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.
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.
After one half-life, 50 percent of the original parents remains; after two, only 25 percent remains, and so on.
Decay curve of a radioactive element with a half-life equal to one time unit.
Scientists determined the Earth's age using a technique called radiometric dating.
Radiometric dating is based upon the fact that some forms of chemical elements are radioactive, which was discovered in 1896 by Henri Becquerel and his assistants, Marie and Pierre Curie.
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.
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.
Recently, rocks over 3.96 billion years old have been dated from northern Canada, Wyoming, and China.