The Precambrian Period: Events & Facts

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  • 0:00 Eons of the Precambrian
  • 0:46 Hadean
  • 1:44 Archean
  • 2:47 Proterozoic
  • 4:16 Lesson Summary
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Lesson Transcript
Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

The earth as we know it today is shaped by what happened during the Precambrian period, which covers four billion years! This lesson covers the events that shaped this important period.

Eons of the Precambrian

The primary way we get an understanding of Earth's history is by studying the rocks and fossils leftover from various time periods. The difficulty in studying the Precambrian period is that many of the rocks from that time have eroded away or metamorphosed so they are not recognizable in their original form.

The beginning of the Precambrian period starts with the formation of Earth about 4.5 billion years ago and ends at the first sign of complex life about 540 million years ago. Though the Precambrian Period is often referred to as a period, it's actually the only supereon, which means that it spans multiple eons. The Precambrian has been divided into three eons: the Hadean, the Archean, and the Proterozoic.

Hadean

The Hadean eon took place 4,500 - 4,000 million years ago. As our planet formed, gravity pulled gas and dust from the solar system into what we know now as Earth. Friction then heated the material that would eventually become Earth into a sphere of hot molten lava. At this time, the earth was a very harsh environment; there was no solid crust, just molten lava covering the surface. All the water that we see in the oceans, lakes, rivers, and glaciers were evaporated in the atmosphere. Just imagine the humidity!

If this wasn't enough, asteroid impacts were a common occurrence on the young planet. Some scientists believe the moon was formed by one such collision. It is theorized that a Mars-sized object crashed into Earth early in the Hadean eon. The theory states that the impact caused a large amount of debris to be ejected from Earth, which later combined to form the moon.

Archean

The Archean eon took place 4,000 - 2,500 million years ago. Over time, as the earth cooled, the outer edge of the planet solidified from molten lava to a solid crust. Although it wasn't perfectly solid, large volcanoes were erupting all over Earth and forming new land. With the crust shifting and cracking below, collisions formed the beginnings of early continents.

As the earth cooled even more, the water in the atmosphere began to rain out. All the water that was evaporated in the atmosphere began to pour down onto the newly formed solid ground. By this eon in the Precambrian, most of the water had rained down from the atmosphere and covered Earth with a global ocean.

It is believed that the first signs of life formed during the Archean eon in these oceans. Carbon dating from fossils found in Western Australia show evidence that a small single-cell organism named cyanobacteria, or more commonly known as blue-green algae, was present in the ocean around 3,000 million years ago.

Proterozoic

The Proterozoic eon took place 2,500 - 541 million years ago. The start of the Proterozoic eon is based off the earliest carbon dating of the continental rock that has not been deformed. By the Proterozoic eon, the earth had cooled even more, and the number of volcanoes had decreased from what was seen in the Archean eon.

Plate tectonics were still very active, and by this eon, there were two supercontinents on either side of the planet. However, these continents look nothing like the ones we recognize today. Not only were they shaped differently, but they were completely void of life.

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