A volcanic degassing event at the explosive-effusive transition

A sequence of five Vulcanian explosions followed a lava dome collapse in July 2003 at Soufrie` re Hills Volcano. Each explosion occurred at similar to t = 190 n (4.)3 where n = 1 - 5 and t is the time ( s) since the decompression rate peak during the collapse. Instead of a sixth explosion at the predicted time, a rapid emission of 97 x 10(3) kg SO2 was observed by a spectrometer network. This event represents the transition from explosive to effusive activity. After the last explosion, high magma ascent rates were maintained, but the critical overpressure explosion criterion was not reached. Instead, degassing and crystallisation in the upper conduit caused horizontal gradients in viscosity and flow rate, and brittle failure at the walls when the rate of shear strain exceeded a critical value. Development of a permeable shear zone allowed gas release, relief of overpressure and a return to effusive lava- dome building.