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

# Short note: Crust

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

- Asked in: 2068 Baisakh, 3 marks
- Video: [Earth's Crust and Mantle Explained: Crash Course Geology #3](https://www.youtube.com/watch?v=bouvNyq8xCY) — CrashCourse
- 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-crust
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-2

## Question

Write a short note on the crust of the earth.

## Short answer

The crust is the outermost thin solid layer (5-70 km), composed of igneous, sedimentary and metamorphic rocks. Continental crust is thicker, granitic (sial, 2.7 g/cm³); oceanic crust is thinner, basaltic (sima, 3.0 g/cm³). It forms the lithosphere with the upper mantle and is where all engineering and tectonic activity occur.

## Answer

The **crust** is the outermost, thin, rigid, solid layer of the Earth, lying above the Mohorovicic (Moho) discontinuity. It makes up less than 1% of Earth's volume.

#### Types
- **Continental crust**: thickness 30-70 km (thickest below the Himalaya); mainly granitic (rich in silica and aluminium, called sial); density about 2.7 g/cm³; old rocks, up to 4 billion years.
- **Oceanic crust**: thickness 5-10 km; basaltic (silica and magnesium, sima); density about 3.0 g/cm³; young, less than 200 million years.

#### Features
- Composed mainly of igneous, sedimentary and metamorphic rocks.
- Most abundant elements: oxygen (46.6%), silicon (27.7%), aluminium (8.1%), iron (5%), calcium, sodium, potassium, magnesium.
- Temperature rises about 25-30 °C per km depth.
- Together with the top of the upper mantle, it forms the **lithosphere**, which is broken into plates.

The crust is the part on which all civil engineering works are built, and it is the layer where earthquakes, folding and faulting occur.

## Check yourself

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

1. Oceanic crust is:
   A. Granitic, 30-70 km thick
   B. Basaltic, 5-10 km thick, about 3.0 g/cm3
   C. Peridotite
   D. Iron-nickel
   Answer: B. Continental is granitic. (Key point: Types)

2. Lithosphere consists of:
   A. Crust only
   B. Mantle only
   C. Crust plus top of upper mantle
   D. Core
   Answer: C. Broken into plates. (Key point: Lithosphere)

3. Most abundant crustal element:
   A. Iron
   B. Silicon
   C. Aluminium
   D. Oxygen
   Answer: D. 46.6% oxygen. (Key point: Elements)