Radiometric Dating
Explain how radiometric dating works — radioactive isotopes decay at a known rate (half-life), so measuring the ratio of parent to daughter isotope in a rock or fossil gives an absolute age; distinguish between carbon-14 (useful up to ~50,000 years) and uranium-lead (useful for millions to billions of years)
What a learner can do afterwards
- Defines half-life as the time for half the radioactive parent isotope to decay to the daughter isotope
- Explains that the parent:daughter ratio in a sample gives an estimate of absolute age
- Distinguishes carbon-14 (for recent organic material) from uranium-lead or potassium-argon (for deep geological time), explaining why carbon-14 cannot be used for dinosaur bones
The lesson
You already know that rock layers stack up in order, so the bottom layer is older than the top. That tells you which rock is older, but not how many years old it is. Radiometric dating gives you an actual number, like 66 million years, using something inside the rock itself: radioactive atoms.
Start with 100 atoms of a radioactive parent isotope. After one half-life, 50 have decayed into the daughter isotope, so the sample now has 50 parent and 50 daughter atoms. Scientists measure this parent-to-daughter ratio in a rock, and since they know the half-life, they can calculate exactly how long ago the rock formed.
Different isotopes have different half-lives, so each one suits a different age range. Carbon-14 has a short half-life, about 5,730 years, so after roughly 50,000 years almost none is left to measure. That makes it useful only for recent organic remains, not dinosaurs. Uranium-lead and potassium-argon have half-lives of millions or billions of years, so they can measure the true age of much older rock.
Dinosaurs died out about 66 million years ago, so any carbon-14 in their bones decayed away long before now. That is why scientists instead date the volcanic rock layers above and below a fossil using uranium-lead, then use those ages to bracket the fossil's age.
Radioactive atoms decay at a fixed half-life, so the parent-to-daughter ratio in a rock reveals its age: fast carbon-14 covers thousands of years, while slow uranium-lead covers the age of the dinosaurs.
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