Radiometric dating is least useful for dating


Therefore, their ages indicate when they were formed.Because all parts of the solar system are thought to have formed at the same time (based on the solar nebula theory), the Earth must be the same age as the moon and meteorites--that is, about 4.6 billion years old.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.



Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years.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 fourth, lead 204, is not the result of 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.

At any moment, the ratio between them is a measure of the time elapsed, as long as the system remains closed.