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.Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time.Third, magnetism in rocks can be used to estimate the age of a fossil site.Younger layers are deposited on top of older layers (principle of superposition).Layers that cut across other layers are younger than the layers they cut through (principle of cross-cutting relationships).Thus, any deformations of strata (Figures 2 and 3) must have occurred after the rock was deposited.Layers of rock are deposited horizontally at the bottom of a lake (principle of original horizontality).
Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's surface to determine the relative ages of geological events preserved in the rock record.There are three general approaches that allow scientists to date geological materials and answer the question: "How old is this fossil?" First, the relative age of a fossil can be determined.The universe is full of naturally occurring radioactive elements.Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms." When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed.Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.