History of Earth | SEE Astronomy Notes

Hypothesis about the origin of the earth:

a) Planetesimal Hypothesis.
b) Nebular Hypothesis.
c) Tidal Hypothesis, etc.

Planetesimal Hypothesis:

This hypothesis was purposed by George Buffon in 1749 A.D. So it is called Buffon Hypothesis.
According to him the earth along with other planets and satellites were formed when sun collided with large comet millions of years ago.
history-of-the-earth-notes

Nebular Hypothesis:

In 1775 AD, a German philosopher Immanuel Kant purposed Nebular Hypothesis. It was Modified by Mathematician Laplace. According to this Hypothesis sun and planets were formed from a large revolving cloud of hot gases and dust.
As the cloud cooled which grew a central mass and broke down to form sun, planets, and satellites, etc.
Astronomy-notes

Tidal Hypothesis:

This Hypothesis was purposed by Sir James Jeans and Sir Harold Jeffery.
According to this hypothesis, in the past, a large star had reached near to the sun with revolving due to the attraction force of the star a tidal, produced from the sun. The materials of tide become cold and broke into planets and satellites.
class-10-history-of-earth

Nebula:

Definition: The collection of hot gases and dust particles is called nebula.

Geological timescale:

Definition: The time scale which covers the earth history from its origin to present days is called Geological Timescale.

Eon:

Definition: The largest unit of geological timescale having more periods is known as Eon.
It is divided into three parts. They are:
a) Azoic eon.
b) Cryptozoic eon.
c) Phanerozoic eon, etc.

Era:

Definition: The geological history of the earth is divided into 4 periods in the basis of major events occurred at the time of origin and evolution of organisms, which is known as Era.
Example of eras are Azoic era, Paleozoic Era, Mesozoic Era, and Cenozoic era, etc.

Summary of Era:

Era Duration Comments
Cenozoic Era 65 million years ago to the present time. a) Evolution of primary mammals and horse.
b) Development of crocodile, bat, and elephants, etc.
c) Evolution of sharp-teethed cats, tigers, rat, and dogs, etc.
d) Evolution of human beings.
Mesozoic era 250 million years ago to 65 million years ago. a) Evolution of dinosaurs, aves, and reptiles, etc.
b) The dominance of reptiles, so it is called the golden era of reptiles.
c) In this era the development if reptiles are at the peak.
d) Angiosperms plants are developed and distributed, etc.
Paleozoic era 540 million years ago to 250 million years ago. a) Evolution of unicellular organisms.
b) Evolution of marine invertebrates, amphibians, and reptiles, etc.
Azoic era 45 billion years ago to 570 million years ago. a) Evolution of the earth.
b) Evolution of the marine algae.
c) Evolution of unicellular micro-organism (e.g: bacteria, virus, etc.)

Fossil:

Definition: Fossil is the impression or imprint of hard parts of plants and animal on sedimentary rock.
The process of fossil formation is called fossilization. It is a time consuming and very slow process and it takes millions of years.
Diagram:
history-of-earth

Causes:
Following are the causes of fossils:
a) Earthquake.
b) Volcanic eruption.
c) Landslides.
d) Flood.
e) The nuclear test, etc.

Importance:
a) Help in the study of the evolution of plants and animals.
b) Estimation of time periods of animals, planets, and rock.
c) Fossil fuel, etc.

Fossil fuel:

Definition: The fuel obtained from fossilization of plant and animal is called fossil fuel.
Its types are:
i. Solid: Coal
ii. Liquid: Petrolium, Diseal, Kerosine, etc.
iii. Gas: LPG (Liquid petrolium gas), etc.

Coal:

Definition: Coal is a fossil fuel which is black in or brown in carbonaceous deposit formed from plants buried under the earth for millions of years ago.
Its types are :
i. Peat: Low quality of coal.
ii. Lignite: Dark brown coal.
iii. Bituminous: Black and hard coal.
iv. Anthracite: best quality of coal, etc.

Formation of coal:

Coal is formed from the fossils of plants, covered by sediments of soil and sand along with water and were protected from decay. Due to high temperature and pressure, chemical changes occurred in the dead bodies of the plants. As a result, coal formed.

Mineral oil:

Definition: The mixture of hydrocarbons (90-95%) and petroleum is called Mineral oil.
Importance:
Following are the importance of mineral oil:
a) Mineral oil is used to run vehicles.
b) It is used as lubricants.
c) It is used in many industries as fuel.
d) Coal tar, a product of petroleum is used for the construction of roads.
e) It is used for cooking purpose, etc.

Answer the following questions:

a) What is the various hypothesis about the origin of the earth?
Ans: The various hypothesis about the origin of the earth are:
i. Planetesimal Hypothesis.
ii. Nebular Hypothesis.
iii. Tidal Hypothesis, etc.

b) How was the earth originated according to the tidal hypothesis?
Ans: The earth originated according to the tidal hypothesis, in the past, a large star had reached near to the sun with revolving due to the attraction force of the star a tidal, produced from the sun. The materials of tide become cold and broke into planets and satellites.

c) What is geological timescale? What is the estimated age of the earth?
Ans: The time scale which covers the earth history from its origin to present days is called Geological Timescale.
The estimated age of the earth is 4.6×105 years.

d) What is Era? Mention the different era of the geological time scale of the earth?
Ans: The geological history of the earth is divided into 4 periods in the basis of major events occurred at the time of origin and evolution of organisms, which is known as Era.
Following are the different eras:
i. Azoic era
ii. Paleozoic Era
iii. Mesozoic Era
iv. Cenozoic era, etc.

e) What are periods? What are the periods of the Mesozoic era?
Ans: The period is the basic unit of geological time in which a single type of rock system is formed.
The period of the Mesozoic era is Triassic, Jurassic, and Cretaceous, etc.

f) What is a fossil? How is fossil formed?
Ans: Fossil is the impression or imprint of hard parts of plants and animal on sedimentary rock.
It is a time consuming and very slow process and it takes millions of years.

Give reasons:

a) Why coal is called fossil fuel?
Ans: Coal is used as fuel in different industries. So, it is called fossil fuel.

b) Why natural gas is called fossil fuel?
Ans: Natural gas is formed from fossil and used as fuel. So it is called fossil fuel.

c) Why did fossil find in sedimentary rock?
Ans: The fossil is formed from the dead body parts one over another million years after in sediments rocks. In the absence of sedimentary rocks, no remnants of dead bodies can be preserved. So, the fossils are only found in the sedementry rocks.