The magma tends to be homogeneous by heat convection, but segregation by gravity, supply by surrounding walls and replenishment by others reservoirs tends to scatter the composition and to give heterogeneous mixtures. These requirements are essential to use the method as a chronometer and are summarized in figure 5. Each fossil species reflects a unique period of time in Earth's history. Precambrian Research. One would think that if this were a good science, then such studies would be done and published, but they are strangely lacking. In reality, the overwhelming majority of measurements on the fossil bearing geologic column are all done using one method, the K-Ar method. This decay occurs at a known rate half-life of approx 1.
Only the parent radioactive element is present in the sample at the beginning.
Martin; S. Once one reversal has been related to the GPTS, the numerical age of the entire sequence can be determined.