Environment

‘True polar wander’ and Earth’s ice ages


Rice University geophysicists have determined Earth shifted relative to its spin axis within the past 12 million years, which caused Greenland to move far enough toward the north pole to kick off the ice age that began about 3.2 million years ago.

The study is based on an analysis of fossil signatures from deep ocean sediments, the magnetic signature of oceanic crust and the position of the mantle “hot spot” that created the Hawaiian Islands. Co-authors Richard Gordon and Daniel Woodworth said the evidence suggests Earth spun steadily for millions of years before shifting relative to its spin axis, causing the geographic locations of the north and south poles to change, or “wander.” Geophysicists refer to this effect as “true polar wander.”

True polar wander occurs when the entire Earth shifts relative to its spin axis. Credit: Victor C. Tsai/Wikimedia Commons

“We’re taking these hot spots as marked trackers of plumes that come from the deep mantle, and we’re using that as our reference frame,” Gordon said. “We think the whole global network of hotspots was fixed, relative to the Earth’s spin axis, for at least 36 million years before this shift.”

Like any spinning object, Earth is subject to centrifugal force, which tugs on the planet’s fluid interior. At the equator, where this force is strongest, Earth is more than 42 km (26 miles) larger in diameter than at the poles. Gordon said true polar wander may occur when dense, highly viscous bumps of mantle build up at latitudes away from the equator.

“It was only about a 3 degree shift, but it had the effect of taking the mantle under the tropical Pacific and moving it to the south, and at the same time, it was shifting Greenland and parts of Europe and North America to the north. That may have triggered what we call the ice age,” Gordon said.

Earth is still in an ice age that began about 3.2 million years ago, authors say. Earth’s poles have been covered with ice throughout the age, and thick ice sheets periodically grow and recede from poles in cycles that have occurred more than 100 times. During these glacial cycles, ice has extended as far south as New York and Yellowstone National Park. Earth today is in an interglacial period in which ice has receded toward the poles.

Gordon said true polar wander is not merely a change in the location of Earth’s magnetic poles. As the planet spins, it’s iron core produces a magnetic field with “north” and “south” poles near the spin axis. The polarity of this field flips several times every million years, and these changes in polarity are recorded in the magnetic signatures of rocks the world over. The paleomagnetic record, which is often used to study the movement of tectonic plates across Earth’s surface, contains many instances of “apparent polar wander,” which tracks the motion of the spin axis and which includes the effects of both plate motion and true polar wander, Gordon said.

He said Earth’s mantle is ever-changing as new material constantly cycles in and out from tectonic plates. The drawing down and recycling of plates via subduction provides a possible explanation for the highly viscous mantle anomalies that probably cause true polar wander.

If the mantle anomalies are massive enough, they can unbalance the planet, and the equator will gradually shift to bring the excess mass closer to the equator. The planet still spins once every 24 hours and true polar wander does not affect the tilt of the Earth’s spin axis relative to the sun. The redistribution of mass to a new equator does change Earth’s poles, the points on the planet’s surface where the spin axis emerges.

Woodworth said the hot spot data from Hawaii provides some of the best evidence that true polar wander was what caused Earth’s poles to start moving 12 million years ago. Islands chains like the Hawaiians are formed when a tectonic plate moves across a hot spot.

“True polar wander shouldn’t change hot spot tracks because the hot spot track is the record of the motion of the plate relative to the hot spot,” Woodworth said.


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Reference:

“Paleolatitude of the Hawaiian Hot Spot Since 48 Ma: Evidence for a Mid‐Cenozoic True Polar Stillstand Followed by Late Cenozoic True Polar Wander Coincident With Northern Hemisphere Glaciation” – Daniel Woodworth,  Richard G. Gordon – First published: October 19, 2018 – https://doi.org/10.1029/2018GL080787

Plain language summary

The Earth has shifted relative to its spin axis over the past 12 million years (Ma). This shift, which geoscientists call true polar wander, caused the Earth’s mantle beneath the tropical Pacific to move southward while causing Greenland to move northward. The latter motion may have contributed to the onset of the current ice age, which began ≈3 Ma before present. These conclusions follow our analysis of the history of motion of the Pacific tectonic plate relative to the spin axis, which is preserved in sediments and rocks on the Pacific seafloor. We also infer the motion of the Pacific plate relative to the solid Earth from the plate’s history of motion relative to hot spots, such as Hawaii. Hot spots are sites of voluminous volcanism, thought to lie over rising plumes of hot rock from deep in the Earth’s mantle. As the Pacific plate moves over the Hawaiian plume, it creates a line of extinct volcanoes that record the motion of the plate relative to the plume. Combining this information, we find that Hawaii and other global hot spots were nearly fixed in latitude from 48 to 12 Ma before present, which marks a 36‐Ma‐long time interval preceding the shift.

Related research from Rice:

Hot spot at Hawaii? Not so fast — Aug. 18, 2017 https://news.rice.edu/2017/08/18/hot-spot-at-hawaii-not-so-fast/

New plate adds plot twist to ancient tectonic tale — Aug. 14, 2017 https://news.rice.edu/2017/08/14/new-plate-adds-plot-twist-to-ancient-tectonic-tale/

Pacific plate shrinking as it cools — Aug. 27, 2014 https://news.rice.edu/2014/08/27/pacific-plate-shrinking-as-it-cools/

Tectonics: Precision is hallmark of 20-year study — March 26, 2010 https://news.rice.edu/2010/03/26/tectonics-precision-is-hallmark-of-20-year-study-2/

Researchers solve mystery that has plagued researchers of tectonics — March 28, 2002 https://news.rice.edu/2002/03/28/researchers-solve-mystery-that-has-plagued-researchers-of-tectonics/

Featured image credit: Diego Marcondes



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