planet

Plate tectonics

The arrangement Earth settled into: its cool outer shell is not one continuous lid but a dozen or so rigid plates that pull apart at mid-ocean ridges, slide past one another along faults, and sink back into the mantle at deep trenches. The part worth remembering is that plates are not rafts dragged along by currents underneath — they *are* the cold upper boundary layer of the convection, and most of the force moving them comes from the weight of their own sinking edge. It is the only regime in the Solar System that exactly one body has.