The Age of the Earth: Aeons

The idea of ​​geological time is quite foreign to most of us who are not geologists, mainly because we usually live less than a century and it is not easy for us to imagine periods of several million years (Ma) or Ga (Giga-annum, thousand million years), because in such great magnitudes of time one life or hundreds of successive lives are not enough to perceive any change in the makeup of the planet. Thus, the geological time scale (TSG) establishes divisions both in the age of the rock strata and in the absolute age of the Earth, thus creating stratigraphic and chronological limits, which allows us to form a clearer idea of ​​the age of the Land.

These limits define the different stages into which the TSG is divided (the names you read below are the chronological names used in the official version published in Spanish in 2016) and the lowest of these stages is an Age, a geochronological subdivision that is usually several thousand to a few million years. In these an Epoch is divided, they usually represent significant changes in the populations of plants and animals; Different epochs form a Period, these span between 30 and 200 Ma and represent very drastic changes in the populations of the planet, such as the Cretaceous which ended with the extinction of the dinosaurs, and which marked the beginning of the Cenozoic Era, in which we we find. The eras are of very different lengths, between 200 and 1,000 Ma, and together they make up the largest formal division of the TSG, an Eon, spanning between several hundred and several billion years.

The history of the Earth is divided into 4 eons: Hadeic, Archaic, Proterozoic and Phanerozoic, and their names come respectively from the Greek for Hell, Origin, Early Life and Evident Life. The Hadeic spans from the origin of the Earth to about 567 Ma later; this period was mainly characterized by the Last Heavy Bombardment, as we call the period during which the Earth was basically a globe of lava bombarded by meteorites. The oldest terrestrial mineral that we have found is a zircon of about 4,400 Ma, about 100 or 150 Ma after the Moon formed when the Earth collided with another celestial body. The first molecules that gave rise to what later became living cells originated in the Hadean, and the RNA molecule and the process of photosynthesis may already have existed.

The Archaic marks the end of the last late bombardment and the origin of life on the planet and covers about 1,200 Ma. We know that the continents were divided in the Archaic so there must already be plate tectonics; The atmosphere contained enormous amounts of greenhouse gases that diminished towards the end of the eon, as larger continents and more complex life forms appeared than the prokaryotic organisms that existed throughout the period, as shown by the oldest fossils. known to us, the 3,700 Ma stromatolites (microbial fossils) discovered in Greenland.

The Proterozoic is characterized by major changes in both the Earth’s crust and atmosphere. The great continental masses were finally defined at the beginning of this stage and oxygen levels (thanks to the action of bacteria and cyanobacteria) increased so much that they gave rise to the event known as Great Oxidation, in which mineral deposits throughout the entire planet began to rust, leaving clearly defined bands of iron in the rock strata. These rocks indicate levels of oxygen in the atmosphere that did not exist before, and this facilitated the emergence of the first more complex forms of life. We believe that the first eukaryotic cells and the first multicellular organisms such as some sponges originated at this stage, but there is no scientific consensus on this fact.

We group these three eons into the only Supereon that exists in the ETG, which we know generically as Precambrian, basically because in these three life was quite uninteresting and tiny, so we established another division based on the first explosion of forms. of life, mainly in the oceans, as well as the origin of green plants and the subsequent colonization of the surface, and corresponds to the last and most interesting of the 4: the Phanerozoic, so we will leave it for the next one.

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Ramón Martínez Leyva


A pale blue dot

He is a Computer Systems Engineer. His areas of knowledge are technologies, science and the environment.

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