Dating of zircons
To see how it works, we'll start at the beginning, using uranium as an example: At left, a zircon crystal in a thin section cut from granite. Crystal structure image adapted fromadapted from Materialscientist CC Attribution-Share Alike 3.0 Unported Tens to hundreds of thousands of years before a major volcanic eruption, magma builds up beneath the surface of the Earth.In the magma, crystals of zirconium silicate (called zircons), as well as other crystals, form.
The answers to all of these questions lie in our understanding of the geologic processes that affect the deposition of radioactive elements.
In other words, the chance that a given atom will decay is constant over time.
Decay rates are measured in half-lives the amount of time in which half of a radioactive element will decay.
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.