Dating fossils and rock
Other techniques include analyzing amino acids and measuring changes in an object's magnetic field.Scientists have also made improvements to the standard radiometric measurements.The rejection of the validity of fossils and of dating by religious fundamentalists creates a problem for them: Fossil sequences were recognized and established in their broad outlines long before Charles Darwin had even thought of evolution.
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.
In the past 150 years they have not found any fossils that Darwin would not have expected.
New discoveries have filled in the gaps, and shown us in unimaginable detail the shape of the great ‘tree of life’.
Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world.
This information has also helped determine the age of the Earth itself.
Some critics, particularly religious fundamentalists, argue that neither fossils nor dating can be trusted, and that their interpretations are better.
Other critics, perhaps more familiar with the data, question certain aspects of the quality of the fossil record and of its dating.
But this sediment doesn't typically include the necessary isotopes in measurable amounts.
Fossils can't form in the igneous rock that usually does contain the isotopes.
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.