Dating very old rocks
Recently, rocks over 3.96 billion years old have been dated from northern Canada, Wyoming, and China.The ages of these oldest rocks still don't tell us how old the Earth is, but they do establish a minimum age.(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.Over time, radioactive isotopes change into stable isotopes by a process known as radioactive decay.Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.
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.
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.