Summary
Activity this month has been dominated by small volume pyroclastic flows down Gages valley, White River valley and Tar River valley. The flows reached the sea in each of these directions and produced a narrow, deep cleft in the dome running approximately east-north-east to west-south-west. Ash was deposited over the whole island, but heavy rains later in the month rapidly cleared any remaining dust from inhabited areas. Seismicity has been dominated by rockfalls and volcano-tectonic earthquakes, the latter occasionally occurring in swarms and with some of the larger events being felt across the island.
Visual Observations
As in the previous month, the volcanic activity has been dominated by intermittent, small volume pyroclastic flows originating from all flanks of the dome. On 2nd November, a few small rockfall events were recorded, some of which were followed by low amplitude tremor.
At 08:21 on 3rd November a larger collapse of the dome occurred generating pyroclastic flows that travelled down the Tar River as far as the sea and down the White River valley as far as the location of the Galway's Soufriere. The ash cloud from this event reached well above 10,000 feet and drifted to the west, depositing most ash south of the Belham valley.
A major collapse of the dome occurred at 21:17 on 5th November, the main pyroclastic flows generated by this event travelled down the White River valley and reached the sea depositing two blocky lobes on the White River delta. The surge cloud associated with this flow climbed half way up the northern slopes of Fergus Mountain. A small, fresh and predominantly fine-grained pyroclastic flow deposit was also observed in Gingoes Ghaut near to Reids Estate. The ash cloud generated by this dome collapse event reached about 20,000 feet, and drifted to the west. The flows originated from a deep gully between Chances Peak and the dome above Galway's. Samples were taken of the southern lobe of the blocky deposit on the White River delta, and also from recent deposits on the Tar River delta.
Two further small pyroclastic flows occurred on 8th November at 09:20 and 9th November at 08:47. These also travelled down the White River, but the ash clouds only reached about 6,000 feet and 10,000 feet above sea level respectively.
On 12th November at 06:07, the largest dome collapse event in the current series occurred. Pyroclastic flows travelled down Gages valley, Tar River valley and the White River valley. The ash cloud reached about 25,000 feet and ash fall was experienced throughout the island. The collapse was followed by vigorous ash venting, the continued ash fall affected mainly the Richmond Hill area. The pyroclastic flows that travelled down Gages valley almost reached the sea in Plymouth, with some burning observed near the port buildings. For the first time, the War Memorial and the Post Office were reached by the pyroclastic flow. Lobes of material also reached into the Amersham area, and a large water tower was transported into the upper parts of Parsons. Pyroclastic flows also reached the sea at the Tar River delta, and reached the old coastline at the bottom of the White River valley.
As a result of this and the previous activity a deep channel was cut into the dome. This gully is about 150 m deep and about 30 m wide and bisects the dome between the top of the Tar River and the top of Gages valley. The sides are extremely steep and, in places, overhanging. Several large cracks are still apparent in various sectors of the dome, including the area above White River and above Tyer's Ghaut.
In the weeks following the large collapse, there were a few small pyroclastic flows and some periods of low amplitude seismic tremor coupled with ash venting.
Temperature measurements were made of the recent deposits. On 16th November, near to the War Memorial, temperatures reached 386 degrees centigrade at 1 metre below the surface.
On the weekend of 28th/29th November, heavy rain resulted in extensive mudflows down all flanks of the volcano. Up to one metre of new material was deposited on the Belham Bridge, and there was further deposition in Plymouth and on the airport runway.
Seismicity
The month began with a swarm of volcano-tectonic earthquakes on the 1st November. The largest of these earthquakes was felt throughout the island. The hypocentres of the earthquakes in this swarm were under Chances Peak to the southwest of the volcano. For most of the rest of the month, rockfall signals and pyroclastic flows dominated seismicity. It was observed again that volcano-tectonic earthquakes beneath the dome often followed very rapidly from the larger dome collapse events. There was a second swarm of volcano-tectonic earthquakes on the 25th November. In this case a pyroclastic flow occurred shortly after the swarm started.
Table 1. Earthquake types
These earthquake counts are of events that triggered the broadband network's event recording system between 00:00 and 00:00 each day (local time).
Date Hybrid LP Rockfall VT 01 November 98 0 0 48 50 02 November 98 0 0 36 4 03 November 98 0 0 20 7 04 November 98 0 0 14 6 05 November 98 1 1 14 11 06 November 98 0 0 52 16 07 November 98 2 0 39 8 08 November 98 1 0 40 13 09 November 98 2 0 47 15 10 November 98 0 0 26 13 11 November 98 0 0 29 17 12 November 98 1 1 27 5 13 November 98 0 0 36 1 14 November 98 0 0 26 16 15 November 98 0 0 28 4 16 November 98 1 0 42 8 17 November 98 0 0 14 7 18 November 98 0 0 25 3 19 November 98 0 0 16 2 20 November 98 0 0 7 3 21 November 98 0 0 0 0 22 November 98 0 0 4 3 23 November 98 0 0 19 2 24 November 98 1 1 30 1 25 November 98 0 0 30 47 26 November 98 0 1 20 4 27 November 98 0 0 22 13 28 November 98 0 0 15 4 29 November 98 0 0 11 4 30 November 98 1 0 10 3
Table 2. Swarms
Start Date/Time Duration Hybrid LP VT 01 November 98 21:09 3.33 0 0 40 25 November 98 20:56 5.25 0 0 42
Ground Deformation
GPS measurements of various sites around the volcano were made in collaboration with the University of Puerto Rico. The usual monthly occupations of the MVO sites also took place in the latter part of the month. The main result from these occupations was the continuation of the eastward movement of Long Ground. This site has now moved by about 4 cm to the east since March 1998.
Volume Measurements
A series of photographs and theodolite measurements were taken from sites around the volcano to help build up an accurate surface map of the dome.
Environmental Monitoring
One measurement of sulphur dioxide flux was measured using the miniCOSPEC machine this month. This gave a value of 740 tonnes per day on 2nd November. This is similar to the fluxes measured over the previous 2 months.
A team from Cambridge University visited MVO in November to continue with FTIR measurements of the volcanic plume.
Table 3. Average daily sulphur dioxide flux measured by miniCOSPEC, November 1998
Date Flux (tonnes/day) 2-Nov-98 740
Sulphur dioxide was also measured at ground level using several sets of diffusion tubes located around the island. The results are shown in Table 4. For the 2 weeks from 16 October to 1 November, the wind was mostly to the north and therefore sulphur dioxide levels were generally higher than usual in areas north of the volcano. In the first 2 weeks in November, the prevailing wind was usually to the west and therefore sulphur dioxide levels north of the Belham River were below detection limits. Overall the levels in the reporting period were lower than over previous months.
Table 4. Sulphur dioxide diffusion tube results, November 1998. Levels in ppb.
16-Oct-98 to 1-Nov-98 1 to 16-Nov-98 Police HQ, Plymouth 48.8 92.8 St. George's Hill 49.2 13.4 Weekes 14.4 1.3 Vue Pointe Hotel 11.1 0.0 Lawyers 6.5 0.0
Water and ash samples were also collected from various sites around the volcano and were sent to the UK for analysis.
MVO Staff Changes
Arrivals:
Lutchman Pollard (Seismic Research Unit)
Willy Aspinall (Independent)
Departures:
Desmond Seupersad (Seismic Research Unit)
Keith Rowley (Independent)
Visitors:
Marie Edmonds (Cambridge University)
Lizzette Rodriguez (University of Puerto Rico)
Carlton "Pyiko" Williams (Seismic Monitoring Unit, St. Vincent)
Director of MVO, Richie Robertson left Montserrat for a short visit to the USA at the end of the month.
The MVO Operational Board Meeting was held in Montserrat on 23 and 24 November. The following officials visited to participate: John Shepherd (Seismic Research Unit, Trinidad), Baldwyn Mootoo (UWI, Trinidad) and Tony Reedman (British Geological Survey).