Below 580 C, if there is a magnetic field imposed on the system, the micro-domains of the magnetite will line up parallel to the Earth s magnetic field. The final piece of the puzzle relates to the Earth s magnetic field. Because of the liquid core, there is a magnetic field that is closely oriented parallel to the rotation axis of the planet. The line of magnetism are shown in the following figure. There are minerals in rocks that behave as small magnets, particularly an iron oxide called magnetite.
Magnetite is found both in sedimentary and igneous rocks.
But magnetite is only magnetic at low temperatures. Above 580 C, magnetite is above its Curie point , a temperature above which the minerals are not magnetic. The thermal agitation of the minerals is too high. Below this temperature, the small domains which constitute microscopic magnets, line up with the Earth s magnetic field. As it cools below 580 C it acquires the Earth s magnetic field.