The Andaman Sea lies between the Andaman Islands, Sumatra, and the Malay Peninsula. There is a broad continental shelf in the east and the north, where the sediment is dredged for cassiterite, an ore of tin. Alcock Rise and Sewell Rise are separated by an area of deep ocean floor, where a spreading center has been pushing the Burma and Sunda microplates apart for the last 3–4 million years. This divergence created the Andaman Sea. The eastern half of the sea lies over the Sunda Plate, which includes most of Sumatra and the Malay Peninsula. The western half includes the Andaman and Nicobar Islands and sits on the Burma Plate, which forms a junction with the Indian Plate at the Sunda Trench. At this subduction zone, the Indian Plate is being overridden by the younger Burma Plate. The southern part of this zone was the source of the 2004 Indian Ocean tsunami.