The answer is Magnetic polarity of the rock. This is especially true to rocks with iron minerals. Iron is strongly ferromagnetic. The atoms are easily aligned in the presence of a magnetic field. Therefore, as they flow out as lava on the earth’s surface, they align themselves with the earth’s magnetic field. Changes in magnetic field of the earth also affect the orientation of these mineral in these rocks. These rocks are therefore used to determine the polarity of the earth at a particular time.