Humans subdivide time into useable units such as our calendar year, months, weeks, and days; geologists also subdivide time. They have created a tool for measuring geologic time, breaking it into useable, understandable segments. For the purposes of geology, the “calendar” is the geologic time scale. One way to distinguish and define each segment of time is by the occurrence of major geologic events and the appearance (and disappearance) of significant life-forms, starting with the formation of Earth’s crust followed by the appearance of ever-changing forms of life on Earth.
The geologic time scale grew out of necessity: organizing the immensity of geologic time and correlating geologic events on a worldwide scale. No one person or expert committee proposed the geologic time scale used today. It grew by trial and error through the efforts of numerous geologists working independently. Today the recognition of formal subdivisions of geologic time is determined by international committees.
NPS Geologic Resources Inventory, 2018.
As technology of dating methods improves, geologists probably will make small but significant changes to the geologic time scale for years to come. Moreover, as geologists discover more complete sections of rock, which preserve evidence of significant portions of geologic time, and as the International Commission on Stratigraphy evaluates and accepts the best worldwide examples of geologic events preserved in sections of rocks, the geologic time scale will be updated and refined.
The divisions of the geologic time scale are organized stratigraphically, with the oldest at the bottom and youngest at the top. GRI map abbreviations for each geologic time division are in parentheses. Boundary ages are in millions of years ago (mya). Major North American life history and tectonic events are included. Compass directions in parentheses indicate the regional locations of events. Bold horizontal lines indicate major boundaries between eras. Graphic design by Trista Thornberry-Ehrlich (Colorado State University) and Rebecca Port (NPS Geologic Resources Division), adapted from geologic time scales published by the U.S. Geological Survey and the International Commission on Stratigraphy.
Last updated: October 5, 2021