Earth’s inner core is deforming—here’s what we know

LOS ANGELES — Scientists have just detected something unusual happening 3,000 miles beneath our feet. New research from an international study reveals that Earth’s inner core, a ball of mostly iron roughly the size of Pluto, isn’t behaving the way scientists thought. It appears to be both changing its spin rate and deforming at its surface, challenging our understanding of what lies at our planet’s center.

Earth’s inner core was long thought to be a solid sphere of mostly iron anchored by gravity within the molten liquid outer core. Scientists have long debated whether this massive ball rotates at a different speed than the rest of Earth, and what else might be happening at its surface. This study, published in Nature Geoscience, provides compelling evidence that rewrites what we know about the inner core’s nature.

“What we ended up discovering is evidence that the near surface of Earth’s inner core undergoes structural change,” says study author John Vidale, dean’s professor of earth sciences at the University of Southern California, in a statement.

Earth’s inner core plays a crucial role in generating our planet’s magnetic field, which protects life from harmful solar radiation and helps animals navigate. It also influences Earth’s rotation, affecting the length of our days by tiny but measurable amounts. Understanding the inner core’s behavior helps scientists better grasp these fundamental planetary processes.

For nearly three decades, scientists have used seismic waves from earthquakes to study Earth’s internal structure. These waves act like sound passing through different materials, changing speed and direction as they encounter various layers and boundaries inside Earth. By analyzing how these waves travel through the planet, researchers can deduce properties of materials they cannot directly observe.

This detection method led to the discovery in the 1990s that Earth’s inner core might rotate slightly faster than the rest of the planet, a phenomenon called super-rotation. However, recent studies suggested this rotation slowed around 2009, leading to debates about what exactly was happening deep inside Earth.

The research team analyzed 121 repeating earthquakes from 42 locations near Antarctica’s South Sandwich Islands between 1991 and 2024. These “earthquake twins” occurred in the same locations with nearly identical characteristics, making them ideal natural experiments for spotting any changes in the inner core over time.

“As I was analyzing multiple decades’ worth of seismograms, one dataset of seismic waves curiously stood out from the rest,” says Vidale. “Later on, I’d realize I was staring at evidence the inner core is not solid.”

The breakthrough came from comparing seismic recordings at two different locations in North America. The Yellowknife Array in Canada and the Eielson Array near Fairbanks, Alaska, are sophisticated facilities designed to detect and analyze seismic waves. Between 2004 and 2008, waves recorded at Yellowknife showed distinct changes while those at Eielson remained unchanged.

This difference proved crucial because of how the seismic waves traveled through Earth’s core. Waves reaching Yellowknife skimmed closer to the inner core’s surface, while those arriving at Eielson penetrated deeper. The contrasting observations suggested localized changes were occurring near the boundary 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,” explains Vidale. “What we’re observing in this study for the first time is likely the outer core disturbing the inner core.”

The discovery builds on decades of evolving understanding about Earth‘s core. When seismologists first identified the inner core in the 1930s, they viewed it as a simple solid sphere. By the 1980s, scientists realized it had complex properties, including possible variations in its structure. This new research reveals it’s even more dynamic than previously thought.

The observed changes result from viscous deformation, the inner core being reshaped by powerful forces. The liquid outer core’s churning motion creates magnetic forces that pull on the inner core, while variations in Earth’s gravity due to mantle density differences also exert influence. These combined effects appear capable of reshaping the inner core’s surface, similar to how extremely hot metal can be deformed under pressure.

Changes in the inner core can affect Earth’s magnetic field, which impacts everything from satellite communications to animal migration. The core’s behavior also influences subtle variations in day length, which, while too small for humans to notice, must be accounted for in precise technological systems like GPS.

Finding evidence for both rotation changes and surface deformation resolves a long-standing scientific debate. Rather than being competing explanations for observed seismic wave variations, both processes appear to be happening simultaneously.

The inner core isn’t simply a rigid sphere rotating within our planet; it’s a complex system that responds to various forces by both changing its spin and deforming at its surface. The next challenge lies in determining what these core changes might mean for Earth’s future.