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: -

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.