Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 38
12 October 1996


The October 1 dome continued to grow at a relatively rapid rate and the eastern face has become quite steep with frequent small rockfalls beginning to occur on the eastern face. The level of seismic activity remained low with rockfalls signals decreasing during the week. The style of seismic activity changed on 10 October with the occurrence of long period earthquakes. Measurement of line lengths to the re-established Castle Peak reflector have been possible from Long Ground and Whites with no systematic trends yet developed. Installation of the new network of broadband seismic instruments was completed during the week.

Visual observations

Visibility was quite poor for most of 06 October, but some views were obtained from the east. The October 1 dome had increased in size and had acquired a more rubbly appearance, in contrast to being quite smooth when it first appeared. The dome was still roughly flat topped and the eastern face had become steeper. Some material had fallen from this face towards the Tar River Valley. The new dome was now over half the height of Castle Peak rock and growing quite fast. Vigorous steaming was observed from the north of the dome and only a few rockfalls were observed. These occurred mostly from the new dome and from the back of the explosion scar.

Views of the dome obtained on 07 October indicated that the October 1 dome was about 3/4 the height of the Castle Peak dome. The dome appeared to be growing as a single cohesive mass and has not produced any of the spines which were quite common during the earlier period of dome growth. The south-east side of the new dome was quite steep.

The eastern face of the October 1 dome had become very steep by 08 October. Vigorous steaming was observed from the southwest of the new dome and from the old dome complex. Several rockfalls were observed, and seemed to be originating from the base of the eastern face of the dome.

Inclement weather on 09 October allowed only brief views of the dome. These indicated that the dome had grown marginally and had experienced some slumping of material from the eastern side. This minor slumping of material was believed to be related to excessive amounts of rainfall which were experienced during the previous night.

Excellent views of the dome from several areas around the volcano and from the helicopter on the 10 October indicated that the new dome was now higher than the top of Castle Peak. The new dome had expanded laterally, filling about one quarter of the large scar produced by the 17/18 September explosion. The dome is a chocolate-brown colour, has an even surface, and nearly rounded conical shape. Vigorous steaming was observed from all round the base of the dome. Many small rockfalls were observed from the dome; these involved only the surface material of the dome, with no large blocks falling. These rockfalls have been associated with in low-amplitude seismic signals.

The new dome was observed from Whites early on 11 October. Regular small cascades of incandescent material from both shoulders of the eastern face of the dome were observed with occasional slippage of material from a central gully which had developed on the eastern side of the dome. Some rockfalls were also observed during the day from the coastline at the mouth of the Tar River. Rockfall activity during the past two days have been quite frequent but have often been too small to be recorded by seismic stations closest to the volcano.

The week ended with generally poor visibility preventing clear views of the dome. Regular small rockfalls were observed from the east face of the dome and an early morning view from Whites suggested that the dome had not grown markedly since the previous day.


The seismicity has remained at a generally low level although the style of activity has undergone some change during the week. Rockfalls signals continue to be the most dominant events with some volcano-tectonic and long period earthquakes beginning to appear during the latter part of the week.

Earthquake types: 06 October to 12 October 1996

These earthquake counts are of events which triggered the seismic network event recording system between 0000 to 2400 each day.

Date          VT        LP    Hybrid         RF     Tremor
06 Oct. 96     0         0         0         22        Low
07 Oct. 96     0         0         19        19        Low
08 Oct. 96     0         1         14        14        Low
09 Oct. 96     0         4         9         9         Moderate
10 Oct. 96     0         5         8         8         Low
11 Oct. 96     1         0         5         5         Moderate
12 Oct. 96     0         0         6         6         Moderate

The reduction in rockfall activity which began at the end of the previous week continued throughout most of the week. Hybrid earthquakes which returned at the end of the previous week continued during the early parts of the week. These events were very small and reduced in number towards the end of the period. There was an increase on the 10 October in the occurrence of large long-period events which had reappeared on the 08 October. One of the larger events which occurred at 14:04 on 10 October was accompanied by a large rockfall from the north face of the new dome which produced a very small ash cloud.

A single volcano-tectonic event located just south of Bethel at a depth of 1.55 km was recorded on 11 October.

The level of broadband tremor has been variable during the week, with higher levels often associated with increased steam venting as a result of rainfall. The tremor is believed to be due to movement of water and steam within the volcano.

A flash flood in Fort Ghaut on 10 October was recorded by the Gages seismic station between 0411 and 0422.

Two regional events were recorded by the seismograph network during the week: at 00:35 on 09 October and at 05:27 on 11 October.

Ground Deformation

The eastern triangle to Castle Peak was measured with the Total Station three times during the week. Low cloud cover did not allow measurement of some of the lines and prevented any measurement of all lines on 06, 09, 11 and 12 October. Measurement of the Whites to Castle Peak line on 07 October showed no change compared with the previous occupation made on 04 October. This line shortened by 8 mm between 07 and 08 October and then lengthened by 6 mm between 08 and 10 October. This line continue to show no consistent trend since reestablishment of the Castle Peak reflector which was destroyed by the 17/18 September eruption. The line from Long Ground to Castle Peak was measured for the first time on 09 October.

A GPS survey on the MVO Bignet which straddles the volcano was conducted on 12 October. This network was last measured just after the 17/18 September explosion. Processing of the results of this survey had not been completed by the end of the week.

Gas and ash measurements

Sampling of the respirable fraction of ash at various places previously affected by heavy ash fall has continued. Preliminary results suggest that the concentration of cristobalite in this ash is high in Plymouth.

Other work

An attempt to conduct a bathymetric survey of the sea in the area of new delta at Tar River was aborted on 11 October due to very choppy conditions. This exercise, which should be attempted during the next week, will allow a more accurate estimate to be made of the volume of material erupted by the volcano during the course of the eruption.

Installation of the new broadband seismic stations was completed during the latter part of the week.

Staff changes
Dr Paul Cole, BGS

Prof. Steve Sparks, Bristol University
Mr Desmond Seupersad, SRU

Visitors & Scientific Collaborators
Drs Hiroshi Shimizu, Volcano Research Center-ERI, Univ. of Tokyo
Dr Setsuya Nakada, Shimabara Earthq. Volc. Obs., Kyushu Univ.
Dr William Ambeh, SRU

Montserrat Volcano Observatory