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.
Recently, rocks over 3.96 billion years old have been dated from northern Canada, Wyoming, and China.
The ages of these oldest rocks still don't tell us how old the Earth is, but they do establish a minimum age.
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.
The discovery gave scientists a tool for dating rocks that contain radioactive elements.