Why is potassium 40 useful in dating igneous rocks
And he hopes the rock has remained sealed until the time he collected his sample.
the geologist only needs to measure the relative amounts of potassium-40 and argon-40 in the rock at the present time to be able to calculate an age for the rock.
Argon, on the other hand, is an inert gas; it cannot combine chemically with anything.
As a result under most circumstances we don't expect to find much argon in igneous rocks just after they've formed.
Finally, we must consider the possibility of argon loss.
He thinks this solves his problem of not knowing the initial quantity of the daughter element in the past and not being able to go back in time and make measurements. He assumes that any argon-40 that he measures in his rock sample must have been produced by the radioactive decay of potassium-40 since the time the rock solidified.
He imagines that his radioactive hour glass sealed when the rock solidified, and his radioactive clock started running.
(However, see the section below on the limitations of the method.) This suggests an obvious method of dating igneous rocks.
If we are right in thinking that there was no argon in the rock originally, then all the argon in it now must have been produced by the decay of Ar in them will be so small that it is below the ability of our instruments to measure, and a rock formed yesterday will look no different from a rock formed fifty thousand years ago.