There was a single precursor long period seismic event at 3km below the dome at 2:22am local time on 20 May. A brief episode of heightened seismic amplitude corresponding to ash venting occurred at 3-3:30am.
During heavy rain, another episode of increased seismic amplitude, interpreted as ash venting, began at 5:52am and this gradually increased and developed into a high amplitude seismic signal. The heavy rain was causing mudflows in Belham River valley at this time. By 06:32 low-level ash clouds were drifting to the northwest of the volcano from the crater area and a steam-rich plume was rising to 6,000ft. Unconfirmed reports suggest that pyroclastic flows first reached the sea at about 06:45.
Regular pulses of pyroclastic flows were reaching the sea down the Tar River valley by 07:20 with major pulses recorded in seismic amplitude at 07:36, 07:43 and between 08:01 and 08:04. Also between 07:30 and 08:10 a number of long-period seismic events were detected.
At 07:40 an ash cloud was reported at nearly 17km (55,000ft) the highest reported ash cloud during the ten years of the eruption. At 07:43 pyroclastic surges were observed spreading across the northeastern flanks of the volcano reaching the Spanish Point area. It was also estimated at this time that surges had spread 3km offshore from Tar River valley, across the surface of the ocean.
By 07:50 lithics (pieces of lava dome) were falling in areas northwest of the volcano, most were less than 3.5 cm across, the largest found in the inhabited area was 6cm across. Six car windscreens were reported broken. The deepest ash fall in inhabited areas was about 3 cm.
Activity began to reduce in intensity after 08:15am and a high amplitude seismic signal remained until about 9am. At this time, residents in the Old Towne and Salem area were subjected to high levels of volcanic gases particularly hydrogen chloride causing some to move north to get out of the volcanic plume. Widespread and noisy mudflows were reported in the Trants area to the northeast of the volcano. Vigorous ash venting from a vent in the western part of the crater continued until about 5:00pm when it began to decline in vigour.
A 1m high tsunami was reported from Deshaies beach in Guadeloupe and swells were detected in Little Bay, Montserrat and at Jolly Harbour and English Harbour, Antigua.
Relatively light ash and steam venting has been continuous since the collapse. Wind direction shifted towards the north late on 21 May causing ash fall and raining mud in most parts of the island.
Scientists remain alert to the possibility of further explosive activity but seismic activity has been low since the event on Saturday.