Scientists today confirm one of the most surprising and controversial ideas in geophysics - that Earth's solid inner core is spinning slightly faster than the rest of the planet.

Four US seismologists used long-term earthquake records to prove the point. They tracked tremors that occur fairly regularly off the South Sandwich Islands in the South Atlantic Ocean and analysed the seismic traces on the other side of the world in Alaska. The results are published in the journal Science.

Because the seismic waves pass through Earth's centre, the scientists were able to relate the varying seismographic data over time to changes taking place inside the planet.

The effect is not large - the inner core of solid iron is rotating 0.3 to 0.5 degrees per year faster than the molten outer core and mantle, meaning that it would take 900 years to gain one full revolution on the outer planet.

But it could be an extremely important observation for understanding Earth.

"For decades, people thought of the Earth's interior as changing very slowly over millions of years," said Paul Richards, a seismology professor at Columbia University, New York.

"This shows that we live on a remarkably dynamic planet.

"Now we need to understand what is driving these changes."

Xiadong Song of the University of Illinois believes the explanation is electromagnetic: "The magnetic field generated in the [molten] outer core diffuses into the inner core, where it generates an electric current. The interaction of that electric current with the magnetic field causes the inner core to spin, like the armature in an electric motor."

www.earthinstitute.columbia.edu