Report on Soufriere Hills (United Kingdom) — 11 December-17 December 2002
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 11 December-17 December 2002
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2002. Report on Soufriere Hills (United Kingdom). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 11 December-17 December 2002. Smithsonian Institution and US Geological Survey.
16.72°N, 62.18°W; summit elev. 915 m
All times are local (unless otherwise noted)
Reports of activity at Soufrière Hills covered the interval 6-13 December. Activity increased during the first 3 days of this interval, peaking in a dome-collapse event on the night of 8 December. Following this event, activity returned to moderate levels for the remainder of the week.
On 6 and 7 December most activity occurred on the N and NE flanks of the active extruded lobe, producing numerous pyroclastic flows in Tuitt's and White's ghauts, and the Tar River Valley. On 8 December, activity was focused NNE, producing numerous small-to-moderate pyroclastic flows in White's Ghaut. A sustained dome collapse began on 8 December at 2045, producing energetic pyroclastic flows down White's Ghaut to the sea at Spanish Point. Ash clouds rose to ~3 km a.s.l. and drifted WNW. In Plymouth and Richmond Hill 4 mm of ash was deposited. Seismic activity returned to background levels on 9 December by 0045 and several days of weak tremor occurred.
The collapse scar formed on the dome's NNE flank was estimated to have had a volume of 4-5 million cubic meters. This was being filled rapidly with freshly extruded lava. Observations on 13 December revealed a large amount of fragmental lava extruded in a northerly direction on the summit. A large spine was also extruded on the NW side of the summit.
SO2 emission rates were generally low during the first 3 days of the report period (280 metric tons per day on average), but following the dome-collapse event, on 9 December, they reached an average of 2,350 tons per day. On 10 December emission rates decreased to an average of 620 tons per day.
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.