Volcanic activity at Soufrière Hills during 21-28 September was similar to the previous week, except for a slight reduction in the number of hybrid earthquakes. Seismicity continued to be dominated by bands of hybrid earthquake swarms and increased rockfall activity with periodicities of approximately 11 to 13 hours. The active lava dome continued to grow at a moderate rate, producing rockfalls and small but energetic pyroclastic flows that traveled E to the upper reaches of the Tar River Valley. Periods of vigorous ash venting were associated with the hybrid-earthquake swarms. Steam-and-ash clouds were visible on satellite imagery rising to ~500 m above the volcano.

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)