Earth spins faster inside than out

An analysis of earthquake waves ends a 9-year-old debate about Earth's core.
By | Published: August 26, 2005
Earth from Apollo 17
The Apollo 17 crew captured this view of Earth during their journey to the moon. It was the first time an Apollo crew had the opportunity to photograph the south polar cap. The photo extends from the Mediterranean Sea to Antarctica. The Arabian Peninsula can be seen at the northeastern edge of Africa. The Asian mainland is on the horizon toward the northeast.
August 26, 2005
Scientists have shown Earth’s core spins faster than its surface, proving a long-suspected notion that our planet’s interior rotates more like a fluid than a solid. The speed-up, somewhere between 0.3º and 0.5º per year, is small but real.

Xiaodong Song, a geology professor at the University of Illinois, Urbana-Champaign, and Paul Richards of Columbia University’s Lamont-Doherty Earth Observatory presented the first evidence for Earth’s fluid-like, or differential, rotation in 1996. But some seismologists, suspecting limitations in the data created the appearance of motion, weren’t convinced.

The new study by Song, Richards, and colleagues, published today in the journal Science, ends this debate. “Extraordinary claims require extraordinary proof,” says Song. “We believe we have that proof.”

Earth’s iron core consists of two layers: a solid inner core about 1,500 miles (2,400 kilometers) across and a fluid outer core about 4,350 miles (7,000 km) wide.

The team compared seismic waves from historical earthquakes that traversed Earth’s fluid and solid cores over the last 35 years. The researchers examined 17 sets of similar seismic waves — called waveform doublets — from earthquakes occurring in the South Sandwich Islands region off the coast of South America.

The doublets, which were recorded at 58 seismic stations in and near Alaska, allowed the scientists to detect time delays experienced by the waves as they passed through Earth. These delays provide compelling evidence for differential rotation of the solid inner core.

“The similar seismic waves that passed through the inner core show systematic changes in travel times and wave shapes when the two events of the doublet are separated in time by several years,” Song explains. “The only plausible explanation is a motion of the inner core.”

The most likely explanation for why the inner core rotates at a different speed, Song says, is electromagnetic coupling.

The inner core plays an important role in driving the dynamo that generates Earth’s magnetic field. “The magnetic field generated in the outer core diffuses into the inner core, where it generates an electric current,” he explains.

The team suggests some sort of electromagnetic twist created by the geodynamo forces the inner core to spin relative to Earth’s mantle and crust. “The interaction of that electric current with the magnetic field causes the inner core to spin, like the armature in an electric motor,” Song says.

The fluid outer core creates little frictional drag, insulating the solid inner core from the motion of Earth’s mantle.

“Differential rotation is a fundamental dynamic process that goes to the heart of the origin of our planet and how it has evolved,” Song says. “There is still much to learn about the inner Earth.”