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Report on Soufriere Hills (United Kingdom) — June 1998

Soufriere Hills

Bulletin of the Global Volcanism Network, vol. 23, no. 6 (June 1998)
Managing Editor: Richard Wunderman.

Soufriere Hills (United Kingdom) Very low levels of activity in late May

Please cite this report as:

Global Volcanism Program, 1998. Report on Soufriere Hills (United Kingdom) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 23:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199806-360050

Soufriere Hills

United Kingdom

16.72°N, 62.18°W; summit elev. 915 m

All times are local (unless otherwise noted)

The following condenses the Monserrat Volcano Observatory's (MVO) Scientific Report for 24 May-1 June 1998. Volcanic activity remained at very low levels during the reporting period. No significant changes in dome morphology occurred and seismic activity was limited to small numbers of volcano-tectonic earthquakes.

Visual observations. There were no pyroclastic flows and only occasional small rockfalls down the upper flanks on the E side of the dome complex. Some small rockfalls originated from the summit area of Galway's dome; they traveled down the SW flanks of the complex.

The temperatures of pyroclastic flow deposits that formed during the 21 September collapse were re-measured on 31 May. At a 2-m depth, maximum temperatures reached 355°C, suggesting that the deposits had not cooled significantly since previously measured on 13 May.

Poor weather conditions hindered visual observations; however, no significant changes are believed to have occurred around the dome complex.

Seismicity. Seismicity was generally low. Intervals of scattered volcano-tectonic earthquakes alternated with intervals of almost complete quiet. One exception was a swarm on 25 May. This swarm consisted of many small signals, most of which did not trigger the networks. The signals were originally considered volcano-tectonic earthquakes because of their frequency content, but they were generally emergent and often had 2-3 velocity maxima. In this sense they differed from the simple rise and decay of most volcano-tectonic earthquakes.

The most striking feature of these signals was the difference in arrival times at different stations (often over 10 seconds). Also the order of arrival at different stations changed from event to event. These observations indicated that the signals were propagated as air-waves and that the source was different each time. Crude time-distance calculations, assuming a velocity of 330 m/s, showed that many of the sources were in the Farms and Upper Gages areas. Thus, it was later concluded that these signals were not volcano-tectonic but presumably caused by small phreatic explosions as water reacted with hot pyroclastic deposits. This conclusion was borne out by comparison with data from July 1995, a time when phreatic activity prevailed and the record showed many seismic events with similar emergent starts and large differences in inter-station arrival times.

Ground deformation. Some of the GPS equipment was on loan to the Seismic Research Unit (Trinidad) who conducted surveys on St. Vincent during this period; thus surveys at Montserrat were reduced. Available data indicated that the motion of the Hermitage site was clearly slowing.

Environmental monitoring. Low volcanic activity kept aerosol levels low, with heavy, typically early morning downpours maintaining some of the lowest airborne dust and ash levels over the last month. A sulfurous haze was visible from, and was occasionally smelled at, the MVO-south station. The haze descended to the W over Fort Ghaut, Gages fan, and Plymouth.

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.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.mvo.ms/).