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Now imagine that you have a rock sample that contains 39% uranium-235 and 61% lead-207. At around 1000 million years (i.e., one billion years), as shown on the graph at right above.Thus, you would calculate that your rock is about a billion years old.Slightly different dating techniques are used with different radioactive elements, but the same basic logic of estimating backwards based on radioactive decay remains the same.The geology behind radioisotopic dating Though the basic logic behind radioisotopic dating relies on nuclear physics and quantum theory, many geologic processes also factor into our ability to date a particular rock. How do they know that the rock isn't contaminated with elements that would throw off the dating?Radioactive decay Radioisotopic dating relies on the process of radioactive decay, in which the nuclei of radioactive atoms emit particles.This releases energy (in the form of radiation) and often transforms one element into another.Thus, when a geologist dates a rock using uranium-lead dating, he or she is actually getting an estimate on the age of its zircon crystals, which formed "shortly" before the volcanic eruption.
The example above describes uranium/lead decay, which happens very slowly; however, different radioactive elements have different half-lives. This allows scientists to date events that are more or less ancient.How can the formation of a rock be correlated with a particular ancient event?The answers to all of these questions lie in our understanding of the geologic processes that affect the deposition of radioactive elements.These zircon crystals are tiny just a tenth of a millimeter long but they are the key to uranium-lead dating.
If these crystals were pure, they would contain just zirconium, silica, and oxygen; however, uranium happens to have a similar arrangement of outer electrons to zirconium, and so as zircons form, "mistakes" are sometimes made, and uranium is substituted for zirconium.For example, as shown at left below, uranium-235 has a half-life of 704 million years.