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Geomagnetism
Shifting compass points

If Earth’s magnetic field reversed today, North on a magnetic compass would switch to South. Scientists know that there have been several hundred reversals in the planet’s history, and some say the field will reverse again soon. However, researchers have a poor understanding of what happens during a reversal event. Knowing how long a reversal takes and the associated changes to the magnetic field could help them better predict when the next reversal will take place. A new study is a step in that direction, as it sheds some light on the processes driving the creation of Earth’s magnetic field.

In the April 8 edition of Nature, Brad Clement, a geologist at Florida International University in Miami, reports that for at least the last four polar reversals, the duration of the event varied depending on latitude. The reversal durations were significantly shorter at low latitudes and longer at middle to high latitudes.

“This is best explained by the presence of a significant non-dipole field,” says Ronald Merrill, a geologist with the University of Washington, Seattle, who wrote an accompanying commentary in Nature. Thus, in addition to Earth’s bar-magnet-like dipole, Earth could have had additional magnetic fields during polar reversals.

Although geologists have widely accepted the existence of magnetic reversals for 40 years, Merrill says, the whole process is very misunderstood. The geologic record documents the instances of Earth’s polarity reversing throughout history, but the reversals seem to occur randomly in time. The shortest interval between reversals was 20,000 to 30,000 years and the longest was 50 million years, Merrill says.

The most recent reversal occurred about 790,000 years ago. After examining magnetic minerals in marine sedimentary cores and exposed terrestrial sequences, Clement reports that this latest reversal took 2,000 years at the equator and 10,000 years at mid-latitude. The minerals recorded the reversal as the sediments formed into rock at the varying latitudes. His research shows similar results across the three previous reversals. Clement chose these four reversals because multiple samples from a wide geographical range were available from those time periods.

One of the problems current models have in estimating duration times, Clement says, is that “different workers have used different criteria.” Thus, he first strictly defined each reversal duration by determining the sequence thickness of reversal transition zones. Clement also limited his research to records that showed a full reversal and transition period and which had sedimentation rate estimates. The resulting average duration of each of the last four reversals shows a strong pattern, Clement says.

There is “a good chance that Clement is correct, within the limits of his working definition for reversal length,” says Robert Coe at the University of California, Santa Cruz. “We need more and higher-resolution reversal records,” he says, but Clement’s findings do add an “interesting generalization” to the current models of reversals. Coe calls the research a “convincing demonstration that Earth’s magnetic field is not dipolar during reversals”; if it were, he says, durations would be independent of latitude.

While Clement’s work is certainly “an important contribution to our understanding of magnetic-field reversals on our planet,” Merrill says that he does not expect it to resolve the controversy associated with reversal transitions. And like Coe, he says that more data are necessary. Clement agrees, saying he already is planning to study older reversals and samples from higher latitudes to further test his model.

Megan Sever

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