An atoll is a ring-shaped island that has a coral rim surrounding a lagoon. Sometimes, there are small coral islands or cays along the rim. Atolls are found in warm tropical or subtropical areas of the oceans and seas where corals can grow. About 440 atolls exist worldwide, and most are located in the Pacific Ocean.
Two scientific models, the subsidence model and the antecedent karst model, explain how atolls form. According to Charles Darwin’s subsidence model, an atoll forms when a volcanic island sinks beneath the ocean. A coral reef grows around the island as it sinks. Over time, the volcanic island disappears completely, and the coral reef becomes a barrier reef that no longer touches the island. Eventually, only the reef and small coral islands remain, with a lagoon where the island once was. The lagoon is not the original volcanic crater. For an atoll to remain, the coral reef must continue to grow at the same rate as the sea level changes, whether the island sinks or the ocean rises.
Another model, the antecedent karst model, explains atoll formation differently. This model begins with a flat, mound-like coral reef forming as an oceanic island (volcanic or nonvolcanic) sinks below sea level. When sea level drops, the reef is exposed to air and turns into limestone. Rain dissolves the limestone, but the dissolving happens fastest in the center of the island and slowest along the edges. This creates a saucer-shaped island with a raised rim. When sea level rises again, the rim becomes a base for new coral growth, forming the islands of the atoll. The flooded center of the saucer becomes the lagoon inside the atoll.
Usage
The word "atoll" comes from the Dhivehi word "atholhu" (pronounced [ˈat̪oɭu]). Dhivehi is a language spoken in the Maldives and belongs to the Indo-Aryan language family. The first recorded use of the word in English was in 1625 as "atollon." Charles Darwin introduced the term in his scientific work, The Structure and Distribution of Coral Reefs. He noted that the word had origins in local languages and described an atoll as a "circular group of coral islets," which is similar to the term "lagoon-island."
Modern definitions describe atolls as "ring-shaped reefs that surround a lagoon with no land areas except for reefs and small islands made from broken coral pieces" or, in a strictly physical sense, as "a ring-shaped ribbon reef that encloses a lagoon."
Distribution and size
There are about 440 atolls in the world. Most of these are found in the Pacific Ocean, especially in areas such as the Caroline Islands, Coral Sea Islands, Marshall Islands, Tuamotu Islands, Kiribati, Tokelau, and Tuvalu. Many are also in the Indian Ocean, including the Chagos Archipelago, Lakshadweep, the atolls of the Maldives, and the Outer Islands of Seychelles. Indonesia has several atolls across its islands, such as in the Thousand Islands, Taka Bonerate Islands, and Raja Ampat Islands. The Atlantic Ocean has very few atolls, except for eight located east of Nicaragua, which belong to the Colombian department of San Andres and Providencia in the Caribbean.
Reef-building corals grow only in warm tropical and subtropical waters, so atolls are found only in these regions. The northernmost atoll is Kure Atoll at 28°25′ N, along with other atolls in the Northwestern Hawaiian Islands. The southernmost atolls are Elizabeth Reef at 29°57′ S and Middleton Reef at 29°27′ S, both in the Tasman Sea and part of the Coral Sea Islands Territory. The next southern atoll is Ducie Island in the Pitcairn Islands Group at 24°41′ S. The atoll closest to the Equator is Aranuka in Kiribati, with its southern tip just 13 km (8 mi) north of the Equator.
Bermuda is sometimes called the "northernmost atoll" at 32°18′ N. However, coral reefs there depend on the warm waters of the Gulf Stream. Bermuda is called a pseudo-atoll because it looks like an atoll but formed differently.
Most atolls have very small land areas compared to their total size. Atoll islands are low, with elevations under 5 meters (16 ft). Lifou, with 1,146 km² (442 sq mi), is the largest raised coral atoll by total area, followed by Rennell Island at 660 km² (250 sq mi). Some sources say Kiritimati is the largest atoll by land area, with 321 km² (124 sq mi) or 575 km² (222 sq mi), along with a main lagoon of 160 km² (62 sq mi) and other lagoons totaling 319 km² (123 sq mi).
A reef knoll is a hill made from the remains of an ancient atoll in a limestone region. Aldabra, with 155 km² (60 sq mi), is the second largest atoll by dry land area. Huvadhu Atoll in the Maldives has the most islands of any atoll, with 255 individual islands.
Formation
In 1842, Charles Darwin described how coral atolls in the southern Pacific Ocean form based on observations from a five-year voyage on the HMS Beagle between 1831 and 1836. Darwin proposed that tropical islands, such as high volcanic islands, barrier reef islands, and atolls, show a gradual change caused by the sinking of an oceanic volcano over time. He explained that a coral reef growing around a volcanic island will rise as the island sinks, forming a barrier reef, like those found at Aitutaki in the Cook Islands or Bora Bora in the Society Islands. The outer edge of the reef stays near sea level due to coral and algae growth, while the inner part sinks, creating a lagoon. Eventually, the volcano sinks below the ocean, leaving only the barrier reef, which becomes an atoll.
According to research by J. E. Hoffmeister, F. S. McNeil, E. G. Prudy, and others, the antecedent karst model suggests that atolls are features from the Pleistocene era, formed when sinking and chemical breakdown of coral reefs occurred during periods of low sea levels in the past. The raised edges of islands created by this breakdown became places where coral grew and formed atolls when sea levels rose again.
Studies by A. W. Droxler, Stéphan J. Jorry, and others support the antecedent karst model. They found that modern atolls do not depend on a submerged volcanic island or a reef attached to a sinking volcano. Instead, the older reefs beneath modern atolls are flat-topped and not atolls. These studies also show that atolls did not form until much later, during the Mid-Brunhes period (MIS-11), long after volcanic islands were covered by flat-topped reefs during the Neogene era.
Atolls form where tropical marine organisms grow, so they are only found in warm tropical waters. Volcanic islands in colder areas, where reef-building organisms cannot survive, sink and become seamounts. Islands in regions with warm enough water for reefs to grow upward as they sink are at the Darwin Point. In colder, polar regions, islands become seamounts or guyots; in warmer, equatorial regions, they become atolls, like Kure Atoll. However, ancient atolls from the Mesozoic era show different growth patterns.
Coral atolls are important because they are places where calcite turns into dolomite. Scientists have proposed several models to explain this process, including evaporative, seepage-reflux, mixing-zone, burial, and seawater models. While the exact source of magnesium for dolomitization is still debated, it is widely accepted that seawater provided the magnesium needed to change calcite into dolomite. Scientists believe that seawater must flow through an atoll for this process to happen.
Royal Society expeditions 1896–98
In 1896, 1897, and 1898, the Royal Society of London performed drilling on Funafuti atoll in Tuvalu to study how coral reefs form. They wanted to find signs of sea creatures that live in shallow water within the coral of Pacific atolls. This research came after the studies on coral reef structure and spread done by Charles Darwin in the Pacific.
The first expedition in 1896 was led by William Johnson Sollas from the University of Oxford. Geologists on this trip included Walter George Woolnough and Edgeworth David from the University of Sydney. David led the expedition in 1897. The third expedition in 1898 was led by Alfred Edmund Finckh.