Report on Soufriere Hills (United Kingdom) — 31 December-6 January 2009
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 31 December-6 January 2009
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2008. Report on Soufriere Hills (United Kingdom). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 31 December-6 January 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 during 26 December-2 January activity from Soufrière Hills lava dome was characterized by significantly increased lava extrusion, ash emissions, and pyroclastic flows. Lava extrusion on the top, N, W, and SW sides of the dome continued, and incandescence on the dome was visible at night when weather was favorable. Pyroclastic flows regularly reached the bottom of Tyers Ghaut (NW); surges associated with the larger flows spilled into the next valley to the W. Deposits filling Tyers Ghaut caused the flows to travel farther, into the upper part of the Belham River. Pyroclastic flows were also noted in valleys to the W. Ash emissions from the top of the lava dome increased; although most pyroclastic flows originated from rockfalls, some originated at the vent. Ashfall was reported in areas 6-7 km NW. Large incandescent blocks, deposited by rockfalls and pyroclastic flows, were visible on multiple occasions at night in the lower parts of Tyers Ghaut. Fires triggered by surges were visible in the neighboring valley. The Hazard Level remained at 4.
Based on analysis of satellite imagery and information from MVO, the Washington VAAC reported large eruptions on 3 January. Ash plumes drifted NE at an altitude of 7.6 km (25,000 ft) a.s.l., E at an altitude of 10.7 km (35,000 ft) a.s.l., S at an altitude of 6.1 km (20,000 ft) a.s.l., and W at an altitude of 2.4 km (8,000 ft) a.s.l. A thermal anomaly was detected. According to news articles, about 70 people were evacuated from Area B, about 6-8 km NW. The next day, steam-and-gas plumes possibly containing ash drifted W and WSW.
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.