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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.
This is not a significant problem for igneous rocks, but it affects the usefulness of the techniques for other forms of rock that have more complex heating histories.
For these other rocks argon-argon (40Ar - 39Ar) dating can be more useful.
Potassium 40 decays into argon 40 through a process known as electron capture.
In electron capture, an electron from the innermost electron shell "falls" into the nucleus, causing a proton to convert into a neutron.
In molten rock almost all of the argon will be released into the atmosphere; so in volcanic material, when the rock cools and hardens, the argon begins to accumulate in the crystals, effectively starting the clock.
In the case of potassium-argon decay, this loss of a proton causes the atom to change from a reactive alkali metal to a non-reactive noble gas, which is an important characteristic.
Because argon is an inert gas, if it is not physically trapped in a rock, it will diffuse into the atmosphere.
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.
There is a possibility of contamination from atmospheric argon, but that can be adjusted for.Another factor that can skew measurements is reheating of the rock, which can partially reset the clock by releasing some of the stored argon into the atmosphere.