Why are some mountains much taller than others?

The tallest mountain ranges usually form at convergent plate boundaries, where crust is crumpled, thickened, and pushed upward. The Himalayas began forming about 50 million years ago when the Indian Plate collided with the Eurasian Plate, and that collision is still ongoing. Mountains can become very high if uplift from plate motion, volcanism, or crustal thickening happens faster than erosion can wear them down. That is why young, active ranges like the Himalayas and Andes are much taller than older ranges that have been eroded for hundreds of millions of years.