A relative age simply states whether one rock formation is older or younger than another formation. The Geologic Time Scale was originally laid out using relative dating principles. Numerical ages have been added to the Geologic Time Scale since the advent of radioactive age-dating techniques.
Many minerals contain radioactive isotopes.
It illustrates how the amount of a radioactive parent isotope decreases with time. This amount is a percentage of the original parent amount. Time is expressed in half-lives. Experiment by dragging on the graph. Note that this half-life can be obtained from the graph at the point where the decay and growth curves cross.