Every 200,000 years or so, the earth’s poles trade places. Typically it takes several thousand years. But when geologists Scott Bogue of Occidental College and Jonathan Glen of the U.S. Geological Survey examined 15-million-year-old Nevada lava, they found evidence that the planet’s magnetic field shifted several thousand times faster than normal at least once.
When lava cools, it locks away a record of the earth’s magnetic field. Examining lavas that cooled in two consecutive years, Bogue and Glen found the field swung 53 degrees from east to north, about 1 degree a week. They thought they had erred, but more detailed tests confirmed the pattern, which they announced in September. The only other evidence for rapid field change comes from Oregon lava analyzed in 1985.
Bogue thinks the quick shift took place near the end of a millennia-long polarity reversal, when a slow magnetic drift accelerated dramatically for reasons unexplained. “I suspect it’s a very herky-jerky, unsteady process,” he says.
Further study could help geologists understand the turbulent motion of the earth’s liquid core, which generates the magnetic field and may initiate its flips.