Paleomagnetism

The Earths magnetic field reverses roughly every half million years. When iron containing rocks are formed (in sediment or in solidified volcanic larva) they may become magnetized, so creating a record of which way the field was pointing.

The reversal of the earth's field is random, rather like a human's fingerprints, and forms a recognizable pattern over time. By comparing these patterns in rocks from different continents we can work out which rocks were formed at the same time. This old magnetism is called paleomagnetism. Study of it was vital in establishing the theory of continental drift.