Report on Soufriere Hills (United Kingdom) — 15 February-21 February 2006
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 15 February-21 February 2006
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2006. Report on Soufriere Hills (United Kingdom). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 15 February-21 February 2006. 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 vigorous ash-and-steam venting at Soufrière Hills on 10 February, a small dark lobe of lava was observed on the western side of the lava dome in the crater. By early on 11 February this lobe had advanced rapidly towards the NE side of the dome and was visible as a steep-sided plateau of lava from inhabited areas around Salem. Photographs from fixed cameras showed continued changes to this lava lobe over the next few days, and the NE margin could be seen glowing at night and shedding rockfalls into the NE part of the crater. The initial growth rate of this lobe was well over 5 cubic meters per second, but the rate declined around 17 February. The new lava lobe began to fill the gap between the lava dome and the northern and western crater walls, raising the possibility that small rockfalls could spill over those areas in coming weeks.
The sulfur-dioxide flux averaged 568 metric tons per day. Data from Fourier Transform Infra Red spectrometry measurements indicated an increase in the hydrogen chloride/sulfur dioxide mass ratio in the gas plume from 2.0 in the last reporting period to an average of 2.5 on 13 February.
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.