Uses of uranium lead dating
For example, carbon-14 decays into nitrogen-14 and has a half-life of just 5,730 years.
Hence, carbon-14 dating can only be used to estimate much younger ages, up to around 60,000 years.
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.
Slightly different dating techniques are used with different radioactive elements, but the same basic logic of estimating backwards based on radioactive decay remains the same.
The geology behind radioisotopic dating Though the basic logic behind radioisotopic dating relies on nuclear physics and quantum theory, many geologic processes also factor into our ability to date a particular rock. How do they know that the rock isn't contaminated with elements that would throw off the dating?
Thus, you would calculate that your rock is about a billion years old.
And in the next 704 million years, it will decay leaving behind ¼ gram, and in the next 704 million years, it will decay leaving behind ⅛ gram and so on.
At the same time, the amount of the element that it decays into (in this case lead-207), will increase accordingly, as shown below. At what point on the graph would you expect the ratio of uranium to lead to be about 39 to 61?
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.