Scientists generally divide coral reefs into four classes: fringing reefs, barrier reefs, atolls, and patch reefs.
Fringing reefs grow near the coastline around islands and continents. They are separated from the shore by narrow, shallow lagoons. Fringing reefs are the most common type of reef that we see.
Barrier reefs also parallel the coastline but are separated by deeper, wider lagoons. At their shallowest point they can reach the water’s surface forming a “barrier” to navigation. The Great Barrier Reef in Australia is the largest and most famous barrier reef in the world.
Atolls are rings of coral that create protected lagoons and are usually located in the middle of the sea. Atolls usually form when islands surrounded by fringing reefs sink into the sea or the sea level rises around them (these islands are often the tops of underwater volcanoes). The fringing reefs continue to grow and eventually form circles with lagoons inside.
Patch reefs are small, isolated reefs that grow up from the open bottom of the island platform or continental shelf. They usually occur between fringing reefs and barrier reefs. They vary greatly in size, and they rarely reach the surface of the water.