Radioactive dating can be done by analyzing the fraction of carbon in organic material that is carbon-14.

The ratio of carbon-14 to carbon-12 in the atmosphere has been roughly constant over thousands of years. A living plant or tree will be constantly exchanging carbon with the atmosphere, and will have the same carbon ratio in its tissues.

When the plant dies, this exchange stops. Carbon-14 has a half-life of about 5730 years; it gradually decays away and becomes a smaller and smaller fraction of the total carbon in the plant tissue.

This fraction can be measured, and the age of the tissue deduced.

Objects older than about 60,000 years cannot be dated this way – there is too little carbon-14 left.

Other isotopes are useful for geologic time scale dating.

Uranium-238 has a half-life of 4.5 x 109 years, and has been used to date the oldest rocks on Earth as about 4 billion years old.

VIDEO: http://www.youtube.com/watch?v=ErgdpG_N9vQ