Geological time scale is a record of earth’s history based on the organisms that lived at different times.
The geological time scale is a system of chronological measurement that related stratigraphy (the study of rock strata, especially the distribution, deposition and age of sedimentary rocks) to time, and is used by the geologists, palentologists and other earth scientists to describe the time and relationship between the events that have occurred throughout earth’s history.
The first geological time scale was proposed in 1913 by the British geologist Arthur Holmes (1890-1965). This was soon after the discovery of the radioactivity and using it Holmes estimated that the earth was about 4 billion years old (evidence from radioactive dating indicates that earth is about 4.5 million years old). This was much greater than previously believed.
The geological time scale is divided into five main eras: Coenozoic, Mesozoic, paleozoic, Proterozoic and Archezoic. Each era is divided into periods and each period is divided into epochs.
It is as follows:
There is another kind of time division used – the eon. The entire interval of the existence of visible life is called the Phanerozoic eon. The great Precambrian expanse of time is divided into the Proterozoic, Archean and Hadean eons in order of increasing age.
The names of the eras in the Phanerozoic eon (the eon of visible life) are the Cenozoic (recent life), Mesozoic (middle life) and Paleozoic (ancient life). The further subdivision of the eras into 12 periods is based on in-identifiable but less profound changes in life-forms.