## The Density of the Earth

The average density of the Earth is aboutCompare this with water – pure water has a density ofIt may seem pretty obvious that the densest materials sink. In fact, over the lifetime of the Earth, they have sunk all the way to the centre, so the density of material at the centre of the Earth is far higher than the density of material at the surface. In fact, as we move towards the centre, the density increases, sometimes sharply. This increase in density reflects three trends:

The densest material tends to be closest towards the centre. A sharp increase in density usually indicates the boundaries between different materials.

As material sinks, it is compressed by the material above it, which tends to increase the density.

As the pressure increases, atoms tends to form molecules which leads to naturally denser materials.

A graph of density and speed of S and P waves against depth is shown below. Notice how density and speed increase together. In fact this is misleading. It can be shown that speed is inversely proportional to the density, so that as the density increases the speed decreases. It tends to increase however with the rigidity of materials. As materials become denser they become more rigid, and this tends to doninate over density to increase the speed of S and P waves.