The Making of Mountains

Back in the days when Grandpa went to grade school, geography teachers had no trouble explaining how the earth's mountains are formed. The earth is cooling and shrinking, they told the kids; its crust has wrinkled into mountain ranges like the skin of a drying apple. Modern geophysicists, who believe that the earth was cold when it started its career, have abandoned this charmingly simple theory. Trouble is, they have had little luck developing a satisfactory substitute.

Now, Physicist Raymond A. Lyttleton of Cambridge University proposes a return to the wrinkle theory of mountain building--but with a difference. The earth was cool and solid when it was formed, says Lyttleton; then radioactivity gradually heated its rocky material. A few billion years ago, the earth's central core got hot enough to change from a plastic solid to a true liquid. Under the enormous pressure that exists near the center of the earth, liquid rock is more compressible than solid rock. So when the core liquefied, it was squeezed into a smaller amount of space, allowing the layers about it to settle down too.

Most of the earth's material is plastic enough to contract evenly, but the thin surface crust is rigid. Instead of contracting smoothly when the core shrank, it cracked and wrinkled, just as in the old theory. Sometimes parts of the earth's crust slid over other parts like sheets of ice in a fast-flowing river. These surface irregularities, much changed by erosion, are the earth's mountains.

Lyttleton figures that the earth's compressible liquid core, which can be studied by means of earthquake waves, has caused the earth to shrink about 400 miles in diameter. Some 20 million square miles of crust have been tucked away in mountainous folds and wrinkles. How long this process will continue, Lyttleton does not know. But mountains are still rising, and Lyttleton estimates that if the entire earth were to liquefy, it would lose another 50 miles of diameter.

The moon and Mars, Lyttleton calculates, are too small to have liquid cores, and this may be why neither of them has mountain ranges. But Venus is about the same size as the earth, is probably made of much the same material, and it may have a shrinking liquid core. As man's space probes continue to study the distant planet, they may discover that it has a pattern of wrinkled, earth-type mountains hidden under its cloud deck.

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