how does fossil forms?-2022



A fossil is the mark or hardened 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 passed away. Most of them are found in sedimentary rocks. Animal fossils range from tiny microscopic to gigantic dinosaur bone in size.

Identification of fossils

Different types of fossils can be identified by comparing the different shapes and parts of organs. Some methods of identification of fossil are mentioned below.

1.    A part of the skeletal system or the whole skeleton preserved in sedimentary rocks is a fossil.

2.    A fossil may be in the form of molds and casts.

3.    Shells of animals, wood of plants, etc. also may be preserved in sedimentary rocks as fossils.

Process of fossil formation

The process of forming fossils is called fossilization. Any organism after its death may or may not be fossilized. However, under favourable conditions, the organisms which are  composed of completely soft parts may be fossilized. For instance, jellyfish have been wholly preserved. On the other hand, even an organism with hard parts will not necessarily be fossilized. A number of factors affect fossilization but the two factors given below play the chief role in this process.

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

2.    The organism must be covered by protective materials shortly after the death. There are many ways of fossilization; some of them are mentioned below:

·       The animals that live in or nearby water sometimes get covered by the sediment like mud, dirt and gravel and harden into rock. The hard parts of the body, like teeth and skeleton are preserved in their original form in the rock. The fossil is exposed at the surface by geographical activities like faulting and surface erosion.

·       Some fossils are formed when animals are frozen in ice and preserved to the core.
·       Organisms are also fossilized in the form of casts in sediments. The water inside the sediment dissolves the hard parts, leaving a hollow space in the sediment. The hollow shape is called mold; the space is then filled with minerals which harden to form a cast.

  • ·       Replacement of mineral instead of organic material of the dead bodies in sediment gets petrified (turns into stone). Fossilization occurs in this way as well.
   Except the above methods, preservation of footprints of animals, tracing of leaves on sedimentary rocks, etc. are also some other ways of fossilization.

Importance of fossils

Fossils are very useful for the following purposes:

1.    They provide evidence of evolution of living beings.

2.    They help to know about the extinct animals and plants.

3.    They show the history of the location from where a fossil is obtained.

4.    Fossil fuels like coal and petroleum are the main sources of energy.

5.    They help to make geological timescale.

Fossil fuels

Fossil fuels are non-renewable sources of energy. Coal, petroleum and natural gas are the three categories of fossil fuels. Fossil fuels are formed from the dead bodies of living beings after millions of years. At present, fossil fuels are being widely used in industries, vehicles, kitchen, etc. as the main sources of energy. Overuse of them brings energy crisis and pollution.


Coal is a black or brown carbonaceous deposit formed by plant fossils. It is found in the form of thick layer inside the earth and is taken out by mining. Its main components are cellulose and lignin. Lignin forms from the wood of plants. Protein, oil, organic acid and salts are also found in it. Especially, there are four types of coal. The types of coal, hardness, colour and other properties are shown in the following table.



Types of Coal


Preselect fossils


Weathering Property









weathers easily

burns with smoke, loosely held particles






slightly hard

weathers easily

burns with less flame, waxy





visible under microscope


Doesnot weather easily

burns with blue flame




black and shiny

visible under microscope


does not weather easily

brittle, burns without smoke

Out of them, lignite is the worst and anthracite is the best type of coal.

Formation of coal

The exact mechanism of formation of coal is not known, but on the basis of geological evidence, it is considered that it was formed from the fossils of plants. In carboniferous period [345-280 million years ago], forest of coal plants (fast growing bushes and giant tree ferns) grew in the swamp of the earth. When they died and fell into quite swamp water, they were protected against rotting. Bacteria and geothermal energy changed some parts of the wood into gases that escaped out; the remaining black mixture containing mostly carbon was left that became a coal seam (Coal layer). In course of time, the increasing pressure from the overlying mud and sand squeezed out most of the liquid, leaving behind a pasty mass which slowly hardened into coal. In this way, coal was formed inside the earth. The layer of coal inside the earth may vary from some inches to several feet.

Importance of coal

Coal is used for the following purposes:

1.            It is used in industries, houses and other places for heating and generating electricity.

2.            Coal tar obtained from coal is used in the manufacture of plastic.

3.            Coal is used as fuel in steam engines to change water into steam.

4.            In some countries, coal is the main source of national economy.

5.           It is used for manufacturing petroleum and synthetic natural gas.

Mineral oil

The word petroleum is derived from Latin word petr which means "rock" and oleum which means oil. So, it is rock oil. Oil and natural gas together are called petroleum. Paraffin, fuel, lubricating oil, kerosene, natural gases, grease, wax, asphalt and many other products are obtained from petroleum. Hydrocarbon covers 90-95% part of petroleum; the rest part is oxygen, sulphur, etc.

Formation of mineral oil

The mechanism of the formation of mineral oil is open for speculation but scientists believe that petroleum was formed from dead bodies of plants and animals. As the plants and animals died, they piled up on the sea bottom. In course of time, millions of tons of sand and mud covered them. Under pressure, the mud and sand changed into rock. The formed rocks created a very high pressure on the dead bodies. Because of high pressure at moderate temperature, scarcity of oxygen, and by the action of bacteria, the dead bodies were converted into a dark liquid trapped in the pore of the rocks, which was crude oil or petroleum. It takes millions of years to form crude oil. The petroleum thus formed is stored between two impervious layers of the rocks. The products obtained after the refining of crude oil and their uses are given below:





Petroleum ether

Solvent, cleaning



Vehicles fuel





Lubricating oil, ease



Vaseline, ease



Paraffin wax

Manufacturing of candle and matches


Petroleum cake, bitumen


Fuel, construction of road





Importance of mineral oil

Some major uses of mineral oil are as the following:

  • ·       It is used as a lubricant.
  • ·       It is used as a fuel for cooking food.
  • ·       It is used in vehicles as a fuel.
  • ·       It is used in many industries as a fuel.
  • ·       It is used in manufacturing of plastics, chemical fertilizers, pesticides, paints, explosives, medicine, etc.


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