Why must geologists establish relative ages before choosing rocks for isotopic dating?

Geologists must establish relative ages before conducting isotopic dating because most rock units cannot be directly dated using absolute methods. Establishing a relative sequence is an essential prerequisite for any precise geochronological program, as it allows geologists to identify the most favorable units for isotopic analysis.1

By first determining the relative order of strata through visual observations and logical deductions—such as the principle of superposition or crosscutting relationships—geologists can place undatable rocks into a broader temporal context. This process ensures that even units that lack suitable minerals for isotopic dating can be assigned a meaningful age based on their position relative to a datable unit. Furthermore, because isotopic dating is complex and expensive, relative dating provides a necessary framework to integrate data from widely spaced outcrops, ensuring that the resulting absolute ages are applied to the correct geologic events.123