Atoll - Distribution and Size

Distribution and Size

The distribution of atolls around the globe is instructive: 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 Laccadive 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 Sea.

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 what 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.

The largest atolls by total area (lagoon plus reef and dry land) are listed below:

• Saya de Malha Bank, Western Indian Ocean (35000 km²) (without separate North Bank), submerged, least depth 7 m,
• Lansdowne Bank, west of New Caledonia (21000 km²), submerged, least depth 3.7 m
• Great Chagos Bank (12642 km², land area only 4.5 km²)
• Reed Bank, Spratly Islands (8866 km²), submerged, least depth 9 m
• Macclesfield Bank, South China Sea (6448 km²), submerged, least depth 9.2 m
• North Bank (Ritchie Bank, north of Saya de Malha Bank) (5800 km²), submerged, least depth <10 m
• Rosalind Bank, Caribbean Sea (4500 km²), submerged, least depth 7.3 m
• Boduthiladhunmathi (Thiladhunmathi-Miladhunmadulu) Atoll, Maldives, (two names, but a single atoll structure) (3850 km², land area 51 km²)
• Chesterfield Islands, New Caledonia (3500 km², land area <10 km²)
• Huvadhu Atoll, Maldives (3152 km², land area 38.5 km²)
• Truk Lagoon, Chuuk (3130 km²)
• Sabalana Islands, Indonesia (2694 km²)
• Nukuoro atoll, Federated States of Micronesia, lagoon, is 40 km², land area of 1.7 km², divided among more than 40 islets that lie on the northern, eastern and southern sides of the lagoon
• Lihou Reef, Coral Sea (2529 km², land area 1 km²)
• Bassas de Pedro (2474.33 km²), submerged, least depth 16.4 m
• Ardasier Bank, Spratly Islands (2347 km²), cay on the south side?
• Kwajalein, Marshall Islands (2304 km², land area 16.4 km²)
• Diamond Islets Bank, Coral Sea (2282 km², land area <1 km²)
• Namonuito Atoll, Chuuk (2267 km², land area 4.4 km²)
• Ari Atoll, Maldives (2252 km², land area 69 km²)
• Maro Reef, Northwestern Hawaiian Islands, 1934 km²
• Rangiroa, Tuamotu Islands (1762 km², land area 79 km²)
• Kolhumadulhu Atoll, Maldives (1617 km², land area 79 km²)
• North Malé Atoll, Maldives (1565 km², land area 69 km²)
• Ontong Java, Solomon Islands (1500 km², land area 12 km²)

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.