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GeographyNCERT Class 7 · Our Environment

Inside Our Earth: Interior, Rocks and the Rock Cycle

How the Earth is built from the inside out — its three concentric layers, the three families of rocks that compose its crust, and the rock cycle that endlessly recycles them.

⏱ 6 min readGS-I7 sections5 memory tricks
Why this matters for UPSC

Physical geography is a Prelims staple, and this chapter supplies high-frequency MCQ and match-the-following fodder: layer composition (SIAL/SIMA/NIFE), rock types with examples, volume shares, and the rock-cycle sequence. For GS-I Mains it underpins geomorphology and resource geography — e.g., why Deccan basalt yields black soil, and why fuel minerals sit in sedimentary basins. Treat it as the foundation chapter for the lithosphere.

Understand the chapter

A Dynamic Earth and Its Layered Interior

The Earth is a dynamic planet that changes constantly within and without, and its interior is arranged in concentric layers like an onion. From outside in these are the crust (thinnest), the mantle, and the core. We cannot reach the centre — the deepest mine (South Africa) is only about 4 km and the deepest drilled hole about 6 km, against a 6371 km radius — so our knowledge of the interior is largely indirect.

  • Crust: about 35 km thick on continents, only about 5 km under oceans — the thinnest layer.
  • Mantle: extends to 2900 km below the crust; about 84% of Earth's volume.
  • Core: radius about 3500 km; nickel + iron (NIFE); very high temperature and pressure.
  • UPSC add-on: crust-mantle boundary = Mohorovicic (Moho) discontinuity; mantle-core = Gutenberg discontinuity.

SIAL, SIMA and NIFE — Composition by Layer

The crust's composition differs between continents and oceans, a distinction UPSC loves to test. Continental crust is built mainly of silica and alumina (SIAL) and is lighter and granitic; oceanic crust is mainly silica and magnesium (SIMA) and is denser and basaltic. The innermost core is NIFE — nickel and ferrous (iron).

  • SIAL = SIlica + ALumina → continental crust (lighter).
  • SIMA = SIlica + MAgnesium → oceanic crust (denser).
  • NIFE = NIckel + FErrous → core (innermost).

Rocks: Definition and the Three Families

Any natural mass of mineral matter forming the Earth's crust is a rock, and rocks differ in colour, size and texture. All rocks fall into three families defined by how they form: igneous (from cooling magma), sedimentary (from compacted sediments), and metamorphic (from heat-and-pressure transformation). Rocks are aggregates of minerals, whereas a single mineral has a definite chemical composition.

  • Igneous — solidified magma or lava; called 'primary rocks'.
  • Sedimentary — hardened layers of sediment; may hold fossils.
  • Metamorphic — existing rock changed by great heat and pressure.

Igneous Rocks — Extrusive vs Intrusive

Igneous rocks form when molten magma cools and solidifies, and are called primary rocks because they crystallise directly from the melt. Where the cooling happens decides the texture. Lava cooling rapidly on the surface gives fine-grained extrusive rocks like basalt — the Deccan plateau is basaltic; magma cooling slowly deep underground gives coarse-grained intrusive rocks like granite.

  • Extrusive: fast surface cooling → fine grains → basalt (Deccan plateau).
  • Intrusive: slow deep cooling → large grains → granite (used in grinding stones).
  • Resource link: weathered Deccan basalt forms black/regur soil, ideal for cotton.

Sedimentary and Metamorphic Rocks

Rocks crack and break into small fragments called sediments, which wind and water transport and deposit; compaction and hardening of these layers form sedimentary rocks such as sandstone. Because they build up in layers at the surface, sedimentary rocks uniquely preserve fossils. When igneous or sedimentary rocks face intense heat and pressure, they recrystallise into metamorphic rocks — clay becomes slate and limestone becomes marble.

  • Sedimentary: e.g., sandstone; the only family that bears fossils; hosts coal and petroleum.
  • Metamorphic: clay → slate; limestone → marble.
  • Monuments: Red Fort = red sandstone; Taj Mahal = white marble.

The Rock Cycle

The three rock families are not fixed end-states; under the right conditions each can transform into another in a continuous loop called the rock cycle. Magma cools into igneous rock, which weathers into sediments that compact into sedimentary rock; heat and pressure convert these into metamorphic rock, which on further heating melts back into magma. The cycle links the Earth's internal (endogenic) heat with external (exogenic) weathering and deposition.

  • Sequence: igneous → sedimentary → metamorphic → magma → igneous.
  • Driven by cooling, weathering/erosion, heat-pressure, and melting.

Minerals and Their Economic Importance

Rocks are made of minerals — naturally occurring substances with definite chemical compositions and characteristic physical properties. Minerals underpin the economy: some are fuels (coal, natural gas, petroleum), while others feed industry, medicine and agriculture. This resource dimension is what carries a Class-7 chapter into UPSC GS-I resource geography.

  • Fuel minerals: coal, natural gas, petroleum.
  • Industrial/metallic minerals: iron, aluminium, gold, uranium.
  • Mineral (definite composition) is not the same as a rock (aggregate, no fixed composition).

Key terms

Crust
Outermost and thinnest layer of the Earth — about 35 km on continents, 5 km under oceans.
Mantle
Layer beneath the crust extending to 2900 km depth; makes up about 84% of Earth's volume.
Core (NIFE)
Innermost layer, radius about 3500 km, made mainly of nickel and iron, with extreme heat and pressure.
SIAL
Continental crust dominated by silica and alumina; lighter, granitic in character.
SIMA
Oceanic crust dominated by silica and magnesium; denser, basaltic in character.
Igneous rock
'Primary' rock formed from solidified magma/lava; extrusive (fine-grained) or intrusive (coarse-grained).
Sedimentary rock
Rock formed by compaction and hardening of sediments; the only type that preserves fossils.
Metamorphic rock
Pre-existing rock transformed by great heat and pressure (clay→slate, limestone→marble).
Rock cycle
Continuous transformation among the three rock families through cooling, weathering, heat-pressure and melting.
Mineral
Naturally occurring substance with a definite chemical composition and fixed physical properties.

Must-know facts exam-ready

  • Crust is the thinnest layer (about 35 km continental, 5 km oceanic); mantle reaches 2900 km below crust; core radius about 3500 km; Earth's radius is 6371 km.
  • Composition codes: continental crust = SIAL (silica+alumina); oceanic crust = SIMA (silica+magnesium); core = NIFE (nickel+iron).
  • Volume share: crust 1%, mantle 84%, core 15%.
  • Three rock types are igneous, sedimentary and metamorphic; igneous are the 'primary rocks'.
  • Extrusive igneous = basalt (fast cooling, fine-grained; Deccan plateau); intrusive igneous = granite (slow cooling, coarse-grained).
  • Sedimentary rocks (e.g., sandstone) form from compacted sediments and uniquely preserve fossils; coal and petroleum occur in sedimentary strata.
  • Metamorphism: clay → slate; limestone → marble under heat and pressure.
  • Monuments: Taj Mahal = white marble (metamorphic); Red Fort = red sandstone (sedimentary).
  • Minerals have a definite chemical composition; fuel minerals = coal, natural gas, petroleum; industrial = iron, aluminium, gold, uranium.
  • Word origins: igneous (Latin ignis = fire), sedimentary (Latin sedimentum = settle down), metamorphic (Greek = change of form).
  • UPSC add-on: crust-mantle boundary = Mohorovicic (Moho) discontinuity; mantle-core boundary = Gutenberg discontinuity.
  • Deccan plateau basalt (extrusive) weathers into black/regur soil, the cotton-growing belt — the key resource-geography link.

Memory tricks remember it for good

SIAL-SIMA-NIFE (top to bottom)
SIAL = Silica+Alumina (continental crust); SIMA = Silica+Magnesium (oceanic crust); NIFE = Nickel+Ferrous (core)
💡 Recall each layer's composition in order from crust down to the core.
'I Saw Magma' = I-S-M
Igneous, Sedimentary, Metamorphic — the three rock families, with Igneous first as the 'primary' rock
💡 Remember all three rock types and that igneous comes first from magma.
EX-fast-Fine, IN-slow-Coarse
EXtrusive cools fast → Fine grains (basalt); INtrusive cools slow → Coarse grains (granite)
💡 Never reverse basalt vs granite — their cooling site and texture.
Fire-Settle-Change
Igneous (ignis=fire), Sedimentary (sedimentum=settle down), Metamorphic (metamorphose=change of form)
💡 Recall the word origin and defining process of each rock type.
1-84-15
Crust 1%, Mantle 84%, Core 15% of Earth's volume
💡 Lock the volume shares — the mantle is overwhelmingly the largest.

Traps to avoid

  • SIAL is continental (lighter, granitic) and SIMA is oceanic (denser, basaltic) — aspirants routinely swap the two.
  • '2900 km' is the mantle's lower limit measured as depth below the crust, while '3500 km' is the core's radius — different yardsticks, not interchangeable figures.
  • The crust is the thinnest layer yet only about 1% of volume; the mantle (84%), not the crust or core, is the largest by volume — don't equate 'thin' with 'small share' or hand 84% to the core.
  • Fossils occur in sedimentary rocks only — never in igneous or metamorphic rocks, where heat/pressure destroys them.
  • Basalt = extrusive (fast cooling, fine grains) and granite = intrusive (slow cooling, coarse grains) — do not reverse the cooling location or texture.
  • Don't swap metamorphic pairs: clay → slate and limestone → marble (not clay → marble).

Exam focus

🧠 Prelims angles

  • Match-the-following: rock type with example — basalt (extrusive igneous), granite (intrusive igneous), sandstone (sedimentary), marble/slate (metamorphic).
  • Layer composition codes SIAL / SIMA / NIFE and which layer each denotes.
  • Numerical facts: crust thickness (35/5 km), mantle depth (2900 km), core radius (3500 km), Earth radius (6371 km), volume shares (1/84/15%).
  • Sequence of the rock cycle and identifying which rock type bears fossils.
  • Discontinuities (Moho between crust and mantle) — a recurring physical-geography MCQ theme.
  • Mineral vs rock distinction (definite chemical composition) and classification of fuel minerals (coal, petroleum, natural gas).

✍️ Mains angles GS-I

  • How does the Deccan's volcanic (basaltic) geology shape India's soils and agriculture?Link extrusive basalt → black regur soil → moisture-retentive, lime-rich → the cotton-growing belt.
  • Explain the rock cycle as the interface of endogenic and exogenic forces shaping the lithosphere.Connect internal heat (cooling, melting) with external agents (weathering, transport, deposition) in a closed, continuous loop.
  • The distribution of India's mineral and energy resources rests on its rock systems.Tie fossil fuels to sedimentary basins and metallic minerals to the igneous/metamorphic peninsular shield.
Practice Geography questions from this syllabus →

Last-minute revision tick as you recall

  • Onion layers: Crust (thinnest) → Mantle (to 2900 km, 84% volume) → Core (radius 3500 km, NIFE).
  • SIAL = continental (silica+alumina); SIMA = oceanic (silica+magnesium); NIFE = core (nickel+iron).
  • Volumes: crust 1%, mantle 84%, core 15%; Earth's radius 6371 km.
  • Igneous = primary; extrusive basalt (fine, Deccan) vs intrusive granite (coarse).
  • Sedimentary = sandstone, holds fossils; metamorphic = clay→slate, limestone→marble.
  • Rock cycle: igneous → (weathering) sedimentary → (heat/pressure) metamorphic → (melting) magma → igneous.
  • Taj Mahal = marble; Red Fort = sandstone.
  • Minerals = definite chemical composition; fuels = coal, natural gas, petroleum.
  • UPSC add-on: Moho = crust-mantle boundary; Gutenberg = mantle-core boundary.

Distilled from NCERT Class 7 · Our Environment for UPSC. Always cross-check facts with the original NCERT.