Rapid Variation of Field Strength during a Geomagnetic Polarity Reversal
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New paleomagnetic results show that the ancient magnetic field intensity changed from 4.5 to 8 uT during the cooling of a 5 meter thick lava flow erupted during a geomagnetic reversal 16.2 million years ago. The lava flow occurs near the base of a thick sequence of flows exposed in the Sheep Creek Range, north central Nevada. Earlier study on samples collected in a vertical profile through the flow showed that the top of the flow was baked and remagnetized by overlying lavas. Samples from just below this level and near the flow base, were cooled and magnetized first. Samples from the flow interior cooled and magnetized later after the field direction had changed 80 degrees. We have documented an identical pattern in paleointensities determined from the same samples using a 15 step double-heating experiment. Upper and lower samples yielded ancient field intensities of 6.7-8 uT; those from the flow interior yielded ancient field intensities of 4.5-5 uT. It therefore appears that the field intensity varied from ~ 8uT to ~ 5uT and then back to ~ 8uT as the reversing field rapidly changed direction.