The geological process responsible for the formation of mountains is called tectonic activity, specifically plate tectonics. Mountains typically form at convergent plate boundaries, where two tectonic plates collide. This collision can cause the Earth's crust to buckle, fold, and uplift, leading to the creation of mountain ranges. An example is the Himalayas, formed by the collision of the Indian and Eurasian plates. Other processes involved in mountain formation can include volcanic activity, where magma pushes up through the Earth's crust, and erosion, which shapes existing mountains over time.