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

# Internal structure of the earth

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

- Asked in: 2080 Baisakh, 3 marks; 2079 Bhadra, 2 marks; 2078 Kartik, 2 marks; 2072 Chaitra, 2 marks; 2068 Chaitra, 3 marks; 2065 Shrawan (old course), 4 marks; 2066 Jestha (old course), 4 marks (asked 7 times)
- 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-the-earth
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-2

## Question

Describe the internal structure of the earth with a neat diagram.

## Short answer

Earth's interior consists of the crust (5-70 km thick, granitic continental and basaltic oceanic), the mantle (to 2900 km, ultramafic rock with a partly molten asthenosphere), and the core (iron-nickel, outer liquid from 2900-5150 km, inner solid to 6371 km). Seismic wave behavior reveals these layers.

## Answer

The Earth is divided into three main concentric layers, crust, mantle and core, on the basis of seismic wave behaviour. Its radius is about 6371 km.

```text
        _______________
      /   Crust (5-70)  \
     /  ______________   \
    /  /   Mantle      \   \
   |  |   (to 2900 km)  |   |
   |  |  __________     |   |
   |  | / Outer core \  |   |
   |  | | (liquid)   |  |   |
   |  | |  Inner     |  |   |
   |  | |  core      |  |   |
   |  | \__(solid)__/   |   |
    \  \______________/   /
     \___________________/
```

#### 1. Crust
- Outermost layer, thickness about 5-10 km under oceans and 30-70 km under continents.
- **Continental crust**: granitic (sial), density about 2.7 g/cm³.
- **Oceanic crust**: basaltic (sima), density about 3.0 g/cm³.
- Separated from the mantle by the **Mohorovicic (Moho) discontinuity**.

#### 2. Mantle
- Extends from the Moho to 2900 km depth; about 84% of Earth's volume.
- Made of ultramafic rock (peridotite), density 3.3-5.5 g/cm³.
- Upper mantle includes the partly molten **asthenosphere**; lithosphere = crust + top of upper mantle.
- Separated from the core by the **Gutenberg discontinuity**.

#### 3. Core
- From 2900 km to 6371 km; mostly iron and nickel (nife).
- **Outer core** (2900-5150 km): liquid, since S-waves do not pass through it.
- **Inner core** (5150-6371 km): solid due to high pressure; density about 13 g/cm³.
- Temperature reaches about 5000-6000 °C at the centre.

The structure is known from the speed and path of earthquake (seismic) waves.

## Check yourself

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

1. The three main layers of Earth are:
   A. Crust, asthenosphere, lithosphere
   B. Mantle, Moho, core
   C. Crust, mantle, core
   D. Sial, sima, nife only
   Answer: C. Divided on seismic behaviour. (Key point: Layers)

2. The outer core is liquid because:
   A. P-waves stop
   B. It is cold
   C. It is made of rock
   D. S-waves do not pass through it
   Answer: D. Shear waves cannot travel through liquids. (Key point: Outer core)

3. The boundary between mantle and core is the:
   A. Gutenberg discontinuity
   B. Moho
   C. Conrad
   D. Lugeon
   Answer: A. Moho separates crust and mantle. (Key point: Discontinuities)

4. Continental crust is:
   A. Basaltic, 3.0 g/cm3
   B. Granitic (sial), about 2.7 g/cm3
   C. Iron-nickel
   D. Peridotite
   Answer: B. Oceanic crust is basaltic and denser. (Key point: Continental crust)

## Related in Engineering Geology I

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

## Related in other subjects

- [8051 internal structure and family chart](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-internal-structure-family) — Embedded System, chapter 9