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The rotation of Earth’s inner core may have halted and may even be reversing, according to new research.
The Earth is made up of the crust, mantle, and inner and outer cores. The solid the inner core is located about 3,200 miles below the earth’s crust and is separated from the semi-solid mantle by the liquid outer core, which allows the inner core to spin at a different speed than the rotation of the earth itself.
With a radius of nearly 2,200 miles, Earth’s core is roughly the size of Mars. It consists mainly of iron and nickel and contains about one third of the mass of the Earth.
In research published Monday in the journal Nature Geoscience, Yi Yang, a research associate at Peking University, and Xiaodong Song, a chair professor at Peking University, studied the seismic waves of earthquakes that have traversed the Earth’s inner core in similar pathways since 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 come to a halt.
“We show startling observations that indicate that the inner core has nearly ceased rotation over the past decade and may be experiencing 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 interact and other processes 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 completely shut down,” he said. The study’s finding, he said, “means that the inner core is now more in tune with the rest of the planet than it was a decade ago, when it was spinning a bit faster.”
“Nothing cataclysmic is happening,” he added.
Song and Yang argue that, based on their calculations, a slight imbalance of electromagnetic and gravitational forces could slow and even reverse the rotation of the inner core. They believe this is part of a seven-decade cycle, and that the turning point before the one they detected in their data around 2009/2010 occurred in the early 1970s.
Tkalcic, who is the author of “The Earth’s Inner Core: Revealed by Observational Seismology,” said the “data analysis from the study is solid.” However, the study’s findings “must be taken 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.
Tkalcic, who devotes an entire chapter of his book to the rotation of the inner core, suggested that the inner core cycle is every 20 to 30 years, rather than the 70 proposed in the latest study. He explained why such variations occur and why it was so difficult to understand what is happening in the outer reaches of the planet.
“The objects of our studies are buried thousands of miles below our feet,” he said.
“We use geophysical inference methods to infer Earth’s internal properties, and caution should be exercised until multidisciplinary results confirm our assumptions and conceptual frameworks,” he explained.
“You can think of seismologists as doctors who study the internal organs of patients’ bodies using imperfect or limited equipment. So, despite progress, our picture of the inner Earth is still hazy and we are still in the discovery stage.
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