How do coral sand cays form on reefs?

Coral sand cays are small, low-lying islands that form on coral reef platforms through the process of accretion. They typically develop on the leeward side of a reef, where wave energy and tidal currents are reduced, allowing suspended sediment and reef debris to settle and accumulate.123

The formation process generally involves the following stages:

• Deposition: Sediment, including coral fragments and finer detritus, is broken off from the reef by waves and storms. These materials are transported by currents and deposited where water velocity drops abruptly.142
• Stabilization: As the deposit builds up, it may be shaped by wind into dunes. The island becomes stabilized through the formation of beach rock—a process where calcium carbonate precipitates and cements the loose grains together.142
• Vegetation: Once the land is sufficiently stable and above high-tide level, it is colonized by plants, which further anchor the sediment and contribute organic material to soil development.142

While these islands can reach heights of 1.5 to 3 meters above high tide, they remain geologically precarious and are subject to significant erosion or destruction by severe storms.142