Ever felt the Earth move under your feet? It’s a wonder more of us don’t: scientists have suspected for a while that Earth’s inner core rotates at a faster speed than the rest of the planet. Now, a new study published in the journal Science Advances suggests that not only does the inner core spin faster, it changes direction every six years.

There is much we still don’t know about the internal construction of our planet. If you want to know what’s at the bottom of the ocean, you can send a submarine; if you want to know what’s on the Moon, you can send a spaceship.

But contrary to the Victorian imaginings of Jules Verne, you can’t just go on a journey to the centre of the Earth – in fact the furthest human beings have ever managed to drill down below the surface is about 12km. Which, if they were hoping to get all the way to Earth’s inner core, left them with about 5,138km to go.

It is only quite recently that we have had the technological knowhow to determine much about what’s going on under our feet.

Based on density measurements, scientists concluded in the late 18th Century that Earth must have a metal core, but it wasn’t until 1936 that Danish geophysicist Inge Lehmann showed that there must in fact be a molten outer core and a solid inner one, while the now widely accepted idea that the inner core rotates at a faster rate only emerged in the 1990s.

More like this

Among those that have provided evidence to support this idea is John E Vidale, a professor of Earth sciences at the University of Southern California. But now Vidale has gone further, claiming that the speed of this rotation varies over time, and that – as has previously been suggested by others, based on tiny fluctuations in both the length of Earth’s day and the strength of its magnetic field – every six years sees a change in direction.

Vidale came to this conclusion after he and fellow researcher Wei Wang compared data gathered by the USAF’s Large Aperture Seismic Array in Montana following a series of nuclear tests in the period 1969-1974.

Studying the shockwaves from earthquakes, volcanoes and nuclear explosions is one of the primary methods by which scientists can determine the physical constitution of the Earth, as waves passing through different materials will be affected in different ways.

Sure enough, by comparing the data gathered after different explosions, Vidale and Wang determined that there had been a change in spin direction during this period – as predicted by those who had hypothesized such a scenario. This marks the first time any observational evidence for the hypothesis has been found.

“The idea the inner core oscillates was a model that was out there, but the community has been split on whether it was viable,” said Vidale.

“We went into this expecting to see the same rotation direction and rate in the earlier pair of atomic tests, but instead we saw the opposite. The inner core is not fixed – it’s moving under our feet, and it seems to be going back and forth a couple of kilometres every six years.”

Read more about the Earth:


Russell Deeks is a freelance writer with nearly 30 years’ journalism experience, working across the fields of music, technology and science – which, he says, cross over more often than you might think. Despite the drawback of holding a degree in English & American Literature, he has been a regular contributor to BBC Science Focus since 2006.