So the Vine–Matthews–Morley hypothesis says that new crust is created by cooling of magma at mid-ocean ridges, and since the magma goes through it's curie point as it cools, it ends up recording the current polarity of the geomagnetic field. As the magnetic poles flip over time, this means the sea floor contains a bunch of different magnetic field directions, going back and forth and thus recording the historic days of the magnetic reversals.

So...