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The Mascarene Plateau

The Mascarene Plateau extends approximately 2,000 km between Seychelles and Mauritius and is one of the few submerged features visible from space.

It covers an area of over 115,000 km² of shallow water with depths ranging from 8 m to 150 m on the plateau and it plunges to depths of over 4000 m at its edges.

Its core is granite, with a mantle of basalt and limestone and it extends west along the Amirantes Ridge that divides the Somalia and Mascarene plates. The limestone banks found on the plateau appear to be the remnants of coral reefs which indicate that the plateau was once a succession of islands. Some of which may have been islands as recently as 18,000 - 6,000 years ago, when sea levels were 130 metres lower than today.

In the north, the Seychelles Plate, is of a continental nature, evidenced by its Precambrian granitic outcrops while the base of the southern part, the Saya de Malha and Nazareth banks is volcanic and was covered with a thick sedimentary cover when above water level.

Gondwana began to break up during the late Triassic to the early Jurassic. Major mantle plumes such as the Karoo-Ferrar Plume first split Gondwana at about 182 million years ago. The Paraná-Etendeka plume 132 million years ago split South America and Africa. The Marion plume 88 million years ago split Madagascar and India and finally the Reunion hotspot split the Mascarene Plateau from India 64 million years ago.

The separation of the Mascarene Plateau was accompanied by a tremendous outpouring of basaltic lava from this hotspot, giving birth to a mass of igneous rock called the Deccan Traps, which today covers much of western India. Because the Mascarene Plateau was just beginning to separate from India at the time, much of this basalt covers the plateau as well, though the eruptions did not smother all of the older granite.

Some of the granite remains still above sea level, forming the granitic islands of the Seychelles, located in the northern tip of the Mascarene Plateau. While the plateau drifted northwards over the Réunion Hotspot a series of now submerged islands were formed. This process started 64 million years ago, after the separation of the Mascarene Plateau and India. The Saya de Malha bank was formed, about 35 million years ago and later the Nazareth bank and Cargados Carajos Shoals (Saint Brandon). Cargados Carajos today still counts 22 small islands. About 7 million years ago the northern islands of Mauritius and Mauritius itself were formed. Reunion only reached the surface about 2 million years ago, and is the youngest island originating from the Réunion Hotspot.

Over time there has been a continuous change in sea levels due to periods of glaciation near the poles (ice ages) that determined the climate and sea levels on earth. The last of the ice ages reached its maximum roughly 18,000 years ago, and then gave way to warming. Sea levels rose rapidly between 18,000 and 6,000 years ago (about 130 m) , drowning almost the entire Mascarene Plateau. Today only a few areas remain un-submerged forming the islands of Saint Brandon, Albatross Island and some offshore islets near Mauritius (incl. Round Island). The rest is submerged, large areas by only about 10-40 meters below current sea level. In the past, plate tectonic processes would cause subsidence, subduction and swelling which would all contribute to the height of the plateau above sea level.

When the Mascarene Plateau split and moved westward toward Africa on its tectonic plate, it would have carried the ancestors of our own species. Other members of the primate family had to wait until India eventually joined Asia, where they may have evolved, diverged and migrated. Those that eventually reached Africa left easily discovered fossils in the changing topography of the Great Rift Valley. These would provide their human finders with evidence to create ascending ladders, and ancestral trees and bushes.

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