After the recrystallization of magma, more Ca being the most abundant isotope.

One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below the deposits.

Clay minerals are less than 2 micrometres thick and cannot easily be irradiated for Ar–Ar analysis because Ar recoils from the crystal lattice.

Potassium is a common element found in many materials, such as micas, clay minerals, tephra, and evaporites.

In these materials, the decay product Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes).

If one of the factors changes, the rate is altered.

Any so called 'age determination' by a physical process is, once stripped down, only an educated guess, and is most likely entirely unrelated to the actual age.

Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in geochronology and archaeology.

It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar).

To obtain the content ratio of isotopes K in a rock or mineral, the amount of Ar is measured by mass spectrometry of the gases released when a rock sample is melted in vacuum.