- The measurement of age in years. The determination of the absolute age of rocks, minerals and fossils, in years before the present, is the basis for the field of geochronology. The measurement of the decay of radioactive isotopes, especially uranium, strontium, rubidium, argon and carbon, has allowed geologists to more precisely determine the age of rock formations. Tree rings and seasonal sedimentary deposits called varves can be counted to determine absolute age. Although the term implies otherwise, "absolute" ages typically have some amount of potential error and are inexact. Relative age, in contrast, is the determination of whether a given material is younger or older than other surrounding material on the basis of stratigraphic and structural relationships, such as superposition, or by interpretation of fossil content.