Topics: Carbon dating reveals earliest origins of zero symbol - BBC
The half-life of uranium-238 is 4500 million years. When it decays it forms thorium-234 which is also unstable. Finally, after a series of radioactive isotopes are formed it becomes lead-206, which is stable. The age of the rock can be calculated if the ratio of uranium to lead is known. As the rock gets older the proportion of lead increases. If half of the uranium-238 has turned into lead-206 the rock will be 4500 million years old.
You're using the Internet Explorer 6 browser to view the BBC website. Our site will work much better if you change to a more modern browser. It's free, quick and easy. Find out more about upgrading your browser here…
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.