Social Science Short note .[3 Marks Each]

• Due to extremely high temperature and extreme pressure the basic properties like structure, layering, composition, colour, etc. of igneous and sedimentary rocks change totally.
• The rocks thus formed are called ‘Metamorphic rocks'. E.g., clay changes into slate and limestone into marble. Moreover quartzite, graphite, diamonds, etc. are also found in metamorphic rocks.

Wind is an active agent of erosion and deposition in a desert.

• The wind in the desert region erodes the lower part of the rocks more easier and faster.
• As a result, the lower part of the rocks is shallow and the upper part remains wider,
• so these rocks make a shape like an Umbrella (Mushroom rocks), which is called 'Cap rock'.
• In the desert region, wind carries sand from one place to another due to its velocity (speed).
• When the speed of the wind is slowed down, the sand which is transported by the wind is deposited somewhere on the land and forms a small hill-like sand ridge, which is called 'Barchans'.
• When the sand particles are and light in weight, the wind carries them hundreds of kilometres away.
• Thus a level plain is formed on large area where these sand particles are transported and deposited by wind, which is called 'Loess' plain. Such plain is developed in the north-western China.

Sometimes due to internal pressure, there are great movements in the semi-liquid matters of the magma in the interior of the earth.

• The water which percolates in the interior is transformed into water vapour due to internal heat.
• The vapour occupies more space than the water.
• So it exerts tremendous pressure on all sides.

Due to such reasons, the land explodes where the rocks are soft.

• Vapour, ash, rock fragments, mud and thereafter the hot magma-lava are thrown out.
• This magma-lava cools down and spreads on all sides and forms sloppy, coneshaped mountain. This is called a 'Volcanic mountain'. Thus, a volcano is formed.