Radiometric dating assumptions
The following quote from The Earth: An Introduction to Physical Geology by Tarbuck & Lutgens, pp.
55-57, (1987), gives us an idea of the tremendous complexity of the processes that occur when magma solidifies.
There is also an exhaustive study of this subject called the (Radioisotopes and the Age of the Earth) Group, a team of six scientists who are investigating the subject in depth, and have published the first of several studies. Carbon-14 dating has limited value for evolution because its half-life is too short.
Such processes can cause the daughter product to be enriched relative to the parent, which would make the rock look older, or cause the parent to be enriched relative to the daughter, which would make the rock look younger.
This calls the whole radiometric dating scheme into serious question.
The general idea is that many different minerals are formed, which differ from one another in composition, even though they come from the same magma.
The mineral makeup of an igneous rock is ultimately determined by the chemical composition of the magma from which it crystallized.
Therefore, since sedimentary rock is the only kind of rock that bears fossils, a relative date is estimated by the position of a sedimentary rock in relation to an igneous outflow.