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Geography NCERT

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Lesson 3, Topic 2
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Chapter – 2 Interior Our Earth

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What is Earth made-up of

  • The earth, our homeland is a dynamic planet.

  • It is constantly undergoing changes inside and outside.

  • Let’s know what lies in the interior of the earth?

  • & What is the earth made up of?

Composition of Earth

  • By mass, the Earth is composed of mostly iron (35 percent), oxygen (30 percent), silicon (15 percent), and magnesium (13 percent).

  • It is made of distinct layers: a thin crust, upper mantle, lower mantle, outer core, and inner core, as well as transition zones.

INTERIOR OF THE EARTH

  • 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.

  • The main mineral constituents of the continental mass are silica and alumina. It is thus called sial (si-silica and al-alumina). The oceanic crust mainly consists of silica and magnesium; it is therefore called sima (si-silica and ma-magnesium).

  • Just beneath the crust is the mantle which extends up to a depth of 2900 km. below the crust.

  • The innermost layer is the core with a radius of about 3500 km. It is mainly made up of nickel and iron and is called nife (ni – nickel and fe – ferrous i.e. iron).

  • The central core has very high temperature and pressure.

  • The crust forms only 1 per cent of the volume of the earth, 84 per cent consists of the mantle and 15 per cent makes the core.

  • The radius of the earth is 6371 km.

ROCKS AND MINERALS

  • The earth’s crust is made up of various types of rocks.
    A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter.

  • Rocks are usually grouped into three main groups: igneous rocks, metamorphic rocks and sedimentary rocks.

  • It is categorized by the minerals included, its chemical composition and the way in which it is formed.

Importance of Rocks & Minerals

  • Rocks and minerals are all around us! They help us to develop new technologies and are used in our everyday lives.

  • Our use of rocks and minerals includes as building material, cosmetics, cars, roads, and appliances.

  • In order maintain a healthy lifestyle and strengthen the body, humans need to consume minerals daily.

  • Rocks are very useful to us. The hard rocks are used for making roads, houses and buildings.

Words’ Origin

  • Igneous: Latin word Ignis meaning fire.

  • Sedimentary: Latin word sedimentum meaning settle down.

  • Metamorphic: Greek word metamorphose meaning change of form.

Igneous Rocks

  • When the molten magma cools, it becomes solid.

  • Rocks thus formed are called igneous rocks.

  • They are also called primary rocks.
  • There are two types of igneous rocks:

    (a) intrusive rocks &
    (b) extrusive rocks.

Sedimentary Rocks

  • Rocks roll down, crack, and hit each other and are broken down into small fragments. These smaller particles are called sediments.

  • These sediments are transported and deposited by wind, water, etc. These loose sediments are compressed and hardened to form layers of rocks. These types of rocks are called sedimentary rocks.

  • For example, sandstone is made from grains of sand. These rocks may also contain fossils of plants, animals and other microorganisms that once lived on them.

Metamorphic Rocks

  • Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means “change in form”.

  • The original rock is subjected to heat and pressure, causing profound physical or chemical change.
    For example, clay changes into slate and limestone into marble.

Rock Cycle

  • The Rock Cycle is a group of changes.

  • Igneous rock can change into sedimentary rock or into metamorphic rock.

  • Sedimentary rock can change into metamorphic rock or into igneous rock. Metamorphic rock can change into igneous or sedimentary rock.

  • Igneous rock forms when magma cools and makes crystals.

  • 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.

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

What are Fossils?

  • Fossils are our window into the past and help us learn about the past.

  • A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age.

  • Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood, oil, coal, etc…

  • Simply we can say – the remains of the dead plants and animals trapped in the layers of rocks are called fossils.

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