Earth's magnetic poles swap places every few hundred thousand years.

The process of magnetic reversal is driven by the geodynamo, which is the movement of liquid iron and nickel in the Earth's outer core. As this molten metal flows, it creates electric currents that generate the planet's magnetosphere. Over time, complex fluid dynamics and chaotic shifts in the core's convection currents cause the magnetic field to lose stability and eventually flip polarity.The most recent major reversal is known as the Brunhes-Matuyama reversal. Based on radiometric dating of volcanic rocks and sediment cores, researchers from institutions like the University of Wisconsin-Madison have pinpointed this event to approximately 780,000 years ago. During a reversal, the magnetic field does not disappear completely but becomes much weaker and more complex, often featuring multiple temporary poles.Geologists discover these historical flips through paleomagnetism. When lava cools and solidifies into basalt, magnetite crystals within the rock align themselves with the Earth's current magnetic field. These 'fossilized' magnetic signatures provide a permanent record of the field's orientation at the time of the rock's formation. Studies of the mid-ocean ridges have shown symmetrical 'stripes' of alternating magnetic polarity on either side of the ridge, proving that reversals are a recurring global phenomenon.