Montserrat

Geography

Montserrat is one of the youngest volcanic islands that make up the Lesser Antilles Island arc and measures 16 km N-S and 10 km E-W. Montserrat’s mountainous slopes are covered with lush tropical rainforest. The island is dominated by four volcanic centres. Decreasing in age from North to South, the centres are: Silver Hills, Centre Hills, Soufriere Hills, and South Soufriere Hills. Prior to the activity at the Soufriere Hills volcano (SHV), the highest peak on the island was Chance’s peak at 909 m above sea level. However, the lava dome that currently forms the summit of the Soufriere Hills volcano is the highest point on the island, reaching 1089 m above sea level. 

The island is surrounded by a shallow submarine shelf that extends down to about 100 m below sea level. The width of the shelf varies considerably around the island. It is widest in the north, extending to 5 km offshore of the Silver Hills, an indication that the present Silver Hills are the eroded remnants of a much more substantial volcanic centre. Around the southern coast of Montserrat, the shelf is considerably narrower, testament to the youthfulness of the Soufriere Hills and South Soufriere Hills volcanic centres. 

The southern part of  Montserrat is cut by a series of normal faults trending N110°E-N130°E that also exhibit left-lateral slip (Feuillet et al., 2010). These on-island faults link up with Montserrat-Havers Fault System to the west and the Bouillante‐Montserrat Fault System (BMFS) to the east, that extends all the way to the island of Basse-Terre, Guadeloupe. These fault systems accommodate both extension and slip from the oblique convergence of the Caribbean and North American tectonic plates. The fault system also controls the alignment of the uplifted areas and the pre-1995 lava domes that constitute the Soufriere Hills and South Soufriere Hills volcanic centres. 

Country Facts

Approximately 5,387 people live on the island of Montserrat. English is the official language. Currency used: Eastern Caribbean Dollar (XCD).

Disaster Management

The Montserrat Volcano Observatory is the official monitoring agency for Soufrière Hills Volcano in Montserrat.

Montserrat Volcano Observatory

Hope Dr Flemmings, Montserrat

Tel: +1 664-491-5647
Email: mvomontserrat@gmail.com 
Website: https://www.facebook.com/mvoms

In the event of an earthquake, volcanic eruption or tsunami the Disaster Management Coordination Agency (DMCA) is the official authority in Montserrat.

Disaster Management Coordination Agency (DMCA)

Sweeney’s Montserrat
Montserrat

Tel: +1 664-491-7166 or 664-491-3076
Email: dmca@gov.ms
Website: http://dmca.gov.ms/

Montserrat is dominated by four volcanic centres, which decrease in age from north to south: Silver Hills (~2.17 – 1.03 Ma), Centre Hills (~1.14 – 0.38 Ma), Soufriere Hills (~0.45 – 0.30 Ma to present), South Soufriere Hills (~128 ka)(Cassidy et al., 2012; Coussens et al., 2017; Hatter et al., 2018; Harford et al., 2002). With the exception of the South Soufriere Hills, the eruptive products of the volcanic centres are predominantly andesitic rocks produced by dome-forming eruptions with associated explosive episodes, lava dome collapses and debris avalanches. The South Soufriere Hills centre is more basaltic than the other centres, producing basaltic to basaltic andesite lava flows, scoria flows and fall out deposits (Cassidy et al., 2012). In addition, there is a small area of uplifted carbonate rocks exposed in the cliffs along the southeast coast of the island within the South Soufriere Hills volcanic centre. 

 The Silver Hills is a deeply eroded andesitic volcanic centre consisting of volcaniclastic sequences, debris avalanche deposits and extensive areas of hydrothermal alteration (Harford et al., 2002). Volcanism at the Silver Hills was dominated by episodic andesite lava dome growth and collapse, produced Vulcanian style eruptions, and experienced periodic sector collapse events (Hatter et al., 2018). The Silver Hills is surrounded by a shallow submarine shelf that extends up to 5 km offshore. 

 The Centre Hills is the largest volcanic centre on the island with its lava domes and volcaniclastic deposits occupying most of the centre of the island. Eruptive products associated with the Centre Hills consist of andesitic block-and-ash flows, pumice-and-ash flows, pumice fall, lahars, fluvial and debris avalanche deposits (Coussens et al., 2017, Harford et al., 2002). Abundant pumiceous deposits exemplify the largest known explosive eruptions on Montserrat (up to magnitude 5) (Coussens et al., 2017). 

The Soufrière Hills consists of a predominantly andesitic dome complex (Chance’s Peak, Gage’s Mountain, Galway’s Mountain, Perches Mountain and the present lava dome) flanked by volcaniclastic deposits. Before the 1995 eruption, Castle Peak (formed 1630 AD) partially filled English’s Crater, but was initial buried and then later destroyed by the growth and collapse of multiple lava domes during the 1995-2010 eruption. Located to the northwest of Soufriere Hills, Garibaldi Hill and St. George’s Hill are tectonically uplifted andesitic volcanic massifs consisting of pyroclastic and epiclastic deposits (Harford et al., 2002). 

 The South Soufrière Hills differs from its northern counterparts in comprising basaltic to basaltic andesite lava flows, with flow collapse breccias and scoria-fall deposits (Harford et al., 2002). The activity that formed these eruptive products occurred over a very short (geologically speaking) interval of approximately 10,000 years between 131–128 ka ago. 

Generalised map of Montserrat showing the main features of the four different volcanic centres

Generalised map of Montserrat showing the main features of the four different volcanic centres, the uplifted areas of Garibaldi Hill, St George’s Gill and Richmond Hill; the pre-1995 lava domes and the current lava dome (brown shading). The orange cross-hatching shows the area affected by rockfalls, pyroclastic flows and lahars during the 1995-2010 eruption. 

The Leeward Islands area, as shown in the epicentral plot, is the most seismically active zone in the Eastern Caribbean and has hosted the largest magnitude earthquakes to have occurred in the region since the 1600’s, when written accounts for the region began. The average number of background earthquakes, i.e. those that recur on a daily/weekly basis, manifest minor fluctuations. The output level sometimes increases in association with the occurrence of a significant magnitude earthquake. This can take the form of foreshocks and aftershocks or only aftershocks.

One such period in 2000-2001 exhibited elevated earthquake activity associated with a magnitude 5.6 event on 2000/10/30 that was located east of Barbuda. However, since 2011, activity in the area has been generally elevated over that seen in previous years. Elevated activity is sometimes precursory to more significant magnitude earthquakes. In the epicentral plot shown for the period since 2011, the moderate to strong earthquakes are labelled. Whether or not the elevation being observed is associated with those events or indicative of a more significant magnitude event to come, only time will tell. It may be worthy of note that on 2011/09/06, there was an earthquake of magnitude 5.6 located at 21.66°N and 60.20°W, which is an unusual area for earthquakes to occur and significantly extends the area of increased activity.

During the instrumental era, since 1953, there have been a number of strong to major earthquakes in the area. On 1974/10/08, there was a magnitude 7.4 earthquake located north-east of Antigua/south-west of Barbuda causing damage at Modified Mercalli Intensity VIII in Antigua and lower intensities in the more distant islands. The damage was confined mainly to larger and older buildings, to a petroleum refinery, and to a deep-water harbour. A few people received minor injuries, but no fatality was reported. Then on 1985/03/16, there was a magnitude 6.3 event located west of Antigua/south-east of Nevis that was strongly felt in Montserrat, Nevis, Antigua and St. Kitts.

During the historical era, which dates back to the 16th Century, the first major earthquake in the area, for which there are written accounts, is estimated to have been located close to Nevis and occurred on 1690/04/05. The damage accounts in Robson (1964) suggest impact at Modified Mercalli Intensity IX: In Antigua, some buildings collapsed into rubble, with some associated deaths. Governor Codrington, the then Governor, lost property with value estimated at £2,000. Aftershocks were felt almost daily for a month.

The region’s great earthquake, at magnitude estimated to be in the range 8.0-8.5, was also located in this area, south-east of Antigua, on 1843/02/08. In this instance also, the maximum intensity, based on damage accounts, has been put at Modified Mercalli Intensity IX. In St. Johns, Antigua and across the island generally all masonry structures were destroyed or severely damaged. Many houses were left with their outer masonry walls collapsed and the inner walls supporting the roof. Wooden houses remained standing. Alluvial ground was fissured and sulphurous water thrown out. Landslides were general on hill slopes. At English Harbour the wharf, built on reclaimed ground, sank and afterwards had an undulating appearance ‘like waves on the sea’. A cloud of dust hung over the island for some minutes after the earthquake. After the earthquake the sea rose four feet, but sank again immediately, remaining calm throughout. The number killed was variously estimated in the range 12-40. The damage at English Harbour was variously estimated at £20,000-£25,000. The total damage in Antigua including the loss of the sugar crop was estimated at £2,000,000.

The occurrence of such events in the past should be taken as clear evidence of the potential for high level damage from large magnitude earthquakes in the Eastern Caribbean. The absence of such events in the 20th Century should serve to motivate to urgently putting measures in place to mitigate the impact of such events that would in all likelihood occur this Century.

Soufrière Hills Volcano

• 16.71°N, 62.18°W
• Elevation – 800m
• Last eruption – 2010

Soufrière Hills Volcano is the only active volcano in Montserrat, producing both dome–forming and explosive eruptions. 

Historical Eruptions

No historical eruptions were recorded on Montserrat until 18 July 1995, Soufrière Hills. This eruption was preceded by three years of elevated seismicity (Robertson et al., 2000). The eruption began with 18 weeks of phreatic activity followed by dome extrusion in November heralding the start of magmatic activity (Kokelaar, 2002). There have been five phases of lava dome extrusion: November 1995 – March 1998; November 1999 – July 2003; August 2005 – April 2007; July 2008 – January 2009 and October 2009 – February 2010 (Wadge et al., 2014). In addition to dome growth, this volcano has exhibited interspersed dome collapses, pyroclastic density currents, tephra fallout and lahars (Nelson, 2017). During these five phases, the activity has been characterized by the growth and destruction of a series of lava domes, along with numerous explosive eruptions. Some notable events during the 1995-2010 eruption include the 1997 Boxing Day collapse and debris avalanche which severely impacted the SW flank of the volcano and the largest historical lava dome collapse in July 2003 when the entire lava dome collapsed in a series of powerful pyroclastic flows and explosions. The lava dome had grown rapidly prior to the event and more than 210 million m3 of material collapsed into the sea. 

Since the end of last extrusive phase in 2010, there has been no significant surface activity. Despite the absence of surface activity monitoring data indicate that the magmatic system is still active. Among other indications, there is ongoing island-wide ground deformation associated with subsurface pressurisation and an anomalously high flux of sulphur dioxide (Scientific Advisory Committee on Montserrat, 2019). These indicators of unrest indicate that future lava extrusion is still a possibility.  

Related Resources

Volcanic Hazard Atlas of the Lesser Antilles

Historical Eruptions (Curating Crises Project)

Geothermal activity, in the form of various soufrieres and fumaroles, has been ever-present throughout the history of Montserrat. Evidence for geothermal activity in the Silver Hills volcanic centre is found in the present of highly weathered and hydrothermally altered deposits and rocks that represent now-extinct soufrieres. 

 Prior to the start of the 1995-2010 eruption, there were several soufrieres, located at Galway’s Lower and Upper Gage’s and in the Tar River valley. These were all destroyed by activity early in the 1995-2010 eruption. 

 Presently, the only active geothermal features associated with the Soufrière Hills Volcano can be found on or adjacent to the present lava dome. Since the end of the last phase of extrusion in February 2010, numerous fumaroles have developed on and immediately adjacent to the lava dome. These fumaroles are routinely monitored by MVO using a thermal infrared camera and thermocouples. Measured temperatures vary from less than 100 °C to more than 500 °C. The distribution of the fumaroles is such that the highest temperature fumaroles are all located on the lava dome, while the majority of the lower temperature fumaroles are located adjacent to or short distances away from the lava dome. 

Photograph looking into the 2010 collapse scar on the lava dome of the Soufriere Hills volcano, Montserrat, showing plumes from many fumaroles on the lava dome.

Photograph of a prominent fumarole in the 2010 collapse scar with a thermal image of the same feature inset. Temperatures of this feature are up to 400 °C

Photograph of two prominent fumaroles on the lava dome, in the rear wall of the 2010 collapse scar, with a thermal image of the same features inset. Temperatures of these features are over 400 °C.

An MVO scientist installs a temperature logger and thermocouple in a sulphur-rich, low-temperature (<100 °C) fumarole area located approximately 600 m from the lava dome at the Soufriere Hills volcano.

A resistivity survey conducted by Tombs and Lee (1976) was amongst the first to evaluate the geothermal resource in Montserrat. The study identified the south-western region as a host to a potential geothermal system. Fumarole fields located on the slopes of Soufrière Hills Volcano with discharge temperatures close to boiling point (98 – 99 ° C) and a thermal spring (Hot Water pond) located on the western side of the island with maximum outlet temperatures of 90 ° C (Chiodini et al., 1996) also provided evidence of the geothermal system in this area. The thermal features on the Soufrière Hills Volcano were destroyed by the 1995 volcanic eruption. The Hot Water pond is now the only natural surface feature associated with this geothermal system. A geothermal exploration project, consisting of geological, geochemical and geophysical surveys, was conducted in 2009 to verify the existence and extent of the geothermal system in the south-west of the island (EGS, 2010). Located in this region are two topographic features, Garibaldi Hill and St. Georges Hill. Together with joint geophysical interpretation studies, a fracture controlled geothermal system was identified within the St. Georges Hill region (Ryan et al., 2013). Subsequent to the exploration programme, two exploratory production wells, MON-1 and MON-2 were drilled in 2013 to total depths of 2298 mbsl and 2870 mbsl (EGS, 2014). The two wells are approximately 500 m apart in the St. Georges Hill region (Figure 1). MON-1 and MON-2 are capable of producing 17 kg/s and 11 kg/s respectively of total flow, both at a wellhead pressure of 7 bar. Based on preliminary production tests it was reported that either of the wells is capable of generating ~2 MWe (Brophy et al., 2014). The peak load on the island is 2.1 MWe. Following the completion and testing of the wells, the Government of Montserrat decided to the expand the production and reinjection capacity of the wellfield. A decision was made to drill an additional well to provide adequate capacity. In 2016, the third geothermal well, MON-3, was drilled to a total depth of 2668 mbsl. It is located ~1500 m and 1250 m WSW from MON-1 and MON-2, respectively. Production tests on MON-3 were not executed because wellbore instability resulted in the partial collapse of the well (ISOR, 2016). Three cores ~7 – 9 m long were recovered from MON-3 at depths 1478, 1700 and 2100 mbsl. Petrological, petrophysical and geophysical laboratory tests on the cores are currently in progress. These include measurements with high temperature – high pressure triaxial apparatus, which enables petrophysical measurements to be made at reservoir temperatures and pressures. These data will help to develop the understanding of parameters such as permeability that control fluid flow in the geothermal reservoir. More importantly, the datasets will help to constrain future exploration studies on the island that will be vital for thefurther development of the geothermal field. At the Cabinet meeting of 30 September, 2021, the Cabinet of Montserrat discussed and approved the preparation of legislation relating to the development and usage of the geothermal resource. The legislation is intended to guide the development and use of geothermal energy on Montserrat and whether it is undertaken publicly or privately.  

Location of Montserrat and the geothermal project area

The Montserrat Volcano Observatory (MVO) operates over 50 monitoring stations using a wide variety of techniques. These include seismology, geology, geochemistry, petrology, deformation and degassing. Many of these are remote, powered by solar energy and telemeter the data back to MVO in real time. MVO scientists and technicians also regularly monitor the volcano using helicopter and boat traverse to access more remote areas and take data.

Helicopter at MVO

Monitoring changes to the lava dome and the fumaroles and mapping the deposits from the activity is the responsibility of the Dome Volume and Geology program and involves the application of a wide range of tools and techniques including: visual observations and photography from the ground and from a helicopter; monitoring the activity with remote digital cameras; use of remote sensing imagery, photogrammetry and field mapping for deposit mapping.

Seismic Monitoring at MVO relies on a network of seismic stations, which use seismometers to measure the ground motion at different sites around the volcano. This network has been continuously improved during the course of the eruption. A network of three stations operated by the Seismic Research Unit (now Seismic Research Centre) was in place prior to the eruption. This was expanded during the first few weeks of the eruption with equipment and assistance from the USGS/VDAP program.

Monitoring the deformation of the surface of the volcano (bulging, sinking, fractures) gives information on the magmatic system architecture, and reflects movements of magma, gas or other fluids, or movements along faults, within the earth crust. MVO has, since the beginning of the eruption in 1995, been using several volcano-deformation monitoring methods, including Global Navigation Satellite System, Strain, and Electronic Distance measurements.

Measurements of the gas emissions from the volcano have been carried out throughout the eruption. These measurements have constituted an important input to the assessment of volcanic risk. Sulphur dioxide (SO₂) and hydrogen chloride (HCl) are the species routinely monitored, primarily by remote sensing techniques.

During volcanic eruptions, energy passes into the ground in the form of seismic waves. But volcanoes can also be noisy places, and energy is also released into the atmosphere in the form of acoustic or sound waves. Infrasound is recorded using low-frequency microphones that measure changes in atmospheric pressure. MVO uses these infrasound sensors at several of its seismic stations, and it is hoped that this network can be expanded as part of the planned upgrade of the seismic network.

Source: Montserrat Volcano Observatory website

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