This week, Cherry Lewis of the University of Bristol presented a talk about the history of dating the Earth as part of the BA Festival of Science in York, England.
Scientists have many ways to discover how old rocks and fossils are.
The ratio of lead versus uranium in the zircon is what is used to determine the age of the rock. As you know, radioisotopes do not decay directly into a stable state; rather they go through stages of radioactive decay until reaching a stable isotope.
This means that any lead found in zircon minerals was made by radioactive decay, after the formation of the mineral.
Zircon also concentrates Uranium (You) and (although less so) Thorium (Th) and excludes lead (Pb), which means it has a very high You/Pb ratio.
In another 710 million years, there would be only one 235U atom for every three 207Pb atoms.
In the uranium series, the half-lives of the minerals are much longer, but the process is the same.The graph will show not only the age of the rocks but also when important geological events occurred in the past.So how do scientists get the zircon and figure out its age? Geologists go out looking for certain types of rock that they know to be older than others.In a report of his findings published in 1913 in the journal , Holmes expressed the less-than-ecstatic reception his findings received: "The geologist who ten years ago was embarrassed by the shortness of time allowed to him for the evolution of the Earth’s crust , is still more embarrassed with the superabundance with which he is now confronted." The Earth's age continued to be hotly debated for decades afterward. In the 1920s, Earth's age crept up toward 3 billion years, making it for a time even older than the universe, which was then estimated to be about 1.8 billion years old.The best estimate for Earth's age is based on radiometric dating of fragments from the Canyon Diablo iron meteorite. Since igneous rocks have no fossils, this makes zircon valuable in dating them.