Radioisotope dating audio
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.
For example, as shown at left below, uranium-235 has a half-life of 704 million years.
Hence, carbon-14 dating can only be used to estimate much younger ages, up to around 60,000 years.
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.
Geologists extract the appropriate minerals from the rock (in this case, zircon crystals) and use a technique called mass spectrometry to figure out the relative amounts of uranium and lead in the zircon.
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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The amount of material involved in these estimates is small, but can be used to generate powerful results.