Question

1) Explain various theories of formation of coral reefs.​

Answer 1

Answer:

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.

ADVERTISEMENTS:

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).

Origin of Coral Reefs according to Charles Darwin

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.

Explanation:

Answer 2

Answer:

Subsidence theory of Darwin.- All coral reefs develop due to growth of polyps at shallow depth,because at greater depth they can't survive. ... -Further subsidence of land and island is completely submerged under water and a ring of coral reefs in the form of atoll is formed.