Social Science [3 Marks Each]QUE-ANS

The earth is made up of several layers on one another like an onion.

• The uppermost layer on the surface of the earth is called 'Crust.
• It is the thinnest of all the layers. It extends up to about 35 km on the continents.
• The continental mass is composed of minerals of mainly 'Silica' (Sand) and 'Alumina' (Aluminium). Thus it is called 'SiAl' (Si - Silica and Al- Alumina).
• The layer underlying the 'SiAl' mainly consists of 'Silica' and 'Magnesium'. So it is called SiMa (Si - Silica and Ma – Magnesium).
• The crust just below SiMa is the 'Mantle'. It extends up to a depth of about 2900 km. The innermost layer of the earth is CORE.
• Its radius is about 3500 km. This layer consists of heavy minerals like nickel and ferrous.
• So it is called NiFe (Ni - Nickel and Fe - Ferrous). The central core has very high temperature, pressure and the density of the matter is very high in the Core.

When the sand particles are small and light in weight, the wind carries thern hundreds of kilometres away. Thus, a level plain is formed on the large area where these sand particles are transported and deposited by wind, which is called a 'Loess plain'.

In the desert regions when the wind blows, it lifts and transports the sand from one place to another due to its velocity. When the wind stops blowing, sand is deposited on the surface and forms a small hill like sand dune which is called 'Barchans'.

In the desert region, the wind crodes the lower parts of the rocks more easily and laster than the upper layers. As a result the lower part of the rocks becomes narrower and the upper part is wider. This forms a shape like an umbrella, hence it is called a Cap rock

• The matter separated from the rocks due to the erosion is carried away by the dynamic forces like river, glacier, sea waves, wind, etc. by them in their flow.
• When their flow becomes slow in due course, this matter is deposited (spread) on the land. This process is called 'deposition'.

At the time of volcanic eruption, the hot magma (lava) from the interior of the earth comes out and spreads on the surface of the earth. This magma rapidly cools down faster and becomes solid. Thus, the rocks which are formed due to the cooling down of the lava found on the crust, are called 'Extrusive igneous rocks'. Basalt is this type rock.

At the time of volcanic eruption, the hot magma (lava) of the interior of the earth comes out and spreads on the surface of the earth. When this magma (lava) cools down and forms the rocks that are formed are called 'Igneous rocks'. There are two types of igneous rocks : (1) Extrusive igneous rocks and (2) Intrusive igneous rocks.