Geology

By analyzing seismic refraction and seismic reflection data from all over the world, geophysicists have identified boundaries that separate three concentric parts of the earth: the crust, mantle, and core. The crust—5 to 50 kilometers (3−30 miles) thick—is essentially the thin, hardened skin of what was once the earth's molten exterior surface. Underlying the crust is the mantle—2,900 kilometers (1,740 miles) thick—a thick zone of much hotter, partially plastic rock. The mantle surrounds the inner and outer core—about 7,000 kilometers (4,200 miles) combined in diameter—the innermost zone of the planet.