Paleomagnetism is the the study of recordings of the Earth's past magnetic fields. Volcanic rocks and some sedimentary rocks containing magnetic minerals are able to fossilize the intensity and orientation of the magnetic field at a given epoch. Magnetite is one of these minerals.
Using paleomagnetism, the shift of the magnetic poles and above all the periodic inversion of the Earth's geomagnetic field were discovered. These two discoveries have played an important part in the origins of the plate tectonics theory. By dating the rocks associated with an inversion, a chronology can be established as shown on the diagram below where epochs such as the Brunhes and the Matuyama are represented.
(Credit: Pierre-André Bourque and Pauline Dansereau).