In the Glatzmaier-Roberts model, a reversal begins with additional north and south poles appearing at the core. These additional poles may not appear at the Earth's surface—at least not initially—though these islands of reversed polarity can weaken the overall magnetic field strength. (One such island has appeared beneath the South Atlantic Ocean.) In this simulation, a weak pole of reversed polarity now exists at the core. This anomaly has not made it to the surface—a compass near this "area" would still point to true north—but the strength of the magnetic field is as much as 30 percent weaker at the surface.
After a short period of instability, the north and south magnetic poles switch polarity. Scientists don't fully understand why this happens.