Radioactive decay and dating
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.
Because lead (the stable daughter of uranium) has a very different arrangement of electrons, it does not make its way into the crystal as it is forming.
The formation of crystals in the magma marks the moment that the radio-isotopic clock starts ticking.
This suite of techniques allows scientists to figure out the dates that ancient rock strata were laid down and hence, provides information about geologic processes, as well as evolutionary processes that acted upon the organisms preserved as fossils in interleaved strata.The zircon formation may have occurred tens to hundreds of thousands of years before the eruption and deposition.However, when dealing with rocks that are hundreds of millions of year old, the time between zircon formation and eruption really is short in comparison.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?