Can anyone explain how coral islands are formed and how long it takes to be formed?

Answer 1

Coral islands, or coral reefs, are the product of living organisms. Before the ecosystem that forms the reef, and later the island exists there must first be the right physical conditions for coral and other reef-dwellers to live. Coral must have shallow water, with high oxygen content. For this reason coral communities thrive best on windward locations where they will get lots of wave action both to oxygenate the water and to circulate and remove sediments. This same circulation brings food to the tiny polyps that catch food by filtering it, and that build the coral.

One type of coral reef that forms beautiful rings with interior lagoons are those that are built on seamounts. These inactive volcanoes often subside slowly over time as they cool down. The coral will build upward at the same rate keeping up with sea level in most cases. If the coral does not grow upward fast enough to keep up, it will eventually die off from lack of food. This is often referred to as ‘drowning’ but is in fact simply starvation.
At this NOAA website they have satellite photos of many of the coral reef atolls in the new Northwestern Hawaiian Island Marine National Monument.
http://www.hawaiireef.noaa.gov/imagery/satimages.html
If you look at the images of Midway Atoll and Kure Atoll you can see excellent examples of how reefs have built around the sides of these ancient volcanoes. You can also identify the windward side of the atoll by the larger accumulations of coral and other material deposited by living organisms. The reef at Midway is about 28 million years old, and is over 500 feet thick (160 m). Below the reef are volcanic basalts deposited when Midway was an active volcano. The subsidence that allowed the reef to grow 500 feet thick was caused by the cooling and shrinking of the volcano after it ceased to be an active volcano.


Another well known carbonate (coral) reef environment is the Bahamas off the coast of Florida. These islands formed on top of a horst graben, which is a geologist’s term for a large block of rock that is elevated above surrounding blocks. Faults formed by the collision of the North American and Caribbean plates about 200 million years ago formed these blocks. The rocks that lie under the carbonate reefs are actually basaltic oceanic crust. These uplifted blocks of oceanic crust formed ideal shallow water environments for corals to colonize, and during several episodes of sea level fall the corals were able to colonize and build on deeper portions of the blocks, growing upward as sea levels rose. The reef rock has been drilled as deep as 5 kilometers where carbonate rock of Jurassic age is found. That makes these reefs over 140 million years old. Since sea level has never been 5 km lower, the reefs have been growing upward for the last 140 million years as the block of oceanic crust they sit on top of has been slowly sinking.


Barbados, in the eastern Caribbean, is an island reef that has been lifted above sea level. As the South American and Caribbean tectonic plates have collided they have formed a belt of folds in the seafloor. One of these folds grew high enough to reach near sea level and coral began to grow on top of it. The coral has grown over 100 meters thick, and the oldest exposed coral reef, on the highest part of the island, is over 600,000 years old. The island has a series of benches, or terraces, formed as the island has risen above sea level. These terraces get younger as they step downward toward the beach, with the youngest living reef rock still being in the sea around Barbados. http://hoopermuseum.earthsci.carleton.ca/cruise/barbados/geology.html


To summarize, coral reefs, and the islands they later become are formed by communities of living organisms, including many species of coral, but also many other reef-dwelling organisms including sponges, various types of carbonate forming algae, sea-grasses, bryozoans, mollusks, and many more. It is a rich and diversified community, and many of the organisms that live in coral reef environment secrete carbonate shells or living-structures that accumulate and build the reef. The time required to build a reef may be as little as half a million years (like Barbados), or it may be as long as 200 million years (like the Bahamas).

Some excellent references:
Harris, P.M, and W.S. Kowalik, editors. 1994. Satellite Images of Carbonate Depositional Settings. Tulsa, Oklahoma: American Association of Petroleum Geologists, 147 pp., includes 32 35mm slides and 3 transparent map overlays in pocket

Scholle, P.A, Don G Bebout, and Clyde H. Moore, editors. 1983. Carbonate Depositional Environments. Tulsa, Oklahoma: American Association of Petroleum Geologists, 708 pp.

!!! My 1000th Answer!!!

Some other reef sites:
Excellent comprehensive site on reef ecology
http://www.uvi.edu/coral.reefer/
NOAA's coral reef site:
http://www.coralreef.noaa.gov/
Coral Reef Alliance:
http://www.coralreefalliance.org/
Coral reefs at Texas A&M:
http://oceanworld.tamu.edu/students/coral/

Answer 2

Coral islands are formed by coral building sea organisms known as polyps. Polyps protect their bodies by building limestone walls around themselves. Over time, these colonies grow large enough to form reefs. Through a process that takes millions of years, these reefs become so expansive that they can cover hundreds of square kilometers. Reefs form the basis for islands. An island is formed if the sea level drops around the reef, or land on which the reef rests, rises, causing the submerged reef to be exposed. Over time, sand and dust accumulate on the reef, eventually forming an island. Although not as numerous as volcanic islands, well-known coral islands are found in the Florida Keys, Zanzibar and the Bahamas.

Answer 3

Coral Islands are just coral reefs that have been uncovered.

Atolls, the circular coral islands are formed by coral building up on the sides of a subsea volcano, the volcano may subside eventually but the coral builds up as the volcano erodes away forming a ring around the original crater.

They take 10'f of thousands of years to form.

Answer 4

Coral islands/reefs are formed when the skeletal material of corals piles up over years and years.

From what I can see, the length of time is dependent upon local conditions, etc. Here's a link to a site with details:-

http://www.answersingenesis.org/creation/v14/i1/coral_reef.asp

Answer 5

coral reefs are living organisms and when they die, they harden forming the perfect base for new coral to grow on. over a period of many years the coral will build up and up until it reaches the waters surface where it will stop as it cannot grow out of water. the amount of time it takes will depend on the depth of water.

Answer 6

1 year or so

Answer 7

Coral Reefs ARE in fact the skeletal remains of sea creatures that have fossilized. When one dies, its skeleton stays, and builds lime deposits, then more creatures live on the reef, and die, causing it to grow.. It IS dependant on the amount of life in that area, but it does in fact take thousands of years to produce an decent size reef

Answer 8

I assume that you are studying or researching this subject? There is 'oceans' of information available on the Web and it is always wise to visit a number of sites to obtain a cross-section of information and opinions.
The following are excellent sites, with lots of pics and diagrams that you may find useful:
http://www.coexploration.org/bbsr/coral/html/body_reef-formation.htm
http://www.biosbcc.net/ocean/marinesci/04benthon/crform.htm
http://www.undersea.com.au/corals/coral_reefs_formation.htm
http://www.gpc.edu/~pgore/students/f95/starmoss/coral.htm
Enjoy exploring this undersea world.

Answer 9

I'm not sure but if they weren't there which will be the case if the seas temperature keeps rising then the islands will be smashed by the unforgiving sea

Answer 10

they are form from coral and they need milions o years to form o just seconds (subteranial eruption, or other masiv impact geological changes)