Rocks become magnetized according to the direction of the Earth’s field at the time of their formation. As the Earth’s plates drift with time, the direction of the remanent magnetism (q.v.) changes and consequently the apparent location of the magnetic pole. The polar wandering curves for different plates differ; see Figure P-8a. The magnetic poles also move with time in addition to plate movement effects, producing secular changes, including the westward drift of the magnetic pole; see Figure P-8b.