Theories related to coral reefs formations !!!

The subject of the origin of coral reefs has been debated for our one and a half centuries and several theories  have been put forward but none has been universally accepted. Some major ones are:-

1) Subsidence theory of Charles Darwin  ►According to it, all coral reefs develop due to growth of polyps at shallow depths because at greater depths they cannot survive.
Fringes -Barrier -Atolls are successive stages of coral reefs development.
First, polyps grow at suitable stage below sea level and develop to form fringing reefs.
After, land is subjected to subsidence due to tectonic forces,thus coral reefs reach greater depths where  they may not survive.
The growth of polyp is retarded near the coastal land but is vigorous at the outer edges of the land. 
Consequently, a lagoon is formed between coastal land and reef, hence the Barrier reef.
Further subsidence of land and island is completely submerged under water and a ring of coral reef in the  form of atoll is formed. Depth of lagoon does not increase because of sedimentation.

The most important idea of this theory is that the rate of sinking of the island is slower than the rate at which  corals are building the reef upwards.

🔵Limitations >
1)If fringing reefs, bar­rier reefs and atoll reefs, as maintained by Darwin, are only three stages  of the evolutionary growth of a reef, then fringing reef and barrier reef should not be found on either side of  the same island at the same level but observations and new discoveries have revealed the existence of such  situations. 
2)If the subsidence theory is accepted then most of the islands of the Pacific Ocean would be submerged.
3)There are also some evidences of the existence of coral reefs associated with the emerg­ing islands. 

2) Glacial control theory of Daly ►this theory was put forward by Daly to explain thee growth of coral reefs  in relation to fluctuating sea levels. He was sure that the coral reefs have been formed after the Pleistocene Ice Age. He argued that due to the locking up of a lot of water into the ice sheets made the sea level to fell by 33 to 38 fathoms.The lowering of the temperatures also resulted in the death of existing reefs. The new sea level also resulted in the erosion of the existing reefs. After the ice age the sea regained its earlier level and the temperatures also became favorable for the growth of corals. The eroded platforms formed by the earlier eroded reefs got submerged and fresh growth of corals started on these platforms. 

 The coral polyps started growing on the seaward edges of the platforms thus forming the fringing reefs  on narrow eroded platforms.

On the broader platforms developed the barrier reefs.

Isolated submerged platforms served as the base for the formation of atolls.

A/q to his theory ,the thickness of the corals would be equal to the lowering in the sea level during the ice age. This theory also explains the flat floors of the lagoons and almost equal depth of all the lagoons behind  the barrier reefs and in the center of the atolls.

🔵Limitations :-->1)According to this theory the depth of all the lagoons of atolls and barrier reefs should be uniform but actual observations do not validate this concept.

2)this theory does not have a very convincing answer to the problem of the removal and disposal of sediment  during the ice age.

3) Stand-Still theory of Murray►According to him, coral reefs formed when accumulating mounds of calcareous marine sediments reached the shallow depths that could support the growth of corals. The platform(wave cut or with submarine deposition),on which the corals build a reef,remain stable.This theory assumes not only a stable base of the coral reefs but also stability in the sea level.The polyps start growing on volcanic peaks at a depth of nearly 30 fathoms(180 feet) and grow upwards forming fringing reefs around the islands.With the passage of time,these fringing reefs will grow outwards on the foundation of their own coral debris and the reef would be extended into the deeper parts of the sea = transformation of fringing reefs into barrier reefs.The theory also says that the lagoon occupying a location between the reef and the coast is formed through solution of inner part of the reef where the dead corals are easily dissolved into the sea water. The Atolls a/q to this theory are formed on the submarine platforms growing outwards in all directions to form a cluster of corals initially, and later on a complete ring of the reef is formed around the lagoon in the center. A/q to Murray,the lagoonward side of atoll is characterized by dead corals while the seaward side has living corals which continuously grow outward. The lagoon is a result of the solution action.With passage of time the ring of the reef grows outwards, and the lagoon grows in size but becomes progressively shallower due to the deposition of the eroded material.

🔵Limitations of murray theory :--1)this theory assumes the pre-existence of a large number of volcanic peaks serving as the foundation on which the corals grow.Although there are large number of such submarine volcanic peaks in the pacific ocean,its not considered a feasible proposition.

2)this theory explains the fringing and barrier reefs of lesser thickness but does not explain the greater thickness of coral formations which is quite significant in a number of places.

3)also, if this theory is accepted, the depth of the corals should not exceed 180 feet and the depth of the lagoon should not be more than 30 fathom. However, deeper lagoons and deep coral reefs do exist.

4) Davis' theory of Coral Reefs  ►He applied the concepts of physiography to the problem of coral reefs. 

According to him corals grow along the subsiding land. He opined that the flatness of the lagoon bottom does not represent the true bottom of the lagoon but is a result of debris deposition in the lagoon. Thus he supported the idea of subsidence.

He argued that the lowered sea level on subsiding islands would create cliffs and spurs, but the reefs would protect most of them from wave attack.Hence the cliffs will not be visible.

Another argument he put forward in support of the subsidence is that the mass or weight of the corals may cause sinking of the foundation and the isostatic adjustments in the substratum may take place.

Thus this theory advocates the old idea of subsidence.It is also comprehensive as it includes the changes in sea level as well as tectonic changes in the landmass.

However, one fact this theory fails to explain is the assumed equal depth of the lagoons. The flat floor of the lagoon and its shallow depth may be attributed to the sedimentation but it does not prove that the original bottom of the lagoon may not be showing variable depth.