Thursday, 5 February 2009


While in no way sexy, paleomagnetic dating stands the test of time for being useful. You probably remember a demo from science class where a sheet of paper is laid over a bar magnet, and iron filings are scattered lightly onto it. The filings line up with the fields generated by the magnet, in patterns that look rather like bottom-to-bottom fireworks bursts. Something similar occurs when rock is formed, whether by heat or by sedimentation:

Microscopic iron particles in the rock line up with the Earth’s magnetic fields as they are oriented at that time. But unlike the iron filings on the paper, which shift if the paper or the magnet is moved, iron particles in solidified rock remain frozen in their alignment unless the rock is reheated to 600°C or more.

This means that even when the Earth’s magnetic poles flip end for end – something they have done several hundred times during Earth’s history – the iron stays aligned as it was when the rock was formed. By comparing the residual magnetism in the rock with the known historical changes in the Earth’s magnetic fields, an age and birthplace for the rock can be easily be estimated — in theory.

The principle behind paleomanetic dating is easy to summerize: the practise is much more complicated. The intervals between pole reversals fluctuate wildly, from as little as one hundred thousand years to tens of millions of years. The time taken for the shifts to complete once started also varies, from a thousand to eight thousand years.

Imagine trying to time events with a grandfather clock that had a pendulum which swung at intervals that could vary from one minute to many hours, while the period of the pendulum swing itself ranged randomly from one second to eight seconds, and you’ll get a sense of the complexity of paleomagnetic dating. While far from a perfect science, paleomagnetic dating provides valuable pieces of the puzzle of Earth’s past.