Interior of the Earth

Why this chapter matters for UPSC: This is Chapter 3 of Fundamentals of Physical Geography (Reprint 2026-27). It has three parts: how we know about the earth's interior, earthquakes and the layered structure they reveal, and volcanoes with their landforms. Learn the wave properties, shadow zones, layer depths, volcano types and intrusive forms, and be able to link the interior to the belts of earthquakes and volcanoes and, for GS3, to earthquake and tsunami preparedness.

Contemporary hook: India's only active volcano, Barren Island in the Andaman Sea (NCERT, India: Physical Environment), has been in an eruptive period since December 2022, with activity recorded in March–April 2024, July and November 2025 and an ash emission on 10 January 2026 (Smithsonian Global Volcanism Program).

🧠 First Principles — Read This First

No one has seen the inside of the earth. The deepest drill hole reached about 12 km, while the centre is more than 6,300 km down. 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. One kind of wave passes through solids, liquids and gases; another passes only through solids. Waves 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, the asthenosphere, is weak and is the main source of molten rock, or magma. Magma that reaches the surface is 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 erosion may later expose.

PART 1 — Quick Reference

Table 1: Sources of information about the interior (NCERT)

NCERT places Kola "in Arctic Ocean"; the borehole is on land, on the Kola Peninsula of Russia. The USGS describes it as "a Soviet hole on the Kola Peninsula" drilled to a depth of 12 km (USGS, The Interior of the Earth). NCERT's drilling names are loose: the programmes were the Deep Sea Drilling Project (1966–1983), the Ocean Drilling Program (1983–2003), the Integrated Ocean Drilling Program (2003–2013) and the International Ocean Discovery Program (2013–2024) (IODP).

Table 2: Earthquake waves

Wave velocity changes 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

Source: NCERT kegy203.pdf pp. 3–4. Use NCERT's angles for NCERT-based questions.

Table 4: Types of earthquakes (NCERT)

Table 5: Measuring earthquakes

Table 6: Structure of the earth (NCERT)

Source: NCERT kegy203.pdf pp. 5–6. NCERT gives the earth's radius as about 6,378 km, the equatorial radius; the standard Preliminary Reference Earth Model (PREM, 1981) uses a mean radius of 6,371 km and a core radius of 3,480 km, which puts the core–mantle boundary about 2,891 km down (arithmetic).

Table 7: Volcano types (NCERT)

Table 8: Intrusive forms (NCERT)

PART 2 — Concepts & Narrative

NCERT opens by noting that our picture of the earth is largely based on estimates and inferences, and that the processes shaping the surface are of two kinds: endogenic (from within) and exogenic (at the surface). Understanding the interior explains why the earth shakes and how a tsunami wave is generated.

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.

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.

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. A quake lasts only a few seconds, but NCERT says its effects are devastating "provided the magnitude of the quake is more than 5 on the Richter scale"; in practice damage depends on depth, distance, ground and buildings as well as magnitude. On frequency, quakes of magnitude 8 and above are rare, about once in 1–2 years, while "tiny" ones occur almost every minute. NCERT leaves the distribution of earthquakes for the next chapter.

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: 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.

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 starts moving 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 at 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 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 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. 1–8 (kegy203.pdf); the epicentre activity is from the 2019-20 edition.

PART 3 — UPSC Integration

Frames that score.

1. Interior, plate margins, hazards. The weak asthenosphere and mantle processes, then the plate boundaries, then the belts of earthquakes, volcanoes and fold mountains.
2. Hazard answer from NCERT's effects list. Use the twelve effects, split into landform effects and effects on life and property, then preparedness.
3. Eruption answer by type. For questions on eruptions and their impact, classify by eruption type (shield, composite, caldera, flood basalt) and state the regional effects: lava, ash, gases, pyroclastic debris.

Exam Strategy

Prelims fact-traps from this chapter

• Shadow zone in NCERT: P 105°–145°; S absent beyond 105°.
• 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 eruptions.
• 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); the epicentre is on the surface.
• Sill = thick, sheet = thin; lapolith = saucer; phacolith = wavy; laccolith = dome with a level base; dyke = vertical wall.
• Tsunamis are generated by tremors; they are not earthquakes themselves.
• Pratt (density differences, no roots) vs Airy (uniform density, roots).

Mains question patterns

• Why fold mountains, earthquakes and volcanoes share the same belts: answer from plate margins.
• A year's eruptions and their regional impact: classify by eruption type and effect.
• India's earthquake vulnerability: use the effects list and India's tectonic setting; seismic zoning is on the page Natural Hazards and Disasters.

Practice Questions

Practice (UPSC-pattern, not past papers). Questions 1 to 4 are NCERT's own exercise MCQs.

Prelims:

1. Which one of the following earthquake waves is more destructive?
   (a) P-waves
   (b) S-waves
   (c) Surface waves
   (d) None of the above

2. Which one of the following is a direct source of information about the interior of the earth?
   (a) Earthquake waves
   (b) Volcanoes
   (c) Gravitational force
   (d) Earth magnetism

3. Which type of volcanic eruptions have caused the Deccan Trap formations?
   (a) Shield
   (b) Flood
   (c) Composite
   (d) Caldera

4. Which one of the following describes the lithosphere?
   (a) Upper and lower mantle
   (b) Crust and upper mantle
   (c) Crust and core
   (d) Mantle and core

5. Consider the following statements, according to NCERT: 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?
   (a) 1 only
   (b) 1 and 2 only
   (c) 2 and 3 only
   (d) 1, 2 and 3

6. 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
   A vertical wall-like body is a dyke; a sill is horizontal.

Mains (practice):

1. What are the effects of the propagation of earthquake waves on the rock mass through which they travel? (NCERT Ex. 3(i), 150 words) Approach: P-waves stretch and squeeze, S-waves make troughs and crests, surface waves displace rock; then reflection and refraction at density changes, and the shadow zones they create.
2. What do you understand by intrusive forms? Briefly describe the various intrusive forms. (NCERT Ex. 3(ii), 150 words)
3. Explain the association between the distribution of fold mountains, earthquakes and volcanoes. (150 words)

Short answers (NCERT Ex. 2, about 30 words): body waves are the P- and S-waves generated at the focus; the direct sources are mining, drilling and volcanic material; shadow zones develop because S-waves cannot cross the liquid outer core and P-waves are refracted at the core; the indirect sources other than seismic activity are temperature, pressure and density, meteors, gravity and magnetic surveys.

📦 Revision Capsule

Sources

• NCERT, Fundamentals of Physical Geography, Textbook for Class XI, ch 3 "Interior of the Earth", Reprint 2026-27: kegy203.pdf; the 2019-20 edition (epicentre activity, layer densities) via the Wayback copy of the book: kegy2dd.zip, 4 May 2019.
• NCERT, India: Physical Environment, Class XI, ch 2 (Barren Island): kegy102.pdf.
• 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, and why are there so many?": usgs.gov.
• NOAA Ocean Exploration, "What is the mid-ocean ridge?": oceanexplorer.noaa.gov.
• IRIS (EarthScope), "Seismic Shadow Zone: Basic Introduction": iris.edu.
• USGS Earthquake Hazards Program, "Determining the Depth of an Earthquake": usgs.gov.
• A.M. Dziewonski and D.L. Anderson, "Preliminary reference Earth model", Physics of the Earth and Planetary Interiors 25 (1981): 297–356: doi:10.1016/0031-9201(81)90046-7.
• International Ocean Discovery Program, "History": iodp.org.
• 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.
• Smithsonian Institution, Global Volcanism Program, "Barren Island" (Wayback copy, 21 May 2026): volcano.si.edu.