The earths magnetic field is in constant flux and undergoes a complete reversal in polarity at irregular intervals every several hundred thousand years or so. That much has been known for some time. But scientists have so far been unable to pin down why polarity reversals occur and how long they take.

Endeavoring to throw some light on the mysterious flips, Florida International University researcher Bradford M. Clement took a fresh look at geological data from the four most recent magnetic field reversals. According to his study, published today in the journal Nature, the average duration of a reversal is close to 7,000 years. The analysis further suggests that the timescale of the transition differs at various latitudes. During the last polarity shift, approximately 790,000 years ago, sites close to the equator underwent the 180-degree change over the course of 2,000 years, but the process took closer to 10,000 years in midlatitude regions. Data from the three other reversals also support the idea of latitude dependence, Clement reports, though they are not sufficiently complete to draw a firm conclusion.

Previous estimates of the duration of polarity reversals have ranged from a few thousand to 28,000 years. In the decade since computing power has advanced enough to perform three-dimensional simulations of the earths magnetic field, Clement explains, numerous models have been devised, each yielding very different results. What geologists need to do is "focus the modeling effort somehow," he says. To that end, Clement hopes the results of his analysis may help narrow the variables researchers use as they continue to mine the earth for its geological secrets. --Alla Katsnelson