Radiometric Dating of Rocks

Lesson notes

Overview

Uranium-238 decays to lead over billions of years, giving ages for the oldest rocks. This topic is rated intermediate and is number 12 of 98 in Earth & Space. Builds on the basics with relationships, methods and trade-offs.

What you are seeing

The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.

In depth

Radioactive isotopes in rocks decay at known rates, so the ratio of parent to daughter atoms gives the time since the rock formed. Uranium-238 decays to lead-206 with a half-life of 4.47 billion years, suitable for the oldest rocks, while potassium–argon and carbon-14 suit younger ones. Dating the oldest Earth minerals gave 4.4 billion years, and meteorites gave 4.54 billion years for the age of the Earth. The method needs a closed system.

Reading the relationship

The expression "N = N₀(½)^(t/4.47 Gyr)" is a compact summary of the idea. It uses the symbols N, N₀, t, G, y, r; name what each one stands for before you start. Notice that it contains a ratio, so increasing the denominator makes the result smaller. Then use the sliders to see how changing one quantity changes the others.

A guided exploration

1. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.

Try this

Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.

Common pitfalls

Earth and space systems work on very different scales of time and distance. Check which scale a number refers to before comparing it with another.

How it connects

It builds on “Geological Time”, the topic before it in Earth & Space. Once you are comfortable, continue with “Weathering and Erosion”.

Why it matters

Earth and space science helps us understand our planet, its climate and the wider universe.