We explain what they are and what the geological eras are. Also, what are eons, periods, epochs and the geological table.
What are geological eras?
The geological eras of the Earth are the different formal temporal units into which geological time is divided and organized that is, the history of the formation of our planet. Its duration corresponds to each erathem, which is the length of time it takes for rocks in a specific layer of soil to form.
Geological eras are the intermediate units between geological eons (major category) and geological periods (minor category). All of this according to the Geological Time Scale (GTS) managed by specialists in the field.
The geological eras are evidenced from the fossil record and the constitution of the sedimentary layers of the Earth's crust and allow us to temporarily classify and date the finds we make through excavations, such as fossils, rocks or minerals.
The length of each era can be very variable, from a few hundred million years to almost a thousand, depending on the case. There are ten different eras since the end of the Hadic eon, the initial and undifferentiated stage of the Precambrian supereon, around 4.6 billion years ago.
The division of the Geological Time Scale into eras began in the 19th century, when the pioneers of geology and paleontology began their excavation and research work, and faced the need to classify the layers of the Earth.
They noticed that the difference between one layer and another responded to certain climatic, geological and even biological conditions, so when digging deeper, they were going back in geological time. The first three eras identified belong to the Phanerozoic eon, and are the eras that comprise life on the planet: Paleozoic, Mesozoic and Cenozoic.
The geological table
As with the rest of the classifications of the Geological Time Scale, the definition of eras responds to conventions of scientists and specialists in the field, since the history of the planet is really a continuity.
However, thanks to this series of conventions, it was possible to establish the geological Table or Geological Time Scale, which is an ordered and hierarchical diagram in which all the divisions of the planet's history are detailed:
- Eons. The major division of time spans, occasionally organized into even larger supereons. Two eons are recognized: Phanerozoic (which begins 541 million years ago and leads to today) and Precambrian (which begins with the formation of the Earth and culminates with the explosion of life in the seas), although the latter can also be understood as a supereon, which contains three distinct eons: Hadic (4,600 to 4,000 million years ago), Archean (4,000 to 2,800 million years ago) and Proterozoic (2,500 to 635 million years ago).
- You were. The ones we have been talking about, which constitute the large-scale divisions of each eon, comprising a few hundred million years each.
- Periods (or systems). Which are the most specific divisions of each era, in which important changes occurred in the biota (life) of the moment.
- Epochs. Subdivisions of the periods, which take into account the general characteristics of the fauna and flora in said period of time.
What are the geological eras?
As we said previously, there are ten geological eras, comprised in four different eons:
- Hadic Aeon It does not present division into eras, since it is a time that is too remote and with conditions that are too primitive in the formation of the planet, to leave salvageable and studyable evidence.
- Archaic Aeon It includes four different geological eras:
- It was Eoarchaic It begins 4,000 million years ago and ends approximately 3,600 million years ago. Its name comes from the Greek words Eo (“dawn”) and Archios (“ancient”), and is where the oldest known rock formations were formed. It is possible that life appeared in its first cellular forms in this era, but there are no fossil records to prove this.
- Paleoarchaic era It begins 3.6 billion years ago and ends 3.2 billion years ago, and it is the era from which the oldest known fossil forms come, such as bacteria and other primitive photosynthetic organisms (anoxygenic, that is, they did not yet produce oxygen).
- Mesoarchaic era It begins 3.2 billion years ago and ends 2.8 billion years ago. This era witnessed the formation and fragmentation of the first supercontinent, called Vaalbará, and the first ice age in history.
- Neoarchaic era It begins 2.8 billion years ago and ends 2.5 billion years ago. It is the era in which microorganisms began oxygenic photosynthesis, that is, producing oxygen, forever changing the composition of the planetary atmosphere.
- Proterozoic Eon It includes three different eras:
- Paleoproterozoic era It begins 2.5 billion years ago and ends 1.6 billion years ago. This era begins with a gigantic environmental change known as the Great Oxidation, a consequence of photosynthesis sustained by cyanobacteria in the sea. The main mountain belts that still survive today also emerged.
- Mesoproterozoic era It begins 1,600 million years ago and ends about 1,000 million years ago. It saw the breakup of the Columbia supercontinent and the formation of another one called Rodinia, as well as the substantial beginning of the fossil record, with red algae and cyanobacteria colonies.
- Neoproterozoic Era It begins about 1,000 million years ago and ends approximately 542 million years ago. The most extensive glaciation known in the geological record takes place here, in which the so-called “Snowball Earth” was formed. Towards its end, the first multicellular organisms appear, including the first aquatic animals.
- Phanerozoic Eon It includes three different eras, which are:
- Paleozoic era. Also called the Primary Era, it begins about 541 million years ago and ends about 252 million years ago. Its name comes from Greek and means “ancient life”, since in this era the most primitive forms of higher life known from the fossil record emerged. It begins after the disintegration of the supercontinent Pannotia and culminates with the formation of another one called Pangea, dominated by the first reptiles and relatively modern plants, such as conifers.
- Mesozoic era Also known as the Secondary Era, it begins about 251 million years ago and ends just 68 million years ago. Its name, as in the previous case, means “intermediate life”, since it is where most of the ancestors of modern life forms appear. This is where the reign of the dinosaurs occurs, from its beginnings to its dramatic extinction, and also great orogenic events, such as the gradual fragmentation of Pangea and the positioning of the continents more or less in their current location.
- Cenozoic era Also called the Tertiary Era, it began about 66 million years ago and extends to the present day. Its name, similar to the two previous cases, means “New Life”, since the world in this period of time reached its current configuration and modern life forms emerged, that is, the reign of mammals. The first higher primates appear in their last 30 million years, and among them the human being, barely 200,000 years ago.
Continue with: Prehistory
References
- “Geological era” on Wikipedia.
- “What were the geological eras like?” (video) on CCH Academic Portal.
- “The geological eras of planet Earth” in History and Biographies.
- “The geological eras and their inhabitants” in the Digital Library of the Latin American Institute of Educational Communication.
- “Geologic Time” in The Encyclopaedia Britannica.
