Radiometric dating is based on
On the surface, radiometric dating methods appear to give powerful support to the statement that life has existed on the earth for hundreds of millions, even billions, of years.
We are told that these methods are accurate to a few percent, and that there are many different methods.
Since Cambrian and later rocks are largely sedimentary and igneous (volcanic) rocks are found in Cambrian and later strata, if these rocks are really 550 million years old, then life must also be at least 550 million years old.
Therefore, my main concern is with rocks of the Cambrian periods and later.
Potassium is about 1/40 of the earth's crust, and about 1/10,000 of the potassium is potassium 40.
Uranium decays to lead by a complex series of steps. Thus we obtain K-Ar dating, U-Pb dating, and Rb-Sr dating, three of the most common methods.
For potassium 40, the half-life is about 1.3 billion years.
It just means that there is enough accuracy in the measurements to compute t to one or two percentage points of accuracy, where t is the time required to obtain the observed ratio of daughter to parent, assuming no initial daughter product was present at the beginning, and no daughter or parent entered or left the system.
In two half-lives, half of the remainder will decay, meaning 3/4 in all will have decayed.