Fossils are generally found in sedimentary rock not igneous rock.Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years.
How does radiometric dating work
So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.
Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium.
In this case, U238 is the “parent” and Pb206 is the “daughter.” Scientists begin by measuring how long it takes for a parent isotope to decay into a daughter isotope.
In this particular case, it takes 4,460,000,000 years for half of a sample of U238 to decay into Pb206.
by Tas Walker A geologist works out the relative age of a rock by carefully studying where the rock is found in the field.
The field relationships, as they are called, are of primary importance and all radiometric dates are evaluated against them.
From the mapped field relationships, it is a simple matter to work out a geological cross-section and the relative timing of the geologic events.
His geological cross-section may look something like Figure 2.
Radiometric dating is a method which scientists use to determine the age of various specimens, mainly inorganic matter (rocks, etc.), though there is one radiometric dating technique, radiocarbon dating, which is used to date organic specimens. Basically, scientists take advantage of a natural process by which unstable radioactive “parent” isotopes decay into stable “daughter” isotopes spontaneously over time.