 Many reefs develop in shallow tropical volcanic islands. In this situation, the reef evolves in a manner first described by Charles Darwin. Initially, very few coral polyps will settle upon the volcanic island. But once the volcano becomes dormant, the reef will rapidly colonize the island in a manner called a fringing reef. In the fringing reef, the reef lies right next to the island itself. As the island becomes further eroded, the reef becomes a barrier reef and is separated from the island by a shallow lagoon. This is often what we think about when we think about a tropical island with a reef. When the island itself becomes completely eroded, we have what is known as an atoll, in which the reef lies along a ring separated by a shallow lagoon in the middle. Many reefs that lie along continents are either fringing reefs where the reef is right next to the land or barrier reefs like the Great Barrier Reef in Australia where the continent and the reef are separated by some distance.