Volcanic and other such activity at the mid-Atlantic ridge extrudes material to fill the gap between separating tectonic plates associated with continental drift. The magnetization of rocks is found to reverse in a coordinated manner with distance from the ridge. What does this imply about the Earth’s magnetic field and how could the knowledge of the spreading rate be used to give its historical record?
When the molten lava comes out from a volcano, it gets solidifies into a rock and its gets magnetized is in the direction of the local magnetic force in that area at that time when it cools down.
So, if a volcano produce many lava flows over a past period than the magnetization of the various flows can be analyzed...