Over the past billion years, the Earth’s poles have shifted tens of degrees relative to the crust and then back to their original position. There were times in the Earth’s deep history when a site at the North Pole wandered close to the equator only to return again to the cold climate of higher latitudes.
Earth from outer space.
IMAGE: Hunor Focze
Scientists have long known about this so-called “polar wander,” but, until now, they have not been able to explain why the Earth’s poles ‘know’ to go back. A new study co-authored by CIFAR Program Director Jerry Mitrovica (Harvard) published in the journal Nature offers a fresh perspective to explain this mysterious mechanism.
The process in which the Earth’s crust moves all at once is driven by a mechanism called “mantle convection.” It is the same process that drives continental drift. Thermal currents from the Earth’s interior rise up and descend like lava lamps, changing the orientation of the crust relative to the pole. But this phenomenon does not explain why the polar wander returns to its original starting point.
Using computer modelling, Dr. Mitrovica and his team identified two mechanisms that in tandem caused the Earth’s poles to return after they had shifted.
The first has to do with the Earth’s shape. The Earth bulges at its equator, and this extra weight at the equator helps to stabilize the Earth’s rotation. Like a toy punching bag, this extra weight acts as a self-righting mechanism when the crust moves.
The second mechanism is an increased stress on the tectonic plates as the Earth’s surface tips over. Movement stresses and deforms the plates causing them to want to return to their original state much like an elastic band.
“Large, polar-wander events would have strongly perturbed the Earth’s climate, sea level and carbon cycle,” says Dr. Mitrovica. “And these perturbations would have had implications for how life evolved on Earth. It will help us to understand how the Earth got to be where it is today.”