A year is divided into months, weeks and days. The geologic time scale is also divided into time categories. What are the time divisions that are used for the geologic time scale and what determines how they are divided?

The earth's history has been divided into a series of time events by geologists that have different time lengths not like hours but they are variable in length. the basis of this is that geological time is divided using earths notable events. A vivid example is the boundary found between the boundary of Permian and Triassic that is marked by global extinction where large proportion of fauna and flora were rendered extinct.

The first principal subdivision is called the eon which is the largest division of the geological time scale. The Phanerozoic and Precambrian are the two major eons, whereby the Precambrian dates back from the earth's formation to the time of multicellular organisms' emergence which is basically a million of years back, then Phanerozoic eon dates up to the present years. Eons are made up of eras, divisions that span time periods of tens to hundreds of millions of years. The three major eras are the Paleozoic, the Mesozoic, and the Cenozoic. The Cenozoic era is the one we are in today. It began 65 million years ago, right about the time that the dinosaurs went extinct. Eons refers to the largest intervals of geological time and dates back to hundred million of years in duration. The Phanerozoic Eon is classified as being the most recent eon which comprises of three subdivisions called eras namely; Cenozoic, Mesozoic and Phanerozoic.

Eras refers to smaller time intervals that are derived from the Eons whereby notable events in the history of the earth form a major theme in determining the boundaries of the eras, by which we have three major eras namely; the Paleozoic, the Mesozoic, and the Cenozoic. The eras are further subdivided into periods.in reference to the time scale, the Paleozoic constitutes of various divisions namely; Ordovician Permian, Devonian, Cambrian, Mississippian, Silurian, and Pennsylvanian, periods.

Lastly, there exists distinct finer subdivisions of time where the periods of Cenozoic are further subdivided into epochs. The achievement of epoch subdivisions is mainly attributed based on the most recent portion of the geological time scale. the design of geologic time scale was aimed at showing the duration of each time unit, this was achieved by positioning a linear time line on the left side of the time columns. The divisions of the time scale are based on the study of earth and the formation of rocks over a long period of time. The study of the rock strata by geologists all over the world. the information gathered  by geologists was penned down over time to make the geological time scale.