Geological eras, basically, are the geochronological measure that represents the time in which the rocks formed within an eratheme developed, a type of unit of measurement belonging to the chronostatigraphic type, which represents the rocks formed during the era geological. It should be noted that the "geological eras" are the second geochronological measure that covers more periods, being exceeded by the super eons, the eons, being their subdivision, and followed by the periods (which divide it), epochs, ages and the crones.
What are geological eras
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To know what they are, it is important to know the concept of era. The history of humanity is divided into time cycles, among which is the era, which is the period of time that refers to an important event that defines its beginning and another of similar relevance that marks its end.
In geology (which is the science that studies the composition and structure of the earth, its rock formations, its processes, characteristics and evolution), the definition of era is the division of the most important transformations that the earth has gone through in its morphology and structure. It should be noted that several geological eras, in turn, make up what is known as eons, which are the largest known units of time (only surpassed by super eons); and, in turn, eras are divided into periods.
The geological history of the earth is made up of four eons, which at the same time are divided into ten eras, the oldest (Hadic) eon being the only one that is not made up of any era, since no rocks are preserved from that eon. It cannot be said exactly how long its duration is, since not all those that are known have the same duration, since it is their events that define it.
What are the geological eras
According to metric units, geological chronometry is classified as:
- The Precambrian, which would be the unit of the super eon, which is divided into Hádic, Archaic, Proterozoic and Phanerozoic. The first aeon was not divided into eras because there are not enough records of it and also, it is estimated that it lasted a short time.
- The archaic eon, meanwhile, is divided into the Eoarchic, Paleoarchic, Mesoarchic and Neoarchic eras.
- The Proterozoic eon in the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic eras.
- Finally, the Phanerozoic eon is classified into the Paleozoic, Mesozoic, and Cenozoic eras.
It was Azoic
Also known as the Azoic Eon or Azoic Period, it is the geological stage that reflects the space of time in which the formation of rocks on the planet occurred before the existence of any vestige of life on earth. The term comes from the Greek "azoikos", whose meaning is "relative to lands where there are no animals"; although it can also come from the Greek a- which means “without” and zoön- which means “animal” or “living being”, which initially meant “without life”.
The Azoic Era was the one that took place about 4.657 million years ago, in which the planet earth was formed and other majestic events took place in the Universe and the Solar System. This stage had its culmination about 4,000 million years ago, and its study has been cumbersome, since there are no fossil remains from which data can be extracted and the rocks formed during it have been modified over the passage of millions of years.. The main events that occurred during this period are:
- The Solar System was generated from stellar dust and gas from the alleged explosion of a massive star (supernova). These particles together with the action of gravity formed the planets, their satellites and asteroids that make up the system.
- The formation of the earth was generated about 4,500 million years ago, and one theory suggests that excess matter from the formation of the sun (dust and gases) was united by gravitational effects, and then cooled and defined its shape.
- It gives rise to the Moon, which is believed to be part of a protoplanet (planetary embryos or very small planets), which would have collided with the earth 4.533 million years ago. One of the remains of this collision was positioned at a distance necessary to orbit around the earth, giving rise to the moon.
- The earth's crust forms and the core cools. Prior to this, there was great volcanic activity and a constant bombardment of celestial bodies such as meteorites, which allowed the contribution of elements that formed the cratons.
It was archaic
This geological stage is divided into several stages, which are Eoarchic, Paleoarchic, Mesoarchic and Neoarchic, each of which has its own events. Namely:
1. Eoarchic: It began about 4,000 million years ago until about 3,600 million years ago, having a duration of approximately 400 million years. This era is the first corresponding to the Archaic Aeon, in turn, belonging to the Precambrian Super Aeon.
- The earth's surface had a solid crust in which the presence of cyanobacteria was evidenced (called blue-green algae in ancient times, which were a type of bacteria that carried out their oxygenic photosynthesis). However, part of the Earth's surface probably had areas made up of lava.
- During this stage the solar system was subjected to a violent asteroid bombardment, also known as " Late Heavy Bombardment ", which took place approximately 4.1 billion years ago to 3.8 billion years ago. It is presumed that during this stellar activity, the moon obtained its craters, since it was one of the most affected bodies during this phenomenon, like the planet Mercury.
- Towards the end of this stage, the hypothetical first super continent called Vaalbará begins to form.
- It is estimated that during it, the first unicellular life forms such as bacteria appeared, although there is no sustainable evidence in the form of microfossils that confirms their dating from this era.
- The Earth's magnetic field was generated along with the crystallization of the Earth's inner core.
- The first self-replicating RNA or ribonucleic acid molecules (similar to DNA) were generated.
2. Paleoarchic: It developed 3.6 billion years ago to 3.2 billion years ago, so its duration was about 400 million years. It was the second era belonging to the Archaic Aeon.
- The first forms of life were verified as fossilized bacteria in microbial mats (multilayer sheet of microorganisms) about 3.48 billion years ago.
- These bacteria developed the ability to self-replicate, that is, to reach a fixed size, according to the species, then reaching binary fission as an asexual form of reproduction.
- Similarly, bacteria develop anoxygenic photosynthesis, as well as the first oxygen-producing bacteria.
- The oldest fossils denoting the first life forms on the planet were stromatolites, which were found in shallow waters.
- In the same belt formed green rocks of Barberton in South Africa due to the collision of an asteroid between 37 and 58 kilometers in diameter.
- The climate was similar to today but lacking in oxygen.
- In the Paleoarchic, the presence and fall of planetesimals (interstellar objects) decreased. This frequency has been maintained since then until today, which is the collision of an object about ten kilometers wide once every hundred million years. This fact facilitated the formation and stability of the new super continents.
- It is presumed that some of the current cratons (continental mass whose structure has not been fragmented by orogenic movements) were formed.
3. Mesoarchic: It took place 3,200 million years ago until 2,800 million years ago, for a total of 400 million years and taking third place in that eon.
- In the Mesoarchic, the first glaciation occurs, probably due to an imbalance as a consequence of the metabolism of the first microorganisms, which is believed to have culminated in the fragmentation of Vaalbará towards the end of the Mesoarchic.
- Other cratons originated, such as that of Tanzania, because the continents were able to increase their size considerably. The collision of several cratons was evidenced, which later originated the super continent Ur.
- The land looked very different from today, since the seas had a green color; while, due to the presence of carbon dioxide, the sky looked reddish.
- The climate suffered alterations due to the emission of gases, however, later it managed to stabilize until it reached temperatures similar to those of today, which facilitated the development of life on earth and the diversification of species. However, the sun had a luminosity of 70% compared to today.
- They give rise to the first reefs, which are believed to come from the stromatolites.
4. Neoarchic: It began 2,800 million years ago until 2,500 million years ago, with an extension of 300 million years. It was the era that culminated the Archaic Aeon.
- He was initiated the oxygenic photosynthesis by bacteria, thus giving start to large emissions of molecular oxygen to the atmosphere. This release of oxygen reacted with minerals and later with greenhouse gases.
- Stromatolites were more concentrated due to cyanobacteria and anaerobic organisms.
- The final stage of the Vaalbará division resulted in great tectonic and volcanic activities, which, according to certain authors, would be the real cause for which the Mesoarchic glaciation came to an end.
- The stabilization of the cratons that exist today is generated, as well as large orogenies (process of formation of mountain ranges by the tectonic movement of plates).
- The combination of the cratons give rise to the super continent Kenorland.
- Oxygen begins to accumulate in the atmosphere, but it is harmful and deadly to all organisms other than cyanobacteria. However, thanks to them, the temperatures were stabilizing, which would later make it easier for other living beings to develop.
Paleozoic era
It represents one of the geological stages that arose 541 million years ago until 251 million years ago, extending for approximately 290 million years. The Paleozoic is the first era of the Phanerozoic Aeon, whose characteristics are:
- Six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
- In the Cambrian Period an important diversification of life was generated known as the Cambrian explosion, in which animal life flourishes in the seas, the first and most of the phyla appear.
- During the Ordovician Period invertebrates dominate and diversify; the first bryozoan corals, starfish, among other taxa appear; and plants and fungi appear on the ground.
- In the Silurian Period the first vascular plants are evident; the first fish with a jaw; sea scorpions grow.
The Devonian period is known for the formation of Euramérica due to the Laurentia and Baltic cratons. Hard-scale fish and amphibians also appear; the first wingless insects; ferns, horsetails and the first seed plants.
- During the Carboniferous Period, large forests with ferns appear, as well as the first flying insects and the first reptiles. Large trees are also formed; and terrestrial vertebrates.
- In the Permian Period, surfaces unite to form the super continent Pangea. Reptiles and parareptiles diversify; the carboniferous flora is replaced by the first plants with true seeds and the first mosses. However, 251 million years ago 95% of life was extinguished, the largest known extinction being called the Permian-Triassic mass extinction.
Mesozoic era
It took place 252 million years ago until 66 million years ago, having an extension of approximately 186 million years. The Mesozoic is the second of the Phanerozoic Aeon and is characterized by:
- It is made up of three periods: Triassic, Jurassic and Cretaceous.
- It is known as the era of dinosaurs and as the era of cycads (ancient group of plants).
- During the Triassic Period, archosaurs (diapsid amniotes or four-legged vertebrate reptiles) dominate the earth in the form of dinosaurs; like ichthyosaurs and notosaurs in the oceans; and like pterosaurs in the air. The first mammals and crocodiles appear.
- In the Jurassic Period, the super continent Pangea split into two forming Gondwana and Laurasia. Gondwana later gives rise to South America, Africa, Australia Zealandia, Hindustan, Madagascar, and Antarctica; while Laurasia is later divided into Eurasia and North America.
- During the Cretaceous Period new types of insects proliferate, the first flowering plants appear; and mammalian animals with placenta appear. Dinosaurs diversify even more and evolve on land.
- After the erosion destroyed the Hercynian (mountain) range, the super continent Pangea was subjected to tension, which caused it to begin to fragment into continents, which began to position themselves in the order they have today.
- The climate was extraordinarily warm, which allowed the evolution and diversification of countless species of animals.
Cenozoic era
It occurred 66 million years ago until today, being the last stage of the Phanerozoic Eon. It is characterized by:
- It is divided into the periods: Paleogene, Neogene and Quaternary.
- The transition from the Mesozoic to the Cenozoic meant the extinction of most of the large reptiles, so that mammals had a greater chance of life.
- The phases of the uplift of the Pyrenees between Spain and France took place, at the same time that sedimentation occurred that filled the Aínsa-Jaca basin in Spain.
- When the sea withdrew, the sedimentation produced the formation of deltas, which became part of the continents, and the erosion also produced changes when the Ebro River made its way to the Mediterranean Sea.
- Karst processes were generated that still take place today.
Quaternary era
This geological stage has taken place from about 2.59 million years ago to the present. It stands out from the rest of the time periods, since it has had a numerous geological activity that is critical for humans. Of this, the deposits of glacial, periglacial and fluvial origin stand out; in addition, glacial sediments of the moraine type are conserved (a hill of unlayered glacial material). Large reliefs are also formed, such as debris cones and slopes debris; also, depressions, like canyons. The Quaternary era is divided into two epochs which are the Pleistocene and the Holocene:
- Pleistocene: This era was considered the era of man, since Homo had its evolution during it. It began 2.59 million years ago, extending until 10,000 BC, that is, about 12,000 years ago.
At this time six great glaciations were generated, and in turn there were interglacial periods in which the climate became warmer. We are currently in the last interglacial period.
- Holocene: It is the most recent time, since it is in force and began in 10,000 BC or 12,000 years ago. At this time the sea level rose considerably, which caused large islands of today to be seen to be separated from their continental shelves.
During it, the ice covered more than a quarter of the earth's surface, reaching the 40th parallel (which is 40º south of the equatorial plane), so that the sea level dropped approximately 100 meters and life had to adapt to the new conditions.
These large blocks of ice were evidenced in Scandinavia to northern Germany, western Russia, and southwest of the British Isles; another system covered much of Siberia; and another in Canada extended to the United States. Almost all existing mountain tops had ice, as did the Arctic and Antarctic.
There were animals such as polar bears, mammoths, reindeer, foxes, elk, bison, saber-toothed tigers, wildcats, rhinos, among others. The flora consisted of tundra, lichens and mosses.
In the same way, the Bering Strait is formed, and what is now known as the Sahara desert began to dry up (which had evidenced rains, pleasant climates and vegetation.
The Holocene is characterized by being warm, so that the fauna and flora are geographically distributed according to the range of climates that exist today, with global temperature variations of 1 ºC. It is thought that the Holocene may end in a new ice age.
In this geological stage, extinctions have been evidenced, which have increased due to human intervention, which ensures that we are in the sixth extinction.
