A Shifty Center

Deep beneath the planet’s surface, Earth’s inner core may not be as unyielding as once thought.

New research suggests the solid core’s surface is shifting over time, reshaping in response to the turbulent outer core that surrounds it. This discovery challenges the traditional view of the inner core as a static, iron-nickel sphere and raises new questions about how Earth’s deepest layers influence the planet’s magnetic field and even the length of a day.

“The interesting thing is that the surface of the inner core is dynamic,” John Vidale, a seismologist at the University of Southern California and the study’s lead author, told Live Science. “It seems to be responding to the changing forces from the rotation and probably rising and falling a noticeable amount.”

For their paper, Vidale and his team analyzed earthquake waves that skimmed the edge of the inner core between 1991 and 2023.

Using seismic data from earthquakes near the South Sandwich Islands in the Atlantic Ocean, they noticed subtle differences in how waves traveled through the core’s surface over time. These variations persisted even when the core rotated into a previously recorded position – suggesting the outermost layer of the inner core may be shifting.

The researchers believe this movement is likely caused by churning currents in the liquid outer core. This molten layer – composed mostly of iron and nickel – surrounds the solid inner core and is responsible for generating Earth’s magnetic field. As the outer core’s flow changes, it may be subtly deforming the surface of the inner 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 explained in a statement. “What we’re observing in this study for the first time is likely the outer core disturbing the inner core.”

Since the 1990s, scientists have debated whether the inner core rotates independently of the rest of the planet. A previous study co-authored by Vidale found that the inner core once spun faster than Earth’s surface but began slowing around 2010, now lagging slightly behind. The new findings suggest that, in addition to rotation, the inner core may experience structural changes driven by external forces.

The implications of these findings extend beyond seismology. The solidification of the inner core plays a crucial role in the flow of heat through the outer core, which in turn sustains Earth’s magnetic field. Understanding the core’s dynamics could offer insight into the planet’s deep history and the evolution of its magnetic shield.

Still, other researchers noted that the current data only covers a small fraction of the core’s surface, with the authors acknowledging that much remains uncertain about the inner core’s behavior.