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
   99   100   101   102   103   104   105   106   107   108   109