Using relative dating the fossil is compared to something for which an age is already known.
For example if you have a fossil trilobite and it was found in the Wheeler Formation.
Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you.
Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old.
In a way this field, called geochronology, is some of the purest detective work earth scientists do.
There are two basic approaches: relative age dating, and absolute age dating.
Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.
The majority of the time fossils are dated using relative dating techniques.By combining multiple geochronological (and biostratigraphic) indicators the precision of the recovered age can be improved.Geochronology is different in application from biostratigraphy, which is the science of assigning sedimentary rocks to a known geological period via describing, cataloguing and comparing fossil floral and faunal assemblages.A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods.More slowly decaying isotopes are useful for longer periods of time, but less accurate in absolute years.To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.