Class 10 Science Note

History Of Earth

The name of hypothesis given about the origin of Earth is:

Old planetesimal hypothesis: This hypothesis states that earth was formed from the collision of the gases mass and comet. When they collides a huge amount of matter was thrown which in later cooled to form planets.

New planetesimal hypothesis: This hypothesis states that when the gaseous mass and comet come close to each other, due to their gravity ,great tides forms on the gaseous mass which remained as large spiral arms. These arms later cooled to form the solid particles called planets.

Dust cloud hypothesis: This hypothesis states that sun and planets were formed from the large amount of cloud and gases. These dust and gaseous combined due to the pull of gravity and form a huge mass of materials. Thus formed mass became sun and solar energy was produced by nuclear fusion. The rotating cloud forms small globes of gases and dust to form planets .

Nebular hypothesis: A German philosopher named Immanuel Kant proposed nebular hypothesis about the origin of the solar system in 1796 AD, and it was improved by a French astronomer named Laplace later in 1796 AD. According to this hypothesis the sun and the planets were formed from a large whirling cloud of gases and dust. When the cloud cooled and grew smaller, it began to spin faster. As the surface of the cloud cooled by radiation, a ring of matter was formed at the equatorial region of it. The ring was escaped out from the surface of the main mass and the first planet was formed. This process was repeated again and again till the entire solar system was formed and the remaining mass was the sun.

Tidal hypothesis: English astronomers Sir James Jeans and Sir Harold Jeffrey proposed the tidal hypothesis in 1917 AD. They also supposed that a passing comet had exerted a tidal pull upon the gaseous mass but the effect was to cause a long filament of gas to be drawn from the mass. The outer part of the filament escaped in to space the inner part came back into the gaseous mass and the middle part formed a series of round structures of different sizes which were the planets and the remaining mass was the sun.

This hypothesis proposed that the sun and the planets were formed from a large cloud of gases and dust. The light of the star pushed the atoms of the gases and dust to form large particles. These large particles were attracted to each other by the pull of gravity and they began to crowd together. Eventually a huge ball of material formed the sun and the solar energy produced in it by the nuclear fusion.

 

Geological time scale: The time scale that includes the earth origin to present is called geological time scale. This age of the earth is divided into three eons they are as follows:

a. Phanerozoic Eon began about 570 million years ago and it is still continuing a present. In this eon vertebrates and phanerogams are developed. This eon is divided into three eras.

Cenozoic era: Cenozoic Era began about 65 million years ago and is still continuing. It is divided into two periods. Quaternary  and tertiary. The wide variety of plants and animals that we know today came into existence during Cenozoic Era.

Mesozoic era: Mesozoic era began from 250 million years ago and ended at 65.5 million years ago. In this era, different types of hills and mountains were formed. It is supposed that the vital conditions for the survival of life on land, water and air were formed. This era is divided into three periods.

Paleozoic era: Paleozoic era was began from 540 million years ago and ended at 250 million years ago. In this era, the plants and animals were found to have developed which was found from the studies of the fossils remained in sedimentary rocks. Similarly, it is also believed that there was a change in atmosphere and whether. This era is divided into six periods.

 

b. Proterozoic Eon was extended from 570 million years ago to about 2500 million years ago. In this eon invertebrates and cryptogams were evolved.

 

c. Archean Eon extended from 200 to 3800 million years ago. In this eon life was originated on the earth. Proterozoic Eon was extended from 570 million years ago to about 2500 million years ago. In this eon invertebrates and cryptogams were evolved. Phanerozoic Eon began about 570 million years ago and it is still continuing a present. In this eon vertebrates and phanerogams are developed.

 

Fossils:

A fossil is the mark or hardening remains of a plant or animal that lived thousands or millions of years ago. Some fossils are leaves, shells or skeletons that were preserved after a plant or animal died.

Fossils can be identified by following ways:

a. The impression of a whole or a part of any organ of dead body remains in hard ground or stone.

b. The whole skeleton of animals and plants are pressed into rocks.

c. Fossils are identified by morphology.

Organisms are fossilized in the form of cast in sediments. The water inside the sediment dissolves the hard parts, leaving a hollow space in the sediments. The hollow space is called mold, the space is then filled with minerals which harden to form a cast.

 

Fossils are mainly formed due to following reasons:

a. If the animal has hard body parts, the chance of fossilization increases.

b. The organism must be covered by the protective materials shortly after the death.

 

Importance of fossils:

They are main source of coal and petroleum.

They help us to know about the animals and plants  once existed on the earth

 

Minerals 

Mineral fuel is the mixture of hydrocarbons (about 90% to 95%) and the other substances like oxygen, nitrogen, sulphur, etc.

Some of the importance of mineral fuel is as follows:

It is used as lubricant.

It is used as fuel for cooking.

It is used in vehicle as fuel.

It is used in construction of road ( black tar or bitumen )

It is used in manufacturing of plastics, chemical fertilizers, pesticides, paints, explosives, medicines, etc.

 

Coal 

Plants and animals, which died millions of years ago, got buried beneath the Earth. They got covered under clay and sands, which prevents oxygen of air from reaching them. Due to high temperature and pressure inside the Earth, those dead bodies got decomposed in the absence of oxygen. This results in the formation of coal.

The importances of coal are:

It is used as fuel in railway transportation, industries, brick factories, iron etc.

It is used to generate electricity, for manufacturing petrol like natural gas etc.

It is also used for making various organic compounds like benzene, phenol etc.


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