how long ago was the last time earth's magnetic field reversed
Geomagnetic reversal The time spans of chrons are randomly distributed with most being between 0.1 and 1 million years[citation needed] with an average of 450,000 years. Most reversals are estimated to take between 1,000 and 10,000 years. The latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago, and may have happened very quickly, within a human lifetime.[1]
Earth's magnetic field The intensity of the field is often measured in gauss (G), but is generally reported in nanoteslas (nT), with 1 G = 100,000 nT. A nanotesla is also referred to as a gamma (γ).[13]  The tesla is the SI unit of the magnetic field, B. The Earth's field ranges between approximately 25,000 and 65,000 nT (0.25–0.65 G). By comparison, a strong refrigerator magnet has a field of about 10,000,000 nanoteslas (100 G).[14]