Radiometric Dating of Earth
Aligned to HS-ESS1-6 — Next Generation Science Standards.
What this lesson teaches
Radioactive elements in rocks decay at a fixed rate called a half-life, acting like built-in clocks. By comparing how much parent isotope has decayed into daughter product, scientists calculate a rock's absolute age and reconstruct Earth's 4.6-billion-year history.
Worked examples
Example 1
Uranium-238 decays to lead-206 with a half-life of 4.5 billion years. A zircon crystal with equal amounts of uranium and lead has gone through one half-life, so it crystallized about 4.5 billion years ago, near the time Earth formed.
Example 2
Carbon-14 cannot date a dinosaur. Its half-life is 5,730 years, so after about 50,000 years there is nothing left to measure.
Example 3
The method assumes no lead escaped or entered. Zircon is used because its crystal structure rejects lead when it forms.
Practice questions
- Observe: The oldest Earth minerals and the oldest meteorites both date to about 4.5 billion years. What does that agreement suggest?
- Model: Draw a decay curve showing a parent isotope halving over one, two, and three half-lives.
- Explain: A rock sample shows three-quarters daughter isotope and one-quarter parent. Determine how many half-lives passed and explain your reasoning.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Datación radiométrica de la Tierra
Los elementos radiactivos en las rocas se desintegran a un ritmo fijo llamado vida media, funcionando como relojes internos. Al comparar cuánto isótopo padre se ha convertido en producto hijo, los científicos calculan la edad absoluta de una roca y reconstruyen los 4.6 mil millones de años de historia de la Tierra.
Ejemplo 1: El uranio-238 se desintegra en plomo-206 con una vida media de 4.5 mil millones de años. Un cristal de circón con cantidades iguales de uranio y plomo ha pasado por una vida media, así que cristalizó hace unos 4.5 mil millones de años, cerca de cuando se formó la Tierra.
Ejemplo 2: El carbono-14 no puede fechar un dinosaurio. Su vida media es de 5,730 años, así que después de unos 50,000 años no queda nada que medir.
Ejemplo 3: El método supone que no entró ni salió plomo. Se usa circón porque su estructura cristalina rechaza el plomo al formarse.
More Space Systems lessons
Teach this lesson today
An AI tutor that guides with questions instead of just giving answers — all K–12 core subjects, every child in the family.
Start free — no card required