We have walked on the moon, sent rovers to Mars, and discovered planets light-years away. But the farthest we have ventured inside our own planet--the 7.6-mile-deep Kola Superdeep Borehole in the Russian Arctic--got us only 0.4 percent of the way to the core. The inner earth has been terra incognita, until now.
Using technologies analogous to medical CAT scans, geologists are virtually peeling back our planet's outer layers and exposing its internal life in exquisite, three-dimensional detail. To create this big-picture view, University of Southern California geophysicist Thorsten Becker used seismic tomography, which tracks waves produced by earthquakes as they travel around and through the earth. The model reveals the structure of the mantle--the thick shell of hot, compressed rock that lies between the crust and the core. The mantle, which makes up more than four-fifths of the earth's volume, is energized by radioactive heating within it and by the molten iron outer core below it. "As the core gives off heat," Becker says, "the mantle convects, moving sluggishly, like a pot of boiling honey." That slow churning, in turn, drags the crust with it.
Seismic waves move through relatively hot regions (red) more slowly and cold regions (blue) more quickly than through the rest of the mantle. By measuring the motions of those waves, Becker was able to create this thermal map, with the sphere in the middle showing the temperature at the deepest point of the mantle. (Lumps delineate the temperature anomalies; gold arrows denote how quickly different regions of the mantle and crust are moving, with the longest indicating four inches a year.) "Mantle convections drive plate tectonics," Becker says. "So if we want to understand anything that happens on the surface geologically, from mountain-building to earthquakes, we need to understand the deep movements of the mantle."
This gallery originally ran in the Invisible Planet special issue in July-August, 2011.