Earth’s 10 geological eras – From the Eoarchean to the Cenozoic

John Henrique Nov 29, 2020

Geological eras are divisions in Earth’s history. Yes, our planet has a history – quite a long one, by the way.

Since the Earth was formed 4.56 billion years ago, it has gone through different phases that geologists (scientists who study our planet) call eras, which are units on the geological scale (or Earth’s time scale). Eras, in turn, are divided into periods, epochs, and ages.

These are the ten eras in Earth’s history:

  1. Eoarchean
  2. Paleoarchean
  3. Mesoarchean
  4. Neoarchean
  5. Paleoproterozoic
  6. Mesoproterozoic
  7. Neoproterozoic
  8. Paleozoic
  9. Mesozoic
  10. Cenozoic

We, humans, live in the Cenozoic Era, which literally means “new life” and started 65 million years ago.

In addition to the eras, there are other units on the geological scale: eons (larger than eras) and, as we mentioned earlier, periods (subdivisions of eras), epochs (subdivisions of periods), and ages subdivisions of epochs).

Each era is determined by events of great importance, such as the formation of rocks, the appearance of life, or the extinction of a group of animals, such as dinosaurs.

Therefore, it is correct to say that the ages are units that scientists use to divide the geological calendar into its most important events.

Just as we divide the year into months, geologists divide the Earth’s calendar into eras.

Geologic time scale covering the Precambrian and Phanerozoic eons with detail down to the epoch.
Geologic time scale covering the Precambrian and Phanerozoic eons with detail down to the epoch. (Credit: United States Geological Survey).

The four eras of the Archean (3.85 to 2.5 billion years ago)

Eoarchean, Paleoarchean, Mesoarchean and Neoarchean

These four eras are part of the Archean Eon, which started 3.85 billion years ago and ended 2.5 billion years ago.

But the history of the Earth does not begin in the Archean. The first eon, actually, is called Hadean, which begins with the formation of the Solar System and ends with the appearance of the first rocks here on Earth.

In fact, the formation of rocks is the event that marks the beginning of the Archean Eon.

Over the four ages of this eon, some important events in Earth’s history have occurred: intense movement of tectonic plates and the formation of the first continents.

In the Paleoarchean Era, Vaalbara was formed, the first continent on Earth. It ended up splitting apart in the next era.

Fossils from this era show that life already existed on Earth.

The oldest forms of life on our planet, stromatolites, appeared at this time. Stromatolites are marine structures (colonies) formed by cyanobacteria (similar to algae).

The three eras of the Proterozoic (2.5 billion years ago to 542 million years ago)

Paleoproterozoic, Mesoproterozoic and Neoproterozoic

These three eras are part of the Proterozoic Eon, which started 2.5 billion years ago.

The last era of this eon, the Neoproterozoic, ended 542 million years ago, which makes this eon the longest of all, with almost 2 billion years of duration.

In the Mesoproterozoic Era, the supercontinent Rodinia was formed, giving rise to all other subsequent continents. On the margins of this unique continent lived the first animals in Earth’s history.

Before the Proterozoic Eon, the atmosphere was composed mostly of carbon dioxide (CO2).

During the Proterozoic, the Earth’s atmosphere was enriched with oxygen, creating the conditions for the emergence of larger and more complex life forms.

The primitive organisms of the first eon gave way, from the Neoproterozoic Era onwards, to multicellular organisms – the Ediacara fauna, also called Ediacara biota.

The Ediacara fauna was formed by multicellular marine animals, consisting of tissues and organs, which could reach 1 meter in length.

The fossils of these animals, which lived about 600 million years ago, were found in the Ediacara region of Australia.

The end of the Neoproterozoic Era marks the end of the Precambrian, a name that was used in the past to refer to the long period of Earth’s history that encompasses the first three eons: the Hadean, the Archean, and the Proterozoic.

Paleozoic era (542 to 248 million years ago)

The Paleozoic Era is the first of the Phanerozoic Eon, in which we live today. It started 542 million years ago – Paleozoic means “ancient life”.

Despite the name, from the point of view of the geological calendar, it is a very recent era.

The Paleozoic Era is marked by very important events in the history of our planet. One of them, known as the Cambrian explosion, is characterized by the diversification and dissemination of life on the planet.

In the Cambrian Period, the first of the Paleozoic Era, various types of invertebrate animals, such as mollusks, appeared.

Animals with shells also appeared during this period, such as brachiopods and fish. More developed fish, with pairs of fins, appear in the period after the Cambrian, the Ordovician.

Although life was predominantly marine, in the Paleozoic Era some land life-forms appeared: the first plants, amphibians, reptiles, and insects.

These animals spread across the Gondwana supercontinent, formed from fragments of the ancient continent Rodinia. In the Devonian Period, the first forests emerged, providing better conditions for life on land to thrive.

At the end of the Paleozoic Era, mass extinction of several marine and land species occurred. It is not yet known whether this event was caused by climate change or increased volcanic activity.

Mesozoic era (248 to 65 million years ago)

The Mesozoic Era (which means “middle life”) comprises Earth’s history from 248 to 65 million years ago.

A remarkable event in the Mesozoic Era was the formation of the supercontinent Pangea, some 200 million years ago.

After the mass extinction of the late Paleozoic, life diversified throughout Pangea. It was in this era that dinosaurs and flying reptiles (pterosaurs) emerged.

Dinosaurs quickly took over the Earth, but were soon extinct.

65 million years ago, in the Cretaceous Period of that era, a huge meteorite put an end to the existence of this group of animals.

During the Mesozoic Era, the first flowering plants and the first birds emerged. The first mammals are also from that time. We are still not talking about the large mammals we know today, but small animals, which were similar to modern mice.

Cenozoic era (65 million years ago to the present day)

The Cenozoic Era (which means “new life”) began 65 million years ago, with the event that caused the extinction of the dinosaurs, and it is the era in which we are living.

If we take into account that our planet was formed 4.56 billion years ago, the Cenozoic Era corresponds to only 1.19% of the Earth’s lifetime.

In the Cenozoic, there were important events that directly influenced the emergence of human beings.

During this era, the Earth’s surface acquired the aspect we know today: the separation of continents, mountain ranges, and the isolation of Antarctica.

In fact, the continents, as we know them today, emerged through the fragmentation of Pangea, a process that began 130 million years ago and is still taking place.

Of course, this movement is slow – a few centimeters a year. But if we look at the geological time scale (which is measured not in days or weeks, but in millions of years), we can predict that in the distant future neither the continents nor the oceans will look as we know them today.

Geologists estimate that a new supercontinent could form 250 million years from now.

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Written by John Henrique

John has a degree in IT and is the founder of Hyperaxion. He is a science enthusiast and can usually be found reading a book or playing role-playing games.


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