How We Know How Old a Rock Is

Step 1 of 5

Layers only tell you the order.

For a long time the best anyone could do was that lower rock is older than the rock above it. That gives a sequence — this before that — but no numbers, and geologists argued for a century about whether Earth was thousands or millions of years old.

Step 2 of 5

Some atoms fall apart on a schedule.

A radioactive atom will, at some unpredictable moment, break into a different atom. You cannot say when any single one will go. But in a large group, exactly half of them go in a fixed period — and that period never changes.

Step 3 of 5

Heat and pressure cannot speed it up or slow it down.

This is what makes it a clock rather than an estimate. Chemistry, weather, temperature, being buried, being melted at the surface — none of it touches the rate. The atom is working from the inside.

Step 4 of 5

So you count both kinds and take the ratio.

When a crystal forms it locks in a known amount of the parent atom and none of the daughter. Every daughter atom you find later got there by one parent decaying. Count both, and the proportion tells you how many half-lives have passed.

Step 5 of 5

Different clocks for different ages.

Carbon runs fast — half gone in 5,730 years — so it dates wood and bone up to about fifty thousand years. Uranium in zircon crystals runs slow enough to date the oldest rocks on Earth, at four and a half billion years.

The age of the Earth was finally pinned down in 1953 by Clair Patterson, using a meteorite because no Earth rock is old enough.