Reefs (Geomorphic)

A reef in the geomorphic sense is a wave-resistant, constructional ridge built chiefly by the framework of reef-building corals and calcareous algae, rising from the sea floor to or near the surface. Darwin’s subsidence model orders three forms: fringing reefs against the shore, barrier reefs separated from land by a lagoon, and atolls, rings of reef around a central lagoon left as the volcanic island subsides. Reef growth is fastest in warm, clear, sunlit water above about 18 degrees Celsius and in the photic zone. Reefs are major bioclastic-sediment factories and serious navigation hazards, charted as the limiting depth on many tropical coasts.