what is a cay

A cay, also spelled caye or key, is a small, low-elevation, sandy island on the surface of a coral reef. They are commonly found in tropical environments throughout the Pacific, Atlantic, and Indian oceans, including in the Caribbean and on the Great Barrier Reef. Cays are formed when ocean currents transport loose sediment across the surface of a reef to where the current slows or converges with another current, releasing its sediment load. Gradually, layers of deposited sediment build up on the reef surface – a depositional node. Such nodes occur in windward or leeward areas of reefs, where flat surfaces sometimes rise around an emergent outcrop of old reef or beach rock. Cay sediments are largely composed of calcium carbonate (CaCO3), primarily of aragonite, calcite, and high-magnesium calcite. They are produced by myriad plants and animals, such as coralline algae, coral, mollusks, foraminifera, and sponges. Sand cays are usually built on the edge of the coral platform, opposite the direction from which the prevailing winds blow. Debris broken from the reef is swept across the platform at high tide but is prevented from washing over the edge by waves produced by the refraction and convergence of waves around the platform itself. Cays are technically different than islands because rather than being formed by volcanic action or continental plates, cays are low-lying islands consisting mostly of sand or coral and situated on top of a coral reef.