Updated: Oct 12, 2021
In our last blog, we talked about corals and several conditions that are necessary for their survival. In this blog, we will study different theories that are given about the origin of corals and how actually they were formed.
Before we move on to theories for the creation of Corals let us first look what are the different types of corals!
Types of coral reefs: A. Fringing Reefs: They grow near the coastline around islands and continents. They are separated from the shore by narrow, shallow lagoons. Fringing reefs are the most common type of reef that we see. They lie close to the shore extending outwards from the mainland. The outer edge grows rapidly because of the splashing waves.
B. Barrier Reef: They are also parallel the coastline but are separated by deeper, wider lagoons. At their shallowest point, they can reach the water’s surface forming a “barrier” to navigation. The Great Barrier Reef in Australia is the largest and most famous barrier reef in the world. The reef is partially submerged. They have narrow gaps at several places to allow the water from the enclosed lagoon to return to the open ocean. C. Atolls: They are circular in shape, enclosing a shallow lagoon without any land in the center. The encircling rings are usually broken in a few places to allow the free flow of water. On the inside of the reefs, sand & limestone debris collect & palm trees like coconuts may grow. Atolls usually form when islands surrounded by fringing reefs sink into the sea or the sea level rises around them (these islands are often the tops of underwater volcanoes). The fringing reefs continue to grow and eventually form circles with lagoons inside.
Theories of creation of corals: Theory 1: Subsidence Theory by Charles Darwin. There are 3 steps as shown in the figure below that explain the subsidence theory.
Step 1: Initially corals developed as fringing reefs along the coasts of the islands.
Step 2: The islands subsided downwards due to the down warping of the earth's crust or tectonic movements. To match with the pace of subsidence the corals continued to grow upwards creating a lagoon in between as shown in the figure above. The growth was more vigorous at the outer edge so the reef widened leading to the formation of the Barrier reef. Remember the island's top is still above the water level in this case. The water body separating the island from coral is called a lagoon.
Step 3: In the 3rd step further subsidence of the land occurred making it completely submerged below the water level. The corals continued to grow to match the subsidence of land. Now only the outer rim of the reefs was seen forming an atoll.
Have a look at this animation to better visualize the concept.
Theory 2: Glacial Control Theory by Daly
It says that at the time of the Pleistocene glaciation, the sea level had gone down by 100–150 m and many pre-existing coral reefs came above the sea level. On coming above the sea level, their tops became almost flat due to erosion and acquired a platform shape.
The lowering of the sea level also led to the formation of platforms and steps created by the work of ocean waves along the continental shelves and the islands. After the Pleistocene glaciation, when the sea level began to rise, coral polyps started developing on these different types of platforms.
Fringing reefs were formed on the narrow wave-cut platforms along the new continental shelves.
When the coral polyps developed on the broad wave-eroded platforms situated away from the coast, it led to the formation of barrier-reefs with deeper and broader lagoons.
Thick horse-shoe-shaped reefs or atolls having steep slopes developed on the platforms situated along with the islands.
So these are two theories associated with the Formation of Coral Reefs
Comparing the two theories one on one
The above diagram sums up both theories conceptually. Hope you are clear on the concepts now.
For better understanding watch this lecture https://bit.ly/3C5lTEE
Link to the previous blog : https://www.thelaymanway.com/post/upsc-specific-facts-about-coral-reefs
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