PYROCLASTIC FLOWS GENERATED BY GRAVITATIONAL-INSTABILITY OF THE 1996-97 LAVA DOME OF SOUFRIERE HILLS VOLCANO, MONTSERRAT

Numerous pyroclastic flows were produced during 1996-97 by collapse of the growing andesitic lava dome at Soufriere Hills Volcano, Montserra t. Measured deposit volumes from these flows range from 0.2 to 9 x 10( 6) m(3). Flows range from discrete, single pulse events to sustained l arge scale dome collapse events. Flows entered the sea on the eastern and southern coasts, depositing large fans of material at the coast. S mall runout distance (<1 km) flows had average flow front velocities i n the order of 3-10 mis while flow fronts of the larger runout distanc e flows (up to 6.5 km) advanced in the order of 15-30 m/s. Many flows were locally highly erosive. Field relations show that development of the fine grained ash cloud surge component was enhanced during the lar ger sustained events. Periods of elevated pyroclastic flow productivit y and sustained dome collapse events are linked to pulses of high magm a extrusion rates.