Inside Our Earth | NMMS Revision Notes | Class 7 Social

  

What is the Earth?

(i) The earth is a dynamic planet and comprises of three layers: crust, mantle and core.

(ii) Constant changes take place inside as well as outside the earth.

Interior of the Earth:

(i) The Earth is made of several concrete layers.

(ii) The uppermost layer over the earth’s surface is called the crust. It is the thinnest of all the layers. It is about 35 km on the continental masses and only 5 km on the ocean floors.

(iii) Silica (Si) and alumina (Al) are the main mineral constituents of the continental mass. It is, thus, called sial.

(iv) The oceanic crust mainly consists of silica (Si) and magnesium (Mg) called sima.

(v) Just below the crust is the mantle up to an extent of 2,900 km.

(vi) The innermost layer is core with a radius of 3,500 km. As it is made of nickel (Ni) and iron (Fe), it is called nife. The central core has very high temperature and pressure.

Rocks and Minerals:

(i) The earth’s crust is made of various types of rocks. Any natural mass of mineral matter that makes up the earth’s crust is called a rock.

(ii) There are three major types of rocks; igneous rocks, sedimentary rocks and metamorphic rocks.

(iii) When the molten magma cools, it becomes solid. Rocks thus formed are called igneous or primary rocks. They are of two types, extrusive igneous rocks and intrusive igneous rocks.

• When molten lava comes on the earth’s surface, it rapidly cools down and becomes solid. Rocks formed in such a way on the crust are called extrusive igneous rocks. They have a very fine grained structure. For example, basalt.
• Sometimes, the molten magma cools down deep inside the earth’s crust. Solid rocks so formed are called intrusive igneous rocks. Since they cool down slowly, they form large grains. Granite is an example of such a rock.

(iv) Rocks roll down and break into small fragments and these smaller particles are called sediments. These sediments are transported, compressed and hardened to form layers of rocks. These types of rocks are called sedimentary rocks.

(v) Igneous and sedimentary rocks can change into metamorphic rocks under great heat and pressure. For example, clay changes into slate and limestone into marble.

(vi) The process of transformation of the rock from one to another is called the rock cycle.

• When the molten magma cools; it solidifies to become igneous rock. These
  igneous rocks are broken down into small particles that are transported and deposited to form sedimentary rocks.
• When the igneous and sedimentary rocks are subjected to heat and pressure, they change into metamorphic rocks.
• The metamorphic rocks which are still under great heat and pressure melt down to form molten magma. This molten magma again can cool down and solidify into
  igneous rocks.

(vii) Rocks are made of different minerals. Minerals are naturally occurring substances which have certain physical properties and definite chemical composition.

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