Today the crust is composed of eight major tectonic plates--the vast, irregular slabs of rock whose movements shift continents and sculpt mountains. One hundred fifty million years ago, the Farallon Plate covered more than one-third of the earth's surface. But at a rate of about an inch a year, the plate plunged beneath the western edge of North America, down into the mantle. The seismic image above, created by geophysicist Karin Sigloch of Ludwig-Maximilians University of Munich and based on data collected by EarthScope seismometers, depicts where the remains of that plate lie now.
Color-coded layers represent depth intervals of about 120 miles; at its deepest, the plate is 1,000 miles underground. Sigloch's study bolsters the theory that the Farallon Plate ground against the underside of North America and drove up the ranges of the Rockies, including the Elk Mountains in Colorado, shown at right. "By studying images of this plate deep underground," Sigloch says, "we can learn about conditions at the surface tens of millions of years ago."