coral reef

Introduction
Structure
- Corals and other reef-building organisms
- Orientation and arrangement
- Sedimentation
- Types of coral reefs
Origin and development of reefs
Reef accumulation
- Tropical water conditions
- Geochemistry of reefs
- Winds, currents, temperature, and salinity
- Biological factors
Threats to coral reefs
- Crown-of-thorns starfish
- Coral bleaching
- Other threats

References & Edit History Quick Facts & Related Topics

Images & Videos

A crown of thorns starfish, Acanthaster planci, with a round body covered in long, sharp orange spines, rests on a coral reef.

What causes coral bleaching on the Great Barrier Reef?

How can coral reefs be protected from sunscreen damage?

Underwater view of a shallow coral reef off Guam showing widespread coral bleaching, with many corals appearing white and lacking vibrant color.

Dense coral reef in the Red Sea with clusters of colorful corals and many small orange fish swimming in clear blue water.

At a Glance

coral reef summary

Quizzes

Taj Mahal, Agra, India. UNESCO World Heritage Site (minarets; Muslim, architecture; Islamic architecture; marble; mausoleum)

Wonders of the World Quiz

Chutes d'Ekom - a waterfall on the Nkam river in the rainforest near Melong, in the western highlands of Cameroon in Africa.

Ecosystems

Related Questions

How are atolls different from barrier reefs?
How can a lagoon remain inside an atoll as the ocean floor sinks?
How can coral skeletons reveal past seawater temperatures?
How can hurricanes both build and destroy coral reef islands?
How can uplift turn a coral reef into a coral island?
How did Charles Darwin explain the formation of atolls?
How did drilling at Enewetak Atoll support Darwin’s theory of atoll formation?
How did melting ice sheets after the Pleistocene Epoch affect coral reefs?
How do algae and protozoans bind coral reef material together?
How do calcareous algae contribute to coral reef structure?
How do carbon isotopes help scientists determine the age of coral reef samples?
How do carbonate sand and mud become part of a coral reef system?
How do changes in sea level affect coral reef growth?
How do coral polyps build the limestone framework of a coral reef?
How do coral sand cays form on reefs?
How do fringing reefs differ from barrier reefs?
How do patch reefs form?
How do plants and animals contribute to soil formation on coral reef islands?
How do prevailing winds affect the location of coral cays?
How do storms and waves create coral reef islands?
How does a barrier reef become an atoll?
How does a fringing reef develop into a barrier reef?
How does calcium carbonate cement help stabilize a coral cay?
How does erosion shape a coral reef that rises above sea level?
How does land subsidence help coral reefs grow upward?
How does plate tectonics help explain the development of Pacific coral atolls?
How does rainwater help turn loose reef debris into solid coral island material?
How does sedimentation turn reef material into rock?
Which organisms besides corals help build coral reefs?
Why are atolls usually ring-shaped?
Why are coral reef islands often low and flat?
Why are coral reef islands vulnerable to changes caused by storms and waves?
Why are corals able to form reefs in shallow sunlit water?
Why are reef-building corals usually colonial animals?
Why can a rapid rise in sea level drown a barrier reef and eliminate its lagoon?
Why can freshwater floods damage inshore coral reefs?
Why do many coral sand cays form on the sheltered side of a reef?
Why do seagrasses and mangroves grow behind some coral reef crests?
Why do tropical coral reefs need warm seawater?
Why does sunlight affect the growth of coral reefs?

Geography & Travel Physical Geography of Water

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External Websites

National Center for Biotechnology Information - Coral Toxicity
NOAA's Coral Reef Information System - What are Coral Reefs
Smithsonian Ocean - Corals and Coral Reefs
Live Science - What are coral Reefs?
NASA - What is a Coral Reef?
UN Environment Programme - Coral Reefs
ACS Publications - Environmental Science and Technology - Toward a New Era of Coral Reef Monitoring

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

coral reef - Student Encyclopedia (Ages 11 and up)

coral reef

geology

Also known as: organic reef

Written and fact-checked by

Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

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Last updated

Oct. 24, 2026 •History

coral reef, ridge or hummock formed in shallow ocean areas by algae and the calcareous skeletons of certain coelenterates, of which coral polyps are the most important. A coral reef may grow into a permanent coral island. Often called the “rainforests of the sea,” coral reefs are home to a spectacular variety of organisms. A coral reef is actually a complex of features, only part of which is a living coral or algal framework, although...

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