Based on satellite imagery and reports from the MVO, the Washington VAAC reported that the Soufrière Hills lava dome collapsed at 1740 on 29 August. Ash venting was seen on satellite imagery prior to the collapse at 0615 and 1245. The plumes reached altitudes of 2.4 km (8,000 ft) a.s.l., drifted N, and then shifted to the W and SW. According to the Antigua Aircraft Tower, the dome collapse produced a plume that rose to an altitude of about 9 km (30,000 ft) a.s.l. The upper portion of the cloud drifted E and the lower portion possibly drifted N and W. Pyroclastic flows reached the sea down the Tar River Valley.

On 30 August, small pyroclastic flows were visible on the NE and S flanks of the lava dome. On 31 August, two vigorous ash-and-steam vents opened on the W and N flanks accompanied by tremor. A pilot reported an ash plume at an altitude of 4.6 km (15,000 ft) a.s.l. drifting W. The Alert Level was raised to 4 (on a scale of 0-5). The Washington VAAC reported continuous gas and ash emissions during 1-4 September; plumes reached altitudes of 3 km (10,000 ft) a.s.l. and drifted W.

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.

Sources: Washington Volcanic Ash Advisory Center (VAAC), Montserrat Volcano Observatory (MVO)