Volcanic activity at Soufrière Hills was generally at high levels during 16-23 May, although there was a slight decrease in intensity on 19 and 20 May. Most activity was focused on the lava dome's NE flank, producing rockfalls and numerous pyroclastic flows along the northern side of Tar River Valley. During the last 3 days of the report period, activity increased on the N flank, with pyroclastic flows traveling into the top of Farrell's Plain and entering the top of Tyre's and Tuitt's ghauts. Pulses of vigorous ash venting were observed from the dome's summit. Intense glow was seen on the summit and NE flank on the evenings of 20 and 21 May. Low-level ash plumes were sometimes visible on satellite imagery.

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