4 Kinds of Coral Reef
 

Summary

• Tiny coral polyps build the massive structure of the reef.
• There are 4 main types of coral reef: fringing reef; barrier reef; atoll; and patch reef.
• As reefs mature, they can develop from fringing reefs to barrier reefs to atolls

Building a reef

Coral reefs are underwater rock outcrops covered by a thin layer of living coral polyps. These outcrops can be huge, sometimes over a kilometer (1 km = 0.7 miles) thick and hundreds of kilometers in length. But the coral polyps that build these massive structures form a layer only a few millimeters thick (1 mm = approximately 1/32").

Reef-building coral polyps live in a protective shell made from calcium carbonate (CaCO₃), and as they divide and grow they form colonies. These colonies form the basic structure of the reef, helped by other organisms, like coralline algae. Building a reef is a slow process because the coral that form reefs grow about 0.3 - 2 cm (5/16 -13/16") each year. It is estimated that coral polyps need about 10 000 years to form a reef, and 100 000 to 30 million years to build a fully mature reef.

The 4 kinds of reef

Polyps are only able to build a reef with the help of single-celled zooxanthellae algae. Polyps provide zooxanthellae with nutrients and shelter, and receive food in return. Zooxanthellae algae require sunlight for photosynthesis, and coral reefs usually begin to form in clear, shallow waters near land because water gradually filters out sunlight with depth.

This relationship with land is used to categorize the different types of coral reef. 4 main types of reef exist:

• Fringing Reef
  This is the most common type of reef. They are located very close to land, and often form a shallow lagoon between the beach and the main body of the reef.

images originally from USGS (http://pubs.usgs.gov/fs/2002/fs025-02/)

• Barrier Reef
  This type of reef resembles a fringing reef, but they are located further from the shore and can be much bigger than fringing reefs.  For example, the Great Barrier Reef in Australia lies 300 – 1000m (328 - 1093 yards) from shore, and 2000km (1243 miles) long

images originally from USGS (http://pubs.usgs.gov/fs/2002/fs025-02/)

• Atoll
  These circular or horseshoe-shaped reefs encircle a lagoon. No apparent landmass is normally associated with an atoll.

images originally from USGS (http://pubs.usgs.gov/fs/2002/fs025-02/)

• Patch Reef
  These outcrops of coral usually lie within a lagoon.

How to make an atoll

Fringing reefs, barrier reefs and atolls are thought to represent stages in coral reef formation. Charles Darwin first proposed this theory of coral reef formation in 1842.

Step 1: A fringing reef forms first, and starts growing in the shallow waters close to a tropical island.

Step 2: Over time, the island subsides and the reef grows outwards, and the distance between the land and the reef increases. The fringing reef develops into a barrier reef.

Step 3: If the island completely subsides, all that is left is the reef. The reef retains the approximate shape of the island it grew around, forming a ring enclosing a lagoon. Darwin speculated that underneath each lagoon should be a bed rock base – the remains of the original island. Subsequent drilling into atolls proved this prediction true.

Reefs require very similar conditions to develop and are subject to similar forces from the open ocean. This means that similar types of reefs form under the same conditions even if they are on opposite sides of the world.

References

Achituv, Y. & Dubinsky, Z. (1990). Evolution and Zoogeography of Coral Reefs. In Ecosystems of the World: 25 Coral Reefs. Dubinsky, Z. (ed.). Elsevier, New York.

Anderson, G. (2003). Coral reef formation. MarineBio.net. Retrieved 5 September 2008 from http://www.marinebio.net/marinescience/04benthon/crform.htm

Birkeland, C. (1997). Introduction. In Life and Death of Coral Reefs. Birkeland, C. (ed.). Chapman and Hall, New York.

Darwin, C. (1842). The structure and distribution of coral reefs. [Reprint by University of California Press, Berkeley 1962]

Hatcher, B.G. (1997). Organic Production and Decomposition. In Life and Death of Coral Reefs. Birkeland, C. (ed.). Chapman and Hall, New York.

NOAA. (2008, March 25). How do coral Reefs form? NOAA National Ocean Service Education. Retrieved March 25 2008 from http://oceanservice.noaa.gov/education/kits/corals/coral04_reefs.html

Wikipedia. (2008, March 25). Coral Reef. Wikipedia. Retrieved March 25, 2008, from http://en.wikipedia.org/wiki/Coral_reefs

Wright, D. & Duncan, B. (2007, June 4). OC/GEO 103 Lecture - Ocean Habitats - Coral Reefs. Department of Geosciences, Oregon State University. Retrieved March 26, 2008, from http://dusk2.geo.orst.edu/oceans/lec25_ocean_hab.html