3 methods of radiometric dating
In the case of carbon dating, it is not the initial quantity that is important, but the initial ratio of C, but the same principle otherwise applies.
Recognizing this problem, scientists try to focus on rocks that do not contain the decay product originally.
One example of this can be found in metamorphic rocks.
This does not mean that all rock samples are unreliable, but it is possible to account for a process which throws off the data for metamorphic rocks.
This can happen due to one of three forces or "interactions": strong, electromagnetic, and weak, in order of decreasing strength.
For these reasons, if a rock strata contains zircon, running a uranium-lead test on a zircon sample will produce a radiometric dating result that is less dependent on the initial quantity problem.
Another assumption is that the rate of decay is constant over long periods of time, which is particularly implausible as energy levels changed enormously over time.
If you had an ensemble of identical particles, the probability of finding a given one of them still as they were - with no decay - after some time is given by the mathematical expression This governs what is known as the "decay rate." The rate is unique to different particles and so to different atomic elements.
This makes different elements useful for different time scales of dating; an element with too short an average lifetime will have too few particles left to reveal much one way or another of potentially longer time scales.