Why are atolls ring shaped?

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Robin Millar

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Why are atolls ring shaped?
« on: 04/08/2009 08:30:03 »
Robin Millar asked the Naked Scientists:
Darwin explained the formation of coral atolls.  Darwin explained that a coral reef would grow around an oceanic island.  Over time the island subsides below the ocean surface leaving the reef ringing a lagoon.
My question: why doesn't the coral expand into the lagoon, leaving a circle of coral, rather than a ring of coral?

What do you think?


Offline JimBob

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Why are atolls ring shaped?
« Reply #1 on: 05/08/2009 04:08:38 »
Two reasons.

Most reef forming corals need high energy to exist. They need a lot of nutrients washed in and out of their feeding space to survive, expand and reproduce.

Corals also need a solid substrate on which to grow. That is why they grow on the rough rock of a newly formed atoll. As the rock is destroyed by weathering it form the sand behind the reef. This sand is not a place that corals can grow on. Yet if some species of siliceous or calcium formed sponges or other hard bodied animals colonize the back reef, corals can affix to the skeletons of these animals and form broad reefs. The Great Barrier Reef is an example of the large reef system. It grew as the mainland was worn away by eons of erosion.
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Offline chris

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Why are atolls ring shaped?
« Reply #2 on: 11/08/2009 11:15:57 »
In the latest episode of the Darwin Festival Podcast Brian Rosen, from the Natural History Museum, London, discusses this very question.

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