Report on Soufriere Hills (United Kingdom) — 6 August-12 August 2008
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 6 August-12 August 2008
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2008. Report on Soufriere Hills (United Kingdom). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 6 August-12 August 2008. Smithsonian Institution and US Geological Survey.
16.72°N, 62.18°W; summit elev. 915 m
All times are local (unless otherwise noted)
MVO reported that seismic levels from Soufrière Hills were relatively low during 1-8 August. Sulfur dioxide emissions (tons per day) were significantly higher than emissions prior to the partial lava-dome collapse on 28 July. Unconfirmed reports indicated the presence of an ash plume on 3 August. A small ash cloud was seen on 7 August.
Further investigation of the 28 July event revealed that the total amount of material that collapsed from the dome was about 200,000-300,000 cubic meters. Satellite radar images indicated that the vent above Gages wall was enlarged by the explosion to about 150 x 60 meters, elongated E-W. Precautionary access restrictions for areas in and around Belham valley were lifted because evidence suggested that the dome had not been destabilized due to the event.
A new Hazard Level System, designed by MVO and Disaster Management Coordination Agency (DMCA) in consultation with the Government of Montserrat and community groups, was implemented on 8 August. The system divides the southern two-thirds of the island into six zones, and includes two Maritime Exclusion Zones. The Hazard Level ranges from 1-5 and is set by the National Disaster Preparedness, Response, and Advisory Committee (NDPRAC) with advice from MVO. The Hazard Level restricts access into each of the zones depending on the number assigned and is unrelated to the Alert Level. The current Hazard Level was 3.
Geologic Background. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.