It is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 (K-40) ,decays to the gas Argon as Argon-40 (Ar-40).By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.When rocks are heated to the melting point, any Ar-40 contained in them is released into the atmosphere.When the rock recrystallizes it becomes impermeable to gasses again.Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials.Geologists have used this method to date rocks as much as 4 billion years old.This is actually a mini-simulator, in that it processes a different sample each time and generates different dates.
The samples were taken from pegmatites of the Ilmeny Mountains and the Ilmeny-Vishnevy Mountains Complex in the South Urals; pegmatites from the Adui granitic pluton and its framework in the Central Urals; gneisses and granulites of the Taratash Complex in the South Urals; and felsic gneisses from the Transangara region of the Yenisei Ridge.
There are some techniques and calculations which can "look through" the post formation event.
These include isochron dating (K-Ar, U-Pb, Rb-Sr and Sm-Nd) or the step heating Ar-Ar technique of the K-Ar method.
The apparent age may be affected by the post-depositional or post-formation history of the rocks.
Natural contamination of chemical sediments with detrital material can also affect the results of dating of diagenesis.