How does erosion shape a coral reef that rises above sea level?

When a coral reef is raised above sea level—whether by tectonic uplift or a drop in sea level—it is subjected to subaerial erosion and solution weathering.12

The specific shaping of these emerged limestone islands depends on several factors:

• Planing: If the reef is exposed to marine erosion, it may be planed off to create a flat-topped submerged platform. If the reef later submerges, a wave-cut terrace often remains around its perimeter.1
• Runoff and Solution: On horizontal surfaces, subaerial erosion is driven by runoff. Continuous subsoil solution in these areas often creates a saucer-shaped interior, while the edges remain as a prominent rim.1
• Case Hardening: On steep, bare surfaces, alternate wetting and drying cause recrystallization of the limestone. This process, known as case hardening, creates durable walls or knifelike edges.1
• Karst Features: Emerged reefs often develop a pitted, sculptured landscape characteristic of karst topography.2

When these islands eventually submerge again, coral and algal growth typically resumes, often favoring the rim and any remaining pinnacles, which can lead to the development of an atoll or annular reef structure.1