A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods.
As radioactive Parent atoms decay to stable daughter atoms (as uranium decays to lead) each disintegration results in one more atom of the daughter than was initially present and one less atom of the parent.
The probability of a parent atom decaying in a fixed period of time is always the same for all atoms of that type regardless of temperature, pressure, or chemical conditions. The time required for one-half of any original number of parent atoms to decay is the half-life, which is related to the decay constant by a simple mathematical formula.
Nevertheless, there is substantial evidence that the Earth and the other bodies of the Solar System are 4.5-4.6 billion years old, and that the Milky Way Galaxy and the Universe are older still.
The principal evidence for the antiquity of Earth and its cosmic surroundings is: Spontaneous breakdown or decay of atomic nuclei, termed radioactive decay, is the basis for all radiometric dating methods.
Burial dating uses the differential radioactive decay of 2 cosmogenic elements as a proxy for the age at which a sediment was screened by burial from further cosmic rays exposure.
Luminescence dating techniques observe 'light' emitted from materials such as quartz, diamond, feldspar, and calcite.
Biostratigraphy does not directly provide an absolute age determination of a rock, but merely places it within an interval of time at which that fossil assemblage is known to have coexisted.
Both disciplines work together hand in hand however, to the point where they share the same system of naming rock layers and the time spans utilized to classify layers within a stratum.
Two methods of paleomagnetic dating have been suggested (1) Angular method and (2) Rotation method.
Magnetostratigraphy determines age from the pattern of magnetic polarity zones in a series of bedded sedimentary and/or volcanic rocks by comparison to the magnetic polarity timescale.
The polarity timescale has been previously determined by dating of seafloor magnetic anomalies, radiometrically dating volcanic rocks within magnetostratigraphic sections, and astronomically dating magnetostratigraphic sections.