Note on Geological Time Scale

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The age of the earth has been estimated about 4.6 billion years. A time scale that covers the earth’s history from its origin to the present is called geological time scale. The chronology is divided into hierarchy of time intervals: Eons, Eras, Periods and Epochs. The total age of the earth is divided into three Eons as follows: -

  1. Phanerozoic Eon [present to 570 million years ago]
  2. Cryptozoic Eon [570 million years ago to 3500 million years ago]
  3. Azoic Eon [3500 million years ago to 4600 million years ago]

Phanerozoic Eon:

This eon began about 570 million years ago and is still continuing at present. In this eon, vertebrates and phanerogams are developed. The eon is further divided into following eras:

  1. Cenozoic era (present life) [present to 65 million years ago]
    This era is divided into two periods: Quaternary and Tertiary. The Quaternary period includes the present time which is divided into further two epochs: Holocene and Pleistocene. The Tertiary period lasted until 2 years ago and is divided into further five epochs: Pliocene, Eocene, Miocene, Oligocene and Paleocene.
  2. Mesozoic era (Middle life)
    This era began about 65 million years ago and ended about 225 million years ago. The era is further divided into three periods: Cretaceous, Jurassic and Triassic. This era was also called the Time of reptiles.
  3. Paleozoic era (past life)
    This era began about 225 million years ago and ended about 570 million years ago. This era is further divided into seven periods: Permian, Pennsylvanian, Mississippian, Devonian, Silurian, Ordovician and Cambrian. Mississippian and Pennsylvanian are jointly called Carboniferous Period.

Cryptozoic eon:

This eon was extended from 570 million years ago to about 3500 million years ago. In this eon, invertebrates and cryptogams were evolved. This eon is further divided into following eras:

  1. Proterozoic era
    It is extended from about 570 million years ago to 2500 million years ago. In this era, marine invertebrates were formed and few of them had shells.
  2. Archeozoic era
    It is 2500 million years ago to 3500 million years ago. In this era, earliest plant like marine, algae were formed.

Proterozoic era and Archeozoic era are conjointly called Precambrian era.

Azoic eon:

This eon is extended from 3500 to 4600 million years ago. In this eon, life was not originated on the earth. But there were rocks, water and air all through this time on the earth.

  • The age of the earth has been estimated to be about 4600 million years ago.
  • Geological time is divided into different eons, eras, periods and epochs.
  • Phanerozoic, Proterozoic and Azoic are the three eons. Similarly, Cenozoic, Mesozoic and Paleozoic are the three eras of Phanerozoic eons.
  • Cenozoic era is divided into two periods: Quaternary and Tertiary.
  • Proterozoic era and Archeozoic era of Cryptozoic eon are conjointly called Precambrian era.
  • Azoic eon is extended from 3500 to 4600 million years ago. In this eon, life was not originated on the earth.
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Very Short Questions

The estimated age of the earth is 4.5 billion years. The age of the earth has been estimated on the basis of the study of rocks, fossils, etc.

The main eras in which the earth's history is studied are:
i. The Azoic era (including archeozoic and proterozoic eras)- about 4 billion years.
ii. The Paleozoic era- about 330 million years.
iii. The Mesozoic era- about 175 million years.
iv. The Cenozoic era- from 65 million years ago till today.

Rocks of Archeozoic era contain fossils of the first and the most primitive life- bacteria and some algae. Proterozoic era's rocks contain fossils of worms, Sponges, jelly fish, corals and other primitive invertebrates. Algae and bacteria remained plentiful.

In the history of the earth, the time period that lasted from 750 million years ago to 240 million years ago is termed as Paleozoic era. This era is subdivided into seven periods- Cambrian, Ordovician, Silurian, Devonian, Carboniferous (Mississippian and Pennsylvania) and Permian. During the Paleozoic era, fern were the most developed plants reaching up to a height of 100 feet. This era also saw the first true seed plants, the conifers. It was rich in marine animals like corals, sea lilies, sea urchin, eurypterids, sharks, armored fish, etc. and reptiles, Amphibious also appeared as first air-breathing animals with backbones to walk on land.

The period that lasted from 240 million years ago to 65 million years ago in the history of the earth is called Mesozoic era. It is divided into three parts: Triassic, Jurassic and Cretaceous.

The initial part of Cenozoic era from 65 million years to 1.8 million years ago is named as tertiary epoch. This epoch has five periods- Paleocene, Eocene, Oligocene, Miocene and Pliocene. Paleocene- Primary mammals and horses evolved. Eocene- Different mammals (Owl, elephant, whale) developed. Oligocene- Sharp teeth cats and monkeys evolved. Miocene- Herbs developed. Pliocene- Human beings evolved.

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  • The first dinosaur was formed about _______.

    225 million years ago


    555 million years ago


    55 million years ago


    125 million years ago


  • In which period of Mesozoic era did dinosaur extinct?

    Triassic period
    Jurassic period
    Mississippian period
    Cretaceous period
  • The earliest man was formed about ______.

    12 million years ago


    6 million years ago


    2o million years ago


    2 million years ago


  • A time scale that covers the earth's history from its origin to the present is called ______.

    geological time scale


    general time scale


    galvanic time scale


    geometrical time scale


  • In which of the eon was invertebrate developed?

    All
    Cryptozoic Eon
    Azoic Eon
    Phanerozoic Eon
  • Which one of them are jointly called Precambrian era?

    Mesozoic era and Archeozoic era
    Cenozoic era and Proterozoic era
    Paleozoic era and Mesozoic era
    Proterozoic era and Archeozoic era
  • Paleozoic era is further divided into ______.

    seven eons


    five periods


    two periods


    seven periods


  • Azoic eon is extended ______.

    present to 570 million years ago


    from 3500 million years ago to 4600 million years ago


    from 570 million years ago to 3500 million years ago


    from 600 million years ago to 1300 million years ago


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charchit don

when were mountains formed?????


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Rukesh yadav

Who is the discoverer of geological time scale?


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Jagadish poharal

Shouldn't we study about periods and epochs?


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