Magnetic field, Earth reversal

Jacobs, J. A., 1984, Reversals of the Earth s Magnetic Fields Adam Hilger Ltd, Bristol. 

In fact, scientists now know that both of these explanations are correct the earth s magnetic poles have wandered from place to place over time and the rocks in which magnetic minerals are found have traveled across the earth s surface. In addition, there is strong evidence that the polarity of the earth s magnetic field has shifted (the north pole changing to the south pole, and vice versa) at least 171 times in the past 76 millions years. These reversals of polarity take place rather slowly, over a period of 5,000-10,000 years. They then remain fixed for a period of up to a million years. 

Idnurm and Schmidt, 1986 Acton and Kettles, 1996). Elsewhere, magnetostratigraphy (the record of dated reversals in the Earth s magnetic field) has been applied to trace the timing of downward progression of the weathering front (e.g. Pillans, 1997 Theveniaut and Freyssinet, 1999). 

The study of palaeomagnetism has extended knowledge of the secular magnetic variation into the geological past and it is clear that the direction of the geomagnetic field has reversed many times. The source of the field and the cause of the variations are not known with any certainty but the source is believed to be associated with dynamo action in the earth s liquid core. 

Fig. 106. (a) Magnetization of a-DyCn measured (i) in the earth s field and (ii) in 4.94 Oe applied at 4.2 K. The arrows indicate the sequence of measurements. (b) Reversible (solid lines) and irreversible (dashed line) susceptibilities, derived from these data (from Coey et al. 1981). 

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