Atolls Fringing Reefs Barrier Reefs
A ring of reefs surrounding a central lagoon. Atolls may be found out at sea in relative isolation. Some atolls have have foundations nearly 400m below the surface, as in the Maldives. However, since the corals themselves require strong sunlight to start growing in the first place, it would be hard to understand how they could have foundations over 400 metres below the suface. This question will be answer in the formation section.
Finging reefs are reefs that grow along the shoreline of a land mass such as a volanic island. These reefs grow on the shallow sea floor that surronds these land masses, and are not very deep. A good examples of fringing reefs are found on the Hawaiian islands.
Barrier reefs are found offshore growing somewhat parallel to the shore line, but not growing along it, as in fringing reefs. They are formed when a landmass sinks, leaving a ring of fringing reefs. The worlds largest reef, the great barrier reef is one example.