This chain of coral-fringed islands forms the Leeward Island chain within the French Society Islands. At bottom right are the islands of Tahaa and Raiatea. They are old, eroded volcanoes, fringed by a coral reef. Northward along the chain, the original central volcanoes are older and more heavily eroded. On Bora Bora (center), the reef is prominently developed and the island significantly eroded. The northernmost island, Tupai, is merely an atoll, having lost any relic of the volcano around which the reef originally grew, except for the shallow floor of the lagoon, showing up in turquoise.
This sequence provides an excellent illustration of the hypothesis first propounded by Charles Darwin to explain the origin of coral reefs in deep oceans. Reef-building corals can only live in shallow waters of 20 meters (66 feet), in temperatures over 21° centigrade (70° Fahrenheit). Initially, corals formed fringing reefs around volcanic islands. Old volcanoes are very rapidly eroded in tropical climates until they reach sea level. Below sea level, the rate of erosion is much slower, and atolls such as Tupai might exist for long periods. If for geological reasons the original volcano subsides below sea level at a slow enough rate, corals will continue to build, thus preserving an atoll at the surface long after the original volcanic edifice has been deeply submerged.
(Caption courtesy of LPL)