BharatNotes Why this chapter matters for UPSC: This is Chapter 3 of Fundamentals of Physical Geography in the current rationalised NCERT (Reprint 2026-27). In the rationalised book it has three parts: how we know about the interior, earthquakes (waves, shadow zones, types, measurement, effects) and the layered structure of the earth, and volcanoes with volcanic landforms. Prelims tests the wave properties, shadow-zone angles, layer depths and the names of volcano types and intrusive forms. Mains GS1 and GS3 have asked about mantle plumes, volcanic eruptions, earthquakes and tsunamis.
Contemporary hook: UPSC Prelims 2026 (GS Paper I) asked in which country the Tungurahua Volcano, declared a UNESCO Global Geopark in 2025, is located (Ecuador).
🧠 First Principles — Read This First
No one has seen the inside of the earth. The deepest drill hole reached about 12 km, while the distance to the centre is over 6,300 km. Almost everything we know about the interior is inferred from evidence that reaches the surface.
The most useful evidence comes from earthquakes. An earthquake is a sudden release of energy when rocks on either side of a fault slip past each other. The energy spreads out as waves. Some waves can pass through solids, liquids and gases; one kind can pass only through solids. Waves also change speed and bend when they enter material of a different density. Seismographs around the world record when and where the waves arrive, and in some zones they do not arrive at all. From those records, scientists worked out that the earth has a thin crust, a thick mantle, a liquid outer core and a solid inner core.
The same interior explains volcanoes. Part of the upper mantle, called the asthenosphere, is weak and is the main source of molten rock, or magma. When magma reaches the surface it is called lava, and the kind of lava decides the kind of volcano: runny basalt builds gentle shields and wide lava plateaus such as the Deccan Traps; thick, sticky lava builds steep, explosive cones. Magma that cools before reaching the surface forms underground rock bodies such as batholiths and dykes, which may be exposed later by erosion.
For UPSC, this chapter links the theory of the interior to two hazards India faces, earthquakes and tsunamis, and to India's own volcanic landform, the Deccan Traps.
PART 1 — Quick Reference
Table 1: Sources of information about the interior (NCERT)
| Type | Source | NCERT detail |
|---|---|---|
| Direct | Surface rocks and mining | Gold mines in South Africa are 3–4 km deep; deeper is too hot |
| Direct | Deep drilling | "Deep Ocean Drilling Project" and "Integrated Ocean Drilling Project"; the deepest drill, at Kola, reached 12 km |
| Direct | Volcanic eruptions | Magma brought to the surface can be analysed, but the depth of its source is hard to ascertain |
| Indirect | Temperature, pressure and density | All increase with depth; rates of change used to estimate values at depth |
| Indirect | Meteors | Not from the earth's interior, but formed from the same or similar material |
| Indirect | Gravitation | g is greater near the poles and less at the equator; gravity anomalies show how mass is distributed in the crust |
| Indirect | Magnetic surveys | Distribution of magnetic materials in the crust |
| Indirect | Seismic activity | "One of the most important sources" |
NCERT locates Kola "in Arctic Ocean". The borehole is on land, on the Kola Peninsula in Russia (USGS).
Table 2: Earthquake waves
| Wave | Type | Travels through | Vibration | Arrival |
|---|---|---|---|---|
| P (primary) | Body wave | Solid, liquid and gas; "similar to sound waves" | Parallel to the direction of the wave; creates density differences (stretching and squeezing) | First |
| S (secondary) | Body wave | Solids only | Perpendicular to the direction of the wave, in the vertical plane; creates troughs and crests | After a time lag |
| Surface waves | Generated when body waves interact with surface rocks | Along the surface | — | Last; most destructive (displace rocks, collapse structures) |
Wave velocity rises with the density of the material; waves are reflected (rebound) or refracted (change direction) when they meet material of different density.
Table 3: Shadow zones
| Item | NCERT | Modern value (USGS) |
|---|---|---|
| Both P and S recorded | Within 105° of the epicentre | — |
| P-wave shadow zone | Band between 105° and 145° from the epicentre | 104°–140° |
| Beyond 145° | P-waves recorded, S-waves not | — |
| S-wave shadow zone | Entire zone beyond 105°; a little over 40% of the earth's surface | S-waves "stopped entirely by the liquid core" |
Sources: NCERT kegy203.pdf p.20; USGS Earthquake Hazards Program, "Shadow Zone" — image and video. Use NCERT's angles for NCERT-based questions.
Table 4: Types of earthquakes (NCERT)
| Type | Cause |
|---|---|
| Tectonic | Sliding of rocks along a fault plane; the most common |
| Volcanic | A special class of tectonic earthquake, confined to areas of active volcanoes |
| Collapse | Collapse of the roofs of underground mines in areas of intense mining; minor tremors |
| Explosion | Explosion of chemical or nuclear devices |
| Reservoir induced | Occur in areas of large reservoirs |
Table 5: Measuring earthquakes
| Scale | Measures | Range (NCERT) | Note (USGS) |
|---|---|---|---|
| Richter (magnitude) | Energy released | 0–10 | Developed in 1935 by Charles Richter; logarithmic; each whole number is a tenfold increase in amplitude and about 31 times more energy; "no upper limit". For large earthquakes seismologists now use moment magnitude (Mw) |
| Mercalli (intensity), after an Italian seismologist | Visible damage | 1–12 | The Modified Mercalli Intensity scale (Wood and Neumann, 1931) has 12 levels in Roman numerals (I–XII); intensity varies from place to place for the same earthquake |
Table 6: Structure of the earth (NCERT)
| Layer | Depth / thickness | State and material |
|---|---|---|
| Oceanic crust | Mean thickness 5 km | Brittle, solid |
| Continental crust | Around 30 km; up to 70 km in the Himalayan region | Brittle, solid |
| Lithosphere (crust + uppermost mantle) | 10–200 km thick | Rigid; all natural earthquakes occur here |
| Mantle | From the Moho to 2,900 km | Higher density than the crust |
| Asthenosphere (upper mantle) | Extends up to about 400 km | "Astheno" = weak; main source of magma |
| Lower mantle | Beyond the asthenosphere to 2,900 km | Solid |
| Outer core | From 2,900 km | Liquid; nickel and iron ("nife") |
| Inner core | Centre | Solid; nickel and iron |
Source: NCERT kegy203.pdf pp.22–23. NCERT gives the earth's radius as "about 6,378 km", which is the equatorial radius (6,378.137 km); the volumetric mean radius is 6,371 km (NASA Earth Fact Sheet).
Table 7: Volcano types (NCERT)
| Type | Lava and eruption | Form and example |
|---|---|---|
| Shield | Basalt, very fluid; low explosivity unless water enters the vent | Largest volcanoes on earth barring basalt flows; not steep; Hawaiian volcanoes; a cinder cone may form at the top of the vent |
| Composite | Cooler, more viscous lava than basalt; explosive | Lava, pyroclastic material and ash pile up in layers near the vent |
| Caldera | The most explosive | Collapses on itself; the collapsed depression is a caldera; implies a huge magma chamber close by |
| Flood basalt provinces | Highly fluid lava flowing long distances | Thousands of sq. km covered; individual flows more than 50 m thick and hundreds of km long; Deccan Traps (most of the Maharashtra plateau) |
| Mid-ocean ridge | Frequent eruptions in the central portion of the ridge | Ridge system more than 70,000 km long through all ocean basins |
Table 8: Intrusive forms (NCERT)
| Form | Shape | Indian example in NCERT |
|---|---|---|
| Batholith | Large dome of magma cooled deep in the crust; granitic; the cooled portion of a magma chamber; exposed only after denudation | Domal granite hills of the Karnataka plateau (laccoliths or batholiths) |
| Laccolith ("lacolith" in NCERT) | Large dome with a level base, fed by a pipe-like conduit from below | Karnataka plateau |
| Lapolith | Saucer-shaped, concave to the sky | — |
| Phacolith | Wavy mass at the base of synclines or top of anticlines, connected to a magma chamber | — |
| Sill / sheet | Near-horizontal body; thick = sill, thin = sheet | — |
| Dyke | Wall-like body solidified almost perpendicular to the ground in cracks and fissures | Most common intrusive form in western Maharashtra; feeders of the Deccan Traps |
PART 2 — Concepts & Narrative
Why the earth shakes
An earthquake is the shaking of the earth caused by a release of energy that sends waves in all directions. The release happens along a fault, a sharp break in the crustal rocks. Rocks on either side tend to move in opposite directions, but the pressure of the overlying strata locks them together by friction. When the tendency to move overcomes the friction, the blocks deform and then slide past one another abruptly.
Focus (hypocentre) and epicentre
The focus, also called the hypocentre, is the point inside the earth where the energy is released. The epicentre is the point on the surface nearest to the focus, directly above it, and the first to experience the waves. NCERT notes that all natural earthquakes occur in the lithosphere, the outer layer up to about 200 km deep.
How the waves map the interior
A seismograph records the waves as a curve with three sections: P-waves first, S-waves after a lag, surface waves last. Because wave velocity and direction change with the density of the material, the pattern of arrival times reveals the layers the waves have crossed.
Why there is a shadow zone
S-waves travel only through solids. Every earthquake produces a large region beyond 105° from its epicentre where no S-waves are recorded: they have been stopped by something that is not solid. This is the evidence that the outer core is liquid.
P-waves do cross the liquid core, but they are refracted (bent) sharply at the core boundary. As a result there is a band between 105° and 145° where no direct P-waves arrive, while stations beyond 145° do receive them. NCERT therefore describes the zone between 105° and 145° as a shadow zone for both waves, and the S-wave shadow as larger, a little over 40% of the earth's surface. Because the shadow zone is measured from the epicentre, each earthquake has its own.
The inner core was identified later. The Danish seismologist Inge Lehmann (1936, paper "P′") found weak P-wave arrivals inside the shadow zone from a 1929 New Zealand earthquake and argued they could be explained only by a solid inner core (Niels Bohr Institute, University of Copenhagen). Gutenberg and Richter estimated its radius at about 1,200 km in 1938. The outer core had been identified earlier, in 1906, by R.D. Oldham from earthquake records (USGS).
Effects of earthquakes
NCERT lists twelve immediate hazardous effects: 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 bear on landforms; the rest are of immediate concern to life and property.
A tsunami occurs only if the epicentre lies below oceanic waters and the magnitude is high enough. NCERT stresses that tsunamis are waves generated by the tremors, not an earthquake in themselves. The quake itself lasts a few seconds, but NCERT says its effects are devastating if the magnitude exceeds 5 on the Richter scale. On frequency, NCERT notes that quakes of magnitude 8 and above are rare, about once in 1–2 years, while "tiny" quakes occur almost every minute.
The layers
The crust is the outermost, brittle, solid layer, thinner under the oceans than under the continents and thickest beneath major mountain systems. Below the Moho discontinuity lies the mantle, denser than the crust and extending to 2,900 km. Its upper portion, the asthenosphere, is weak and supplies magma to volcanoes; the lower mantle is solid. The core begins at the core–mantle boundary at 2,900 km. Earthquake wave velocities revealed it: the outer core is liquid, the inner core solid, and both are made mostly of nickel and iron, which is why the core is sometimes called the "nife" layer.
Lithosphere vs crust
The crust is a layer defined by composition. The lithosphere is the crust plus the uppermost part of the mantle, 10–200 km thick in NCERT (Exercise 1(iv): the lithosphere is "crust and upper mantle"). Tectonic plates are slabs of lithosphere, not of crust alone. Beneath them, the asthenosphere (to about 400 km in NCERT) is the weak zone of the upper mantle.
Discontinuities and modern figures
- Mohorovičić (Moho) discontinuity: crust–mantle boundary, detected by a sharp increase in the speed of earthquake waves; named after the Croatian scientist Andrija Mohorovičić (USGS). NCERT uses the name "Moho's discontinuity".
- Gutenberg discontinuity: the name commonly given to the mantle–core boundary (2,900 km in NCERT). NASA's Earth Fact Sheet gives a core radius of 3,485 km, which puts the boundary about 2,886 km below the mean surface (arithmetic).
- Lehmann discontinuity (outer–inner core): the inner core has a radius of about 1,220 km (Niels Bohr Institute), so the boundary lies roughly 5,150 km deep (6,371 − 1,220; arithmetic).
- Volume: the mantle is about 84% of the earth's volume, the core 15% and the crust 1% (USGS).
- Composition of the core: principally iron, with about 10% of oxygen, sulphur or nickel, or a combination (USGS). NCERT's "nickel and iron" is the school-level summary.
- Temperature of the inner core: estimated at about 5,400°C, around the temperature of the sun's surface (Niels Bohr Institute).
Terms such as the "Conrad" and "Repetti" discontinuities appear in some Indian guidebooks but are not in NCERT; they were removed from this page because their depths could not be confirmed from a primary source.
Isostasy: Pratt vs Airy (not in NCERT Ch.3)
Isostasy is the state of gravitational equilibrium between the crust and the mantle, as if the lighter crust were floating on the heavier mantle. Surveys in India in the 1800s showed that the Himalayas are compensated in this way. Two models explain it (UC San Diego, IGPP lecture notes):
- Pratt: blocks of different density reach down to a uniform depth of compensation. Mountains stand high because they are made of less dense rock. There are no roots.
- Airy: the crust has uniform density but varying thickness. Higher mountains have deeper roots into the mantle.
Confused pair: "roots" belong to Airy, "density differences with a level base" to Pratt.
Volcanoes
A volcano is a place where gases, ash and/or molten rock (lava) escape to the ground. A volcano is active if such material is being released or has been released in the recent past. The material comes from the asthenosphere. In the upper mantle it is called magma; once it moves towards the crust or reaches the surface it is called lava. What reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust, and gases such as nitrogen compounds and sulphur compounds, with minor amounts of chlorine, hydrogen and argon.
Volcanoes are classified by the nature of the eruption and the form that develops at the surface (Table 7). The key control is the lava. Fluid basalt spreads out and builds broad, gentle forms (shields, flood basalts); cooler, viscous lava traps gas and erupts explosively, building layered cones or, at the extreme, collapsing into a caldera. NCERT notes that the Deccan Traps probably covered a much larger area originally than they do now.
Volcanic landforms: intrusive forms
Lava cools into igneous rock. If it cools on reaching the surface it forms volcanic rocks; if it cools within the crust it forms plutonic rocks. The shapes taken by lava that cools within the crust are called intrusive forms (Table 8). The difference between them is mainly geometry. Batholiths and laccoliths are domes, the laccolith with a level base and a feeder pipe. Lapoliths are saucers. Phacoliths follow the waves of folded rock. Sills and sheets lie nearly horizontal along weak planes, and dykes cut almost vertically through fissures. India gives NCERT two examples: the exfoliated granite domes of the Karnataka plateau, and the dykes of western Maharashtra that fed the Deccan Traps.
Source for Part 2: NCERT, Fundamentals of Physical Geography, Class XI, Ch.3, Reprint 2026-27, pp.19–25 — kegy203.pdf.
PART 3 — UPSC Integration
PYQ pattern
Mains (from the site's GS1/GS3 bank, 2013–2024):
- gs1-pyq-2018-06 (GS1 2018, 10 marks): "Define mantle plume and explain its role in plate tectonics."
- gs1-pyq-2014-09 (GS1 2014, 10 marks): "Why are the world's fold mountain systems located along the margins of continents? Bring out the association between the global distribution of fold mountains and the earthquakes and volcanoes."
- gs1-pyq-2021-09 (GS1 2021, 10 marks): "Mention the global occurrence of volcanic eruptions in 2021 and their impact on regional environment."
- gs3-pyq-2017-38 (GS3 2017, 15 marks): the December 2004 tsunami — factors responsible for tsunamis, effects, and NDMA preparedness guidelines.
- gs3-pyq-2015-76 (GS3 2015, 12.5 marks) and gs3-pyq-2021-10 (GS3 2021, 10 marks): India's earthquake vulnerability and preparedness.
Prelims (2026 bank): prelims-2026-gs1-021, location of Tungurahua Volcano (Ecuador).
Pattern: Prelims tests named facts and locations; Mains links the interior (mantle, plumes) to the surface distribution of earthquakes and volcanoes, and then to hazard management in GS3.
Mains frameworks
- Interior → plate margins → hazards. Weak asthenosphere and mantle processes → plate boundaries → belts of earthquakes, volcanoes and fold mountains (2014, 2018).
- Hazard answer from NCERT's effects list. Use the twelve effects, split into landform effects and effects on life and property, then preparedness (GS3 2015, 2017, 2021).
- Eruption answer by type. For "volcanic eruptions and their impact" (2021), classify by eruption type (shield, composite, caldera, flood basalt) and state the regional effects: lava, ash, gases, pyroclastic debris.
Cross-paper relevance
- GS1 — interior, seismic waves, volcano types and landforms; distribution of earthquakes and volcanoes.
- GS3 — disaster management: earthquake vulnerability (2015, 2021) and tsunami preparedness (2017).
Exam Strategy
Prelims fact-traps from this chapter
- Shadow zone: NCERT 105°–145° (P), S absent beyond 105°; USGS gives 104°–140° for P. Figures such as 103°–142° match neither source.
- S-waves stop at the outer core, showing it is liquid. The inner core is solid.
- The direct source among earthquake waves, volcanoes, gravity and magnetism is volcanoes (NCERT Ex. 1(ii)).
- Surface waves are the most destructive (Ex. 1(i)).
- Flood basalt eruptions formed the Deccan Traps (Ex. 1(iii)), not shield or caldera.
- Lithosphere = crust + upper mantle (Ex. 1(iv)); 10–200 km thick. Asthenosphere to 400 km (NCERT).
- Richter = magnitude (energy), 0–10 in NCERT; Mercalli = intensity (damage), 1–12.
- Focus = hypocentre (inside the earth); epicentre is on the surface.
- Sill = thick, sheet = thin; lapolith = saucer; phacolith = wavy; laccolith = dome with level base; dyke = vertical wall.
- Tsunamis are generated by tremors; they are not themselves earthquakes.
- Pratt (density differences, no roots) vs Airy (uniform density, roots).
Mains question patterns
- Association of fold mountains with earthquakes and volcanoes (2014): answer from plate margins.
- A year's volcanic eruptions and regional impact (2021): classify by eruption type and effects.
- India's earthquake vulnerability (2015, 2021): use the effects list and India's tectonic setting; seismic-zone details are on the BharatNotes page Natural Hazards and Disasters (India: Physical Environment).
Practice Questions
- Mains GS1 2014 (gs1-pyq-2014-09): "Why are the world's fold mountain systems located along the margins of continents? Bring out the association between the global distribution of fold mountains and the earthquakes and volcanoes."
- Mains GS1 2018 (gs1-pyq-2018-06): "Define mantle plume and explain its role in plate tectonics."
- Prelims 2026 (prelims-2026-gs1-021): "Tungurahua Volcano, which was declared a Global Geopark by UNESCO in 2025, is situated in which one among the following countries? (a) Ecuador (b) Peru (c) Bolivia (d) Colombia." (Answer: a)
- Practice (UPSC-pattern, not a past paper): Consider the following statements: 1. S-waves are not recorded beyond 105° from the epicentre. 2. The P-wave shadow zone is caused by the refraction of P-waves at the core. 3. The shadow zone of P-waves is larger than that of S-waves. Which of the statements given above is/are correct according to NCERT? (a) 1 only (b) 1 and 2 only (c) 2 and 3 only (d) 1, 2 and 3. (Answer: b)
- Practice (UPSC-pattern, not a past paper): Consider the following pairs (intrusive form : shape): 1. Lapolith : saucer-shaped, concave to the sky 2. Sill : thin vertical wall-like body 3. Laccolith : dome with a level base. Which of the pairs given above is/are correctly matched? (a) 1 only (b) 1 and 3 only (c) 2 and 3 only (d) 1, 2 and 3. (Answer: b — a vertical wall is a dyke)
📦 Revision Capsule
Hard Facts
- Direct sources: mines (South African gold mines 3–4 km), drilling (Kola, 12 km), volcanic eruptions; indirect: temperature/pressure/density, meteors, gravity anomalies, magnetic surveys, seismic waves
- P-waves: fastest, through solid, liquid and gas, vibrate parallel to the wave; S-waves: solids only, vibrate perpendicular; surface waves: last and most destructive
- Shadow zone (NCERT): P 105°–145°; S beyond 105° (a little over 40% of the surface); USGS P-shadow: 104°–140°
- Earthquake types: tectonic, volcanic, collapse, explosion, reservoir induced
- Richter magnitude 0–10 (NCERT; USGS: no upper limit); Mercalli intensity 1–12
- Crust: oceanic 5 km, continental ~30 km, Himalaya up to 70 km; lithosphere 10–200 km; asthenosphere to ~400 km; core–mantle boundary 2,900 km
- Outer core liquid, inner core solid; nickel and iron ("nife"); inner core identified by Inge Lehmann (1936)
- Volcano types: shield (Hawaii), composite, caldera (most explosive), flood basalt (Deccan Traps), mid-ocean ridge (>70,000 km)
- Intrusive forms: batholith, laccolith, lapolith (saucer), phacolith (wavy), sill (thick)/sheet (thin), dyke (vertical; feeders of the Deccan Traps)
Core Concepts
- Seismic waves change speed and direction with density; the shadow zones show the outer core is liquid
- All natural earthquakes occur in the lithosphere, by sudden slip along faults
- The asthenosphere is the main source of magma; lava type controls volcano form and explosivity
- Magma cooled at the surface forms volcanic rock, within the crust plutonic rock and intrusive forms
Confused Pairs
- Focus/hypocentre (inside) vs epicentre (surface)
- Magnitude (Richter, energy) vs intensity (Mercalli, damage)
- Crust vs lithosphere (crust + uppermost mantle)
- Sill (thick) vs sheet (thin) vs dyke (vertical)
- Pratt (density varies, no roots) vs Airy (thickness varies, roots)
PYQ Pattern
- Mains GS1: mantle plume (2018), fold mountains with earthquakes and volcanoes (2014), volcanic eruptions of 2021; GS3: earthquake preparedness (2015, 2021) and tsunami (2017); Prelims 2026: Tungurahua Volcano location
Sources
Sources
- NCERT, Fundamentals of Physical Geography, Textbook for Class XI, Chapter 3 "Interior of the Earth", rationalised edition, Reprint 2026-27 — kegy203.pdf.
- USGS Earthquake Hazards Program, "Shadow Zone" (image description, c.2021) — usgs.gov/media/images/shadowzonegif.
- USGS, The Interior of the Earth (General Interest Publication) — pubs.usgs.gov/gip/interior.
- USGS, The Severity of an Earthquake (General Interest Publication) — pubs.usgs.gov/gip/earthq4/severity_text.html.
- USGS FAQ, "Moment magnitude, Richter scale – what are the different magnitude scales…" (updated 10 Sep 2024) — usgs.gov.
- NASA, Earth Fact Sheet (equatorial radius 6,378.137 km; mean radius 6,371.000 km; core radius 3,485 km) — nssdc.gsfc.nasa.gov.
- Niels Bohr Institute, University of Copenhagen, "P′ or the discovery of the Earth's inner core" (Inge Lehmann) — nbi.ku.dk.
- UC San Diego, Institute of Geophysics and Planetary Physics, tectonics course Lecture 2 "Gravity, Isostasy and Flexure" — igppweb.ucsd.edu.
- PYQ texts: BharatNotes Mains GS1/GS3 bank (2014, 2015, 2017, 2018, 2021) and Prelims 2026 GS Paper I bank.
