by the creation, destruction, relative movement and interaction of a dozen or so crustal plates - comprising the lithosphere - which slide over the less rigid asthenosphere below. Collisions between plates produce folded mountains and zones of seismic activity are concentrated along the plate boundaries. Seismic waves generated during earthquakes reveal the internal structure of the Earth by the way they propagate. At the centre, there is a molten metallic core of iron and nickel, possibly with a solid core at the very centre. The central temperature is around 4,000АC. A silicate mantle overlies the core. The outermost crust is about 10 kilometres (6 miles) thick under the oceans and 30 kilometres (20 miles) thick where there are continents. In planetary terms, the surface of the Earth is very young. The basaltic rocks forming the ocean floors are among the youngest. The Precambrian shields - which occupy about 10 per cent of the surface - are the oldest, and the nearest approximation to the cratered terrain that forms a large part of other planetary surfaces. Weathering has removed all but a few traces of whatever impact craters there were. The molten metallic core gives rise to the Earth's magnetic field and magnetosphere. A layer of electrically charged particles between heights of about 50 and 600 kilometres (30 and 400 miles) form the ionosphere. The funnelling of charged particles by the magnetic field to regions between latitudes of 60А and 75А creates the phenomenon of the aurorae. Satellite measurements have shown that the Earth is also an intense source of radio waves at kilometre wavelengths, though these are generated high up and are not detectable at ground level.