It is known from the magnetic properties of old rocks that the North and South magnetic poles of the Earth have changed places in the past. This doesn't mean that anything you see on a map has moved, but just that the places where the Earth's magnetic field is strongest has changed. These places, called the magnetic poles, happen to be near the true north and south magnetic poles, which are the points the Earth revolves around. Why the magnetic poles changed places in the past was a mystery for a long time, but it is believed that the Earth's magnetic field is caused by the flowing of molten rock in the core of the Earth. As the molten rock swirls, it creates a magnetic field. This flow, while very slow, is also turbulent, and changes with time. Sometimes, the currents of the molten rock reverse, and this makes the magnetic poles change place.

This has been a great mystery in the Earth Sciences but it was a very important piece of evidence in the theory of plate tectonics.Early on, while people were still doubting the idea of plate tectonics, the lack of explanation for why the magnetic poles reverse caused some scientists to call this field of geo-magnetism, geo-mysticism. Even today, this process is poorly understood and very complex because of the physics involved. One recent theory is that as the Earth spins, the liquid iron core of the Earth spins as well, generating a magnetic field but the liquid in the core spins much slower than the Earth's rotation because of friction and viscosity. In order to balance the difference in rotation velocities between the whole Earth, and the material in its core, the magnetic pole periodically must reverse.