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.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.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.
The fourth, lead 204, is not the result of radioactive 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.(The nucleus of an atom is made up of protons and neutrons.) For example, the element carbon, which always has six protons in its nucleus, has three isotopes: one with six neutrons in the nucleus, one with seven, and one with eight.Some isotopes are stable, but some are unstable or radioactive.
If, however, the rock is subjected to intense heat or pressure, some of the parent or daughter isotopes may be driven off.