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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For example, over time, uranium atoms lose alpha particles (each made up of two protons and two neutrons) and decay, via a chain of unstable daughters, into stable lead.

Although it is impossible to predict when a particular unstable atom will decay, the decay rate is predictable for a very large number of atoms.

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.

Of course, in this case "shortly" is meant in terms of geologic timescales.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.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 zircon formation may have occurred tens to hundreds of thousands of years before the eruption and deposition.