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.

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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.

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