How convection moves plates Large-scale convection within the mantle causes new crust to rise at rifts and diverge, eventually plunging at trenches. A problem with the convection theory Analysis of earthquake waves reveals that the layered mantle structure would be destroyed by large-scale convection. How gravity moves plates from the ridge axis Mantle plumes elevate oceanic ridges, and gravity causes plates to slide off oceanic ridges and into trenches. How gravity aids in subduction Since the descending slab is actually denser than the surrounding asthenosphere, it can be pulled downward. The next time Africa moves toward us, oceanic crust will likely subduct beneath North America, rather than ram into us.
Faults fracture with movement.
When the stress along a fracture becomes great enough on one side of a fracture relative to the other, motion happens. At greater depths, rocks normally flow and fold rather than fracture. How elastic rebound causes quakes- Friction prevents motion along fault. This snap is the earthquake. The epicenter lies just above the focus.