Geologic Age

the age of rocks. A distinction is made between absolute and relative geologic ages. Absolute geologic age is the age of rocks as expressed in absolute time units; it is established on the basis of studying the disintegration of radioactive elements (uranium, thorium, potassium, rubidium, and others) contained within minerals. It is usually estimated in millions of years. The term is employed conditionally, since none of the numbers obtained are “absolute,” and absolute age is often given in the first approximation (with a minimal error of ± 5 percent). Relative geologic age is the age of rocks established on the basis of the positions of strata in a cross section with respect to one another. If the strata are dipping, the lower ones are older, and the upper ones are younger (the sequential law of stratification). Comparison of sedimentary layers from regions that are distant from each other has made possible the creation of a general stratigraphic scale, subdivided into a number of segments (systems), which are characterized by specific complexes of plant and animal remains. By analyzing the fossils found in strata, one can identify the place of deposits on the general scale, that is, determine the relative geologic age.