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.