The method depends upon uranium and its sister element thorium going through long decay chains until finally the stable element, Lead 206 is reached. In this process, alpha particles are given off. In fact, eight alpha particles are given off from each Uranium 238 atom that decays to Lead 206 . In evaluating the uranium dating method, the fact is that uranium minerals always exist in open systems. This causes us to immediately question the validity of the method.
Similar difficulties prevail with pitchblend veins.
Here again widely diverging ages can be measured on samples from the same spot. The lead ages, for example, have given consistently older dates then the others. This method is the most popular method for dating rocks, and is used extensively to give general dates for fossils found in some rock formations. Potassium minerals are found in igneous and in some sedimentary rocks, therefore the Potassium-Argon method isn't as restricted in its use as is uranium. The K-Ar system behaves a little differently from the uranium series above.