MVO reported that during 12-19 December activity from Soufrière Hills lava dome was characterized by increased lava extrusion, ash venting, rockfalls, and pyroclastic flows. Frequent pulses of ash rose from multiple places on the NW face of the lava dome and from a low on the dome behind Gages Mountain (as seen from Salem). On 13 December a pyroclastic flow traveled E down the Tar River Valley and reached the sea. Nighttime incandescence from the NW face was present during 16-19 December. Frequent rockfalls and several small pyroclastic flows descended Gages Valley. The largest pyroclastic flow, on 17 December, produced an ash cloud that rose to an altitude of 3 km (10,000 ft) a.s.l. On 18 December, observations of the lava dome confirmed significant growth on the SW flank. Photographs showed that most of the growth had taken place since 8 December; lava was filling in the area between the lava dome and Chance's Peak. Initial calculations suggested that the dome grew at a rate of 1 cubic meter per second during this time. Two small pyroclastic flows descended Galway's Valley on 19 December.

Based on analysis of satellite imagery and information from MVO, the Washington VAAC reported that during 19-23 December ash plumes drifted W, WSW, SW, and S. Thermal anomalies were detected on satellite imagery on 19 and 21 December. A pilot observed an ash plume at an altitude of 4.3 km (14,000 ft) a.s.l. on 20 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)