Examples of isotopes used in radiometric dating
Crystallization of a mineral is a good way to close a system. Any disturbance of the system effectively resets the clock to zero by allowing decay products to escape or reshuffling the abundances of elements.
Weathering and metamorphism are the two most common ways to disturb a system.
Sedimentary rocks are generally hard to date because common cements like silica don't have datable radioisotopes, and minerals like glauconite that are common in sedimentary rocks are very prone to resetting.
If only there were long-lived isotopes of silicon, calcium, and magnesium!
The driveway was poured in 1950, and the coins are all dated 1920. Radiometric dating generally requires that a system be closed - in other words, has not had material added or removed.
Uranium-lead dating methods often use this approach because some of the minerals used in dating lose the lead decay products over time.
It's amazing how often people fail to realize that you can't date materials if they don't have the necessary ingredients. You can't use carbon-14 to date an arrowhead with no carbon in it.
What radioactive materials actually do is decay according to a law: Decays/Time = K * Number of atoms K is a constant called the decay constant.
Let t stand for time and N(t) stand for the number of atoms at time t .