Why are coral reef islands often low and flat?

Coral reef islands, often referred to as cays, are typically low and flat because they are geologically tied to the structure of the coral reef system upon which they form.1

Their formation generally occurs through two primary processes:

• Accretion: Islands develop when reef rock, broken off by storms and waves, is mixed with finer detritus and deposited by currents or waves where water velocity decreases. These deposits accumulate on the relatively flat top of the reef system, often near the high-tide mark. Wind may further shape these deposits into low dunes, and the material is eventually cemented together by calcium carbonate to form stable land.12
• Uplift: In some cases, crustal movements raise a portion of a reef system above sea level. This creates a low, plateaulike feature that is subject to marine erosion and subaerial weathering, which tends to plane the surface into a flat-topped platform.13

Because these islands are essentially small, emergent parts of a larger, mostly submerged reef structure, they remain low-lying and precarious, often rising only a few meters above sea level.1