Another line of evidence is based on the present-day abundances of the various isotopes of lead found in the Earth's crust. Three of these isotopes (lead 206, 207, 208) result from radioactive decay of isotopes of thorium and uranium.
The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.
These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay.
Once scientists have determined the parent-daughter ratio, they can use this measurement along with half-life of the parent to calculate the age of a rock containing the radioactive isotope.
Radiometric dating has shown that very old rocks--3.5 billion years or older--occur on all the continents.
If, however, the rock is subjected to intense heat or pressure, some of the parent or daughter isotopes may be driven off.
Therefore, scientists perform radiometric dating only on rocks or minerals that have remained closed systems.Ideally, the mineral crystals in igneous rocks form a closed system--nothing leaves or enters the crystal once it is formed.This means that as radioactive parent elements decay, they and their daughters are trapped together inside the crystal.Over time, radioactive isotopes change into stable isotopes by a process known as radioactive decay.Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years.Radiometric dating works best on igneous rocks, which are formed from the cooling of molten rock, or magma.