The rocks that can be studied on the earth's surface tell us much about the earth's uppermost crust but very little about the other 99 percent of the planet. Drilling has reached a maximum depth of about 12 kilometers (7 miles). Samples of rock from the deeper crust and mantle are sometimes included as xenoliths in deep‐seated intrusives that moved along structural zones to the surface. Rare segments of ultramafic rocks in complex tectonic settings are thought to have originated from the lower crust or upper mantle.
Fortunately, the field of geophysics—the application of the laws of physics to the dynamics of the earth—provides compelling data that allow us to interpret how the inner earth is constructed. The principal characteristics that geophysicists study are seismic waves, gravity, heat flow, magnetism, and electrical conductivity. When integrated, the data allow us to construct a realistic picture of how the inner earth works.