Problems with radiogenic dating
The atoms of some chemical elements have different forms, called isotopes.
These break down over time in a process scientists call radioactive decay.
Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.
If the proportions of parent and daughter isotopes of these decay series can be measured, periods of geological time in millions to thousands of millions of years can be calculated.Radiometric dating uses the decay of isotopes of elements present in minerals as a measure of the age of the rock: to do this, the rate of decay must be known, the proportion of different isotopes present when the mineral formed has to be assumed, and the proportions of different isotopes present today must be measured.This dating method is principally used for determining the age of formation of igneous rocks, including volcanic units that occur within sedimentary strata.The table below shows characteristics of some common radiometric dating methods.
Geologists choose a dating method that suits the materials available in their rocks. Measuring isotopes is particularly useful for dating igneous and some metamorphic rock, but not sedimentary rock.It is also possible to use it on authigenic minerals, such as glauconite, in some sedimentary rocks.