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# How Do Coral Reefs Form?

## Corals Tutorial

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 directly from the  shore, forming borders along the shoreline and surrounding islands.  Barrier reefs also border shorelines, but at a greater distance. They  are separated from their adjacent land mass by a lagoon of open, often  deep water. 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. Atolls are usually circular or oval, with a  central lagoon. Parts of the reef platform may emerge as one or more  islands, and gaps in the reef provide access to the central lagoon.  

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. 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.  Depending on their size, barrier reefs and  atolls can take from 100,000 to 30,000,000 years to fully form.

All three reef types — 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 horizontal and vertical zones of corals, algae  and other species. These zones vary 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.

Corals usually develop into one of three characteristic structures: fringing reefs, barrier reefs or atolls. Learn more and view a larger image.

As coral reefs grow, they establish characteristic biogeographic patterns. Learn more and view an animation.

### Corals Lessons

              - Welcome
              - What are Corals?
              - Zooxanthellae What's That?
              - How Do Stony Corals Grow?
              - How Do Coral Reefs Form?
              - Where Are Coral Reefs Found?
              - How Do Corals Reproduce?
              - Importance of Coral Reefs
              - Natural Threats to Coral Reefs
              - Anthropogenic Threats to Corals
              - Coral Diseases
              - Protecting Coral Reefs
              - References
              - Roadmap to Resources 
              - Subject Review (PDF)

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