Geomagnetic reversals are events where the Earth's magnetic field changes direction, causing the magnetic north and south poles to switch places. These reversals occur over thousands to millions of years and are recorded in the geological layers of rocks, particularly in oceanic crust where magnetic minerals align with the Earth's magnetic field as they cool. The last major reversal, known as the Brunhes-Matuyama reversal, happened about 780,000 years ago. While the exact cause of these reversals is not fully understood, they are believed to be linked to the dynamics of the Earth's outer core, where molten iron generates the magnetic field.