Paleomagnetism is the study of the earth's past magnetic field. Paleomagnetism is one of the strongest pieces of evidence to support Plate Tectonics. Evidence has shown, for example, that some rocks in Alaska have magnetic minerals oriented in such a way that they must have been laid down at or near the equator.
The drills brought back rocks from the bottom of the ocean that we had no way of accessing before. Scientists used radioactive dating to figure out how long ago the rocks formed. They eventually realized that the further you go away from the mid-ocean ridge , the older the rocks get. So the Atlantic Ridge is formed from new rock under the Mantel.