Activity at Soufrière Hills remained at elevated levels (table 63), similar to that previously reported (BGVN 30:12), a state that culminated with a dome collapse on 20 May 2006. Although that event took away considerable portions of the dome (and caused a small tsunami), photographs revealed post-collapse dome growth focused over a broad SE sector extending from the SW around to the NE. Numerous rockfalls continued from the S, E, and NE flanks of the lava dome. The NE-side rockfalls added talus to the upper reaches of the Tar River valley and were visible at night.

Table 63. Soufrière Hills seismicity during 28 December 2005 to 12 May 2006. * Due to weather conditions, gas measurements were not made. ** As a result of the collapse, instrumentation was lost and gas measurements were not able to be measured. Courtesy of MVO.

A central spine was first observed on 17 January 2006 when clouds briefly cleared from the dome. On 22 January, two new relatively thin, vertical planar spines were seen on the SE flank of the lava dome and collapsed on 29 January. Helicopter and field observations indicated continued dome growth, particularly in the SE (figure 64).

Figure 64. A photo showing the growing dome on SoufriPre Hills as viewed from Tar River at the seaward (E) end of the delta. Photo taken 23 January 2006 along the SW coastline. Courtesy of Montserrat Volcano Observatory (MVO).

On 10 February, MVO reported increased activity to the Washington VAAC. Satellite imagery showed a prominent hotspot at the volcano and a NW-drifting ash plume at an altitude of ~3 km. A small dark lobe of lava was observed on the western side of the lava dome in the crater. Steaming and venting were observed throughout the day. A photo appears as figure 65.

Figure 65. A 10 February 2006 photo taken at Soufriere Hills showing ash and steam venting from the dome. This view is from the SE; the ash cloud drifted N. Courtesy of MVO. Courtesy of MVO.

By early 11 February, this lobe had advanced rapidly towards the NE side of the dome and was visible as a steep-sided plateau of lava from inhabited areas around Salem. Photographs from fixed cameras showed continued changes to this lava lobe over the next few days, and the NE margin could be seen glowing at night and shedding rockfalls into the NE part of the crater. Ash-and-gas emissions continued through 15 February, producing plumes to an altitude of ~2.7 km. The initial growth rate of this lobe surpassed 5 cubic meters per second, but the rate declined around 17 February. The new lava lobe began to fill the gap between the lava dome and the N and W crater walls, raising the possibility that small rockfalls could spill over those areas in coming weeks. After 22 February, incandescent rockfalls were visible at night, coursing down the N, E, and SW sides of the dome and into the Tar River Valley (figure 66).

Figure 66. A Soufriere Hills photo showing the incandescent rockfalls at night taken from Perches Mountain, SE of the volcano. This photo was taken on 22 February 2006. Courtesy of MVO.

On 26 February, rapid vertical growth of the lava dome at Soufrière Hills was visible on camera images, and by 27 February a large spine about 30 m wide and at least 30 m high had developed at the dome's summit. By 28 February this spine had split into two parts and was leaning precariously to the NE. At about 2115 on 28 February the overhanging parts of the spine disintegrated and generated pyroclastic flows that traveled down the Tar River Valley almost as far as the coast. A low-level ash cloud drifted W. Additional changes to the shape of the spines and the upper NE flank of the volcano were noted in the following days as they disintegrated further. Rockfalls were visible on the N, NE, and E flanks of the volcano. Some fumaroles were observed on the upper outside part of Gages Wall (W of the lava dome) on 27 February, suggesting movement of fluids in this area.

During 3-17 March, lava-dome growth continued and the dome reached an altitude of ~950 m. The active lava lobe shed rockfalls and small pyroclastic flows to the W, N, and E. A vigorous gas vent was seen on the W side of the lava dome on 8 March, above Gages valley. Small fumaroles were visible at the top of Gages valley and below the lava dome remnant that stands at the top of Gages Valley.

Observations during 17 March-7 April revealed that lava-dome growth was focused in the summit area and towards the E and NE (figure 67). The N side of the lava dome showed little change. Rockfalls and pyroclastic flows were restricted to the Tar River Valley and were numerous on 19-20 March. The largest pyroclastic flows traveled as far as 2 km.

Figure 67. A Soufriere Hills photo of the growing lava dome taken on 30 March 2006. The photographer stood on Jack Boy Hill and looked NE. Courtesy of MVO.

Lava extrusion continued during 7-21 April. Growth occurred to the E and N, and an eastward-facing lobe developed on the NE side of the dome. Numerous small rockfalls continued from the active eastern flanks of the dome, adding to the talus in the upper reaches of the Tar River valley. Rockfalls were accompanied by minor ash venting. Due to unusual wind conditions, plumes were predominately transported N and NW, shifting to the E on 20 April. As a result of this process, light ashfall occurred over much of Montserrat. Thermal images taken on 27 April indicated some very hot areas on the E flank of the dome.

Deposits from a series of pyroclastic flows occurring on 4 May extended as far as the Tar River delta. Northerly directed winds during the reporting period resulted in light ashfall in areas north of the Belham valley. The dome volume was approximately 80 million cubic meters and the average growth rate through April was about 8 cubic meters per second.

On 18 May, a survey conducted on the southern half of the dome was carried out using a terrestrial laser scanner and showed that the summit of the dome had reached a height of 1,006 m, this is 83 m higher than Chance's Peak (figure 68).

Figure 68. The SE side of the Soufriere Hills lava dome as viewed from Galways Mountain on 11 May 2006. A new shear lobe forms the highest point of the dome and is growing toward the S. Chance's Peak is in the back left and Centre Hills in the back right. Courtesy of MVO.

20 May collapse. A major lava dome collapse took place on the morning of 20 May (figure 69). A helicopter flight in the afternoon confirmed that most of the lava dome had gone, together with some remnants of the 2003 lava dome, leaving a broad, deep, eastward-sloping crater at the summit of the volcano. The volume of the lava dome was believed to be about 90 million cubic meters and most of this collapsed over a period of less than three hours. Views of the W part of the crater where ash venting is continuing were not possible but it is unlikely that there is significant dome material remaining there.

Figure 69. A set of photos taken 1600 on 20 May 2006 after the lava dome collapse. (A) A shot taken from the E showing an overview of the delta, Tar River Valley, and dome complex. (B) The crater as viewed from the NE above the Tar River Valley. Ash emission continued from a vent on the W side of the crater and rose to an altitude of 1.8 km. (C) A photo taken from E of the steaming summit crater showing most of the lava dome, including parts of the remaining 2003 dome. (D) A photo shot from MVO showing the towns of Flemings, Hope, and Salem in the early afternoon as the ash-and-gas cloud dissipated. Belham River Valley, Old Towne, and Garibaldi Hill remained obscured by the cloud of ash and gas. Courtesy of MVO.

At 0222 on 20 May there was a single precursor, a long-period seismic event located 3 km below the dome. A brief episode of heightened seismic amplitude corresponding to ash venting occurred during 0300-0330. During heavy rain, another episode of increased seismic amplitude, interpreted as ash venting, began at 0552, and it developed into a high-amplitude seismic signal. The heavy rain caused mudflows in Belham River valley. By 0632 low-level ash clouds were drifting to the NW of the volcano from the crater area and a steam plume was rising to 6,000 ft (~1800 m). Unconfirmed reports suggested that pyroclastic flows first reached the sea at about 0645. Regular pulses of pyroclastic flows were reaching the sea down the Tar River valley by 0720 with major pulses recorded in seismic amplitude at 0736, 0743, and between 0801 and 0804. Also between 0730 and 0810 a number of long-period seismic events were detected. At 0740 an ash cloud was reported at nearly 17 km, altitude the highest reported ash cloud during the ten years of the eruption. At 0743, pyroclastic surges were observed spreading across the NE flanks of the volcano reaching the Spanish Point area. It was also estimated at this time that surges had spread 3 km offshore from Tar River valley, across the surface of the ocean.

By 0750, lithics were falling in areas NW of the volcano; most were less than 3.5 cm across, and the largest found in the inhabited area was 6 cm 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 0815 and a high-amplitude seismic signal remained until 0900. 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 N (figure 69). Widespread and noisy mudflows were reported in the Trants area to the NE of the volcano. Ash venting from the W of the crater continued until about 1700 when it began to decline.

A 1-m-high tsunami was reported from Deshaies beach in Guadeloupe and swells were detected in Little Bay, Montserrat, and at Jolly and English Harbour, Antigua. Relatively light but continuous ash-and-steam venting followed the collapse.

The weeks after the 20 May collapse. Wind direction shifted towards the N late on 21 May causing ash fall and raining mud in most parts of the island. Scientists remained alert to the possibility of further explosive activity but seismic activity was at low levels after the event on 20 May.

Since the May collapse, the lava dome continued to grow. As of 9 June it was approximately 20 million cubic meters in size. This is similar to the size of the dome in early January 2006. The average growth rate since the dome collapsed on 20 May was close to 10 cubic meters per second, well above the average growth rate of 6 cubic meters per second noted between January and April 2006.

By the end of the report period the dome was broad and flat-topped with a growing talus slope extending E. The lava on the summit of the dome is blocky, which is typical of lava extruded at a high rate. Vigorous ash and gas emitted by a vent W of the lava dome occurred during the week of 2 June. The venting is accompanied by a roaring sound that is sometimes audible in the Salem area. Prevailing winds have taken most of this ash and gas to the west over Plymouth. Satellite imagery on 4 June showed a thin area of ash out to St. Croix. In addition, there were multiple SFC and pilot reports of ash over the E portion of Puerto Rico and the Virgin Islands. Mudflows were reported on the 11 and 13 June during heavy rainfall.

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), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).