Earth's inner core rotation may have paused and possibly reversed

Changes in the rotation might occur on a decadal scale and could aid our understanding of how processes deep in the Earth affect its surface.
Thomas Velasquez
Digital image of Earth's inner core
Earth's inner core


The rotation of the Earth’s solid inner core may have recently paused and could be reversing, according to a study published in Nature Geoscience this week. The authors show evidence that oscillation in the rotation of the inner core coincides with periodic changes in the Earth’s surface system and that there is an interaction between different layers of the Earth.

The inner core rotation and its effect

The Earth’s inner core is separated from the rest of the solid Earth by a liquid outer core, enabling it to rotate differently from the rotation of the Earth itself. The spin of the inner core is driven by the magnetic field generated in the outer core and balanced by the gravitational effects of the mantle. Knowing how the inner core rotates could illuminate how these layers interact. However, the speed of this rotation, and whether it varies, is debated.

Researchers Yi Yang and Xiaodong Song analyzed the difference in the waveform and travel time of seismic waves from near-identical earthquakes that have passed through the Earth’s inner core along similar paths since the 1960s. They found that since around 2009, paths that previously showed significant temporal variation have exhibited little change, suggesting that the inner core rotation has paused. They also identified that this might be associated with a reversal of the inner core rotation as part of a seven-decade oscillation with a previous turning point occurring in the early 1970s. The authors indicate that this variation correlates with changes in geophysical observations at the Earth’s surface, such as the magnetic field and the length of day. The reversal of the inner core rotation would shorten the length of the day by a fraction of a millisecond over the course of a year and might have a small effect on Earth's magnetic field but wouldn’t affect life on the surface. 

Earth's inner core rotation may have paused and possibly reversed
Pulsed electromagnetic frequencies

Seismologists' analysis

“In theory, it has been going on for a long time but we only have observations over a few decades,” said Xiaodong Song, a seismologist at Peking University in Beijing and a co-author of the study.

Understanding how the inner core rotates can help scientists figure out how the Earth's different layers interact. Dr. Song and co-author Yi Yang, also a seismologist at Peking University, analyzed seismic waves from similar earthquakes. Dr. Yang said. “It is like we are doing a CT scan for the Earth [when] we have those repeating earthquakes that happen at the same location.” They found that between 2009 and 2020, that rotation stopped and might have reversed direction.

Competing theories

John Vidale, an American-born seismologist who specializes in examining seismograms to explore features within the Earth, thinks there may be other interpretations of the seismic data.

“The changes they noticed are valid although what’s actually happening isn’t so clear,” Dr. Vidale said in an interview with WSJ. “They have a very good analysis and the theory they put in the papers is probably as good as anything at the moment, but there are several competing ideas as well.”

Some scientists theorize that the inner core changes its rotation at shorter intervals than the 70-year cycle described by the new study’s authors. Other scientists' theories include that the inner core rotation may be influenced by the Earth’s surface system, such as the movement of tectonic plates and ocean currents.

The impact of inner core rotation on Earth's surface

This doesn’t mean that the Earth is going to stop spinning. However, the discovery of a possible pause and reversal in the inner core rotation could have significant implications for our understanding of the Earth’s surface system. The inner core rotation is believed to play a role in the formation of the Earth’s magnetic field, which protects life on the surface from harmful solar radiation. It could also affect the length of the day and the Earth's distribution of heat.

These findings have opened up new avenues for research and how we understand the Earth’s surface system. The scientific community will continue collecting data and conducting research as they debate and explore the various theories surrounding this complex and fascinating topic.

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