Earth’s solid outer layers have tipped relative to the planet’s spin axis in at least four rapid episodes over the past 320 million years, submerging some regions and raising others above water, according to a study published Thursday in the journal Science.
The process, known as true polar wander, shifts the geographic poles, the points around which Earth rotates, rather than the magnetic poles that guide a compass. It happens when the crust and mantle slide together over the planet’s core as Earth redistributes its mass to stay balanced. “The crust and the mantle effectively slip over the core during a true polar-wander event,” Mathew Domeier, the study’s lead author and a geoscientist at Norway’s University of Oslo, told New Scientist.
Three of the four episodes the team identified unfolded while dinosaurs roamed the planet: in the Early Jurassic, about 200 million to 190 million years ago; from the Late Jurassic into the Early Cretaceous, about 150 million to 140 million years ago; and in the mid-Cretaceous, about 100 million to 90 million years ago, Gizmodo reported. The fourth came between 30 million and 20 million years ago.
Clues in ancient shorelines
Most earlier evidence for true polar wander came from magnetic signals locked in ancient rocks. That record is patchy and has led to conflicting conclusions, including arguments that the process was large but steady rather than sudden.
Domeier’s team, all from the University of Oslo’s Centre for Planetary Habitability, tested the idea another way. Because Earth spins, its oceans and its rock both bulge outward around the equator. If the solid planet rolls, landmasses are dragged through that ocean bulge, and since seawater responds faster than rock, the result looks like a sea-level change even though it is the land that has moved. Two diagonally opposite sectors of the globe flood, and the other two emerge from the sea.
Scientific American compared the resulting signature to a four-leaf clover, while New Scientist likened it to the panels of a beach ball. “This is a very strange pattern that is not caused by other Earth processes,” Domeier told New Scientist.
The researchers examined reconstructions of continental flooding, spaced 10 million years apart, and found the telltale pattern in four time windows. Giovanni Muttoni, a University of Milan geologist who was not part of the research, told Scientific American the method offers an independent check on the magnetic data. The strongest signal, from 150 million to 140 million years ago, lines up with a period when magnetic records had already hinted at rapid wander, he said.
Why it matters
Domeier told New Scientist that the team cannot pin down exact speeds, but it can identify events in which the poles moved faster than about 0.6 degrees per million years. He said that is quicker than most tectonic plates move and fast enough to potentially affect climate.
“During true polar wander, continents can move rapidly across the climate belts even though their positions relative to one another change much less,” Muttoni told Scientific American. Shifts like that could reshape ocean currents and ecosystems, the magazine noted. The authors wrote that true polar wander should be treated as an episodic driver of sea-level change and likely of other environmental and biological shifts, according to Gizmodo.
Sabin Zahirovic of the University of Sydney told New Scientist that few people realize Earth’s outer shell moves relative to its spin axis. “This is a very exciting development that helps us link very deep Earth processes to changes in long-term climate and sea level on the planet,” he said.
The process has not stopped. Satellite measurements show the poles drifting about 10 centimeters, or roughly 4 inches, a year, mostly because of melting ice caps, Domeier told Scientific American. In the paper, the researchers noted that today’s rate is faster than the average speed at which tectonic plates move relative to one another.
What’s next
Scientists still do not know what sets off the fast episodes or how far the poles traveled during each one. Domeier said working out why the events happened when they did is the next step. The authors also wrote that similar bursts probably occurred earlier in Earth’s history, a possibility future studies could test.
