The Mantle-Crust System

Abstract

The principal subdivisions of the Earth’s interior are based on the depth distribution of seismic wave velocities (Fig. 1.1). The crust is defined as the region extending from the surface to the Mohorovicic Discontinuity, (Moho) which lies at a depth of 30–50 km beneath most continents and about 10–12 km beneath most oceans. Underlying the crust, and extending to a depth of about 400 km, lies the upper mantle. Below this depth, extending between 400 and 1000 km, lies the transition zone. This is characterized by high-velocity gradients on the average and by the presence of two or more seismic velocity discontinuities. In contrast, the lower mantle, extending from 1000 to 2900 km, is characterized by a moderate and relatively uniform increase of seismic velocities with depth. The boundary of the Earth’s core is reached at a depth close to 2900 km. This is marked by a major first-order seismic discontinuity for P waves and by cessation of transmission of S waves, implying a liquid state. The core is divided into two regions, the liquid outer core and the inner core, as shown in Figure 1.2. The latter region transmits S waves as well as P waves and is therefore believed to be solid. The dimensions and masses of the individual regions are given in Table 1.1.