Report on Soufriere Hills (United Kingdom) — October 2004
Bulletin of the Global Volcanism Network, vol. 29, no. 10 (October 2004)
Managing Editor: Richard Wunderman.
Soufriere Hills (United Kingdom) Heavy rains cause frequent mudflows and increased seismicity
Please cite this report as:
Global Volcanism Program, 2004. Report on Soufriere Hills (United Kingdom) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 29:10. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200410-360050.
16.72°N, 62.18°W; summit elev. 915 m
All times are local (unless otherwise noted)
Table 58, taken from reports of the Monserrat Volcano Observatory (MVO), summarizes activity at Soufrière Hills between 1 October and 26 November. The activity level remained elevated during much of this time period due to increases in seismicity, gas emission, rainfall, and mudflows.
|Date||Activity Level||Hybrid EQ's||Mixed EQ's||Volcano-tectonic EQ's||Long-period EQ's||SO2 emissions (tons/day)||Rockfalls|
|01 Oct-08 Oct 2004||elevated||8||--||--||2||187-1144||1|
|08 Oct-15 Oct 2004||elevated||9||--||--||--||156- 553||1|
|15 Oct-22 Oct 2004||elevated||49||--||1||--||250-1100||4|
|22 Oct-29 Oct 2004||elevated||40||--||1||--||320-370||--|
|29 Oct-05 Nov 2004||elevated||33||--||39||--||140- 440||1|
|05 Nov-12 Nov 2004||--||21||--||14||--||147- 225||3|
|12 Nov-19 Nov 2004||--||12||--||40||5||1111||3|
|19 Nov-26 Nov 2004||--||25||--||5||1||125-330||3|
Heavy rains during the first six weeks of the reporting period led to steam venting, which triggered an increase in hybrid and volcanic-tectonic earthquakes. A large number of hybrid and volcano-tectonic (VT) earthquakes was recorded during most of October and early November. The most intense seismicity occurred during 2106-2216 on 12 November and 1335-1436 on 14 November.
Following the rains of 5-12 November, several fumaroles developed along the former Tuitt's Bottom and Pea Ghauts, but by 12 November, drier conditions prevailed and fumaroles diminished. Sulfur dioxide emissions remained low throughout most of the reporting period, however two surges in SO2 flux occurred during the weeks of 1 October and 15 October. Mudflows occurred since May. As heavy rainfall continued during October and November, more mudflows occurred. Nine separate mudflow events were recorded for this reporting period. The flows of 15, 19, 21, 22-29 October and 1, 3, 9, and 11 November were minor, though one of the flows, which traveled down the NW flank, reached the Belham River. A much heavier flow began around 0620 on 19 November, with a pulse occurring at 1138.
One MVO scientist deemed mudflows the "ongoing legacy of this [the 1995] eruption." Montserrat's rainy season typically continues until December, and more mudflows may occur in coming months. Mudflows have proven to be destructive, whether they have arisen from short, intense downpours or from a buildup over several rains. The example was given of mudflows after two hours of heavy rain on the afternoon of 21 May, which led to burial of the gateway to the Radio Antilles' offices.
MVO personnel made two observation flights during the reporting period (on 28 October and 4 November). Both flights confirmed the presence of the pond seen 30 August in the pit formed by the 3 March dome collapse. Looking into the crater, MVO scientists found no evidence of ongoing dome-building.
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.
Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).