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Outer Core sediments, which are eventually buried under
The outer core is made of liquid iron and nickel more sediment and changed by heat and pres-
and is between 3,500°C – 4,000°C. As the liquid sure.
metal swirls around, it induces a current that Some rocks are even formed from the remains of
generates the earth’s magnetic field. Heat from living things.
the core powers the convection currents in the Types of Rocks
mantle. It is 2,900 km thick. Different types of rocks are formed different
Inner Core ways. Thus, they have different characteristics
The inner core is the hottest part of the earth with regard to their composition, colour, size and
reaching temperatures between 4,000°C – texture. On the basis of their mode of formation,
4,700°C, which are as hot as the surface of the there are three major types of rocks—igneous,
sun. It is made of solid iron and nickel that are sedimentary and metamorphic.
under so much pressure they cannot melt. It is Igneous Rocks
1,200 km thick and heavy radioactive elements Igneous rock may form with or without crystalli-
within the core generate the intense heat as they
sation. Below the Igneous rock, there is one of
decay.
the three main rock types. Igneous rock is formed
MATERIALS OF THE EARTH’S CRUST through the cooling and solidification of magma
(ROCKS AND MINERALS) or lava on the surface as intrusive (plutonic)
The lithosphere is made up of rocks and miner- rocks or on the surface as extrusive (volcanic)
als. Rocks are those materials which are obtained rocks.
from the earth’s crust. They include not only the
hard materials such as granite and marble, but
also soft and loose materials such as chalk, clay,
sand and salt.
How the Rocks are Formed?
Rocks are made up of substances called minerals.
Any naturally occurring solid substance with a
definite chemical composition is called a mineral.
All rocks are formed in the earth’s lithosphere,
which includes the earth’s crust and the top part Igneous Rocks
of its mantle, where partially molten rock called
This magma can be derived from partial melts of
magma flows very slowly under the crust.
existing rocks in either a planet’s mantle or crust.
Some rocks are formed when magma gets close Typically, the melting is caused by one or more of
to the surface and slowly cools. three processes— an increase in temperature, a
Some of that red-hot magma breaks through the decrease in pressure and a change in composi-
earth’s crust to form fiery volcanoes, releasing tion.
lava to cool quickly on the surface or even Igneous rocks have following characteristics :
underwater.
g These rocks are hard and compact.
Other rocks form as a result of the weathering of
g These rocks do not occur in distinct beds or
older rocks and erosion, creating layers of
stratus.
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