Coral reefs are structures built by millions of tiny organisms that create an environment favorable to the thousands of varieties of fish and other sea creatures that make their home on the reef. The reef structure builds up over time as individual coral polyps die, leaving behind a residue of calcium carbonate. New corals grow on top of the residue, slowly creating a wave-resistant framework. Algae, sponges, and other creatures symbiotically contribute to the reef as it develops.
Charles Darwin, famous for his theory of evolution, also wrote a monograph about the structure and distribution of coral reefs, his theory of the development of coral reefs, in 1842. He described three major types of coral reef structures – fringing reefs, barrier reefs, and atolls. He believed that most reefs begin as fringing reefs, growing in the shallow water surrounding volcanic islands. As the island mass gradually begins to subside into the sea, the reef continues growing until it forms a barrier reef, a solid ring around the slowly disappearing island. Finally, the island vanishes completely below the surface, leaving a generally circular coral reef around a lagoon, called an atoll.
There are three primary characteristics of an atoll. The first is the outer reef, which faces the action of the waves and may extend steeply hundreds of feet down the slope of what was once a volcanic mountain.
Second, at the top of the slope, is the reef rim, what is seen as the roughly circular shape in an aerial photograph. The rim can vary substantially in width and can consist of reef flats and reef islands. The reef flats are often partially submerged during high tides. Reef islands begin to develop from sediment carried in by incoming waves. Further growth can occur as vegetation takes hold and a land mass builds above the tide line.
Third is the lagoon. Most atolls have breaks in the reef rim where tidal movement causes a water exchange between the ocean and the lagoon. Water brought in by the tide carries nutrients and encourages the growth of patch reefs inside the lagoon. In contrast, atolls with limited breaks must rely on wave action to carry water into the lagoon and are less likely to support patch reefs. In atolls with very shallow lagoons, incoming sediment can gradually fill in the lagoon and create a larger land mass, as in these NASA photographs of the Nukutavake and Pinaki atolls in French Polynesia.
All types of coral reefs, including atolls, are found only in warm tropical waters. There are many atolls in the Pacific and Indian oceans, and a smaller number in the Caribbean region. Familiar examples include the Bikini Atoll in the Pacific, the Maldives in the Indian Ocean, and the Lighthouse and Turneffe Atolls off the coast of Belize.