Earth's inner core is rapidly changing and now "rotating backwards"

It’s not much of a stretch to say that Earth’s inner structure, especially the innermost spherical core, has stupefied scientists for generations. It sits over 3,000 miles below the surface, smaller than the Moon yet massive enough to play a big part in Earth’s magnetic field.

A recent study in Nature Geoscience points to changes that started around 2010, suggesting Earth’s inner core’s spin was slowing.

In effect, it was found to be to be reversing and backtracking relative to the planet’s surface. This discovery has sparked fresh conversations about how the planet’s interior behaves over time.

Geophysicists have used earthquake signals from nearly every continent to probe the inner core, which is surrounded by a swirling layer of liquid metal. Findings point to structural shifts in the iron sphere that raise new questions.

After two decades of debate, investigators feel more confident linking these signals to an actual slowdown rather than an imagined event.

John Vidale, Dean’s Professor of Earth Sciences at the USC Dornsife College of Letters, Arts and Sciences, worked on the project with Wei Wang of the Chinese Academy of Sciences.

Innermost layers of Earth’s interior

Earth’s interior can be divided into several layers: the crust, the mantle, the liquid outer core, and the solid inner core at the center. Many scientists have studied how the inner core moves relative to the mantle and crust.

Some found evidence that the core once rotated faster than the rest of Earth. Others noticed it might be easing up.

The latest data points to “unambiguous evidence” of a slower pace starting around 2010, landing it at a speed slightly below that of Earth’s surface.

“When I first saw the seismograms that hinted at this change, I was stumped. But when we found two dozen more observations signaling the same pattern, the result was inescapable,” Vidale explained.

“The inner core had slowed down for the first time in many decades. Other scientists have recently argued for similar and different models, but our latest study provides the most convincing resolution.”

Records from earthquakes and nukes

Researchers rely on waves generated by earthquakes to examine the farthest corners of our planet. These waves pass through the inner core, reflecting minute details about its movement and shape.

For their analysis, the team gathered readings from 121 repeating earthquakes originating near the South Sandwich Islands between 1991 and 2023.

They also tapped into data from Soviet nuclear tests conducted between 1971 and 1974, as well as repeated French and American nuclear tests.

Vidale attributes the pace shift to the “churning of the liquid iron outer core that surrounds it, which generates Earth’s magnetic field, as well as gravitational tugs from the dense regions of the overlying rocky mantle.”

Investigators think these forces may slightly affect day length. “It’s very hard to notice, on the order of a thousandth of a second, almost lost in the noise of the churning oceans and atmosphere.”

Earth’s inner core is “backtracking”

Some prior research indicated that the inner core might be reversing itself. The new results add weight to that possibility by suggesting the core’s rotation has slowed and even begun shifting direction compared to Earth’s outer layers.

Vidale “didn’t set out to define the physical nature of the inner core.” Yet the project unexpectedly uncovered more than just a slowdown in rotation.

“What we ended up discovering is evidence that the near surface of Earth’s inner core undergoes structural change,” Vidale said.

Shifting layers deep underground might explain certain fluctuations in day length, but these effects are so slight that most people never detect them.

The team believes dynamic interactions between the liquid outer core and dense mantle zones drive these shifts, making the inner core’s movement appear to switch course from time to time.

Surprising clues from seismograms

“But as I was analyzing multiple decades’ worth of seismograms, one dataset of seismic waves curiously stood out from the rest,” Vidale said.

He and his colleagues analyzed waveforms picked up by monitoring stations in places like Fairbanks, Alaska, and Yellowknife, Canada.

“At first the dataset confounded me,” Vidale said.

Modern techniques have since sharpened the picture, indicating that the inner core’s boundary may go through slow changes in shape caused by viscous deformation.

“What we’re observing in this study for the first time is likely the outer core disturbing the inner core,” Vidale said.

Rather than acting like a rigid ball locked in place, the inner core seems to adapt over intervals that current seismic devices are finally able to capture.

Scientists suspect this iron center may shift in ways that influence the planet’s magnetic field, given how much of that field is generated by the liquid outer region.

Why does Earth’s inner core matter?

Experts at USC aim to track the core’s patterns more precisely in the years ahead. They plan to zero in on subtle signals that show how the inner core’s shape evolves.

They hope this effort might offer a clearer picture of the underlying forces behind any reversing motion, including interactions that appear to adjust the core’s boundaries.

While Earth’s inner core is often portrayed as static and solid, these fresh observations add nuance. Repeating earthquakes provide a valuable peek at what’s happening in an environment most people can only imagine.

The research team believes that following waveforms from future quakes could unlock details about how Earth’s center changes over time.

Vidale said the focus is on mapping these movements in ways that help scientists see what triggers the shifts in speed and form.

The new waveforms expand the broader conversation on Earth’s mechanics. Certain aspects, like magnetic fluctuations and temperature gradients, may link back to the inner core’s pace.

Though the planet’s center remains out of reach, these findings reveal a place that is much more active than once believed.

The full study from 2024 was published in the journal Nature.

The full study from February 2025 was published in the journal Nature Geoscience.

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