Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 49
11 January 1997

This report covers the 7-day period to 24:00 on 11 January.


The main volcanic activity during this period has been continued rapid growth of the October 1 dome, with a high level of rockfall and pyroclastic flow activity in the Tar River valley. Dome volume measurements on 5 January showed that the dome had again reached its maximum volume, of 28 million cubic metres (dense rock equivalent). The dome collapse reached a peak on the evening of 8 January, when at least one pyroclastic flow reached the fan at the bottom of the Tar River Valley.

Visual observations

Growth of the October 1 dome continued during the week, although the rate of growth seemed to have reduced by the end of the period. The activity was concentrated in the east and north-east parts of the dome, where the dome has built upwards and outwards, and much of Castle Peak has been eroded along the main pyroclastic flow chute. The north-west of the dome was also active, where the new material is contained within the scar formed by the September 1996 explosion.

On 5 January, a continuous ash plume was present, with ash rising from the flanks of the dome, rather than from individual rockfalls. The erosive gully through Castle Peak was first formed around this time, and small pyroclastic flows from a steep section of the dome above Castle Peak occurred every few minutes. The flows only travelled a few hundred metres from the dome. The level of rockfall activity increased during 5 January, with the north-east dome becoming more active- rockfalls in this area spread out, due to the lack of a clear channel. On January 6, the activity continued to increase, with pyroclastic flows from the Castle Peak chute travelling as far as the bench in the middle of the Tar River valley, near the site of the Tar River Soufriere. Excellent viewing conditions on the evening of 7 January showed that a 150( sector of the dome was active, from Castle Peak in the east to the remnant of the scar left by the July 1996 dome collapses on the northern flank of the dome. There were at least five different areas of activity, some of which developed into erosive chutes in the following days. The activity was pulsed, with peaks every 5 to 10 minutes. In the afternoon of 8 January, pyroclastic flows reached as far as level with the Tar River estate house (about 1.75 km from the dome). The flows tended to occur in groups of 2 or 3, with periods of low activity in between.

Sometime between 8 and 10 January, at least one pyroclastic flow reached the fan at the bottom of the Tar River valley, and the surge cloud from this flow probably entered the sea. This activity was not observed, but was probably coincident with a peak in the seismic energy just before midnight on 8 January. A small ash-cloud surge was detached from the main flow on the northern side of the Tar River valley close to the estate house. This surge covered some temperature patches to the west of the estate house, and a temperature I n excess of 200 degrees C was recorded 10 metres inside the surge cloud.

The rockfall and pyroclastic flow activity was at a lower level from 9 to 11 January. A "butterfly" structure of fresh slabs of lava was seen at the top of the Castle Peak chute on 10 January, and appeared to be quite unstable. The strike of the structure was oriented 270 degrees.

A sampling trip was made to the fan at the base of Tar River valley on 11 January. The deposits from the recent pyroclastic flow were examined; these were similar to earlier deposits in that they consisted of angular blocks of fairly dense juvenile material, unlike the pumiceous samples from the 19 December collapse (MVO scientific report 47/48). There were two lobes of new deposits, an earlier fines-rich lobe overlain by a coarser deposit. Temperatures in both of the lobes were about 400 degrees C at 25-45 cm depth.


Table 1: Earthquake types: 5 to 11 January 1997
These earthquake counts are of events that triggered the short-period seismic network event recording system between 0000 and 2400 each day.

Date	      	VT      LP	Hybrid  RF	Galway's Wall Landslides

05 JAN 97	1	5	0	59	0
06 JAN 97 	1	4 	1 	106	0
07 JAN 97 	3	1	5 	134	0
08 JAN 97 	0	4	35 	165	0
09 JAN 97 	0	7	30 	149	0
10 JAN 97 	2	7	9  	98	0
11 JAN 97	6 	1	2 	71	0

The main seismic activity during the week was rockfall and pyroclastic flow signals. These signals continue to be quite varied in character- most of the signals are emergent and dominated by energy with a frequency of 3 to 5 Hz. However some signals have fairly impulsive starts, sometimes with the initial part of the signal dominated by low frequency energy.

Ground Deformation and Gravity Measurements

No Electronic Distance Meter (EDM) measurements of the eastern triangle have been possible, because the reflector on Castle Peak is covered by ash. The northern triangle was measured on 6 January, and showed no changes in line length.

Measurement of the GPS network covering the whole of the volcano was made on 4 January. No changes were detected since the network was last measured on 7 December.

Microgravity stations last occupied in July 1996 were remeasured. A radial line extending from Brodericks to Chances Peak indicated gravity increases at all stations. The maximum difference was observed at between 600 and 700 metres elevation. The changes are localised, as a traverse from Brodericks to Galway's Soufriere immediately to the south of Chances Peak indicated no change since 28 June 1996. The Brodericks to Chances Peak radial line was also extended further down towards the shoreline.

The tiltmeter on Chances Peak continued to show oscillations along the axis radial to the crater. The oscillations had a period of about 8 hours at the beginning of the week, and the inflation along this axis slightly preceded peaks in rockfall activity. The amplitude and period of the oscillations decreased with time, and then became less regular after 7 January, when daily temperature changes seemed to exert a stronger influence on the tiltmeter.

The extensometer across one of the cracks of Chances Peak has been transmitting continuous data to the Observatory. The data show continued slight extension of the crack, at a rate of about 0.5 mm per day. No landslides were observed from the Galway's Wall, which supports the assumption that the rate of deformation of the wall has slowed markedly in the last month.

Dome Volume Measurements

The recent dome extrusion and related fragmental deposits were surveyed on 5 and 11 January using a combination of GPS-laser range-finding from the helicopter, fixed location photographs and theodolite measurements. The total dense rock equivalent volume on 5 January was determined to be 6.9 million cubic metres total erupted since approximately the evening of 25 December, of which approximately 3 million cubic metres is talus and block and ash flow deposits, and the remainder (about 57%) is lava. This indicates an average effusion rate over the first 11 day period of the new extrusion of about 7.3 cubic metres per second. Approximate calculations of the volume on 28 December indicate only a slightly higher initial effusion rate, about 8.3 cubic metres per second over the first 2.5 days of growth, therefore implying an average of about 7 cubic metres per second over the 8 day period to 5 January. The measurements indicate that since about 28 December the lava has grown slowly in volume relative to the talus, showing that the lava front was becoming less stable. This trend continued through 11 January. The total dense rock equivalent volume for 11 January is 10.2 million cubic metres, of which approximately 5.8 million cubic metres is talus and the remainder (about 43%) is lava. The lava volume has grown by only about 400,000 cubic metres from 5 to 11 January whereas the talus has grown by about 2.9 million cubic metres, consistent with the observations that the extrusion height and lateral extent have been changing only slowly, whereas the talus has been observed to accumulate rapidly through frequent rock falls and pyroclastic flows. For the 6 day period from 5 to 11 January the extrusion rate has been 6.3 cubic metres per second. The results indicate a slight easing with time of effusion rate during this spurt of growth, but the overall average of about 600,00 cubic metres per day over 17 days is the highest level of sustained magma supply, by a factor of about 2, exhibited by this eruption.

Gas Measurements

Measurements of sulphur dioxide flux were made on 7, 9, 10 and 11 January, by driving the COSPEC instrument along the road to the south-west of the volcano. The results were average values of 314, 1041, 389 and 410 tonnes per day, respectively. The background rate appears to be 300 to 400 tonnes per day, which is similar to recent measurements. The peak of 1041 tonnes per day was measured on the morning after a sizeable collapse of the dome. Similar high values were recorded in July and August 1996 after major dome collapses, suggesting that collapses lead to high levels of sulphur dioxide production.

Environmental Monitoring

The routine collection of water samples and deployment of sulphur dioxide diffusion tubes continues at sites downwind of the volcano. Sampling of airborne dust levels was restarted this week. Analysis of recent samples has been completed, and will be reported in the next scientific report.

Staff Changes
Lloyd Lynch, SRU
Richard Herd, BGS (after a short break)
Angus Miller, BGS (after a short break)

Randy White, USGS
Chan Ramsingh, SRU

Montserrat Volcano Observatory