SOME 16 million years ago, north became south in a matter of years. Such fast flips are impossible, according to models of the Earth’s core, but this is now the second time that evidence has been found.
The magnetic poles swap every 300,000 years, a process that normally takes up to 5000 years. In 1995 an ancient lava flow with an unusual magnetic pattern was discovered in Oregon. It suggested that the field at the time was moving by 6 degrees a day – at least 10,000 times faster than usual. “Not many people believed it,” says Scott Bogue of Occidental College in Los Angeles.
Now Bogue and his colleague Jonathan Glen of the United States Geological Survey in Menlo Park, California, say they have found a second example in Nevada. The lava rock suggests that in one year, Earth’s magnetic field shifted by 53 degrees (Geophysical Research Letters, DOI: 10.1029/2010GL044286). At that rate, a full flip would take less than four years, but there could be another interpretation. “It may have been a burst of rapid acceleration that punctuated the steady movement of the field,” says Bogue.