ISLAND PROFILES
St. Lucia - Geology
Saint Lucia is made up almost entirely of volcanic rocks. Like all of the islands of the Lesser Antilles, Saint Lucia began its life as a series of submarine volcanoes.
After many eruptions over millions of years these volcanoes built large topographic
features that slowly rose above the surface of the water, joined with neighbouring
volcanic islands, and grew to the island we see today.
Newman (1965) divided the volcanic centres in Saint Lucia into 3 broad groups based
on age and geographic distribution. From oldest to youngest these groups are the
Northern, Central and Southern series. This subdivision is somewhat confusing, as
several of the centres within the Northern Series are actually located in the south
of the island. Furthermore, subsequent age dates obtained for the volcanic rocks
of Saint Lucia show that several centres that were originally classed as part of
the youngest Southern Series more likely correlate with the older centres of the
Northern Series. We prefer to use a slightly revised version of the original subdivision,
grouping the volcanic rocks of Saint Lucia as follows:
Group 1: Eroded basalt1 and andesite2 centres (a revision of the
‘Northern Series’ of Newman, 1965)
Group 2: Dissected andesite centres (called the ‘Central Series’
by Newman, 1965)
Group 3: The Soufrière Volcanic Centre (a revision of the Southern
Series of Newman, 1965)
Geological map of Saint Lucia (modified from OAS, 1984).Click on map for larger view
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The eroded basalt and andesite centres are the oldest rocks on Saint Lucia and are
located in the northern and southernmost parts of the island (see map above). Age
dates for the centres in the north range from 18 - 5 Ma3 (Briden et al., 1979; Le
Guen de Kerneizon et al., 1983). The centres in the south, including Mt. Gomier,
Morne Caillandre/Victorin, Moule a Chique/Maria islands, Savannes, Beauséjour, St.
Urbain and Mt. Tourney, have published ages ranging from 10.1 Ma (lava near De Mailly)
to 5.2 Ma (lava from Savannes). The age of these eroded centres indicates that they
are unlikely to erupt again. However, there is some shallow seismicity and cold
fumarolic4 activity associated with some of the southern centres and these centres
should be monitored closely for any signs of reactivation.
The dissected andesite centres comprise the central and eastern part of Saint Lucia
and are somewhat younger than the eroded dominantly basaltic centres to the north
and south. Age dates for these centres range from 10.4 Ma (lavas west of Dennery)
to 2.8 Ma (lavas from Derriere Dos). These old ages indicate that these centres
are unlikely to be the site of future volcanic activity. The youngest volcanic activity
in Saint Lucia produced the rocks of the
Soufrière Volcanic Centre.
Glossary
1 Basalt. A type of volcanic rock with 45-55wt.% SiO2
2 Andesite. A type of volcanic rock with 55-63 wt.% SiO2
3 Ma = Millions of years
4 Fumarolic. A fumarole is a crack in the surface of the Earth
(usually associated with volcanoes) from which gases and steam are emitted.
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