|MadSci Network: Earth Sciences|
There are probably a large number of ways in which rocks can be dated. The dates of some rocks, for instance, might be estimating by noting the age of the layer of rock in which the rock was found. I believe the method you are asking about is referred to as carbon dating. An excellent educational site on carbon dating is the following http://members.aol.com/dsfrink/ocr/ocrpage.htm which also provides links to other related sites you may be interested in. I hope this helps. Thanks for your question. sid
Try the links in the MadSci Library for more information on Earth Sciences.