Scientists at Columbia University's Lamont-Doherty Earth Observatory have found that the Earth's inner core is rotating faster than the planet itself.
The motion of the inner core has never before been detected or measured. The finding, reported July 18 in the journal Nature, will likely advance understanding of how the Earth's magnetic field is created and why it reverses periodically; how heat flows through the planet, and how the Earth's multi-layered interior has evolved.
The inner core rotates in the same direction as the Earth and slightly faster, completing its once-a-day rotation about two-thirds of a second faster than the entire Earth. Over the past 100 years that extra speed has gained the core a quarter-turn on the planet as a whole, the scientists found. Such motion is remarkably fast for geological movements -- some 100,000 times faster than the drift of continents, they noted. The scientists made their finding by measuring changes in the speed of earthquake-generated seismic waves that pass through the inner core.
The research was conducted by Xiaodong Song and Paul G. Richards, seismologists at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y. Dr. Song is the Storke-Doherty Lecturer and Dr. Richards the Mellon Professor of Natural Sciences at Columbia.
"For decades, the motion of the inner core has been the realm of theoreticians," Dr. Richards said in an interview. "For the first time, we have a hard piece of observational evidence, an actual measurement, of what's happening down there."
The discovery of such a fundamental property will advance planetary understanding, the scientists said. It will spark new research to explain the observed pattern of changes in Earth's magnetic field, including the way the north and south poles have "wandered" and reversed periodically over Earth's history. It will yield new knowledge about temperatures at the center of the Earth and the flow of planetary heat that ultimately drives the motions of tectonic plates at Earth's surface to create mountains and oceans, split continents and cause earthquakes.
The Lamont-Doherty researchers' findings culminate a period of rapid discovery about the inner core. In the late 1980s and early 1990s, several scientists, including Dr. Song when he worked with colleagues at the California Institute of Technology, observed that seismic waves traveling through the inner core along a roughly north-south pathway moved faster than those traveling closer to the equator. The scientists theorized that the enormous pressure in the Earth's core, which is millions of times higher at the inner core surface than the atmospheric pressure at the Earth's surface, has aligned iron atoms in a distinctive crystalline structure that slows down waves in some directions and creates a "fast track" for seismic waves in a near north-south direction. More recent studies also indicate that the "fast-track axis" is not exactly north-south but is tilted slightly from the axis around which the Earth is rotating. The core's "fast axis" would emerge at an imaginary point on Earth's surface up to 10 degrees from the north pole, where the spin axis would emerge.
The Earth and the core are rotating on the same spin axis, but because the inner core rotates just a bit faster than the planet as a whole, the "fast axis" through the core moves eastward. Over the years, it traces a circular path around the north pole and moves to different positions relative to the Earth's mantle and crust. This basic feature allowed the Lamont scientists to make their discovery.
Dr. Song and Dr. Richards studied seismic waves from 38 earthquakes that occurred between 1967 and 1995 near the South Sandwich Islands at the bottom of the globe. They measured the speed of waves that traveled up through the inner core to receiving seismographs in Alaska at the top of the globe and found that the waves arrived about 0.3 seconds sooner in the 1990s than they did in the 1960s.
The speed of the waves steadily increased over the period because the inner core's "fast axis" -- the fastest possible route through the inner core -- was gradually becoming more closely aligned with the actual pathway traveled by the waves between South Sandwich and Alaska, the scientists said. The change in the waves' speed showed that the fast axis was in motion relative to the Earth, proving that the core is spinning faster than the Earth, they said.
As added proof, the Lamont-Doherty scientists measured travel times of seismic waves from earthquakes at the Kermadec Islands near New Zealand that were received in Norway. These waves took longer to travel through the inner core in the 1990s than they did in the 1980s because the core's fast axis had moved away from the seismic wave pathway between Kermadec and Norway over that period.
The research was funded by the National Science Foundation. The NSF also has supported Lamont-Doherty's archive of long-term, worldwide seismological records, which hold key data for measuring the inner core's rotation.
Dr. Song and Dr. Richards were intrigued by preliminary results last fall from a theoretical computer model simulating the dynamo that creates Earth's magnetic field. The model, created by Gary Glatzmaier of the Los Alamos National Laboratory and Paul Roberts of the University of California at Los Angeles, suggested that the Earth and its inner core may rotate at different rates.
To prove the theory, the Lamont-Doherty seismologists identified a pathway that aligned closely with the inner core's fast axis. On one end was a seismic monitoring station; on the other, a quake-prone region that consistently generated many seismic waves to study. They compared the arrival times of seismic waves that traveled through the inner core and those that passed merely through the outer core. The latter waves remained constant over the 28-year span, but those that traversed the inner core arrived slightly but steadily sooner over the three decades, they found.
Dr. Song and Dr. Richards calculated that over a year, the inner core rotates about one longitudinal degree more than the Earth's mantle and crust. The inner core makes a complete revolution inside the Earth in about 400 years.
The Earth's core was formed very early in Earth's history as heavier molten iron sank toward the center of the planet. As the Earth cooled and dissipated its internal heat toward the surface, some molten iron began to solidify to create the dense, solid inner core at the center of the planet. Enormous pressure keeps the inner core solid in a region of temperatures in the range of 7,000 degrees Fahrenheit and possibly much higher.
Fluid iron in the outer core has continued to solidify at the boundary between the two cores, so that over a billion years, the inner core has grown steadily to its present diameter of 1,500 miles. (The inner and outer core together are 4,350 miles wide and the Earth's diameter is about 7,900 miles.) The electrical currents and convecting movements within the Earth's liquid core generate the planet's powerful magnetic field, which in turn drives the inner core in the way that can now be observed.
About a billion amps of current is flowing into and out of the inner core, across the boundary between the inner and outer cores. This current, in the presence of a magnetic field, results in forces being applied to the inner core, which then rotates. Essentially, the inner core rotates because it is part of a vast electric motor.
The inner core has a diameter about three-quarters that of the moon, and a mass density almost 13 times greater than the density of water. The mass of the inner core is about one hundred million million million tons -- which is about 30 per cent greater than the mass of the moon.