Many are familiar with the idea of magnetic poles flipping every 200,000 years or more. Evidence for this was found in the 1960’s when scans by aircraft looking for submarines showed some strange anomalies in the strength of the magnetic field at different locations. It was later realised that basalt, a volcanic rock containing magnetite, fixes the current magnetic orientation of the earth’s magnetic field as it cools down. The younger the rock the closer it aligns to the current magnetic north, older basalt rock shows evidence of previous magnetic fields and that the magnetic poles flip so north becomes south. This can be seen when taking magnetic readings of the basalt rock emanating from the mid Atlantic ridge where the tectonic plates are being forced apart as volcanic rock pushes up from the upper mantle. The process of millions of years of new rock being pushed out, like the rings of a cut tree, show lines indicating different polarities of earths history. The image below shows the magnetic striping of the sea bed of the mid Atlantic ridge, the darker tone, the closer it is to out current North / South orientation, the lighter tone shows the opposite polarity. In effect the sea bed is like a tape recording of the earths magnetic past, as the theory was accepted a new area of plate tectonics was born called Magnetostratigraphy.
different magnetic fields emanating from the mid Atlantic ridge
Further anomaly’s have since been mapped by NOAA, this image shows more subtle variations in the earths magnetic field strength.