As the two plates move away from each other in a divergent boundary, the magma from the mantle beneath rises to replace the void created. The magma then cools into a new crust. Before the magma cools, the iron minerals align themselves with the magnetic fields of the earth due to their ferromagnetic properties. These cause the rocks after they are cooled to seem to have bands.
Due to magnetic reversals of the earth's magnetic fields (i.e change in magnetic north and south poles) over several hundred thousand years, these bands will orient differently depending on the then earth magnetic polarity when the magma was cooling.
