page_title: Atoll
section_title: Distribution and size
hierarchical_section_title: Atoll / Distribution and size
caption_attribution_description: English: Aerial view of Guraidhoo in Kaafu Atoll, Maldives. The view is facing west and Guraidhoo is the inhabited island at the bottom of the photo.
context_page_description: An atoll, sometimes known as a coral atoll, is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. 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. The lagoon forms over the volcanic crater or caldera while the higher rim remains above water or at shallow depths that permit the coral to grow and form the reefs. For the atoll to persist, continued erosion or subsidence must be at a rate slow enough to permit reef growth upward and outward to replace the lost height.
context_section_description: There are approximately 440 atolls.
  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 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 only found in the tropics and subtropics. The northernmost atoll of the world is Kure Atoll at 28°24′ N, along with other atolls of the Northwestern Hawaiian Islands. The southernmost atolls of the world are Elizabeth Reef at 29°58′ S, and nearby Middleton Reef at 29°29′ 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°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. However, Bermuda is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a very different mode of formation. 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.
  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 meters (9). Measured by total area, Lifou (1146 km²) is the largest raised coral atoll of the world, followed by Rennell Island (660 km²).  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 km² land area; according to other sources even 575 km²), 160 km² main lagoon, 168 km² other lagoons (according to other sources 319 km² 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. The largest atoll in terms of island numbers is Huvadhu Atoll in the south of the Maldives with 255 islands.
caption: Kaafu Atoll in Maldives
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