When the eruption occurs, zircons are released in the ash and lava, which then become rocks like rhyolite.
How can the formation of a rock be correlated with a particular ancient event?
The answers to all of these questions lie in our understanding of the geologic processes that affect the deposition of radioactive elements.
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.
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.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.Radioisotopic dating is a key tool for studying the timing of both Earth's and life's history.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.In other words, the chance that a given atom will decay is constant over time.