A fossil’s age can be determined by a method known as Relative Dating. If the age of the rock beneath the layer in which the fossil is found is known, then the fossil will be younger than that rock according to the Law of Superposition. Or if the rock layer above is known, then the fossil will be older than that layer. Radiometric Dating uses the rate at which radioactive isotopes decay in igneous and metamorphic rocks. The rate of decay is known by the half-life of the isotope. The half-life is the average amount of time an atom of a radioactive element remains in its original form. When the half-life elapses, half of the original igneous or metamorphic rock will have decayed and formed a different type of rock such as a sedimentary rock.
