On 25 June 1997, an unsteady, retrogressive, partial collapse of the lava dome at Soufrière Hills Volcano lasted 25 minutes and generated a major pulsatory block-and-ash flow, associated pyroclastic surges and a surge-derived pyroclastic flow that inundated an area of 4 km2 on the north and NE flanks of the volcano. Three main pulses are estimated to have involved 0.78, 2.36 and 2.36 x 106m3 of debris and the average velocities of the fronts of the related block-and-ash flow pulses were calculated to be 15 ms-1, 16.1 ms-1 and 21.9 ms-1 respectively.Deposits of block-and-ash flow pulses 1 and 2 partially filled the main drainage channel so that material of the third pulse spilled out of the channel at several places, inundating villages on the eastern coastal plain. Bends and constrictions in the main drainage channel, together with depositional filling of the channel, assisted detachment of pyroclastic surges from the pulsatory block-and-ash flow. The most extensive pyroclastic surge detached at an early stage from the third block-and-ash flow pulse, swept down the north flank of the volcano and then climbed 70 m in elevation before dissipating. Rapid sedimentation from this surge generated a high-concentration granular flow (surge-derived pyroclastic flow) that drained westwards into a valley not anticipated to be at high risk.Observations support the hypothesis that the interior of the Soufrière Hills Volcano lava dome was pressurized and that pyroclastic surge development became more substantial as deeper, more highly pressurized parts of the dome were incorporated into the pyroclastic flow. Surge development was at times so violent that expanded clouds detached from the block-and-ash flow within a few tens of metres of the lava dome.
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