Formation of the Rocky Mountains: Learn Interesting Secrets About Mountain Geology that Caused Them to Form

An Introduction to the Rocky Mountains

Mount Bonneville in the Rocky Mountains Range

Orogeny is the process of mountain building. These processes are complex and take millions of years to shape and form mountains. The formation of the Rocky Mountains is believed to have taken place in the Early Cenozoic and Mesozoic eras some 50 to 80 million years ago during a period known as Cordilleran orogeny.

The remnants of the old mountain system occurred in a series of pulses some one million years ago. Erosion has since exposed the crystalline cores and carved out valleys and peaks.

One of the most important processes in forming the magnificent chain is plate tectonics. Plate tectonics is a theory that states that beneath the surface of the Earth are plates upon which continents sit. These plates are constantly moving and at some point the entire landmass of the Earth was one giant super continent called Pangaea. When Pangaea broke apart, the plates shifted and this shifting caused the collision of the plates into one another. The Rocky Mountains are believed to have been formed during these active movements of the plates.

The entire process took place in three major uplifts or mountain building stages and the final uplift, the Laramide orogeny, is responsible for raising the Rocky Mountains. Let us examine these three stages and see how the Rocky Mountains were formed.

First Uplift

Slabs of Sedimentary Rocks in the Rocky Mountains

The first uplift generally refers to the deposition of ancestral rocks in the Rocky Mountain region. Sandstone, shale and siltstone were the original rocks deposited in the region by a prehistoric sea some two billion years ago.

Precambrian metamorphic rock is the oldest rock and there are also deposits of volcanic rocks. Argilite, a sedimentary rock, is also found in the region dating back 1.7 billion years. The sedimentary rocks were trapped between the tectonic plates some 1.7 billion years ago.

As a result of the collision, these sedimentary rocks were transformed within the core of a Proterozoic mountain range into metamorphic rocks by large amounts of heat and pressure, The Proterozoic mountains, over a period of 1.3 billion to 500 million years ago, gradually eroded to expose the metamorphic rocks and granite.

Second Uplift

Nearly 500 million years ago, the flattened region was covered by shallow seas. Hundreds of thousands of feet of Paleozoic sedimentary rocks were deposited onto the surface. During the Pennsylvanian sub-period, the rocks of the southern Rocky Mountains were disturbed due to mountain building processes. These processes created the Ancestral Rocky Mountains. Two large mountainous islands were created consisting of Precambrian metamorphic rock. These mountains, however, eroded leaving large deposits of sedimentary rocks. Towards the end of the Mesozoic, approximately 10,000 to 15,000 feet of sediment had accumulated in the region. The region now known as the Rocky Mountain Park was eroded again and for a brief period covered by seas from the middle of the Permian period to the end of the Cretaceous period some 65 million years ago.

Third Uplift

Shallow Subduction Laramide Orogeny

The main collisions of the major tectonic plates occurred some 130 million years ago on the western edge of North America. This uplifting affected the area surrounding the Colorado Rockies.

The Laramide orogeny was responsible for raising the Rocky Mountains. In this process, the angle of subduction became shallow resulting in the underlying oceanic lithosphere (crust) to cause a drag on the overlying continental lithosphere. The shallow angle of the subducting plate resulted in a major increase in friction and interactions with the continental mass above.

Thrust faults are breaks within the surface of the Earth where rocks of lower strata are pushed above rocks of higher strata. Due to the friction of the subducting plate with the continental mass above, huge thrusts were generated, which caused sheets of crust to pile on top of one another resulting in the formation of the extraordinary Rocky Mountains.

Current Rockies

The present state of the Rockies is due to processes that lasted millions years of years. Erosion due to wind and rain is constantly redefining the terrain.

The Rockies are a popular tourist attraction and are visited by millions of visitors each year. People like to go hiking on the mountain ranges such as Pikes Peak or visit some of the popular national parks such as Yellowstone National Park and Banff National Park. Skiing is quite also popular during the winter and there are many resorts located in the United States as well as in Canada around the mountain ranges.

The Rockies represent some of the ancestral rocks that formed them. There are extensive collections of granite dating back to the Ancestral Rockies such as Precambrian mudstones along with Paleozoic limestones and dolomites. They are a reminder of the awesome forces of nature that have shaped this extensive and spectacular mountain range.


1. Topinka, Lyn, "America's Volcanic Past," USGS,

2. Cameron, Ward, "The Formation of the Rocky Mountains", Mountainnature,

3. Perkins, Sid, "How the Rocky Mountains Got its Midsection", ScienceNow,

Image Credits:

Bonneville Basin Mount Bonneville by Philthy 45 at en.Wikipedia under CCA 3.0

Roxborough by Attribution: Montano336 at the English Language Wikipedia under CCA 3.0

Shallow subduction Laramide orogeny by Melanie Moreno and used courtesy of the U.S. Geological Survey under Public Domain