1. Summary

In this video, we see how the Rocky Mountains were created by the movement of the Pacific and North American plates. We learn that parts of the range are up to 1000 miles away from the plate boundaries, which is unusual because plate boundaries are where mountain ranges are typically found. This video will explain how the remarkable movement of the Pacific and North American plates, which formed the Southern Rocky Mountains, differs from typical mountain building plate tectonics.

2. Why Watch This Video?

1. Have you ever wondered how mountains were made?

This video has helpful graphic representations of plates colliding to form mountain ranges, and it also has two different scientists who explain it in their own way. We get to see how fold mountains happened in different locations, and the variations on these occurrences.

2. Would you like to know how the Rockies are different than other mountain ranges?

In this video, geologists show us that although the general principles of convergent plate margins and subduction occurred when the Rocky Mountains were formed, some event triggered an unusual manifestation of these things.

3. Have you ever been confused by how plate movement affects the formation of mountains?

There are three different ways tectonic plates interact. They move past one another at transform margins, move away from one another at divergent margins, and move towards one another in convergent margins. In this video they show us that the Rocky Mountain belt occurred because of compression at convergent margins between the Pacific and North American plates, but they also outline how this compression was not typical.

3. Key Terms

• Convergent plate boundary – where two tectonic plates meet, and are moving towards each other
• Subduction – the movement of a tectonic plate which pushes the edge of it downward towards the mantle beneath another plate
• Continental crust – the layer of rock (igneous, sedimentary and metamorphic) which make up the outermost part of the earth. It also includes the parts of the seabed which meet the oceanic crust

4. Loose Ends

Loose end #1: Are the Rocky Mountains fold or fault block mountains?

The answer is that the Rockies are fold mountains. Fold mountains occur when two tectonic plates, in this case the Pacific and North American, are pushed together and the resulting compression and force creates an elevated area. The imagery in the first portion of this video shows the granite being pushed to the surface in a block, which can be confusing. The best illustration is later in the video where the geologist puts the cloth over his students, demonstrating the “folding” of the landscape.

Loose end #2: How are mountain ranges along plate boundaries typically formed?

This video highlights the anomaly of the Rocky Mountains, with a focus on those in the south. Mountain ranges like the Himalayas, Andes, and Alps are all active fold mountains that formed along the plate boundaries. These mountains formed when two plates converged and subduction occurred, with the difference being that one plate was pulled down more vertically towards the mantle (25-45 degrees) which created a range right where the plates met. The basic steps leading to theses mountain ranges being formed are the same, it’s the behavior of the subducted plates that differentiates them. The Rockies were formed during the Laramide Orogeny, which began 70 to 80 million years ago, and ended 35 to 55 million years ago.

Loose end #3: Why is there so much granite exposed in the Southern Rockies, but the Canadian Rockies are made of sedimentary rocks?

It can be confusing because they share a name, and on a map they look like they follow a similar path. The Canadian Rockies (which technically go into Montana) did not go through the same subduction process as their neighbors to the south, however. As shown in the video, the more horizontal subduction of the Pacific plate in the south pushed granite through the sedimentary layer on the surface over the course of millions of years. Conversely in what is now Canada, the Pacific plate was pulled down more vertically towards the mantle and the resulting fold was of the sedimentary rock closer to the top of the continental crust.

5. Self-Test Questions