subdivision of geological time; shorter than an eon and longer than a period
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An era is a major chunk of Earth's history that scientists use to organize the geological timescale, fitting between the even-larger divisions called eons and the smaller divisions called periods. Understanding eras helps us make sense of how life and Earth itself have changed over billions of years, since each era is marked by distinct differences in the types of organisms that lived during that time and the geological conditions that existed.
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The geologic time scale, proportionally represented as a log-spiral with some major events in Earth's history. A megaannum (Ma) represents one million (10) years.
The geologic time scale or geological time scale describes how geologic time is divided into standardised intervals. It uses the rock record together with the principles of chronostratigraphy to place rock sequences into their relative age positions, and geochronology techniques, such as radiometric dating, to precisely date the boundaries between them. It is used primarily by Earth scientists (including geologists, paleontologists, geophysicists, geochemists, and paleoclimatologists) to describe the timing and relationships of events in geologic history. The time scale has been developed through the study of rock layers and the observation of their relationships and identifying features such as lithologies, paleomagnetic properties, and fossils. The definition of standardised international units of geological time is the responsibility of the International Commission on Stratigraphy (ICS), a constituent body of the International Union of Geological Sciences (IUGS), whose primary objective is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart (ICC) that are used to define divisions of geological time. The chronostratigraphic divisions are in turn used to define geochronologic units.