Slow mantle circulation and the pull of sinking oceanic slabs help move tectonic plates.
Heat inside Earth makes mantle material circulate slowly. Hotter material expands, becomes less dense and rises. Near the top it spreads and cools. Cooler, denser material sinks. This circulation is called convection.
Flowing mantle interacts with plates above it. In the school model, rising material spreads beneath plates and helps move them apart. Downward flow is associated with plates coming together. Real mantle circulation is more complex than the two loops in a simple diagram.
Slab pull begins when cold, dense oceanic lithosphere sinks beneath another plate. This sinking is subduction. Gravity pulls the slab down, and the attached plate is pulled along behind it. The sinking material is part of the plate itself.
Step by step
Heat and rise
Hotter mantle expands, becomes less dense and rises.
Cool and sink
Cooler mantle is denser and sinks, completing slow circulation.
Slab pull
Gravity pulls down a dense oceanic slab, dragging its attached plate along.
Worked example: Follow the force
Imagine a dense oceanic plate already descending beneath a continent. To explain further convergence, connect the words: gravity acts on the sinking slab, so it pulls the rest of the oceanic plate towards the trench.
Watch out for this
Slab pull happens because the continental plate pushes the oceanic plate down.
The key force in slab pull is gravity acting on the dense sinking slab. Name the force, the material it acts on and the resulting movement.
Check your understanding
Which explanation correctly links slab pull to movement?
- The ocean above pushes the entire plate sideways.
- Hot magma pulls the continent into the core.
- A dense slab sinks under gravity and pulls its attached plate along.