In bold new research, a group of scientists in France believes that it has established a link between two of the great discoveries in 20th-century geophysics – plate tectonics and the fact that the Earth’s magnetic field has reversed direction many times throughout the planet’s history. The researchers from Ecole Normale Supérieure, CNRS and Institut de Physique du Globe in Paris, argue that during a given geological period the location of continents is linked with the frequency of magnetic field reversals. The findings have been met with cautious excitement by other geophysicists in the field.
The Earth’s magnetic field is produced by the flow of molten iron in the planet’s outer core – the Coriolis force helps to create a convection pattern in this zone, leading to a geodynamo. By studying the orientation of magnetic minerals in rocks at the Earth’s surface, geophysicists know that the main dipolar component of the field has reversed direction many times since the field became established early in Earth’s history.
It has long been recognized, however, that the average rate of these magnetic field reversals has varied throughout the past. For example, during the last 25 million years the average reversal rate has been once every 250,000 years; compared with once every 600,000 years during the preceding 25 million years. Most geophysicists agree that the frequency of reversals must be related to slow changes in the conditions at the boundary between the outer core and the overlying mantle – a region some 2900 km beneath the Earth’s surface. Modelling work has shown that more reversals occur when there is an asymmetry between conditions in the outer core in the northern hemisphere and those in the south.
“Symmetry breaking”
In this latest work, the French team suggests that these conditions at the core–mantle boundary may be correlated with the mantle-wide convection. These enormous convection cells result in the circulation of near-molten material within the mantle and they ultimately provide the driving force behind plate tectonics. The team speculates that this “symmetry breaking” deep within the Earth may be reflected in the distribution of continents – resulting in more landmass in one of the hemispheres than the other.
To test the theory, the team quantified the north–south symmetry of continents throughout the Earth’s history. The researchers did this using reconstructed locations of continents projected onto a 2D map and enclosing all continents with a so-called complex envelope – this enabled them to observe the symmetry about the equator. They then compared the changing symmetry with the well-established rate of magnetic-field reversals over the past 300 million years.
Publishing their findings in Geophysical Research Letters, the researchers report a correlation between the rate of field reversals and the extent of asymmetry among continents. Both phenomenons occur on a timescale of roughly 100 million years. “The point is that 100 million years is not a long time for mantle flow because it is flowing very slowly,” team member François Pétrélis told physicsworld.com, pointing out that a tectonic plate will typically take the same amount of time to move a few thousand kilometres.
A French revolution?
Ronald Merrill, a geomagnetic researcher at the University of Washington, says that the paper is likely to receive attention in international circles but that it will not be viewed as revolutionary. While he does not dismiss the link, he is concerned that it is difficult to establish a clear link between heat flow at the core and the distribution of continents. He points out that the processes that lead to the creation of new continental material on top of the tectonic plates are far from trivial.
Ulrich Christensen, a geophysics researcher at the Max Planck Institute for Solar System Research, takes a similar position. “The correlation is good enough to further explore the idea, but is more suggestive than compelling,” he says. Christensen believes that the idea will be strengthened with more evidence from dynamo simulations that the north–south symmetry at the core–mantle boundary is indeed the most important controlling factor for reversal frequency.
Pétrélis says this is one of the ways in which he intends to develop this research and he will also look for more information about the reversal process in paleomagnetic records.
