According to basic geography, mountains and volcanoes were formed by tectonic plates moving under the earth's surface. There are three motions, first is the plate pull away, second is they collide head on and third they collide and one slips over the other which causes an eruption from the earth's core, which is magma, leading to the formation of mountains and volcanoes.
The proof behind tectonic plates is the fact that south America and Africa when put side by side fit into each other perfectly, this can be seen occurring numerous times with numerous countries. If the plates are moving and the Earth is flat, there is no where for the plates to go. They would fall off the edge of the Earth.
If we completely disregard the existence of tectonic plates, how are there any volcanoes or mountains. If the Earth is flat, why is the entirety of the earth's crust not flat?