ROCKY MOUNTAINS

From this website's page, "Tectonic Plates," you may recall that the Earth's rocky, outermost layer, the lithosphere, comprises seven major sections called tectonic plates. These rigid tectonic plates are massive slabs of rock underlying our continents and oceans. The tectonic plates rest — or, in a sense, "float" — on the hot, softer, semi-solid rock beneath them. You can imagine North America as a giant, slow-moving raft floating on molten (semi-melted) rock. Beneath this raft are massive, powerful forces pushing and pulling over millions of years. These forces cause tectonic plates to move around at an average rate similar to human fingernail growth, about two inches per year. However, the movement of tectonic plates can range from less than half an inch to nearly six inches per year.

     About 80 to 55 million years ago, during the Laramide Orogeny,* something unusual happened: a tectonic plate underlying the Pacific Ocean — the Farallon Plate — began sliding underneath the western edge of North America. When a tectonic plate slides under another tectonic plate, the plate underneath usually descends into Earth's mantle — gradually and steeply — at or near the edge of the upper plate.

       But in the case of the Farallon Plate, it sank under the North American Plate not at a steep angle but at a very shallow angle, "scraping" against North America's underside as it (the Farallon Plate) moved east. The pressure, stress, and friction between these two tectonic plates reached deep into the North American continent — hundreds of miles inland. That underground pressure, stress, and friction significantly deformed Earth's crust in the American West — faulting, folding, crumpling, and shoving it up. That pressure, stress, and friction raised vast blocks of land and created the Rocky Mountains.

     Later, erosion shaped the mountains further — wind, rain, glaciers, and rivers carved them into the rugged, dramatic peaks we know today.

* An orogeny is the process of mountain formation, especially by a folding and faulting of the earth's crust.