Report on Soufriere Hills (United Kingdom) — 6 July-12 July 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 6 July-12 July 2005
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2005. Report on Soufriere Hills (United Kingdom). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 6 July-12 July 2005. Smithsonian Institution and US Geological Survey.
16.72°N, 62.18°W; summit elev. 915 m
All times are local (unless otherwise noted)
During 1-8 July, seismic and volcanic activity at Soufrière Hills remained at elevated levels. The seismic network recorded 15 hybrid earthquakes, 11 long-period earthquakes, 9 volcano-tectonic earthquakes, and 11 rockfalls. Periodic ash venting continued and an explosion occurred on 3 July at 0130, which was similar to an explosion on 28 June. The reversal of deformation to an inflationary trend that began in mid-July continued during the report period. The daily recorded sulfur-dioxide flux varied from 241 metric tons per day (t/d) on 4 July to 1700 t/d on 1 July, with an average of 767 t/d for the week. This was above the long-term average for the eruption of 500 t/d.
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.