Geological scale: Eons, Eras, Periods and Epochs

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A Geological Scale demonstrates the main events that occurred on planet Earth, from its origins to the present day, and their duration periods.

The planet is approximately 4.6 billion years old, divided into a chronological scale to better organize the evolution of events.

These time intervals are known in geology as chronostratigraphic units which are divided into:

  • Eons
  • Ages
  • Periods
  • Seasons
  • Ages

Eon is the name of a large period of geological time, so long that it is practically indeterminate.

As the Earth's geological age is approximately 4.6 billion years, the best interpretation of this passage is made by transforming these years into four Aeons:

  • Hardean
  • Archaean
  • Proterozoic
  • Phanerozoic

One It was geological corresponds to the way in which the continents and oceans were distributed and how the living beings on Earth met each other.

Periodgeological it is a division of the Age. Era is a smaller interval of time within the Period. Already the Age corresponds to the smallest division of Geological Time and has a maximum duration of 6 million years.

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Hadean Aeon

The geological time called the Hadean Eon marks the first phase of the Earth and is characterized by the formation of the Solar System. In its formation, the Earth was condensed material orbiting the Sun.

Due to the force of gravity, this material was merging into different layers and as the planet cooled, acquired its current structure, with an iron core, silicate mantle and an outer crust slim.

This geological period ends with the formation of the oldest rocks preserved on the earth's surface.

The name Hadeano comes from Hades, from the underworld of Greek mythology, and represents conditions considered hellish on Earth during its first part of history.

In this geological passage, much of the planet was molten. As the Earth cooled, it acquired the structure we know today, an iron core, silicate mantle, and thin outer crust.

Archean Aeon

This is when life first appears on Earth. There are no continents yet, just small islands and a shallow ocean.

The word Archean means ancient. This geological period began to form when the Earth had cooled, 4 billion years ago.

The Earth's atmosphere was composed of volcanic gases, nitrogen, hydrogen, carbon and low levels of oxygen. The first oceans begin to form and, in them, the first unicellular organisms - prokaryotes It is eukaryotes.

The Archean Eon is divided into four ages:

  • Eoarchean (3.8 to 3.6 billion years);
  • Paleoarchean (3.6 to 3.2 billion years);
  • Mesoarchean (3.2 to 2.8 billion)
  • Neoarchean (2.8 to 2.5 billion years old).

In these four Eras, the Earth still suffered from intense meteorite bombardment. A supercontinent appears, called Vaalbara, and the first bacteria.

Proterozoic Eon

The Proterozoic Eon is characterized by the emergence of the first multicellular beings. Therefore, the name comes from the combination of the Greek words protect (first) and zoico (life). This is the last stage Precambrian, 3.7 billion years ago.

The first forms of life, green and red algae, begin to develop photosynthesis. The end of the Proterozoic Eon is marked by the glaciation widespread.

The continents were grouped into a single mass called Rodinia, which fragmented and gave rise to the paleocontinents: Laurentia, Baltica, Siberia, Kazakhstan and Gondwana.

The Proterozoic Eon is divided into three ages:

  • Paleoproterozoic (2.5 to 1.6 billion years ago), marked by the emergence of eukaryotic beings;
  • Mesoproterozoic (from 1.6 to 1 billion years ago), when the supercontinent Rodinia was formed and sexual reproduction;
  • Neoproterozoic (1 billion years to 542 million years), when there are already multicellular marine animals.

Phanerozoic Eon

This is the Aeon in which we live and it began 542 million years ago. The word Phanerozoic derives from the Greek and means apparent (zoic) life (phaneros).

The Phanerozoic Eon is divided into three ages:

  • Cenozoic Era
  • Mesozoic Era
  • Paleozoic Era

Paleozoic Era and its Periods

The Paleozoic Era is between 542 and 241 million years ago. From Greek, "paleo" means "ancient" and "zoica" is life. This Era represents two important events in life on Earth, being marked by the first safe record of animals with mineral parts - shells and carapaces.

The second event occurs at the end, 248.2 million years ago, when the largest mass extinction of life on Earth occurs. The Paleozoic Era is divided into six geological periods:

  • Cambrian
  • Ordovician
  • Silurian
  • Devonian
  • Carboniferous
  • Permian

Cambrian Period

This is the first period of the Paleozoic Era and occurred between 545 and 495 million years ago. During this period, the Earth already had animals with exoskeleton, in addition to filamentous microorganisms. It is the beginning of the exploration of abundant and diverse life.

Ordovician period

The Ordovician Period lasted from 495 to 443 million years. This is when the invertebrate fauna and primitive fish appear - without jaws and with pairs of fins.

The so-called Cambrian explosion occurred, with the definition of marine life and the appearance of the first terrestrial organisms, which were lichens and bryophytes. The largest mass extinction of the Paleozoic Era also occurs as a result of the formation of large glaciers.

Silurian Period

It occurred 443 to 417 million ago. This period is marked by the abundance of marine life and recovery from the glaciation of the Ordovician period.

The fauna is made up of jawed fish, freshwater fish and insects such as spiders and centipedes. The flora is marked by terrestrial plants, which appear for the first time.

Devorian Period

The Devorian period began 416 million years ago and ended 359.2 million years ago. It's called "Pisces Period". The Devonian world was populated by plants and animals - most of them extinct.

Terrestrial life also begins to become refined, with the appearance of vascular plants, arthropods and the first tetrapods in shallow waters.

Carboniferous Period

The Carboniferous period lasted from 354 to 290 million years and is named after the vast coal layers that stretch across northern Europe, Asia and North America. It is during this geological period that the Appalachian Mountains and large forests emerge.

In the Carboniferous period, reptiles acquired the capacity for internal reproduction with the presence of shelled eggs. Tropical seas are now home to a great diversity of life, including branchiopods, bryozoans, molluscs and echinoderms.

On land, the first winged insects appeared and the plants already carried seeds. There were ferns, as well as plants with a significant trunk.

Permian Period

It is the last period of the Paleozoic Era and began 299 million years ago, ending 251 million years ago. During this period, the Earth was inhabited by a great diversity of terrestrial insects and vertebrates.

Among the insects were cicadas, lice, beetles, flies, wasps and moths. The Earth's continents are grouped into a single continent, Pangea. The end of the period is marked by the mass extinction of 95% of all life on Earth.

Mesozoic Era and its Periods

The Mesozoic geological era begins when there is only one continent on Earth, Pangea. It lasted between 241 million and 65.5 million ago, comprising the periods: Triassic, Jurassic and Cretaceous.

This Era was marked by intense volcanism and the fragmentation of Pangea into two continents, Laurasia, to the North, and Gondwana, to the South.

Triassic Period

The Triassic period began 251 million years ago and ended 199.6 million years ago. Between recovery from the worst mass extinction at the end of the Permian period.

Life in the Triassic took time to recover and biological diversity was favored by the heat, which reached even the polar regions, and the hot and arid climate.

The first appear dinosaurs and oviparous mammals, marking the repopulation of the Planet. In addition to dinosaurs, the first flying reptiles (pterosaurs), turtles, frogs and mammals appeared.

In the oceans, invertebrates and corals evolve into new species. The variety of molluscs, such as clams and snails, increases, and the first sharks and marine reptiles appear.

Jurassic Period

The Jurassic period lasted between 205.7 and 142 million years ago. The fauna during this period is quite varied, and the waters invade the continents, forming large intercontinental seas.

Among the fauna examples are crustaceans, fish with modern structure, amphibians and the first birds and small marsupial mammals appear.

The seas are filled with an immense variety of sharks, bony fish, marine crocodiles and other animals of all sizes.

Reptiles extend throughout the Earth's domain. That is why this period was called the "Age of Dinosaurs". There were also flies, butterflies and dragonflies. Much of the Earth was covered in trees and flowering plants.

Cretaceous Period

The world underwent significant changes during the Cretaceous period, which was between 145.5 million and 65.5 million years ago. This period is the heyday of dinosaurs.

The Earth was also dominated by plants such as ferns and conifers. Marine diversity is great and there are not many differences in the fauna recorded in the Jurassic period.

The fractures on the Pangea continent are visible, the continents assume their current shape and this condition is fundamental for the change of life on Earth.

Dinosaurs became extinct as a result of the fall of a meteorite measuring 10 kilometers in diameter on the Yucatán peninsula, in Mexico.

The event left the Earth covered in dust for months and killed plants, preventing photosynthesis and exterminating the dinosaurs.

Among the reptiles, only crocodiles, lizards and turtles remained. The Cretaceous period is also marked by the emergence of placental mammals.

Cenozoic Era and its Periods

The Cenozoic Era is current geological time, beginning 65 million years ago. The term comes from the Greek, kaines (recent) and zoica (life). It is divided between Periods Paleogene, Neogene It is Quaternary.

Paleogene Period

The Paleogene period begins 65 million years ago and ends 23 million years ago. It was during this period that modern mammals appeared. The fauna, however, does not differ much from what occurred in the Cretaceous period.

The Paleogene is divided into three Epochs: Paleocene, Eocene It is Oligocene. It is during these times that the formation of mountain ranges in North America takes place.

The marine fauna displays specimens of pelecypods, gastropods, echinoids and foraminifera. As remnants of the Cretaceous, on the planet, there are squids, octopuses, turtles, snakes and crocodiles.

It is during this period that small mammals, ancestors of current rodents, appear, more precisely in the Paleocene Epoch.

Marine life experienced intense diversification during the Eocene Epoch (58 to 33.9 million years ago) when tectonic plates were also stabilized.

Birds undergo important diversification. Bony fish and ancestors of ostriches, rhinos, horses, whales and manatees appear.

In the Oligocene the first forms of monkeys and great primates appear.

Lasting from 33.9 to 23 million years, the Oligocene is marked by the development of dogs and large cats, such as the saber-toothed tiger.

Neogene Period

The Neogene lasted from 23 to 2.6 million years and was divided into two Epochs: Miocene and Pliocene.

The diversification of fauna and flora is intense in these two Eras. The Miocene lasted from 23 to 5.3 million years and the Pliocene was from 5.3 to 2.6 million years.

During these times, seals, sea lions and whales appear. In the terrestrial environment, mammals such as hyenas, giraffes, cattle, bears and mastodons live.

Large mammals, such as horses, rhinos, camels and antelopes, still appear in the Miocene - the longest period of the Cenozoic Era. The variety is favored by the change in ocean circulation, which also led to the evolution of marine vertebrates.

The hallmark of the Pliocene Epoch is the appearance of hominids, more precisely, the Australapithecus, in South Africa.

Quaternary Period

This period began 2.6 million years ago and continues to the present day. In the Quaternary, the continental masses were already more or less as they are today. It has two Eras: the Pleistocene and the Holocene.

There is an intense development of mammals, the Homo sapiens, there is greater diversity of flora and fauna, in addition to intense climate changes.

Pleistocene Epoch

In this geological period, intense periods of glaciation and interglaciation occurred, characterized by periods of extreme cold or hot and dry conditions and lasted from 2.6 million to 11,700 years.

The study of this Era is important for understanding the climate direction of Planet Earth. It is very common for researchers to study the Pleistocene to go to Antarctica to obtain samples of air bubbles from this Era to analyze the atmosphere of the past.

At this time comes the Homo sapiens (350 to 200 thousand years ago) that occupied, throughout the Quaternary, almost all the continents of the world.

Holocene Epoch

Holocene is the geological term that covers the last 11,500 years of Earth's history. Therefore, this is when man appears.

The term comes from the combination of the Greek words holo (whole) and kainos (recent). This is considered the most important geological moment on Earth, with significant changes in the climate regime, which directly impacts the consolidation of biological development.

Bibliographic references

POTAPOVA, M. S. Geology as a historical science of nature. Terrae Didatica, v. 3, no. 1, p. 86–90, 1968.

RICCARDI, A. W. Chronostratigraphic Scale. Magazine of the Argentine Geological Association, v. 73, no. 2, p. 292–293, 2016.

CASTILHO, Rubens. Geological scale: Eons, Eras, Periods and Epochs.All Matter, [n.d.]. Available in: Access at:

See too

  • Cenozoic Era
  • Paleozoic Era
  • Mesozoic Era
  • Precambrian
  • Jurassic Period
  • Archaeozoic Era
  • Ice age
  • Proterozoic
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