Atoll

coral atollatollsDarwin Pointcoral atollsatoll reefatoll poolcoral islandisland atollMidway
An atoll, sometimes called a coral atoll, is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely.wikipedia
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Lagoon

lagoonscoastal lagoonlagoonal
An atoll, sometimes called a coral atoll, is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely.
Lagoons are commonly divided into coastal lagoons and atoll lagoons.

The Structure and Distribution of Coral Reefs

coralsformation of such coral reefsthe formation of coral reefs
Charles Darwin recognized its indigenous origin and coined, in his The Structure and Distribution of Coral Reefs, the definition of atolls as "circular groups of coral islets" that is synonymous with "lagoon-island".
The Structure and Distribution of Coral Reefs, Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836, was published in 1842 as Charles Darwin's first monograph, and set out his theory of the formation of coral reefs and atolls.

Tuamotus

Tuamotu ArchipelagoTuamotuTuamotu Islands
Most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago and the Outer Islands of the Seychelles).
The Tuamotus, also referred to in English as the Tuamotu Archipelago or the Tuamotu Islands (Îles Tuamotu, officially Archipel des Tuamotu), are a French Polynesian chain of almost 80 islands and atolls forming the largest chain of atolls in the world.

Islet

isletsmoturocks
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" or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".

Marshall Islands

MarshallsRepublic of the Marshall IslandsMarshall
Most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago and the Outer Islands of the Seychelles).
The country's population of 53,158 people (at the 2011 Census ) is spread out over 29 coral atolls, comprising 1,156 individual islands and islets.

Maldivian language

DhivehiMaldivianDivehi
The word atoll comes from the Dhivehi (an Indo-Aryan language spoken on the Maldive Islands) word atholhu (Dhivehi: އަތޮޅު, ), meaning an administrative subdivision.
The English words atoll (a ring of coral islands or reefs) and dhoni (a vessel for inter-atoll navigation) are anglicised forms of the Maldivian words ' and '.

Tokelau

Tokelau IslandsUnion IslandsTK
Most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago and the Outer Islands of the Seychelles).
It consists of three tropical coral atolls (Atafu, Nukunonu and Fakaofo), with a combined land area of 10 km².

Tuvalu

Ellice IslandsElliceEllice group
Most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago and the Outer Islands of the Seychelles).
It is composed of three reef islands and six true atolls spread out between the latitude of 5° to 10° south and longitude of 176° to 180°, west of the International Date Line.

Atolls of the Maldives

Miladhummadulhu Atollnatural atollsatoll of the Maldives
Most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago and the Outer Islands of the Seychelles).
The Maldives are formed by a number of natural atolls plus atolls in the form of a few islands and isolated reefs today which form a pattern stretching from 7 degrees 10′ North to 0 degrees 45′ South.

Coral island

coralcoral cluster islandcoral islands
There may be coral islands or cays on the rim.
This causes the coral to grow into an atoll with a shallow lagoon in the middle.

Chagos Archipelago

ChagosChagos IslandsChagos Island
Most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago and the Outer Islands of the Seychelles).
The Chagos Archipelago or Chagos Islands (formerly the Bassas de Chagas, and later the Oil Islands) are a group of seven atolls comprising more than 60 individual tropical islands in the Indian Ocean about 500 kilometres (310 mi) south of the Maldives archipelago.

Caroline Islands

CarolinesCarolineWestern Carolines
Most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago and the Outer Islands of the Seychelles).
Most of the islands comprise low, flat coral atolls, but some rise high above sea level.

Kure Atoll

KureGreen Island, HawaiiKure Island
The northernmost atoll of the world is Kure Atoll at 28°24′ N, along with other atolls of the Northwestern Hawaiian Islands.
Kure Atoll (Mokupāpapa) or Ocean Island is an atoll in the Pacific Ocean 48 nmi beyond Midway Atoll in the Northwestern Hawaiian Islands at 28.41667°N, -178.33333°W.

Charles Darwin

DarwinDarwinianCharles Robert Darwin
Charles Darwin recognized its indigenous origin and coined, in his The Structure and Distribution of Coral Reefs, the definition of atolls as "circular groups of coral islets" that is synonymous with "lagoon-island".
He theorised that as the land rose, oceanic islands sank, and coral reefs round them grew to form atolls.

Northwestern Hawaiian Islands

Northwest Hawaiian IslandsHawaiiHawaiian island
The northernmost atoll of the world is Kure Atoll at 28°24′ N, along with other atolls of the Northwestern Hawaiian Islands.
The Northwestern Hawaiian Islands or Leeward Islands are the small islands and atolls in the Hawaiian island chain located northwest (in some cases, far to the northwest) of the islands of Kauai and Niihau.

Maldives

Maldivianthe MaldivesRepublic of Maldives
The word atoll comes from the Dhivehi (an Indo-Aryan language spoken on the Maldive Islands) word atholhu (Dhivehi: އަތޮޅު, ), meaning an administrative subdivision.
Political infighting during the '70s between Nasir's faction and other political figures led to the 1975 arrest and exile of elected prime minister Ahmed Zaki to a remote atoll.

Ducie Island

DucieWestward IsletAcadia Islet
The next southerly atoll is Ducie Island in the Pitcairn Islands Group, at 24°40′ S. Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°24′ N. At this latitude coral reefs would not develop without the warming waters of the Gulf Stream.
Ducie Island is an uninhabited atoll in the Pitcairn Islands.

Aranuka

While there is no atoll directly on the Equator, the closest atoll to the Equator is Aranuka of Kiribati, with its southern tip just 12 km north of the Equator.
Aranuka is an atoll of Kiribati, located just north of the equator, in the Gilbert Islands.

Kiritimati

Christmas IslandKiritimati IslandKiritimati (or Christmas Island)
More sources however list as the largest atoll in the world in terms of land area Kiritimati, which is also a raised coral atoll (321.37 km 2 land area; according to other sources even 575 km 2 ), 160 km 2 main lagoon, 168 km 2 other lagoons (according to other sources 319 km 2 total lagoon size).
The island has the greatest land area of any coral atoll in the world, about 388 km2; its lagoon is roughly the same size.

Pseudo-atoll

However, Bermuda is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a very different mode of formation.
A pseudo-atoll, like an atoll, is an island that encircles a lagoon, either partially or completely.

Seamount

seamountssea mountseamount chain
The coral of the atoll often sits atop the rim of an extinct seamount or volcano which has eroded or subsided partially beneath the water.
In tropical zones extensive coral growth results in the formation of coral atolls late in the seamount's life.

Aldabra

Aldabra AtollAldabra IslandMalabar Island
The second largest atoll by dry land area is Aldabra with 155 km 2.
Aldabra is the world's second-largest coral atoll.

Raised coral atoll

raised coralraised coral islandraised coral islands
Measured by total area, Lifou (1146 km 2 ) is the largest raised coral atoll of the world, followed by Rennell Island (660 km 2 ).
A raised coral atoll or uplifted coral atoll is an atoll that has been lifted high enough above sea level by tectonic forces to protect it from scouring by storms and enable soils and diverse – often endemic – species of flora and fauna to develop.

Macclesfield Bank

Zhongsha
Macclesfield Bank is an elongated sunken atoll of underwater reefs and shoals in the South China Sea.

Outer Islands (Seychelles)

Outer IslandsCoralline SeychellesOuter Islands (Coralline Seychelles)
Most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago and the Outer Islands of the Seychelles).
They are generally low-lying, and often form atolls around a central lagoon.