Chapter 4 Rocks And Minerals The Earths Crust Rocks
Do you know?
- Our planet earth is more than 5000 million years old.
- The earth comprises of three main regions-the lithosphere (which is solid), the hydrosphere (which is the liquid portion) and the atmosphere (or gaseous portion).
- The outermost surface of the earth is made up of hard soil, sand, silt, clay and rocks like granite, etc.
- The pressure and temperature under the earth’s surface are extremely high.
- Within the earth the temperature increases with depth at the rate of 1°C for every 32 m of depth, giving an average temperature of about 5000°C at the centre. At such high temperatures, the rocks remain in a molten state which is known as magma.
- During volcanic eruptions, such molten magma erupts on the surface of the earth and flows down the slope of the surface. Then it is called lava.
- Outside the crater of a volcano, the lava gets solidified by cooling and in this way, primary rocks are formed.
- Rocks differ from one another in colour and texture.
Read and Learn More WBBSE Notes For Class 6 General Science and Environment
Chapter 4 Rocks And Minerals Different Types Of Rocks
Different Types Of Rocks
On the basis of the mode of formation.
The rocks are classified as
- Igneous rocks.
- Sedimentary rocks and
- Metamorphic rocks.
Igneous Rocks
The word ‘igneous’ means ‘fire’, derived from the Latin word ‘ignis’ or its Sanskrit word ‘Agni’. Igneous rocks are formed by the process of solidification of hot molten lava.
They are referred to as primary rocks. Basalt, Granite and Pumice are three common examples of igneous rocks. The black-coloured stones you see near railway tracks or stone chips that are used for the construction of asphalt roads are nothing but pieces of granite.
Note that no holes are seen in a granite rock. But in a pumice stone, you can see many holes. Usually, pumice is formed by the process of quick cooling and solidification of bubbling foam from the upper portion of molten magma.
When the bubbles escape into the air, the holes are formed in the solid pumice.
Sedimentary Rock
The word ‘sedimentary’ is derived from a Latin word which means ‘settling down’. Sediments of sand, gravel, slit and clay are deposited in water bodies like seas, rivers and lakes over millions of years.
Due to the high pressure and temperature of the interior layers of the earth, these sediments get consolidated (or hardened) into rocks which are known as sedimentary rocks. These are also called secondary rocks.
Sometimes fossilized remains of plants and animals are found between the layers of sediments. As shown in Sandstone, Shale and Limestone are three common examples of sedimentary rocks.
Sandstone is formed by the consolidation of the deposits of sand, whereas deposits of clay get hardened to form shale.
Metamorphic Rocks
The word ‘metamorphism’ means ‘change of form’. Metamorphic rocks are rocks which are formed by the process of metamorphism of pre-existing igneous and sedimentary rocks.
During the process of metamorphism, the original structure of pre-existing rocks gets altered into new minerals which combine together to form metamorphic rocks.
Metamorphic rocks are harder and more resistant than the original rocks.
Marble, Slate, Gneiss, Quartzite, Schist, etc. are examples of metamorphic rocks.
For example:
Limestone gets metamorphosed to form marble. Shale gets altered to form Slate. Granite gets metamorphosed into Gneiss. Quartzite is the metamorphosed form of Sandstone.
Chapter 4 Rocks And Minerals Minerals And Ores
The earth’s crust is made up of rocks of various types. Do you know that a rock is a mixture of various minerals in different combinations. About 2000 minerals are known to exist in the earth’s crust.
However, minerals are made up of some chemical elements. Among these elements, in the earth’s crust oxygen is the most abundant, and silicon is the second most abundant element.
Other elements are aluminium, iron, calcium, sodium, copper, magnesium, zinc, etc. You must have an idea about some objects made from metals that we use in our everyday life.
Follow this chart:
| Metal | Uses |
| Aluminium | To make utensils, furniture, electrical wires, picture frames, and the body of planes. |
| Iron | For making machines, rods, rail lines, utensils, lamp posts, and railings. |
| Copper | In the manufacture of electrical wires, cables, electrotyping, electric motors, electric cells, coins. |
| Zinc | In galvanizing iron and steel, to prepare a painted zinc white, in a dry cell. |
| Silver | Utensils, ornament manufacturing. |
| Gold | In manufacturing valuable ornaments. |
Most of the metals are found to occur in nature in combination with other elements forming compounds mixed with soil and sand. These compounds are known as minerals.
Example:
Aluminium is found as different compounds, namely, Bauxite, Cryolite, Felspar, and Diaspore-which are minerals of aluminium. The minerals of iron are Haematite, Magnetite, Copper has the minerals Copper glance, Copper pyrites, and Cuprite.
Metal extraction is the process by which a metal is extracted from its minerals.
The particular mineral from which a metal can be extracted conveniently and economically is called the ore of that metal.
Example:
- The ore of Aluminium is Bauxite, of Iron it is Haematite, of Copper it is a Copper glance.
- All ores are minerals but all minerals are not ores for a metal.
A metal may have more than one mineral, but one or two ores from which the metal can be extracted conveniently and economically.
For example:
Aluminium has the minerals Bauxite, Cryolite, Felspar, and Diaspore but its ore is only Bauxite. Because Aluminium is extracted economically from Bauxite.
Chapter 4 Rocks And Minerals Alloys
You are familiar with some solid substances like bronze, brass, steel, stainless steel, bell- metal, etc. These are not pure metals, but rather, a mixture of metals. These are called alloys.
Definition Of Alloys :
An alloy is a mixture of two or more metals as a solid solution.
Why do we prefer alloys than pure metals?
- Alloys are harder than pure metals.
For example, a bridge built of pure iron cannot withstand heavy load but a bridge built of steel (an alloy of iron and carbon) can do that easily. - Alloys are used to protect the metal from corrosion by air or moisture.
For example, an iron spoon gets rusty easily but a spoon made of stainless steel (an alloy of iron and chromium) never rusts. - The purpose of alloying is to lower the melting point.
For example, an alloy of lead and tin, called solder has a melting point lower than lead or tin. An electric fuse is also made out of the alloy of lead and tin in a 3:1 ratio. - An alloy is used for superior casting.
For example, statues, and medals are made out of brass (an alloy of copper and zinc), and bronze (an alloy of copper and tin).
Chapter 4 Rocks And Minerals Fossils
A fossil is a skeletal impression (or trace) of once-living plants and animals from the past geological age. These skeletal impressions get preserved in between the layers of sedimentary rocks.
Formation of fossils :
When a plant or animal died in a watery environment (one day which were alive) and got buried in mud and slit, their body parts underwent a lot of changes over millions of years. First of all their soft tissues had been decomposed.
For animals, only hard bones or shells were left behind and for plants, traces of leaves were left behind. Over time many chemical changes also took place and the organic remains got preserved over the top and hardened into rocks.
These rock-like traces are called fossils. Fossils help us in fixing the relative ages of rocks. Besides this, the footprints of extinct animals help us to study about them.
Chapter 4 Rocks And Minerals Fossil Fuel
Fuels are the substances which produce heat on burning. Our domestic use fuels include cow-dung cakes, matchstick, wood, hay, paper, etc. We also use fossil fuels like coal, petroleum and natural gas.
These are called fossil fuels because they are formed by the chemical decomposition of dead remains of plants and animals over millions of years which got buried underneath the earth.
Coal :
Coal is found deep under the earth’s surface. Coal was formed when huge forest areas got buried under the earth’s surface about 200 million of years ago.
Due to the very high pressure and temperature of the interior layers of the earth and chemical decomposition reactions, the remains of plants got compacted to form a stony residue, called coal.
Different varieties of coal, e.g. peat, lignite, and bituminous are formed. They differ in their carbon content and volatile compounds.
Uses of coal :
Coal is the most common fossil fuel that we use. On burning coal, heat energy is produced. This heat energy is used to generate electricity in thermal power stations.
When coal is burnt in absence of air, the volatile impurities are expelled, leaving behind a liquid residue (called tar) and a porous residue (called coke).
The coke contains 98% of free carbon and does not produce any smoke on burning. So, coke is a better fuel than coal. Coal is also used as a source of some organic compounds such as benzene, toluene, naphthalene, etc.
Chapter 4 Rocks And Minerals Petroleum And Natural Gas
Millions of years ago, the dead bodies of extremely small sea animals and plants got covered by mud and sand. Due to intense heat and pressure under the earth, the sediments changed into sedimentary rocks.
After a lot of chemical decomposition, the bodies of dead animals and plants decayed into a product of petroleum and natural gas.
Petroleum occurs at depths 500-2000 m between two layers of impervious rocks and the natural gas always remains above petroleum trapped in between the rock cap and the petroleum layer.
Uses of petroleum :
Crude petroleum is refined by the process of fractional distillation at different temperatures. The products of refining of petroleum are kerosene, petrol, diesel, LPG, paraffin wax, lubricating oil, asphalt, petroleum gas, naphtha, etc.
- Kerosene is used as a household fuel in wick stoves.
- Petrol is used as fuel for light vehicles like cars, scooters, etc.
- Diesel is used to running heavy vehicles like buses, trucks, tractors, etc.
- LPG (Liquid Petroleum Gas) is used as a household fuel.
Uses of natural gas :
Natural gas contains 95% methane. Under high pressure, natural gas is stored in cylinders. It is commonly called Compressed Natural Gas (or CNG) and is used as fuel for homes, transports and industries.
Natural gas can be piped from production wells to homes in the same way as water supply. Natural gas causes less pollution on burning, than vehicles run on diesel. So, natural gas is a very clean and efficient fuel.
WBBSE Notes For Class 6 General Science and Environment
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- Chapter 1 Interdependence of Organisms and the Environment
- Chapter 2 Phenomena Around Us
- Chapter 3 Element, Compound and Mixture
- Chapter 4 Rocks and Minerals
- Chapter 5 Measurement
- Chapter 6 Primary Concept of Force and Energy
- Chapter 7 Statics and Dynamics of Fluid
- Chapter 8 The Human BodyChapter 9 Common Machines
- Chapter 10 Biodiversity and its Classification
- Chapter 11 Habits and Habitats of Some Important Animals
- Chapter 12 Waste Products