MVO reported that during 16-23 March activity at the Soufrière Hills lava dome was at a low level, although seismicity increased. Two swarms of volcano-tectonic earthquakes occurred, the first between 1604 and 1651 on 22 March (49 events) and the second between 0310 and 0527 on 23 March (54 events). Earthquakes in the second swarm were markedly larger than those in the first. Several changes on the volcano were observed on 23 March; fumarolic activity had increased and a new fumarole had appeared on the NW face of the lava dome behind Gages Mountain. In addition a vent producing pulsing steam emissions with a small amount of ash had formed in the back of the February 2010 collapse scar. Ash plumes rose 1.8 km (6,000 ft) a.s.l. and very light ashfall occurred on the W flank of the volcano. Audible roaring associated with the venting was heard intermittently from MVO, 5.75 km NW of the volcano. The Hazard Level remained at 2.

Based on a METAR weather report and analyses of satellite imagery, the Washington VAAC reported that on 24 March a gas-and-ash plume drifted 65 km WSW. Later that day the VAAC reported a detached area of ash drifting NW and a second small emission of ash drifting WNW. The next day haze and vog W of Soufrière Hills was detected in satellite imagery and a pilot reported ash at altitudes of 1.5-3 km (5,000-10,000 ft) a.s.l. drifting NW.

On 26 March MVO noted that activity returned to a low level following the ash-venting on 23 March. The report noted that Zone C on the volcanic risk map, which was temporarily closed the previous week due to an increase in volcano-tectonic earthquakes below Soufrière Hills, re-opened for day-time entrance on 27 March. Zone C is to the WNW of Soufrière Hills and includes Cork Hill, Weekes, Foxes Bay, Richmond Hill, and Delvins, and extends 500 m out to sea.

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)