The rotation of Earth’s inner core may have stopped and may even reverse, new research suggests. (Cigdem Simsek, Alamy)
Estimated reading time: 3-4 minutes
ATLANTA – The rotation of Earth’s inner core may have stopped and may even reverse, new research suggests.
The earth consists of the crust, the mantle and the inner and outer core. Located about 3,200 miles below Earth’s crust, the solid inner core is separated from the semi-solid mantle by the liquid outer core, allowing the inner core to spin at a different speed than Earth itself.
With a radius of nearly 2,200 miles, Earth’s core is about the size of Mars. It consists mostly of iron and nickel and contains about a third of Earth’s mass.
In a study published Monday in the journal Nature Geoscience, Yi Yang, an associate research scientist at Peking University, and Xiaodong Song, a chair professor at Peking University, examined seismic waves from earthquakes traveling along similar paths through the inner core since the 1960s have walked the Earth conclude how fast the inner core is spinning.
What they found was unexpected, they said. Since 2009, seismic records that had previously changed over time showed little difference. This indicates that the rotation of the inner core has stopped.
“We show surprising observations that suggest the inner core has almost stopped rotating over the past decade and may be undergoing a reversal,” they wrote in the study.
“If you look at the decade between 1980 and 1990 you see clear changes, but if you look at 2010 to 2020 you don’t see big changes,” Song added.
The spin of the inner core is driven by the magnetic field generated in the outer core and is balanced by the gravitational effects of the mantle. Knowing how the inner core spins could shed light on how these layers interact and other processes deep within the Earth.
However, the speed of this rotation and whether it varies is a matter of debate, said Hrvoje Tkalcic, a geophysicist at the Australian National University who was not involved in the study.
“The inner core doesn’t stop,” he said. The result of the study, he said, “means that the inner core is now more synchronized with the rest of the planet than it was a decade ago when it was spinning slightly faster.”
“Nothing cataclysmic is happening,” he added.
Song and Yang argue that based on their calculations, a small imbalance in the electromagnetic and gravitational forces could slow and even reverse the inner core’s rotation. They believe this is part of a seven-decade cycle, and that the tipping point before what they detected in their data around 2009/2010 occurred in the early 1970s.
Tkalcic, author of The Earth’s Inner Core: Revealed by Observational Seismology, said the study’s “data analysis is robust.” However, the results of the study “should be viewed with caution” as “more data and innovative methods are needed to shed light on this interesting problem”.
Song and Yang agreed that more research was needed.
Examination of 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 cycle occurs every 20 to 30 years instead of the 70 years suggested in the latest study. He explained why such fluctuations occur and why it has been so difficult to understand what is happening in the innermost reaches of the planet.
“The objects of our study are buried thousands of kilometers beneath our feet,” he said.
“We use geophysical inference methods to infer the Earth’s interior properties, and caution is warranted until multidisciplinary results confirm our hypotheses and conceptual framework,” he explained.
“You can think of seismologists as doctors examining the internal organs of patients’ bodies with imperfect or limited equipment. Therefore, despite advances, our picture of inner Earth is still blurry and we are still in the discovery phase.”