The location of Earth’s geographic poles has changed several times over the past 320 million years in a process known as true polar wander, according to measurements of ancient flooding brought about by sea level changes resulting from our planet’s crust shifting relative to Earth’s spin axis.
Such a shift can affect climate, both on local scales and globally, as regions that were by the poles find themselves moving down to lower latitudes and vice versa. This can then affect atmospheric circulation patterns, which affect climate across the world.
The changes would have affected the environments in which life developed and evolved, from the age of the dinosaurs through to some of our oldest primate ancestors.
True polar wander is not the same as a change in the rotational axis of the Earth. Currently, Earth spins at an angle of 23.4 degrees relative to the plane of its orbit around the sun, and this rotational axis can vary periodically over tens of thousands of years as a result of the Milankovitch cycles caused by the gravitational pull of the sun and planets.
True polar wander is different. Here, the angle of Earth’s spin axis does not change relative to its orbital plane. Instead, Earth’s crust and mantle shift relative to that rotational axis. This is because Earth is not perfectly spherical. Instead, it is oblate, with Earth’s rotation causing the equator to bulge out and the poles to slightly flatten. Because Earth is not spherical, it has multiple axes of inertia, the largest one being aligned with the rotational axis.
These axes need to be balanced to prevent Earth’s rotation from becoming chaotic. However, plate tectonics and indeed the Milankovitch cycles can upset that balance of mass, for example by causing more continental crust to congregate in one hemisphere than another.
To rebalance things, Earth’s crust and mantle must become decoupled from our planet’s core, which stays aligned with the rotational axis. In physical terms, if there is more continental crust in one hemisphere, the crust and mantle might rotate in that direction relative to the rotational axis and the Earth’s core in order to balance itself. This sees the geographical locations of the poles change, hence the name ‘true polar wander’.
Currently, true polar wander takes place at a rate of 10 centimeters (4 inches) per year, but figuring out when and how fast true polar wander has occurred in the past is not without its difficulties, not least since plate tectonics can mask and even mimic the signal of true polar wander.
Previous studies relied on paleomagnetism, the study of residual magnetic traces in ancient rocks, since the orientation of those magnetic traces would have varied depending upon where that rock was located relative to Earth’s magnetic field. Alas, it’s not a terribly accurate means of measuring true polar wander and scientists arrived at substantially different conclusions using this technique.
Nevertheless, based on paleomagnetism, scientists had concluded that true polar wander had been negligible, or slow to proceed, in the distant past. However, new results from researchers at the Centre for Planetary Habitability at the University of Oslo in Norway, led by Mathew Domeier, have shown this not to be the case.
Domeier’s team looked through the geological record to track how sea level has changed across millions of years. True polar wander would bring about an identifiable pattern in sea-level changes as the oceans reacted in response to changes in the mass distribution of the planet. The sea level changes would be equal but opposite between hemispheres — sea level rises on one side of one hemisphere would be matched by rises on the other side of the opposite hemisphere, creating a ‘quadrupolar’ effect.
By using statistical modeling on maps of sea level changes across intervals of 10 million years, Domeier’s team identified four periods of true polar wander: in the early Jurassic period 190—200 million years ago, the late Jurassic — early Cretaceous period 140—150 million years ago, during the mid-Cretaceous 90—100 million years ago and from the Oligocene period 30 million years ago through to the Miocene 20 million years ago.
The results show that, contrary to established opinion, true polar wander can happen in bursts that are relatively fast on geological timescales, with the results showing that during the episodes of true polar wander the Earth’s surface shifted by as much as 0.6 degrees per million years relative to the spin axis. This is slow by human standards, but fairly fast on geological timescales, certainly quicker than plate tectonics.
Domeier’s team were also able to rule out a suspected case of true polar wander around 300 million years ago when the last great supercontinent, Pangea, migrated northwards. It turns out that Pangea’s movements were entirely due to plate tectonics.
The findings were published on Oct. 1 in the journal Science.
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