submarine plateau, made up of a very shallow, extensive ridge in the Indian Ocean that forms a crescent through the Seychelles and Amirante islands. The ridge extends from latitude 4° to 21° S and from longitude 54° to 63° E. It is believed to be a small continental outlier similar to Madagascar and separated from the continent. The granitic Seychelles islands, which rise from the ridge, are considered anomalous in an ocean basin because most oceanic rocks are basaltic. The southern part of the plateau is, however, volcanic.