Earth's core may start rotating in reverse

New research published on January 23 in the journal Nature Geoscience reveals that changes in the rotation of the Earth's inner core could occur within the next few decades.

In the mid-1990s, Xiaodong Song of the SinoProbe Laboratory at the School of Earth and Space Sciences at Peking University, China, in collaboration with Paul Richards of Columbia University, provided the first evidence of independent rotation of the inner core. They found that the inner core rotates from west to east, although slightly faster than the other rigid layers such as the mantle and crust. This rotation is mainly driven by magnetic and electric effects within the surrounding liquid outer core , as well as gravitational interactions with the mantle.

Simulation of cross-section of the Earth's layers. (Photo: iStock)

But after that discovery, there were still some unresolved questions about the direction of the core's rotation, taking into account factors such as rotation speed. Such questions prompted Song and his colleague Yi Yang, also at SinoProbe, to collect more data over longer periods of time to test different models, leading to the latest prediction. In the newly published study, they analyzed seismic waves generated by natural earthquakes that have traveled through the Earth's core since the 1960s.

"Seismic waves from two repeat earthquakes often have similar waveforms coming from the same station," the team said. " However, when waves from repeat earthquakes interact with the Earth's inner core, they exhibit different waveforms and arrival times because they reflect different structures in the inner core."

Using seismic waves, the team was able to deduce the inner core's rotation pattern over the past few decades, revealing new details about the process and how it relates to the Earth's other layers. Their findings show that the inner core rotated faster than the Earth's mantle and crust in a west-to-east direction from the early 1970s until about 2009. Then, the rotation seemed to pause between 2009 and 2011. Since then, the rotation has appeared to gradually reverse direction. These changes are likely part of a seven-decade oscillation.

The Earth consists of several layers. The thin outer layer is the crust, which is mostly solid rock and is about 32–48 km thick in continental areas, although the oceanic region averages about 6.4 km in thickness. Beneath the crust is the mantle, which is about 2,900 km thick and makes up 84% of the Earth's total volume. This layer is composed of rocky material that is denser than the crust and is mostly solid, although rocks in some areas are molten under high pressure.

Beneath the mantle is the Earth's core, which is divided into an inner core and an outer core. The outer core is about 2,253 km thick and is mostly liquid iron and nickel. At the center of the Earth is the extremely dense inner core, about 1,207 km thick and made up of iron, small amounts of nickel, and many other elements. There are two main forces that influence the inner core. The first is electromagnetic force. The Earth's magnetic field is created by the movement of the liquid in the outer core. The magnetic field causes the inner core to spin. The other force that influences the inner core is gravity. The inner core and the mantle have significantly different physical properties, so the gravitational force between the two structures tends to pull the inner core toward equilibrium.

'If the two forces are not balanced, the inner core will either speed up or slow down,' the researchers said. 'The inner core's 70-year oscillation cycle is likely influenced by electromagnetic and gravitational forces.' The oscillation corresponds to periodic changes in other geophysical observations, such as changes in the magnetic field or the length of the day.

However, Song and his colleagues note that there are some limitations to the study, including the limited time period of available seismic data. "We have not observed the full cycle of the 70-year oscillation with data collected over less than six decades. Modern digital seismic stations began to be deployed globally in the 1990s. Many ancient seismic data are recorded on paper, but they are quite scarce and difficult to access," the team shared.