How are the reversals of the earth's magnetic field recorded on the seafloor?
In the polarity of the rocks.
The seafloor is spread by a mid-ocean ridge. Rocks farther away are older, and rocks closer to the ridge are newer. As the magma from the ridge cools, some of it's molecules orient themselves with the magnetic field. When the field reverses, the polarity of the rocks also reverses. By interpreting the width of bands of differently polarized rock and knowing the rate at which it spreads, we can determine how long magnetic reversals lasted.