How fast are the tectonic plates moving?

How do magnetic stripes on the sea floor provide evidence that tectonic plates are are moving apart at the mid-oceanic ridges?

• Answer:

  New oceanic crust is being created at the mid-ocean rift zones, a global divergent plate boundary system where uprising magma fills in the spaces being created by the pulling apart of plates. Part of the newly forming crust follows the direction of one plate, and part of the newly forming crust follows the direction of the other plate; therefore, the age of the rock created from the magma has a mirror image on either side of the rift. As new oceanic crust is continuously being formed at the mid-ocean ridges by upwelling molten rock, the direction of Earth's magnetic field is recorded in the rock by the orientation of magnetic minerals, which align themselves in the direction of the magnetic field before the rock completely crystallizes. Earth's magnetic field is known to change its orientation with frequency on a geologic time scale, thus a pattern of stripes of magnetic orientation is formed which can be detected with a magnetometer. The discovery of magnetic stripes on the seafloor have allowed researchers to provide additional proof that new crust is being created, and that magnetic reversals have occurred and will occur in the future. They tell us that over the course of thousands of years, Earth's polarity will reverse itself. The stripes are the magnetic "footprint" of Earth's shifting magnetic field.