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Every NCERT book that matters for UPSC — open a book, then a chapter. Learn each chapter's concepts, then how UPSC tests them — with diagrams, flowcharts and the past questions to attempt.
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← Fundamentals of Physical Geography — all chaptersInterior of the Earth
Start here: why this chapter matters
In simple wordsThink of the earth as a boiled egg: a thin hard shell (the crust), a thick middle (the mantle) and a dense centre (the core). This chapter answers three questions a curious student would ask: How do we know what is inside if no one has been there? Why does the ground shake? Why does molten rock come out of volcanoes?
The shape of the land you see — mountains, plateaus, valleys — is made by two sets of forces. Exogenic forces work from outside (rivers, wind, glaciers wearing land down). Endogenic forces work from inside (earthquakes, volcanoes, the slow movement of rock). You cannot fully understand a region's landscape, or the risks people living there face, without the inside story. That is why geography starts with the earth's interior.
- Comes after Chapter 2, which explained how the earth formed and settled into layers.
- Leads into Chapter 4 (plate tectonics), which explains where earthquakes and volcanoes happen and why.
- Feeds GS3 Disaster Management — earthquakes and tsunamis are core topics there.
| Word | What it means here |
|---|---|
| Lithosphere | The rigid outer shell: the crust plus the very top of the mantle |
| Density | How tightly packed a material is — denser layers sit deeper |
| Seismic waves | Vibrations sent through the earth by an earthquake (Greek seismos = shaking) |
| Fault | A crack in rocks along which blocks have moved |
| Magma / lava | Molten rock — called magma underground, lava once it moves up or reaches the surface |
| Igneous rock | Rock formed when magma or lava cools (Latin ignis = fire) |
| Discontinuity | A boundary where the earth's properties change sharply, e.g. the Moho between crust and mantle |
At a glance — the six ideas to carry away
- We read the interior mostly through indirect evidence — earthquake waves are the most important.
- S-waves travel only through solids; their large shadow zone shows the outer core is liquid.
- Layers: crust → mantle (asthenosphere) → outer core (liquid) → inner core (solid).
- Magnitude (Richter, 0–10) = energy released; intensity (Mercalli, 1–12) = damage seen.
- Volcano type depends on lava: fluid basalt builds gentle shields; viscous lava explodes.
- Magma cooling inside the crust makes intrusive forms — the Deccan Traps were fed by dykes.
1. How do we know what is inside?
In simple wordsA doctor can't open you up to check every organ, so they use X-rays and ultrasound — signals that pass through the body and come back changed. Geologists do the same with the earth: they study a few samples they can actually touch, and read the rest from signals and measurements.
The earth's radius is about 6,378 km, yet the deepest hole ever drilled is about 12 km — less than 0.2% of the way to the centre. So almost everything we know about the interior is inferred. Only a small part comes from material we can examine directly.
| Direct sources (we hold the material) | Indirect sources (we infer from properties) |
|---|---|
| Mining — South African gold mines reach 3–4 km; deeper is too hot to work. | Temperature, pressure and density all rise with depth. Knowing how fast they rise and how thick the earth is, scientists estimate values at every depth. |
| Deep drilling — the Deep Ocean Drilling Project and the Integrated Ocean Drilling Project; the Kola drill reached 12 km. | Meteors — their material is not from our interior, but they formed from the same stuff as the earth, so they hint at its make-up. |
| Volcanic eruptions — magma brought to the surface can be tested in labs, though we can't tell exactly how deep it came from. | Gravity — stronger at the poles than at the equator (the poles are closer to the centre). Readings that differ from the expected value are gravity anomalies; they reveal how mass is spread through the crust. |
| Magnetic surveys — show where magnetic materials lie in the crust. | |
| Seismic waves — the most important source; they give a complete picture of the layers. |
Beyond NCERTThe Kola Superdeep Borehole is on the Kola Peninsula in Russia's Arctic north (on land, not in the sea); it reached about 12.2 km in 1989. The earth is slightly flattened: about 6,378 km to the centre at the equator and about 6,357 km at the poles — which is exactly why gravity is a little stronger at the poles.
Exam trapA volcano is a direct source (NCERT exercise Q1-ii). Earthquake waves, gravity and magnetism are indirect. Meteors are indirect too — their material is not from the earth's interior.
⤵ Sources of information — at a glance
The earth's interiorNo one can reach it — radius ≈ 6,378 km
Directthe material itself
Mines3–4 km (S. Africa)
Deep drillingKola 12 km · DODP · IODP
Volcanic eruptionsmagma analysed in labs
Indirectinferred from properties
T, P, density gradientsall increase with depth
Meteors · gravity · magnetismgravity anomaly → mass distribution
Seismic waves ★the most complete picture
2. Earthquakes: why the earth shakes
In simple wordsPush two rough wooden blocks against each other. At first they stick; you keep pushing; suddenly they jerk past each other with a snap. Rocks along a crack in the crust do the same — and the ‘snap’ sends vibrations through the earth. That vibration is an earthquake.
An earthquake is the shaking of the earth caused by a sudden release of energy. It happens along a fault — a sharp break in crustal rocks. Rocks on either side try to move in opposite directions, but the weight of the rock above presses them together and friction locks them. When the stress finally beats the friction, the blocks deform and then slip abruptly, and energy radiates as waves in all directions.
- Focus / hypocentre — the point where the energy is released.
- Epicentre — the point on the surface nearest the focus, directly above it; it feels the waves first.
- All natural earthquakes take place in the lithosphere (up to ~200 km deep).
- A seismograph records the waves reaching the surface.
⤵ From stress to damage
Stress builds along a faultblocks pushed in opposite directions
Friction locks the blocksrocks deform, storing energy
Stress beats friction → abrupt slipenergy released at the focus
Body waves (P, S) spread through the earthreach the epicentre first
Body waves meet surface rocks → surface wavesthe most destructive
On landshaking, landslides, liquefaction, structural collapse, fires…
Under the ocean + high magnitudetsunami — waves generated by the tremor
3. Earthquake waves
In simple wordsStretch a slinky and push one end: a squeeze travels along it — that is how a P-wave moves. Now flick a rope up and down: a hump travels along it — that is an S-wave. You can push a squeeze through water or air, but you can't flick a hump through water — which is why S-waves stop at liquids.
There are two families. Body waves start at the focus and travel through the body of the earth. When they interact with rocks at the surface they create surface waves, which travel along the surface. Waves move faster through denser material, and they reflect (bounce back) or refract (bend) when they meet material of a different density — which is how they betray the layers inside.
Reading NCERT Figure 3.1 — Earthquake Waves (page 20)
- One seismograph line, read left to right as time: three distinct bursts — P-waves, then S-waves, then surface waves.
- The arrows marked arrival times show each family reaching the station at a different moment — P first, S after a gap, surface last.
- The amplitude (height of the wiggles) grows from P to S to surface waves: the last to arrive shake the ground the most — which is why they do the most damage.
- The gap between P and S arrival grows with distance from the epicentre; seismologists use it to work out how far away the quake was.
| P-waves (primary) | S-waves (secondary) | Surface waves | |
|---|---|---|---|
| Arrival | First (fastest) | After a time lag | Last |
| Travel through | Solids, liquids and gases (like sound waves) | Solids only | Along the surface |
| Particle motion | Parallel to the direction of travel — compression and stretching create density differences | Perpendicular to travel, in the vertical plane — crests and troughs | Displace rocks |
| Damage | Low | Moderate | Most destructive — buildings collapse |
Beyond NCERTNear the surface P-waves travel at roughly 6 km/s and S-waves at roughly 3.5 km/s. Surface waves are of two kinds — Love waves (side-to-side) and Rayleigh waves (rolling, like ocean waves).
Why S-waves matterBecause S-waves cannot pass through liquid, the places where they don't arrive tell us where the liquid is. That one property let scientists map the core.
4. Shadow zones: how the core was found
In simple wordsShine a torch at a ball: a dark patch appears behind it, the shadow. An earthquake is the torch, the core is the ball, and the patch of the earth's surface where certain waves never arrive is the shadow zone. From the size of the shadow, you can work out the size and nature of the ball.
Seismographs far from an earthquake record its waves — except in certain areas, the shadow zones. Every earthquake has its own shadow zone, placed relative to its epicentre, so if you know the epicentre you can draw its shadow zone.
Reading NCERT Figure 3.2 (a) and (b) (page 21)
- The rays are curved, not straight — waves keep bending (refracting) because the earth gets denser with depth.
- (a) P-waves: rays leaving the quake reach every point up to 105° directly. Rays that hit the core are bent sharply and come out beyond 145° (‘P-waves received here’). In between, on both sides, is a band marked ‘no direct P-waves’ — the P-wave shadow zone, 105°–145°.
- (b) S-waves: rays reach up to 105° and none enter the core. Everything beyond 105° is shaded ‘no direct S-waves received here’ — a much bigger zone than for P-waves.
- The angles are measured from the epicentre around the earth's centre, so the zones are rings around the globe, not points.
- Within 105° of the epicentre: both P and S waves are recorded.
- Beyond 145°: P-waves are recorded, S-waves are not.
- P-wave shadow zone: a band 105°–145° from the epicentre.
- S-wave shadow zone: the entire area beyond 105° — larger, a little over 40% of the earth's surface.
⤵ The reasoning UPSC expects
S-waves travel only through solidsa property of the wave
No S-waves beyond 105°something in the way stops them
That layer must be liquid→ the liquid outer core (from 2,900 km)
P-waves bend sharply at the core boundary→ a P-wave gap at 105°–145°, then P again
5. Types, measurement and effects
| Type of earthquake | Cause |
|---|---|
| Tectonic (most common) | Sliding of rocks along a fault plane |
| Volcanic | A special class of tectonic quake, confined to areas of active volcanoes |
| Collapse | Roofs of underground mines collapse in intensely mined areas — minor tremors |
| Explosion | Explosion of chemical or nuclear devices |
| Reservoir-induced | Occur in areas of large reservoirs (the weight and seeping water of a big dam lake) |
In simple words — magnitude vs intensityMagnitude is like the wattage of a bulb — one fixed number for the quake (energy released). Intensity is how bright the light looks where you stand — it changes from place to place (damage seen). One earthquake has one magnitude but many intensities.
| Magnitude | Intensity | |
|---|---|---|
| Scale | Richter | Mercalli (an Italian seismologist) |
| Measures | Energy released | Visible damage caused |
| Range | 0–10 | 1–12 |
12 immediate hazards (NCERT list): ground shaking · differential ground settlement · land and mud slides · soil liquefaction · ground lurching · avalanches · ground displacement · floods from dam and levee failures · fires · structural collapse · falling objects · tsunami. The first six change landforms; the rest threaten life and property directly.
- A quake lasts only seconds, but is devastating if the magnitude is above 5 on the Richter scale.
- Tsunami occurs only if the epicentre is under the ocean and the magnitude is high enough. A tsunami is waves generated by the tremor, not an earthquake itself.
- High-magnitude quakes (8+) are rare — about once in 1–2 years; tiny ones occur almost every minute.
Reading the photos and map on page 22
- The collage shows the 26 December 2004 Indian Ocean tsunami: wrecked vehicles on Marina Beach, Chennai, flattened homes in Colombo, Sri Lanka, and destruction in Thailand and Indonesia.
- The map places the epicentre off Sumatra with circles spreading across the Bay of Bengal and Indian Ocean — the waves crossed an entire ocean within hours, reaching India, Sri Lanka, the Maldives, Thailand, Malaysia, Myanmar and Bangladesh.
- Its death-toll figures (‘10,000 approx’) were early reports from the day. The final count was over 2 lakh people across 14 countries (Beyond NCERT) — which is why UPSC asked about it in Mains 2017.
- The photo of Aman Setu at Uri on the Line of Control shows a bridge broken by an earthquake — the 2005 Kashmir earthquake (Beyond NCERT). It illustrates ‘structural collapse’ from the hazards list.
Beyond NCERT (asked by UPSC)The Richter scale is logarithmic: each whole number up means 10× the wave amplitude and roughly 32× the energy — not 100×. UPSC tested exactly this in 2006. Today seismologists mostly report the moment magnitude. India is divided into seismic zones II–V (Zone V most severe — the Himalaya, the North-East, Kutch and the Andaman & Nicobar Islands).
6. Structure of the earth
In simple wordsHeavy things sink, light things float. When the young earth was molten, heavy iron and nickel sank to the centre to form the core, lighter rock floated up to form the crust, and the mantle sits in between. Each layer is denser than the one above it.
Reading NCERT Figure 3.3 — The interior of the earth (page 23)
- It is a wedge cut from the surface to the centre; the point of the wedge is the centre, labelled 6,378 km (the radius).
- The lithosphere is bracketed as the crust and uppermost solid mantle; the asthenosphere is the band just beneath it.
- The core starts at 2,900 km. The figure splits it into liquid (outer) and solid (inner), with the boundary at 5,100 km — a number that appears only in the figure, not in the text.
- The crust is labelled ‘0–100 km thick’. Use the text's figures for exams — about 5 km under oceans, about 30 km under continents, up to 70 km under the Himalaya.
- The relief drawn on top (mountains) reminds you the crust is thickest under mountain ranges.
| Layer | Depth / thickness | State | Remember |
|---|---|---|---|
| Crust | Oceanic ~5 km · continental ~30 km · up to 70 km under the Himalaya | Solid, brittle | Outermost layer; oceanic crust is thinner |
| Lithosphere | 10–200 km | Solid | Crust + uppermost mantle |
| Asthenosphere | Up to ~400 km | Weak (astheno = weak) | Main source of magma |
| Mantle | Moho discontinuity → 2,900 km | Lower mantle solid | Denser than the crust |
| Outer core | 2,900 km → 5,100 km | Liquid | Nickel + iron |
| Inner core | 5,100 km → centre (6,378 km) | Solid | Nickel + iron — the ‘nife’ layer |
Beyond NCERT (useful for Prelims)Boundaries have names: Moho (crust | mantle), Gutenberg (mantle | core, ~2,900 km), Lehmann (outer | inner core, ~5,100–5,150 km). Older books split the crust into SIAL (silica + aluminium, the lighter continents) over SIMA (silica + magnesium, heavier, the ocean floors) — UPSC asked about SIMA in 1985 and 1986. The inner core is solid despite being the hottest part because the pressure there is enormous.
7. Volcanoes
In simple wordsThink of a soda bottle. Gas dissolved under pressure stays calm until you open it; shake it first and it erupts violently. Magma behaves the same way: runny magma lets gas escape gently and oozes out; thick, sticky magma traps gas until it blasts out explosively.
A volcano is a place where gases, ash and/or molten rock escape to the ground. It is active if material is being released now or was in the recent past. Molten rock in the upper mantle is magma; once it moves towards or reaches the surface it is lava. What comes out: lava flows, pyroclastic debris, volcanic bombs, ash and dust, and gases — nitrogen compounds, sulphur compounds, and small amounts of chlorine, hydrogen and argon.
Reading the volcano photos (pages 23–24)
- Shield volcano: a broad, low, dark lava field — note how flat the land around the vent is; fluid basalt spreads far before it cools.
- Cinder cone: a small, steep, rounded mound — built from fragments thrown up when lava fountains out of the vent of a shield.
- Composite volcano: a tall, steep, symmetric, snow-capped cone rising high above the plain — the classic ‘volcano shape’ built from alternating layers of lava and ash.
- Compare the slopes: gentle (fluid lava, shield) versus steep (sticky lava and ash, composite).
| Type | Lava / eruption | Form and example |
|---|---|---|
| Shield | Very fluid basalt; low explosivity (explosive only if water gets into the vent) | Largest volcanoes after basalt flows; not steep · Hawaii. Fountaining lava can build a cinder cone at the vent |
| Composite | Cooler, more viscous lava; explosive | Lava + pyroclastic material + ash pile up in layers |
| Caldera | Most explosive of all | Collapses on itself into a depression — a caldera; the magma chamber is huge and close |
| Flood basalt | Highly fluid lava over long distances | Thousands of sq km of flows, each can be 50 m+ thick · Deccan Traps (Maharashtra plateau), once far larger |
| Mid-ocean ridge | Frequent eruptions in the central part | A ridge system over 70,000 km long through all ocean basins |
Beyond NCERT — IndiaBarren Island in the Andaman Sea is India's only active volcano (UPSC 2018); Narcondam nearby is dormant. The Deccan Traps formed around 66 million years ago (end of the Cretaceous); their weathering produced the black cotton (regur) soil of the Deccan (UPSC 2021).
8. Volcanic landforms
In simple wordsPour melted wax into a box of sand. Some wax reaches the top and cools there; some gets trapped in cracks and pockets inside and cools in odd shapes — sheets, domes, saucers, walls. Dig the sand away later and those shapes appear. Magma does the same inside the crust: its frozen shapes are the intrusive forms, revealed when erosion strips the rock above.
Lava cools into igneous rock. If it cools at the surface it forms volcanic rock; if it cools inside the crust it forms plutonic rock. The shapes it takes inside the crust are the intrusive forms.
Reading NCERT Figure 3.4 — Volcanic Landforms (page 24)The block diagram labels 17 features. The text explains only a few of them; here is every label, explained.
| Label in Figure 3.4 | Where | What it is |
|---|---|---|
| Batholith | Deep underground | Huge granite body — the cooled magma chamber; seen only after erosion |
| Stock | Underground | A smaller, batholith-like body (Beyond NCERT: under about 100 sq km exposed) |
| Laccolith / eroded laccolith | Underground / exposed | Dome with a flat base fed by a pipe; once erosion exposes it, it stands as a domal hill (Karnataka plateau) |
| Sill | Between rock layers | Horizontal sheet of cooled magma (thin ones are ‘sheets’) |
| Dyke | Cutting across layers | Vertical wall of cooled magma; feeders of the Deccan Traps (western Maharashtra) |
| Volcanic conduit | Underground | The pipe through which magma rises to the vent |
| Country rock | All around | The existing rock that the magma pushes into |
| Diatreme | Underground pipe | A pipe blasted open by gas and filled with broken rock (Beyond NCERT: kimberlite diatremes carry diamonds) |
| Volcanic neck with radiating dykes | Surface | Magma frozen inside an old vent, left standing after the softer cone erodes, with dykes spreading out like spokes |
| Lava flow | Surface | A stream of lava that has cooled on the ground |
| Lava plateau | Surface | A wide, high, flat area built by many layers of fluid lava (the Deccan) |
| Lava mesa | Surface | A flat-topped hill capped by hard lava after the land around it eroded |
| Composite volcano | Surface | Steep cone of alternating lava and ash layers |
| Cinder cone | Surface | Small steep cone of lava fragments around a vent |
| Ash cone with volcanic dome | Surface | A cone of ash whose crater holds a dome of thick, sticky lava |
| Caldera with cinder cone on floor | Surface | A collapsed crater in which a new small cone has grown (Beyond NCERT: e.g. Crater Lake, USA) |
| Ash flows | Surface | Fast, hot avalanches of ash and gas poured down the slopes |
| Intrusive form | Shape | Remember |
|---|---|---|
| Batholith | Huge dome deep in the crust, several km deep | Granite; the cooled magma chamber; exposed only after erosion |
| Laccolith | Dome with a level base, fed by a pipe from below | Like a buried composite-volcano dome; granite domal hills of the Karnataka plateau are laccoliths or batholiths |
| Lapolith | Saucer shape, concave to the sky | Lava spread along a weak horizontal plane |
| Phacolith | Wavy mass at the base of synclines or top of anticlines | In folded country; connected to a magma chamber below |
| Sill / sheet | Near-horizontal body | Thick = sill, thin = sheet |
| Dyke | Wall-like, almost perpendicular to the ground | Most common in western Maharashtra; feeders of the Deccan Traps |
⤵ Where the magma goes
Magma in the asthenospherethe weak zone of the upper mantle
Rises through weak zones and cracksmagma → lava once it moves up
Cools inside the crustplutonic rock
Intrusive formsbatholith · stock · laccolith · lapolith · phacolith · sill · dyke
Reaches the surfacevolcanic rock
Volcanoes and surface formsshield · composite · caldera · flood basalt · lava plateau · cinder cone
Exam trapMagma solidified horizontally between rock layers is a sill, not a dyke. UPSC 2001 used exactly this as the false statement.
NCERT exercise — answer outlines
Try each question yourself first, then open the outline to compare. These are the points an examiner looks for.
What are body waves? (30 words)
Waves produced at the focus when energy is released; they travel through the body of the earth in all directions. Two kinds: P-waves (fastest; solids, liquids, gases) and S-waves (solids only). On reaching the surface they generate surface waves.
Name the direct sources of information about the interior of the earth. (30 words)
Surface and mined rocks (South African gold mines, 3–4 km); deep drilling projects (Deep Ocean Drilling Project, Integrated Ocean Drilling Project; Kola, 12 km); and volcanic eruptions, whose magma can be analysed.
Why do earthquake waves develop a shadow zone? (30 words)
S-waves cannot pass through liquid, so the liquid outer core blocks them — no S-waves beyond 105°. P-waves refract at the core–mantle boundary (density change), leaving no direct P-waves between 105° and 145°.
Briefly explain the indirect sources of information other than seismic activity. (30 words)
Rising temperature, pressure and density with depth; meteors of similar composition; gravity anomalies showing mass distribution; and magnetic surveys showing magnetic materials in the crust.
What are the effects of propagation of earthquake waves on the rock mass through which they travel? (150 words)
Intro: energy from the focus travels as body and surface waves, vibrating the rocks they pass through.
P-waves: vibrate parallel to travel → alternate compression and stretching → density differences in the rock.
S-waves: vibrate perpendicular, in the vertical plane → crests and troughs; they cannot cross liquids.
Surface waves: displace rocks at the surface → collapse of structures; the most damaging.
Changes in path: speed rises in denser rock; waves reflect and refract at boundaries — the basis of shadow zones.
On landforms: ground shaking, differential settlement, landslides, soil liquefaction, ground lurching, avalanches.
Conclude: the same waves that damage rock also reveal the earth's layered interior. Add a small P/S motion sketch.
P-waves: vibrate parallel to travel → alternate compression and stretching → density differences in the rock.
S-waves: vibrate perpendicular, in the vertical plane → crests and troughs; they cannot cross liquids.
Surface waves: displace rocks at the surface → collapse of structures; the most damaging.
Changes in path: speed rises in denser rock; waves reflect and refract at boundaries — the basis of shadow zones.
On landforms: ground shaking, differential settlement, landslides, soil liquefaction, ground lurching, avalanches.
Conclude: the same waves that damage rock also reveal the earth's layered interior. Add a small P/S motion sketch.
What do you understand by intrusive forms? Briefly describe the various intrusive forms. (150 words)
Define: igneous bodies formed when magma cools within the crust (plutonic rocks), as opposed to volcanic rocks cooled at the surface.
Forms: batholith — huge granite dome, the cooled magma chamber; laccolith — dome with a flat base fed by a pipe (Karnataka plateau hills); lapolith — saucer shape, concave up; phacolith — wavy body in folds; sill/sheet — horizontal, thick/thin; dyke — vertical wall (western Maharashtra; feeders of the Deccan Traps).
Significance: seen at the surface only after erosion; they form hills and ridges and host minerals.
Diagram: one labelled cross-section like Figure 3.4 earns marks.
Forms: batholith — huge granite dome, the cooled magma chamber; laccolith — dome with a flat base fed by a pipe (Karnataka plateau hills); lapolith — saucer shape, concave up; phacolith — wavy body in folds; sill/sheet — horizontal, thick/thin; dyke — vertical wall (western Maharashtra; feeders of the Deccan Traps).
Significance: seen at the surface only after erosion; they form hills and ridges and host minerals.
Diagram: one labelled cross-section like Figure 3.4 earns marks.
What to remember
Numbers
Earth's radius≈ 6,378 km
Deepest mines (South Africa, gold)3–4 km
Deepest drill (Kola)12 km
Oceanic crust / continental crust~5 km / ~30 km
Crust under the Himalayaup to 70 km
Lithosphere10–200 km (crust + uppermost mantle)
Asthenosphereup to ~400 km
Core–mantle boundary2,900 km
Outer core | inner core boundary (Figure 3.3)5,100 km
P and S both recordedwithin 105° of the epicentre
P-wave shadow zone105°–145°
S-wave shadow zonebeyond 105° — just over 40% of the surface
Richter (magnitude, energy)0–10
Mercalli (intensity, damage)1–12
Devastating quakemagnitude > 5
Mid-ocean ridge system> 70,000 km
One-liners
- Lithosphere = crust + uppermost mantle (not the crust alone).
- ‘Astheno’ = weak; the asthenosphere is the main source of magma.
- Core = nickel + iron = ‘nife’. Outer core liquid, inner core solid.
- Surface waves are the most destructive; P-waves are the fastest.
- Gravity is greater at the poles than at the equator.
- Magma below the surface; lava once it moves up or reaches it.
- Deccan Traps = flood basalt; their feeders were dykes.
- One earthquake: one magnitude, many intensities.
Traps to avoid
- Volcanoes are a direct source; earthquake waves, gravity, magnetism and meteors are indirect.
- Magnitude ≠ intensity: Richter measures energy, Mercalli measures damage.
- A tsunami is not an earthquake; it is waves generated by an under-sea tremor.
- Horizontal intrusion = sill (thick) or sheet (thin); vertical = dyke.
- Lapolith is saucer-shaped (concave up); laccolith is a dome with a flat base.
- Richter +1 = 10× amplitude and ~32× energy, not 100× energy.
- The P-wave shadow zone is a band (105°–145°); the S-wave shadow zone is everything beyond 105°.
- The 5,100 km boundary comes from Figure 3.3, not the text — read the figures.
How to strategise
Prelims — what UPSC asks
- Statement-based concept checks are the norm: how P and S waves move and what they pass through (1987, 2023), what causes earthquakes (1981, 2004), magnitude vs intensity and the Richter scale (1979, 2001, 2006).
- NCERT lines are lifted almost verbatim: the 2024 question listed pyroclastic debris, ash and dust, nitrogen and sulphur compounds — the exact NCERT list of volcanic products. Read the lists in this chapter word for word.
- Terms and definitions: caldera (1990), SIMA (1986), sill vs dyke (2001).
- India applications: Barren Island volcano (2018), black cotton soil from Deccan basalt (2021), age of the Deccan Traps (1997), the 1991 Garhwal earthquake (1992).
- Volcanoes in the news (map work): Mt St Helens (2005), Tungurahua (2026). Keep a running list of volcanoes and their countries from current affairs.
Mains — where it appears and how to answer
- GS1 (Geography): volcanic eruptions and their regional impact (2021), the natural resource potential of the Deccan Trap (2022), tsunamis — how and where they form (2025).
- GS3 (Disaster management): India's earthquake preparedness (2015), tsunami and the NDMA guidelines (2017), India's vulnerability to earthquakes (2021).
- Answer frame: define → mechanism (draw the fault/focus or shadow-zone sketch) → distribution (link to plate boundaries, Chapter 4) → India examples → impacts → mitigation (building codes, early warning, NDMA guidelines).
- A 30-second sketch earns marks: the layered interior, the shadow zone, or a volcano type. Practise drawing them small and labelled.
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. Which one of the following earthquake waves is more destructive?
2. Which one of the following is a direct source of information about the interior of the earth?
3. Which type of volcanic eruptions have caused Deccan Trap formations?
4. Which one of the following describes the lithosphere?
5. Seismographs beyond 145° from an epicentre record…
6. According to NCERT Figure 3.3, the boundary between the liquid outer core and the solid inner core lies at about…
7. One earthquake is reported as ‘magnitude 6.5’ and ‘intensity VIII near the epicentre, IV in the next state’. This is possible because…
UPSC questions from this chapter
Open any question to attempt it in the Question bank — your attempt is saved to your progress.