The Montserrat Volcano Observatory (MVO) reported that during mid-May through mid-September 2002, seismicity at Soufrière Hills was dominated by rockfall signals. Four volcano-tectonic (VT) earthquakes were reported during the first week of June and nine during the week of 9-16 August. SO₂ emission rates were measured using Differential Optical Absorption Spectrometers (DOAS). SO₂ fluxes generally remained at moderate levels. High fluxes occurred at times, such as during rockfall activity on 12 August (up to 690 t/day). On 6 September SO₂ emissions were low at 42-170 t/day, although levels increased to 170-518 t/day through 13 September (table 41).

Table 41. Seismicity at Soufrière Hills during 10 May-13 September 2002. "--" indicates that the information was not reported. Courtesy MVO.

During mid-May, growth of the summit lava dome continued to be concentrated on the E flank, giving rise to numerous rockfalls and small pyroclastic flows in the upper reached of the Tar River Valley. Pyroclastic flows were observed moving NE in the uppermost part of Tuitt's Ghaut during an observation flight on the morning of May 13. This was the first indication that pyroclastic flows generated on the NE flank of the active dome were able to flow into this drainage system. This new direction of flow was possible after the 29 July collapse scar had become largely buried on this side of the dome. The summit region of the active dome was visible briefly on several occasions during late May. It had a broad blocky appearance, and growth seemed to have become concentrated on the SE, giving rise to rockfalls and small pyroclastic flows on the SE flank of the dome. There was little activity on the NE flank of the dome during the last week of May.

Very clear conditions during 31 May-3 June provided the first good views of the summit region for several months, revealing that since early April a large lobe had been extruded on the dome's upper SE side. The lobe was ~150 m across and reached 1,023 m altitude. The upper surface of the lobe had a spiny though slab-like appearance. Since the dome was last seen, it had developed a small lobe-like protrusion on the summit's W side. Minor June rockfalls occurred on the dome's E and W sectors.

During mid-June, although the dome was mostly covered by clouds, photos of the summit area were captured on many days by the remote digital camera at White's Yard. Despite the low level of rockfall and seismic activity, the massive extrusion lobe on the SE side of the dome continued to grow steadily. Most of the upper surface of the active lobe had the smooth form of a whale's back; it also contained a low-angle spine directed upwards towards the SE. The free face at the front of the lobe on the SE side was steep and blocky in appearance. A theodolite survey of the dome taken during a brief period of clear weather on 11 June measured these altitudes: the general summit area of the active lobe stood at 1,025-1,030 m, and the top of the spine, at 1,048 m.

Rockfall activity increased abruptly on the night of 14 June and remained moderately high until the 18th, when it declined once more. Rockfalls and small pyroclastic flows were produced by material collapsing off the E face of the dome. Several small pyroclastic flows were also produced on the NE flank and were observed flowing into the upper part of Tuitt's Ghaut. By late June, growth of the extrusion lobe on the SE side of the dome appeared to have stagnated. Rockfall activity decreased abruptly on the afternoon of 22 June and declined to very low levels during 25-28 June.

No change in dome morphology occurred during early to mid-July. Rockfall activity on the dome increased slightly on the morning of 3 July, and a small, low ash cloud drifted over Plymouth around 1000. This followed several hours of heavy rain during the night, which was associated with substantial mudflows in the center of Plymouth. Rockfalls increased slightly during 6-8 July, before decreasing to very low levels through 12 July.

Observations of the dome on 15 July suggested that dome growth was continuing at a very low rate. Growth was concentrated on the SE part of the dome, at the lobe that was active during mid- to late June. The level of rockfall activity from this active lobe increased slightly on 15 July, with a small pyroclastic flow at 0800 directed down the Tar River Valley.

A swarm of low-amplitude long-period (LP) earthquakes began on 19 July and increased in strength during the following four days. The swarm continued at an elevated level until it began to decrease slightly during 31 July-2 August.

Observations of the dome on 21 July indicated that significant growth had recommenced, with the extrusion of a new lobe on the NE side of the summit region. Growth of the new extrusion lobe gave rise to rockfalls and small pyroclastic flows off the NE flank of the dome. On the morning of 23 July a minor collapse produced small but continuous pyroclastic flows for about an hour. These mainly flowed into the upper parts of Tuitt's Ghaut and down White's Ghaut for about half the distance to the coast. A few also flowed into the upper part of the Tar River Valley. A similar event, lasting for ~20 minutes, occurred in the early hours on the morning of 26 July.

On the morning of 1 August observations revealed that the new extrusion lobe on the N side of the summit had a broad whaleback form. Growth of this lobe was directed N and, around 2-4 August, the lobe crumbled repeatedly, producing rockfalls and small pyroclastic flows in Tuitt's Ghaut. Limited activity occurred on the NW part of the dome, although one small pyroclastic flow descended the notch between the central and NW buttresses. Individual rocks also reached upper Tyre's Ghaut (behind Gage's Mountain). During 6-9 August, rockfall activity declined substantially due to the lobe becoming more coherent and not collapsing. By mid-August, talus had accumulated in the upper reaches of Tuitt's Ghaut and small pyroclastic flows occurred in both Tuitt's and White's Ghauts. The active lobe also shed more talus into the notch in the NW sector of the old dome, which leads towards Tyre's Ghaut.

Rockfall talus continued to accumulate in the upper reaches of Tuitt's Ghaut during 16-23 August, and there were overspills of talus from the N side of the Tar River Valley into the two tributaries of White's Ghaut. The NE buttress, a remnant of the old dome complex from mid-1997, was now completely buried. Erosion of the E edge of the central buttress continued. Talus continued to slowly accumulate in the notch in the NW sector of the old dome, which leads towards Tyre's Ghaut. During intense rainfall early on 21 August, a small collapse occurred in the Tar River Valley of the talus that had accumulated on the SE sector of the dome during April-May 2002.

During late August, small pyroclastic flows were mainly concentrated on the NE flank where they had been channeled into the upper reaches of Tuitt's Ghaut; although some had spilled eastwards along the N side of the Tar River Valley. Talus also continued to accumulate in the notch in the NW sector of the old dome, which leads towards Tyre's Ghaut. Torrential rainfall produced mudflows in the Belham Valley in the early hours of 28 August.

During early September, growth continued to be focused on the N side of the dome complex although it had become more centralized and the summit height now exceeded 1,050 m. Otherwise the focus of activity remained concentrated on the NE flank, with frequent rockfalls and small pyroclastic flows. Most of these were channeled into the upper reaches of Tuitt's Ghaut; although some had spilled eastwards along the N side of the Tar River Valley.

During mid-September, dome growth remained centralized, and the summit height exceeded 1,050 m. Otherwise the focus of activity remained concentrated on the E flank, with frequent rockfalls and small pyroclastic flows. Around 6-8 September most of these spilled eastwards along the N side of the Tar River Valley, although by 12-13 September activity appears to have refocused northwards onto Tuitt's Ghaut, with subordinate amounts continuing to spill eastwards into the Tar River Valley.

During the reporting interval, the daytime entry zone (DTEZ) remained open, weather permitting. MVO warned that activity could increase suddenly, with dangerous situations developing quickly. Protective masks were to be worn in ashy conditions and the Belham Valley was to be avoided during and after heavy rainfall due to the possibility of mudflows. Access was prohibited to Plymouth, Bramble airport, and points closer to the volcano; including a marine exclusion zone around the southern part of the island ~3 km beyond the coastline, extending from Trant's Bay in the E to Garibaldi Hill on the W.

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: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).