Off the Beaten Track
in Vol. 14 - March Issue - Year 2013
True Polar Wander
A greatly-exaggerated diagram of true polar wander
Volcanic island chain formation
OK, so try to imagine an object of enormous weight forming on the surface of the Earth; say, for example, a gigantic volcano or a large land mass. If this object were to materialize far from the Earth’s equator, it would create an imbalance in the spinning motion of our planet. The centrifugal force of Earth’s rotation would correct this imbalance by gradually pulling the huge object away from the Earth’s axis and relocating the object to a point along the equator. But the object would not be the only one to move, for it would take with it the entire crust of the Earth, to which it is attached.
Sound like science fiction? A group of scientists based in Norway and Germany published a study in late 2012 which suggests that the Earth’s crust actually moves as a single piece over its underlying mantle. This phenomenon is called true polar wander and, although it’s considered a geophysical theory, in recent years it has been gaining credibility since first coming to the attention of researchers and scientists in the mid 1800’s.
True polar wander is not to be confused with plate tectonics, which defines the movement of land plates on the Earth’s crust, nor with precession, by which the Earth’s axis of rotation slowly moves and traces a circle among the stars.
To understand the basis for this theory, we need to understand how our planet is made. Far from being a solid mass, the Earth actually consists of five concentric layers, each with very different characteristics. The outermost layer, called the crust, is formed by rocky plates of aluminum-silicate. These plates float on a molten outer mantle, which in turn surrounds a more fluid inner mantle. Beneath this inner mantle there’s a liquid outer core, composed mostly of nickel and iron and, at the very center of our planet, there’s a solid inner core made of iron.
The outer mantle, just beneath the crust, is where most of Earth’s heat is stored. This enormous volume of dense, hot liquid moves by convection, creating a circular flow similar to what is observed in a pot of boiling water. There is a continuous upward movement of hot liquid from the deeper levels to just beneath the crust, where it cools and returns downwards. The plates which make up the Earth’s crust react to this movement and wind up creating movements of their own, bumping and grinding against each other and creating tectonic processes often responsible for earthquakes and other cataclysmic effects.
The theory is that if surface or internal mass distributions become uneven, either due to tectonic processes or for whatever other reason, the centrifugal force of the Earth’s spin would push the mass anomalies towards the equator.
In this case, the defining difference would be that the entire crust of the Earth would slide as a single piece over the molten mantle beneath until it finds its new balance, much as a loose-fitting cape could easily be rotated over the body of the wearer.
But how to prove that the Earth’s crust is moving when any point of observation is moving along with it? The group of researchers focused their attention on so-called hotspots, volcanic regions which receive molten liquid from the outer mantle through fissures in the Earth’s crust. Thanks to a computer model which tracks the formation of certain volcanic island chains and the presumed movement of the hotspots which generated these islands, the researchers claim that they have successfully demonstrated the existence of true polar wander. According to their calculations, the Earth went through two distinct periods of true polar wander from ninety to forty million years ago, when the position of the Earth’s crust moved by about nine degrees with respect to its axis of rotation.
Not only, they also state that our planet is now going through a period of true polar wander, probably caused by the retreat of the Laurentide ice sheet over 20,000 years ago which caused an imbalance in the distribution of surface land mass.
But before we start worrying about a roller coaster ride on our green planet as it hurdles through space, we can take solace in the fact that true polar wander is a very slow and imperceptible process which can cover a time span from one million to one hundred million years!
By Giovanni Gregorat, Contributing Editor MFN & Sales Manager, Pometon Abrasives
Author: Giovanni Gregorat