True polar wander

One of the powerful bits of evidence that support continental drift are the plots of magnetic pole positions determined from rocks of different ages exposed on a modern continent, relative to that of the present pole position. Comparing such plots from different continents sometimes reveals similarities in their shapes over long periods of time, so that the plots partly match when they are superimposed.  In such fits, other parts of the plots diverge considerably.  Such comparisons are best explained respectively by the former unity of the two modern continents and their movement together, and their separation to drift independently.  The plots are illusory, and are called apparent polar wander paths.  For most of the Phanerozoic Aeon such palaeomagnetic data tie in well with other evidence for the formation of composite continental masses, such as Pangaea, and plate movements since the Triassic.  That provides confirmation of the basic assumption in palaeomagnetic studies that the Earth’s magnetic poles remain close to those of its rotation, bar some circulation around the axis and magnetic reversals.  It is tempting to use the same assumption for earlier times, in the absence of  easy fitting of the margins of continental segments and the sea-floor magnetic stripes that are the key to plate tectonics since the early Mesozoic.  If magnetic poles did move well away from the poles of rotation at any time in the past, that would play havoc with continental reconstruction.  True polar wander is something that many tectonicians “Dinnae care to speak aboot”!  That is not surprising, for another reason.  Use of the term imply mean two things: a long-term shift in the Earth’s magnetic polarity relative to its axis of rotation (to me that warrants the adjective “true”, and ); a shift in the relative position of the whole crust and mantle relative to the core, whose dynamism determines the magnetic field.

A recent review (Irion, R. 2001.  Slip-sliding away.  New Scientist 18 August 2001, p. 34-37) concentrates on evidence for the second usage.  There is evidence that suggests a 20° shift of all continents over a period of 2 Ma, in the Cretaceous.  This is dwarfed by a suggestion of a 90° shift in 15 Ma that span the time of the Cambrian Explosion, so that a continent could have moved from the pole to the equator at a rate far faster then anything known from Mesozoic to Recent sea-floor spreading.  One explanation is destabilization of the Earth’s angular momentum by concentration of all crustal mass and the effect of a massive mantle plume beneath it at high latitudes.  That would distort the Earth’s shape.  A planet’s rotation is most stable when it its shape is fat around the equator.  The opposite, a prolate spheroid, is least stable, and a polar supercontinent could result in such instability, restoring steady state if the whole caboodle slipped to lower latitudes.  That is what is proposed to explain some odd palaeomagnetic pole positions newly and accurately gathered from early Cambrian rocks.  Such a notion takes on its own momentum, because of its association in time with the explosive diversification of animals with hard parts.

It is not a fundamentally new idea, for Alfred Wegener suggested that the mechanism for his hypothesis of continental drift was Pohlflucht (flight from the poles) of continental mass.


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