Because the method is primarily useful for volcanic materials, and fossils more commonly lie in sedimentary layers, the material that is dated is usually from volcanic strata (commonly volcanic ash layers) above and below that of the fossil.
The fossil is given an age limit between the dated volcanic layers.
The first Hominid fossil dated using this method was an Australopithecus boisei (initially called Zinjanthropus boisei) specimen discovered by Mary Leakey in the Olduvai Gorge in 1959.
Volcanic ash layers are relatively common along the Great Rift Valley of east Africa.
Such a wealth of information didn’t arise by chance.
Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials.
When the rock recrystallizes it becomes impermeable to gasses again.
As the K-40 in the rock decays into Ar-40, the gas is trapped in the rock.
Geologists have used this method to date rocks as much as 4 billion years old.
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).
Potassium argon (40K-40Ar) dating is a form of radiometric dating widely used because of the range of dates for which it is useful.
The technique can be used for dates ranging from earth's beginning, 4550 mya (4.55bn in US terminology) to about 100,000 years ago.
Argon can easily escape by diffusion which will result in too low ages.