NCERT Class 7 Geography Chapter 2: Inside our Earth Completes Notes Part 1

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Inside Our Earth

Onion Peel: Demonstrating Earth’s Interior

Image of Demonstrating Earth’s Interior

Image of Demonstrating Earth’s Interior

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Now we will start with a kind of comparative analysis. If you have ever work around with peeling of an Onion You would see there are concentric rings. Similar to the structure of the onion you have the earth’s interior. So as you can see there are different layers which are visible in an onion when you have a cross section. Similarly inside an earth there are three major layers. The outer most what is known as the crust. Next two crusts are the mantle and the inner most is known as the core. Now each of this three which is known as the crust, mantle and the core are further subdivided into two sections that is outer layers and inner layers. We have covered crust, mantle and core in details Inver offer you tube lecture. You have the link here. So you can refer that for more details.

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Interior of the Earth: 3 Divisions and 5 Discontinuities

What will be discussing today is will be briefing understanding these components and what are rocks and mineral. Now let’s first start with crust.


  • Continental - Silica & Aluminum (SIAL) – 35 km

  • Oceanic – Silica & Magnesium (SIMA) – 5 km

  • B/w upper & lower crust – Conrad Discontinuity

  • B/w crust & mantle – Moho Discontinuity

  • Jigsaw puzzle

  • Thinnest Layer

As I said crust is the outer most layer so when you have the kind of outer most layer that is visible on the earth surface which includes the mountain, Plateaus and Plans itself is known as the crust. Now this crust can be of two times I can classified at as one that forms at the continents or landmarks and other that forms at the Ocean. Now one which forms at the Ocean is predominately silica and magnesium and one that forms on the continent is predominately silica and Aluminium.

At the continent you have much higher thickness which is around 35kms in contrast to ocean which is nearly 5kms deep. However the despite it is 35kms and 5kms as we have seen. Crust is considered to one of thinnest layer among all the three layers that is crust, mantle and core. So you have crust which is thinnest of all and the arrangement is like a jigsaw of puzzle. So it’s kind of pieces which are fitted together. One into another and as you can see another model the plate-tectonic and you have the kind of arrangement of pieces which could be seen one over the other. So for example under the continental tripe theory and plate-tectonic if we see it was previously considered that earth is kind of single landmass.

So you have parts of South America that closely fit with the parts of Africa and so on and so for so you have a kind of jigsaw puzzle arrangement, which can be seen over the earth’s crust. Now as I said crust can be further sub-divided into upper crust and lower crust. The boundary that separates upper crust and lower crust is known as Conrad boundary and finally between crust and mantle you have another layer of discontinuity what we call this discontinuities because there is change in a structure that is seen from one to another. So layer between crust and mantle is known as Moho discontinuity so this are the some of the common discontinuities that you must be familiar with. Now we will start with the next part that is mental.