NCERT CLASS-7 GEOGRAPHY
CHAPTER-2 [INSIDE OUR EARTH]
- The earth, our homeland is a dynamic planet. It is constantly undergoing changes inside and outside.
- INTERIOR OF THE EARTH-
● Just like an onion, the earth is made up of several concentric layers with one inside another.
● The uppermost layer over the earth’s surface is called the crust.
● It is the thinnest of all the layers.
● it is about 35 km.
● on the continental masses and only 5 km on the ocean floors.
- SIAL / SIMA /NIFE
● The main mineral constituents of the continental mass are silica and alumina.
● It is thus called sial (si-silica and al-alumina).
● The oceanic crust mainly consists of silica and magnesium; it is therefore called sima (si-silica and ma-magnesium).
● Just beneath the crust is the mantle which extends up to a depth of 2900 km. below the crust.
● The innermost layer is the core with a radius of about 3500 km.
● It is mainly made up of nickel and iron and is called nife (ni – nickel and fe – ferrous i.e. iron).
● The central core has very high temperature and pressure.
- ROCKS AND MINERALS-
● The earth’s crust is made up of various types of rocks.
● Any natural mass of mineral matter that makes up the earth’s
crust is called a rock.
● Rocks can be of different colour, size and texture.
● There are three major types of rocks: igneous rocks,
sedimentary rocks and metamorphic rocks.
● when the molten magma cools, it becomes solid.
● Rocks thus formed are called igneous rocks.
● They are also called primary rocks.
- IGNEOUS ROCKS-
There are two types of igneous rocks:
Intrusive Rocks and Extrusive Rocks
● Lava is actually fiery red molten magma coming out from the interior of the earth on its surface.
● When this molten lava comes on the earth’s surface, it rapidly cools down and becomes solid. Rocks formed in such a way on the crust are called extrusive igneous rocks.
● They have a very fine grained structure.
● For example, basalt. The Deccan plateau is made up of basalt.
- Sometimes the molten magma cools down deep inside the earth’s crust.
- Solid rocks so formed are called intrusive igneous rocks.
- Since they cool down slowly they form large grains.
- Granite is an example of such a rock.
- Grinding stones used to prepare paste/powder of spices and grains are made of granite.
- SEDIMENTARY ROCKS-
● Rocks roll down, crack, and hit each other and are broken down into small fragments.
● These smaller particles are called sediments.
● These sediments are transported and deposited by wind, water, etc.
● These loose sediments are compressed and hardened to form layers of rocks.
● These types of rocks are called sedimentary rocks.
● For example, sandstone is made from grains of sand.
● These rocks may also contain fossils of plants, animals and other microorganisms that once lived on them.
- METAMORPHIC ROCKS-
● Igneous and sedimentary rocks can change into metamorphic rocks under great heat
and pressure.
● For example, clay changes into slate and limestone into marble.
- ROCK CYCLE-
- one type of rock changes to another type under certain conditions in a cyclic manner.
- This process of transformation of the rock from one to another is known as the rock cycle.
- the molten magma cools; it solidifies to become igneous rock.
- These igneous rocks are broken down into small particles that are transported and deposited to form sedimentary rocks.
- When the igneous and sedimentary rocks are subjected to heat and pressure they change into metamorphic rocks.
● The metamorphic rocks which are still under great heat and pressure melt down to
form molten magma.
● This molten magma again can cool down and solidify into igneous rocks.
- MINERALS AND THEIR USES-
● Rocks are made up of different minerals. Minerals are naturally occurring
substances which have certain physical properties and definite chemical
composition.
● Minerals are very important to humankind. Some are used as fuels.
● For example, coal, natural gas and petroleum.
● They are also used in industries – iron, aluminium, gold, uranium, etc, in medicine,
in fertilisers, etc.
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