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Barrier reefs traditionally refer to fringing systems that have been separated from the land because of two processes: lateral growth and subsidence of the land. An island bordered by fringing reefs that eventually evolve to a barrier reef system is exemplified by Mangaia (#37) and Aitutaki (#37), Cook Island. The land is still emergent in the lagoon in Aitutaki, while an atoll would be without any central land. Sometimes, islands where the land occupies a small fraction of the lagoon are called pseudo-atoll.
The rim of a Tuamotu, Cook or Marshall atoll can be considered the final stage of a barrier reef. However, the rim of Maldivian atoll would not be a barrier reef because the biological, geological, tectonic and eustatic processes that created the atolls are different on this region than in Tuamotu or Marshall. However, considering their modern geomorphology and their benthic zones, there is no real justification to not consider Maldivian reef rims barrier reef systems. On the other hand, some reefs sometimes presented as barrier reef (Andros, Bahamas or even the Florida Keys, #1) are really not modern barrier reefs for two reasons. They present a succession of small margin reefs (Florida Keys #1) or patch reefs whose geological history is different than true barrier reef. Plus, their modern morphology and benthic zonations is very different than true barrier reefs and they don't make a physical barrier at all.
Typical barrier reefs include long uninterrupted reefs such as those encountered along New Caledonia coasts (#26, #27, #28, #29) or Belize (#5). The Great Barrier Reef, including Torres Strait reefs, is actually made of a dense matrix of reefs (#32, #33) whose formation does not follow the typical barrier reef scheme, though in some cases they clearly create a physical barrier (#31 Pompei Reefs, #34 Ribbon Reefs). Individually, none of the Great Barrier Reef reefs can compare in length to the New Caledonia barrier reefs and they are far smaller than many atoll rims. The longest uninterrupted barrier reef worldwide would be along the giant Pacific atoll rims.
Geomorphologically, the Great Barrier Reef provides a unique complexity of reef structures. The unique latitudinal gradient of the Great Barrier Reef offers a variety of seascapes and processes that have resulted in a tremendous number of lagoon and reef flats configurations. Numerous local or regional scales processes modulate the basic barrier reef scheme worldwide. For instance, islands may occur within the barrier reef system such as in Palau (#39) or Mayotte. Barrier reefs may form almost closed annular-shaped structures (Palau #40). Barrier reef may connect to fringing reefs making a continuous shore-ocean structure like in Wallis (#38). A barrier reef may occur within a larger barrier reef system (Lizard Island reefs within the Great Barrier Reef system) like annular faros occur in atolls. Some barrier reef sections can be drowned. Finally, barrier reefs may be double (#28) or even triple.