The only rocks that geologists can study directly in place are those of the crust; and Earth's crust is but a thin skin of rock, making up less than 1% of Earth's total volume. Mantle rocks brought to Earth's surface in basalt flows, in diamond-bearing kimberlite pipes, and also the tectonic attachment of lower parts of the oceanic lithosphere to the continental crust, give geologists a glimpse of what the underlying mantle might look like. Meteorites also give clues about the possible composition of the core of the Earth. But, to learn more about the deep interior of Earth, geologists must study it indirectly, largely by using tools of geophysics - that is, seismic waves and the measurement of gravity, heat flow, and earth magnetism.

The evidence from geophysics suggests that Earth is divided into three major layers - the crust on Earth's surface, the rocky mantle beneath the crust, and the metallic core at the center of Earth. The study of plate tectonics has shown that the crust and uppermost mantle can be conveniently divided into the brittle lithosphere and the plastic asthenosphere.

You will learn in this chapter how gravity measurements can indicate where regions of the crust and upper mantle are being held up or held down out of their natural position of equilibrium. We will discuss Earth's magnetic field and its history of reversals. We will show how magnetic anomalies can indicate hidden ore and geologic structures. We close with a discussion of the distribution and loss of Earth's heat.

Expanded Readings From Chapter 17

Mantle Xenolith A Peek at the Deep