What reef type is separated from land by a lagoon?

A reef separated from land by a lagoon is a barrier reef.

Barrier reefs generally run parallel to a coastline but are positioned farther offshore than fringing reefs. Between the reef and the mainland or an island lies a lagoon, which may be shallow or very deep and can contain patch reefs, sand cays and seagrass habitats. The reef’s outer face meets open-ocean waves, while the lagoon provides relatively sheltered water.

The Great Barrier Reef is the best-known example. It extends along the coast of Queensland, Australia, and is separated from the mainland by a broad, complex lagoon system. Barrier reefs can also surround islands rather than continental shores.

A fringing reef grows directly against the coast, so it lacks the broad intervening lagoon typical of a barrier reef. Atolls are another common source of confusion: they are ring-shaped reef systems surrounding a lagoon, usually with no central high island. “Barrier” describes the reef’s relationship to land, not simply its size.