Final answer:
The numerical age of rocks is determined through radiometric dating, which uses radioactive clocks to measure the decay of isotopes like uranium-238, obtaining accurate ages for rocks.
Step-by-step explanation:
Numerical ages for rocks are calculated from radioactive clocks. Using radiometric dating, geologists can determine the numerical age of rocks, minerals, and some fossils by analyzing the ratios of parent and daughter isotopes of radioactive elements.
This method calculates how many half-lives have passed since the rock or fossil formed by measuring the rate of decay. Radiometric dating assumes there were no daughter atoms at formation and that the system remained closed. While uranium-238 is used to date rocks, carbon-14 is used for organic materials through a similar process known as radiocarbon dating.