An Atoll or a Coral Atoll is a circular or Oval shaped non uniform coral reef that has a higher coral surface like an island that surrounds a lagoon in center. The higher coral surface ring which forms a ring shaped island can have gaps which connect the lagoon with the ocean or seas. As waves erode the limestone from this coral reef, it forms sand and slowly other elements are deposited or washed ashore over this ring shaped island reef. These elements like sand, seeds, coconuts and few minerals form the round island around the lagoon. This type of island is called an Atoll.
Atolls are formed by an underwater volcano that forms a volcanic island that rises above the surface of the ocean or sea.
Then the free swimming corals that are looking for a place of a new settlement get attached to this volcanic island and start growing and expanding. Thus, forming a coral reef around the volcanic island. These corals which are formed around this volcanic island are called hermatypic coral, these are a type of hard structured corals.
These hard Hermatypic corals later form a structure that can grow above the sea level forming an island, this structure is mainly made up of limestone. This coral reef that surrounds the volcanic island is called a fringing reef. There is a shallow water body in the middle of this ring shaped coral reef, this waterbody is called a lagoon.
With passage of time, the volcanic island around which these coral reefs were formed, starts eroding and slowly sinks on the ocean or sea floor. This process of subsiding can take more than millions of years.
After this volcanic island is completely submerged and subsided into the ocean bed. Later with passage of more thousands of years, the top of this sunken island becomes flat surface by facing ocean waves and water current. This flat surface is called Guyot.
When this volcanic island finally becomes Guyot, the corals that were growing around it continue to further expand into a coral reef formation called barrier reef. This ring shaped coral barrier reef expands and continues to grow. When fully formed, this barrier reef will protect the central lagoon from winds and strong waves of ocean or seas.
The submerging of this volcanic island will create minor changes to the topography of the ocean or the sea, these minor fluctuations in the oceanic topography can make major changes in the characteristics of the coral reef. The outer ring of this coral reef lives as a live oceanic ecosystem and the coral reef inside of this ring and lagoon slowly starts to die and erode. The erosion and decay of the coral reefs inside the lagoon are caused by the lack of nutrients that corals can feed upon.
Lagoons get their bright blue and light teal shade from the decay of the limestone of the reef that was present under the surface of the lagoon.
Darwin point is a spot where ocean temperature is warm and ideal for Hermatypic Corals to grow and expand, as the Hermatypic corals only grow in warm oceans or seas. The Darwin Point is named after the legendary naturalist Charles Darwin.
The theory of formation of Atolls was first outlined by Charles Darwin.