When the larvae of tropical corals start free swimming in the waters, they start searching for a new considerable place to settle. This is the planktonic stage of the life cycle of a coral. When these larvae if coral finds a suitable spot for settlement, usually underwater rocks or hard submerged surfaces, they attach themselves to it. With time, these coral larvae continue to grow and continue to expand non-stop.
Sometimes when a volcano erupts underwater and forms a volcanic island by its lava, these free-swimming corals get attached to the hard submerged surfaces of this volcanic island, considering it a suitable settlement spot.
1. As per Charles Darwin, The formation of an atoll begins with a volcanic island that subsides (Sinks down into the ocean floor)
2. The Corals continue to grow and build a fringing coral reef around the subsiding volcanic island.
This process of expansion and growth of corals is slow. In fact very slow. The massive and hard corals can grow at a rate of 0.5 to 3 centimetres per year, and the Branching corals or Staghorn corals grow more quickly at the rate of 20 centimetres annually. Thus, these Atolls are not only one of the most unique and interesting geographical features of this planed but also are the oldest ones. It can take around 100 Thousand years to even 30 Million Years for the formation of an Atoll.
3. When the volcanic island submerges into the ocean floor, the corals that were growing around it don’t subside and stay in the same form.
With passage of time, these corals continue to expand and further grow towards water surface around this volcanic island. As the corals grow and form a fringing reef, the volcanic island that used to be hard once can start to submerge in the ocean again and slowly become extinct.
4. As the volcanic island continues to sink below into the ocean, the fringing reef continues to expand and grow larger.
When the volcanic island sinks below the sea level completely, the corals which were settled on it once and formed a fringing reef around the volcanic island don’t get affected and further continue to grow towards the ocean surface and expand. That is how atolls are formed.
5. When the island sinks below the sea, the lagoon in middle is formed and becomes deeper.
Usually these atolls are oval or circular in shape, this shape depends upon the outline of the submerged volcanic island upon which they were formed. Atolls always have a lagoon in the centre, formed by filling the ocean water over the place where the volcanic island used to be.
6. Finally the coral fringing reef becomes an island enclosing a lagoon in middle, it is called Atoll.
The corals that rise higher than the sea surface and are denser will take the form of an island. The corals that are not high enough will stay submerged and appear as gaps in the atoll through which the ocean and lagoon will stay connected. These gaps will act as the access between the ocean and the central lagoon of the atoll.
The primary characteristics of corals are the shape of the body, size of coral, colours, tentacles or branches.
1. The bottom topography of the ocean or sea
2. The depth of the sea or ocean
3. The speed and force of waves
4. The strength of water current
5. The amount of light underwater surface
6. The temperature of the water
7. The suspended sediments of ocean or sea
• Caroline Islands
• Lighthouse Atoll