If a large mass developed on one of the Earth's poles today, after 5 to 20 million years, the mass would be located at the equator. Credit: Adam Maloof
Eight hundred million years ago, the world flipped.
In research recently published in the Geological Society of America Bulletin, scientists argue that a sudden imbalance of mass on the Earth caused the whole planet to tilt 60° from its axis of rotation. This event, called "true polar wander," may have moved entire land masses, shifting some continents into the tropics and others out of them.
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The research team—which included Princeton geoscientist Adam Maloof and Galen Halverson of the Université Paul Sabatier in France, and was funded by the National Science Foundation—based its findings on observations of sediment in Svalbard, Norway. Limestone deposits from the region contained minerals that during sedimentation orient themselves along Earth's magnetic field, and in some samples, the researchers noticed that the minerals showed two sudden changes in orientation—each corresponding to a change in the position of the magnetic north pole relative to the rocks.
The researchers deduced that about 800 million years ago the Earth's continents shifted with respect to the planet's spin axis and magnetic field.
"There were sudden rotations in the position of the magnetic north pole with respect to Svalbard of about 60 degrees," said Maloof, who noted that according to plate tectonic theory, a rotation of that magnitude would take 30 to 50 million years.
"We had no evidence that there is that much time missing from the geologic record," he said.
The magnetic changes the researchers observed appeared to have happened in less than 10 to 20 million years. Instead, the researchers suspected that the Earth might have experienced true polar wander—a change in the Earth's orientation relative to the axis it spins around.
True polar wander occurs when a rotating object develops an imbalanced mass, resulting in either too much near its poles or too little near its equator. Any deformable rotating body in rotational equilibrium bulges at the equator, said Maloof. If a substantial new mass—such as a large volcano— suddenly appears near a pole, the body stabilizes itself by shifting the mass away from the axis of rotation and toward the equator.
"It actually doesn't have to be a huge mass," Maloof said, before referencing Thomas Gold's famous thought experiment: "If the Earth were a perfect sphere or a perfect football, you could literally stick the weight of an insect on the pole and cause true polar wander."
True polar wander does not change the orientation of the body's spin axis relative to external objects—in this case, the sun or other planets—nor does it involve, as plate tectonics does, continents moving relative to one another.
The Earth is far from the perfectly symmetrical sphere or football shape ideal for true polar wander, and such a large shift is unlikely to happen in the planet's foreseeable future, Maloof said.
While the magnetic data first tipped the scientists off to the possibility of true polar wander, their hypothesis is supported by two other lines of evidence: The researchers noted corresponding changes in sea level at the time of the polar wander event. Also, if the land had shifted into and out of the tropics, as they suspected, they predicted they would see changes in the carbon cycle—when land lies in tropical regions, carbon is buried more efficiently. Indeed, they did observe enriched carbon levels.
"If the argument were based solely on the paleomagnetic poles, I would be skeptical because there are many ways that one can get spurious results, especially from rocks nearly a billion years old," said Texas A&M geological oceanographer William Sager via email. "Altogether, the three lines of evidence make Maloof et al.'s hypothesis stronger because one has to come up with an alternative that explains the three linked observations."