St. Lucia
Flag
Location
13.88°N, 60.97°W
Area
616.00 sq km
Capital
Castries
Timezone
Eastern Caribbean (UTC-4)
Population
166,637 (2021 est.)
Full Country Name
Saint Lucia
Geography
Dominated by high peaks and rain forests in the interior, Saint Lucia is well known for the twin peaks of Gros Piton and Petit Piton on the southwestern coast, its beaches and natural harbours. Mount Gimie, the highest peak, is located in the central mountain range and rises to 958 meters (3,143 ft) above sea level. Saint Lucia has a tropical, humid climate moderated by northeast trade winds that allow for pleasant year-round conditions. Mean annual temperatures range from 26 °C (78.8 °F) to 32 °C (89.6 °F). Annual rainfall is approximately 1,999 mm with most precipitation occurring during the June to December wet season.
Country Facts
One third of the population resides in Castries. Gros Islet, Soufrière and Vieux Fort are the other major towns. English is the official language but Saint Lucian Creole French, colloquially referred to as Patois, is spoken by the majority of the population. Currency used: Eastern Caribbean Dollar (XCD) or US Dollar (USD).
Disaster Management
In the event of an earthquake, volcanic eruption or tsunami the National Emergency Management Office is the official authority in Saint Lucia.
National Emergency Management Office
PO Box 1517
Bisee
Castries
Saint Lucia
Tel: +1 758-452-3802
Email: admin@nemo.gov.lc
Website: www.nemo.gov.lc/
Seismicity Overview
Saint Lucia has an intermediate seismic hazard for the Eastern Caribbean region. The island lies in a transition zone where the rate of seismic activity is climbing. The island’s seismic hazard is not as low as St. Vincent’s but it is not as high as Martinique’s. There are more than 1,500 earthquakes in the defined Saint Lucia seismic zone. – 4 was the annual average number of events greater than magnitude 2.1 up to 2010 that occurred in this specified area; it is now 39. This zone is not as active as areas in the northern Leewards and the Trinidad and Tobago area, but does have a history of a major event. There have been at least five swarms of shallow earthquakes in Saint Lucia in the last 100 years. These occurred in 1906, 1986, 1990, 1999 and 2000. A sixth burst may have occurred in early 1998 when a number of earthquakes were reported felt but for which there were no seismograph recordings. At least three of these swarms, those of 1906, 1990 and 2000, seem to have been triggered by a larger tectonic earthquake. The last tectonic earthquake of note was of Magnitude 7.75 in 1953. The earthquake caused partial collapse of buildings previously damaged by fire, and some damage to other buildings in the capital city of Castries. New buildings designed to resist earthquakes were undamaged.
Volcanism Overview
The Soufrière Volcanic Centre is located near the town of Soufrière in southwest Saint Lucia. This is the youngest and only ‘live’ (likely to erupt again) volcanic centre in the island. About 30,000 years ago violent, large explosive eruptions generated large pyroclastic flows and extensive ashfall. The high cliffs at Choiseul Beach were produced by these eruptions. Since then several lava domes have formed in the Soufrière region, producing smaller eruptions similar to those of the Soufrière Hills Volcano in Montserrat. The most recent eruption associated with the Soufrière Volcanic Centre was a phreatic or steam eruption in 1766. Since then, there has only been hot spring and fumarolic (gas/steam vent) activity. The spectacular Gros and Petit Piton are remnants of two large dacitic lava domes that formed about 200-300 thousand years ago. They represent the steep inner core (“volcanic plug”) of two lava domes after almost all the loose rubbly material that normally aprons lava domes has been removed by efficient erosion due to the wind and the sea.
With the exception of some minor sedimentary rocks of lower Miocene age cropping out on the east coast, Saint Lucia almost entirely volcanic. Lindsay (2005) categorized these rocks based on age and distribution into 3 main groups which are described below from oldest to youngest.
Figure 1: Geological Map of Saint Lucia (Lindsay 2005)
Eroded basalt and andesite centres
The eroded basalt and andesite centres are the oldest rocks in Saint Lucia. They crop out in the northern and southern-most parts of the island, and rocks of similar age and composition probably underlie most of the younger rocks found elsewhere on Saint Lucia. The oldest radiometric ages obtained from these basalts are 15 – 18 Ma (Briden et al. 1979, Le-Guen de Kerneizon et al. 1983), although shallow marine volcaniclastic deposits in the northern part of Saint Lucia potentially date back to the Eocene.
Dissected andesite centres
The central part of the island, extending down the southeast coast contains many andesitic lavas and volcaniclastic deposits that appear younger than the deformed basaltic rocks to the north, yet are dissected enough not to appear recent. These forested and largely inaccessible central highlands have resulted in limited dating capabilities for these dissected andesitic centres. Available eruption ages indicate activities between 10.4 and 1 Ma (Le-Guen de Kerneizon et al., 1983).
Soufrière Volcanic Centre
Located in the south-western part of the island, the Soufrière Volcanic Centre is the focus of the most recent volcanic activity. It comprises a series of extensive pyroclastic flow deposits, lava flows, domes with associated block-and-ash-flow deposits, and explosion craters that are located in and around the Qualibou depression. This depression, interpreted to be a caldera, is a large arcuate structure about 10 km in diameter that formed about 300,000 years ago. Published ages for volcanic activity at the Soufrière Volcanic Centre range from 5 Ma to 20 ka (Lindsay et al. 2013).
In the Lesser Antilles island arc, Saint Lucia lies just north of the low seismicity segment from Grenada to St. Vincent and the Grenadines. For several decades, the background seismicity in this area was at a relatively low level, as may be seen from the annual magnitude distribution plot of earthquakes since 2000. However, on 2008/09/02 a magnitude 4.7 earthquake occurred east of Saint Lucia that was associated with several other earthquakes on the same day, with a total of 37 located events by the end of September. Activity in the area decayed over time, but has persisted with a steady output of events, including episodic bursts. There have been more than 400 events in that east of Saint Lucia area, since 2008. In addition, there is an area just north of Saint Lucia that has been outputting low magnitude events during the instrumental era. The largest was of magnitude 4.5 and occurred on 1990/-05/19; for 2017, the largest in that area was of magnitude 3.1. The dense clusters of epicentral plot are as a result of these areas of heightened output. It is in the general area near the north coast of Saint Lucia that one of the major earthquakes of the 20th Century was located. On 1953/03/19, a magnitude 7.7 event occurred near the north-west coast of Saint Lucia. The damage impact was at the MMI VII. In Castries there was partial collapse of buildings previously damaged by fire and some damage to other buildings. New buildings designed to resist earthquakes were undamaged Similar levels of shaking were experienced during the 1843/02/08 great earthquake, estimated magnitude 8.0-8.5, which was located north-east of Guadeloupe, at a distance of more than 200 km. In Castries the Roman Catholic Church, Custom House, gaol and other masonry buildings were cracked, but none were severely damaged.
Along with the earthquake activity shown in the epicentral plot, on 2015/07/17, there were two strong earthquakes, of magnitude 6.3 and 6.6, in a sequence of hundreds of events that was located about 200 km directly east of Saint Lucia.
In the past, Saint Lucia has experienced damage from a major earthquake close to the island. All indicators suggest that such large earthquakes, and possibly larger, are also in its future.
In addition to the earthquakes that arise from tectonic processes, volcanic processes can generate earthquakes. These volcanic processes produce a background level of such earthquakes, some of which may be seen in the cluster of events in southern Saint Lucia. An elevation in the number of volcanic earthquakes being seen is symptomatic of heightened activity within the system and is used as an early warning sign of possibly important changes in the volcano requiring a pre-determined response.
Soufriere Volcanic Centre
- 13.83°N, 61.05°W
- Elevation – 777m
- Last eruption – 1766
The Soufrière Volcanic Centre, located in the southwestern region of the island, is the focus of the most recent volcanic activity in Saint Lucia. It comprises a series of different volcanic vents and a vigorous high-temperature geothermal field manifested at the Sulphur Springs area. It is located within the Qualibou depression, an arcuate structure that formed about 300 thousand years ago due to an extremely large gravity slide.
The oldest dated rocks of the Soufrière Volcanic Centre are 5-6 million years old lava exposed near the coast at Jalousie and Malgretoute. This probably correlates with the basaltic activity of similar age further to the southeast. About 2 million years ago a major phase of volcanism led to the formation of Mt. Gimie and its neighbouring mountains. The spectacular Pitons are the remnants of two large lava domes that formed about 200-300 thousand years ago.
Petit and Gros Piton. The Pitons are the remants of two lava domes that formed about 200-300 thousand years ago.
An intense and violent phase of volcanic activity occurred at the Soufrière Volcanic Centre between 40 and 20 thousand years ago when a series of major eruptions produced numerous dacitic pyroclastic flows and surges that flowed down all major valleys in the southern half of Saint Lucia and produced the deposits that now make up the southern slopes of the island. The deposits formed during these explosive eruptions have been divided into two main groups: the Choiseul and the Belfond pumice deposits (Wright et al. 1984).
It has been proposed that these explosive eruptions occurred from within the Qualibou depression, and led to the formation of a semi-circular volcanic collapse feature known as the Qualibou caldera (Wohletz et al. 1986). Other workers claim that the radial distribution of the numerous pyroclastic flow deposits in southern Saint Lucia suggests that they did not come from within the Qualibou depression at all, rather from small vents in the Central Highlands (e.g. Mt. Grand Magazine and Piton St. Esprit) (Roobol et al. 1983 and Wright et al. 1984).
The nature of the Choiseul and Belfond pyroclastic flow deposits indicate a particular style of eruption. They were formed by explosive eruptions that generated a buoyant eruption column which subsequently collapsed to produce pyroclastic flows. Such eruptions are particularly devastating, because the pyroclastic flows that are generated can travel out from the vent in all directions.
After the phase of explosive activity that formed the Choiseul and Belfond pyroclastic deposits a series of small lava domes (e.g. Terre Blanche, Belfond) and explosion craters (e.g. La Dauphine estate) formed near the centre of the depression. Some minor dome-collapse pyroclastic flow deposits (block and ash flow deposits) are associated with the lava domes, indicating a history of dome growth and collapse. Thin deposits of pyroclastic material surround the explosion craters, and these probably formed during minor, short-lived, explosive events. Field relations indicate that the explosion craters are younger than the adjacent Belfond lava dome. Unfortunately none of these domes or craters has been dated and it is therefore impossible to say with certainty when the last magmatic eruption occurred in Saint Lucia.
The presence of the relatively young (< 20,000 years) lava domes and craters together with the active geothermal field at Sulphur Springs indicates that the Soufrière Volcanic Centre is potentially active and may erupt again.
Historical Eruptions
Historical eruptions in the Caribbean are generally regarded as those that have occurred since European settlement and the introduction of written records of the region. In Saint Lucia European settlement began in the early 1600s but was intermittent for most of the 17th century, changing hands several times between the French and English.
There have been no historical magmatic eruptions in Saint Lucia, i.e. eruptions involving the effusive or explosive ejection of magma at the surface of the Earth. There have, however, been several minor phreatic (steam) explosions from the Sulphur Springs area in historic times. The last one occurred in about 1766 and was described by Lefort de Latour (1787) as a ‘minor explosion, which spread a thin layer of cinders far and wide’. These ‘cinders’ (ash) probably represented fragments of old rock blasted apart by expanding steam rather than fragments of new magma.
Future Eruptions & Conclusion
The occurrence of occasional swarms of shallow earthquakes together with the vigorous hot spring activity in southern Saint Lucia indicate that this area is still potentially active and the island can therefore expect volcanic eruptions in the future.
The Soufrière Volcanic Centre is the most likely location for future eruptions in Saint Lucia. There are four different scenarios for future activity at this centre; in order of decreasing probability these are: 1) a phreatic (steam) or hydrothermal eruption from the Sulphur Springs area; 2) a small explosive magmatic eruption forming an explosion crater in the Belfond area; 3) an effusive magmatic dome-forming eruption within the Qualibou Caldera and 4) a large explosive magmatic eruption from either the Central Highlands or from within the Qualibou Caldera.
The most likely activity is a phreatic (steam) or hydrothermal eruption from the Sulphur Springs geothermal field. Such an eruption would be relatively small, and would only affect the area directly surrounding Sulphur Springs. Phreatic and hydrothermal eruptions do not erupt fresh magma, and tend to be short lived (a few hours or days). A prolonged series of large phreatic eruptions may, however, herald the onset of an actual magmatic eruption.
The most likely activity is a phreatic (steam) or hydrothermal eruption from the Sulphur Springs geothermal field. Such an eruption would be relatively small, and would only affect the area directly surrounding Sulphur Springs. Phreatic and hydrothermal eruptions do not erupt fresh magma, and tend to be short lived (a few hours or days). A prolonged series of large phreatic eruptions may, however, herald the onset of an actual magmatic eruption.
Related Resources
The well-known Sulphur Springs of Saint Lucia is the hottest and most active geothermal area in the Lesser Antilles. The main area of Sulphur Springs comprises numerous hot springs, bubbling mud pools and fumaroles (steam vents) in an area of strongly hydrothermally altered clay-rich rock approximately 200 m x 100 m in size. Many fumaroles have temperatures 100°C or hotter, and temperatures of up to 172°C have been recorded. Numerous studies have been carried out over the past 50 years to investigate the geothermal energy potential of Sulphur Springs, to date, however, no attempt at exploitation has been made.
Sulphur Springs geothermal field. The constant emission of steam does not serve as a “safety valve” for the volcano.
Geothermal systems such as Sulphur Springs form when rainwater seeps into the ground where it is heated by hot rock. The hot water becomes buoyant, and rises back to the surface along cracks. In some places the water is heated so much that it rises as steam. The heat source for the Sulphur Springs geothermal system is probably the cooling magma body responsible for the young volcanism of the Soufrière Volcanic Centre (SVC). The diagram below illustrates this process.
Currently, activity at Sulphur Springs is concentrated on the western side of the Sulphur Springs Road. However, extensive areas of hydrothermally altered ground together with stunted vegetation on the eastern side of the road (i.e. on the flanks of Terre Blanche) clearly show that this area was once active. Furthermore, the area beneath the viewing platform, including Gabriel’s crater, does not appear on a map of Sulphur Springs from the 1950s (Robson and Willmore 1955), indicating that this area of activity is relatively recent. It is possible that, over time, activity at Sulphur Springs might continue migrating to the south and west.
Such migration of activity in geothermal systems such as Sulphur Springs is quite normal. The area should, however, be watched closely for signs of further migration, as this may have a significant long-term impact on nearby residences and structures, such as the viewing platform. Migrating geothermal activity into areas of steep slopes may also increase the likelihood of landslides triggered by extensive hydrothermal alteration. Any significant changes in the geothermal features at Sulphur Springs should be reported to the National Emergency Management Organisation (NEMO) and/or the Seismic Research Centre.
Hazards from Sulphur Springs
Scientists believe there is only a very slim chance of a magmatic eruption from the SVC in the next 100 years. However, there are serious ever-present hazards at Sulphur Springs of which visitors should be aware.
Hot geothermal systems such as Sulphur Springs, emit large amounts of harmful gases. Two of the more dangerous gases are carbon dioxide and hydrogen sulphide. Breathing air with greater than 20% carbon dioxide can cause death. The gas is heavier than air and tends to accumulate in hollows in the ground, displacing the breathable air. Since it is colourless and odourless, people and animals are unable to notice that it is there and may suffocate. In areas of hot or cold Soufriere people should not enter low-lying hollows to remove dead animals in case there is a build-up of carbon dioxide.
Hydrogen sulphide has a very strong and unpleasant smell, like rotten eggs. It is extremely toxic. Breathing low concentrations causes headaches, fatigue, dizziness, and irritation of the eyes and upper respiratory tract. Inhalation of hydrogen sulphide can seriously aggravate sinus and respiratory systems and can cause bronchitis.
Other dangers at Sulphur Springs include landslides, boiling pools, and phreatic (steam) eruptions.
