Why the Earth’s Core Is Spinning Backward (Relatively)

Recent headlines regarding the Earth’s inner core suggest a scenario ripped from a science fiction movie. However, new findings published in the journal Nature by seismologists at the University of Southern California (USC) reveal a nuanced reality. The solid metal ball at the center of our planet has indeed slowed down, and in relation to the surface we stand on, it appears to be moving backward.

The USC Study: Confirming the Slowdown

In June 2024, researchers led by John Vidale, a Dean’s Professor of Earth Sciences at USC Dornsife, provided what they consider unambiguous evidence of the core’s behavior. The study analyzed seismic data from repeating earthquakes to track the movement of the inner core over three decades.

The findings contradict previous models suggesting the core rotates steadily faster than the planet’s surface. Instead, the data indicates the inner core is backtracking.

Understanding Relative Rotation

It is crucial to clarify what “spinning backward” actually means in this context. The Earth’s inner core is not literally spinning in the opposite direction of the planet like a rogue top. It is still rotating in the same direction as the crust.

However, its speed has changed. For decades, the inner core rotated slightly faster than the Earth’s mantle and crust. This is known as “super-rotation.” According to Vidale’s research, around 2010, the inner core slowed down. It is now rotating slower than the mantle.

Think of it like two race cars on a track:

• The Crust (Us): Driving at 100 mph.
• The Inner Core: Driving at 90 mph.

From the perspective of the driver in the faster car (us on the surface), the slower car (the core) appears to be moving backward, even though both are moving forward. This is the “backtracking” phenomenon scientists have observed.

How Scientists Measured the Impossible

Because we cannot drill 3,000 miles down to observe the core directly, scientists rely on seismology. The USC team, including researcher Wei Wang, utilized a specific method involving “repeating earthquakes.”

These are seismic events that occur at the exact same location at different times to produce identical seismograms. The team analyzed data from:

• 121 repeating earthquakes occurring around the South Sandwich Islands between 1991 and 2023.
• Twin Soviet nuclear tests occurring between 1971 and 1974.
• French and American nuclear tests from other archival data.

By looking at how seismic waves from these events traveled through the Earth’s center, the researchers could spot changes. If the inner core was stationary relative to the surface, the waves would look the same every time. Instead, the arrival times of the waves changed, indicating the medium they passed through (the inner core) had shifted position.

The data showed a clear pattern:

1. 2003 to 2008: The core was super-rotating (spinning faster).
2. 2008 to 2023: The core began sub-rotating (spinning slower).

Why Is This Happening?

The movement of the inner core is a constant tug-of-war between two massive forces deep inside the planet.

1. The Magnetic Fluid: The outer core is made of molten iron and nickel. As it churns, it generates the Earth’s magnetic field. This magnetic field grabs onto the solid inner core and spins it. This is usually the force that drives rotation.
2. The Mantle’s Gravity: The rocky mantle, which sits above the outer core, is immense. Its gravity exerts a drag on the inner core and tries to slow it down.

When these forces are unbalanced, the speed of the inner core fluctuates. Vidale notes that the recent slowdown is likely caused by the churning of the liquid outer core surrounding the inner solid ball, combined with gravitational tugs from the dense regions of the overlying rocky mantle.

Impact on the Length of Days

While the slowing core sounds alarming, the physical effects on the surface are minimal. The most measurable impact is on the length of a day.

The Earth’s rotation determines our 24-hour cycle. However, the conservation of angular momentum means that if one part of the Earth slows down (the core), the rest (the crust) must adjust slightly.

The slowing of the inner core alters the length of a day by mere fractions of a second. Vidale estimates the change is on the order of a thousandth of a second. This is roughly the same variability caused by the churning of the oceans and the atmosphere. While atomic clocks might need a tiny adjustment, human circadian rhythms will remain unaffected.

Settling the Scientific Debate

This study helps settle a long-standing debate in the geophysics community.

• Steady Rotation Model: Some earlier theories posited the core always spins faster by a constant degree (about 1 degree per year).
• Oscillation Model: A 2023 study by Peking University researchers Yi Yang and Xiaodong Song suggested the core oscillates on a roughly 70-year cycle, switching directions every 35 years.

The USC data supports the oscillation theory but provides much higher resolution on the timeline. Vidale’s team posits that the core is currently in the “slowing” phase of this decades-long cycle.

Frequently Asked Questions

Is the Earth’s core going to stop completely? No. The core is simply rotating slightly slower than the crust. It continues to spin in the same direction as the rest of the planet.

Will this affect the magnetic field? The magnetic field is generated by the liquid outer core, not the solid inner core. While the interaction between the two is complex, the slowing of the inner core does not signal a collapse of the magnetic field.

Does this cause earthquakes? The slowing of the core is identified using earthquake data, but it does not cause earthquakes on the surface. Tectonic plates in the crust move independently of the deep core’s rotation speed.

How big is the inner core? The inner core is a solid sphere of iron and nickel roughly the size of the Moon, sitting nearly 3,000 miles beneath our feet.

Why is this discovery important? Understanding the core helps scientists understand how the Earth formed, how it generates its magnetic field (which protects us from solar radiation), and how the planet’s interior evolves over millions of years.