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
1 · Read
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 work out about 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.
More to read
22.3 Half Life and Radiometric Dating
Some atoms are wobbly and break apart over time. Scientists call this decay. The half life is the time it takes for half of a sample to change into something new. After two half lives, only a quarter of the first atoms remain. Each material has its own fixed half life, like a built in clock.
Rock clocks let us date ancient layers. By measuring how much of the first material is left and how much new material has formed, we can work out the age. Different clocks suit different ages, which is why one method cannot date everything.
21.3 Radioactive Decay
Atoms of the same element can come in heavier and lighter forms. The heavy forms may be unstable and shed bits until they turn into a new element. The starting form is called the parent, and the end form is called the daughter. Count the parents against the daughters and you hold a clock.
Short lived clocks date young things, and long lived clocks date old rocks. Carbon 14 works for recent bones and wood, but it ticks too fast for dinosaurs. Uranium and potassium clocks tick slowly enough for deep time.
Teaching radioactive decay & radiometric dating: an analog activity based on fluid dynamics
You will model decay without any real radiation. Water draining from one cup to another follows the same math as atoms breaking apart. Each minute the flow shrinks by a steady fraction, just like each half life halves the parents. Measure, plot, and watch the curve bend.
Then link the model to real rocks. The cup level stands for parent atoms, and the drained water stands for daughters. Read the ratio and estimate the age.
Explore more
2 · Watch
En anglaisTake it off screen
Where it sits
Learn first
Optional connections
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
24 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.