Initiation of Magma Fragmentation Leading to Vulcanian Explosions

The SHV volcano on Montserrat island in the Caribbean has been active since 1995. More than half the island has been made uninhabitable because of continuing dome collapses, pyroclastic flows and vulcanian explosions. In 2003, a small network of borehole strainmeters was installed at ~5 km to ~9 km from the volcano. Since then the strain changes caused by many vulcanian explosions have been recorded. These explosions are short-lived, and result from a sudden decompression of high pressure, vesiculated magma in the conduit. The strain data indicates a decrease in conduit pressure throughout the fragmentation and ejection process that typically occurs in less than 200 seconds. The dense rock equivalent material ejected is usually in the range <400,000 to 1,100,000 cubic meters. Persistence of magma with high-pressure vesicles requires a sealed conduit. In laboratory simulations of fragmentation, the process is usually triggered by a rapid reduction of the gas pressure above a hot rock column, e.g. Spieler et al, 2004. The strain data revealed that at SHV fragmentation is preceded by a rapid (<10 seconds) pressure reduction in the conduit of ~0.5 MPa. This is followed by a strong strain change and vigorous seismic disturbance as is usually observed during vulcanian explosions. The initial pressure reduction is rather similar to the pressure threshold determined for equivalent conditions in the laboratory study. We postulate that a crack in the plug sealing the conduit allows gas to escape, decreasing the pressure at the top of the magma column sufficiently to initiate fragmentation.