Geological Time Scale – Chart, Events, Geological Time Scale UPSC Notes PDF

By BYJU'S Exam Prep

Updated on: November 14th, 2023

The geological time scale categorizes the evolutionary history of the Earth into numerous periods from the beginning of the origin of the world. It presents a record of the major events that occurred during the specific time period. The events of geological time scale brought forth major alterations in the physical, biological, and chemical constituents of the Earth and contributed immensely to the evolution of the Earth. These changes in Earth occurred over millions of years. Hence, it is also referred to as mya. The geological time scale is also referred to as the calendar, which is used for inference and reference of major events in the history of evolution.

The geological time scale presents a relationship between the time or period with the occurrence of the events. The concept was proceeded further by James Hutton and Willliam Smith. The scale is segmented into various units of time. There are numerous eons in the geological time scale, such as the archean eon, proterozoic eon, and phanerozoic eon. It is a highly crucial topic for the UPSC exam; the candidates can get the complete details of the topics to be able to complete the syllabus efficiently in the designated time span.

What is Geological Time Scale?

The Geological time scale depicts the complete picture of the evolutionary history of the Earth. Eon is divided into eras which are further segregated into periods subdivided into epochs. Scientists require to depict the time of the events that occurred in the past, which is a crucial aspect of the research. Hence Geological time scale is of high essentiality to them. It is also referred to as the geological calendar, as the primary purpose of the GTS is to facilitate the organized record of the events of geology.

The fossil records present a crystal clear picture of the events of the geological time scale that can be related to the initiation of a period and the emergence and extinction of the fauna and flora. There is a prevalence of particular and specific flora and fauna during a specific span of the geological time scale.

Division of Geological Time Scale

The geological time scale comprises numerous subunits that assist us in comprehending GTS. These terms represent different time periods and intervals that can be used to specifically refer to a certain period of time. Check the complete details as enlisted here to gain a complete understanding of the terms.

  • Eons represent the largest time period of the GTS. Eons refer to many hundred million years.
  • Eons are further segmented into eras. For example, the phanerozoic era is segmented into Cenozoic, Mesozoic, and Phanerozoic. It is referred to as the tens to hundreds of millions of years.
  • Eras further undergo divisions into periods. An impeccable example of this is the Mesozoic era has been further divided into Triassic, Jurassic, and cretaceous. Periods refer to millions of years to tens of millions of years.
  • Epochs are the further divisions of the Geological time scale. The Cenozoic period is further subdivided into epochs. Epochs refer to hundreds of thousands of years to tens of millions of years.

Geological Time Scale with Events

The various periods and time intervals of the geological time scale are marked by major events. The Geological time scale is segmented into eons such as Archean, Hadean, Proterozoic, and Phanerozoic. The initial three can be combined to be called the Precambrian supreon. The list of the eons has been presented here; check the complete details of the eons.

Hadean Eon

In this eon of geological time scale, oxygen was not present. The time period of the eon accounts for 4540-4000 mya of geological time scale. Fossil records obtained help in the inference of the events of geological time scale. The major events that occurred in the Haldean eon are as follows-

  • The temperature of the Earth was extremely high during this period. The majority of the surface was present in the molten form.
  • Frequent collisions and extreme volcanism also marked their presence in this eon. Abundant radioactive elements having short lifespans were also present during this time interval.
  • On the basis of the pieces of evidence, it is believed that the moon emerged after the collision of a huge impact with the body named Theia. This happened around 4.5 billion years ago on the geological time scale.
  • Certain theories support the occurrence of the events such as the collision of asteroids with numerous planets; Venus, Mercury, Mars, and Earth.
  • Volcanic outgassing led to the formation of the primordial atmosphere and then further led to the creation of the oceans. This marked an essential event in the origin and evolution of the Earth.
  • At an extremely high temperature of 230 degrees Celsius, water still existed in liquid form. The reason supporting this occurrence was the high levels of carbon dioxide in the atmosphere, which created an atmospheric pressure of above 27 atmospheres.

Archean Eon

This eon marked the beginning of life on Earth in geological time scale. Henceforth this holds supreme importance in the pages of history and evolutionary biology. The presence of life forms was restricted to the emergence of single-celled organisms like nuclei.

  • Oxygen was absent in this eon as well. The crust of the Earth started cooling, which further led to the formation of the continents.
  • The carbon dioxide was dissolved in the ocean water, surging its acidity. The end of the eon witnessed the formation of plate tectonics which shares similarities with the modern Earth.
  • Water was found in liquid form. The ocean basins were prevalent in this eon.
  • The earliest stromatolites were unearthed in Western Australia and were discovered in 3.48 billion years old sandstones of geological time scale. The first recognized fossils contained stromatolites.
  • The stromatolites are the microbial mats that are formed by cyanobacteria in shallow water.

Proterozoic Eon

This is the last eon of the supreon Precambrian of geological time scale. It expands from the emergence of oxygen in Earth’s atmosphere to the formation of complex forms on Earth. The bacteria started producing oxygen and embarked on the emergence of life forms. Eukaryotes marked their emergence in this eon, along with a few multicellular organisms.

The predecessors of fungi were also present in this period. According to theories of geological time scale, this eon might have witnessed the Snowball Earth periods marked with extreme glaciation and low temperatures. It highlighted the occurrence of orogeny, meaning mountain building.

Phanerozoic Eon

The microorganisms were there until 580 million years ago in the geological time scale. The initial phanerozoic era of the geological time scale witnessed the emergence of plants. Vertebrates became the dominant species of the oceans.

  • The formation of Pangea occurred during this time of the geological time scale, which was later segmented into Laurasia and Gondwana. Life forms of fungi, plants, insects, and animals started evolving on land.
  • The successors of dinosaurs, the birds, also embarked on its emergence. The evolution of modern animals and human beings was also initiated during this period.
  • The demarcation of the proterozoic and phanerozoic was marked by the presence of the fossils of trilobites.
  • The phanerozoic eon is further segmented into three eras on the basis of the dominant species during the time period.
  1. Paleozoic – Amphibians, fishes, arthropods
  2. Mesozoic – Reptiles, the extinction of dinosaurs, led to the evolution of birds and mammals.
  3. Cenozoic – Witnessed the rise of mammals.

Geological Time Scale Chart

The Geological time scale is a crucial topic for numerous competitive exams. The main eras of the geological time scale are the Cenozoic, Mesozoic, and paleozoic. These were further segregated into major periods. The chart elaborates on the dominance of the species and the occurrence of major events during particular periods.

Major Events of Geological Time Scale
MYA Era Period Geological Time Scale in Order from Oldest to Youngest
541-251 million years ago Paleozoic Cambrian First fishes and first Chordates emerged.
Ordovician Diversification of animal life.
Silurian Occurrence of first vascular plants.
Devonian Fishes are diverse, and amphibians emerged.
Carboniferous Occurrence of trees, first reptiles, ferns.
Permian This period witnessed the huge extinction of several species and the diversification of reptiles.
251- 65 mya Mesozoic Triassic The first mammals and first dinosaurs occurred during this time interval.
Jurassic First birds and diversification of dinosaurs
Cretaceous This led to the extinction of the dinosaurs. This period witnessed the emergence of primates and flowering plants.
66 to 0


Cenozoic Tertiary Diversification of mammals
Quaternary Evolution of mammals

Importance of Geological Time Scale

The geological time scale is the chronological dating system. It is of great importance to scientists to decode the dominance of species at various time intervals. It presents a clear picture of the evolutionary history, hence referred to as a calendar. The study of fossils has facilitated essential information about the geological time scale and traces the history of the emergence, extinction, and evolution of species at certain time intervals.

Today, we can easily trace the emergence, and diversification of mammals, plants, ferns, and reptiles, thanks to the elaborated geological time scale. Researches in the field of Evolutionary biology and geology rely upon GTS.

Geological Time Scale UPSC

The Geological time scale presents essential information about geological events. It is segmented into numerous time units to represent the time scale. It comprises a list of the major events with reference to the time period. This is the way to track the dominance of flora and fauna of a specific region and trace its history. The questions related to this can be asked in the Prelims and Mains exam. Prepare a timetable for preparing for the IAS exam. Clear the concepts and move on the path of achieving excellence and proficiency in the topic.

The candidates can also refer to the geology books for UPSC to get a complete and deep understanding of the events. Practice previous year’s questions to analyze the level of difficulty and the types of questions asked in the exam.

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