An example of radiometric dating
They each rely on a couple of different (thoroughly verified) principles.First, that radioactive isotopes have a fixed half-life (totally independent of their environment).The reason carbon dating works is that the fresh carbon-14 gets mixed in with the rest of the carbon in the atmosphere and, since it’s chemically identical to regular carbon, gets worked into whatever is presently absorbing atmospheric carbon.In particular: plants, things that eat plants, things that eat things that eat plants, and breatharians.
Since uranium-238 (the isotope comprising more than 99% of natural uranium) has a half-life of billions of years, it’s useful for figuring out the age of (among other things) zircons that crystallized billions of years ago. And, not for nothing, it’s also caused a thousandfold increase in lead contamination in the bodies (or bones at least) of everything that breathes and/or eats.And second, that the elements they were before and after the radioactive decay have different chemical properties. If an atom doesn’t interact chemically in the right way, then it won’t be incorporated into a forming crystal.As water freezes and each molecule falls into place, atoms that don’t fit in the forming ice crystal are excluded. For example, zircon (a crystal) is perfectly happy to incorporate uranium, but excludes lead.Scientists now have accurate methods (see below ) for dating fossils.
methods rely on characteristic faunal and geological patterns to bracket the period when the fossil existed.
Fortunately, the stuff ancient civilization leave lying around tend to be found in clumps called “middens”.