The magnitude and direction of this magnetization conform to the magnetic field that existed at a given point on the earthâs surface when the rock was formed, that is, millions or hundreds of millions of years ago. Paleomagnetism permits scientists to study the geomagnetic fieldâs evolution, which is ârecordedâ in the magnetization of rocks. Every rock contains grains of ferromagnetic or ferrimagnetic minerals, such as magnetite, titanomagnetites, hematite, ilmenites, maghemite, and pyrrhotite.