Geologic assessment of active tectonism depends on two key measures: the age and the amount of deformation of a given stratigraphic unit.The amount of deformation can normally be measured with greater accuracy than the age.These techniques can be grouped as numerical, relative dating, and correlation.Numerical techniques are best, but datable materials are often lacking, and in these cases age estimation must be made using relative-dating or correlation techniques.Other critics, perhaps more familiar with the data, question certain aspects of the quality of the fossil record and of its 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.
Earth is constantly changing—nothing on its surface is truly permanent.
Rocks that are now on top of a mountain may once have been at the bottom of the sea.
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.