A cay is a small, low elevation, sandy island formed on the surface of coral reefs. Cay is pronounced as ‘key’ and is sometimes spelled key or quay.
Cays are typically located in tropical waters such as the tropical parts of the Pacific Ocean, Indian Ocean, and Atlantic Ocean. For example, there are many cays throughout the Great Barrier Reef – the largest coral reef in the world. Some popular cays in the area include Green Island, Michaelmas Cay, Upolu Cay, and Heron Island.
Cays are mostly made up of biogenic sediment from coral reef ecosystems. Biogenic sediment refers to the skeletal remains of plants and animals. Ocean currents transport the sediment across the reef surface, then deposit it in a place where currents converge or slow down. Weather and tidal patterns can impact on the formation and eradication of cays.
Some of the smaller cays often appear to be no more than a sandbar, while larger or older cays will have trees and other foliage growing on them.