Soufrière Hills was relatively quiet through April and early May 2005, with activity increasing somewhat through June and several explosive events in late June and in July (BGVN 30:06). Table 61 summarizes activity during 8 July thorough 26 August 2005. Further text brings this report through 5 September, with the comment that slow dome growth continued.

Table 61. A summary of the weekly number of earthquakes (EQs), rockfalls, and averaged spot measurements of SO₂ flux at Soufriere Hills during July and August 2005. Cases of "mixed earthquakes" were unreported during the reporting interval. Date ranges go from noon on the starting day to noon on the end day. Courtesy of MVO.

On 6 August a vigorous eruption sent a plume to ~ 1.8 km above the volcano. Evidence of uplift and fracturing were observed on the crater floor, and an area of blocky lava resembling a small lava dome was observed. Due to poor visibility further observations will be necessary to determine if the feature is a new dome or was caused by the collapse, or uplift, of old dome rock.

Volcanic and seismic activity remained at elevated levels at Soufrière Hills during 12-19 August. Periodic ash venting continued, with a vigorous episode occurring on 18 August at 1800. On 16 August, the presence of a small blocky lava dome with talus slopes was confirmed. There was some ash venting from the dome, but no significant rockfalls were seen. Activity at Soufrière Hills remained at elevated levels during 2-9 September, the end of this reporting period. Observations made on 5 September suggested that slow lava-dome growth continued.

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), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).