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Geophysics

A magnetic reversal record

Nature volume 389, pages 678–679 (1997)Cite this article

The Earth's magnetic field has reversed polarity hundreds of times in the past, the most recent, well-documented event (the Brunhes/Matuyama reversal) occurring about 780,000 years ago. The duration of a reversal is generally thought to be between 1,000 and 6,000 years, during which time the average intensity of the field at Earth's surface is estimated to decrease to about a quarter of its usual value1,2. What happens to the direction of the field during a reversal is controversial, and on page 712 of this issue3 Channell and Lehman set out to tackle the issue. They present some of the highest resolution data ever obtained from marine-sediment records of two reversals — the Brunhes/Matuyama, and the one preceding it which occurred about 990,000 years ago.

Earth's main magnetic field is associated with electric currents in the iron-rich outer core, which occupies a depth range between 2,900 and 5,150 km. Nevertheless it is convenient, and for present purposes2 is mathematically justifiable, to describe the field as originating from dipole and nondipole sources at Earth's centre. The dipole source can be pictured as a bar magnet, currently tilted 11° with respect to Earth's rotation axis, and accounts for about 80 per cent of the present field at Earth's surface. The rest essentially comes from the more complex nondipole sources. The geomagnetic poles are the two points on Earth's surface that lie along the axis of the dipole. The field is said to have normal polarity when the north geomagnetic pole lies above 50° N latitude, to have reverse polarity when it lies below 50° S, and to be transitional in between.

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References

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  1. the Geophysics Program, University of Washington, Box 351650, Seattle, 98195-1650, Washington, USA

    Ronald T. Merrill

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  1. Ronald T. Merrill
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Merrill, R. A magnetic reversal record. Nature 389, 678–679 (1997). https://doi.org/10.1038/39482

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