Mechanisms of lava dome instability and generation of rockfalls and pyroclastic flows at Soufrière Hills Volcano, Montserrat

Abstract

AbstractBetween 3 January 1996 and 1 December 1998 at Soufrière Hills Volcano, Montserrat, there were 27150 seismic signals attributable to rockfalls and pyroclastic flows. Large (1-4 x 106m3) and major (>4 x 106m3) dome collapses began to occur after the extrusion rate reached 2 m3 s-1 and the lava dome exceeded 30 x 106m3 in volume. Large to major collapses occurred on 26 occasions and were usually associated with periods of elevated extrusion rate (6 13 m3 s-1), intense hybrid earthquake swarms and/or inflation-deflation cycles of crater-rim ground deformation. During these cycles, gas-rich pulses of magma, 140 000 320 000 m3 in volume, were intruded into the dome and, soon thereafter, dome collapses from the headwalls of shear lobes were generated. Large dome collapses also occurred after 10 March 1998, when magma extrusion ceased, but these had no seismic precursors. These events represent structural failures from over steepened canyon-like walls and were followed by intense degassing, suggesting that gas pressure build-up within the relict dome may have played some role. Rockfall counts and durations established from seismic data show variations that correlate with extrusion rate. Using pyroclastic flow runout and rockfall duration data as proxies for event magnitude, power law relationships between frequency (total number of events) and magnitude have been found. Seismic signals associated with rockfalls and pyroclastic flows commonly comprise both high-frequency and long-period components. Intense degassing from the dome is interpreted as the source for the long-period component. These results indicate that rockfalls and pyroclastic flows generated by dome collapse at Montserrat are not simply the result of passive dome failure, but are intimately related to discharge of pressurized gas and pulses of magma extrusion. Pyroclastic flows were usually sourced from lobe headwalls, where lava was hot and gas-rich and where fragmentation of the micro vesicular andesite lava occurred more readily.