During 14-21 December, volcanic activity at Soufrière Hills remained high. Observations of the lava dome on 16 December revealed that it had increased in volume since it was last seen in late November. The top of the dome was broadly rounded and had a blocky appearance. Most of the growth appeared to occur on the W side of the dome, but rockfalls and small pyroclastic flows also occurred on the E and S flanks. Periods of weak, but sustained ash-venting occurred on most days and seismicity was dominated by rockfall signals. Intense rainfall on the morning of 21 December produced large mudflows NW of the volcano in Belham Valley.

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)