This chapter introduces the main ideas of geology, and explains briefly the technical terms used in this book. We deal with the overall structure of the Earth, the theory of plate tectonics, the main mineral groups and rock types (igneous, sedimentary and metamorphic), the deformation of rocks into folds and faults, the processes of erosion and formation of the landscape, and the geological time scale.

MODEL OF THE EARTH

The Earth is a generally spherical planet, slightly flattened at the poles. Inside the Earth are three components (see Figure 2.2): a central, hot core with a 3470 km radius, composed mainly of iron and nickel with a density of 10 000–13000 kg/m3 (10–13 t/m3), solid in the centre and liquid in the outer core; a mantle 2900 km thick, composed of dense (3–6 t/m3) rocky material in a hot plastic state; and an outer, cool, solid crust with a density of 2–3 t/m3. The core and mantle make up most of the Earth – 99% by volume – while the crust is a relatively thin skin.

The core-mantle boundary is a zone of fundamental importance. Circulation in the liquid outer core is the dynamo that produces the Earth's magnetic field. A combination of thermal circulation and gravitation, as heavier constituents settle towards the centre, fuels this engine of the Earth's magnetism. The core–mantle boundary has thin zones that slow seismic waves by 10–30%, but no–one knows why these thin zones occur. Recent work suggests that these ultra-low-velocity zones are caused by irregular patches of liquid material at the base of the mantle that contain impurities driven out as the core continues to cool and crystallise.