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.The main point is that the ages of rock formations are rarely based on a single, isolated age measurement.On the contrary, radiometric ages are verified whenever possible and practical, and are evaluated by considering other relevant data.
Unbeknownst to the scientists engaged in this controversy, however, geology was about to be profoundly affected by the same discoveries that revolutionized physics at the turn of the 20th century.
They observed that every rock formation, no matter how ancient, appeared to be formed from still older rocks.
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.
Second, the rock or mineral must not lose or gain either potassium or argon from the time of its formation to the time of analysis.
By many experiments over the past three decades, geologists have learned which types of rocks and minerals meet these requirements and which do not.
For example, a method based on a parent isotope with a very long half-life, such as C method can only be used to determine the ages of certain types of young organic material and is useless on old granites.