Geology

Regional Setting

Saint Lucia lies in the Lesser Antilles arc between the islands of Martinique in the north and St. Vincent in the south (Figure 1).  The islands of the Lesser Antilles form an arcuate line along the eastern margin of the Caribbean Sea that stretches ~700 km from Sombrero in the north to Grenada in the south and that marks the boundary between the North American and Caribbean plates.  The islands of the Lesser Antilles have formed over millions of years by volcanic processes related to the westward subduction (under thrusting) of the North American plate beneath the Caribbean plate.  These processes are still on-going today.  When the North American plate reaches depths of about 100 km, the crust is melted to form magma.  This magma is less dense than the surrounding rock, and rises up to the surface where it erupts to form volcanoes.  This process happens all the way along the plate boundary, and the line of volcanoes that is produced forms the arc.

Geology

With the exception of some minor sedimentary rocks of lower Miocene age cropping out on the east coast, Saint Lucia is made up entirely of volcanic rocks.  Lindsay et al. (2002) proposed a revised version of Newman’s (1965) subdivision of the volcanic rocks of Saint Lucia.  They propose the following groupings:

Eroded basalt and andesite centres (a revision of the ‘Northern Series’ of Newman 1965)

Dissected andesite centres (called the ‘Central Series’ by Newman 1965)

The Soufrière Volcanic Centre (a revision of the Southern Series of Newman 1965)

Caribbean map

The Soufrière Volcanic Centre is the focus of the most recent volcanic activity in Saint Lucia (Lindsay et al. 2002).  It comprises a series of volcanic vents and a vigorous high-temperature geothermal field and is associated with the Qualibou depression, a large arcuate structure that formed about 300,000 years ago as a result of an extremely large gravity slide or structural collapse.  Published ages for volcanic activity at the Soufrière Volcanic Centre range from 5 Ma to 20 ka, although the youngest volcanic activity, a series of lava domes and small explosion craters, has not been dated.  The presence of relatively young (<20 ka old) 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.

Soufrière Volcanic Centre

Previous studies on the geology of Saint Lucia all recognised that the youngest volcanoes lie in the southwest, near the town of Soufrière.  Numerous studies have been carried out in this area, both on the volcanic geology and on the geothermal system at Sulphur Springs.  Despite these studies, there is considerable confusion amongst the public of Saint Lucia as to the nature and actual location of the ‘volcano’ at Soufrière, and opinions are even divided in the scientific literature.

The most detailed and comprehensive geological study of the Soufrière area was carried out by Tomblin (1964).  He interpreted the cirque-shaped depression in this area as a caldera that formed more than 40,000 years ago at the end of a period of extremely violent volcanic activity (Tomblin, 1964; Tomblin, 1965; Robson and Tomblin, 1966; Westercamp and Tomblin, 1979).  Figure 2(a) shows Tomblin’s (1965) interpretation of the Soufrière depression.  Tomblin and co-workers defined calderas as ‘large volcanic depressions, more or less circular or cirque-like in form, the diameters of which are many times greater than those of the included vent or vents, no matter what the steepness of the walls or form of the floor’.  Their interpretation of the Soufrière depression as a caldera was based on several lines of evidence:

1. The depression has a distinct cirque-like, steep-walled topography,

2. The depression is partially infilled by younger lava domes and craters, and

3. The landscape surrounding the depression is coated by a great thickness of pyroclastic deposits, estimated by Tomblin (1964) to be about 40,000 years old, which could have been produced during a caldera-forming event. 

Geo areas

Studies carried out in the early 1980s led to a redefinition of the Qualibou depression based primarily on new age dates.  Basalts at Malgretoute and Jalousie located within the depression south of the town of Soufrière were dated as being 5 - 6 million years old.  The nearby Gros and Petit Piton, also located within the structure, were dated at 230 - 290 thousand and 260 thousand years, respectively.  These dates constrain the age of the depression.  This interpretation implies that the depression formed sometime after 5-6 million years ago.  This provides an upper age constraint for the age of the depression. The Pitons, on the other hand, lie undisturbed on the floor of the structure, which indicates the depression must have formed before them, i.e. earlier than 290 thousand years ago.  This provides a lower age constraint for the age of the depression. The depression therefore formed sometime between 5-6 million and 290 thousand years ago.

A recent study of the seafloor off Saint Lucia revealed a series of large-scale debris avalanche deposits off the south-western coast of the island that may be related to the formation of the Qualibou depression (Deplus et al., 2001).  An estimated age of <100–200 thousand years based on the thickness of overlying sediments was given for these deposits, which further constrains the age of the depression.  It suggests that the collapse probably occurred much closer to 290 thousand years ago than the upper age constraint of 5 – 6 million years.  Based on this data, Lindsay et al (2002) estimated that the Qualibou depression formed about 300 thousand years ago.  This categorically rules out early suggestions by Tomblin and co-workers that the depression formed by caldera collapse associated with the eruption of the thick sequence of pyroclastic rocks found surrounding the depression, as these were deposited much later, between 20 and 40 thousand years ago.

Extensive geothermal exploration drilling and geophysical surveys were carried out in the Soufrière region between 1974 and 1984, and these led Wohletz et al. (1986) to yet another interpretation of the depression.  They did not dispute the interpretation of the large, Soufrière depression as some sort of structural depression, but they believed there was convincing structural and stratigraphic evidence that a little over half (about 12 km2) of the area within the Soufrière depression is occupied by a caldera, which they termed the Qualibou caldera.  They believed that the extensive pyroclastic deposits in southwest Saint Lucia were indeed sourced from the caldera during a series of violent eruptions between 20 and 40 thousand years ago, and not from the volcanoes in the Central Highlands as proposed by Wright et al. (1984).  Their reinterpretation of the Soufrière depression is shown in Fig. 2(b). 

In addition to the geologic studies related to the Qualibou depression, much work has been carried out over the past 20 years in the Sulphur Springs area for the purpose of evaluating its potential as a geothermal power source.  Most structural, hydrogeologic and geophysical data obtained from these studies are consistent with the Wohletz et al. (1986) model of a small caldera restricted to the central part of the Qualibou depression.  These geothermal investigations reached the following similar conclusions:

1. The Sulphur Springs is the surface manifestation of a high-temperature, sub-surface geothermal field with good energy-producing potential. 

2. The geothermal field is related to young volcanic activity within the NE-SW trending Qualibou depression.  Geophysical surveys have revealed a possible magma body beneath the Belfond/Terre Blanche area (Gandino et al., 1985) which probably represents the heat source for the Sulphur Springs geothermal field.

3. The Qualibou depression formed by a combination of down-faulting along NE-SW trending regional faults and possible caldera subsidence related to volcanic activity (note that the subsequent results of Deplus et al. 2001 show that there was probably also a major gravity slide component).

REFERENCES

Deplus, C., Friant, A.L., Boudon, G., Komorowski, J.C., Villemant, B., Harford, C., Ségoufin, J. and Cheminée, J., 2001. Submarine evidence for large-scale debris avalanches in the Lesser Antilles Arc.  :. Earth and Planetary Science Letters, 192: 145-157.

Gandino, A., Piovesana, F., Rossi, R. and Zan, L., 1985. Preliminary evaluation of Soufriere geothermal field, St. Lucia (Lesser Antilles). Geothermics, 14(4): 577-590.

Lindsay, J., David, J., Shepherd, J. and Ephraim, J., 2002. Volcanic Hazard Assessment for St. Lucia, Lesser Antilles. Unpublished report presented to the Government of St. Lucia, August 2002.

Lindsay, J., Robertson, R., Shepherd, J. and Ali, S. (Editors), 2005. Volcanic Hazard Atlas of the Lesser Antilles. Seismic Research Unit, University of the West Indies, St. Augustine, 279 pp.

Newman, W.R., 1965. A Report on General and Economic Geological Studies St. Lucia, West Indies., United Nations report prepared for the Government of St. Lucia.

Robson, G.R. and Tomblin, J.F., 1966. Catalogue of the Active Volcanoes of the World, Part XX: West Indies. International association of Volcanology, Rome, 56 pp.

Tomblin, J.F., 1964. The Volcanic History and Petrology of the Soufriere region, St. Lucia. Unpublished PhD Thesis, University of Oxford, 213 pp.

Tomblin, J.F. 1965. The Geology of the Soufrière Volcanic Centre, St. Lucia. Fourth Caribbean Geological Conference. Trinidad.

Westercamp, D. and Tomblin, J.F., 1979. Le volcanisme recent et les eruptions historiques dans la partie centrale de l’arc insulaire des Petites Antilles. Bulletin du Bureau de Recherches Geologiques et Minieres, Series 2, 3-4(1979): 293-321.

Wohletz, K., Heiken, G., Ander, M., Goff, F., Vuataz, F.D. and Wadge, G., 1986. The Qualibou Caldera, St. Lucia, West Indies. Journal of Volcanology and Geothermal Research, 27(1-2): 77-115.