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At any moment, the ratio between them is a measure of the time elapsed, as long as the system remains closed.
One line of evidence involves rocks from outside the Earth--meteorites and moon rocks.
Radiometric dating shows that almost all meteorites are between 4.5 and 4.7 billion years old.
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.
Ideally, the mineral crystals in igneous rocks form a closed system--nothing leaves or enters the crystal once it is formed.
This means that as radioactive parent elements decay, they and their daughters are trapped together inside the crystal.
(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.
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.Tags: Adult Dating, affair dating, sex dating