Hello! Welcome to the Archean Eon, also referred to as the Archeozoic eon. Its name comes from the Greek word Arkhe, meaning "beginning, origin". In most texts, the beginning of this eon will be about 3.8 Billion years ago, or Ga, and the end about 2.5 years ago. However, the Archean eon has no strictly defined starting point; its beginning coincides with the oldest known rocks. Because rocks have been discovered that are 4 billion years old, the beginning of the Archean has technically been pushed back, but most sources still refer to it as beginning 3.8 billion years ago.
In older literature, the Archean Eon was a part of the Hadean eon, but that is no longer the case. The Archean is one of the four principal eons of Earth's history, and begins the second division of Precambrian time. It is directly followed by the Proterozoic.
The Archean Eon has been divided into several smaller eras. Read about each era in the section below:
The Eoarchean
The earliest era of the Archean eon was the Eoarchean. It began at the start of the Archean and ended 3.6 billion years ago. During the Eoarchean, the crust continued to form and stabilize from the end of the Hadean. No rocks survive from before the Eoarchean- they were either beaten away or recycled back into the mantle. However, rocks do survive from the Eoarchean itself. In fact, the era is best known through the Isua Greenstone Belt, a rock formation containing metamorphic and sedimentary rock located in Southwestern Greenland.
The Earth was a very different place during the Eoarchean. It was almost entirely water, with a volcano or volcanic island scattered here or there. Even the oceans were unrecognizable; because of dissolved iron compounds, they were green. During this time, Earth produces about 3 times as much heat internally than it does now, which made the planet very geoactive and compensated for a sun that was only 70-75% as bright as it is now.
Life, probably based around methane or other sulfates, may have first appeared at this time. While the first incontrovertible fossils came later, this is the era where we see the first signs of life.
The Paleoarchean
The Paleoarchean was the second era in the Archean, and was the time between 3.6 Ga and 3.2 Ga. This era is defined chronometrically, which means that there are no specific rock layers for this era. However, this was a very important era for life. It is thought that this is the era when the last universal common ancestor of all life on Earth lived. During this era, archea and eubacteria evolved, and the earliest stromatolites (oxygen-producing colonies of blue-green algae, or cyanobacteria) appeared.
The Mesoarchean
The Mesoarchean is another chronometrically defined era from 3.2-2.8 Ga. Stromatolites were increasingly common in coastal waters, and there was oxygen flow in the air for the first time (not much, but some). This is also the era when the first incontrovertible fossils appear. While there is evidence of life, it cannot be completely proven at this time that it was in fact life and not some other abiotic factors.
Continent formation continued, and by the end of the era, the two cratons Kaapral (located in South Africa) and Polbara (located in Australia) were separated. Continental grown during this era eventually led to larger continents such as Pangea, and, eventually, our continents.
The Neoarchean
The final era in the Archean, the Neoarchean began 2.8 billion years ago and ended 2.5 billion years ago. In this era, cyanobacteria started producing huge amounts of oxygen, which finally started getting into the air. In fact, there was so much oxygen that it poisoned life in the early Proterozoic.
Large continents with modern plate tectonics appeared at this time. Continents grew by getting lighter and though through re-melting and reformation of old rocks. Lighter rocks are more buoyant, and so they can more easily float over the liquid mantle.
Not only did the end of the Archean end an era, it also ended an eon. But all joking aside, the end of the Archean marked the end of a time when geologic factors ruled the Earth, and the beginning of one where life took charge.
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