The Geologic Record: Definition & Timeline

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  • 0:00 Geologic Record
  • 0:34 Hadean & Archean Eons
  • 2:20 Proterozoic Eon
  • 3:17 Phanerozoic Eon
  • 5:13 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Have you ever wondered how our Earth is subdivided, time-wise? The geologic record has four eons that help divide important parts of our Earth's history, and this lesson goes over them.

Geologic Record

Your life can be divided into major stages, like childhood, your adult years, and your later years. And each of those stages can be divided into divisions, like infancy, teenage years, middle years, and so forth.

Geologists have done something similar with the history of the Earth. They've created the geologic record, a standard time scale that partitions the Earth's history into four eons and their subdivisions of eras, periods, and epochs.

Let's go over these eons and their important subdivisions and events.

Hadean & Archean Eons

Why don't we enter a time machine and travel back to the first eon? As we dial in the year -4.6 billion years ago, we step out of the machine into a hellish scene!

The first eon, called the Hadean, lasted from the origin of the Earth roughly 4.6 billion years ago and ended about 4 billion years ago. At this point in time, the Earth was very hot and had a partially molten surface. It was like hell on Earth, almost literally, hence the name Hadean, which comes from Hades, the ancient Greek god of the underworld. The Hadean time is not a true geologic period because with the exception of meteorites, there were no rocks on Earth at the time.

Since I don't like hot weather, and because the concept of Hades scares me, let's jump back into our time machine and travel to -4 billion years from today. As we step out of the time machine this time around, no pun intended, we start to suffocate because there is so little oxygen in the atmosphere.

We're now in the second eon, the Archean eon, where the Earth cooled enough for the continental plates and rocks to form but had an atmosphere composed of gases toxic to most life-forms of today.

The Archean eon started 4 billion years ago and ended 2.5 billion years ago. During this time, life came into existence, but the only life around was that of prokaryotic cells. Prokaryotes are single-celled organisms that do not have a membrane-bound nucleus or organelles. So, the likes of bacteria are prokaryotes.

Proterozoic Eon

Since the Hadean is too hot, and the Archean eon has nothing for us to breathe, I don't want to stick around here for long either. Let's get back into the time machine and travel to the third eon, the Proterozoic. To get to this eon, we have to dial the machine to -2.5 billion years since it started about 2.5 billion years ago and ended roughly 542 million years ago.

As we leave the time machine this time around, we can breathe a bit better because the concentration of atmospheric oxygen increased quite a bit. It's also a more interesting time because eukaryotic cells have appeared. These are cells that have a nucleus and organelles contained within membranes. These are the cells that make up our own body. It is during this eon that we also notice that algae and soft-bodied invertebrate animals appear on Earth.

Phanerozoic Eon

This is all very exciting, but all the cool stuff appears a bit later. So, let's get back into the time machine one last time to a time roughly 542 million years ago, the start of the fourth and present eon, the Phanerozoic.

The Phanerozoic eon is divided into three eras, the Paleozoic, Mesozoic, and Cenozoic, with each one lasting roughly hundreds of millions of years. And each one of these eras is further subdivided into periods, like the Cambrian, Jurassic, or Neogene to give a few examples. These periods last roughly tens of millions of years each. The periods are further divided into epochs, lasting about a few million to tens of millions of years each.

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