Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 06
10 January 1996

Activity at the Soufriere Hills volcano remained at a relatively low level for the first part of the past week, but seismicity picked up on 6 January and was accompanied by new dome and spine growth. No deformation above the level of detection by the EDM network is occurring at present.

Seismic activity began the week at the same level as it had been for several weeks previously, with a very low level of volcano-tectonic earthquake activity, some long-period events recorded on most of the seismic network and short periods of low amplitude, broadband tremor recorded only on the Gages seismic station. Late on 5 January, broadband tremor picked up slightly in amplitude at Gages and was then mixed with small long-period events for the next 12 hours or so before being replaced by a swarm of hybrid events located at less than 3 km depth beneath the volcano. Over 300 such events occurred at regular intervals for a period of about 8 hours, after which rate of occurence and then amplitude dropped off. These events were virtually identical in waveform and must represent the repetitive occurrence of the same process at the same place. Lower amplitude, regular hybrid events continued through to 8 January at a rate of 1 every 1 to 2 minutes. Several periods of low to moderate amplitude, broadband tremor and small long-period events were recorded at Gages during this period. The level of activity decreased somewhat for the remainder of the week, although the number of volcano-tectonic and moderate long-period events was constant throughout the week. One significant complex event was recorded on most seismometers in the network at 12:52 on 9 January; this is interpreted to be a major rock avalanche event, possibly accompanied by small explosions.

Two small regional earthquakes were recorded by the local seismic network during this week.

EDM lines around the volcano continue to be measured on a routine basis. None of the lines showed any changes during the week beyond the level of accuracy of the measurements. The dry-tilt sites at Amersham and Brodericks on the west side of the volcano were re-occupied during the early part of the week; neither showed any change since the last occupation in October. The Long Ground tiltmeter continues to show no measurable deformation.

Visual observations were possible at the begining and end of the week, and a total clearance on 8 January enabled good viewing from the crater rim at Chance's Peak and from altitude in the helicopter. Active dome growth appeared to be very slow early in the week, with evidence for small amounts of rock avalanching seen on the northern and eastern flanks of the new dome. The good overview observations on 8 January suggested some general swelling of the new dome and the addition of some new material in its central part. Also noted was the moving and tilting of the September spine by pressure from the new dome. Observations on 10 January revealed the growth of a new spine within the area of dome growth noted above. Initial impressions indicate that growth rate is similar to that observed for previous spines. A small ash fall occurred in Plymouth early on 4 January; this is thought to have been generated by a small rock avalanche.

The good weather conditions and relative quiesence of the new dome on 8 January enabled the helicopter to hover low over the northeastern part of old Castle Peak Dome, and Dr Simon Young was able to collect a large rock sample from the part of the new dome within the 18 July vent. This rock is thought to have been extruded in late November. It is crystal rich and likely to be silicic andesite, comprising dominantly plagioclase with subordinate pyroxene and hornblende and very little glassy material. Field inspection suggests that it is very similar to old Castle Peak Dome rock. Parts of the sample have been sent to 4 laboratories for complete geochemical and petrological characterisation.

Fieldwork undertaken during the week included visits to both Hot River and Fort Ghaut, where new exposures gave further insight into the recent eruptive history of the Soufriere Hills volcano. It is now clear that several charcoal-bearing pyroclastic units were erupted during the growth of Castle Peak Dome, intermixed with lahar-type units. Dating of charcoal should reveal the length of time for the activity at Castle Peak. Further work in both drainages is required to fully characterise the sequence.

The onset of repetitive hybrid events located at shallow depths beneath the crater followed within a few days by the confirmation of new dome growth has been seen on at least two occassions previously during this crisis, and the present style of dome growth now appears to be well characterised. Growth rate of the dome remains low, and rock avalanches from the actively growing areas have not yet had sufficient mobility to move out of the crater area.

Montserrat Volcano Observatory