In order to understand atolls and their formation we must first examine how coral reefs are formed since an atoll is actually a coral reef that builds around an island which subsides. Coral is often mistaken for a rock or a plant; however, it is actually composed of tiny, fragile animals called coral polyps. Coral actually refers to these animals and the skeletons they leave behind after they die. Coral reefs are massive structures made of limestone that is deposited by living things; they are multi-story structures with holes and crevices shared by various creatures. As the polyps of stony corals grow, they produce limestone for their skeletons. When they die, their skeletons are left behind and are used as foundations for new polyps, which build new skeletons over the old ones. An actual coral mound or tree is composed of layer upon layer of skeletons covered by a thin layer of living polyps.
Darwin's studies examined fringing reefs and classified them as coral reefs that encircle an island and went further to define atolls as "rings" of coral with a lagoon in the middle. Darwin was intrigued by the formation of atolls and their resemblance to fringing reefs surrounding volcanic islands. He suggested that atolls represent a latter stage of volcanic island development where the island sinks into the ocean over time. As the island sinks, the corals continue to grow around the fringe of the island until the island disappears leaving only the corals, a "perfect atoll".
As Island volcanoes grew quite and died the island stopped growing and slowly began to sink of its enormous weight. Waves, rain and wind meanwhile, lashed and gnawed at the land and giant landslides may have carved chunks from its flanks. Over millions of years the island sank, shrank and almost disappeared. As the island subsided living reefs grew upward and remained in the zone of warmth and light that provided the perfect reef building conditions.