Geomagnetic field flip-flops in a flash

Just north of a truck stop along Interstate 80 in Battle Mountain, Nev., lies evidence that the Earth’s magnetic field once went haywire.

Magnetic minerals in 15-million-year-old rocks appear to preserve a moment when the magnetic north pole was rapidly on its way to becoming the south pole, and vice versa. Such “geomagnetic field reversals” occur every couple hundred thousand years, normally taking about 4,000 years to make the change. The Nevada rocks suggest that this particular switch happened at a remarkably fast clip.