Report on Soufriere Hills (United Kingdom) — 19 October-25 October 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 19 October-25 October 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, 19 October-25 October 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 14-21 October, elevated seismic and volcanic activity continued at Soufrière Hills. A majority of the volcano-tectonic earthquakes recorded during the week occurred during a 36-hour period over 17-18 October when a lobe on the S part of the lava dome was developing. At this time there were also a large number of rockfalls, which continued until 20 October. The daily sulfur-dioxide flux averaged 658 metric tons per day (t/d), above the long-term eruption average of 500 t/d. The hydrogen-chloride versus sulphur-dioxide ratio increased to about 1.15. Limited camera observations suggested that the lava dome was growing at a rate of less than 2 cubic meters per second.
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.