Some reefs found in the national marine sanctuaries fit obviously into one of these categories. In other cases, the classification scheme does not clearly apply. Fringing reefs closely follow shorelines at a short distance with at most a narrow, shallow lagoon between reef and land. Barrier reefs also generally follow a shoreline, however, they are separated by a greater distance and a deeper lagoon. The distance may vary from about 1.5 to 40 km and the lagoon may be deeper than 18 m.
Atolls are horseshoe or ring-shaped, open sea reefs that surround a lagoon.
They are common in the Indian and Pacific Oceans in areas of submerged volcanic islands. Bank reefs are also open sea reefs but have no central lagoon. They are surrounded by deep water and are miles from any land mass. Patch reefs are small, isolated reefs. Their size and the depth at which they are found vary.