Report on Soufriere Hills (United Kingdom) — 3 March-9 March 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
3 March-9 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, 3 March-9 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)
Volcanic and seismic activity increased significantly at Soufrière Hills during 27 February to 5 March, with a lava-dome collapse on 3 March. Low-level tremor occurred from around 1900 on 2 March until 1444 on 3 March when seismicity increased significantly and an explosion and lava-dome collapse event lasting about 10 minutes occurred. According to the Washington VAAC, the ash cloud produced by the explosion reached a height of ~6 km a.s.l. During 1445-1500, pyroclastic flows swept NE down the Tar River, reaching the sea at the Tar River Fan at least twice. By 1525 seismicity had returned to background levels, although vigorous ash venting continued until around 0700 on 4 March. Low-level tremor began soon after the main event, lasting ~18 hours. Several hybrid earthquakes occurred during the evening of 3 March.
No ash fell in populated areas, rather it drifted SW over the southern parts of Plymouth, Amersham, and areas farther S. Visual observations suggested that the explosion removed the small lava dome that had grown in the collapse scar in late July 2003. A portion of the north-western remnant of the 1995-1998 lava dome also collapsed during the event. A small explosion on 5 March at 1009 was followed by ash venting. During the report period, the sulfur-dioxide flux reached a maximum value of 820 metric tons around 1 March, before falling to 540 metric tons on 2 March. As of 5 March seismic and volcanic activity remained at elevated levels.
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.