This is why ridges are higher than the surrounding crust. As the oceanic crust is pulled away from the spreading center, it cools and sinks. Also, the farther from the spreading center, the older the crust. The Earth's field is a great magnet which switches its polarity often and without warning. As crust forms, it retains a record of the magnetic field at the time.
New lithosphere is created at the central ridge and is spreads away.
Crust acquires the magnetization of the prevailing field and blocks of alternate polarity add or subtract from the ambient Earth's magnetic field resulting in lineated stripes of anomalously high and low values of the total magnetic field. The magnetic field averages to approximately the rotational axis of the Earth. If one plots the location of the magnetic pole as viewed from a continent that is drifting, that pole appears to move. Thus, the apparent polar wander paths of continents record continental drift. The theory of plate tectonics as described above makes a number of testable predictions.