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Report on Soufriere Hills (United Kingdom) — 26 November-2 December 2008

Soufriere Hills

Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 26 November-2 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, 26 November-2 December 2008. Smithsonian Institution and US Geological Survey.

Weekly Report (26 November-2 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 that during 21-28 November the activity level at the Soufrière Hills lava dome remained low, and there was no evidence of lava extrusion. Rockfalls were detected by the seismic network. The lava dome continued to emit steam vigorously from multiple places, including new fumarolic areas on the W and S sides. Long-exposure photographs revealed several hot spots on the dome. The vertical cliff face on the W side of the dome was cracked in several places and erosion was evident at the base.

At approximately 2135 on 2 December, an explosion occurred on the W side of the lava dome without any precursory seismicity. Large blocks were ejected up to 1 km from the dome and incandescent blocks were scattered over the NW side of Gages Mountain (about 1 km WNW). A pyroclastic flow traveled to the W down Gages valley into Plymouth (about 5 km W), and further to the sea. Buildings in Plymouth caught fire and could be seen burning from Salem (about 4 km N of Plymouth) for several hours afterwards. Ash plumes were accompanied by lightning strikes and drifted W. The Hazard Level remained at 3.

Based on analysis of satellite imagery, the Washington VAAC reported that on 28 November a puff of ash and steam drifted SW and S.

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)