![]() The Pacific Proving Grounds, a series of 2,000 atolls and other islands under U.S. The first hydrogen bomb, for instance, was tested by the United States at Bikini Atoll, part of the Marshall Islands in the Pacific Ocean. ![]() In the 20th century, this isolation made them attractive as testing sites for nuclear weapons from the United States, Britain, and France. (Desert does not refer to the islands' climate, but their "deserted" or uninhabited status.) Many are remote and difficult to reach. Atolls are often uninhabited "desert" islands. Shipwrecks from the 18th century through World War II lie at the bottom of atolls such as Kwajalein, part of the Marshall Islands. The Kon-Tiki was eventually hauled out of Raroia, but atoll wrecks are popular dive sites throughout the Pacific. Raroia was uninhabited, but nearby Native islanders in canoes eventually rescued the Europeans on the washed-up wreck. It wrecked on the shallow coral of the second, Raroia atoll. The quick-moving currents around atolls prevented the Kon-Tiki from docking at the first Polynesian island it encountered. It was the atolls of Polynesia, the final part of their journey. The most difficult challenge of the journey was not the waves, currents, or trade winds of the open ocean. The Kon-Tiki successfully sailed 6,980 kilometers (4,340 miles) from Peru to the South Pacific. The Kon-Tiki was a large balsa raft built and sailed by explorer Thor Heyerdahl and his crew in 1947. The Kon-Tiki, probably the most famous raft in history, became one of these atoll casualties. Thousands of ships, from ancient Polynesian canoes to sophisticated American warships, have been stranded and wrecked on hidden atolls. Often, their low-lying elevation has proved perilous. The famous naturalist was the first to outline how atolls form.Ītolls and People The rocky or sandy shores of atolls have been important sites throughout human history. An island that is located where ocean temperatures are just warm enough to support hermatypic corals is said to be at the “ Darwin point," named after Charles Darwin. Hermatypic corals only live in warm water. This material, including organic matter such as plant seeds, form a ring-shaped island or islets. This sand and other material deposited by waves or wind pile up on the reef. They pound, break, and erode the coral into tiny grains of sand. In the final stage of an atoll’s formation, ocean waves break apart pieces of the limestone reef. The limestone decays, changing the color of the lagoon from deep ocean blue to bright teal. The algae that corals need to survive face much more competition for fewer nutrient resources. ![]() Corals on the inner, lagoon-facing side, however, begin to slowly decay. The outer, ocean-facing side of the reef remains a healthy marine ecosystem. Subsidence brings slight differences in ocean chemistry that change the reef radically. The barrier reef protects the lagoon from the harsh winds and waves of the open ocean. A barrier reef is farther from shore, and has a deeper lagoon. As the island subsides to become a guyot, its ring-shaped fringing reef turns into a barrier reef. As it subsides, the flat-topped seamount is called a guyot. The seamount erodes into the sea, its top made flat by the constant pounding of powerful ocean waves. Over millions of years, the volcanic island erodes and sinks to the seafloor. ![]() The thin, shallow strip of water between the fringing reef and the island is the lagoon. This coral reef, called a fringing reef, surrounds the island just below the ocean surface. Billions of these limestone exoskeletons are the reef. Hermatypic corals create a hard exoskeleton of limestone (calcium carbonate). The type of corals that build reefs are called hermatypic corals, or hard corals. In the next stage, tiny sea animals called corals begin to build a reef around the island. The top of the volcano becomes an oceanic island. As the volcano continues to erupt, the seamount's elevation grows higher, eventually breaking the surface of the water. First, the volcano erupts, piling up lava on the seafloor. Atolls develop with underwater volcanoes, called seamounts. Channels between islets connect a lagoon to the open ocean or sea. Sometimes, atolls and lagoons protect a central island. An atoll surrounds a body of water called a lagoon. An atoll is a ring-shaped coral reef, island, or series of islets.
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