The Geological Time Scale
Image Above: A geological clock that displays the relationship between definitive events and times that have occurred in Earth's history. As Earth's history is extremely long, you cannot see at this scale the two million year Quaternary period, the time of recognizable as the beginning of life for humans.
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The geological time scale (GTS) is a system of time measurements that outline the Earth's history. It is used to describe the relationship between events and periods of time that have occurred in Earth's history. In fact, the GTS is organized with units of time based on these events and changes in strata (layers of rock) and the characteristics of the periods of time. Different spans of time are determined by the major events and changes to the Earth that happened, such as mass extinctions. For example, the change over from the Cretaceous period to the Paleogene period is marked by the Cretaceous–Paleogene extinction event, which was when non-avian dinosaurs and many other groups of life demised. Geologists, among other earth scientists including paleontologists, use this timescale to date rocks and other samples they find on the Earth. Geologic time tells the 4.54 billion year history of the Earth.
Earth's history is divided into various measurements of time, a supereon, eons, eras, periods, epochs and ages. The largest defined unit of time is a supereon, which is composed of eons. Eons are divided into eras. These eras are then further divided into periods, which then divide into epochs. Epochs turn to our smallest unit of time, ages. The timescale can be displayed in several ways, including graphically with images, but most commonly in tables. Some are displayed below, above and to the left. |