Report on Soufriere Hills (United Kingdom) — 2 October-8 October 2002
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 2 October-8 October 2002
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2002. Report on Soufriere Hills (United Kingdom). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 2 October-8 October 2002. Smithsonian Institution and US Geological Survey.
16.72°N, 62.18°W; summit elev. 915 m
All times are local (unless otherwise noted)
Volcanic and seismic activity at Soufrière Hills increased significantly during 26 September to 4 October following the previous week's major switch in lava-dome extrusion direction. On the 27th a 4-hour-period of heightened activity occurred in the afternoon and evening, with small semi-continuous pyroclastic flows traveling down the N flanks and eastwards into the upper portions of Tuitts Ghaut and then into Whites Bottom Ghaut. A newly extruded lobe was visible on the 28th almost directly to the NW with a broad headwall over the N, NW, and W flanks. On the evening of the 29th there was another period of heightened activity on the northern flanks that lasted 1.5 hours, with pyroclastic flows just reaching the sea along Whites Bottom Ghaut. It was estimated that during this small event only 2-3 million m3 of the N edge of the active NW lobe was shed. Observations on 1 October revealed that re-growth of the collapsed area had occurred. A brief period of heavy rain on the 2nd triggered a moderate-sized mudflow down the Belham Valley. Analysis of seismic data suggested that pyroclastic-flow activity on the 2nd began at 13:10 and sustained dome collapse continued for 6 hours. Low-energy pyroclastic flows were observed reaching the sea on the Tar River's flanks throughout the collapse, and ash clouds were produced that drifted to the NW. Heavy ashfall occurred in the residential areas of Salem, Old Towne, and Olveston, with deposits up to 9 mm thick. Subsequent observations revealed that this collapse was confined to the volcano's eastern flanks, and that this was again a relatively small event (less than 5 million m3 of material was shed off of the eastern side of the dome complex).
According to the Washington VAAC, after daybreak on 3 October there were several reports of ashfall in Puerto Rico, and visible satellite imagery at 1115 confirmed that an ash cloud at a height of around 2.4 km a.s.l. covered most of the island. At 1615 the area of very thin ash was not visible on satellite imagery. By the next day, ash from the previous day's emissions had drifted W and around 0902 it was located over southern Puerto Rico nearest to the city of Ponce. A thin plume of ash also extended SSW of St. Croix island.
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.