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.
Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date.
Uranium is present in many different rocks and minerals, usually in the form of uranium-238.
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.