Earth's True North and South Poles Have Wandered Four Times Over the Past 320 Million Years, Study Suggests

You may have heard that Earth’s magnetic poles can wander away from the planet’s true North and South Poles. But these geographic points are also mobile. In fact, they seem to have gone on four big journeys over the past 320 million years, according to a study published on October 1 in the journal Science.

Earth’s solid, outermost shell—the crust—shifts more than you might think. It floats atop a viscous rock layer called the mantle, which surrounds the planet’s core. Movements in the crust and mantle can change the distribution of the planet’s mass and disrupt Earth’s balance around its rotational axis.

To reestablish balance, the crust and mantle reorient themselves around the core. Earth’s geographic poles slide around in the process, a phenomenon called true polar wander. True polar wander is even taking place right now.

But investigating past episodes of this phenomenon is difficult because the movement of tectonic plates can be mistaken for true polar wander. Earlier studies have had inconsistent results, and researchers haven’t been sure whether the North and South Poles make continuous slow, steady shifts or take great strides in quick bursts. So, Domeier and colleagues used a new strategy to track down rapid true polar wander: ancient sea-level changes.

It works because our planet isn’t a perfect sphere. Its rock and ocean are fatter around the equator. When true polar wander takes place, the bulging belt of water stays still while landmasses move, meaning some dry regions of crust plunge underwater and some previously submerged regions become exposed to air.

The affected areas form a telltale pattern with four quadrants, somewhat resembling a four-leaf clover, per Scientific American. The researchers searched for this signature when running computer simulations of flooding dynamics on Earth over the past 320 million years. Four time points stood out: the Oligocene to Miocene about 20 million to 30 million years ago, the mid-Cretaceous about 90 million to 100 million years ago, the Late Jurassic to Early Cretaceous about 140 million to 150 million years ago, and the Early Jurassic about 190 million to 200 million years ago.