November 13, 1997
But Kenneth Creager, an associate professor of geophysics, has also found that the inner core, which has a diameter three-quarters that of the moon, is not as agile as thought. Its rotation rate, relative to the Earth's two outer layers, the crust and the mantle, is four to 12 times slower than previously estimated.
Creager's latest findings on the behavior and structure of the most hidden and enigmatic part of the planet are being published in tomorrow's (Nov. 14) edition of Science.
The new data, he says, is "clear confirmation" that the inner core is rotating at a faster rate than the rest of the planet. "But we have only a snapshot in time and cannot say how fast the inner core was spinning millions or even hundreds of years ago," Creager says.
The inner core's independent rotation is thought to be caused by a process called convection in the molten iron outer core that surrounds the inner core and that produces the Earth's magnetic field. This process is driven in part by the energy transferred as the entire core loses heat to the mantle.
Last year Xiaodong Song and Paul Richards of Lamont-Doherty Earth Observatory in Palisades, N.Y., analyzed travel times of waves generated from South Atlantic earthquakes and recorded over a period of 30 years by a seismographic station in Alaska. They found that the time it takes these waves to pass through the Earth, including the inner core, gradually decreased by three-tenths of a second between the 1960s and the 1990s.
The complex explanation for this, says Creager, is that the solid inner core is anisotropic: its iron crystals are aligned in such a way that they produce a grain like that of wood. Seismic waves flow swiftly with the grain, but slowly against the grain.
Waves traveling roughly parallel to the polar axis on which the Earth spins move about 3 percent faster than those traveling perpendicular to the axis. The fastest direction, however, seems to be tilted about 10 degrees from the spin axis. Song and Richards reasoned that if the inner core rotates faster than the mantle by 1 degree a year, the waves traveling from the South Atlantic to Alaska were more closely aligned with the grain of the inner core during the 1990s than during the 1960s, opening up a faster travel pathway.
The Lamont-Doherty researchers assumed that the alignment of the crystals is the same throughout the inner core. In contrast, Creager, studied waves generated by three earthquakes in the South Atlantic in 1991 and recorded by an array of seismometers in Alaska. This allowed him to take a snapshot of the inner core and produce a detailed map showing a substantial change in speed as the waves traveled across a region of only 300 miles.
Because of this "dramatic" change in speed over such a short distance, says Creager, the inner core does not have to rotate very far to cause a three-tenths of a second change in travel time. He estimates that the inner core is rotating with respect to the mantle at a rate that is four times slower than that calculated by the Lamont-Doherty team.
At the current pace of rotation, Creager theorizes, it would take more than 1,000 years for the inner core to make one compete revolution with respect to the mantle and crust. By comparison, Song and Richards estimated that the inner core has made more than a quarter of a complete revolution just since the beginning of this century.
"Though we do not know what causes the iron crystals to align," says Creager, "new clues are being revealed every year." For example, he cites the recent discovery that crystals in the western hemisphere of the inner core appear to be aligned in a different fashion from those in the eastern hemisphere, and that the formations change over a region as small as a few hundred miles. This may be caused by convection within the inner core.
"The key observation is that something deep within the Earth is changing on the scale of human lifetimes," says Creager. That, he notes, "is something new to seismologists."