When we look at sand in an hourglass, we can estimate how much time has passed based on the amount of sand that has fallen to the bottom.
Radioactive rocks offer a similar “clock.” Radioactive atoms, such as uranium (the parent isotopes), decay into stable atoms, such as lead (the daughter isotopes), at a measurable rate.
Uranium-Lead dating is a radiometric dating method that uses the decay chain of uranium and lead to find the age of a rock.
Thus all the lead in the zircon can be assumed to be radiogenic.
To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*.
By definition, D* = N-1) (2) Now we can calculate the age if we know the number of daughter atoms produced by decay, D* and the number of parent atoms now present, N.
Uranium minerals themselves are too uncommon to be very useful in dating.
The most common dating method involves the use of minerals like zircon and monazite that are relatively common in granitic rocks.
Yet this view is based on a misunderstanding of how radiometric dating works.