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.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.The level of nitrogen gradually reduces as the bone decays.
These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages.
This form of uranium usually decays into a stable lead isotope but the uranium atoms can also split – a process known as fission.
During this process the pieces of the atom move apart at high speed, causing damage to the rock or mineral.
Fossils and other objects that accumulate between these eruptions lie between two different layers of volcanic ash and rock.
An object can be given an approximate date by dating the volcanic layers occurring above and below the object.
Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date.