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Atoll is defined as a ring-shaped coral reef, island, or series of islets.[1]. Reefs are typically grouped into 3 categories: atoll, barrier, and fringe reef.[2].A fringing reef is formed and surrounds the island. This forms just below the surface of the ocean.[1]

View of Atoll


Atoll is located northwest of Majuro on the northern fringe of the Marshall Islands and is composed of more than 23 islands and islets. In 1946, Bikini Atoll was the first site in the Marshall Islands used for nuclear weapon testing by the US. During this time, the 167 inhabitants were moved to neighboring islands. By July 1958 however, the nuclear testing in the Marshall Islands was terminated. 23 weapons testing over this period of time. In 1968, a number of radiological surveys were conducted on the island. [3]

Charles Darwin

Charles Darwin's view on the natural progression of life, and the evolution of coral atolls, and if his statements were correct, Darwin developed the theory that coral atolls grow as reefs and stretch toward sunlight while the ocean islands slowly sink beneath the sea surface. Darwin's theory was believed for a very long time, but a study published May 9 in the journal Geology proved different than what Darwin thought.[4]

Another theory of their formation Darwin came up with was that the formation process having its roots in volcanic activity. These theories came from personal observations he made during his 6 year voyage around the southern section of the Pacific Ocean. Atoll was the result of the gradual sinking of a volcanic island that has ling since cooled, leaving behind an open crater in the middle section of the island. As the island sinks, the surrounding territory of the island falls.[5]


Atoll's develop with underwater volcanoes, called seamounts. Sea animals called corals also play a part in the role of this development. A fringing reef is formed and surrounds the island. This forms just below the surface of the ocean. Over millions of years, the volcanic island erodes and sinks to the seafloor. With this subsidence, the flat-topped seamount is called a guyot. The island becomes this guyot, or a ring-shaped fringing reef and turns into a barrier reef. Ocean waves also break pieces of the limestone reef, causing the atoll to form as well.[1]

Formation of Atoll

Distribution and development

Atoll's form from an volcanic island or a guyot. An atoll's formation depends on the conditions of water temperature, erosion, salinity, sedimentation, and its sinking rate. The hydrological and geological conditions that occur in these different areas cause these reefs to occur in different areas of the tropics. Reefs are typically grouped into 3 categories: atoll, barrier, and fringe reef. Atoll formation requires strong light and warm waters. A large percentage of the world's atolls are in the central and southwestern pacific and in parts of the Indian oceans, and a number are found, mainly on the continental shelves, in the Caribbean.[2]

Effect and threats

Groundwater flow into Atoll's is impacted by the structure and the temperature. The inward and upward circulation of water into these atoll's was studied and the negative temperature gradients were measured and the salinity increased the flow rate by 35%. When the growth of the atoll is stimulated the platform thickness increases the velocity of the water flow.[6]

The effects of global warming could be causing the species of the GBR to possibly shift southwards. There are around 150 different species, meaning the corals are species are rich and well diverse. Scientists need satellite images to study the ecological health of the reefs. The rising sea levels is a current problem for coral atolls. This rise in sea level is a result of the global temperature increasing. Along with the rising sea levels, the changes in storm intensity, rainfall, and ocean water temps also have an impact.[7]

Ten most captivating atolls

These atolls are found in the tropics and the subtropics. The top 10 Atolls in the world are (from 10-1), Funafuti, Bikini Atoll, Tubbatha Reef, Lighthouse Reef, Tikehau, Caroline Island, Aitutaki Atoll, Aldabra, Rangiroa, and Atolls of the Maldives.[8]

See also

Other closely related articles in this wiki include:


  1. 1.0 1.1 1.2 Atoll. (2012, August 28). Retrieved March 03, 2016, from
  2. 2.0 2.1 Shultz, N (2007, January 24). Retrieved February 27, 2016 from
  3. International Atomic Energy Agency. (1998, 2016). Conditions at Bikini Atoll. Retrieved February 26, 2016, from
  4. Oskin, B. (2013, May 13). How Coral Atolls Form | Coral Reefs. Retrieved February 27, 2016, from
  5. What is an Atoll? (2016). Retrieved February 27, 2016, from
  6. Leclerc, A.-M., Jean-Baptiste, P., Texier, D., & Broc, D. (1999, July 13). Density-induced water circulations in atoll coral reefs: A numerical study. Limnology and Oceanography, 44(5), 1268–1281.
  7. Luntz, S. (2008, April). Rising Seas Already Threatening Coral Atolls. Australasian Science, 29(3), 10. Retrieved February 27, 2016.
  8. 10 Most Captivating Atolls in the World. (2016, January 13). 10 Most Captivating Atolls in the World. Retrieved February 27, 2016, from

External links