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K–Ar dating
Potassium–Argon dating or 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).
Potassium is a common element found in many materials, such as micas, clay, tephra, and evaporites. In these materials, the decay product ^{40}Ar 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 ^{40}Ar to the amount of ^{40}K remaining. The long halflife of ^{40}K allows the method to be used to calculate the absolute age of samples older than a few thousand years^{[1]} The quickly cooled lavas that make nearly ideal samples for K–Ar dating also preserve a record of the direction and intensity of the local magnetic field as the sample cooled past the Curie point of iron. The geomagnetic polarity time scale was calibrated largely using K–Ar dating.^{[1]}
References
 ↑ ^{1.0} ^{1.1} McDougall, Ian and T. Mark Harrison 1999. Geochronology and thermochronology by the ^{40}Ar/^{39}Ar method. Oxford. ISBN 0195109201 .

