2 types of radiometric dating

04 Sep

Radiometric dating works best on igneous rocks, which are formed from the cooling of molten rock, or magma.As magma cools, radioactive parent isotopes are separated from previously formed daughter isotopes by the crystallization process.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.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.

The ages of these oldest rocks still don't tell us how old the Earth is, but they do establish a minimum age.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.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.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 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.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.