This MCQ module is based on: Earth’s Layered Structure, Volcanoes & Exercises
TOPIC 6 OF 29
Earth’s Layered Structure, Volcanoes & Exercises
🎓 Class 11
Social Science
CBSE
Theory
Ch 3 — Interior of the Earth
⏱ ~25 min
🌐 Language: [gtranslate]
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3.9 The Structure of the Earth
The Earth is built up of three main concentric layers — the crust, the mantle and the core. Each was discovered or refined through the seismic-wave evidence introduced in the previous part of this chapter. The boundaries between them are called discontinuities, named after the scientists who first identified them.
📍 What This Lesson Covers
Crust (continental & oceanic) → Mohorovičić, Gutenberg and Lehmann discontinuities → mantle, asthenosphere, lithosphere → outer (liquid) and inner (solid) core → volcanoes (shield, composite, caldera, flood basalt, mid-ocean ridge) → intrusive forms (batholith, laccolith, lapolith, phacolith, sill, dyke) → all NCERT exercises.
3.10 The Crust
The crust is the outermost solid part of the Earth. It is brittle in nature. The thickness of the crust is not uniform — it varies under the oceans and under the continents.
- Oceanic crust is thinner. Its mean thickness is about 5 km.
- Continental crust is thicker. Its mean thickness is around 30 km. Beneath major mountain systems the continental crust is even thicker — it reaches as much as 70 km in the Himalayan region.
The boundary between the crust and the layer beneath it is called the Mohorovičić discontinuity?, or Moho for short. It marks the place where the velocity of seismic waves jumps abruptly because the rock changes from the lighter materials of the crust to the denser ultramafic rocks of the mantle.
3.11 The Mantle
The portion of the interior beyond the crust is called the mantle. It extends from the Moho discontinuity down to a depth of about 2,900 km. The mantle is denser than the crust.
The upper portion of the mantle is called the asthenosphere?. The word astheno means "weak", and the layer behaves plastically — it can flow over geological time. It is considered to extend up to a depth of about 400 km. The asthenosphere is the principal source of magma that finds its way to the surface during volcanic eruptions. Below the asthenosphere lies the lower mantle, which extends down to the core–mantle boundary; despite being hot, the lower mantle remains in a solid state because of the enormous pressure.
The crust together with the uppermost (rigid) part of the mantle is called the lithosphere. Its thickness ranges from about 10 to 200 km. All natural earthquakes occur in this rigid outer shell.
📖 Definition — Magma vs Lava
Magma is the molten rock material in the upper mantle. The moment that material starts moving towards the crust, or actually reaches the surface, it is renamed lava. What erupts from a volcano is therefore not simply lava: it includes lava flows, pyroclastic debris, volcanic bombs, ash and dust, and gases such as nitrogen compounds, sulphur compounds and small amounts of chlorine, hydrogen and argon.
3.12 The Core
As we saw in the previous part of this chapter, the velocities of earthquake waves first revealed the existence of the core. The core–mantle boundary is located at a depth of 2,900 km and is called the Gutenberg discontinuity?. Within the core itself, a deeper boundary called the Lehmann discontinuity? separates a liquid outer core from a solid inner core.
- The outer core is in a liquid state. It blocks the passage of S-waves, producing the S-wave shadow zone that you studied in the last lesson.
- The inner core is in a solid state, kept solid by the immense pressure of the overlying material.
The core is made up of very heavy material — mostly nickel and iron — and is sometimes referred to as the NIFE layer (from the chemical symbols Ni and Fe).
Cross-Section of the Earth's Interior
Bloom: L2 UnderstandFigure 3.4 (after NCERT 3.3): the Earth's interior is divided by three principal discontinuities — Moho, Gutenberg and Lehmann.
| Layer | Depth Range | State | Composition / Notes |
|---|---|---|---|
| Oceanic crust | 0 – ~5 km | Solid, brittle | Thinner than continental; basaltic |
| Continental crust | 0 – ~30 km (up to 70 km in Himalayas) | Solid, brittle | Thicker; granitic |
| Asthenosphere (upper mantle) | ~0 – 400 km | Plastic / partly molten | Source of magma |
| Lower mantle | 400 – 2,900 km | Solid | Denser; high pressure |
| Outer core | 2,900 – 5,150 km | Liquid | Blocks S-waves |
| Inner core | 5,150 – 6,378 km | Solid | Nickel + iron (NIFE) |
SOURCE WORK — Reading the Layered Earth
Bloom: L2 Understand
Use Figure 3.4 above and the table next to it to answer: (a) Which discontinuity separates the asthenosphere from the rest of the upper mantle? (b) Which two layers together make up the lithosphere? (c) Why is the inner core solid even though it is the hottest part of the Earth?
✅ Answers
(a) None of the three named discontinuities separates the asthenosphere from the lower mantle; the asthenosphere is just the weak upper part of the mantle and the change is gradual. The Moho is above the asthenosphere; the Gutenberg is below the lower mantle. (b) The lithosphere is made of the crust + uppermost (rigid) mantle. (c) The inner core is solid because of the extreme pressure of all the overlying material; pressure raises the melting point above the actual temperature there.
3.13 Volcanoes
A volcano is a place where gases, ashes and/or molten rock material — lava — escape to the ground. A volcano is called an active volcano when these materials are being released, or have been released, in the recent past. Volcanoes are classified on the basis of (a) the nature of the eruption and (b) the form developed at the surface.
3.13.1 Shield Volcanoes
Barring the basalt flows, shield volcanoes are the largest volcanoes on the Earth. The Hawaiian volcanoes are the most famous examples. They are mostly made up of basalt, a type of lava that is very fluid when erupted. Because the lava flows easily, these volcanoes are not steep — their slopes spread broadly like a warrior's shield laid on the ground (hence the name). They are usually characterised by low explosivity, but they can become explosive if water somehow gets into the vent. The upcoming lava moves like a fountain and throws out a cone at the top of the vent, developing into a cinder cone.
3.13.2 Composite Volcanoes
Composite volcanoes erupt cooler and more viscous lavas than basalt. These eruptions are often explosive. Along with the lava, large quantities of pyroclastic material and ashes reach the ground. This material accumulates in the vicinity of the vent openings and forms layers — making the mounts appear "composite" (alternating layers of lava and pyroclastics).
3.13.3 Caldera
Calderas are the most explosive of the Earth's volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than build any tall structure. The collapsed depressions are called calderas. Their explosiveness indicates that the magma chamber supplying the lava is not only huge but also lies in close vicinity to the surface.
3.13.4 Flood Basalt Provinces
These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of square kilometres of thick basalt lava flows. There can be a series of flows, with some flows attaining thicknesses of more than 50 m; individual flows may extend for hundreds of kilometres. The Deccan Traps of India — covering most of the Maharashtra plateau today — are a much larger flood-basalt province; geologists believe that the original trap formations covered an even larger area than the present.
3.13.5 Mid-Ocean Ridge Volcanoes
These volcanoes occur in oceanic areas. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins. The central portion of this ridge experiences frequent eruptions. Mid-ocean ridge volcanism produces new oceanic crust — a process you will study in detail in the next chapter on plate tectonics.
Five Types of Volcanoes
Figure 3.5: profile view of the five major volcano types described in the NCERT chapter.
3.14 Volcanic Landforms — Intrusive Forms
The lava released during volcanic eruptions cools to form igneous rocks. Cooling can occur either when lava reaches the surface (giving volcanic rocks) or while the lava is still inside the crust (giving plutonic rocks). The shapes that lava takes when it solidifies inside the crust are called intrusive forms. Six of them are commonly described in NCERT Figure 3.4.
3.14.1 Batholiths
A batholith? is a large body of magmatic material that cools deep within the crust to form huge domes. They appear at the surface only after long denudation has stripped away the overlying rock. Batholiths cover large areas and may extend several kilometres in depth. They are granitic bodies — the cooled remains of magma chambers themselves.
3.14.2 Laccoliths
Laccoliths? are large dome-shaped intrusive bodies with a level base, connected to a deeper source by a pipe-like conduit. A laccolith resembles the surface dome of a composite volcano, but at greater depth. It can be regarded as a localised source of lava that finds its way to the surface. The Karnataka plateau is dotted with domal hills of granite — many of them, now exfoliated, are examples of laccoliths or batholiths.
3.14.3 Lapolith, Phacolith and Sills
As lava moves upwards through the crust, a portion of it may move horizontally wherever it finds a weak plane and rest in different forms.
- Lapolith — when the intruded mass takes a saucer shape, concave to the sky.
- Phacolith — a wavy mass of intrusive rock found at the base of a syncline or at the top of an anticline in folded igneous country, with a definite conduit to a source magma chamber (subsequently developed as a batholith) beneath.
- Sills and sheets — near-horizontal intrusive bodies. The thinner ones are called sheets; the thicker horizontal deposits are called sills.
3.14.4 Dykes
When lava makes its way through cracks and fissures developed in the land, it solidifies almost perpendicular to the ground. It cools in the same vertical position to develop a wall-like structure. Such structures are called dykes. They are the most commonly found intrusive forms in the western Maharashtra area, and are considered the feeders of the eruptions that built up the Deccan Traps.
Intrusive Forms — Where They Sit in the Crust
Figure 3.6 (after NCERT 3.4): batholiths sit deep; laccoliths form rounded domes; sills are horizontal sheets; dykes cut vertically through crust; lapoliths are saucer-shaped; phacoliths are wavy folds linked to deeper magma chambers.
Crust Thickness — Oceanic vs Continental vs Himalayan (km)
The continental crust beneath major mountain systems can be 14× thicker than typical oceanic crust.
IMAGINE — A Walk through the Layers
Bloom: L6 Create
Imagine you are an indestructible mole that can dig at the rate of 1 km per hour from the surface to the centre of the Earth. Write a 60-word travelogue of your six imaginary "stops" — describe the layer you are in, the discontinuity you have just crossed, the temperature/state and what surrounds you. Use the names: Moho, asthenosphere, lower mantle, Gutenberg, outer core, Lehmann, inner core.
✅ Sample Travelogue
Hour 30: just crossed the Moho — basalt above, plastic asthenosphere ahead. Hour 400: leaving the asthenosphere; entering the firm lower mantle, dark and hot. Hour 2,900: Gutenberg discontinuity — and suddenly I am swimming through a sea of liquid iron in the outer core. Hour 5,150: a Lehmann doorway opens; the iron is now solid again. Hour 6,378: the very centre — pressure pinning every atom in place.
3.15 NCERT Exercises
1. Multiple Choice Questions
(i) Which one of the following earthquake waves is more destructive?
- (a) P-waves
- (b) S-waves
- (c) Surface waves
- (d) None of the above
Answer: (c) Surface waves. They travel along the surface, displace rocks and cause structural collapse — making them the most destructive of the three families of seismic waves.
(ii) 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
Answer: (b) Volcanoes. Volcanic eruptions throw out lava and pyroclastics that can be analysed in the laboratory — physical material from inside the crust. Earthquake waves, gravity and magnetism are indirect sources because they reveal properties, not material.
(iii) Which type of volcanic eruptions have caused Deccan Trap formations?
- (a) Shield
- (b) Flood
- (c) Composite
- (d) Caldera
Answer: (b) Flood. The Deccan Traps are a flood-basalt province — vast outpourings of highly fluid lava that travelled hundreds of kilometres and built up thick stacked sheets, each often more than 50 m thick.
(iv) 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
Answer: (b) crust and upper mantle. The lithosphere is the rigid outer shell consisting of the crust and the rigid uppermost portion of the mantle — its thickness varies between 10 and 200 km.
2. Answer the following questions in about 30 words
(i) What are body waves?
Answer: Body waves are seismic waves generated at the focus that travel through the body of the Earth in all directions. They are of two types — P-waves (primary, faster, longitudinal) and S-waves (secondary, slower, transverse, solid-only).
(ii) Name the direct sources of information about the interior of the earth.
Answer: The direct sources are surface rocks, samples obtained from mining operations (such as the deep gold mines of South Africa), cores from ultra-deep boreholes (Kola — about 12 km), and lava and pyroclastic material erupted by volcanoes.
(iii) Why do earthquake waves develop shadow zones?
Answer: Because the Earth's interior is layered, seismic waves bend (refract) and reflect at boundaries. P-waves are refracted by the liquid outer core into a band-shaped shadow between 105° and 145°; S-waves cannot travel through the liquid outer core at all, so they leave a far larger shadow beyond 105°.
(iv) Briefly explain the indirect sources of information of the interior of the earth other than those of seismic activity.
Answer: Indirect sources (other than seismic) include: (1) the rate at which temperature, pressure and density change with depth in mines; (2) gravitational measurements — gravity anomalies reveal mass distribution in the crust; (3) magnetic surveys, which reveal the distribution of magnetic minerals; and (4) meteors, whose layered composition mimics the Earth's interior.
3. Answer the following questions in about 150 words
(i) What are the effects of propagation of earthquake waves on the rock mass through which they travel?
Answer: When seismic waves move through the rocks of the Earth, they cause vibration of every particle they touch. The exact effect depends on which type of wave is passing.
P-waves vibrate parallel to their direction of travel. They alternately push and pull the rock; each particle is first compressed and then stretched. As a result the wave creates regular pulses of high and low density along its path.
S-waves, by contrast, vibrate perpendicular to the direction of propagation. They generate sideways shaking and form troughs and crests in the rock. Because they require shear stiffness, they cannot pass through fluids — they are absorbed entirely by the liquid outer core.
Surface waves, generated when body waves reach the surface, also vibrate perpendicular to their direction. They are slow but the most destructive — they displace rocks at the surface and cause buildings to collapse. Reflection and refraction at boundaries between layers of different density account for the speed and direction changes recorded on seismographs.
P-waves vibrate parallel to their direction of travel. They alternately push and pull the rock; each particle is first compressed and then stretched. As a result the wave creates regular pulses of high and low density along its path.
S-waves, by contrast, vibrate perpendicular to the direction of propagation. They generate sideways shaking and form troughs and crests in the rock. Because they require shear stiffness, they cannot pass through fluids — they are absorbed entirely by the liquid outer core.
Surface waves, generated when body waves reach the surface, also vibrate perpendicular to their direction. They are slow but the most destructive — they displace rocks at the surface and cause buildings to collapse. Reflection and refraction at boundaries between layers of different density account for the speed and direction changes recorded on seismographs.
(ii) What do you understand by intrusive forms? Briefly describe various intrusive forms.
Answer: Lava that solidifies inside the crust — without ever reaching the surface — produces plutonic rocks shaped by the spaces it filled. These shapes are called intrusive forms.
Batholiths are huge granitic bodies that cool deep in the crust as massive domes; they cover large areas and are exposed only after long denudation.
Laccoliths are large dome-shaped intrusions with a level base, fed by a pipe-like conduit from below; the granite domes of the Karnataka plateau are classic examples.
Lapoliths form when lava settles into a saucer shape, concave to the sky.
Phacoliths are wavy intrusions found at the base of synclines or the top of anticlines in folded igneous country, with a definite conduit to a deeper magma source.
Sills and sheets are near-horizontal bodies — sheets are the thinner deposits, sills the thicker ones.
Dykes are wall-like vertical intrusions formed when lava solidifies in cracks and fissures; they are abundant in western Maharashtra and are believed to have been the feeders of the Deccan Traps.
Batholiths are huge granitic bodies that cool deep in the crust as massive domes; they cover large areas and are exposed only after long denudation.
Laccoliths are large dome-shaped intrusions with a level base, fed by a pipe-like conduit from below; the granite domes of the Karnataka plateau are classic examples.
Lapoliths form when lava settles into a saucer shape, concave to the sky.
Phacoliths are wavy intrusions found at the base of synclines or the top of anticlines in folded igneous country, with a definite conduit to a deeper magma source.
Sills and sheets are near-horizontal bodies — sheets are the thinner deposits, sills the thicker ones.
Dykes are wall-like vertical intrusions formed when lava solidifies in cracks and fissures; they are abundant in western Maharashtra and are believed to have been the feeders of the Deccan Traps.
4. Project Work
PW Locate the following on the outline world map and the map of India:
- Two continental volcanoes (e.g. Mt. Vesuvius — Italy; Mt. Fuji — Japan)
- One mid-ocean ridge volcano (e.g. Surtsey — North Atlantic)
- The Deccan Traps (Maharashtra plateau, India)
- The Kola superdeep borehole (Arctic, Russia)
- The Himalayan crust-thickening zone
Hint: Use a blank political world map. Mark Vesuvius near the Bay of Naples (Italy), Mt. Fuji on the south-east coast of Honshu (Japan), Surtsey to the south of Iceland, the Deccan Traps shading most of Maharashtra and parts of MP/Karnataka, the Kola borehole on the Kola Peninsula, and shade the Himalayan arc from Kashmir to Arunachal Pradesh.
3.16 Key Terms — Quick Recap
| Term | Quick Definition |
|---|---|
| Lithosphere | Crust + uppermost rigid mantle (10–200 km); locus of all natural earthquakes |
| Asthenosphere | Weak upper mantle, ≤400 km, source of magma |
| Mohorovičić discontinuity | Boundary between crust and mantle |
| Gutenberg discontinuity | Boundary between mantle and outer core (2,900 km) |
| Lehmann discontinuity | Boundary between liquid outer core and solid inner core |
| NIFE layer | Nickel + Iron core, very heavy |
| Caldera | Collapsed depression on the most explosive volcanoes |
| Flood basalt | Highly fluid lava sheets covering 1,000s of sq km, e.g. Deccan Traps |
| Batholith / Laccolith | Deep dome-shaped intrusions (granitic / fed by pipe) |
| Sill / Dyke | Horizontal intrusive sheet / vertical wall-like intrusion |
⚖️ Assertion–Reason Questions — Structure & Volcanism
Options:
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
Assertion (A): The continental crust is thicker beneath the Himalayas than beneath the Indian plains.
Reason (R): The mean thickness of the continental crust is around 30 km, but it can reach 70 km in major mountain systems like the Himalayas.
Answer: (A) — Both true; R is the precise explanation. Mountain roots make the crust extra thick.
Assertion (A): Hawaiian shield volcanoes have very gentle slopes.
Reason (R): They are built up of basalt, a lava that is highly viscous and traps explosive gases.
Answer: (C) — A is true. R is false: basalt at the temperatures of Hawaiian eruptions is one of the most fluid (least viscous) lavas, which is why it spreads broadly and the volcanoes have gentle slopes.
Assertion (A): Dykes are nearly vertical wall-like intrusive bodies.
Reason (R): Dykes form when lava forces its way through cracks and fissures and solidifies almost perpendicular to the ground.
Answer: (A) — Both true; R is the correct mechanism. Dykes of western Maharashtra are believed to be feeders of the Deccan Traps eruptions.
📝 Competency-Based Questions — Structure & Volcanic Landforms
A geology field-school is touring western Maharashtra. The students stand on a cliff of layered basalt that is hundreds of metres thick, and notice tall vertical dark walls of rock cutting straight through the cliff face.
Q1. Which volcanic landform are the students most likely to identify in (a) the layered basalt cliff itself, and (b) the tall vertical dark walls?
L2 UnderstandModel Answer: (a) The layered basalt cliff is part of the flood basalt province known as the Deccan Traps — sheets of highly fluid basaltic lava, sometimes more than 50 m thick. (b) The vertical dark walls are dykes, which formed when lava solidified inside cracks; they are believed to be the feeders that fed the trap eruptions.
Q2. Why does the asthenosphere — and not the crust — produce the magma that erupts from volcanoes?
L3 ApplyModel Answer: The asthenosphere is the upper, weak portion of the mantle (down to ~400 km). The combination of high temperature and slightly lower pressure here produces partial melting; the resulting molten rock is called magma. The crust, by contrast, is brittle and solid. So almost all magma ascends from the asthenosphere through fractures and conduits to feed surface volcanoes.
Q3. Compare a composite volcano with a shield volcano in terms of (a) lava viscosity, (b) explosivity and (c) shape of the cone.
L4 AnalyseModel Answer: (a) Shield volcanoes erupt very fluid basaltic lava; composite volcanoes erupt cooler, more viscous lava. (b) Shields show low explosivity (unless water enters the vent); composites are explosive and throw out pyroclastics and ash. (c) Shields have low, broad slopes (like a warrior's shield); composites build steep, layered cones of alternating lava and pyroclastic material.
HOT Q. A scientist measures gravity carefully along a 2,000-km transect across a continent and finds an unexpected positive gravity anomaly along part of the path. What might this tell her about the rocks below the surface?
L6 CreateHint: A positive gravity anomaly means more mass per unit volume than the textbook value — the rocks beneath that stretch are denser than expected. Possibilities include a buried ultramafic intrusion, a concealed batholith of dense rock, or a portion of dense oceanic-type crust trapped within the continent. Indirect signals like this one led geologists to map deep crustal structure long before drilling could reach it.
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