Atolls are ring-shaped reef formations with a central lagoon. They are rings of coral reefs ranging from 1 to 20 miles in diameter that are the result of the full submersion of a volcanic island which was surrounded by fringing and barrier reefs. Oftentimes, atolls are uninhabited desert islands relatively untouched and unexplored by humans. Although some grow beyond short reef formations, forming substantial landmass. These islands and islets have become the home of several island nations including the Republic of the Marshall Islands in the Pacific Ocean and the Maldives in the Indian Ocean.
Atolls are the third and final stage of Darwin’s Subsidence Theory. After the corals around the island have colonized, a newly developing island forming a fringing reef, the volcano gradually sinks as it cools and condenses, leaving the ring of corals behind. This middle stage where the top of the island still emerges from the ocean but a lagoon has formed in the separation of the reef and island is known as a barrier reef. The corals continue to build upwards to stay in the sunlit range. Eventually the island will submerge completely, leaving one large lagoon and a ring of reef formations which is an atoll.
Atolls can range in size from very small to expanding over 130 km with land and lagoon areas of up to 3,000 sq. km. They also vary greatly in rim width. Some ranging only a few hundred meters while others form large islands. Additionally, because atolls are so old (they began as fringing reefs), their reef formations are built on thick layers of dead coral reefs. Scientists have drilled up to 1400 meters towards the ocean floor before hitting volcanic rock.
References
1. NationalGeographic
2. Wikipedia
3. Britannica.com
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