When reading about the Archean Eon, it is easy to get confused.  Many of the events that were attributed to the Archean Eon have been moved back to the Hadean Eon due to recent discoveries.  While the earth is much less chaotic than it was before, there is still a lot of change in the climate.

Important Vocabulary

Craton:

A large portion of the crust forming a major part of a continent.  These are often characterized by base rock being exposed at the surface.

Glaciation:

A period of time, generally at least thousands of years where the temperature is colder and so glaciers will grow.

Supergroup:

A set of two or more groups of rock formations that share certain characteristics.

Important Dates

Beginning: 4 Billion Years Ago

End: 2.5 Billion Years Ago

Discussion

There was a lot going on during the Archean Eon.  This Eon has been divided into 4 recognized eras that I will discuss below.  There are no significant events that mark the passage of these different eras.  These eras are mostly defined for ease in communication when comparing ancient timelines.

Eoarchean Era: 4.0 – 3.6 Billion Years Ago

Evidence of the Air In The Rocks

Most evidence from this time period suggested that there was very little to any breathable oxygen in the atmosphere.  For this reason, many scientists believed that photosynthetic life had not yet developed.  Recent studies have found that plants are able to differentiate between Carbon-12 and Carbon-13 in their different chemical properties.  Since these are both stable isotopes of carbon, you cannot assume that a specific ratio in their concentration are due to decay.  Certain ratios of these two carbon isotopes found in eoarchean rock have suggested that photosynthetic cyanobacteria already existed and would have been producing some amount of breathable oxygen.

Late Heavy Bombardment

Another event that often defines this era is called the Late Heavy Bombardment.  This event is characterized by a disproportionately large amount of asteroids colliding with Mercury, Venus, Earth, and Mars.  Evidence of this event has been found in samples of rock brought back from the moon by Apollo astronauts.  A theory called the Nice Model supposes that the planets in our solar system may not have always been found in their current configuration.  This model suggests that movements of the outer gas giants like Neptune and Uranus could have destabilized many asteroids and caused them to change orbit and collide with the inner rocky planets.

The First Continents

The crust is mostly formed at this point, but likely with a few large lava oceans.    Several notable cratons like the Superior Craton begin to form.  The Superior Craton comprises the better part of the  Canadian Shield and is located just north of Lake Superior. have begun to form the nucleus of several continents.

Paleoarchean Era: 3.6 – 3.2 Billion Years Ago

Vaalbara

One of the most notable events in the paleoarchean era is the existence of supercontinents.  Most children have heard of the supercontinent called  Pangeaea, but that is still several billion years in the future.  The first supercontinent that we know of was a combination of the Kaapvaal and Pilbara cratons, which are now found in South Africa and Western Australia respectively.  The names from these cratons were combined to form the name of our oldest known supercontinent Vaalbara.

The Largest Asteroid to Ever Impact The Earth

In the late paleoarchean era, approximately 3.26 billion years ago, it is believed that an asteroid of about 23-36 miles wide collided with the earth somewhere near the Kaapvaal craton.  This is suspected to be the largest object to collide with the earth’s crust.  This asteroid would have been much larger than Mount Everest and would have left a crater about 300 miles across.  There is no known evidence of this crater left today, but we find formations in the barberton greenstone belt (located east of Johannesburg) that suggest a significant impact.  This impact is suspected to have significantly affected the tectonic plate movement of the earth’s crust, and likely helped make the earth what it is today.

Mesoarchean Era: 3.2 – 2.8 Billion Years Ago

Stromatolite

Stromatolite is a rock formation that comes from the existence of life like cyanobacteria that creates mineral layers that get cemented together over time.  Having had a saltwater aquarium, I am well aware of how cyanobacteria forms a film over a surface and can create mineral layers.  Structures like this have been found dating to the mesoarchean era.  This supports the idea of a very healthy population of cyanobacteria on the earth at this time.

Pongola Glaciation

Up until this point in the earth’s history, the world has been quite warm.  This is part of why the previous eon is called the Hadean, a reference to hell.  Whatever the reason, the earliest known cooling of the earth occurred during the Mesoarchean Era, about 2.9 Billion Years Ago.  Some hypothesize that this glaciation occurred because of the abundance of cyanobacteria that changed the atmosphere and caused a freezing.  This earliest glaciation is named for the area in South Africa where we find evidence of it.  The Pongola supergroup is a part of the Kaapvaal craton.

Neoarchean Era: 2.8 – 2.5 Billion Years Ago

The Kenorland Supercontinent

The Kenorland supercontinent formed about 2.72 billion years ago.  By this time our other known supercontinent Vaalbara is believed to be breaking up.  Kenorland is made up of cratons that form the core of parts of North America, Greenland, The Baltic States, Western Australia, and Kalaharia.  It is important to note that supercontinents like this would form and fall apart across millions of years.  The most stable parts of these continents are the cratons that make them up.  One example of this is the Yigarn craton that makes up part of Western Australia and contains zircon crystals that date back to the Hadean Eon.

A World Of Rust

The Neoarchean era is the first time where we find significant evidence of oxygen in earth’s atmosphere.  While, up to this point, we have seen much older evidence of photosynthetic life like cyanobacteria, the neoarchean era is marked by oxidized compounds found in the minerals.  While we generally think of photosynthesis as producing oxygen gas, not all forms of photosynthesis do this.  All photosynthesis means is to absorb energy from the sun, but it does not have to produce oxygen.  It is therefore believed that many of the forms of photosynthetic life before this period were anoxygenic.

Conclusion

The end of the Archean Eon shows a very different world from the earth at the end of the Hadean Eon.  There is no doubt at this point that the earth is very much alive, and the life on it is having a significant effect on the environment.  At this point, though, the most complicated life that we know of are still basically bacteria and slimy cyanobacteria.  The temperature of the earth is also much cooler and likely closer to the temperature that we know today.  It is doubtful that any of us could survive in the Archean atmosphere.  I imagine it would have looked somewhat like the hot pools at Yellowstone National Park.