As with any dating technique, there are some significant limitations.
The calcium-potassium age method is seldom used, however, because of the great abundance of nonradiogenic calcium in minerals or rocks, which masks the presence of radiogenic calcium.
On the other hand, the abundance of argon in the Earth is relatively small because of its escape to the atmosphere during processes associated with volcanism.
Lava flows that lie above and below rock beds with ancient human fossils are a good—and true—example.
The mineral sanidine, the high-temperature form of potassium feldspar, is the most desirable.
The potassium-argon dating method has been used to measure a wide variety of ages.
The potassium-argon age of some meteorites is as old as 4,500,000,000 years, and volcanic rocks as young as 20,000 years old have been measured by this method.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.That is, a fresh mineral grain has its K-Ar "clock" set at zero.The rock sample to be dated must be chosen very carefully.As the K-40 in the rock decays into Ar-40, the gas is trapped in the rock.