---
title: "Short note: Mantle"
subject: "Engineering Geology I"
chapter: 2
key: "semester_3/engineering_geology_i:short-note-mantle"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/short-note-mantle
site: BCT Engineering
---

# Short note: Mantle

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

- Asked in: 2075 Chaitra, 2 marks
- Video: [The Earth's Mantle is Nothing Like You Thought](https://www.youtube.com/watch?v=RLQpuPqDEfM) — SciShow
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/short-note-mantle
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-2

## Question

Write a short note on the mantle of the earth.

## Short answer

The mantle is Earth's thickest layer (to 2900 km, 84% volume), made of solid peridotite (Mg-Fe silicates) that flows slowly. It includes the plastic asthenosphere; P and S waves pass through it. Convection currents in the mantle drive plate tectonics.

## Answer

The **mantle** is the thick layer of the Earth between the crust and the core. It lies below the Mohorovicic discontinuity (about 5-70 km) and extends to 2900 km, where the Gutenberg discontinuity separates it from the core. It forms about 84% of Earth's volume.

- **Composition**: dense ultramafic silicate rock, mainly peridotite, rich in magnesium and iron (olivine, pyroxene).
- **Density**: about 3.3 g/cm³ at the top to 5.5 g/cm³ at the base.
- **Temperature**: about 1000 °C at the top to over 3000 °C at the base.
- **Divisions**: upper mantle (to about 700 km, including the plastic asthenosphere) and lower mantle (700-2900 km).
- Slow convection currents in the mantle are believed to move the lithospheric plates.
- Both P and S waves pass through it, so it is solid, though it flows very slowly.

## Check yourself

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

1. The mantle extends to:
   A. 2900 km
   B. 700 km
   C. 5150 km
   D. 6371 km
   Answer: A. Gutenberg discontinuity. (Key point: Extent)

2. The mantle is mainly:
   A. Granite
   B. Peridotite
   C. Basalt
   D. Iron-nickel
   Answer: B. Ultramafic silicate. (Key point: Composition)

3. Why is the mantle considered solid?
   A. S-waves stop in it
   B. Only P passes
   C. Both P and S waves pass through it
   D. It is molten
   Answer: C. S-waves travel only in solids. (Key point: Solid)