Report on Soufriere Hills (United Kingdom) — 30 September-6 October 2009
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 30 September-6 October 2009
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2009. Report on Soufriere Hills (United Kingdom). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 30 September-6 October 2009. 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 a short volcano-tectonic earthquake swarm from Soufrière Hills lava dome was detected at 2100 on 4 October. A period of tremor and vigorous ash venting followed about an hour later. The resulting ash plume drifted WNW across the island and out to sea, causing ashfall in Old Towne and Olveston. The seismic signals indicated no explosive activity or pyroclastic flows, but only two rockfalls after the ash-venting event. During midnight to 0600 on 5 October, intermittent ash venting produced ash plumes that drifted WNW. Two more "ash venting" events occurred at 1035 and 1325, without precursory seismicity, producing ash plumes that rose to altitudes of 3-4.6 km (10,000-15,000 ft) a.s.l. Ash fell S of inhabited areas. Based on information from MVO and analyses of satellite imagery, the Washington VAAC reported that on 6 October several ash clouds rose to altitudes of 3.7-5.5 km (12,000-18,000 ft) a.s.l. and drifted W.
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.