Study: Earth’s Inner Core May Be Changing Speed

By Andrew Smith
26 January 2023

A recent study suggests that the rate at which the Earth's center is turning is slowing down compared to the outer parts of the planet.

Earth scientists, or geologists, say that the center or core of the Earth can rotate, or turn, at a different rate than the outer areas.

The latest research appeared in the scientific publication Nature Geoscience.

The writers of the research are Xiaodong Song and Yi Yang of China's Peking University in Beijing. They used information about seismic waves from earthquakes to study the core.

Earth's inner core is a ball of hot iron that is a little smaller than the Moon. Because it floats in the liquid metal of the outer core, it can spin at a different speed than the rest of the planet.

In the 1930s, scientists discovered the Earth's core by studying changes in the speed of seismic waves as they travelled through the planet. The changes in speed showed that the core was a solid center made mostly of iron.

The new research by Song and Yang studied what they called "repeated seismic waves from the early 1990s." They looked for changes in the amount of time seismic waves take to travel over the same paths inside the Earth.

The scientists suggest changes in the spin of Earth's core can explain the changes in travel times.

"We believe the inner core rotates, relative to the Earth's surface, back and forth, like a swing," they told The French news agency AFP. They added that its relative rotation changes about once every 35 years.

They also said these changes correspond with very small changes in "length of day"—the exact amount of time it takes for Earth to rotate on its axis.

Different research results

In June 2022, John Vidale and Wei Wang, Earth scientists at the University of Southern California (USC), published a study in Science Advances.

They reported that between 1969 and 1971 the core rotated more slowly than the outer layers. They also said that after 1971, the core rotated faster than the outer layers. In their study, Wang and Vidale used data on seismic waves from nuclear test explosions.

Earth scientists say the Earth's core is difficult to understand and study. They disagree about how much, and how often, the spin might change. Some scientists argue that the speed of the spin does not change at all. They say the changes in seismic waves are caused by changes on the surface of the inner core instead of differences in how fast the core is spinning.

Hrvoje Tkalcic is a geophysicist at the Australian National University. He thinks scientists will disagree about the latest research.

He compared seismologists, who study earthquakes and seismic waves, to doctors "who study the internal organs of patients' bodies using imperfect or limited equipment." He added: "Our image of the inner Earth is still blurry."

Song and Yang said scientists need years of seismic data to answer questions about Earth's core.

Vidale of USC said, "Something's happening and I think we're going to figure it out. But it may take a decade."

I'm Andrew Smith.

Andrew Smith adapted this story for Learning English based on reporting from Agence France-Presse

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Words in This Story

core –n. the very center of something

spin –v. to turn

seismic –adj. related to earthquakes

swing –v. to move back and forth between two positions

correspond with –v. (phrasal) to have a direct relationship to or with something or someone

imperfect –adj. having some fault or incomplete part or area

blurry –adj. unclear

figure (something) out –v. (phrasal) to understand or find something by thought

decade –n. a 10-year period