Report on Soufriere Hills (United Kingdom) — 8 August-14 August 2012
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 8 August-14 August 2012
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2012. Report on Soufriere Hills (United Kingdom). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 8 August-14 August 2012. 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 during 3-10 August activity at the Soufrière Hills lava dome was mostly at a low level. The seismic network detected two small swarms of volcano-tectonic earthquakes on 7 and 8 August. Scientists at MVO observed a period of ash venting that began at 1700 on 8 August, less than two hours after the second swarm. Roaring sounds were heard at the same time. The ash plume drifted W over Plymouth at an altitude of about 1 km (3,280 ft) a.s.l., and a small amount of ashfall was reported by a fisherman offshore. The source of the venting appeared to be the gas vent in the floor of the 11 February 2010 collapse scar, and not the crater created on 23 March 2012.
On 9 August the inside of the collapse scar was partially visible during a helicopter flight. Fumarolic activity in the 23 March crater had increased compared to two weeks ago, and some other fumaroles were also more active. A change in wind direction shifted the volcanic plume N for much of the day and the odor of volcanic gas was noticeable in some inhabited areas. The Hazard Level remained at 2 (on a scale of 1-5).
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.