These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio.This age is computed under the assumption that the parent substance (say, uranium) gradually decays to the daughter substance (say, lead), so the higher the ratio of lead to uranium, the older the rock must be.
Ordinarily, these reactions are not complete so that various amounts of each of these minerals may exist at any given time.
The right branch of the reaction series is a continuum in which the earliest formed calcium-rich feldspar crystals react with the sodium ions contained in the melt to become progressively more sodium rich.
Most scientists today believe that life has existed on the earth for billions of years.
This belief in long ages for the earth and the existence of life is derived largely from radiometric dating.
As the crystallization process continues, the composition of the melt (liquid portion of a magma, excluding any solid material continually changes.