Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.
Index fossils are fossils that are known to only occur within a very specific age range.
To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*.
By definition, D* = N-1) (2) Now we can calculate the age if we know the number of daughter atoms produced by decay, D* and the number of parent atoms now present, N.
These isotopes break down at a constant rate over time through radioactive decay.
By measuring the ratio of the amount of the original (parent) isotope to the amount of the (daughter) isotopes that it breaks down into an age can be determined.
The Wheeler Formation has been previously dated to approximately 507 million year old, so we know the trilobite is also about 507 million years old.
Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of 415 to 420 million years.
There are two main methods determining a fossils age, relative dating and absolute dating.
Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.
After the passage of two half-lives only 0.25 gram will remain, and after 3 half lives only 0.125 will remain etc.