5 - The core

Abstract

This chapter provides information on the structure, composition, and evolution of the Earth's core. Detailed analysis of travel times of seismic waves reflected from and transmitted through the core indicate that the outer liquid core is relatively homogeneous and well mixed, probably because of mixing by convection currents. Seismic-velocity data indicate that the radius of the core is 3485 ± 3 km and that the outer core does not transmit secondary, or shear, waves. This observation shows that the outer core is in a liquid state. The inner core with a radius of 1220 km transmits S-waves at low velocities, suggesting that it is a solid near the melting point or partly molten. Although seismic data indicate that the inner core is solid, on geologic timescales it may behave like a fluid undergoing solid-state convection like the mantle. Three lines of evidence indicate that the core is composed chiefly of iron. First, the internal geomagnetic field must be produced by a dynamo mechanism, which is only possible in a liquid metal outer core. Because the fluidity and electric conductivity of the mantle are too low to produce the Earth's magnetic field, the outer core must be liquid metal for the geodynamo to operate. Second, the calculated density and measured body-wave velocities in the core are close to those of iron measured at appropriate pressures and temperatures. Third, iron is by far the most abundant element in the solar system that has the seismic properties resembling those of the core.