Earth cross section showing the classic distinction between crust, mantle and core. But new research ... [+] shows Earth's structure is far more complicated and may change over time.gettyAnalyzing an unusual dataset of seismic waves, a team of geophysicists discovered evidence that Earths inner core is less solid than previously thought, and its surface changed in just a few decades.When an earthquake happens, seismic waves spread from its point of origin in all directions. The speed at which the waves travel depends on the type of wave, but also on the density and elasticity of the material through which the waves pass. Seismographic stations record these different waves, and on the basis of these recordings, geophysicists can draw conclusions about the structure and composition of the Earth and examine the processes that take place inside it.Based on seismic records of the past 100 years, scientists compiled the classic cross-section of Earth, showing a solid crust, a viscous mantle, a liquid outer core, and an inner solid core. A study published in 1995 even argued that the inner core is a gigantic crystal made of iron.A very schematic cross-section of the Earth showing the sub-surface layers and main mineral ... [+] composition.D.BressanBut the surface of the Earths inner core may be changing at a surprisingly fast rate, as shown by a new study published by a team of geophysicists from the University of Southern California (USC), the Chinese Academy of Sciences and University of Utah, suggesting that the core is less solid than previously thought.MORE FOR YOUThe original aim of the scientists was to chart the rotation 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," study lead author John Vidale, Deans Professor of Earth Sciences at USC, explains.The study utilized the seismic data of 121 earthquakes occurring between 1991 and 2024 near Antarcticas South Sandwich Islands. 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. The seismic waves show changes in their travel direction that the cores rotation can not explain."At first the dataset confounded me," Vidale admits. It wasnt until his research team improved the studys resolution technique did it become clear the seismic waveforms were influenced by the shape of the inner core.The results indicate that the surface of the inner core may undergo a slow viscous deformation, a bit like tar, changing its shape over time. The most likely cause of this deformation is interaction between the inner and outer core.What were observing in this study for the first time is likely the outer core disturbing the inner core, explains Vidale. Turbulence in Earths molten outer core disrupts the boundary layer of the inner core, and as the decades-long seismic data shows, this happens at a surprisingly fast rate.This discovery opens a door to reveal previously hidden dynamics deep within Earths core, and may lead to better understanding of how Earths core generates its magnetic field.The study, "Annual-scale variability in both the rotation rate and near surface of Earths inner core," was published in the journal Nature Geoscience.Additional material and interviews provided by Will Kwong, USC Media Relations.