Photo: Satellite image of a ring-shaped island with a bright blue central lagoon. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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The limestone decays, changing the color of the lagoon from deep ocean blue to bright teal. They pound, break, and erode the coral into tiny grains of sand. This sand and other material deposited by waves or wind pile up on the reef. This material, including organic matter such as plant seeds, form a ring-shaped island or islets.
This is an atoll. Hermatypic corals only live in warm water. The famous naturalist was the first to outline how atolls form. Atolls and People The rocky or sandy shores of atolls have been important sites throughout human history. Often, their low-lying elevation has proved perilous. Atolls are often hidden by ocean waves. Thousands of ships, from ancient Polynesian canoes to sophisticated American warships, have been stranded and wrecked on hidden atolls.
The Kon-Tiki , probably the most famous raft in history, became one of these atoll casualties. The Kon-Tiki was a large balsa raft built and sailed by explorer Thor Heyerdahl and his crew in The most difficult challenge of the journey was not the waves, current s, or trade wind s of the open ocean. It was the atolls of Polynesia, the final part of their journey. The quick-moving currents around atolls prevented the Kon-Tiki from docking at the first Polynesian island it encountered. It wrecked on the shallow coral of the second, Raroia atoll.
Raroia was uninhabited, but nearby native islanders in canoes eventually rescued the Europeans on the washed-up wreck. The Kon-Tiki was eventually hauled out of Raroia, but atoll wrecks are popular dive sites throughout the Pacific. Atolls are often uninhabited "desert" islands. Desert does not refer to the islands' climate, but their "deserted" or uninhabited status. Many are remote and difficult to reach. In the 20th century, this isolation made them attractive as testing sites for nuclear weapon s from the United States, Britain, and France.
The Pacific Proving Grounds, a series of 2, atolls and other islands under U. France continued nuclear testing on the atoll of Moruroa until Nations throughout Polynesia, including the " nuclear-free zone " of New Zealand, protested extended nuclear testing.
Reefs were being destroyed, and some tests dropped toxic fallout onto nearby inhabited islands. After Castle Bravo, the first hydrogen bomb test, the U. The same elements that make atolls popular for nuclear testing also make them attractive to tourists.
Atolls are sparse ly populated, low-lying islands whose white, sandy beaches and placid lagoons are ideally suited to the tourism industry. Tourism is a key factor in both the Maldivian and Kiritbati economies. Many atolls, however, have few tourists and are among the world's underdeveloped countries. Atolls have few natural resource s. Soil quality on atolls is very poor, and erosion is a constant threat.
Most native residents on atolls practice subsistence agriculture and fishing. Almost all food and fuel is imported, often at great cost. Fisheries and support for the shipping industry help support communities on remote atolls. There's nothing better than swimming around in the lagoon all day.
Neil: I love looking at the coral reef around the lagoon , too. We had to groan when we overheard Dorothy's bad pun. Of course, if you're not familiar with geography , then her joke might have gone right over your head. Why was it such a bad pun? As geography buffs know, ring-shaped coral reefs, islands, or groups of islets surrounding a central lagoon are known as atolls.
The formation of an atoll is a slow process that can take millions of years. It begins when an underwater volcano erupts, creating a buildup of lava on the seafloor. With continued eruptions, the lava continues to build until it breaks through the ocean's surface and forms an island. Over time, hard corals , also known as hermatypic corals, begin to build a reef around the island.
What we see as a coral reef actually consists of millions upon millions of the hard exoskeletons made out of calcium carbonate more commonly called limestone created by the corals. At this point, the reef is called a fringing reef. It surrounds the island just below the surface of the water. Over millions of years, the ocean waves begin to erode the volcanic island in a process known as subsidence.
Eventually, the island erodes and disappears below the surface , leaving a lagoon of water surrounded by a coral reef. At this point, the reef is called a barrier reef because it protects the lagoon from the waves and winds of the ocean.
Finally, over time, ocean waves begin to break up parts of the coral reef. Pieces of coral get eroded into tiny grains of sand that pile up on the reef to form tiny islets or a ring-shaped island. Examples of atolls include the islands of the Maldives, Polynesia, and Micronesia. Although some atolls are inhabited , many are deserted because they're remote, difficult to reach, and have few natural resources. Soil quality tends to be poor and erosion is an ongoing threat.
Their pristine beaches and crystal-clear waters can be attractive to tourists, though. Unfortunately, they can also be quite perilous. Coral reefs surrounding atolls often cannot be seen by boats, leading to countless shipwrecks on atolls over the course of history.
In fact, some atolls are famous as dive sites where tourists dive along the reefs to explore old shipwrecks. Some atolls are also famous for another reason. Their isolation and desolation makes them attractive sites to test nuclear weapons, which countries such as the United States, Great Britain, and France have done many times throughout the last century.
Between and , the U. Some of these areas remain too radioactive to inhabit even today. Join us in Wonderopolis tomorrow for a steaming-hot Wonder of the Day sure to crack you up!
Are you ready to learn more about unique geographical features of Earth? Be sure to explore the following activities with a friend or family member:. They are truly amazing and beautiful to look at, Tristyn! The formations seem to be random, as they occur over an amount of time. They do make some interesting shapes, though! A great question! We encourage you to take a little Wonder Journey to find out more, Damian.
Let us know what you come up with!
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