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Volcano Alert Status Report
Tuesday, February 13, 1996

Contact: Claude E S Hogan
Government Information Service Tel: (809) 491-/4851/7164
Chief Minister's Office Fax: (809) 491-7227
P.O.Box 292
Plymouth, Montserrat
West Indies December 14, 1995

Volcano Alert: Status Report
Thursday, February 13, 1996

Eruptive Events, Monitoring Procedures,
Impact & Forecast
January 31, 1996

Plymouth Amidst the hurricanes, tropical storms and floods of the Caribbean's 1995 hurricane season the 10,000 people of Montserrat battled the effects of an erupting volcano. The island's Soufriere Hills Volcano climaxed into a spate of ash and steam eruptions on July 18, 1995 following swarms of earthquakes dating back to 1992. Local newspapers carried reports of earthquakes by the hundreds in November, 1994. The reports were published by the Trinidad-based Seismic Research Unit, University of West Indies where the Cameroon born Chief Scientist, Dr William Ambeh observes, in hindsight, that the 1994 earthquake swarm "probably represented one of the first stages towards the creation of a path for molten rock or magma to actually move to the surface" at the Soufriere Hills Volcano in 1995. The ash showers of July 18, 1995 was a phenomenon new to the island's current generation. The initial ashfalls affected mainly residential areas of the island's southwest. Some residents chose to spend the evening of July 18 away from home.

The Government and people of Montserrat have been through a roller-coaster of events including having to twice relocate a majority of the population to a designated safe area north of the capital, Plymouth in phased exercises August 21-23 and December 1-2, 1995. The delineation of the safe-area is based on current analysis of scientific data, data collected by the Seismic Research Unit since 1985 and supported by a 1987 Wadge & Isaacs Scientific Study. The study forecast and developed eruption models emanating from English's Crater. Residents of the far eastern village of Long Ground to Tar River have been relocated at least three other times ahead of the rest of the population and remain the people at highest risk in the ongoing crisis. The following is an overview and status report of Montserrat's pre-occupation with a volcano which has placed tremendous pressure on the scientific community and has shaken the economy and social services of this 39.5 square mile island.

The Montserrat Volcano

Soufriere Hills Volcano lies in the south-central part of the British colonial island of Montserrat, at the northern end of the Lesser Antilles volcanic arc. The volcano is listed as 1600-05 in The Catalogue of Active Volcanoes in the World. The volcanic centre consists mainly of a closely-spaced cluster of five domes around English's Crater covering an area of about 35 sq. km at a height of 915 metres. The furthest distances from its highest point are near Plymouth (5 km) and near W. H. Bramble Airport (5.5 km). The crater, which is breached on the eastern side, is partly filled by a dome (Castle Peak) and now includes several additional spines which made their intrusion between September, 1995 and January, 1996. Increases in seismic activity and changes to local soufriere conditions have occurred on three occasions within the past 110 years, namely in 1897-98, 1933-37 and 1966-67. None of these seismic crises developed into actual eruptive activity and were characterized mainly by low to significant earthquake and fumarolic activity. Some increase in seismicity was noted at the Soufriere Hills by the Seismic Research Unit (SRU), University of the West Indies in 1992 and this activity increased markedly in November 1994, with a large number of relatively deep (10 to 20 km) earthquakes being recorded by a local seismic monitoring network on Montserrat. Scientists believe the current crisis may be giving "valuable planning and preparation time ... before a potential eruption."

Eruptive Activity

Eruptive activity at the Soufriere Hills Volcano began on July 18, 1995 with the opening of a steam and ash vent on the northwest side of Castle Peak Dome (a prehistoric lava dome which partially fills the horseshoe-shaped English's Crater). Several radiocarbon dates from pyroclastic flow units within the Tar River valley, which drains the open, eastern side of English's Crater, give a consistent date for the last phase of explosive activity at around 1600 AD. Activity between July 18 and August 21 comprised a number of phreatic eruptions of steam and finely ground andesitic ash from the initial vent area and from a number of newly opened vents dotted across the Castle Peak Dome and in the moat which surrounds it. Some of these eruptions deposited small volumes of ash on the capital of the island, Plymouth, which lies 4-5 km west (downwind) of the active crater area.

Relocation & Re-Occupation

A phased relocation of residents (6,000) from the southern half of the island was ordered immediately following a large phreatic eruption on August 21, 1995. Ash from the eruption-cloud and from a density current which flowed down the flanks of the volcano caused darkness in Plymouth and surrounding areas for a considerable period at around 8:02am , although total ash thickness in the city was only a few millimetres. A reduction in the likelihood of magmatic eruption at the volcano enabled the relocation order to be partially lifted on September 3rd a day before the passage of Hurricane Luis. Since that time, the volcano has been through several periods of relative quiescence, intermixed with periods of intense seismicity, thought to be associated with shallow magmatic intrusion, and periods of regular phreatic eruptions depositing several millimetres of ash in Plymouth. A change in eruptive style occurred in mid-November, and a second relocation was ordered on December 1, 1995 (4,000) soon after an active lava dome was seen for the first time to be developing on the west side of Castle Peak Dome. Two distinct intrusive events have occurred, both associated with development of areas of new doming on the western side of Castle Peak and eventual extrusion of a spine. Deformation measurements during the first of these episodes in late September included EDM, dry tilt and electronic tiltmetre observations. None of these methods indicated significant deformation of the lower flanks of the volcano. The second intrusive episode, in mid-November, had a distinct deformation event as recorded by changes in EDM line length from Tar River to Castle Peak Dome of the order of fourteen centimetres (14 cm) over the four days immediately prior to onset of increased seismicity. Confirmation of the volcano having again entered a new phase of activity was gained on November 30 with the first sighting of fresh magma at the surface within a lava dome. This dome showed two areas of main growth. One within the large crater formed by repeated phreatic eruptions since July 18 and the other adjacent to the September and November cryptodomes. It is thought that lava dome growth started about November 16, 1995 and is continuing at a rate of about 0.5 m3/sec. As of December 6, 1995 the two dome structures have been coalescing slowly into what scientists now refer to as a "composite dome". The intermittent glowing of protruding spines at the top of the mountain have been a night-time attraction for residents from around the island who converge in the eastern village of Harris' for the spectacular views which have included small flows of hot rock within the confines of the crater. There has been no official off-island evacuation.

Monitoring The Volcano

Monitoring of the Soufriere Hills Volcano is being undertaken by the Seismic Research Unit (SRU) in conjunction with the British Geological Survey (BGS), University of Puerto Rico with assistance from The United States Geological Survey (USGS), Observatoire Volcanologique Guadeloupe and a number of individual researchers.

The expanding seismic network currently comprise 9 stations, the closest (Gages) less than 1.5 km from the crater. The seismic data from these stations, along with data from two electronic tiltmeters, are telemetered to the observatory in Old Towne, within the designated safe zone, and are inspected in real time using both digital and analogue recorders. A total of thirteen EDM (electronic distance measurement) lines are measured daily or twice weekly along four triangles of Castle Peak, Farrell's, Gages and Chances Peak. These cover both the lower and upper flanks of the volcano on all but the southern side. A geodetic GPS (global positioning system) net of three permanent stations records baseline changes right across the volcano up to the end of December, 1995. One of the stations was moved on an occasional basis to either the high point of the volcano (Chances Peak) to form a more proximal net or to one of the locations elsewhere around the volcano to vary the lines measured. Dry tilt surveys continue to be undertaken at several sites on the flanks of the volcano. Severl new stations are being established as the MVO continues to inprove its monitoring network.

Gas monitoring of the established soufrieres is undertaken weekly. Correlation Spectrometer (COSPEC) measurements of Sulphur Dioxide (SO2 ) content in the eruptive gas plumes were undertaken during the early stages of the eruption. A network of gas monitoring devices is also being established in Plymouth for collection of data related to human health. Petrological and geochemical analysis of ash samples from the phreatic eruptions have been undertaken by a number of research groups. Direct sampling of gas or rock from within the active crater area was made possible on January 8, 1996 by a daring helicopter flight to within English's Crater area and the retrieval of juvenile rock by MVO Scientist, Dr Simon Young. The rocks are being analysed to provide scientists with further insight on the ongoing volcanic activity at Montserrat's Soufriere Hills Volcano. Chief Scientist, Dr William Ambeh says that a petrological study "may tell you that what has actually come out now is not completely new [material] ... but could have been lying there since 400 years ago." Visual observation of the crater area remains an important monitoring activity. This is undertaken either on foot from the crater rim below Chances Peak (although this practice was once discontinued while the lava dome was actively growing) or from the air by helicopter.

Other fieldwork includes inspection of newly exposed stream sections with a view to obtaining a more thorough understanding of the geological history and different eruptive styles of the volcano. Monitoring activities are expected to continue on a 24-hour basis for many more months, and certainly whilst there is any chance of further eruptive activity at the volcano. Scientists have guaranteed the government at least twenty four hours notice ahead of any major eruption. Residential areas from Spanish Point eastwards to Long Ground were re-occupied January 15, 1996 on the understanding that they may be asked to move within a short time if the hazard level warrants it. Plans are moving ahead for the establishment of a permanent observatory on Montserrat.

Environmental Impact

The ballistic projectiles and the significant phreatic eruptions experienced during the initial part of the current eruption of the Soufriere Hills volcano are actually characterized by tephra falls. Tephra consists of rock fragments which are ejected into the air and which fall according to wind direction.

There has been volcanic gases which have created some stir among people. The most significant of these gases has been sulphur dioxide. The damage to the vegetation of the surroundings hills including the Gages and Chances mountain ranges are quite self-evident. Just looking from Plymouth towards Gages one could see the damage to the vegetation. The ecological damage has been mainly due to sulphur dioxide combining with water droplets and then falling as acid rain. Other volcanic gases such as carbon dioxide have also been detected. Carbon dioxide is an odorless, colourless gas that can accumulate in 'toxic' amounts in depressions. If one moves within these areas one may be suffocated because carbon dioxide is denser than air and tends to hug the topography. The scientists have already warned residents about this effect and have dis-encouraged casual visits to areas within 1.5 kilometres of the volcano. In terms of percentage volume however, the largest amount of volcanic gas emitted is actually water-vapor. There has been no significant damage that can be attributed to the earthquakes being monitored by the observatory since July 18, 1995, few of which are felt by the population at large. A number of earthquakes have been felt during the current crisis and even before but there are no reports of damage to structures caused as a result of volcanic earthquakes. In most cases the volcanic earthquakes are very small. The largest recorded so far during the crisis is probably magnitude three point five (3.5) on the Richter Scale. One would expect an earthquake of probably magnitude five (5) to cause any significant damage to a structure that is relatively well-designed. Tsunamis are not a hazard of major consideration in the current situation on Montserrat but obviously there is a potential hazard from Tsunamis should there be a major collapse of material which flow rapidly into the sea. This would cause displacment of a certain volume of water and only then you may have tsunamis or tidal waves being generated.


While economic data have not yet been quantified and analysed, some general observations can be made concerning the impact of the volcanic crisis on the socio-economic state of Montserrat. On the positive side the island's tourism industry has been catapulted forward as an eco-tourism product; the decentralization thrust of the island's Physical Development Plan has been energized; and farmers of the East and South in particular continue to focus on feeding the country amidst expectations that the ash will, over time, make the soil more fertile and improve yields. The continued relocation of the Glendon Hospital as a precautionary measure has been previously highlighted. The acquisition of new school buses will also bolster emergency response capabilities if another relocation should become necessary. In general terms, the people of Montserrat continue to demonstrate extra-ordinary resilience and flexibility in the ongoing volcanic crisis in which interactive information sharing continues to play a prominent role in preparatory efforts.

Towards The Future

Forecasting activity at the Soufriere Hills Volcano remains the biggest challenge for the MVO team of scientists. According to Chief Scientist, Dr William Ambeh, forecasting volcanic eruptions is mainly an exercise in recognizing different patterns in the activity and "the more we monitor the volcano then the better we seem to understand it and we tend to have more confidence in conclusions that we draw." Other questions germane to the situation in Montserrat have been answered by Dr Ambeh as follows.:

Q. What is the main focus of scientists now six months after the first phreatic eruption?

A. We are not worried about small eruptions that simply result in the generation of ash. Obviously, there's that inconvenience. There's that psychological effect in terms of the fear and there are other effects in terms of the environmental damage that it causes but in most cases, unless you decided to actually panic, then you wouldn't be killed by something like that. What we are worried about is the nature and timing of a potentially destructive event happening.

Q. How would you describe a destructive event?

A. A destructive event, will be, in this case, either a major collapse of the dome resulting in pyroclastic flows or debris flows and possibly associated ash surges or after the quiet growth, you suddenly have a major explosive eruption as well that would generate huge eruption columns which may ultimately collapse to again generate pyroclastic flows or surges and things like that. So we come to a state where the scientists, the volcanologists, or the volcano-seismologists or those actually monitoring the volcano are put under a certain amount of pressure because they may have a poor quality of data but the public would expect them to say something because they depend on the scientists. So you have a situation where there is pressure on the scientists but the scientist thinks that the quality of his or her data is not of the type that they would make some decisions with confidence and that may lead to two different things. Obviously, they'd have to make a decision some way. So you may have a situation where they will play safe, and say, 'OK, evacuate." That way you're on the safe side. People are out of the region of danger and then you wait and you see what happens and then you move on. Or you may have what is known as the stay-put type of mentality where you say, "OK immaterial of what is happening, we are not moving." Both of these can lead to disasters, in terms of the economics because you can have as much damage being done to the economy of the country by having an unnecessary evacuation as compared to having a stay-put policy where you may actually eventually have an explosive eruption and then people are killed. We have several cases in the literature where this has actually happened where you have the two extremes where you've had unnecessary evacuation. The classic one is the '76-'77 eruption of the Soufriere in Guadeloupe and the evacuation of the southern part. More than 70,000 people were moved to the northern part of Guadeloupe for 4 months and the cost of that was about four hundred million dollars. So that in itself was a disaster to a certain extent. But obviously you may have a situation where you could stay put as well and then people get killed. So those are the types of things that we have to look at when we make decisions based on the scientific information that we get in terms of whether we advise the authorities to move people out or to stay put.

Q. Would you please describe the activity at the volcano and the current rate of dome growth?

A. For this particular volcano, we've gone through the gamut of pre-eruption activity in terms of earthquake sequences from 1992 to about June 1995. We've gone through the periods of phreatic eruptions with significant ash and steam emissions and then their consequent inconvenience or consequential destruction to the environment. Now we are in a period where since late September when you had the first dome, things settled down and then in late November, as well, you had the present episode of dome growth actually starting so we have what we consider at the moment to be essentially slow dome growth. The rate is variable. Sometimes, for example, the times when we pick up a significant amount of seismic activity, then after things quiet down, maybe in the next day or two, you'd see a spine appear, grow rapidly on the order of 10 to 15 metres a day and then collapse. But essentially, the overall rate at which this dome is growing is considered to be slow.

Q. What are the possible ramifications of slow dome growth?

A. If the dome that we have within English's Crater at the moment, keeps on growing at the present rate, it would probably take several months before it reaches a state where it would constitute a significant hazard. But with time, obviously, that hazard would increase and you'd have after maybe months or even years, you'd have the sides of the dome collapsing and when that occurs, you'd have rock avalanches, pyroclastic flows. The preferential pathway of this would be to the open part to the East. If you have that dome growing to such an extent that its height is greater than the rim of the crater, then collapses would cause over-spills that may not necessarily just result in the small ash falls that we've experienced in Plymouth. Obviously, it's greater for the East but there's also the potential for that hazard to be significant for the western area once we get the size of the dome growing to a size that is far higher than the crater rim.

Q. So what is the potential now for a destructive event?

A. Based on the geological records, I think the large major eruptions that occurred in Montserrat were probably greater than 10,000 years ago when you had what we would consider big eruptions and that period ceased. And then, about 400 years ago, you had the start of what seemed to be another cycle of activity in terms of the eruptions that occurred before 1632 which was a small eruption. A small pyroclastic flow was produced and then a dome emplacement also occurred so at the moment, this current dome growth may be part of that pattern of small dome growth rather than major explosive eruptions although we cannot totally exclude that possibility of a major explosive eruption. Based on geological records, empirical evidence and looking through several cases in the literature, the longer the dome grows quiescently, then the less likely it is that you'd have a major explosive eruption later on. So it is based on things like that, again including the fact the magma seems to be considerably de-gassed coming out, we interpret that would suggest that this activity may continue in slow mode for several months or even years to come. It may not necessarily culminate in a large explosive eruption. Obviously, we have to be cautious in the way we look at things so that's why the monitoring program, at the present intense level, is going to continue. It may be months, it may be years, but it is going to continue at the present level until we feel satisfied that the activity has actually reduced to a state where we can actually reduce the level of monitoring.

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