1. the mountain building and other changes associated, over hundreds of millions of years, with global-level plate tectonics. denudation and weathering processes, both those operating at the present time and those which have operated, with stunning long-term effect, over tens of millions of years.
2. Mountains are most often formed by movement of the tectonic plates in the Earth's crust. Great mountain ranges like the Himalayas often form along the boundaries of these plates. Tectonic plates move very slowly. It can take millions and millions of years for mountains to form.
