In the 1960s Hess proposed that the crust separates along the rifts in the mid ocean ridges and that new seafloor forms by upwelling of hot new crust into these cracks. Magnetic anomaly patterns on the sea floor provide evidence. They are peculiar banded magnetic patterns found on the sea floor. By using the age of magnetic reversals they could calculate the rate of seafloor spreading using the equation speed=distance/time. Another line of evidence is the fact that rocks closest to the spreading ridge are younger and older rocks are further away.
