Reference Work Entry

World Regional Geology

Part of the series Encyclopedia of Earth Science pp 300-301

Grenada

  • Rhodes W. Fairbridge
  • , William J. Rea

The southernmost of the British Windward Islands in the Lesser Antilles, Grenada is situated at about 12°N and 62°W. It was formerly part of the British West Indies, becoming independent in 1974. The island was discovered by Columbus in 1498 and originally named “Concepción.” It was settled by the French in 1650 but occupied by Britain after 1783. The capital is St. George's, where there is a good harbor.

Climatically the islands lie in the NE trade wind belt, but are rarely struck by hurricanes. The precipitation is very variable, with a maximum on the windward side, ranging there to over 4000 mm, dropping to only 1000 mm in the S. The rainy season is from May to December. The greatest heat occurs from July to September, the maximum being 32.4°C and the minimum 19.9°C.

The coastline is strongly cliffed, especially on the western side, but the southern coast is more gentle, dissected into 20 long, drowned valleys (up to 1 km across and as much as 200 m deep).

Part of the inner volcanic arc of the Caribbean belt, Grenada is a spectacular volcanic island. Measuring 33 by 19 km, and covering 280 km2 (125 sq mi), it has a mountainous spine of heavily forested ridges and gorges, rising to 838 m. There are two crater lakes; one of them, Grand Etang, in the center of the island, is at 530 m, and overflows into a stream. The other, 2 m below sea level, occurs in a 95-m-deep crater and is 0.5 km across.

The basement of the island consists of a folded Eocene to Oligocene volcano-sedimentary succession. The post-Oligocene rocks include rare limestones but are mainly volcanic. Two suites of volcanic rocks are recognized (Sigurdsson, H. Sigurdsson et al., 1973): (1) a strongly under-saturated suite of olivine-rich alkali picrites, basanites, and alkali basalts, occurring mainly as lava flows with associated explosion craters; and (2) a suite of calc-alkaline andesites. Mount St. Catherine (840 m), at the northern end of Grenada, is the youngest volcanic structure on the island. It probably formed during the Pleistocene. No eruptions have been recorded in historic times, but thermal springs occur at six localities around the flanks of the mountain (Robson, G. R. Robson and Tomblin, J. F. Tomblin, 1966).

Grenada itself is the largest and most southerly island in the Grenadine Group , which consists of 8 larger and 125 smaller islands and rocks, covering in all about 100 km2. They stretch for 95 km in a SSW-NNE trend between Grenada and Saint Vincent (q.v.) and rise from a submarine bank that is mainly less than 50 m in depth. The southern Grenadines are administered by Grenada; here the largest island is Carriacou. The northern Grenadines come under Saint Vincent. The Grenadines are mainly volcanic, partly coral reefs; there are both cliffed islands and some with sandy beaches, because of their generally low relief (max. elevation 335 m).

On Carriacou , which is 15 km long and covers 34 km2 with a maximum elevation 297 m, there are reef limestones of Middle to Upper Oligocene age overlying and overlain by tuffs and agglomerates. These are followed by the “Carriacou Limestone” of Aquitanian age. After a Lower Miocene orogenic phase, there followed the Miocene “Grande Baie Beds,” deep-water shales and tuffs.

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© Dowden, Hutchinson & Ross, Inc. 1975
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