Layers of the Earth – Complete Exam Notes
The Earth is composed of three main concentric layers based on chemical composition and physical properties:
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Crust
-
Mantle
-
Core
1️⃣ Crust
🔹 Key Points
- Outermost layer of the Earth
Oceanic crust: 5–10 km
Composed mainly of Silica (Si) + Aluminium (Al) → called SIAL
Density: ~2.7 g/cm³
Temperature: up to 400°C
Contains life-supporting elements
Rich in igneous, sedimentary, and metamorphic rocks
🔹 Important Facts for Exams
Oceanic crust is denser than continental crust
Oceanic crust is mainly made of Basalt
Continental crust is rich in Granite
2️⃣ Mantle
🔹 Key Points
- Lies below the crust
Constitutes about 84% of Earth's volume
Rich in Silica + Magnesium → called SIMA
Density: ~3.3–5.5 g/cm³
Temperature: 500°C to 4,000°C
🔹 Subdivisions
Upper MantleIncludes Asthenosphere
Semi-molten and plastic
Responsible for plate tectonics
🔹 Important Facts
-
Magma originates from the upper mantle
-
Asthenosphere allows lithospheric plates to move
3️⃣ Core
🔹 Key Points
-
Innermost layer
-
Thickness: ~3,500 km
-
Rich in Nickel (Ni) + Iron (Fe) → called NIFE
-
Temperature: up to 6,000°C
-
Density: ~9.5–13 g/cm³
🔹 Subdivisions
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Outer Core
-
Liquid
-
Responsible for Earth’s magnetic field
-
-
Inner Core
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Solid
-
Extremely high pressure
-
🔹 Important Facts
-
Core generates Geomagnetic Field
-
S-waves do not pass through the outer core
🔑 Discontinuities (Very Important for Exams)
| Discontinuity | Separates |
|---|---|
| Conrad | Upper & Lower Crust |
| Mohorovičić (Moho) | Crust & Mantle |
| Gutenberg | Mantle & Core |
| Lehmann | Outer Core & Inner Core |
🧠 Seismic Wave Evidence (Highly Asked)
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P-waves: Travel through solids, liquids, gases
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S-waves: Travel only through solids
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Absence of S-waves in outer core → proves it is liquid
📘 MCQs: Layers of the Earth (4 Options)
Q1. Which layer of the Earth contains SIAL?
A. Core
B. Mantle
C. Crust
D. Asthenosphere
✅ Answer: C
Q2. The Earth’s magnetic field is generated by:
A. Inner core
B. Outer core
C. Mantle
D. Crust
✅ Answer: B
Q3. Which discontinuity separates the crust and mantle?
A. Gutenberg
B. Lehmann
C. Conrad
D. Mohorovičić
✅ Answer: D
Q4. Asthenosphere is located in:
A. Crust
B. Upper Mantle
C. Lower Mantle
D. Outer Core
✅ Answer: B
Q5. Which seismic waves cannot travel through the outer core?
A. P-waves
B. Surface waves
C. S-waves
D. Love waves
✅ Answer: C
Q6. Which layer has the highest temperature?
A. Crust
B. Mantle
C. Outer core
D. Inner core
✅ Answer: D
Q7. Basalt is mainly found in:
A. Continental crust
B. Mantle
C. Oceanic crust
D. Core
✅ Answer: C
Q8. Gutenberg discontinuity lies between:
A. Crust & Mantle
B. Mantle & Outer Core
C. Outer Core & Inner Core
D. Upper & Lower Mantle
✅ Answer: B
Q9. The plastic behavior of the mantle is due to:
A. Low pressure
B. High temperature
C. Semi-molten rocks
D. Presence of iron
✅ Answer: C
Q10. Which layer forms the largest volume of Earth?
A. Crust
B. Mantle
C. Outer Core
D. Inner Core
✅ Answer: B