During 20-27 September, activity at Soufrière Hills' dome complex increased in comparison to the previous week, with a major change in direction of extrusion following a hybrid earthquake swarm the previous week. Growth of the previously active NE lobe stagnated during the 21st to 22nd. A near vertical spine was extruded in the central area around the 21st, possibly indicating a switch in growth direction. Observations on the 26th revealed a large new lobe that had extruded towards the W in the area previously known as Gages Wall. Material spalling off of this lobe produced rockfalls and small pyroclastic flows down Gages Valley for up to 1 km. The most notable events were pyroclastic flows on the evening of the 25th and the morning of the 27th. Growth and rockfall activity then changed towards the northern flanks, suggesting a possible stagnation of the recently extruded western lobe. Spectacular incandescence and semi-continuous rockfall activity were observed on the NE and N flanks of the dome on the night of the 26th and the early hours of the 27th. The Washington VAAC reported that a low-level ash cloud from an emission on the 29th at 1510 was visible over E Puerto Rico on satellite imagery through the following day. On the 30th a light dusting of white ash fell in E Puerto Rico at Roosevelt Roads Naval Air Station.

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)