Report on Soufriere Hills (United Kingdom) — 9 July-15 July 2003
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 9 July-15 July 2003
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2003. Report on Soufriere Hills (United Kingdom). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 9 July-15 July 2003. Smithsonian Institution and US Geological Survey.
16.72°N, 62.18°W; summit elev. 915 m
All times are local (unless otherwise noted)
High levels of pyroclastic-flow activity began at Soufriere Hills on 12 July. A hybrid earthquake swarm that began on 9 July merged at 0700 on 12 July into a continuous tremor signal. Prolonged and heavy rainfall occurred on the 12th during 0600-0900, causing mudflows into the Belham Valley. Pyroclastic flows began to travel into the Tar River Valley, with a moderate-sized flow occurring at 0653. A series of similar-sized pyroclastic flows traveled in the Tar River Valley throughout the morning. The first pyroclastic flow reached the sea at 1045. Pyroclastic-flow activity increased slowly through the afternoon until it became almost continuous. Flows also occurred into Tuitt's and White's ghauts. The activity picked up markedly at 1827, with more energetic pyroclastic flows. The level of activity fluctuated thereafter, with several smaller pyroclastic flows into the Tar River Valley, before escalating again at 2005 with another phase of near-continuous pyroclastic flows. The flows increased in size and several surges traveled 2 km over the sea at the mouth of the Tar River Valley. Pyroclastic flows also reached the sea in White's Ghaut and the Spanish Point area. These flows resulted in heavy ashfall and accretionary lapilli, particularly between Salem and Woodlands. A number of explosive events took place during this collapse, with the largest occurring between 2300 and 2400. The Washington VAAC reported that ash clouds rose to a maximum height of ~15 km a.s.l.
Heavy falls of ash and rock fragments occurred over all of the inhabited parts of Montserrat. The ashfall deposit was 115 mm thick at Lime Kiln Bay. The ash burden resulted in the collapse of several wooden buildings in the Salem area. Vegetation damage was extensive with downed trees and branches broken from many others. Many birds were killed by the ash or trapped alive in it. Ashfall from this event was reported on the islands of Nevis, St Kitts, Anguilla, and St Maarten, and resulted in the closure of several airports. At 0910 on 13 July an explosive eruption occurred, following 2 hours of very low seismic activity. The Washington VAAC estimated a cloud height of ~12 km a.s.l.
During a helicopter reconnaissance flight on the morning of 14 July, a large collapse scar was seen in the lava dome directed down the Tar River Valley. The Tar River Valley was extensively modified and eroded with a deep canyon gouged by the pyroclastic flows. The fan had been extended eastwards into the sea and northwards along the coast. The area north of the Tar River Valley extending to Killyhawk Ghaut was devastated.
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.