Activity at the Soufriere Hills volcano has recently shown a significant change in style which dates from 10 November, 1995. In the 10 days previous to that time, three large phreatic eruptions occurred, depositing significant ash (c. 2mm in each event) on the capital of Montserrat, Plymouth, 4.5 km to the west of the active vent area. These eruptions followed a period of relative quiescence during most of October.
Four days of significant deformation in the upper part of the volcanic edifice as recorded by EDM and GPS measurements of Castle Peak Dome and Chances Peak were followed by a high level magmatic intrusion, causing intense seismic activity, on 15 November. The style of seismicity of this intrusive event was very similar to that experienced in late September, which culminated in the formation of a cryptodome and later a possibly juvenile spine on the west side of Castle Peak Dome. The surface manifestation of this later intrusive event was seen on 17 and 18 November as a second area of doming to the northwest of the September spine. For the following 10 days, no good visual observations were possible. Glimpses through steam and cloud cover suggested further doming and rock avalanching over a wide area on the northwest side of Castle Peak Dome including the edge of the July 18 crater. This surface deformation was accompanied by continued high levels of seismic activity and by a clear increase in the production of acidic aerosols in the summit area.
Good visual observations were possible on 30 November, when the presence of fresh juvenile material at the surface was confirmed for the first time in the form of a lava dome which had formed within the 18 July vent (the oldest and most active phreatic vent during the current crisis); other possible areas of active dome growth were also noted. Since that time, visual monitoring of the lava dome has been possible on a daily basis. The dome is gradually coalescing into a single feature, having started as two discrete bodies, one wholly confined to the 18 July vent area and the other covering an area to the northwest of the September cryptodome. All of the active dome growth is so far restricted to the western part of Castle Peak Dome.
The dome has a blocky surface with a number of small (estimated less than 5 m high) spines protruding from it. Two of these spines are now becoming quite tall and have clearly grown significantly over the past days. Other spines have broken off the dome, but none of the ensuing gravitational collapses has yet produced a significant debris avalanche or pyroclastic flow. Other rock avalanches have occurred, producing small ash puffs which may contain some hot material. The surface of the dome has been seen to be locally incandescent. Estimates of the growth rate of the dome from 16 November (estimated date of first juvenile material at surface) to the present are of the order of 0.5 m3/sec.
Seismicity has been at an elevated level since 15 November, with a few relatively quiet periods. The style of seismicity changed significantly on 27 November, with the loss of discreet, locatable events and the onset of a period of low amplitude tremor intermixed with periods of intense, low amplitude, long-period events at rates of up to 5 per minute recorded only on the closest seismic station (MGAT, Upper Gages). Tremor has increased somewhat over the past couple of days on other stations further from the crater (MLGT, Long Ground and MBET, Bethel) and the amplitude of events at Gages has also increased. Locatable events have intensified and are again occurring predominantly beneath the crater area, and the general level of seismicity as recorded by RSAM is now as high as it has been since 15 November.
Deformation of the volcano has been at a very low rate since 14 November, with daily shortening of the order of a few millimetres on most EDM and GPS lines, even those measuring the upper flanks of the volcano. No distinct deformation events have been recorded by the electronic tiltmeters on the eastern and western flanks.
Gas and acid aerosol production has been at an enhanced level over the past 2-3 weeks although no measurements of SO2 levels are available. Vegetation damage has been noted over a wide area downwind of the active area; this area extends for about 3km downwind and 1.5km laterally. The damage to trees especially is quite severe on the upper western flank of the volcano. Gases sampled at three of the soufrieres around the volcano continue to show no change in composition.