What is uranium 235 dating
The amount of material involved in these estimates is small, but can be used to generate powerful results.
A grapefruit sized piece of granite may contain a thousand zircons just a few milligrams of material and only a tiny amount of each crystal (a few hundred parts per million) is actually uranium or lead and useful in dating the rock.
Scientists usually express this as an age range (e.g., one billion years plus or minus half a million years), meaning that they are very confident that the true date falls somewhere within that range.
With modern techniques, these ranges have gotten narrower and narrower, and consequently, even very ancient rocks can be dated quite precisely.
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.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.When the eruption occurs, zircons are released in the ash and lava, which then become rocks like rhyolite.Geologists hunt for these particular sorts of rock to date the volcanic eruption in which the rock formed.
For practice, use the graph above to estimate the age of a rock sample that contains 10% uranium and 90% lead.