Earth's Core Rotates Faster Than Its Crust, Scientists Say

“Extraordinary claims requireextraordinary proof,” said Xiaodong Song, a professor of geology at theUniversity of Illinois at Urbana-Champaign and corresponding author ofa paper to appear in the Aug. 26 issue of the journal Science. “Webelieve we have that proof.”

Earth’s iron core consists of asolid inner core about 2,400 kilometers in diameter and a fluid outercore about 7,000 kilometers in diameter. The inner core plays animportant role in the geodynamo that generates Earth’s magnetic field,and an electromagnetic torque from the geodynamo is thought to drivethe inner core to rotate relative to the mantle and crust.

Thefirst observational evidence for differential rotation was presented in1996 by Song and Paul Richards, a seismologist at the Lamont-DohertyEarth Observatory of Columbia University. For the past nine years, someseismologists have suspected that flaws, or artifacts, in the data wereresponsible for the purported movement.

By comparing historicalseismic waves traversing Earth’s fluid and solid cores, Song and hiscolleagues found compelling evidence for differential rotation of thesolid inner core. The researchers reported observations of 17 sets ofsimilar seismic waves – called waveform doublets – from earthquakesoccurring in the South Sandwich Islands region off the coast of SouthAmerica.

The doublets, which were recorded at up to 58 seismicstations in and near Alaska with a time separation of up to 35 years,allowed the researchers to detect temporal changes along the samplingpaths.

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

Themost likely explanation for why the inner core is rotating at adifferent speed, Song said, is electromagnetic coupling. “The magneticfield generated in the outer core diffuses into the inner core, whereit generates an electric current. The interaction of that electriccurrent with the magnetic field causes the inner core to spin, like thearmature in an electric motor.”

The fluid outer core decouplesthe solid inner core’s movement from the mantle. Because the fluidouter core is not very viscous, frictional drag is small.

“Differentialrotation is a fundamental dynamic process that goes to the heart of theorigin of our planet and how it has evolved,” Song said. “There isstill much to learn about the inner Earth.”

In addition to Songand Richards, co-authors are Illinois graduate students Yingchun Li andXinlei Sun and Columbia graduate student Jian Zhang and researchscientist Felix Waldhauser. The work was funded by the U.S. NationalScience Foundation and the Natural Science Foundation of China.