101 Things You Don't Know About Science and No One Else Does Either
by James Trefil
Top Ten Problems in Science
Astronomy and Cosmology
Earth and Planetary Sciences
How Does the Earth's Mantle Move?
The modern picture of the earth holds that the motion of the continents is driven by heat generated by radioactivity deep inside the planet. This heat rises to the surface through convection (the same process you see in a pot of boiling water, in which hot water rises from the bottom, then cools off and sinks to repeat the cycle). Over hundreds of millions of years, rocks deep underground go through the same cycle, with the plates and continents riding along the top of the moving rock like bubbles on boiling water.
But although there is superficial agreement among scientists to this point, the arguments start when we get into details. In particular, there is continuing debate about how deep the convection process extends into the earth's interior.
Some background: after the earth was first formed, it went through a period of melting all the way through. Heavy materials sank to the center, and the rest of the planet acquired a layered structure. Outside of the nickel-iron core and extending to within a few miles of the surface is a 2,000-mile-thick layer of heavy rock called the mantle. It is the convection of this rock that moves the continents.
Scientists cannot study the interior of the earth directly, but they can observe the arrival of seismic waves at observatories on the surface. Every time a major earthquake occurs, waves travel through the earth, and by measuring how long it takes them to get to different points on the surface, scientists can build up an amazingly detailed picture of the earth's interior. This technique can be compared to the medical procedure of a CAT scan, in which pictures of the body are built up by measuring the absorption of X-rays.
Seismic studies reveal one outstanding fact about the mantle: a layer about 400 miles down seems to mark a transition between the upper and lower mantle. Seismic waves apparently speed up when they pass down through this transitional layer, which can be explained only by assuming that the atoms in the lower mantle are arranged differently from those in the upper mantle. For the last thirty years a controversy has raged among geologists over the question of whether the roiling convection that drives the continents extends down only to this layer, or whether it goes all the way to the bottom of the mantle. As often happens with long-standing controversies in the sciences, the basic problem is that there are two sets of data, from two different disciplines, that cannot be reconciled.
On one side, geochemists point to the presence on the earth's surface of a number of chemical elements whose abundance on the surface seems out of proportion to their abundance in the upper mantle. The geochemists argue that this kind of imbalance can occur only if material from deeper in the mantle is brought up occasionally, which would happen if convection mixed the two mantle layers. On the other side, seismologists argue that they see no evidence for such mixing in their "CAT scans" of the earth.