Various theories have been put forth to explain the mode of origin of coral reefs, taking into account the fluctuation of the Pleistocene sea level and the stability of the land concerned. The latter fact analyses three conditions—a subsiding island, a stationary island, and an emerging land with reefs along with them.
Out of the three types of reefs, the fringing reef is perhaps the most simple and easiest to explain. Corals in the past established themselves along with suitable submarine structures, within 30 fathoms (around 50 metres) of depth. Upward growth, however, ceased when the reef reached the low tide level because coral polyps cannot stand a long exposure to the atmosphere, but the outward growth towards the sea continued.
The material eroded by waves was consequently deposited on its surface. The origin of the other two reefs, the barrier, and the atoll, is not so easy to explain. Hence, there are different views on their origin.
All the theories of reef formation can be broadly categorized into two groups:
Darwin’s Subsidence Theory
This theory was put forth by Charles Darwin in 1837 and modified in 1842, during his voyage on the Beagle when it became clear to him that coral polyps could grow only in shallow waters.
Darwin assumes that along a suitable platform, coral polyps flocked together and grew upward towards a low water level. The resulting reef, in this stable condition, would be a fringing reef. But, at the same time, Darwin assumes, the seafloor and the projecting land in coral seas started submerging, and the living corals found themselves in deeper waters. Hence, an urge to grow upward and outward would be balanced by the subsidence of the land.
As a result of this, Darwin postulated that the fringing reef, barrier reefs, and atolls are only three stages in the evolutionary growth of a reef (Fig. 3.16). As the land subsides, the fringing reef would grow upwards and outwards, resulting in the formation of a shallow lagoon.
Further subsidence would convert it into a barrier reef with wide and comparatively deeper lagoon. The width of the reef is increased due to the rapid outward growth of the reef and the deposition of coral debris along it. The last stage of submergence (comparable to thousands of feet) results in partial or complete disappearance of the land and the existence of a coral ring enclosing a lagoon.
In spite of continued subsidence, Darwin maintains that the shallowness of the lagoon would be due to the deposition of the sediment from the nearby subsiding land. Hence, the lagoon always remains flat and shallow.
The theory, though simple in its presentation, implies that the barrier reef and atoll can occur only in the areas of submergence, and the great amount of vertical thickness of coral material is primarily due to the subsidence of land and consequent upward growth of coral polyps.
Evidence in Support of the Theory:
There is much evidence of subsidence in coral areas. For example, submerged valleys in the east of Indonesia and the coastal areas of Queensland. Had there been no subsidence, the sediment produced by the erosion of coral reefs would have filled the lagoons and caused the death of corals.
The material produced by erosion gets continuously accumulated at the subsiding lagoon bottom. That is why the lagoons are shallow. During an experimental boring, done to a depth of 340 m in the island atoll of Funafuti, dead corals were discovered at these depths.
Only subsidence can explain the existence of corals at this depth because, generally, corals cannot grow below 100 metres. Also, these dead corals showed evidence of their having got ‘dolomitised’ which is possible only in shallow waters. All this evidence goes to prove the subsidence theory.
Evidence against the Subsidence Theory:
Many scientists, like Agassiz and Semper, have argued that the corals have developed in places where there is no evidence of subsidence. Timor is one such area. Similarly, lagoons, with depths of 40m to 45m and many kilometres wide, cannot be explained on the basis of subsidence.
Also, the question arises as to why there is uniform subsidence in the tropical and sub-tropical areas and not so in other areas. Kuenon has described some areas where the fringing and barrier reefs are found close to each other.
This is not possible if the subsidence has been a continuous process. Finally, if it is supposed that the coral islands are a product of subsidence, we will have to assume the existence of a vast area in the Pacific Ocean which has submerged, leaving behind corals as islands. There is no evidence of the existence of such a vast land area in the Pacific Ocean that existed in ancient times.
Murray’s Stand Still theory
John Murray was against the idea of coral formation due to the subsidence of submarine platforms. As per him, any submarine platform could be lowered by erosion or built up by deposition until it was at a suitable height for corals to grow.
Then the corals will start growing on this platform leading to the formation of a fringing reef. Due to the increased growth on the outward margin of the reef, it will turn into a barrier reef. Atolls are formed due to the outward growth of corals in all directions at the top of the submarine platform. In the Lagoon side of the Atoll, dead corals are found which get dissolved making the lagoon deeper and on the other side, we’ll find living corals.
He argued that either by the erosion of volcanic hills rising above sea level or by the deposition of sediments on those lying below sea level, it was possible to have an adequate number of shallow submarine platforms on which the reef-building corals could grow.
He explained the existence of corals bellow 30-fathom depth by saying that above this depth reef will be formed by living corals, while at greater depths mostly coral debris will be found which will be cemented by ocean water.
His theory has been criticized due to the following reasons:
Existence of submarine platforms everywhere is doubtful.
It is difficult to accept lagoon formation by solution as seawater is not a good solvent.
Reefs are found below the depth of 30 fathoms.
The assumption that both erosion and deposition are active at the depth of 30 fathoms, does not seem logical.
Daly’s Glacial Control Theory
Daly, while studying the coral reefs of Hawaii, was greatly impressed by two things. He observed that the reefs were very narrow and there were marks of glaciations. It appeared to him that there should be a close relationship between the growth of reefs and temperature.
According to Daly’s hypothesis, in the last glacial period, an ice sheet had developed due to the fall in temperature. This caused a withdrawal of water, equal to the weight of the ice sheet. This withdrawal lowered the sea level by 125-150 m.
The corals which existed prior to the ice age had to face this fall in temperature dining this age and they were also exposed to air when the sea level fell. As a result, the corals were killed and the coral reefs and atolls were planed down by sea erosion to the falling level of sea in that period.
When the ice age ended, the temperature started rising and the ice sheet melted. The water returned to the sea, which started rising. Due to the rise in temperature and sea level, corals again started growing over the platforms which were lowered due to marine erosion.
As the sea level rose, the coral colonies also rose. The coral colonies developed more on the circumference of the platforms because food and other facilities were better available there than anywhere else.
Hence, the shape of coral reefs took the form of the edges of submerged platforms, A long coral reef developed on the continental shelf situated on the coast of eastern Australia. Coral reefs and atolls developed on submerged plateau tops. After the ice age, the surface of platforms was not affected by any endogenetic forces and the crust of the earth remained stationary.
Evidence in Support of Daly’s Hypothesis:
The experimental borings done on the Funafuti atoll provide evidence in support of Daly’s hypothesis. Also, in the ice age, all the platforms were cut down to the sea level by marine erosion. Hence, the depth of these platforms and that of lagoons with barrier reefs and coral atolls were almost equal.
Study shows that the depths of the platforms and of lagoons are equal at all places. The greatest merit of this hypothesis is that it needs no subsidence of the crust, as is the case with Darwin’s hypothesis. Finally, the sea waves and currents could have easily cut down the islands and converted them into low platforms.
Evidence against Daly’s Hypothesis:
There are some platforms which are so long and broad that their formation cannot be considered as the work of marine erosion alone. One such platform is the Nazareth Platform—350 km long and 100 km wide. It is about 600 m high everywhere.
Also, Daly could not explain the existence s of coral colonies at depths of 100 metres. He had to admit local subsidence to be able to explain coral colonies in some deeper areas. Daly had also calculated that the fall of sea level during the ice age was around 80 metres.
It appears that this calculation is not correct. In fact, the fall of sea level can be correctly measured by the angle of walls of submerged V-shaped valleys. If calculation is done on this basis, the sea level should have fallen by more than 80m. Finally, Daly had stated that the temperature was lowered during the ice age. It must have caused the death of corals, but there is no evidence of this phenomenon.
From the above discussion, it appears that the hypotheses of Darwin and Daly are not contradictory but complementary. Both together throw a lot of light on the phenomenon.
Davis’ Application of Physiography to the Problem of Origin of Coral Reefs:
Davis gave his explanation in order to revive and re-establish the old idea of submergence as applied to the coral reef problem. In 1928, he attempted to give concrete physiographic evidence to explain various problems hitherto unsolved.
To begin with, Davis reasserted the validity of submergence. He stressed that the indented and embayed coastlines found in the coral seas demonstrate the submergence of the land. According to him, the flatness does not denote the true bottom of the lagoon, but is only due to the deposition of debris. Similarly, the shallowness of the lagoon illustrates the subsidence of the land.
Davis has also taken into consideration the facts of changing sea level. According to him, lowered sea level on subsiding islands would also create cliffs and spurs, but most of them would be protected by reefs along the shores from wave attack, hence cliffs would not be seen. Further, subsidence would also drown such cliffs if they were formed.
Thus, this theory advocates the old idea of subsidence with renewed application of physiography. It is also comprehensive in its application as it includes the changes of the sea -level as well as the tectonic changes of the landmass.
In spite of the above evidence, one fact is left unexplained, viz. the assumed equal depth of the lagoons. The flat floor of the lagoon and its shallow depths may be attributed to the sedimentation, but this in no case proves that the original bottom of the lagoon, concealed beneath, may not be showing different depths.