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. 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.
Some isotopes are stable, but some are unstable or radioactive.
Over time, radioactive isotopes change into stable isotopes by a process known as radioactive decay. 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. Decay curve of a radioactive element with a half-life equal to one time unit. Note that at time 0, the time of the mineral's formation, the crystal contains only parent atoms. Radiometric dating works best on igneous rocks, which are formed from the cooling of molten rock, or magma.