MVO reported that during 19-26 December activity from Soufrière Hills lava dome was characterized by increased lava extrusion, rockfalls, and pyroclastic flows. Lava extrusion on the N, W, and SW sides of the dome continued and incandescence on the dome was visible at night when weather was favorable. Rockfall events increased by 80 percent compared to the previous week. Pyroclastic flows began to enter Tyers Ghaut (NW) on 20 December and likely reached the bottom of the ghaut (ravine) on 21, 23, and 25 December. On 22 December, the Hazard Level was increased to 4 due to the repeated occurrences of pyroclastic flows in the lower part of Tyers Ghaut. On 24 December, a large pyroclastic flow that reached Plymouth (about 5 km W), and possibly the sea, generated an ash plume to an altitude of 3 km (10,000 ft) a.s.l. Large incandescent blocks, deposited by rockfalls and pyroclastic flows, were visible on multiple occasions at night in the upper and middle parts of Tyers Ghaut.

Based on analysis of satellite imagery, the Washington VAAC reported that during 26-30 December ash plumes drifted W, WSW, SW, and S. Intermittent thermal anomalies were detected on satellite imagery on 27 December. Plumes rose to altitudes of 2.1-4.9 km (7,000-16,000 ft) a.s.l. on 28 and 30 December.

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)