THEORIES of MOTION


Push South America and Africa together, and the fit leaves no room for argument. They were locked together as part of a large landmass, Pangaea, until some 200 million years ago, when it began to break into the mobile continents of modern times.


Ever since, the Atlantic Ocean has widened along a hot, crust-producing seam that runs through Iceland toward Antarctica. This process of seafloor spreading adds about an inch a year between the Eastern and Western Hemispheres.


The currents driving these vast tectonic changes sweep through the underlying man-tle rock about as fast as fingernails grow. The idea that solid rock can flow is hard to grasp because rock seems immutable in the human time frame. Yet over millions of years rock can move like glacier ice.


Heat makes the rock in the mantle flow. Most comes from decaying radioactive isotopes in the layer itself. Some comes from the outer core, as from the bottom of a double boiler.


Experts agree on all these points. Yet as Caltech geophysicist Don Anderson points out, “The paradigm behind the science of geology is, ‘Life is a mess.’ “ And scientists have yet to sort out completely the mess in the mantle.


Its pattern of movement remains the most contentious issue. Some researchers believe that the upper and lower mantles convect separately, exchanging little or no material. Others think currents sweep through the whole mantle. Increasingly, though, scientists see in the newest studies evidence for both patterns. Richard Carlson, a geochemist at the Carnegie Institution, compares the earth to a layered drink. “It’s like alcohol and orange juice. If you stir them gently, they will form layers, but with a lot of stirring they’ll mix.”



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