The Earth's Interior

Abstract

Seismologists have developed many methods for studying the structure and composition of the Earth's interior. Tens of thousands of earthquakes occur every year and each one sends out seismic waves in all directions. The bigger events send waves into the deep interior and these are used to infer the three-dimensional structure of our planet. Large earthquakes also generate surface waves which travel repeatedly around the surface and constructively interfere to generate the free oscillations, or normal modes, of the Earth. The velocities of seismic waves depend on composition, mineralogy, temperature and pressure and arc useful as tools for studying these parameters. The new science of seismic tomography (meaning to 'slice the Earth') provides images which are useful in mapping convection. While we can sample the crust of the Earth by a variety of techniques, the rest of the interior is largely inaccessible. Geophysical data, such as the mass and moment of inertia of the Earth and the velocities of seismic waves, are not adequate unambiguously to infer the chemistry of the mantle and core. We know from the distribution of elements in the Solar System which elements are likely to be most important in the Earth. We also obtain valuable clues by studying meteorites, the properties of the other planets in the Solar System, and the Sun (see also Chapters 1 and 2).