The University of the West Indies Seismic Research Centre

Soufrière Volcanic Centre
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 (see Figure 1 below).

Figure 1. Map of the main vents of the Soufrière Volcanic Centre.

The oldest dated rocks of the Soufrière Volcanic Centre are 5-6 million year old basaltic1 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 dacitic2 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 surges3 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).

Petit and Gros Piton. The Pitons are the remants of two lava domes that formed about 200-300 thousand years ago.

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 column4 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 eruption5 occurred in Saint Lucia.

The steep slopes behind the northern
end of the town of Soufrière represent
the northern margin of the Qualibou Depression.

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 effusive6 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
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.

Glossary
1 Basalt. A type of volcanic rock with 45-55 wt.%SiO2.

2 Dacite. A type of volcanic rock with 63-70% SiO2.

3 Pyroclastic flows and surges. A pyroclastic flow is a hot (100-600 0C), fast-moving (>100km/hr) mixture of ash, rock fragments and gas. They usually travel down valleys and cause total destruction of the area over which they flow. Pyroclastic flows have been the main cause of destruction and loss of life in Montserrat since 1995. A pyroclastic surge is a dilute turbulent cloud of gases and rock debris that moves above the ground surface at great speeds. These form in a similar way to pyroclastic flows, but their effects are more widespread. Pyroclastic surges can be either hot or cold.

4 Eruption column. Explosive eruptions generate abundant ash and other volcanic particles which are carried up into the atmosphere by expanding hot gases to produce a buoyant eruption column.

5 Magmatic eruption. Eruptions involving the release of magma (molten rock) at the surface of the Earth. Magmatic eruptions may be either explosive or effusive. Explosive magmatic eruptions occur when dissolved gases in a rising magma expand to form gas bubbles which then burst as the magma nears the Earth's surface, leading to explosive fragmentation of the magma.

6 Effusive eruption. Effusive eruptions occur when molten rock (lava) reaches the Earth's surface and erupts passively. The products of these eruptions are lava flows and lava domes. They generally occur when the gas content of the magma is low.