Magnetic field reversal. Artwork illustrating reversal of the Earth's magnetic field. In top image, the magnetic north pole is geographically north (top) and the magnetic south pole is south (bottom). In the bottom image, the poles are reversed. Palaeomagnetic evidence suggests that in the past 10 million years, 4 or 5 reversals have occurred every million years. This is shown by the orientation of magnetic minerals in rocks. The cause of such reversals may be irregular flow movements in the Earth's outer core.