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Skip Navigation LinksHome : Island Profiles : St. Vincent : Geology
ISLAND PROFILES
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.