CAPF AC 2021 Q105: Geography – Physical Geography & Geomorphology
Convection currents, which move the tectonic plates, are found in which layer of the earth?
- (a) Crust
- (b) Mantle
- (c) Outer core
- (d) Inner core
Answer: (b) Mantle
Explanation
Where the convection currents are
How to think: tectonic plates are slabs of the lithosphere.
They float and move on the hot, slowly flowing rock below them.
That rock belongs to the mantle.
| Layer | State | Role |
|---|---|---|
| Crust | Solid, thin | Forms the top of the plates |
| Mantle | Hot, slowly flowing rock (asthenosphere in the upper part) | Convection currents move the plates |
| Outer core | Liquid iron and nickel | Its flow creates the Earth's magnetic field |
| Inner core | Solid iron and nickel | Hottest part |
How it works: heat from radioactive decay makes hot rock rise.
It spreads sideways under the plates, cools and sinks again.
This idea was put forward by Arthur Holmes in the 1930s.
Answer: (b) Mantle.
Why each option is right or wrong
- (a) Incorrect. The crust is the rigid top layer that is carried along; it does not convect.
- (b) CORRECT. Convection currents in the mantle drive the tectonic plates.
- (c) Incorrect. Flow in the outer core produces the magnetic field, not plate movement.
- (d) Incorrect. The inner core is solid.
Key point
Convection currents in the mantle (Arthur Holmes, 1930s) drive plate movement. Lithosphere = crust + uppermost solid mantle; it rests on the weak asthenosphere. Outer core flow → geomagnetism.
Remember
Mantle convection moves the plates.
Common mistakes
- Marking the outer core because it is the liquid layer.
- Thinking the plates move because the crust itself flows.
- Mixing up the cause of plate movement with the cause of the magnetic field.
Exam tip
Educated guess: the plates must be moved by the layer directly beneath them. Below the crust lies the mantle.
Sources
- NCERT Class XI Geography — Fundamentals of Physical Geography, Chapter 4: Distribution of Oceans and Continents (convectional current theory)
- NCERT Class VII Geography — Inside Our Earth