A MODEL OF THE EARTH

The Earth is a near-spherical planet slightly flattened at the poles (see Figure 2.1b). Our understanding of its inner nature comes largely from interpreting records of seismic waves generated by earthquakes that have passed through it. Three separate layers, with different compositions, occur within the Earth (see Figure 2.1a):

1. • a central, hot core, with a 3470 km radius, composed mainly of iron and nickel, with a density of 10–13 t/m3 (10000–13000 kg/m3), solid in the inner part but liquid in the outer part

2. • a mantle, 2900 km thick, composed of dense (3–6 t/m3), rocky material in a hot, semiplastic state, except for its uppermost layer, which is solid

3. • an outer, cool, solid crust, generally 8–50km thick, with a density of 2–3 t/m3

These three layers are separated by discontinuities identified from the passage of seismic waves. The dense and heavy core and mantle make up most of the Earth (99 per cent by volume), whereas the light crust in comparison forms a very thin skin.

The base of the crust is marked by a major seismic discontinuity where the speed at which earthquake waves travel is abruptly increased. Since the speed of such waves is directly dependent on the density of the material in which they are propagated, the crust–mantle boundary marks a major density contrast between material within the crust and that within the mantle. Crustal rocks are richer in silicon, a light element, and poorer in iron and magnesium, heavier elements, than those of the mantle. The boundary is referred to as the Mohorovičić discontinuity, or the Moho for short, after the Croatian seismologist who discovered it, in 1909. For the continents the crust averages 40 km thick but is much thinner for the ocean, at only 8 km. Depth to the Moho has been mapped for the Australian continent and averages 38 km.