Buried bones absorb chemicals, such as uranium and fluorine, from the surrounding ground and absorb more of these chemicals the longer they remain buried.The rates of absorption depend on a number of factors which are too variable to provide absolute dates.Fossils and other objects that accumulate between these eruptions lie between two different layers of volcanic ash and rock.
The heat from a volcanic eruption releases all the argon from the molten rock and disperses it into the atmosphere.
Argon then starts to re-accumulate at a constant rate in the newly formed rock that is created after the eruption.
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.
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.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.Instead, other methods are used to work out a fossil’s age.These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages.This technique is, however, useful for providing relative dates for objects found at the same site.