Is the earth’s inner core stopping its rotation? What does that mean?

A new study published this week suggests that the earth’s inner core – the spinning mass at the centre of the planet – may have stopped rotating.

Scientists have previously hypothesised that the planet’s outer layer spins at a different rate to the inner core, which is suspended within a layer of molten iron. It is thought that the inner core’s relative speed plays a role in maintaining the magnetic fields on planet earth, but the new findings suggest that something is changing beneath our feet.

Seismologists at Peking University in Beijing found that the inner core previously spun faster than the rest of the planet, but stopped doing so sometime in the last 15 years. One of the authors of the study, Xiaodong Song, described the inner core as “a planet within a planet, so how it moves is obviously very important”.

What is the earth’s inner core?

The earth’s core was discovered in 1936 after researchers investigated how earthquakes spread across the planet. Scientists found that the planet has a 7,000km-wide core, which consists of a solid centre surrounded by a layer of molten iron and other elements. The inner core is essentially suspended with a layer of liquid and scientists believe that this allows the centre to spin independent of the rest of the planet.

In 1996 Song was part of a study comparing how earthquakes travel through different parts of the planet. Researchers found that the travel time of seismic waves could change and they surmised that the inner core was rotating at a different speed to the rest of the planet.

They believes that the inner core was moving in a ‘super-rotation,’ i.e. one that is faster than the speed at which the earth’s surface rotates. The difference was only around one-tenth of a degree per year but it was enough to influence how earthquakes travel, and is thought to be related to the earth’s magnetic fields.

Earth’s inner core moves to ‘sub-rotation’ rate

The recent study draws together data collected between 1995 and 2021 and suggests that the inner core’s rotation has been slowing in recent decades. They found that the super-rotation stopped sometime around 2009, but now believe that it could slip into a sub-rotation.

A ‘sub-rotation’ is a rate of movement that is slower than that on the earth’s core. This could be the result of changes in the earth’s magnetic and gravitational forces but further research is needed before that can be confirmed.

So what does that mean for us? The change in the rate is so slight as to be imperceptible, but Nature suggests that it could have an impact on the length of a day if it were to slow the rotation of the earth’s outer core. Beyond any major impact in the short-term, however, keeping track of changes to the earth’s movement is critical to better understanding the planet.

A “long history of continuous recording of seismic data is critical for monitoring the motion of the heart of the planet”, Song wrote in the study.