The inner core of our planet is changing, to the surprise of many scientists. Earth’s core was previously assumed to be solid and unmoving, although that assumption was long-debated. In a groundbreaking study, researchers used seismic waves from earthquakes to determine the shape of the core by analyzing how the waves travel to the opposite side of the planet. This illuminated how the core was truly changing shape.
A history of earthquakes
The research team compiled data from 121 earthquakes that occurred between 1991 and 2023. Then they examined the seismic waves, also known as shockwaves, that went from the South Sandwich Islands from the southern Atlantic Ocean all the way to North America. They compared the quakes on this “pathway” to different years, when the core was in the same rotational position. “They’re basically exactly the same earthquakes that hit years apart — the same place, the same fault,” says John Vidale, a seismologist at the University of Southern California and lead author of the study, to the Washington Post.
The shifting surface of Earth’s inner core
From these comparisons, they discovered the core had shifted. Instead of staying centered as it rotates, it moves slightly, creating hills and dips. “We’ve been able to learn about these changes in the inner core because we have decades of records, from seismic stations run by diligent scientists and engineers, which let us see how the inner core might be changing over human timescales,” says geophysicist Jessica Irving, who was not involved in the study. One theoretical cause is the outer core tugging on the inner.
Between Earth’s inner and outer core
This study has also shed light on another mysterious aspect of our planet’s layers, namely the region between the outer and inner core. The outer core contains liquid iron that crystallizes onto the inner core, and this churning process sustains the planet’s magnetic field. The region is believed to be a combination of liquid and solid. “Even though that inner-core part is really solid, [this boundary] is really soft,” said Guanning Pang, a co-author and geophysicist at Cornell University. “Maybe as soft as jelly.” Therefore, it allows fluctuation in the inner core’s shape.
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A long-standing debate
The Earth is made of several layers, starting with the crust, mantle, outer core, and the inner core. Previous research has shown the inner core is spinning more slowly than previously recorded, and then faster. In this 2024 study, the team had suggested the variations in seismic waves are actually a result of different speed cycles. However, some scientists disagree with this theory, saying that their data could be a result of changes in the shape of the core’s surface.
Changing shape or speed?
“Both these phenomena can affect the travel times of the inner-core-sensitive seismic waves equally,” said Tkalcic, a researcher at the Australian National University. “However, there has been an ongoing debate in the deep-Earth-seismology community about which one is real.” But this new study, published in Nature Geoscience, explains that both changes in speed and shape could be the case, although further research is needed to prove this.
What does this mean for humanity?
As of now, these findings don’t really impact the lives of people on the Earth’s surface. But Vidale and the rest of the team hope their discoveries can correlate to broader implications. “This is kind of the first time we’ve seen the evidence for this kind of motion,” says Vidale. “The surface of the inner core is moving around in ways we hadn’t detected and still don’t understand very well.”
The coming days…
The inner core is responsible for the length of days, because it rotates within the Earth like a planet within a planet. However, scientists are skeptical that the core’s shifting will impact days as we know them. “In science, we generally try to look at things until we understand them,” Vidale says to BBC. “In all likelihood, this finding doesn’t affect our daily lives one iota, but we really want to understand what’s happening in the middle of the Earth.”
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