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In the method, the travel times and paths of seismic waves -- produced naturally in earthquakes, or in explosions -- are tracked as they bounce through the Earth. There are two major kinds of seismic waves: P-waves, which oscillate in the direction that the wave is traveling (like an accordion), and slower S-waves, which oscillate at right angles to the direction of motion (like a slithering snake). The waves will speed up or slow down, and be reflected or refracted, depending on the type of material they pass through. Both types of waves travel fastest through cold, dense rock, and slower through hotter regions (the velocities increase as you probe deeper into the mantle). By analyzing these millions of criss-crossing rays, says James, "we can determine what parts of the Earth are 'fast,' and what parts are 'slow,' and get a picture of the structures under a particular area." Using this same type of data, geophysicist Ronald Cohen, also of Carnegie, has made computer models of the Earth's inner core -- and determined that most of the inner core is a single crystal of iron. For decades, geologists and geophysicists have also been using a more hands-on approach to deciphering the secrets of the inner Earth. Laboratory experiments are used to mimic the conditions in the mantle and core. Researchers, for example, study the composition of minerals believed to be common deep within the earth by using hydraulic presses and diamond anvils. Samples of rock are squeezed to extraordinary pressures -- sometimes millions of pounds per square inch -- and heated with laser beams to extreme temperatures, 5,000 degrees Fahrenheit or more. Other techniques involve blasting minerals with projectiles, to briefly change their crystal structure so that they resemble materials common in the mantle, for example. In this way, researchers at Sandia National Laboratories and Caltech recently arrived at an estimate of the temperature of the lower mantle -- no more than 7,300 degrees Fahrenheit. The measurement couldn't be taken directly -- no one, after all, has ever devised a way to stick a thermometer hundreds of miles into the Earth.
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