Chapter 5 - The Core

Abstract

Earth's core is responsible for the generation of Earth's magnetic field. The core contains information regarding the earliest history of accretion of the planet. When the core formed, some thermal and compositional features were established which controlled the subsequent evolution of the core. These features also influenced the evolution of the mantle, crust, and atmosphere. Seismic velocity data indicate that the radius of the core is 3485 km and that the outer core does not transmit S waves, interpreting it is in a liquid state as also supported radio astronomical measurements of Earth's normal modes of free oscillations. The inner core, with a radius of 1220 km, transmits S waves at very low velocities, suggesting that it is a solid near the melting point or partly molten. Detailed analysis of the travel times of seismic waves reflected from and transmitted through the core indicates that the outer liquid core is relatively homogeneous and well mixed. The core is composed chiefly of iron as indicted by three points: the internal geomagnetic field must be produced by a dynamo mechanism, which is only possible in a liquid-metal outer core; the calculated density and measured body wave velocities in the core are close to those of iron measured at appropriate pressures and temperatures; iron is the most abundant element in the solar system that has seismic properties resembling those of the core.