The geologic time scale is a system of chronological measurement that relates stratigraphy to time, and is used by geologists, paleontologists, and other earth scientists to describe the timing and relationships between events that have occurred throughout Earth’s history. Earth was created around 4.6 billion years ago. Since then there have been supercontinents, dinosaurs, shark eating reptiles, birds of terror, and humans. To keep track of the earth’s history, the timeline was divided into Eras, Periods, and Epochs. There are three Eras, the Paleozoic, Mesozoic and Cenozoic. These are then broken into Periods, which are then divided into the Epochs. Different spans of time on the GTS are usually delimited by changes in the composition of strata which correspond to them, indicating major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the dinosaurs and many other groups of life. Older time spans which predate the reliable fossil record (before the Proterozoic Eon) are defined by absolute age..
The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. The terms eonothem, erathem, system,series, and stage are used to refer to the layers of rock that correspond to these periods of geologic time in Earth’s history.
Geologists qualify these units as Early, Mid, and Late when referring to time, and Lower, Middle, and Upper when referring to the corresponding rocks. For example, the Lower Jurassic Series inchronostratigraphy 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.”
Geologic units from the same time but different parts of the world often look different and contain different fossils, so the same period was historically given different names in different locales.
The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth were made in the late 18th century. The most influential of those early attempts (championed by Abraham Werner, among others) divided the rocks of Earth’s crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a “Tertiary Period” as well as of “Tertiary Rocks.” Indeed, “Tertiary” (now Paleogene and Neogene) and “Quaternary” (now Pleistocene and Holocene) remained in use as names of geological periods well into the 20th century.
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