Measuring age by one of the long-lived radioisotopes requires a closed
system. Usually this is some kind of crystal formed in a period of time
that is short, compared to the time that has elapsed since, and that has
remained unchanged since it formed. Specifically, neither the parent
isotopes can have been added nor the daughter isotopes removed by any
process other than radioactive decay.
The earth is a dynamic system, however. Things are always changing and
moving—not very rapidly, perhaps, but fast enough, in geologic time, to
raise mountains and shift oceans. Solutions are moving around,
dissolving something here and depositing it again somewhere else.
Temperatures are changing as one place is denuded by erosion and another
area buried under layers of sediment. Under such conditions, few systems
remain closed. It is perhaps surprising that we find any closed systems
at all. Let us look at a few that are known to be reliable. (They are
listed in Table I on page 4.)