New electronic magnetometers developed in the 1950s allowed scientists (who were looking for oil) to map the magnetism of rocks. On land the pattern seemed jumbled, with no meaningful order. But when they mapped the ocean floor, the magnetism was in long strips running parallel to the mid ocean ridges (discovered by Harry Hess). Further, the magnetic strips were the same on both sides spreading out from the ridge. The sea-floor was giant tape-recording of the Earth's global magnetic field - every half million years or so, the Earth's magnetic poles reverse. Nobody knows why. But the last reversal was more than 800,000 years ago.

When molten rock cools and turns solid, the iron crystals will align themselves with the direction of the Earth's magnetic field, becoming weakly magnetised. Many rocks contain iron (Fe) particularly basalt. Do you remember the composition of oceanic crust? Learn more here.

Earth's magnetic field is generated by convection currents in the liquid outer core (which is a good conductor of electricity). This is called dynamo theory and is not completely understood, but recently scientists have modelled what happens to the magnetic field during a pole reversal.