Absolute dating: radioactive decay is a clock for rocks

0:00Rock layers tell us which fossils are older. But how do we find out an actual age, in years?

0:07When rock forms, it traps some radioactive atoms. We will call them parent atoms.

0:14Over time, each radioactive atom changes into a different kind of atom. This is called radioactive decay.

0:22Nothing speeds it up or slows it down. Not heat, not pressure, not water. So it works like a clock.

0:31The time it takes for half of the parent atoms to change is called the half-life. At the start, all sixteen atoms here are parent atoms.

0:40After one half-life, half of the parent atoms are left. Eight out of sixteen.

0:47After two half-lives, half of those have changed too. Only four are left. That is a quarter.

0:54After three half-lives, just two are left. That is an eighth.

1:00Scientists measure how many parent atoms and new atoms a rock holds. That tells them how many half-lives have passed.

1:08Suppose, for this kind of atom, one half-life is one hundred million years.

1:14Three half-lives have passed. Three times one hundred million. So the rock is three hundred million years old.

1:23Fossils are dated from layers of volcanic rock just above and below them. So this fossil is about three hundred million years old too.

1:32Radioactive atoms decay at a steady rate. Counting them gives rocks, and the fossils near them, an age in years.