The absolute age of the earth can be accurately obtained by isotope determination  For a long time, it has been found that radionuclide in rocks spontaneously and at a constant rate gradually decay into non radioactive daughter isotopes, releasing energy  As long as the temperature, pressure and other factors unchanged, people can get an accurate value Of course, this method has the disadvantage that it is difficult to eliminate the influence of late thermal metamorphism when isotopic determination is carried out
If the sample is weathered rock and is much different from the parent rock, the determined age can not represent the true age of the rock layer
Accurate sampling is one of the keys to getting an accurate geochronology of an area by isotopic determination.