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 older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted.
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 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.
Instead, other methods are used to work out a fossil’s age.