→ Both outside and inside of the earth, change is going on constantly.

→ Interior of the earth:

• Many concentric layers which are one inside the another maked the earth.
• The crust is the uppermost layer over the earth’s surface. Among all the layers, it is the thinnest. It is about 35 km on the continental masses and only 5 km on the ocean floors.
• The main mineral components of the continental mass are silica and alumina and known as sial.
• The oceanic crust mainly consists of silica and magnesium and known as sima.
• The mantle is just under the crust which extends upto a depth of 2900 km below the crust.
• The innermost layer is mainly consists of nickel and iron and is known as nife which is the core with a radius of about 3500 km and has very high temperature and pressure.

→ Rocks and Minerals:

• A rock is the natural mass of mineral matter that makes up the earth’s crust. It can be of different shape, colour, size and texture.
• Three types of rocks are there:
  • igneous rocks
  • sedimentary rocks
  • metamorphic rocks.

→ Igneous rocks are also called as primary rocks. There are two types of igneous rocks:

• Intrusive rocks – They cool down slowly and thus they form large grains such as Granite.
• Extrusive rocks – They have a very fine grained structure such as basalt. The Deccan plateau is made up of basalt rocks.

→ The sediments are transported and deposited by wind, water, etc. Layers of rocks are formed by these loose sediments which are compressed and hardened. These types of rocks are known as sedimentary rocks such as sandstone which is made from grains of sand. These rocks may also contain fossils of plants, animals and other micro-organisms that once lived on them.

→ Igneous and sedimentary rocks changes into metamorphic rocks under great heat and pressure such as clay changes into slate and limestone into marble.
The process of transformation of the rock from one to another rock is called the rock cycle.
Rocks are made up of different minerals and are naturally occurring substances which have certain physical and chemical properties. Some are used as fuels, some in medicines, industries, fertilisers, etc.

JAC Class 7 Social Science Notes