Inside Our Earth

This is Chapter 2 of Our Environment (Class 7 Geography): just 5 pages (7–11). It describes the earth's three layers (crust, mantle, core) with the terms sial, sima and nife, defines a rock, explains the three rock families — igneous (intrusive and extrusive), sedimentary and metamorphic — and the rock cycle, and closes with minerals and their uses. It has 4 numbered figures (a painted globe, a crust block, a rock photograph and a rock-cycle painting), two ‘Do you know?’ boxes, a ‘Word Origin’ box, a glossary, two ‘Let's do’ activities (with photographs of the Red Fort and the Taj Mahal) and five exercises ending in a ‘For fun’ picture. A short chapter, so this page has 8 concept sections without padding.

• Goes further in kegy203 (Interior of the Earth, Class 11: how we know the interior, seismic waves, shadow zones, the asthenosphere, the 5,100 km inner-core boundary, volcanoes and intrusive landforms). Read this chapter first, that one next.
• The rock families matter later: the Class 11 ‘Minerals and Rocks’ chapter has been dropped from the current book (kegy205 notes this), so this Class 7 chapter is now NCERT's main account of the three rock types. kegy205 (Geomorphic Processes) takes up weathering — how sediments are made; kegy206 (Landforms) shows limestone and other rocks shaping landscapes; kegy204 (Distribution of Oceans and Continents) moves the crust about.
• Minerals: jess105 (Minerals and Energy Resources, Class 10) and legy205 (Class 12) treat minerals as resources; kegy102 (Structure and Physiography) explains the Deccan's basalt and older rocks.
• Next: gess203 (Our Changing Earth) — plates, earthquakes, volcanoes and the landforms of rivers, wind, ice and sea.
• UPSC, honestly: GS1 physical geography. Prelims asked these basics directly in the early years — the three rock groups (1982), fossils in sedimentary rocks (1980), extrusive and intrusive as igneous (1986), SIMA as the heavy rocks (1986), a sial–sima match (1985) — and again in 2001 (sedimentary rocks; quartzite) and 2009 (the core is mainly iron). We link 8. Earthquake waves, magma and volcano questions sit on kegy203.

• Layers: crust (thinnest: ~35 km continents, ~5 km oceans) → mantle (to ~2,900 km deep) → core (radius ~3,500 km). Radius 6,371 km.
• Sial = silica + alumina (continents); sima = silica + magnesium (ocean floors); nife = nickel + iron (core).
• Volume: crust 1%, mantle 84%, core 15%. Deepest mine ~4 km (South Africa); centre unreachable.
• Igneous (primary) rocks from cooled magma: extrusive = fast, fine grains (basalt, Deccan); intrusive = slow, large grains (granite).
• Sedimentary rocks from compressed sediments, in layers, with fossils (sandstone). Metamorphic: heat + pressure — clay → slate, limestone → marble.
• Rock cycle: magma → igneous → sediments → sedimentary → metamorphic → magma. Mineral = natural, definite chemical composition.

The earth is made of several concentric layers, one inside another (Fig. 2.1). The top layer, the crust, is the thinnest of all: about 35 km on the continents and only about 5 km on the ocean floors. Beneath it lies the mantle, which reaches down to a depth of about 2,900 km. The innermost layer, the core, has a radius of about 3,500 km; it is made mainly of nickel and iron and so is called nife (ni for nickel, fe for iron). The centre has very high temperature and pressure. A ‘Do you know?’ box adds that the crust is only 1 per cent of the earth's volume, the mantle 84 per cent and the core 15 per cent, and that the earth's radius is 6,371 km.

The crust is not the same everywhere. Under the continents it is thicker (about 35 km), and its main mineral constituents are silica and alumina — hence sial (si + al). Under the oceans it is thinner (about 5 km) and made mainly of silica and magnesium — hence sima (si + ma) (Fig. 2.2). Sima is the heavier of the two; sial the lighter.

A ‘Do you know?’ box on page 7 says the deepest mine in the world, in South Africa, is about 4 km deep; that engineers searching for oil have dug a hole about 6 km deep; and that to reach the centre you would have to dig a hole about 6,000 km deep on the ocean floor — which is not possible. Exercise 4(i) asks why. The reasons: the distance (over 6,000 km of rock), the rising temperature and pressure with depth (the book stresses the very high temperature and pressure of the core), and the fact that rock deep down is hot enough to flow or melt.

The crust is made of many kinds of rocks. A rock is any natural mass of mineral matter that makes up the earth's crust; rocks differ in colour, size and texture. There are three major types: igneous, sedimentary and metamorphic. When molten magma cools it becomes solid; the rocks formed this way are igneous rocks, also called primary rocks (the others form, ultimately, from them). There are two kinds. Extrusive igneous rocks form when lava — fiery red molten magma that has come out of the earth's interior onto the surface, as in a volcano — cools rapidly on the crust; they have a very fine-grained structure, like basalt, of which the book says the Deccan plateau is made. Intrusive igneous rocks form when magma cools deep inside the crust; since they cool slowly they form large grains, like granite — the stone of the grinding stones used to make spice pastes and flour.

Rocks roll down slopes, crack and hit each other, and break into small fragments called sediments. Wind, water and other agents carry these sediments and lay them down elsewhere. The loose sediments are then compressed and hardened into layers of rock — sedimentary rocks. Sandstone, for example, is made from grains of sand. Sedimentary rocks may also contain fossils — the remains of dead plants, animals and micro-organisms trapped in the layers (the book's glossary).

Igneous and sedimentary rocks can change into metamorphic rocks under great heat and pressure (Fig. 2.3). The book's examples: clay changes into slate, and limestone into marble. The word comes from the Greek for change of form: the rock is transformed while it stays solid.

One type of rock changes into another under certain conditions in a cyclic way — the rock cycle. Molten magma cools and solidifies into igneous rock; igneous rocks are broken into small particles that are transported and deposited to form sedimentary rocks; igneous and sedimentary rocks under heat and pressure become metamorphic rocks; metamorphic rocks still under great heat and pressure melt into magma; and the magma can cool into igneous rock again (Fig. 2.4).

Rocks are useful: hard rocks build roads, houses and buildings, and stones are used in children's games — seven stones (pitthoo), hopscotch (stapu/kit kit) and five stones (gitti). A ‘Let's do’ shows the Red Fort built of red sandstone and the Taj Mahal of white marble. Rocks are made of different minerals — naturally occurring substances with certain physical properties and a definite chemical composition. Minerals matter greatly to people: some are fuels (coal, natural gas, petroleum); others are used in industry (iron, aluminium, gold, uranium), in medicine and in fertilisers.

The opening questions, the games question, and the two ‘Let's do’ activities.

Seven short answers, five ticks, a match, three reasons and a ‘For fun’. Try first, then open.

Numbers

One-liners

• The earth has three concentric layers: crust, mantle and core.
• The crust is the thinnest layer — thicker under continents, thinner under oceans.
• Continental crust is sial; oceanic crust is sima; the core is nife.
• A rock is a natural mass of mineral matter; a mineral has a definite chemical composition.
• Igneous rocks form when magma or lava cools; they are the primary rocks.
• Fast cooling gives fine grains (basalt); slow cooling gives large grains (granite).
• Sedimentary rocks are compressed layers of sediments and may hold fossils.
• Heat and pressure change rocks into metamorphic rocks without melting them.
• In the rock cycle, any rock can become any other kind over time.
• Minerals serve as fuels and in industry, medicine and fertilisers.

Traps to avoid

• The mantle reaches about 2,900 km below the surface, not below the crust.
• Sima (silica + magnesium) is the heavier crust of the ocean floors; sial is the lighter crust of the continents.
• Extrusive rocks are fine-grained; intrusive rocks are coarse-grained.
• Fossils occur in sedimentary rocks, not igneous ones.
• Shale (made of clay) becomes slate; limestone becomes marble; sandstone becomes quartzite.
• The Deccan Traps are basalt; much of the rest of the Deccan plateau is granite and gneiss.
• Strictly, coal is a sedimentary rock, though the book calls it a mineral.
• The deepest borehole is about 12 km (Kola, 1989), not 6 km.
• The earth's radius is 6,371 km (mean) or 6,378 km (equatorial); both appear in NCERT.

Prelims — what UPSC asks

• From this chapter: the three rock groups (1982), fossils in sedimentary rocks (1980), extrusive and intrusive rocks as igneous (1986), SIMA as the heavy rocks of the lithosphere (1986, also on kegy203), a sial–sima match (1985), statements on sedimentary rocks (2001), quartzite from sandstone (2001) and the core as mainly iron (2009).
• Elsewhere: magma, sills and dykes (2001) and black cotton soil from basalt (2021) on kegy203; gold and copper in old igneous rocks (1992) on jess105 and legy205; the Taj Mahal's marble and air pollution (1984) on legy209; the red sandstone of Mathura sculpture (1988) on lehs104.
• Method: these are definition questions — learn the pairs (sial–light–continents, sima–heavy–oceans; extrusive–fine–basalt, intrusive–coarse–granite; limestone–marble, sandstone–quartzite, shale–slate) and the order of the layers.

Mains — where it appears and how to answer

• No Mains question asks this chapter's content directly. It is the foundation for GS1 questions on the interior and tectonics (mantle plume, GS1 2018, on kegy204) and on resources (jess105, legy205).
• Use the rock cycle and the igneous–sedimentary–metamorphic link when explaining soils (black soil from Deccan basalt), mineral occurrence, or building stone and heritage (marble and sandstone monuments). A small labelled sketch of the layers or the rock cycle earns marks.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

Open any question to attempt it in the Question bank — your attempt is saved to your progress.