The theory of plate tectonics states that the earth's crust is separated into plates that float on the semi molten mantle below. Mountain ranges are formed at convergent plate margins where an oceanic plate subducts beneath a Continental plate. The continuous compression over time cause the rocks to be faulted and folded upwards into ranges. The Andes was formed in this way.
Continental plates also collide. When this happens no plate subducts instead they are both folded and faulted upwards. The Himalayan mountain range was formed this way.
