13.Magnetic Reversals and Crustal Overturn

Magnetic Reversals and Crustal Overturn

The Earth's magnetic field periodically changes, but no theory effectively explains why or when this happens.

Crustal overturn could provide part of this answer, and also help calculate the thickness of the crust that moves.

If the magnetic field is produced by the earth's core, the most likely location, it is very unlikely that whatever mechanism produces it that it is going toflip! The core is simply too massive.

The Earth's core produces a powerful magnetic field, and this will induce an aligned magnetic field in the crust. This will align any magnetic domains in the crust with the internal field and will form a permanent magnet, producing a  crustal magnetic field aligned with the internal field.

When the crust overturns, the permananet magnetic field of the crust isno longer aligned with the internal magnetic field, and now shields the internal field from the core.
The magnetic field on the outer surface of the crust reflects the crustal field, so there is no immediate magnetic flip from the crustal overturn.

Instead, the internal magnetic field will slowly re-align the crustal field. As the crustal field loses its strength in its original orientation, the processof realignment will accelerate. This realignment process will not be uniformover the Earth's surface. As the internal field comes to predominate at thesurface  in one region, a new north and south pole will appear at thesurface in that region. For  a time there will be multiple north andsouth poles scattered over the Earth's surface.

The Earth's magnetic field will not decay to zero and reform in the opposite direction! Regions showing the current crustal N-S alignment will shrink insize as regions showing the new S-N alignment grow. These multiple poles willappear to drift towards the  axis of rotation poles, until the magneticfield becomes uniform with an opposite polarity to that of the present, anda  magnetic flip will have taken place.

The magnetic flip will be complete when the internal magnetic field hasinduced a matching field in the crust, but because the two fields will bein alignment, the total magnetic field will be much stronger than it is now.

This theory also explains why the present crustal magnetic field is centred over northern Canada, some distance from the present geographic pole. When the crustal and core magnetic fields are in equilibrium the magnetic pole and the rotational pole should be in alignment. The difference between the two at present is a measure of how much the crust shifted in the last crustal overturn.

This information is copyright Peter Thomson 2001-2004