St. Lucia - Volcanism
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
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 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
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.
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.
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.