Fossils and other objects that accumulate between these eruptions lie between two different layers of volcanic ash and rock.
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.
Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date.
A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age.
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.This newer method converts a stable form of potassium (potassium-39) into argon-39.Measuring the proportions of argon-39 and argon-40 within a sample allows the age of the sample to be determined.The technique can, however, provide the relative ages of bones from the same site.Most fossils are found in sedimentary rocks deposited in layers.This damage is in the form of tiny marks called fission tracks.