Dating rocks and soils

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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.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.This means that all of the lead 204 on the Earth has been around since the formation of the Earth.Based on extensive sampling of the Earth's crust, scientists determined the present-day abundances of the four isotopes of lead relative to each other and to the parent isotopes that produced three of them.

Decay curve of a radioactive element with a half-life equal to one time unit.One way to think about the closed system of the crystal is to compare it to an hourglass.The grains of sand in the top half of the hourglass are the radioactive parents, and those falling to the bottom are the stable daughters.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.

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