Inside Our Earth (NCERT Class 7 Geography Chapter 2 Notes)

Interior of Earth
Earth is a dynamic planet and undergoes constant changes both inside and outside its surface. It is made up of several concentric layers with one over another.

Three Major Layers of Earth
There are three major layers of Earth, namely, 1. Crust, 2. Mantle and 3. Core.

  1. The uppermost layer of earth is known as the crust. It is the thinnest layer of the earth, measuring 35 km in depth on continental masses and 5 km on the ocean floor.
  2. The main mineral constituents of continental crust are silica and alumina. Thus, it is known by the name of Sial (si-silica and al-alumina).
  3. The main mineral constituents of oceanic crust are silica and magnesium. Thus, it is known by the name of Sima (si-silica and ma-magnesium).
  4. It only forms one per cent of the total earth volume.

  1. The mantle is the intermediate layer after crust. It extends to the depth of 2900 km.
  2. It is the largest and most massive layer of the Earth.
  3. It makes up 84 per cent of the earth volume.

  1. The core is the innermost layer of the earth. It is of radius 3500 km.
  2. It is mainly made up of nickel and iron. Thus, it is known by the name of Nife (ni-nickel and fe-iron).
  3. It has very pressure and temperature.
  4. It forms 15 per cent of the earth volume.

Rocks and Minerals
  1. The earth crust is made up of a variety of rocks. Any natural mass of minerals which makes up earth crust is known as rock. It can be of various colour, shape and size.
  2. Minerals have great importance to humankind. Some are used as fuels such as coal and some are used in industry such as iron and aluminium.
  3. There are three major types of rocks, namely, igneous rocks, sedimentary rocks and metamorphic rocks.

Igneous Rocks
  1. When molten magma cools, it solidifies to become igneous rock. These are also known as primary rocks. There are two types of igneous rocks: intrusive igneous rocks and extrusive igneous rocks.
  2. When molten lava comes out on the earth surface and quickly cools down to become rock. Such igneous rocks are known as extrusive igneous rocks. These have a very fine grain structure. Basalt is the example of extrusive igneous rocks. Basalt is commonly found in Deccan Plateau.
  3. When molten lava does not come out and slowly cools down under earth surface to form rock. Such igneous rocks are known as intrusive igneous rocks. They have large grains. Granite is an example of intrusive igneous rocks.

Sedimentary Rocks
  1. The rocks can wear and tear with time and broken into smaller particles known as sediments. These sediments are transported and deposited by wind or water on a spot. When these sediments are exposed to external pressure, they harden to form sedimentary rocks.
  2. Sandstone is an example of sedimentary rocks. It is made from grains of sand.
  3. The sedimentary rocks are known for fossil deposits.

Metamorphic Rocks
  1. Igneous and sedimentary rocks when exposed to heat and pressure, transform into metamorphic rocks.
  2. Slate and Marble are examples of metamorphic rocks. Slate is the result of clay metamorphic and Marble is the result of limestone metamorphosis.

Rock Cycle
Rock Cycle
Rock Cycle
  1. Under certain conditions, one type of rock changes into another type of rock in a cyclic manner. This phenomenon is known as the rock cycle.
  2. The molten magma upon cooling becomes igneous rock. The wear and tear of igneous rock lead to the formation of sediments. These sediments on pressure become sedimentary rocks. When igneous and sedimentary rocks are exposed to heat and pressure. They become metamorphic rocks. On further heat and pressure, metamorphic rocks can convert into molten magma. This molten magma again cools down to become igneous rock.
Source: NCERT Class 7 Geography
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