Lab - Paleomagnetism and Oceanic Expansion

Official Extract:
Plate Tectonics: The Story of a Model (1-B)

Scientific Problematic:
At the end of the 1950s, it was discovered that the Earth's magnetism could be recorded in certain rocks at the time of their formation.
Discoveries of variations in thermal flux (first measurements in the early 1950s) and oceanic topography (early 1960s) led to the idea of oceanic expansion through the accretion of material rising at the axis of mid-ocean ridges.
How did the information from the magnetism of certain rocks validate the hypothesis of an expansion of oceanic floors at the mid-ocean ridges?