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NCGT Journal March 2013

19 May 2013

David Pratt has his own web site but has published an article in the March 2013 NCGT journal and provides some interesting information you won't find in mainstream sources … problems concerning the consensus model of Pole movement – see www.ncgt.org/newsletter.php.

Most palaeomagneticians in the 1950s concluded that it is mainly the continents that have wandered over the Earth's surface rather than the magnetic or geographic Poles having moved. That was just after Wegener's theory of continental drift was brought out of the shadows, dusted down and presented as the bees knees. However, the present consensus is that there has also been a certain amount of true polar wander – a shift of the rotation axis relative to the entire Earth. Palaeomagnetic Poles are used to specify palaeolatitude but palaeolongitude cannot be constrained from palaeopoles alone.

Alfred Wegener's theory was that continents ploughed slowly through the denser oceanic crust. Plate Tectonics which developed out of continental drift theory differs and postulates that new lithosphere is generated at 'spreading ridges' and is consumed at 'subduction zones', and that moving lithospheric plates carry the continents with them. Apparent polar wander paths tend to consist of long gently curved tracks linked by sharp blips known as cusps. The curved track is assumed to correspond with a period of constant plate motion while the cusps are assumed to correspond to sudden changes in plate motion (plate reorganisation).

The consensus interpretation of Palaeomagnetic data is therefore founded on two basic assumption, which are i) that when rocks are formed they are magnetised in the direction of the geomagnetic field existing at the place of their formation. The aquired magnetisation is retained in the rocks over geological time (but it was quickly found this was only partially true) and ii) the geomagnetic field averaged for any time period of the order of hundreds of thousands of years is a dipole field orientated along the Earth's rotation axis.

Pratt says both these assumptions are questionable. Palaeomagnetism is plagued with uncertainties. For example, rocks can be remagnetised, especially sedimentary rocks. Another complicating factor is that it is not always certain whether the geomagnetic field at any given time in the past was of normal or reversed polarity (which direction was north and which direction was south). Rock magnetism is also subject to modification by weathering, thermal effects, metamorphism, chemical changes, and tectonic deformation. Inclination shallowing (resulting from sediment compaction), horizontal and vertical block rotations, and other crustal motions have been identifed as potential sources of error (Butler, 2004). So has magnetostriction – the alteration of the direction of magnetisation by directed stress (Graham et al 1957, Jeffreys 1976).

The full article can be read online (above) for free. It can also be downloaded and printed out to read at your leasure. The point is, catastrophism would generally cloud the issue, just by looking at that list of uncertainties.

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