Activity at Soufriere Hills has been high over recent months, culminating in the collapse of a major dome and explosive activity during 12-13 July 2003. A summary of reports by the Montserrat Volcano Observatory (MVO) from 27 June to 12 September 2003 is provided below, with sulphur dioxide emissions and activity data (table 48).

Table 48. Summary of activity at Soufriere Hills, 2 May-12 September 2003. Activity occurred as summarized above, with the addition of three explosion signals during 11-18 August. Courtesy of the Montserrat Volcano Observatory.

Activity was generally at a moderate level in early May, increasing over 7-9 May and remaining high through 23 May. Activity mainly focused towards the NE, with rockfalls and numerous pyroclastic flows along the N side of the Tar River and in the Tar River Valley. On 12 and 13 May, flows were seen on the N and NW flanks in the area of Farrell's Plain and the upper reaches of Tyre's Ghaut. During 21-23 May there was increased activity on the N flanks, with a number of pyroclastic flows into the top of Farrell's Plain, Tyre's Ghaut, and Tuitt's Ghaut. Pulses of vigorous ash venting were observed at the summit, and intense glow on the summit and NE flanks was seen on the nights of 20 and 21 May. Sulfur emissions varied during May, with a high of 744 metric tons/day (t/d) (8.6 kg/s) on 14 May and a low of 300 t/d (3.4 kg/s) on 18 May. Extreme highs of 850 t/d (9.9 kg/s) and 820 t/d (9.5 kg/s) occurred on 4 and 9 May, respectively.

During the first week of June, activity was variable, generally declining to a moderately low level. Most activity through 6 June was focused on the E and NE flank, producing rockfalls and numerous pyroclastic flows in the Tar River Valley and occasionally in White's Ghaut and Tuitt's Ghaut. Activity during the week ending 13 June decreased to a low level, and remained low through 27 June, increasing over 26-27 June on the N flanks. Hybrid earthquake activity developed into a diffuse swarm on 22-23 June, some events at depths of 3 km below the lava dome. SO₂ emissions were relatively stable in June, varying between 240 t/d (2.8 kg/s) and 540 t/d (6.3 kg/s).

Sulfur emissions varied between 260 t/d (3 kg/s) and 585 t/d (6.8 kg/s) in July, but jumped to 840 t/d (9.7 kg/s) on 2 July. This could be related to increased activity during the first week of July, with pyroclastic flow and rockfall activity focused on the N flanks of the dome. Most flows occurred in Tuitt's Ghaut, with some in Tyre's Ghaut and White's Ghaut. Sporadic flows also occurred on the W side of the dome in the Gages area.

Activity remained high over the week ending 11 July, with a swarm of several thousand small hybrid earthquakes, at a rate of 1-2 per minute, commencing in the early hours of 9 July. While the size of these earthquakes increased slowly, individual events were below the normal recording threshold. The swarm of hybrid earthquakes intensified slightly over the night of 11 July, with events becoming larger and more closely spaced. Glimpses of the N part of the dome complex on 10 and 11 July confirmed that dome growth switched to the N, as was also shown by the northerly focus to the rockfalls and pyroclastic flows. Pyroclastic flows occurred most frequently in White's Ghaut, Tar River Valley, and Tuitt's Ghaut, with several small flows in Tyre's Ghaut earlier in the week.

By the morning of 12 July, events in the earthquake swarm merged into a continuous tremor signal. A period of prolonged and heavy rainfall between 0600 and 0900 caused mudflows in the Belham Valley. Small pyroclastic flows, the first of which were pale and weakly convective, occurred in the Tar River Valley. Flow activity built slowly through the afternoon until it was almost continuous. There were marked increases in the intensity of the activity at 1827 and again at 2007. Some flows traveled more than 2 km over the surface of the sea at the mouth of the Tar River Valley. Pyroclastic flows also reached the sea in White's Ghaut and the Spanish Point area. These flows resulted in the extremely heavy fallout of ash and accretionary lapilli over the island, particularly S of Woodlands.

A number of explosive events took place towards the end of the dome collapse of 12 July, with the largest occurring between 2300 and midnight. Showers of rock fragments fell on the island, with dense rocks up to 60 mm in diameter recorded. The Washington Volcanic Ash Advisory Center (VAAC) provided a column height of around 16 km for this event. The activity persisted at a high level until around 0200 on 13 July. It began subsiding slowly, declining to very low levels by the following morning, when a sudden Vulcanian explosion occurred from the lava dome. Two more explosions occurred in the next two days, producing pumice that reached 15 cm in size at Richmond Hill (~5 km W) and 4 cm in Olveston. Heavy ashfall from the collapse was experienced over all the inhabited parts of Montserrat, with the greatest thickness (over 15 cm) recorded at Vue Pointe Hotel. North of St Peter's the thickness was less than 1 cm.

The bulk of the dome structure was removed in the collapse, and pyroclastic flows impacted the area between Tar River Valley and Spanish Point. The activity destroyed GPS sites at White's Yard and Hermitage, and a camera site at White's Yard. Solar panels were smashed by falling rocks at Spring Estate GPS site and at Garibaldi Hill. After the collapse, sulfur-dioxide emissions jumped to highs between 1,030 t/d (12 kg/s) and 1,720 t/d (20 kg/s), much higher than any other readings over the past several weeks.

Activity was extremely low through 1 August with only a few events triggering the seismic network. The restrictions of the October 2002 exclusion zone were lifted on 1 August. The pattern of earthquakes through the week of 25 July indicated that dome growth within the explosion crater probably restarted, although it was not possible to confirm this visually due to low clouds. Intense activity began at 0608 on 1 August with an episode of powerful ash venting. There were many strong bursts of gas release and jets of ash; the plume rose to over 3.2 km. This activity declined to very low levels about 0730. Another episode of gas venting began at 0834.

Over the next week activity fluctuated, with periods of relative quiet separating episodes of intense degassing and hybrid earthquake activity. At the beginning of the week the volcano was extremely active with intense ash venting from the explosion crater. It was then fairly quiet with occasional rockfalls and hybrid earthquakes. A good view of the new dome was obtained from the air on 5 August, showing a small southerly directed lobe growing extremely slowly, if at all. Earthquake activity increased on the evening of 7 August with eight large hybrid events occurring overnight.

Through 22 August activity was at low levels; the dome remained a small lobe just over 100 m across. Several small slumps from the interior wall of the 12 July collapse scar produced small rockfalls, light ash in the plume, and the formation of some large fumaroles. By 29 August new fumaroles opened SE of the main explosion crater, towards the upper parts of the Tar River Valley. A strong sulphurous smell and blue haze N of the volcano did not reflect increased activity. SO₂ emissions in August were again variable, with a low of 450 t/d (5.4 kg/s) on 6 August and highs approaching 2,500 t/d (29 kg/s) the following week.

Through the latter part of the week ending 5 September, the gas plume was out of reach of the spectrometer network due to winds from Hurricane Fabian. Activity remained low through 12 September, but several episodes of ash venting occurred with a few small earthquakes.

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.

Information Contacts: Richard Herd, Montserrat Volcano Observatory, Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).