The three classic forms of coral reef, distinguished by their position relative to the coast and the presence of a lagoon, first explained in a single subsidence theory by Charles Darwin.
- Fringing reef: grows directly along and attached to the shore (or with only a very narrow, shallow lagoon), the most common and youngest type; example, reefs of the Gulf of Mannar in India.
- Barrier reef: a broad reef lying offshore, separated from the coast by a wide, deep lagoon; the Great Barrier Reef off Queensland, Australia, is the largest in the world.
- Atoll: a roughly circular or horseshoe-shaped ring of reef enclosing a central lagoon, typically built on a sinking volcanic island; the Lakshadweep islands of India are atolls.
- Darwin's subsidence theory links the three: as a volcanic island slowly sinks while the coral grows upward, a fringing reef becomes a barrier reef and finally an atoll once the island disappears.
- Reefs need warm (about 20 to 25° Celsius), shallow, clear, sunlit, saline tropical water; warming seas cause coral bleaching (loss of symbiotic algae).
The fringing-barrier-atoll triad, Darwin's subsidence theory linking them, the Great Barrier Reef and Lakshadweep atolls, and reef growth conditions are recurring oceanography facts.
Fringing (attached to shore) versus barrier (offshore with a wide deep lagoon) versus atoll (ring around a lagoon, no central island); the order of evolution is fringing then barrier then atoll as the land subsides.
Three reef types: fringing (shore-attached), barrier (offshore with deep lagoon), atoll (ring around a lagoon), linked by Darwin's subsidence theory.