We explain what the geological eras are, the eons that group them and their periods. Also, what are its characteristics and importance.

geological eras
Geological eras are a type of division in the long history of the Earth.

What are geological eras?

The geological eras are a type of division of the geological time scale, that is, of the scale used to measure the history of the planet and of life from its very beginnings. It is one of the largest units of the scale, which can be found within each eon, that is, the eons are divided into geological eras.

There are ten geological eras, throughout which the process of formation and transformation of the Earth began, as well as the emergence of life, its complexity, its diversification throughout the world and its evolution. Its most recent point is the appearance of human beings and the rise of our civilization.

See also: Primitive man

the eons

The Archean Eon began around 4 billion years ago.

The geologic time scale represents everything. the time elapsed since the origin of the planet until the present.

That enormous span of time, estimated at just over 4.5 billion yearsis divided into four large periods known as eons.

These are:

  • Hadic Aeon. It started about 4.5 billion years ago.
  • Archean Eon. It began around 4 billion years ago.
  • Proterozoic eon. It started about 2.5 billion years ago.
  • Phanerozoic Eon. It began approximately 541 million years ago and continues until today.

The first three aeons, moreover, informally compose a supereon known as the Precambrian.

these eons represent the great periods of earth’s history. Each one is divided into Geological Eras, minor time units, of variable number depending on the eon. These, moreover, can be divided into Periods, even shorter; and these in turn into Epochs. The Epochs in Geological Ages, and the latter in Chronos or Chronozones.

Origin of the geological time scale

Also called geological time scale wave international chronostratigraphic tableis a time frame of reference to chronologically order events in Earth’s history and life about it. For this he is guided by the nature of the rocks and the continents, establishing divisions and periods.

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Based on your relative age and in the superimposition of the layers of rocks and other materials that make up the soil. The transition from one layer to the next represents the equivalent of a trip back in time.

Previously different scales were used depending on where the geologist came fromespecially during the 19th century, when there were numerous discoveries on the subject, due to the rise of mineral exploitation in the world.

To overcome differences this scale was internationalized and unified from 1974, thanks to the creation of the International Committee on Stratigraphy of the International Union of Geological Sciences. There is an approximate equivalent for other known stars, such as the Moon or Mars.

The geological eras of the Earth

Cenozoic era
The Cenozoic era begins about 66 million years ago.

Up to now, there is talk of ten geological eras, distributed differently over the four eons of the geological time scale. These eras are:

  • Hadic Aeon. It does not contain geological eras.
  • Archean Eon. Four geological eras:
    • Eoarchaic. It begins about 4,000 million years ago.
    • paleoarchaic. It begins about 3,600 million years ago.
    • Mesoarchaic. It begins about 3,200 million years ago.
    • Neoarchaic. It begins about 2,800 million years ago.
  • Proterozoic eon. Three geological eras:
    • Paleoproterozoic. It begins about 2,500 million years ago.
    • Mesoproterozoic. It begins about 1,600 million years ago.
    • Neoproterozoic. It begins about 1,000 million years ago.
  • Phanerozoic Eon. Three great geological eras:
    • paleozoic. It begins about 541 million years ago.
    • Mesozoic. It begins about 252 million years ago.
    • Cenozoic. It begins about 66 million years ago.

Division of geological eras

Geological eras, in turn, are divided into periods. These are much more numerous and are the following:

  • It was Eoarchaic. No periods or subdivisions.
  • It was Paleoarchaic. No periods or subdivisions.
  • It was Mesoarchaic. No periods or subdivisions.
  • It was Neoarchaic. No periods or subdivisions.
  • Paleoproterozoic era. Four Periods:
    • Sideric. Started about 2,500 million years ago.
    • Riacic. Started about 2.3 billion years ago.
    • Orosiric. Started about 2.050 million years ago.
    • stateric. Started about 1.8 billion years ago.
  • It was Mesoproterozoic. Three periods:
    • calimic. Started about 1.6 billion years ago.
    • ectasic. Started about 1.4 billion years ago.
    • sthenic. Started about 1.2 billion years ago.
  • Neoproterozoic era. Three periods:
    • Tonic. Started about 1,000 million years ago.
    • cryogenic. Started about 850 million years ago.
    • Ediacaran. Started about 635 million years ago.
  • paleozoic era. Six periods:
    • Cambrian. Started about 541 million years ago.
    • Ordovician. Started about 485 million years ago.
    • Silurian. Started about 443 million years ago.
    • Devonian. Started about 419 million years ago.
    • Carboniferous. Started about 385 million years ago.
    • Permian. Started about 298 million years ago.
  • Mesozoic era. Three periods:
    • triassic. Started about 252 million years ago.
    • Jurassic. Started about 201 million years ago.
    • Cretaceous. Started about 145 million years ago.
  • Cenozoic era. Three periods:
    • paleogene. Started about 66 million years ago.
    • Neogene. Started about 23 million years ago.
    • Quaternary. Started about 2.5 million years ago.
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Importance of Geological Eras

fossil geological ages
The eras allow to classify and order the findings of geological excavations.

The classification of geological time in Eras, on the one hand, allows you to name and order rocks, stones and fossils based on clear criteria found in paleontological excavations. The context in which they were found is used as an indication of when the rock was formed or transformed, or when the creature would have been alive.

Second, these studies provide more and better information regarding the past stages of our planet and the conditions in which life was formed, as well as the changes it must have gone through to be able to reach today.

Main Geological Eras

paleozoic era
The Paleozoic is also known as Ancient Fauna.

The Geological Ages of Phanerozoic Eon were the first to be identified and formulated, by the pioneers of XIX century geology, such as the British Charles Lyell.

Initially, the first identified Ages were called Primary, Secondary and Tertiarybut they soon came to be called Paleozoic, Mesozoic and Cenozoic, understood respectively as “Ancient Fauna” or “Age of Invertebrates”, “Middle Fauna” or “Age of Reptiles” and “New Fauna” or “Age of Mammals”. .

Ages of the Archean Eon

The Hadic Aeon can hardly be investigated, since it covers the formation of the planet. The next most remote is the Archean Eon and, due to its antiquity, the study of its respective four Eras is difficult and demanding.

Rocks have gone through many changes and transformations throughout 4 billion years of history. The eventual fossils of microbial life that arose in this period left hardly any recognizable traces.

In these Ages, therefore, the slow cooling of the Earth occurred to levels that allowed the emergence of life in the first oceans. The atmosphere was very different from the one we know today: ammonia and methane were abundant in it.

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Nevertheless, photosynthesis began towards the end of the last era. That gave rise, over the next eon, to the Great Oxidation of the atmosphere.

Eras of the Proterozoic Eon

eukaryotic cell
Initially, in the Proterozoic Eon, eukaryotic cells arose.

Throughout these three eras oxygen abounded thanks to photosynthesis. Along with the cooling of the planet, it allowed the development of more complex forms of life.

Initially they were eukaryotic cells but they soon arose, at least in the seas, the first forms of multicellular life (metazoans). The continents had already formed and their dispersion and reunion had begun, forming various supercontinents throughout history, the last of which was Pangea.

Climate changes became more evident in the Neoproterozoic Era, when the first glaciation in the planet’s history occurred (in the Cryogenic). Secondly, allowed the conquest of Earth by the first forms of plant life.

Eras of the Phanerozoic Eon

In these last three stages shaped the world as we know it. Initially the Earth was conquered by the first forms of amphibious and later reptilian life.

This is how it happened new evolutionary dynamics that resisted the great mass extinctionsespecially those from the Permian-Triassic (more than 90% of life extinct) and the Cretaceous-Paleogene (about 75% of life extinct).

The Mesozoic was the Age of Dinosaursin which they flourished, prospered, diversified and eventually died out, leaving only birds and small mammals as witnesses.

these ways of life claimed the world as their own in the Cenozoic, when the supercontinent Pangea broke up. Life flourished again in a version more similar to how we understand it today.

Appearance of the Human Being

Cenozoic era
The struggle between hominin species culminated in the arrival of Homo sapiens.

The first ancestors of the human being arose in the Cenozoic Era, towards the end of the Neogene period and the beginning of the Quaternary, in the middle of the Ice Age. At the end of the latter, the struggle between the various species of hominins culminated in the arrival of the Homo sapiens.

Human civilization began when this species, Later, he discovered agriculture and abandoned the nomadic life.


  • “Geological Age” on Wikipedia.
  • “Geological eras”in GeoEncyclopedia.
  • “What were the geological ages like?” (video) in CCH Academic Portal.
  • “Era (geologic time)” in The Encyclopaedia Britannica.
  • “Geologic Eras & Periods & Epochs” at Memorial University (Canada).

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