Saint Lucia Geology
Saint Lucia is located within the Windward Islands of the Lesser Antilles, which are situated along the edge of one of the more volatile sections of the earth’s crust where the great American Plate meets with and submerges under the Caribbean Plate.
The result of this powerful action is the formation of a chain of volcanic islands created where molten rock material rises up through the fractured and weakened crust.
St. Lucia Has Volcanic Origins
Initially growing on the sea floor, the volcanic land masses reached the surface of the ocean to form islands. For a distance of 700 kilometers, from Anguilla in the north to Grenada in the south, these volcanic islands form an arc along the eastern border of the Caribbean plate.
While most of these islands are volcanic in origin there are exceptions. Barbados, which lies approximately 100 miles east of this arc, is a coral limestone island which was formed by the build up of layers of the skeletons of the tiny coral polyps which grow upon one another in a never-ending cycle of life and death.
Marie Galante was also formed in the same manner. The eastern half of Guadeloupe is limestone while the western half is volcanic; this pattern continues north with those islands on the east edge of the arc being of limestone origin while the islands to their west all are volcanic. This pattern of a mix of limestone and volcanic structures continues to the Virgin Islands.
The island of Saint Lucia is almost entirely of volcanic origin with exception of a small area of coral reef formations in the north.
The volcanic events which formed the island had several centers of activity and took place over a number of geological time periods. In general, parts of the island north of the Roseau Valley are older exhibiting volcanic features which are quite eroded making identification of the centers of volcanic activity impossible. To the south, on the other hand, the younger volcanic landforms are well preserved and very evident.
Volcanic Activity Timeline
Geologists believe Saint Lucia was formed during four principal periods of volcanic activity:
- 19 to 9 million years ago: A series of small volcanoes form an island of what is now from Castries to the northern tip.
- 6 to 5 million years ago: Volcanic activity forms a separate land mass that is now the southern part of the island.
- 2 to 1 million years ago: Intense volcanic activity centered in the south central section around Mount Gimie connects the northern and southern sections and builds up the island.
- Less than 1 million years ago: Another series of volcanoes become active around Soufriere Bay forming the present day Sulphur Springs and the Pitons.
Saint Lucia Geology of the Sulphur Springs
The Sulphur Springs are located within the Qualibou Caldera, a volcanic valley bordered by Morne Soufriere to the east and Rabot Ridge to the west.
Morne Soufriére, the dome shaped mountain which is clearly visible from the town and most other vantage points in the area, is 1,500 meters in diameter and 450 meters in elevation. It formed as a classic volcanic dome and due the fact it shows very little weathering is thought to be among the youngest features in the area.
The Rabot Ridge extends north to south between the Sulphur Springs and the west coast road and is thought to have formed at the same time as the Pitons.
The slopes facing the Sulphur Springs on both Morne Soufriere and Rabot Ridge are affected by a number of faults (fractures in the earth’s crust) which produced crustal movements that shaped the surface topography in the area. These fractures also allow rain water to infiltrate deep into the earth where it comes in contact with hot rocks. The pressure created by the heating causes the water/steam to rise back to the surface erupting as either fumaroles (mostly steam and gases) or hot springs (mostly water).
The volcanic surface features of the Sulphur Springs are confined to an area of approximately 100 x 250 meters and are located within blocks of the crust which slumped down as part of the activity of the faults on the Rabot side. The most common features are the hot spring pools filled with bubbling black water.
The color comes from hydrogen sulphide that is brought to the surface in the steam and gives the area its characteristic pungent smell. The temperatures of the hot springs range from 63ºC to 96 ºC.
Amid the hot spring pools are fumaroles which are open cavities which discharge steam and gases. The temperatures of the fumaroles range from just above the boiling point to 171ºC.
As a result of the activity of the hot springs and fumaroles over the centuries, the Sulphur Springs area is covered with extensive deposits of minerals such as kaolin, sulphur, ferrous sulphide and gypsum. Most of the minerals are white in color with the sulphur being greenish-yellow. Around the margins of the active hot features orange and reddish deposits of iron oxides are found.
Directly related to the activity of the Sulphur Springs is the existence of numerous other hot springs in the area. The best known is the one feeding the mineral baths on Diamond Estate. Its therapeutic values had been recognized in the 18th century by the doctors of King Louis XVI who recommended the use of the spring for the recuperation of French troops.
The features found at the Sulphur Springs and the surrounding areas are the only visible volcanic features present in Saint Lucia today.
The last significant volcanic eruptions occurred between 3,700 and 20,000 years ago at Belfond, which is located above the Sulphur Springs to the south. Historic records indicate the activity of the hot springs and fumaroles have maintained a fairly constant tempo throughout the past 240 years.
The most recent documented episode of more intense activity occurred in 1766 when what was described as a ‘steam explosion that spread a thin layer of cinders far and wide’ occurred. Local lore also holds that less intense steam explosions occurred during the 1800s but with no significant impact.
Volcanic Activity Close to St. Lucia
Saint Lucia’s immediate island neighbors have not been so lucky in more recent times. On May 8, 1902, the town of St. Pierre in Martinique was destroyed by a powerful eruption of La Montagne Pelee. Within minutes the volcanic mountain was blown to pieces sending a wall of flames (glowing cloud of hot gases) over the town killing 28,000 inhabitants. Only two persons survived the event.
On the island of St. Vincent, the first documented eruption of the Mount Soufriére volcano occurred in 1718. Another eruption in 1812 lasted three days and destroyed property and lives. In February, 1902, the mountain started rumbling once again, a warning sign which was totally ignored by the inhabitants. On May 7, a violent eruption occurred which destroyed many villages and killed 2,000 people. On April 13, 1979, the volcano exploded again blasting ash, stones and steam many thousands of feet into the air. Over the ensuing 12 days there were 19 more explosive eruptions. Luckily, people evacuated quickly at the on-set of the events and no lives were lost but there was considerable damage to homes and property.
On the St. Augustin campus of the University of the West Indies in Trinidad, seismologists continuously monitor all volcanic activity in the Caribbean region with their main concern being the volcanoes in St. Vincent, Martinique and Guadeloupe.
Geothermal as an Energy Source?
Over the past 60 years scientists have explored the possibility of tapping the geothermal energy sources found in Saint Lucia’s Soufriére region. A team of Scandinavian scientists funded by the United Nations first examined the possibilities in the area of the Sulphur Springs in the 1950s. In the 1970s the United Kingdom Ministry of Overseas Development drilled seven shallow exploratory wells; steam was found in four of the seven boreholes.
Later studies speculated that geothermal fluids and gases should be found in abundance at a depth of between 3,000 and 6,000 feet under the central and southern caldera area where porous rocks and faults allow greater fluid movement. A major drilling project, funded by the United Nations, US AID and the Government of Saint Lucia, was launched in 1986.
A successful well needed to show three characteristics:
- Temperatures above 180ºC
- Fractures within the rocks
- The existence of water/steam
The first well, drilled at Belfond to a depth of 7,261 feet, found temperatures in excess of 240ºC but an absence of rock fractures resulted in disappointment.
A second well was then drilled just above the Sulphur Springs. At a depth of 4,636 feet temperatures of 293ºC and the right formation of rock fractures were found. It was calculated that enough steam could be produced to generate 3 – 4 megawatts of electricity, which would be sufficient to power much of the southern part of the island.
In the past few years a few foreign companies have made proposals to further the development of viable geothermal energy on the island but no projects have been initiated. In the meantime, Saint Lucia has the assurance that there is an alternative source of energy available which could make the island energy independent in the future.