Final answer:
Atolls are formed through a process called subsidence, where coral organisms build their reefs on a sinking volcanic island. The Great Barrier Reef is an example of an atoll formed over millions of years.
Step-by-step explanation:
An atoll is a ring-shaped coral reef that surrounds a lagoon partially or completely. Atolls are formed over a long period of time through a process called subsidence. The process starts with the formation of a volcanic island, usually from a hotspot or tectonic activity. Over time, as the volcano becomes inactive and the island sinks, the coral organisms continue to build their reefs upward towards the ocean surface, forming the characteristic ring shape of an atoll.
For example, let's consider the formation of the Great Barrier Reef in Australia. The reef started forming approximately 20 million years ago when it was an isolated volcanic island. As the volcanic activity ceased and the island subsided, coral organisms began building their reefs on the volcanic foundation. Over millions of years, the coral reefs kept growing towards the surface, forming the extensive reef system we see today.