The rotation of Earth’s inner core may have stopped and may even be reversing, new research suggests.
Earth consists of a crust, a mantle, and an inner and outer core. The solid 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, allowing the inner core to rotate at a different rate than the rotation of the Earth itself.
With a radius of almost 2,200 miles, Earth’s core is roughly the size of Mars. It consists mostly of iron and nickel and contains about a third of the mass of the Earth.
In research published in the journal Nature Geoscience on Monday, Yi Yang, an associate research scientist at Peking University, and Xiaodong Song, a professor at Peking University, studied seismic waves from earthquakes that traveled through Earth’s inner core along similar paths from 1960s to deduce how fast the inner core rotates.
What they found was unexpected, they said. Since 2009, the previously time-varying seismic records show little difference. This, they said, suggests that the rotation of the inner core has stopped.
“We show surprising observations that indicate that the inner core has almost stopped rotating over the last decade and may be experiencing retrograde,” 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 rotation 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 rotates can 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 research.
“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 bit faster.”
“Nothing catastrophic is happening,” he added.
Song and Yang say that based on their calculations, a small imbalance in the electromagnetic and gravitational forces can 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 found 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 analysis of the survey data is reliable.” However, the study’s findings “should be taken with caution” as “more data and innovative methods are needed to shed light on this interesting issue.”
Song and Yang agreed that more research is needed.
Studying the Earth’s Core
Tkalcic, who devotes an entire chapter of his book to the rotation of the inner core, suggests that the inner core cycles every 20 to 30 years, rather than the 70 suggested in the latest study. He explained why such variations occur and why it is so difficult to understand what is happening in the deepest parts of the planet.
“The objects of our research are buried thousands of kilometers beneath our feet,” he said.
“We use geophysical inference methods to infer Earth’s internal properties and must be cautious 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 using imperfect or limited equipment. So, despite the progress, our picture of the inner Earth is still unclear and we are still in the discovery stage.”
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