Closed system radiometric dating
Radiometric dating is based on the decay rate of these isotopes into stable nonradioactive isotopes.
Since the half-life of carbon-14 is 5730 years, scientists can measure the age of a sample by determining how many times its original carbon-14 amount has been cut in half since the death of the organism.
Radiometric dating--the process of determining the age of rocks from the decay of their radioactive elements--has been in widespread use for over half a century.
There are over forty such techniques, each using a different radioactive element or a different way of measuring them.
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.