How Do Coral Atolls Form

An atoll is a ring-shaped coral reef, an island or a series of islets. An atoll surrounds a body of water called a lagoon. Sometimes atolls and lagoons protect a central island. The canals between islets connect a lagoon to the sea or the sea. Atolls develop with submarine volcanoes, called seamounts. First, the volcano erupts and accumulates lava on the seabed. As the volcano continues to erupt, the height of the seamount becomes higher and eventually breaks the surface of the water. The top of the volcano becomes an oceanic island. In the next phase, tiny marine animals, corals, a reef begin to build around the island. The type of corals that build the reefs is called hermatypic corals or stony corals.

Hermatypic corals form a hard exoskeleton of limestone (calcium carbonate). Billions of these calcareous exoskeletons are the reef. This coral reef, called fringing reef, surrounds the island just below the surface of the sea. The thin strip of shallow water between the fringing reef and the island is the lagoon. For millions of years, the volcanic island eroded and sank to the bottom of the sea. This process is called lowering. The seamount erodes in the sea, its tip becomes flat due to the constant pounding of the powerful waves of the sea. When it subsides, the flat seamount is called Guyot.

When the island becomes a guyot, its ring-shaped fringing reef turns into a barrier reef. A coral reef is farther from the shore and has a deeper lagoon. The coral reef protects the lagoon from strong winds and ocean waves. Subsidence brings slight differences in ocean chemistry that radically alter the reef. The outer face of the reef, facing the sea, remains a healthy marine ecosystem. However, the corals on the inner side facing the lagoon are slowly beginning to decompose. The algae that corals need to survive face much stronger competition for fewer nutrient resources. The limestone disintegrates, changing the color of the lagoon from the deep blue of the ocean to bright blue-green. In the final phase of atoll formation, ocean waves break parts of the limestone reef.

They beat, break and erode corals into tiny grains of sand. This sand and other materials deposited by waves or wind accumulate on the reef. This material, including organic matter such as plant seeds, forms a ring-shaped island or island. It is an atoll. Hermatypic corals live only in warm water. An island where ocean temperatures are just warm enough to support hermatypic corals would be located at Darwin Point, named after Charles Darwin. The famous naturalist was the first to sketch how atolls are formed. Thank you for your interest in a new version! This HTML code is preformatted to comply with our guidelines, including: attribution of author and Knowable magazine; retention of all hyperlinks; Inclusion of the canonical link to the original article in the article`s metadata. The text of the article (including the title) may not be changed without the prior permission of Knowable Magazine staff. Photos and illustrations are not included in this license. For more information, see our full policies. On January 6, 2009, U.S.

President George W. Bush announced the creation of the Pacific Remote Islands Marine National Monument, which includes several islands and atolls under U.S. jurisdiction. [27] [28] Barrier reefs and atolls are not only among the most beautiful and biologically diverse habitats in the ocean, but also among the oldest. At growth rates of 0.3 to 2 centimeters per year for massive corals and up to 10 centimeters per year for branched corals, it can take up to 10,000 years before a coral reef forms from a group of larvae. Depending on their size, it can take 100,000 to 30,000,000 years for barrier reefs and atolls to fully form. Most of the world`s atolls are located in the Pacific Ocean (with concentrations in the Caroline Islands, Coral Sea Islands, Marshall Islands, Tuamotu Islands, Kiribati, Tokelau and Tuvalu) and the Indian Ocean (Chagos Archipelago, Lakshadweep, Maldives Atolls and Outer Seychelles Islands). The Atlantic Ocean has no large groups of atolls, with the exception of eight atolls east of Nicaragua, which belong to the Colombian department of San Andres and Providencia in the Caribbean. Reef-building corals thrive only in warm tropical and subtropical waters of oceans and seas, and so atolls are found only in tropical and subtropical regions.

The northernmost atoll in the world is Kure Atoll at 28°25′ N, as well as other atolls in the northwestern Hawaiian Islands. The southernmost atolls in the world are Elizabeth Reef at 29°57′ S and nearby Middleton Reef at 29°27′ S in the Tasman Sea, both of which are part of the territory of the Coral Sea Islands. The nearest southern atoll is Ducie Island in the Pitcairn Islands group at 24°41′ S. The closest atoll to the equator is Aranuka of Kiribati. Its southern tip is only 8 miles (13 km) north of the equator. All this data helped Droxler and Jorry get a detailed picture of the creation of the Maldives. It all starts with a flat rock bank of carbonate minerals such as limestone. Many of these banks formed in many parts of the tropical ocean about 5 to 2.5 million years ago, when Earth`s climate was relatively warm and sea levels didn`t change much. In this stable environment, the skeletons of the Dead Sea creatures drifted toward the seafloor and slowly and steadily gathered into large carbonate rock formations. In recent geological times, the planet plunges into the deep cold of an ice age about every 100,000 years (due to the shape of Earth`s orbit around the sun).

Huge layers of ice form and spread across continents, trapping much of Earth`s water and lowering global sea levels. Then, when Earth`s orbit causes it to receive more radiation from the sun again, the planet warms, the ice sheets melt, and sea levels rise. “You get this yo-yo of sea level changes,” Droxler says. Therefore, scientists have used the Maldives to support their theory of atoll formation. Over the years, companies such as Royal Dutch Shell have drilled into some islands and the surrounding seabed in search of oil and gas. There have also been a number of expeditions, including two ocean drilling expeditions and a sonar beam survey that revealed the topography of the seafloor around the capital island of Malé, including ancient reef terraces that were once exposed but gradually drowned at the end of the last ice age. Some atolls are also famous for another reason. Their isolation and devastation make them attractive locations for nuclear weapons testing, as countries like the United States, Britain and France have done repeatedly over the past century. The advent of the nuclear age has also provided some clues about the need to reverse the Darwin Atoll idea, Droxler adds. Beginning in the 1940s, research teams drilled into Pacific atolls such as Bikini and Eniwetok in the Marshall Islands to better understand their structure before they were destroyed by atomic bombs.

These expeditions discovered volcanic rock beneath carbonate atolls – but far too deep to support Darwin`s idea of a sinking volcano leaving an atoll behind. (Today, the residents of Bikini and Eniwetok still live with the radioactivity left by U.S. nuclear tests and the deteriorating structure of the reefs destroyed by the bombs.) Corals are made of limestone. The glue you use to make cement. Nevertheless, corals break off during severe tropical storms, drift and can pile up on top of other corals just below the surface to form a chain of islands around a central atoll. Unfortunately, they can also be very dangerous. The coral reefs surrounding the atolls often cannot be seen by boats, resulting in countless shipwrecks on the atolls throughout history. In fact, some atolls are known as dive sites, where tourists dive along reefs to explore ancient shipwrecks. This animation shows the dynamic process of coral atoll formation. Corals (depicted in beige and purple) begin to settle and grow around an oceanic island, forming a fringing reef. It can take up to 10,000 years for a fringing reef to form.

Over the next 100,000 years, if conditions are right, the reef will continue to expand. As the reef expands, the inner island usually begins to sink and the fringing reef turns into a barrier reef. When the island sinks completely underwater, leaving behind a growing coral ring with an open lagoon in the middle, it is called an atoll. The process of atoll formation can take up to 30,000,000 years. In most cases, the area of an atoll is very small compared to the total area. The islands of the atoll are low, with their heights of less than 5 meters (16 feet). In terms of total area, Lifou (1,146 km2, 442 square miles) is the largest high coral atoll in the world, followed by Rennell Island (660 km2, 250 square miles). [16] However, other sources cite Kiritimati as the largest atoll in the world. It is also a high coral atoll (321 km2, 124 square miles in area; according to other sources, even 575 km2, 222 square miles), 160 km2 (62 square miles) main lagoon, 168 km2 (65 square miles) other lagoons (according to other sources 319 km2, 123 square miles total lagoon size). What happens when changes in sea level come into play? Let`s take Hawaii as an example.