The Andaman Islands and the Nicobar Islands to the south form part of a range of submarine mountains, 1130 km long, running from Cape Negrais in the Arakan Yoma range of Burma, to Achin Head in Sumatra. This range separates the Bay of Bengal from the Andaman Sea, and it contains much that is geologically characteristic of the Arakan Yoma.

The older rocks are early Tertiary or late Cretaceous. The newer rocks are in Ritchie’s Archipelago chiefly, and contain fossils of radiolarians and foraminifera. There is coral along the coasts everywhere, and the Sentinel Islands are composed of the newer rocks with a superstructure of coral. A theory of a still continuing subsidence of the islands was formed by Kurz in 1866 and confirmed by Oldham in 1884. Signs of its continuance are found on the east coast in several places.