Every few years, new geologic time scales are published, providing the latest dates for major time lines.Older dates may change by a few million years up and down, but younger dates are stable.
For example, Uranium-238, an unstable radioisotope, decays into Lead-206, a stable chemical element, with thirteen intermediate unstable radioisotopes in between (Uranium-238 decays into Thorium-234, which decays into Protactinium-234, and so on to Lead-206).In this case, Uranium-238 is called the "parent" and Lead-206 is called the "daughter." Radiometric Dating Techniques Three Basic Assumptions Radiometric dating (with the exception of Carbon Dating) can only be used to date igneous and metamorphic rock.Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates.A key point is that it is no longer necessary simply to accept one chemical determination of a rock’s age.For example, it has been known since the 1960s that the famous Cretaceous-Tertiary boundary, the line marking the end of the dinosaurs, was 65 million years old.
Repeated recalibrations and retests, using ever more sophisticated techniques and equipment, cannot shift that date. With modern, extremely precise, methods, error bars are often only 1% or so.
Some critics, particularly religious fundamentalists, argue that neither fossils nor dating can be trusted, and that their interpretations are better.
Other critics, perhaps more familiar with the data, question certain aspects of the quality of the fossil record and of its dating.
The fossil record is fundamental to an understanding of evolution.
Fossils document the order of appearance of groups and they tell us about some of the amazing plants and animals that died out long ago.
Age estimates can be cross-tested by using different isotope pairs.