How is age estimated in radiometric dating?

• A. Comparing current rock color to modern rocks
• B. Measuring parent-daughter ratios and using half-lives
• C. Observing fossil succession only
• D. Analyzing magnetic polarity

Answer: (B)

The key idea is that age comes from the steady rate of radioactive decay. By measuring how much parent isotope remains and how much daughter isotope has formed, you can work backward to the time that has passed because the decay rate is known and constant. The ratio of daughter to parent changes in a predictable way as time passes, governed by the decay constant (related to the half-life). Using the measured ratio and the known half-life, you solve for time elapsed to get the age since the mineral or rock formed. In practice, scientists check that the system has stayed closed (no loss or gain of isotopes) and may use multiple samples or the isochron method to account for any initial daughter present, which helps ensure a reliable age.

Color comparisons don’t provide a time from decay, fossil succession gives relative ages based on the order of appearance of organisms, and magnetic polarity records Earth's field reversals but does not by itself yield a precise numerical age without radiometric data.