The rotation of Earth’s inner core may have stopped and could even go backwards, new research suggests. (Cigdem Simsek, Alamy)
Estimated reading time: 3-4 minutes
ATLANTA — The rotation of Earth’s inner core may have stopped and could even reverse, new research suggests.
The Earth is made up of the crust, the mantle, and the inner and outer cores. The solid inner core is located about 3,200 miles below the Earth’s crust and is separated from the semisolid mantle by the liquid outer core, which allows the inner core to rotate at a different speed than the rotation of the Earth itself.
With a radius of nearly 2,200 miles, Earth’s core is about the size of Mars. It is made up mainly of iron and nickel, and contains about a third of the Earth’s mass.
In research published Monday in the journal Nature Geoscience, Yi Yang, an associate research scientist at Peking University, and Xiaodong Song, a senior professor at Peking University, studied seismic waves from earthquakes that passed through the inner core of the Earth. Earth along similar paths from the 1960s to infer how fast the inner core is spinning.
What they found was unexpected, they said. Since 2009, seismic records, which previously changed over time, showed little difference. This, they said, suggested that the rotation of the inner core had stopped.
“We show striking observations indicating that the inner core has almost stopped rotating in the last decade and may be undergoing a reversal,” they wrote in the study.
“When you look at the decade between 1980 and 1990, you see a clear change, but when you look at 2010 to 2020, you don’t see much change,” Song added.
The spin of the inner core is driven by the magnetic field generated in the outer core and balanced by the gravitational effects of the mantle. Knowing how the inner core spins could shed light on how these layers and other processes interact deep within the Earth.
However, the speed of this rotation and whether it varies is debated, said Hrvoje Tkalcic, a geophysicist at the Australian National University, who was not involved in the study.
“The inner core doesn’t stop completely,” he said. The study’s finding, he said, “means that the inner core is now more in sync with the rest of the planet than it was a decade ago, when it was spinning a little faster.”
“Nothing catastrophic is happening,” he added.
Song and Yang argue that, according to their calculations, a small imbalance in electromagnetic and gravitational forces could slow down and even reverse the rotation of the inner core. They believe this is part of a seven-decade cycle and that the tipping point before the one they detected in their data around 2009/2010 occurred in the early 1970s.
Tkalcic, who is the author of “Earth’s Inner Core: Revealed by Observational Seismology,” said the study’s “data analysis is robust.” However, the study’s findings “should be taken with caution” as “more data and innovative methods are needed to shed light on this exciting problem.”
Song and Yang agreed that more research was needed.
Studying the Earth’s core
Tkalcic, who devotes an entire chapter of his book to the rotation of the inner core, suggested that the inner core cycles every 20 to 30 years, instead of the 70 proposed in the latest study. He explained why such variations occur and why it was so difficult to understand what happens at the far reaches of the planet.
“The objects of our studies are buried thousands of kilometers under our feet,” he said.
“We use geophysical inference methods to infer the internal properties of the Earth, and caution should be exercised until multidisciplinary findings confirm our hypotheses and conceptual frameworks,” he explained.
“You can think of seismologists as doctors who study the internal organs of patients’ bodies with imperfect or limited equipment. So despite progress, our picture of Earth’s interior is still blurry and we’re still at the stage of discovery”.
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