Seismologists Discover Earth's Inner Core May Have Paused Its Rotation Pattern

Deep beneath our feet, the solid iron core of our planet is behaving in unexpected ways. Recent data from earthquake waves shows that the inner center of the Earth paused its rotation and is now slightly reversing its spin rate relative to the surface. Let us look at the exact science behind this major geological discovery.

The Solid Iron Sphere Inside Our Planet

To understand what is happening, you have to look at the anatomy of the Earth. About 3,200 miles beneath the crust lies the inner core. It is an extremely hot, incredibly dense sphere made mostly of solid iron and nickel. Measuring roughly 1,500 miles wide, this core is about the size of Pluto.

Surrounding this solid center is the outer core, a layer of liquid metal. Because the inner core floats within this liquid bath, it is physically separated from the solid mantle and crust above it. This separation allows the inner core to spin independently. For decades, scientists believed the core rotated slightly faster than the rest of the planet, a concept known as super-rotation.

How Seismologists Track the Core's Spin

Scientists cannot drill down 3,200 miles to watch the core spin. The deepest hole ever dug, the Kola Superdeep Borehole in Russia, only reached about 7.6 miles down. Instead, geologists rely on earthquakes.

When a large earthquake occurs, it sends shockwaves rippling through the entire planet. Seismologists track a specific type of data called repeating earthquakes. These are seismic events that happen in the exact same location but at different times.

For example, an earthquake might happen in the South Sandwich Islands in 1991, and another identical quake might hit that exact spot in 2023. Seismometers in Alaska record the waves as they travel straight through the center of the Earth. By comparing the shape and travel time of the wave from 1991 to the wave from 2023, scientists can detect changes in the inner core. If the solid iron center has rotated, the wave will pass through a different angle of the iron structure, altering the data recorded on the surface.

The Great Pause of 2010

In early 2023, seismologists Yi Yang and Xiaodong Song from Peking University published a groundbreaking study in the journal Nature Geoscience. They analyzed decades of earthquake data and noticed something strange. Around the year 2010, the seismic wave patterns stopped changing.

The inner core had caught up to the rotation speed of the Earth’s surface and essentially paused. It was no longer outpacing the mantle.

This finding was heavily supported by a June 2024 study led by John Vidale at the University of Southern California (USC). Vidale and his team analyzed data from 121 repeating earthquakes recorded between 1991 and 2023. They also looked at historical data from French, Soviet, and American nuclear tests from the 1970s. The USC data confirmed the Peking University findings: the inner core definitely hit the brakes around 2010.

Understanding the Reversal in Spin Rate

The new data suggests the core is not just pausing, but slightly reversing its spin rate. However, you do not need to worry about the Earth suddenly spinning backward.

The core is not physically spinning in the opposite direction in space. Instead, it is now rotating slightly slower than the Earth’s surface. This is a matter of perspective. Imagine you are driving a car on the highway at 70 miles per hour. If the car next to you slows down to 65 miles per hour, that car appears to be moving backward from your point of view, even though both of you are still moving forward.

Because the mantle and crust are now spinning slightly faster than the inner core, the core appears to be rotating in reverse from our viewpoint on the surface.

A Natural 70-Year Cycle

Researchers believe this changing spin rate is completely normal. The current consensus points to a 70-year oscillation cycle.

The magnetic field generated by the liquid outer core pulls on the solid inner core, trying to make it spin faster. At the same time, the massive gravitational force of the Earth’s rocky mantle pulls on the inner core, trying to slow it down. This constant tug-of-war causes the core’s speed to fluctuate over time.

According to the data from Song and Yang, the cycle swings back and forth roughly every 35 years.

• The last time the core paused and reversed direction was in the early 1970s.
• It hit its peak forward speed around the early 1990s.
• It paused again around 2010.
• Scientists expect the next major shift to happen around the mid-2040s.

How This Affects the Surface

This shifting iron center will not cause catastrophic events. It has been happening for millions of years. However, the changing momentum deep inside the planet does have a few measurable effects on our daily lives.

The most notable impact is a change in the length of a day. Because the inner core is so massive, its shifting speed slightly alters the rotation of the entire planet. When the core slows down, the Earth’s rotation adjusts, changing the length of a 24-hour day by fractions of a millisecond.

Additionally, the tug-of-war between the core layers can cause microscopic changes to the Earth’s magnetic field. Some geologists also suspect this 70-year cycle might link to tiny historical fluctuations in global temperatures and sea levels, though more research is required to prove a direct connection.

Frequently Asked Questions

Will the reversing core cause more earthquakes? No. The rotation of the inner core does not directly trigger earthquakes on the surface. Earthquakes are caused by tectonic plates shifting in the Earth’s crust, which is entirely separate from the solid iron center.

Does the core ever stop spinning completely? The inner core never stops spinning relative to space. When scientists say it “paused,” they mean its rotation speed temporarily matched the exact rotation speed of the Earth’s surface.

How hot is the inner core? The temperature of the solid inner core is estimated to be around 9,392 degrees Fahrenheit (5,200 degrees Celsius). This is nearly as hot as the surface of the sun. It remains solid despite the heat due to the immense pressure at the center of the Earth.

How do scientists know the core is made of iron? Scientists determine the composition of the core by studying how fast seismic waves travel through it, combined with observations of the Earth’s magnetic field and the density required to maintain the planet’s overall gravitational pull. Iron and nickel are the only elements that fit all the mathematical models perfectly.