Earth’s geographic poles have undergone four rapid shifts over the past 320 million years, according to a study published on October 1 in the journal Science. Researchers tracked these dramatic movements of the planet’s outer crust by analyzing ancient sea-level fluctuations.
Our planet’s outer shell is far more mobile than it appears. While tectonic plates crawl across the globe at a leisurely pace, Earth’s solid crust and molten mantle occasionally undergo a much more dramatic reorientation around the planet’s core. This phenomenon, known as true polar wander, shifts the geographic positions of the North and South Poles relative to Earth’s spin axis.
The process is distinct from the regular movement of Earth’s magnetic poles or the cyclic variations in the planet’s tilt caused by Milankovitch cycles. Instead, true polar wander occurs when the distribution of Earth’s mass becomes unbalanced—often due to continental crust congregating in one hemisphere. To restore equilibrium, the crust and mantle decouple from the stable core and rotate. The geographic poles literally slide across the surface.
Tracking Ancient Flooding in the Geological Record
For years, scientists studying this planetary phenomenon relied on paleomagnetism, examining residual magnetic traces preserved in ancient rocks. That traditional approach yielded inconsistent results, leading many researchers to conclude that true polar wander was either negligible or agonizingly slow throughout Earth’s history. A team led by geophysicist Mathew Domeier at the University of Oslo in Norway decided to test that assumption using an entirely different geological archive.
Because Earth bulges around the equator due to its rotation, any major shift in the outer crust leaves a distinctive signature on the oceans. The massive belt of equatorial water stays relatively stable while the landmasses beneath it move. Dry regions plunge beneath the waves while previously submerged areas emerge into the air.
This is a very strange pattern that is not caused by other Earth processes.
Mathew Domeier, geophysicist at the University of Oslo, via New Scientist
By applying statistical modeling to maps of global flooding, Domeier’s team identified four distinct episodes of rapid true polar wander spanning the age of dinosaurs and the rise of early primates. During these bursts, the Earth’s surface shifted by as much as 0.6 degrees per million years relative to the spin axis—a rapid clip on geological timescales.
Four Major Shifts Across 320 Million Years of Earth History
- The Early Jurassic period, occurring 190 to 200 million years ago
- The Late Jurassic to Early Cretaceous period, spanning 140 to 150 million years ago
- The mid-Cretaceous period, taking place 90 to 100 million years ago
- The Oligocene through Miocene epochs, stretching from 20 million to 30 million years ago
The researchers also evaluated a suspected event from roughly 300 million years ago, when the supercontinent Pangea migrated northward. Their analysis ruled out true polar wander for that era, confirming that Pangea’s journey was driven entirely by standard plate tectonics.
Refuting Previous Assumptions About Crustal Stability
The new findings challenge long-held beliefs about how the planet’s outer layers behave over vast stretches of time.
These findings refute the view of true polar wander as negligible or persistently slow.
Study authors, via Science
The authors highlight the need to consider true polar wander as an episodic driver of sea-level fluctuations, ocean currents, and biological evolution. Sabin Zahirovic, a geoscientist at the University of Sydney in Australia who did not participate in the research, noted that the study bridges a critical gap in earth sciences.
True polar wander is not confined to the distant past. Satellite measurements confirm that true polar wander takes place at a rate of 10 centimeters—about four inches—per year today, driven primarily by contemporary melting ice caps. Yet researchers still lack answers regarding what triggered the massive ancient bursts or how far the geographic poles traveled during each event.
Trying to understand why these events happened at these times would be the next logical step.
Mathew Domeier, lead study author, via Scientific American