Era (geology)

A geologic era is a subdivision of geologic time that divides an eon into smaller units of time.[1] The Phanerozoic Eon is divided into three such time frames: the Paleozoic, Mesozoic, and Cenozoic (meaning "old life", "middle life" and "recent life") that represent the major stages in the macroscopic fossil record. These eras are separated by catastrophic extinction boundaries, the P-T boundary between the Paleozoic and the Mesozoic and the K-Pg boundary between the Mesozoic and the Cenozoic.[2] There is evidence that catastrophic meteorite impacts played a role in demarcating the differences between the eras.

The Hadean, Archean and Proterozoic eons were as a whole formerly called the Precambrian. This covered the four billion years of Earth history prior to the appearance of hard-shelled animals. More recently, however, the Archean and Proterozoic eons have been subdivided into eras of their own.

Geologic eras are further subdivided into geologic periods, although the Archean eras have yet to be subdivided in this way.[3]

List of geological eras in Earth's history

Eon Era Time frame (Ma = million years ago)
Phanerozoic Cenozoic 66 million years ago to present
Mesozoic 251.902 to 66 million years ago
Paleozoic 541 to 251.902 million years ago
Proterozoic Neoproterozoic 1,000 to 541 million years ago
Mesoproterozoic 1,600 to 1,000 million years ago
Paleoproterozoic 2,500 to 1,600 million years ago
Archean Neoarchean 2,800 to 2,500 million years ago
Mesoarchean 3,200 to 2,800 million years ago
Paleoarchean 3,600 to 3,200 million years ago
Eoarchean 4,000 to 3,600 million years ago
Hadean (unofficial) not officially divided into eras Formation of Earth to 4,000 million years ago

See also


  1. ^ "Chapter 9. Chronostratigraphic units". Stratigraphic guide. International Commission on Stratigraphy. Retrieved 2 August 2018.
  2. ^ Erwin D.H. (1994). "The Permo–Triassic Extinction" (PDF). Nature. 367: 231–236.
  3. ^ International Commission on Stratigraphy. "International Chronostratigraphic Chart v2018/07" (PDF). Retrieved 2 August 2018.
Laramide orogeny

The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute. The Laramide orogeny occurred in a series of pulses, with quiescent phases intervening. The major feature that was created by this orogeny was deep-seated, thick-skinned deformation, with evidence of this orogeny found from Canada to northern Mexico, with the easternmost extent of the mountain-building represented by the Black Hills of South Dakota. The phenomenon is named for the Laramie Mountains of eastern Wyoming. The Laramide orogeny is sometimes confused with the Sevier orogeny, which partially overlapped in time and space.

The orogeny is commonly attributed to events off the west coast of North America, where the Kula and Farallon Plates were sliding under the North American plate. Most hypotheses propose that oceanic crust was undergoing flat-slab subduction, i.e., with a shallow subduction angle, and as a consequence, no magmatism occurred in the central west of the continent, and the underlying oceanic lithosphere actually caused drag on the root of the overlying continental lithosphere. One cause for shallow subduction may have been an increased rate of plate convergence. Another proposed cause was subduction of thickened oceanic crust.

Magmatism associated with subduction occurred not near the plate edges (as in the volcanic arc of the Andes, for example), but far to the east, called the Coast Range Arc. Geologists call such a lack of volcanic activity near a subduction zone a magmatic gap. This particular gap may have occurred because the subducted slab was in contact with relatively cool continental lithosphere, not hotter asthenosphere. One result of shallow angle of subduction and the drag that it caused was a broad belt of mountains, some of which were the progenitors of the Rocky Mountains. Part of the proto-Rocky Mountains would be later modified by extension to become the Basin and Range Province.

List of museums in Tasmania

This list of museums in Tasmania, Australia, contains museums which are defined for this context as institutions (including nonprofit organizations, government entities and private businesses) that collect and care for objects of cultural, artistic, scientific or historical interest and make their collections or related exhibits available for public viewing. Also included are non-profit art galleries and university art galleries.

To use the sortable table, click on the icons at the top of each column to sort that column in alphabetical order; click again for reverse alphabetical order.

Raibl Formation

The Raibl Formation is a Middle Triassic geologic formation in the Northern and Southern Limestone mountain ranges of the Eastern Alps, in Central Europe.It preserves fossils dating back to the Norian Middle Triassic sub-period of the Triassic period, during the Mesozoic Era.

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Geologic time
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