A Brief Overview of the Archean Eon for Middle School Science Classes

Dec 3, 2024

3 min read

Earth history is a fascinating topic! It is also a massive topic! The reading passage below will teach you all about the Archean Eon. If you want to learn more (and why wouldn't you?!) you can check out my Earth History page. I also have all of my passages available at Teachers Pay Teachers. They come with so many extras to get your students thinking about the content! I also recommend scrolling to the bottom of the page to check out my digital picture book on the Archean Eon!

About 4 billion years ago, during the Hadean Eon, the Earth was pummeled by rocks from space. Uranus and Neptune’s orbits were pushed outwards by the interaction between Jupiter and Saturn. This caused the asteroid belt, currently located between Mars and Jupiter, to move inwards. As a result, Earth and the moon were hit with thousands of asteroids.

The impact craters from this event are still visible on the moon. The Hadean Eon ended after the Late Heavy Bombardment. The Archean Eon began around 4 billion years ago with the formation of Earth’s crust. While we have evidence that solid rocks existed during the Hadean Eon, none of these rocks are around today. The oldest rocks on Earth formed during the beginning of the Archean Eon. Most were igneous rocks.

The first organisms on Earth didn’t breathe oxygen. Instead, they got energy from the chemicals in the environment. Earth’s atmosphere was filled with methane, nitrogen, carbon dioxide, and ammonia. These chemicals made the sky red instead of blue. Organisms that get energy without oxygen are called anaerobic. Oxygen was toxic to these anaerobic prokaryotes. Around 3.5 billion years ago, the first aerobic organisms appeared. Oxygen is not toxic to aerobic organisms. These aerobic organisms were called cyanobacteria. They lived on the ocean floor. As they made energy, they would release oxygen into the water. However, the oxygen couldn’t get into the atmosphere because it would bond with the iron that was dissolved in the oceans.

During the Hadean and Archean Eons, there was more volcanic activity than there is today. When they erupt, volcanoes release iron. As a result, a lot of iron dissolved in the oceans. Oxygen from the aerobic prokaryotes and iron from the volcanoes interacted to form hematite. Hematite is red. You can still see a band of red hematite in rocks from the Archean Eon.

The beginning of the Archean Eon was also marked by the first life on Earth. Life first appeared in the oceans. It was prokaryotic. A prokaryote is a simple organism with no organelles. Bacteria are prokaryotic.

While oxygen was being produced, not much of it was getting to the atmosphere. That changed about 2.7 billion years ago with the formation of Kenorland. Kenorland was a massive supercontinent. Before Kenorland, some small islands had formed, but most of the Earth was covered in water. As Kenorland

formed, it pushed the seafloor up around it. The cyanobacteria that had been on the ocean floor

were now exposed to the Sun. The cyanobacteria were able to photosynthesize or make energy using sunlight. As a result, they pumped more oxygen into the water. The cyanobacteria began producing

more oxygen than could be bonded to the dissolved iron in the ocean, so the oxygen was forced into the atmosphere.

While cyanobacteria are single-celled organisms, they clump together to form mats. These mats trap sediments that eventually form sedimentary rock. These rock formations are called stromatolites. Today, we can see fossilized stromatolites from the Archean Eon. There are also modern stromatolites formed by living cyanobacteria in Shark Bay in Australia.

The Archean Eon ended just before one of the greatest extinction events in Earth’s history. The cyanobacteria, a tiny, one-celled organism, would change the fate of everything on Earth.

Archean Eon Picture Book

Looking for another way to learn about the Archean Eon? Check out this picture book version. The pages are a part of my Earth History bundle on Teachers Pay Teachers.