Compare relative dating and radioactive dating
Let's say that Paul the Paleontologist found an iguanodon fossil in the light green layer shown above.And, he also found a coelophysis fossil in the yellow layer. Of course, the coelophysis, which means that coelophysis came before iguanodon.
The key in relative dating is to find an ordered sequence.In 1905, Ernest Rutherford figured out that we could use radiation to establish the ages of rocks.By studying how the mass of uranium changed with radioactive decay, Rutherford was able to determine the age of a rock containing a uranium mineral. It meant that scientists could suddenly establish the actual ages of all their rocks and fossils!Scientists piece together a story of how one event came before or after another.Relative dating cannot tell us the actual age of a rock; it can only tell us whether one rock is older or younger than another.