Geologists often need to know the age of material that they find.They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years.Others measure the subatomic particles that are emitted as an isotope decays.Some measure the decay of isotopes more indirectly.Unfortunately, none of the original rocks still exist, so scientists had to use less direct evidence to determine the age of the Earth.Another line of evidence is based on the present-day abundances of the various isotopes of lead found in the Earth's crust.After one half-life, 50 percent of the original parents remains; after two, only 25 percent remains, and so on.
These break down over time in a process scientists call radioactive decay.
The discovery gave scientists a tool for dating rocks that contain radioactive elements.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.
Radiometric dating works best on igneous rocks, which are formed from the cooling of molten rock, or magma.
As magma cools, radioactive parent isotopes are separated from previously formed daughter isotopes by the crystallization process.