During 15-22 December, moderate lava-dome growth at Soufrière Hills continued and was focused on the SW part of the dome. Rockfalls and pyroclastic flows occurred on the S side of the dome and in the upper parts of both White's and Tuitt's Ghauts. Based on satellite imagery and pilot reports, the Washington VAAC reported that steam emissions with variable amounts of ash drifted W on 23 and 24 December. A pilot reported that an ash plume rose to an altitude of 3 km (10,000 ft) a.s.l. and drifted W on 24 December. The MVO reported that the emissions originated from a vent on the W side of the dome. A small pyroclastic flow was visible NW in the upper reaches of Tyre's Ghaut and venting became more energetic. The alert level was raised to 4 (on a scale of 0-5). At night, incandescent rockfalls were visible from the NW and W.

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)