36×106 m3. A series of thick, blocky lobes were extruded from a central vent. In addition, several short-lived spines and two large shear lobes were also extruded. Significant PDC activity resulted in substantial valley filling of up to 108 m. The large pre-existing dome significantly influenced the growth of lobes, such that many block-and-ash flows were generated from viscous lobes draped over the summit and upper slopes. Geomorphic changes caused by rapid filling of the surrounding valleys aided in both flow avulsion and the emplacement of deposits up to 6 km from the dome. These geomorphic changes have important consequences for hazards from PDCs.