Radioactive isotopes used carbon 14 dating
My purpose here is not to review and discuss all of the dating methods in use.
Instead, I describe briefly only the three principal methods. These are the three methods most commonly used by scientists to determine the ages of rocks because they have the broadest range of applicability and are highly reliable when properly used.
Comparing these rocks with the products of present erosion, sedimentation, and earth movements, these earliest geologists soon concluded that the time required to form and sculpt the present Earth was immeasurably longer than had previously been thought.Sometimes these decay schemes are used individually to determine an age (e.g., Rb-Sr) and sometimes in combinations (e.g., U-Th-Pb).Each of the various decay schemes and dating methods has unique characteristics that make it applicable to particular geologic situations.Radiometric dating is based on the decay of long-lived radioactive isotopes that occur naturally in rocks and minerals.
These parent isotopes decay to stable daughter isotopes at rates that can be measured experimentally and are effectively constant over time regardless of physical or chemical conditions.
The main point is that the ages of rock formations are rarely based on a single, isolated age measurement.