Coral reefs begin to form when free-swimming coral larvae attach to submerged rocks or other hard surfaces along the edges of islands or continents. As the corals grow and expand, reefs take on one of three major characteristic structures-fringing, barrier or atoll. Fringing reefs, which are the most common, project seaward from the shore, forming borders along the shoreline and surrounding islands. Barrier reefs also border shorelines, but distance.
They are separated from juxtapose land mass by a lagoon of open, often deep water. If a fringing reef forms around a volcanic island that severe completely below sea level while the coral continues to grow upward, an atoll forms. Atolls are usually circles or oval, with a central lagoon. Parts of the reef platform may appear as one or more islands, and gaps in the reef provide access to the central lagoon.
All three reef type-fringing, barrier and atoll-share similarities in their biogeographic profiles. Bottom topography, depth, wave and current strength, light, temperature, and suspended sediments all act to create characteristic parallel and perpendicular zones of corals, algae and other species. These zones very according to the location and type of reef. The major divisions common to most reefs, as they move seaward from the shore, are the reef flat, reef crest or algal ridge, buttress zone, and seaward slope.Coral reefs change through time. They begin with new tropical islands and are produced by an oceanic hot spot or at a plate boundary.
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