Radiometric dating is largely done on rock that has formed from solidified lava.
Lava (properly called magma before it erupts) fills large underground chambers called magma chambers.
This commonly occurs if the system has been heated or otherwise disturbed, causing a loss of some of the lead daughter atoms.
Because Pb are chemically identical, they are usually lost in the same proportions. Wetherill has shown that the two points on the Concordia curve intersected by this straight line will represent the time of initial crystallization and the time of the subsequent lead loss.
A pioneering investigation into the crystallization of magma was carried out by N. At successively lower temperature, other minerals begin to crystallize as shown in Figure 3.6.