K–Ar dating
Potassium is a common element found in many materials, such as micas, clay, tephra, and evaporites. In these materials, the decay product 40Ar is able to escape the liquid (molten) rock, but starts to build up when the rock solidifies (recrystallises). Time since recrystallization is calculated by measuring the ratio of the amount of 40Ar to the amount of 40K remaining. The long half-life of 40K is more than a billion years, so the method is used to calculate the absolute age of samples much older than a few thousand years.