That’s because zircon is super tough – it resists weathering. Each radioactive isotope works best for particular applications.
The half-life of carbon 14, for example, is 5,730 years.
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.
Are there repairs or cracks in the sidewalk that came after the sidewalk was built?It’s based either on fossils which are recognized to represent a particular interval of time, or on radioactive decay of specific isotopes. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.After all, a dinosaur wouldn’t be caught dead next to a trilobite.On the other hand, the half-life of the isotope potassium 40 as it decays to argon is 1.26 billion years.So carbon 14 is used to date materials that aren’t that old geologically, say in the tens of thousands of years, while potassium-argon dating can be used to determine the ages of much older materials, in the millions and billions year range.Archaeology dating techniques can assure buyers that their item is not a fake by providing scientific reassurance of the artefact's likely age.Archaeological scientists have two primary ways of telling the age of artefacts and the sites from which they came: relative dating and absolute dating.Relative dating in archaeology presumes the age of an artefact in relation and by comparison, to other objects found in its vicinity.Limits to relative dating are that it cannot provide an accurate year or a specific date of use.But the most accurate forms of absolute age dating are radiometric methods. Sedimentary rocks in particular are notoriously radioactive-free zones.This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.