The Thompson Report

The eruption since 1995 of the Soufrière Hills Volcano (SHV) can be reduced to a relatively simple story. After some preliminary throat-clearing explosions, the characteristic very viscous andesite lava of the volcano has emerged from various points near the summit of the pre-eruption volcano, dormant for about 400 years. The emerging lava has built a series of domes because its high viscosity prevents the lava from flowing sideways. The old crater into which these domes have formed is incomplete, with an open segment facing the sea to the east down the Tar River valley. This means that the domes have all been unsupported and potentially likely to collapse on their eastern sides. At times they have also overtopped the other walls of the old crater and thus menaced other sectors around SHV. The volcanic events that have happened have either been related to collapses of a dome or to violent explosions when gases trapped within the lava deep below the surface have had an opportunity to decompress rapidly and thus blast towards the surface. Dome collapses down the Tar River valley have ranged from rock falls to massive events, involving most of a dome. Collapses of domes in other directions (over the old crater walls) have been smaller but dangerous because they approached inhabited areas. A third type of collapse (with huge accompanying explosion) occurred on 26 December 1997, when a dome pushed down a weak segment of the old crater wall.

Until recently, gravity and the Tar River valley have acted as controls on these lava domes because, when they grew large enough, they simply collapsed into the sea to the east. The accompanying explosions were enormous but did nothing worse to the island population than showering them with volcanic ash. But the latest dome has so far (January 2009) behaved differently. It developed slightly to the NW of previous ones and its partial collapse, with explosions, over the old crater wall on 8 January 2007 sent a pyroclastic flow and surge down the Belham River valley to within a mile of residents. Since then, this large dome has refused to follow the pattern set by its predecessors and collapse wholesale down Tar River valley to the east. There is no obvious reason why this has happened – it remains a mystery and scientific opinion is that such a collapse is almost inevitable at any moment. The problem for Montserrat residents and government is that this dome is so large that, if the part overlooking the Belham River valley was all to collapse (together with an accompanying explosion), computer modeling of such an event suggests that the hot debris (pyroclastic flow) and accompanying hot gas blast (pyroclastic surge) might possibly reach part or all of the populated area between the Belham and Nantes rivers.

Throughout 2007 and much of 2008 the problem was minimized by a pause in the lava extrusion, so that the dome just sat there and slowly cooled. Then both explosive and non-explosive lava emission on the dome started again at the end of July 2008 and have continued (with pauses) since. At first the new activity was concentrated on the west flank of the dome, above the ruins of Plymouth. Then, at the end of 2008, the action switched to the NW slopes of the dome. New dome lava accumulating in this sector collapses down Tyre’s Ghaut and could eventually smooth the landscape below so much that debris from future collapses and explosions reach too far down the Belham River valley for comfort. The labeled photograph (from the June 2007 helicopter series) is useful to refer to when reading recent MVO weekly reports. It shows clearly how small differences in the sites of eruptive vents on the W and NW sides of the present dome can lead to pyroclastic flows and surges passing either down Gages Valley (towards Plymouth) or down Tyre’s Ghaut, towards the Belham Valley.

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