This report covers the interval from 9 March to 17 August 2001 and chronicles ongoing dome growth, including a vigorous episode of dome collapse and mass wasting on 28-29 July. As reported in BGVN 26:02, on 25 February 2001, the direction of the continuing dome growth changed markedly, shifting its predominant growth from the volcano's E side towards the S side. Then, as also reported in the Bulletin, the character of the seismicity changed dramatically in early March with the number of hybrid earthquakes exceeding 300/week (table 37). However, by mid-March, seismic activity had decreased significantly. Dome growth with attendant rockfalls, pyroclastic flows, and ash clouds continued at low levels until early-May. A small pyroclastic flow occurred on 9 May and traveled ~2.5 km down the White River to the S of the dome. The number of rockfalls increased substantially in the following week and remained at higher levels until early August. Observations during the week of 11-18 May indicated that the main dome growth was still concentrated in the S sector of the dome, and a lobe of new lava was observed over Galway. Reports from the week of 8-15 June noted that the summit over Galway appeared to contain the highest point on the dome.

Table 37. Seismic and SO₂ data from Soufriere Hills during 16 February to 17 August 2001. Courtesy of MVO.

Two notable events occurred during the week of 29 June-6 July. First, on the morning of 30 June, there were prolonged rockfalls that involved ~0.5 x 10⁶ m³ of material transported down the N side of the talus apron in the Tar River valley. Second, on the evening of 4 July, two small pyroclastic flows passed down the W flank of the volcano in the Amersham area, stopping ~1 km short of the sea. Following the pyroclastic flows in the Amersham area, the daytime entry zone (DETZ) was closed until further notice and has remained that way through at least 17 August.

Lava dome collapse. Shortly after 1700 on 29 July, a large pyroclastic flow passed down the Tar River valley on the volcano's E flank and a continuous, dense plume of ash developed and blew W. Pyroclastic-flow output increased gradually over the next three hours, with many of the flows reaching the sea. The downwind plume deposited substantial amounts of wet ash with accretionary lapilli over the residential areas of Salem, Isles Bay, and Olveston.

Pyroclastic-flow activity peaked at ~1950, when surge clouds associated with the largest flow moved out over the sea, followed by rock fragments falling over a wide area in the NW of the island in the sector between Salem and St. Peters. Some fragments were pumiceous, although the majority consisted of angular, dense lithic fragments generally less than a few centimeters in size, but with maximum dimensions of 6 cm. A second peak in pyroclastic-flow output took place shortly after 2200, when another large flow entered the sea and extended out from the shore for 0.5 km or more and rock fragments fell in the Salem area again. After about 0200 on 30 July seismic signals indicated that this dome collapse had largely finished, and the activity level declined rapidly. The ash plume from the collapse dispersed for considerable distances to the NW. Ash was deposited as far away as Puerto Rico and the Virgin Islands.

Observation flights indicated that a large portion of the dome had collapsed. The general summit region dropped ~150 m and there was a complex, amphitheater-shaped scar several hundred meters deep incised into the core of the dome at the head of the Tar River valley. Within this scar, a new dome began extruding. Observations indicated that minor pyroclastic flows also occurred in the upper reaches of White's, Tuitt's, and Gages ghauts, and also on the southern flanks of the dome in the upper reaches of White River. The main pyroclastic flows in the Tar River were highly erosive; they incised a deep canyon extending across the delta region to the shore and split the delta into two distinct lobes. Analysis of seismic data indicated that the two most intensive periods of pyroclastic-flow activity were associated with explosive events related to the collapse of the largest fragments of the dome.

Reports after 3 August noted that activity at Soufriere Hills was at a low level, and it continued that way to the end of the reporting period (17 August). Small-scale rockfalls and minor pyroclastic flows occurred, but clear views of the upper parts of the volcano were hampered by clouds. Occasional views of the dome noted that it was continuing to grow in the scar produced by the 29 July collapse.

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/).