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Probing the Depths

Probing the Depths

by Kathy Svitil

In a remote northern outpost of Russia, not far from the border with Finland, Russian researchers have drilled a remarkable hole, called the Kola well. The hole, which penetrates over eight miles into the crust, is the deepest ever made. It was drilled to help researchers understand geophysical processes such as how ores are formed and the stresses that deform rock at those depths.

But although Kola is an amazing hole, it has contributed very little to scientists' understanding of the Earth below the crust -- the upper and lower mantle, and the outer and inner core. On the thinnest of continents, the mantle lies at least 15 miles below the surface; the core, thousands. (See The Hot Zones animation, 8K. You will need the free Flash plug-in to view this animation.) The Kola well is deep, but not near deep enough.

It's much easier to reach the mantle by boring through the oceanic plates, particularly at the mid-ocean ridges. There, the crust is only about three miles thick. One research ship, the JOIDES Resolution, had drilled more than half-way to the mantle through a ridge in the equatorial Pacific seafloor. Japanese researchers are now planning a project, slated to begin around 2003, that should make it the whole way.

Since scientists haven't yet managed to get into Earth's interior by digging there, they've devised other techniques to take a look. The Earth has provided some help. South Africa, for example, is home to deep natural conduits, called kimberlite pipes (from which diamonds are mined). Although they haven't been drilled, "they are like boreholes," says geophysicist David James of the Carnegie Institution of Washington in Washington D.C. "They bring up chunks of material from the mantle from two hundred to two hundred and fifty kilometers' depth. They give us a good picture of what the mantle looks like, so you don't really need to drill a hole."

Researchers like James also commonly rely on a technique called seismic tomography to visualize the inner Earth. Seismic tomography gives an x-ray-like picture of the Earth -- showing the boundary of the mantle and core, for example, or hot plumes of magma under a volcanic island -- just as a CAT-scan of a medical patient creates a three-dimensional image of the body.

Tomography Image

A seismic tomographic image of the mantle beneath central Eurasia. Hotter areas are red; cooler, blue.

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.

Tomographic image courtesy of Dr. Ivan Koulakov of the United Institute of Geology, Geophysics, and Mineralogy,  Siberian Branch of the Russian Academy of Sciences.

Article: The Earth at Work | Sidebar One: Probing the Depths | Sidebar Two: "Black Smokers" | Sidebar Three: Ring of Fire | ANIMATION
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