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No. 108: Nov-Dec 1996

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The motor of the world*

The innards of our planet, as presently visualized, consist of an inner core of solid iron about 2,400 kilometers in diameter. Surrounding this is a fluid outer core, which in turn is wrapped onionlike by the mantle and outer crust. Earthquakes are always sending seismic waves through these regions and jostling seismometers installed all over the globe. From this wealth of seismic signals, geophysicists have found that the inner core, lubricated by the fluid outer core, rotates about 1.1 per year faster than the mantle and crust. The inner core interacts with the geomagnetic field and is, in effect, like the rotor of a slow, ponderous induction motor. Expanding upon this vision of the earth as a colossal electrical machine, E. Stokstad writes:

"Electric currents of about a billion amps flow across the boundary between the solid inner core and the fluid outer core that lies around it. In the presence of the Earth's magnetic field, these currents generate massive forces that tug on the inner core. And because the outer core has a relatively low viscosity, the inner core can spin freely."

(Stokstad, Erik; "Earth's Heart Is in a Spin," New Scientist, p. 18, July 20, 1996. The basic paper is: Song, Xiadong, and Richards, Paul G.; "Seismological Evidence for Differential Rotation of the Earth"s Inner Core," Nature, 382:221, 1996)

Comments. Awesome as this gigantic natural electric motor may be, it doesn't challenge any paradigms; in fact, it reinforces current notions concerning the origin of the geomagnetic field. The anomalist, however, inevitably asks questions and makes iconoclastic connections.

  1. Why couldn't this planet-size piece of rotating machinery actually be a generator rather than a motor? The kinetic energy of the faster-spinning inner core might actually create the geomagnetic field.
  2. What happens when the geomagnetic field reverses, as it has often done according to the magnetostratigraphic record? Does the earth's motor go into reverse?
  3. If the inner core can slip relative to the crust and mantle, why cannot the crust-plus-mantle slip, too -- perhaps catastrophically? Pole-shift proponents will like this idea!

*Title idea from A. Rand's Atlas Shrugged. Of course, Rand's hero was talking about a different kind of motor.

From Science Frontiers #108, NOV-DEC 1996. 1996-2000 William R. Corliss