The earth’s magnetic field polarity reverses roughly every 700,000 years. Magnetite crystals caught up in basaltic lava flows from the mid-Atlantic ridge align with the polarity of the magnetic field at the time they are deposited. Geologists collecting cores of the basalts leading away from the rift see parallel bands of basalt with alternating polarity. Radiometric dating of the rocks indicates that the bands can be correlated and demonstrate the plate movement away from the rift system.