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How Do Reefs Form?


Coral Reef in St. Croix, U.S. Virgin Islands / NOAA

Healthy coral reef in St. Croix, U.S. Virgin Islands

NOAA, http://www.noaa.gov/features/climate/coralreefwatch.html
Question: How Do Reefs Form?
Reefs are centers of biodiversity, where you'll find many types of fish,invertebrates and other marine life. But did you know that coral reefs are living also?

Coral reefs are composed of animals called stony corals. Corals are made up of tiny organisms called polyps. The polyp sits inside a calyx, or cup, made of limestone (calcium carbonate). As the polyps live, reproduce, and die, they leave their skeletons behind.

A coral reef is ultimately built up by layers of these skeletons covered by living polyps, which are further cemented together by organisms like coralline algae, and physical processes like waves washing sand into spaces in the reef.

There Are 3 Types of Coral Reefs:

  • Fringing reefs, which grow close to the coast in shallow waters.
  • Barrier reefs, which are large, continuous and are separated from land by a lagoon (the Great Barrier Reef is the largest example of a barrier reef.)
  • Atolls, which are ring-shaped and located near the sea surface on top of underwater islands or inactive volcanoes.
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