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The ocean plates spread and grow in opposite directions, so rocks that are equidistance from the center have the same magnetic polarity and age.That is, until they get subducted and recycled under less-dense oceanic or continental crust.
The lone outlier (the bright splash of purple that you see north of Africa) to all of this is the Mediterranean Sea.The two approaches are often complementary, as when a sequence of occurrences in one context can be correlated with an absolute chronlogy elsewhere.Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled.A coin, vessel, or other common artifact could link two archaeological sites, but the possibility of recycling would have to be considered.
It should be emphasized that linking sites together is essential if the nature of an ancient society is to be understood, as the information at a single location may be relatively insignificant by itself.(You may have heard this fact before, and while true, there is a logical explanation as to why.) Seafloor mapping, in its earliest, most primitive form, consisted of lowering weighted lines and measuring how far the sunk.This was done mostly to determine near-shore hazards for navigation.Every so often (it has occurred over 170 times over the past 100 million years), the poles will suddenly switch.