Final answer:
The Earth's magnetic field last reversed roughly 780,000 years ago, a phenomenon that typically occurs every 200,000 years due to the movement of liquid metals in the planet's outer core.
Step-by-step explanation:
The last time Earth's magnetic field reversed was approximately 780,000 years ago, an event known as the Brunhes–Matuyama reversal.
Earth's magnetic field flips direction roughly every 200,000 years, which is part of a complex process involving the flow of liquid metals in Earth's outer core that generate electric currents and thus the magnetic field. Currently, we experience what is called normal polarity, with the magnetic north pole near the geographic north pole. However, during reversed polarity, the magnetic north pole would be near the geographic south pole. The exact mechanism driving these reversals remains an area of ongoing research, as scientists strive to understand the reasons behind the magnitude and frequency of the fluctuations in the magnetic field.