What force pushes upward on the continental crust?
Convection currents form from energy released from the core via nuclear processes that heat the mantle and cause it to rise in a convection current.
Convection currents, are thought to be one of the contributing factors in continental drift, but there is also the more likely theory of slab pull, which as the crust is subducted and cools, it pulls the the plate a long with it.
Hope this helps.
One factor is subduction zones.
As an ocean crust is pushed and pulled under a continental crust the continental crust is pushed upward.
Examples of the uplift caused by a subduction zone are the Andes, mountains, of South America, the Cascades of North America and the Sierra Nevada of California.
What caused the formation of mountains in the center of continents such as the Urals and Rocky mountains are much less certain. Also how the thick ocean sediments in the middle of continents were forced upwards from sea level is not understood in terms of present plate tectonic theories.