Final answer:
Mountain building causes metamorphism by pushing shallow rocks to depths where they get heated.
Step-by-step explanation:
Mountain building causes metamorphism because mountains are heavy and they push shallow, cool rocks to depths where they get heated. When two continental masses collide, the intense pressure and folding of the Earth's crust can drag rocks deep below the surface, subjecting them to high temperatures and pressures. This process, known as tectonic or regional metamorphism, can cause the minerals in the rocks to rearrange and form new minerals.