Describe how paleomagnetism is

Used to calculate the rate of sea floor spreading?

Every 50,000 years or so, the magnetic poles of the Earth reverse. As basalt is erupted from sea floor spreading centers, the iron in the basalt aligns itself to the magnetic pole orientation. Paleomagnetism is simply studying the magnetic orientation of iron in old rocks. So, if you have a section of sea floor and you find three zones that record magnetic pole reversals, you know that by the rate of those reversals you are looking at roughly 150,000 years of time.