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When the eruption occurs, zircons are released in the ash and lava, which then become rocks like rhyolite.Geologists hunt for these particular sorts of rock to date the volcanic eruption in which the rock formed.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.Scientists usually express this as an age range (e.g., one billion years plus or minus half a million years), meaning that they are very confident that the true date falls somewhere within that range.With modern techniques, these ranges have gotten narrower and narrower, and consequently, even very ancient rocks can be dated quite precisely.Zircons are nearly perfect clocks because we can be relatively certain that when the crystal formed, no lead was present and that means that when we discover ancient zircons in rocks today, we can be relatively confident that any lead present is the result of radioactive decay.
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.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.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.
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.