For example, if two fossils are found at the same site, but at different depths, scientists assume that the fossil found at the greater depth is older. Stratigraphic dating is based on the tendency of lower strata (layers) of rock to be older than higher strata.
The deeper something is found, the older it generally is. Volcanic activity, earthquakes and the unpredictability of rock formation and accumulation can make the chronology established by this method inaccurate.
For example, by radiocarbon dating bits of bone found next to a spear head.
Without relative and radiometric dating techniques, no one would know whether dinosaurs lived 5,000 or 150,000,000 years ago.
The rate by which these unstable elements decay are generally expressed in terms of half-life.