Theories Of Origin Of Coral Reef - Magadh University

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Theories of Origin of Coral ReefDr. Meenakshi PrasadAssistant ProfessorP.G. Deptt. OfGeographyMagadh UniversityBodh GayaSource : google images

DisclaimerThe material for the presentation has beencompiled from various books and online sources& it is for general information & educationalpurpose only. While the author makes anendeavor to keep the information up to date andcorrect she makes no representation of any kindabout the completeness and accuracy of thematerial. The information shared through thispresentation should be used for educationalpurposes only.

Content Introduction Stand Still Theory CriticismsMURRAYSource : google images

Introduction There is no dispute about the origin of fringingcoral reefs, but the base of barrier reefs and atollsis sometimes at such a depth where it is notpossible for the coral polyps to survive. To solvethis problem, some theories have been givenabout the origin of coral reefs, out of which threetheories have special recognition. These are – Land Subsidence Theory by Darwin Stand Still Theory by Murray & Glacial Control Theory by Daly

The most widely recognised & accepted amongst thethree theories is the Land Subsidence Theory. It isfollowed by the Glacial Control Theory & Stand StillTheory respectively.Source : google images

Stand Still Theory Murray propounded his theory of the formation of coralreefs in the year 1880 on the basis of the informationreceived during the Challenger expedition (1872-76). According to Murray, the development of different types ofcoral structure is based on the topography of shallow oceanand the effect of lagoon water. This view has also been supported by thinkers such asSempler (1881) and Agassiz (1906). According to this theory, for the development of coral reefsit is necessary to have sub-marine platforms whose depthshould not exceed 54-55 m.

This suitable depth is obtained by two processes. If the sub-marine platforms are higher than this, they aredegraded by erosion & solution action and if they arelocated below this depth, they are subjected to sub-marinepelagic deposition and attain the required depth. Coral polyps develop on such platforms in large coloniesand together these colonies form the coral reef. Initially the reef develops in the form of fringing reef and itslagoon is shallow. The coral polyps on the outer edge of the platform growsrelatively quickly.

After some time, the coral reef extends beyond a depth of 54–55 m, and after attaining this boundary, small pieces of coralare broken down and deposited on the base of coral reefs.Thus, without the subsidence of land the coral reef extendsinto the deep ocean. The coral polyps living on this pile of debris makes itsdevelopment outward. On the other hand the coral polyps of the inner part of thereef dies due to lack of food. The fresh water of the river flows into the lagoon of thefringing reef.

The water of the lagoon slowly begins to dissolve the limedominated structure of the reef. When solution action begins in the lagoon, the lagoon beginsto deepen and the internal gradient of the coral structureintensifies. In such a situation a barrier reef is developed. When the solution action becomes more intense, the depth ofthe lagoon further increases and the internal gradient of thecoral structure becomes even more intense. This type offeature is called an atoll.

Stand Still Theory of Murray

Regarding the construction of the atolls, Murray saidthat they were built independently due to the circulargrowth of coral polyps at the top of sub-marineplatforms and hills that are located at suitable depths. In this situation also, the coral polyps on the outeredge of the reef develop at a rapid pace and soonreach the surface to form the ring-shaped coral reef oratoll within which a lagoon is formed. Dead coral on the side of the lagoon dissolvescontinuously in the water, which causes the lagoon toexpand, but the outward development of the coral reefcontinues which increases the size of the atoll i.e. itsthickness and width.

Criticisms According to this theory, for the construction of coral reefs,innumerable sub-marine platforms & peaks should exist ata depth of 54-55 m, which is not possible. According to this theory, the surface of the lagoon is theresult of solubility and the maximum depth of the coral reefcan be 54–55 m and the depth of the lagoon will notexceed 54–55 m. This assumption does not apply in mostcircumstances. Firstly, the soluble capacity of saline water of the sea is low. Second, there is evidence of deposition instead of solublechemical action at the bottom of the lagoon and

Third, the depth of many lagoons is 100 m or more. It seems impossible to have both erosion anddeposition processes at the same depth. On the basis of these facts, this theory has beenalmost rejected and at present it has only theoreticaland historical recognition.

References Sharma, R.C. & Vatal, M. : Oceanography forGeographers, Chaitanya Publishing House, Allahabad,1995 Singh. S : Physical Geography, Prayag Pustak Bhawan,Allahabad,2012

After some time, the coral reef extends beyond a depth of 54- 55 m, and after attaining this boundary, small pieces of coral are broken down and deposited on the base of coral reefs. Thus, without the subsidence of land the coral reef extends into the deep ocean. The coral polyps living on this pile of debris makes its development .

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