MVO reported that on the morning of 20 May a major lava-dome collapse at Soufriére Hills occurred over a time period of less than three hours. Approximately 90 million cubic meters of the lava dome material was shed from the summit leaving a broad, deep, eastward-sloping crater. Pyroclastic flows traveled E down the Tar River Valley and were estimated to extend out to 3 km over the sea. Lahars due to excessive rain were produced NW in the Belham River Valley, N in the Trants area, and to the NE. An ash cloud reached 16.8 km (55,000 ft) a.s.l. by 0740, the highest reported ash cloud during the 10 years of the eruption, and traveled NW. Lithics (average size of 3.5 cm across) fell NW of the volcano. On 21 May, ash and mud fell on the northern parts of the island. Prior to the lava-dome collapse, during 12 May and 19 May, lava extrusion had continued.

The Washington VAAC reported that the ash plume from the 20 May dome collapse initiated at approximately 0700. On 21 May, the remnant ash cloud from 20 May was at a height of ~11.3 km (37,000 ft) a.s.l. along the northern coast of South America and the Southern Caribbean. An ash cloud at a height of ~7 km (23,000 ft) a.s.l. extended S of Puerto Rico and the Dominican Republic. According to news reports, the ash cloud on 20 May forced the suspension of some international flights in areas of the Caribbean through 21 May. On 22 May, multi-spectral imagery indicated that an ash plume at a height of ~3 km (10,000 ft) a.s.l. extended over the islands of Anguilla, St. Martin, and St. Kitts. On 23 May, a thin ash plume was visible on satellite imagery and moved WNW.

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), Associated Press, Montserrat Volcano Observatory (MVO)