Palaeomagnetic Methods used in Correlation and Dating.

The Earth's magnetic field approximates to that of a magnetic dipole, with (at the present day) the horizontal component (or declination) pointing towards local magnetic north. The vertical component (or inclination) of the field varies systematically over the Earth largely as a function of latitude.  In the southern hemisphere, the magnetic field vector points away from the Earth's surface, and in the northern hemisphere it points towards the surface. Over periods of relatively short time (10s to 1000s of years), the declination and inclination drift. This drift or secular variation is the foundation of archaeomagnetic dating.

Palaeomagnetic results from rocks and sediments show that through geologic time, the Earth's magnetic field direction has not been constant, but has periodically reversed in direction ( ~10k to 30Ma years duration). In fact, the field has reversed polarity many times. During a reversal of the magnetic field, the present day ('normal') polarity is switched, so that magnetic north lies near the South Pole. Such reversals are known to have occurred periodically throughout geological history. Through an evaluation of these polarity reversals recorded within sediments (the magnetostratigraphy), sequences can either be correlated to the same age sequences elsewhere, or assigned a chronostratigraphic age based on a correlation to the magnetic polarity time scale. On a longer time scale (>20Ma) because of continental drift, the palaeomagnetic poles also drift, which can also provide a simple means of dating.

Dr. Mark Hounslow for further details (m.hounslow using address: @lancs.ac.uk)