The technique can, however, provide the relative ages of bones from the same site.
Most fossils are found in sedimentary rocks deposited in layers.
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 age of volcanic rocks and ash can be determined by measuring the proportions of argon (in the form of argon-40) and radioactive potassium within them.
Each volcanic eruption produces a new deposit of ash and rock.
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.
Uranium is present in many different rocks and minerals, usually in the form of uranium-238.This relatively new technique was developed in order to achieve more accurate dates than those obtained from the potassium-argon method.The older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted.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.