Bottom topography, depth, wave and current strength, light, temperature, and suspended sediments all act to create characteristic horizontal and vertical zones of corals, algae and other species. Depending on their size, barrier reefs and atolls can take from 100,000 to 30,000,000 years to fully form.Īll three reef types - fringing, barrier and atoll - share similarities in their biogeographic profiles. With growth rates of 0.3 to 2 centimeters per year for massive corals, and up to 10 centimeters per year for branching corals, it can take up to 10,000 years for a coral reef to form from a group of larvae. In addition to being some of the most beautiful and biologically diverse habitats in the ocean, barrier reefs and atolls also are some of the oldest. Parts of the reef platform may emerge as one or more islands, and gaps in the reef provide access to the central lagoon. Atolls are usually circular or oval, with a central lagoon. If a fringing reef forms around a volcanic island that sinks completely below sea level while the coral continues to grow upward, an atoll forms. They are separated from their adjacent land mass by a lagoon of open, often deep water. Barrier reefs also border shorelines, but at a greater distance. ![]() ![]() Fringing reefs, which are the most common, project seaward directly from the shore, forming borders along the shoreline and surrounding islands. As the corals grow and expand, reefs take on one of three major characteristic structures - fringing, barrier or atoll. 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.
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