Very slow currents in the relatively plastic lower mantle, or aesthenosphere, are thought to push the crustal plates along and drive the process of plate tectonics. These currents are caused by convection, with the mantle rock being heated from below via radioactive decay and thus becoming less dense than the rock above. These hotter rocks move slowly upward, displacing the rock above and forcing it down in an elliptical motion.
The movement of plates is extremely slow, from a human perspective, with each plate moving relative to the others only centimeters or less a year. Seven major moving plates are recognized by plate tectonics. Each boundary between the plates has characteristic features based on how they move relative to each other. Oceanic plates, or the crustal plates at the bottom of the oceans, spread apart from one another. This produces a chain of volcanoes winding through the oceans of the world, such as the mid-Atlantic ridge, where mantle material leaks up through the separation of the plates.
Where the spreading oceanic plates encounter continental plates, they tend to plunge under, creating ocean trenches at what are known as subduction zones. When two continental plates meet, they tend to wrinkle and create mountain ranges, such as the Himalayas or the Appalachian Mountains.