concept

Slab pull

The dominant force moving tectonic plates, and it comes from nowhere but the plate's own weight. Oceanic crust cools with age and after twenty or thirty million years is denser than the mantle beneath it. Once it starts to sink, its basalt metamorphoses into denser eclogite — heavier by some 400 kilograms per cubic metre — so the descending sheet hauls the rest of the plate along behind it. It accounts for over eight-tenths of the total force, which is why a plate's speed tracks the length of its sinking edge.