5.2 What is a Coral Atoll? Darwin’s Hypothesis on the Origin of Coral Island Atolls

Coral atolls are ring-shaped structures that often extend several thousand feet below sea level. What causes atolls to form, and how do they attain such a great thickness?

Corals are colonial animals about the size of an ant that feed with stinging tentacles, and are related to jellyfish. Most corals protect themselves by creating a hard external skeleton made of calcium carbonate. When corals reproduce and grow over many centuries their skeletons fuse into large structures called coral reefs. Other corals, as well as sponges and algae, begin to attach to the reef, enlarging it further. Eventually fishes, sea slugs, octopus (octopi), and other organisms are attracted to these diverse and productive habitats.

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Corals require specific environmental conditions to grow. For example, reef-building corals grow best in waters with an average annual temperature of about 75 degrees Fahrenheit. They cannot survive prolonged exposure to temperatures below 64 degrees or above 86 degrees. In addition, reef-builders require an attachment site, usually other corals, and clear sunlit water. Consequently, the limiting depth of most active reef growth is only about 150 feet.

The restricted environmental conditions required for coral growth creates an interesting paradox; how can corals, which require warm, shallow, sunlit water no deeper than a few dozen meters to live, create thick structures such as coral atolls that extend into deep water?

The naturalist Charles Darwin was one of the first to formulate a hypothesis on the origin of atolls. From 1831 to 1836 he sailed aboard the British ship HMS Beagle during its famous circumnavigation of the globe. In various places that Darwin visited, he noticed a progression of stages in coral reef development from (1) a fringing reef along the margins of a volcano, (2) a barrier reef with a volcano in the middle and (3) an atoll, which consists of a continuous or broken ring of coral reef surrounded by a central lagoon. The essence of Darwin’s hypothesis was that as a volcanic island slowly sinks, the corals continue to build the reef complex upward.

Darwin’s hypothesis explained how coral reefs, which are restricted to shallow water, can build structures that now exist in much deeper water. During Darwin’s time, however, there was no plausible mechanism to account for how an island might sink.

Today, plate tectonics helps explain how a volcanic island can become extinct and sink to great depths over long periods of time. Volcanic islands often form over a relatively stationary mantle plume, which causes the lithosphere to be buoyantly uplifted. Over a span of millions of years these volcanic islands become inactive and gradually sink as the moving plate carries them away from the region of hotspot volcanism.

Furthermore, drilling through atolls has revealed that volcanic rock does indeed underlie the oldest and deepest coral reef structures, confirming Darwin’s hypothesis. Thus, atolls owe their existence to the gradual sinking of volcanic islands containing coral reefs that build upward through time.

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