Report on Soufriere Hills (United Kingdom) — 17 March-23 March 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
17 March-23 March 2004
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2004. Report on Soufriere Hills (United Kingdom). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 17 March-23 March 2004. Smithsonian Institution and US Geological Survey.
16.72°N, 62.18°W; summit elev. 915 m
All times are local (unless otherwise noted)
Seismic and volcanic activity at Soufrière Hills during 12-19 March increased in comparison to the previous week. Seismicity was dominated by moderate-to-strong tremor beginning around 0300 on 15 March, with five episodes of moderate tremor. The tremor was accompanied by gas-and-ash venting. At 1622 on 15 March an increase in the energy level of the tremor was recorded and a convecting ash cloud rose to ~2 km above the volcano. At 1745 emergent pulsating ash clouds were observed, although no eruption column was established. Ash drifted WSW away from populated areas over Plymouth, Amersham, and areas farther S. Tremor remained at elevated levels throughout the night peaking around 2245, coincident with a vigorous venting episode that produced an ash cloud to ~4.5 km above the volcano accompanied by lightning. Vigorous gas-and-ash venting continued through 16 March. The amount of ash in the gas plume decreased during 17 March and ash venting had all but ceased by the end of the day. The sulfur-dioxide flux remained at moderate levels during the report period.
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.