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A method for dating the age of rocks.

Potassium occurs in reasonably large amounts in all common rocks. One of the isotopes of potassium, potassium-40 is radioactive, and may decay into Argon-40.

Given the half-life of Potassium-40 and the corresponding amount of argon-40 present in the rock, the date of formation can be established. However, this date is reset whenever the rock is subjected to metamorphic processes, since the argon gas can escape.

The surrounding minerals will also give an indication of the last reset of the geological clock. For example, biotite will lose argon completely at 300 Celsius, while hornblende will only lose all of its argon at temperatures greater than 500 Celsius.