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 example
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.
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: 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.
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