Paleomagnetism is the study of the
magnetic properties of rocks which began in the early 1950s.
Almost immediately after it was adopted by geologists, two major observations were made which had important consequences. Igneous rocks commonly contain small quantities of magnetic minerals such as magnetite, and when these minerals cool in the Earth's magnetic field, they become magnetised in the direction of the Earth's magnetic field at that time.
Studies on numerous basalt flows around plate margins all revealed sequences of flows in which the direction of the magnetic field 'encoded' into the rocks alternated between the current orientation of the Earth's magnetic field and a reversed orientation.
The second observation was that older flows, dating from the pre-Pleistocene era, showed orientations which differed greatly from that of todays. Some Jurassic dolerites in Australia show an orientation which pointed towards a magnetic pole very close to Tasmania.
From these two observations, geologists were able to map both the course of continental drift and the movement of the magnetic poles throughout geological time. This was a very important discovery, coming as it did at the beginning of a time where the theory of continental drift was starting to be seriously looked at by the academic community, and an explanation as to how the continents were moving was actively sought after.
These discoveries proved to be the foundation of the current theories on seafloor spreading and lead to the formation of the theory of plate tectonics.