It is said that Darwin, throughout his many marine biological observations, had an opportunity to study coral fringing reefs, barrier reefs and atolls in 1836 while in the Indian Ocean. His voyage on The Beagle presented him with the opportunity to observe and document some of the ocean's mysteries such as atolls and their formation. In his essay on the theory of atoll formation, Darwin writes:.
"Now as the island sinks down, either a few feet at a time or quite insensibly, we may safely infer from what is known of the conditions favorable to the growth of coral, that the living masses, bathed by the surf on the margin of the reef, will soon regain the surface. The water however, will encroach little by little on the shore, the island becoming lower and smaller, and the space between the inner edge of the reef and the beach proportionally broader.We can now at once see why encircling barrier-reefs stand so far from the shores which they front. We can also perceive, that a line drawn perpendicularly down from the outer edge of the new reef, to the foundation of solid rock beneath the old fringing-reef, will exceed by as many feet as there have been feet of subsidence, that small limit of depth at which the effective corals can live: -- the little architects having built up their great wall-like mass, as the whole sank down, upon a basis formed of other corals and their consolidated fragments.As the barrier-reef slowly sinks down, the corals will go on vigorously growing upwards; but as the islands sinks, the water will gain inch by inch on the shore -- the separate mountains first forming separate islands within one great reef -- and finally, the last and highest pinnacle disappearing. The instant this takes place, a perfect atoll is formed.".
Darwin's classification scheme of tropical corals reefs--fringing reefs, barrier reefs and atolls--is still used today.
What is an atoll and how did it come to be?.
