The formation of barrier reefs and atolls has created a lot of controversies as they have been found at far greater depths, in some areas exceeding 1000 meters; a level where polyps cannot survive.
As a result, relevant theories have been put forward to explain this anomaly. That is subsidence theory, deglaciation theory and antecedent theory.
Subsidence theory by Charles Darwin 1842
Charles Darwin explained that the process of coral formation was gradual and occurred due to the subsidence of a volcanic island. Darwin’s theory explains that volcanic eruption formed a volcanic island on the ocean floor.
The volcanic island slowly subsided due to isostatic re-adjustments that followed the eruption. Such subsidence increased the depth of water beyond the level at which coral polyps could survive.
Consequently, some polyps died while others survived on the flanks/ sides and started to grow to keep pace with the changes in the water depth.
The polyps that survived grew vigorously upwards and outwards and in the process transformed into barrier reefs and eventually into atolls when the volcano completely submerged.
N.B. the diagrams should show the upward ad outward growth.
Deglaciation theory by Daly
Daly based his theory on the sea level changes during and after the ice age, not the subsidence of the volcanic island.
According to Daly, before the ice age or glaciation, fringing corals colonized the edge of a Marine Island due to warm water and other conditions that favor coral growth and development.
During the ice age/glaciation, a lot of water was locked up in ice sheets and caused a fall in the sea level. The cold conditions killed some coral polyps while maximum erosion removed the top of the island and the reef to form a wave-cut platform.
After the ice age, the return of warm conditions resulted in deglaciation, a rise in the sea level and the growth of corals.
As sea level increased, the polyps that survived on the flanks of the wave cut platform grew vigorously upwards and outwards to keep pace with the changes in the water depth and be maintained at the surface water.
Through this process, coral reefs that colonized the flanks gradually transformed into barrier reefs and finally into atolls when the island/ wave cut platform submerged completely.
Antecedent theory by sir john Murray
According to Murray, there existed stable submarine platforms on which pelagic deposits including corals accumulated at a depth below 60 meters. Barrier reefs and atolls began to form on these platforms as fringing reefs.
As reefs grew upwards and outwards, they were pounded by waves such that masses of coral fragments accumulated on the seaward side; cemented and consolidated into hard reefs .
The polyps inside the reef however died due to lack of food and their skeletons dissolved in water to form a lagoon inside the reef. This changed the barrier reef into atoll.