Earth’s inner core is less solid than previously thought

The surface of the Earth’s inner core may be changing, as shown by a new study from USC scientists that detected structural changes near the planet’s center, published today in Nature Geoscience.

The changes of the inner core has long been a topic of debate for scientists. However, most research has been focused on assessing rotation. John Vidale, Dean’s Professor of Earth Sciences at the USC Dornsife College of Letters, Arts and Sciences and principal investigator of the study, said the researchers “didn’t set out to define the physical nature of the inner core.”

“What we ended up discovering is evidence that the near surface of Earth’s inner core undergoes structural change,” Vidale said. The finding sheds light on the role topographical activity plays in rotational changes in the inner core that have minutely altered the length of a day and may relate to the ongoing slowing of the inner core.

Redefining the inner core

Located 3,000 miles below the Earth’s surface, the inner core is anchored by gravity within the molten liquid outer core. Until now the inner core was widely thought of as a solid sphere.

The original aim of the USC scientists was to further chart the slowing of the inner core. “But as I was analyzing multiple decades’ worth of seismograms, one dataset of seismic waves curiously stood out from the rest,” Vidale said. “Later on, I’d realize I was staring at evidence the inner core is not solid.”

The study utilized seismic waveform data — including 121 repeating earthquakes from 42 locations near Antarctica’s South Sandwich Islands that occurred between 1991 and 2024 — to give a glimpse of what takes place in the inner core. As the researchers analyzed the waveforms from receiver-array stations located near Fairbanks, Alaska, and Yellowknife, Canada, one dataset of seismic waves from the latter station included uncharacteristic properties the team had never seen before.

“At first the dataset confounded me,” Vidale said. It wasn’t until his research team improved the resolution technique did it become clear the seismic waveforms represented additional physical activity of the inner core.

Deformed inner core

The physical activity is best explained as temporal changes in the shape of the inner core. The new study indicates that the near surface of the inner core may undergo viscous deformation, changing its shape and shifting at the inner core’s shallow boundary.

The clearest cause of the structural change is interaction between the inner and outer core. “The molten outer core is widely known to be turbulent, but its turbulence had not been observed to disrupt its neighbor the inner core on a human timescale,” Vidale said. “What we’re observing in this study for the first time is likely the outer core disturbing the inner core.”

Vidale said the discovery opens a door to reveal previously hidden dynamics deep within Earth’s core, and may lead to better understanding of Earth’s thermal and magnetic field.

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About the study: In addition to Vidale, other study authors include Ruoyan Wang of USC Dornsife, Wei Wang of the Chinese Academy of Sciences, Guanning Pang of Cornell University and Keith Koper of the University of Utah.

This research was supported by the National Science Foundation (EAR-2041892), the National Natural Science Foundation of China (42394114), the National Key R&D Program of China (Grant 2022YFF0503203) and the Key Research Program of the Institute of Geology & Geophysics (IGGCAS-201904, IGGCAS-202204).