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 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.
This form of uranium usually decays into a stable lead isotope but the uranium atoms can also split – a process known as fission.
During this process the pieces of the atom move apart at high speed, causing damage to the rock or mineral.
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.
This relatively new technique was developed in order to achieve more accurate dates than those obtained from the potassium-argon method.
The older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted.
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.
Using these methods, geologists have created a geologic time scale for organizing past times in earth’s history.
Highland County igneous rock intrudes sedimentary rock (Photograph by Stan Johnson) This light-colored Highland County igneous intrusion cuts through the darker sedimentary rock.
This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.