Earth's solid shell rolled over at least four times since the dinosaur age, new study finds

Several times since the age of dinosaurs, the entire solid outer shell of Earth—crust and mantle together—has lurched sideways relative to the planet's spin axis, flooding some continents and hoisting others out of the sea. A study published in Science has now identified at least four such episodes in the past 320 million years, and finds the process is still going on today.

The phenomenon is called true polar wander. Earth is not a perfect sphere—it bulges at the equator—and when shifts in the distribution of mass inside the planet throw off its rotational balance, the solid outer layers tilt and slide around the liquid inner core until balance is restored. The geographic poles physically move across the surface as a result. What the new research challenges is the long-held view that this process is either negligible or persistently slow.

The findings took me quite by surprise.— Mathew Domeier, geoscientist, University of Oslo

Domeier, the study's lead author, and his team took what they describe as a fundamentally different approach to detecting true polar wander. Rather than relying on paleomagnetic records—signals preserved in ancient rocks that have yielded inconclusive and often contradictory results—they turned to extensive maps of continental flooding stretching back 320 million years, analyzing them at 10-million-year intervals.

The logic of the method rests on a distinctive geometric fingerprint. Because Earth's spin makes both rock and water bulge at the equator, when the solid Earth tips out of alignment it is driven through that watery bulge. Two opposing quarters of the globe flood while the other two dry out, producing what the researchers describe as a four-leaf-clover pattern of sea-level change. Crucially, the water stays put—it is the continents that move through it, making the effect look like sea-level rise or fall when it is actually something else entirely.

The water stays put while the continents move.— Mathew Domeier, geoscientist, University of Oslo

That four-leaf-clover pattern appears in the flooding record at four distinct intervals: around 200–190 million years ago in the Early Jurassic; 150–140 million years ago spanning the Late Jurassic to Early Cretaceous; 100–90 million years ago in the mid-Cretaceous; and most recently between 30 and 20 million years ago during the Oligocene to Miocene. The strongest signal, from 150 to 140 million years ago, matches a period for which magnetic data had already hinted at rapid wander, providing what one outside researcher called an independent test of the existing evidence.

The particularly elegant aspect of this study is the independent test it provides.— Giovanni Muttoni, geologist, University of Milan

The consequences of these episodes extend well beyond shifting coastlines. Because the solid Earth moves while the climate belts tied to the spin axis stay fixed, continents can sweep rapidly across different climate zones even as their positions relative to one another change comparatively little. According to Muttoni, who was not involved in the research, this rapid movement could reshape ocean currents, upend ecosystems, and perhaps even influence Earth's magnetic field—generated by the convective flow of the planet's liquid outer core.

During true polar wander, continents can move rapidly across the climate belts even though their positions relative to one another change much less.— Giovanni Muttoni, geologist, University of Milan

The paper argues that the phenomenon deserves recognition as an episodic control on sea-level change and likely on other global environmental and biological dynamics. It also suggests that similar rapid episodes probably punctuated earlier stretches of Earth history not yet captured by the new analysis.

True polar wander is distinct from the more familiar drift of Earth's magnetic poles, which reflects changes in the planet's liquid core rather than movement of the solid shell. It is also separate from plate tectonics, the continuous shuffling of crustal plates relative to one another—a motion that has historically made true polar wander difficult to isolate and detect.

The process has not stopped. Based on satellite measurements, true polar wander is occurring today at roughly 10 centimeters per year, driven mainly by the redistribution of mass as ice caps melt, according to Domeier. What remains unknown is what triggers the rapid bursts identified in the study and how far the poles actually moved during each episode. Domeier says understanding why these events happened when they did is the next logical step.

Trying to understand why these events happened at these times would be the next logical step.— Mathew Domeier, geoscientist, University of Oslo

Why it matters — Understanding that Earth's solid shell has repeatedly lurched on its axis—reshaping coastlines, climate belts, and potentially ecosystems during the age of dinosaurs—adds an overlooked variable to how scientists reconstruct past environments and interpret the planet's biological history.

Reported by gizmodo.com, newscientist.com, scientificamerican.com