The long-standing belief that Earth’s inner core is a solid, unchanging sphere has been challenged by a groundbreaking revelation by scientists who discovered that the core is deforming, undergoing significant shape changes, indicating profound implications about how it affects Earth’s magnetic field and even the length of the day in the future.
When researchers analyzed how seismic waves from earthquakes travelled from the South Sandwich Islands in the South Atlantic Ocean to seismometers in Alaska and Canada, on the other side of the planet, they were surprised to find that between 2004 and 2008, the waves changed as they passed through the inner core, suggesting that the core is deforming, not just rotating as previously thought.
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The waveforms, or shapes, of some of the seismic signals changed because the waves briefly penetrated Earth’s inner core — which was changing shape, the scientists explain. “For the first time we’re seeing that it’s deforming,” says John Vidale, a seismologist at the University of Southern California in Los Angeles, who reported the findings in Nature Geoscience.
The study also sheds light on a long-running scientific debate about whether changes in seismic waves are caused by the core’s rotational shifts or physical deformations at its boundary. The new evidence suggests both factors are at play. Some of the waveform changes are linked to the core’s rotation, while others appear to result from the outer boundary of the inner core developing bulges. Since iron melts at around 1,500°C, Vidale believes the boundary between the inner and outer core may constantly melt and refreeze, forming new structures.
Another possibility is that iron is bubbling up from the inner core’s surface like a “burp.” It’s also possible that the surface of the inner core is becoming rougher and more uneven due to the pull of gravity and the movement of the surrounding molten outer core.
“The changes near the inner-core boundary most likely result from viscous deformation driven by coupling between boundary topography and mantle density anomalies or traction on the inner core from outer-core convection,” the researchers wrote in the abstract of the published paper.
Seismic Data Unveils Core’s Shifting Topography
Earth’s inner core sits about 5,100 kilometers below the surface, encased in a molten outer core primarily made of iron and nickel. The inner core also grows slowly over time as iron crystallizes onto it from the outer core. This process helps sustain Earth’s magnetic field, which influences the length of a day. Previous studies have shown that the inner core rotates at a slightly different speed than the rest of the planet.
Studying Earth’s inner core is challenging as it lies deep beneath the surface—a 2,500-kilometer-wide sphere of solid iron at an intense 5,200°C — and no instruments can reach it. So, the team of scientists relied on seismic waves from earthquakes to understand its structure.
When earthquakes occur, their waves either pass through the inner core or bounce off its surface. By comparing waves from earthquakes in the same location but at different times, researchers can detect changes happening deep inside the planet. So, the team led by seismologist Vidale analyzed 168 pairs of earthquakes recorded between 1991 and 2023 in the South Sandwich Islands, a seismic hotspot in the Southern Ocean. These waves traveled through Earth to sensors in Alaska and Canada.
The scientists focused on earthquake pairs that occurred when the inner core was in the same rotational position. This allowed them to rule out rotation as a cause for any variations they observed. Their findings showed that between 2004 and 2008, waves traveling to Canada’s Yellowknife station changed unexpectedly, while those reaching Alaska remained consistent. This suggests that the inner core is not a perfect sphere but instead shifts over time, with certain areas rising and falling by kilometers within a few years. These deformations deflect seismic waves, altering their paths too.
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Although scientists had theorized that the core might change shape, this study provides the first clear evidence. “Basically, we are seeing it for the first time,” Vidale said.”Ideally, we’d like to tie all these things that we’re seeing together to better understand Earth’s deep interior,” Vidale said.
Besides discovering new insights, the study raises further questions about how changes in the core might impact the planet’s magnetic field and even the length of a day. As Vidale cautioned, “We’re not 100% sure we’re interpreting these changes correctly,” there’s more to study on Earth’s inner core.