1.   As ferrous metals cool from a melt, they record the magnetic dominance of that moment.

2.   But over the eons, magnetic dominance has flipped many times to the south pole, a change called geomagnetic reversal.

3.   Since the dates of magnetic reversals are known from other studies, researchers can date a rock by determining its magnetic dominance.

4.   Since dates of magnetic reversals are known from other studies, researchers can date a rock by determining its magnetic dominance.

5.   When ferrous metals cool after melting, they record the magnetic dominance at that moment.

6.   When ferrous metals cool from a melt, they record the magnetic dominance at that moment.