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The amount of material involved in these estimates is small, but can be used to generate powerful results.A grapefruit sized piece of granite may contain a thousand zircons just a few milligrams of material and only a tiny amount of each crystal (a few hundred parts per million) is actually uranium or lead and useful in dating the rock.
Hence, carbon-14 dating can only be used to estimate much younger ages, up to around 60,000 years.
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.
For practice, use the graph above to estimate the age of a rock sample that contains 10% uranium and 90% lead.
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.
For example, as shown at left below, uranium-235 has a half-life of 704 million years.