Hypothesis about the origin of the earth
- Planetesimal Hypothesis
- Nebular Hypothesis
- Tidal Hypothesis
Planetesimal Hypothesis
George Wofan proposed this hypothesis in 1749 A.D.
According to this hypothesis, the earth along with other planets and satellites was formed when a comet moving around the universe stroke the sun millions of years ago.
Nebular Hypothesis
Immanuel Kant put forward the nebular hypothesis in 1755 AD. It was later improved by Laplace in 1796 A.D.
According to this hypothesis, the solar system developed from a huge revolving nebula. When the nebula cooled and contracted, it condensed to form rings of matter with the central huge mass. The central mass became the sun and the escaped mass changed into planets and satellites. Nebula is a cloud of gas and interstellar dust.
Tidal hypothesis
James Jeans and Harlod Jeffry proposed the tidal hypothesis in 1917 A.D.
According to this hypothesis, a big star came closer to the sun exerting a tidal pull. Because of this tidal pull, a long filament of the fragmented matter was drawn from the sun. The fragmented matter started revolving around the sun and thus planets and satellites were formed.
Geological Time Scale
The geological time scale is the time scale that covers the earth’s history from its origin to the present day.
Eon: It is the longest period of time in geological history. It is divided into two divisions: Cryptozoic and Phanerozoic eon.
Era: It is a division of an eon.
Cryptozoic eon and Pre-cambrian era
- Duration: 4500 to 570 million years ago.
- Invertebrates and cryptogams were evolved in this eon.
Phanerozoic eon
- Duration: 570 million years ago to present.
- Evolution of vertebrates and phanerogams took place.
- It is further divided into: Paleozoic era, Mesozoic era, Cenezoic era.
Paleozoic era
- Duration: 570-250 years ago
- Formation of sedimentary rocks.
- Invertebrate and vertebrate animals were developed in the ocean and the land.
- Amphibian plants and animals were in abundant number. Therefore, this era is also called the era of amphibians.
- This era is futher divided into six periods.
Mesozoic era
- Duration: 250-65.5 years ago.
- Formation of hills mountains.
- Reptiles were the dominant creature of this era. Therefore, this era is also called the age of reptiles.
- This era is further divided into three periods: Triassic, Jurassic, and Cretaceous periods.
- Dinosaurs, the most famous organisms of this period evolved in the Triassic period. They became abundant and dominant in the Jurassic period and became extinct in the cretaceous period.
- Conifers, the earliest angiosperms, and the first mammals appeared.
Cenozoic era
- Duration: 65.5 years ago to present.
- Breaking of rocks and volcanic eruptions took place in this era.
- The temperature was decreased and mountain caps were covered with ice.
- Human beings became the dominant organisms. Therefore, this era is also called the era of human beings or the recent era.
- This era is divided into two periods: tertiary and quaternary.
- It is divided into seven epochs(shortest time in geological time scale).
Fossil
Fossils are the impressions or hardened remnants of dead plants and animals. Fossils are generally found in sedimentary rocks.
Identification of fossils
Some of the ways for the identification of fossils are:
- Traces of dead bodies found on the sedimentary rocks are fossils.
- Fossils may be in the form of molds or casts.
- Hard parts of organisms like skeletons, teeth, bones, shells, etc. are preserved in the sedimentary rocks as fossils.
Importance of fossils
- Help to get an idea about the origin of life on the earth.
- Help to know about the extinct animals and plants.
- Provide evidence of the evolution of organisms.
- Help to study the ancient geography and climate of a country.
- Fossil fuels like coal and petroleum are the main sources of energy.
Fossil fuels
The fuels formed underground from the remains of dead plants and animals are called fossil fuels. Examples: coal, petrol, diesel, kerosene, natural gases, etc.
Coal
Coal is a brownish-black hard substance formed from the plants buried under the earth millions of years ago.
Types of coal
- Anthracite: It is the coal of the best quality. It contains high percentage of carbon and burns without smoke.
- Bituminous: It is black and hard coal mostly used for domestic purposes. It burns with more heat and little smoke.
- Lignite: It is dark brown coal. It contains less percentage of carbon.
- Peat. It is a low-quality coal. It burns with little heat and more smoke.
Formation of coal
The plants during the Carboniferous period were buried under the soil due to natural calamities like landslides, floods, earthquakes, etc. They were covered by sediments of soil and were protected from decaying. Due to the high pressure and temperature, chemical changes occurred in the dead bodies of the plants. As a result, some gases escaped out, the liquid part squeezed out leaving hard solid residue of carbon i.e. coal.
Importance of coal
- Thermal coal is used to produce electricity.
- Coal is used in the production of steel.
- It is used to make various organic compounds like benzene, phenol, etc.
- Activated charcoal is used to make cosmetic products.
Mineral Oils
Mineral oil is the mixture of hydrocarbons(about 90-95%) and petroleum. It is also called rock oil because it is found between the layers of the rocks under the earth’s surface.
Formation of mineral oil
Mineral oil is formed due to the decomposition of remains of dead organisms buried under the sea millions of years ago. The dead bodies got covered with the layers of sand and clay. Lack of air, high temperature, and high pressure transformed dead organisms into petroleum and natural gases.
Main uses
- Natural gas: As a fuel
- Petroleum ether: As a solvent, for cleansing purposes.
- Diesel: Vehicle fuel
- Lubricating oil or grease: Lubrication
- Paraffin wax: Making candles and matches
- Petrol, kerosene: As a fuel
Importance of mineral oil
- Mineral oil is used to run vehicles.
- It is used as lubricant.
- It is used in various industries and factories as fuel.
- Coal tar, a product of petroleum is used for the construction of roads.