How an Atoll Forms From Coral into an Island
These Coral Islands are Millions of Years Old
Apr 30, 2009
Atolls are islands created on the backs of animals – or rather, coral. These islands are generally circular or oval in shape with a ring of land circling a central lagoon. Since they are made from coral reefs, atolls are generally found in tropical latitudes with most existing in the Pacific Ocean. These islands can make for some of the most beautiful dive sites and beaches in the world. They also have an interesting story behind their geology.
Atoll Formation Starts with a Volcano
The famous Charles Darwin is credited with figuring out how an atoll forms and this was quite an achievement at the time since atolls possess a couple puzzling features. Atolls are made from coral reefs with coral skeletons going down for kilometers, yet corals can only grow in the top 100 meters of the ocean where light penetrates. Nobody knew how there could be coral skeletons at such depths. Darwin proposed that atolls begin with a coral reef growing around a volcanic island. Over thousands of years, as sea levels change and as the volcano subsides, or sinks, the coral reef grows upwards. This dual process of the volcano sinking and the corals growing eventually lead to the familiar circle shape of an atoll. It is now known that an atoll can take as long as 30 million years to form.
Atolls are Beautiful Islands with Sand and Trees
A person on an atoll may not realize the island is made of dead coral. The island part has sandy beaches, plants, and a living reef can be seen off the beach. The coral did not make the sand and plants on its own. Rather, much of the sand on these beaches is created by parrotfish. The primary component of a parrotfish’s diet is algae extracted from coral chomped off the reef. This coral is crushed by the fish’s teeth in order to access the algae and the undigested coral is excreted as beautiful white sand. Plants travel to islands in a variety of ways. Sometimes they are carried by wind, sometimes by currents, and occasionally by a visiting bird or animal.
Visiting an Atoll Can Be a Fun and Interesting Experience
Atolls come in all sorts of sizes varying from a few kilometers to hundreds of kilometers across. Some atolls are great dive sites accessed only by live-aboard boats, while other atolls are small nations. Tuvalu is one such nation. The CIA World Factbook reports that Tuvalu is a group of nine atolls halfway between Hawaii and Australia. This country consists of 26 square kilometers of land and a population of 12,300 people. The Maldives are another island nation, this time in the Indian Ocean and sport some of the best diving in the world. Some atolls are recognized as UNESCO World Heritage Sites such as the Aldabra Atoll in the Seychelles. Another famous atoll is the Bikini Atoll in the Marshall Islands, the site of American nuclear tests in the 1940s and 1950s.
Truly these beautiful islands, created on the backs of animals, are an interesting geological feature and many of the individual islands themselves hold stories worth looking into.
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