The Earth: Shape, Size, and Structure
Delhi Police & SSC Exams Preparation
1. Introduction
Our Earth is the third planet from the Sun and the only known planet to support life. It has a unique atmosphere, moderate temperature, and presence of water and oxygen that make life possible.
Key Study Areas:
- Shape: Oblate Spheroid/Geoid
- Size: Diameter, Circumference, Surface Area
- Structure: Crust, Mantle, Core
- Key Concepts: Discontinuities, Layers
Exam Significance:
Key concepts often asked in Delhi Police & SSC exams
Exam Trick: "Earth is not flat, it's a fat sphere that supports life everywhere!"
2. Shape of the Earth
Key Facts:
- The Earth is not a perfect sphere
- Equatorial diameter is slightly larger than polar diameter
- True shape is called an Oblate Spheroid or Geoid
Proofs of Earth's Spherical Shape:
Satellite photographs show round Earth
Ship disappears from bottom to top on horizon
Circumnavigation - travel around the world
Earth's shadow on Moon during lunar eclipse
Exam Trick: "S-C-S-E → Satellite, Curvature, Ship, Eclipse = Proofs of Spherical Shape"
3. Size of the Earth
| Measurement | Approx. Value | Note |
|---|---|---|
| Equatorial Diameter | 12,756 km | Largest at equator |
| Polar Diameter | 12,714 km | Shorter due to flattening |
| Difference | 42 km | Causes oblateness |
| Mean Radius | 6,371 km | Important for exams |
| Circumference (Equatorial) | 40,075 km | Around the equator |
| Total Surface Area | 510 million sq km | 71% water, 29% land |
Memory Line: "6-3-7-1 → Earth's radius fun run!"
Exam Trick: "Flattened at poles, fat at equator"
4. Structure of the Earth
The Earth's interior is divided into three major layers — Crust, Mantle, and Core.
| Layer | Depth Range | Composition | Key Points |
|---|---|---|---|
| Crust | 0–35 km | Rocks – SiAl (continental), SiMa (oceanic) | Thinnest, solid outer layer |
| Mantle | 35–2900 km | Silicates of iron and magnesium | 83% of Earth's volume |
| Core | 2900–6371 km | Nickel & Iron (NiFe) | Hottest and densest part |
Earth's Internal Layers Diagram
Crust (0–35 km)
Mantle (35–2900 km)
Core (2900–6371 km)
Exam Trick: "C–M–C → Crust, Mantle, Core → Cold, Molten, Core-hot"
5. Important Terms
| Term | Meaning |
|---|---|
| Lithosphere | Crust + uppermost rigid mantle |
| Asthenosphere | Semi-molten part below lithosphere |
| Moho Discontinuity | Between Crust & Mantle |
| Gutenberg Discontinuity | Between Mantle & Core |
| Lehmann Discontinuity | Between Outer & Inner Core |
Exam Trick: "MGL → Moho, Gutenberg, Lehmann = Mind Great Layers!"
6. Comparison Table – Crust, Mantle, Core
| Property | Crust | Mantle | Core |
|---|---|---|---|
| State | Solid | Semi-solid | Solid + Liquid |
| Density | Least dense | Moderate | Most dense |
| Material | SiAl, SiMa | Silicates | Nickel, Iron |
| Temperature | Coolest | Hot | Hottest |
PYQs (Delhi Police + SSC + State Exams)
The actual shape of the Earth is —
Options: (a) Perfect sphere (b) Oblate spheroid (c) Ellipsoid (d) Cylinder
Category: Earth's Shape
Show Answer
b) Oblate spheroid
The mean radius of the Earth is approximately —
Options: (a) 5,500 km (b) 6,371 km (c) 7,000 km (d) 5,800 km
Category: Earth's Size
Show Answer
b) 6,371 km
The difference between the equatorial and polar diameters of Earth is about —
Options: (a) 21 km (b) 42 km (c) 100 km (d) 500 km
Category: Earth's Size
Show Answer
b) 42 km
The outermost layer of the Earth is called —
Options: (a) Mantle (b) Crust (c) Core (d) Lithosphere
Category: Earth's Structure
Show Answer
b) Crust
The boundary between the Earth's Crust and Mantle is called —
Options: (a) Mohorovičić discontinuity (b) Gutenberg discontinuity (c) Lehmann discontinuity (d) Lithosphere
Category: Discontinuities
Show Answer
a) Mohorovičić discontinuity
The innermost layer of the Earth is —
Options: (a) Mantle (b) Core (c) Crust (d) Lithosphere
Category: Earth's Structure
Show Answer
b) Core
The core of the Earth is mainly made up of —
Options: (a) Iron and Nickel (b) Silica and Aluminum (c) Silica and Magnesium (d) Iron and Sulphur
Category: Composition
Show Answer
a) Iron and Nickel
The layer just below the crust is known as —
Options: (a) Lithosphere (b) Mantle (c) Asthenosphere (d) Outer core
Category: Earth's Structure
Show Answer
b) Mantle
The portion of the Earth where life exists is called —
Options: (a) Lithosphere (b) Atmosphere (c) Hydrosphere (d) Biosphere
Category: Earth's Spheres
Show Answer
d) Biosphere
Which one of the following statements is correct about Earth's shape?
Options: (a) Earth is a perfect sphere (b) Earth is flat at equator (c) Earth is slightly flattened at poles (d) Earth is cube-shaped
Category: Earth's Shape
Show Answer
c) Earth is slightly flattened at poles
Final Exam Tips
Remember all values and boundaries (Moho, Gutenberg, Lehmann)
Focus on composition and depth order
Diagrams and tricks can help in fast recall
Trick line: "Crust floats, Mantle flows, Core glows."
Master Geography for Delhi Police Exam!
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