Radiocarbon dating, developed in the 1940s, is based on the presence of a variant form—isotope—of the element carbon that exists in rocks.

Many elements have a variety of atomic isotopes. For instance, most carbon atoms have a mass of 12 units, but the carbon atoms used in radiocarbon dating have a mass of 14 units. This isotope is known as carbon-14.

Isotopes are measured by the rate of their radioactive decay, which begins as soon as the rock is formed. The isotopes used in dating decompose very slowly, over millions of years, at a predictable rate called a half-life. As they break down, they leave behind what is known as the decay product.

For instance, when carbon-14 decays, nitrogen-14 takes its place. When one half-life is over, the sample of rock has 50 percent carbon-14 and 50 percent nitrogen-14.

Each successive half-life reduces the remaining amount of the isotope by 50 percent, so after the second half-life, the sample shows 25 percent carbon-14 and 75 percent nitrogen-14.

The half-life of carbon-14 is 5,700 years. Therefore, if a sample has 50 percent carbon-14 and 50 percent nitrogen-14, we know it is 5,700 years old. A measurement of 25 percent carbon-14 and 75 percent nitrogen-14 means the rock is 11,400 years old.

Carbon dating is used for rocks between 100 and 70,000 years old. For older rocks, isotopes with longer half-lives are necessary. Potassium-40 has a half-life of 1.3 billion years and is used for dating rocks older than 70,000 years.