The following points highlight the four main theories of coral reefs and atolls. The theories are: 1. Subsidence Theory of Darwin 2. Stand Still Theory of Murray 3. Glacial Control Theory of Daly 4. Concept of W.M. Davis.
1. Subsidence Theory of Darwin:
Charles Darwin postulated his subsidence theory first in 1837 and modified it in the year 1842 during his Voyage on the ‘Beagle’. After close observation of different types of reefs in the oceans Darwin was convinced that coral polyps could grow only in shallow oceanic waters though coral reefs were found at greater depths where coral polyps could not survive at any cost.
Darwin postulated his theory in order to solve the riddle of this contradiction i.e., confinement of coral polyps to shallow depth but their occurrence, in practice, at greater depth. According to him the land or island involved in the origin and growth of coral reefs is seldom stationary rather it undergoes gradual subsidence. According to him fringing reefs, barrier reefs and atolls are successive stages of the development of coral reefs.
First of all coral polyps flock together along a suitable submarine platform (fig. 30.4, A1-A1 and 30. 5B) and grow upward and ultimately reach sea-level and fringing reef is formed. Thus, fringing reef is formed in stable condition of the land. After this, the land is subjected to subsidence because of tectonic forces and thus coral polyps also reach greater depth where they may not survive.
Consequently, they grow upward and outward at much faster rate so that they can get food for their survival. The growth of polyps is retarded near the shore of the land but it is very phenomenal and vigorous at the outer edge of the land. Consequently, a lagoon is formed between the coast and fringing reef and barrier reef is formed (fig. 30.4, A2 – A2 and 30.5 C).
There is further subsidence of the land and the island is completely submerged under water and a ring of coral reef in the form of atoll is formed (fig. 30.5D). It may be pointed out that Darwin did not invoke sudden and rapid subsidence of land rather he conceived gradual and slower rate of land subsidence than the rate of upward growth of corals so that they could never find themselves in deeper waters.
It may be noted that the depth of lagoon does not increase inspite of gradual subsidence of the land because there is continuous sedimentation in the lagoon.
The following evidences and points strongly support the validity of Darwin’s subsidence theory:
(i) The shallowness of lagoons indicates gradual subsidence of land. If the land is taken to be stable, the lagoon would be filled due to deposition of sediments.
(ii) The absence of cliffs along the coral islands validates the idea of subsidence of land because cliffs are found along only those coral islands which are stationary.
(iii) The coasts and the islands of the Pacific Ocean having raised beaches (indicative of emergence of land) are devoid of barrier and atoll reefs.
(iv) The islands having atolls are characterized by very steep slopes. It may be mentioned that very steep slopes are found only along the upper parts of the islands. This fact also denotes subsidence of the land.
(v) The thickness of coral reefs increases downward. This feature reveals the fact that coral reefs are formed along the subsiding base of submarine platforms.
Evaluation of the Theory:
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. If the subsidence theory is accepted then most of the islands of the Pacific Ocean would be submerged. There are also some evidences of the existence of coral reefs associated with the emerging islands.
2. Stand Still Theory of Murray:
Theories based on the concept of non-subsidence or stand-still situation of land fall in two categories. According to first group corals grow upon suitable stable submarine platforms with unchanging sea level while according to the second group necessary suitable submarine platforms become available due to lowering of sea-level and consequent erosion of land by sea-waves but the land always remains stable. The theory of Murray belongs to the first group.
Murray propounded his theory of the formation of coral reefs in the year 1880 on the basis of the information received during the Challenger Expeditions (1872-76). According to him coral polyps can live upto the depth of 30 fathoms (180 feet). Sea-level and submarine platforms are stable. Several submarine platforms, volcanic peaks, islands are present below sea-level.
If the submarine land platforms are above the permissible depth for the survival of coral polyps (180 feet) they are subjected to wave erosion so that their heights are lowered down. On the other hand, if the submarine platforms are below the required depth of sea of 180 feet, their height is increased due to deposition of marine sediments. After getting suitable foundation at required depth of 180 feet coral polyps begin to grow upward along the coasts and fringing reef is formed.
The coral polyps after some time also grow outward on the foundation of their own debris. Thus, the continuously outward growing fringing reef is transformed into barrier reef in due course of time. The lagoon is formed between the land and barrier reef because of dissolution of dead corals. Atolls are formed due to outward growth of corals in all directions at the top of submarine platforms.
Thus, a ring of coral reef is formed around the solution lagoon (fig. 30.6). According to Murray the lagoon-ward side of atoll is characterized by dead corals while the seaward side has living corals which continuously grow outward. The dead corals are gradually dissolved and thus the lagoon is continuously widened. The lagoons also become shallower because of deposition of dissolved dead corals.
Evaluation of the Theory:
The non-subsidence theory of Murray acclaimed wide popularity in the beginning but later on it was severely criticised on the following grounds:
(1) Murray’s theory requires the existence of numerous suitable submarine platforms the depth of 180 feet but the existence of such features is not possible.
(2) Murray has described two contradictory views of marine erosion and deposition at the depth of 30 fathoms (180 feet) at the same time over different submarine peaks. Such proposition is not possible.
(3) A limit of 30 fathoms for deposition and erosion cannot be accepted.
(4) According to Murray, the lagoon is formed due to solution of dead corals. This mechanism is also doubtful because if the lagoon may be formed due to solution of corals, the pelagic deposits laid down on the submarine platforms or peaks would also be dissolved.
(5) If the land or submarine platforms and peaks are stable then the lagoons would be completely filled up with the marine sediments and thus the lagoons would disappear.
(6) According to Murray coral reefs cannot be found beyond the depth of 30 fathoms but these have also been found at greater depths.
3. Glacial Control Theory of Daly:
Daly propounded his theory of coral formation in the year 1915 after he was convinced that coral reefs were formed after Pleistocene Ice Age. According to him sea-level fell by 33 to 38 fathoms (198 feet to 228 feet) due to glaciation (confinement of sea water in the form of ice on the continents) during Pleistocene Ice Age. The existing corals died due to lowering of temperature of marine water.
Wave-cut platforms were formed along continental coasts and islands due to abrasion by a sea waves. After the end of ice age the sea-level again rose by 33 to 38 fathoms due to return of sea water which was imprisoned on the continents in the form of ice during the ice age. In other words, the ice melted due to rise of temperature and the melt- water after reaching the oceans raised their levels to previous stage.
Thus, the wave-cut platforms were submerged under sea water up to the depth of 33 to 38 fathoms. The corals which could survive during the glacial period and new coral polyps began to grow and establish their colonies on the seaward edges of submerged platforms. Thus, fringing reefs were formed on narrow wave-cut platforms while barrier reefs were formed on broad wave-eroded platforms.
Atolls were formed around isolated wave eroded island peaks. Lagoons of uniform depth were formed between the reefs and the land because of uniform lowering of sea-level due to glaciation during Pleistocene Ice Age (fig. 30.7).
Evaluation of the Theory:
The glacial control theory of Daly is criticised on the following grounds:
(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. According to W.M. Davis the depth of different lagoons varies considerably. Not only this, even the depth at different parts of the same lagoons ranges between 120 to 300 feet. In some lagoons the depth varies between 20 feet to 600 feet.
(2) The coasts might have been cliffed due to wave erosion during the phase of the lowering of the sea-level during Pleistocene Ice Age. Thus, the cliffs formed during glacial period should also be present now but they are seldom found. In fact, the corals might have protected the coasts from being cliffed.
(3) If all the marine islands were eroded up to 33 to 38 fathoms then there should not be islands between the coasts and coral reefs but numerous such islands are found.
4. Concept of W.M. Davis:
The famous American geomorphologist W.M. Davis postulated his concept of the origin of coral reefs in 1914-18 and revived the subsidence theory as postulated by Darwin and others. He presented several physiographic evidences in support of subsidence theory and to explain hither to unsolved several problems related to the formation of coral reefs. According to him corals grow along the subsiding land.
The presence of indented and embayed coast lines found in the coral seas validates the subsidence and consequent submergence of land. According to him the flatness of the bottoms of lagoons and their uniform depth are not due to uniform lowering of sea level and these (bottoms) are not true bottoms but they are due to deposition of marine sediments.
The shallowness of lagoons is because of deposition of debris. If the submarine platforms are taken to be stable then the deposition of marine sediments would fill up the lagoons and the overflowing of water would kill the living corals on the seaward side of the reefs. On the other hand, any amount of debris may be accommodated in the lagoons on the basis of subsidence theory because the bottom is subjected to continuous subsidence. Davis has presented many more evidences in support of subsidence theory.