Welcome to You Ask Andy

David Walters, age 12, of Portland, Ore., for his question:

HOW ARE CORAL REEFS FORMED?

Some 200 million years ago, much of North America was submerged by shallow seas. Patiently a long coral reef formed along its southern shores. Nowadays this fossilized coral is buried below west Texas  and its porous rocks hold reservoirs of precious petroleum. Meantime, other coral reefs formed in other warm, shallow seas.

We call them coral reefs, though the sea dwelling coral creatures could never build them without help. Stony algae and other seaweeds also add building materials. So do tiny chalk building creatures, sea snails, shellfish and a vast variety of other stodgy sea dwellers. Whenever and wherever

ocean conditions are suitable, teams of these builders patiently add bits of solid material and gradually construct a rocky reef in the water.

The right conditions call for clean, clear sea water, as warm or warmer than 70 deg. The project begins on a solid, submerged ledge, no deeper than 120 to 150 feet below the waves and tossing tides. The coral polyps that build their limy houses depend on algae that need sunlit water, and both need minerals from clean, clear water, plus moving waves to provide oxygen.

Reefs cannot form in silty river deltas, where the busy builders would suffocate. Nor can they form where water is too cold or too calm or where the solid foundation is too deep for sunlight to penetrate. So called fringing reefs are built on offshore ledges. Through the ages they extend out to sea, and the edge by the shore decays.

After a few hundred thousand years, a project of this type forms a barrier reef several miles from the shore. Other reefs form coral isles called atolls, shaped like rings or crescents around blue lagoons. These formations are built on the crests of old undersea volcanic cones. Other reefs form flat tables on the submerged tops of old sea mountains.

    Right from the beginning, the underwater building is occupied. For the reef is an ecosystem, a small world of its own populated with a multitude of assorted plants and animals that depend upon each other. Living coral polyps wave petal tinted tentacles from windows in their stony homes. Colorful snails, worms and shellfish shelter in coral crevices and gaudy little fishes flit through the sunny water.

 

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