How is radiometric dating used to determine the age of rocks?

• A. By counting layers of rock buildup
• B. By measuring fossils in the rock
• C. By measuring rock color
• D. By measuring parent/daughter isotopes and applying known half-lives

Answer: (C)

Radiometric dating works by looking at how radioactive isotopes decay over time. A mineral in the rock contains a parent isotope that slowly changes into a daughter isotope at a specific rate, described by its half-life. By measuring how much parent and how much daughter are present today, and knowing the decay rate, we can calculate how long the decay has been happening. This lets us estimate the rock’s age. Examples include uranium-238 turning into lead-206 and potassium-40 turning into argon-40. The method relies on the system staying closed since formation, so no parent or daughter has escaped or been added. Color, how many fossils are in the rock, or how the rock layers look do not give an absolute age.