The Earth´s magnetic field is powerful and protects the planet from incoming radiation (see figure here). If it was not for the magnetic field the compass would not have been invented. The magnetic field has been around for billions of years extending through Earth on both ends. However, it has not been constant but, occasionally the north end has become the south end and vice versa. Such magnetic reversals are paramount for geology as this behaviour is recorded in lavas. And in Iceland there are thousands of lavas, mostly of basalt composition. Such lavas have iron rich minerals, magnetite, that take up the Earth´s magnetic field as they crystallize. Generally, lavas can keep their magnetic signature for many hundred million years. Because, crystallization takes place just after the lava erupts its original magnetic signature can be measured today as either “normal” (N) or “reverse” (R) depending on whether it is like that of Earth´s present magnetic sphere or that of the opposite (reverse). And if you have, for example, a sequence of lavas in a gully, say 30 lavas, and measure these upwards as N – R – N (say 10 lavas each) you come up with a column that can potentially be correlated with the Geomagnetic Time Scale (GTS) that has been studied for Earth and extends back a few hundred million years. However, to make the final step in the correlation with the GTS you need age dating of rocks and thus magnetic measurements and age dating go hand in hand in the correlation process. To measure the lava magnetic signature one can do that in the field with a handheld magnetometer although the final and most reliable magnetic measurements are done in the laboratory, epsecially if the rocks have undergone alterations of hydrothermal fluids.