The Earth's chemistry

The bulk of the Earth is made from iron, oxygen, magnesium and silicon. More than 80 chemical elemnts occur naturally in the Earth and its atmosphere.

The crust is made mostly from oxygen and silicon, with aluminium, iron, calcium, magnesium, sodium, potassium, titanium and traces of 64 other elements.

The upper mantle is made up of iron and magnesium silicates; the lower is silicon and magnesium sulphides and oxides.

The core is mostly iron, with little nickel and traces of sulphur, carbon, oxygen and potassium.

fig. 2 - This diagram shows the percentages of the chemical elements that make up the Earth.

Evidence for the Earth's chemistry comes from analysing densities with the help of earthquake waves, and from studying stars, meteorites and other planets.

When the Earth was still semi-molten, dense elemnts such as iron sank to form the core. Lighter elements such as oxygen floated up to form the crust. Some heavy elemnts, such as uranium, ended up in the crust because they easily make compounds with oxygen and silicon.

Large blobs of elemnts that combine easily with sulphur, such as zinc and lead, spred through the mantle. Elemnts that combine with iron, such as gold and nickel, sank to the core.