The number of tracks increases over time at a rate that depends on the uranium content.It is possible to calculate the age of a sample by measuring the uranium content and the density of the fission tracks.This form of uranium usually decays into a stable lead isotope but the uranium atoms can also split – a process known as fission.
This technique is, however, useful for providing relative dates for objects found at the same site.
Another useful chemical analysis technique involves calculating the amount of nitrogen within a bone.
Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.
Uranium is present in many different rocks and minerals, usually in the form of uranium-238.
Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date.
A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age.
This problem is now reduced by the careful collection of samples, rigorous crosschecking and the use of newer techniques that can date minute samples.
Volcanic rocks – such as tuff and basalt – can be used in dating because they are formed at a particular moment in time, during an eruption.
Where the rocks are not strongly folded or tilted it is possible to work out the order in which the layers were formed.
The oldest rocks and fossils are at the bottom and the youngest are on top.
The technique can, however, provide the relative ages of bones from the same site.