How, exactly, is dating done?

There are a few things to keep in mind when you're dating objects using isotopic methods such as the one you describe.What you do with isotopic dating is to count the number of atoms of the "parent" nuclide and the number of atoms of the "progeny" (also called "daughter") nuclide. This is what can tell you the age of the rock. Say, for example, you are dating a rock using the uranium-lead dating method. To find the age, you will simply count the number of uranium atoms and the number of lead atoms. There are two primary isotopes of uranium, U-235 (with a half-life of about 700 million years) and U-238 (with a half-life of 4.38 billion years). U-235 decays to lead (Pb-207, to be precise). If you count exactly the same number of atoms of both U-235 and Pb-207, then you know that the uranium has been in the rock for exactly one half-life because, after one half-life, one half of the U-235 atoms will have decayed to lead, and the other half will still be uranium-235. So, if you count equal numbers of parent and progeny atoms, you know that the rock is exactly one half-life old. In this case, it would be 704 million years in age.As the rock gets older, you have fewer and fewer parent atoms and more progeny. So, a rock with a 3:1 ratio of progeny to parent atoms would be 2 half-lives old (because there would be 1/4 of the parent atoms left, so 3/4 would be progeny, giving a 3:1 ratio).For more information about isotopic dating, you can look in just about any high-school or college-level geology text book. One that is particularly good is "Earth", by Press and Siever. Another book that is outstanding at discussing isotopic dating is "The Age of the Earth", by Brent Dalrymple. And, on a more technical level, there are two college textbooks that are also outstanding. One is called "Isotope Geology" by Gunter Faure, and the other is called "Radiogenic Isotope Geology", by Alan Dickin.[This message has been edited by JIM, 11-17-2003]