In this way, multiple trees can be used to build a master chronology for a forested region.

European oak trees have been used to build a 12,000-year chronology.

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Once the rock hardens, however, all the Argon-40 is trapped in the sample, giving us an accurate record of how much Potassium-40 has decayed since that time.

So, if we find a rock with equal parts Potassium-40 and Argon-40, we know that half the Potassium-40 has decayed into Argon-40, and that the rock hardened 1.3 billion years ago.

Radiometric dating requires that one understand the initial ratio of the two elements in a given sample by some means.

In this case, Argon-40 is a gas that easily bubbles out and escapes when it is produced in molten rock.

A tree’s age can be found by simply counting its rings.

By comparing the pattern of thick and thin rings to weather records, scientists can verify that the method is accurate.

This method can even be used on dead trees that fell in a forest long ago.

For example, the last 200 rings in the dead tree might match up with 200 rings early in the life of the living tree, so the two trees together can count back many years.

The expansion of the universe gives an age for the universe as a whole: 13.7 billion years old.