History of Earth

Age of Earth: Age of Earth is 4.5 billion/4500 million years

Hypothesis of origin of earth

1) George Wufon hypothesis.

This hypothesis was developed by George Wufon in 1749 AD according to which a “huge comet moving around sun collided with it so that large mass of it was scattered which later cooled, condensed and planets were formed including earth.”

2) Jeans and Jeffrey hypothesis (Tidal hypothesis)

This hypothesis was developed in 1917 AD according to which a huge star approached close to the sun. This caused a very large tide on sun which later fragmented, cooled and condensed forming planets including earth.

3) Kant and Laplace hypothesis (Nebular hypothesis)

This hypothesis was developed by Kant in 1755 AD which was later improved by Laplace in 1796 AD according to which “a very large whirling nebula of gas and dust was formed. Its central part condensed and sun was formed and outer part changed into rings. The rings cooled, fragmented from which planets Including earth were formed.”

Geological time scale: The time scale that covers the history of earth from its origin to present is called geological  time scale.

Duration of geologic time scale: Its duration is 4.5 billions or 4500 millions years. Divisions of geological time scale

1. Eons
2. Era
3. Period
4. Epoch

a. Nome Eons of geological time scale.

Eons of geological time scale are:

1. Hadean Eon
2. Archean Eon
3. Proterozoic Eon
4. Phanerozoic Eon

b. Nome different era and write their duration.

Different era are:

1. Pre Cambrian era = 4500 – 57O MYA
2. Paleozoic era = 570-250 MYA
3. Mesozoic era = 250-65 MYA
4. Cenozoic era  = 66 MYA to now

c. Write major events that occurred in precambrian era.

The major events that occurred in precambrian era are:

1. The suitable conditions for life i.e. water, atmosphere, temperature etc were formed.
2. Bacteria and algae evolved.
3. Unicellular and multicellular organisms evolved at the end of this era.

d. When were bacteria and algae evolved ?

Bacteria evolved 3.8 billions year ago and algae evolved 3.2 billion years ago.

e. Write major events that occurred in Paleozoic era.

The major events that occurred Paleozoic era are:

1. Marine animals evolved and developed.
2. Many invertebrates and vertebrates evolved.
3. First fish evolved.
4. Non-flowering plants i.e. fern, wing bearing insects evolved and developed.
5. Amphibians and reptiles evolved.

f. Write major events that occurred in mesozoic era.

The major events that occurred in mesozoic era:

1. Dinosaur evolved, become dominant and extinct at the end of this era.
2. Flowering plants i.e. cone bearing plant evolved and developed.
3. Hills and mountains were formed.

g. Write major events that occurred in Cenozoic era.

Major events that occurred in Cenozoic era are

1. Fist mammals and horse evolved.
2. Elephant, whale, owl etc were evolved.
3. Sharp toothed animals like cat, monkey evolved and developed.
4. Grazing animals evolved and developed.
5. Ancient human evolved.
6. Modern human and other mammals developed and became dominant.
7. Himalaya was formed.

Fossils: The remaining of plants and animals that lived many years ago are called fossils.

Importance of fossils:

1. It is used to know history of the earth.
2. It is used to make geological time scale.
3. It support theory of organic evolution.
4. It is used to know past climate and weather.
5. It is used as fuel.

a. How is fossil formed?

The plants and animals that lived many years ago were buried on the bed of ocean due to various geological cause millions o layers were formed in millions of year. The upper layer and water pressed lower level for long duration so the soft part of them decomposed and hard part of them remained preserved in the sedimentary rocks in form of skeleton, body parts, traces etc which are called fossils. In this way, fossil is formed.

Identification of fossils: Fossils are identified

1. by their skeleton
2. by the body parts
3. by their traces or imprints.
4. by their moulds.

b. Fossils are found only in sedimentary rocks but not in igneous rocks. Why?

Sedimentary rocks are formed in the bed of ocean due to the sedimentation of different materials including dead organisms from which fossils are formed but igneous rocks are formed from lava in which all body organisms get burnt. Therefore, fossils are found only in sedimentary rocks but not in igneous rocks.

Fossil fuel: The source of energy obtained from fossils is called fossil fuel. They are: coal, mineral oil, Natural gas.

Coal: The black-brownish hard sedimentary rock that mainly contains carbon is called coal. Types of coal

1. Peat-least quality coal
2. Lignite
3. Bituminous
4. Anthracite (best quality coal)

Q) How is coal formed?

The plant that lived many years ago were buried in swamps due to various geological changes the upper layers pressed lower layers with high pressure and temperature for millions of years. This caused chemical changes in them forming hard black brownish sedimentary rocks that mainly contains carbon. This is called coal. In this way, coal is formed.

Importance of coal

1. It is used as fuel in industries and factories.
2. It is used as fuel in train.
3. It is used to generate electricity.
4. It is used to make varicose ceramic compounds.
5. It is used to make rubber.

Mineral oil: The dark colored dense liquid found liquid beneath the earth surface which mainly contains hydrocarbon is called mineral oil.

Crude oil: The mineral oil obtained from mine is called crude oil.

Q) How is crude oil purified?

Crude oil is purified by fractional distillation method.

Q How is mineral oil formed? Explain.

The plants and animals that Lived many years ago were sedimented on the base of ocean due to various geological causes. The millions of layers were formed in millions of years which causes high pressure and temperature in lower layers. Those pressure and temperature causes chemical change in them that formed dense and dark liquid that mainly contain hydrocarbons. This is called mineral oil. In this way, it is formed.

Products of mineral oil:

1. Petrol
2. Diesel
3. Kerosene
4. Lubricants
5. Parafin wax
6. Vaseline
7. Tar

Formation of mineral oil.

Importance of mineral oil:

1. It is used as domestic fuel as LPG.
2. It is used as fuel in vehicles and industries.
3. It is used to generate electricity.
4. It is used to construct roads.
5. It is used to manufacture medicines, pesticides, fertilizer etc

Solved exercise

a. Who propounded the “Tidal hypothesis” on the origin of earth and solar system

Sir Jeans and Sir Harold Jeffrey propounded the Tidal hypothesis on the origin of earth and solar system.

b. According to “The world book encyclopedia” what is the estimated age of the earth? 

According to “The world book encyclopedia” the estimated age of the earth is 4.5 billion years

c. In which era did dinosaurs evolve, flourish and disappear from earth?

In Mesozoic era, dinosaurs evolved, flourished and disappeared from earth.

d. In which era did human beings evolve?

Human beings evolved in Cenozoic era

e. How do scientists determine evolutionary trend of organisms on earth.

Scientists determine evolutionary trend of organisms on earth by radioactive dating and fossil dating of rocks and fossils.

f. What is paleontology?

Paleontology is the study of fossils.

g. Where is coal found in Nepal?

Coal is found in Dang.

h. What is carbonization?

The process of forming coal from ancient plants due ta action of high pressure and temperature in them is called carbonization.

i. Write two differences between Mesozoic and Cenozoic era

j. When did fern evolve and flourish? Write two animals developed in this era.

Fern evolved and flourished in Paleozoic era. Two animals developed in this era are: fish and wing bearing insects.

k. Why were certain organisms seen in particular era but become extinct in next era?

Certain organisms were seen in particular era but extinct in next era because extinction is most probably due to glaciation or volcanism or asteroids impact.

Answer the questions:

a. What is the estimated age of earth? Was the earth as it is today when it had originated?Explain

Earth wasn’t as it is today when it had originated. The earth was very hot and there wasn’t suitable conditions for life i.e suitable temperature, atmosphere and water.

b. How is the geological time scale divided and determined?

Geological time scale is divided and determined by study of rocks and fossils.

c. When did first life appear on the earth? What was it like?

The first life appeared on earth at 3.8 billion years ago and it was Bacteria.

d. When did primitive primates appear?

Primitive primates appeared in Cenozoic era

e. What are the ways by which remains of plants and animals were preserved in the rocks?

The plants and animals that lived many years ago were buried on ocean bed. Millions of layer were formed in millions of years so that upper layer pressed lower layer forming sedimentary rocks in which plants and animals were preserved.

f. Why did dinosaurs appear earlier than human beings? Explain

According to theory of organic evolution, less developed organism evolved earlier than more developed organism. The dinosaurs belong to reptiles which is less developed than mammals. It is supported by study of fossils. Therefore, dinosaurs appeared earlier from than humans.

g. Write differences Cenozoic and Paleozoic era.

h. Write differences Mesozoic and Precambrian era.

Notes:

1. Cenozoic era is known as age of mammals because mammals evolved and flourished in this era.
2. Mesozoic era is known as era of reptiles because reptiles like dinosaurs evolved and flourished in this era.
3. Paleozoic era is known as era of amphibians because amphibians evolved and flourished in this era.