365 Days of Climate Awareness 207 – Coral Islands and Atolls

Both coral islands and atolls are made from reefs, the hard calcium carbonate structures left behind by coral polyps. Both occur in tropical and semitropical ocean water. Atolls are (roughly) ring-shaped and surround a central open water patch, the lagoon. The two are related, though there are competing theories on how they form.

Animation of the Darwin theory of atolls.

Both start with ocean volcanoes, like Hawaii, which occur over mantle hot spots. While the volcano is located over the hot spot and continues to erupt, it continues to grow. But as the crust moves tectonically, the volcano eventually becomes extinct, and a new volcano begins erupting at a different spot on the crust. Erosion now takes over on the now-extinct volcanic island, and over the course of a few tens of millions of years it erodes below sea level and becomes a seamount.

Penrhyn Atoll, Pacific Ocean.

According to Charles Darwin’s theory, if the volcano occurred in tropical water, and a system of fringing coral reefs grew up around it, those reefs will continue growing, as successive generations of coral polyps develop and build their exoskeletons on top of the old, staying within the euphotic (shallow, well-lit) ocean zone. So the old extinct volcano erodes away but the fringing coral reef remains. The fringing reef continues growing into a larger barrier reef, and the central lagoon, being sheltered behind the coral breakwater, starts to develop a different ecosystem from the surrounding ocean. The barrier reef becomes large enough to accumulate detritus and so become a ring-shaped island.

Ihuru and Vabbin, Maldives.

A more recent theory of J.E. Hoffmeister and others holds that atolls are the exposed remains of larger platform reefs which stood well above sea level and eroded in the middle during glacial lowstands, when glaciers covered much of the northern hemisphere, and sea levels were as much as 120 m below present. Once the glaciers melted and sea level rose again, the hollowed-out platform reefs became ring-shaped islands.

Structure of an atoll.

In the Darwin version, coral islands are atolls whose lagoons have been filled in with sediment. In the Hoffmeister theory, atolls are coral islands whose interiors have been scooped out by erosion when the ocean was lower than now. In both cases, the structures grew up on eroded hotspot volcanoes and are composed of calcium carbonate coral structures. They are found only in warm ocean water.

Roughly 440 atolls are known to exist worldwide. Most coral islands and atolls occur in the Pacific and Indian Oceans.

Tomorrow: tropical islands and sea level rise.

Be brave, and be well.