The Rocky Mountains were primarily formed by tectonic plate collisions and the resulting uplift of the Earth's crust.

The Rocky Mountains' formation is primarily attributed to a period of mountain building called the Laramide Orogeny, which occurred roughly 80 to 55 million years ago. This process involved the collision and subduction (one plate sliding under another) of tectonic plates beneath the North American plate. This compression and uplift of the Earth's crust resulted in the formation of the massive mountain range we know today as the Rockies.