Scientists in France believe they have established a link between plate tectonics and magnetic reversals.
Publishing their findings in Geophysical Research Letters (16 Oct 2011) the researchers, from Ecole Normale Supérieure, CNRS, and Institut de Physique du Globe in Paris, argue that the frequency of magnetic field reversals has depended on the distribution of tectonic plates on the surface of the globe (the earth’s mantle) during any given geological period.
Their modelling shows that more reversals occur when there is an asymmetry, when there is more landmass in one of the hemispheres than the other, which occurs on a timescale of roughly 100 million years.
Some 200 million years ago, Pangaea, the supercontinent that encompassed almost all of the Earth’s land masses, began breaking into a multitude of smaller pieces that shaped the Earth as we know it today. By comparing the surface area of continents in the Northern hemisphere to those in the Southern hemisphere, the researchers were able to calculate a degree of asymmetry (with respect to the equator) in the distribution of the continents during that period.
The amount of asymmetry has varied at the same rhythm as the magnetic reversal rate (number of reversals per million years). The two curves evolved in parallel to such an extent that they can almost be superimposed. In other words, the further the center of gravity of the continents moved away from the equator, the faster the rate of reversals.
The scientists envisage two scenarios.
In the first scenario, terrestrial plates could be directly responsible for variations in the frequency of magnetic reversals: after plunging into the Earth’s crust at subduction zones, the plates could descend until they reach the core, where they could modify the flow of iron.
In the second scenario, the movements of the plates may only reflect the mixing of the material taking place in the mantle and particularly at its base. In both cases, the movements of rocks outside the core would cause flow asymmetry in the liquid core and determine reversal frequency.
The title of this paper – “Plate Tectonics May Control Geomagnetic Reversal Frequency” – implies that tectonic plate movement triggers the increase in magnetic reversals, but I think they have it backward.
As I say in “Not by Fire but by Ice,” I think electromagnetic forces drive the plates, not the other way around.
Thanks to Owen, Kay Garden, Wanda and Peter Lamb for these links
More information: Plate Tectonics May Control Geomagnetic Reversal Frequency. F. Pétrélis, J. Besse, J.-P. Valet. Geophysical Research Letters, 16 October 2011.