Activity at Soufrière Hills during 21-28 March remained at moderate levels. A lava dome continued to grow centrally from its summit region. Activity was dominated by rockfalls and pyroclastic flows that spilled off the active summit in a broad arc extending from the S and around the E flanks to the NW. Most activity was towards the NW, with pyroclastic flows occurring in the Tar River Valley. Small pyroclastic flows also traveled down the dome's N flank into White's Ghaut, Tuitt's Ghaut, the upper reaches of Tyre's Ghaut, and on Farrell's Plain. After a brief, intense rainstorm there was a 4- to 5-hour-long period of increased pyroclastic-flow and rockfall activity. Observations on 27 and 28 March revealed that rockfalls and small pyroclastic flows had begun to spill off the dome's S flank and a large vertical spine had extruded on the S side of the summit. Sulfur dioxide emission rates fluctuated considerably during the report week. The Washington VAAC reported that low-level ash plumes were 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)