Cay

A cay (/ˈkiː/ or /ˈkeɪ/), also spelled caye or key, is a small, low-elevation, sandy island on the surface of a coral reef. Cays occur in tropical environments throughout the Pacific, Atlantic and Indian Oceans (including in the Caribbean and on the Great Barrier Reef and Belize Barrier Reef).

Formation and composition

A cay forms when ocean currents transport loose sediment across the surface of a reef to a depositional node, where the current slows or converges with another current, releasing its sediment load. Gradually, layers of deposited sediment build up on the reef surface. Such nodes occur in windward or leeward areas of reef where surfaces sometimes occur around an emergent outcrop of old reef or beach rock.

The island resulting from sediment accumulation is made up almost entirely of biogenic sediment – the skeletal remains of plants and animals – from the surrounding reef ecosystems. If the accumulated sediments are predominantly sand, then the island is called a cay; if they are predominantly gravel, the island is called a motu.