What force pushes upward on the continental crust?

The continental crust is pushed upward as an oceanic crust is tugged and pushed beneath it.

The Sierra Nevada in California, the Cascades in North America, and the Andes mountains in South America are a few examples of uplift brought on by a subduction zone.

Less is known about the processes that led to the formation of the mountains in the middle of continents, like the Urals and Rocky Mountains, and how the thick ocean sediments in the middle of continents were driven upward from sea level, according to current plate tectonic theories.

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.

-Charlie

Buoyant force or upthrust.