St. Vincent - Geology
St. Vincent is entirely volcanic and Rowley (1978a) observed that there is "roughly
an even distribution of lava flows and pyroclastics". However, recent mapping suggest
that there is much variation in this distribution, so that at some centres (e.g.
South-East Volcanics) lavas are dominant while at others (e.g. Grand Bonhomme Volcanic
Centre), volcaniclastics predominate. Rowley (1978b) estimated that 55% of the island
was mantled by well-bedded, pyroclastic fall deposits (the Yellow Tephra Formation),
produced by eruptions of the Soufrière volcano during the late Pleistocene.
The rock types exposed on the island fall into four compositional groups: basalts;
basaltic-andesites; andesites, and xenoliths of coarse-grained plutonic and metamorphic
character. Basalt is generally the most abundant rock type in the most southern
parts of the island while basaltic-andesites dominate further north. Basaltic magmas
are most common during the early stages in the evolution of individual volcanic
centres and andesites are most common as dykes and as the last stage dome or central
plugs, which occupy the vent at some centres.
Structurally the island is aligned along a north-south axis. Slope gradients along
the west of the central axis of the island are significantly greater than gradients
on the east. No field evidence has been found of faulting but almost all the major
river courses on the island appear to be structurally controlled. An emergent coastline
found along the east coast has been suggested by Rowley (1978a) to be due to Plio-Pleistocene
uplift. Erosion has severely dissected the southern volcanic centres and original
structures cannot be readily identified. Arcuate scarp features located at Grand
Bonhomme, Morne Garu and the Soufrière volcano have been attributed to relict caldera
or collapse structures (Rowley, 1978a; Sigurdsson and Carey, 1990; Geotermica Italiana,
1992). A number of cold mineral springs are located in the southern parts of the
island but fumarolic
activity is confined to the Soufrière volcano.
St. Vincent has been divided into four major geologic regions: the South-East Volcanics,
and the Grand Bonhomme, Morne Garu and Soufrière Volcanic Centres (Robertson 2003);
based an examination of of the topography field geology, geochemistry and previous
work undertaken on the island. The geology of the Pre-Soufriere Volcanic centtres
are described below. Information on the Soufriere Volcanic Centre can be found here.
Geological map of St. Vincent (from Robertson 2003).
Photograph of southern St. Vincent showing the typical landscape of the SEV
Milikin's Bay, St. Vincent showing to spatter cones at Brighton.
The Pre-Soufrière Volcanic centres of St. Vincent
The pre-Soufrière Volcanic centres of St. Vincent consists of the South-East Volcanics
and the Grand Bonhomme and Morne Volcanic Centres (Robertson, 2003).
The South-East Volcanics is the most southerly geologic region
on the island. It is a dissected landscape of rounded hills with low topography
(<210 m), which extends from the Warrawarrow River in the west to the extensive
Yambou lava flow in the east. The area is dominated by red scoriaceous basaltic
spatter interbedded with and often overlying, massive to well-jointed basaltic lava
flows, which are intruded by dykes. It contains the oldest rocks exposed on the
island (2.74 ± 0.11 Ma; Briden, et al., 1979) and is mostly overlain by fine-grained
yellow ash, which are correlated with late Pleistocene Yellow Tephra erupted by
the Soufrière volcano (Hay, 1959, Rowley, 1978b). The youngest deposits exposed
in the area are alluvial silt, sand and gravels found in the river valleys.
The Grand Bonhomme Volcanic Centre extends from Argyle to Colonarie
in the east and Sion Hill Bay to Chateaubelair in the west. It is the largest geologic
region on the island and is interpreted as a large stratovolcano with interbedded
sequences of block and ash pyroclastic flow deposits, ashfall deposits, lava flows
and subordinate domes. The landscape is heavily forested and the interior inaccessible
and composed of deeply weathered lavas and volcaniclastic deposits. This volcanic
centre is a composite of several eruptive centres that are now represented by the
topographic highs of Grand Bonhomme (970 m), Petit Bonhomme (747 m), Mount St. Andrews
(735 m) and an unnamed peak (1021 m). These peaks are central domes or plugs of
volcanoes that coalesced to form a large composite volcanic centre. Previous dating
of lavas from the western flank of the Grand Bonhomme Volcanic Centre by Briden
et al (1979) obtained ages of 1.33 ± 0.09 and 1.18 ± 0.10 Ma respectively for lava
flows at Westwood and Chateaubelair.
The Morne Garu Volcanic Centre occurs immediately to the north
of Grand Bonhomme and consists of Mount Brisbane (932 m) to the east and Richmond
Peak (1074 m) to the west. These two peaks are the remnants of an eroded Morne Garu
crater or caldera that is estimated to have been 3 km in diameter (Sigurdsson, et
al., in prep). Morne Garu is largely inaccessible and the underlying volcanics are
extensively covered with fine-grained yellow ashfall deposits. Recent ages obtained
by Heath et al. (1998, 1998) from lavas at Indian Estate (11 ± 14 ka) and Black
Point (180 ± 20 ka) on the western flank of Mount Brisbane indicate that volcanism
may have been much younger at this centre and may have overlapped with the Soufrière
Volcano to the north. The major formations exposed are lava flows, undifferentiated
volcaniclastics, red scoria bombs and yellow ashfall deposits. Reworked alluvial
deposits occur in the major river valleys.
Monogenetic spatter cones
A number of rounded spatter cones composed of a poorly consolidated sequence of
clast-supported, pumice lapilli airfall, scoria bombs and ash resting on old lava
flows occur mainly in the southeast of St Vincent although some are found further
north. Eruptive centres were identified at Kings Hill, Diamond (S) and Rose Cottage.
Eruptions produced abundant scoria bombs, which fell close to these centres and
formed thick, and sometimes welded deposits. Ash and small projectiles deposited
further from the vents produced discrete beds.
The best exposure of spatter cones in the northern part of the island occurs at
Belleisle Hill where a thick sequence (>20 m) of interbedded grey lapilli-sized
ash and red scoria is overlain by yellow ash. The red scoria clasts are composed
of olivine microphyric basalts but the scoria beds also contain angular basaltic-andesite.