How did scientists discover the age of the rocks on the ocean floor? Which are older? Younger?
The age of basaltic layers can be estimated using radioactive decay of Potassium, Argon. The age of the sedimentary can only be determined relatively.
Sedimentary layers can only be dated using relative methods such as the rules of superimposition and the assumed ages of index fossil evidence. Since the advent of the theory of plate tectonics it is assumed that the sedimentary layers closest to the divergent boundaries are the youngest and the layers further from the divergent boundaries are the oldest.
Samples of the basalt under the sediments can be dated using Potassium, Argon. These dates are only estimates as Argon gas can easily escape the crystal changing the age, as well as Potassium which is very chemically reactive being removed from the crystal. Samples can be assumed to be older or younger based on radioactive dating even if absolute ages are difficult to achieve.
The oldest sedimentary layers of the ocean are destroyed pushed back into the mantle at subduction zones. So the closer a sedimentary ocean layer is to a subduction zone the older the sedimentary layer is .