fbpx
Wikipedia

Atoll

An atoll ( /ˈæt.ɒl, -ɔːl, -l, əˈtɒl, -ˈtɔːl, -ˈtl/)[1][2] is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim.[3] : 60 [4] Atolls are located in warm tropical or subtropical oceans and seas where corals can grow. Most of the approximately 440 atolls in the world are in the Pacific Ocean.

The atoll of Teti'aroa in French Polynesia

Two different, well-cited models, the subsidence and antecedent karst models, have been used to explain the development of atolls.[5] According to Charles Darwin's subsidence model,[6] the formation of an atoll is explained by the subsidence of a volcanic island around which a coral fringing reef has formed. Over geologic time, the volcanic island becomes extinct and eroded as it subsides completely beneath the surface of the ocean. As the volcanic island subsides, the coral fringing reef becomes a barrier reef that is detached from the island. Eventually, reef and the small coral islets on top of it are all that is left of the original island, and a lagoon has taken the place of the former volcano. The lagoon is not the former volcanic crater. For the atoll to persist, the coral reef must be maintained at the sea surface, with coral growth matching any relative change in sea level (subsidence of the island or rising oceans).[5]

An alternative model for the origin of atolls, called the antecedent karst model, was first proposed by J. E. Hoffmeister,[7][8] later revised by E. G. Prudy,[9] and evaluated and modified using a compilation of seismic reflection and drillhole data from various atolls by A. W. Droxler and others.[5] In the antecedent karst model, the first step in the formation of an atoll is the development of a flat top, mound-like coral reef during the subsidence of an oceanic island of either volcanic or nonvolcanic origin below sea level. Then, when relative sea level drops below the level of the flat surface of coral reef, it is exposed to the atmosphere as a flat topped island which is dissolved by rainfall to form limestone karst. Because of hydrologic properties of this karst, the rate of dissolution of the exposed coral is lowest along its rim and the rate of dissolution increases inward to its maximum at the center of the island. As a result, a saucer shaped island with a raised rim forms. When relative sea level submerges the island again, the rim provides a rocky core on which coral grow again to form the islands of an atoll and the flooded bottom of the saucer forms the lagoon within them.[5][9]

Usage

The word atoll comes from the Dhivehi word atholhu (Dhivehi: އަތޮޅު, [ˈat̪oɭu]), Dhivehi is an Indo-Aryan language spoken in 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".[6]: 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"[10] or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".[11]

Distribution and size

 
Penrhyn atoll
 
NASA satellite image of some of the atolls of the Maldives, which consists of 1,322 islands arranged into 26 atolls
 
Nukuoro from space. Courtesy NASA
 
Los Roques Archipelago in Venezuela, the largest marine national park in Latin America,[12] from space. Courtesy NASA
 
View of the coast of Bikini Atoll from above

There are approximately 440 atolls in the world.[13]

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.

Largest atolls by total area (lagoon plus reef and dry land)[14]
Name
Position
Location
Area (km2)
Remarks
Great Chagos Bank 6°10′S 72°00′E / 6.17°S 72.00°E / -6.17; 72.00
12,642
Land area 4.5 km2
Reed Bank 11°27′N 116°54′E / 11.45°N 116.90°E / 11.45; 116.90
8,866
Submerged, at shallowest 9 m
Macclesfield Bank 16°00′N 114°30′E / 16.00°N 114.50°E / 16.00; 114.50
6,448
Submerged, at shallowest 9.2 m
North Bank (Ritchie Bank, north of
the Saya de Malha Bank)
9°04′S 60°12′E / 9.07°S 60.20°E / -9.07; 60.20
5,800
Submerged, at shallowest <10 m
Rosalind Bank 16°26′N 80°31′W / 16.43°N 80.52°W / 16.43; -80.52
4,500
Submerged, at shallowest 7.3 m
Thiladhunmathi (Boduthiladhunmathi) 6°44′N 73°02′E / 6.73°N 73.04°E / 6.73; 73.04
3,850
Land area 51 km2
Chesterfield Islands 19°21′S 158°40′E / 19.35°S 158.66°E / -19.35; 158.66
3,500
Land area <10 km2
Huvadhu Atoll 0°30′N 73°18′E / 0.50°N 73.30°E / 0.50; 73.30
3,152
Land area 38.5 km2
Truk Lagoon 7°25′N 151°47′E / 7.42°N 151.78°E / 7.42; 151.78
3,152
[15]
Sabalana Islands 6°45′S 118°50′E / 6.75°S 118.83°E / -6.75; 118.83
2,694
Nukuoro Atoll 3°51′N 154°56′E / 3.85°N 154.94°E / 3.85; 154.94
40
Land area 1.7 km2 in 40 islets
Lihou Reef 17°25′S 151°40′E / 17.42°S 151.67°E / -17.42; 151.67
2,529
Land area 1 km2
Bassas de Pedro 13°05′N 72°25′E / 13.08°N 72.42°E / 13.08; 72.42
2,474
Submerged, at shallowest 16.4 m
Ardasier Bank 7°43′N 114°15′E / 7.71°N 114.25°E / 7.71; 114.25
2,347
Cay on the south side?
Kwajalein Atoll 9°11′N 167°28′E / 9.19°N 167.47°E / 9.19; 167.47
2,304
Land area 16.4 km2
Diamond Islets Bank 17°25′S 150°58′E / 17.42°S 150.96°E / -17.42; 150.96
2,282
Land area <1 km2
Namonuito Atoll 8°40′N 150°00′E / 8.67°N 150.00°E / 8.67; 150.00
2,267
Land area 4.4 km2
Ari Atoll 3°52′N 72°50′E / 3.86°N 72.83°E / 3.86; 72.83
2,252
Land area 69 km2
Maro Reef 25°25′N 170°35′W / 25.42°N 170.59°W / 25.42; -170.59
1,934
Rangiroa 15°08′S 147°39′W / 15.13°S 147.65°W / -15.13; -147.65
1,762
Land area 79 km2
Kolhumadulhu Atoll 2°22′N 73°07′E / 2.37°N 73.12°E / 2.37; 73.12
1,617
Land area 79 km2
Kaafu Atoll (North Malé Atoll) 4°25′N 73°30′E / 4.42°N 73.50°E / 4.42; 73.50
1,565
Land area 69 km2
Ontong Java Atoll 5°16′S 159°21′E / 5.27°S 159.35°E / -5.27; 159.35
1500
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 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).[16] 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

Formation

 
Aerial view of Bora Bora
 
Bikini Atoll

In 1842, Charles Darwin[6] 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,[7] F. S. McNeil,[17] E. G. Prudy,[9] 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[5] 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.[18][19]

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.[20][21][22]

Investigation by the Royal Society of London

 
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.[23] The third expedition in 1898 was led by Alfred Edmund Finckh.[24][25][26]

United States national monuments

On January 6, 2009, U.S. President George W. Bush announced the creation of the Pacific Remote Islands Marine National Monument, covering several islands and atolls under U.S. jurisdiction.[27][28]

See also

References

  1. ^ Pronunciation heard in old video of Bikini Atoll
  2. ^ "atoll". Dictionary.com Unabridged (Online). n.d.
  3. ^ Blake, Gerald Henry, ed. (1994). World Boundary Series. Vol. 5 Maritime Boundaries. Routledge. ISBN 978-0-415-08835-0. Retrieved 12 February 2013.
  4. ^ Migoń, Piotr, ed. (2010). Geomorphological Landscapes of the World. Springer. p. 349. ISBN 978-90-481-3055-9. Retrieved 12 February 2013.
  5. ^ a b c d e Droxler, A.W. and Jorry, S.J., 2021. The Origin of Modern Atolls: Challenging Darwin's Deeply Ingrained Theory. Annual Review of Marine Science, 13, pp.537-573.
  6. ^ a b c Darwin, Charles R (1842). 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. Darwin Online. London: Smith Elder and Co.
  7. ^ a b Hoffmeister, J.E., 1930. Erosion of elevated fringing coral reefs. Geological Magazine, 67(12), pp.549-554.
  8. ^ Hoffmeister, J.E., and Ladd, H.S., 1935. "The foundations of atolls: a discussion." The Journal of Geology, 43, no. 6, 653-665.
  9. ^ a b c Purdy, E. G., 1974. Reef configurations, cause and effect. In Laporte, L. F. (ed.), Reefs in Time and Space. Society of Economic Palaeontologists and Mineralogists Special Publication 18, pp. 9–76.
  10. ^ McNeil (1954, p. 396).
  11. ^ Fairbridge (1950, p. 341).
  12. ^ (in Spanish). Caracas, Venezuela: Instituto Nacional de Parques (INPARQUES). 2007. Archived from the original on 2008-04-24. Retrieved 27 February 2013.
  13. ^ Watts, T. (2019). "Science, Seamounts and Society". Geoscientist. August 2019: 10–16.
  14. ^ "Atoll Area, Depth and Rainfall" (spreadsheet). The Geological Society of America. 2001.
  15. ^ . conserveonline.org. Micronesia Program Office. Archived from the original on 2011-09-27. Retrieved 2007-12-05.
  16. ^ "Misinformation about Islands". worldislandinfo.com.
  17. ^ MacNeil, F.S., 1954. The shape of atolls; an inheritance from subaerial erosion forms. American Journal of Science, 252(7), pp.402-427.
  18. ^ Bialik, Or M.; Samankassou, Elias; Meilijson, Aaron; Waldmann, Nicolas D.; Steinberg, Josh; Karcz, Kul; Makovsky, Yizhaq (January 2021). "Short-lived early Cenomanian volcanic atolls of Mt. Carmel, northern Israel". Sedimentary Geology. 411: 105805. Bibcode:2021SedG..41105805B. doi:10.1016/j.sedgeo.2020.105805. S2CID 228873675.
  19. ^ El‐Yamani, Mahmoud S.; John, Cédric M.; Bell, Rebecca (16 May 2022). "Stratigraphic evolution and karstification of a Cretaceous Mid‐Pacific atoll (Resolution Guyot) resolved from core‐log‐seismic integration and comparison with modern and ancient analogues". Basin Research. 34 (5): 1536–1566. doi:10.1111/bre.12670. hdl:10044/1/98098. S2CID 248223664.
  20. ^ Budd, D.A., 1997. Cenozoic dolomites of carbonate islands: their attributes and origin. Earth-Science Reviews, 42(1-2), pp.1-47.
  21. ^ Wheeler, C.W., Aharon, P. and Ferrell, R.E., 1999. Successions of late Cenozoic platform dolomites distinguished by texture, geochemistry, and crystal chemistry; Niue, South Pacific. Journal of Sedimentary Research, 69(1), pp.239-255.
  22. ^ Suzuki, Y., Iryu, Y., Inagaki, S., Yamada, T., Aizawa, S. and Budd, D.A., 2006. Origin of atoll dolomites distinguished by geochemistry and crystal chemistry: Kita-daito-jima, northern Philippine Sea. Sedimentary Geology, 183(3-4), pp.181-202.
  23. ^ David, Cara (Caroline Martha) (1899). Funafuti or Three Months On A Coral Atoll: an unscientific account of a scientific expedition. London: John Murray. ISBN 978-1-151-25616-4.
  24. ^ Finckh, Dr. Alfred Edmund (11 September 1934). "TO THE EDITOR OF THE HERALD". The Sydney Morning Herald. NSW: National Library of Australia. p. 6. Retrieved 20 June 2012.
  25. ^ Cantrell, Carol (1996). Finckh, Alfred Edmund (1866–1961). Australian Dictionary of Biography at Australian National University. National Centre of Biography. Retrieved 23 December 2012.
  26. ^ Rodgers, K A; Cantrell, Carol (1987). "Alfred Edmund Finckh 1866–1961: Leader of the 1898 Coral Reef Boring Expedition to Funafuti". Historical Records of Australian Science. 7 (4): 393–403. doi:10.1071/HR9890740393. PMID 11617111.
  27. ^ "Presidential Proclamation 8336" (PDF). The White House. 6 January 2009.
  28. ^ (PDF). 12 January 2009. Archived from the original (PDF) on March 1, 2009.
  • 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.

External links

  • 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
  • World Atolls, Goldberg 2016: A global map containing all atolls

atoll, other, uses, disambiguation, atoll, ɔː, ɔː, ring, shaped, island, including, coral, that, encircles, lagoon, partially, completely, there, coral, islands, cays, located, warm, tropical, subtropical, oceans, seas, where, corals, grow, most, approximately. For other uses see Atoll disambiguation An atoll ˈ ae t ɒ l ɔː l oʊ l e ˈ t ɒ l ˈ t ɔː l ˈ t oʊ l 1 2 is a ring shaped island including a coral rim that encircles a lagoon partially or completely There may be coral islands or cays on the rim 3 60 4 Atolls are located in warm tropical or subtropical oceans and seas where corals can grow Most of the approximately 440 atolls in the world are in the Pacific Ocean The atoll of Teti aroa in French Polynesia Two different well cited models the subsidence and antecedent karst models have been used to explain the development of atolls 5 According to Charles Darwin s subsidence model 6 the formation of an atoll is explained by the subsidence of a volcanic island around which a coral fringing reef has formed Over geologic time the volcanic island becomes extinct and eroded as it subsides completely beneath the surface of the ocean As the volcanic island subsides the coral fringing reef becomes a barrier reef that is detached from the island Eventually reef and the small coral islets on top of it are all that is left of the original island and a lagoon has taken the place of the former volcano The lagoon is not the former volcanic crater For the atoll to persist the coral reef must be maintained at the sea surface with coral growth matching any relative change in sea level subsidence of the island or rising oceans 5 An alternative model for the origin of atolls called the antecedent karst model was first proposed by J E Hoffmeister 7 8 later revised by E G Prudy 9 and evaluated and modified using a compilation of seismic reflection and drillhole data from various atolls by A W Droxler and others 5 In the antecedent karst model the first step in the formation of an atoll is the development of a flat top mound like coral reef during the subsidence of an oceanic island of either volcanic or nonvolcanic origin below sea level Then when relative sea level drops below the level of the flat surface of coral reef it is exposed to the atmosphere as a flat topped island which is dissolved by rainfall to form limestone karst Because of hydrologic properties of this karst the rate of dissolution of the exposed coral is lowest along its rim and the rate of dissolution increases inward to its maximum at the center of the island As a result a saucer shaped island with a raised rim forms When relative sea level submerges the island again the rim provides a rocky core on which coral grow again to form the islands of an atoll and the flooded bottom of the saucer forms the lagoon within them 5 9 Contents 1 Usage 2 Distribution and size 3 Formation 4 Investigation by the Royal Society of London 5 United States national monuments 6 See also 7 References 8 External linksUsage EditThe word atoll comes from the Dhivehi word atholhu Dhivehi އ ތ ޅ ˈat oɭu Dhivehi is an Indo Aryan language spoken in 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 6 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 10 or in an exclusively morphological sense as a ring shaped ribbon reef enclosing a lagoon 11 Distribution and size Edit Penrhyn atoll NASA satellite image of some of the atolls of the Maldives which consists of 1 322 islands arranged into 26 atolls Nukuoro from space Courtesy NASA Los Roques Archipelago in Venezuela the largest marine national park in Latin America 12 from space Courtesy NASA View of the coast of Bikini Atoll from above Raa Atoll in Maldives Kaafu Atoll in Maldives There are approximately 440 atolls in the world 13 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 Largest atolls by total area lagoon plus reef and dry land 14 Name Position Location Area km2 RemarksGreat Chagos Bank 6 10 S 72 00 E 6 17 S 72 00 E 6 17 72 00 Indian Ocean 12 642 Land area 4 5 km2Reed Bank 11 27 N 116 54 E 11 45 N 116 90 E 11 45 116 90 Spratly Islands 8 866 Submerged at shallowest 9 mMacclesfield Bank 16 00 N 114 30 E 16 00 N 114 50 E 16 00 114 50 South China Sea 6 448 Submerged at shallowest 9 2 mNorth Bank Ritchie Bank north ofthe Saya de Malha Bank 9 04 S 60 12 E 9 07 S 60 20 E 9 07 60 20 North of Saya de Malha Bank 5 800 Submerged at shallowest lt 10 mRosalind Bank 16 26 N 80 31 W 16 43 N 80 52 W 16 43 80 52 Caribbean 4 500 Submerged at shallowest 7 3 mThiladhunmathi Boduthiladhunmathi 6 44 N 73 02 E 6 73 N 73 04 E 6 73 73 04 Maldives 3 850 Land area 51 km2Chesterfield Islands 19 21 S 158 40 E 19 35 S 158 66 E 19 35 158 66 New Caledonia 3 500 Land area lt 10 km2Huvadhu Atoll 0 30 N 73 18 E 0 50 N 73 30 E 0 50 73 30 Maldives 3 152 Land area 38 5 km2Truk Lagoon 7 25 N 151 47 E 7 42 N 151 78 E 7 42 151 78 Chuuk FSM 3 152 15 Sabalana Islands 6 45 S 118 50 E 6 75 S 118 83 E 6 75 118 83 Indonesia 2 694Nukuoro Atoll 3 51 N 154 56 E 3 85 N 154 94 E 3 85 154 94 Pohnpei FSM 40 Land area 1 7 km2 in 40 isletsLihou Reef 17 25 S 151 40 E 17 42 S 151 67 E 17 42 151 67 Coral Sea 2 529 Land area 1 km2Bassas de Pedro 13 05 N 72 25 E 13 08 N 72 42 E 13 08 72 42 Lakshadweep India 2 474 Submerged at shallowest 16 4 mArdasier Bank 7 43 N 114 15 E 7 71 N 114 25 E 7 71 114 25 Spratly Islands 2 347 Cay on the south side Kwajalein Atoll 9 11 N 167 28 E 9 19 N 167 47 E 9 19 167 47 Marshall Islands 2 304 Land area 16 4 km2Diamond Islets Bank 17 25 S 150 58 E 17 42 S 150 96 E 17 42 150 96 Coral Sea 2 282 Land area lt 1 km2Namonuito Atoll 8 40 N 150 00 E 8 67 N 150 00 E 8 67 150 00 Chuuk FSM 2 267 Land area 4 4 km2Ari Atoll 3 52 N 72 50 E 3 86 N 72 83 E 3 86 72 83 Maldives 2 252 Land area 69 km2Maro Reef 25 25 N 170 35 W 25 42 N 170 59 W 25 42 170 59 Northwestern Hawaiian Islands 1 934Rangiroa 15 08 S 147 39 W 15 13 S 147 65 W 15 13 147 65 Tuamotu Islands 1 762 Land area 79 km2Kolhumadulhu Atoll 2 22 N 73 07 E 2 37 N 73 12 E 2 37 73 12 Maldives 1 617 Land area 79 km2Kaafu Atoll North Male Atoll 4 25 N 73 30 E 4 42 N 73 50 E 4 42 73 50 Maldives 1 565 Land area 69 km2Ontong Java Atoll 5 16 S 159 21 E 5 27 S 159 35 E 5 27 159 35 Solomon Islands 1500 Land area 12 km2In 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 16 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 reefsFormation EditSee also Coral reef Formation Aerial view of Bora Bora Tarawa Atoll Bikini Atoll In 1842 Charles Darwin 6 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 7 F S McNeil 17 E G Prudy 9 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 5 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 18 19 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 lagoonCoral 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 20 21 22 Investigation by the Royal Society of London Edit 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 23 The third expedition in 1898 was led by Alfred Edmund Finckh 24 25 26 United States national monuments EditOn January 6 2009 U S President George W Bush announced the creation of the Pacific Remote Islands Marine National Monument covering several islands and atolls under U S jurisdiction 27 28 See also Edit Islands portalBaratal limestone sometimes described as the oldest known atoll Low island Nukuoro Atoll Kapingamarangi AtollReferences Edit Pronunciation heard in old video of Bikini Atoll atoll Dictionary com Unabridged Online n d Blake Gerald Henry ed 1994 World Boundary Series Vol 5 Maritime Boundaries Routledge ISBN 978 0 415 08835 0 Retrieved 12 February 2013 Migon Piotr ed 2010 Geomorphological Landscapes of the World Springer p 349 ISBN 978 90 481 3055 9 Retrieved 12 February 2013 a b c d e Droxler A W and Jorry S J 2021 The Origin of Modern Atolls Challenging Darwin s Deeply Ingrained Theory Annual Review of Marine Science 13 pp 537 573 a b c Darwin Charles R 1842 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 Darwin Online London Smith Elder and Co a b Hoffmeister J E 1930 Erosion of elevated fringing coral reefs Geological Magazine 67 12 pp 549 554 Hoffmeister J E and Ladd H S 1935 The foundations of atolls a discussion The Journal of Geology 43 no 6 653 665 a b c Purdy E G 1974 Reef configurations cause and effect In Laporte L F ed Reefs in Time and Space Society of Economic Palaeontologists and Mineralogists Special Publication 18 pp 9 76 McNeil 1954 p 396 Fairbridge 1950 p 341 Archipielago de Los Roques in Spanish Caracas Venezuela Instituto Nacional de Parques INPARQUES 2007 Archived from the original on 2008 04 24 Retrieved 27 February 2013 Watts T 2019 Science Seamounts and Society Geoscientist August 2019 10 16 Atoll Area Depth and Rainfall spreadsheet The Geological Society of America 2001 A Blueprint for Conserving the Biodiversity of the Federated States of Micronesia conserveonline org Micronesia Program Office Archived from the original on 2011 09 27 Retrieved 2007 12 05 Misinformation about Islands worldislandinfo com MacNeil F S 1954 The shape of atolls an inheritance from subaerial erosion forms American Journal of Science 252 7 pp 402 427 Bialik Or M Samankassou Elias Meilijson Aaron Waldmann Nicolas D Steinberg Josh Karcz Kul Makovsky Yizhaq January 2021 Short lived early Cenomanian volcanic atolls of Mt Carmel northern Israel Sedimentary Geology 411 105805 Bibcode 2021SedG 41105805B doi 10 1016 j sedgeo 2020 105805 S2CID 228873675 El Yamani Mahmoud S John Cedric M Bell Rebecca 16 May 2022 Stratigraphic evolution and karstification of a Cretaceous Mid Pacific atoll Resolution Guyot resolved from core log seismic integration and comparison with modern and ancient analogues Basin Research 34 5 1536 1566 doi 10 1111 bre 12670 hdl 10044 1 98098 S2CID 248223664 Budd D A 1997 Cenozoic dolomites of carbonate islands their attributes and origin Earth Science Reviews 42 1 2 pp 1 47 Wheeler C W Aharon P and Ferrell R E 1999 Successions of late Cenozoic platform dolomites distinguished by texture geochemistry and crystal chemistry Niue South Pacific Journal of Sedimentary Research 69 1 pp 239 255 Suzuki Y Iryu Y Inagaki S Yamada T Aizawa S and Budd D A 2006 Origin of atoll dolomites distinguished by geochemistry and crystal chemistry Kita daito jima northern Philippine Sea Sedimentary Geology 183 3 4 pp 181 202 David Cara Caroline Martha 1899 Funafuti or Three Months On A Coral Atoll an unscientific account of a scientific expedition London John Murray ISBN 978 1 151 25616 4 Finckh Dr Alfred Edmund 11 September 1934 TO THE EDITOR OF THE HERALD The Sydney Morning Herald NSW National Library of Australia p 6 Retrieved 20 June 2012 Cantrell Carol 1996 Finckh Alfred Edmund 1866 1961 Australian Dictionary of Biography at Australian National University National Centre of Biography Retrieved 23 December 2012 Rodgers K A Cantrell Carol 1987 Alfred Edmund Finckh 1866 1961 Leader of the 1898 Coral Reef Boring Expedition to Funafuti Historical Records of Australian Science 7 4 393 403 doi 10 1071 HR9890740393 PMID 11617111 Presidential Proclamation 8336 PDF The White House 6 January 2009 Weekly Compilation of Presidential Documents PDF 12 January 2009 Archived from the original PDF on March 1 2009 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 External links Edit 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 World Atolls Goldberg 2016 A global map containing all atolls Retrieved from https en wikipedia org w index php title Atoll amp oldid 1136154857, wikipedia, wiki, book, books, library,

article

, read, download, free, free download, mp3, video, mp4, 3gp, jpg, jpeg, gif, png, picture, music, song, movie, book, game, games.