Radioactive dating shows that

Note that at time 0, the time of the mineral's formation, the crystal contains only parent atoms.

At time 1, 50% of the parent atoms remain; at time 2, only 25% remain, and so on.

Scientists determined the Earth's age using a technique called radiometric dating.

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.

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.

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.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.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.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.

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