Page 104 - SST Class 07
P. 104
Igneous rocks contain only minerals and ROCK CYCLE
substances, but not organic matter. When igneous
rocks break and erode, it may be mixed up with You have seen that old rocks change into new
organic matter. The remains of dead plants and ones. This is a slow but continuous process. The
magma is liquid rock material which cools down
animals are called organic matter or fossils . When
organic matter forms layers, it turns into coal, and solidifies into solid igneous rocks on or below
the earth’s surface. The igneous rocks are broken
which is also a sedimentary rock.
down into small particles by the wind or water
F ind Outind Out movement and are transported to another place,
where they form into sedimentary rocks. When
Which type of minerals do we get from the rocks igneous and sedimentary rocks are subjected to
which are of great economic value? pressure and heat, they change into metamorphic
Metamorphic Rocks rocks. These rocks can melt under great heat and
pressure and thus magma can be created. Thus,
These rocks are originally igneous or sedimentary
the rock cycle starts all over again.
rocks which have changed due to pressure, heat
or action of water. This process is called
metamorphism , and the rocks thus formed are
called metamorphic rocks. In this process, small
crystals combine together and form larger crystals.
Different minerals and materials too can combine
and form into newer minerals and materials. Thus,
this process changes clay into slate, limestone into
marble, coal into graphite and granite into gneiss.
This is a long process and can take hundreds or
thousands of years.
Rock cycle
Just Think!
Does the process of rock formation affect us in any
way?
Quiz Time
Write True or False :
Formation of metamorphic rocks 1. The igneous rocks are broken down into big
particles by the wind or water movement._____
Activity
2. The remains of dead plants and animals are
Are any rocks used around you in roads, called fossils. _____
buildings, houses? Name any three rocks used in 3. Sandstone is an igneous rock. _____
them and their types :
4. The rocks which come out on the earth’s surface
(a) _________________ _________________ are called extrusive rocks. _____
(b) _________________ _________________
MINERALS
(c) _________________ _________________ Rocks can be very hard, such as granite or marble;
or these can be very soft, such as sand or gravel.
Contemporary Social Science-7
104
