What this lesson teaches
I can describe Earth's layers and explain how convection and slab pull move tectonic plates.
Syllabus K335, Topic 4.1, Key Question 1. What is the plate tectoni theory?
Make a guess
An ocean plate is sinking into the mantle at one edge. How does that help move the whole plate?
- The continent pushes the ocean plate down.
- It does not; ocean currents move the plate instead.
- Gravity pulls the slab, which drags the plate.
- Melting at the edge makes the plate lighter.
Show the answer
Gravity pulls the slab, which drags the plate.
Cold, dense ocean plate sinks under gravity at a subduction zone. This slab pull drags the rest of the plate behind it.
Slow circulation in the mantle and the pull of sinking ocean plates move tectonic plates.
Heat inside Earth makes the mantle circulate very slowly. Hot rock expands, becomes less dense and rises. Near the top, it spreads out and cools. Cooler, denser rock sinks. This circulation is called convection.
The moving mantle drags on the plates above it. Where it rises and spreads, it helps push plates apart. Where it sinks, plates tend to move together. Real mantle flow is more complex than a simple diagram.
Slab pull happens when cold, dense ocean plate sinks under another plate. This sinking is called subduction. Gravity pulls the sinking slab down. The rest of the plate is dragged along behind it.
- Convection
- Hot mantle rises, cools and sinks in a slow loop.
- Subduction
- A dense ocean plate sinks under another plate.
- Slab pull
- Gravity on the sinking slab drags its plate along.
Step by step
Heat and rise
Hot mantle expands, becomes lighter and rises.
Cool and sink
Cooler, denser mantle sinks again.
Slab pull
Gravity pulls a sinking slab and drags its plate.
Real examples to use
Nazca Plate and the Peru-Chile Trench
Off South America, the ocean Nazca Plate sinks under the South American Plate along the Peru-Chile Trench. The sinking slab helps pull the Nazca Plate east towards the trench.
Worked example: Follow the force
An ocean plate is already sinking under a continent. Gravity pulls on the dense sinking slab. So the slab drags the rest of the ocean plate towards the trench.
Watch out for this
Slab pull happens because the continent pushes the ocean plate down.
The key force is gravity pulling on the dense sinking slab.
Check your understanding
Which explanation links slab pull to plate movement correctly?
- Hot magma under the continent pulls the slab towards the core.
- The weight of ocean water pushes the plate sideways.
- A dense slab sinks under gravity and drags its plate.
Show the answer
A dense slab sinks under gravity and drags its plate.
Yes. Gravity on the slab is the key force.