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.
Geologic time scale showing the geologic eons, eras, periods, epochs, and associated dates in millions of years ago (MYA). The time scale also shows the onset of major evolutionary and tectonic events affecting the North American continent and the Northern Cordillera (SCAK, south-central Alaska; SEAK, southeast Alaska; NAK, northern Alaska; CAK central Alaska). The following subdivisions and events are included on the time scale, from oldest to youngest. The oldest subdivision of the time scale is the Precambrian (symbolized by PC, X, Y, or Z in the GRI GIS data). The Precambrian is split into three eons: Hadean (4600-4000 MYA), Archean (4000-2500 MYA), and Proterozoic (2500-541 MYA). Global evolutionary and tectonic events that occurred during the Precambrian include (organized from oldest to youngest and including the eon in which the event occurred): formation of the Earth 4,600 MYA (Hadean); formation of the Earth’s crust (Hadean); origin of life (Hadean); oldest known Earth rocks (Archean); early bacteria and algae (stromatolies; Archean); simple multicelled organisms (Proterozoic); Kanektok Metamorphic Complex (oldest known rocks in Alaska; Proterozoic); and complex multicellular organisms (Proterozoic). The next subdivision of the timescale is the Phanerozoic Eon (541.0 MYA-present). The Phanerozoic Eon is split into three eras: Paleozoic (541.0-252.2 MYA; symbolized by PZ in the GRI GIS data), Mesozoic (252.2-66.0 MYA; symbolized by MZ in the GRI GIS data), and Cenozoic (66.0 MYA-present; symbolized by CZ in the GRI GIS data). The Paleozoic Era is split into seven periods (organized from oldest to youngest and including the geologic map symbol used in the GRI GIS data): Cambrian (541.0-485.4 MYA; C); Ordovician (485.4-443.4 MYA; O); Silurian (443.3-419.2 MYA; S); Devonian (419.2-358.9 MYA; D); Mississippian (358.9-323.2 MYA; M); Pennsylvanian (323.2-298.9 MYA; PN; the Mississippian and Pennsylvanian are also collectively known as the Carboniferous); and Permian (298.9-252.2; P). Major evolutionary and tectonic events that occurred during the Paleozoic include (organized from oldest to youngest and including the period in which the event occurred): Wales Orogeny (SEAK; Cambrian); early shelled organisms (Cambrian); rise of corals (Cambrian); trilobite maximum (Ordovician); primitive fish (Ordovician); mass extinction (Ordovician-Silurian); first land plants (Silurian), Kakas Orogeny (SEAK; Silurian); first forests (evergreens; Devonian); extensive plutonism and volcanism in the Yukon-Tanana and Brooks Range (Devonian); first amphibians (Devonian); mass extinction (Devonian); Ellesemerian Orogeny/Antler Orogeny (Devonian-Mississippian); ancestral Rocky Mountains (Mississippian); first reptiles (Mississippian); sharks abundant (Pennsylvanian); coal-forming swamps (Pennsylvanian); supercontinent Pangaea and Tethys Ocean (Pennsylvanian-Permian); and mass extinction (end Permian). The Mesozoic Era is split into three periods (organized from oldest to youngest and including the geologic map symbol used in the GRI GIS data): Triassic (252.2-201.3 MYA; Tr), Jurassic (201.3-145.0 MYA; J), and Cretaceous (145.0-66.0 MYA; K). Global evolutionary and tectonic events that occurred during the Mesozoic include (organized from oldest to youngest and including the period in which the event occurred): flying reptiles (Triassic); first dinosaurs and first mammals (Triassic); breakup of Pangaea begins (Triassic); mass extinction (end Triassic); Talkeetna arc (Jurassic); dinosaurs diverse and abundant (Jurassic); Brookian Orogeny (Jurassic-Cretaceous); early flowering plants (Cretaceous); opening of the Canada Basin and rotation of Arctic Alaska (Cretaceous); placental mammals (Cretaceous); exhumation of the Nome Complex (Cretaceous); extensive plutonism (Cretaceous), mass extinction (end Cretaceous). The Cenozoic Era is split into three periods (organized from oldest to youngest and including the geologic map symbol used in the GRI GIS data): Paleogene (66.0-23.0 MYA; PG), Neogene (23.0-2.6 MYA; N; together the Paleogene and Neogene are also known as the Tertiary [T]), and Quaternary (2.6 MYA-present; Q). The Paleogene is split into three shorter subdivisions called epochs: Paleocene (66.0-56.0 MYA; EP), Eocene (56.0-33.9 MYA; E), and Oligocene (33.9-23.0 OL). The Neogene is split into two epochs: Miocene (23.0-5.3 MYA; MI), and Pliocene (5.3-2.6 MYA; PL). The Quaternary is split into two epochs: Pleistocene (2.6-0.01 MYA; PE), and Holocene (0.01 MYA-present, H). Global evolutionary and tectonic events that occurred during the Mesozoic include (organized from oldest to youngest and including the epoch in which the event occurred): early primates (Paleocene); slab-window subduction (SCAK; Paleocene-Eocene), start of Bering Sea Volcanic eruptions (Oligocene); Alaska Range uplift (CAK; Oligocene); spread of grassy ecosystems (Miocene-Pliocene); modern humans (Pleistocene); ice age glaciations (Pleistocene); extinction of large mammals and birds (end Pleistocene); end of the ice age (end Pleistocene).
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.
Notes
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