# Dating rocks using radioactive decay

The parent isotope is the original unstable isotope, and daughter isotopes are the stable product of the decay. In the first 5,730 years, the organism will lose half of its C-14 isotopes.

Half-life is the amount of time it takes for half of the parent isotopes to decay. In another 5,730 years, the organism will lose another half of the remaining C-14 isotopes.

This process continues over time, with the organism losing half of the remaining C-14 isotopes each 5,730 years.

Fossils are collected along with rocks that occur from the same strata.

Segment from A Science Odyssey: "Origins."Geologists have calculated the age of Earth at 4.6 billion years.

But for humans whose life span rarely reaches more than 100 years, how can we be so sure of that ancient date? Even the Greeks and Romans realized that layers of sediment in rock signified old age.

As the uranium in rocks decays, it emits subatomic particles and turns into lead at a constant rate.

By measuring the amount of carbon-14 left in dead organic material the approximate time since it died can be worked out.

The results showed that Ötzi died over 5000 years ago, sometime between 33 BC. Uranium has a very long half-life and so by measuring how much uranium is left in a rock its approximate age can be worked out.

Geologist Ralph Harvey and historian Mott Greene explain the principles of radiometric dating and its application in determining the age of Earth.

There's a small amount of radioactive carbon-14 in all living organisms.

When they die no new carbon-14 is taken in by the dead organism.