The Structure of Earth and Its Constituents

Inferring Earth’s composition is a prerequisite to understanding its evolution and dynamics as well as those of planets like it. One might think that the composition of Earth can be easily inferred from the rocks that we can see on Earth’s surface. However, it immediately becomes obvious that these rocks cannot be the major constituent of Earth’s interior because the densities of typical rocks on Earth’s surface, such as granite or basalt, are so small, even if the effects of compression on density are taken into account. Therefore, materials in the deep Earth (and most other planets) are different from those on the surface. What materials are there, and how do we infer the composition of the deep interior of Earth (and other planets)? You may want to drill into Earth, but the deepest drilled hole in the world is in the Kola peninsula in Russia, which is only ∼ 12 km deep (remember that Earth’s radius is 6,370 km). Although some kinds of volcanoes bring materials from the deep mantle, this sampling is usually limited to ∼ 200 km deep. Therefore, our inference of Earth’s internal structure must be based largely on indirect information. In this connection, both geochemical and geophysical observations are particularly relevant. In geochemistry, the scientist measures the chemical composition of various materials, then uses the chemical rules that govern the distribution of various chemical elements to infer the chemical composition of Earth. In geophysics, one measures physical properties such as seismic wave velocities and density, and infers the composition and structure of Earth’s interior based on the physical principles that control the variation of physical properties with thermodynamic conditions (pressure and temperature). In this chapter, I will first summarize these basic observations, and then explain several models of Earth that have been proposed on the basis of geo-