Geological Time Scale
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: -
- Phanerozoic Eon [present to 570 million years ago]
- Cryptozoic Eon [570 million years ago to 3500 million years ago]
- Azoic Eon [3500 million years ago to 4600 million years ago]
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:
- 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.
- 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.
- 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.
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:
- 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.
- 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.
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.
Things to remember
- 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.
- It includes every relationship which established among the people.
- There can be more than one community in a society. Community smaller than society.
- It is a network of social relationships which cannot see or touched.
- common interests and common objectives are not necessary for society.
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Questions and Answers
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.