The explosive decompression of a pressurized volcanic dome: the 26 December 1997 collapse and explosion of Soufrière Hills Volcano, Montserrat

Abstract: We calculate the gas flux through a pressurized and permeable lava dome as magma slowly ascends to the surface and degasses. This provides an estimate of the mass of gas and the associated energy that may be released following the explosive collapse of such a dome. We estimate that during the 26 December 1997 dome failure of Soufrière Hills Volcano, up to 10¹⁶ J of energy was released. We then develop a model of the dome break-up driven by the pressure jump across a fragmentation wave. We predict that the fragmentation front advances into the dome at a speed of order 10 ms^-1, while the gas and solid fragments issue from the dome at speeds of order 50-60 ms^-1. This fragmented material forms a dense fountain of gas and solid fragments. This may rise 100-200 m above the dome and then spread out to form a dense pyroclastic density current, with initial density of order a few hundred kilograms per cubic metre. We anticipate that such flows will become stratified in density and mean grain size. Some new experiments on stratified gravity currents illustrate that the flow propagation speed is similar to a well mixed current of the same total buoyancy, although the flow tends to become laterally stratified, with the dense lower region out-running the fluid higher in the current.