Age dating of rocks
In a report of his findings published in 1913 in the journal , Holmes expressed the less-than-ecstatic reception his findings received: "The geologist who ten years ago was embarrassed by the shortness of time allowed to him for the evolution of the Earth’s crust , is still more embarrassed with the superabundance with which he is now confronted." The Earth's age continued to be hotly debated for decades afterward. In the 1920s, Earth's age crept up toward 3 billion years, making it for a time even older than the universe, which was then estimated to be about 1.8 billion years old.
The best estimate for Earth's age is based on radiometric dating of fragments from the Canyon Diablo iron meteorite.
In 1898, Marie Curie discovered the phenomenon of radioactivity, in which unstable atoms lose energy, or decay, by emitting radiation in the form of particles or electromagnetic waves.
By 1904 physicist Ernest Rutherford showed how this decay process could act as a clock for dating old rocks.
For example, the lower Jurassic Series in chronostratigraphy corresponds to the early Jurassic Epoch in geochronology.
The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Evidence from radiometric dating indicates that Earth is about 4.54 billion years old.
The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon.From the fragments, scientists calculated the relative abundances of elements that formed as radioactive uranium decayed over billions of years."It was not until the 1950s that the age of the universe was finally revised and put safely beyond the age of the Earth, which had at last reached its true age of 4.56 billion years," Lewis said.Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon.The three million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.
The geology or deep time of Earth's past has been organized into various units according to events which took place.