Read more about the changes in the Earth's magnetic field over time
Beneath the Earth's crust and mantle lies a complex internal structure believed to be a dynamic core of molten iron, perhaps mixed with nickel and sulphur. With a temperature maintained by nuclear processes, the molten iron moves slowly creating a system of circulating electric currents in the core. It is this motion which is believed to be responsible for the generation of the Earth's magnetic field.
An ever changing field
The resulting magnetic field evident on the surface of the Earth is far from static. There are variations in intensity and direction across the surface. These variations need to be taken into account whenever using a compass. Over a time scale of millions of years there appear to have been many major changes to the direction of the Earth's magnetic field, with up to 10 complete direction reversals in the last five million years. The last complete reversal occurred about 780 000 years ago.
Scientists make use of samples of igneous rock to help chart these magnetic changes. When molten volcanic rock emerges it rapidly cools. This cooling process traps within the rock evidence of the prevailing magnetic field direction of the Earth at the time of the eruption.