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Report on Soufriere Hills (United Kingdom) — 3 December-9 December 2008

Soufriere Hills

Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 3 December-9 December 2008
Managing Editor: Sally Sennert.

Please cite this report as:

Global Volcanism Program, 2008. Report on Soufriere Hills (United Kingdom) (Sennert, S, ed.). Weekly Volcanic Activity Report, 3 December-9 December 2008. Smithsonian Institution and US Geological Survey.

Weekly Report (3 December-9 December 2008)

Soufriere Hills

United Kingdom

16.72°N, 62.18°W; summit elev. 915 m

All times are local (unless otherwise noted)

MVO reported a total of four explosive events from the Soufrière Hills lava dome during 2-5 December. The first event was triggered by a small dome collapse on 2 December, occurred without precursory seismicity, and was followed by a pyroclastic flow on the W flank. Resultant ash plumes, accompanied by lightning strikes, rose to an altitude of 12.2 km (40,000 ft) a.s.l. and drifted W. Explosions ejected incandescent blocks up to 1.6 km away from the dome that landed on Gages Mountain (about 1 km WNW), leaving impact craters. The pyroclastic flow also generated multiple pyroclastic surges that traveled S and N, setting fire to trees and bushes.

On 3 December another explosion scattered incandescent blocks all over the NW side of Gages Mountain. The third eruptive event, forceful emissions of ash on 4 December, resulted in ash plumes that rose to an altitude of 4.6 km (15,000 ft) a.s.l. The fourth explosive event occurred on 5 December and ejected incandescent blocks that were deposited on the NW side of Gages Mountain. A pyroclastic flow traveled to the W down Gages valley into Plymouth (about 5 km W) and an ash plume drifted NW.

On 3, 4, and 5 December small, relatively slow moving pyroclastic flows traveled no more than 3.2 km down the Gages valley. In periods between the events, near-continuous emissions of ash-laden steam were noted. The Hazard Level remained at 3.

According to the Washington VAAC, MVO reported eruptions on 6 December. Ash was seen on satellite imagery expanding in multiple directions, then to the E, SE, and W.

Geological Summary. 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.

Sources: Montserrat Volcano Observatory (MVO), Washington Volcanic Ash Advisory Center (VAAC)