Atoll

The word atoll comes from the Dhivehi word atholhu (Dhivehi: އަތޮޅު, [ˈat̪oɭu]). Dhivehi is an Indo-Aryan language spoken in the Maldives. The word's first recorded English use was in 1625 as atollon. Charles Darwin coined the term in his monograph, The Structure and Distribution of Coral Reefs. He recognized the word's indigenous origin and defined it as a "circular group of coral islets", synonymously with "lagoon-island".[5]^: 2

More modern definitions of atoll describe them as "annular reefs enclosing a lagoon in which there are no promontories other than reefs and islets composed of reef detritus"[9] or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".[10]

There are approximately 440 atolls in the world.[11]

Most of the world's atolls are in the Pacific Ocean (with concentrations in the Caroline Islands, the Coral Sea Islands, the Marshall Islands, the Tuamotu Islands, Kiribati, Tokelau, and Tuvalu) and the Indian Ocean (the Chagos Archipelago, Lakshadweep, the atolls of the Maldives, and the Outer Islands of Seychelles). The Atlantic Ocean has no large groups of atolls, other than eight atolls east of Nicaragua that belong to the Colombian department of San Andres and Providencia in the Caribbean.

Reef-building corals will thrive only in warm tropical and subtropical waters of oceans and seas, and therefore atolls are found only in the tropics and subtropics. The northernmost atoll in the world is Kure Atoll at 28°25′ N, along with other atolls of 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 Coral Sea Islands Territory. The next southerly atoll is Ducie Island in the Pitcairn Islands Group, at 24°41′ S. The atoll closest to the Equator is Aranuka of Kiribati. Its southern tip is just 13 km (8 mi) north of the Equator.

Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°18′ N. At this latitude, coral reefs would not develop without the warming waters of the Gulf Stream. However, Bermuda is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a very different origin of formation.

In most cases, the land area of an atoll is very small in comparison to the total area. Atoll islands are low lying, with their elevations less than 5 metres (16 ft). Measured by total area, Lifou (1,146 km², 442 sq mi) is the largest raised coral atoll of the world, followed by Rennell Island (660 km², 250 sq mi).[12] More sources, however, list Kiritimati as the largest atoll in the world in terms of land area. It is also a raised coral atoll (321 km², 124 sq mi land area; according to other sources even 575 km², 222 sq mi), 160 km² (62 sq mi) main lagoon, 168 km² (65 sq mi) other lagoons (according to other sources 319 km², 123 sq mi total lagoon size).

The remains of an ancient atoll as a hill in a limestone area is called a reef knoll. The second largest atoll by dry land area is Aldabra, with 155 km² (60 sq mi). The largest atoll in terms of island numbers is Huvadhu Atoll in the south of the Maldives, with 255 islands.

Map from Charles Darwin's 1842 The Structure and Distribution of Coral Reefs showing the world's major groups of atolls and coral reefs

Aerial view of Bora Bora

Tarawa Atoll

Bikini Atoll

In 1842, Charles Darwin[5] explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard HMS Beagle from 1831 to 1836. Darwin's explanation suggests that several tropical island types: from high volcanic island, through barrier reef island, to atoll, represented a sequence of gradual subsidence of what started as an oceanic volcano. He reasoned that a fringing coral reef surrounding a volcanic island in the tropical sea will grow upward as the island subsides (sinks), becoming an "almost atoll", or barrier reef island, as typified by an island such as Aitutaki in the Cook Islands, and Bora Bora and others in the Society Islands. The fringing reef becomes a barrier reef for the reason that the outer part of the reef maintains itself near sea level through biotic growth, while the inner part of the reef falls behind, becoming a lagoon because conditions are less favorable for the coral and calcareous algae responsible for most reef growth. In time, subsidence carries the old volcano below the ocean surface and the barrier reef remains. At this point, the island has become an atoll.

As formulated by J. E. Hoffmeister,[6] F. S. McNeil,[16] E. G. Prudy,[8] and others, the antecedent karst model argues that atolls are Pleistocene features that are the direct result of the interaction between subsidence and preferential karst dissolution that occurred in the interior of flat topped coral reefs during exposure during glacial lowstands of sea level. The elevated rims along an island created by this preferential karst dissolution become the sites of coral growth and islands of atolls when flooded during interglacial highstands.

The research of A. W. Droxler and others[4] supports the antecedent karst model as they found that the morphology of modern atolls are independent of any influence of an underlying submerged and buried island and are not rooted to an initial fringing reef /barrier reef attached to a slowly subsiding volcanic edifice. In fact, the Neogene reefs underlying the studied modern atolls overlie and completely bury the subsided island are all non-atoll, flat-topped reefs. In fact, they found that atolls did not form doing the subsidence of an island until MIS-11, Mid-Brunhes, long after the many the former islands had been completely submerged and buried by flat topped reefs during the Neogene.

Atolls are the product of the growth of tropical marine organisms, and so these islands are found only in warm tropical waters. Volcanic islands located beyond the warm water temperature requirements of hermatypic (reef-building) organisms become seamounts as they subside, and are eroded away at the surface. An island that is located where the ocean water temperatures are just sufficiently warm for upward reef growth to keep pace with the rate of subsidence is said to be at the Darwin Point. Islands in colder, more polar regions evolve toward seamounts or guyots; warmer, more equatorial islands evolve toward atolls, for example Kure Atoll. However, ancient atolls during the Mesozoic appear to exhibit different growth and evolution patterns.[17][18]

• 
  Darwin's theory starts with a volcanic island which becomes extinct
• 
  As the island and ocean floor subside, coral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef
• 
  As the subsidence continues the fringing reef becomes a larger barrier reef farther from the shore with a bigger and deeper lagoon inside
• 
  Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon

Coral atolls are important as sites where dolomitization of calcite occurs. Several models have been proposed for the dolomitization of calcite and aragonite within them. They are the evaporative, seepage-reflux, mixing-zone, burial, and seawater models. Although the origin of replacement dolomites remains problematic and controversial, it is generally accepted that seawater was the source of magnesium for dolomitization and the fluid in which calcite was dolomitized to form the dolomites found within atolls. Various processes have been invoked to drive large amounts of seawater through an atoll in order for dolomitization to occur.[19][20][21]

Aerial overview of the Wake Island atoll, part of the Pacific Remote Islands Marine National Monument

In 1896, 1897 and 1898, the Royal Society of London carried out drilling on Funafuti atoll in Tuvalu for the purpose of investigating the formation of coral reefs. They wanted to determine whether traces of shallow water organisms could be found at depth in the coral of Pacific atolls. This investigation followed the work on the structure and distribution of coral reefs conducted by Charles Darwin in the Pacific.

The first expedition in 1896 was led by Professor William Johnson Sollas of the University of Oxford. Geologists included Walter George Woolnough and Edgeworth David of the University of Sydney. Professor Edgeworth David led the expedition in 1897.[22] The third expedition in 1898 was led by Alfred Edmund Finckh.[23][24][25]

• Baratal limestone, sometimes described as the oldest known atoll
• Low island
• Nukuoro Atoll
• Kapingamarangi Atoll

• Dobbs, David. 2005. Reef Madness: Charles Darwin, Alexander Agassiz, and the Meaning of Coral. Pantheon. ISBN 0-375-42161-0
• Fairbridge, R. W. 1950. "Recent and Pleistocene coral reefs of Australia". J. Geol., 58(4): 330–401.
• McNeil, F. S. 1954. "Organic reefs and banks and associated detrital sediments". Amer. J. Sci., 252(7): 385–401.

Wikimedia Commons has media related to Atoll.

• Formation of Bermuda reefs
• Darwin's Volcano – a short video discussing Darwin and Agassiz' coral reef formation debate
• NOAA National Ocean Service Education - Coral Atoll Animation
• NOAA National Ocean Service - What are the three main types of coral reefs?
• Research Article: Predicting Coral Recruitment in Palau's Complex Reef Archipelago Archived 2021-09-20 at the Wayback Machine
• World Atolls, Goldberg 2016: A global map containing all atolls