This clock representation shows some of the major units of geological time and definitive events of Earth history. The informal Hadean eon represents the time before fossil record of life on Earth, its upper boundary is now regarded as 4. Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Paleaozoic, Mesozoic and Cainozoic are eras of the Phanerozoic eon. The two million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale. The geologic time scale is a chronologic schema or idealized model relating stratigraphy to time that is used by geologists , paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic time spans presented here agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy , and uses the standard color codes of the United States Geological Survey.
Geological - definition of geological by The Free Dictionary
Radiometric dating often called radioactive dating is a way to find out how old something is. The method compares the amount of a naturally occurring radioactive isotope and its decay products, in samples. The method uses known decay rates. It is used to date many kinds of natural and man-made materials. Fossils may be dated by taking samples of rocks from above and below the fossil's original position. Radiometric dating is also used to date archaeological materials, including ancient artifacts. Radiometric dating methods are used to establish the geological time scale.
The ratification process is still ongoing, and thus a date remains to be decided definitively, but the peak in radionuclides fallout consequential to atomic bomb testing during the s has been more favoured than others, locating a possible beginning of the Anthropocene to the detonation of the first atomic bomb in , or the Partial Nuclear Test Ban Treaty in An early concept for the Anthropocene was the Noosphere by Vladimir Vernadsky , who in wrote of "scientific thought as a geological force". Stoermer subsequently used "anthropocene" with a different sense in the s  and the term was widely popularised in by atmospheric chemist Paul J. Crutzen ,  who regards the influence of human behavior on Earth's atmosphere in recent centuries as so significant as to constitute a new geological epoch.
Radiometric dating , radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. Together with stratigraphic principles , radiometric dating methods are used in geochronology to establish the geologic time scale. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.