Earth's wobbles reveal deep secrets

The boundary between the Earth's core and the mantle is an active one

By BBC News Online science editor Dr David Whitehouse

Tiny deviations, less than a millimetre in size, in the predicted wobble of the Earth's axis are providing clues to what happens 2,900 km (1,800 miles) beneath us, at the boundary between the planet's mantle and its iron core.

A new idea suggests that iron-rich sediments are floating to the top of the Earth's core where they then settle at the base of the mantle.

The sediments create a drag that throws the Earth's wobble off by a small but detectable millimetre or two over a period of about 18.6 years.

"The wobble is explained by metal patches attached to the core-mantle boundary," says Raymond Jeanloz, professor of geology and planetary science at the University of California, Berkeley, US.

"As the outer core turns, its magnetic field is deflected by the patches and the core material gets slowed down, just like mountains rubbing against the atmosphere slows the Earth down," he added.

Seismic slowing

The theory of core-mantle drag was first proposed a decade ago by Bruce Buffett of the University of British Columbia. It also helps to explain the observed slowing of seismic waves that traverse the core-mantle boundary.

The Earth's major wobble is caused by the Moon and Sun tugging on the Earth's bulging equator, creating an effect that lasts 25,800 years called precession. Other processes create smaller wobbles called nutations.

The principal components of the nutation are caused by the Earth's annual circuit of the Sun and the 18.6 year precession of the Moon's orbit.

While these nutations have been studied for many years, extremely precise measurements of the pointing direction of the Earth's axis have revealed unexplained deviations from the predicted nutation.

'Pulled and tugged'

The origin of this effect may lie at the boundary between the mantle, made up of viscous rock that extends 2,900 km (1,800 miles) below the crust, and the outer core, which is thought to be liquid iron with the consistency of water. The inner core, made of very pure, solid iron, rotates along with the outer core, dragging the Earth's magnetic field with them.

"The Earth is getting pulled and tugged at regular periods, but we observe a difference in the way the Earth responds to these tugs and pulls and what we predict," Buffett said.

The sediment layer would have to be less than a km (half a mile) thick in order to have the observed effect, and would probably cover only patches of the outer core.

"Think of it as a fuzzy boundary between the mantle and the core, with patches perhaps 10 to 20 km (6 - 12 miles) across and up to a 1,000 metres (3,280 ft) thick," Jeanloz said.

Scientists speculate that the silicates stuck to the mantle might also be caught up in mantle convection and carried to the surface, accounting for reports of core material in lava erupting from hot, spot plumes, like those under Hawaii.