A small, low, sandy island formed on the surface of a coral reef is called a cay.
Cays form when waves and currents move reef debris—especially fragments of coral, shells, coralline algae, and other carbonate-producing organisms—across a reef flat. Where water movement slows, the sediment accumulates into a low depositional mound that may eventually rise above ordinary tides. Soil, vegetation, and seabird guano can later develop on its surface.
The word has a Caribbean history. It derives from the Lucayan word cairi, meaning “island,” which became cayo in Spanish and then cay in English. The same word family explains caye and the American-English spelling key.
A cay is not the same as a mesa, fjord, or tombolo. A tombolo is a sediment ridge connecting an island to a mainland or another island, while a cay is specifically a small reef-top island. Cays occur in places including the Caribbean, the Great Barrier Reef, and the Belize Barrier Reef.