This damage is in the form of tiny marks called fission tracks.When volcanic rocks and minerals are formed, they do not contain fission tracks.
The level of nitrogen gradually reduces as the bone decays.
Absolute dating is not possible with this method because the rate at which the nitrogen content declines depends on the surrounding temperature, moisture, soil chemicals and bacteria.
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.
Only one sample is required for this method as both the argon-39 and argon-40 can be extracted from the same sample.
In special cases, bones can be compared by measuring chemicals within them.
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.
The number of tracks increases over time at a rate that depends on the uranium content.