---
title: "Deep Time: Radiometric Dating and the Geological Column"
description: "Radioactive decay gives a clock that started when a mineral crystallised, and choosing the right isotope pair for the age being measured is most of the skill. Hanging numbers on the relative sequence "
canonical: https://lightmysky.com/learn/science/deep-time-radiometric-dating-and-the-geological-column-mt_w5DL8yRghy
source: https://lightmysky.com/learn/science/deep-time-radiometric-dating-and-the-geological-column-mt_w5DL8yRghy.md
retrieved: 2026-09-12
---

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# Deep Time: Radiometric Dating and the Geological Column

Radioactive decay gives a clock that started when a mineral crystallised, and choosing the right isotope pair for the age being measured is most of the skill. Hanging numbers on the relative sequence is what turned the geological column into a timescale with an age for the Earth.

Subject: Science · Area: Earth's Systems · Ages 15 to 16
Page: https://lightmysky.com/learn/science/deep-time-radiometric-dating-and-the-geological-column-mt_w5DL8yRghy

## Ready when they can

- Calculate an age from a parent-to-daughter ratio and a half-life
- Explain why one isotope pair suits young material and another suits ancient rock
- State what a radiometric date is actually dating, and give a case where that is not the event of interest

## Lesson: Clocks inside rocks

Parent atoms turn into daughter atoms at a fixed pace. One half-life is the time for half the parents to change. So count what is left: one half still there means one half-life passed, one quarter means two, one eighth means three. Multiply the count by the half-life to get the age. A crystal with one eighth left and a 700 million year half-life went through 3 halvings, so it is 2100 million years old.

Pick a clock that fits the span. After about ten half-lives less than one thousandth of the parent remains, too little to measure well. Carbon-14, with a half-life near 5,700 years, reads young things and dies out near 50,000 years. A 500 million year rock needs a billion-scale clock instead. For a rock near 4 billion years old, uranium-238 turning to lead-206, half-life near 4.5 billion years, is the smart choice.

**Example.** Ask what your answer actually marks. The clock starts when the mineral seals its atoms in as it crystallises. A zircon age from a lava flow therefore marks the crystal forming as the flow cooled and locked up. That sealing moment may differ from the event you care about, so always name what sealed before you quote the number.

**Tip.** Solve every dating problem the same way. Turn the leftover fraction into a halvings count, multiply by the half-life, check the pair suits the age, and state what sealed. Fraction to count to years to meaning: that chain keeps the clock honest.

**Recap.** Count half-lives for the age, match the clock to the span, and name what sealed.

## Practice

13 questions on this page, each with its working shown.

## Needs first

- [Radiometric Dating](https://lightmysky.com/learn/science/radiometric-dating-mt_bHbpLW1HUg)
- [Reading a Rock Sequence: Superposition, Fossils and Correlation](https://lightmysky.com/learn/science/reading-a-rock-sequence-superposition-fossils-and-correlation-mt_gIwqGoQsag)
- [Half-Life and Activity](https://lightmysky.com/learn/science/half-life-and-activity-mt_S5snTqUjyi)

## Opens up

- [The Neutral Theory and Reading a Molecular Clock](https://lightmysky.com/learn/science/the-neutral-theory-and-reading-a-molecular-clock-mt_yIidVpa703)