Summary
Chapter 2 of the Class 7 Geography NCERT textbook (Our Environment), "Inside Our Earth", explains that the earth's interior has three concentric layers — crust, mantle, and core — and that its crust is composed of three types of rocks (igneous, sedimentary, and metamorphic) which continuously transform through the rock cycle.
- The layered structure of the earth — The chapter describes the planet as three nested shells — a thin crust, a vast mantle making up most of its volume, and a nickel-iron core — with continental sial and oceanic sima forming the outer surface.
- The three families of rocks — It classifies rocks by how they form: igneous from cooled magma, sedimentary from compressed layers that can trap fossils, and metamorphic reshaped by heat and pressure — each with distinct traits.
- The rock cycle as continual change — It shows rocks are never permanent, cycling from magma to igneous to sediment to metamorphic and back to magma, emphasising that the solid earth is in constant slow transformation.
- Minerals and their uses — The chapter introduces minerals as naturally occurring substances with definite composition, valued as fuels like coal and petroleum and as industrial materials such as iron, gold, and uranium.
Key points & formulas
- 01Earth has three layers: crust (1% of volume), mantle (84%), and core (15%); the crust is the thinnest at about 35 km on continents and 5 km on ocean floors.
- 02Continental crust is called sial (silica + alumina); oceanic crust is called sima (silica + magnesium).
- 03The innermost core has a radius of about 3500 km and is made of nickel and iron, hence called nife.
- 04Three types of rocks: igneous (from cooled magma, also called primary rocks), sedimentary (from compressed sediments), and metamorphic (under heat and pressure).
- 05Extrusive igneous rocks (e.g., basalt) form on the surface and have fine grains; intrusive igneous rocks (e.g., granite) cool slowly deep inside and have large grains.
- 06Sedimentary rocks can contain fossils — remains of dead plants and animals trapped in rock layers.
- 07The rock cycle is the continuous transformation: magma → igneous → sediments → sedimentary → heat/pressure → metamorphic → magma again.
- 08Minerals are naturally occurring substances with definite chemical composition, used as fuels (coal, natural gas, petroleum) and in industries (iron, aluminium, gold, uranium).
Frequently asked questions
01What are the three layers of the earth?
The three layers are the crust (outermost and thinnest), the mantle (extends up to 2900 km below the crust), and the core (innermost, with a radius of about 3500 km).
02What is the difference between sial and sima?
Sial refers to the continental crust, which is mainly made of silica and alumina. Sima refers to the oceanic crust, which mainly consists of silica and magnesium.
03How thick is the earth's crust?
The crust is about 35 km thick on continental masses and only about 5 km thick on ocean floors.
04What is nife and where is it found?
Nife is the name for the earth's innermost core. It is called nife because it is made of nickel (ni) and iron (fe, from the Latin word ferrous). The core has a radius of about 3500 km.
05What are igneous rocks and how are they formed?
Igneous rocks (also called primary rocks) are formed when molten magma cools and solidifies. When lava cools rapidly on the surface, fine-grained extrusive rocks like basalt form. When magma cools slowly deep inside the crust, large-grained intrusive rocks like granite form.
06What is the difference between extrusive and intrusive igneous rocks?
Extrusive igneous rocks form when lava cools rapidly on the earth's surface; they have a very fine grained structure (e.g., basalt — the Deccan Plateau is made of basalt). Intrusive igneous rocks form when magma cools slowly deep inside the crust, forming large grains (e.g., granite, used for grinding stones).
07How are sedimentary rocks formed?
Rocks break down into small fragments called sediments. These sediments are transported and deposited by wind and water, then compressed and hardened over time into layers of sedimentary rocks. Sandstone is an example, formed from grains of sand.
08What are fossils?
Fossils are the remains of dead plants and animals trapped in the layers of rocks. Sedimentary rocks often contain fossils of plants, animals, and other microorganisms.
09What are metamorphic rocks? Give two examples.
Metamorphic rocks are formed when igneous or sedimentary rocks change under great heat and pressure. For example, clay changes into slate, and limestone changes into marble.
10What is the rock cycle?
The rock cycle is the continuous process by which rocks transform from one type to another. Molten magma cools to form igneous rocks; these break down into sediments that form sedimentary rocks; heat and pressure convert them into metamorphic rocks; metamorphic rocks under extreme conditions melt back into magma, and the cycle continues.
11Why can we not go to the centre of the earth?
To reach the centre of the earth, one would need to dig a hole 6000 km deep on the ocean floor, which is not possible. The deepest mine in the world (in South Africa) is only about 4 km deep, and the deepest oil drilling hole reaches only about 6 km.
12What percentage of earth's volume does each layer make up?
The crust forms only 1% of earth's volume, the mantle makes up 84%, and the core makes up 15%.
13What are minerals and what are they used for?
Minerals are naturally occurring substances with certain physical properties and definite chemical composition. They are used as fuels (coal, natural gas, petroleum), in industries (iron, aluminium, gold, uranium), in medicine, and in fertilisers.
14Is the NCERT Class 7 Chapter 2 'Inside Our Earth' PDF available for free download?
Yes — the NCERT Class 7 Social Science Geography 'Our Environment' Chapter 2 PDF is available for free with no sign-up required.
More chapters in Our Environment
Read Chapter 2 of Our Environment, the Class 7 Social Science NCERT textbook (2026-27 edition), online for free: the complete chapter as published by NCERT with every diagram, solved example and exercise, with a chapter summary, question answers and revision notes. Open the NCERT PDF above, or browse all NCERT Class 7 textbooks.
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