Dating of rocks
Attempts to transform these ratios into dates are where this becomes problematic.
Assigning a date requires that the rate at which the parent decays into the daughter element has been the same throughout the rock’s history.
Because of this decay, the helium content of any mineral or rock capable of retaining helium will increase during the lifetime of that mineral or rock, and the ratio of helium to its radioactive progenitors then becomes a measure of , helium dating provided most of the dates used in the early geologic time scales.
Helium ages, however, tend to be too low because the gas escapes from the rock.
Geologists do not directly measure the age of a rock.