Introduction

The branch of science which deals with the study of earth is called geology and the scientists who study the earth are called geologists.

Some hypothesis about origin of the earth     

The earth is the only planet where life is possible. It has favorable conditions to support life. The earth was originated about 4.5 billions of years ago. Different scientists have given the different hypothesis about origin of the earth. Some of them are as follows.

Planetesimal hypothesis:

In 1745 AD. A French philosopher George Wofon put forward this hypothesis about the origin of the earth. According to this hypothesis, the sun and another star collided together, as a result of which a very large amount of matter was thrown out of the sun. These hot matters from the sun cooled to form the earth and the other planets.

Nebular hypothesis:

In 1755 AD, a German philosopher Immanuel Kant proposed this theory. According to this hypothesis, the sun and the planets were formed from the large whirling cloud of hot gas and dust particles called nebula. As the cloud cooled and grew smaller, it began to spin faster causing rings of matter to break away from the outer edge of the cloud. The main part of the cloud formed the sun and the rings formed the earth and the planets.

Tidal or gaseous hypothesis:

•  Sir James and Jeffery in 1919 proposed this hypothesis. According to this hypothesis, a large star reached near the sun while it was revolving. Due to the attraction power of the star, a tide was produced from the sun. The material of the tide became cool and broke into planets and satellites.
• The origin and age of the earth have been explained on the basis of different scientific researches. The age of the earth has been estimated on the basis of study of fossils and rocks.
• The earth evolved about 4.5 billion years ago. At that time the earth was very hot and there was no suitable environment for the evolution of living beings. After millions of years, the earth became cold and suitable for the evolution of living beings. Hence, living beings were evolved after millions of years of the formation of the earth. Some facts about the earth

Geological time

The age of the earth from its origin to the present is called geological time. The geological time is divided into different time periods on the basis of changes on the earth and organisms. These are eon, era, periods and epoch. Eons are the longest duration of time period i.e. billions of years. There are four eons. They are

• Hadean eon (4.5 to 4 billion years ago)
• Archean eon (4 to 2.5 billion years ago)
• Proterozoic eon (2500 to 541 million years ago)
• Phanerozoic eon (541 million years ago to present)

The entire geological time period is divided into four eras. They are Pre Cambrian (azoic era), Paleozoic era, Mesozoic era and Cenozoic era.

1. Pre Cambrian(Azoic era)

• This era began about 4.5 billion years ago and ended about 570 million years ago. The main events of this era are.
• The origin of the earth took place.
• The earth was very hot and no possibility of life.
• The igneous rocks, iron and gold mixed genesis and granite were formed.
• At the end of the era, some tracts of soft organisms were believed to be found.

2. Paleozoic era

• This era began about 570 million years ago and ended about 240 million years ago. The main events of this era are.
• Changes occurred in the atmosphere and weather but the earth was still hot.
• Formation of sedimentary rocks took place.
• Fossils of different types of plants and animals were formed.
• Marine shelled animals first appeared. Aquatic and terrestrial invertebrates like winged insects, fishes and amphibians were evolved.
• Different plants like blue algae, club mosses, ferns and conifers were originated.
• Different invertebrates and mammals like reptiles became extinct.
• The amphibians were dominant in this era so it is called era of amphibian.

Periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian

3. Mesozoic era

• This era began about 240 million years ago and ended about 65 million years ago. The main events of this era are
• Different types of hills and mountains were formed
• Climatic conditions became suitable for the evolution and development of different organisms.
• The plants like fern algae, confers etc. formed in Paleozoic era continued to exist in this era.
• Flowering plants life appeared in this era.
• Very large dinosaurs and other reptiles evolved and became extinct at the end of this era.
• The first warm blooded animals like mammals and bird appeared first.
• In this era dinosaurs/reptiles were abundant and dominant on the earth. Therefore this era is called age of reptiles.

Periods: Triassic, Jurassic and Cretaceous.

4. Cenozoic era

• This era began about 65 million years ago to the present.  The main events of this era are.
• Ice accumulated in the Himalayan region and polar region and the earth became suitable for the existences of different types of new plants and animals.
• The Alps, Andes and Himalayan Mountains were formed breaking and expansion of rocks, volcanic eruption etc. took place.
• The mammals and horses first appeared on the earth.
• The sharp teethed cats, dogs and monkeys were originated and elephant and whale were developed.
• Various herbs or grasses and other new plants were evolved.
• Large mammals became extinct and human beings first evolved on the earth.
• This era is dominated by human beings and herbaceous plants.

Periods: tertiary and quaternary

Fossils

The fossils are the dead remains imprints of the plants and animals that were buried under the earth millions of years ago. Fossils are found in the sedimentary rocks.

Sedimentary rocks are formed by the slow deposition of sediments like sands, silts or volcanic ash on the bottom of the ponds, lakes or oceans. Sediments cover the entire bodies or parts of dead plants and animals. The soft parts decay leaving only the hard parts like bones, scales, shells etc. Slowly over long period of time the deposited materials are changed into sedimentary rocks. Thus fossils are found in the sedimentary rocks.

Identification of fossils

The fossils can be identified as

• By the study of impression or imprints of whole body or parts of any organ of dead organisms.
• By the study of skeleton preserved in the rocks.
• B the study of the casts or moulid found in sandstone or soil.

Formation of fossils

The fossils are formed in sedimentary rocks. The process of formation of fossils is called fossilization. The fossils are formed by different ways.

               When organisms die, their parts are covered by soil and sand on the sea bed or during volcano, earthquake, flooding etc. the soft or delicate parts of the plants and animals decay and other parts remain compressed on mud or soil. The compressed parts form impression and preserve in the sedimentary rocks. Sometimes the hard parts like skeleton remain preserved in the rock. When dead bodies are covered in by soil, sometimes the decaying parts form hollow spaces in the soil. Such vacant or hollow spaces are filled with other materials or minerals to form casts or moulds. In this way fossils are formed.

Conditions for formations of fossils

• The bodies of plants and animals must be covered by protective material like soil, sand etc shortly after the death.
• The buried organisms must have hard parts on their bodies.

Importance of fossils

• They provide the evidence of evolution of organisms.
• They give information about the development of geological eras and periods.
• They help to know about extinct animals and plants.
• They (like coal, mineral oil) are the main sources of energy.

They help to know about different strata of rocks.

Fossils fuel

The fuels formed by the decomposition of plants and animals that were buried under the earth are called fossils fuels. They are coal, mineral oils and natural gases.

Coal

Coal is formed by the remains of plants remain buried in the earth for millions of years. It is a mixture of carbon, hydrogen and oxygen. It is found forming layers with stones and sedimentary rocks.

Formation of coal

The coal is formed from the dead remains of plants. Millions of year ago, the giant plants were buried under marshy or swamp land due to various reasons like earthquakes, volcanoes etc. When the plants were covered with sediments, they were avoided from the action of bacteria. They buried under the earth for millions of year. Under high pressure and temperature, the parts of plants were converted into carbon rich substances called coal. In this way the coal is formed by the decomposition of plants buried under the earth million of years.

Importance of coal

• It is used as fuel in houses, industries etc.
• It is used as fuel in railway engines.
• It is used to generate electricity in thermal power station.
• It is source of organic compounds like benzene, phenol etc.
• It is the source of income. When it is plenty, it can be sold to other countries to earn money.

Mineral oil

The petroleum products are called mineral oils. They mainly contain hydrocarbons other substances present in mineral oils are oxygen, Sulphur, nitrogen etc. It is obtained by drilling hole deep into the earth.

Formation of mineral oil

The mineral oil is formed by the decomposition of remains of the plants and animals, buried under the earth for millions of years. When the small plants and animals in the sea died they piled up in the bottom of the sea and covered with mud and sand for millions of years. Over millions of years, the mud and sand become hard and thick and created very high pressure on dead bodies. Due to high pressure and bacteria, they are changed into hydrocarbons by chemical reaction. In the way mineral oil is formed in crude form.

               The crude mineral oil is obtained by making deep holes in the earth’s crust. This is refined to obtained different minerals as follows.

Petroleum gas	Used as fuel
Petroleum ether	Used as solvent and for cleaning
Deiseal oil	Used as fuel in vehicle
Kerosene	Used as fuel
Lubricating oil	Used as lubricant's
Grease or vaseline	Used as lubricants
Paraffin wax	Used to make candles, matches etc..
Petroleum wax	Used as fuel

Importance of mineral oil

• It is used as fuel in industries, vehicles etc.
• It is used in heating, cooking and lighting purpose.
• It is used in generating electricity.
• It is used in manufacturing fertilizers, plastics, insecticides, explosives etc.

• The branch of science which deals with the study of earth is called geology and the scientists who study the earth are called geologists.
• The age of the earth from its origin to the present is called geological time. The entire geological time period is divided into four eras. They are Pre Cambrian (azoic era), Paleozoic era, Mesozoic era and Cenozoic era.
• The fossils are the dead remains imprints of the plants and animals that were buried under the earth millions of years ago. Fossils are found in the sedimentary rocks.
• The fuels formed by the decomposition of plants and animals that were buried under the earth are called fossils fuels.
• Coal is formed by the remains of plants remain buried in the earth for millions of years. It is a mixture of carbon, hydrogen and oxygen. It is found forming layers with stones and sedimentary rocks.
• The petroleum products are called mineral oils. They mainly contain hydrocarbons other substances present in mineral oils are oxygen, Sulphur, nitrogen etc.