What moves the plates

Not floating on lava

Many people picture plates floating on a sea of molten magma. That isn't right. The mantle is almost all solid rock. Recall from Year 8:

• The lithosphere is the rigid outer shell: the crust plus the top of the mantle, roughly 100 km thick. It is broken into the tectonic plates.
• Below it, the asthenosphere is solid mantle rock so hot that it can slowly flow, a few centimetres a year, like very stiff toffee or the way a glacier flows. Only a tiny fraction of it is melted.

Plates still behave a bit like floating objects, because what matters is density. Continental crust is mostly granite-like rock (about 2.7 g/cm³), ocean crust is basalt (about 3.0 g/cm³), and the mantle below is denser again (about 3.3 g/cm³). So continents sit high, like a thick block of wood in water, and ocean crust sits lower. As ocean lithosphere moves away from a ridge it cools, shrinks and thickens, until after tens of millions of years it is dense enough to sink back into the mantle at a trench.

The forces on a plate

The energy that moves plates is the Earth's internal heat. It comes from heat left over from the planet's formation and from the radioactive decay of uranium, thorium and potassium in rocks (Chapter 1). Three effects turn that heat into plate movement:

• Mantle convection. Hot mantle rock is less dense, so it slowly rises. Near the top it spreads out, cools, becomes denser and sinks again. These slow convection currents carry heat out of the Earth, and the flowing mantle can drag on the base of the plates (mantle drag).
• Ridge push. At a mid-ocean ridge the hot lithosphere stands high. Gravity makes the plate slide gently down and away from the raised ridge, like a sheet sliding off a slight hump.
• Slab pull. At a subduction zone, old, cold, dense ocean lithosphere sinks into the mantle under its own weight (gravity) and drags the rest of the plate behind it, like a tablecloth sliding off a table when one end hangs over the edge. Most geologists now think this is the largest force.

The evidence for slab pull is the speed of different plates. Plates with long subducting edges, such as the Pacific, Nazca and Cocos plates, move fastest (7–10 cm a year). Plates with no slab pulling them, such as the African, Eurasian and Antarctic plates, move slowest (1–2 cm a year). The Australian Plate is unusually fast for one carrying a large continent because old ocean floor is sinking along its northern edge, under Indonesia.