The past and present geomagnetic field

The magnetic field of the Earth approximates the field that would be expected from a large bar magnet embedded within it inclined at an angle of about 11° to the spin axis. The actual cause of the geomagnetic field is certainly not by such a magnetostatic process, as the magnet would have to possess an unrealistically large magnetization and would lie in a region where the temperatures would be greatly in excess of the Curie temperature.

The geomagnetic field is believed to originate from a dynamic process, involving the convective circulation of electrical charge in the fluid outer core, known as magnetohydrodynamics (Section 4.1.3). However, it is convenient to retain the dipole model as simple calculations can then be made to predict the geomagnetic field at any point on the Earth.

The geomagnetic field undergoes progressive changes with time, resulting from variations in the con-vective circulation pattern in the core, known as secular variation. One manifestation of this phenomenon is that the direction of the magnetic field at a particular geographic location rotates irregularly about the direction implied by an axial dipole model with a periodicity of a few thousand years. In a paleomagnetic study the effects of secular variation can be removed by collecting samples from a site which span a stratigraphic interval of many thousands of years. Averaging the data from these specimens should then remove secular variation so that for the purposes of paleomagnetic analysis the geomagnetic field in the past may be considered to originate from a dipole aligned along the Earth's axis of rotation.

Paleomagnetic measurements provide the intensity, azimuth and inclination of the primary remanent magnetization, which reflect the geomagnetic parameters at the time and place at which the rock was formed. By assuming the axial geocentric dipole model for the geomagnetic field, discussed above, the inclination I can be used to determine the paleolatitude ^ at which the rock formed according to the relationship 2 tan ^ = tan I. With a knowledge of the paleo-latitude and the azimuth of the primary remanent magnetization, that is, the ancient north direction, the apparent location of the paleopole can be computed. Such computations, combined with age determinations of the samples by radiometric or biostratigraphic methods, make possible the calculation of the apparent location of the north magnetic pole at a particular time for the continent from which the samples were collected. Paleomagnetic analyses of samples of a wide age range can then be used to trace how the apparent pole position has moved over the Earth's surface.

It is important to recognize that remanent magnetization directions cannot provide an estimate of paleolongitude, as the assumed dipole field is axisym-metric. There is a consequent uncertainty in the ancient location of any sampling site, which could have been situated anywhere along a small circle, defined by the paleolatitude, centered on the pole position.

If a paleomagnetic study provides a magnetic pole position different from the present pole, it implies either that the magnetic pole has moved throughout geologic time, that is, the magnetic pole has wandered relative to the rotational pole, or if the poles have remained stationary that the sampling site has moved, that is, continental drift has occurred. It appears that wandering of the magnetic pole away from the geographic pole is unlikely because all theoretical models for the generation of the field predict a dominant dipole component paralleling the Earth's rotational axis (Section 4.1.3). Consequently, paleomagnetic studies can be used to provide a quantitative measure of continental drift.

An early discovery of paleomagnetic work was that in any one study about half of the samples analysed provided a primary remanent magnetization direction in a sense 180° different from the remainder. Although the possibility of self-reversal of rock magnetism remains, it is believed to be a rare phenomenon, and so these data are taken to reflect changes in the polarity of the geomagnetic field. The field can remain normal for perhaps a million years and then, over an interval of a few thousand years, the north magnetic pole becomes the south magnetic pole and a period of reversed polarity obtains. Polarity reversals are random, but obviously affect all regions of the Earth synchronously so that, coupled with radiometric or paleonto-logic dating, it is possible to construct a polarity timescale. This subject will be considered further in Chapter 4.

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