The explosive eruption of Soufriere Hills Volcano, Montserrat, West Indies, 17 September, 1996

On 17 September 1996 the Soufriere Hills Volcano started a 9 hour period of dome collapse involving 11.7 x 10(6) m(3) (DRE) of lava. After 2.5 hours of quiescence a sustained explosive eruption began. Estimated eruption parameters are: plume height at least 11.3 km and a maximum of 15 km; 180 m/s for launch velocities of ballistic clasts; peak explosion pressure of 27.5 MPa; magma water contents of 2.5-5%; magma discharge rates from 2300 to 4300 m(3)/s; ejecta volume of 3.2 (+/- 0.9) x 10(6) m(3) (DRE). Ejecta consists of pumice (rho = 1160 kg/m(3)), higher density vesiculated ejecta (rho = 1300 to 2000 kg/m(3)), dense glassy clasts (rho = 2600 kg/m(3)), breccias cut by tuffisite veins and hydrothermally altered lithics. The ejecta are interpreted as a conduit assemblage with evacuation of the conduit down to depths of about 3 to 5 km. The eruption was triggered by unloading of a gas pressurised conduit due to dome collapse.