Facts about Geological Time Scale: important for UPSC IAS exam

By Nitin Singhal|Updated : September 16th, 2020

The Geological Time Scale (GTS) helps us to understand the earth's evolutionary history. The GTS is very significant in identifying the factors which led the emergence of life forms and major events happened on earth. This article is important for UPSC and State Services Examinations like UPPSC, BPSC, MPPSC, UKPSC, WBPSC, OPSC, RPSC and so on.

Facts about Geological Time Scale: Important for UPSC IAS

What is the Geological Time Scale (GTS)?

  • GTS is a system of chronological dating which is used by various geologists, paleontologists, and other earth scientists to describe the timing and relationships of events that have occurred on earth during earlier times.


  • The earth’s history got divided by GTS into various periods which are based upon various events that transformed the earth’s physical, chemical and biological features.

Time Divisions

  • GTS is basically divided into various time scales like eons, Hadean, Archean, Proterozoic and Phanerozoic. 
    • The first three of these can be referred to collectively as the Precambrian supereon.
    • The eon can be divided into eras, which in turn are divided into periods, which are further divided into epochs.

Hadean Eon

  • This era (4,540 – 4,000 mya) represents the time before a reliable (fossil) record of life.
  • There was no oxygen on earth.
  • In this era, temperatures on earth are extremely hot, and much of the surface was molten due to frequent collisions with other bodies, extreme volcanism and the abundance of short-lived radioactive elements.
  • It is believed that the moon was formed when a giant impact collided with a planet-sized body named Theia (approximately 4.5 billion years ago).
  • A disproportionately large number of asteroids are theorised to have collided with the early terrestrial planets in the inner Solar System, including Mercury, Venus, Earth, and Mars.
  • The outgassing of volcanoes created the primordial atmosphere and then the ocean.
  • Despite excessive temperature of 230° C, the liquid water oceans exist because, at an atmospheric pressure of above 27 atmospheres, which is caused by the heavy carbon- dioxide, the water remains in liquid state.

Archean Eon

  • In this time period, there was the beginning of life on earth.
  • Life was limited to simple single-celled organisms like nuclei (e.g; Prokaryota).
  • There was no oxygen.
  • The cooling of earth’s crust led the formation of continents.
  • In this period, the ocean water was more acidic (due to dissolved carbon dioxide) compared to the Proterozoic period.
  • By the end of this eon, the formation of plate tectonics was as similar as that of the modern Earth.

Proterozoic Eon

  • It is the last eon of the Precambrian “supereon”.
  • It spans from the time of the appearance of oxygen in Earth’s atmosphere to just before the proliferation of complex life (such as corals) on the Earth.
  • Bacteria start producing oxygen that leads to the emergence of life forms.
  • Eukaryotes (have a nucleus), emerge, including some forms of soft-bodied multicellular organisms.
  • Earlier forms of fungi formed around this time.

Precambrian Supereon 

  • It was also during the Proterozoic that the first symbiotic relationships between mitochondria (found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved.

Phanerozoic Eon

  • In this time period, the living beings remained mostly small and microscopic until about 580 million years ago.
  • In the early Phanerozoic eon, plant life appears on the land.
  • Other complex life creatures like vertebrates start dominating the ocean bodies.
  • Later on Pangaea forms and divided into Laurasia and Gondwana.
  • Gradually, life expands to land and all familiar forms of plants, insects, animals and fungi begin appearing.
  • Birds, the descendants of dinosaurs, and more recently mammals emerge.
  • Various other modern animals which also includes human beings also start evolving in this period.

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