Chapter 2 - Inside Our Earth

• Earth is a solid ball with a radius of 6371 km.
• Constant changes occur inside and in addition outside the earth.

Interior of the Earth

Like an onion, the Earth is made up of several layers with one inside another.

1. The crust
   1. forms only 0.5 per cent of the total volume of the earth
   2. is the uppermost layer of the earth’s surface
   3. is the thinnest of all the layers and has less than 1% of the Earth's volume
   4. The continental crust is 10 to 30 kms thick, and it is mainly composed of silica and alumina (hence called SIAL)
   5. The land under oceans or the oceanic crust is 5 to 10 kms thick, and mainly consists of silica and magnesium (hence called SIMA)
2. The mantle
   1. forms 16 per cent of the total volume of the earth
   2. extends up to a depth of 2900 kms below the crust
   3. is the middle and the thickest layer of earth
   4. consists of hot, dense and semi solid rocks
   5. contains most of the mass of the Earth
3. The core
   1. forms 83 per cent of the total volume of the earth
   2. innermost layer with a radius of about 3500 kms
   3. The central core has very high temperature and pressure
      1. Outer Core
         1. is a liquid layer of about 2,266 kms
         2. is mainly composed of nickel and iron (called NIFE)
      2. Inner Core
         1. is the Earth's innermost part

Rocks and Minerals

Rocks are composed of different minerals which are naturally occurring substances which have certain physical properties and definite chemical composition.

The earth’s crust is mostly made up of various types of rocks. There are three major types of rocks.

Igneous Rocks (In Latin Ignis means fire)

• are formed through the cooling and solidification of magma or lava
• are also called primary rocks
• There are two types of igneous rocks:
• Extrusive igneous rocks:
  • are formed by the rapid cooling of molten magma on the Earth's surface
  • are smooth, crystalline, and fine-grained
  • also known as volcanic rocks
  • Example - Basalt
  • The Deccan plateau is made up of basalt rocks
  • Intrusive Igneous Rocks
    • are formed from magma that cools and solidifies inside the earth’s crust
    • are coarse-grained as they are formed by slow-cooling magma
    • Example – Granite

Sedimentary Rocks (In Latin sedimentum means settle down)

• are formed by the deposition of sediments (broken down fragments of rocks) at the Earth's surface and within water bodies
• may also contain fossils (remains of the dead plants and animals trapped in the layers of rocks ) of plants, animals and other microorganisms that once lived on them

Metamorphic Rocks (In Greek metamorphose means change of form)

• Igneous and sedimentary rocks can change into metamorphic rocks under great heat and pressure
• are formed as a result of changes in igneous or sedimentary rocks, in a process called metamorphism
• Example – Clay changing to slate, limestone changing to marble

Rock Cycle

The process of transformation of the rock from one to another is known as the rock cycle.

• Rocks come from lava and go back to lava, but in the process they might change from one type to another.
• When the molten magma cools, it solidifies to become igneous rock.
• The 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.

Usefulness of Rocks

Rocks are very important and provide us with

• material for construction purposes
• precious stones like gold and silver
• minerals for various industries
• chemicals for medicine and fertilizer industries
• fuel like crude oil and coal