## The Density of the Earth

The average density of theEarth is aboutComparethis with water – pure water has a density ofItmay seem pretty obvious that the densest materials sink. In fact,over the lifetime of the Earth, they have sunk all the way to thecentre, so the density of material at the centre of the Earth is farhigher than the density of material at the surface. In fact, as wemove towards the centre, the density increases, sometimes sharply.This increase in density reflects three trends:

The densest material tendsto be closest towards the centre. A sharp increase in density usuallyindicates the boundaries between different materials.

As material sinks, it iscompressed by the material above it, which tends to increase thedensity.

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

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