Carbon-14 is a method used for young (less than 50,000 year old) sedimentary rocks.This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon-14 by all living things.By dating rocks of known ages which give highly inflated ages, geologists have shown this method can’t give reliable absolute ages.Many geologists claim that radiometric “clocks” show rocks to be millions of years old.It may be used to date a wide range of natural and man-made materials.Fossils may be dated by taking samples of rocks from above and below the fossil's original position.
For example, a study of the Amitsoq gneisses from western Greenland used five different radiometric dating methods to examine twelve samples and got agreement to within 30 million years on an age of 3,640my.
Since the 1950s, geologists have used radioactive elements as natural "clocks" for determining numerical ages of certain types of rocks. "Forms" means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism cools off.
It's this resetting process that gives us the ability to date rocks that formed at different times in earth history.
The method works best if neither the parent nuclide nor the daughter product enters or leaves the material after its formation.
Anything which changes the relative amounts of the two isotopes (original and daughter) must be noted, and avoided if possible.