Potassium argon method of dating igneous rocks
Because argon is inert, it cannot be chemically incorporated in the minerals when they are formed, but it can be physically trapped in the rocks either during or after formation. If the source of this argon is atmospheric contamination, then we can correct for this.
The reasoning is as follows: the atmosphere does not only contain Ar as being atmospheric argon.
The severity of this problem decreases as the accuracy of our instruments increases.If the mineral composition of the two sample is different, so that the sample for measuring the potassium is richer or poorer in potassium than the sample used for measuring the argon, then this will be a source of error.Another concern with K-Ar dating is that it relies on there being no Ar in the rock when it was originally formed, or added to it between its formation and our application of the K-Ar method.Since the 1950s, geologists have used radioactive elements as natural "clocks" for determining numerical ages of certain types of rocks. "Forms" means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism cools off.It's this resetting process that gives us the ability to date rocks that formed at different times in earth history.