Interior of the Earth: Crust, Mantle, Core Notes & 25 High-Yield MCQs for UPSC/SSC

Educational Earth structure diagram highlighting crust, mantle, outer core, and inner core, designed for UPSC and competitive exam geography revision.

Earth is not a solid ball of uniform material but a layered planet with distinct zones, each having unique composition, temperature, and behavior. Understanding these layers helps explain earthquakes, volcanoes, mountain building, and continental movement. The study of Earth’s interior connects geography, geology, and environmental science in practical, observable ways.

From the thin crust we live on to the deep, metallic core, Earth’s structure controls natural processes that shape daily life. Soil formation, mineral resources, groundwater storage, and tectonic hazards all depend on internal layering. Scientists use seismic waves, gravity data, and magnetic studies to understand hidden regions beneath feet.

How This Topic Is Useful for UPSC & Other Exams
Knowledge of Earth’s structure is essential for UPSC and other competitive exams because it links geography, environment, disaster management, and resource studies. Questions often test layers, seismic waves, plate tectonics, volcanoes, and earthquakes. Conceptual clarity improves map skills, analytical thinking, and understanding of current events related to natural hazards globally.

Cross-sectional diagram of Earth illustrating its four major layers and internal boundaries important for geography and competitive exams.

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Earth Layers – Comparison (Structure, Temperature & State)

Layer	Key Details
Crust	Thin outer surface; 0–700°C; solid rock.
Mantle	Thick moving layer; 700–4,000°C; semi-solid magma.
Outer Core	Metal layer: 4,000–5,500°C; liquid iron-nickel.
Inner Core	Central zone; 5,500–6,000°C; solid iron.
Note	The text should be visible in the compose view also.

Keyword Definitions (UPSC / SSC / RRB / SEBI / IBPS / NDA Exams)

• Crust: The outermost solid layer of Earth where humans live, containing continents and ocean floors. It is thin compared to other layers and composed mainly of silicate rocks rich in aluminum and silica.
• Mantle: The thick middle layer beneath the crust extending to great depths. It is composed of semi-solid silicate rocks rich in magnesium and iron, and convection currents here drive plate tectonic movements.
• Core: The innermost region of Earth divided into outer and inner core. It is mainly composed of iron and nickel, responsible for Earth’s magnetic field and internal heat generation.
• Outer Core: A liquid metallic layer surrounding the inner core. Movement of molten iron and nickel here generates Earth’s magnetic field through the geodynamo process, protecting life from harmful solar radiation.
• Inner Core: The solid central sphere of Earth composed mainly of iron and nickel. Extreme pressure keeps it solid despite very high temperatures comparable to the surface of the Sun.
• Seismic Waves: Energy waves produced by earthquakes or explosions that travel through Earth. Their speed and behavior help scientists understand internal structure, boundaries between layers, and material properties.
• P-Waves: Primary waves that travel fastest through Earth and can pass through solids, liquids, and gases. Their movement helps identify different internal layers and earthquake epicenters.
• S-Waves: Secondary waves that move slower than P-waves and can travel only through solids. Their absence in the outer core proves that the outer core is liquid.
• Moho Discontinuity: The boundary between the crust and mantle discovered through seismic studies. It marks a sudden change in rock composition and density, affecting wave velocity.
• Convection Currents: Circular movements of semi-molten material in the mantle driven by heat differences. These currents cause movement of tectonic plates, leading to earthquakes, volcanoes, and mountain formation.
• Plate Tectonics: Theory explaining movement of Earth’s lithospheric plates over the asthenosphere. It accounts for continental drift, ocean floor spreading, earthquakes, volcanic activity, and formation of major landforms.
• Asthenosphere: A semi-fluid layer of the upper mantle beneath the lithosphere. It allows tectonic plates to move slowly over it due to its plastic, deformable nature.

Dear Students, Mastering Earth’s structure builds strong basics for geography and disaster topics. Stay curious, revise diagrams, and connect concepts with real-world events like earthquakes and volcanoes.

MCQs on Earth Structure

1. Which layer of Earth is the thinnest?
A. Mantle
B. Core
C. Crust
D. Outer core

Answer & Explanation

The crust is the outermost and thinnest Earth layer, forming continents and ocean floors. Its small thickness compared to mantle and core makes it geologically fragile. Answer: C

2. Which layer is responsible for plate movement?
A. Inner core
B. Mantle
C. Crust only
D. Outer core

Answer & Explanation

Convection currents in the mantle generate forces that move tectonic plates above. Heat-driven circulation of semi-molten materials causes continental drift, earthquakes, and volcanic activity. Answer: B

3. Earth’s magnetic field mainly originates from:
A. Crust
B. Mantle
C. Outer core
D. Inner core

Answer & Explanation

The outer core contains moving molten iron and nickel. Their motion generates electric currents, producing Earth’s magnetic field through the geodynamo mechanism. Answer: C

4. S-waves cannot travel through:
A. Solids
B. Liquids
C. Rocks
D. Minerals

Answer & Explanation

S-waves require rigid material for transmission and cannot pass through liquids. Their absence in the outer core confirms its liquid state. Answer: B

5. The boundary between crust and mantle is called:
A. Gutenberg discontinuity
B. Moho discontinuity
C. Lehmann boundary
D. Core boundary

Answer & Explanation

The Mohorovičić discontinuity marks the boundary between crust and mantle, showing abrupt changes in seismic wave velocities due to different rock compositions. Answer: B

(MCQs 6–25 follow same Blogger-safe structure, covering P-waves, core composition, asthenosphere, mantle thickness, geothermal gradient, lithosphere, plate boundaries, and seismic evidence.)

6. Which seismic waves travel the fastest?
A. S-waves
B. Surface waves
C. P-waves
D. Love waves

Answer & Explanation

P-waves are primary seismic waves and travel fastest through Earth. They move in compressional motion and pass through solids, liquids, and gases, helping detect internal layers. Answer: C

7. The thickest layer of Earth is:
A. Crust
B. Mantle
C. Outer core
D. Inner core

Answer & Explanation

The mantle is Earth’s thickest layer, extending about 2,900 kilometers. It lies between crust and core, composed mainly of magnesium and iron-rich silicate rocks. Answer: B

8. The inner core is mainly composed of:
A. Silica and aluminum
B. Iron and nickel
C. Basalt
D. Magnesium silicate

Answer & Explanation

The inner core consists mainly of iron and nickel. Extreme pressure keeps it solid despite high temperatures, forming Earth’s dense central sphere. Answer: B

9. The semi-fluid layer below lithosphere is:
A. Mesosphere
B. Asthenosphere
C. Exosphere
D. Core

Answer & Explanation

The asthenosphere is a semi-molten, plastic layer beneath the lithosphere. It allows tectonic plates to move slowly, supporting continental drift theory. Answer: B

10. Which layer generates geothermal heat mainly?
A. Crust
B. Mantle
C. Core
D. Lithosphere

Answer & Explanation

Heat from radioactive decay and residual formation energy mainly comes from Earth’s core and mantle, driving geothermal gradient and tectonic processes. Answer: C

11. Continental crust is mainly made of:
A. Basalt
B. Sima
C. Sial
D. Iron

Answer & Explanation

Continental crust is rich in silica and aluminum, often called Sial. It is thicker and lighter than oceanic crust, forming continents. Answer: C

12. Oceanic crust is primarily composed of:
A. Granite
B. Basalt
C. Sandstone
D. Limestone

Answer & Explanation

Oceanic crust mainly consists of basaltic rocks rich in magnesium and iron. It is thinner and denser than continental crust. Answer: B

13. Gutenberg discontinuity separates:
A. Crust and mantle
B. Mantle and outer core
C. Outer and inner core
D. Lithosphere and asthenosphere

Answer & Explanation

The Gutenberg discontinuity marks the boundary between mantle and outer core, where seismic waves slow and S-waves disappear. Answer: B

14. Lehmann discontinuity lies between:
A. Crust and mantle
B. Mantle and outer core
C. Outer core and inner core
D. Lithosphere and mesosphere

Answer & Explanation

The Lehmann discontinuity separates outer core and inner core, discovered by seismic wave reflections indicating a solid inner core. Answer: C

15. Lithosphere includes:
A. Crust only
B. Mantle only
C. Crust and uppermost mantle
D. Core

Answer & Explanation

Lithosphere consists of crust and rigid uppermost mantle. It forms tectonic plates moving over the asthenosphere. Answer: C

16. Which wave causes maximum destruction?
A. P-wave
B. S-wave
C. Surface wave
D. Body wave

Answer & Explanation

Surface waves travel along Earth’s surface and cause maximum shaking and damage during earthquakes due to larger amplitudes. Answer: C

17. Convection currents occur in:
A. Crust
B. Mantle
C. Inner core
D. Atmosphere

Answer & Explanation

Mantle convection currents circulate semi-molten materials due to heat differences, driving plate tectonics and geological activities. Answer: B

18. Temperature increases with depth due to:
A. Solar radiation
B. Geothermal gradient
C. Wind pressure
D. Gravity

Answer & Explanation

The geothermal gradient refers to rising temperatures with depth, caused by internal heat from radioactive decay and core heat. Answer: B

19. Plate tectonics explains:
A. Weather changes
B. Mountain formation
C. Ocean tides
D. Seasons

Answer & Explanation

Plate tectonics theory explains mountain building, earthquakes, volcanoes, and continental drift through movement of lithospheric plates. Answer: B

20. Which layer is liquid?
A. Inner core
B. Mantle
C. Outer core
D. Crust

Answer & Explanation

The outer core is liquid due to extremely high temperatures melting iron and nickel, confirmed by S-wave absence. Answer: C

21. Main element in mantle rocks:
A. Iron
B. Magnesium
C. Silicon
D. All of these

Answer & Explanation

Mantle rocks contain silicates rich in iron, magnesium, and silicon, forming dense mineral structures. Answer: D

22. Which layer has highest density?
A. Crust
B. Mantle
C. Inner core
D. Asthenosphere

Answer & Explanation

The inner core has the highest density due to immense pressure compressing iron-nickel metals tightly. Answer: C

23. Evidence for Earth’s layers mainly comes from:
A. Satellites
B. Seismic waves
C. Soil study
D. Ocean tides

Answer & Explanation

Seismic wave behavior during earthquakes reveals Earth’s internal structure through changes in speed, reflection, and refraction. Answer: B

24. Which force drives mantle convection?
A. Wind
B. Solar heat
C. Internal heat
D. Water currents

Answer & Explanation

Heat from Earth’s core and radioactive decay causes density differences, driving mantle convection currents. Answer: C

25. The solid outer shell of Earth is called:
A. Asthenosphere
B. Lithosphere
C. Mesosphere
D. Core

Answer & Explanation

The lithosphere is the rigid outer shell comprising crust and uppermost mantle, forming tectonic plates. Answer: B

Conclusion 

Understanding Earth’s structure reveals how internal layers influence surface features, natural disasters, and resource distribution. From crust to core, each layer plays a vital role in shaping geography and environmental processes. Strong conceptual clarity helps in competitive exams and builds awareness of real-world geological events affecting human life globally.

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