In radiometric rock dating, how do we know what the starting isotope is in order to zero our geological clock?

It is always mentioned very matter-of-factly in textbooks that when volcanic magma cools and solidifies, the parent isotope is such and such, e.g. uranium 238. But they never elaborate or qualify this statement, or explain, for instance, the physics behind why lava starts off with a particular heavy, unstable isotope. I would have thought that this would be the most crucial piece in presenting augments supporting the validity of the technique. All the stuff about half-lives is merely the clockwork mechanism after all. But what determines the initial state and what is the supporting evidence. Can anyone elucidate please?