Both plate boundaries and plates move over time. As previously described, plates can change the locations of trenches and subduction zones, as well as the positions of midoceanic ridges and transform faults. For example, subduction at a convergent boundary can stop in one location and begin nearby in another. Plates can become larger or smaller over time depending on the generation rates of new crust at spreading centers and the rates of subduction.
Convection currents. Some geologists favor convection currents in the mantle as the best explanation for plate tectonic movement. It is reasonable to assume that the heat radiated from the core creates convection currents in the mantle, and the mantle rocks begin to move plastically. Convection movement in the uppermost layers of the mantle may pull on the lithospheric rocks, breaking them into huge plates that move slowly on the more plastic, lubricated surface of the asthenosphere. Another possibility is that the 670‐kilometer boundary in the mantle breaks the convection pathways into an upper and lower part; convection in the lower part may induce the convection currents in the upper part (less than 670 kilometers deep) that move the plates. Still others believe that plate motion on the surface creates the underlying mantle convection—that is, when plates diverge, hotter mantle rock rises upward to fill the space between the plates, helping to push them apart; as the plates move away from the spreading center they cool and begin to sink, creating downward currents.
Mechanisms of plate movement and subduction. Three mechanisms (in order of importance) have been proposed to explain why plates move apart and subduct: slab‐pull, ridge‐push, and trench‐suction. Slab‐pull is the result of a plate subducting at a steep angle through the mantle; this downward motion tends to pull the other side of the plate away from the ridge crest. The ridge‐push theory maintains that new crust cools as it moves away from the ridge, becoming more dense, sinking, and forming a slope on the midoceanic ridge. A parallel slope may develop underneath the plate at the base of the lithosphere. These surfaces are zones of weakness that help the plate move away from the ridge. When a plate is subducted at a steep angle, it also creates trench‐suction that pulls the overlying plate, and the trench, toward the ridge.