The Caribbean Sea is geologically separate from the North and South American continents. Together with Central America, it forms a tectonic plate which is moving W at about 4 cm a year. At the same time, the North American and South American plates, which lie under the continents and the eastern half of the Atlantic Ocean, are moving W. Most of the Caribbean islands are close to the boundaries of the Caribbean plate. Plate boundaries are geologically active; this is why most parts of the Caribbean experience earthquakes from time to time, and why there are a number of active volcanoes in the region. The Windward islands, along the eastern boundary of the plate, are the most volcanically active area.
This is because they lie over a subduction zone, where the Caribbean plate is being pushed E, over the edge of the Atlantic portions of the North American and South American plates. Sediments from the ocean floor are drawn down below the surface. They melt, and move up towards the surface as magma. Where a plume of magma reaches the surface, a volcanic island is formed. There are active volcanoes on Montserrat (where the local population has been evacuated from the S to the N because of eruptions since 1995); St Vincent and Guadeloupe (both called the Soufrire, because of the strong smell of sulphur); and Martinique, of course, has Mont Pele, which wiped out the city of St Pierre on 8 May 1902.
There is also a submarine volcano with the picturesque name of Kick Em Jenny, to the N of Grenada. All the Windwards, and most of the Leeward Islands, have clear signs of geologically recent volcanic activity, and could become active again in the future. There are recognisable volcanic craters, hot springs, solfatara's (of which St Luci as famous drive-in-volcano, also called Soufrire, is one example), and the famous boiling lake on Dominica. Barbados is not a volcanic island. It lies more directly o the plate boundary, and is formed by a wedge of sediments which are being pushed upwards as the plates move together.
The surface rocks in most of Barbados are the remains of old coral reefs which grew as the water over this wedge of sediments became shallow. The eastern part of Guadeloupe, Grande Terre, and the small island of Marie Galante were formed in much the same way. The oldest rocks in the Greater Antilles: Jamaica, Hispaniola, and Puerto Rico, were formed about 70 million years ago, when the Caribbean plate was moving N, and there was a line of volcanic islands along the plate boundary. There are no active volcanoes in this part of the Caribbean now, because the plate is no longer moving N. But there is intense faulting and fracturing of the crust as the Caribbean plate moves E, past the southern boundary of the North American plate.
This faulting has thrust these three large islands up above sea level. The rocks in the Greater Antilles have also been folded by earth movements. The combination of folding and faulting has produced a hilly and sometimes mountainous landscape. Large areas of Puerto Rico and Jamaica are also covered by limestone, which was formed about 30 million years ago when this part of the earths crust was below sea level. The island of Jamaica is now being gradually tilted to the S the N coast is being pushed up above sea level, and the S drowned. Off the coast of southern Jamaica, there are large areas of relatively shallow sea which were land when sea levels were about 30 m lower than they now are, during the ice ages of the past million years.
Cuba and the Bahamas are part of the North American plate. Southern Cuba is mountainous, and strongly affected by the plate boundary. But the rest of Cuba and the Bahamas are geologically quite stable, and are formed mainly of limestone. The Bahamas are on a section of crust which has been stretched and weakened over the past 120 million years as the North American plate moved away from the African plate and the Atlantic Ocean became wider.
For the whole of this period, the Bahamas has formed a shallow tropical sea. Evaporation from the warm sea surface causes the concentration of calcium carbonate on the water to become very high, so tiny grains of this mineral, ooliths, form and collect on the sea bed. These grains form a rock known as oolite. The Bahama Banks are a platform of oolite several kilometres thick.
During the glacial periods, sea level fell, and the Banks became enormous islands. Sand dunes which formed in the ice ages solidified, and remained above sea level when the ice melted and the sea rose again, to form the present-day Bahama islands. To the S, Trinidad and Tobago were joined to the South American mainland when sea levels were low in the ice ages, hence the richness and variety of their plant and animal life. The boundary between the South American and Caribbean plate actually runs through N Trinidad, so these islands are another earthquake zone. There are also signs of early volcanic activity on Tobago, though not from a geologically recent period. Many Caribbean coastlines are being pushed upwards by earth movements: Barbados, the N coast of Jamaica, NW Haiti for example.
Along many of these emergent coastlines, the land rises in a series of steps, each one marking an old coastline and a fossil coral reef. Where the land is being slowly submerged, as along the S coast of Jamaica, in the Bahamas, Antigua, and many of the Windwards, there is an indented coast with many offshore islands. Some shallow bays in these areas are being filled in by mud and other sediments; this makes interesting wetland wildlife. The Guianas on the South American mainland are by contrast geologically very stable, and are formed of ancient rocks several thousands of millions of years old.
Guianas gold and diamonds are derived from these ancient rocks. In the coastal belt of the Guianas, however, there is a layer of geologically more recent river-borne and marine sediments over the ancient rocks. This includes fertile silt, disastrously infertile white silica sands, and, below the surface in some areas, valuable bauxite deposits.