About Grenada

Abstract

The island of Grenada is composed of a mixture of monogenetic and polygenetic volcanic centres, none of which has erupted during the historical period. The abundance of reworked volcaniclastic deposits throughout the island limits the reconstruction of the island’s volcanic history and hampers hazard assessment. Monogenetic explosion craters define a NE-SW trend across the island, and it is possible that similar craters may develop along this line in the future. Mt. St. Catherine is considered to be the only live polygenetic volcano on Grenada based on its well-preserved morphology, the presence of associated fumaroles on its flanks and the occurrence of volcanic earthquakes in the past. Its volcanic deposits indicate that both effusive (dome-building) and explosive eruptions are possible. The types of volcanic activity expected in the future are steam venting, phreatic explosions and lava dome-building eruptions from a vent located within the breached crater at the summit. Such activity may result in pyroclastic flows and surges, lahars and ash fall which will likely affect villages located to the east and west of the volcano. Explosive eruptions would impact a much wider area but are considered to be less likely to occur in the near future.

Introduction

In assessing the volcanic hazard likely to be posed by future eruptions on Grenada, this chapter takes an empirical approach utilising both published reports on the geology of the island and fieldwork conducted by the author. In attempting to define the most likely styles of eruption possible on the island, consideration is given to the historical seismicity and geothermal activity along with the geologic record of past eruptions. The style of activity exhibited by volcanoes elsewhere in the region is also considered.

Geographical Setting

Grenada is an oval shaped island, elongate in a north-east to south-west direction. It is approximately 34 km long and 19 km wide with a total area of 312 km2. It has a rugged topography reaching a maximum height of 910 m at the Mt. St. Catherine volcanic centre. Central peaks stretch along the length of the island. Apart from Mt. St. Catherine, the principal peaks from north to south are Mt. Granby (683 m), Mt. Qua Qua (640 m), South East Mt. (703 m), Mt. Lebanon (700 m), Mt. Sinai (703 m) and Mt. Maitland (522 m). The island is asymmetric, with the western side of the island being well dissected with deep, sharply cut valleys forming a well-indented coastline, while the east coast slopes gently to the sea. The population is 97,000, with the majority located in and around the capital of St. George’s. The economy is based largely on agriculture and tourism.

Andesite domes of Levera Island and Green Island (background), part of the Northern Domes Volcanic Centre

Relief map of Grenada

Geology

A characteristic feature of the geology of Grenada is the large surface area of secondary or reworked volcanic material (Arculus 1973) caused by erosion of primary volcanic deposits. This has made it difficult to determine the lateral extent and evolution of volcanic centres. Past efforts at reconstructing the volcanic geology of the island (e.g. Arculus 1976; Geotermica Italiana 1991) have been based on geological observations, the degree of dissection of the topography, radiometric ages and photogeological interpretation.

Grenada is unusual in the Caribbean in its abundance of monogenetic basaltic explosion craters and is unusual among arc volcanoes in general in its abundance of highly magnesian and silica-undersaturated basalts. It consists of a folded lower Tertiary volcaniclastic and sedimentary basement (the Tufton Hall formation) overlain by an approximately 900 m (2500 ft) thick sequence of eroded but tectonically undisturbed Miocene to Recent volcanic rocks (Arculus 1976). Arculus (1973, 1976) divided the Miocene to Recent volcaniclastic rocks into a number of volcanic centres including: the Northern Domes, Southwest Grenada, Southeast Grenada, Mt. Sinai, Mt. Moritz, Mt. Maitland; Mt. Granby-Fedon’s Camp, Mt. St. Catherine and the Explosion Craters. Geotermica Italiana (1991) also subdivided the volcanic formations but used slightly different boundaries. The description used in this chapter follows that of Arculus (1973, 1976) with the incorporation of information provided by Geotermica Italiana (1991).