So how does this all relate to relative dating? These laws allow scientists to put sediments/rock units and geological events in order and potentially link strata from multiple locations. If fossil A is found at the top of a sequence and fossil B is found at the bottom of the sequence, it can at least be determined that fossil A is younger than fossil B. While useful, relative dating cannot determine the exact age of a fossil and, in order to do so, scientists use absolute dating methods.
Most absolute dates are obtained through radiometric dating methods, which is based on radioactive isotopes that decay over time. Isotopes are elements that have the same number of protons but a different number of neutrons. When an isotope is not stable, it will decay at a fixed rate, which is different for each isotope. One of the most common isotopic dating methods used is radiocarbon dating, which relies on the decay of C-14 (a radioactive isotope of carbon) to N-14 (stable isotope of nitrogen). With this method, scientists can only date organic material that contains carbon like bone or charcoal. Also, because of the time it takes for C-14 to decay, this dating method can only date material that are younger than about 40,000 years old!
Another common radiometric dating method used is K-Ar (Potassium-Argon) or 40Ar-39Ar (Argon-Argon), which dates minerals (e.g., feldspar) found in volcanic material deposited during the time the fossil was living. A great example of this can be seen with our dear friend “Lucy.” Geologists were able to extract feldspar minerals from a layer of volcanic ash right below the stratigraphic layer that Lucy was found in and used them to date the deposit. Using 40Ar/39Ar as the radioactive clock, the deposits were dated to roughly 3.18 million years old. Without that volcanic layer and radiometric methods, we would have never known how old Lucy really was!