Things are moving slower inside the earth than expected
Only little is known about the structure of the Earth's core. According to subsequent theories, it consists of solid, white-hot iron whose small crystals are surprisingly well oriented. Driven by heat currents, the core rotates faster than the rest of the globe - but not quite as fast as geophysicists previously assumed. Kenneth Creager, professor of geophysics at the University of Washington, has discovered that Earth's inner core, about three-quarters the size of the Moon, is not as fast as previous calculations had suggested (Science 14 November 1997).
The new data, he says, clearly confirm that the inner core is rotating faster than the rest of the planet."However, we only have a snapshot from the present day and we cannot judge how fast the inner core was spinning millions or even hundreds of years ago," says Creager.
The independent rotation of the inner orifice is believed to be due to convection within the molten outer core, a process that also creates the Earth's magnetic field. Convection is driven in part by the energy loss of the entire core to the mantle. But how can you measure the speed at which the earth's interior rotates?
Last year, Xiaodong Song and Paul Richards of the Lamont-Doherty Earth Observatory studied the propagation velocities of waves generated by earthquakes in the South Atlantic and recorded by a seismographic station in Alaska. They evaluated the data from the last 30 years and found that the waves needed three tenths of a second less time to travel through the earth including its inner core. According to Creager, the explanation lies in the fact that the solid inner core is anisotropic: its iron crystals are aligned in such a way that they produce a grain similar to that in wood. Seismic waves travel quickly with the grain but slowly across it. Waves traveling roughly parallel to the Earth's axis are about 3 percent faster than waves traveling perpendicular to the axis. However, the fastest direction appears to be tilted about ten degrees away from the axis of rotation. Song and Richards concluded that when the inner core rotates one degree per year faster than the mantle, the waves traveling from the South Atlantic to Alaska were more aligned with the grain of the inner core in the 1990s than in the 1960s, so they are moving faster today than they used to.
The Lamont-Doherty researchers assumed that the orientation of the crystals is the same throughout the inner core. In contrast, Creager studied waves generated by three 1991 South Atlantic earthquakes and recorded by a series of seismometers in Alaska. This gave him a kind of snapshot of the inner core, from which he could make a detailed map. This shows a substantial change in the speed at which the waves traveled a distance of just 300 miles.
Because of the relatively large variations in velocity over such a short distance, Creager believes that the inner core need not rotate very far to produce the three-tenths of a second time difference that his colleagues have observed. He estimates that the inner core is rotating relative to the mantle four times slower than calculated by the Lamont-Doherty team.
At the current rate of rotation, Creager says it would take more than 1000 years for the inner core to complete a full rotation relative to the mantle and crust. By comparison, Song and Richards estimated that the inner core has already completed more than a quarter of a full rotation since the beginning of this century.
"The most important observation is that changes in the Earth's interior occur over the course of a human lifetime," enthuses Creager. He is convinced that "this is something new for seismologists."
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