Potassium-argon or K-Ar dating is a geochronological method used in many geoscience disciplines. It is based on measuring the products of the radioactive decay of potassium (K), which is a common element found in materials such as micas, clay minerals, tephra and evaporites.

Potassium (K) exisits in 3 isotopes - ³⁹K (92.23%), ⁴⁰K (0.00118%), ⁴¹K (6.73%).

The radioactive isotope ⁴⁰K decays to ⁴⁰Ar and ⁴⁰Ca with a half-life of 1.26x10⁹ years. ⁴⁰Ca is the most common form of Ca, however, so the increase in abundance due to K decay results in a negligible increase in total abundance making it less useful as a geochronometer. The ⁴⁰Ar isotope is much less abundant however, and is therefore a more useful isotope. As argon is a gas, it is able to escape from molten rock. However, when the rock solidifies, the decayed ⁴⁰Ar will begin to accumulate in the crystal lattices. In order to determine the ⁴⁰Ar content of a rock, it must be melted and the isotopic composition of the released gas measured via mass spectrometry. It is also necessary to separately measure the amount of ⁴⁰K in the sample. This can be measured using flame photometry or atomic absorption spectroscopy. The ratio between the ⁴⁰Ar and the ⁴⁰K is related to the time elapsed since the rock was cool enough to trap the Ar. This is the key principle behind K-Ar dating.