Why Atolls?

Geologists find coral atolls as fascinating as detective stories. The clues lie strung through the earth's warm seas in festoons of ringlike islands, like Wake Island in the Pacific (see cut). And for more than a century the geologists have been debating what the clues really mean. The most familiar theory is that atolls started as coral reefs fringing a small island. When the island sank (or the sea rose), the ring of coral kept growing upward, eventually forming an atoll with a lagoon where the island used to be.

In the latest American Journal of Science, F. Stearns MacNeil of the U.S. Geological Survey adds up the old clues to get a new theory: the rings were formed on dry land and later sank below the sea. He believes that coral and other sea organisms, growing on a shallow bottom, will build up a flat-topped reef (like many that exist today). In some cases, he says, such reefs were raised above the water, probably by changes of sea level because of ice ages, to become full-fledged islands. Then furious tropical rain went to work on the porous coral, dissolving it. The center of the island eroded faster than the rim, particularly if it had picked up a layer of soil. Reason: the soil contributed acids that attacked the limestone.

The soft coral of the rim, by alternate solution and recrystallization, was "casehardened" into solid rock that eventually stood in a high wall around most of the island. Then after the once-flat coral reef had eroded into a saucer, MacNeil believes, the sea rose again and flooded the low center. When the sea rose high enough, more coral grew on the high rim, building it up and forming the familiar shape of an atoll.

Geologist MacNeil is prepared to offer two kinds of evidence to support his stand. First, there are actually many islands, standing well above sea level, whose high rims and comparatively low centers could very well have been formed by the process he describes. Second, and even more convincing, the theory has survived a realistic laboratory test. A block of limestone, he reports, sprayed with dilute hydrochloric acid to approximate the effect of long-continued rain, erodes into a shallow saucer with a raised rim.

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