Notes, Exercises, Videos, Tests and Things to Remember 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: -
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:
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 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.
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.
The first dinosaur was formed about _______.
55 million years ago
125 million years ago
225 million years ago
555 million years ago
The earliest man was formed about ______.
6 million years ago
2o million years ago
12 million years ago
2 million years ago
A time scale that covers the earth's history from its origin to the present is called ______.
galvanic time scale
general time scale
geological time scale
geometrical time scale
Paleozoic era is further divided into ______.
Azoic eon is extended ______.
from 570 million years ago to 3500 million years ago
from 600 million years ago to 1300 million years ago
present to 570 million years ago
from 3500 million years ago to 4600 million years ago