Atoll

An atoll is a coral reef that encircles a central lagoon. The types of corals that form atolls live only in warm water, so most atolls are located in tropical regions in the Pacific and Indian Oceans. One or more small islands may form on top of atolls. Although many atolls are uninhabited, some do support human populations. The processes that form atolls take place over millions of years. In modern times, climate change, rising sea levels, and extreme weather all pose risks to atolls.

Background

The formation of an atoll actually begins with the formation of a volcanic island. Volcanic islands form when volcanoes on the seafloor erupt. Over time, layers of lava accumulate on top of one another, causing the volcano to grow higher and higher. Before the volcano's top breaks the surface of the water, the volcano is called a seamount. A volcanic island forms once the top of the volcano rises above the surface of the water.

Once a volcanic island or seamount forms, a coral reef may begin to grow around it. A coral reef is a marine ecosystem made mostly from the hard exoskeletons of living and dead corals. Corals secrete calcium carbonate, the primary component of limestone, to form their hard exoskeletons. When groups of corals come together, they form a coral reef. Coral reefs typically form in warm water that receives plenty of sunlight. For this reason, the optimum depth for the growth of coral reefs is no more than about 80 feet (25 meters). Coral reefs are among the most diverse ecosystems on the planet. Thousands of ocean species make their homes in and around coral reefs. The formation of a coral reef in a ring around a volcanic island begins the development of an atoll.

Overview

The formation of an atoll can take millions of years. First, coral reefs form a fringe reef around a volcanic island. During this time, corals settle and grow on the parts of the island that are underwater. A narrow strip of water between the island's shoreline and the fringe reef forms a lagoon. As the fringe reef continues to grow, the central island begins to subside, or sink. As the island sinks, coral reef continues to build upward, toward the warmer waters near the ocean's surface. The fringe reef gradually becomes a barrier reef. A barrier reef is a coral reef that runs approximately parallel to the shoreline. In the case of atoll formation, the barrier reef forms a ring around the central island. The lagoon separating the reef from the island's shores becomes wider and deeper. Eventually, subsidence causes the island to disappear beneath the surface of the water. The result is an atoll, or a coral reef encircling a central lagoon.

In time, one or more small islands, called islets, may form on top of the atoll. Islets form as a result of wind and wave action. Wave action breaks apart the limestone reef, forming sand, gravel, and other materials. Waves and wind then deposit these materials on top of the reef, forming islets. The same forces may deposit plant and animal matter on the islets. Plants such as coconut palms, breadfruit trees, and screw pines often take root and grow on these islets.

Atolls, and the lagoons they surround, vary in size. For example, the lagoon area of Kwajalein in the Marshall Islands is among the largest in the world. It is 75 miles (120 kilometers) long and 15 to 20 miles (24 to 36 kilometers) wide. In total, it covers an area of 840 square miles (2,180 square kilometers). Rose Atoll, which lies within American Samoa, a US territory, is among the smallest atolls in the world. Its combined land and lagoon area is just 2.7 square miles (7 square kilometers). Lagoon depths also vary. Most lagoons in the Marshall Islands are about 100 to 300 feet (30 to 90 meters) deep. However, some atoll lagoons are much shallower.

Atolls often have poor soil made of coral sand, gravel, and a mixture of other materials. Many have a small amount of land and little freshwater. For this reason, they support limited plant and animal life. Many remain uninhabited by humans. Because of their often remote locations, atolls were used as sites for nuclear-weapons testing in the past. For example, the United States conducted twenty-three nuclear tests on Bikini Atoll in the Marshall Islands between 1946 and 1958. As a result of the tests, a small number of people who lived in nearby areas had to be relocated. Still, many developed health issues related to radiation exposure. In time, radiation levels at Bikini Atoll decreased, and people began to visit the area to snorkel and scuba dive. However, the soil and water remain contaminated, so people cannot live there full-time. In 2017, a team of researchers from Stanford University began studying the effects of the nuclear tests on plants and animals on Bikini Atoll.

Many atolls feature crystal clear water and diverse marine life and have become popular destinations for snorkelers and scuba divers. Kiribati and the Maldives are island nations, each made up of several coral atolls. Kiribati is located in the Pacific Ocean between Hawaii and Australia. Its Caroline Island is one of the most picture-perfect tropical islands on Earth. The atolls of the Maldives, situated in the Indian Ocean, offer countless opportunities for snorkeling and scuba diving. Both nations also have permanent populations who live there year-round. Aldabra, an atoll in Seychelles, which is located northeast of Madagascar in the Indian Ocean, remains mostly untouched by humans. However, it is home to the world's largest population of giant tortoises. In the Western Hemisphere, a popular atoll for divers is Lighthouse Reef in Belize, which features the Great Blue Hole, a giant underwater sinkhole.

The world's atolls offer some of the most diverse marine ecosystems on the planet and some of the most pristine tropical beaches in the world. However, atolls, especially those located just a few meters above sea level, face many risks. Rising sea levels caused by warmer global temperatures and melting glaciers threaten to swallow up some atolls. Likewise, rising ocean temperatures can damage corals and be harmful to other sea creatures that depend on the reef. In addition, more frequent extreme weather events, which are known to cause damaging winds and powerful waves, could also damage atolls through erosion.

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