This report covers activity from 26 May to 21 July 2000. During this interval, the lava dome continued to grow; however, between 26 May and 2 June, the direction of the dome's growth changed. Although it continued to grow vertically, the majority of growth appears to have redirected from the E and NE to the S and possibly the W.

Visual observations were severely limited due to clouds throughout the early part of this period. However, during the week of 23-30 June a "rough, spiny area" appeared high on the E face of the dome at the top of the Tar River Valley. The week of 9-16 June, the dome grew to about 914 m. By 25 June, the dome had surpassed the height attained prior to the 20 March 2000 collapse. During this event, instruments for measuring dome volume were damaged. Observations from 30 June through 7 July showed that the area of dome growth had changed to a more slab-like appearance. A new area of spiny growth was first seen on 10 July. This growth appeared on the NE flank at 940 m elevation, which was thought to be the highest point on the dome. On 17 July, a large area of new growth was reported on the S and W sectors of the dome, attaining a height of 950 m.

Pyroclastic flows were reported to the ENE in the Tar River, between 9 and 16 June. The following week, pyroclastic flows were reported in the Gages valley to the W. Additional pyroclastic flows during the week of 7 July went NE into the upper Tar valley; some, if not all, of the flow material originated from the remains of the 1995-98 dome. On 21 July at 0620, there was a small pyroclastic flow with an explosive start. During an observation flight later that day, evidence of pyroclastic flows was observed to the SW in the upper region of the White River valley.

Rockfalls occurred throughout the reporting period (table 34). However, the week of 23 to 30 June was characterized by nearly constant rockfalls and small pyroclastic flows. These rockfalls were concentrated on the E side of the dome and talus accumulated much more slowly to the S above the White River. Prior to this, during the week of 9 to 16 June, the rockfalls occurred almost exclusively in the Tar River valley. During 30 June to 14 July, rockfalls occurring to the E of the dome were infrequent despite the presence of large blocks at the top of the steep E slope. The majority of the rockfall events at this point were occurring to the S and to the W of the dome.

Table 34. Seismic data for Soufriere Hills during 26 May-21 July 2000. Courtesy of MVO.

Seismic records (table 1) revealed a sharp increase in the number of long-period (LP) earthquakes after 2 June. The frequency of LP events continued to increase until its peak during 23-30 June. This same week marked the low point in the number of hybrid earthquakes. The number of volcano-tectonic earthquakes increased towards the end of the reporting period.

A steady production of ash during the week of 9-16 June maintained a dilute ash plume that moved W towards Plymouth and off the coast. Neither this ash plume nor the smaller ash clouds produced by rockfalls during the preceding weeks affected the inhabited parts of the island. During the week of 30 June to 7 July, abundant steaming was observed on the W flanks of the dome. The following week, steaming occurred on the N side between the main masses of the old dome. During this same week, ash venting was also observed from the S side of the dome.

The sharp increase in the number of LP and hybrid earthquakes after 2 June was taken to indicate increasing pressure in the dome. In addition, the dome's filling in of the crater on all sides suggests that rockfalls and pyroclastic flows will increase in the future. These events are expected to affect not only the Tar River valley, but also several other surrounding valleys, particularly Tuitt's Ghaut, White River valley, and Gages valley. These observations also lead to increased concern over the possibility of a substantial dome collapse in the near future.

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.mvomrat.com/).