Last magnetic pole reversal

Earth's magnetic field is generated by moving molten iron in the outer core, and every few hundred thousand years, on average, it weakens, wanders and flips, so that a compass would point south. The last full reversal, called the Matuyama-Brunhes reversal, took place about 773,000 years ago. The record is locked in volcanic rocks and sediments, whose magnetic minerals line up with the field as they form. The names honor Bernard Brunhes, a French physicist who in 1906 found rocks magnetized in the opposite direction, and Motonori Matuyama, a Japanese geophysicist who tied such reversals to age in 1929.

The flip is not instant. Studies suggest the field weakened and grew complicated for thousands of years during the change. The reversal gives scientists a worldwide timestamp: any rock or sediment that records the switch can be matched to the same moment, wherever it lies. Archaeologists use it to date early human sites, such as Gran Dolina in Spain, where the layers of the Homo antecessor fossils lie below the boundary and so are older than 773,000 years. Because the field also shields the planet from charged particles from space, reversals have attracted attention, but no mass extinction has been tied to one.