Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old.To determine the ages of these specimens, scientists need an isotope with a very long half-life.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.But this sediment doesn't typically include the necessary isotopes in measurable amounts.
So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash.Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world.The most widely known form of radiometric dating is carbon-14 dating.This is what archaeologists use to determine the age of human-made artifacts. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old.
Each of them typically exists in igneous rock, or rock made from cooled magma.
Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock.