Earth’s inner core can change shape

WASHINGTON, DC – Earth’s inner core, a solid metal ball spinning inside the molten outer core, may be slowing down and changing shape.

Recent analyzes of earthquake waves have suggested that about 15 years ago, the rotation of the inner core may have slowed so much that it appeared to stop or change direction relative to the surface. But a new analysis suggests that something more must be changing at the center of the Earth.

The most likely explanation is that the inner core isn’t just spinning differently — its surface is probably transforming as well, geophysicist John Vidale of the University of Southern California in Los Angeles reported Dec. 9 at a meeting of the American Geophysical Union. The discovery could help settle a long-running debate over what is changing at the inner core.

No instrument can physically probe the Earth’s core. So researchers study it using seismic waves from earthquakes. Scientists typically use earthquakes that occur in the South Sandwich Islands near Antarctica, which rest on the opposite side of the planet from instrument clusters in Alaska. Earthquake waves travel through the planet like sonar waves through water, with some passing through the inner core on their way to Alaska. Instruments there then record the waves as sharp signatures called waveforms, which contain information about what the waves encountered on their journey through Earth.

For robust detections of changes in the inner core, researchers compare similar-sized earthquakes that occurred in the same location but at different times. Such twin oscillations, known as doubles, should generate the same waveforms if their journeys through Earth were identical. But researchers have noticed that some pairs in the South Sandwich Islands generate different waveforms in Alaska, indicating that something in the core had changed between the time the two earthquakes occurred in those pairs.

In 2023, geophysicists reported that the changes in the waveform came from the rotation of the inner core slowing down so much that it appeared to have stopped moving—or even turned back—relative to the surface sometime around 2009 (SN: 1/23/23). Then, earlier this year, Vidale’s team apparently confirmed the change. They were able to match several waveforms before and after the twist, identifying times when the inner core had regained a previous orientation towards the surface (SN: 25.6.24).

For the new study, Vidale and colleagues analyzed nearly 200 pairs of earthquakes that occurred from 1991 to 2024. They examined matched pairs of pre- and post-reversal waveforms recorded at two separate receiver arrays located near Fairbanks, Alaska. and Yellowknife, Canada.

Intriguingly, 10 pairs showed subtle changes in the Yellowknife waveforms that were not present in the Fairbanks waveforms. The team knew that waves reaching these clusters, which are roughly 1,600 kilometers apart, take slightly different paths through Earth: Waves reaching Fairbanks penetrate deep into the inner core while those reaching Yellowknife graze the exterior. her.

“The simplest explanation is deformation in the shallow inner core,” says Vidale.

It is possible that the entire geoid-shaped inner core is being deformed, like a football being reshaped, so that its edges point in two new directions. Otherwise, he says, patches of the core’s inner surface can swell or shrink. That would be like the little bumps and scratches that form on the metaphorical football. It is also possible that both processes are occurring. Such changes could be driven by the gravitational pull of the mantle — Earth’s innermost layer — or by material flowing into the outer core, Vidale says.

This is not the first time researchers have reported that the surface of the inner core changes over time. In 2006, geophysicist Lianxing Wen of Stony Brook University in New York reported that parts of the region’s surface can rise or fall by hundreds of meters per decade, possibly due to material being ejected from the inner core as it cools. But unlike Vidale, Wen and colleagues such as geophysicist Xin Zhang of the University of Science and Technology of China in Hefei maintain that the inner core does not rotate unlike Earth. “Surface changes can fully explain all the results,” says Zhang.

Geophysicist Xiaodong Song of Peking University in Beijing, who was one of the first to report that the inner core rotates differently from the rest of the planet, generally agrees with Vidale’s findings. While the changes in waveform are probably caused primarily by rotational changes, other processes such as surface changes may also occur, he says. “It’s not either or.”

As for how all this affects life on Earth, “we don’t know that this will affect anything on the surface,” Vidale says. “But we can’t say for sure until we understand what’s going on.”