Atoll refers to a horse-shoe or ring-shaped coral reef. An atoll is a unique geological formation that is found in tropical oceans. It is formed by the growth of coral reefs on top of submerged volcanic islands. Over time, as the volcanic island subsides or erodes, the coral reefs continue to grow upwards, creating a circular or horseshoe-shaped structure with a central lagoon.

The formation of an atoll can be divided into several stages:

1. Volcanic Island Formation: Atolls start as volcanic islands that rise from the seafloor. These islands are formed by volcanic activity and are usually surrounded by clear, shallow waters.

2. Fringing Reef Stage: As the volcanic island grows, coral reefs start to develop around its edges. These reefs are known as fringing reefs and are formed by the accumulation of calcium carbonate skeletons secreted by coral polyps.

3. Barrier Reef Stage: Over time, the volcanic island subsides or erodes, causing the fringing reef to grow outwards and form a barrier reef. The barrier reef is separated from the island by a lagoon, which is a shallow body of water.

4. Atoll Formation: As the volcanic island continues to sink or erode, the barrier reef eventually becomes a complete circle or horseshoe shape with a central lagoon. This circular or horseshoe-shaped structure is called an atoll. The lagoon in the center of the atoll is often deeper than the surrounding waters and can provide a habitat for a variety of marine life.

Atolls are typically found in tropical regions with warm waters and high levels of sunlight. They are home to diverse ecosystems and support a wide range of marine organisms, including corals, fish, and other marine animals. Atolls also provide protection for coastal areas by acting as natural barriers against waves and storms.

In conclusion, an atoll is a ring-shaped or horseshoe-shaped coral reef that forms around a submerged volcanic island. It goes through several stages of formation, starting as a volcanic island and eventually becoming a circular or horseshoe-shaped structure with a central lagoon. Atolls are important ecosystems and provide valuable habitats for marine life.