Compasses would have pointed south for much of Earth's history.
Earth's magnetic north and south poles swap places, at random, every few hundred thousand years on average. The last full flip was about 780,000 years ago. We know because volcanic rock and the seafloor record the field like a tape recorder, in stripes of alternating direction.
Earth's magnetic field is generated by molten iron churning in the outer core, a process known as dynamo action. Its polarity is not fixed. There have been at least 183 reversals over the last 83 million years, around one every 450,000 years on average, though the timing appears random. The most recent, the Brunhes–Matuyama reversal, happened about 780,000 years ago. Estimates of how long a flip takes vary widely, from a couple of thousand years to around 12,000, and possibly far longer.
The first hints came in the early 20th century, when Bernard Brunhes noticed volcanic rocks magnetised in the opposite direction to the modern field. In the late 1920s Motonori Matuyama found that the reversed rocks were all early Pleistocene or older. Better radiometric dating in the 1950s let Allan Cox, Richard Doell and Brent Dalrymple build the first magnetic polarity timescale in 1959.
Then came a spectacular confirmation. Ships towing magnetometers found regular stripes of alternating magnetism running along the ocean floor. In 1963 Frederick Vine and Drummond Matthews explained them: new seafloor forms at mid-ocean ridges and freezes in whatever field exists at the time, so spreading creates mirror-image stripes on either side. The Canadian Lawrence Morley reached the same idea independently, but Nature and another journal rejected his paper. The stripes became key evidence for seafloor spreading and plate tectonics.
The rhythm is irregular. At times the field flipped several times in a million years; at others it held steady for tens of millions, in 'superchrons' such as the Cretaceous Normal Superchron, which lasted about 37 million years. Scientists have looked for links between reversals and mass extinctions, but statistical analysis finds no correlation. Computer simulations of the core can reproduce reversals on their own, suggesting no outside trigger is needed.
Source: Wikipedia — Geomagnetic reversal · Text summarised from Wikipedia (CC BY-SA 4.0)
