The rate of decay of the geomagnetic field sets an upper limit of 10,000 years on the age of the earth.
There are two components to the magnetic field of the Earth: a dipole field and a non-dipole field. According to Ronald Ecker,
only the dipole-field strength has been 'decaying' for a century and a half...the strength of the nondipole field (about 15 percent of the total field) has increased over the same time span, so that the total field has remained almost constant. [The] assumption of a steady decrease in the field's strength throughout history is also irreconcilable, of course, with the paleomagnetic evidence of fluctuation and reversals [in the geomagnetic field]. (Ecker 1990:105)
Evidence of the reversals Ecker mentions have been found in the magnetic orientation of rock on the sea floor.
Ecker R. 1990. Dictionary of Science and Creationism. Buffalo: Prometheus.
Last updated: 21 Mar 2008
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