---
title: "Internal structure of earth and basis of study"
subject: "Engineering Geology I"
chapter: 2
key: "semester_3/engineering_geology_i:internal-structure-of-earth-and"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/internal-structure-of-earth-and
site: BCT Engineering
---

# Internal structure of earth and basis of study

Engineering Geology I · Chapter 2: Basic Reviews of the Earth

- Asked in: 2074 Asoj, 2+1 marks
- Video: [Different Layers of the Earth | It's Interior, Structure and Composition](https://www.youtube.com/watch?v=hmgR4PiGp1E) — Amit Sengupta
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/internal-structure-of-earth-and
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-2

## Question

Describe the internal structure of the earth with a suitable diagram. What are the bases of the study of the internal structure?

## Answer

#### Internal structure
The Earth consists of three concentric shells: crust, mantle and core.

```text
  Crust   : 5-70 km    (granitic/basaltic)
  ---------- Moho ----------
  Mantle  : to 2900 km (peridotite)
  ------- Gutenberg -------
  Outer core: 2900-5150 km (liquid Fe-Ni)
  Inner core: 5150-6371 km (solid Fe-Ni)
```

- **Crust**: thin outer rock layer; continental crust is granitic and thick (30-70 km), oceanic crust is basaltic and thin (5-10 km).
- **Mantle**: thick layer extending to about 2900 km depth; ultramafic rocks; the upper part (asthenosphere) is plastic and allows plate movement.
- **Core**: iron-nickel; outer core liquid, inner core solid; highest density and temperature.

#### Bases of study
The interior cannot be seen directly, so it is studied by:
1. **Seismic waves**: change of speed and direction at boundaries; S-waves stop at the outer core (liquid), P-waves are refracted and make shadow zones.
2. **Density**: the mean density of the Earth (5.5 g/cm³) is much higher than that of surface rocks (2.7 g/cm³), so the interior must be made of heavier material.
3. **Meteorites**: believed to represent the material of the Earth's interior.
4. **Volcanic eruptions** and xenoliths: samples from the mantle.
5. **Deep drilling and mines**: show temperature rise with depth (geothermal gradient).
6. **Magnetism and gravity**: the magnetic field indicates a liquid iron core.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. S-waves stopping at the outer core indicates it is:
   A. Solid
   B. Gas only
   C. Liquid
   D. Rocky
   Answer: C. Shear waves cannot pass liquids. (Key point: Shadow zone)

2. Earth's mean density 5.5 vs surface 2.7 suggests:
   A. Interior is lighter
   B. Earth is hollow
   C. Density is uniform
   D. Interior is heavier
   Answer: D. Denser interior. (Key point: Density evidence)

3. Earth's magnetic field indicates:
   A. A liquid iron core
   B. A solid mantle only
   C. A granitic crust
   D. No core
   Answer: A. Moving liquid iron generates field. (Key point: Magnetism)

4. Direct samples of mantle come from:
   A. Deep drilling to core
   B. Volcanic eruptions and xenoliths
   C. Satellites
   D. Fossils
   Answer: B. Drilling cannot reach the mantle. (Key point: Samples)

## Related in Engineering Geology I

- [Internal structure of the earth](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ainternal-structure-of-the-earth) — Engineering Geology I, chapter 2, asked 7×

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