Rock – The hard, solid material that forms part of the surface of the earth is called rock.
The outer layer of the earth is called crust.
Most of the Earth’s crust is made of solid rocks.
Rocks are made up of chemical substances called minerals.
Rocks may contain only one type of mineral or may have many minerals in them.
Minerals are found in mixture called ores.
Rocks are also present beneath the ocean-water.
Kind of rocks –
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Igneous Rocks – The rocks formed by the solidification on the cooling of molten magma, are called igneous rocks.
They are formed by cooling and hardening of molten rocks.
Magma or molten rocks in the innermost layer (or core) of the earth is pushed upwards by other rocks.
When it reaches the earth’s surface, it flows out through the cracks and is called lava.
Some of the lava cools and hardens below the earth’s crust and forms igneous rocks.
The type of igneous rock formed depends on the type of minerals the magma contains and the rate at which it cools down.
Different types of igneous rocks are –
Granite
It is the most common igneous rock.
Granite is a hard rock.
Granite is of different colours.
White, grey, pink and red are the most common.
Granite contains basically three minerals.
These are quartz, mica and feldspar.
It is used to make building and floors.
Basalt
Basalt is formed by quick lava cooling on the earth’s surface.
It is fine-grained and almost black in colour.
Pumice
It is a light weight rock.
Pumice is formed by lava froth.
Air is trapped inside the pumice stone.
This makes it very light.
It is used as a building material like concrete blocks, as a teeth polishing powder by dentists and as a scrubber to remove dead skin cells from the body.
Sedimentary Rocks – Those rocks are formed by the hardening and compression of sediments is called sedimentary rocks.
All rocks on the earth’s surface slowly break down into small pieces.
These small pieces are called sediments.
Sediments are moved or deposited at one place, by running water, wind, waves and gravity.
They get deposited in layers at the bottom of lakes and oceans.
Layers of sediment cemented together to form sedimentary rocks.
Pressure of upper layers and cement change sediments into sedimentary rocks.
Sedimentary rocks are very useful to know about ancient plants and animals.
Scientists study these rocks because there are traces of ancient plants and animals called fossils in the layers of these rocks.
Different types of sedimentary rocks are:
Sandstone
Sandstone is made from grains of sand which have been naturally cemented together.
It is of many colours.
The colour of depends on the colour of depositing sand.
It contains grains of mineral quartz which lends it pink or reddish colour.
The famous Red Fort and Qutb Minar in Delhi are made of red sandstone.
Conglonterate
Conglomerate is formed when gravel, pebbles and stones get cemented together by minerals.
It is uneven and coarse in texture.
It is used in construction work.
Shale
Shale is made by the deposition of clay and mud particles.
It is a soft rock.
It is mainly used to make bricks and cement.
Limestone
This sedimentary rock is different from other types as it is not formed by deposits of mud and sand.
Limestone is made up of a mineral ‘calcite’ which is present in the shells of sea animals.
When they die, the shells stick to the bottom of the sea and form layers there.
It is used in making cement and bricks, Chalk are also one form of limestone.
Metamorphic Rocks – The rocks that are formed when the igneous and sedimentary rocks are subjected to great heat and pressure are called metamorphic rocks.
OR
The rocks which are formed by the action of heat and pressure are called metamorphic.
Both igneous and sedimentary rocks can be changed into metamorphic rocks.
Different kinds of metamorphic rocks are:
Slate
Slate is formed from soft shale.
It can be broken into thin sheets and is used for making ‘slates’ and blackboards.
It is also used for making roofs of the buildings.
Marble
Marble is the most commonly used metamorphic rock.
Marble is made from limestone and is found in various colours.
It can be cut and polished easily.
Taj Mahal in Agra is made of white marble.
Gneiss
The metamorphic rock made from granite is called gneiss.
It has parallel bands of minerals and glitter because of mica in it.
It is used as a building stone.
Quartzite
This metamorphic rock is formed from sandstone that is rich in the mineral quartz.
It is a very hard and weather resistant rock Quartzite is used in glass and ceramic industry.
Minerals – Naturally occurring substances which have certain physical properties and a definite chemical composition are called minerals.
Minerals are chemical substances that occur naturally.
They are the building blocks of all the rocks.
Minerals are identified by their hardness, colour, density and the way they reflect light.
Minerals may be metallic or non metallic.
Metallic minerals such as iron, aluminium, copper and zinc are obtained from their ores.
These are used for making machines, coins, wires, utensils etc.
Metals like silver and gold are used for making jewellery.
Magnesium, calcite, mica etc. are non-metallic minerals.
These are used for industrial purposes.
Our body too requires some minerals such as calcium, sodium, iron etc. in small quantities
Uses of rocks and minerals
Almost all the materials we use in our daily life are made of minerals and rocks.
For example, the house where we live is constructed by bricks, cement, marble and ceramic tiles.
The utensils we use for cooking and eating are made of iron, aluminium, silver or ebonite.
All are made of minerals.
The bulk of minerals are used in industrial needs such as for the manufacture of housing materials and automobiles.
Precious rocks containing ruby, diamond, emerald, sapphire etc. are used as gemstones.
Fossil fuels like coal and petroleum are also obtained from rocks.
These are very useful to us.
Coal – A Fossil Fuel
Coal is formed from the remains of plants.
Millions of years ago there were thick forests on the earth, growing in swamps.
As the trees of the forests died, they got buried in the swamps.
More and more trees died on top, squeezing those below.
Due to heat and pressure they gradually changed into coal.
The sun’s energy makes plants grow.
Coal thus has stored energy of the sun from millions of years ago.
When we burn coal as a fuel we use this trapped energy of the sun.
Petroleum – Another Fossil Fuel
When plants and animals living in the sea died, their bodies sank to the bottom.
More and more sediments collected on the top.
The plants and animals were squeezed and slowly changed into petroleum over millions of years.
Like coal, petroleum also contains stored energy of the sun millions of years ago.
Coal and petroleum are used as a fuel.
Petroleum is used for making chemicals, cosmetics, fertilizers, plastics and other things.
Need to conserve natural resources
All the minerals, coal and petroleum are valuable natural resources.
However they are limited.
If we keep using them as fast as we are doing today they will not last very long.
Coal and petroleum are used excessively.
Also, burning so much of fuel releases harmful substances such as smoke and poisonous gases in the air, causing pollution, which can result in dangerous diseases.
We can conserve these fossil fuels by using them judiciously and encouraging renewable sources of energy like the sun, water and wind.
These are unlimited and do not cause pollution on being used.
