CLASS-VII GEOGRAPHY
The
earth, our homeland is a dynamic planet. It is constantlyundergoing 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.
CRUST
:
•
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.
•
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)
•
The crust forms only 1 per cent of the
volume of the earth, 84 per cent consists of the mantle and 15 per cent makes
the core.
•
The radius of the earth is 6371 km.
MANTLE:
•
Just
beneath the crust is the mantle which extends up to a depth of 2900 km. below
the crust.
CORE:
•
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.
IGNEOUS
ROCKS :
•
When the molten magma cools, it becomes
solid. Rocks thus formed are called igneous rocks.
•
They are also called primary rocks. There
are two types of igneous rocks:
Intrusive
Rocks and Extrusive Rocks.
•
When fiery red molten magma coming out from
the interior of the earth on its surface. 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 rocks.
•
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.
•
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
micro organisms that once lived on them.
•
Fossils:
The remains of the dead plants and animals trapped in the layers of rocks are
called fossils.
•
Igneous and sedimentary rocks can change
into Metamorphic Rocks under great heat and pressure.
•
For example, clay changes into slate and
limestone into marble.
USE
OF ROCKS:
•
Rocks are very useful to us.
•
The hard rocks are used for making roads,
houses and buildings. You use stones in many games.
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.
•
When 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.
………The
end………
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