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Which decay series is used to date very old rocks, based on the ratio of a parent isotope to its final stable product?
AUranium-238 decaying into lead-206
BCarbon-14 decaying into nitrogen-14
CIodine-131 decaying into xenon-131
DCobalt-60 decaying into nickel-60
Answer & Solution
Correct answer: A. Uranium-238 decaying into lead-206
1. Carbon-14 dating is useful for organic material but has too short a half-life for extremely old rocks.
2. Rocks are instead dated using the decay of uranium-238, whose decay series ends with lead-206.
3. Comparing the ratio of uranium-238 to lead-206 in a rock reveals how long it has been solid.
4. Iodine-131 and cobalt-60 have half-lives measured in days, far too short for dating ancient rock.
5. So uranium-238 decaying into lead-206 is correct.
_Source: OpenStax Physics (CC BY 4.0), Ch 22 "The Atom", section 22.3 Half Life and Radiometric Dating_
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