Geomorphology of Aldabra Atoll

Abstract

1. Regional setting Aldabra Atoll (latitude 9° 24' S, longitude 46° 20' E) is situated 420 km northwest of Madagascar and 640 km from the East African mainland, in the southwest Indian Ocean (figure 1). It forms one of a group of slightly elevated coral reefs to the north of Madagascar, and is thus distinguished from the sea-level coral reefs of the Farquhar group, the Amirantes, and the central Indian Ocean. The raised reefs of Aldabra, Assumption, Cosmoledo and Astove are situated on the summits of mountains approximately 4000 m high, rising from a fairly flat sea floor between 4000 and 4300 m deep. Aldabra and Assumption cap two neighbouring peaks, which are distinct at depths shallower than 2500 m, and Cosmoledo and Astove another pair, distinct above the 2000 m level (figure 2). The general bottom topography round these islands is based on surveys by H.M.S. Owen in 1962. More detailed surveys have been made of Aldabra itself, by H.M.S. Owen in 1962 and H.M.S. Vidal in 1967, and these soundings are contoured in figure 3.