During 21-28 December volcanic activity increased at Soufrière Hills, with a large number of rockfalls, an increase in SO2 flux, and periods of continuous ash emission. Most lava-dome growth was concentrated on the E side of the dome, where spectacular incandescent rockfalls were seen on the nights of 26 and 27 December. Rockfall seismicity increased throughout the week. There was a large increase in SO2 flux; an average emission rate of 851 metric tons was measured on 27 December in comparison to 181 metric tons on 19 December. The increase in SO2 flux coincided with an increase in rockfalls. According to the Washington VAAC, continuous ash-and-steam venting beginning on 27 December at 0315 produced an ash plume that remained below ~3 km a.s.l. A large area of dense ash below 3 km a.s.l. was observed on satellite imagery from an emission that began on 28 December at 1330. On 1 January a high number of rockfalls generated low-level ash clouds (up to 1.5 km).

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)