MVO reported that during 15-22 September the lava dome at Soufrière Hills continued to grow at a moderate rate, slower than earlier in the month. Growth appears to have occurred predominantly in the summit area and on the S and E sides of the dome. The vent situated in the Gages Wall is still active, with minor explosive activity seen during an observation flight on 19 September. An intense 30-minute episode of volcanic tremor on 19 September was accompanied by rockfall activity that caused minor pyroclastic flows down the N and NE flanks of the lava dome. On 22 September the volume of the dome was about 80 million cubic meters. Seismicity was dominated by rockfalls with a significant drop in earthquake activity relative to the previous reporting period. The alert level was reduced to 3 (on a scale of 0-5) on 21 September. Aviation ash advisories during this period noted continuous ash emissions.

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), Montserrat Volcano Observatory (MVO)