Marine invertebrates that belong to the phylum Cnidaria are called corals. Usually, they are found in small, crowded colonies made up of several similar tiny animals called polyps. Included in this group are the significant reef builders called stony corals found in tropical waters that release calcium carbonate to create a rigid skeleton. There are three types of coral reefs fringing reef, barrier reef, and atoll.

Charles Darwin first proposed the theory of subsidence in 1837, and he revised it in 1842 while on his “Beagle” voyage. Darwin found that coral polyps could only develop in shallow oceanic waters, even though coral reefs may be found at deeper depths, after closely observing many types of reefs in the ocean. The fringing reefs, barrier reefs, and atoll are the three stages of a coral reef’s development.

Before the formation of a fringing reef, coral polyps first gather along an appropriate underwater substrate, grow higher, and eventually reach sea level, the land becomes stable. Coral polyps might not survive in deeper depths due to tectonic stress. They expand and climb higher at a far quicker rate to obtain food for survival. Polyp growth is slowed down close to the coast, but it picks up speed and intensity at the outer edge of the land. Thus barrier reef and lagoon are created between the coast and the fringing reef. The subsidence further causes the island to be fully submerged underwater. This forms a ring called an atoll.

Conclusion

Darwin’s theory of subsidence is supported by evidence such as shallow lagoons, lack of cliffs, absence of barrier reefs and atoll from the raised beaches of the Pacific ocean, and steep slopes of atolls. However, new discoveries contradict the development stages of coral reefs as fringing reefs and barrier reefs are found on opposite sides of the same island at the same level. If the subsidence theory is true then most islands of the Pacific ocean would be underwater. Additionally, coral reefs may be connected to developing islands.