Why is u 238 used for dating rocks
Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.
Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U).
Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.
Unfortunately, these elements don't exist in dinosaur fossils themselves.
Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world.