---
title: "Earth cross-section: composition, density, temperature"
subject: "Engineering Geology I"
chapter: 2
key: "semester_3/engineering_geology_i:earth-crosssection-composition-density-temperature"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/earth-crosssection-composition-density-temperature
site: BCT Engineering
---

# Earth cross-section: composition, density, temperature

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

- Asked in: 2059 Chaitra (old course), 3+2 marks
- Video: [An Overview of Earth’s Layers](https://www.youtube.com/watch?v=n-ab1YPBdj8) — Professor Dave Explains
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/earth-crosssection-composition-density-temperature
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-2

## Question

Draw a neat cross-section of the interior of the earth and mention the composition, density and temperature variation within the earth.

## Answer

```text
  Surface
  +-----------------------------+
  | Crust    5-70 km            |
  |-----------Moho--------------|
  | Mantle   to 2900 km         |
  |---------Gutenberg-----------|
  | Outer core  (liquid)        |
  | Inner core  (solid)         |
  +-----------------------------+
```

| Layer | Depth (km) | Composition | Density (g/cm³) | Temperature |
|---|---|---|---|---|
| Crust | 0-10 (ocean) / 0-70 (continent) | Granite (Si, Al) / basalt (Si, Mg) | 2.7-3.0 | Rises to about 500-900 °C at the base |
| Upper mantle | to about 700 | Peridotite (Mg, Fe silicates) | 3.3-4.0 | About 1000-1500 °C |
| Lower mantle | 700-2900 | Dense Mg-Fe silicates and oxides | 4.0-5.5 | About 2000-3500 °C |
| Outer core | 2900-5150 | Liquid iron and nickel, some S/O | 9.9-12 | About 4000-5000 °C |
| Inner core | 5150-6371 | Solid iron and nickel | 12.8-13.5 | About 5000-6000 °C |

#### Variations
- **Composition**: changes from light silicate rocks outside to heavy metals inside.
- **Density**: increases with depth due to compression and heavier materials, from 2.7 to about 13 g/cm³; the mean is 5.5.
- **Temperature**: rises with depth. In the crust the geothermal gradient is about 25-30 °C per km; the rate drops below the crust, reaching about 5000-6000 °C at the centre.

## Check yourself

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

1. From outside to inside, composition changes from:
   A. Heavy metals to light rocks
   B. All silicate
   C. Light silicate rocks to heavy metals
   D. All gas
   Answer: C. Fe-Ni core. (Key point: Composition trend)

2. Density with depth:
   A. Decreases
   B. Constant
   C. Falls at core
   D. Increases from 2.7 to about 13 g/cm3
   Answer: D. Compression and heavier materials. (Key point: Density)

3. Crustal gradient is about:
   A. 25-30 °C per km
   B. 2-3 °C per km
   C. 250 °C per km
   D. 0
   Answer: A. Rate drops below crust. (Key point: Geothermal gradient)

## Related in Engineering Geology I

- [Apparent dip in cross-section drawing](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aapparent-dip-in-crosssection-drawing) — Engineering Geology I, chapter 5
- [Physical features of the earth's surface](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aphysical-features-of-the-earths) — Engineering Geology I, chapter 2, asked 3×

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