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Science Frontiers ONLINE No. 93: May-Jun 1994 |
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The Giant Crystal At The Heart Of The Earth
Geophysicists have been forced to consider this possibility because of two anomalies:
- Seismic waves from earthquakes pass through the earth's core faster when they travel parallel to the earth's axis than when they travel in the plane of the equator. The transit time difference is 2-4 seconds. Apparently, the earth's core is not perfectly spherical or its properties are different in different directions.
- The natural vibration or "ringing" frequencies of the earth are "split," that is, instead of a series of single "tones" we detect a series of closely paired frequencies. This is symptomatic of a core that is anisotropic; that is, its properties are different in different directions.
J. Tromp, of Harvard, may have de-anomalized both sets of observations with a single theory:
"For the shape of the core alone to explain the observations, he says, the shape of the inner core would have to be very unrealistic. Instead, he claims that the inner core behaves like a giant asymmetric crystal, aligned with the Earth's axis so that seismic waves travel faster in that direction. Tromp's analysis fits neatly with suggestions that the inner core is made of a high-pressure phase of iron in which the atoms are close-packed in hexagons, because such a 'sigma' phase is anisotropic."
But, asks Tromp, how and why did the core material assume this crystalline character?
(Hecht, Jeff; "The Giant Crystal at the Heart of the Earth," New Scientist, p. 17, January 22, 1994.)
Comment. How does this big iron crystal jibe with the dynamo theory of the earth's magnetic field? Could Tromp's "giant asymmetric crystal" produce a permanent magnetic field, thereby forcing the supposed dynamo to play second magnetic fiddle?
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